Validation and prediction of solubility parameters of ionic liquids for CO2 capture

Separation and Purification Technology 97 (2012) 51–64

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Validation and prediction of solubility parameters of ionic liquids for CO2 capture Yamini Sudha Sistla, Lucky Jain, Ashok Khanna ⇑ Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India

a r t i c l e

i n f o

Article history: Available online 17 February 2012 Keywords: CO2 capture Ionic liquids Solubility parameter Molecular simulations Group contribution correlation

a b s t r a c t The present study attempts to screen a number of ionic liquids (ILs) for CO2 capture and its separation from flue gas mixture based on Hildebrand solubility parameter (d). Solubility parameters of various ILs have been computed through molecular dynamic simulations in Material Studio. Initially the density and solubility parameters of seven ILs have been validated with experimental values available in the literature and are found to be in close correspondence. A group contribution correlation for d of IL is developed and expressed as the weighted sum of attractions and repulsions of cation and anion as: d2IL ¼ 0:6dca dan  0:2d2ca  0:2d2an . This correlation was used to find out the d values of cation and anion. The solubility parameters of 21 cations and 10 anions have successfully been determined by using d values of 31 ILs and the correlation. Furthermore, using d values of 21 cations and 10 anions and the group contribution correlation, the d values of 210 ILs have been predicted. The d values obtained by correlation match well with the d values obtained by direct molecular simulations for twenty seven ILs, suggesting that correlation equation is reliable to predict the d values of several ILs. Furthermore, comparison of the literature solubility data of different gases in ILs with that of solubility parameters in the present study reveals that the d values are able to successfully explain the experimental solubility trends. Of all ILs studied, d values of phosphonium cation based ILs (d = 20.7–18.3 MPa0.5) and ILs containing tris(nonafluorobutyl)trifluorophosphate ([bFAP]) anion (d = 20.5–18.3 MPa0.5) are observed to be closer to that of CO2 (d = 17.85 MPa0.5) and are significantly far from that of other flue gases like CH4, N2, H2 and H2O. Thus, by using the fluorinated anion and phosphonium cation based ILs, CO2 can be more selectively separated from the major flue gases. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Rising earth’s temperature due to increasing emissions of greenhouse gases such as carbon dioxide (CO2), water vapor, nitrous oxide and methane is of major concern today. Of all the culprit gases, CO2 has major share towards global warming. The major industries producing CO2 rich flue gas include coal fired power plants, vehicles, manufacturing plants for cement, limestone, ammonia [1]. The flue gas from coal fired power plants mainly consist of 13% CO2, 68% N2, 16% H2O, 3% O2 and lower concentrations of other gases like H2S, CH4, NOx and SOx [2]. Since the amount of CO2 is large, there is a high need to efficiently separate and capture CO2 from a flue gas mixture. A typical biogas consists of around 60% methane and 40% CO2. This high percent of CO2 need to be removed in order to improve the efficiency of a biogas based power plant. Many industries are using amine scrubbing process to treat acid gases like CO2, H2S and SO2. Amines absorb CO2 through chemical reaction by forming carbamate [3]. In spite of their good CO2 solubility, amines are highly corrosive due to their basic nature and have low thermal stability, which reduces the reusability of ⇑ Corresponding author. Tel.: +91 512 2597117; fax: +91 512 259 0104. E-mail address: [email protected] (A. Khanna). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2012.01.050

these amines and making the desorption process difficult at large scale. These drawbacks along with high heat of dissolution with CO2 make the amine scrubbing process highly energy intensive. Blanchard et al. [4] in 1999 reported the solubility of CO2 in the ionic liquid (IL), [bmim][PF6], initiating the scope for ILs as promising solvents for CO2. The ILs having large organic cations and small inorganic/organic anions are known for their non volatility, high liquid range, high thermal stability and tunability. The properties of ILs depend on the type of cation and anion and thus ILs are regarded as ‘‘designer solvents’’ [4,5]. The solubility of CO2 in ILs is generally through physical absorption involving the weak association between CO2 and IL, although some of the ILs absorb through chemical reaction. The low heat/enthalpy of absorption of CO2 in ILs (around 12 kJ/mol) adds another advantage, i.e., minimum energy requirement for solvent regeneration [6,7]. The ILs can also absorb CO2 selectively over the other gases such as CH4, N2 and O2. Finotello et al. [8] have reported the experimental Henry’s constants of CO2, N2 and CH4 in IL mixtures. The Henry’s constant was observed to be lowest (i.e., highest solubility) for CO2, followed by CH4 and N2. Anderson et al. [9] studied solubility of various flue gases in different ILs. It was reported that the solubility of CO2 and SO2 is much higher compared to that of other gases such as CH4 and O2. This was attributed to the increase in degree of

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ordering with CO2 and SO2 in ILs compared to that of CH4 and O2. Many researchers reported experimental data on the solubility and selectivity of CO2/CH4 and CO2/N2 in ionic liquids [10–20]. Bates et al. [21] developed ‘‘task specific ionic liquids’’ by tethering amine functional groups to imidazolium ion to improve the CO2 solubility and found that amine tethered imidazolium ILs exhibit higher CO2 solubility compared to imidazolium ILs. It was observed that the absorption of CO2 in amine tethered ILs is through chemical reaction with the formation of carbamate. Thus, amine tethered ILs can be used for the increased CO2 separation from a flue gas mixture. However, the main limitation of these ILs is their high viscosity, which further increases with dissolved CO2. The search for most prominent ILs for CO2 capture and its effective separation from other gases such as CH4, N2 and water vapor, acid gases like H2S, NOx and SOx is still under progress. Because of the availability of thousands of ionic liquids, theoretical methods like activity coefficient models, group contribution methods, COSMO-RS and molecular dynamic simulations can successfully be used to predict various physical properties in order to reduce the cost of experimentation [22]. Activity coefficient models and group contribution methods need some prior experimental results to optimize their parameters. The quantum mechanical methods like COSMO-RS also need some preliminary experimental results in order to optimize some of the parameters of the code. Whereas, molecular dynamic studies do not require prior experimental data for the estimation of physical properties, thus effectively reducing the experimental costs for screening of various solvents. Besides, molecular simulations can successfully predict various physical properties of liquid systems [20,23]. The solubility parameter can be used as an effective screening tool for selecting the most preferable solvent as it works on the basic principle ‘‘like dissolves like’’ [24,25]. A solute is considered to be soluble in a solvent when the solubility parameters of solute and solvent approach each other. This means the closer the solubility parameter of a solute to the solubility parameter of a solvent, the higher will be the solubility of that solute in that particular solvent. Since different gases have different solubility in ILs, the above principle can be used to find out an IL which can selectively separate CO2 from a mixture of flue gases. Thus, the present study aims to discover an IL, whose solubility parameter is close to that of CO2 and far from that of other flue gases. However, the negligible vapor pressure of ILs makes it difficult to experimentally determine the solubility parameters of ILs. Various experimental and theoretical methods are available for the estimation of solubility parameters. Experimentally, the solubility parameter can be estimated by direct and indirect methods [26]. The direct methods include measuring the heat of vaporization by calorimetry or measuring the solubility/miscibility of compounds of interest in solvents of a known solubility parameter. However, these direct methods are difficult to employ for ILs since ILs are thermally very stable compounds with negligible vapor pressure. Solubility parameter estimation can also be achieved by indirect experimental methods: inverse gas chromatography (IGC), using melting temperatures of ILs; using intrinsic viscosity measurements, or by using the activation energy of viscosity [24,25]. Solubility parameters can also be estimated by using theoretical methods, which include: PC-SAFT and Non-random Hydrogen Bonding (NRHB) models, Regular Solution Theory, and lattice energy model [27–30]. Some of these theoretical methods do require some parameters based on the experimental data. Furthermore, the available solubility parameters of ILs by using the above methods are very limited. On the other hand, molecular simulations are one of the convenient tools to predict solvent properties of ionic liquids and do not need any experimental data for their estimation [31]. Thus the present work uses commercially available software Accelrys Material Studio [32] to compute the solubility parameters

