Kinetics of Ethylene Epoxidation on a Promoted Ag/α-Al₂O₃ Catalyst—The Effects of Product and Chloride Co-Feeds on Rates and Selectivity

The overall chloriding effectiveness factor (Z*), defined as the ratio of ethyl chloride concentration in parts per million to the sum of ethylene and ethane concentration in mole percent multiplied by a weighting factor to account for their efficacy in removing chlorine-adatoms from the surface, was used as a parameter to account for the effects of chlorine on the kinetics of ethylene epoxidation on a highly promoted 35 wt % Ag/α-Al₂O₃ catalyst. An increase in O₂ order (≈0.7 to 1) and a decrease in C₂H₄ order (≈0.5 to <0) with increasing Z* (Z*=2.1, 3.4, 5.2, and 8.9) was observed implicating kinetic relevance of O₂ activation on chloride-promoted silver catalysts. Carbon dioxide co-feed (1–5 mol %) was found to promote ethylene oxide selectivity as CO₂ co-feed reversibly inhibits CO₂ synthesis rates (−0.6 order) more than ethylene oxide synthesis rates (−0.49 order) at all Z* values. Ethylene oxide and CO₂ rates were found to be invariant with ethylene oxide (0–0.5 mol %) and acetaldehyde (0–1.7 ppm) co-feeds, suggesting that there is minimal product inhibition under reaction conditions. A model involving a common reaction intermediate for ethylene oxide and carbon dioxide synthesis and two types of atomically adsorbed oxygen species—nucleophilic and electrophilic oxygen—is proposed to plausibly describe the observed reaction rate dependencies and selectivity trends as a function of the chloriding effectiveness.