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Proceeding Paper

Efficient Solvent Extraction of Phenol Using Imidazolium-Based Ionic Liquids †

1
Laboratoire de Catalyse et Synthèse en Chimie Organique, Faculté des Sciences, Université de Tlemcen, B.P. 119, Tlemcen 13000, Algeria
2
Faculté de Médecine, Université de Tlemcen, 12 B P 123 Hamri Ahmed, Tlemcen 13000, Algeria
3
Laboratoire de Chimie Moléculaire et Thioorganique, UMR CNRS 6507, INC3M, FR 3038, ENSICAEN et Université de Caen Normandie, 14050 Caen, France
*
Author to whom correspondence should be addressed.
Presented at the 27th International Electronic Conference on Synthetic Organic Chemistry (ECSOC-27), 15–30 November 2023; Available online: https://ecsoc-27.sciforum.net/.
Chem. Proc. 2023, 14(1), 25; https://doi.org/10.3390/ecsoc-27-16151
Published: 15 November 2023

Abstract

:
Ionic Liquids (ILs) have gained significant attention in both industry and academia due to their unique properties and diverse applications. With countless possible combinations of cations (e.g., Ammonium, Sulfonium, Phosphonium) and anions (Organic or Inorganic), the variety of ILs is immense. Among them, Imidazolium salts form the largest IL family, making them a focal point in this study. Phenolic compounds, essential in the chemical industry, become hazardous to both human and aquatic life when released into the environment. Traditional separation methods for phenolics unfortunately involve environmentally problematic processes. This research focuses on synthesizing and characterizing Imidazolium-functionalized ILs ([EtO2C2mim]Br, [EtO2C2mim]BF4, [EtO2C2mim]PF6) and studying their phenol extraction abilities. The results revealed [EtO2C2mim]Br as the most effective, extracting 99% of phenol and offering a promising alternative for the efficient extraction of phenols from real coal liquefaction oil.

1. Introduction

The phenol, an organic compound, plays a pivotal role in various industrial processes, serving as a vital component in the production of phenolic resins and related chemicals. It also boasts versatility as a solvent, antiseptic, and an additive in disinfectants [1,2]. However, phenol’s classification as a toxic and carcinogenic substance [3,4] raises significant concerns for both human health and the environment when it is released into natural ecosystems.
The conventional approach for phenol extraction relies on the use of potent alkaline and acidic chemicals. Unfortunately, this method generates substantial volumes of wastewater laden with phenol [5,6]. Consequently, there exists an urgent need for an environmentally friendly and highly efficient alternative method to extract phenols, thus mitigating the environmental impact associated with the traditional approach.
Over the past decades, there has been a burgeoning interest in harnessing the potential of room temperature ionic liquids (ILs) for the removal of phenolic compounds from both oil mixtures and aqueous solutions [7,8,9].
This study aims to present the advancements in utilizing ILs for phenolic compound extraction. To achieve this, a series of Imidazolium-functionalized Ionic Liquids (ILs: [EtO2C2mim]Br (a), [EtO2C2mim]BF4 (b), and [EtO2C2mim]PF6 (c)) were synthesized, characterized, and their phenol extraction capabilities were investigated using a UV-visible spectrophotometry titration method. Additionally, the effects of IL structural characteristics on phenol extraction will be discussed.

2. Materials and Methods

2.1. Instruments and Reagents

N-methylimidazole, ethyl bromoacetate, sodium tetrafluoroborate, potassium hexafluorophosphate, phenol, and solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received.
UV–visible absorption spectra were measured with an Agilent Cary 60 spectrophotometer.
For IR spectroscopy, solid samples were taken neat on a Thermo Scientific IR200 FT-IR spectrophotometer; only significant absorptions are listed.
1H NMR spectra were performed on a Brucker AscendTM-400 spectrometer at 298 K in DMSO solutions. Chemical shifts were reported relative to TMS as an internal standard.