of various ILs through molecular simulations. A correlation is also developed to obtain the solubility parameters of large number of ILs based on the solubility parameters of only few ILs. A comparison is made between the solubility parameters of ILs with that of gases like CO2, CH4, N2, H2, NOx and SOx for the purpose of CO2 capture alone as well as for the selective CO2 separation from a mixture of flue gases. 2. Theory section 2.1. Solubility parameter theory and background [33] Solubility parameter d is defined as the square root of the cohesive energy density. Cohesive energy E is the energy required to break the interactions between molecules. The relation between Hildebrand solubility parameter and cohesive energy E and molar volume Vm is given as:

sffiffiffiffiffiffiffi   E DHv  RT 0:5 d¼ ¼ Vm Vm

ð1Þ

where DHv the heat of vaporization, and RT is an ideal gas PV term. The process of mixing of a solute in a solvent is spontaneous when the Gibbs free energy change of mixing DGmixing is zero or negative. Free energy change for the solvation process can be given by the relation

DGmixing ¼ DHmixing  T DSmixing

ð2Þ

where, DHmixing is the heat of mixing and DSmixing is the entropy change of mixing. Hildebrand and Scott [33 and references therein] proposed the heat of mixing by the relation

DHmixing ¼ /1 /2 V m ðd1  d2 Þ2

ð3Þ

where /1 and /2 are the volume fraction of solute and solvent, respectively. d1 and d2 are the solubility parameters of solute and solvent, respectively. According to Eq. (3), only positive heat of mixing is allowed, which has been cited as drawback of this theory. Patterson and Delmas [34] showed that right hand side of the Eq. (3) is the non-combinatorial (residual) term of the free energy of mixing, not the total free energy of mixing. Therefore Eq. (3) can be redefined as: 2 DGmixing noncomb ¼ /1 /2 V m ðd1  d2 Þ

ð4Þ

The residual free energy of mixing includes all energy terms and the combinatorial term includes the entropy terms of mixing. Now, the total free energy of mixing can be written as the sum of residual and combinatorial terms:

DGmixing ¼ DGmixing  T DSmixing res comb

ð5Þ

By combining the two Eqs. (4) and (5), free energy of solution can be written as

DGmixing ¼ /1 /2 V m ðd1  d2 Þ2  T DSmixing comb

ð6Þ

For a mixing process to be spontaneous, total free energy of mixing must be either zero or negative. More negative the value of total free energy of mixing more will be the solubility of solute in solvent. For total free energy of mixing to be highly negative, solubility parameters of solute and solvent must be close to each other. The combinatorial entropy of mixing is always positive for a mixing process. When the residual term is zero, the combinatorial entropy term, will take part in total free energy of mixing. It can be clearly seen from Eq. (6) that combinatorial entropy change multiplied by absolute temperature is an important term for mixing. So higher the temperature, more negative will be total free energy of mixing and more will be the solubility. But increasing the

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temperature does not always lead to high solubility. In some cases, increasing the temperature leads to insolubility. The maximum value of free energy for a spontaneous mixing process is zero. In that case, difference between the solubility parameters of solute and solvent will be maximum. This will be the upper bound of the difference in solubility parameters for mixing to be spontaneous

jd1  d2 jmax

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T DSmixing ¼ /1 /2 V m

53

role in the calculation of solubility parameter. The optimized structures of [emim]+ and [Tf2N] along with partial charges are shown in Fig. 1. These partial charges on all atoms of all the optimized structures of cations and anions have been kept fixed for further simulations in Material Studio. 2.3. Density calculations through NPT simulations

ð7Þ

For solubility of solute in solvent to be feasible, the following inequality must be followed:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T DSmixing 0 6 jd1  d2 j 6 /1 /2 V m

ð8Þ

The total cohesive energy is the energy required to break all cohesive bonds. Generally it is measured in terms of the energy required to completely evaporate the liquid, i.e., heat of vaporization of liquid. Charles M. Hansen proposed that total heat of vaporization consist of (atomic) dispersion forces, (molecular) permanent dipole-permanent dipole forces and (molecular) hydrogen bonding (electron exchange). Hansen Solubility parameter is given by Eq. (9) in terms of dispersion (dD), dipole–dipole (dP) and hydrogen bond (dH) contributions [31,32]

d2 ¼ d2D þ d2P þ d2H

ð9Þ

For the present study, Hansen solubility parameters for various flue gas components like CO2, CH4, N2, etc. have been taken from the literature [33]. The values of Hansen solubility parameters for CO2 and other flue gas components at 298.15 K and at atmospheric pressure are given in the Table 1. The solubility parameters of various ILs have been computed through molecular simulations in Material Studio. 2.2. Computational details The IL has been considered as an equimolar mixture of cation and anion. The structures of cations and anions used in the present study are given in Table 2. The molecular structures of individual cations and anions were constructed using visualization software MOLDEN [35]. The geometry optimization for all the cations and anions has been done using DFT calculation using Gaussian03 [36] with exchange correlation function B3LYP [37] and basis set 6-311 + G⁄ [38]. Electrostatic partial charges (ESP) were calculated using ESP analysis in Gaussian03 for all the optimized structures. Partial charge on each atom of the molecule plays an important

Table 1 Hansen solubility parameters of various gases at 298 K and 1 atm [33]. Compound

dD

dP

dH

d

Water Ammonia Chlorine Sulfur dioxide Carbon dioxide (CO2) Carbon monoxide Ethane Ethylene Helium Hydrogen Hydrogen sulfide Methane Nitrogen oxide Nitrogen Nitrous oxide Oxygen Acetylene

15.5 13.7 17.3 15.8 15.7 11.5 15.6 15.0 1.0 5.1 17.0 14.0 11.5 11.9 12.0 14.7 14.4

16.0 15.7 10.0 8.4 6.3 4.9 0.0 2.7 0.0 0.0 6.0 0.0 20.0 0.0 17.0 0.0 4.2

42.3 17.8 0.0 10.0 5.7 0.0 0.0 2.7 0.0 0.0 10.2 0.0 0.0 0.0 0.0 0.0 11.9

47.81 27.40 19.98 20.50 17.85 12.50 15.60 15.48 1.00 5.10 20.71 14.00 23.07 11.90 20.81 14.70 19.15

