François Fajula
email: ffajula@enscm.fr

Anne Galarneau
email: anne.galarneau@enscm.fr

Combining phase separation with pseudomorphic transformation for the control of the pore architecture of functional materials: A review

Keywords: Spinodal separation, pseudomorphic crystallization, monolitic hierarchical zeolites, catalysis, strontium capture

Chemical phase separation and pseudomorphic transformation are very powerful tools for fine tuning the porous architecture of inorganic oxides. In this short review the basic principles of the preparation of centimetric bodies made of zeolites is introduced. The synthesis of monoliths with homogeneous distributions of interconnected macropores with skeleton made of SOD, LTA and FAU crystals is described. Their unique hierarchical porous texture featuring the structural micropores, intercrystalline mesopores and flow-through macropores leads to remarkable hydrodynamic behavior in separation, catalysis and ion exchange processes operated in continuous-flow mode. In the base-catalyzed carbon-carbon bond formation, productivities twice those achievable with fixed-beds of packed particles are observed. In the capture of strontium present in radioactive effluents their efficiency is three orders of magnitude that of traditional powder in batch.

Introduction

The control of the pore architecture of inorganic materials is of paramount importance for the optimization and intensification of most processes operated in continuous flow mode in the fields of catalysis, adsorption or separation [1] and for applications requiring selfstanding bodies [2,3] with tunable and controlled pore sizes such as Li-ion batteries [4] or super thermal insulators [5,6]. Over the years many synthesis and post-synthesis strategies, involving possibly the use of reusable or sacrificial porogens, have been developed to address this challenge. In the frame of our research activities aiming at producing catalysts and sorbents with improved mass transfer characteristics we have explored an original approach enabling, not only to precisely tune the textural features of the materials but also to cope with the macroscopic shape constraints attached to their uses. This strategy is based on a combination of chemical spinodal phase separation, to produce monolithic centimetric macroporous bodies with flow through interconnected homogeneous macropores, and pseudomorphic transformation to generate secondary networks of homogeneous and ordered mesopores and/or micropores in the rigid skeleton of the monoliths, generating a large and readily functionalizable active surface. In this review we will introduce the basic principles underlying the chemistry involved in the preparation and modification of these materials, we will describe their physico-chemical properties and highlight their performances in some applications.

Spinodal phase separation

Chemical phase separation has been used for the first time by the group of Nakanishi in Kyoto [7,8] for producing silica chromatographic columns featuring a network of flowthrough macropores with a narrow pore size distribution adjustable between 2 and 10 µm and diffusive mesopores in the skeleton struts with sizes adjustable in the range 6 -20 nm.

Compared to classical packed-bed columns, they combine enhanced mass transfer, lower pressure drop and higher separation efficiency owing to a high hydrolytic permeability and short diffusion path lengths. For instance, monoliths with a macropore diameter and skeleton thickness of 2 µm demonstrated the same efficiency as beds of particles of 3 µm (plate height, H min = 11 µm) but with a pressure drop of only 2 bar/cm instead of 35 bar/cm, thus allowing much higher flow rates for ultrafast separation [9]. These materials have been commercialized by Merck in the 2000's under the names of Chromolith for macroscopic pieces or Chromolith CapRod for capillary columns.

Silica monoliths are formed by two concurrent processes, namely a phase separation between silicates and water promoted by a polymer additive and a hydrolytic sol-gel condensation in acid medium. The organic polymer (usually a polyethylene oxide chain) induces the destabilization, and the phase separation, of the initial silicate oligomers-water system by decreasing the hydrophilicity of the silicates and increasing the replusive forces during silicate polymerization. The phase separation is then frozen by a sol-gel transition which kinetics are controlled by the temperature or the amount of water and acid (Fig. 1).

Fig. 1: Schematic representation of a spinodal-phase separation [10]

After the acid step, the resulting monolith features a narrow distribution of macropores, and a poorly condensed silica skeleton. To consolidate the later a post-treatment in mild alkaline conditions is performed (usually in ammonia solution), which leads to the generation of a secondary network of mesopores with diameters adjustable in the range 6 to 20 nm [11,12,13]. Calcined silica monoliths feature a remarkable mechanical strength. Thus a material (Fig. 2) with 5 µm macropores, a macropore volume of 2 mL/g, a surface area of 700 m 2 /g, and a mesoporous network consisting of 10 nm mesopore diameters and a mesopore volume of 1 mL/g showed a flexural strength of 0.96 MPa and a compressive strength of 2.67 MPa [14]. To date, the chemical spinodal phase separation procedure has been employed successfully to prepare macroporous monoliths with flow through interconnected macropores made of alumina [15], titania [16,17], zirconium phosphate [18], polysilsesquioxanes [19], magnesium oxide [20], iron oxide [21]. Fig. 2. SEM picture, photograph and mercury porosimetry (two intrusion/extrusion cycles up to 4000 bar) of silica monoliths (0.6 cm x 10 cm) with macropores of 5 µm and mesopores of 10 nm diameters.

Besides separation, the unique textural and structural features of these materials, and more particularly their high and accessible surface area, offer significant potential for catalysis. The interskeleton macropores enable high pressure-driven flows leading to fast transport of reactants and products while the intraskeleton mesopores provide extended surface to active sites readily accessed by diffusion. Moreover, compared to traditional packed-beds of porous particles, microreactors consisting in pore-flow-through monoliths favor enhanced exchange between the moving and adsorbed phases, with minimal, if any stagnant fluid, and homogeneous contact times within the catalytic beds. These aspects, and the versatility of the surface activation of silica monoliths, have been illustrated in the catalytic synthesis of fine chemicals. For instance, monoliths functionalized with organosilanes or inorganic precursors operated in continuous flow mode allowed productivities and selectivities significantly larger than fixed bed and batch reactors in the Knoevenagel condensation (with grafted amino groups), the transesterification of triacetine (with grafted sulfonic groups), the Diels-Alder addition (with alumina deposited on the surface), the Friedländer reaction (with occluded CuBTC MOF nanoparticles) and the selective hydrogenation of alcynes (with deposited Pd nanoparticles) [1,20,21,22,23,24,25]. More recently C-N doped monolithic titanium oxide photocatalysts revealed a remarkable potential for the continuous flow treatment of wastewater under visible light [27].

With these liquid-solid catalytic systems, optimal catalytic performance was achieved with materials with 5 µm macropores and 2-3 µm skeleton walls and a ratio monolith diameter to monolith length 1:5 or above [1,24]. Monoliths proved also very efficient as supports, for enzyme immobilization, with for instance 1000 times enhanced productivities compared to the native enzyme for the transformation of sucrose to glucose and fructose [28] and for ionic liquids confinement for the removal of aromatic sulfur compounds from fuels [29].

A further benefit of the unique characteristics of the silica monoliths is the ability of their skeleton to be re-engineered into more ordered porous systems, involving ordered mesoporous structutres and zeolites, while preserving the macroporous architecture. This is achieved by a pseudomorphic transformation of the original silica skeleton, as addressed below.

Pseudomorphic transformation

Pseudomorphism is a common phenomenon in the mineral world; it corresponds to the precipitation of an altered mineral with a morphology unrelated to its crystallographic symmetry by dissolution of a parent mineral [START_REF] Sinlankas | Mineralogy[END_REF][START_REF] Garcia | [END_REF]. The resulting material, called a pseudomorph, assumes the outward crystal morphology of the parent one. The transformation occurs at a non constant matter content by reaction of a mineralizing solution that exchanges cations and/or anions with the existing mineral and allows precipitation of the new structure.

In nature, pseudomorphic transformation is a slow process controlled by the rates of water infiltration and soil erosion. Our first attempts to use pseudomorphism for the preparation of materials with ordered porosities and pre-defined shapes were directed to the synthesis of ordered mesoporous materials [32,33,34]. Preshaped silica and silica-alumina particles (5 -800 µm particle sizes) were gradually locally dissolved and then re-precipitated into MCM-41 materials with the same morphology using the templating effect of surfactant micelles (Fig. 3). The key point in preserving the morphology of the parent silica particles during the transformation process is to tune the rate of mesostructure precipitation to that of silica dissolution. As soon as some silicates species become available they should re-precipitate into MCM-41. Alkalinity, dilution, nature and amount of surfactants, temperature and the presence of auxiliary swelling agents must be thus adapted to the composition and porosity of the parent solid [35,36]. Moreover, fine control of the kinetics of the transformation allowed to prepare beads of MCM-48 by a double pseudomorphic transformation of native beads of MCM-41 [34]. The properties of beads (5 and 10 µm diameter) MCM-41 and MCM-48 pseudomorphs functionalized with octyl chains have been evaluated as stationary phases for normal and reversed-phase liquid chromatography using mixtures of alkylbenzenes, dibutyl and diethyl-phtalates as solutes. Compared to benchmarck commercial materials with the same sizes, mesostructured silicas exhibited increased retentions factors enabling separation of poorly retained solutes not achievable with traditional supports [37,38].

Regarding the pseudomorphism crystallization of zeolites, the procedure has been used for long, initially with the objective of reducing possible deleterious impact of binders or to provide mechanical strength in catalysts and sorbents [see eg. 39,40,[START_REF] Schwieger | Proceedings 12th Intern. zeolite Conf[END_REF][START_REF] Plee | [END_REF]43]. More recently pseudomorphism has been used to produce materials with hierarchical distributions of pores, from macro to micropores, using diatomeous earths [44,45,46] or structured monoliths [47,48] as parent bodies. Combining chemical phase separation with pseudomorphic transformation has now opened a new, versatile and very powerful route towards a fine tuning of the structural and textural features of functional porous materials.

This will be illustrated in the following.

Zeolites with hierarchical porosity produced by spinodal phase separation and pseudomorphic crystallization.

Sodalite monoliths

Pseudomorphic transformation of the amorphous silica skeleton of macroporous monoliths into zeolites proved very powerful for the binderless shaping of zeolite nanocrystals. First attempts were made for the production of inorganic stable and efficient basic solid catalysts for process intensification by crystallization of the amorphous skeleton of a silica monolith (hereafter MonoSil) into sodalite (MonoSil-SOD) [49]. Because of the small size of its pore apertures (flexible aperture between 0.20 to 0.28 nm [50]), sodalite is hardly considered as a catalyst. However, Shanbhag et al [51] showed that sodalite with a hierarchical mesoporous/microporous structure synthesized using an organosilane surfactant exhibited a higher basicity and a much higher catalytic activity than that of CsNaX or KAlMCM-41 for various base catalyzed reactions involving bulky and small substrates, such as Knoevenagel condensation, Claisen-Schmidt condensation in liquid phase, and acetonylacetone cyclization in vapor phase. Interestingly the catalyst showed longer lifetime than CsNaX and KAlMCM-41.

A parent MonoSil (7.8 cm length, 6.14 mm diameter, 6 µm macropores, 3 µm skeleton thickness, macropore volume 1.75 mL/g, surface area 750 m 2 /g, 7 nm mesopore diameter, mesopore volume 0.96 mL/g) was impregnated with an alkaline solution of sodium aluminate and tetrapropylammonium hydroxyde to achieve a molar composition of 1 SiO 2 /0.15 NaOH/2.6 NaAlO 2 /33 H 2 O/0.004 TPAOH in a Teflon-lined stainless steel autoclave which was hydrothermally treated at 150 °C for 18 hours. The recovered monolith, washed and calcined at 550 °C for 8 h, preserved its macroscopic shape and exhibited the XRD pattern of pure sodalite. 29 Si NMR and 27 Al NMR spectra featured single peaks (at -91 and 59 ppm, respectively) characteristic of SOD with Si/Al = 1. SEM images (Fig. 4) showed the preservation of the macroporous network, with the amorphous skeleton of the original monolith transformed into an aggregation of crystals of 0.5 to 1.5 µm in size.

