EFFECT OF PREHYDROLYSIS ON THE TEXTURAL AND CATALYTIC PROPERTIES OF TITANIA SILICA AEROGELS

When applied to the preparation of mixed oxides, sol-gel chemistry aff ords considerable control over the extent of component mixing: well-mi xed oxides result from a preparation in which precursor reactivities a re evenly matched. In this work, we describe the preparation and chara cterization of two sets of mixed titania-silica aerogels covering the entire composition range, one set prepared using prehydrolysis of the silicon precursor to promote homogeneous component mixing and a second set prepared without prehydrolysis. Compared to the prehydrolyzed mix ed aerogels, samples prepared without prehydrolysis of the silicon pre cursor exhibit low 1-butene isomerization activities, low acid site de nsities, and low fractional Bronsted site populations. These results s uggest that the silica and titania components are segregated in the no nprehydrolyzed samples, with inactive silica-rich clusters obscuring m uch of the surface of a titania-enriched ''core.'' The dependence of i mportant catalytic properties upon composition and homogeneity are exp lained in terms of the relative number and location of M-O-M' (cation- oxygen-cation) linkages that exist in a given sample. (C) 1994 Academi c Press, Inc.