A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS

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1 A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS D. Klvana, J. Chaouki, M. Repellin-Lacroix, G. Pajonk To cite this version: D. Klvana, J. Chaouki, M. Repellin-Lacroix, G. Pajonk. A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS. Journal de Physique Colloques, 1989, 50 (C4), pp.c4-9-c4-3. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1989 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 REVUE DE PHYSIQUE APPLIQUÉE Colloque C4, supplément au n 4. Tome 4, avril 1989 Cf-9 A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS D. KLVANA, J. CHAOUKI, M. REPELLIN-LACROIX* and G.M. PAJONK* Ecole Polytechnique de Montreal, GGnie de Physique, Montreal, Canada 'University Claude Bernard, Lyon I, CNRS-ISM UA-31, F-696 Villeurbanne Cedex, France Résumé - Pour conserver les propriétés texturales des gels encore imbibés de solvant au moment de leur séchage il existe la possibilité d'évacuer le solvant dans les conditions hypercritiques (procédé à haute température conduisant aux aérogels) et aussi celle de sublimer ledit solvant qui conduit alors à l'obtention de cryogels (procédé à basse température). Une série de cryogels d'oxydes mixtes de nickel et d'aluminium ont été ainsi élaborés. Leurs propriétés texturales ont été déterminées par adsorption-désorption de N, par la méthode BET, la poroslmétrie au mercure tandis que leur structure a été étudiée par les rayons X. Ces cryogels sont comparés aux aérogels de formulation chimique identique. Finalement un échantillon de cryogel a été testé dans la réaction catalytlque d'hydrogénation du toluène en méthylcyclohexane qui constitue un procédé potentiel de stockage de l'hydrogène pour les systèmes de transport. Abstract - In order to preserve the textural properties of a wet gel one possibility is to evacuate the solvent under hypercritical temperature conditions (high temperature process leading to the well known aerogel materials) and another one is to sublimate the solvent frozen at low temperature conditions (low temperature process leading to cryogels). The preparation of a series sfeeveral nickel oxide on alumina cryogels is described in details. Their surface and textural properties are measured by physical adsorption-desorption of N, by the BET method and also by mercury porometry. The corresponding data are compared to those belonging to aerogels of same chemical compositions. Cryogels (as aerogels) appeared to be amorphous from their XRD patterns. Finally one sample of a nickel on alumina cryogel is tested as a potential catalyst in a flow reactor for the reaction of hydrogenation of toluene into methylcyclohexane which is one possible means to store hydrogen in transport systems. 1 - INTRODUCTION It is well known that during the drying-step of a wet gel the formation of a liquid-vapour meniscus in the porous network is responsible for its collapse. The aerogel method (see ref. /I/ and 111 for instance) is a good means to preserve the textural properties of the gel but it suffers from some drawbacks such as (in general) high pressure, high temperature and impossibility to use water as a solvant which must be replaced (directly or not) by flammable organic compounds such as alcohols. In principle the same purpose can be reached through the means of lyophylisatlon or freeze drying which is based on the sublimation of the solvant once it has been frozen /3/. To obtain cryogels necessitates two steps : first freeze the gel and second sublimate the solvant. It is important in this last stage to avoid carefully any melting of the solvant especially when a heat source is used to accelerate the sublimation step (see ref. /, 4/). The results presented in this paper are to be considered as preliminary ones at the time of this Symposium. Article published online by EDP Sciences and available at

