Selective Crystallization of Phosphoester Coordination Polymer for the Separation of Neodymium and Dysprosium: A Thermodynamic Approach.

Thermodynamics of the formation of coordination polymers (CPs) or metal-organic frameworks (MOFs) has not been focused on, whereas many CPs or MOFs have been synthesized in a solution. With a view of separating Nd and Dy in an aqueous solution, we demonstrate that crystallization of the CPs of Nd and Dy based on dibutyl phosphoric acid (Hdbp) can be thermodynamically described; crystallization yields of [Ln(dbp)] (Ln = Nd or Dy) complex are predicted well using a simple calculation, which takes the apparent solubility products (K) for [Ln(dbp)] and the acid dissociation constant of Hdbp into account. The K values of [Nd(dbp)] and [Dy(dbp)] are experimentally determined to be (1.3 ± 0.1) × 10 and (2.9 ± 0.4) × 10 M, respectively, at 20 °C. The ratio of these K values, that is, ca. 4500, is significantly larger than the ratio of the solubility products for inorganic salts of Nd and Dy. Therefore, Nd and Dy are selectively crystallized in an aqueous solution via the formation of CPs. Under optimized conditions, Dy crystallization is preferable, whereas Nd remains in the solution phase, where the ratio of the Dy molar content to the total metal content (i.e., Nd + Dy) in the crystal is higher than 0.9. The use of acids, such as HCl or HNO, has no practical impact on the separation in an aqueous solution.