Solubilities and Thermodynamic Properties of Carbon Dioxide in

Mar 31, 2017 - In this work, the solubility data of CO2 in three kinds of guaiacol-based deep eutectic solvents (DESs) at 293.15, 303.15, 313.15, and ...
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Solubilities and Thermodynamic Properties of Carbon Dioxide in Guaiacol-Based Deep Eutectic Solvents Xiaobang Liu, Bao Gao, Yaotai Jiang, Ning Ai, and Dongshun Deng* Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China ABSTRACT: In this work, the solubility data of CO2 in three kinds of guaiacol-based deep eutectic solvents (DESs) at 293.15, 303.15, 313.15, and 323.15 K and pressures below 600.0 kPa were determined. The DESs were prepared from guaiacol and quaternary ammonium salts (QASs, including choline chloride, acetylcholine chloride, and diethylamine hydrochloride) with a molar ratio of guaiacol to QAS of 3:1, 4:1, and 5:1. Henry’s law was used to correlate the solubility data, and the thermodynamic properties, such as standard Gibbs free energy, standard enthalpy, and standard entropy of these systems were derived. The solubility of CO2 increased with increasing molar ratio of guaiacol to QAS in each kind of DES. The dissolution enthalpies were negative at all conditions. The dissolution capacities of present DESs for CO2 were further compared with those of other DESs and several common ionic liquids.

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extractant in liquid−liquid separation23,24 with satisfactory performances. Recently, DESs have also been reported as new CO2 absorbents because the predominantly ionic species in them have interesting affinity to CO2. Li et al.25 reported the solubility data of CO2 in the DES of ChCl and urea at 313.15, 323.15, and 333.15 K and pressures close to 13 MPa. Leron et al.26−28 determined the solubility of CO2 in the DESs of ChCl paired with urea, glycerol, or ethylene glycerol. Francisco et al.19 reported the CO2 capture by natural DES of 1:2 mol (ChCl + lactic acid). Ali et al.29 modeled experimental solubility of CO2 at 298.15 K and 10 bar in serials of phosphonium- and ammonium-based DESs using the cubic Peng−Robinson equation. Ullah et al.30 systematically studied the CO2 capturing by DES of ChCl−levilunic aicd using experimental and molecular simulation approaches. Su et al.31 and Lin et al.32 presented CO2 solubility in aqueous DES and also investigated the influence of water. In order to improve the dissolution capacity of CO2, the task-specific DESs33 and DES blended with monoethanolamine34 were further developed. Our previous works reported the solubility of CO2 measured in the DESs of ChCl paired with the HBDs including phenol, furfuryl alcohol, dihydroic alcohols, and levulinic acid.35−37 As a continuation of our study, guaiacol was selected as HBD to form DES as a new CO2 absorbent on the basis of the following considerations. The benzene ring and ether group in guaiacol are helpful for binding CO2 according to literature.38,39 Guaiacol can be derived or extracted from biomass,40,41 which is consistent with the ideas of “green chemistry”.42,43 In the present work, we hoped to measure the solubility data of

INTRODUCTION The global warming problem has attracted public concern worldwide. CO2 is regarded as a chief contributor to cause global warming.1 Hence, carbon capture and storage (CCS) has become the research focus.2,3 Although the absorption of CO2 by traditional amine-based absorbents is an industrially used technology to remove CO2 from a gas stream, they are not environmentally friendly owing to solvent loss, toxicity, partial degradation, and high energy demand during the regeneration process. Therefore, the research for promising and economical alternatives to amine-based absorbents is still desirable. Ionic liquids (ILs) have attracted extensive attention as gas absorbents because they are highly thermal and chemically stable, nonvolatile, and tunable liquids.4−6 In the literature,7−13 ILs are largely reported as potential alternatives to conventional CO2 absorbents. However, the relatively high price and complicated preparative technology decrease the attraction of ILs when compared with traditional solvents. Hopefully, deep eutectic solvents (DESs), known as new ILs analogues, emerge and seem to be good candicates.14,15 DES has both solvent properties simlar to those of ILs and the merits of low cost, good renewability, and low toxicity. DES could be conveniently prepared by simply mixing a hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) with a suitable composition. The formed eutectic mixture through hydrogen-bond networks has a lower melting point than HBD and HBA. Generally, the inexpensive quaternary ammonium or phosphium salts have been popularly selected as HBA, with choline chloride (ChCl) as the most widely used material. The scope of HBD includes amides, carboxylic acids, and alcohols.16−19 Furthermore, DES can be manufactured on a large scale from cheap, available, and high-purity raw materials. DESs were extensively reported as solvents in chemical reaction20,21 and electrodeposition,22 and © XXXX American Chemical Society

