Journal of Molecular Liquids 306 (2020) 112911
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Efﬁcient extraction of phenol from low-temperature coal tar model oil via imidazolium-based ionic liquid and mechanism analysis Xin Xu a, Ao Li a, Tao Zhang a, Lianzheng Zhang a, Dongmei Xu a, Jun Gao a,⁎, Yinglong Wang b a b
College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China College of Chemical and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
a r t i c l e
i n f o
Article history: Received 8 January 2020 Received in revised form 8 March 2020 Accepted 14 March 2020 Available online 16 March 2020 Keywords: Low-temperature coal tar Phenol Extraction Ionic liquid Hydrogen bond
a b s t r a c t Since phenol is a high value-added compound in chemical industry, which is separated from coal tar, in this work, the imidazolium-based ionic liquid, 1-ethyl-3-methyl imidazolium lactate ([EMIM][LAC]) was prepared as extractant to separate phenol from the model oil. The prepared [EMIM][LAC] was characterized by 1H NMR. The effects of extraction temperature, time, and IL/model oil mass ratio were explored. The results showed that [EMIM] [LAC] can extract phenol efﬁciently. Then, the separation mechanism between phenol and [EMIM][LAC] was investigated by discussing the surface-charge density distribution obtained by the COSMO-SAC model and intermolecular interaction obtained by quantum chemical calculations. The results showed that [EMIM][LAC] was a suitable extractant due to hydrogen bond formed by the IL and phenol. Moreover, the chemical bond for [EMIM][LAC] with phenol was further validated by FT-IR spectra. © 2020 Elsevier B.V. All rights reserved.
1. Introduction Coal tar, as a by-product generated by coal pyrolysis [1,2], can be divided into low-temperature coal tar (LTCT), medium-temperature coal tar (MTCT) and high-temperature coal tar (HTCT) according to different coking temperature. Phenolic compounds are 20–30% approximately in LTCT  with more than 500 constituents . Therefore, it is of signiﬁcance to separate phenolic compounds for efﬁcient utilization of LTCT. Notably, phenolic compounds are widely utilized in producing chemicals, such as phenolic resins , engineering plastics and medicines . So far, among the technologies for separation of phenolic compounds from LTCT, caustic washing method [7–9] with large amount of consumption of acid and alkali is usually applied in industry, which leads to discharge of phenolic wastewater . Therefore, it is necessary to explore an efﬁcient and energy saving method to extract phenol from LTCT. Nowadays, liquid-liquid extraction as an alternative is applied in separating phenolic compounds from coal tar by using extractants of organic solvents [7,10,11], biological reagents  and deep eutectic solvents [9,13,14]. Compared with these extractants, ionic liquids (ILs) attract much attention to separate phenolic compounds from LTCT because of the advantages of environment friendly, non-toxicity, less ⁎ Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.molliq.2020.112911 0167-7322/© 2020 Elsevier B.V. All rights reserved.
volatility and high thermal stability [15–19]. Hou et al.  adopted imidazolium-based ILs to extract phenol from hexane and found that the extraction efﬁciency of 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]) reached 99%. However, the IL can cause corrosion to the equipment due to its halogen ion  and is more soluble in coal tar, which can lead to the loss of the IL. Yao et al.  synthesized several dual-functionalized ionic liquids (DFILs) to extract phenol and the best extraction efﬁciency of the prepared ILs can reach to 99.1%. Nevertheless, the prepared DFILs are solid at room temperature, which may need additional energy for extraction process and is not suitable for industrial application. Hence, it is essential to ﬁnd an efﬁcient and environmentally friendly extractant to extract phenol from coal tar at room temperature. Meanwhile, the mechanism for extracting phenolic compounds from coal tar was explored in the literatures. Zhang et al.  adopted choline derivative salts [N1,1,nC2OH]Cl (n = 1, 4, 6, 8) to extract phenolic compounds from toluene. They found that the important driving force to extract was the -OH group in phenols and the Cl atom in [N1,1,nC2OH]Cl to form O-H···Cl hydrogen bond. Meng et al.  used ethanolammonium based ILs with formate or acetate anion to extract phenol from model oil. And they conﬁrmed that the O-H···O hydrogen bond was formed for phenol with the anions. Gai et al.  used 1,1,3,3tetramethylguanidinium 2-pyrrolidinecarboxylate ([TMG][Pro]) to separate phenol from hexane. They proved that the key to extract was the O-H···O bond formed between phenol and [TMG][Pro]. From the above literatures, it can be concluded that –OH in phenol forms a
X. Xu et al. / Journal of Molecular Liquids 306 (2020) 112911
Table 1 Detailed information of the chemicals. Component
1-Methyl-1H-imidazole Bromoethane Hexane Toluene Phenol Lactic acid Sodium hydroxide Silver nitrate Ethyl acetate p-Xylene
616-47-7 74-96-4 110-54-3 108-88-3 108-95-2 50-21-5 1310-73-2 7761-88-8 141-78-6 106-42-3
C4H6N2 C2H5Br C6H14 C7H8 C6H6O C3H6O3 NaOH AgNO3 C4H8O2 C8H10
Shanghai Bidepharm Chemical Co., Ltd. Shanghai Macklin Biochemical Co., Ltd Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Tianjin Hongyan Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Chengdu Kermel Chemical Co., Ltd.
