Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters
Guangming Cai, Ya-Huei Cathy Chin
Abstract
This study unravels the catalytic effects of adjacent protons in redox catalysis of bifunctional Keggin-type phosphomolybdic acid clusters (H 3 PMo 12 O 40 ). 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 [HOCH 2 ···H···O*] ‡ and [OH···HOCH 2 ···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···HOCH 2 –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···HOCH 2 ···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.