Solubilities and Thermodynamic Properties of Carbon Dioxide in

Article pubs.acs.org/jced

Solubilities and Thermodynamic Properties of Carbon Dioxide in Some Biobased Solvents Xiaofeng Li,*,† Xiaobang Liu,‡ Yaotai Jiang,‡ and Dongshun Deng*,‡ †

Department of Chemistry, Zhejiang University, Hangzhou 310027, China College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China

‡

S Supporting Information *

ABSTRACT: Using the isochoric saturation method, solubilities of CO2 in five biobased solvents (BBSs) have been determined at temperatures between 293.15 and 323.15 K with 10 K intervals and pressure scope of (0−600.0) kPa. The BBSs were selected from glyceryl triacetate (GT), glycerol formal (GF), DL-1,2-isopropyli-deneglycerol (GAK), triethyl citrate (TEC), and acetyl triethyl citrate (ATEC). Henry’s constants and thermodynamic properties such as Gibbs free energy, enthalpy, and entropy of dissolution were derived from the solubility data. The gravimetric solubilities of CO2 in BBSs changed with the sequence of GT > ATEC > TEC ≈ GAK > GF. All the dissolution enthalpies were negative at each condition. The dissolution capacities of BBSs for CO2 were further compared with those of several common absorbents as well as ionic liquids. It was shown that they were similar to polyethylene glycol dimethyl ether (NHD) which is an industrial CO2 absorbent. dimethylformamide (DMF),13 methanol,14 surfactants,15 poly(ethylene glycol) (PEG),16 and polyethylene glycol dimethyl ether (NHD).17 In practical application, high boiling point absorbents are preferred because of less absorbent loss during regeneration, energy-savings, and consumption reduction. The structure−property relationship also dominates the solubility of CO2 in various organic absorbents.18 It is reported that the absorbents rich in carbonyl, ester, and ether groups usually demonstrate good dissolution capacity for CO2.17,19,20 Recently, “green chemistry” was developed rapidly, with emphasis on replacing harmful solvents by green ones in the chemical industry.21,22 On the basis of the above-mentioned ideas, five biobased solvents (BBSs), glyceryl triacetate (GT), glycerol formal (GF), DL-1,2-isopropyli-deneglycerol (GAK), triethyl citrate (TEC), and acetyl triethyl citrate (ATEC), were selected in the present work as CO2 absorbents, and the solubility data were determined. The selected five BBSs have low volatility and high boiling point, chemical and thermal stability, low toxicity, and noncorrosivity. These properties make them superior in regeneration of absorbent, safety in production, and equipment selection. Moreover, the abundant CO2-philic groups in the molecules of these BBSs are helpful for CO2 dissolution. In this study, new experimental solubility data of CO2 in the selected five BBSs were reported at the temperature range of (293.15 to 323.15) K with 10 K intervals under a pressure of (0 to 600.0) kPa. Henry’s constants were obtained by linear correlation of the pressure dependence of solubility. The thermodynamic parameters such

1. INTRODUCTION Carbon dioxide (CO2) is the most important anthropogenic greenhouse gase. Global increases in CO2 concentrations are due primarily to massive burning of fossil fuel. The changes in CO2 concentration and aerosols, land cover, and solar radiation alter the energy balance of the climate system, thus resulting in serial environmental and ecological problems.1 In the past decades, how to reduce the emission of CO2 at its source has become a focus topic of global interest and attention. Capture and sequestration (CCS) of CO2 is truly a great challenge for scientists and governments all over the world.2 At present in industry, a chemical absorption method based on an aqueous alkanolamine solution is widely used to capture CO2 because of its rapid and effective performance.3 But meanwhile, this method also encounters the troubles of secondary pollution, equipment corrosion, energy consumption, and degradation of alkanolamine.4 These limitations need more attention for improvement. In the past 20 years, ionic liquid (ILs) have emerged because of their unique and interesting properties such as negligible vapor pressure, high thermal stability, and structure diversity.5 They were also introduced into the gas separation field as a class of green and potential medium.6 Up to date, many ILs were reported as new CO2 absorbents in the literature.7−10 However, high cost and viscosity and relatively low gravimetric absorption ability are still obstacles to their industrial application.11 Conversely, a physical absorption method using organic solvents with relatively low molecular weights and high boiling points has low energy consumption, low volatility, and high gravimetric absorption capacity.12 Actually, many absorbents have been reported in the literature, including propylene carbonate (PC),12 N-methylpyrrolidinone (NMP),12 dimethyl sulfoxide (DMSO),13 © 2016 American Chemical Society

Received: May 16, 2016 Accepted: July 29, 2016 Published: August 11, 2016 3355

DOI: 10.1021/acs.jced.6b00399 J. Chem. Eng. Data 2016, 61, 3355−3362

Journal of Chemical & Engineering Data

Article

The measured densities were tabulated and graphically compared with those taken from the literature in the Supporting Information. The density relative deviations (defined as the deviation between experiment and literature density divided by experimental value) are within 1.0%, meaning that the determined density data are consistent with those from the literature. Viscosity of the BBSs was obtained at 298.15 K and 101.3 kPa by the Pinkevitch method with the relative standard uncertainty of 2%. High purity ethylene glycol was used to calibrate the viscometer. The determined viscosity of 16.36 mPa s agreed well with 16.31 mPa s in the literature23 for GT at 298.15 K. 2.2. Experimental Apparatus. The solubilities of CO2 in the BBSs were measured using the isochoric saturation method. Detailed description of the apparatus is available in our previous works.24 It was composed mainly of gas equilibrium cell (EC) and a gas reservior (GR), with the volumes of 141.61 cm3 and 370.99 cm3, respectively. The temperature of solutions in the EC and GR was maintained at a certain value using a thermostatic water bath with the standard uncertainty of 0.05 K. Two pressure transmitters (Fujian WIDEPLUS Precision Instruments Co., Ltd., WIDEPLUS-8) with the standard

as dissolution Gibbs free energy, enthalpy, and entropy were further deduced. Finally, the comparisons of absorption ability in present BBSs with those in several other common solvents and some ILs were systematically carried out.

2. EXPERIMENTAL SECTION 2.1. Materials. CO2 was supplied by Jingong Special Gas Co., Ltd. Acetyl triethyl citrate (ATEC), glyceryl triacetate (GT), glycerol formal (GF), triethyl citrate (TEC), DL-1,2-isopropylideneglycerol (GAK) were purchased from Aladdin Industrial Corporation. Glycerol formal is available as a mixture of 5-hydroxy1,3-dioxane and 4-hydroxymethyl-1, 3-dioxolane (66:34, molar ratio, determined by 1H NMR). Table 1 lists information in detail on the chemicals used. Some thermophysical properties of the BBSs from literature were included in Table 2. An electronic balance (Mettler-Toledo AL204) with the standard uncertainty of 0.0002 g was used to weight the mass of solvents during experiment. Density of the BBSs were determined at T = (293.15, 303.15, 313.15, and 323.15) K under 101.3 kPa using the pycnometer method with the standard uncertainty of 0.001 g.cm−3. Double distilled water was used as a calibrating substance. Table 1. Description of Chemicals Used in Present Work chemicals

abbreviation

CAS No.

source

purifica-tion method

mass fraction purity

analysis method

carbon dioxide glyceryl triacetate glycerol formal DL-1,2-isopropyli-deneglycerol triethyl citrate acetyl triethyl citrate

CO2 GT GF GAK TEC ATEC

124-38-9 102-76-1 5464-28-8 4740-78-7 100-79-8 77-93-0 77-89-4

Jingong Special Gas Co., Ltd. Aladdin Industrial Co., Ltd. Aladdin Industrial Co., Ltd. Aladdin Industrial Co., Ltd. Aladdin Industrial Co., Ltd. Aladdin Industrial Co., Ltd.

none none none none none

0.999 0.985 0.980 0.980 0.980 0.970

GCa GCa GCa GCa GCa

a

Gas−liquid chromatography.

