Cation functional group effect on SO2 absorption in amino acid ionic liquids

Introduction: The effect of the functional group of the cation on SO2 acidic gas absorption by some designed amino acid ionic liquids (AAILs) was studied.

Methods: An isolated pair of glycinate anion and pristine imidazolium-based cation, as well as decorated cation functionalized by hydroxyl (OH), amine (NH₂), carboxylic acid (COOH), methoxy (OCH₃), and acetate (CH₃COO) groups, were structurally optimized by density functional theory (DFT) using split-valence triple-zeta Pople basis set.

Results and Discussion: The binding and Gibbs free energy (ΔG_int) values of SO₂ absorption show the AAIL functionalized by the COOH group is the most thermodynamically favorable green solvent and this functional group experiences the closest distance between anion and captured SO₂ and vice versa in the case of cation … SO₂ which may be the main reason for being the best absorbent; in addition, the highest net charge-transfer amount of SO₂ is observed. Comparing the non-covalent interaction of the systems demonstrates that the strongest hydrogen bond between captured gas and anion, as well as π-hole, and van der Waals (vdW) interaction play critical roles in gas absorption; besides, the COOH functional group decreases the steric effect while the CH₃COO functional group significantly increases steric effect after absorption that declines the hydrogen bond.

Introduction

Looking at the environmental effects caused by sulfur dioxide (SO₂) gas, effective technology for flue gas desulfurization (FDG) is of great practical importance. SO₂, as an acidic gas, is released from the combustion of fossil fuels containing sulfur in power plants, incinerators, and boilers; the roasting of sulfide ore in metallurgy and sulfuric acid industry are the major sources of this atmospheric pollution as well as the main component of acid rain and fog. The high toxicity of this gas leads to many problems for human health and due to the presence of moisture in the atmosphere, the oxides in the air react with SO₂ and produce sulfuric acid, which is the main precursor for acid rain (Cui et al., 2021a; Zhu et al., 2021; Mohammadi et al., 2022; Zhang et al., 2022). The emission of SO₂ leads to the formation of acid rain, photochemical smog (Li et al., 2021a), and has adverse effects on the quality of the ecosystem. From the other side of view, SO₂ is a stronger acid than carbon dioxide gas, and the presence of sulfur dioxide in small amounts in the flue gas has adverse effects on the removal of CO₂ gas. Moreover, SO₂ has an impact on the post-combustion CO₂ capture applications and reduces the CO₂ absorption capacity and lifetime of the absorbent as well as increasing the operating costs (Zhang et al., 2020). As a result, SO₂ capturing is of great practical importance.

Conventional desulfurization methods, such as ammonia/amine scrubbing, wet washing, and limestone scrubbing, lead to the production of low-value by-products, solid gypsum waste, wastewater, highly polluted water, and volatile organic compounds (VOCs) caused by solvent evaporation (Jiang et al., 2020; Sheng et al., 2021). In other words, they create secondary pollution and have limited application due to low reversibility. A prerequisite for a successful gas capturing technology is low price and low energy consumption. SO₂ gas reacts with amines and creates irreversible salts, accordingly it reduces the lifetime of the absorber and increases the operating cost. Therefore, effective, clean, and green technology should be explored.

Attributable to the properties of thermal stability, low vapor pressure, physical and chemical stability, adjustability, non-volatility, tuneability, and great tendency to capture SO₂, ionic liquids (ILs) are a hot research topic in the field of gas absorption as green and clean solvents (Mondal and Balasubramanian, 2016; Yang et al., 2017; Cui et al., 2021b; Rashid, 2021; Liu et al., 2022a). In addition, functional ILs have shown efficient SO₂ absorption (Ren et al., 2018; Wang et al., 2020; Geng et al., 2022). Doblinger et al. (2021) have experimentally measured polar gas SO₂ solubility in non-functionalized IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and the functionalized alkyl side-chain of cation by hydroxyl, cyanide, and methyl benzyl. SO₂ was physically dissolved in target ILs. To answer why functionalized ILs are applied for absorption one should state that typical ILs have physical absorption and therefore limited absorption capacity; adding functional groups such as hydroxyl, amine, and ether leads to more absorption of SO₂ based on physical and chemical absorption (Liu et al., 2022b). ILs based on imidazolium (Li et al., 2021a; Doblinger et al., 2021), guanidinium (Wang et al., 2007; Geng et al., 2022), and pyridinium (Zeng et al., 2014; Yan et al., 2019) can physically dissolve the gas. In addition to experimental efforts (Huang et al., 2014; Cui et al., 2020; Wang et al., 2021a; Cui et al., 2021b; Geng et al., 2021; Liu et al., 2022b; Hou et al., 2022) and theoretical and computational studies (Wang et al., 2007; Wang et al., 2022a; Mohammadi et al., 2022) to investigate the solubility of gases in ILs, ab initio and first principle investigations have also been conducted to inspect the structure and mechanism of the complex of IL and SO₂ gas (Gu et al., 2013; Herrera et al., 2017; Cui et al., 2020; Wang et al., 2020; Li et al., 2021a; Li et al., 2021b; Yin et al., 2021; Zhu et al., 2021; Liu et al., 2022b). For example, Wang et al. (2007) simulated the solubility of CO₂ and SO₂ in guanidium-based IL and Zhang et al. (2020) investigated the stability effect of ILs during the CO₂ absorption process in the presence of SO₂. Due to the ability of chemical removal of SO₂ by ILs, the chemical reaction of SO₂ gas has been taken into consideration through ab initio quantum computations (Piacentini et al., 2022).

