Elementary reactions in the oxidative coupling of methane over Mn/Na{sub 2}WO{sub 4}/SiO{sub 2} and Mn/Na{sub 2}WO{sub 4}/MgO catalysts
- Texas A and M Univ., College Station, TX (United States). Dept. of Chemistry
Although it is possible to achieve 20% CH{sub 4} conversion and 80% C{sub 2+} selectivity during the oxidative coupling of CH{sub 4} (OCM) over Mn/Na{sub 2}WO{sub 4}/SiO{sub 2} and Mn/Na{sub 2}WO{sub 4}/MgO catalysts at 800 C, these materials are considerably less active than a Sr/La{sub 2}O{sub 3} catalyst. Similarly, the SiO{sub 2{minus}} and MgO-based catalysts are less active in the formation of CH{sub 3{minus}} 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 C{sub 2}H{sub 4} to CO{sub 2} at 800 C is largely a result of a heterogeneous reaction, due to the inhibiting effect of CH{sub 4} on the gas phase oxidation reaction. Pulse experiments demonstrated that the presence of gas phase O{sub 2} is essential for obtaining an active catalyst. Presumably, an endothermic equilibrium reaction involving O{sub 2} results in a form of surface oxygen that is capable of abstracting a hydrogen from CH{sub 4}. The unusually large activation energy of ca. 63 kcal/mol for both catalysts may result, in part, from this oxygen equilibrium.
- OSTI ID:
- 672469
- Journal Information:
- Journal of Catalysis, Vol. 179, Issue 1; Other Information: PBD: 1 Oct 1998
- Country of Publication:
- United States
- Language:
- English
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