Mercury porosimetry and nitrogen sorption revealed a macroporous network of uniform size around 3 µm with a macroporous volume of 1.00 mL g -1 (or 0.34 mL/cm 3 of monolith in comparison to 0.47 mL/cm 3 for original MonoSil). The smaller macropore volume per unit volume of monolith results from the growth of the SOD crystals. Large secondary pores between 4 to 100 nm are identified by the upper volume uptake in mercury porosimetry (0.07 mL g -1 ) and by the vertical hysteresis loop above 0.8 P/P 0 in nitrogen isotherm revealing intercrystalline pores between crystals and embedded inside the SOD crystals. The MonoSil-SOD was cut into 1 cm pieces and cladded with a heat-shrinkable DERAY-PTFE gain to be mounted in a recirculating flow system (Fig. 5). The Na + cations present in as-synthesized MonoSil-SOD were exchanged by K + cations (91% exchange) by circulating a KCl solution through the microreactor. After washing with water and activation at 140°C under reduced pressure the Knoevenagel condensation was performed at 80°C with a contact time of 2 min between 4-isopropylbenzaldehyde (4-IPB) and ethylcyanoacetate (ECA) to give 4-isopropylstyrylethyl-cyanoacetate.

LTA monoliths

LTA-monoliths (MonoSil-LTA) with skeletons formed by an association of faceted cubic-like crystals with a broad distribution of sizes in the range 0.2 -1.5 µm together with 1 µm spherical microcrystals were obtained by pseudomorphic transformation of parent silica monoliths prepared by using PEO polymers of either 20 kDa (macropores 5 µm, skeleton thickness 3 µm) or 35 kDa (macropore 11 µm, skeleton thickness 7 µm) at 100 °C for 24 h using a mixture with the overall molar ratio: 1 SiO 2 / 0.15 NaOH/ 2.6 NaAlO 2 / 33 H 2 O/ 0.004 TPAOH [52]. Compared to classical procedures used for LTA synthesis [START_REF]Verified syntheses of zeolitic materials[END_REF] a lower amount of NaOH and a higher amount of aluminum were engaged in the crystallization mixture to slow down the rate of dissolution of the parent MonoSil in order to preserve the monolithic skeleton structure during its transformation into LTA. The micropore volume of the MonoSil-LTA pseudomorph after ion exchange with calcium cations in order to allow access of nitrogen to the structural micropores was equal to 0.18 mL/g indicating that MonoSil-LTA consisted in at least 70 wt% of zeolite. As previously described for MonoSil-SOD, a basic MonoSil-LTA catalyst microreactor was prepared by exchanging the sodium cations by potassium cations (85% exchage) and evaluated in the Knoevenagel condensation reaction. At contact times of 2 min and 5 min, steady state conversions of 55% and 99%, respectively, were obtained, with no loss of activity after 20 hours on stream.

MonoSil-LTA has been evaluated for the removal of radioactive cations from complex aqueous effluents mimicking contaminated seawater effluents of the Fukushima case.

Traditional zeolite-based procedures for the selective removal of Cs + and Sr 2+ generate huge volumes of contaminated waste and give rise to severe health hazards due to the handling of powders. A MonoSil-LTA has been used for the continuous-flow removal of Sr 2+ (10 mg/L) contained in a solution of sodium chloride (0.1 M) at flow rates of 0.5 and 0.15 mL/min (Fig. 6). At the lower flow rate, the breakthrough curve shows a complete retention of strontium amounting to 0.37 mmol/g and a total exchange capacity of 0.86 mmol/g (Fig. 6). Under the same conditions, ion-exchange resins retained merely 0.05 mmol/g whereas in batch experiments, LTA powder retained only 0.22 mmol/g after 24h [START_REF] Sachse | [END_REF]. Experiments were then performed using a simulated Fukushima seawater containing radioactive 90 Sr + (5.5 ng/L, 28 kBq/L) in the presence of large amounts of several competing cations (Na + 9.6 g/L, Mg 2+ 1.28 g/L, Ca 2+ 0.4 g/L, K + 0.5 g/l non radioactive Sr + 0.008 g/L).

Compared to LTA powder in batch, the MonoSil-LTA was able to process 4 times more volume of the highly salty radioactive solution per unit of mass and allowed to reach much higher decontamination levels with the additional benefit of a significant safety improvement.

In depth analysis of the above data revealed that the overall efficiency of the decontamination process could be significantly enhanced -particularly for high flow rates -by improving the transfer of the fluid through the monolith skeleton and, consequently, the kinetics of the of ion exchange reaction. Introducing intercrystalline mesopores and decreasing the size of the crystals have been therefore attempted. The methodology used consisted in separating the nucleation and crystallization steps of the pseudomorphic transformation [55].

Silica monoliths (6 mm diameter, 5 or 10 cm length) with hierarchical porosity (macropore diameter 5 µm, macropore volume 2 mL/g, mesopore diameter 13 nm, mesopore volume 1 mL/g) were used as starting materials. The monoliths were soaked in an alcaline aluminate solution with the overall composition indicated above which was aged for 24 h at 40 °C. The amorphous skeleton of MonoSil was then crystallized into LTA at 70 °C for 24 h. SEM images (Fig. 7) reveal a uniform macroporous network of ca 4 µm and a skeleton thickness of ca 2.5 µm formed by an aggregation of nanocrystals of 300-500 nm at the outer surface and of 100 nm in the core. This MonoSil-LTA consists of ca 95wt% of zeolite LTA and features a uniform hierarchical trimodal porosity with structural micropores of 0.4 nm, inter crystal mesopore diameter of 35 nm and macropore diameter of 4 microns In the capture of strontium present in salty solutions, the trimodal MonoSil-LTA proved superior to bimodal MonoSil-LTA and 1000 times more efficient than packed beds of LTA powders. Actually 4200 bed volumes (BV) of solution could be treated with 1 mL of monolith at a flow rate of 34 BV/h with no trace of Sr 2+ detectable by ionic chromatography in the effluent [56].

FAU monoliths

Direct pseudomorphic crystallization of silica monoliths into faujasite X (FAU) by reaction of the parent MonoSil with an alkaline mixture containing the aluminium source could not be achieved as was the case of the preparation of MonoSil-SOD and MonoSil-LTA due to a too fast rate of silica dissolution compared to the rate of FAU crystallization. To reach our goal, a double pseudomorphic transformation was operated [57]. MonoSil was first transformed into MonoSil-Al by incorporation of alumina in the silica skeleton. To this end, the silica monoliths have been transformed into silica-alumina monoliths (0.25 < Al/Si < 0.40) at low NaOH concentration ([NaOH] = 0.24 M) at 40 °C for 24 h. This allowed to decrease the solubility of the original skeleton while preserving the macroscopic structure of the macroporous network. The self-standing binderless FAU-X monoliths (MonoSil-FAU)

were then formed at higher NaOH concentration ([NaOH] = 2.2 M) by a two steps crystallization procedure involving an aging period at 40 °C for 4 days and a crystallization step at 100 °C for 24 h. The incorporation of aluminium in silica monolith results in a significant increase of the density of the monolith with a decrease of the intergrain mesoporosity and surface area. After crystallization the micropore volume measured by nitrogen adsorption at 77 K and the XRD pattern (Fig. 8) indicate that the material consists in more than 90% FAU zeolite.

Several families of MonoSil-FAU could be produced with this protocol. SEM images (Fig. 8) reveal that MonoSil-FAU feature macropores with diameters adjustable from 3 to 20 µm (similar to the parent silica monolith). The skeleton of the FAU-X monoliths is formed by an aggregation of two populations of FAU-X nanocrystals (100 -200 nm / 400 -500 nm) generating a secondary porosity between the nanocrystals of 30 to 1000 nm in diameter, centered at 300 nm. MonoSil-FAU materials present therefore three levels of readily interconnected porosities with a macropore volume of ca. 1.0 mL/g, a secondary pore volume of ca. 0.40 mL/g and a micropore volume of 0.30 mL/g.

The performances of MonoSil-FAU are being evaluated in the capture and sequestration of Cs ions from contaminated aqueous streams and the separation of xylenes.

Conclusions

Laboratory experiments have definitely demonstrated the potential of synthesis strategies based on phase separation and pseudomorphic crystallization for the preparation of materials with tunable hierarchical porosities. The present review highlights how macroporous monoliths with homogeneous and interconnected networks of micro, meso and macropores may contribute to process intensification through textural control and opens very stimulating directions of research. Future efforts should be directed to the production of higher silica zeolites, the scale-up of the procedures and the seamless cladding of the monoliths.

ZSM-5 monoliths featuring large macropores (> 400 µm) have been obtained using a monolithic polyuretane foam as template. The materials led to higher performance than its pelletized counterpart in the methanol to olefins reaction but with a limited productivity gain however due to the large pores and thick walls. Transformation of MonoSil type monoliths into ZSM-5 has been reported by Lei et al [48] but the material exhibited a very low micropore volume (< 0.02 mL/g) indicating only marginal crystallization. Regarding scale-up, while the production of large tubular species or disks of macroporous silicas by spinodal phase separation is readily achievable, the homogeneous crystallization of their skeleton throughout the whole piece is more tricky as stirring cannot be employed calling for imaginative new procedures. Finally, proper cladding of the monolithic rods to produce self standing reactors or sorbents is very important to expand the fields of applications. The use of thermoshrinking polymers is very convenient and easy to handle at laboratory scale but limits the range of operation conditions to low temperature and low pressure systems. Cladding into stainless-steel tubing using epoxy resin adhesive [58] or chemical resistant cements are alternatives.

Fig. 3 .

 3 Fig. 3. Pseudomorphic transformation of silica beads (LiChrospher 100, from Merck) into MCM-41. A and B: SEM images and XRD patterns for the LiChrospher and MCM-41, respectively. C: Nitrogen isotherms at 77 K for the calcined product recovered after a) 15 min, b) 30 min, c) 1 h, d) 1 day and e) 6 days of transformation (Adapted from ref. [32]).

Fig. 4 .

 4 Fig. 4. SEM images of a) the parent silica monolith and of b) its pseudomorphic transformation into Sodalite.

Fig. 5 .

 5 Fig. 5. Picture of a cladded MonoSil-SOD (0.6 x 1 cm) with glass connections.

Fig. 6 .

 6 Fig. 6. Breakthrough curves for MonoSil-LTA for an initial Sr 2+ concentration of 10 mg/L in a 0.1 M solution of NaCl. Filled squares 0.5 mL/min, open circles 0.15 mL/min.

Fig. 7 .

 7 Fig. 7. SEM images (A,B), XRD pattern (C) and photograph (D) of MonoSil-LTA with trimodal porosity.

Fig. 8 .

 8 Fig. 8. SEM image, picture and XRD pattern of MonoSil-FAU.

Table 1

 1 summarizes the main physico-chemical characteristics of the monoliths at different stages of the synthesis protocol.

Table 1 .

 1 Physico-chemical characteristics of the monoliths at different stages of the synthesis protocol.

Acknowledgments:

The authors thank the St Nikon Foundation for constant and stimulating support.


