Designing Ionic Liquids with Dual Lewis Basic Sites to Efficiently

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Designing Ionic Liquids with Dual Lewis Basic Sites to Efficiently Separate Phenolic Compounds from Low-temperature Coal Tar Hengjun Gai, Lin Qiao, Caiyun Zhong, Xiaowei Zhang, Meng Xiao, and Hongbing Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02119 • Publication Date (Web): 17 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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ACS Sustainable Chemistry & Engineering

Designing Ionic Liquids with Dual Lewis Basic Sites to Efficiently Separate Phenolic Compounds from Low-temperature Coal Tar Hengjun Gai, Lin Qiao, Caiyun Zhong, Xiaowei Zhang, Meng Xiao, Hongbing Song∗ College of Chemical Engineering, Qingdao University of Science and Technology, Zhengzhou Road No.53, Qingdao, 266042, China ∗

Corresponding author Dr. H. B. Song, E-mail: [email protected]

ABSTRACT Although separation of phenolic compounds from coal tar has the good practical application value in industry, the traditional separation method can result in serious environmental problems. In the present work, three ionic liquids (ILs) with dual Lewis basic sites were designed and synthesized by neutralizating 1,1,3,3-tetramethylguanidine (TMG) and corresponding acids (L-proline, acetate acid and tetrafluoroboric acid), which were employed to separate phenolic compounds from model oil and coal tar. The basicity of TMG-based ILs were characterized by probe molecule through 1H NMR and quantum chemical calculations. Moreover, the influence of factors on separation efficiency, such as stirring time, extraction temperature, and the ratio of ILs to model oil, was investigated in details. The results showed that [TMG][BF4] showed the best extraction efficiency of 98.2%, at a extraction temperature of 30 ℃, stirring time of 35 min, and extraction ratio of 1.1 g/4 mL. After separating, these ILs were recovered through back extraction. Furthermore, the separation mechanism was determined by analyzing the hydrogen bond and chemical bond for ILs and phenolic compounds via UV-vis, FT-IR and quantum chemical calculations. Thus, TMG-based ILs can be effective in separating phenolic compounds from coal tar and as an alternative extractant in future. KEYWORDS: Phenolic compounds, Low-temperature coal tar, Separation, Probe molecule, Ionic liquid INTRODUCTION

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Low-temperature coal tar is one of the byproducts of coal dry distillation.1-3 Moreover, it has a high amount of high added-value chemicals. Phenolic compounds,4 as a typical example, are among the most versatile and important industrial organic chemicals, and exhibit a wide range of applications, such as for the production of bisphenol A, phenolic resin, engineering plastics, and synthetic fiber.5 However, phenolic compounds can increase hydrogen consumption in the hydrogenation of coal tar during the production of fuel oils, and the water produced by the reaction can adversely affect the activity and life of the catalyst.6-7 Therefore, separating phenolic compounds from coal tar is highly crucial for coal chemical industry.8 The traditional method for separating phenolic compounds from coal tar, namely, caustic washing method, has brought tremendous environmental pollution and has thus seriously restricted the development of clean, high value-added chemicals from coal tar.9 Recently, new extraction methods have been employed to separate the phenolic compounds from coal tar instead of caustic washing.10 Among them, the ionic liquids (ILs) have gathered attention due to their wide application prospects. For instance, Hou et al. adopted imidazolium-based ILs to efficiently separating phenols from oil.11 And Gao et al. employed three acidic imidazolium-based ILs to separate basic N-compound pyridine from coal tar.12 Meanwhile, they developed different ILs to investigate liquid-liquid extraction in separation field under mild conditions.13-15 More efficient and environmental friendly methods are needed for the separation of high-added chemicals from coal tar. However, these ILs have high oil solubility. Thus, it is desired to find some alternatives for ILs that have low oil solubility, high phenol extraction efficiency and high recovery rate. In terms of chemical property, phenolic compounds are both a Brønsted acid and a Lewis base because of their positively charged H-atom on -OH group and the lone-pair electron on O-atom. Therefore, when separating phenols from coal tar, acid-base interactions may occur. In this article, we designed suitable chemical structures of ILs that consisted of 1,1,3,3-tetramethylguanidinium (TMG) cation and different anions to improve the phenol extraction rate. The designed TMG-based ILs containing double Lewis basic sites were synthesized by neutralizing 1,1,3,3-tetramethylguanidine and corresponding acids (L-proline, acetate acid and tetrafluoroboric acid) with different pKa values.16 Moreover, the basicity of TMG-based ILs was analyzed by probe molecule through 1H NMR quantum chemical calculations. Furthermore, the separation mechanism of TMG-based ILs was confirmed by UV-vis, FT-IR, and quantum chemical