Received: December 7, 2016 Accepted: March 23, 2017

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DOI: 10.1021/acs.jced.6b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 1. Specifications of the Chemicals Used in This Study

a

chemicals

CAS No.

source

mass fraction purity

choline chloride (ChCl) diethylamine hydrochloride (DH) acetylcholine chloride (ACC) guaiacol (GC) carbon dioxide (CO2)

67-48-1 660-68-4 60-31-1 90-05-1 124-38-9

Jinan Hualing Pharmaceutical Co., Ltd. Aladdin Industrial Corp. Co., Ltd. Aladdin Industrial Corp. Co., Ltd. Aladdin Industrial Corp. Co., Ltd. Jingong Special Gas Co., Ltd.

>0.985 ≥0.99 ≥0.99 ≥0.99 >0.999a

Volume fraction purity as stated by the supplier.

mass conservation. The detailed operation steps were also illustrated as previously noted.35−37 In a typical operation, the temperature of GR was always maintained at 303.15 K. After loading a certain amount of DES (approximately 60−80 g, the exact mass, w, was determined by electronic balance) into the EC, the whole system was evacuated to pressure p0. Then, opening the valve between GR and CO2 cylinder and closing the valve linked GR and EC, CO2 was loaded into GR from the cylinder until pressure p1. A thermostatic water bath was used to warm EC to the experimental temperature of T. With the valve between GR and EC opened, CO2 was slowly charged into EC to be absorbed by the DES under magnetic stirring. The absorption was regarded to reach equilibrium if the pressure of EC did not change within 2 h. The stable pressures were written down as p2 for GR and p3 for EC. The equilibrium partial pressure of CO2 in the EC was denoted as p3 − p0. The amount of CO2 loaded into the EC was the difference between the masses of CO2 in the GR before and after transfer operation. The mass of dissolved CO2 was derived from the mass of CO2 transferred into the cell by subtracting the mass of gaseous CO2 at absorption equilibrium. The next experimental points were obtained by transferring a greater amount of CO2 into EC from GR with the same procedure until the pressure was close to 600.0 kPa.

CO2 in three kinds of guaiacol-based DESs prepared from guaiacol and quaternary ammonium salts (QASs, including choline chloride, acetylcholine chloride, and diethylamine hydrochloride) with molar ratios of guaiacol to QAS of 3:1, 4:1, and 5:1at the temperature scope of 293.15−323.15 K and pressures below 600.0 kPa. Henry’s law was applied to correlate the experimental data, and the standard Gibbs free energy, standard enthalpy, and standard entropy of the CO2−DES systems were further derived according to the relationship between temperatures and Henry’s law constants. Finally, the absorption capacities in guaiacol-based DESs and other absorbents were compared.

