Oxidative-Coupling-Assisted Methane Aromatization: A Simulation Study

This study simulates a high-temperature reaction in a plug-flow reactor (PFR) for the aromatization of methane via oxidative coupling of methane (OCM) using a state-of-the-art gas-phase chemical kinetic mechanism. Benzene is formed from a methane-oxygen (CH4-O2) feed via formation of ethylene through OCM followed by homogeneous gas-phase aromatization of C2H4 after O2 depletion. Because both OCM and C2H4 aromatization are exothermic reactions, the process is advantageous over an endothermic nonoxidative methane aromatization reaction. For the OCM reaction, the previously reported mechanism in which the catalyst achieves the quasi-equilibrated formation of OH• from an H2O-O2 mixture is included in the gas-phase combustion chemistry reaction network. It is evident that OH• formation increases benzene yield as a consequence of enhanced C2H4 yield from the OCM. The influence of temperature, CH4/O2 ratio, and contact time on benzene yield is elucidated, and reaction pathways leading to aromatic formation are analyzed. The maximum benzene yield on a carbon basis at a total pressure of 1 atm reaches 10% at CH4/O2 ratios from 3 to 6 and temperatures of 800-900 °C (isothermal). Our analysis on the differential rates of production suggests that benzene is formed from the benzyl radical via toluene and from the reaction between allyl and propargyl radicals. Simulations show that using the exothermicity of the process enables adiabatic reactor operation, which is beneficial for reducing the external heat supply (i.e., inlet temperature) by utilizing the exothermic reactions.