Pretreatment effect in the oxidative dehydrogenation of ethane on magnesia in the presence and absence of tetrachloromethane

The effects of a small addition of tetrachloromethane (TCM) into the feedstream for the oxidative dehydrogenation of ethane have been investigated over magnesia at reaction temperatures between 723 and 1098 K. Both the conversion of ethane and the selectivity to ethylene at reaction temperatures of 723-823 K were markedly increased on addition of TCM, while at temperatures between 998 and 1098 g little or no changes in these quantities were observed. Pretreatment experiments in which MgO was exposed to TCM + O-2 or TCM + O-2 + H2O prior to introduction of the reactant stream showed that the chlorided surface which formed during the pretreatment contributed to the aforementioned improvement. The concentration of surface chloride, in whatever form, produced during the pretreatment was influenced by the introduction of water but showed no dependence on the temperature, the partial pressure of TCM in the pretreatment gas or the duration of exposure. Excess chlorination of the surface tended to reduce the beneficial effect especially in the presence of TCM in the feedstream.