Role of surface chlorine species in the oxidative coupling of methane in the presence of tetrachloromethane on magnesium phosphate and sulphate

The oxidative coupling of methane was carried out over Mg₃ (PO₄)₂ and MgSO₄ in the presence of tetrachloromethane (TCM) in order to compare the catalytic activities, the interaction between TCM and each catalyst and the effect of the nature of the anion. The conversion of methane on the sulphate increased with increase in the partial pressure of TCM and of reaction temperature, while with the phosphate the presence of TCM had little effect under the same conditions as those on the sulphate. With both catalysts the addition of TCM increased the selectivities to ethylene. Relatively large quantities of chlorinated species were detected on the catalyst surface of the used phosphate by XPS, although none were detected by XRD analyses. In contrast, only small quantities of chlorinated species were formed on the sulphate catalyst and magnesia was detected by XRD in the bulk phase of the used sulphate. The interactions between TCM and the catalyst and, in particular, the effect of the nature of the anion are discussed.