SO2 Absorption by Carboxylate Anion-Based Task-Specific Ionic

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SO2 Absorption by Carboxylate Anion-Based Task-Specific Ionic Liquids: Effect of Solvents and Mechanism Junhai Zhao,† Shuhang Ren,† Yucui Hou,‡ Kai Zhang,† and Weize Wu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 10029, China Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

‡

S Supporting Information *

ABSTRACT: Task-specific ionic liquids (TSILs) have been widely observed to effectively absorb low-concentration SO2 from flue gas by chemical interaction. However, the interaction between SO2 and TSILs is still unclear. The addition of solvents can decrease the viscosity of TSILs and promote mass transfer, but whether the solvents have an impact on absorption is unknown, as is the effect on absorption mechanism. To solve these issues, we synthesized several types of TSILs containing carboxylate anion to capture SO2 from simulated flue gas. The mechanism of absorption of SO2 by the TSILs was investigated in detail using Fourier transform infrared (FT-IR), 1H nuclear magnetic resonance (NMR), and 13 C NMR. The results show that chemical interactions can be found between the carboxylate anions of TSILs and SO2. Simultaneously, the effect of several solvents on SO2 absorption capacity of TSILs was studied. The results indicate that ethylene glycol (EG) in TSILs has a strong influence on SO2 absorption by guanidinium- and alkanolaminium-based TSILs, but for quaternary ammonium-based TSILs, EG has no effect. Finally, the interaction between EG and TSILs was investigated by FT-IR, and the absorption mechanism was studied. It has been found that the addition of EG can improve the basicity of guanidinium- and alkanolaminium-based TSILs to increase the absorption capacity.

1. INTRODUCTION

partial pressures. Otherwise, the IL is just a normal IL, and its absorption efficiency is low. The mechanism of absorption of SO2 by ILs has also been studied in the literature. Generally, there is no doubt that physical interaction exists between the normal ILs and SO2. However, there are different understandings about the absorption mechanism of TSILs. Han et al.11 synthesized the first TSIL, 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), to capture SO2. By comparing the Fourier transform infrared (FT-IR) and 1 H nuclear magnetic resonance (NMR) spectra of [TMG]L before and after SO2 absorption, they suggested that SO2 reacted with the N−H group of the cation to form a new N−S band, and the [TMG]L:SO2 stoichiometric ratio is 1:1. Wu et al.12,13 concluded that 1 mol of monoethanolammonium lactate ([MEA]L) could chemically capture 0.5 mol of SO2, and they distinguished the physical and chemical absorption capacity of SO2. The studies described above show that SO2 reacts with TSILs on the cation but the stoichiometric ratio is different, whereas other studies indicate SO2 reacts with the anion of TSILs. Wang et al.14,15 used theoretical research to study the solubility of SO2 in [TMG]L. They found both the anion and the cation interacted with SO2, which formed a S···O band with the

Sulfur dioxide (SO2) emitted from the burning of fossil fuels is a kind of atmospheric pollutant. It not only threatens the health of people but also has a strong damaging effect on soil and buildings. Today, flue gas desulfurization (FGD) is regarded as an effective solution for capturing SO2 from the burning of fossil fuels.1,2 Several processes, including wet FGD and dry FGD, have been developed to make flue gases clean.3−5 However, some disadvantages, such as byproduct (calcium sulfate) production and solvent volatilization, may cause secondary pollution. Consequently, recyclable solvents with low volatilities and high absorption capacities are proposed as excellent absorbents for capturing SO2. Recently, ionic liquids (ILs), because of their unique properties, especially their low vapor pressure and designable structure,6−8 have been regarded as environmentally benign solvents for separation of acidic gases, especially SO2. Recently, a number of ILs have been studied for the purpose of SO2 capture.9 These ILs can be separated into normal ILs and task-specific ILs (TSILs) on the basis of the interaction between ILs and SO2. It was reported that the pKa value of organic acids, which were used to synthesize ILs as the anions, could be used to identify TSILs and normal ILs for SO2 capture.10 They proposed that if the organic acid is weaker than sulfurous acid [pKa(acid) > 1.81], the ILs containing the organic acid as the anion can be called TSILs for SO2 absorption and it can absorb SO2 effectively with low SO2 © XXXX American Chemical Society

