The catalytic conversion of methane to higher hydrocarbons

Considerable progress has been made in the development of catalysts for the partial oxidation of methane to ethane and ethylene. Several of the more active and selective catalysts are composed of metal oxides (e.g. MgO, CaO, ZnO, Mg6MnO8 and Sm2O3) which are modified with either lithium or sodium ions. While operating in a conventional fixed-bed flow reactor, at atmospheric pressure and at temperatures in excess of 650°C, combined ethane and ethylene (C2) yields of 20% have been achieved. Typically, at a conversion level of 40% one may expect a C2 selectivity of 50%. The Li/MgO and Na/CaO catalysts have been studied extensively with respect to the reaction mechanism and the nature of the site which is responsible for the activation of methane. Methyl radicals are formed on the surface, but they subsequently enter the gas phase where they may either couple to form ethane or react with oxygen to initiate homogeneous reactions. The extent to which these homogeneous chain-branching reactions participate in the selective and non-selective oxidation of CH4 remains to be determined. Centers of the type [Li+O−] or [Na+O−] are responsible for the formation of CH3 at T<720°C, but at higher temperatures alkali metal oxides or peroxides may also be involved in the activation of CH4. The early members of the lanthanide oxide series also are active for the conversion of methane, but the selectivities vary greatly across the series. Secondary reactions of CH3 radicals with CeO2, Pr6O11 and Tb4O7 result in complete oxidation, but these secondary reactions can be minimized by modifying the surface with Na2CO3. Thus, it appears that alkali metal ions may play several roles in forming an effective partial oxidation catalyst.