Science

NNadir

(33,510 posts) Sat Feb 15, 2020, 02:43 PM Feb 2020

Extraction and Separation of Lanthanides Using Hydrophobic Ionic Liquids.

The paper I'll discuss in this post is this one: Synergistic Enhancement of the Extraction and Separation Efficiencies of Lanthanoid(III) Ions by the Formation of Charged Adducts in an Ionic Liquid (Hiroyuki Okamura et al. Ind. Eng. Chem. Res. 2020, 59, 1, 329-340).

The importance of the lanthanide elements to modern technology cannot be over estimated. Among many other systems, their importance to so called "renewable energy," particularly the wind industry, is dependent on access to these elements since the wind industry depends on the use of permanent magnets, which are in turn, dependent access to the elements neodymium and dysprosium. The majority of these elements are obtained in China, often under appalling environmental and health and safety conditions.

I'll jump here to the text of the paper which has a nice summary of the value of these elements:

Lanthanoid (Ln) elements are widely used in permanent magnets, lasers, lamp phosphors, rechargeable batteries, and other cutting-edge technology products.(1,2) To achieve metal sustainability, the development of highly efficient chemical processes for the recycling of Ln from secondary resources is crucially important. Liquid–liquid extraction is one of the most effective methods for separating and purifying metal ions.(3?5) However, selective extraction and separation of individual Ln ions remains a challenging task because of their similar chemical properties in aqueous solutions. To overcome these challenges, researchers have begun developing synergistic extraction techniques to improve the extractability of Ln(III) ions. For example, the addition of a hydrophobic neutral ligand such as tributyl phosphate (TBP; Figure S1) or trioctylphosphine oxide (TOPO; Figure 1) to organic solvents containing an acidic chelating reagent such as ?-diketone has been demonstrated to enhance the extraction efficiency of Ln(III) ions.(6,7) These synergistic effects have proven to be quite effective for increasing the extractability of Ln(III) ions but often results in a decrease in their separability because of their superior extractability for every Ln(III) ion. Other methods employing bidentate amines and crown ethers as a synergist have been found to improve not only the extractability but also the separability of Ln(III) ions.(8,9)

Recently, ionic liquids (ILs) have attracted considerable interest in green and sustainable chemistry and engineering(10,11) and in applications such as functional materials,(12,13) catalysts,(14?16) and pharmaceuticals.(17)Typical ILs are composed of an asymmetric and bulky organic cation and a halide-containing inorganic or organic anion. The ILs consist entirely of ions, and their properties, such as extremely low vapor pressure and incombustibility, are thus quite different from those of molecular diluents. The physicochemical properties of ILs uniquely depend on both the cation and anion.(18) Thus, the polarity, hydrophobicity, and miscibility can be tuned by varying the constituent ions. These unique features offer ILs great potential as functional extraction media for solvent extraction.(19,20)...

The authors discuss, with references, to some of the complexing agent classes that have been utilized in the current organic solvent technology that is currently used in this technology, and looks at some different members of this class for use in their ionic liquid.

The structures of these components are shown:

The caption:

Figure 1. Chemical structures of the acidic chelating ligand, hydrophobic neutral ligand, and ionic liquid employed in this study.

The "Htta" reagent, (2-thenoyltrifluoroacetone) is an acidic reagent, and as such, its properties vary with pH. It has been widely utilized in the extraction chemistry of europium, a feature to which I'll allude below. "TOPO" is trioctylphosphine oxide.

The authors discuss their approach using these reagents.

...In the present study, the extraction behavior of Ln(III) ions with Htta in [C4mim][Tf2N] was investigated in the presence or in the absence of TOPO. The IL [C4mim][Tf2N] has the cation with a short 1-alkyl chain. Thus, the extraction of cationic complexes is favorable, and its solubility in water is lower compared to that of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2mim][Tf2N]). These findings were then compared to corresponding extraction behavior in benzene to reveal the solvent effect of IL on the synergism; [C4mim][Tf2N] was found to enable improved extraction and separation efficiencies. The extraction equilibrium of La(III), Nd(III), Eu(III), Dy(III), and Lu(III) was studied in detail using three-dimensional (3D) analysis to determine the composition and extraction constants of Ln(III) complexes extracted into [C4mim][Tf2N]. Then, the adduct formation constants of the corresponding Ln(III)–tta– chelates with TOPO in [C4mim][Tf2N] were calculated, and the differences among the Ln(III) ions were clarified. Furthermore, the coordination structures of the adducts in the IL were studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS)(32,33,39)and electrospray ionization mass spectrometry (ESI-MS) using the Eu–Htta–TOPO–[C4mim][Tf2N] system. Finally, the stripping of the extracted metals and the recycling of the [C4mim][Tf2N] phase were investigated.

Some experimental details:

The extraction of all Ln(III) ions, except Pm(III), was carried out with 1.0 × 10–2mol dm–3 Htta in the presence or in the absence of 1.0 × 10–3 mol dm–3 TOPO in [C4mim][Tf2N]. The Ln(III) ions extracted into the IL phase were quantitatively back-extracted into 1 mol dm–3 HCl or HNO3. In addition, the extraction of Ln(III) ions with only 1.0 × 10–3 mol dm–3 TOPO in [C4mim][Tf2N] was almost negligible, which is in accordance with the extraction from the nitrate aqueous solution.(43)Figure 2 shows the extraction curves of La(III), Nd(III), Eu(III), Dy(III), and Lu(III) with Htta in the absence (a) and in the presence (b) of TOPO in [C4mim][Tf2N] as a function of the aqueous phase pH.

