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Article

Extraction of Lanthanides(III) from Nitric Acid Solutions with N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide into Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids and Their Mixtures with Molecular Organic Diluents

by
Alexander N. Turanov
1,
Vasilii K. Karandashev
2,
Vladimir E. Baulin
3,
Yury M. Shulga
3,* and
Dmitriy V. Baulin
4
1
Osipyan Institute of Solid State Physics, Russian Academy of Sciences (ISSP RAS), Chernogolovka 142432, Russia
2
Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences (IMT RAS), Chernogolovka 142432, Russia
3
Federal Research Center for Problems of Chemical Physics and Medicinal Chemistry, Russian Academy of Sciences (FRC PCP MC RAS), Chernogolovka 142432, Russia
4
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences (IPCE RAS), Moscow 119071, Russia
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(11), 1167; https://doi.org/10.3390/min14111167
Submission received: 30 September 2024 / Revised: 1 November 2024 / Accepted: 14 November 2024 / Published: 17 November 2024
(This article belongs to the Special Issue Solvent Extraction of Rare-Earth Elements with Ionic Liquids and Deep Eutectic Solvents)
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Abstract

:
The extraction of lanthanides(III) from aqueous nitric acid solutions with novel unsymmetrical diglycolamide extactant, N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide (DMDCHDGA) into bis(trifluoromethylsulfoyl)imide-based ionic liquids (ILs), namely 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C8mim][Tf2N]), benzyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N222Bn][Tf2N]) methyltrioctylammonium bis(trifluoromethylsulfonyl)imide ([N1888][Tf2N]), and their mixtures with molecular organic diluent 1,2-dichloroethane (DCE), is studied. DMDCHDGA has been shown to interact with components of the IL [C4mim][Tf2N]. The effect of HNO3 concentration in the aqueous phase on the extraction of Ln(III) ions is studied. The stoichiometry of the extracted complexes is determined, and the mechanism of Ln(III) extraction in a system with [C4mim][Tf2N] is discussed. It is shown that the efficiency and intragroup selectivity of the extraction of Ln(III) ions with DMDCHDGA into [C4mim][Tf2N] is significantly higher than when using its symmetric analog TODGA.

Graphical Abstract

1. Introduction

Due to their unique physical and chemical properties, individual rare earth elements (REEs) and their compounds are widely used in various modern engineering fields such as optics, electronics, energy storage systems, catalysts, solid-oxide fuel cells, magnets, etc., as well as for the production of radiopharmaceuticals. Technological progress in the modern world leads to an increase in the consumption of REEs. At the same time, an increase in the production of REEs is faced with the depletion of mineral resources and the accumulation of environmentally unsafe waste. This necessitates the processing of low-profit mineral resources with a low REE content, as well as industrial waste, such as spent catalysts, end-of-life electronics, etc. [1,2,3]. In most cases, when using REEs, their high purity is required. Meanwhile, the separation of REEs is a complex task due to the similarity of the physicochemical properties of these elements. Among the known methods of REE separation, liquid extraction methods are distinguished by their efficiency, productivity, and the possibility of organizing a multistage process. Currently, these methods are widely used for the recovery, preconcentration, and separation of REEs in hydrometallurgy [4,5,6,7]. In addition, the solvent extraction technique is the main method for the separation of actinides and lanthanides from high-level nuclear waste generated during the spent nuclear fuel reprocessing.
The main requirements for extractants are their high ability to selectively bind metal cations, sufficient solubility in organic solvents, and environmental safety. Carboxylic and organophosphorus acids, as well as neutral mono- and polydentate organophosphorus compounds, are used as extractants for the extraction and separation of REEs from raw materials of natural and technogenic origin. However, organophosphorus extractants are toxic compounds that pollute the environment since their disposal produces phosphorus-containing waste.
In recent decades, interest has increased in the use of alkyl-substituted diglycolic acid diamides-diglycolamides (DGAs) as extractants. Compared with bidentate neutral organophosphorus compounds, DGAs have such advantages as ease of synthesis, more lenient purification requirements, and a harmless chemical composition of completely combustible elements. These compounds contain three oxygen atoms in their composition, so they act as tridentate ligands when complexed with Ln(III) ions. DGAs exhibit high extraction efficiency with respect to actinide and lanthanide ions in nitric acid media [8,9,10,11,12,13,14,15,16]. The effect of the DGA structure on their extraction efficiency and selectivity has been studied in detail [8,9,10,11,12]. It has been established that an increase in the length of alkyl substituents in the DGA molecule leads to a significant increase in the solubility of the extractant in organic solvents but is accompanied by a decrease in its extraction efficiency with respect to Am(III) and Eu(III) [8,12]. In order to find more effective and selective DGAs, a number of polyfunctional compounds containing two, three, or four DGA fragments were synthesized. In a number of cases, such compounds exhibit higher extraction efficiency and selectivity compared to unmodified DGAs [6]. Among the DGA compounds, N,N,N′,N′-tetraoctyl diglycolamide (TODGA) and N,N,N′,N′-tetra(2-ethylhexyl) diglycolamide (T2EHDGA) have a fairly high lipophilicity and solubility in various organic diluents; therefore, their extraction properties have been studied in most detail.
The efficiency of lanthanides(III) extraction with DGA compounds largely depends on the nature of the organic diluent [11]. A significant disadvantage of DGAs is their tendency to aggregate in solutions of nonpolar organic solvents, which leads to the formation of a third phase during the extraction of metal ions from a nitric acid medium. This seriously complicates the process of REE extraction. To eliminate this phenomenon, various modifiers are added to the organic phase, such as high molecular weight alcohols, tri-n-butyl phosphate, and others [14].
Much less work has been devoted to the study of REE extraction with unsymmetrical DGAs, in which different alkyl chains are attached to the amidic N atoms [17,18,19,20,21,22]. However, such compounds, for example, N,N′-dimethyl-N,N′-dioctyldiglycolamide (DMDODGA), showed higher extraction capacity with respect to REEs than TODGA [18]. In addition, the use of unsymmetrical DGAs as extractants allows us to surmount the problem of the third-phase formation during the Ln(III) extraction from nitric acid solutions [20].
Another important component of the extraction system is the organic diluent. Its nature affects the efficiency and selectivity of metal ion extraction, the kinetics of the process, the solubility of the extractant in the organic phase, and its capacity for the extracted metal complexes, as well as the transfer of the extractant into the aqueous phase, i.e., contamination of the rafinate of the extraction process with it.
In recent years, ionic liquids (IL), due to their extremely low volatility, incombustibility, and excellent solvating properties, have been considered possible substitutes for conventional molecular organic diluents in various extraction areas, including spent nuclear fuel reprocessing [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. In addition, it has been found that simple replacement of traditional molecular solvents with IL [24]. An undoubted advantage of using ILs as a solvent is the absence of the formation of a third phase during the extraction of actinides and REEs from nitric acid solutions with TODGA [14].
A number of works are devoted to the study of actinides and lanthanides extraction with DGA compounds into ILs [37,38,39,40,41]. 1-Allkyl-3-methylimidazolium hexafluorophosphates ([Cnmim][PF6]) and 1-allkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][Tf2N]) (n = 4, 6, 8, and 10) was studied as ILs. Since [Cnmim][PF6] turned out to be poorly stable in nitric acid media, [Cnmim][Tf2N] were used as diluents in most published works.
It was shown that solutions of DGA extractants in [Cnmim][Tf2N] effectively extract actinides and lanthanides from aqueous solutions under low acidity conditions, while solutions of DGAs in traditional organic solvents, such as chloroform or isooctane, show insignificant extraction [37,38]. However, a sharp decrease in intragroup selectivity was found in the extraction of REEs from nitric acid media by DGA solutions in ILs compared to solutions of these DGAs in molecular solvents [38]. This significantly limits the use of DGA–IL systems for the separation of REEs.
In previous works, we have established that the addition of even small amounts of ILs to a solution of neutral extractants in conventional molecular diluents leads to a significant increase in the efficiency of the extraction of metal ions. A significant synergistic effect was observed in the extraction of actinides, lanthanides, and alkaline earth cations from nitric acid solutions in the presence of small amounts of [C4mim][Tf2N] in the organic phase containing DGA ligands [42,43,44,45], carbamoylmethyl-phosphine oxide type ligands [46], and pillar[5]arene-based phosphine oxide [47].
The use of mixtures of ILs and neutral donor extractants in molecular diluents for the extraction of metal ions allows us to reduce the viscosity of the organic phase and significantly reduce the transition of ILs into the aqueous phase compared to undiluted ILs, as well as to reduce the cost of the extraction process. In some cases, the use of such mixtures leads to an increase in the efficiency and selectivity of REE extraction, as well as a simplification of the process of reextraction of metal ions. We believe that the development of this direction of using ILs in solvent extraction has great prospects.
A significant disadvantage of imidazolium ILs is their significant solubility in aqueous solutions, which leads to losses of ILs during the extraction of metal ions and creates additional environmental problems. This limits the widespread use of ILs as diluents [26]. One of the ways to reduce the transition of ILs into the aqueous phase may be the use of alkylammonium-based ILs for the extraction of metal ions [36]. Such ILs are characterized by greater hydrophobicity compared to imidazolium ILs. In addition, they are easier to prepare and more accessible.
In previous work, we investigated the extraction of lanthanides(III) from aqueous nitric acid solutions with TODGA into methyltrioctylammonium bis(trifluoromethylsulfonyl)imide ([N1888][Tf2N]) IL. It has been shown that the high hydrophobicity of this IL leads to a significant decrease in the transfer of IL components into the aqueous phase compared to imidazolium-based ILs [48]. Additionally, the use of [N1888][Tf2N] as a diluent leads to a significant increase in the efficiency and selectivity of lanthanides(III) extraction with TODGA compared to conventional organic solvents.
This paper describes the results of our studies on the simultaneous extraction of lanthanides(III) from nitric acid solutions with novel unsymmetrical diglycolamide extactant, N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide (DMDCHDGA) into bis(trifluoromethanesulfonyl)imide-based ionic liquids ([C4mim][Tf2N], [C8mim][Tf2N], [N222Bn][Tf2N], and [N1888][Tf2N]) and their mixtures with molecular organic diluent 1,2-dichloroethane. A comparison of the extraction properties of DMDCHDGA and its symmetrical analogue TODGA was carried out. The structural formulas of the extractants and ILs used are shown in Scheme 1.

