Combining KA Oil Oxidation and O2 Reduction to H2O2 Enables Kinetically and Energetically Favored Adipic Acid Production Pathways

Adipic acid (AA), produced via oxidation of KA oil (a mixture of cyclohexanol and cyclohexanone), is a crucial monomer for nylon-6,6 synthesis. Electrochemical oxidation of KA oil eliminates the use of the nitric acid oxidant in the conventional process, offering a greener approach. However, electrochemical KA oil oxidation is slow; in particular, the 6e– oxidation of cyclohexanone to AA is sluggish, increasing electrical energy consumption. Here, we demonstrate that pairing KA oil oxidation with the 2e– oxygen reduction reaction (ORR) mitigates energetic and kinetic limitations. By replacing the commonly used hydrogen evolution reaction with the ORR, the operating voltage is greatly reduced. More importantly, we discovered that H2O2 generated from the 2e– ORR can oxidize cyclohexanone to 6-hydroperoxyhexanoic acid (HPHA), a reaction not previously reported in the literature. HPHA can then be further electrochemically oxidized to AA with significantly faster kinetics than cyclohexanone, effectively eliminating the kinetic bottleneck of electrochemical cyclohexanone oxidation. This discovery enables a three-step pathway for AA production: electrochemical oxidation of cyclohexanol to cyclohexanone, chemical oxidation of cyclohexanone to HPHA by H2O2, and electrochemical oxidation of HPHA to AA. The selectivity and efficiency of each reaction, as well as those of the overall process, are investigated in detail.