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

Abstract

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 CO_x 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.