Tetraethylammonium Amino Acid Ionic Liquids and CO2 for

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Tetraethylammonium Amino Acid Ionic Liquids and CO2 for Separation of Phenols from Oil Mixtures Youan Ji, Yucui Hou, Shuhang Ren, Congfei Yao, and Weize Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02166 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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Tetraethylammonium Amino Acid Ionic Liquids and CO2 for Separation of Phenols from Oil Mixtures Youan Jia, Yucui Houb, Shuhang Rena, Congfei 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, Shanxi 030619, China

ABSTRACT: Phenols have wide applications and much commercial value, and they are obtained from oil mixtures by separation. However, the previous separation agents have low separation efficiency or corrosive halide ions or difficult to be regenerated. In this work, we designed several tetraethylammonium amino acid (TAAA) ionic liquids (ILs) without corrosive halide ions, and found that the ILs could separate phenols from oil mixtures with much high extraction efficiency and could be regenerated using CO2. The effects of separation time, initial phenol content, TAAA type, water content in TAAA, and phenols’ type on separation were investigated. It has been found that TAAA can separate phenols with high separation efficiencies and the maximum separation efficiency of phenol can reach up to 99.0% at a TAAA:phenol mole ratio of 0.60. Meanwhile, ultimate phenol contents can reach as low as 1.40 g/dm3. The initial phenol content almost has no influence on the ultimate phenol contents. For real coal tar oil mixture, the separation efficiency of phenols can reach up to 98.6%. The TAAAs can be regenerated and reused without significant decreases in separation efficiency of phenols. The separation mechanism has also been proposed based on chemical reactions. Keywords: Tetraethylammonium amino acid, Phenols separation, Ionic liquids, Chemical reaction, CO2.

*

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

ACS Paragon Plus Environment

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1. INTRODUCTION Phenols are extensively used to produce synthetic fibers, engineering plastics, medicines, and other industrial organic chemicals.1-3 As documented in the previous reports4-8, many kinds of oils (coal pyrolysis oil, coal liquefaction oil, biomass pyrolysis oil, and petroleum) contain a certain amount of phenols. For example, low-temperature coal tar distillates typically have phenols’ contents of 20–30 wt%,1 and bio-oils usually have over 20 wt%.6,9 Therefore, it is important to separate phenols from these oil mixtures before further treatment. In their chemical structures, phenols contain one or more acidic hydroxyl groups.10 Hence, chemical separation method, also called alkali (such as NaOH solution) washing and acid (such as H2SO4 or H2CO3) releasing method,11-13 is regarded as the efficient method to separate phenols. However, this method uses large amounts of both strong alkalis (like NaOH) and acids (like H2SO4), and produces a large amount of phenols-containing wastewater, which excessively costs a lot and seriously pollutes the environment. The improvement of environmental protection requirements no more allows the use of the above chemical separation method. In recent years, ionic liquids (ILs)14-20 and deep eutectic solvents (DESs)21-25 were proposed to separate phenols from oil mixtures. Separations of phenols using these methods are mostly based on the hydrogen bonds formed by separation agents and phenols. For IL method, Hou et al.26,27 synthesized several imidazolium-based ILs and used them to separate phenol from toluene. The separation efficiency could reach 99.9%. Xiong et al. synthesized poly ILs,28 and our group synthesized several dicationic ILs29,30 to separate phenols from oil mixtures. These ILs showed separation efficiencies over 90%. For DES method, quaternary ammonium salts were first proposed to separate phenols via forming DESs with phenols.31,32 Inspired by this, Zhang et al.33 proposed several other choline derivative salts to separate 2


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phenols via forming DESs. The separation efficiencies are mostly over 90%. The IL and DES methods overcome the shortcomings of the chemical separation method. However, some new problems appeared along with the separation process: (1) Most of the separation agents contain halide ions, such as Cl– and Br–, which may cause serious corrosion to the steel devices;34-36 (2) The separation agents that have high phenols’ separation efficiency are very difficult to be regenerated. For example, tetraethylammonium chloride shows phenol separation efficiency over 99%, but unfortunately, it cannot be regenerated. To solve the problems, Yao et al.37 used two quaternary ammonium-based zwitterions to separate phenols. However, the remaining phenols in oil were a little high (about 14 g/dm3). Therefore, it is important to explore separation agents that can be regenerated, and contain no halide ions. The mechanism of the traditional chemical separation method is shown in eq. (1) and eq. (2).

OH + NaOH =

ONa + H2 O

(1)

2

ONa + H2O + CO2 = 2

OH + Na2CO3

(2) Chemical separation method usually has high separation efficiency. For a toluene + phenol oil mixture with a phenol content of 100.0 g/dm, over 99% of phenol can be separated using NaOH. The critical issue of this method is that NaOH cannot be regenerated (the route of Na2CO3 → NaOH cannot be realized), thus increasing the cost and seriously polluting the environment. If the separation agent separates phenols based on chemical reaction, and can be regenerated, the above problems may be solved. Amino acids are environmentally friendly and biodegradable agents and contain no 3


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halide ions. Amino acid-based ionic liquids have been studied for separation of mixtures by many researchers.38-42 The acidity of phenols is stronger than that of neutral and alkaline amino acids. Phenols may have a chemical reaction with amino acid-based ILs because a weak acid can be replaced by a strong acid. Then, we designed several tetraethylammonium amino acid (TAAA) ILs to separate phenols from oil mixtures. The structures of these TAAAs are shown in Scheme 1, and the method to obtain these ILs is shown in Scheme 2. As can be seen, the neutralization of tetraethylammonium hydroxide ([N2222][OH]) with amino acid produces TAAA. All these TAAAs contain no halide ions. If they are feasible to separate phenols, serious corrosion by halide ions can be easily avoided. Also, for IL and DES methods, diethyl ether was used to regenerate the separation agents.31,32 In this work, because of the stronger acidity of CO2 (or carbonate acid) than that of phenols, the regeneration process may be achieved by using CO2, which is much more environmentally friendly than diethyl ether. As reported, the absorbed CO2 in TAAAs can be easily released by heating and vacuum treatment.40 Thus, the regeneration process of TAAA can be reasonably developed. In this work, as shown in eq. (3), we proposed the separation of phenols from oil mixtures using TAAAs based on acid-base chemical reaction. The effects of separation time, separation temperature, initial phenol content, water content in TAAA, and TAAA type on phenol separation were thoroughly investigated. The results indicate that the maximum separation efficiency of phenols can reach up to 99.0%, and the ultimate phenols content can reach as low as 1.40 g/dm3. O N

NH2

OH

O =

+

N

O

HO

O

[N2222][L-Ala]

NH2

+

Phenol

[N2222][Phe]

L-Ala

4


(3)

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2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemical materials used in this experiment included phenol, o-cresol, m-cresol, acetone, [N2222][OH], toluene, L-alanine (L-Ala), L-proline (L-Pro), sarcosine (Sar), glycine (Gly), L-lysine (L-Lys), nitrogen (N2) and carbon dioxide (CO2). Their specifications and suppliers are indicated in Table 1. The water content in original [N2222][OH] was analyzed by a moisture analyzer (ZDY-502, INESA, Shanghai, China), and the water content was 76.06 wt%. The other purities in the table shown in mass fraction were provided by the suppliers. All reagents were of analytical reagent grade and used without further purification. The real coal tar oil was 453−503 K distillate fraction of coal liquefaction, and was supplied by Huanghua Coal Chemical Industry Co., Ltd., Hebei, China. 2.2. Preparation of TAAAs. TAAAs were synthesized following the previous literature.38-42 Amino acid and equal molar [N2222][OH] were added to a beaker with a known amount of water, and the system was magnetically stirred for 30 min to ensure a completed reaction as shown in Scheme 2. After that, the obtained mixture was transferred to a tube. Nitrogen with a 150 cm3/min flow rate was bubbled through the mixture at 393 K for 24 h to remove water, through which we could get the TAAAs. The water content in the TAAAs was analyzed by the moisture analyzer. The water contents in [N2222][L-Ala], [N2222][Gly], [N2222][L-Pro], [N2222][Sar], and [N2222][L-Lys] were 2.11 wt%, 2.43 wt%, 2.49 wt%, 1.97 wt%, and 2.27 wt%, respectively. 2.3. Separation processes and analytical methods. The separation processes were identical for different oil mixtures, and were wholly conducted in a draught cupboard. An oil mixture of 10 cm3 with an initial phenol content of C0 was added to a graduated tube with a magnetic stirrer. A known amount of TAAA was added to the graduated tube. Then the graduated tube was put in a constant temperature water bath of 298.2 K, and magnetically 5


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stirred for some time. After settling down for a while, there is a clear boundary between the upper oil phase and the lower IL phase, and the two phases were observed clearly. The upper oil phase was removed from the lower IL phase, and the volume of the upper oil phase (VL, dm3) was measured. Then, a small amount of the oil sample was taken and analyzed by gas chromatography (GC, Shimadzu, GC-2014). The GC was equipped with an RTX-5 capillary column and a flame ionization detector (FID). The analytical program of GC was referred to our previous reports.29,37 The phenols content detected by GC was recorded as Cphe, g/dm3. The phenols’ separation efficiency was calculated through eq. (4). SE = (0.01·C0 – VL·Cphe) / (0.01·C0)

(4)

where SE refers to the separation efficiency of phenols, %; C0 is the initial content of phenols, g/dm3; Cphe refers to the content of phenols in the oil phase, g/dm3; VL is the volume of the liquid oil phase, dm3. 2.4. Regeneration of the TAAAs. The upper oil phase was removed to regenerate the TAAAs after separation. About 20 cm3 acetone was added to this graduated tube mainly to dissolve the separated phenols. CO2 with a 150 cm3/min flow rate was bubbled through the regeneration system for 30 min, which ensured that as much CO2 as possible could react with the system. Then, the upper acetone phase was removed and treated with a distillation process, through which we could get the phenols product and recycle acetone. N2 with a 150 cm3/min flow rate was bubbled through the TAAAs phase at 373 K for 4 h, through which we could get the regenerated TAAAs. Both the regenerated TAAAs and acetone were used to the next cycle. 2.5. The separation of real coal tar oil. In this subsection, a known amount of real coal tar oil (phenols content, 33.5 wt%) was added to a 250 cm3 beaker. In this real coal tar oil, the molar mass of cresol (108 g/mol) was approximately considered to be the average molar 6


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mass of phenols. Then [N2222][L-Ala] with a 0.6 time molar amount of phenols 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 298.2 K for about 30 min. Then the magnetic stirrer was turned off for settling down a while, and two layers appeared clearly. The phenols content in the oil phase was determined following China national standard method (GB/T 24200-2009, China). 2.6. Characterization. The synthesized TAAAs were characterized by 1H NMR to verify their chemical structures. The regenerated and the original TAAAs were analyzed by 1

H NMR for comparison. In order to explore the mechanism of the separation, the obtained

solid was washed with anhydrous ethanol and analyzed by 1H NMR and

13

C NMR. The

regenerated [N2222][L-Ala] was analyzed by 13C NMR to study the separation mechanism. All the spectra of 1H NMR and

13

C NMR were performed on an NMR spectrometer (Bruker,

AVANCE III, Germany, 400MHz, D2O).

