Highly Efficient Separation of Phenolic Compounds from Oil Mixtures

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Highly Efficient Separation of Phenolic Compounds from Oil Mixtures by Imidazolium-Based Dicationic Ionic Liquids via Forming Deep Eutectic Solvents Youan Ji, Yucui Hou, Shuhang Ren, Congfei Yao, and Weize Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01793 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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Highly Efficient Separation of Phenolic Compounds from Oil Mixtures by Imidazolium-Based Dicationic Ionic Liquids via Forming Deep Eutectic Solvents Youan Jia, Yucui Houb, Shuhang Rena, Confei Yaoa, Weize Wua,* a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology,

Beijing 100029, China b

Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

ABSTRACT: The separation of phenolic compounds from oil mixtures is an important step for further refining or applications of the oil mixtures. In this work, three imidazolium-based dicationic ionic liquids (DILs, 1,2-bis[N-(N’-methylimidazolium)]ethane dibromide, DIL1; 1,3-bis[N-(N’-methylimidazolium)]propane

dibromide,

DIL2;

and

1,4-bis[N-(N’-

methylimidazolium)]butane dibromide, DIL3) were synthesized and used to separate phenolic compounds from oil mixtures. The effects of time, temperature, DIL:phenol mole ratio, and initial phenol concentration on separation performance were investigated in detail. It was found that the removal efficiency of phenol followed the order of DIL1 < DIL2 < DIL3. DIL3 showed the maximum phenol removal efficiency of 96.6 % and the minimum ultimate phenol concentration of 3.9 g/dm3. Initial phenol concentration had little influence on the ultimate phenol concentration. It was less than 0.4 in DIL:phenol mole ratio needed to obtain the highest phenol removal efficiency, and the separation process could be completed within 5 min. These DILs could be reused without decrease in removal efficiency of phenol, and the properties of DILs did not change after 4 cycles. Also, DIL3 was demonstrated to separate phenolic compounds from real coal tar oil with the removal efficiency of 93.1 %. Importantly, these DILs showed much smaller solubility in oil mixtures than mono-cationic ILs. In addition, these DILs showed higher thermal stability than mono-cationic ILs, which provides a broader range of operation temperature than ever. *

Corresponding author; Email: [email protected]; Tel./Fax: +86 10 64427603. 1

ACS Paragon Plus Environment

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1. INTRODUCTION Phenolic compounds are a kind of important chemicals in the chemical industry, and broadly used in engineering, agriculture, medicine, and so on.1-3 Phenolic compounds can be derived from coal pyrolysis oil, coal liquefaction oil, and biomass pyrolysis oil as reported in the literatures.1,3,4 Actually, the existence of phenolic compounds in the oil mixtures makes the further processing of oil mixtures more difficult. Also, it is a waste of valuable materials (phenolic compounds) by hydrogenation of the oil mixtures to yield fuels.5-7 Therefore, for further refining or applications, the separation of phenolic compounds from oil mixtures shows an increasing significance. The presently used method for the separation of phenolic compounds in industry is chemical extraction using NaOH aqueous solution and then acidification of the extract by mineral acids.8-10 The disadvantages of this process not only consume large amounts of both strong alkalis and acids but also produce excessive amounts of waste water containing phenols. Therefore, it is expected to separate phenols from oil mixtures using a non-aqueous method with high extraction efficiency. In recent years, many solvents, including deep eutectic solvents (DESs) and ionic liquids (ILs), have been proposed to solve the above problems. DES is composed of hydrogen bond acceptor (HBA, such as quaternary ammonium salts, QASs) and hydrogen bond donor (HBD, such as carboxylic acid),11 and DES has a melting point much lower than either of the individual components.11-13 In recent years, advances have been made in many directions for applications of DESs.11,14 DESs have the advantages of easily synthesis, wide liquid temperature range and chemical stability towards water, and hence the extraction of phenolic compounds from oil mixtures using DES is continuously developed. In 2012, our group15-17 reported that phenolic compounds could be separated from oil mixtures by QASs via forming DESs. Besides, Jiao et al. proposed several extractants, 2


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including imidazole and its analogous compounds,18 and amide compounds,19 to separate phenolic compounds via forming DESs. Similarly, Zhang et al.20 used several choline derivative-based deep eutectic solvents to separate phenolic compounds from model oil with separation efficiency up to 94.7 %. Gu et al.21 proposed that choline chloride based DESs could serve as novel extraction media for phenolic compounds from model oil. Also, Yao et al.22 used two quaternary ammonium-based zwitterions, betaine and L-carnitine, to separate phenolic compounds via forming DESs with high removal efficiencies more than 90 %. In addition, there are some other reports focused on the phenolic compounds separation via forming DESs.23-25 ILs have the properties of structural feasibility, excellent solvent power, and low vapor pressure, and show much potential for the separation of phenolic compounds. Hou et al.26,27 found that imidazolium-based ILs, especially 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), had excellent extraction efficiencies (up to 99.9 %). Meng et al.28 compared twelve extractants in extraction performance, and found that monoethanolammonium formate is the most promising extractant. Zhuang et al.29 studied the extraction of phenolic compounds by using four imidazolium-based ILs, and they also studied the selectivity of o-cresol and m-cresol. However, these extractants mentioned above are small in structure, and they are much likely to dissolve in oil mixtures. Gao et al.5 revealed that some extractants are partially oil-soluble (such as imidazole and its analogous compounds, and amide compounds), and Meng et al.28 found that some extractants could be dissolved in oil mixtures with concentrations up to 1000 ppm (mono-ethanol amine). The high concentration of extractants in oil mixtures may contaminate oil mixture, waste extractants, and complicate the process. Another disadvantage of these extractants is the low thermal stability under high temperatures. Therefore, it is expected to found the extractants that do not dissolve in 3


