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

摘要

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 ethylene 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 ethylene yields are consistent with the large C2H4/C2H6 ratios that are attained 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 not poison the catalyst. In addition, the surface areas of the chlorided catalysts are greater than those which contain a comparable amount of Li, but no chloride ions. Based upon the activity results, CO2 temperature-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 is available. The active centers are believed to be associated with a thin (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, multilayers 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 converted to Li2CO3, which is inactive for the OXD reaction. (C) 1995 Academic Press, Inc. [References: 26]