Anionic MOFs Embedded in Anion-Exchange Membranes for the Separation of Lithium/Magnesium Cations

Abstract

Effective separation of lithium (Li⁺) and magnesium (Mg²⁺) is essential for extracting lithium from brines with high Mg/Li ratios. Metal-organic framework (MOF)-based hybrid membranes as an emerging architecture have attracted extensive attention in ion separation owing to their combined advantages of both MOFs and polymers. Here, we reported anionic MOFs as the porous fillers of an anion-exchange membrane (AEM)-based matrix for efficient Li⁺/Mg²⁺ separation. In the poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO)-based AEM matrix, the Li⁺/Mg²⁺ cations can be transferred via the electrostatic attraction of exchanged anions and separated by the electrostatic repulsion of quaternary ammonium groups. After incorporating anionic MOFs, the resulting HSO₃-UiO-66@QPPO-20% exhibits an improved Li⁺/Mg²⁺ selectivity (5.92) and Li⁺ permeability (0.238 mol·m^–2·h^–1) in diffusion dialysis, increased by 48% and 114% relative to the original QPPO, respectively. The embedded anionic MOFs can provide additional fast pathways for cation transfer and sieve Li⁺/Mg²⁺ by using their regular frameworks. Molecular dynamics simulations show that Li⁺ with fewer charges and looser hydrated shells, as well as weaker interactions with sulfonate groups, exhibits higher mobility in HSO₃-UiO-66 frameworks relative to Mg²⁺. This work, as an example, offers a new strategy for efficient cation separations using AEM-based MOF hybrid membranes.