Supported ionic liquid membrane contactor with crown ether functionalized polyimide membrane for high-efficient Li+/Mg2+ selective separation

Worldwide demand for lithium is increasing rapidly. Implementing an efficient extraction for lithium from salt-lake brines with low concentration and high Mg/Li (m/m) ratio remains challenging. A supported ionic liquid membrane (PI@14C4-SILM) contactor was developed in this work using a crown ether functionalized polyimide membrane material (Poly(DAB14C4-6FDA)) as the support and tributyl phosphate (TBP) and (1-butyl-3-methylimidazolium bis-trifluoromethylsulfonimide salt) ([C4mim][NTf2]) mixture as the liquid phase for high-efficiency Li recovery. The results show that when the feed phase Mg/Li ratio is 35 and the stripping phase is 0.5 mol/L HCl, the Li+ penetration rate, initial flux and Li/Mg selectivity of PI@14C4-SILM contactor are 0.354 μm/s, 0.128 mol/m2·h and 19.1, respectively. Molecular dynamics (MD) simulation results show that Poly(DAB14C4-6FDA) and TBP have greater interaction with Li+ than other ions (Na, Mg, K). Meanwhile, the binding energies of Poly(DAB14C4-6FDA)-Li-NTf2 and TBP-Li-NTf2 were calculated by density functional theory (DFT), which were −163.26 and −182.1 kcal/mol, respectively. The higher binding energies of the complexes indicated that their structures were more stable. The mechanism of the PI@14C4-SILM contactor's excellent Li+ mass transfer performance may be the formation of a fast Li+ transfer channel between the solid phase containing the crown ether structure and the liquid mobile phase in the pore structure. Besides, the TBP·Li·NTf2 and TBP·H·NTf2 structures in the liquid membrane phase effectively maintained the cyclic operation of the SILM system. This study provides an effective solution for selective separation of Mg/Li and utilization of Li resources in salt lake water.