Switching the basicity of ionic liquids by CO2

www.rsc.org/greenchem | Green Chemistry COMMUNICATION Switching the basicity of ionic liquids by CO2 † Wenjing Li, Zhaofu Zhang, Buxing Han,* Suqin Hu, Jinliang Song, Ye Xie and Xiaosi Zhou Received 8th July 2008, Accepted 4th September 2008 First published as an Advance Article on the web 22nd September 2008 DOI: 10.1039/b811624e The basicity of several basic ionic liquids is studied quantitatively for the first time, and the basicity of the ionic liquids can be switched repeatedly by bubbling CO2 and N2 through the solution alternately. Ionic liquids (ILs) have some unusual properties, such as extremely low vapor pressure, wide liquid temperature range, high thermal and chemical stability, and ability to dissolve a wide range of organic and inorganic chemical compounds.1,2 Moreover, ILs can be functionalized by designing different cations and anions. Researchers have developed large amounts of functional ILs for particular applications.3–5 For example, acidic ILs have been proven to be efficient catalysts for many acid-catalysed organic reactions.6,7 Basic ILs with amino groups were synthesized and used to capture CO2 8,9 and H2 S10 , and to promote hydrogenation of CO2 .11 1,1,3,3-tetramethylguanidinebased ILs have been found to be very effective to aldol reaction and absorb SO2 from mixed gases.12–14 Some basic ILs containing biodegradable components, including ILs derived from natural amino acids15–17 and a deep eutectic mixture of choline chloride and urea,18 have aroused increasing research interest and found promising application in different fields, such as separation,19 catalysis20 and material synthesis.21 Acidic or basic ILs represent new classes of acids or bases. The study of the properties of these ILs is of great importance from both fundamental and practical points of view. Thomazeau et al. characterized the Hammett acidity scale for Brønsted acid in non-chloroaluminate ILs.22 Yang and Kou studied the Lewis acidity of ILs consisting of imidazolium chloride and metal chloride by monitoring the IR spectra of probes.23 However, to the best of our knowledge, study of the basicity of ILs has not been reported. Determining and tuning the basicity of ILs is a new and interesting topic because the efficiency of many processes depends on the basicity of the media or can be controlled by the basicity of the media. Moreover, considering the wide application of ILs, reversible tuning of the basicity of ILs would be more interesting and offer great advantages in applications. As an environmentally benign gas, CO2 has been used to develop switchable nonpolar-to-polar solvents,24,25 gelators26 and surfactants.27 In this work, we first studied the basicity of some amino-contained ILs (Scheme 1), choline chloride/urea (1 : 2 molar ratio, CH/urea), 1-aminoethyl-3-methyl imidazolium tetrafluorobo- Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, PR China. E-mail: hanbx@iccas.ac.cn; Fax: 86-10-62559373; Tel: 86-10-62562821 † Electronic supplementary information (ESI) available: Preparation of ILs and experimental details. See DOI: 10.1039/b811624e 1142 | Green Chem., 2008, 10, 1142–1145 Scheme 1 Structure of the ILs investigated in this work. (a) CH/urea, (b) [AEMIM][BF4 ], (c) [TMG][ClO4 ], (d) [BMIM][BF4 ]. rate ([AEMIM][BF4 ]), and 1,1,3,3-tetramethylguanidinium perchlorate ([TMG][ClO4 ]). The basicity is evaluated by the Hammett function. The Hammett function of the commonly used IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4 ]) was also determined for comparison. Then, we proposed a method to tune the basicity of the ILs using CO2 . It was found that adding CO2 of ambient pressure to the ILs could reduce the basicity of the ILs significantly, and the basicity of the ILs was readily recovered after removing CO2 by bubbling N2 through the solutions. This novel, simple, green and reversible method to tune the basicity of ILs has potential application in different fields. The Hammett function was proposed by Hammett and Deyrup in the early 1930s to evaluate the acidity and basicity outside the pH range in water and the acidity in nonaqueous