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EXPERIMENTAL SECTION Materials. Table 1 summarized the reagents used with their purities and sources. All the reagents were directly used without further purification. CO2 gas (>0.999, volume fraction purity as stated by the supplier) was purchased from Jingong Special Gas Co., Ltd. (Hangzhou, China). Preparation of Guaiacol-Based DESs. The DESs were simply prepared by mixing GC with ChCl, ACC, or DH at suitable temperature and atmospheric pressure until a homogeneous phase was formed. Then, the obtained DESs were further dried at 353 K under vacuum for 48 h before use. The concentration of water in each DES was determined using Karl Fischer analysis (SF-3 Karl Fischer titration, Zibo Zifen Instrument Co. Ltd.), with the result of less than 1.2 × 10−3 (mass fraction). Densities of the DESs were systematically measured at the temperatures corresponding to solubility experiments and atmospheric pressure using a 5.567 ± 0.004 cm3 pycnometer. The volume of the pycnometer at each experimental temperature was calibrated using double-distilled water. An electronic balance (Mettler-Toledo AL204 with the standard uncertainty of 2 × 10−4 g) was used to weight the mass of the DESs. Apparatus and Procedure. A stainless apparatus for solubility determination was illustrated in our previous works.35−37 It was mainly composed of a gas equilibrium cell (EC, 141.61 cm3) and gas reservoir (GR, 370.99 cm3). Two calibrated platinum resistance thermometers inserted into the cells with standard uncertainty of 0.10 K were used to measure the temperatures, and two pressure transmitters (Fujian WIDEPLUS Precision Instruments Co., Ltd., WIDEPLUS-8, 0−600.0 kPa) with the standard uncertainty of 0.6 kPa were applied to record experimental pressures. The solubilities of CO2 in DESs were measured using an isochoric saturation method,44 which was similar to the constant-volume method.45 The method was based on the following principle. A measured quantity of a gas was brought into contact with a weighted quantity of gas-free absorbent while stirring; the amount of the remaining gas was measured after the gas−liquid equilibrium was established. The gas solubility in the absorbent was then calculated according to

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RESULTS AND DISCUSSION Table 2 lists the density data of three kinds of guaiacol-based DESs at four temperatures and 101.3 kPa. The DES of GC-DH has the lowest density, while the other two kinds of DESs have similar densities. The density increased evidently with the increasing mole percentage of GC in the DES. In the isochoric saturation method, the shifted amount of CO2 from GR to EC was divided into gaseous and dissolved Table 2. Densities (ρ) of DESs at Different Temperatures and 101.3 kPaa ρ/(g·cm−3) DES

293.15 K

303.15 K

313.15 K

323.15 K

nChCl:nGC = 1:3 nChCl:nGC = 1:4 nChCl:nGC = 1:5 nDH:nGC = 1:3 nDH:nGC = 1:4 nDH:nGC = 1:5 nACC:nGC = 1:3 nACC:nGC = 1:4 nACC:nGC = 1:5

1.1533 1.1556 1.1497 1.0954 1.1024 1.1061 1.1496 1.1493 1.1527

1.1475 1.1477 1.1434 1.0931 1.0983 1.1035 1.1461 1.1448 1.1481

1.1412 1.1406 1.1362 1.0837 1.0898 1.0937 1.1398 1.1381 1.1425

1.1341 1.1324 1.1280 1.0758 1.0841 1.0835 1.1338 1.1317 1.1351

a The standard uncertainties u are u(T) = 0.10 K and u(p) = 0.60 kPa; the combined expanded uncertainty Uc is Uc(ρ) = 0.002 g·cm−3 (0.95 level of confidence), and the standard uncertainty of mole ratio of QASs to GC is 0.001.

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DOI: 10.1021/acs.jced.6b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. Experimental CO2 Mole Fraction (x1) and Molality (m1) in DESs at Temperature (T) and Equilibrium Pressure (p)a 293.15 K

303.15 K

p/kPa

x1

m1/(mol·kg−1)

