Influence of alkali and chloride promoters on silver catalysts in ethylene epoxidation

Commercial ethylene epoxidation catalysts consist of Ag particles supported with α-alumina and contain a mixture of promoters. High activity catalysts typically contain Cs in combination with other alkali promoters and a gaseous organochloride. We studied a series of Ag catalysts that were individually promoted with different alkali species (Li, Na, K, Rb and Cs) at equimolar loadings of 2.5 mol% (mol_alkali/(mol_alkali + mol_Ag)). O₂-TPD experiments revealed that the absorption strength of oxygen onto the silver surface was influenced by the presence of the alkali species. Ethylene epoxidation experiments were performed both in the absence and presence of the industrially vital ethyl chloride promoter. Without the chloride, the alkali-promoted catalysts were less active than the silver reference catalyst and showed a decreased primary ethylene oxide (EO) selectivity. Introducing ethyl chloride to the feed increased the primary EO selectivity of all catalysts to ca. 80% without sacrificing the activity for the alkali-promoted catalysts. Systematic EO isomerization experiments revealed that the secondary reactions of EO are inhibited by alkali promoters on the silver surface rather than on the support. Larger/less electronegative alkali species such as Rb and Cs inhibited EO degradation to a greater extent than Li and Na. However, when ethyl chloride was introduced to the feed the EO conversion of all catalysts was blocked. This study hence demonstrates how the combination of chloride and alkali promoters affects the Ag sites, specifically suppressing both undesired combustion and promoting desired ethylene epoxidation when increasing the conversion.