98.0% 98.0% 97.0% 99.5% 99.0% 99.0% 96.0% 99.8% 99.5% 99.0%
hydrogen bond with an electronegative atom. Because 1-ethyl-3methyl imidazolium lactate ([EMIM][LAC]) contains three oxygen atoms, it is possible to form O-H···O hydrogen bond with phenol, and [EMIM][LAC] is non-corrosive to the equipment. Therefore, [EMIM] [LAC] was adopted as extractant for extracting phenol from the model oil and its extraction mechanism was explored, which have not been reported in the literatures. For extracting phenol from the prepared model oil (mass ratio of hexane to toluene, 7:1), [EMIM][LAC] was synthesized and characterized by 1H NMR to conﬁrm its structure. The extraction experiment was performed and the extraction efﬁciency (EE) was calculated to evaluate the extraction capacity of [EMIM][LAC]. Moreover, to explore the mechanism of extracting phenol by [EMIM][LAC], the bond length, interaction energy and electron densities were obtained by quantum chemical calculations, and the COSMO-SAC model was applied in calculating the surface-charge density distribution (σ-proﬁle) to explore the intermolecular interaction between [EMIM][LAC] and phenol. Finally, the extraction mechanism was also veriﬁed by FT-IR spectra. 2. Experimental section 2.1. Materials The analytical reagents employed in this study were purchased commercially. Their purities were provided by the suppliers. The physical properties of the chemicals are given in Table 1. 2.2. Preparation of model oil Considering the complex components in coal tar, phenol is adopted to represent the phenolic compounds in coal tar, and hexane and toluene were used to represent the linear alkanes and the aromatic compounds in coal tar, respectively. Based on the actual compositions of the LTCT [24,25], the mass ratio of phenol, hexane and toluene was 2:7:1. Each component was precisely weighed by an electronic balance and the model oil was prepared with phenol content of 20.02 wt%. 2.3. Synthesis of [EMIM][LAC] [EMIM][LAC] was synthesized by referring to the reported literatures [8,26]. Firstly, 41.09 g of 1-methyl-1H-imidazole (0.50 mol) reacted with 59.93 g of bromoethane (0.55 mol) for 15 h at 55 °C in a round ﬂask to prepare [EMIM][Br]. Secondly, [EMIM][Br] was added to the prepared silver lactate solution until no AgBr precipitation formed. The supernatant was removed and puriﬁed through a rotary evaporator for approximately 1 h at 90 °C in vacuum condition. [EMIM][LAC] was obtained after vacuum drying for 24 h at 90 °C. The 1H NMR spectrum of [EMIM][LAC] is presented in Fig. S1 in the Supplementary materials and the analysis is as follows: 1H NMR (DMSO-d6, δ, ppm): 1.02 (3H, s, CH3); 1.19 (3H, s, NCH2CH3); 3.83 (1H, s, CH); 3.64 (3H, s, NCH3); 3.98 (2H, s, NCH2); 7.41 (1H, s, NC_CH); 7.56 (1H, s, NC_CH); 8.93 (1H, s, N_CH). The water content
of the prepared IL [EMIM][LAC] was 0.33 wt% determined by Karl Fischer Moisture Meter (KLS701). 2.4. Experiments and analysis method The experiment equipment and detailed procedures required for liquid-liquid extraction were given in the previous work [4,27,28]. After extracting, the upper layer samples were removed and the composition was measured using gas chromatography (GC, Lunan SP-7820). Meanwhile, p-xylene was adopted to be an internal standard. The detection conditions of GC are shown in Table 2. All samples were measured more than three times to reduce error and the average values were adopted in this work. The extraction performance of [EMIM][LAC] was evaluated by calculating EE, which is described in Eq. (1): EEphenolic compounds ¼ ½ðC0 −C f Þ=C0 100%