Table 2. Thermophysical Properties of the Selected Five BBSsa

a The data were obtained from SciFinder. bAt 6 Pa/0.045Torr. cAt 293.15 K. dDetermined values at 298.15 K and 101.3 kPa, Standard uncertainties u for temperature and pressure are u(T) = 0.05 K and u(p) = 0.6 kPa. Relative standard uncertainties of viscosity is ur (η) = 0.02; eSaturated vapor pressure at 298.15 K; fThe data were taken from Chemical Toxicity Database, oral for rate. gPredictive value.

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By combining eqs 2 and 3, the quality of CO2 dissolved in the solvents is then given as follows:

uncertainty of 0.6 kPa were used to record the experimental pressures. 2.3. Experimental Procedure. At beginning of the measurement, a certain amount of BBSs (w) was introduced into the EC and the whole system was evacuated to pressure p1. Then, GR was charged with CO2 from the cylinder to a pressure p2. With the needle valve between the GR and EC opened, CO2 was brought into the EC from the GR to be absorbed by solvents while under magnetic stirring. Gas liquid equilibrium is assumed to be reached if the pressure of EC remains stable at least 2 h. The final pressure was recorded as p3 for GR and p4 for EC. Then, the equilibrium partial pressure of CO2 in the EC was denoted as follows,

pe = p4 − p1

nCO2 = [(ρg (p2 , T ) − ρg (p3 , T ))VGR − (VEC − Vsolvent)] /(1 − ρg (pe , T )V 2∞)

Furthermore, the solubility data were calculated and compared using the above-mentioned simplified and precise method, with the results demonstrating that the difference was within 1%. In the present work, we trended to calculate the solubility with the precise approach. The liquid phase molality (mCO2) and mole fraction (xCO2) of CO2 were then calculated according to two formulas as below,

(1)

The amount of absorbed CO2 (nCO2) was calculated by the following equation, nCO2 = ρg (p2 , T )VGR − ρg (p3 , T )VGR − ρg (pe , T )(VEC − Vliquid) (2)

where ρg (pi,T) is the density of CO2 in g·cm−3 under temperature T and pressure pi (i = 2, 3, e), and can be found from NIST database.25 VGR and VEC present the volumes of GR and EC, respectively. Vliquid is the volume occupied by the liquid solution. The simplified values of Vliquid can be directly calculated by the mass and density at equilibrium temperature for each absorbent. It is well-known that the volume of the absorbent will change due to the dissolution of CO2. Then, the precise values of Vliquid need to be corrected with the consideration of the above-mentioned influence. According to the method in the literature,26,27 such correction can be expressed as the following equation, Vliquid = Vsolvent + nCO2V 2∞

(4)

mCO2 = nCO2 /w

(5)

xCO2 = nCO2 /(nCO2 + nsol)

(6)

where nsol is mole amounts of the solvent used and can be obtained as the quotient of the mass (w) divided by molecular weight.

3. RESULTS AND DISCUSSION 3.1. Solubilities of CO2 in the Solvents. The solubilities of CO2 in five BBSs were determined at 293.15, 303.15, 313.15, and 323.15 K, and under the pressure range up to 600.0 kPa. The experimental results were listed in Tables 3−7, including equilibrium pressure (p), molality (m1), and molar fraction (x1) of CO2 in the liquid phase. Figure 1 illustrated the pressure dependence of the solubility (expressed as mCO2 or xCO2) at various temperatures for GT. The solubility profile of CO2 in five BBSs at 303.15 K were demonstrated in Figure 2. As shown in Figures 1 and 2, the solubility of CO2 increased with increasing pressure or decreasing temperature, meaning that CO2 dissolves into the present BBSs through a physical process. The uncertainties of the measurement include system errors from pressure, temperature, mass, and volume. According to the estimation method for uncertainty,29 the relative standard

(3)

where Vsolvent presents the volume of pure solvent, which can be calculated from the mass and density value at the equilibrium temperature. V∞ 2 is the partial molar volume of CO2 in the solvent at infinite dilution, which can be estimated according to the literature method,28 with the results of 39, 40, 41, and 41 cm3·mol−1 at 293.15, 303.15, 313.15, and 323.15 K, respectively.

