Djaouidaallam

Simona Bennici

Lionel Limousy

Smain Hocine

Improved Cu,Zn-based catalysts for CO 2 hydrogenation to methanol Catalyseurs améliorés à base de Cu et Zn pour l'hydrogénation de CO 2 en méthanol

Keywords: CO 2 valorization, polyol synthesis, methanol synthesis, oxide catalysts

Dans cette étude, des catalyseurs binaires et ternaires (CuO-CeO 2 , ZnO-CeO 2 , CuO-ZnO-CeO 2 et CuO-ZnO-Al 2 O 3 ) ont été utilisés pour l'hydrogénation CO 2 en méthanol à pression atmosphérique. L'influence de la dispersion des métaux, de la formation de spinelles et des propriétés de surface sur les performances catalytiques ont été étudiées.

Les catalyseurs, préparés par la méthode polyol en utilisant du polyéthylène glycol en tant que solvant, présentent des propriétés améliorées en termes de dispersion des oxydes métalliques, de morphologie (forme de type éponge pour les catalyseurs contenant CeO 2 ) et de variétés d'espèces métalliques et d'oxydes métalliques à la surface. De plus, les catalyseurs CuO-ZnO-CeO 2 et Cu-ZnO-Al 2 O 3 présentent une activité et une sélectivité supérieures à celles de catalyseurs préparés par des procédés plus conventionnels, pour la synthèse du méthanol par hydrogénation de CO 2 .

Introduction

The concentration of carbon dioxide (a greenhouse gas) inexorably increases in the atmosphere. Various methods are currently examined to decrease the CO 2 concentration or to convert it into valuable chemical products. The utilization of CO 2 as a feedstock for the synthesis of high added value chemicals is a promising alternative for CO 2 abatement [1,2].

The simplest way to utilize carbon dioxide is the hydrogenation into valuable compounds, such as methanol (MeOH) and DME (dimethyl ether) [START_REF] Matzen | Methanol and dimethyl ether from renewable hydrogen and carbon dioxide: Alternative fuels production and life-cycle assessment[END_REF][START_REF] Bos | A novel condensation reactor for efficient CO 2 to methanol conversion for storage of renewable electric energy[END_REF][START_REF] Wang | Recent advances in catalytic hydrogenation of carbon dioxide[END_REF][START_REF] Witoon | Direct synthesis of dimethyl ether from CO 2 hydrogenation over Cu-ZnO-ZrO 2 /SO 4 2-ZrO 2 hybrid catalyst: effects of sulfur-to-zirconia ratios[END_REF][START_REF] Olah | Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons[END_REF][START_REF] Waugh | Methanol synthesis[END_REF]. Such conversion is often performed on copper-based catalysts. Industrial methanol synthesis is performed by catalytic hydrogenation of syngas (H 2 /CO/CO 2 ) over Cu/ZnO/Al 2 O 3 -type catalyst. Unfortunately, the industrial Cu/ZnO/Al 2 O 3 catalyst are not active nor selective in CO 2 hydrogenation. Previous published research works demonstrated that the use of industrial catalysts brings to very low hydrogenation conversions of CO 2 to methanol [START_REF] Hansen | Methanol synthesis, Handbook of heterogeneous catalysis[END_REF]. This issue can be overcome by developing suitable catalysts, which can effectively convert carbon dioxide to methanol [START_REF] Arakawa | Selective conversion of CO 2 to methanol by catalytic hydrogenation over promoted copper catalyst[END_REF][START_REF] Saito | Development of Cu/ZnO-based high performance catalysts for methanol synthesis by CO 2 hydrogenation[END_REF][START_REF] Jun | Concurrent Production of Methanol and Dimethyl Ether from Carbon Dioxide Hydrogenation: Investigation of Reaction Conditions[END_REF][START_REF] Ushikoshi | A 50 kg/day class test plant for methanol synthesis from CO 2 and H 2[END_REF]. Even if copper based materials are promising catalysts for the hydrogenation of CO 2 [START_REF] Witoon | CO 2 hydrogenation to methanol over Cu/ZnO nano catalysts prepared via a chitosan-assisted coprecipitation method[END_REF][START_REF] Dong | CO 2 hydrogenation to methanol over Cu/ZnO/ZrO 2 catalysts prepared by precipitation-reduction[END_REF][START_REF] Li | Highly selective hydrogenation of CO 2 to methanol over CuO-ZnO-ZrO 2 catalysts prepared by a surfactant-assisted co-precipitation method[END_REF][START_REF] Guo | CO 2 hydrogenation to methanol over Cu/ZnO/ZrO 2 catalysts prepared via a route of solid-state reaction[END_REF][START_REF] Ateka | Performance of CuO-ZnO-ZrO 2 and CuO-ZnO-MnO as metallic functions and SAPO-18 as acid function of the catalyst for the synthesis of DME co-feeding CO 2[END_REF][START_REF] Deerattrakul | CO 2 hydrogenation to methanol using Cu-Zn catalyst supported on reduced graphene oxide nanosheets[END_REF], further investigations are required for developing new catalytic materials able to give high conversion of CO 2 and improved selectivity to MeOH.

In order to improve the catalytic performance for methanol synthesis from H 2 /CO 2 feeding gas, CuO/ZnO catalysts have been widely modified by adding various activators or other metals (Zr, Si, La, Ti, Cr, Ga, Ce, Fe, Nb, Pd...) [START_REF] Zhang | Effect of promoter TiO 2 on the performance of CuO-ZnO-Al 2 O 3 catalyst for CO 2 catalytic hydrogenation to methanol[END_REF][START_REF] Huang | Catalytic hydrogenation of CO 2 to methanol: study of synergistic effect on adsorption properties of CO 2 and H 2 in CuO/ZnO/ZrO 2 system[END_REF][START_REF] Liaw | Liquid-phase synthesis of methanol from CO 2 /H 2 over ultrafine CuB catalysts[END_REF][START_REF] Prasad | Fischer-Tropsch Synthesis by Carbon Dioxide Hydrogenationon Fe-Based Catalysts[END_REF][START_REF] Inoue | Support Effect of Zinc Oxide Catalyst on Synthesis of Methanol from CO 2 and H 2[END_REF][START_REF] Melian-Cabrera | CO 2 hydrogenation over Pd-modified methanol synthesis catalysts[END_REF]. The effect of the support was also extensively studied. The type of support affects both CO 2 conversion and methanol selectivity, and, in general, basic oxides such as La 2 O 3 , Sm 2 O 3 , Nb 2 O 5 , In 2 O 3 and ThO 2 [START_REF] Denise | Oxide-supported copper catalysts prepared from copper formate: Differences in behavior in methanol synthesis from CO/H 2 and CO 2 /H 2 mixtures[END_REF][START_REF] Ramaroson | Reaction of CO-H 2 and CO 2 -H 2 on copperzinc catalysts promoted by metal oxides of groups III and IV[END_REF], used as supports, favor the methanol formation. The preparation methods have also a considerable influence on the catalytic performance [START_REF] Arena | Synthesis, characterization and activity pattern of Cu-ZnO/ZrO 2 catalysts in the hydrogenation of carbon dioxide to methanol[END_REF][START_REF] Liu | Recent Advances in Catalysts for Methanol Synthesis via Hydrogenation of CO and CO 2[END_REF][START_REF] Hong | CO 2 Hydrogenation to Methanol Over Cu/ZnO/Al 2 O 3 Catalysts Prepared by a Novel Gel-Network-Co-precipitation Method[END_REF]. Several methods such as coprecipitation [START_REF] Chiang | Direct synthesis of formic acid via CO 2 hydrogenation over Cu/ZnO/Al 2 O 3 catalyst[END_REF][START_REF] Nitta | Effect of starting salt on catalytic behaviour of Cu-ZrO 2 catalysts in methanol synthesis from carbon dioxide[END_REF][START_REF] Słoczynski | Catalytic activity of the M/(3ZnO•ZrO 2 ) system (M = Cu, Ag, Au) in the hydrogenation of CO 2 to methanol[END_REF][START_REF] Słoczynski | Effect of Mg and Mn oxide additions on structural and adsorptive properties of Cu/ZnO/ZrO 2 catalysts for the methanol synthesis from CO 2[END_REF][START_REF] Raudaskoski | The effect of ageing time on coprecipitated Cu/ZnO/ZrO 2 catalysts used in methanol synthesis from CO 2 and H 2[END_REF], impregnation [START_REF] Choi | The role of ZnO in Cu/ZnO methanol synthesis catalysts-morphology effect or active site model[END_REF][START_REF] Qi | Effect of Titanium on Methanol Synthesis from CO 2 Hydrogenation over Cu/γ-Al2O 3[END_REF][START_REF] Köppel | Copper-and silver-zirconia aerogels: preparation, structural properties and catalytic behavior in methanol synthesis from carbon dioxide[END_REF], and sol-gel [START_REF] Köppel | Copper-and silver-zirconia aerogels: preparation, structural properties and catalytic behavior in methanol synthesis from carbon dioxide[END_REF][START_REF] Carnes | The catalytic methanol synthesis over nanoparticle metal oxide catalysts[END_REF] have been developed to prepare copper-based oxide catalysts. Moreover, the co-precipitation synthesis was improved by addition of reducing agents, such as chitosane [START_REF] Witoon | CO 2 hydrogenation to methanol over Cu/ZnO nano catalysts prepared via a chitosan-assisted coprecipitation method[END_REF], and NaBH 4 [START_REF] Dong | CO 2 hydrogenation to methanol over Cu/ZnO/ZrO 2 catalysts prepared by precipitation-reduction[END_REF]. Surfactant-assisted co-precipitation [START_REF] Li | Highly selective hydrogenation of CO 2 to methanol over CuO-ZnO-ZrO 2 catalysts prepared by a surfactant-assisted co-precipitation method[END_REF], solvent-free routine combustion [START_REF] Lei | Hydrogenation of CO 2 to CH 3 OH over CuO/ZnO/Al 2 O 3 catalysts prepared via a solvent-free routine[END_REF], and micro-fluidic co-precipitation [START_REF] Angelo | Catalyst synthesis by continuous co-precipitation under micro-fluidic conditions: Application to the preparation of catalysts for methanol synthesis from CO 2 /H 2[END_REF] are novel synthesis methods that allow obtaining a good repeatability of the synthesis, and an improved homogeneity of the phases present in the catalyst. Other methods, such as impregnation and sol-gel, can also produce catalysts with large specific surface areas and high CuO dispersion [START_REF] Díez-Ramírez | Hydrogenation of CO 2 to methanol at atmospheric pressure over Cu/ZnO catalysts: influence of the calcination, reduction, and metal loading[END_REF]. Polyol synthesis represents a good alternative to the classical synthesis methods. In particular, it presents many advantages, such as the possibility to precisely modulate the stoichiometric ratio, the homogeneous mixing of the various components, the low cost and the short reaction time. Therefore, polyol synthesis reveals to be an attractive technique for the synthesis of nano-oxide powders.