where C0 and Cf represent the concentration of phenol in the model oil phase before and after reaction, respectively. The FT-IR spectra was recorded using a Nicolet-IS50 spectrometer to verify the bonding mechanism of phenol and [EMIM][LAC] [8,29]. 3. Results and discussion In this work, for exploring the extraction capacity of [EMIM][LAC] for phenol, the inﬂuence of extraction temperature, time and IL/oil mass ratio on the extraction efﬁciency of phenol were investigated. 3.1. Extraction temperature The temperature effect on the extraction of phenol from the model oil was investigated at temperature within the range of 25–55 °C. And a series of extraction experiments were explored at [EMIM][LAC]/ model oil mass ratio of 1:5 and extraction time of 30 min. The EE and phenol content in the rafﬁnate phase for extracting phenol varied with temperature is presented in Fig. 1. The EE value decreases with increasing the extraction temperature and the content of phenol in the rafﬁnate phase increases slightly when increasing the temperature. Table 2 GC analysis conditions for the model oil. Name
Characteristic Description for model oil
Carrier gas Injector Oven
Type Pressure Temperature Temperature
Capillary column PEG-20M (50 m × 0.25 mm × 0.5 μm, Kromat Technologies) Nitrogen 0.18 MPa 523.15 K Initial temp, 353.15 K (1 min); heating rate, 40 K·min−1; ﬁnal temp, 493.15 K (6 min) 543.15 K
X. Xu et al. / Journal of Molecular Liquids 306 (2020) 112911
Fig. 1. Inﬂuence of extraction temperature on phenol extraction efﬁciency and phenol content in the rafﬁnate phase. Conditions: [EMIM][LAC]/model oil mass ratio, 1:5; extraction time, 30 min; standing time, 1 h.
The EE value decreased from 94.4% to 88.8%, which is due to the weakening of the interaction between [EMIM][LAC] and phenol  when the extraction temperature increased from 25 to 55 °C. Also, increasing the temperature can increase the solubility of phenol in hexane and toluene. Thus, the temperature 25 °C was adopted for the following extraction experiments.
Fig. 3. Inﬂuence of mass ratio [EMIM][LAC]/model oil on phenol extraction efﬁciency and phenol content in the rafﬁnate phase. Conditions: extraction temperature, 25 °C; extraction time, 30 min; standing time, 1 h.
extraction time 30 min. As can be seen in Fig. 3, when increasing the mass ratio, the EE of phenol increases dramatically and the content of phenol in the rafﬁnate phase decreases. The EE reaches the highest value of 99.9% when the mass ratio is 1:1. For balancing the extraction economic beneﬁt and extraction efﬁciency , the mass ratio of 1:5 was selected with phenol extraction efﬁciency of 94.4%. 3.4. Mechanism analysis
3.2. Extraction time Extraction time was explored within the range of 5–60 min, namely, 5, 20, 30, 40 and 60 min. The extraction process was conducted at [EMIM][LAC]/the model oil mass ratio of 1:5 and temperature of 25 °C. As shown in Fig. 2, with increasing the extraction time, the extraction efﬁciency of phenol and the content of phenol in the rafﬁnate phase nearly keep constant, which indicates that phenol can quickly transfer from the organic phase into the IL phase. To separate phenol from the model oil high efﬁciently as well as ensure the balance of the extraction process [14,30], 30 min was adopted as the appropriate extraction time. 3.3. Mass ratio of IL to model oil
To explore the extraction mechanism of phenol from the model oil using [EMIM][LAC], the interaction between [EMIM][LAC] and phenol were investigated by quantum chemical calculations in this work. The Dmol3 code [31–34] was applied for the theoretical calculations. The details were reported in the literatures [27,35,36]. The basis set superposition error (BSSE) was corrected by application of the counterpoise correction method . And formulas of calculating interaction energy are presented in Eqs. (2) and (3): △Einteraction ¼ EAB −EA −EB þ EBSSE
EBSSE ¼ EA −EA;AB þ EB −EB;AB
Dosage of extractant is of importance for extraction in industry. So, the inﬂuence of the [EMIM][LAC]/model oil mass ratio within the range of 1:10–1:1 was explored at extraction temperature 25 °C and
where EAB refers to the energy of the A and B complex in the A, B basis set, EA and EB refer to the energy of one component, respectively. EA,
Fig. 2. Inﬂuence of extraction time on phenol extraction efﬁciency and phenol content in the rafﬁnate phase. Conditions: [EMIM][LAC]/model oil mass ratio, 1:5; extraction temperature, 25 °C; standing time, 1 h.
