Rhenium promotion of Ag and Cu–Ag bimetallic catalysts for ethylene epoxidation

Rhenium remains the least understood promoter of silver catalysts for the epoxidation of ethylene. While a large number of patents can be found claiming the use of rhenium to yield ethylene oxide selectivities over 80%, very little is present in the open literature to explain these results. In this work we report initial studies of rhenium promoted Ag and Cu–Ag bimetallic catalysts under chlorine-free operating conditions. Rhenium addition improves the selectivity of both the monometallic and bimetallic catalysts, while decreasing activity in each case. Optimization of preparation procedures shows maximum ethylene oxide selectivities are obtained with rhenium loadings of 25 ppm for the monometallic catalysts and between 50 and 100 ppm for Cu–Ag bimetallic catalysts. The reaction kinetics for rhenium-promoted Ag and unpromoted Ag catalysts suggest that the catalysts containing rhenium present a more uniform distribution of sites for oxygen adsorption than do unpromoted catalysts, thus enhancing the ethylene oxide selectivity.