Unusual lanthanide ion exchange selectivity modulated via crystalline morphology change of a mixed-metal coordination polymer.

Abstract

The separation of lanthanides is critical for ensuring supply security of lanthanide elements, and modulating separation selectivity is a significant approach to control this process. The author's research has explored modulating lanthanide ion (Ln³⁺) exchange selectivity through crystallographic transitions in phosphate ligand-based crystalline coordination polymers (CCPs). This review reports Ln³⁺ ion exchange in two systems: LnL1, a CCP formed by bis(2-ethylhexyl) phosphate (L1) with Ce³⁺, Nd³⁺, and Sm³⁺, and LnL2, a CCP formed by bis(4-nitrophenyl) phosphoric acid (L2) with Ce³⁺. In these systems, Ln³⁺ selectivity deviated from the conventional pattern corresponding to the atomic number in the lanthanide series. This phenomenon can be attributed to spatial constraints of ion exchange sites within the CCP framework: the ion exchange reaction is influenced by both electrostatic interactions and steric effect. When Ln³⁺ is incorporated into the CCP surface, structural distortion due to the coexistence of two Ln³⁺ types induces a change in crystal morphology. This change acts as a gate-opening mechanism, facilitating subsequent ion exchange reactions toward the inside of CCP. The combination of two Ln³⁺ species likely determines the occurrence of a structural transition, finally influencing Ln³⁺ ion exchange selectivity. While it is challenging to directly apply the systems in this study to a practical separation system, this CCP-specific mechanism involving distortion and transition of the crystal's higher-order structure is expected to contribute to developing a new Ln separation method in the future.