Fig. 4. The σ-proﬁles of [EMIM][LAC], phenol, hexane and toluene.
X. Xu et al. / Journal of Molecular Liquids 306 (2020) 112911
Fig. 5. Bond lengths for [EMIM][LAC] with phenol (a), hexane (b), toluene (c).
AB and EB, AB refer to the energy of A or B in the A, B basis set, respectively. The geometry optimization and energy optimization for [EMIM] [LAC], phenol, toluene and hexane was performed for calculating the subsequent σ-proﬁles, bond length, interaction energy and two different electron densities using Dmol3 code.
3.4.1. σ-Proﬁles The σ-proﬁles for [EMIM][LAC], phenol, toluene and hexane were obtained by the COSMO-SAC model. There are two red vertical lines (σ = ±0.0084 e/Å2) shown in Fig. 4, which divide the σ-proﬁle into three regions: hydrogen bond donor region on the left side of the cut off value −0.0084 e/Å2, hydrogen bond receptor region on the right side of 0.0084 e/Å2 and non-polar region between the two cut off values. The further the σ-proﬁle is from the cut off value of −0.0084 e/Å2 in the left hydrogen donor region, the stronger the hydrogen bond donor ability of the component is. Conversely, if the σ-proﬁle is further from the cut off value of 0.0084 e/Å2 in hydrogen receptor region, the stronger the hydrogen bond receptor ability of the component is. As can be seen in Fig. 4, the σ-proﬁle for [EMIM][LAC] is widely distributed with two clear peaks near −0.011 e/Å2 and +0.021 e/Å2, indicating that the IL performs the capacity as hydrogen bond donor and acceptor. Since the peak in receptor region is further than the cut off value, indicating that [EMIM][LAC] has the stronger ability as hydrogen bond acceptor. The σ-proﬁle for phenol is more extensive than that of hexane and toluene in hydrogen bond donor region, which shows that phenol has stronger donor ability than hexane and toluene. So, [EMIM][LAC] as a stronger hydrogen bond acceptor is easier to interact with phenol as a stronger hydrogen bond donor to form hydrogen bond. 3.4.2. Bond length The bond lengths between [EMIM][LAC] and (phenol/hexane/toluene) are shown in Fig. 5, which are 1.708 Å, 4.100 Å and 3.435 Å, respectively. The bond length between [EMIM][LAC] and phenol is shorter than the sum of the van der Waals radii of the two mutually attracted atoms (H atom from -OH group in phenol and O atom from -COO group in [EMIM][LAC]), 3.02 Å, since the van der Waals radii are 1.30 and 1.72 for H and O atoms, which indicates that hydrogen bond interaction occurred between [EMIM][LAC] and phenol. The bond length for the H atom in hexane with O atom in the IL is longer than the sum of their van der Waals radii, which shows that there is no hydrogen bond interaction between hexane and [EMIM][LAC]. For [EMIM][LAC] and toluene, there may be weak π-π interaction formed by the imidazole ring in the IL and aromatic ring in toluene . Therefore, the formation of hydrogen bond between phenol and [EMIM][LAC] is helpful for extracting phenol from the model oil using [EMIM][LAC].
