Measurement and Correlation of CO2 Solubility in 1-Ethyl-3

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

pubs.acs.org/jced

Measurement and Correlation of CO2 Solubility in 1‑Ethyl-3methylimidazolium ([EMIM]) Cation-Based Ionic Liquids: [EMIM][Ac], [EMIM][Cl], and [EMIM][MeSO4] Joon-Hyuk Yim,† Seung-Jae Ha,‡ and Jong Sung Lim*,‡ †

Doosan Heavy Industries & Construction, Seongsan-gu, Changwon, Gyeongnam 51711, Korea Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Korea

‡

ABSTRACT: The solubility of carbon dioxide (CO2) was investigated using three different 1-ethyl-3-methylimidazolium ([EMIM]) cation-based ionic liquids: 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]), 1-ethyl-3-methylimidazolium chloride ([EMIM][Cl]), and 1-ethyl-3-methylimidazolium methyl sulfate ([EMIM][MeSO4]). The CO2 solubility data for the three ionic liquids were produced by measuring the bubble-point and cloud-point pressures of the CO2 + ionic liquid mixtures. The temperature range was from 303.15 to 403.15 K, and the pressure range was from 0.45 to 48.6 MPa. The experimental data provided results showing that the solubility of CO2 in ionic liquids increased with pressure, decreased with temperature, and was also affected by the different anions used in the experiment. The solubility is determined by CO2 mole fraction in ionic liquids, and the order of magnitude of the CO2 solubility was found to be [EMIM][Ac] > [EMIM][MeSO4] > [EMIM][Cl]. For the correlation and calculation of the experimental data, we used the Peng−Robinson equation of state, the conventional van der Waals one-fluid mixing rule, and the modified Lydersen−Joback−Reid method. The average absolute deviations of pressure were 0.0231 for CO2 + [EMIM][Ac], 0.0141 for CO2 + [EMIM][Cl], and 0.0275 for CO2 + [EMIM][MeSO4] systems.

1. INTRODUCTION Lignocellulose is a cell wall material commonly found in woody plants. It is a porous structure mainly consisting of cellulose, hemicellulose, and lignin.1 Lignin has gained much attention for conversion into polymer chemicals usually formed from fossil fuels, while cellulose has attracted attention for conversion to biofuels and biopolymers.2 Furthermore, lignocellulosic biomass is the most abundant plant material of our planet. It is extensively used as a renewable resource in many industrial fields such as energy, chemicals, and materials.3 Lignocellulosic biomass has a wide range of applications such as in natural polymers with better mechanical properties and high thermal stabilities obtained using nanotechnology. It is also used to create natural polymers to formulate biodegradable plastics and other composites with lignocellulosic fibers.4 For processing lignocellulosic materials, many methods are employed, such as organic solvents, dilute acids, lime pretreatments, and steam explosion.5 However, such traditional methods normally have a negative effect on the environment because of the formation of toxic degradation byproducts.6 Moreover, in these methods, it is difficult to separate and dissolve lignocellulosic materials because of the unique structure of cellulose and lignin.7 To overcome these shortcomings, ionic liquids (ILs) have been widely recognized as novel solvents for the pretreatment and dissolution of lignocellulosic materials.8 ILs have attracted strong interest as alternative solvents for various applications such as biomass processing, green processes, polymer formation, and modifications.9 They are also © XXXX American Chemical Society

gradually being recognized as green, environmentally friendly solvents for the dissolution and pretreatment of cellulose.5 Various kinds of ILs have been studied as novel environmentally friendly chemicals because of their unique properties such as high thermal stability, nonflammability, good ionic conductivity, and near-zero vapor pressure.2,10 As an alternative solvent, carbon dioxide is also used in many industrial fields because of its relatively low cost, low critical pressure, low temperature, nonpolarity, nontoxicity, and nonflammability.11,12 Therefore, in the field of green processes, ILs, carbon dioxide, and IL + carbon dioxide systems are highly advantageous as a novel solvent for processing materials to create new products such as polymers and organic products.10,12 In addition, new processing methods can be developed by adjusting the solubility, phase behavior, and pressure of the polymeric components of biomass in IL + carbon dioxide systems.13 Therefore, in lignocellulosic material processing, it is very important to know the solubility of CO2 for the designated IL. IL + carbon dioxide solvent systems for the lignocellulosic material have also been considered as an environmentally friendly medium by many research groups.14−18 In this regard, 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], and 1-allyl-3-methylimidazolium [AMIM] and Received: June 10, 2017 Accepted: February 8, 2018

A

DOI: 10.1021/acs.jced.7b00532 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemical Structure and Purity Data of Ionic Liquids and CO2

