Abstract
A thorough structural characterization of the La(NO3)3 salt dissolved into several mixtures of ethyl ammonium nitrate (EAN) and methanol (MeOH) with EAN molar fraction χEAN ranging from 0 to 1 has been carried out by combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS). The XAS and MD results show that changes take place in the La3+ first solvation shell when moving from pure MeOH to pure EAN. With increasing the ionic liquid content of the mixture, the La3+ first-shell complex progressively loses MeOH molecules to accommodate more and more nitrate anions. Except in pure EAN, the La3+ ion is always able to coordinate both MeOH and nitrate anions, with a ratio between the two ligands that changes continuously in the entire concentration range. When moving from pure MeOH to pure EAN, the La3+ first solvation shell passes from a 10-fold bicapped square antiprism geometry where all the nitrate anions act only as monodentate ligands to a 12-coordinated icosahedral structure in pure EAN where the nitrate anions bind the La3+ cation both in mono- and bidentate modes. The La3+ solvation structure formed in the MeOH/EAN mixtures shows a great adaptability to changes in the composition, allowing the system to reach the ideal compromise among all of the different interactions that take place into it.
Highlights
Lanthanides and their derivatives have attracted much attention in the last years due to the emergence of novel application fields, including organic synthesis, catalysis, and medicine.[1−3] In nuclear power technology and industrial processes, lanthanide 3+ (Ln3+) ions are separated by dissolving them into solvents with different polarities,[4] and organic solvents have been widely used in the past to extract Ln3+ ions from the aqueous phase
We used molecular dynamics (MD) in combination with X-ray absorption spectroscopy (XAS) to investigate the La(NO3)[3] salt dissolved into several mixtures of ethyl ammonium nitrate (EAN) and MeOH with the EAN molar fraction ranging from 0 to 1
EXAFS spectra and Fourier transform (FT) have been obtained for mixtures with similar compositions, such as those with χEAN = 0.1 and χEAN = 0.2, or with χEAN = 0.8 and χEAN = 1.0, suggesting that the structure of the La3+ ion first solvation shell is very similar in these composition ranges
Summary
Lanthanides and their derivatives have attracted much attention in the last years due to the emergence of novel application fields, including organic synthesis, catalysis, and medicine.[1−3] In nuclear power technology and industrial processes, lanthanide 3+ (Ln3+) ions are separated by dissolving them into solvents with different polarities,[4] and organic solvents have been widely used in the past to extract Ln3+ ions from the aqueous phase In this respect, the possibility of employing ionic liquids (ILs) has been considered due to their several excellent properties and to their “green” characteristics.[5−10] An accurate atomistic description of the Ln3+ ion solvation structures in IL media can be important to select the best performing solvent. A typical consequence is the presence in EAN of a strong three-dimensional hydrogen bond network similar to the tetrahedral one that can be found in Received: May 6, 2021 Published: July 8, 2021
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