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

a

p

K

kPa

293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

55.3 99.3 179.9 264.9 361.7 455.1 532.4 62.0 115.4 195.9 287.4 383.4 481.2 547.9

m1 mol·kg

−1

0.0755 0.1346 0.2474 0.3692 0.5097 0.6481 0.7652 0.0687 0.1284 0.2200 0.3252 0.4384 0.5564 0.6378

T

p

m1

x1

K

kPa

mol·kg−1

x1

0.0162 0.0285 0.0512 0.0746 0.1001 0.1239 0.1431 0.0148 0.0272 0.0458 0.0663 0.0873 0.1083 0.1222

313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

62.8 123.4 205.7 291.0 386.3 492.2 565.6 70.9 126.4 208.0 297.5 397.0 504.5 554.2

0.0580 0.1131 0.1915 0.2738 0.3662 0.4702 0.5444 0.0535 0.0959 0.1599 0.2311 0.3126 0.3988 0.4390

0.0125 0.0241 0.0401 0.0564 0.0740 0.0931 0.1062 0.0116 0.0205 0.0337 0.0480 0.0639 0.0801 0.0875

Standard uncertainties u are u(T) = 0.05 K, u(p) = 0.6 kPa, and relative standard uncertainties of solubility are ur (x) = 0.02, ur (m) = 0.02. 3357



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Table 4. Experimental CO2 Mole Fraction (x1) and Molality (m1) in GF at Temperature (T) under Equilibrium Pressure (p)a

a

T

p

m1

T

p

m1

K

kPa

mol·kg−1

x1

K

kPa

mol·kg−1

x1

293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

77.8 130.0 219.7 316.1 414.8 515.1 575.7 77.1 132.6 223.6 317.7 419.1 517.0 578.9

0.0403 0.0687 0.1166 0.1680 0.2224 0.2788 0.3119 0.0319 0.0559 0.0942 0.1335 0.1768 0.2192 0.2443

0.0042 0.0071 0.0120 0.0172 0.0226 0.0282 0.0314 0.0033 0.0058 0.0097 0.0137 0.0181 0.0223 0.0248

313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

87.2 138.0 230.4 315.4 430.7 515.5 582.8 81.0 138.0 223.9 336.9 422.8 520.4 575.1

0.0290 0.0470 0.0786 0.1095 0.1509 0.1823 0.2064 0.0251 0.0406 0.0656 0.0995 0.1247 0.1514 0.1657

0.0030 0.0049 0.0081 0.0113 0.0155 0.0186 0.0210 0.0026 0.0042 0.0068 0.0103 0.0128 0.0155 0.0170

Standard uncertainties u are u(T) = 0.05 K, u(p) = 0.6 kPa, and relative standard uncertainties of solubility are ur (x) = 0.02, ur (m) = 0.02.

Table 5. Experimental CO2 Mole Fraction (x1) and Molality (m1) in GAK at Temperature (T) under Equilibrium Pressure (p)a

a

T

p

m1

T

p

m1

K

kPa

mol·kg−1

x1

K

kPa

mol·kg−1

x1

293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

70.9 156.0 218.1 294.5 402.7 499.0 559.2 72.2 120.5 215.7 311.2 413.9 511.6 573.2

0.0782 0.1775 0.2500 0.3392 0.4701 0.5871 0.6603 0.0653 0.1094 0.1982 0.2903 0.3898 0.4845 0.5447

0.0102 0.0229 0.0320 0.0429 0.0585 0.0720 0.0803 0.0086 0.0142 0.0255 0.0370 0.0490 0.0602 0.0672

313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

81.4 140.3 225.1 320.7 416.4 523.7 584.5 80.9 138.0 229.1 365.4 430.1 517.6 577.9

0.0621 0.1071 0.1733 0.2497 0.3377 0.4284 0.4802 0.0518 0.0898 0.1482 0.2403 0.2842 0.3437 0.3825

