Ether-Functionalized Imidazolium Hexafluorophosphate Ionic Liquids

Ind. Eng. Chem. Res. 2007, 46, 7389-7392

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Ether-Functionalized Imidazolium Hexafluorophosphate Ionic Liquids for Improved Water Miscibilities Henri S. Schrekker,*,†,‡ Marcelo P. Stracke,† Clarissa M. L. Schrekker,† and Jairton Dupont*,† Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gonc¸ alVes 9500, Porto Alegre-RS, CEP:91501-970, P.O. Box 15003, Brazil, and Laboratory of Technological Processes and Catalysis, Institute of Chemistry, UFRGS, AV. Bento Gonc¸ alVes 9500, Porto Alegre-RS, CEP:91501-970, P.O. Box 15003, Brazil

The hexafluorophosphate anion of 1-alkyl-3-methylimidazolium ionic liquids is responsible for their poor water miscibility. Transformation of the hexafluorophosphate ionic liquids into water miscible liquids by structural modifications in the imidazolium cation could result in new applications. Improved water miscibilities were achieved with 1-alkyl ether-3-methylimidazolium hexafluorophosphate ionic liquids. Especially, ionic liquid 1-triethylene glycol monomethyl ether-3-methylimidazolium hexafluorophosphate 5 showed a strongly increased water miscibility range at 30 °C, which was further enhanced in the presence of ethanol as cosolvent. Besides, a complete water miscibility of 5 was observed at an elevated temperature of 50 °C. This knowledge may facilitate the predictive development of new task-specific ionic liquids. 1. Introduction Ionic liquids (ILs) that are liquid at or below 25 °C are a special class of ILs and are referred to as room-temperature ionic liquids (RTILs).1 In general, ILs consist of a large organic cation together with an organic or inorganic anion. Especially, the class of imidazolium cation-based ILs has proven to be highly attractive and versatile. Frequently encountered favorable characteristics of imidazolium ILs are, for instance, high thermal stability, being liquid over a wide temperature range, air and moisture stability, very low vapor pressure, wide electrochemical window, high conductivity and ionic mobility, easy recycling, and being a good solvent for a wide variety of organic and inorganic chemical compounds.1-5 Besides, imidazolium ILs are “designable” because structural modifications in both the cation (especially the 1 and 3 positions of the imidazolium ring) and anion permit the tuning of properties like, e.g., miscibility with water and organic solvents, melting point, and viscosity.1 As a result, applications of imidazolium ILs are numerous and found in the fields of extraction and separation processes,3,6,7 synthetic chemistry,2,3 catalysis (organometallic2,4,8,9/transitionmetal nanoparticle8-13/bio),14 materials science,3,15 and electrochemistry.16,17 The use of imidazolium ILs in combination with water can provide beneficial circumstances. Imidazolium ILs are being adopted as replacements for volatile organic solvents.1 Water is, without a doubt, the most green solvent. As a consequence, an important property for the design of new processes with ILwater mixtures is their miscibility.18-22 Dependent on the application, either a monophasic or a biphasic system could be desired. Mainly the 1-alkyl-3-methylimidazolium ILs have been studied to understand the relation between structural modifications and water miscibility.23-27 This relation is characterized by a highly pronounced anion effect. Halide-, nitrate-, etha* Corresponding authors. E-mail: [email protected] (H.S.S.); [email protected] (J.D.). Tel.: +55-51-3308-6284 (H.S.S.); +55-513308-6321 (J.D.). Fax: +55-51-3308-7304 (H.S.S.); +55-51-3308-7304 (J.D.). † Laboratory of Molecular Catalysis, Institute of Chemistry. ‡ Laboratory of Technological Processes and Catalysis, Institute of Chemistry.

