Screening ionic liquids for developing advanced immobilization technology for CO2 separation

Introduction

Excessive CO₂ emissions have led to serious problems and received great concern (Figueres et al., 2018). According to the report from International Energy Agency, the amount of CO₂ emissions in 2020 was already around 30 Gt (Iea, 2020), and the excessive CO₂ emissions have led to environmental problems, such as global warming, glacial melting, and seawater acidification (Clark et al., 2020; Singh and Polvani, 2020; Hanna et al., 2021). To reduce CO₂ emissions, carbon capture and storage (CCS) has been proposed as one of the important options, in which CO₂ separation is often needed to capture CO₂. Technologies have been developed for CO₂ separation, which can be divided into four categories, absorption, adsorption, membrane, and cryogenic (D'Alessandro et al., 2010), while the energy usage and cost of these traditional technologies are still high (Figueroa et al., 2008). Hence, developing energy-efficient and cost-effective technology for CO₂ separation is necessary, and novel absorbent development is one of the research focuses.

Ionic liquids (ILs) are molten salts with cation and anion as constituents while in the liquid state at room temperature (Zhang et al., 2017; Chen et al., 2020). ILs have the advantages of low vapor pressure, high thermal stability, and designable ability. Some ILs possess relatively high CO₂ solubility and selectivity over other gases (e.g., N₂ and CH₄) as well as low regeneration temperature and desorption enthalpy, making them desirable absorbents for CO₂ separation (Zhang et al., 2012; Wang et al., 2020). Many ILs have been designed and synthesized for CO₂ separation, such as pyrrolidinium-, imidazolium-, quaternary ammonium-, and quaternary phosphonium-based ILs (Blanchard et al., 2001; Anderson et al., 2007; Kilaru et al., 2007). However, their high cost and high viscosity (i.e., low CO₂ mass transfer rate) are still the current drawbacks hindering industrial applications of ILs on a large scale (Goodrich et al., 2010; Li et al., 2013).

Immobilizing ILs on porous materials is an effective strategy to overcome the above deficiencies (Yan et al., 2011). Zhang et al. found that the CO₂ absorption rate in the tetrabutylphosphonium amino acid salts immobilized on silica (SiO₂) was much higher than that in the bulk phase (Zhang et al., 2006). Wang et al. immobilized 1-ethyl-3-methylimidazolium amino acid on the surface of the polymethylmethacrylate microspheres. It was found that the CO₂ sorption equilibrium could be reached within 15 min (Wang et al., 2013b), which is also much faster compared to that in the bulk phase. It is widely accepted that the intensification of CO₂ absorption rate is owing to the large mass transfer area after IL immobilization (Zhang et al., 2009; Vicent-Luna et al., 2013; Wang et al., 2013a; Khan et al., 2014). However, the abnormally high CO₂ absorption capacity in the immobilized ILs was also observed. For instance, Zhang et al. noticed that the CO₂ absorption capacity in the 1-butyl-3- methylimidazolium tetrafluoroborate [(C₄mim) (BF₄)] confined in mesoporous silica gels was improved by about 1.5 times (Zhang et al., 2010). Wu et al. observed that the CO₂ absorption capacity of 1-hexyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([C₆mim][Tf₂N]) was increased from 0.031 to 0.386 mol-CO₂/mol-IL after immobilized on the surface of titanium dioxide (Wu et al., 2017). Therefore, the enhanced CO₂ absorption capacity can be another important reason to improve the CO₂ absorption performance, and immobilizing ILs on the porous materials is a promising way to promote the development of IL-based technologies for CO₂ separation.

To develop IL-immobilized sorbent for CO₂ separation, it is desirable to make a prior theoretical screening based on the properties of ILs, owing to the huge amount of ILs (up to 10¹⁸) that can be potentially synthesized. Normally, CO₂ absorption capacity and selectivity as well as desorption enthalpy are considered to evaluate the CO₂ separation performance. Among them, CO₂ absorption capacity directly shows the ability of ILs to capture CO₂, and desorption enthalpy reflects the energy usage for regeneration. Consequently, these two properties can be used to primarily evaluate the performance of ILs and are often used as the index in IL screening (Maiti, 2009; Palomar et al., 2011; Lee and Lin, 2015; Zhang et al., 2016; Taheri et al., 2021), which is valid for the technologies where the bulk ILs are used. While as discussed in the previous paragraph, when ILs are immobilized, the IL properties will change, causing a difference in CO₂ absorption capacity from its bulk. For example, based on molecular simulations, Pinilla et al. found that the density of 1,3-dimethylimidazolium chloride in a confined space is twice that in the bulk phase (Pinilla et al., 2005), Sha et al. observed a liquid-to-solid phase transition monolayer when 1,3-dimethylimidazolium chloride was confined between the graphite walls (Sha et al., 2008) and confirmed its higher melting point (Sha et al., 2009). This evidenced that when IL is immobilized, due to the asymmetric and strong interaction between the IL molecule and solid surface, the properties of ILs may be very different from its bulk phase, which need to be considered in screening immobilized ILs for CO₂ separation.

To consider the special properties of the immobilized ILs, the quantity related to the density change can be used as an additional index. Both advanced experiments and computer simulations have evidenced a higher density of the immobilized ILs compared to the bulk (Shi and Sorescu, 2010; Shi and Luebke, 2013). In particular, as reported by Gubbins et al., the molecules in a fluid or solid film adsorbed on a solid substrate experience strong compression, which is equivalent to a pressure up to several thousand bar (Gubbins et al., 2018). Based on these observations, it can be inferred that the enhanced CO₂ absorption capacity in the immobilized ILs comes from the complex interaction between the IL and substrate, which may be reflected by the density change. In other words, the compressibility of ILs may be an essential factor in determining the CO₂ absorption capacity, i.e., the higher the compressibility, the higher the potential to pressurize IL (via the interaction between IL and substrate) to increase density, and the higher the CO₂ absorption capacity due to the increased density. Therefore, besides the CO₂ absorption capacity and desorption enthalpy, the compressibility of IL at the pressure that is equivalent to the asymmetric and strong interaction with the substrate, might be an additional index in screening ILs when developing immobilized-ILs for CO₂ separation.

