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. 2020 Sep 1;5(36):23090–23098. doi: 10.1021/acsomega.0c02863

Separation of m-Cresol from Coal Tar Model Oil Using Propylamine-Based Ionic Liquids: Extraction and Interaction Mechanism Exploration

Xiaobin Bing †, Zenghui Wang †, Feng Wei †, Jun Gao †, Dongmei Xu †,*, Lianzheng Zhang †,‡, Yinglong Wang §
PMCID: PMC7495739  PMID: 32954159

Abstract

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m-Cresol is an important chemical material, which is mainly derived from low-temperature coal tar. In this work, for separating m-cresol from coal tar model oil, two propylamine-based ionic liquids (ILs) propylamine formate ([PA][FA]) and propylamine acetate ([PA][Ac]) were selected as extractants. The selected ILs were synthesized and characterized by Fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance (NMR) spectroscopy. The effects of temperature, mass ratio of IL to model oil, and separation time on the separation efficiency of m-cresol were explored. The separation efficiency (SE) and distribution coefficient (D) were calculated from the experimental data to assess the separation performance of [PA][FA] and [PA][Ac]. The results showed that propylamine formate was a promising extractant with the separation efficiency of 97.8% and distribution coefficient of 27.59 at 298.15 K and mIL/moil = 0.2. In the meantime, molecular dynamics (MD) simulations were employed to comprehend the interaction mechanism, from which the noncovalent interaction energy (IE), radial distribution function (RDF), spatial distribution function (SDF), and averaged noncovalent interaction (aNCI) were calculated. The results showed that both cation and anion formed hydrogen bonds with m-cresol and the anions played a leading role with electrostatic interaction energy in separating m-cresol. In addition, the regeneration and reuse of the ionic liquids were explored.

1. Introduction

Phenolic compounds are valuable chemical resources with a mass fraction between 20 and 30% in low-temperature coal tar (LTCT),1−4 among which m-cresol is an important component that can be extensively used to produce organic chemicals.5,6 Therefore, it is of significance to separate m-cresol from LTCT for further applications. Generally, in industrial applications, a large amount of corrosive aqueous solutions (such as NaOH and H2SO4 aqueous solutions) are used to recover phenolic compounds from LTCT,7−9 which leads to environmental problems because of the phenolic wastewater.10,11 Thus, it is important and necessary to separate m-cresol from LTCT using a highly efficient and environmentally benign method.

Until now, new solvents such as ionic liquids (ILs)12−15 have been widely applied as highly efficient and green extractants in the separation field.16−20 Especially, various ILs have been used to separate phenolic compounds from LTCT. Hou et al.21 used four imidazolium-based ionic liquids to separate phenol from model oil, and 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) displayed the highest extraction efficiency. Ji et al.11 designed several tetraethylammonium amino acid (TAAA) ionic liquids to separate phenol from an oil mixture; the maximum separation efficiency of phenol was 99%, and the final phenol content in the oil phase could reach 1.40 g/dm3. Yao et al.22 synthesized four dual-functionalized ionic liquids to separate phenolic compounds from model oil. The lowest content of phenol in the oil phase was 5.9 g/m3 and the highest extraction efficiency was 98.5%. Sidek et al.23 synthesized the ILs 1-allyl-3-benzylimidazolium chloride, 1,3-dibenzylimidazolium chloride, and 1-benzyl-3-vinylimidazolium chloride to remove phenolic compounds from model oil, and the prepared ionic liquids presented high removal efficiency toward phenolic compounds even after six regeneration cycles. Liu et al.24 studied the extraction of m-cresol from model oil using 1-ethyl-3-methylimidazolium acetate with the extraction efficiency of 98.85%. Kalidindi et al.25 explored the separation of phenols from coal tar oil using a binary mixture of an ionic liquid and acetonitrile as the extractant instead of pure organic solvent tetraethylene glycol, as reported by Venter et al.,26 and the extraction efficiency of phenol and p-cresol exceeded 96%. In addition, more results of separating phenolic compounds using ILs as extractants were reported in the literature.6,27−29 To our knowledge, m-cresol has the highest content (13.41 wt %) in phenol oil from low-temperature coal tar,30 but fewer literature studies have reported the separation of m-cresol as a representative phenolic compound from model oil since phenol or p-cresol is usually selected as the typical phenolic compound.6,21,31−33 Hence, it is meaningful to explore the separation of m-cresol from LTCT by means of ionic liquids.

