Efficient and Reversible Absorption of Sulfur Dioxide of Flue Gas by

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Efficient and reversible absorption of SO2 of flue gas by environmentally benign and stable quaternary ammonium inner salts in aqueous solutions Kai Zhang, Shuhang Ren, Lingyuan Meng, Yucui Hou, Weize Wu, and Yuyun Bao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02953 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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Efficient and reversible absorption of SO2 of flue gas by environmentally benign and stable quaternary ammonium inner salts in aqueous solutions Kai Zhanga, Shuhang Rena, Lingyuan Mengb, Yucui Houc, Weize Wua,*, Yuyun Baoa a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China; b Tang Aoqing Honors Program in Science, College of Chemistry, Jilin University, Changchun 130012, China c Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

*

Corresponding author. Tel./Fax: +86 10 64427603. E-mail address: [email protected]

(W. Wu) 1

ACS Paragon Plus Environment

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ABSTRACT A novel kind of environmentally benign and stable absorbents, quaternary ammonium inner salts betaine (Bet) and L-carnitine (L-car) in aqueous solution, was designed and applied to efficiently and reversibly capturing SO2 in flue gas. The effects of inner salt concentration, absorption temperature, SO2 concentration and reuse cycle on the absorption of SO2 by the two absorbents were investigated. The results show that SO2 absorption capacities of the absorbents with 50 % inner salts mass fraction were 0.155 mol SO2 / mol Bet for Bet aqueous solutions and 0.599 mol SO2 / mol L-car for L-car aqueous solutions at 40 oC with a SO2 concentration of 2 %, and the absorption capacities of absorbents did not change after five absorption/desorption cycles. Furthermore, FT-IR and 1H NMR results demonstrate that the SO2 absorption follow the law that a strong acid (H2SO3) replaces a weak acid (–COOH). The absorbents could be used for more than one week without any change in structure, indicating the promise for industrial applications in desulfurization of flue gas.

Keywords: Absorption; SO2; quaternary ammonium inner salts; mechanism

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1. INTRODUCTION Sulfur dioxide (SO2) in flue gas, mainly issued from the combustion of fossil fuels, is one of the dominant air pollutants, threatening human health and environment1. Currently, flue gas desulfurization (FGD) is widely used in the industry to control of SO2 emission by limestone as an absorbent2. However, the drawbacks of the technique are the non-renewability of absorbents and the generation of a huge amount of wastewater, from which useful SO2 is not recovered. Consequently, the development of renewable and efficient absorbents for removal and recovery of SO2 is particularly significant to our society. In recent years, different absorbents like inorganic salts, organic solvents and ionic liquids (ILs) have been developed for the desulphurization of flue gas. Among inorganic salts, sodium sulfite as the absorbents can remove SO2 efficiently3. Unfortunately, the easy oxidability of the absorbents results in degradation of absorbents

performance.

The

organic

solvents

like

dimethylacetamide4,

methyldiethanolamine5 and ethylenediamine6 also have been found to be good absorbents for SO2 removal. But these absorbents are highly toxic, volatile, nondegradable and not environmentally-friendly7, 8. ILs, especially functional ILs, which have considerable solubility for SO2 with low concentration, have been attracting scientific and technological attention for SO2 removal and recovery due to their unique properties, such as negligible vapor pressure, wide liquid temperature range, and tunable structure9, 10. Han et al.11 reported the first functional IL 1,1,3,3tetramethlylguanidinium lactate ([TMG][La]) to remove SO2 with a low concentration. Later, functional ILs based on quaternary phosphonium12, imidazolium13, guanidinium14, hydroxyl-ammonium15 were also designed and 3


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testified to capture SO2 with high solubility. However, the disadvantages of these functional ILs, such as toxicity16, poor biodegradability17, 18 complicated preparation, high viscosity19, high expenses20 and short-term thermal stability21, 22, may limit their large-scale adoption in SO2 removal. Therefore, development of absorbents that are non-volatile, cheap, easy preparation, environmentally-benign and able to achieve rapid and reversible SO2 capture with high capacities is always highly desired. Jia et al. reported six environmentally-friendly water-soluble amino acids as absorbents to apply on SO2 absorption, and SO2 saturation uptake of α- or β-alanine solution was very considerable23. α- or β-Amino acids are inner salts, and the H atom on –COOH transfers on –NH2 to form a zwitterion. Its structure is similar to that of a protonated functional IL (e.g., [TMG][La]), as shown in Figure 1, where – COO– as a conjugate base plays an important role on the absorption of SO224. However, α- or β-amino acids are readily polymerized to form polypeptide, to some extent, which reduces the absorption performance of absorbents, and makes the absorbents unstable. Industrially, absorbents for SO2 capture are expected to endure high temperatures especially during regeneration of absorbents and have a long period of stability, for example several months. Though, functional ILs and amino acids are not stable for a long time of uses, they inspired us to design new absorbents. Theoretically, due to no active hydrogen, quaternary ammonium structure is much more stable than amino structure. Quaternary ammonium lactate or formate ILs, for instance [N2222][La] as shown in Figure 1, are functional absorbent for SO2 capture. In our previous work24, it was found that the pKa of organic acids formed the anion 4


