We report here mechanistic interpretations of oxidative dehydrogenation (ODH) and dehydration rates of  CH3OH-O2 mixtures on Mo- and W-based POM clusters, with different central atoms (H3P Mo12O40, H4Si Mo12O40) or addenda atoms (H3+nPVn=1-3Mo12-n O40, H4PV1W11 O40). Such materials exhibit well-defined atomic connectivity, making them well-suited for combining rigorous kinetic and isotopic studies with density functional theory (DFT) estimates of their redox and acid properties and the energetics of the relevant elementary steps.ODH reactions involve kinetic coupling between steps that reduce and oxidize POM clusters. Reduction steps that typically limit rates for most O2/CH3OH reactant ratios depend on the ability of oxide clusters to accept electrons during H-abstraction from molecularly adsorbed CH3OH, in a step mediated by a late transition state (TS). As a result, TS energies correlate strongly with H-addition energies (HAE) to catalysts' O-atoms (slope of ETS versus HAE ~ 0.8), which probe energetics of reduction analogously to the H-abstraction step. HAE values change with location on clusters and need exponentially weighted reaction-averaging (HAEavg) over all locations. HAEavg decrease with changing POM clusters' addenda-atoms from W to Mo and their V-substitution suggesting improved ability to accept electrons in reduction step, consistent with higher measured ODH rate constants.Re-oxidation of clusters is kinetically silent but their details are relevant to elementary steps that complete the redox turnovers and accessible through accurate detection of reduced centers in UV-vis spectra. Spectra from theory and experiments indicate small extents of reduction; mechanistic interpretations of their dependence on reactant pressures are consistent with fast re-oxidation rates that increase with the extents of reduction. H2O inhibits ODH reactions via competitive adsorption with CH3-OH on POM lattice O-atoms.Dependence of dehydration rates on composition reflects ionic and covalent interactions that must be overcome in removing proton from the clusters (i.e., deprotonation energy), and their influence on stability of ion-pair dehydration TS.

Oxidation and acid reactions of methanol on bifunctional polyoxometalate clusters: Experiments and theory

248th National Meeting (2014)

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