Common Active Sites and Oxidants within Ethylene and Propylene Epoxidation on Promoted Silver Catalysts

Selective propylene epoxidation on K-promoted Ag/CaCO 3 in the presence of NO, CO 2, and C 3 H 5 Cl occurs over a catalytic surface that is highly selective and stable for subsequent ethylene epoxidation ( S ≈ 90%), indicating a generalized promotional effect of surface intermediates generated from NO, CO 2, and C 3 H 5 Cl for light olefin epoxidation on supported K-promoted silver catalysts. Crucial to epoxidation selectivity is the NO-derived selectivity promoter proposed to be K–NO x surface moieties, whose removal reversibly reduces the epoxidation-to-combustion ratio for ethylene and propylene epoxidation by approximately an order of magnitude. K–NO x moieties are observed to be significantly less stable during propylene epoxidation than during ethylene epoxidation, attributed to orders of magnitude higher rates of O abstraction via combustion cascades during propylene epoxidation. Selective ethylene epoxidation is also shown to require Cl co-promotion, which significantly enhances selectivity ( S ≈ 74 to 93%) and overall oxidation rates (∼2×)─effects similar to those previously reported on Cs/Re/Cl co-promoted Ag formulations. Co-epoxidation reactions reveal selective ethylene ( S ≈ 85–90%) and propylene ( S ≈ 55%) epoxidation compete for a shared oxidant on shared active sites. These studies demonstrate mechanistic similarities of oxidation and promotion within ethylene and propylene epoxidations on promoted silver catalysts.