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of ILs could be used to differentiate functional ILs from normal ILs for the capture of acidic gases from flue gas. If the pKa of an organic acid is larger than that of sulfurous acid, the IL formed by the organic acid can be called functional ILs for SO2 capture and it can have a high absorption capacity of SO2 with low concentrations. The pKa of carboxylate acid (3.7−4.8) is larger than that of sulfurous acid (1.85). Hence, the salts formed by carboxylate ion will be functional ILs or inner salts for SO2 capture. In this work, based on above information, we designed two biodegradable quaternary ammonium inner salts, betaine (Bet) and L-carnitine (L-car), as stable and functional absorbents for the capture of SO2 in flue gas. They are expected to be more stable than functional ILs due to the higher bond energy of covalent than ionic bond and more stable than α- or β-amino acids that readily polymerize to form polypeptide. As quaternary ammonium inner salts, Bet and L-car are non-volatile. Their molecular structure is similar to the structure of aprotic functional IL (e.g., [N2222][La]), as shown in Figure1, both of which contain –COO– group, and are reasonably functional for capturing SO2 with low concentration. A problem for quaternary ammonium inner salts is that they are solid at flue gas temperatures. As we know, water always exists in flue gas, and quaternary ammonium inner salts are hydrophilic. It suggests that quaternary ammonium inner salts can react with water to form an aqueous solution or a deep eutectic solvent, which is favorable for SO2 absorption. Obviously, these quaternary ammonium inner salts are cheap, readily available and environmentally friendly. Moreover, the addition of water will greatly reduce the viscosity of the absorbents and increase the transportation property of absorbent. So, water is suitably adopted to Bet and L-car 5


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to form liquid absorbents. Consequently, Bet and L-car aqueous solutions are stable, cheap, biodegradable, easy preparation and environmentally-benign absorbents for SO2 capture. In this work, the effect of inner salt concentration, absorption temperature and SO2 concentrations on the absorption of SO2 were investigated. The absorption mechanism of SO2 was studied through FT-IR and 1H NMR. The results showed that the prepared absorbents could absorb SO2 with low concentrations efficiently and be regenerated easily.

2. EXPERIMENTAL SECTION 2.1. Materials. SO2 (99.95 %), CO2 (99.999 %) and N2 (99.999 %) were obtained from Beijing Haipu Gases Co., Ltd., Beijing, China. The simulated flue gas with different SO2 concentrations was obtained by mixing SO2 and N2 in a high pressure gas cylinder. Betaine (98%) was purchased from J & K Scientific Co., Ltd., Beijing, China. Lcarnitine (98%) was obtained from Saipuruisi Co., Ltd., Beijing, China.

2.2. SO2 Absorption and Desorption Experiments. The schematic diagram of experimental system is shown in Figure 2. It mainly consisted of a gas cylinder, a rotometer, a water glass tube, an absorbent glass tube, a water (or boil) bath, a temperature controller, and a tail gas absorption device. The absorbents with different inner salt concentrations were prepared by adjusting the proportions of inner salts and water, and winner salt represents the mass percent of inner salt. The SO2 absorption and desorption experiments were conducted using the absorbent glass tube with a length of 100 mm and an inner diameter of 15 mm under the ambient pressure. Typically, 2.00 g water and 2.00 g absorbent were loaded into 6


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the glass tube with water and the absorbent glass tube, respectively. Then the two glass tubes were immersed in the water bath. The simulated flue gas of 100 cm3/min bubbled through water in the glass tube with water before the absorption in order to offset the water evaporated from the absorption tube. The desorption experiment was carried out with a similar way to the above absorption experiment, and only the gas bubbled through the two tubes was changed to pure N2. The temperatures of the water bath and the oil bath were maintained within ± 0.5 oC. The concentrations of SO2 in absorbents were determined by an iodine titration method (HJ/T 56–2000, a standard method of State Environmental Protection Administration of China). The absorption/desorption efficiency (DE) of the sorbents was determined using the following equation (1).