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oil mixtures and are of high thermal stability. To solve these two problems, Xiong et al.30 synthetized poly ILs to adsorb phenol from oil mixtures, and the results revealed that the poly ILs have good thermal stability. However, the poly ILs are difficult to synthesize. Also, the poly ILs have very poor mass transfer because of the high degree of polymerization. Recently, more and more attention has been paid to dicationic ionic liquids (DILs) because of their high thermal stability.31-33 In addition, much work has been done in the application of imidazolium-based DILs.34,35 DILs are a kind of novel ionic liquids that contain one dication and two single anions, and they can also be considered as dipolymer analogues, which may reduce the concentration of extractants in oil mixture. Considering that [Bmim]Cl has a high extraction efficiency towards phenolic compounds, we expect that DILs that have the similar structure to [Bmim]Cl can also separate phenolic compounds from oil mixtures. In this way, the separation of phenolic compounds using imidazolium-based DILs can allow us a more broad range of operation temperature. Also, the concentration of extractants in oil mixtures may be reduced to a large extent because of the large structure of DILs. In this article, three symmetric imidazolium-based DILs (including 1,2-bis[N-(N’methylimidazolium)]ethane dibromide, DIL1; 1,3-bis[N-(N’-methylimidazolium)]propane dibromide, DIL2; and 1,4-bis[N-(N’-methylimidazolium)]butane dibromide, DIL3) were synthesized. The structures of these DILs are shown in Scheme 1. Usually, lower melting points are preferred for ILs, but that does not exclude applications of high melting ILs.32 These studied DILs are solid at room temperature. Most interestingly, they can act as HBA to form DESs with phenol (HBD), and thus they are proposed to separate phenolic compounds from oil mixtures via forming DESs. The effect factors of extraction process, such as extraction time, extraction temperature, initial phenolic compound concentration, 4


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and types of phenolic compound, were studied. Of the DILs, DIL3 showed the highest separation efficiency up to 96.6 % and the minimum ultimate phenol concentration of 3.9 g/dm3. We studied the extraction mechanism by FT-IR, and measured their melting points and decomposition temperatures. These DILs showed much smaller solubility in oil mixtures and were more stable than mono-cationic ILs. N

N

2 N

N

2Br

DIL1

N

N

3 N

N

2Br

DIL2

N

N

4 N

N

2Br

DIL3

Scheme 1. The structures of DILs used in this experiment.

2. EXPERIMENTAL 2.1. Chemical materials The chemical materials used in this experiment include methylimidazole, 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, diethyl ether, phenol, toluene, n-hexane, o-cresol, m-cresol, [Bmim]Br, and acetonitrile. All the chemicals were of analytical reagent grade and used as received without further purification. The details of these chemical materials are indicated in Table 1. 2.2. Preparation of model oils Due to the complexity of real oil mixture,36,37 we selected some representative compounds (phenol, o-cresol, and m-cresol) as phenolic compounds, and toluene or n-hexane as oil. In this experiment, phenolic compounds + toluene mixture and phenol + n-hexane mixture were used as model oils. The preparation of phenol + toluene mixture is shown to indicate the preparation of model oils. Toluene was continuously added to a 100 cm3 volumetric flask, which contained 10.0 g phenol. Then, the volumetric flask was shaken until all phenol was dissolved in toluene. In this way, phenol + toluene mixture with an initial phenol concentration of 100 g/dm3 was prepared. Similar procedure was adopted 5


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to the preparation of the other model oils. 2.3. Preparation of imidazolium-based DILs These imidazolium-based DILs were synthesized based on the previous reports.34,38 1,2-Dibromoethane,

1,3-dibromopropane,

or

1,4-dibromobutane

(0.10

mol)

and

methylimidazole (0.24 mol, a slight overdose of methylimidazole was to ensure that all dibromoalkane could react with methylimidazole) were mixed in a 100 cm3 round bottom flask. Then 50 cm3 acetonitrile, which was considered as solvent, was added to the flask. The system was placed in an 80 oC oil bath, and it was equipped with a reflux, and magnetically stirred for 6 h. Then, enough diethyl ether was used to remove the unreacted reactants and solvent. The obtained solids, which were called imidazolium-based DILs, were dried in a vacuum at 80 oC until that the weight of the solids was constant. Due to the high hydrophilic property, these DILs were stored in a desiccator to avoid water absorption. 2.4. Extraction process and analyzing methods The whole extraction process was conducted in a draught cupboard. We added V0 cm3 oil mixture with an initial phenol concentration of C0 g/dm3 to a graduated test tube. An amount of DIL was then added to the oil mixture. The oil mixture system was placed in a constant temperature water bath that was equipped with a magnetic stirring system (524 G, Shanghai Meiyingpu Instrument and Meter Manufacturing Co., Ltd., Shanghai, China), magnetically stirred for some time, and settled down for several minutes. Then, two layers clearly appeared, where the upper phase was phenol-removed oil, and the lower phase was called the DES phase. For the oil mixtures of phenolic compound and toluene, a sample of the upper phase was taken to analyze the phenol concentration by gas chromatography (GC, Shimadzu, GC-2014, Japan). The conditions are listed as follows: column, RTX-5 capillary column (30 m × 0.25 mm × 0.25 µm); detector, flame ionization detector (FID); solvent, 6


