Thermodynamic and Kinetic Studies of CO2 Capture by Glycol and

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data 2018, 63, 2671−2680

Thermodynamic and Kinetic Studies of CO2 Capture by Glycol and Amine-Based Deep Eutectic Solvents Mohd Belal Haider, Divyam Jha, Balathanigaimani Marriyappan Sivagnanam, and Rakesh Kumar* Department of Chemical Engineering, Rajiv Gandhi Institute of Petroleum Technology, Jais 229304, India

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S Supporting Information *

ABSTRACT: Anthropogenic CO2 emissions into the atmosphere are responsible for the global warming, therefore, it is essential to reduce these emissions at the source. Recently, deep eutectic solvents (DESs) have shown great potential to absorb the CO2. In the current study, 15 different types of amine- and glycol-based deep eutectic solvents were synthesized and investigated for CO2 absorption. In general, aminebased solvents have shown higher CO2 absorption as compared to glycol based solvents. In particular, the highest CO2 absorption was observed for the tetrabutyl ammonium bromide and methyldiethanol amine (TBAB/4MDEA) system having a CO2 solubility of 0.29 (mol CO2/mol solvent) at 1 MPa and 303.15 K. Thermophysical properties of all synthesized DESs were estimated using the modified Lydersen−Joback−Reid method and Lee−Kesler mixing rule. Experimental CO2 solubility data were well fitted using the nonrandom two liquid and the Peng−Robinson thermodynamic models. Apart, CO2 solubility data were correlated with Henry’s law, and Henry’s constant was calculated for all DESs. The kinetic modeling of CO2 absorption in DESs was also studied and rate constants were evaluated. Abbott et al. synthesized the first DES using choline chloride (ChCl) and urea with molar ratio of 1:2, with both of the components being less toxic and biodegradable.14 The most popular choice for the HBA is quaternary salts that are readily available and are inexpensive, as well as nontoxic. The properties of DESs can be tuned depending upon the application by changing the choice of HBD and HBA.16−18 Therefore, like ILs these DESs can be treated as designer solvents. DESs have been investigated in different areas of applications such as organic synthesis, nanotechnology, separation process, and many others.16,17,19−23 The possibility of DESs for the application of CO2 capture was first explored by Li et al. in the eutectic mixtures of choline chloride (ChCl) and urea with a molar ratio of (1:1.5), (1:2), and (1:2.5) at different temperatures and pressures.24 Their study showed that CO2 solubility in DESs varies with change in molar composition of HBA-to-HBD. Leron et al. studied the CO2 solubility in different combination of ChCl with urea, glycerol, and ethylene glycol and suggested that the nature of HBDs has a key role on the CO2 solubility in DESs.25−27 Ali et al. developed and studied CO2 solubility at 298.15 K and 1 MPa in 17 different ammonium and phosphonium-based DESs.28 The first task-specific DES system composed of ChCl, glycerol, and a superbase was reported by Sze et al.29 The superbase was used for formation of active alkoxide anions by deprotonation of the OH groups in glycerol and ChCl. The effect of water present in the DESs on CO2 solubility was studied by Su et

1. INTRODUCTION Anthropogenic carbon dioxide (CO2) emissions are considered to be a major cause of climate change.1,2 Carbon capture and storage (CCS) emerged as an important method to reduce CO2from point sources.2 Out of many technologies developed, CO2 capture by liquid solvent is considered to be the most mature with a technology readiness level (TRL) of 7.3,4 Both physical (purisol, rectisol, selexol) and chemical solvents (aqueous amine solutions) have been used for CO 2 absorption.2 Aqueous amine solutions (monoethanol amine, methyldiethanol amine) carry a major share in the industry to remove CO2 from flue gas.5,4 However, aqueous amines are not environment friendly due to their high vapor pressure, corrosive behavior, toxicity, partial degradation, and high energy requirement for solvent regeneration.4,6 Therefore, there is a need to search for a suitable alternative to the aminebased solvents. Ionic liquids (ILs) have shown great potential for replacing the aqueous amine solutions.4,6−8 ILs are considered to be green solvents because of their negligible volatility, high thermal and chemical stability, and tunability.7−11 However, ILs require complicated preparative technology which adds cost to ionic liquids, and they have poor biodegradability.9,12,13 Recently, deep eutectic solvents (DESs) sharing physicochemical properties of ILs but with lower cost, have been developed.14−18 DESs can be prepared by mixing a suitable molar ratio of hydrogen bond acceptor (HBA) with hydrogen bond donor (HBD).16,17 The formed mixture will have a melting point lower than individual components due to the hydrogen-bond network. DESs are economical and promising alternatives to ILs because of their ease of synthesis. © 2018 American Chemical Society

