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Highly efficient and reversible SO2 capture by halogenated carboxylate ionic liquids† Cite this: RSC Adv., 2015, 5, 60975

Guokai Cui,*a Yanjie Huang,a Ruina Zhang,b Fengtao Zhanga and Jianji Wang*a Because of the unique properties of ionic liquids, it has been suggested that ionic liquids, especially functionalized ionic liquids, could be used as good solvents for the capture of acidic gases such as SO2. In this work, a kind of carboxylate ionic liquid with a halogen atom on the alkyl chain of the carboxylate anion was developed for highly efficient and reversible capture of SO2 through multiple-site interactions. It was found that these halogenated carboxylate ionic liquids improved SO2 capture performance as well as being reversible. Spectroscopic investigations and quantum chemical calculations show that the Received 24th May 2015 Accepted 9th July 2015

enhancement in SO2 capacity originated from the halogen sulfur interaction between the halogen group on the carboxylate anion and SO2. Furthermore, the captured SO2 was easy to release by heating or

DOI: 10.1039/c5ra09752e

bubbling N2 through the SO2-saturated ionic liquids. This highly efficient and reversible process using halogenated carboxylate ionic liquids through adding a halogen group to the carboxylate anion provides

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an excellent alternative to current SO2 capture technologies.

Introduction Controlling and minimizing emissions of SO2 is very important, as it is a signicant source of atmospheric pollution that threatens the environment and human health. Although several conventional removal processes, such as limestone scrubbing and ammonia scrubbing, have been developed for ue gas desulfurization (FGD), the inherent disadvantages of these technologies should not be ignored, including the production of large quantities of wastewater and useless by-products.1 Accordingly, the development of new materials and processes for the efficient, reversible, and economical capture of these gases is highly desired and of critical importance. Ionic liquids (ILs) have been proposed as better gas absorbents due to their unique properties, such as extremely low vapor pressure, wide liquid temperature range, non-ammability, chemical stability, and tunable properties.2 Here, we show that both enhanced capacity and improved reversibility can be achieved by adding a halogen sulfur weak interaction for the capture of SO2. Acid gases such as SO2 (ref. 3) and CO2 (ref. 4) are expected to have a high solubility in ILs, especially in functionalized ILs. Given the fact that the reaction could be occurred between the

a

Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China. E-mail: [email protected]; jwang@ htu.cn

b

School of Mathematics and Information Science, Henan Normal University, Xinxiang, Henan 453007, China † Electronic supplementary information (ESI) available: NMR and IR data of ionic liquids, Table S1 and S2. See DOI: 10.1039/c5ra09752e

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acid gas and basic ion, thus lots of ILs with various basicity were developed to capture these acid gases in order to achieve higher absorption capacities, including those basic ILs based on amino cations,5 aprotic heterocyclic anions,6 thiocyanate anions,7 and phenolate anions.8 Normally, chemical absorptions always have high capacity for gas absorption along with high absorption enthalpy, which results in difficult desorption as well as high energy demand for regeneration.6c,9 Several methods including tuning the basicity,6b and substituents on the anion8a,10 were developed for tuning the absorption enthalpy, however, which oen lead to the reduced capacity due to the decrease of the interaction between IL and gas. Carboxylate ILs have been widely investigated to capture acid gases, especially SO2. The rst carboxylate IL used for SO2 absorption is 1,1,3,3-tetramethylguandinium lactate ([TMG][L]), which absorbed about 1.0 mole SO2 per mole of IL at 1 bar with 8% SO2 in a gas mixture of SO2 and N2.11 Maginn et al.12 studied the capture of CO2 in the 1-butyl-3-methylimidazolium acetate ([Bmim][CH3COO]) for the rst time and proposed a mechanism for the chemical reaction of the gas with the IL. Subsequently, other kinds of carboxylate ILs, including those with lactate,13 formate,14 acetate,15 amino acid anions,16 dicarboxylate,17 benzoate,18 and their substituted derivatives as anion, have also been developed and used for acid gas absorption. It was found that an organic acid that has a larger pKa value than sulfurous acid (pKa ¼ 1.81, 25 C) would be chosen and neutralized with a strong base in order to synthesize a functional IL to chemically capture SO2.19 However, these carboxylate ILs turn out to be some difficult in desorption as well as high energy demand for regeneration.15c,20 More recently, Wang et al. reported a dual method to improve SO2 capture through