3. RESULTS AND DISCUSSION 3.1 The 1H NMR results of the TAAAs. The 1H NMR spectra of the synthesized TAAAs are shown in Figure 1. The peak positions and other information were listed as follows. [N2222][L-Ala] (400 MHz, D2O): δ = 0.93 ppm (3H, CH3CN), δ = 1.10 ppm (12H, CH3C), δ = 2.80 ppm (1H, CH), δ = 3.15 ppm (8H, CH2N); [N2222][Gly] (400 MHz, D2O): δ = 1.17 ppm (12H; CH3C), δ = 3.07 ppm (2H; CH2CO2), δ = 3.18 ppm (8H; CH2N); [N2222][L-Pro] (400 MHz, D2O): δ = 1.06 ppm (12H; CH3C), δ = 1.55−1.94 ppm (4H; CH2-Pro), δ = 2.57−2.90 ppm (2H; CH2NH), δ = 3.09 ppm (8H; CH2N), δ = 3.27 ppm (H; CH); [N2222][Sar] (400 MHz, D2O): δ = 1.07 ppm (12H; CH3C), δ = 2.14 ppm (3H; CH3N), δ = 2.95 ppm (2H; CH2CO2), δ = 3.07 ppm (8H; CH2N); [N2222][Lys] (400 MHz, D2O): δ = 1.07 ppm (12H; CH3C), δ = 1.05−1.45 ppm (4H; CH2-Lys), δ = 2.41 ppm (2H; CH2NH2), δ = 2.98 7


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ppm (1H; CHNH2), δ = 3.07 ppm (8H; CH2N). These results verify the chemical structures of the ILs as shown in Scheme 1. 3.2. Effect of time on phenol separation. Usually, it is expected that the separation process takes a short time. In this subsection, [N2222][L-Ala] and [N2222][Gly] were chosen as the separation agents to study the effect of time on phenol separation. The result is shown in Figure 2. As shown in Figure 2, for both [N2222][L-Ala] and [N2222][Gly], the separation efficiency of phenol increases sharply as the time increases from 0 to 5 min, and then keeps as a constant value as the time further increases. The separation efficiencies for [N2222][L-Ala] and [N2222][Gly] at an equilibrium state are 98.5% and 98.7%, respectively. When the separation systems reached equilibrium and settled down for 5 min, white solids were observed at the bottom of the both graduated tubes. These white solids were characterized with NMR, and the NMR results are shown in Figure 3. The peaks’ information is listed as follows. In Figure 3(a), 1

H NMR (400 MHz, D2O): δ = 3.10 ppm (2H; CH2-Gly), δ = 1.38 ppm (3H; CH3-Ala), δ =

3.70 ppm (1H; CH-Ala); In Figure 3(b), 13C NMR (400 MHz, D2O): δ = 44.7 ppm (CH2-Gly), δ = 181.5 ppm (COOH-Gly), δ = 15.2 ppm (CH3-Ala), δ = 50.5 ppm (CH-Ala), δ = 175.3 ppm (COOH-Ala). The peak at δ = 4.73 ppm in Figure 3(a) is assigned to the hydrogen peak of H2O. The results indicated that the white solid was L-Ala for [N2222][L-Ala] as a separation agent, and was Gly for [N2222][Gly] as a separation agent according to the NIST database. These white solids result from the chemical reaction between TAAAs and phenol, yielding [N2222][Phe] and amino acid (solid), as shown in eq. (3). The separation time in the following work was set to 20 min to ensure sufficient time for separation. 3.3. Effect of temperature on phenol separation. Separation temperature usually affects the mutual solubility, and it is expected that the temperature has no influence on separation efficiency. Then the separation temperature was varied from 298.2 K to 343.2 K to 8