Recently, molecular dynamics (MD) simulations have gained more and more attention for exploring the intermolecular interaction mechanism between ionic liquids and separated substances.34 Dehury et al.35 investigated the removal of 1-butanol from water with ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([OMIM][Tf2N]) as the extractant using MD simulations and found that the cation [OMIM]+ played a leading role in separating 1-butanol. Iwahashi et al.36 explored the interface layering between 1-butanol and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) by MD simulations. Oliveira et al.37 found that the π–π interaction between the imidazole ring and thiophene provided the driving force in the desulfurization process by MD simulations. Singh et al.38 investigated the molecular arrangement and extraction mechanism for desulfurization from hydrocarbon fuel using imidazolium-based ionic liquids by MD simulations. Based on the above, for extracting m-cresol from LTCT using ILs, it is necessary to explore the interactions between the ILs and m-cresol by MD simulations.

In this work, to explore the separation of m-cresol from LTCT model oil, two propylamine-based ionic liquids propylamine formate ([PA][FA]) and propylamine acetate ([PA][Ac]) were prepared due to their high efficiency of flue-gas desulfurization,39 low viscosity, and low toxicity.40 The factors that can affect the separation efficiency of m-cresol including temperature, mass ratio of IL to model oil, and separation time were systematically explored. In the meantime, the noncovalent interaction energy (IE), radial distribution function (RDF), spatial distribution function (SDF), and averaged noncovalent interaction (aNCI) were calculated by MD simulations to explore the interaction mechanism. Finally, the regeneration and reuse of the prepared ionic liquids were investigated.

2. Results and Discussion

2.1. Characterization of Ionic Liquids by 1H Nuclear Magnetic Resonance (NMR) Spectroscopy

The synthesized ionic liquids [PA][FA] and [PA][Ac] were characterized by a 1H NMR spectrometer (Bruker, Germany, 300 MHz) with dimethyl sulfoxide (DMSO)-d6 as an internal reference solvent. The 1H NMR spectra of the ILs [PA][FA] and [PA][Ac] are shown in Figures S1 and S2. The 1H NMR spectra for [PA][FA] and [PA][Ac] consist of the following peaks: [PA][FA] (DMSO-d6, 300 MHz, ppm): δH = 8.31 (1H, s, −C–H), 7.67 (3H, t, −NH3+), 2.56 (2H, d, −CH2), 1.43 (2H, d, −CH2), and 0.75 (3H, t, −CH3). [PA][Ac] (DMSO-d6, 300 MHz, ppm): δH = 0.86 (3H, t, −CH2–CH3), 1.44 (2H, d, −CH2–CH2), 1.75 (3H, s, −CH3), 2.58 (2H, d, −CH2–CH2), and 6.79 (3H, t, −NH3). The purity of the prepared [PA][FA] and [PA][Ac] exceeded 99% by comparing the ratios of the integral peak areas with the ratios of the number of hydrogen atoms using Bruker TopSpin 3.5 software.32,41

2.2. Exploration of the Optimal Extraction Conditions

2.2.1. Temperature

The temperature was investigated from 298.15 to 333.15 K at a mass ratio of 0.2 (mIL/moil) and the phase contact time of 30 min. Figure 1 shows the temperature effect on the m-cresol separation efficiency and distribution coefficient. The SE and D values of two ILs decrease as the temperature increases due to the increased solubility of m-cresol in hexane.42,43 The IL [PA][FA] shows higher separation efficiency and distribution coefficient compared to [PA][FA]. The highest SE and D values are 97.8% and 27.59 for [PA][FA] and 94.1% and 15.95 for [PA][Ac], respectively, at 298.15 K. Therefore, in view of the highest separation efficiency, the optimum temperature was determined to be 298.15 K for the following experiments.

Figure 1.

Figure 1

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Effect of temperature on extracting m-cresol from model oil: (a) separation efficiency and (b) distribution coefficient.

