Complex metal halide oxides .1. Bismuth chloride oxides having various structures as catalysts for oxidative coupling of methane

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 X(2) type structure, was most active for the methane conversion but less selective for the formation of C-2 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 C-2 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 C-2 selectivity, whereas the Bi3-xLaxClO4 catalysts treated with NaCl brought about a maximum yield of C-2 hydrocarbons.