Artificial Cation‐Chloride Co‐transporters for Chloride‐Facilitated Lithium/Magnesium Separation

Inspired by nature, many artificial ion sieving materials have been developed, shedding light on the next‐generation ion, e.g., Li+, extraction applications. Artificial co‐transporters remain notoriously difficult to construct since they have a much more complex ion sieving property. For example, the cation‐chloride co‐transporters have both alkaline ion and chloride ion selectivity but no alkaline ion/chloride ion selectivity. We here demonstrate a method to construct artificial co‐transporters, using a porous organic framework membrane which has relatively disordered stacking structure and rich quaternary ammonium groups paired with counter‐ions. This imparts the membrane with extremely narrow pores (~0.3 nm) and almost no surface charge, enabling size‐based high alkaline ion selectivity against other cations, high Cl‐ selectivity against other anions, but almost no alkaline ion/Cl‐ selectivity. Such synchronized sieving property further allows us to enhance the extraction of high‐value cations (Li+) by simply feeding excessive low‐value anions (Cl‐). As a demonstration, we realized high‐flux (0.44 mol m‐2 h‐1, driven by only concentration gradient) and highly selective (selectivity: 185) Li+/Mg2+ separation by reversing the current industrial brine‐based lithium extraction process, i.e., sieving Li+ before removing NaCl.