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BY 4.0 license Open Access Published by De Gruyter Open Access April 28, 2022

Molecular screening of ionic liquids for CO2 absorption and molecular dynamic simulation

  • Xingang Jia EMAIL logo , Xiaoling Hu , Kehe Su , Wenzhen Wang and Chunbao Du
From the journal Open Chemistry


Though ionic liquids (ILs) are considered potential materials for CO2 capture because of their unique properties, it is time-consuming and costly to choose task-specific and suitable IL using the traditional “try-and-error” method. From the point of molecular design view, 25 cations and 20 anions are combined and screened using COSMOtherm to predict CO2 solubility at 298 K and 100 kPa. The prediction result showed that ILs with bFAP(tris(nonafluorobutyl)trifluorophosphate) anion could dissolve more CO2 than any others. To further understand the absorption performance of CO2 in ILs, molecular dynamic simulations are carried out to explore the interactions between CO2 and the four selected ILs, namely, [EMPyr][bFAP](1-ethyl-2-methylpyrazolium tris(nonafluorobutyl)trifluorophosphate), [B(Hex)3P][bFAP](butyl-trihexyl-phosphonium tris(nonafluorobutyl) trif-luorobutyl trifluorophosphate), [(Me)5isobuGua][bFAP](n,n,n,n,n-pentamethyl-n-isopropylguanidinium tris(nona-fluorobutyl)-trifluorophosphate), and [BEIM][bFAP] (1-butyl-3-ethyl-imidazolium tris(nonafluorobutyl)trifluo-rophosphate), at the atomic and molecular levels.

1 Introduction

Human emissions of carbon dioxide are continuously increasing and have been a primary driver of global climate change [1,2]. The Intergovernmental Panel on Climate Change (IPCC) predicted that the global surface temperature change at the end of the twenty-first century would likely exceed 1.5°C [3]. It is a vast and urgent challenge for human beings to cope with global warming [4]. Consequently, CO2 capture and storage/utilization (CCS/CCU) have been widely accepted as an effective way to reduce greenhouse gas emissions. New materials such as ionic liquids (ILs) have been researched and developed for cost-effective CO2 capture and utilization. Except for its own unique physical characteristics, the properties of ILs can also be adjusted and designed for specific applications. Since Blanchard et al. [5] first reported that CO2 has high solubility in [BMIM][BF6] in 1999, a quite large number of studies have been focused on the application of the possibility of ILs in CO2 absorption processes [6,7,8,9,10]. Aki et al. studied the effects of the alkyl chain length from butyl to octyl on CO2 solubility [11]. The result showed that the longer the chain length was, the more soluble it was. Pankaj et al. [12,13] studied the role of anions in ILs and found ILs containing different anions had different CO2 solubility. The mechanism of CO2 solubility in ILs is generally categorized into physical absorption and chemical absorption. Some functionalized ILs are capable of chemically binding to CO2 where covalent bonds are formed during CO2 absorption in ILs [14,15]. The two types of absorptions have their own advantages and disadvantages. Compared with chemical absorption, physical absorption has the advantage of less energy required for desorption [16]. Whether ILs are classified into physical or chemical absorption, as a potential alternative for CO2 capture, the possible number of ILs was theoretically huge up to 1018. Obviously, it was time-consuming and costly to develop novel ILs used for specific applications from the vast sea of ILs by the traditional “try and error” method. The Conductor-like Screening Model for Real Solvents (COSMO-RS) used to predict the thermodynamic properties of ILs has been successfully proven to be effective in many literature [17,18,19,20,21]. It combined statistical thermodynamics and quantum chemistry and can be used as a valuable tool to screen molecules for a specific application based on the molecular structure.

The aim of this work is to systematically develop a way by using COSMOtherm to quickly screen 500 different ILs, which are combined by 25 cations and 20 anions, and get CO2 solubility in ILs. Molecular dynamic (MD) simulation had been proved to be a powerful tool in studying and predicting the physical properties of IL-related systems [22,23,24]. It can provide a fundamental relationship between microscopic and macroscopic properties. To further have a deep understanding of the structural properties and the interactions of ILs and CO2, simulation was also carried out in this work. To achieve the goal, we investigate the solvation energy and transport properties of CO2 in four different ILs which shared a common anion based on the screened results. It is hoped that our study could help provide a better understanding of the CO2 absorption mechanism by ILs and facilitate the novel ILs design for CO2 absorption.

