Co-N and pyridinic nitrogen species in Co/NC synergistically promote oxidative esterification of furan aldehydes/alcohols

The Co/NC catalyst is recognized as a key material for the oxidative esterification of furfural/alcohols to 2,5-furandicarboxylic acid dimethyl ester (FDMC). However, nitrogen species leaching results in the detachment of Co from Co-N_x active sites and the aggregation of Co into nanoparticles, which significantly decreases catalytic activity. Herein, we report a highly active Co/NC-IE catalyst featuring abundant Co-N and pyridinic nitrogen species, in which Zn-N bonds surrounding Co-N sites effectively inhibit Co aggregation and nitrogen loss. The synergy between Co-N and pyridinic nitrogen dramatically accelerates the rate-determining step of HMF → HMFM, raising the reaction rate from 0.13 to 1.92 h−1. Under mild conditions (80 °C, 8 bar O₂, and base-free), the catalyst achieves a 99% FDMC yield with a reaction rate of 10.9 (Formula presented). 1H NMR adsorption studies indicate that the increased pyridinic nitrogen enhances the adsorption of the hydroxyl group in HMF. Quenching experiments and EPR analysis reveal that superoxide radicals serve as the active oxygen species responsible for the oxidation. The high density of Co-N sites imparts a negative charge to the catalyst, facilitating electron transfer to O₂. Electrocatalytic oxygen reduction studies suggest that Co-N sites preferentially activate O₂ via a 2-electron transfer pathway, in which superoxide abstracts hydrogen to form -OOH, followed by further hydrogen abstraction to generate -OOHH. By contrast, once Co nanoparticles form, oxygen activation proceeds via a slower 4-electron transfer pathway, leading to catalyst deactivation.