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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 CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 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 3• radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH 3• radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 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 CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O 22− ions. In addition to the oxidative coupling of CH4, 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.

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When methane was passed over MgO at temperatures of approximately 500 O C , methyl radicals were produced on the surface, released into the gas phase, and trapped downstream in a solid argon matrix where they were analyzed by EPR spectroscopy. Significant differences in initial activity were observed, depending on whether the MgO was pretreated under vacuum or a flow of oxygen. Vacuum conditioning led to essentially no activity while oxygen conditioning resulted in substantial radical production. The oxidant of choice was also critical. Nitrous oxide resulted in a continuous decline of activity while in the presence of oxygen the formation of radicals was at a steady state. Doping of MgO with lithium, sodium, or iron was also examined. Lithium was found to greatly increase the activity up to a doping level of approximately 15.0 wt %. Two pathways are believed to be responsible for the radical formation. Over pure MgO, intrinsic cation vacancies react with molecular oxygen to give an 0-center which can abstract a hydrogen atom from methane to produce the methyl radical. For the lithium-doped samples, substitutional Li' ions react with molecular oxygen to form a [Li'O-] center which is also capable of abstracting a hydrogen atom from methane.