Magnitude and origin of the deuterium kinetic isotope effect during methane coupling and related reactions over Li/MgO catalysts

The deuterium kinetic isotope effect during methane coupling over a Li/MgO catalyst at 746°C has been determined as a function of methane concentration (10-80%) at two different oxygen concentrations (5 and 10%). In contrast to one earlier report, the overall rate ratio, CH ₄ versus CD ₄, appears constant (1.59 ± 0.1) over this range of conditions. The isotope effect to ethane is slightly greater than the mean value. Those to ethylene and carbon monoxide are considerably greater still due to a second kinetic isotope effect in their production. The isotope effect to carbon dioxide is correspondingly less. Similar experiments have been carried out for the oxidation of ethylene at 660°C. The overall isotope effect, C ₂H ₄ versus C ₂D ₄, is 1.45 ± 0.12, and as with methane it is greater for carbon monoxide production than for carbon dioxide production. Methane oxidation has been modeled by coupling a simple literature model for surface steps involving O _s ^- to a model for the subsequent gas-phase reactions of surface generated methyl radicals. The calculations show that a parameter set suggested for the surface processes in earlier work does not provide a good fit to the observed kinetic orders in methane or oxygen. The set also underestimates the extent of ¹⁶O ₂/ ¹⁸O ₂ mixing during methane coupling. Alternative parameter sets which are more consistent with the kinetic and exchange data predict a small dependence of the expected kinetic isotope effect on pressure which falls within the bounds of the experimental measurements. It is concluded that over the range of conditions used here the rate of methane oxidation over Li/MgO catalysts is largely governed by the rate of bond breaking in methane. However that rate is sufficiently close to the rate at which surface oxidation sites are being created that the latter could be rate influencing under substantially different conditions as for example when using N ₂O as the oxidant.