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adding an electro-withdrawing interaction site on the anion of IL.18a,21 Herein, we designed and prepared a kind of carboxylate ILs with a halogen atom on the alkyl chain of the anion for improving SO2 capture (see Scheme 1 for these ILs' structures). Through a combination of absorption experiment, quantum chemical calculation, and spectroscopic investigation, we show that both enhanced capacity and improved reversibility for SO2 capture can be achieved due to the adding halogen sulfur weak interaction, resulting in highly efficient and excellent reversible capture.

Experimental Materials and characterizations 6-Bromo-n-hexanoic acid (6-BrC5H10COOH), 2-bromo-n-hexanoic acid (2-BrC5H10COOH), 6-chloro-n-hexanoic acid (6ClC5H10COOH), and n-hexanoic acid (C5H11COOH) were purchased from Sigma-Aldrich. Trihexyl(tetradecyl)phosphonium bromide ([P66614][Br]) were purchased from Strem Chemicals. A series of gases with different SO2 partial pressure were prepared by mixing SO2 (99.95%) and N2 (99.9993%), which were obtained from Beijing Oxygen Plant Specialty Gases Institute Co., Ltd. An anion-exchange resin (Amersep 900 OH) was obtained from Alfa Aesar. All chemicals were obtained in the highest purity grade possible, and were used as received unless otherwise stated. 1H NMR and 13C NMR spectra were recorded on a Bruker spectrometer (400 MHz) in DMSO-d6 or CDCl3 with tetramethylsilane (TMS) as the standard. FT-IR spectra were recorded on a Nicolet 470 FT-IR spectrometer. Viscosity was measured by a Brookeld DV2T viscometer. Melting points and decomposition temperatures were measured with a DSC Netzsch 204F1 meter and TGA Netzsch STA 449F3 instrument under N2 atmosphere at a heating rate of 10 C min1, respectively. Preparation of carboxylate ILs In a typical synthesis of carboxylate IL [P66614][C5H11COO], equimolar C5H11COOH was added to the phosphonium hydroxide ([P66614][OH]) solution in ethanol, which prepared from [P66614][Br] by the anion-exchange method.22 The mixture was then stirred at room temperature for 24 h. Subsequently, ethanol and water were distilled off at 60 C under reduced pressure. The obtained carboxylate ILs were dried under high vacuum for 24 h at 60 C to reduce possible traces of water. The structures of these ILs were conrmed by NMR and IR

Scheme 1 The structures of the cation and the halogenated carboxylate anions employed in this work for SO2 capture.

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spectroscopy (see the ESI†); no impurities were found by NMR. The water content of these ILs was determined with a Karl Fisher titration (Mettler Toledo DL32, Switzerland) and found to be less than 0.1 wt%. The residual bromide content of the IL was determined by a semi-quantitative Nessler cylinder method, which showed that bromide content was lower than 0.15 wt%. Absorption and desorption of SO2 In a typical absorption of SO2, SO2 of atmospheric pressure was bubbled through about 1.0 g IL in a glass container with an inner diameter of 10 mm, and the ow rate was about 60 ml min1. The glass container was partly immersed in a circulation water bath of desirable temperature. The amount of SO2 absorbed was determined at regular intervals by the electronic balance with an accuracy of 0.1 mg until the weight remained constant. The amount of SO2 absorbed could be calculated by subtracting the amount of IL. During the absorption of SO2 under reduced pressure, SO2 was diluted with N2 in order to reduce the partial pressure of SO2 passing through the system. The SO2 partial pressure was controlled by changing the ow rate ratio of SO2 and N2. The standard deviations of the absorption loadings under 1.0 bar is 0.05 mole SO2 per mole IL. Desorption of SO2 from SO2-saturated ILs was carried out and monitored in an analogous way as for the described absorption method. The ILs were regenerated by heating or bubbling N2 at 120 C through the SO2-saturated ILs. In a typical desorption of SO2, N2 of atmospheric pressure at a ow rate of about 60 ml min1 was bubbled through about 1.0 g SO2-saturated ILs in a glass container with an inner diameter of 10 mm, which was partly immersed in a circulation oil bath of 120 C. The amounts of SO2 desorption were measured at regular intervals by using an electronic balance with an accuracy of 0.1 mg until the weight was constant. The amount of SO2 desorbed could be calculated by subtracting the amount of IL.