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calculations. The results indicated that hydrogen bonding and coordination occur between the -OH of the phenol and the double Lewis basic sites of the anions and cations from TMG-based ILs. Additionally, TMG-based ILs are low-cost, environmentally friendly, and non-corrosive, and fully conform to the concepts of green chemistry. Thus, the developed TMG-based ILs have economic and environmental benefits. This work is highly instructive for improving the environmental protection and efficiency of extraction processes. EXPERIMENTAL Reagents and materials. 1,1,3,3-Tetramethylguanidine (TMG, 99%) , L-proline (Pro, 99%), acetate acid (HAc, 99%) and tetrafluoroboric acid (HBF4, 99%) were purchased from Aladdin Reagent Co., Ltd. For the model oil, the n-hexane and phenol were purchased from Sinopharm Chemical Reagent Co., Ltd. After those other solvents such as diethyl ether (99%), ethyl acetate (99%) and ethanol (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used for all experiments. Synthesis of TMG-based ILs. All ILs were synthesized and modified according to methods in literature.17 The synthesis involved a 1:1 M ratio of TMG to L-Proline, and HAc and HBF4. In a typical experiment, L-proline (0.1 mol, 11.5 g) in a 50 mL round bottom flask, which L-proline was divided into four parts during the experiment, then flask was placed in an ice-water bath. Afterward, TMG (0.1 mol, 11.5 g) was placed in a constant-pressure dropping funnel, and stirred, and the dropping funnel was adjusted to start the TMG drip. After the TMG had dripped, the reaction was allowed to remain for another 6 h. Ethyl acetate was used to extract unreacted TMG and L-proline, and the rest of ethyl acetate was removed by using a rotary evaporator and drying in a vacuum dryer at 40 °C for 12 h. Finally, the product, labeled [TMG][Pro], was obtained. [TMG][Ac] and [TMG][BF4] were derived using the same method, but with acetate acid and tetrafluoroboric acid instead of L-proline, respectively. The synthetic names and structural



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formulae of TMG-based ILs are shown in Table 1. Analysis of 1H NMR and FT-IR spectra of [TMG][Pro] are as follows. For [TMG][Pro], 1H NMR (DMSO-d6, δ, ppm): 1.54-1.59 (2H, m, -CH2), 1.76-1.84 (2H, m, -CH2), 2.69-2.8 (H, m, -CH), 2.82 (12H, d, -CH3), 2.96-3.04 (2H, m, -CH2), 3.58 (1H, s, -NH), 4.63 (2H, s，-NH2+), FT-IR (υ, cm−1): 3413 (-OH), 1615 (-NH), 1411, 1201, 1093 (Supporting Info, Figures S1 and S2). For [TMG][Ac], 1H NMR (CDCl3, δ, ppm): 1.95 (3H, s, -CH3), 2.98 (12H, s, -CH3), 8.84 (2H, s， -NH2+), FT-IR (υ, cm−1): 3354 (-OH), 1605, 1566, 1408, 1337, 1096 (Supporting Info, Figures S3 and S4). For [TMG][BF4], 1H NMR (DMSO-d6, δ, ppm): 2.77 (12H, s，-CH3), 4.4 (2H, s，-NH2+), FT-IR (υ, cm−1): 3286 (-OH), 3106 (-NH2), 2283, 1645, 1607, 1563, 1434 (Supporting Info, Figures S5 and S6). Table 1. Name and structure of TMG-based ILs TMG-based ILs

Structure

1,1,3,3-tetramethylguanidinium

Abbreviation [TMG][Pro]

2-pyrrolidinecarboxylate

1,1,3,3-tetramethylguanidinium acetate

[TMG][Ac]

1,1,3,3-tetramethylguanidinium tetrafluoroborate

[TMG][BF4]

Characterization and analysis method. Analysis of phenol extraction rate from the model oil was performed using a gas chromatograph equipped with a DB-5 capillary column, moreover,