ILs also face toxicity, high viscosity, and high cost of production that limit their application in FGD technology. However, to enhance their efficiency, task-specific ILs, i.e., amino acid ionic liquids (AAILs) were proposed which are environmentally friendly, easily available, biodegradable, non-toxic, and due to having two amino and carboxylic groups, they are suitable for the desulfurization process (Wang et al., 2020; Piacentini et al., 2022). An experimental investigation has shown that single-amino amino acids, especially glycine, have good absorption performance and gas saturation uptake increases to 0.461 g/g (Wang et al., 2022b). Herrera et al. (2017) confirmed by DFT computation that glycinate anion in the case of [EMIM][GLY] has a stronger interaction with captured SO₂ (E_int = −126.8 kJ mol⁻¹ between anion and SO₂ in comparison to −37.0 kJ mol⁻¹ between [EMIM]⁺ and gas). Yin et al. (2021) have shown that the interaction energy is directly related to the bond length and bond angle of SO₂ and this gas capturing by silica-based porous IL is performed due to the charge transformation. Gu et al. (2013) have demonstrated that combination energy of SO₂ and [BMIM][MeSO₄] IL is equal to 10.86 kcal/mol and the absorption occurs due to the reducing aromaticity of the imidazolium ring and electrophilicity of SO₂. Li et al. (2021b) have stated that carboxylic groups in the structure of IL increase absorption performance; moreover, absorption energy close to or equal to −123 kJ mol⁻¹ leads to an easy release of the SO₂ in desorption conditions.

Due to the significant potential of ILs and AAILs for SO₂ capturing application as well as the ability to utilize computational methods to reliably simulate the molecular properties of these green and safe solvents, the important objective of the current research is to examine the physical absorption of SO₂ in tunable imidazolium-based AAILs. The foundation for such simulations is providing a molecular understanding of the process of SO₂ physical absorption by some imidazolium/amino acid ILs to evaluate the effect of the functional group of imidazolium cation alkyl chain on the absorption of SO₂. Though many researchers have studied the absorption of acid gases both experimentally and theoretically, as mentioned above, the gas absorption by the AAILs is still obscure. Consequently, various structural factors affecting the absorption of SO₂ are discussed based on the present results.

Computational details

Density functional theory (DFT) simulations using Gaussian09 reversion A.01 (Frisch et al., 2009) were conducted to further understand the effect of the cation functional group on SO₂ capturing by AAILs based on imidazolium cation. As DFT is one of the most efficient methods for characterizing molecular structures, conformational properties, and hydrogen bond (HB) interaction for this class of compounds, here, all computations were performed by DFT (Cao et al., 2016; Ebrahimi and Moosavi, 2018; Haddad et al., 2020). Carrying out the DFT computation leads to a precise quantification of short-range interactions (Herrera et al., 2017) between AAIL and SO₂ gas; as a result, binding energy and favorable interaction sites will be discovered. In this line, screening and the most suitable design of ions will be performed.