Due to the equivalent pressure is extremely high (up to several thousand bar), it is hard to determine the compressibility of ILs by using experimental measurements, and thus model prediction can be a viable option. A lot of theoretical models have been developed to predict the properties of ILs, which can be used as theoretical tools to screen ILs, such as Conductor-like Screening Model for Real Solvents (COSMO-RS), COSMO segment activity coefficient model (COSMO-SAC), Soave Redlich Kwong, Universal Quasi–Chemical Model, and so on (Maiti, 2009; Gonzalez-Miquel et al., 2011; Lee and Lin, 2015; Farahipour et al., 2016; Kamgar and Rahimpour, 2016; Theo et al., 2016). However, none of them can be used at high pressures. In our previous work (Ji and Adidharma, 2008; Ji and Adidharma, 2010; Ji et al., 2012; Shen et al., 2015; Ji and Held, 2016; Shen et al., 2018; Sun et al., 2020), electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) has been developed with ion-specific parameters, and, in particular, the model can be used up to high pressures owing to the consideration of the dispersion between IL-cations and IL-anions. The model performance has been verified extensively (Ji and Adidharma, 2012; Ji et al., 2013; Ji et al., 2014; Shen et al., 2015; Bülow et al., 2019; Sun et al., 2020; Sun et al., 2021). All these make ePC-SAFT a powerful tool for predicting the compressibility of ILs in a wide pressure range.

In this work, for the first time, the compressibility of IL at high pressures was proposed as a new index, which was then combined with the CO₂ absorption capacity and desorption enthalpy to screen ILs for developing immobilized-ILs for CO₂ separation. The ePC-SAFT model was used to predict the properties in a wide temperature and pressure range to provide systematic data for screening ILs step by step. In addition, to verify the screening results, the CO₂ separation performance of the screened ILs was compared with the experiments.

Theory

ePC-SAFT

ePC-SAFT was developed by Cameretti and Sadowski (Cameretti et al., 2005), as an extension of PC-SAFT proposed by Gross and Sadowski (Gross and Sadowski, 2000; Gross and Sadowski, 2001). In ePC-SAFT, the dimensionless residual Helmholtz energy (A^res) is expressed as

${\mathit{A}}^{\mathit{res}}={\mathit{A}}^{\mathit{hc}}+{\mathit{A}}^{\mathit{disp}}+{\mathit{A}}^{\mathit{ion}}(1)$

Where A^hc and A^disp are the contributions from the hard chain and dispersive terms, respectively, and the expressions can be obtained from the literature (Gross and Sadowski, 2001). The ionic term ($A^{ion}$) was represented by the Debye-Hückel theory (Gross and Sadowski, 2001)

$A^{ion}=-\frac{\kappa }{12\pi {\epsilon }_{\tau }{\epsilon }_0}\cdot {\displaystyle \sum }_j{x}_j{q}_j^2{\chi }_j(2)$

Where κ is the inverse Debye-screening length with a unit of reciprocal meter, ${\epsilon }_{\tau }$ is the relative dielectric constant of the medium, ${\epsilon }_0$ is the dielectric constant of vacuum, $x_j$ is the mole fraction of ion j, and $q_j$ is the charge of ion j. The units of ${\epsilon }_0$, κ, and $q_j$ are F/m, reciprocal meter, and coulomb, respectively. The definitions of κ and ${\chi }_j$ have been described in detail in the original ePC-SAFT (Gross and Sadowski, 2001).

In 2012, ePC-SAFT was extended to predict the properties of ILs, where each IL was assumed to be fully dissociated into IL-anion and IL-cation (Ji et al., 2012). Each IL-ion with three parameters, i.e., segment number, the segment diameter, and the segment energy, while ${\epsilon }_{\tau }$ was set to be unity for pure ILs. In particular, dispersive interaction exists between IL-cation and IL-anion, which is different from the ordinary aqueous electrolyte solutions.

In modeling, the parameters of ePC-SAFT for each IL-ion were taken from the literature, which were fitted to the experimental liquid-density of pure ILs or estimated with the linear equation based on the molar weight of IL-ions (Ji and Held, 2016).

Following ePC-SAFT, the density can be obtained from the dimensionless residual Helmholtz energy numerically at different temperatures and pressures, and then other thermodynamic properties can be further derived, such as compressibility, fugacity coefficient, etc. The combination of thermodynamic properties and phase equilibria can be used to predict the gas solubility, such as CO₂ solubility in ILs, and the relevant properties, such as desorption enthalpy, can be further obtained.

Compressibility

Following ePC-SAFT, the isothermal compression coefficient (${\kappa }_T$) of ILs can be estimated with Eqn. 3:

${\kappa }_T^{-1}=\rho {\left(\frac{\partial P}{\partial \rho }\right)}_T(3)$

Where P is the pressure in bar, T is the temperature in Kalvin, and $\rho$ is the density of ILs obtained from Eqn. 4.

$P={\rho }^2{\left(\frac{\partial A^{res}}{\partial \rho }\right)}_T(4)$

CO₂ solubility

Following our previous study (Ji et al., 2012), the vapor pressure of ILs is negligible, and the phase equilibrium for CO₂ in a CO₂-IL system can be represented by the following equation:

${\phi }_{C{O}_{\mathit{2}}}^V\left(T,v^V\right)=x_{C{O}_{\mathit{2}}}\cdot {\phi }_{C{O}_{\mathit{2}}}^L\left(T,v^L,x_{C{O}_{\mathit{2}}}\right)(5)$

Where $x_{C{O}_2}$ is the mole fraction of CO₂ in the liquid phase, ${\phi }_{C{O}_2}^L$ and ${\phi }_{C{O}_2}^V$ are the fugacity coefficients for CO₂ in the liquid and vapor phases, respectively, and $v^L$ and $v^V$ are the molar volumes of liquid and vapor phases, respectively. In this work, ${\phi }_{C{O}_2}^L$, ${\phi }_{C{O}_2}^V$, $v^L$, and $v^V$ were calculated with ePC-SAFT, where the parameters of CO₂ were taken from the original PC-SAFT (Gross and Sadowski, 2001).