solvents by determining the ionization ratio of indicators in a solution.28,29 For a basic solution, the Hammett function measures the tendency of the solution to capture protons. When weak acids were chosen as indicators, the Hammett function H is defined by the following equation. H - = pK(HI) + log([I- ]/[HI]) (1) where pK(HI) is the thermodynamic ionization constant of the indicator in water, [I- ] and [HI] stand for the molar concentrations of anionic and neutral forms of the indicator, respectively. Clearly, a medium with larger H - value has stronger basicity. Usually, an indicator is only applicable to a narrow H - range. The commonly used method to study the Hammett function in wide range is to use different indicators because different indicators give approximately the same Hammett value for a solution, especially when using indicators of the same charge type and similar structure.28–31 In this work we used three indicators, 4-nitrobenzylcyanide, 4-nitrophenol and 2,5dinitrophenol to determine the basicity of the ILs with and without CO2 at 25 ◦ C. Their pK(HI) values are 13.43,30 7.1532 and 5.15,33 respectively. The absorption of the neutral form of the indicators is negligible at the wavelength monitored in this work. Therefore, This journal is © The Royal Society of Chemistry 2008 Table 1 H - values for hydrazine aqueous solution measured in this work and reported in the literature Hydrazine (wt%) H - (measured, ±0.01) H - (literature) 10 15 20 25 30 11.55 11.86 12.18 12.62 12.97 11.55 11.93 12.29 12.72 13.15 the ratio [I- ]/[HI] can be known from UV-Vis spectra by applying Beer’s law.22,28–31 [I- ]/[HI] = e/(eI - - e) (2) where e is the measured extinction coefficient of the indicator at a given wavelength, and eI - represents the extinction coefficient at the same wavelength of the indicator completely in the anionic form. We first determined the H - values of hydrazine aqueous solutions of different concentrations to verify the reliability of our experimental method, and the results are presented in Table 1 together with those reported by other authors. It can be observed that the results determined in this work agree well with the literature values.30 The basicity of the ILs was determined with 4-nitrobenzylcyanide and 2,5-dinitrophenol. The absorption maximum of the anionic form of was 4-nitrobenzylcyanide at about 540 nm in CH/urea and [AEMIM][BF4 ]. For [TMG][ClO4 ] and [BMIM][BF4 ], 2,5-dinitrophenol was used. Value of eI - is necessarily measured in solutions with sufficient high basicities.28,31 In this work, eI - was determined in the mixed solution of the ILs and ethylenediamine (30 wt%) in which the absorption band of the neutral form of the indicator disappeared completely, indicating that the indicator existed entirely in the anionic form in the solution. The H - values of the ILs determined are presented in Table 2. CH/urea and [AEMIM][BF4 ] behave as bases and exhibit high H - values, while [TMG][ClO4 ] have much lower value similar to that of the neutral IL [BMIM][BF4 ]. It should be noted that the [BF4 ] anion might be hydrolysed in the presence of water, which may have some effect on the H - measurements. However, we have found that the H - values of ILs remained constant after the dried ILs were preserved for several days. Guanidine and its derivatives are known to be very strong organic bases of the order of strength of potassium hydroxide.34 Therefore, the neutralization of 1,1,3,3-tetramethylguanidine by the strong perchloric acid leads to a neutral product. It is known that most ILs are hygroscopic and water could affect the properties of ILs, such as density, viscosity and polarity.35,36 The ILs used here were dried under vacuum for at least 24 h at 70 ◦ C prior to use and the water content determined by Karl Fischer titration was less than 100 ppm. To study the effect of the remaining water on the H - values of the dried ILs, we measured the H - values of ILs with 1 wt% and 3 wt% added water, and the results are also presented in Table 2. It can be seen that water slightly reduces the basicity of CH/urea and [AEMIM][BF4 ], due to the hydrogen bonding between