PEG200 Mixtures and

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Absorption of SO2 in Furoate Ionic Liquids/PEG200 Mixtures and Thermodynamic Analysis Yaotai Jiang, Xiaobang Liu, and Dongshun Deng* Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China ABSTRACT: Carboxylate ionic liquid (IL) and polyethylene glycol 200 (PEG200) mixtures are suitable for the absorption of acidic SO2 because of their unique properties. In the present work, tetraethylammonium furoate ([N2222][FA]) and choline furoate ([Ch][FA]) were blended with PEG200 to form 40 wt % ILs/PEG200 mixtures as SO2 absorbents. The solubilities of SO2 in these two ILs/PEG200 mixtures were measured at T = (303.15, 313.15, 323.15, and 333.15) K and pressure up to 1.2 bar. [N2222][FA]/PEG200 demonstrated the high SO2 absorption capacity of 7.224 mol per kg absorbent at 303.15 K and 1.0 bar. The chemisorption mechanism of SO2 by mixture was studied by Fourier transform infrared and nuclear magnetic resonance spectra, and the influence of cations on the SO2 absorption capacity was analyzed. Furthermore, the reaction equilibrium thermodynamic model was used to correlate the experimental data, and Henry’s law constant (H), reaction equilibrium constant (K0), and thermodynamic properties (ΔrG0m, ΔrH0m, and ΔrS0m) were derived. The absorption enthalpies of the two binary mixtures are −46.39 and −23.07 kJ·mol−1 for the two mixtures, respectively. Considering the low cost, biodegradability of the materials, high capacity, and low absorption enthalpy, the [N2222][FA]/PEG200 mixture was regarded to be an attractive and promising alternative to pure ILs and ordinary solvent as a desulfurizer.

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INTRODUCTION With the rapid development of the economy, the sulfur dioxide (SO2) emission in China since 2000 is of increasing concern.1 Sulfur dioxide is the primary cause of acid precipitation, which adversely affects natural systems, agriculture, building materials, and human health.2 Therefore, how to manage SO2 emission effectively has become the research forefront. Recently, Córdoba overviewed the flue gas desulfurization (FGD) technologies used to reduce sulfur discharges from coal-fired power plants such as wet, dry, and/or semidry FGD.3 Even to this day, FGD is still regarded as the most effective method in controlling and reducing the emission of SO2,4 with wet limestone FGD as the most widely used technology.3 But the FGD process produces substantial FGD−gypsum and filtered water (effluent) from gypsum slurry filtration. Therefore, it is still desirable to explore new environmentally friendly processes with high SO2 capturing efficiency, low operation cost, and negligible byproducts. Nowadays, many new substances and materials have been studied to capture SO2 from flue gas such as organic solvents,5−7 deep eutectic solvents (DESs),8 metal−organic frameworks (MOFs),9 supported ionic liquid phase materials,10,11 etc. However, the practical utilization of these are subject to secondary pollution, high cost of the operation processes, unknown toxicity, loss of structural integrity, limited reusability of the absorbents, and so on. Meanwhile, since Wu et al.12 first synthesized the task-specific ionic liquid (TSIL) of 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) as effective © XXXX American Chemical Society

SO2 absorbent, IL itself as a green solvent is a consecutive research hotpot in chemistry, especially in desulfurization field. Many functionalized ILs13−22 are continually synthesized and investigated as potential absorbents for SO2 capture. However, high viscosity is an inherent defect for most of them, which will enhance the difficulty in transportation and restrain the kinetics of absorption. One simple and cheap solution is to blend ILs with low viscous organic solvents such as polyethylene glycol (e.g., PEG200,23,24 PEG40025,26), sulfolane,20 or water27 for SO2 or CO2 absorption. As we know, furans and their derivatives are chemical building blocks common in plant biomass and are abundantly used in food, medicines, and industrial processes.28 In our previous work,29 six furoate anion-functionized ILs were prepared and applied as new SO2 absorbents. It was found that tetraethylammonium furoate ([N2222][FA]) and choline furoate ([Ch][FA]) possessed higher absorption capacity based on mass scale than other quaternary phosphonium furoate, but they were solid or viscous liquid. To make up for such deficiencies, [N2222][FA] and [Ch][FA] were diluted by PEG200 to form 40 wt % ILs/PEG200 mixtures as SO2 absorbents in this work. Furthermore, [Ch]+ came from biodegradable, cheap, and nontoxic choline chloride. [N2222] + was a typical quaternary ammonium cation with low molecular Received: March 31, 2017 Accepted: January 16, 2018