DE =

Ra − Rd ×100% Ra

(1)

where Ra is the mole ratio of SO2 to inner salt when the absorbent was saturated, Rd is the mole ratio of SO2 to inner salt after desorption. The removal efficiency (RE) is determined using the following equation (2).

RE =

mtabs ×100% mtin

(2)

where mtabs is the mole of SO2 in absorbent at determination time and mtin is the mole of SO2 in simulated flue gas charged into the absorbent glass tube at determination time.

2.3. FT-IR and 1H NMR Analyses. NMR spectra can provide evidence to speculate the absorption mechanism. NMR of virgin absorbents and SO2 absorbed absorbents were conducted by a 400 7


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MHz Bruke Avance III spectrometer. We put D2O into a capillary tube (10 cm × ϕ0.9 mm) and inserted the tube to a NMR tube (17.8 cm × ϕ5 mm) containing a sample in order to eliminate the influence of D2O on the spectrum. IR spectra of samples was recorded on a Fourier transform spectrometer with wavenumbers from 400 to 4000 cm−1.

3. RESULTS AND DISCUSSION 3.1. Effect of Inner Salt Concentration on the Absorption of SO2. The effect of inner salt concentration of the two absorbents on the absorption of SO2 at 40 oC with a SO2 concentration of 2 % (2 % SO2 in atmospheric N2) is shown in Figure 3. The inner salt concentration has almost no effect on SO2 solubility in absorbents in a unit of mol SO2 / mol absorbent, such as the solubility of SO2 are 0.606, 0.592, 0.603, 0.598 mol SO2 / mol L-car at wL-car = 50 %, 40 %, 30 %, 20 %, respectively. The result shows that inner salts play a role of active absorption species in absorbents. Interestingly, the gravimetric absorption capacity of SO2 in absorbents is increased with increasing inner salt concentration due to the decrease of water concentration, such as the gravimetric absorption capacities of SO2 are 0.0475, 0.0719, 0.0941, 0.121 g SO2 / g absorbent at wL-car = 20 %, 30 %, 40 %, 50 %, respectively. However, when the mass fraction of L-car is increased to 60 %, L-car is not completely dissolved in the water solution, suggesting its concentration is higher than its solubility. Hence, the following work was mainly carried out at winner salt = 50 % for the highest mass ratio of SO2 to absorbent.

3.2. Effect of Temperature on the Absorption of SO2. Figure 4 shows the effect of temperature on the absorption of SO2 with a 8


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concentration of 2 % and winner salt = 50 %. It can be seen that both of the absorbents can absorb low-concentration SO2 efficiently, and the solubility of SO2 decreases with increasing temperature. For example, the solubility of SO2 in L-car aqueous solutions decreases from 0.716 mol SO2 / mol L-car at 30 oC to 0.478 mol SO2 / mol L-car at 50 oC. The SO2 absorption capacities of two absorbents from 30 to 50 oC shows that L-car aqueous solution has a higher absorption capacity than Bet does, such as 0.599 mole ratio of SO2 to L-car at 40 oC for the former, and 0.160 for the latter. The results demonstrate the absorbents with saturated SO2 can be regenerated by increasing temperature.

3.3. Effect of SO2 Concentration on the Absorption of SO2. The effect of SO2 concentration on the absorption capacities of two absorbents (winner salt = 50 %) is shown in Figure 5. The mole ratio of SO2 to Bet and L-car decrease from 0.805 and 1.222 to 0.160 and 0.599, respectively, when the concentration of SO2 decreases from 1 to 2 % at 40 oC, which means SO2 concentration has a significant influence on the absorption capacity. It suggests that the absorbents can be reused by reducing the SO2 concentration. In addition, it can be seen that the solubility data is not related linearly with SO2 concentration, indicating that the absorption does not conform to Henry’s law, and the absorption process is not physical dissolution. As expected, the absorption of SO2 is contributed by chemical interaction between SO2 and absorbed inner salts. The high solubility of SO2 (0.382 mol SO2 / mol L-car) is obtained with a very low concentration of SO2 (0.37 %), which is in the concentration range of flue gas SO2. The result suggests the absorbent is suitable to apply on the desulphurization of flue gas. 9