Results and discussion Absorption of SO2 To investigate the absorption of SO2 by carboxylate ILs with a halogen atom on the alkyl chain of the anion, several different carboxylic acids, such as 6-BrC5H10COOH, 2-BrC5H10COOH, 6ClC5H10COOH, and C5H11COOH were selected. These carboxylate ILs were prepared by the acid–base neutralization between different carboxylic acids and a solution of [P66614][OH] in ethanol, which was obtained by the anion-exchange method. Physical properties of these carboxylate ILs including density, viscosity, melting point, and thermal stability were also determined and the data were listed in Table S1.† In general, halogenated carboxylate ILs had higher viscosities and densities in comparison with their corresponding halogen-free counterpart. All carboxylate ILs decompose at temperatures (Tdec) higher than 200 C, implying that they have enough thermal stability for application in SO2 capture, and the halogenated carboxylate ILs exhibited higher thermal stability because of the decreased basicity.6e For example, the decomposition temperature of


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[P66614][6-BrC5H10COO] is 279 C, while that of [P66614] [C5H11COO] is 240 C. The effect of chemical structure of these carboxylate ILs on the performances of SO2 absorption was investigated, which was listed in Fig. 1. It was seen that SO2 absorption capacities by [P66614][6-BrC5H10COO], [P66614][6-ClC5H10COO], [P66614][2BrC5H10COO], and [P66614][BrCH2COO] were 4.34, 4.28, 3.97, and 3.89 (ref. 18a) mole SO2 per mole IL, respectively, while that by [P66614][C5H11COO] and [P66614][CH3COO] were 3.82 and 3.48 (ref. 18a) mole SO2 per mole IL. The higher absorption capacity for [P66614][C5H11COO] with longer alkyl spacer length are due to the increased van der Waals interaction between the alkyl groups and SO2.23 Clearly, compared to halogen-free ILs, halogenated carboxylate ILs such as [P66614][6-BrC5H10COO], [P66614] [6-ClC5H10COO] and [P66614][BrCH2COO] with a Br atom or Cl atom on the terminal of alkyl of the anion exhibited higher SO2 absorption capacities. For [P66614][2-BrC5H10COO] with a Br atom located near the carbonyl group, although a little higher capacity was achieved during the SO2 absorption compared with that by [P66614][C5H11COO], the absorption capacity was lower than by [P66614][6-BrC5H10COO]. As the concentration of SO2 in ue gas is low,24 the effect of the SO2 partial pressure on the absorptions by halogenated carboxylate IL [P66614][6-BrC5H10COO] and halogen-free IL [P66614][C5H11COO] has been investigated (Fig. 2a). It was seen that the molar ratios of SO2 to IL for [P66614][6-BrC5H10COO] and [P66614][C5H11COO] decreased 4.34 to 1.61 and 3.82 to 1.70, respectively, when the SO2 partial pressure decreased from 1.0 to 0.1 bar. Clearly, the SO2 absorption capacity by [P66614][6BrC5H10COO] is signicantly higher than that by [P66614] [C5H11COO] at 1 bar. Fig. 2b shows the temperature dependence of the SO2 absorption of [P66614][6-BrC5H10COO] and [P66614] [C5H11COO] at 1 bar. As can be seen, the SO2 absorption capacity by [P66614][6-BrC5H10COO] decreased signicantly than that by [P66614][C5H11COO] with the increase of the temperature. As can be seen, the SO2 absorption capacity by [P66614][6-BrC5H10COO] and [P66614][C5H11COO] decreased from 4.34 to 1.72 and 3.82 to 1.74 mole SO2 per mole IL, respectively, when the temperature increased from 20 C to 80 C. Clearly, the difference for SO2 capture by [P66614][6-BrC5H10COO] and [P66614] [C5H11COO] becomes smaller and smaller with the decreasing of the SO2 partial pressure or the increasing of the temperature.