2.2.2. Mass Ratio of Ionic Liquid to Model Oil

The mass ratio of ionic liquid to model oil (mIL/moil) is an important parameter in liquid–liquid extraction. In this work, the mass ratio of ionic liquid to model oil was investigated in the range of 1:2 to 1:15 at 298.15 K with the phase contact time of 30 min. As shown in Figure 2, the SE values of two ILs decrease slowly before mIL/moil = 1:5 and decrease rapidly after mIL/moil = 1:5. For the IL [PA][FA], the SE values decrease from 98.5 to 97.8% before mIL/moil = 1:5 and further decrease to 79.8% at mIL/moil = 1:15. The D values decrease with increasing mass ratio of ionic liquid to model oil. Hence, considering the consumption of ionic liquid and separation efficiency, mIL/moil = 1:5 was chosen for further experiment exploration.

Figure 2.

Figure 2

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Effect of mIL/moil on extracting m-cresol from model oil: (a) separation efficiency and (b) distribution coefficient.

2.2.3. Separation Time

The separation time was investigated in the range of 0–30 min to explore the mass transfer time of m-cresol between two immiscible phases. As presented in Figure 3, the SE and D values for the two ILs increase quickly within 1 min and increase slowly with the increase of separation time. The maximum values of SE and D are 97.8% and 27.57 for [PA][FA] and 94.1% and 15.95 for [PA][Ac], respectively, at 30 min. Therefore, the separation time of 30 min was determined to be optimal in view of the reasonable time consumption in industrial operation and higher separation efficiency.

Figure 3.

Figure 3

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Effect of separation time on extracting m-cresol from model oil: (a) separation efficiency and (b) distribution coefficient.

2.3. Extraction Mechanism Exploration by MD Simulations

2.3.1. Noncovalent Interaction Energy

The noncovalent interaction energy (kJ/mol) for the ternary system (ILs + m-cresol + hexane) was calculated and is reported in Table 1. The total interaction energy (Etotal) is the aggregation of electrostatic interaction energy (Eelec) and van der Waals (EvdW) interaction energy. As can be seen from Table 1, for the systems (ILs + m-cresol + hexane), the total interaction energy of IL–m-cresol is stronger than those of IL–hexane and hexane–m-cresol, implying that less m-cresol exist in the hexane-rich phase after separation. Moreover, the electrostatic interaction energy in the IL + m-cresol system is stronger than the van der Waals interaction energy, which means that the electrostatic interaction energy is the governing factor. Finally, the total interaction energy of [PA][FA]-m-cresol (−136.20 kJ/mol) is higher than that of [PA][Ac]–m-cresol (−105.62 kJ/mol), which means the stronger interaction and higher separation efficiency of [PA][FA].

Table 1. Noncovalent Interaction Energy of Different Molecular Pairs Calculated from MD Simulations at 101.325 kPa and 298.15 Ka.
molecular pair electrostatic interactions (Eelec) van der Waals interactions (EvdW) total noncovalent interactions (Etotal)a
[PA][FA] + m-Cresol + Hexane
[PA]+–m-cresol –32.95 –15.88 –48.83
[FA]−–m-cresol –81.75 –5.62 –87.37
[PA][FA]–m-cresol –114.7 –21.50 –136.20
[PA]+–hexane –1.30 –5.70 –7.00
[FA]−–hexane –0.74 –1.60 –2.34
[PA][FA]–hexane –2.04 –7.30 –9.34
hexane–m-cresol –0.08 –5.53 –5.61
[PA][Ac] + m-Cresol + Hexane
[PA]+–m-cresol –19.56 –14.98 –34.54
[Ac]−–m-cresol –67.13 –3.95 –71.08
[PA][Ac]–m-cresol –86.69 –18.93 –105.62
[PA]+–hexane –1.30 –6.23 –7.53
[Ac]−–hexane –0.69 –3.66 –4.35
[PA][Ac]–hexane –1.99 –9.89 –11.88
hexane–m-cresol –0.08 –5.27 –5.35
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a

Etotal = Eelec + Evdw.

2.3.2. Radial Distribution Function

The site–site radial distribution function (RDF) was employed to investigate the structural packing of different molecular pairs in the system at a microscopic level. As given in Figure 10, the atoms OF1 and OF2 of formate, OA1 and OA2 of acetate, NP1 of propylamine, and OM1 of m-cresol were selected to generate the RDF plots. The results are presented in Figure 4. As shown in Figure 4, the RDF peaks of oxygen atoms of anions and the hydrogen group of m-cresol are acquired at 0.25 nm, which indicates a strong hydrogen bond between anions and m-cresol. However, the height of summit g(r) for formate is higher than that of acetate, which implies that the interaction between [PA][FA] and m-cresol is stronger than that between [PA][Ac] and m-cresol. Likewise, similar results can be found for the cation and m-cresol. Hence, hydrogen bond interactions exist in the systems (ILs + m-cresol), and a stronger interaction between [PA][FA] and m-cresol means a higher separation efficiency.

