Acidic properties of silica-containing mixed oxide aerogels: Preparation and characterization of zirconia-silica and comparison to titania-silica

In this work we describe our preparation and characterization of zirconia-silica aerogels having 2:1, 1:1, and 1:2 Zr:Si atomic ratios. By preparing samples both with and without silicon precursor prehydrolysis, we also probe different extents of component mixing. Prehydrolyzed samples display improved molecular-scale homogeneity at all compositions as evidenced by total acid site densities, fractional Bronsted populations, and 1-butene isomerization activities that are higher than in corresponding nonprehydrolyzed materials. Acid site densities of our zirconia-silicas are among the highest reported in the literature for comparable materials prepared by a number of different methods, suggesting that the aerogel preparation delivers samples that, even in its nonprehydrolyzed variant, are among the most homogeneously mixed. The effects of prehydrolysis on the catalytic properties of zirconia-silica parallel those we previously reported for titania-silica, illustrating the general applicability of the technique for preparation of well-mixed, two-component oxides. We do, however observe some important differences between the two oxide pairs. Zirconia-silica is a more active l-butene isomerization catalyst than titania-silica and, as a function of composition, its maximum activity occurs at a higher silica content (50 mol% silica vs 33 mol%). Within the framework of the Tanabe model of mixed oxide acidity, we explain these observations in terms of the number and transition metal coordination of the Si-O-M (M = Ti, Zr) linkages accessible in each mixed oxide pair. (C) 1996 Academic Press, Inc.