Kinetic Modeling of Ethylene Epoxidation Kinetics as a Function of Chlorine Coverage over a Highly Promoted Silver Catalyst

Abstract

Kinetic modeling of previously reported data demonstrate that chlorine exhibits an electrostatic influence, rather than solely ensemble site blocking, to improve the selectivity of ethylene epoxidation over promoted silver catalysts. Kinetic models that neglect the impact of surface chlorine content on either oxygen dissociation or ethylene adsorption enthalpies are unable to describe observed changes in the apparent reaction orders of ethylene consumption. Models that employ solely ensemble effects to adjust product selectivity with chlorine content falsely predict a simultaneous maximum in ethylene oxide (EO) and carbon dioxide synthesis rates with respect to chlorine coverage at 10–15% of a monolayer of chlorine. Incorporating electronic effects of chlorine on the transition states of the selectivity-determining steps into the model, however, properly describes the measured rate trends with surface chlorine content. Specifically, the model reflects the maxima observed in ethylene oxide synthesis rate with the concomitant monotonic decrease in combustion rates with increasing chloride content. Slight improvements to model predictions by combining ensemble and electronic effects on EO selectivity with increasing Cl coverage suggest that both effects may be relevant in combination but that ensemble effects alone cannot describe the observed behavior.