The effect of chloride ions on a Li{sup +}-MgO catalyst for the oxidative dehydrogenation of ethane
- Texas A&M Univ., College Station, TX (United States)
The addition of chloride ions to a Li{sup +}-MgO catalyst at a ratio of Cl/Li {>=} 0.9 significantly improves the yields of ethylene that can be achieved during the oxidative dehydrogenation (OXD) of ethane. At 620{degrees}C, C{sub 2}H{sub 4} 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 C{sub 2}H{sub 4}/{sub 2}H{sub 6} ratios that are attained over these catalysts during the oxidative coupling of CH{sub 4}. The activity of the catalysts with Cl/Li {>=} 0.9 is partly a result of the fact CO{sub 2} 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, CO{sub 2} 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 Li{sub 2}O that partially covers the LiCl crystallites. This Li{sub 2}O is capable of activating {sub 2}H{sub 6}, 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 Li{sub 2}O form on the surface of LiCl and/or on the MgO. In the presence of CO{sub 2}, this Li{sub 2}O is extensively converted to Li{sub 2}CO{sub 3}, which is inactive for the OXD reaction. 20 refs., 9 figs., 6 tabs.
- OSTI ID:
- 75803
- Journal Information:
- Journal of Catalysis, Vol. 151, Issue 1; Other Information: PBD: Jan 1995
- Country of Publication:
- United States
- Language:
- English
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