I would like to gratefully thank the funding support from the European Union's Horizon 2020 research program under grant agreement N. 720783-MULTI2HYCAT. This program includes six different partners across Europe, including Universita del Piemonte Orientale (UPO, IT), CSC (ES), University of Southampton (UK), CNRS (FR), Solvay (FR, BE), Almirall (ES) and CAGE Chemicals (IT).

First and foremost, I would like to express my best appreciation to my supervisor, Prof. Marc PERA-TITUS, for his continuous and patient support during my PhD study, who gave me the golden opportunity to work on this project. I would like to thank him for his creative and comprehensive supervision to my scientific research and wise mentoring, which made me feel warm and confident along my thesis. This is a big treasure for me that will stay with me all my life.

I would also like to thank all the colleagues in the E2P2L lab (Eco-Efficient Product Process Laboratory, UMI 3464 CNRS-Solvay, Shanghai, China), during the past three years, for the excellent facilities and environment, as well as insightful industrial expertise that helped for developing my activities. In particular, I would like to thank Dr Stéphane STREIFF, director of the E2P2L laboratory, for his welcome and continuous support, and Dr Fan JIANG for her support and advice. My sincere thanks also go to all other colleagues in the team for their suggestions and kind help along my research work.

Next, I want to express my gratitude to all the partners of the MULTI2HYCAT project, who provided me great support during my thesis. In particular, I would like to thank Dr Ivana MILETTO and Prof Enrica GIANOTTI (UPO) for their support and advice on catalyst characterization, as well as Dr Marta MEAZZA, Prof Ramon RIOS and Prof Robert RAJA for providing benzimidazole-silica catalysts. I also appreciate the warm support of Dr Zhengkun JIANG and Loic BAUSSARON for their support in the modeling studies. I am sincerely thankful to the jury members: Prof Anne PONCHEL, Prof Robert RAJA, Prof Anne Giroir-Fendler, Dr Belen ALBELA and Dr Stéphane STREIFF for evaluating my PhD manuscript and participating in my jury.

My final thanks go to my family for your strong support and encouragement along my life.

Abstract

Biomass is a renewable and clean resource, and great interest is devoted to its conversion into fuels and fine chemicals. In particular, furfural emerges as a key platform molecule. In this view, it is an urgent need to develop eco-efficient and intensified processes affording the conversion of furfural into added-value chemicals in a minimum number of steps. In this PhD thesis, we engineered single-reactor tandem reactions, where furfural is transformed into target chemicals in a one-pot fashion without separation of reaction intermediates. To this aim, multifunctional catalysts, showing high activity, selectivity and reusability, are required. As a first example, we conceived a single-reactor tandem process for the aldol condensation/crotonization reaction between furfural and methyl isobutyl ketone (MIBK), followed by hydrogenation with molecular H2. Two robust bifunctional catalysts were developed and optimized based on Pd/HPSAPO-5 with hierarchical porosity and Brønsted acidity, and Pd/Al2O3, with Lewis acidity. The catalysts exhibited high yield to THF-derivatives and were reusable in multiple steps without decrease of catalytic activity. As a second example, we conceived a single-reactor tandem process for the benzoin condensation of furfural to furoin, followed by reductive amination of the as-generated furoin with NH3 and molecular H2. A high yield of 2-amino-1,2-di(furan-2-yl)ethan-1-ol could obtained over combined silica-supported benzimidazole and Ru/Al2O3. Keywords: Biomass, Furfural, Aldol condensation, Hydrogenation, Amination

Résumé

La biomasse est une ressource renouvelable et propre, et un grand intérêt est consacré à sa conversion en carburants et en produits de chimie fine. Tout particulièrement, le furfural est l'une des molécules plate-forme les plus importantes. Dans ce contexte, il s'avère nécessaire de développer des procédés éco-efficaces et intensifiés permettant la conversion du furfural en produits chimiques à haute valeur ajoutée dans un nombre réduit d'étapes. Dans cette thèse, nous avons développé des procédés à étapes multiples dans un seul réacteur, où le furfural est converti en plusieurs molécules de spécialité en un seul pot sans séparation des intermédiaires de réaction. Dans ce but, des catalyseurs multifonctionnels, réutilisables, sont nécessaires. Dans une première partie, nous avons conçu un procédé en seul pot pour une réaction de condensation aldolique/crotonisation entre le furfural et la méthylisobutylcétone (MIBK), suivie d'une hydrogénation avec l'H2 moléculaire. Deux catalyseurs bifonctionnels robustes ont été développés et optimisés : un catalyseur Pd/HPSAPO-5 à porosité hiérarchique et acidité de Brønsted, et un catalyseur Pd/Al2O3 avec acidité de Lewis. Ces catalyseurs offrent des hauts rendements vers des dérivés tetrahydrofuraniques, and sont réutilisables sur plusieurs cycles catalytiques sans réduction apparente d'activité. Dans une deuxième partie, nous avons conçu un procédé en seul pot pour une réaction de condensation benzoïne du furfural vers la furoïne, suivie par l'amination réductrice de la furoïne avec l'ammoniac et l'H2 moléculaire. Le produit 2-amino-1,2-di(furan-2-yl)ethan-1-ol a été obtenu avec haut rendement utilisant un mélange de catalyseurs benzimidazole-silice et Ru/Al2O3.

Mots clés:

Biomasse, Furfural, Condensation aldolique, Hydrogénation, Amination

Overview of biomass and biomass energy

In recent years, the gradual reduction of reserves of non-renewable resources such as oil, coal and natural gas, combined with the warming change in the climate, has boosted the development of green strategies for economic growth and environmental protection. Current efforts by governments, industry and academia focus on the utilization of renewable energy sources, including solar, wind, biomass, hydrogen, geothermal, tidal and 'nuclear' energies. Biomass has attracted increasing attention because it is environmentally friendly, has low cost, and is carbon neutral. [1][2][3][4][5][6][7] It is estimated that the yearly amount of biomass produced by photosynthesis alone on Earth amounts to 144-180 billion tons (dry weight), and its energy is approximately 3 to 8 times the world energy consumption in the early 1990s. 8 However, to date, biomass has been largely employed as firewood, which is inefficient and affects the environment. Other biomass energy sources are mainly derived from starch-based ethanol and biodiesel. For these processes, there is still lack of suitable cheap raw materials, and it is difficult to avoid the status of "competing with the people for food". 9 Therefore, biomass energy alternatives needs to be further explored and utilized efficiently, which special focus on the synthesis of added-value chemicals and fuels.

Platform chemicals from lignocellulose 1.2.1 Cellulose, hemicellulose and lignin

At present, biomass raw materials (Figure 1.1) that can be used as raw materials to produce fuels and chemicals belong mainly to three types: lignocellulose, lipids and proteins. 10 Major streams of biomass are lignocellulose and lipids. Lignocellulose, which is composed of cellulose, hemicellulose and lignin, constitutes the most abundant source of biomass. Lipids contain triglycerides issued from animal fats, plants, microalgae and turpentine streams. Finally, there is a smaller group of protein-based fractions of animal or vegetable origin such as glutamic acid and phenylalanine. 11 The structure of these molecules determines the possible transformation routes into fuels, chemicals and materials. 12 Lignocellulose is the biomass source with the largest reserves and fastest growth. It constitutes the most important biomass source for preparing fuels and chemicals. [13][14][15][16][17][18][19][20][21][22] Cellulose is one of the main components of plant cell wall, and is built by D-glucose molecules linked through β-1,4glycosidic bonds to form a macromolecular linear polymer. The molecular weight can reach 50,000~2,500,000 or 300~15,000 glucose groups. Therefore, cellulose is difficult to dissolve in water and organic solvents. The cellulose content in lignocellulosic biomass is about 40-80%, while the content in cotton is close to 100%, which is the purest natural source of cellulose. Cellulose is considered as the most abundant biopolymer containing linear β (1-4) glucose chains, which is an important raw material for papermaking and textiles. [23][24] Hemicellulose is a type of heteropolysaccharide that coexists with cellulose in the cell walls of most plants. It is composed of five-carbon sugars (such as xylose, arabinose, etc.) and six-carbon sugars (such as glucose, galactose, mannose, etc.) linked through β-1,4 or β-1,6 groups, as well as other polymers linked by different glycosidic bonds. Hemicellulose has a random, noncrystalline structure and low strength. The most abundant hemicellulose type is xylan (Figure 1.2). The structure of xylan is a kind of polypentasaccharide, which is connected by β-1,4xylosidic bonds and carries a variety of substituents. Partial degradation of xylan can form xylooligosaccharides, and complete degradation can yield five-carbon monosaccharides: xylose, ferulose, arabinose, etc., of which xylose is the main one. [25][26][27] Among the three components of lignocellulose, lignin is the only high-molecular polymer with a 3D network containing aromatic rings (Figure 1.3). The network is cross-linked with three basic phenyl propane monomer units known as monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 1.4), making it an alternative raw material for the petrochemical industry for preparing aromatic compounds. 28 In plant cell walls, lignin fills the space between cellulose and hemicellulose to link carbohydrate polymers together and maintain the stability and rigidity of the entire system structure.

Biomass refining process

In the 1990s, scientists popularized the concept "biorefinery". The U.S. Renewable Energy Laboratory defines a biorefinery as a collective term that integrates biomass conversion processes and equipment for the production of biomass fuels, energy and chemicals. [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] By producing multiple products, a biorefinery can take advantages of the availability of different biomass streams and process intermediates and maximize the value derived from the feedstocks.

In biomass biorefineries, pretreatment methods include hydrothermal treatment, gas explosion, pyrolysis and so on. Normally, the effective technology for biomass pretreatment should meet the following requirement: a) break the complex structure of lignocellulose; b) reduce the crystallinity of cellulose; c) preserve monomer from lignocellulose; d) minimize the formation of degradation products, which will inhibit subsequent conversion, e) use as little energy as possible and produce as many products as possible; and f) to generate the minimum amount of wastewater. [START_REF] Mika | [END_REF][31] Since the composition of the feed stream largely depends on the pretreatment methods and the characteristics of the feedstock (such as the basic structure and the type of lignin), each biorefinery should be specifically adapted to the biomass in the local environment. Gallezot proposed three potential processes (Figure 1.5): 1) direct gasification or pyrolysis of lignin to syngas, and further processing to produce raw materials; 2) functional monomers from lignin are separated to prepare simple aromatic compounds (including phenol, xylene, toluene, etc.), and then the existing methods in oil industry are used to prepare bulk chemicals; and 3) use of specific functional groups and high-efficiency catalysts to prepare high-added value chemicals (such as vanillin) in one step. Because none of the three processes produces unique products, downstream product separation and purification is very important, which is also a major factor restricting the development of biorefineries. For processes 2 and 3, catalysts with high activity and high selectivity are required, which constitutes a major technical barrier. [START_REF] Holladay | In Top Value-Added Chemicals from Biomass -Volume Ii Results of Screening for Potential Candidates from Biorefinery Lignin[END_REF][START_REF] De Jong | In Industrial Biorefineries & White Biotechnology[END_REF][START_REF] Chew | [END_REF][35][36][37] Biorefineries can access multiple chemicals by fractioning an initial biomass raw material into multiple intermediates (carbohydrates, proteins, triglycerides) that can be further converted into valueadded products. The use of biomass as feedstock can provide benefits by reducing the impact on environment, such as lower emission of pollutants and hazardous products. [38][39][40][41] Figure 1.5 Lignocellulosic biorefinery scheme with particular emphasis on the lignin stream.

Furfural

Furfural is a very important platform molecular (Table 1.1) issued from biomass. Furfural is a colorless liquid with an aldehyde group in the 2 position of furan. It can only be converted from lignocellulosic biomass and cannot be produced from oil. Furfural was first isolated in 1821 (published in 1832) by the German chemist Johann Wolfgang Döbereiner, who produced a small sample as byproduct of formic acid synthesis. [42][43] In 1845, George Fownes named this oil as "furfurol" from furfur (bran), and oleum (oil). 44 In 1901, Carl Harries a German chemist resolved the furan structure by synthesizing it from succindialdehyde, thereby also confirming the furfural structure. [45][46] Today, furfural, with an annual output of about 200,000 tons, is still produced from agricultural by-products such as sugarcane bagasse and corn cobs. The main countries producing furfural are the Dominican Republic, South Africa and China. 47 Table 1.1 Platform molecules, revisited 48

Ethanol Xylitol

Glycerol Furfural

Lactic acid HMF

3-Hydroxypropanoic acid 2,5-furandicarboxylic acid

Levulinic acid Succinic acid

Sorbitol

Bio-hydrocarbons

Furfural is produced by dehydration of xylose, which is in turn issued from hemicellulose, as shown in Scheme 1.1. Furfural is a key intermediate for the full utilization of biomass raw materials (bagasse, corn cob, straw, hardwood) that are rich in hemicellulose. Furfural is commercially produced using aqueous solutions of mineral acids (e.g., HCl, HNO3, H2SO4) to catalyze the dehydration of xylose. For instance, the typical process commercialized by Quaker Oats using diluted H2SO4 as catalyst affords about 50% furfural together with solid humins. The use of homogeneous inorganic acids as catalysts faces many limitations relying mostly on poor recycling and reactor corrosion. In this view, current research focus on the use of solid acid catalysts. Solid acid catalysts have the following advantages: 1) they are easy to separate and recycle from the reaction system; 2) they can be used without reactor corrosion on the basis of preparation; and 3) solid acids can be prepared with tunable acidity to improve the product selectivity.

Scheme 1.1 Acid-catalyzed dehydration of xylan to furfural.

Catalytic conversion of furfural and its derivatives

Chemically, furfural (C5H4O2, furan-2-carbaldehyde, 2-furaldehyde) is a heteroaromatic furan ring with an aldehyde functional group. Both functionalities (aldehyde group and aromatic ring) are responsible for its chemical reactivity. Furfural can undergo the typical reactions of aldehydes, such as acetalization, [49][50] acylation, 51 aldol and Knoevenagel condensations, [52][53] reduction to alcohols, 54 reductive amination to amines, [55][56] decarbonylation, 57 oxidation to maleic acid / anhydride, [58][59] and Grignard reactions. 60 Besides, the aromatic furan ring can be subjected to alkylation, hydrogenation, oxidation, halogenation, and nitration reactions. [61][62][63][64][65][66] Figure 1.6 shows the broad spectrum of furfural-based chemistry. Currently, the most relevant use of furfural as chemical feedstock is in the production of furfuryl alcohol and 5-membered ring heterocycles (e.g., furan, methylfuran, furfurylamine, furoic acid). However, a large number of transformations of furfural are possible, and indeed, the use of renewable furfural to produce fine and commodity chemicals and fuels is still today an underexplored area. [67][68][69][70][71][72][73] Figure 1.6 Simplified reductive and oxidative catalytic transformations of furfural to added value chemicals.

Long chain conversion (C-C coupling reactions)

Furfural can be subjected to C-C coupling reactions to increase the molecular size and diversity of furfural. Among the different possible strategies, we list below aldol condensation/ crotonization with ketones and furoin condensation reactions, which have been used in our research to build derivatives from furfural. We address below the key aspects of these reactions and the corresponding catalytic formulations.

Aldol condensation reactions of furfural

Aldol condensation/crotonization reactions constitute an important class of C-C reactions that are widely applied to organic synthesis. The aldol condensation of furfural with ketones is commonly employed in the food industry, and can be used to prepare longer-chain value-added liquid fuels starting from a variety of ketones. In this reaction, and enol or enolate intermediate reacts with a carbonyl compound to form either β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration (crotonization) to give the corresponding α,β-unsatureatd carbonyl moiety (Scheme 1.2). Scheme 1.2 Reaction mechanism for aldol condensation.

Commercial aldol condensation reactions can be catalyzed by homogeneous bases such as NaOH, KOH and Ca(OH)2. Dumesic and coworkers studied furfural with various ketones in a biphasic system with NaOH as catalyst, and found the final distribution and yield of products can be controlled by adjusting the ketone-to-furfural and NaOH-to-furfural molar ratios. 74 Also, one-pot processes have been designed starting from carbohydrates. 75 Homogeneous bases are highly active for aldol condensation/crotonization reactions, but hardly recyclable. Besides, they can promote corrosion, self-condensation reactions, and the generation of salts after neutralization.

To overcome these shortcomings, heterogeneous basic and acid catalysts have been developed, which are listed and described in the following lines, and compiled in Table 1.2. A variety of ketones have been employed, including acetone, cyclopentanone and levulinic acid.

Heterogeneous basic catalysts

Recent work has targeted the design of heterogeneous basic catalysts based on mixed oxides enriched with sites of medium strength (e.g., Mg-Zr, Mg-La), [75][76][77][78][79][80][81][82][83][84] hydrotalcites and layered double hydroxides (Co-Al spinels, Mg-Al), [85][86][87][88] KF/aluminas, [89][90] titania, 91 ZnO (in the presence of water), and alkaline/N-doped zeolites/zeotypes (Table 1.2). [92][93][94][95] Among the different catalysts, MgO-ZrO2 was found to be the most active one (Scheme 1.3), affording 96% furfural conversion, 67% yield of furfurylidene and 29% yield of difurfurylidene acetone. 76 Cejka and coworkers found the presence of K2O species as strong basic sites in K-impregnated USY zeolites enhanced both the activity and the occurrence of the second condensation step, and about 44% conversion could obtained at 100 °C for 2 h with the K-IMP-Y(15). 96 Most of the catalysts show low activity under the evaluated conditions. To reach at least 90% furfural conversion often requires long reaction times, very high catalyst loadings (Fur/cat ratio < 3) or high temperature, except in the case of Co-Al spinel, Mg-Al hydrotalcite, and dolomites. 97 While these catalysts are active, they may suffer from deactivation by water due to the formation of hydroxides (e.g., Mg(OH)2 from MgO, Ca(OH)2 from CaO).

Heterogeneous acid catalysts

Besides basic catalysts, some examples have been reported on heterogeneous acid catalysts for the aldol condensation/crotonization of furfural with ketones, but operating usually at higher temperatures (Table 1.2). Most of the examples comprise catalysts enriched with Brønsted acid catalysts relying on zeolites with purely microporous 98,99 and hierarchical architectures. [100][101] In particular, Kukicka and coworkers evaluated the catalytic performance of NH4-ZSM-5 (23), HZSM-5 (23), NH4-ZSM-5(50), HZSM-5(50), NH4-MOR, HMOR (20), NH4-BEA (25), HBEA (25), NH4-BEA (38), HBEA (38), HSDUSY (80) and HSUSY( 5)) for the aldol condensation of furfural with acetone in batch reaction conditions at 20-100 °C for a reaction time between 0-24 h. 102 Among the different catalysts, HBEA (25) displayed the best performance with 38% furfural conversion at 100 °C. Apart from zeolites, metal oxides with acid properties have also been reported for the reaction. In particular, hydrated ZnO exhibited high activity and selectivity towards β-furfurylidenelevulinic acid in the aldol condensation of furfural with levulinic acid at 85 °C. 103 Besides these examples, few studies have been reported on Lewis-acid type catalysts for aldol condensation reactions, relying mainly on Nb2O5 or Nb2O5 encapsulated in MFI zeolites, [104][105] Sn/BEA, 106 and Hf-MOFs. 107 While heterogeneous acid catalysts are active for aldol condensation/crotonization reactions encompassing furfural, they can suffer from deactivation due to deposition of oligomers along the reaction, which can promote coke generation. In this view, the development of robust acid catalysts with high stability against coking in aldol condensation reactions is an unsolved challenge. Al2O3 is very a very common acidic solid material, usually used as a catalyst support.

There are very few reports in the literature that alumina is used in the condensation reaction of furfural and acetone, and its catalytic effect is very unsatisfactory.

Benzoin condensation of furfural

Benzoin condensation is a well-known C-C coupling reaction that has been used to promote the self-or homocondensation on benzaldehyde or the heterocondensation of benzaldehyde with other aldehydes. The reaction is catalyzed by nucleophiles such as cyanide or a N-heterocyclic carbene. [108][109][110] Furfural and 5-hydroxymethylfurfural (HMF) can also self-condense to generate 5,5'-dihydroxymethyl furoin (DHMF) using a N-heterocyclic carbene (NHC) organocatalyst. 111- 114 The reaction also works well for furfural and 5-methylfurfural. 113 Scheme 1.4 shows the proposed mechanism for NHC-catalyzed self-coupling of furfural into furoin, which operates via umpolung condensation as proposed by Breslow for benzoin condensation of benzaldehyde. 115- 117 In this mechanism, the carbene organocatalyst is first activated by deprotonation with a base, followed by the nucleophilic attack of one furfural molecule by the carbene, generating a zwitterionic adduct. Next, proton transfer occurs, leading to the formation of the Breslow/Enders intermediate that attacks a second molecule of furfural. After a proton transfer and the release of the carbene catalyst, the furoin product is obtained. As mentioned above, this mechanism is also termed umpolung condensation that converts an electrophilic carbonyl carbon to a nucleophilic center as an acyl anion equivalent, encompassing a polarity inversion. [118][119][120][121] Scheme 1.4 Proposed mechanism for NHC-catalyzed benzoin condensation of furfural into furoin.

The benzoin condensation of furfural has been attempted on a variety on NHC organocatalysts.

