Measurements of kinetic isotope effects and hydrogen/deuterium distributions over methane oxidative coupling catalysts

The kinetic isotope effect for CH₄ compared to that for CD₄ has been measured for the oxidative coupling reaction of methane over Li MgO, SrCO₃, and Sm₂O₃ catalysts in a flow reactor. Each catalyst gave results consistent with CH bond breaking being the slow step. For temperatures between 680-780 °C over Li MgO, k_H K_D decreased slightly with temperature. The isotope effect for ethane production was more sensitive to the level of conversion and declined from 1.8 at low conversion to near unity under conditions where the ethylene to ethane ratio was high (~1). Selectivities to hydrocarbons were lower with CD₄ and did not change with decreased flow rates, implying that either CO_x and C₂ products arise by totally separate slow steps or, if a common step with CH₃ radicals is involved, then CO_x formation occurs on the catalyst. Experiments with CH₄ CD₄ mixtures showed that CH₃CD₃ and CH₂CD₂ were the dominant mixed products. The distribution of the ethanes always reflected the relative concentrations of CH₃ and CD₃ determined by the kinetic isotope effect. At low ethylene to total C₂ ratios (~0.2) this was also true for ethylene; but at higher ratios substantial exchange to produce ethylenes other than C₂H₄, CH₂CD₂, and C₂D₄ occurred. The concentration of the exchanged methanes correlated with total methane conversion but was dependent on the surface. Exchange in the ethylenes also correlated with exchange in the methanes and purely gas phase processes appear at least partially responsible. H₂: HD: D₂ ratios are always at equilibrium and exchange also occurs between CD₄ and H₂.