Synthesis of ethylene via oxidative coupling of methane I. Determination of active catalysts

Ethylene and ethane (C2's) have been synthesized by catalytic oxidative coupling of methane at atmospheric pressure and temperatures of 500–1000°C. A great number of metal oxides, supported on an α-alumina support, have been screened for activity and selectivity in the normal, concurrent feeding mode of reactants, as well as, in the sequential or feed programming mode. The concurrent feeding experiments gave low selectivities of 0–20%, while feed programming experiments gave selectivities of ~50%. In the latter case, however, a stainless-steel reactor became catalytically active for burning of methane to carbon oxides. Although a stainless-steel reactor was used in most of the work reported here, a quartz reactor was found to be inert. The most active catalysts for C2 formation were the oxides of Sn, Pb, Sb, Bi, Tl, Cd and Mn, while Li, Mg, Zn, Ti, Zr, Mo, Fe, Cr, W, Cu, Ag, Pt, Ce, V, B and Al showed little or no activity. The low C2-forming activity of Pt and Ce, however, may be due to the secondary burning of C2's on the stainless-steel reactor walls. This also makes it difficult to quantify the C2-forming selectivity of the active metal oxides. The active metals seem to exhibit a common characteristic: they can cycle between at least two oxidation states. Although there are differences in selectivities in C2 formation and carbon oxides formation, no correlation seems to exist with the free-energy changes in the oxidation states. A possible mechanism for C2 formation from methane is proposed.