In particular, benzimidazolium (bim) salts with one or two long-chain aliphatic substituents at the nitrogen atoms in the imidazole ring have been reported as good candidates for the reaction. [122][123] Bim catalysts have been supported over silica and polymers, affording the engineering of active and recyclable heterogeneous catalysts. [124][125][126] Chen and coworkers combined features of homogeneous and heterogeneous catalysts by intercalating benzimidazolium cations [BI] + into the nanogalleries of Na + /montmorillonite (MMT) clay leading to generation of recyclable supported precatalysts [BI] + /MMT (Figure 1.7). 127 Upon treatment with a base, the hybrid material could catalyze furfural self-condensation into furoin in almost at a yield higher than 96% over the three consecutive runs.

Furil can be prepared from furoin in the presence of air or an oxidant.

Hydrogenation of furfural

In the utilization of biomass, hydrogenation reactions are often used to reduce unsaturations. Furfural and HMF can produce various hydrogenated products as a function of the type of metalsupported catalyst (Figure 1.8). 130 Indeed, the mechanism of reactant adsorption and hydroconversion are sensitive to the type of metal, including noble metals (e.g., Pt, Pd, Ru, Au, Rh, Ir) and non-noble metals (e.g., Ni, Cu, Co, Mo, Fe), type of support and presence of additives on catalytic performances. The key to the success of this catalyst was attributed to the confinement of Pd nanoparticles within the channels (from 5 to 6 Å) of pure-silica MFI-type support. This specific configuration leads to a high Pd dispersion and to a strong interaction between the porous silica and Pd particles.

Functionalizing bio-based furanic derivatives in a minimum number of synthetic steps is of huge interest to increase the molecular diversity and complexity. In this view, sequential reactions have been developed targeting the synthesis of branched alkanes as jet fuels and monomers for polyester and epoxy resins starting from the aldol condensation reaction of furfural with ketones (e.g., acetone, MIBK, cyclopentanone), followed by hydrodeoxygenation (HDO) at solvent-free conditions. [135][136][137][138] Also, furoin and furil intermediates from furfural can be hydrogenated to THF-derivatives and subjected to ring opening by HDO over Pd/C + La(OTf)3 / Eu(OTf)3 catalysts, affording the synthesis of diesel fuels with 10-12 carbon atoms. [139][140] Furoin and furil intermediates can also be used for preparing biofuels with at least 12 carbon atoms by subjecting the intermediates to esterification and etherification reactions. 141 Scheme 1.5 Reaction pathways for the total hydrogenation of furfural to tetrahydrofurfuryl alcohol.

Amination of furfural

Amines are important intermediates in the chemical industry due to their nucleophilic properties, conferring them high reactivity. [142][143][144] In particular, primary amines are valuable chemicals, which serve as versatile feedstocks for the synthesis of advanced chemicals, life science molecules and polymers. 142,[145][146] The industrial synthesis of amines usually involves hazardous reagents (e.g., HCN) or generates hazardous by-products (e.g., HCl). Moreover, the preparation of primary amines by direct nucleophilic substitution of organic halides suffers most often from a lack of selectivity, the reaction usually rendering a mixture of primary, secondary and tertiary amines. The use of acid catalysts to activate the alcohol group can also be responsible for the generation of alkenes through uni/bimolecular elimination mechanisms. 147 An alternative process based on the addition of ammonia or amines on alkenes has also been proposed for the synthesis of tertiary amines over acid catalysts (e.g., t-butylamine from isobutene), but with low activity. 148 Finally, amines can also be produced by the reduction of amides, but generally under harsh conditions. 149 Reductive amination of aldehydes or ketones is the most widespread method for preparing amines at industrial scale using heterogeneous catalysts based on supported Cu, 150 Ni, 151 Pt, 152 Ru, 153 Rh, 154 Pd, 155 due to their easy separation and recyclability, and lack of use of toxic reagents and/or stoichiometric reductants (Figure 1.9). [156][157][158] In particular, reductive amination using NH3 and H2 has been explored in the field of Organic Chemistry, giving access to a variety of amines. [159][160] However, reductive amination reactions can suffer from overalkylation and the reduction of the aldehydes/ketones to alcohols, decreasing the selectivity to primary amines. 161- 162 Hence, from an atom-economical and environmental standpoint, there is a need to develop selective and efficient catalysts for producing biobased primary amines, which is the prime task of this investigation. Furfural can be converted into amino-compounds by reductive amination for preparing polymers, surfactants, biologically active molecules and pharmaceutical intermediates, such as antiseptic agents, antihypertensives and diuretics (e.g. furosemide). [163][164][165] The synthesis of furfurylamines has been investigated from furfural and 5-hydroxymethylfurfural by reductive amination using homogeneous and heterogeneous catalysts. 166-170 THF-derived amines can be further generated by deep hydrogenation of the furan ring for the synthesis of polybutadiene rubber with a precise control of the microstructure, 170-172 as well as for the synthesis of poly(2-vinyl-4,4dimethylazlactone) (pVDMA)-derived (co)polymers with lower critical solution temperature. 173- 176 The reductive amination of furfuryl alcohol to THF-amines can be achieved over Raney Ni at 180 °C and 0.1 MPa H2. [177][178] In this context, the discovery of catalytic systems that enable the one-pot reductive amination of furfural to THF-amines at mild conditions with high selectivity would be of great interest.

Cascade reactions using furfural

A cascade reaction, also known as a domino reaction or tandem reaction, is a chemical process that comprises at least two consecutive reactions such that each subsequent reaction occurs only in virtue of the chemical functionality formed in the previous step. In cascade reactions, isolation of intermediates is not required, as each reaction composing the sequence occurs spontaneously under required condition. Cascade reactions can be used to engineer one-pot / single-reactor processes. In this view, it would be desirable to devise simple, easily accessible and robust catalytic systems affording multi-step reactions to engineer intensified processes and avoiding the separation of intermediates.

The possibility of combining all or some reaction steps in the same reactor in a one-pot fashion has attracted great attention. As key innovation, the aldol condensation reaction of furfural with a ketone combined with a sequential reaction been proposed for engineering tandem reactions in a single reactor, avoiding the isolation of reaction intermediates and any change of solvent. To this aim, combinations of catalysts or multifunctional catalysts with different catalytic centers are required. For instance, one-pot preparation of biofuels and monomer precursors has been reported by combining a first aldol condensation step of furfural with acetone, methyl isobutyl ketone (MIBK) or cyclopentanone, followed by hydrodeoxygenation or hydrogenation catalyzed by Pd or Pt supported over acid/base supports, 136, 179-182 or Pt encapsulated in acid zeolites. [183][184] In particular, Faba et al. prepared a bifunctional Pd/MxNyO catalyst for aldolizationhydrogenation of furfural-acetone mixtures, where the basic support catalyzed the aldol condensation/ crotonization reaction of furfural and acetone, and Pd nanoparticles catalyzed the hydrogenation of the as-prepared aldol intermediates (Scheme 1.6). 185 Scheme 1.6 Reaction network for one-pot aldol-condensation of furfural and acetone, followed by hydrogenation.

Dumesic and coworkers proposed a catalytic process for preparing high-quality diesel fuels using furfural (or HMF) and acetone. 74 The process includes aldol condensation and hydrodeoxygenation, which increases the molecular complexity of furfural (Scheme 1.7). In a two-phase reactor, the aldol condensation reaction of furfural (or HMF) and acetone is carried out under the catalysis of an aqueous NaOH solution at room temperature, and the C9 product obtained by the reaction is extracted into the organic phase. The C9 product in the organic phase can continue to react with a molecule of furfural to generate C15 furan compounds. These products are catalyzed by bifunctional Pt/Al2O3 and Pd/SiO2-Al2O3 catalysts, and finally converted into linear C9 and C15 hydrocarbons through ring opening, dehydration and oxygen deoxygenation reactions. Finally, in the entire reaction process, the overall yield of liquid C9 and C15 hydrocarbons can reach 73%.

Scheme 1.7 Tandem reaction from furfural condensation and hydrogenation reaction.

One-pot reactions have also been reported for the synthesis of furan-and THF-derived amines by the one-pot aldol condensation reaction of furfural with ketones, followed by reductive amination with NH3 and H2, over combined Amberlyst-26 and Ru/C or Pd/Al2O3 catalysts. 186- 187,196,197 Key to these developments is the compatibility of the different active centers, which can show antagonistic behavior in the different steps. As example, the reaction of furfural with MIBK afforded the in situ formation of a carbonyl-conjugated vinylfuran intermediate, which was further subjected to reductive amination to give the corresponding furan-and THF-derived amines depending on the catalyst used (Scheme 1.8). Using Pd/Al2O3 as catalyst and H2 (2.0 MPa) as reductant, an overall yield of 98% of THF-amine could be achieved at 120 °C after 20 h. These examples open up a new route for upgrading biomass-derived intermediates, which could promote the economy and environmental footprint of the process. Further eco-efficient process one-pot conversion of carbohydrates to value added chemicals is highly desired.

Objectives of the thesis

Furfural, issued from lignocellulosic biomass, is an important platform molecule with high potential for preparing renewable and green fine chemicals and fuels. To increase the molecular complexity of furfural, it is convenient to combine C-C coupling reactions with hydrogenation and reductive amination steps. In this PhD thesis, we aim at preparing long-chain derivatives from furfural using eco-efficient processes. A main objective is to engineer single-reactor tandem multistep processes affording a high degree of intensification without separation of reaction intermediates.

Herein, we will show two examples of single-reactor (two-step) tandem reactions encompassing a first C-C coupling step and a subsequent hydrogenation or reductive amination step: (1) aldol condensation/crotonization of furfural with MIBK, followed by hydrogenation with molecular H2, and (2) benzoin condensation of furfural, followed by reductive amination with NH3 and H2 (Figure 1.10). In these examples, the reaction intermediate (i.e. aldol product or furoin/furil) is not isolated. In chapters 3 and 4, we present two different types of bifunctional catalysts for conducting the single-reactor tandem aldol condensation/crotonization of furfural with MIBK, followed by hydrogenation. The catalysts consist of crystalline silico-aluminophosphate Pd/HPSAPO-5 with hierarchical porosity and variable silicon content (chapter 3), and Pd/Al2O3 catalyst. Besides, in chapter 5, we engineered a single-reaction tandem reaction encompassing the benzoin condensation of furfural to furoin, followed by the reductive amination of the as-generated furoin to the alcohol-amine.

Finally on the guidance of the results provided in Chapters 3-5, a series of conclusions will be provided at the end of the manuscript together with further perspectives. The Al-doped SBA-15 catalyst with a nominal SiO2/Al2O3 molar ratio of 76 was prepared by the evaporation-induced self-assembly (EISA) method. The preparation method can be found elsewhere. 1 Briefly, 2.0 g of Pluronic P-123 triblock co-polymer was dissolved in 38 mL of anhydrous ethanol. Then, 4.16 g (20 mmol) of tetraethylorthosilicate (TEOS) and 0.054 g (0.26 mmol) of aluminium isopropoxide [(CH3)2CHO]3Al and deionized water were added under vigorous stirring. After stirring for 24 h at room temperature, the mixture was poured into a Petri dish to initiate the EISA process. The as-obtained gels were calcined at 550 °C for 6 h under static air with a heating rate of 1 °C•min -1 .

Synthesis of microporous SAPO-5 with variable Si loading

Aluminium isopropoxide (7.0 g) was added slowly to deionized water (10 mL) and triethylamine (TEA, 2.77 mL) under stirring, and the mixture was stirred for 1 h. Tetraethylorthosilicate (TEOS, 2.31 mL, 4.62 mL and 6.92 mL for Si=0.30, Si=0.60 and Si=0.90, respectively) was added dropwise under stirring, along with 7.29 mL of deionized water, and the mixture was further stirred for 2 h. Phosphoric acid (2.18 ml, 85 wt% in H2O) was added dropwise under stirring. The gel was vigorously stirred for 30 min to produce a white gel with the composition:

1 Al : 0.93 P : xSi : 0.58 TEA : 28 H2O

where x = 0.3; 0.6; 0.9.

The gel was transferred to a Teflon-lined stainless-steel autoclave and subjected to hydrothermal synthesis at 200 °C for 60 h under autogenous pressure. The solid product was then recovered from the autoclave by filtration, washed with water, dried in air at 100 °C and finally calcined in a tube furnace at 600 °C for 16 h under airflow to generate a white crystalline powder.

For the sake of clarity, the nomenclature SAPO-5/xSi was used, where 'x' refers to the molar Si/Al ratio in the white gels.

Synthesis of hierarchical HPSAPO-5 with variable Si loading

Aluminium isopropoxide (7.0 g) was added slowly to deionized water (10 mL) and TEA (2.77 mL) under stirring, and the mixture was stirred for 1 h. CTAB-containing MCM-41 (1.03 g, 2.06 g and 3.09 g for Si=0.30, Si=0.60 and Si=0.90, respectively) was slowly added, and the mixture was further stirred for 2 h. Phosphoric acid (2.18 mL, 85 wt% in H2O) was added dropwise under stirring. The mixtures with three variable Si content were vigorously stirred for 30 min to produce white gels with the composition 1 Al: 0.93 P: x Si: y CTAB:0.58 TEA: 28 H2O, where x = 0.3 and y = 0.033; x = 0.6 and y = 0.066; x = 0.9 and y = 0.099.

The gels were transferred to a Teflon-lined stainless-steel autoclave and crystallized at 200 °C for 60 h under autogenous pressure. The solid product was recovered from the autoclave by filtration and washed with water, dried in air at 100 °C and calcined in a tube furnace under airflow at 600 °C for 16 h to remove the organic surfactant and the micropore template, producing a white crystalline solid.

For the sake of clarity, the nomenclature HPSAPO-5/xSi was used, where 'HP' refers to 'high porosity', and 'x' refers to the molar Si/Al ratio in the white gels.

Synthesis of Pd/HPSAPO-5 Si=0.9, Pd/Al2O3, Ru/Al2O3 and Ru/SiO2

In this thesis, the noble metal catalysts were all prepared by the wet impregnation method using an solution of the metal precursors. The precursors were palladium acetate and ruthenium chloride for Pd and Ru catalysts, respectively. In a typical preparation, 5% Pd/HPSAPO-5, 0.9Si was prepared by dispersing 4.9 g of HPSAPO-5, 0.9Si in a solution of 0.54 g of palladium acetate in 30 mL acetone. The final suspension was sonicated for 30 min, and the acetone was evaporated at 40 °C under vigorous stirring. The sample was then dried in an oven overnight at 80 °C, calcined at 400 °C for 4 h and reduced at 200 °C for 4 h under H2.

Bim-silica catalyst

The catalyst came from the partner University of Southampton, UK, stored in a dryer.

Characterization methods

Powder X-ray diffraction (PXRD)

X-ray diffraction is a versatile and non-destructive analytical method for the determination of material properties such as phase composition, structure and texture of powders and crystal solids. Depending on the composition, X-ray radiation will diffract in samples according to Bragg's law as a function of the distribution of atoms in a crystal. The diffraction pattern provides information about the phase, crystal structure and crystal size. The reflections in the XRD pattern obey to Bragg's law

nλ = 2dsinθ (2.1)
where λ is the wavelength of the x-ray, d is the spacing of the crystal layers (path difference), θ is the incident angle (the angle between incident ray and the scatter plane), and n is an integer.

The Scherrer equation, in X-ray diffraction and crystallography, is a formula that relates the size of sub-micrometre crystallites in a solid to the broadening of a peak in a diffraction pattern. It is used in the determination of size of crystals in the form of powder. Scherred equation can be expressed as follows

𝜏 = 𝐾𝜆 βcosθ (2.2)
with τ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size, which may be smaller or equal to the particle size;

K is a dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the X-ray wavelength; β is the line broadening at half the maximum intensity, after subtracting the instrumental line broadening, in radians. This quantity is also sometimes denoted as Δ(2θ) θ is the Bragg angle.

The PXRD patterns were recorded on a PANalytical X'Pert PRO diffractometer provided with Cu-Kα radiation (λ = 1.54062 Å) generated at 45 kV and 30 mA. The spectra were collected in the range 5° ≤ 2θ ≤ 40° with a step size of 0.02°. The patterns were indexed using the Joint Committee on Powder Diffraction (JCPDS) database and interpreted using MDI JADE 5.0 software.

Thermogravimetric Analysis (TGA)

Thermogravimetry is a technique where the mass of a sample is monitored as a function of temperature in a specified atmosphere. To magnify the steps in the thermogravimetric profiles, the derivative thermogravimetric (DTG) curve is frequently used. Differential thermal analysis (DTA) is a technique where the difference in temperature between the sample and a reference material is monitored as a function of the temperature in a specific atmosphere. The DTA curves are generally a plot of the difference of temperature as ordinate and the temperature of the reference as abscissa.

In our experiments, the thermogravimetric measurements were performed using a TA Instruments SDT Q600 apparatus. The solid was treated from room temperature to 900 °C (10 °C.min -1 ) under an atmospheric environment with a flow rate of 75 mL(STP).min -1 . Approximately, 10 mg of sample was heated in an open Pt pan.

Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) is a microscopy technique where a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid.

For TEM analysis, a JEOL 2100 instrument with a LaB6 filament and an acceleration voltage of 200 kV, and equipped with a Gatan 832 CCD camera, was used. Prior to TEM characterization, the samples were dispersed in ethanol solution with ultrasonic treatment for 5 min and then dropped onto a carbon film on a copper grid. The particle size distributions were measured from manual analysis of micrographs using Image software.

H2 temperature programmed reduction (H2-TPR)

The H2-TPR profiles were collected for metal-containing catalysts on a Micromeritics AutoChem II2920 system equipped with a quartz U-type tubular reactor and a TCD detector. A cold trap was used before the detector to avoid any interference of water in the TPR plot. In each test, 100 mg of the given catalyst was loaded into the reactor and purged with 30 mL(STP)/min He at 300 °C for 1 h to remove adsorbed moisture and vapors, and then cooled down to room temperature. The temperature was then increased up to 750 °C at a heating rate of 10 °C•min -1 under a H2 flow diluted in Ar (10 v/v %). The system was calibrated using an Ag2O standard (>99% purity) to measure the H2 consumption in the TPR profiles. All the profiles were normalized to a constant mass of wafer.

Pulse CO chemisorption analysis

Chemisorption study was carried out using an AutoChem 2920 instrument (Micromeritics) via the pulse method by introducing 5% CO and 95% He over a sample maintained at 25 °C. To calculate metal dispersion, an adsorption stoichiometry of CO/Ru = 1 was assumed. Metal dispersion:

The metal dispersion is calculated using the equation below, where PD is the percent metal dispersion, Vs is the volume adsorbed (cm 3 at STP), SFcalc is the calculated stoichiometry factor, SW is the sample weight and GMWcalc is the calculated gram molecular weight of the sample [g/g-mole].

PD = 100 × ( 𝑉 𝑠 ×𝑆𝐹 𝑐𝑎𝑙𝑐 𝑆 𝑤 ×22414 ) × GM𝑊 𝑐𝑎𝑙𝑐 (2.3)
The active particle size is estimated using the equation below, where Dcalc is calculated metal density (g/cm 3 ), Ws is the sample weight, GMWcalc is the calculated gram molecular weight (g/gmole), and SAcacl is the calculated specific surface area (per gram of metal)

APS = 6 (𝐷 𝑐𝑎𝑙𝑐 )×( 𝑊 𝑠 𝐺𝑀𝑊 𝑐𝑎𝑙𝑐
)×(6.023×10 23 )×(𝑆𝐴 𝑐𝑎𝑙𝑐 )

(2.4)

NH3-Temperature-programmed desorption (NH3-TPD)

NH3-TPD was used to measure the density and strength of acid sites in the catalysts. The measurements were performed on a Micromeritics AutoChem 2920 system equipped with a quartz U-type tubular reactor and a thermal conductivity (TCD) detector. A typical test was carried out as follows: 100 mg of catalyst was pre-treated at 250 °C for 2 h under a He flow [40 mL(STP)/min] to remove adsorbed moisture and vapors, and then cooled down to room temperature. Subsequently, NH3 was chemisorbed at 100 °C using pulsed injections until saturation. The temperature was then increased up to 1000 °C at a heating rate of 10 °C.min -1 and the TPD profiles were recorded from 100 to 1000 °C. The quantification method used for measuring the number of acid sites can be found elsewhere. All the profiles were normalized to a constant mass of wafer.

Specific area measurement (BET)

BET (BET = Brunauer, Emmett, Teller) is an acronym that refers to the initials of the three people who developed the mathematics of the method for the measurement of specific surface areas of solids, as well as the pore size distribution from gas adsorption at cryogenic conditions. The volume of gas (usually N2) adsorbed on the surface of the particles is measured at the boiling point of N2 (-196 °C). At this temperature, N2 is below the critical point and can condense on the surface of the particles. The Brunauer-Emmett-Teller (BET) theory aims to explain the physical adsorption of gas molecules on a solid surface. 2 The textural properties of the catalysts were inspected by N2 and Ar physisorption at -196 °C in the relative pressure (P/P 0 ) range from 1x 10 -6 to 1 using a Quantachrome ASiQ apparatus. Prior to the analyses, the samples were outgassed at 300 °C for 24 h (residual pressure lower than 1.3 х10 -6 mbar. The specific surface areas were measured by the Brunauer-Emmett-Teller (BET) method in the P/P 0 range from 0.01 to 0.1. The desorption branch was analyzed by the NLDFT (non-local density functional theory) method to obtain the pore size distributions of the different samples.

