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Low viscosity amino acid ionic liquids with asymmetric tetraalkylammonium cations for fast absorption of CO2 Hong Yu, You-Ting Wu,* Ying-Ying Jiang, Zheng Zhou and Zhi-Bing Zhang* Received (in Montpellier, France) 12th July 2009, Accepted 16th September 2009 First published as an Advance Article on the web 9th October 2009 DOI: 10.1039/b9nj00330d Fifteen novel amino acid ionic liquids (AAILs) were prepared by the combination of several tetraalkylammonium cations with four amino acid anions ([Gly], [L-Ala], [b-Ala] and [Val]). The asymmetry of the tetraalkylammonium cations is shown to have a significant influence on the viscosity of the ionic liquids composed of amino acid anions, especially for the four triethyl- butylammonium ([N2224])-based ionic liquids that have viscosities of lower than 60 mPa s, with the lowest being only 29 mPa s. The low viscosity tetraalkylammonium-based AAILs are further demonstrated to improve apparently the reaction and mass transfer rates of CO2 in the ionic liquids. Ionic liquids (ILs) or molten salts have attracted much attention from industrial and academic communities as novel solvents or liquid materials for green chemistry.1 ILs have mostly been designed using a series of cationic derivatives, such as imidazolium,2,3 phosphonium,4,5 and cyclic or non-cyclic quaternary ammonium,6–8 due to their easy and convenient chemical modification. Among these cations, 1-ethyl-3-methyl- imidazolium (EMI) is found to more frequently form low viscosity, low melting ILs with various anions,9 primarily owing to its flat chemical configuration, unsaturated nature and extended charge distribution. However, imidazolium-based ILs are fairly expensive and not readily available, which limits their large-scale applications in industry. In contrast, TAAs are cheap, readily available and widely used as mediums or phase-transfer catalysts in chemical synthesis. Because of this, it is a good step to use TAA as the potential cationic component for a variety of ILs. However, it is regrettable that most cheaply and readily available TAA salts with total carbon number less than 16 melt at temperatures far above room temperature,10,11 i.e., above 100 1C. Till now, only a fraction of those TAA salts formed with several expensive anions, such as [Tf2N] 12 and [TSAC],13 have been found to be ionic liquids of low melting points, not to mention the fact that the TAA-based ILs still have high viscosities. As the viscosity of ILs is one of the most important transport properties that determine their real applications in industry, new and readily available anions for the preparation of TAA-based ILs of low cost and viscosity are eagerly required. After Fukumoto et al.14 reported for the first time ILs composed of imidazolium cations and amino acid (AA) anions in 2005, amino acids have been used to act as a platform for the preparation of functionalized ILs, such as AAs as anions,14–16 AAs as cations17 or using AA derivatives.18 In 2006, Ohno et al.15 succeeded in synthesizing amino acid-based ionic liquids (AAILs) from phosphonium cation and found that these tetrabutylphosphonium-based (TBP or [P4444]) AAILs were even less viscous than [P666 (14)][Tf2N] (450 mPa s at 25 1C).19 Zhang et al.16 also prepared [TBP][amino acid]s and indicated that this type of AAILs still had viscosities larger than 200 mPa s and had to be supported on porous silica gel for reversible CO2 absorption. In a preliminary investigation, our group has reported that four of nine AAILs prepared from a symmetric tetraalkylammonium cation ([Nnnnn], n = 1 to 4) are shown to have lower viscosities than the AAILs mentioned above. In particular, tetraethylammonium a-alanine ([N2222][L-Ala]) is found to have the lowest viscosity among all nine reported [TAA][amino acid]s, down to 81 mPa s,20 which is