Elucidating the Mechanism for Oxidative Coupling of Methane Catalyzed by La₂O₃: Experimental and Microkinetic Modeling Studies

Oxidative coupling of methane (OCM) has been widely proposed to be one of the most promising methods for the direct conversion of methane to C-2 products, such as ethane and ethene. Highly active free radicals play a crucial role, while accurate identifications are limited. To probe these free radicals and reveal their reactions, experiments focused on the OCM catalyzed by La2O3 were designed to be carried out in a packed bed reactor at low-pressure conditions over a wide temperature range. Dozens of species, including methyl radical, ethyl radical, and formaldehyde, were observed by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A microkinetic model that coupled a detailed gas-phase and surface mechanism was developed and validated against the experimental results, especially to reveal the crucial roles of free radicals in the formation of C-2 products as well as the oxygenated intermediates. The prediction results of the kinetic model agreed well with the experimental measurements. Rate of production and sensitivity analysis were performed to reveal the complex reaction network and key reactions of the OCM. Methyl was confirmed to play a key role based on both experimental and modeling perspectives, while ethyl is crucial in the transformation of C-2 species and the formation of C-3-C-4 species. This indicates that the selective regulation of free radicals such as methyl and ethyl in OCM is worth paying attention to. The present work provides more detailed chemistry of OCM reactions, which would be helpful to improve product selectivity of OCM.