CuO/ZnO/Al 2 O 3 -type catalysts obtained by polyol synthesis have been successfully applied in the Reverse Water Gas Shift Reaction RWGS [START_REF] Lortie | Synthesis of CuNi/C and CuNi/Al 2 O 3 Catalysts for the Reverse Water Gas Shift Reaction[END_REF], in the alcohol-assisted low temperature methanol synthesis from syngas [START_REF] Jeong | Preparation of Cu/ZnO catalyst using a polyol method for alcohol-assisted low temperature methanol synthesis from syngas[END_REF], and in methanol reforming [START_REF] Wu | Kinetics of hydrogen production of methanol reformation using Cu/ZnO/Al 2 O 3 catalyst[END_REF].

The polyol method [START_REF] Park | Synthesis and size control of monodisperse copper nanoparticles by polyol method[END_REF][START_REF] Altinçekiç | Influence of synthesis conditions on particle morphology of nanosized Cu/ZnO powder by polyol method[END_REF] permits to synthesize nano-sized metallic powders with uniform size distribution and shape [START_REF] Park | Synthesis and size control of monodisperse copper nanoparticles by polyol method[END_REF][START_REF] Altinçekiç | Influence of synthesis conditions on particle morphology of nanosized Cu/ZnO powder by polyol method[END_REF][START_REF] Bobadilla | Influence of PVP in magnetic properties of NiSn nanoparticles prepared by polyol method[END_REF]. In recent years, the polyol method has been studied by many researchers [START_REF] Lu | Effects of the ratio of Cu/Co and metal precursors on the catalytic activity over Cu-Co/Al 2 O 3 prepared using the polyol process[END_REF][START_REF] Chuang | Copper catalysts prepared via microwaveheated polyol process for preferential oxidation of CO in H 2 -rich streams[END_REF][START_REF] Song | Preparation of colloidal silver nanoparticles by chemical reduction method[END_REF][START_REF] Byeon | A novel polyol method to synthesize colloidal silver nanoparticles by ultrasonic irradiation[END_REF]. These investigations showed that the crystallites' size and shape [START_REF] Park | Synthesis and size control of monodisperse copper nanoparticles by polyol method[END_REF] can be controlled by varying the reduction temperature, the pH, and the nucleation-protective agent concentration [START_REF] Lu | Effects of the ratio of Cu/Co and metal precursors on the catalytic activity over Cu-Co/Al 2 O 3 prepared using the polyol process[END_REF][START_REF] Chuang | Copper catalysts prepared via microwaveheated polyol process for preferential oxidation of CO in H 2 -rich streams[END_REF][START_REF] Song | Preparation of colloidal silver nanoparticles by chemical reduction method[END_REF][START_REF] Byeon | A novel polyol method to synthesize colloidal silver nanoparticles by ultrasonic irradiation[END_REF][START_REF] Jeong | Preparation of Cu/ZnO catalyst using a polyol method for alcohol-assisted low temperature methanol synthesis from syngas[END_REF]. Up to now, and at our knowledge, no attempts to prepare Cu/ZnO catalyst for methanol production using the polyol method have been done. In a typical polyol synthesis, polyols (ethylene glycol, diethylene, glycerol, and tetraethylene glycol) act at the main time as the reaction medium and as the reducing agent.

The metal precursor is reduced through a redox reaction between the metal precursors and the polyolic species. Therefore, the reaction temperature is an important parameter, because the oxidation potential of polyol chemicals decreases with the increasing of the reaction temperature [49, [START_REF] Boz | Altınçekiç Liquid phase hydroxylation of benzene to phenol over Cu/ZnO catalysts[END_REF][START_REF] Bayrakdar | Effects of PVP on the preparation of nanosized Al 2 O 3 supported Ni catalysts by polyol method for catalytic partial oxidation of methane[END_REF]. Nucleation-protective chemicals such as polyvinylpyrrolidone (PVP) are occasionally employed to prevent sintering and agglomeration of metal particles [START_REF] Boz | Altınçekiç Liquid phase hydroxylation of benzene to phenol over Cu/ZnO catalysts[END_REF][START_REF] Bayrakdar | Effects of PVP on the preparation of nanosized Al 2 O 3 supported Ni catalysts by polyol method for catalytic partial oxidation of methane[END_REF].

The various published researches indicate that the choice of the preparation conditions strongly affects the activity of catalysts prepared by polyol method.

In the present work, the so-called "polyol method" has been used to obtain improved catalytic materials for the hydrogenation reaction of CO 2 to methanol. The influence of the metal dispersion, spinel formation and surface properties of binary and ternary catalysts (CuO-CeO 2 , -ZnO-CeO 2 , CuO-ZnO-CeO 2 and CuO-ZnO-Al 2 O 3 ) , prepared by polyol method using polyethylene glycol as solvent, is evaluated in the CO 2 hydrogenation to methanol at atmospheric pressure, used as test reaction.

Experimental

Preparation of binary and ternary polyol catalysts

Two binary catalysts (labeled ZnO-CeO 2 , and CuO-CeO 2, with molar ratio of Zn/Ce and Cu/Ce equal to 1) and two ternary catalysts (labeled CuO-ZnO-CeO 2 , and CuO-ZnO-Al 2 O 3, with Cu/Zn/Ce and Cu/Zn/Al molar ratios of 1/1/2) were prepared by polyol method.

The reaction temperature and the choice of the solvent were selected referring to the available investigations reported in the literature [START_REF] Jeong | Preparation of Cu/ZnO catalyst using a polyol method for alcohol-assisted low temperature methanol synthesis from syngas[END_REF][START_REF] Wu | Kinetics of hydrogen production of methanol reformation using Cu/ZnO/Al 2 O 3 catalyst[END_REF]. These experimental conditions seem to favor the formation of nanocrystallites. Differently from the synthesis reported in these already published works, nitrate-base precursors were used in the present research instead of Meacetates. In fact, nitrates do not participate to secondary reactions like those operated by acetates that can react to form methyl acetate and ethyl acetate. Firstly, copper nitrate Cu(NO 3 ) 2 .2.5H 2 O (Sigma-Aldrich 98%), and/or zinc nitrate Zn(NO 3 ) 2 .6H 2 O (Sigma-Aldrich 99%) were dissolved in polyethylene glycol (C 2 H 6 O 2 ) n (Sigma-Aldrich 99%, d=1.13g/ml).

The solution at pH=2 was then heated up, step by step, and kept under stirring for 1 hour at 70°C, 2h at 120°C, and 2h at 180°C. Cerium and/or aluminum nitrates (Ce(NO 3 ) 3 .6H 2 O and/or Al(NO 3 ) 2 .1H 2 O from (Sigma-Aldrich 99%) were then added to the initial solution under vigorously stirring for 4 hours at the same temperature (180°C). The formation of a precipitate was observed. The suspension was cooled down at 100°C, and NH 4 OH was added dropwise to reach a pH of 7. The resulting suspensions were kept under vigorous stirring at 100°C for 24 h. Finally, the formed gel was dried at 180°C for 72 h, and then calcined in air at 450°C for 5h.

Characterization techniques

Thermogravemetric analyses (TGA) were performed with a TGA/DSC 1 LF 1100 STARS system from METTER TOLEDO. The samples were heated-up at 5°C/min under air flow (100 mL/min) from room temperature to 500°C.

The XRD patterns were acquired with a PANalytical MPD X'Pert Pro diffractometer operating with Cu Kα radiation, λ = 0.15406 nm at 40 mA and 45 kV with a scan rate of 2° min -1 . The data were collected in the 5-60° 2θ range. The diffraction patterns were analyzed using the Joint Committee on Powder Diffraction Standard (JCPDS). CuO, ZnO, and CeO 2 crystallite size were calculated by means of the Scherrer equation:

d = . ƛ * ( 1
)
where d is the diameter of the crystallite, ƛ the X-ray wave length, and is the width at half height (FWHM) of the peak.

N 2 -adsorption isotherms of the samples were acquired at -196°C in a Micromeritics ASAP2420 apparatus. 0.25 g of catalyst was pretreated at 300°C during 10 h under vacuum (~50 m Torr). The specific surface area was calculated applying the multi-Pont BET method [START_REF] Brunauer | Absorption of gases in multi molecular layers[END_REF], while the pore size distribution was obtained by applying the BJH method [START_REF] Barrett | The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms[END_REF] to the desorption branch of the isotherm.

Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX) was performed on a PHILIPS XL 30 instrument (electrons acceleration voltage of 15 kV).