Desorption enthalpy

In this work, the ILs that physically absorb CO₂ were considered, and thus, the CO₂ desorption enthalpy (ΔH) can be calculated with the following equation:

$\mathrm{\Delta }H=R\left(\frac{\partial ln{H}_{C{O}_{\mathit{2}}}\left(T\right)}{\partial \left(1/T\right)}\right)(6)$

where $H_{C{O}_2}\left(T\right)$ is the Henry’s constant of CO₂ in the IL. In this work, the value of $H_{C{O}_2}\left(T\right)$ was calculated with Eqn. 7

$H_{C{O}_{\mathit{2}}}\left(T\right)=\underset{x\to 0}{\mathrm{l}\mathrm{i}\mathrm{m}}\frac{P{\phi }_{C{O}_{\mathit{2}}}^V}{x_{C{O}_{\mathit{2}}}}(7)$

In Eqn. 7, the fugacity coefficient (${\phi }_{C{O}_2}^V$) of CO₂ in the vapor phase was calculated with Eqs. 8–10:

$ln{\phi }_{C{O}_{\mathit{2}}}^V=\frac{{\mu }_{C{O}_{\mathit{2}}}^{res}\left(T,v^V\right)}{kT}-lnZ(8)$

$\frac{{\mu }_{C{O}_{\mathit{2}}}^{res}\left(T,V\right)}{kT}=A^{res}+\left(Z-1\right)+{\left(\frac{\partial A^{res}}{\partial x_{C{O}_{\mathit{2}}}}\right)}_{T,V}-x_{C{O}_{\mathit{2}}}{\left(\frac{\partial A^{res}}{\partial x_{C{O}_{\mathit{2}}}}\right)}_{T,V}(9)$

$Z=P{v}^V/RT(10)$

Where, ${\mu }_{C{O}_2}^{res}\left(T,v^V\right)$ is the chemical potential of CO₂, k is the Boltzmann constant (1.380649 × 10^–23 J/K), Z is the compressibility factor, and R is the gas constant [8.314J/ (mol·K)].

Results and discussion

To screen ILs for developing IL-immobilization technology, in this work, it was achieved step by step. Firstly, compressibility was used to reflect the potential in density increase for enhancing the CO₂ absorption, and then ILs were primarily screened. Subsequently, the ILs with high compressibility were further screened based on the CO₂ absorption capacity. Additionally, the desorption enthalpy of the screened ILs was predicted for further verification. Finally, the screened ILs were verified with the available experimental data.

The screening is based on the properties predicted theoretically with ePC-SAFT. In predicting compressibility, to represent the interaction between the IL molecule and solid surface, we set the pressures ranging from 1 to 6,000 bar. Generally, for a CO₂ separation process, the absorption can be from room temperature to 313.15 K. Considering the heat release during absorption, the temperature was set from 298.15 to 323.15 K. Meanwhile, according to the applicability of ePC-SAFT and practical applications, the conditions in predicting the CO₂ absorption capacity and desorption enthalpy were set to be 298.15 to 323.15 K and 1 to 50 bar.

Compressibility

The studied ILs contain the IL-cations of [C_nmim]⁺, [C_npy]⁺, [C_nmpy]⁺, [C_nmpyr]⁺, and [THTDP]⁺, and the IL-anions of [Tf₂N]^-, [PF₆]^-, [BF₄]^-, [tfo]^-, [DCA]^-, [SCN]^-, [C₁SO₄]^-, [C₂SO₄]^-, [eFAP]^-, Cl⁻, [Ac]^-, and Br⁻.

The ePC-SAFT model with the available parameters was used to predict the isothermal compression coefficient of ILs. The results at 298.15 K and 1–6,000 bar were illustrated as an example as depicted in Figure 1. The green lines in Figure 1 represent the ILs with good compressibility, while the red ones indicate the ILs with poor compressibility. As shown in Figure 1, the compressibility of ILs can be significantly different, and the highest compressibility is around 3 times of the lowest one at atmospheric pressure. The IL compressibility with the significant difference makes it essential to consider the compressibility when screening ILs for developing the IL-immobilized absorbent.

FIGURE 1

FIGURE 1. κ_T of 272 ILs predicted with ePC-SAFT at 298.15 K.

Furthermore, it was found that the isothermal compression coefficient decreases with increasing pressure, indicating that it becomes more difficult to further increase the IL density at high pressures. The same phenomena were observed at other temperatures, as depicted in Supplementary Figures S1–S5. The IL compressibilities at different temperatures are listed in Supplementary Table S1. To further illustrate the effect of temperature on the compressibility, [C₂py][SCN], [C₁₀mim][DCA], and [C₂mpyr][eFAP], which represent the ILs with poor, medium, and good compressibility, respectively, were selected as three representatives. The results are shown in Figure 2, evidencing that the compressibility is slightly affected by temperature. Other ILs also showed the same phenomena.

FIGURE 2

FIGURE 2. κ_T of ILs at different temperatures.

To illustrate the influences of anions and cations, the compressibilities of [C₂mpyr]⁺, [C₂mpy]⁺, [C₂mim]⁺, [C₂py]⁺ and [THTDP]⁺ at 298.15 K were further analyzed as examples to illustrate the effect of anions. The results are depicted in Figure 3 and Supplementary Figures S6–S9. As shown in Figure 3, it was found that the compressibility of ILs with the cation of [C₂mpyr]⁺ decreased with the order of [eFAP]^- > [Tf₂N]^- > Br⁻ > [tfo]^- > [C₁SO₄]^- > [BF₄]^- > [PF₆]^- > Cl⁻ > [DCA]^- > Ac⁻ > [C₂SO₄]^- > [SCN]^-. For the other cations, the effect of anions is quite similar to that of [C₂mpyr]⁺. Therefore, it can be inferred that the anions play the important role on the compressibility of ILs.

FIGURE 3

FIGURE 3. κ_T value of ILs with [C₂mpyr]⁺.