over 4 times less viscous than tetrapentyl- ammonium bis(trifluoromethylsulfonyl)imide ([TPA][Tf2N]) (430 mPa s at 25 1C).21 It is evident from our preliminary investigation that a combination of AAs with tetraalkyl- ammonium cations may give ionic compounds with desirable properties, especially low viscosity. Since AAILs are multi- functionalized (with amino and carboxyl groups and chiral center), biodegradable and of high biological activity, they are expected to have potential use as green solvents for a wide variety of applications such as reactions and separations of chemical, biological, and pharmaceutical substances. However, since real industrial applications usually require ILs with additional features, such as low cost and low viscosity, it is of particular interest and critical importance to discover more TAA-based AAILs of low cost and viscosity. Herein, as a continuation of our research, fifteen novel AAILs were prepared by coupling five tetraalkylammonium cations ([N2222], [N4444], [N2221], [N1114] and [N2224]) with four amino acid anions ([Gly], [L-Ala], [b-Ala] and [Val]). Our emphasis was put on how the asymmetry of TAA cations and AA anions influences the properties of the AAILs, especially the viscosity. In addition, the low viscosity AAILs prepared were further demonstrated to improve apparently the absorption rate of CO2. Experimental Trimethylbutylammonium chloride ([N1114][Cl], white solid) and triethylmethylammonium chloride ([N2221][Cl], white solid), both with a minimum purity of 99.5%, were purchased from Jintan Chemical Research Institute (Jiangsu, China). Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China. E-mail: ytwu@nju.edu.cn; Fax: (+86) 25-83593772 This journal is �c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2385–2390 | 2385 LETTER www.rsc.org/njc | New Journal of Chemistry D ow nl oa de d by C hi na u ni ve rs ity o f P et ro le um (E ast C hin a) on 28 M ay 20 12 Pu bl ish ed o n 09 O ct ob er 2 00 9 on h ttp :// pu bs .rs c.o rg | d oi: 10 .10 39/ B9 NJ 003 30D View Online / Journal Homepage / Table of Contents for this issue Anion exchange resin �711 (Cl) of analytical grade, as well as tetraethylammonium hydroxide and tetrabutylammonium hydroxide of electronic grade, was produced by Nanjing Chem. Industry Corporation (Nanjing, China). Other reagents such as triethylamine ((C2H5)3N), butyl bromide (C4H9Br), glycine, L-alanine, b-alanine, valine and ethanol were of analytical grade and used without any further purification. In the experiments, the synthesis of AAILs with symmetric tetraalkylammonium cations followed the procedure described in our previous work.20 As for the preparation of AAILs with asymmetric tetraalkylammonium cations, a three-step procedure was adopted: the halide precursor (if not available) was synthesized in the first step followed by the anion exchange and neutralization. Triethylbutylammonium bromide ([N2224][Br]) was prepared via the alkylation of (C2H5)3N (35.0 g) with C4H9Br (47.5 g) in ethanol (100 ml) under reflux and vigorous stirring for 12 h. The solvent and unreacted components were removed by rotary evaporation, and [N2224][Br] (80 g) white solid was obtained after being dried at 60 1C under vacuum for 24 h. The tetraalkylammonium hydroxides ([N2224][OH], [N1114][OH], [N2221][OH]) were obtained by anion exchange. All aqueous solutions were prepared with deionized water, and anion exchange resin �711 (Cl-type) was pretreated with hydrochloric acid (2 M) before use. The resin was transformed from Cl-type into OH-type by passing NaOH solution (5 M, 10 ml/min) through the resin column (l = 100 cm, r = 3 cm) until Cl� could not be detected with AgNO3–HNO3 solution. As the resin (OH-type) is not stable at temperatures higher than 40 1C, NaOH solution must be used after it is cooled. Excess NaOH solution was washed off using deionized