2. Experimental

2.1. Materials and Methods

Unless otherwise noted, solvents and starting materials were obtained from commercial suppliers. All chemicals used were of reagent grade without further purification before use.
All starting materials for the synthesis of the extractant DMDCHDGA and ILs were purchased from Sigma-Aldrich (St. Louis, MO, USA) and were used without further purification. Chemical grade chloroform, 1,2-dichloroethane, and benzene were purchased from Vekton (Yekaterinburg, Russia). Benzene was dehydrated by distillation over metallic sodium. Stock aqueous solutions of lanthanides(III) were prepared from single-element standard solutions (High Purity Standards, North Charleston, SC, USA). High-purity nitric acid (65%) was purchased from Merck, Germany. All aqueous solutions were prepared using deionized water from NANOPURE purification system (Thermo Scientific, North Charleston, SC, USA) with the specific resistivity of 18 MΩ.
Carbon, hydrogen, and nitrogen content was analyzed on a C, H, N analyzer (Carlo Erba Strumentazione, Milano, Italy).
1H NMR spectra were recorded on a Bruker CXP-200 spectrometer; standards: TMS (internal). The IR spectra were obtained with a resolution of 4 cm−1, and 32 scans were recorded at room temperature in the range of 4000–600 cm−1 on a Perkin-Elmer “Spectrum Two” FTIR spectrometer (Waltham, MA, USA) with an ATR attachment.
The concentrations of Ln(III) in the initial and equilibrium aqueous solutions were determined using inductively coupled plasma mass spectrometry (ICP-MS) on an XSeries II mass spectrometer (Thermo Scientific, Waltham, MA, USA). The concentration of the Tf2N− anion in equilibrium aqueous solutions was determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES) on an iCAP-6500 Duo spectrometer (Thermo Scientific, Waltham, MA, USA). The concentration of HNO3 in the equilibrium aqueous phase was determined by potentiometric titration of H+ with a standard NaOH solution.
The solvent extraction experiments were performed using the method described in the previous work [48]. In these experiments, all lanthanides(III) (except for Pm) were presented in the initial aqueous phase. The initial concentration of each Ln(III) was 2 × 10−6 M. The concentration of HNO3 in the aqueous phase varied in the range of 0.1–7 M.

2.2. Synthesis

The data of elemental analysis and 1H NMR spectra of previously described compounds correspond to literature data.
N,N,N′,N′-tetra(n-octyl)diglycolamide (TODGA) was synthesized and purified by the well-known method [49]. This route deals with the direct reaction of 1 mol of 2,2′-oxydiacetyl chloride with 2 mol of dioctylamine in the presence as a basis of triethylamine dissolved in dry benzene. DMDCHDGA was synthesized in a similar manner, firstly used here for this compound with a yield of 78% (Scheme 2).
To a solution of 11.8 g (60 mmol) of 2,2′-oxydiacetyl chloride in 80 mL of dry benzene were successively added 13.6 g (120 mol) of methylcyclohexylamine and 12.1 g (120 mmol) of triethylamine. The resulting mixture was boiled for 4 h, and the solvent was evaporated in vacuo. The residue was added to 80 mL of diluted (1:2) HCl, and the mixture was extracted with benzene (3 × 20 mL); the resulting extract was washed with diluted (1:2) HCl (2 × 20 mL) and distilled water (2 × 20 mL) and evaporated in vacuo. The residue was chromatographed on a silica gel column (100 ÷ 200 μm), eluent CHCl3, CHCl3: i-PrOH (10:1). The yield of DMDCHDGA was 15.2 g, 78%, m.p. 48–49 °C (hexane). For C18H32N2O3 calculated, %: C, 66.63; H, 9.94; N, 8.63. Found, %: C, 66.32, 66.35; H, 9.79, 9.71; N, 8.32, 8.39. 1H NMR spectrum (200 MHz, CDCl3, 25 °C, TMC, δ, ppm): 0.98–1.95 m (20H, cyclohexyl), 2.83 s (6H, 2CH3), 3.49 m (1H, cyclohexyl), 4.19–4.50 m (5H, 2OCH2 + 1H, cyclohexyl).
ILs 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N], 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C8mim][Tf2N], and trioctylmethylammonium bis(trifluoromethylsulfonyl)imide [N1888][Tf2N] were synthesized according to the procedure described elsewhere [50].
Benzyltriethylammonium bis(trifluoromethylsulfonyl)imide ([N222Bn][Tf2N]) was synthesized by the method [51] first used here for this compound. This route deals with the direct reaction of equivalent quantities of bromide benzyltriethylammonium and lithium bis(trifluoromethanesulfonyl)imide in water with a yield of 94% (Scheme 3).
To prepare [N222Bn][Tf2N], 10 g (37 mmol) of bromide benzyltriethylammonium and 11.5 g (40 mol) of Li(NTf2) were consistently dissolved in 50 mL of distilled water. The mixture was stirred for 0.5 h at 23 °C and then extracted with chloroform (2 × 20 mL). The extract was washed with distilled water (2 × 20 mL) and carefully evaporated in a vacuum. The yield of [BTEA][NTf2] was 14.4 g, 94%, as a pale-yellow liquid. For C15H22F6N2O4S2 calculated, %: C, 38.13; H, 4.69; N, 5.93; Found, %: C, 38.02, 37 85; H, 4.49, 4.51; N, 5.62, 5 79. 1H NMR spectrum (200 MHz, CDCl3, 25 °C, TMC, δ, ppm): 1.39 m (9H, 3CH2CH3), 3.16 m (6H, 3CH2CH3), 4.30 s (2H, CH2Ph), 4.71 m (5H, Ph-H).

