Online Kinetics Study of Oxidative Coupling of Methane over La₂O₃ for Methane Activation: What Is Behind the Distinguished Light-off Temperatures?

Abstract

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly to valuable C2 products (ethane and ethylene). The main difficulties from further investigation of this reaction are due to the nature of its high operating temperature and the severe reaction exothermicity. In this work, an especially designed online characterization setup is applied for this reaction, which achieved both precise bed temperature control and real-time product measurement. High-resolution temperature-dependent product rates of OCM (CO2, ethane, and ethylene) were obtained, and their behavior vs reaction conditions as well as the activation energy barriers above their onset temperatures are clearly differentiated over a recently reported high performance nanorod La2O3 catalyst. Different from general expectation, CO2, resulting from full methane oxidation, dominates all the products in the lower temperature region, whereas the C2 species are only formed at much higher temperatures. Further analysis indicates that the selectivity and apparent activation energy for both COx and C2 products are strongly influenced by the oxygen concentration and temperature. In combination with density functional theory calculations and additional experimental measurements, significant insights are brought to this high-temperature reaction of wide interest.