Silver catalysts for ethylene epoxidation: The role of support and promoters

In this project, α-alumina-supported silver catalysts were investigated for the ethylene epoxidation reaction. Ethylene oxide, the desired end product, is an important feedstock for products such as antifreeze and plastics. In this reaction, CO₂ is the main byproduct, resulting from further combustion reactions of ethylene oxide. Therefore, it is crucial to understand how CO₂ formation can be suppressed by studying the catalyst. We first prepared α-alumina support material using carbohydrate-based templates. Both templates resulted in support materials with a much higher surface area than commercially available materials, which is essential for making the catalyst more efficient. The purity of the α-alumina depended on the template size and calcination time. Pure α-alumina is critical for overall selectivity toward the desired ethylene oxide. Silver particles were then added to these supports to study how particle size and preparation methods affect catalyst performance. The main conclusion: not only is the size of the silver particles important, but also the ratio between the active silver surface and the surface area of the support. This influences how much of the desired product is formed and how much ethylene oxide further reacts to CO₂. In addition, we studied how the addition of rhenium- or alkali-based (such as lithium, sodium, or cesium) promoters changes the catalyst’s performance. Rhenium oxide helped increase selectivity by suppressing overall activity. However, rhenium oxide in combination with the industrially important gaseous ethyl chloride promoter led to undesirable side reactions. On the other hand, alkali promoters helped suppress these side reactions, even at higher conversions, especially larger ones like cesium. Experiments with these “solid” promoters were carried out with and without the gaseous ethyl chloride. This research highlights how different promoters interact with the active sites and emphasizes the importance of carefully combining and studying promoters.