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

When applied to the preparation of mixed oxides, sol-gel chemistry affords considerable control over the extent of component mixing: well-mixed oxides result from a preparation in which precursor reactivities are evenly matched. In this work, we describe the preparation and characterization 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 mixed aerogels, samples prepared without prehydrolysis of the silicon precursor exhibit low 1-butene isomerization activities, low acid site densities, and low fractional Bronsted site populations. These results suggest that the silica and titania components are segregated in the nonprehydrolyzed samples, with inactive silica-rich clusters obscuring much of the surface of a titania-enriched ''core.'' The dependence of important catalytic properties upon composition and homogeneity are explained in terms of the relative number and location of M-O-M' (cation-oxygen-cation) linkages that exist in a given sample. (C) 1994 Academic Press, Inc.