Lithium-promoted magnesium oxide was used as a catalyst for the partial oxidation of methane by molecular oxygen in a conventional flow reactor. Ethane and ethylene (C2 compounds) were obtained with 50% selectivity at a 38% conversion of CH4 over 4 g of 7 wt % Li-promoted MgO at 720 °C. The initial reactant mixture had a CH4 to 02 ratio of ca. two. The other products were CO and CO2. In this reaction system both the C2 selectivity and the CH4 conversion increased with an increase in reaction temperature up to ca. 700 °C. All catalysts doped with 1 to 26 wt % Li were effective in the C2 production, while undoped and 0.2 wt % Li-promoted MgO caused nonselective oxidation. EPR spectroscopy revealed that [Li+O-] centers probably were the active sites responsible for initiating the reaction by abstracting a hydrogen atom from CH4. The [Li+O-] centers exist in an equilibrium state only at high temperatures in the presence of oxygen. Based on kinetic analyses a mechanism has been proposed in which most of the products, including CO and CO2, are produced via CH3.; the coupling of two CH3. leads to C2H6, which is partly oxidized to C2H4. Below 700 °C CO and CO2 are produced by reactions of CH3. with either surface O2- ions or gaseous O2. The latter reaction leads to chain reactions in which additional methyl radicals are generated. The Li-promoted MgO contains no transition metals; thus it is a new type of oxidation catalyst on which thermally generated O~ ions activate methane.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry