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 Mg²⁺/Li⁺ concentration ratios and keep stable in ultrahigh Mg²⁺-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⁺/Mg²⁺ selectivity up to 10³ since Mg²⁺ 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 Mg²⁺/Li⁺ mole ratio of the feed solution increases from 0.2 to 30, the membrane Li⁺/Mg²⁺ 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 Mg²⁺ and three monovalent metal ions (K⁺, Na⁺, and Li⁺, referred to as M⁺) in the feed solutions shows a significant improvement in Li⁺/Mg²⁺ separation efficiency. The Li⁺/Mg²⁺ selectivity can go up to 1114 when the Mg²⁺/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 Mg²⁺/Li⁺ concentration ratio. These results indicate the feasibility of the MOFCMs for direct lithium extraction from brines with Mg²⁺ concentrations up to 3.5 M. This study provides an alternative strategy for designing efficient MOF membranes in extracting valuable minerals in the future.