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 usi ng nitrogen adsorption, X ray diffraction, Raman and DRIFT spectroscop y, and n-butane isomerization. Using the technique of prehydrolysis, w e explored the role of silica, and the degree to which it was mixed wi th zirconia on the catalytic and acidic properties. The presence of si lica, both dispersed in zirconia (well mixed) and segregated on the su rface of zirconia (poorly mixed), retarded sintering caused by heat tr eatment, thereby increasing the surface area of the aerogels. On poorl y mixed zirconia-silica, tungstate effectively dispersed onto the surf ace regardless of the existence of silica-rich patches. The ensuing re duction in specific activity in n-butane isomerization, compared to zi rconia-tungstate, established that tungstate on these patches did not enhance active sites. On well-mixed zirconia-silica, the presence of s iloxane species in the support further reduced catalytic activity by e xtending the influence of silica in zirconia-tungstate. The lower cata lytic activities of these materials correlated with the presence of we aker Bronsted acid sites than those of zirconia-tungstate, This work d emonstrates that silica as a dopant is effective in increasing the sur face area of zirconia-tungstate but not in enhancing n-butane isomeriz ation activity. (C) 1998 Academic Press