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Using Ionic Liquid Mixtures To Improve the SO2 Absorption Performance in Flue Gas Wen Li, Ying Liu,* Lihong Wang, and Guanjun Gao School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia 010021, People’s Republic of China S Supporting Information *

ABSTRACT: The functional ionic liquids (ILs) [NH2emim][OAc] and [NH2emim][BF4] were synthesized for SO2 capture. Although functional ILs have potential advantages in SO2 absorption, the high viscosity of a single IL markedly affects the SO2 absorption separation process. To improve the SO2 absorption performance, [NH2emim][OAc] and [NH2emim][BF4] were mixed with low-viscosity imidazolium-based ILs [bmim][OH]/[bmim][BF4]. When the mole ratio of [NH2emim][OAc]/ [bmim][OH] is about 1:1, the binary mixture has the best SO2 absorption capacity. The SO2 absorption performance was investigated in the simulated flue gas, where SO2 partial pressure was about 0.2 kPa. Under the low SO2 partial pressure, the absorption capacity of the [NH2emim][OAc] + [bmim][OH] mixture was more than 0.7 mol of SO2/mol of IL, which was superior to that of a single IL. The regeneration test of [NH2emim][OAc] + [bmim][OH] showed that the binary mixture had high reversibility and the SO2 absorption capacity almost kept constant after 12 recycles. The viscosity and density of the IL mixture changed slightly, which only have a 2−3% increase in each cycle.

1. INTRODUCTION As an air pollutant, a large amount of sulfur dioxide (SO2) is mainly emitted into the atmosphere by burning fossil fuel and energy conversion. This issue has attracted much more attention.1,2 Limestone, ammonia scrubbing, and organic solvent absorption are the conventional materials for SO2 removal.3,4 However, ammonia scrubbing and organic materials often lead to the volatilization of solvents, and a large amount of calcium sulfate from limestone scrubbing needs to be retreated. Therefore, environmentally friendly and economically viable SO2 capture for large-scale reduction is becoming important. Ionic liquids (ILs) have many favorable properties, including good chemical and physical stability, low vapor pressure, nonflammability, and tunable properties.5 Han et al. have found that the functional IL tetramethylguanidinium lactate ([TMG][L]) could absorb SO2 by chemisorption.6 Several other functional ILs, such as hydroxyl ammonium ILs,7,8 imidazolium ILs,9−11 TMG-based ILs,12 phenolate-based ILs,13 imidazolium poly(azolyl)borate ILs,14 and supported ionic liquid membranes (SILMs),15 also exhibited relatively high SO2 absorption capacities. In addition, a new strategy was developed to improve SO2 capture through multiple-site absorption by tuning the interaction between the basic anion and acidic SO2. Thus, the high capacity of SO2 capture and good reversibility were achieved.16,17 However, absorption capabilities of these ILs were usually determined in the simulated flue gas of the laboratory, where SO2 partial pressure was much higher than that of real flue gas.18 In fact, the SO2 concentration of flue gas was less than 0.2 vol %, resulting in a relatively low partial pressure (99.99 wt %) were obtained from Beifen (China) Gas Technology Company. 1-Butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4], 99 wt %) and 1-butyl-3methylimidazolium bromide ([bmim]Br, 98 wt %) were purchased from Sigma-Aldrich Chemical Co. 2-Bromoethylamine hydrobromide (C2H7Br2N, 98%), 1-methylimidazole (C4H6N2, 99 wt %), acetic acid (CH3COOH, 99 wt %), sodium acetate anhydrous (CH3COONa or NaOAc, 98 wt %), potassium hydroxide (KOH, 99 wt %), and sodium borate (NaBF4, chemically pure) were obtained from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Preparation of ILs. [NH2emim][BF4] and [NH2emim][OAc] were synthesized according to the method of the literature.22,28,29 The cation of ILs was assembled by the reaction of 1-methylimidazole with 2-bromoethylamine hydrobromide in ethanol for 24 h under reflux. Subsequent ion exchange with NaBF4 or NaOAc/CH3COOH in ethanol gives the product IL, and the ethanol is removed in vacuum. [bmim][OH] was prepared by the modification of a reported procedure.30 Solid potassium hydroxide (2.3 g) was added to a solution of [bmim]Br (8.8 g) in dry methylene chloride (20 mL), and the mixture was stirred vigorously at room temperature for 10 h. Precipitated KBr was filtered off, and the filtrate was evaporated to leave the crude [bmim][OH] as a viscous liquid that was washed with ether (2 × 20 mL) and dried at 363 K for 10 h to prepare the pure IL for use. The structures of ILs were confirmed by proton nuclear magnetic resonance (1H NMR, Bruker WB400 AMX spectrometer).28,29 Methanol-d4 (CD3OD) was used as a solvent, and tetramethylsilane (TMS) was used as an internal standard. [bmim][OH] was also verified by Fourier transform infrared (FTIR) spectra (Nicolet 4700). NMR and infrared (IR) data of ILs are listed in the Supporting Information. The density of the IL was determined using a pycnometer with a certainty of ±1 kg m−3 at the constant temperature. The viscosity was measured by a rotatory viscometer with a certainty of ±1 mPa s (SNB-1 digital display) provided by Shanghai Bilon Instrument Co., Ltd., China. 2.3. SO2 Absorption/Desorption Processes. All of the SO2 absorption and desorption experiments were carried out in a 20 mL capacity stainless-steel reactor immersed in a water bath, and the water temperature was controlled to range from 293 to 353 K (Figure 1). In this work, the simulated flue gas was the mixture of N2 and SO2. In a typical absorption process, 0.2 vol % SO2 and 99.8 vol % N2 were mixed in the gas storage. The intake speed of simulated flue gas was controlled at approximately 60 mL/min, and the total pressure was controlled at 101.3 kPa. At the beginning of the experiment, 10 mL IL mixtures were introduced into the reactor. Inlet and outlet SO2 concentrations were determined by a gas analyzer (MRU NOVA2000) with a certainty of ±1 ppm of SO2. The gas analyzer was used at a desirable temperature (e.g., 298 K), and SO2−N2 gas with atmospheric pressure went through the analyzer. The amount of absorbed SO2 could be calculated by the following equation:31

