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

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/O-2 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 O-2, CH4, and H2O on rates and selectivities for both CH4 conversion and for subsequent reactions of C2H6, C2H4 and C-3 products; they are also consistent with measured kinetic and thermodynamic isotope effects for C-H bond activation mediated by surface and OH radicals. Kinetic isotope effects and isotopic scrambling studies (CD4/CH4; D2O/H2O;O-18(2)/O-16(2)) indicate that C-H bond activation is irreversible and kinetically-relevant. O-2 dissociation is quasi-equilibrated, but becomes irreversible as H2O/O-2 ratios increase with increasing conversion and residence time. Competitive reactions of (CH4)-C-13/O-2 with (C2H6)-C-12, (C2H4)-C-12, and (C3H6)-C-12 with and without added H2O show that H-abstraction from hydrocarbons is much less sensitive to C-H bond strength when OH radicals are used to abstract hydrogen instead of oxide surfaces. Maximum C2+ yields require conditions that favor OH-mediated pathways while maintaining equilibrium oxygen surface coverages and OH radical concentrations. OH-mediated pathways are more sensitive to O-2 pressure than surface-mediated pathways; thus, low O-2 pressures and staging strategies that maintain stoichiometric O-2 requirements and low local O-2 pressures can improve C2+ selectivities but only when OH radicals are maintained at equilibrium concentrations via catalytic H2O-O-2 reactions. These findings and interpretations indicate that intermediate O-2 pressures give maximum C2+ yields, but that their optimal value depends sensitively on prevalent H2O concentrations as they vary with conversion along the reactor. These predictions about the consequences of various operating strategies have become feasible because of the detailed and quantitative nature of the mechanism-based kinetic networks reported here for the first time.