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investigate the effect of temperature on the separation process. [N2222][L-Ala] and [N2222][Gly] were chosen as the separation agents, and the results are shown in Figure 4. As shown in Figure 4, as the temperature increases from 298.2 K to 343.2 K, the separation efficiency of phenol decreases slightly from 98.5% to 98.0% for [N2222][L-Ala], and from 98.7% to 97.9% for [N2222][Gly]. As expected, separation temperature has almost no influence on SE. The slight decrease of SE maybe result from an exothermic reaction between [N2222][L-Ala] and phenol. Therefore, there is no need to heat or cool the oil mixtures, and the separation can be performed at room temperature. In the following experiment, the separation temperature was set to 298.2 K. 3.4. Effect of TAAA:phenol mole ratio on phenol separation. The amount of TAAA used in the separation process greatly affects phenol separation efficiency. It is usually expected to obtain the highest phenol separation efficiency by using an amount of separation agent as small as possible. Therefore, it is important to find a proper TAAA:phenol mole ratio for phenol separation, and thus the effect of TAAA:phenol mole ratio on phenol separation efficiency was investigated. The results are shown in Figure 5. As shown in Figure 5, both [N2222][L-Ala] and [N2222][Gly] shows excellent separation performance towards phenol. For both [N2222][L-Ala] and [N2222][Gly], as TAAA:phenol mole ratio increases from 0 to about 0.4, the separation efficiency of phenol sharply increases from 0 to a certain value, and the phenol content sharply decreases from 100.0 g/dm3 to a certain value. As TAAA:phenol mole ratio increases further from 0.4 to 0.6, the separation efficiency of phenol and the phenol content both keep almost constant values The highest separation efficiencies for [N2222][L-Ala] and [N2222][Gly] are 99.0% and 98.7%, respectively, and the ultimate phenol contents for [N2222][L-Ala] and [N2222][Gly] are 1.4 g/dm3 and 1.9 g/dm3, respectively. It is noted that the amount of TAAA depends on the mole ratio of TAAA to phenol in the oil, which is greatly different from the traditional separation on the volume ratio 9


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of separation agent to oil. Moreover, by adding TAAA to the oil mixture, more and more white solids could be observed at the bottom of graduated tubes. As expected, the reaction shown in eq. (3) occurred. TAAA can react with phenol to yield amino acid, which does not dissolve in TAAA IL solution. Thus the reaction results in the increase of the number of white solids and the decrease of phenol content in oil mixtures. Due to the dependence of SE on phenol content as shown in eq. (4), the decrease of phenol content leads to increasing SE. When TAAA:phenol mole ratio was more than a certain value (such as 0.5 shown in Figure 5), the addition of more TAAA could not further react with phenol because the reaction shown in eq. (3) reached equilibrium. 3.5. Effect of initial phenol content on phenol separation. The effect of initial phenol content on the separation is important since different kinds of oil mixtures may have different contents of phenol. It is expected that the initial phenol content has no influence on phenols’ separation. Then, the TAAAs can be applied to separate phenols from many kinds of oil mixtures. In this work, initial phenol content was varied (50.0 g/dm3, 100.0 g/dm3, and 200.0 g/dm3) to study the effect of initial phenol content on phenol separation. [N2222][L-Ala] was chosen as a separation agent, and the results are shown in Figure 6. As shown in Figure 6, the phenol content decreases with the increase of [N2222][L-Ala]:phenol mole ratio from 0 to about 0.4, and then keeps almost constant as the [N2222][L-Ala]:phenol mole ratio further increases from about 0.4 to 0.6. As expected, the ultimate phenol contents can reach as low as 1.4 g/dm3, 1.5 g/dm3, and 1.9 g/dm3 for the initial phenol concentrations of 50.0 g/dm3, 100.0 g/dm3, and 200.0 g/dm3, respectively. The results indicate that the initial content of phenol has almost no influence on the ultimate phenols content. Phenol is extracted through the reaction shown in eq. (3). As the reaction reaches 10


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equilibrium,

there

is

a

reaction

equilibrium

constant,

K,

which

is

equal

to

C[N2222][Phe]/(C[N2222][L-Ala]·Cphenol). When TAAA:phenol mole ratio was more than a certain value (such as 0.5 shown in Figure 6), C[N2222][Phe] and C[N2222][L-Ala] almost do not change with adding more TAAA, which results in the almost constant Cphenol. Hence, the ultimate phenols content is not influenced by the initial content of phenol. 3.6. Effect of TAAA type on separation. The above results indicate that both [N2222][L-Ala] and [N2222][Gly] have high separation ability for phenol. How about the ILs using other common amino acids as anions? Then, new TAAAs ([N2222][L-Pro], [N2222][Sar], [N2222][L-Lys]) were synthesized using the same method and were used to separate phenol from oil. Owing to their different structures, they may have different separation efficiencies for phenols. Based on the previous study, TAAA:phenol mole ratios of 0.60–0.61 were selected. The effect of TAAA type on phenol separation is shown in Figure 7. As shown in Figure 7, all the studied TAAAs show excellent performance in separating phenol from oil. The separation efficiencies of phenol are 97.9%, 98.1%, 98.7%, 99.0%, and 98.1% for [N2222][L-Pro], [N2222][Sar], [N2222][Gly], [N2222][L-Ala], and [N2222][L-Lys], respectively, and all the separation efficiencies of phenol not lower than 97.9%. In the separation process, white solids were observed for all these TAAAs, indicating that all the TAAAs separate phenol from oil mixture by chemical reaction with phenol. Phenol is more acidic than amino acid, indicating that amino acid in TAAAs can be replaced by phenol yielding amino acid. 3.7. Effect of phenol type on separation. There are many kinds of phenols in real oil mixtures, and typical phenols are phenol and cresol. Because the separation mechanism of this work is based on chemical reaction, it is expected that the difference in the structure of phenols may have little influence on the separation. Therefore, it is necessary to study the separation of different phenols from oil mixtures. In this section, phenol, o-cresol, and 11