Renshun Xu

Hervé Nabet

Audrey Breton

Patrick Baldoni-Andrey

Nicolas Lesage

Thomas Cacciaguerra

Vasile Hulea

Francois Fajula

Anne Galarneau
email: anne.galarneau@enscm.fr

Highly efficient mesoporous Mg/γ-Al 2 O 3 catalysts for ozonation of saline petroleum effluents

Keywords: Catalytic ozonation, Water treatment, Saline effluents, Alumina, Basicity, Oxidation

γ-Al 2 O 3 is promising for catalytic ozonation because it produces HO° radicals. To improve its basicity and/or its oxidative character, highly mesoporous γ-Al 2 O 3 was doped with different metals M alone or in mixture (M = Mg, Ca, Zn, Fe, Ba, Zr, Cu, Co, Ni, Ce, Ti, with 0.05 < M/Al < 0.5 molar ratio) prepared either by incipient wetness deposition or by direct sol-gel.

Tests to determine the basic and oxidative character of the catalysts have been set up. Results

show that Mg-doped alumina materials exhibited the highest basicity and Cu,Mg-doped alumina materials the highest oxidative character. These materials (5 g/L) were tested in the catalytic ozonation of a synthetic saline (NaCl = 50 g/L) petroleum effluent such as produced water (TOC = 216 mg/L) containing phenols, acetic acid and polycyclic aromatic hydrocarbons. Among these catalysts, Mg-doped alumina with molar ratio Mg/Al = 0.1, 0.2 prepared by incipient wetness deposition were the more efficient catalysts. Under reuse the Mg-doped alumina catalyst (Mg/Al = 0.1) exhibited the highest ozonation rate with 98% TOC removal in 5 h. This remarkable behavior was attributed to the in-situ formation during ozonation of spinel (MgAl 2 O 4 ) nanoparticles at the surface of Mg-doped alumina particles.

INTRODUCTION

Catalytic ozonation is expected to become a central technology for the processing of contaminated aqueous effluents containing non-biodegradable toxic organic residues (For a recent review see eg [1]). Compared to established methods, such as adsorption, coagulation, flocculation, incineration, membrane separation or biological oxidation, catalytic oxidation should allow higher degradation efficiency with lower energy consumption, lower footprint and, accordingly, lower operation and investment costs.

Catalytic ozonation, as part of advanced oxidation processes, can be operated at ambient temperature and pressure with total mineralization of the organic substrate into carbon dioxide and water. This is a significant advantage compared to single ozonation of organic matter, which usually leads ultimately to the formation of aldehydes and carboxylic acids, which do not react with ozone. Though the mechanism of the reaction is not fully resolved and subject to debate [2], the role of the ozonation catalyst is, schematically speaking, to promote, on the one hand, the formation of hydroxyl radicals, able to degrade all types of organics and intermediates by ozone decomposition and, on the other hand, the adsorption of ozone and the organics present in the effluent, both effects contributing to enhanced kinetics. Regardless the catalytic system under consideration, homogeneous or heterogeneous, catalytic ozonation reveals very complex and the chemical pathways involved depend on many factors such as the experimental conditions, the nature of the substrate and of the intermediate oxidation by-products, the chemical nature, the texture, the stability and the amount of catalyst. With respect to heterogeneous catalytic ozonation, supported metals (Cu, Pt, Ce, Ru, Mn,…), metal oxides (MnO 2 , MgO, TiO 2 , Al 2 O 3 , CeO 2 , …), modified zeolites (MFI, LTA, FAU) and mesostructures (SBA-X, CMK, M41S), activated carbons and combinations of the above have been reported [1,2]. Raw minerals such as perovskites, bauxite and goetite have been used also in catalytic ozonation [3][4][5]. As a common requirement for these materials to be active is the necessity to adsorb ozone and organic matter at their surface. Though gas phase adsorption of ozone and organics is well described, adsorption of ozone on polar or apolar surfaces in the presence of an excess of water is far from being documented and clearly understood. For this reason, several mechanisms and catalytic active sites have been suggested to account for the activity of solid surfaces to decompose ozone such as (i) oxidized or reduced forms of supported metals [6], (ii) Lewis centers of metal oxides [7], (iii) nondissociated hydroxyl groups of metal oxides [8][9][10] and (iv) basic centers of the catalyst surface [11][12][13].

In a precedent study [13] we have investigated the catalytic ozonation of 2,4-dimethyl phenol (2,4-DMP) and of a model petrochemical effluent containing a mixture of phenol, acetic and naphtenic acids, pyrene, and naphtalene using various catalytic materials featuring an acidic (zeolites, mesoporous alumino-silicates), basic (ion-exchanged zeolites), amphoteric (γalumina), oxidative (titanium oxide) or neutral (silica) surfaces. Among them, γ-alumina proved the most efficient.

Conflicting conclusions are reported in literature regarding the activity of γ-alumina for ozonation. For instance, the ability of γ-alumina to generate free radicals by interaction of ozone with hydroxyl groups is put forward in several papers [6,8,[13][14][15][16][17][18], but the chemical nature of the catalytic active sites is subject of debate. Neutral OH groups [17,19] or Brönsted acidic sites [20] have been proposed. This hypothesis is supported by the fact that nitrates or sulfates inhibit the activity of the surface by substitution of reactive hydroxyls. On the contrary, Oyama et al. [21] reported no activity for mere alumina surfaces and suggested that it could play a single role of support for metal or metal oxides with Lewis acidity or oxidative properties. As an alternative hypothesis, the generation of hydroxyl radicals has been attributed to reaction of ozone with adsorbed molecules [18,22]. In that case, ozonation activity would be controlled by the adsorption of the organic substrates and intermediate oxidation products on the surface of γ-alumina rather than by the presence of reactive hydroxyl groups.

The difficulty in clarifying the catalytic role of alumina surface in the ozonation reaction stands in the fact that, not only the surface may undergo chemical and structural modification during the reaction, but also, as said above, the process is highly dependent on the operation conditions. Among others, the conditions of the activation of the catalyst, the nature of the substrate and the pH of the medium have a dramatic impact on activity. For instance, nonpolar substrates such as aromatic hydrocarbons [23] and ethers [24] do not adsorb and react in the presence of alumina under conditions where ozone is readily decomposed and able to oxidize fulvic and humic acids, which showed a high affinity for the alumina surface [18].

The influence of pH is complex as it can affect the overall process. Thus, depending on pH value, hydroxyl radicals can be consumed by condensation to generate hydrogen peroxide [15] and strong adsorption of intermediate carboxylates may occur for pH values lower than the pH zpc resulting in lower activity and low stability of the catalyst.

By combining ozonation experiments with surface characterization by infra-red spectroscopy and gravimetric sorption measurements, we have proposed that the activity of γ-alumina results essentially from the basic character of its hydroxylated surface and to its high capacity in adsorbing carboxylic acids [13]. The use of tert-butanol as radical scavenger suggested the involvement of hydroxyl radicals in the reaction [13]. The addition of NaCl neutralized 70% of HO° during the catalytic ozonation of 2,4-dimethyl phenol (2,4-DMP) as chloride ions present a very strong scavenging effect on radicals [13]. However the addition of NaCl to the aqueous effluent containing polycyclic aromatic hydrocarbons (PAHs), phenol and acetic acid allowed to increases the total organic carbon removal from 50% to 90%. The results suggest that sodium cations prevent carboxylic acids adsorption on the basic sites of γ-alumina whereas chloride anions give Cl 2 or chlorine radicals prone to oxidize PAHs, which in turn produce HO° by intermediate in-situ formation of H 2 O 2 [13]. The latter is then able to react with acetic acid to form peracetic acid, a very powerful oxidant.

In order to deepen our understanding of the catalytic role of γ-alumina in the ozonation reaction we have undertaken a systematic study of the modification of its surface using a series of metal promoters or modifiers (Mg, Ca, Zn, Fe, Ba, Zr, Cu, Co, Ni, Ce, Ti) either by incipient wetness deposition or by direct sol-gel (sg) synthesis. These metals have been selected in order to improve the basicity and/or the oxidative character of the alumina surface.

Two specific tests, for measuring the basicity of the materials in water and for estimating their redox behavior have been developed. The performances of characteristic materials have been then evaluated in the ozonation of a model petrochemical effluent.

EXPERIMENTAL

Materials

Synthesis of metal-doped aluminas by incipient wetness deposition on γ-alumina

γ-Al 2 O 3 (Alfa Aesar, powder obtained by grinding the original particles) was used as a support for all syntheses. In a typical incipient wetness impregnation, 2 g of γ-Al 2 O 3 were mixed with 2-3 mL of aqueous metallic salt (x g) (Table 1) solutions for 24 hours. The samples were then dried for 24 h at 80 °C and calcined in air at 450, 600 or 750 °C for 5 h.

Methods

Basicity measurement by benzoic acid adsorption

Aqueous solutions of different concentrations of benzoic acid (solubility 2.9 g/L or 0.020 mol/L) have been prepared and characterized by UV spectroscopy (Fig. S1). At pH < 2

(

Determination of the oxidative character of the materials

The oxidative dehydrogenation of benzyl alcohol into benzaldehyde (Scheme 1) was chosen as reaction model for determining the oxidative properties of the catalysts prepared by impregnation and sol-gel method.

Scheme 1. Catalytic oxidation of benzyl alcohol to benzaldehyde

In a typical oxidation reaction, the catalyst (0.3 g) was dispersed in 15 mL of cyclohexane followed by the addition of benzyl alcohol (517 µL, 4.97 mmol) in an autoclave reactor. The reaction was carried out under stirring for 24 h at 150 °C and air pressure of 2 bar.

Periodically reaction samples were recovered, filtered and analyzed by GC-MS (Agilent, GC-2010) with a ZB-35 HT column. The column temperature increased from 80 to 180 °C with the rate of 15 °C/min, injector temperature was 250 °C, column flow was 20 mL/min.

Calibration curves have been plotted between the amount of benzaldehyde/benzyl alcohol and GC area (Fig. S2). The amount of the benzyl alcohol has been taken into account for determining the reaction conversion and the benzaldehyde yield has been calculated.

Benzaldehyde is the only product, no traces of benzoic acid have been obtained under these conditions.

Materials Characterization

All measurements were performed at "Plateau Technique, Pole Chimie Balard Montpellier".

X-Ray Diffraction (XRD) patterns of the materials were performed using a Bruker D8 The ozonation reactions were carried out at 30 °C in a bubble column reactor (7 cm diameter, 25 cm height) (Fig. S3) using 1 L of solution and 5 g of catalyst (previously dehydrated at 105

°C for 16 h). At t = 0 min the flow of ozone was started. Aliquots (8 mL) were taken at t = 5, 10,15,20,30,60,90,120,150,180,240,300 COD measurements were not recorded as too imprecise due to the presence of the high amount of salt.

TOC measurements were performed with a TOCmeter Shimadzu TOC Vppn with a method developed at TOTAL SA (adapted from NF 1484) using acidification at pH 2 and degassing of the sample. Solutions were filtered before analysis (filter 0.45 µm) and diluted 1/10 with water to decrease the alkalinity.

The consumed ozone by oxidized carbon was measured from the difference of concentration of gaseous ozone in the input and in the output of the reactor (Fig. S3). The flow rate of gas in the reactor is known (0.2 L/min), the measurements of ozone concentration in the input and output of the reactor allow to calculate the total ozone mass transferred in the liquid. The with O 3,consumed the amount (mol) of consumed ozone during the reaction, C org,oxidized the amount (mol) of organic carbon oxidized during the reaction, O 3,gaz,input (g/L) and O 3,gas,output (g/L) the amount of ozone in the gas phase in the input and output of the reactor, respectively, t x (min) the time needed to degrade X% of organic carbon, X percentage of organic carbon degraded, Q g the gas flow rate (L/min) at the input of the reactor, C org,initial the amount (mol/L) of initial organic carbon, V the reactor volume (L), M O3 the molar mass of ozone (48 g/mol).

