Interfacial Ion Association Governs Selective and Regenerable Samarium Recovery by a β-Diketonate Ionic Liquid

Abstract Following chemical precipitation, naphthenic-acid extraction for deep Al removal can coextract substantial Sm, generating an Al-rich process stream for investigating selective lanthanide transfer. Here, we report a quaternary ammonium β-diketonate ionic liquid, [N1888][TTA], that selectively recovers Sm from simulated and industrial feeds. Extraction proceeds through ion association at a weakly solvated interface rather than classical ion exchange. At equilibrium, the organic-phase [N1888][TTA]/Sm molar ratio was 4:1; after six sequential contacts, the Sm loading reached 5.9 g·L–1 and the ratio decreased to approximately 3.5:1. Raman and UV–Vis titration show that SmL4 dominates under ligand excess, whereas higher metal loading promotes lower-ligand-number species. EXAFS fitting resolves two Sm–O contributions with effective coordination numbers of approximately 5.6 and 1.7, consistent with an eight-coordinate Sm center. Theoretical calculations and free-energy analysis further support the proposed extraction pathway. Compared with HTTA, [N1888][TTA] combines stronger Sm extraction with easier stripping. The regenerated extractant retained its extraction and separation performance over five cycles. Following sodium oxalate precipitation and calcination, an Sm2O3-rich oxide product was obtained from the real process feed in 85.6% yield. These results establish a molecular basis for linking ion association and Sm coordination speciation to reversible extraction and stripping in β-diketonate ionic liquid media.