Diane, Justyna, Hanaa, Ashraf, Wenzhang, Ayman, Wissam Quoc Khaled

An

Soraya

Kamila

Bhagyesh

Natalia Alexandre

Aleksandra

Miguel Lucie

Dolores

Shefali, Oleksandra... Antonio Amar

Laurent Djakovitch

Alexander Sorokin

Michèle Besson

Ana Rita

Morgado Prates

Zahraa Shahin

Marwa Abo Hamdan

Thomas

Eugénie Alyssa Santiago

Corentin, Rémy, Christophe ... Hors Sabine Ecaterina

Youssry, Islam Melissa Farah

Keywords: Zinc oxide, zinc aluminium oxide, Cu/ZnAlOx, hydrogenolysis, sol-gel, DPU, 1,2-PDO

This work focuses on the synthesis of novel hybrid ZnO and ZnAl2O4 NPs by sol-gel method as support for the copper based catalysts and the evaluation of their catalytic performance in the hydrogenolysis of glycerol into 1,2-propanediol in aqueous phase using batch reactor.

The copper-based catalysts were prepared by deposition precipitation with urea (DPU method) and the commercial ZnO was used as benchmark to study the effect of the elaborated supports. The effect of the synthesis method was studied and a one pot method, similar to coprecipitation, was applied for the elaboration of these catalysts. The effects of the synthesis parameters were studied and the catalysts elaborated were robust and competitive in terms of conversion (96%) and selectivity (83%) comparing to the catalysts reported in the literature.

Moreover, hybrid ZnO and ZnAl2O4 presented high specific surface area (in the range of 100 and 100 -220 m².g -1 ), only few being reported in the literature with the sol-gel method, with nanoparticles self-assembled in 3D spherical shapes resistant to high calcination temperatures as shown by TEM.

Résumé :

Ce travail de thèse porte sur le développement de nanomatériaux hybrides de ZnO et de ZnAl2O4 utilisés comme supports de catalyseurs à base de Cu° pour l'hydrogénolyse du glycérol en 1,2-propanediol dans un réacteur batch et en phase liquide.

Plusieurs voies de synthèses ont été optimisées dont la méthode de dépôt précipitation en présence d'urée (DPU) et du ZnO commercial a été utilisé comme référence pour évaluer les caractéristiques des supports élaborés. Une méthode de dépôt innovante dite « one-pot », qui consiste à élaborer le support et le catalyseur dans le même réacteur, a été mise au point pour pallier des difficultés de reproductibilité par la méthode DPU. Cette voie de synthèse a permis d'élaborer des catalyseurs robustes et très compétitifs en terme de sélectivité (83%) et de conversion (96%) par rapport aux catalyseurs trouvés dans la littérature. Les supports et les catalyseurs élaborés ainsi présentent des caractéristiques physico-chimiques importantes surtout au niveau de leurs grandes surfaces spécifiques (de l'ordre de 100 -220 m².g -1 ), peu courantes dans la littérature, permettant une dispersion importante des sites catalytiques.

Mots-clés : Oxyde de zinc, oxyde de zinc aluminium; Cu/ZnAlOx; hydrogénolyse; sol-gel; DPU; 1,2-PDO.

List of Abbreviations

Introduction

The depletion of fossil fuel reservoirs, together with the indispensable shift of the feedstock towards a biorefinery approach, necessitates the use of renewable resources on a large scale for the production of chemicals and fuel. Thus, the development of efficient and environmentally friendly technologies has become an imperative. The limitation of this concept is the high cost of the production of biofuel. In order to valorize this production, the co-products should be recovered and valorized. In the case of the production of biofuel via the transesterification of vegetables oils, glycerol is produced in large quantity (10 molecules of glycerol for 1 molecule of biodiesel) estimated to reach 42 billion liters in 2020 1 .

Glycerol is one of the most versatile and valuable chemical substances known, with more than thousands of uses and applications 2 . Its physical and chemical properties make it a versatile organic compound. Food products, cosmetics, toiletries, and drugs are just a few of the products in which glycerol is employed either as a constituent of the processing or as a major reactant.

Glycerol can be converted by catalytic hydrogenolysis into ethylene and propylene glycols (1,2-propanediol and 1,3-propanediol), which are important chemical platform for various applications 3 . Hydrogenolysis reactions, which results in the cleavage of C-C and C-O bonds, offer promising possibilities for future biorefinery processes. This reaction is currently intensively discussed in the context of the transformation of glycerol to ethylene and propylene glycols, but it also has great potential in regard to the conversion of biomassderived polyols, such as sugars or sugar alcohols (sorbitol, xylitol, erythritol…). As multiple C-O and C-C bonds cleavage can occur, along with other side-reactions, selectivity in hydrogenolysis reactions remain a major challenge.

In the literature different systems have been studied for the conversion of glycerol depending on the desired product. The hydrogenolysis reaction can be done using water as solvent, in a batch reactor under hydrogen pressure and with the addition of noble metal catalysts. Rh 4 , Ru 5 and Pt 6 show high yield of conversion and variable selectivity toward 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO). Noble-free systems have the advantage of limiting the C-C bond cleavage, in favor of C-O bond cleavage 7,8 .

In our case we focused on the hydrogenolysis of glycerol into 1,2-propanediol (1,2-PDO), due to the large application of this molecule. 1,2-PDO is used as chemical feedstock for the production of unsaturated polyester resins. Related to this application, 1,2-PDO reacts with propylene oxide to give oligomers and polymers that are used to produce polyurethanes. 1,2-PDO is used in water based acrylic architectural paints to extend drying time due to its slower evaporation rate compared to water 9 . It is also used as a humectant, solvent and preservative in food and for tobacco products. In addition, it is one of the major ingredients, along with glycerol, of the e-liquid and cartridges used in electronic cigarettes where it is aerosolized in the atomizer. Last but not least, it is used in model railway and model boat applications to simulate steam or smoke 10 . Besides, it can be used in various consumable items such as coffee-based drinks, liquid sweeteners, ice cream, whipped dairy products and soda and as a solvent in many pharmaceuticals, including oral, injectable and topical formulations, such as for diazepam and lorazepam which are insoluble in water 11 .

In literature noble-metal free systems have attracted attention for the transformation of glycerol into 1,2-PDO in the last decades, due to their low cost and abundance. Cu-based nanoparticles (NPs) have generated a great interest in recent years, especially in the field of catalysis. The possible modification of the chemical and physical properties of these nanoparticles using different synthetic strategies and conditions and/or via post-synthetic chemical treatments has been largely responsible for the rapid growth of interest in these nanomaterials and their applications in catalysis 12 . Copper is a non-noble metal, known for its selectivity for the cleavage of the C-O bond and its low price compared to noble metals.

Additionally, Cu's high boiling point makes it compatible with high temperature and pressure chemical reactions. Such properties are important for the development of reactive, selective and stable catalytic systems 13 . Zinc oxide is a semiconductor which is used in extensive domains of industrial applications such as catalysis, antibacterial coating, UV sunscreen formulations… These applications require high specific surface area (nanometric materials) and the development of reproducible, efficient and easy-scalable industrial production processes 14 .

Our novel patented industry-capable (in terms of legislation concerns) and cost effective chemical solution approach (PCT WO 2016/038317 A1) 15 from ZnEt 2 and polyacrylic acid (PAAH) led to mesospheric self-assembly hybrid ZnO-PAAH nanomaterials that can be used as original nanostructured and very high surface area ZnO support (> 100 m².g -1 ).

Therefore, the main objective of our project was to study the interest our new hybrid ZnO-PAAH supports in the elaborations Cu/ZnO-or Cu/ZnAlO x based noble free catalysts that could be used for the hydrogenolysis of glycerol.

In the first chapter, a review on the literature was done starting from the properties of zinc oxide as a material, to its different preparation methods and uses. In a second time, the hydrogenolysis of glycerol over varied catalysts, with a focus on Cu/ZnO catalysts, was reviewed to have a general idea on what had already been done and how it would be possible to enhance the catalytic properties of the catalyst.

The second chapter is dedicated to the experimental part where we describe briefly the synthesis methods for ZnO and the Cu catalysts, the characterization tools and the catalytic setup.

In the third chapter, the synthesis of ZnO-PAAH supports is detailed. Study of different parameters of synthesis, that can influence the properties of the ZnO-PAAH material such as the molecular weight (MW) of the modifying agent PAAX [X = 2 (MW = 2 000 g.mol -1 ) or 5 (MW = 5 000 g.mol -1 ], the synthesis temperature (between 0 and 40 °C), the effect of washing (EtOH, H 2 O, various volumes and mixture) and the temperature of calcination (between 300 and 400 °C) are addressed. The aim of this chapter was to demonstrate how to get ZnO-PAAH or ZnO with high specific surface area and well-defined morphology in order to use them as support for copper catalysts.

The fourth chapter presents the catalytic results with various Cu/ZnO catalysts. These latter are synthesized on commercial ZnO by deposition precipitation with urea (DPU) method in order to optimize the parameters of Cu deposition. In a second time, ZnO-PAAX materials (described in chapter 3) are used as support for copper deposits. Finally, catalysts are synthesized by a "one pot method" in order to gain time and develop more reproducible catalysts. All the catalysts are tested for the hydrogenolysis of glycerol into 1,2-PDO.

In the fifth chapter, the effect of aluminium addition in the support is studied. Elaboration of a series of ZnAlO-PAAH supports and further Cu/ZnAlO x catalysts are carried out changing different synthetic parameters (quantity of urea, nature of PAAH, calcination of the support …). After morphological and structural characterizations, Cu/ZnAlO x catalysts are tested for the hydrogenolysis of glycerol into 1,2-PDO. Elaboration of a highly active and robust Cu/ZnAlO x catalysis achieved through a novel "one pot-two steps" route.

Chapter I

State of the Art

Table of contents

I. ZnO

Zinc oxide is known to be a material combining interesting chemical and physical properties.

It is an oxide with high chemical stability, high electrochemical coupling coefficient, high photostability and broad range of radiation absorption 1,2 . Zinc oxide is classified as a semiconductor in Groups IIb VIa (12 16) in materials science, whose covalency lies at the boundary between ionic and covalent semiconductors. It is an interesting candidate in laser technologies and optoelectronics applications due to its wide band gap (E g ~ 3.37 eV), large exciton binding energy (60 meV), high mechanical stability at room temperature and strong thermal stability at high temperatures 3,4 .

In addition, it can be used as a converter, sensor, energy generator and photocatalyst in hydrogen production due to its piezoelectric properties 5,6 . Its low toxicity, biodegradability and biocompatibility makes it an interesting material for biomedicine and pro ecological systems. It can also be used in the ceramic industry due to its hardness and rigidity [7][8][9] . Due to its nanometric structures variety, it can be used in many applications in nanotechnology (as additives in cosmetics 10 , tyre industry 11 …).

Zinc oxide can be elaborated as one dimensional (1D), two dimensional (2D) or three dimensional (3D) structures:

1D structures represent the largest structural group, including nanorods 12 , needles 13 , rings 14 , wires 15 , tubes [START_REF] Chen | Preparation of nanosized ZnO using brass[END_REF] 2D structures such as nanoplate/nanosheet and nanopellets [START_REF] Chiu | Photocatalytic study of two dimensional ZnO nanopellets in the decomposition of methylene blue[END_REF] , 3D structures of ZnO include flower, snowflakes, dandelion, coniferous urchin like, etc. [START_REF] Bitenc | Synthesis and characterization of crystalline hexagonal bipods of zinc oxide[END_REF] .

Some examples of the various morphologies are included in Figure 1 1.

Crystal structures

ZnO as an element of IIb VIa group, crystallizes in two structures either cubic or hexagonal. In both structures, each Zn atom is surrounded by four O at the corners of a tetrahedron, and vice versa. This tetrahedral coordination is typical of the sp 3 covalent bonding, but at the same time, ZnO has a strong ionic character. The difference in electronegativity between the oxygen atom and the zinc atom places ZnO at the boundary between covalent polar semiconductor and ionic semiconductor. The crystal structures known for ZnO are cubic rocksalt (B1), cubic zinc blende (B3) and hexagonal wurtzite (B4) as shown in Figure 1 2. This specificity is related to the mixed nature of the bonds within the crystal. High pressures are needed to obtain the rocksalt structure (NaCl), growth on cubic substrates stabilize the zinc blende structure while the wurtzite form is widely preferred under normal temperature and pressure conditions. This latter is formed by the stacking of negatively charged oxygen atom planes and positively charged zinc atom planes. More precisely, it is composed of two interpenetrated sub networks of the compact hexagonal type, which are displaced relative to each other along the c axis with a translation coefficient (u) of 0.375 in an ideal structure. Zinc oxide crystallizes in the space group P63mc with lattice parameters a = 3.250 Å and c = 5.207 Å.

Chemical, electronic and optical properties

Zinc oxide is almost insoluble in pure water (in the order of one millionth in mass proportion or 0.42x10 3 g per 100 g of pure water) and does not absorb carbon dioxide from the air.

Nevertheless, it is an amphoteric oxide which is soluble in acid (Zn 2+ form) and basic [Zn(OH) 4 2 zincate anions] solutions.

Zinc oxide is a 3.3 eV band gap semiconductor that therefore does not absorb visible light, and explains its transparency. White powder at room temperature, oxygen deficiencies can form in this material at high temperature, which explains why zinc oxide turns yellow when heated.

This effect is called thermochromy. ZnO is one of the most popular piezoelectric materials due to its hexagonal crystal structure which does not have a center of symmetry. Zinc oxide defects, which can be impurities, crystallinity defects, extended defects or adsorbed molecules, will have a significant impact on the electronic properties of this compound. ZnO is also known as n type conductive material. In order to have good applications in optoelectronics, ZnO is generally doped with [START_REF] Chen | The effect of Al doping on the morphology and optical property of ZnO nanostructures prepared by hydrothermal process[END_REF][START_REF] Park | Synthesis of highly crystalline Ga doped zinc oxide nanoparticles for hybrid polymer solar cells[END_REF][START_REF] Yuen | Room temperature deposition of p type arsenic doped ZnO polycrystalline films by laser assist filtered cathodic vacuum arc technique[END_REF][START_REF] Brauer | Activities towards p type doping of ZnO[END_REF] :

N type doping from group III like Ga, Al, In as Zn substitution and group VII as O substitution P type doping, from group V and group I, harder to reproduce.

Transition metals as Co, Fe, Ni and Mn have also been widely studied as dopant for ZnO materials [START_REF] Lin | Magnetophotoluminescence properties of Co doped ZnO nanorods[END_REF][START_REF] Pearton | ZnO Doped With Transition Metal Ions[END_REF] .

In order to determine the quality of as prepared ZnO nanostructures, studying the optical response of such nanomaterial can be crucial. One important technique to characterize these NPs is Low Temperature Photo Luminescence (PL). By studying carefully, the presence of different peaks corresponding to free exciton, biexcitons or green emission one can easily determine the quality of ZnO nanoparticles/structures. At very low temperature, around 10 K, the presence of free exciton peaks along with green emission has been shown an efficient

way to analyze the quality of ZnO nanostructures [START_REF] Chen | The effect of Al doping on the morphology and optical property of ZnO nanostructures prepared by hydrothermal process[END_REF] . Sometimes the presence of biexcitons peaks in the low temperature PL could also tell about the sample quality. So a careful analysis of all the peaks in the PL spectra is very much necessary.

Another important technique is Room Temperature Photo Luminescence (RTPL), which is very useful to identify the presence of defects in the sample. Since the intensity of defects emission is dependent on the temperature in a way that, higher the temperature is, more defects vibrations there are. This results in the origin of one or more peaks in the visible region of ZnO PL spectra in addition to the characteristic UV emission peaks. Although the exact origin of different peaks in visible region is not very clear, reports proposed origin of these to be attributed to zinc valency [START_REF] Silvestre | Food packaging based on polymer nanomaterials[END_REF] , oxygen vacancy [START_REF] Lozada | Photoluminescence emission at aging in ambient air of ZnO:Ag nanocrystals obtained by ultrasonic spray pyrolysis[END_REF] , zinc or oxygen interstitials [START_REF] Liu | Growth mechanism and properties of ZnO nanorods synthesized by plasma enhanced chemical vapor deposition[END_REF] , hydroxyl groups [START_REF] Qiu | The growth mechanism and optical properties of ultralong ZnO nanorod arrays with a high aspect ratio by a preheating hydrothermal method[END_REF] , and many other surface impurities and defects.

Even if the ratio of UV to visible emission intensity could be used as a tool to verify the quality of the sample, this ratio is dependent on the excitation intensity and so is not a sufficient proof of good quality [START_REF] Shi | Influence of excitation density on photoluminescence of zinc oxide with different morphologies and dimensions[END_REF] . Most of the time the quality of the material is highly dependent on the synthesis method used.

Two main processes are used to synthesis ZnO: metallurgical and chemical processes.

Metallurgical process

Zinc oxide can be obtained by metallurgical processes by roasting zinc ore.

Two types of zinc oxide can be obtained by this process [START_REF]Rubber Compounding Ingredients-Zinc Oxide-Test Methods[END_REF] :

Type A obtained by direct process known as American process

Type B obtained by indirect process known as French process

The American process, developed by Samuel Wetherill, takes place in a single production cycle. Zinc ore is reduced by coal heating in the first step, and zinc vapor is then oxidized in the same reactor in the second step. The process takes place in a furnace, in which the first layer consists of a coal bed and the second layer is in the form of zinc ore mixed with coal above the first. Carbon monoxide, resulting from blowing air to the reactor, is responsible for the reduction of zinc. Heating occurs to oxidize the reduced zinc into zinc oxide. As the raw material is impure (traces of iron, cadmium or calcium may be found in the zinc ore), the resulting zinc oxide (type A) contains impurities in the form of compounds of other metals.

The particles are mainly needle shaped, sometimes spheroidal and the color is not permanently white due to impurities. The process is illustrated in the scheme 1 1.

Scheme 1 1: American process for elaborating ZnO from zinc ore

The French process, developed by LeClaire, consists of melting metallic zinc in a furnace until it evaporates, after heating to 1000 °C. Unlike the American process, the French process takes place in separated locations: vertical furnaces, vertical load, refining column, vaporizer and rotary combustion chamber [START_REF] Tsuzuki | Mechanochemical synthesis of nanoparticles[END_REF] . The production of ZnO occurs by the immediate reaction of zinc vapor with oxygen in the air. A simplified scheme of the process is illustrated in scheme 1 2.

Scheme 1 2: French process for elaborating ZnO from zinc ore Finally, with the two processes, the particles of zinc oxide are transported through a cooling duct and collected in a bag filtration station. The particles are mainly spheroidal in shape and range in size from 0.1 to a few micrometers assembled into agglomerates [START_REF] Mahmud | Nanostructure of ZnO fabricated via French process and its correlation to electrical properties of semiconducting varistors[END_REF] . However, type B has a higher degree of purity than type A since the starting material is purely metallic zinc.

Chemical process

Due to the interesting properties of zinc oxide, many researchers have worked on the development of different synthesis methods to elaborate this material. Different chemical processes are used for the production of ZnO; the following paragraphs describe the most common ones.

Mechanico chemical process

The process involves high energy dry grinding, where the reaction is initiated through ballpowder impacts in a ball mill at low temperature.

Anhydrous zinc chloride and carbonate sodium are the usual starting materials for this process 35,36 . However other precursors can be used, such as oxalic acid (C 2 H 2 O 4 .2H 2 O) instead of carbonate sodium [START_REF] Stankovi | Controlled mechanochemically assisted synthesis of ZnO nanopowders in the presence of oxalic acid[END_REF] . A solvent in solid form (usually NaCl) is added to the system to separate the formed nanoparticles.

ZnCO 3 formed after the first step is then calcined at a temperature of 400 °C to 800 °C. The process can be divided into two reactions:

ZnCl 2 + Na 2 CO 3 ZnCO 3 + 2NaCl ZnCO 3 ZnO + CO 2
The milling time and the calcination temperature can affect the particle size; some examples are presented in the following table (Table 1 1). The paper of Ao et al. [START_REF] Ao | Mechanochemical synthesis of zinc oxide nanocrystalline[END_REF] reports an in depth study on the effect of the milling time and the calcination temperature. At 600 °C, the increase in the milling time leads to a decrease in the crystallite size attributed to the existence of a critical effect during mechanical milling. After 6 hours the crystallite size remain stable till 10 hours. On the other side, fixing the milling time to 6 hours and increasing the calcination temperature lead to an enormous increase in the crystallite size to 36 nm at 800 °C. Unfortunately, the article do not show the effect of the milling time and temperature on the specific surface area of the material. However in the article of Tsuzuki et al., [START_REF] Shi | Influence of excitation density on photoluminescence of zinc oxide with different morphologies and dimensions[END_REF] authors obtain spherical particles of 27 nm size leading to a medium surface area of ZnO = 47 m 2 .g 1 prepared by using the precursors cited above (ZnCl 2 and Na 2 CO 3 ). Actually, as the surface area is related to the particle size, in this technique the experimental conditions control the specific surface area of the material but it is usually low to medium (below 60 m 2 .g 1 ) .

One disadvantage of this method is the difficulty to attain a uniform grinding of the powder.

Moreover, the reduction of the grains to the desired size requires higher energy and longer milling time resulting in an increase in the amount of impurities in the sample. Advantages of this method are the production on a large scale and at low costs of nanoparticles of small size, with low tendency to agglomerate, as well as the high homogeneity of the crystal structure and morphology.

Hydro(solvo) thermal method

The hydro(solvo) thermal method consists in adding the reactants to the solvent which can be water or other organic solvent (alcohols, DMF, acetone…) in an autoclave and to start the synthesis under high pressure and temperature. It is called hydrothermal when the solvent is water and solvothermal if a nonaqueous solvent is used.

The method guarantees a narrow particle size distribution of small crystalline particles, high purity and good chemical homogeneity through homogeneous nucleation during the process. This is achieved by controlling the desired parameters such as reaction time and temperature, pH of the solution, use of additives and the type of solvent in case of solvothermal method [START_REF] Anas | Studies on the evolution of ZnO morphologies in a thermohydrolysis technique and evaluation of their functional properties[END_REF] .

In addition, the particles are obtained with all these properties without additional calcination [START_REF] Lee | Crystallization behavior of nano ceria powders by hydrothermal synthesis using a mixture of H 2 O 2 and NH 4 OH[END_REF] . Compared to the co precipitation method, hydrothermal technique leads to particles with a higher crystallinity and various (interesting) morphologies.

Some studies focus on the advantages and drawbacks of this method [START_REF] Chen | The effect of Al doping on the morphology and optical property of ZnO nanostructures prepared by hydrothermal process[END_REF][START_REF] Podrezova | Comparison between ZnO nanowires grown by chemical vapor deposition and hydrothermal synthesis[END_REF][START_REF] Ehrentraut | Solvothermal growth of ZnO[END_REF][START_REF] Piticescu | Hydrothermal synthesis of zirconia nanomaterials[END_REF][START_REF] Mihaiu | Thermal behavior of ZnO precursor powders obtained from aqueous solutions[END_REF] . First, it is an inexpensive method where all the precursors are generally cheap and the main cost comes from the heating. In case of using water as a solvent it is considered as eco friendly because no waste would be dangerous. In term of chemistry, it allows the synthesis of single crystals and the direct precipitation of thin films coatings onto the surfaces of substrates. This synthetic pathway is widely used to obtain ZnO nanowire carpets onto 2D support [START_REF] Lecarme | Nanowires as a promotor of high photoinduced efficiency and voltage gain for cathode battery recharging[END_REF] .

Nevertheless, it does not have the ability to handle the nucleation growth of the particles and since it is in a closed autoclave, the medium cannot be visible. It is also hard to manipulate it quickly so for example if a cooling down is necessary during the synthesis it will take time.

Controlled precipitation

Controlled precipitation of ZnO is achieved by reacting zinc salt (acetate, chloride, sulphate…)

with basic reactants that release OH groups. The nature and the quantity of the basic reactant (sodium borohydride, hydrazine hydrate, urea, potassium hydroxide…) [START_REF] Debbarma | Effect of reducing agents on the structure of zinc oxide under microwave irradiation[END_REF] , the pH [START_REF] Lamdab | Effect of a pH controlled co precipitation process on rhodamine B adsorption of MnFe 2 O 4 nanoparticles[END_REF] , the temperature [START_REF] Raj | Effect of temperature on structural, optical and photoluminescence studies on ZnO nanoparticles synthesized by the standard co precipitation method[END_REF] , the solvent [START_REF] López | Synthesis and characterization of ZnO nanoparticles: effect of solvent and antifungal capacity of NPs obtained in ethylene glycol[END_REF] and the precipitation time [START_REF] Hong | Synthesis and surface modification of ZnO nanoparticles[END_REF] affect the particle size. After precipitation, the material is recovered and a thermal treatment followed by grinding is necessary to obtain pure ZnO.

The growth of ZnO particles can also be controlled by adding surfactants or ligands to the reactants as they affect coagulation and particles flocculation [START_REF] Li | Growth of well defined ZnO microparticles with additives from aqueous solution[END_REF] . The controlled precipitation method with additives involves usually the chelation of the metal cations of the precursor, in an aqueous environment, to control its reactivity. The large use of this method is due to the good reproducibility of the resulting material.

Sol gel method

The sol gel method is one of the most applied methods for the elaboration of metal oxide nanoparticles and mixed oxide composites in various rheologies (sol, gel or precipitate).

A sol is a stable dispersion of colloidal (or nanometric) particles in a solvent. Nanoparticles can interact through van der Waals forces or hydrogen bonds. A gel is a three dimensional continuous network that contains a liquid phase. There are two types of gel: colloidal or polymeric. In a colloidal gel, the solid network consists of agglomeration of colloidal particles, these gels can be reversible to give back a sol. In a polymer gel, the particles are aggregated trough strong interactions and these gels are irreversible. Depending on the type of drying, xerogels are obtained by removing the liquid from the medium under atmospheric or aerogel by supercritical conditions.

Figure 1 3 shows various morphologies of (nano)materials that can be reached by sol gel method: such as film from a colloidal sol (Figure 1

Hydrolysis reaction: M OR + H 2 O M OH + XH

Polycondensation reactions:

M OH + M OR M O M + ROH alcoxolation M OH + M OH M O M + H 2 O oxolation M OH + M(ROH) M (OH) M + ROH
The sol gel process is considered to be kinetically controlled and process parameters such as the nature of the metal and its oxidation state, the nuclearity (m) of the complex, the amount of water, the temperature and others will strongly influence the characteristics of the final material. In the case of transition metals, the hydrolysis kinetics of the sol gel process is extremely fast which generally leads to amorphous materials that require annealing. Process control requires working on precursor engineering to slow hydrolysis in this case. In the case of silicon, on the other hand, the phenomenon must be accelerated by the addition of a catalyst such as HCl, NaOH, NH 4 F…

The process is considered simple, inexpensive, reproducible and relatively easy to implement under soft synthetic conditions. In addition, the sol gel method allows the surface of zinc oxide to be modified with selected organic compounds by adding a surfactant [polymers like poly(methyl methacrylate), polyacrylic acid, Pluronic 123… ], esters, glycerol, or even biosurfactants (sophorolipids, Emulsan…) during the synthesis. On the other hand, it allows morphological and structural controls and creation of porous material by the control of the drying and aging step leading to an increase in the specific surface area of ZnO compared to the other methods. The main drawback of sol gel is that most of the precursors are generally expensive and sensitive to air which can limit the scaling up. Sol gel process is also known to be time consuming especially if the ageing and drying steps have to be controlled. Finally, some structural modifications can happen at the densification stage (shrinking phenomenon) creating some undesired defects.

ZnO nanoparticles obtained by the sol gel method are involved in many fields of application because the process gives the possibility to modify the surface or texture of ZnO by adding different additives in situ or by post modification. [START_REF] Mahato | Nanocrystalline zinc oxide for the decontamination of sarin[END_REF] The scheme below lists a certain type of modification depending on the final application of the material (Scheme 1 3). (R= Et, iPr, cyclohexyl) as a precursor of choice, which in the case of ZnEt 2 is a commodity product (especially for ZnO deposits in the gas phase such as CVD, ALD...) [START_REF] You | Investigation of the sol gel method on the flexible ZnO device[END_REF][START_REF] Najafi | Resistivity reduction of nanostructured undoped zinc oxide thin films for Ag/ZnO bilayers using APCVD and sputtering techniques[END_REF] .

Syntheses of ZnO from zinc alkyl precursors were previously illustrated by the works of groups of Klabunde et al. [START_REF] Carnes | Synthesis, isolation, and chemical reactivity studies of nanocrystalline zinc oxide[END_REF] and Chaudret et al. [START_REF] Monge | Room Temperature organometallic synthesis of soluble and crystalline ZnO nanoparticles of controlled size and shape[END_REF] Recently, we have studied the effect of polymeric modifying agent, such as polyacrylic acid (PAAH), for the synthesis of ZnO from ZnEt 2 58 . Sol gel method was applied at RT, the ZnEt 2 precursor being added directly and under stirring into aqueous solution of PAAH at different concentrations. TEM images showed hybrid mesospherical ZnO PAAH agglomerates elaborated in presence of PAAH (2 000 and 5 000 g.mol 1 ) at 0.63 wt%. The particle size of the agglomerated particles was between 150 200 nm and 50 100 nm, respectively. Increasing the chain length to 100 000 and 240 000 g.mol 1 resulted in different morphologies as platelets and undefined aggregates for the two systems. Very recently, Daniele et al. reported the doping of ZnO PAAH with different metals (Ni, Co, Al, Cu…) and at three different concentrations (0.1, 1 and 5%) using the same approach [START_REF] Zhang | Doping of ZnO inorganic organic nanohybrids with metal elements[END_REF] . The results showed that the increase in dopant concentration led to quenching of visible luminescence for ZnO nanoparticles. Nevertheless, at low doping concentration, this phenomenon could be avoided.

Miscellaneous methods

Other approaches to get different shapes of ZnO via chemical processes are possible but will not be detailed in this manuscript because outside the scope of this work, it is:

Growing ZnO crystals from a gas phase described by Grasza et al. [START_REF] Grasza | Effect of annealing atmosphere on the quality of ZnO crystal surface[END_REF] Thin layer of ZnO by pulsed layer deposition (PLD) synthesized by Wei et al. [START_REF] Wei | Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target[END_REF] Ultrapure particles of ZnO obtained by Zhao et al. via an aerosol pyrolysis [START_REF] Zhao | Acetate derived ZnO ultrafine particles synthesized by spray pyrolysis[END_REF] Connected rods of ZnO using a sonochemical process and microwave heating applied by Hu et al. [START_REF] Hu | Sonochemical and microwave assisted synthesis of linked single crystalline ZnO rods[END_REF] Ellipsoids and flakes of ZnO obtained by precipitation in an emulsion system described by Jesionowski et al. [START_REF] Ko Odziejczak Radzimska | Structural characterisation of ZnO particles obtained by the emulsion precipitation method[END_REF] ZnO is a very interesting material, applied in different fields as we will address in the following sections. From 2020, ZnO world production is estimated to be 10 5 tons per year. For 2013, the producing countries and the fields of application are given in Figure 1 5 [START_REF] Ko Odziejczak Radzimska | Zinc oxide From synthesis to application: A review[END_REF] :

ZnO as additive in rubber industry

ZnO is used as an additive in the manufacture of cross linked rubber products 66 . In fact, the thermal conductivity of typical pure silicone rubber is low. In order to improve it, thermal conductivity fillers such as metal powders, metal oxides and inorganic particles can be added.

ZnO can improve the thermal conductivity of silicone rubber while maintaining its high electrical resistance. However, ZnO tends to aggregate and to form larger particle size in the polymer matrix. In addition, ZnO is commonly used as a cross linking agent for carboxylated elastomers. It can also be used to produce vulcanizates with high tensile strength.

Unfortunately, zinc oxide crosslinked carboxylic elastomers have poor bending properties and high compression set.

In order to solve these problems, ZnO particles can be modified by adding different compounds to fit the upcoming criteria:

Improve the interaction between ZnO nanoparticles and the polymer to have a better dispersion Produce vulcanizates with better mechanical properties and higher crosslink density Increase the interphase between elastomer carboxylic groups and ZnO nanoparticles mainly in form of snowflakes

The disadvantage of using ZnO in the rubber industry is its impact on the environment after the end of the product's life as zinc element has a toxic effect on aquatic organisms when releases in excess into the water [START_REF] Baysal | Physicochemical transformation of ZnO and TiO 2 nanoparticles in sea water and its impact on bacterial toxicity[END_REF] .

a) b)

ZnO as antibacterial agent for food industry and medical applications

ZnO nanoparticles have been developed as a successful candidate in the food industry [START_REF] Silvestre | Food packaging based on polymer nanomaterials[END_REF][START_REF] Espitia | Zinc oxide nanoparticles: Synthesis, antimicrobial activity and food packaging applications[END_REF] .

Some of the main advantages of ZnO NPs are their capacity to inhibit bacterial growth when added on food packaging. The use of ZnO as an antibacterial agent against foodborne pathogens in food industry has required interdisciplinary research linking physicists, biologists, chemists, and medicine.

ZnO is also used for medical applications, as an oral medication for epilepsy and diarrhea [START_REF] Jiang | The Advancing of zinc oxide nanoparticles for biomedical applications[END_REF] . It can be used in dermatological substances against itching and inflammation due to its properties that accelerate wound healing. It can have a peeling effect when applied in higher concentration. ZnO is used in various types of food supplements, nutritional products, nail fortifiers and as component of dental pastes [START_REF] Espitia | Zinc oxide nanoparticles: Synthesis, antimicrobial activity and food packaging applications[END_REF] .

ZnO in electronic and technological applications

The wide band gap of ZnO and the high binding energy at room temperature make ZnO a perfect candidate for photoelectronic and electronic equipments. Due to its luminescent properties, ZnO is used in field emission display (FED) equipment such as TV and laptops [START_REF] Mansouri | Analytic model for ZnO thin film transistor under dark and UV illumination[END_REF] .

With regard to its conductivity and visible ray permeability, it is possible to obtain the production of light permeable electrodes in solar batteries. Nevertheless, it is a promising material for UV emitting device and has potential for use as a transparent electrode in photovoltaic [START_REF] Wang | Electrodeposition zinc oxide inverse opal and its application in hybrid photovoltaics[END_REF] . It is also found in gas sensors, lighting protectors, electrical equipment for protection against network overvoltages and production of varistors [START_REF] Djuriši | ZnO nanostructures for optoelectronics: Material properties and device applications[END_REF] .

