Mechanism of Cooxidative Methane Dimerization Catalysis: Kinetic and Thermodynamic Aspects

Abstract

A general mechanism for the direct oxidative conversion of methane is emerging from recent experimental work. Considerable evidence indicates that heterogeneous production of methyl radicals followed by gas-phase coupling to form ethane initiates the production of higher hydrocarbons during the selective catalytic oxidation of methane. The definitive experiments are those of Lunsford and co-workers (Driscoll et al. 1985; Campbell and Lunsford 1988, 1989; Driscoll, Campbell, and Lunsford 1987; Driscoll, Zhang, and Lunsford 1986), who use matrix-isolated electron paramagnetic resonance (MIEPR) measurements to determine the distribution of methyl radicals downstream of a Li/MgO catalyst bed. These results have been confirmed by other workers using mass spectrometric (Amorebieta and Colussi 1988, 1989), spectroscopic (Lee, Yu, and Lin 1990; Gulcicek, Colson, and Pfefferle 1990), and other MIEPR (Sinev, Korchak, and Krylov 1988) methods. Isotopic exchange experiments (Nelson, Lukey, and Cant 1988, 1989; Nelson and Cant 1990; Cant et al. 1988; Mims et al. 1989) also support the view that the methyl radical is the primary intermediate in the production of ethane. Several reports indicate that the reactor geometry influences the product distribution and that microreactors without catalysts (van Kasteren, Geerts, and van der Wiele 1988; Lo, Agarwal, and Marcelin 1988; Lane and Wolf 1988; Asami et al. 1988; Yates and Zlotin 1988) can selectively produce ethane and ethene. These observations affirm the significant role of homogeneous kinetics in all aspects of the reaction, including its initiation.