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m-cresol are chosen as phenolic compounds to study the effect of phenol types on separation. The contents of the three phenols are all 50.0 g/dm3, and [N2222][L-Ala] was also chosen as the separation agent. The effect of phenol type on separation is shown in Figure 8. As shown in Figure 8, the contents of phenols decrease sharply with the increase of [N2222][L-Ala]:phenol mole ratio from 0 to about 0.4, and then keeps almost constant as the [N2222][L-Ala]:phenol mole ratio increase from about 0.4 to 0.6. As expected, the ultimate phenols contents can reach as low as 1.2 g/dm3, 1.3 g/dm3, and 1.3 g/dm3 for phenol, o-cresol, and m-cresol, respectively, and they are almost equal. [N2222][L-Ala] shows little selectivity towards phenol, o-cresol, and m-cresol, and the ultimate phenols contents are almost identical due to their similar structures and all following the reaction shown in eq. (3). 3.8. Effect of water content in TAAAs on phenol separation. In this work, there was about 2 wt% water in each kind of studied TAAA, which can reduce its viscosity, and may favor the chemical reaction between TAAA and phenol. However, because of the interaction between water and TAAA, the addition of water reduces the concentration of TAAA, and may have some negative influence on phenol separation. The effect of water content in TAAAs on phenol separation was studied to find a proper amount of water. [N2222][L-Ala] was also chosen as the separation agent, and an [N2222][L-Ala]:phenol mole ratio of 0.60 was used. The results are shown in Figure 9. As shown in Figure 9, as expected, the separation efficiency of phenol decreases from 98.5% to 90.9% as the water content in TAAA increases from 2.1 wt% to 53.0 wt%. The addition of water reduces the viscosity of [N2222][L-Ala] favoring for the mass transfer, but at the same time, the water in [N2222][L-Ala] dilutes [N2222][L-Ala], thus decreasing its separation ability because the extraction equilibrium is related to the concentration of TAAA as discussed in the subsection 3.5. Therefore, less water in TAAA is preferable for relative high separation efficiencies. 12


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3.9. Reuse of the separation agents. The above results show that these TAAAs have high separation ability towards phenol. Considering the rational use of resources and environmental protection, these TAAAs should be regenerated for reuse. Therefore, it is expected that these TAAAs can be regenerated without changes in properties. As mentioned in the Introduction, for most of the separation agents, diethyl ether was considered to be an effective anti-solvent.27,31 Because of the different mechanism between this method with other methods, diethyl ether could not be further used in this study. As shown in eq. (2), the traditional chemical method uses CO2 to regenerate phenols from aqueous C6H5ONa solution. In this work, as shown in eq. (5), CO2 was used to react with [N2222][Phe] and obtain phenol, yielding [N2222][L-Ala-CO2]. Phenol was collected by acetone because acetone does not dissolve in the TAAAs and phenols can dissolve in acetone. Then, [N2222][L-Ala] can be regenerated by simple heating and vacuum treatment of [N2222][L-Ala-CO2], and the regenerated [N2222][L-Ala] can be used to the next cycle. This process was proved to be an effective way to regenerate TAAAs, and both [N2222][L-Ala] and [N2222][Gly] were used to study the effect of cycle time on separation process.

O N

O

NH2 + CO2 =

+ HO

[N2222 ][Phe]

OH O

NHCOOH +

N O

L-Ala

[N2222][L-Ala-CO2 ]

Phenol

(5)

The separation efficiency of phenol as a function of cycle time is shown in Figure 10. As shown in Figure 10, the separation efficiency of phenol almost keeps constant with the reused cycle. For [N2222][L-Ala], the separation efficiency of phenol decreases slightly from 99.0% to 95.8% after 4 cycles. For [N2222][Gly], the separation efficiency of phenol decreases slightly from 98.7% to 96.2% after 4 cycles. This slight decrease of phenol removal efficiency may result from the slight loss of TAAA during regeneration. The 1H NMR spectra of original TAAA and regenerated TAAA were measured for comparison, and indicated in Figure 11. The 13


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peaks at δ = 3.42 ppm and 3.77 ppm in Figure 11(a) are assigned to active hydrogen. The results show that the 1H NMR spectra peaks of the regenerated TAAAs slightly shifted to low field compared to those of the original TAAAs. For example, the peak at δ = 3.27 ppm in the 1

H NMR spectrum of original [N2222][L-Ala] shifted to δ = 3.32 ppm in the 1H NMR spectrum

of regenerated [N2222][L-Ala], indicating that CO2 in [N2222][L-Ala] was not totally released by heating for 4 h. However, as shown in Figure 10, this chemical shift does not influence phenol separation efficiency. 3.10. The separation of phenols from real oil mixture. The above results indicate that all the studied TAAAs, especially [N2222][L-Ala] and [N2222][Gly], can successfully separate phenol from model oil mixtures with high separation efficiencies. However, the separation of phenols from real coal tar oil also shows much importance. In this subsection, [N2222][L-Ala] was used to separate phenols from coal tar oil. According to China national standard method (GB/T 24200-2009, China), we measured the amount of phenols in the original coal tar oil, and that was 33.5 wt%. [N2222][L-Ala] with a 0.6 time mass amount of phenols was added to coal tar oil, and the upper oil phase became much clearer than the original real coal tar oil after stirring and settling down. The results showed that the content of phenols in the phenols-removed real oil was 0.9 wt%, and the separation efficiency of phenols was 98.6%. Hence, [N2222][L-Ala] could also separate phenols from real coal tar oil. 3.11. Mechanism of phenol separation by TAAAs. The separation mechanism using TAAA is important for not only understanding how TAAAs interact with phenol, but also designing new separation agents. For phenols, one or more hydroxyl groups are directly connected to their aromatic rings. The oxygen in phenols (–OH) is a strongly electronegative atom, thus resulting in the weak acid of phenol (Brønsted acid). In this work, it is expected that these TAAAs react with phenols to keep the high separation efficiency like NaOH. Taking phenol and [N2222][L-Ala] for example, phenol may have a chemical reaction 14