RESULTS AND DISCUSSION

Materials characterization

Materials prepared by incipient wetness impregnation

The XRD pattern of the parent γ-Al 2 O 3 shows three broad peaks at about 37, 46 and 67° corresponding to a cubic spinel with defects (Fig. 1). For Mg/γ-Al 2 O 3 (Mg/Al = 0.05), the calcination temperature from 450 to 750 °C does not affect the crystalline structure (Fig. S4). Only slightly thinner peaks are observed for a calcination temperature of 750 °C resulting from some particle sintering. Consequently, the increase of calcination temperature leads to a slight decrease in specific surface area from 246 to 196 m 2 /g, a slight increase of mesopore diameter from 10 to 11 nm and mesoporous volume from 0.56 to 0.64 mL/g. This is consistent with the same behavior observed for γ- S3).

Al 2 O 3 in
The increase of Cu/Al ratio 0.1 < Cu/Al < 0.5 during the impregnation on γ-Al 2 O 3 (calcination at 600 °C) leads to the increase of XRD peaks intensity of CuO NPs (Fig. 2) at the surface of γ-Al 2 O 3 . For Cu/Al ≤ 0.2 the textural properties of γ-Al 2 O 3 are maintained, whereas for larger amount of Cu, CuO NPs are blocking the entrance of some mesopores leading to a decrease of surface area and mesopore volume of the materials (Table S4). CuO NPs are of interest as they are known for their oxidative properties [29]. The increase of calcination temperature to 800 °C leads to the appearance of a well crystalline phase, a cubic spinel phase CuAl 2 O 4 [30] for Cu/Al ≤ 0.3 and for Cu/Al ≥ 0.4 in addition to the spinel phase some CuO NPs (Fig. S8). We can suggest that for a calcination at 600 °C the CuO NPs are at the surface of a spinel phase Cu x Al 2-x O 4 . The increase of temperature at 800 °C lead the increase of crystallinity and to the change of textural properties of the Cu/γ-Al 2 O 3 materials (Table S5) with a drastic decrease in surface area from 200 m 2 /g to 100 and 40 m 2 /g for Cu/Al instead of 116 m 2 /g for Cu/Al = 0.5 (Table S6). XRD patterns of Al 2 O 3 synthesized by sol-gel method and calcined at different temperatures (Fig. S9) show that at 600 °C alumina exhibits a poorly crystallized gamma structure. A temperature higher than 750°C is necessary to reach the gamma crystalline phase of alumina.

Alumina synthesized by the sol-gel method and calcined at 600 °C features high surface area, mesopore volume and mesopore diameter of 440 m 2 /g, 0.84 mL/g and 7 nm, respectively (Table S7). The increase of calcination temperature to 800 °C to reach γ-Al 2 O 3 structure leads to a decrease of surface area and mesopore volume to 233 m 2 /g and 0.65 mL/g, respectively, similar to commercial γ-Al 2 O 3 used for impregnation.

The sol-gel method has been applied to synthesize metal doped-alumina materials. A Ti/Al 2 O 3 material has been synthesized with a Ti/Al molar ratio of 0.25. TiO 2 is generally known for its high oxidative character, especially when it features the anatase crystalline phase. However, the resulting mixed oxides is mostly amorphous and the gamma alumina phase is not well crystallized even for a calcination at 750 °C (Fig. S10). The mixed-oxide Ti/Al 2 O 3 synthesized by sol-gel presents very different textural properties in comparison to alumina: small mesopore diameter of 5 nm, a low mesopore volume of 0.27 mL/g and a surface area of 240 m 2 /g (Table S8).

Cu/Al 2 O 3 (Cu/Al = 0.2) materials have been prepared by sol-gel and calcined at 600 and 800 °C. XRD pattern (Fig. S11) of the material calcined at 600 °C shows mainly an amorphous phase with two weak peaks at 2ϴ = 35 and 40° due to few CuO NPs. The textural properties of Cu/Al 2 O 3 calcined at 600 °C (Table S9) are close to the one of Ti/Al 2 O 3 . An increase of calcination temperature to 800 °C reveals the formation of a cubic spinel phase CuAl 2 O 4 (Fig. S11). The addition of Mg (Mg/Al = 0.2) to form a mixed Cu/Mg/Al (0.2/0.2/1) material leads after calcination at 600 °C to more crystallized CuO NPs (Fig. S12). This observation is similar to that made for the Cu/Mg/Al (0.2/0.2/1) material obtained by impregnation due to the preferential formation of Mg/Al 2 O 3 spinel phase. This material shows a higher mesopore volume (0.44 mL/g) and surface area (300 m 2 /g) (Table S10) in comparison to Ti/Al 2 O 3 and Cu/Al 2 O 3 materials prepared by sol-gel. By increasing the amount of Mg and Cu (Cu/Mg/Al = 0.5/0.5/1) the amount of CuO NPs increases evidenced by the increase of the intensity of XRD peaks (Fig. S12). Nitrogen sorption isotherm shows the formation of less homogeneous size distributions of mesopores suggesting inhomogeneous sizes of NPs. The addition of Ca instead of Mg (Cu/Ca/Al = 0.2/0.2/1) leads to the formation of CuO NPs (Fig. S13) and a drastic loss of the textural properties with a decrease of surface area to 74 m 2 /g (Table S11).

Ca is presumably more prone to form CaAl 2 O 4 spinels in comparison to Mg. The increase of calcination temperature to 800 °C, leads to a mixture of poorly crystallized spinel phase and some CuO NPs.

Basicity of the materials in water

The basicity of all materials prepared either by incipient wetness impregnation or by sol-gel precipitation has been investigated. Basic sites can be analyzed in the gas phase for example by CO 2 adsorption monitored by FTIR as we have done in the previous publication using γ- However, interpretation of the UV spectrum of benzoic acid in a water solution is not straightforward since peaks intensity and position vary with the pH of the solution and cannot be solely examined at 272 nm. At a basic pH, the UV bands are shifted towards lower wavelengths and the extinction coefficient of the band decreases resulting in a difficult quantification of benzoic acid in solution. The pKa of benzoic acid is 4.2. At pH < pKa -2, therefore pH = 2.2, benzoic acid is under its protonated molecular form C 6 H 5 -COOH, and at pH > pKa + 2, therefore pH = 6.2, benzoic acid is under its anionic form C 6 H 5 -COO -. For 2.2 < pH < 6.2, there is an equilibrium between these two forms. The shape of the UV spectrum of the remaining benzoic acid in solution is complex (Scheme 2). For accurate quantitative estimation of the benzoic acid remaining in solution after separation from the solid, it is necessary to adjust the pH below 2.2 or above 6.2 to analyze always the same state of benzoic acid.

Al
We have therefore developed a new methodology to quantify precisely the amount of benzoic acid remaining in solution by UV-Vis measurements to characterize the basicity of the materials and the amount of benzoic acid adsorbed in the solid. First the pH of the benzoic acid solution in contact with the solid has been measured, then the pH of the solution (after filtration) has been decreased below pH 2 to obtain the same position of UV bands, the intensities of the bands were then solely dependent on the benzoic acid concentration.

Basicity of doped-alumina prepared by incipient wetness impregnation

The initial pH of the benzoic acid solution was 3. S12). The pH of the solutions were 6.6 and 8.1

for Mg/Al = 0.1 and 0.2, respectively (Fig. 4, Table S12). The basic character of γ-Al Materials prepared with other alkaline-earth metals (Ba, Ca) (M/Al = 0.05) deposited on γ-Al 2 O 3 led to an increase of the pH values of the benzoic acid solution (Fig. 4, Table S12) with the highest pH found for Ca/γ-Al 2 O 3 (pH = 7.0). All the pH values of benzoic solutions were above 6.2 for Mg/, Ca/ and Ba/γ-Al 2 O 3 materials and benzoic acid was under its anionic form C 6 H 5 -COO -. It means that alkaline-earth metals really improve the basicity of γ-Al 2 O 3 . For Mg/, Ca/ and Ba/γ-Al 2 O 3 materials, no benzoic acid was adsorbed on these materials. (Fig. 4, Fig. S14, Table S12).

Besides alkaline-earth metal, other elements (Fe, Co, Ni, Cu, Zn and Ce) have been deposited on γ-Al 2 O 3 and calcined at 600 °C. The pH values of the resulting solutions with their mixture with benzoic acid solutions were higher (4.2 < pH < 5.4) than γ-Al 2 O 3 (pH = 4.0) except for Fe/γ-Al 2 O 3 , which pH was 3.8 (Fig. 4, Table S12). Among the series Co/, Ni/, Cu/, Zn/, Ce γ-Al 2 O 3 , the highest pH values were obtained for Cu/γ-Al 2 O 3 with pH = 5.1 to 5.4 for Cu/Al = 0.05 to 0.5, respectively. Co/, Zn/, Cu/γ-Al 2 O 3 materials adsorbed a similar amount of benzoic acid (0.23 mmol/g), slightly less than on γ-Al 2 O 3 (0.37 mmol/g) (Fig. S14, Table S12).

Similarly S12). Increasing the temperature of calcination to 800 °C for Cu/γ-Al 2 O 3 does not decrease the pH (pH = 5.4) as for Mg/γ-Al 2 O 3 (pH = 4.3), meaning that Cu remains in the surface of the particles, but the amount of adsorbed benzoic acid decreases to 0.10 mmol/g. This is due to the decrease of the specific surface area of the material coming from the formation of dense spinel CuAl 2 O 4 phase at 800 °C (Fig. 4, Table S12).

The materials formed by co-impregnation of Mg and Cu on γ-Al 2 O 3 with molar ratios Mg/Cu/Al = 0.1/0.1/1 and Mg/Cu/Al = 0.2/0.2/1 show an increase of the pH of the benzoic acid solution with values of 5.9 and 6.8, respectively, which are intermediate of those of Mg/γ-Al 2 O 3 (pH = 6.6 and 8.1 for Mg/Al = 0.1 and 0.2, respectively) and those of Cu/γ-Al 2 O 3 (pH = 5.2 for Cu/Al = 0.1 and 0.2). The amount of benzoic acid adsorbed is low (0.05 mmol/g) for Mg/Cu/Al = 0.1/0.1/1 and no adsorption of benzoic acid is measured for Mg/Cu/Al = 0.2/0.2/1 (Fig. S14, Table S12) as expected for such pH values (Fig. S15).

In summary, the materials prepared by incipient wetness impregnation featuring the highest basicity are Mg/γ-Al 2 O 3 (Mg/Al = 0.2) > Ca/γ-Al 2 O 3 (Ca/Al = 0.07) > Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1) > Mg/γ-Al 2 O 3 (Mg/Al = 0.12). In suspension in benzoic acid solution they lead to pH values higher than 6.2 (Fig. 4) with no benzoic acid adsorption (Fig. S15).

These materials are therefore suitable catalysts for sustainable ozonation.

Basicity of the materials prepared by sol-gel

The basicity of alumina and different metal/alumina-based materials prepared by sol-gel precipitation was probed following the same protocol as above.

Similar amounts of benzoic acid were adsorbed on the commercial γ-Al 2 O 3 and the sol-gel Al 2 O 3 (0.37 mmol/g compared to 0.33 mmol/g) (Table S12). The pH value of the benzoic acid solution after mixing with sol-gel Al 2 O 3 was slightly higher (pH = 5) than that obtained with the commercial γ-Al 2 O 3 (pH = 4).