The three ionic liquids and high-grade CO2 gas were used without further treatment. 2.2. Experimental Apparatus and Procedure. To measure the solubility of CO2 in ionic liquids, we adopted the same equipment which was used in our previous work.22−27 That was a high pressure experimental apparatus equipped with a variable-volume view cell. Figure 1 illustrates a schematic

their salts with Br-, Cl-, MeSO4, and acetate have attracted much interest as they can dissolve cellulose and lignin.19−21 To use these types of ILs + CO2 systems for biomass processing and modifications, measurement of CO2 solubility in the selected IL is essential. Therefore, in this study, we measured the solubility of CO2 in [EMIM] cation-based IL to investigate the phase behavior of CO2 in these three ILs, which are 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]), 1-ethyl-3-methylimidazolium chloride ([EMIM][Cl]), and 1-ethyl-3-methylimidazolium methyl sulfate ([EMIM][MeSO4]). Recently, Kiran et al.13 reported experimental results for the miscibility of CO2 in 1-ethyl-3-methylimidazolium acetate ([EMIM][Ac]) at 298.15, 323.15, 348.15, and 373.15 K, but the work mainly focused on the volumetric property and compressibility data. In the current study, we reported new experimental data in terms of CO2 solubility, which depends on its mole fraction in [EMIM][Ac] from 303.15 to 373.15 at 10 K intervals. Moreover, the experimental solubility data of CO2 in [EMIM][Cl] was reported for the first time. The solubility of CO2 was determined by measuring the bubble-point pressure (or cloud-point pressure) at a fixed temperature. The temperature range for the experimental measurements of [EMIM][MeSO4] was same as that for [EMIM][Ac] with additional measurements for 353.15−403.15 at 10 K intervals for [EMIM][Cl] due to its higher melting temperature. The experimental data results were correlated with the Peng− Robinson equation of state (PR-EoS) using the conventional van der Waals one-fluid mixing rules. We used the modified Lydersen−Joback−Reid method to estimate the critical properties and acentric factor of the ILs.

Figure 1. A schematic diagram of the experimental apparatus: (1) water for pressing; (2) pressure generator; (3) pressure gauge; (4) piston; (5) sapphire window; (6) magnetic bar; (7) stirrer; (8) air bath; (9) variable-volume view cell; (10) light source; (11) borescope; (12) CCD camera; (13) monitor; (14) temperature gauge; (15) heater; (16) heating controller.

diagram of the experimental apparatus. The cell has a dimension of 16 mm i.d. × 70 mm o.d. and an internal working volume of about 31 cm3. A pressure generator (High Pressure Equipment Co. model 50-6-15) was used to pressurize water, thereby displacing the piston. A sapphire window was inserted into the view cell for visual observation of the cell interior. The system pressure was measured with a high-precision pressure

2. EXPERIMENTAL SECTION 2.1. Materials. The ionic liquids [EMIM][Ac], [EMIM][Cl], and [EMIM][MeSO4] were purchased from FutureChem (South Korea). The chemical structure and the purity data of ionic liquids used in the present research are shown in Table 1. The HPLC analysis purity data of the three ionic liquids were provided by the supplier. Before measurement, the ionic liquids were evacuated with a vacuum pump during a 24 h period to remove water content and other gases. After evacuation, to analyze the final water content, coulometric Karl−Fischer titration (Metrohm model 684) was performed. All water content of ionic liquids after evacuation was less than 24 ppm. We used high-grade CO2 (99.999 mass %) for measuring CO2 solubility.

Table 2. Critical Properties and Acentric Factor of CO2 from Literature31 and Ionic Liquids Calculated from the Modified Lydersen−Joback−Reid Method32 M/(g·mol−1) CO2 [EMIM][Ac] [EMIM][Cl] [EMIM][MeSO4] B

44 170.21 146.62 222.26

Tb/K

Tc/K

Pc/MPa

ω

548.15 512.27 689.8

304.21 770.04 748.61 1053.61

7.3847 3.0436 3.4171 4.5917

0.2239 0.5666 0.4164 0.3400



Article

Table 3. Solubility Data for the [EMIM][Ac] + CO2 Systema mole fraction of CO2, x1

standard uncertainty in CO2 mole fraction

0.295

0.0017

0.327

0.0016

0.357

0.0016

0.395

0.0016

a

T/K

P/ MPa

phase behavior

mole fraction of CO2, x1


303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

0.45 0.57 0.74 0.93 1.18 1.44 1.70 2.01 0.62 0.82 1.06 1.28 1.63 2.01 2.43 2.85 0.85 1.08 1.35 1.73 2.16 2.65 3.18 3.76 1.26 1.61 2.02 2.47 3.00 3.71 4.37 4.96

bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

0.461

0.0013

0.535

0.0012

0.557

0.0011

0.575

0.0012

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.31 2.81 3.49 4.23 5.07 6.05 7.17 8.29 3.88 4.92 6.12 7.56 9.06 11.12 13.28 15.42 4.72 6.14 7.70 9.61 11.95 14.41 17.30 20.20 5.87 7.63 9.46 11.82 14.61 17.85 20.96 24.64

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b cc c

T/K

P/MPa

phase behavior

353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15

11.21 12.67 14.13 15.55 16.99 18.51 13.09 14.94 16.84 18.77 20.64 22.53 15.27 17.70 20.25 22.77 25.28 27.77 18.36 21.28 24.15 27.11

b b b b b b b b b b b b b b b cc c c c c c c

Standard uncertainties u are u(T) = 0.1 K; ur(p) = 0.01 (1% of the measured value). bBubble point. cCloud point.

