468447 Dioxygen Activation Pathways in Selective Oxidations Catalyzed By Metal Oxides
Propene epoxidation (PO) products were detected from O2-C3H6 reactants only in the presence of CH3OH on Mo-based Keggin clusters, consistent with the involvement of O2-derived species generated on reduced centers that form as CH3OH ODH intermediates. Two possible activated dioxygen intermediates, peroxo (OO*) and hydroperoxo (HOO*) species, may form via O2 activation at reduced centers consisting of O-vacancies (*) and surface hydroxyls (OH*), respectively. DFT indicates that these two pathways differ in their kinetic response to O2 concentrations, because OO* formation is limited by the formation of the O-vacancy formation step, which does not depend on O2 pressure; in contrast, HOO* formation is mediated by H-transfer from hydroxyls to O2 as the kinetically-relevant step, making HOO* concentrations proportional to O2 pressure. Measured PO formation rates from CH3OH-O2-C3H6 reactants (453 k, 0.1-4 kPa CH3OH, 1-60 kPa O2, 1-35 kPa C3H6) were proportional to O2 pressures at low pressures (10 < kPa), suggesting that HOO* act as the main O2-derived epoxidation intermediates. DFT calculations also indicate the low activation barrier (∆E‡ = 32 kJ/mol) for C3H6 reactions with HOO* sites. These HOO* intermediates can also react with OH* to form OO* and H2O, but such steps occur with much higher barriers (∆E‡ = 81 kJ/mol) than for OOH* reactions with C3H6. In contrast, OO* species, formed via either O2 reactions with vacancies or from OOH*, preferably activate C-H bonds in CH3OH (∆E‡ = 31 kJ/mol) over O-insertion reactions with C3H6 (∆E‡ = 39 kJ/mol). The DFT-derived barriers for CH3OH C-H activation on OO* (∆E‡ = 31 kJ/mol) are much lower than for the respective reaction with O* species (∆E‡ = 69 kJ/mol) for a give O-atom in a POM cluster, suggesting that such steps occur rapidly after O2 activation to consume the OO* species and complete a catalytic ODH turnover. Alternate steps involving O-atom migration from OO* to reduced centers require multiple steps, each with significant larger barriers (∆E‡ = 104 kJ/mol) than for direct C-H activation in CH3OH by OO* .
The first-order dependence of PO formation rates from CH3OH-O2-C3H6 mixtures on C3H6 and O2 pressures at low pressures, approached a zero-order kinetic regime on both C3H6 and O2; rates also increased with increased CH3OH pressures along with the concomitant increase in ODH rates. These trends are consistent with the proposed reaction steps for PO occurring on the HOO* sites that are generated from hydroxyl centers by O2, where the formation rate is in steady-state with consumption rates of OOH* via PO or decomposition to OO* and H2O. These data, and their mechanistic interpretation, indicate that the ratio of O-insertion to C-H activation rates for a given C3H6/CH3OH reactant ratio would increase as the HOO*/OO* increases, because of their relative reactivities in these two reactions. Indeed, the presence of H2O increased measured PO/ODH ratios, because O-vacancies (*) react with H2O to form OH*, thus causing a concomitant increase in HOO*/OO* ratios during O2 activation on oxide surfaces. These mechanistic inferences, derived from the scavenging of O2-derived species by propene, taken together with theoretical treatments, illustrate their complexity and selectivity consequences, while providing guidance for catalysts and materials that can enhance or inhibit the relative contributions from pathways mediated by HOO* and OO* intermediates. They also provide fundamental insights into how isolated vacancies, formed as intermediates in ODH reactions, react with the O2 co-reactants to complete ODH catalytic turnovers.
The authors acknowledge the financial support of the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC05-76RL0-1830) and computational resources from EMSL at Pacific Northwest National Laboratory (PNNL) and from XSEDE supported by the National Science Foundation.
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