All the calculations were performed using the Becke-three-parameter (B3) for the exchange part and the Lee-Yang-Parr (LYP) gradient-corrected functional with split-valence triple-zeta Pople basis set beside the polarization and dispersion functions, 6-311++G(d,p) basis set, in vacuum for the ground state optimization. Harmonic vibrational frequencies were computed at the same level to confirm that all studied geometries do not have imaginary frequency, i.e., they are corresponding to the local minima on the potential energy surfaces. Optimized structures were applied to find the binding and Gibbs Free energies, and Gaussian NBO version 3.1 (Fogarty et al., 2018) has been utilized to calculate partial atomic charges, atomic orbital occupancies, and its contribution to bonding interaction and delocalization of electron density within the SO₂ and AAIL complexes. Determination of the atomic charges was performed at the same level of theory that was used for the geometry optimization without any symmetry constraint. Afterward, the binding energy was calculated at the same level of theory. The binding energy (BE) of AAIL-gas complexes was obtained with the following relation:

$BE=E_{AAIL...S{O}_2}-E_{AAIL}-E_{gas}(1)$

where $E_{AAIL...S{O}_2}$ represents the total energy of the optimized AAIL-gas complexes and $E_{AAIL}$ and $E_{gas}$ are the total energy of the optimized isolated AAIL and gas molecule, respectively. The optimized configuration which had the lowest binding energy was selected for further investigation and discussion.

The net charge-transfer amount (NCTA) of SO₂ absorbed by each AAIL was analyzed by the NBO population and calculated with the following relation:

$NCT{A}_{S{O}_2}=C_{AAIL...S{O}_2}-C_{gas}(2)$

among them, $C_{AAIL...S{O}_2}$ is the charge of the gas absorbed by each AAIL and $C_{gas}$ illustrates the charge of the optimized gas molecule which is zero since it is a neutral molecule in the isolated state (free gas).

Six different initial configurations or binding sites for gas in geometry optimized AAILs were constructed, wherein the SO₂ molecule was kept near the cation, close to the anion (both carboxylic and amine groups, separately), between the AAIL ion pairs, on the cation chain, and near hydrogen atoms of imidazolium ring sites and all these configurations were optimized. Each optimized minimum on the potential energy surface was confirmed via frequency analysis. Toward this end and to better understand the cation functional group effect on the interactions between the AAIL and SO₂ from the atomic point of view, DFT computations were conducted at the same procedure to determine the BEs and NCTAs. The structures under investigation are 1-propyl-3-methylimidazolium glycinate, [C₃MIM][GLY] besides to the cation propyl chain functionalized by five different functional groups including hydroxyl (−OH), amine (−NH₂), carboxylic acid (−COOH), methoxy (−OCH₃), and acetate (−CH₃COO), represented as [C₃OHMIM][GLY], [C₃NH₂MIM][GLY], [C₃COOHMIM][GLY], [C₃OCH₃MIM][GLY], and [C₃OOCCH₃MIM][GLY], respectively; to have accessibility to task-specific AAIL for gas absorption and explore the role of cation functional group on gas absorption capacity and perform a micro-mechanism analysis. After carrying out all computations, non-covalent interaction reduced density gradient (NCI-RDG) (Johnson et al., 2010; Otero-De-La-Roza et al., 2012) by Multiwfn (Lu and Chen, 2012) as well as VMD (Humphrey et al., 1996) software and electrostatic surface potential (ESP) were computed and also applied for some visual analyses.

Results and discussion

To examine the effect of the AAIL compound besides its structure on the SO₂ capturing, which is critically important in separation performance, quantum chemistry calculations were carried out. To select the most stable structure, first of all, the cation alkyl chain length was changed from methyl to hexyl, [C₁MIM][GLY] to [C₆MIM][GLY], and it was found that [C₃MIM][GLY] has the strongest interaction with SO₂ gas. As a result, this AAIL was selected as the base of the current study. After that, six different configurations of SO₂ concerning ion pairs, as mentioned in the previous section were optimized. To shed light on this point, the most stable configuration of captured gas with respect to AAIL was deeply studied and all the most stable structures are shown in Figure 1.

FIGURE 1

FIGURE 1. The most stable geometry for studied AAIL…SO₂ complexes. The white, gray, blue, red, and yellow denote H, C, N, O, and S atoms, respectively. The closest distance (Å) between cation…SO₂ and anion…SO₂ is represented in each case. (A) [C₃MIM][GLY], (B) [C₃COOHMIM][GLY], (C) [C₃NH₂MIM][GLY], (D) [C₃OCH₃MIM][GLY], (E) [C₃OHMIM][GLY], (F) [C₃CH₃COOMIM][GLY].

