A General Nonhydrolytic Sol-Gel Route to Oxides
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SYLVIE ACOSTA, PASCAL ARNAL, ROBERT J.P. CORRIU, DOMINIQUE LECLERCQ, P. HUBERT MUTIN AND ANDRE VIOUX. UMR 44, CNRS/ Rh6ne-Poulenc/ Universit6 Montpellier II, case 007, Universitd Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 5, France. ABSTRACT A nonhydrolytic sol-gel route based on the condensation between chlorides and oxygen donors such as ethers and alkoxides is presented. Four examples, silica, alumina, titania and binary oxides in the Al/Si system show that this is a general route. The mechanism of this condensation is completely different from the one of classical sol-gel process, since it implies nucleophilic substitution at the carbon center instead of the metal center. As a consequence, the differences in reactivity between different metals are reduced. In addition, the structure of the precursors may be retained in the gel. Thus, the nonhydrolytic sol-gel process is very efficient for the preparation of homogeneous bicomponent oxides. Futhermore, nonhydrated gels are formed, which allowed us to prepare amorphous aluminas with high surface areas. INTRODUCTION In the sol-gel processing from metal alkoxides, inorganic polymerization occurs in two steps: the formation of hydroxyl groups by nucleophilic attack of H20 at the metal center with cleavage of the M-OR bond. Their condensation with each other or with alkoxide groups takes place by a similar mechanism. (RO)nM + H 20 2 (RO)n. 1MOH 4 (RO)nIMOH + (RO)nM
(RO)nIMOH + ROH
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((RO)n-IM) 2 0 + H 20
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((RO)n_1 M)2 0 + ROH
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Thus, hydrolysis and condensation of tetraalkoxysilanes (RO) 4 Si readily gives monolithic gels, i.e. gels extending throughout the initial volume of the liquid. On the other hand, metal alkoxides are much more reactive, and the general trend is the formation of precipitates [1]. To obtain monolithic metal oxide gels sophisticated processing is required; it may be performed by decreasing the reactivity of the metal alkoxide precursors by chemical modification (alcohol exchange, addition of carboxylic acids or 03-diketones) or by control of the local concentration of water (alcohol dehydration, esterification, reversed micelles microemulsions) [1-3]. Futhermore, due to the different reactivities of metal alkoxides toward hydrolysis and condensation, especially in the case of silicon and transition metal alkoxides, the control of the composition and the homogeneity of binary oxides is difficult to achieve by the sol-gel process [4-7]. As a result of the extreme hydrolytic reactivivity of metal alkoxide compounds, new methods of formation of metal oxides based on alternative chemistries are desired. One possibility is to use acetone as a condensation reagent, as reported with zinc alkoxides [8]. We have focused our efforts on elaborating a novel inorganic polymerization reaction based on the condensation of metal halides with metal alkoxides. There are some reports, mainly in the organic litterature, which suggest this possibility. For example, the reaction of silicon chloride with tetrabenzyloxysilane gives silica and ben
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