X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos

Axis Ultra DLD spectrometer equipped with a monochromatic micro focused Al Kα (1486.6

eV and Mg Kα=1253.6 eV) X-ray exciting source. The pass energy of the analyzer was set at 40 eV. The adventitious C1s peak (284.6 eV) was used as internal reference with an accuracy of ±0.3 eV. XPS was used to evaluate the oxidation state of copper, zinc and cerium species.

O 1s, Cu 2p, Zn 2p, Ce 3d and Al 2p species were quantified by analysis of the survey XPS spectra. The binding energy of C 1s, O 1s, Cu 2p, Zn 2p Ce 3d, and Al 2p were analyzed in order to identify the associated surface species and the relative atomic concentration.

Catalytic tests

The catalytic hydrogenation of carbon dioxide to methanol was evaluated in a fixedbed continuous flow reactor. During each test, 0.3 g of catalyst was pretreated with hydrogen at 300°C for 3 hours. After the reduction step, the catalyst was exposed to a H 2 /CO 2 (1/9 volume ratio) flow at different reaction temperatures (190-240°C). The feeding mixture and the reaction products were analyzed online with a gas chromatograph (Shimadzu GC-2014) equipped with FID and TCD detectors. CO 2 conversion (X) and products' selectivity were calculated as follows, using the quantitative correction factor kf, and peaks area (S) for the different species:

X CO % = !" × 100 × kf () (2)
CH + OH selectivity % = × 100 × kf (7 9 (4)

Results and discussions

Characterization of catalysts

Thermal behavior

The catalysts thermal behavior, before and after calcinations, is shown in Fig. 1. The samples mass continuously decreased with the temperature. For the catalysts before calcination, the TGA curves showed three main events connected to the residual precursors decomposition (Fig. 1a) (indeed, the most part of the precursors has been eliminated by the thermal treatment during the synthesis).

A first mass variation in the 0.5-2.5% range was observed up to 250 °C, and was attributed to the loss of water (physi-and chemi-sorbed) as well as to the decomposition of residual nitrates from the precursors. The second and highest mass loss (0.5-3%), between 250 and 350°C, was probably related to the elimination of polyethylene glycol. The third step, at temperature higher than 350°C, corresponded to the lowest mass loss (0.5%), and was attributed to the decomposition of hydroxyl groups, leading to metal oxide formation. After calcinations (Fig. 1b), the catalyst showed a low mass loss (below 2%), attributed to the release of physisorbed water (under 150 °C), and the decomposition of carbonates formed by reaction with atmospheric CO 2 . The presence of carbonates was observed also by XPS (as described in section 3.1.3).

Structure and morphology of the catalysts

The X-ray diffractograms of the calcined bimetallic and trimetallic catalysts are displayed in Fig. 2. For the three calcined catalysts, CuO-CeO The metal oxide crystallite dimensions were evaluated applying the Scherrer equation to the main diffraction peak of each phase, and the results are reported in Table 1. ZnO crystallites size of 20-30 nm was calculated in the binary ZnO-CeO 2 catalyst. CuO crystallites, in the 10-20 nm dimension range, were detected for the CuO-CeO 2 catalyst (Table 2). The CuO (6 nm) and ZnO (12 nm) crystallites were much smaller for the ternary CuO-ZnO-CeO 2 sample, indicating that the presence of zinc and copper oxides together ameliorate the oxides distribution and dispersion during the synthesis procedure.

The nitrogen adsorption-desorption isotherms of the various polyol samples (acquired at -196 °C) are shown in Fig. 3a. The catalysts isotherm shows a continuous increase of adsorbed nitrogen over the whole P/P0 range, with a relatively steep increase at P/P0 ≥ 0.85.

For the all catalysts, the N 2 adsorption isotherms (Fig. 3b) were of type (II) [START_REF] Witoon | Tuning of catalytic CO 2 hydrogenation by changing composition of CuO-ZnO-ZrO 2 catalysts[END_REF] with no plateau at high P/P0 values, which is usually observed for materials with macropores or interparticular mesoporosity [START_REF] Aguado | Influence of the thermal treatment upon the textural properties of sol-gel mesoporous γ-alumina synthesized with cationic surfactants[END_REF]. H4-type hysteresis was observed for the CuO-ZnO-Al 2 O 3 sample, and H3-type for the ZnO-CeO 2 , CuO-CeO 2 , and CuO-ZnO-CeO 2 samples [START_REF] Deerattrakul | CO 2 hydrogenation to methanol using Cu-Zn catalyst supported on reduced graphene oxide nanosheets[END_REF]. These types of hysteresis are typical of lamellar compounds or slit-shaped pores [START_REF] Prinetto | Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-precipitated samples[END_REF].

All materials presented relatively low porous volumes (Table 1, 3 th column), as expected for this kind of preparation that produces bulk oxides. The presence of copper oxide seems to improve the specific surface area of the catalysts (in the 30-38 m 2 .g -1 range), as reported in Table 1. The catalyst with the lower surface area (18 m 2 .g -1 ) was ZnO-CeO 2. This behavior is probably connected to the improved pore volume that can be obtained in the presence of the copper precursor, that end up into a higher internal surface area. Indeed, ZnO-CeO 2 presented the lowest pore volume, and the hysteresis between the adsorption and desorption isotherms was almost absent. EDX analysis showed a very homogeneous distribution of the different oxides in all samples.

Chemical composition and surface properties

Photoelectron spectroscopy was used to evaluate the oxidation state of copper, zinc and cerium species in the CuO-ZnO-CeO 2 and CuO-ZnO-Al 2 O 3 calcined catalysts, as well as to evaluate their surface chemical composition (Tables 2). All elements (Cu, Zn, Ce, Al, and O) were present at the samples' surface (Table 2). The survey XPS acquisition permitted to quantify the various atoms (O 1s, Cu 2p, Zn 2p, Ce 3d and Al 2p) for the two trimetallic catalysts. Clear differences in the surface elements were observed for the two samples. Zn concentration was particularly low on the surface of the CuO-ZnO-CeO 2 sample (0.53 %). At the contrary a big amount of carbon was detected on its surface (24.31 %). Because the most part of carbon on the surface of CuO-ZnO-CeO 2 could be assigned to residual contamination or hydrocarbon chains, as described later in this section, these results can suggest that the carbon pollution presented a higher affinity for Zn, thus selectively covered at the extreme surface. Less dramatic differences were identified in the CuO-ZnO-Al 2 O 3 sample that showed similar concentrations of copper and zinc on the surface.

No N1s XPS peak (generally placed around 400. 2, as well as the associated surface species and their atomic concentration.

The Cu 2p spectrum of both CuO-ZnO-CeO 2 and CuO-ZnO-Al 2 O 3 (Fig. 5) catalysts presented the characteristic spin-orbit split Cu 2p 1/2 and Cu 2p 3/2 peaks, with their shake-up satellites of Cu 2+ [START_REF] Zhu | The rise of calcination temperature enhances the performance of Cu catalysts: contributions of support[END_REF]. Binding energies, in the 932.0-932.8 and 933.2-934.6 eV range, are characteristic of Cu + and Cu 2+ , respectively [START_REF] Liu | Methyl formate synthesis from methanol on titania supported copper catalyst under UV irradiation at ambient condition: Performance and mechanism[END_REF][START_REF] Dong | CO 2 hydrogenation to methanol over Cu/Zn/Al/Zr catalysts prepared by liquid reduction[END_REF]. These two contributions could be identified in the CuO-ZnO-CeO 2 sample (upper spectrum in Fig. 5). The large and strong peak centered at 932.7 eV was then attributed to Cu 2+ and/or Cu + species, while the small and broad peak centered at 934.9 eV was assigned to Cu 2+ species [START_REF] Zhang | Modification of the supported Cu/SiO2 catalyst by alkaline earth metals in the selective conversion of 1, 4-butanediol to γbutyrolactone[END_REF]. For the CuO-ZnO-Al 2 O 3 sample (Fig. 5), the Cu 2p 3/2 peak presented two contributions characteristic of Cu 2+ species; the first one was observed at 933.61 eV and related to copper oxide, while a second contribution, at a higher binding energy (935.66 eV), was assigned to Cu 2+ in the CuAl 2 O 4 -like environment (the presence of the spinel structure, CuAl 2 O 4, was also observed by XRD analysis). Indeed, the shift to higher binding energy is indicative of a charge transfer from Cu 2+ towards Al 2 O 4

2- [START_REF] Murcia-Mascarós | Oxidative methanol reforming reactions on CuZnAl catalysts derived from hydrotalcite-like precursors[END_REF][START_REF] Sun | Direct synthesis of DME over bifunctional catalyst: surface properties and catalytic performance[END_REF]. The intensity ratio of the satellite peak to the related main peak (I sat /I pp ) was of 0.17, for the CuO-ZnO-CeO 2 catalyst, and of 0.44 for CuO-ZnO-Al 2 O 3 . The lower I sat /I pp value is characteristic of well dispersed copper oxide species in an octahedral coordination environment, while the higher value is symptomatic of a coordination change, most probably due to the formation of the spinel [START_REF] Friedman | Characterization of CuAl 2 O 3 catalysts[END_REF]. In addition, the contribution at around 529 eV of the O 1s spectra can also be assigned to the presence of Cu 2+ and Cu + (see Table 2). The presence of carbon was detected on the surface of both samples. The C 1s spectra is reported in Fig. 6, for the CuO-ZnO-CeO 2 and CuO-ZnO-Al 2 O 3 catalysts. In both cases the spectra were composed of two contributions; the first, centered at 289.5 eV was assigned to carbonate species, while the more intense band, at around 285 eV, to residual contamination or hydrocarbon chains, probably deriving from the polyethylene glycol used during the catalyst synthesis. Indeed, cerium oxide and reduced cerium are known to react with CO 2 (present in the atmosphere, for example), and give rise to carbonate species on the surface [START_REF] Yang | Carbon dioxide adsorption on CeO 2 (110): an XPS and NEXAFS study[END_REF]. The carbonate band was much less intense for the CuO-ZnO-Al 2 O 3 catalyst, and the corresponding atomic percentage was about 1.3% instead of 7.2 % for the CuO-ZnO-CeO 2 catalyst. The presence of carbonates was also confirmed by the O 1s XPS band centered at high binding energy (531.9 eV and 531.4 eV, respectively for CuO-ZnO-CeO 2 (Fig. 7a) and CuO-ZnO-Al 2 O 3 (Fig. 7b).