noate-, and trifluoroacetate-based 1-alkyl-3-methylimidazolium ILs are fully miscible with water at ambient temperature. However, water miscibility is drastically reduced to immiscible for their “hydrophobic” hexafluorophosphate- and bis(trifluoromethanesulfonyl)imide-equivalents, including IL 1 (Figure 1).28,29 An intermediate behavior has been observed for the tetrafluoroborate ILs.18 Their miscibility can be fine-tuned by the alkyl chain length of the imidazolium cation. Tetrafluoroborate ILs with a short 1-alkyl chain (up to butyl) are completely miscible, and those with a longer 1-alkyl chain form biphasic systems. Biphasic IL-water systems have been the basis for innovative liquid-liquid separation and extraction techniques.3,6,7 Interestingly, alcohols like ethanol have a strong cosolvent effect, which furnishes fully miscible ternary IL-water-ethanol mixtures.30,31 However, the use of a cosolvent could be undesired and is not necessarily a general solution for all “hydrophobic” anionspecific (e.g., hexafluorophosphate and bis(trifluoromethanesulfonyl)imide) applications that require monophasic IL-water mixtures. Avoidance of a cosolvent would require the transformation of a hydrophobic IL into a hydrophilic one by structural modifications in the imidazolium cation. ILs that contain a specific functionality covalently incorporated in either the cation or the anion are denominated task-specific ILs.32-35 This task-specific IL approach is the reason for the rapidly expanding application scope of ILs. For example, improved HgCl2 solubilities and extractions of HgCl2 from aqueous solutions have been reported for ethylene glycol functionalized imidazolium ILs (Figure 1: cations [C2O1MIm], [C3O1MIm], and [C5O2MIm]) and ethylene glycol bridged bis-imidazolium ILs (Figure 1: cation [MImC6O2MIm]) when compared to their alkyl analogs.36,37 The ethylene glycol spacer of bis-imidazolium bis(trifluoromethanesulfonyl)imide IL 6 (Figure 1) was responsible for an increased saturating water content.37 An increased polarity and a slightly higher saturating water content were determined for the monoethylene glycol monomethyl etherfunctionalized bis(trifluoromethanesulfonyl)imide IL 3b (Figure 1).25,38 Branco et al. reported about the full miscibility of the monoethylene glycol- and monoethylene glycol monomethyl ether-functionalized hexafluorophosphate ILs 2 and 3a in water (Figure 1).36 However, attachment of the longer diethylene

10.1021/ie0709685 CCC: $37.00 © 2007 American Chemical Society Published on Web 09/26/2007

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Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 Table 1. Solubility of Water in Imidazolium Hexafluorophosphate ILs entry

IL [cation]a

Tb (°C)

EtOH (µL)

sol. %c (v/v)

sol. %c,d (w/w)

H2O MFe

1 2 3 4 5 6 7

1 [C4MIm] 1 [C4MIm] 3a [C3O1MIm] 3a [C3O1MIm] 5 [C7O3MIm] 5 [C7O3MIm] 5 [C7O3MIm]

30 50 30 50 30 30 50

0 0 0 0 0 90 0

3.5 5.3 12.8 17.1 29.4 66.7f miscg

2.6 3.9 9.2 12.5 23.3 59.4f miscg

0.29 0.38 0.59 0.69 0.86 0.97f miscg

a Ionic liquid (1.0 mL). b Temperature. c Solubility. d Density: 1 ) 1.37;9a 3a ) 1.45;17 5 ) 1.37.17 e Mole fraction. f Ethanol not included. g Fully miscible.

Figure 1. Imidazolium Ionic Liquids with “Hydrophobic” Anions.

