(521ax) Reaction Pathways for the Electrochemical Oxidation of Cyclohexane to KA Oil

About 93% of adipic acid, a nylon precursor, is produced from KA oil, a mixture of cyclohexanone (K) and cyclohexanol (A). KA oil is produced from cyclohexane oxidation at 0.8-2 MPa and 140-180 ^â—¦C in the presence of transition metal catalysts and atmospheric oxygen. Because KA oil is more easily oxidized than cyclohexane, cyclohexane oxidation conversion is typically limited to below 9% to ensure high selectivity. An electrochemical approach to cyclohexane oxidation is potentially a safer, more efficient approach to synthesis of KA oil.

A limited number of studies have recently demonstrated the viability and improved performance of electrochemical cyclohexane oxidation to KA oil using annular, two-electrode flow reactors to achieve >99% selectivity for KA oil production. Although these results are promising, fundamental questions about the reaction mechanism and even the identity of the reactants remain. Lack of reference electrode in previous two-electrode studies prevents relation of operating potentials at the anode and cathode to thermodynamic references to quantify kinetic overpotentials, transport limitations, and ohmic losses. Furthermore, complex, undivided flow reactors introduce more variables that make analysis and reproducing results across groups more difficult.

In this work, we elucidate the mechanism of electrochemical cyclohexane oxidation on different electrode materials using a batch, divided cell with a reference electrode. Using chronoamperometry, isotopic labeling, and chemical analysis, we determine voltage dependence and the role of water and O₂ in cyclohexane conversion to KA oil. Electrode characterization before and after operation reveals structural changes, such as corrosion, from high voltage. Additionally, we demonstrate that crossover from the counter electrode needs to be considered when studying KA oil production. Our findings provide foundational insights for creating guidelines on electrochemical oxidation of alkanes and other petrochemicals. Advancing fundamental understanding in this field can help inform the broader field of C-H bond activation and organic electrosynthesis.