Received: July 22, 2016 Revised: November 22, 2016 Accepted: November 28, 2016

A

DOI: 10.1021/acs.iecr.6b02801 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research lactate anion and a N−H···O band with the [TMG]+ cation. However, the S···O interaction is much stronger than N−H···O hydrogen bonding, which suggests that the chemical absorption process was mainly due to the strong interactions between SO2 and the lactate anion. Highly thermal stable ILs, tetraethylammonium lactate ([N2222]L) and 1-butyl-3-methylimidazolium lactate ([Bmim]L), are synthesized to capture SO2 at high temperatures,16 and the results indicate that SO2 reacts with the hydroxyl (-OH) of the lactate anion to form a new bond. Wu et al.17 synthesized six kinds of carboxylic acid-based ionic liquids, triethylbutylammonium dicarboxylates. They proposed that interactions between the oxygen atom in the anions could form a strong structure with the sulfur atom in SO2 based on FTIR spectra and structural calculations. These carboxylic acidbased TSILs that do not contain -OH can also chemically capture SO2 with low SO2 partial pressures, which is incompatible with a former work.16 Although the mechanism of SO2 capture by TSILs has been investigated by several research groups, the mechanism is still unclear. The high viscosity of TSILs seriously affects the mass transfer of the absorption process. Wu et al.18 have determined the viscosity of pure [TMG]L, which is 663 mPa s at 40 °C, and the viscosity increases with an increase in the SO2 content of the IL. As the high viscosity is unfavorable for the practical use of ILs, many studies focus on the technologies, such as supported ILs,19 polymerized ILs,20 and aqueous IL solutions,21 to reduce the viscosity or prevent the direct use of TSIL. Wu et al.16,22 found that water had an effect on the absorption of SO2 by TSILs, and the absorption mechanism was different from the absence of water. However, the effect of other solvents on the capture of SO2 has not been studied. Carboxylate is a basic anion in ILs.23 Because of the high pKa of carboxylate acids, almost all ILs with the anion of carboxylate can be called TSILs for SO2 capture. They can absorb SO2 on the basis of physical interaction and chemical interaction,11,13 and the physical effect is almost negligible at low SO2 partial pressures. The chemical absorption is dominated for capturing SO2 at low concentrations in gases.12 In this work, we synthesized several carboxylic acid-based TSILs and studied the absorption mechanism of SO2 by FT-IR, 1H NMR, and 13C NMR spectra. Meanwhile, the effect of several solvents on the absorption of SO2 by TSILs has been studied. Finally, the interaction between ethylene glycol as a solvent and TSILs was investigated by FT-IR, and their absorption mechanism was analyzed.

