A highly efficient bifunctional copper catalyst for Baeyer-Villiger oxidation of cyclohexanone: Two different active centers and the reaction path investigation

Polycaprolactone is a significant alternative to nondegradable plastic, and the synthesis of its monomer ε-caprolactone (ε-CL) has obtained increasing attention. Here, one kind of Cu-SiO2 bifunctional catalyst with highly dispersed copper species and various metal valences was developed and applied in Baeyer − Villiger (B-V) oxidation for cyclohexanone (Cy=O) to produce ε-CL with O2/benzaldehyde. The Lewis acidity and the different valence of the copper species could be adjusted by two simple post-treatment methods. The optimal Cu-SiO2-60-1.5 exhibits > 99 % conversion of Cy=O and > 99 % selectivity to ε-CL with high efficiency of BEA. The enhancement primarily originates from the synergism of Cu2+, Cu+, and the Lewis acidity of the catalysts. In addition, the possible mechanisms and realistic reaction path were proposed and verified by combing the characterization and DFT calculations. It clarifies the not well-understood reaction mechanism in B-V oxidation of Cy=O. And it reveals that the formation of the Ph-CO3H intermediate is rate-limiting step, and it is beneficial for the vital "Criegee" intermediates formation thanks to its lower activation energy. This work provides a promising strategy for designing suitable oxidative catalysts and clarifies realistic reaction path of B-V oxidation of Cy=O, which could improve the development of degradable plastics.