Densities of various ILs have been computed through molecular simulations in isothermal-isobaric ensemble (NPT) in which number of moles (N), pressure (P) and temperature (T) are conserved. All the structures optimized in Gaussian03 were imported in Material Studio. Models for bulk ionic liquid were made using Amorphous cell Module in Material Studio. To make the model resembling a liquid at room temperature, density of ionic liquid at room temperature has is required. Since room temperature densities for all ILs considered here, are not known, initial simulations have been run to compute the density of each ionic liquid. An amorphous cell has been constructed with periodic boundary conditions containing 20 cations and 20 anions. A sample model of the amorphous cell is shown in the Fig. 2. Since the density of IL is greater than that of water, initial guess for the density is taken as that of water, i.e., 1 g/ml. NPT simulations at constant temperature, pressure and number of molecules have been run at atmospheric pressure and 298 K temperature with an equilibration time of 100 ps and a production time of 150 ps. Prior to the molecular dynamic (MD) simulations, geometric optimization has been done using Forcite module for all the models constructed using amorphous cell module. All the MD simulations have been done using Forcite module. Electrostatic and van der Waals terms were considered using Ewald summation method with an accuracy of 105 kcal/mol. For Van der Waals term, the repulsive cut-off was 0 chosen as 6 Å A. All MD simulations have been done using COMPASS version 2.7 force field with charge calculated using DFT calculations in Gaussian03. For NPT molecular dynamic simulations, Andersen thermostat and Berendsen barostat were chosen [39]. The density, temperature and energy profiles for an IL [emim][Tf2N] are shown in the Supporting information. 2.4. Cohesiveenergy density calculations through NVT simulations [40] The cohesive energy density and solubility parameter were computed through canonical ensemble simulations (NVT) where number of moles (N), volume (V) and temperature (T) are conserved. These simulations were done at atmospheric pressure and 298 K for the same amorphous cell consisting of 20 cations and 20 anions by using the density, which has been computed by NPT simulations. NVT simulations were run for an equilibration time of 100 ps followed by 150 ps production runs by using the same module and force field which has been used for NPT simulations. For NVT molecular dynamic simulations, Andersen thermostat was chosen. The NVT simulations compute the cohesive energy density and solubility parameters arising due to van der Waals and electrostatic contributions. The electrostatic contribution resulting from molecular simulations is essentially the sum of the coulombic and hydrogen bonding contributions since most of the force fields do not contain a term for hydrogen bonding. Thus molecular simulations yield van der Waals and electrostatic contributions. A direct relation between electrostatic contributions to solubility parameter and coulombic- and hydrogen bondingcontributions to solubility parameter can be given as d2ES ¼ d2C þ d2H . The cohesive energy density of ILs computed from NVT simulations includes only inter-molecular interactions. It does not consider the intra-molecular interactions between cation and anion of a particular IL molecule. As the cations and anions have been defined as separate species with positive and negative charge,

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Table 2 Structures of cations and anions considered. Name of the ion

Abbreviation

Cation 1-Methyl-3-methylimidazolium; R1 = CH3, R2 = CH3 1-Ethyl-3-methylimidazolium; R1 = C2H5, R2 = CH3 1-Propyl-3-methylimidazolium; R1 = C3H7, R2 = CH3 1-Butyl-3-methylimidazolium; R1 = C4H9, R2 = CH3 1-Pentyl-3-methylimidazolium; R1 = C5H11, R2 = CH3 1-Hexyl-3-methylimidazolium; R1 = C6H13, R2 = CH3 1-Octyl-3-methylimidazolium; R1 = C8H17, R2 = CH3 1-Propyl-3-butylimidazolium; R1 = C3H7, R2 = C4H9 1-Aminopropyl-3-butylimidazolium; R1 = C3H8N, R2 = C4H9 1-Dimethylaminopropyl-3-butylimidazolium; R1 = C5H12N, R2 = C4H9 1-Vinylbenzyl-3-butylimidazolium; R1 = C3H7, R2 = C4H9

[mmim]+ [emim]+ [pmim]+ [bmim]+ [pemim]+ [hmim]+ [omim]+ [pbim]+ [apbim]+ [dmapbim]+ [vbbim]+

1-N-Butyl-3-methylpyridinium; R = C4H9 1-N-Hexyl-3-methylpyridinium; R = C6H13

[bmpy]+ [hmpy]+

O-Ethyl-N,N,N0 ,N0 -tetramethylisouronium; R = C2H5 O-Buthyl-N,N,N0 ,N0 -tetramethylisouronium; R = C4H9

[ETU]+ [BTU]+

S-Ethyl-N,N,N’,N’-tetramethylisothiouronium; R = C2H5 S-Propyl-N,N,N0 ,N0 -tetramethylisothiouronium; R = C3H7

[ETT]+ [PTT]+

(p-Vinylbenzyl)trimethylammonium

[VBTMA]+

[2-(Methacryloyloxy)ethyl]trimethylammonium

[MATMA]+

Structure

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Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64 Table 2 (continued) Name of the ion

Abbreviation

Structure

+

Tetrabutylphosphonium

[P(C4)4]

Trihexyltetradecylphosphonium

[P(14)666]+

Hexamethylguanidinium; R1 = CH3, R2 = CH3

[HMG]+

Anions Lactate

[Lactate]

Methyl sulfate; R = CH3 Ethyl sulfate; R = C2H5 Octyl sulfate; R = C8H17

[MeSO4] [EtSO4] [OctSO4]

Trifluoroacetate

[CF3COO]

Bis(trifluoromethylsulfonyl)imide

[Tf2N]

Nonafluorobutylsulfonate

[NfO]

(continued on next page)

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Table 2 (continued) Name of the ion

Abbreviation

Tris(trifluoromethyl)trifluorophosphate; R = CF3 Tris(pentafluoroethyl)trifluorophosphate; R = C2F5 Tris(heptafluoropropyl)trifluorophosphate; R = C3F7 Tris(nonafluorobutyl)trifluorophosphate; R = C4F9

[mFAP]  [eFAP]  [pFAP]  [bFAP]

Hexafluorophosphate

[PF6]

Tetrafluoroborate

[BF4]

Trifluoromethyltrifluoroborate

[mFAB]

Pentafluoroethyltrifluoroborate

[Feb]

calculation of cohesive energy density should include both the inter-molecular and intra-molecule terms. Intra-molecular term arises due to interaction between cation and anion of same molecule. Calculation of this intra-molecular interaction term (binding energy) has been done using ‘‘Blends’’ module with same force field and partial charges used for MD simulations. To get the accurate value of solubility parameter of an IL, intra-molecular term should be added to the cohesive energy density. 3. Results and discussion Initially, the density and solubility parameters of seven ionic liquids obtained through NPT and NVT molecular simulations, respectively. These data have been validated with the experimental literature values [25,41] as shown in Table 3. It can be seen from Table 3 that both simulation and experimental values are in close correspondence, suggesting that the predictions of solubility parameters are reliable and quite accurate (RMSD for density is 0.025 and for d is 1.92). 3.1. Group contribution correlation Since, the number of existing ILs is in a few thousands, it is very difficult to calculate the solubility parameter of each and every IL through molecular simulations. This is because of long

Structure



computational times required for the simulations. Thus, we have developed the following described approach to predict the solubility parameters of various ILs. As the IL can be considered as a mixture of cation and anion and also using the concept of group contribution, the total solubility parameter of any IL can be divided into the contributions of cation and anion. Since cation and anion are the charged species, it is not possible to calculate the individual contributions to solubility parameter directly from molecular simulations. The main reason is that the calculation of solubility parameter needs the density prediction of the compound. As the density prediction of a charged species (cation/anion) is not feasible, molecular simulations can not be directly used for solubility parameter estimation of charged species. Therefore a correlation has been developed for the d of IL as a function of contribution of dcation and danion. Squaring the solubility parameter gives cohesive energy density. Cohesive energy is the energy required to break the interactions, which binds the molecules with each other. These interactions in an ionic liquid can be divided into three types: (a) anion–anion interactions; (b) anion–cation interactions; and (c) cation–cation interactions. Interactions between anion–anion and cation–cation will always be negative due to repulsive forces and interactions between anion–cation will be positive due to attractive forces. Based on these assumptions a correlation equation has been proposed as follows:

