Partial oxidation of methane and ethane over metal oxide catalysts

Li{dollar}sp+{dollar}/MgO and supported sodium-manganese oxide are two of the most promising catalysts for the conversion of methane via the oxidative coupling reaction. The nature of the active species was explored by a combination of kinetic measurements and characterization of the physical properties of the catalysts.; The addition of Cl{dollar}sp-{dollar} ions to a Li{dollar}sp+{dollar}/MgO catalyst at a ratio of Cl/Li {dollar}ge{dollar} 0.9 significantly improves the yields of ethylene that can be achieved during the oxidative dehydrogenation (OXD) of ethane. This high activity for the OXD of ethane is responsible for the large C{dollar}sb2{dollar}H{dollar}sb4{dollar}/C{dollar}sb2{dollar}H{dollar}sb6{dollar} ratio achieved in the coupling reaction over the same catalyst. The enhanced activity and selectivity of the catalyst mainly results from the fact that Cl{dollar}sp-{dollar} removes most of the strongly basic sites and promotes formation of weakly basic sites. The formation of multilayers of Li{dollar}sb2{dollar}CO{dollar}sb3{dollar} is, therefore, inhibited. The sintering of the catalyst and the CO{dollar}sb2{dollar} poisoning effect is also minimized. The active centers for ethane OXD are believed to be associated with a thin layer of Li{dollar}sb2{dollar}O that partially covers the LiCl crystallites. This Li{dollar}sb2{dollar}O is a weakly basic site but is capable of activating ethane. In most respects, the effects of Cl{dollar}sp-{dollar} ions on the Li{dollar}sp+{dollar}/ZnO catalyst parallel those that were observed for the Li{dollar}sp+{dollar}/MgO catalyst.; Carbon dioxide, either added to the reagents or produced during the reaction, significantly improves the hydrocarbon selectivity in the oxidative coupling of methane (OCM) and the OXD of ethane. Carbon dioxide inhibits secondary reactions of CH{dollar}sb3cdot{dollar}radicals and C{dollar}sb2{dollar}H{dollar}sb4{dollar} with the surface, probably at the strongly basic sites, of the Li{dollar}sp+{dollar}/MgO catalyst. Regarding this positive effect on the selectivity, chloride ions play a similar role to that of carbon dioxide.; A Mn/Na{dollar}sb2{dollar}WO{dollar}sb4{dollar}/MgO catalyst was developed. At 800{dollar}spcirc{dollar}C, 1 atm, and using a CH{dollar}sb4{dollar}/O{dollar}sb2{dollar} ratio of ca. 8/1, a methane conversion of 20% was achieved at a C{dollar}sb{lcub}2+{rcub}{dollar} selectivity of {dollar}ge{dollar}80%, with no diluent in the reagents, which is among the best catalytic results yet achieved for the OCM reaction. Similar catalytic results were observed over the corresponding SiO{dollar}sb2{dollar}-supported catalysts, demonstrating that a common active site is involved. Sodium ions concentrate on the surface of the catalyst and are required for the high selectivity. Manganese ions, which are highly dispersed in a Na{dollar}sb2{dollar}O/Na{dollar}sb2{dollar}O{dollar}sb2{dollar}/Na{dollar}sb2{dollar}CO{dollar}sb3{dollar} surface phase, are believed to activate a surface form of oxygen. The bulk lattice oxygen is not involved in the catalytic reaction.