Fig. 1 SO2 absorption by carboxylate ILs as a function of time at 20 C and 1 bar under SO2 (60 ml min1).


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However, the absorption capacities of these two carboxylate ILs are very close at relatively low pressure and high temperature, which showed the chemical interaction between the carboxyl group in the anion of IL and SO2. The difference between these two ILs is the former has a Br atom in the anion, which results in the higher absorption capacity at atmospheric pressure and room temperature, because of the weak physical interaction between the Br atom in the anion and S atom in the SO2. These results indicated that there are the combinations of physical absorption and chemical absorption in these carboxylate anionfunctionalized ILs. The effect of water on SO2 absorption by carboxylate ILs was also investigated, which was shown in Table S2.† The results showed that the SO2 absorption capacity decreased when water was added to the IL, presumably due to reprotonation of the anion.25 For example, SO2 absorption capacity of [P66614] [C5H11COO] was 3.28 mole SO2 per mole IL at 20 C and 1 bar under 100% humidity SO2, while 3.82 mole SO2 per mole IL could be achieved under dry SO2. Desorption of SO2 The desorptions of SO2 by these carboxylate ILs were investigated, which were shown in Fig. 3. As can be seen, the desorptions of SO2 by halogenated carboxylate ILs such as [P66614][6BrC5H10COO] and [P66614][2-BrC5H10COO] at 120 C were complete, while the desorption of SO2 by halogen-free IL [P66614] [C5H11COO] is not complete, where about 0.5 mole SO2 per mole IL remained, thus resulting in the available absorption capacity of 3.3 mole per mole IL during the SO2 absorption. It indicates that the Br atom in the anion of halogenated carboxylate ILs is important to the release of SO2. Thus, the halogenated carboxylate ILs exhibited the higher available capacities of SO2 capture due to the high absorptions as well as the facile desorptions. Reversibility of ILs The absorption/desorption recycle of IL is a critical property for gas absorption with direct effect on the economics, because it determines the frequency of IL's replacement.26 Fig. 4 shows the comparison with SO2 absorption/desorption process by [P66614]

Fig. 2 The effect of SO2 partial pressure at 20 C (a) and temperature at 1 bar (b) on SO2 capture by halogenated carboxylate IL [P66614] [6-BrC5H10COO] ( ) compared with halogen-free IL [P66614] [C5H11COO] ( ).

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SO2 absorption by [P66614][6-BrC5H10COO] for 30 cycles. SO2 absorption was carried out at 20 C and 1 bar under SO2 (60 ml min1), and desorption was performed at 120 C and 1 bar under N2 (60 ml min1). Fig. 5

The effect of halogen group in halogenated carboxylate ILs on SO2 absorption and desorption as a function of time. SO2 absorption was carried out at 20 C and 1 bar under SO2 (60 ml min1), and desorption was performed at 120 C and 1 bar under N2 (60 ml min1). [P66614][6-BrC5H10COO]: absorption, ; desorption, . [P66614][2BrC5H10COO]: absorption, ; desorption, . [P66614][C5H11COO]: absorption, C; desorption, B. Fig. 3

[6-BrC5H10COO] and [P66614][C5H11COO]. As shown, all the absorption capacities and rapid absorption rates of carboxylate ILs remained steadily, while the desorption of SO2 by halogenfree IL [P66614][C5H11COO] is not complete during these cycles. Furthermore, the stability of these ILs was also investigated under typical desorption condition (120 C, 1.0 bar, 60 ml min1 N2) for 30 h, which was shown in Fig. S1.† It can be seen that both of [P66614][6-BrC5H10COO] and [P66614][C5H11COO] lost about 0.3% weight aer 30 h, indicating the high stability of carboxylate ILs. We selected [P66614][6-BrC5H10COO] as a sorbent material to investigate the stability of SO2 absorption during the recycling of halogenated carboxylate IL. The 30 absorption/desorption cycles by [P66614][6-BrC5H10COO] are shown in Fig. 5. It was clear that [P66614][6-BrC5H10COO] could be repeatedly recycled for more than 30 times without a loss of absorption capability, indicating that the process of SO2 absorption by these halogenated carboxylate ILs is highly reversible. Considering both enhanced absorption and easier desorption by these halogenated carboxylate ILs, we believe that