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57

taken as equal, i.e., k2. Initially, solubility parameters of 16 ILs formed by combinations of four different cations ([emim]+, [bmim]+, [hmim]+ and [omim]+) and four different anions ([Tf2N], [PF6], [CF3COO] and [eFAP]) were computed using direct molecular simulations as described in Section 2.2. The detailed sample calculation of solubility parameter for an ionic liquid is provided in the Supporting information. The d values of 16 ILs obtained from MD Simulations along with the values of density are shown in Table 3. By using these solubility parameters of 16 ILs, applying Eq. (10) for the above 16 ILs, we can get a set (sixteen) of equations with a total of 10 unknowns: k1, k2, demim, dbmim, dhmim, domim, dTf2N, dPF6, dCF3COO and deFAP. Furthermore, a normalization condition for Eq. (10) was considered and is given by Eq. (11). Since k1 and k2 are the weighting factors or relative contributions to different types of interactions, the sum of the weightage factors must be equal to 1.0. Thus, the normalization condition can be written as:

k1 þ k2 þ k2 ¼ 1:0 ) k1 þ 2k2 ¼ 1:0

ð11Þ

Now the set of 16 equations obtained by using Eq. (10) for 16 ILs and Eq. (11) were solved simultaneously and iteratively by using the method of least squares. A MATLAB code has been developed to calculate the 10 unknown variables by using least square fitting method. To run a MATLAB code, initial guesses for these variables are required. Many initial guess values have been tried. Initial guess values with least mean error in solubility parameter have been chosen to get the values of k1, k2 and other variables. The k1 and k2 values obtained are 0.6084 and 0.1958, respectively. The Eq. (10) will now become:

d2IL ¼ 0:6084ðdcation danion Þ  0:1958ðd2cation Þ  0:1958ðd2anion Þ Fig. 1. Optimized structures of cation and anion of ionic liquid with ESP partial charge: (a) [emim]+ and (b) [Tf2N].

ð12Þ

The d values of the four cations and four anions are given in Table 4. Now by using the above equation, solubility parameters for any new ionic liquid can be obtained if dcation and danion are known. 3.2. Cation and anion contributions To calculate the solubility parameter of any IL using the Eq. (12), cation and anion contribution of that IL is needed. The d contributions of some common cations and anions have been calculated from molecular simulations and the correlation Eq. (12).

ð10Þ

3.2.1. Procedure for dcation While calculating the d contribution of cations, the danion is fixed as d[Tf2N]. Solubility parameter of an IL ([Tf2N] anion and various cations), dIL, is obtained from the molecular simulation as described in Section 2.2. By substituting dIL and danion = d[Tf2N] in Eq. (12), the cation contribution can then be calculated as dcation. The same procedure has been adopted for the calculation of dcation of 21 cations – [emim]+, [pmim]+, [bmim]+, [pemim]+, [hmim]+, [omim]+, [bmpy]+, [VBTMA]+, [MATMA]+, [apbim]+, [pbim]+, [dmapbim]+, [vbbim]+, [ETT]+, [PTT]+,[ETU]+, [hmpy]+, [P(C4)4]+, [HMG]+, [BTU]+ and [P(14)666]+. Results are shown in Table 5. The dcation of the above mentioned cations has also been found by fixing other anions such as [CF3COO] and [eFAP] in order to see if the reference anion chosen makes any difference on the dcation value. A table of comparison is provided in the Supporting information. Comparison of dcation obtained by fixing [TF2N] anion with dcation values obtained by fixing the other anions ([CF3COO] and [eFAP]) showed similar dcation values with a relative error of 2.5%, suggesting that dcation estimation is independent of reference anion chosen.

where k1 and k2 are the weighting factors, accounting for relative contributions of different types interactions exist between ions. As the number of cation–cation interactions is equal to the number of anion–anion interactions, the coefficients of d2cation and d2anion are

3.2.2. Procedure for danion While calculating the d contribution of anions, the dcation is fixed as d[bmim]+. The solubility parameter dIL of an IL consisting of [bmim]+ cation and various anions has been computed from

Fig. 2. Model of bulk ionic liquid [emim]+[Tf2N] constructed using ‘Amorphous Cell’ Module in Material Studio.

d2IL ¼ k1 ðdcation danion Þ  k2 ðd2cation Þ  k2 ðd2anion Þ

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Table 3 Density and solubility parameters for ionic liquids calculated using molecular simulations and comparison with literature values. Ionic liquid +



[emim] [Tf2N] [bmim]+[Tf2N] [hmim]+[Tf2N] [omim]+[Tf2N][emim]+[PF6] [bmim]+[PF6] [hmim]+[PF6] [omim]+[PF6] [emim]+[CF3COO] [bmim]+[CF3COO] [hmim]+[CF3COO] [omim]+[CF3COO] [emim]+[eFAP] [bmim]+[eFAP] [hmim]+[eFAP] [omim]+[eFAP] RMSD ⁄

qms (g/cm3)⁄

qexp (g/cm3)

dms (MPa0.5)⁄

dexp (MPa0.5)[25,41]

1.555 1.468 1.392 1.354 1.492 1.389 1.287 1.240 1.338 1.225 1.18 1.131 1.783 1.674 1.609 1.556

1.519 1.436 1.373 1.317 – 1.369 1.302 1.233 – – – – – – – – 0.025

29.1 27.8 26.2 25.9 36.7 33.3 31.1 29.5 36.5 33.3 31.1 29.7 26.4 24.7 24.4 23.7

27.6 26.7 25.6 25.0 – 29.8 28.6 27.8 – – – – – – – – 1.92

ms – from simulations in Material Studio, exp – literature experimental values.

3.3. Prediction of solubility parameters

Table 4 Solubility parameters of 4 cations and 4 anions. Cations +

[emim] [bmim]+ [hmim]+ [omim]+

dcation (MPa0.5) 74.0 64.5 59.2 56.1

Anions 

[Tf2N] [PF6] [CF3COO] [eFAP]

danion (MPa0.5) 55.8 85.8 84.7 48.8

molecular simulations. Now by substituting dIL and dcation = d[bmim]+ in Eq. (12), the values of danion can be obtained. Similar procedure has been adopted for the prediction of danion of 10 anions: [CF3COO], [PF6],[eFAP], [EtSO4], [OctSO4], [NfO], [mFAP], [pFAP] and [bFAP]. Results are shown in Table 6. The danion of various anions has also been found by fixing other cations such as [emim]+ and [hmim]+. A table of comparison is provided in the Supporting information. Comparison of danion obtained by fixing [bmim]+ cation with danion values obtained by fixing other two cations ([emim]+ and [hmim]+) showed similar danion values with a relative error of 2.5%, suggesting that danion estimation is independent of reference cation chosen.

Solubility parameters of 210 (10  21) ionic liquids have been predicted using Eq. (12) by using the d values of 10 anions and 21 cations and are shown in Table 7. To calculate the solubility parameters of these 210 ionic liquids, molecular simulations have been run only for 31 (as shown in Table 5 (21) and Table 6 (10)) ionic liquids. Using the d values of these 31 ionic liquids and Eq. (12), dcation of 21 cations and danion of 10 anions have been calculated using the procedure described before. The quality of solubility parameter correlation has been assured by comparing the solubility parameters of ILs determined using the correlation with those obtained by direct molecular simulation. To this end, 27 ionic liquids, which were not used previously either in the calculation of danion or in the calculation of dcation, are randomly selected. Comparison between correlation (Eq. (12)) values and direct molecular simulations for 27 ILs is presented in Table 8. It is clear that solubility parameters obtained from correlation Eq. (12) match well with those obtained from direct molecular simulations with an RMSD of 0.5 and a relative error of 1.5%. It indicates that the correlation Eq. (12) developed above is reliable and very useful in pre-

Table 5 Density and solubility parameters of ILs used in calculating dcation.