0.0081 0.0140 0.0224 0.0319 0.0427 0.0536 0.0597 0.0068 0.0117 0.0192 0.0308 0.0362 0.0434 0.0481


Table 6. Experimental CO2 Mole Fraction (x1) and Molality (m1) in TEC at Temperature (T) under Equilibrium Pressure (p)a

a

T

p

m1

T

p

m1

K

kPa

mol·kg−1

x1

K

kPa

mol·kg−1

x1

293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

70.7 137.5 203.4 300.5 421.3 510.2 566.9 76.3 131.7 222.2 314.2 451.6 522.1 576.8

0.0765 0.1489 0.2206 0.3300 0.4675 0.5717 0.6384 0.0727 0.1258 0.2119 0.2957 0.4212 0.4881 0.5427

0.0207 0.0395 0.0574 0.0836 0.1144 0.1364 0.1499 0.0197 0.0336 0.0553 0.0755 0.1042 0.1188 0.1304

313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

71.1 133.2 223.3 348.7 454.1 579.7 51.2 137.0 272.9 343.5 396.7 456.3 533.6

0.0548 0.0999 0.1701 0.2675 0.3477 0.4571 0.0391 0.0868 0.1742 0.2217 0.2563 0.2958 0.3482

0.0149 0.0269 0.0449 0.0688 0.0876 0.1121 0.0107 0.0234 0.0459 0.0577 0.0661 0.0755 0.0878

Standard uncertainties u are u(T) = 0.05 K, u(p) = 0.6 kPa, and relative standard uncertainties of solubility are ur (x) = 0.02, ur (m) = 0.02. 3358



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Table 7. Experimental CO2 Mole Fraction (x1) and Molality (m1) in ATEC at Temperature (T) under Equilibrium Pressure (p)a

a

T

p

m1

T

p

m1

K

kPa

mol·kg−1

x1

K

kPa

mol·kg−1

x1

293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15

68.0 121.8 205.0 299.7 430.3 511.1 568.4 69.0 127.3 207.0 345.5 419.6 514.6 583.3

0.0841 0.1551 0.2613 0.3873 0.5614 0.6772 0.7395 0.0677 0.1254 0.2051 0.3455 0.4381 0.5383 0.6287

0.0261 0.0470 0.0768 0.1097 0.1516 0.1773 0.1905 0.0211 0.0384 0.0613 0.0991 0.1224 0.1463 0.1668

313.15 313.15 313.15 313.15 313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15

83.6 145.0 207.5 302.6 405.0 502.8 572.8 74.2 136.6 219.5 359.8 425.4 513.3 583.4

0.0702 0.1230 0.1763 0.2589 0.3495 0.4361 0.4970 0.0555 0.1000 0.1595 0.2616 0.3110 0.3762 0.4292

0.0219 0.0377 0.0532 0.0761 0.1001 0.1219 0.1366 0.0174 0.0309 0.0483 0.0769 0.0901 0.1070 0.1202


Figure 2. Solubilities of CO2 in five solvents at 303.15 K: (a) expressed as molality; (b) expressed as molar fraction. ▽, GF; ☆, GAK; ○, TEC; □, GT; △, ATEC.

Figure 1. Solubilities of CO2 in glyceryl triacetate: (a) expressed as molality; (b) expressed as molar fraction. □, 293.15 K; △, 303.15 K; ○, 313.15 K; ▽, 323.15 K; , linear fit.

ur (m) =

uncertainty of CO2 solubility can be calculated by the following equations, ur (x) =

=

u(x) = x

u(m) = m

∑ u(mliq )2 u(n)2 + u(n1)2 + u(n2)2 + nCO2 2 mliq 2 (8)