According to Figure 1, in all cases, the distance between trapped gas and anion is less than SO₂ and cation. As the distance between the S atom of SO₂ and O atom of the anion, S…O distance, is smaller than O…H distance, the distance between the O atom of anion and H atom of the methyl group in cation, it is understood that anion has the main role in capturing SO₂ gas. However, in both cases, the distance is smaller than the sum of van der Waals (vdW) radii of S (1.80 Å), O (1.52 Å), and H (1.20 Å) atoms reported by Gu et al. (2013) for S…O and O…H that verifies both electrostatic and HB interactions are observed in this binding, i.e., the gas capturing is occurred due to the stronger physical interaction between anion and SO₂ gas. In all cases, except AAIL functionalized by the acetate functional group (Figure 1F), the imidazolium ring of cation interacts with the trapped gas from its methyl side chain. In addition, the closer distance between anion and absorbed gas in comparison to cation illustrates that the SO₂ molecule interacts more strongly with the anion; in other words, the moderate interaction causes a closer distance to the carboxylic acid group of [GLY]⁻. In a similar way to the anion, SO₂ is an acceptor molecule here. Anion has extra electrons or negative charge; then, it plays the role of a donor species. Accordingly, SO₂ tends to be near the anion instead of the cation and is an electrophile compound.

Distance between anion and cation before and after the absorption process is reported in Table 1. It is observable that cation-anion distance increases through gas absorption except that it does not change by absorption in [C₃COOHMIM][GLY] AAIL. While the aforementioned distance in other systems is affected by absorption of SO₂, the cation-anion distance of [C₃COOHMIM][GLY] AAIL is unchanged and [C₃CH₃OOMIM][GLY] AAIL experiences the greatest increase which agrees well with the result of NCTA of absorbed SO₂. There is a specific charge transfer interaction between SO₂ and the anionic species of AAILs. The higher the anion basicity, the greater the interaction with SO₂ and the greater the AAILs capacity for gas absorption (Mohammadi et al., 2022), which follows the same trend as cation-anion distance. It is more pronounced that E_gap = E_LUMO-E_HOMO is the lowest one if HB interaction is formed between trapped gas and AAIL. Based on Figure 1 and Table 1, the absorbed SO₂ gas distance is close to the cation of [C₃CH₃COOMIM][GLY] AAIL while it is at the uttermost distance from the anion in comparison with the other AAILs and the smallest SO₂ absorbed NCTA has occurred; moreover, its position is different from the other target AAILs.

TABLE 1

TABLE 1. Distance (Å) between anion and cation (r_cation-anion) before and after absorption as well as SO₂ adsorbed NCTA (e) and E_gap (eV).

In general, COOH functional group increases the distance between cation and SO₂ which may be the main reason for being the best absorbent in this study. In addition, the NCTA of SO₂ in that system is the highest; the CH₃COO functional group decreases SO₂…cation distance which leads to the lower interaction energy; as can be seen, the amount of charge transfer is also the lowest value. SO₂…anion distance is also the highest for this functional group which is the main reason for lower absorption energy; it will be discussed in the following paragraphs.

The atomic charge of the center of the atom or the atomic charge is the simplest and most intuitive concept to describe the charge distribution in a chemical system. This characteristic has important claims, such as helping to determine the state of atoms in different chemical environments, checking molecular properties, site of reaction predictions, etc. Because the atomic charge affects the dipole moment, polarizability, electronic structure, acid-base behavior, and many other molecular properties of the system, charge calculation plays an important role in the application of quantum chemical computations. Natural population analysis (NPA) is widely used for AAILs. If the charge variation in gas from the pure state to the absorbed one is small, it means that no significant charge transfer occurs. In the case of AAIL functionalized by acetate, the lowest NCTA value of SO₂ adsorbed shows the weakest AAIL and SO₂ interaction which leads to the lowest Wiberg bond index that enjoys strong correlations with each other (Ge et al., 2019).

Interestingly, the BE and the interaction Gibbs Free energy (ΔG_int) of all systems demonstrate quantitatively the AAIL capacity in gas absorption; see Figure 2 for more details.

FIGURE 2

FIGURE 2. SO₂ BE and ΔG_int values of absorption for the studied AAIL systems.