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DOI: 10.1021/acs.jced.7b00306 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Scheme 1. Synthesis Schemes for [N2222][FA] and [Ch][FA]

was a yellow transparent liquid with an extremely high viscosity of 7559 mPa·s at room temperature. Characterization of ILs. The structures of ILs were characterized using 1H NMR and FTIR spectra in our previous work.29 The chemical shift data of 1H NMR were again listed below for subsequent discussion. [N2222][FA]. 1H NMR (500 MHz, DMSO-d6, 25 °C, TMS) δ 7.46 (1H, dd, O−CHCH), 6.53 (1H, dd, CH−CHC), 6.35−6.34 (1H, t, CHCH−CH), 3.23−3.19 (8H, m, 4 × NCH2), 1.17−1.13 (12H, t, 4 × CH3). [Ch][FA]. 1H NMR (500 MHz, DMSO-d6, 25 °C, TMS) δ 7.74 (1H, dd, O−CHCH), 6.98−6.97 (1H, dd, CH−CH C), 6.54−6.53 (1H, t, CHCH−CH), 3.86−3.84 (2H, t, OCH2), 3.44−3.42 (2H, t, NCH2), 3.13 (9H, s, 3 × NCH3), 2.51−2.50 (1H, s, COH). The purity of ILs was determined with NMR spectroscopy with the results of 0.99 and 0.98 for [N2222][FA] and [Ch][FA], respectively. Prior to preparation of ILs/PEG200 mixtures, the water content of ILs was measured by Karl Fischer analysis (SF-3 Karl Fischer Titration, Zibo Zifen Instrument Co. Ltd.) with low values of 735 and 1450 ppm, respectively. PEG200 was also dried with 4 Å molecular sieve before mixing. Measurement of Physical Properties. The densities of two ILs/PEG200 mixtures were determined at T = (303.15, 313.15, 323.15 and 333.15) K under 101.3 kPa using pycnometer method with the standard uncertainty of 0.001 g· cm−3. Double distilled water was used as a calibrating substance. The viscosities were measured by Pinkevitch method with relative standard uncertainty of 0.02. High purity ethylene glycol was used to calibrate the viscometer. Apparatus and Procedure to Determine Solubility of SO2 in ILs/PEG200. As shown in Figure 1, the solubility determination experiments were performed using an isochoric

mass used to prepare ILs of [N2222][diglutarate] as absorbent of SO2 with satisfactory performance in literature.20 PEG200 was chosen as a solvent and coabsorbent on account of its low toxicity, chemical stability, low price, low vapor pressure, low melting point, extensive sources, as well as high biodegradability.30 As a continuous and further research, the physical properties, SO2 solubility data at various temperatures, and pressures up to 1.2 bar in 2 mixtures were systematically determined. Thermodynamic properties were also calculated by correlating the solubility data with the reaction equilibrium thermodynamic model (RETM). The chemisorption mechanism was proposed according to the nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy analysis. The recyclability was also investigated.

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EXPERIMENTAL SECTION Materials. Sulfur dioxide (SO2, 0.999, mass fraction purity, the same below besides special statement) and nitrogen (N2, 0.999) were supplied by Jingong Special Gas Co., Ltd. (Hangzhou, China). 2-Furoic acid (FA, 88-14-2, AR grade, 0.98), tetraethylammonium chloride ([N2222]Cl, 56-34-8, AR grade, 0.98), and polyethylene glycol (PEG200, with the average molecular weight of 200 g·mol−1, AR grade) were obtained from Aladdin Industrial Co., Ltd. (Shanghai, China). Choline hydroxide ([Ch]OH, 123-41-1, AR grade, 0.998) was purchased from Jinan Jinhui Chemical Co., Ltd. (Jinan, China). Anion-exchange resin (201 × 7(OH), the exchange capacity was 3.41 mmol·g−1, dry basis) was supplied by The Chemical Plant of NanKai University. All of these chemicals were used without any further purification. Preparation of ILs. Tetraethylammonium furoate ([N2222][FA]) and choline furoate ([Ch][FA]) were prepared by direct neutralization reaction of 2-furoic acid with corresponding bases. The synthesis scheme was illustrated as following. Particularly, [N2222]OH ethanol solution was prepared using an anion-exchange method from [N2222]Cl according to the literature.19 As an example, the synthetic procedure of [N2222][FA] is described here: 0.1 mol of 2-furoic acid was loaded into a 500 cm3 flask and dissolved with 50 cm3 absolute ethanol. The flask was placed in a water bath of room temperature and stirred magnetically. Equimolar [N2222][OH] ethanol solution was then dropwise added into the flask. The reaction lasted for 12 h. Subsequently, water and ethanol were removed by evaporation at 343 K under reduced pressure, and the resulting crude residue was washed with tert-butyl methyl ether (3 × 30 mL) to remove unreacted reactants. The resulting product was dried under vacuum at 353 K for 48 h, and a hygroscopic white solid was obtained (with the melting point of 334 K). The synthetic procedure of [Ch][FA] was similar to that of [N2222][FA] (Scheme 1). However, [Ch][FA]

Figure 1. Schematic diagram of the SO2 solubility apparatus. 1, SO2 gas cylinder; 2, 3, thermostatic water bath and magnetic stirrer; 4, SO2 gas reservoir (GR); 5, SO2 gas equilibrium cell (EC); 6, vacuum; 7, 8, pressure transmitter; 9, 10, 11, 12, valve; 13, digital indicator. B