⎛ u(nCO ) ⎞2 ⎛ u(nCO ) + u(nliq ) ⎞2 2 ⎟ 2 ⎟⎟ ⎜⎜ ⎟ + ⎜⎜ n ⎠ ⎝ ⎝ nCO2 ⎠ CO2 + nliq

in which u(n1), u(n2), and u(n) can be estimated by

u(n)2 + u(n1)2 + u(n2)2 + u(nliq )2 u(n)2 + u(n1)2 + u(n2)2 + 2 nCO2 (nCO2 + nliq )2

u(ni) =

(7) 3359

2 ⎛ u(Vi ) ⎞2 ⎛ u(Ti ) ⎞2 ni ⎛ u(Pi) ⎞ ⎜ ⎟ +⎜ ⎟ +⎜ ⎟ R ⎝ Pi ⎠ ⎝ Vi ⎠ ⎝ Ti ⎠

(9)



Article

and u(nliq) is estimated by u(nliq ) =

u(mliq ) (10)

MW

The measurement errors of temperature, pressure, and volume are u(T) = 0.05 K, u(p) = 0.6 kPa, u(V) = 0.05 mL, u(mliq) = 2·10−4 g. 3.2. Henry’s Constant. As above-mentioned, the solubility of CO2 demonstrated a linear relationship with temperature or pressure. Correspondingly, Henry’s law30 can be applied to quantitatively describe the absorption behaviors for these BBSs. Thus, Henry’s constant on the basis of molar fraction (Hx) can be expressed as follows, Hx(p , T ) ≡ lim

Figure 3. Linear correlation of Henry’s constants (based on mole fraction) with temperatures. △, GF; ○, GT; ◊, TEC; ▽, ATEC; □, GAK; , linear fit.

f 2liq (p , T , x 2) x2

x2 → 0

(11)

It is well-known that Henry’s law always provides an excellent approximation when the molar fraction of gas does not exceed about 0.03. Although the limitation of solubility appears to hold to be much larger for CO2,31 the mole fractions of CO2 in present solvents are sometimes more than 0.1 and may lead to nonideality. Thus, the effect of such nonideality on the accuracy of the Henry’s constants needs to be well estimated. Carvalho et al.32 had studied the nonideality of CO2 solution in low volatile solvents (vapor pressure at room temperature inferior to 100 Pa) and proposed that the accuracy of Henry’s constants deviated from the ideal solution, was within the uncertainty of the experimental data, and was solvent independent for pressures up to 5 MPa and temperatures up to 363 K. In the present work, the pressures and temperatures are far lower than the abovementioned values. Therefore, the influence of nonideality of the solutions on the Henry’s constants can be neglected. According to definition, the values of Hx are calculated on the basis of the mole quantity rather than molecular mass. Therefore, they are directly related to information on the molecular structure of each BBS. It is well-known that the gas solubility ability of an absorbent generally came from the total effect of intermolecular interaction and molar volume. As seen from Table 9, the solubility in present BBSs follows the sequence of ATEC > TEC ≈ GT > GAK > GF at the same temperature. This means

where f 2 (p, T, x2), p and x2 represents the fugacity, equilibrium pressure, and molar fraction of CO2 in the liquid phase, respectively. According to equilibrium criteria, the fugacity of CO2 in the liquid phase and gas phase must be the same when CO2 reaches the dissolving equilibrium. Then, f 2liq (p , T , x 2) = f 2vap (p , T , y2 ) = y2 pϕ2(p , T , y2 )

(12)

where y2 and ϕ2 denote the gas phase molar fraction and fugacity coefficient of CO2, respectively. At experimental temperature, the gas phase can be regarded as pure CO2 because the vapor pressure of each solvent is low enough to be neglected. Given that pCO2 was low, ϕ2 (p,T) was approximately equal to unity. Thus, Henry’s constant at a very diluent region of CO2 in the liquid phase could be calculated by combing eqs 11 and 12 as follows, Hx(p , T ) = lim

f2 (p , T , x 2)

x2 → 0

x2

≅

pϕ2(p , T ) x2

≈

pCO

2

x2 (13)

Similarly, Henry’s constant (Hm) based on molality could be determined using following equation, Hm(p , T ) ≡ lim

m2 → 0

f2 (p , T , m2) m2

≅

pϕ2(p , T ) m2

≈

pCO

2

m2

Table 9. Thermodynamic Properties of CO2 Dissolution in BBSs Based on Molar Fraction at 0.1 MPa and 303.15 K