ZnO as photocatalyst

ZnO is used as photocatalyst by being irradiated by light with an energy higher than its Figure 1 6: Basic mechanism of ZnO photocatalysis. Ehv is the irradiated photon energy, Eg is the energy gap, VB is the valence band, CB is the conduction band, R is the electron acceptor and D is the electron donor [START_REF] Qi | Review on the improvement of the photocatalytic and antibacterial activities of ZnO[END_REF] .

The main drawback of ZnO photocatalysts is their limited effectiveness in the use of sunlight, as they are only active under UV irradiation and do not absorb visible light because of their wide band gap [START_REF] Aponiene | Effective combination of LED based visible light, photosensitizer and photocatalyst to combat Gram ( ) bacteria[END_REF] [START_REF] Sirelkhatim | Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism[END_REF][START_REF] Awasthi | Facile synthesis of ZnO flowers modified graphene like MoS2 sheets for enhanced visible light driven photocatalytic activity and antibacterial properties[END_REF][START_REF] Pan | TiO 2 -ZnO composite sphere decorated with ZnO clusters for effective charge isolation in photocatalysis[END_REF] . In fact, doping metal or nonmetal atoms can modify the photoelectric properties of ZnO to extend its spectral response to the visible light region as it can effectively reduce the band gap of semiconductors [START_REF] Yu | New insight into the enhanced photocatalytic activity of N , C and S doped ZnO photocatalysts[END_REF] . Doped metal or nonmetal atoms change the coordination environment of the Zn atom and tune the electronic structure of the ZnO by adding localized electronic energy levels in the band gap (Figure 1 7). Photogenerated charge carriers are then trapped and photocatalytic activities are consequently increased in visible range [START_REF] Kuriakose | Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method[END_REF][START_REF] Liau | Energy level variations of Cu doped ZnO fabricated through sol gel processing[END_REF] .

ZnO as support for heterogeneous catalysts

In addition to the use of ZnO as photocatalyst in different reactions as in water splitting and waste water treatment, ZnO is used in catalytic reactions as a support due to his wide morphology and its interesting properties. We will list some reactions and discuss more in detail the role of ZnO in the transformation of biomass.

ZnO for CO oxidation

The most used catalyst in this reaction with ZnO as support is Au as active metal. Many studies showed the impact of the morphology of ZnO on this reaction.

Carabineiro et al. 87 studied three different ZnO for this reaction. They compared between two commercial ZnO (one purchased from Evonik and the second one from Strem Chemicals referred as ZnO EV and ZnO SC , respectively). They prepared their own ZnO by chemical vapour deposition (CVD). 1 wt% of gold was supported on the three supports to compare them and explore which sample will show a strong metal support interaction. They found that the ZnO prepared with CVD (ZnO CVD ) is the one showing the highest activity for CO oxidation with 2 mol CO /g Au 1 .h 1 . When they compared the morphology of the three samples, they found that Au on ZnO CVD is in the form of nanocrystals epitaxially oriented on the ZnO rods leading to a strong metal support interaction and resulting in an increase in the catalytic activity. Au supported on ZnO EV and ZnO SC did not have a define morphology resulting in lower interaction between gold and ZnO NPs leading to a decrease in the activity of the catalyst compared to the Au/ZnO CVD .

ZnO for methanol synthesis

For methanol synthesis by the hydrogenation of CO 2 , the most active system is Cu/ZnO/Al 2 O 3 .

As for CO oxidation, many studies focused on the effect of the morphology of ZnO on the activity of the catalyst. For example, Liao et al. [START_REF] Liao | Morphology Dependent interactions of ZnO with Cu nanoparticles at the materials' interface in selective hydrogenation of CO 2 to CH 3 OH[END_REF] prepared copper catalysts supported on ZnO nanorods or ZnO nanoplates and Al 2 O 3 . The difference in morphology between the two ZnO led to difference in the interaction between copper particles and ZnO. Under different catalytic conditions (CO 2 :H 2 molar ratio and temperature of reaction), the catalysts prepared with ZnO nanoplates had a higher selectivity toward methanol (around 70 % for all the reaction) than the copper supported on ZnO nanorods resulting in a selectivity of 40 %. The difference came from the higher concentration in oxygen vacancies with the ZnO nanoplates.

Actually, in the case of the nanoplates a strong interaction between the (002) facet of ZnO and Cu existed due to the transfer of the electrons of ZnO to the Cu Fermi level.

Cu/ZnO (filament like) and Cu/ZnO (rod like) were studied by Lei et al. [START_REF] Lei | Hydrogenation of CO 2 to CH 3 OH over Cu/ZnO catalysts with different ZnO morphology[END_REF] . Once again the higher interaction between ZnO and Cu due to the higher exposure of (002) ZnO facet resulted in higher activity of the Cu/ZnO (filament like) with 16 % of CO 2 conversion and 78 % of selectivity toward methanol compared to 8% of conversion and 62 % of selectivity toward methanol with Cu/ZnO (rod like) catalysts.

ZnO for methanol steam reforming

As for methanol synthesis, Cu/ZnO catalysts were extensively studied for the methanol steam reforming reaction. In the following example [START_REF] Lin | Nanostructured zinc oxide nanorods with copper nanoparticles as a microreformation catalyst[END_REF] , the authors showed the importance of the high specific surface area of ZnO on the catalytic activity. They designed a core shell nano structured catalyst by elaborating nanorod of ZnO enclosed in Cu nanoparticles shell and compared the activity of their catalysts to a commercial one. The designed catalyst showed 93 % methanol conversion versus 88 % conversion with the commercial one but the main difference comes after the recyclability test where the conversion with the designed catalyst decreased only to 82 % while it decreased till 55 % with the commercial one. The authors explained the higher activity and resistance toward the catalytic reaction conditions of their system by the higher specific surface area compared to the commercial catalyst resulting in a better dispersion of copper particles on the ZnO support and facilitating the contact between methanol and the catalyst.

ZnO for biomass transformation

Since the conversion of biomass is attracting much attention, the researches related to this subject were extended to new innovative systems. Nevertheless, ZnO was always a good candidate for these reaction due to its interesting properties (surface modification, amphoteric character…). In the following sections, few articles about biomass transformations using ZnO as support are described:

a. Cu ZnO for the hydrogenolysis of sorbitol into polyols

In the paper of Gallezot et al [START_REF] Blanc | Starch derived polyols for polymer technologies: preparation by hydrogenolysis on metal catalysts[END_REF] [START_REF] Wang | Efficient conversion of microcrystalline cellulose to 1,2 alkanediols over supported Ni catalysts[END_REF] . Among all these supports, ZnO showed the highest conversion due to his amphoteric character leading in accelerating the dehydration reaction (the 1 st reaction in the mechanism). The authors showed that increasing the amount of nickel to 20 wt% achieved a total conversion till 30 wt% of Ni, which decreased when the amount of nickel was further increased. The authors explained this decrease in activity by the decrease of the specific surface area of the catalyst at higher loading of nickel (from 22 m 2 .g 1 to 16 m 2 .g 1 ).

c. Additional examples

One of the most interesting reactions in biomass conversion is the production of bio diesel by transesterification of oils and fats. In the paper of Liu et al. [START_REF] Liu | Controllable growth of 'multi level tower' ZnO for biodiesel production[END_REF] , authors produced biodiesel by transesterification of soybean oil in the presence of dispersed ZnO catalyst in methanol. The paper showed the effect of the pH of the solution on the nucleation growth of the ZnO particles and how it had an effect on the properties of ZnO and later on its catalytic activity.

In the paper of Morales et al. [START_REF] Morales | Role of exposed surfaces on zinc oxide nanostructures in the catalytic ethanol transformation[END_REF] , authors studied the transformation of ethanol into aldehydes with ZnO nanoparticles. They found that (i) the ZnO with the highest specific surface area resulted in higher conversion and (ii) the catalyst with highest acidic sites resulted in higher selectivity toward ethylene. Many other examples exist in the literature for the use of ZnO as support in heterogeneous catalysts specially in hydrogenation reactions such as: hydrogenation of fructose to mannitol [START_REF] Toukoniitty | Effect of ultrasound on catalytic hydrogenation of D Fructose to D Mannitol[END_REF] , hydrogenation of furfural to 2 methylfuran and furan [START_REF] Zheng | Towards understanding the reaction pathway in vapour phase hydrogenation of furfural to 2 methylfuran[END_REF] , hydrogenation of butyric acid into butanol [START_REF] Lee | Direct Hydrogenation of Biomass Derived Butyric Acid to n Butanol over a Ruthenium Tin Bimetallic Catalyst[END_REF] .

In conclusion, ZnO is a valuable support for the deposition of active metals in the form of nanoparticles for many applications. The quest now is to know how to deposit the catalyst in order to obtain high performance in glycerol hydrogenolysis.

II.

Heterogeneous catalysts: preparation methods

Heterogeneous catalysis, which consists of carrying out a reaction in the gas or liquid phase on a solid catalyst, is at the heart of modern energy and chemical industries. Most chemical processes are performed using functional nanomaterials as catalysts [START_REF] Louis | Synthesis of heterogeneous catalysts from "Catalyst preparation: science and engineering[END_REF] .

Moreover, heterogeneous catalysts encounter challenges during preparation. Having appropriate physico chemical properties specific to the desired application is challenging. To have these properties, the catalyst must have active metal particles, a high dispersion of these particles on the support, a high resistance to avoid leaching problems and a high interaction with the support. At the same time, the support must have good distribution and high porosity, thermal stability to withstand harsh reaction parameters, acidity or basicity appropriate to the catalytic reaction. All these properties can lead to an increase in the activity, selectivity and stability of the support, and then of the catalyst. At the same time, as mentioned in LePage's book [START_REF] Page | Developing industrial catalysts from[END_REF] , this catalyst should be manufactured on an industrial scale without losing its properties and activity.

In order to produce the most efficient catalyst, many preparation methods have been developed. The following section describes the most common methods for synthetizing copper based catalysts. When possible, the examples will focus on Cu/ZnO and the effect of the preparation method on the specific surface area of the catalyst and the stability of copper particles during the catalytic reactions. We will see how the surface area of ZnO plays a major role in the dispersion of Cu particles and their size and how is it possible to control these parameters. Most of the research conducted to elaborate copper based catalysts were made for methanol synthesis 100-102 .

Scheme 1 4: Simplified scheme for the preparation of catalysts by impregnation method Impregnation is one of the easiest methods of preparing heterogeneous catalysts. It consists of dissolving a metal salt in water and adding a certain volume of this aqueous solution to the powder support followed by heating and stirring. If the volume of the aqueous solution of the metal precursor is greater than the pore volume of the support, the impregnation is called wet impregnation. At the end of the first step, the water is removed by a rotary evaporator. If the volume of the aqueous solution of the metal precursor is equal to the pore volume of the support, the method is called dry impregnation and the sample is just dried after the first step.

In both cases, the sample has to undergo heat treatment (calcination and/or reduction) to transform the precursor into active metal particles. Scheme 1 4 presents an overlook of the preparation of a catalyst by impregnation method. One of the advantages of this method is the large choice of supports that can be used, and the ease and low cost of preparation. It can be however hard to achieve high loading of active metals.

In the Wang's paper cited above [START_REF] Wang | Efficient conversion of microcrystalline cellulose to 1,2 alkanediols over supported Ni catalysts[END_REF] The elaboration of non noble metal catalysts (nickel and copper) with high metal loadings using deposition precipitation (DP) as a synthesis method, was developed by Geus et al 104,105 .

The method involved the precipitation of a metal hydroxide on the surface of a support. In practice, an aqueous solution of the metal precursor and an alkaline precipitating agent [e.g.

NaOH, NH 4 OH, CO(NH 2 ) 2 ] were added to a suspension of the support powder. The method consisted in converting a highly soluble inorganic metal salts into another less soluble compound which precipitated on the support. The conversion of the metal precursor into low soluble compound was achieved by raising the pH of the solution. Figure 1 8 shows the principle of the method. In a first step, the metal precursor was converted into his low soluble compound which was above the saturation curve appointed as S and in a second step it precipitated in the area where the concentration was higher than the supersolubility (supersaturation) curve appointed as SS. In order to precipitate the metal precursor on the surface of the substrate and not in the solution, an interaction was mandatory. Therefore, the SS curve was shifted to the lower concentration in the presence of the support and it was called SS support . The general mechanism of the DP method is illustrated in scheme 1 5.

Figure 1 8: Diagram for precipitation of metal precursor. S= solubility curve, SS support = supersolubility curve of the metal precursor in presence of the support and SS= supersolubility curve in liquid [START_REF] Louis | Synthesis of heterogeneous catalysts from "Catalyst preparation: science and engineering[END_REF] .

In order to have homogeneous dispersion of nucleates onto the support, the precipitating agent must be added gradually, otherwise the concentration above the SS curve increased drastically leading to a fast nucleation phenomenon of the precipitate in the solution. The nucleation barrier could be decreased by enhancing the interaction of the precursor with the support and this could be done if the nucleation at the surface of the support was occurring when the concentration was between S and SS. In the following section we will see some examples of catalysts prepared by deposition precipitation. The general experimental method consists in adding the carrier to the solution of the metal precursor under stirring; the precipitating agent is then added. The temperature and time of heating depend on the precipitating agent and the precursors. Most of the time, the longer the deposition time, the better the dispersion of the metal on the support.

As with the impregnation method, the sample after the synthesis is then washed, dried and subjected to a heat treatment (reduction) to be active. Several groups have used this preparation method for non noble metals [105][106][107] , and later it was developed for the production of catalysts containing noble metals, in particular gold 108 . The main advantage of the deposition precipitation method is the possibility of depositing a large quantity of metal via a controlled deposition. Thanks to the good interaction between the support and the precursor, the metal is highly dispersed, which gives a very active phase after heat treatment. As expected, the homogeneous deposition precipitation method had resulted in higher copper dispersion and hence smaller copper particle size. Nevertheless, the catalytic properties of the catalyst prepared by heterogeneous deposition precipitation showed a better conversion. It is unfortunate that the authors did not compare the characteristics of the catalysts after the reaction. However, they observed that the catalyst prepared by homogeneous deposition precipitation was not stable and that after the catalysis, the metal particles were sintered into large aggregates.

Deposition Precipitation with urea (DPU)

An interesting deposition precipitation method is by using urea as precipitating agent, noted as DPU method. When urea is added to the medium, the pH is increasing gradually after heating the medium because the decomposition temperature of urea is around 60 °C following the reaction:

Thus, compared to the traditional DP method, DPU allow to separate the addition of the base and the moment when the base starts to act, urea is known as a delay base. An important parameter to consider for the DPU method is the PZC (zero charge point) of the support. Even if the deposition of the metal begins before reaching the PZC of the substrate, the maturation step begins when the pH increases and reaches a plateau higher than the PZC of the substrate. In the first report, 1% by weight of Cu/ZnO was synthetized by dissolving copper nitrate in an aqueous solution (Conc Cu 2+ = 2.4 x 10 3 M) and adding it to 3 g of commercial ZnO (Kadox 911, BET = 8 m 2 .g 1 ) in a glass reactor. 112 The mixture was heated at 80 °C and 0.96 g of urea was added to the solution and kept under heating and stirring for 20 h. The solution was centrifuged and the solid was washed several times with water and dried for 24 hours at room temperature under vacuum. After synthesis, the solid was calcined at 400 °C under oxygen for 2 h and reduced under hydrogen at 350 °C for 2 hours. The thermal treatment had a big impact on the catalyst and they consider that for the first time a Cu 3 Zn alloy was formed in Cu/ZnO sample. In addition to the thermal effect, the study compared the effect of the preparation method on the behavior of the catalyst. For that reason, in parallel for Cu/ZnO prepared by DPU, Cu ZnO was prepared by incipient wetness impregnation. It was found that by DPU method Cu 0 nanoparticles of 4 nm were obtained compared to 22 nm by the impregnation method.

In the second report, different catalysts were prepared by changing the concentration of the copper precursor. 113 The same preparation method: 0.8 wt% and 1.55 wt% of Cu/TiO 2 were prepared by mixing 1 g of TiO 2 (P25 Evonik -BET = 50 m 2 .g 1 ) in suspension in 100 mL of water and a copper nitrate solution. The mixture was heated at 80 °C for 20 hours after adding 5 g of urea. The drying procedure of the sample was the same: centrifugation followed by washing and drying. The sample was directly reduced under H 2 at 350 °C for 2 hours, without calcination. After studying the effect of the heating treatment on the catalysts, the pre calcination step was shown to affect the size distribution of the metal but only when the precursor is different than copper nitrate. For that reason, in this study, authors abandoned the calcination step. This paper focused on studying the mechanism of DPU and the effect of Cu loading, time reaction and pH on the catalyst. Due to the high surface area of TiO 2 , Cu 0 particles with a size smaller than 1 nm were obtained. A high specific surface area therefore seems a prerequisite for obtaining very small nanoparticles and therefore a high dispersion.

Scheme 1 6: Simplified scheme for the preparation of catalysts by co precipitation method

The co precipitation method is quite similar to the deposition precipitation one; the difference being that the support is formed at the same time as the deposit. The method consists of adding solutions of both precursors at the same time to ensure nucleation and growth of the metal on the support and adding a precipitating agent as for deposition precipitation. These compounds have low solubility and precipitate, a separation step is performed to recover the precipitate and heat treatments are used to activate the material.

Solutions of metal precursors are often highly soluble (chlorides, sulfides and nitrates) and are likely to combine with basic reagents in the form of hydroxides and alkali carbonates to form metal hydroxides and metal carbonates. The conditions of synthesis such as the temperature, the stirring time, the nature of precursors, the washing, the drying and the method of mixing affect highly the structure of the catalyst and its surface. The high supersaturation levels, the changes in the solid phase during the process, the chemical changes during the aging of the precipitates and the low stability of supersaturated solutions make the study of the mechanism of this method difficult. In summary, the process consists on mixing precursors solutions, the formation of the primary particles by nucleation and crystal growth and the aggregation of these particles. Later the filtration, or any kind of separation, is done to remove the excess of solutions followed by thermal treatment to activate the catalyst. Scheme 1 6

gives an overview of the preparation of the catalyst by the co precipitation method.

Anyway, this method has many advantages, and the main one is that it guarantees small particle sizes even with a high metal loading (up to 80 % sometimes). This high content of metal on the support cannot be achieved by another preparation method. In one hand, the process is fast and it involves simple steps contributing in an easy control of the particle size and the composition of the catalyst. On the other hand, it gives high output and recovery and there are different ways to modify the surface of the particles leading to a high surface area.

Unfortunately, this method results in large quantities of salt solutions after the filtration of the product. Another drawback is that the catalyst will be in powder form while for some catalytic reactions it is better to have it in a defined shape as tablet or extrudate.

In the review of Schlögl et al. 114 , copper and zinc nitrates are combined and alkali carbonates have been used as precipitating agent to produce Cu/ZnO. They showed that the atomic distribution of the different species within the solid solution was strongly related to the crystalline nature of the metal precursor. In addition, the choice of a pure and crystalline phase of the precursors made it possible to control the synthesis of the catalyst and thus to increase the reproducibility of the synthesis.

In this context, the objective was to find a composition that optimized the dilution of active species in a refractory matrix species. As a general rule, a 1:1 ratio should be advanced to reach high inter dispersion. But in this example [zincian malachite: (Cu,Zn) 2 (OH) 2 CO 3 ], the 1:1 ratio is not compatible with the miscibility range of the target precursor phase. Authors also noted that the meso structure of the precursor compound must be taken into account.

Platelets, needles structures led to a large specific surface area accessible by creating porous aggregates. Ultimately, the heat treatment was carried out in a very specific way (the flow rate and heating temperature must be strongly controlled) and with particular attention. The low calcination and reduction temperatures saved the large interface between the matrix and the final phase.

Alternative anions

In order to improve the contact between Cu and ZnO, an upgrade of the classical co precipitation method was done and the method is known as alternative anions. In this The effect of the calcination temperature on the activity of the catalyst for methanol synthesis showed that the first three catalysts in Table 1 3 had a lower activity than the others which were calcined in four steps. The result was explained by the increase of Cu particles sizes obtained after the one step calcination. The increase was due to the exothermic decomposition of the precursor which resulted in a significant increase in CuO size and sintering of the Cu particles. With the same calcination steps, catalysts with a higher Cu specific surface area showed higher activity. The catalysts prepared by this method were compared to another one prepared by standard co precipitation. The catalyst prepared by the co precipitation method has the largest Cu specific surface area after calcination at 320 °C but did not show the best activity. The last three entries of the table show that the addition of Al resulted in an increase in the BET surface area of the catalyst prepared by alternative anion method but it was not as high as that of the co precipitated catalyst.

In conclusion, while the Cu dispersion is an important criterion for catalyst activity, the preparation method and the interaction between the catalyst components are also crucial parameters for controlling the activity.

Another type of precursors was used by Epple and his coworkers for the preparation of Cu ZnO catalysts 117 . Two types of preparation were used: the batch precipitation and the continuous overflow methods (Figure 1 10). In the batch precipitation method, [Zn(CN) 4 ] 2-was prepared by mixing aqueous solutions of zinc sulfate and potassium cyanide. Copper(II) sulfate was then added under stirring and copper zinc cyanide was obtained as precipitate. In the continuous overflow precipitation method, the aqueous solutions of K 2 [Zn(CN) 4 ] and CuSO 4 were pumped at different flow rates into a stirred precipitation vessel at room temperature. The overflowing suspension was immediately filtered. The precipitate was air dried overnight at room temperature.

In both preparation methods, the mixed coordination compound Zn[Cu(CN) 3 ] were heated at high temperature (300 °C to 500 °C) to give a 1:1 mixture of CuO and ZnO which was then reduced to Cu/ZnO under a mixture of H 2 /N 2 (5/95) at 350 °C. 117 The specific surface area obtained by this method was not reported but the particles size of copper was 3 nm and the catalyst particles were spherical which meant that the specific surface area should be high. First, a combination of sol gel synthesis and photochemical oxidation was applied to obtain a mixture of CuO/ZnO nanocrystals. Sol gel consisted of the precipitation of precursors in methanol by adding hydroxide in the presence of silane. Photochemical oxidation was responsible for the removal of organic substituents from the silane and contributed to the elaboration of an amorphous SiO 2 layer leading to a significant increase in the specific surface area of the catalyst (from 15 to 120 m 2 .g 1 ).

The second sol gel method was done by using propylene oxide as gelation initiator to elaborate xerogels and aerogels of ZnO/Al 2 O 3 , CuO/Al 2 O 3 , and CuO/ZnO/Al 2 O 3 . For the aerogel preparation, the solvent was extracted with supercritical CO 2 . Using the same synthesis parameters, aerogels had a higher specific surface area and a higher Cu specific surface area than the corresponding xerogels. These catalysts were used for methanol synthesis and aerogels catalysts showed a higher activity. By comparing catalysts with and without ZnO, the presence of ZnO revealed an increase in the catalytic activities of xerogel and aerogel due to stronger support metal interaction. Scheme 1 7: One pot synthesis method for the elaboration of the ligand stabilized Cu and ZnO nanoparticles 118 . Many changes in the solution color appeared during synthesis, proving the exchange of ligands between Zn and Cu complexes and leading to copper alkyl species and zinc carboxylate. This technique led to the development of an active catalyst (as active as the commercial one) with small Cu particle size (1 4 nm) and organometallic zinc compounds. One of the advantages of this method is that no heating was used for synthesis and reproducible catalysts were prepared due to the simplicity of the preparation. The catalytic results are summarized in the Table 1 5. III.

Glycerol as source for propanediols production

Glycerol is a component of triglyceride which can be found in most plant and animal oils and fats. Several routes contribute to glycerol production; it can be produced from saponification processes and from the biodiesel production as byproduct. In the first process, the hydrolysis of fats and vegetable oils in presence of an excess of sodium hydroxide or potassium hydroxide results in carboxylate salts (soap) and glycerol. The process is known as the traditional soap manufacturing and it is shown in Scheme 1 8. In the 2 nd process, the transesterification of triglyceride under acid, basic or enzymatic catalysis leads to the formation of biodiesel but with a large excess of glycerol, in which glycerol represents about 10 wt% of the total biodiesel produced 121 . Scheme 1 9 shows the production of glycerol from the transesterification process.

Scheme 1 8: Glycerol production by saponification process.

Scheme 1 9: Glycerol production from biodiesel.

In addition, biological fermentation processes for glycerol production from sugar are becoming useful route based on the inexpensive and abundant materials and cellulolytic derivatives used for this production 122,123 . Glycerol has become more readily available as a renewable resource due to the increased production and application of biomass on one hand and the geopolitical benefit of this production on the other 124 . As a result, there is

Dehydration of glycerol into 1,2 PDO and 1,3 PDO

The dehydration mechanism was first proposed by the group of Suppes 141 in 2005 using a copper chromite (Cu Cr) catalyst with 80 wt% glycerol solution, 14 bar at 200 °C and after 24 hours. Glycerol was first dehydrated to acetol, which reacted further with hydrogen and formed 1,2 PDO (Scheme 1 10). The catalyst was reduced in the reactor just before adding the reactants, which means that no surface hydroxyl groups could participate to the reaction.

In order to validate their mechanism, they split the reaction into two steps. Initially, they isolated the acetol formed solely by the presence of the catalyst (without adding hydrogen).

In the second step they added hydrogen to the acetol generated in the first step, and conducted the reaction with the same catalyst as previously. The yield of 1,2 PDO (around 50%) obtained by combining the two steps confirmed the mechanism.

This mechanism was also known for the acidic catalysts 142,143 or with metals catalysts with the addition of an acidic function as reported in the paper of Tomishige et al. 144 . The addition of acidic catalysts (SO 4 2 /ZrO 2 , MFI, BEA, Amberlyst…) on Ru/C metal catalysts increased the conversion of glycerol and the selectivity toward 1,2 PDO. The higher conversion was obtained with the addition of amberlyst leading to an increase in the conversion from 6 % to 15 % with an increase in the selectivity toward 1,2 PDO from 18 % to 53 % at 20 wt% of glycerol, 80 bar of H 2 , 10 hours of reaction and at 180 °C in presence of 150 mg of catalyst. The reduction of the catalyst was done under hydrogen for 3 hours at 300 °C with a heating rate of 2 °C.min 1 . The copper loading was 11.6 wt% and the copper metallic surface area was 28 m 2 .g 1 . The conditions of the catalytic reactions were the following: 4 grams of catalyst, a flow 110 ml.h 1 of 0.11 mol.l 1 of glycerol solution at 260 °C and under 10 and 40 bar of H 2 . The activity and the selectivity toward 1,2 PDO increased with the increase of the H 2 pressure from 6.37 mol glycerol .h 1 .g Cu 1 x 10 3 at 10 bar to 12.4 mol glycerol .h 1 .g Cu 1 x 10 3 at 40 bar with a selectivity toward 1,2 PDO of 63% and 84.5%, respectively after 65 hours of reaction.

Comparing the two studies results in a similar behavior of the two catalysts, both showing a deactivation due to a sintering phenomenon. Nevertheless, the difference in surface area between the two supports may be responsible for the higher activity of Cu/C catalyst (1400 m 2 .g 1 ) compared to copper Raney catalyst (20 50 m 2 .g 1 depending on the supplier and / or the preparation method).

Other studies have shown a different pathway with bimetallic catalysts under the following reaction conditions: 67 wt% glycerol, 120 °C, 80 bar of H 2 , 150 mg of catalyst and for 12 hours.

The direct hydrogenolysis was proposed by Tomishige's group over Ir ReO x /SiO 2 141 and Rh M/SiO 2 142 (M = Re, Mo or W) catalysts. Authors reported that glycerol was adsorbed at the interface between Ir or Rh metal surface and ReO x cluster to form two terminal alkoxides: 2,3 dihydroxypropoxide and 1,3 dihydroxyisopropoxide. In a second step, the C O bond was cleaved by nucleophilic attack on hydride, activated by iridium metal, on the positions 2 or 3 of dihydroxypropoxide (Figure 1 11). The last step was the release of the propanediols by hydrolysis of the resulting alkoxide. In this reaction, 1,3 PDO was obtained with a higher selectivity than 1,2 PDO which could be explained by the greater stability of the transition state of the intermediate alkoxide leading to 1,3 PDO.

Glycerol hydrogenolysis reaction has been extensively studied over catalysts based on Rh, Pt, Pd, Ru, Ni and the results have been summarized in different reviews [149][150][151][152] . Table 1 6 shows a non exhaustive list of some results from the literature using the metals cited above in a batch reactor. The main drawback of the noble metals catalysts is their high price compared to the transition metals but also their activity toward C C cleavage resulting in high conversion of glycerol but low selectivity toward 1,2 PDO as seen in Table 1 As we reported in the introduction, copper based catalysts also have been extensively studied for glycerol hydrogenolysis and have a high selectivity towards 1,2 PDO under mild conditions.

This can be explained by the low activity of Cu for C C bond cleavage and high efficiency for C O bond cleavage 141,142,157 compared to noble metal catalysts.

Cu Cr catalysts

The first report of copper as a catalyst for glycerol hydrogenolysis was from a Cu Cr system by Adkins' group in 1932 158 . The conversion was 55% and the selectivity to 1,2 PDO 85 % but the reaction conditions were harsh: 250 °C and 200 bar of H 2 .

Later, less severe conditions were applied as in 2005 by ) via impregnation of Cu(NO 3 ) 2 and Cr(NO 3 ) 3 [Cu/Cr molar ratio = 1/5] on carbon followed by removal of the carbon template. Authors investigated the effect of the Cu/Cr ratio and found that the optimum molar ratio Cu/Cr was 1:5 in order to reach a conversion of 51% and a selectivity of 97.1% to 1,2 PDO, at 200 °C, under 14 bar H 2 and after 10 hours. This study highlighted that an increase in specific surface area led to an increase in the conversion of glycerol.

Later, other methods of preparation were studied. In 2010, Kim et al. reported different Cu/Cr catalysts prepared by a NaOH co precipitation method 160 . They studied the effect of the molar ratio by first testing a pure Cu catalyst (a conversion of 28% was obtained with a selectivity of 60% to 1,2 PDO) while a pure chromium catalyst was not very active (10% conversion) and not selective. The most active catalyst was a Cu/Cr catalyst with molar ratio 1:2 containing mainly the spinel phase of CuCr 2 O 4 and achieving a 80.3% conversion with a selectivity of 83.9% toward 1,2 PDO. All catalysts were studied under the same conditions (50 g of concentrated glycerol at 90%, 220 °C, 80 bar of H 2 in presence of 1 g of catalyst and after 12 hours). It seems that the optimal metal ratio depends on the method of preparation.

In the two studies showing the spinel phase of CuCr 2 O 4 , the effect of increasing the acidity of the catalyst due to that phase was reflected in higher conversion of glycerol. 161 studied the coprecipitation method with NH 4 OH in order to prepare Cu/Cr catalysts. They also added Ba, Zn and Al as promoter where the conversion was low (16%) without Ba and reached 34% after adding the promoter with a selectivity of 82% to 1,2 PDO. The reaction conditions were 220 °C under 52 bar of H 2 and 5 hours using 20 wt% glycerol in 2 propanol and 1 g of catalyst. Adding a promoter to the catalyst seems to be mandatory in some cases to enhance the activity of the catalyst due to the increase of surface area or the acidity by the promoting groups.

Mane et al. in 2011

A new preparation method was developed by Xiao et al. in 2012 162 through an alkoxide free sol-gel process. The aim was to produce small particle size of Cu and Cr, using Cu(NO 3 ) 2 and Cr(NO 3 ) 3 as precursors and precipitated by propylene oxide. Once again, the reaction conditions were mild (42 bar H 2 , 210 °C, 60 wt% aqueous glycerol solution, 5 wt% catalyst and 10 hours of reaction) and the conversion was 85.9% and the selectivity was 98.5% toward 1,2 PDO (only traces of propanol and acetol). This method allowed the conservation of the small particles even after the catalytic conditions. The exact size is not given but from the XRD patterns, it could be seen that the only change is a slight increase in the intensity of the main copper peak (at 43°) after test.

The last mention of Cu Cr as catalyst for this liquid phase reaction was made by Yi et al. in 2016 163 . The study focused on theoretical calculation to understand the mechanism of the reaction but they also compared a pure Cu catalyst with a Cu Cr one prepared using the co precipitation method. As other studies have showed, the Cu catalyst was less active than Cu Cr. Indeed, the conversion increased from 19.6% to 32% and selectivity in 1,2 PDO increased from 52.6% to 73.8% by adding Cr under the same conditions (220 °C, 40 bar of H 2 , for 12 hours, 1 g of catalyst and 40 wt% glycerol solution).

Cu/ZnO catalysts

As mentioned above, the activity of Cu Cr for this reaction was significant but the toxicity of chromium contributed to the search for a substitute. In order to maintain a high selectivity towards 1,2 PDO, Cu/ZnO proved to be an interesting candidate. This system has been widely applied in different industrial processes such as methanol synthesis, water gas shift reaction and methanol steam reforming as it was cost effective and easy to handle. Table 1 8 reports in chronological order the most significant results for the hydrogenolysis of glycerol over Cu Zn systems and Cu based catalysts.

The first use of a copper zinc catalyst for glycerol hydrogenolysis was reported in 2004 by Chaminand et al 164 at IRCELYON. However, the activity of the catalyst was low with respect to the high catalyst loading and the long reaction time, the selectivity for 1,2 PDO was 100% at 19% of conversion. The reaction conditions are mentioned in the first entry of the Table 1 found that the 1:1 molar ratio gave the highest activity for the production of 1,2 PDO.

Compared to the Cu Cr system mentioned above, the activity of these catalysts was low.

Balaraju et al. 145,157 , have also studied the effect of the weight of the catalyst on the conversion and the selectivity. The catalytic reaction was carried out at 200 °C, under 20 bar H 2 and 20 wt% of glycerol for 8 hours. The increase of the catalyst weight from 0.6 g to 1.8 g led to an increase in the conversion from 13% to 26% (not proportional) and an increase in the selectivity toward 1,2 PDO from 87% to 90%.

In 1 8. They have also studied the effect of water as deactivating factor and replaced it with 1,2 butanediol and found that the conversion was increased from 5% to 55% under the same conditions with a selectivity of 86%. This was due to sintering phenomenon which were more important (75 nm) in presence of water than 1,2 butanediol (47 nm), (Table 1 8, entry 5).