p/kPa

x1

51.5 138.1 236.7 329.2 427.5 529.4

0.0021 0.0055 0.0094 0.0131 0.0169 0.0215

0.0161 0.0432 0.0742 0.1039 0.1346 0.1714

55.3 132.3 243.8 335.5 433.8 533.2

0.0020 0.0045 0.0082 0.0113 0.0146 0.0178

52.1 143.2 231.9 326.5 430.0 526.0

0.0022 0.0060 0.0097 0.0136 0.0180 0.0221

0.0173 0.0469 0.0764 0.1082 0.1431 0.1765

100.3 151.3 264.2 341.7 437.4 545.9

0.0035 0.0054 0.0094 0.0121 0.0156 0.0194

52.8 139.8 246.1 331.1 419.5 528.4

0.0024 0.0059 0.0108 0.0145 0.0185 0.0233

0.0188 0.0469 0.0862 0.1165 0.1488 0.1885

46.9 145.2 239.4 330.4 431.8 538.9

0.0017 0.0052 0.0088 0.0122 0.0161 0.0200

45.1 137.1 222.8 320.6 414.9 517.7

0.0024 0.0065 0.0106 0.0154 0.0200 0.0249

0.0197 0.0545 0.0892 0.1299 0.1693 0.2120

53.9 133.6 241.5 339.1 428.8 513.5

0.0024 0.0060 0.0103 0.0143 0.0180 0.0216

49.6 134.6 224.4 323.1 411.3 516.1

0.0025 0.0067 0.0111 0.0159 0.0205 0.0259

0.0209 0.0558 0.0936 0.1344 0.1736 0.2212

61.7 142.6 227.5 345.8 425.4 524.8

0.0028 0.0063 0.0100 0.0152 0.0186 0.0229

48.8 131.9 245.3 326.5 419.2 522.3

0.0026 0.0071 0.0129 0.0172 0.0222 0.0275

0.0218 0.0584 0.1077 0.1440 0.1862 0.2321

57.0 148.3 244.9 341.2 424.4 520.1

0.0027 0.0067 0.0112 0.0156 0.0194 0.0237

53.7 144.3 234.1 331.7 428.8 528.6

0.0026 0.0066 0.0111 0.0160 0.0207 0.0256

0.0183 0.0476 0.0803 0.1158 0.1510 0.1873

53.8 139.6 227.1 329.5 431.7 531.2

0.0023 0.0057 0.0093 0.0134 0.0175 0.0214

55.1 138.4 227.6 326.4 422.1 524.9

0.0026 0.0068 0.0111 0.0158 0.0205 0.0252

0.0194 0.0507 0.0828 0.1182 0.1544 0.1903

54.8 140.6 263.7 338.6 431.8 558.6

0.0022 0.0056 0.0108 0.0139 0.0177 0.0228

49.3 137.3

0.0023 0.0065

0.0171 0.0491

52.0 138.8

0.0023 0.0059

313.15 K

m1/(mol·kg−1)

p/kPa

nChCl:nGC = 1:3 0.0153 57.5 0.0351 145.7 0.0644 233.2 0.0892 330.3 0.1157 429.6 0.1419 529.4 nChCl:nGC = 1:4 0.0275 53.4 0.0422 142.5 0.0746 235.9 0.0956 335.5 0.1241 431.8 0.1549 542.9 nChCl:nGC = 1:5 0.0137 55.3 0.0412 140.7 0.0704 235.9 0.0979 331.8 0.1289 434.8 0.1610 536.4 nDH:nGC = 1:3 0.0199 59.2 0.0499 182.9 0.0862 271.2 0.1203 357.5 0.1525 436.4 0.1834 524.6 nDH:nGC = 1:4 0.0233 54.2 0.0525 141.1 0.0837 242.2 0.1279 346.8 0.1577 431.8 0.1947 529.2 nDH:nGC = 1:5 0.0220 58.8 0.0557 156 0.0930 273.6 0.1300 336.3 0.1629 428.6 0.1995 550.7 nACC:nGC = 1:3 0.0164 54.1 0.0407 147.8 0.0667 235 0.0969 338.0 0.1267 434.5 0.1559 535.3 nACC:nGC = 1:4 0.0163 55.5 0.0419 140.8 0.0804 235.8 0.1041 330.2 0.1327 438.8 0.1722 531.5 nACC:nGC = 1:5 0.0171 49.9 0.0445 145.0

C

323.15 K

x1

m1/(mol·kg−1)

p/kPa

x1

m1/(mol·kg−1)

