Ionic Liquids - American Chemical Society

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Chapter 5

Ionic Liquids as Novel Diluents for Solvent Extraction of Metal Salts by Crown Ethers Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch005

Richard A . Bartsch, Sangki Chun, and Sergei V . Dzyuba Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, T X 79490-1061

The efficiency and selectivity in solvent extraction of alkali metal salts, alkaline earth metal salts and lead salts from aqueous solutions into solutions of dicyclohexano-18-crown-6 (DC18C6) in 1-alkyl-3-methylimidazoliun hexafluorophosphates, [C -mim]PF , have been determined. For competitive extractions of alkali metal salts and alkaline earth metal salts and single cation extractions of lead salts, the influence o f systematically varying n in the room temperature ionic liquid from 4 to 9 and of anion variation in the metal salts is assessed. n

6

Room temperature ionic liquids (RTILs) are receiving intense attention as solvents for a wide variety of organic reactions (7-5). Applications of RTDLs in separations processes has received much less attention (4-11). In solvent extraction, the negligible vapor pressure and low flammability o f RTILs are important advantages over conventional organic diluents. In metal ion separations, Dai and co-workers (7) observed large distribution coefficient values for extraction of strontium nitrate from aqueous solutions into solutions

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© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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DC18C6

[C -mim]PF n

6

Figure 1. Structures ofdicyclohexano~l8-crown-6 and 1 -alkyl-3-methylimidazolium hexafluorophosphates. of dicyclohexano-18-crown-6 (DC18C6) (Figure 1) in disubstituted imidazolium hexafluorophosphates and bis[(trifluoromethane)sulfonyl]amides. Subsequently, Rogers and co-workers (9) reported the extraction of sodium, cesium and strontium nitrates from aqueous solutions into 18-crown-6, DC18C6 and 4,4 (5 )-di-(^r/-butylcyclohexano>18-crown-6 in 1-butyl, 1-hexyl- and 1-octyl3-methylimidazolium hexafluorophosphates. Very recently, "task specific" RTILs containing metal ion chelating units and their use in the solvent extraction o f C d and H g were described by Rogers and co-workers (10). To probe the influence of systematic structural variation within RTILs upon metal salt extraction by DC18C6, we developed an improved preparation of 1 -alkyl-3-methylimidazolium hexafluorophosphates, [C -mim]PF , with η « 4-9 (Figure 1). The influence of this structural variation within the R T I L and of the anion of the metal salts upon the efficiency and selectivity in competitive alkali metal salt and alkaline earth metal salt extractions by DC18C6 was determined. In addition, the single cation solvent extraction of lead salts by DC18C6 in the RTILs was investigated. ,

,

2

2 +

n

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Improved Synthesis of l-Alkyl-3-methylimidazolium Hexafluorophosphates Although [C -mim]PF RTELs have been utili ed as solvents in many processes (i-5), no convenient method has been reported for the preparation of such solvents in large quantities and with sufficient purity for analytical studies. We have developed a convenient, two-step procedure for the synthesis of [C -mim]PF6 (Figure 2). In the first step, the neat reaction of 1-methylimidazole and equimolar 1-bromoalkane at 140 °C gave an essentially quantitative yield of ft

6

n

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

60 the [C-mim]Br (II). A n aqueous solution of the imidazolium bromide in a plastic bottle was stirred in an ice bath and one equivalent of 60 % aqueous hexafluorophosphoric acid was added slowly. After stirring in the ice bath and then at room temperature, the mixture was transferred to a separatory funnel containing water and one equivalent of triethylamine. The [C -mim]PF was separated, washed with water, and dissolved in dichloromethane. The solution was washed with water and the dichloromethane was evaporated in vacuo. Traces o f water were removed from the residue by azeotropic distillation with benzene using a Dean-Stark trap. The benzene was evaporated in vacuo and the resultant oil was dried at elevated temperature in vacuo. The overall yields of [C -mim]PF with η = 4-9 were 64-87 % for the two-step process and increased as the 1-alkyl group was elongated (12). n