Table 4. Solubility Data for the [EMIM][Cl] + CO2 Systema mole fraction of CO2, x1


0.098

0.0021

0.142

0.0019

0.195

0.0019

0.243

0.0018

T/K

P/MPa

phase behavior



353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15

2.53 2.76 3.00 3.26 3.50 3.74 4.07 4.51 4.98 5.44 5.87 6.31 6.23 6.91 7.63 8.35 9.04 9.76 8.42 9.62 10.85 12.09

bb b b b b b b b b b b b b b b b b b b b b b

0.283

0.0018

0.306

0.0016

0.328

0.0015

0.347

0.0016

C



Article

Table 4. continued mole fraction of CO2, x1

a


T/K

P/MPa

phase behavior

393.15 403.15

13.30 14.49

b b



T/K

P/MPa

phase behavior

393.15 403.15

30.09 33.14

c c

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

4.31 5.37 6.86 8.73 10.56 13.14 16.02 19.03 5.71 7.39 9.83 12.77 15.85 19.57 23.26 27.18 7.69 11.71 15.69 19.96 24.47 29.04 33.34 37.55 16.31 21.44 26.70 32.01 36.53 40.96 44.81 48.55

b b b b b b b b b b b b b b b b cc c c c c c c c c c c c c c c c


Table 5. Solubility Data for the [EMIM][MeSO4] + CO2 Systema mole fraction of CO2, x1


0.089

0.0028

0.145

0.0026

0.213

0.0026

0.296

0.0022

a

T/K

P/MPa

phase behavior



303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

0.63 0.75 0.87 1.14 1.43 1.71 2.17 2.51 1.19 1.39 1.65 2.22 2.71 3.17 3.91 4.82 2.11 2.56 3.19 4.06 4.87 5.83 7.14 8.31 3.38 4.10 5.01 6.43 7.71 9.36 11.36 13.52


0.339

0.0023

0.378

0.0022

0.419

0.0021

0.455

0.0021


gauge (Dresser Heise model CC-12-G-A-02B, ±0.05 MPa accuracy, ±0.01 MPa resolution), which was placed between the pressure generator and the view cell. The system temperature was measured within ±0.1 K with an RTD temperature probe that was inserted into the cell. A detailed explanation for the experimental apparatus and procedure can be found in our previous publication.23−27 Acentric factor is calculated by critical properties and normal boiling temperature (Tb for Pb = 0.1 MPa). All of the calculated critical properties for the normal boiling temperature and the acentric factors of the ionic liquids are indicated in Table 2 along with the values for CO2. As the pressure increased, the CO2 and ionic liquid mixture in the cell reached a single phase, after which the pressure was slowly reduced until the first CO2 bubble was observed from the solution. At this point in time, we measured the pressure of the cell and noted the reading as the bubble-point pressure at a fixed CO2 mole fraction and temperature. The uncertainty analysis was performed according to the ISO guidelines28 for the composition measurements of each

component (x1), as shown in Tables 3−5. A detailed procedure for the uncertainty analysis can also be found in our previous work.29

3. CORRELATION The experimental solubility data of the three CO2 + ionic liquid system were correlated with the PR-EoS.30 The conventional van der Waals one-fluid mixing rules were used for the calculation of the mixture parameters in ionic liquid phase. We also used the PR-EoS and the conventional van der Waals onefluid mixing rules in our previous research articles.25−27 The PR-EoS can be written as follows: P=

α (T ) RT − V−b V (V − b) + b(V − b)

(1)

The binary parameters are calculated from the following quadratic mixing rules: a = ∑i ∑j xixjaij D

(2) DOI: 10.1021/acs.jced.7b00532 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

aij = (aijaij)1/2 (1 − kij)

(3)

b = ∑i ∑j xixjbij

(4)

⎛ bi + bj ⎞ bij = ⎜ ⎟(1 − lij) ⎝ 2 ⎠

(5)

bi =

0.457235R2Tci2 Pci

⎡ ⎛ ⎢1 + (0.37464 + 1.54226wi − 0.26992wi2)⎜⎜1 − ⎢⎣ ⎝

T Tci

⎞⎤ ⎟⎟⎥ ⎠⎥⎦

(7)

To calculate the parameters of the PR-EoS, data are needed for the critical temperature (Tc, K), critical pressure (Pc, bar), normal boiling temperature (Tb), and acentric factor (ω) of the ionic liquids and CO2. In case of CO2, we used the critical properties and acentric factor in NIST database.31 However, we estimated the critical properties of the ionic liquids because those values for the ionic liquids were not readily available. To estimate the critical temperature (Tc) and critical pressure (Pc) for the ionic liquids, we used the modified Lydersen−Joback− Reid group contribution method.32

