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.

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Three kinds of coordination polymers ([M(dehp)3], M = Ce, Fe, or Al) were prepared by mixing the sodium form (Na(dehp)) of di(2-ethylhexyl) phosphoric acid and MCl3 in an ethanol-water binary mixture. They have monoclinic crystalline structure with similar lattice parameters. The lanthanide ion (Ln(3+) = La(3+), Sm(3+), Dy(3+), or Yb(3+)) exchange properties were studied in a 20 : 80 vol% ethanol-water binary mixture containing 2 mM Ln(NO3)3 at room temperature. The rate of Ln(3+) adsorption is relatively slow; it requires over 3 weeks to reach equilibrium. [M(dehp)3] has different Ln(3+) affinities depending on the kind of central metal ions: the affinity order at 3 week adsorption is Yb(3+) < La(3+) < Dy(3+) < Sm(3+) for [Ce(dehp)3], La(3+) < Sm(3+) < Dy(3+) < Yb(3+) for [Fe(dehp)3], and La(3+) < Sm(3+), Dy(3+), Yb(3+) for [Al(dehp)3]. The difference in affinity order can be explained by two factors: the coordination preference and steric strain caused by the polymeric structure. The chemical and structural analyses suggested that the Ln(3+) adsorption progresses first by the central M(3+)/Ln(3+) exchange, followed by a morphological change to a rod-like or fibrous form by a solid phase reaction. In the case of [Fe(dehp)3], the eluted Fe(3+) may be hydrolyzed and precipitated as amorphous iron hydroxide.

Coordination polymers can be attractive ion exchange materials because of their crystallinity and semi-flexibility, which are rather opposing properties, and play integral and synergistic roles in introducing unique ion-exchange behavior. In this paper, Ln(3+)/Ce(3+) exchange (Ln(3+) = Nd(3+), Gd(3+), Dy(3+), or Lu(3+)) in a coordination polymer, [Ce(dehp)3], based on di(2-ethylhexyl)phosphoric acid (Hdehp) is studied by distribution coefficient measurements, ion-exchange isotherms, Kielland plot analysis, and morphology observation. The ion-exchange selectivity is in the order Nd(3+) < Gd(3+) < Dy(3+) < Lu(3+) when a small amount of Ln(3+) is loaded, but Lu(3+) ≈ Nd(3+) < Gd(3+) ≈ Dy(3+) for a high loading ratio. The Kielland plot suggests that a steric effect is involved in the reactions, which becomes stronger in the order of Nd(3+)/Ce(3+) < Gd(3+)/Ce(3+) < Dy(3+)/Ce(3+) < Lu(3+)/Ce(3+) for exchange systems. This trend is attributable to the differences in the ionic sizes between an incoming Ln(3+) and original Ce(3+). Scanning electron microscopy observations reveal the generation of a new phase via the Ln(3+)/Ce(3+) exchange. Such a phenomena results from solid-solid transformation, rather than dissolution-recrystallization. The small steric strain in the Nd(3+)/Ce(3+) system leads to the formation of a Nd(3+)-and-Ce(3+) solid-solution, whereas the morphological change is possibly restrained by the strong strain caused by loaded Ln(3+) with an ionic size significantly smaller than the original Ce(3+).