2 Methods

2.1 Screened ILs and COSMO-RS calculations

ILs can theoretically be designed by combining different anions and cations; 25 cations and 20 anions are selected to combine 500 IL solvents. Their structures and names are listed in Tables S1 and S2 of the supporting information. The cations include pyrazolium, phosphonium, ammonium, imidazolium, pyridinium, pyrrolidinium, guanidinium, and piperidinium. The anions include acetate, phosphate, sulfate, and dicyanamide. COSMOtherm was used to predict the solubility of CO2 in the above-mentioned ILs. The standard operating procedure of COSMOtherm contains two steps. First, quantum calculation of the involved molecule species is performed by Gaussian or Turbmole suit program. Second, statistical thermodynamics calculation is carried out within COSMOtherm. All screened IL structures could be seen as the combination of ion pairs, namely, cation and anion. The structures of ion pairs, which had been early optimized using four different theoretical levels and incorporated into the program, were directly loaded from the database. As suggested in the study by ref. [25], a molecular model of counter ions was applied in COSMOtherm calculations. ILs are treated as equimolar mixtures of cations and anions. The parameterization (BP_TZVPD_FINE_20.ctd) contains the intrinsic parameters for the calculation and is used. The σ-profile, which contains the screening charges on the molecular surface, was used as an input in the COSMOtherm to calculate the solubility of CO2 in ILs at near ambient pressure and temperature.

2.2 Molecular dynamics simulations

MD simulations were carried out using GROMACS 2018.2 program package [26]. The force field, General AMBER Force Field (GAFF), was used in the MD simulations. Each anion or cation in ILs was geometrically optimized at B3LYP/6-311 + G(d,p) in Gaussian 16 package [27]. The atom RESP charges of ILs were calculated using Multiwfn 3.7 program [28,29]. The bonded and non-bonded parameters used in the simulation are obtained by ACPYPE online server [30]. A leap-frog algorithm was used to integrate Newton’s equations of motion, and the time step of 2fs was adopted in all the simulations. The temperature was controlled by a velocity rescaling thermostat with a coupling constant of 0.4 ps [31]. The pressure was maintained by Parrinello–Rahman barostat with a coupling constant of 2.0 ps [32]. The periodic boundary conditions were applied in all directions. The bonds were constrained with the LINCS algorithm [33]. The cut-off for the short-range electrostatics and Lennard–Jones interactions were both set to 1.0 nm, and the particle mesh Ewald algorithm [34] was used to deal with the long-range electrostatic interactions. For each system, energy minimization of the simulation system was first performed with the steepest descent algorithm to generate an appropriate starting configuration and then equilibrated for 1 ns at isothermal and fixed volume (NVT) ensemble to reach the reference temperature of 298.15 K and the reference pressure of 1 bar, respectively. Then, 20 ns trajectory was produced at the isothermal and isobaric (NPT) ensemble, and the last 10 ns trajectory was used for statistical analysis.

Free energy of CO2 in pure water and ILs was calculated using the BAR (Bennett Acceptance Ratio) method with 21 equidistant sets of λ values from λ = 0 to λ = 1. TIP4P water model was used. Coordinates, velocities, and δH/δλ were saved to disk at every ten steps for the post-processing of BAR. Each node was subjected to energy minimization with 20,000 steps of the steepest descent method. The simulation was then pre-equilibrated at NVT ensemble with no position restrains. After NVT equilibration, the process was continued with 2 ns simulations at NPT at 298 K for all systems. Data collection was done from the last 1 ns.

3 Solubility data and simulation models

3.1 CO2 solubility in ILs

The solubility of CO2 in 500 ILs is calculated at 298 K and 100 kPa using the method mentioned above, and the results are tabulated in Table S3. Figure 1 shows the trend and difference of CO2 solubility in the screened ILs.

Figure 1 
                  CO2 relative solubility in ILs calculated by COSMOtherm.
Figure 1

CO2 relative solubility in ILs calculated by COSMOtherm.

