Oxidation of C₂ 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.