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n-hexane and 2,5-dibromotoluene were chosen as solvent and internal standard, respectively. The conditions are as follows: DB-5 capillary column (30 m × 0.25 mm × 0.25 µm), detector using flame ionization detector (FID). The temperature of the GC started at 130 °C, held for 1 min, and increased to 250 °C at a rate of 20 °C·min−1, and then retained for 1 min. The total analysis time was 8 min. The temperature of the injection port was 250 °C, and the temperature of FID was 280 °C. The phenol concentration detected by the GC was recorded as the Peak area. TU-1810 UV-vis spectrometer was used to analyze the difference in maximum absorption peaks of IL and phenol. Fourier transform infrared (FT-IR) spectroscopy of the samples was recorded with a Bruker Tensor 27 FT-IR spectrometer. Nuclear magnetic resonance (NMR) spectroscopy of the samples were analyzed with Bruker AVANCE III at 500 MHz. Gaussian software and its corresponding Gaussian View software were used in quantum chemistry calculation to theoretically calculate the experimental results. Basicity characterization of ILs. The principle of basicity characterization of ILs was as follows. Phenol, as a probe molecule, was combined with basic ILs, and the basicity characterization of ILs was performed using quantum chemical calculations and probe molecule by 1H NMR. The principle of the analysis was mainly to obtain the chemical shift between the phenol and ILs through 1H NMR, and then the interaction between the phenol and ILs was calculated by quantum chemical calculations. The combination of the two methods can then demonstrate the basicity of ILs. Separation process in model oil. In a typical experiment, TMG-based ILs (0.5 g) was added into a 25 mL breaker containing 4 mL model oil and stirred for 30 min. Afterward, the mixture was allowed to separate into layers. The extraction rate of phenol was determined by using GC. The



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removal efficiency was calculated using Equation (1), as shown below: The removal efficiency of phenol % =

× 100% (1)

where Co and Cf represent the phenol concentration in the original model oil and the upper layer after the reaction, respectively. Next, the optimum conditions were obtained by optimizing the conditions of stirring time, reaction temperature, and extraction ratio. Theoretical calculation. All the theoretical calculations were implemented through using the default convergence criterion of the Gauss 09 program.18 The first step was to optimize the molecular structure using DFT at the B3LYP/6-31G+** level.19 Meanwhile, the zero-point energy of the optimized molecules was calculated, and then the global minimum was used as subsequent calculations.20

Second,

single-point

energy

calculations

were

performed

at

the

B3LYP/6-311G++** level, and the calculated single point was corrected by zero point energy to obtain the total energy.21 Finally, the interaction energies between phenol and TMG-based ILs, the lowest surface average local ionization energies between phenol and ions were calculated. Reverse extraction of phenol and ILs recovery. In this section, the back extraction rate of phenol in ILs by using diethyl ether and the regeneration capacity of ILs are studied. In a typical experiment, 1.1 g of ILs and 4 mL of the model oil were sequentially added into a 25 mL breaker and stirred at 30 °C for 35 min. After removing the upper layer, the obtained lower layer was continuously extracted thrice by diethyl ether (each time with 10 mL). Then the lower layer containing a little diethyl ether was placed in a vacuum dried at 40 °C for 24 h. Finally, ILs were recovered. Effect of reverse extraction on phenol and recovery were verified by 1H NMR and GC, respectively.


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RESULTS AND DISCUSSION The basicity characterization of TMG-based ILs Due to phenolic compounds are Brønsted acids, the extraction mechanisms are highly dependent on the basicity of ILs, and increasing the basic site of ILs may effectively enhance extraction efficiency. Thus, we tune and increase the basic site of ILs for improving the efficiency of the extraction process. Three ILs with dual Lewis basic sites were designed and synthesized by neutralizating 1,1,3,3-tetramethylguanidine (TMG) and corresponding acids (L-proline, acetate acid and tetrafluoroboric acid), labeled [TMG][Pro], [TMG][Ac], and [TMG][BF4], respectively. The basicity of synthetic ILs were confirmed by investigating the interaction between TMG-based ILs 1

and an acidic probe molecule, phenol, which was analyzed by H NMR and quantum chemistry.22-24 1

H NMR analysis

To shed light on the nature of the interaction between designed TMG-based ILs and phenol, 1H NMR spectroscopy was employed. Figure 1 shows the 1H NMR spectra for addition of phenol to [TMG][BF4]. The OH and NH signals are too broad to be observed due to the rapid proton exchange and intramolecular hydrogen-bonding interactions.25 The NH2+ protons of the inequivalent amino cation in [TMG][BF4] were monitored, and they were found to exhibit one broad single peak around 4.44 ppm, and single peak of the OH protons in phenol were observed at 5.58 ppm. In the case of phenol addition, significant downfield shifts of NH peaks were observed and OH peaks disappeared. As expected, the signals for other protons in phenol and [TMG][BF4] also exhibit somewhat shift upon phenol addition. Which indicated that the [TMG][BF4] and phenol existed hydrogen-bonding interactions.



