(574b) Kinetics of Ethylene Epoxidation and Chlorine Moderation over Promoted silver catalysts

High ethylene oxide (EO) selectivity (~90%) in ethylene epoxidation over promoted Ag/α-Al₂O₃ catalysts transpires with the concurrent propagation of two distinct catalytic cycles – one involving oxidation of ethylene to EO and CO₂ and another involving chlorine deposition and removal – the chlorine promoter catalytic cycle being facilitated by the continuous addition of a gas phase alkyl chloride promoter in ppm levels and an alkane moderator. Here, we examine the identity and rates of molecular events occurrent in each catalytic cycle to elucidate the interdependencies among them. The kinetics of chlorine deposition and removal probed through steady state measurements of propyl chloride formation rates by co-feeding propane and ethyl chloride in the epoxidation feedstream are congruent with a mechanistic description involving two rate determining steps that require the presence of surface-bound oxygen adatoms: deposition of chlorine from an alkyl chloride and removal of chlorine with an alkane. Conversely, a kinetic description of ethylene oxidation necessitates assessment of surface chlorine coverages because changes in oxygen reaction order (1 to 0.4) and ethylene reaction order (1 to 0.1) for EO formation with increasing chlorine coverage, evaluated through steady state measurements of EO formation rates as a function of chlorine coverage at different reactant pressures, reflect the role of chlorine in altering ethylene oxidation kinetics in addition to improving EO selectivity. We combine this mechanistic and kinetic description of ethylene oxidation and chlorine moderation catalytic sequences to formulate a packed bed reactor model that enables accurate predictions of EO rate and selectivity and allows for tuning the reaction conditions to achieve high EO selectivity.