Selective propylene epoxidation on K-promoted Ag/CaCO3 in the presence of NO, CO2, and C3H5Cl 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, CO2, and C3H5Cl for light olefin epoxidation on supported K-promoted silver catalysts. Crucial to epoxidation selectivity is the NO-derived selectivity promoter proposed to be K–NOx surface moieties, whose removal reversibly reduces the epoxidation-to-combustion ratio for ethylene and propylene epoxidation by approximately an order of magnitude. K–NOx 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.
Esposito et al. (Tue,) studied this question.