Isotopic Studies\nof Reaction Pathways within Propylene\nEpoxidation over Promoted Silver Catalysts

Kinetic analysis of steady-state\nrates with isotopically\nlabeled\nC<sub>3</sub>D<sub>6</sub> and <sup>18</sup>O<sub>2</sub> reagents\nis used to probe epoxidation and combustion pathways in propylene\nepoxidation by O<sub>2</sub> over promoted Ag catalysts. When utilizing\nC<sub>3</sub>D<sub>6</sub>, combustion rates decrease (∼1.1\nto 1.2×) while epoxidation selectivity (∼40 to 50% to\n∼70 to 75%) and rates (by 2.1–2.6×) increase significantly.\nThe decrease in combustion rates is entirely offset by an increase\nin epoxidation rates on an oxygen atom basis, suggesting kinetic isotope\neffects influence only selectivity-relevant branching, to epoxidation\nor combustion, after a shared rate-determining step which generates\nan oxidant but does not involve propylene. Equimolar mixtures of C<sub>3</sub>H<sub>6</sub> and C<sub>3</sub>D<sub>6</sub> feature identical\nrates of C<sub>3</sub>H<sub>6</sub>O and C<sub>3</sub>D<sub>6</sub>O formation despite an overall inverse kinetic isotope effect in\nepoxide formation, further suggesting that the shared rate-determining\nstep does not feature an intermediate containing propylene. The active\noxidant formed from <sup>18</sup>O<sub>2</sub> is shown to readily\nexchange with coprocessed C<sup>16</sup>O<sub>2</sub>, rendering the\noxygen isotopic content of products identical to cofed CO<sub>2</sub>, likely via the generation of unstable carbonate intermediates.\nIsotopic scrambling of the active oxidant does not scramble gaseous\nO<sub>2</sub>, indicating the irreversible transformation of O<sub>2</sub> to an active oxidant. The description of overall oxidation\nrates on an O-basis reveals the primary promoting effect of Cl is\nto alter selectivity-relevant branching steps with mild inhibition\nof overall oxidation rates.