In eq 4, bii = bi and bjj = bj, and in eqs 3 and 5, kij and lij are the binary interaction parameters. aii =

0.077796RTci Pci

2

(6)

Figure 3. P−x1 diagram of CO2 solubilities of the ionic liquid + CO2 system: (a) [EMIM][Ac] + CO2. The symbols are temperature; (●) 303.15 K, (△) 313.15 K, (■) 323.15 K, (▽) 333.15 K, (★) 343.15 K, (◊) 353.15 K, (◑) 363.15 K, (⧫) 373.15 K. (b) [EMIM][Cl] + CO2. The symbols are temperature; (●) 353.15 K, (△) 363.15 K, (■) 373.15 K, (▽) 383.15 K, (★) 393.15 K, (◊) 403.15 K. (c) [EMIM][MeSO4] + CO2. The symbols are temperature; (●) 303.15 K, (△) 313.15 K, (■) 323.15 K, (▽) 333.15 K, (★) 343.15 K, (◊) 353.15 K, (◑) 363.15 K, (⧫) 373.15 K; (−) calculated by the PR-EoS.

Figure 2. P−T diagram of CO2 solubilities of the ionic liquid + CO2 system: (a) [EMIM][Ac] + CO2. The symbols are CO2 mole fraction; (●) 0.295, (△) 0.327, (■) 0.357, (▽) 0.395, (⧫) 0.461, (◑) 0.535, (◊) 0.557, (◪) 0.575: (b) [EMIM][Cl] + CO2; (●) 0.097, (△) 0.142, (■) 0.195, (▽) 0.243, (⧫) 0.283, (◑) 0.306, (◊) 0.328, (◪) 0.347: (c) [EMIM][MeSO4] + CO2; (●) 0.089, (△) 0.145, (■) 0.213, (▽) 0.296, (⧫) 0.339, (◑) 0.378, (◊) 0.419, (◪) 0.455. E


Journal of Chemical & Engineering Data Tb = 198.2 + Tc =

Pc =

Article

∑ nΔTbM

movement of a stirring bar in the variable-volume view cell was no longer visually observed, and we defined this separation pressure as the cloud-point pressure in this study. Tables 3−5 show the experimental solubility data for the CO2 + [EMIM][Ac], CO2 + [EMIM][Cl], and CO2 + [EMIM][MeSO4], respectively. These experimental data tables show the measured bubble-point or cloud-point pressures at fixed temperatures and CO2 mole fractions. For the CO2 + [EMIM][Ac] system, when the CO2 mole fraction is less than 0.557, the bubble-point behavior was observed through all the temperature ranges studied in this work. When the CO2 mole fraction is 0.575, the bubblepoint behavior was observed at the temperature below 353.15 K, and cloud-point behavior begins to appear from 363.15 K. In the CO2 + [EMIM][Cl] system, the bubble-point behavior was observed through all the temperature ranges when the CO2 mole fraction is less than 0.306. When the CO2 mole fraction is 0.328, cloud-point behavior was observed at the temperature above 383.15 K, and when the CO2 mole fraction is higher than 0.347, cloud-point behavior was observed through all the temperature ranges. For the CO2 + [EMIM][ MeSO4] system, when the CO2 mole fraction is less than 0.378, the bubble-point behavior was observed all through the temperature ranges. When the CO2 mole fraction is 0.419, cloud-point behavior was observed at the temperature above 323.15 K, and when the CO2 mole

(8)

Tb 0.5703 + 1.0121∑ nΔTM − (∑ nΔTM)2

(9)

M 2 [0.2573 + ∑ nΔPM]

(10)

where M is the molecular weight of the ionic liquid. The groups considered for the modified Lydersen−Joback−Reid method are presented in the literature.32 The acentric factor was calculated from the normal boiling temperature and the critical properties estimated by the modified Lydersen−Joback−Reid group contribution method.32 ω=

⎡P ⎤ (Tb − 43)(Tc − 43) log⎢ c ⎥ (Tc − Tb)(0.7Tc − 43) ⎣ Pb ⎦ −

⎡P ⎤ (Tc − 43) ⎡ Pc ⎤ log⎢ ⎥ + log⎢ c ⎥ − 1 (Tc − Tb) ⎣ Pb ⎦ ⎣ Pb ⎦

(11)