As can be seen, by changing the functional group, three different regimes are observed for the studied systems: 1) COOH functional group increases the BE and ΔG_int of SO₂ in AAIL, 2) CH₃COO functional group dramatically decreases these values, and 3) inserting the functional groups of NH₂, OCH₃, and OH into the AAIL structure does not make an observable change in BE and ΔG_int. Apart from the BE, the thermodynamic parameter ΔG_int is a negative value indicating that the absorption process is a spontaneous process. The results of ΔG_int also exhibit that COOH functional group intensifies the absorption while CH₃COO decreases ΔG_int significantly which implies the carboxylic functional group thermodynamically improves the absorption capacity of SO₂ by [C₃COOHMIM][GLY] AAIL. In addition, the NH₂, OCH₃, and OH functional groups do not cause a substantial change in the SO₂ capturing. Additionally, according to the BE and ΔG_int variations through gas absorption, it is proved that [C₃COOHMIM][GLY] AAIL enjoys the highest stability which is in agreement with Wang et al. (2021a) results that the carboxyl group is responsible for this high amount of energy. Yin et al. (2021) have computed the interaction energy between porous ILs (PILs) and SO₂; inspired by this work, the interaction energy is less than the values obtained in the current study. In other words, the largest interaction energy reported by Yin et al. (2021) is less than the current results showing that AAILs have more capacity in SO₂ capturing in comparison to PILs.

The related values of BE present quantitively the magnitudes of interaction between AAIL and SO₂. It is noticeable that the BE values are in the range of −67.90 to −90.34 kJ mol⁻¹ demonstrating a weak interaction between the absorbent and the SO₂ gas molecule. In the case of amine, ether, and hydroxyl functional groups, the BE values are to some extent equal and the S atom of SO₂ has also equal NCTA. AAIL with the carboxylic acid functional group is the most stable structure; it may be that SO₂ breaks the HB between anion and cation through charge transfer of SO₂⋅⋅⋅anion elucidating that the complex has a good ability to absorb SO₂ gas. To evaluate charge transfer in these complexes, atomic site charges were obtained by using the NBO method in the gas phase. The charge of the SO₂ gas after absorption is negative confirming the charge transfer from the AAIL to the gas. Based on Figure 3, there is a direct relationship between BE and $\Delta q_{S{O}_2}$; as a consequence, gas dissolution in the AAIL has enthalpic nature (Mondal and Balasubramanian, 2016).

FIGURE 3

FIGURE 3. SO₂ BE values dependency on NCTA of absorbed SO₂ with the variation of cation functional group.

The current BE values between AAIL and SO₂ are stronger than the values obtained at the same basis set in the case of pyridinium-based ILs (Zeng et al., 2014) elucidating the higher absorption capacity of imidazolium-based AAILs functionalized the cation by ether, amine, hydroxyl, carboxylic acid, and carboxylate functional groups. The rate of change in BE due to the addition of a functional group is 33% and the overall change of the system is related to this energy, the more negative the NCTA of absorbed SO₂, the greater the interaction between trapped SO₂ gas and AAIL.

The structural properties of SO₂ both in the pure state and the optimized structure of each target AAIL… SO₂ complex are described in Table 2 which contains both S=O bond lengths (shown by r_S=O1 and r_S=O2 bond lengths to discriminate these changes) besides the O=S=O bond angle.

TABLE 2

TABLE 2. Structure parameters (S=O bond lengths and the O=S=O bond angle) for SO₂ absorption.

According to the table, if the variation in S=O bond length and O=S=O bond angle due to the absorption is significant, it shows that the interaction between SO₂ and AAIL is considerable. Interestingly, intramolecular parameters in gas SO₂ are in excellent agreement with Yin et al. reported value (Yin et al., 2021). As Table 2 shows, [C₃CH₃COOMIM][GLY] experiences the lowest interaction between the captured gas and AAIL functionalized by the acetate functional group because of the slightest variation in SO₂ bond parameters. In all cases, the bond length change is less than 0.5 Å which confirms a reversible physical absorption. The highest BE is related to the [C₃COOHMIM][GLY] AAIL and the weakest one occurs in the case of [C₃CH₃COOMIM][GLY] AAIL; nonetheless, in both cases, the value of BE is greater than the interaction energy between SO₂ and aqueous glycine (−47.82 kJ mol⁻¹) (Wang et al., 2021b) and [C₄MIM][MeSO₄] (−45.438 kJ mol⁻¹) (Gu et al., 2013), respectively. Considering these facts, if the BE value of AAIL…SO₂ under environmental conditions is appreciated, the AAIL can capture the gas in harsh conditions. In addition, the target AAILs can release the captured gas with physical variations such as temperature or pressure change. As the characteristics of [C₃NH₂MIM][GLY], [C₃OCH₃MIM][GLY], and [C₃OHMIM][GLY] AAILs are similar to each other, in the next paragraphs, it is only concentrated on [C₃MIM][GLY], [C₃COOHMIM][GLY], [C₃CH₃COOMIM][GLY] AAILs complexed with SO₂.

