A conductive chlorine ion-imprinted polymer threaded in metal-organic frameworks for electrochemically selective separation of chloride ions

Abstract Highly selective ion separation is crucial to engineering applications. In this work, inspired by the working mechanism of biological protein ion channels, inclusion of ion-imprinted conductive polymers inside metal-organic frameworks (MOFs) with size selective effect was considered for the effective separation of ions. In-situ polymerization of polypyrrole (PPy) with anion exchange behavior in the UiO-66 cages was performed to obtain a conductive chlorine ion-imprinted polymer film electrode (Cl-IIP@UiO-66) for the electrochemically selective separation of chloride ions (Cl−) from wastewater. Herein, the chlorine ion-imprinted conductive PPy chains not only tuned the pore size of UiO-66, but also provided ion and electron transport channels at the molecular scale. As a result, the Cl− ion exchange capacity of the Cl-IIP@UiO-66 coated electrode reached to 52.16 mg g−1 with an adsorption equilibrium time less than 4 h under the extra electric field. Meanwhile, the Cl-IIP@UiO-66 film exhibited high separation factors of 9.02, 10.29, 13.41 and 18.50 for Cl−/Br−, Cl−/F−, Cl−/SO42− and Cl−/PO43−, respectively, which was mainly attributed to the anion-imprinting effect and the nanostructure for the selective recognition for Cl− ions. Remarkably, based on the controlled swelling property of MOFs, the Cl-IIP@UiO-66 maintained superior electrochemical reversibility and durability over 98% even after 1000 uptake/release cycles. Hence, it is believed that such a novel electroactive Cl− ion-imprinted film electrode could be a promising candidate for wastewater treatment. Also, it opened a new pathway to develop novel MOF-based electroactive ion exchange materials for various ion separations.