Received: January 5, 2018 Accepted: July 9, 2018 Published: July 20, 2018 2671

DOI: 10.1021/acs.jced.8b00015 J. Chem. Eng. Data 2018, 63, 2671−2680

Journal of Chemical & Engineering Data

Article

al.30 They observed that the solubility of CO2 in ChCl−urea (molar ratio 1:2) system decreases with increase in the water content. Sarmad et.al. investigated the CO2 solubility in a series of ammonium and phosphonium DESs at 298.15 K and pressure up to 2 MPa.31 They reported that 15 synthesized DESs have higher CO2 solubility than the conventional ILs and therefore DESs are one of the potential candidates for CO2 capture. Liu et al. studied CO2 solubility in Guaiacol-based DESs at various temperatures ranging from 293.15 to 323.15 K and pressures below 0.6 MPa.32 Their results showed that CO2 absorption in DESs follows a physical behavior. Chen et al. studied CO2 solubility in DESs composed of ChCl as HBA and dihydric alcohols as HBD.33 The highest CO2 solubility (∼0.15 mol/kg at 293.15 K and 0.51 MPa) was shown by ChCl/4(2,3butanediol) DES33 among different dihydric alcohol-based DESs,. Recently, Bhawna et al. studied the role of superbase in DESs for the CO2 capture ability.34 They found that the addition of superbases not only enhance the CO2 solubility due to lower steric hindrance but also help in attaining the equilibrium faster due to covalent association. According to Sarmad et al.,35 DESs have huge potential to replace conventional solvents for CO2 capture. In the present work CO2 solubility, in a series of ammonium based DESs, has been studied experimentally. The CO2 solubility was measured at a fixed temperature (303.15 K) and varying pressure (up to 1.2 MPa). To understand and predict the vapor liquid equilibrium of the CO2-DES system, the experimentally measured CO2 solubility was correlated using the Peng−Robinson equation of state and the nonrandom two liquids (NRTL) thermodynamic model. In addition, the kinetics of CO2 absorption for in-house synthesized DESs was also studied and rate constants were evaluated.

acceptor (HBA). The mixture was heated at 348.15 K for a period of 3 h under constant agitation of 500 rpm until a homogeneous colorless liquid was formed. The moisture present in DESs was removed by drying DES overnight at 333.15 K under vacuum.29 The molar composition and abbreviation used for the synthesized DESs are shown in Table 2. Table 2. Deep Eutectic Solvents Synthesized in This Worka hydrogen bond acceptor choline chloride (ChCl)

tetrabutyl ammonium bromide (TBAB)

chemical name

source

CASRN

choline chloride tetrabutyl ammonium bromide ethylene glycol diethylene glycol diethanol amine methyl diethanolamine carbon dioxide

SD Fine Chem Ltd. SD Fine Chem Ltd.

67-48-1 1643-19-2

≥98 ≥99

N/A N/A

Merck SPECTROCHEM SD Fine Chem Ltd. Sigma-Aldrich

107-21-1 111-46-6 111-42-2 105-59-9

≥99 ≥99 ≥98 ≥99

≤0.30 ≤0.10 ≤0.20 ≤0.30

Sigma Gases

124-38-9

≥99.9

N/A

ethylene glycol (EG) diethylene glycol (DEG)

diethanol amine (DEA) ethylene glycol (EG) diethylene glycol (DEG) methyldiethanol amine (MDEA) diethanol amine (DEA)

name

ChCl/2EG

DES1

ChCl/3DEG ChCl/4DEG ChCl/6MDEA ChCl/7MDEA ChCl/6DEA

DES2 DES3 DES4 DES5 DES6

TBAB/2EG TBAB/3EG TBAB/4EG TBAB/2DEG TBAB/3DEG TBAB/4DEG TBAB/3MDEA TBAB/4MDEA TBAB/6DEA

DES7 DES8 DES9 DES10 DES11 DES12 DES13 DES14 DES15

a

Standard uncertainty u in compositions (x) in moles of DES mixtures of u(x) = 0.005.

