Complex Metal Halide Oxides (1): Bismuth Chloride Oxides Having Various Structures as Catalysts for Oxidative Coupling of Methane

Abstract Bismuth chloride oxides having four distinct structural types and lanthanum-substituted bismuth chloride oxides were tested as catalysts for the oxidative coupling reaction of methane. Their catalytic properties were compared in relation with their structure. BiClO, which has a layer structure with a double chlorine sheet, so-called X2 type structure, was most active for the methane conversion but less selective for the formation of C2 hydrocarbons. The catalyst structure was quite unstable, so that the activity decreased markedly until reaching a constant level, while the selectivity increased. During these changes of the catalytic performance BiClO changed by liberating chlorine to Bi24Cl10O31, the so-called Arppe’s phase, and then finally to a highly stable and selective Bi3ClO4 phase which was constructed with a double metal oxygen sheet and a single chlorine sheet. When more chlorine was removed from Bi3ClO4, Bi12Cl2O17, which has quintuple oxide sheets and a single chlorine sheet, was formed as an intermediate phase to be less selective for C2 hydrocarbons and ultimately Bi2O3 was formed with no chlorine, to be nonselective. The substitution of Bi in Bi3ClO4 for La caused a decrease of the C2 selectivity, whereas the Bi3−xLaxClO4 catalysts treated with NaCl brought about a maximum yield of C2 hydrocarbons.