Alkylated Imidazole Moieties in a Cross-Linked Anion Exchange Membrane Facilitate Acid Recovery with High Purity

An anion exchange membrane (AEM) is applied for acid recovery through the diffusion dialysis (DD) process. However, a specially designed AEM is required to achieve a high dialysis coefficient of acid (UH+) and separation factor (S). The design of our membrane for acid recovery and metal ion separation is based on the optimization of water swelling, structural stabilization (cross-linking), and introduction of cationic moieties where the delocalization of charge in a ring system (imidazole) is possible. Based on the above design, herein, preparation of a cross-linked AEM using a poly(acrylonitrile)-co-poly(vinyl imidazole) copolymer for the separation of metal ions and acid (HCl) is reported. The AEMs have been prepared by quaternization of the copolymer followed by cross-linking. The effect of a quaternizing agent (alkyl chain length) and copolymer composition on the performance of the membranes has been evaluated. The membrane (AEM2-Bu-2) prepared by the quaternization of the copolymer (AN/VIm = 75:25 mol/mol) with 1-bromobutane followed by cross-linking exhibits the highest acid recovery performance with a UH+ of 0.047 m/h and S value of 130 during the separation of the 1 M HCl + 0.18 M FeCl2 mixture. The quaternized imidazole ring provides stabilization of the cationic charge, which enhances the performance of the membrane. This work provides an insight to design a high-performance AEM for acid recovery through the DD process.