Oxidative coupling of methane over cobalt—magnesium and manganese—magnesium mixed oxide catalysts

Abstract

A series of Mn—Mg oxides and Co—Mg oxides were investigated as catalysts for the oxidative coupling of methane in a CH 4/O 2 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 MnO 2/MgO or Co 3O 4/MgO, calcined at 950°C, were selective to C 2's whereas co-precipitated, calcined mixtures were amorphous and non-selective. A maximum in C 2-yield of 17% was obtained at 750°C, CH 4/O 2=2 and 101 kPa for the manganese—magnesium oxide prepared by mixing, at a manganese oxide content equivalent to 0.8 mol-% MnO 2 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 C 2-yield to be dependant of the lithium compound, decreasing in the order LiCl> Li 2CO 3LiOH. The Li 2CO 3/MgO had a C 2-yield of 17%. It is suggested that the C 2-yield as a function of manganese or cobalt oxide content reflects the predominance of O 2− or O − on the catalyst. At low cobalt or manganese oxide concentrations C 2 selectivity is high and O − is expected to be dominant whereas, at higher concentrations, selectivities toward CO x are high and O 2− is expected to be dominant. The performance of the manganese—magnesium and cobalt—magnesium oxide measured over a range of temperatures (700–800°C), CH 4/O 2 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 CO x and ethane is produced via two parallel reactions and ethene is the dehydrogenation product of ethane.