The increasing contamination of organic micropollutants in environmental and biological systems is one of the serious issues faced by humanity. The potentially harmful effects of organic micropollutants (pesticides, pharmaceuticals, and dyes) on human health and aquatic ecosystems have raised worldwide concerns. The releasing of organic micropollutants to freshwater causes serious health hazards because most of them are mutagenic or carcinogenic in nature. Comparing with absorbents, membrane technology possesses merits of high separation performance, low energy consumption, mild operation temperature, cost efficiency, and no cross-contamination, etc.
Here, we propose 2D covalent organic framework (COF) membranes for molecular separation. Azobenzene derivatives act as linkers, which can be photo-switched between trans- and cis-isomers by irradiation under ultraviolet (UV) or visible light. The resultant smart COF membrane has nanopores that can be switched between On- (big pores) and Off-state (small pores) valves for molecular separation. The COF membranes were tested in different organic solvents and exhibit permeances as high as 56.7 L m-2 h-1 bar-1, in the following order: acetonitrile > acetone > methanol > ethanol > isopropanol > DMF. Interestingly, the smart cis-COF membranes with Off-state valves under UV show higher rejection for dye molecules than the trans-COF analogue with On-state valves. Different from the traditional membranes with immutable pore channels, the pore size of COF membranes can be remotely tuned by controlling the trans-to-cis isomerization of azobenzene via UV/Vis light, therefore, the permeance and separation factor can be adjusted as needed.
The ingenious design strategy endows traditional membranes with new fascination and opens up new perspectives for separation science and technology. The development of COF based smart membranes with a periodic honeycomb lattice could be a promising solution for a wide variety of separations such as catalyst and solvent recycle, wastewater treatment (organic micropollutant or heavy metal removal), valuable drug purification from organic solvents in pharmaceutical industry (vitamin or antibiotic refinement), oil extraction and purification in food industry, and other challenging separations (hemodialysis or chiral pharmacology separation).

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