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a

DMSO

dbf

ec

ec

dbf

k

8

7

6

5

4

3

2

δ (ppm)

Figure 1. 1H NMR of the complex of the [TMG][BF4] and phenol The 1H NMR spectra of [TMG][Pro] and [TMG][Ac] were similar to that of [TMG][BF4], thus, the following simple results were obtained. For [TMG][Pro], when [TMG][Pro] and phenol reacted, the hydroxyl peak of the phenol disappeared from the 1H NMR spectra, and the active hydrogen on the benzene ring changed (Supporting Info, Figure S7). As for phenol, after the formation of the complex, the chemical shift of the active hydrogen at the benzene ring changed, and the hydroxyl peak of the phenol disappeared from the 1H NMR spectra, thereby indicating that the [TMG][Ac] and phenol underwent chemical reaction (Supporting Info, Figure S8). The results proved that [TMG][Pro] and [TMG][Ac] possessed basicity. Theoretical analysis At the microscopic level, an effective guide to interpreting and predicting the electrostatically reactive ability of molecules is the most-negative-surface electrostatic potential (Vs, min) of the investigated molecule.26-30 Although electrostatic interaction is a key factor in the analysis of the strength of the basicity between phenol and TMG-based ILs, it is not the only factor. In fact, except electrostatic interactions, covalent interactions (i.e., electron delocalization), also have a great influence on the basicity of the molecule. Similar to the case of the electrostatic interaction, an


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effective guide to interpreting and predicting the electron-transfer ability of molecules is also defined at the molecular surface, i.e., the lowest surface average local ionization energy (Is, min) of the investigated molecule.28, 31-32 Therefore, Vs, min and Is, min between TMG-based ILs and phenol were calculated.

For TMG-based ILs composed of various anions, the corresponding absolute value of Vs, min for phenol+[BF4]- is 647 kJ/mol (Figure 2), whereas the values of Vs,

min

for phenol+[Ac]- and

phenol+[Pro]- are 140 and 238 kJ/mol, respectively (Supporting Info, Figures S9 and S11). Similar -

to the case of Vs, min, the corresponding absolute value of Is, min for phenol+[BF4] is 6.71 eV (Figure

3), whereas the values of Is,

min

for phenol+[Ac]- and phenol+[Pro]- are 1.45 and 2.47 eV,

respectively (Supporting Info, Figures S10 and S12). Generally, TMG-based ILs with different cations is in good accordance with that expected from the quantum chemical parameters Vs, min and Is, min. It can be demonstrated that the basicity of three TMG-based ILs are very strong, and the orders of the absolute values of Vs, min and Is, min are both [Ac]- < [Pro]- < [BF4]-. Although the basicity of [TMG][Pro] is stronger than that of the [TMG][Ac], the molecular size of [TMG][Pro] is larger than that of [TMG][Ac]. Therefore, the influence of steric hindrance should be taken into account in the real experimental process of extraction, which illustrated in section of reaction mechanism.



Figure 2. Computed electrostatic potential at the isodensity contour (0.001 electron/bohr3) surface of [TMG][BF4] and phenol with their corresponding optimized geometries calculated by Gaussian. Color ranges are as follows: The scale spans -157 (red) through 0.0 (green) to 157 (blue) (unit: kJ/mol). The position of Vs,

min

for an amino group and fluoride ion is denoted by a cyan

hemisphere. The value of Vs, min for an amino group and fluoride ion is shown in the column graph, in kJ/mol.

Figure 3. Computed electrostatic potential at the isodensity contour (0.001 electron/bohr3) surface


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of [TMG][BF4] and phenol with their corresponding optimized geometries calculated by Gaussian. Color ranges are as follows: The scale spans -5.86 (red) through 0.0 (green) to 5.86 (blue) (unit: eV). The position of Is, min for an amino group and fluoride ion is denoted by a cyan hemisphere. The value of Is, min for an amino group and fluoride ion is shown in the column graph, in eV.

Factors affecting the extraction efficiency Effect of the anion structure of TMG-based ILs The influences of various anion substituents ([Pro]-, [Ac]-, and [BF4]-) on the performance of the TMG-based ILs during the extraction process were investigated. Figure 4 shows the phenol removal efficiency of the three TMG-based ILs. Among them, [TMG][Pro] exhibited appropriate removal efficiency of phenol (59%) from the model oil at the phase ratio of 1 g/4 mL in 5 min. Whereas [TMG][BF4] and [TMG][Ac] exhibited a stronger extraction ability in the same condition, the removal efficiency of phenol was 88% and 86%, respectively. The substituent groups of the TMG-based ILs from the best to worst performance followed the order of [BF4]- > [Ac]- > [Pro]-. The large difference between the extraction rates of IL and phenol was mainly due to the structure of the IL and the electronegativity of the anion having different abilities to adsorb phenol. The large size of [TMG][Pro] produced steric hindrance to adsorb phenol, whereas the electronegativity of the other two ILs, especially [TMG][BF4], were more conducive to the formation of hydrogen bonds between ILs and phenol, and was favorable for the adsorption reaction.