Fourier-transform infrared spectroscopy (FTIR)

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.

FTIR spectra of self-supporting pellets were collected under vacuum conditions (residual pressure <10 -5 mbar) using a Bruker Equinox 55 spectrometer equipped with a pyroelectric detector (DTGS type) with a resolution of 4 cm -1 . NH3 and 2,6-di-tert-butylpyridine (2,6-dTBP) were adsorbed at room temperature using specially designed cells permanently connected to a vacuum line to perform adsorption-desorption in situ measurements. FTIR spectra were normalized with respect the pellet weight and, whenever specified, are reported in differencemode by subtracting the spectrum of the sample in vacuum from the spectrum of the adsorbed molecules. The total number of accessible Brønsted acid sites (N) was estimated using the Lambert-Beer law in the form A = εNρ, where A is the integrated area of the bands of protonated species (cm -1 ), ε is the molar extinction coefficient (cm 2 •µmol -1 ), N is the concentration of the vibrating species (µmol•g -1 ), and ρ is the density of the disk (mass/area ratio of the pellet, mg•cm -2 ).

Solid-state (SS) NMR spectra

Solid-state NMR (ssNMR) spectroscopy is a special type of nuclear magnetic resonance (NMR) spectroscopy, characterized by the presence of anisotropic (directionally dependent) interactions. Compared to the more common solution NMR spectroscopy, ssNMR usually requires additional hardware for high-power radio-frequency irradiation and magic-angle spinning.

Solid-state (SS) NMR spectra were acquired on a Bruker Avance III 500 spectrometer and a wide bore 11.7 T magnet with operational frequencies for 1 H, [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si, 31 P and 27 Al of 500.13, 99.35, 202.45 and 130.33 MHz, respectively. A 4-mm triple resonance probe in double resonance mode with magic angle spinning (MAS) was employed in all the experiments, and the samples were packed on a Zirconia rotor and spun at a MAS rate of 15 kHz. The magnitude of radiofrequency fields were 100, 83 and 42 kHz for 1 H, 31 P and [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si respectively. The 27 Al MAS spectra were acquired on large sweep width with small pulse angle (π/12) to ensure quantitative interpretation. In the case of [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si, 31 P and 27 Al MAS NMR, high-power proton decoupling was applied. The relaxation delay, d1, between accumulations was 5, 1, 20, and 60 s for 1 H, 27 Al, 31 P and [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si MAS NMR spectroscopy, respectively. All chemical shifts were reported using δ scale, and were externally referenced to TMS for 1 H and [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si NMR, Al(H2O)6 3+ ion in 1.0 m AlCl3 solution for 27 Al NMR and H3PO4 (85%) for 31 P NMR. The chemical shifts reported for 27 Al NMR were not corrected for second-order quadrupole effects. The 1 H MAS NMR spectra were fitted with DMFIT functions for quantitative deconvolution of overlapping peaks. The samples were packed into a NMR rotor and dehydrated at 573 K under vacuum (1x10 -4 mbar) for 2 h prior to the loading into the magnet and recording of the NMR spectrum.

Products identification and quantification

Gas chromatography (GC)

The analysis of reactants and products was carried out by gas chromatography on a Varian Bruker 450 GC equipped with a flame ionization detector (FID) and a HP-5MS column (L = 30m, Ø = 0.25mm, e = 0.25μm, temperature from 80 °C to 250 °C) (Figure 2.1). The analytical methods were adjusted for the different mixtures depending on the boiling point and polarity of the compounds. For all methods, the injector and detector temperatures were set at 250 °C and 300 °C, respectively, and the sample injection volume was 2 μL. The calibration was performed using biphenyl/n-dodecane as the internal standard.

High performance liquid chromatography (HPLC)

High-performance liquid chromatography (HPLC) is a technique in analytical chemistry used to separate, identify and quantify components in a mixture. The technique relies on pumping a pressurized liquid solvent containing the sample mixture through a column filled with a solid adsorbent material. Each component in the sample interacts differently with the adsorbent material, causing different flow rates for the different components and leading to the separation of their separation. The technique is adapted to mixtures with low boiling point compounds, or compounds sensible to temperature.

The conversion of furoin/furil was measured on an Agilent 1200 series HPLC high-performance liquid chromatograph equipped with an Agilent XDB-C18 column (5 μm, 4.6 mm × 150 mm) and a UV diode array detector. In detail, the injection volume of the solution mixture was 2 μL, and the mobile phase (acetonitrile, methanol) was flown at 1 mL.min -1 for the detection of products. The column temperature was 45 °C. The wavelength of the UV detector was set at 214 nm and 251 nm for detecting products.

General calculation methods in GC and HPLC

In each experiment, the conversion, selectivity, yield and the initial reaction rate of formation (ri,0) of each product, as well as the carbon balance (CB), were defined using the following expressions (given for the FF-MIBK system as example)

FF conversion = 1 - n FF n FF 0 x100
(2.5)

Selectivity i = n i n FF 0 -n FF x100 (2.6) Yield i = n i n FF 0 x100 (2.7) r i,0 = 1 W dn i dt | t=0
(2.8)

CB FF = Σn i + n FF n FF 0 x100
(2.9)

CB FF+MIBK = Σn i + n FF + n MIBK n FF 0 + n MIBK 0 x100 (2.10)
where ni is the number of moles of product i, and W is the catalyst loading.

The TOF (the turnover frequency) is defined as the number of the catalytic cycles per unit of time. 3 It is calculated as the number of molecules converted per active site per time. The TON (the turnover number) is a measure of catalyst stability. In the words of Kozuch and Martin 4 "the TON deals with its lifetime robustness". It is calculated as the number of catalytic cycles a site can perform until it deactivates completely, meaning that the catalytic test must be perform until the catalyst is completely deactivated.

Gas chromatography mass spectrometry (GC-MS)

Gas chromatography-mass spectrometry (GC-MS) 5 is an analytical method that combines the features of gas chromatography and mass spectrometry to identify different compounds within a test sample. The GC-MS analyses were carried out on an Agilent 6890N/5973 inert GC/MSD instrument equipped with a HP-5MS column (L = 30m, Ø = 0.25mm, e = 0.25μm, temperature from 50 °C to 250 °C, 10 °C/min).

Liquid chromatography mass spectrometry (LC-MS)

Liquid chromatography-mass spectrometry (LC-MS) 6 is an analytical technique that combines the physical separation capabilities of liquid chromatography (or HPLC) with the mass analysis capabilities of mass spectrometry (MS). The LC-MS analyses were carried out in a LC-QExactive Mass Spectrometry (Thermo) instrument (Source Type ESI, Scan Begin 50 m/z, Scan End 1000 m/z, Ion Polarity Positive).

Nuclear Magnetic Resonance spectroscopy (NMR)

Nuclear magnetic resonance spectroscopy (NMR) 7 is an analytical technique that can be used for measuring the structure of organic compounds. Each compound can be well characterized by several factors such as chemical shift, spin multiplicity, coupling constants, and peak integration. In this study, liquid 1 H and 13 C NMR analyses were used to identify the structure of products in the different reactions. NMR spectra were recorded on a Bruker Advance DPX 400 spectrometer.

Experimental procedures

General procedure for the aldol condensation of furfural and MIBK

The aldol condensation/crotonization reaction was conducted in a stainless steel autoclave reactor (30 mL) from Taiatsu. In a typical test, the autoclave was charged with 0.2 g (2 mmol) of distilled furfural, 3.8 g (38 mmol) of MIBK, 40 mg of catalyst and 180 °C in the reactor with a magnetic stirring bar. The mixture was heated to the desired temperature under a stirring speed of 1000 rpm, and the kinetic profiles were measured out for 1-24 h. Preliminary experiments were conducted ensuring the absence of external mass transfer limitations on the reaction rate.

A series of experiments was also carried out on a Top Industrie reactor. In a typical test, the reactor was charged with 2 g (20 mmol) of distilled furfural, 38 g (380 mmol) of MIBK, 400 mg of catalyst and 120 °C in the reactor with a mechanical stirring. The mixture was heated to the desired temperature under a stirring speed of 1000 rpm.

Preliminary experiments were conducted to assess the reproducibility of the experiments at comparable conditions on both reactors using Siral HPV as catalyst (Figure 2.2). The comparison of the kinetic plots for FF conversion and ALD-1 selectivity on both reactions clearly point out good reproducibility of the experiments.

Procedure for single-reactor tandem aldol condensation / hydrogenation

The single-reactor tandem aldol condensation / hydrogenation reaction was conducted by performing first the aldol condensation/crotonization reaction between FF and MIBK in an autoclave (30 mL) at the conditions described above, followed by H2 introduction at the desired pressure (1.0-2.0 MPa), and the hydrogenation reaction was conducted at 80 °C-160 °C for 6-12 h ensuring complete reduction of the Pd phase according to the H2-TPR profiles of the catalysts.

Procedure for single-reactor tandem benzoin coupling / reductive amination

The benzoin condensation of furfural was carried out over a supported bim-silica catalyst in an autoclave using DMF solvent, and diazabicycloundecene (DBU) as catalyst. Typically, first put CHAPTER 3: Single-reactor tandem aldol condensation-hydrogenation of furfural and MIBK over bifunctional Pd/HPSAPO-5

Introduction

Considerable research has been carried out in the last years to engineer basic catalysts for conducting aldol condensation reactions of FF and ketones, including metal and mixed oxides (e.g., ZrO2, ZnO, MgO-ZrO2) 1-3 , zeolites [4][5] and metalsubstituted zeotypes. However, fewer examples have been reported on acid catalysts, relying on acid oxides (e.g., ZrO2 and ZnO), acid zeolites [6][7] and metal-organic frameworks. Zeolites are particularly attractive due to their tunable acid properties, but can exhibit poor activity due to mass transport limitations within micropores, affecting the accessibility of the reagents to the acid sites, as well as low hydrothermal stability and deactivation due to coke deposition. To overcome these shortcomings, hierarchical acid zeolites have been developed using different methodologies, some of which have recently shown promising results in aldol condensation reactions. In particular, hierarchical silicoaluminophosphates (SAPOs) acid zeo-type catalysts represent a viable alternative. Indeed, microporous SAPOs have been extensively used in acid catalyzed reactions for hydrocarbon conversion. In particular, SAPO-5, with an AFI framework, a pore diameter of 7.3 Å and moderate acidity, has been used as heterogeneous catalyst in methanol-to-olefin conversion, toluene alkylation and xylene isomerization. Recently, hierarchical SAPO catalysts combining the micropores of the zeolite framework and an additional mesoporous network, have been synthesized using both topdown and bottom-up strategies.

Herein, a novel bottom-up approach was used to synthesize hierarchical SAPO-5 catalysts (HPSAPO-5). This method avoids postsynthetic demetallation under acid or basic conditions as commonly found in top-down methods, resulting in a localized collapse of the zeolite framework.

A series of HPSAPO-5 catalysts with variable Si loadings were prepared using cetyl trimethylammonium bromide (CTAB) encapsulated within MCM-41, serving both as Si source and mesoporogen. This particular synthetic methodology mitigates the use of expensive and sophisticated surfactants to induce mesoporosity in the microporous framework, and benefits from a superior retention of the acid properties of microporous SAPOs, whilst simultaneously enhancing mass transfer. The Si loading allows a modulation of the acid density and strength by a partial substitution of framework P with Si, resulting in the formation of Brønsted acid sites of medium acid strength through a type-2 substitution mechanism (SM2). An increase of the Si loading can also induce the formation of silica islands in SAPO frameworks, altering the acid strength. Besides, by impregnating Pd over HPSAPO-5, a bifunctional catalyst can be designed, affording the formation of furan-and tetrahydrofuran-derivatives in an efficient single-reactor tandem process starting from FF and ketones. These products could be applied as potential biofuels or biosolvents.

Results and discussion

Aldol condensation/crotonization of FF with MIBK over acid zeolites

In a first step, we investigated the catalytic performance of a library of reference acid catalysts in the aldol condensation/crotonization reaction of FF and MIBK, including ZrO2, ZnO and a series of acid zeolites with variable topologies, i.e. ZSM-5 (Si/Al = 23-280), BEA (Si/Al = 150), MOR (Si/Al = 6.1) and SAPO-5 with variable Si loadings (SAPO-5/xSi, x = 0.3, 0.6, 0.9). In all cases, the main product was (E)-1-(furan-2-yl)-5-methylhex-1-en-3-one (ALD-1) (Scheme 3.1), whereas (E)-and (Z)-3-(furan-2-ylmethylene)-4-methylpentan-2-one (ALD-2a and ALD-2b, respectively) were obtained as by-products. The alcohol intermediate issued from the aldol condensation (ALD-1-OH) was only observed at trace levels for a FF conversion higher than 10%. Besides, no products issued from the aldol self-condensation/crotonization of MIBK were detected. Table 3.1 compiles the FF conversion, ALD-1 and ALD-2a / ALD-2b selectivities and ALD-1 yield, as well as the carbon balance with respect to FF, for the different catalysts. Reaction conditions: 0.2 g FF, 3.8 MIBK, 40 mg catalyst, 180 °C, 3 h ZnO and ZrO2 (entries 1 and 2) display only 18% and 25% FF conversion, respectively, with an ALD-1 selectivity of 76% and 32%. Regardless of the Si/Al molar ratio, the ZSM-5 zeolites (channel size = 5.5 Å) exhibit poor FF conversion (range 10-20%) and an ALD-1 selectivity up to 60% for ZSM-5 (Si/Al = 80) (entries 3-6). Similar results are obtained for BEA (channel size = 5.5 Å) and MOR (channel size = 6.7 Å) (entries 7 and 8). In contrast, Y zeolite (cavity size = 7.4 Å) displays a higher FF conversion (40%) and 52% selectivity to ALD-1 (entry 9). The microporous SAPO-5/xSi zeolites with a comparable micropore size (7.3 Å), show similar FF conversion, especially for x = 0.6 and x = 0.9, with values of 49% and 40% (entries 12 and 14), respectively, while the FF conversion for SAPO-5/0.3Si is only 23% (entry 10). In parallel, the ALD-1 selectivity is about 60% for SAPO/0.6Si and SAPO-5/0.9Si, approaching the value obtained for ZSM-5 (Si/Al = 80) and Y zeolites, whereas SAPO-5/0.3Si shows a much lower ALD-1 selectivity of 47%. The selectivity of the aldolization by-products ALD-2a and ALD 2b is similar for the different SAPO-5/xSi samples with a value in the range 8-10%.

We further investigated the catalytic activity of the hierarchical HPSAPO-5/xSi zeolites (x = 0.3, 0.6, 0.9). A neat increase of the FF conversion is observed for HPSAPO-5/0.9Si (entry 15) and HPSAPO-5/0.3Si (entry 11) compared to the parent SAPO-5/0.9Si (73% vs. 49%) and SAPO-5/0.3Si (45% vs. 23%), respectively, whereas no apparent change is observed for HPSAPO-5/0.6Si (49%, entry 13). The ALD-1 selectivity for HPSAPO-5/0.9Si and HPSAPO-5/0.6Si is comparable to that obtained for the parent zeolites, but the selectivity is much higher for HPSAPO-5/0.3Si compared to SAPO-5/0.3Si (72% vs. 47%). The selectivity of the aldolization byproducts ALD-2a and ALD-2b is similar for the different HPSAPO-5/xSi samples with a value about 13%, which is slightly higher than the value obtained on the parent SAPO-5/xSi samples. To assess for the role of mesoporosity on the catalytic properties, we measured the catalytic activity for Al-SBA-15 (Si/Al = 76) (entry 16). Unlike the HPSAPO/xSi catalysts, especially HPSAPO/0.9Si, the FF conversion and ALD-1 selectivity are lower, with values in line to those obtained for Y zeolite.

Overall, the results presented above point out an important effect of the Si loading and the presence of a hierarchical architecture on the catalytic properties of acid SAPO-5 catalysts in the aldol condensation/crotonization reaction of FF with MIBK. The acid properties and pore architecture of HPSAPO-5/xSi make these catalysts attractive for the reaction unlike microporous zeolites and other reference acid catalysts. These encouraging results prompted us to study in detail the reaction kinetics of the HPSAPO-5/xSi at different temperatures.

Kinetic profiles for FF + MIBK aldolization/crotonization over HPSAPO-5/xSi

The kinetic profiles were measured for the HPSAPO-5/xSi and the parent SAPO-5/xSi catalysts in the temperature range 140-180 °C. The initial FF:MIBK molar ratio was set to 1:18 (5.2 wt% FF) while keeping the FF-to-catalyst weight ratio at a constant value of 5.0 to promote the formation of ALD-1 and keep a high carbon balance with respect to FF (Figure 3 Regardless of the temperature, HPSAPO-5/0.3Si exhibits a monotonous increase of the FF conversion with the reaction time until a plateau value lower than 100%, which increases with the temperature from ca. 30% at 140 °C (Figure 3.2C1) to 60% at 180 °C (Figure 3.2C3) after 24 h. An analogous trend is observed for the parent SAPO-5/0.3Si, with an almost complete superposition of the kinetic curves at longer reaction times for the different temperatures, although a markedly lower conversion is observed for times below 12 h. The presence of a plateau in the FF conversion can be explained by a potential catalyst deactivation during the reaction, which appears to be unaffected by the porous texture of the catalysts. As a matter of fact, the carbon balance with respect to FF decreases along the reaction with a final down to 80% (Figure 3.5). However, the total carbon balance (including MIBK) is higher than 95%. Unlike HPSAPO-5/0.3Si, HPSAPO-5/0.6Si and HPSAPO-5/0.9Si do not display a plateau for the FF conversion at longer reaction times, even if the reaction proceeds slowly in the former case after 12 h, and reach higher FF conversions. The FF conversion evolves from 56% at 140 °C (Figure 3.2B1) to 65% at 180 °C (Figure 3.2B3) for HPSAPO-5/0.6Si, and from 60% at 140 °C (Figure 3.2A1) to full conversion at 180 °C (Figure 3.2A3) for HPSAPO-5/0.9Si, after 24 h. These results suggest that, whereas both samples show partial deactivation during the reaction, this is much less prominent on HPSAPO-5/0.9Si.

A similar conclusion can be drawn from the kinetic plots on the parent SAPO-5/0.6Si and SAPO-5/0.9Si, but with two divergent behaviors compared to the hierarchical counterparts: SAPO-5/0.6Si exhibits higher FF conversions compared to HPSAPO-5/0.6Si, especially at 160 °C with a FF conversion reaching 75% after 24 h, whereas SAPO-5/0.9Si exhibits much lower FF conversions compared to HPSAPO-5/0.9Si, even at the highest temperature (180 °C) and long reaction times. This different behavior points out a positive effect of the hierarchical architecture on the catalytic activity for HPSAPO-5/0.9Si, whereas the effect is low and even detrimental for HPSAPO-5/0.6Si. Turning now our attention into the selectivity, irrespective of the temperature, the microporous SAPO-5/xSi catalysts exhibit a sharp increasing trend for the ALD-1 selectivity until a plateau value at about 60%, especially for SAPO-5/0.3Si. This becomes clearly visible in the selectivityconversion plots, especially at lower temperature (140 °C) (Figure 3.3A2-C2). In contrast, the hierarchical HPSAPO-5/xSi catalysts show a less prominent increase of the ALD-1 selectivity with the FF (Figure 3.3A1-C1), being almost constant for HPSAPO-5/0.9Si. In all cases, the aldol by-products ALD-2a and ALD-2b show similar selectivities for the hierarchical and parent SAPO-5 catalysts with a selectivity up to 15% at 180 °C after 24 h.

The increasing trend of the ALD-1 selectivity for SAPO-5/xSi suggests a hindering effect of internal mass transfer on the reaction rate due to a poor accessibility of FF and MIBK to the acid centers in the zeolite micropores. This accessibility is promoted at higher temperature, enhancing the diffusivity of the reactants and ALD-1 within the micropores. In the case of hierarchical HPSAPO-5/xSi catalysts, the presence of mesopores enriched with Si-OH groups on the external surface is expected to favor mass transfer between SAPO-5 particles and promote MIBK adsorption (vide infra), enhancing the reaction at lower FF conversion. Besides, the presence of a plateau for the ALD-1 selectivity (<100%) at high FF conversion can be attributed to catalyst deactivation, blocking the accessibility to the acid centers in the micropores.

Reaction rates, reaction order and activation energies for ALD-1 synthesis

The initial reaction rates of ALD-1 formation were measured from the time-evolution of the ALD-1 yield (Figure 3.6, 3.7) using Eq 2.8 and rational polynomial functions for fitting the kinetic curves. The parity plots are collected in Figure 3.8. Figure 3.9 plots the rates on HPSAPO-5/xSi and SAPO-5/xSi as a function of the temperature and the Si loading (x). Among the SAPO-5/xSi catalysts (Figure 3.9B), the initial rate of ALD-1 formation is sensibly higher on SAPO-5/0.6Si, especially at 140 °C, evolving from 2.6 mmol•g -1 •h -1 at 140 °C to 12 mmol•g - 1 •h -1 at 180 °C. In contrast, among the HPSAPO-5/xSi catalysts (Figure 3.9A), the highest rates are observed on HPSAPO-5/0.9Si for all temperatures, reaching a value as high as 110 mmol•g - 1 •h -1 at 180 °C. When comparing the initial reaction rates of ALD-1 formation on HPSAPO-5/xSi and SAPO-5/xSi catalysts (Table 3.2), the highest enhancement is observed for HPSAPO-5/0.9Si with ratios of 30, 16 and 17 at 140 °C, 160 °C and 180 °C, respectively. This observation points out a remarkable effect of the hierarchical architecture on the reaction rate of ALD-1 formation. In contrast, these ratios are much lower for HPSAPO-5/0.3Si (range 5.6-9.8), and especially for HPSAPO-5/0.6Si, showing ratios lower than 2 and accordingly an almost negligible effect of the hierarchical architecture on the catalytic properties. With these results in hand, we measured the activation energies of ALD-1 formation by representing the Arrhenius plots for the HPSAPO-5/xSi and SAPO-5/xSi catalysts (Figure 3.10).