2.3. Characterization. A Mettler Toledo (NewClassic MS) balance was used to measure the mass. The density of the samples was measured using a portable density meter (Kyoto Electronics, Japan, DA-130N). The water contents of all DESs were measured using Metrohm Karl Fisher Titrator (852 titrando) and were found to be ≤0.003 (mass fraction). Differential scanning calorimetry (DSC 4000 PerkinElmer) was used to measure the freezing point of the DES. FTIR analysis of the synthesized DES was done using PerkinElmer Spectrum-2 spectrometer. All samples were scanned over a wavenumber range of 450−4000 cm−1. The quality of the background signal was checked before each measurement. 2.4. CO2 Solubility Measurements. The details of experimental setup has been reported in our previous work.36 CO2 solubility apparatus consists of a gas reservoir attached to a stainless steel high pressure equilibrium cell of capacity 25 mL. Pressure transducers (DiGi gauge TX-430, 0− 6.5 MPa, accuracy = 0.25%) were attached to measure the pressure inside the equilibrium cell and gas reservoir. Constant stirring of 300 rpm was provided to the equilibrium cell using a magnetic stirrer to ensure the uniformity of gas in the solvent. The entire system was maintained at a constant temperature of 303.15 K using an IKA-make chiller. In a typical experiment, a gas reservoir was degassed using vacuum pump. CO2 gas was then introduced into known volume of reservoir and initial pressure was recorded using pressure gauge. The initial number of moles of CO2 was calculated by the following PVT relation:

Table 1. List of Chemicals Used for DESs Synthesis water content (%)

molar composition

methyldiethanol amine (MDEA)

2. EXPERIMENTAL SECTION 2.1. Materials. Choline chloride (≥98%), tetrabutyl ammonium bromide (≥99%), ethylene glycol (≥99%), and diethylene glycol (≥99%), were purchased from S D FineChem Limited. Methyldiethanol amine (≥99%), was purchased from Sigma-Aldrich Pvt. Ltd. Carbon dioxide was supplied by Sigma Gases Pvt. Ltd. (≥99.9%). All materials were used for the preparation of DESs without further purification. The list of chemicals used along with their purities are listed in Table 1. 2.2. Synthesis of DESs. The procedure for synthesis of DESs was similar to that described by Abbott et al.14 In general, DESs were synthesized by combining the fixed molar ratios of hydrogen bond donor (HBD) and hydrogen bond

purity (%)

hydrogen bond donor

i nCO = 2

2672

PV i res i ZCO RTi 2

(1) DOI: 10.1021/acs.jced.8b00015 J. Chem. Eng. Data 2018, 63, 2671−2680


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where Pi and Ti represent initial pressure and temperature respectively, and Vres, ZCO2 represent the volume of reservoir, compressibility factor of CO2 at Pi,Ti. Thereafter, known volume of DES was loaded into equilibrium cell and the system was degassed using a vacuum pump. Then CO2 was fed to equilibrium cell using a valve, which connects the reservoir, and equilibrium cell. The pressure of the equilibrium cell was recorded periodically which gradually reduces due to CO2 dissolution in the DES. A constant stirring of the mixture was done using a magnetic stirrer during the CO2 absorption and the system was allowed to reach equilibrium conditions (∼6 h). The equilibrium point was observed when the pressure of the system becomes stable. The pressure was measured at the equilibrium condition and the moles of CO2 left in the gas phase were calculated by the following equation: equil nCO = 2

Pc =

(7)

M 0.2573 + (∑ niΔPMi)2

(8)