NCI-RDG analyses were performed for each AAIL…SO₂ complex to take into consideration the nature of non-covalent interactions. Furthermore, the goal of NCI evaluation is to find the weak HB, vdW, and steric effect interaction domains from wave-function calculations. This analysis represents a three-dimensional (3D) scheme of interactions in 3D space. Figures 4A–C display the color-filled isosurfaces of the interactions of pristine AAIL, [C₃MIM][GLY], and ones functionalized by carboxylic acid and acetate, i.e., [C₃COOHMIM][GLY] and [C₃CH₃COOMIM][GLY] AAILs, with trapped SO₂. If the colored-filled isosurface is between AAIL and SO₂, gas capture has occurred. Whenever this domain is green, there is a vdW interaction, blue domain shows an HB interaction which is present in [C₃MIM][GLY] and [C₃COOHMIM][GLY] AAILs; in general, the red shows repulsive interactions. According to the figures, blue isosurfaces in the region between the S atom of SO₂ and the O atom of the anion point to considerable electrostatic interactions. Captured SO₂ gas is near to the methyl group of the cation if there is a vdW interaction between gas and cation that is in agreement with Li et al. (2021a) results. It is observable that the interaction between anion and SO₂ for the AAIL functionalized by COOH is the strongest in comparison with two other AAILs ([C₃MIM][GLY] and [C₃COOHMIM][GLY]). In addition, the S atom of SO₂ orients towards the anion, and the two O atoms of SO₂ rotate toward the cation in [C₃MIM][GLY] and [C₃COOHMIM][GLY] AAILs (Figures 4A, B). While only one of the O atoms of SO₂ gas rotates towards the cation, the other orients toward the anion (Figure 4C). This orientation may cause weaker interaction between SO₂ and absorbent. As can be seen, COOH functional group improves SO₂/AAIL interaction because of the strong interaction of the S atom with the anion and the O atoms with the cation. Consequently, these observations confirm the considerable sensitivity of AAIL factionalized by COOH to SO₂ gas in comparison to the other target AAILs.

FIGURE 4

FIGURE 4. NCI-RDG analyses of AAIL complexed with SO₂ (A) [C₃MIM][GLY], (B) [C₃COOHMIM][GLY], and (C) [C₃OOCCH₃MIM][GLY].

Figure 5 exhibits the localized orbital locator (LOL) and ESP analysis of the interaction between each AAIL and SO₂ studied in the gas phase at B3LYP/6-311++G(d,p) level of theory. Normally, a great value of the LOL points to the covalent bond and a small value indicates the electrostatic interactions (Nkungli and Ghogomu, 2017).

FIGURE 5

FIGURE 5. LOL and ESP analyses of the interaction between the AAILs and SO₂: (A) LOL of anion and cation in [C₃MIM][GLY]…SO₂ complex, (B) LOL of SO₂ and anion of [C₃MIM][GLY]…SO₂ complex, (C) ESP of [C₃MIM][GLY]…SO₂ complex, (D) LOL of anion and cation of [C₃COOHMIM][GLY]…SO₂ complex, (E) LOL of SO₂ and anion of [C₃COOHMIM][GLY]…SO₂ complex, (F) ESP of [C₃COOHMIM][GLY]…SO₂ complex, (G) LOL of anion and cation of [C₃OOCCH₃MIM][GLY]…SO₂ complex, (H) LOL of SO₂ and anion of [C₃OOCCH₃MIM][GLY]…SO₂ complex, (I) ESP of [C₃OOCCH₃MIM][GLY]…SO₂ complex.

According to Figure 5, LOL values are insignificant confirming an electrostatic interaction between the AAILs and the SO₂. In addition, ESP analysis is a representation of the most stable configuration and a guide for molecular structure optimization. It can be applied to detect the reactive sites of a molecule in the systems (Politzer and Murray, 2002). The values of the ESP on the 3D map surface follow the trend of red < orange < yellow < green < blue. The blue regions depict electron deficiency (nucleophilic reactivity) and the red regions point to the relative abundance of electrons (electrophilic reactivity). Consistent with Figure 5C, SO₂ is a reactive nucleophilic center for coordination with [C₃MIM][GLY] AAIL and ESP around SO₂ in the [C₃COOHMIM][GLY] AAIL (Figure 5F) is green color meaning that the electron density is balanced. However, in the case of AAIL functionalized by CH₃COO, the electron density is somewhat out of equilibrium (Figure 5I). Studying the non-covalent interactions including vdW, HB, and electrostatic is of great importance and can be also carried out by ESP analysis which is suitable for a qualify interaction analysis. HB is the main factor in gas absorption by AAIL. The results demonstrate that the positive charge on the S atom (around 1.60 e) of SO₂ and the negative charge of the O atom from glycinate anion (about −0.76 e) lead to interaction between AAIL and SO₂. When SO₂ interacts with AAIL, in agreement with Yin et al. (2021) study, SO₂ interaction with cation from O atom by HB occurs and its interaction with the anion from S atom belongs to electrostatic interaction. As a result, it seems that increasing the number of carboxylate groups in AAIL structure is an efficient parameter for SO₂ absorption by AAIL.

