The effect of alkali metal salts on ZnOα-Al2O3 and MnO2 catalysts for the oxidative coupling of methane Original Research Article

A series of alkali metal halides/ZnO(60 wt.-%)/α-Al2O3 and NaClMnO2 were tested for the oxidative coupling of methane to ethylene and ethane. Sodium halides such as NaF, NaCl, NaBr and NaI were used as alkali metal halides. Recorded activities were associated with the apparent molal enthalpy of formation of sodium halides. The activity order was NaBr > NaCl > NaI > NaF. NaCl and NaBr played a role as promoters. NaF and NaI played a role as inhibitors. A kinetic study showed the reaction over NaCl(30 wt.-%)/ZnO(60 wt.-%)/α-Al2O3 was of the dual site Langmuir-Hinshelwood type. The obtained activation energy for the methyl radical formation was ca. 39 kcal/mole. The active oxygen species responsible for methyl radical formation seems to be a diatomic oxygen such as O22− or O2− adsorbed on the surface of the catalyst.