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.电催化氧化（Electrocatalytic oxidation）因反应条件温和、安全性高，成为工业相关氧化工艺的新兴替代方案。其催化性能不仅与催化剂结构密切相关，更与界面离子微环境（interfacial ionic microenvironment）息息相关。本研究以电催化烯烃环氧化（electrocatalytic olefin epoxidation）为典型范例，通过动力学、电容分析、原位光谱（in situ spectroscopy）及理论计算的组合研究，阐明了界面离子环境对间接氧化与直接氧化两种不同反应路径的差异化调控机制。在间接环氧化体系中，反应路径涉及介体（mediator）的亲水活化，以及其随后在疏水环境附近与烯烃的进一步反应；阴离子（anions）的亲疏水性（hydrophilicity/hydrophobicity）特征可调控界面以分散溶剂域与活性物种，而两亲性磺酰亚胺阴离子（amphipathic sulfonimide anions）可实现最优催化性能。在直接环氧化体系中，大体积烯烃底物需穿透致密堆积的阴离子双电层，才能与电极表面原位生成的表面氧物种接触并发生环氧化反应，此时限制因素变为双电层的拥挤程度或阴离子尺寸。相较于其他大体积阴离子，体积更小的四氟硼酸盐阴离子（tetrafluoroborate anions）对传质（mass transfer）的影响极小，因此表现更优。本研究不仅凸显了界面离子环境在调控有机电合成（organic electrosynthesis）中的关键作用，更强调了不同反应路径下微环境所产生的差异化影响。