Supported Silver Catalysts for Ethylene Epoxidation

Abstract

This thesis is focused on the preparation and catalytic performance of supported silver catalysts. The catalytic performance of supported silver catalysts decreases with ‘time-on-stream’. This decrease is in performance is attributed to the increase of the average silver particle size. Optimizing the performance of the catalyst by designing the support material for high selectivity, activity and stability proves an interesting approach. When the particle size of silver particles on different types of support can be controlled, the effect of particle size can be separated from the effect the support. We discuss the preparation silver catalysts supported on industrially relevant α-alumina. By varying the gas atmosphere during the decomposition of the silver precursor by heat treatment, the average silver particle size can be tuned to a desired particle size. Using this method, the particle size can be tailored independently of the support specific surface area or the weight loading of the active metal. We showed that the selectivity towards ethylene oxide was independent of the particle size. The synthesis of an ordered macroporous or 3DOM α-alumina support is discussed. These foam-like structures with pore sizes of circa 200 nm are ideal for facilitating the large particles required for active ethylene epoxidation catalysts. These materials were obtained by a hard-templating method. By increasing the amount of aluminium hydroxide in the pores of the template, a higher yield is obtained and the pore structure of the resulting macroporous material is better maintained. By applying a two-step heat treatment, one in inert atmosphere and one in oxidizing atmosphere, the ordered morphology is even better maintained. The resulting 3DOM α-alumina has maintained the structure imposed by the hard template better than 3DOM α-alumina produced via a single heat treatment step. The thermal stability of silver supported on ordered macroporous α-alumina was investigated. The average silver particle size of α-alumina supported silver increases relatively easily in oxidizing atmosphere, while the particles are quite stable in reducing atmosphere during thermal stability tests. The rate of sintering is less on higher surface area supports, due to a more effective separation of the silver particles. On the high surface area, ordered macroporous support, the silver particles are even less prone to sintering, when the silver particles are larger than the pore windows of the foam-like morphology of the support. The catalytic stability of silver particles during ethylene epoxidation is increased when the particles are supported on the high surface area material, compared to silver particles supported on low surface area α-alumina. While high support specific surface areas are beneficial for the stability of support particles, the specific surface area of the support material in industrial ethylene epoxidation catalysts is typically very low (<5 m2/g) to prevent side reactions which are co-catalysed by the support material. This is believed to be due to the isomerization of the desired product, ethylene oxide, to acetaldehyde. We show the detrimental role of the support material on the selectivity towards ethylene oxide was shown.