The XPS spectrum of Ce 3d for the CuO-ZnO-CeO 2 sample is depicted in Fig. 8 and presents a complex behavior. The split of the band into numerous peak is due to the hybridization between the final state Ce 4f orbitals and the O 2p oxygen orbital [START_REF] Óvári | Near ambient pressure XPS investigation of the interaction of ethanol with Co/CeO 2 (1 1 1)[END_REF]. In Fig. 8 the peaks are identified by V and U labels, indicating respectively the spin orbit coupling 3 d 3/2 and 3d 5/2 [START_REF] Pfau | The electronic structure of stoichiometric and reduced CeO 2 surfaces: an XPS, UPS and HREELS study[END_REF], by applying the convention introduced by Burroughs et al. [START_REF] Burroughs | Satellite structure in the X-ray photoelectron spectra of some binary and mixed oxides of lanthanum and cerium[END_REF] in 1976.

Ce3d spectrum can be decomposed in 5 doublets, (U''', V'''), (U'', V''), (U', V'), (U°, V°), and (U,V), corresponding to the emissions from the spin-orbit split 3d 3/2 and 3d 5/2 core levels [START_REF] Pfau | The electronic structure of stoichiometric and reduced CeO 2 surfaces: an XPS, UPS and HREELS study[END_REF][START_REF] Burroughs | Satellite structure in the X-ray photoelectron spectra of some binary and mixed oxides of lanthanum and cerium[END_REF][START_REF] Nelin | Relationships between complex core level spectra and materials properties[END_REF]. The five doublets were assigned to different final states of tetravalent (Ce 4+ ) or trivalent (Ce 3+ ) in Ce compounds (see

Catalytic tests

In order to verify the catalytic performances of the binary and ternary polyol catalysts in relation to the chemical composition, oxide structure and surface properties, CO 2 hydrogenation to methanol was used as test reaction. The tests were carried-out at atmospheric pressure by feeding the tubular reactor with a H 2 /CO 2 =1/9 mixture. Methanol and methane were the main carbon containing products, while only traces of carbon monoxide were detected.

The activity of the four catalysts was compared at 240 °C (Fig. 9). The binary catalysts, CuO-CeO 2 and ZnO-CeO 2, did not show any catalytic activity (CO 2 conversion below 2%).

The ternary CuO-ZnO-CeO 2 and Cu-ZnO-Al 2 O 3 catalysts showed similar conversion curves as a function of time, with stabilization of their activity after 4h reaction. The CuO-ZnO-CeO 2 catalyst showed the best result with a maximum conversion of about 20%, while Cu-ZnO-Al 2 O 3 reached a conversion not exceeding 14%. The enhanced activity of the ternary catalysts is attributed to the synergistic effect between CuO and ZnO, as already known from the literature [START_REF] Frost | Junction effect interactions in methanol synthesis catalysts[END_REF][START_REF] Burch | Synergy between copper and zinc oxide during methanol synthesis. Transfer of activating species[END_REF][START_REF] Burch | Synergy at a distance in the synthesis of methanol over copper catalysts[END_REF]. The higher activity of the CuO-ZnO-CeO 2 catalyst can be attributed to both the basicity of CeO 2 [START_REF] Li | On the role of Ce in CO 2 adsorption and activation over lanthanum species[END_REF], which favors the adsorption of CO 2 (that is an acid molecule), and the enhanced reducibility of the system, leading to the increase of the number of active sites after the reducing pretreatment operated in-situ, before the reaction tests. This transformation explains also the change in slope of the CO 2 conversion curve in Fig. 10a; at first the slope of the 240 °C curve followed that of the curves at lower temperature, while when the transformation to CH 4 started, the slope decreased and the rate for reaching the stationary state slowed-down.

Conclusions

The catalysts prepared by the polyol method, and reported in the present research, presented improved properties in terms of metal oxide dispersion, and morphology (i.e. sponge like shape for the CeO 2 -containing catalysts), and a variety of the metal and metal oxide species on the surface. Moreover, the CuO-ZnO-CeO 2 and Cu-ZnO-Al 2 O 3 catalysts exhibited a higher activity and selectivity in the methanol synthesis by CO 2 hydrogenation than those displayed by catalysts prepared by more conventional methods, as summarized in Table 3. The results obtained for the catalysts prepared in the present work are also reported for comparison. The most part of the catalysts listed in Table 3 presented CO 2 conversion lower than 20 % and selectivity to methanol lower than 65%, even when the reaction was performed at relatively high pressure (1.5-5.0 MPa), a more favorable condition than that used in the present research. It is worth to notice that on catalysts prepared by impregnation [START_REF] Díez-Ramírez | Hydrogenation of CO 2 to methanol at atmospheric pressure over Cu/ZnO catalysts: influence of the calcination, reduction, and metal loading[END_REF], the reaction performed at atmospheric pressure, and with a H 2 /CO 2 ratio of 9, allowed to obtain only 12% conversion and 70% selectivity to methanol at 240°C, confirming that polyol method is promising for the preparation of active catalysts for CO 2 hydrogenation (20% CO 2 conversion and 90% selectivity to MeOH).

  Fig. 3b represents the pore size distribution of the catalysts. All catalysts presented a certain mesoporosity centered in the 17-23 nm range. The presence of macropores (up to 150 nm) was observed for the three samples containing CeO 2 , while even larger pores were present in the CuO-ZnO-Al 2 O 3 sample.

  The morphology of the different calcined CuO-CeO 2 , ZnO-CeO 2 , CuO-ZnO-CeO 2 , CuO-ZnO-Al 2 O 3 samples was investigated by scanning electron microscopy, and the acquired images are presented in Fig.4. The catalysts containing CeO 2 showed a sponge-like morphology, as reported in Fig.4for the CuO-CeO 2 (a-c), ZnO-CeO 2 (d-f), and CuO-ZnO-CeO 2 (g-i) samples. The three ceria-containing samples were characterized by the presence of round-shaped cavities and pores of various dimensions, well distributed in the sample catalyst structure. CuO-ZnO-Al 2 O 3 presented a different morphology (Fig.4 j-l), with well-developed and thin plate-shaped bonded sheets in the 1 to 5 µm dimension interval. The presence of macropores was confirmed by these images and linked to the formation of gas bubbles during the preparation step, due to the decomposition of the metal oxide precursors.

  3 eV), related to nitrate (NO 3 -) species (salt precursors) has been identified for the CuO-ZnO-Al 2 O 3 and CuO-ZnO-CeO 2 catalysts, as an indication of the complete decomposition of nitrated after calcination. The binding energy of C 1s, O 1s, Cu 2p, Zn 2p Ce 3d, and Al 2p of the two trimetallic catalysts are listed in Table

Fig. 5 . 6 Fig. 6 . 3 Fig. 7 . 3 Fig. 8 .Fig. 10 .

 566373810 Fig.5. Cu 2p XPS spectra of CuO-ZnO-CeO 2 and CuO-ZnO-Al 2 O 3 samples Spectres XPS Cu 2p des échantillons CuO-ZnO-CeO 2 et CuO-ZnO-Al 2 O 3

  For the ZnO-CeO 2 and CuO-ZnO-CeO 2 samples, diffractions at 31.7, 34.5, 36.2, CuAl 2 O 4 and ZnAl 2 O 4 spinel could be identified in the CuO-ZnO-Al 2 O 3 sample. The diffraction peak of CuAl 2 O 4 and ZnAl 2 O 4 were superposed and placed at 31.2, 36.8, 55.6, 59.3, and 65.2° 2θ. No diffraction peaks relative to ZnO could be identified in CuO-ZnO-Al 2 O 3; all zinc was present as spinel, CuZnAl 2 O 4 .

	62.9 and 69.0° 2θ indicated the presence of hexagonal ZnO [31,58,60-61] and corresponded
	respectively to the (100), (002), (101), (103) and (201) planes. The cubic CuO phase
	[31,58,60], present in the CuO-CeO 2 and CuO-ZnO-CeO 2 samples, was characterized by
	broad diffraction lines at 35.5 and 38.7° 2θ corresponding to the (-111) and (111) planes.

