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

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⁺, Ca²⁺, and Mg²⁺) 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 Ca²⁺ and Mg²⁺ 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.