Figure 10.

Figure 10

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Optimized structures of the investigated molecules.

Figure 4.

Figure 4

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RDF plots of different molecules: (a) [PA][FA]–m-cresol and (b) [PA][Ac]–m-cresol.

2.3.3. Spatial Distribution Function

The spatial distribution function (SDF) was employed to probe the distribution sites of m-cresol around anions and the cation. The TRAVIS44 package was employed to calculate the SDF plots, and the results are presented in Figure 5. As shown in Figure 5a,b, the entire carbonyl groups of formate and acetate are tightly surrounded by m-cresol (red surface), which implies that the electrostatic attractive interaction plays the leading role between the anions and m-cresol. In Figure 5c, m-cresol surface is attracted by the entire propylamine, which implies that both electrostatic attractive interaction and van der Waals interaction34 are present in the system cation + m-cresol.

Figure 5.

Figure 5

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SDF plots: m-cresol (red surface) around acetate (a), formate (b), and propylamine (c) at an isovalue of 6.4 particle nm–3.

2.3.4. Averaged Noncovalent Interaction

Averaged noncovalent interaction (aNCI) analysis45,46 is used to characterize the weak noncovalent interaction in fluctuating environments, which provides a rich representation of van der Waals interactions, hydrogen bonds, and steric repulsion.47 The aNCI plots for formate, acetate, and propylamine were generated using Multiwfn,48 which are presented in Figure 6. As shown in Figure 6a,b, the blue surface is tightly attracted by the carbonyl group of anions, which indicates the formation of a strong hydrogen bond between m-cresol and anions. In Figure 6c, the blue surface appears around the ammonium group of the cation, while the entire carbon chain is surrounded by a green surface. It can be inferred that a hydrogen bond is formed between m-cresol and the ammonium group, whereas there is a van der Waals interaction effect on m-cresol and the entire carbon chain. Thus, the aNCI plots for formate, acetate, and propylamine confirm the strong hydrogen bond interaction formed between IL and m-cresol.

Figure 6.

Figure 6

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aNCI plots for formate (a), acetate (b), and propylamine (c). The surfaces are colored on a blue–green–red scale. Blue refers to a strong, attractive interaction, green refers to a van der Waals interaction, and red refers to a strong nonbonded overlap.

3. Intermolecular Interaction Mechanism Exploration by Fourier Transform Infrared (FT-IR) Spectroscopy

FT-IR spectroscopy was employed to verify the interaction mechanism between ILs and m-cresol. Pure m-cresol, ILs ([PA][FA] and [PA][CA]), and complexes ([PA][FA] + m-cresol and [PA][CA] + m-cresol) were characterized using a Nicolet FT-IR spectrometer. The spectra are given in Figure 7. As shown in Figure 7, the peak at 3497.90 cm–1 is ascribed to −OH in pure m-cresol, whereas the peaks at 3451.39 and 3445.56 cm–1 are ascribed to −NH3+ in the pure ILs [PA][FA] and [PA][Ac], respectively.6 When the complexes formed, the characterization peak shifted to 3438.51 cm–1 for complex 1 ([PA][FA] + m-cresol) and shifted to 3438.32 cm–1 for complex 2 ([PA][Ac] + m-cresol), which indicated that hydrogen bonds were formed between m-cresol and the ILs.

Figure 7.

Figure 7

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FT-IR spectra of (a) m-cresol, [PA][FA], and complex 1 ([PA][FA] + m-cresol) and (b) m-cresol, [PA][Ac], and complex 2 ([PA][Ac] + m-cresol).

4. Regeneration and Reuse of the Ionic liquids

For reusing the ILs [PA][FA] and [PA][Ac], the ILs were regenerated by diethyl ether after the extraction experiment. The separation performance of recovered ILs was explored. As shown in Figure 8, the separation efficiency is above 96.1% for [PA][FA] and above 93.0% for [PA][Ac] after five uses, where [PA][FA] still shows a higher separation efficiency compared to [PA][Ac]. Furthermore, the structural characteristics of the regenerated ILs and fresh ILs were validated using FT-IR and 1H NMR spectra, which are shown in Figures 9, S3 and S4. As noted from FT-IR and 1H NMR spectra, there is no significant change for the characteristic peaks of regenerated ILs and fresh ILs. The two ionic liquids [PA][FA] and [PA][Ac] can be regenerated completely with a purity of higher than 99%.