3.4.3. Bond energy The bond energy between [EMIM][LAC] and (phenol/hexane/toluene) was calculated and is listed in Table 3. From Table 3, the bond energy between the extractant and phenol is −51.11 kJ/mol, which is higher than those between the IL and (toluene/hexane). Meanwhile, the value of the bond energy between [EMIM][LAC] and phenol is more than 40 kJ/mol, which indicates that strong hydrogen bond interaction exists for the IL with phenol. Hence, when [EMIM][LAC] is added into the model oil, phenol can be preferentially extracted into the IL phase due to the formation of strong hydrogen bond for phenol with [EMIM][LAC].
3.4.4. Electron densities To verify further the interaction mechanism between phenol and [EMIM][LAC], the calculated total electron density between the IL and phenol were calculated, which are described in Fig. 6. The obvious contact could be found between the electron area of the -OH group in phenol and the -COO group in [EMIM][LAC]. This morphology shows that the O-H···O hydrogen bond was formed between the two molecules. Meanwhile, the region varies with the different isovalues. When the isovalue is 0.2, the O-H···O bond formed between [EMIM][LAC] and phenol is still clear. Moreover, the similar result can be observed in deformation charge density maps, which are presented in Fig. 7. The three slices of the deformation density were formed on the cross section where the O atom in carboxyl group of [EMIM][LAC] co-exists with H atom in phenol, hexane and toluene, respectively. It can be observed that the electron receiving region surrounded by the O atom in the lactate anion is represented in red, and the electron donor region surrounded by the H atom in phenol is shown in blue. The accumulation of the region formed between [EMIM][LAC] and phenol is clear, which is caused by the formed OH···O hydrogen bond between the two molecules.
Table 3 Bond energy for [EMIM][LAC] with phenol, hexane and toluene in the model oil. System
Phenol Hexane Toluene [EMIM][LAC] [EMIM][LAC] + phenol [EMIM][LAC] + hexane [EMIM][LAC] + toluene
−307.564887 −237.118313 −271.628173 −687.994087 −995.585474 −925.139368 −959.641012
−0.001028 −0.000293 −0.000407
−51.110282 −0.210060 −4.342995
1 hartree = 27.211 eV = 627.509 kcal/mol = 2625.753 kJ/mol.
X. Xu et al. / Journal of Molecular Liquids 306 (2020) 112911
Fig. 6. Total density maps between [EMIM][LAC] and phenol under different isovalues.
Fig. 7. Deformation charge density maps between [EMIM][LAC] and phenol (a), hexane (b), toluene (c).
3.5. FT-IR veriﬁcation For validation of the hydrogen bond formation, FT-IR was applied since the formed hydrogen bond can be reﬂected through the vibration of ʋ-OH. In this work, the infrared spectra of the prepared [EMIM][LAC], [EMIM][LAC] + phenol complex and phenol were measured at 400–4000 cm−1 wavenumber interval at room temperature. In general, the vibration of ʋ-OH varies within the range of 3200–3600 cm−1. As shown in Fig. 8, the absorption peak caused by the ʋ-OH of pure phenol was at 3381 cm−1 shifted to 3450 cm−1 with adding [EMIM][LAC]. This chemical shift accounts for the hydrogen bond formed between [EMIM] [LAC] and phenol.
calculating the σ-proﬁles of [EMIM][LAC], phenol and the components in the model oil. The results of σ-proﬁles showed that [EMIM][LAC] interacted with phenol to form hydrogen bond. Meanwhile, the hydrogen bond formed between phenol and [EMIM][LAC] was validated by the FT-IR spectra. Based on the experimental results and theoretical
4. Conclusion In this work, for extracting phenol from the model oil, the IL [EMIM] [LAC] was prepared and characterized by 1H NMR. The extraction experiment was performed to explore the extraction capacity of the prepared IL. The extraction temperature, time and the dosage of [EMIM][LAC] were investigated on the extraction efﬁciency for phenol. The highest extraction efﬁciency of 99.9% was reached at 25 °C for 30 min with the [EMIM][LAC]/model oil mass ratio of 1:1. To explore the extraction mechanism, the intermolecular interactions were calculated by quantum chemical calculations. The results indicated that O-H···O hydrogen bond was engendered for -COO group in the lactate anion with -OH group in phenol. In addition, the COSMO-SAC model was applied in
Fig. 8. FT-IR spectra of [EMIM][LAC], [EMIM][LAC] + phenol complex and phenol.