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Moreover, the solubilities of pure CO2 in two absorbents has also been measured and they are 0.0288 mol of CO2 / mol of L-car, and 0.0263 mol of CO2 / mol of Bet at 40 oC and winner salt = 50 %. While the solubilities of pure SO2 in two absorbents are 1.22 mol SO2 / mol L-car and 0.80 mol SO2 / mol Bet, as shown in Figure 5. The result indicates that the selectivity of SO2/CO2 for the absorbents is very high. In addition, for comparison, the solubility of SO2 in two absorbent studied in this work and ILs reported in the literature are listed in Table 1. The solubility of SO2 with different concentrations in several functional ILs demonstrates that the solubility of SO2 in two absorbents studied in this work are comparable with those of functional ILs reported in the literature.

3.4. Absorption and Removal Efficiency with Time. Figure 6 shows the absorption curves and removal efficiencies with time for Lcar-H2O and Bet-H2O absorbents. As can be seen from this figure, the absorption curves at initial absorption period are almost linear and the RE of two absorbents can reach 97 %, which demonstrates that SO2 charged into the absorption tube is highly captured, although the simulated flue gas is simply bubbled through the absorbents. The reason that RE decreases at the later absorption period is that the absorbents reach closely their saturated absorption capacities. In practical application, the absorption of SO2 in flue gas is performed in an absorption column, and RE is influenced by stage number and flow ratio of flue gas to absorbent. Obviously, an increase in stage number and a decrease in flow ratio of flue gas to absorbent are favorable to increase RE.

3.5. Regeneration of Absorbents and Their Long Time Stability. 10


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Bet (wBet = 50 %) and L-car (wL-car = 50 %) aqueous solutions saturated with SO2 (2 vol %) at 40 oC were treated with 100 ml/min N2 at 100 oC. Figure 7 and Figure 8 show the regeneration of absorbents during five absorption/desorption cycles. The result demonstrates that the solubility of SO2 in two absorbents have no obvious loss, such as the solubility of SO2 in L-car aqueous solution (wL-car = 50 %) in 5 cycles are 0.581, 0.587, 0.578, 0.596 and 0.598. In addition, the DEs on five absorption/desorption cycles are high, such as 95.5 %, 95.9 %, 94.7 %, 96.4 % and 96.6 % for Bet-H2O absorbent, and 73.1 %, 73.8 %, 75.1 %, 75.6 % and 75.9 % for L-car-H2O absorbent. As a result, the absorbents can be regenerated to capture SO2 with high capacity and stability. To investigate their long time stability, Bet (wBet = 50 %) and L-car (wL-car = 50 %) aqueous solutions was first saturated with SO2 (2 vol %) at 40 oC and kept for 12 h, and then increased to 100 oC keeping for another 12 h, lastly, they were treated with 100 ml/min N2 at 100 oC and cooled to 40 oC, repeated for 9 days. As shown in Figure 9, the 1H NMR spectra of two absorbents after 9 days of use indicates that there were no any new chemical peak appeared, which indicates the absorbents have long-term stability.

3.6. Absorption Mechanism. The above results indicate that the solubility of low concentration SO2 in two inner salt aqueous solutions is much high, hinting that the absorption process is not a physical dissolution. For comprehending the absorption mechanism, two absorbents and absorbents + SO2 were studied by 1H NMR and FT-IR. The 1H NMR spectra of Bet + H2O and L-car + H2O with and without SO2 are shown in Figure 10 and Figure 11, respectively. As can be seen, the typical peak of –CH2– in 11


