Crystallization Behavior of Tungstate on Zirconia and Its Relationship to Acidic Properties

Abstract

Zirconia-tungstate aerogels were prepared by two methods, a one-step sol–gel synthesis and an incipient wetness impregnation. The effects of preparation parameters on their physical and chemical properties were studied. The catalytic activity and surface acidity were characterized by a set of chemical probes, includingn-butane isomerization and pyridine adsorption, which were developed into an acidity scale for the evaluation of solid acids in general. Maximum catalytic activity inn-butane isomerization occurred at saturation monolayer coverage of zirconia by tungstate. The one-step synthesis required a more elevated activation temperature in order to expel tungstate from the bulk of zirconia before dispersion on the surface. Variation of the activation temperature allowed study of the transition between a catalytically inactive and active material and consequently identification of the active species. Activity inn-butane isomerization coincided with the presence of (i) a larger population of Brønsted sites and (ii) stronger Brønsted sites on the surface. Infrared spectroscopy indicated that the surface active tungstate species was identical regardless of preparative route. We conclude that the preparation method affected the activation behavior of zirconia-tungstate materials but not the active species inn-butane isomerization. Furthermore, zirconia-tungstate can be placed in a meaningful Brønsted acid strength scale which has the potential as a tool for ranking acid catalysts.