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dichloromethane; internal standard, 2-nitrotoluene; injection volume of sample, 0.6 µL. The temperature program of the GC was started at 80 oC, settled for 1 min, and then increased at a rate of 40 oC min−1 until the temperature reached 220 oC and settled for 3 min. The total analysis time was 7.50 min. The temperature of the injection port was 250 oC, and the temperature of FID was 280 oC. In the following part of the work, except special illustration, oil mixtures refer to mixtures formed by phenolic compounds and toluene. For the oil mixture of phenol and n-hexane, a known amount sample of the upper phase was taken to determine the phenol concentration by a UV-vis spectrophotometer (TU-1901, Beijing Purkinje General Equipment Co., Ltd., China), and the wavelength of 270.5 nm was selected for the determination of phenol concentration in oil mixtures. Both the concentrations of phenolic compound after extraction were recorded as Cphe. The volume of the upper phase and the lower phase were recorded as Vu and Vl, respectively. The following equation (1) shows the calculation process of removal efficiency. RE = (1 – Cphe·Vu / C0·V0) ·100%

(1)

where RE is the removal efficiency of phenolic compounds, %; Cphe refers to the concentration of phenolic compounds in phenol-removed phase after separation, g/dm3; C0 refers to the initial concentration of phenolic compounds in the initial oil mixture, g/dm3; V0 is the volume of the initial oil mixture, dm3; Vu is the volume of the phenol-removed phase after separation, dm3. After forming DES, the phenol-removed oil phase was removed, and phenol and DIL are expected to be separated by adding anti-solvent. In this work, diethyl ether was chosen as anti-solvent, and 150 cm3 diethyl ether (a small amount of diethyl ether cannot totally regenerate DIL) was added to the lower DES phase to regenerate DIL and recover phenolic compounds. The diethyl ether and DES system was magnetically stirred for 30 min with a rotating speed of 800 rpm. After setting down for 20 min, there was a solid DIL appearing 7


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clearly at the bottom of the beaker. The solid DIL was collected and dried in a vacuum oven at 80 oC for about 24 h after filtering. The filtrate containing phenolic compounds was collected in another 200 cm3 beaker, and treated with a distillation process, through which phenolic compounds and recycled diethyl ether were obtained. 2.5. Separation of phenolic compounds from real oil In this work, a known amount (246.90 g) of real coal tar oil (180-230 oC distillate fraction of real coal tar oil, supplied by Huanghua Coal Chemical Industry Co., Ltd., Hebei, China) was added to a 1000 cm3 beaker. In this real coal tar oil, the molar mass of cresol (108 g/mol) was approximately considered as the average molar mass of the phenolic compounds. Then DIL3 with a 0.5 time molar amount of phenolic compounds was added to the beaker. The beaker was placed in a constant temperature water bath that was equipped with a magnetic stirrer, and was magnetically stirred at 25 oC for about 30 min. Then the magnetic stirrer was turned off for settling down a while and two layers, of which the lower phase was called real DES and the upper phase was phenols-removed oil phase, appeared clearly. The real DES phase was separated from the upper phase by a liquid separatory funnel. The phenolic compounds concentration in the upper phase was determined following the method of National Standard of P. R. China (GB/T 24200-2009). 2.6. Characterization methods The synthesized DILs and recovered DILs were confirmed by 1H NMR (Bruker, AVANCE III, Germany, 400MHz, D2O). In order to analyze the mechanism of the separation, FT-IR spectra of phenol, DILs, and DESs were performed on a Fourier transform spectrometer (Nicolet 6700, USA, KBr). A differential scanning calorimeter (Mettler Toledo, Switzerland, DSC1) was used to determine the melting points of DESs and individual chemicals. A thermogravimetric analyzer (Mettler Toledo, Switzerland, TGA/DSC1) was used to 8


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determine the thermal decomposition temperatures, including the start temperatures (Tstart) and the onset temperatures (Tonset). All samples were run in a nitrogen atmosphere at the heating rate of 10 oC/min.

3. RESULTS AND DISCUSSION 3.1. Synthesized DILs and Melting points The yields of DIL1, DIL2, and DIL3 were 84.8 %, 81.3 %, and 82.4 %, respectively. All the DILs synthesized in this experiment are white and solid powder at room temperature.38,39 The 1H NMR spectra of the DILs are shown in Figure 1, which are highly consistent with the results reported in the literature.39 The melting points of individual DILs and DESs formed by DIL and phenol are listed in Table 2. The melting points of DILs are 209 oC for DIL1, 167 oC for DIL2, and 139 oC for DIL3. The melting point of phenol is 41 oC. The melting points of DESs formed by DIL1, DIL2, or DIL3 and phenol are 13 oC, 7 oC, and –14 oC, respectively. The melting points of DESs are far below than those of corresponding individuals, which indicates that the formed solvent in the lower phase was deep eutectic solvent.15 3.2. Effect of time on phenol separation The effect of time on phenol separation has been studied with the initial phenol concentration of 100 g/dm3 at a DIL:phenol mole ratio of 0.5 at 25 oC, and the results are shown in Figure 2. The concentration of phenol decreases sharply with the increase of separation time, and then, keeps almost identical with the continuous increase of time from 5 min to 30 min. The whole separation process could be completed within 5 min, which indicates that there is a rapid mass transfer for phenol. The separation time was set as 30 min in the following experiments to ensure that the extraction could reach equilibrium. 3.3. Effect of temperature on phenol separation Figure 3 shows phenol concentration and phenol removal efficiency as a function of 9


