(576f) Dioxygen Activation Pathways in Mars-Van Krevelen Redox Cycles

Oxidative dehydrogenation (ODH) on reducible metal oxides occurs via Mars-van Krevelen redox cycles at lattice O-atoms (O*), which abstract H-atoms from organic reactants at the kinetically-relevant step, followed by re-oxidation of reduced centers by O_2. Compared to relatively well-established reduction part of the cycle, the re-oxidation steps have received much less attention in spite of their possible consequences for ODH selectivity. Here, we investigate O₂ activation steps during CH₃OH ODH on Mo-based Keggin polyoxometalate (POM) using experimental and theoretical methods, which employ strategies to scavenge active oxygen intermediates with C₃H₆ to form epoxidation (PO) products.

H-abstractions in CH₃OH by vicinal O* centers form HCHO, leaving a two-electron reduced center in form of two hydroxyls (H/OH*) or an O-vacancy (*), upon recombinative H₂O desorption. O₂ activation at * forms electrophilic peroxo (OO*) species via an inner-sphere route, which can insert one of its O-atoms to C=C in C₃H₆ to form epoxide. Consistently, PO products were detected from O₂-C₃H₆ only in the presence of CH₃OH, which is required to form a reduced center that acts as precursor for O₂ activation. PO/ODH rate ratios reflect the fraction of reduced centers that forms bound OO*, which would otherwise reoxidize via outer-sphere routes that circumvent OO* formation. Such ratio decreased with H₂O pressure, indicating that outer-sphere routes become favorable as H₂O caps * to H/OH* that reacts with O₂ to form H₂O₂(g). H₂O₂(g) can react with another reduced center, acting as a molecule shuttle to kinetically couple two oxidation events at a remote location. The relative contribution between inner and outer sphere routes influences the product selectivity, because OO* intermediates can mediate O-insertion to form oxygenated products that are more susceptible for CO₂ formation. These results shed lights on O₂ activation routes in Mars-van Krevelen redox cycles and their unexplained role in ODH product selectivity.