In 2011, Bienholz et al. reported that by adding Ga to a Cu/ZnO catalyst, the catalyst activity improved significantly 165 . In their report, compared to the use of the Cu/ZnO catalyst prepared by NaOH coprecipitation, when Ga was added, the glycerol conversion increased from 84% to 96%; the experimental conditions were 220 °C under 50 bar H 2 ; 80% aqueous glycerol was loaded and the reaction lasted 7 hours. However, Ga is an expensive metal, which will result in a higher production cost (Table 1 A summary of the listed results shows that the catalysts used for glycerol hydrogenolysis were mostly prepared by coprecipitation. In the case of Cu/ZnO, the most used zinc precursors were zinc nitrate and zinc acetate. Some groups used also commercial ZnO and elaborated inhomogeneous catalysts with undefined structure. The specific surface area of the catalyst showed an important effect on the conversion combined with the particle size of the Cu active phase.

In general, a high surface areas have resulted in smaller copper particle sizes and better dispersions leading to an increase in the catalytic activity. The solvents have also played an important role on the activity of the catalyst, and water seems to inhibit the activity of the catalyst by deactivating the copper particles. Nevertheless, adding a promoter to the catalyst, especially Al results in an increase in the acidity of the catalyst leading to a higher conversion.

IV. Objectives of the thesis

In our previous work 58 , a sol gel route from reacting ZnEt 2 with an aqueous polyacrylic acid solution (PAAH) was developed and patented to obtain either highly luminescent ZnO nanocomposites or stable ZnO nano suspensions with high quantum yield to be applied in LED The starting point of our project was the use of our new hybrid ZnO based nano structured objects as supports for copper based catalysts for glycerol hydrogenolysis. The study of different parameters such as the temperature, the washing setup, the chain length of the PAAH, the calcination temperature after synthesis and the weight percentage of PAAH had to be optimized in order to understand their synthesis mechanism, to control their morphologies and to reach high specific surface areas. This will be addressed in chapter 3 of this manuscript.

Actually, one of the main property we had to optimize was the specific surface area because most of our materials elaborated by the sol gel method were having small surface areas with an average of 20 m 2 .g 1 .

Comparing to the literature as mentioned above, the Cu/ZnO catalysts were mostly prepared by co precipitation. In our project, we started by preparing catalysts by deposition precipitation with urea (DPU) in order to have small Cu particle size and well dispersed on our hybrid ZnO matrix. Commercial ZnO support was first used and then substituted later by the novel hybrid ZnO PAAH one. In a second time, a "one pot" synthesis was applied to decrease the preparation time and to limit the precipitation effects. These results will be presented in chapter 4.

The elaborated catalysts were used for the hydrogenolysis of glycerol into 1.2 PDO and the first catalytic tests were carried out with Cu/ZnO in order to increase the conversion of glycerol before adding a promoter. Since many studies showed the impact of adding promoters to the Cu/ZnO catalysts, we decided to add Al species to enhance the properties of our catalysts by increasing the acidity of the catalyst and the specific surface area (chapter 5).

The conversion of glycerol into 1,2 PDO with high selectivity is very challenging because of side reactions observed in literature leading to 1,3 PDO, ethylene glycol, acetol… Developing a catalyst selective toward 1,2 PDO was also one of our main target. Before developing the experimental conditions of each synthesis, a short list of the commercial products applied for the synthesis of ZnO, ZnAl oxides, the copper based catalysts and the glycerol is resumed in the Table 2 1. The procedure described in this paragraph is the classical method to synthesize 2 grams of ZnO (Figure 2 1):

CHAPTER II

Experimental part

Table of contents

2.52 g of PAA1.8 (Mw PAAH = 1800 g.mol 1 ), PAA2 (Mw PAAH = 2000 g.mol 1 ), PAA5 (Mw PAAH = 5000 g.mol 1 ) and PAA100 (Mw = 100 000 g.mol 1 ) initially concentrated at 50 wt% were dissolved in 200 mL of distilled water to obtain 0.63 wt% PAAX/water. The solution was stirred at 1000 rpm under inert atmosphere (argon) at room temperature (15 25 °C).

Then, 14.4 mL of DEZ was added in one shot with a syringe. The reaction is exothermic and fast, and produce ethane that is eliminated through a bubbler connected to the schlenk flask. After one hour of stirring, the solution was centrifuged at 8000 r.p.m for 20 minutes, washed (with water or ethanol) and the wet powder was dried in an oven under air at 80 °C.

To prepare 6 grams of ZnO, 600 mL of PAAX solution were used instead of 200 mL and 43.2 mL of DEZ were added under the same experimental conditions. For each synthesis one equivalent of DEZ (14.4 mL) was added under argon at room temperature as described before. After synthesis, the suspension was centrifuged and the powder was washed with 90 mL of ethanol for half an hour and dried at 80 °C under air.

The synthesis was conducted at three other temperatures: 0, 10 and 40 °C. Considering that the classical method was conducted at room temperature, i.e. around 20 °C.

The syntheses were done using PAA2 and PAA5, as before. In a schlenk flask of 1 L, 600 mL of PAA2 or PAA5 at 0.63 wt% were added. One equivalent of a solution of DEZ (43.2 mL, 0.16 mol) was added to the flask under argon.

For 0 °C, the flask was emerged in an ice bath for 30 minutes before adding the DEZ. The temperature of solution was measured before adding the reactant and it was stable after 20 minutes.

For 10 °C, the flask was emerged in a mixture of water and ice in order to maintain the temperature stable at 10 °C.

For 40 °C, the flask was maintained in a metallic bath and heated at 40 °C.

The transformation of DEZ into ZnO was immediate at the three different temperatures, this is why it's important to have the temperature inside the flask stable before adding the reactant. As the solution was stirred for one hour, the temperature of the bath was maintained the same for one hour.

After synthesis, the suspensions were centrifuged and washed with 90 mL of ethanol for half an hour and dried at 80 °C under air.

For ZnO/PAA5, 60 °C was also added due to the experimental results and the procedure was always the same.

Some of the materials were calcined after drying. The materials were synthesized using the standards parameters: 0.63 wt% PAAH, room temperature, washing with water and centrifugation at 4700 r.p.m for 20 minutes. After drying, the materials were calcined in a static oven for four hours at different temperatures, starting from RT till attending the desired temperature with a heating rate of 5°C.min 1 . The choice of the temperature was related to the loss of the organic compound shown by TGA results, which occurs between 300 °C and 400 °C.

The study was done using PAA2 and PAA5:

For ZnO/PAA2, four temperatures were studied: 300, 325, 350 and 400 °C. For ZnO/PAA5, three temperatures were studied: 300, 350 and 400 °C.

Synthesis of ZnO ZnAl 2 O 4 by hydrolysis of ZnET 2 and AlMe 3

In order to enhance the catalytic properties of the support, aluminium was added by using trimethyl aluminium AlMe 3 (trimethylaluminium or TMA) as precursor. For the different synthesis, the hydrolysis was done at 0 °C. The effect of the molar ratio Zn:Al and the presence of PAA2 were studied.

1) Synthesis of ZnO ZnAl 2 O 4 with a theoretical molar ratio Zn:Al of 1:2 in presence of PAA2.

In a schlenk flask of 500 mL, 200 mL of a solution of PAA2 at 0.63 wt% was added. The flask was immerged in an ice bath for 30 minutes in order to decrease the solution temperature to 0 °C. On the other hand, in a 50 mL schlenk flask, 3.1 mL of TMA (97% from sigma Aldrich) was added to 14.4 mL of DEZ under argon. The transfer of the mixture of TMA and DEZ to the 500 mL flask was done slowly under argon with a syringe. In the seconds following the transfer, ethane and methane gases were generated and evacuated by the bubbler connected to the 500 mL flask. The reaction is highly exothermic and the color of the solution turned into white in a few minutes. After one hour, the stirring was stopped and the suspension was centrifuged with a rate of 8000 r.p.m for 30 minutes. 90 mL of ethanol was added to the wet powder obtained after centrifugation and the mixture was kept under stirring at room temperature for 30 minutes. A last centrifugation round was done as previously. To finish, the centrifuge tube was kept in the oven at 80 °C overnight and the powder obtained is divided in 3 batches: the as synthesized one was used as support for catalyst, a few mg were calcined at 400 °C and 600 °C for characterization.

2) Synthesis of ZnO ZnAl 2 O 4 with a molar ratio Zn:Al of 1:2 without PAA2

The same procedure was followed but without PAA2. Centrifugation, washing and drying steps were same as the 1 st synthesis.

3) Synthesis of ZnO ZnAl 2 O 4 with a molar ratio Zn:Al of 1:2 without PAA2

The first bottle of TMA had been opened for a long time, or it is readily oxidized. For that reason, a new bottle of TMA was purchased from Alfa Aesar (25 wt% in hexane). The synthesis steps were same as the previous one but the corresponding volume of TMA was 4 times higher since the bottle was not pure (12.27 mL).

4) Synthesis of ZnO ZnAl 2 O 4 with a molar ratio Zn:Al of 1:2 in presence of PAA2

The synthesis was done now with PAA2. No change in the procedure was done.

5) Synthesis of ZnO ZnAl 2 O 4 with a molar ratio Zn:Al of 2:1 in presence of PAA2

This time we synthetized the material by adding more DEZ than TMA to obtain a molar ratio of 2:1. Instead of 12.27 mL of TMA, 3.06 mL of the new bottle of TMA was added to the schlenk flask containing 14.4 mL of DEZ under argon. The transfer, centrifugation and washing steps were the same as before.

Synthesis of supported Cu catalysts by deposition precipitation

A commercial ZnO (Alfa Aesar) was first used to set up the parameters of the synthesis. precursor into copper oxide, at 400 °C for four hours in a static oven with a heating rate of 5 °C.min 1 . Once the heating was over, the sample was removed from the oven after cooling down. The second step was the reduction to transform Cu 2+ to Cu 0 ; the powder was transferred to a cell as in the scheme below. First the cell was purged with argon for 10 minutes before H 2 flow with an output of 50 mL.min 1 and the oven was heated till 300 °C or 350 °C for one hour with a heating rate of 5 °C.min 1 .

After cooling down under argon and at room temperature, a passivation step was needed in order to form a protective oxide layer on the surface of the catalyst. The passivation was conducted for half an hour with a mixture of gas flow of 1% O 2 /N 2 with 50 mL.min 1 flow rate. The tables below show the different catalysts synthetized using the commercial support: After synthesis the filtration and thermal treatment were as described above.

Deposition precipitation with ZnO/PAAX :

The hybrid ZnO (ZnO PAAX uncalcined) were used after drying at 80 °C. The same DPU method was used with the hybrid ZnO as previously.

However, TGA results showed that 50% of weight was lost by heating at 400 °C. The syntheses were conducted before calcination, hence in order to respect the same quantities (wt% Cu/ZnO) as for the commercial ZnO, two grams of hybrid ZnO were used instead of 1 g of commercial ZnO. Catalysts with different Cu 0 weight percentage were synthesized (from 10 wt% to 30 wt%). Table 5 2 resumes the studied parameters of ZnO/PAAX used as support for copper catalysts. The deposition was also conducted on calcined ZnO.

As the aim is to compare the results between commercial support and the synthesized one, the same nominal Cu weight percentage was aimed.

For that reason, the same quantity of ZnO was supposed to be added in order to have the same weight percentage. Once again, that was a little bit hard because the weight loss calculated by TGA can have a 5% error. As the catalyst will be calcined at 400 °C after synthesis, the weight of ZnO initially added to prepare the catalyst should be calculated according to the TGA results in order to obtain in the end one gram of ZnO. The easier way was to calcine 2 g of ZnO at the selected temperature and after calcination adding the powder to the flask containing the copper and the urea to prepare the catalyst.

The protocol was the same as previously, different volume of Cu 2+ solution at 0.157 M were added to the calcined ZnO with 4 g of urea and different volumes of water depending on the weight percentage of Cu in order to have the same concentration as before (20 mmol.L 1 ). The table below shows different parameters studied for ZnO/PAA2 and ZnO/PAA5 calcined samples. As it was the last part of the work, all the parameters were fixed. For that reason, all the syntheses were done with 20 wt% of Cu. The different kind of ZnO ZnAl 2 O 4 synthesized were used as support for catalysts. In addition to the as synthesized support, a deposition precipitation was done with a calcined material.

For the as synthesized one without PAA2, 1.1 g of support was suspended in 163 mL of water in presence of urea and 25 mL of Cu 2+ solution of 0.157 M was added under stirring.

For the supports synthesized with PAA2, based on TGA results, 1.8 g of the supports were added to obtain 1 g after calcination at 400 °C. The volume of water and Cu 2+ were kept the same. For the support calcined at 400 °C, 1 g was used. In the 2 nd family of catalysts aluminium was incorporated. The precursor used was the old bottle of AlMe 3 purchased from sigma Aldrich (97% in hexane). Different methods of syntheses were used. In a first trial, 30 wt% of copper was added, for that 37.5 mL of Cu 2+ solution was mixed with the solution of PAA2 and urea. As for Cu/ZnO, 14.4 mL of DEZ was added to the hot solution of Cu 2+ and PAA2 under argon, followed by 5.11 mL of TMA at 60 °C, the stirring took place for 3 hours.

Synthesis of Cu/ZnO ZnAl 2 O 4 by one pot

At the beginning the color of the solution was blue as before and it turned into green at the end of the reaction indicating the presence of Al in the suspension. The washing and thermal treatment were the same as described above. . . .

. sin (1)

With P : Pressure at equilibrium (mmHg); P 0 : Saturation pressure of adsorption of the sample (mmHg);

C :constant obtained from adsorption and desorption rates;

Vads : Volume adsorbed at 77K (cm 3 );

V m : Volume of adsorbed monolayer (cm 3 ).

The determination of the steering coefficient as well as the intercept at the origin by plotting as a function of allow us to access the value of V m . Then the total area of the solid is then deduced by using equation (4):

4.35 V m (4)
This surface relative to the mass of the degassed solid used during the analysis is called specific surface area (S.S.A). It is expressed in m 2 g 1 . The BET specific surface area of the samples was determined by N 2 physisorption at 77K, using an ASAP 2020 Micromeritics apparatus. Prior to the measurements, catalysts were degassed at 150 °C for 3 h under nitrogen (10 4 mbar) with a heating rate of 5 °.min 1 in with Smart VacPrep Micrometrics.

After each catalyst preparation, the chemical analysis is used in order to access the exact content of metal deposited on the support, and to detect any leaching of the active species into solution during the chemical reaction. In the case of copper, zinc and aluminum, solid sample is dissolved in a mixture of H 2 SO 4 and HNO 3 under heating temperature between 250 and 300 °C. Sample preparation involves grinding of the material in ethanol in a mortar and depositing a drop of the suspension on a Cu TEM holey carbon grid for samples with no copper, whereas for the latter Ti TEM holey carbon grids were used. In case of the supported catalyst, this procedure allows for a very thin layer of the sample to deposit on the grid, avoiding overlap between the catalyst particles and making it easier to image the metal particles on the support.

A picture and scheme of the TEM equipment is showed in Figure 2 After the hydrogenolysis of glycerol, the 1,2 PDO formation was measured using a Shimadzu GC 2010 system equipped with a flame ionization detector and an autosampler AOC 20i. A Phenomenex FFAP 30 x 0.25 μm is used for the separation of products. The carrier gas is N 2 , the injector and detector temperature are 250 °C and 300 °C respectively.

1,4 Butan diol was choosen as internal standard for this reaction and a list of the possible products was calibrated (Appendix 1 11). Their retention time is shown in the table below.

CHAPTER III

ZnO PAAX: study of synthesis parameters for LED applications 1 . The starting point of this PhD project was to evaluate the interest of such system as support for copper based catalyst for glycerol hydrogenolysis.

Table of contents

One of the first aim of this study was to find synthesis conditions that allow a control of the specific surface area of the final ZnO PAAH support and in particular to obtain high values in order to disperse Cu 0 effectively. Several parameters were studied such as the molecular weight and the concentration of the polymer, the washing and the drying phase and the synthesis temperature. We have also studied the thermal behaviour of ZnO PAAH hybrid supports to evaluate the evolution of their characteristics after calcination. Another important parameter that we focused on was the reproducibility of these syntheses.

I.

Effect of the synthesis parameters on ZnO PAAX

The samples will be labelled "ZnO PAAX", where X denotes the molecular weight of the polymer (X = 1.8 for PAA with Mw = 1 800 g.mol 1 , 2 for PAA with Mw = 2 000 g.mol 1 , 5 for PAA with Mw = 5 000 g.mol 1 and 100 for PAA with Mw = 100 000 g.mol 1 ). ZnO without PAAX will be used as a reference.

First, it should be noted that the synthetic yields compared to zinc moles are a function of the molecular weight of the PAAH (under the same operating conditions) with the particularity of being quantitative without PAAH and with PAA1.8, of being greater than 100% with PAA2 and PAA100 and of being only 50% with PAA5. For this latter, the centrifugation step only allowed to precipitate half of the solid, the rest remaining in suspension in water. The materials characterized in the following section are those resulting from centrifugation and rinsing. A study on the stable suspension obtained for PAA5 will also be discussed at the end of this sub chapter.

X ray powder diffractograms (XRD) for hybrid ZnO PAAX nanoparticles synthesized by sol gel method at room temperature with different PAAX (X = 1.8, 2, 5, 100) at a concentration of 0.63 mol.l 1 are shown in Figure 3 1. XRD patterns showed either amorphous ZnO, crystallized ZnO (hexagonal phase: zincite, PDF 01 083 6338 and Appendix 1) or Zn(OH) 2 (orthorhombic phase: wulfingite PDF 04 012 2300 and Appendix 2). Average crystallite sizes were calculated using the Debye Scherrer equation on the diffrac.eva software with peaks at 47.5, 57 and 63°.

Well crystallised ZnO with average crystallite sizes of 16 or 11 nm were obtained for ZnO without PAAX and ZnO PAA1.8 respectively (Figure 3 1 a, b). Moreover, diffraction peaks exhibited similar relative intensity between the two samples, hence there was no significant effect of PAA1.8 on the crystallinity of the material.

On the other hand, the use of PAA2 (Figure 3 A well crystallized mixture of ZnO and Zn(OH) 2 was observed when PAA100 was added to the solution with an average crystallite size of 12 nm for ZnO calculated with the peaks at 32, 35 and 63°and 30 nm for Zn(OH) 2 calculated with the peaks at 41 and 53° (Figure 3 1 e). In this last case, it was hard to know whether ZnO was transforming into Zn(OH) 2 or if Zn(OH) 2 was obtained first by the hydrolysis step of ZnEt 2 followed by a condensation. The second proposal seemed more likely with the presence of such a long polymer chain that partially blocked the further dehydration of Zn(OH) 2 especially when the synthesis was done at room temperature.

The XRD results have shown that all the crystallized samples had similar crystallite average size but the main difference was the crystallinity of the samples and the appearance of Zn(OH) 2 with PAA100. respectively. The molecular weight of the polymer seemed to have an impact on the size of the self assembled aggregates but we did not know whether this could be attributed to lower size of the ZnO crystallites. At a larger scale, ZnO PAA2 and ZnO PAA5 showed a combination of two morphologies spherical particles and 2D nano sheets; moreover some particles agglomerated had higher particle size in Figure 3 3 proving that the two materials were not so homogeneous.

Finally, ZnO PAA100 (Figure 3 3 e) exhibited two morphologies: irregular platelets and spherical aggregates of different sizes between 10 and 50 nm, which could be related to the presence of two phases as seen in XRD. TG analyses, carried out under air till 700 °C at a rate of 5 °C.min 1 , demonstrated that polyacrylic acid was present for all the sample but in different quantities (Figure 3 4). In theory, if all the polymer was integrated into the hybrid system, we should have a weight loss of about 46% above 200 °C. This value was very close to the ones for the samples obtained with PAA2 and PAA5 while for PAA100 and in particular PAA1.8, this one was quite far. Indeed, ZnO PAA2 and ZnO PAA5 showed weight loss of 40% and 41% above 200 °C, respectively, while ZnO PAA100 exhibited a weight loss of 30%. ZnO PAA1.8 displayed a low organic weight loss indicating that a very small amount of PAA1.8 was in interaction with ZnO NPs (Figure 3 4). The comparison between the TGA spectra of pure polymers (Figure 3 5) and hybrid materials (Figure 3 4) showed that these latter were more thermally robust because the temperatures onset of weight loss were much higher (320 °C instead of 200 °C). Evaporation of water/solvent occured below 200 °C and above 200 °C, TGA of the two pure polymers showed several weight losses corresponding to their pyrolysis. While degradation was complete at 500 °C for PAA2, 20 wt% of residue were still present for PAA5 and the total degradation occurred at temperature higher than 800 °C. The remaining 5 wt% was found as black carbon residue in the crucible. The FTIR spectra of the samples (Figure 3 6) showed that polyacrylic acid was mainly present as polyacrylate confirming the presence of Class II hybrid systems (in sol gel classification) and a strong interaction (via a deprotonation reaction) of the polymer on the surface of the ZnO nanoparticles. It should be noted that the use of polyacrylate ester (PAAMe) instead of polyacrylic acid during synthesis led to ZnO without any organic content and confirmed the lack of interaction between the ester and the surface of ZnO. In the case of polyacrylic acid, this interaction was strong and resulted in a very different flocculation phenomenon depending on its molecular weight. One possible explanation would be that partial deprotonation of polyacrylic acid led to hydrophobic systems, which could be noted as ZnO (PAAH) x (PAA) y , which precipitated in aqueous solution as self assembled spherical hybrid nano objects. These objects would only be obtained if the final hydrophobic/hydrophilic balance was appropriate, which would not be the case for a polymer that it was too short or too long. The synthetic yield with PAA5 was only of 50%. In order to collect all the ZnO material during the synthesis of ZnO PAA5, the aqueous suspension obtained after centrifugation was freeze dried. The powder obtained was naturally less dense than the one obtained after drying. In addition, the XRD analysis of the lyophilized powder of ZnO PAA5 (Figure 3 7) showed the two broad peaks at 33 and 59° associated with the unknown phase previously observed in Figure 3 TGA of the two ZnO PAA5 fractions (precipitate and suspension), either dried or freeze dried, did not show any significant differences in terms of thermal stability (onset temperature of degradation around 380 °C) and organic/inorganic ratio (about 50% of each). It could therefore be assumed that the two fractions were very similar in nature and composition.

Aqueous solutions of PAA2 and PAA5 polymers at the same concentration as those of the syntheses were freeze dried and the powders obtained were studied by XRD. The data showed an amorphous structure with two broad peaks at 20 and 38° (Figure 3 9).

It seemed that the unknown phase found on some XRD spectra of ZnO PAA2 and ZnO PPA5 might come from an hybrid species Zn(OH) PAAX. Indeed, on the EVA software, the two files Zn(OH) 2 (PDF 04 012 2300) and Zn 5 (CO 3 ) 2 (OH) 6 (PDF 04 036 1475, in appendix 6) were the ones that fit the most and two of the peaks fit those of the unknown phase. The BET analysis of the freeze dried ZnO PAA5 solid showed a smaller specific surface area of about 16 m 2 .g 1 , compared to 23 m 2 .g 1 after washing. Moreover the images obtained by the TEM and SEM analyses showed few mesospheres and a continuous polymer film containing nanocrystallites smaller than 100 nm (Figure 3 10). This morphology was different from ZnO PAA washed presented in Figure 3 2 d. At this stage of our study, we could assume that the unidentified phase present on XRD spectra could be associated with a 2D structure combining PAAH and nanoparticles of ZnO or Zn(OH) 2 . Therefore, the drying mode was also an important parameter to consider when synthesizing the different ZnO PAAX materials in order to generate materials with high specific surface area. The impact of freeze drying on ZnO nanomaterials has already been mentioned in the literature for a synthesis of ZnO by reaction between zinc nitrate and sodium carbonate 3 .

After synthesis, the batch was separated into two parts; one part was dried in an oven at 110

°C and the other one was freeze dried. The powders obtained were then calcined at different temperatures and the results are summarized in the table below: The results presented in this study were quite similar to those obtained for ZnO PAA5. Freeze drying was accompanied by a decrease in specific surface area associated with an increase in particle size (Figure 3 10). In the rest of our study, only conventional drying in an oven at 80 °C for 14 hours will be used.

The original spherical morphology of the 3D hybrids ZnO PAA2 and ZnO PAA5 nano structured aggregates was further investigated to determine the synthesis parameters, such as washing/rinsing process, concentration of polymer and synthesis temperature, that would influence the final characteristics of these hybrid objects and in particular their specific surfaces that were sought to be maximized for use as supports for copper based catalysts.

Washing steps are known to be an important step in the synthesis of hybrid nanoparticles as they can affect the specific surface area by removing organic matter that would not be effectively grafted onto the surface 4 . In a typical synthesis, after one hour of reaction, the ZnO PAAX slurry was centrifuged to obtain a wet powder of ZnO PAAX which was rinsed for a few minutes with a small amount of ethanol. Due to a lack of reproducibility of the structural characteristics (BET for instance from 20 to 40 m².g 1 ), we decided to carry out different washing by using two solvents (water and/or ethanol) and different washing volumes. Wet samples were then washed with a specific volume of solvent (from 30 to 90 mL for an initial volume of 200 mL of PAAX solution) for 30 minutes under stirring. When the volumes of PAAX solution were higher, the volume of solvent was proportionally increased, as was the washing time. For all samples (unwashed and washed), a centrifugation step and oven drying at 80°C in ambient air were performed to obtain the final powder.

ZnO PAA2 system

The ZnO PAA2 samples obtained without washing or after washing with different volumes of ethanol (30 mL, 60 mL, 90 mL) or ethanol/water (60 mL) have shown very similar XRD diffractograms (Figure 3 11) and ATG curves (Figure 3 12). Only the intensity of the peak at 33° (unidentified phase) seemed to increase with the amount of ethanol. It would therefore be concluded that the washing step did not impact the crystallinity of the sample, the ZnO crystallite size, or the amount of organic matter (Table 3 3). The latter was predictable due to the insolubility of PAA2 in ethanol. Nevertheless, the BET and TEM results show that the samples were structurally different.

Indeed, the BET results (Table 3 3) showed that washing with ethanol significantly increased the specific surface area of ZnO PAA2 samples from 20 to 91 m².g 1 , especially when the volume of ethanol was large. TEM images before and after washing with 90 mL of ethanol provided a possible explanation for this increase in specific surface area (Figure 3 13). These showed that ethanol washing induced the "appearance" of a larger quantity of 2D nano sheet structures composed of zinc and polymer according to EDX.

If we combined all these data, we could consider that ethanol only removed water whose presence would result in folding these 2D structures onto themselves via hydrogen interactions, for example. The elimination of water would therefore make it possible to unfold these structures which would be at the origin of the increase in the specific surface area observed after washing. The presence of water when washing with a water/ethanol mixture (1:1) confirmed this effect with a lower specific surface area of 41 m².g 1 .

ZnO PAA5 system

The effect of ethanol washing on the properties of ZnO nanoparticles was extended to ZnO PAA5. As before, the XRD diffractograms showed no impact on the crystallinity of the material as shown in Figure 3 14. ZnO PAA5 washed with different volumes of EtOH was amorphous like ZnO PAA5 rinsed with EtOH (Figure 3 1 d). Similarly, the TGA also showed an average loss of 50% for the three volumes (Figure 3 15, Table 3 4) as for the ZnO PAA2 samples (Table 3 3).

As for ZnO PAA2, there was an increase in specific surface area with the increase in the volume of EtOH from 20 to 75 m².g 1 (Table 3 4). The length of the polymer chain having shown a significant impact on the morphology of the ZnO PAA material, it was interesting to study the effect of the mass percentage of PAA2, only, during the synthesis on the material.

As mentioned in the experimental section, the polyacrylic acid solution used to produce the majority of ZnO PAAX nanoparticles had a concentration of 0.63 wt% (the commercial solution having a concentration of 63wt%). Different PAA2 solutions were therefore prepared with concentrations ranging from 0.3 to 1.2 wt% and a synthesis without PAAX was carried out as a reference.

As the washing step improved the properties of the material, all samples were washed with 90 mL EtOH. As shown in Figure 3 16, peaks of the unknown phase at 33° and 59° were present when PAA2 was added, compared to ZnO without PAA2. The increase in the weight percentage of PAA2 resulted in a decrease in the crystallinity of the material to a quasi amorphous structure with 1.2 wt% of PAA2 (Figure 3 16). The BET specific surfaces are summarized in Table 3 5 and the values are plotted according to the weight percentage of PAA2 in Figure 3 17. The results showed that the increase in the specific surface area of ZnO PAA2 hybrid NPs followed a Gaussian with a maximum around 75 m².g 1 between 0.3 and 0.6 wt%. For higher PAA2 concentrations, the specific surface area decreased to 17 m 2 .g 1 for 1.2 wt%. TEM images for 0.3 wt% and 1.2 wt% of PAA2 (Figure 3 18) showed that a high initial concentration of PAA2 generated large aggregates in size with some mesospheres but no more 2D nano structures such as nano sheets.

The same behaviour was found in an article studying the effect of raw rice as a bio model for the synthesis of ZnO by hydrothermal method using zinc acetate and sodium hydroxide as reactants 6 . The authors found that the increase in raw rice concentration led to an increase in the specific surface area of ZnO nanoparticles up to a maximum where it then decreased. The TGA curves under air (Figure 3 21) showed that weight losses between 200 and 600 °C (Table 3 5) were proportional to the weight percentage of PAA2 added initially. With 0.3 wt% of PAA2, the proportion of inorganic components was 67%, while it is only 39% for 1.2 wt% of PAA2. It should be noted that the current specific surface area (60 m 2 .g 1 ) associated with ZnO PAA2 synthesized with 0.63wt% did not correspond to that indicated in previous works in Table 3 3 (90 m 2 .g 1 ), whereas it should be the same. These two sets of samples (Table 3 5 and Table 3 3) were synthesized within a few months of each other. This suggested the influence of another parameter that had been neglected so far, namely the synthesis temperature. One of the major problems with ZnO PAAX synthesis was undoubtedly the reproducibility of the results and, in particular, the measurement of the specific surface area which seemed extremely sensitive to subtle changes of parameters.

Therefore, after having set some parameters in the previous sections, the synthesis temperature was finely controlled in order to minimize its impact on the structural properties of the hybrid ZnO. All the experiments described earlier in this chapter were performed at room temperature (between 15 and 20 °C), so we focused on studying the influence of a temperature range of 0 to 40 °C on the structural properties of ZnO PAAX materials (X = 2 and 5). Other parameters such as washing with 90 mL EtOH and 0.63wt% of PAA2 concentration were kept constant.

Effect of the synthesis temperature on ZnO PAA2

For ZnO PAA2, syntheses were performed at 0 °C, 10 °C and 40 °C. The XRD results showed no difference in crystallinity and crystallite size between the three materials (Figure 3 22, Table 3 6) which were comparable to those of ZnO PAA2 synthesized at room temperature (Figure 3 11).

BET specific surface area measurements showed differences between materials (Table 3 6), with the highest surfaces (80 90 m².g 1 ) being obtained at low temperature (0 10 °C), in the same range than values obtained previously at room temperature. On the other hand, a temperature of 40 °C had the effect of reducing by two the specific surface area (37 m².g 1 ).

The TG analyses (Figure 3 23) did not show any effect of the synthesis temperature on the inorganic/organic ratios since they were identical for materials produced at the same concentration of PAA2 of 0.63 wt%. Temperature therefore did not affect the thermodynamics of the system since the same raw material was formed overall but it affected the material's morphological evolution through the 60 minutes of synthesis. In order to have a general idea of the behaviour of ZnO PAAX NPs and the relationship between nucleation growth and synthesis temperature, the study was extended to ZnO PAA5 to verify if there was also an effect of the nature of the polymer.

Effect of the synthesis temperature on ZnO PAA5

The results obtained for ZnO PAA5 were in the opposite direction to those obtained with ZnO PAA2. Indeed, based on XRD measurements (Figure 3 25), the structure of ZnO crystallites was strongly influenced by the synthesis temperature. At low temperatures, ZnO PAA5 nanoparticles were mainly amorphous or very small, while an increase in temperature led to an increase in the degree of crystallinity or size of crystallites. The synthesis temperature of 60 °C was added to the three previously selected values to visualize the evolution in terms of structure over a larger temperature range. Table 3 7 summarizes the different physico chemical characteristics of the four samples and shows that the specific surface area tends to increase with the synthesis temperature with a maximum of 120 m².g 1 at 40 °C, which is the opposite for PAA2. The latter surface area was the highest value that has been obtained so far in our study. The average crystallite sizes of ZnO around 20 nm and the percentages of organic TEM and SEM analyses (Figure 3 27 and Figure 3 28) of the samples synthesized at 0 °C and 40 °C showed that this increase in specific surface area was once again accompanied by the appearance of a 2D nano sheet phase, a phenomenon already observed in the ZnO PAA2 system study when washing with 90 mL ethanol (Section I.2.1) or with 0.3 wt% of PAA2 (Section I.3). TEM images showed that at 0 °C, the material was essentially composed of spherical aggregates of an average size of 60 nm associated with a very interesting monodispersity (Figure 3 27 a). This observation was confirmed by the SEM images obtained on this same material where the monodispersity was visualized on a larger scale (Figure 3 28).

At 40 °C, there was a majority of spherical aggregates on average smaller (40 nm) but also more polydispersed. This smaller size could also explain the increase in specific surface area, but it also seemed plausible to explain it by the presence of 2D nano sheet structures. SEM images of the material obtained at 40 °C showed that there was an intimate mixture between spherical aggregates and 2D nano sheets. This could be very interesting for different applications but also delicate in order to develop a robust method for the production of catalyst supports with a high specific surface area.

Nevertheless, these hybrid supports will have to undergo calcination steps between 300 and 400 °C when developing copper based catalysts, so it is also necessary to evaluate the thermal behavior of these structures in the temperature range up to 400 °C.

As shown in the TG analyses in Figure 3 4, ZnO PAA2 and ZnO PAA5 began to degrade thermally at around 300 °C. We therefore studied the thermal behaviour of ZnO PAA2 and ZnO PAA5 substrates between 300 and 400 °C since they will be calcined at 400 °C after copper deposition. ZnO PAAX powders were thus characterized after calcination at 300, 325, 350 and 400 °C under air in a static furnace at a heating rate of 5 °C.min 1 until the desired temperature was reached and maintained for 4 hours.

It is important to mention that this thermal behaviour study was carried out on substrates obtained at room temperature with a PAAX mass concentration of 0.63 wt% and rinsing with ethanol. It should therefore be dissociated from the previous sections on synthesis parameters but it will nevertheless make it possible to evaluate, in a relative way, the thermal behaviour of the different ZnO PAA2 and ZnO PAA5 structures.