∑ niΔVMi

(9)

where M is the molar mass (g/mol) of the molecule and ni represents the number of appearances of the ith group in the molecule. ΔTbM, ΔTM, ΔPC, ΔVM represent the contribution of group of atoms in the molecule to their boiling point (K), critical temperature (K), critical pressure (bar), and critical molar volume (cm3/mol), respectively. After the prediction of critical properties of HBD and HBA, the Lee-Kesler43 mixing rule was used to estimate the critical properties of DESs. The equations used are as follows; TCM =

(2)

Vt here is total volume of system, VDES represents the volume of deep eutectic solvent fed to the equilibrium cell, Pf represent the equilibrium pressure reached, and ZfCO2 represents the compressibility factor of CO2 at Pf and Ti. The difference between CO2 mole values corresponds to the amount of gas absorbed in the DES. The number of moles CO2 absorbed in the DESs is calculated by abs i equil nCO = nCO − nCO 2 2 2

0.5703 + 1.0121 ∑ niΔTMi − (∑ niΔTMi)2

Vc = 6.75 +

Pf (Vt − VDES) f ZCO RTi 2

Tb

Tc =

1

∑ ∑ xixjVC1/4TC

VC1/4 M

ij

i

ij

(10)

j

where TCij = kij*(TCi*TCj)1/2

(3)

VCij =

1 1/3 (VCi + VC1/3 )3 j 8

VCM =

∑ ∑ xixjVC

(11)

(12)

ij

i

(13)

j

And

3. MODELING OF CO2 ABSORPTION 3.1. Kinetic Modeling. Kinetics of the CO2 absorption process in DESs was studied assuming physical absorption. For the physical absorption process, the rate of absorption at any time t can be written as37 dn t = k(ne − nt ) dt

PCM =

ωM =

∑ niΔTbMi

∑ xiωi

(15)

I

(4)

3

TCM (K), PCM (bar), VCM (cm /mol) represent the critical temperature pressure and volume of DESs, respectively. ωM and kij represent the acentric factor and binary interaction parameter of DESs. The value of kij range from 1.0 to 1.3 for mixture of nonpolar substances and for polar substances value ranges from 0.95 to 1.06.43 In the present study, because of the absence of any experimental data, the kij value is assumed to be unity for all calculations. 3.2.2. Thermodynamic Models. The CO2 solubility in DESs was modeled by the Peng−Robinson equation of state and NRTL method. The Peng−Robinson equation44 of state (PR-EOS) is given by

(5)

3.2. Thermodynamic Model. 3.2.1. Critical Properties Estimation. To calculate the thermodynamic properties, critical properties of the DESs were first estimated. DESs comprise HBD and HBA; therefore, individual properties of DESs precursors were evaluated using the modified Lydersen− Joback−Reid (LJR) method.38 The modified LJR method was developed by Valderrama et al.38−40 by combining the Lydersen41 and Joback−Reid42 methods. The modified LJR method is specifically defined for high molecular weight compounds and was used to evaluate the critical properties of ionic liquids.38,40 The following equations were used to estimate the critical properties of HBD and HBA: Tb = 198.2 +

(14)

where

where ne and nt represent the number of moles of CO2 absorbed at equilibrium and at time t, respectively, and k represents the rate constant, respectively. Integration of the above equation reduces to the following expression: ij n − ne yz zz = kt lnjjj t j n0 − ne zz k {

(0.2905 − .085ωCM )RTCM VCM

P=

RT a − v−b v(v + b) + b(v − b)

(16)

where a and b are the binary interaction parameters and are the functions of critical properties a=

0.45724(RTC)2 [1 + m(1 − Tr0.5)]2 PC

b = 0.0778

(6) 2673

RTC PC

(17)

(18) DOI: 10.1021/acs.jced.8b00015 J. Chem. Eng. Data 2018, 63, 2671−2680

Journal of Chemical & Engineering Data m = 0.37464 + 1.5422ω − 0.2699ω 2

Article

(19)

3.2.3. Nonrandom Two Liquids. The equation for NRTL model is ln γi =

∑j xjτjiGji ∑k xkGki

+

∑ j

ij y jjτ − ∑m xmτmjGmj zzz jj ij z ∑k xkGkj zz ∑k xkGki j k { xjGji

(20)

where Gij = exp( −αijτji) τji = aij +

bij T

+ eij ln(T ) + fij T

(21)