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Table 1. Densities (ρ) and Viscosities (η) of the 40 wt % ILs/PEG200 Mixtures at Different Temperatures and 101.3 kPaa ρ/g·cm−3

η/mPa·s

temperature/K

[N2222][FA]/PEG200

[Ch][FA]/PEG200

[N2222][FA]/PEG200

[Ch][FA]/PEG200

303.15 313.15 323.15 333.15

1.1175 1.1086 1.1042 1.1011

1.1762 1.1662 1.1597 1.1545

85.70 51.77 29.69 19.40

143.79 85.05 50.84 28.78

The standard uncertainties u are u(T) = 0.10 K and u(p) = 0.006 bar; the combined expanded uncertainty Uc is Uc(ρ) = 0.002 g·cm−3 (0.95 level of confidence), and the relative standard uncertainty of viscosity is ur(η) = 0.02.

a

method under atmospheric pressure, in which the two processes were implemented at 303.15 and 343.15 K, respectively. In a typical run, an accurate mass of ILs/ PEG200 mixture (about 3.0 g) was added into a glass tube immersed in a thermostatic water bath with the desired temperature. The inner diameter of the glass tube was 12 mm, and the length was 200 mm. SO2 was bubbled into the mixture at a flow rate of 40 cm3·min−1 using a long stainless needle. The total weight of glass tube as well as the needles was determined at regular intervals by a electronic balance (Mettler-Toledo AL204) until a constant value. The uncaptured SO2 was introduced to an off-gas absorber containing NaOH aqueous solution. In release experiments, pure N2 with a flow rate of 100 cm3·min−1 went through a 4 Å molecular sieve column and then bubbled through the saturated ILs/PEG200 mixtures. A detailed description of the experimental process is available in our previous work.33

saturation method in a stainless apparatus. A detailed description of the device and the experimental procedure are accessible in our previous work.31 The apparatus was composed of a SO2 cylinder (1), two constant temperature water baths (2, 3) with standard uncertainty of 0.10 K, a gas reservoir (4, GR, 370.99 cm3), a gas equilibrium cell (5, EC, 141.61 cm3) with a magnetic stirrer, a vacuum pump (6), two pressure transmitters (7, 8), and a digital indicator (13). Among them, the pressure transmitters (Fujian WIDEPLUS Precision Instruments Co., Ltd. WIDEPLUS-8) with standard uncertainty of 0.006 bar were used to record pressure changes during absorption processes, and the constant temperature water baths were used to control the temperatures of GR and EC at the desired values. In addition, an electronic balance (Mettler-Toledo AL204) with standard uncertainty of 0.0002 g was used to determine the mass of absorbents added into the EC. In a typical run, a known amount of absorbent (w) was placed into EC, and the air in the whole system was wiped off using the oil pump (6). The temperature of water bath 2 for GR was always kept at 300.15 K, while the other for EC was kept at each desired temperature. The pressure in EC was recorded to be p1 at beginning. Then, SO2 from its gas cylinder was fed into GR with the pressure of p2. The needle valve (11) between the two chambers was turned on to introduce SO2 into EC. Absorption equilibrium was thought to be achieved when the temperatures and pressures of the two chambers remained constant for at least 2 h. The equilibrium pressures were recorded as p3 for GR and p4 for EC. Therefore, the SO2 partial pressure in EC was calculated as the following: ps = p4 − p1 (1)

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RESULTS AND DISCUSSION Physical Properties of ILs/PEG200 Binary Mixtures. The densities and viscosities of two 40 wt % ILs/PEG200 mixtures were systematically measured. The temperature ranged from 303.15 to 333.15 K with 10 K intervals, and the data are presented in Table 1 and Figures 2 and 3. As can be

Then, the total SO2 captured by the binary mixture nt could be calculated by eq 2: ⎛ w⎞ ⎟⎟ nt = ρg (p2 , T )VGR − ρg (p3 , T )VGR − ρg (ps , T )⎜⎜VEC − ρsol ⎠ ⎝ (2) −3

where ρg(pi,T) represents the density of SO2 in mol·cm at pi(i = 2,3,s) and temperature T, and it can be obtained according to NIST Standard Reference Data.32 VGR and VEC represent the volumes of GR and EC in cm3, respectively. ρsol is the density of the absorbent in g·cm−3 at temperature T. Continuous determination of solubility data at higher pressure was proceeded by introducing more SO2 into EC to reach new equilibrium. After the experiment, SO2 remaining in the chambers was introduced into an off-gas absorber containing aqueous solution of NaOH in case of SO2 leaking into the atmosphere. Apparatus and Procedure to Study Absorption and Desorption of SO2 in ILs/PEG200. The absorption and desorption experiments were carried out using a bubble

Figure 2. Densities of 40 wt % ILs/PEG200 mixtures (■, [N2222][FA]/PEG200; ●, [Ch][FA]/PEG200; lines, fitting results).

