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

Abstract

A series of supported WO 3 catalysts were synthesized by incipient wetness impregnation of ammonium metatungstate aqueous solutions onto Al 2O 3, TiO 2, Nb 2O 5, and ZrO 2 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 (CH 3OH-TPSR and steady-state CH 3OH dehydration). The molecular structures of the dehydrated supported tungsten oxide phase were determined to be monotungstate and polytungstate surface WO x species below monolayer surface coverage (<4.5 W/nm 2), crystalline WO 3 nanoparticles (4.5–9 W/nm 2) and large bulk-like WO 3 crystals (>9 W/nm 2). The electronic structure for the different tungsten oxide species, E g, was independent of the specific support and decreased monotonically with increasing tungsten oxide domain size (W/nm 2). The solid acid catalytic activity, however, did not correlate with either the molecular or electronic structures because of the dominant contribution by the surface WO x species to the overall catalytic performance of the supported WO 3 catalysts and its reactivity dependence on the specific support. For supported WO 3/Al 2O 3, the surface WO x monolayer was more active than the crystalline WO 3 phases and, consequently, the TOF decreased with increasing surface W/nm 2 density. For tungsten oxide supported on Nb 2O 5, TiO 2 and ZrO 2, the surface WO x monolayer is less active than the crystalline WO 3 phases and, consequently, the TOF increases with surface W/nm 2 density. These reactivity trends reflect the influence of the specific support cation electronegativity on the acid character of the bridging W–O-Support bond.