Some results are shown graphically:

Figure 2. Extraction curves of Ln(III) with Htta in the absence (a) or in the presence (b) of TOPO in [C4mim][Tf2N] as a function of pH in the aqueous phase. [Htta]IL = 1.0 × 10–2 mol dm–3, [TOPO]IL = 1.0 × 10–3 mol dm–3. [(Na,H)Cl] = 1.0 × 10–1 mol dm–3, [Ln3+] = 1.0 × 10–5 mol dm–3, VIL/Vaq = 1/3. (a) Htta alone: La, blue open diamond; Nd, purple open up triangle; Eu, red open circle; Dy, orange open down triangle; Lu, green open square. (b) Htta–TOPO: La, blue filled diamond; Nd, purple filled up triangle; Eu, red filled circle; Dy, orange filled down triangle; Lu, green filled square.

Figure 3. Comparison of the pHD=1 values for the extraction of Ln(III) ions with Htta in the absence or in the presence of TOPO in [C4mim][Tf2N] and benzene. The values of pHD=1 in benzene were calculated from the extraction constants(44) and the adduct formation constants.(45) [Htta]ext = 1.0 × 10–2 mol dm–3, [TOPO]ext = 1.0 × 10–3 mol dm–3. [C4mim][Tf2N]: Htta alone, ○; Htta–TOPO, ●. Benzene: Htta alone, △; Htta–TOPO, ▲.

Since the DU editor no longer allows exponents, and because the captions are quite busy, captions for figures 4 through 7 are posted as graphics objects. A cubic decimeter (dm^(-3) is a fancy word (a bow to SI units) for "liter.”

The caption for the above figure:

This next graphic is a 3D representation of the synergistic effects of the two extractant agents TOPO and htta-

The caption for the above figure:

Of course the interesting thing about this system is that not only can it extract the lanthanides, but can do so in an environment which also allows for their separation.

This graphic shows an example:

The caption:

Figure 8. Plots of separation factors between Lu and La (?Lu/La) calculated from the extraction constants as functions of tta– concentration in the aqueous phase and TOPO concentration in the [C4mim][Tf2N] phase in the Htta system (a) and the Htta–TOPO system (b).

The structure of the complexes was investigated, and showed, for europium, that a

The lower lanthanides, from lanthanum up to and including gadolinium are also components of used nuclear fuels, and the fast separation and recovery of these elements is desirable. The high energy density of nuclear fuels means that the amount of these elements obtainable from nuclear fuels is low compared to natural sources, but they still have economic value, and those which retain significant radioactivity over relatively long periods of time, significantly samarium and europium, may have particularly high value to accomplish certain environmentally important tasks, owing to this activity.

It is interesting to note that samarium and europium, along with cerium are the lanthanides which exhibit multiple oxidation states. Europium has a well known +2 oxidation state, stable in aqueous solution, samarium a +2 oxidation state in the solid phase and non-aqueous solution, and cerium a +4 oxidation state under a wide variety of conditions, making it useful for thermochemical water and carbon dioxide splitting among many other redox situations, including self cleaning ovens. These three elements are also the three lanthanides in nuclear fuel whose radioactivity remains for appreciable periods of times, samarium for a few centuries owing to its 151 isotope (t(1/2) = 88.8 years), europium for about a century owing to its 152 isotope (t(1/2) = 13.5 years) and 154 isotope (t(1/2 = 8.5 years), and cerium for less than a decade (t(1/2) = 284.9 days). The quantities of samarium and europium isotopes are generally low because of the high neutron capture cross sections of the isotopes of these elements. (Pure non-radioactive europium can be obtained by allowing samarium 151 isolated from used nuclear fuel to decay into this stable daughter nuclide.) Although europium is a valuable element, and tends to be somewhat depleted in many lanthanide ores, the small amounts possible to isolate from decayed samarium-151 is small, and not likely to have tremendous economic value.

It is not clear how this system might operate in the separation of used nuclear fuels to recover the valuable constituents. The radiation stability of ionic liquids has been extensively studied, notably by a scientist whose work I follow quite closely, Jim Wishart, at Brookhaven National Lab and also by his frequent co-author, Ilya Shkrob at Argonne National Labs. The imidazolium ions are known to degrade in radiation fields, albeit (as Wishart has noted) not necessarily at a rate that effects its performance as a solvent, apparently because of the ability to solvate electrons. (I became familiar with Dr. Wishart when attending a lecture of his on electron solvation.) In some systems, degradants of the widely used TBP (tributyl phosphate) extractant in existing nuclear fuel reprocessing schemes can give rise Zr and Pu (and other metals) upon degradation. The TOPO reagent is sort of an analogue, although TBP is a phosphate and TOPO is a phospine. Htta has been used as an extractant for other metals as well.

However, the wonderful thing about ionic liquids is that their composition is tunable to fit purposes. There is an entire class of ionic liquids that use phosphinium ions as cations, by the way. Another feature is that, being ionic, they are useful for the performance of electrochemistry. Owing to the aforementioned variable oxidation states of cerium, samarium, and europium, it is possible to imagine very fast facile separations of these elements by exploiting solubility differences between oxidation states, as well as extraction into liquid metal cathodes or deposition on solid electrodes.

This paper is certainly not the last word in these types of separations, but it's a lovely paper along a route to a sustainable world.

I hope you're enjoying your weekend.