Poly(alkyl-biphenyl pyridinium) anion exchange membranes with a hydrophobic side chain for mono-/divalent anion separation

A series of poly(alkyl-biphenyl pyridinium) anion exchange membranes (AEMs) with a hydrophobic side chain were prepared for mono-/divalent anion separation using electrodialysis (ED). A poly(alkyl-biphenyl pyridinium) polymer was synthesized via superacid-catalyzed polymerization, and then quaternization was conducted using Menshutkin reactions with 1-bromopentane. The obtained quaternized product had excellent solubility in common organic solvents, making it flexible to form homogeneous membranes by a solution casting method. The introduction of a hydrophobic side chain resulted in a microphase separation structure in the membrane, which is favorable to the active transport of Cl⁻ (higher Cl⁻ flux of up to 3.37 mol m⁻² h⁻¹ at a 10 mA cm⁻² current density) compared with that of SO₄²⁻ ions giving a high permselectivity of 11.9 in a mixed salt (NaCl/Na₂SO₄) system. In addition, the prepared membrane exhibited excellent alkaline stability in successive ED tests. It showed an OH⁻ flux of up to 3.6 mol m⁻² h⁻¹ with a permselectivity of 361.2 between OH⁻ and WO₄²⁻, which is much higher than that of Neosepta ACS membrane. The ED results manifest that the poly(alkyl-biphenyl pyridinium) AEMs can be promising candidates for practical mono-/divalent anion separation in industry.