The benzoic acid solution in contact with the sol-gel Ti/Al 2 O 3 featured a slightly lower pH value (pH = 4.7) than that attained with sol-gel alumina and showed a lower amount of benzoic acid adsorbed (0.18 mmol/g). The sol-gel Cu/Al 2 O 3 (Cu/Al = 0.2) led to a pH value (pH = 5.2) similar to that observed with the sol-gel alumina and the Cu/γ-Al 2 O 3 prepared by impregnation with a slightly lower amount of benzoic acid adsorbed (0.19 mmol/g) (Table S12). The highest basicity was reached with the sol-gel Cu/Mg/Al 2 O 3 with a pH of benzoic acid solution of 7.9, close to the one obtained for the more basic material prepared by impregnation (Mg/γ-Al 2 O 3 with Mg/Al = 0.2; pH = 8.1) (Fig. 4), and a low amount of benzoic acid adsorbed (0.06 mmol/g).

As shown above, most of metal-doped alumina materials prepared either by incipient wetness deposition or by sol-gel (sg) method increased the basicity of alumina. (Fig. 4, Fig. S14). The

Oxidative character of the materials

As a test for scaling the oxidative character of the materials, we have chosen a catalytic model reaction consisting in the oxidative dehydrogenation of benzyl alcohol into benzaldehyde in the presence of air [29]. In literature [29], the reaction was performed at 100 °C for 24 h under atmospheric pressure of air, using CuO NPs as catalyst in toluene with 0.33 mol/L of benzyl alcohol and 20 g/L of catalyst, corresponding to 75 mol% Cu with respect to benzyl alcohol. CuO NPs were synthesized from CuCl 2 .2H 2 O in water in presence of N-cyclohexyl-3-amino-propane sulfonic acid as chelating agent with pH adjustment at 11 with KOH, afforded, after washing and drying, a high benzaldehyde yield (> 99% conversion and selectivity).

First, the reaction conditions (solvent, catalyst amount, reaction time, temperature, air pressure) were optimized with the material prepared by incipient wetness impregnation consisting of CuO NPs supported on γ-Al 2 O 3 and calcined at 600 °C, Cu/γ-Al 2 O 3 (Cu/Al = 0.2). Toluene as the solvent can be oxidized into benzaldehyde and therefore it was replaced by cyclohexane. Without catalyst, benzyl alcohol could also be converted into benzaldehyde with 6% conversion for air pressure superior to 2 bar. The air pressure was therefore set at 2 bar. The highest conversion of benzyl alcohol conversion into benzaldehyde was obtained with 0.3 g of catalyst in 15 mL (20 g/L) of cyclohexane for 4.97 mmol of benzyl alcohol (0.33 mol/L), for a reaction temperature of 150 °C and a reaction time of 24 h. Benzaldehyde was the only product obtained, no traces of benzoic acid was detected.

The oxidative properties of all the materials have been evaluated using the optimal conditions defined above for Cu/γ-Al 2 O 3 (Cu/Al = 0.2). Benzaldehyde yields are reported in Fig. 5 for all alumina-based materials prepared by incipient wetness impregnation and sol-gel method. The highest benzaldehyde yields were obtained on copper containing materials prepared by incipient wetness impregnation on γ-Al 2 O 3 and calcined at 600 °C. In the case of Cu/γ-Al 2 O 3 materials, the benzaldehyde yield increased to 68% when the molar ratio Cu/Al increased from 0.1 to 0.2. Further increase of the amount of Cu on γ-Al 2 O 3 (Cu/Al = 0.4 and 0.5) had an unfavorable effect on the catalytic activity with a decrease of benzaldehyde yield to 30 %. This is probably due to an increase in size of the CuO NPs by sintering, leading to a smaller intrinsic surface area of the NPs. Interestingly and surprisingly, the materials prepared by co-impregnation of Cu and Mg, Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.1/0.1/1 and 0.2/0.2/1) led to higher benzaldehyde yields of 68 and 95%, respectively (Fig. 5). As explained above, Mg is more prone than Cu to enter in γ- for metal NPs in catalysis as they bring a structural promoting role towards interaction with the active metal allowing higher metal dispersion by stabilizing smaller nanoparticles [33][34][35][36][37].

To conclude, the materials with the highest oxidative character are Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1) > Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.1/0.1/1) = Cu/γ-Al 2 O 3 (Cu/Al = 0.2). Notably, Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1) features both high oxidative character and high basicity.

Catalytic ozonation

Reaction mechanisms

The mechanism of ozonation has been investigated in previous works [13,38] using various model substrates in the presence or absence of catalyst. Main results are summarized in Tables 4 and5. With 2,4-dimethylphenol (DMP) as starting substrate, intermediate molecules have been identified, and final by-products consisted of acetic acid > formic acid >> oxalic acid.

Acetic acid is formed rapidly similarly for single and catalytic ozonation, though in lower amount in the presence of catalyst. Acetic acid is the main final product as it cannot be oxidized by O 3 or HO°. The use of catalysts significantly increases the efficiency of the ozonation process as illustrated in runs 1, 4, 7 and 9, Table 4. For γ-Al 2 O 3 initial rate of degradation is increased by a factor 5.3 and TOC removal by a factor 3 to 4. This is due to the occurrence of the radical pathway, as evidenced by the addition of OH° scavengers such as NaCl and t-butanol in the reaction medium (runs 5, 6 and 8, Table 4). In both cases the initial rate of TOC removal is significantly decreased as well as the final extent of TOC removal. In the case of the synthetic effluent simulating produced water from off-shore oil extraction (phenol (200 mg/L), polycyclic aromatic hydrocarbons (PAHs) as naphtenic acid (25 mg/L), pyrene (0.05 mg/L) and naphthalene (0.95 mg/L), acetic acid (200 mg/L)), the use of γ-Al 2 O 3 as catalyst resulted in a strong (6.4 fold) increase of the initial rate of TOC removal (runs 10 and 12, Table 5) compared to direct ozonation, as observed with the DMP substrate. The mechanism is therefore similar and is a mixed mechanism O 3 + HO° + adsorption (γ-Al 2 O 3 ).

In both systems (direct and catalytic ozonation), however, the extent of TOC removal limits at 50-60%, due to the high initial content of refractory acetic acid in the starting solution. The presence of salt (NaCl) in the synthetic effluent (runs 10, 11, 12 and 13, Table 5) has similar scavenging effect for HO° as for DMP substrate for the initial rate of TOC removal for direct ozonation (with a decrease by a factor 1.7) and slightly less for catalytic ozonation (with a decrease by a factor 1.4). But, the main difference with DMP substrate is that the decrease of initial reaction rate resulted in a dramatic increase of the level of TOC removal, the latter being above 90% after 300 min reaction. The extent of TOC removal achieved, implying decomposition of acetic acid, led to the conclusion that a somewhat different, and more complex mechanism, was operating in this case. By reference to literature data it was therefore suggested [13] that PAH were oxidized by reaction with chlorine or chlorine radicals, within in addition, in-situ generation of HO° and hydrogen peroxide in the reaction medium, able to degrade acetic acid via the transient formation of peracetid acid, a powerful oxidant able to degrade PAH. This suggests two pathways for HO° generation: one from the hydroxyls of the catalysts and one from the decomposition of PAH. The following reactions are suggested:

1-production of HO° by the catalyst:

2-degradation of phenols into carboxylic acids 3-scavenging effect of HO° by Cl -:

4-reactions of Cl -derivatives with PAH:

Catalyst-OH + O 3 HO° (fast)
Phenols + HO° or O 3 acetic acid > formic acid >> oxalic acid

Cl -+ HO° HOCl -° HOCl -° + H + Cl° + H 2 O Cl -+ O 3 O 2 + ClO - Cl -+ ClO -+ 2H + Cl 2 + H 2 O Pyrene + Cl 2 dihydroxypyrene + H 2 O 2 Pyrene-dione + Chloropyrene + HO° CH 3 COOH + H 2 O 2 CH 3 COOOH + H 2 O CH 3 COOOH + PAH
degradation products of PAH (carboxylic acids)

5-final degradation of carboxylic acids:

We suggest that a faster production of HO° by the catalyst could predominate on the reaction rate of the scavenging by Cl -and reach higher initial degradation rate for saline produced water as obtained in the present work for γ-Al 2 O 3 doped with Mg (runs 15 to 18, Table 5) and presented below.

Catalytic ozonation of saline synthetic produced water

The catalytic properties γ-Al 3): specific surface areas close to 200 m 2 /g, mesopore volumes of 0.4-0.7 mL/g, mesopore diameters of 10 nm and C BET = 100-120 characteristic of hydroxylated surfaces. No increase of temperature was noticed during catalytic ozonation reactions. The pH of the effluent increased from pH = 3 -4 to 5 -7 when catalysts (freshly dried at 105°C) were added to the reaction mixture (Fig. S16). A brown color appeared as soon as ozone was introduced in the reactor and disappeared after 20 -50 min (Fig. S17) when the RedOx potential jumped steeply from 350 to 1000 mV (Fig. S18). Similar brown color has already been reported

during the oxidation of phenol with HO° radicals into ortho-and para-benzoquinone = 0.2/0.2/1), TOC removal reached 95% after 5 h (Fig. 6, runs 15, 16, Table 5). The evolution of TOC with these materials reveals that the main TOC reduction occurs within the first 2 h (> 90%).

Uncalcined γ-Al 2 O 3 was not active in catalytic ozonation, whereas γ-Al 2 O 3 calcined at 600 °C was active. This observation could explain why some authors consider γ-Al 2 O 3 as excellent in catalytic ozonation due to the generation of HO° [13,41,42], while some others found that γ-Al 2 O 3 does not catalyze the removal of pollutants such as hydrocarbons [21,41,42]. γ-Al 2 O 3 needs activation before to be used in catalytic ozonation. A good reproducibility of the results of literature was obtained with γ-Al 2 O 3 calcined at 600 °C (runs 13 and 14, Table 5).

The catalyst with only oxidative character such as Cu/γ-Al 2 O 3 (Cu/Al = 0.2) was poorly active in catalytic ozonation of the saline effluent as TOC removal was only around 65% after 5 h ozonation (Fig. 6) inferior to the TOC removal by single ozonation [13]. Cu leaching was also observed during catalytic ozonation as after reaction Cu/γ-Al 2 O 3 revealed a ratio Cu/Al of 0.12 instead of 0.20. On the contrary, all catalysts featuring high basicity properties proved very active, except the catalyst prepared by sol-gel, Mg/Cu/Al 2 O 3 (sg) (Cu/Mg/Al = 0.2/0.2/1). This catalyst was destroyed and densified during catalytic ozonation, its surface area has decreased from 300 to 20 m 2 /g (Table 3), which explains its low activity.

Catalysts prepared by incipient wetness impregnation exhibited a slight decrease (~ 25%) of surface area and pore volume after ozonation (S = 150-190 m 2 /g, V = 0.40-0.60 mL/g) (Table 3) with a preservation of their molar composition, except Mg/Cu/γ-Al 2 O 3 (Cu/Mg/Al = 0.2/0.2/1). In this case a large amount of Cu was leached during catalytic ozonation leading to 0.003 < Cu/Al < 0.07 measured by EDS on the solid. Therefore, only the basic character of this catalyst accounted for its high catalytic activity. Mg/Cu/γ-Al 2 O 3 (Cu/Mg/Al = 0.2/0.2/1) showed the same activity as Mg/γ-Al 2 O 3 (Mg/Al = 0.2) (Fig. 6).

