PREPARATION OF MIXED CHROMIUM AND ALUMINUM-OXIDES BY SOL-GEL PROCESS

Gels of mixed chromium and aluminum oxide were prepared by sol-gel process. This method involves the hydrolysis and condensation of an homogeneous stoichiometric mixture of Al(O-s-Bu)3 and Cr(acac)3 with a mass ratio Al/Cr = 10 in sec-Buranol. Geiation occured by adding acetic acid to the preceding mixture. The nature of the product, as well as the structural and catalytic properties of the xerogels obtained by calcination at temperatures up to 440 degrees, depend on the amount of added acetic acid represented by the ratio k=[CH3COOH] / [Al(O-s-Bu)(3)]. For small values of k ( k < 2), gelatinous precipitates are obtained which are transformed upon heating into dense materials with low surface areas and pore volume resulting from intramolecular condensation of polymers with many O-s-Bu ligands and little unreacted acetic acid trapped inside the polymer matrix. In contrast, gels were produced for k greater than or equal to 2, which transformed upon heating into granulated material with higher surface areas and pore volumes resulting from intermolecular condensation of polymers with OH and acetate ligands and more unreacted acetic acid trapped inside the polymer matrix. The pore size increases with the amount of unreacted acetic acid.