The Catalytic Oxidative Coupling of Methane

Abstract One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH 4 is first oxidatively converted into C 2 H 6 , and then into C 2 H 4 . The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH 4 on a metal oxide surface, and a homogeneous gas‐phase component, which includes free‐radical chemistry. Ethane is produced mainly by the coupling of the surface‐generated CH radicals in the gas phase. The yield of C 2 H 4 and C 2 H 6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C 2 H 4 , both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH 4 conversion with 80% combined C 2 H 4 and C 2 H 6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH 4 on certain catalysts. There is spectroscopic evidence for surface O − or O ions. In addition to the oxidative coupling of CH 4 , cross‐coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross‐coupling reaction also appears to involve surface‐generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.