2.2. Absorption and Desorption. Pure SO2 and N2 gas were mixed together at high pressures in a 40 L gas cylinder to prepare a simulated flue gas with 3% SO2 by volume. The uncertainty in SO2 content was estimated to be less than ±2%. The absorption experiment and desorption were performed at an ambient pressure. The apparatus in this work are the same as our previous work.10,13,18,22 In a typical experiment, the gas mixture was bubbled through the IL loaded in the test tube, the absorption of SO2 was treated with SO2-containing gas at a rate of 50 cm3/min, and the desorption of SO2 was treated with 100 cm3 of N2/min, which was monitored by a rotameter and calibrated by a soap-film flow meter. The test tube was partly immersed in the oil bath. After a certain period of absorption, the content of absorbed SO2 in the IL was calculated by the weight difference. Saturated SO2:IL mole ratios represent the absorption capacities of SO2 in ILs, which is defined as Rs, the SO2:IL mole ratio when the IL is saturated. The desorption efficiency, DE, is defined as (Rs − Rt)/Rs × 100%, where Rt is the SO2:IL mole ratio at the determination time. The reproducibility of the contents of absorbed SO2 in ILs was better than ±2.5%, and the data had an estimated standard uncertainty of 5%. 2.3. FT-IR ,1H NMR, and 13C NMR Characterizations. 1H NMR and 13C NMR spectra were used to identify a physical or chemical procedure of SO2 absorption by the displacement of the wavelength in the 1H NMR and 13C NMR spectra before and after absorption.27 1H NMR and 13C NMR spectra were recorded using a NMR spectrometer (AV 600, Bruker), and FT-IR spectra were recorded using a FT-IR spectrometer (Nicolet 6700). The reproducibilities of the chemical shift in NMR were better than ±0.5%, and it was estimated that the data had a standard uncertainty of 1%.

3. RESULTS AND DISCUSSION 3.1. Absorption of 3% SO2 by Carboxylic Acid-Based TSILs. Several carboxylic acid-based TSILs were investigated to absorb 3% SO2. Figure 1 shows their structures. Absorption capacities of SO2 by the different ILs at ambient pressure and 40.0 °C are listed in Table 1. Obviously, all of the carboxylic acidbased ILs can absorb low-concentration SO2 with high absorption capacities. In addition, their absorption capacities of SO2 are different from each other, because of the properties of ILs and the absorption mechanism. For comparison, the absorption capacity of SO2, at a SO2:[TMG]L mole ratio of 0.53 at 40 °C, is slightly higher than 0.51 at 40.0 °C, as reported by Ren et al.18 The reason is that 3% SO2 level in this work is greater than the 2% level used by the authors. Moreover, the absorption capacities of SO2 in carboxylic acid-based TSILs in this work are also compared with those of carboxylic acid-based TSILs reported in the literature, which are listed in Table S1. 3.2. Effect of Chloroform-d on the NMR Spectra of ILs before and after SO2 Absorption. In conventional 1H NMR experiments, deuterated reagents are mixed with samples as the internal references, and then the chemical shifts of atoms can be analyzed. Deuterated solvents may affect the interaction between ILs and SO2 or interact with SO2 and then affect the chemical shifts.28 As a result, the effect of chloroform-d on the NMR spectra of ILs before and after SO2 absorption needs to be studied. The NMR experiments were performed with the internal and external references. A sample mixed in CDCl3 was an internal reference. For external references, the sample and the deuterated reagent (CDCl3) were injected into capillary tubes (ϕ0.9 mm × 10 cm) and NMR tubes (ϕ5 mm × 17.8 cm),

2. EXPERIMENTAL SECTION 2.1. Materials and Analysis Method. SO2 (99.95%) and N2 (99.99%) were obtained from Beijing Haipu Gases. Monoethanolamine (99%), 1,1,3,3-tetramethylguanidine (99%), tetraethylammonium hydroxide (25% aqueous solution), DL-lactic acid (80−85%), propanoic acid (99.5%), succinic acid (99.5%), malic acid (99%), ethylene glycol (EG, 99%), polyethylene glycol (PEG, average Mn of 200), dimethyl sulfoxide (DMSO, 99.5%), and N-methyl pyrrolidone (NMP, 99.5%) were supplied by Aladdin Chemical Co., Ltd. (Shanghai, China). All acids and solvents were analytical reagent grade. All carboxylic acid-based TSILs in this work were synthesized and characterized according to the literature.11,24,25 A sweeping method26 was used to dry the ILs in this work, and their water contents measured by Karl Fischer analysis were [BMIM]L > [TMG]L > [MEA]L. Under the condition that includes the same cation, the larger pKa the carboxylate anion, the stronger the basicity for the ILs, further corresponding to a stronger absorption capacity of SO2. Because the basicity of [MEA]L is lower than that of [MEA]P, [MEA]P E

