Metal‐Hydroxide‐Porphin Framework Membrane for Almost Perfectly Selective Li ⁺ Retention

Abstract Membrane separation of Li + from other ions, particularly alkaline ions, offers a green way toward direct lithium extraction from various brines. Here we report a metal‐hydroxide‐porphin framework membrane, allowing the passage of all major competing cations (Na + , K + , Ca 2+ , and Mg 2+ ) while only retaining Li + . The crystalline nature of the membrane allows periodically arranged porphin molecules, enabling stable molecular recognition effect. By density functional theory calculations, we found that the porphin molecules had a strong interaction with Li + and prevented their translocation through the porphin cavity. As a result, the membrane allows Na + and K + transport but rejects the transport of Li + completely. Notably, due to the much higher mobility of divalent ions, the membrane also allows the passage of Ca 2+ and Mg 2+ while still retaining Li + under a low external voltage (0.8 V). We expect that this work establishes metal‐hydroxide‐porphin framework membrane as a promising and versatile platform for ion sieving studies and applications.