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(displacement reaction) with [N2222][L-Ala], yielding L-Ala and [N2222][Phe], because phenol is more acidic than L-Ala. As expected, as shown in Figure 3, for [N2222][L-Ala] as a separation agent, the white solid during the separation process was proved to be L-Ala. As shown in eq. (3), a chemical reaction occurred, producing [N2222][Phe] and amino acid. As we know, CO2 is more acidic than phenols. According to eq. (2), we developed a route to recover phenol by using CO2 (eq. (5)). Jiang et al. used [N2222][β-Ala] to absorb CO2, and found the existence of carbamate carbonyl carbon,38 which indicated the formation of –NHCOOH group. Similarly, in the

13

C NMR spectrum of regenerated [N2222][L-Ala] (shown in Figure 12), we

also found the characteristic peak of carbamate carbonyl carbon (δ = 160 ppm, –NHCOOH; δ = 177 ppm, –COO–), which also indicated the formation of –NHCOOH group during regeneration process. Then, the intermediate [N2222][L-Ala-CO2] was proposed in the regeneration process. As shown in eq. (5), another replacement reaction occurred, producing [N2222][L-Ala-CO2] and phenol. Phenol can be separated by phase separation or dissolving in solvents (like acetone used in this work). As we know, –COOH is an electron withdrawing group, which reduces the density of the electron cloud around the hydrogen nucleus. Therefore, as shown in Figure 11, the 1H NMR spectra peaks of the regenerated TAAAs shifted to low field compared to those of the original TAAAs, which also indicated that a very small amount of CO2 was not released in the regenerated TAAA. According to literature,38,43,44 TAAAs can be used to absorb CO2, and can also be regenerated by heating at low pressure. Inspired by this idea, N2 with a 150 cm3/min flow rate was bubbled through [N2222][L-Ala-CO2] at 393.2 K for 4 h, and as shown in eq. (6), the regenerated [N2222][L-Ala] was obtained.

O

O

NHCOOH

NH2

N

N O [N2222][L-Ala-CO2]

+ CO2 O

[N2222][L-Ala]

(6) 15


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3.12. Comparison between this method with other methods. Compared to the traditional alkali washing method,11,12 this method shows many advantages. Firstly, no strong alkali and acid are used. Secondly, all the TAAAs used in this work are biodegradable,45 and can be regenerated without producing wastewater containing phenol. Compared to the IL method and DES method, TAAAs also show some advantages listed as follows: (1) TAAAs shows the similar separation efficiency to tetraethylammonium chloride, but they can be regenerated and reused; (2) These TAAAs contain no halide ions, such as Cl– and Br–, and they will not cause serious corrosion to the steel devices. This method is highly efficient and environmentally benign.

4. CONCLUSIONS Several TAAAs were designed to separate phenols from oil mixtures based on chemical reaction. The effects of separation time, initial phenol content, TAAA type, water content in TAAA, and phenols’ type on separation were investigated in detail. The results indicate that temperature has almost no influence on separation efficiency and the extraction can be performed at room temperature. All the designed TAAAs can separate phenols with high separation efficiencies, and the maximum separation efficiency of phenol from toluene + phenol mixture is 99.0 %. Initial phenols’ content has almost no influence on the ultimate phenols content, which can reach as low as 1.40 g/dm3. Water in TAAA has a negative effect on phenol separation. For real coal tar oil mixture, the separation efficiency of phenols can reach up to 98.6%. It is also found that the studied TAAAs can be regenerated using CO2 and reused without significant decreasing in separation efficiency of phenols. The separation mechanism has also been proposed.

AUTHOR INFORMATION Corresponding Author 16


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* E-mail: [email protected]. Tel./Fax: +86 10 64427603. ORCID Weize Wu: 0000-0002-0843-3359 Yucui Hou: 0000-0002-9069-440X Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Professors Zhenyu Liu and Qingya Liu for their help. This work is financially supported by the National Key R&D Program of China (2017YFB0602401).

17


Energy & Fuels 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

NOMENCLATURE IL

ionic liquids

DES

deep eutectic solvent

DILs

dicationic ionic liquids

TAAA

tetraethylammonium amino acid

[N2222][OH]

tetraethylammonium hydroxide

L-Ala

L-alanine

L-Pro

L-proline

Sar

sarcosine

Gly

glycine

L-Lys

L-lysine

C0

initial phenol concentration

VL

the volume of the upper oil phase

FID

flame ionization detector

GC

gas chromatography

Cphe

the concentration of phenolic compound detected by GC

SE

the separation efficiency of phenols

[N2222][L-Ala]

tetraethylammonium L-alanine ionic liquid

[N2222][Gly]

tetraethylammonium glycine ionic liquid

[N2222][L-Pro]

tetraethylammonium L-proline ionic liquid

[N2222][Sar]

tetraethylammonium sarcosine ionic liquid

[N2222][L-Lys]

