NHPI catalyzed air oxidation of cyclohexanone: Mechanism and kinetics study

As an important monomer material for synthesizing polycaprolactone, ε-caprolactone (ε-CL), is currently produced through the Baeyer–Villiger oxidation of cyclohexanone. As a green and promising process to produce ε-CL, oxidizing cyclohexanone with air in the presence of sacrificial agents has attracted extensive attention from academia and industry. However, the slow reaction kinetic and the weak oxidation capacity of air limits the application of this green technology. Herein, we used N-hydroxyphthalimide (NHPI) as a higdhly efficient homogeneous catalyst for the air oxidation of cyclohexanone. Through investigating the influence of reaction conditions such as temperature, gas velocity, aldehyde-ketone ratio, and solvent ratio on the conversion rate of cyclohexanone and selectivity of ε-CL, an optimum reaction condition was obtained and the ε-caprolactone production rate is as high as 3.43 mmol/gcat/min with a selectivity of 90%. The reaction mechanism is investigated through in-situ UV–visible spectra, electron paramagnetic resonance (EPR) spectra, and starch/KI experiment, and a plausible mechanistic mode involving three pivotal reactions was proposed. The dynamic model was constructed to validate the proposed mechanism and provide good predictions for the actual reaction rate. This work provides meaningful insights into NHPI-catalyzed air oxidation of cyclohexanone, and the constructed kinetic model provides theoretical guidance for scale-up processes.