DOI: 10.1021/acs.iecr.6b02801 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research capture and weakens the interaction between SO2 and -COO− of the carboxylate anion to reduce the enthalpy for the capture of SO2. For example, as shown in Figure 7, quaternary ammonium

Figure 8. Absorption of SO2 by several solvents at 40 °C with 3% SO2.

absorption capacity of SO2 by the mixed absorber that consists of an IL and a solvent. 3.4.2. Effect of Solvents on the Absorption of 3% SO2. As we all know, the content of water in flue gases is very high. The presence of H2O during the absorption process has been studied by many groups. Deng et al.34 and Zhang et al.30 found that the SO2 absorption capacities of [C4bet][SCN] and [C4by][SCN] in the presence of water exhibit almost no change. Ren et al.18 and Tian et al.16 also investigated the effect of water on SO2 capture by carboxylic acid-based TSILs ([TMG]L and [N2222]L), and they proposed that the absorption mechanism would be changed when the water exists in ILs. They demonstrated that SO2 reacts with water and forms H2SO3, and then the IL reacts with H2SO3 to yield lactic acid because lactic acid (pKa = 3.86) is weaker than H2SO3 as an acid (pKa = 1.89). Although water can reduce the viscosity of ILs, the evaporation of water happens upon desorption of SO2, resulting in energy consumption. Therefore, in this work, we added other nonvolatile and stable solvents and investigated their effects on the absorption of SO2 by TSILs. Figure 9 shows the trends of SO2 absorption capacity

Figure 7. Desorption of SO2 in the four TSILs at 100 °C as a function of time.

malate ILs have desorption efficiencies higher than those of quaternary ammonium succinate ILs, indicating that the existence of -OH of the carboxylate anion slightly improved the desorption efficiency of SO2. On the basis of the discussions presented above, we proposed a new mechanism for the chemical capture of SO2 by carboxylic acid-based TSILs, as presented in Scheme 1. The absorption Scheme 1. Proposed Mechanism of SO2 Absorption by Carboxylic Acid-Based TSILs

process is practically reversible, different from the absorption by sodium hydroxide (NaOH). There is a chemical equilibrium in this absorption process that relates to the absorption temperature, the SO2 partial pressure, and the properties of the absorbent. 3.4. Effect of Solvent on the Capture of SO2 by TSILs. The high viscosity of ILs seriously affects the mass transfer of the absorption process. Therefore, it is necessary to take some measures to improve the transportation. The main method is to add water or other solvents in ILs, but does adding the solvents have an effect on absorption capacity? 3.4.1. Absorption of 3% SO2 by Solvents. First, we choose several nonvolatile or stable solvents to investigate their effect on the absorption ability of 3% SO2. As shown in Figure 8, alcohols (EG and PEG) and water almost cannot absorb SO2 at a low concentration of 3%,33 while dimethyl sulfoxide (DMSO) and Nmethyl pyrrolidone (NMP) have very small absorption capacities of 3% SO2 (0.062 and 0.104 mol of SO2/mol of solvent, respectively). Hence, it is necessary to subtract the absorption capacity of SO2 by the added solvent when we determine the

Figure 9. Effect of solvent on the absorption of SO2 by [TMG]L at 40 °C with 3% SO2.

in [TMG]L with different mass fractions of solvent to [TMG]L at 40 °C. We can see that when EG or PEG was added to the IL, the absorption capacity of SO2 in [TMG]L increased greatly with the mass fraction of the added solvent. Comparatively, DMSO and NMP have almost no effect on the absorption capacity of SO2 by [TMG]L. Therefore, the addition of EG (or PEG) may affect the chemical absorption equilibrium because EG (or PEG) cannot capture SO2 at a low concentration of 3% (see Figure 8). F

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Figure 10. Effect of EG on the absorption of SO2 by (a) [MEA]L and (b) [TMG]L at 40 °C with 3% SO2.

