Critical Role of Chlorinated Hydrocarbons in Propylene Epoxidation over K-Ag/CaCO₃

Selective propylene epoxidation over K-promoted Ag/CaCO₃ features promotion by surface-bound chlorine adatoms (Cl*) deposited by trace alkyl chlorides co-fed continuously in a fashion similar to ethylene epoxidation over promoted Ag/α-Al₂O₃ catalysts. Steady-state propylene epoxidation over K-Ag/CaCO₃ in the presence of gaseous promoters (NO, CO₂, and C₂H₅Cl) at varied contact times in flow reactors as well as transient, low-conversion batch epoxidation reveal allyl chloride, formed via propylene oxychlorination, to be a highly unstable intermediate. In situ allyl chloride formation and decomposition are in turn observed to control Cl* coverages and induce substantial bed-scale Cl* gradients during propylene epoxidation by (i) reducing Cl* coverage upstream in the catalyst bed via propylene oxychlorination and (ii) increasing downstream Cl* coverages by allyl chloride decomposition. This is supported by postreaction batch titrations of K-Ag/CaCO₃ beds after steady state propylene epoxidation at varying contact times which reveals bed-average Cl* coverages increasing from ∼0.03 to ∼0.2 monolayers (moles Cl per surface Ag) with propylene conversion increasing from ∼0.05% to 2.3%. Control of Cl* coverages by in situ generated allyl chloride is highly consequential for selective epoxidation with epoxide selectivity (∼25 to 55%) and site-time yield (∼20 to 54 nmol g_cat^-1 s^-1) increasing substantially with bed-averaged Cl* coverages as propylene conversion increases (∼0.05% to 2.3%). Results presented herein reveal a key difference in the mechanistic pathways dictating activity- and selectivity-salient Cl* coverages within propylene epoxidation compared with ethylene and 1,3-butadiene epoxidation directly related to the unique propensity of propylene to remove Cl* from Ag surfaces during epoxidation to generate allyl chloride.