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temperature with the initial phenol concentration of 100 g/dm3 at a DIL:phenol mole ratio of 0.5. For all these DILs, phenol concentration in oil mixture slightly increases with increasing temperature, and phenol removal efficiency decreases slightly with increasing temperature. As indicated in section 3.9, there is a hydrogen bond between DIL and phenol. This hydrogen bond is much stronger than the molecule interaction between oil and phenol, thus leading to the separation of phenol. Generally, an increase in temperature can increase the mutual solubility,40 which is unfavorable for separation. Therefore, room temperature is the best choice for the phenol separation, and there is no need to cool or heat the oil mixtures. In the following experiments, separation temperature was set to 25 oC. 3.4. Effect of DIL:phenol mole ratio on phenol separation At first, a small amount of DILs could completely dissolve in oil mixture, and two phases did not appear. With the addition of more DILs, oil mixture became turbid, and after a few minutes of setting down, two phases was gradually formed. The lower phase was called DES phase. The minimum DILs:phenol mole ratios (Rmin) for the formation of two phases at the initial phenol concentration of 100 g/dm3 were 0.009 for DIL1, 0.012 for DIL2, and 0.016 for DIL3. Different amounts of DILs can result to different removal efficiencies of phenol, which was indicated in the previous reports.16,17 The effect of DIL:phenol mole ratio on phenol concentration and phenol removal efficiency with the initial phenol concentration of 100 g/dm3 is shown in Figure 4. Phenol concentration in oil phase after separation decreases sharply with the increase of DIL:phenol mole ratio from Rmin to 0.3, and keeps almost identical at DIL:phenol mole ratios from 0.3 to 0.6. The ultimate phenol concentrations were found to be 13.5 g/dm3 for DIL1, 6.0 g/dm3 for DIL2, and 3.9 g/dm3 for DIL3. In addition, phenol removal efficiency increases sharply with the increase of DIL:phenol mole ratio from Rmin to 0.3. With the continual addition of DILs, solid DIL was observed at the 10


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bottom of the graduated test tube, and phenol removal efficiency keeps almost identical at DIL:phenol mole ratios from 0.3 to 0.6. The maximum phenol removal efficiencies are found to be 88.1 % for DIL1, 94.7 % for DIL2, and 96.6 % for DIL3. Among these three DILs, they exhibit the order of DIL1 > DIL2 > DIL3 in ultimate phenol concentration, and the order of DIL1 < DIL2 < DIL3 in phenol removal efficiency. 3.5. Effect of initial phenol concentration on phenol separation Due to the difference in phenol concentrations of oil mixtures, it is necessary to study phenol separation from oil mixtures with different initial phenol concentrations. Figure 5 shows the effect of initial phenol concentration, 50 g/dm3, 100 g/dm3, or 200 g/dm3, on phenol removal using DIL3. As shown in Figure 5, phenol concentration decreases sharply with the increasing phenol:DIL mole ratio from Rmin to 0.3, and tends to be stable with the further increase of phenol:DIL mole ratio from 0.3 to 0.6. It is found that the ultimate phenol concentrations are identical (about 4.0 g/dm3) despite of the difference in initial concentrations. The minimum DIL:phenol mole ratio needed to reach the lowest phenol concentration is 0.3. 3.6. Effect of types of phenolic compound on separation Real oil mixtures contain many kinds of phenolic compounds.1 Typical phenolic compounds are phenol and cresol. DIL3 was chosen as extractant to separate phenolic compounds, and the effect of types of phenolic compound, including phenol (50 g/dm3), o-cresol (50 g/dm3), and m-cresol (50 g/dm3), on extraction using DIL3 is shown in Figure 6. The concentrations of phenol, o-cresol, and m-cresol decreases with the increase of phenols:DIL mole ratio. The minimum concentrations for phenol, o-cresol, and m-cresol are 2.1 g/dm3, 4.0 g/dm3, and 2.9 g/dm3, respectively. 3.7. Separation of phenol from n-hexane In oil mixtures in industry, both aromatic hydrocarbons and aliphatic hydrocarbons 11


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exist. The separation of phenol from aliphatic hydrocarbons is very necessary to study. In this section, n-hexane was chosen as the model oil, and DIL3 was chosen as the extractant. Phenol removal efficiency as a function of DIL3:phenol mole ratio with the initial phenol concentration of 10 g/dm3 using DIL3 is shown in Figure 7. As shown in Figure 7, the removal efficiency of phenol decreases with the increase of DIL3:phenol mole ratio. Although the initial phenol concentration is as low as 10 g/dm3, the maximum phenol removal efficiency can reach 98.7% at DIL3:phenol mole ratio of 0.5, and the minimum phenol concentration is only 0.13 g/dm3, which is much more lower than that in the separation of phenol from toluene. Because of the non-polarity of n-hexane, phenol removal from n-hexane was more easily than that from toluene. 3.8. Reuse of DILs These DILs show high removal efficiency of phenolic compounds according to the results mentioned above. Considering the rational use of resources and environmental protection, these DILs must be anti-extracted from DES for further use. In this experiment, diethyl ether was found to be an effective solvent for the recovery of DILs. Diethyl ether has a strong interaction with phenolic compound resulting in high solubility of phenolic compound in diethyl ether, and DIL can be easily regenerated from DESs. With low boiling point, diethyl ether is easily re-obtained by simple distillation from the mixture of phenolic compounds + diethyl ether. Then, phenolic compound products are obtained, and the re-obtained diethyl ether can be used for the next cycle. In addition, the studied DILs do not dissolve in diethyl ether.32,39 Therefore, no DIL is wasted and no cross contamination of product is produced. All the DILs were anti-extracted by diethyl ether and were used to the next cycle for four times at the same conditions. The removal efficiency of phenol in each cycle at a DIL:phenol mole ratio of 0.5 with an initial phenol concentration of 100 g/dm3 is shown in 12