Figure 1. Schematic diagram of the apparatus used for SO2 absorption and desorption. and ρSO2 is the density of SO2. The test of desorption similar to the absorption experiment, where pure N2 flowed into the reactor, and SO2 concentration at the outlet were also detected by the gas analyzer. In addition, the process of SO2 desorption from the IL mixtures was carried out at a constant temperature of 353 K.

3. RESULTS AND DISCUSSION 3.1. SO2 Solubility and Absorption Capacity in a Single IL. The data of SO2 solubility in a single IL are listed in Table 1, which were determined at 298 K and 101.3 kPa Table 1. Properties of the Synthesized ILs

t2

A SO2 =

absorption (mol of SO2/mol of IL)

density, ρ (kg m−3)

viscosity, η (mPa s)

[NH2emim][OAc] [NH2emim][BF4] [bmim][OH] [bmim][BF4]

0.185 0.130 0.115 0.095

1436 1460 1175 1153

2810 3320 125 165

(partial pressure of SO2 was approximately 0.2 kPa). As a result of the low partial pressure of SO2 in the simulated flue gas, the absorption capacity of conventional IL ([bmim][BF4] or bmim[OH]) is lower than 0.2 mol of SO2/mol of IL. A high viscosity of [NH2emim]+-based IL also lead to poor results. ILs with smaller viscosity would be good absorbents. However, although the viscosity of [bmim][OH] and [bmim][BF4] is low, these ILs did not absorb more SO2. The results indicate that effective capture of SO2 from flue gas requires strong chemical absorption. The ions of [bmim][OH] and [bmim][BF4] would not provide such strong effects. 3.2. Effects of the Pressure and Temperature on SO2 Absorption. The effect of the pressure on the SO2 absorption capacity by [NH2emim]+- and [bmim]+-based ILs was investigated (Figure 2a). As the partial pressure was increased from 0.2 to 10 kPa, the mole ratios of SO2/IL for [NH2emim][BF4] and [NH2emim][OAc] increased from 0.13 to 0.25 and from 0.19 to 0.30, respectively. Clearly, the SO2 capacity of [NH2emim]+-based ILs would be affected by the change of the partial pressure. By comparison, the absorption capacity of [bmim][BF4] and [bmim][OH] is almost constant with increasing the pressure, indicating that [bmim]+-based ILs mainly depend upon the chemical

MILρSO Q∫ (C0 − CSO2(t )) dt 2

IL

t1

mILMSO2

where ASO2 is the molar absorption capacity of SO2 in IL, Q is the flow rate of the gas stream, C0 and CSO2 are the SO2 concentrations at inlet and outlet streams during the whole absorption process, respectively, t1 is the beginning time of the absorption process, t2 refers to the time as the SO2 concentration at the outlet stream returns to the initial concentration, mIL is the mass weight of the ILs used for absorption, B

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absorption performance of mixed systems [NH2emim][BF4] + [bmim][BF4]/[bmim][OH]. Figure 3 shows that the mixture [NH2emim][OAc] + [bmim][OH] at X[NH2emim][OAc] = 0.5 has the highest SO2 absorption capacity (0.72 mol of SO2/mol of IL). The density and viscosity of the [NH2emim] [OAc] + [bmim][OH] system with different [NH2emim][OAc] mole ratios are shown in Figure 4, when the absorption temperature and pressure are 298 K and 101.3 kPa, respectively. The density of the mixtures increased with increasing the molar fraction of [NH2emim][OAc]. It is worth noting that the viscosity of the mixtures rose smoothly and increased greatly when X[NH2emim][OAc] reached 0.5. As mentioned above, the SO2 absorption capacity increased rapidly when X[NH2emim][OAc] was less than 0.5 and, subsequently, increased slowly. In other words, the SO2 absorption capacity of the [NH2emim][OAc] + [bmim][OH] mixture achieved an inflection point when X[NH2emim][OAc] was 0.5. It is no doubt that the density and viscosity of the mixed system affect the absorption performance to a certain extent by improving the efficiency of SO2 mass transfer from gas to liquid phase. Clearly, the SO2 absorption performance of the IL mixture is not only affected by the viscosity. The SO2 absorption capacity, density, and viscosity of the [NH2emim][OAc] + [bmim][OH] system at different temperatures were determined when X[NH2emim][OAc] was 0.5 at absorption pressure of 101.3 kPa (Figure 5). As shown, the temperature has an effect on the SO2 absorption performance of the IL mixture. The [NH2emim][OAc] + [bmim][OH] system had a lower SO2 absorption capacity with the gradual increase of the temperature. In Figure 5, the density and viscosity of the IL mixture also decreased with the increase of the temperature. The changes in the density and viscosity of the [NH2emim][OAc] + [bmim][OH] system with the temperature were consistent with those of the SO2 absorption performance. However, the lower viscosity of [NH2emim][OAc] + [bmim][OH] at a higher temperature do not lead to a better SO2 absorption capacity, once again indicating that the absorption performance of the IL mixture should not be explained only from the point of view of viscosity. The interactions of a single IL with SO2 were further investigated by 1H NMR spectroscopy (Figure 6). In comparison to the 1H NMR spectrum of the fresh IL [bmim][OH], new resonance peaks at 8.10 ppm were observed in the spectrum of [bmim][OH] after SO2 absorption (SO2treated), indicating the formation of SO···H. Meanwhile, the typical peaks of the anion [OH]− in the 1H NMR spectrum (10.51 ppm) disappeared. These results show that the reaction [bmim][OH] + SO2 → [bmim][HSO3] occurred during the SO2 capture. For the single IL [NH2emim][OAc], after the absorption of SO2, the typical peaks of −COOH in the 1H NMR spectrum moved upfield from 12.75 to 11.83 ppm and the resonance peaks of H (unsaturated C−H in the imidazole ring, with N connected to the left and right) were observed at 7.52 ppm, indicating that the proposed interaction between [NH2emim][OAc] and SO2 had occurred (Scheme 1). According to the interactions of a single IL and SO2, the [NH2emim][OAc] + [bmim][OH] system would exhibit two kinds of interaction sites. SO2 reacts with the NH2 group on the [NH2emim]+ cation, while SO2 also probably forms [HSO3]− with the [OH]− anion of [bmim][OH]. These interactions

Figure 2. Effect of the (a) pressure and (b) temperature on SO2 absorption by ILs: (blue ■) [NH2emim][OAc], (red ●) [NH2emim][BF4], (green ▲) [bmim][OH], and (magenta ◆) [bmim][BF4].

absorption to capture SO2 molecules. Figure 2b shows the relationship between the temperature and SO2 absorption at 10 kPa of SO2 partial pressure. The absorption capacities of [NH2emim]+-based ILs decreased significantly as the temperature increased, indicating that a high temperature is not favorable for the absorption of SO2. However, the results also imply that captured SO2 could be relatively easily removed by heating or bubbling N2 through the ILs. 3.3. SO2 Absorption Properties in IL Mixtures. Although [NH2emim][OAc] and [NH2emim][BF4] have many favored properties, the SO2 absorption capacities of these ILs are relatively low. Figure 3 illustrates the SO2

Figure 3. SO2 absorption capacity of mixtures at different X[NH2emim][OAc] and X[NH2emim][BF4]. SO2 concentration at the inlet, 0.2 vol % (∼2000 ppm); absorption temperature, 298 K.

absorption capacities of IL mixtures at different mole fractions of [NH2emim][OAc] or [NH2emim][BF4]. Here, X is defined as the molar ratio of the functional IL (e.g., [NH2emim][OAc]) to the IL mixture (e.g., [NH2emim][OAc] + [bmim][OH]). The SO 2 absorption capacity increased quickly when X[NH2emim][OAc] was less than 0.5 but increased slowly more than 0.5. The same trend was also observed for the SO2 C

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Figure 4. Density and viscosity of the [NH2emim][OAc] + [bmim][OH] system with different X[NH2emim][OAc].

Figure 5. Absorption, density, and viscosity of the [NH2emim][OAc] + [bmim][OH] system at different temperatures. X[NH2emim][OAc], 0.5; SO2 concentration at the inlet, 0.2 vol %.

a N2−SO2 mixture gas that contained 2160 ppm of SO2.32 In addition, when attempts were made to obtain 10% SO2 gas (10 mol % in N2, 100 kPa, and 293 K) absorbed in the ILs to simulate flue gas removal, only 0.007, 0.064, 0.061, 0.080 mol of SO2/mol of IL was absorbed in [bmim][BTA], [TMG][BF4], [TMG][BTA], and [TMGB2][BTA], respectively.12 3.4. SO2 Absorption/Desorption Processes. Figure 8 compares the SO2 absorption/desorption processes exhibited by [NH2emim][OAc] + [bmim][OH] and [NH2emim][BF4] + [bmim][BF4] mixtures. To investigate the absorption rates of IL mixtures, SO2 absorption processes were carried out at 298 K and 101.3 kPa. Here, pure SO2 gas was used to shorten the equilibrium time and to determine the absorption capacity of IL mixtures at a high SO2 partial pressure. Desorption was performed at 353 K under pure N2 with atmospheric pressure. As shown, all of the absorption capacities of [NH2emim][OAc]

might help in improving the SO2 absorption capacity of IL mixtures. The SO2 absorption efficiency of IL could be determined by the SO2 concentrations of the absorption apparatus. The relationship between the SO2 concentration at the outlet and time for the [NH2emim][OAc] + [bmim][OH] mixture with X [NH 2 emim][OAc] = 0.5 is shown in Figure 7. The SO 2 concentration at the inlet was 0.2 vol % (∼2000 ppm) in this study, indicating that the SO2 partial pressure was about 0.2 kPa. The IL mixtures could capture almost all SO2 from the simulated flue gas within 0−5 h under such a low SO2 concentration atmosphere. An equilibrium was reached at about 15 h (SO2 at the outlet of ∼2000 ppm). At the equilibrium, the mole ratio of SO2 in the IL mixture (XSO2) was 0.42 (∼0.72 mol of SO2/mol of IL). In contrast, the capacity of TMGL−SiO2 was 0.42 mol of SO2/mol of TMGL in 17 h with D

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Figure 6. 1H NMR spectra of a single IL before and after the reaction with SO2: (a) [bmim][OH] and (b) [NH2emim][OAc].

Scheme 1. Proposed Reactions between the Single IL [NH2emim][OAc] and SO2

+ [bmim][OH] remained steady (∼1.9 mol of SO2/mol of IL), and the absorption and desorption rates were high in pure SO2 and N2 gas. For [NH2emim][BF4] + [bmim][BF4], it is observed that the desorption of SO2 in the mixture was not complete (∼0.2 mol of SO2/mol of IL remained), resulting in ∼1.2 mol of SO2/mol of IL net absorption capacity during the SO2 absorption process. It suggests that the anion of the IL mixture would markedly influence the release of SO2. In contrast, the desorption of SO2 for [TMG][lactate] and 1butyl-3-methylimidazolium acetate ([Bmim][OAc]) is not easy at 353 K. Particularly, as a result of the low stabilities of the ILs and their strong interactions with SO2, [TMG][lactate] and [bmim][OAc] absorption capacities will decrease during the regeneration performance.16 3.5. Regeneration Performance of the IL Mixture. The regeneration performance of an IL determines the frequency of the replacement of the IL, and the absorption/desorption recycling directly impacts the cost of the SO2 absorption

Figure 7. SO2 concentration at the outlet versus time. Absorption pressure, 101.3 kPa; SO2 concentration at the inlet, 0.2 vol % (∼2000 ppm); and absorption temperature, 298 K.

performance. During the regeneration performance, [NH2emim][OAc] + [bmim][OH] (X[NH2emim][OAc] = 0.5) was selected to investigate the capacity and stability of SO2 absorption. Figure 9 illustrate the results of absorption/ desorption cycles in [NH2emim][OAc] + [bmim][OH]. SO2 absorption was carried out at 298 K for 13 h, and absorption E

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Figure 10. Density and viscosity of the [NH2emim][OAc] + [bmim][OH] system in 12 absorption/desorption cycles.

Figure 8. Comparison of SO2 absorption/desorption processes for different IL mixtures. [NH2emim][OAc] + [bmim][OH], (red ●) absorption and (blue ▼) desorption; [NH2emim][BF4] + [bmim][BF4], (orange ●) absorption and (navy ▼) desorption.

with a low SO2 concentration (0.7 mol of SO2/mol of IL) of the mixture [NH2emim][OAc] + [bmim][OH] was achieved, which is significantly superior to that of a single IL. Captured SO2 was easy to release by heating or bubbling N2 through the IL mixtures. The [NH2emim][OAc] + [bmim][OH] system has relatively high absorption capacity and reversibility. Mixing conventional ILs and functional ILs is an effective method to improve the SO2 absorption of ILs. We believe that more efficient and more reversible IL mixtures for SO2 capture can be developed in the future by mixing the appropriate single ILs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02884. Data of NMR, IR, and thermal stability for the ILs (PDF)

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Figure 9. SO2 absorption by [NH2emim][OAc] + [bmim][OH] for 12 cycles.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-471-4992982. Fax: +86-471-4992981. E-mail: [email protected].

pressure was 101.3 kPa with ∼2000 ppm of SO2 concentration at the inlet. Desorption was performed at 353 K under pure N2 gas for 30 min. Clearly, high absorption capacities and rapid desorption rates were well-maintained during the 12 cycles. Experiments also showed that the absorption ability of the IL mixture was 75% after 22 times. The IL mixture has highly reversible property. Density and viscosity of the IL mixture after regeneration were investigated, and the results were shown in Figure 10. As the number of regeneration cycles increased, the viscosity and density value of the [NH2emim][OAc] + [bmim][OH] mixed system would rise slightly. This increase was in the range of 2− 3% for each cycle.

ORCID

Ying Liu: 0000-0001-7246-8997 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (21266015 and 20806036).

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REFERENCES

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4. CONCLUSION The functional ILs [NH2emim][OAc] and [NH2emim][BF4] were synthesized. The single IL ([bmim][BF4]/[bmim][OH]) was mixed with the functional ILs to improve SO2 absorption performance of the functional ILs. When the simulated flue gas F

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DOI: 10.1021/acs.energyfuels.6b02884 Energy Fuels XXXX, XXX, XXX−XXX