100

The removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

60

40

20

0

[TMG][BF4]

[TMG][Ac]

[TMG][Pro]

Type of TMG-based ILs Figure 4. Effect of TMG-based ILs structure on the removal efficiency of phenol.

Effect of phase ratio on the extraction rate of phenol The phase ratio is an important factor for the extraction process. In order to analyze the effect of phase ratio on the extraction rate of phenol, the extraction experiment was designed as follows. The ratios of ILs to model oil were 0.5 g/4 mL, 0.7 g/4 mL, 0.9 g/4 mL, 1.1 g/4 mL, 1.2 g/4 mL, and 1.3 g/4 mL, respectively. Figure 5 shows that as the ratio increases, the IL on the extraction rate of phenol gradually increases. The highest extraction rate was 97%, which was obtained via using [TMG][BF4] at the phase ration of 1.1 g/4 mL. The extraction performance of [TMG][Pro] was not good, mainly due to its much greater anion in ILs. In conclusion, the optimum phase ratio for extractions should be controlled at 1.1 g/4 mL in the succeeding experiments.


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100

2000

90

1750


80 1500 70 1250

60

[TMG][BF4]

50

[TMG][Ac] [TMG][Pro] [TMG][Pro] [TMG][Ac] [TMG][BF4]

40 30

1000 750 500

20

Phenol content (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


250

10 0

0 0.4

0.6

0.8

1.0

1.2

1.4

Phase ratio(ILs/model oil=x:4ml)

Figure 5. Effect of phase ratio on the removal efficiency of phenol

Effect of stirring time on the extraction rate of phenol Stirring time of the mixture is another important factor for the extraction process, because it can affect the equilibrium between the two phases. In order to observe the effect of time on phenol extraction, the extraction experiment was designed as follows: The effect of stirring time was investigated comprehensively from 5 to 45 min at 25 °C using the phase ratio of 1.1 g/4 mL. Figure 6 shows that the extraction rate of phenol from ILs gradually increased as extraction time increased. In particular, when the stirring time was 35 min, the extraction rate of [TMG][BF4] reached 98%. The extraction rate of [TMG][Pro] was lower than those of the other two due to its weak interaction with phenol, and reached equilibrium after stirring for 35 min. Therefore, the optimal stirring time was 35 min.



100

2000

90

1750


80 1500 70


1250

60 50

[TMG][Pro] [TMG][Ac] [TMG][BF4]

1000

40

[TMG][BF4]

750

30

[TMG][Ac] [TMG][Pro]

500

20 250

10 0

0 0

10

20

30

40

50

Reaction time (min)

Figure 6. Effect of reaction time on the removal efficiency of phenol

Effect of temperature on the extraction rate of phenol The effect of temperature on phenol extraction was investigated from 25 °C to 45 °C. Figure 7 shows that the temperature hardly affected the phenol extraction rate. When the temperature increased, the removal efficiency of phenol displayed almost no change. Therefore, the extraction process was performed at 30 °C.

100

2000 1750


80 1500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1250

60

[TMG][BF4] [TMG][Ac] [TMG][Pro] [TMG][BF4]

40

[TMG][Ac] [TMG][Pro]

20

1000 750 500 250

0

0 25

30

35

o

40

45

Reaction temperature ( C)

Figure 7. Effect of reaction temperature on the removal efficiency of phenol

Reaction mechanism


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As for the designed TMG-based ILs, both anions and cations are basic ions that can react chemically with phenols, and the designed anion has a greater electronegativity and can form strong hydrogen bonds with phenol. The hydrogen bond formation and the chemical reaction between phenol and [TMG][BF4] were analyzed using UV-vis and FT-IR.

UV-vis analysis of the reaction mechanism UV-vis can determine whether a reaction has changed by detecting the changes in the maximum absorption wavelength of the substance. For phenol and [TMG][BF4], the maximum absorption wavelengths of the pure substances of phenol and [TMG][BF4] were measured, and then phenol and [TMG][BF4] mixed at a mole ratio of 1:1. Afterward, the reactant was dissolved in ethanol, and its maximum absorption wavelength was measured after the reaction. Figure 8 shows that the maximum absorption wavelength changed greatly after the reaction. [TMG][Pro] and [TMG][Ac] showed similar results in UV-vis spectroscopy. (Supporting Info, Figures S13 and S14). The UV-vis only made preliminary analysis of the reaction, and was consist well with qualitative analysis through FT-IR spectroscopy.

