The Complex Chlorination Effects on High Selectivity Industrial EO Catalysts: Dynamic Interplay between Catalyst Composition and Process Conditions

Under industrially relevant conditions, commercial ethylene oxide catalysts exhibit a complex response with respect to chlorination: Overall rates increase with gas phase chlorine potential while selectivity goes through a maximum. Commercial plants typically track the gas phase chlorine potential using relatively simple functions based solely on the concentration of chlorinating (alkyl chloride) and dechlorinating agents (alkenes and alkanes). These functions are used to fine-tune the chlorine promotion of the catalyst bed during plant operation to achieve maximum selectivity. This approach involves trial and error for each plant, since chlorination is a complex function of process conditions, catalyst composition, and catalyst age. Thus, the chlorination metrics found in patent and scientific literature are incomplete descriptors of chlorine effects. This perspective aims to provide a mechanistic view of the chlorination process toward creating a quantitative description of how the chlorine chemistry impacts the rates of ethylene oxide (EO) and CO2 formation on high selectivity industrial EO catalysts. Additionally, the dynamic interplay between process conditions and the optimal chlorine level is emphasized. Apart from the concentrations of chlorinating and dechlorinating species, temperature, and partial pressures of oxygen, water and carbon dioxide are identified to be the relevant process variables affecting the catalyst response to chlorine. This framework can help to quantify the relative importance of reactor concentration and temperature gradients on the chlorination state of the catalyst. Understanding chlorine dynamics on EO catalysts and incorporating this knowledge into kinetics and reactor models can have widespread utility for EO manufacturers─from designing more robust plants and operating conditions for current EO catalyst formulations to guiding the research and development efforts aimed at more efficient ethylene epoxidation.