Effect of Oxidants on the Oxidative Coupling of Methane over a Lead Oxide Catalyst

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Abstract
Oxidative coupling of methane was studied over a PbO/MgO catalyst using a variety of oxidants such as N2O, NO, CO2, and SO2. While N2O showed both high activity and selectivity for the title reaction, NO produced CO2, exclusively. The coupling reaction was assumed to proceed via the redox cycle of Pb and PbO on each oxidant mentioned above. Carbon dioxide produced small amounts of C2 hydrocarbons and CO, while SO2, was inactive for the reaction. Oxygen, N2O, CO2 could oxidize the Pb/MgO which had been formed by the reaction of PbO/MgO with methane at 1023 K. Thus-prepared PbO/MgO produced C2 hydrocarbons from methane. Even NO, which gave no C2 hydrocarbons in the CH4–NO cofeed reaction, converted the Pb/MgO to PbO/MgO and the PbO/MgO gave C2 hydrocarbons exclusively upon reacting with CH4. NO seems to oxidize the methyl radical, which is an intermediate of the coupling reaction to CO2. The ineffectiveness of SO2 as an oxidant was attributed to the formation of PbS, which is inactive in the methane activation.