OXIDATIVE COUPLING OF METHANE OVER OXIDE-SUPPORTED SODIUM-MANGANESE CATALYSTS

The oxidative coupling of methane over Mn/Na2WO4/SiO2, Mn/Na2WO4/MgO, and NaMnO4/MgO catalysts was studied using both a cofeed flow system a nd a pulse reactor. At 800 degrees C and 1 atm, and using a CH4/O-2 ra tio of ca. 8/1, a methane conversion of 20% was achieved at a C-2+ sel ectivity of greater than or equal to 80%, with no diluent in the reage nts. The similar catalytic behaviors of the three catalysts suggest th at a common active site, consisting of an Na-O-Mn species, may be invo lved. Results from a pulse reaction sequence (an O-2 pulse followed by a series of pure CH4 pulses) indicate that the active species are not stable under reaction conditions unless gas phase O-2 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 relat ionship between the specific activity for CH4 conversion and the conce ntration of surface Mn, which is believed to be responsible for the ac tivation of O-2. The resulting form of oxygen then abstracts a hydroge n atom from CH4. Sodium is essential for preventing the complete oxida tion of CH4, perhaps by isolating the Mn ions. The tungstate ions appe ar to impart stability to the catalysts. (C) 1995 Academic Press, Inc.