OXIDATIVE COUPLING OF METHANE OVER COBALT MAGNESIUM AND MANGANESE MAGNESIUM MIXED-OXIDE CATALYSTS

A series of Mn-Mg oxides and Co-Mg oxides were investigated as catalysts for the oxidative coupling of methane in a CH4/O2 co-feed, tubular reactor. The performance of the mixed oxides depended on the preparation procedure and their manganese oxide or cobalt oxide content. Physical mixtures of MnO2/MgO or Co3O4/MgO, calcined at 950°C, were selective to C2's whereas co-precipitated, calcined mixtures were amorphous and non-selective. A maximum in C2-yield of 17% was obtained at 750°C, CH4/O2=2 and 101 kPa for the manganese-magnesium oxide prepared by mixing, at a manganese oxide content equivalent to 0.8 mol-% MnO2 in MgO. The corresponding yield of the cobalt-magnesium oxide was 12% at a cobalt oxide content equivalent to 0.35% CoO in MgO. Comparison of these catalysts with a series of Li/MgO catalysts showed the latter to be unstable at the conditions investigated and the C2-yield to be dependant of the lithium compound, decreasing in the order LiCl> Li2CO3>LiOH. The Li2CO3/MgO had a C2-yield of 17%. It is suggested that the C2-yield as a function of manganese or cobalt oxide content reflects the predominance of O2- or O- on the catalyst. At low cobalt or manganese oxide concentrations C2 selectivity is high and O- is expected to be dominant whereas, at higher concentrations, selectivities toward COx are high and O2- is expected to be dominant. The performance of the manganese-magnesium and cobalt-magnesium oxide measured over a range of temperatures (700-800°C), CH4/O2 ratios (1-5) and GHSV (2160-7260 ml/g cat./h) are also discussed in terms of a reaction sequence in which the majority of COx and ethane is produced via two parallel reactions and ethene is the dehydrogenation product of ethane. © 1990.