Formation and Reactions of Methyl Radicals over Metal Oxide Catalysts

Abstract

Current interest in the oxidative coupling of methane has focused attention on the formation and reactions of methyl radicals. Although other intermediates may contribute to the formation of minor amounts of ethylene (Hutchings, Woodhouse, and Scurrell 1989), it is generally agreed that the reaction sequence involves the coupling of CH3. radicals to form C2H6, followed by the dehydrogenation of C2H6 to form the desired product C2H4 (Nelson, Lukey, and Cant 1988; Mims et al. 1989). Within this rather simple mechanistic framework many ancillary issues remain to be reconciled, as will be evident in this and subsequent chapters of the book. Some of these issues include the site at which the initial CH3. radicals are formed, the role of surface-initiated gas-phase radical reactions, and the origin of carbon oxides (CO and CO2). The diversity of catalysts and operating conditions makes it difficult to generalize; however, we attempt to show that surface-generated gas-phase radicals are important intermediates in the catalytic process and that the fate of these radicals determines, in part, the selectivities that are obtained.