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2009, 113, 4181–4183 Published on Web 02/27/2009
Ultrastable Superbase-Derived Protic Ionic Liquids Huimin Luo,† Gary A. Baker,‡ Je Seung Lee,‡ Richard M. Pagni,§ and Sheng Dai*,‡ Nuclear Science and Technology and Chemical Sciences DiVisions, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996 ReceiVed: February 12, 2009
Protic ionic liquids are synthesized via proton transfer from acids to organic bases. One of the key issues associated with conventional protic ionic liquids is the thermal instability resulting from temperature-induced decomposition via reverse proton transfer. This shortcoming significantly hampers the use of these protic ionic liquids in separations, electrochemical capacitors, fuel cells, and so forth. Herein we show that it is possible to prepare protic ionic liquids with thermal stabilities approaching those of common aprotic ionic liquids. Our new class of protic ionic liquids, derived via integrated neutralization and metathesis of superbasic phosphazenes or guanidines, exhibits exceptionally low vapor pressures at 150 °C while being stable to strong alkali agents such as aqueous KOH, suggesting potential in energy-related applications, including electrochemical capacitors and PEM-type fuel cells. Proton-exchange membranes hold immense potential for mobile fuel-cell applications, but the current hydrated fluoropolymer-based technology has serious technical limitations including a truncated operational temperature range due to evaporative loss of water at temperatures much beyond 80 °C. Supported protic ionic liquids (PILs),1-5 however, offer intriguing promise for electrochemically stable, anhydrous, protonconducting fuel-cell electrolytes that can perform at temperatures approaching 150 °C. Although the proton-conducting mechanism of these PILs differs from that of the conventional PEMFCs and does not depend on water, there are several substantive issues which must be resolved for these materials to become viable. One key hurdle is the volatility of available PILs at the operating temperature of fuel cells (>100 °C).6,7 This intrinsic vapor pressure results from the retro-proton transfer between the constituent ions, generating the parent Brønsted acid and base species which lack strong Coulombic interactions, leading to marked vaporization at elevated temperature.7 Other factors, such as the decomposition of cations and anions through their intrinsic thermal reactions, can also play a role in the vaporization of PILs under high-temperature conditions. The electrolyte loss at high temperatures is detrimental in terms of not only fuel-cell operational lifetime and performance but also system safety. The vapor pressure of a PIL is largely determined by the equilibrium concentrations of the neutral acid and base forms from which the ionic liquid is prepared. This is, in turn, determined by mutual proton transfer between the uncharged Brønsted components. On the basis of pioneering work by Angell and co-workers,4,5,8 the thermodynamic driving force for PIL formation can be correlated to ∆pKa [)pKa(BH+) * To whom correspondence should be addressed. E-mail:
[email protected]. † Nuclear Science and Technology Division, Oak Ridge National Laboratory. ‡ Chemical Sciences Division, Oak Ridge National Laboratory. § University of Tennessee.
10.1021/jp901312d CCC: $40.75
pKa(HA)], where HA and B signify the acid and base precursors, respectively. Thus, in a qualitative sense, PILs made from stronger acid/base couples can be expected to yield lower equilibrium concentrations of acid and base, and thus generate lower vapor pressures. We note that although fluorinated superacids have been widely explored in PILs, no attempt has been made to make use of superbase moieties in PIL preparation. In this Letter, we report on a new family of conductive, lowvolatility PILs following the strategy of pairing organic superbasic proton acceptors with appropriate superacid-based fluorous anions. These new PILs exhibit thermal stabilities superior to all known PILs with decomposition temperatures, Tdcp, similar to those of archetypal aprotic ionic liquids (AILs). Superbases, neutral organic bases with proton affinities so high that their protonated conjugate acids (BH+) cannot be deprotonated by hydroxide ion, have recently found unique utility in catalysis and stoichiometric asymmetric synthesis.9 Inspired by work from the Angell group, we have indeed discovered that the huge proton affinities of organic superbases enable the preparation of PILs with exceptional thermal stability. In this work, five superbasic phosphazenes, two bicyclic guanidines (MTBD, HTBD), and 1,1,3,3-tetramethylguanidine (HNC(dma)) were explored (see Chart 1 for structures and designations). In all cases, their pKa(BH+) values measured in acetonitrile exceed 20.9 The new PILs were easily synthesized in high yield (>98%) in a single pot in two steps (see Supporting Information Scheme S1 and Table S1). We note that Watanabe and co-workers,10 among others,1-5 have prepared PILs by neutralization of a wide range of amines with superacids, particularly hydrogen bis(trifluoromethylsulfonyl)imide (HTf2N). Most recently, Huang et al.11 have developed general and less costly one-pot procedures for elaborating PILs based on protonation using HNO3, followed by direct metathesis using lithium salts of [Tf2N-] or bis(perfluoroethylsulfonyl)imide, [beti-], within aqueous media. Using the PIL [MTBDH][Tf2N] for illustration (see Supporting 2009 American Chemical Society
4182 J. Phys. Chem. B, Vol. 113, No. 13, 2009
Letters
CHART 1: Superbases Used as Building Blocks to Derive Novel PILs
Figure 1. Isothermal TGA scans conducted under N2 at 150 °C showing the superior thermal stability of [EtP2(dma)H][Tf2N] and [MTBDH][Tf2N] compared to the conventional PIL [Et3NH][Tf2N].