3. Results and Discussion

3.1. Interaction of DMDCHDGA with Components of Ionic Liquid [C4mim][Tf2N]

Unlike conventional organic solvents, ILs are active components of extraction systems. The hydrophobic anion Tf2N− ions participate in the formation of extractable complexes during the extraction of metal ions with neutral donor extractants in the presence of ILs. In addition, the interaction of ILs with neutral extractants, such as [C4mim][Tf2N] and tri-n-butyl phosphate (TBP) [52], is observed. To better understand the mechanism of REE ion extraction with DGA solutions in ILs, it is of interest to study the interaction of DMDCHDGA and [C4mim][Tf2N] components. Since such interaction is accompanied by the formation of complexes that are more hydrophobic than the original components, the method of Tf2N− ion distribution between the equilibrium organic and aqueous phases was used to study the processes of interaction of DMDCHDGA and IL components.

3.1.1. Distribution of Tf2N− Ions in the DMDCHDGA—[C4mim][Tf2N]—1,2-dichloroethane (DCE)—Water System

It was shown earlier [42] that the distribution of the [C4mim][Tf2N] components between 1,2-dichloroethane and water is described by the equation
C4mim+ + Tf2N− ⇌ C4mimTf2N(org)
where components with the index (org) and those without an index refer to the organic and aqueous phase, respectively. The partition constant of [C4mim][Tf2N] between DCE and water is expressed as
KIL = [C4mimTf2N](org)/[C4mim+][Tf2N−]
and KIL is 7940 [42].
In the presence of DMDCHDGA in the organic phase, an increase in the distribution ratio of Tf2N− ions (DTf2N) is observed with an increase in the concentration of DMDCHDGA in the organic phase (Figure 1).
This may be due to the interaction of the IL and DMDCHDGA in the organic phase, leading to the formation of an adduct that is more hydrophobic than the original IL. The equilibrium of this process can be described by the following equation:
C4mimTf2N(org) + nL (org) ⇌ LnC4mimTf2N(org)
The equilibrium constant is expressed as
β = [LnC4mimTf2N](org)/[C4mimTf2N](org)[L]n(org)
The dependence of DTf2N on the concentration of DMDCHDGA in the organic phase can be described by the following equation:
DTf2N = ([C4mimTf2N](org) + [LnC4mimTf2N](org))/[Tf2N−] =
[C4mimTf2N](org) (1 + β[L]n(org))/[Tf2N−] = Do(1 + β[L]n(org))
where n is the stoichiometric ratio of DMDCHDGA:IL:in the adduct and Do is the distribution ratio of Tf2N− ions in the absence of DMDCHDGA in the organic phase. This equation can be transformed as
DTf2NDo−1 − 1 = β[L]n(org)
In logarithmic form, Equation (6) can be represented as
log(DTf2NDo−1 − 1) = logβ + nlog[L](org)
The plot of log(DTf2NDo−1 − 1) versus log[L] shows a straight line with a slope close to 1 (Figure S1). Therefore, the LC4mimTf2N adduct is formed in the organic phase, and the β value is 17.5 ± 1.2.
It is desirable to verify the idea of the interaction of DGA and [C4mim][Tf2N] using IR data. Figure 2 shows the IR spectra of DMDCHDGA, IL, and 0.1 M DMDCHDGA/IL solution. According to known literature sources (see, for example, ref. [53]), the absorption bands in the range between 2850 and about 3000 cm−1 are due to stretching vibrations of aliphatic C–H bonds, and the structure visible between 3100 and 3200 cm−1 is attributed to aromatic C–H bonds. Apparently, there are no aromatic bonds in the ligand. The most intense peak in the spectrum of the ligand is associated with stretching vibrations of C=O bonds.
In the spectrum of the IL, we note three groups of bands caused by the anion. These are the ranges of 1300–1400 cm−1, 1200–1240 cm−1, and 1010–1080 cm−1, which are associated with asymmetric stretching vibrations (νas) of the SO2, CF3, and SNS groups, respectively. The assignment is based on publications [54,55].
From the presented spectra, one can draw an unambiguous conclusion that the concentration of the free DMDCHDGA (spectrum 1) in the 0.1 M DMDCHDGA/IL solution (spectrum 3) is quite small, judging by the most intense peak in the DMDCHDGA spectrum (1646 cm−1), which is marked with an asterisk in the spectrum of the solution.
The main question that naturally arises is whether any chemical reactions occur when the DMDCHDGA is dissolved in the IL. To answer this question, we (a) compared spectra 2 and 3 and (b) prepared two more solutions with a higher ligand content (0.5 M and equimolar mixture of DMDCHDGA and IL). Figure 3 shows fragments a and b of the compared spectra, where the greatest changes are observed. The increase in the intensity of the peaks with maxima at 2941 and 1645 cm−1 can be associated simply with the addition of the ligand without any interaction. However, the disappearance of the peak at 1724 cm−1 clearly indicates that the introduction of the ligand is accompanied by some chemical interactions. The presence of a low-intensity peak at 1724 cm−1 in the spectrum of the IL has not been described in the literature. Based on its position, this peak can be attributed to the amide C=O group, which, in principle, can be formed during the oxidation of ILs during storage, despite its known thermodynamic stability [56,57]. It can be thought that this group is restored as a result of chemical processes upon introduction of the DMDCHDGA.
Let us now consider the spectra of the solution with different DMDCHDGA contents (Figure 4). It is evident that with increasing ligand content, the intensity of the broad absorption band in the range from 3650 to 3300 cm−1, with a maximum of 3460 cm−1 increases. The bands of the stretching vibrations of the C–H bonds of the ligand are shifted toward higher frequencies by 5–15 cm−1 upon dissolution, and this shift increases with decreasing ligand concentration. The intensity of the peak of the stretching vibrations of the C=O bonds in the ligand naturally increases with increasing DMDCHDGA concentration in the solution. The position of this peak (1647 cm−1) is practically the same as in the initial solid state (1645 cm−1). Let us note here other peaks, the intensities of which increase with increasing DMDCHDGA content: 1452, 1412, 1259, 1088, and 895 cm−1. We associate these peaks with the ligand in the solution. It is difficult to find a match for them in the spectrum of the ligand in the solid state. Note that the intensities of the peaks caused by the IL decrease with increasing concentration of the dissolved ligand.
In general, the analysis of the spectra shown in Figure 2, Figure 3 and Figure 4 indicates that the IR spectrum of the ligand molecules in the IL differs from that in the solid, which is natural since the immediate environment of the ligand molecules changes. It cannot be ruled out that these changes are related to the interaction of DMDCHDGA and IL.

