Isotopic evidence for direct methyl coupling and ethane to ethylene conversion during partial oxidation of methane over Li/MgO

The products of the reaction of a CH₄/CD₄/O₂ mixture over a Li-promoted MgO (Li/MgO) catalyst at 750°C have been determined by mass spectrometry and high-resolution Fourier transform infrared spectroscopy. Under these conditions there was no significant hydrogen exchange between CH₄ and CD₄. C₂H₆, CD₃CH₃, and C₂D₆ were the only ethanes detected, and C₂H₄, CD₂CH₂, and C₂D₄ were the only ethylenes. The relative concentrations of the hydrocarbon products could be predicted on the basis of the relative concentrations of CH₃ and CD₃ determined by the kinetic isotope effect. The results suggest that the reaction proceeds almost exclusively by gas-phase methyl coupling and that ethylene is produced by gas-phase conversion of ethane.