Sol-gel chemistry as a synthetic tool for studying the effects of homogeneity on mixed oxide catalytic materials.

摘要

ixed oxides play an important role in chemical and petrochemical processing as catalysts and catalyst supports. Homogeneity--how well the components are mixed on a molecular level--is an important determinant of a mixed oxide's catalytic properties. Unlike many conventional preparation methods, sol-gel chemistry affords considerable control of molecular-scale mixing. Homogeneity is linked to the relative reactivities of the alkoxide precursors used in a sol-gel synthesis--we expect good component mixing when precursor reactivities are evenly matched and poor mixing when they are not.;High density of surface acid sites, high specific activity for reactions that require acid catalysts and delay of component oxide crystallization are, in general, characteristic of well-mixed zirconia-silicas and titania-silicas. Comparison of the two mixed oxide pairs demonstrates that formation of hetero linkages (M-O-Si; M = Ti, Zr) is a necessary, but not sufficient, condition for generation of acid sites in mixed oxides. The hetero linkages must also possess the necessary 'excess charge.' Our results support the essential feature of the Tanabe model of mixed oxide acidity--the 'excess charge' calculation--as a simple and powerful tool for predicting the appearance and type (Bronsted vs. Lewis) of acid sites.;This work establishes sol-gel chemistry as a key experimental tool for studying the structure-activity relationship in mixed oxide catalytic materials.;By carefully adjusting precursor reactivities, we use the sol-gel method to explore homogeneity's impact on the textural, structural, and acidic properties of zirconia-silica and titania-silica aerogels. Of the various methods available to overcome a reactivity mismatch for promotion of homogeneous mixing, prehydrolysis of the less reactive precursor is the most flexible. By varying a single preparative parameter, the molar ratio of water to silicon precursor, we use prehydrolysis to access a 'continuum of mixing states' in a set of 50 mol% zirconia-50 mol% silica aerogels.