Dg. Barton et al., Structural and catalytic characterization of solid acids based on zirconiamodified by tungsten oxide, J CATALYSIS, 181(1), 1999, pp. 57-72
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.