2 , ZnO-CeO 2 , and CuO-ZnO-CeO 2 , only the diffraction peaks relative to the CuO, ZnO, and CeO 2 phases could be clearly identified. The diffraction peaks centered at 28.6, 33.1, 47.5, 56.3, 59.1 and 69.0° 2θ, corresponding respectively to the (111), (200), (220), (311), (222) and (400) diffraction planes of cubic CeO 2 [58-59], were observed for the ZnO-CeO 2 , CuO-CeO 2 , and CuO-ZnO-CeO 2 catalysts. Broad diffraction lines at 2θ = 35.5 38.7, 61.5 and 68.1° indicated the presence of CuO and corresponds respectively to the (-111), (111), (-202), (-113) and (-220) planes. In addition to CuO,

Table 1 :

 1 morphological and structural properties of the catalysts

	Sample	S BET (m 2 .g -1 )	V pore (cm 3 .g -1 ) b	D pore (nm)	Crystallite size with XRD analysis (nm) b CuO ZnO CeO 2 Cu,ZnAl 2 O 4
	CuO-CeO 2	32	0.07	17	10-20	-	5	-
	ZnO-CeO 2	18	0.03	20	-	20-30	5	-
	CuO-ZnO-CeO 2	38	0.12	22	6	12	5	NA
	CuO-ZnO-Al 2 O 3	30	0.04	23	20-30	-	-	30-40

a Determined from desorption branch of N 2 adsorption isotherm by BJH model b Crystallite size calculated by Scherrer equation

Table 2 :

 2 Surface elements' presence as determined by XPS analysis XPS spectra for the two trimetallic catalysts was characterized by one defined peak around 1021-1022eV that could be assigned to Zn2+ species. The presence of ZnO have been also confirmed by the O 1s band centered at 530.8 eV [73-74]. The XPS signal of CuO-ZnO-CeO 2 was much less intense than that of CuO-ZnO-Al 2 O 3 , indicating the higher ZnO concentration on the CuO-ZnO-Al 2 O 3 surface (8.68 % atom for CuO-ZnO-Al 2 O 3 and 0.53 % atom for CuO-ZnO-CeO 2 catalysts.

	Orbital	CuO-ZnO-CeO 2			CuO-ZnO-Al 2 O 3
		B.E.	Species	At	B.E.	Species	At
		(eV)		(%)	(eV)		(%)
		284.98	C-C	16.54	285.01	C-C	2.46
	C 1s	286.53	C-OR	0.60	286.56	C-OR	0.28
		287.98	C=O	2.55	288.01	C=O	0.40
		289.48	O=C-O	4.62	289.47	O=C-O	0.89
		529.49	Cu-O	6.97	529.80	Cu-O	9.02
	O 1s	529.43	Ce 4+ /Cu-O 29.07	531.71	Al-O	33.22
		530.90	Ce 3+ /Zn-O 4.17	530.80	Zn-O	8.51
		531.90	(CO 3 ) 2-	7.04	531.40	(CO 3 ) 2-	2.64
		932.75	Cu 2+ /Cu +	5.87	933.61	Cu 2+	4.31
	Cu 2p	934.91 940.92	Cu 2+ /Cu + Cu 2+ /Cu +	0.97 0.99	935.66 940.87	Cu 2+ Cu 2+	1.42 1.68
		943.54	Cu 2+ /Cu +	0.78	943.43	Cu 2+	1.65
	Zn 2p	1021.56	ZnO	0.53	1021.87	ZnO	7.71
		881.00 (V 0 )
		899.39 (U 0 ) Ce 3+	0.92	-	-	-
		882.44 (V) 900.76 (U)	Ce 4+	4.42	-	-	-
		884.95 (V I )
		903.20 (U I ) Ce 3+	3.80	-	-	-
		888.70 (V II )	Ce 4+
		907.36 (U II )		5.33	-	-	-
	Ce 3d	898.16 (V III ) Ce 4+	4.82	-	-	-

Table 3

 3

	); U''' (916.6 eV) and V''' (898.2 eV) were due to a Ce

  Moreover, in the Cu-ZnO-Al 2 O 3 catalyst, copper and zinc oxides were not fully available due to the formation of the CuAl 2 O 4 and ZnAl 2 O 4 spinels, as shown by XRD. Indeed, zinc and copper trapped into the spinel cannot enter in intimate contact and their synergistic catalytic effect[START_REF] Burch | Synergy between copper and zinc oxide during methanol synthesis. Transfer of activating species[END_REF][START_REF] Burch | Synergy at a distance in the synthesis of methanol over copper catalysts[END_REF][START_REF] Li | On the role of Ce in CO 2 adsorption and activation over lanthanum species[END_REF] cannot be deployed.On the most active catalyst (CuO-ZnO-CeO 2 ), further tests, at different reaction temperature, were performed. The CO 2 conversion as a function of time is plotted in Fig.10afor the tests performed in the 190-240 °C temperature range. At the beginning the curves presented an activation step, characterized by a steep section of the conversion curve; after 1 h reaction, the change of slop indicated that the stationary state is going to be reached up (maximum CO 2 conversion). A slightly different behavior was observed for the test performed at 240 °C in which the approach to the maximum conversion was slower and characterized by a change in slope. By increasing the temperature, the selectivity to CH 4 increased, while the methanol selectivity decreased simultaneously (Fig.10b). Methanol selectivity was favored at low temperature; MeOH is indeed unstable at high temperature and is transformed into CH 4 and H 2 O (in presence of hydrogen) through the following successive reactions: CO 2 + H 2 → CH 3 OH + H 2 → CH 4 + H 2 O

Table 3 :

 3 catalytic performances in the hydrogenation of CO 2 , obtained on the catalysts prepared

	Type of catalyst	Preparation method	T (°C)	P (MPa)	R (H 2 /CO 2 )	CO 2 (%) conversion	CH 3 OH (%) Selectivity	Ref.
	CuO-ZnO-Al2O 3	Polyol method	240 0.1	9.0	14	86	this
	CuO-ZnO-CeO 2	Polyol method	240 0.1	9.0	20	90	work
	CuO/ZnO/Al 2 O 3	Solvent free routine	240 3.0	3.0	16	64	[40]
	CuO/ZnO/ZrO 2	Surfactant co-precipitation method	240 3.0	3.0	12	33	[16]
	CuO-ZnO-ZrO 2 (M) Co-precipitation microfluidic	240 5.0	3.0	9	47	[41]
	CuO-ZnO-ZrO 2 (pH) Co-precipitation at controlled pH				14	50
	CZZ0 CZZ3	Precipitation/reduction method (NaBH 4 ) 230 5.0	3.0	17 15	67 62	[15]
	CZZ5					15	67
	Cu/ZnO	Co-precipitation	240 3.0	3.0	17	78	[84]
	10Cu-/CeO 2					4	66
	0.5Pd-10Cu/CeO 2 2Pd-10Cu/CeO 2	Coprecipitation + impregnation	230 3.0	3.0	6 15	49 29	[85]
	CuO-ZnO-TiO 2 -ZrO 2 Co-precipitation (oxalate)	240 3.0	3.0	17	44	[61]
	Cu/ZnO	Impregnation	240 0.1	9.0	5	12	[42]
	Cu-Zn/SiO 2	Impregnation	250 2.0	3.0	2	66	[1]
	5%CuZn/rGo					14	3
	10%CuZn/rGo 20%CuZn/rGo	Incipient wetness impregnation	250 1.5	3.0	26 19	5 9	[19]
	CuO/ZnO/Al 2 O 3 (Cp) With Internal cooling water CuO/ZnO/Al 2 O 3 (Cp) Without Internal cooling water	240 3.0	3.0	52	69	[86]
	Cu/ZrO 2 +CaO	Wetness impregnation	250 0.1	3.0	3	1	[87]

Acknowledgements

All physico-chemical characterizations were performed by the technical platforms of IS2M.

The authors are very grateful to S. Hajjar, L. Josien, L. Michelin, H. Nouali for their contribution.


Dylan Chaillot

Jocelyne Miehé-Brendlé

Simona Bennici

Insights on the influence of the precursors on the sol-gel synthesis of hybrid organic-inorganic saponite-like materials

Keywords: Clays, Hybrids, Hydrotalcite, Materials, Saponite, Sol-gel, Synthesis

Lamellar organic-inorganic hybrid materials, especially talc and saponite-like structures, have been broadly investigated over the past decades due to the combination of the thermal and mechanical properties of the inorganic part with the possibility to finely tune hydrophilic-hydrophobic balance, ion exchange capacity and properties through the choice of the silicon source. These compounds are generally produced by co-precipitation or hydrothermal synthesis methods, allowing the formation of well-crystallized particles but time and energy consuming. More recently, hybrid organic-inorganic lamellar materials have been synthesized by using sol-gel method, which is attractive as it is a one-step method performed under mild conditions, with low energy consumption and short formation times. The originality of this research stands in the study of the influence of the reactants on the formation of saponite-like hybrids and especially on the role of the aluminium source in the sol-gel synthesis of hybrid organic-inorganic lamellar materials. The obtained materials have been characterized by XRD, FT-IR, solid-state NMR and TGA, and it was shown that hydrotalcite-like compounds can be obtained beside saponite-like compounds depending on the aluminum source.

Résumé

De nombreuses études ont été menées récemment sur les matériaux hybrides organique-inorganiques de structure lamellaire, type talc et saponite notamment, suite à la combinaison des propriétés thermiques et mécanique de la fraction inorganique d'une part, avec la possibilité d'adapter la balance hydrophile/hydrophobe, la capacité d'échange cationique, ainsi que les propriétés associées au choix de la source de silicium. Ces composés sont généralement synthétisés par co-précipitation ou par synthèse hydrothermale qui permettent la formation de particules bien cristallisées, mais demandent beaucoup de temps et d'énergie. Plus récemment, ces matériaux ont été synthétisés par la méthode sol-gel qui permet d'obtenir des matériaux de grande pureté en des temps très courts avec une faible nécessité énergétique. L'originalité de ce travail repose sur l'étude de l'influence des réactifs sur la formation de matériaux hybrides organiqueinorganiques de structure de type saponite, notamment le rôle de la source d'aluminium dans la synthèse par voie sol-gel de matériaux hybrides organique-inorganique de structure lamellaire. Les matériaux synthétisés ont été analysés par DRX, IR-TF, RMN solide, et ATG, ce qui a permis de découvrir que des composés similaires à des hydrotalcites peuvent être obtenus selon la source d'aluminium employée.

