(113d) Isotopic Investigation of Reaction Pathways within Propylene Epoxidation By O2 on Silver Catalysts

Oxidation pathways over Cl- and K-promoted Ag/CaCO₃ in the direct epoxidation of propylene with molecular oxygen were probed using ¹⁸O₂ and C₃D₆ isotopic reagents at steady state catalytic conditions. Reactions with C₃D₆ reveal inverse kinetic isotope effects (>2× increase) in epoxide synthesis rates and normal kinetic isotope effects (~1.2× decrease) in combustion, indicating epoxidation and combustion share a rate-determining step. Overall oxidation rates on an O-basis are unchanged when using C₃D₆, suggesting the shared rate-determining step generates the sole oxidant for epoxidation and combustion. The rates of C₃H₆O and C₃D₆O formation are identical for an equimolar mixture of C₃H₆ and C₃D₆, requiring incorporation of propylene to form oxidation products to occur after the rate-determining step. Reactions with co-fed ¹⁸O₂ with varying levels of co-fed C¹⁶O₂ reveal the active oxidant exchanges oxygen readily with C¹⁶O₂ without scrambling ¹⁸O₂ to form ¹⁶O¹⁸O or ¹⁶O₂, indicating the active oxidant within propylene oxidation is irreversibly derived from ¹⁸O₂. These isotopic studies suggest propylene epoxidation occurs via a rate-determining step generating an active oxidant—capable of facile exchange of oxygen with carbon dioxide—which subsequently interacts with propylene, either initiating epoxidation or combustion via C-H bond cleavage. This mechanistic scheme for oxidation is utilized to rationalize the influence of various promoters (i.e., trace gaseous nitric oxide and surface-adsorbed Cl) on rates and selectivity of propylene epoxidation over Ag.