Within the limits of the experimental error, all the catalysts except SAPO-5/0.3Si, irrespective of their porous architecture, exhibit comparable apparent activation energies at about 100 kJ/mol. This observation points out an identical catalytic mechanism for the different samples with the absence of internal mass transfer effects on the measured reaction rates. In this view, the higher activity observed on HPSAPO-5/0.9Si should be attributed, at first sight, to a higher density of accessible acid sites with the right strength (see chapter 3.2.7 below).

Catalyst recyclability and reuse

The catalyst recyclability and reuse was assessed for HPSAPO-5/0.9Si in four consecutive catalytic runs for the aldol condensation/crotonization reaction of FF with MIBK at 160 °C for 3 h (Figure 3.11). The first run results in a FF conversion of 65% and ALD-1 and ALD-2a,b selectivities of 61% and 13%, respectively. After the reaction, the catalyst was separated, washed with ethanol and reused for a further catalytic test. The FF conversion decreases to 36%, whereas the ALD-1 selectivity increases to 72% at the expense of the ALD-2a,b selectivity with a value of 8%. A slight decline of the FF conversion together with an increase of the ALD-1 selectivity is observed between the 2nd-4th runs. These results reinforce the idea pointed out above of partial catalyst deactivation during the reaction, most likely by the formation of oligomers or coke in the micropores. In this view, after the fourth run, the catalyst was separated, calcined at 400 °C under airflow and reused again. The catalytic activity is recovered, with a FF conversion of 58% and ALD-1 and AlD-2a,b selectivities of 58% and 8%, respectively.

Single-reactor tandem aldol condensation/crotonization of FF with MIBK, followed by hydrogenation

Given the high catalytic activity obtained over HPSAPO-5/0.9Si, we further investigated the single-reactor tandem aldol condensation/crotonization reaction of FF with MIBK, followed by hydrogenation (Scheme 3.2). To this aim, we impregnated HSAPO-5/0.9Si with Pd at three different loadings (0.3 wt%, 0.8 wt%, 4.8 wt%). First, we conducted the aldol condensation/ crotonization reaction over 0.3%Pd/HSAPO-5/0.9Si and 4.8%Pd/HSAPO-5/0.9Si in the temperature range 140-180 °C. Figure 3.12 plots the kinetic profiles obtained for the FF conversion and selectivity of ALD-1 and ALD-2a,b byproducts. For comparison, the kinetic profiles for the parent HPSAPO-5/0.9Si are also represented. Unlike HPSAPO-5/0.9Si, 0.3% Pd/HSAPO-5/0.9Si and 4.8%Pd/HSAPO-5/0.9Si exhibit a sustained increase of the FF conversion until 100 % after 12 h. The ALD-1 selectivity also exhibits a plateau, but at a higher value than in the case of HPSAPO-5/0.9Si (75% vs. 62%). The initial reaction rate of ALD-1 formation is lower in the case of 0.3%Pd/HSAPO-5/0.9Si and 4.8%Pd/HSAPO-5/0.9Si (9 mmol•g -1 •h -1 at 160 °C), whereas the rate measured on HSAPO-5/0.9Si is 60 mmol•g -1 •h -1 (Figure 3 With these results in hand, we performed the single-reactor tandem aldol condensation/ crotonization reaction of FF with MIBK, followed by hydrogenation, over the Pd-loaded HPSAPO-5/0.9Si catalysts. Two reaction products were obtained, that is the furanderivative issued from the hydrogenation of the conjugated C=C bond (F-ALD-1), and the THF-derivative issued also from the hydrogenation of the furan ring (THF-ALD-1). However, the relative selectivity to both products, as well as the total yield, depends on the hydrogenation conditions (Table 3.3). Noteworthy, neither furan nor THF-derivatives issued from the hydrogenation of the C=O bond are detected. The best result is obtained over 4.8%Pd/ HSAPO-5/0.9Si, affording the synthesis of THF-ALD-1 with 45% yield with respect to FF. F-ALD-1 is also obtained with 5% yield. Additional hydrogenated products, including products issued from ALD-2a,b, are obtained with an overall yield of 14%.

Understanding the structure and acid properties of HPSAPO-5/xSi catalysts

The catalytic results presented above point out an enhanced activity and selectivity of HPSAPO-5/0.9Si in the aldol condensation/crotonization reaction of FF with MIBK, as well as the high efficiency of 4.8%Pd/HPSAPO-5/0.9Si for the synthesis of THFALD-1. In light of these results, we explored the framework structure, porous architecture, acidity and acid strength of this catalyst compared to HPSAPO-5/0.6Si and HPSAPO-5/0.3Si, as well as their microporous counterparts. We also characterized the nature of the Pd phase over HPSAPO-5/0.9Si.

Structural and textural properties of HPSAPO-5/xSi catalysts

The XRD patterns of HPSAPO-5/xSi and the microporous SAPO-5/xSi counterparts reveal the typical reflections belonging to the AFI framework (Figure 3.15). This suggests a high phase purity and crystallinity of all the samples regardless of their Si loading and porous architecture, both for the fresh and spent catalyst after reaction.

The presence of the hierarchical architectures, with micro-and mesoporosity, in the HPSAPO-5/xSi catalysts was evidenced by N2 adsorption/desorption analysis at 77 K (Figure 3.16A). All the hierarchical catalysts exhibit Type IV isotherms with a hysteresis loop in the P/P0 range 0.7-1.0, which is indicative of the presence of mesopores with an average size of ca. 30 Å as inferred using the NLDFT method (Figure 3.16B). Besides, a peak centered at ca. 10 Å can be visualized, which is typical of tensile strength effect due to N2 adsorption. 8 The green distribution in panel Fig. S11B was obtained from the Ar adsorption/desorption isotherm at 77 K on HP SAPO-5/0.6Si to highlight the microporous range typical of SAPO-5, revealing a micropore size at around 7.3 Å as expected for the AFI structure.

Table 3.4 lists the BET surface area, as well as the total, microporous and mesoporous specific surface areas and pore volumes measured by the NLDFT method. Both the BET and NLDFT surface areas increase with the Si loading. The mesoporous surface area (Smeso) and mesopore volume (Vmeso), together with total pore volume (Vtot), also increase with the Si loadings, while the contribution of the microporous surface area (Smicro) and the micropore volume (Vmicro) are very similar in all samples. Volumetric data strongly supports the successful preparation of hierarchical architectures, and indicates the coexistence of multiple levels of porosity within HPSAPO-5.

Chemical environment of HPSAPO-5/xSi catalysts

The chemical environment of the framework atoms in the HPSAPO-5/xSi and the microporous SAPO-5/xSi catalysts was probed by 27 Al, 31P, and 29Si MAS NMR (Figure 3.17-3.19). In all cases, a unique resonance band at ca. 37 ppm is observed in the 27 Al NMR spectra (Figure 3.17), which can be unambiguously ascribed to tetrahedrally-coordinated Al atoms in Al(OP)4. This observation suggests in turn the absence of Lewis acid sites in all the catalysts. A single band at about -30 ppm is also observed in the 31 P NMR spectra (Figure 3.18), which can be assigned to tetrahedrallycoordinated P atoms. Since both the 27 Al and 31 P NMR spectra exhibit a single resonance, it is possible to confirm a strict alternation of Al and P at the T-positions of the aluminophosphate framework. 9 In the case of the samples with the highest Si loading (i.e. x = 0.9), the 31 P NMR spectra exhibit a broad foot beneath the main band at ca. -24 ppm that can be assigned to P-OH units. Finally, the [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF]

Acidity and acid strength of HPSAPO-5/xSi catalysts

1 H MAS NMR

1 H MAS NMR is an ideal tool to investigate the nature and strength of the acid sites present in SAPO-5/xSi and HPSAPO-5/xSi catalysts with different Si loadings and complements the studies performed with FTIR spectroscopy of adsorbed probe molecules (see below). 1 H MAS NMR spectra render direct information on the nature and density of protonic species present in the samples (Figure 3.20, Table 3.5). In all cases, two well separated bands centered at around 3.5 and 4.6 ppm are visible in the spectra, which can be ascribed to two different Al(OH)Si Brønsted acid sites (BAS1 and BAS2) located in the 12-and 6-membered ring channels inside the AFI framework, respectively. 9 The intensity of both bands decreases with the Si loading for HPSAPO-5/xSi catalysts, while no definite trend is observed for their microporous counterparts.

Whereas the intensity of both bands is similar for HPSAPO-5/0.6Si and SAPO-5/0.6Si, and for HPSAPO-5/0.3Si and SAPO-5/0.3Si, HPSAPO-5/0.9Si exhibits much lower intensity for both species compared to SAPO-5/0.9Si (21 vs. 12 for the species at 3.5 ppm, and 14 vs. 5 for the species at 4.6 ppm). Apart from Brønsted acid sites, a signal at ca. 1.7 ppm is visible in all the 1 H MAS NMR spectra, which can be attributed to isolated Si-OH groups, together with a band centered at around 2.6 ppm due to Al-OH and/or P-OH defects with very weak acidity. [10][11][12] The intensity of the former band exhibits a neat increase with the Si loading for HPSAPO-5/xSi catalysts from 4 (x = 0.3) to 19 (x = 0.9), whereas the increase is more moderate for SAPO-5/xSi catalysts (from 11 for x = 0.3 to 15 for x = 0.9). When comparing the SAPO-5/xSi and HPSAPO-5/xSi catalysts at the same Si loading, the intensity is similar for x = 0.6 and x = 0.9, while a much lower intensity is observed for HPSAPO-5/0.3Si compared to SAPO-5/0.3Si (11 vs. 4). In contrast, the latter bands are systematically enhanced for HPSAPO-5/xSi catalysts compared to the SAPO-5/xSi counterparts (20 vs. 15 for x = 0.3, 27 vs. 19 for x = 0.6 and 33 vs. 12 for x = 0.9). Besides, for HPSAPO-5/xSi catalysts, the intensity increases monotonously with the Si loading from 20 for x = 0.3 to 33 for x = 0.9. Overall, all these results point out that, by increasing the Si loading, more defects such as Si-OH, Al-OH and/or P-OH are produced in HPSAPO-5/xSi catalysts at the expense of Brønsted acid sites.

In situ vibrational FTIR spectroscopy using NH3 and 2,6-dTBP as basic probes.

The FTIR signature of O-H species in SAPO-5/xSi catalysts was reported in a previous study. 13 Briefly, the FTIR spectra are constituted by two main bands, one at 3680 cm -1 that can be assigned to the O-H stretching mode of free P-OH defects, and a pronounced band at 3745 cm -1 that can be attributed to Si-OH groups, pointing out the presence of defects in the AFI framework upon incorporation of Si.

The FTIR spectra in the OH stretching region of HPSAPO-5/xSi display two distinctive bands centered at 3630 and 3513 cm -1 (Figure 3.21A), which can be associated to Al(OH)Si Brønsted acid sites located in the 12-ring and 6-ring channels of the AFI framework. [10][11][12][13] Additional signals arising from isolated SiOH (3745 cm -1 ) and isolated POH (3678 cm -1 ) sites are also visible, pointing out the presence of defects in the AFI framework upon incorporation of Si. In the FTIR spectra of microporous SAPO-5 (Figure 3.21B), the bands due to the stretching of BAS are not visible due to the high scattering profile of the microporous SAPO-5 and only the contribution of SiOH (3745 cm -1 ) can be appreciated. 13 The nature and accessibility of Brønsted acid sites in the HPSAPO-5/xSi catalysts can be elucidated using basic probe molecules with variable steric hindrance, such as NH3 and 2,6-ditertbutyl-pyridine (2,6-dTBP), adsorbed at room temperature. The total amount of Brønsted acid sites was measured using NH3, which, due to its small kinetic diameter (0.26 nm), is usually considered as a suitable probe molecule for titrating acid hydroxyl groups accessible through pores, channels or windows in zeolites with a size < 4 Å. 14 In contrast, 2,6-di-tert-butyl-pyridine (2,6-dTBP), with a kinetic diameter of 1.05 nm, is not expected to enter the micropores, and can therefore only probe the more accessible acid sites sitting on the mesopores or in the pore mouth of micropores. 15 ), whereas it shows a higher value for SAPO-5/0.9Si (52.30 µ•mol•g -1 ) (Table 3.7). This observation points out a higher degree of Si incorporation in the SAPO-5 framework at the highest Si loading (x = 0.9). In the case of hierarchical HPSAPO-5/xSi catalysts, HPSAPO-5/0.6Si and HPSAPO-5/0.9Si contain a higher density of Brønsted acid sites with 46.25 and 45.32 µ•mol•g -1 , respectively, compared to HPSAPO-5/0.3Si, with 42.79 µ•mol•g -1 . However, HPSAPO-5/0.9Si displays a lower density of Brønsted acid sites than SAPO-5/0.9Si, as also evidenced by 1 H MAS NMR (Table 3

.5).

Proton transfer with more accessible Brønsted acid sites in hierarchical catalysts also occurs when 2,6-dTBP is adsorbed and a band at 1618 cm -1 (ν8a), due to protonated 2,6-dTBPH + species, can be clearly observed. Table 3.6 lists the density of accessible Brønsted acid sites (N) in HPSAPO-5/xSi catalysts titrated using 2,6-dTBP compared to the density of accessible sites titrated by NH3. These data evidence that only a small fraction of Brønsted acid sites are accessible to the bulkier probe molecules. The accessibility factor (AF), 16 defined as the density of sites titrated by 2,6-dTBP divided by the density of Brønsted acid sites titrated by NH3, reveals a higher value for HPSAPO-5/0.3Si (AF = 0.049), while HPSAPO-5/0.6Si and HPSAPO-5/0.9Si show a similar value (AF = 0.025). SiOH density (sites/nm 2 ) d A a (cm -1 ) N b (mol.g -1 ) A a (cm -1 ) N b (mol g - a Iintegrated area A of the 1450 cm -1 and 1618 cm -1 was obtained by using the spectra of the catalysts outgassed at 298 K to eliminate the contribution of NH3 interacting with Si-OH b  = 0.147 cm 2 mol -1 for NH4 + band, 16 and 0.50 cm 2 mol -1 for 2,6-dTBP 15 c In parentheses, acid sites per nm 2 of microporous surface area b SiOH sites per nm 2 of mesoporous surface area a The integrated area A of the 1450 cm -1 band was obtained by using the spectra of the catalysts outgassed at 298K to eliminate the contribution of NH3 interacting with Si-OH b  = 0.147 cm 2 mol -1

By assuming that the Brønsted acid sites titrated by 2,6-dTBP are preferentially located on the mesoporous surface, we can estimate the density of Brønsted acid sites located in the micropores by subtracting the density of acid sites titrated by 2,6-dTBP from that titrated by NH3. Interestingly, regardless of the Si loading, the density of Brønsted acid sites is similar in the different HPSAPO-5/xSi catalysts, with a value of 0.13 µmol.m -2 or 0.08 sites.nm -2 of microporous surface area. In line with the results from 1 H MAS NMR, this observation points out that the density of Brønsted acid sites is not affected by the Si loading in the HPSAPO-5/xSi catalysts, and that a higher Si loading contributes to the generation of Si-OH moieties at the expense of acid Si-O(H)-Al moieties for microporous SAPO-5/xSi, sitting mostly on the mesopores. Despite their different location on the catalyst, the interconnectivity of micro-and mesopores in the HPSAPO-5/xSi catalysts favors the cooperativity between the Brønsted acid sites and Si-OH groups during acid-catalyzed reactions, as recently unveiled by combining spectroscopic (inelastic neutron scattering, FTIR and MAS NMR) and computational analyses on HPSAPO-5/xSi catalysts. 18 This property is expected to facilitate the activation of MIBK and further attack to FF, catalyzing the aldol condensation/crotonization reaction (Scheme 3.3), considering also that the SiOH density is similar for HPSAPO-5/xSi catalysts with 0.6 and 0.9 Si content and it is low for HPSAPO-5/0.3Si (Table 3.6). In light of these considerations, the highest catalytic activity observed for HPSAPO-5/0.9Si can be attributed to the synergistic effect between the amount of OH sites and its hierarchical structure (highest mesoporous area and pore volume, Table 3.4).

The interaction of MIBK via H-bond with the Si-OH was confirmed by MIBK adsorption on the hierarchical catalysts, followed by FTIR spectroscopy. Upon MIBK adsorption (Figure 3.24), the band ascribed to the O-H stretching of Si-OH groups (3745 cm -1 ) vanishes, whereas characteristic bands attributed to the vibrational features of MIBK appear at lower wavenumbers.

On the contrary, the bands ascribed to Brønsted acid sites (3630 and 3513 cm -1 ) are visible, suggesting that Brønsted sites are still available. The shift of the C-O stretching mode of adsorbed MIBK with respect to that in liquid phase confirms that MIBK and SiOH interact by Hbonding (Table 3.8). We also measured FF adsorption after MIBK outgassing. Interestingly, the bands due to the O-H stretching of Brønsted sites completely disappear. However, the bands ascribed to Si-OH groups are visible, pointing out a preferential adsorption of FF in the micropores (Figure 3

NH3-TPD profiles

To further rationalize the higher activity of HPSAPO-5/0.9Si compared to reference acid catalysts, we measured the NH3-TPD profiles for the series of catalysts listed in Table 3 NH3-TPD measurements were also performed on the fresh and spent HPSAPO-5/0.9Si after the aldol condensation/crotonization reaction to gain a better insight into the density distribution and strength of acid sites (Figure 3.26, Table 3.9). The NH3-TPD profile of the fresh catalysts exhibits two main regions (Figure 3.26A): (1) a low-temperature region (100-350 °C), and (2) a high-temperature region (450-650 °C). The first region can be deconvoluted into four bands (I-IV) that can be attributed to weakly acid sites in P-OH and Si-OH groups in line with the 27 Al, 31 P and [START_REF] Himmel | Biomass recalcitrance : deconstructing the plant cell wall for bioenergy[END_REF] Si NMR MAS findings. Besides, NH 4+ •nNH3 associations promoted by H-bonding cannot be excluded. The second region is constituted by a broad band (VI) that is indicative of strong acid sites and that, on the guidance of the FTIR spectra using NH3 as molecular probe, can be ascribed to Brønsted acid sites. By integrating the NH3-TPD profile, the density of weak acid sites belonging to bands I-IV is 775 mmol.g -1 , whereas the density of strong acid sites is 32.7 mmol.g -1 . The latter value compares well with the density of sites measured by FTIR using NH3 as probe molecule (45.3 mmol.g -1 ), confirming that band VI can be assigned to strong Brønsted acid sites. 3.9.

The key features of the NH3-TPD profile on HPSAPO-5/0.9Si are essentially kept after the aldol condensation/crotonization reaction at 180 °C (Figure 3.26B). Nonetheless, the low-temperature region expands to 450 °C due to the apparition of a new broad band (V) centered at 363 °C and a drastic decline of band I. The density of weak acid sites belonging to bands I-V is 1076 mmol.g -1 , which is larger than that of the fresh catalyst, pointing out the generation of additional weakly acid centers (band V) during the reaction. The generation of new acid centers can be attributed to partial hydroxylation of the external surface of HPSAPO-5/0.9Si during the reaction, 20 resulting most likely in a preferential location near the micropore mouths not far from Al atoms. The presence of new weak acid centers is not expected to impact the catalytic activity and catalyst deactivation. In contrast, band VI ascribed to strong acid sites exhibits a slight decline from 32.7 to 12.7 mmol. g -1 , which can be attributed to partial blockage due to oligomers or coke species during the reaction as inferred by thermogravimetric analysys (TGA) (Figure 3.29). Indeed, a characteristic weight loss is observed between 200 and 600 °C on the spent HPSAPO-5/0.9Si that can be attributed to the formation of aliphatic coke at the level of 160 mgC/g. The presence of partial catalyst deactivation can explain the presence of a plateau in the kinetic curves (Figure 3.2) and selectivity-conversion plots (Figure 3.3) for the ALD-1 selectivity at high conversion.

Nature of the Pd phase over HPSAPO-5/xSi

To gain insight into the Pd dispersion on HPSAPO-5/0.9Si, we visualized the 0.3%/HPSAPO-5/0.9Si and 4.8%/HPSAPO-5/0.9Si samples by HR-TEM (Figure 3.30). In both cases, a high degree of Pd dispersion is observed with Pd nanoparticles smaller than 2.5 nm for 0.3%/HPSAPO-5/0.9Si, while two particle distributions centered at 5 nm and 14 nm are observed for 4.8%/HPSAPO-5/0.9Si. The H2-TPR profiles revealed in both cases the presence of a reduction band centered at a temperature up to 60 °C, confirming the fast and complete reduction of the palladium phase in the single-reactor tandem reaction just upon introduction of H2 to the reactor after ALD-1 formation. 3.9.

