Separation of cresol from coal tar by imidazolium-based ionic liquid [Emim][SCN]: Interaction exploration and extraction experiment

Separation of cresol from coal tar by imidazolium-based ionic liquid [Emim][SCN]: Interaction exploration and extraction experiment

Fuel 264 (2020) 116908 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Separatio...

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Fuel 264 (2020) 116908

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Separation of cresol from coal tar by imidazolium-based ionic liquid [Emim] [SCN]: Interaction exploration and extraction experiment

T



Ao Lia, Xin Xua, Lianzheng Zhanga, Jun Gaoa, , Dongmei Xua, Yinglong Wangb 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

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Cresol Ionic liquid Coal tar Extraction Interaction mechanism

Cresol is a valuable chemical raw material which is mainly separated from coal tar. In this work, imidazoliumbased ionic liquid (IL) 1-ethyl-3-methyl imidazolium thiocyanate ([Emim][SCN]) was synthesized and used to separate cresol from coal tar. The extraction effect of IL on p-cresol was investigated, and the key experimental factors such as mass ratio of extraction agent to model oil, extraction temperature, and extraction time were studied and optimized. Under optimal conditions, the extract efficiency can reach up to 98.25%. Furthermore, quantum chemical calculation was used to explore the interaction of cresol with IL at the molecular level. And the calculation results were verified by infrared spectrum analysis. The results show that H-bond between the cresol and IL [Emim][SCN] are the main driving force of this separation process, and the interaction with pcresol is the strongest.

1. Introduction Cresol is one of the phenolic compounds and is mainly derived from coal tar [1–3], which is a high value-added chemical in chemical industry and widely applied in epoxy resins, dye intermediates, light-



resistant antioxidants, synthetic fibers and phenolic resins [4,5]. Therefore, separation of cresol from coal tar can increase the economic value of coal tar and reduce the hydrogen consumption in the subsequent hydrogenation of coal tar [6,7]. Usually, the traditional caustic washing method is used for separating phenols from coal tar in industry

Corresponding author. E-mail address: [email protected] (J. Gao).

https://doi.org/10.1016/j.fuel.2019.116908 Received 11 November 2019; Received in revised form 3 December 2019; Accepted 16 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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the reaction was performed under magnetic stirring and condensation reflux for 15 h. After the reaction, ethyl acetate was used to remove the unreacted reactants. Secondly, the prepared [Emim][Br] reacted with sodium thiocyanate with acetone as solvent to synthesize [Emim] [SCN]. This reaction lasted for 48 h. Finally, dichloromethane was used to extract unreacted reactants. The rest of the dichloromethane was removed using a rotary evaporator. Then the prepared [Emim][SCN] was dried in a vacuum dryer at 443.15 K for 12 h. The reaction route to synthesize [Emim][SCN] is presented in Scheme 1. The prepared IL [Emim][SCN] was characterized by 1H NMR spectrum, which is presented in Figs. S1 and S2 in the Supplementary Material. Analysis of 1H NMR spectra of [Emim][SCN] is as follow [Emim][SCN], 1H NMR (DMSO‑d6, δ, ppm): 1.21 (3H, t, NCH2CH3); 3.65 (3H, s, NCH3); 3.99 (2H, d, NCH2); 7.49 (1H, s, NC = CH); 7.57 (1H, s, NC = CH); 8.90 (1H, s, N = CH). The water content of the IL [Emim][SCN] was 0.26 wt%, which was determined by Karl Fischer Moisture Meter (KLS701).