The previous studies have investigated cation and anion effects on CO2 solubility [35,36,37]. The combination of cation and anion can affect CO2 solubility. Generally, CO2 solubility in ILs strongly depends on the choice of the anion. Aki et al. also studied the influence of the anion with the same 1-butyl-3-methylimidazolium cation [11]. They proved that the fluorination of the anion can improve CO2 solubility. The trend of CO2 solubility in ILs predicted by COSMOtherm is consistent with the previous work. It was concluded from Figure 1 that ILs with B(Hex)3P cation have much higher CO2 solubility than any other ILs, and CO2 solubility in ILs with bFAP anion was also much higher than any other ILs. CO2 solubility in various ILs is much different. Among the 500 screened ILs, the highest CO2 solubility is 0.084 mole fraction in [B(Hex)3P][bFAP], while the lowest CO2 solubility is 0.002 mole fraction in [MPrAm][NO3] at 298 K and 100 kPa.

The previous experiments explored ILs containing nitrile anion that had a higher CO2 solubility [38,39]. Their work showed that CO2 solubility in [emim][B(CN)4] (1-ethyl-3-methyl imidazolium tetracyanoborate) is 0.027 mole fraction. Compared with that, ILs containing [bFAP] anion have much higher CO2 solubility.

3.2 MD simulations of ILs and ILs/CO2

A few of ILs were selected according to the CO2 solubility data. Specifically, bFAP anion was paired with four different cations, namely, [EMPyr](1-ethyl-2-methylpyrazolium), [B(Hex)3P](butyl-trihexyl-phosphonium), [(Me)5isobuGua](n,n,n,n,n-pentamethyl-n-isopropyl-guanidinium), and [BEIM](1-butyl-3-ethyl-imidazolium). Two kinds of MD simulations were performed. The first set was to examine CO2 solvation free energy in ILs. The second was to mimic the equilibrium and dynamic properties of pure ILs and the IL/CO2 system. For the calculation of CO2 solvation free energy in IL, 1 CO2 molecule was put into the center of the cubic box. For the pure IL simulation system, a cubic box of 8 nm × 8 nm × 8 nm was created with 256 pairs of cation and anion. For IL/CO2 system, the simulation box was constructed by 10 CO2 molecules and 256 ion pairs. The initial configurations of the MD simulations were built by the Packmol package [40].

4 Results and discussions

4.1 Henry constant and solvation free energy of CO2 in ILs

Henry constant, as another measure of CO2 solubility in ILs, could also be computed using COSMOtherm. The computed Henry constants of CO2 in pure water and the four selected ILs are listed in Table 1. Additionally, to understand and compare the CO2 absorption ability in ILs and water from the point of solvation free energy, the CO2 solvation free energy in pure water was also computed by the BAR method and COSMOtherm. The values of the solvation free energy of CO2 (abbreviated as ΔG) in pure water and four different ILs are listed in Table 1. The calculated results using the BAR method are 3.80 kJ mol−1 in pure water, −10.18 kJ mol−1 in [EMPyr][bFAP], −11.37 kJ mol−1 in [BEIM][bFAP], −12.21 kJ mol−1 in [(Me)5isobuGua][bFAP], and −10.43 kJ mol−1 in [B(Hex)3P][bFAP], respectively. Compared with the solvation free energy of CO2 in pure water, the negative values clearly showed that the CO2 molecule is more easily dissolved in IL than in water.

Table 1

Henry constant and solvation free energy of CO2 in ILs

Entry ILs H CO 2 ( MPa ) ΔG (kJ mol−1)
BAR method COSMOtherm
1 H2O 342.6 3.80 ± 0.10 0.54
2 [EMPyr][bFAP] 1.74 −10.18 ± 0.37 −4.26
3 [BEIM][bFAP] 1.46 −10.43 ± 0.34 −4.44
4 [(Me)5isobuGua][bFAP] 1.16 −11.37 ± 0.75 −4.84
5 [B(Hex)3P][bFAP] 0.86 −12.21 ± 0.08 −4.86

Though the four ILs could dissolve CO2 more easily than water, it was concluded that there were different CO2 solubility capabilities among the four selected ILs. Though the values of ΔG obtained by COSMOtherm were much smaller than that obtained by the BAR method, they all showed the same trend. The solvation free energy of CO2 in studied ILs followed the increasing order: [EMPyr][bFAP] < [BEIM][bFAP] < [(Me)5isobuGua][bFAP] < [B(Hex)3P][bFAP], which agrees with the relative CO2 solubility predicted by COSMOtherm.

