Pathway-Dependent Ion Effects for Electrocatalytic Olefin Epoxidation

Electrocatalytic oxidation is an emerging substitute for industrially relevant oxidation processes due to its mild conditions and high safety, and the catalytic performance is not only associated with the catalyst structure but also closely related to the interfacial ionic microenvironment. In this work, by using electrocatalytic olefin epoxidation as a representative example, we elucidated the different influencing mechanisms of the interfacial ionic environment toward two distinct mechanisms of indirect oxidation and direct oxidation through a combinatory study via kinetics, capacitance analysis, in situ spectroscopy, and theoretical calculation. In the indirect epoxidation system, the reaction pathway involves the hydrophilic activation of the mediator and its further reaction with olefin near the hydrophobic environment. The hydrophilicity/hydrophobicity characteristics of the anions tailor the interface for dispersing the solvent domains and active species, and the amphipathic sulfonimide anions create optimal performance. In the direct epoxidation system, the large olefin substrate must penetrate into the densely packed anion double layer to contact the surface oxygen species generated in situ on the electrode to be epoxidized, and the limiting factor turns into the crowdedness of the double layer or the anion size. The smaller tetrafluoroborate anions outperformed other larger anions by minimally impacting mass transfer. This work not only highlights the key role of the interfacial ionic environment in modulating organic electrosynthesis but also emphasizes the distinct influences of the microenvironment under different reaction pathways.