To study the influence of cations on the compressibility of ILs, [eFAP]^-, [Tf₂N]^-, [BF₄]^-, [PF₆]^-, [C₂SO₄]^-, and [SCN]^- were selected for investigation. The results of their isothermal compressibilities are shown in Figure 4 and Supplementary Figures S10–S14. As depicted in Figure 4, for the ILs with the cation of [Tf₂N]^-, the compressibility of ILs follows the order of [C₂mpyr]⁺ > [C₂mpy]⁺ > [THTDP]⁺ > [C₁₂mim]⁺ > [C₁₀mim]⁺ > [C₃mpyr]⁺ > [C₈mim]⁺ > [C₆mim]⁺ > [C₃mpy]⁺ > [C₄mim]⁺ > [C₃mim]⁺ > [C₄mpy]⁺ > [C₄mpyr]⁺ > [C₂mim]⁺ > [C₆mpy]⁺ > [C₆py]⁺ > [C₄py]⁺ > [C₈py]⁺ > [C₃py]⁺ > [C₂py]⁺ > [C₁₀py]⁺ > [C₁₂py]⁺ > [C₆mpyr]⁺. While for the other anions, the order of different cations is a bit different. With increasing n, the compressibility of [C_nmim]⁺ increases, while that of [C_nmpyr]⁺ decreases. On the other hand, the cation of [THTDP]⁺ always shows higher compressibility. The compressibilities of [C_nmpy]⁺ and [C_npy]⁺ are not related to the value of n. In contrast, according to the literatures, in the bulk phase, the physical CO₂ solubility can be improved by increasing the alkyl chain length on the cation (Anthony et al., 2005). This indicates that, if the CO₂ solubility is the only index for screening ILs, the ILs with the cation of long alkyl chain length will be selected. However, when the compressibility of ILs is considered, the screening result may be different.

FIGURE 4

FIGURE 4. κ_T value of ILs with [Tf₂N]^-.

According to the results shown in Figure 3 and Figure 4, and Supplementary Table S1, we can find that the compressibility of the ILs with [C₂mpyr]⁺ (IL-cation) at 1 bar is from 0.31 to 0.68 Gpa⁻¹, i.e., with a change of 0.37 Gpa⁻¹, while the compressibility of ILs with the anion of [Tf₂N]^- at 1 bar only changes from 0.46 to 0.66 Gpa⁻¹, i.e., the difference is only 0.20 Gpa⁻¹. This observation indicates that the influence of IL-anion on compressibility is more than that of IL-cation. Similar results can be observed for other cations and anions. Also, for the ILs with the same anion, their compressibility changes much less compared with the ILs with the same cation (Supplementary Figures S6–S14). Therefore, we can conclude that the compressibility of ILs is mainly affected by anion.

According to Figure 1, the κ_T value for some ILs is not sensitive to the pressure, which makes it unobvious to perform screening. In order to differ the compressibility of ILs intuitively, the values of Δκ_T (Δκ_T = κ_T,1 bar-κ_T,6000 bar) ranging from 298.15 to 323.15 K were calculated, and then ILs were screened. As depicted in Figure 5, 30 ILs with excellent compressibility are illustrated together with the worst one, i.e., [C₂py][SCN]. With the increasing temperature, the Δκ_T values of ILs increase, and ILs with the anion of Br⁻ are most sensitive to the temperature. Based on the screened 30 ILs, we observed that the ILs containing the cations of [C₂mpyr]⁺, [C₂mpy]⁺, [THTDP]⁺, [C₃mpyr]⁺, [C₃mpy]⁺, [C₁₂mim]⁺, [C₁₀mim]⁺ or the anions of [eFAP]^-, [Tf₂N]^- have better compressibility than other ILs.

FIGURE 5

FIGURE 5. Δκ_T value of 30 ILs with better compresibility and the worst one.

CO₂ capacity

After the preliminary screening based on the compressibility, ePC-SAFT was also used to predict the CO₂ absorption capacity for these 30 ILs. The results of CO₂ absorption capacity are shown in Figure 6 and Supplementary Figures S15–S19 and listed in Supplementary Table S2.

FIGURE 6

FIGURE 6. CO₂ absorption capacity in 30 ILs at 298.15 K.

As illustrated in Figure 6, the CO₂ absorption capacity in ILs increases with increasing pressure. Most of ILs with the anion of [Tf₂N]^- have better CO₂ absorption capacity than the ILs with the anion of [eFAP]^-. For the ILs with the cation of [C_nmim]⁺, the CO₂ absorption capacity increases with increasing the chain length. For example, the order of CO₂ absorption capacity is in the order of [C₁₂mim][Tf₂N] > [C₁₀mim][Tf₂N] > [C₈mim][Tf₂N] > [C₆mim][Tf₂N] > [C₄mim][Tf₂N]. This is consistent with the experimental observations (Anthony et al., 2005), which also indicates that the results predicted by the ePC-SAFT model are reliable. Furthermore, not all ILs with high compressibility show good CO₂ solubility. For example, the CO₂ absorption capacities of [C₂mpy]Br and [C₂mpyr]Br are not as high as expected. Based on the results at 298.15 K, 7 ILs with higher CO₂ absorption capacity were further screened, i.e., [C₁₂mim][Tf₂N], [C₁₀mim][Tf₂N], [C₈mim][Tf₂N], [C₁₂mim][eFAP], [C₆mim][Tf₂N], [C₁₀mim][eFAP], and [THTDP][Tf₂N].

Desorption enthalpy

CO₂ desorption enthalpy affects the energy demand in the desorption unit. H₂O is the physical absorbent for biogas upgrading (i.e., CO₂ removal), while 30 wt% monoethanolamine (MEA) is a chemical absorbent for CO₂ separation from flue gases (Kim and Svendsen, 2011; Gupta et al., 2013; Chen et al., 2020). In this section, the CO₂ desorption enthalpy of the above-screened 7 ILs was predicted, and the results were compared with the traditional solvents, as depicted in Figure 7 and listed in Supplementary Table S3. It can be seen that, among the screened ILs, [THTDP][Tf₂N] has the lowest CO₂ desorption enthalpy, and the ILs with the anion of [eFAP]^- show lower desorption enthalpy than that with the anion of [Tf₂N]^-. For example, the CO₂ desorption enthalpy in [C₁₂mim][eFAP] is lower than [C₁₂mim][Tf₂N], and the desorption enthalpy in [C₁₀mim][eFAP] is also lower than [C₁₀mim][Tf₂N]. Anyhow, the desorption enthalpy of all the screened ILs is around −12 kJ/mol, confirming the physical interaction between CO₂ and the selected ILs. Therefore, CO₂ desorption enthalpy can be out of consideration when screening the physical ILs. Compared with H₂O and 30 wt% MEA, the desorption enthalpy of the screened ILs is approximately reduced by 30 and 85%, respectively.