(22)

αij = cij + dij(T − 273.15)

(23)

τii = 0

(24)

Gii = 1

(25)

Figure 2. FTIR spectra of ethylene glycol and DESs.

where aij, bij, eij, and f ij are interaction parameters. 3.2.4. Henry’s Law Constant. The physical absorption of CO2 in DESs can be correlated using Henry’s law as given by the following expression; H = lim

x→0

f liq (p , T , x) x

(26)

where H is Henry’s law constant, x is mole fraction of CO2 dissolved in DESs and f liq is the fugacity of CO2 absorbed in the DESs.

4. RESULT AND DISCUSSION 4.1. Phase Diagram of DES. Mixing of the substituent according to the molar ratios listed in Table 1 result in the

Figure 3. Solubility of CO2 in DES 1 (ChCl:2EG).

Figure 4. Solubility of CO2 in ChCl:3DEG and ChCl:4DEG.

formation of a clear homogeneous liquid with a decrease in freezing point due to hydrogen bonding. The images of all

Figure 1. Freezing point of choline chloride with (a) ethylene glycol and (b) diethanolamine. 2674



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Figure 8. Solubility of CO2 in DES6 and DES15. Figure 5. Solubility of CO2 in TBAB/xEG.

Figure 9. FTIR Spectra of DES4 (a) before CO2 absorption (b) after CO2 absorption.

Figure 6. Solubility of CO2 in TBAB/xDEG.

Figure 7. Solubility of CO2 in ChCl/xMDEA.

synthesized DES have been shown in the Supporting Information as Table S1. Phase diagrams of choline chloride with ethylene glycol and choline chloride with diethanolamine were established by measuring the freezing points (temperature at which liquid begins to solidify) of mixtures using differential scanning calorimetry (DSC 4000 PerkinElmer) as shown in the Figure 1. A eutectic point could be observed for

Figure 10. FTIR Spectra of DES6 (a) after CO2 absorption; (b) before CO2 absorption.

DES1 and DES6 at molar ratios of 2 and 6, respectively. The freezing points of eutectic solvents are considerable lower than 2675



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shown in the Figure 2. The absorption band of ethylene glycol at 3275 cm−1, which is attributed to the stretching of −OH group moved toward the higher wavenumber region of 3345 cm−1. This could be ascribed to the formation of hydrogen bonds which confirms the formation of DES.16 Similar behavior was observed in the case of TBAB/EG in which the −OH stretching shifted from 3275 to 3382 cm −1 confirming the formation of a hydrogen bond. The FTIR spectra of other synthesized DESs are shown in the Supporting Information as Figures S1 to S5. 4.3. CO2 Solubility in DES. All the CO2 absorption experiments were conducted at a constant temperature of 303.15 K and pressure range from 0.1 to 2.0 MPa. First of all, CO2 solubility was measured in DES1 compared with the reported literature27 as shown in Figure 3. The experimentally measured solubility of CO2 in DES1 was found in good agreement with the reported one. Further, experiments were conducted for our own synthesized DESs. 4.3.1. CO2 Solubility in Glycol-Based DESs. ChCl/GlycolBased DESs. The CO2 solubility of DES1, DES2, and DES3 are shown in the Figure 4. It was observed that CO2 solubility decreases by changing the HBD from ethylene glycol to diethylene glycol. The lower CO2 solubility in DES2 and DES3 could be due to over saturation of hydrogen bond donor group in the ChCl/DEG system.44 Strong hydrogen bond donor selfinteractions are responsible for a decrease in the free volume.31,46 The decrease in the free volume causes the lower CO2 solubility in the ChCl/DEG-based DESs.47A slight enhancement in CO2 solubility was observed upon increasing the mole ratio of DEG from 2 to 3. It was found that the increase in HBD/HBA molar ratio increases the CO2 solubility in DES due to the strength of hydrogen bond decreases in DES.23,31 CO2 solubility follows a linear trend with the pressure in DES1, DES2, and DES3. Therefore, ChCl-based DES of ethylene glycol and diethylene glycol can be considered as the potential physical solvents. TBAB/Glycol-Based DESs. Further, CO2 solubilities in TBAB/EG and TBAB/DEG-based DESs were investigated as shown in the Figure 5 and Figure 6, respectively. An increase in the molar ratio of HBD in DESs considerably increases the CO2 solubility as shown in Figures 5 and 6, which means CO2 solubility depends on the type of HBD and salt/HBD ratio. On

Figure 11. Solubility of CO2 in TBAB/xMDEA.

