Understanding the Roles of Electrogenerated Co3+ and Co4+ in Selectivity-Tuned 5-Hydroxymethylfurfural Oxidation.

The Co-based electrocatalyst is among the most promising candidates for electrochemical oxidation of 5-hydroxymethylfurfural (HMF). However, the intrinsic active sites and detailed mechanism of this catalyst remains unclear. We combine experimental evidence and a theoretical study to show that electrogenerated Co3+ and Co4+ species act as chemical oxidants but with distinct roles in selective HMF oxidation. It is found that Co3+ is only capable of oxidizing formyl group to produce carboxylate while Co4+ is required for the initial oxidation of hydroxyl group with significantly faster kinetics. As a result, the product distribution shows explicit dependence on the Co oxidation states and selective production of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 2,5-furandicarboxylic acid (FDCA) are achieved by tuning the applied potential. This work offers essential mechanistic insight on Co-catalyzed organic oxidation reactions and might guide the design of more efficient electrocatalysts.