(14)

where m2 is the liquid phase molality of CO2. In the present work, Hx or Hm for each mixture were obtained from the slope of p−x1 or m1 lines. Henry’s constants of CO2 in five BBSs are listed in Table 8, and the values of Hx and Hm lie in the scope of (2.87−33.42) MPa and (0.70−3.43) MPa·kg mol−1, respectively. The Hx for five BBSs at various temperatures were graphically presented in Figure 3. It is evident that the values of Hx increase linearly with increasing temperatures.

ΔdisG0 solutions GT + CO2 GF + CO2 GAK + CO2 TEC + CO2 ATEC + CO2

ΔdisH0

−1

−1

kJ mol

kJ mol

−14.10 −15.65 −14.02 −12.89 −13.26

9.55 13.73 11.19 9.50 8.95

ΔdisS0 J mol−1 K−1 −78.02 −96.92 −83.18 −73.86 −73.25

Table 8. Henry’s Constants (Hm, Based on Molality and Hx, Based on Molar Fraction) of CO2 in Selected Solvents at Different Temperatures Hm/MPa·kg mol−1

Hx/MPa

solvents

293.15 K

303.15 K

313.15 K

323.15 K

293.15 K

303.15 K

313.15 K

323.15 K

GT GF GAK TEC ATEC

0.70 ± 0.01 1.86 ± 0.03 0.85 ± 0.02 0.89 ± 0.02 0.77 ± 0.02

0.87 ± 0.01 2.37 ± 0.05 1.06 ± 0.04 1.07 ± 0.4 0.95 ± 0.03

1.05 ± 0.03 2.85 ± 0.08 1.23 ± 0.04 1.27 ± 0.05 1.16 ± 0.05

1.27 ± 0.05 3.43 ± 0.10 1.51 ± 0.05 1.54 ± 0.06 1.37 ± 0.07

3.66 ± 0.09 18.31 ± 0.35 6.92 ± 0.17 3.71 ± 0.10 2.87 ± 0.14

4.42 ± 0.11 23.23 ± 0.58 8.49 ± 0.20 4.33 ± 0.13 3.48 ± 0.15

5.27 ± 0.16 27.81 ± 0.71 9.82 ± 0.20 5.14 ± 0.19 4.10 ± 0.20

6.27 ± 0.22 33.42 ± 0.82 11.93 ± 0.24 6.05 ± 0.20 4.76 ± 0.26

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Table 10. Hm Comparison with Other Absorbents at 313.15 K

that the molecular structure with more carboxyl, ester, or ether groups possesses the higher solubility ability for CO2, as similar to that reported by Gui.17 On the other hand, GF demonstrates the lowest solubility ability for CO2 among five BBSs because it has the smallest molar volume (85.97 cm3·mol−1 at 298.15 K). Similarly, ATEC possesses the best solubility ability because of its highest molar volume (279.61 cm3·mol−1 at 298.15 K). From the viewpoint of practical application and atomic economy, the capture ability based on weight of absorbent is more valuable and comparable. Herein, the gravimetric solubility sequence (as shown with Hm) with the consideration of molecular mass was selected and shown as GT > ATEC > TEC ≈ GAK > GF for these BBSs. Interestingly, the two sequences based on Hx and Hm are almost the same. When comprehensively taking into account the viscosity, volatility, and safety as illustrated in Table 2, GT maybe more competitive than the others as a CO2 capturer. 3.3. Thermodynamic Properties. Thermodynamic parameters are important parameters to understand the gas dissolving behavior and calculation basis in the design of equipment for gas absorbent operation. With the dependence of Henry’s constants on temperature at hand, it is convenient to further calculate three thermodynamic properties of ΔdisG, ΔdisH, and ΔdisS according to following equations,33 ⎛ ∂ ln(H(T , p)/p0 ) ⎞ ⎟⎟ ΔdisH = R ⎜ ∂(1/T ) ⎝ ⎠