Introduction

Saponite belongs to the 2:1 clay mineral family with one octahedral sheet made of magnesium sandwiched between two tetrahedral sheets containing silicon partly substituted by aluminum. These structural substitutions induce a charge deficit that is compensated by counterions in the interlayer space (generally sodium Na + , calcium Ca 2+ , or potassium K + ). In the case of organicinorganic hybrids having a saponite-like structure, an organoalcoxysilane is used as silicon source, by this way, an organic chain is covalently bounded to the silicon and thus, to the inorganic framework. The general chemical formula materials can be denoted as:

Cx[Mg 3 [(RSi) 4 -xAlx]O 10 (OH) 2 ]
Where x represents the tetrahedral substitution rate and C the counterions used to compensate the charge deficit induced by the substitutions in the tetrahedral sheets into the structure [START_REF] Sparks | Environmental Soil Chemistry[END_REF][START_REF] Zhang | [END_REF][3][4][5][6][7] as shown in Fig. 1. Synthesis methods play an important role on the crystallinity, shape, and particle size of these compounds, so the preparation methods remain an important research subject in the field of layered materials [3][4][5][6]. Many methods have been developed over the last decades, but the main ones include co-precipitation, hydrothermal synthesis [3,5,[8][9][10][11], and sol-gel method [12][13][14][15][16][17][18]. However, the morphology and crystallinity of such materials are related to the synthesis conditions. Sol-gel method is becoming a new way to synthesize hybrid organic-inorganic lamellar materials. It consists in dissolving inorganic salts containing divalent and trivalent cations, such as magnesium and aluminium salts for example, into a solvent and increasing the pH by adding NaOH to promote the condensation that occurs preferentially in basic media [12][13][14][15][16][17][18]. The solution is kept under constant stirring and fixed temperature, generally room temperature, during several hours. This process allows obtaining high purity materials in short times with low energy consumption, but the products generally present lower crystallinity than those obtained using co-precipitation or hydrothermal synthesis methods [3][4][5][6].

The novelty of this study stands on the sol-gel synthesis conditions and mechanisms bringing to the formation of different lamellar materials depending on the precursors, especially the use of different aluminum source. These materials were intended as materials presenting acidic properties, and a series of analysis has been performed (X-Ray Diffraction, Fourier Transform Infrared, and solidstate Nuclear Magnetic Resonance spectroscopy) to characterize such materials.

Experimental

Reactants

N-Propyltriethoxysilane C 9 H 22 O 3 Si (ABCR, 97 wt%), Trimethoxyphenylsilane C 9 H 14 O 3 Si (TCI Chemicals, >98 wt%), magnesium chloride hexahydrate MgCl 2 .6H 2 O (Sigma-Aldrich, BioXtra, ≥99.0%), magnesium nitrate hexahydrate Mg(NO 3 ) 2 .6H 2 O, aluminum acetylacetonate Al(C 5 H 7 O 2 ) 3 (Sigma-Aldrich, ReagentPlus®, 99%), aluminum chloride hexahydrate AlCl 3 • 6H 2 O (Fluka Analytical, ≥99,0%), ethanol CH 3 CH 2 OH (Carlo Erba Reagents, ≥97%), sodium hydroxide NaOH (Carlo Erba Reagents, 99.9%), and distilled water were used as received during the study.

Synthesis of the lamellar materials

The synthesis of organic-inorganic saponite-like hybrid materials involve magnesium chloride or magnesium nitrate, aluminium acetylacetonate or aluminum chloride, and N-propyltriethoxysilane or trimethoxyphenylsilane as sources of the hybrid materials framework. The sol-gel synthesis process of hybrid organic-inorganic materials can be described as follow: the required amount of magnesium salt and aluminium source are respectively dissolved into ethanol, according to the desired Si/Al substitution rate in the saponite structural formula, Cx[Mg 3 [(RSi) 4-x Al x ]O 10 (OH) 2 ] with 0.5 ≤ x ≤ 1.2, and their molar masses. A fixed quantity of organo-alcoxysilane (2g in this work) is added under stirring, and then sodium hydroxide (NaOH) 1M is added until the pH reaches 10. The amounts of the different precursors are detailed in the Table 1. The solution is left under stirring at room temperature for 24h, before being centrifuged at 11000 rpm for 10 min, washed 3 times with 30mL of ethanol, and dried for 24h in an oven at 60°C for further analysis. Samples are labelled as DCHxSy (for saponite-like hybrid samples) and DCHT00y (for talc-like hybrid samples), with x the theoretical Si/Al substitution rate according to the structural formula, and y the number of the sample.

Samples characterization

Firstly, the structure of the as synthesized samples was investigated by X-Ray Diffraction, using a Panalytical X'Pert PRO MPD with Cu-Kα radiation (λ = 1.5418 Å), and performed from 2 to 70°2θ with a step of 0.017°2θ, on randomly oriented powder samples.

Potassium Bromide (KBr) pellets were prepared to perform Fourier Transform Infrared (FT-IR) spectroscopy, made with 5mg of the samples mixed with 100mg of KBr. The measurements were performed in the 400-4000 cm -1 region using a Bruker Equinox 55 spectrometer with a DTGS detector. The number of scan was fixed at 100 scans with a resolution of 4 cm -1 .

Thermogravimetric analysis of the synthesized samples were performed with a Mettler-Toledo TGA/DSC1 LF1100 apparatus under argon, in alumina sample holders, with a flow rate of about 100 mL.min -1 from 25 to 600°C and a heating rate of 5°C.min -1 . An empty sample holder was recorded as reference to correct baseline deviation. 29 Si and 1 H decoupled 29 Si magic angle spinning (MAS and single pulse MAS, respectively) Nuclear Magnetic Resonance (NMR) spectroscopy spectra were recorded on a Bruker AVANCE II 300 MHz spectrometer at 59.6 MHz. Samples were analyzed in a 7 mm diameter cylindrical rotor, at a spinning frequency of 4 kHz, and recorded during 24h. 1 H decoupled 29 Si NMR experiments have been performed with a π/4 pulse duration of 2.57 µs with a recycle time of 90 s. 27 Al magic angle spinning (MAS) Nuclear Magnetic Resonance (NMR) spectroscopy spectra were recorded on a Bruker AVANCE II 400 MHz spectrometer at 104.3 MHz. Samples were analyzed in a 2.5 mm diameter cylindrical rotor at a spinning frequency of 25 kHz, and recorded during 8h. 27 Al spectrum were recorded with a delay time of 1 s by using 4 µs single pulse, corresponding to a pulse angle of π/12.

Results and discussion

Syntheses with aluminum chloride

Syntheses with aluminum chloride have first been performed during this study. The compositions and molar ratios of the synthesized samples are reported in Table 1, with experimental Si/Al, Al/Mg and Si/Mg molar ratio calculated according to the molar masses of the different precursors.

Diffraction patterns of lamellar materials generally shows (00l) reflections due to their 2dimensional structure, which allows to estimate the c-cell parameter (thickness of the lamellar material) by adding the interlayer distance and the thickness of a single layer. Fig. 2 The broadness of these peaks indicates a low organization of the structure (crystallinity) due to the synthesis process and the presence of an organic chain linked to the organo-alcoxysilane, but these positions are typical from a trioctahedral 2:1 clay-like structure according to the positions of (001) and (060).

Fourier Transform Infra-Red has been performed in order to detect typical bounds between aluminum, magnesium, and silicon. The results are shown in Fig. 3, with a large wide band around 3500 cm -1 corresponding to the presence of water in the structure. However, the sample DCH0,5S014 (with the lower amount of aluminum in the structure) shows two vibrational bands at 3715 and 3620 cm -1 corresponding to stretching Si-Al. These bands tend to disappear with increasing aluminum content into the samples, so they correspond to stretching Si-Al bounds. The two small bands at 3072 and 3049 cm -1 correspond to the C-H stretching vibrational bands, and the presence of stretching Si-C around 1151 cm -1 indicates the silicon precursor has efficiently been incorporated in the structure with the organic chain. Additional small bands between 1300 and 1200 cm -1 proves the presence of magnesium into the structure of the materials, but also aluminum with Al-OH vibration band at 860 cm -1 . Moreover, two intense combination bands at 1967 and 1760 cm -1 are visible in these spectra and are related to the incorporation of aluminum and magnesium into the structure.

To deeply analyze the samples, solid-state Nuclear Magnetic Resonance (NMR) spectroscopy has been performed to probe the local environment of 27 Al and 29 Si into the samples. Regarding 29 Si NMR (CP-MAS and single pulse MAS), clay-like materials generally exhibit 3 main resonances between -40 and -80 ppm, called T n environments where T stands for trifunctional silane and n the number of oxygen bounded to the probed silicon atom, according to the formula of the silicon part RSi(OM)n(OH)3-n with M corresponding to silicon or magnesium. Thus, T 3 to very well condensed silicon atoms without surrounding OH groups and generally located at the center of the tetrahedral sheets (RSi(OM) 3 ), whereas T 2 and T 1 are related to silicon atoms at the border of these sheets, with respectively 1 and 2 surrounding OH groups [4][5][6]14,17]. Fig. 4 (A) shows that the samples exhibit these three main features: around -76, -67, and -60 ppm, corresponding respectively to T 3 , T 2 , and T 1 environments. It also shows an evolution of the condensation of silicon into the tetrahedral sheet: T 3 environment tends to decrease with increasing aluminum amount into the samples, which induces more defects into the structure. 27 Al MAS NMR has also been performed to probe the position of this element into the structure, which depends on its coordination state. According to Fig. 4 (B), two resonances can be seen: around 8.5 and 53 ppm, corresponding respectively to aluminum into octahedral (Al VI) and tetrahedral (Al IV) sheets. As the four samples exhibit those positions, aluminum is located in both sheets, which does not correspond to pure theoretical saponite. However, the area of the first peak around 53 ppm seems higher than the second one, which may indicate a more important presence of aluminum into the tetrahedral sheet.