3.5 3.0

Phenol [TMG][BF4]

2.5

Complex 2.0

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.0

-0.5 200

250

300

350

-1

Wavenumber (cm )


400


Figure 8. The UV-vis spectrocopy of reaction mechanism between [TMG][BF4] and phenol

FT-IR analysis of the reaction mechanism [TMG][BF4] formed a complex with phenol due to the hydroxyl group in phenol, which formed acid-base coordination and hydrogen- bonding with the functional group of [TMG][BF4]; therefore, an effective method to verify the acid-base coordination and hydrogen bonding information was FT-IR.33-34 The results of the analysis are shown in Figure 9.

Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3371

3173

3286 3106 [TMG][BF4] Complex

4000

3500

3000

2500

2000 -1

1500

1000

Wavenumber (cm )

Figure 9. Infrared spectra of reaction of phenol and [TMG][BF4] The peak in the range of 3200 to 3500 cm-1 was ascribed to the stretching vibration of OH (ν-OH) in phenol, whereas the peak in the range of 3100 to 3500 cm-1 was due to the stretching vibration of NH2 (ν-NH2) in TMG. As shown in Figure 9, the peak at 3106 cm-1 was attributed to the ν-NH2 of pure [TMG][BF4]. When the complex was formed, the peak shifted to 3173 cm-1; this phenomenon was attributed to the fact that the amino functional groups of [TMG][BF4] exhibited acid-base coordination with the hydroxyl of the benzene ring. Moreover, the fluorine atoms of the [TMG][BF4] exhibited large electronegativity, easily formed hydrogen bonds with


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phenol. The analysis revealed that the ν-OH stretching vibration of pure phenol was re-evaluated at wavenumber of 3336 cm-1 (Supporting Info, Figure S15). When the complex was formed, the wavenumber shifted to 3371 cm-1. Thus, hydrogen bonds formed between [TMG][BF4] and phenol. Therefore, two kinds of interaction existed between [TMG][BF4] and phenol, namely, hydrogen bond and acid-base coordination. The infrared spectra of the results of [TMG][Pro] and [TMG][Ac] were similar to that of [TMG][BF4], the simple results are as follows. Due to steric hindrance, the reaction effect of [TMG][Pro] was poor, however, hydrogen bonds were generated due to the presence of NH2 functional groups on the five-membered ring. The analysis revealed that the ν-OH stretching vibration of pure phenol was re-evaluated at wavenumber 3336 cm-1 (Supporting Info, Figure S15). When the complex was formed, the wavenumber shifted to 3340 cm-1. As can be seen from the wavenumber of the blue shift, the force of hydrogen bonding was highly different from that of [TMG][BF4]. Moreover, due to the presence of NH2 functional groups on proline, it also facilitates the reaction with phenol. As seen from the infrared spectrum, the ν-NH2 deformation vibration of pure [TMG][Pro] was re-evaluated at wavenumber 1615 cm-1. When the complex was formed, the wavenumber shifted to 1603 cm-1, compared with the wavenumbers of [TMG][Pro] and complex (Supporting Info, Figure S16). As for [TMG][Ac], the analysis revealed that the ν-OH stretching vibration of pure phenol was re-evaluated at wavenumber 3336 cm-1. When the complex was formed, the wavenumber shifted to 3326 cm-1. Hydrogen bonds were formed between [TMG][Ac] and phenol (Supporting Info, Figure S17).

Quantum chemical calculations between phenol and TMG-based ILs Quantum chemistry calculation of intermolecular forces can further verify the accuracy of the



experiment.35 At present, many scholars use quantum chemistry as a theoretical calculation to provide basis for the accuracy of experimental data.36 In this article, the reaction mechanism between phenol and three TMG-based ILs was proven by FT-IR and UV-vis experiments. Then quantum chemistry was used to calculate the interaction between three phenol and TMG-based ILs to verify the accuracy of the experiment. The definition of quantum chemistry calculation is to use the basic principles of quantum chemistry and calculate the properties of molecules by means of computer programs.37 The core of the computational theory is the Schrodinger equation.38 The calculation methods mainly include semi-empirical calculations, ab initio calculations, and density functional theory methods.39 The calculation accuracy and computational cost of different methods are quite different. Density functional theory is a quantum mechanics method for studying the electronic structure of multi-electron systems. The main goal is to use electron density instead of wave function as the basic amount of research. Density functional theory has been widely used in solid physics calculations since 1970.40 In most cases, the density functional theory using local density approximation gives satisfactory results compared with other methods for solving quantum mechanical problems. In this paper, density functional theory is used to optimize the IL structure and the influence of anion and cation structure on the interaction between anions and cations is discussed.41 The specific density functional method is selected by the selected exchange and associated potentials to consider factors such as calculation accuracy and computational efficiency.42 Becker’s three-parameter hybrid exchange functional with Lee-Yang-Parr correlation functional (B3LYP)


