Rational design of novel carboxylic acid functionalized phosphonium based ionic liquids as high-performance extractants for rare earths

Developing the novel ionic liquids as the potential substitutes for conventional organic solvents in extraction of the rare-earth metals is highly desirable but challenges still remain.In this study,the welldesigned carboxylic acid functionalized phosphonium based ionic liquids,(4-carboxyl)butyl-trioctylphosphonium chloride/nitrate,were synthesized and characterized.The as-prepared samples were tested as the undiluted hydrophobic acidic extractant for rare-earth metal ions,affording the maximal loading of 3 mol/mol towards Nd(Ⅲ)in aqueous solution and the remarkable stripping performance.The results also reveal their excellent extractability and selectivity for Sc(Ⅲ)in the mixtures of six rare-earth ions,as well as the outstanding separation properties between rare-earth and first row transition-metal ions(i.e.,La/Ni,Sm/Co).Moreover,the extraction mechanism indicates that the extracted rare-earth complex via a proton exchange in the ionic liquid phase is structurally similar to the complexes obtained with neutral extractants.This work presents a prototype for the fabrication of the hydrophobic cation-functionalized ionic liquids for highly efficient rare-earth extraction and provides the future application in recycling of rare-earth metals from the spent magnets.