Methane Activation by Diatomic Molybdenum Carbide Cations

Abstract

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas-phase and condensed-phase studies. Herein, methane activation by the diatomic cation MoC(+) is presented. MoC(+) ions have been prepared and mass-selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time-of-flight mass spectrometer. Bare metal Mo(+) and MoC2H2(+) ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C2H4 elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC(+) with CH4. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C-C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC(+) plays a key role in methane activation because it becomes sp(3) hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C-H bond activation and C-C coupling involved in methane transformation over molybdenum carbide-based catalysts.