Cationic covalent organic framework membranes with stable proton transfer channel for acid recovery

Diffusion dialysis (DD) based on the anion exchange membranes (AEMs) is considered as a promising technology for acid recovery. Two-dimensional (2D) cationic covalent organic frameworks (COF) with uniform pores and intrinsic cationic-type skeleton structures are excellent candidates for the synthesis of the AEMs. However, the sustaining of crystallinity and porosity in acid environments is still a key challenge for the existing COF, which severely restricts their applicability in the acid recovery. Herein, by introducing -OH functionalities adjacent to the Schiff base centers, we synthesized the acid-stable cationic COF (DhaTGCl) layers with intralayer hydrogen bond on the surface of hydrolytic polyacrylonitrile (HPAN) ultrafiltration membrane. The strong electrostatic repulsion, endowed by cationic guanidine-based knots, as well as well-aligned proton channel, constructed by -OH functionalities, rendered extremely high separation factor (S) of 4589 +/- 684.2 for HCl recovery from HCl/ FeCl2 solution. The membrane maintained high selectivity toward H+/Fe2+ after 10 cycles during DD process, while the one without -OH degraded during cycling experiments. Our results imply that building the intralayer H-bonding interactions could be a promising approach for expanding the application of the COF membranes for acid recovery.