FIGURE 7

FIGURE 7. CO₂ desorption enthalpy in 7 ILs compared with traditional solvents.

Verification

Among the screened 7 ILs, [C₆mim][Tf₂N] immobilized on the surface of titanium dioxide with a particle size of 25 nm (P25) has been studied in the previous study (Wu et al., 2017), showing desirable CO₂ separation performance. As shown in Figure 8, the CO₂ absorption capacity of [C₆mim][Tf₂N]/P25 is about ten times that in the bulk [C₆mim][Tf₂N] at 298.15 K and atmospheric pressure. Banu et al. (Banu et al., 2013) studied CO₂ in the [C₂mim][Tf₂N] and [C₆mim][Tf₂N] confined in the ceramic porous membrane at 298.15 K and atmospheric pressure. Figure 9 shows that after immobilization, the CO₂ absorption capacities in [C₂mim][Tf₂N] and [C₆mim][Tf₂N] were enhanced by about 1.6 and 2 times, respectively, and the CO₂ absorption capacity intensification in [C₆mim][Tf₂N] is higher than that in [C₂mim][Tf₂N]. According to the result in Figure 4, the compressibility of [C₆mim][Tf₂N] is much higher than that of [C₂mim][Tf₂N], which is consistent with the trend of the CO₂ absorption capacity. Therefore, it can be concluded that the index proposed in this work is reasonable, and the compressibility of ILs is an important index in screening ILs for developing IL-immobilized absorbent for CO₂ separation.

FIGURE 8

FIGURE 8. CO₂ absorption capacity of P25-[C₆mim][Tf₂N] and [C₆mim][Tf₂N].

FIGURE 9

FIGURE 9. Henry’s constants of CO₂ in [C₂mim][Tf₂N] and [C₆mim][Tf₂N].

As only limited experimental results on the immobilized ILs are available, in the future, the CO₂ separation performance of IL-immobilized absorbents will be determined experimentally to systematically verify the screen method and results. In addition, the CO₂ separation performance is a combination of thermodynamics and kinetics, and some ILs can absorb CO₂ chemically. In the future, the research on the screen will be extended to include CO₂ chemical absorption and consider the kinetic contribution to the CO₂ separation performance.

Conclusion

In this study, a new additional index, i.e., the compressibility of ILs, was proposed to screen ILs for developing IL-immobilization technology for CO₂ separation. The developed ePC-SAFT model was used as a theoretical tool to predict reliable and systematic data for screening. From 272 physical ILs, 30 ILs were selected firstly based on the compressibility. Then, 7 ILs, i.e., [C₁₂mim][Tf₂N], [C₁₀mim][Tf₂N], [C₈mim][Tf₂N], [C₆mim][Tf₂N], [C₁₂mim][eFAP], [C₁₀mim][eFAP], and [THTDP][Tf₂N], were finally screened based on the CO₂ absorption capacity and desorption enthalpy. Finally, the CO₂ separation performance with [C₆mim][Tf₂N] was compared with the experimental results and discussed, verifying the reliability of the IL screening. In the future, systematic experiments will be designed to further verify this screen index, and the quantitative relationship between compressibility and CO₂ absorption capacity will be studied to further understand the mechanism of the enhanced CO₂ absorption capacity in the immobilized ILs.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

XL and XJ contributed to conception and design of the study. ZD performed the calculation, statistical analysis and wrote the first draft of the manuscript. YS provide technical support. ZZ wrote sections of the manuscript. YC and XJ reviewed and edited the manuscript.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We would like to thank the National Natural Science Foundation of China (No. 21838004, 22108115) and Joint Research Fund for Overseas Chinese Scholars and Scholars in Hong Kong and Macao Young Scholars (No. 21729601). ZD thanks the Postgraduate Research and Practice Innovation Program of Jiangsu Province (Grant Nos. KYCX21_1180), YC thanks China Postdoctoral Science Foundation funded project (No. 2021M691554) and Kempe foundation (SMK21-0020) in Sweden, and XJ thanks Swedish Energy Agency.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2022.941352/full#supplementary-material

References

Anderson, J. L., Dixon, J. K., and Brennecke, J. F. (2007). 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, 1208–1216. doi:10.1021/ar7001649

PubMed Abstract | CrossRef Full Text | Google Scholar

Anthony, J. L., Anderson, J. L., Maginn, E. J., and Brennecke, J. F. (2005). Anion effects on gas solubility in ionic liquids. J. Phys. Chem. B 109, 6366–6374. doi:10.1021/jp046404l

PubMed Abstract | CrossRef Full Text | Google Scholar

Banu, L. A., Wang, D., and Baltus, R. E. (2013). Effect of ionic liquid confinement on gas separation characteristics. Energy Fuels 27, 4161–4166. doi:10.1021/ef302038e

CrossRef Full Text | Google Scholar

Blanchard, L. A., Gu, Z., and Brennecke, J. F. (2001). High-pressure phase behavior of ionic liquid/CO₂ systems. J. Phys. Chem. B 105, 2437–2444. doi:10.1021/jp003309d

CrossRef Full Text | Google Scholar

Bülow, M., Ji, X., and Held, C. (2019). Incorporating a concentration-dependent dielectric constant into ePC-SAFT. An application to binary mixtures containing ionic liquids. Fluid Phase Equilibria 492, 26–33. doi:10.1016/j.fluid.2019.03.010

CrossRef Full Text | Google Scholar

Cameretti, L. F., Sadowski, G., and Mollerup, J. M. (2005). Modeling of aqueous electrolyte solutions with perturbed-chain statistical associated fluid theory. Ind. Eng. Chem. Res. 44, 3355–3362. doi:10.1021/ie0488142