Figure 11. Effect of EG on the absorption capacity of SO2 by (a) [TMG]P, (b) [TEA]P, (c) [N2222]L, and (d) [N2222]P at 40 °C with 3% SO2.

Because EG had a strong effect on the SO2 absorption by [TMG]L, we mainly studied the effect of EG on the SO2 absorption by carboxylic acid-based TSILs. Figure 10 shows the trends of SO2 absorption as a function of time and the

absorption capacities of SO2 in [MEA]L and [TMG]L with different mass fractions of EG at 40 °C with 3% SO2. It can be seen that the addition of EG not only shortens the absorption time to equilibrium but also increases the absorption capacities of G

DOI: 10.1021/acs.iecr.6b02801 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 12. 1H NMR spectra of (a) [TMG]P and (b) [N2222]P before and after SO2 absorption (CDCl3): (A) IL, (B) IL and EG, (C) IL and SO2, and (D) IL, EG, and SO2.

SO2 with an increase in the EG mass fraction. For example, when EG was not added to [MEA]L, the SO2:[MEA]L mole ratio was 0.28. When the mass fraction of EG increased to 0.3 and 0.6, the SO2:[MEA]L mole ratio increased to 0.38 and 0.51, respectively. The results indicate that the EG present in [MEA]L and [TMG]L plays a positive role in SO2 absorption capacity. Whether EG has the same impact on SO2 absorption by other carboxylic acid-based TSILs has also been investigated, and the results are shown in Figure 11. As expected, EG has the same effect on [MEA]P and [TMG]P absorbing SO2. Interestingly, the SO2 absorption capacity of the quaternary ammonium-based TSILs ([N2222]P and [N2222]L) does not obviously change with the addition of EG. Therefore, the presence of EG in TSILs has a strong influence on absorption by guanidinium- and alkanolaminium-based TSILs, but for quaternary ammonium-based TSILs, EG has no effect. The phenomena are explained in the following section. 3.5. Effect of EG on the Mechanism of Absorption of SO2 by TSILs. As mentioned above, EG has affected the SO2 absorption equilibrium of guanidinium-based TSILs. For further study, we compared the 1H NMR spectra of the mixed absorber consisting of ILs and EG before and after SO2 absorption with that of pure ILs, and the results are shown in Figure 12. Via comparison of IL (A) with IL and EG (B), it can be seen that EG has almost no influence on the 1H NMR spectra of the ILs before SO2 absorption. For the 1H NMR spectra of the ILs after SO2 absorption, when EG was not added to [TMG]P, the typical peak of -CH2- of the propionate anion moves upfield from 2.13 to 2.06 ppm (see spectra A and C in Figure 12a). When EG was added to [TMG]P, the typical peak of -CH2- moves downfield from 2.1 to 2.24 ppm (see spectra B and D in Figure 12a). While we cannot see a similar phenomenon in Figure 12b, the addition of EG has almost no influence on the 1H NMR spectra of [N2222]P before and after SO2 absorption. To further investigate the mechanism, the FT-IR spectra of ILs with different mass ratios of EG were characterized. The results are shown in Figures 13 and 14. In Figure 13, the increase in the level of EG can enhance the stretching vibrational band of the hydroxyl hydrogen atom in EG, which shifts from 3255 to 3363 cm−1. This means that the intermolecular hydrogen bond in EG is gradually replaced by the new hydrogen bond between EG and [TMG]P. The formation of new hydrogen bond reduces the frequency of the stretching vibration, makes the infrared absorption peak move to a low wavenumber, and enhances the intensity of the absorption band. In Figure 14, the increase in the

Figure 13. FT-IR spectra of EG (1) and [TMG]P (2): (a) w1 = 1, (b) w1 = 0.5, and (c) w1 = 0.2.

