Acidic Properties of Silica-Containing Mixed Oxide Aerogels:Preparation and Characterization of Zirconia–Silica andComparison to Titania–Silica

Abstract

In this work we describe our preparation and characterization ofzirconia–silica aerogels having 2 : 1, 1 : 1, and1 : 2 Zr : Si atomicratios. By preparing samples both with and without siliconprecursor prehydrolysis, we also probe different extents ofcomponent mixing. Prehydrolyzed samples display improvedmolecular-scale homogeneity at all compositions as evidenced bytotal acid site densities, fractional Brønsted populations, and1-butene isomerization activities that are higher than incorresponding nonprehydrolyzed materials. Acid site densities ofour zirconia–silicas are among the highest reported in theliterature for comparable materials prepared by a number ofdifferent methods, suggesting that the aerogel preparationdelivers samples that, even in its nonprehydrolyzed variant, areamong the most homogeneously mixed. The effects of prehydrolysison the catalytic properties of zirconia–silica parallel thosewe previously reported for titania–silica, illustrating thegeneral applicability of the technique for preparation ofwell-mixed, two-component oxides. We do, however, observe someimportant differences between the two oxide pairs.Zirconia–silica is a more active 1-butene isomerizationcatalyst 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 ofmixed oxide acidity, we explain these observations in terms ofthe number and transition metal coordination of the Si–O–M(M=Ti, Zr) linkages accessible in each mixed oxide pair.