seen from Figures 2 and 3, [N2222][FA]/PEG200 possesses density and viscosity lower than those of [Ch][FA]/PEG200 at each temperature. The viscosities of two ILs/PEG200 binary mixtures are less than 90 mPa·s at 313.15 K. Particularly, at 303.15 K, the viscosity of [N2222][FA]/PEG200 is only 85.70 mPa·s. It is well-known that the low viscosity will promote the mass transfer during the SO2 absorption and desorption processes. As expected, the densities and viscosities of the two binary absorbents decrease with increasing temperature, while the C



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chemical absorption type, which indicate that chemical reaction may take place during SO2 absorption and further increase in capacity at high pressures due to physical absorption. Furthermore, the absorption of SO2 in the ILs/PEG200 mixtures at 313.15 K was presented in Figure 6. It revealed that the cations of ILs remarkably affected the SO2 absorption capacities of the mixtures. For instance, the absorption capacities of [N2222][FA]/PEG200 and [Ch][FA]/PEG200 at 0.1 bar were 1.954 and 0.548 mol·kg−1 respectively, with a different value of 1.406 mol·kg−1. When the SO2 partial pressure increased to be 1.0 bar, the capacities changed to be 5.957 and 3.224 mol·kg−1 respectively, with a lager different value of 2.733 mol·kg−1. This phenomenon will be discussed later on the basis of absorption mechanism. The solubilities of SO2 in [N2222][FA]/PEG200 and [Ch][FA]/PEG200 at 303.15 K and 1.0 bar are 7.224 mol· kg−1 (0.463 g·g−1) and 4.735 mol·kg−1 (0.303 g·g−1), respectively. Even when the SO2 partial pressure is reduced to 0.004 bar, the solubility is 0.718 mol·kg−1 (0.046 g·g−1) for [N2222][FA]/PEG200. It means that present absorbents have the potential to capture SO2 from flue gases. Given that pure PEG200 is only a physical absorbent for SO2, the solubility of SO2 in PEG200 under low pressure is very small.30 For instance, solubility of SO2 in PEG200 at 303.15 K and 0.039 bar is merely 0.3075 mol·kg−1 (0.0197 g·g−1). Therefore, ILs with chemisorption play a vital role in capturing SO2 under low partial pressures for ILs/PEG200 binary mixtures. Herein, the chemisorption mechanism was further investigated with [N2222][FA]/PEG200 mixture as a candidate due to the similar absorption behavior of the two systems. 1H NMR, 13C NMR, and FT-IR were applied to characterize the [N2222][FA]/PEG200 and [N2222][FA]/PEG200 + SO2 at SO2 partial pressure of 0.005 bar. It is well-known that PEG200 is a physical solvent, and the interaction between SO2 and PEG200 is very weak at low pressure region.30 Then, spectroscopic information mainly reflects the chemisorptions of SO2 by [N2222][FA]. From the 1H NMR and 13C NMR spectra of [N2222][FA] /PEG200 and [N2222][FA]/PEG200 + SO2 as shown in Figure 7, the chemical shifts of the hydrogen atoms in the ring moved downfield from 7.63, 6.88, and 6.50 ppm to 7.93, 7.28, and 6.74 ppm, respectively. The peak of −COO− in furoate moved upfield from 163.74 to 160.48 ppm, indicating the interaction of SO2 with −COO−. For the carbon atom adjacent to carboxyl group, a larger upfield shift from 152.96 to 146.94 ppm means the synergistic interaction of SO2 by −COO− and oxygen atom in the anion. The π···S interaction between furoate ring and SO2 results in downfield shifting of other carbons from 143.31, 112.40, and 111.25 ppm to 146.33, 117.56, and 112.46 ppm, respectively. The chemical shifts of the hydrogen atoms in cation change much less, while those of carbon atoms remain unchanged. Therefore, the chemisorption of SO2 by [N2222][FA]/PEG200 is assumed to strong acid− base interaction between acidic SO2 and Lewis-based −COO− as well as auxiliary interaction of SO2 with furoate ring, as illustrated in Scheme 2. Such a mechanism can be further verified by the FT-IR spectra, as shown in Figure 8. The peaks at 1358 and 1597 cm−1 associated with the symmetric and asymmetric stretching bands of −COO− in [N2222][FA]/ PEG200, respectively, means that the negative charge is dispersed on two oxygen atoms of −COO−. Compared with [N2222][FA]/PEG200 + SO2, a new absorption band at 1705 cm−1 indicates the negative charge is concentrated onto one oxygen atom due to the strong interaction between acidic SO2

Figure 3. Viscosities of 40 wt % ILs/PEG200 mixtures (■, [N2222][FA]/PEG200; ●, [Ch][FA]/PEG200; lines, fitting results).

changing trends of density and viscosity are different. Then, the density data were fitted using a second-order polynomial, and the viscosity data were fitted by the Arrhenius equation as follows: ρ = A + BT + CT 2

(3)

⎛E ⎞ η = A exp⎜ a ⎟ ⎝ RT ⎠

(4)

The fitting results are shown graphically in Figures 2 and 3, and the obtained model parameters are included in Table 2. Table 2. Fitting Results of the Experimental Densities and Viscosities of the 40 wt % ILs/PEG200 Mixtures parameters