4. RESULTS AND DISCUSSION In this work, the solubility of carbon dioxide in [EMIM][Ac], [EMIM][MeSO4], and [EMIM][Cl] was measured at the desired temperature. To use these types of ILs + CO2 systems for biomass processing and modifications, measurement of CO2 solubility in the selected IL is vital. Most studies related to ILs + CO2 systems were performed in the relative low pressure region; therefore, we measured the solubility of three ionic liquid + CO2 systems for enlargement of solubility database of lignocellulosic processing and biomass area. The CO2 gas + ionic liquid mixture in the variable-volume view cell was pressurized steadily until it became a homogeneous single phase. Next, the CO2 + ionic liquid homogeneous mixture was depressurized very slowly until we observed the first gas bubble appearing from the homogeneous solution. This separation point was regarded as the bubble-point pressure. On the other hand, the cloud-point behavior was observed at a set of specific experimental conditions: above the highest condition (CO2 mole faction, temperature, and pressure) at which the bubble-point behavior could exist. The detailed experimental conditions related to bubble point and cloud point for each system are listed in Tables 3−5, respectively. At a cloud-point pressure, the phase separation from a homogeneous liquid phase into liquid−liquid two phases induces the solution in the variable-volume view cell to become cloudy. At this point, the

Table 7. Average Absolute Deviations of Pressure (AAD-P %) between Experimental Data and Calculated Values for the Ionic Liquid + CO2 Systemsa AAD-P %b temperature (K)

[EMIM][Ac]

[EMIM][MeSO4]

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 383.15 393.15 403.15 average

3.36 2.23 2.15 1.85 1.68 1.90 2.94 2.40

2.95 3.37 3.31 2.17 2.56 2.58 2.92 2.17

2.31

2.75

[EMIM][Cl]

1.14 1.04 1.21 1.45 1.72 1.89 1.41

a

Standard uncertainties u are u(T) = 0.1 K; ur(p) = 0.01 (1% of the measured value). bAverage absolute deviation in percentage:

AAD% =

exp calc 1 N |Pi − Pi | ∑ Piexp N i=1

× 100 (N = number of data points).

Table 6. Binary Interaction Parameters (k12 and l12) for the Ionic Liquid Systems [EMIM][Ac]

[EMIM][MeSO4]

[EMIM][Cl]

temperature (K)

k12

l12

k12

l12

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 383.15 393.15 403.15

0.2580 0.2654 0.2640 0.2676 0.2690 0.2756 0.2875 0.2975

0.2618 0.2662 0.2633 0.2642 0.2613 0.2617 0.2667 0.2714

0.1153 0.1237 0.1302 0.1237 0.1230 0.1251 0.1178 0.1157

0.0432 0.0472 0.0496 0.0417 0.0388 0.0373 0.0293 0.0249

F

k12

l12

0.2412 0.2618 0.2827 0.3046 0.3301 0.3571

0.0342 0.4185 0.0491 0.0566 0.0661 0.0758



Article

Table 8. Solubility Data for the [EMIM][Ac] + CO2 System in Molality Scalea CO2 molality mCO2/mol·kg−1

standard uncertainty in CO2 molality

2.4656

0.0010

2.8579

0.0009

3.2637

0.0009

3.8453

0.0009

a

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

0.45 0.57 0.74 0.93 1.18 1.44 1.70 2.01 0.62 0.82 1.06 1.28 1.63 2.01 2.43 2.85 0.85 1.08 1.35 1.73 2.16 2.65 3.18 3.76 1.26 1.61 2.02 2.47 3.00 3.71 4.37 4.96


CO2 molality mCO2/mol·kg−1


5.0406

0.0008

6.7628

0.0007

7.3964

0.0006

7.9610

0.0007

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

2.31 2.81 3.49 4.23 5.07 6.05 7.17 8.29 3.88 4.92 6.12 7.56 9.06 11.12 13.28 15.42 4.72 6.14 7.70 9.61 11.95 14.41 17.30 20.20 5.87 7.63 9.46 11.82 14.61 17.85 20.96 24.64

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b cc c

T/K

P/MPa

phase behavior

353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15

11.21 12.67 14.13 15.55 16.99 18.51 13.09 14.94 16.84 18.77 20.64 22.53 15.27 17.70 20.25 22.77 25.28 27.77 18.36 21.28 24.15

b b b b b b b b b b b b b b b cc c c c c c


Table 9. Solubility Data for the [EMIM][Cl] + CO2 System in Molality Scalea CO2 molality mCO2/mol·kg−1


0.7385

0.0014

1.1306

0.0013

1.6532

0.0013

2.1917

0.0012

T/K

P/MPa

phase behavior

353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15 383.15 393.15 403.15 353.15 363.15 373.15

2.53 2.76 3.00 3.26 3.50 3.74 4.07 4.51 4.98 5.44 5.87 6.31 6.23 6.91 7.63 8.35 9.04 9.76 8.42 9.62 10.85

bb b b b b b b b b b b b b b b b b b b b b G



2.6946

0.0012

3.0116

0.0011

3.3335

0.0010

3.6259

0.0011



Article

Table 9. continued CO2 molality mCO2/mol·kg−1

a


T/K

P/MPa

phase behavior

383.15 393.15 403.15

12.09 13.30 14.49

b b b



T/K

P/MPa

phase behavior

383.15 393.15 403.15

27.11 30.09 33.14

c c c


Table 10. Solubility Data for the [EMIM][MeSO4] + CO2 System in Molality Scalea CO2 molality mCO2/mol·kg

a

−1


0.4419

0.0013

0.7678

0.0012

1.2208

0.0012

1.8962

0.0010

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

0.63 0.75 0.87 1.14 1.43 1.71 2.17 2.51 1.19 1.39 1.65 2.22 2.71 3.17 3.91 4.82 2.11 2.56 3.19 4.06 4.87 5.83 7.14 8.31 3.38 4.10 5.01 6.43 7.71 9.36 11.36 13.52