From the results above, it appears that surface basicity is the only requirement to generate active and stable catalysts for catalytic ozonation of saline petroleum effluents while oxidative character do not play a significant role. 5). After 2 h reaction, the levels of TOC removal were 80.5, 90.3 and 93.9% and after 5 h they reached 92% for γ-Al 2 O 3 and 95% for Mg/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2) (Fig. 6). The stability of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) catalyst was evaluated in a second run of ozonation (runs 17, 18, Table 5). The catalyst recovered by filtration after a first run has been washed with demineralized water and dried at 105 °C. 3.3 g of catalyst were thus recovered.

Mg

During the second run, Mg/γ-Al 2 O 3 (Mg/Al = 0.1) catalyst (3.3 g/L) exhibited an outstanding improvement of activity, which reached 98% TOC removal after 5 h reaction (Fig. 7). For a better comparison with the fresh catalyst Mg/γ-Al 2 O 3 (Mg/Al = 0.1), an ozonation test with the same amount of fresh catalyst (3.3 g/L) was performed and revealed a much lower efficiency with 87 % TOC removal after 5 h. To understand the origin of the increase of efficiency of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after an ozonation cycle, the textural and structural features of the used catalysts were examined by nitrogen adsorption, TGA, SEM (Fig. 8), XRD (Fig. 9) and TEM (Fig. 10111213).

Textural and structural characterization of the catalysts after ozonation

After 1 and 2 ozonation runs the catalyst Mg/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2) preserved their textural properties: specific surface areas, mesopore volumes and mesopore diameters are similar (Table 3). at the surface (Fig. 9). TEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after one ozonation showed the classical morphology of γ-Al 2 O 3 with, in addition, a large population of nanoparticles (100 nm) (Fig. 10). Some of the nanoparticles started to segregate from the grains surface. After the second ozonation run, the segregation of the nanoparticles from the surface increased and some separate discrete phase appeared, formed by an aggregation of nanoparticles (Fig. 11). TEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.2) after one ozonation run showed the typical morphology of γ-Al 2 O 3 , some nanoparticles (100 nm) at the surface of the grains, with, in addition, some large plates characteristic of a lamellar structure (Fig. 12). are identical, revealing a same level of dehydroxylation, so a similar amount of surface -OHs (Fig. S23). However a slightly lower dehydroxylation degree was noticed for the Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after the second ozonation run, suggesting less surface OHs, whereas the ozonation efficiency was improved. A segregation of Mg at the surface of the alumina particles for MgO/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2) could be at the origin of the activity enhancement through the formation of Mg rich phases as spinel and LDH at the surface. This segregation might be favored under strong oxidizing reaction conditions as ozonation since the spinel amount increased upon successive ozonation.

The promotional effect of surface spinels on the catalytic ozonation activity has been reported several times and attributed to the generation of oxygen vacancies. Two hypothesis have been put forward to account for this observation: either (i) a higher amount of active-OH groups on the surface by water adsorption producing more HO° radicals [50,51] or (ii) reaction of ozone with vacancies producing very active superoxide radicals, which can in turn lead to formation of HO° radicals as well [52,53]. In both cases the flow of radicals is increased. In TGA profiles (Fig. S23) the weight loss occurring between 150 and 200 °C could be attributed to some water molecules trapped into the spinel structure, supporting the first hypothesis.

The proposed mechanisms operate most probably for phenols and PAHs oxidation, but presumably not for other types of pollutants such as chlorinated compounds, which are not interacting with HO° but rather degraded by direct ozonation of adsorbed ozone with adsorbed pollutants at the surface of the catalyst. In that case the process requires hydrophobic catalysts such as ZSM-5 zeolites with high Si/Al ratio [41]. A mixture of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) catalyst with silicalite-1 could represent a versatile catalyst for catalytic ozonation.

CONCLUSION

  adjusted with one drop of HCl), the benzoic acid concentration[C] shows a linear relationship with the maximum intensity (I) of the most intense band at 273 nm:I = 985.011 [C] -0.0741This equation was used to determine the amount of benzoic acid remaining in solution after adsorption on the different materials. Typically, 0.035 g of solid was added into 10 mL of 0.002 mol/L benzoic acid solution (pH = 3.3) under vigorous stirring for 4 h to insure adsorption equilibrium (no difference in UV intensity was observed after 2 and 24 h of stirring). Then the solution and the solid were separated by filtration. The pH of the solution was measured and adjusted at pH < 2 value with HCl. The solution is characterized by UV spectroscopy and the concentration of benzoic acid was determined. The amount of benzoic acid adsorbed on the solid was calculated by difference with the initial concentration.

  dissolved ozone, measured by the indigo method, was subtracted from the total ozone mass to access to the amount of ozone consumed during the chemical reaction. By dividing this amount by the amount of mineralized carbon measured by TOC, the amount of consumed ozone by oxidized carbon has been calculated by the following equation [27]:

  this temperature range.XRD patterns of Ca/γ-Al 2 O 3 and Ba/γ-Al 2 O 3 with M/Al = 0.05 calcined at 600 °C (Fig.S5)show the same peak positions as γ-Al 2 O 3 or Mg/γ-Al 2 O 3 (Mg/Al = 0.05), but with a decrease in peaks intensity suggesting a decrease in crystallinity when the ionic radius of the cation increases (Ba > Ca > Mg). This suggests that the incorporation of these cations into the spinel structure of γ-Al 2 O 3 induces a larger deformation for the larger cation (Ba). The incorporation of Ba and Ca in γ-Al 2 O 3 does not affect the textural properties (TableS1), which remain close to those of γ-Al 2 O 3 . XRD patterns (Fig. S6) of M/γ-Al 2 O 3 samples with M/Al = 0.05 calcined at 600 °C for M = Fe, Co, Ni, Cu, Zn, Zr, Ce show globally the same crystallographic structure as γ-Al 2 O 3 . XRD patterns of Fe/, Co/, Ni/, Zn/γ-Al 2 O 3 are similar to γ-Al 2 O 3 crystallographic structure suggesting the incorporation of these cations in the spinel structure of γ-Al 2 O 3 with slight decrease in crystallinity in the order Zn > Ni > Co > Fe. These cations are known to form MAl 2 O 4 spinel phases at high temperature. XRD patterns of Cu/, Zr/, Ce/γ-Al 2 O 3 show additional peaks suggesting the formation of oxides nanoparticles (NPs) at the surface of γ-Al 2 O 3 . For Zr/γ-Al 2 O 3 an additional broad peak at 2ϴ = 30° suggests the formation of some tiny ZrO 2 NPs at the surface of γ-Al 2 O 3 . Similarly for Ce and Cu additional peaks corresponding to oxide NPs are observed. Peaks at 29 and 57° in 2ϴ for Ce/γ-Al 2 O 3 are characteristic of CeO 2 NPs and peaks at 36 and 39° in 2ϴ for Cu/γ-Al 2 O 3 are characteristic of CuO NPs with monoclinic structure. The impregnation with Co on γ-Al 2 O 3 for a larger amount of Co (Co/Al = 0.5) leads to additional XRD peaks (Fig. S7) appearing at 2ϴ = 32, 37, 45, 59, 66° characteristic of Co 3 O 4 NPs formation at the surface of spinel Co/γ-Al 2 O 3 leading to a decrease of surface area and mesopore volume (Table S2). A co-addition of Mg, Ca or Fe with Co does not change the XRD patterns, whereas a co-addition of Cu leads to the formation of supplementary CuO NPs. N 2 adsorption-desorption isotherms of the different elements (Fe, Co, Ni, Cu, Zn, Ce, Zr with M/Al = 0.05) deposited on γ-Al 2 O 3 show little variation of textural properties of γ-Al 2 O 3 except for Ce/γ-Al 2 O 3 , which shows a decrease of surface area and mesopore volume suggesting the blocking of some mesopores by CeO 2 NPs, while CuO NPs does not affect textural properties of γ-Al 2 O 3 (Table

≤

  0.2 and 0.3 ≤ Cu/Al ≤ 0.5, respectively. It is to notice that spinel-type materials CuAl 2 O 4 and CuMgAl 2 O 4 have already been used efficiently in catalytic ozonation for the degradation of clofibric acid, a blood lipid regulator [31].

Fig. 2 .

 2 Fig. 2. (a) XRD patterns and (b) nitrogen sorption isotherms at 77 K of Cu/γ-Al 2 O 3 samples prepared by incipient wetness impregnation with different Cu/Al molar ratios and calcined at 600 °C.

Fig. 3 .

 3 Fig. 3. (a) XRD patterns and nitrogen sorption isotherms at 77 K of Mg/Cu/γ-Al 2 O 3 samples prepared by incipient wetness impregnation with different molar ratios and calcined at 600 °C.

  Scheme 2. Example of UV spectra of benzoic acid obtained in acidic and basic conditions

  3. By adding γ-Al 2 O 3 , the pH increased to 4. At pH = 4 there was a mixture of C 6 H 5 -COOH and C 6 H 5 -COO -and the surface of γ-Al 2 O 3 had a majority of Al-OH species with few Al-O -groups. Benzoic acid was adsorbed on the alumina surface with an amount of 0.37 mmol/g due mainly to Al-OH/C 6 H 5 -COOH interactions. The addition of Mg/γ-Al 2 O 3 materials in the benzoic solution led to an increase of pH values from 5 to 8 depending on Mg amount. The pH of the solution increased with the increase of Mg content and in parallel benzoic acid adsorption decreased with no adsorption observed for Mg/Al > 0.06 (Fig. S14, Table

  2 O 3 was therefore improved by the incorporation of Mg on γ-Al 2 O 3 . As no adsorption of benzoic acid was observed for Mg/Al > 0.06, Mg/γ-Al 2 O 3 materials should prove effective for sustainable catalytic ozonation. The increase of pH of the benzoic solution is related to the increase of basicity of the materials and is accompanied by a decrease of benzoic acid adsorption (Fig. S15), which is the opposite of what was suggested in literature [32]. In literature [32], nitrogen-doped porous carbons have been studied, which are weakly basic materials leading presumably to no pH increase. In this case the maximum benzoic acid adsorption was found for the more basic materials as only C-N/C 6 H 5 -COOH interactions are involved. When the pH increases above a value of 4, the amount of C 6 H 5 -COO -increases, leading to less adsorption of benzoic acid due to electrostatic repulsion with Al-O -groups on the surface.

Fig. 4 .

 4 Fig. 4. Basicity test: pH of remaining benzoic acid solution after 4 h contact with metal-doped aluminas materials.

  Ce and Zr deposited on γ-Al 2 O 3 show a small increase of pH of the benzoic acid solution with pH = 4.35 for Ce/γ-Al 2 O 3 and pH = 4.22 for Zr/γ-Al 2 O 3 and lead to an amount of benzoic acid adsorbed of 0.27-0.28 mmol/g. These results are in accordance with the relationship between the pH of solution and the adsorbed benzoic acid amount (Fig.S15) correlated to the amount of anionic form of benzoic acid, which increases with the increase of pH accordingly to its pKa value. An exception is observed for Fe/γ-Al 2 O 3 , which shows a slight decrease of pH (pH = 3.8) in comparison to γ-Al 2 O 3 (pH = 4.0), and no adsorption of benzoic acid (Fig.S15). We suggest that Fe interacts and covers the basic sites of γ-Al 2 O 3 .Cu/γ-Al 2 O 3 features the highest basicity among non alkaline-earth metals deposited on γ-Al 2 O 3 , but its basicity remains below alkaline-earth/γ-Al 2 O 3 as Mg/γ-Al 2 O 3 with pH of benzoic acid equal to 8.1 for Mg/Al = 0.2 (Fig.4, Table

  highest basicity was revealed by the materials containing alkaline-earth metals. The basicity decreases in the following order: Mg/γ-Al 2 O 3 (Mg/Al = 0.2) ≥ Mg/Cu/Al 2 O 3 (sg) (Mg/Cu/Al = 0.2/0.2/1) > Ca/γ-Al 2 O 3 (Ca/Al = 0.07) > Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1) > Mg/γ-Al 2 O 3 (Mg/Al = 0.12). The basicity brought by the transition metals is lower and it decreases in the order: Cu > Co > Ce > Zn > Ni = Zr > Fe.