tetraethylammonium L-lysine ionic liquid

[N2222][Phe]

tetraethylammonium phenol ionic liquid

[N2222][β-Ala]

tetraethylammonium β-alanine ionic liquid

18


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prussian

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analogues

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liquid/water/MgCl2 and application in electrochemical reduction of CO2. Green Chem. 2016, 18, 518-520. (18) Ranu, B. C.; Banerjee, S.; Jana, R. Ionic liquid as catalyst and solvent: The remarkable effect of a basic ionic liquid, [Bmim]OH on Michael addition and alkylation of active methylene compounds. Tetrahedron 2007, 63, 776-782. (19) Hapiot, P.; Lagrost, C. Electrochemical reactivity in room-temperature ionic liquids. Chem. Rev. 2008, 39, 2238-2264. 20


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(20) Salargarcía, M. J.; Ortizmartínez, V. M.; Hernándezfernández, F. J.; Ap, D. L. R.; Quesadamedina, J. Ionic liquid technology to recover volatile organic compounds (VOCs). J. Hazard. Mater. 2017, 321, 484-499. (21) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. Deep eutectic solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids. J. Am. Chem. Soc. 2004, 126, 9142-9147. (22) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. 2003, 1, 70-71. (23) Hizaddin, H. F.; Sarwono, M.; Hashim, M. A.; Alnashef, I. M.; Hadj-Kali, M. K. Coupling the capabilities of different complexing agents into deep eutectic solvents to enhance the separation of aromatics from aliphatics. J. Chem. Thermodyn. 2015, 84, 67-75. (24) Salleh, Z.; Wazeer, I.; Mulyono, S.; Blidi, L. E.; Hashim, M. A.; Hadj-Kali, M. K. Efficient removal of benzene from cyclohexane-benzene mixtures using deep eutectic solvents–COSMO-RS screening and experimental validation. J. Chem. Thermodyn. 2016, 104, 33-44. (25) Hou, Y. C.; Yao, C. F.; Wu, W. Z. Deep eutectic solvents: Green solvents for separation applications. Acta Phys. Chim. Sin. 2018, 34, 873-885. (26) Hou, Y. C.; Peng, W.; Yang, C. M.; Li, S. Y.; Wu, W. Z. Extraction of phenolic compounds from simulated oil with imidazolium based ionic liquids. J. Chem. Ind. Eng. (China) 2013, 64, 118-123. (27) Hou, Y. C.; Ren, Y. H.; Peng, W.; Ren, S. H.; Wu, W. Z. Separation of phenols from oil using imidazolium-based ionic liquids. Ind. Eng. Chem. Res. 2013, 52, 18071-18075. (28) Xiong, J. L.; Meng, H.; Lu, Y. Z.; Li, C. X. Synthesis of 1,4-bis(chloromethyl) benzene crosslinked poly(vinyl imidazolium) and its adsorptivity for phenol from coal tar. Chem. J. Chinese U. 2014, 35, 2031-2036. 21


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(29) Ji, Y. A.; Hou, Y. C.; Ren, S. H.; Yao, C. F.; Wu, W. Z. Highly efficient separation of phenolic compounds from oil mixtures by imidazolium-based dicationic ionic liquids via forming deep eutectic solvents. Energ. Fuel. 2017, 31, 10274-10282. (30) Ji, Y. A.; Hou, Y. C.; Ren, S. H.; Yao, C. F.; Wu, W. Z. Highly efficient extraction of phenolic compounds from oil mixtures by trimethylamine-based dicationic ionic liquids via forming deep eutectic solvents. Fuel Process. Technol. 2018, 171, 183-191. (31) Guo, W. J.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Tian, S. D.; Marsh, K. N. Separation of phenol from model oils with quaternary ammonium salts via forming deep eutectic solvents. Green Chem. 2012, 15, 226-229. (32) Pang, K.; Hou, Y. C.; Wu, W. Z.; Guo, W. J.; Peng, W.; Marsh, K. N. Efficient separation of phenols from oils via forming deep eutectic solvents. Green Chem. 2012, 14, 2398-2401. (33) Zhang, Y.; Li, Z. Y.; Wang, H. Y.; Xuan, X. P.; Wang, J. J. Efficient separation of phenolic compounds from model oil by the formation of choline derivative-based deep eutectic solvents. Sep. Purif. Technol. 2016, 163, 310-318. (34) Zhu, J. J.; Zhu, C. F.; Wei, W. J. Analysis and evaluation of causes leading to corrosion of coal tar rectification tower. Mater. Prot. 2007, 40, 71-73. (35) Wang, X. Research on corrosion mechanism and anti-corrosion technology for coal tar refinery. Fuel Chem. Process. 2013, 44, 50-53. (36) Gao, J. J.; Dai, Y. F.; Ma, W. Y.; Xu, H. H.; Li, C. X. Efficient separation of phenol from oil by acid-base complexing adsorption. Chem. Eng. J. 2015, 281, 749-758. (37) Yao, C. F.; Hou, Y. C.; Ren, S. H.; Wu, W. Z.; Zhang, K.; Ji, Y. A.; Liu, H. Efficient separation of phenol from model oils using environmentally benign quaternary ammonium-based zwitterions via forming deep eutectic solvents. Chem. Eng. J. 2017, 620–626. 22