[N2222][FA]/PEG200

[Ch][FA]/PEG200

density A B C R2 A Ea R2

2.7443 −0.00976 1.54 × 10−5 0.990 viscosity 5.28 × 10−6 41 852.98 0.99847

2.6051 −0.00835 1.2 × 10−5 0.997 5.40 × 10−6 43 102.50 0.99848

SO2 Absorption in ILs/PEG200 Mixtures. Absorption Capacity and Absorption Mechanism. The solubilities of SO2 in two ILs/PEG200 mixtures were measured at T = (303.15, 313.15, 323.15, and 333.15) K and pressure up to 1.2 bar. The experimental results are presented in Tables 3 and 4, where p stands for SO2 equilibrium pressure above the absorbents and mt presents the molality of SO2 in the liquid phase. mt can be easily calculated by the following equation, n mt = t (5) w where nt represents the total SO2 uptake by the absorbent and can be calculated by eq 2. w is the mass of absorbents used in each experiment. Figures 4 and 5 show SO2 solubility profiles in ILs/PEG200 mixtures at different temperatures. It is observed that both blended absorbents display a similar absorption behavior. The solubility of SO2 grows significantly with the increasing of SO2 partial pressure at low pressure range (below 0.1 bar) while increasing slowly at high pressure range (0.1−1.2 bar). This phenomenon was very evident in the [N2222][FA]/ PEG200 mixture. The absorption isotherms are typically D



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Table 3. Experimental SO2 Molality (mt) in 40 wt % [N2222][FA]/PEG200 Mixture at Temperature (T) and Equilibrium Pressure (p)a 303.15 K

a

313.15 K

323.15 K

333.15 K

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

0.021 0.043 0.067 0.101 0.150 0.196 0.310 0.401 0.499 0.597 0.702 0.814 0.895 0.996 1.107 1.198

1.406 1.693 1.893 2.125 2.426 2.715 3.492 4.029 4.587 5.114 5.655 6.205 6.603 7.154 7.720 8.195

0.024 0.055 0.084 0.114 0.154 0.204 0.307 0.398 0.508 0.599 0.693 0.804 0.908 1.008 1.096 1.206

1.276 1.597 1.826 2.036 2.286 2.572 3.088 3.510 3.974 4.348 4.714 5.138 5.522 5.940 6.256 6.713

0.027 0.077 0.104 0.139 0.193 0.297 0.418 0.516 0.605 0.698 0.806 0.909 1.012 1.100 1.181

1.140 1.532 1.686 1.863 2.113 2.533 2.972 3.297 3.581 3.868 4.183 4.481 4.813 5.059 5.285

0.025 0.045 0.071 0.098 0.127 0.158 0.200 0.293 0.415 0.515 0.607 0.695 0.796 0.890 0.991 1.092 1.198

1.015 1.169 1.316 1.438 1.557 1.677 1.816 2.110 2.453 2.712 2.945 3.150 3.386 3.598 3.822 4.039 4.260

Standard uncertainties u are u(T) = 0.10 K, u(p) = 0.006 bar; relative standard uncertainty ur(mt) = 0.03.

Table 4. Experimental SO2 Molality (mt) in 40 wt % [Ch][FA]/PEG200 Mixture at Temperature (T) and Equilibrium Pressure (p)a 303.15 K

a

313.15 K

323.15 K

333.15 K

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

p/bar

mt/(mol·kg−1)

0.032 0.063 0.096 0.136 0.180 0.232 0.298 0.397 0.497 0.607 0.695 0.805 0.901 0.997 1.081 1.209

0.556 0.776 0.972 1.187 1.405 1.649 1.945 2.373 2.789 3.225 3.573 3.999 4.373 4.741 5.086 5.574

0.040 0.070 0.093 0.129 0.151 0.208 0.300 0.405 0.496 0.605 0.700 0.804 0.898 1.007 1.104 1.202

0.394 0.523 0.618 0.753 0.830 1.022 1.307 1.611 1.876 2.178 2.430 2.702 2.961 3.254 3.520 3.782

0.030 0.066 0.100 0.140 0.219 0.298 0.393 0.493 0.591 0.714 0.811 0.909 1.011 1.117 1.209

0.300 0.455 0.576 0.699 0.927 1.133 1.362 1.592 1.811 2.077 2.282 2.482 2.691 2.907 3.094

0.036 0.070 0.107 0.141 0.202 0.305 0.401 0.512 0.606 0.708 0.811 0.904 1.004 1.100 1.208

0.260 0.374 0.483 0.570 0.714 0.932 1.117 1.321 1.488 1.661 1.833 1.983 2.144 2.295 2.462

Standard uncertainties u are u(T) = 0.10 K, u(p) = 0.006 bar; relative standard uncertainty ur(mt) = 0.03.