4.2 Structural properties

Some important structural properties of the simulated systems were investigated through MD simulations. The density, solvent-accessible area (SAS), and free volume (FV) of all simulated systems are summarized in Table 2.

Table 2

Calculated density and other properties for the simulated systems

System Density (g cm−3) SAS (m2 cm−3) FV (%) V (nm3 mol−1)
[BEIM][bFAP] 1.752 45 35.99 109
[B(Hex)3P][bFAP] 1.431 81 38.03 162
[EMPyr][bFAP] 1.844 62 36.05 99
[(Me)5isobuGua][bFAP] 1.706 67 36.01 114
[BEIM][bFAP]/CO2 1.697 113
[B(Hex)3P][bFAP]/CO2 1.357 165
[EMPyr][bFAP]/CO2 1.774 103
[(Me)5isobuGua][bFAP]/CO2 1.660 119

The maximum density corresponds to [EMPyr][bFAP] (1.844 g cm−3), and the minimum density corresponds to [B(Hex)3P][bFAP] (1.431 g cm−3). It was noticed that the density of the IL/CO2 binary system was a little smaller than that of the corresponding pure IL. FV within ILs was correlated to the gas solubility. CO2 molecules were preferentially solvated within the vicinity of the anions [41]. Figure 2 shows the representative vicinity within ILs. It was noticed that the four ILs with bFAP anion all had a larger FV of more than 35%. The larger the FV was, the more CO2 molecules were solvated. It was consistence with the CO2 solubility in the four ILs.

Figure 2 
                  Representative snapshot of the FV space (green) within bFAP-based ILs: (a) [BEIM][bFAP], (b) [B(Hex)3P][bFAP], (c) [EMPyr][bFAP], and (d)[(Me)5isobuGua][bFAP].
Figure 2

Representative snapshot of the FV space (green) within bFAP-based ILs: (a) [BEIM][bFAP], (b) [B(Hex)3P][bFAP], (c) [EMPyr][bFAP], and (d)[(Me)5isobuGua][bFAP].

4.3 Radial distribution function of ILs and ILs/CO2 system

To study the influence of CO2 absorption on the overall IL structure, the equilibrium structures of ILs and CO2 are presented by the radial distribution function g(r). The g(r) of the constituent compounds are calculated to study cation–anion, cation–cation, and anion–anion interactions in IL and shown from a to c in Figure 3. The g(r) of cation, anion, and CO2 in the ILs/CO2 system is also calculated and shown from (d) to (i) in Figure 3.

Figure 3 
                  Radial distribution functions in ILs and IL/CO2 systems at 298 K and 100 kPa: (a) cation–anion in IL, (b) cation–cation in IL, (c) anion–anion in IL, (d) cation–anion in IL/CO2, (e) cation–cation in IL/CO2, (f) anion–anion in IL/CO2, (g) CO2–cation in IL/CO2, (h) CO2–anion in IL/CO2, and (i) CO2–CO2 in IL/CO2.
Figure 3

Radial distribution functions in ILs and IL/CO2 systems at 298 K and 100 kPa: (a) cation–anion in IL, (b) cation–cation in IL, (c) anion–anion in IL, (d) cation–anion in IL/CO2, (e) cation–cation in IL/CO2, (f) anion–anion in IL/CO2, (g) CO2–cation in IL/CO2, (h) CO2–anion in IL/CO2, and (i) CO2–CO2 in IL/CO2.

