Influence of Ag particle size and Ag : Al₂O₃ surface ratio in catalysts for the chloride-promoted ethylene epoxidation

Ethylene epoxidation is catalyzed by alpha-alumina supported silver catalysts. The influence of silver particle size has been a topic of debate, and was typically investigated without the industrially essential chloride promoter. We studied the catalyst behavior in the presence of chloride. Transient behavior was observed in the first tens of hours on stream, not as a result of particle growth, but due to the gradual change in the nature of the active silver site in the presence of chloride. Different strategies were used to tune the particle size: either varying the silver loading or varying the decomposition atmosphere. Increasing the particle size from 13 to 50 nm by changing the Ag loading from 2 to 15 wt% increased the selectivity from 35 to 80%. However, increasing the 15 wt% Ag particle size from 48 to 184 nm by varying the heat treatment led to a decrease in selectivity from 80 to 50%. Changing the Ag particle size with both strategies also changes the Ag : Al2O3 surface ratio. The ethylene oxide selectivity is actually correlated to the Ag : Al2O3 surface ratio, rather than to the particle size in this size range. This can be explained by its influence on the probability of a formed ethylene oxide molecule to subsequently further react over support surface groups.