Synthesis of Novel γ-Carbon-Substituted Dialkylphosphinic Acids (P355/P227-355): Unraveling Structural Symmetry Effects on Middle Rare Earth Separation and Extraction Mechanism

In this study, two γ-carbon-substituted dialkylphosphinic acids—symmetrical di-(3,5,5-trimethylhexyl)phosphinic acid (P355) and unsymmetrical (2-ethylhexyl)(3,5,5-trimethylhexyl)phosphinic acid (P227-355)—were synthesized via a precise free radical addition method. Their chemical structures were fully characterized using ESI-HRMS,1H NMR,31P NMR, and FT-IR. Their middle REE extraction/separation performance, anti-emulsification behavior, and underlying mechanisms were investigated. Key results showed that P355 had better Dy saturation capacity (357.51 mg/L) and good selectivity for middle REEs (their average value ofβN + 1/N= 3.18), while P227-355 showed higher back-extraction efficiency (≈90% Dy stripping at ≥0.02 mol/L H2SO4). Methyln-pentyl ketone (MNPK) eliminated emulsification and boosted saturation capacity (324.18 mg/L Sm and 357.51 mg/L Dy for P355). Mechanistically, the extraction followed cation exchange (Sm3++ 2(HL)2↔ Sm·L3·(HL) + 3H+); MNPK formed hydrogen-bonded associates (HL·MNPK) with free extractants, slightly reducing the effective concentration of (HL)2but not altering the core cation exchange mechanism.