Efficient and stable anion exchange membrane: Tuned membrane permeability and charge density for molecular/ionic separation

• Fine tuning of porous nature of anion-exchange membrane with drying temperature.
• Stable AEM by “green” chloromethylation of polysulfone for electrodialysis.
• Nono-porous positively charged membrane for salt separation.
• Absence of nucleophilic substitution/Hofmann elimination in alkaline conditions.

Herein, we are reporting improved process for chloromethylation of polysulfone (PS) by in situ Friedel–Craft reaction without the use of chloromethyl methyl ether (CME). Further, in situ amination (Menshutkin reaction) was carried out to prepare quaternized ammonium polysulfone (QAPS). QAPS of varied degree of chloromethylation (DCM) (44.01–65.67%) was used for preparing anion-exchange membranes (AEMs) and it was observed that their pore morphology was highly dependent on membrane drying conditions (temperature and RH). As representative case, QAPS membrane (DCM: 44.01% and named as AEM-1(NF)x: x indicates drying temperature) dried at low temperature (30 °C (RH: 75%); 45 °C (RH: 54%) and 60 °C (RH: 12%)) showed nano-porous nature with 5.28–1.89 L m−2 h−1 bar−1 permeability corresponding to 59.31–80.23% salt rejection. Further, permeability of AEM–1(NF)x followed the trend: AEM–1(NF)30>AEM-1(NF)45>AEM-1(NF)60. Formation of porous structure of these membranes was attributed to the phase inversion of the QAPS polymer due to presence of atmospheric water at low temperature (high RH).In addition, QAPS membranes (different DCM values) dried at 80 °C (RH:<5%), exhibited completely dense nature with negligible permeability and named as AEM-1, AEM-2 and AEM-3 (DCM; 44.01; 53.87 and 65.67, respectively). Especially, AEM-3 (DCM: 65.67%) was designed to possess all the required properties such as high water uptake (22.20%), ion-exchange capacity (1.597 mequiv g−1), and counter-ion transport number (0.96), along with reasonable conductivity (8.45 mS cm−1) due to quaternary ammonium group functionality. Chronopotentiometric and i–v studies of AEMs revealed excellent electro-transport property for AEM-3, while its electrodialysis (ED) performance confirmed the suitability for electro-membrane applications. Reported method is novel for preparing stable alkaline membrane with tuneable permeability and charged nature for separations of molecules/ions by pressure and/or electro-driven technologies.

Tunable membrane pore structure, permeability and charge density without phase inversion in non-solvent for molecular/ionic separation.Figure optionsDownload high-quality image (200 K)Download as PowerPoint slide