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Bet moved downfield from 3.80 ppm to 3.90 ppm, and the peak of –CH2– connected –COO– in L-car moved downfield from 2.23 ppm to 2.45 ppm, and the typical peak of –CH3 in Bet and L-car is barely changed, which demonstrate that SO2 acts with –COO– in the absorption process. It also suggests that there are no covalent bond formation between SO2 and two absorbents because of the moderate chemical shift. As a result, we speculate SO2 dissolved in two absorbents is a process that SO2 dissolved in water to form H2SO3, which ionizes to H+ and HSO3–, and then H+ transfers on –COO– in two inner salts and formed –COOH due to the pKa of monocarboxylic acid (RCOOH) is larger than that of H2SO324. Thus, the solubility of SO2 in two absorbents depends on the interaction force between H+ and –COO–. The stronger the interaction force is, the more SO2 dissolved. The group – COO– in two inner salts both connect the electron-donating group –CH2–, however, an electron-withdrawing group –N+(CH)3 is next to –CH2– in Bet, which results the electron density of –COO– in L-car to be larger than those in Bet. Thus, the binding capacity of L-car with H+ is stronger than that of Bet, which promotes more dissolution of SO2 in L-car. Meanwhile, it explains why the absorption capacity of SO2 in L-car is higher than that in Bet at same absorption condition. On the other hand, increasing temperature reduces the binding force between H+ and inner salt, which accounts for the solubility of SO2 decreasing with increasing temperature. The proposed absorption mechanism could be further testified through the FTIR. The FT-IR spectra of absorbents and absorbents + SO2 are shown in Figure 12 and Figure 13. As we know, COOH dissociates one H atom to yield –COO– with a negative charge, and the negative charge is not concentrated upon one oxygen atom, but equally spread on two oxygen atoms, which results in the two carbon–oxygen bonds to be equivalent. Take L-car + H2O as example, it can be found that there are 12


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strong peaks centered at 1575 cm−1 and 1399 cm−1, assigned to the symmetric and asymmetric stretching frequencies of carboxylate –COO–, respectively, while there is no peak at about 1715 cm−1, assigned to C=O of carboxylic acid –COOH 25. This demonstrates that the two carbon–oxygen bonds in two inner salts aqueous solutions are equivalent. Compared with the FT-IR spectra of Bet + H2O and L-car + H2O, the FT-IR spectra of Bet + H2O + SO2 and L-car + H2O + SO2 show new absorption bands at 1732 cm−1 and 1715 cm−1, respectively. This indicates that the negative charge is deflected from one oxygen atom to another one after SO2 absorption because of the formation of –COOH. Meanwhile, it can be also found that there are new absorption bands at 1172 cm−1 in Bet + H2O +SO2 and 1177 cm−1 in L-car + H2O + SO2, which are assigned to HSO3–. It demonstrates that HSO3– is formed after SO2 absorption in two inner salts aqueous solutions. However, the bond intensity of 1172 cm−1 in Bet + H2O +SO2 is weaker than that of 1177 cm−1 in L-car + H2O + SO2, which is attributed to the higher solubility of SO2 in L-car + H2O than that in Bet + H2O. As a result, the absorption mechanism of SO2 in two inner salts aqueous solutions is proposed as that the strong acid (H2SO3) replaces the weak acid (– COOH), which promotes SO2 dissolution (Figure 14). The equations also explain why the SO2 solubility increases with increasing SO2 concentration. The concentration of HSO3– increases with increasing SO2 concentration, which promotes the SO2 absorption according to the following equations in Figure 14.

4. CONCLUSION Two quaternary ammonium inner salts betaine (Bet) and L-carnitine (L-car) aqueous solutions were used as absorbents for SO2 capture. The absorption of SO2 13


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in two absorbents was studied at various inner salt concentrations, temperatures and SO2 concentrations. The results show that the solubility of SO2 increases with decreasing temperature and with increasing concentration, and does not change with inner salt concentrations. There was no obvious loss of SO2 absorption capacities during the regeneration cycles. Two kinds of absorbent can absorb low concentration SO2 efficiently, and the solubility of SO2 was 0.382 mol SO2 / mol Lcar with wL-car = 50 % at 40 oC. The absorption mechanism of SO2 in two inner salts aqueous solutions was speculated to be the strong acid (H2SO3) replacing the weak acid (–COOH). Two quaternary ammonium inner salts aqueous solutions based on Bet and L-car as solute are cheap, stable, biodegradable, easy to prepare and environmentally-benign, which make them promising absorbents in the application to SO2 capture.

ACKNOWLEDGMENTS The authors are grateful to Prof. Zhenyu Liu and Prof. Qingya Liu for their help. The project is supported financially by the National Natural Science Foundation of China (No. 21176020 and 21306007), the Research Fund for the Doctoral Program of Higher Education of China (No. 20130010120005) and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT).

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(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, (18), 2415-2417. (12) Qu, G. F.; Zhang, J.; Li, J. Y.; Ning, P., SO2 absorption/desorption characteristics of two novel phosphate ionic liquids. Sep. Sci. Technol. 2013, 48, (18), 2876-2879. (13) Wang, C. M.; Cui, G. K.; Luo, X. Y.; Xu, Y. J.; Li, H. R.; Dai, S., Highly efficient and reversible SO2 capture by tunable azole-based ionic liquids through multiple-site chemical absorption. J. Am. Chem. Soc. 2011, 133, (31), 11916-11919. (14) Shang, Y.; Li, H. P.; Zhang, S. J.; Xu, H.; Wang, Z. X.; Zhang, L.; Zhang, J. M., Guanidinium-based ionic liquids for sulfur dioxide sorption. Chem. Eng. J.

