PROMOTER EFFECTS OF DICHLOROMETHANE ON THE OXIDATIVE COUPLING OF METHANE OVER MNMGO CATALYSTS

Small amounts of manganese oxide incorporated on MgO enhanced C-2+ hyd rocarbon selectivity during the oxidative coupling of methane (OCM). B oth C-2+ selectivity and CH4 conversion were also improved by adding a lkaline metal halides. Dosing CH2Cl2 pulses into the CH4/O-2 feed stre am over alkali promoted Mn-MgO catalysts led to an improvement in the yield to C-2+ hydrocarbons. The extent of this effect and the stabilit y of the alkali phases were found to depend on the type of promoter. P hotoelectron spectroscopy (XPS) revealed that all the used catalysts r etained important amounts of chlorine in the form of Cl- ions, and sim ultaneously manganese was oxidized to Mn4+. In the light of quantitati ve XPS data, a redox mechanism consisting of the decomposition of halo compound at catalyst surface through formation of an oxyhalide of the type MnO2-x/2Clx has been proposed. The different patterns for CH4 con version and C-2+ selectivity indicate a key role of the alkali promote r and its distribution on the surface, on the formation of this Cl-con taining species.