Rare Earth Separations: Kinetics and Mechanistic Theories

Picayo, Gabriela A.; Jensen, Mark P.

doi:10.1016/bs.hpcre.2018.10.002

Cited by 9 publications

(3 citation statements)

References 174 publications

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“…The solution chemistry of the f elements is of ever-increasing importance because lanthanides and actinides are found in all aspects of modern technology − and are critical in clean energy technology from windmill magnets over batteries to nuclear power. The primary area of concern is the separation and isolation of pure f elements, which requires understanding and control of the speciation in solvent extraction processes. − Investigation of the f-element speciation and structure in simple acids started with Choppin and Spedding and Habenschuss , and has continued with greater success for the 5f elements − rather than for the 4f elements, − although lanthanide luminescence is a strong tool that indirectly informs on the solution structure. − The main issue for the 4f elements is the purely electrostatic bonding and rapid ligand exchange that make most observed properties an average from several species in solutionan average that changes as a function of the concentration, pH, temperature, ionic strength, etc. , Further, if the time scale of the experiment is slow compared to the rate of ligand exchange, the possibility that an observation is a weighted average has to be considered. The number of possible structures in solution must be reduced and determined in order to build structure–property relationships, and this can be done by using multidentate chelators that form kinetically inert complexes with trivalent lanthanide ions. , …”

Section: Introductionmentioning

confidence: 99%

Including and Declaring Structural Fluctuations in the Study of Lanthanide(III) Coordination Chemistry in Solution

2019

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The physicochemical properties of lanthanide-(III) ions are directly linked to the structure of the surrounding ligands. Rapid ligand exchange prohibits direct structure−property relationships from being formed for simple complexes in solution because the property measured will be an average over several structures. For kinetically inert lanthanide(III) complexes, the simpler speciation may alleviate the problem, yet the archetypical complexes formed by ligands derived from cyclen are known to have at least four different forms in solutioneach with a variation in the crystal field that gives rise to significantly different properties. Slow interchange between forms has been engineered, so that a single complex geometry can be studied, but fast or intermediate interchange between forms is much more commonly observed. The rapid structural fluctuation can report on the changing chemical environment and can be disregarded if a specific property of a lanthanide(III) complex is exploited in an application. However, if we are to understand the chemistry of the lanthanide(III) ions in solution, we must include the structural fluctuation that takes place even in kinetically inert lanthanide(III) complexes in our studies. Here, we have scrutinized the processes that determine the speciation of lanthanide(III) complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA)-like ligands, in particular the processes that enable exchange between forms that have different physicochemical properties, exemplified by the exchange between the diastereomeric capped square-antiprismatic (cSAP) and capped twisted-square-antiprismatic (cTSAP) forms of DOTA-like lanthanide(III) complexes. In the characterization of a kinetically inert f-element complex, understanding the structural fluctuation in the system is critical because a single observed property can arise from a weighted average, from all forms present, or from a single form with a dominating contribution. Further, the experimental condition will influence both the distribution of lanthanide(III) species in solution and the rates of the processes that change the coordination sphere of the lanthanide(III) ions. This is highlighted using data from a series of cyclen-derived ligands with different pendant arms and different denticity. The data were obtained in experiments that take place on different time scales to show that the rate of the process that results in a structural change must be considered against the time of the experiment. We conclude that the structural fluctuations must be taken into account and that they cannot be predicted from the ligand structure. Thus, an estimate of the exchange rates between forms, the relative concentrations of the specific forms, and the effect of the specific structure of each form of the complex must be included in the description of the solution properties of f-element chelates.

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Section: Introductionmentioning

confidence: 99%

Including and Declaring Structural Fluctuations in the Study of Lanthanide(III) Coordination Chemistry in Solution

2019

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“…As such, further development of kinetic-based separations remains a challenging but compelling research opportunity. Indeed, only a few examples the application of kinetic factors to RE separations have been reported. , …”

Section: Introductionmentioning

confidence: 99%

Redox-Driven Chelation and Kinetic Separation of Select Rare Earths Using a Tripodal Nitroxide Proligand

et al. 2019

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Separation of the rare-earth (RE) elements (Sc, Y, La–Lu) is challenging because of their similar chemical properties, but is necessary for their applications in renewable energy and electronic device technologies. The development of separation processes driven by kinetic factors represents a new area for this field. Herein, we disclose a novel method of separating select rare earths by reacting RE cyclopentadienides with the triradical species tris(2-tert-butylnitroxyl)benzylamine (1). The key proligand 1 was characterized using a variety of techniques including X-ray crystallography, magnetometry, and EPR spectroscopy. When applied to an equimolar mixture of La:Y cyclopentadienide complexes, different rates of chelation of these organometallic precursors by 1 were observed, affording a separation factor of 26 under the reported conditions.

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“…Inclusion of N-donor atoms on extractants results in an An/Ln separation factor to the order of 100, whereas the separation factor is on the order of 1 for O-donor atom extractants . Adjacent f -element separationsfor both the An and Lncan take advantage of the slight decrease in ionic radii across both 4 f and 5 f periods. − Many different size-selective extractants with mixed N,O-donor sites have been investigated for f -element separations, with the notable differences revolving around the flexibility of the binding site and steric effects on the alkyl substituted groups. , …”

Section: Introductionmentioning

confidence: 99%

Enhancing f-Element Separations with ADAAM-EH: The Impact of Phase Modifiers and a DGA Aqueous Complexant

Peroutka,

Fontan,

Jackson

et al. 2024

Ind. Eng. Chem. Res.

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Recent investigations have used a 2-ethylhexyl diamide amine (ADAAM-EH) for Am/Cm separations in combination with N,N,N′,N′-tetraethyldiglycolamide as an aqueous complexant to achieve an unprecedented separation factor of 41. The aim of this research effort is to understand the speciation of trivalent lanthanide (Ln) and actinide (An) ions in the organic phase of an ADAAM-EH extraction system, both with and without phase modifiers (PM) (1-octanol and tri-n-butyl phosphate (TBP)). Leveraging spectroscopic techniques in combination with distribution ratio measurements provides an understanding of organic phase f-element ligand complexation. In the absence of PM, Ln is extracted in a stoichiometric 1:1 [M(ADAAM-EH) 1 (NO 3 ) x (H 2 O) 1 ](NO 3 ) 3−x complex. The addition of 1-octanol at 20 vol % results in multiple species present. One of the species is the same as the no PM case, and the other species results in an increased −OH coordination to the inner sphere, potentially displacing some NO 3 . In the case of TBP, increasing concentration results in additional red-shifted bands in the UV− visible spectra, suggesting the complexation of additional ligands of either ADAAM-EH or TBP. The new system knowledge obtained by and spectroscopic experiments will provide benchmarking information for computational studies of the inner-and outer-sphere coordination environments of f-element cations and insights into ADAAM-EH adduct formation with PM, like 1octanol and TBP.

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Rare Earth Separations: Kinetics and Mechanistic Theories

Cited by 9 publications

References 174 publications

Including and Declaring Structural Fluctuations in the Study of Lanthanide(III) Coordination Chemistry in Solution

Including and Declaring Structural Fluctuations in the Study of Lanthanide(III) Coordination Chemistry in Solution

Redox-Driven Chelation and Kinetic Separation of Select Rare Earths Using a Tripodal Nitroxide Proligand

Enhancing f-Element Separations with ADAAM-EH: The Impact of Phase Modifiers and a DGA Aqueous Complexant

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