Fig. 4 The comparison with SO2 absorption/desorption process by [P66614][6-BrC5H10COO] and [P66614][C5H11COO]. SO2 absorption was carried out at 20 C and 1 bar under SO2 (60 ml min1), and desorption was performed at 120 C and 1 bar under N2 (60 ml min1). For [P66614] [6-BrC5H10COO]: absorption, ; desorption, . For [P66614] [C5H11COO]: absorption, C; desorption, B.

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halogen groups such as Br and Cl on the anion, especially on the terminal alkyl of the anion, exhibited the dual-tuning role for improving the capture of SO2. To the best of our knowledge, [P66614][6-BrC5H10COO] exhibits superior available absorption than other ILs up to date (Table S3†). Generally, an electronwithdrawing substituent on the anion would reduce the interaction between the anion and gas, resulting in the decreased capacity. Why do these halogenated carboxylate ILs exhibit such a different behavior for SO2 capture? We believe that such a different behavior of halogen group in these ILs may be contributed to the following two factors. Firstly, there lies in weak halogen sulfur interaction between halogen atom on the alkyl chain of the anion and SO2, leading to the increase of absorption capacity. Secondly, halogen group is a kind of electron-withdrawing group, which disperses the negative charge of the O atoms on the anion and decreases the enthalpy for SO2 absorption, resulting in the improved desorption. Therefore, halogen group should play a dual role both as an added interaction site and as an electron-withdrawing group, which improves signicantly the capture of SO2.

Quantum chemical calculations To investigate the dual role of halogen group on the anion in these halogenated carboxylate ILs, we calculated the Mulliken atomic charge of the oxygen and halogen atoms in the anion using the Gaussian 09 program27 as the literature.28 As can be seen in Table 1, compared with the Mulliken atomic charge of O atom in the halogen-free anion, that in the halogenated anions decreased because halogen group is a kind of electronwithdrawing group, resulting in the reduced interaction. For example, the Mulliken atomic charge of O atom in [6-BrC5H10COO] is 0.580, while that in [C5H11COO] is 0.609. On the other hand, halogen group shared the negative charge of O atom in the anion, which enhanced halogen sulfur interaction between halogen group and SO2, leading to the increase of absorption capacity. For example, the Mulliken atomic charge of Br atom in [6-BrC5H10COO] is 0.184, while that in 6-bromon-hexanoic acid is 0.127. To further investigate the interaction between carboxylate ILs and SO2, and the dual role of halogen group on the capture of SO2, we calculated the geometry optimization for the free


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RSC Advances The effect of different carboxylate ILs on SO2 absorption capacity and SO2 absorption enthalpy


Mulliken atomic chargec Ionic liquid

Absorption capacitya

Absorption enthalpyb,c

O

X

[P66614][6-BrC5H10COO] [P66614][6-ClC5H10COO] [P66614][2-BrC5H10COO] [P66614][C5H11COO] [P66614][BrCH2COO]d [P66614][CH3COO]d

4.34 4.28 3.97 3.82 3.89 3.48

25.0 16.9 35.4 — 35.9 —

0.580 0.603 0.573 0.609 0.583 0.627

0.184 0.053 0.306 — 0.237 —

a Reaction conditions: SO2 absorbed at 20 C and 1 bar for 30 min under SO2 (60 ml min1), mole SO2 per mole IL. b Interaction enthalpies of the complexes with halogen group on the anion with the closest SO2 molecule, kJ mol1. c Carried out at B3LYP/6-31++G(d,p) level. d Values are cited from the ref. 18a.