CO2 molality mCO2/mol·kg

−1


2.3114

0.0010

2.7435

0.0010

3.2479

0.0009

3.7585

0.0009

T/K

P/MPa

phase behavior

303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15 303.15 313.15 323.15 333.15 343.15 353.15 363.15 373.15

4.31 5.37 6.86 8.73 10.56 13.14 16.02 19.03 5.71 7.39 9.83 12.77 15.85 19.57 23.26 27.18 7.69 11.71 15.69 19.96 24.47 29.04 33.34 37.55 16.31 21.44 26.70 32.01 36.53 40.96 44.81 48.55

b b b b b b b b b b b b b b b b cc c c c c c c c c c c c c c c c


[EMIM][Ac] and (c) CO2 + [EMIM][MeSO4] systems at 8 fixed temperatures from 303.15 to 373.15 K and for the (b) CO2 + [EMIM][Cl] system at 6 fixed temperatures from 353.15 to 403.15 K. Each figure represents the solubilities of CO2 in each ionic liquid as a function of pressure at various temperatures. At a comparatively low CO2 mole fraction area, the equilibrium pressures were fairly low; however, the pressures rose very steeply when the mole fraction of CO2 was further increased. Figure 3 also indicates that the CO2 solubility increases with increasing pressure at a fixed temperature, while the CO2 solubility decreases with increasing temperature at a fixed pressure. In Figure 3, the calculated solubility data correlated with the experimental results are also illustrated. We used the PR-EoS and the van der Waals conventional simple mixing rules for the correlation. In Table 6, the calculated binary interaction parameters for [EMIM][Ac], [EMIM][MeSO4], and [EMIM][Cl] systems at eight or six equally spaced temperatures are

fraction is higher than 0.455, cloud-point behavior was observed all through the temperature ranges studied here. At various CO2 mole fractions, the relationship between the temperature and pressure in equilibrium state are represented in Figure 2. At a fixed CO2 mole fraction, the bubble-point or cloud-point pressure shows a linear increase with increasing temperature. This implies that as the system temperature increases, the CO2 solubility in the ionic liquids decreases. The slope of the bubble-point or cloud-point pressure versus temperature curves, (∂P/∂T)x1, increased along with an increase in the CO2 mole fraction. This also means a rise of the CO2 mole fraction reduces the CO2 solubility in ionic liquids. When the CO2 mole fraction increased at a fixed temperature, the bubble-point or cloud-point pressure increased significantly. The bubble-point or cloud-point pressure vs mole fraction of CO2 curve, P−x1 diagram, obviously shows this. Figure 3 illustrates the P−x1 diagrams for the (a) CO2 + H



Article

study. The sequence of magnitude for the solubility of CO2 was [EMIM][Ac] ≫ [EMIM][MeSO4] > [EMIM][Cl]. Therefore, the solubility of CO2 in [EMIM][Ac] was the highest and [EMIM][Cl] was the lowest within the three ionic liquids that we studied in this work. Normally, it is assumed that a conventional ionic liquid with acidic or basic functionalities will highly influence the absorption of CO2.33 In this case, acetate anion increased CO2 solubility, and chloride anion decreased CO2 solubility in the [EMIM] cation-based ionic liquid system. Figure 5 also shows a comparison of CO2 solubility according to the number of cyanide anions in the ionic liquids. These data are from our previous work.23 The CO2 solubility in [EMIM][C(CN)3] ionic liquid is the highest; the CO2 solubility in [EMIM][N(CN)2] is the middle among these ionic liquids, and the CO2 solubility in [EMIM][SCN] ionic liquid is the lowest at each fixed pressure. In these experimental data, the CO2 behaves as a Lewis acid, while the cyanide anion in the ionic liquid behaves as a Lewis base.23 Therefore, the more cyanide included in the anion, the greater the increase in CO2 solubility. In addition, Figure 5 also illustrates a comparison of CO2 solubility in ionic liquids having the same cation and containing different fluorine atoms in anion. Among the three ionic liquids, the solubility of CO2 in [EMIM][eFAP] is higher than that other two ionic liquids as follows: [EMIM][eFAP]34 > [EMIM][Tf2N]35 > [EMIM][PF6].36 This can be explained by the presence of a larger amount of fluorine atoms in the [eFAP] anion than in the [Tf2N] and [PF6] anions.37 Further, the fluoroalkyl groups in [Tf2N], which are known to be CO2-philic, affect the CO2 solubility.35 Also, the [Tf2N] anion is more highly fluorinated than the [PF6] anion.38 As a result, the order of magnitude of the solubility of CO2 turned out to be [EMIM][eFAP]34 > [EMIM][Ac] ≈ [EMIM][Tf2N]35 > [EMIM][C(CN)3]23 > [EMIM][N(CN)2]23 ≈ [EMIM][PF6]36 > [EMIM][SCN]23 ≈ [EMIM][MeSO4] > [EMIM][Cl]. For the reliability of our data, we compared our experimental data with previously published literature data, as shown in Figures 6 and 7. Shiflett et al.39 previously reported the

