Extraction separation of Nd(III) with a novel dicarboxylate Temperature-Responsive ionic liquid

The development of ionic liquids (ILs)-based extractants for the solvent extraction of rare earth elements (REEs) is of great importance but still faces challenges. Based on the molecular structure and thermodynamic characteristics of temperature-responsive ILs (TRILs), a design method for TRILs was developed with the COSMO-SAC solvation model and quantum chemical calculations. A novel dicarboxylate TRILs, 1,6-bis(carboxyethyl)-N,N,N’,N’-tetramethyl-1,6-hexanediamine bis(trifluoromethanesulfonyl)imide ([CM-TMHDA][Tf 2 N] 2 ) was successfully synthesized, along with dihydroxy TRILs 1,6-Bis(hydroxyethyl)-N,N,N’,N’-tetramethyl-1,6-hexanediaminium bis(trifluoromethanesulfonyl)imide ([HE-TMHDA][Tf 2 N] 2 ) and diester TRILs 1,6-Bis(ethyl acetate)-N,N,N’,N’-tetramethyl-1,6-hexanediaminium bis(trifluoromethanesulfonyl)imide ([EA-TMHDA][Tf 2 N] 2 ) reported in literatures. By combining experimental and molecular simulation analyses, the phase behavior of [CM-TMHDA][Tf 2 N] 2 and the effects of acids and inorganic salts on its phase behavior were systematically studied. Additionally, a temperature-controlled phase transition system composed of TRILs and aqueous solutions was developed for the extraction and separation of Nd(III) from aqueous solutions. The results indicated that the homogeneous extraction system constructed with TRILs reduced interfacial mass transfer resistance, significantly enhancing the extraction kinetics of Nd(III). Various experimental parameters were optimized, revealing that under the conditions of 80 °C for 30 min, the carboxylic functionalized IL achieved optimal extraction efficiency for Nd(III), reaching 56 %. After four cycles, the extraction performance of [CM-TMHDA][Tf 2 N] 2 maintained. This study provides new insights for improving the separation and extraction techniques of REEs, demonstrating substantial potential for applications in this field.