Oxidative dimerization of methane over well defined lithium-promoted magnesium oxide catalysts

Abstract

Pure MgO samples having greatly different morphologies and lithium-promoted catalysts derived from these materials have been examined by electron microscopy. In addition, their ability to generate and react with gas-phase methyl radicals has been determined using e.s.r. in combination with a matrix-isolation technique. Lithium caused the MgO to sinter severely at 700 °C, but the resulting material showed improved activity for the generation of methyl radicals. These factors gave rise to catalysts which were moderately active and selective for the oxidative dimerization of CH₄ at 700 °C (up to 20% conversion, 66% C₂+ selectivity). The results are particularly significant in that catalysts which had been prepared such that the lithium was initially present as Li₂O on the surface exhibited almost constant C₂+ production as the Li₂O was converted to Li₂CO₃. Thus, neither Li₂O nor Li₂CO₃ is the active phase for the selective conversion of CH₄. A catalyst prepared from Li₂CO₃ and MgO sintered much less extensively, produced more CH₃˙ radicals and gave higher yields of C₂+ hydrocarbons. A comparison of these catalytic properties suggests that surface area and related morphological factors do not have an adverse effect on the oxidative dimerization reaction, provided the surface can be rendered inert to secondary reactions with methyl radicals.