Figure 14. FT-IR spectra of EG (1) and [N2222]P (2): (a) w1 = 1, (b) w1 = 0.5, and (c) w1 = 0.2.

level of EG has no obvious effect on the stretching vibrational band of the hydroxyl in EG, indicating that no new hydrogen bond was formed between EG and [N2222]P. We inferred that EG can promote absorption of SO 2 by guanidinium- and alkanolaminium-based TSILs on the basis of formation of a hydrogen bond between -OH of EG and -NH3+ of the cation. As we know, the basicity of the aliphatic amine was affected jointly by electronic and solvation effects. The formation of a hydrogen bond between the EG and aliphatic amine can generate the solvation effect, and its extent increases with the number of H atoms on the N of the aliphatic amine. After that, ammonium cations become more stable, and the basicity of the amine H

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(3) Jeong, S. M.; Kim, S. D. Removal of NOX and SO2 by CuO/γ-Al2O3 Sorbent/Catalyst in a Fluidized-Bed Reactor. Ind. Eng. Chem. Res. 2000, 39, 1911−1916. (4) Ma, X. X.; Kaneko, T.; Tashimo, T.; Yoshida, T.; Kato, K. Use of Limestone for SO2 Removal from Flue Gas in the Semidry FGD Process with a Powder-Particle Spouted Bed. Chem. Eng. Sci. 2000, 55, 4643− 4652. (5) Tokumura, M.; Baba, M.; Znad, H. T.; Kawase, Y.; Yongsiri, C.; Takeda, K. Neutralization of the Acidified Seawater Effluent from the Flue Gas Desulfurization Process: Experimental Investigation, Dynamic Modeling, and Simulation. Ind. Eng. Chem. Res. 2006, 45, 6339−6348. (6) Earle, M. J.; Seddon, K. R. Clean Solvents: Alternative Media for Chemical Reactions and Processing. Pure Appl. Chem. 2002, 819, 10− 25. (7) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. (8) Rogers, R. D.; Seddon, K. R. Ionic Liquids-Solvents of the Future? Science 2003, 302, 792−793. (9) Lei, Z. G.; Dai, C. N.; Chen, B. H. Gas Solubility in Ionic Liquids. Chem. Rev. 2014, 114, 1289−1326. (10) Ren, S. H.; Hou, Y. C.; Tian, S. D.; Chen, X. M.; Wu, W. Z. What Are Functional Ionic Liquids for the Absorption of Acidic Gases? J. Phys. Chem. B 2013, 117, 2482−2486. (11) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J. Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415−2417. (12) Jin, M. J.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Tian, S. D.; Xiao, L.; Lei, Z. G. Solubilities and Thermodynamic Properties of SO2 in Ionic Liquids. J. Phys. Chem. B 2011, 115, 6585−6591. (13) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Liu, Q. Y.; Xiao, Y. F.; Chen, X. T. Properties of Ionic Liquids Absorbing SO2 and the Mechanism of the Absorption. J. Phys. Chem. B 2010, 114, 2175−2179. (14) Wang, C. M.; Zheng, J. J.; Cui, G. K.; Luo, X. Y.; Guo, Y.; Li, H. R. Highly Efficient SO2 Capture through Tuning the Interaction between Anion-Functionalized Ionic Liquids and SO2. Chem. Commun. 2013, 49, 1166−1168. (15) Wang, Y.; Wang, C. M.; Zhang, L. Q.; Li, H. R. Difference for SO2 and CO2 in TMGL Ionic Liquids: a Theoretical Investigation. Phys. Chem. Chem. Phys. 2008, 10, 5976−5982. (16) Tian, S. D.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Qian, J. G. Absorption of SO2 at High Temperatures by Ionic Liquids and the Absorption Mechanism. Bull. Korean Chem. Soc. 2014, 35, 2791. (17) Huang, K.; Wang, G. N.; Dai, Y.; Wu, Y. T.; Hu, X. B.; Zhang, Z. B. Dicarboxylic Acid Salts as Task-Specific Ionic Liquids for Reversible Absorption of SO2 with a Low Enthalpy Change. RSC Adv. 2013, 3, 16264−16269. (18) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Chen, X. T.; Fan, J. L.; Zhang, J. W. Effect of H2O on the Desulfurization of Simulated Flue Gas by an Ionic Liquid. Ind. Eng. Chem. Res. 2009, 48, 4928−4932. (19) Zhang, Z. M.; Wu, L. B.; Dong, J.; Li, B. G.; Zhu, S. P. Preparation and SO2 Sorption/Desorption Behavior of an Ionic Liquid Supported on Porous Silica Particles. Ind. Eng. Chem. Res. 2009, 48, 2142−2148. (20) An, D.; Wu, L. B.; Li, B. G.; Zhu, S. P. Synthesis and SO2 Absorption/Desorption Properties of Poly (1, 1, 3, 3-Tetramethylguanidine Acrylate). Macromolecules 2007, 40, 3388−3393. (21) Xia, Y. Z.; Du, S. M.; Wei, J. M.; Liu, J.; Li, J. Z.; Shi, S. X. Preparation of Poly(1,1,3,3-Tetramethylguanidine Acrylate) and Its Absorption-Desorption Properties to SO2 Chin J. Guocheng Gongcheng Xuebao 2012, 12, 583−589. (22) Qian, J. G.; Ren, S. H.; Tian, S. D.; Hou, Y. C.; Wang, C. X.; Wu, W. Z. Highly Efficient and Reversible Absorption of SO2 by Aqueous Triethylenetetramine Tetralactate Solutions. Ind. Eng. Chem. Res. 2014, 53, 15207−15212. (23) Hajipour, A.; Rafiee, F. Basic Ionic Liquids. a Short Review. J. Iran. Chem. Soc. 2009, 6, 647−678. (24) Gao, H. X.; Han, B. X.; Li, J. C.; Jiang, T.; Liu, Z. M.; Wu, W. Z.; Chang, Y. H.; Zhang, J. M. Preparation of Room-Temperature Ionic Liquids by Neutralization of 1, 1, 3, 3-Tetramethylguanidine with Acids