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n

6

6

Br (95-100%)

PF

6

(64-87%)

Figure 2. Synthesis of l-alkyl-3~methylimidazolium hexafluorophosph

Solvent Extraction of Metal Salts by Dicydohexano-18-crown-6 in l-Alkyl-3-methylimidazoliuin Hexafluorophosphates Competitive Alkali Metal Salt Extractions To compare with solvent extraction results obtained for the RTILs, competitive solvent extractions of alkali metal chlorides by DC18C6 were performed in the frequently encountered diluents of chloroform, nitrobenzene and 1-octanol. From contact of 5.0 m L of 2.0 m M (in each) aqueous solution o f the five alkali metal chlorides with 2.0 m L of 20 m M DC18C6 in the diluent, extraction of the alkali metal chlorides into the organic phase was undetectable (12). This low extraction efficiency is attributed to the high hydration energy of chloride ion (13). Results from competitive solvent extractions of 2.5 m L of 2.0 m M (in each) aqueous solutions of the five alkali metal chlorides with 1.0 gram of 20 m M solutions of DC18C6 in [C -mim]PF with η = 4-9 are presented in Figure 3. The level of extraction is given as the percentage of the crown ether that is complexed (loaded) by the metal ion. Since its loading did not reach 1 %, points n

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In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

61 +

for L i are omitted from the graph. (The data are corrected for the levels of alkali metal cations that are extracted by the RTE, in the absence of DC18C6.) Immediately apparent is the very appreciable extraction of alkali metal cations by DC18C6 in the RTIL diluents under conditions that gave no measurable alkali metal cation extraction into chloroform, nitrobenzene or 1-octanol (72).

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η

Figure 3. Influence of η variation in competitive alkali metal cation extractionfromaqueous solutions by DC18C6 in [C -mim]PF n

&

With the exception of [Cô-mimJPFô, there is a smooth decrease in the extraction efficiency for each alkali metal cation species as the 1-alkyl group is elongated. (Unusual behavior of [C -mim] has been noted previously by others (14).) Thus, increasing the lipophilicity of the RTBL decreases the propensity for alkali metal cation extraction by DC18C6. The observed extraction selectivity of K > Rb > Cs > Na > L i is consistent with die relative complexing abilities of 18-crown-6 ligands for the alkali metal cations (15). The influence of varying the 1-alkyl group of the RTIL diluent on the K /Cs and K /Rb selectivities for competitive alkali metal cation extractions +

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In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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62 by DC18C6 is shown in Figure 4. With the exception o f [C -mim]PF , the K / C s selectivity and, to a lesser extent, the K / R b selectivity increase as the 1alkyl group of the R T I L is elongated. Thus, the decrease in extraction efficiency as the 1-alkyl group of the RTIL is lengthened (Figure 3) is coupled with an increase in extraction selectivity (Figure 4). 6

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η

Figure 4. Influence of η variation upon K^/Cs* and K^/Rb* selectivities in competitive alkali metal cation extraction by DC18C6 in [C -mim]PF n

&

To evaluate the counterion influence in extractions involving RTIL diluents, competitive extractions o f alkali metal chlorides, nitrates and sulfates from aqueous solutions by DC18C6 in [Cg-mim]PF were performed. A comparison of the crown ether loadings for the five alkali metal cations as the anion was varied from chloride to nitrate to sulfate is presented in Table L It is immediately apparent that the efficiencies and selectivities of alkali metal cation extraction are unaffected by variation of the aqueous phase anion. This differs markedly from results reported for competitive alkali metal salt extractions by DC18C6 in chloroform and 1-octanol (16,17). For these molecular solvents, the metal ion loading was strongly affected by anion variation. This further 6

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

63 Table I. Effect of Anion on Loading for Competitive Solvent Extractions of Alkali Metal Salts from Aqueous Solutions by DC18C6 in [C -mim | P F 8