water. Tetraalkylammonium halide solution (2 M) was then loaded into the column, and transformed into tetraalkylammonium hydroxide ([N2224][OH], [N1114][OH], or [N2221][OH]) solution. The OH� concentrations of the resulting solutions were determined using titration with HCl solution. The tetra- alkylammonium hydroxide solutions were then reacted with a slight excess of amino acid through neutralization at room temperature for 3 h. Water was evaporated to generate a residual solution that contained the required ionic liquid. After being further dried at 60 1C under vacuum, the residue was diluted with ethanol to precipitate the excess amino acids. After filtration, the ethanol was removed by evaporation. Finally, the products were dried at 60 1C under vacuum for 2 days before being used for differential scanning calorimetry (DSC, Perkin-Elmer DSC 7), thermogravimetric analysis (Perkin-Elmer TG/DTA, 2010), viscosity (HAAKE Rheostress 600) and density (DMA 5000 density meter) measurements. 1H NMR spectroscopy (Varian XL-300) and elemental analysis (Elementar Vario EL) were performed to determine the structure of the ammonium amino acids. The amount of water was measured to be less than 0.05 w/w% using Karl Fisher coulometric titration (Brinkmann Metrohm 756 KF Coulometer) for all the AAILs. Table 1 summarizes the properties of fifteen [TAA][amino acid]s prepared in this work, three of them with symmetric TAA ([Nnnnn]) and twelve species with asymmetric TAA ([N1114], [N2221], [N2224]). The colors of these fifteen AAILs are shown in Fig. 1, twelve of them being colorless or of light color while the other three are yellowish. All AAILs should be colorless in nature, and the yellow color may result from slight oxidation of the amino group during the removal of water and ethanol from the samples at high temperature. As shown in Table 1, the melting points (Tm) of fifteen products are lowered to room temperature or below and all these AAILs show a glass transition temperature. Five of these AAILs have a melting point whilst no melting events for the remaining ten are detectable in the temperature range scanned. In contrast to the findings that ILs containing TAA cations are usually solids and have high melting points, all fifteen TAA-based products are ILs with melting points of lower than 30 1C. From the comparison of all AAILs that have a detectable value (see Table 1 of this paper and Table 1 in our previous paper)20 it is apparent the AAILs having an asymmetric TAA cation ([N2224]) melt at a temperature of 10 to 30 1C lower than those having symmetric [TAA] cations ([Nnnnn], n= 1, 2, 4) no matter which amino acids are assigned to be the counter ions. All these observations may be due to the alkyl chain length and symmetry of the cation, that is, the longer the alkyl and the more asymmetric the cation, the lower the melting point. For example, the trend of the highest to lowest Tm values is [N1111][Val] (40 1C) 4 [N4444][Val] (25 1C) E [N2222][Val] (22 1C) 4 [N2224][Val] (10 1C), which Fig. 1 Prepared tetraalkylammonium-based amino acid ionic liquids. Above (left to right): [N4444][L-Ala], [N2222][Gly], [N2222][Val], [N2224][Val], [N2224][Gly], [N2224][L-Ala], [N2224][b-Ala]. Below (left to right): [N1114][Gly], [N1114][b-Ala], [N1114][L-Ala], [N1114][Val], [N2221][Gly], [N2221][b-Ala], [N2221][L-Ala], [N2221][Val]. Table 1 Properties of tetraalkylammonium-based amino acid ionic liquids prepared in this work IL Tg/1C Tm/1C Tdec/1C r/g cm �3 Z/mPa s [N1114][Gly] �93 NDa 176 0.989 158 [N1114][L-Ala] �92 NDa 175 0.969 62 [N1114][b-Ala] �84 NDa 175 0.990 283 [N1114][Val] �100 NDa 177 NAb NAb [N2222][Gly] �79 14 180 1.064 129 [N2222][Val] �78 22 186 1.006 163 [N4444][L-Ala] �76 NDa 177 0.958 210 [N2221][Gly] �87 NDa 182 1.051 151 [N2221][L-Ala] �93 NDa 179 0.994 84 [N2221][b-Ala] �94 NDa 178 1.008 171 [N2221][Val] �95 NDa 180 