Modification of the Acid Properties of Silica–Zirconia Aerogels by in Situ and ex Situ Sulfation

Sulfated silica–zirconia mixed oxides containing 33 mol% Zr were prepared by sol-gel routes with the sulfate/zirconia molar ratio varying between 0.2 : 1 and 0.3 : 1. The influence of the added sulfate introduced by both in situ and ex situ methods on the bulk and surface properties was examined and in particular the role of sulfate in modifying the acid site density. Both silica and sulfate (in situ) were well distributed throughout the solid, thus delaying the onset of crystallization of zirconia phases. Ex situ sulfuric acid treatment resulted in extraction of a proportion of the zirconia to the surface to form small amorphous zirconia clusters, thus increasing Lewis acidity at the expense of Brønsted acid sites. As the sulfate concentration was increased beyond an estimated 2.82 SO42− per square nanometer of exposed zirconia, this Lewis acidity was replaced by Brønsted acid sites. The maximum number of Brønsted acid sites which could be created by sulfate treatment was only marginally greater than the site density on a nonsulfated sample, although the site strength was much greater for the former. The distribution of components and in particular the role played by segregated, amorphous zirconia at the surface is discussed in terms of the modification of both Lewis and Brønsted acid sites.