Oxidative Coupling of Methane over Oxide-Supported Sodium-Manganese Catalysts

Abstract

The oxidative coupling of methane over Mn/Na2WO4/SiO2, Mn/Na2WO4/MgO, and NaMnO4/MgO catalysts was studied using both a cofeed flow system and a pulse reactor. At 800°C and 1 atm, and using a CH4/O2 ratio of ca. 8/1, a methane conversion of 20% was achieved at a C2+ selectivity of ≥ 80%, with no diluent in the reagents. The similar catalytic behaviors of the three catalysts suggest that a common active site, consisting of an Na-O-Mn species, may be involved. Results from a pulse reaction sequence (an O2 pulse followed by a series of pure CH4 pulses) indicate that the active species are not stable under reaction conditions unless gas phase O2 is present, and that bulk lattice oxygen does not participate in the methane coupling reaction when carried out in the cofeed mode. There is a linear relationship between the specific activity for CH4 conversion and the concentration of surface Mn, which is believed to be responsible for the activation of O2. The resulting form of oxygen then abstracts a hydrogen atom from CH4. Sodium is essential for preventing the complete oxidation of CH4, perhaps by isolating the Mn ions. The tungstate ions appear to impart stability to the catalysts.