3.1.2. Extraction of HTf2N Acid with DMDCHDGA into DCE

When C4mimTf2N is contacted with aqueous acid solutions, Tf2N− ions are transferred into the aqueous phase [58]. N-H acid bis[(trifluoromethyl)sulfonyl]imide (triflimide, also abbreviated as HTf2N) is a monoprotic acid, which can react with neutral donor extractants. The interaction of triflimide with several neutral organophosphorus compounds, such as triphenylphosphine, triphenylphosphine oxide, etc., gives rise to phosphonium salts of triflimide, which was confirmed by 1H, 13C, 19F, 31P NMR, and IR [59]. In previous work, we showed that the DGA ligand TODGA dissolved in dichloroethane forms the 1:1 complexes with triflimide [43].
First, we studied the extraction of HTf2N with DMDCHDGA into DCE in the absence of [C4mim][Tf2N] in the organic phase.
Data on the distribution of HTf2N between its aqueous solutions and a solution of DMDCHDGA (L) in dichloroethane indicate that in the studied range of HTf2N concentrations, complexes of the extractant (L) with one HTf2N molecule pass into the organic phase (Figure 5). Therefore, the extraction of HTf2N can be described by the equation
H+ + Tf2N− + L(org) ⇄ LHTf2N(org)
The concentration constants of HTf2N extraction (KHTf2N) are expressed as
KHTf2N = [LHTf2N]/([L][H+][Tf2N−]
where [L] is the equilibrium concentration of free extractant in the organic phase. The logKHTf2N value for DMDCHDGA in 1,2-dichloroethane, calculated from the data in Figure 5, is 4.21 ± 0.12. This value is close to the KHTf2N value for TODGA in DCE (logK = 4.15 ± 0.3 [43]).
In reviewing the literature, no data was found on the reaction of HTf2N with DGA ligands. It is possible to hypothesize that two kinds of species could be formed, namely [DMDCHDGAH]+[Tf2N]− salt and H-complex, DMDCHDGA·HTf2N, as in the case of nitric acid [39]. We hypothesize that proton hydrates H3O+ interact with C=O groups of the DMDCHDGA molecules via hydrogen bonding, resulting in the formation of a complex cation.
The IR spectrum of the DMDCHDGA-HTf2N complex, which is a highly viscous liquid, is shown in Figure 6. First of all, the spectrum is characterized by the appearance of a slanted background, which we associate with the electron conductivity of the liquid under study. Further, the complex lacks absorption bands associated with the stretching vibrations of OH bonds. It can be assumed that the complex lacks water and hydrated protons in a noticeable concentration. If we compare the presented spectrum with the spectrum of the ligand, which is shown in Figure 2, we can see that the ν(C–H) peaks are shifted toward higher frequencies by 5–10 cm−1. The ν(C=O) peak in the complex broadens, but its position does not change. In the region of asymmetric stretching vibrations of SO2 groups νas(SO2), two peaks with maxima at 1348 and 1330 cm−1 are observed, which, according to the works [60,61,62], should be considered as evidence of the presence of Tf2N anions in the sample.

3.1.3. Distribution of Tf2N− Ions in the DMDCHDGA–[C4mim][Tf2N]–DCE//Aqueous HNO3 Solutions System

As can be seen from Figure 7, during the distribution of Tf2N− ions between DCE containing DMDCHDGA and acidic aqueous solutions, the value of DTf2N increases with increasing concentration of DMDCHDGA in the organic phase and to a greater extent than at the extraction from neutral solutions (Figure 1).
This is due to the interaction of DMDCHDGA and [C4mim][Tf2N] in the organic phase, leading to the formation of a complex LHTf2N that is more hydrophobic than the initial IL. The equilibrium of this process can be described by the following equation:
C4mimTf2N(org) + L(org) + H+ ⇌ LHTf2N(org) + C4mim+
The equilibrium constant is expressed as
K1 = [LHTf2N](org) [Tf2N−]/[C4mimTf2N](org)[L](org)[H+] = [LHTf2N](org) /KIL[Tf2N−][L](org)[H+] = KHTf2N/KIL
The dependence of DTf2N on the concentration of DMDCHDGA in the organic phase can be described by the following equation:
DTf2N = ([C4mimTf2N](org) + [LHTf2N](org))/[Tf2N−] = [C4mimTf2N](org) (1 + K1[L](org)[H+] [C4mim+]−1)/[Tf2N−] = Do(1 + KIL K1[L](org)[H+]/DTf2N)
At a constant concentration of H+ ions in the aqueous phase, the plot of (DTf2NDo−1 − 1) versus [L](org)/DTf2N shows a straight line with a slope close to 1 (Figure S2). The value of K1 calculated from the data in Figure 5 is 17,900 ± 1800 and corresponds to the value of K1 = KHTf2N/KIL = 16,200 determined from independent experimental data. This indicates the adequacy of the proposed model of the distribution of the Tf2N− ion in the C4mimTf2N–L–DCE system.
In addition to the extraction of HTf2N, when an organic phase containing ILs and DMDCHDGA is contacted with aqueous acid solutions, the extractant also interacts with HNO3, HCl, etc. As a result, an increase in the concentration of the acid in the aqueous phase above 1 M leads to a decrease in the equilibrium concentration of the free extractant in the organic phase and, accordingly, to a decrease in the DTf2N value (Figure 8).
Since HCl is extracted with DGA solutions to a significantly lesser extent than HNO3, the suppression of HTf2N extraction occurs in the system with HCl to a significantly lesser extent, which corresponds to higher DTf2N values in this system compared to extraction from HNO3 solutions at an equal concentration of the extractant in the organic phase (Figure 8).

3.1.4. Distribution of Tf2N− Ions in the DMDCHDGA—Undiluted [C4mim][Tf2N]//Aqueous HNO3 Solutions System

As can be seen from the data in Figure 9, an increase in the concentration of HNO3 in the aqueous phase leads to an increase in the concentration of Tf2N− in the aqueous phase. This may be due to a partial transition of [C4mim][Tf2N] upon contact with HNO3 solutions into the form [C4mim][NO3]. The equilibrium of this process can be described by the following equation:
H+ + NO3− + [C4mim][Tf2N](org) ⇌ [C4mim][NO3](org) + Tf2N− + H+
The presence of significantly less hydrophobic [C4mim][NO3] in the IL phase results in the concentration of C4mim+ ions in acidic aqueous solutions being greater than the concentration of Tf2N− ions, whereas at [HNO3] = 0, the concentrations of C4mim+ and Tf2N− ions in the aqueous phase are equal to each other [58].
The interaction of the components of the IL with the extractant in neutral and protonated form leads to the fact that the transition of Tf2N− ions from the IL to the aqueous phase is noticeably reduced in the presence of the extractant (Figure 9). At a constant concentration of HNO3 in the aqueous phase, an increase in the concentration of DMDCHDGA in undiluted C4mimTf2N is accompanied by a decrease in the concentration of Tf2N− ions in the equilibrium aqueous phase. Based on the data obtained, it can be assumed that DMDCHDGA is present in the IL phase in the form of [LC4mim+][Tf2N−] complexes and after contact with the HNO3 solution in the form of LHTf2N complexes. In addition, in the region of high HNO3 concentration in the aqueous phase, DMDCHDGA can be in the form of LHNO3 and L(HNO3)2 complexes [63].
The introduction of nitric acid into the solution of DMDCHDGA in ILs (Figure 10) increases the intensity of the peak at 3559 cm−1, and a new peak appears at 3645 cm−1. Most likely, these peaks are associated with vibrations of the NO3− x nH2O clusters. The second feature that can be observed when introducing nitric acid is a sharp decrease in the intensity of the peak at 1644 cm−1, one might say, its almost complete disappearance. We associated this peak with the stretching vibrations of the C=O groups of the ligand. Since the ligand could not disappear, we must look for the compound into which it has transformed. It is noteworthy that simultaneously with the disappearance of the peak at 1644 cm−1, a number of new peaks appear: 1540, 1514, 1300, 1286, 991, 966, and 544 cm−1. It is possible that the disappearance of the peak at 1644 cm−1 can be associated with L-HNO3 complexes of different compositions, which appear in this series. However, the peaks at 1286 and 991 cm−1 are most likely due to asymmetric and symmetric stretching vibrations of the N=O bonds of the NO2 group. When nitric acid is added, the low-intensity peak at 950 cm−1 also disappears. The assignment of this peak is still unclear to us. It is obvious that the introduction of nitric acid is accompanied by reactions of several types.