Effect of calcination temperature on ZnO PAA2

The XRD diffractograms associated with the four samples obtained after calcination of ZnO PAA2 at 300 °C, 325 °C, 350 °C and 400 °C (Figure 3 30) showed an increase in crystallinity with increasing calcination temperature. The crystallinity was calculated by Rietveld refinement with FullProf software and the results are presented in Table 3 8. Indeed, at 300 °C, ZnO PAA2 was 90% amorphous and the 10% of the crystallized phase corresponded to hexagonal ZnO will a crystal size between 9 nm (calculated with peaks at 47°, 57° and 63°) and 15 nm (calculated with peaks at 31.5°, 34° and 36°). The increase in calcination temperature to 325 °C resulted in a 90% increase in crystallinity with small crystallites of an average size of 4.4 nm.

For temperatures above 325 °C, ZnO PAA2 was fully crystallized with the same hexagonal structure with an average size of 12 nm for ZnO PAA2 calcined at 350 °C and 19 nm after calcination at 400 °C calculated with all the peaks. The increase in the size and degree of crystallinity of the crystallites with the increase in the calcination temperature has been reported in several studies on ZnO nanoparticles and ZnO films. 8,9 It therefore seemed that 350 °C was a transition temperature for the ZnO PAA2 system, which was confirmed by the TG analyses since the organic content became very low above 325 °C. Indeed, the TGA curves (Figure 3 31) confirmed the presence of a large part of the polymer after calcination at 300 °C and 325 °C with respective total weight losses of 47 and 53% for ZnO PAA2 (Table 3 8 Compared to as prepared ZnO PAA2 sample, the BET surface area increased from 20 m 2 .g 1 to 28 and 38 m 2 .g 1 after calcination at 300 °C and 325 °C, respectively. BET showed an increase in specific surface area with the increase in calcination temperature up to a maximum of 51 m 2 .g 1 after calcination at 350 °C. At 400 °C, the specific surface area was the lowest, indicating that ZnO nanoparticles started sintering.

The increase in the specific surface area of ZnO nanoparticles for low calcination temperatures followed by a decrease at higher temperatures was reported by Shohel et al. 9 They prepared ZnO by electrolysis of zinc plates in a sodium bicarbonate solution for 1 hour under a fixed current of 2A. The precipitates were then centrifuged, air dried and crushed in a mortar and calcined at 200 800 °C. Table 3 9 summarizes the values obtained in their article for the effect of the calcination temperature on the specific surface area of ZnO NPs. The specific surface area of ZnO increased from 40 m 2 .g 1 to 51 m 2 .g 1 with an increase in the calcination temperature from room temperature to 300 °C followed by a decrease in the specific surface area for temperatures above 400 °C (Table 3 9). Indeed, at low calcination temperatures, the particles were smaller than at high temperatures, which led to an increase in pore volume and specific surface area. At higher calcination temperatures, the particles tended to sinter due to inter crystalline growth resulting in a decrease in pore volume and therefore a decrease in specific surface area. The decrease in the specific surface area of ZnO nanoparticles when calcined at temperature above 400 °C seemed to be frequent, it had also been reported in Bai's article 10 . Authors prepared ZnO NPs by sol gel method by reacting a zinc acetate solution with a potassium hydroxide solution in the presence of tetraethylorthosilicate as a surface modifier. The colloid as prepared was separated from the unreacted molecules by centrifugation and washed with methanol and water. Figure 3 33 shows the relationship between the calcination temperature of ZnO nanoparticles and the specific surface area. The specific surface area decreased for calcination temperature above 400 °C. Between 200 °C and 400 °C, the specific surface area was approximately 65 m 2 .g 1 and decreased to 27 m 2 .g 1 at 800 °C. Again, the authors attributed this behaviour to the increasing size of crystallites at higher temperatures. The TGA curves of solids calcined at 300, 350 and 400 °C showed one weight loss, between 200 and 600 °C, of less than 10%, confirming the almost total degradation of the polymer from 300 °C and a slight evolution of the overall composition (Organic/Inorganic) of the materials obtained at these three temperatures (Figure 3 36). The same result as for ZnO PAA2 was found with an onset of weight loss temperature that was lowered by 100 °C, proving again that the organic component was of a totally different nature from the original PAA5.

c d

Compared to ZnO PAA2, the structure of the ZnO PAA5 hybrid material has appeared less robust. This observation could be correlated to the smaller size of the aggregates for PAA5 and therefore to a larger area of exposure to thermal degradation. Table 3 10 summarizes the characteristics of the materials obtained. But as mentioned above, these hybrid materials will be used as a support for the development of heterogeneous catalysts for glycerol hydrogenolysis via the deposition of copper molecular compounds followed by a first calcination step at 400 °C. If we refer to the specific surfaces obtained at this temperature, they were around 20 m².g 1 for the two supports developed. It was therefore the impact of the morphology and the nature of the interactions between the different hybrid carriers ZnO PAA2 and ZnO PAA5 and copper precursors that must be studied in detail and these results will be described in Chapter IV.

II. Conclusion

In conclusion of this chapter, it was demonstrated that the development of reproducible batches of ZnO PAAX hybrid nanomaterials with well defined properties (large specific surface area in particular) involved the fine control of many parameters such as synthesis temperature, polymer quantity and powder washing.

Weight concentrations of PAA2 or PAA5 in the range of 0.5 to 0.63 wt% were appropriate to obtain a 3D mesospheric morphology following the self assembly phenomena of the ZnO PAAH hybrid nanoparticles. Their syntheses were generally accompanied by a secondary phase in the form of 2D nano sheets. The latter, which seemed to be preferred at high temperature for PAA5 and low temperature for PAA2, led to a significant increase in the specific surface area of the final material. Thus, for ZnO PAA2, low temperature synthesis (0 10 °C) was associated with a high specific surface area of 91 m 2 .g 1 and for ZnO PAA5, a maximum specific surface area of 120 m 2 .g 1 was obtained at 40 °C.

Ethanol washing seemed to affect the surface dehydration of aggregates, which led to a decrease in inter aggregate (or even inter nano sheet) interactions of Van der Waals (hydrogen interactions), allowing an increase in the specific surface area.

The thermal stability of the ZnO PAA2 and ZnO PAA5 systems depended on the size of the aggregates and therefore on the molecular weight of the polymer. ZnO PAA5, forming smaller aggregates, was less thermally stable with total degradation of the hybrid system from 300 °C while the mesospheres of ZnO PAA2 were still observable at 325°C.

In the following chapters, ZnO PAA2 and ZnO PAA5 will be used as starting materials for copper based catalysts in order to study the effect of certain support synthesis parameters on the catalytic results (conversion, selectivity) in the hydrogenolysis of glycerol. Specific attention will be given to ZnO PAA2 synthesized at 0 °C, washed with ethanol and not calcined as a support for Cu nanoparticles. This chapter is dedicated to the synthesis of Cu/ZnO catalysts and their use for the hydrogenolysis of glycerol into 1,2 PDO. We first started by synthesizing Cu/ZnO with different weight percentage and concentration of copper by deposition precipitation with urea method (DPU). This first screening was done using a commercial ZnO in order to choose the synthesis parameters for copper deposition that led to the highest conversion.

CHAPTER IV

Development of Cu/ZnO catalysts: support effect

Table of contents

In a second time, the home made ZnO/PAAH was used as support in order to check the effect of the synthesis parameters of ZnO on the activity of the catalyst. We ended the chapter by elaborating Cu ZnO with a novel one pot method.

Commercial ZnO as support

The first set of catalysts was developed using commercial ZnO purchased from Alfa Aesar as a support. Figure 4 1 shows the XRD patterns of this material, it was very well crystallized with the hexagonal phase (PDF number 01 083 6338) of zincite. The size of the crystallites calculated using the Scherrer equation showed that crystallites had a size of about 50 nm when calculated with all the peaks, except with the one at 33°, corresponding to the crystallographic plane (002), i. e. 72 nm. On the other hand, TEM images (Figure 4 2) showed large particles ranging from 50 to 200 nm with a heterogeneous shape. The high particle size was reflected in the specific surface area measured by BET which was very low with a value of 13 m 2 .g 1 .

Figure 4 1: XRD diffraction pattern of commercial ZnO The pH of the solution before adding urea was neutral and reached 8 at the end of the deposition step. After heating, the solution was filtered, washed with water and dried in the oven overnight in air. The dry powder mentioned as "After DPU" was calcined at 400 °C for 4 hours in air and reduced at 300 °C under H 2 for 1 hour followed by a passivation step with 1% O 2 /N 2 flow at room temperature during 30 minutes.

The reaction of deposition of copper on ZnO support in presence of urea is proposed in scheme 4 1 (the stoichiometry is not shown). T° corresponds to the temperature of deposition precipitation and it was chosen according to the temperature of degradation of urea (> 60 °C). on Diffrac.Eva software. This phase would result from a partial dissolution of ZnO in basic media followed by co precipitation. After calcination, the later phase transformed into monoclinic CuO (PDF number 00 045 0937 appendix 8) where the main peak was at 38°.

Hydrogen reduced CuO to cubic Cu 0 (PDF number 04 013 9963 appendix 9) with the two main peaks at 43° and 51°. A third peak for Cu 0 appeared at 74° for higher copper loadings. The intensity of the peaks depended on the weight percentage of copper. It is worth noting that the ICP results (Cu exp ) are consistent with the theoretical weight percentage proving that the DPU method (in our conditions) was guarantying the total deposition of copper onto the support (Table 4 1). The crystallite sizes of Cu 0 are also shown in Table 4 1. The estimation of the crystallite size might not always be accurate, as the peaks were broad and small, especially at low Cu loading but the error did not exceed 5 % of the value. At the same weight percentage, the crystallite main size showed a decrease in the size when increasing the Cu 2+ concentration. This phenomenon did not seem to be random because it occurred for different copper loadings. To our knowledge, there is no study showing the effect of precursor concentration at the same weight percentage for the synthesis of Cu catalysts by DPU. At higher concentration and lower volume, the kinetic was different and the precursor should precipitate more rapidly. Or the temperature and pH increased with time; therefore, the precipitation might occur at lower temperature and / or lower pH at higher concentration. This might result in the formation of a larger number of nuclei and / or a better support/precursor interaction, which would result on the formation of smaller particles. At low weight percentage (2.5%) of copper, the conversion was very low, only 2% of glycerol was converted, and thus independently of the different initial concentrations of Cu 2+ . The increase in the weight percentage of copper to 5% resulted in an increase in catalyst conversion, as the same mass of catalyst was used for all the reactions (500 mg). Nevertheless, there was no homogeneous behavior for the catalyst at this percentage where conversion increased and/or decreased without having a defined relationship with Cu 2+ concentration.

When the weight percentage of copper was higher, the catalyst began to behave according to a defined trend. The increase in the concentration of the Cu 2+ solution for 10 wt% Cu/ZnO resulted in an increase, followed by a decrease in catalyst conversion, with an optimum of 25% for a Cu 2+ concentration of 20.8 mM. At higher weight percentage of copper (15% and 20%), the support began to be highly charged and might be inaccessible to glycerol molecules, which prevented an increase in conversion. Actually, the surface density 2 of Cu per nm 2 was around 8 atoms/nm for a weight percentage of copper higher than 8 wt%, calculated by considering an average surface area of the catalyst of 20 m 2 .g 1 . A monolayer of copper had a surface density around 8 atoms/nm 2 2 which meant that the catalyst's surface might be saturated by copper and ZnO might be inaccessible. As shown in scheme 4 2, the catalyst was bifunctional and the first step of the mechanism involved the acidic properties of ZnO. That was why, if the surface of the support was highly charged with copper, the interaction between the molecules of glycerol and ZnO particles decreased leading to a decrease in the glycerol conversion.

An important point to mention was that all catalysts were highly selective (> 90%) towards 1,2 PDO as shown by GC (see appendix 10) and the remaining 10% was mainly EG and traces of ethanol and unknown product. This was in agreement with the literature 3,4 where Cu/ZnO were known to exhibit high selectivity to 1,2 PDO under these reaction conditions, as shown in chapter I (Table 1 8).

In the same paper cited above, showing the formation of the copper nitrates after DPU, the effect of the weight percentage of copper in the solution during synthesis was also studied for Cu/TiO 2 1 and authors observed an increase in the copper crystallite size with the increase in the copper weight percentage. It was a common phenomenon and we got the same results.

The effects of copper weight percentage on catalyst properties and structures were then studied in depth using the "20.8 mM Cu 2+ concentration" series (in grey in Table 4 2). As we could not find a published work dealing with the effect of the precursor concentration in the solution at the same weight percentage, the effect of [Cu 2+ ] will then be discussed in more details for 10 wt% Cu/ZnO. As we already observed, the copper weight percentage measured by ICP corresponded to the theoretical percentage indicating the total copper deposition on ZnO. With regard to the specific surface area of the catalyst, there was no change to note since all catalysts were prepared with the same commercial support having a surface area around 13 m 2 .g 1 . In the last stage of catalyst elaboration, the specific surface area increased to an average of 20 m 2 .g 1 due to copper deposition and heat treatment. percentage but the size of the crystallite decreased slightly from 58 nm at 2.3 wt% to 48 nm at 19.2 wt% of copper. For copper contents less than or equal to 15 wt%, the size of Cu 0 crystallites (C.S.) before catalysis was constant (ca. 14 nm). On the other hand, at 20 wt%, the size of Cu 0 crystallites increased from 14 to 20 nm. When the weight percentage of copper was high (19.2 wt%), there were not only an increase in the size of the crystallite, but also in the intensity of the Cu 0 peaks. The evolution of the conversion and selectivity to 1,2 PDO versus time are shown in Figure 4 5. For all the catalysts the conversions were low with a maximum of 25% after 24 h for 10%Cu/ZnO. These catalysts had identical kinetic profiles, with a linear increase in conversion with the increase in reaction time (Figure 4 5) suggesting a zero order reaction. The coefficient R 2 of the equation was greater than 0.99 for all catalysts.

At low conversion (2 %; associated with 2.3%Cu/ZnO), 1,2 PDO was the only product detected by GC analysis; the selectivity is presented in Figure 4 5 by the grey curve. The selectivity toward 1,2 PDO was high for all the other catalysts ( 90 %) with selectivity toward EG 10% (black curve, Figure 4 5). It would therefore appear that between 15 and 20 wt% of copper, copper particles saturate the catalyst surface, resulting in a drop in conversion. Since the support played a major role in this reaction by hydrolyzing glycerol to acetol in a first step, as we already concluded, saturation of its surface led to a decrease in reaction rate. Another factor contributing to this low conversion was the sintering of copper and ZnO particles. Indeed, Figure 4 6 shows the diffractograms associated with 2.3%Cu/ZnO and 19.2%Cu/ZnO after test. The crystallite sizes after reaction were reported for all the catalysts in Table 4 3. For the catalysts with low Cu loading, the crystallites did not show any strong structural changes. However, for the catalysts with 15 and 20 wt% of copper, the crystallite sizes showed a greater sintering of ZnO and, more specifically, Cu 0 nanoparticles. This behavior was reported by Gao et al. 6 who demonstrated that increasing the Cu/Zn molar ratio from 0 to 1.86 led to an increase in glycerol conversion from 0 to 48% followed by a decrease to 33% with increasing copper content. Nevertheless, selectivity towards 1,2 PDO increased continuously up to 67% with increasing copper content. The results published by the authors are listed in Table 4 4.

Table 4 4: Effect of Cu/Zn molar ratio on the conversions and selectivities of the catalysts from reference 6.

At this point of our study, it would therefore be desirable to investigate the impact of the concentration of the copper precursor solution for the same final content of 10 wt%, which led, under our experimental conditions, to the best glycerol hydrogenolysis Cu/ZnO catalyst.

their catalyst (Cu Zn/MgO) without being retreated for the same catalytic reaction. The question of whether or not the new phase was carbonaceous was not discussed in this article.

However, some peaks were not attributed and the mention of methanol as a by product indicated a C C cleavage. In our case, ethylene glycol and traces of ethanol and methanol (less than 0.001 M) were detected by GC analysis for the most efficient catalyst (10%Cu/ZnOcom

2 ), confirming C C cleavage under our experimental conditions. The phase containing carbon was not observed after reaction for the less active 10%Cu/ZnOcom 8 . The TGA curves of the catalysts before catalysis in Figure 4 9 showed a similar thermal behavior, in terms of weight loss. A slight shift in thermal events to higher temperatures could be noted for 10%Cu/ZnOcom 8 . Theoretically, no weight loss should be recorded because the catalysts were already calcined at 400 °C and reduced at 300 °C and the TGA was performed under nitrogen. Nevertheless, the weight gain after 300 °C could be attributed to the contamination of the TG gas line with O 2 leading to CuO x . Then, the weight remained constant until high temperatures (900 °C) where some carbon species could be degraded, contributing to the small weight loss of 2%. From a global point of view, these two catalysts with 10% by weight of copper were thermally stable up to 900 °C.

The TGA curves also showed differences in thermal behavior between the two catalysts after reaction. As shown in Figure 4 9, for 10%Cu/ZnOcom 8 the data were very close to those of the starting catalyst. However, for 10%Cu/ZnOcom 2 there was a significant and unprecedented loss of weigt of about 11% between 200 and 600 °C, confirming the presence of CO 3 and OH on the used catalyst. The small specific surface area of the support led to deactivation of the catalyst for copper loads greater than 10 wt% due to sintering of the copper particles during catalysis and saturation of the surface.

The increase in the concentration of the Cu 2+ precursor solution allowed a better dispersion of copper nanoparticles, resulting in smaller copper crystallite size, but this did not assure a higher activity at higher copper weight percentage.

When highly active, C C bond cleavages occurred and it was accompanied by a structural change of the catalyst after reaction.

At low conversion (2%), the catalyst was highly selective (99%) towards 1,2 PDO while at higher conversion (25%), selectivity decreased to 90% with the appearance of ethylene glycol.

We have then decided to study the different ZnO PAAX hybrid systems (X = 2, 5, 100) with high specific surface area, described in chapter 3, as supports instead of commercial ZnO, using the optimal concentration of Cu 2+ (20.8 mM).

II. ZnO PAAX (X = 2, 5, 100) as supports

The catalysts were prepared from a non calcined ZnO PAA2 support, washed with 90 mL EtOH and by adding copper according to the DPU method described in the upper section, with [Cu 2+ ] = 20.8 mM. We will address the effect of the synthesis temperature of the support and the weight percentage of copper loaded on the different support on the catalytic conversion of Cu/ZnO PAA2 catalysts.

Effect of the synthesis parameters (support and copper wt%)

The catalysts were synthesized with different wt% of Cu, using different hybrid ZnO supports, following scheme 4 3 identical to the DPU method described in the first section with the commercial ZnO.

Table 4 6 presents some of the properties of the catalysts, as well as the conversion and selectivity at 24 h. The ZnO PAA2 were synthesized at different temperature (0, 10, RT and 40 °C). The specific surface areas presented in Table 4 6 were for the support after drying. After DPU, calcination and reduction, all the catalysts exhibited the same specific surface area of about 25 m 2 .g 1 . The difference in specific surface area for supports synthesized at the same temperatures came from the reproducibility problems when elaborating the ZnO supports, as discussed in Chapter III, because the supports were not synthesized at the same time.

The catalysts are labelled XCu/ZnO PAA2y z where X is the theoretical weight percentage of copper, y is the temperature of synthesis of the support and, when needed, z is the number of the experiment (to differ between two catalysts having the same loading of copper and the same synthesis temperature of support but not produced from the same batch). For example, 10Cu/ZnO PAA2 0 2 is the catalyst prepared with 10 wt% of copper, with a support elaborated at 0 °C and different from 10Cu/ZnO PAA2 0 1 .

The selectivities towards 1,2 PDO were similar for all the catalysts, independently of the support. It went from 98% towards 1,2 PDO at low conversation, down to 86 90% at high conversion. Ethylene glycol was formed as a by product.

When comparing the effect of the copper percentage on the conversion, we noticed an increase in the conversion with the increase of copper loading under the same parameters till 20 wt% of copper. This was attributed to the higher ratio n Cu /n glycerol at higher loading of copper since the catalytic reaction conditions were the same for all the catalysts (weight of catalyst and glycerol loading). 20Cu/ZnO PAA2 RT 1 and 20Cu/ZnO PAA2 RT 2 had very similar properties: The two batches of ZnO PAA2 were synthesized at room temperature and exhibited very similar S.S.A. (80 and 82 m 2 .g 1 ). Moreover, Cu crystallite sizes were in the same range (19 21 nm) however both catalysts showed different conversions (44% vs. 55%) due to the difference in actual copper loading (18.0% vs. 21.2%). When comparing the catalysts based on support synthesized at 0 °C (Cu/ZnO PAA2 0 ), Cu 0 sizes were around 12 nm for 5, 10, and 20 wt% of Cu, and the conversion increased from 4% to 70% when increasing Cu loading.

However, Cu crystallite size increased up to 20 nm when increasing the loading to 38 wt% (40Cu/ZnO PAA2 0 ). This was associated with a decrease in conversion from 70% to 4%. The high copper load could have blocked the active sites on the ZnO and caused the activity to decrease compared to a lower weight percentage of copper. If we calculate the number of atoms of copper occupying the surface of ZnO PAA2 with a SSA = 80 m 2 .g 1 , 41.6 wt% of copper correspond to 8 atoms of Cu per nm 2 which was more or less the number of atoms before saturating the surface. The second main reason for this low conversion might be the deactivation of 40Cu/ZnO PAA2 0 due to the sintering of copper particles during the catalysis as Cu 0 increased from 20 nm to 40 nm. An important result was the possibility of depositing a very high copper charge, about 38 wt%, on the surface of such supports. Unfortunately, catalyst activity decreased strongly for this high loading, but this could be beneficial for other catalytic reactions where ZnO had no role in the reaction mechanism. Compared to commercial ZnO, the same phenomenon was observed but at lower copper loading (15 wt%) due to its lower specific surface.

Regarding the effect of the synthesis temperature of the support. If we compared 10Cu/ZnO PAA2 RT and 10Cu/ZnO PAA2 0 , the support specific surface area and the Cu dispersion were slightly higher for the second one. The catalyst with the support synthesized at 0 °C was more active (13% vs. 48% of conversion). The same trend was observed for the catalysts with 20 wt% of copper. Comparing the support synthesized at 0 °C (20Cu/ZnO PAA2 0 ), 10 °C (20Cu/ZnO PAA2 10 ), RT (20Cu/ZnO PAA2 RT ) and 40 °C (20Cu/ZnO PAA2 40 ), when the temperature decreased, the specific surface area increased (37, 80, 91, 105 m 2 .g 1 ), and the crystallite size decreased (from 21 to 12 nm). This resulted in an increase in the conversion, from 27% to 70%. If we looked back to the effect of the synthesis temperature on the NPs of ZnO, the main difference laid in the morphology of the material. It appeared that the presence of 2D nano sheets at low temperature generated a higher dispersion of Cu particles than the support synthesized at high temperature. Figure 4 10 shows the XRD diffractograms after reaction of the two catalysts 10Cu/ZnO PAA2 0 and 10Cu/ZnO PAA2 RT synthesized at 0 °C and RT (20 25 °C), respectively. The main difference laid in the size of the copper nanoparticles after the reaction and in the appearance of the new phase for the catalyst with the highest conversion as already observed with 10%Cu/ZnO com 2 . 10Cu/ZnO PAA2 0 1 showed a conversion of 48% with no increase in the copper crystallite size after reaction while 10Cu/ZnO PAA2 RT 1 displayed 13% of conversion and a drastic sintering of copper crystallite from 15 to 40 nm. Once again, the phase containing carbon was associated with good activity (48% conversion) and cleavage of C C bonds.

Catalyst

Selectivity at 1,2 PDO was 90% and the main by product was ethylene glycol. Generally, the increase in the specific surface area of the support led to a decrease of the deposited metal particle size and an increase in glycerol conversion. The most active catalyst, with a 70% glycerol conversion after 24 hours of reaction at 30 bar of hydrogen and 200 °C, was the one synthesized with ZnO elaborated at 0 °C and loaded with 20 wt% of copper (20Cu/ZnO PAA2 0 ) which was also the one with the highest specific surface area of 105 m 2 .g 1 . This remarkable value in terms of specific surface area for a ZnO based system allowed a high dispersion of copper and the achievement of the smallest Cu 0 sizes around 12 nm.

Moreover, an increase in Cu loading was also associated with an increase in activity up to 20

Cu wt%. The results obtained showed that there was a compromise to be found in terms of copper percentage in order to accommodate both a reasonable amount of copper and a high dispersion.

Comparing the properties of Cu/ZnO com to Cu/ZnO PAA2, the two families of catalysts showed similar copper crystallite size which ranged between 11 and 20 nm depending on the copper weight percentage. However, under the same conditions, the conversion was always higher when the support used was the one developed instead of the commercial one.

As the highest conversion was obtained at 20 wt% of copper and 20.8 mM Cu 2+ with ZnO PAA2 synthesized at 0 °C and washed with 90 mL of EtOH (20Cu/ZnO PAA2 0 , Table 4 6), the synthesis was up scaled from 1 gram of catalyst to 3 grams. Two catalysts were synthesized using two different batches of ZnO/PAA2 supports, synthesized under the above conditions.

The Cu loading were determined by ICP and are included in Table 4 7 and the XRD diffractograms are shown in Figure 4 11. The main difference between the two reduced catalysts was the size of the Cu 0 crystallites which was 20 nm for the 20.6 wt% Cu catalyst and 28 nm for the 19.8 wt% Cu catalyst. Two reactions were performed with 20.6%Cu/ZnO and 19.8%Cu/ZnO using 1 g under the same reaction conditions (0.23 M glycerol, under 30 bar H 2 , 200 °C and 24 hours). Despite the difference in Cu main crystallite size, the two catalysts exhibited the same kinetic profiles up to 24 hours as shown in Figure 4 12 and a final conversion of ca. 63%. Figure 4 13 shows the XRD patterns associated with 20.6%Cu/ZnO and 19.8%Cu/ZnO after reaction using 1 g where the two catalysts exhibits the carbonic phase (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 . Another reaction was conducted with 1 g of 19.8%Cu/ZnO but this time the reaction was stopped after 6 h and the catalyst was recovered, dried and analyzed by XRD (Figure 4 13). After 6 hours of reaction, the catalyst was not showing any structural changes, the carbonic phase was not present. This confirmed the hypothesis that the phase was appearing at high conversion due to the formation of products resulting from C C cleavage. However, after all the reactions, the ZnO crystallite sizes increased from ca. 20 nm to 33 nm (after 6h) and ca. 39 nm (after 24h). This showed that despite the thermal stability of the catalysts (as shown by TGA under N 2 , Figure 4 9) the hydrothermal stability of the ZnO particles was limited.

Reactions were then conducted using 0.3 g, 0.5 g, 1 g and 1.5 g of the catalysts. We could observe an increase in conversion (from 15% to 73%) with an increase in mass, which was expected as the ratio n Cu /n glycerol increased. This increase was proportional between 0.3 g and 1 g, however the result was lower than expected with 1.5 g. As before, the increase in conversion was followed by a slight decrease of selectivity in 1,2 PDO.

The catalytic results obtained with 20Cu/ZnO PAA2 0 synthesized in a batch of 1 g (Table 4 6) or 3 g (Table 4 7) were quite different (70% vs. 32%). Hence the up scaling of the method of synthesis was accompanied with problems of reproducibility.

Recyclability of Cu/ZnO PAA2

The first recyclability test was carried out with 20Cu/ZnO PAA2 0 . The selected catalyst, the one with the highest activity indicated in Table 4 6 (section II.1.1.), was collected after a first reaction with 0.5 g, filtered and dried in an oven at 80 °C and reused without pre treatment.

Only 0.25 g was recovered from the first reaction but the ratio n glycerol /n catalyst (7.8) was respected for the second reaction, by diluting the solution. While the fresh catalyst gave 70% conversion, its reuse led to a decrease to 34% conversion. It was difficult to know if the results were comparable, as even if the ratio n glycerol /n catalyst was the same, the amount of substrate and catalyst were lower in the second reaction. Deactivation due to water molecules has been reported in different studies such as in the article by Jean et al. 9 where the conversion decreased with increasing water content in glycerol solution with concentration varying from 80 to 100 wt% in water.

In order to conduct the recyclability test with the same mass, the catalyst in Table 4 7 (section II.1.2.) used for the reaction with 1.5 g was recovered. The conversion of this fresh catalyst was around 40 % for 0.5 g and 73% with 1.5 g (Table 4 8). The catalyst was recycled without pre treatment and once again the catalyst was not as active as the freshly reduced one. The Table 4 8 shows the properties and the conversion of the recycled catalysts. The conversion decreased to 8 % and the selectivity toward 1,2 PDO decreased to 75 % while the selectivity toward EG increased till 22 % with appearance of many unknown products. In a second time, the used catalyst was calcined at 400 °C and reduced at 350 °C. The conversion was as expected 48 % instead of 8 % and the selectivity toward 1,2 PDO went back to its initial value of 90 %. XRD diffractograms (Figure 4 14) showed that the catalyst after the first reaction (reused B.

T.) and after the second reaction (reused A. T.) exhibited the same diffractograms, i.e. there was the presence of (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 phase. However, after calcination and reduction, the retreated catalyst had the same structure as the fresh catalyst. This result meant that this Cu/ZnO catalyst could be regenerated and this was proven by maintaining the conversion at the same level. The catalyst reused without treatment was not very active, hence (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 phase might be detrimental for the activity. Although the conversion results were excellent for some of the catalysts presented in section II.1, their reproducibility in term of S.S.A. of the support and Cu crystallite size were nevertheless sometime random which was time consuming and difficult to manage. This major difficulty for rational application in catalysis seemed inherent to the use of the ZnO PAA2 hybrid as support for Cu deposed by DPU method. To improve the reproducibility of the results, we have therefore decided to use ZnO PAA2 supports calcined at different temperatures (300 400 °C) in the next section.

The table below summarizes the results obtained for glycerol hydrogenolysis using calcined ZnO PAA2 supports synthesized at room temperature (the non calcined ZnO PAA2 is used as reference). The value of the specific surface area corresponded to the support before calcination. The supports were then calcined at 300, 325, 350 or 400 °C. The final catalysts were synthesized by depositing either 10 or 20 wt% of Cu. The specific surface area of the catalyst were not measured but the specific surface area of all the catalysts should be in the same range after calcination and reduction, around 25 m 2 .g 1 , as in chapter II.1.1.

The DPU method was the same as before but the commercial ZnO and the ZnO PAA2 were substituted that time by calcined ZnO PAA2, noted ZnO PAA2, as presented in scheme 4 4. Catalyst are labeled according to the theoretical copper weight percentage and the calcination temperature of the support. For example, 10Cu/ZnO PAA2 300 is the catalyst presented in the first entry of the table with a theoretical copper weight percentage of 10 wt% and prepared using the ZnO PAA2 calcined at 300 °C. It was difficult to accurately manage the amount of final copper based on the speculated remaining organic weight loss, which was why the copper ICP results were far from the theoretical targets of 10 and 20 wt% of copper (Cu exp , Table 4 9). Nevertheless, a comparison of the last two entries in the table showed that the catalyst based on a support calcined at 400 °C had a similar activity to the non calcined support; their characteristics were identical.

We could therefore deduce that the calcination of the support at 400 °C before DPU or at the end of the synthesis of the catalyst produced the same properties. Figure 4 The lower conversion observed for 20Cu/ZnO PAA2 350 in comparison with 20Cu/ZnO PAA2 400 might be attributed to the higher Cu 0 crystallite size. If we compared the effect of the temperature of calcination of the support with 10 wt%Cu, the best conversion of 54% after 24 hours of reaction was obtained for a support calcined at 350 °C (10Cu/ZnO PAA2 350 , in grey), associated with a low Cu 0 size (13 nm) and a high specific surface area of the support (72 m 2 .g 1 ). Below 350 °C, the retention of a large quantity of organic matter did not seem to be favorable for good activity even with low Cu 0 size (14 15 nm). The lower activity of 10Cu/ZnO PAA2 400 compared to 10Cu/ZnO PAA2 350 might be attributed to the larger Cu particles and the lower Cu loading. The selectivities toward 1,2 PDO and EG were almost the same for all the catalysts, 90% and 10% respectively.

In conclusion, the effect of the calcination of the support on the catalytic activity of Cu/ZnO seemed a little delicate because it was difficult to control the remaining organic content.

Calcining the ZnO PAA2 support at 400 °C appeared to be the best way to obtain a good conversion in agreement with the study on non calcined supports and this in a more reproducible way. Indeed, different batch of 20Cu/ZnO PAA2 400 were prepared and they exhibited the same structural properties and gave the same conversion (results not shown).

Non Calcined ZnO PAA5

As mentioned in Chapter III, the synthesis conditions and in particular the synthesis temperature could strongly influence the specific surface area of ZnO PAA5 supports. 20 wt% of copper were deposited on ZnO PAA5 synthesized at 0 °C, RT and 40 °C for the synthesis of catalysts. A fourth sample was synthesized with 10 wt% of Cu, on ZnO PAA5 synthesized at room temperature. The samples were labeled as previously. As we mentioned in the previous chapter, the yield of ZnO PAA5 obtained by sol gel method was around 50 wt%. For that reason, the number of experiments done with that support compared to ZnO PAA2 was lower.

The surface area associated with the support, the crystallite size of Cu before and after reaction, the Cu weight percentage, along with the catalytic results are included in Table 4 10.

The results showed that the S.S.A. of the support and Cu crystallite size had little effect on the conversion of Cu/ZnO catalysts, the best conversion being obtained with 20Cu/ZnO PAA5 RT 2, the catalyst having neither the highest specific surface area nor the smallest copper crystallite size. It was difficult at this stage to explain this lack of trend, but several observations could be made. First, despite extremely high specific surfaces (120 135 m².g 1 ), the size of copper crystallites remained relatively important (18 20 nm). This could be explained by weak interactions between copper precursors and ZnO PAA5. Therefore, the morphology of the support might affect the activity of the catalyst since at room temperature the ZnO PAA5 was mostly spherical while at 40 °C it was aggregated in 2D nanosheets. It was worth noting that the catalyst obtained from the support synthesized at 0 °C was extremely selective with respect to the 1,2 PDO (99%) at high conversion (64%). This was not the case when using ZnO commercial or ZnO PAA2. Hence, the combination of high surface area of the support and clean morphology (20Cu/ZnO PAA5 0 ) led to 64 % conversion with almost total selectivity toward 1,2 PDO.

