Control of Reactivity in C−H Bond Breaking Reactions on Oxide Catalysts:  Methanol Oxidation on Supported Molybdenum Oxide

Abstract

Oxidation of organic substrates on metal oxide catalysts can be viewed as entailing a transfer of electrons from the organic moeity to the catalytic center, and is expected to involve the electron-accepting levels in the metal center. This investigation deals with the study of the electron-transfer processes associated with the C-H bond breaking reaction in adsorbed methoxide species in the course of methanol oxidation on supported MoO 3 . It is shown that the activity of a series of catalysts duly correlates with the unoccupied density of electronic states of the parent metal oxides. The unoccupied levels are probed with near-edge X-ray absorption spectroscopy (NEXAFS). The particular system chosen for study is that of methanol oxidation on supported molybdenum oxide, a reaction in which C-H bond breaking is rate-determining. The findings of this study are in good agreement with the known facts about the reaction and allow a deeper understanding of various reported effects. For example, the positive relationships between the catalytic activity and the decreasing electronegativity of the support ion, the increasing reducibility of the dispersed molybdenum centers, and the increasing size of the molybdenum ensembles can all be explained by the increasing ease of accommodation of charge by the metal centers. The findings in this study should be of importance in understanding other C-H bond breaking reactions and multicomponent catalyst systems.

Kinetics and Mechanism of Dimethyl Ether Oxidation to Formaldehyde on Supported Molybdenum Oxide Domains

“…38 A similar argument seems plausible for methanol oxidation, which occurs via rate-determining C-H bond activation steps, normally associated with the formal reduction of metal centers. Support effects were substantiated when Oyama et al 39 showed that the activity of a catalyst correlates with the density of unoccupied electronic states; MoO x deposited on support materials that readily accept electron density from reducing C-H bond cleavage steps in methanol oxidation result in higher conversion rates. 39 This reasoning becomes less unequivocal for the observed correlations between reducibility and DME oxidation rates, because H-abstraction appears to be kinetically irrelevant and C-O bond cleavage, which limits reaction rates, does not require the formal reduction of Mo 6+ centers.…”

“…Support effects were substantiated when Oyama et al 39 showed that the activity of a catalyst correlates with the density of unoccupied electronic states; MoO x deposited on support materials that readily accept electron density from reducing C-H bond cleavage steps in methanol oxidation result in higher conversion rates. 39 This reasoning becomes less unequivocal for the observed correlations between reducibility and DME oxidation rates, because H-abstraction appears to be kinetically irrelevant and C-O bond cleavage, which limits reaction rates, does not require the formal reduction of Mo 6+ centers. It appears, however, that electron density at cation sites increases gradually along the reaction sequence, instead of occurring sharply during the formal reduction assumed to occur at the H-abstraction step.…”

Using DFT in Search for Support Effects During Methanol Oxidation on Supported Molybdenum Oxides

“…One of these similarities is the support effect, which is assumed to be related to the supporting metal oxide and strongly influences the reaction turn-over frequency (TOF). For methanol oxidation it is found that the TOF increases when changing the support from: [2][3][4][5] MoO x /SiO 2 < MoO x /Al2O 3 ! MoO x /TiO 2 < MoO x /ZrO 2 .…”

“…[12] These observations together with various kinetic studies, principally conducted through the combined efforts of research groups at Virginia Tech and Tokyo Institute of Technology, led to the proposal of a schematic reaction mechanism [Eqs. (1) and (2)]: [2,3,13] CH 3 OHþMoþO ! MoÀOCH 3 þOÀH ð1Þ…”

“…This mechanism is suggested to be identical for all the supports above. [2,3,13] . At low methanol coverage the kinetics of this scheme are compatible with Equation (3): [14] r CH 2 O ¼ K ads k z P CH 3 OH ¼ k app P CH 3 OH ð3Þ…”