Thermochemical and Kinetic Correlations of Redox and Lewis Sites on Cobalt–Molybdenum Oxides: Illustrated with Alkanol-O2 Catalysis

Abstract

Electronic properties of redox and Lewis acid sites on bifunctional metal oxides are inherently correlated to each other, a phenomenon that has long been recognized but not yet explicitly and quantitatively illustrated. Using alkanol oxidative dehydrogenation (ODH) and inter- and intramolecular dehydration (inter- and intra-DEH) kinetics as the respective thermochemical/electronic proxies for redox and Lewis acid sites, we elucidate the thermochemical and electronic correlations of these two types of sites on Co_yMoO_x domains with Co-to-Mo atomic ratio (y) varying from 0 to 1. At redox sites (O*), alkanol ODH occurs via a late, kinetically relevant C_α–H scission transition state [O···H···RHCO···Mⁿ⁺]^‡, involving a net H atom (H^•), arising from an electron (e^–) and a proton (H⁺) transfer to a redox site, making hydrogen addition energy (HAE) as the kinetic descriptor, encapsulating the negative of electron (–EA_MO) and proton (–PA_O^–) affinities of catalysts. At Lewis acid sites, alkanol inter-DEH proceeds via S_N2-type substitution with the [O^δ−···H···RH₂CO···CH₂R···OH···M^δ+]^‡ transition state, while intra-DEH, whether uni- or bimolecular, occurs via E2-type elimination through the [O^δ−···H···R′HCH₂C^⊕···^⊖OH···M^δ+]^‡ and [RH₂C(H)O^δ−···H···R′HCH₂C^⊕···^⊖OH···M^δ+]^‡ transition states. These DEH pathways involve C–O scission in their respective transition states, where an electron and a ^•OH radical transfer as a ^⊖OH group to the Lewis acid center (M^δ+–O^δ–). Consequently, the negative ^⊖OH affinity (–HA_⊖OH) serves as an incomplete kinetic descriptor, encapsulating the same negative electron affinity and the negative ^•OH affinity (–HA_•OH) of catalysts. The common electron transfer during the evolution of all these transition states in alkanol ODH and DEH entails the electron affinity of metal oxides to determine their relative activation enthalpies. On Co_yMoO_x, introducing Co cations as electronic perturbators increases the electron affinity of these oxides, thereby reducing both HAE at redox sites and –HA_⊖OH at Lewis acid sites, which proportionally decreases the activation enthalpies for C_α–H scission in methanol, ethanol, n-propanol, and n-butanol ODH; C–O formation in methanol and ethanol inter-DEH; and C_β–H scission in uni- and bimolecular n-propanol and n-butanol intra-DEH, as the Co-to-Mo atomic ratio increases. These linear kinetic correlations in activation enthalpies between alkanol ODH and DEH explicitly illustrate the thermochemical and electronic correlations between redox and Lewis acid sites and the resulting interplay between their turnover rates after accounting for activation enthalpy–entropy compensations. The mechanistic interpretation and framework established here correlate the kinetic, thermochemical, and electronic properties of redox and Lewis acid sites, providing insights into reactivity couplings between redox and Lewis acid catalysis on other bifunctional domains.