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+, 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.