(708b) Selective Epoxidation of Propylene on Promoted Ag/CaCO3 catalysts

Using a combination of steady and transient chemical kinetics studies coupled with structural characterization by SEM and chemical characterization by N₂O adsorption and chlorination/dechlorination cycles, we report the kinetic influence of solid (K and Mo) and gas phase (CO₂, NO, and C₂H₅Cl) promoters on propylene epoxidation by molecular oxygen over Ag/CaCO₃ formulations with ~60% epoxide selectivity. Akin to selective (S ~ 90%) ethylene epoxidation on promoted Ag/α-Al₂O₃ formulations, epoxidation of propylene in the presence of C₂H₅Cl features concurrent propagation of two catalytic sequences: unselective/selective oxidation of propylene and deposition/removal of surface chlorine (Cl*). Distinctive in propylene epoxidation is the direct participation of propylene in Cl* removal—enabled by its significantly weaker allylic C-H bonds (BDE ~ 365 kJ/mol) than the vinylic C-H bonds of ethylene (460 kJ/mol)—as well as the key role of the resulting allyl chloride (C₃H₅Cl) intermediate in subsequent redeposition of chlorine. We elucidate these parallel unselective Cl-mediated pathways, their kinetic role in unselective oxidation, and the role of promoters in altering Cl* coverages and reaction pathways to enable selective epoxidation of propylene on promoted Ag/CaCO₃ catalysts.