0.0017 0.0041 0.0065 0.0093 0.0122 0.0150

0.0133 0.0320 0.0512 0.0735 0.0962 0.1187

63.1 160.6 239.9 341.4 442.6 536.5

0.0014 0.0039 0.0057 0.0083 0.0107 0.0128

0.0110 0.0304 0.0447 0.0653 0.0847 0.1013

0.0014 0.0044 0.0072 0.0103 0.0132 0.0165

0.0112 0.0347 0.0566 0.0817 0.1049 0.1311

54.7 143.6 238.5 336.7 435.2 535.3

0.0014 0.0038 0.0063 0.0090 0.0116 0.0141

0.0109 0.0300 0.0493 0.0713 0.0921 0.1121

0.0019 0.0045 0.0076 0.0107 0.0142 0.0175

0.0150 0.0361 0.0608 0.0854 0.1139 0.1405

48.5 146.3 237.5 334.9 432.0 529.2

0.0015 0.0040 0.0065 0.0094 0.0120 0.0148

0.0115 0.0314 0.0519 0.0750 0.0962 0.1182

0.0024 0.0068 0.0100 0.0131 0.0159 0.0189

0.0201 0.0565 0.0840 0.1103 0.1344 0.1602

56.6 143.4 246.0 331.9 428.9 525.4

0.0019 0.0047 0.0080 0.0106 0.0137 0.0168

0.0160 0.0388 0.0665 0.0893 0.1154 0.1421

0.0020 0.0053 0.0090 0.0130 0.0162 0.0199

0.0163 0.0442 0.0759 0.1099 0.1371 0.1691

58.9 148.4 230.1 337.7 430.4 526.8

0.0019 0.0050 0.0076 0.0113 0.0144 0.0175

0.0156 0.0415 0.0641 0.0952 0.1217 0.1483

0.0024 0.0062 0.0109 0.0133 0.0170 0.0219

0.0196 0.0514 0.0904 0.1109 0.1425 0.1842

59.9 193.1 253.3 333.8 431.5 535.8

0.0020 0.0068 0.0088 0.0117 0.0152 0.0190

0.0166 0.0563 0.0732 0.0975 0.1270 0.1592

0.0019 0.0051 0.0081 0.0117 0.0150 0.0185

0.0134 0.0363 0.0586 0.0844 0.1089 0.1347

55.9 143.5 239.4 337.9 438.0 533.0

0.0018 0.0046 0.0075 0.0105 0.0135 0.0163

0.0132 0.0329 0.0536 0.0756 0.0978 0.1179

0.0022 0.0053 0.0088 0.0122 0.0161 0.0195

0.0163 0.0393 0.0651 0.0914 0.1207 0.1470

53.0 165.3 241.6 339.3 437.4 536.0

0.0017 0.0053 0.0078 0.0109 0.0142 0.0173

0.0125 0.0392 0.0581 0.0815 0.1058 0.1297

0.0017 0.0054

0.0131 0.0406

48.6 144.3

0.0016 0.0048

0.0122 0.0360

DOI: 10.1021/acs.jced.6b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 3. continued 293.15 K

303.15 K

p/kPa

x1

m1/(mol·kg−1)

p/kPa

x1

230.0 325.5 421.7 522.2

0.0112 0.0160 0.0208 0.0255

0.0845 0.1218 0.1590 0.1960

222.9 333.2 428.0 528.2

0.0095 0.0143 0.0184 0.0226

313.15 K

m1/(mol·kg−1) nACC:nGC 0.0716 0.1083 0.1401 0.1730

323.15 K

p/kPa

x1

m1/(mol·kg−1)

p/kPa

x1

m1/(mol·kg−1)

= 1:5 237.0 335.1 432.8 526.5

0.0088 0.0124 0.0161 0.0195

0.0664 0.0940 0.1224 0.1489

238.7 341.3 436.2 535.8

0.0078 0.0112 0.0146 0.0179

0.0587 0.0845 0.1108 0.1363

a Standard uncertainties u are u(T) = 0.10 K and u(p) = 0.6 kPa; relative standard uncertainties ur are ur(x) = 0.03 and ur(m) = 0.03. The standard uncertainty of mole ratio of QASs to GC is 0.001.

parts. Therefore, the CO2 dissolved in the DES can be calculated by following equation, nCO2 = ρg (p1 , Ti )VGR − ρg (p2 , Ti )VGR − ρg (ps , Ti )(VEC − w/ρDES )