glycol monomethyl ether functionality resulted in a drastically reduced solubility of IL 4 in water (Figure 1). This supported us to study the water miscibilities of the 1-alkyl and 1-alkylether3-methylimidazolium hexafluorophosphate ILs 1, 3a, and 5 (Figure 1). Interestingly, a strongly enhanced hydrophilic nature was observed for the triethylene glycol monomethyl etherfunctionalized hexafluorophosphate IL 5. 2. Experimental Section All experiments were carried out under an argon atmosphere in dried glassware using standard Schlenk, syringe, and septa techniques. Ethanol 96% (v/v) was purchased from VETEC Quı´mica Fina LTDA and used without further purification. Deionized water was used from Easy pure LF. Procedures reported previously in the literature were used for the synthesis of 1,39 3a,36,40-42 and 5,40-42 and the spectral data were in accordance with the literature data. 2.1. Determination of the Solubility of Water in the Hexafluorophosphate Ionic Liquids 1, 3a, and 5 and the Solubility of 5 in Water. Water was added under magnetic stirring to 1, 3a, and 5 (1.0 mL) at 30 or 50 °C until phase separation occurred. The addition of water under magnetic stirring at 30 °C to ionic liquid 5 was continued until the phase separation disappeared. The endpoints were visually determined. 2.2. Determination of the Solubility of Water in Ionic Liquid 5 in the Presence of Ethanol. Water was added under magnetic stirring to a mixture of 5 (1.0 mL) and ethanol (90 µL) at 30 °C until phase separation occurred. The endpoints were visually determined. 2.3. Determination of the Solubility of the Hexafluorophosphate Ionic Liquids 1 and 3a in Water in the Presence of Ethanal. Ethanol was added under magnetic stirring to a biphasic system of 1, 3a, and 5 (1.0 mL) and water (6.7 mL) at 30 °C until the phase separation disappeared. The endpoints were visually determined. 3. Results and Discussion The imidazolium ILs 1, 3a, and 5 (Figure 1) were prepared in a straightforward manner and produced high yields, using a simple and practical method for the synthesis of halide-free

ILs.36,39,40 A synthetic procedure that avoids the presence of halide impurities is important because these exert a strong influence on the physical properties of ILs.23 Two properties that constitute the miscibility of an IL and water are as follows: (1) the solubility of water in the IL and (2) the solubility of the IL in water. The previously reported miscibilities of ethylene glycol-functionalized imidazolium hexafluorophosphate ILs and water were solely based on the solubilities of the ILs 2, 3a, and 4 in water (Figure 1).36 On this basis, the solubilities of water in the ILs 1, 3a, and 5 were studied first. These solubilities were determined by the addition of water to 1.0 mL of IL until phase separation occurred, and the results are presented in Table 1. The 2.6% (w/w) solubility of water in IL 1 at 30 °C is in the expected range when compared to previously reported values (Table 1, entry 1).21,23,24 A strongly enhanced water solubility was observed in IL 3a, the ether-functionalized equivalent of IL 1 (Table 1, entry 3). The same effect was reported previously for their bis(trifluoromethanesulfonyl)imide equivalents, which caused an increase of the water solubility from 1.4% (w/w) to 3.0% (w/w) with 3b.25 In general, a longer alkyl-substituent (decyl vs butyl) correlates to a lower water solubility.23-27 Interestingly, functionalization of the imidazolium ring with the longer triethylene glycol monomethyl ether tail as in IL 5 resulted in a further enhanced water solubility of 23.3% (w/w). This strongly diminished hydrophobic character of the ether-functionalized ILs could be due to their higher polarity and/or hydrogenbonding participation.38 Ethanol showed to have a strong cosolvent effect.30,31 The presence of a small amount of ethanol expanded the water solubility range in IL 5 at 30 °C from 23.3% (w/w) to 59.4% (w/w) (Table 1, entry 5 vs entry 6). Another parameter that affects the miscibility of water and ILs is the temperature.21,23,27 As expected, increased water solubilities were observed at an elevated temperature of 50 °C (Table 1, entries 2, 4, and 7). The IL-dependent water solubility increased in the same order as observed at 30 °C: 1 [C4MIm] < 3a [C3O1MIm] < 5 [C7O3MIm]. However, the differences in water solubility were larger at 50 °C. Under these conditions, IL 5 and water are fully miscible (Table 1, entry 7),43 which shows that manipulation of the temperature could be used to realize IL-specific applications (shift between monophasic and biphasic conditions). Besides, this demonstrates the possibility to transform a hydrophobic IL into a hydrophilic IL by the ether-functionalization of the imidazolium ring. Second, the solubilities of the ILs 1, 3a, and 5 in water were studied at 30 °C, and the results are presented in Table 2. The addition of water to the biphasic IL 5-water mixture that was obtained above 23.3% (w/w) water (Table 1, entry 5) was continued until phase separation disappeared, which indicated a solubility of 13.5% (w/w) for IL 5 in water (Table 2, entry

