PROCESS FOR THE SYNTHESIS OF THE IONIC LIQUID TETRAOCTYLAMMONIUM DI(2-ETHYLHEXYL)-OXAMATE (IL-5), PRODUCT OBTAINED AND ITS USE IN SELECTIVE METAL EXTRACTION

Abstract

a) The present invention falls within the area of the synthesis of ionic liquids, namely it concerns a process of synthesis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) with a high degree of purity and its use in the extraction and selective separation of metals, namely lanthanides. Thus, it is the object of the present invention, a process for the synthesis of a pure ionic liquid using in its constitution only the elements carbon, hydrogen, oxygen and nitrogen (CHON), assuming itself as a “green” alternative in the recovery of metals, thus reducing the environmental impact in the way they are recovered, as well as the guarantee of a more efficient extraction of these metals.

Claims

1. Process of synthesis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) characterized by comprising the following steps: a) preparation of di(2-ethylhexyl)-methyl oxamate by reacting di(2-ethylhexyl)amine with methyl-oxalyl chloride; b) adding an aqueous solution of sodium hydroxide to di(2-ethylhexyl)-methyl oxamate obtained in step a) dissolved in a non-halogenated organic solvent with some polarity; c) stirring the solution obtained in the previous step at a temperature between 20 and 30° C. and removing the solvent by vacuum, obtaining a solid residue; d) dissolving the residue obtained in the previous step in a short-chain ether; e) simple filtration with filter paper of the solution obtained in the previous step, removal of the solvent from the filtrate by vacuum, obtaining sodium di(2-ethylhexyl)-oxamate as a white solid; f) adding a solution of tetraoctylammonium chloride in water to a solution of sodium di(2-ethylhexyl)oxamate dissolved in an ether prepared from the product obtained in the previous step; g) stirring the solution; h) separation of the aqueous phase from the organic phase; i) washing the organic phase with water; j) removing the solvent from the organic phase under vacuum; k) obtaining the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) as a slightly yellowish viscous liquid.

2. Process according to claim 1, characterized by, in step b), the solvent used is tetrahydrofuran.

3. Process according to claim 1, characterized by, in step c), the reaction mixture is made at a temperature between 20 and 30° C. for 5 days.

4. Process according to claim 1, characterized by, in step d), the ether is diethyl ether.

5. Process according to claim 1, characterized by, in step g), the reaction mixture is carried out at a temperature between 20 and 30° C. for 5 days.

6. Ionic liquid tetraoctylammonium di(2-ethylhexyl-oxamate (IL-5) obtained through a process of synthesis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) comprising the following steps: l) preparation of di(2-ethylhexyl)-methyl oxamate by reacting di(2-ethylhexyl)amine with methyl-oxalyl chloride; m) adding an aqueous solution of sodium hydroxide to di(2-ethylhexyl)-methyl oxamate obtained in step a) dissolved in a non-halogenated organic solvent with some polarity; n) stirring the solution obtained in the previous step at a temperature between 20 and 30° C. and removing the solvent by vacuum, obtaining a solid residue; o) dissolving the residue obtained in the previous step in a short-chain ether; p) simple filtration with filter paper of the solution obtained in the previous step, removal of the solvent from the filtrate by vacuum, obtaining sodium di(2-ethylhexyl)-oxamate as a white solid; q) adding a solution of tetraoctylammonium chloride in water to a solution of sodium di(2-ethylhexyl)oxamate dissolved in an ether prepared from the product obtained in the previous step; r) stirring the solution; s) separation of the aqueous phase from the organic phase; t) washing the organic phase with water; u) removing the solvent from the organic phase under vacuum; v) obtaining the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) as a slightly yellowish viscous liquid. the ionic liquid characterized by having the molecular structure: ##STR00002##

7. Ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) ionic liquid according to claim 6, characterized by having a proton and carbon nuclear magnetic resonance spectrum of .sup.1H NMR (300 MHz, CDCl.sub.3, δ ppm): 3.48-2.92; (m, 4H+8H, H.sup.III from oxamate and H.sup.1 from [N.sub.8888].sup.+), 1.71-1.50; (m, 2H+8H, H.sup.IV from oxamate and H.sup.2 from [N.sub.8888].sup.+), (m, 16H+40H, H.sup.V-VII, IX from oxamate and H.sup.3-7 from [N.sub.8888].sup.+), 0.76-0.93; (m, 12H+12H, H.sup.VIII,X from oxamate and H.sup.8 from [N.sub.888].sup.+); .sup.13C NMR (75 MHz, CDCl.sub.3, δ ppm); 172.29 (C.sup.I), 169.52 (C.sup.II), 58.91 (C.sup.1), 50.95, 50.80, 44.89, 44.65 (C.sup.III), 37.19, 37.13, 36.61, 36.52 (C.sup.IV), 31.80 (C.sup.6), 30.78, 30.76, 30.72, 30.63 (C.sup.V), 29.25, 29.14 (C.sup.4,5), 29.10, 29.03, 28.99 (C.sup.IX), 26.43 (C.sup.2,3), 23.93, 23.80, 23.68, 23.22, 23.19, 23.16, 22.70, 22.23 (C.sub.X) 14.27 14.25, 14.23 (C.sup.VIII), 14.15 (C.sup.8), 11.21, 11.18, 10.90 (C.sup.X) 10.85.

8. Ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) according to claim 6 characterized by its total thermal decomposition at 270° C.

9. Use of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) according to claim 6 in the selective extraction of lanthanides.

Description

DESCRIPTION OF THE FIGURES

[0022] FIG. 1—representation of the schematic formula of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5).

[0023] FIG. 2—representation of the proton nuclear magnetic resonance spectrum, .sup.1H NMR, of the ionic liquid tetraoctylamonium di(2-ethylhexyl)-oxamate in CDCl.sub.3.

[0024] FIG. 3—representation of the carbon 13 nuclear magnetic resonance spectrum, .sup.13C NMR, of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate in CDCl.sub.3.

[0025] FIG. 4—representation of the absorption spectrum in the infrared region of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0026] FIG. 5—graphical representation of the differential thermal analysis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0027] FIG. 6—graphical representation of the thermogravimetric analysis of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0028] FIG. 7—graphical representation of the percentage of extraction of each lanthanide using the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0029] FIG. 8—graphical representation of the percentage of extraction of each lanthanide as a function of the variation in the mixing time using the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0030] FIG. 9—graphical representation of the percentage of extraction of each lanthanide as a function of pH variation using the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0031] FIG. 10—graphical representation of the percentage of extraction of each lanthanide as a function of the variation of the ionic strength of the medium using the ionic liquid tetraoctylamonium di(2-ethylhexyl)-oxamate.

[0032] FIG. 11—graphical representation of the percentage of extraction of each lanthanide as a function of the variation in the ratio between the ionic liquid and the existing metal using the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate.

[0033] FIG. 12—graphical representation of the percentage of retroextraction of each lanthanide to an aqueous phase using acidic solutions of HNO.sub.3.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The synthesis process of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) comprises the following steps: [0035] a) preparation of di(2-ethylhexyl)-methyl-oxamate by reacting di(2-ethylhexyl)amine with methyl-oxalyl chloride; [0036] b) addition of an aqueous solution of sodium hydroxide to di(2-ethylhexyl)-methyl-oxamate, obtained in step a), dissolved in a non-halogenated organic solvent with some polarity; [0037] c) stirring the reaction mixture obtained in the previous step at a temperature between 20 and 30° C. and subsequent removal of the solvent by vacuum, obtaining a solid residue; [0038] d) dissolving the solid residue obtained in the previous step in a short chain ether; [0039] e) simple filtration with filter paper of the solution obtained in the previous step and removal of the solvent from the filtrate by vacuum, obtaining sodium di(2-ethylhexyl)-oxamate as a white solid; [0040] f) addition of an aqueous solution of tetraoctylammonium chloride to an ether solution of sodium di(2-ethylhexyl)oxamate prepared from the product obtained in the previous step; [0041] g) stirring the solution; [0042] h) separation of the aqueous phase from the organic phase; [0043] i) washing the organic phase with water to completely remove salts and new separation of the two phases; [0044] j) removal of the solvent from the organic phase under vacuum; [0045] k) obtaining the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5) in the form of a slightly yellowish viscous liquid.

[0046] Di(2-ethylhexyl)-methyl-oxamate is the starting compound for the synthesis of the IL-5 ionic liquid, object of the present invention. It is a reagent not commercially available in the chemical industry, so its preparation is carried out by reacting di(2-ethylhexyl)amine with methyl-oxalyl chloride as described on page 200 of the PhD thesis of Axel Braam, PhD thesis, Philips-Universitat Marburg, 2015.

[0047] In a preferred mode of the invention the solvent used in step b) is tetrahydrofuran, since through the use of this solvent a higher yield is obtained.