Loading m Cs r 13.6 4.8 22.8 4.6 13.2 22.3 13.6 22,7 5.0 with a standard deviation o f ± 0.3 %. +

Anion LC Na chloride 0.2 0.4 nitrate 0.2 0.4 0.2 sulfate 1,0 N O T E : Average from triplicate extractions

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underscores the unusual characteristics o f RTILs as diluents in solvent extraction. Competitive Alkaline Earth Metal Salt Extractions For comparison with solvent extraction results obtained for the RTELs, competitive solvent extractions of four alkaline earth metal chlorides by DC18C6 were conducted in the molecular solvents of chloroform, nitrobenzene and 1-octanol. After contact of 5.0 m L of 2.0 m M (in each) aqueous solution o f the four alkaline earth metal chlorides with 2.0 m L of 20 m M DC18C6 in the diluent, no extraction of alkaline earth metal cations into the organic phase was evident. Results from competitive solvent extractions of 2,5 m L of 2.0 m M (in each) aqueous solutions of the four alkaline earth metal chlorides with 1.0 gram of 20 m M solutions o f DC18C6 in [C„-mim]PF with η = 4-9 are presented in Figure 5. Since its loading did not reach 1 %, points for M g are omitted from the graph. A s can be seen, very appreciable levels o f Sr and B a extraction are observed for the RTIL diluents; whereas there was no observable extraction of any alkaline earth metal cation with DC18C6 in the molecular solvents. For R a and S i , there is a pronounced decrease in die extraction efficiency as the 1-alkyl group in the RTIL is elongated. Increasing the lipophilicity of the RTIL markedly diminishes the alkaline earth metal cation extraction efficiency. Sensitivity of the extraction efficiency for the alkaline earth metal cations (Figure 5) is much greater than that for alkali metal cations (Figure 3). The observed extraction of B a > S r » C a > M g is in agreement with the relative complexing abilities of 18-crown-6 ligands for the alkaline earth metal cations (75). The influence of varying the 1-alkyl group in the R T I L diluent on the Ba /Sr selectivity is shown in Figure 6. (With η > 7, the loading with S r was too low to accurately determine the selectivity ratio.) A s was noted earlier for the alkali metal cation extractions, the loss in extraction efficiency with elongation o f the 1-alkyl group (Figure 5) is coupled with increased selectivity (Figure 6). In Table II is shown the effect of varying the anion of the metal salt in competitive extractions o f alkaline earth metal chlorides, nitrates and perchlorates from aqueous solutions into DC18C6 solutions of [C -mim]PF . A s 6

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In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Table II. Effect of Anion on Loading for Competitive Solvent Extractions of Alkaline Earth Metal Salts front Aqueous Solutions by DC18C6 in [C -mint]PF« 4

Anion

Mgl

chloride nitrate perforate NOTE: Average

QsL

Slfl

ML—

0 0 20.3 29.6 0.4 0.6 21.1 29.6 Q.l 0_4 212 22J from triplicate extractions with a standard deviation of ± 0.3 %.

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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6

4

5

η

6

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Figure 6. Influence of η variation on selectivity in solvent extraction of alkaline earth metal cations from aqueous solutions by DC18C6 in [C^mimJPF^.

was observed for competitive extractions of alkali metal cations by DC18C6 in [C8-mim]PF (Table I), variation of the aqueous phase anion has no effect on the efficiency or selectivity of competitive alkaline earth metal ion extraction by DC18C6 in [C -mim]PF . 6

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Lead Salt Extractions 2+

DC18C6 has been utilized for complexation o f Pb in homogeneous media (IS) and transport of lead salts across synthetic membranes (18). For comparison with the solvent extractions results obtained for the RTILs, single metal salt extractions of lead nitrate by DC18C6 with molecular solvents of chloroform, nitrobenzene and 1-octanol ware performed. From contact of 5.0 m L o f 2 0 m M aqueous solutions of lead nitrate with 2.0 m L o f 5.0 m M