3.2. Efficiency and Selectivity of Ln(III) Extraction with DMDCHDGA and TODGA into C4mimTf2N Ionic Liquid and Molecular Organic Solvent

At a moderate concentration of HNO3 in the equilibrium aqueous phase, a comparison was made of the efficiency and selectivity of Ln(III) extraction with unsymmetrical DGA-DMDCHDGA and its symmetrical analogue, TODGA, into C4mimTf2N IL, as well as the molecular organic solvent 1,2-dichloroethane (DCE).
As can be seen from the data in Figure 11, the extraction of Ln(III) with DMDCHDGA and TODGA into DCE increases as the atomic number (Z) of lanthanides increases. Overall, this order of extractability in the lanthanides series is typical for the DGA ligands [15] in nitric acid systems. This trend in the dependence of logDLn − Z was explained by an increase in the positive charge density of Ln3+ ions as their ionic radii decreased with increasing Z. At the extraction of La(III)–Nd(III) with DMDCHDGA and TODGA into DCE, the DLn values for these extractants differ little; however, with increasing Z, DMDCHDGA extracts heavy Ln(III) ions significantly more efficiently than TODGA (Figure 11).
This may be due to lower steric hindrances to the complexation of Ln3+ ions with DGA ligands in the case of DMDCHDGA.
The selectivity of Ln(III) extraction with DGA compounds can be characterized by the separation factor of Lu over La (SFLu/La = DLu/DLa). From the data on the Ln(III) extraction from the 3 M HNO3 solution with DMDCHDGA and TODGA into DCE, it follows that the systems with DMDCHDGA are characterized by higher selectivity. The SFLu/La value in systems with DMDCHDGA is 2040, while in the system with TODGA, the SFLu/La value is 275.
Replacement of DCE with IL [C4mim][Tf2N] as a solvent for DMDCHDGA results in a noticeable increase in DLn, especially for the extraction of heavy Ln(III). In the Ln(III) series, the DLn(IL)/DLn(DCE) value increases from 2.2 for La(III) to 25 for Gd(III) and then changes little. The increase in DLn in the system with ILs is apparently associated with the participation of Tf2N− ions in the formation of extractable Ln(III) complexes, which are more hydrophobic than Ln(III)–DGA complexes containing nitrate ions. In addition, replacement of DCE with [C4mim][Tf2N] results in a significant increase in the selectivity of Ln(III) extraction with DMDCHDGA. The SFLu/La value in a system with DCE is 2040, while in the system with [C4mim][Tf2N], the SFLu/La value is 23,400, i.e., an order of magnitude higher.
In contrast, replacing DCE with the IL [C4mim][Tf2N] as a TODGA solvent leads to an increase in DLn only for La(III)–Gd(III) extraction, while for the remaining Ln(III), a decrease in DLn is observed (Figure 11). As a result, the selectivity of TODGA in the system with [C4mim][Tf2N] is significantly reduced. The SFLu/La value in the system with DCE is 275, while in the system with [C4mim][Tf2N], the SFLu/La value is 0.76 only. Previously, it was shown that the selectivity of Ln(III) extraction from HNO3 solutions with N,N,N′,N′-tetrabutyldiglycolamide into [C4mim][Tf2N] is significantly lower than when octanol as a diluent was used [38]. Therefore, the unsymmetrical diglycolamide DMDCHDGA exhibits significantly higher extraction efficiency and selectivity for Ln(III) ions in nitric acid media than its symmetrical analogue TODGA.

3.3. Effect of the Composition of the Aqueous Phase on the Extraction of Ln(III) with DMDCHDGA into [C4mim][Tf2N]

It is known that at the Ln(III) extraction with symmetric and unsymmetric DGA compounds into conventional organic solvents, the increase in DLn with an increase in the concentration of HNO3 in the aqueous phase is observed [11,13]. Therefore, in this case, the Ln(III) extraction with DGA compounds into the organic phase from nitric acid media via a solvation mechanism can be described as
Ln3+ + 3NO3− + (s-n)L(org) + nHNO3L(org) ⇌ LnLs-n(HNO3L)n(NO3)3(org)
where s is a solvation number.
The character of the logDLn − log[HNO3] dependence changes sharply at the extraction with DGA solutions in ILs. The effect of the HNO3 concentration in the equilibrium aqueous phase on the extraction of Ln(III) ions with DMDCHDGA into [C4mim][Tf2N] is shown in Figure 12.
An increase in the concentration of HNO3 in the aqueous phase leads to a decrease in the distribution ratios of Ln(III). A similar dependence of DLn on the acidity of the aqueous phase was previously observed at the extraction of Ln(III), alkaline earth metals, and U(VI) with neutral DGA extractants into [C4mim][Tf2N] [37,38,39,40]. The extraction of Ln(III) ions with the neutral ligand DMDCHDGA into [C4mim][Tf2N] via a cation exchange mechanism can be described by the following equilibrium:
Ln3+ + sL(org) + 3[C4mim][Tf2N](org) ⇌ LnLs(Tf2N)3(org) + 3C4mim +
It follows from this equation that formally the efficiency of Ln(III) extraction by unbound DMDCHDGA (L) does not depend on the concentration of HNO3 in the aqueous phase. The decrease in the DLn values with increasing nitric acid concentration can be explained by a decrease in the concentration of unbound DMDCHDGA in the IL phase due to the interaction of DMDCHDGA and HTf2N (Equation 8). It should be noted that the effect of the concentration of H+ and NO3− ions in the aqueous phase on the change in the DLn values varies significantly. As can be seen from the data in Figure 13, at a constant acidity of the aqueous phase, an increase in the concentration of NO3− ions in the aqueous phase does not lead to a significant change in the distribution ratios of Ln(III). Some decrease in DLn with an increase in the concentration of nitrate ions can be due to the shift of equilibrium (13) to the right, leading to an increase in the concentration of [C4mim][NO3] in the organic phase and, consequently, to a decrease in DLn.
On the other hand, an increase in the acidity of the aqueous phase at a constant concentration of NO3− ions leads to a sharp decrease in the distribution ratios of Ln(III) (Figure 14). This may be due to the involvement of the LHTf2N complex in the extraction process. The plot of logDLn versus log[H+] shows a straight line with a slope close to −3 (Figure 14).
Therefore, the process of extraction of Ln(III) ions with DMDCHDGA into [C4mim][Tf2N] IL via the cation exchange mechanism can be described by the following equilibrium:
Ln3+ + sLHTf2N(org) ⇌ LnLs(Tf2N)3(org) + sH+
Previously, Qi et al. [40] showed that the change of acid anion (NO3−, Cl−, SO42−) in aqueous phase has little effect on the DNd value during the extraction of Nd(III) with N,N′-dimethyl-N,N′-dioctyl-3-oxadiglycolamide (DMDODGA) into [C4mim][Tf2N], i.e., acid anion does not participate in the extraction reaction.

