THE EFFECT OF CHLORIDE IONS ON A LI-MGO CATALYST FOR THE OXIDATIVE DEHYDROGENATION OF ETHANE()

The addition of chloride ions to a Li+ -MgO catalyst at a ratio of Cl/ Li greater than or equal to 0.9 significantly improves the yields of e thylene that can be achieved during the oxidative dehydrogenation (OXD ) of ethane. At 620 degrees C, C2H4 yields of 58% (75% conversion, 77% selectivity) have been maintained for up to 50 h on stream. These eth ylene yields are consistent with the large C2H4/C2H6 ratios that are a ttained over these catalysts during the oxidative coupling of CH4. The activity of the catalysts with Cl/Li greater than or equal to 0.9 is partly a result of the fact that CO2 formed during the reaction does n ot poison the catalyst. In addition, the surface areas of the chloride d catalysts are greater than those which contain a comparable amount o f Li, but no chloride ions. Based upon the activity results, CO2 tempe rature-programmed desorption data, and X-ray photoelectron spectra, a model has been proposed in which lithium is mainly present as LiCl on the MgO support, provided a nearly stoichiometric amount of chloride i s available. The active centers are believed to be associated with a t hin (atomic) layer of Li2O that partially covers the LiCl crystallites . This Li2O is capable of activating C2H6, but its basic strength has been modified so that it does not form carbonate ions at 620 degrees C . When the amount of chloride is limited, or is not present at all, mu ltilayers of more strongly basic Li2O form on the surface of LiCl and/ or on the MgO. In the presence of CO(2)z, this Li2O is extensively con verted to Li2CO3, which is inactive for the OXD reaction. (C) 1995 Aca demic Press, Inc.