⁄

Ionic liquid

Dsim (g/cm3)⁄

dIL,sim (MPa0.5)⁄

d[Tf2N] (MPa0.5)

dcation (MPa0.5)

[bmpy]+[Tf2N] [P(C4)4]+[Tf2N] [HMG]+[Tf2N] [BTU]+[Tf2N] [VBTMA]+[Tf2N] [MATMA]+[Tf2N] [pmim]+[Tf2N] [pemim]+[Tf2N] [apbim]+[Tf2N] [pbim]+[Tf2N] [dmapbim]+[Tf2N] [vbbim]+[Tf2N] [ETT]+[Tf2N] [PTT]+[Tf2N] [ETU]+[Tf2N] [hmpy]+[Tf2N] [P(14)666]+[Tf2N] [emim]+[Tf2N] [bmim]+[Tf2N] [hmim]+[Tf2N] [omim]+[Tf2N]

1.41 1.235 1.422 1.418 1.42 1.431 1.494 1.432 1.415 1.394 1.357 1.383 1.449 1.416 1.419 1.382 1.082 1.555 1.468 1.392 1.354

26.67 23.18 26.65 25.86 27.26 27.8 28.48 27.53 27.01 26.54 25.43 25.56 26 26.48 26.21 26.37 20.62 29.12 27.78 26.17 25.92

55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83 55.83

59.93 45.97 58.83 55.25 61.99 65.30 70.46 63.59 60.64 58.29 53.52 54.05 55.86 58.01 56.79 57.50 39.47 78.06 65.20 56.60 55.52

sim – from direct MD simulations.

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Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64 Table 6 Density and solubility parameters of ILs used in calculating danion. Ionic liquid +



[bmim] [Tf2N] [bmim]+[PF6] [bmim]+[CF3COO] [bmim]+[eFAP ] [bmim]+[NFO] [bmim]+[mFAP ] [bmim]+[pFAP ] [bmim]+ [EtSO4] [bmim]+[OctSO4] [bmim]+[bFAP] *

Dsim (g/cm3)*

dIL,sim (MPa0.5)*

d[bmim]+ (MPa0.5)

danion (MPa0.5)

1.468 1.389 1.225 1.674 1.448 1.559 1.699 1.171 1.075 1.726

27.78 33.28 33.29 24.67 25.98 28.27 22.15 31.94 28.53 20.4

64.46 64.46 64.46 64.46 64.46 64.46 64.46 64.46 64.46 64.46

55.83 82.48 82.62 47.43 50.67 57.38 42.11 73.00 58.27 38.96

sim – from direct MD simulations.

Table 7 Predicted solubility parameter using group contribution correlation Eq. (12). Cat./An. +

[emim] [pmim]+ [bmim]+ [pemim]+ [hmim]+ [omim]+ [apbim]+ [pbim]+ [vbbim]+ [dmapbim]+ [VBTMA]+ [MATMA]+ [PTT]+ [ETT]+ [ETU]+ [BTU]+ [bmpy]+ [hmpy]+ [P(C4)4]+ [HMG]+ [P(14)666]+

[CF3COO]

[PF6]

[EtSO4]

[OctSO4]

[mFAP]

[Tf2N]

[NfO]

[eFAP]

[pFAP]

[bFAP]

37.3 35.1 33.3 32.7 29.7 29.2 31.5 30.5 28.4 28.2 32.0 33.3 30.3 29.3 29.8 29.0 30.8 30.1 23.7 30.7 18.5

37.3 35.1 33.3 32.7 29.7 29.2 31.5 30.5 28.4 28.2 32.0 33.3 30.3 29.3 29.8 29.0 30.7 30.1 23.7 30.7 18.5

35.1 33.4 31.9 31.4 29.0 28.6 30.5 29.7 28.0 27.8 30.9 32.0 29.6 28.8 29.1 28.5 29.9 29.4 24.2 29.9 20.1

30.2 29.3 28.5 28.2 26.7 26.5 27.7 27.1 26.1 25.9 27.9 28.6 27.1 26.5 26.8 26.4 27.3 26.9 23.5 27.3 20.7

29.8 29.0 28.3 28.0 26.5 26.3 27.4 26.9 25.9 25.7 27.7 28.3 26.9 26.4 26.6 26.2 27.1 26.7 23.4 27.0 20.7

29.1 28.5 27.8 27.5 26.2 25.9 27.0 26.5 25.6 25.4 27.3 27.8 26.5 26.0 26.2 25.9 26.7 26.4 23.2 26.6 20.6

26.7 26.4 26.0 25.8 24.8 24.6 25.4 25.1 24.3 24.2 25.6 26.0 25.0 24.7 24.8 24.6 25.2 24.9 22.4 25.2 20.2

24.9 24.9 24.7 24.6 23.8 23.6 24.3 24.0 23.4 23.3 24.4 24.7 24.0 23.7 23.8 23.6 24.1 23.9 21.7 24.1 19.8

21.4 22.0 22.2 22.1 21.8 21.7 22.1 21.9 21.6 21.5 22.1 22.2 21.9 21.7 21.8 21.7 22.0 21.9 20.4 22.0 18.9

19.0 20.0 20.4 20.5 20.4 20.4 20.5 20.5 20.3 20.3 20.5 20.4 20.5 20.4 20.4 20.4 20.5 20.5 19.5 20.5 18.3

Table 8 Comparison of d values predicted using correlation with the d values calculated using direct MD simulations. S. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Ionic liquid +



[PTT] [pFAP] [pemim]+[bFAP] [hmpy]+[eFAP] [bmim]+[OctSO4] [hmpy]+[CF3COO] [pmim]+[EtSO4] [pbim]+[OctSO4] [apbim]+[NFO] [pbim]+[pFAP] [pmim]+[NFO] [apbim]+[EtSO4] [PTT]+[EtSO4] [ETU]+[mFAP] [Pemim]+[mFAP] [dmapbim]+[EtSO4] [hmpy]+[mFAP] [Pemim]+[bFAP] [dmapbim]+[NFO] [PTT]+[NFO] [bmim]+[pFAP] [BTU]+[NFO] [vbbim]+[bFAP] [ETT]+[pFAP] [ETT]+[OctSO4] [HMG]+[OctSO4] [HMG]+[pFAP] [BTU]+[EtSO4] RMSD

Density (g/cm3)

dgc (MPa0.5) (from, Eq. (12))

dmd (MPa0.5) (MD simulations)

1.452 1.685 1.583 1.075 1.140 1.212 1.043 1.421 1.631 1.520 1.155 1.173 1.527 1.533 1.099 1.472 1.685 1.347 1.424 1.702 1.413 1.626 1.660 1.083 1.048 1.657 1.166

21.9 20.4 23.9 28.5 30.1 33.4 27.1 25.4 21.9 26.4 30.5 29.6 26.6 28.0 27.8 26.7 20.5 24.2 25.0 22.2 24.6 20.3 21.7 26.5 27.3 22.0 28.5

22.0 20.5 23.2 27.6 29.5 33.6 26.6 26.0 21.5 27.5 31.0 30.1 26.9 28.2 28.5 26.3 20.2 24.2 25.2 22.2 24.5 19.5 20.9 26.3 27.0 21.5 29.0 0.51

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Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64

38

[CF3COO]

36

[PF6]

-

-

[EtSO4]

Solubility Parameter (MPa

0.5 )

34

-

[OctSO4]

32

-

[mFAP] [Tf2N]

30

-

[NFO] [eFAP] [pFAP] [bFAP]

28 26 24 22 20 18 2

3

4

5

6

7

8

Alkyl chain length of methylimidazolium cation Fig. 3. Solubility parameter versus alkyl chain length on 1-alkyl-3-methylimidazoliumcation for different anions.

