Elementary reactions in the oxidative coupling of methane over Mn/Na2WO4/SiO2 and Mn/Na2WO4/MgO catalysts

摘要

Although it is possible to achieve 20% CH4 conversion and 80% C2+ selectivity during the oxidative coupling of CH4 (OCM) over Mn/Na2WO4/SiO2 and Mn/Na2WO4/MgO catalysts at 800 degrees C, these materials are considerably less active than a Sr/La2O3 catalyst. Similarly, the SiO2- and MgO-based catalysts are less active in the formation of CH3. radicals that emanate into the gas phase and in the secondary reaction of these radicals with the catalysts. During the OCM reaction, ethane is the primary product; ethylene is produced only in small amounts at short contact times. As a secondary product, however, ethylene competes with methane in the formation of carbon dioxide. The conversion of C2H4 to 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 reaction. Pulse experiments demonstrated that the presence of gas phase O-2 is essential for obtaining an active catalyst. Presumably, an endothermic equilibrium reaction involving O-2 results in a form of surface oxygen that is capable of abstracting a hydrogen atom from CH4. The unusually large activation energy of ca. 63 kcal/mol for both catalysts may result, in part, from this oxygen equilibrium. (C) 1998 Academic Press. [References: 18]