0.973 244 [N2224][Gly] �89 NDa 173 0.957 38 [N2224][L-Ala] �92 5 176 0.940 29 [N2224][b-Ala] �88 8 174 0.948 44 [N2224][Val] �88 10 175 0.939 59 NA = not available.a The data at 30 1C. b The data are from Yaws.33 2386 | New J. Chem., 2009, 33, 2385–2390 This journal is �c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 D ow nl oa de d by C hi na u ni ve rs ity o f P et ro le um (E ast C hin a) on 28 M ay 20 12 Pu bl ish ed o n 09 O ct ob er 2 00 9 on h ttp :// pu bs .rs c.o rg | d oi: 10 .10 39/ B9 NJ 003 30D View Online is similar to the case of the lowest to highest degree of alkyl chain length and asymmetry of cations [N1111] o [N4444] E [N2222]o [N2224]. The Tg values of AAILs having asymmetric TAA cations are also found to be 10 to 20 1C lower than those having symmetric tetraalkylammonium cations, which can also be explained in terms of the symmetry and flexibility degree of the cations. Fig. 2 shows the DSC graphs of four ionic liquids ([N2224]-L-Ala, [N2222]Val, [N2221]-L-Ala and [N1114]-L-Ala) as examples to represent two types of phase transitions observed in all the 15 AAILs in the work. In addition, [TAA][amino acid]s are shown to have lower density than traditional ILs (such as imidazolium-based ILs22), with their values ranging from 0.94 to 1.1 g cm�3. It is also evident from Table 1 that large and asymmetric TAA cations usually generate AAILs of low densities. The [TAA][amino acid]s obtained in this work are thermally stable up to 170 to 190 1C as shown in Table 1 and exemplifed in Fig. 3. The Tdec values are generally found to decrease with increasing size and asymmetry of the cation, and [N2224] and [N1114] represent two cations of lower thermal stability. Changing the anion from Val to Gly, b-Ala or L-Ala, all with the same cation ([N2224], [N2221] or [N1114]), made little difference to the thermal stability, indicating that cations appear to play a more significant role than anions. In fact, since the amino acid anions are slightly basic, TAA cations of larger size and asymmetry undergo easier b-elimination,21 leading to lower thermal stability. The viscosities of [N2224], [N1114] or [N2221][amino acid]s at 25 1C are found to be much lower than those of [emim][amino acid]s14 and [TBP][amino acid]s.15 Among the AAILs in Table 1, all [N2224][amino acid]s, containing Gly, Val, L-Ala or b-Ala, show the lowest viscosities in comparison to the symmetric [TAA]-based AAILs and other asymmetric [TAA]-based AAILs. In particular, the viscosity of [N2224][L-Ala] is 29 mPa s, only about 1/3 of the viscosity of [N2222][L-Ala] (81 mPa s at 25 1C).14 It is even less viscous than many conventional ILs, such as ethylmethylimidazolium bis(trifluoromethanesulfonyl)- amide ([emim][Tf2N], 34 mPa s at 25 1C) 22 and ethylmethyl- imidazolium trifluoromethanesulfonate ([emim][CF3SO3], 45 mPa s at 25 1C).22 In addition, although [N2222][L-Ala] has the lowest viscosity among the symmetric TAA-based AAILs,20 the viscosities of [N2221][L-Ala] (84 mPa s) and [N1114][L-Ala] (62 mPa s) are also similar to or lower than that of [N2222][L-Ala], implying that the asymmetry of the cation does have a significant impact on the viscosity. Besides the asymmetry of the cations, the molecular size and asymmetry of the anions also apparently influence the viscosity of [TAA][amino acid]s. It is seen in Table 1 that the viscosities of [TAA][amino acid]s generally decrease in the order of [Val] 4 [b-Ala] 4 [Gly] 4 [L-Ala], no matter which tetraalkyl- ammonium is used as the counter cation. Since the viscosity of ILs generally decreases with decreasing molecular weight of the anion, it is reasonable that the viscosity decreases in the order of [TAA][Val] 4 [TAA][b-Ala] 4 [TAA][Gly]. However, although [TAA][L-Ala] has the same molecular weight as [TAA][b-Ala], [L-Ala] is more asymmetric than [Gly] and [b-Ala], which accounts for the viscosity