Remarkably High Li+ Adsorptive Separation Polyamide Membrane by Improving the Crown Ether Concentration and Electron Density

To meet the boost demanding of lithium, it is highly desirable to develop a green and efficient method to recover Li+ resources from salt lake brine. In this work, a novel polyamide (14C4 PA) containing crown ethers in the main chain with a loading of up to 1.46 mmol g–1 was prepared by the polycondensation of diaminobenzo-14-crown-4 (DAB14C4) with 2,2-bis-(4-carboxyphenyl)-hexafluoropropane. The structure of 14C4 PA was confirmed by NMR and FT-IR. A high mechanical strength (6.5 MPa) and highly porous (72.7%) polyamide membrane was prepared by the non-solvent-induced phase separation (NIPS) method. The membrane displayed an exceptional high adsorption capacity for Li+ (Qm = 40.1 mg g–1) and good repeatability. The Li+ was adsorbed to the membrane by monolayer chemical adsorption that was proved by its adsorption kinetics and isotherms. The 14C4 PA membrane with a high selectivity adsorption of Li+ was also observed in the presence of three interfering ions (Na+, K+, and Mg2+), and the selective separation factors of Li+ to Na+, K+, and Mg2+ were up to 14.0, 25.7, and 8.47, respectively. The 14C4 PA membrane showed a ∼25% higher Li+ adsorption than that of the corresponding 14C4 polyimide (PI) membrane with a 7.3% higher crown ether density. The simulation calculation results indicated that the higher Li+ adsorption amount of 14C4 PA is attributed to its larger Li+ binding energy (−507.9 vs −467.3 kJ/mol) owing to the relatively higher charge density of the polyamide repeat unit (−0.157 to −0.329 a.u.) than that of PI (−0.111 to −0.311 a.u.). In summary, our current study provides a feasible approach to synthesize functional polymeric membrane materials for the adsorption and green extraction process for the selective separation of Li+ and Mg2+.