3.4. Stoichiometry of Extracted Ln(III) Complexes

The stoichiometric ratios of Ln(III):DMDCHDGA in the extracted complexes were determined using the slope analysis. The plot of logDLn versus log[DMDODGA] shows straight lines with a slope close to 3 (Figure 15). This indicates that three DMDCHDGA molecules are coordinates to a Ln(III) ion in the IL phase.
At the same time, Qi et al. [40] showed that during the extraction of Nd(III), Sm(III), Gd(III), and Yb(III) with DMDODGA into [C4mim][Tf2N], the stoichiometric ratio of Ln(III):DMDODGA in the extracted complexes is 1:4. Since increasing the concentration of nitrate ions in the aqueous phase has almost no effect on the efficiency of the Ln(III) extraction with DMDCHDGA into the IL phase, it can be assumed that Ln(III) are extracted in the Ln(III)–HNO3–DMDCHDGA–IL system in the form of LnL3(Tf2N)3 complexes. The presence of Tf2N− anions in such complexes leads to a significant increase in their hydrophobicity compared to complexes formed with nitrate ions. We assume that Tf2N− anions, which have a weak coordination ability [64], are located in the outer coordination sphere of the extracted complexes. Large Tf2N− anions are incompatible with the tightly bound structure of water in the aqueous phase, which makes their transition to the organic phase energetically more favorable compared to smaller nitrate anions.

3.5. IR Spectra Study of the Extracted Species

After saturation of the organic phase with Eu(III) in the range of 4000 to 2500 cm−1, where the stretching vibrations of the C–H and O–H bonds fall, almost nothing changed (Figure 16).
Only a slight decrease in the intensity of the peak at 3569 cm−1 can be noted, which we associate with clusters. This fact serves as a basis for asserting that NO3− ions participate in complexation with EuL3 only to an insignificant extent. The formation of EuL3(Tf2N)3-x(NO3)x complexes is advantageous from the point of view of increasing entropy. The main change in the IR spectrum is associated with the appearance of a new peak at 1612 cm−1. We associate this peak with stretching vibrations of the C=O bonds in the formed EuL3(Tf2N)3-x(NO3)x complexes. A red shift of the ν(C=O) absorption band was also observed during the complexation of Eu-TODGA in chloroform [11], as well as during the complexation of REEs with DMDODGA [21]. Comparison of the IR spectra of rare earth metal complexes and the ligand L studied by us with those for the better-studied TODGA ligand seems reasonable due to the similarity of their coordination centers. Another example of the red shift of the carbonyl peak can be taken from the work [65]. The peak related to the stretching vibration of the C=O group of the free ligand of TODGA was detected at 1645 cm−1 in the spectrum of the composite with ILs. After Nd(III) extraction, this peak shifted toward lower wavenumbers to 1615 cm−1.
Thus, the data obtained by us are consistent with the literature data that in the IR study of the process of complexation of REE with ligands with a coordination unit of the DGA type, the main effect is a large (several tens of cm−1) red shift of the absorption band of the ν(C=O) ligand. In some cases, the absorption of the carbonyl group can be masked by the absorption of the solvent in which the reaction is carried out. This occurs when carrying out the reaction described by us in dichloroethane. In this case, the difference spectrum can be used. Indeed, when subtracting the spectrum of sample 4 from the spectrum of sample 5, we see that in the difference spectrum (Figure 17, curve 1), the peak at 1612 cm−1 has the maximum intensity. However, if the complexation process is associated simply with the shift of the absorption band, then in the difference spectrum, in place of the disappeared band at 1644 cm−1, we should have seen a positive peak of equal height. We certainly see a small positive peak in place of the disappeared peak at 1644 cm−1, but its intensity is much less than the intensity of the peak at 1612 cm−1. Consequently, during complex formation, not only does the absorption band of the ν(C=O) ligand undergo a red shift, but also an increase in the intensity of this band.
The spectra of samples 6 (0.1 M DMDCHDGA/DCE//3 M HNO3) and 7 (0.1 M DMDCHDGA/DCE//3 M HNO3 + Eu3+) are shown in Figure S3. It is not possible to isolate the absorption band of the ν(C=O) ligand from the overall spectrum. The difference spectrum of samples 7 and 6 (Figure 17, curve 2) is similar in nature to the difference spectrum of samples 5 and 4. The most intense absorption band in the difference spectrum of samples 7 and 6 is at 1604 cm−1, which, by analogy with the above analysis, we associate with the position of the ν(C=O) absorption in the Eu(L)3 complex. Thus, we propose to use the difference IR spectra to identify complexation in similar cases involving REE.

3.6. Effect of the Cationic Part of the Ionic Liquid on the Extraction of Ln(III) with DMDCHDGA

The effect of the cationic part of the ionic liquid on the extraction of Ln(III) with DMDCHDGA from nitric acid solutions is considered. It can be seen from Figure 18 that an increase in the length of the alkyl radical in the cation of imidazonium ILs leads to some decrease in the DLn values, although less noticeable than in the case of Ln(III) extraction with DMDODGA into [C4mim][Tf2N] and [C8mim][Tf2N] [40].
During Nd(III) extraction from 3 M nitric acid solution with DMDODGA, replacing [C4mim][Tf2N] as a diluent with [C8mim][Tf2N] leads to a decrease in the DNd value by an order of magnitude [40], whereas a similar replacement in the case of DMDCHDGA has little effect on the change in the DNd value (Figure 18). The reason for such behavior in systems with these two asymmetric DGAs, differing in substituents at the amide groups (octyl and cyclohexyl), requires additional study. For ammonium ILs, an unusual effect of the nature of the cationic part of the IL on the efficiency of Ln(III) extraction with DMDCHDGA was also noted. The DLn values in the system with the more hydrophobic IL [N1888][Tf2N] are an order of magnitude higher than in the case of the less hydrophobic IL [N222Bn][Tf2N] (Figure 18). This may be partly due to the fact that during Ln(III) extraction according to the reaction
Ln3+ + sL(org) + 3[Nnnnn][Tf2N](org) ⇌ LnLs(Tf2N)3(org) + 3Nnnnn+
an increase in the concentration of the IL cation in the aqueous phase leads to a shift in equilibrium (17) to the left, i.e., to a decrease in the transition of REE ions into the IL phase. In the case of the more hydrophobic IL [N1888][Tf2N], the transition of its cations into the aqueous phase occurs to a significantly lesser extent than in the case of [N222Bn][Tf2N], which may be the reason for the higher DLn values in the system with [N1888][Tf2N].