(1)

where ρg(pi,Ti) denotes the density of CO2 in mol·cm−3 under pressure pi (i = 1, 2, s) and temperature Ti (including the temperature of GR and equilibrium temperature T), and is found out from the database.46 VGR and VEC represent the volumes of GR and EC in cm−3, respectively. ρDES refers to the density of DES at various temperatures in g·cm−3; the density change of the DES owing to the CO2 solubility is negligible and the determined ρDES is used to denote the CO2-saturated DES.47 The solubilities of CO2, expressed as molar fraction (xCO2) and molality (mCO2) of CO2 in the DESs, were calculated according to following equations, xco2 = nco2 /(nco2 + nDES)

(2)

mco2 = nco2 /w

(3)

Figure 2. Solubilities of CO2 (molar fraction) in DES of nDH:nGC = 1:5: ◊, 303.15 K; Δ, 313.15 K; ○, 323.15 K; ▽, 333.15 K; , linear fit.

where nCO2 is the mole number of CO2 absorbed in the DES and nDES and w are mole number and mass of DES, respectively. The solubility of CO2 in the DESs was determined at T = 293.15, 303.15, 313.15, and323.15 K and pressures below 600.0 kPa. The molar ratios of ChCl, DH, or ACC to GC were fixed at 1:3, 1:4, and 1:5. Table 3 included the experimental data such as gas phase equilibrium pressure (p), liquid phase molality (m2), and molar fraction (x2) for CO2. Figures 1 and 2 described the typical influence of pressure on the solubility (m2 and x2) at different temperatures with the DES of nDH:nGC = 1:5 as a representative. Figure 3 showed the isothermal CO2 solubility at 303.15 K in all the DESs. Because

Figure 3. Solubility of CO2 (molality) in various guaiacol-based DESs at 303.15 K: Δ, nChCl:nGC = 1:3; ○, nACC:nGC = 1:3; ☆, nACC:nGC = 1:5; □, nDH:nGC = 1:3; ▽, nDH:nGC = 1:5.

Figure 1. Solubilities of CO2 (molality) in DES of nDH:nGC = 1:5: ◊, 303.15 K; Δ, 313.15 K; ○, 323.15 K; ▽, 333.15 K; , linear fit.

of the overlap, the profiles of the DESs with the molar ratio of 1:4 and nChCl:nGC = 1:5 were omitted. As shown in Figures 1−3, the solubility of CO2 in DESs demonstrated linear increasing with the increasing pressure at each temperature, which means that the dissolution behavior obey Henry’s law within the range of experimental pressure. Moreover, it was evident that the solubility profiles for CO2 in the DESs basically pass through the origin of the coordinate axis, which indicates that dissolving of CO2 in the DESs is a typical physical process at the investigated pressure.48,49 Henry’s Law Constant. As discussed above, Henry’s law can conveniently model the physical dissolving of CO2 in the DESs, leading to a quantitative parameter of Henry’s law constant. Henry’s law constant based on molar fraction (Hx) can be expressed as follows,50 D

DOI: 10.1021/acs.jced.6b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 4. Henry’s law constants (Hm, Based on Molality and Hx, Based on Molar Fraction) of CO2 in DESs at Different Temperatures Hm/(MPa·kg·mol−1)

Hx/MPa

DES

293.15 K

303.15 K

313.15 K

323.15 K

293.15 K

303.15 K

313.15 K

323.15 K

nChCl:nGC = 1:3 nChCl:nGC = 1:4 nChCl:nGC = 1:5 nDH:nGC = 1:3 nDH:nGC = 1:4 nDH:nGC = 1:5 nACC:nGC = 1:3 nACC:nGC = 1:4 nACC:nGC = 1:5

3.14 3.00 2.82 2.45 2.36 2.26 2.84 2.75 2.67

3.76 3.54 3.36 2.80 2.70 2.61 3.40 3.25 3.06

4.48 4.13 3.84 3.25 3.15 3.01 3.99 3.62 3.54

5.27 4.76 4.49 3.71 3.55 3.40 4.49 4.14 3.96

24.94 23.90 22.71 20.79 20.07 18.69 20.75 20.69 20.43

29.79 28.11 26.97 23.68 22.88 21.90 24.72 24.44 23.35

35.40 32.75 30.71 27.41 26.61 25.15 28.89 27.12 26.96

41.60 37.66 35.88 31.18 29.97 28.34 32.44 31.04 30.09

f 2liq (p , T , x 2)

