(666d) Reaction Mechanism of Electrochemical Alkene Epoxidations in Aqueous-Organic Electrolyte

Electrochemical alkene epoxidations offer a promising alternative to existing epoxidation technologies. By applying an anodic potential (>2.0 V_RHE) at ambient conditions, H₂O acts as the oxygen source for the epoxidation of alkenes. Prior studies on C₂₊ alkene electroepoxidations utilized batch reactors and offer limited product analysis, catalyst characterization, and stability examination. To date, a mechanistic understanding of electroepoxidations remains elusive. Here, we probe the reaction mechanism for 1-hexene (C₆H₁₂) electroepoxidation with H₂O to 1,2-epoxyhexane (C₆H₁₂O) in an aqueous-organic electrolyte by combining kinetic studies varying applied potential, [H₂O], and [C₆H₁₂] with in situ Raman spectroscopy to examine the kinetically relevant intermediate.

Cyclic voltammetry in a flow cell reveals that C₆H₁₂ electroepoxidation occurs at potentials greater than 2.0 V_RHE and 1.96 V_RHE on Au and MnO_x, respectively. Current densities at potentials greater than the onset of H₂O oxidation decrease by a factor of three with increasing C₆H₁₂ activities (0.02 < a_C6H12 < 1.4), indicating that C₆H₁₂ epoxidation competes with H₂O oxidation. Complementary measurements show current densities increase logarithmically as H₂O activities rise above 4 and remain constant with further increases above ~30, suggesting the competing reactions both require O-atoms derived from an active surface species. We propose that C₆H₁₂ epoxidation and H₂O oxidation share a pathway to form active oxygen-containing surface species. This species either reacts with C₆H₁₂ to form C₆H₁₂O for epoxidation or proceeds through the H₂O oxidation mechanism to form O₂. Bulk electrolysis demonstrates that soluble MnO₄^- forms from MnO_x at oxidizing potentials, and these complexes may react with C₆H₁₂ in the fluid-phase to form C₆H₁₂O in stoichiometric processes. On both Au and MnO_x, FE values for epoxidations increase as potentials decrease. Scanning electrochemical measurements quantify the standard heterogeneous rate constant and charge transfer coefficient. Insights from this study can be used to optimize selectivity (electron, carbon) and reaction conditions.