Structure Activity Relationship Approach toward the Improved Separation of Rare-Earth Elements Using Diglycolamides

Abstract

The separation of adjacent lanthanides continues to be a challenge worldwide because of the similar physical and chemical properties of these elements and a necessity to advance the use of clean-energy applications. Herein, a systematic structure−performance relationship approach toward understanding the effect of N-alkyl group characteristics in diglycolamides (DGAs) on the separation of lanthanides(III) from a hydrochloric acid medium is presented. In addition to the three most extensively studied DGA complexants [N,N,N′,N′-tetra(n-octyl)diglycolamide, TODGA; N,N,N′,N′-tetra(2-ethylhexyl)diglycolamide, TEHDGA; N,N′-dimethyl-N,N′-di(n-octyl)diglycolamide, DMDODGA], 12 new extracting agents with varying substitution patterns were designed to study the interplay of steric and electronic effects that control rare-earth element extraction. Subtle changes in the structure around diglycolamide carbonyl oxygen atoms result in dramatic shifts in the lanthanide extraction strength and selectivity. The effects of the chain length and branching position of N-alkyl substituents in DGAs are elaborated on with the use of experimental, computational, and solutionstructure characterization techniques.

Study on the extraction of lanthanides by isomeric diglycolamide extractants: an experimental and theoretical study

“…12 The DGAs with branched chain alkyls such as 2-ethylhexyl and 3,7-dimethyloctyl can not only increase the lipophilicity by virtue of increased van der Waals dispersion forces, 13,14 but also exclude some interfering metal ions. 15 Due to the existence of two kinds of alkyl substituents attached on the amide N atoms, many developed ADGAs balance superior performances in terms of alleviation of the third phase formation, excellent extractability, enough solubility in aliphatic hydrocarbon diluent and high selectivity. 7,16 For example, N,N 0dimethyl-N,N 0 -dihexyl-diglycolamide exhibits high extraction ability and good separation performance in the meantime.…”

Advancing Rare-Earth Separation by Machine Learning

“…It is known that subtle changes to the size of N , N ′-alkyl groups affect DGA performance in Ln(III) separation. 8 The performance of DGAs that incorporate N , N ′-alkyl substituents with branching is rather underexplored, for example, the substituents at γ (e.g., 2 and 3 ) and δ (e.g., 1 ) positions as opposed to α 35 and β 36 positions with respect to the amide nitrogen. Additionally, the introduction of structure-rigidifying elements in DGA, such as the δ-lactam motif in ligand 4 , opens new possibilities for chemically modifying the diglycolamide backbone to further alter separation behavior.…”

Molecular Forces in Liquid–Liquid Extraction

“…As noticed by Nash and co-workers, when it comes to classical solvents, more volatile solvents are more efficient at separation . Finally, once expressed as Δ tr , the extraction of lanthanides is linked to the ionic radius by a simple relation that combines the ion surface charge and a preferred radius of polar cores . Nevertheless, the road is long, and better theories need to be developed for every aspect of colloidal soft-matter approaches, especially if they are to be applied to the nonequilibrium transfer of ions, where the hysteresis of back-extraction cannot be obtained.…”