Covalent organic polymers for aqueous and organic solvent nanofiltration

Here we present the preparation of a novel positively charged covalent organic polymer (COP) based nanofiltration membrane. The porous COP selective layer grows on top of a polybenzimidazole (PBI) support from coupling reaction of 1,3,5-tris(bromomethyl)benzene and 4,4′−dipyridyl in alkaline conditions. Chemical and morphological analyses confirm the formation of a thin layer of COP (less than 50 nm), which is also evidenced by its high water permeance and salt retentions. Moreover, it is shown that COP membranes can form hydrogen bonds with HNO3, leading to a tighter membrane pore size that increases the NaCl retention from 46% to 75% without significantly losing its permeance. Further, the composite membranes demonstrated exceptional solvent and pH stabilities. The COP membranes showed a molecular weight cut off between 400 and 1000 g mol−1 together with pure solvent permeances of 0.91, 2.4, 3.3, 7.1 and 9.5 Lm−2h−1bar−1 toward dimethylformamide (DMF), ethanol, acetone, methanol an acetonitrile, respectively. The remarkable performance together with the stability in harsh environments make the newly developed COP membranes an attractive candidate for extreme nanofiltration.