Ind. Eng. Chem. Res., Vol. 46, No. 22, 2007 7391 Table 2. Solubility of Imidazolium Hexafluorophosphate ILs in Water at 30 °C entry 1 2 3 4

IL [cation]a

EtOH (µL)

sol. %b (v/v)

sol. %b,c (w/w)

IL MFd

1 [C4MIm] 3a [C3O1MIm] 5 [C7O3MIm] 5 [C7O3MIm]

6400 3200 0 90

13.0e 13.0 10.2e 13.2e

17.0e 17.8 13.5e 17.2e

0.0128e 0.0134 0.0074e 0.0099e

a Ionic liquid (1.0 mL). b Solubility. c Density: 1 ) 1.37;9a 3a ) 1.45;17 5 ) 1.37.17 d Mole fraction. e Ethanol not included.

3). As a consequence, IL 5 and water form a biphasic system at 30 °C when the water content is between 23.3 and 86.5% (w/w). The ethanol cosolvent effect is less pronounced for the solubility of IL 5 in water; however, the immiscibility range is drastically reduced in the presence of a small amount of ethanol (Table 1, entry 6 and Table 2, entry 4). Now, a biphasic system is formed when the water content is between 59.4 and 82.8% (w/w). Transformation of the biphasic IL-water mixtures into homogeneous ternary IL-water-ethanol mixtures, using similar water contents (Table 2, entries 1, 2, and 4), showed to be ILdependent, and the necessary amount of ethanol increased drastically in the following order: 5 [C7O3MIm] < 3a [C3O1MIm] < 1 [C4MIm]. Apparently, a consistent correlation exists between the solubilities of the ILs 1, 3a, and 5 in water-ethanol mixtures and the solubilities of water in the same ILs. Etherfunctionalized IL 3a is characterized by a higher solubility in comparison to its alkyl-substituted equivalent 1. Besides, extension of the monoethylene glycol chain of IL 3a to the triethylene glycol chain of IL 5 causes a further solubility increase. Surprisingly, the previously reported results about the solubility of IL 3a (fully water miscible) and IL 4 (lower solubility in comparison to IL 1) are contradictory. 4. Conclusions In conclusion, the “hydrophobic” nature of imidazolium hexafluorophosphate ILs can be strongly affected by the attachment of ether functionalities to the imidazolium ring. The “hydrophilicity” of the studied hexafluorophosphate ILs increases in the following order: 1 [C4MIm] < 3a [C3O1MIm] < 5 [C7O3MIm]. Although IL 1 has a very low water saturation content, IL 5 has a broad water miscibility range at 30 °C, which can be enlarged in the presence of a small amount of ethanol as cosolvent. Interestingly, IL 5 is fully water miscible at 50 °C. The results reported herein open the possibility to develop ILwater monophasic processes, using ether-functionalized ILs with hydrophobic anions, and will contribute to predict the behavior of new task-specific ILs in terms of their miscibilities with water. Acknowledgment The authors thank the CNPq and CAPES for financial support. H.S.S. thanks the CNPq for a visiting scientist fellowship. Supporting Information Available: 1H and 13C NMR of 3a and 5. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; VCHWiley: Weinheim, Germany, 2002. (2) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. ReV. 1999, 99, 2071.

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ReceiVed for reView July 16, 2007 ReVised manuscript receiVed August 31, 2007 Accepted September 18, 2007 IE0709685