and Lewis-based −COO−.8 The new peaks at 553 and 968 cm−1 are ascribed to be scissor bending vibration and S−O stretches of the captured SO2.34 The original bands at 1184 and 1300 cm−1 are enhanced into 1174 and 1292 cm−1 after absorption of SO2, which are assigned to be the asymmetric stretching and symmetric vibration of SO bonds. As mentioned above, the absorption capacities between [N2222][FA]/PEG200 and [Ch][FA]/PEG200 showed an evident difference even with the same anion. Such phenomenon was ascribed to the effect of cation−anion interaction on the chemisorptions of [FA]−. When ILs was contacted with electron deficient SO2, SO2 and cations interacted with [FA]− competitively. From the 1H NMR data of pure ILs in characterization, the chemical shifts of hydrogen atoms in furoate of [Ch][FA] were larger than those in [N2222][FA], indicating [Ch]+···[FA]− interacted stronger than [N2222] +···

[FA]−. Thus, [Ch][FA]/PEG200 demonstrated absorption capacity much lower than that of [N2222][FA]/PEG200 at the same pressure. Recycling of Absorbents. As far as we know, the recyclability of an absorbent is one of the most important evaluating indicators because it directly relates to the operation cost and equipment investment in practice. To estimate the recyclability of the ILs/PEG200 mixtures utilized in this work, the SO2saturated [N2222][FA]/PEG200 mixture was heated to 343.15 K and bubbled with 100 cm3·min−1 dry N2 at atmospheric pressure to release the dissolved SO2. Then, the renewed [N2222][FA]/PEG200 was reused to capture SO2 under 303.15 K and 1.0 bar. Such recycling experiments were carried out five times with the results illustrated in Figure 9. No obvious decline of equilibrium absorption capacity was found after five cycles. E



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behavior, especially in low pressure region. Such phenomenon was attributed to the chemical interaction between SO2 and ILs. Namely, for such absorption process, the interactions between SO2 and ILs/PEG200 mixtures could be divided into two parts, physical dissolution and chemical reaction. They were interdependent and controlled by dissolution equilibrium and reaction equilibrium expressed as eqs 6 and 7, respectively. H

SO2 (g ) → SO2 (l)

(6)

K10

SO2 (l) + IL(l) → SO2 IL(l)

(7)

Thus, the overall reaction is expressed as eq 8, Figure 4. Solubilities of SO2 in 40 wt % [N2222][FA]/PEG200 mixture (■, 303.15 K; ●, 313.15 K; ▲, 323.15K; ▼, 333.15 K; lines, RETM fit).

K0

SO2 (g ) + IL(l) → SO2 IL(l)

(8)

where g and l represent the existing states (gas and liquid) of a species. Here, Henry’s law constant (H), equilibrium constant (K01), and overall reaction equilibrium constant (K0) were used to qualitatively describe eqs 6−8, respectively. On the basis of the literature,20 the RETM could be used to correlate the experimental solubility data. When the SO2 absorption isotherms were fitted using eq 9, H and K10 were obtained. Subsequently, K0 was calculated using eq 10, ⎛ ⎞ 1⎟ ⎜ m IL0 mt = ⎜ + ×p H H⎟ + p 0 ⎝ ⎠ K1 K0 =

Figure 5. Solubilities of SO2 in 40 wt % [Ch][FA]/PEG200 mixture (■, 303.15 K; ●, 313.15 K; ▲, 323.15K; ▼, 333.15 K; lines, RETM fit).

(9)

K10 × p0 H

(10)

where mt is total concentration of SO2 in ILs/PEG200 in mol· kg−1, mIL0 is the initial concentration of ILs in ILs/PEG200 in mol·kg−1, which should be a constant, p refers to the SO2 partial pressure in bar, and p0 refers to the standard pressure (1.0 bar). Fitting curves for [N2222][FA]/PEG200 and [Ch][FA]/ PEG200 are displayed in Figures 4 and 5, respectively (R2 > 0.99). Also, the calculated values of K0 and H for ILs/PEG200 mixtures are presented in Table 5. It can be seen that [N2222][FA]/PEG200 has a larger value of K0 and a smaller value of H than those of [Ch][FA]/PEG200 at same temperature, indicating that [N2222][FA] has an interaction with SO2 stronger than [Ch][FA] both chemically and physically. The Henry’s law constants in terms of molality in pure ILs and PEG200 could be obtained by following equations,

Figure 6. Influence of cations on the absorption of SO2 in the 40 wt % ILs/PEG200 mixtures at 313.15 K (■, [N2222][FA]/PEG200; ●, [Ch][FA]/PEG200).

