Structural and catalytic characterization of solid acids based on zirconiamodified by tungsten oxide

Tungsten oxide species form strong acid sites on ZrO2 supports and inhibit ZrO2 crystallite sintering and tetragonal to monoclinic structural transfor mations. W-L-I X-ray absorption near-edge spectra suggest that the W center s are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WOx-ZrO2 samples(2-21 wt.% W) oxidized at 1073 K. Maximum o-xylene isomerization turnover rates (per W atom) on WOx-ZrO2 solids occur at WOx surface densities (10 W nm(-2)) that exceed the theoretical monolay er capacity of ZrO2. Similar turnover rates are obtained on WOx-ZrO2 sample s with similar WOx surface densities (W nm(-2)) over a large range of oxida tion temperatures (773-1223 K) and WOx concentrations (5-21 wt.% W). UV-vis ible spectra suggest an increase in WOx domain size with increasing surface density. High isomerization turnover rates appear to require the presence of WOx domains of intermediate size on ZrO2 surfaces. WOx domains of interm ediate size appear to provide a compromise between reducibility and accessi bility of WOx centers. These domains are necessary to delocalize a temporar y charge imbalance that forms Bronsted acid sites in the presence of H-2 an d stabilizes carbocation intermediates. The presence of H-2 during o-xylene isomerization increases turnover rates and prevents rapid deactivation. Sl ow D-2/o-xylene exchange reactions indicate that H atoms from H-2 are not f requently involved in the activation or desorption of xylenes. H-2 is requi red, however, in order to reverse the occasional desorption of H atoms duri ng o-xylene isomerization reactions. These desorption processes lead to the destruction of Bronsted acid sites by the formation of strongly adsorbed u nsaturated species in the absence of H-2. After promotion with Pt (0.3 wt.% ), WOx-ZrO2 solids catalyze n-heptane isomerization in the presence of H-2 at 400-500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WOx-ZrO2, efficient hydrogen transfe r steps prevent extensive cracking of adsorbed carbocations by limiting the ir surface lifetimes. (C) 1999 Academic Press.