3.7. Extraction of Lanthanides(III) from Aqueous Nitric Acid Solutions with Mixtures of DMDCHDGA and Ionic Liquids in Molecular Organic Diluent

In this work, we consider the effect of bis(trifluoromethanesulfonul)imide-based ionic liquids on the extraction of Ln(III) ions with solutions of DMDCHDGA in molecular organic diluent. The effect of the HNO3 concentration in the equilibrium aqueous phase on the extraction of Eu(III) ions with a mixture of DMDCHDGA and [C4mim][Tf2N] in DCE, as well as DMDCHDGA alone, is shown in Figure 19.
Addition of [C4mim][Tf2N] to the organic phase leads to a significant increase in the DLn values. Since the IL itself does not extract Ln(III) under these conditions, it can be said that a synergistic effect is observed in this system. This effect is often used in solvent extraction to improve the efficiency of metal ion extraction [66]. The magnitude of the synergistic effect in the DMDCHDGA–[C4mim][Tf2N] system can be determined using the synergism coefficients (SC), calculated as
SC = D(L+IL)/(D(L) + D(IL))
where D(L), D(IL), and D(L+IL) are the distribution ratios of the metal ion with DMDCHDGA and [C4mim][Tf2N] taken separately and with their mixtures, respectively.
It can be seen from Figure 19 that during the extraction of Eu(III) from nitric acid solutions, the SC value decreases from 7400 to 1.6 with an increase in the HNO3 concentration from 0.5 to 5 M. The same character of the SC–[HNO3] dependence was observed earlier in the extraction of Ln(III) ions with mixtures of TODGA and [C4mim][Tf2N] in DCE [44].
At the extraction of Ln(III) with DMDCHDGA–[C4mim][Tf2N] mixture from 3 M HNO3 solution, the SC values increase from 1.2 for La(III) to 7.2 for Gd(III) and then change little with an increase in Z (Figure 20). It is obvious that the extraction of more hydrated heavy Ln(III) ions is most sensitive to the participation of hydrophobic Tf2N− anions in the formation of extractable Ln(III) complexes. In addition, the intragroup selectivity of Ln(III) extraction in the presence of ILs increases significantly. The SFLu/La values during extraction from the 3 M HNO3 solution with DMDCHDGA solutions in DCE without and with [C4mim][Tf2N] are 2040 and 8510, respectively.
It should be noted that Ln(III) ions are also effectively extracted with mixtures of DMDCHDGA and ILs from solutions of other mineral acids, such as HCl, although in the absence of ILs, the extraction of Ln(III) from HCl solutions is very low (Figure 20).
Accordingly, the SC values in the system with HCl are significantly higher than during extraction from nitric acid solutions. Indeed, the formation of extractable complexes in the presence of ILs can be formally represented by the anion exchange reaction
LnLsA3 + 3[C4mim][Tf2N] ⇄ LnLs(Tf2N)3 + 3C4mim+
where A are anions of mineral acid (NO3− or Cl−).
Such an exchange is more energetically favorable in the case of less stable, i.e., less extractable complexes containing chloride anions in their composition.
The effect of the nature of the cationic part of the ionic liquid on the extraction of Ln(III) with mixtures of DMDCHDGA and ILs is considered. As can be seen from the data in Figure 21, an increase in the hydrophobicity of imidazolium ILs leads to a decrease in the DLn values. For ammonium ILs, an increase in their hydrophobicity leads to a less noticeable decrease in the DLn values. Note that DMDCHDGA extracts Ln(III) ions in the system with [N1888][Tf2N] more efficiently than TODGA (Figure 21). In addition, the intragroup selectivity of Ln(III) extraction in the DMDCHDGA–[N1888][Tf2N]–DCE system is higher than in the TODGA–[N1888][Tf2N]–DCE system. At the Ln(III) extraction from the 3 M HNO3 solution, the SFLu/La values for these systems are 3400 and 1120, respectively.

4. Conclusions

The extraction of lanthanides(III) from aqueous nitric acid solutions with novel unsymmetrical diglycolamide extactant, N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide (DMDCHDGA) into bis(trifluoromethylsulfonyl)imide-based ILs ([C4mim][Tf2N], [C8mim][Tf2N], [N222Bn][Tf2N], and [N1888][Tf2N]) and their mixtures with molecular organic diluent 1,2-dichloroethane is studied. Distribution and IR spectroscopy methods have shown that DMDCHDGA interacts with [C4mim][Tf2N], forming a 1:1 complex. When in contact with aqueous solutions of nitric acid, the neutral donor extractant DMDCHDGA (L), dissolved in the IL, forms the LHTf2N complex. The stoichiometry of the extracted complexes is determined, and the effect of the composition of the aqueous phase is studied. It is shown that the efficiency of Ln(III) extraction with DMDCHDGA solutions in ILs decreases with increasing acidity of the aqueous phase. It is suggested that the LHTf2N complex is involved in the extraction of Ln(III) ions. It is established that the unsymmetrical diglycolamide DMDCHDGA demonstrates a significantly higher extraction efficiency with respect to Ln(III), as well as intragroup selectivity in nitric acid media, than its symmetrical analogue TODGA. It was shown that replacing [C4mim][Tf2N] with a more hydrophobic IL [N1888][Tf2N] does not lead to a decrease in the efficiency and selectivity of Ln(III) extraction with DMDCHDGA, while the loss of ILs during the extraction process is sharply reduced.
This study demonstrates a significant synergistic effect in the extraction of lanthanides(III) from aqueous solutions of HNO3 with DMDCHDGA dissolved in 1,2-dichloroethane in the presence of bis(trifluoromethylsulfonyl)imide-based ionic liquids. This is due to the high hydrophobicity of IL anions, Tf2N−, which are involved in the formation of the extracted complexes as counter ions. The use of mixtures of DMDCHDGA and ILs in conventional organic solvents for Ln(III) extraction allows for a reduction in the viscosity of the organic phase and the consumption of ILs. The results of this work can be used to develop technologies for the recovery and separation of REEs from various objects, for example, from industrial end-of-life wastes.

Supplementary Materials

he following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14111167/s1, Figure S1. Dependence of log(DTf2NDo−1 − 1) versus log[DMDCHDGA]; Figure S2. Dependence of (DTf2NDo−1 − 1) versus [DMDCHDGA](org)/DTf2N; Figure S3. IR spectra of samples 6 (0.1 M DMDCHDGA/DCE//3 M HNO3) and 7 (0.1 M DMDCHDGA/DCE//3 M HNO3 + Eu3+).

Author Contributions

Conceptualization, A.N.T.; methodology, A.N.T., V.K.K., Y.M.S., and V.E.B.; investigation A.N.T., D.V.B., V.K.K., and Y.M.S.; formal analysis, A.N.T., V.K.K., and V.E.B.; writing–original draft preparation, A.N.T., V.K.K., and V.E.B.; writing–review and editing, A.N.T., V.K.K., and V.E.B.; supervision, A.N.T., V.K.K., and V.E.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

