Mechanistic relationships in the activation of methane and the conversion of methanol on heteropoly oxometallates

Abstract

Heteropoly oxometallates have been employed in the heterogeneous catalysis of the conversion of methanol to hydrocarbons and the partial oxidation of methane. In the former process methanol is first protonated at Brønsted-acid sites, but at higher temperatures the heteropoly anions are partially methylated subsequent to scission of the C—O bonds in protonated methanol. With silica-supported 12-molybdophosphoric acid the exchange of the protons by cations effectively poisons the catalyst for the conversion of methane. Introduction of a chloro-additive to the feedstream in the latter process produces changes in the conversion and selectivity which are markedly different for the molybdenum- and tungsten-containing catalysts. With the former the conversion of methane is increased while the selectivity to partial oxidation products is decreased. With the latter the former comments are also applicable but selectivities to methyl chloride reach as high as 90 mol %. The results from cation exchange and the addition of a chloro-additive are interpreted and a mechanism with common features is proposed for the methanol and methane conversion processes.