Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration.

Abstract

Tungstate domains supported on ZrO2, Al2O3, TiO2, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO3. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO3 crystallites depend strongly on the underlying support (e.g., WOx/C catalysts are 10-50-fold more active than WOx/Al2O3 catalysts). The electrical double layer (EDL) surrounding the negatively charged WOx domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol-1 K-1), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.