H2,m(T , p) =

lim

mPEG200 → 0

p 0.4p 0.6p = + H H1,m H2,m

The result means that the ILs/PEG200 mixtures are satisfactory for repeated SO2 absorption/desorption uses. Thermodynamic Analysis. Thermodynamic analysis is helpful for us to have a deeper understanding of the absorption behavior between the solute and solvent. In this section, the Henry’s law constants (H), reaction equilibrium constants (K0), thermodynamic properties such as molar reaction Gibbs energy (ΔrG0m), molar reaction enthalpy (ΔrH0m), and molar reaction entropy (ΔrS0m) were derived for the present two mixtures. According to the above discussion, the relation between the solubility and the partial pressure deviates away from ideal

f gas mPEG200

≈

p mPEG200 /m0

(11)

(12)

where p is the partial pressure of SO2 at equilibrium in bar and H is the Henry’s law constant of SO2 absorption in ILs/ PEG200 in bar. H1,m and H2,m are the Henry’s law constants of SO2 absorption in pure ILs and PEG200 in bar, respectively. fgas is the gas phase fugacity of SO2 in kPa. mPEG200 is the molality of SO2 captured by pure PEG200 in mol·kg−1 and can be obtained in our previous work.23 m0 is the standard molality (1 mol·kg−1). According to the definition of molar fraction and F



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Figure 7. 1H NMR (upper) and 13C NMR (below) spectra of 40 wt % [N2222][FA]/PEG200 before and after absorption of SO2.

Scheme 2. Proposed Chemisorption Mechanism between [N2222][FA] and SO2

Figure 9. SO2 absorption in 40 wt % [N2222][FA]/PEG200 mixture for 5 cycles (absorption: 303.15 K, SO2, 1.0 bar, 40 mL·min−1; desorption: 343.15 K, N2, 1.0 bar, 100 mL·min−1).

Figure 8. FT-IR spectra of 40 wt % [N2222][FA]/PEG200 before (black) and after (red) absorption of SO2.

molality, the relationship between H1,x and H1,m can be easily deduced as following, H1,x = H1,m

[N2222][FA] are smaller than those in [Ch][FA], meaning that [N2222][FA] has a physical interaction with SO2 stronger than that of [Ch][FA], which is consistent with the experimental results of ILs/PEG200 mixtures. To get a deeper understanding of the gas dissolution and reaction equilibrium between SO2 and the ILs/PEG200 mixtures, thermodynamic functions such as molar reaction Gibbs energy ΔrG0m, molar reaction enthalpy ΔrH0m, and molar

M1 m0

(13)

where H1,x is the Henry’s law constant of SO2 absorption in pure ILs in terms of molar fraction in bar, M1 is the molar mass of ILs in mol·kg−1. The estimated H1,m and H1,x of pure ILs were presented in Table 6. Both the H1,m and H1,x values in G



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Table 7. Molar Reaction Gibbs Energy (ΔrG0m), Molar Reaction Enthalpy (ΔrH0m) and Molar Reaction Entropy (ΔrS0m) of SO2 Absorption in 40 wt % ILs/PEG200 Mixtures at 313.15 K

Table 5. Standard Equilibrium Constants of Overall Reaction (Ko) and Henry’s Law Constants of SO2 Absorption (H) in 40 wt % ILs/PEG200 Mixtures [N2222][FA]/PEG200 o

temperature/K

K

303.15 313.15 323.15 333.15

180.8 112.5 56.2 36.1

2

H/bar

R

0.179 0.232 0.317 0.452

0.9985 0.9972 0.9990 0.9972

[Ch][FA]/PEG200 K

o

4.5 2.4 2.3 1.8

H/bar

R2

0.311 0.522 0.729 1.081

0.9967 0.9953 0.9948 0.9945

mixed absorbents [N2222][FA]/ PEG200 [Ch][FA]/ PEG200

ΔrG0m/kJ·mol−1

ΔrH0m/kJ·mol−1 (R2)a

ΔrS0m/J·mol−1·K−1

−12.30

−46.39 (0.9924)

−108.86

−2.25

−23.07 (0.9477)

−66.48

R is the correlation coefficient for ΔrH0m.

a 2

Table 6. Estimated Henry’s Constants of SO2 Absorption for Pure ILs in 40 wt % ILs/PEG200 Mixtures (H1,m, Henry’s Law Constants in Terms of Molality in bar; H1,x, Henry’s Law Constants in Terms of Molar Fraction in bar) [N2222][FA]

values indicate that the absorption processes are exothermic and favorable from the view of enthalpy. The absolute value of ΔrH0m in [Ch][FA]/PEG200 is less than that in [N2222][FA]/ PEG200, suggesting that [Ch][FA] has a weaker interaction with SO2. This is consistent with the above-mentioned effect of cations on the absorption capacity. In other words, the absorbed SO2 can be stripped out more easily. The negative values of ΔrG0m indicate that the capture of SO2 by ILs/PEG200 is a spontaneous process. From molecular points, the negative values of ΔrS0m mean that the systems of SO2 saturated ILs/ PEG200 have an ordering degree higher than that of fresh ILs/ PEG200. Comparison with Other ILs. To comprehensively evaluate the present two renewable ILs as SO2 absorbents, the suppositional solubilities of SO2 in pure ILs were calculated using eq 17 according to Huang et al.20 and compared with other ILs reported in literature with the results summarized in Table 8.

