Selectivity Origin of Organic Electrosynthesis Controlled by Electrode Materials: A Case Study on Pinacols
Cuibo Liu, Rui Li, Wei Zhou, Liang Yu, Yanmei Shi, Runlai Li, Yangfang Ling, Yifu Yu, Jianxin Li, Bin Zhang
Abstract
Unveiling the origin of an electrode’s ability to control the reaction outcome and identifying key factors to explore a promising electrode for selective synthesis of value-added chemicals are highly desirable for minimizing the reliance on the dominant trial-and-error screening mode of electrode materials. Here, the electroreductive pinacol coupling of aromatic carbonyl compounds in an alkaline solution was selected as a model; a hydrogen adsorption free energy (ΔGH*) far from 0, the specific aryl ring adsorption of substrates, and the facile desorption of products were proposed factors that make a material ideal for pinacol synthesis. These factors made carbon fiber paper (CP) optimal for hydrobenzoin synthesis with 99% selectivity, 96% Faraday efficiency, and the reaction rate of 0.6 mmol cm–2 h–1. The aryl ring adsorbed on the CP surface promoted electron transfer and endowed the pinacol production with high selectivity over a wide range of potentials and current densities. Furthermore, hydrogen bonding was also crucial in reducing the reaction energy for generating the ketyl radical, a key intermediate for pinacols, in the alkaline solution. The CP could be reused for 10 cycles while maintaining 99% selectivity for pinacol. This electrochemical method showcased a wide substrate scope, which could be developed for the paired anodic fabrication of benzaldehyde and cathodic production of pinacol using the CP as bifunctional electrodes, and gram-scale synthesis of pinacol in a membrane reactor, highlighting its great promise.