of [TAA][L-Ala] being least. The fact that L-Ala anion forms even less viscous ILs with TAA cations than b-Ala reveals the significant impact of anion asymmetry on the viscosity. It is concluded that a carefully selected combination of molecular size and asymmetry for both the TAA cations and amino acid anions is essential to reduce the viscosity of [TAA][amino acid]s. In fact, both the five TAA cations and the four amino acid anions used in this work are among the simplest and most readily available species. Even though other amino acids of simple molecular structure, such as leucine ([Leu]), isoleucine ([Ile]), serine ([Ser]), cysteine ([Cys]) and methionine ([Met]), may also be used to form [N2224], [N2221] or [N1114]-based AAILs of possible low viscosity, it is believed that the viscosities of these AAILs may not be lowered to the level of [N2224][Val], [N2221][Val] or [N1114][Val] primarily due to the larger molecular size or the additional presence of polar groups (such as –OH, –SH) in such amino acid anions. On the other hand, it is also believed that there is little possibility of obtaining low viscosity AAILs by using more complex TAAs of carbon number larger than [N4444]. A possible way of preparing more AAILs of low viscosity is to use asymmetric TAA cations of carbon number not larger than 12, such as [N1223], [N2225], and [N1224]. However, since such asymmetric TAA cations are not commercially available nowadays and have to be prepared in the laboratory, the search for more AAILs of low viscosity is quite time-consuming but well worth doing. Nevertheless, the Fig. 2 Representative DSC traces at a heating rate of 10 1C/min. (a) [N2224]-L-Ala: melting point (Tm) and glass transition (Tg); (b) [N2222]Val: melting point (Tm) and glass transition (Tg); (c) [N2221]-L-Ala: single glass transition (Tg); (d) [N1114]-L-Ala: only glass transition(Tg). Fig. 3 Thermogravimetric analysis traces of six AAILs. This journal is �c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 2385–2390 | 2387 D ow nl oa de d by C hi na u ni ve rs ity o f P et ro le um (E ast C hin a) on 28 M ay 20 12 Pu bl ish ed o n 09 O ct ob er 2 00 9 on h ttp :// pu bs .rs c.o rg | d oi: 10 .10 39/ B9 NJ 003 30D View Online four [N2224]-based AAILs that utilize the four simplest and most readily available amino acids as the counter anions are of low cost and viscosity, and are thought to meet the criteria for engineering purposes that concern mass and heat transfer. The global warming due to the increased atmospheric CO2 concentration results primarily from the excessive consumption of fossil fuels and is becoming an important environmental issue today.23–25 Carbon sequestration, which captures CO2 from large point sources such as fuel gas, natural gas, water gas and waste gas from electrical power plants and stores it in geological formations, has been proposed as a solution to this problem. However, aqueous amines that are currently used most frequently in industry for large scale CO2 capture suffer from many technical difficulties, i.e., the uptake of water into the gas stream, high energy consumption during the regeneration of the absorbing solution, and the volatile loss of amine sequestering agent. As a result, innovative task-specific amino- terminated26,27 or amino acid-based ionic liquids16 were developed and proposed to be stable and non-volatile CO2 absorbents28 for the replacement of traditional aqueous amines. However, all these new ILs reported in the literature still have high viscosity, which disfavors the heat and mass transfer during the applications. Therefore, efficient separation of CO2 using low viscosity [TAA][amino acid]s is not only a potential addition to economically viable sequestration efforts, but also a good system for the verification of enhanced CO2 mass transfer benefiting from low viscosity. The absorption