Structure and function of strong solid acid catalysts based on tungsten oxides supported on zirconia

Tungsten oxide species form strong acid sites on ZrO 2 supports and inhibit ZrO2 crystallite sintering and tetragonal to monoclinic structural transformations. After promotion with Pt, WO x-ZrO2 solids catalyze n-heptane isomerization in the presence of H2 at 400--500 K with much higher selectivity than sulfated oxides or zeolitic acids. Alkane isomerization proceeds via bimolecular reactions involving hydrogen transfer from alkanes or H2. Surface isomerization steps limit n-heptane isomerization turnover rates and efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetime.;High isomerization turnover rates require the presence of WOx clusters of intermediate size on ZrO2 surfaces. Maximum o-xylene isomerization turnover rates on WOx-ZrO2 solids occur at WOx surface densities (∼10 W-atom/nm2) that exceed the polytungstate monolayer coverage on ZrO2. These clusters are necessary to delocalize a temporary charge imbalance that forms Bronsted acid sites in the presence of H2 and stabilizes carbocation intermediates. The presence of H2 during o-xylene isomerization increases turnover rates and prevents rapid deactivation. H2 is required in order to reverse the occasional desorption of H-atoms during o-xylene isomerization reactions. These desorption processes lead to the destruction of Bronsted acid sites and to the formation of strongly adsorbed unsaturated species during reactions in the absence of H2.;The structure and catalytic activity of WOx species on ZrO 2 is controlled only by WOx surface density (W-atoms/nm 2), irrespective of WOx concentration, oxidation temperature, and ZrO2 surface area. X-ray absorption spectra at the W-L I edge suggest the predominant presence of distorted octahedral WO x species in all WOx-ZrO2 samples (2--15 W-atoms/nm2). UV-visible absorption edge analysis shows that three distinct regions of WOx coverage on ZrO2 supports appear with increasing WOx surface density: a sub-monolayer region (0--4 W-atoms/nm2), a polytungstate growth region (4--8 W-atoms/nm2), and a polytungstate/crystalline WO3 co-existence region (>8 W-atoms/nm2). The sub-monolayer region is characterized by distorted octahedral WOx species that are well dispersed on the ZrO2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few acid sites that can isomerize o-xylene isomerization at 523 K. At intermediate WOx surface densities, the absorption edge energy decreases, WOx domain size increases, WOx species become easier to reduce, and o-xylene isomerization rates increase with increasing WOx surface density. At high WOx surface densities, the growth of monoclinic WO 3 crystallites leads to lower o-xylene isomerization rates because WO x species become inaccessible to reactants and because WOx species may lose oxygens and become unable to stabilize the additional negative charge required for the formation of Bronsted acid sites.