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Figure 8. The results show that these DILs can be reused without significantly decrease in removal efficiency of phenol and regenerated rate of DIL (regenerated rate of DIL was defined as mass of the regenerated DIL/initial mass of the DIL). We determined the structures of the DILs before and after use by 1H NMR. The 1H NMR spectra of the original DILs and regenerated DILs are shown in Figure 9. It is found that there is no change between the 1H NMR spectra of original DILs and regenerated DILs, indicating that the DILs could be completely recovered and the structures of the DILs were not changed. 3.9. Separation of phenolic compounds from real oil The above results indicate that DIL1, DIL2, and DIL3 can successfully extract phenol from model oils. However, the separation of phenolic compounds from real oil also shows much importance. In this section, DIL3 was used to separate phenolic compounds from coal tar oil. The concentration of phenolic compounds in the original coal tar oil was measured, and was 33.5 % in mass fraction. When DIL3 with a 0.5 time mass amount of phenolic compounds was added in the coal tar oil, two phases were also found clearly in the beaker after stirring and settling down. Analysis result showed that the concentration of phenolic compounds in the extracted coal tar oil was 2.91 % in mass fraction, and separation efficiency of phenolic compounds was 93.1 %. The results demonstrate that DIL3 can separate phenolic compounds from real coal tar oil. 3.10. Separation mechanism analysis Separation mechanism, with which one can design new extractants, is desired to propose for phenolic compound separation. Due to the strong electron-negative of oxygen atom, the hydrogen in –OH group of phenolic compound is of high activity. There is a high chance that ions with saturated electrons can share electron with the hydrogen atom. Each of the studied DILs in this experiment contains two Br– and these Br–, which can share an electron with the hydrogen in –OH group to form hydrogen bond. Hence, it is expected that 13


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these DILs separate phenol via forming hydrogen bond with phenol.41 FT-IR was usually considered to be an effective method to study hydrogen bond information.42 Figure 10 shows the FT-IR spectra of DILs and DESs to analyze the formation of DESs. The stretching vibration of ν-OH of phenol can be observed in 3322 cm–1. Taking DIL1 for example, the absorption peak of ν-OH stretching vibration is shifted from 3322 cm–1 to 3189 cm–1 after phenol forming DES with DIL1, as shown in Figure 10. This blue shift is because that the hydrogen bond in DES weakens the stretching vibration of ν-OH. Similarly, the spectrum of ν-OH stretching vibration also moves to lower wavenumbers of 3179 cm–1 for DES2 and 3158 cm–1 for DES3. In addition, a large chemical shift represents strong hydrogen bond interaction between DIL and phenol, and thus the separation ability follows the order of DIL3 > DIL2 > DIL1, which is consistent with their chemical shifts. As shown in Figure 5, although the initial phenol concentrations were different, the variation tendencies in phenol concentration were similar. At DIL3:phenol mole ratios from 0.3 to 0.6, the concentration of phenol was constant despite the difference in initial phenol concentration. Based on the previous reports16,17 and the above results, we put forward a mechanism for phenol separation as shown in Eq. 2. Phenol (L) + DIL (S)

KC

DES (L)

(2)

where KC, the equilibrium constant of Eq. 2, is equal to CDES/Cphe. CDES is the phenol concentration in DES, and Cphe is the concentration of phenolic compound in oil phase. KC is determined by the nature of this reaction. It is a function of temperature and extractants, not initial concentration of phenol. For different DILs show KC different, Cphe are different, as shown in Figure 4. For different initial phenol concentrations, as shown in Figure 5, at DIL3:phenol mole ratios more than 0.3, there was solid DIL3 observed in the DES phase, which indicated that CDES was constant. Besides, KC is constant at that temperature. Therefore, according to Eq. (2), Cphe keeps constant although the initial phenol 14


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concentrations are different. As shown in Figure 3, Cphe has a slight decrease with temperature increasing, because KC decreases with increasing temperature. For the reused DILs, as shown in Figure 8, they had no difference from the original DILs. Therefore, they have the same separation ability as the original DILs. 3.11. Comparison between DILs and mono-cationic extractants In the previous reports,26,27 one of the most representative bromide based monocationic ILs is [Bmim]Br, which has the similar structure to DIL3 used in this experiment as shown in Scheme 1. The comparison between [Bmim]Br and DIL3 in phenol removal efficiency with the initial phenol concentration of 100 g/dm3 at a temperature of 25 oC in oil mixture of toluene + phenol is shown in Figure 11. As shown in in Figure 11, with the increase of DIL:phenol mole ratio, both [Bmim]Br and DILs show the increasing tendency in removal efficiency of phenol. [Bmim]Br shows the maximum removal efficiency of phenol of 97.4 %, while DIL3 shows an almost same value of 96.6 %. The mole ratio of DIL3 to phenol used to reach the highest phenol removal efficiency was 0.3, which was much smaller than 0.6 for [Bmim]Br. The reason is that there are the two bromides in DIL3, and every DIL can form two hydrogen bonds with two phenolic compounds compared to [Bmim]Br. Therefore, the mole amount of these DILs used to reach a maximum removal rate of phenol was much smaller than that of the mono-cationic ILs. As expected, DILs show lower solubility in oil than mono-cationic ILs. For an oil mixture of 100 g/dm3 of phenol, the results showed that 0.3462 g of [Bmim]Br (1.58×10–3 mole of [Bmim]Br) could completely dissolve in 10 cm3 oil mixture. However, for DIL3, it was only 0.0666 g (1.75×10–4 mole of DIL3). To further compare them, we added separately 0.1 mole of [Bmim]Br and 0.1 mole of DIL3 to 100 dm3 pure toluene. It was found that the concentration of [Bmim]Br in toluene (1.45×10–3 mol/dm3) was 25.3 time more than that of DIL3 (5.71×10–5 mol/dm3). 15


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Tstart is the temperature at which the decomposition starts. Tonset is the intersection of the weight baseline at the beginning and the tangent of the weight vs temperature curve as decomposition occurs. Tstart and Tonset temperatures for thermal decomposition of DILs and mono-cationic ILs are listed in Table 3. It is found that both Tstart and Tonset of DILs are higher than mono-cationic ILs, thus allowing us a broader range of operation temperatures than mono-cationic ILs. As one kind of new materials, at present these DILs cannot be purchased from market directly, and should be prepared by chemical synthesis. Due to that the synthesis process of DILs is longer than those of the previous extractants (such as [N2222]Cl, choline chloride, and [Bmim]Br in references 16, 17, and 26, respectively), DILs show a little higher cost. In addition, the raw materials of DILs are a little more expensive than those of mono-cationic ILs. Therefore, these DILs are a little more expensive than the latter extractants. However, with the merits of DILs to overcome the disadvantages of the previous extractants, DILs show bright application in the separation of phenolic compounds from oil mixtures.