We also measured the NH3-TPD profile of the unreduced 4.8% Pd/HPSAPO-5/0.9Si catalyst (Figure 3.31, Table 3.8entry 3). The key bands on the parent HPSAPO-5/0.9Si are essentially kept after Pd impregnation. However, bands I and II become more intense with a density of acid sites belonging to bands I-IV of 1163 mmol.g -1 compared to 775 mmol.g -1 for HPSAPO-5/0.9Si. This observation points out the generation of additional weakly acid centers upon palladium impregnation. In contrast, band VI ascribed to strong acid sites exhibits a slight shift to lower temperature (from 540 °C to 400 °C) together with an increase of the density of acid sites from 32.7 to 56.1 mmol.g -1 , which can be attributed to additional Lewis acidity ascribed to PdO. This additional acid site density vanishes upon reduction (not shown).

Conclusion

A series of crystalline silicoaluminophosphate SAPO-5 catalysts with hierarchical porosity and variable silicon content were synthesized using a bottom-up method starting from presynthesized MCM-41, with the surfactant (CTAB) inside the mesopores. In particular, HPSAPO-5/0.9Si catalyst exhibited a remarkable increase of the catalytic activity compared to the microporous counterpart in the aldol condensation/crotonization reaction between furfural and methyl isobutyl ketone.

HPSAPO-5/0.9Si exhibited a lower density of Brønsted acid centers compared to microporous SAPO-5/0.9Si, as inferred by combined 1H MAS NMR and in situ FTIR spectroscopy using NH3 as probe molecule. However, HPSAPO-5/0.9Si displayed a higher mesoporous specific surface area combined with the presence of Si-OH moieties on the external surface. The interconnectivity of micro-and mesopores in HPSAPO-5/xSi catalysts is expected to favor the cooperativity between strong Brønsted acid sites located in the SAPO-5 channels, which are characterized by a band centered at 540 °C in the NH3-TPD profile, and external Si-OH groups, facilitating the activation of MIBK and further attack to FF.

By impregnating Pd, 4.8% Pd/HPSAPO-5/0.9Si afforded the synthesis of hydrogenated THFderivatives with 45% yield in the single-reactor tandem aldol condensation/crotonization reaction between furfural and methyl isobutyl ketone, followed by hydrogenation with molecular H2. The catalytic activity declined after the reaction mostly due formation of carbon deposits, but could be recovered after calcination. were optimized to achieve high conversion and selectivity, as well as a high level of reusability.

The nature, strength and accessibility of the acid sites on alumina were characterized by combining NH3-TPD, FT-IR spectroscopy using NH3 as probe molecule, and solid-state 1 H and 27 Al MAS NMR. The structural and morphological properties of the catalysts were investigated using X-ray diffraction (XRD) and HR-TEM. A Langmuir-Hinshelwood-type model was proposed for rationalizing the catalytic surface mechanism on Al2O3 for the aldol condensation reaction by contrasting experimental and fitted kinetic data.

Scheme 4.1 Two-reactor vs. single reactor tandem process for the synthesis of hydrogenated derivatives from FF.

Theory

Kinetic modeling

In collaboration with Dr, Z. Jiang (Solvay, RICL), a kinetic model based on Langmuir-Hinshelwood type kinetics was developed and contrasted to kinetic experiments to rationalize the influence of the operation variables on the catalytic performance for the aldol condensation/ crotonization of FF with MIBK. The model was further implemented in the reactor mass balances and integrated over the reactor volume. The mass balance an isothermal batch reactor can be expressed by Eqs 4.1 and 4.2

dc i dt = - c i V L dV L dt + ∑ ν i,j r j n 𝑟 j (4.1) 1 V L dV L dt = ∑ v ̅ i (∑ ν i,j r j n 𝑟 j ) n 𝑠 i (4.2)
where ci is the concentration of species i (i.e. FF, MIBK, ALD-OH-1, ALD-1), VL is the liquid volume in the reactor, ns and nr are the number of species and chemical reactions (ns = 4, nr = 1), respectively, υi,j is the stoichiometric coefficient of species i for reaction j, rj is the reaction rate of the chemical reaction j, and v ̅ i is the molar volume of species i.

Numerical approach and fittings

The kinetic model was incorporated into the mass balances (Eqs 4.1 and 4.2), resulting in a system of coupled ordinary differential equations (ODEs). The ODEs were solved using the Matlab ® functions ode15s with a relative tolerance of 0.1% for the local discretization errors.

The model parameters were fitted using a non-linear least-square method using a hybrid

where nexp is the number of experiments, nspe is the number of samples in the experiment, and y(e,f) is the output molar fraction vector measured under the input vector e,,f, which corresponds to the f th system input vector of the e th experiment, y(e,f, ), is the predicted output molar fraction vector, and y is the variance-covariance matrix for the experimental molar fractions.

The confidence interval of the fitted parameters, were used as descriptors to assess the quality of the kinetic model. The 95% confidence interval for the optimized parameters (CIj) was calculated using Eq 4.4

CI j = ±t 1-0.05/2√ cov jj = 1.96 √ cov jj (4.4)
where t1-0.05/2 refers to the Student-t (1.96 for 95% confidence interval), and covjj are the diagonal elements of the variance-covariance matrix (Eq 4.5)

cov = SSE N Y -N p (J T × J) -1 (4.5)
where J is the Jacobian matrix and (Ny -Np) is the number of degrees of freedom, Ny and Np referring to the number of experiments and parameters, respectively.

Results and Discussion

Aldol condensation/crotonization of FF with MIBK over alumina catalysts

In a first step, we investigated the catalytic performance of different silica-aluminas and aluminas for the aldol condensation/crotonization reaction of FF and MIBK (Table 4.1). All the catalysts exhibit high selectivity to (E)-1-(furan-2-yl)-5-methylhex-1-en-3-one (ALD-1) (Scheme 4.2) with a yield up to 89% for alumina spheres (1.8/210) at 180 °C after 3 h (entries 1-8). The calcination temperature does not affect the selectivity of alumina, but the FF conversion declines slightly for a calcination temperature higher than 400 °C (entries 5-7). This catalytic performance opposes that observed on hydrated ZnO, Al-SBA-15 (Si/Al = 76) and hierarchical HPSAPO-5 with only Brønsted acid centers (entries 9-11) (see ref [68]), where (E)-and (Z)-3-(furan-2-ylmethylene)-4-methyl-pentan-2-one (ALD-2a and ALD-2b, respectively) are also formed as by-products with a selectivity <15%. This contrasting behavior suggests a key role of Lewis acidity in alumina on the selectivity towards ALD-1. Besides, in all cases, no by-products issued from the aldol self-condensation/crotonization of MIBK are observed.

Kinetic profiles for FF + MIBK aldolization/crotonization over Al2O3

The kinetic profiles were measured on the Al2O3 spheres in the temperature range 120-180 °C. In line with our previous study in chapter 3, 1 the initial MIBK/FF and FF/cat weight ratios were set to 19 (5.2 wt% FF) and 5.0, respectively, to promote ALD-1 formation and keep a high carbon balance. Clearly, regardless of the reaction conditions, the ALD-1 selectivity displays a plateau (>90%) at a FF conversion higher than 20%. The ALD-1-OH selectivity reaches 50% at very low FF conversion (5%), but decreases sharply to 10% while increasing the FF conversion from 5% to 20%. Beyond this value, the ALD-1-OH selectivity exhibits a sustained decline until 5% at full FF conversion. No evidence of formation of ALD-2a,b is observed irrespective of the FF conversion.

Overall, this body of results points out that Al2O3 (spheres) is an active and selective catalyst for preparing ALD-1 from the aldol condensation/crotonization reaction of FF with MIBK. The main by-product is ALD-OH-1, but the yield is lower than 5% at a FF conversion higher than 20%.

Reaction rates and activation energies for ALD-1 synthesis

The initial reaction rates of ALD-1 formation over Al2O3 (spheres) were measured from the time-evolution of the ALD-1 yield using rational polynomial functions for fitting the kinetic curves (see plots and fittings in Figure 4.2). The highest rate (110 mmol.g -1 .h -1 ) was measured at 180 °C for a MIBK/FF ratio of 19.0 (w/w) (Figure 4.4), which is comparable to the value measured on hierarchical SAPO-5 (see chapter 3). A decrease of the MIBK/FF ratio (i.e. higher FF concentration) results in a lower reaction rate, suggesting a possible inhibition of adsorbed FF.

We also measured the apparent activation energy for ALD-1 formation by representing the Arrhenius plots (Figure 4.4). The apparent activation energy is about 38 kJ/mol, which is much lower than the value measured on microporous and hierarchical SAPO-5 catalysts (~100 kJ/mol).

This observation combined with the much higher ALD-1 selectivity for Al2O3 suggests a different catalytic mechanism for aldol condensation/crotonization of FF with MIBK when comparing Al2O3 and SAPO-5 catalysts.

Catalytic mechanism for ALD-1 synthesis

The high activity and selectivity of Al2O3 for the aldol condensation/crotonization reaction of FF with MIBK prompted us to study in more detail the catalytic mechanism. To this aim, we screened a series of Langmuir-Hinshelwood kinetic models [2][3][4] to rationalize the experimental trends, with emphasis on the inhibition effect of FF on the reaction rate. In line with the experimental observations, the following assumptions were considered: The kinetic expressions for r3 and r4 combined with the expressions for adsorption equilibrium (steps 1, 2, 5-7) were implemented to mass balances in Eq 6-7, the differential equations were solved numerically, and the parameters of the kinetic model (i.e. Ea3, Ea4, A3, A4, K1, K2, K5, K6, K7) were fitted by comparison of experimental and predicted trends.

Catalyst recyclability and reuse

The catalyst recyclability and reuse was assessed in six consecutive catalytic runs for the aldol condensation/crotonization reaction of FF with MIBK at 180 °C for 3 h (Figure 4.7). The 1 st run results in a 90% FF conversion and an ALD-1 selectivity of 98%. After the reaction, the catalyst was separated, washed with ethanol and reused in the next run. The FF conversion declined slightly to 82%, whereas the ALD-1 selectivity kept unchanged. The FF conversion and ALD-1 selectivity were almost constant for the 3rd-5th runs, revealing high catalyst stability during the reaction even if some oligomers might generate. Further catalyst calcination at 400 °C under airflow after the 5th run resulted in a total recovery of the initial FF conversion (82%).

Single-reactor tandem process

The high catalytic activity and selectivity of Al2O3 (spheres) prompted us to implement this catalyst in a single reactor for conducting the tandem aldol condensation/crotonization reaction [7][8] of FF with MIBK, followed by hydrogenation. To this aim, we prepared three metal-supported catalysts, i.e. Pd/Al2O3, Pt/Al2O3 and Ru/Al2O3, at 5 wt% nominal metal loading, by impregnating the corresponding metal precursor over Al2O3 spheres in acetone. First, we investigated ALD-1 hydrogenation over at 180 °C for 3 h and 2.0 MPa H2 pressure (Table 4.4

entries 1-3). Two reaction products are obtained, that is the furan-derivative issued from the hydrogenation of the conjugated C=C bond (F-ALD-1), and the sequential THF-derivative issued from the hydrogenation of the furan ring in F-ALD-1 (THF-ALD-1). Neither furan-nor THF-derivatives issued from the hydrogenation of the C=O bond were detected. The highest selectivity is obtained over Pd/Al2O3 (92%), and accordingly this catalyst was used hereinafter for engineering a single reactor process.

We then investigated the catalytic performance of unreduced PdO/Al2O3 catalyst in the aldol condensation/crotonization reaction of FF with MIBK at 180 °C and MIBK/FF and FF/cat ratios With these results in hand, we engineered a single-reactor tandem process [9][10] over Pd/Al2O3 (Figure 4.9, Table 4.3 entries 7-8). In this process, the aldol condensation/crotonization reaction was first conducted at 180 °C for 3 h and, after this period, H2 (2.0 MPa) was introduced to the reactor, and the hydrogenation reaction was further carried out at 180 °C for 6 h. A maximum THF-ALD-1 yield about 86% is achieved 180 °C after 6 h for 2.0 MPa H2 pressure. Besides, the fully hydrogenated product of ALD-3 (i.e. THF-ALD-3) is also observed at low yield (<10%).

For comparison, when the hydrogenation reaction was conducted at 180 °C for 6 h and 2.0 MPa H2 after the aldol condensation/crotonization reaction, the F-ALD-1 and THF-ALD-1 yields decline drastically at the expense of ALD-3, the latter reaching a maximum yield of 32% (Table 4.4, entry 9). Besides, the yield of THF-ALD-3 increases slightly to 12%.

Understanding the structure and acid properties of Pd/Al2O3

The catalytic results presented above point out an enhanced activity and selectivity of Pd/Al2O3 (spheres) for the synthesis of THF-ALD-1 in a single-reactor tandem aldol condensation/ crotonizationhydrogenation process. In light of these results, we explored the structural and textural properties of the catalyst, as well as the acidity and acid strength.

Structural and textural properties of Pd/Al2O3 and Al2O3

The The chemical environment of Al atoms in Al2O3 (crushed spheres) was probed by 27 Al MAS NMR spectroscopy (Figure 4.11) [11][12] . In both cases, near resonance bands are observed centered at 10 ppm and 68 ppm, which can be unambiguously ascribed to octahedrally and tetrahedrally However, three new bands appear at 3500 cm -1 , 3370 cm -1 and 3308 cm -1 that can be ascribed to adsorbed NH3 in Lewis acid centers, preferentially in contact with surface oxygen atoms by H-bonding. These bands are neat for Al2O3 and Pd/Al2O3, but vanish for PdO/Al2O3, suggesting a lower degree of H-bonding on the latter sample. For all samples, three characteristic bands appear in the low-frequency region at 1695 cm -1 , 1485 cm -1 and 1397 cm -1 that are distinctive of adsorbed NH 4+ on Al-OH groups (Figure 4.14B). Four additional bands are also visible in the same region centered at 1620 cm -1 and 1245 cm -1 that can be assigned to NH3 adsorbed on Lewis acid centers. These species are retained after outgassing at room temperature for the three samples, pointing out preferential interaction with surface oxygen atoms by H-bonding. Further outgassing by increasing the temperature results in a decline of all the bands for all the samples (Figure 4.14C, Figure 4.15). Finally, a band is also visible at 1100 cm -1 upon contact of the samples with NH3 that be attributed to physisorbed NH3 on Al-OH groups, which vanishes after outgassing at room temperature.

NH3-TPD

NH3-TPD measurements were also performed on the crushed Al2O3 (fresh) to gain a better insight into the density distribution and strength of acid sites (Figure 4.16, Table 4.5). To this aim, two samples were used grinded at 100 µm and 200 µm. The NH3-TPD profile of both samples exhibits three main regions: (1) a low-temperature region (100-350 °C), (2) a medium temperature region (400-650 °C) and (3) a high-temperature region (700-850 °C). The first region can be deconvoluted into three/four bands that can be attributed to weakly acid sites. The second region is constituted by two/three bands that is indicative of medium-strong acid sites and that, on the guidance of the FTIR spectra using NH3 as molecular probe, can be ascribed to Lewis acid sites. Finally, the third region comprises two bands attributed to strong acid sites, and that are also indicative of Lewis acid sites. By integrating the NH3-TPD profiles, both samples display a similar density of acid sites for the three zones. In particular, regions 2 and 3 show densities of Lewis acid sites about 202/135 µmol•g -1 and 78/91 µmol•g -1 , respectively.

Conclusion

Pd/Al2O3 was found to be a highly efficient catalyst for conducting the single-reactor tandem aldol condensation/crotonization reaction of furfural with MIBK, followed by hydrogenation. Unlike HPSAPO-5/0.9Si, Al2O3 provides high activity and selectivity towards the aldol intermediate, even at low temperature (120 °C), with excellent carbon balances. Noteworthy, the intermediate ALD-2a,b is not observed. Lewis acidity, which was characterized by NH3-FTIR and NH3-TPD, offers more selectivity in the aldol condensation reaction compared to catalysts enriched with Brønsted acid sites. By impregnating Pd, 5%Pd/Al2O3 afforded the synthesis of THF-derivatives with ca. 90% yield in the single-reactor tandem process.

CHAPTER 5: Single-reactor tandem benzoin condensation -Reductive amination over bimsilica + Ru/Al 2 O 3

Introduction

Amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of NH3, wherein one or more hydrogen atoms are replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. There are few reports for amines synthesis from biomass because of lack of suitable methods and catalytic materials.

An upgrading process of FF comprises C-C bond coupling and amination, increasing the molecular diversity of amines. In this view, it is desirable to design multistep processes with high degree of intensification and high activity/selectivity to the desired amines. One-pot reactions have been reported for the synthesis of furan-and THF-derived amines by the one-pot aldol condensation reaction of FF with ketones, followed by reductive amination with NH3 and H2, over combined Amberlyst-26 (A26) and Ru/C or Pd/Al2O3 catalysts. [1][2] For example, the reaction of FF with MIBK afforded the in situ formation of a carbonyl-conjugated vinylfuran intermediate over A26, which was further subjected to reductive amination with NH3 and H2 over 5% Ru/C and 5% Pd/Al2O3, to give the corresponding furan-and THF-derived amines, respectively. In particular, using Pd/Al2O3 as catalyst and H2 (2.0 MPa) as reductant, 98% overall yield of THF-amine was achieved at 120 °C after 20 h. 2 Benzoin condensation is a C-C coupling reaction that can promote self-condensation of HMF towards 5,5'-dihydroxymethylfuroin (DHMF) derivatives using a silica-or polymer-supported benzimidazole (bim) catalysts with one/two long-chain aliphatic substituents at the N-atoms in the imidazole ring (Scheme 5.1A). [3][4] In the presence of a base, supported bim catalysts can catalyze FF self-condensation into furoin with more than 96% yield over the three consecutive runs. 3 Furoin can be easily oxidized to the diketone product (i.e. furil) under benzoin condensation conditions (i.e. presence of bim and base) at room temperature under air. 5

Scheme 5.1 Two-reactor vs. single reactor tandem process for the synthesis of hydrogenated derivatives from furfural.

Sequential reactions have been developed encompassing furoin derivatives targeting the synthesis of branched alkanes, targeting the synthesis of THF-derivatives and C10-C12 diesel fuels. [3][4] As a way forward, herein we report the synthesis of alcohol-amines and diamines from furoin and furil derivatives over Ru/Al2O3. Such products show potential industrial interest as ligands for homogeneous catalysts. We further engineered a single-reactor tandem process by combining silica-supported bim (catalyst 1) and Ru/Al2O3 (catalyst 2) to access alcohol-amines and diamines starting from FF (Scheme 5.1B). This process encompasses two reactions, namely the benzoin self-condensation of FF, and the reductive amination of the furoin intermediate with NH3 and H2. DMF and THF were considered as solvents of choice for the single-rector tandem process, since both are effective for the benzoin condensation reaction of FF over silicasupported bim. 3

Catalyst-free reductive amination of furoin/furil

The chemistry of furoin is complex and a panel of cyclic products can be generated upon exposure of furoin to NH3 or ammonium containing reactants. 6 To elucidate the nature of the different possible compounds generated, we carried out a preliminary study under reaction conditions that are compatible to those used for preparing alcohol-amines and diamines from furoin and furil.

Upon exposure of furoin or furil to NH3 using DMF as solvent, both reagents begin to react slowly at a temperature higher than 80 °C for 2 h, and the reaction can even proceed further after cooling down to room temperature. The solution becomes gradually darker along the reaction, and turns completely black after standing overnight at room temperature. This observation can be explained by the formation of large oligomers, which can hardly be separated and identified by GC or GC-MS. Comprehensive analysis of the reaction system clearly reflects the presence of cyclic compounds such as 2,3,5,6-tetra(furan-2-yl)pyrazine, 2,2′-bipyridine-3,3′-diol, 2-amino-1,2-di(furan-2-yl)ethan-1-ol and 1,2-di(furan-2-yl)ethane-1,2-diamine (Scheme 5.2), which are in line with an earlier report. 6 This observation prompted us to identify in more detail the different products and conditions favoring their formation. In the remainder of this chapter, all these cyclic compounds will be referred as 'by-products', which might appear during the amination of furoin and furil in the presence of a metal-supported catalyst. For clarity, 2-amino-1,2-di(furan-2-yl)ethan-1-ol and 1,2-di(furan-2-yl)ethane-1,2-diamine, which are target products of this project, have been labeled hereinafter as 'alcohol-amine' and 'diamine'.

Scheme 5.2 Main cyclic compounds (i.e. "by-products") and target products (i.e. alcohol-amine and diamine) generated upon exposure of furoin / furil with NH3

Catalyst-free furoin amination

Furoin was first reacted with both NH3 and ammonium acetate in DMF, generating 2,3,5,6tetra(furan-2-yl)pyrazine for a temperature higher than 130 °C (Scheme 5.3). The yield of 2,3,5,6-tetra(furan-2-yl)pyrazine was slightly better starting from ammonium acetate, i.e. 15% vs. 18% at full furoin conversion, at 140 °C for 4 h, using 0.06 g furoin, 6 mL DMF and 1 g ammonium acetate (or 0.22 g NH3). The slightly higher yield combined with the easier operation of ammonium acetate compared to NH3 prompted us to use the former reagent in the forthcoming experiments.

