Catalysis science of the solid acidity of model supported tungsten oxide catalysts

A series of supported WO3 catalysts were synthesized by incipient wetness impregnation of ammonium metatungstate aqueous solutions onto Al2O3, TiO2, Nb2O5, and ZrO2 supports as a function of tungsten oxide loading. The resulting solid acid catalysts were physically characterized with in situ Raman and UV–vis spectroscopy and chemically probed by methanol dehydration to dimethyl ether (CH3OH-TPSR and steady-state CH3OH dehydration). The molecular structures of the dehydrated supported tungsten oxide phase were determined to be monotungstate and polytungstate surface WOx species below monolayer surface coverage (<4.5 W/nm2), crystalline WO3 nanoparticles (4.5–9 W/nm2) and large bulk-like WO3 crystals (>9 W/nm2). The electronic structure for the different tungsten oxide species, Eg, was independent of the specific support and decreased monotonically with increasing tungsten oxide domain size (W/nm2). The solid acid catalytic activity, however, did not correlate with either the molecular or electronic structures because of the dominant contribution by the surface WOx species to the overall catalytic performance of the supported WO3 catalysts and its reactivity dependence on the specific support. For supported WO3/Al2O3, the surface WOx monolayer was more active than the crystalline WO3 phases and, consequently, the TOF decreased with increasing surface W/nm2 density. For tungsten oxide supported on Nb2O5, TiO2 and ZrO2, the surface WOx monolayer is less active than the crystalline WO3 phases and, consequently, the TOF increases with surface W/nm2 density. These reactivity trends reflect the influence of the specific support cation electronegativity on the acid character of the bridging W–O-Support bond.