halogen-free anions, the halogenated anions, the free SO2, and the complex of the halogen with the closest SO2 at the B3LYP/631++G(d,p) level.29 The optimized structures and the energetics, which reect the carbonyl sulfur interactions (C]O/S) and halogen sulfur interaction (X/S), were shown in Fig. 6 and Table S4.† It can be seen from Fig. 6a and b that the intermolecular O/S distances between O atoms of carboxyl group and S atom of SO2 in [C5H11COO]–2SO2 complex were both predicted ˚ Additionally, hydrogen bonds were formed to be 2.270 A. between the C–H bonds of the carboxylate anion and the S]O bond of the SO2. As can be seen from Fig. 6b, there are two S] O/H–C hydrogen bonds formed between SO2 and [C5H11COO] anion, and the bond distances were both predicted to be 2.531 ˚ 30 Furthermore, the calculated absorption enthalpies for A. [C5H11COO]–SO2 and [C5H11COO]–2SO2 complexes were 102.6 and 60.2 kJ mol1, respectively, indicating the chemical interactions between the carboxylate ILs and SO2.

Fig. 6c shows the intermolecular Br/S distances in [6BrC5H10COO]$SO2 complex ([6-BrC5H10COO] anion with the ˚ which closest SO2 molecule) was predicted to be 3.365 A, corresponds to a reduction of approximately 7.8% of the sum of van der Waals radii31 of the two interacting atoms. Moreover, the close contact between nucleophilic Br group in [6-BrHexCOO] anion and electrophilic S atom in SO2 involved the lateral sides of the halogen, and the calculated C(sp3)–Br/S angle amounts to 85.4 .32 The absorption enthalpy of bromide sulfur interaction for [6-BrC5H10COO]$SO2 complex was found to be 25.0 kJ mol1 while that for [6-BrC5H10COOH]$SO2 complex was 12.6 kJ mol1, indicating that the halogen sulfur interaction between [6-BrC5H10COO] anion and acidic SO2 is strong, resulting in the increase of absorption capacity. Furthermore, we also calculated the gas-phase reaction energetics between the anion [6-BrC5H10COO] and SO2. As can be seen in Fig. 6d–f, the calculated absorption enthalpies for [6BrC5H10COO]–SO2, [6-BrC5H10COO]–2SO2, and [6-BrC5H10COO]–3SO2 complexes were 102.0, 60.2, and 13.8 kJ mol1, respectively, indicating that the similar chemical absorption capacities could be reached by [P66614][6-BrC5H10COO] and [P66614][C5H11COO], and Br group on the [6-BrC5H10COO] anion as an added interaction site leads to the increase of physical absorption capacity. Spectroscopic investigations The interactions of SO2 and these carboxylate ILs were further investigated by FT-IR and NMR spectroscopy to support the

Fig. 6 Optimized structures of [C5H11COO]–SO2 (a–b) and [6-BrC5H10COO]–SO2 (c–f) complexes at B3LYP/6-31++G(d,p) level. (a–b) Multiple-site interactions between [C5H11COO] and SO2 molecules: (a) [C5H11COO]–SO2, DH ¼ 102.6 kJ mol1; (b) [C5H11COO]–2SO2, DH ¼ 60.2 kJ mol1; (c) Br atom in [6-BrC5H10COO] anion with the closest SO2 molecule, DH ¼ 25.0 kJ mol1; (d–f) multiple-site interactions between [6-BrC5H10COO] and SO2 molecules: (d) [6-BrC5H10COO]– SO2, DH ¼ 102.0 kJ mol1; (e) [6-BrC5H10COO]–2SO2, DH ¼ 60.2 kJ mol1; (f) [6-BrC5H10COO]–3SO2, DH ¼ 13.8 kJ mol1. Note that van der Waals radii (in A ˚ ) are 1.70 (C), 1.20 (H), 1.52 (O), 1.55 (N), 1.80 (S), 1.75 (Cl), 1.85 (Br).31 O, red; S, yellow; N, blue; C, gray.


