The partial oxidation of methane by chlorine promoted manganese oxides

Abstract

The partial oxidation of methane over metal oxides to form C2+ hydrocarbons is known to take place by two different mechanisms: (i) abstraction of a hydrogen at an alkali metal stabilised surface oxygen anion (O-), and; (ii) abstraction of a hydrogen at a reducible metal cation. Both of these mechanisms release methyl radicals (CH3•) into the gas phase, where they form ethane. Subsequent reactions produce ethene and low levels of propene. Introduction of chlorine containing species into the gas phase improves selectivity to ethene. Mn3O4, α-Mn2O3, γ-MnO2, β-MnO2, λ-MnO2, LiMn2O4, Li4Mn5O12, MnAsO4.H2O, LiMnAsO4.OH, MnPO4.H2O and LiMnPO4.OH have been investigated for their ability to convert methane to C2+ hydrocarbons when promoted with gas phase chlorine in the form of CHCl3. When chlorine promoted, all catalysts selectively form ethene at high temperatures (> 600°C). α-Mn2O3, λ-MnO2, β-MnO2, λ-MnO2, LiMn2O4, MnAsO4.H2O, LiMnAsO4.OH, MnPO4.H2O and LiMnPO4.OH are able to form propene yields of ca. 8% with high selectivity at low temperatures (300 → 600°C). There is no obvious correlation between manganese valence state, the structure of the metal oxide and propene formation. Propene formation occurs by a mechanism that is not understood. The mechanism is different to that proposed for other metal oxides by previous researchers and appears to involve catalyst surface adsorption of methane without release of methyl radicals.