| Tungstated zirconia materials (WOx-ZrO 2) are an alternative to sulfated zirconia catalysts for solid acid-catalyzed reactions, exhibiting improved thermal stability at typical reaction conditions with only slightly lower acid strength. Extensive investigations of the catalytic performance of WOx-ZrO2 for light alkane isomerizations and alcohol dehydrations in the literature have not reached consensus regarding the nature of the active site. The current work probes the identity of the active site through a material synthesis approach, producing both conventional and nanoparticle-supported WOx-ZrO 2 analogues. Results from the systematic characterization of textural, structural, and electronic features of each material are correlated with catalytic performance for steady state methanol dehydration and temperature programmed surface reaction (TPSR) of chemisorbed methanol. Results are discussed in terms of surface density (rhosurf), and a review of rhosurf determination is included to clarify discrepancies between research groups. Observations are placed in context with current state-of-the-art active site models.; Multiple conclusions confirm and clarify results from other research groups, and significant new details are identified that provide unique insight into WOx-ZrO2 materials for acid catalysis. Maxima in turnover rates (TOR) correspond with intermediate rho surf in the range of ∼5.5-6.5 W-atom·(nm 2 composite)-1 for WOx supported on zirconium oxyhydroxide. WO3 (nano)crystallites form exclusively as a function of surface density, occurring between 6.7-7.1 W-atom·(nm 2 composite)-1 for the same materials. Raman reveals that WOx molecular surface structures are not simply a function of rhosurf but also depend upon treatment temperature and type of support. Oxygen dependence was found to be zero order, implying the redox character of W6+ is not involved in dimethyl ether formation. Monotungstates and polytungstates coexist at all WOx surface coverages, and general Raman spectroscopy features are identified that correlate with the highest methanol dehydration TORs. No connection was observed between steady state TOR and TPSR-determined acid strength. Arguments for acid catalysts with a broad acid site distribution are made. |