[0048] In another preferred mode of the invention the reaction mixture described in step c) is made at a room temperature between 20 and 30° C. for 5 days.

[0049] In another preferred mode of the invention, the ether used in step d) is diethyl ether since a higher yield is obtained through the use of this type of ether.

[0050] In step e) of the process described above, simple filtration with filter paper is performed in order to remove the non-soluble fraction and then the ether of the filtrate is removed by vacuum to obtain sodium di(2-ethylhexyl)-oxamate.

[0051] In another preferred mode of the invention the reaction mixture described in step g) is made at a room temperature between 20 and 30° C. for 4 hours.

[0052] From the point of view of its use, the present invention, presents a more efficient use and a more efficient recovery of the lanthanides through a great selectivity of extraction among the several lanthanides.

[0053] This selectivity is due to the type of coordination that the anion of the ionic liquid establishes with the different metals.

Example 1

[0054] Di(2-ethylhexyl)-methyl-oxamate was prepared as described on page 200 of the Doctoral thesis by Axel Braam, PhD thesis, Philips-Universitat Marburg, 2015. A sodium hydroxide solution (0.235 g; 59 mmol) in 7.5 ml of ultrapure Millipore water (ISO 3696) was slowly added to a solution of di(2-ethylhexyl)-methyl oxamate (1.542 g; 47 mmol) dissolved in tetrahydrofuran (7.5 ml). The reaction mixture was stirred at room temperature for 5 days. After stirring the reaction mixture, the solvent was removed under vacuum and a solid residue was obtained, which was dissolved in 15 ml of diethyl ether and filtered. The solvent was again removed under vacuum to obtain a white solid with a yield of 79% (1.35 g) compared to the initial di(2-ethylhexyl)-methyl-oxamate. Subsequently, tetraoctilammonium chloride (1.802 g, 36 mmol) dissolved in Millipore water (90 mL) was slowly added to a solution of sodium di(2-ethylhexyl)oxamate (1.253 g, 37 mmol) in diethyl ether (125 mL) and the previous solution was vigorously stirred for 4 hours at room temperature. After stirring, the aqueous phase was separated from the organic phase, washing the latter with Millipore ultra-pure water (1×50 mL). The organic solvent was removed under vacuum and the desired ionic liquid was obtained as a slightly yellowish viscous liquid with a yield of 84% (2.412 g) relative to the sodium salt used.

[0055] The ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5), object of the present invention obtained by the described process has the following molecular structure:

##STR00001##

[0056] In addition, the ionic liquid IL-5 was characterized through various analytical techniques to ascertain its purity and verify some of its fundamental properties for different types of use.

[0057] The degree of purity of the IL-5 obtained allows to obtain non contaminated extracts and thus to clearly analyze its performance in the extraction of lanthanides.

[0058] Proton (FIG. 2) and carbon (FIG. 3) nuclear magnetic resonance spectra were performed, of which the most relevant data are presented below: .sup.1H NMR (300 MHz, CDCl.sub.3, δ ppm): 3.48-2.92; (m, 4H+8H, H.sup.III from oxamate and H.sup.1 from [N.sub.8888].sup.+), 1.71-1.50; (m, 2H+8H, H.sup.IV from oxamate and H.sup.2 from [N.sub.8888].sup.+), (m, 16H+40H, H.sup.V-VII,IX from oxamate and H.sup.3-7 from [N.sub.8888].sup.+), 0.76-0.93; (m, 12H+12H, H.sup.VIII,X from oxamate and H.sup.8 from [N.sub.8888].sup.+); .sup.13C NMR (75 MHz, CDCl3, δ ppm); 172.29 (C.sup.I), 169.52 (C.sup.II), 58.91 (C.sup.1), 50.95, 50.80, 44.89, 44.65 (C.sup.III), 37.19, 37.13, 36.61, 36.52 (C.sup.IV), 31.80 (C.sup.6), 30.78, 30.76, 30.72, 30.63 (C.sup.V), 29.25, 29.14 (C.sup.4,5), 29.10, 29.03, 28.99 (C.sup.IX), 26.43 (C.sup.2,3), 23.93, 23.80, 23.68, 23.22, 23.19, 23.16, 22.70, 22.23 (C.sup.7+C.sup.VI,VII) 14.27 14.25, 14.23 (C.sup.VIII), 14.15 (C8), 11.21, 11.18, 10.90 (C.sup.X) 10.85.

[0059] These data make it possible to identify the different magnetic resonances corresponding to the different hydrogen and carbon atoms in the ionic liquid, all of which are assigned according to the numbering in FIG. 1.