Hx(p , T ) = lim

x2 → 0

x2

Henry’s law constant Hm is a universal scale to evaluate the dissolution capacity of CO2 in different absorbents. Thus, the values of Hm at 313.15 K in the present three kinds of guaiacolbased DESs and some ILs as well as other DESs from literature were systematically compared. As shown in Table 5, the studied

(4)

where f liq 2 (p,T,x2), p, T, and x2 represent fugacity, pressure, temperature, and molar fraction of CO2 in the liquid phase at gas−liquid equilibrium, respectively. According to the gas−liquid equilibrium criterion, CO2 must possess the same fugacity in the gas and liquid phases at equilibrium state. Thus, f 2liq (p , T , x 2) = f 2vap (p , T , y2 ) = y2 pϕ2(p , T , y2 )

Table 5. Comparison of Hm in present DESs with some ILs and other DESs at 313.15 K

(5)

where f vap 2 (p,T,y2) and y2 are the gas phase fugacity and molar fraction of CO2, respectively. ϕ2 is the fugacity coefficient of CO2 at equilibrium condition. As suggested in the literature,51 the vapor pressures of DESs at experimental temperatures are negligible as compared to the equilibrium partial pressure of CO2. Therefore, the gas phase above the DES in the EC was regarded to be pure CO2 and y2 was simplified to be unity. The two-term virial equation is applied to calculate the ϕ2 because of the relatively low pressure. Then, at infinite dilution region of CO2 in liquid phase, Hx can be expressed as the following, Hx(p , T ) = lim

f 2liq (p , T , x 2)

x2 → 0

x2

= lim

x2 → 0

pϕ2(p , T ) x2

≅

pϕ2(p , T ) x2 (6)

Similarly, Henry’s law constant on the basis of molality (Hm) can be derived as Hm(p , T ) ≡ lim

m2 → 0

f 2liq (p , T , m2) m2

≅

pϕ2(p , T ) m2

(7)

where m2 is the liquid phase molality of CO2. The values of Henry’s law constant were obtained as the slope by linear correlation of fugacity versus molar fraction or molality of CO2 for each isotherm. The results were listed in Table 4, with Hx and Hm laying in the ranges of 18.69−41.60 MPa and 2.26− 5.27 MPa·kg mol−1, respectively. It is clear that GC-DH systems demonstrate the best absorption capacity among three kinds of DESs, while GC-ChCl presents slightly lower absorption capacity than GC-ACC at the same molar ratio and temperature. It also implied that the ester group in ACC was more helpful for dissolving CO2 than the hydroxyl group in ChCl, which agreed well with the conclusion drawn by Li et al.52 Furthermore, the solubility of CO2 in the same kind of DES increased with the increasing mole percentage of guaiacol, meaning that guaiacol plays a main role in the dissolution of CO2 into DES.

absorbents

Hm/(MPa·kg·mol−1)

nChCl:nGC = 1:3 nChCl:nGC = 1:4 nChCl:nGC = 1:5 nDH:nGC = 1:3 nDH:nGC = 1:4 nDH:nGC = 1:5 nACC:nGC = 1:3 nACC:nGC = 1:4 nACC:nGC = 1:5 [hhemel]53 [hhemea]53 [dmim][Me2PO4]55 [hmim][BF4]54 [bmim][Tf2N]56 nChCl:nurea = 1:2.525 nChCl:nurea = 1:226 nChCl:nglycerol = 1:227 nChCl:nethylene glycol = 1:228 nlevulinic acid:ncholine chloride = 3:137 nfurfuryl alcohol:ncholine chloride = 3:137 1:2nChCl:nlactic acid = 19 nChCl: n1,4‑butanediol = 1:435

4.48 4.13 3.84 3.25 3.15 3.01 3.99 3.62 3.51 5.38 4.65 2.35 1.81 1.06 1.37 1.29 1.70 2.71 2.62 3.54 4.00 4.18