2011, 175, 324-329. (15) Yuan, X. L.; Zhang, S. J.; Lu, X. M., Hydroxyl ammonium ionic liquids: synthesis, properties, and solubility of SO2. J. Chem. Eng. Data 2007, 52, (2), 596599. (16) Biczak, R.; Pawłowska, B.; Bałczewski, P.; Rychter, P., The role of the anion in the toxicity of imidazolium ionic liquids. J. Hazard. Mater. 2014, 274, 181-190. (17) Pham, T. P. T.; Cho, C.-W.; Yun, Y.-S., Environmental fate and toxicity of ionic liquids: a review. Water Res. 2010, 44, (2), 352-372. (18) Peric, B.; Sierra, J.; Martí, E.; Cruañas, R.; Garau, M. A.; Arning, J.; BottinWeber, U.; Stolte, S., (Eco) toxicity and biodegradability of selected protic and aprotic ionic liquids. J. Hazard. Mater. 2013, 261, 99-105. (19) Paduszyński, K.; Domańska, U., Viscosity of ionic liquids: an extensive database and a new group contribution model based on a feed-forward artificial neural network. J. Chem. Inf. Model. 2014, 54, (5), 1311-1324.

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Energy & Fuels

(20) Chen, L.; Sharifzadeh, M.; Mac Dowell, N.; Welton, T.; Shah, N.; Hallett, J. P., Inexpensive ionic liquids:[HSO4]−-based solvent production at bulk scale. Green Chem. 2014, 16, (6), 3098-3106. (21) Maton, C.; Vos, N. D.; Stevens, C. V., Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools. Chem. Soc. Rev. 2013, 42, 59635977. (22) Cao, Y.; Mu, T., Comprehensive Investigation on the Thermal Stability of 66 Ionic Liquids by Thermogravimetric Analysis. Ind. Eng. Chem. Res. 2014, 53, (20), 8651-8664. (23) Deng, R. P.; Jia, L. S.; Song, Q. Q.; Su, S.; Tian, Z. B., Reversible absorption of SO2 by amino acid aqueous solutions. J. Hazard. Mater. 2012, 229, 398-403. (24) 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, (8), 2482-2486. (25) Lee, K. Y.; Gong, G. T.; Song, K. H.; Kim, H.; Jung, K. D.; Kim, C. S., Use of ionic liquids as absorbents to separate SO2 in SO2/O2 in thermochemical processes to produce hydrogen. Int. J. Hydrogen Energy 2008, 33, 6031-6036. (26) Huang, J.; Riisager, A.; Wasserscheid, P.; Fehrmann, R., Reversible physical absorption of SO2 by ionic liquids. Chem. Commun. 2006, 38, 4027-4029. (27) Shiflett, M. B.; Yokozek, A., Separation of carbon dioxide and sulfur dioxide using room-temperature ionic liquid [bmim][MeSO4]. Energy Fuels 2010, 24, 10011008. (28) Huang, K.; Lu, J. F.; Wu, Y. T.; Hu, X. B.; Zhang, Z. B., Absorption of SO2 in aqueous solutions of mixed hydroxylammonium dicarboxylate ionic liquids. Chem. Eng. J. 2013, 215, 36-44.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

List of Table Caption Table 1. Solubility of SO2 in Two Absorbents (winner salt = 50 %) Studied in This Work and Some ILs at Different SO2 Concentrations

List of Figure Caption Figure 1. Chemical structures of amino acid, Bet, L-car, a protonated functional IL [TMG][La] and an aprotic functional IL [N2222][La].

Figure 2. Schematic diagram of experimental system. 1, gas cylinder; 2, pressure reducing valve; 3, rotometer; 4, water glass tube; 5, absorbent glass tube; 6, water or boil bath; 7, temperature controller; 8, tail gas absorption device.

Figure 3. Effect of inner salt concentration on the SO2 absorption at a temperature of 40 oC by the two absorbents: ■, L-car; ▲, Bet.

