Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide

Abstract

Tungsten oxide species form strong acid sites on ZrO2supports and inhibit ZrO2crystallite sintering and tetragonal to monoclinic structural transformations. W-LIX-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WOx–ZrO2samples (2–21 wt.% W) oxidized at 1073 K. Maximumo-xylene isomerization turnover rates (per W atom) on WOx–ZrO2solids occur at WOxsurface densities (10 W nm−2) that exceed the theoretical monolayer capacity of ZrO2. Similar turnover rates are obtained on WOx–ZrO2samples with similar WOxsurface densities (W nm−2) over a large range of oxidation temperatures (773–1223 K) and WOxconcentrations (5–21 wt.% W). UV–visible spectra suggest an increase in WOxdomain size with increasing surface density. High isomerization turnover rates appear to require the presence of WOxdomains of intermediate size on ZrO2surfaces. WOxdomains of intermediate size appear to provide a compromise between reducibility and accessibility of WOxcenters. These domains are necessary to delocalize a temporary charge imbalance that forms Brønsted acid sites in the presence of H2and stabilizes carbocation intermediates. The presence of H2duringo-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D2/o-xylene exchange reactions indicate that H atoms from H2are not frequently involved in the activation or desorption of xylenes. H2is required, however, in order to reverse the occasional desorption of H atoms duringo-xylene isomerization reactions. These desorption processes lead to the destruction of Brønsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H2. After promotion with Pt (0.3 wt.%), WOx–ZrO2solids catalyzen-heptane isomerization in the presence of H2at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WOx–ZrO2, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.