Figure 8.

Figure 8

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Separation efficiency vs number of reuses of the regenerated ILs: (a) [PA][FA] and (b) [PA][Ac].

Figure 9.

Figure 9

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FT-IR spectra of the regenerated ILs: (a) [PA][FA] and (b) [PA][Ac].

5. Conclusions

To separate m-cresol from LTCT model oil using liquid–liquid extraction with ionic liquids, two propylamine-based ILs [PA][FA] and [PA][Ac] were prepared and characterized by 1H NMR and FT-IR spectroscopy. The temperature, mass ratio of IL to model oil, and separation time were investigated. The highest separation efficiency and distribution coefficient were 97.8% and 27.59 for [PA][FA] and 94.1% and 15.95 for [PA][Ac], respectively, at 298.15 K with the separation time of 30 min and mIL/moil = 0.2. In the meantime, MD simulations were performed to explore the intermolecular interaction mechanism between the ILs and m-cresol. The noncovalent interaction energy, radial distribution function, spatial distribution function, and averaged noncovalent interaction were calculated. The results showed that the electrostatic interaction energy played a leading role in separating m-cresol from model oil and [PA][FA] interacted more strongly with m-cresol. The major interaction sites of ILs are the carbonyl group of anions and the ammonium group of the cation. Strong hydrogen bonds were formed between the carbonyl group and the ammonium group of ILs and the hydroxyl group of m-cresol. In addition, the hydrogen bond interactions between ILs and m-cresol were verified by FT-IR spectroscopy. Finally, the ionic liquids [PA][FA] and [PA][Ac] were regenerated and reused with high separation efficiency at least five times.

6. Materials and Methods

6.1. Chemicals

All of the chemicals used in the experiments were analytical grade reagents and obtained commercially, which were used without further treatment. The detailed information is listed in Table 2.

Table 2. Information of the Chemicals.

chemicals CAS no. supplier purity (wt %)a
propylamine 107-10-8 Shanghai Macklin Biochemical Co., Ltd. ≥99%
formic acid 64-18-6 Shanghai Macklin Biochemical Co., Ltd. ≥99%
acetic acid 64-19-7 Chengdu Kelong Chemical Co., Ltd. ≥99%
m-cresol 108-39-4 Shanghai Macklin Biochemical Co., Ltd. ≥99%
hexane 110-54-3 Chengdu Kelong Chemical Co., Ltd. ≥97%
benzaldehyde 100-52-7 Shanghai Titan Scientific Co., Ltd. ≥99%
anhydrous ethanol 64-17-5 Yantai Far Eastern Fine Chemical Co., Ltd ≥99.7%
diethyl ether 60-29-7 Chengdu Kelong Chemical Co., Ltd. ≥99%
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a

Provided by suppliers.

6.2. Preparation of Ionic Liquids

The ionic liquids [PA][FA] and [PA][Ac] were synthesized by a one-pot method.49,50 The preparation of [PA][FA] and [PA][Ac] was performed using our previously reported procedures.51 First, equimolar propylamine and the acid were mixed, and anhydrous ethanol was introduced to dilute the solution. The reaction lasted for 20 h at 283.15 K in a thermostat water bath (DF-101D/Z, Lichen, Shanghai). Afterward, the reaction solution was rinsed three times with anhydrous ethanol and dried in a vacuum drying oven at 353.15 K for 50 h under −0.1 MPa to remove volatile impurities. Finally, the pure ionic liquids were stored in a desiccator to prevent moisture. The water content of the prepared ILs was measured using a Karl Fischer Moisture Meter (KLS701), which was 0.32 wt % for [PA][FA] and 0.29 wt % for [PA][Ac].

6.3. Preparation of Model Oil

Since the mass fraction of phenolic compounds in LTCT is nearly 20–30%, to simplify the composition of LTCT, a model oil mixture of m-cresol and hexane with a mass ratio of 2:8 was prepared using an electronic balance (FA3204C, Techcomp, Shanghai) with an accuracy of ±0.0001g.

