High ethylene oxide (EO) selectivity (~90%) over promoted Ag/α-Al2O3 catalysts transpires with concurrent propagation of two 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 (~0.5 mol%). 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 chlorine, in addition to improving EO selectivity, also influences reaction orders of oxygen and ethylene for EO formation. We combine this mechanistic and kinetic description of ethylene epoxidation and chlorine moderation catalytic sequences to formulate a packed bed reactor model to describe the changing surface coverage of chlorine along the length of the bed which in turn alters the EO rate and selectivity.

Kinetics of ethylene oxidation and chlorine moderation over promoted Ag/α-Al2O3 catalysts

ACS Spring Meeting 2021

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