Total selectivity at high conversion when using copper based catalysts has been reported in few studies but in presence of organic solvents as in the article by Mizugaki et al. 10 where authors used copper nanoparticles supported on AlO x . The reaction conditions were the following: 0.35 mmol of glycerol with 25 mg of catalyst (0.19 mmol of copper), in presence of

Calcined ZnO PAA5

As with ZnO PAA2, the effect of the calcination temperature of the support on the catalytic activity was studied. After being synthesized at RT, the supports were calcined at 300 °C and 350 °C. Since ZnO PAA5 was already completely transformed after calcination at 350 °C, no calcination was conducted at 400 °C, in contrary to ZnO PAA2.

The results are reported in Table 4 11. The conversions after 24 h were very low, 4 % and 27 % for 10 wt% and 17 wt% of copper, respectively, compared to non calcined supports (48 70% of conversion; Table 4 10). The decrease in conversion might again be due to the morphological change of the support. The specific surface areas of the catalysts were not measured but should be around 20 30 m 2 .g 1 (cf Chapter 3, section I.5.2.). In any case, it appeared that the calcination of ZnO PAA5 before copper deposition had a negative impact on catalyst activity. For ZnO PAA5, the catalyst activities were higher when the support was not calcined, which meant that the polymer (PAA5) played a beneficial role in depositing copper and it might either contribute to the formation of new active sites or it could help the Cu ZnO interaction.

Two catalysts were synthesized with ZnO PAA100 at 10 and 20 wt% of copper. None activity were detected even with a material with high specific surface area (70 m 2 .g 1 ) and small copper crystallite size (11 nm) as shown in the After reaction, the XRD diffractogram (Figure 4 17) showed that the catalyst was unchanged which was linked to the inactivity of the catalyst. A slight increase in copper particle size occurred from 13 to 18 nm due to the reaction conditions.

III. Cu ZnO by a "one pot" method

In order to save time and in an attempt to obtain reproducible Cu/ZnO catalysts, a one pot method was implemented. Two approaches, a "one step" and a "two steps", were investigated and the various synthesis parameters optimized. Since the best catalytic results

were obtained with the catalysts elaborated with ZnO PAA2 supports, only PAA2 was studied in this section.

The first test of Cu ZnO production by a one pot method was performed by mixing all the precursors in the same schlenk flask. At room temperature, copper precursor, urea and PAA2 solution were mixed together and heated till 60 °C. After 20 minutes at 60 °C, the ZnEt 2 was added to the solution under argon as described in the experimental part of Chapter 2. After 3 hours of heating at 60 °C, the suspension was centrifuged and the wet powder was washed with 90 mL of EtOH (for 1 g of catalyst). As before, the powder underwent calcination and reduction to elaborate the final catalyst labelled as Cu ZnO to differ it from the one prepared by DPU (Cu/ZnO). A representative scheme of the "one pot one step" approach is shown in scheme 4 5.

Scheme 4 5: Synthesis of Cu ZnO by "one pot one step" approach

We called it a "one pot one step" approach to distinguish it from the next approach where we divided the synthesis into two steps.

Table 4 13 summarizes the catalyst properties at different stages. The specific surface after synthesis was similar to the ZnO PAA2 support synthesized at high temperature (40 °C) since the elaboration of the ZnO PAA2 was happening at 60 °C leading to a small surface area of 28 m 2 .g 1 . The catalyst was tested: 41 % of conversion with a selectivity of 88% towards 1,2 PDO were obtained after 24h (Table 4 15). An increase from 24 m 2 .g 1 to 43 m 2 .g 1 occurred after the catalytic reaction associated with the formation of the new carboxylate phase. The size of the crystallite of ZnO was almost the same during all stages of synthesis, even after the reaction.

Table 4 13: Properties and catalytic results of "one pot one step approach Cu ZnO" at different steps

The XRD diffractograms of the catalyst prepared by this approach are shown in Figure 4 18.

Compared to Cu/ZnO com (ca. 60 nm, Figure 4 3) or Cu/ZnO PAA2 (ca. 20 nm), the catalyst after synthesis had an average ZnO crystallite size of 11 nm. After calcination CuO crystallites were around 15 nm and remained in the same range after reduction (13 nm) and catalysis (14 nm).

The results indicated that neither ZnO, nor Cu sintered during the reaction. However, as for Cu/ZnO PAA2 prepared by DPU, Cu ZnO prepared by "one pot -one step" method was showing a phase change after the catalytic reaction.

The catalyst could be compared to 20Cu/ZnOPAA2 with ZnO synthesized at 0 °C, 10 °C, RT and 40 °C (Table 4 6). The conversion was 41%, higher than 20Cu/ZnOPAA2 Since the most outstanding properties of ZnO PAA2 were obtained at low synthesis temperature and the highest conversion was obtained with 20Cu/ZnO PAA2 0 , the second one pot method was divided into two steps. The first step was the in situ preparation of the support at 0 °C in presence of the copper solution and urea and the second step was the decomposition of urea at 60 °C. As for the "one pot one step" approach, centrifugation, washing and thermal treatments were needed to elaborate the final catalyst. scheme 4 6

shows the "one pot two steps" approach.

not the same, the XRD of the catalyst after test did not show any phase change (Figure 4 19).

The new synthesis method seemed to be beneficial for the elaboration of a robust catalyst, partially stable under hydrothermal condition. The conversion was higher (61%) with the "one pot two steps" approach compared to 41% for the one pot -one step. It could be attributed to a stronger interaction between copper and ZnO since the first step of the synthesis, where the surface area was relatively high (89 m 2 .g 1 ) leading to a conservation of the ZnO nanoparticles after the catalytic test and just resulting in sintering of the particles. Nevertheless, the one pot -one step catalyst also seemed to be a robust catalyst, even that the ZnO is transforming, no sintering is occurring.

That can be explained by an almost total coverage of the surface of ZnO by the copper particles leading to a protection one by the other.

Comparing our results to studies found in literature for Cu ZnO prepared by co precipitation, considering the one pot similar to that synthesis method, we can say that the activity of our system can be considered in the middle or relatively high.

Some studies showed low activity of the system as in the article of Wang et al. 11 In conclusion, our new "one pot two steps" succeeded to elaborate a robust system with high conversion and high selectivity toward 1,2 PDO competitive for the hydrogenolysis of glycerol under mild conditions.

As we mentioned in the literature, the Cu/ZnO catalysts were often accompanied by a promoter as Al 13 , Ga 12 … and these systems showed higher activity compared to the simple Cu/ZnO. For that reason, the addition of Al on the Cu/ZnO catalyst will be studied in the following chapter.

IV. Conclusions

The synthesis of copper based catalysts using commercial ZnO has shown that the activity of these catalysts was strongly influenced by the weight percentage of copper and the concentration of Cu 2+ . The small specific surface area of the commercial support led to sintering of the copper particles with high loadings. The highest conversion of 25% was obtained when using this support with 10 wt% of copper and 20. In the case of ZnO PAA5, the best catalytic results were obtained with the support synthesized at 0 °C as the selectivity was almost total for 1,2 PDO at 64% conversion, which was not the case with all the other supports studied. These results show that the nature of the polymer used to develop ZnO nanoparticles seemed to have a significant impact on the catalyst activity.

In comparison, the catalyst synthesized with ZnO PAA100 as support was totally inactive.

The recyclability tests conducted with Cu/ZnO PAA2 gave similar conversions when the catalysts were reactivated and retested under the same conditions as for the fresh catalyst. In parallel, scaling up was possible but the catalyst prepared by the DPU method was not easily reproducible.

This was why a "one pot" method was developed to avoid this problem. At first, a "one pot one step" approach was not very interesting because the activity decreased compared to that of the conventional DPU and the only improvement was avoiding the sintering problem of copper and ZnO particles. On the other hand, a "one pot two steps" approach helped to maintain conversion and selectivity with a more robust catalyst showing an increase in ZnO crystallite size but without changing the phase.

In the next chapter, this last method will be further developed by adding alumina to increase specific surface area and then catalyst activity. After obtaining promising results on the hydrogenolysis of glycerol with Cu/ZnO catalysts prepared either by DPU or by a "one pot" method, we were interested in adding aluminium to this system in an attempt to increase the conversion. As shown in Chapter 1, Cu Zn Al system is well known for this reaction and many Zn:Al ratios have been investigated. Some studies showed that the 1:2 Zn:Al 1 molar ratio gave the highest conversion while others found that 1:0.5 Zn:Al gave better results 2 .

CHAPTER V Cu Zn Al: Effect of Al on the hydrogenolysis

In this chapter, we will start by synthesizing Zn x Al y O oxide supports and study the effect of the synthesis parameters on the properties of the support. In a second step, we will add Cu to the Zn x Al y O mixed support using the previously optimized DPU method and we will test the catalyst activity on glycerol hydrogenolysis. Finally, we will synthesize the Cu Zn x Al y O catalyst by a "one pot" method and will also study the effect of the synthesis parameters on the activity of the catalyst for the same reaction.

In the literature, mixed oxides of ZnAl have been produced by different approaches such as hydrothermal [3][4][5] or Sol gel 6 methods. Sol gel did not lead to specific surface area as high as when produced by hydrothermal or other sophisticated methods. This is why, in this project, we have tried to make a compromise between the good properties of the materials obtained by hydrothermal and the ease of the sol gel method. For this reason, and after setting the ZnO PAA2 synthesis parameters in Chapter III, the same conditions were applied to produce Zn Al with two different molar ratios.

The samples will be named Zn x Al y O PAA2 z , with:

x, y : molar content of zinc and aluminium, corresponding to the actual loading z : the calcination temperature after synthesis.

For example, in the following chapter, the as prepared sample will be named Zn x Al y O PAA2 and the sample calcined at 600 °C, Zn x Al y O PAA2 600 .

I. Synthesis of Zn x Al y O PAA2: Study of the molar ratio of metals

As the material will be used as a support for copper catalysts, the molar ratio Zn:Al is an important factor to study. Nevertheless, for reasons of time remaining, we carried out this study only with two stoichiometries Zn:Al (1:2 and 2:1) keeping in mind that the 1:2 stoichiometry would be likely to give the spinel phase ZnAl 2 O 4 .

As mentioned in the experimental part, the bottle of AlMe 3 used for some of these syntheses was pure (97% in hexane), purchased from Sigma Aldrich, however it had been opened for a long time. For that reason the theoretical molar ratio of Zn:Al was not equal to the experimental one because the bottle was partially oxidized. For Zn Al, two samples were prepared using this bottle and used later as supports for Cu/Zn Al. While for Cu Zn Al (last section of the chapter, prepared by the "one pot" method), seven catalysts were elaborated with the old bottle. The concerned samples will be appointed in the chapter.

Initially, the molar ratio used to synthesize the Zn Al support was 1:2 in the presence of aqueous PAA2 solution. As described in the experimental section, the synthesis was carried out as for ZnO PAA2 after setting the synthesis parameters at 0 °C and 0.63 wt% of PAA2. For this purpose, the PAA2 solution was cooled in an ice bath before adding a mixture of (ZnEt 2 + AlMe 3 /old bottle) with a respective ratio Zn:Al of 1:2. The powder obtained after washing with 90 mL EtOH and drying steps (centrifugation and night drying in the oven) was calcined at 400 and 600 °C in order to study the effect of calcination on the nature of the material. Indeed, a spinel phase of ZnAl 2 O 4 might form at high calcination temperature 3,7 . No EDX measurements were made on the samples but the molar ratios were measured by ICP analysis for the sample calcined at 400 °C and gave formula of Zn 1.95 AlO 4.67 . Therefore, this sample was labelled Zn 2 Al 1 O PAA2 z . Data are summarized in Table 5 1. . This ratio did not reflect the ICP Zn:Al ratio of 2:1, which could be due to the fact that part of the solid was amorphous. For clarification, we should also mention that the proportions obtained with Diffrac.Eva software were semi quantitative and should mainly be used for comparing the samples between each other.

For the upcoming diffractograms we will just label the peaks associated with ZnAl 2 O 4 phase as ZnO one was present in most samples. In the following XRD patterns (Figure 5 1), all peaks of ZnAl 2 O 4 are represented, even those that are not well crystallized and/or overlap with ZnO, in order to distinguish between the two crystallographic phases. Actually, the two main peaks of ZnAl 2 O 4 at 31° (Intensity: 80%) and 37° (100%) were really close to the major peaks of ZnO at 32° (71%) and 36° (100%). Therefore, an easy way to distinguish between the two phases was the presence of a peak at 34° (56%) which was associated with ZnO and peaks at 59° (40%) and 65° (40%) which were associated with ZnAl 2 O 4 . Compared to the aluminium free system, the materials Zn 2 Al 1 O PAA2 z had much higher specific surfaces after calcination; indeed at 400 °C we obtained a specific surface area of the order of 234 m 2 .g 1 which dropped to 98 m 2 .g 1 after calcination at 600 °C. While ZnO PAA2 after calcination at 400 °C resulted in a surface area of 18 m 2 .g 1 (Chapter III, Table 3 8). In terms of similarities, those as prepared with or without aluminium were similar (between 70 and 90 m 2 .g 1 ) and the same temperature dependent evolution was observed for both systems. The result showed that the addition of aluminium during synthesis was responsible for increasing the specific surface area of the materials.

TG analyses were performed as for ZnO PAAX (under air at a heating rate of 5 °C per minute up to 700 °C) to study the content of organic compounds in the material before and after calcination, as shown in Figure 5 meaning that ZnO could be present.

2) Or that ZnAl 2 O 4 was having the exact stoichiometry of 1:2 and amorphous Al oxides were present in small quantities. It was also possible that some Al was incorporated into ZnO as a dopant.

Once again, we would like to remind that XRD analysis and calculation on Diffrac.Eva are semi quantitative. The decrease in the molar ratio Zn:Al (and therefore an increase in the proportion of Al) resulted in a significant increase in the specific surface area for all materials. The hybrid as prepared material (Zn 1 Al 2 O PAA2) had an already very high specific surface area of 223 m 2 .g 1 . As before, calcination at 400 °C increased the specific surface area to 333 m 2 .g 1 (confirmed by repeating the synthesis twice with a difference of only 10 m 2 .g 1 ), which was a very high value unreported till now for ZnAl 2 O 4 obtained by sol gel method. Once again, at 600 °C a decrease in the specific surface area to 137 m 2 .g 1 was observed.

Thermogravimetric analyses of the compounds Zn 1 Al 2 O PAA2 z (Figure 5 5), showed that the change in the molar ratio Zn:Al did not have a major impact on their thermal behaviour. The weight loss of the compound Zn 1 Al 2 O PAA2 between 200 and 600 °C was about 30% (Table 5 2) slightly lower than that of Zn 2 Al 1 O PAA2 (35%, Table 5 summarized in Table 5 3 showed a higher abundance of Zn on 2D nano sheets (zones 1, 11 and 12) with a molar ratio Zn:Al (1:1) on these spots, suggesting in that case a combination of ZnO and Al oxydes as proposed after the XRD analysis. While a higher Al content was observed within spherical shapes (zones 2, 10 and 13) resulting in a higher stoichiometry for ZnAl 2 O 4 . It seemed that the two hypothesis proposed after XRD analysis were possible.

It could therefore be suggested that the 2D nano sheets were more loaded with the ZnO phase as seen for ZnO PAA2 without adding aluminium (Chapter III, Figure 3 2) but contained as well some Al oxides. The nanoparticles could be attributed to the ZnAl 2 O 4 phase.

In conclusion, the addition of aluminium in greater or lesser quantities during synthesis induced a loss of crystallinity of ZnO, an increase in the proportion of ZnO of 2D nano sheet morphology combined with Al content which was accompanied by a significant increase in the specific surface area. This addition also led at 600 °C to the appearance of the ZnAl 2 O 4 phase regardless of the initial stoichiometry used. The next parameter to be studied was the absence of the PAA2 polymer during synthesis since the 3D spherical self assemblies were no longer formed.

As the objective was to see the effect of the absence of PAA2 on the final material, the sample was prepared under the same operating conditions as Zn:Al with PAA2 (section I. XRD patterns shown in Figure 5 7 showed that the absence of PAA2 had a significant effect on the crystallinity and nature of the final material. After synthesis, the solid was biphasic and consisted of 90% of well crystallized ZnO and 10% of ZnAl 2 O 4 after calculation with Diffrac.Eva.

The size of the crystallites calculated using the same peaks as previously gave an average size of 22 nm for ZnO and 11 nm for ZnAl 2 O 4 . Contrary to the system with PAA2, this one evolved very little according to the temperature since the particle sizes of ZnO and ZnAl 2 O 4 remained unchanged and the proportion of ZnAl 2 O 4 went from 10% to 17% (at 400 °C) and finally 23%

(at 600 °C). Unlike the system with PAA2, the specific surface area was not significantly influenced by the calcination temperature, the average specific surface area being 80 m 2 .g 1 for the 3 samples (Table 5 4). However, it followed the same trend as with PAA2 and a maximum of 90 m 2 .g 1 was obtained after calcination at 400 °C.

In conclusion, the absence of PAA2 during synthesis in the presence of aluminium led to a could therefore act as a dispersing agent to homogenize the Zn/Al mixture at the molecular scale but also as a surfactant to increase the specific surface area. Our results could be compared with a study on the synthesis of ZnAl 2 O 4 with and without surfactant 8 . In an article by Chaudary et al. 8 , ZnAl 2 O 4 was obtained by a two steps sol gel process. A ZnAl precursor was obtained by dissolving aluminium isopropoxide and anhydrous zinc acetate in toluene and heating at reflux for 3 hours. The mixture was then evaporated under vacuum to give a white solid. This solid, considered as the mixed precursor ZnAl, was dissolved in a mixture of isopropanol and distilled water. The resulting gel was separated by centrifugation and washed with water and acetone. After drying, the powder was sintered at 500 °C for 2 hours. In order to study the surfactant effect during the hydrolysis step, a solution of cetyltrimethylammonium bromide (CTAB) or sodium lauryl sulfate (SLS) was added. The particle size decreased from 12 nm without the addition of surfactants to 6 nm with both types of surfactants. The specific surface area increased from 79 m 2 .g 1 without surfactant to 130 m 2 .g 1 and 82 m 2 .g 1 with CTAB and SLS, respectively.

Thermogravimetric analysis of the as prepared material showed that the only weight loss of (Figure 5 8). The materials calcined at 400 and 600 °C showed no weight loss.

In conclusion, the absence of PAA2 led to the appearance of a ZnO + ZnAl 2 O 4 biphasic system that evolved slightly, in terms of specific surface area and particle size, as a function of temperature. Surprisingly, this synthesis produced the ZnAl 2 O 4 spinel phase at very low temperatures, whereas it was generally obtained at temperatures of 600 °C 9 . It was therefore interesting to see what this synthesis pathway gave when the starting stoichiometry is similar to that of ZnAl 2 O 4 . The sample illustrated in this section was elaborated with the new bottle of AlMe 3 as for Zn:Al 5 2) confirming that PAA2 was previously totally removed after calcination at 400 °C. Therefore, the two materials prepared with the new bottle of AlMe 3 , in presence and absence of PAA2 showed an experimental molar ratio very similar to the theoretical one. This proved that the difference in stoichiometry is mainly attributed to the oxidation of the old bottle of AlMe 3 and not to the Al leaching under the operating conditions. In terms of specific surface area, the as prepared material had a higher specific surface area (274 m 2 .g 1 ) than the one synthesized with PAA2 (223 m 2 .g 1 ). As in the previous case without PAA2, it remained stable after calcination at 400 °C (260 m 2 .g 1 ) then dropped to 169 m 2 .g 1 at 600 °C.

Thermogravimetric analysis of the solid after synthesis showed a weight loss up to about 400 °C, which suggested the possible presence of metal hydroxide in large quantities such as Al(OH) 3 or AlO(OH). These amorphous phases would not be visible in XRD but could also explain the proportion of ZnAl 2 O 4 obtained by XRD, not compatible with the stoichiometry obtained by ICP. For the solid calcined at 400 °C, the weight loss of 16 wt% between 25 and 200 °C would confirm the loss of most metal hydroxides. In the paper of Kanari et al. 9 , zinc carbonate hydroxide started decomposing from 150 °C proving the decomposition of such compounds at low temperatures. For the one at 600 °C the weight loss was very low (3 wt%), in accordance with a system mainly composed of crystallized ZnAl 2 O 4 . Following this study, we therefore had a range of mixed oxide supports ZnAl with two fixed and controlled Zn/Al ratios and a specific surface area ranging from 89 to 333 m 2 .g 1 at 400 °C, the maximum temperature that the Cu/ZnAl catalyst will be subjected to. In the following section, copper depositions on all these supports were performed and the effects of nature, compositions and morphologies of the supports on the catalytic results were studied.

II. Synthesis of Cu/ZnAl by DPU: study of the Zn Al effect

The first set of catalysts Cu/ZnAl was synthesised by the DPU method as described in Chapter II using Zn The XRD diffractograms of the two catalysts with 20 wt% of copper (Figure 5 11) were similar to the supports without copper calcined at 400 °C, the only difference being due to the additional presence of copper peaks. As previously observed, the presence of PAA2 decreased the crystallinity of the compounds. The crystallite sizes of ZnO did not vary so much with or without copper and the Cu 0 crystallite sizes were 11 and 17 nm for Cu/Zn 2 Al 1 O PAA2 and Cu/Zn 2 Al 1 O, respectively, calculated by using the three peaks of copper at 43°, 51° and 74.5°.

However, the specific surface area of the support with PAA2 dropped from 234 to 111 m².g 1 for the catalyst (i.e. after copper deposition) while it remained the same for the two systems without PAA2 (89 and 90 m 2 .g 1 ). It therefore appeared that the Zn 2 Al 1 O PAA2 support had not undergone the same structural evolution with and without copper. The decrease in surface area could be related to a strong interaction of the copper with the PAA2 during the deposition that influenced the surface of the support or the decomposition of the PAA2.

Comparing to Cu/ZnO, the surface area was much higher (S.S.A of 25 -40 m 2 .g 1 were obtained for all the Cu/ZnO using the commercial and the synthesized support as shown in chapter IV).

In conclusion, addition of aluminium led to increase of specific surface area of the catalysts as for the supports.

With regard to the performance of these catalysts, the catalyst Cu/Zn performance might be due to the difference in Cu crystallite size. It was also accompanied by a difference in stability. Indeed, structural analyses by BET showed that the catalyst prepared from the support without PAA2 presented a huge increase in the specific surface area after catalysis, suggesting a significant transformation of the catalyst. The metal contents of the catalysts were measured by ICP AES and were compared to the ones of the support after calcination at 400 °C, to be as close as possible to the temperature at which the catalysts were prepared. The content in copper is presented as a fixed value (x)

to highlight the difference in molar ratio of Zn:Al between the support and the catalyst.

The results, summarized in Table 5 7 showed that the major difference was due to the zinc content, which was 20 to 30% lower for the catalyst than for the support, regardless the presence or absence of PAA2. In order to verify if zinc was removed by filtration during the synthesis of the catalyst by DPU two references test were done. First, for a catalyst prepared in the presence of urea, a determination of zinc in the solution obtained after filtration was carried out by ICP AES. The presence of 12.4 wt% of zinc proved a leaching phenomenon during catalyst synthesis by our DPU method. This phenomenon could be due to the pH (about 8 under our conditions) which could lead to the solubilization of part of the zinc. For this purpose, another catalyst synthesis was carried out without urea under the same conditions (3 hours of heating at 95 °C) to check the impact of the pH of the solution. The pH of the solution was 7 and the ICP results of the solution after filtration showed a zinc loss twice as high with 20.6 wt% of zinc content in the solution in the absence of urea. The ZnO solubility diagram in Figure 5 12 10 The properties of the catalysts are summarized in Table 5 8. The XRD diffractograms of the three catalysts before reaction (Figure 5 The metal contents of the supports and corresponding catalysts are summarized in Table 5 9.

The values showed that only the Zn 1 Al 2 O PAA2 support kept the same proportion of zinc after DPU. The use of the other two PAA2 free substrates for copper deposition resulted in zinc losses of up to 50%. In this aluminium rich system, the presence of PAA2, when in contact with copper, seemed to avoid a significant loss of zinc. It could be assumed that this system contained more amorphous zinc oxide or hydroxide soluble in basic conditions during urea decomposition or that PAA2 served as a buffer compound due to its carboxylic acid functions. The specific surfaces of the aluminium rich Zn 1 Al 2 catalysts (Table 5 8) were significantly larger than those of the Zn 2 Al 1 catalysts (Table 5 6) with values ranging from 170 to 230 m².g 1 , the maximum value being obtained without PAA2 as in the Zn 2 Al 1 system.

In terms of reactivity, the catalyst without PAA2 proved to be the most efficient with a 24 hour conversion of 83% associated with a slightly lower selectivity towards 1,2 PDO (86%).

However, it was difficult to extract from the previous data a major factor in these performances, except that the best catalyst seemed the most robust in terms of copper nanoparticles size (even if they are the biggest) and specific surface area (Table 5 8). However, XRD diffractograms showed the appearance of peaks at 34 and 37 ° attributed to ZnO that was not present in the catalyst before test, and that appeared due to the catalytic reaction conditions (Figure 5 12).

The other two catalysts showed lower performances and displayed weak modifications on the XRD diffractogarms with the appearance of either weak proportion of carbonaceous phases [(Cu,Zn) 5 (CO 3 ) 2 (OH) 6 and Zn 5 (CO 3 ) 2 (OH) 6 ] (which were favourable for the Zn 2 Al 1 system) or the sintering of Cu° particles. The evolution of specific surface areas was more difficult to correlate with reactivity.

TEM images of catalysts with PAA2 after reaction (Figure 5 images. However, the crystalline phase of ZnAl 2 O 4 was visible in Figure 5 14 c and d and had an average particle size similar to that of the crystallites calculated by XRD of 6 nm for both catalysts. This loss of zinc during the DPU process led, during our study, to a variability in the characteristics of our catalysts, which we tried to avoid by setting up a "one pot" synthesis route by adding copper in the PAA2 solution when elaborating the ZnO and the ZnAlO.

III. Synthesis of Cu Zn x Al y O z by a "one pot method": Study of the synthesis parameters

This new one pot method was divided into two routes: a "one pot one step" and a "one pot two step" in relation to the synthesis strategies; further steps of calcination and reduction were still needed to get the final Cu ZnAl catalyst.

As mentioned at the beginning of the chapter, the old bottle of AlMe 3 used to elaborate ZnAl oxides, was also used for the elaboration of the catalysts by the "one pot" method.

Scheme 5 2: Overview of the one pot -one step method

As for Cu ZnO, all the precursors were added in the same flask and the method was called "one pot one step". Scheme 5 2 shows a brief sum up of the steps, this one pot consists on mixing the copper precursor with the PAA2 solution and urea at room temperature, heating at 60 °C before adding the mixture of ZnEt 2 and AlMe 3 stored in a schlenk flask. After 3 hours, the slurry was centrifuged, washed with 90 mL of EtOH and after drying overnight, the powder underwent thermal treatment to elaborate the catalyst.

In the case of this sample, the molar ratio Zn:Al targeted was 1:2 due to the high activity reported with this molar ratio in Cu Zn Al for the hydrogenolysis of glycerol The materials were characterized by BET and XRD after each step and the results are shown in Table 5 10. The specific surface area of the catalyst after synthesis had a low value reminding the one obtained for Cu ZnO prepared by one pot -one step method (Chapter IV, section III.1). Calcining the sample led to an increase of the surface area from 28 to 54 m 2 .g 1 .

However, after reduction the specific surface area was low, similar to that obtained with Cu ZnO PAA2 (24 m 2 .g 1 ) and much lower than Cu/Zn x Al y O z obtained by DPU. The increase in the surface area after calcination seemed to be related to the ZnO and not to the crystallization of Al because it was followed by a decrease after reduction.

XRD diffractograms (Figure 5 0937) (unassigned on the figure but characterized by the main peaks at 35° and 39°) with an average crystallite size of 16 nm. After reduction, the catalyst was essentially composed of the ZnO phase and Cu 0 . This latter one had average sizes of about 13 nm calculated with the three peaks at 43, 51 and 74.5°. The size of ZnO was not changing after each step while ZnAl 2 O 4 was mostly amorphous, the crystallite size could not been determined. Comparing to the catalyst obtained by DPU method in presence of PAA2 (Table 5 6) and with same molar ratio of Zn:Al The catalytic results will be presented in the following III.3. section, with the catalyst obtained by the "one pot-two steps" route in order to compare the effect of the synthesis method on the behaviour of the catalyst.

As for Cu ZnO (Chapter IV, section III.2), the "one pot" method was modified into a two steps approach. The main difference was that the synthesis was conducted at two temperatures. As before, the copper precursor solution is mixed with PAA2 solution and urea at room temperature but the Zn and Al precursors are added at 0 °C instead of 60 °C. Scheme 5 3 shows a brief sum up of the syntheses steps, the centrifugation, 90 mL EtOH washing step and the Chapter V 208 thermal treatments for the elaboration of the final catalyst. As it was explained in the fourth chapter, the idea from the "one pot -two steps" route was to allow the precipitation of Zn x Al y O z PAA2 before increasing the temperature. Once the heating began, urea started decomposing and led to an increase in the pH.

The "one pot -two steps" route in this chapter was studied in depth and the scheme below shows the first attempt for the elaboration of Cu Zn Al via this approach englobing all the steps and / or the additives (PAA2, urea). In the upcoming section (III.4.), some parameters will be changed.

Scheme 5 3: Overview of the one pot -two steps method Once again, the catalyst was prepared with the old bottle of AlMe 3 as for the catalyst elaborated by the "one pot -one step" approach.

Table 5 11 summarizes the properties of the materials after each step and those of the catalyst regenerated and recycled. Nevertheless, the separation of the synthesis into two steps resulted in higher surface area after each step compared to the catalyst prepared by the "one pot -one step" route. The surface area of the catalyst after synthesis (62 m 2 .g 1 ) was in the range of ZnO PAA2 elaborated at 0 °C presented in chapter III, section I.4.1. Calcining the sample did not show an increase in the specific surface area which was not a common behavior nor for ZnO neither for ZnAl 2 O 4 .

However, the reduction step resulted in an increase till 97 m 2 .g 1 , showing a value in the same range as the Cu/ZnAl catalysts prepared by DPU with molar ratio Zn:Al (2:1) shown in Table 5 6.

Comparing the S.S.A value of the reduced catalyst obtained by the "one pot -one step" route (25 m².g 1 ) and "one pot -two steps" route (97 m².g 1 ) demonstrated again the importance of the synthesis temperature even in the presence of aluminium.

XRD in Figure 5 17 shows a crystallized phase of hexagonal ZnO for the material after synthesis with an average crystallite size of 35 nm calculated with the peaks at 36°, 42° and 62°. A peak corresponding to ZnAl 2 O 4 was seen at 35° but the crystallite size could not be calculated due to the wide diffraction of the peak. After calcination, the catalyst had a similar structure as the catalyst prepared by the "one pot -one step" route however the CuO phase was less intense despite the similar crystallite size of 21 nm. Once again ZnAl 2 O 4 was not present after calcination and that was also expected since no increase in surface area occurred after calcination. ZnO was less crystallised than the catalyst prepared by the "one pot -one step" route and the crystallite size could not be calculated.

After reduction, the catalyst was a combination of the three phases with similar percentage for ZnO (24%) and ZnAl 2 O 4 (29%). Even that ZnAl 2 O 4 was highly present compared to the catalyst prepared in one step, the peaks were too large and the crystallite size could not be calculated. Cu 0 calculated with the three peaks at 43, 51 and 74.5° showed an average value of 13 nm similar to the "one step" route. The TEM images of the material are shown in Figure 5 18 and the associated EDX measurements in Table 5 12. The images showed highly dispersed particles with mesospheric shapes. The EDX spot 6 of Figure 5 18 suggested the presence of a large copper particle. The molar ratio obtained by ICP (Zn:Al, 1:1.4) could be associated with the spots 1 and 11.

Moreover, the area number 1 was representative of the ZnAl 2 O 4 with the oriented fringes. The catalytic behaviours of the two catalysts elaborated by "one pot" routes are shown in the Table 5 13. The catalyst obtained by the "one step" approach showed a lower conversion (83%) compared to the one elaborated by the "two steps" approach (96%) but with selectivity toward 1,2 PDO in the same range of ca. 80%.

The catalyst obtained by the "one step" route with a molar ratio Zn:Al (1.4:1) and When we compared the catalysts synthesised with (UHP) or without (HP) urea, with 5%, 10% or 20% of Cu, no urea exhibited higher conversion. For each Cu loading (5, 10, 20), the specific surface area and Cu 0 crystallite sizes were similar for the UHP and HP catalysts. In order to understand the effect of urea on the properties of the catalysts, XRD and SEM were carried out with CZA 20UHP (66% conversion) and CZA 20HP (98% conversion). XRD diffractograms of the catalysts after tests in Figure 5 21 showed similar structure for both catalysts with differences in the percentage of the phases calculated by Difrrac.Eva software. It could be seen that the CZA 20HP exhibited a higher stability compared to CZA 20UHP as ZnO phase was not transformed into hydrozincite or copper zinc hydroxide. Table 5 16 resumes the properties of the 2 catalysts before and after test. SEM images for the catalysts before and after test are shown in Figure 5 22. The catalysts before test did not have the same morphology with irregular spheres for CZA 20UHP and rod like assemblies for CZA 20HP. After catalysis, the difference in the morphology of the catalysts was more pronounced. Figure 5 22 d presenting CZA 20HP after catalysis shows particular needles of ZnO and ZnAl 2 O 4 where copper particles can be easily dispersed. This was reflected by the total conversion of glycerol with CZA 20HP with a selectivity toward 1.2 PDO of 88% and 11% for EG compared to 66% conversion with CZA 20UHP and a same range of selectivity.

Therefore, the use of urea had a deleterious effect on the conversion rates of copper based catalysts, regardless of the copper content. This could be explained by a decrease of the acidity of the catalyst or the precipitation mechanism of the metals. Since the reactants were so active and the formation of the material was occurring at the first step of the synthesis (at 0 °C), the presence of urea at that stage was leading to structural changes and might be responsible for differences in the precipitation phenomenon and the crystal growth leading to a difference in the catalysts activity. For the same copper content of 20 wt%, we have also studied the increase of aluminium content for Zn/Al ratio of 1:1 and 1:4 (two last entries; Table 5 15) and found that increasing Al content led to an increase of the specific surface area without changing the size of the Cu° NPs (ca. 15 nm). Nevertheless, the higher specific surface area did not affect the performance, where the two catalysts had similar results and were amongst the best of our study (above 80% of conversion and selectivity). And increase in the acidity (bronsted and Lewis ) of the catalyst 14 . However due to lack of time, the acidity of these catalysts were not characterised.