X. Xu et al. / Journal of Molecular Liquids 306 (2020) 112911
analysis, the prepared IL [EMIM][LAC] can be adopted as an efﬁcient extractant for extracting phenol from the model oil and hydrogen bond plays an important role in extraction. CRediT authorship contribution statement Xin Xu: Data curation, Writing - original draft. Ao Li: Data curation, Investigation. Tao Zhang: Formal analysis. Lianzheng Zhang: Validation. Dongmei Xu: Supervision. Jun Gao: Conceptualization, Methodology. Yinglong Wang: Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper. Acknowledgements The authors are grateful for the support of the National Natural Science Foundation of China (21878178), Natural Science Foundation of Shandong Province (ZR2019BB066). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2020.112911. References  F.F. Dai, K. Xin, Y.H. Song, M.D. Shi, H.P. Zhang, Q.S. Li, Liquid–liquid equilibria for the extraction of phenols from alkane using ethylene glycol, Fluid Phase Equilib. 419 (2016) 50–56.  X.-l. Wang, J. Shen, Y.-x. Niu, Q.-t. Sheng, G. Liu, Y.-g. Wang, Solvent extracting coal gasiﬁcation tar residue and the extracts characterization, J. Clean. Prod. 133 (2016) 965–970.  X.K. Liu, X.L. Zhang, Solvent screening and liquid-liquid measurement for extraction of phenols from aromatic hydrocarbon mixtures, J. Chem. Thermodyn. 129 (2019) 12–21.  L.Z. Zhang, D.M. Xu, J. Gao, S.X. Zhou, L.W. Zhao, Z.S. Zhang, Extraction and mechanism for the separation of neutral N-compounds from coal tar by ionic liquids, Fuel 194 (2017) 27–35.  W.J. Guo, Y.C. Hou, W.Z. Wu, S.H. Ren, S.D. Tian, K.N. Marsh, Separation of phenol from model oils with quaternary ammonium salts via forming deep eutectic solvents, Green Chem. 15 (2013) 226–229.  Y.C. Hou, Y.H. Ren, W. Peng, S.H. Ren, W.Z. Wu, Separation of phenols from oil using imidazolium-based ionic liquids, Ind. Eng. Chem. Res. 52 (2013) 18071–18075.  H.J. Gai, L. Qiao, C.Y. Zhong, X.W. Zhang, M. Xiao, H.B. Song, A solvent based separation method for phenolic compounds from low-temperature coal tar, J. Clean. Prod. 223 (2019) 1–11.  C.F. Yao, Y.C. Hou, S.H. Ren, Y.A. Ji, W.Z. Wu, H. Liu, Efﬁcient separation of phenolic compounds from model oils by dual-functionalized ionic liquids, Chem. Eng. Process. 128 (2018) 216–222.  L. Yi, J. Feng, W.Y. Li, Z.Y. Luo, High-performance separation of phenolic compounds from coal-based liquid oil by deep eutectic solvents, ACS Sustain. Chem. Eng. 7 (2019) 7777–7783.  S.T. Ma, Q. Yu, Y.F. Hou, J.F. Li, Y. Li, Z.H. Ma, L.Y. Sun, Screening monoethanolamine as solvent to extract phenols from alkane, Energy Fuel 31 (2017) 12997–13009.  J.J. Gao, Y.F. Dai, W.Y. Ma, H.H. Xu, C.X. Li, Efﬁcient separation of phenol from oil by acid–base complexing adsorption, Chem. Eng. J. 281 (2015) 749–758.  Y.A. Ji, Y.C. Hou, S.H. Ren, C.F. Yao, W.Z. Wu, Separation of phenolic compounds from oil mixtures using environmentally benign biological reagents based on Brønsted acid-Lewis base interaction, Fuel 239 (2019) 926–934.  T.T. Jiao, C.S. Li, X.L. Zhuang, S.S. Cao, H.N. Chen, S.J. Zhang, The new liquid–liquid extraction method for separation of phenolic compounds from coal tar, Chem. Eng. J. 266 (2015) 148–155.
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