The non-covalent interaction (NCI) analysis can be used to determine the interactions based on electron density and the sign of the second derivative in the perpendicular direction of the bond (λ₂) (Geng et al., 2022). This analysis provides a clear description of the attractive and repulsive interactions between AAILs and SO₂. A large positive value of sign (λ₂ ρ) points to steric repulsion (Figure 6), a large negative value of sign (λ₂ ρ) refers to the HB (Figure 6), and the value near to zero (λ₂ ≤ 0) denotes the vdW interactions (Figure 6). By comparing the NCI of these three systems, it sheds light on the interaction types and the spatial positions between AAIL and SO₂ at the atomic and molecular levels; it is observable that the COOH functional group decreases the steric effect after absorption of SO₂ while the CH₃COO functional group significantly increases the steric effect after absorption. COOH functional group does not change HB and vdW interactions whereas the CH₃COO functional group shrinkages this interaction. It is worth mentioning that before absorption of SO₂ the steric effects of all systems are similar and absorption of SO₂ significantly decreases the steric effect; in all systems, the vdW and HB experience a slight decrease after absorption. The absorption mechanism systematically investigated from the quantum chemical point of view shows that RDG analysis is a clear visualization method for weak interaction sites. It is crystal clear that vdW interaction between [C₃CH₃COOMIM][GLY] AAIL and SO₂ is not absent and the weakest is present while the strongest vdW interaction is seen in [C₃COOHMIM][GLY]…SO₂ complex system; therefore, [C₃COOHMIM][GLY] AAIL has the ability to inhibit the interaction of SO₂ with other gases and improve its absorption rate.

FIGURE 6

FIGURE 6. The non-covalent interaction (NCI) analyses for studied AAILs and AAIL…SO₂ complexes. (A) [C₃MIM][GLY] AAIL, (B) [C₃MIM][GLY]…SO₂, (C) [C₃COOHMIM][GLY], (D) [C₃COOHMIM][GLY]…SO₂, (E) [C₃CH₃COOMIM][GLY], and (F) [C₃CH₃COOMIM][GLY]…SO₂.

The above facts designate that weak interaction magnitude is related to BE but the interaction can not be observed graphically and tangible. Therefore, Johnson et al. (2010) have suggested RDG analysis method that can display the non-covalent interaction graphically, Figure 6. According to these figures, a spike near zero and the right side is a non-covalent bond. A λ₂ > 0 is a sign of non-bonding interaction and λ₂ < 0 shows a bonding interaction, where λ₂ is the eigenvalue of the electron density (Hessian) second derivative matrix. The multiplication of electron density and λ₂ has a value with a sign that shows the interaction type and its intensity. A value of $\rho \mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}\left({\lambda }_2\right)<-0.02$ means a strong interaction (HB, halogen bond, etc.), if $-0.01<\rho \mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}\left({\lambda }_2\right)<+0.01$, it shows a vdW non-covalent interaction, and $\rho \mathrm{s}\mathrm{i}\mathrm{g}\mathrm{n}\left({\lambda }_2\right)>0.01$ is a mutual exclusion such as potential resistance effect in a ring and cage, i.e., strong non-bonding overlap.

