Pore functionalization of cationic covalent organic frameworks membrane: A case towards acid recovery

•Cationic COF was used as selective layer towards H+/Fe2+ separation.•High selectivity of COF membrane was achieved based on electrostatic repulsion.•Methoxy group acted as pump to enhance H+ transport.•Pore-environment engineering was a new strategy to enhance membrane performance. Covalent organic frameworks (COFs), which are a new class of porous organic materials, are considered as potential candidates for separation membranes because of their regular channels and tailored functionality. The imine-linked COF was synthesized using reversible condensation reactions, resulting in its un-stabilization in the acid environment. Thus, it is a formidable challenge to develop stable COFs membranes for applications in harsh environments (e.g. strong acid). Herein, we demonstrated the in-situ construction of acid-stable COF separating layer (TMTATGCl) on the surface of the hydrophilic hydrolyzed polyacrylonitrile (HPAN) substrate via acid-catalyzed interfacial polymerization. Triaminoguanidine chloride (TGCl), a cationic monomer, was used to construct cationic nanochannels in the COF layer, conferring its screening feature toward cations, especially toward divalent cations. Besides, implanted methoxy groups could facilitate H+ diffusion along the pore pathway of the as-prepared COF. As a result, the methoxy functionalized cationic nanochannels of the DMTATGCl/HPAN enhanced H+ transfer, while other ions (e.g. Fe2+) were obstructed, allowing for high selectivity (S = 1248.9) and permeability (UH+ = 0.0118 m/h). Moreover, the membranes (TMTATGCl/HPAN) exhibited remarkable acid-stability, which maintained the high separation performance after 10 cycles under strong-acid conditions. This study provides an effective strategy for developing the ion channels membrane with specific interaction sites for effective ion separation.