Figure 12. Solubility of CO2 in TBAB/xDEG.

either of their constituents (mp choline chloride = 575.15 K, ethylene glycol = 260.25 K, and diethanolamine = 301.15 K). This depression in melting point arises due to interaction between the choline chloride and the HBD (ethylene glycol and diethanolamine) group.45 4.2. FTIR Spectra of Synthesized DESs. The FTIR spectra of ethylene glycol and synthesized DES1and DES7 are

Table 3. Estimated Critical Properties and Density of DESs Measured at 298.15 K Temperature and at p = 0.1 MPaa DESs

TC (K)

VC (cc/mol)

PC (MPa)

ω

density calcd (kg/m3)

density expt (kg/m3)

absolute percentage error

DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9 DES10 DES11 DES12 DES13 DES14 DES15

601.99 658.57 661.21 671.11 672.15 693.44 655.36 637.84 628.05 712.32 700.81 694.28 709.25 702.84 717.72

259.70 342.457 336.21 396.67 395.77 384.11 420.90 356.01 318.72 528.73 471.33 437.89 537.05 506.43 458.85

4.156 3.375 3.432 2.883 2.887 3.008 2.752 3.153 3.459 2.352 2.581 2.743 2.258 2.362 2.589

0.9155 0.9684 0.9822 1.0396 1.0453 1.0925 0.9518 0.9620 0.9681 0.9816 0.9956 1.0039 1.0319 1.0427 1.1080

1219.3 1207.9 1212.9 1114.5 1114.1 1047.1 1191.8 1209.4 1220.0 1172.2 1187.3 1196.4 1096.3 1099.3 1035.3

1116.8 1116.9 1112.1 1046.8 1053.8 1098.4 1065.8 1069.8 1072.7 1075.4 1078.2 1081.8 1051.3 1056.9 1092.7

09.18 08.15 09.06 06.47 05.72 04.68 11.83 13.05 13.74 08.99 10.12 10.59 04.28 04.01 05.25

a

Standard uncertainty of temperature is u(T) = 0.2 K, u(p) = 10 kPa and relative standard uncertainty ur are ur(ρ) = 0.01. 2676



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Table 4. Calculated NRTL Parameters for the DES-CO2 System component i

component j

aij

aji

bij

bji

cij

DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9 DES10 DES11 DES12 DES13 DES14 DES15

CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2

0.212 0.630 2.279 1.142 −1.468 3.545 0.808 1.834 −0.007 2.98 2.667 1.858 363 −0.654 2.497

4.81 0.363 −0.492 −0.611 1.285 −1.404 0.38409 −0.481 5.047 −1.197 −1.1557 −0.983 −0.636 −0.218 8.031

−14.325 −24.325 −18 −24.45 −24.36 −27 −11.21 −17.324 −30.45 −12.312 −21.54 −14.23 20.45 −17.542 23.368

21.52 −28.45 −18 −29.78 −30.23 −27 −17.23 12.123 21.36 −16.245 −12.345 −16.321 −16.54 −12.315 −288.041

0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Figure 13. Kinetics of CO2 solubility in DESs. Figure 14. Fugacity vs mole fraction of CO2 dissolved in DESs.

