ORIGIN OF KINETIC ISOTOPE EFFECTS DURING THE OXIDATIVE COUPLING OF METHANE OVER A LI+ MGO CATALYST
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Kinetic isotope effects (KIEs) during the oxidative coupling of methane over a Li+/MgO catalyst at 700C have been determined using several different experimental methods and by a reaction model that includes both heterogeneous and homogeneous reactions. By maintaining a constant partial pressure of methane of 190 Torr and changing the partial pressure of oxygen, a variation in the H/D KIE, based on differing rates of CH4 and CD4 conversion, was observed. In addition, a similar variation in KIE was found from the isotopic distribution of H and D in the ethane product when CH4 and CD4 were co-fed into the reactor. The decrease in KIE with increasing methane-to-oxygen reactant ratio is evidence that a unique rate-limiting step does not exist; rather, the rates of methane activation and of oxygen incorporation, as individual steps, are comparable. Other factors, such as the absolute partial pressures of the reagents may become even more important than the methane-to-oxygen ratio under some conditions. When N2O was used as the oxidant instead of O2 and normal catalytic conditions were employed, the observed KIE was unity, even though the N2O was present in excess. In this case, a unique rate-limiting step was operative, viz., the incorporation of oxygen into the lattice. The KIE was also determined separately for the production of methyl radicals, but these experiments were carried out at much lower reagent partial pressures (<1 Torr). With O2 as the oxidant, KIEs of 1.5 0.2 were observed, although the methane-to-oxygen ratios were large (>40). This result confirms that there is a KIE associated with the activation of methane on the surface but that this is not necessarily a unique rate-limiting step. With N2O as the oxidant, it was possible to vary the methane-to-nitrous oxide ratio over a large range (from 0.29 to 44). At low ratios, the KIE was 1.9 0.2. The former appears to be a case where methane activation at the surface is uniquely rate-limiting. In general, the oxidative coupling of methane over Li+/MgO is not characterized by a unique rate-limiting step; however, rate-limiting cases may be found, particularly with N2O as the oxidant. 1993 American Chemical Society.