⎛ H(T, p) ⎞ ⎟ ΔdisG = RT ln⎜ 0 ⎝ p ⎠

p

common solvents

water

PC

Hm/MPa

2.95 PEGhigh boil point solvents 400 Hm/MPa ionic [hmim] liquids [BF4]

1.91

1.81 Hm/MPa present solvents GT Hm/MPa a

0.87

0.72 TX100a 1.68 [bmim] [Tf2N]

methanol

DMSO

0.49 FAPE1215 1.70 [bheaa]

0.85 EEA 0.75 [emim] [Et2PO4]

NHD 1.05

1.06 GF

2.38 GAK

1.80 TEC ATEC

2.37

1.06

1.07

0.95

Henry’s constant Hm were calculated at T = 308.15 K.

tetrafluoroborate ([hmim][BF4]),38 and 1-ethyl-3-methylimidazolium diethylphosphate ([emim][Et2PO4]).39 Water40 is a cheap and clean solvent used anywhere, but its absorption capacity for CO2 demonstrates far lower than that present for BBSs. To our knowledge, compared with traditional absorbents and new emerging ILs, the present BBSs possess low toxicity, high biodegradability, and low volatility. Therefore, they can be regarded as a kind of potential candidates for CO2 capture in the future.

4. CONCLUSIONS In the study, the data of CO2 solubility in five BBSs were reported at 293.15, 303.15, 313.15, and 323.15 K under pressures of (0−600.0) kPa using the isochoric saturation method. Henry’s law constants and thermodynamic properties were systematically derived. The results indicate that the dissolution of CO2 in these solvents follows a physical mode and ATEC possesses the highest gravimetric dissolving ability of CO2 among the five solvents. The process of desorption was energy-efficient because of the low dissolution enthalpy. The positive value of ΔdisG shows the process of dissolution is not spontaneous and mainly pushed by pressure.

(15)

(16)

Δdis H − ΔdisG (17) T 0 where p is the pressure at standard state. ΔdisH is the standard dissolution enthalpy which quantitatively reflects the liquid phase intermolecular interaction between CO2 and absorbents. ΔdisG presents the standard dissolution Gibbs free energy which reaches the minimum at the gas−liquid equilibrium. ΔdisS denotes the standard dissolution entropy reflecting the order of solution after absorption. In Table 9 were listed the ΔdisG, ΔdisH, and ΔdisS at 303.15 K and 0.1 MPa. As shown in Table 9, the negative ΔdisH values suggest that heat is released during absorption of CO2 and there are strong interactions between CO2 and the solvents. The negative values of ΔdisS mean that the solutions become less chaotic because of less movement of CO2 molecules at random after dissolution of CO2 on the molecular level. The positive values of ΔdisG at the studied temperature show that the process of dissolution is not spontaneous and mainly pushed by pressure. 3.4. Comparison with Literature Solvents. To systematically evaluate the present five BBSs as CO2 absorbents, the dissolving ability of CO2 in these solvents was compared with that of other physical absorbents, including common solvents, high boiling point solvents, and some new emerging ILs. Hm provided a convenient and uniform scale for different absorbents owing to its definition on mass. In Table 10 were listed the detailed results of comparison at 313.15 K. As shown in Table 10, the values of Hm in selected solvents are almost near unity, which is lower than that in methanol,14 similar to PC,12 DMSO,12 2-(2-ethoxyethoxy)ethyl acetate (EEA),34 and NHD,17 but evidently higher than those in higher boiling point solvents and ordinary ILs, such as TX-100,15 PEG-400,35 fatty amine polyoxyethylene ether 1215 (FAPE-1215),36 bis(2-hydroxyethyl)ammonium acetate ([bheaa]),37 1-hexyl-3-methylimidazolium Δdis S =

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00399. The experimental values (Table S1) and comparison results (Figure S1) with those from the literature for the densities of present solvents at different temperatures and 101.3 kPa (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Solubilities and Thermodynamic Properties of Carbon Dioxide in

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