Thermochemical Correlations of Redox and Brønsted Sites on Bifunctional Polyoxometalate Clusters and Their Kinetic Consequences in Methanol-O2 Catalysis

Abstract

Kinetic interconnectivities of methanol oxidative dehydrogenation and dehydration are manifestation of the underlying thermochemical/electronic correlations between redox and Brønsted sites on bifunctional Keggin-type polyoxometalate (POM) phosphomolybdic acid clusters with their electronic properties perturbed by sodium cation exchange (H_xNa_3–xPMo, x = 3–0). As sodium exchange increases, activation free energies for the elementary C–H scission in methanol oxidative dehydrogenation, occurring at isolated redox sites (O*) or Brønsted acid-redox site pairs (OH/O*), and for the first-order C–O formation in methanol dehydration, occurring at Brønsted sites, increase proportionally within 10–11 kJ mol^–1 at 433 K, while their activation enthalpies exhibit an inverse correlation. A Born–Haber thermochemical analysis reveals the reasons behind the site interconnectivities by establishing their respective kinetic-thermochemical relationships. The kinetically relevant C–H scission involves a late transition state, either [HOCH₂···H···O*]^‡ at O* or [OH···HOCH₂···H···O*]^‡ at OH/O*, with the transfer of an electron (e^–) and a proton (H⁺) as an H atom (H•) from the methyl fragment to redox sites, where hydrogen addition energy (HAE), comprising the negative electron affinity (−EA_POM) and proton affinity (−PA) of POM clusters, is a kinetic descriptor. The parallel methanol C–O formation features a late carbocationic transition state, [(CH₃OH···CH₃⁺···H₂O)···POM^–]^‡, involving proton transfer from POM clusters to adsorbed methanol species, where the deprotonation energy (DPE) of the Brønsted site serves as a kinetic descriptor. Notably, hydrogen addition energy decreases by ∼23 kJ mol^–1, while deprotonation energy increases by 80–230 kJ mol^–1, as sodium exchange increases. This slight negative thermochemical correlation arises from the inherent opposing proton transfers during redox (−PA) and Brønsted acid catalysis (DPE), modulated by the energetic effect of electron transfer (−EA_POM) upon sodium exchange on H_xNa_3–xPMo clusters (x = 3–1). The mechanistic interpretation and framework established here explicitly correlate the kinetic, thermochemical, and electronic properties of redox and Brønsted sites, offering insights into their intrinsic reactivity couplings, and are applicable to other bifunctional catalysts.