CZA

Comparing to the literature, the results were acceptable, as we mentioned in the introduction of this chapter the Zn:Al molar ratio with the highest conversion was changing from a study to another. For example, in the paper of Tan et al. On the other hand, a study of kim el al 15 showed that Cu:Zn:Al molar ratio 2:2:1 was showing the highest conversion with 5 wt% of catalyst and 20 wt% of glycerol, under 17 bar H 2 at 200 °C and after 20 hours of reaction, the conversion for this system was 73% with 76% selectivity for 1,2 PDO and 7.6% selectivity for EG.

IV. Conclusions

This chapter showed many interesting results that allowed understanding the behaviour of the catalysts. In term of Zn Al synthesis, the effect of adding PAA2 during the synthesis led to a remarkable specific surface area (333 m 2 .g 1 ) of ZnO ZnAl 2 O 4 calcined at 400 °C and obtained with (Zn:Al) molar ratio (1:2).

The deposition precipitation method was not easy to reproduce. Many parameters should be finely controlled and studied before. Using Cu/Zn 2 Al 1 PAA2 prepared by DPU, the conversion of glycerol was 92% into 1,2 PDO (with 500 mg of catalyst, 4.2 wt% of glycerol in 100 mL of water, under 30 bar H 2 at 200 °C and after 24 hours) with 78% of selectivity toward 1,2 PDO.

In addition, the effect of the support was easily confirmed in this section where the molar ratio of Zn:Al had an impact on the conversion. It was proved that each catalyst has its own characteristics and these characteristics where influencing the conversion of glycerol.

However, it was hard to decorrelate the influence of each parameter separately.

The conversion obtained with the catalyst elaborated by the "one pot -one step" approach was 83% with 78% selectivity toward 1,2 PDO versus 96% conversion and 83% selectivity toward 1,2 PDO with the catalyst elaborated by the "one pot -two step approach".

On the other hand, the "one pot method" was showing a define trend for each studied parameter. Once again, separating the method into two steps allowed the elaboration of well defined catalysts with high specific surface area of 185 m 2 .g 1 even after calcination/reduction.

The study of the effect of urea during the "one pot" synthesis showed that removing urea (at the same Zn:Al molar ratio and copper weight percentage), allowed an increase of the conversion.

A very good catalyst was obtained in presence of PAA2 without heating and gave competitive results comparing to the same system obtained with heating in term of conversion (79%) and selectivity (90%) resulting in more ecological and time saving process. This result proved the importance of having PAA2 in the "one pot" method as well. Changing the molar ratio of Zn:Al had also an impact on the conversion. Taking in consideration the cited results, the most active catalyst was obtained for a molar ratio Zn:Al of 2:1 but as mentioned above it depended on many parameters and the lack in experiments for other molar ratio prevented us of generalizing this result. Anyway, the conversion obtained with this catalyst (loaded at 20 wt% of copper) was 98% with a selectivity of 88% toward 1,2 PDO.

Furthermore, comparing the catalytic activity of the Cu Zn Al systems and Cu ZnO obtained in the fourth chapter, the effect of adding Al to the catalyst was approved and the conversions were much higher in the case of the former system, under the same synthesis and catalytic conditions.

General Conclusion

In conclusion, our work opens up promising results in terms of hybrid ZnO materials as support for copper-based catalyst for the hydrogenolysis of glycerol.

We showed that if only the synthesis of ZnO-PAA2 and ZnO-PAA5 resulted in 3D selfassembled mesospherical morphology, batches were also accompanied by 2D ZnO-based nano-sheets. Therefore, studies of synthesis parameters were exclusively done for ZnO-PAA2

and ZnO-PAA5 systems and demonstrated that precise synthetic conditions were required to give materials with high specific surface area and small crystallites. In the case of ZnO-PAA2, nanohybrids mesospherical particles with an average of specific surface area of 90 m 2 .g -1 (BET)

and aggregate size between 100-200 nm (TEM) were obtained at low temperature (0-10 °C)

with a PAA2 weight concentration between 0.5-0.63 wt% and washed with 90 mL of ethanol.

For ZnO-PAA5, a high specific surface area (120 m 2 .g -1 ) and smaller aggregates around 40 nm were elaborated at 40 °C with a PAA5 weight percentage of 0.63% and washed as well with 90 mL of ethanol. The increase of specific surface areas was generally associated with appearance of 2D nanosheets in addition to the 3D self-assembled mesospheres. The calcination and thermal behavior of the ZnO-PAA2 and ZnO-PAA5 materials were, unfortunately, studied before elaborating the material with the highest surface areas. Nevertheless, starting from a material with a surface of 20-30 m 2 .g -1 , the calcination temperature at 350 °C resulted in an increase of the specific surface. For ZnO-PAA2, the mesospherical morphology was intact till 325 °C calcination temperature but the higher surface area was obtained for ZnO calcined at 350 °C with S.S.A = 51 m 2 .g -1 and smaller aggregate size 10-20 nm while ZnO-PAA5 showed higher thermal sensitivity and the mesospheres transformed into ZnO with undefined shapes after calcining at 300 °C.

Before using these diverse ZnO-PAAX as supports for the elaboration of copper catalysts, an optimization of the DPU method was done while using commercial ZnO as support. The effect of the weight percentage of copper and the concentration of Cu 2+ in the solution showed an optimum at 20.8 mM for 10 wt% of copper. At higher copper percentage, it seemed that the low surface area of the commercial ZnO (around 13 m 2 .g -1 ) resulted in saturation of ZnO supports leading to a decrease in the glycerol conversion. The most active catalyst elaborated with the commercial ZnO resulted in 25% conversion and 90% selectivity toward 1,2-PDO after

7 II. 1 . 42 III. 1 .

 71421 Chapter I .................................................................................................................................. 5 I. ZnO .................................................................................................................................. Impregnation ................................................................................................ II.2. Deposition Precipitation (DP) ....................................................................... Deposition Precipitation with urea (DPU) .............................................. II.3. Co precipitation method ............................................................................... Alternative anions .................................................................................. II.4. Other preparation methods of Cu ZnO .......................................................... III. Glycerol as source for propanediols production ........................................................... Different mechanisms for the hydrogenolysis of glycerol into propanediol ... Dehydration of glycerol into 1,2 PDO and 1,3 PDO ................................ Dehydrogenation of glycerol to 1,2 PDO ................................................ III.2. List of copper catalysts for glycerol hydrogenolysis ....................................... Cu Cr catalysts ....................................................................................... Cu/ZnO catalysts .................................................................................... Cu/MgO catalysts ................................................................................... Other Cu based catalysts........................................................................ IV. Objectives of the thesis ................................................................................................. 58 V. References ..................................................................................................................... 60

Figure 1 1 :

 1 Figure 1 1: SEM images of some examples of various morphologies of ZnO nanostructures: (a), (b) tetrapod structures; (c) variable diameter structures; (d) nanosheets; (e) nanoshells; (f) multipods; (g), (h), and (i) nanorods.[START_REF] Djuriši | ZnO nanostructures for optoelectronics: Material properties and device applications[END_REF]

Figure 1 2 :

 2 Figure 1 2: ZnO crystal structures: (a) cubic Rocksalt (B1), (b) cubic Zinc blende (B3) and (c) hexagonal Wurtzite (B4). (Black circles: O atoms, grey circles: Zn atoms) 20

  3 a), or powder from a colloidal sol transformed into a gel (Figure 1 3 b).

Figure 1 3 :

 3 Figure 1 3: Examples of synthesis by the sol gel method: (a) films from a colloidal sol; (b) powder from a colloidal sol via a gel.[START_REF] Znaidi | Sol-gel deposited ZnO thin films: A review[END_REF] Sol gel process is also known for its large ability on controlling and modifying the surface and the textural properties of the elaborated (nano)materials. The process can be described as formation of an oxide network through hydrolysis polycondensation reactions of a molecular precursor (usually metal alkoxides and alkoxysilanes) in an organic solvent. The general formula of metal alkoxides is [M(OR) n ] m where M is the metal or non metal element, R an alkyl or aryl group and m the nuclearity such as Ti(O i Pr) 4 , [Zr(OEt) 4 ] 4 , Si(OEt) 4 ... In the first step, the precursor is hydrolyzed into hydroxide species which undergo polycondensation step leading to the elaboration of an oxide /hydroxide network.

Scheme 1 3 :

 3 Scheme 1 3: Modification of ZnO (Type of modifiers and applications)

  This latter one has studied different synthesis parameters reacting bisalkyl zinc precursor [Zn(c C 6 H 11 ) 2 ] in different types of solvants and ligands. Leaving a solution of dicyclohexylzinc (II) in THF at room temperature in open air led to agglomerated and crystalline zincite ZnO particles with undefined shapes and different sizes. Therefore, when the synthesis was done under argon and in presence of ligands as long chain amines, different morphologies of ZnO depending on the synthesis conditions were obtained as shown in Figure 1 4. The ligands used in the synthesis were: dodecylamine (DDA), hexadecylamine (HAD) and octylamine (OA) and the three types of solvents were THF, toluene and heptane.

Figure 1 4 :

 4 Figure 1 4: TEM micrographs of various ZnO nanorods, which were prepared using 1 equiv HDA (a), DDA (b), or OA (c) ligands in the absence of solvent; d) long ZnO nanorods obtained in the presence of 2 equiv OA as ligand, in the absence of solvent 57

Figure 1 5 :

 5 Figure 1 5: ZnO a) productive countries and b) applications fields. Redrawn from reference 65

  bandgap energy (3.3 eV). The mechanism is explained by the absorption of the light energy that triggers the charge separation and excites electrons from the valence band (VB) to the conduction band (CB). Holes are left in the VB 72 . The photogenerated e /h + carriers then move to the surface of the ZnO photocatalysts. Generally, the electrons play the role of reducing agents by transforming the O 2 molecules into O 2 . super oxide radical and the holes play the role of oxidative agents by transforming the reactant into his oxidative. A recombination phenomenon between e and h + can occur before attending the surface of ZnO and cause the deactivation of the photocatalyst. The Figure 1 6 illustrates the photodegradation mechanism of polluants by using ZnO 73 .

Figure 1 7 :

 7 Figure 1 7: Schematic of the comparison of the band structures of pure, metal doped ZnO, and nonmetal doped ZnO 73 .

Figure 1 9

 1 Figure 1 9 shows the different steps before obtaining the active catalyst:

Figure 1 9 :

 9 Figure 1 9: Preparation scheme of Cu/ZnO catalyst and TEM images of the different stages of preparation 114 .

Figure 1 10 :

 10 Figure 1 10: Setup of the batch precipitation method (a) and the continuous overflow method (b).117

  considerable interest in developing new technologies to convert glycerol into value added chemicals such as propanediols and glycols with the following representation: Glycerol hydrogenolysis has then been extensively studied in the recent years125,126 , since main products obtained from this reaction are 1,2 propandiol (1,2 PDO) and 1,3 propanediol (1,3 PDO), isopropanol, n propanol, ethylene glycol, propylene and propane. 1,2 propanediol is a synthetic organic compound widely used in industrial applications: polymer production, cosmetics, food industry, tobacco products and as de icing fluid in aircrafts. In 2013 the global production of 1,2 PDO was 2.2 Million tons mainly in the USA and China 127 . 1,3 propanediol (1,3 PDO)128 , is mainly used as a monomer for the production of: polytrimethylene terephthalate, a biodegradable polyester used in the manufacture of carpet and textile, polyethers and polyurethanes. The largest amount of propanediols is obtained by oxidation of propylene to propylene oxide. Nevertheless, more economical and less complicated routes can be applied.The catalytic hydrogenolysis of glycerol consists in adding hydrogen to the substrate in order to break a C-C or C-O bond. Since the H H bond is thermally stable, the addition of a metal catalyst is necessary in order to dissociate the H 2 molecule. The hydrogen adsorbed on the surface later reacts with the substrate and gives rise to new products. In the case of glycerol, different products are obtained after hydrogenolysis depending on the catalyst and experimental conditions. Glycerol hydrogenolysis has been extensively studied in the recent decades and interesting results have been obtained125,126 . Some groups studied the mechanism of this reaction, others focused on improving the activity and/or the selectivity toward propanediols. A summary of reported mechanisms is presented in the following pages, with a focus on 1,2 PDO as the main product. Regarding the hydrogenolysis of glycerol to propanediols, catalysts based on noble metals (Pt, Pd, Rh and Ru) 129-132 and transition metals (Ni 133-135 and Cu 136-139 ) have been studied in recent years. Depending on the experimental parameters (nature of the metal catalyst, nature of the support and solvent), different types of reaction mechanisms have been proposed in the literature. The most frequently cited are a 2 steps mechanism (dehydration followed by hydrogenation) and a 3 steps mechanism (dehydrogenation followed by dehydration and hydrogenation). The dehydration hydrogenation mechanism is promoted by the presence of an acidic catalyst helping the first step of the mechanism to occur. The most used catalysts are Cu supported catalysts that can combine the acidic and the hydrogenation functions. While the 2 nd mechanism is favored in neutral and alkaline conditions and it is more known with metal noble catalyst as (Ru, Pt…) 140 .

Scheme 1 10 :

 10 Scheme 1 10: Hydrogenolysis of glycerol with an aldol intermediate.141

Figure 1 11 :

 111 Figure 1 11: Model structures of the transition states of the hydride attack to the adsorbed substrate in the glycerol hydrogenolysis 141 .

  devices. The co precipitation method from zinc salts (acetate, nitrate, sulphate) in basic conditions (NaOH, KOH, LiOH) and in the presence of PAAH failed to reach the same performances. The efficiency of the resulting nanomaterials in terms of luminescence were correlated to the elaboration of hybrid systems ZnO/PAAH which self assembled in 3D spherical morphologies.Commercial ZnO have usually non homogeneous shape with large particle size distributed randomly from 50 nm to 300 nm. Depending on the supplier, the properties of the material may change but most of them have very low specific surface areas of around 10 m 2 .g 1 .

81 III. 2 .

 812 CHAPTER II ...................................................................................................................... Synthesis of ZnO by hydrolysis of ZnEt 2 in a solution of PAAH at 0.63% ................ I.1. Effect of the molecular weight of PAAH ........................................................ I.2. Study of the influence of the washing set up after synthesis ........................ I.3. Study of the influence of the concentration of PAAH .................................... I.4. Synthesis of ZnO/PAAH at different temperature ......................................... I.5. Calcination after synthesis ............................................................................ Synthesis of ZnO ZnAl 2 O 4 by hydrolysis of ZnET 2 and AlMe 3 .................................. Synthesis of supported Cu catalysts by deposition precipitation ........................... III.1. Deposition precipitation of Cu on commercial ZnO ....................................... Synthesis of Cu/ZnO commercial with different concentration of Cu 2+ and wt% Cu° Synthesis of Cu/ZnO with the synthesized ZnO ............................................. Deposition precipitation with ZnO/PAAX : ............................................. Deposition precipitation with the calcined ZnO: .................................... Deposition precipitation of Cu with ZnO ZnAl 2 O 4 ................................... Synthesis of Cu/ZnO ZnAl 2 O 4 by one pot ................................................................ IV.1. Synthesis of Cu ZnO and Cu/ZnO ZnAl 2 O 4 in one pot one step: .................... IV.2. Synthesis of Cu/ZnO and Cu/ZnO ZnAl 2 O 4 in one pot two steps: ..................

Figure 2 1 :

 1 Figure 2 1: Synthesis of ZnO NPs

7. 6 g

 6 Scheme 2 1: Exemple of synthesis of Cu/ZnO by DPU

Scheme 2 2 :

 2 Scheme 2 2: Glass cell for reducing the catalyst under a gas flow

Cu

  ZnO were synthetized by mixing all the precursors at once: In order to have 20 wt% of Cu, 25 mL of Cu 2+ solution (157 mmol/L or 10 g/L of Cu 2+ ) prepared by dissolving 7.6 g of Cu(NO 3 ) 2 .3H 2 O in 200 mL of water were added in a flask of 500 mL; 4 g of urea was added to this solution to keep the same parameters as before; 2.52 g of PAA2 (50 wt% in water) dissolved in 200 mL of water was added to prepare the ZnO suspension inside the mixture. The temperature was increased to 60 °C (the temperature of decomposition of urea) before adding 14.4 mL of the DEZ and stirring , 350 and 400 °C 300 and 350 °C under argon. The heating continued till 60 °C for 3 hours. During this time, the solution was having the same color (blue) during all the process. After three hours, the solution was centrifuged at 8000 r.p.m for 20 minutes and washed with 90 mL of ethanol and centrifuged again before drying under air at 80 °C for one night. After synthesis, the powder was calcined at 400 °C for 4 hours under air and reduced with H 2 flow at 350 °C for one hour, as previously. The color of the dried and calcined powder is same as the catalyst obtained by DPU method showed in the picture below.

Figure 2 2 :

 2 Figure 2 2: Color of the sample a) in solution, b) after drying and c) after calcination, for Cu ZnO

Figure 2 3 :

 3 Figure 2 3: Color of the sample a) in solution, b) after drying and c) after calcination for Cu ZnO ZnAl 2 O 4The temperature of the synthesis can have a big impact on the properties of the ZnO, so other one pot syntheses were done by dividing the synthesis into 2 steps.

Figure 2 4 :

 4 Figure 2 4: Color of the sample for the two steps method; a) in solution at 0°C, b) in solution at 60 °C and c) after drying

Figure 2 5

 2 Figure 2 5: a) Calcined CuO/ ZnO ZnAl 2 O 4 and b) reduced Cu/ZnO ZnAl 2 O 4

Figure 2 6 :

 6 Figure 2 6: Dependance of the color on the weight percentage of copper

Figure 2 7 :

 7 Figure 2 7: Photo (left) and scheme (right) of the batch reactor

  After mineralization, the resulting sample solution is then nebulized into the core of inductively coupled argon plasma, where temperatures of approximately 9000 K are attained. At such high temperatures, the nebulized solution is vaporized, and the analyte species are atomized, ionized and thermally excited. The analyte species can then be detected and quantitated with an optical emission spectrometer (OES), which measures the intensity of radiation emitted at the element specific, characteristic wavelength from thermally excited analyte atoms or ions. Intensity measurements are converted to elemental concentration by comparison with calibration standards. The device used is ACTIVA spectrophotometer by Horiba JOBIN YVON. Transmission electron microscopy (TEM) is a technique which allow the direct observation of metal particles. The size distribution of these particles can be established as well as the dispersion of the metal can be calculated. TEM images are obtained on a JEOL 2010 LaB 6 microscope operating at 200 kV. The resolution of electronic microscopy is much higher than the optical ones due to the very short De Broglie wavelength of electrons (resolution of 0.19 nm in Jeol 2010 microscopy).

8 .

 8 An electron beam is emitted by an electron gun containing an emission source (LaB 6 single crystal in JEOL 2010) connected to a high voltage source (200 Kv in JEOL 2010) under vacuum. The beam is converged by the different electromagnetic lenses of the TEM, typically the condenser, objective and projector lenses. The condenser lenses are responsible for primary beam formation and focus the beam on the sample. When passing through the sample, electrons are scattered due to collisions with the atoms in the sample and the beam is focused by the objective lenses to form the image of the sample. The projector lenses magnify the image and project them on an imaging device (a CCD camera in the case of JEOL 2010). The images are then recorded using Digital Micrograph software.

Figure 2 8

 2 Figure 2 8 Photo (left) and scheme (right) of JEOL 2010 TEM apparatus.

99 I. 1 .I. 3 .I. 4 .I. 5 .

 991345 CHAPTER III ............................................................................................................................ 98 I. Effect of the synthesis parameters on ZnO PAAX ......................................................... Effect of the molecular weight of PAAH ........................................................ I.2. Effect of the washing process after hydrolysis............................................. ZnO PAA2 system ................................................................................ ZnO PAA5 system ................................................................................ Effect of PAA2 concentration during synthesis ............................................ Effect of the synthesis temperature ............................................................ Effect of the synthesis temperature on ZnO PAA2 ............................... Effect of the synthesis temperature on ZnO PAA5 ............................... Effect of calcination temperature after synthesis ........................................ Effect of calcination temperature on ZnO PAA2 ................................... Effect of calcination temperature on ZnO PAA5 ................................... II. Conclusion ................................................................................................................... 136 III. References ................................................................................................................... 137As mentioned in Chapter I, ZnO nanoparticles can be synthesized using different approaches and the surface can be modified with many types of additives. The method chosen in our study is sol gel with the addition of polyacrylic acid (PAAH) as a modifying agent during the synthesis of ZnO PAAH hybrid systems as described in our publication1 . The hydrolysis of diethyl zinc in an aqueous PAAH solution is following the reaction below: Previous works done in our laboratory demonstrated that such ZnO PAAH hybrid systems may have original spherical self assembly structures, depending on the nature and the concentration of the polyacrylic polymer, that could be used as efficient nano luminophors

  1 c) led to a mixture of a poorly crystalline zincite phase with an average crystallite size of 19 nm calculated with the peaks at 47.5, 57 and 63° and an unidentified phase with two peaks at 33 and 59°. ZnO PAA5 exhibited mainly an amorphous ZnO structure and again the unidentified phase with two broad peaks at 33 and 59° (Figure 3 1 d).

Figure 3 1 :Figure 3 2 .

 132 Figure 3 1: XRD diffraction patterns of (a) ZnO without PAAX, (b) ZnO PAA1.8, (c) ZnO PAA2, (d) ZnO PAA5 and (e) ZnO PAA100

Figure 3 2 :Figure 3 3 :Figure 3 4 :

 23334 Figure 3 2: TEM images of: a) ZnO without PAAX, b) ZnO PAA1.8, c) ZnO PAA2, d) ZnO PAA5 and e) ZnO PAA100

Figure 3 5 :

 5 Figure 3 5: TGA curves of pure PAA2 and PAA5

Figure 3 6 :

 6 Figure 3 6: FTIR spectra for the different ZnO PAAX

Figure 3 7 :Figure 3 8 :

 738 Figure 3 7: XRD diffraction patterns of ZnO PAA5 dried after washing or lyophilized

Figure 3 9 :

 9 Figure 3 9: XRD diffraction patterns of PAA2 and PAA5

Figure 3 10 :

 10 Figure 3 10: Images of ZnO PAA5 after freeze drying obtained by: a) SEM and b) TEM

Figure 3 11 :

 11 Figure 3 11: XRD patterns of ZnO PAA2 using different washing

Figure 3 12 :

 12 Figure 3 12: TGA curves of ZnO PAA2 using different washing

Figure 3 13 :Figure 3 15 : 4 :

 3133154 Figure 3 13: TEM images of ZnO PAA2: a,c) unwashed and b,d) washed with 90 mL of EtOH

75 Figure 3 16 :

 75316 Figure 3 16: XRD diffraction patterns for ZnO PAA2 synthesized with different weight percentage of PAA2

Figure 3

 3 Figure 3 19 shows the relationship between the specific surface area of ZnO nanoparticles and the rice concentration. They also observed a modification of the surface associated with a change in specific surface area of the nanoparticles when the additive concentration was

Figure 3 18 :

 18 Figure 3 18: TEM images of ZnO PAA2 with: a) 0.3 wt% and b) 1.2 wt% of PAAH

Figure 3 17 :

 17 Figure 3 17: Relation between specific surface area and PAA2 weight percentage

Figure 3 20 :

 20 Figure 3 20: SEM images of ZnO prepared with a) 0 g of rice and b) 8 g of rice 6

Figure 3 21 :

 21 Figure 3 21: TGA curves for ZnO PAA2 synthesized with different weight percentage of PAA2Table 3 5: Effect of the weight percentage of PAA2 on the properties of ZnO NPs.

Figure 3 23 :Figure 3 24 :

 23324 Figure 3 23: TGA curves for ZnO PAA2 synthesized at different temperatures (0.63wt% of PAA2)

Figure 3 25 :

 25 Figure 3 25: XRD diffraction patterns of ZnO PAA5 at different synthesis temperature (0.63 wt% of PAA5)

Figure 3 26 :

 26 Figure 3 26: TGA cuves of ZnO PAA5 at different synthesis temperature (0.63 wt% of PAA5)

Figure 3 27 :Figure 3 28 :Figure 3 29 :

 327328329 Figure 3 27: TEM images of ZnO PAA5 synthesized at: a) 0 °C and b) 40 °C

Figure 3 30 :

 30 Figure 3 30: XRD diffraction patterns of ZnO PAA2 calcined at different temperatures .

  ). The increase in the calcination temperature led to an increase in the degradation of the polymer. After calcination at a temperature above 350 °C, ZnO PAA2 had almost completely lost its organic component. A weight loss of less than 10% was observed at 350 °C and 400 °C by TG analysis. Another change occurred at high calcination temperature, the onset of degradation temperature shift to a lower value (250 °C instead of 350 °C) indicating a change in the ZnO PAA interaction.

Figure 3 31 :

 31 Figure 3 31: TGA curves for ZnO PAA2 calcined at different temperatures and PAA2

Figure 3 32 :

 32 Figure 3 32: TEM images of ZnO PAA2 calcined at: a) 300 °C, b) 325 °C, c) 350 °C and d) 400 °C

Figure 3 33 :

 33 Figure 3 33: Relation between ZnO NPs specific surface area and calcination temperature 10

Figure 3 34 : 30 m 2 .g 1 )

 343021 Figure 3 34: XRD diffraction patterns of ZnO PAA5 calcined at different temperature

III. 1 .III. 2 .

 12 CHAPTER IV .......................................................................................................................... 138 Commercial ZnO as support ........................................................................................ 139 I.1. Effect of Cu 0 weight percentage .................................................................. I.2. Effect of Cu 2+ concentration ........................................................................ II. ZnO PAAX (X = 2, 5, 100) as supports .......................................................................... 154 II.1. ZnO PAA2 non calcined ............................................................................... Effect of the synthesis parameters (support and copper wt%) ............. ....................................................... Recyclability of Cu/ZnO PAA2 .............................................................. II.2. ZnO PAA2 calcined ...................................................................................... II.3. ZnO PAA5 as support .................................................................................. Non Calcined ZnO PAA5 ....................................................................... Calcined ZnO PAA5 .............................................................................. II.4. ZnO PAA100 as support .............................................................................. III. Cu ZnO by a "one pot" method .................................................................................. 170 Cu ZnO synthesis by a "one pot one step" approach ................................. Cu ZnO synthesis by a "one pot two steps" approach ............................... IV. Conclusions .................................................................................................................. 177 V. References ................................................................................................................... 178

Figure 4 2 :

 2 Figure 4 2: TEM images of commercial ZnO support

Scheme 4 1 :

 1 Scheme 4 1: Synthesis of Cu/ZnO catalysts by DPU on commercial ZnO As a first screening, twenty catalysts were synthesized using various [Cu 2+ ] during the synthesis to get different final Cu loading (Table 4 1). The XRD patterns shown in Figure 4 3 correspond to the crystallographic states of 5 wt% Cu/ZnO, synthesized with [Cu 2+ ] = 20.8 mmol.L 1 , at different stages of the synthesis (after DPU, after calcination and after reduction), asrepresentative examples of all the catalysts. The hexagonal phase of ZnO was present for all three steps with an average crystallite size of 60 nm (based on the three mains peaks). After DPU, a new peak appears at 28° corresponding to the monoclinic phase Cu 2 (NO 3 )(OH) 3 (PDF number 00 045 094). In this first stage copper ions precipitated onto the support in presence of urea. As it was mentioned in the first chapter, urea played a role of a delay base by decomposing first into ammonium and cyanate leading to an increase in the pH. Hence, Cu ions precipitated with the nitrates ions in the form of Cu 2 (NO 3 )(OH) 3 onto the surface of ZnO, as previously reported by Louis et al. over TiO 2 1 . We cannot exclude the formation of (Cu , Zn)(CO 3 )(OH) 2 (PDF number 00 036 1475 appendix 7) which overlaid with Cu 2 (NO 3 )(OH) 3

Figure 4 3 :

 3 Figure 4 3: Representative XRD diffraction patterns of Cu/ZnO at different step of catalyst synthesis

Figure 4 4

 4 Figure 4 4 shows the XRD diffractograms of the Cu/ZnO materials before catalysis with a zoom on the main Cu 0 peaks. Sizes of the ZnO crystallites before catalysis were between 45 and 58 nm in accordance with the size of the starting support (50 nm) indicating no sintering during catalyst synthesis. Intensities of ZnO peaks were not affected by the change in copper weight

Figure 4 4 :

 4 Figure 4 4: XRD diffraction patterns of Cu/ZnO with different copper weight percentage

Figure 4 5 :

 5 Figure 4 5: Conversion and selectivity in function of time for Cu/ZnO with different copper weight percentage.Grey curve: selectivity for 2.3%Cu/ZnO; black curve: selectivity for the other catalysts

Figure 4 6 :

 6 Figure 4 6: XRD patterns of Cu/ZnO at 2.3 wt% and 19.2 wt% after catalysis

Figure 4 7 :

 7 Figure 4 7: XRD patterns of 10%Cu/ZnO before and after catalysis

Figure 4 8 :

 8 Figure 4 8: TEM images of 10%Cu/ZnO catalysts: a,c) 10%Cu/ZnOcom 2 b,d) 10%Cu/ZnOcom 8 before and after catalysis, respectively

Figure 4 9 :

 9 Figure 4 9: TGA curves under nitrogen of Cu/ZnO catalysts at 10 wt% of Cu 0 before and after catalysis

Figure 4 10 :

 10 Figure 4 10: XRD diffraction patterns of 10Cu/ZnO PAA2 RT 1 and 10Cu/ZnO PAA2 0 after catalysis

Figure 4 12 :

 12 Figure 4 12: 24 hours' kinetic profile with 1g of 19.8%Cu/ZnO and 20.6%Cu/ZnO catalysts

Figure 4 14 :

 14 Figure 4 14: XRD patterns of the fresh and reused catalysts (20%Cu/ZnO PAA2). (B.T. : before test and A.T. : after test)

Scheme 4 4 :

 4 Scheme 4 4: Synthesis of Cu/ZnO catalysts by DPU of copper on calcined ZnO PAA2

Figure 4 17 :

 17 Figure 4 17: XRD diffraction patterns of 22.4%Cu/ZnO PAA100 before and after reaction. (B.T. : before test and A.T. : after test)

Figure 4 18 :

 18 Figure 4 18: XRD patterns at different step of the "one pot one step approach Cu ZnO"

Figure 5 1 :

 1 Figure 5 1: XRD diffraction patterns of Zn 2 Al 1 O PAA2 z

2 .

 2 It showed that the same amount of the polymer PAA2 was incorporated into the hybrid mixed material Zn 2 Al 1 O PAA2 and ZnO PAA2 with a total 51% weight loss between 200 and 600 °C. Weight loss at low temperature(60 200 °C) corresponded to the evaporation of the solvent. At higher temperatures, degradation could be attributed to the decomposition of the formed oxides into ZnO x and/or Zn x Al y O z . Organic compounds started to decompose at the same temperature (around 300 °C) as for ZnO PAA2 (Figure34 in Chapter III on page 103), suggesting no impact of Al on the synthesis and the thermal behaviour of the mixed oxide.After calcination at 400 °C, the TGA showed that the compound Zn 2 Al 1 O PAA2 400 had lost a large part of the organic matter. The decomposition of the organic residue began at a temperature below that of Zn 2 Al 1 O PAA2, of about 150 °C and the final total loss of weight was of 13%. The increase in calcination temperature to 600 °C logically resulted in a decrease in the amount of organic compounds present in the compound with a total weight loss of 6%, indicating an absence of residue for Zn 2 Al 1 O PAA2 600 .

Figure 5 2 :

 2 Figure 5 2: TGA curves of ZnO PAA2 and Zn 2 Al 1 O PAA2 z

Figure 5 3 :

 3 Figure 5 3: TEM images of a,c) Zn 2 Al 1 O PAA2 400 and b,d) Zn 2 Al 1 O PAA2 600Again, a correlation between the presence of 2D nano sheets and a large specific surface area could be highlighted.

Figure 5 4 :

 4 Figure 5 4: XRD diffraction patterns of Zn 1 Al 2 O PAA2 z ICP measurements of the compound Zn 1 Al 2 O PAA2 400 yielded a formula ZnAl 2.46 O 7.79 with an excess of aluminium in comparison to the nominal stoichiometry but much closer than the stoichiometry obtained with the old bottle.Table 5 2 summarizes the properties of the

Figure 5 5 :Figure 5 6

 55 Figure 5 5: TGA curves of Zn 1 Al 2 O PAA2 z

Figure 5 6 :

 6 Figure 5 6: TEM images of a,c) Zn 1 Al 2 O PAA2 and b,d) Zn 1 Al 2 O PAA2 400

Figure 5 7 :

 7 Figure 5 7: XRD diffraction patterns of Zn 2 Al 1 O z

Figure 5 8 :

 8 Figure 5 8: TGA curves of Zn 2 Al 1 O z

( 1 : 2 )

 12 with PAA2 (section I.2). The physico chemical characteristics of the different Zn 1 Al 2 O z materials are summarized in

Figure 5 9 :

 9 Figure 5 9: XRD diffraction patterns of Zn 1 Al 2 O z

Figure 5 10 :

 10 Figure 5 10: TGA curve of Zn 1 Al 2 O z

Figure 5

 5 Figure 5 11 also shows the XRD data of the two catalysts after the catalytic reaction. As already observed in the last chapter, the catalyst with high conversion showed the appearance of new Zn based carbonaceous phases [monoclinic aurichalcite (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 , PDF 00 038 0154 and monoclinic hydrozincite Zn 5 (CO 3 ) 2 (OH) 6 , PDF 00 019 1458] indicating C C cleavage. The Cu/Zn 2 Al 1 O catalyst was also transformed since only the ZnAl 2 O 4 phase was visible. Both catalysts exhibited a ZnAl 2 O 4 phase with an average crystallite size of 6 nm.

Figure 5 11 :

 11 Figure 5 11: XRD diffraction patterns of Cu/Zn 2 Al 1 O materials before and after catalysis. (B.T.: before test and A.T.: after test)

Figure 5 12 :

 12 Figure 5 12: Solubility diagram of ZnO copied from reference 10

Figure 5 13 :Figure 5 14 :

 13514 Figure 5 13: XRD diffraction patterns of Cu/Zn 1 Al 2 O before and after catalysis (B.T.: before test and A.T.: after test)

Figure 5 15 :

 15 Figure 5 15: TEM images of: a,c) Cu/Zn 1 Al 2 O PAA2, b,d) Cu/Zn 1 Al 2 O PAA2 400 after catalysis

( 1 . 4 : 1 )

 141 , Cu 0 crystallites were in the same range (11 nm).

