Crystallization behavior of tungstate on zirconia and its relationship to acidic properties - II. Effect of silica

Zirconia-silica-tungstate aerogels were prepared by sol-gel chemistry followed by supercritical drying. The materials were characterized using nitrogen adsorption, X ray diffraction, Raman and DRIFT spectroscopy, and n-butane isomerization. Using the technique of prehydrolysis, we explored the role of silica, and the degree to which it was mixed with zirconia on the catalytic and acidic properties. The presence of silica, both dispersed in zirconia (well mixed) and segregated on the surface of zirconia (poorly mixed), retarded sintering caused by heat treatment, thereby increasing the surface area of the aerogels. On poorly mixed zirconia-silica, tungstate effectively dispersed onto the surface regardless of the existence of silica-rich patches. The ensuing reduction in specific activity in n-butane isomerization, compared to zirconia-tungstate, established that tungstate on these patches did not enhance active sites. On well-mixed zirconia-silica, the presence of siloxane species in the support further reduced catalytic activity by extending the influence of silica in zirconia-tungstate. The lower catalytic activities of these materials correlated with the presence of weaker Bronsted acid sites than those of zirconia-tungstate, This work demonstrates that silica as a dopant is effective in increasing the surface area of zirconia-tungstate but not in enhancing n-butane isomerization activity. (C) 1998 Academic Press