To get a better insight into the absorption of SO₂, the Frontier Molecular Orbital (FMO) analysis was also performed and is plotted in Figure 7. HOMO is a bond orbital or lone pair while LUMO is an anti-bond orbital. Based on Fukui’s FMO theorem, whenever HOMO or other filled orbitals are near LUMO or other unoccupied orbitals, electron exchange from HOMO to LUMO of the other molecule is easier and the attraction is strong. For a pure AAIL (which is shown by AAIL in Figure 7) HOMO of [C₃MIM][GLY] and [C₃COOHMIM][GLY] AAILs is composed of a balanced distribution of the orbitals on the anion and ring of the cation. Whereas, HOMO of [C₃CH₃COOMIM][GLY] is distributed on the anion. After adding the SO₂ (which is shown by AAIL/SO₂ in Figure 7), the orbitals are concentrated on anion in all systems. Meanwhile, the orbital of SO₂ shows a slightly bigger overlap with the AAIL functionalized by the COOH group. While it shows the lowest overlap with AAIL functionalized by the acetate group. 2p atomic orbitals of C, O, and N formed the highest occupied molecular orbital (HOMO) of the AAILs which is delocalized over the SO₂ gas. Furthermore, the LUMO of the system involving [C₃COOHMIM][GLY] AAIL and SO₂ shows an overlap between the cation, anion, and SO₂ while the other system shows anion and SO₂ overlap. All these observations point out the favorability of the COOH functional group for the absorption of SO₂. Therefore, RDG besides ESP designate that HB and electrostatic interactions of O…H and S…O, respectively, play the role in the absorption gas process. The electron transform from HOMO of anion to LUMO of SO₂ arises throughout the gas capturing. As a result, S…O interaction is a π-hole interaction since the V-shaped SO₂ molecule with a π delocalized bond has an interaction with lone pair electrons of the O atom in the carboxylic group of the anion. This interaction appropriately matches with π-hole bonding interaction (Zhu et al., 2021).

FIGURE 7

FIGURE 7. Frontier molecular orbitals for pure AAILs and AAIL…SO₂ complexes.

The isosurface of electron densities is the regions with increasing and decreasing electron density after SO₂ accommodation in AAIL with an isovalue of (−0.15 and 0.15 a.u.). These regions with an increase in density are shown in red and a decrease in electron density, electron density difference (EDD), is shown in blue in Figure 8.

FIGURE 8

FIGURE 8. EDD for the complex of AAIL…SO₂.

As the figure illustrates, SO₂ experiences an enhancement in electron density. Wherever the distance is minimum, the EDD is the maximum, and SO₂ absorbed NCTA is also confirmed. Additionally, the anion charge depletion is greater than the cation which verifies the anion’s critical role in gas absorption. In summary, the solubility of SO₂ in AAILs with different functionalities demonstrates that electron-withdrawing groups such as carboxylic acid reduce the chemical absorption enthalpy as well the reconstruction of electricity consumption will be disrated.

Conclusion

Conventional ILs face economic issues, toxicity, and poor biodegradability; consequently, it is pivotal to replace them with task-specific ion pairs and make them more suitable for acidic gas capturing. The synergistic effect of cation functionalized by electron-donating groups for SO₂ absorption in AAILs based on imidazolium cation was under consideration. For this purpose, the glycine amino acid played role as AAIL anion and effect of the functional groups on cation with propyl alkyl chain length functionalized by hydroxyl (OH), amine (NH₂), carboxyl (COOH), methoxy (OCH₃), and acetate (CH₃COO) was under evaluation. In order to intuitively understand the magnitude of the force more, the binding energy (BE), the captured gas distance concerning the cation and anion, and SO₂ structure change were calculated. It was made clear that the carboxylic acid functional group has a great contribution in the absorption of SO₂ by AAIL while CH₃COO dramatically decreases, and adding the functional group of NH₂, OCH₃, and OH does not affect the absorption energy of SO₂ in the target AAILs. The Gibbs free energy of SO₂ absorption shows that the AAIL functionalized by the carboxylic acid group (COOH) is a thermodynamically favorable solvent. COOH functional group decreases the distance between anion and SO₂ which may be the main reason for being the best absorbent. In addition, the number of charge transfers of SO₂ in that system was the highest. Non-covalent interaction analysis investigates the nature of interactions. Comparing the NCI demonstrates that the COOH functional group decreases the steric effect. However, the CH₃COO functional group significantly increases the steric effect after absorption. To distinguish the weak interaction between AAIL and captured gas, the RDG map was used; the acetate functional group make diminutions in HB interaction followed by reduction in NCI-RDG, LOL, ESP, EDD, and HOMO-LUMO results.

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 author.

Author contributions

HS: data curation, analysis, and/or interpretation of data, writing- original draft preparation MR: conceptualization, methodology, software, revising the draft and editing FM: visualization, investigation, supervision, writing- reviewing and editing.

Funding

This research project was financially supported by Ferdowsi University of Mashhad, Iran (Grant No. 3/57434).

Acknowledgments

The computations were partly carried out in the High-Performance Computing (HPC) Center at Ferdowsi University of Mashhad. The authors would like to appreciate the HPC cooperation.

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.

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