40

-

[CF3COO] -

[PF6]

-

[mFAP] [Tf2N]

0.5

Solubility parameter (MPa )

35

-

[NFO] [eFAP] [pFAP] [bFAP]

30

25

15

[emim]+ [pmim]+ [bmim]+ [pemim]+ [hmim]+ [omim]+ [apbim]+ [pbim]+ [vbbim]+ [dmapbim]+ [VBTMA]+ [MATMA]+ [PTT]+ [ETT]+ [ETU]+ [BTU]+ [bmpy]+ [hmpy]+ [P(C4)4]+ [HMG]+ [P(14)666]+

20

Fig. 4. Comparison of solubility parameters of ILs with fluorination on anions.

Table 9 Contributions of d from van der Waals and electrostatic interactions for ILs with [bmim]+ cation and various anions. Ionic liquid

dvan

[bmim]+[lactate] [bmim]+[EtSO4] [bmim]+[OctSO4] [bmim]+[PF6] [bmim]+[TF2N] [bmim]+[NFO] [bmim]+[eFAP]

17.86 18.66 18.33 17.30 19.90 16.51 17.72

der Waal

dcoulombic + hydrogen

bonding

47.69 44.28 36.94 47.01 35.92 36.25 31.13

dicting solubility parameters of several ILs just based only on the d values of few ILs. It indeed enormously reduces the computational time required for the prediction of solubility parameters of ILs.

3.4. Effect of anion on solubility parameter The solubility parameters of various anions for methylimidazolium cation-based ILs are presented in Fig. 3. It is observed from Fig. 3 that for a particular methylimidazolium cation, the inorganic ([PF6]) and non fluorinated ([EtSO4], [OctSO4]) anions have higher d values compared to those for organic anions. In case of fluorinated organic anions (Fig. 3), as the fluorination of anions increases, the d value decreases. The d values are observed to decrease in the following order with respect to fluorination: [CF3COO] (3F) > [Tf2N] (6F) > [mFAP] (12F) > [eFAP] (18F) > [pFAP] (24F) > [bFAP] (30F). The number of fluorine atoms present in the anion is given in the parentheses. This type of trend with fluorination is observed not only for alkyl methylimidazolium cations-based ILs but also found for other type of cation-based

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Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64

36

[E tS O 4 ][O c tS O 4 ][T f2 N ][N fO ]-

0.5

Solubility parameter (MPa )

34 32 30 28 26 24 22

[HMG]+

[P(14)666]+

[hmpy]+

[P(C4)4]+

[BTU]+

[bmpy]+

[ETT]+

[ETU]+

[PTT]+

[VBTMA]+

[MATMA]+

[dmapbim]+

[pbim]+

[vbbim]+

[omim]+

[apbim]+

[hmim]+

[bmim]+

[pemim]+

[pmim]+

18

[emim]+

20

Fig. 5. Comparison of solubility parameters of ILs with sulfonation on anions.

(pyridinium, uronium and phosphonium) ILs. This observation can be clearly seen in Fig. 4. To understand the difference between the solubility parameters of inorganic/non-fluorinated anions and organic fluorinated anions, the individual contributions of van der Waals and electrostatic contributions to the total solubility parameter of various ionic liquids comprising [bmim]+ cation and different anions are computed and are presented in Table 9. From Table 9, it is clear that the differences are mainly due to higher electrostatic contribution (equal to the collective contribution of coulombic and hydrogen bonding) for non fluorinated and inorganic anions compared to that for organic fluorinated anions. The solubility parameter from the van der Waals interaction contribution doesn’t show any significant differences between inorganic, non fluorinated anions and organic fluorinated anions. As for the application of ILs for CO2 capture, the comparison of the d value (17.85) of CO2 with the d values (18 to 37) of ILs presented in Table 7 reveals that the d values of [bFAP] and [pFAP] anions-based ILs are closer to that of CO2, indicating that CO2 absorption or solubility will be more effective in [bFAP] and [pFAP]-anions based ILs compared to other anion-based ILs. In fact, this observation is consistent with the experimental CO2 solubility data reported in the literature [9]. Anthony et al. [16] showed that the experimental H value of CO2 at 25 °C decreases when the anion is changed from [PF6] (53.4 bar) to [Tf2N] (33 bar) for bmim cation-based IL. As the Henry’s constant and solubility of gas are inversely proportional to each other, the CO2 solubility is higher in [bmim][Tf2N] compared to [bmim][PF6]. The d values of [bmim][Tf2N] and [bmim][ PF6] obtained in the present study at 25 °C are 27.8 and 33.3 MPa0.5 (Table 7), respectively. Clearly, the d value of CO2 (17.85 MPa0.5, Table 1) is closer to that of [bmim][Tf2N], suggesting that [bmim][Tf2N] is superior to [bmim][PF6] for CO2 absorption, which is in line with Anthony et al. [16]. On the other hand, although the solubility parameter of ionic liquids decreases with increasing fluorination of anions, the ions such as [Tf2N] and [NfO] have low fluorination compared to the anion [mFAP], but d’s of the ILs with anions [Tf2N] and [NfO] are not higher than that of ILs with anion [mFAP]. This could be due to the presence of sulfur atom in anions [Tf2N] and [NfO], which is also observed to play a role. Therefore, the d values of

ILs with anions [Tf2N] and [NfO] are slightly lower than the d values of ILs with [mFAP] anion. The combined effect of the presence of sulfur and fluorine atoms in anions, on solubility parameter of ionic liquids can be seen in Fig. 5. Solubility parameters of ILs with anions – [EtSO4], [OctSO4], [Tf2N] and [NfO] decrease in the following: [EtSO4] (1S) > [OctSO4] (1S) > [Tf2N] (6F,2S) > [NfO] (9F,1S). 3.5. Effect of cation and alkyl chain length of imidazolium cation on solubility parameter The effect of the alkyl group attached at R2 position of the methylimidazolium cation on solubility parameter is presented in Fig. 3 and Table 7 for different anion-containing ILs by considering alkyl groups ranging from ethyl (n = 2) to octyl (n = 8) groups. It is observed that the solubility parameter decreases as chain length of alkyl group of methylimidazolium cation increases from ethyl (n = 2) to octyl (n = 8) for most of the anions studied ([PF6], [CF3COO], [EtSO4], [OctSO4], [NfO], [mFAP] and [eFAP]), except that the solubility parameters of [pFAP] and [bFAP] anionsbased ILs did not show any effect on solubility parameter with increasing chain length of alkyl group of methylimidazolium cation. These two anions showed similar d values for different alkyl chain lengths (from n = 2 to 8) of methylimidazolium cation. This can be explained from the definition of solubility parameter d ¼ ½DHVv mRT 0:5 . Since all the ILs have negligible vapor pressure, we can assume that the heat of vaporization of all the ILs as equally large. Then the difference in solubility parameters comes from the difference in the molar volumes. Higher the molar volume of IL, lower will be the solubility parameter. As the fluorination of anion increases, the molecular size of the anion also increases due to repulsive forces between fluorine atoms. The molecular size of [bFAP] anion is much higher than the molecular size of [emim]+ and [omim]+ cations, which was confirmed by COSMO-RS studies (not shown for brevity). For highly fluorinated ILs such as [pFAP] and [bFAP], fluorine atoms play a critical role and thus diminish the effect of alkyl chain length of methylimidazolium cation on the solubility parameter. Consequently, [pFAP] and [bFAP] anion-based ILs show relatively constant d values irrespective of alkyl chain length of methylimidazolium cation-based ILs. Solubility