CrossRef Full Text | Google Scholar

Chen, Y., Sun, Y., Yang, Z., Lu, X., and Ji, X. (2020). CO₂ separation using a hybrid choline-2-pyrrolidine-carboxylic acid/polyethylene glycol/water absorbent. Appl. Energy 257, 113962. doi:10.1016/j.apenergy.2019.113962

CrossRef Full Text | Google Scholar

Clark, T. D., Raby, G. D., Roche, D. G., Binning, S. A., Speers-Roesch, B., Jutfelt, F., et al. (2020). Ocean acidification does not impair the behaviour of coral reef fishes. Nature 577, 370–375. doi:10.1038/s41586-019-1903-y

PubMed Abstract | CrossRef Full Text | Google Scholar

D'Alessandro, D. M., Smit, B., and Long, J. R. (2010). Carbon dioxide capture: prospects for new materials. Angew. Chem. Int. Ed. 49, 6058–6082. doi:10.1002/anie.201000431

CrossRef Full Text | Google Scholar

Farahipour, R., Mehrkesh, A., and Karunanithi, A. T. (2016). A systematic screening methodology towards exploration of ionic liquids for CO₂ capture processes. Chem. Eng. Sci. 145, 126–132. doi:10.1016/j.ces.2015.12.015

CrossRef Full Text | Google Scholar

Figueres, C., Le Quéré, C., Mahindra, A., Bäte, O., Whiteman, G., Peters, G., et al. (2018). Emissions are still rising: ramp up the cuts. Nature 564, 27–30. doi:10.1038/d41586-018-07585-6

PubMed Abstract | CrossRef Full Text | Google Scholar

Figueroa, J. D., Fout, T., Plasynski, S., Mcilvried, H., and Srivastava, R. D. (2008). Advances in CO₂ capture technology—the U.S. Department of energy's carbon sequestration program. Int. J. Greenh. Gas Control 2, 9–20. doi:10.1016/s1750-5836(07)00094-1

CrossRef Full Text | Google Scholar

Gonzalez-Miquel, M., Palomar, J., Omar, S., and Rodriguez, F. (2011). CO₂/N₂ selectivity prediction in supported ionic liquid membranes (SILMs) by COSMO-RS. Ind. Eng. Chem. Res. 50, 5739–5748. doi:10.1021/ie102450x

CrossRef Full Text | Google Scholar

Goodrich, B. F., De La Fuente, J. C., Gurkan, B. E., Zadigian, D. J., Price, E. A., Huang, Y., et al. (2010). Experimental measurements of amine-functionalized anion-tethered ionic liquids with carbon dioxide. Ind. Eng. Chem. Res. 50, 111–118. doi:10.1021/ie101688a

CrossRef Full Text | Google Scholar

Gross, J., and Sadowski, G. (2000). Application of perturbation theory to a hard-chain reference fluid: an equation of state for square-well chains. Fluid Phase Equilibria 168, 183–199. doi:10.1016/s0378-3812(00)00302-2

CrossRef Full Text | Google Scholar

Gross, J., and Sadowski, G. (2001). Perturbed-Chain SAFT: an equation of state based on a perturbation theory for chain molecules. Ind. Eng. Chem. Res. 40, 1244–1260. doi:10.1021/ie0003887

CrossRef Full Text | Google Scholar

Gubbins, K. E., Gu, K., Huang, L., Long, Y., Mansell, J. M., Santiso, E. E., et al. (2018). surface-driven high-pressure processing. Engineering 4, 311–320. doi:10.1016/j.eng.2018.05.004

CrossRef Full Text | Google Scholar

Gupta, M., Da Silva, E. F., Hartono, A., and Svendsen, H. F. (2013). Theoretical study of differential enthalpy of absorption of CO₂ with MEA and MDEA as a function of temperature. J. Phys. Chem. B 117, 9457–9468. doi:10.1021/jp404356e

PubMed Abstract | CrossRef Full Text | Google Scholar

Hanna, R., Abdulla, A., Xu, Y., and Victor, D. G. (2021). Emergency deployment of direct air capture as a response to the climate crisis. Nat. Commun. 12, 368. doi:10.1038/s41467-020-20437-0

PubMed Abstract | CrossRef Full Text | Google Scholar

Iea (2020). Global energy-related CO2 emissions, 1900-2020. [Online]. Available: https://www.iea.org/data-and-statistics/charts/global-energy-related-co2-emissions-1900-2020 (Accessed April 30, 2020).

Google Scholar

Ji, X., and Adidharma, H. (2008). Ion-based SAFT2 to represent aqueous multiple-salt solutions at ambient and elevated temperatures and pressures. Chem. Eng. Sci. 63, 131–140. doi:10.1016/j.ces.2007.09.010

CrossRef Full Text | Google Scholar

Ji, X., and Adidharma, H. (2012). Prediction of molar volume and partial molar volume for CO₂/ionic liquid systems with heterosegmented statistical associating fluid theory. Fluid Phase Equilibria 315, 53–63. doi:10.1016/j.fluid.2011.11.014

CrossRef Full Text | Google Scholar

Ji, X., and Adidharma, H. (2010). Thermodynamic modeling of CO₂ solubility in ionic liquid with heterosegmented statistical associating fluid theory. Fluid Phase Equilibria 293, 141–150. doi:10.1016/j.fluid.2010.02.024

CrossRef Full Text | Google Scholar

Ji, X., and Held, C. (2016). Modeling the density of ionic liquids with ePC-SAFT. Fluid Phase Equilibria 410, 9–22. doi:10.1016/j.fluid.2015.11.014

CrossRef Full Text | Google Scholar

Ji, X., Held, C., and Sadowski, G. (2012). Modeling imidazolium-based ionic liquids with ePC-SAFT. Fluid Phase Equilibria 335, 64–73. doi:10.1016/j.fluid.2012.05.029

CrossRef Full Text | Google Scholar

Ji, X., Held, C., and Sadowski, G. (2014). Modeling imidazolium-based ionic liquids with ePC-SAFT. Part II. Application to H₂S and synthesis-gas components. Fluid Phase Equilibria 363, 59–65. doi:10.1016/j.fluid.2013.11.019

CrossRef Full Text | Google Scholar

Ji, Y., Ji, X., Lu, X., and Tu, Y. (2013). Modeling mass transfer of CO₂ in brine at high pressures by chemical potential gradient. Sci. China Chem. 56, 821–830. doi:10.1007/s11426-013-4834-8