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and the polarizable continuum model (PCM) were adopted in this study, and Select 6-31G++** base group for optimization.43 The specific optimization steps are as follows: Different atomic and molecular properties can be reflected on the basis of the electrostatic potential, including intermolecular interaction energy, sites of action, electro negativities (chemical potentials).44 By studying the intermolecular force through electrostatic potential, you can turn the otherwise complex number into a visually observable figure. Electrostatic potential map showed that electrostatic potential was different at different points. The negative electrostatic potential areas were susceptible to electrophilic reagent attacks, and electrostatic potential for positive region was susceptible to nucleophilic attack.45

Figure 10. The optimized structures of the electrostatic potential mapped on to the 0.0004 density subsurface of ILs and phenol. The optimized structures of the electrostatic potential are shown in Figure 10. The red and blue colors indicated the negative and positive regions, respectively. The darker the color, the greater the absolute value of the electrostatic potential. Figure 10 showed strong negative potential of functional groups of hydroxyl of the phenol, which was vulnerable to attack



groups with positive potential. In addition, the force was directly related to the electrostatic potential. Through the comparison of the electrostatic potential map of three kinds of ILs, the intermolecular interaction force between [TMG][BF4] and phenol was concluded to be the strongest, followed by [TMG][Ac], and the weakest was [TMG][Pro]. For all the optimization objects, the harmonic vibration frequency and total energy were calculated, and the total energy through the zero point correct 46. The overall research object is the interaction energy, which is calculated as follows:

∆EeV = 27.21 × [#$% &' − )#$ &' − #% &'*]℃℃ The EAB is the total energy; EA and EB are the energy under the pure composition object. The greater the absolute value of the △E, the more stable the structure is, and the specific analysis is as follows: Figure 11 showed that the intermolecular interaction energy of the complexes was between 0.8 and 7.7 eV (1 eV = 96.5 kJ/mol), indicating that these ILs formed strong coordination bonds with phenol. The theoretical predictions provided good insight into the interaction between ILs and phenol. Compared with the other two ILs, the electron energy between phenol and [TMG][BF4] was the largest; thus, the theory proved that the interaction between the three ILs was consistent with the experimental results.


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800 Interaction energy and Electron energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

0

[TMG][Ac]+phenol [TMG][BF4]+phenol [TMG][Pro]+phenol

Interaction energy/kJ/mol 666 749 83

Electron energy/ev 69 77 8

Figure 11. The comparisons of intermolecular interaction energy and electron energy between TMG-based ILs and phenols, units: kJ/mol. (For comparison, the electron volts was to expand 10 times.) Reuse of TMG-based ILs In this experiment, the [TMG][BF4] with the best extraction rate was used as a regeneration example. According to the designed experiment procedure, first, back extraction rate was analyzed by using GC and the results were shown in Table 2 under the condition of stirring for 30 min at 30 °C with extraction ratio of 1.1 g/4 mL. After three cycles, the phenol in the ILs was almost extracted with diethyl ether. Therefore, diethyl ether achieved good back-extraction effect on the complex of phenol and [TMG][BF4]. Table 2. Phenol content in ILs after three cycles

Phenol content mg/L

Phenol content in

Once

Twice

three times

the extraction phase

extraction

extraction

extraction

972

426

431

112

The original [TMG][BF4] and those after 5 cycles were characterized by using 1H NMR. Figure 12 shows the 1H NMR spectra of [TMG][BF4] before and after use. As shown in Figure 12, the 1H



NMR spectra of [TMG][BF4] before and after use did not change, which indicated that [TMG][BF4] extracted phenol from the model oils and was totally regenerated by anti-extraction with diethyl ether. Afterwards, the phenol extraction rate of [TMG][BF4] after recycling several times was analyzed by using GC. The analysis results are recorded in Figure 13.

[TMG][BF4] Water Reused

Water Original

8

7

6

5

4

3

2

δ (ppm)

Figure 12. 1H NMR spectra of [TMG][BF4] before and after use.

