(683g) The Role of Gas-Phase Promoters within Propylene Epoxidation By O2 over Ag

Using a combination of steady-state and transient chemical kinetics studies coupled with in-situ enumeration of surface chlorine content, we report the key role of allyl chloride in autocatalytic promotion of propylene epoxidation over K-promoted Ag/CaCO₃ formulations with >50% epoxide selectivity. Akin to selective (S ~ 90%) ethylene epoxidation on promoted Ag/α-Al₂O₃ formulations, epoxidation of propylene in the presence of alkyl chloride gas phase promoters features concurrent propagation of two catalytic sequences: unselective/selective oxidation of propylene as well as deposition/removal of selectivity-enhancing surface chlorine (Cl*). Distinctive in propylene epoxidation, however, is the direct participation of propylene in Cl* removal to rapidly generate gaseous allyl chloride, reducing steady-state promoting Cl* coverages. Discussed results show the key role of decomposition of allyl chloride—generated in-situ from C₂H₅Cl or co-fed directly—in maintaining appreciable Cl* coverages for selective propylene epoxidation. Additionally, results varying partial pressures of co-fed gaseous promoters (such as NO and CO₂) highlight the roles of well-known promoters in accrual of surface-bound Cl*, and therefore provide new insights into relationships between process conditions and catalyst performance over promoted Ag/CaCO₃.