Oxidative coupling of methane over alkali-promoted manganese molybdate catalysts

MnMoO{dollar}sb4{dollar} and alkali-promoted (Li, Na, or K) MnMoO{dollar}sb4{dollar} have been examined for the methane oxidative coupling reaction. These catalysts have been studied through BET surface area measurements, X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, temperature programmed reduction using hydrogen, temperature programmed desorption of methane, oxygen, carbon dioxide, and carbon monoxide, and isotopic oxygen exchange studies. Steady-state experiments were also used to compare the catalysts both at equal conversion levels and using equal gas feed flow rates, as well as through the addition of excess carbon dioxide to the feed gas. Isotopic labelling studies have also been performed under steady-state reaction conditions using {dollar}sp{lcub}18{rcub}{dollar}O{dollar}sb2{dollar} or {dollar}sp{lcub}13{rcub}{dollar}CH{dollar}sb4{dollar} to examine the roles of gas-phase oxygen and lattice oxygen and determine the surface residence times of the methane reactant and carbon containing products.; An increase in both the catalytic activity and the selectivity to C{dollar}sb2{dollar} hydrocarbons was observed for the alkali-promoted catalysts as compared to the unpromoted MnMoO{dollar}sb4{dollar}, with a significantly higher selectivity to C{dollar}sb2{dollar} hydrocarbons found for the K-promoted MnMoO{dollar}sb4{dollar}. The highest activity for methane conversion was found over the Na-promoted MnMoO{dollar}sb4{dollar}, followed by the Li-promoted MnMoO{dollar}sb4{dollar}. Information gained through the characterization and isotopic labeling studies has been used to understand the different function of the promoter ions for the promoted catalysts. The higher activity observed for the Li- and Na-promoted catalysts was attributed to incorporation of the promoter ions into the lattice structure, increasing the mobility of bulk oxygen for site regeneration. However, these catalysts were also found to be more active for oxygen insertion, leading to higher yields of HCHO and CO{dollar}sb{lcub}rm x{rcub}{dollar}. The results from the isotopic oxygen exchange experiments and the temperature programmed desorption experiments showed that different oxygen species can form on the surface of the K-promoted catalyst as compared to the other catalysts studied. The surface oxygen on the K-promoted catalyst may be less active toward the adsorbed methane, allowing the desorption and subsequent coupling of the methyl radicals.