given. Table 7 shows the average absolute deviations of pressure (AAD-P %) between the experimental and calculated solubility data at each temperature. The overall AAD-P values were 0.0231 for CO2 + [EMIM][Ac], 0.0141 for CO2 + [EMIM][Cl], and 0.0275 for CO2 + [EMIM][MeSO4] systems. These AAD-P values are slightly higher than the standard uncertainty (ur(p) = 0.01); however, these values are relatively small and acceptable. Therefore, we determined that the estimated calculation results by using the PR-EoS with the van der Waals one-fluid mixing rules well correlated with the experimental solubility data. These CO2 solubility data in three ionic liquids will be useful with volumetric database such as [EMIM] cation-based ionic liquid + organic solvent system by Kiran et al.2 for determination of process and operation condition to utilize lignocellulosic material from biomass material. In addition, Tables 8−10 also show the experimental solubility data of CO2 in the three ionic liquids, [EMIM][Ac], [EMIM][Cl], and [EMIM][MeSO4] in molality scale for practical and industrial applications. Figures 4 and 5 show the effect of different anions on the solubility of CO2 in [EMIM] cation-based ionic liquids at 353.15 K.

Figure 4. P−x1 diagram of CO2 solubilities of the [EMIM] cation-based ionic liquid + CO2 system at 353.15 K for comparison of our experimental data: (red and black circle) [EMIM][Ac], + CO2, (yellow triangle) [EMIM][MeSO4], + CO2, (green and black square) [EMIM][Cl] + CO2.

Figure 6. P−x1 diagram of CO2 solubilities of the [EMIM][Ac] + CO2 system at 323.15 K for comparison of experimental data and literature data: (red and black circle) [EMIM][Ac] + CO2 (this study) and (white triangle) [EMIM][Ac] + CO2 (literature data).39

Figure 5. P−x1 diagram of CO2 solubilities of the [EMIM] cation-based ionic liquid + CO2 system at 353.15 K with various anions: (black circle) [EMIM][eFAP],34 (red and black circle) [EMIM][Ac], (white triangle) [EMIM][TF2N],35 (white circle) [EMIM][C(CN3)],23 (black star) [EMIM][N(CN)2],23 (white square) [EMIM][SCN],23 (white diamond) [EMIM][PF6],36 (yellow triangle) [EMIM][MeSO4], (green and black square) [EMIM][Cl].

[EMIM][Ac] + CO2 system at relatively low pressure region under 2 MPa, and they also determined that [EMIM][Ac] exhibits strong chemical absorption for CO2 at low pressure area under 0.5 MPa. Figure 6 shows the CO2 solubility in the [EMIM][Ac] + CO2 system at 323.15 K for comparison of our

Figure 5 also includes some results from our previous work.23 Figure 4 shows only our experimental data measured in this I



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ACKNOWLEDGMENTS

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REFERENCES

Article

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF2016R1D1A1B01013707). This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (Grant 20174010201150).