Information Figure S1), MTBD (pKa(BH+) ) 25.4) was initially dissolved in water and chilled in an ice bath, followed by careful titration with 10.6 N HNO3 to form aqueous [MTBDH][NO3]. Anion exchange was achieved by adding one equivalent of LiTf2N in water, resulting in spontaneous segregation of the dense, lower [MTBDH][Tf2N] phase. After careful washing with bidistilled water several times, rotary evaporation resulted in a nearly colorless free-flowing PIL in essentially quantitative yield. In this case, following vacuum drying at 70 °C overnight, [MTBDH][Tf2N] contained some 110 ppm water (by Karl-Fischer Coulometric titration) and showed a room temperature ionic conductivity (σ) of 1.49 mS cm-1 (Supporting Information Table S2). The thermophysical properties of our new superbase-derived PILs are compiled within Supporting Information Table S1. Of the 16 newly synthesized PILs, only five possess melting points outside the classic ionic liquid range. However, even in these cases, the liquidus temperature can be greatly reduced by simple eutectic mixing with conventional AILs. For instance, the borderline PIL [t-BuP1(dma)H][beti], which has a Tfus value of 108 °C, forms a liquid system near ambient temperature when mixed with 28 vol % [bmim][Tf2N] (bmim ) 1-butyl-3methylimidazolium). The DSC curves for these two systems are compared in Supporting Information Figure S2. The thermal properties of the 16 superbase-derived PILs were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As seen from Supporting Information Table S1, their thermal decomposition temperatures (Tdcp) fall in the 300-380 °C range, considerably above the operating temperature of PEM fuel cells. With this in mind, selected PILs were tested at 150 °C for a period of several hours using TGA and the results are shown in Figure 1 (see also Supporting Information Figure S3). Clearly, the weight losses of these superbase PILs are significantly lower than those of conventional ammonium-based PILs bearing the same anion, such as [Et3NH][Tf2N]. Quite remarkably, the superbase-derived PIL [MTBDH][Tf2N] produced a kinetic TGA scan very similar to that of [bmim][Tf2N] (Supporting Information Figure S4). In fact, based on the linear (zero-order) evaporative mass losses for [MTBDH][Tf2N] at several temperatures well below Tdcp (Figure 2), an enthalpy of vaporization, ∆vapH, of 89 kJ mol-1
Figure 2. Time-dependent isothermogravimetry of [MTBDH][Tf2N] showing the linear mass losses at five discrete temperatures.
Figure 3. [Et3NH][Tf2N] (left vial) and [MTBDH][Tf2N] (right vial) PILs dyed with Nile Red before (A) and after (B) addition of concentrated KOH(aq). The inset shows Nile Red in various water-toEtOH volume ratio solutions.
