Surface reaction and lattice oxygen transfer in chemical looping oxidative coupling of methane: Molecular dynamics simulations

Abstract

Chemical-looping oxidative coupling of methane (CL-OCM) is a promising methodology for ethylene production from methane. This article utilizes molecular dynamics (MD) simulation to assess the performance of eight metal oxide catalytic oxygen carriers in CL-OCM reactions. It also investigates the impact of reaction time and particle size on the efficiency of the most effective Mn₂O₃ COC. The results indicate that extending the reaction time appropriately enhances C₂H₄ selectivity and a C/O ratio of 1 is found to be the optimal size for Mn₂O₃-based CL-OCM. Furthermore, surface reactions and lattice oxygen transfer are analyzed by MD simulation in Mn₂O₃-based CL-OCM, providing deeply insights into the reaction mechanism. The findings reveal that the gas-phase dimerization of ${\text{CH}}_{\text{3}}^{\text{*}}$ to form C₂H₆ serves as the primary carbon coupling pathway in CL-OCM. In addition, there are two other carbon coupling pathways, both initiated by ${\text{CH}}_{\text{2}}^{\text{*}}$. Methanol formation through surface combination of ${\text{CH}}_{\text{3}}^{\text{*}}$ and OH* represents an initial step in CL-OCM side reactions. Therefore, inhibiting methanol formation is crucial for enhancing C₂ selectivity in CL-OCM. There exists a transformation of lattice oxygen and surface lattice oxygen plays a key role in methane activation. The quantity of lattice oxygen and difference in bulk lattice oxygen migration resistance are major factors influencing CH₄ conversion and C₂ selectivity. This study provides a new way to reaction mechanism exploration related to CL-OCM catalytic oxygen carriers.