Scheme 5.3 Catalyst-free furoin amination reaction with NH3 and ammonium acetate.

We explored in detail the effect of the operation parameters on the yield of 2,3,5,6-tetra(furan-2yl)pyrazine at 140 °C for 4 h. Among the different solvents tested, DMF, THF and 2methylterahydrofuran exhibited the highest yield with a value in the range 18-21% (Table 5.1). The reaction could not proceed in water due to the poor solubility of furoin. We further studied the effect of the temperature using THF as solvent and keeping the otherwise reaction conditions unchanged (Table 5.2). The yield of 2,3,5,6-tetra(furan-2-yl)pyrazine displayed a sustained increase with the temperature in the range 130-160 °C with a maximum value at about 30% at full furoin conversion. A further increase of temperature to 170 °C promoted the formation of other by-products (not shown), limiting the yield of 2,3,5,6-tetra(furan-2-yl)pyrazine to 30%. Finally, we explored the effect of the mass of ammonium acetate at 160 °C in THF (Table 5.

3).

The yield of 2,3,5,6-tetra(furan-2-yl)pyrazine was promoted for a mass of ammonium acetate above 0. We also explored the potential effect of water on the reaction. To this aim, we conducted a series of tests at 160 °C for 3 h using wet and dry THF as solvent (Table 5.4). Water was found to slightly affect the yield of 2,3,5,6-tetra(furan-2-yl)pyrazine. The best yield (44%) was achieved using undried THF and dry ammonium acetate, as well as dry THF and ammonium acetate (entries 2 and 3). The addition of water might hinder the first step of the reaction mechanism (Scheme 5.4). a Reaction condition: 0.03 g furoin, 0.1 g ammonium acetate, 3 mL THF, 3 h, 160 °C

Catalyst-free furil amination

In a next step, we investigated the formation of 2,2′-bipyridine-3,3′-diol. This by-product can be generated upon exposure of furil to NH3 at a temperature higher than 160 °C (Scheme 5.5). The reaction proceeds fast in the presence of DMF, achieving a yield about 22% at 170 °C for 4 h at full furoin conversion (Table 5.5). As the temperature rises from 160 °C to 200 °C, the yield of 2,2′-bipyridine-3,3′-diol increases first until 180 °C with a maximum value of 36%, and decreases further to 24% at 200 °C), which can be attributed to partial product or substrate decomposition at higher temperature. Scheme 5.5 Catalyst-free furil amination with NH3.

The formation of 2,2′-bipyridine-3,3′-diol can be rationalized on the basis of the mechanism depicted in Scheme 5.6. Furil reacts fast with NH3 generating a diimine intermediate. This intermediate is very unstable and may be easily activated by a proton in solution, favoring ring activation and intramolecular rearrangement leading to pyridine rings. The diimine intermediate can also suffer from reversible polymerization, as observed for other sugar derivatives upon exposure to NH3 such as diformylfuran. 7 a Reaction condition: 0.03 g furil, 0.8 g NH3, 3 mL DMF, 3 h Scheme 5.6 Possible mechanism for by-2,2′-bipyridine-3,3′-diol formation from the reaction of furil with NH3.

To prevent polymerization of the diimine intermediate, we explored the effect of short-chain aliphatic amines as additives. The rationale behind this idea relies on the fast formation of secondary imines with the amine additive, and then slowly react with NH3 to generate the primary imine. In line with previous reports, 8 we selected n-butylamine as polymerization inhibitor at variable n-butylamine : furil molar ratios (Table 5.7). The addition of n-butylamine exerted a positive effect of the yield of 2,2′-bipyridine-3,3′-diol, but this was moderate and only observed in the presence of large excess of n-butylamine (more than 4 equiv).

Furion and furil amination in the presence catalyst

Furoin amination

With the results above, we investigated the reductive amination of furoin with NH3 and H2 in an autoclave. The tests were first conducted at 80 °C and 100 °C both either in the absence of catalyst, or in the presence of commercial alumina-supported Pd and Ru catalysts (5 wt% metal), using DMF as solvent (Figure 5.1-5.2). The use of Pd/Al2O3 as catalyst resulted in a broad distribution of products, especially at lower temperature (Figure 5.1). However, in the case of Ru/Al2O3, the GC chromatograms clearly showed the presence of three main peaks both at 80 °C (Figure 5.1) and 100 °C (Figure 5.2) that can be assigned to the keto-imine intermediate (18.2 min), keto-amine or alcohol-imine intermediate (17.6 min), and alcohol-amine product (17.2 min), as inferred by GC-MS and 1 H NMR (presence of two characteristic doublets centered at 4.7 ppm and 5.4 ppm, Figure 5.3). Note however that the last peak is complex, including different products and intermediates besides the alcohol-amine product (Scheme 5.7). The alcohol-amine product was obtained as main product until 140 °C with no evidence of formation of diamine. The keto-amine and alcohol-imine intermediates, which can rapidly interconvert by tautomerization (see Scheme 5.4), could be hardly distinguished by GC-MS (an identical peak at m/z = 189 is observed in both cases), and accordingly could coexist together in the reaction system. Finally, different by-products were clearly observed at higher temperature that can be attribute to cyclic by-products, in particular 2,3,5,6-tetra(furan-2-yl)pyrazine, as well as oligomers (Scheme 5.7). We quantified the yield of the alcohol-amine product achieved over a series of 5%M/Al2O3 catalysts (M = Pd, Ru, Rh, Pt) at 160 °C for 2 h under 2.0 MPa H2 (Table 5.8). 5%Pd/Al2O3, 5%Rh/Al2O3 and 5%Pt/Al2O3 (entries 1-3) exhibit a poor alcohol-amine yield (up to 28%), whereas the yield over 5% Ru/Al2O3 shows the highest value (47%) (entry 7). The yield was found to be very sensitive to the Ru loading over Al2O3 (entries 4-6), but was not affected by the type the support (i.e. silica, carbon) at the same Ru loading (5 wt%) (entries 8-9). Overall, 5% Ru/Al2O3 exhibits the best performance towards the alcohol-amine. HR-TEM characterization on this catalysts revealed an average size of Ru nanoparticles about 2.2 nm measured by pulsed CO chemisorption. The catalyst could be completely reduced at 150 °C according to the H2-TPR profiles (not shown).

In light of the promising performance of Ru/Al2O3, we assessed the effect of the temperature and H2 pressure over this catalyst in the reductive amination of furoin. The alcohol-amine yield increases with the temperature in the range 80-140 °C at constant H2 pressure (4.0 MPa) with a maximum value of 47%, and keeps almost constant beyond this value (Figure 5.4). The yield also increases drastically with the H2 pressure in the range 0.5-4.0 MPa at constant temperature (160 °C), and keeps constant beyond this value (Figure 5.5).

The preliminary study shown in Figure 5.1-5.2 clearly points out the formation of keto-imine and keto-amine / alcohol-imine intermediates in the reductive amination of furoin with NH3 and H2 over Ru/Al2O3 at low temperature (80 °C and 100 °C). Interestingly, at higher temperature, the keto-imine intermediate is not observed, but the keto-amine / alcohol-imine intermediates are still observed at short reaction times (Figure 5.6), which can be hydrogenated to generate the alcohol-amine product (Scheme 5.8).

Furil amination

We further studied the reductive amination of furil with NH3 and H2 over Ru/Al2O3. The alcohol-amine intermediate is preferentially generated together with cyclic by-products and oligomers (Table 5.9). To favor the formation of the diamine, we attempted the use of additives, including n-butylamine, A-type zeolite to capture water, titanium isopropoxide, 9 and 2,2,2trifluoroethanol. In particular, titanium isopropoxide was found to exert a remarkable effect of the catalytic performance, favoring the formation of diamine. A possible stabilization mechanism driven by titanium isopropoxide in depicted in Scheme 5.9. 9 To assess the stability of bim-silica, the catalyst was extracted from the reaction system after the [10][11] , washed with HCl and DMF and retested in the benzoin condensation of FF. The catalyst was found to be active (see GC chromatogram in Figure 5.9), pointing out that the bim moiety was not detached from the support during the tandem reaction. TG analyses on the fresh and spent bim-silica catalysts confirmed the stability of bim-silica during the tandem reaction (Figure 5.10).

We also surveyed the stability of the bim-silica + Ru/Al2O3 catalysts after three consecutive runs for the single-reactor tandem reaction (Table 5.10). The catalyst mixture performed well in the first two runs with only moderate decline of the alcohol-amine yield (from 42% to 38%). However, the yield was only 4% after the 3 rd run, most likely due to inactivation of the bim moieties after subjecting the bim-silica catalyst to successive alkaline / acid conditions.

Conclusions

Along this chapter, we studied the reductive amination of self-condensation products from furfural (i.e. furoin and furil) with NH3 and H2. Without catalyst, furoin reacts with NH3 and ammonium acetate, generating 2,3,5,6-tetra(furan-2-yl)pyrazine with 39% yield. Furil also reacts with NH3 and ammonium acetate, generating 2,2'-bipyridine-3,3'-diol with 44% yield. When Ru/Al2O3 and H2 are added to the reaction system, furoin and furil easily react to alcohol-amine with 47% and 34% yield, respectively. The diamine product can be generated from furil in the presence of titanium isopropoxide. We also attempted a single-reaction tandem reaction encompassing the benzoin condensation of furfural to furoin, followed by the reductive amination of the as-generated furoin to the alcohol-amine. The combination of bim-silica and Ru/Al2O3 afforded and overall yield of 42%.

Along this PhD thesis, we have presented different examples of single-reactor (two-step) tandem reactions encompassing a first C-C coupling step and a subsequent hydrogenation or reductive amination step. Our work focused on bio-based furfural, a cheap and industrially available platform molecule issued from lignocellulose. Two examples have been developed: (1) aldol condensation/crotonization of furfural with MIBK, followed by hydrogenation with molecular H2, and (2) benzoin condensation of furfural, followed by reductive amination with NH3 and H2.

In these examples, the reaction intermediate (i.e. aldol product or furoin / furil) is not isolated.

In chapters 3 and 4, we have developed two different types of bifunctional catalysts for the aldol condensation/crotonization of furfural with MIBK, followed by hydrogenation. First, in chapter 3, we synthesized a series of crystalline silico-aluminophosphate SAPO-5 with hierarchical porosity and variable silicon content using a bottom-up method starting from pre-synthesized MCM-41, with the surfactant (CTAB) inside the mesopores. A HPSAPO-5/0.9Si (0.9 wt% Si) catalyst enriched with Brønsted acid sites of medium strength exhibited a remarkable increase of the catalytic activity compared to the microporous counterpart in the aldol condensation/ crotonization reaction between furfural and MIBK. However, the carbon balance was poor (about 30%). The impregnation of HPSAPO-5/0.9Si with Pd afforded the synthesis of THFderivatives with 45% yield in the single-reactor process. The catalytic activity declined after the reaction mostly due formation of carbon deposits, but could be recovered after calcination.

In chapter 4, we prepared a bifunctional Pd/Al2O3 catalyst enriched with Lewis acid centers for the single-reactor tandem aldol condensation/crotonization reaction of furfural with MIBK, followed by hydrogenation. Al2O3 provides higher activity and selectivity towards the aldol intermediate compared to HPSAPO-5/0.9Si, even at low temperature (120 °C), with excellent carbon balance. By impregnating Pd, 5%Pd/Al2O3 afforded the synthesis of THF-derivatives with ca. 90% yield in the single-reactor tandem process.

Finally, we studied the reductive amination of self-condensation products from furfural (i.e. furoin and furil) with NH3 and H2. The reductive amination of furoin and furil with NH3 and H2 over 5%Ru/Al2O3 generates the alcohol-amine product with 47% and 34% yield, respectively. The diamine product can only be generated from furil using titanium isopropoxide as additive.

The single-reaction tandem reaction encompassing the benzoin condensation of furfural to furoin, followed by the reductive amination of the as-generated furoin to the alcohol-amine was conducted over a mixture of benzimidazole-support silica and Ru/Al2O3, resulting in an overall yield of 42%.

Overall, the results presented in this study confirm the good credentials of single-reactor tandem processes for conducting multistep reactions starting from furfural. Techno-economic and Life Cycle Assessment studies carried out under the Multi2hycat program point out a benefit about 10% in terms of unit cost and environmental footprint for single-reactor tandem processes compared to two-reactor processes.

Figure 1 . 1

 11 Figure 1.1 Overview of the prominent fractions of the biomass feedstock. 10

Figure 1 . 2

 12 Figure 1.2 Chemical structure of xylan.

Figure 1 . 3

 13 Figure 1.3 Chemical structure of lignin.

Figure 1 . 4

 14 Figure 1.4 Structure of monolignols, primary building blocks of lignin.

Figure 1 . 7

 17 Figure 1.7 Outlined synthesis of [BI] + /MMT.

Figure 1 . 8

 18 Figure 1.8 Representative hydrogenation molecule product map.

Figure 1 . 9

 19 Figure 1.9 Diagram of representative amines derived from platform molecules with emphasis on furfural.

Scheme 1 . 8

 18 Scheme 1.8 Cascade reaction of aldol condensation and reductive amination.

Figure 1 . 10

 110 Figure 1.10 Catalytic conversion of furfural to value-added chemicals (graphical abstract).

Figure 2 . 1

 21 Figure 2.1 Typical GC method used for the analysis of product mixtures.

Figure 2 . 2

 22 Figure 2.2 Time-evolution of FF conversion, ALD-1 selectivity and carbon balance in the aldol condensation/crotonization reaction of FF with MIBK over silica-alumina (Siral 5 HPV) in Taiatsu (open symbols) and Top Industrie (filled symbols) reactors. Reaction conditions: MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w).

Scheme 3 . 1

 31 Scheme 3.1 Main products obtained in the aldol condensation/crotonization reaction of FF with MIBK over acid zeolites.

. 1 ). Figure 3 . 2

 132 plots the curves obtained for the FF conversion and the ALD-1 and ALD-2a, b selectivites, whereas Figure 3.3 plots the corresponding selectivity conversion curves. Additional kinetic curves at lower temperature (120 °C) can be found in the Figure 3.4.

Figure 3 . 1

 31 Figure 3.1 Time-evolution of the (A) FF conversion, and (B) ALD-1 and ALD-2a,b selectivities, in the aldol condensation/crotonization reaction of FF with MIBK over SAPO-5/0.6Si as a function of the initial FF : MIBK molar ratio at constant FF-to-catalyst weight ratio (5.0). Reaction conditions: 0.2-4.0 g FF, 2.0-3.9 g MIBK, 20-400 mg catalyst, 160 °C. The straight and dashed curves are a guide to the eye.

Figure 3 . 2

 32 Figure 3.2 Time-evolution of the FF conversion, ALD-1 and ALD-2a,b selectivity in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/xSi (filled symbols, straight lines) and SAPO-5/xSi (empty symbols, dotted lines) at different temperature: (A) x = 0.9, (B) x = 0.6 and (C) x = 0.3. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 3 h, (A1-C1) 140 °C, (A2-C2), 160 °C and (A3-C3) 180 °C.

Figure 3 . 3

 33 Figure 3.3 Selectivity-conversion plots in the aldol condensation/crotonization reaction of FF with MIBK over (A1-C1) HPSAPO-5/xSi and (A2-C2) SAPO-5/xSi: (A) x=0.9, (B) x=0.6 and (C) x=0.3. Reaction conditions as in Figure 3.2.

Figure 3 . 4

 34 Figure 3.4 Time-evolution of the FF conversion, ALD-1 and ALD-2a,b selectivity in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/xSi and SAPO-5/xSi: (A) x=0.9, (B) x=0.6 and (C) x=0.3. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 120 °C, 3 h. The straight and dashed curves are a guide to the eye.

Figure 3 . 5

 35 Figure 3.5 Time-evolution of the FF and FF+MIBK carbon balances in the aldol condensation/ crotonization reaction of FF with MIBK over HPSAPO-5/xSi and SAPO-5/xSi: (A) x=0.9, (B) x=0.6 and (C) x=0.3. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 3 h, (A1-C1) 140 °C, (A2-C2), 160 °C and (A3-C3) 180 °C. The straight and dashed curves are a guide to the eye.

Figure 3 . 6

 36 Figure 3.6 Time-evolution of the ALD-1 yield in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/xSi and SAPO-5/xSi: (A1, A2) x=0.9, (B1, B2) x=0.6 and (C1, C2) x=0.3. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 140-180 °C, 3 h.

Figure 3 . 7

 37 Figure 3.7 Time-evolution of the ALD-1 yield in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/xSi and SAPO-5/xSi: (A) x=0.9, (B) x=0.6 and (C) x=0.3. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 120 °C, 3 h.

Figure 3 . 8

 38 Figure 3.8 Parity plots for the fitted and experimental ALD-1 yield in the aldol condensation/ crotonization reaction of FF with MIBK over HPSAPO-5/xSi (left) and SAPO-5/xSi (right) in the temperature range 120-180 °C. Other reaction conditions as in Figure 3.4.

Figure 3 . 9 3 . 1 .

 3931 Figure 3.9 Evolution of the initial reaction rate of ALD-1 formation as a function of the temperature and the Si loading in the aldol condensation/crotonization reaction of FF with MIBK over (A) HPSAPO-5/xSi and (B) SAPO-5/xSi, with x=0.3, 0.6, 0.9. Reaction conditions as in Figure 3.1. The curves are a guide to the eye.

Figure 3 . 10

 310 Figure 3.10 Arrhenius plots for the reaction rate of ALD-1 formation in the aldol condensation/ crotonization reaction of FF with MIBK over HPSAPO-5/xSi (filled symbols) and SAPO-5/xSi (empty symbol), with x=0.3, 0.6, 0.9. Reaction conditions as in Figure 3.2.

Figure 3 . 11

 311 Figure 3.11 Catalyst recyclability and reuse for HPSAPO-5/0.9Si after washing with ethanol and calcination at 400 °C under airflow. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 160 °C, 3 h.

. 13 ,Scheme 3 . 2 .

 1332 Scheme 3.2. Main products obtained in the single-reactor tandem aldol condensation/ crotonization reaction of FF with MIBK, followed by hydrogenation.

Figure 3 .

 3 Figure3.12 Time-evolution of (A) FF conversion, and (B) ALD-1 and ALD-2a,b selectivity at 160 °C in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/0.9Si, 0.3%Pd/HPSAPO-5/0.9Si and 4.8%Pd/HPSAPO-5/0.9Si. Reaction conditions as in Figure1. The curves are a guide to the eye.

Figure 3 .

 3 Figure 3.13 Time-evolution of the ALD-1 yield in the aldol condensation/crotonization reaction of FF with MIBK over HPSAPO-5/xSi, 0.3Pd%/HPSAPO-5/xSi and 4.8%Pd/HPSAPO-5/xSi. Reaction conditions as in Figure 3.4.

Figure 3 . 14

 314 Figure 3.14 Parity plots for the fitted and experimental ALD-1 yield in the aldol condensation/ crotonization reaction of FF with MIBK over HPSAPO-5/xSi, 0.3Pd%/HPSAPO-5/xSi and 4.8%Pd/HPSAPO-5/xSi. Reaction conditions as in Figure 3.4.

Figure 3 .

 3 Figure 3.15 PXRD patterns of (A) microporous SAPO-5/xSi and (B) HPSAPO-5/xSi, with x=0.30 (black patterns), x=0.6 (red patterns) and x=0.9 (blue patterns).

  Si MAS NMR spectra reveal multiple signals in the range -90 to -110 ppm due to tetrahedrally-coordinated framework Si atoms bonded to four, three, two, one and zero Al atoms (Figure 3.19).

Figure 3 .

 3 Figure 3.16 (A) N2 adsorption/desorption isotherms at 77 K on HPSAPO-5/xSi with x=0.30 (black curve), x=0.6 (red curve) and x=0.9 (blue curve), and (B) pore size distribution of the different samples in the mesopore range. The orange curve in panel B refers to the pore size distribution in the micropore range of HPSAPO-5/0.6Si obtained by Ar adsorption at 77 K.

Figure 3 .

 3 Figure 3.17 Solid-state 27 Al NMR MAS spectra on HPSAPO-5/xSi (A) and microporous SAPO-5/xSi (B) catalysts, with x=0.3 (black curves), 0.6 (red curves) and 0.9 (blue curves).

Figure 3 .

 3 Figure 3.18 Solid-state 31 P NMR MAS spectra on HPSAPO-5/xSi (A) and microporous SAPO-5/xSi (B) catalysts, with x=0.3 (black curves), 0.6 (red curves) and 0.9 (blue curves).

Figure 3 .

 3 Figure 3.19 Solid-state 29 Si NMR MAS spectra on HPSAPO-5/xSi (A) and microporous SAPO-5/xSi (B) catalysts, with x=0.3 (black curves), 0.6 (red curves) and 0.9 (blue curves).

Figure 3 .

 3 Figure 3.20 1 H MAS NMR spectra of (A) HPSAPO-5/xSi and (B) microporous SAPO-5/xSi with x = 0.30 (black curvess), x = 0.6 (red curves) and x = 0.9 (blue curves).

Figure 3 .

 3 Figure 3.21 FTIR spectra of calcined HPSAPO-5/xSi (A) and microporous SAPO-5/xSi (B) catalysts, with x=0.3 (curves a), 0.6 (curves b) and 0.9 (curves c).

Figure 3 .

