The oxidative dehydrogenation of ethane over chlorine-promoted lithium-magnesium oxide catalysts

Abstract

The addition of chlorine to a Li +MgO catalyst improves considerably the ethylene yield that may be attained during the oxidative dehydrogenation (OXD) of ethane. While operating at 650°C, Li +MgO catalysts, produced via a sol-gel method or by adding HCl to an unsintered Li +MgO catalyst, promoted a C2H6 conversion of 75–79% at a C2H4 selectivity of 70% after 50 hr on stream. Even after 250 hr the ethylene yield was 45%. Chlorine was slowly lost from the catalysts as a result of reaction with H 2O, but the evidence indicates that the improved activity and selectivity is largely a result of heterogeneous rather than homogeneous chlorine-promoted reactions. Over the modified catalysts the rate of C 2H 6 conversion was increased relative to the rate of C 2H 4 conversion. The presence of chlorine in the solid significantly decreased the amount of C0 2, a poison for the OXD reaction, that was taken up by the catalyst. Thus, C0 2 formed during the OXD of C2H6 may be less effective in poisoning the active centers on a chlorine-modified catalyst. The presence of chlorine also may alter the reactive forms of oxygen on the surface (e.g., O − ions) so that they are capable of activating the weaker CH bond in ethane, but are less effective in activating the stronger CH bond in ethylene. With respect to the oxidative coupling of CH 4, these results demonstrate that at 650°C the primary step, which includes the activation of CH 4 and reactions involving CH 3 · radicals, is responsible mainly for the CO x products, not the subsequent oxidation of CH 4 and C 2H 6. This would be the case even at relatively high partial pressures of C 2H 4 and C 2H 6.