New insights into the nature of the acidic catalytic active sites present in ZrO2-supported tungsten oxide catalysts

An extensive series of supported WO3/ZrOx(OH)(4-2x) catalysts (WZrOH) were synthesized by standard aqueous impregnation of ammonium metatungstate into an amorphous ZrOx(OH)(4-2x) metastable support, followed by high-temperature calcination (at 773-1173 K). The supported WZrOH catalysts were also compared with well-defined model supported WO3/ZrO2 catalysts (WZrO2) consisting of a thermally stable crystalline m-ZrO2 support. Both series of supported tungsten oxide catalysts were physically characterized (by XRD, XPS, TEM, in situ Raman, and in situ UV-vis spectroscopy) and chemically probed by methanol dehydration (i.e., TPSR spectroscopy and steady-state catalytic studies). Monolayer surface WOx coverage was found to occur at similar to 4.5-5 W-atoms/nm(2) for both catalytic systems. Whereas the dehydrated model supported WZrO2 series contained only surface WOx species below monolayer coverage, the dehydrated supported WZrOH series had the same surface WOx species, as well as some Zr-stabilized distorted WO3 nanoparticles (NPs). Above monolayer coverage, the model supported WZrO2 catalysts contained only ordered crystalline WO3 NPs, but the supported WZrOH catalysts had both ordered WO3 NPs and Zr-stabilized distorted WO3 NPs. The comparative methanol dehydration to dimethyl ether acidity study revealed that the Zr-stabilized distorted WO3 NPs were the catalytic active sites in supported WZrOH catalysts. These findings represent a new model for the origin of the enhanced solid acidity of supported WZrOH catalysts. (C) 2008 Elsevier Inc. All rights reserved.