Selective extraction performance and mechanism of lithium using Lewis base ligands containing phosphoryl/carbonyl groups

[Display omitted] •The Li selective extraction ability of various Lewis base ligands was investigated.•The ligand basicity was quantitively determined through potentiometric titration etc.•The relationship of ligand structures and Li+ extraction selectivity was established.•The ligand with lower basicity is more favorable for the selective extraction of Li+. With the rapidly increasing demand for lithium resources, lithium extraction from low-grade and secondary resources has received widespread attention. The combination of tributyl phosphate (TBP) and FeCl3 is the most studied and reported separation system. However, the selective extraction mechanism for lithium is not clear. In this study, the extraction systems consisting of different Lewis base ligands (L) with phosphoryl/carbonyl groups and FeCl3 were employed for lithium extraction from the LiCl-MgCl2-H2O system. The extraction efficiency and selectivity of lithium by various Lewis base ligands with different branched structures were investigated. The interactions between Li+ and the ligands were deeply analyzed by spectroscopic characterization and quantum chemistry calculations. Meanwhile, the relationship between the ligand structures and the selective binding ability of Li+ was innovatively established. The results show that the extraction efficiency and selectivity of lithium by various ligands descend in the order: TIBP > TBP > N503 > MIBK > P350 > TBPO > TOA. The extracted complexes [LiLn]+·[FeCl4]− are formed through the non-covalent interaction between Li+ and the O atoms of the ligands as well as the Cl atoms of [FeCl4]−. The ligand with low basicity, strong Mulliken electronegativity, and high average localized ionization energy minimum value is preferable for the selective extraction of Li+. On the contrary, the ligand is more likely to be protonated. The study provides a theoretical basis for deeply understanding the relationship between the ligand structures and their selective binding ability towards Li+, which is also conducive to developing more efficient systems for lithium recovery from complex solutions.