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

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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 3+ and Dy 3+ in an aqueous solution, we demonstrate that crystallization of the CPs of Nd 3+ and Dy 3+ based on dibutyl phosphoric acid (Hdbp) can be thermodynamically described; crystallization yields of [Ln(dbp) 3 ] (Ln = Nd or Dy) complex are predicted well using a simple calculation, which takes the apparent solubility products ( K sp a ) for [Ln(dbp) 3 ] and the acid dissociation constant of Hdbp into account. The K sp a values of [Nd(dbp) 3 ] and [Dy(dbp) 3 ] are experimentally determined to be (1.3 ± 0.1) × 10 –14 and (2.9 ± 0.4) × 10 –18 M 4, respectively, at 20 °C. The ratio of these K sp a values, that is, ca. 4500, is significantly larger than the ratio of the solubility products for inorganic salts of Nd 3+ and Dy 3+ . Therefore, Nd 3+ and Dy 3+ are selectively crystallized in an aqueous solution via the formation of CPs. Under optimized conditions, Dy 3+ crystallization is preferable, whereas Nd 3+ 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 3, has no practical impact on the separation in an aqueous solution.