Mechanistic control of surface chlorine within propylene epoxidation

Propylene epoxidation over K-promoted Ag/CaCO₃ yields maximally reported selectivity to propylene oxide (S ∼ 55%) only in the presence of trace allyl chloride. Steady state plug flow propylene epoxidation over K-Ag/CaCO₃ reveals co-fed allyl chloride decomposes readily over Ag, rendering significant bed-scale gradients which are eliminated herein via the use of a gradientless flow reactor with continuous stirred tank reactor hydrodynamics. Conversion of allyl chloride is measured on-stream to enumerate coverages of surface-deposited Cl* in-situ during propylene epoxidation, revealing oxidation-relevant Cl coverages that range linearly (∼0–0.15 mol Cl per mole surface Ag) with allyl chloride partial pressure (∼0.2–2 Pa C₃H₅Cl) and correspond with substantial improvement in steady state propylene oxide selectivity (S ∼ 5% to 55%) and rates (2 to 33 nmol g_cat^-1 s⁻¹). Varying oxygen (∼2–8 kPa O₂) and propylene (∼5–20 kPa C₃H₆) partial pressure yields no significant change to oxidation-relevant Cl coverages, revealing allyl chloride controls such Cl* coverages without significant bias from reactant partial pressures. Oxidation rates at controlled chlorinating conditions feature near-linear dependence on O₂ partial pressure (∼0.7–0.9 order) and are independent of propylene partial pressure while epoxidation selectivity under these conditions is independent of both oxygen and propylene partial pressure, providing mechanistic insight into oxidation and chlorination pathways within promoted Ag-catalyzed propylene epoxidation.