Figure 5 16 :

 16 Figure 5 16: XRD of Cu x Zn 1.4 Al 1 O 4.1 prepared by the "one pot one step" method

Figure 5 17 :

 17 Figure 5 17: XRD diffraction patterns of "one pot two steps" Cu x Zn 1 Al 1.4 O 4.2 (after test corresponds to after the 1 st run)

Area 5

 5 showed higher content in Al compared to Zn, suggesting again the possibility of elaborating Al oxides or Zn x Al y O z with high Al content, as seen for ZnAl oxides for a molar ratio Zn:Al (1:2) (section I.2).

Figure 5 18 :

 18 Figure 5 18: TEM images of Cu x Zn 1 Al 1.4 O 4.2 one pot two steps before catalysis

80 86 14 °C for 3

 143 hours. Without this heating, a copper default was observed (CZA 20UP and CZA 20P), which could be explained by a partial precipitation of the copper.In addition, unlike the DPU process, all Zn/Al ratios were very close to 2 except, again, if no heating to 60 °C was performed. In conclusion, the step of heating to 60 °C for 3 hours was mandatory to have a control on the stoichiometry of Cu:Zn:Al. Heating was also mandatory if urea was used, otherwise its non decomposition led to deactivation of the catalyst by probable poisoning of its surface (14% conversion for the CZA 20UP catalyst).

Figure 5

 5 Figure 5 20 shows the XRD diffractograms of CZA 5HP, CZA 5UHP, CZA 20HP and CZA 20UHP as examples. As before, peaks associated with ZnAl 2 O 4 and Cu are labelled on the figure, the peaks without labels correspond to ZnO. For all the catalysts, the Cu 0 crystallite sizes are included in Table 5 15 and they are within the range 12 17 nm, with an average of 15 nm, calculated with the three peaks at 43, 51 and 74.5°. Moreover, small crystallites of ZnO (7 nm) were observed (calculated with the peaks at 57, 63 and 69°) along with very small crystallites of ZnAl 2 O 4 (size undetermined by Diffrac.Eva). CZA 20H was the exception with 30 nm for Cu 0 and 19 nm for ZnO. Therefore, another important factor to mention is the use of PAA2 by comparing CZA 20HP with CZA 20H, the conversion decreased to 46% without PAA2 due to the sintering of the copper particles during the synthesis. Keeping the polymer during the synthesis helps the elaboration of the homogeneous mesospheres of ZnO ZnAl 2 O 4 leading to a better dispersion of Cu 0 more active in catalysis.

Figure 5 20 :

 20 Figure 5 20: XRD diffractograms of CZA 20H and CZA [XUHP, XHP/ (X= 5 and 20)] before catalysis

Figure 5 21 :

 21 Figure 5 21: XRD diffractograms of CZA 20UHP and CZA 20HP (Zn:Al ; 2:1) after catalysis (A.T.: after test)

Having a higher content

  of Al, at similar wt% of copper (20UHP Zn:Al 2:1 and 20UHP last entry with Zn:Al 1:4), led to an increase of the activity of the catalyst from a conversion of 66% to 80%, respectively, and this could be related to three main factors: Increase in the specific surface area of the catalyst 12 ,Stabilization of ZnO caused by Al at higher loading13

Table 1 1

 1 : Effect of milling time and calcination temperature on the ZnO particle size

	Precursor	Milling time (hour)	Calcination temperature (°C)	Particle Size (nm)	References
	Na 2 CO 3	2	600	25	38
		6	600	21
		10	600	21
		6	400	18
		6	600	21
		6	800	36
	Na 2 CO 3	1	400	26	36
	Na 2 CO 3	9	300	27	35
		9	450	56
	C 2 H 2 O 4 .2H 2 O	4	450	40	37

  [START_REF] Kim | Visible light sensitive Na doped p type flower like ZnO photocatalysts synthesized via a continuous flow microreactor[END_REF]

  , authors investigated the role of CuO ZnO (33 : 65) catalyst purchased from United catalysts Inc. in the hydrogenolysis of sorbitol derived from starch.

They obtained an almost total conversion (98 %) of sorbitol under 130 bar of H 2 with 35 % selectivity to deoxyhexitol (hexane 1,2 diol, hexane 1,6 diol, hexane 1,2,3 triol …) at 240 °C. At 180 °C under the same H 2 pressure, conversion decreased to 92 % but selectivity increased till 63 % toward deoxyhexitol. An important point to mention from this study was the leaching of zinc ions in the solution after catalysis due to the acidity of the reaction medium. Working in basic conditions reduced the amount of Zn 2+ in solution from 30 to 13 ppm at 180 °C. b. Ni/ZnO for the conversion of cellulose into 1,2 alkanediols Wang et al. compared the effect of different oxide supports (ZnO, MgO, SiO 2 , TiO 2 , ZrO 2 ) and activated carbon with Ni based catalysts for the conversion of cellulose

  , Ni/ZnO was prepared by incipient wetness impregnation using nickel nitrate [Ni(NO 3 ) 2 .6H 2 O] as precursor. The catalyst elaborated after being capped 12 hours at room temperature, calcined at 400 °C and reduced at 500 °C for 5 hours showed a very low specific surface area of 12 m 2 .g 1 and high crystallite size of Ni around 23 nm. The

	performance in terms of conversion were lower than compared to the same system prepared
	by co precipitation.
	Most of the studies elaborating metal based catalyst (copper in particular) using ZnO as
	support, combine it with Al 2 O 3 in order to increase the specific surface area of the catalyst.
	Tanaka et al. 103 prepared Cu/ZnO/Al 2 O 3 by impregnating Al 2 O 3 support with an aqueous
	solution of copper and zinc nitrates. After calcination at 500 °C for 3 hours, the catalyst
	reached an average specific surface area of 115 m

2

.g 1 . A high specific surface area is not always enough when the impregnation method is applied because the metal particles are usually inhomogeneous, in terms of size and morphology, due to the random nucleation growth that occurs during the drying step. Scheme 1 5: Simplified scheme for the preparation of catalysts by Deposition Precipitation (DP) method

Table 1 2

 1 : Texture of the calcined Cu/SiO 2 samples and results for the glycerol conversion

	The first use of Cu catalysts prepared by deposition precipitation for the hydrogenolysis of
	glycerol was reported by Xia et al in 2012 109 . Two different catalysts of Cu/SiO 2 were prepared,
	the first by homogeneous deposition precipitation and the second by heterogeneous
	deposition precipitation. In the first case, aqueous solutions of ammonia aqua copper
	complex ([Cu(NH 3 ) 4 (H 2 O) n ]SO 4 with a pH 11) and silica (SiO 2 40 wt%) were mixed under stirring
	at room temperature for 3 hours. The mixture was then heated to 80 °C and kept at that
	temperature for 20 hours to slowly remove the ammonia. The resulting gel was then filtered,
	washed in hot distilled water, dried at 120 °C overnight, and later calcined at 450 °C under air
	for 3 h. For the second catalyst, a suspension of dried SiO 2 gel was poured into an aqueous
	solution of Cu(NO 3 ) 2 and stirred for 3 hours at room temperature. Then, an aqueous NaOH
	solution (up to pH = 10) was added to quickly precipitate Cu(OH) 2 species on SiO 2 . The
	temperature was then raised to 80 °C for 4 hours. The solution was washed at neutral pH and
	dried at 120 °C overnight and air calcined at 450 °C for 3 hours. Table 1 2 shows the effect of

Catalyst Properties Homogeneous deposition precipitation Heterogeneous deposition precipitation CuO a (wt%)

	30.5	31.0
	BET

surface area (m 2 .g 1 ) 366 334 Pore volume b (cm 3 .g 1 )

		0.72	0.85
	Cu dispersion c (%)	24.6	16.2
	Cu particle size c (nm)	4.1	6.2
	Conversion (%)	22.1	35.0
	Selectivity for 1,2 PDO (%)	98.0	93.7
	a Measured by XRF, b BET Method, c Calculated from dissociative N 2 O adsorption 109 .

Table 1 3

 1 method, citrate is used instead of carbonate. The method was developed by Courty et al. and was then used by different groups for the preparation of Cu/ZnO/Al 2 O 3 115 . : Effect of the calcination temperature and the compositions of the catalysts on the Cu surface determined by N 2 O chemisorption copied from 116

	It consists of mixing Cu(NO 3 ) 2 .xH 2 O, Zn(NO 3 ) 2 6H 2 O, Al(NO 3 ) 3 9H 2 O with an appropriate
	amount of citric acid. The solution is heated to 85 °C for 2 hours to obtain a homogeneous
	mixture. Solvent evaporation is carried out in a rotary evaporator under pressure (700 Pa) and
	at the same temperature overnight. After drying at 110 °C for 4 hours, a calcination step is
	carried out in flowing air and the final catalyst is obtained. In this study, the effect of the

Catalyst Composition CuO/ZnO/Al 2 O 3 (wt%) Temperature of calcination (°C) Cu S.S.A. (m 2 .g 1 ) S.S.A./BET catalyst (m 2 .g 1 ) CuZnAl

		65/28/07	400	6.6	n.d.
	CuZnAl	65/28/07	350	11	n.d.
	CuZnAl	65/28/07	320	7.6	n.d.
			Four step
	CuZnAl	65/28/07	calcination up to	10	n.d.
			350
			Four step
	CuZnAl	65/28/07	calcination up to	13	44.4
			320
			Four step
	CuZn	70/30/0	calcination up to	5.0	15.0
			320
			Four step
	CuAl	70/0/30	calcination up to	7.0	48.2
			320
	CuZnAl
	prepared by co	65/28/07	320	24	53.7
	precipitation
	n.d. = Not determined.

  The same group worked on a different precursor: the mixed Cu/Zn L tatrates 118 which contained both metals in a single solid phase and decomposed at moderate temperature (50 °C) because inorganic counter ions such as alkali metals or carbonate were avoided. The BET surface area increased with increasing ZnO content (from 2 m 2 .g 1 without ZnO to 45 m 2 .g 1 at 90 % molar of ZnO). The preparation method made it possible to obtain Metal L tartrates (MTTs) with different Cu/Zn ratios. It consisted in precipitating the aqueous solution of copper acetate and zinc acetate with L tartaric acid (H 2 TT) according to this equation: Sol gel methods were also used by this group for the preparation of Cu/ZnO 119 . This paragraph lists two different ways to use metal nitrates as precursors [Cu(NO 3 ) 2 .3H 2 O, Zn(NO 3 ) 2 .6H 2 O and Al(NO 3 ) 3 .9H 2 O]:

Table 1 4

 1 : BET surface area, pore volume and catalytic activity of Cu/Zn/Al calcined xerogels and aerogels

119 Sample Cu content (wt%) Nature BET S.S.A. catalyst (m 2 .g 1 ) Cu / S.S.A. (m 2 .g 1 ) Catalytic activity in methanol synthesis (%) CuO/Al 2 O 3

		50.9	Xerogel	34	3.7	2.0
	CuO/Al 2 O 3	52.7	Aerogel	222	5.2	4.0
	CuO/ZnO/Al 2 O 3	39.3	Xerogel	57	2.6	4.9
	CuO/ZnO/Al 2 O 3	34.2	Aerogel	175	6.0	22
	Organometallic complexes can be prepared by mixing equimolar quantities of copper bis
	stearate and diethyl zinc in toluene at room temperature. This preparation method was
	carried out by Williams et al 120 for the synthesis of methanol as shown in the Scheme 1 7.

Table 1 5

 1 : Effect of the weight ratio of ZnO:Cu on the catalytic activity for CO 2 hydrogenation to methanol 120 Table15 shows that the increase in the Zn content led to an increase in the methanol conversion from 39 mmol.g ZnOCu 1 h 1 with the 30:70 expected ZnO:Cu molar ratio to 60 mmol.g ZnOCu 1 h 1 with the 55:45 molar ratio followed by a decrease at higher content of ZnO.

	This means that there was an optimal value of ZnO:Cu weight ratio.

In conclusion, there are a multitude of methods for preparing Cu/ZnO catalysts, each with advantages and disadvantages. In general, the preparation and heat treatment conditions have a remarkable effect on the final structure of the catalyst and ultimately on its activity, but other conclusions can be summarized: the co precipitation method is one of the simple and the most widely used, but its disadvantage is the high risk of non replication of the synthesis. DPU leads to very small Cu crystallites and strong metal support interaction Sol gel process gives usually high metal homogeneity and high specific surface area Al 2 O 3 seems to be an essential promoter for increasing catalyst activity by: the increase in its surface area, increasing the long term stability of the catalyst, the increase in surface acidity, the decrease in particle size of the active metal The post treatment steps, i.e. washing, drying, calcination, reduction, must always be carried out carefully, whatever the method of preparation. Entry n° Molar ratio precursors ZnEt 2 : Cu(stearate) 2 Theoretical weight ratio ZnO:Cu Methanol maximum activity / mmol.g ZnOCu 1 h 1 Methanol activity after 12h/ mmol.g ZnOCu 1 h 1

	1	1:3	30:70	39	39
	2	1:1	55:45	60	58
	3	2:1	70:30	43	37
	4	7:1	90:10	18	13

Table 1 6

 1 : Short list for glycerol hydrogenolysis over noble metals and Ni supported catalysts

			Glycerol	Conversion	Selectivity
	Catalyst	T(°C)/P H2 (bar)/t(h)				Ref
			(wt%)/Cat(g)	(%)	(%)
	Rh/ZnO	180/20/12	2.5/0.2	98	28	129
	Pt/TiO 2	210/60/6	4.5/1	25	60	153
	Pd/Fe 2 O 3	180/0/24	12/0.6	100	94	154
	Ru/Al 2 O 3	240/80/5	1.27/0.9	69	38	155
	Raney Ni	190/10/44	8/2	97	71	156

Table 1 7

 1 54.8 % with a selectivity of 85 % to 1,2 PDO. They studied different parameters for glycerol hydrogenolysis using this catalyst and proved the dehydration hydrogenation mechanism by isolating the intermediate acetol. A summary of the best results, obtained in this publication, in term of conversion and selectivity is listed in the Table 1 7. All the results were obtained after 24 hours of reaction and at 80% weight concentration of glycerol for CuO x (Cr 2 O 3 ) y catalyst. : Effect of the synthesis parameters and reaction conditions on the hydrogenolysis of glycerol 141 In 2009, Liang et al. 159 prepared Cu Cr catalysts with high specific surface area (up to 88 m 2 g

	Catalyst (wt%)	P (bar)	T (°C)	Water content (wt%)	Conversion (%)	Selectivity 1,2 PDO (%)
	5	14	200	20	54.8	85.0
	20	14	200	20	78.5	62.0
	5	14	260	20	87	8.8
	5	14	200	0	69.1	71.9
	5	20.7	200	20	65.3	89.6
	1

Dasari et al. 141

. First, they tested different commercial catalysts including palladium, ruthenium, nickel, copper, and copper chromite in the form of metal oxides or supported metal on C or alumina at 200 °C and 14 bar of H 2 for 24 hours. The most active was a copper chromite supplied by Sud Chemie, it showed a conversion of

Table 1 8

 1 8. : Different Cu based catalyst reported for glycerol conversion

  The catalyst preparation method appeared to have a significant contribution and effect on the results. The best results were obtained with Cu ZnO prepared by the co precipitation method as mentioned by the following groups: Wang & Liu in 2007 (Table18, entry 2) 145 and Balaraju et al. in 2008 (Table 1 8, entry 3) 157 . They studied different Cu/Zn molar ratio but both groups

		Ref				168			169	170	171	172	166
	Selectivity	1,2-PDO	(%) a	0	17.5		60.2	44.6	95.5	97.1	74.2	94.5	92.1	95	76.4
	Conversion	(%) a	0.4	4.9		16.2	24.6	91.2	56.7	77.2	31.3	85.8	94	72.6
	Duratio	n	(h)	8	8		8	8	20	10	6	8	10	8	20
	T	(°C)	200	200		200	200	180	180	200	200	180	200	200
	P	(bar)	24	24		24	24	30	20	40	25	40	25	17
	C glycerol	(wt%)	80	80		80	80	75	75	80	4	80	4	20
	W catalyst(g) /	w glycerol (g)	0.4/40	0.4/40		0.4/40	0.4/40	1/7.5	1/8	0.8/16	0.4/20	0.4/20
		Entry Catalyst		ZnO	Cu	9	Cu/ZnO	Cu/Cu2O/ZnO	10 Cu0.4/Mg5.6/Al2 O8.6	11 Cu0.4/Mg5.6/Al2 O8.6	12 Cu/MWCNT	13 CuO/ZrO2	Cu/ZrO2	8 Cu/Zn/Al	a at final time

  8, entry 6).In 2013, Tan et al.166 reported a new preparation method for a Cu/ZnO/Al 2 O 3 catalyst, called evaporation induced self assembly (EISA), in which Pluronic P123 was used as a structure directing agent, copper acetate as the precursor of CuO, zinc nitrate as the precursor of ZnO and aluminum isopropoxide as the precursor of Al 2 O 3 . The experimental conditions were reported to be 180 °C and 40 bar of H Cu 0.4 /Mg 5.6 /Al 2 O 8.6 catalyst prepared using NaOH and Na 2 CO 3 coprecipitation method, was reported as the optimal molar ratio. Under experimental conditions such as 180 °C, under 30 bar of H 2 , 75% aqueous glycerol, the glycerol conversion was 91.2% and the 1,2 PDO selectivity 95.5% after 10 h. However, a significant copper 10 wt%

2 ; 80% glycerol was loaded and 3 wt% of catalyst was used; within 10 hours the glycerol conversion was 85.8% and the selectivity to 1,2 PDO was 92.1% (Table

1

8, entry 7).

In 2015, Kim et al.

167

synthetized Cu Zn Al catalysts by microwave process. They have also studied the molar ratio of the materials and the effect of reduction after synthesis. Once again, reducing the catalyst led to increase the conversion and the selectivity to 1,2 PDO. The highest reported yield of 1,2 PDO was 76.4% and a conversion of 72.6% with a molar ratio Cu/Zn/Al of 2:2:1, at 200 °C, under 17 bar of H 2 and after 20 hours. (Table

1

8, entry 8).

In 2017, Mariscal et al.

168

studied two types of preparation: i) Coprecipitation method to elaborate: CuZn oxides with different atomic ratio of Cu and Zn and two monometallic oxides of Cu and Zn, and ii) Physical mixing of precursors: bimetallic catalyst with a molar ratio Cu/Zn = 2.5:1. Reaction conditions were chosen to be interesting for industrial application (solution of glycerol 80 wt% in water similar to crude glycerol from biodiesel production). All tests were done at 200 °C under 24 bar of H 2 and for 8 hours. ZnO as catalyst showed no activity, monometallic Cu showed a low conversion (5%) and low selectivity (17.5%) for 1.2 PDO. In the case of bimetallic oxide catalyst CuZn, the conversion of glycerol increased with the increase of the atomic ratio of Cu/Zn. Unlike the selectivity of the different products (1,2 PDO, acetol and ethylene glycol), no correlation was observed between selectivity and atomic ratio. The maximum selectivity values observed for 1,2 PDO were 63.8%, corresponding to the catalyst with 0.2 molar ratio Cu/Zn. The highest yield of 1,2 PDO (24.6% of conversion and 44.6% selectivity) was obtained with the Cu/Cu 2 O/ZnO catalyst with a molar ratio Cu/Zn = 1:1 (Table

1

8, entry 9).

Cu/MgO catalysts

Recently, Cu/MgO catalysts have been reported for this reaction. It was found that the basicity of the catalyst also played an important role in its performance. In 2011, Yuan et al.

169

reported for the first time a Cu/MgO/Al 2 O 3 catalyst for a glycerol hydrogenolysis process to produce 1,2 PDO. A loading of catalyst was required as shown in (Table

1

8, entry 10). It was also reported that when 5% to 10% NaOH were added to the reaction mixture, glycerol conversion was higher and 1,2 PDO selectivity decreased due to increased ethylene glycol production.

To improve the activity of the Cu/MgO/Al 2 O 3 catalyst, another metal was added. In 2011, Xia et al.

170

reported that when Pd was added, the catalyst was active under milder conditions compared to a Cu/MgO/Al 2 O 3 catalyst. In this report, the reaction conditions were: 180 °C, under 20 bar of H 2 , 75% aqueous glycerol and for 10 hours and a glycerol conversion of 56.7% and a 1,2 PDO selectivity of 97.1% were obtained using the Cu/MgO/Al 2 O 3 catalyst (Table

1

promotion effect of Pd was proposed to be due to H 2 spillover from Pd to Cu. Other Cu based catalysts

  In 2018, Muhler et al.172 studied the effect of the reduction on Cu/ZrO 2 catalysts. They used the co precipitation method to prepare different series with different metal loadings of catalysts. The reduction of the catalyst after the calcination step led to an increase in the conversion from 31% with 18% CuO/ZrO 2 to 95% with the same catalyst but reduced. The reaction conditions are shown in the last entry of the Table1 8.

8, entry 11

). The addition of 4 wt% Pd increased the glycerol conversion and 1,2 PDO selectivity to 76.9% and 97.2%, respectively. The Some copper based supported catalysts prepared by impregnation method have also been reported. However, the activities of supported Cu based catalysts were not as high as those prepared by coprecipitation method. The most frequently reported catalysts were Cu supported on Al 2 O 3 173,174 , on SiO 2 138,142,175,175,176 and other supports (MgO, Al hydrotalcite, ZrO 2, carbone nanotube…) 171,177,178 . In 2011, Wu et al. 171 reported different Cu catalysts supported on multi wall carbon nanotubes (MWCNT), which showed high activity for glycerol hydrogenolysis to produce 1,2 PDO. When the experimental conditions were 200 °C under 40 bar of H 2 , 80 wt% aqueous glycerol with a reaction time of 6 hours, the glycerol conversion was reported to be 31.3% and the 1,2 PDO selectivity was 91.1% (Table 1 8, entry 12).

Table 2 1

 2 : Properties of the chemical applied for the syntheses

	Product	Formula	Acronym	Supplier	Purity (%)
	Polyacrylic acid	CH 3 (CH 2 CHCOOH) n CH 3	PAAX	Sigma	50
	Diethyl Zinc	Zn(CH 2 CH 3 ) 2	DEZ	Sigma	15
	Trimethyl aluminium	Al(CH 3 ) 3	TMA	Sigma	97
				Alfa Aesar	25
	Copper nitrate	Cu(NO 3 ) 2 3H 2 O	C.N	Sigma	99.9
	Zinc oxide	ZnO	ZnO	Alfa Aesar	99.9
	Urea	CH 4 N 2 O	U	Sigma	99
	Glycerol	C 3 H 8 O 3	GLY	Sigma	99.5
	1,2 Propanediol	C 3 H 8 O 2	1,2 PDO	Sigma	99.5

The first step of the work was the synthesis of the support (ZnO). Different parameters were studied such as the molecular weight of PAAH, the washing process, the weight concentration of PAA2… Synthesis

of ZnO by hydrolysis of ZnEt 2 in a solution of PAAH at 0.63%

The synthesis of ZnO was done via sol gel method, based on our previous work, by hydrolyzing one equivalent of ZnEt 2 (diethylzinc or DEZ) in a solution of PAAH under argon (schlenk system) due to the pyrophoric state of diethyl zinc.

The PAAH solutions were prepared using commercial PAAH with different molecular weight diluted in water. The commercial products used for ZnO synthesis and their detailed properties are listed in the table below:

Table 2 2

 2 : Commercial products used for the synthesis of ZnO

	Properties	Diethyl zinc	Polyacrylic acid (PAAH)
	Supplier	Sigma Aldrich	Sigma Aldrich
	CAS No	557 20 0	9003 01 4
	Formula	ZnC 4 H 10	CH 3 (CH 2 CHCOOH) n CH 3
	Mw (g.mol 1 )	123.5	1800 (PAA1.8), 2000
			(PAA2), 5000 (PAA5), 100000
			(PAA100)
	Boiling T° (°C)	118	/
	Melting T° (°C)	28	/
	Density (g.cm 3 )	1.2	/
	Aspect	Liquid	Viscous liquids
	Dangers	C*, F*, N*	Xi*
	Purity	15% in toluene	50% in water

C*: corrosive, F*: inflammable, N*: dangerous for the environment, Xi*: irritant.

Table 2 3

 2 : Weight of added PAA2 for different concentration

	Percentage of PAA2	Weight of PAA2 added
	(wt%)	in 200 mL of water (g)
	0	0
	0.3	1.2
	0.5	2.0
	0.63	2.52
	0.8	3.36
	1	4.2
	1.2	4.8

Table 2 4

 2 : Volume of the added solution for each copper concentration

Table 2 5

 2 : Different ZnO used as supports for catalysis

	Cu° weight percentage (%)	Added volume of Cu 2+ solution (mL)
	2.5	2.02 of solution 1
	5	5.27 of solution 2
	10	11.1 of solution 2
	15	17.7 of solution 2
	20

Deposition precipitation with the calcined ZnO:

Table 2 6

 2 Synthesis parameters for Cu/calcined ZnO catalyst

Deposition precipitation of Cu with ZnO ZnAl 2 O 4

Table 2 7

 2 : Possible products of hydrogenolysis of glycerol and their retention time

	Products	Formula	M.W.	Retention time
			(g.mol 1 )	(min)
	Glycerol	C 3 H 8 O 3	92.08	22.8
	1.2 PDO	C 3 H 8 O 2	76.09	15.2
	1.3 PDO	C 3 H 8 O 2	76.09	17.5
	Lactic acid	C 3 H 6 O 3	90.07	21.7
	Ethylen glycol	C 2 H 6 O 2	62.06	15.8
	n butanol	C 4 H 10 O	74.12	8.7
	Acetol	C 3 H 6 O 2	74.07	11
	EtOH	C 2 H 5 OH	46.07	4.4
	MeOH	CH 3 OH	32.04	3.9
	1.4 BDO	C 4 H 10 O 2	90.12	18.9
	2 propanol	C 3 H 8 O	60.10	4.3
	The conversion and the selectivity were calculated based on these equations:
	%		100
	With :
	C 0 substrate : Initial concentration of substrate ( in mol.L 1 );

C t substrate : Concentration of substrate at time t (in mol.L 1 ). N C product i : Number of carbon atoms in the product i; N C substrate : Number of carbon atoms in the substrate.

Table 3 1

 3 showed that the largest specific surface area of 50 m 2 .g 1 was obtained with ZnO PAA100. ZnO PAA2 and ZnO PAA5 had similar values around 20 m 2 .g 1 . ZnO without PAAX and ZnO PAA1.8 had similar specific surface area of around 40 m 2 .g 1 . The main pore diameter of the three samples ZnO PAA2, ZnO PAA5 and ZnO PAA100 was around 4 nm proving that all the NPs were mesoporous (Appendix 3 5). No data has been published on the relationship between the specific surface area of ZnO and the length of PAAH, but related study such as S. Park et al.2 , demonstrated that an increase in the chain (carbon or nitrogen) of the added fuel resulted in an increase in the specific surface area

	of the ZnO NPs: 27 m 2 .g 1 with urea as additive (CH 4 N 2 O), 32 m 2 .g 1 with carbohydrazide
	(CH 6 N 4 O) and 120 m 2 .g 1 with glycine (C 2 H 5 NO 2 ). The main characteristics of the ZnO PAAX are
	compiled in Table 3 1.

Table 3 1

 3 : ZnO PAAX characteristics.

  1.

			Pore		Aggregate	TGA
		S.S.A./ BET		Crystallite size
	Sample	(m 2 .g 1 )	diameter /	/ XRD (nm)	size / TEM	weight loss
			BET (nm)		(nm)	[200 600 °C] (%)
	ZnO without	43		16	20 30	2
	PAAX
	ZnO PAA1.8	38		11	15 25	5
	ZnO PAA2	20	3	19	150 200	40
	ZnO PAA5	23	4	Amorphous	50 100	41
	ZnO PAA100	50	3 10	ZnO: 12 Zn(OH) 2 : 30	10 50	30

Table 3 2

 3 : ZnO NPs properties after different treatments3

Table 3 3

 3 : Effect of EtOH volume on ZnO PAA2 NPs properties Morphological changes in ZnO nanoparticles due to the ethanol/water ratio in a double solvent system were reported by Yin et al. in 20155 . The study showed that spherical ZnO was obtained without adding water while the addition of different volumes of water transformed the morphology into hexagonal discs with a low water content (2 mL) and micro rods with 8 mL of water.

	Solvent volume	S.S.A. /	ZnO crystallite size /	TG / weight loss
	(mL)	BET (m 2 .g 1 )	XRD (nm)	[200 600°C] (%)
	Un washed	20 40	19	40
	30 EtOH	60	18	50
	60 EtOH/H 2 O (1:1)	41	16	51
	60 EtOH	68	22	52
	90 EtOH	91	20	50

2 .g 1 ) ZnO crystallite size / XRD (nm) Agregate size / TEM (nm) TG / weight loss 200 600 (%)

	0	44	16	20 30	3
	0.30	75	18	100 150	33
	0.50	75	19	X	45
	0.63	60	19	X	50
	0.80	36	13	X	54
	1.00	22	17	X	59
	1.20	17	Amorphous	250 300	61

PAA2 (wt%) S.S.A. BET (m Figure 3 22: XRD diffraction patterns for ZnO PAA2 synthesized at different temperatures (0.63 wt% of PAA2)

Table 3 7

 3 : Effect of the synthesis temperature on ZnO PAA5 characteristics

	: not determined

Table 3 9

 3 : Effect of the calcination temperature on ZnO NPs properties9

	Sample	Surface area	Total pore	Particle size	Mean pore
		(m 2 .g 1 )	volume (cm 3 .g 1 )	(BET; nm)	diameter (nm)
	ZnO air dried	40.32	0.4029	26.56	39.99
	ZnO 200	49.70	0.4759	21.53	38.29
	ZnO 300	51.11	0.5601	20.92	43.81
	ZnO 400	40.63	0.4848	26.38	47.79
	ZnO 500	19.95	0.4744	53.68	95.19
	ZnO 600	14.31	0.2073	74.65	57.84
	ZnO 700	10.81	0.1176	98.88	43.47
	ZnO 800	2.82	0.0151	379.56	21.42

Table 3

 3

	10: properties of ZnO PAA5 calcined at different temperature
					TG / Weight
	Calcination T°	S.S.A. / BET	ZnO crystallite size	ZnO particle size /
	(°C)	(m 2 .g 1 )	/ XRD (nm)	TEM (nm)	loss [200 600]
					(%)
	300	26	20	20 30	6
	350	31	18	20 30	5
	400	20	24	20 30	2

Table 4 1

 4 : Cu/ZnO catalysts with different initial [Cu 2+ ] and copper loadings. The conversions after 24 hours obtained with all the catalysts prepared with the commercial ZnO with different copper weight percentage and [Cu 2+ ] are summarized in Table42. For all the catalysts studied in this chapter, the carbon balance was always above 95%.

	Cu theo (wt%) a		2.5		5		10		15		20
	[Cu 2+ ]	C.S. b	Cu exp c	C.S.	Cu exp	C.S.	Cu exp	C.S.	Cu exp	C.S.	Cu exp
	(mmol.L 1 )	(nm)	(wt%)	(nm)	(wt%)	(nm)	(wt%)	(nm)	(wt%)	(nm)	(wt%)
	5.2	20	2.4	24	5.1	23	11.1	23	15.5
	10.4	15	2.4	22	5.0	20	10.3	20	16.5
	20.8	13	2.3	13	4.9	15	9.7	14	15.5	20	19.2
	40.8	13	2.3	13	5.0	15	10.3	14	14.3
	81.6			12	4.7	11	9.3	16	14.2

a Theoretical copper weight percentage b Copper crystallite size based on Scherrer equation c Experimental copper weight percentage obtained by ICP analysis After characterizing the main properties of these catalysts, the conversion of glycerol into 1,2 propanediol (1,2 PDO) was studied in a batch reactor, under 30 bar of H 2 and 200 °C, for 24 hours. 100 mL of an aqueous solution of 0.23 M glycerol and 500 mg of catalyst were used. As it was mentioned in the chapter I, when using Cu/ZnO in water, the reaction follows the dehydration hydrogenation route as shown in Scheme 4 2. As a first step, the dehydration takes place on the surface of ZnO due to its acidic properties resulting in acetol as intermediate, detected in our case as well. It is then transformed over the copper surface into 1,2 propanediol (1,2 PDO) as major product and ethylene glycol (EG) as secondary product with low selectivity (< 10 %), depending on the conversion. Scheme 4 2: Mechanism of glycerol hydrogenolysis over Cu/ZnO catalysts

Table 4 2: Conversions of glycerol after 24hrs for Cu/ZnO catalysts with different initial [Cu 2+ ] and copper loadings

Table 4 3

 4 summarizes data on elemental copper analysis, specific surface area, size of ZnO and Cu 0 crystallites of catalysts prepared with a Cu 2+ concentration of 20.8 mM and different copper weight percentage.

Table 4 3

 4 : Characteristics of the catalysts synthesized at 20.8 mM of Cu 2+ (B.T. : before test and A.T. : after test)

Table 4 6

 4 : Properties of the support ZnO PAA2, the catalysts Cu/ZnO PAA2 and the associated catalytic results after 24h.

(B.T. : before test and A.T. : after test)

Table 4 7

 4 : Cu loading, ZnO and Cu 0 size for two batches of Cu/ZnO PAA2 and their associated catalytic results

	after 24h using different mass.
	Cu exp (wt%)	Mass of catalyst (g)	ZnO size (nm) B. T. A. T.	Cu 0 size (nm) B.T. A. T.	Conversion (%)	Selectivity (%) 1,2 PDO EG
		0.3	21	39	20	38	15	96	4
	20.6	1	21		20	24	66	88	11
		1.5	21	35	20	25	73	85	14
		0.5	21	38	28	30	32	99	1
	19.8	1 0.5	21 21	40 33	28 28	35 29	60 12 (6h)	90 99	8 1
	( : not determined)

Figure 4 11: XRD patterns of Cu/ZnO catalysts with 19.8 wt % and 20.6 wt% of copper, before reaction

Table 4 8

 4 : Structural properties and activity of reused Cu/ZnO catalysts

	Catalysts	ZnO size (nm) Cu 0 size (nm)	/ 24 hrs (%) Conversion	Selectivity (%)
		B. T.	A. T.	B. T. A. T.		1,2 PDO	EG
	Fresh / 1.5 g	21	35	20	25	73	85	14
	Reused / 0.5 g	35	46	25	26	8	75	22
	Retreated and
	reused / 0.5 g	22	35	20	24	48	90	10

Table 4 9

 4 : Effect of the calcination temperature of ZnO PAA2 on the catalysts structural properties and the catalytic results

  15 shows the XRD of Cu/ZnO calcined at 400 °C with the highest conversion (20Cu/ZnO PAA2 400 ) before and after test and compared to the reference catalyst 20Cu/ZnO PAA2 RT 2 . XRD patterns are showing the same phases and crystallite size for the two catalysts, before and after reaction.

