The Functional Role of Promoters within Silver-Catalyzed Propylene Epoxidation

Abstract

Silver-catalyzed propylene epoxidation utilizing dioxygen as an oxidant—a potential alternative to relatively inefficient commercial processes—occurs at state-of-the-art selectivity (S ~ 50-60%) only in the presence of several co-promoting additives: solid potassium and gaseous nitric oxide, carbon dioxide, and ethyl chloride. Within this thesis, the roles of co-promoters are investigated by decoupling secondary promoter-driven reaction pathways and their influence on the catalytic surface from concurrent primary oxidative reaction pathways of epoxidation and undesired propylene combustion. Isotopic and kinetic observations reveal rate-determining generation of an oxidant shared between subsequent epoxidation and combustion pathways, with combustion initiated via C-H bond cleavage. Co-fed ethyl chloride is demonstrated to generate allyl chloride—a key intermediate formed in only part-per-million concentrations which improves epoxidation selectivity by an order of magnitude via deposition of Cl adatoms on the catalyst surface. Nitric oxide and carbon dioxide are shown to generate competing alkali-stabilized surface moieties—proposed to be nitrate and carbonate, respectively—which enhance epoxidation selectivity by an order of magnitude. Surface-adsorbed Cl and alkali-nitrate moieties are shown to be crucial for selective propylene epoxidation and are furthermore demonstrated to render state-of-the-art selectivity for ethylene epoxidation (S ~ 90%)—indicating mechanistic generality in oxidative pathways and effects of surface-adsorbed promoters within olefin epoxidation. These studies therefore reveal the interactions of solid and gas phase promoters with the silver catalytic surface and with other promoters which generate surface-adsorbed chloride and alkali-oxyanion species required for selective propylene epoxidation by dioxygen.