Highly Cation Permselective Metal–Organic Framework Membranes with Leaf‐Like Morphology

Abstract Highly cation permselective metal–organic framework (MOF) membranes are desirable for the extraction of valuable metal cations. However, fabrication of defect‐free and stable permselective MOF membranes is technically challenging, owing to their arduous self‐assembly and poor water resistance, respectively. A simple and readily scalable method has been developed for the controlled in situ smart growth of UiO‐66‐NH 2 into leaf‐like nanostructures with tunable density of the leaves and the surface layer thickness. The self‐assembly approach reproducibly fabricates seamless, ultrathin (<500 nm) UiO‐66‐NH 2 membranes at the surface of anodic aluminum oxide. The membranes contain nanosized interstices among the MOF leaves, which enable maximum admission of ions within the membrane, and angstrom‐sized inherent pores in every single UiO‐66‐NH 2 crystal, which efficiently regulate the cation permselectivity. Consequently, the highest ever reported cation separations (Na + /Mg 2+ >200 and Li + /Mg 2+ >60) and excellent membrane stability during five sequential electrodialysis cycles are achieved. These characteristics position the fabricated MOF membranes as potential candidates for efficient extraction of pure lithium and sodium ions from salt lakes and seawater, respectively.