Steric effect involved in Ln3+/Ce3+ exchange in a coordination polymer based on di(2-ethylhexyl)phosphoric acid

Abstract

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, Ln³⁺/Ce³⁺ exchange (Ln³⁺ = Nd³⁺, Gd³⁺, Dy³⁺, or Lu³⁺) 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 Nd³⁺ < Gd³⁺ < Dy³⁺ < Lu³⁺ when a small amount of Ln³⁺ is loaded, but Lu³⁺ ≈ Nd³⁺ < Gd³⁺ ≈ Dy³⁺ 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 Nd³⁺/Ce³⁺ < Gd³⁺/Ce³⁺ < Dy³⁺/Ce³⁺ < Lu³⁺/Ce³⁺ for exchange systems. This trend is attributable to the differences in the ionic sizes between an incoming Ln³⁺ and original Ce³⁺. Scanning electron microscopy observations reveal the generation of a new phase via the Ln³⁺/Ce³⁺ exchange. Such a phenomena results from solid–solid transformation, rather than dissolution–recrystallization. The small steric strain in the Nd³⁺/Ce³⁺ system leads to the formation of a Nd³⁺-and-Ce³⁺ solid-solution, whereas the morphological change is possibly restrained by the strong strain caused by loaded Ln³⁺ with an ionic size significantly smaller than the original Ce³⁺.