Kinetics of Ethylene Epoxidation on a Promoted Ag/α-Al2 O3 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-Al2 O3 catalyst. An increase in O2 order (≈0.7 to 1) and a decrease in C2 H4 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.