4. CONCLUSIONS Three imidazolium-based DILs were synthesized and used to separate phenolic compounds from oil mixtures. The performances of the DILs on phenol separation were studied. It was found that all the DILs could separate phenolic compounds and their separation ability follows the order of DIL1 < DIL2 < DIL3. DIL3 showed the maximum phenol removal efficiency of 96.6 % and the ultimate phenol concentration of 3.9 g/dm3. Also, these DILs could be reused without decrease in phenol removal efficiency, and the structures of DILs did not change after 4 cycle. The whole process could be completed within 5 min. It was also found that DIL3 showed much smaller solubility in oil mixtures than [Bmim]Br. It was only 0.3 in DIL3:phenol mole ratio needed to obtain the highest phenol removal efficiency, which was also much smaller than 0.6 for [Bmim]Br. In addition, 16


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these DILs show higher thermal stability than [Bmim]Br, which provides a broader range of operation temperatures than mono-cationic ILs. It was demonstrated that DIL3 could separate phenolic compounds from real coal tar oil with the removal efficiency of 93.1 %. In the separation process with DILs, there is no wastewater produced no strong alkali or acid are used, and both extractants and solvent can be recycled.

ACKNOWLEDGMENTS We sincerely thank Professors Zhenyu Liu and Qingya Liu for their help. This work is financially supported by the National Basic Research Program of China (2011CB201303), Specialized

Research

Fund

for

the

Doctoral

Program

of

Higher

Education

(20120010110005) and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT).

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REFERENCES (1) Schobert, H. H.; Song, C. Fuel. 2002, 81, 15-32. (2) Guo, S. C., Coal chemical technology. Second Ed.; Chem. Ind. Press: Beijing, 2006; p 80-135. (3) Amen-Chen, C.; Pakdel, H.; Roy, C. Biomass Bioenerg. 1997, 13, 25-37. (4) Li, J. H.; Wang, C.; Yang, Z. Y. J. Anal. Appl. Pyrol. 2010, 89, 218-224. (5) Gao, J. J.; Dai, Y. F.; Ma, W. Y.; Xu, H. H.; Li, C. X. Chem. Eng. J. 2015, 281, 749-758. (6) Erdmann, K.; Mohan, T.; Verkade, J. G. Energ. Fuel. 1996, 10, 378-385. (7) Li, D.; Li, Z.; Li, W. H.; Liu, Q. C.; Feng, Z. L.; Fan, Z. J. Anal. Appl. Pyrolysis. 2013, 100, 245-252. (8) Matsumura, A.; Sato, S.; Kodera, Y.; Saito, I.; Ukegawa, K. Fuel Process. Technol. 2000, 68, 13-21. (9) Sato, S.; Matsumura, A.; Ikuo, S.; Ukegawa, K. Energ. Fuel. 2002, 16, 1337-1342. (10) Scouten, C. G. Removal of phenols from phenol-containing streams. US4256568, 1986. (11)Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. J. Am. Chem. Soc. 2004, 126, 9142-9147. (12) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Chem. Commun. 2003, 1, 70-71. (13) Smith, E. L.; Abbott, A. P.; Ryder, K. S. Chem. Rev. 2014, 114, 11060-11082. (14) Abbott, A. P.; Cullis, P. M.; Gibson, M. J.; Harris, R. C.; Raven, E. Green Chem. 2007, 9, 868-872. (15) Guo, W. J.; Hou, Y. C.; Ren, S. H.; Tian, S. D.; Wu, W. Z. J. Chem. Eng. Data. 2013, 58, 866-872. 18


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(16) Guo, W. J.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Tian, S. D.; Marsh, K. N. Green Chem. 2012, 15, 226-229. (17) Pang, K.; Hou, Y. C.; Wu, W. Z.; Guo, W. J.; Peng, W.; Marsh, K. N. Green Chem. 2012, 14, 2398-2401. (18) Jiao, T. T.; Li, C. S.; Zhuang, X. L.; Cao, S. S.; Chen, H. N.; Zhang, S. J. Chem. Eng. J. 2015, 266, 148-155. (19) Jiao, T. T.; Zhuang, X. L.; He, H. Y.; Li, C. S.; Chen, H. N.; Zhang, S. J. Ind. Eng. Chem. Res. 2015, 54, 2573-2579. (20) Zhang, Y.; Li, Z. Y.; Wang, H. Y.; Xuan, X. P.; Wang, J. J. Sep. Purif. Technol. 2016, 163, 310-318. (21) Gu, T. N.; Zhang, M. L.; Tan, T.; Chen, J.; Li, Z.; Zhang, Q. H.; Qiu, H. D. Chem. Commun. 2014, 50, 11749-11752. (22) Yao, C. F.; Hou, Y. C.; Ren, S. H.; Wu, W. Z.; Zhang, K.; Ji, Y. A.; Liu, H. Chem. Eng. J. 2017, 620–626. (23) Hou, Y. C.; Kong, J.; Ren, Y. H.; Ren, S. H.; Wu, W. Z. Sep. Purifi. Technol. 2017, 174, 554-560. (24) Ji, Y. A.; Hou, Y. C.; Ren, S. H.; Yao, C. F.; Wu, W. Z. Fluid Phase Equilib. 2016, 429, 14-20. (25) Lin, Z. Q.; Hou, Y. C.; Ren, S. H.; Ji, Y. A.; Yao, C. F.; Niu, M. G.; Wu, W. Z. Fluid Phase Equilib. 2016, 429, 67-75. (26) Hou, Y. C.; Ren, Y. H.; Peng, W.; Ren, S. H.; Wu, W. Z. Ind. Eng. Chem. Res. 2013, 52, 18071-18075. (27) Hou, Y. C.; Peng, W.; Yang, C. M.; Li, S. Y.; Wu, W. Z. J. Chem. Ind. Eng. (China) 2013, 64, 118-123. (28) Meng, H.; Ge, C. T.; Ren, N. N.; Ma, W. Y.; Lu, Y. Z.; Li, C. X. Ind. Eng. Chem. Res. 19