Table 5. Calculated Rate and Henry’s Constants DESs DES1 DES2 DES3 DES4 DES5 DES6 DES7 DES8 DES9 DES10 DES11 DES12 DES13 DES14 DES15

molecular weight (g/mol) 87.921 114.496 112.821 122.086 121.720 110.066 148.845 127.242 114.145 178.204 156.024 149.370 169.965 159.804 136.173

Henry’s Law constant (MPa) 21.278 29.246 28.435 10.324 06.162 07.155 20.549 21.236 23.002 08.947 09.579 09.379 04.456 04.342 06.299

rate constant (sec‑1) 3.49 3.54 3.64 6.35 4.90 2.43 2.97 3.31 3.81 2.87 3.23 3.79 4.24 4.56 2.70

× × × × × × × × × × × × × × ×

in ChCl/amine-based DESs is due to amine affinity toward CO2. This is also evident from the fact that aqueous MDEA has significantly higher CO2 solubility as compared to pure DEG, as shown in the Supporting Information, Figure S6. Increasing the molar ratio of ChCl/MDEA from 1:6 to 1:7 does not show significant effect on CO2 solubility. A similar trend was also observed for CO2 solubility in the DEA-based DESs as shown in the Figure 8. The FTIR spectra of DES4 and DES6 before and after CO2 absorption are shown in Figure 9 and Figure 10, respectively. The CO2 solubility in MDEAbased DESs show physical absorption behavior as evident from their FTIR spectra. As evident from Figure 9, after the absorption of CO2 a band at 2340 cm−1 can be observed which is attributed to asymmetric OCO stretching.36 The remaining spectra in the case of DES4 remained unaltered which meant the absorption of CO2 in DES4 is physical. However, in the case of DES6 new peaks in the fingerprint region, specifically at 1441 cm−1 can be observed which is attributed to CO vibration of carbamate species, thus confirming the reactive nature of CO2 absorption in DES6.48,49 This can be explained based on the molecular structure of MDEA and DEA. MDEA is a trisubstituted amine and hence does not form carbamates since there is no hydrogen which can be replaced by the CO2. However, DEA being a secondary amine absorbs CO2 by the formation of carbamic acids.49 TBAB/Amine-Based DESs. CO2 solubility in TBAB//DEA and TBAB/MDEA based DESs are shown in Figure 8 and 11, respectively. CO2 uptake in TBAB/amine-based DESs

−03

10 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03 10−03

changing the TBAB/DEG molar ratio from 1:2 to 1:4, CO2 solubility increases by 1.3 times. Furthermore, it was observed that for same TBAB/4HBD molar ratio, DEG-based HBD has a higher (∼2.2 times) CO2 solubility as compared to the EG based HBD. 4.3.2. CO2 Solubility in Amine-Based DESs. ChCl/AmineBased DESs. The solubility of CO2 in ChCl/amine-based DESs have been shown in Figures 7 and 8. It was found that ChCl/amine-based DESs have more CO2 uptake as compared to ChCl/glycol-based DESs (Figure 4). Higher CO2 solubility 2677



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Figure 15. Comparison of CO2 solubility in synthesized DESs with previously reported DESs at 303 K and 0.5 MPa (solid and partially shaded line show CO2 solubility in the present work and previously reported work, respectively).

4.5. Kinetic Parameters. The kinetic behavior of some DESs is shown in Figure 13. The calculated value of the apparent rate constant is tabulated in the Table 5. It can be observed from the figure that glycol-based DESs normally have lower apparent rate constant as compared to amine-based DESs. The lower apparent rate constant in glycol implies that the rate of CO2 absorption in glycol-based DESs is slow as compared to amines-based DESs. The CO2 absorption rate was found to be the highest in DES4 (∼6.35 × 10−3) and lowest in DES6 (2.43 × 10−3) among the ChCl-based DESs. Similarly, in TBAB-based DESs; highest apparent rate constant was found in the DES14(∼4.56 × 10−3) and the lowest in the DES15(∼2.70 × 10−3). Furthermore, with increase in HBA/ HBD molar ratio, the apparent rate constant also increases for the same HBA and HBD. A notable exception was DES6 and DES7 where an increase in HBD molar ratio decreases the apparent rate constant. The rate kinetics depends upon the density, viscosity, reactivity, and equilibrium solubility. Therefore, an in depth study of these parameters is important to understand the behavior of apparent rate constant in various DESs. To determine the Henry’s law constant; eq 26 was fitted to first order polynomials as shown in the Figure14. The Henry’s law constant obtained for various synthesized DESs has been given in Table 5. The lower is the value of the Henry’s law constant, the greater will be the solubility of CO2. It was observed that amine-based DESs have a lower value of Henry’s law constant compared to glycol-based DESs, which results in higher CO2 absorption in amine-based DESs compared to glycol-based DESs. Also, TBAB-based DESs have higher solubility compared to ChCl-based DESs. The lowest Henry’s law constant was found to be 4.342 MPa for DES14 and highest for DES2 (∼29.25 MPa). Therefore, the highest CO2 uptake is for DES14 and lowest for DES2. Furthermore, Henry’s law constant decreases with increased HBD concentrations in DESs which suggests that increasing HBD concentration would enhance the CO2 uptake of the DESs. The DES synthesized in the current work has been compared with the DES reported in the literature, as shown in Figure 15.29,33,50,51 The result obtained for CO2 solubility for choline-based DESs are comparable to the previously reported data.