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Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64

parameters of ILs having different imidazolium cations are compared in Fig. 6. It is observed that for a particular anion, IL with 1-propyl-3-butylimidazolium ([pbim]+) cation has lower d value compared to IL with 1-propyl-2-methylimidazolium ([pmim]+) cation. Therefore the solubility parameter decreases with increasing the chain length of alkyl group on position ‘3’ of the imidazolium ring. It can also be observed from the Fig. 6 that for a particular anion, the addition of tertiary amine group (dimethyl amino propyl) on butylmethyl imidazolium cation ([dmapbim]+) reduces the value of solubility parameter compared to that of pure IL propyl butylimidazolium cation ([pbim]+). Similarly, the solubility parameters of ionic liquids with different anions and various cation groups like pyridinium, ammonium, uronium and phosphonium have been compared in Fig. 7. For all the cationic groups, it is clear that the value of d decreases with increasing alkyl chain length. This decreasing trend arises due to the increase in molar volume of IL with increasing alkyl chain length. The ILs with phosphonium cations does not seem to follow the same trend as that observed for ILs with other cationic groups. For phosphonium cation-based ILs, the non fluorinated [EtSO4] anion has high solubility parameter compared to all the other anions studied. However, the effect of

+

[emim] + [pmim] + [bmim] + [pemim] + [hmim] + [omim] + [apbim] + [pbim] + [dmapbim] + [vbbim]

35

30

25

[bFAP] -

[pFAP] -

[eFAP] -

[NfO] -

[Tf2N] -

[OctSO4] -

[EtSO4] -

[PF6] -

[mFAP] -

20

[CF3COO]-

Solubility Parameter (MPa0.5)

40

Anions

(a)

32

Solubility Parameter (MPa0.5)

Fig. 6. Comparison of solubility parameters of ILs of various imidazolium cations.

30

(b) 30

Anions

(c)

(d) Solubility Parameter (MPa0.5)

25 +

[P(C4)4] 24

+

[P(14)666]

23 22 21 20 19

[bFAP] -

[pFAP] -

[eFAP] -

[NfO] -

Anions

34 +

32

[VBTMA] + [MATMA]

30 28 26 24 22 20

Anions

Fig. 7. Effect of alkyl chain length on d of ILs formed from different cations: (a) pyridinium, (b) uronium, (c) phosphonium and (d) ammonium.

[bFAP] -

[pFAP] -

[eFAP] -

[NfO] -

[Tf2N] -

[mFAP] -

[OctSO4] -

[EtSO4] -

[PF6] -

[bFAP] -

[pFAP] -

[eFAP] -

[CF3COO]-

Anions

[NfO] -

[Tf2N] -

[mFAP] -

[OctSO4] -

[EtSO4] -

[PF6] -

18 [CF3COO]-

Solubilty Parameter (MPa0.5)

20

[bFAP] -

[pFAP] -

[eFAP] -

[NfO] -

[Tf2N] -

[mFAP] -

[OctSO4] -

[EtSO4] -

[PF6] -

[CF3COO]-

20

22

[Tf2N] -

22

24

[mFAP] -

24

26

[OctSO4] -

26

28

[CF3COO]-

28

+

[ETU] + [BTU]

[EtSO4] -

[bmpy] + [hmpy]

[PF6] -

Solubility Parameter (MPa0.5)

+

Y.S. Sistla et al. / Separation and Purification Technology 97 (2012) 51–64 +

[omim] + [dmapbim] + [VBTMA] + [PTT] + [BTU] + [hmpy] + [HMG] + [P(14)666]

0.5 Solubility Parameter (MPa )

32

30

28

26

24

22

20

[bFAP]-

[pFAP]-

[eFAP]-

[NFO]

[Tf2N]-

[mFAP]-

[OctSP4]-

[EtSO4]-

[PF6]-

[CF3COO]-

18

Fig. 8. Comparison of solubility parameters of ILs of different types of cations.

fluorination of organic anions on the solubility parameters of phosphonium-based ILs show the same trend as that of other cation-group based ILs i.e., the d value decreases with increasing fluorination. Solubility parameters of ionic liquids with different cation groups have been compared in Fig. 8. Ionic liquids with all cation types except phosphonium type cation follow the same pattern of solubility parameter for anions: [PF6], [CF3COO], [eFAP], [EtSO4], [OctSO4], [NfO], [mFAP], [pFAP] and [bFAP]. The Phosphonium cation IL ([P(14)666]+) showed different behavior compared to all other cations (Fig. 8). Furthermore, the solubility parameters of all the other cations show a large variation over the whole spectrum of anions ([CF3COO] to [bFAP]), whereas the d values of phosphonium ILs are clustered in a limited solubility parameter range of 18 to 20. This shows that the effect of anion is not significant for phosphonium ILs perhaps because of its large size. Furthermore, the solubility parameters of phosphonium-based ILs are much lower than that of other cation-based ILs. For CO2 capture application, the d value (17.85) of CO2 is compared with the d values of all the ILs studied (Table 7). It is observed that phosphonium cation-based ILs have d values closer to that of CO2, suggesting that phosphonium cation-based ILs can be the promising ILs for CO2 capture. Specifically, for phosphonium cation-based ILs the most preferable anions are [pFAP] and [bFAP]. For other cation-group based ILs, the effective CO2 capture is still possible but the anion combination should be with [pFAP] and [bFAP] anions. Finotello et al. [30] studied the effect of alkyl chain length on CO2 solubility for [Tf2N] anion-based IL. When the cation of [Tf2N] anion-based IL is changed from [emim]+ (ethyl) to [hmim]+ (hexyl), the experimental H value of CO2 decreases from 39 to 34 atm, indicating that the CO2 solubility is higher for [hmim][Tf2N] than that for [emim][Tf2N]. Comparison of the d values of [hmim][Tf2N] (26.2 MPa0.5) and [emim][Tf2N] (29.1 MPa0.5) obtained (Table 7) in the present study with that of CO2 (17.85 MPa0.5) also reveals that the solubility of CO2 in [hmim][Tf2N] is higher than in [emim][Tf2N], since the d of CO2 is closer to that of [hmim][Tf2N]. It reiterates that the solubility parameters computed are reliable and are able to successfully explain the solubility trends.

