Mechanistic insights into the oxidative coupling of methane over a Li/MgO catalyst: an experimental and microkinetic modeling study

This study investigates the oxidative coupling of methane (OCM) using a Li/MgO catalyst in a packed bed reactor. Experiments were conducted at a pressure of 110 Torr over a temperature range of 873-1173 K. Stable products, including alcohols, aldehydes, ketones, and C3 hydrocarbons, were quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Methyl and ethyl radicals were definitively identified, emphasizing their critical role in the formation of C2-C3 hydrocarbons. Based on these speciation results, a microkinetic model for the OCM reaction over Li/MgO was developed. The reaction network analysis revealed that the main pathway for COx generation involves the conversion of CH3 to CH3O(s)/CH3O through surface and gas-phase reactions. Gas-phase reactions also promote the deep oxidation of C2H4. Sensitivity analysis indicated that methane activation is primarily governed by surface reactions, while the coupling of CH3 to form C2H6 is mainly driven by gas-phase reactions. Both surface and gas-phase reactions contribute equally to the formation of C2H4. Additionally, the pressure dependence analysis demonstrated that the high-pressure limit of the CH3 coupling reaction restricts the increase in C2 yield as pressure rises. In conclusion, this study provides a comprehensive investigation into the OCM speciation pool, develops a microkinetic model aligned with experimental findings, and elucidates the reaction network driven by free radicals. These insights offer valuable guidance for optimization reaction conditions and catalyst performance. This study investigates the oxidative coupling of methane (OCM) using a Li/MgO catalyst in a packed bed reactor.