Isotopic Studies of Reaction Pathways within Propylene Epoxidation over Promoted Silver Catalysts

Kinetic analysis of steady-state rates with isotopically labeled C₃D₆ and ¹⁸O₂ reagents is used to probe epoxidation and combustion pathways in propylene epoxidation by O₂ over promoted Ag catalysts. When utilizing C₃D₆, combustion rates decrease (∼1.1 to 1.2×) while epoxidation selectivity (∼40 to 50% to ∼70 to 75%) and rates (by 2.1-2.6×) increase significantly. The decrease in combustion rates is entirely offset by an increase in epoxidation rates on an oxygen atom basis, suggesting kinetic isotope effects influence only selectivity-relevant branching, to epoxidation or combustion, after a shared rate-determining step which generates an oxidant but does not involve propylene. Equimolar mixtures of C₃H₆ and C₃D₆ feature identical rates of C₃H₆O and C₃D₆O formation despite an overall inverse kinetic isotope effect in epoxide formation, further suggesting that the shared rate-determining step does not feature an intermediate containing propylene. The active oxidant formed from ¹⁸O₂ is shown to readily exchange with coprocessed C¹⁶O₂, rendering the oxygen isotopic content of products identical to cofed CO₂, likely via the generation of unstable carbonate intermediates. Isotopic scrambling of the active oxidant does not scramble gaseous O₂, indicating the irreversible transformation of O₂ to an active oxidant. The description of overall oxidation rates on an O-basis reveals the primary promoting effect of Cl is to alter selectivity-relevant branching steps with mild inhibition of overall oxidation rates.