6.4. Experimental and Analysis Methods

For extraction experiment, 1 g of ionic liquid was added into 5 g of coal tar model oil in a 15 mL test tube and magnetically stirred for 30 min at a constant temperature of 298.15 K. Afterward, the mixture was allowed to settle for 60 min at 298.15 K, and two layers were separated clearly.

To measure the concentration of m-cresol in the upper layer after extraction, a certain amount of m-cresol was taken out and analyzed by gas chromatography (GC, Techcomp, Shanghai) using N2 (99.999 wt %) as a carrier gas. The programmed temperature was applied, which was as follows: the initial temperature of the oven was set at 353.15 K, then the temperature was increased to 453.15 K with a heating rate of 30 K/min. After that, the temperature was increased to 493.15 K with a heating rate of 40 K/min. In view of different boiling points of benzaldehyde, m-cresol, and hexane, benzaldehyde was selected as an internal substance to calculate the content of m-cresol in the upper phase.33 The temperatures of both the injection port and the detector were set at 493.15 K. All of the supernatant samples were analyzed five times, and the mean value was used to calculate the separation efficiency.

6.5. Separation Efficiency (SE) and Distribution Coefficient (D)

For evaluating the separation performance of the prepared ILs, separation efficiency and distribution coefficient index were adopted and are defined as follows

6.5. 1
6.5. 2

where Co is the mass fraction of m-cresol in model oil before separation, CIL is the mass fraction of m-cresol in lower phase, and Cf is the mass fraction of m-cresol in the upper phase after separation.

7. MD Simulation Details

The original structures of the cation [PA]+, anions [FA]− and [Ac]−, m-cresol, and hexane were drawn using Avogadro software52 and optimized with B3LYP theory and def2-SVP basis53,54 using the ORCA package.55 The optimized structures are shown in Figure 10. The partial charges for different species were calculated by the restrained electrostatic potential (RESP)56,57 module of Multiwfn48 and are reported in Tables S1 and S2. The force field parameters of all investigated molecules were generated according to the generalized Amber force field (GAFF)58,59 using ANTECHAMBER60 since GAFF has been successfully applied for the systems, ILs + organic compounds.61,62 The veracity of the employed force field (GAFF) was validated by calculating the deviation between MD-predicted and experimentally measured densities for [PA][FA], [PA][Ac], m-cresol, and hexane at 298.15 K,34 which is given in Table S3. As shown in Table S3, the deviations are lower than 1%, which proves that the GAFF is applicable for this exploration.

For all MD simulations, the GROMACS package63 was used and performed at 298.15 K and 101.325 kPa using the Langevin thermostat64 and Parrinello–Rahman barostat.65 All of the bonds that contain hydrogen atoms were constrained by the LINCS algorithm.66,67 The particle mesh Ewald (PME) method38 was used to treat the Lennard-Jones and short-range Coulombic interactions and long-range interactions with a cutoff radius of 1.2 nm. A time integration step of 0.001 ps and periodic boundary conditions (PBCs) were used for all simulations.68

The original simulated system composed of 120 molecules of IL, 120 molecules of m-cresol and 450 molecules of hexane was generated using PACKMOL.69 First, the molecules of IL and hexane were placed into two independent but connected cube boxes, namely, the IL-rich phase and the hexane-rich phase. Thereafter, the m-cresol molecules were randomly distributed in the hexane-rich phase. The generated simulation system was energy-minimized to remove unwanted contact using the steepest descent method.62 Then, the system was equilibrated for 3.5 ns in the constant pressure–constant temperature (NPT) ensemble. Finally, the production run was performed for 50 ns under the constant temperature (NVT) ensemble to achieve a clear separation phase. The Visual Molecular Dynamics (VMD) package70 was used to analyze the trajectory of m-cresol moved from the hexane phase to the IL phase.

Acknowledgments

The authors are grateful for the support of the National Natural Science Foundation of China (No. 21878178), Natural Science Foundation of Shandong Province (No. ZR2019BB066), and Open Project of Chemistry Department, Qingdao University of Science and Technology (No. QUSTHX202007).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02863.

  • The detailed procedures of RESP charge of different atoms, density obtained from MD simulations, and 1H NMR spectra of the fresh and recovered ILs (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02863_si_001.pdf (195.7KB, pdf)

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