Anal. Chem. 2009, 81, 400–407
Water as an in Situ NMR Indicator for Impurity Acids in Ionic Liquids Yoshiro Yasaka, Chihiro Wakai, Nobuyuki Matubayasi, and Masaru Nakahara* Institute for Chemical Research, Kyoto University, Kyoto 611-0011, Japan A sensitive in situ NMR spectroscopic method for detecting acids contaminating ionic liquids (ILs) has been developed. The chemical shift and the spectral width of water added to ILs were used as indicators to measure the impurity acid level. Owing to the high resolution power of NMR, the detection limit is below the level of 10-3 mol kg-1. A new method is applicable to a number of commonly used ILs such as the imidazolium- and ammonium-based ILs except for those composed of acidic cations or anions. The method was utilized to monitor the purification efficiency in the recrystallization of a typical hydrophilic IL, 1-butyl-3-methylimidazolium methanesulfonate from acetone. It was demonstrated that impurity acids can be almost perfectly removed by single or double recrystallization. It is widely recognized that small amounts of impurities in ionic liquids (ILs) can affect their physical and chemical properties, such as thermodynamics, surface chemistry, transport properties, and reaction kinetics and pathways.1-7 Common contaminants of ILs are water, acids, halide anions, metal cations, parent amines, and unidentified colored substances.1,5,7,8 Of these impurities, acids can be the most detrimental to the study of solvent effect of ILs. Impurity acids can be introduced with the addition of anions (A-) through HA (free acid) or RA (haloalkanes, sulfonate or phosphate esters) that are to hydrolyze to form HA. Despite the popularity, titration and electrochemical methods are not sufficiently sensitive techniques. Acid impurity test is commonly carried out by dissolving the IL into water and measuring the pH of the aqueous phase.1 This indirect method is, however, low in sensitivity because of the following: (i) impurity acids contained in ILs are undesirably diluted in the aqueous phase, and (ii) the pH of unbuffered solution (typically in the pH range of 4-7) is easily affected by the absorption of atmospheric carbon dioxide and other acidic gases. * To whom correspondence should be addressed. E-mail: nakahara@ scl.kyoto-u.ac.jp. (1) Ionic Liquids in Synthesis I Wassercheid, P., Welton, T., Eds.; Wiley-VCH, Weinheim, 2007. (2) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275. (3) Widegren, J. A.; Laesecke, A.; Magee, J. W. Chem. Commun. 2005, 1610. (4) Widegren, J. A.; Saurer, E. M.; Marsh, K. N.; Magee, J. W. J. Chem. Thermodyn. 2005, 37, 569. (5) R.-Rubero, S.; Baldelli, S. J. Am. Chem. Soc. 2004, 126, 11788. (6) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (7) Yasaka, Y.; Wakai, C.; Matubayasi, N.; Nakahara, M. J. Phys. Chem. A 2007, 111, 541. (8) Earle, M. J.; Gordon, C. M.; Plechkova, N. V.; Seddon, K. R.; Welton, T. Anal. Chem. 2007, 79, 758; 2007, 79, 4247.
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Analytical Chemistry, Vol. 81, No. 1, January 1, 2009
In this paper we report a highly sensitive analytical method for detecting impurity acids using the 1H NMR chemical shift of indicator water. The chemical shift of water added is very sensitive to the coexistence of other chemical species that have dissociative protons. In our method, we employ a minimal quantity of water (less than 1% by weight against IL) as an NMR spectroscopic acid indicator. The chemical shift of water changes more sensitively than the color (absorption) shift of dyes depending on the H+ concentration in ILs. The use of an NMR-spectroscopic impurity indicator allows us to overcome the above mentioned difficulties by in situ analysis without sample dilution. Furthermore, NMR is noninvasive, and ILs subjected to the NMR impurity test can be directly used after drying. The high-sensitivity NMR analysis for impurity acids investigated here works as a powerful method for controlling impurities in ILs. Although a few purification procedures have been proposed,1 the most effective has not yet been established. Recently, recrystallization by organic solvents,7,9 zone-melting,10 decolorization,8 and acid neutralization11 by a column were reported, and the purity improvement was confirmed by neutralization titration, NMR spectroscopy, and cyclic voltammetry.7,9 By employing NMR-spectroscopic impurity analysis ILs can be purified by recrystallization from a dipolar aprotic organic solvent, acetone. The difficulty of removing impurity acids from ILs depends on the hydrophobicity or hydrophilicity (phase-separation behavior with water) of the IL, which is controlled by the component anion. Hydrophobic ILs composed of such a large anion as bis(trifluoromethanesulfonyl)imide, [(CF3SO2)2N]-, can be sufficiently free from impurity acids after they are washed with water and dried by heating under reduced pressure.1 On the other hand, hydrophilic ILs are freely miscible with water and thus the extraction method is inapplicable. Thus, there are few good purification methods available in the literature. Recrystallization, however, is a versatile purification method for most substances. It is more efficient than distillation that is often applied to purify classical organic solvents and also some of the thermally resistant ILs with a low vapor pressure.12 EXPERIMENTAL SECTION The IL, 1-butyl-3-methylimidazolium methanesulfonate ([bmim]+[CH3SO3]-), was obtained from Solvent Innovation and (9) Cassol, C. C.; Ebeling, G.; Ferrera, B.; Dupont, J. Adv. Synth. Catal. 2006, 348, 243. (10) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A. J. Am. Chem. Soc. 2005, 127, 16792. (11) Lungwitz, R.; Spange, S. New J. Chem. 2008, 32, 392. (12) Earle, M. J.; Esperanca, J. M. S. S.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831. 10.1021/ac801767u CCC: $40.75 2009 American Chemical Society Published on Web 11/21/2008
was purified by recrystallization as described below. The chloride salt, [bmim]+[Cl]-, was obtained from Solvent Innovation and purified by recrystallization as in ref 7. These two ILs were dried under reduced pressure at 90 °C for one day. The ILs, 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim]+ [NTf2]-) and 1-methyl-3-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([mppp]+[NTf2]-), were obtained from Kanto Kagaku (special grade for Ionic Liquid Research Associate of Japan) and were used as received. The water content of all the ILs were analyzed by NMR (the detection limit: 1-5 mmol kg-1); obtained values are 25 mmol kg-1 for [bmim]+[Cl]- and