[8]. This process consumes large amounts of strong acid and alkali, and causes serious environmental pollution [9,10]. Therefore, it is necessary to find a more environmentally friendly and efficient separation method to recover cresol from coal tar. Comparatively, extraction have been applied to separated cresol from coal tar due to its energy saving, where different organic solvents (polyols and ethanolamine) [11,12], complexing agents (urea and hexamethylenetetramine) [2,13] and deep eutectic solvents (DES) [14–17] were adopted as extraction agents. Recently, ionic liquids (ILs) have been reported as extractants to separate N-compounds [18,19], Scompounds [20,21], asphaltenes [22,23] and naphthenic acids [24] from coal tar due to the advantages of non-volatility, good thermal stability, designability and environment friendly [25]. The ILs with imidazole group as cation and halogen as anion showed good extraction effect on separation of cresol [26–28], where [Bmim][Cl] displayed the best extraction efficiency of about 99.0% [26], and the separation mechanism was explored [28,29]. But the ILs with halogen anions can cause serious corrosion to steel equipment and the viscosity is high that is not suitable for industrial applications. In addition, Jiao et al [2,3] also designed an extraction process for separating phenols from coal tar using DES and recovered the DES. However, there are few separation conceptual designs for extracting cresol using ILs. In view of the advantages and disadvantages of the above ILs, it is worthwhile to further explore the suitable IL as extractant for separation of cresol from coal tar, which is harmless to equipment and suitable for industrial application. In this work, the IL 1-ethyl-3-methyl imidazolium thiocyanate ([Emim][SCN]) with low viscosity was synthesized and applied to extract cresol from coal tar. The extraction performance of [Emim][SCN] was investigated. Meanwhile, the quantum chemical calculation, which can further verify the accuracy of the experiment [30–33], and Fourier Transform Infrared (FT-IR) spectra were used to investigate the interactions between [Emim][SCN] and cresol.

2.3. Extraction experiment Because there are a lot of compounds in coal tar [36], p-cresol were dissolved in toluene and n-hexane with continuous shaking to prepare the model oil. The mixing ratio by mass was set at 2:1:7 in the bulk density as their content ratio in coal tar [12]. The prepared IL was put into the model oil for extraction experiments. The detailed information of the liquid-liquid extraction apparatus and procedures were described in our previous work [18,37,38]. The p-cresol extraction efficiency (EE) is calculated by Eq. (1).

EEcresol = [(C0 −Cf )/ C0] × 100%

(1)

where C0 refers to the concentration of p-cresol in the model oil. Cf refers to the concentration of p-cresol in the model oil after extraction. 2.4. Analysis method

2. Experimental section

The concentration of p-cresol analysis was carried out in a gas chromatography (GC SP-7900, Lunan), which was equipped with a capillary column (SE-30, 30 m × 0.32 mm × 0.25 μm) and a flame ionization detector (FID). Benzaldehyde was adopted as internal standard. The detailed analysis conditions and temperature program are presented in Table 2.

2.1. Materials All reagents used in this work are analytical grade. The reagents with the purities reported by the suppliers were used directly. The names, CAS numbers, formulas and purities for the chemicals are listed in Table 1.

3. Results and discussion

2.2. Synthesis of [Emim][SCN]

3.1. Extraction temperature

The IL [Emim][SCN] was synthesized and purified with the methods in literatures [34,35]. Firstly, 1-methl-1H-imidazole (0.5 mol) and bromoethane (0.55 mol, a slight overdose of bromoethane to ensure all 1-methl-1H-imidazole can react with bromoethane), were mixed in a round-bottom flask to prepare 1-ethyl-3-methylimidazolium bromide ([Emim][Br]). The reaction temperature was set at 333.15 K, as well as

Extraction temperature is important for the separation process. The influence of extraction temperature was explored for separation of pcresol from the model oil. The extraction experiments were performed for 30 min at temperatures of 298.15, 303.15, 308.15, 318.15 and 328.15 K with a mass ratio of IL to model oil of 0.2. The results of extraction efficiency and mass fraction of p-cresol in the raffinate phase

Table 1 CAS Number, formula, provider, and purity of the used chemicals. Component

CAS.no

Molecular formula

Suppliers

Purity/wt%

Bromoethane Sodium thiocyanate 1-Methl-1H-imidazole P-cresol Hexane Toluene Acetone Dichloromethane Ethyl acetate Benzaldehyde

74-96-4 540-72-7 616-47-7 106-44-5 110-54-3 108-88-3 67-64-1 75-09-2 141-78-6 100-52-7

C5H5Br NaSCN C4H6N2 C7H8O C6H14 C7H8 C3H6O CH2Cl2 C4H8O2 C7H6O

Shanghai Macklin Biochemical Co., Ltd. Shanghai Macklin Biochemical Co., Ltd. Shanghai Bide Pharmatech Co., Ltd. Shandong Xiya Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Chengdu Kelong Chemical Co., Ltd. Tianjin Kemiou Chemical Reagent Co., Ltd. Shanghai Titan Scientific Co., Ltd.

