Development of high crystalline and robust ionic COFs nanofiltration membranes for dyes/salts separation via facile microwave technique

Abstract

Fabrication of high crystalline, robust and continuous covalent organic frameworks (COFs) nanofiltration membranes based on green solvents is promising yet highly challenging. Here, we prepared robust COFs membranes by facile microwave-assisted technique using water as the solvent involved in the reaction. Initially, we prepared TbPa-SO₃H COFs suspensions containing cetyltrimethylammonium bromide (CTAB) by Schiff base reaction. Subsequently, using household microwave-assisted solvent evaporation technology, the TbPa-SO₃H suspension could be deposited on the polyacrylonitrile (PAN) support membrane to form high crystalline TbPa-SO₃H separation layer in just a few minutes, thus transforming it from amorphous to crystalline state. Due to its high crystallinity, the obtained TbPa-SO₃H membranes exhibited high rejection for common dyes (96% for Congo red, MW 696.5 Da), low salts rejection (1.21% for NaCl) and high pure water flux (109 L m⁻²h⁻¹bar⁻¹). To our surprise, the obtained COFs membranes remained their original structure after numerous cycles of use, continuous filtration and high pressure (7 bar) operating conditions. Microwave-assisted preparation of COFs membranes strategy lays a solid foundation for a new generation of crystalline membrane materials and precise separation.