The g(r) provided deep insights into the interactions that occurred in the studied system. It was revealed from Figure 3(a–c) that cation–anion interaction of both ILs is stronger than those of cation–cation or anion–anion interactions. Figure 3a shows the interaction between the cation and anion in ILs. Similar observations were found that the weaker interaction between the cation and the anion affected the CO2 solubility in ILs [42]. Taking [B(hex)3][bFAP] as an example, the intense cation–anion g(r) peak appears at about 0.67 nm. The g(r) peak of cation–anion in [BEIM][bFAP] was about 0.56 nm, while in Figure 3b and c, the cation–cation and anion–anion g(r) of [B(hex)3][bFAP] peaks appear at 1.20 and 1.10 nm, respectively. It was clear in Figure 3d that the cation–anion g(r) shape in IL/CO2 systems did not change obviously compared with pure IL systems. It was consistent with the previous simulation studies which showed that the addition of CO2 molecules does not change the structures of ILs [36,43]. The interactions between CO2 and ILs ions are shown in Figure 3(g and h). The CO2–anion interactions in the four IL/systems all display g(r) peaks at 0.54 nm. It is obvious that CO2 molecules are interacting stronger with the anions as compared with the cations since the CO2–anion peak is located at a lower distance than that of the CO2–cation peak.

The electrostatic and the van der Waals energies between ions and CO2 interactions were also computed from the MD simulation systems and are listed in Table 3.

Table 3

Electrostatic and the van der Waals interaction energies computed from MD simulation (kJ mol−1)

System Electrostatic interaction (Coulomb) Van der Waals interaction (LJ)
[c]…[c] [c]…[a] [a]…[a] [c]…CO2 [a]…CO2 [c]…[c] [c]…[a] [a]…[a] [c]…CO2 [a]…CO2
[BEIM][bFAP]/CO2 43651.3 −95239.5 188,402 −120.28 −6.47 −4389.09 −29561.8 9993.66 −105.28 −266.25
[B(Hex)3P][bFAP]/CO2 288,630 −537,241 408,878 −64.85 −6.33 −18377.8 −43958.1 18228.7 −171.65 −189.98
[EMPyr][bFAP]/CO2 292,123 −614,795 449,418 −125.76 9.85 −1764.45 −23068.1 7274.88 −78.19 −282.65
[(Me)5isobuGua][bFAP]/CO2 346,539 −761,467 521,422 −81.92 −3.97 −6261.04 −30233 10860.8 −117.08 −244.85

For IL and IL/CO2 systems, the total interaction energies of cation–cation and anion–anion mainly showed unfavorable interactions. As for cation–anion, it was favorable interaction and electrostatic interaction made more contributions to the total interaction than Van der Waals interaction. Electrostatic interaction of cation and CO2 was larger than that of anion and CO2. Van der Waal’s interaction of anion and CO2 was larger than that of cation and CO2. It showed that the interaction of cation and CO2 was mainly electrostatic interaction, while the interaction of anion and CO2 was mainly Van der Waals interaction.

4.4 Mean-squared displacement (MSD) and Diffusion coefficients

The dynamic properties of ILs and IL/CO2 systems are quantified using MSD and diffusivity. Figure 4(a–e) show the MSDs of cation, anion, and CO2 in ILs and ILs/CO2 at 298 K.

Figure 4 
                  Comparison of the mean square displacement of of cations, anions, and CO2 in IL and IL/CO2 at 298 K: (a) [BEIM] and [bFAP] in pure [BEIM][bFAP], (b) [B(Hex)3P] and [bFAP] in pure [B(Hex)3P][bFAP], (c) [EMPyr] and [bFAP] in pure [EMPyr][bFAP], (d) [(Me)5isobuGua] and [bFAP] in pure [(Me)5isobuGua][bFAP], and (e) CO2 in the four ILs.
Figure 4

Comparison of the mean square displacement of of cations, anions, and CO2 in IL and IL/CO2 at 298 K: (a) [BEIM] and [bFAP] in pure [BEIM][bFAP], (b) [B(Hex)3P] and [bFAP] in pure [B(Hex)3P][bFAP], (c) [EMPyr] and [bFAP] in pure [EMPyr][bFAP], (d) [(Me)5isobuGua] and [bFAP] in pure [(Me)5isobuGua][bFAP], and (e) CO2 in the four ILs.

The initial rapid increase in MSD within 1 ns showed that a longer simulation time was required to evaluate the self-diffusion coefficients of ions or CO2 in both IL and IL/CO2 systems. The slopes of the MSDs for the cations are steeper than those for the anions in IL and IL/CO2 systems. It was possibly due to the fact that a larger anion size resulted in a slower movement than the cation.