becomes stronger. Finally, the absorption capacity of SO2 is improved.

4. CONCLUSIONS In this work, several types of TSILs containing carboxylate anion were synthesized to capture SO2 of simulated flue gas. The mechanism of absorption of SO2 by the TSILs was thoroughly understood by FT-IR, 1H NMR, and 13C NMR. Results show that there are chemical interactions between the -COO− of the carboxylic acid anion of TSILs and SO2. In addition, when the carboxylic acid anions have a hydroxyl group, the -OH group can be not only an added interaction site but also an electronwithdrawing group, which enhances the capture of SO2 and weakens the interaction between SO2 and -COO− of the carboxylate anion, improving the desorption efficiency of SO2. Meanwhile, the effect of various solvents on the absorption of SO2 by TSILs has been studied. The presence of EG in TSILs has a strong influence on the absorption by guanidinium- and alkanolaminium-based TSILs, but for quaternary ammoniumbased TSILs, EG has no effect. Finally, the interaction between EG and TSILs was studied by FT-IR, and the effect on the absorption mechanism was analyzed. It was demonstrated that the addition of EG can improve the basicity of guanidiniumbased ILs and then improve SO2 absorption capacity. This work makes clear the roles of the carboxylate anion and solvents in the absorption of SO2, which can provide guidance in further designing more effective absorbents.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02801. One additional table, 10 additional figures, and additional references (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone and fax: +86 10 64427603. ORCID

Weize Wu: 0000-0002-0843-3359 Author Contributions

J.Z. and S.R. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Zhenyu Liu and Prof. Qingya Liu for their discussions and suggestions. This project is financially supported by the National Natural Science Foundation of China (21176020 and 21306007), the Research Fund for the Doctoral Program of Higher Education of China (20130010120005), and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC(BUCT).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02801 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02801 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX