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Physicochemical Properties and Investigation of azole-based deep eutectic solvents as efficient and reversible SO2 absorbents Dongshun Deng, Xiaobang Liu, and Bao Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02478 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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Industrial & Engineering Chemistry Research

Physicochemical Properties and Investigation of Azole-Based Deep Eutectic Solvents as Efficient and Reversible SO2 Absorbents Dongshun Deng,* Xiaobang Liu and Bao gao

Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China.

ABSTRACT: Four azole-based DESs from acetyl choline chloride (ACC) and azoles (imidazole, Im, and 1,2,4-triazole, Tri) were prepared and utilized for SO2 capture. Their physicochemical properties, including density, viscosity, TGA, freezing point and pH were determined. The influences of temperature and pressure on the SO2 absorption performance of the DESs were systematically conducted. The results showed that all the DESs exhibited efficient and reversible absorption for SO2 and ACC-Im possessed higher absorption capacity than ACC-Tri, with the maximum value of 0.381 g of SO2 per g of ACC-Im (1:2) DES at 303.15 K and 0.1 bar. The possible absorption mechanism was investigated using 1HNMR and FTIR spectroscopy. Furthermore, the ACC-Tri (1:2) was used to capture SO2 at low partial pressure, showing high available capacity of 0.116 g of SO2 per g of DES for 3300 ppm SO2. Present DESs emergy as promising absorbents for SO2 due to easy preparation, high absorption capacity and good reversibility. Keywords: Deep eutectic solvent; SO2; Absorption; Mechanism.

1. INTRODUCTION With the combustion of coals in fossil-fuel-fired power plants, a large amount of sulfur dioxide (SO2) is released into atmosphere. SO2 produces serious hurt to public health, ecological balance and natural environment in the form of smog and acid rain.

ACS Paragon Plus Environment


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The stringent environmental regulation clearly controls the emission concentration of SO2 in flue gas,1 and then it is necessary to remove or reduce SO2 from the exhaust flue gases. Lime/limestone is by far the utmost used absorbent in industrial Flue Gas Desulfurization (FGD) technology.2 However, lime/limestone technology still suffers from plenty of obstacles such as the production of wastewater, high energy penalty and equipment corrosion. Furthermore, the valuable sulfur resource is wasted as useless CaSO4, which is inconsistent with the concept of sustainable development and circular economy. Although some organic solvents have also been reported as SO2 absorbents, the second pollution arisen from their high volatility is a simultaneous problem.3-5 In the past two decades, Ionic liquids (ILs) have been recognized as promising absorbents for acidic gases owing to their attractive and unique properties such as negligible vapour pressure, high thermal stability, good chemical inertness, and extensive structure tunability.6-8 Since Wu et al first reported task-specific 1, 1, 3, 3-tetramethylguanidinium

lactate

([TMG][L]) as

SO2 absorbent with

high

chemisorption capacity and good recycling performance,9 functional ILs have attracted continuous concerns in the field of SO2 removal.10-15 Nevertheless, the process for manufacturing ILs is far to be environmentally friendly.16 Such drawback together with the high cost substantially limits ILs as practical gas absorbent from lab to industry.17 Interestingly, the emerging deep eutectic solvents (DESs) are presented as versatile alternatives to common ILs because they share most of their advantages, but


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overcome some of their limitations. DESs can be directly prepared by mixing hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) in an appropriate molar ratio, which are readily available, cheap, and toxicologically well characterized.18,19 As new green solvents, DESs have received extensive applications in several aspects, such as reaction medium,20,21 extraction separation,22,23 and absorbent for CO2,24-29 etc. Recently, Yang30 and co-workers first reported renewable DESs of choline chloride (ChCl)-glycerol (Gly) with different molar ratios as SO2 absorbent, with the highest absorption capacity of 0.678 g of SO2/ g of DES at 293.15 K and 1 bar with nChCl: nGly = 1:1. Liu et al.31 prepared five thiocyanate-amide DESs to absorb SO2, and pure SO2 solubility in acetamide-KSCN (3:1) can reach 0.588 g of SO2/g of DES at 293.15 K. Sun et al.32 determined the absorption capacities of SO2 in four ChCl-based DESs at different temperatures and SO2 partial pressures, and ChCl−thiourea (1:1) DES demonstrated the highest value of 2.96 mol of SO2 per mol of DES at 293 K and 1 atm. Li et al.16 theoretically investigated the interaction nature between ChCl-Gly DES and SO2, and found that the charge-transfer phenomenon was responsible for the high absorption capacity of SO2 in the DES. It needed to point out that the abovementioned common DESs lacked of the ability to capture SO2 with low pressure. Thus, Zhang et al.33 designed two functional DESs based on betaine or L-carnitine with ethylene glycol for efficient absorption of SO2 at low concentration, with the highest absorption capacity of 0.151 g of SO2/g of L-carnitine-ethylene glycol (1:3) for 2.0 vol% SO2 at 40℃. However, the chemisorptions was so strong that the desorption residue was almost one third of the original absorption capacity



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even regenerated under 90℃. For developing DES as efficient SO2 absorbent, it is necessary to introduce functional HBA or HBD. Bara et al.34 prepared eleven imidazole (Im) solvents with various electron-donating groups at the carbons in Im ring and investigated their capability to absorb SO2. The results illustrate that Im solvents have high SO2 solubilities by direct chemical interaction between Im solvents and SO2. The reason why Im solvents can capture SO2 with low pressure is that base N atom plays a crucial role in SO2 absorption. Based on this reason, Im was selected as functionalized HBD to form DES. The similar 1,2,4-triazole (Tri) was also chosen as a HBD because of its special performance in ILs with anion of Tri for SO2 capture. At the beginning, the frequently used HBA of choline chloride (CC) was used to pair with Im or Tri, unfortunately, it was difficult to obtain DESs with eutectic point less than room temperature. Then, the attention was transferred to acetyl choline chloride (ACC) as HBA. Interesting, four DESs were obtained suitable for SO2 absorbent. Their physicochemical properties have been comprehensively determined. The absorption performance of SO2 in four azole-based DESs was investigated at different molar ratio of HBA to HBD, temperature and SO2 partial pressure. Regeneration experiment of ACC-Im DES was also conducted. A mechanism of acid-base interaction between SO2 and DES was proposed according to 1H NMR and FTIR spectroscopy. 2. EXPERIMENTAL SECTION 2.1. Materials. Pure SO2 (0.999, mass fraction, the same below), 10% and 1% SO2 (volume fraction) gases were supplied by Hangzhou Jingong Special Gas Co., Ltd.


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Anhydrous ACC (AR grade, 0.99, mass fraction, the same below), imidazole (AR grade, 0.99) and 1,2,4-triazole (AR grade, 0.995) were obtained from Shanghai Aladdin Chemical Company. All the chemicals were used as received without any further purification. 2.2. Preparation of azole-based DESs. An electronic balance (Mettler-Toledo AL204) with the accuracy of ± 2*10−4 g was used to determine the weight of the materials. The DESs were simply prepared by mixing ACC with Im or Tri in a 50 cm3 air-tight bottle at 353 K and atmospheric pressure till a homogenous, colorless and transparent liquid phase was formed. Then, the obtained DESs were further dried under vacuum at 353 K for 48 h before use. The water content of each DES was determined by Karl Fischer analysis (SF-3 Karl-Fischer Titration, Zibo Zifen Instrument Co. Ltd.), with the result of less than 1.5·10−3 (mass fraction) in all cases. 1

H NMR spectra were recorded on a Bruker AVANCE Ⅲ (500 MHz) spectrometer

in CDCl3 with tetramethylsilane (TMS) as the standard. FTIR spectra were recorded on a Nicolet 6700 FTIR spectrometer. 2.3. Determination of physical properties. Densities of the DESs were systematically measured at temperature range of (303.15-363.15) K and ambient pressure using pycnometer method with the standard uncertainty of 0.002 g.cm-3. Double distilled water was used as calibrating substance. Viscosities of the DESs under the same condition as the density determination were obtained by Pinkevitch method with the relative standard uncertainty of 2.0%. High purity ethylene glycol was used to calibrate the viscometer. Thermogravimetric analysis (TGA) was



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performed on a PerkinElmer instrument Q50 under N2 atmosphere with a heating rate of 10 K.min-1, and the decomposition temperatures (Td) were determined according to the TGA curves. 2.4. Absorption and desorption of SO2 in DESs. The SO2 absorption and desorption performance in these azole-based DESs were investigated using weighting method. In a typical run, an accurate mass of DES (about 2.0 g) was added into a glass tube with an inner diameter of 12 mm and length of 200 mm. SO2 was bubbled into the DES at a flow rate of 80 cm3.min−1 through a long stainless needle, while the glass tube was immersed in a thermostatic water bath with the desired temperature. The total weight of glass tube and needle was determined at regular intervals during the absorption process by electronic balance (Mettler-Toledo AL204). 10% and 1% SO2 were used from cylinders as received from the suppliers. The 3300 ppm SO2 was prepared online by adjusting the volume flows of 1% SO2 and dried N2 according to the literature method.12 In the desorption of SO2, atmospheric N2 was bubbled through the solutions at 363.15 K at the flow rate of 100 cm3.min−1. The total weight of the glass tube was also determined at regular intervals during desorption process by the electronic balance. 3. RESULTS AND DISCUSSION 3.1. Preparation of DESs. In present work, the DESs of ACC-Tri (1:1), ACC-IM (1:1.5), ACC-IM (1:2) and ACC-IM (1:2) were prepared and used for SO2 capture. Their information on composition, freezing points and pH values were described in Table 1. In the preparation of azole-based DESs, the other DESs with molar ratios of


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3:1, 2:1, 1:2, 1:3 for ACC-Tri and 3:1, 2:1, 1:1 for ACC-Im were also tried. However, they demonstrated as solid or viscous gel phase at room temperature and were not suitable for SO2 absorption. Table 1. Description of azole-based DESs and their freezing points and pH values at 303.15 K. DESs abbreviation

HBA

HBD

Molar ratio of HBA to HBD

freezing point / K

pH value

ACC-Tri (1:1)

ACC

Tri

1:1

207.90

8.10

ACC-Im (1:1.5)

ACC

Im

1:1.5

210.93

9.36

ACC-Im (1:2)

ACC

Im

1:2

209.96

9.52

ACC-Im (1:3)

ACC

Im

1:3

206.91

9.56

3.2. Physicochemical properties. Density and viscosity data are basic physicochemical properties in defining the nature of substance and important parameters related to equipment designing and selection. The determined density and viscosity data of present DESs at atmospheric pressure and temperature ranging from 303.15 K to 363.15 K are plotted in Figure 1. The raw data for density and viscosity are shown in Supporting Information.



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Figure 1. Density (a) and viscosity (b) data of four DESs at different temperatures. ☆, ACC-Tri (1:1); □, ACC-Im (1:1.5); ○, ACC-Im (1:2); ◊, ACC-Im (1:3). The lines are linear fit and Arrhenius equation fit for density and viscosity, respectively. As expected, the density and viscosity of the DESs decreased with increasing temperature. The temperature dependence of density demonstrated linear relationship, while the viscosity and temperature profiles indicate non-linear behavior. The viscosity values for DES of ACC-Tri are higher than those of ACC-Im systems. For ACC-Im systems the viscosity decreased with increasing molar composition of Im in the DESs. The density behavior was fitted using a linear two-parameter and viscosity data was correlated by the Arrhenius equation as following,

ρ = a + bT

(1)

η = η0 *exp( Ea / R / T )

(2)

where ρ and η demonstrate the experimental density in g.cm-3 and viscosity in mPa.s for DES, respectively. T denotes the experimental temperature in Kelvin, and a and b are correlating constants. η0 is pre-exponential constant in mPa.s. Ea represents the activation energy in kJ.mol-1 for viscosity. R is the universal gas constant of 8.314 J.mol-1 K-1. The best fitted values of abovementioned parameters are listed in Table 2. As shown in Table 2, the correlation coefficient, R2, is more than 0.997, which means that the linear and Arrhenius equations can satisfactorily describe the temperature dependence of density and viscosity, respectively. Table 2. Fitting results of the experimental densities and viscosities of the prepared DESs. Parameters

ACC-Tri (1:1)

ACC-Im (1:1.5)

ACC-Im (1:2)


ACC-Im (1:3)

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Density a

1.2932

1.3297

1.2820

1.2773

b*104

-4.536

-7.1259

-5.0377

-5.0179

R2

0.995

0.998

0.994

0.998

η0*107

2.186

1.656

5.297

14.907

Ea / kJ.mol-1

53.11

53.09

48.08

48.48

R2

0.999

0.998

0.996

0.997

Td / K

448

399

416

406

Viscosity

TGA

The thermal stability of the four DESs was determined by thermogravimetric analysis (TGA), with the results shown in Figure 2. The corresponding thermal decomposition temperature (Td, i.e., the temperatures at which the weight loss of a compound in an aluminum pan under an N2 atmosphere reached 5 wt%) was also listed in Table 2.35 The Td values fall approximately 400 K, meaning that DESs are basically stable under the regeneration temperature.

Figure 2. The scanning TGA results for the azole-based ILs with a 10℃.min-1 temperature heating rate to 390℃ under N2 atmosphere. Red, ACC-Im (1:1.5); blue, ACC-Im (1:2); Magenta, ACC-Im (1:3); black, ACC-Tri (1:1).



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3.3. Absorption of SO2 by functional azole-based DESs. Behaviors of SO2 absorption in four azole-based DESs were investigated. As shown in Figure 3, the time dependence of absorption capacity at 303.15 K and 0.10 bar for each DES was presented. The results demonstrate that the absorption process needed almost 40 minutes to complete, such slightly slower absorption rate in present DESs than conventional ILs and organic solvents mainly resulted from the strong and dense hydrogen-bond network in the DESs.16,19 Moreover, the structure of azole has evident influence on the SO2 absorption performance for azole-based DESs, with ACC-Im DES possessing higher absorption capacity of SO2 than ACC-Tri DES. The result is related to the basicity of the azoles, which are the main components responsible for the absorption of SO2 in azole-based DESs. It is well known that the pKa values of Im and Tri are 18.6 and 14.8, respectively.36 Thus the basicity of the former is stronger than that of the latter. Correspondingly, ACC-Im demonstrated higher ability to donor electron to SO2 than ACC-Tri, which is consistent with the sequence of pH values in Table 1. For ACC-Im systems, it can be seen that the absorption capacity increased from 0.355 to 0.381 g of SO2 per g of DES when the molar ratio of Im to ACC reduced from 3:1 to 2:1. However, when the molar ratio further reduced to 1.5:1, the absorption capacity kept at the similar value of 0.383 g of SO2 per g of DES. Obviously, the ACC-Im (1:2) exhibited the best absorption performance under 0.10 bar SO2 with the comprehensive consideration of thermophysical properties and absorption capacity.


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Figure 3. Time-dependence of SO2 absorption behaviors for four DESs at 303.15 K and 0.10 bar. Black ☆, ACC-Im (1:1.5); red ○, ACC-Im (1:2); blue ◊, ACC-Im (1:3); black □, ACC-Tri (1:1). In the following investigation, ACC-Im (1:2) was selected as the representative DES. In order to evaluate present azole-based DESs for SO2 capture, we compared them with other DESs and some functionalized ILs reported in the literatures on the basis of gravimetric absorption capacity at 303.15 K and 0.10 bar, with the results listed in Table 3. Because three amide-based DESs lack the data under 0.1 bar, the absorption capacities under 1.0 bar are then applied. It is evident that azole-based DESs demonstrate higher absorption ability than conventional ones because the former have introduced the functionalized HBD of Im or Tri. For the functionalized ILs, the similar result is obtained. Herein it is interesting to compare the difference of absorption capacities between azole-based ILs and azole-based DESs. Present absorption capacity at SO2 pressure of 0.1 bar includes chemical and physical parts. Although ILs with strong basic anions should exhibit higher chemisorption capacity for SO2 than DESs with weak basic HBDs, the larger molecular weights of



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azole-based ILs discount their gravimetric absorption capacities. For example, the molar absorption capacities of ACC-Im (1:2) and [P66614][Im] are 1.89 and 2.07 mol of SO2 per mole of absorbent, respectively. Because the molecular weight of [P66614][Im] (550) is higher than that of ACC-Im (1:2) (317.8), then, the gravimetric absorption capacity of ACC-Im (1:2) (0.381 g of SO2/g of DES) is evidently larger than that of [P66614][Im] (0.241 g of SO2/g of IL). Furthermore, the ester in ACC will be helpful for enhancing the physical absorption amount in azole-based DESs. Therefore, the comprehensive result is that the azole-based DESs absorb more SO2 than azole-based ILs. Table 3. Absorption capacities of SO2 in some DESs and functionalized ILs. Absorption

Absorbents a,b

T(K)

P (bar)

ACC-Im (1:1.5)

303.15

0.1

0.356

This work

ACC-Im (1:2)

303.15

0.1

0.381

This work

ACC-Im (1:3)

303.15

0.1

0.383

This work

ACC-Tri (1:1)

303.15

0.1

0.227

This work

ACC-Im (1:2)

303.15

1.0

0.989

This work

ChCl-EG (1:2)

303.15

0.2

0.158

Sun32

ChCl-glycerol (1:1)

293.15

0.1

0.153

Yang30

Acetamide–KSCN (3 : 1)

303.15

1.0

0.460

Liu31

CPL-imidazole (1:1)

303.15

1.0

0.625

Liu37

CPL-TBAB (1:1)

298.15

1.0

0.640

Guo38

[P66614][Tetz]

293.15

0.1

0.178

Wang12

[P66614][Im]

293.15

0.1

0.241

Cui39

[N2224][dimalonate]

313.15

0.1

0.152

Huang40

[P66614][DAA]

293.15

0.1

0.181

Cui41

capacity c


References

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[P66614][C5H11COO]

293.15

0.1

0.182

Cui42

[C4bet][SCN]

293.15

0.1

0.276

Yang35

a

The ratio in bracket was molar ratio of each DES. The full names of abbreviations in the table: ChCl and CC, choline chloride; EG, ethylene glycerol; LA, levulinic acid; CPL, caprolactam; TBAB, tetrabutyl ammonium bromide; [P66614][Tetz], trihexyl(tetradecyl)phosphonium tetrazole; [P66614][Im], trihexyl(tetradecyl)phosphonium imidazole; [N2224][dimalonate], triethylbutylammoniumdimalonate; [P66614][DAA], trihexyl(tetradecyl)phosphonium diacetamide; [P66614]-[C5H11COO], trihexyl(tetradecyl)phosphonium hexanoic acid; [C4bet][SCN], butyl betaine thiocyanate. c g of SO2/g of absorbent b

3.4. Desorption of SO2 and recycling of DES. The effects of temperature and partial pressure on SO2 absorption capacity were further investigated and illustrated in Figure 4 using ACC-Im (1:2) and ACC-Tri (1:1) as candidates. It can be seen from Figure 4(a) that the mass ratio of SO2 to ACC-Im (1:2) or ACC-Tri (1:1) decreased from 0.381 to 0.131 and 0.233 to 0.060, respectively, when the temperature increased from 303.15 to 353.15 K. As shown in figure 4(b), the SO2 absorption capacity increased with the increasing partial pressure of SO2 at 303.15 K, for example, the mass ratio of SO2 to DES increased from 0.381 to 0.989 and 0.227 to 0.766 when the pressure changed from 0.1 to 1.0 bar for ACC-Im (1:2) and ACC-Tri (1:1), respectively. These results implied that the SO2 saturated DESs could be easily regenerated by heating or bubbling N2 operation.



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Figure 4. The effect of temperature at 0.1 bar (a) and partial pressure of SO2 at 303.15 K (b) on SO2 absorption capacity by ACC-Im (1:2) (○) and ACC-Tri (1:1) (□). Recycling performance was another important property related to a gas absorbent. Then, the recycling process was conducted, with the result presented in Figure 5. For the DES of ACC-Tri (1:1), the absorbed SO2 could be completely stripped out by bubbling N2 at 363.15 K and the absorption performance was maintained after five cycles. However, the DES of ACC-Im (1:2) demonstrated different result. Although its high absorption capacity could be maintained during the recycling, approximate 0.025g SO2 per g DES was still remained after regeneration. The residue of desorption was attributed to the chemisorption between SO2 and DES because the SO2 concentration in the regeneration condition was very low and related to the basicity of DES. As mentioned above, ACC-Im (1:2) demonstrated higher pH value than ACC-Tri (1:1). Thus, ACC-Im (1:2) demonstrated larger desorption residue of SO2 than ACC-Tri (1:1). In fact, such phenomenon was also reported in the functional


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DESs of betaine or L-carnitine and ethylene glycol, their desorption residues discounted about 30% of the absorption capacities.33

Figure 5. Absorption and desorption cycles of 0.10 bar SO2 in two DESs. In each cycle, SO2 was absorbed at 303.15 K and desorbed at 363.15 K and N2 flow of 100 cm3.min−1. Black column and sparse, ACC-Im (1:2); blue column, ACC-Tri (1:1). 3.5. Spectroscopic investigation and absorption mechanism. As is well known, spectroscopy can be used as a direct way to deeply understand the insitu structural information during gas absorption process.43 For investigating the interactions between SO2 and present DESs, FTIR and 1HNMR spectra of virgin ACC-Im (1:2) and SO2-saturated ACC-Im (1:2) at 0.1 bar were recorded and shown in Figures 6 and 7. For the other DESs of ACC-Im (1:1.5), ACC-Im (1:3) and ACC-Tri (1:1), the FTIR and 1HNMR spectra are similar as shown in Supporting Information.



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Figure 6. FTIR spectra of ACC-Im (1:2) (black) and ACC-Im (1:2) + SO2 (red).

Figure 7. 1H NMR spectra of ACC-Im (1:2) (black) and ACC-Im (1:2) + SO2 (red). By comparing two FTIR spectra in Figure 6, the original bands of C=N, =C-H and –NH at 1574, 3021 and 3109 cm-1 are red shifted to be 1585, 3053 and 3135 cm-1 after saturated by SO2, indicating the chemical interaction between SO2 and nitrogen atoms of Im ring. Moreover, two enhanced absorption bands at 1314 (from original 1325) and 1140 (from original 1137) cm-1 are assigned to the asymmetric stretching and symmetric vibration of S=O bond, respectively. Another new peak at 529 cm-1 is assigned to the scissor bending vibration (δ) of dissolved SO2.44 Furthermore, new band at 1992 cm-1 should be assigned to the S=O…H-N chemical interaction between imidazole and SO2. When considering the FTIR bands in the ACC, the bands of –CH3 changes from 2922 to 2951 cm-1. The bands of ester change slightly from 957 and


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1062 to 955and 1058 cm-1, respectively. As illustrated in Figure 7, the chemical shift of H in NH moved from 12.28 ppm in virgin ACC-Im (1:2) to 13.90 ppm in SO2-saturated ACC-Im (1:2), this demonstrates that imidazole reacted with SO2 to form inner salt. The chemical shifts of H atoms connected to C(2) and C(4, 5) shift downfield from 7.26, 6.41 ppm to 7.66, 6.63 ppm, respectively, means that the acid-base interaction between SO2 and NH reduced the electron density of Im ring. On the contrary, the chemical shifts of hydrogen atoms in the ACC moved very slightly (1.21 ppm-1.30 ppm, 2.56 ppm-2.65 ppm, 3.14 ppm-3.20 ppm, 3.69 ppm-3.83 ppm), which shows that there is almost no chemical reaction between ACC and SO2. But ACC can promote the SO2 absorption by forming hydrogen bond between ester group and SO2. It is well known that the physical dissolution of SO2 in an absorbent is directly proportional to the gas pressure. Then, in present SO2 with low pressure of 0.1 bar, the absorption capacity basically reflects chemical interaction between SO2 with the DES. As a result, the absorption mechanism of SO2 in ACC-Im (1:2) is prospected as a chemical reaction between acidic SO2 and basic N in Im as presented in Scheme 1. SO2 + N

Absorption NH

Desorption

N

H O N S O

Scheme 1. Proposed chemical mechanism between ACC-Im (1:2) and SO2. 3.6. Absorption for SO2 with low partial pressure. Because of the low concentration of SO2 in the flue gas, the neat absorption capacity under low SO2 partial pressure is a vital parameter to evaluate the availability of an absorbent in FGD. For conventional ILs and DESs, the absorption capacity for SO2 at low pressure was



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very small because the capture process accorded with physical dissolution. For dissolving such problem, the easy way is to introduce groups or anions with strong basicity into IL or functional component into DESs, however, this strategy often lead to high desorption residue and intensive energy consumption. As the DES of ACC-Im (1:2) demonstrated satisfactory performance and low desorption residue for 10% SO2, its absorption ability for SO2 with lower concentration was further evaluated. As shown in Table 4, ACC-Im (1:2) exhibited good available capacity of 0.116 g of SO2 per g of DES under 3300 ppm SO2, which was slightly higher than that of other solvents reported in the literatures, except for [P4442][Tetz] (0.18 g of SO2 per g of IL, 293.15 K). However, the synthesis of [P4442][Tetz] involves multistep reactions and many chemicals such as tributylphosphine, bromoethane, MeCN and anion-exchange resin, which results in high cost of the IL. Considering the high absorption capacity and low cost, these azole-based DESs have the potential in application as SO2 absorbent. Table 4. Comparision of SO2 absorption by present DESs and other solvents under low partial pressure. Absorbents a,b

SO2 T(K)

Available capacity c

References

concentration ACC-Im (1:2)

313.15

10000 ppm

0.128

This work

ACC-Im (1:2)

313.15

3300 ppm

0.116

This work

L-car-EG (1:3)

313.15

20000 ppm

0.101

Zhang33

L-car-EG (1:3)

313.15

3700 ppm

0.119d

Zhang33

Bet-EG (1:3)

313.15

20000 ppm

0.06

Zhang33

Bet-EG (1:3)

313.15

3700 ppm

0.034d

Zhang33

[N2222]L

298.15

3440 ppm

0.11

Zeng45


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[TMG]L

298.15

3440 ppm

0.16

Zeng45

[P4442][Tetz]

293.15

2000 ppm

0.18

Chen46

[N2224][disuccinate]

313.15

4000 ppm

0.12

Huang40

[N2222][diglutarate]

313.15

2000 ppm

0.11

Huang47

a

The ratio in the bracket was molar ratio of each DES. The full names of abbreviations in the table: L-car, L-carnitine; EG, ethylene glycerol; Bet, betaine; [N2222]L, tetraethylammonium lactate; [TMG]L, 1,1,3,3-tetramethylguanidinium lactate; [P4442][Tetz], tributyl(ethyl)phosphonium tetrazole; [N2224][disuccinate], triethylbutylammonium disuccinate; [N2222][diglutarate], tetraethylammonium diglutarate. c Equal to the absorption capacity minus the desorption residue, g of SO2/g of absorbent. d The absorption capacity. b

4. CONCLUSIONS In summary, four azole-based functionalized DESs were prepared and evaluated for SO2 capture from the physicochemical property, absorption performance, absorption mechanism, and comparison with other DESs as well as ILs. The capture of SO2 was significantly affected by the choice of azole and molar ratio. ACC-Im (1:2) demonstrated the maximum absorption capacity of 0.381 and 0.116g of SO2 / g of DES at 313.15 K under 0.1 bar and 3300 ppm SO2, respectively. The SO2-saturated DES can be regenerated easily by heating or bubbling N2 through the DES and recycled five times without evident loss of performance. Considering the low cost, easy preparation, and good performance, these azole-based DESs were regarded as attractive alternatives for SO2 capture.

ASSOCIATED CONTENT

Supporting Information



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Experimental densities (ρ) along with their standard deviations (SD) of the prepared DESs at different temperatures (T); Experimental viscosities (η) along with their standard deviations (SD) of the prepared DESs at different temperatures (T); FTIR spectra of ACC-Im (1:1.5) (black) and ACC-Im (1:1.5) + SO2 (red); FTIR spectra of ACC-Im (1:3) (black) and ACC-Im (1:3) + SO2 (red); FTIR spectra of ACC-Tri (1:1) (black) and ACC-Tri (1:1) + SO2 (red); 1H NMR spectra of ACC-Im (1:1.5) (black) and ACC-Im (1:1.5) + SO2 (red); 1H NMR spectra of ACC-Im (1:3) (black) and ACC-Im (1:3) + SO2 (red); 1H NMR spectra of ACC-Tri (1:1) (black) and ACC-Im ACC-Tri (1:1) + SO2 (red).

AUTHOR INFORMATION

Corresponding Author *Dongshun Deng. Tel/Fax: +86-571-8832-0892. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEGEMENTS

The research was supported by the Natural Science Foundation of Zhejiang Province (LY17B060010).

REFERENCES

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thermodynamic analysis. Chem. Eng. J. 2014, 237, 478–486.



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For Table of Contents Only

N HN

SO2 absorption HN

N

O

O

N

O H S O N N

N

O

O H S O O N N

SO2 desorption

3300ppm SO2 available absorption capacity: 0.116 g of SO2 per g of DES


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