Unlocking Direct Lithium Extraction in Harsh Conditions through Thiol-Functionalized Metal–Organic Framework Subnanofluidic Membranes

Metal–organic framework (MOF) membranes with high ion selectivity are highly desirable for direct lithium-ion (Li + ) separation from industrial brines. However, very few MOF membranes can efficiently separate Li + from brines of high Mg 2+ /Li + concentration ratios and keep stable in ultrahigh Mg 2+ -concentrated brines. This work reports a type of MOF-channel membranes (MOFCMs) by growing UiO-66-(SH) 2 into the nanochannels of polymer substrates to improve the efficiency of MOF membranes for challenging Li + extraction. The resulting membranes demonstrate excellent monovalent metal ion selectivity over divalent metal ions, with Li + /Mg 2+ selectivity up to 10 3 since Mg 2+ should overcome a higher energy barrier than Li + when transported through the MOF pores, as confirmed by molecular dynamics simulations. Under dual-ion diffusion, as the Mg 2+ /Li + mole ratio of the feed solution increases from 0.2 to 30, the membrane Li + /Mg 2+ selectivity decreases from 1516 to 19, corresponding to the purity of lithium products between 99.9 and 95.0%. Further research on multi-ion diffusion that involves Mg 2+ and three monovalent metal ions (K +, Na +, and Li +, referred to as M + ) in the feed solutions shows a significant improvement in Li + /Mg 2+ separation efficiency. The Li + /Mg 2+ selectivity can go up to 1114 when the Mg 2+ /M + molar concentration ratio is 1:1, and it remains at 19 when the ratio is 30:1. The membrane selectivity is also stable for 30 days in a highly concentrated solution with a high Mg 2+ /Li + concentration ratio. These results indicate the feasibility of the MOFCMs for direct lithium extraction from brines with Mg 2+ concentrations up to 3.5 M. This study provides an alternative strategy for designing efficient MOF membranes in extracting valuable minerals in the future.