Oxidative coupling of methane over chloride-modified magnesium oxide catalysts

The oxidative coupling of methane, to produce ethane and ethylene, is catalyzed by a number of metal oxide systems. Of particular interest is lithium-doped magnesium oxide, Li{dollar}sp+{dollar}-MgO, which has been extensively investigated as an oxidative coupling catalyst. The incorporation of chloride ions into Li{dollar}sp+{dollar}-MgO results in a more active catalyst that gives ethylene-to-ethane product ratios that are significantly higher than for Li{dollar}sp+{dollar}-MgO. High ethylene yields are desirable due to the value of ethylene as a feedstock for the chemical industry.; Exceptionally large C{dollar}sb2{dollar}H{dollar}sb4{dollar}/C{dollar}sb2{dollar}H{dollar}sb6{dollar} ratios are obtained at Cl/Li ratios of near unity. These high C{dollar}sb2{dollar}H{dollar}sb4{dollar}/C{dollar}sb2{dollar}H{dollar}sb6{dollar} ratios are a result of the enhanced activity for the dehydrogenation of ethane over these Li{dollar}sp+{dollar}-MgO-Cl{dollar}sp-{dollar} catalysts. The enhancement by chloride is dependent on the Cl/Li ratio; not the absolute amount of chloride present. At Cl/Li ratios below unity, the promotional effect of chloride rapidly declines and the C{dollar}sb2{dollar}H{dollar}sb4{dollar}/C{dollar}sb2{dollar}H{dollar}sb6{dollar} product ratio drops to the level of the unpromoted Li{dollar}sp+{dollar}-MgO catalyst. Chloride modifies the catalyst, reducing the basicity, so that CO{dollar}sb2{dollar} does not poison Li{dollar}sp+{dollar}-MgO-Cl{dollar}sp-{dollar} as it does Li{dollar}sp+{dollar}-MgO. This poisoning by CO{dollar}sb2{dollar} has a major effect on the reaction kinetics, significantly reducing the intrinsic activity of Li{dollar}sp+{dollar}-MgO. No such poisoning takes place over Li{dollar}sp+{dollar}-MgO-Cl{dollar}sp-{dollar}. Although the Cl/Li ratio is of primary importance for enhanced C{dollar}sb2{dollar}H{dollar}sb4{dollar} selectivity, long term catalyst stability depends on an excess of Li and Cl to provide a reservoir. Under typical reaction conditions at 650{dollar}spcirc{dollar}C, both are slowly lost from the catalyst surface. It is possible to restore the activity and selectivity to a partially deactivated Li{dollar}sp+{dollar}-MgO-Cl{dollar}sp-{dollar} catalyst by addition of HCl to the feed stream.