 3 Figure 3.22 plots the differential FTIR spectra of NH3 and 2,6-dTBP adsorbed on HPSAPO-5/xSi catalysts in the low frequency zone, where the signals of protonated species are observed. For comparison, NH3 was also probed on microporous SAPO-5/xSi to compare the amount of Brønsted acid sites in microporous and hierarchical catalysts (Table 3.6, Figure 3.23). NH3 adsorption induces proton transfer from the Brønsted acid sites to NH3, resulting in the formation of NH 4+ species that can be visualized in the band appearing at 1450 cm -1 (bending mode) (Figure 3.22A). The density of accessible Brønsted acid sites (N) titrated by NH3 is similar for microporous SAPO-5/0.3Si (46.80 µ•mol•g -1 ) and SAPO-5/0.6Si (44.08 µ•mol•g -1), whereas it shows a higher value for SAPO-5/0.9Si (52.30 µ•mol•g -1 ) (Table3.7). This observation points out a higher degree of Si incorporation in the SAPO-5 framework at the highest Si loading (x = 0.9). In the case of hierarchical HPSAPO-5/xSi catalysts, HPSAPO-5/0.6Si and HPSAPO-5/0.9Si contain a higher density of Brønsted acid sites with 46.25 and 45.32 µ•mol•g -1 , respectively, compared to HPSAPO-5/0.3Si, with 42.79 µ•mol•g -1 . However, HPSAPO-5/0.9Si displays a lower density of Brønsted acid sites than SAPO-5/0.9Si, as also evidenced by1 H MAS NMR (Table3.5).

Figure 3 .

 3 Figure 3.22 Differential FTIR spectra of (A) NH3 and (B) 2,6-dTBP adsorption on calcined HPSAPO-5/xSi with x=0.3 (black curve), x=0.6 (red curve) and x=0.9 (blue curve) after outgassing the probe molecules at room temperature.

Figure 3 .

 3 Figure 3.23 FTIR difference spectra of NH3 adsorption on calcined microporous SAPO-5/xSi with x=0.3 (black curve), x=0.6 (red curve) and x=0.9 (blue curve) after outgassing the probe molecules at room temperature.

Scheme 3 . 3 .

 33 Scheme 3.3. Possible catalytic mechanism in the aldol condensation/crotonization reaction of FF with MIBK catalyzed by HPSAPO-5/0.9Si.

Figure 3 .

 3 Figure 3.24 FTIR difference spectra of calcined HPSAPO-5/0.9Si upon adsorption of MIBK vapor pressure (red curve) and after outgassing MIBK at room temperature (blue curve).

Figure 3 .

 3 Figure 3.25 FTIR difference spectra of calcined HPSAPO-5/0.9Si after outgassing MIBK at room temperature (blue curve) and upon adsorption of FF (green curve) also at room temperature.

. 1 (Figure 3 . 26 ,

 1326 Figure 3.27A-B). ZrO2 and ZnO exhibit bands mainly located in the low temperature region (in blue), whereas the three acid zeolites (i.e. BEA, Y and MOR) also display bands in the high-temperature region (in red), but centered at a temperature lower than 550 °C. The deconvoluted NH3-TPD profile of Al-SBA-15 shows three bands in the high-temperature region centered at 500 °C, 605 °C and 730 °C (Figure3.28C), being mainly attributed to Lewis acid centers. These observations point out that strong Lewis acid centers are detrimental to the catalytic activity.

Figure 3 .

 3 Figure 3.26 NH3-TPD profiles of (A) fresh and (B) spent HPSAPO-5/0.9Si catalysts. The peak deconvolution can be found in the Table3.9.

Figure 3 .

 3 Figure 3.27 NH3-TPD profiles of fresh (A) BEA(150), (B) Y(30) and (C) MOR(6.1) catalysts. In blue, low-temperature region (100-450 °C); in red, high-temperature region (450-650 °C).

Figure 3 .

 3 Figure 3.28 NH3-TPD profiles of fresh (A) ZrO2, (B) ZnO and (C) Al-SBA-15 catalysts. In blue, low-temperature region (100-450 °C); in red, high-temperature region (450-650 °C).

Figure 3 .

 3 Figure 3.29 TGA profiles of the spent HPSAPO-5/0.9Si after the aldol condensation/ crotonization reaction. Reaction conditions: 0.2 g FF, 3.8 g MIBK, 40 mg catalyst, 180 °C, 3 h.

Figure 3 .

 3 Figure 3.30 HR-TEM micrographs of (A1,A2) 0.3%Pd/HPSAPO-5/0.9Si and (B1,B2) 4.8%Pd/HPSAPO-5/0.9Si.

Figure 3 .

 3 Figure 3.31 NH3-TPD profiles of fresh (A) HPSAPO-5/0.9Si and (B) 4.8%PdO/HPSAPO-5/0.9Si catalysts. The peak deconvolution can be found in the Table 3.9.

CHAPTER 4 : 3 4. 1 Introduction

 431 Single-reactor tandem aldol condensation-hydrogenation of furfural and MIBK over bifunctional Pd/Al 2 O Immediately after the third chapter, herein we report a robust catalytic system based on a bifunctional Pd/Al2O3 catalyst for conducting the single-reactor tandem aldol condensation / hydrogenation of FF and MIBK in a batch reactor, in the absence of solvent. The process comprises two in situ consecutive steps (Scheme 4.1): (1) aldol condensation/crotonization of FF with MIBK, and (2) selective hydrogenation of the aldol intermediate. The catalytic conditions

  optimization algorithm combining global optimization with a genetic algorithm (ga) and local search with an interior-point algorithm (fmincon), by comparison of the experimental and predicted concentrations of FF, MIBK, ALD-OH-1 and ALD-1. Herein, the average weighted least square criterion, expressed by Eq 4.3, was used for parameter estimation 𝛏 e,f ) -𝐲 ̂(𝛏 e,f , 𝛉)] T Σ 𝐲 -1 [𝐲(𝛏 e,f ) -𝐲 ̂(𝛏 e,f , 𝛉)]

Scheme 4 . 2

 42 Scheme 4.2 Possible products obtained in the aldol condensation/crotonization reaction of FF with MIBK over acid catalysts (see chapter 3).

Figure 4 . 1

 41 plots the time-evolution of the FF conversion and ALD-1 selectivity. The FF conversion increases monotonously along the reaction, reaching almost full conversion at 180 °C after 6 h. The ALD-1 selectivity increases with the temperature at short reaction times (<3 h), and attains a plateau value higher than 95% at longer times. Noteworthy, the by-product ALD-2a,b is not observed along the reaction. However, the alcohol intermediate (ALD-1-OH), issued from the aldol condensation reaction, is observed at low yield (<5%). Irrespective of the temperature, the carbon balance is excellent with a value higher than 95% at full FF conversion (Figure4.2).

Figure 4 . 1

 41 Figure 4.1 Time-evolution of the FF conversion (dashed curves) and ALD-1 selectivity (straight curves) at variable temperatures in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 spheres. Reaction conditions: MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w). The curves are a guide to the eye.

Figure 4 . 2 .

 42 Figure 4.2. Time-evolution of (A) ALD-1 yield, (B) TON for ALD-1 formation and (C) parity plots for the fitted and experimental TON in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 spheres. Reaction conditions: (A1-C1) MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w), T = 120-180 °C; (A2-C2) MIBK/FF ratio = 4.0-19.0 (w/w); FF/cat ratio = 3.8-20.3 (w/w), T = 160-165 °C.

Figure 4 . 3

 43 Figure 4.3 Selectivity-conversion plots in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 spheres. Reaction conditions: T = 120-180 °C; MIBK/FF ratio = 4.0-19.0 (w/w); FF/cat ratio = 4.0-20.0 (w/w).

Figure 4 . 4

 44 Figure 4.4 Arrhenius plots for the initial reaction rate of ALD-1 formation in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 spheres. Reaction conditions as in Figure 4.2, 4.3.

Figure 4 . 5 4 . 6 .

 4546 plots representative fittings of kinetic curves and a parity plot, whereas the parity plots for all the experiments are shown in the Figure The fitted parameters are listed in Table 4.3. The model reproduces conveniently the experimental trends and confirms the inhibiting role of FF on the reaction rate. Also noteworthy, the activation energies for steps 3 and 4 (i.e. 40.4 kJ/mol and 34.9 kJ/mol, respectively) compare well with the apparent activation energy 5-6 measured from the experimental Arrhenius plots (i.e. 38 kJ/mol).

Figure 4 . 5

 45 Figure 4.5 (A) Experimental and fitted curves using the model depicted in Table 2; (B) parity plot. Additional parity plots can be found in Figure 4.6.

Figure 4 . 6 .

 46 Figure 4.6. Parity plots for experimental and fitted FF, MIBK, ALD-1 and ALD-1-OH weight concentrations in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 spheres. Reaction conditions: MIBK/FF ratio = 4.0-19.0 (w/w); FF/cat ratio = 3.8-20.3 (w/w), T = 120-180 °C.

Figure 4 . 7

 47 Figure 4.7 Catalyst recyclability and reuse for Al2O3 spheres in six consecutive runs after washing with ethanol and calcination at 400 °C (6th run) under airflow. MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w), T = 180 °C, time = 3 h. The carbon balance was >95% for all the runs.

of 19 .(Scheme 4 . 3 ,Scheme 4 . 3

 194343 Scheme 4.3 Products obtained in the aldol condensation/crotonization reaction of FF with MIBK over unreduced PdO/Al2O3 (spheres).

Figure 4 . 8

 48 Figure 4.8 Time-evolution of the FF conversion (dashed curves) and ALD-1 / ALD-3 selectivities (straight curves) in the aldol condensation/crotonization reaction of FF with MIBK over Al2O3 and Pd/Al2O3 spheres. Reaction conditions: MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w), T = 180 °C. The curves are a guide to the eye.

Figure 4 . 9

 49 Figure 4.9 Time-evolution of the ALD-1, ALD-3, F-ALD-1 and THF-ALD-1 yield in a singlereactor tandem aldol condensation/crotonizationhydrogenation process. Reaction conditions: (step 1) MIBK/FF ratio = 19.0 (w/w); FF/cat ratio = 5.0 (w/w), T = 180 °C, 3 h; (step 2) T = 180 °C, H2 pressure = 2.0 MPa. The curves are a guide to the eye.

  XRD patterns of crushed Al2O3 and PdO/Al2O3 spheres reveal the typical reflections of the Al2O3 phase (Figure 4.10-top). Neat reflections belonging to PdO are visible at 2θ = 34°, 43° and 55°, which are indicative of the presence of PdO nanoparticles. The average size of PdO nanoparticles is about 3.4 nm as inferred from HR-TEM (Figure 4.10-bottom), with ca. 25% population of particles larger than 4.4 nm (not shown).

Figure 4 .

 4 Figure 4.10 PXRD patterns of crushed Al2O3 and PdO/Al2O3 spheres (top) and size distribution of PdO nanoparticles on unreduced PdO/Al2O3 measured by HR-TEM (bottom).

4. 6 . 2

 62 Acidity and acid strength of crushed Al2O3 and Pd/Al2O3 4.6.2.1 27 Al NMR MAS

  coordinated Al atoms, i.e. [Al VI (OAl)6] and [Al IV (OAl)4], respectively. A weak band is also visible at 37 ppm that can be attributed to penta-coordinated Al species, pointing out the presence of Lewis acid sites in the catalysts. This becomes weakened after impregnating palladium at the expense of the tetrahedrally coordinated Al atoms, suggesting a low yet detectable decline of the density of Lewis acid-type species on Al2O3.

Figure 4 . 1 H

 41 Figure 4.11 Solid-state 27 Al NMR MAS spectra on crushed Al2O3 and Pd/Al2O3 spheres.

Figure 4 . 4 . 6 . 2 . 3 Figure 4 .

 446234 Figure 4.12 1 H MAS NMR spectra of crushed Al2O3 and Pd/Al2O3 spheres.

Figure 4 .

 4 Figure 4.13 FTIR spectra of crushed Al2O3, PdO/Al2O3 and pre-reduced Pd/Al2O3 crushed.

Figure 4 .

 4 Figure 4.14 plots the differential FTIR spectra of adsorbed NH3 on the different samples upon contact with 30 mbar NH3 at room temperature. The original bands in the high-frequency zone exhibit a clear decline for the three samples, revealing a competition between NH3 and adsorbed Al-OH groups (Figure4.14A). However, three new bands appear at 3500 cm -1 , 3370 cm -1 and 3308 cm -1 that can be ascribed to adsorbed NH3 in Lewis acid centers, preferentially in contact with surface oxygen atoms by H-bonding. These bands are neat for Al2O3 and Pd/Al2O3, but vanish for PdO/Al2O3, suggesting a lower degree of H-bonding on the latter sample. For all samples, three characteristic bands appear in the low-frequency region at 1695 cm -1 , 1485 cm -1 and 1397 cm -1 that are distinctive of adsorbed NH 4+ on Al-OH groups (Figure4.14B). Four additional bands are also visible in the same region centered at 1620 cm -1 and 1245 cm -1 that can be assigned to NH3 adsorbed on Lewis acid centers. These species are retained after outgassing at room temperature for the three samples, pointing out preferential interaction with surface oxygen atoms by H-bonding. Further outgassing by increasing the temperature results in a decline of all the bands for all the samples (Figure4.14C, Figure4.15). Finally, a band is also visible at 1100 cm -1 upon contact of the samples with NH3 that be attributed to physisorbed NH3 on Al-OH groups, which vanishes after outgassing at room temperature.

Figure 4 .

 4 Figure 4.14 Differential FTIR spectra of adsorbed NH3 on crushed Al2O3 (black curves), PdO/Al2O3 (red curves) and Pd/Al2O3 (blue curves).

Figure 4 .

 4 Figure 4.15 Differential FTIR spectra of crushed Al2O3, PdO/Al2O3 and pre-reduced Pd/Al2O3 spheres in the presence of adsorbed NH3 after different outgassing temperatures.

Figure 4 .

 4 Figure 4.16 NH3-TPD profiles of (A) fresh and (B) spent HPSAPO-5/0.9Si catalysts. The peak deconvolution can be found in the Table 4.5.

4 .

 4 1 g. A possible reaction mechanism of 2,3,5,6-tetra(furan-2-yl)pyrazine formation from the reaction of furoin with ammonium acetate / NH3 is depicted in Scheme 5.The first step consists of the nucleophilic attack of NH3 to the carbonyl group in furoin, generating an alcohol-imine intermediate and releasing one water molecule. The alcohol-imine intermediate can undergo tautomerization to an imine-enamine intermediate and further to a keto-amine intermediate by tautomerization.The self-condensation of the keto-amine intermediate, followed by dehydration and oxidation, results in the formation of 2,3,5,6-tetra(furan-2-yl)pyrazine. No clear evidence of such intermediates was found from the analysis of the solutions after reaction. However, evidence of these intermediates was found for reductive amination experiments conducted over Ru/Al2O3 (see chapter 5.4).

Scheme 5 . 4

 54 Scheme 5.4 Proposed mechanism for 2,3,5,6-tetra(furan-2-yl)pyrazine formation from the reaction of furoin with ammonium acetate/NH3.

Figure 5 . 1 .

 51 Figure 5.1. Main peaks obtained in the GC chromatograms in the reducitve amination of furoin with NH3 and H2. Reaction conditions: furoin (0.12 g, 0.625 mmol), pre-reduced catalyst (24 mg), DMF (2 g), NH3 (0.7 g), 80 °C, 4.0 MPa H2, 2 h. From top to bottom: Pd/Al2O3, Ru/Al2O3, neat conditions (room temperature) without NH3 and H2.

Figure 5 . 2 .

 52 Figure 5.2. Main peaks observed in the GC chromatograms in the reductive amination of furoin with NH3 and H2. Reaction conditions: furoin (0.12 g, 0.625 mmol), pre-reduced catalyst (24 mg), NH3 (0.7 g), DMF (2 g), 100 °C, 4.0 MPa H2, 2 h. From top to bottom: 5%Pd/Al2O3, 5%Ru/Al2O3, neat conditions (room temperature) neither NH3 nor H2.

Figure 5 . 3 .Scheme 5 . 7

 5357 Figure 5.3. 1 H NMR spectrum of the reaction system in the reductive amination reaction of furoin with NH3 and H2. Reaction conditions: furoin (0.12 g, 0.625 mmol), pre-reduced 5%Ru/Al2O3 (24 mg), DMF (2 g), NH3 (0.7 g), 140 °C, 4.0 MPa H2, 2 h.

Figure 5 . 4 Scheme 5 . 8

 5458 Figure 5.4 Effect of temperature on the reductive amination of furoin with NH3 and H2 over 5%Ru/Al2O3. Reaction conditions: furoin (0.06 g), NH3 (0.45 g), 5%Ru/Al2O3 (0.012 g), DMF (2 mL), 2 h, 2.0 MPa H2.

Figure 5 . 5

 55 Figure 5.5 Effect of the H2 pressure on the reductive amination of furoin with NH3 and H2 over 5%Ru/Al2O3. Reaction conditions: furoin (0.06 g), NH3 (0.45 g), 5%Ru/Al2O3 (0.012 g), DMF (2 mL), 160 °C, 2 h, 2.0 MPa H2.

Figure 5 . 6

 56 Figure 5.6 Effect of the H2 pressure temperature on the reductive amination of furoin. Reaction conditions: 0.06 g furoin, 0.45 g NH3, 0.012 g 5%Ru/Al2O3, 2 mL DMF, 2 h, 4.0 MPa H2.

Figure 5 . 7

 57 Figure 5.7 Catalyst recycling and reuse. Reaction condition: 0.06 g furoin, 0.45 g NH3, 0.012 g 5% Ru/Al2O3, 2 mL DMF, 2 h, 4.0 MPa H2.

Scheme 5 . 9 5 . 4

 5954 Scheme 5.9 Possible reaction path for the synthesis of alcohol-amine by reductive amination of furoin with NH3 and H2 over Ru/Al2O3.

Figure 5 . 8 .

 58 Figure 5.8. Main peaks obtained in the GC chromatograms after the amination reaction of furoin with NH3 and H2 over Ru/Al2O3 (top) and in the single-reactor tandem process over bim-silica + Ru/Al2O3 (bottom). Reaction conditions: (top) furoin (0.12 g, 0.625 mmol), pre-reduced catalyst (24 mg), DMF (2 g), NH3 (0.7 g), 140 °C, 4.0 MPa H2, 2 h; (bottom) (1) Benzoin condensation: FF (0.096 g, 1 mmol), DBU (0.026 g, 0.17 mmol), bim-silica (0.16 g), pre-reduced catalyst (24 mg), DMF (2 g), 30 °C, 20 h; (2) Reductive amination: 0.7 g NH3, 4.0 MPa H2, 140 °C, 2 h.

Figure 5 . 9 .

 59 Figure 5.9. Main peaks obtained in the GC chromatograms after the single-reactor tandem reaction over recycled bim-silica + Ru/Al2O3. Reaction conditions: FF (0.1 g, 1 mmol), DBU (0.02 g, 0.13 mmol), bim-silica + Ru/Al2O3 (0.16 g), DMF (2 g), 30 °C, 20 h.

Figure 5 . 10 .

 510 Figure 5.10. TG profiles of the bim-silica catalyst before and after the furoin amination reaction. Reaction conditions as in Figure 5.9.

Table 1 . 2

 12 Catalytic activity of different solid catalysts for the furfural-acetone aldol condensation reaction

	Catalytic centers	Catalyst	Furfural/ Cat. (wt)	Pressure (MPa)	T (°C)	Time (min)	Furfural Con. (%)	Sel. FAc & DFAC (%)	Ref
		MgO-ZrO2	3.7	0.1	120	1560		78	76
		MgO-ZrO2	1.25	5 (He)	120	1440		95	77
		Co-Al spinel	6	0.1	140	300		99	78
		WO3-ZrO2	1	1 (He)	80	1800		n/a	79
		MgO-ZrO2	1	1 (He)	80	1800		n/a	79
		Chitosan	0.96	0.1	150	240		~100	80
	Solid Basic	MgO-ZrO2	1.5	1 (N2)	50	1440		~100	88
	Catalyst	Mg-Zr aerogels	13	0.1	60	180		90	84
		MgO/NaY	0.8	0.1	85	480		98	92
		Mg-Al hydrotalcite	3.25	0.1	100	120		90	86
		MgO-ZrO2 /graphite	1.5	1 (N2)	50	1440		85	76
		Dolomite	11	0.1	140	60		~100	97
		K-Y(15)	3.3	Aut. P	100	120		35	96
		MCM-22	6.5	Aut. P	100	120		89	98
	Solid Brønsted acid catalyst	HBEA Hf-MOF-808	3.25 10	Aut. P Aut. P	100 100	480 720		~100 92	102 107
		Nb2O5	4.8	1 (N2)	130	240		69	104
	Solid Lewis acid catalyst	Sn-MFI/Sn-BETA	1.2	Aut. P	140	360		58	106
	Scheme 1.3 Aldol condensation/crotonization of furfural with acetone.