(1) Brandt, A.; Ray, M. J.; To, T. Q.; Leak, D. J.; Murphy, R. J.; Welton, T. Ionic liquid pretreatment of lignocellulosic biomass with ionic liquid-water mixtures. Green Chem. 2011, 13, 2489−2499. (2) Dickmann, J. S.; Hassler, J. C.; Kiran, E. J. Modeling of the volumetric properties and estimation of the solubility parameters of ionic liquid + ethanol mixtures with the Sanchez-Lacombe and SimhaSomcynsky Equations of State: [EMIM]Ac + Ethanol and [EMIM]Cl + Ethanol Mixtures. Supercrit. J. Supercrit. Fluids 2015, 98, 86−101. (3) Reddy, P. A Critical Review of ionic liquids for the pretreatment of lignocellulosic biomass. S. Afr. J. Sci. 2015, 111, 11−12. (4) Satyanarayana, K. G.; Arizaga, G. G. C.; Wypych, F. Biodegradable composites based on lignocellulosic fibers - An Overview. Prog. Polym. Sci. 2009, 34, 982−1021. (5) Wang, Y.; Radosevich, M.; Hayes, D.; Labbé, N. Compatible ionic liquid-cellulases system for hydrolysis of lignocellulosic biomass. Biotechnol. Bioeng. 2011, 108, 1042−1048. (6) Vancov, T.; Alston, A.-S.; Brown, T.; McIntosh, S. Use of ionic liquids in converting lignocellulosic material to biofuels. Renewable Energy 2012, 45, 1−6. (7) Liu, C.-Z.; Wang, F.; Stiles, A. R.; Guo, C. Ionic liquids for biofuel production: opportunities and challenges. Appl. Energy 2012, 92, 406− 414. (8) Dadi, A. P.; Varanasi, S.; Schall, C. A. Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment step. Biotechnol. Bioeng. 2006, 95, 904−910. (9) Wang, H.; Gurau, G.; Rogers, R. D. Ionic liquid processing of cellulose. Chem. Soc. Rev. 2012, 41, 1519−1537. (10) Yim, J.-H.; Lim, J. S. CO2 Solubility measurement in 1-hexyl-3methylimidazolium ([HMIM]) cation based ionic liquids. Fluid Phase Equilib. 2013, 352, 67−74. (11) Blanchard, L. A.; Brennecke, J. F. Recovery of organic products from ionic liquids using supercritical carbon dioxide. Ind. Eng. Chem. Res. 2001, 40, 287−292. (12) Yim, J.-H.; Song, H. N.; Yoo, K-. P.; Lim, J. S. Measurement of CO2 solubility in ionic liquids: [BMP][Tf2N] and [BMP][MeSO4] by measuring bubble-point pressure. J. Chem. Eng. Data 2011, 56, 1197− 1203. (13) Grandelli, H.; Kiran, E. Volumetric properties of mixtures of ionic liquid [EMIM]Ac with CO2 as potential processing fluid for lignocellulosic materials at high pressures. Cartegena de Indies (Colombia) 2013, 1−5. (14) FitzPatrick, M.; Champagne, P.; Cunningham, M. F. The effect of subcritical carbon dioxide on the dissolution of cellulose in the ionic liquid 1-ethyl-3-methylimidazolium acetate. Cellulose 2012, 19, 37−44. (15) Minnick, D. L.; Scurto, A. M. Reversible and non-reactive cellulose separations from ionic liquid mixtures with compressed carbon dioxide. Chem. Commun. 2015, 51, 12649−12652. (16) Gu, T.; Held, M. A.; Faik, A. Supercritical CO2 and ionic liquids for the pretreatment of lignocellulosic biomass in bioethanol production. Environ. Technol. 2013, 34, 1735−1749. (17) Lopes, J. M.; Sánchez, F. A.; RodríguezReartes, S. B.; DoloresBermejo, M.; Á ngelMartín; JoséCocero, M. Melting point depression effect with CO2 in high melting temperature cellulose dissolving ionic liquids. Modeling with group contribution equation of state. J. Supercrit. Fluids 2016, 107, 590−604.

Figure 7. P−x1 diagram of CO2 solubilities of the [EMIM][MeSO4] + CO2 system for comparison of experimental data and literature data: (yellow triangle) [EMIM][MeSO4] + CO2 at 313.15 K (this study) and (white star) [EMIM][MeSO4] + CO2 at 313.3 K (literature data).40

experimental data and Shiflett et al.’s data39 at 323.15 K. Mejia et al.40 recently studied the ([EMIM][MeSO4] + CO2 system at 299, 313, and 333 K and under 9 MPa. Figure 7 shows the comparison of our experimental data with Mejia et al.’s work40 for the solubility of CO2 in the [EMIM][MeSO4] + CO2 system at 313 K. Our experimental data correspond well with the previously published literature data.

5. CONCLUSIONS In this manuscript, the experimental solubility data of CO2 in ionic liquid was measured to discover the effect of different anions on the CO2 solubility in [EMIM] cation-based ionic liquids to utilize IL + CO2 systems for biomass processing and modifications such as lignocellulosic material processing. The solubility of carbon dioxide in [EMIM][Ac], [EMIM][MeSO4], and [EMIM][Cl] was measured at the desired experimental condition. The temperature range was from 303.15 to 403.15 K, and the pressure range was from 0.45 to 48.6 MPa. The PR-EoS with the van der Waals one-fluid mixing rules was used for the correlation of the experimental solubility of CO2 in three ionic liquid data sets. Further, the modified Lydersen−Joback−Reid method was used for estimation of normal boiling temperature, acentric factor, and critical properties of the three ionic liquids. The experimental data results agree relatively well with the calculated results using the PR-EoS. Furthermore, the experimental solubility data of CO2 increase with an increase in pressure and a decrease in temperature. Also, the sequence of magnitude for the solubility of CO2 was [EMIM][Ac] ≫ [EMIM][MeSO4] > [EMIM][Cl]. Regarding the effect of anions, the acetate anion increased CO2 solubility, and the chloride anion decreased CO2 solubility in the [EMIM] cation-based ionic liquid system. In addition, for the reliability of our data, we compared our experimental data with previously published literature data. The result shows our experimental data correspond well with the published literature data.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 02 7058918; E-mail: [email protected]. ORCID

Jong Sung Lim: 0000-0002-1826-6216 Notes

The authors declare no competing financial interest. J



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K


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