2.2. Synthesis of ILs (a, b and c)

Imidazolium-based ionic liquids, [EtO2C2mim]Br (a), [EtO2C2mim]BF4 (b), and [EtO2C2mim]PF6 (c), were prepared based on procedures reported in the literature [10].
[EtO2C2mim]Br: Yield: 95%. IR (ν, cm–1): 1744 (C=O); 1709 (C=N). 1H NMR (400MHz, DMSO, TMS) δppm: 9.34 (s, 1H, NCHN), 7.49 (s, 2H, NCHCHN), 5.12 (s, 2H, NCH2COOEt), 3.92 (q, 2H, J = 7.5 Hz, -COOCH2CH3), 3.77 (s, 3H, NCH3), 0.93 (t, 3H, J = 7.5 Hz, -COOCH2CH3).
[EtO2C2mim]BF4: Yield: 86%. IR (ν, cm–1): 1749 (C=O);1708 (C=N).
1H NMR (400MHz, DMSO, TMS) δppm: 9.10 (s, 1H, NCHN), 7.73 (s, 2H, NCHCHN), 5.2 (s, 2H, NCH2COOEt), 4.22 (q, 2H, J = 7.2 Hz, -COOCH2CH3 ), 3.93 (s, 3H, NCH3), 1.25 (t, 3H, J = 7.2 Hz, -COOCH2CH3).
[EtO2C2mim]PF6: Yield: 82%. IR (ν, cm–1): 1754 (C=O); 1712 (C=N). 1H NMR (400MHz, DMSO, TMS) δppm: 9.06 (s, 1H, NCHN), 7.70 (s, 2H, NCHCHN), 5.15 (s, 2H, NCH2COOEt), 4.22 (q, 2H, J = 6.6 Hz, -COOCH2CH3), 3.90 (s, 3H, NCH3), 1.25 (t, 3H, J = 6.6 Hz, -COOCH2CH3).

2.3. UV-Vis Titrations

In an Erlenmeyer flask, a solution (phenol/hexane) was prepared by dissolving 1.26 g of phenol in 500 mL of hexane (C = 0.026 × 10−2 M). The mixture was agitated until the phenol was fully dissolved. In test tubes, 10 mL of the prepared solution and a specific amount of the ionic liquid (according to precise IL/phenol ratios in hexane) were added. After stirring for 2 min, the phenol content in the various IL-hexane solutions was analyzed using UV-vis spectroscopy. The extraction efficiency was calculated via the difference of phenol contents before and after extraction.

3. Results and Discussion

The synthesis of imidazolium-based ionic liquids: [EtO2C2mim]Br (a), [EtO2C2mim]BF4 (b), and [EtO2C2mim]PF6 (c) is depicted in Scheme 1. The reaction of N-methylimidazole with one equivalent of ethyl bromoacetate at 0 °C led to the formation of [EtO2C2mim]Br (a) in a 95% yield. Subsequently, treating [EtO2C2mim]Br (a) with one equivalent of NH4BF4 or KPF6 in acetone at room temperature for 22 h resulted in the formation of [EtO2C2mim]BF4 (b) and [EtO2C2mim]PF6 (c) in yields of 86% and 82%, respectively.
The ILs (a–c) were identified by 1H-NMR and infrared spectroscopy (as detailed in the experimental section). All the spectra are consistent with the proposed molecular structures.
In this study, we aimed to utilize imidazolium-based functionalized ionic liquids for the extraction of phenol. A model solution containing hexane and phenol was employed for testing. Additionally, to investigate the impact of the anion type on the efficiency of our ionic liquid, we conducted a comparative study of phenol extraction using three ionic liquids prepared previously: [EtO2C2mim]Br (a), [EtO2C2mim]BF4 (b), and [EtO2C2mim]PF6 (c).
The interactions between the ionic liquid and phenol in hexane were examined through titration conducted via UV-visible spectrophotometry, a sensitive method for tracking the progress of the extraction. The spectra of the initial (free) phenol and the solutions obtained after each addition of the ionic liquid were recorded in the UV range between 220 nm and 320 nm.
In the following figure, the UV absorption spectrum of phenol in hexane is presented (Figure 1). The spectrum is characterized by the presence of three main absorption bands between 260 and 280 nm, with the most intense band occurring at 271 nm.
The experimental spectra obtained after the gradual addition of the ionic liquid to a phenol solution (IL/phenol ratios change from 0 to 1.4 in the case of ILs [EtO2C2mim]Br and [EtO2C2mim]BF4, and from 0 to 1.8 in the case of [EtO2C2mim]PF6) exhibit a characteristic spectral change for each experiment, which varies from one ionic liquid to another. Typically, these variations are characterized by a reduction in the maximum absorbance, resulting from the decrease in phenol concentration (Figure 2, Figure 3 and Figure 4).
It can be noticed that Figure 2 illustrates a gradual decrease in absorbance with each addition of the ionic liquid, reaching an IL/Phenol ratio of 1 (maximum absorbance for the last spectrum = 0.1). This suggests that one equivalent of [EtO2C2mim]Br (a) was adequate for phenol elimination in hexane. Similarly, during phenol extraction by [EtO2C2mim]BF4, the addition of the ionic liquid leads to a reduction in absorption intensity (maximum absorbance for the last spectrum = 0.16), and the spectra overlap after an IL/Phenol ratio of 1.2. In contrast, the ionic liquid [EtO2C2mim]PF6 exhibits low extraction efficiency due to its limited solubility in the phenol–hexane solution. Hence, it can be inferred that the halogen anion in quaternary ammonium salts plays a pivotal role in phenol extraction by establishing hydrogen bonds with the phenol’s OH group. However, both [EtO2C2mim]BF4 and [EtO2C2mim]PF6 lack free halogen atoms, instead featuring polyatomic anions. Furthermore, we observed poor solubility of [EtO2C2mim]PF6 in hexane at room temperature, which likely affects phenol extraction.
To determine the new phenol concentration after each addition of the ionic liquid, we used the Beer–Lambert law, which provides the relationship between absorbance (A) and concentration (C): A = ε l C.
To calculate the extraction efficiency, we determined the ratio between the final phenol concentration remaining in the solution and the initial phenol concentration before the addition of the ionic liquid (R% = C_final/C_initial). The results obtained are summarized in the following diagram (Figure 5).