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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DC18C6 in the diluent, Pb loading was 1,1 % with nitrobenzene and less with chloroform and 1-octanol. Résulte from single cation solvent extractions o f 2.5 m L of 2.0 m M aqueous solutions of lead chloride, nitrate and perchlorate with 1.0 gram of 5.0 m M solutions of DC18C6 in [C -mim]PF with η = 4-8 are presented in Figure 7. (The data arc corrected for the levels of lead salt extraction by die R T I L in the absence o f DC18C6.) Since the formation o f precipitates was noted for attempted extractions of lead chloride and lead nitrate by DC18C6 in [C R b > C s > N a > L i . The extraction efficiency decreases, but the selectivity is enhanced, when η in [C -mim]PF is increased. For competitive extractions from aqueous solutions containing four alkaline earth metal chlorides, die selectivity is B a > S r » C a > M g . Again the extraction efficiency is observed to decrease, but the selectivity increases, when the 1-alkyl group in the RTIL is elongated. Solutions of DC18C6 in RTILs also effectively extract P b . In this case, the extraction efficiency is found to depend upon the anion of the lead salt with a higher level of lead chloride extraction than for lead nitrate and perchlorate. Further investigation of the applications of RTILs in separation processes is definitely warranted. n

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Acknowledgment This research was supported by a grant from the Texas Higher Education Coordinating Board Advanced Research Program.

References 1. Seddon, K. R. J. Chem. Tech. Biotechnol. 1997, 68, 351-356. 2. Welton, T. Chem. Rev. 1999, 112, 3926-3945. 3. Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. Engl. 2000, 39, 37723789. 4. Huddleston, J. G.; Willauer, H . D.; Swatlowski, R. P.; Rogers, R. D . J. Chem. Soc., Chem. Commun. 1998, 1765-1766.

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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68 5. Blanchard, Α.; Hancu, D.; Beckman, Ε. J.; Brennecke, J. F. Nature, 1999, 399, 28-29. 6. Armstrong, D . W.; He, L . ; Liu, Y.-S. Anal. Chem. 1999, 71, 3873-3878. 7. Dai, S.; Ju, Y. H . ; Barnes, C . E . J. Chem. Soc., Dalton Trans. 1999, 12011202. 8. Visser, A. E . ; Swatloski, R. P.; Rogers, R. D . Green Chem. 2000, 1, 1-4. 9. Visser, A. E . ; Swatloski, R. P.; Reichert, W . M.; Griffin, S. T.; Rogers, R. D . Ind. Eng. Chem. Res., 2000, 39, 3596-3606. 10. Visser, A. E . ; Swatloski, R. P.; Reichert, W . M.; Mayon, R.; Sheff, S.; Wierzbick, Α.; Davis, J. H . , Jr., Rogers, R. D . J. Chem. Soc., Chem. Commun. 2001, 135-136. 11. Dzyuba, S. V.; Bartsch, R. A. J. Heterocycl. Chem. 2001, 38, 265-268. 12. Chun, S.; Dzyuba, S. V.; Bartsch, R. Α., Anal. Chem., 2001, 73, 3737-3741. 13. Smith, D . W. J. Chem.Educ.1977, 54, 540. 14. Carmichael, A. J.; Seddon, K. R. J. Phys.Org.Chem. 2000, 13, 591-595. 15. Izatt, R. M.; Pawlak, K . ; Bradshaw, J. S. Chem. Rev. 1991, 91, 1721-2086. 16. Hankins, M. G.; K i m , Y. D.; Bartsch, R. A. J. Am. Chem. Soc. 1993, 115, 3370-3371. 17. Hankins, M. G.; Bartsch, R. Α.; Olsher, U. Solv. Extr. Ion Exch. 1995, 13, 983-995. 18. Schow, A. J.; Peterson, R. T.; Lamb, J. D . J. Membr. Sci. 1996, 111, 291295.

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.