Figure 4. Effect of temperature on the absorption of SO2 with a concentration of 2 % (a) in Bet aqueous solution (wBet = 50 %) and (b) in L-car aqueous solution (wL-car = 50 %).

Figure 5. Effect of SO2 concentration on SO2 absorption at a temperature of 40 oC by the two absorbents: ■, L-car aqueous solution (wL-car = 50 %); ▼, Bet aqueous solution (wBet = 50 %).

Figure 6. Absorption curve and removal efficiency for (a) L-car-H2O and (b) BetH2O at a SO2 concentration of 3700 ppm, a temperature of 40 oC and winner salt = 50 %.

Figure 7. Absorption and desorption cycles of SO2 in Bet aqueous solution (wBet = 50 %). In each cycle, SO2 (2 vol %) was absorbed at 40 oC and desorbed at 100 oC. ■, Absorption; □, Desorption.

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Energy & Fuels

Figure 8. Absorption and desorption cycles of SO2 in L-car aqueous solution (wL-car = 50 %). In each cycle, SO2 (2 vol %) was absorbed at 40 oC and desorbed at 100 oC. ■, Absorption; □, Desorption.

Figure 9. 1H NMR spectra for Bet and L-car for different days; (a) Bet, (b) L-car. Figure 10. 1H NMR spectra of Bet aqueous solution (wBet = 50 %) (a) and Bet + SO2 aqueous solution (b).

Figure 11. 1H NMR spectra of L-car aqueous solution (wL-car = 50 %) (a) and L-car + SO2 aqueous solution (b).

Figure 12. FT-IR spectra of Bet aqueous solution (wBet = 50 %) (a) and Bet + SO2 aqueous solution (b).

Figure 13. FT-IR spectra of L-car aqueous solution (wL-car = 50 %) (a) and L-car + SO2 aqueous solution (b).

Figure 14. Proposed mechanism between SO2 and L-car aqueous solution.

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Page 20 of 34

Table 1. Solubility of SO2 in Two Absorbents (winner salt = 50 %) Studied in This Work and Some ILs at Different SO2 Concentrations Absorbent

T/oC

L-car + EG

40

L-car + EG

CSO2/%

absorption capacity

Ref.

1

100

1.222a; 0.486b

This work

40

1

2

0.599a; 0.238b

This work

L-car + EG

40

1

0.37

0.382a; 0.152b

This work

Bet + EG

40

1

100

0.806a; 0.441b

This work

Bet + EG

40

1

2

0.160a; 0.0875b

This work

Bet + EG

40

1

0.37

0.06a; 0.0328b

This work

[TMG][L]

40

1.2

100

1.700c; 0.531d

[11]

[TMG][BF4]

20

1

100

1.270c; 0.402d

[26]

[BMIM][BF4]

20

1

100

1.500c; 0.424d

[26]

[BMIM][OAc]

25

1

100

1.910c; 0.614d

[27]

40

1

100

1.580c; 0.407d

[28]

[P66614][Tetz]

20

1

10

1.54c; 0.178d

[13]

[P66614][Im]

20

1

10

2.07c; 0.241d

[13]

[N2222]L

60

1

3

0.791c; 0.23d

[10]

[Bmim]L

60

1

3

0.676c; 0.189d

[10]

[Hmim]L

60

1

3

0.653c; 0.162d

[10]

[TMG]L

60

1

3

0.414c; 0.129d

[10]

[MEA]L

60

1

3

0.230c; 0.0968d

[10]

[N2224][disuccinate]

P/atm

a: mol SO2 / mol inner salt; b: g SO2 / g inner salt; c: mol SO2 / mol IL; d: g SO2 / g IL

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Energy & Fuels

NH3

O

O

O

R

OH

N+

N+

-O

O

Amino acid

-O

Bet

L-car N

O

O

OH

NH2

N O

N

O

OH

[N2222][La]

[TMG][La]

Figure 1. Chemical structures of amino acid, Bet, L-car, a protonated functional IL [TMG][La] and an aprotic functional IL [N2222][La].

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Figure 2. Schematic diagram of experimental system. 1, gas cylinder; 2, pressure reducing valve; 3, rotometer; 4, water glass tube; 5, absorbent glass tube; 6, water or boil bath; 7, temperature controller; 8, tail gas absorption device.

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Page 23 of 34

0.7

Mole ratio of SO2 to inner salt

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Energy & Fuels

0.6 0.5 0.4 0.3 0.2 0.1 0.0 20

30

40

50

Inner salt content / wt %

Figure 3. Effect of inner salt concentration on the SO2 absorption at a temperature of 40 oC by the two absorbents: ■, L-car; ▲, Bet.