4. Conclusions

Imidazolium-based ionic liquids were investigated for the extraction of phenol from a model solution composed of hexane and phenol. The molar amount of ionic liquid (IL) used was either equal to or close to that of phenol in the model solution. The results indicated that the choice of IL-anion significantly influences the efficiency of phenol extraction in the hexane solution.
For the same cation, [EtO2CO2mim]+, the extraction efficiency follows this order: [EtO2C2mim]Br > [EtO2C2mim]BF4 > [EtO2C2mim]PF6. [EtO2C2mim]Br exhibits the highest extraction efficiency, reaching approximately 99% at room temperature.

Author Contributions

Conceptualization, I.H. and B.M.-K.; methodology, I.H.; software, I.H.; validation, I.H.; formal analysis, I.H.; investigation, I.H.; resources, I.H. and S.B.; data curation, I.H. and S.B.; writing—original draft preparation, I.H.; writing—review and editing, I.H. and B.M.-K.; visualization, I.H.; supervision, D.V. and B.M.-K.; project administration, I.H. and B.M.-K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors are grateful to the General Directorate for Scientific Research and Technological Development (DGRSDT) and the University of Tlemcen.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Scheme 1. Synthesis route of Ionic Liquids (ac).
Scheme 1. Synthesis route of Ionic Liquids (ac).
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Figure 1. UV absorption spectrum of phenol in hexane at a concentration C ≈ 0.026 M.
Figure 1. UV absorption spectrum of phenol in hexane at a concentration C ≈ 0.026 M.
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Figure 2. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]Br for IL/phenol ratios: 0 (the red spectrum) ≤ R ≤ 1.4 (black spectra) at 25 °C.
Figure 2. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]Br for IL/phenol ratios: 0 (the red spectrum) ≤ R ≤ 1.4 (black spectra) at 25 °C.
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Figure 3. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]BF4 for IL/phenol ratios: (the red spectrum) ≤ R ≤ 1.4 (black spectra) at 25 °C.
Figure 3. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]BF4 for IL/phenol ratios: (the red spectrum) ≤ R ≤ 1.4 (black spectra) at 25 °C.
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Figure 4. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]PF6 for IL/phenol ratios: (the red spectrum) ≤ R ≤ 1.6 (black spectra) at 25 °C.
Figure 4. UV absorption spectra related to the extraction of phenol by the IL [EtO2C2mim]PF6 for IL/phenol ratios: (the red spectrum) ≤ R ≤ 1.6 (black spectra) at 25 °C.
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Figure 5. Effect of ILs structure on the phenol extraction efficiency.
Figure 5. Effect of ILs structure on the phenol extraction efficiency.
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MDPI and ACS Style

Hamzi, I.; Bouchakour, S.; Mostefa-Kara, B.; Villemin, D. Efficient Solvent Extraction of Phenol Using Imidazolium-Based Ionic Liquids. Chem. Proc. 2023, 14, 25. https://doi.org/10.3390/ecsoc-27-16151

AMA Style

Hamzi I, Bouchakour S, Mostefa-Kara B, Villemin D. Efficient Solvent Extraction of Phenol Using Imidazolium-Based Ionic Liquids. Chemistry Proceedings. 2023; 14(1):25. https://doi.org/10.3390/ecsoc-27-16151

Chicago/Turabian Style

Hamzi, Imane, Souad Bouchakour, Bachir Mostefa-Kara, and Didier Villemin. 2023. "Efficient Solvent Extraction of Phenol Using Imidazolium-Based Ionic Liquids" Chemistry Proceedings 14, no. 1: 25. https://doi.org/10.3390/ecsoc-27-16151

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