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3.6. Flue gas separation Comparison of the solubility parameters of various flue gases such as CO2, methane, water, nitrogen, nitrogen oxide, suphur dioxide and oxygen (shown in Table 1) with those of various ILs shown in Table 7 reveals that the d values of highly fluorinated anion based ILs and phosphonium cation based ILs are close to that of CO2 and are far from those of CH4, H2O, H2 and N2. Thus, an ionic liquid containing a highly fluorinated anion with phosphonium cation combination is recommended to separate CO2 with high selectivity from a mixture of flue gases. The effectiveness of this solubility parameter as a tool for the separation of CO2 from a flue gas mixture using ILs is further accentuated by comparing the literature experimental solubility data of gas mixtures in ILs with the solubility parameters obtained in the present study. For instance, the experimental Henry’s constant values reported by Anderson et al. [9] reveals that the Henry’s constant (H) of different gases in [hmpy][Tf2N] ionic liquid at 25 °C shows the following trend: HCO2 (32.8 bar) < HC2H4 (58 bar) < HC2H6 (72 bar) < HCH4 (300 bar) < HO2 (463 bar) < HN2 (3390 bar). The solubility follows the opposite trend, which means CO2 is more soluble in [hmpy][Tf2N] compared to the other gases such as C2H4, C2H6, CH4, O2 and N2. Interestingly, comparison of the solubility parameter (d) of [hmpy][Tf2N] (26.4 MPa0.5, from Table 7) obtained at 25 °C in the present study with the solubility parameters of different gases (CO2 (17.85 MPa0.5), C2H4 (15.48 MPa0.5), C2H6 (15.6 MPa0.5), CH4 (14.0 MPa0.5), O2 (14.7 MPa0.5), N2 0.5 (11.9 MPa )) reveals the same solubility trend as mentioned above, since the closer the d value of a particular gas to the d of IL the higher the solubility of that gas in IL. Finotello et al. [30] reported the experimental H values of different gases in [emim][Tf2N] and [hmim][Tf2N] ionic liquids. It was observed that the H value of different gases in both the ILs at different temperatures follows the order: HCO2 < HCH4 < HN2 < HH2 (solubility follows reverse trend). The comparison of d values of both ILs from Table 7 with those of CO2, CH4, N2 and H2 (see Table 1) shows that the solubility trend exactly matches with the experimental solubility trend [30]: CO2 > CH4 > N2 > H2. Furthermore, the experimental H values of gases were smaller for [hmim][Tf2N] than those for [emim][Tf2N] [30]. It implies that the solubility of gases is higher in [hmim][Tf2N] than in [emim][Tf2N]. From Table 7, comparison of the d values of [hmim][Tf2N] (26.2 MPa0.5) and [emim][Tf2N] (29.1 MPa0.5) indeed confirms that the solubility of gases in [hmim][Tf2N] is higher than in [emim][Tf2N]. Similar trends were also observed for solubility of gases (CO2, N2O, C2H4, C2H6, O2, etc.) in ILs such [bmim][Tf2N] and [bmim][PF6] [16]. Thus, the above comparisons clearly demonstrated that the solubility parameter (d) is a very useful tool for the judicious selection of an IL from a large set of ILs for the separation of gases without an extensive experimentation. Although, this solubility parameter does not provide the quantitative solubility, the solubility parameters comparison is very effective in choosing an appropriate IL not only for the separation of gas mixture but also for a particular gas. Furthermore, the methodology or correlation developed in the present study is able to successfully predict the solubility parameter of several ILs based on the d values of few ILs. This significantly reduces the computational effort required in predicting the d values for a large number of ILs.

4. Conclusions The solubility parameters d of various ILs have successfully been computed through molecular dynamic simulations using Material Studio. The computed d values of seven ILs initially have been validated with the available experimental data in the literature

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and are observed to be in close correspondence. A group contribution correlation has been developed to compute the individual contributions of cations and anions. The solubility parameters of 21 cations and 10 anions have successfully been determined using d values of 31 ILs. Using these d values of 21 cations and 10 anions, the d values of 210 ILs have been predicted by the group contribution correlation. The d values obtained by correlation match well with the d values obtained by direct molecular simulations for five ILs, suggesting that correlation equation is reliable to predict the d values of several ILs. This correlation tremendously reduces the computational time required for the prediction of d values of ILs. It was observed that increasing fluorination of anions decreases the d value significantly. Also, increasing the alkyl chain length of cation is observed to decrease the d value. Among the anions studied, the highly fluorinated [bFAP] anion based ILs have d values closer to the d of CO2 irrespective of the cation. Among the cations studied the phosphonium cation-based ILs have d values closer to the d of CO2 irrespective of the anion. Furthermore, the d value of phosphonium-bFAP ILs are much closer to the d value of CO2 and far from that of other flue gases such as CH4, N2, H2 and O2. Thus, the present study findings suggest that by employing the fluorinated anion and phosphonium cation based ILs, CO2 can more selectively be separated from most of the major flue gases. Furthermore, comparison of the literature solubility data of different gases in ILs with that of solubility parameters in the present study reveals that the d values are able to successfully explain the experimental solubility trends. Thus, the solubility parameter is an effective tool for the judicious selection of an IL not only for the CO2 capture application but also for the separation of mixture of gases. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.seppur.2012.01.050. References [1] X. Xu, C. Song, R. Wincek, J.M. Andresen, B.G. Miller, A.W. Scaroni, Separation of CO2 from power plant flue gas using a novel CO2 ‘‘Molecular Basket’’ adsorbent, Fuel Chem. Div. Prepr. 48 (1) (2003) 162. [2] J.F. Brennecke, B.E. Gurkan, Ionic liquids for CO2 capture and emission reduction, J. Phys. Chem. Lett. 1 (2010) 3459. [3] N. Palmeri, S. Cavallaro, J.C.J. Bart, Carbon dioxide absorption by MEA: a preliminary evaluation of a bubbling column rector, J. Therm. Anal. Calorim. 91 (1) (2008) 87. [4] L.A. Blanchard, L.D. Hancu, E.J. Beckman, J.F. Brennecke, Green processing using ionic liquids and CO2, Nature 399 (1999) 28. [5] J.L. Anthony, S.N.V.K. Aki, E.J. Maginn, J.F. Brennecke, Feasibility of using ionic liquids for carbon dioxide capture, Int. J. Environ. Technol. Manag. 4 (2004) 105. [6] C. Cadena, J.L. Anthony, J.K. Shah, T.I. Morrow, J.F. Brennecke, E.J. Maginn, Why CO2 so soluble in imidazolium based ionic liquids, J. Am. Chem. Soc. 126 (2004) 5300. [7] R.E. Baltus, B.H. Culbertson, S. Dai, H. Luo, D.W. Depaoli, Low pressure solubility of carbon dioxide in room temperature ionic liquids measured with a quartz crystal microbalance, J. Phys. Chem. B 108 (2004) 721. [8] A. Finotello, J.E. Bara, S. Narayan, D. Camper, R.D. Noble, Ideal gas solubilities and solubility selectivities in a binary mixture of room temperature ionic liquids, J. Phys. Chem. B 112 (2008) 2335. [9] J.L. Anderson, J.K. Dixon, J.F. Brennecke, Solubility of CO2, CH4, C2H6, C2H4, O2 and N2 in 1-hexyl,3-methylpyridinium bis(trifluoromethylsulfonyl) imide: comparison to other ionic liquids, Acc. Chem. Res. 40 (2007) 1208. [10] J.E. Bara, T.K. Carlisle, C.J. Gabriel, D. Camper, A. Finotello, D.L. Gin, R.D. Noble, Guide to CO2 separations in imidazolium bases room temperature ionic liquids, Ind. Eng. Chem. Res. 48 (2009) 2739. [11] D. Camper, J.E. Bara, C. Koval, R.D. Noble, Bulk-fluid solubility and membrane feasibility of Rmim based room temperature ionic liquids, Ind. Eng. Chem. Res. 45 (2006) 6279. [12] S. Hanioka, T. Maruyama, T. Sotani, M. Teramoto, H. Matsuyama, K. Nakashima, M. Hanaki, F. Kubota, M. Goto, CO2 separation facilitated by task specific ionic liquids using a supported liquid membrane, J. Memb. Sci. 314 (2008) 1.

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