DESs demonstrated better absorption capacities of CO2 than the ammonium-based ILs such as 2-hydroxy-N-(2-hydroxyethyl)-N-methylethanaminium acetate ([hhemea]) and 2-hydroxyN-(2-hydroxyethyl)-N-methylethanaminium lactate ([hhemel]),53 while evidently poorer than the imidazolium-based ILs such as 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]),54 1, 3-dimethylimidazolium dimethylphosphate ([dmim][Me2PO4]),55 and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]).56 To our knowledge, DESs have a greater advantage than imidazoliumand phosphonium-based ILs from the comparisons of biodegradability, toxicity, and price. Moreover, present DESs possess a lower absorption capacity than those composed of ChCl with urea,25,26 glycerols,27 levulinic acid,37 furfuryl alcohol37 and an almost similar performance with ChCl + E

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Table 6. Standard Gibbs Free Energy (ΔdisG0), Enthalpy (ΔdisH0), and Entropy (ΔdisS0) of Dissolution of CO2 in DESs at 0.1 MPa and 313.15 K DESs

ΔdisG0/(kJ·mol−1)

ΔdisH0/(kJ·mol−1)

ΔdisS0/(J·mol−1·K−1

nChCl:nGC = 1:3 nChCl:nGC = 1:4 nChCl:nGC = 1:5 nDH:nGC = 1:3 nDH:nGC = 1:4 nDH:nGC = 1:5 nACC:nGC = 1:3 nACC:nGC = 1:4 nACC:nGC = 1:5

15.28 15.08 14.91 14.61 14.54 14.39 15.05 14.93 14.86

−13.44 −11.95 −11.83 −10.72 −10.66 −10.95 −11.81 −10.37 −10.28

−91.71 −86.32 −85.40 −81.91 −80.47 −79.78 −85.78 −80.80 −80.27

lactic acid19 and ChCl + 1,4-butanediol35 mixtures. Besides the absorption capacity of CO2, the developing of DES-based capturing technology needs to screen the kinds of HBA and HBD, the most suitable HBA−HBD combination, contributing to the best physicochemical property (e.g., low viscosity, weak corrosivity, and good thermal and chemical stability).57 Thermodynamic Properties. Thermodynamic properties can supply abundant information on understanding the dissolution of a gas into a liquid. They are also helpful for designing the industrial equipment for gas absorption operation. Three thermodynamic properties of dissolution Gibbs free energy (ΔdisG0), dissolution enthalpy (ΔdisH0), and dissolution entropy (ΔdisS0) at standard condition are derived by fitting Henry’s law constant with temperature by the following equations, ∂ ln(H(T , p)/p0 ) Δdis H 0 = R ∂(1/T )

(8)

Δdis G 0 = RT ln[H(T , p)/p0 ]

(9)

Δdis S 0 =

Δdis H 0 − Δdis G 0 T

because of the low value of dissolution enthalpy. The dissolution of CO2 into DESs is nonspontaneous.

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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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REFERENCES

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where p0 refers to the standard pressure of 0.1 MPa. ΔdisH0 reflects molecular interaction between the dissolved CO2 and DESs, and ΔdisS0 shows the change of the order degree in liquid phase owing to the dissolution of CO2. The calculated values of ΔdisG0, ΔdisH0, and ΔdisS0 at 313.15 K and 0.1 MPa are included in Table 6. For CO2 dissolution in the present DESs under each condition, the negative ΔdisH0 shows that the dissolving of CO2 in the DES is an exothermic process. However, the relatively small absolute values of ΔdisH0 mean the easy regeneration of CO2-rich DESs. The negative values of ΔdisS0 indicate that the solution becomes a higher ordering degree when dissolved into the DES from a molecular level. Finally, the positive values of ΔdisG0 mean that the dissolution of CO2 into DESs is a nonspontaneous process.

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CONCLUSIONS New solubility data of CO2 in three kinds of guaiacol-based DESs were determined at temperatures ranging from 293.15 to 323.15 K under pressures below 600.0 kPa. Henry’s law constants and thermodynamic properties of CO2 in the DESs were obtained. The results show that the dissolution of CO2 in the DESs follows a physical mode and the DES of diethylamine hydrochloride and guaiacol (1:5) has the highest absorption capacity for CO2. The process of desorption was advantageous F

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H

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