Interfacial Anion-Induced Dispersion of Active Species for Efficient Electrochemical Baeyer–Villiger Oxidation

Abstract

Degradable polymers are an effective solution for white plastic pollution. Polycaprolactone is a type of degradable plastic with desirable mechanical and biocompatible properties, and its monomer, ε-caprolactone (ε-CL), is often synthesized by Baeyer–Villiger (B–V) oxidation that demands peroxyacids with low safety and low atom-efficiency. Herein, we devised an electrochemical B–V oxidation system simply driven by H₂O₂ for the efficient production of ε-CL. This system involves two steps with the direct oxidation of H₂O₂ into •OOH radicals at the electrode surface and the indirect oxidation of cyclohexanone by the generated reactive oxygen species. The modulation of the interfacial ionic environment by amphipathic sulfonimide anions [e.g., bis(trifluoromethane)sulfonimide (TFSI^–)] is highly critical. It enables the efficient B–V oxidation into ε-caprolactone with ∼100% selectivity and 68.4% yield at a potential of 1.28 V vs RHE, much lower than the potentials applied for electrochemical B–V oxidation systems using water as the O sources. On hydrophilic electrodes with the action of sulfonimide anions, hydrophilic H₂O₂ can be enriched within the double layer for direct oxidation while hydrophobic cyclohexanone can be simultaneously accumulated for rapidly reacting with the reactive oxygen species. This work not only enriches the electrified method of the ancient B–V oxidation by using only H₂O₂ toward monomer production of biodegradable plastics but also emphasizes the critical role of the interfacial ionic environment for electrosynthesis systems that may extend the scope of activity optimization.