OXIDATION OF C-2 HYDROCARBON PRODUCTS DURING THE OXIDATIVE COUPLING OF METHANE OVER A LI/MGO CATALYST

The contribution of oxidation of C2 hydrocarbons to carbon oxide formation during the oxidative coupling of methane over a Li/MgO catalyst has been investigated. Small quantities of 13C2H6 and 13C2H4 were added to CH4/O2 in a recycle reactor and the 12C and 13C contents of C2H6, C2H4, CO, and CO2 were determined by mass spectrometry and Fourier transform infrared spectroscopy. At temperatures greater than 740°C, C2 oxidation was responsible for the formation of 30-80% of the carbon oxides. However at lower temperatures (<700°C) C2 oxidation accounted for less than 10% of the carbon oxides. Possible mechanisms for C2 oxidation are competition with CH4 for O2 in a surface-promoted reaction and/or a purely homogeneous process involving competitive reactions of the methyl radical. Calculations based on known rate constants and experimental observations of increased C3H6 formation when 13C2H4 is added to the CH4/O2 mixture show that the abstraction and combination reactions, CH3 + C2H6 → CH4 + C2H5 (6), CH3 + C2H4 → CH4 + C2H3 (7), and CH3 + C2H4 → C3H7 → C3H6 + H (8), compete effectively with the methyl coupling reaction. The 13C content of C2H6 and C2H4 confirms that secondary reactions of C2H6 are responsible for C2H4 formation. Results for the 13C content of CO and CO2 are consistent with CO oxidation being the source of CO2 but are not precise enough for this conclusion to be definitive. The experiments also show that there is an exchangeable pool of CO2 on the catalyst, and the formation of Li2CO3 under reaction conditions is probably responsible. © 1990 American Chemical Society.