CrossRef Full Text | Google Scholar

Kamgar, A., and Rahimpour, M. R. (2016). Prediction of CO₂ solubility in ionic liquids with QM and UNIQUAC models. J. Mol. Liq. 222, 195–200. doi:10.1016/j.molliq.2016.06.107

CrossRef Full Text | Google Scholar

Khan, N. A., Hasan, Z., and Jhung, S. H. (2014). Ionic liquids supported on metal-organic frameworks: remarkable adsorbents for adsorptive desulfurization. Chem. Eur. J. 20, 376–380. doi:10.1002/chem.201304291

PubMed Abstract | CrossRef Full Text | Google Scholar

Kilaru, P., Baker, G. A., and Scovazzo, P. (2007). Density and surface tension measurements of imidazolium-, quaternary phosphonium-, and ammonium-based room-temperature ionic liquids: data and correlations. J. Chem. Eng. Data 52, 2306–2314. doi:10.1021/je7003098

CrossRef Full Text | Google Scholar

Kim, I., and Svendsen, H. F. (2011). Comparative study of the heats of absorption of post-combustion CO₂ absorbents. Int. J. Greenh. Gas Control 5, 390–395. doi:10.1016/j.ijggc.2010.05.003

CrossRef Full Text | Google Scholar

Lee, B.-S., and Lin, S.-T. (2015). Screening of ionic liquids for CO₂ capture using the COSMO-SAC model. Chem. Eng. Sci. 121, 157–168. doi:10.1016/j.ces.2014.08.017

CrossRef Full Text | Google Scholar

Li, L., Zhao, N., Wei, W., and Sun, Y. (2013). A review of research progress on CO₂ capture, storage, and utilization in Chinese Academy of Sciences. Fuel 108, 112–130. doi:10.1016/j.fuel.2011.08.022

CrossRef Full Text | Google Scholar

Maiti, A. (2009). Theoretical screening of ionic liquid solvents for carbon capture. ChemSusChem 2, 628–631. doi:10.1002/cssc.200900086

PubMed Abstract | CrossRef Full Text | Google Scholar

Palomar, J., Gonzalez-Miquel, M., Polo, A., and Rodriguez, F. (2011). Understanding the physical absorption of CO₂ in ionic liquids using the COSMO-RS method. Ind. Eng. Chem. Res. 50, 3452–3463. doi:10.1021/ie101572m

CrossRef Full Text | Google Scholar

Pinilla, C., Del Pópolo, M. G., Lynden-Bell, R. M., and Kohanoff, J. (2005). Structure and dynamics of a confined Ionic liquid. Topics of relevance to dye-sensitized solar cells. J. Phys. Chem. B 109, 17922–17927. doi:10.1021/jp052999o

PubMed Abstract | CrossRef Full Text | Google Scholar

Sha, M., Wu, G., Fang, H., Zhu, G., and Liu, Y. (2008). Liquid-to-solid phase transition of a 1,3-dimethylimidazolium chloride ionic liquid monolayer confined between graphite walls. J. Phys. Chem. C 112, 18584–18587. doi:10.1021/jp8079183

CrossRef Full Text | Google Scholar

Sha, M., Wu, G., Liu, Y., Tang, Z., and Fang, H. (2009). Drastic phase transition in ionic liquid [Dmim][Cl] confined between graphite walls: new phase formation. J. Phys. Chem. C 113, 4618–4622. doi:10.1021/jp810980v

CrossRef Full Text | Google Scholar

Shen, G., Held, C., Lu, X., and Ji, X. (2015). Modeling thermodynamic derivative properties of ionic liquids with ePC-SAFT. Fluid Phase Equilibria 405, 73–82. doi:10.1016/j.fluid.2015.07.018

CrossRef Full Text | Google Scholar

Shen, G., Laaksonen, A., Lu, X., and Ji, X. (2018). Developing electrolyte perturbed-chain statistical associating fluid theory density functional theory for CO₂ separation by confined ionic liquids. J. Phys. Chem. C 122, 15464–15473. doi:10.1021/acs.jpcc.8b04120

CrossRef Full Text | Google Scholar

Shi, W., and Luebke, D. R. (2013). Enhanced gas absorption in the ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) confined in silica slit pores: a molecular simulation study. Langmuir 29, 5563–5572. doi:10.1021/la400226g

PubMed Abstract | CrossRef Full Text | Google Scholar

Shi, W., and Sorescu, D. C. (2010). Molecular simulations of CO₂ and H₂ sorption into ionic liquid 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([hmim][Tf2N]) confined in carbon nanotubes. J. Phys. Chem. B 114, 15029–15041. doi:10.1021/jp106500p

PubMed Abstract | CrossRef Full Text | Google Scholar

Singh, H. A., and Polvani, L. M. (2020). Low antarctic continental climate sensitivity due to high ice sheet orography. npj Clim. Atmos. Sci. 3, 39. doi:10.1038/s41612-020-00143-w

CrossRef Full Text | Google Scholar

Sun, Y., Zuo, Z., Laaksonen, A., Lu, X., and Ji, X. (2020). How to detect possible pitfalls in ePC-SAFT modelling: extension to ionic liquids. Fluid Phase Equilibria 519, 112641. doi:10.1016/j.fluid.2020.112641

CrossRef Full Text | Google Scholar

Sun, Y., Zuo, Z., Shen, G., Held, C., Lu, X., and Ji, X. (2021). Modeling interfacial properties of ionic liquids with ePC-SAFT combined with density gradient theory. Fluid Phase Equilibria 536, 112984. doi:10.1016/j.fluid.2021.112984

CrossRef Full Text | Google Scholar

Taheri, M., Zhu, R., Yu, G., and Lei, Z. (2021). Ionic liquid screening for CO₂ capture and H₂S removal from gases: the syngas purification case. Chem. Eng. Sci. 230, 116199. doi:10.1016/j.ces.2020.116199

CrossRef Full Text | Google Scholar

Theo, W. L., Lim, J. S., Hashim, H., Mustaffa, A. A., and Ho, W. S. (2016). Review of pre-combustion capture and ionic liquid in carbon capture and storage. Appl. Energy 183, 1633–1663. doi:10.1016/j.apenergy.2016.09.103

