Control of Prehydrolysis Ratio for Sol-Gel Preparation of Homogeneous Zirconia-Silica Aerogels

Abstract

Catalytic researchers have long been interested in the preparation of mixed oxides because of their industrial relevance. Among the different preparation methods, sol-gel synthesis has received increasing attention lately due to its potential in delivering samples that are better mixed on a molecular scale. Within sol-gel chemistry, component mixing is linked to the relative reactivities of the alkoxide precursors in the hydrolysis/condensation sequence that forms to the oxide network-evenly matched reactivities generally produce a well-mixed (or homogeneous) oxide; poorly matched precursor reactivities produce segregated materials. Compared to the alkoxides of transition metals, silicon alkoxides are unreactive. Therefore, to promote homogeneity in silica-containing mixed oxides, silicon alkoxide precursors are often allowed to react with water before addition of a transition-metal alkoxide. This approach, known as {open_quotes}prehydrolysis,{close_quotes} gives the less reactive silicon precursor a {open_quotes}head-start{close_quotes} in the reaction sequence. Upon their addition, the transition metal alkoxides can condense with prehydrolyzed Si-OH (silanol) groups to form the Si-O-M (M = transition metal) linkages that are characteristic of a well-mixed sample. 13 refs., 2 figs., 1 tab.