These series of analyses prove the formation of a saponite-like hybrid organic-inorganic material with different aluminum amounts into the structure, as expected in the beginning of this work. To go further in this study, the aluminum precursor has been changed in order to characterize its impact on the structure of the synthesized materials.

Syntheses with N-Propyltriethoxysilane

Two different magnesium salts, magnesium nitrate and magnesium chloride, have been tested as precursors during this experiment. The composition and calculated molar ratios are reported in Table 2.

X-Ray diffractogramms of these samples are shown in the Fig. 5. Six main reflections are observed: 11.4, 22.8, 34.6, 38.6, 46, and 60.8-61.8°2θ. Even if these samples are poorly crystallized according to the broadness of their peaks, their position does not correspond to hybrid organicinorganic saponite-like materials [12][13][14][15][16][17][18]. According to the general indexation of hydrotalcite diffraction patterns in the literature, these peaks are assigned to (003), (006), (012), (015), (018) and (110) reflections, respectively [19][20][21][22][23]. As these patterns show no (001) reflection, this parameter can be estimated by multiplying the distance d 003 by 3, which corresponds to the distance d 001 according to the periodicity observed in the diffraction patterns. According to the Bragg's law 2d hkl .sin(θ) = n.λ, with d hkl the distance of the plane (hkl), θ the diffraction angle, λ the Kα radiation used and n the periodicity index, the first peak indicates a distance d 003 of about 7.9-7.7 Å, which corresponds to a c- Sol-gel synthesis parameters of the samples. Theoretical molar ratios have been calculated according to the amounts of each precursor. DCHT006 is a talc-like organic-inorganic hybrid to show the impact of the aluminum source.

cell parameter of about 2.57 Å. As a comparison, saponite-like materials have a basal distance of about 2 times the thickness of tetrahedral sheets plus the octahedral one and the interlayer distance, which correspond to a final d 001 value of about 11-15 Å under a relative humidity of 80%, sometimes above depending on the size of the counterions and the structure content [7,8,17,18]. The distance observed in these diffraction patterns is much lower than the basal distance of saponite-like materials (around 10 Å plus the length of the organic chain), indicating that the structure of the synthesized materials cannot correspond to hybrid organic-inorganic saponite-like structures but to hydrotalcite-like compounds. Moreover, an interesting feature is generally observed in the low angle region: lamellar materials exhibit a peak around 60°2θ corresponding to the (060;330) reflection. The position of this peak depends on the occupancy of the octahedral sheet: high angle values indicate small distances, typically dioctahedral nature (around 1.49 Å, Al 3+ or Fe 3+ with a vacant site for example), and the contrary indicates a trioctahedral nature (around 1.52 Å, Mg 2+ or Fe 2+ for example). In this case, the width of this peak indicate the presence of two different features: one for the trioctahedral character around 1.51 Å, and the second peak around 1.54 Å (quite visible in the sample DCH0.5S010) for the dioctahedral nature of the synthesized materials. This indicates the presence of both divalent and trivalent cations into the octahedral sheet, respectively Mg 2+ and Al 3+ considering the synthesis reactants in our case. Another interesting point is that no other phases are observed, impurities being generally represented by thin peaks in X-Ray diffractograms. Moreover, the pattern in Fig. 5 look quite similar whatever the Si/Al substitution rate (between 0.5 and 1.2 in the structural formula of hybrid organic-inorganic saponite-like materials), whereas a shift of the main peaks (001) and ( 060) is generally observed in diffractograms of saponite-like materials related to changes in the structure of these lamellar compounds [12][13][14][15][16][17][18].

Considering these observations, Fourier Transformed Infrared (FT-IR) analysis have been performed on the synthesized samples using potassium bromide pellets. As seen from Fig. 6, the spectra obtained show several characteristic bands: the broad and strong absorption band in the range of 3700-3300 cm -1 correspond to the O-H stretching vibration of the surface and interlayer water molecules, as the O-H bending vibration centered at 1660 cm -1 . The three bands observed in the range of 2950-2900 cm-1 are related to the C-H stretching vibration, and the presence of the stretching Si-C vibration band around 1130 cm -1 proves the integrity of silicon and its organic part into the sample. Additional bands in the range of 1300-1200 cm -1 show the presence of magnesium in the materials. It is worthy to note that the positions of the vibrational bands do not change whatever the Si/Al substitution rate into the samples, and stretching Si-Al bands are not observed in the 3500-3800 cm -1 range, similar to Mg-Al Layered Double Hydroxide samples.

The samples DCH0,5S009 and DCH1,2S009 have been analyzed using 27 Al and 29 Si solid-state NMR, as shown by Fig. 7. A third sample has been analyzed by 29 Si CPMAS NMR, DCHT005, a talc-like hybrid sample synthesized without aluminum in order to compare the T-type environments in these materials. Fig. 7 (A) shows three separated peaks: -67.9, -56.9 and -48.3 ppm, whose correspond respectively to T 3 , T 2 , and T 1 environments, but in a very low amount according to the single pulse MAS analyses. This means that only a very few amount of silicon atoms are located into the structure of the synthesized materials. However, T 3 environments seem to be more important than T 2 and T 1 ones, indicating a high condensation of silicon species into the tetrahedral sheets except in the sample DCH1,2S009 which is supposed to have the higher substitution rate and a lower Si/Al molar ratio. For 27 Al NMR spectra, shown in the Fig. 7 (B), a sample has been prepared without silicon precursor (DCH AlMg01), corresponding to the chemistry of a pure hydrotalcite phase. All the samples exhibit one strong peak around 9.1 ppm, and a very small one at 61.3 ppm is observed in the sample DCH1,2S009. The position of the first peak indicates the presence of aluminum in octahedral positions (Al VI), as proved by the sample without silicon source, whereas the second shows the same element in tetrahedral environment (Al IV), which is not possible in pure hydrotalcite phases. According to the results obtained by 27 Al and 29 Si NMR, it can be assumed that the synthesized materials are made of two different compounds: the first is constituted of traces of saponite-like structure, with silicon and aluminum in tetrahedral sheets, and the second part is essentially made of aluminum in an octahedral environment.

The thermal stability of the samples has been tested using Thermogravimetric Analysis (TGA) from 25 to 600°C with a slope of 5°C.min -1 , and is shown in Fig. 8. In all cases, the curves can be divided into 2 main regions: the first one, ranging from 25 to 250°C, is related to the dehydration (loss of the surface water) of the samples. It corresponds to a mass loss of approximately 16-18% in the samples. The second region is located between 250 and 600°C and related to the dehydroxylation and decomposition of the organic part, with a final mass loss of almost half of the starting mass (around 43-47%). Moreover, the sample DCHT006, a talc-like hybrid material synthesized the same way without aluminum, presents higher thermal stability than other materials, which can be explained by the insertion of aluminum into the structure of these lamellar materials inducing structural defects and cavities.

By using this synthesis method in our study, Mg-Al Layered Double Hydroxide (LDH) has however been surprisingly formed during the synthesis of hybrid organic-inorganic saponite-like materials. Hydrotalcite belongs to the Layered Double Hydroxide (LDH) family with a structure based on brucite (Mg(OH 2 ))-like layers (similar to octahedral sheets in natural clays), in which a part of the divalent cations are substituted by trivalent ones, for example a part of Mg 2+ ions are replaced by Al 3+ ions [19][20][21][22][23][24][25][26][27][28][29][30][31]. These structural substitutions generate positive charges within the stacked layers, which are kept electronically neutral by the intercalation of anions and water among the interlayer spaces, as seen in Fig. 9. The general structural formula of layered double hydroxides can be denoted as:

[M x II M y III (OH)(2x+2y)](A n-)(y/2).mH 2 O
Where the divalent and trivalent cations (M II and M III , respectively), the stoichiometric coefficient values (x and y, 2 ≤ x/y < 4), the interlayer anions (A n-= (CO 3 ) 2-, (SO 4 ) 2-, OH -, Cl -for example) and the water content (m) can be varied over a wide range, leading to the Layered Double Hydroxides (LDHs) family; also called anionic clays (contrary to regular clays which present negatively-charged layer, the deficit being compensated by interlayer cations [19,[23][24][25]). A great focus is made on hydrotalcite thanks to the complex layered structure, wide range of chemical compositions, high ion exchange capacity linked to its interlayer space, and reactive surface [19,22,28,29].

To go further in the formation of this material, a different organoalcoxysilane has been tested in the same conditions.

Syntheses with trimethoxyphenylsilane

Table 3. Sol-gel synthesis parameters of the samples synthesized with trimethoxyphenylsilane. Theoretical molar ratios have been calculated according to the amounts of each element.

Synthesis carried out with trimethoxyphenylsilane were performed using magnesium chloride hexahydrate salt as magnesium source. The composition and calculated molar ratios are reported in Table 3.