water and amino group. On the other hand, the addition of water results in slight increase of H - values in the case of [TMG][ClO4 ] and [BMIM][BF4 ]. The values of H - for the IL/water mixtures did not change over three days, indicating that the hydrolysis was negligible under the conditions. The basicity of mixed ILs was also determined. Fig. 1 shows the H - values of binary mixtures of basic ILs and neutral [BMIM][BF4 ] with different concentrations. The three indicators described before were used in a different H - range. x2 represents the mole fraction of the basic ILs, CH/urea and [AEMIM][BF4 ]. As illustrated in Fig. 1, the basicity of the mixtures increases significantly with the increase of x2 . The change in the basicity is more rapid in the neutral IL-rich region and basic IL-rich region, which is similar to the basic aqueous system.30 Fig. 1 The H - values of CH/urea + [BMIM][BF4 ] and [AEMIM][BF4 ] + [BMIM][BF4 ] mixed ILs as a function of the mole fraction of basic ILs x2 . It is very interesting to switch the basiciy of ILs repeatedly using simple and green method. In this work, we studied the effect of CO2 on the H - values of the ILs. The above data were obtained using 4-nitrobenzylcyanide as the indicator. Our experiments showed that the addition of CO2 of ambient pressure to [AEMIM][BF4 ] and CH/urea made the bands of the indicator at 540 nm disappear completely. This means that the basicity of the two ILs is reduced significantly after adding CO2 Table 2 H - values of the ILs with and without additives at 25 ◦ C H - (±0.02) Additives CH/urea [AEMIM][BF4 ] [TMG][ClO4 ] [BMIM][BF4 ] Without additive 1 wt% water 3 wt% water CO2 of 1 atm 10.86a 10.77a 10.65a 6.25 11.75a 11.65a 11.49a 4.96 4.35 4.40 4.47 4.35 5.06 5.12 5.20 5.06 a The values were determined by 4-nitrobenzylcyanide, the other values were determined by 2,5-dinitrophenol. This journal is © The Royal Society of Chemistry 2008 Green Chem., 2008, 10, 1142–1145 | 1143 and the indicator is not applicable to determine the H - values of the ILs in the presence of CO2 . Therefore, we use another indicator, 2,5-nitrophenol, which is suitable for lowering H values. The absorbance of the indicator in the corresponding neat ILs was used as the reference absorbance of the total anionic form because only the absorbance of the anionic form can be observed in the neat ILs for the indicator. The H values of the ILs after equilibration under ambient CO2 pressure are presented in Table 2. It can be observed that the basicity of [AEMIM][BF4 ] and CH/urea decreased significantly after adding CO2 . Especially, the H - value of [AEMIM][BF4 ] after CO2 treatment is nearly the same as the neutral [BMIM][BF4 ]. This is because that CO2 reacts with primary and secondary amines to form ammonium carbamates,8,25,26 which reduces the basicity. As expected, the effect of CO2 on the basicity of [TMG][ClO4 ] and [BMIM][BF4 ] is negligible, because there is no chemical reaction between the ILs and CO2 . Our experiments showed that the H - value of IL samples with absorbed CO2 returned to that of the neat ILs after bubbling N2 through the solutions. As example, Fig. 2 demonstrates the change of H - values of CH/urea by bubbling CO2 and N2 through the solution over three cycles. Clearly, the basicity of the ILs can be switched repeatedly and reversibly by bubbling CO2 and N2 alternatively. This is understandable because the CO2 absorbed by amino group can be released.8,26 This novel method to tune the basicity of ILs may find wide application in different fields, such as separation, chemical reactions and smart materials. As an example, we studied the switching of absorbance of 4-nitrophenol in CH/urea. 4-Nitrophenol in CH/urea is bright yellow. It quickly became colourless after was exposed to CO2 of ambient pressure for five minutes under stirring. After bubbling N2 at 60◦ C for thirty minutes, the solution reverted back to its original colour. The photographs of this process are shown in Fig. 3. Fig. 2 The H - value of CH/urea as a function of time during three cycles of treatment with CO2 at 25 ◦ C followed by N2 at 60 ◦ C. In summary, the basicity of several ILs were determined. Most importantly, the basicity of the amino-group containing ILs can be tuned repeatedly and reversibly by bubbling CO2 and N2 through the solutions alternately. We believe that this novel, simple, green and reversible method to tune the basicity of ILs has potential applications in different fields. 