CHAPTER 2: Experimental section 2.1 Materials 2.1.1 Reagents and solvents Table 2.1

  List of chemical reagents used along this thesis with the corresponding suppliers. All the reagents were used as received without further purification (except furfural)

	170. García-Ortiz, A.; Vidal, J. D.; Climent, M. J.; Concepción, P.; Corma, A.; Iborra, S., ACS
	Sustainable Chemistry & Engineering 2019, 7 (6), 6243-6250.
	171. X. He, A. H. C., J. A. Koutsky
	172. T. Ayusawa, S. M., T. Aoki, R. Hamana US4598159, 1986.
	173. Y. Han, X., Chen, M. Lu, CN101845109A, 2010.
	174. M, L., Y. Ji, J. Dong. Y. Han, T. Yang, , CN102358757A, 2012. Chemical CAS Grade	Supplier
	175. Z. Xiao, J. Z., F. Lv, CN103508986B, 2015. Acetone 67-64-1	98%	Sigma-Aldrich
	Ammonium acetate 176. L. Lin, W. C., Y, Sun, CN106349195A, 2017. 631-61-8 Aluminum isopropoxide 555-31-7 177. Zhu, Y.; Quek, J. Y.; Lowe, A. B.; Roth, P. J., Macromolecules 2013, 46 (16), 6475-6484. 98 % SINOPHARM ≥ 98 % Sigma-Aldrich Ammonia 7664-41-7 99.99 % Air liquid
	178. Zhu, Y.; Batchelor, R.; Lowe, A. B.; Roth, P. J., Macromolecules 2016, 49 (2), 672-680. Biphenyl 92-52-4 ≥ 99 % Sigma-Aldrich
	179. Li, C.; Ding, D.; Xia, Q.; Liu, X.; Wang, Y., ChemSusChem 2016, 9 (13), 1712-1718. Chloroform-d 865-49-6 99.8 atom %D Sigma-Aldrich
	Dimethyl sulfoxide-D6 180. Gu, M.; Xia, Q.; Liu, X.; Guo, Y.; Wang, Y., ChemSusChem 2017, 10 (20), 4102-4108. 2206-27-1 99.8 atom %D Sigma-Aldrich Dodecane 112-40-3 ≥ 99 % Sigma-Aldrich 181. Li, M.; Xu, X.; Gong, Y.; Wei, Z.; Hou, Z.; Li, H.; Wang, Y., Green Chemistry 2014, 16 Di-Water 7732-18-5 Sigma-Aldrich (9), 4371-4377. Ethanol 64-17-5 98 % Sigma-Aldrich 182. Faba, L.; Díaz, E.; Ordóñez, S., ChemSusChem 2014, 7 (10), 2816-2820. Furoin 552-86-3 98 % Sigma-Aldrich
	183. Cho, H. J.; Kim, D.; Li, J.; Su, D.; Xu, B., Journal of the American Chemical Society 2018, Furil 492-94-4 98 % Sigma-Aldrich
	140 (41), 13514-13520. Furfural	98-01-1	97 %	Sigma-Aldrich
	184. Cho, H. J.; Kim, D.; Xu, B., ACS Catalysis 2020, 10 (8), 4770-4779. Hydrogen 1333-74-0 99.99 % Air liquid
	185. Faba, L.; Díaz, E.; Ordóñez, S., Catalysis Today 2011, 164 (1), 451-456. Isopropanol 67-63-0 98 % Sigma-Aldrich
	Isoamyl alcohol 186. Jiang, S.; Ma, C.; Muller, E.; Pera-Titus, M.; Jérôme, F.; De Oliveira Vigier, K., ACS 123-51-3 ≥ 98 % Sigma-Aldrich Catalysis 2019, 9 (10), 8893-8902. Methanol 67-56-1 ≥ 99 % Sigma-Aldrich
	Methyl isobutyl ketone 187. Jiang, S.; Ramdani, W.; Muller, E.; Ma, C.; Pera-Titus, M.; Jerôme, F.; De Oliveira 108-10-1 98 % Sinopharm N,N-Dimethylformamide 68-12-2 98 % Sigma-Aldrich Vigiera, K., ChemSusChem 2020, 13 (7), 1699-1704. Palladium(II) acetate 3375-31-3 98 % Sigma-Aldrich
	Phosphoric acid	7664-38-2	85% in H2O	Sigma-Aldrich
	Ruthenium(III) chloride trihydrate	13815-94-6	98 %	Sigma-Aldrich
	Tetrahydrofuran	109-99-9	98 %	Sigma-Aldrich
	Toluene	108-88-3	98 %	Sigma-Aldrich
	Triethylamine	121-44-8	95.5%	Sigma-Aldrich
	Tetraethyl orthosilicate	78-10-4	99 %	Sigma-Aldrich
	1-Butanol	71-36-3	≥ 99 %	Sigma-Aldrich
	2,6-di-tert-butylpyridine	585-48-8	≥ 97 %	Sigma-Aldrich
	2,2'-Bipyridine-3,3′-diol	36145-03-6	98 %	Sigma-Aldrich
	2-methyltetrahydrofuran	96-47-9	99 %	Sigma-Aldrich
	Note: Commercial furfural contains impurities, since it slowly oxidizes and turns black under air.
	Furfural needs to be distilled under reduced pressure before use, and stored in the fridge at 4 °C.

2.1.2 Catalysts Table 2.2

  List of catalysts used along this thesis with the corresponding suppliers

	Name	CAS	Grade	Supplier
	Amberlyst 26	39339-85-0	-	Sigma-Aldrich
	Zinc Oxide	1314-13-2	≥ 97 %	Sigma-Aldrich
	Zirconium dioxide	1314-23-4	98%	Sigma-Aldrich
	Al2O3-SiO2 (Siral 5 HPV)	-	-	Sasol
	Al2O3-SiO2 (Siral 30)	-	-	Sasol
	Al2O3 (SCCa-5/170)	-	-	Sasol
	Al2O3 (SCFa140)	-	-	Sasol
	Al2O3 (SBa90)	-	-	Sasol
	Magnesium Oxide	1309-48-4	99.9%	Sigma-Aldrich
	H-BEA-150	-	-	Clariant
	MOR-6.1	-	-	ITQ
	ZSM-5 (Si/Al = 23)	-	-	Zeolyst
	ZSM-5 (Si/Al = 80)			Zeolyst
	ZSM-5 (Si/Al = 200)			Zeolyst
	ZSM-5 (Si/Al = 280)			Zeolyst
	Y (Si/Al = 80)			Zeolyst
	Ru/Al2O3 (5 wt%)	-	-	Johnson Matthey
	Pd/Al2O3 (5 wt%)	-	-	Johnson Matthey
	Pt/Al2O3 (5 wt%)	-	-	Johnson Matthey
	Rh/Al2O3 (5 wt%)	-	-	Johnson Matthey
	2.2

Catalyst preparation 2.2.1 Synthesis of Al-SBA-15 (Si/Al = 76)

Table 3 . 1

 31 Summary of catalytic properties for the different zeolites tested in this study

	Entry Catalyst	Conv FF	Sel ALD-1	Sel by-products	Yield ALD-1	CBFF
			(%)	(%)	(%)	(%)	(%)
	1	ZnO	25	76	14	19	87
	2	ZrO2	18	32	5	6	83
	3	ZSM-5 (Si/Al = 23)	14	25	5	4	80
	4	ZSM-5 (Si/Al = 80)	20	60	6	12	83
	5	ZSM-5 (Si/Al = 200)	10	46	4	5	85
	6	ZSM-5 (Si/Al = 280)	13	28	6	4	82
	7	BEA (Si/Al = 150)	26	42	4	11	80
	8	MOR (Si/Al = 6.1)	16	64	4	10	82
	9	Y (Si/Al = 30)	40	52	5	20	84
	10	SAPO-5/0.3Si	23	47	9	11	83
	11	HPSAPO-5/0.3Si	45	72	13	32	81
	12	SAPO-5/0.6Si	49	57	10	28	79
	13	HPSAPO-5/0.6Si	49	61	13	30	81
	14	SAPO-5/0.9Si	40	60	8	24	87
	15	HPSAPO-5/0.9Si	73	62	14	46	76
	16	Al-SBA-15 (Si/Al=76)	38	45	7	17	84

Table 3 . 2 .

 32 Ratio between the initial reaction rate of ALD-1 formation on HPSAPO-5/xSi and on SAPO-5/xSi as a function of the temperature and the Si loading (x)

	x	120 °C	140 °C	160 °C	180 °C
	0.30	2.4	9.8	5.6	6.4
	0.60	2.8	1.8	1.1	1.9
	0.90	13	30	16	17

Table 3 . 3 .

 33 Main products obtained in the single-reactor tandem aldol condensation/crotonization reaction of FF with MIBK, followed by hydrogenation, over Pd-loaded HPSAPO-5/xSi

	Reaction conditions
	1 st step *	2 nd step	Pd (wt%)	Yield F-ALD-1	Yield THF-ALD-1	Yield Other products
	160 °C, 12 h 160 °C, 12 h, 2.0 MPa H2 0.3%	22%	11%	18%
			0.8%	9%	26%	13%
			4.8%	11%	22%	15%
	180 °C, 3 h	160 °C, 12 h, 2.0 MPa H2 0.3%	12%	40%	11%
			4.8%	5%	45%	14%
	180 °C, 3 h	120 °C, 3 h, 2.0 MPa H2 0.3%	7%	42%	14%
	180 °C, 3 h	80 °C, 3 h, 2.0 MPa H2	4.8%	12%	8%	20%
	* Other reaction conditions: 0.2 g FF, 3.8 MIBK, 40 mg catalyst

Table 3 .

 3

		4. Textural properties of HPSAPO-5/xSi catalysts with variable Si loading
			Specific surface areas (m 2 .g -1 )		Pore volumes (cm 3 .g -1 )
	x	SSABET SSANLDFT	Smicro †	Smeso *	Vtot NLDFT	Vmicro †	Vmeso
	0.30	267	375	315	60	0.304	0.079	0.225
	0.60	340	450	327	123	0.323	0.077	0.246
	0.90	464	552	333	219	0.448	0.075	0.373

* Smeso = SDFT -Smicro

Table 3 .

 3

	1 H Chemical shift δ (ppm)	Assignment	HPSAPO-5/xSi x=0.3 x=0.6 x=0.9	Micro SAPO-5/xSi x=0.3 x=0.6 x=0.9
	1.7-2.1	SiOHiso	4	16	19	11	16	15
	2.2-2.4	AlOH, POH H-bonded	20	27	33	15	19	12
	3.5-3.9	BAS1	22	16	12	24	17	21
	4.6-4.9	BAS2	19	11	5	13	9	14

5.

Intensity of protonic species measured from single-pulse 1 H MAS NMR spectroscopy in HPSAPO-5/xSi and microporous SAPO-5/xSi, with x = 0.3, 0.6, 0.9

Table 3 . 6 .

 36 Integrated area (A) of the δasym NH4 + (1450 cm -1 ) and ν8a 2,6-dTBPH + (1618 cm -1 ) bands, density of accessible Brønsted sites (N), accessibility factor (AF) and density of acid sites in micropores measured using NH3 and 2,6-dTBP probe molecules for HPSAPO-5/xSi catalysts with x= 0.3, 0.6 and 0.9

	NH3 adsorption	2,6-dTBP adsorption	Acid sites in
	x	AF	micropores
			(mol g -1 ) c

Table 3 . 7

 37 Integrated area (A) of the δasym NH4 + (1450 cm -1 ) band and concentration of accessible

	Brønsted acid sites (N) monitored by adsorbing NH3 as probe molecule on HPSAPO-5/xSi and
	SAPO-5/xSi
	x	SAPO-5/xSi A a (cm -1 ) N b (mol.g -1 )	HPSAPO-5/xSi A a (cm -1 ) N b (mol.g -1 )
	0.3	6.88	46.8	6.29	42.8
	0.6	6.48	44.1	6.80	46.2
	0.9	7.70	52.3	6.66	45.3

Table 3 . 8

 38 Vibrational modes of MIBK in liquid phase and MIBK adsorbed on HPSAPO-5/0.9Si

	Vibrational mode	MIBK in liquid phase	MIBK@HPSAPO-5/0.9Si
	νasym CH3	2960	2960
	νasym CH2	-	2933
	νsym CH3	2875	2875
	ν CO	1720	1680
	δasym CH3	1470	1470
	δasym CH2	-	1415
	δsym CH3	1370	1370

Table 3 . 9

 39 Results for band deconvolution of NH3-TPD profiles measured on the fresh and spent HPSAPO-5/0.9Si. In parentheses, standard deviation for the temperature

	Entry	Catalyst		Band I	Band II	Band	Band	Band V	Band	Band	Band
						III	IV		VI	VII	VIII
	1	HPSAPO-5/ 0.9Si_Fresh	T (°C)	(20) 151	(19) 188	(20) 230	(47) 252	-	(47) 540	-	-
			µmol/	248	147	79	300	-	32.7	-	-
			g
	2	HPSAPO-5/ 0.9Si_Spent a	T (°C)	(21) 102	(41) 162	(20) 230	(35) 280	(47) 363	(19) 520	-	-
			µmol/	40	284	186	180	385	12.7	-	-
			g
	3	4.8%PdO/HPSAP O-5/0.9Si_Fresh	T (°C)	(18) 112	(19) 150	(36) 200	(47) 276	-	(50) 400	-	-
			µmol/	462	216	312	175	-	56.1	-	-
			g
	4	Al-SBA-15_Fresh	T (°C)	(23) 120	-	-	(40) 315	(51) 390	(53) 500	605 (53) 730 (70)
			µmol/	467	-	-	113	250	260	208	112
			g

A Spent catalyst after 4 runs was recovered, washed by ethanol for 3 times and dried, calcined at 400 °C for 2 h

Table 4 . 1

 41 Summary of catalytic properties for the different aluminas tested in this study a

	Entry Catalyst	Conv FF (%)	Sel ALD-1 (%)	Sel ALD-2a,b (%)	Yield ALD-1 (%)	CBFF (%)
	1	SiO2-Al2O3 (Siral 5 HPV)	75	97	-	73	96
	2	SiO2-Al2O3 (Siral 30)	67	89	-	60	90
	3	Al2O3 (SCCa-5/170)	65	98	-	62	96
	4	Al2O3 (SBa90)	42	96	-	40	96
	5	Al2O3 (SCFa140) b	62	96	-	60	96
	6	Al2O3 (SCFa140)	72	95	-	68	95
	7	Al2O3 (SCFa140) c	48	94	-	45	92
	8	Al2O3 spheres 1.8/210	92	97	-	89	95
	9	ZnO	25	76	14	19	87
	10	Al-SBA-15 (Si/Al = 76)	38	45	7	17	84
	11	H-SAPO-5	73	62	14	46%	76

a Reaction conditions: MIBK/FF = 19.0 (w/w), FF/cat = 5.0 (w/w), 180 °C, 3 h, catalysts calcined at

Table 4 .

 4 2 lists the different elementary steps with the corresponding rate equations and equilibrium expressions. In Table4.2, A and Ea represent the activation energy and preexponential factors, respectively, whereas θi and θ * refer to the surface coverage of species i and empty actives sites, respectively, which can be expressed as follows

	θ * =	1 1+K FF C FF +K MIBK C MIBK +K ALD1OH C ALD1OH +K ALD1 C ALD1 +K H 2 O C H 2 O	(4.5)

Table 4 . 2

 42 Elementary steps for the best model and rate equations

	Step	Elementary reaction	Rate equation
	1	FF + *	K MIBK ⇔	FF *	K 1 =	θ FF * c FF θ *
	2	MIBK + *	K MIBK ⇔	MIBK *	K 2 =	θ MIBK * c MIBK θ *
	3	FF * + MIBK *	𝑘 1 → ALD1OH *	r 3 = A 3 exp (-	Ea 3 RT	) θ MIBK * θ FF *
	4	ALD1OH *	𝑘 2 → ALD1 * + H 2 O	r 4 = A 4 exp (-	Ea 4 RT	) θ MFOH *
	5	ALD1OH + *	K ALD1OH ⇔	ALD1OH *	K 5 =	θ ALD1OH * c ALD1OH θ *
	6	ALD1 + *	K ALD1 ⇔	ALD1 *	K 6 =	θ ALD1 * c ALD1 θ *
	7	H 2 O + *	K H 2 O ⇔ H 2 O *	K 7 =	θ H 2 O * c H 2 O θ *

Table 4 . 3 .

 43 Elementary steps for the best model and rate equations

	Parameter	Estimated value	CI (95%)

Table 4 . 5

 45 Results for band deconvolution of NH3-TPD profiles measured on the fresh and spent Al2O3 and PdO/Al2O3 crushed spheres. In parentheses, standard deviation for the temperature

	Entry Catalyst		Band I	Band II	Band III	Band IV	Band V	Band VI	Band VII	Band VIII	Band IX	Band X
	1	Al2O3 (crushed, 100 m)	T ( o C) µmol/g	-144 195 -290 -505 607 750 796 -45 104 -188 -96 39 48 43
	2	Al2O3 (crushed, 200 m)	T ( o C) µmol/g	85 120 182 258 300 418 436 500 670 800 66 52 112 19 76 96 30 164 32 46
	3	Al2O3 (crushed, 200 m)_spent	T ( o C) µmol/g	--	-189 240 298 376 469 494 -800 -42 31 64 71 14 53 -7.1
	a Spent catalyst after 4 runs was recovered , washed by ethanol for 3 times and dried, calcined at 400 o C
	for 2 h

Table 5 . 1

 51 Effect of the solvent on furoin amination with ammonium acetate a Reaction condition: 0.06 g furoin, 1 g ammonium acetate, 6 mL solvent, 140 °C, 4 h.

	Solvent	Yield 2,3,5,6-tetra(furan-2-yl)pyrazine (%)
	Methanol	12
	Ethanol	7
	Isopropanol	16
	Toluene	5
	DMF	18
	Tetrahydrofuran	21
	2-methyltetrahydrofuran	18
	Isoamyl alcohol	12
	H2O	-

a

Table 5 . 2

 52 Effect of the temperature on furoin amination a

	T (°C)	Yield 2,3,5,6-tetra(furan-2-yl)pyrazine (%)
	130	17
	140	19
	150	27
	160	31
	170	30
	a Reaction condition: 0.03 g furoin, 0.1 g ammonium acetate, 3 mL THF, 3 h.

Table 5 . 3

 53 Effect of the mass of ammonium acetate on furoin amination a

	Ammonium acetate	Yield 1
	(g)	(%)
	0.06	25
	0.08	30
	0.10	39
	0.12	38
	0.14	37
	a Reaction condition: 0.03 g furoin, 0.1 ammonium acetate, 3 mL THF, 160 °C, 3 h.

Table 5 . 4

 54 Furoin amination with different amount of water

	Entries Water content

Table 5 . 5

 55 Effect of the solvent on furil amination

	Solvent	Yield (%)
	Methanol	9
	Ethanol	9
	Isopropanol	15
	Toluene	-
	DMF	22
	Tetrahydrofuran	14
	2-methylterahydrofuran	12
	Isoamyl alcohol	16
	H2O	-
	a Reaction condition: 0.06 g furil, 0.6 g NH3, 6 mL solvent, 4 h, 170 °C

Table 5 . 6

 56 Effect of temperature on furil amination

	T / °C	Yield (%)
	160	24 %
	170	28 %
	180	36 %
	190	33 %
	200	24 %

Table 5 . 7

 57 Effect of the n-butylamine : furil molar ratio on furil amination

	N-butylamine : furil	Yield
	(mol/mol)	(%)
	0	36
	0.5	34
	1	37
	2	38
	3	40
	5	41
	a Reaction condition: 0.03 g furil, 0.8 g NH3, 3 mL DMF, 2 h, 180 °C

Table 5 . 8

 58 Survey of catalysts for the reductive amination of furoin with NH3 and H2

	Entry Catalyst	Yield (%)
	0	-	-
	1	5%Pd/Al2O3	28
	2	5%Rh/Al2O3	10
	3	5%Pt/Al2O3	14
	4	0.1%Ru/Al2O3	30
	5	1%Ru/Al2O3	35
	6	2%Ru/Al2O3	40
	7	5%Ru/Al2O3	47
	8	5%Ru/SiO2	35
	9	5%Ru/C	40

a Reaction condition: 0.06 g furoin, 0.45 g NH3, 0.012 g catalyst, 2 mL DMF,160 °C, 2.0 MPa H2, 2 h.

Table 5 . 9

 59 Catalyst results for furil amination in the presence of additives a

	Additives	Alcohol-amine yield (%)	Diamine yield (%)	Cyclic by-products yield (%)
	-	34	-	6
	n-butylamine	40	-	2
	A-type zeolite (4 Å)	43	-	-
	Titanium isopropoxide	-	38	20
	2,2,2-trifluoroethanol	58	-	-

a Reaction condition: 0.06 g furoin, 0.45 g NH3, 0.012 g 5% Ru/Al2O3, 2 mL DMF, 2 h, 4.0 MPa H2.

Table 5 .

 5 10 Catalytic results in consecutive runs a

	Run	Alcohol-amine yield (%)
	1	42%
	2	38%
	3	4%

a Reaction condition: bim-silica + Ru/Al2O3, solvent: 2 g DMF, step 1: 30 °C, 24 h, N2 protected step 2: 140 °C, 2 h, NH3: 0.45 g, H2 pressure: 4.0 MPa

Chapter 3: Furfural aldol condensation (SAPO-5)

Acknowledgments

CHAPTER 1: Literature Review the catalysts (bim-silica + Ru/Al2O3) into the reactor, add furfural, DMF, DBU, and then replace the air with nitrogen. the reaction was conducted at 30 °C for 20 h. After this period, NH3 and H2 were added, and the reductive amination reaction was conducted at a given temperature. 101