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Energy & Fuels

Mole ratio of SO2 to Bet

0.24

(a)

0.20 0.16 0.12 0.08 0.04 0.00

30

35

40

45

50

Temperature/°C

0.8

Mole ratio of SO2 to L-car

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

0.7

(b)

0.6 0.5 0.4 0.3 0.2 0.1 0.0

30

35

40

45

50

Temperature/°C

Figure 4. Effect of temperature on the absorption of SO2 with a concentration of 2 % (a) in Bet aqueous solution (wBet = 50 %) and (b) in L-car aqueous solution (wL-car = 50 %).

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1.5

Mole ratio of SO2 to inner salt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Bet L-car

1.2 0.9 0.6 0.3 0.0 0

20

40

60

80

100

SO2 concentration /%

Figure 5. Effect of SO2 concentration on SO2 absorption at a temperature of 40 oC by the two absorbents: ■, L-car aqueous solution (wL-car = 50 %); ▼, Bet aqueous solution (wBet = 50 %).

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Energy & Fuels


0.5

100

0.4

(a)

80

0.3

60

0.2

40

0.1

20

0.0 0

50

100

150

200

250

300

Removal efficiency /%

0

Time /min

100


0.06 0.05

80

(b)

0.04

60

0.03 40 0.02 20

0.01 0.00 0

10

20

30

40

50

60

70

80

Removal efficiency /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 90

Time /min

Figure 6. Absorption curve and removal efficiency for (a) L-car-H2O and (b) BetH2O at a SO2 concentration of 3700 ppm, a temperature of 40 oC and winner salt = 50 %.

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0.16


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Energy & Fuels

0.12

0.08

0.04

0.00

1

2

3

4

5

Cycles

Figure 7. Absorption and desorption cycles of SO2 in Bet aqueous solution (wBet = 50 %). In each cycle, SO2 (2 vol %) was absorbed at 40 oC and desorbed at 100 oC. ■, Absorption; □, Desorption.

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Energy & Fuels

Page 28 of 34

0.6 0.5 0.4 0.3 0.2 0.1 0.0

1

2

3

4

5

Cycles

Figure 8. Absorption and desorption cycles of SO2 in L-car aqueous solution (wL-car = 50 %). In each cycle, SO2 (2 vol %) was absorbed at 40 oC and desorbed at 100 oC. ■, Absorption; □, Desorption.

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Energy & Fuels

(a) 9 day 7 day 5 day 3 day Virgin

5.5

5.0

4.5

4.0 δ/ppm

3.5

3.0

2.5

(b)

9 day 7 day 5 day

3 day

Virgin

5.5

5.0

4.5

4.0

3.5 3.0 δ/ppm

2.5

2.0

1.5

Figure 9. 1H NMR spectra for Bet and L-car for different days of reuse: (a) Bet, (b) L-car.

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Page 30 of 34

3.21 3.90

(b)

3.19 (a)

5.5

3.80

5.0

4.5

4.0 δ/ppm

3.5

3.0

2.5

Figure 10. 1H NMR spectra of Bet aqueous solution (wBet = 50 %) (a) and Bet + SO2 aqueous solution (b).

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Energy & Fuels

3.08 (b) 3.34

4.47

2.45 3.05

(a) 3.24

4.37

5.5

5.0

4.5

4.0 3.5 δ/ppm

3.0

2.23

2.5

2.0

Figure 11. 1H NMR spectra of L-car aqueous solution (wL-car = 50 %) (a) and L-car + SO2 aqueous solution (b).

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Energy & Fuels

1619

1397

1732

Absorbance / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

1172

(b)

(a)

3500

3000

2500

2000

Wavenumber / cm

1500

1000

-1

Figure 12. FT-IR spectra of Bet aqueous solution (wBet = 50 %) (a) and Bet + SO2 aqueous solution (b).

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Page 33 of 34

1715

Absorbance / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1575 1399 1177

(b)

(a)

3500

3000

2500

2000

Wavenumber / cm

1500

1000

-1

Figure 13. FT-IR spectra of L-car aqueous solution (wL-car = 50 %) (a) and L-car + SO2 aqueous solution (b).

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 14. Proposed mechanism between SO2 and L-car aqueous solution.

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Efficient and Reversible Absorption of Sulfur Dioxide of Flue Gas by

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