3 - EXPERIMENTAL Preparation of NiO-A1 0 cryogels The method used in the sol-gel process was derived from the one already applied for the preparation of aerogels 151. Nickel acetate was dissolved in ethylene glycol in a proportion comprised between 10 to 15 % in weight with respect to alumina. The required amount of water fox the hydrolysis was added to this first solution. A second one consisting of secondary butylate of aluminium in -methyl- propanol was added to the first solution under vigourous mixing. The so-obtained mixed alcogel was then frozen in a acetone bath cooled with liquid nitrogen at - 96'C in thin layers and then transferred in a Labconco unit to sublimate the solvents. The solvents were evacuated in this way within 4 hours. The resulting cryogel appeared to be a very fine green powder. Three cryogels were prepared with the same proportion of Ni (in weight) but with three different reactant to solvent ratios as indicated in table 1. They are labelled NAC. Cryogel Reactant to solvent ratios in % Apparent densities in g/cm 3 NAC 4 NAC 5 NAC 6 Table 1 - Composition and apparent densities of the NAC cryogels. Textural and structural properties Table 1 shows that the higher the proportion of the reactants used with respect to the solvents the greater the apparent densities. However the BET areas of these cryogels were not significantly different as it is shown in table. Cryogel Degassing temperature in "C Weight loss in % S (BET) in m /g NAC 4 NAC 5 NAC 6 Table. - BET specific areas of the NAC cryogels. The adsorption-desorption N isotherms for NAC 4 and for a comparable aerogel (NA) are show? on figure 1 and their most important feactures are given in tables 3 and 4 where VpN is the micro- and meso-pore volumes measured by physical adsorption of N r is the mean pore radius and St is the derived specific area given by the corresponding t sots of figure. From these data it can be concluded that the NAC 4 cryogel is, from the point of view of its texture, a little bit less developed than the aerogel. Both types of powder are nevertheless essentially macroporous (and amorphous as demonstrated by their XRD patterns). Th5ir macropore volumes Vp were m asured with a mercury porosimeter and their VpH were 0.8 cm /g 5 for the cryogel antg 5.4 cm /g for the aerogel. It is probably the grea%est difference recorded between these two kinds of dried gel powders. Experiments are in progress to improve the porosity of the cryogels.

4 Fig. 1 - Adsorption desorption N isotherm Fig. - "t" plots - Sample S (BET) in m /g. VpN in cm /g r = VpN/S in (A) Cryogel 348 Aerogel 579 Table 3 - Comparison of the textural properties between the cryogel and the aerogel Sample Type of isotherm Capillary condensation Hysteresis St in m /g Cryogel Aerogel I1 I1 Table 4 - Table 3 continued. Application to a catalytic reaction The hydrogenation of toluene in the presence of NAC 4 was operated in a small stainless steel U tube reactor in flow conditions. The binary cryogel was reduced at 400 C in flowing hydrogen for 16 hours and then cooled down to the reaction temperature of 130 C. The only product detected by gas chromatography was methylcyclohexane, the toluene conversion being of %. Experiment details are given in ref. /6/. Preparation of SiO cryogels Sodium metasilicate in water was acidified by HC1 at room temperature. After washing the aquagel it was frozen in thin layer at - 40 C and evacuated by sublimation in the Labconco unit. Preliminary results show that the SiO cryogels developed textural and structural properties as high as those corresponding to the similar aerogels of silica. As in the case of the NiO-A1 0 mixed oxides the aerogels exhibited a VpN again twice higher than that of the similar cryogels 171. But contrary o the Ni0-A1 0 cryogels the Si0 cryogels exhibited 5 large BET specific areas close to 700 m /g and equi$a?ent to those measured for the similar silica aerogels (prepared from the sodium metasilicate precursor)/7/.

5 REVUE DE PHYSIQUE APPLIQUEE CONCLUSIONS At the present time the cryogels can be prepared either under the form of NiO-A1 0 or an aquagel Si0 in the sol-gel step. an alcogel The textural and structural characteristics of the so obtained cryogels are close to those exhibited by the aerogels of the same gross chemical composition. But the preliminary experiments showed that in the case of the binary oxides Ni0-A103 the cryogels have smaller total porous volumes as well as specific areas than the aerogels. For silica, the cryogels develop surface areas equal to those of the aerogels but they have a micro and meso porosity which is half as large as that of the aerogels. 111 "Aerogels", J. Fricke Ed., Springer Verlag, Belin Teichner, S.J. in op cit ref. 1, p. Pajonk, G.M. and Teichner, S.J., in op cit ref. 1, p Schnettler, F.J., Monforte, F.R. and Rhodes, W.W. in "Science of Ceramics", Vol. 4. The British Ceramic Society Stoke on Trent, 1968, p Beyer, L.A., Kalnas, C.E., Roy, C.E. and Lloyd, I.K., Am. Ceram. Soc. Bull., 66 (1987) Gardes, G.E.E., Pajonk, G. and Teichner, S.J., Bull. Soc. Chim. France, 1876, Klvana, D., Chaouki, J., Kuschorsky, D., Chavarie, C. and Pajonk, G.M., Appl. Cat., 6 (1988) Abouarnadasse, S., Chaouki, J., Klvana, D., Chavarie, C. and Pajonk, G.M., in press. The authors are grateful to NSERG (Canada) and to Centre Jacques Cartier (Lyon) for funding this research.