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2014, 53, 355-362. (29) Zhuang, X. L.; Li, Z. Q.; Wang, E. Q. Adv. Mater. Res. 2014, 906, 137-141. (30) Xiong, J. L.; Meng, H.; Lu, Y. Z.; Li, C. X. Chem. J. Chinese U. 2014, 35, 2031-2036. (31) Han, X. X.; Armstrong, D. W. Org. Lett. 2005, 7, 4205-4208. (32) Bhatt, D. R.; Maheria, K. C.; Parikh, J. K. Rsc Adv. 2015, 5, 12139-12143. (33) D'Anna, F.; Gunaratne, H. Q.; Lazzara, G.; Noto, R.; Rizzo, C.; Seddon, K. R. Org. Biomol. Chem. 2013, 11, 5836-5846. (34) Zhang, Z. X.; Zhou, H. Y.; Li, Y.; Tachibana, K.; Kamijima, K.; Jian, X. Electrochim. Acta. 2008, 53, 4833-4838. (35)Payagala, T.; Huang, J.; Breitbach, Z. S.; Sharma, P. S.; Armstrong, D. W. Chem. Mater. 2007, 19, 5848-5850. (36) Wang, P. F.; Jin, L. J.; Liu, J. H.; Zhu, S. W.; Hu, H. Q. Fuel. 2013, 104, 14-21. (37)Jiao, T. T.; Gong, M. M.; Zhuang, X. L.; Li, C. S.; Zhang, S. J. J. Ind. Eng. Chem. 2015, 29, 344-348. (38) Zhao, D.; Liu, M.; Zhang, J.; Li, J.; Ren, P. Chem. Eng. J. 2013, 221, 99-104. (39) Aher, S. B.; Bhagat, P. R. Res. Chem. Intermediat. 2016, 42, 5587-5596. (40) Apelblat, A.; Manzurola, E. J. Chem. Thermodyn. 1987, 19, 317-320. (41) Yang, Q. W.; Xing, H. B.; Su, B. G.; Bao, Z. B.; Wang, J.; Yang, Y. W.; Ren, Q. L. Aiche J. 2013, 59, 1657-1667. (42) Asprion, N.; Hasse, H.; Maurer, G. Fluid Phase Equilib. 2001, 186, 1-25. (43) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. J. Chem. Eng. Data. 2004, 49, 954-964.

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List of table captions Table 1. Specifications of the chemicals used in this experiment Table 2. Melting points of DESs and individual chemicals Table 3. Start and onset temperatures for thermal decomposition of DILs and mono-cationic ILs

List of figure captions Figure 1. 1H NMR spectra of DIL1, DIL2, and DIL3. Figure 2. Phenol concentration as a function of separation time. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; DIL:phenol mole ratio, 0.5; temperature, 25 oC. Figure 3. Effect of temperature on (a) phenol concentration and (b) phenol removal efficiency. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; DIL:phenol mole ratio, 0.5. Figure 4. Effect of DIL:phenol mole ratio on (a) phenol concentration and (b) removal efficiency of phenol. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; temperature, 25 oC. Figure 5. Effect of initial phenol concentration on phenol separation using DIL3. Conditions: oil mixture, toluene + phenol; temperature, 25 oC. Figure 6. Effect of type of phenolic compound on separation with the initial concentrations of 50 g/dm3 for phenol, 50 g/dm3 for o-cresol, and 50 g/dm3 for m-cresol using DIL3. Conditions: oil mixture, toluene + phenol; temperature, 25 oC. Figure 7. Effect of DIL3:phenol mole ratio on phenol separation from n-hexane. Conditions: oil mixture, n-hexane + phenol; initial phenol concentration, 10 g/dm3; temperature, 25 oC. Figure 8. Removal efficiency of phenol (a) and regenerated rate (RR) of DILs (b) as a 21


Energy & Fuels

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function of cycle times. Condition: DIL:phenol mole ratio, 0.5; initial phenol concentration, 100 g/dm3; oil mixture, toluene + phenol; temperature, 25 oC. Figure 9. The 1H NMR spectra of the original DILs and recovered DILs: (a) DIL1, (b) DIL2, (c) DIL3. Figure 10. FT-IR spectra of DILs and DESs formed by DILs and phenol at a phenol:DIL mole ratio of 2.0. Figure 11. Comparison between [Bmim]Br and DIL3 in phenol removal efficiency at the same condition. Conditions: temperature, 25 oC; initial phenol concentration, 100 g/dm3; oil mixture, toluene + phenol.

22


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Energy & Fuels

Table 1. Specifications of the chemicals used in this experiment

Name

CAS number

Purity in mass fraction a

Source

methylimidazole

616-47-7

99 %

Aladdin Chemical Co., Ltd., Shanghai, China

1,2-dibromoethane

106-93-4

99 %


1,3-dibromopropane

109-64-8

99 %


1,4-dibromobutane

110-52-1

99 %


diethyl ether

60-29-7

99.5 %

Beijing Tongguang Fine Chemicals Co., Ltd., Beijing, China

phenol

108-95-2

98 %


toluene

108-88-3

99%

Beijing Tongguang Fine Chemicals Co., Ltd., Beijing, China

[Bmim]Br

85100-77-2

97 %


o-cresol

95-48-7

98 %


m-cresol

108-39-4

98 %


n-hexane

110-54-3

97 %


acetonitrile

75-05-8

99 %


a

The purities of chemicals were stated by the suppliers, and the chemicals were used

without purification.

