来源：ACS Publications
The rare-earth elements (REEs) are classified as light REEs (LREEs), middle REEs (MREEs), and heavy REEs (HREEs) based on their ionic radii and chemical behavior. The selective separation of MREEs from complex REE mixtures is challenging. We report the application of the dual-size-selective chelator macrotripa for selective aqueous precipitation-based separations of MREEs. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements confirmed that macrotripa adopts distinct conformations for large and small REEs in aqueous solution, resulting in weak binding to MREEs. On the basis of this selectivity, macrotripa retains LREEs and HREEs in solution while enabling the oxalate precipitation of MREEs. In ternary La3+/Gd3+/Lu3+ mixtures, separation factors (SFs) of up to 92.9 for La3+/Gd3+ and 12.3 for Lu3+/Gd3+ were achieved. This approach selectively enriches Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, and Y3+ from REE mixtures with the largest SFLa/Eu of 287.