	Entry	S.S.A. BET (m 2 .g 1 )	Cu 0 size (nm) B. T. A. T.	Cu exp (wt %)	Conversion / 24 hrs (%)	Selectivity (%) 1,2 PDO EG
	10Cu/ZnO PAA2 300	34	14	21	16.5	18	88
	10Cu/ZnO PAA2 325	57	15	18	16.4	18	87
	10Cu/ZnO PAA2 350	72	13	14	13.6	54	90	9
	20Cu/ZnO PAA2 350	80	24	30	23.4	35	90
	10Cu/ZnO PAA2 400	72	16	18	9.2	10	88
	20Cu/ZnO PAA2 400	80	19	21	22.1	55	88
	20Cu/ZnO PAA2 RT 2	80	21	25	21.2	55	88

Figure 4 15: XRD patterns for 20Cu/ZnO PAA2 RT 2 and 20Cu/ZnO PAA2 400 before and after catalysis. (B. T. : before test, A. T. : after test)

Table 4 11

 4 : Effect of the calcination of the support ZnO PAA5 on the Cu/ZnO catalytic results

Table 4

 4 12.

	Calcination temperature (°C)	Cu 0 size (nm)	Cu (wt %) ICP	Conversion / 24 hrs (%)	Selectivity (%) 1,2 PDO EG
	300	13	17.0	27	90	10
	350	17	10.3	4	95	5

Table 4 12

 4 : Properties and catalytic results of Cu/ZnO PAA100

	Synthesis	S.S.A. / BET	Cu 0 size	Cu ICP	Conversion
	temperature	(m 2 .g 1 )	(nm)	(wt %)	/ 24 hrs (%)
	RT	70	11	12.9	0.9
	RT	49	13	22.4	1

Table 4 15

 4 : Catalytic results associated with the one pot Cu ZnO

  Figure 4 19: XRD patterns at different steps for the "one pot two steps approach Cu ZnO"Other studies showed very high activity like in the article of Bienholz et al.12 where authors have prepared Cu ZnO catalysts by co precipitation with the same copper and zinc precursors as Wang et al. but the precipitating agent was sodium carbonate instead of urea. The

			Selectivity
	Catalysts Cu/ZnO	Conversion / 24h (%)	/ 24h (%) 1,2 PDO EG
	"one pot one step" approach	41	88	11
	"one pot two steps" approach	61	89	11
	hydrogenolysis of glycerol was done with 90 wt% of glycerol (177 g of glycerol in 200 mL of
	water), under 50 bar of H 2 , in presence of 2.8 g of catalyst (n glycerol /n catalyst = 44) and after 7
	hours. At 200 °C, the conversion was 55 % with 82% selectivity toward 1,2 PDO while the same
	catalyst at 220 °C gave a conversion of 84% with maintaining the selectivity (81%) under the
	same catalytic conditions cited above. The recyclability test resulted in a decrease in the
	conversion from 55% to 38% but the selectivity stayed in the same range (79%).
				where authors
	have prepared Cu ZnO by co precipitation of copper nitrate and zinc nitrate in presence of
	urea. For a molar ratio of Cu:Zn (1:1) and under the following catalytic conditions: 200 °C, 42
	bars of H 2 , glycerol content of 20 wt% (15 g of glycerol in 65 mL of water), in presence of 7.5
	mM of copper [equivalent of 1.1 g of catalyst, resulting in n glycerol /n catalyst = 9.4 a bit higher than
	our system (7.8)] and after 12 hours, the conversion of their system was 22.5% with a
	selectivity toward 1,2 PDO of 83.6%. The crystallite size of ZnO increased from 19 to 28 nm
	and those of copper from 10 to 33 nm.

  8 mM Cu 2+ after 24 hours and at 200 °C and under 30 bar H 2 . The selectivity to 1,2 PDO in this case was 90% and 10% for the ethylene glycol (EG). The catalyst after the catalytic reaction had a new carbon containing phase, associated with C C cleavage. By replacing commercial ZnO with ZnO PAA2, glycerol conversion increased. Moreover, it was possible to charge the catalyst with 35 wt% by weight of copper.

Table of contents

 of CHAPTER V ........................................................................................................................... 179 I. Synthesis of Zn x Al y O PAA2: Study of the molar ratio of metals .................................. 180

	I.1. Zn:Al (2:1) molar ratio in presence of PAA2 (Zn 2 Al 1 O PAA2 z ) ....................... 181
	I.2

. Zn:Al (1:2) molar ratio in presence of PAA2 (Zn 1 Al 2 O PAA2 z

) ......................

. 185 I.3. Zn:Al (2:1) molar ratio without PAA2 (Zn 2 Al 1 O z

) ........................................

. 190 I.4. Zn Al (1:2) molar ratio without PAA2 (Zn 1 Al 2 O z

  II. Synthesis of Cu/ZnAl by DPU: study of the Zn Al effect .............................................. 196

) ......................................... 192

II.1. Deposition of 20 wt% of copper on Zn 2 Al 1 O PAA2 and Zn 2 Al 1 O materials by DPU 197 II.2. 20 wt% of copper on Zn 1 Al 2 O PAA2 and Zn 1 Al 2 O materials by DPU ............. 200 III

  . Synthesis of Cu Zn x Al y O z by a "one pot method": Study of the synthesis parameters

205 III.1. Synthesis of Cu Zn x Al y O z by a "one pot one step" route ............................ 205 III.2. Synthesis of Cu Zn x Al y O z by a "one pot two steps" route ........................... 207 III.3. Catalytic results over Cu Zn x Al y O z prepared by "one pot" routes ................ 211 III.4. Effect of synthesis parameters on Cu Zn x Al y O z by "one pot two steps" route:213 IV

  . Conclusions .................................................................................................................. 220 V. References ................................................................................................................... 222

Table 5

 5 ICP results showed a higher content of zinc from what was expected, with a molar ratio 2:1 instead of 1:2. At first, we did not know if the difference was coming from the synthesis (i.e. leaching of Al) or if the bottle was oxidized resulting in a lower content of Al.Powder X ray diffractograms of Zn 2 Al 1 O PAA2 , Zn 2 Al 1 O PAA2 400, Zn 2 Al 1 O PAA2 600 are shown in Figure51. XRD pattern indicates that as prepared material is composed of crystalline ZnO (hexagonal phase: zincite, PDF number 01 083 6338) with an average crystallite size of 22 nm, calculated by using the peaks at 47°, 57° and 63°, very similar to the ones obtained without aluminium (19 nm) in chapter III section I. The main difference comes from the absence of peaks at 33 and 59° from the "unidentified" phase.The calcination of the powder at 400 and 600 °C, at a heating rate of 5 °C per minute for 4 hours in a static furnace, led to an increase in the crystallinity of ZnO and the appearance of large peaks corresponding to ZnAl 2 O 4 (cubic phase, PDF number 04 014 1594 appendix 12), slightly visible at 400 °C. At 600 °C, the mean crystallite sizes of ZnO decreased to 9 nm and the one of ZnAl 2 O 4 was estimated at 12 nm with peaks at 59°, 65° and 74°.For the material Zn 2 Al 1 O PAA2 600 , calculations with Diffrac.Eva software gave a proportion of 58% ZnO and 42% ZnAl 2 O 4

	1: Characteristics of Zn 2 Al 1 O PAA2 z
	Temperature	S.S.A. /	Crystallite size	Particle	Metal loading	Zn:Al	TG / weight
	(°C)	BET	/ XRD (nm)	size	(weight %)	(molar	loss [200 600
		(m 2 .g 1 )	ZnO ZnAl 2 O 4	/ TEM (nm)	Zn	Al	ratio)	°C]
								(%)
	As prepared	71	22					35
	400	234	8		10 100	55.8 11.8	2:1	4
	600	98	9	12	5 20			1
	( : not determined)

Table 5 2

 5 summarizes the properties of the materials obtained. Once again, the ratio obtained by ICP was not in agreement with the phase calculations made on Diffrac.Eva. Indeed, for an ICP ratio of Zn:Al (1:2), ZnO was not supposed to be present in XRD patterns. Therefore, two hypotheses can be given.1) The spinel phase was more loaded in Al and exhibited a stoichiometry higher than 1:2 but with similar peaks (for example pdf 01 077 0732 of Zn 0.3 Al 2.4 O 4 appendix 13),

Table 5 2

 5 : Characteristics of Zn 1 Al 2 O PAA2 z

( : not determined, A: Amorphous)

  1). The TGA of calcined solids (Zn 2 Al 1 O PAA2 400 and Zn 2 Al 1 O PAA2 600 ) showed a slightly higher weight loss compared to Zn 1 Al 2 O PAA2 400 and Zn 1 Al 2 O PAA2 600 . These suggested that the combustion of PAA2 did not occur exactly in the same way with high aluminium content.

	Temperature	S.S.A. /	Crystallite size	Metal loading	Zn:Al	TG / weight loss
	(°C)	BET		/ XRD (nm)	(weight %)	(molar	[200 600 °C]
		(m 2 .g 1 )	ZnO ZnAl 2 O 4	Zn	Al	ratio)	(%)
	As prepared	223	A	A			30
	400	333			25.2	26.5	1:2.4	8
	600	137		13

Table 5 3

 5 : EDX measurements of Zn 1 Al 2 PAA2 400

	Elements	Zn	Al	O
	Area	(molar %)	(molar %)	(molar %)
	1	19.3	25.3	55.4
	2	5.9	31.4	62.7
	10	8.9	35.8	55.2
	11	21.4	23.0	55.6
	12	15.8	22.4	61.8
	13	7.2	31.1	58.7

  1) using the old AlMe 3 bottle, except that the PAA2 solution was replaced by pure water. The physico chemical characteristics of Zn 2 Al 1 O z are summarized in Table 5 4. The ICP analysis of the material at 400 °C gave a formula of Zn 2.1 AlO 5.1 very close to the stoichiometry obtained for Zn:Al with PAA2 but once again not corresponding to the theoretical one of 1:2.

Table 5 4

 5 : Characteristics of Zn 2 Al 1 O z

( : not determined)

more heterogeneous system with less ZnAl 2 O 4 phase and low specific surface area. PAA2

	Calcination T°	S.S.A. /	Crystallite size /	ICP		Zn:Al	TG/
		BET	XRD (nm)	(wt%)	(molar	weight loss
	(°C)	(m 2 .g 1 )	ZnO	ZnAl 2 O 4	Zn	Al	ratio)	[200 600 °C] (%)
	As synthesized	81	22	11				6
	400	89	24	12	56.5	10.9	2.1:1	1
	600	70	25	12				1

Table 5 5

 5 . The ICP analysis of the material at 400 °C gave a formula of ZnAl 2.43 O 6.9 very close to the initial stoichiometry suggesting a total precipitation of both metal without PAA2. It is also similar to Zn 1 Al 2 O PAA2 400 (Table

Table 5 5

 5 : Characteristics of Zn 1 Al 2 O z ( : not determined).XRD patterns in Figure59 showed that the absence of PAA2 had again a significant effect on the crystallinity and nature of the final material. After synthesis, the solid was again biphasic ZnO + ZnAl 2 O 4 but in this case the proportion of ZnAl 2 O 4 raised to 47%, as estimated with Diffrac.Eva. The size of the ZnO crystallites calculated using the same peaks as previously gave an average size of 26 nm for ZnO. Contrary to the Zn 2 Al 1 O z system, this one evolved much more according to the temperature since at 600 °C, the crystallite sizes of ZnO increased to 35 nm, while remaining unchanged for ZnAl 2 O 4 at 6 nm. The proportion of ZnAl 2 O 4 increased up to 82%, which is similar to Zn 1 Al 2 PAA2 600. As it was suggested for Zn:Al (1:2) with PAA2 , the ZnO phase could overlap with Al 2 O 3 , some Al could be incorporated into ZnO, or ZnAl 2 O 4 could be more charged in Al.

	Calcination T°	S.S.A.	Crystallite size /	ICP		Zn:Al	TG/
	(°C)	/ BET	XRD (nm)		(weight %)	(molar	weight loss [200
		(m 2 .g 1 )	ZnO ZnAl 2 O 4	Zn	Al	ratio)	600 °C] (%)
	As synthesized	274	26					12
	400	260	22	6	27.3	27.1	1:2.4	5
	600	169	35	6				2

  2 Al 1 O PAA2, Zn 2 Al 1 O , Zn 1 Al 2 O PAA2 , Zn 1 Al 2 O PAA 400 and Zn 1 Al 2 O synthesized in the first section as supports. As the highest glycerol conversion was obtained with Cu/ZnO PAA2 at 20 wt% of copper and [Cu 2+ ] = 20.8 mM, the first tests were performed with the same copper content on the new supports. The DPU and catalytic reaction conditions were the same as in the chapter IV (200 °C, 30 bar H 2 and 4.2% by weight of glycerol for 24 hours). Briefly, DPU process summarized in The properties of the catalyst obtained using Zn 2 Al 1 O PAA2 and Zn 2 Al 1 O as supports are summarized in Table 5 6.

the scheme 5 1 consists on adding copper nitrate solution to a suspension of ZnAl as prepared supports (with and without PAA2) in the presence of urea. The heating step (95 °C/3 hrs) was mandatory for the decomposition of urea, the filtration step helped to eliminate the non precipitated ions and the heating treatments (calcination/400 °C and reduction/350 °C) led to the elaboration of the catalyst. As a reminder, after calcination, the catalyst lost all the polymer. Scheme 5 1: Overview of DPU method on different Zn x Al y O z supports

Table 5 6

 5 : Properties of Cu/Zn 2 Al 1 O PAA2 and Cu/Zn 2 Al 1 O and their associated catalytic results after 24h

B.T. : before test and A.T. : after test

  2 Al 1 O PAA2 was found to be much more active than Cu/Zn 2 Al 1 O. Conversion increased from 44% to 92% when PAA2

		ZnO	Cu	C.S. A. T. (nm) ZnAl 2 O 4 Cu	S.S.A. (m 2 .g 1 ) B.T. A.T.	Conver sion/ 24h (%)	Selectivity (%) 1,2 PDO EG
	Cu/Zn 2 Al 1 O PAA2	9	11	6	17	111 115	92	78	22
	Cu/Zn 2 Al 1 O	30	17	6	13	90	219	44	81	19

was added to the support, with no effect on selectivity, both catalysts having an average selectivity of 80% to 1,2 PDO and 20% to ethylene glycol (EG) (Table

5 6

). This difference in Catalysts C.S. B. T.

(nm)

Table 5 7

 5 : Metal contents for Zn 2 Al 1 supports and the resulting copper based catalysts (*: calculated for the support after calcination at 400 °C, : not determined)

  has shown that ZnO solubility is low at pH between 9 and 12. Below pH 9 the solubility of ZnO increases, so urea could maintain a pH close to the area of low zinc solubility.

		Support metal			Catalysts
	Support	content* / ICP (wt%)	Formula Support	metal content / ICP (%wt)	Formula Catalyst
		Zn	Al		Cu	Zn	Al
	Zn 2 Al 1 O PAA2	55.8	11.8	Zn 1.9 AlO 4.7 26.1	37.2	11.5	Cu x Zn 1.4 AlO 3.7
	Zn 2 Al 1 O	56.5	10.9	Zn 2.1 AlO 5.1	24.4	34.7	9.5	Cu x Zn 1.5 AlO 5.6
	Reference tests	Solution Zn content / ICP (wt%)		Catalysts metal content / ICP (%wt) Cu Zn Al
	DP with urea	12.4			22.9 39.3 12.0 Cu x Zn 1.4 AlO 3.7
	DP without urea	20.6			13.2

Table 5 8

 5 : Properties of Cu/Zn 1 Al 2 O PAA2, Cu/Zn 1 Al 2 O PAA2 400 , and Cu/Zn 1 Al 2 O and their associated catalytic

	results after 24h
	Catalyst	ZnAl 2 O 4 size (nm) B.T. A.T.	Cu 0 size (nm) B.T. A.T.	S.S.A. (m 2 .g 1 ) B.T. A.T.	Conver sion /24h (%)	Selectivity (%) 1,2 PDO EG
	Cu/Zn 1 Al 2 O PAA2	7	8	9	26	167 110	51	92	7
	Cu/Zn 1 Al 2 O PAA2 400		14	8	19	185	239	64	88	12
	Cu/Zn 1 Al 2 O	7	7	24	24	228	239	83	86	14
	(B.T. : before test and A.T. : after test)

results showed a significant difference in copper size when using the substrate with PAA2, since an average size of 8, 9 and 24 nm was found for Cu/Zn 1 Al 2 O PAA2 400, Cu/Zn 1 Al 2 O PAA2 and Cu/Zn 1 Al 2 O, respectively. This was true even if PAA2 was removed at 400 °C before Cu deposition, suggesting a beneficial structural effect of PAA2 during support synthesis for copper dispersion.

  13) were similar but slightly different from the initial supports without copper and calcined at 400 °C, the difference being the total disappearance of the peaks corresponding to ZnO. The average size of ZnAl 2 O 4 was estimated at about 7 nm for Cu/Zn 1 Al 2 O PAA2 and Cu/Zn 1 Al 2 O, calculated with the peak at 65°, and the peaks at 31, 37 and 65°, respectively. For Cu/Zn 1 Al 2 O PAA2 400 , the peaks overlapped too much to determine a relevant average size of ZnAl 2 O 4 . The copper nanoparticles were well crystallized for all three samples and their size was calculated using the three peaks at 43, 51 and 74.5°. The

Table 5 9

 5 : Metal loading for supports and the resulting copper based catalysts (*: calculated for the support after calcination at 400 °C)

  15) showed similar morphologies and the increase in copper crystallite size measured by XRD could not be observed on these ZnAl 2.4 O 6.9 22.7 17.4 22.4 Cu x ZnAl 3.1 O 8.8

		Support metal	Formula	Catalyst metal	Formula
		content*/	Support		content/		Catalyst
		(ICP wt%)			(ICP wt%)
		Zn	Al		Cu	Zn	Al
	Zn 1 Al 2 O PAA2	25.5	25.9	ZnAl 2.5 O 7.8 16.5	22.4	22.2	Cu x ZnAl 2.4 O 7.1
	Zn 1 Al 2 O PAA2 400	21.2	22.6	ZnAl 2.6 O 10.8	24.4	10.6	24.1	Cu x ZnAl 5.5 O 15.9
	Zn 1 Al 2 O	27.3	27.1

  1 . The ICP analysis of the catalyst after reduction, gave metal content of: Cu= 31.1 wt%, Al = 10.1 wt%, and Zn= 34.6 wt% resulting in a formula Cu x Zn 1.4 Al 1 O 4.1 . As for the ZnAl oxides, the catalyst has a lower content in Al compared to the theoretical one due to the oxidized bottle of AlMe 3 .

Table 5 10

 5 : Properties of "one pot one step" Cu Zn x Al y O z

(A: amorphous, : not determined).

  16) fitted well with the BET data showing an amorphous phase of ZnO after synthesis reminding the ZnO PAA2 obtained in the chapter III resulting in such low specific surface area. As expected, the calcination step did not allow the crystallization of Al species but it resulted in well crystallized phase of monoclinic CuO (PDF number 00 045

	Catalyst	S.S.A.	ZnO size	Cu° size
		BET (m 2 .g 1 )	(nm)	(nm)
	After synthesis	28	A
	After calcination	54	17	CuO
	After reduction	25	17	13
	After reaction	114		16

Table 5 11

 5 : Properties of "one pot two steps" route for Cu Zn x Al y O z The ICP analysis of the catalyst after reduction, gave metal content of: Cu= 25.1 wt%, Al = 16.5 wt%, and Zn = 28.8 wt% resulting in a formula Cu x Zn 1 Al 1.4 O 4.2 . As expected from the old bottle, the stoichiometry did not reflect the expected one.

Table 5 12

 5 : EDX measurements of Cu x Zn 1 Al 1.4 O 4.2 one pot two steps before catalysis.

Table 5 13

 5 : Catalytic results obtained with Cu Zn x Al y O z catalysts prepared by "one pot" routes

  31.1 wt% of amount of copper percentage and labelled as CZA XUHP where: CZA: Cu ZnO ZnAl 2 O 4 , X is the theoretical weight percentage of copper, U means the use of urea, H heated at 60 °C for 3 hours and P means the use of PAA2.So for example, the catalyst labelled CZA 5UHP was a catalyst loaded with 5 wt% of copper, made with urea and PAA2 and heated at 60 °C while a catalyst labelled CZA 20H was a catalyst loaded with 20 wt% of copper elaborated without urea and without PAA2 but heated at 60 °C for 3 hours.

	classified by increasing
	Elements	Zn	Al		Cu
	Area	(molar %)	(molar %)	(molar %)
	1	20.7	34.5		54.9
	5	5.8	32		62.2
	6	0	4.2		97.0
	11	19.5	41.8		38.7
	Catalyst	Run	Conversion 24h (%)	Selectivity (%) 1,2 PDO EG
	One step	1 st run	83	78	21
	Two steps	1 st run	96	83	17
	Two steps	2 nd run	93	83	17
	Two steps	3 rd run	90	84	16

Table 5 15

 5 : Effect of synthesis parameters on CZA catalysts prepared by "one pot two steps" route By analyzing the data in Table515 and for the same Zn/Al ratio of 2:1, several conclusions can be drawn:

High specific surfaces of the catalysts were obtained by either using urea (above 76 m².g 1 ) or a low copper content (above 140 m².g 1 ). The metallic copper contents measured by ICP were generally very close to the theoretical target percentages (5, 10 and 20 wt%) suggesting that complete copper deposition could be done even without urea but only when heating at 60 CZA catalysts

		Cu content / ICP (wt%)	S.S.A. / BET (m 2 .g 1 )	Cu 0 size (nm)	Formula	Conversion /24 (%)	Selectivity (%) 1,2 PDO EG
	5UHP	4.35	143	13	Zn 2.2 Al 1 O 5.0	34	97	2.6
	5HP	5.21	123	15	Zn 2.0 Al 1 O 4.7	50	91	8.5
	10UHP	9.7	154	12	Zn 2.1 Al 1 O 4.5	44	85	16
	10HP	12.31	141	13	Zn 1.9 Al 1 O 4.1	66	89	12
	20UHP	19.2	76	17	Zn 2.1 Al 1 O 4.9	66	83	16
	20HP	19.8	70	14	Zn 1.9 Al 1 O 4.3	98	88	11
	20UP	14.3	79	13	Zn 1.4 AlO 4.9	14	99	0.5
	20P	15.6	70	11	Zn 1.2 AlO 4.2	79	90	10
	20H	19.2	64	30	Zn 2.1 Al 1 O 4.9	46	91	9
	20HP	15.2	135	15	ZnAl 1.1 O 3.3	83	83	17
	20UHP	17.8	185	16	ZnAl 3.7 O 11.5

Table 5 16

 5 : CZA 20UHP and CZA 20HP (Zn:Al ; 2:1) summarized properties before and after test.

  1 , the highest conversion of 86% with selectivity toward 1,2 PDO (92%) and 2% for EG was obtained with Cu/ZnO/ZnAl 2 O 4 catalyst (with a molar ratio of Cu:Zn:Al equal to 1:3:6 resulting in Zn:Al 1:2). The conditions of the catalytic test were, 80 wt% of glycerol in 20 mL of water, under 40 bar H 2 at 180 °C and with a catalyst weight: glycerol/Cu ratio of 200 after 10 hours of reaction.

Scheme 4 3: Synthesis of Cu/ZnO catalysts by DPU of copper on hybrid non calcined ZnO PAA2

Acknowledgments

This thesis was carried out at the Institute of Research on Catalysis and the Environment of Lyon (IRCELYON, France) within the CDFA (Chimie

In the following section, the two catalysts studied are at the same percentage by weight of copper (10 wt%) but synthesized with different Cu 2+ concentrations (20.8 and 81.6 mM). They are labeled by the weight percentage and the first number of their concentrations (10%Cu/ZnO com 2 designates the catalyst at 10 wt% of copper synthesized with [Cu 2+ ] = 20.8 mM). The characteristics and the catalytic results obtained with these two catalysts are summarized in Table 4 5. XRD diffractograms shown in Figure 4 7 for catalysts before catalysis showed fairly similar crystallite sizes for ZnO and Cu 0 . 10%Cu/ZnOcom 8 has an average size of ZnO crystallites of 55 nm which was slightly larger than with 10%Cu/ZnOcom 2 (46 nm) but Cu 0 was smaller with an average size of 11 nm with 10%Cu/ZnOcom 8 instead of 15 nm with 10%Cu/ZnOcom 2 .

The two catalysts did not have the same behavior for the hydrogenolysis of glycerol. 10%Cu/ZnOcom 2 showed a higher conversion (25%) versus 16% for 10%Cu/ZnOcom 8 . In order to understand the difference in catalyst activity, since no significant difference could be observed for freshly prepared catalysts, the same characterizations were made for catalysts after use (mentioned as after test). The catalyst characterized after test was simply recovered by filtration and dried in an oven at 80 °C under air overnight.

The XRD diffractograms after catalysis presented in Figure 4 The difference between our catalyst and the other Cu/ZnO catalysts reported could come from our original support ZnO PAA5 which made it highly selective even in the presence of water. Figure 4 16 shows the XRD diffractograms before and after catalysis using Cu/ZnO with the highest activity (20Cu/ZnO PAA2 RT 2 ). The copper particles were not sintered after the reaction but once again, the phase (Cu,Zn) 5 (CO 3 ) 2 (OH) 6 was present at 62%, based on a semi quantitative analysis. Table 4 14 shows the catalyst properties after different steps. The specific surface area after synthesis (89 m 2 .g 1 ) was much higher than the one elaborated with the one step method (28 m 2 .g 1 ) reminding the surface area of ZnO PAA2 synthesized at 0 °C. Nevertheless, after reduction the specific surface area were similar (ca. 23 m 2 .g 1 ). However, the conversion was higher with 61 % and selectivity of 89 % for 1,2 PDO and 11 % for EG (Table 4 15). with the later catalyst (92%) was higher than the one obtained by the one pot process (83%).

The main difference between the two catalysts was the copper weight percentage which was higher in the case of the catalyst prepared by the "one pot" method. The higher copper percentage could be responsible for the decrease in conversion for the one pot catalyst due to the surface saturation phenomenon as seen in chapter IV with different catalysts.

The "two steps" catalyst showed higher conversion (96%) compared to the "one step" route.

If we compared the properties of the two catalysts, the "two steps" catalyst had a higher amount of Al with a molar ratio Zn:Al (1.1.4), lower amount of Cu (25% vs. 31%), higher surface area (97 m 2 .g 1 after reduction) and similar Cu 0 crystallite size (13 nm). The catalysts obtained by DPU could not be compared to the "two steps" catalyst because none of them had the same Zn:Al molar ratio. But in term of behaviour, the higher Al content in the "one pot" method seemed to be more favourable compared to the DPU where the conversion decreased at a higher content of Al (Table 5 8). Moreover, it seemed that having a Cu loading above 30% was detrimental for the activity.

A three run recyclability test was performed with the catalyst developed using the "two steps" route. The catalyst properties and the conversion shown in Table 5 13 proved that the recyclability test worked very well. There was no copper leaching during the reaction and the conversion and selectivity were not influenced by recyclability. For the sake of clarity, before each test, the catalyst was regenerated by calcination and reduced because, as with Cu ZnO, when no regeneration was performed, the conversion decreased from 96% to 43%.

XRD showed a more robust system for the "two steps" catalyst (Figure 5 16) than the "one step" catalyst (Figure 5 15). The specific surface areas remained similar, however there was a slight increase in Cu crystallite size. As mentioned previously, synthesis parameters have a significant impact on catalyst properties leading to a change in glycerol conversion. In this section, we have studied different synthesis parameters such as the molar ratio Zn:Al, the presence of urea, the Cu wt%, the temperature and the presence of PAA2.

As seen in the upper part, the catalyst prepared by the "one pot -two steps" route showed a higher conversion than the one prepared by "one pot -one step". For this reason, the synthesis of following batches were done only through the "two steps" route. and 500 mg of catalyst in 100 mL of water.

In a second time, all the ZnO-PAAX systems elaborated in the third chapter were tested under the same catalytic conditions. Results have shown that the conversion increased when the commercial ZnO support was substituted by the ZnO-PAAX one. In addition, it was possible to load the catalyst with higher copper content (37.9 wt%) since the specific surface area of the ZnO-PAAX was higher (135 m 2 .g -1 ), without decreasing conversion neither sintering the copper particles.

Nevertheless, most of the catalysts (with the two types of supports), at high conversion (> 20%) have shown structural modifications of the catalysts. In the case of the commercial ZnO, the catalysts showed sintering of copper particles and a phase change of the ZnO toward a carbonaceous phase or hydrozincite. In the case of the ZnO-PAAX supports, the catalysts were mostly having the phase change but the sintering problem was not as important as with the commercial ZnO. The highest conversions obtained for the three systems were 25, 70 and 70%

for commercial ZnO, ZnO-PAA2 and ZnO-PAA5, respectively, with similar selectivities around 90% toward 1,2-PDO. ZnO-PAA100 resulted in a total deactivation of the catalyst with less than 1% conversion.

The calcination of ZnO-PAA2 support before adding copper resulted in average to a lower conversion compared to the catalysts elaborated with the hybrid and uncalcined support (highest conversion obtained with the calcined support was 55% vs 70% for the uncalcined one). For ZnO-PAA5, the conversion decreased drastically from 70% to 27% with the calcined support. This results proved that the precipitation of copper onto the support, in presence of the PAAH polymer, allowed a better interaction of copper with the ZnO support resulting in highly dispersed material. Nevertheless, all the catalysts were highly selective toward 1,2-PDO with an average of 85-99% depending on the conversion and the major by-product was ethylene glycol (EG) with selectivities between 2-14%.

Even if the catalysts elaborated with the ZnO-PAAX supports seemed to be more active than the one elaborated with the commercial ZnO, catalysts showed some problems of reproducibility when up-scaling was done. The increase of the catalyst's weight led to an increase in the glycerol conversion to 60% with 90% selectivity toward 1,2-PDO with 1 g of catalyst after 24 hours. Recyclability tests have shown that the conversion and the selectivity of these catalysts were recovered when they were regenerated. Regarding the low cost of our systems, this result may be interesting for an application on large scale.

Due to the reproducibility problems, a "one pot method" was developed in order to have a better control on the properties of the catalyst. In term of conversion, this route did not show any improvement but it allowed the elaboration of a robust catalyst highly resistant towards the catalytic conditions compared to all the series of other catalysts (with commercial ZnO and hybrid ZnO) with no change of phase after catalysis.

Based on the literature, the activity of Cu/ZnO systems could be enhanced by adding a promoter such as aluminium. We studied its addition to the catalyst under different synthesis conditions. As for ZnO, we started by elaborating the ZnAlOx-PAA2 hybrid support (only PAA2 was used) oxide with two different molar ratio of Zn/Al (1:2 or 2:1) and deposited copper NPs by DPU method. We studied the effect of the polymer by elaborating ZnAl oxides with and without PAA2 at 0 °C and with 90 mL of ethanol washing at the end of the syntheses. In terms of materials, the as-synthesized ZnAl oxide with 1:2 molar ratio elaborated in presence of PAA2 showed the highest surface area of 222 m 2 .g -1 . Calcining the material at 400 °C resulted in an increase in the specific surface area up to 333 m 2 .g -1 followed by a decrease to 137 m 2 .g - 1 at 600 °C. An interesting point was that the Zn-Al systems elaborated with/without PAA2 had the same behavior. As already observed in the literature, this led then to an increase of the specific surface area of the resulting catalysts and also, depending of the Zn/Al ratio, to the appearance of the spinel phase of ZnAl2O4.Both seemed to play a major role in enhancing the properties of copper-based catalysts.

The DPU method was used to elaborate the catalysts by following the same conditions as before and we compared then different catalytic systems. We demonstrated the remarkable properties of these supports, in term of high specific surface area, small particles (3-12 nm for ZnAl2O4, 8-35 nm for ZnO) and good thermal behavior. The conversion of the glycerol increased to 96% with the catalyst elaborated with Zn:Al molar ratio 1:2 with a selectivity toward 1,2-PDO of 83%. Nevertheless, a leaching of zinc occurred during the DPU process resulting in a lower content of Zn in the catalyst compared to the one in the support. The change in the phase after catalysis was also seen for these new Cu/Zn-Al catalysts.

Therefore, a "one pot" method was applied to elaborate Cu/Zn-Al catalysts. Its optimization was made by study any synthetic parameters such as the presence of urea, of PAA2 and the heating step. The results showed that PAA2 is mandatory to obtain highly dispersed systems with interesting properties by a better control of the pH of the solution. While, in the opposite, the presence of urea in this synthesis method did not seem to be beneficial and resulted in a decrease in the conversion. The heating step seemed also to be important and the most active catalyst was the one prepared with 20 wt% of copper, without urea, in the presence of PAA2 with a molar ratio Zn:Al of (2:1). It resulted in an almost total conversion (98%) of glycerol with 88% selectivity towards 1,2-PDO.

To enclose this part, a three run recyclability test was done with the catalyst elaborated with Zn:Al showing a molar ratio 1:1 (obtained by DPU) and it resulted in the conservation of the conversion (90-93%) and the selectivity (83-84%). After each regeneration the catalyst seemed to recover the properties of the freshly reduced catalyst.

Perspectives

The study opens up perspectives towards different subjects. In term of ZnO and ZnAl2O4, the interesting properties of these materials can be applied as we saw in the literature in varied domain of applications as in nanotechnological applications, optoelectronics, photocatalysis…

Since our previous study on ZnO was done in order to enhance the luminescent properties of the material, the new hybrid system elaborated at low temperatures resulting in homogeneous dispersed materials may be more interesting for such application.

In the case of ZnAl2O4, the spinel phase is known to have a high mechanical resistance and high chemical and thermal stability making this material a nice candidate for applications in optical coatings, host matrix or in ceramics syntheses. Since we were interesting in its catalytic behavior, no mechanical characterizations were done. Nevertheless, the thermal stability was proved by TGA and the resistance of the material toward the catalytic conditions allow to consider it as promising system for these applications.

Moving to the catalysts, the Cu/ZnO is highly studied for the hydrogenation of CO2 into methanol. In a first step, and with the collaboration of some researchers in our institute