Mechanistic Aspects and Reaction Pathways for Oxidative Coupling of Methane on Mn/Na2WO4/SiO2 Catalysts

Abstract

Kinetic and isotopic methods were used to determine the identity, rate constants, and reversibility of elementary steps for primary and secondary reactions involved in the oxidative coupling of methane (OCM) on Mn/Na2WO4/SiO2. We provide evidence in this study for parallel C−H bond activation pathways, in which H-abstraction is mediated by either oxygen species on surfaces or by OH radicals formed via H2O/O2 equilibration on catalyst surfaces. OCM rates and C2+ yields are higher when H2O is present and OH-mediated pathways prevail, because of the high reactivity of OH radicals and of their lesser sensitivity to the energy of the C−H bond containing the hydrogen abstracted. These coupled homogeneous-catalytic sequences account for all observed kinetic effects of O2, CH4, and H2O on rates and selectivities for both CH4 conversion and for subsequent reactions of C2H6, C2H4 and C3 products; they are also consistent with measured kinetic and thermodynamic isotope effects for C−H bond activation mediated by sur...