The active site of promoted ethylene-epoxidation catalysts

The cooperative effect of chlorine moderation and alkali promotion on the initial selectivity of silver-catalysed epoxidation of ethylene have been investigated. To this end a study was made of the conversion ethylene catalysed by silver doped with alkali and non-doped in the presence and absence of chlorine. The silver powders were characterized by temperature programmed reduction as well as by oxygen adsorption studies. Also, the exchange reaction of C2H40 and C2D4 was studied. The data were interpreted with the epoxidation model according to which the elementary step of the selective reaction is electrophylic attack of an adsorbed oxygen atom to the pi-bond of ethylene and the non-selective reaction occurs by electro-positive attack of a different atomic oxygen species to the CH bond of ethylene. The role of alkali appears to be stabilization of that silver oxychloride phase that contains predominantly atomic electrophilic oxygen.