Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters

Abstract

This study unravels the catalytic effects of adjacent protons in redox catalysis of bifunctional Keggin-type phosphomolybdic acid clusters (H3PMo12O40). Isolated redox sites (O*) and Brønsted acid-redox site pairs (OH/O*) catalyze methanol oxidative dehydrogenation (ODH), a redox reaction, via the identical elementary steps and the formation of the kinetically relevant [HOCH2···H···O*]‡ and [OH···HOCH2···H···O*]‡ transition states, but with different kinetic requirements, established from selective site inactivation, product tracking, dynamic pyridine/2,6-di-tert-butylpyridine titrations, and kinetic assessments. The presence of adjacent protons interacts with and stabilizes the methanol precursor in the OH···HOCH2-H···O* adsorbed state through additional H-bonding interactions by 57 kJ mol-1 in adsorption enthalpy and by 144 J mol-1 K-1 in adsorption entropy. These additional interactions, stabilizing the [OH···HOCH2···H···O*]‡ transition state, lead to a decrease in apparent methanol activation enthalpy of 50 kJ mol-1 and in activation entropy of 97 J mol-1 K-1, resulting in an overall increase in methanol ODH turnovers. The kinetic consequences of protons established here enable the rationalization of the redox reactivity on bifunctional POM clusters and display a nontraditional confinement effect to stabilize transition state energies.