100

The phenol extraction rate (%)

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80

60

40

20

0 1

2

3

4

5

Recycled times

Figure 13 The extraction efficiency of recyclable [TMG][BF4]


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The experimental results showed that [TMG][BF4] can be recovered by using diethyl ether back-extraction. Simultaneously, the recyclable [TMG][BF4] can be used to extract phenol, and the extraction rate can reach the same effect as the original. CONCLUSIONS TMG-based ILs developed by neutralizing TMG with L-proline, acetate acid, and tetrafluoroboric acid were employed to extract phenolic compounds from model oils and coal tar oil. The basic properties of ILs were investigated by inserting probe molecules in ILs combined with 1H NMR and quantum chemistry calculation. Among them, [TMG][BF4] showed the best extraction efficiency of 98.2%, at a extraction temperature of 30 ℃, contact time of 35 min, and extraction ratio of 1.1 g/4 mL. Moreover, the mechanism of coordination and hydrogen bonding of phenol with ILs were analyzed through FT-IR and UV-vis. Furthermore, quantum chemistry was applied to calculate the interaction forces and electron energies of the three ILs, and the calculation results were in a good agreement with the experimental results. As for the regeneration of ILs, phenol in ILs could be recovered via adding diethyl ether. Thus, the designed TMG-based ILs can be effective in separating phenolic compounds from coal tar and as an alternative extractant in future. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org. This file contains the spectrum of 1H NMR, IR, UV-vis, and electrostatic potential. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENTS



We thank the support provided by the National Key R&D Program of China (No. 2017YFB0602804), the National Natural Science Foundation of China (No. 21406123), the Key Scientific and Technological Project of Shanxi Province (No. MH2014-10) and the National Key Technology Support Program of China (No. 2014BAC10B01), Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education-Open Fund (2016KY11-072). REFERENCES Schobert, H. H.; Song, C. Chemicals and materials from coal in the 21st century. Fuel. 2002, 81, DOI 10.1016/S0016-2361(00)00203-9. 2. Liu, F.-J.; Wei, X.-Y.; Fan, M.; Zong, Z.-M. Separation and structural characterization of the value-added chemicals from mild degradation of lignites: A review. Appl. Energ. 2016, 170, DOI 10.1016/j.apenergy.2016.02.131. 3. Sun, M.; Chen, J.; Dai, X.-m.; Zhao, X.-l.; Liu, K.; Ma, X.-x. Controlled separation of low temperature coal tar based on solvent extraction–column chromatography. Fuel. Process. Technol. 2015, 136, DOI 10.1016/j.fuproc.2014.09.005. 4. Sharma, A.; Joshi, N.; Kumar, R. A.; Agrawal, P. K.; Prasad, S. High performance liquid chromatographic analysis of phenolic compounds and their antioxidant properties from different cultivars of Cyamopsis tetragonaloba (L.) Taub. Microchem. J. 2017, 133, DOI 10.1016/j.microc.2017.04.020. 5. Gallart℃Ayala, H.; Núñez, O.; Moyano, E.; Galceran, M. T. Field℃amplified sample injection℃micellar electrokinetic capillary chromatography for the analysis of bisphenol A, bisphenol F, and their diglycidyl ethers and derivatives in canned soft drinks. Electrophoresis. 2010, 31(9), DOI 10.1002/elps.200900606. 6. Gai, H.; Guo, K.; Xiao, M.; Zhang, N.; Li, Z.; Lv, Z.; Song, H. Ordered mesoporous carbons as highly efficient absorbent for coal gasification wastewater – A real case study based on the Inner Mongolia Autonomous coal gasification wastewater. Chem. Eng. J. 2018, 341, DOI 10.1016/j.cej.2018.02.005. 7. Niu, M.; Sun, X.; Gao, R.; Li, D.; Cui, W.; Li, W. Effect of Dephenolization on Low-Temperature Coal Tar Hydrogenation To Produce Fuel Oil. Energy. Fuel. 2016, 30(12), DOI 10.1021/acs.energyfuels.6b01985. 8. Shadabi, S.; Ghiasvand, A. R.; Hashemi, P. Selective separation of essential phenolic compounds from olive oil mill wastewater using a bulk liquid membrane. Chem. Pap. 2013, 67(7), DOI 10.2478/s11696-013-0373-1. 9. Bilen, K.; Ozyurt, O.; Bakırcı, K.; Karslı, S.; Erdogan, S.; Yılmaz, M.; Comaklı, O. Energy production, consumption, and environmental pollution for sustainable development: A case study in Turkey. Renew. Sust. Energ. Rev. 2008, 12(6), DOI 10.1016/j.rser.2007.07.009. 10. Meng, H.; Ge, C.-T.; Ren, N.-N.; Ma, W.-Y.; Lu, Y.-Z.; Li, C.-X. Complex Extraction of

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A brief (~ 20 word) synopsis： The designed TMG-based ILs has realized effective in separating phenolic compounds from coal tar via combination of experimental with theoretical study.


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Designing Ionic Liquids with Dual Lewis Basic Sites to Efficiently

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