ELEMENTARY REACTIONS IN THE OXIDATIVE COUPLING OF METHANE OVER MN NA2WO4/SIO2 AND MN/NA2WO4/MGO CATALYSTS/

Although it is possible to achieve 20% CH4 conversion and 80% C2+ sele ctivity during the oxidative coupling of CH4 (OCM) over Mn/Na2WO4/SiO2 and Mn/Na2WO4/MgO catalysts at 800 degrees C, these materials are con siderably less active than a Sr/La2O3 catalyst. Similarly, the SiO2- a nd MgO-based catalysts are less active in the formation of CH3. radica ls that emanate into the gas phase and in the secondary reaction of th ese radicals with the catalysts. During the OCM reaction, ethane is th e primary product; ethylene is produced only in small amounts at short contact times. As a secondary product, however, ethylene competes wit h methane in the formation of carbon dioxide. The conversion of C2H4 t o CO2 at 800 degrees C is largely a result of a heterogeneous reaction , due to the inhibiting effect of CH4 on the gas phase oxidation react ion. Pulse experiments demonstrated that the presence of gas phase O-2 is essential for obtaining an active catalyst. Presumably, an endothe rmic equilibrium reaction involving O-2 results in a form of surface o xygen that is capable of abstracting a hydrogen atom from CH4. The unu sually large activation energy of ca. 63 kcal/mol for both catalysts m ay result, in part, from this oxygen equilibrium. (C) 1998 Academic Pr ess.