X-Ray diffraction patterns of the samples synthesized with trimethoxyphenylsilane are shown in Fig. 10. Even if some impurities are visible, six main reflections are observed: 5.7-7, 11.4, 22.8, 34.8, 38.5, and 60.7°2θ, which are quite similar to the ones observed in the previous samples. Moreover, this first diffraction peak at small angle tends to shift to lower angle values when the substitution rate of silicon by aluminum increases. According to the Bragg's Law and the general indexation of hybrid lamellar materials in literature, this first reflection corresponds to basal reflection values (001) between 13 (sample DCH0,5S013) and 15.5 Å. Even if the samples exhibit hydrotalcite-like features, the presence of this basal distance (001) plus the (060) diffraction peak at 1.52 Å also indicates the formation of a saponite-like material, with octahedral sheets containing divalent cations (magnesium according to our precursors). Solid-state NMR experiments have also been performed on these samples to probe the local environment of silicon and to study the position of aluminum (whether aluminum is present in tetrahedral sheets (Al IV) and/or in octahedral sheets (Al VI) in the structure). Fig. 11 (A) corresponds to the 29 Si CPMAS analyses of the samples DCH0,5S013 and DCH1.2S013, the two extreme samples of the series according to the Si/Al substitution rates. These two samples exhibit three main local environments: -75.5, -68, and -60.7 ppm for T 3 , T 2 , and T 1 environments, respectively. These values are slightly above the ones observed in the previous samples but are still in the range of T-types environments. However, T 3 environments seem to be of greater importance in the sample DCH0.5S013 with the lowest Si/Al substitution rate whereas T 2 and T 1 environments are dominant in the sample DCH1.2S013, indicating a lower condensation of silicon in the structure, and more structural defects than the first sample. Regarding the 27 Al MAS NMR on the four samples, as shown in Fig. 11 (B), the four spectra display two resonances: a first one around 9 ppm, and a smaller one at 54 ppm, which indicates the presence of aluminum in both octahedral (Al VI) and tetrahedral (Al IV) positions respectively.

The results obtained with these materials synthesized using trimethoxyphenylsilane confirm our previous observations with the N-propyltriethoxysilane: X-Ray diffraction data proves the formation of two different lamellar materials, and 27 Al Nuclear Magnetic Resonance analyses show the presence of aluminum in both octahedral and tetrahedral sheets, which is respectively observed in hydrotalcite and saponite-like materials. Moreover, it seems that the increase of the aluminum content in the samples induce more defects in the structures, and a greater ratio of Layered-Double Hydroxide formed during the sol-gel synthesis. The most plausible hypothesis to the formation of such material instead of the expected hybrid organic-inorganic saponite-like product is the non-formation of tetrahedral Si-Al sheets, which may be related to the precursors. According to the results mentioned above, NMR data shows the synthesized materials are mainly made of aluminum in octahedral positions, even if some traces of tetrahedral silicon is detected. The XRD patterns of the synthesized materials are similar to Mg-Al LDHs called hydrotalcite, and the remaining silicon into the samples can be related to the presence of some layers of lamellar hybrid organic-inorganic, as expected at the beginning of this study.

Conclusion

As hybrid organic-inorganic saponite-like materials were expected by the use of the sol-gel synthesis process, the experiments have been performed according to M. Jaber et al. [6]. Depending on the precursors, X-Ray diffraction and solid-state NMR results shows the formation of another type of lamellar material made of magnesium and aluminum, even if silicon has been used during the synthesis process. The presence of small amounts of silicon into the samples showed by 29 Si solidstate NMR proves the formation of some saponite-like layers with silicon and aluminum in tetrahedral positions, but the samples are mainly made of a stacking of brucite layers with magnesium partly substituted by aluminum, also called Mg-Al hydrotalcite or layered double hydroxide. Aluminum is present in octahedral sheets (Al VI), and the basal distance between the layers is estimated at 7.7Å according to X-Ray Diffraction analyses, which is much lower than the values expected with saponite-like materials plus the length of the organic chain. Aluminum acetylacetonate is known in the synthesis of lamellar materials, but is not always the best choice depending on the type of material expected.

By changing the different precursors, especially the aluminum source, the analyses showed the formation of (1) almost pure Mg-Al Layered Double Hydroxides, also called hydrotalcites, or (2) a mixed phase between hydrotalcite and saponite-like structure, or finally (3) pure saponite-like materials by changing the aluminum salt, as outlined in Fig. 12. The changes in the formation of these lamellar materials may be related to the affinity of the aluminum precursors over the organoalcoxysilane: aluminum acetylacetonate might have a lower affinity to N-propyltriethoxysilane and trimethoxyphenylsilane than aluminum chloride, and this interaction reduces hydrolysis and condensation rates during the addition of sodium chloride, as shown by A. E. Danks et al. [32]. This is the reason why the choice of the appropriate precursors for the sol-gel synthesis of lamellar materials is an important parameter to take into account. Further studies concerning the formation mechanisms of these materials instead of saponite-like hybrids are under progress. Spectres Infrarouges des échantillons S014 enregistrés de 400 à 4000 cm -1 . Spectres RMN du 29 Si en CPMAS (A) et du 27 Al en MAS (B) des échantillons S014. Spectres infrarouges des séries d'échantillons S009 et S010 enregistrés de 400 à 4000 cm -1 .

(A) (B) Fig. 7. 29 Si CPMAS NMR spectra (A) of the samples DCH0,5S009 (red) and DCH1,2S009 (blue) compared to a synthesized talc-like hybrid (green), and 27 Al MAS NMR spectra (B) of the samples S009 compared to a sample synthesized only with aluminum and magnesium (green).

Spectres RMN du 29 Si en CPMAS (A) des échantillons DCH0,5S009 (en rouge) et DCH1,2S009 (en bleu) en comparaison avec un hybride de structure de type talc (en vert), et spectres RMN du 27 Al en MAS (B) des échantillons S009 en comparaison avec un échantillon synthétisé en absence d'organo-alcoxysilane (en vert). Résumé des différentes conditions de synthèses et de leur impact.

  2d hkl .sin(θ) = n.λ (With d hkl the distance of the plane (hkl), θ the diffraction angle, λ the Kα radiation used and n the periodicity index).

Fig. 2 .

 2 Fig. 2. XRD patterns of the samples S014 synthesized with aluminum chloride by sol-gel method, washed 3 times with ethanol.Diffractogrammes de Rayons X des échantillons S014 synthétisés avec du chlorure d'aluminium hexahydraté par voie sol-gel, lavés trois fois à l'éthanol.

Fig. 3 .

 3 Fig. 3. FTIR spectra of the samples S014 recorded from 400 to 4000 cm -1 .

Fig. 1 .Fig. 4 .

 14 Fig. 1. Schematic representation of organic-inorganic saponite-like hybrid. Schéma de la structure d'un hybride organique-inorganique de structure de type saponite.

Fig. 5 .

 5 Fig. 5. XRD patterns of the samples S009 (down) and S010 (up) synthesized by sol-gel method with rising Si/Al substitution rates from 0.5 to 1.2 for each series, washed 3 times with ethanol.Diffractogrammes de Rayons X des échantillons S009 (en bas) et S010 (en haut) synthétisés par voie sol-gel, avec des taux de substitution Si/Al croissants de 0,5 à 1,2 pour chaque série, lavés trois fois à l'éthanol.

Fig. 6 .

 6 Fig. 6. FTIR spectra of the series of samples S009 and S010 recorded from 400 to 4000 cm -1 .

Fig. 8 .Fig. 9 .

 89 Fig. 8. Thermogravimetric Analyses of the samples S009 and S010, from 25 to 600°C. DCHT006 is a talc-like hybrid synthesized the same way, without aluminum.Analyses thermogravimétriques des échantillons S009 et S010, de 25 à 600°C. DCHT006 est un hybride de structure de type talc synthétisé par la même méthode, en absence d'aluminium.

Fig. 10 .

 10 Fig. 10. XRD patterns of the samples synthesized with trimethoxyphenylsilane by sol-gel method, washed 3 times with distilled water.Diffractogrammes de Rayons X des échantillons synthétisés avec du phényltrimethoxysilane par voie sol-gel, lavés trois fois à l'eau distillée.

Fig. 11 .Fig. 12 .

 1112 Fig.11.29 Si CPMAS NMR spectra (A) of the samples DCH0,5S013 (red) and DCH1,2S013 (blue), and27 Al MAS NMR spectra (B) of the samples S013.Spectres RMN du29 Si en CPMAS (A) des échantillons DCH0,5S013 (en rouge) et DCH1,2S013 (en bleu), et spectres RMN du27 Al en MAS (B) de la série d'échantillons S013.

  environments correspond

	Sample	Mg amount (g)	Al amount (g)	Si amount (g)	Si/Mg Molar ratio	Si/Al Molar ratio	Mg/Al Molar ratio	pH Mass obtained (g)
	DCH0,5S014	1,757	0,352	1,998	1,166	6,911	5,928	11	1,42
	DCH0.75S014	1,893	0,560	2,003	1,084	4,355	4,014	10,5	1,38
	DCH1S014	2,05	0,811	2,004	1,002	3,009	3,002	11	1,32
	DCH1,2S014	2,195	1,042	1,999	0,934	2,336	2,502	10,5	1,28

Table 2 .

 2

Table 1 .

 1 Sol-gel synthesis parameters of the samples synthesized with aluminum chloride, trimethoxyphenylsilane, and magnesium chloride hexahydrate. Theoretical molar ratios have been calculated according to the amounts of each element.

	Sample	Mg amount (g)	Al amount (g)	Si amount (g)	Si/Mg Molar ratio	Si/Al Molar ratio	Mg/Al Molar ratio	pH Mass obtained (g)
	DCH0,5S014	1,757	0,352	1,998	1,166	6,911	5,928	11	1,42
	DCH0.75S014	1,893	0,560	2,003	1,084	4,355	4,014	10,5	1,38
	DCH1S014	2,05	0,811	2,004	1,002	3,009	3,002	11	1,32
	DCH1,2S014	2,195	1,042	1,999	0,934	2,336	2,502	10,5	1,28

Table3. Sol-gel synthesis parameters of the samples synthesized with aluminum chloride, trimethoxyphenylsilane, and magnesium chloride hexahydrate. Theoretical molar ratios have been calculated according to the amounts of each element.

Acknowledgements

This work was supported by Université de Haute Alsace and École Doctorale Physique et Chimie-Physique (ED 182). DRX, FT-IR, TGA, and RMN measurements were performed on the technical platforms of IS2M. The authors are very grateful to L. Michelin, H. Nouali and S. Rigolet for their contributions.