[Ch][FA]

temperature (K)

H1,m (bar)

H1,x (bar)

H1,m (bar)

H1,x (bar)

303.15 313.15 323.15 333.15

0.16 0.16 0.22 0.31

0.67 0.68 0.92 1.27

4.04 13.96 21.92 29.25

18.77 64.85 101.82 135.88

reaction entropy ΔrS0m were calculated. The logarithms of the overall reaction equilibrium constants K0 versus the inverse of temperatures are presented in Figure 10. Thus, the molar reaction enthalpy ΔrH0m can be obtained using the van’t Hoff equation by linear correlation of ln K0 with 1/T. 0

ΔH ∂ln K 0 =− r m ∂(1/T ) R

(14)

mIL =

mt − 0.6mPEG200 0.4

(17)

The values were given both based on weight and molarity, but the former is more valuable in practical application. It can be found that [N2222][FA] has a superior capacity of SO2 among most of the ILs both in ordinary and low pressure. Meanwhile, taking [NEt2C2Py][SCN] as an example,15 though it has a remarkable capacity of capturing SO2, its absolute value Table 8. Comparison of the Suppositional Solubility of SO2 in Pure ILs in the Present Work with Other ILs SO2 absorption capacitya

Figure 10. Linear fit of lnK and 1/T (■, [N2222][FA]/PEG200; ●, [Ch][FA]/PEG200). 0

Subsequently, ΔrG0m and ΔrS0m can be calculated from eqs 15 and 16. Δr Gm0 = − RT ln K 0

(15)

Δr Hm0 − Δr Gm0 (16) T The thermodynamic property changes at 313.15 K were presented in Table 7. Among the parameters, ΔrH0m is the most worthy of attention because it reveals the interaction between SO2 and absorbents and relates directly to the energy consumption of gas desorption process. As can be seen, the ΔrH0m values are −46.39 and −23.07 kJ·mol−1 for [N2222][FA]/ PEG200 and [Ch][FA]/PEG200, respectively. The negative Δr Sm0 =

ionic liquid

T/K

1.0 bar

0.1 bar

[N2222][FA] [Ch][FA] [N2222][diglutarate]20 [DMEA][diglutarate]20 [choline][LA]23 [MAPNH][MOAc]13 [DMAPNH][EOAc]13 [hmim][4-CH3O-Ben]18 [hmim][2-CH3-Ben]18 [P66614][3-CHO-Indo]17 [P66614][2-CHO-Pyro]17 [Et2NEMIm][SCN]14 [NEt2C2Py][SCN]15 [E2Py]Cl16 [Na(TX-10)][Im]22

313.15 313.15 313.15 313.15 313.15 303.15 303.15 333.15 333.15 293.15 293.15 313.15 293.15 293.15 293.15

0.570 (2.147) 0.132 (0.444) 0.511 (2.085) 0.400 (1.382) 0.517 (1.770) 0.39 (1.05) 0.35 (1.12) 0.33 (1.64) 0.35 (1.66) 0.43 (4.24) 0.46 (4.15) 0.725 (2.720) 1.06 (3.958) 1.155 (3.924) 0.567 (6.65)

0.266 (1.001) 0.041 (0.137) 0.135 (0.551)b 0.107 (0.370)b 0.282 (0.964) 0.125 (0.340) 0.087 (0.274)c 0.11 (0.56)d 0.09 (0.42)d 0.20 (1.92) 0.21 (1.87) NAe 0.37 (1.411) 0.486 (1.650)f 0.238 (2.79)

a c

H

Values given in units of g SO2/g IL (mol SO2/mol IL). bAt 0.004 bar. At 0.118 bar. dAt 0.01 bar. eNA: not available. fAt 0.2 bar. DOI: 10.1021/acs.jced.7b00306 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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of molar reaction enthalpy (63.9 kJ·mol−1) was much larger than that of the present absorbents. Therefore, with the overall consideration of viscosity, absorption capacity, regeneration energy consumption, cost, and biodegradability, [N2222][FA]/ PEG200 and [Ch][FA]/PEG200 are considered as attractive alternatives to pure ILs for SO2 absorption.

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CONCLUSIONS In this work, two anion functionalized ionic liquids [N2222][FA] and [Ch][FA] were synthesized and blended with PG200 to form binary absorbents. Physical properties and SO2 absorption capacities of the mixtures were determined systematically. Furthermore, an RETM was used to correlate the experimental data, and Henry’s law constants (H), reaction equilibrium constants (K0), and thermodynamic properties (ΔrG0m, ΔrH0m, and ΔrS0m) were derived. Results indicate that [N2222][FA] has an interaction with SO2 stronger than that of [Ch][FA] both chemically and physically. The chemisorption mechanism of SO2 and the influence of cations on the SO2 absorption capacity for ILs/PEG200 mixture was analyzed. Due to low cost, biodegradability of the materials, high capacity, and low enthalpy of the absorption process, the [N2222][FA]/PEG200 mixture was shown to be an attractive candidate of SO2 absorbents.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dongshun Deng: 0000-0001-7125-1833 Funding

Financial support from the Natural Science Foundation of Zhejiang Province (Grant LY17B060010) is deeply appreciated. Notes

The authors declare no competing financial interest.

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