98.0 98.5 98.0 97.0 97.0 99.5 99.5 99.9 99.5 99.0

2

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N

N

+

CH3CH2Br

N

Br

N

CH2CH3 N

N

Br

+

NaSCN

N

N

CH2CH3

SCN

+ NaBr

CH2CH3 Scheme 1. Reaction to synthesize [Emim][SCN].

Table 2 GC analysis conditions. Name

Characteristic

Description for model oil

Column

Type Specification Type Pressure Temperature Temperature

Capillary column SE-30 (30 m × 0.32 mm × 0.25 μm) Nitrogen 0.1 MPa 523.15 K Initial temp, 80 °C (1 min); heating rate, 40 °C/ min; then temp 120 °C (0.5 min); heating rate, 40 °C /min; final temp, 220℃ (1 min) Flame ionization detector (FID) 543.15 K

Carrier gas Injector Temperature program Detector

Type Temperature

Fig. 2. Effect of extraction mass ratio of IL to model oil on EE and mass fraction of p-cresol in the raffinate phase.

view of the extraction effect and economic benefit, the optimal mass ratio was determined at 0.2 with the extraction efficiency of 98.25%. 3.3. Effect of extraction time Extraction time is another important factor for the extraction process. In order to observe the effect of time on separating p-cresol from the model oil, the extraction experiments were performed within the time range from 5 to 60 min with the determined mass ratio (IL: model oil) and extraction temperature. The extraction efficiency and mass fraction of p-cresol in the raffinate phase at different extraction time are shown in Fig. 3. From Fig. 3, the extraction efficiency is near constant from 5 min to 60 min, which indicates that p-cresol in the organic facies can migrate quickly to the IL phase. Such a quick equilibrium suggests a

Fig. 1. Effect of extraction temperature on EE and mass fraction of p-cresol in the raffinate phase.

are presented in Fig. 1. As seen from Fig. 1, the extraction efficiency of p-cresol extracted by the IL [Emim]SCN decreases slightly with increasing the temperature and the mass fraction of p-cresol in the raffinate increases as the temperature increases, since increasing the temperature can increase the mutual solubility. The maximum extraction efficiency value is 98.25% at 298.15 K. Thus, room temperature is the best choice for the cresol separation, the optimal extraction temperature was determined at 298.15 K and applied in the following extraction experiments. 3.2. Mass ratio of IL to model oil Considering the economy of the extraction operation, it is important to find a proper amount of extraction solvent for cresol removal. Therefore, the influence of the mass ratio (IL: model oil) on the extraction performance was investigated within the mass ratio range of 0.1 to 1.0. The extraction experiments were carried out for 30 min at the optimized temperature. As a result, Fig. 2 shows the extraction efficiency of p-cresol increases with increasing the mass ratio (IL: model oil). When the mass ratio is 0.2 or more, the extraction can almost reach equilibrium and the extraction efficiency is close to the maximum. In

Fig. 3. Effect of extraction time on EE and mass fraction of p-cresol in the raffinate phase. 3

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Fig. 4. Bond lengths for [Emim][SCN] + p-cresol (a), [Emim][SCN] + m-cresol (b), [Emim][SCN] + o-cresol (c).

deformation electron density indicates that a certain degree interaction between the O⋯H and N⋯C could be observed from the profiles of the deformation density. According to the distribution of electrons around the atom, there exists a hydrogen bond interaction between cresols and the anions of IL [Emim][SCN]. Furthermore, the hydrogen bonding information can be verified with FT-IR spectrum [40,41]. As shown in Fig. 6, the vibration at 3349 cm−1 attributes to the ν–OH of p-cresol. When [Emim][SCN] interacted with p-cresol, the peak shifted to 3466 cm−1. The vibrational displacement of the ν–OH demonstrates the presence of hydrogen bonds between the complex of the IL [Emim][SCN] and p-cresol. Based on the interaction exploration between [Emim][SCN] and cresols using quantum chemical calculations and FT-IR analysis, the N atom of [Emim][SCN] can easily form the hydrogen bond with cresols and p-cresol shows strongest interaction with [Emim][SCN], which is the main driving force to extract p-cresol from the model oil by the IL [Emim][SCN].

