OXIDATIVE COUPLING OF METHANE OVER A LI+/MGO CATALYST USING N2O AS AN OXIDANT

Nitrous oxide is an effective oxidant for the conversion of CH4 to C2H4 and C2H6 over a Li+/MgO catalyst, although the rate of oxygen incorporation into the lattice and the regeneration of the active centers is much slower with N2O than with O2. Thus, except at very low CH4 partial pressures, the rate-limiting step is oxygen incorporation, rather than the activation of CH4 at the surface. Carbon dioxide is a poison for the production of CH3. radicals and for the conversion of CH4. By extrapolation of the rate data, the rate of CH4 reaction over the unpoisoned catalyst was determined, and the rate was shown to be first order with respect to N2O. A kinetic model, based on competitive surface reactions, in addition to gas-phase reactions, adequately accounts for the conversion as a function of reagent concentrations and temperature, as well as the selectivity for ethane and ethylene. At comparable levels of conversion O2 is a less selective oxidant than N2O, not because of more gas-phase oxidation, but because of surface reactions involving reactive oxvgen species. The presence of CH4 significantly inhibits the decomposition of N2O over the catalyst. It is suggested that CH4 and N2O are activated by a common intermediate (e.g., O- S ions) and that CH4 reacts more rapidly with this species. © 1993 Academic Press, Inc.