Fig. 5 .

 5 Fig. 5. Oxidation test: yields of benzaldehyde with the different M/Al 2 O 3 catalysts. The reaction condition: benzyl alcohol (517 µL), cyclohexane (15 mL), 0.3 g catalyst, 150 o C for 24 h, under air 2.0 bar.

  Al 2 O 3 spinel structure and consequently leads to the formation of larger size CuO NPs at the surface. However, in this case the growth of the size of the NPs has a favorable impact on the catalytic activity. This indicates that the support for CuO NPs plays a role in the reaction. The strong interaction between a strongly basic support, which is electron donor, as Mg/γ-Al 2 O 3 , with CuO NPs prevents their migration on the surface and their sintering. The higher activity is presumably due to a better dispersion of CuO NPs at the surface of Mg/γ-Al 2 O 3 in comparison to γ-Al 2 O 3 . Mg/γ-Al 2 O 3 mixed-oxides are often preferred to γ-Al 2 O 3 as supports

  For γ-Al 2 O 3 initial rate of degradation is decreased by a factor 2.2 and TOC removal by a factor 1.6. Without scavengers, phenol removal results from direct ozonation with O 3 , from reaction with OH° radicals and from adsorption of by-products on the catalyst in the case of γ-Al 2 O 3 . FTIR and 13 C MAS NMR experiments demonstrated that the active catalytic sites of γ-Al 2 O 3 are surface basic sites.The use of scavengers allowed to determine the initial rate of degradation for each mechanism: for the mixed mechanism O 3 + HO° + adsorption (γ-Al 2 O 3 ) the initial rate of degradation was 5.3 times higher than for a mechanism with O 3 alone, and the mixed mechanism O 3 + adsorption (γ-Al 2 O 3 + scavenger) was 2.4 times higher than direct O 3 ozonation.

  2 O 3 modified with different metal oxides were evaluated in the ozonation the synthetic produced water. As shown in the experimental section the main characteristics of this effluent are: pH = 3.3 -4.3, COD = 668 mg O 2 /L, TOC = 216 mg/L, RedOx potential = 350 mV, Cl -= 30 g/L. After preliminary investigations, we chose to probe the more basic materials, ie, Mg/γ-Al 2 O 3 (Mg/Al = 0.2), Mg/Cu/Al 2 O 3 (sg) (Cu/Mg/Al = 0.2/0.2/1) > Mg/Cu/γ-Al 2 O 3 (Cu/Mg/Al = 0.2/0.2/1), Mg/γ-Al 2 O 3 (Mg/Al = 0.1) and the materials featuring the highest oxidative character, ie, Mg/Cu/γ-Al 2 O 3 (Cu/Mg/Al = 0.2/0.2/1) > Cu/γ-Al 2 O 3 (Cu/Al = 0.2) (Fig. 6). All the materials have been activated at 600 °C. Commercial γ-Al 2 O 3 calcined and uncalcined have been used for comparison. Note that all catalysts featured similar textural properties (Table

Fig. 6 .

 6 Fig. 6. TOC removal (%) of saline petroleum effluent (initial TOC = 216 mg/L) as a function of ozonation time for the catalysts (5 g/L): (n) uncalcined γ-Al 2 O 3 , (☐) γ-Al 2 O 3 calcined at 600 °C and materials calcined at 600 °C with (+) Mg/Cu/γ-Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1), (Δ) Mg/γ-Al 2 O 3 (Mg/Al = 0.1), (◊) Mg/γ-Al 2 O 3 (Mg/Al = 0.2), (¢) Cu/γ-Al 2 O 3 (Cu/Al = 0.2), (X) Mg/CuO/Al 2 O 3 (Mg/Cu/Al = 0.2/0.2/1) prepared by sol-gel (sg). Dashed line represents the ozonation without catalysts taken from ref. 13.

  [39]. The transient color change is attributed to the formation of dihydroxylated ring intermediates, which can react with their own quinones to generate charge-transfer complexes, which take a dark color at low concentrations. Para-benzoquinone has been used as HO° radicals scavenger in the catalytic ozonation of AO7 (C 16 H 11 N 2 NaO 4 S) dye with Cu/Al 2 O 3 catalyst [40].Benzoquinone drastically inhibited the catalytic activity of Cu/Al 2 O 3 due to rapid reaction with HO°. Therefore we may assume that in our case phenol is first oxidized through direct ozonation into benzoquinone, which then rapidly reacts with HO° to give oxidized products. When all benzoquinone is transformed, the coloration disappears and the concentration of ozone in solution increases. During the first 20 to 50 min, the TOC values decreased but in a different way, depending on the catalyst: 20% of TOC removal for CuO/γ-Al 2 O 3 (Cu/Al = 0.2), Mg/Cu/Al 2 O 3 (sg) (Cu/Mg/Al = 0.2/0.2/1) and 30-50% of TOC removal for γ-Al 2 O 3 calcined at 600 °C, Mg/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2), Mg/Cu/γ-Al 2 O 3 (Cu/Mg/Al = 0.2/0.2/1) (Fig.S18).TOC removal level increased with ozonation time (Fig.6) to reach 70-80% after 5 h for Cu/γ-Al 2 O 3 (Cu/Al = 0.2), Mg/Cu/Al 2 O 3 (sg) (Cu/Mg/Al = 0.2/0.2/1) and uncalcined γ-Al 2 O 3 . For γ-Al 2 O 3 calcined at 600 °C and Mg based catalysts prepared by incipient wetness impregnation on γ-Al 2 O 3 (Mg/γ-Al 2 O 3 , Mg/Al = 0.1 and 0.2, and Mg/Cu/γ-Al 2 O 3 , Cu/Mg/Al

  /γ-Al 2 O 3 (Mg/Al = 0.1) exhibited similar but slightly lower catalytic activity in comparison to Mg/γ-Al 2 O 3 (Mg/Al = 0.2), due to its slightly lower basicity, evidenced by a slightly lower pH increase during ozonation (Fig. S19). After 5 h of catalytic ozonation, the effluents featured pH values of 8.6 and 9.2 for Mg/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2), respectively. However Mg/γ-Al 2 O 3 (Mg/Al = 0.1) was the most efficient catalyst in terms of amounts of ozone consumed for the degradation of pollutants. It allowed less ozone consumption in comparison to all other catalyst with 12 g of consumed ozone per g of carbon mineralized (Fig. S20, S21) corresponding to 3 moles of O 3 per mole of C mineralized. This catalyst Mg/γ-Al 2 O 3 (Mg/Al = 0.1) might be considered among the most efficient catalysts for catalytic ozonation of saline produced water.Catalysts solely composed of MgO nanoparticles have already proven good catalysts for catalytic ozonation of quinolone by generating HO° radicals[43]. They have shown relatively good stability in reuse, but with a progressive decrease of activity after each reuse, leading to a decrease of degradation level of quinolone from 100 to 93% after 5 cycles. This loss of activity was attributed to the increase of the size of the MgO NPs from 17 to 37 nm by sintering during ozonation, reducing its overall activity[43].

Fig. 7 .

 7 Fig. 7. TOC removal (%) of saline petroleum effluents (initial TOC = 216 mg/L) as a function of ozonation time for Mg/γ-Al 2 O 3 (Mg/Al = 0.1) calcined at 600 °C with (¢) 3.3 and (Δ) 5 g/L of catalyst and ( ) in reuse with 3.3 g/L of catalyst.

Fig. 8 .

 8 Fig. 8. SEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after two catalytic ozonations.

Fig. 9 .

 9 Fig. 9. TEM pictures of γ-Al 2 O 3 (calcined 600 °C) after one catalytic ozonation.

Fig. 10 .

 10 Fig. 10. TEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after one catalytic ozonation.

Fig. 11 .

 11 Fig. 11. TEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after two catalytic ozonations.

Fig. 12 .

 12 Fig. 12. TEM pictures of Mg/γ-Al 2 O 3 (Mg/Al = 0.2) after one catalytic ozonation.

Fig. 13 .

 13 Fig. 13. XRD pattern of catalysts after catalytic ozonation: (grey) Mg/γ-Al 2 O 3 (Mg/Al = 0.1) after 1 and 2 ozonation (reuse) and Mg/γ-Al 2 O 3 (Mg/Al = 0.2) after 1 ozonation. For comparison in each graph (black) calcined γ-Al 2 O 3 (600 °C) after 1 ozonation. Diffraction peak for orthorhombic spinel (arrows) and LDH -d 100 = 0.78 nm (*). Schematic representation of the orthorhombic spinel structure.

  Previously, we have demonstrated that γ-Al 2 O 3 was an efficient catalyst for ozonation as it produces HO° radicals from the reaction of O 3 with Al-OH basic sites. In the case of saline effluents, chloride ions play usually a very strong scavenging effect on HO° radicals, which increases with the amount of NaCl and account for 70% of HO° neutralization for NaCl = 50 g/L. However, when the effluents contained polycyclic aromatic hydrocarbons (PAHs) the opposite effect was observed. It was proposed that PAHs react with Cl° or Cl 2 , coming from reaction of Cl -with HO° and Cl -with O 3 , respectively, to produce new HO° and H 2 O 2 , which are able to degrade carboxylates into CO 2 and HO -leading to an increase of pH. In the case of petroleum saline (NaCl 50 g/L) effluents containing acetic acid (30%), phenols and PAHs, the combination of O 3 , NaCl and PAHs allow a mineralization of 90% of the pollutants in 5 h including the mineralization of acetic acid, which is the most resistant carboxylic acid to oxidize. The addition of γ-Al 2 O 3 increases the rate of mineralization in comparison to O 3 with the same level of final mineralization.In this study, in order to further improve catalytic ozonation, mesoporous alumina-based materials containing different promoters (Mg, Ca, Zn, Fe, Ba, Zr, Cu, Co, Ni, Ce, Ti) have been prepared either by incipient wetness on γ-Al 2 O 3 particles or by direct sol-gel (sg) precipitation. These metals have been chosen to improve either the basicity and/or the oxidative character of alumina. A basicity scale in water has been developed based on the amount of benzoic acid adsorbed and the pH of the resulting solutions. An oxidative test has been developed based on the oxidation of benzyl alcohol into benzaldehyde in cyclohexane at 150 °C. The more basic solids were Mg-based materials with molar ratios: Mg/Al (0.2/1), Cu/Mg/Al (0.2/0.2/1)(sg) > Cu/Mg/Al (0.2/0.2/1), Mg/Al (0.1/1). The materials featuring the higher oxidative properties were CuO nanoparticles-based materials with Cu/Mg/Al (0.2/0.2/1) > Cu/Al (0.2/1), Cu/Mg/Al (0.1/0.1/1). All materials present high surface area, mesopore volumes and mesopore diameters similar to γ-Al 2 O 3 (200 m 2 /g, 0.6 mL/g, 11 nm diameter). These materials were tested in the catalytic ozonation of a synthetic petroleum saline effluent representative of a produced water containing phenols, acetic acid and PAHs. The results show that: (1) γ-Al 2 O 3 leads to an improvement of ozonation rate only if it is calcined at 600°C, (2) materials with only oxidative properties as Cu/Al (0.2/1) are not efficient, (3) basic materials are the most efficient. The rate of ozonation increases with the increase of basicity of the material, so with the Mg content. After 5 h ozonation, the pH of the solution initially at 3.3-4.3 has increased to 8.5 for Mg/Al (0.1/1) and to 9.2 for Mg/Al (0.2/1). Ozonation efficiency is only related to the basicity of the materials.Materials prepared by incipient wetness on γ-Al 2 O 3 particles maintain their textural characteristics (mesopore volume, diameter and surface area), whereas sol-gel materials are destroyed during ozonation. Mg/Al (0.2/1), Cu/Mg/Al (0.2/0.2/1), Mg/Al (0.1/1) catalysts lead to the highest rate of ozonation with a mineralization level > 90% after 2 h and 95% after 5 h. Ozonation leads to the leaching of copper in Cu/Mg based materials, whereas the composition of Mg/Al materials remains constant. Mg is the only efficient metal. Mg/γ-Al 2 O 3 (Mg/Al = 0.1) is the most efficient catalyst in terms of mineralization level and consumption of O 3 to oxidize the pollutants. Furthermore, this catalyst in reuse shows a remarkable improvement of catalytic activity with the highest level of mineralization (98%). This increase of catalytic ozonation activity in reuse has never been observed before and is attributed to the formation of nanoparticles of orthorhombic spinel structure (MgAl 2 O 4 ) at the surface of the grains of γ-Al 2 O 3 generating probably more HO° radicals. A catalyst formed only by nanoparticles of orthorhombic spinel structure MgAl 2 O 4 could be an outstanding catalyst for ozonation. For the catalyst with higher Mg content Mg/γ-Al 2 O 3 (Mg/Al = 0.2), some layered double hydroxide (LDH) are formed at the expense of the spinel. Both Mg/γ-Al 2 O 3 oxides (Mg/Al = 0.1 and 0.2) catalysts are promising catalysts for ozonation of real saline petroleum effluents containing PAHs, such as produced water.