The diffusion of cation, anion, and CO2 in ILs and ILs/CO2 systems has also been determined. The diffusion coefficients of the constituent compounds were obtained by the linear fitting of the slope of MSD. Table 4 reports the diffusivities of cations, anions, and CO2 in the system.

Table 4

Diffusion coefficient of cation, anion, and CO2 at 298 K and 100 kPa

System Diffusion coefficient (10−8cm2 s−1)
D cation D anion D CO 2
[BEIM][bFAP] 0.55 0.41
[B(Hex)3P][bFAP] 0.68 0.57
[EMPyr][bFAP] 0.56 0.15
[(Me)5isobuGua][bFAP] 1.19 0.46
[BEIM][bFAP]/CO2 0.60 0.27 9.93
[B(Hex)3P][bFAP]/CO2 0.41 0.31 13.9
[EMPyr][bFAP]/CO2 1.06 0.39 7.16
[(Me)5isobuGua][bFAP]/CO2 1.15 0.21 9.65

The diffusion of cation, anion, and CO2 in ILs and ILs/CO2 systems have also been determined. The diffusion coefficients of the constituent compounds were obtained by the linear fitting of the slope of MSD. Table 4 reports the diffusivities of cations, anions, and CO2 in the system. When CO2 molecules were dissolved in the IL, it was synergistic competition. It is clear from Table 4 that the diffusion coefficient of CO2 was much higher than those of cations and anions. It was mainly due to the small size of CO2 molecules. For CO2 molecule in IL/CO2 system, the self-diffusion coefficient followed the decreasing order: [B(Hex)3P][bFAP]/CO2 > [(Me)5isobuGua][bFAP]/CO2 > [BEIM][bFAP]/CO2 > [EMPyr][bFAP]/CO2, which agreed with the predicted CO2 solubility in ILs.

5 Conclusion

Using COSMOtherm, 500 ILs were selected and screened to predict the CO2 solubility at 298 K and 100 kPa. CO2 solubility in various ILs is much different. Among the 500 screened ILs, CO2 solubility is in the range of 0.002 and 0.084 mole fraction. Anions have much more effect on the solubility of CO2 than cations. ILs with bFAP anion could dissolve more CO2 than any other ILs. The molecular understanding of structural properties of ILs and ILs/CO2 system was further studied using MD simulation. The calculated results of CO2 solvation free energy were −10.18 kJ mol−1 in [EMPyr][bFAP], −11.37 kJ mol−1 in [BEIM][bFAP], −12.21 kJ mol−1 in [(Me)5isobuGua][bFAP], and −10.43 kJ mol−1 in [B(Hex)3P][bFAP], respectively. In IL/CO2 system, CO2 molecules are interacting stronger with the anions as compared with the cations according to the analysis of g(r). The interaction of anion and CO2 is mainly from Van der Waals interaction. The more dissolved in ILs CO2 molecules are, the greater the self-diffusion coefficient is. Basically, the self-diffusion coefficient of cation is higher than that of anion in the IL and IL/CO2 system.

  1. Funding information: This research was supported by the National Natural Science Foundation of China (No. 52073228 and No. 22002117). The authors also appreciate the modern analysis and testing center of Xi’an Shiyou University for its hyper computation service.

  2. Author contributions: Conceptualization, Xingang Jia; methodology, Xiaoling Hu; software, Xingang Jia; validation, Kehe Su; formal analysis, Xingang Jia; investigation, Xiaoling Hu; re-sources, Wenzhen Wang; data curation, Chunbao Du; writing – original draft preparation, Xingang Jia; writing – review and editing, Kehe Su; visualization, Xingang Jia; supervision, Xiaoling Hu; project administration, Xiaoling Hu and Wenzhen Wang; funding acquisition, Wenzheng Wang and Chunbao Du. All authors have read and agreed to the published version of the manuscript.

  3. Conflict of interest: Authors declare there is no conflict of interest.

  4. Ethical approval: The conducted research is not related to either human or animal use.

  5. Data availability statement: All CO2 solubility in 500 ILs predicted by COSMOtherm and the structures of cations and anions are included in supplementary information files.


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Received: 2022-01-02
Revised: 2022-03-22
Accepted: 2022-04-04
Published Online: 2022-04-28

© 2022 Xingang Jia et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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