was estimated for this PIL (Supporting Information Figure S5), a number ca. 25% below the value determined for [bmim][Tf2N] using the same isothermogravimetric approach.12 As illustrated in Figure 3 (and Supporting Information Figure S6), [MTBDH][Tf2N] remains stable when contacted with 1.0 N KOH solution, an observation consistent with the superbasicity of MTBD. In sharp contrast, for the conventional PIL [Et3NH][Tf2N], the biphase degrades almost instantaneously to form an aqueous solution under these conditions, developing autogenous pressure during the process if a sealed vessel is used. Using the positive solvatochromic probe Nile Red (NR),13 it is also noted that the polarity of [MTBDH][Tf2N] is somewhat lower than that of conventional PILs such as [Et3NH][Tf2N]
Letters (ENR values are 214.0 and 209.1 kJ mol-1, respectively). This trend can be discerned by visual inspection of water/EtOH mixtures containing NR (Figure 3B, inset). However, both PILs remain more polar than [bmim][Tf2N] which has an ENR akin to EtOH (218 kJ mol-1).13 In summary, we report a facile one-pot method for generating members of a novel family of hydrophobic PILs derived from pairings between diverse superbases and the superacid-derived anions, [Tf2N-] or [beti-]. As a class, these PILs exhibit the highest thermal stabilities yet observed for any PIL, suggesting potential in PEM-type fuel cells at 150 °C and beyond. Acknowledgment. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, and was sponsored by the Laboratory Directed Research and Development Program at Oak Ridge National Laboratory. Supporting Information Available: An enlarged version of Chart 1, a scheme exemplifying the synthesis of superbasederived PILs, tables summarizing the thermal properties of superbase-derived PILs and physical properties for select PILs, and figures showing 1H NMR spectra, DSC thermograms, an isothermal TGA scan, TGA profiles, and the temperature dependence of the mass loss, each for various PILs or their mixtures. Also provided are side-by-side photos comparing the appearance of neat [MTBDH][Tf2N] and [MTBDH][Tf2N] in the presence of pH indicator-tinted 1.0 N aqueous KOH. This material is available free of charge via the Internet at http:// pubs.acs.org.
J. Phys. Chem. B, Vol. 113, No. 13, 2009 4183 References and Notes (1) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206. (2) Fernicola, A.; Panero, S.; Scrosati, B.; Tamada, M.; Ohno, H. ChemPhysChem 2007, 8, 1103. (3) Nakamoto, H.; Watanabe, M. Chem. Commun. 2007, 2539. (4) Belieres, J. P.; Angell, C. A. J. Phys. Chem. B 2007, 111, 4926. (5) Belieres, J. P.; Gervasio, D.; Angell, C. A. Chem. Commun. 2006, 4799. (6) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Lopes, J. N. C.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831. (7) Rebelo, L. P. N.; Lopes, J. N. C.; Esperanca, J.; Lachwa, H.; Najdanovic-Visak, V.; Visak, Z. P. Acc. Chem. Res. 2007, 40, 1114. (8) Yoshizawa, M.; Xu, W.; Angell, C. A. J. Am. Chem. Soc. 2003, 125, 15411. (9) (a) Room, E. I.; Kutt, A.; Kaljurand, I.; Koppel, I.; Leito, I.; Koppel, I. A.; Mishima, M.; Goto, K.; Miyahara, Y. Chem. Eur. J. 2007, 13, 7631. (b) Kaljurand, I.; Koppel, I. A.; Kutt, A.; Room, E. I.; Rodima, T.; Koppel, I.; Mishima, M.; Leito, I. J. Phys. Chem. A 2007, 111, 1245. (c) Kolomeitsev, A. A.; Koppel, I. A.; Rodima, T.; Barten, J.; Lork, E.; Roschenthaler, G. V.; Kaljurand, I.; Kutt, A.; Koppel, I.; Maemets, V.; Leito, I. J. Am. Chem. Soc. 2005, 127, 17656. (10) (a) Susan, M.; Noda, A.; Mitsushima, S.; Watanabe, M. Chem. Commun. 2003, 938. (b) Noda, A.; Susan, A. B.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2003, 107, 4024. (11) (a) Huang, J. F.; Luo, H. M.; Dai, S. J. Electrochem. Soc. 2006, 153, J9. (b) Huang, J. F.; Baker, G. A.; Luo, H. M.; Hong, K. L.; Li, Q. F.; Bjerrum, N. J.; Dai, S. Green Chem. 2006, 8, 599. (c) Huang, J. F.; Luo, H. M.; Liang, C. D.; Sun, I. W.; Baker, G. A.; Dai, S. J. Am. Chem. Soc. 2005, 127, 12784. (12) Luo, H. M.; Baker, G. A.; Dai, S. J. Phys. Chem. B 2008, 112, 10077. (13) (a) Jin, H.; O’Hare, B.; Dong, J.; Arzhantsev, S.; Baker, G. A.; Wishart, J. F.; Benesi, A. J.; Maroncelli, M. J. Phys. Chem. B 2008, 112, 81. (b) Ogihara, W.; Aoyama, T.; Ohno, H. Chem. Lett. 2004, 33, 1414. (c) Carmichael, A. J.; Seddon, K. R. J. Phys. Org. Chem. 2000, 13, 591.
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