Table 1 .

 1 List of metal-doped aluminas prepared by incipient wetness with 2 g γ-Al 2 O 3

	Catalyst	Metal salt	Metal salt	Metal/Al
			g	molar ratio
	γ-Al 2 O 3	none	0	0
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	2.0	0.2
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	1.18	0.12
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	0.60	0.06
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	0.30	0.03
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	0.15	0.015
	Mg/γ-Al 2 O 3	Mg(NO 3 ) 2 .6H 2 O	0.076	0.007
	Ca/γ-Al 2 O 3	CaCl 2	0.22	0.07
	Zn/γ-Al 2 O 3	Zn(NO 3 ) 2 .6H 2 O	0.38	0.04
	Fe/γ-Al 2 O 3	(NH 4 ) 2 Fe(SO 4 ) 2	0.59	0.05

Synthesis of metal doped-alumina by sol-gel precipitation

Alumina and metal doped-alumina have been prepared by a sol-gel method developed previously for pure alumina

[25]

. 20 g of Al(O sec Bu) 3 were poured into a glass vessel containing 2-butoxyethanol (BuOEt) (162 g) under vigorous stirring. Then 0.3 g of dodecylamine (DDA) was added into the mixture solution under stirring. When DDA was fully dissolved, 7.6 mL of water or aqueous metal (Me) salt solutions were added dropwise.

The obtained sols were vigorously stirred and kept in static at 40 °C for 24 h. The obtained solids were filtered, washed several times with 2-BuOEt, dried at 80 °C and finally calcined at 600, 750 or 800 °C for 5 h. The molar composition of the mixture is 1 Al / 0.04 DDA/ 5.27 H 2 O/ 17 BuOEt/ x Me, x = 0.1 -0.5 (Table

2

).

Table 2 .

 2 List of metal doped-alumina prepared by sol-gel method for 20 g Al(O sec Bu) 3

	Catalyst	Metal salt	Metal salt	Me/Al	T calcination
			g	molar ratio	°C
	Al 2 O 3	/	0	/	/
	Ti-Al 2 O 3	Titanium isopropoxide	6.143	0.25
	Cu-Mg-Al 2 O 3	Cu (NO 3 ) 2 and Mg(NO 3 ) 2	3.91 and 4.15	0.2
	Cu-Mg-Al 2 O 3	Cu (NO 3 ) 2 and Mg(NO 3 ) 2	3.91 and 4.15	0.2
	Cu-Al 2 O 3	Cu (NO 3 ) 2	3.96	0.2
	Cu-Al 2 O 3	Cu (NO 3 ) 2	3.96	0.2
	Cu-Ca-Al 2 O 3	CaCl 2 and Cu (NO 3 ) 2	1.82 and 3.96	0.2
	Cu-Ca-Al 2 O 3	CaCl 2 and Cu (NO 3 ) 2	1.82 and 3.96	0.2
	Cu-Mg-Al 2 O 3	Cu (NO 3 ) 2 and Mg(NO 3 ) 2	9.9 and 10.5	0.5
	Ce-Al 2 O 3	Ce(NO 3 ) 3 . 6H 2 O	3.56	0.1

Table 3 .

 3 Textural properties determined from N 2 sorption isotherms at 77 K of catalysts before and after ozonation: γ-Al 2 O 3 , Cu/γ-Al 2 O 3 , Mg/γ-Al 2 O 3 and Mg/Cu/γ-Al 2 O 3 synthesized by incipient wetness impregnation and sol-gel (sg) and calcined at 600 °C.

	The same crystallographic structure was

Table 4 .

 4 Ozonation of 2,4-dimethylphenol. (Temperature : 25 °C, O 3 flow rate : 40 L/h, Inlet O 3 concentration : 2 g/Nm 3 , [2,4-DMP] = 50 ml/L, Solution volume : 1.5 L)

	Run #	Catalyst	Radical scavenger	Relative initial	% TOC	Ref.
				reaction rate	removal after
					300 min
	1	None	None	1	14	38
	2	None	NaCl (50 g/L)	0.7	10	38
	3	None	t-BuOH (25 mg/L)	0.8	12	13
	4	Na-LTA (2 g/L)	None	4.0	34	38
	5	Na-LTA (2 g/L)	NaCl (50 g/L)	0.9	12	38
	6	Na-LTA (2 g/L)	t-BuOH (25 mg/L)	1.3	15	38
	7	γ-Al 2 O 3 (2 g/L)	None	5.3	47	13
	8	γ-Al 2 O 3 (2 g/L)	t-BuOH (25 mg/L)	2.4	30	13
	9	γ-Al 2 O 3 (5 g/L)	None	5.3	57	13

  The formation of orthorhombic spinel phase MgAl 2 O 4 and possibly LDH phase are therefore responsible for the outstanding activity of the catalysts Mg/γ-Al 2 O 3 (Mg/Al = 0.1) in catalytic ozonation of saline petroleum effluent. In general, the catalytic use of cubic spinel MgAl 2 O 4 has received significant attention as an alternative to traditional alumina-supported catalysts because of its high thermal stability and strong basicity [45]. MgAl 2 O 4 is an important ceramic material due to its attractive combination of physical, chemical, optical and magnetic properties. It is mainly obtained as dense structures featuring very low surface areas. Efforts have been paid to synthesize ultrafine or nanosized powders of MgAl 2 O 4 [44]. Nano-sized Al 2 O 3 , both featuring highest efficiencies than γ-Al 2 O 3 . After 40 min reaction, COD removal was 34, 46 and 52% for γ-Al 2 O 3 , Mg/γ-Al 2 O 3 and Mg-Ce/γ-Al 2 O 3 , respectively. Increasing the Mg amount (Mg/Al = 0.0077) did not change the efficiency. Authors have shown that Mg deposited on γ-Al 2 O 3 enhanced further HO° generation and that direct ozonation operated as well. The active catalytic surface promote formation of HO° from ozone, accelerating chemical degradation. Authors state that the density of surface OH groups stand in the order: γ-Al 2 O 3 > Mg/γ-Al 2 O 3 > Mg-Ce/γ-Al 2 O 3 [49], suggesting that the strength of the basic sites was more important than the number of OH groups in determining catalytic efficiency. Interestingly, it was shown that Mg 2+ was well dispersed on the γ-Al 2 O 3 surface with strong interactions between Mg and γ-Al 2 O 3 support and that the surface content of MgO was greater than that of the bulk [49]. In our case, TGA profiles and surface areas of γ-Al 2 O 3 and Mg/γ-Al 2 O 3 (Mg/Al = 0.1 and 0.2)

	MgAl 2 O 4 powders have been produced at 1150 °C for 4 h [46] or in an ultrasonic oxygen
	atomizer followed by a combustion at 2000 °C [44]. Round nanoparticles (< 60 nm) have
	been obtained featuring surface areas of 30-80 m 2 /g [44]. Spinel MgAl 2 O 4 can be produced at
	lower temperature (700 °C) by chlorination, while in air a temperature above 930 °C was
	necessary [47].
	Recently, mesoporous flake-like cubic spinel MgAl 2 O 4 nanoparticles (10 nm) featuring high
	surface area of 264 m 2 /g with a mesopore diameter of 6 nm have been produced by the sol-gel
	process using nitrate salts of Al and Mg in the presence of propylene oxide and a calcination
	of 700 °C [48]. This material should be worth evaluation in catalytic ozonation. Such a flake-
	like morphology with a high surface area of 169 m 2 /g is similar to that of the confidential
	commercial catalysts used by Sun et al. [42] for ozonation of non saline petroleum effluents,
	and could be therefore a spinel CuAl 2 O 4 . To the best of our knowledge, no spinel MgAl 2 O 4
	catalysts have been used in catalytic ozonation. Only CuAl 2 O 4 and Cu x Mg (1-x) Al 2 O 4 (20 m 2 /g)
	spinels have been reported, with the limitation of Cu leaching [31]. Authors showed slightly
	higher activity and stability for CuMgAl spinels in comparison to HDLs based materials
	(MgAl, MgFeAl and MgCuAl) [31].
	In our study, spinel MgAl 2 O 4 nanoparticles (100 nm) are produced at the surface of γ-Al 2 O 3
	under strong oxidizing conditions at ambient temperature from Mg/γ-Al 2 O 3 (Mg/Al = 0.1)
	materials previously calcined at 600 °C. In recent literature, Mg/γ-Al 2 O 3 (Mg/Al = 0.0025)
	was prepared by incipient wetness impregnation followed by a calcination at 550 °C and used
	in the catalytic ozonation (with a catalyst amount of 5 g/L) of non saline petroleum effluents
	featuring a pH of 8.15 containing 27% of organic alcohol and 25% of heterocyclic
	compounds [49]. Slightly higher efficiencies were found for Mg-Ce/γ-Al 2 O 3 compared to
	Mg/γ-

Acknowledgments

This research was funded by the TOTAL SA through a grant from Fondation Franco-Chinoise pour la Sciences et ses Applications (FFCSA). Authors thank Anne-Sophie Caen and Patrick Van Effenterre from FFCSA for their support. Authors thank Didier Tichit and Dan Lerner for fruitful discussions. The authors wish to acknowledge the support from the Chemistry Plateform of Campus in Montpellier (Plateform MEA University Montpellier), on which SEM, TEM and EDS experiments have been performed.