Strategies To Study Reaction Kinetics, Particle Size Effects, And Site Requirements Over Silver-Catalyzed Ethylene Epoxidation

Abstract

Ethylene oxide (EO), an important chemical intermediate, is manufactured using ethylene epoxidation over a Ag/α-Al2O3 catalyst, consisting of solid promoters such as Cs, Re, S, etc. and gas phase chlorine promoters, to achieve an EO selectivity of ~90%. The ethylene epoxidation reaction involves the simultaneous propagation of multiple catalytic cycles - ethylene oxidation, chlorine moderation, and EO degradation that interact with each other as they share common reactant and product species. This work examines the kinetics of each catalytic cycle independently to reveal that the promoter chlorine significantly changes ethylene and oxygen reaction orders for EO formation, whereas it has negligible impact on the kinetics of EO degradation. The formulation of a packed-bed reactor model that integrates these kinetic nuances enables accurate predictions of EO rate and selectivity thereby presenting an avenue to improve catalyst performance by tailoring the process conditions. EO rates and selectivity also depend on the material properties of the Ag/α-Al2O3 catalyst, specifically the Ag particle size. In this study, a structure-function relationship is developed between the observables, i.e., Ag particle size distribution and the measured EO rates and chlorine coverages. This description reveals that small particles are covered by multiple monolayers of chlorine which partly explain the low EO rates measured over them and that certain large particle sizes (100-150 nm) maximize EO rates and selectivity likely because they exhibit sub-monolayer Cl coverages. In summary, these studies describe how process parameters and material characteristics of the Ag catalyst impact catalytic performance that will aid in designing better ethylene epoxidation catalysts.