Transforming liquid flow fuel cells to controllable reactors for highly-efficient oxidation of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid at low temperature

Highly-efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) at low temperature with air as the oxidant is still challenging. Herein, inspired by the respiratory electron transport chain (ETC) of living cells mediated by electron carriers, we constructed artificial ETCs and transformed liquid flow fuel cells (LFFCs) to flexible reactors for efficient oxidation of HMF to produce FDCA under mild conditions. This LFFC reactor employed an electrodeposition modified nickel foam as an anode to promote HMF oxidation and (VO₂)₂SO₄ as a cathode electron carrier to facilitate the electron transfer to air. The reaction rate could be easily controlled by selecting the anode catalyst, adjusting the external loading and changing the cathodic electron carrier or oxidants. A maximal power density of 44.9 mW cm⁻² at room temperature was achieved, while for FDCA production, short-circuit condition was preferred to achieve quick transfer of electrons. For a single batch operation with 0.1 M initial HMF, FDCA yield reached 97.1%. By fed-batch operation, FDCA concentration reached 144.5 g L^–1 with a total yield of 96%. Ni²⁺/Ni³⁺ redox couple was the active species mediating the electron transfer, while both experimental and DFT calculation results indicated that HMFCA pathway was the preferred reaction mechanism.