CrossRef Full Text | Google Scholar

Vicent-Luna, J. M., Gutiérrez-Sevillano, J. J., Anta, J. A., and Calero, S. (2013). Effect of room-temperature ionic liquids on CO₂ separation by a Cu-BTC metal–organic framework. J. Phys. Chem. C 117, 20762–20768. doi:10.1021/jp407176j

CrossRef Full Text | Google Scholar

Wang, H., Ma, C., Yang, Z., Lu, X., and Ji, X. (2020). Improving high-pressure water scrubbing through process integration and solvent selection for biogas upgrading. Appl. Energy 276, 115462. doi:10.1016/j.apenergy.2020.115462

CrossRef Full Text | Google Scholar

Wang, X., Akhmedov, N. G., Duan, Y., Luebke, D., Hopkinson, D., and Li, B. (2013a). Amino acid-functionalized ionic liquid solid sorbents for post-combustion carbon capture. ACS Appl. Mat. Interfaces 5, 8670–8677. doi:10.1021/am402306s

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, X., Akhmedov, N. G., Duan, Y., Luebke, D., and Li, B. (2013b). Immobilization of amino acid ionic liquids into nanoporous microspheres as robust sorbents for CO₂ capture. J. Mat. Chem. A 1, 2978. doi:10.1039/c3ta00768e

CrossRef Full Text | Google Scholar

Wu, N., Ji, X., Xie, W., Liu, C., Feng, X., and Lu, X. (2017). Confinement phenomenon effect on the CO₂ absorption working capacity in ionic liquids immobilized into porous solid supports. Langmuir 33, 11719–11726. doi:10.1021/acs.langmuir.7b02204

PubMed Abstract | CrossRef Full Text | Google Scholar

Yan, X., Zhang, L., Zhang, Y., Yang, G., and Yan, Z. (2011). Amine-modified SBA-15: effect of pore structure on the performance for CO₂ capture. Ind. Eng. Chem. Res. 50, 3220–3226. doi:10.1021/ie101240d

CrossRef Full Text | Google Scholar

Zhang, J., Zhang, Q., Li, X., Liu, S., Ma, Y., Shi, F., et al. (2010). Nanocomposites of ionic liquids confined in mesoporous silica gels: preparation, characterization and performance. Phys. Chem. Chem. Phys. 12, 1971–1981. doi:10.1039/c001176m10.1039/b920556j

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhang, J., Zhang, S., Dong, K., Zhang, Y., Shen, Y., and Lv, X. (2006). Supported absorption of CO₂ by tetrabutylphosphonium amino acid ionic liquids. Chem. Eur. J. 12, 4021–4026. doi:10.1002/chem.200501015

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhang, S., Zhang, J., Zhang, Y., and Deng, Y. (2017). Nanoconfined ionic liquids. Chem. Rev. 117, 6755–6833. doi:10.1021/acs.chemrev.6b00509

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhang, X., Zhang, X., Dong, H., Zhao, Z., Zhang, S., and Huang, Y. (2012). Carbon capture with ionic liquids: overview and progress. Energy Environ. Sci. 5, 6668. doi:10.1039/c2ee21152a

CrossRef Full Text | Google Scholar

Zhang, Y., Ji, X., Xie, Y., and Lu, X. (2016). Screening of conventional ionic liquids for carbon dioxide capture and separation. Appl. Energy 162, 1160–1170. doi:10.1016/j.apenergy.2015.03.071

CrossRef Full Text | Google Scholar

Zhang, Z., Wu, L., Dong, J., Li, B.-G., and Zhu, S. (2009). Preparation and SO₂ sorption/desorption behavior of an ionic liquid supported on porous silica particles. Ind. Eng. Chem. Res. 48, 2142–2148. doi:10.1021/ie801165u

CrossRef Full Text | Google Scholar

Nomenclature

Abbreviations

ILs Ionic liquids

ePC-SAFT Electrolyte perturbed-chain statistical associating fluid theory

[C_nmim]⁺ 1-n-alkyl-3-methylimidazolium cation

[C_npy]⁺ 1-n-alkyl-pyridinium cation

[C_nmpy]⁺ 1-n-alkyl-3-methylpyridinium cation

[C_nmpyr]⁺ 1-methyl-1-n-alkylpyrrolidinium cation

[THTDP]⁺ Trihexyltetradecylphosphonium cation

[Tf₂N]^- Bis(trifluoromethyl-sulfonyl)amide anion

[PF₆]^- Hexafluorophosphate anion

[BF₄]^- Tetrafluoroborate anion

[tfo]^- Trifluoromethansulfonate anion

[DCA]^- Dicyanamide anion

[SCN]^- Thiocyanate anion

[C₁SO₄]^- Methylsulfate anion

[C₂SO₄]^- Ethylsulfate anion

[eFAP]^- Tris(pentafluoroethyl)trifluorophosphate anion

Cl⁻ Chloride anion

[Ac]^- Acetate anion

Br⁻ Bromide anion

Symbols

κ_T Isothermal Compression Coefficient

ΔH Enthalpy

P Pressure

T Temperature

A Helmholtz energy

κ Inverse Debye-screening length

$\epsilon$ _τ Relative dielectric constant of the medium ₀ Dielectric constant of vacuum

$\epsilon$ _τ Relative dielectric constant of the medium ₀ Dielectric constant of vacuum

$x_j$ Mole fraction of ion j

$q_j$ Charge of ion j

$\rho$ Density

${\phi }_{C{O}_2}^L$ Fugacity coefficients for CO₂ in the liquid phases

${\phi }_{C{O}_2}^V$ Fugacity coefficients for CO₂ in the vapor phases

$v^L$ Molar volumes of liquid phases

$v^V$ Molar volumes of vapor phases

${\mathit{H}}_{C{O}_2}$ Henry coefficient of CO₂

R Gas constant

Z Compressibility factor

P_c Critical pressure

T_c Critical temperature

k Boltzmann constant

${\mu }_{C{O}_2}^{res}\left(T,v^V\right)$ Chemical potential of CO₂