Continuous flow experiments for the oxidative coupling of methane in the absence of catalyst and at low methane conversion were carried out in empty tubular quartz reactors at atmospheric pressure, temperatures from 873 to 1123 K, and inlet molar ratios of CH,/O2 from 4 to 10 and of He/CH4 from 0 to 1.25. The methane conversion varied from 2 to 15% and the oxygen conversion from 10 to 100%. A reaction network was constructed on the basis of elementary free-radical reactions. Arrhenius parameters were estimated for the most important reactions by regression of experimental data. The effects of the process conditions on the conversions of methane and oxygen as well as on the selectivities toward products were simulated adequateliy by considering 33 elementary reactions.Ethane is mainly formed from the recombination of methyl radicals arising from degenerate branched chains involving OH and H 0 2 as main chain carriers. Etlhene originates from ethane mainly via a pyrolytic chain, while the oxidative dehydrogenation contributes to a much lower extent. Carbon monoxide originates from the oxidation of methyl radicals, whereas the contribution of the consecutive oxidation is not significant a t the conversion levels investigated. IntroductionThe oxidative coupling of methane aimed at the production of higher hydrocarbons has attracted much attention during the past years. This reaction is usually carried out in the presence of catalysts and requires temperatures up to 1150 K. At these temperatures, noncatalytic reactions in the gas phase, however, play an essential role in the formation of higher hydrocarbons. It is generally accepted that the most widely studied alkali metal/&aline earth metal oxide and the rare earth metal oxide catalysts act as a methane activator and in particular as a producer of methyl radicals, with the subsequent reactions taking place in the gas phase (Ito et al., 1985). Whether or not the reducible multivalent metal oxides catalyze the reactions in the same way is still in debate.The importance of the gas-phase reactions has been well recognized and is reflected in several recent papers on the oxidative coupling of methane in the absence of catalyst (Labinger and Ott, 1987;Lane and Wolf, 1988;Geerts et al., 1990;Zanthoff and Baerns, 1990). Furthermore, in a recent review, Lunsford (1990) has pointed out the importance of branched-chain reactions in the gas phase with respect to the generation of methyl radicals. Consequently, Lunsford proposed that catalysts might be an important initiator of the chain reaction, but not a major source of methyl radicals. Kinetic models based on the free-radical mechanism have been set up in order to understand the role played by the gas-phase reactions and the interaction between the gas-phase reactions and the reactions on the catalyst surface. The Arrhenius parameters were selected from data bases in the literature that originate mainly from combustion kinetics (Tsang and Hampson, 1986; Warnatz, 1984). While the dependence of the coupling selectivity on co...