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Page 24 of 37

Table 2. Melting points of DESs and individual chemicals

a

Individual chemicals

Melting point of DILs/oC

phenol

41

-

DIL1

209

13

DIL2

167

7

DIL3

139

–14

Melting point of DES formed by DIL and phenol/oC a

The mole ratios of phenol to DILs in the DESs are 6.00.

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Energy & Fuels

Table 3. Start and onset temperatures for thermal decomposition of DILs and mono-cationic ILs Compound

Tstart a/oC

Tonset/oC

[Bmim]Cl b

150

264

[Bmim]Br

215

273

DIL1

219

301

DIL2

283

311

DIL3

281

301

a

Tstart was considered to be the temperature at which the weight loss of the sample was 2%.

b

The thermal data of [Bmim]Cl and [Bmim]Br were obtained from the previous report.43

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Page 26 of 37

DIL1

DIL2

DIL3 14

12

10

8 6 δ/ppm

4

2

Figure 1. 1H NMR spectra of DIL1, DIL2, and DIL3.

26


0

Page 27 of 37

100

DIL type: DIL1 DIL2 DIL3

-3

80

Cphe/gdm

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

Energy & Fuels

60 40 20 0

0

5

10

15

Time/min

20

25

30

Figure 2. Phenol concentration as a function of separation time. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; DIL:phenol mole ratio, 0.5; temperature, 25 oC.

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Energy & Fuels

100

100


-3

80 60

80

RE/%

Cphe/gdm

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

(a)

60

(b)

40

40

20

20

0

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0

10

20

30

o

40

0

50

DIL type: DIL1 DIL2 DIL3 0

10

20

30

40

50

o

Temperature/ C

Temperature/ C

Figure 3. Effect of temperature on (a) phenol concentration and (b) phenol removal efficiency. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; DIL:phenol mole ratio, 0.5.

28


Page 29 of 37

100

100


-3

80 60

80

(a)

RE/%

Cphe/gdm

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

Energy & Fuels

40

60

(b) 40

DIL type: 20 0 0.0

DIL1 DIL2 DIL3

20

0.1

0.2

0.3

0.4

0.5

0.6

0.7

DIL:phenol mole ratio

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

DIL:phenol mole ratio

Figure 4. Effect of DIL:phenol mole ratio on (a) phenol concentration and (b) removal efficiency of phenol. Conditions: oil mixture, toluene + phenol; initial phenol concentration, 100 g/dm3; temperature, 25 oC.

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200

Initial phenol concentration: 3 50g/dm 3 100g/dm 3 200g/dm

-3

160

Cphe/gdm

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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120 80 40 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

DIL3:phenol mole ratio

Figure 5. Effect of initial phenol concentration on phenol separation using DIL3. Conditions: oil mixture, toluene + phenol; temperature, 25 oC.

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Phenolic compound type: phenol o-cresol m-cresol

50

-3

40

Cphe/gdm

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

Energy & Fuels

30 20 10 0

0.0

0.1

0.2

0.3

0.4

0.5

0.6


Figure 6. Effect of type of phenolic compound on separation with the initial concentrations of 50 g/dm3 for phenol, 50 g/dm3 for o-cresol, and 50 g/dm3 for m-cresol using DIL3. Conditions: oil mixture, toluene + phenol; temperature, 25 oC.

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Energy & Fuels

100 80

RE/%

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

Page 32 of 37

60 40 20 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7


Figure 7. Effect of DIL3:phenol mole ratio on phenol separation from n-hexane. Conditions: oil mixture, n-hexane + phenol; initial phenol concentration, 10 g/dm3; temperature, 25 oC.

32


Page 33 of 37

DIL1

DIL2

DIL3

100

100

80

98

60

RR/%

RE/%

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

Energy & Fuels

(a)

96

(b) DIL1

40

DIL3

5

20 0

DIL2

First

Second

Third

0

Fourth

First

Cycle

Second

Third

Fourth

Cycle

Figure 8. Removal efficiency of phenol (a) and regenerated rate (RR) of DILs (b) as a function of cycle times. Condition: DIL:phenol mole ratio, 0.5; initial phenol concentration, 100 g/dm3; oil mixture, toluene + phenol; temperature, 25 oC.

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(a)

Page 34 of 37

(b)

Original Original

Reused 14

12

Reused 10

8

6

4

2

0

14

12

10

8

6

4

2

0

δ/ppm

δ/ppm

(c)

Original

Reused 14

12

10

8

6

4

2

0

δ/ppm

Figure 9. The 1H NMR spectra of the original DILs and recovered DILs: (a) DIL1, (b) DIL2, (c) DIL3.

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Energy & Fuels

Phenol DIL1 DIL2 DIL3 DIL1+phenol DIL2+phenol DIL3+phenol 4000

3500 3000 2500 2000 1500 1000

500

-1

Wavenumber/cm

Figure 10. FT-IR spectra of DILs and DESs formed by DILs and phenol at a phenol:DIL mole ratio of 2.0.

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Energy & Fuels

100 80

RE/%

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

Page 36 of 37

60 40 20

[Bmim]Br DIL3

0 0.0

0.2

0.4

0.6

0.8

IL:phenol mole ratio

Figure 11. Comparison between [Bmim]Br and DIL3 in phenol removal efficiency at the same condition. Conditions: temperature, 25 oC; initial phenol concentration, 100 g/dm3; oil mixture, toluene + phenol.

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Highly Efficient Separation of Phenolic Compounds from Oil Mixtures

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