Anion exchange membranes with excellent monovalent anion perm-selectivity for electrodialysis applications

In recent years, ion exchange membranes with monovalent ion perm-selectivity are highly desirable. Here, two strategies were adopted simultaneously to prepare anion exchange membranes with excellent monovalent anion perm-selectivity. One is to adjust membranes' hydrophobicity by grafting single long chain tertiary amines of different alkyl chain lengths (N,N-dimethylhexadecylamine (DMA16), N,N-Dimethylhexylamine (DMHA), and trimethylamine (TMA)) onto bromomethylated poly (phenylene oxide) (BPPO) backbones and regulating the molar ratio of the tertiary amine and the bromobenzyl in BPPO. Results show that the obtained membrane C16-0.5, which was grafted with the longest tertiary amine (DMA16) and has the lowest molar ratio of DMA16 and the bromobenzyl in BPPO, exhibits the lowest water uptake (WU) and highest monovalent anion perm-selectivity (P-SO4(Cl)) between Cl- and SO42- (12.84). The other is to regulate membranes' compactness through post-treatments, including heat treatment and surface modification by forming a polypyrrole layer on the membrane surface. Results show that membrane C16-0.75-H120, which was under the heat treatment of 120 degrees C, shows a higher P-SO4(Cl) (7.58) than membrane C16-0.75 (5.02) without any heat treatment; moreover, membrane C16-0.75-pr, which was coated with a polypyrrole layer on the surface, has a higher P-SO4(Cl) (9.0) than membrane C16-0.75 (5.02) without any surface modification. In addition, membranes C16-0.5, C16-0.75-H120 and C16-0.75-pr have more excellent P-SO4(Cl) than the commercial membrane ASV (6.52). To sum up, results in this study suggest that both of the two strategies are highly effective to improve membranes' perm-selectivity. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.