1144 | Green Chem., 2008, 10, 1142–1145 Fig. 3 The color change of 4-nitrophenol in CH/urea. (a) in neat CH/urea, (b) after bubbling CO2 for five minutes at 25 ◦ C, (c) after bubbling N2 for half an hour at 60 ◦ C. The concentration of the dye is 5 ¥ 10-5 mol L-1 . Acknowledgements The authors are grateful to the National Natural Science Foundation of China (20533010) and Chinese Academy of Sciences (KJCX2.YW.H16). Notes and references 1 P. Wasserscheid and T. Welton, Ionic Liquids in Synthesis, WileyVCH, Weinheim, Germany ,2003. 2 R. D. Rogers and K. R. Seddon, Ionic Liquids: Industrial Applications to Green Chemistry, ACS, Washington, DC, USA, 2002. 3 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107, 2615. 4 T. L. Greaves and C. J. Drummond, Chem. Rev., 2008, 108, 206. 5 S. G. Lee, Chem. Commun., 2006, 1049. 6 Y. L. Gu, J. Zhang, Z. Y. Duan and Y. Q. Deng, Adv. Synth. Catal., 2005, 347, 512. 7 A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver, D. C. Forbes and J. H. Davis, Jr., J. Am. Chem. Soc., 2002, 124, 5962. 8 E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis, Jr., J. Am. Chem. Soc., 2002, 124, 926. 9 J. M. Zhang, S. J. Zhang, K. Dong, Y. Q. Zhang, Y. Q. Shen and X. M. Lv, Chem.–Eur. J., 2006, 12, 4021. 10 J. H. Davis, Jr., Green Industrial Applications of Ionic Liquids, NATO Science Series, vol. 92, ed. R. D. Rogers, K. R. Seddon and S. Volkov, Kluwer Academic Publishers, Dordrecht, 2000. 11 Z. F. Zhang, Y. Xie, W. J. Li, S. Q. Hu, J. L. Song, T. Jiang and B. X. Han, Angew. Chem., Int. Ed., 2008, 47, 1127. 12 H. X. Gao, B. X. Han, J. C. Li, T. Jiang, Z. M. Liu, W. Z. Wu, Y. H. Chang and J. C. Zhang, Synth. Commun., 2004, 34, 3083. 13 A. L. Zhu, T. Jiang, B. X. Han, J. C. Zhang, Y. Xie and X. M. Ma, Green Chem., 2007, 9, 169. 14 W. Z. Wu, B. X. Han, H. X. Gao, Z. M. Liu, T. Jiang and J. Huang, Angew. Chem., Int. Ed., 2004, 43, 2415. 15 K. Fukumoto, M. Yoshizawa and H. Ohno, J. Am. Chem. Soc., 2005, 127, 2398. 16 G. H. Tao, L. He, N. Sun and Y. Kou, Chem. Commun., 2005, 3562. 17 S. Q. Hu, T. Jiang, Z. F. Zhang, A. L. Zhu, B. X. Han, J. L. Song, Y. Xie and W. J. Li, Tetrahedron Lett., 2007, 48, 5613. 18 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Chem. Commun., 2003, 70. 19 A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and P. Shikotra, Inorg. Chem., 2005, 44, 6497. 20 A. L. Zhu, T. Jiang, D. Wang, B. X. Han, L. Liu, J. Huang, J. C. Zhang and D. H. Sun, Green Chem., 2005, 7, 514. 21 E. R. Cooper, C. D. Andrews, P. S. Wheatley, P. B. Wedd, P. Wormald and R. E. Morris, Nature, 2004, 430, 1012. 22 C. Thomazeau, H. Olivier-Bourbigou, L. Magna, S. Luts and B. Gilbert, J. Am. Chem. Soc., 2003, 125, 5264. 23 Y. L. Yang and Y. Kou, Chem. Commun., 2004, 226. 24 P. G. Jessop, D. J. Heldebrant, X. W. Li, C. A. Eckert and C. L. Liotta, Nature, 2005, 436, 1102. 25 T. Yamada, P. J. Lukac, M. George and R. G. Weiss, Chem. Mater., 2007, 19, 967. 26 M. George and R. G. Weiss, J. Am. Chem. Soc., 2001, 123, 10393. This journal is © The Royal Society of Chemistry 2008 27 Y. X. Liu, P. G. Jessop, M. Cunningham, C. A. Eckert and C. L. Liotta, Science, 2006, 313, 958. 28 M. A. Paul and F. A. Long, Chem. Rev., 1957, 57, 1. 29 L. P. Hammett, Chem. Rev., 1935, 16, 67. 30 N. C. Deno, J. Am. Chem. Soc., 1952, 74, 2039. 31 R. J. Gillespie and T. E. Peel, J. Am. Chem. Soc., 1973, 95, 5173. 32 R. G. Bates and G. Schwarzenbach, Helv. Chim. Acta, 1955, 38, 699. This journal is © The Royal Society of Chemistry 2008 33 S. J. Broderius, M. D. Kahl and M. D. Hoglund, Environ. Toxicol. Chem., 1995, 14, 1591. 34 T. C. Davis and R. C. Elderfield, J. Am. Chem. Soc., 1932, 54, 1499. 35 K. R. Seddon, A. Stark and M. J. Torres, Pure Appl. Chem., 2000, 72, 2275. 36 S. N. V. K. Aki, J. F. Brennecke and A. Samanta, Chem. Commun., 2001, 413. Green Chem., 2008, 10, 1142–1145 | 1145