Porous cross-linked cation exchange membranes for efficient alkali recovery in biomass-based 5-hydroxymethylfurfural alkaline electrocatalytic systems

The alkaline electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a promising route for biomass valorization, yet the coexistence of FDCK (the potassium salt of FDCA) with excess alkali remains a key challenge, making efficient alkali recovery essential. To address this issue, an asymmetric porous cross-linked cation exchange membrane (CEM), featuring a dense ultrathin selective layer supported on a highly porous substrate, was fabricated via a two-step approach: first, preparing a porous base membrane from a chloromethylated/sulfonated polyethersulfone (CMPES/SPES) blend by non-solvent induced phase separation (NIPS); second, functionalizing it through crosslinking with tetraethylenepentamine (TEPA) to form a sub-3 μm selective layer and then post-sulfonation with propane sultone (PS) for charge regulation. The optimal membrane exhibits remarkable diffusion dialysis (DD) alkali recovery performance, achieving a KOH permeability ( U O H − ) of 23.8 × 10 −3  m/h and a KOH/FDCK separation factor ( S O H − / F D C 2 − ) of 55.6. These values represent enhancements by factors of 40–119 and 4–9, respectively, relative to the typical ranges of commercial dense CEMs. Moreover, it maintains stable performance over eight consecutive cycles. This work provides a novel membrane strategy for HMF electrocatalytic oxidation, enabling efficient alkali recovery while preventing valuable FDCA loss, thereby promoting resource efficiency in biomass refining.