(308f) Enhancing Selectivity in Oxidative Coupling of Methane Using Catalytic Solid Oxide Membrane Reacting Systems

Oxidative coupling of methane (OCM) is a chemical process in which methane is converted directly into ethylene and ethane (C₂) at high temperatures (600-900°C) in the presence of oxygen and typically an active catalyst. The main challenge with this process is the formation of thermodynamically stable over-oxidation products, mainly CO and CO₂, which limits the C₂ selectivity. This selectivity problem is particularly significant in conventional reactors (co-fed with methane and oxygen) due to the presence of large gas-phase oxygen concentrations at the reactor inlet, which accelerates the unselective, deep oxidation reactions.¹ Catalytic solid oxide membrane reactors can minimize the unselective reactions by separating the methane and oxygen feed, and allowing the selective passage (conduction) of oxygen ions (O^2-) from the oxygen side to the methane side where the reaction occurs.^2,3 By allowing the distributed and controlled delivery of O^2- to the active sites, gas-phase oxygen can be significantly minimized, thereby limiting the occurrence of deep oxidation reactions. These membrane systems offer additional benefits including better control of reactor temperature and simultaneous separation of pure oxygen from other components in air thereby simplifying downstream separation. Herein, we have synthesized and tested catalytic solid oxide membrane systems in OCM. We show that a BaCe_0.8Gd_0.2O_3-δ (BCG) membrane system exhibits stable OCM performance and high carbon tolerance at extremely low oxidizing conditions. We also demonstrate that the BCG membrane system operated in membrane mode (separated and distributed oxygen feed) gives significantly higher C₂ selectivity compared to the same system co-fed with methane and oxygen, at comparable methane conversions. These results provide reliable experimental evidence of the improved OCM performance of a catalytic membrane system over a conventional co-fed system.

References

(1) Farrell, B. L.; Igenegbai, V. O.; Linic, S. ACS Catal. 2016, 6 (7), 4340–4346.

(2) Farrell, B. L.; Linic, S. Catal. Sci. Technol. 2016, 6, 4370–4376.

(3) Igenegbai, V. O.; Meyer, R. J.; Linic, S. Appl. Catal. B Environ. 2018, 230, 29–35.