This work was performed within the framework of the government assignment for the ISSP RAS, IMT RAS (No 075-00296-24-00), FRC PCP MC RAS (Nos 124013000757-0, 124013000744-0, FFSG-2024-0019), and IPCE RAS.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. The structures of the studied DGAs and ILs.
Scheme 1. The structures of the studied DGAs and ILs.
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Scheme 2. The scheme of N,N′-dimethyl-N,N′-dicyclohexyl-3-oxadiglycolamide (DMDCHDGA) synthesis.
Scheme 2. The scheme of N,N′-dimethyl-N,N′-dicyclohexyl-3-oxadiglycolamide (DMDCHDGA) synthesis.
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Minerals 14 01167 sch003
Scheme 3. The scheme of benzyltriethylammonium bis(trifluoromethylsulfonyl)imide synthesis.
Scheme 3. The scheme of benzyltriethylammonium bis(trifluoromethylsulfonyl)imide synthesis.
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Figure 1. The effect of DMDCHDGA concentration in the organic phase on the distribution ratio of Tf2N− anion between 0.01 M C4mimTf2N solutions in DCE and water.
Figure 1. The effect of DMDCHDGA concentration in the organic phase on the distribution ratio of Tf2N− anion between 0.01 M C4mimTf2N solutions in DCE and water.
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Figure 2. IR spectra of DMDCHDGA (1), IL (2), and 0.1 M DMDCHDGA/IL solution (3). The most intense peak of the ligand (curve 1) is marked with the * symbol on the IR spectrum of the solution.
Figure 2. IR spectra of DMDCHDGA (1), IL (2), and 0.1 M DMDCHDGA/IL solution (3). The most intense peak of the ligand (curve 1) is marked with the * symbol on the IR spectrum of the solution.
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Figure 3. (a,b) Fragments of IR spectra of IL (curve 2) and 0.1 M DMDCHDGA/IL solution (curve 3).
Figure 3. (a,b) Fragments of IR spectra of IL (curve 2) and 0.1 M DMDCHDGA/IL solution (curve 3).
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Figure 4. IR spectra of solutions 0.1 M L/IL (1), 0.5 M DMDCHDGA/IL (2), and equimolar mixture of DMDCHDGA and IL (3).
Figure 4. IR spectra of solutions 0.1 M L/IL (1), 0.5 M DMDCHDGA/IL (2), and equimolar mixture of DMDCHDGA and IL (3).
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Figure 5. Extraction of HTf2N with 0.01 M DMDCHDGA solutions in DCE as a function of equilibrium HTf2N concentration in the aqueous phase.
Figure 5. Extraction of HTf2N with 0.01 M DMDCHDGA solutions in DCE as a function of equilibrium HTf2N concentration in the aqueous phase.
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Figure 6. IR spectrum of the complex DMDCHDGA-HTf2N.
Figure 6. IR spectrum of the complex DMDCHDGA-HTf2N.
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Figure 7. The effect of DMDCHDGA concentration in the organic phase on the distribution ratio of Tf2N− anion between 0.01 M C4mimTf2N solutions in DCE and 0.1 M HNO3 solutions.
Figure 7. The effect of DMDCHDGA concentration in the organic phase on the distribution ratio of Tf2N− anion between 0.01 M C4mimTf2N solutions in DCE and 0.1 M HNO3 solutions.
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Figure 8. The effect of HNO3 and HCl concentration in the aqueous phase on the distribution ratio of Tf2N− anion between 0.02 M C4mimTf2N solutions in DCE and 0.02 M C4mimTf2N solutions in DCE containing 0.02 M DMDCHDGA.
Figure 8. The effect of HNO3 and HCl concentration in the aqueous phase on the distribution ratio of Tf2N− anion between 0.02 M C4mimTf2N solutions in DCE and 0.02 M C4mimTf2N solutions in DCE containing 0.02 M DMDCHDGA.
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Figure 9. The effect of HNO3 concentration− in the aqueous phase on the transfer of Tf2N− ions into the aqueous phase from IL phase in the presence of DMDCHDGA.
Figure 9. The effect of HNO3 concentration− in the aqueous phase on the transfer of Tf2N− ions into the aqueous phase from IL phase in the presence of DMDCHDGA.
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Figure 10. IR spectra of samples 3 (0.1 M DMDCHDGA/IL) and 4 (0.1 M DMDCHDGA/IL//3 M HNO3).
Figure 10. IR spectra of samples 3 (0.1 M DMDCHDGA/IL) and 4 (0.1 M DMDCHDGA/IL//3 M HNO3).
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Figure 11. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA and TODGA solutions in [C4mim][Tf2N] and DCE.
Figure 11. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA and TODGA solutions in [C4mim][Tf2N] and DCE.
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Figure 12. The effect of HNO3 concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N].
Figure 12. The effect of HNO3 concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N].
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Figure 13. The effect of NO3− (HNO3 + NH4NO3) concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N]. [H+] = 2 M.
Figure 13. The effect of NO3− (HNO3 + NH4NO3) concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N]. [H+] = 2 M.
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Figure 14. The effect of H+ concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N]. [NO3−] = 5 M.
Figure 14. The effect of H+ concentrations in the aqueous phase on the extraction of Ln(III) with 0.01 M solutions of DMDCHDGA in [C4mim][Tf2N]. [NO3−] = 5 M.
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Figure 15. The effect of DMDCHDGA concentration in [C4mim][Tf2N] on the extraction of lanthanides(III) from 3 M HNO3 solutions.
Figure 15. The effect of DMDCHDGA concentration in [C4mim][Tf2N] on the extraction of lanthanides(III) from 3 M HNO3 solutions.
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Figure 16. IR spectra of samples 4 (0.1 DMDCHDGA/IL//3 M HNO3) and 5 (0.1 DMDCHDGA/IL//3 M HNO3 + Eu3+).
Figure 16. IR spectra of samples 4 (0.1 DMDCHDGA/IL//3 M HNO3) and 5 (0.1 DMDCHDGA/IL//3 M HNO3 + Eu3+).
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Figure 17. Difference IR spectra of samples 5 and 4 (curve 1) and 7 and 6 (curve 2).
Figure 17. Difference IR spectra of samples 5 and 4 (curve 1) and 7 and 6 (curve 2).
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Figure 18. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA solutions in undiluted ILs.
Figure 18. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA solutions in undiluted ILs.
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Figure 19. The effect of HNO3 concentrations in the aqueous phase on the extraction of Eu(III) with 0.01 M solutions of DMDCHDGA in DCE and DCE containing 0.01 M C4mimTf2N.
Figure 19. The effect of HNO3 concentrations in the aqueous phase on the extraction of Eu(III) with 0.01 M solutions of DMDCHDGA in DCE and DCE containing 0.01 M C4mimTf2N.
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Figure 20. The extraction of lanthanides(III) from 3 M HNO3 and 3 M HCl solutions with 0.01 M DMDCHDGA solutions in DCE and DCE containing 0.01 M C4mimTf2N. At the Ln(III) extraction from 3 M HCl solutions in the absence of C4mimTf2N in the organic phase, the DLn values are <10−2.
Figure 20. The extraction of lanthanides(III) from 3 M HNO3 and 3 M HCl solutions with 0.01 M DMDCHDGA solutions in DCE and DCE containing 0.01 M C4mimTf2N. At the Ln(III) extraction from 3 M HCl solutions in the absence of C4mimTf2N in the organic phase, the DLn values are <10−2.
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Figure 21. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA and TODGA solutions in DCE containing 0.01 M ionic liquids.
Figure 21. The extraction of lanthanides(III) from 3 M HNO3 solutions with 0.01 M DMDCHDGA and TODGA solutions in DCE containing 0.01 M ionic liquids.
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Turanov, A.N.; Karandashev, V.K.; Baulin, V.E.; Shulga, Y.M.; Baulin, D.V. Extraction of Lanthanides(III) from Nitric Acid Solutions with N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide into Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids and Their Mixtures with Molecular Organic Diluents. Minerals 2024, 14, 1167. https://doi.org/10.3390/min14111167

AMA Style

Turanov AN, Karandashev VK, Baulin VE, Shulga YM, Baulin DV. Extraction of Lanthanides(III) from Nitric Acid Solutions with N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide into Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids and Their Mixtures with Molecular Organic Diluents. Minerals. 2024; 14(11):1167. https://doi.org/10.3390/min14111167

Chicago/Turabian Style

Turanov, Alexander N., Vasilii K. Karandashev, Vladimir E. Baulin, Yury M. Shulga, and Dmitriy V. Baulin. 2024. "Extraction of Lanthanides(III) from Nitric Acid Solutions with N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide into Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids and Their Mixtures with Molecular Organic Diluents" Minerals 14, no. 11: 1167. https://doi.org/10.3390/min14111167

APA Style

Turanov, A. N., Karandashev, V. K., Baulin, V. E., Shulga, Y. M., & Baulin, D. V. (2024). Extraction of Lanthanides(III) from Nitric Acid Solutions with N,N′-dimethyl-N,N′-dicyclohexyldiglycolamide into Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids and Their Mixtures with Molecular Organic Diluents. Minerals, 14(11), 1167. https://doi.org/10.3390/min14111167

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