Fig. 7 The FT-IR spectrum of the fresh IL [P66614][C5H11COO] (gray area) and [P66614][C5H11COO] after the absorption of SO2 (red line).

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Fig. 8 The compared FT-IR spectrum of SO2-saturated [P66614][6BrC5H10COO] (red line) and SO2-saturated [P66614][C5H11COO] (gray area).

experimental and computational results. Fig. 7 shows the IR spectrum of [P66614][C5H11COO] before and aer the absorption of SO2. As can be seen, compared with the FT-IR spectra of the fresh IL [P66614][C5H11COO], new bands at 1327 and 1145 cm1 produced, which can be assigned to asymmetric stretching and symmetric vibration of S]O bonds, respectively, and that at 938 cm1 attributable to S–O stretches,6c,11 formed via the absorption of SO2. In addition, the band at 1578 cm1 should be assigned to the C]O vibration in carboxyl group (–COO) of fresh IL [P66614][C5H11COO], where this mode was blue-shied by 141 cm1 in SO2-saturated [P66614][C5H11COO] at 1719 cm1 because of the electrophilic environment due to SO2.30 Meanwhile, another new band at 1044 cm1 should be assigned to the vibration of O–S stretching mode of –OSO2 (Fig. S2†),15c,33 indicating the strong CO/SO2 interactions between carboxyl group on the anion and SO2. Furthermore, new band at 2462 cm1 should be assigned to the S]O/H–C hydrogen bonding interactions between [C5H11COO] anion and SO2.34 To further study the interaction mode of the SO2 absorption by [P66614][6-BrC5H10COO], the compared FT-IR spectra of SO2saturated [P66614][6-BrC5H10COO] and SO2-saturated [P66614] [C5H11COO] was investigated (Fig. 8). As can be seen, compared with S]O stretching bands in neat SO2 at 1360 and 1150 cm1 (ref. 15c and 35) and in SO2-saturated [P66614][C5H11COO] at 1327 and 1145 cm1, SO2-saturated [P66614][6-BrC5H10COO]

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exhibits a red shi of the asymmetric stretching and symmetric vibration of S]O bonds at 1314 and 1127 cm1, respectively, indicating the presence of the halogen sulfur interaction between the anion and SO2.18a,36 The typical peak of the carboxyl group on the anion of [P66614] [C5H11COO] in 13C NMR spectra moved up-eld from 179.3 ppm to 177.1 ppm, while that the peak of carbon adjacent to the carboxyl group also moved up-eld from 39.6 ppm to 34.2 ppm (Fig. 9), indicating the strong CO/SO2 interaction between carboxyl group on the anion and SO2. On the basis of previous reports18a,30 and the observed product, the SO2 absorption mechanism for carboxylate IL is presented in Fig. 6, which exhibits multiple-site interactions between the carboxylate anion and SO2.

Conclusions In summary, new kinds of carboxylate ILs were developed for SO2 capture through adding an other interaction site such as halogen group on the anion to improve the SO2 absorption performance. These halogenated carboxylate ILs exhibited both enhanced capacity and easier desorption than halogen-free ILs, resulting in highly efficient and excellent reversible SO2 capture. Spectroscopic investigations and quantum chemical calculations showed that the capacity increased because of the presence of halogen sulfur interaction between halogen group and SO2, while the desorption improved due to the role as an electron-withdrawing group. This kind of carboxylate ILs developed in this work allows us to have a new insight into gas capture, which opens a door to achieve high capacity as well as excellent reversibility to capture other gas such as H2S and CO2 by ILs.

Acknowledgements We acknowledge the support from the National Natural Science Foundation of China (No. 21403059, 21133009), the Natural Science Foundation of Henan Province (No. 142300413213), the S&T Research Foundation of Education Department of Henan Province (No. 14A150031), and the Doctoral Scientic Research Foundation of Henan Normal University (No. qd13007).

Notes and references

Fig. 9 The 13C NMR spectrum of the fresh IL [P66614][C5H11COO] (blue) and [P66614][C5H11COO] after the absorption of SO2 (red). The NMR data were measured at 20 C in CDCl3.

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