Rapid in-situ growth of covalent organic frameworks on hollow fiber substrates with Janus-like characteristics for efficient organic solvent nanofiltration

Chemically stable covalent organic frameworks (COFs) with uniform and tailorable channels fast in-situ grown on polymeric hollow fiber substrates were reported to efficiently separate organic solvents under a low transmembrane pressure. A tris(4-aminophenyl)amine (TAPA) aqueous solution and a benzene-1,3,5-tricarboxaldehyde (BTCA) organic solution were circulated along the shell and lumen sides of hollow fiber modules, respectively. An imine-based COF was synthesized on the cross-linked polyimide (cPI) substrates because TAPA reacted with BTCA at their interface. The synthesis of COF took place on the inner surface of substrates and also generated crystallites in the matrix, forming an interpenetrating Janus-like network combining the cPI polymer and microporous COFs. The interpenetrating polymer networks not only reinforces the self-mechanical properties of the hollow fiber membranes, but also creates Janus-like characteristics so that the membranes have dramatically enhanced solute retention and super-high permeability in polar and nonpolar solvents. In addition, compared to the traditional synthesis of COF membranes, it took less than 1/6 duration (i.e., 240 min) to fabricate the inner-selective COF composite hollow fiber membranes (HFMs). The optimal composite hollow fiber has a molecular weight cut-off (MWCO) of 784 g mol−1 in isopropanol. This study opens a new strategy to rapidly grow COFs on polymeric hollow fiber substrates and obtain Janus-like characteristics for efficient purification of various organic solvents.