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.