strong interaction between p-cresol and IL. To ensure p-cresol can be extracted completely, in this work, 30 min is chosen as the optimal extraction time. 3.4. Intermolecular interaction For investigating the interactions between the IL and p-cresol, the bond length and bond energy between p-cresol and IL were explored in this work by quantum chemical calculation using Materials Studio. The detailed procedures for the calculation are included in the Supplementary Material. The optimized complex geometry of cresols and IL [Emim]SCN are plotted in Fig. 4. As shown in Fig. 4, the interaction distance between the anion of [Emim][SCN] and p-cresol, mcresol and o-cresol are 1.705 Å, 1.705 Å, and 1.767 Å, which are less than 3.13 Å, the sum of the van der Waals’ radii of H,1.30 Å and N, 1.83 Å. It is shown that there is a strong intermolecular interaction between the anion of IL and three cresols, which can be expressed by the bond length. As results, there exist O–H⋯N hydrogen bonds between three cresols and the IL. The interactions between the molecules can be further explored by bond energy. Generally, the value of the hydrogen bond energy is in the range of 25–40 kJ/mol. The strong hydrogen bond interaction is formed when the bond energy is greater than 40 kJ/mol. The stronger bond energy indicates better separation effect. The bond energy between IL and cresols was calculated directly by BSSE-corrected interaction energies [39]. The calculated results of interaction energy between the molecules are listed in Table 3. As listed in Table 3, the calculated values of interaction energy of [Emim][SCN] with three cresols are −65.0662, −60.5709 and −57.6274 kJ/mol, which are greater than 40 kJ/mol. The larger bond energy means that cresols are inclined to interact with the [Emim] [SCN]. The hydrogen bond between p-cresol and the IL is the strongest, which indicates that p-cresol can interact with [Emim][SCN] strongly than m-cresol and o-cresol. Meanwhile, similar results can also be expressed by deformation electron density. As shown in Fig. 5, the electron donating area surrounding the H atom is expressed in blue and the electron withdrawing encircling around the N atom or O atom are expressed in red. The

4. Conclusion For extraction of cresols from coal tar using ionic liquids, imidazolium-based ionic liquid [Emim][SCN] was synthesized and characterized by 1H NMR in this work. Considering the compounds in coal tar, the model oil was adopted and prepared with the mass ratio of 7:1:2 for n-hexane, toluene, p-cresol. The extraction performance of [Emim] [SCN] was investigated and the extraction conditions were optimized. The optimal extraction performance of p-cresol was achieved with the extract efficiency of 98.25% at the extraction temperature, 298.15 K, mass ratio (IL: model oil), 0.2 and extraction time, 30 min. To explore the extraction mechanism of the IL [Emim][SCN], the interaction between [Emim][SCN] and cresols were calculated using the Dmol3 in the Materials Studio. The calculated results showed that high extraction efficiency of p-cresol by [Emim][SCN] was due to the hydrogen bond interaction between the hydroxyl group of p-cresol and the N atom of the IL. Furthermore, the FT-IR spectrum shows that vibration of OH (ν–OH) has moved after [Emim][SCN] interacted with p-cresol and the peak shifted to 3466 cm−1, which indicates the presence of hydrogen bonds between the complex of the IL [Emim][SCN] and p-cresol.

Table 3 Revised BSSE interaction energy of cresol and IL.

CRediT authorship contribution statement

System

E(hartreesa)

EBSSE(hartreesa)

△E(kJ/mol)

p-cresol m-cresol o-cresol [Emim][SCN] [Emim][SCN] + p-cresol [Emim][SCN] + m-cresol [Emim][SCN] + o-cresol

−346.9092 −346.9047 −346.8790 −836.0135 −1182.8836 −1182.8856 −1182.8779

– – – – −0.0009 −0.0012 −0.0005

– – – – −65.0662 −60.5709 −57.6274

a

Ao Li: Data curation, Writing - original draft. Xin Xu: Data curation. Lianzheng Zhang: Visualization, Investigation. Jun Gao: Formal analysis. Dongmei Xu: 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

1 hartrees = 27.211 eV = 627.509 kcal/mol = 2625.753 kJ/mol. 4

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Fig. 6. FT-IR spectrum for p-cresol, [Emim][SCN] and IL/p-cresol complex.

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