Kinetics of Primary Product Generation in Catalytic Oxidative Coupling of Methane

Abstract

Kinetic analysis of methyl radical consumption pathways in oxidative coupling of methane (OCM) suggests that the near-universal trend of the primary selectivity to C2 products increasing with increasing temperature arises due to different stoichiometries of C2 and COx formation steps with respect to CH3•. Higher temperatures favor higher rates of CH3• generation in kinetically relevant C–H activation steps, while subsequent reactions of radicals to form primary products (C2H6, CO, CO2) have low barriers and are relatively insensitive to temperature. Higher radical concentrations induced by higher temperatures therefore favor second-order coupling over first-order deep oxidation in OCM. Different reaction orders for C2 and COx formation with respect to CH3• partial pressure lead to different responses of C2 and COx rates to changes in process parameters such as temperature, CH4 partial pressure, and H2O partial pressure, which influence the CH3• generation rate without significantly altering rate constants or oxidant activities for C2 and COx formation from CH3•. Simple kinetic models predict that apparent activation energies and apparent CH4 reaction orders for C2 formation (at differential conversion) should each be 2× those for COx formation, so long as CH3• consumption steps to form C2H6 and COx are barrierless and independent of CH4 partial pressure. Increases in rate and C2 selectivity in H2O cofeeds over alkali tungstate catalysts are explained by H2O introducing a low-barrier path that increases the CH3• generation rate alone. Model predictions are validated using OCM data from prior experimental studies over rare earth oxides, promoted Group II oxides, alkali-promoted titanates, and supported alkali tungstates. These models provide a mechanistic framework to evaluate rate and selectivity trends in OCM and illustrate that OCM activity and primary selectivity are not competing objectives; conditions that generate CH3• radicals more rapidly also direct a greater fraction of them toward C2 products.