increases almost to three fold compared to TBAB/glycol-based DESs due to greater amine affinity toward CO2 molecules. Also, increasing the molar ratio of MDEA increases the CO2 solubility as shown in the Figure 11. The increased solubility on increasing the MDEA molar ratio was reflected due to amine affinity toward CO2. Like ChCl/MDEA-based DESs, TBAB/MDEA-based DESs also show physical absorption, whereas TBAB/DEA based DESs show chemisorption behavior as explained earlier. The FTIR spectra of other DESs are given in the Supporting Information. 4.3.3. Effect of Nature of HBAs. Nature of the hydrogen bond acceptor (ChCl and TBAB) was also studied as shown in the Figure 12. The increased CO2 solubility in TBAB-based DESs can be explained due to increase in free volume. The increase in alkyl chain length in TBAB-based DESs consequently increases free volume resulting in enhanced CO2 solubility as compared to ChCl.47 The similar behavior can also be seen in the Figure 8 where DES15 has more CO2 solubility compared to DES6. 4.4. Thermophysical Properties and VLE Prediction. The calculated critical temperature, pressure, volume, and acentric factor are shown in Table 3. The densities of DESs were also calculated and found to be in good agreement with the experimentally measured densities. The highest deviation was shown by the TBAB/DEG based DESs. The estimation of critical properties is required for calculation of binary interaction parameters in the Peng−Robinson equation of state. The vapor−liquid equilibrium behavior of CO2-DESs system were modeled using both Peng−Robinson equation of state and NRTL model. The DESs were assumed to be nondissociating compounds with negligible vapor pressure. The experimental data with goodness of fit for PR-EOS and NRTL has been shown in the Supporting Information Figure S7. Both property methods (NRTL as well as PR-EOS) well correlate the experimental solubility data of CO2. However, the NRTL model was found better compared to PR-EOS, showing lower standard deviations. The binary interaction parameters obtained after fitting the experimental data for the NRTL model are tabulated in Table 4. The tabulated binary interaction parameters for Peng−Robinson model are given in Table S2. 2678



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5. CONCLUSIONS In the present study, we investigated the CO2 absorption capacity in 15 different types of DESs at 30 °C and at varying pressure. In general, it was found that CO2 solubility in aminebased DESs is greater compared to that in glycol-based DESs. Moreover, the highest CO2 solubility (0.29 mol CO2/mol solvent) was shown by DES14 at 1.0 MPa and 303.15 K. Experimental CO2 solubility data were well fitted using the NRTL and Peng-Robinson thermodynamic models. Henry’s constant was calculated for all DESs and the lowest value (4.342 MPa) was obtained for DES14. However, the apparent rate constant was found to be highest in DES4.

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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.8b00015. FTIR spectra of DESs, images of synthesized DESs, experimental data fitting curve, and PR-EOS binary interaction parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 9450352043. E-mail: [email protected]. ORCID

Mohd Belal Haider: 0000-0002-3228-391X Funding

We would like to thank Rajiv Gandhi Institute of Petroleum Technology (RGIPT), RaeBareli and Science and Engineering Research Board (SERB) (SB/FTP/ETA-285/2012), New Delhi, India, for providing financial support. Notes

The authors declare no competing financial interest.

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