[0060] Thus, from the analysis carried out to the proton and carbon nuclear magnetic resonance spectra, it appears that there are no other signals in the spectra, so that the IL-5 ionic liquid synthesized by the previously described process is obtained as a pure compound.

[0061] The infrared spectrum of the compound under study is also presented (FIG. 4). This spectrum clearly identifies the functional groups existing in the ionic liquid and together with the nuclear magnetic resonance spectra presented before, prove the purity of the compound.

[0062] Differential thermal analysis (FIG. 5) and thermogravimetric analysis (FIG. 6) of the ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate were also performed.

[0063] The first analysis also proves that the product is obtained pure by this synthesis process, since it does not contain in the studied temperature range (−80° C. to 80° C.) any other detectable compound nor does it show any phase change. The thermogravimetric analysis also shows that the compound is pure because its thermal decomposition does not leave any residue and in addition guarantees that at 270° C. the decomposition of the ionic liquid is total. These analyses ensure that its decomposition does not leave a residue, which together with the fact that its constitution contains only carbon, hydrogen, oxygen and nitrogen (LHON), guarantees that all ionic liquid is transformed by low temperature combustion into volatile compounds with low environmental impact.

[0064] The ionic liquid tetraoctylammonium di(2-ethylhexyl)-oxamate (IL-5), object of the present invention, obtained by the described process has favorable stereochemical characteristics for its use in the selective separation of metals and among these rare earths (lanthanides).

[0065] The following example exemplifies the use of the ionic liquid IL-5 in the selective separation of lanthanides.

Example 2

[0066] An aqueous solution was prepared containing several lanthanides with an approximate content of 100 ppm for each lanthanide and a pH of about 4. The initial concentration of each lanthanide in the solution was measured by inductively coupled plasma mass spectrometry (ICP-MS) as shown in table 1. Selective separation was carried out by adding 1 ml of a solution of toluene with 400×7 ppm of ionic liquid tetraoctylamonium di(2-ethylhexyl)-oxamate (IL5) to 1 ml of the solution of lanthanides previously prepared and stirred on a vortex mixer for 15 minutes, followed by centrifugation for an additional 5 minutes. At the end, the organic phase was separated from the aqueous phase and the final concentration of the various lanthanides in the aqueous phase was measured using ICP-MS, the percentage of extraction of each one of the lanthanides was calculated. The results are presented in Table 1 and illustrated in FIG. 7. The studied lanthanides uniformly cover the entire series of them and therefore the conclusions drawn here can be generalized to all lanthanides.

TABLE-US-00001 TABLE 1 Results of the percentage of extraction of each lanthanide according to the described process. Ppm Ce Nd Sm Gd Dy Er Yb Initial 100.70 100.10 102.00 88.80 95.30 100.20 103.60 value Final 99.60 96.50 77.00 56.40 28.00 18.00 9.50 value % 1.09 3.60 24.51 36.49 70.62 82.04 90.83 extraction

[0067] The extraction efficiency was studied as described in example 2, varying the extraction contact time, that is, vortex agitation time, with agitation times of 5, 10, 15 and 30 minutes being studied. The analysis of the results obtained represented in FIG. 8 shows that there is no significant difference in the percentage of extraction with this variable.

[0068] The efficiency of the extraction was studied as described in example 2, varying the value of the acidity of the medium (pH), having been studied solutions with a pH of 2, 4 and 6. The analysis of the results obtained represented in FIG. 9 demonstrate that the extraction is most effective with a pH medium of 4.

[0069] The extraction efficiency was further studied as described in example 2, varying the ionic strength of the medium, with four aqueous solutions being studied, three of which with different amounts of sodium nitrate, which are:

[0070] 0.0085 g (1×10 .sup.−4 mols), 0.0425 g (5×10 .sup.−4 mols) and 0.085 g (1×10.sup.−3 mols).

[0071] The extraction efficiency was studied as described in example 2, varying the ratio between the ionic liquid and the existing metals, in the molar ratio ionic liquid/metal of 2:1 and 4:1.

[0072] After a selective extraction to a non-aqueous phase containing the ionic liquid as described above, it is important to check whether it is possible to put the extracted metals back into the aqueous phase, thus completing the recovery cycle. For this purpose, an extraction of the toluene solution obtained in the process described in example 2 was carried out using nitric acid in three different concentrations: 0.5 M, 1 M and 2 M to see how the concentration influenced this retroextraction. The obtained results are shown in FIG. 12.