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(38) Jiang, Y. Y.; Wang, G. N.; Zhou, Z.; Wu, Y. T.; Geng, J.; Zhang, Z. B. Tetraalkylammonium amino acids as functionalized ionic liquids of low viscosity. Chem. Commun. 2008, 8, 505-507. (39) Saravanamurugan,

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Amine-functionalized amino acid-based ionic liquids as efficient and high-capacity absorbents for CO2. Chemsuschem 2014, 7, 897-902. (40) Sistla, Y. S.; Khanna, A. CO2 absorption studies in amino acid-anion based ionic liquids. Chemical Engineering Journal 2015, 273, 268-276. (41) Zhang, J. M.; Zhang, S. J.; Dong, K.; Zhang, Y. Q.; Shen, Y. Q.; Lv, X. M. Supported absorption of CO2 by tetrabutylphosphonium amino acid ionic liquids. Chemistry 2006, 12, 4021-4026. (42) Aronu, U. E.; Svendsen, H. F.; Hoff, K. A. Investigation of amine amino acid salts for carbon dioxide absorption. Int. J. Greenh. Gas Con. 2010, 4, 771-775. (43) Yang, Q. W.; Wang, Z. P.; Bao, Z. B.; Zhang, Z. G.; Yang, Y. W.; Ren, Q. L.; Xing, H. B.; Dai, S. New insights into CO2 absorption mechanisms with amino-acid ionic liquids. Chemsuschem 2016, 9, 806-812. (44) Yu, H.; Wu, Y. T.; Jiang, Y. Y.; Zhou, Z.; Zhang, Z. B. Low viscosity amino acid ionic liquids with asymmetric tetraalkylammonium cations for fast absorption of CO2. New J. Chem. 2009, 33, 2385-2390. (45) Shi, H. B.; Peng, Q. R.; Yang, M.; Pan, J. W.; Wang, D. X. Recent application of amino acids ionic liquids. New Chem. Mater. 2013, 41, 161-163.

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O

NH2

O

Page 24 of 39

O

NH2

N

N

N

O

O

O

[N2222][Gly]

[N2222][L-Ala]

N H

[N2222][L-Pro] O

O N

NH2

N

N H

O

O

NH2

[N2222][L-Lys]

[N2222][Sar]

Scheme 1. Structures of TAAAs used in this experiment.

24


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H

H N

HOOC OH

C

N

NH2

OOC

C R

R

Scheme 2. The reaction to obtain TAAAs.

25


NH2

H 2O

Energy & Fuels 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 26 of 39

Table1. Chemical materials used in this study Chemical name

CAS number

Purity

Supplier

phenol

108-95-2

99%

Aladdin Chemical Co., Ltd., Shanghai, China

acetone

67-64-1

98%

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

o-cresol

95-48-7

98%


m-cresol

108-39-4

98%


[N2222][OH]

77-98-5

23.94%


L-alanine

56-41-7

99%

Shanghai Yuanye Biotechnology Co., Ltd., Shanghai, China

L-proline

147-85-3

99%


sarcosine

107-97-1

99%


glycine

56-40-6

99%


toluene

108-88-3

98%

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

L-lysine

56-87-1

98%


N2

29817-79-6

99.99%

Beijing Haipu Gases Co., Ltd., Beijing, China

CO2

124-38-9

99.999%

Beijing Haipu Gases Co., Ltd., Beijing, China

26


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[N2222][Lys] [N2222][Sar] [N2222][L-Pro] [N2222][Gly]

[N2222][L-Ala]

4

3

2

1

Chemical shift/ppm

Figure 1. 1H NMR spectra of the synthesized TAAAs.

27


0

Energy & Fuels

100 80

SE/%

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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60 40

TAAAIL type: [N2222][L-Ala]

20

[N2222][Gly] 0

0

4

8

12

16

20

Time/min

Figure 2. Separation efficiency of phenol as a function of stirring time. Conditions: initial phenol content, 100.0 g/dm3; temperature, 298.2 K; TAAA:phenol mole ratio, 0.60; model oil, toluene + phenol.

28


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(b)

(a)

[N2222][Gly] as separation agent [N2222][Gly] as separation agent

[N2222][L-Ala] as separation agent [N2222][L-Ala] as separation agent

12

10

8

6

4

2

0

200

Chemical shift/ppm

160

120

80

Chemical shift/ppm

Figure 3. 1H NMR spectra (a) and 13C NMR spectra (b) of the white solids.

29


40

0

Energy & Fuels

100 90

SE/%

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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80 70

TAAAIL type: [N2222][L-Ala]

60

[N2222][Gly] 50 290

300

310

320

330

340

350

Temperature/K

Figure 4. Separation efficiency of phenol as a function of stirring temperature. Conditions: initial phenol content, 100.0 g/dm3; TAAA:phenol mole ratio, 0.60; model oil, toluene + phenol.

30


100

60

60

40

40

20

20

0.1

0.2

0.3

0.4

0.5

0.6

100

(b)

SE/% －3 Cphe/(gdm )

80

－3

80

0 0.0

100

100

(a)

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

80

80

60

60

40

40

20

20

0 0.0

0 0.7

0.1

0.2

0.3

0.4

0.5

0.6

SE/%

Page 31 of 39

0 0.7

Mole ratio

Mole ratio

Figure 5. Separation efficiency of phenol as a function of TAAA:phenol mole ratio for the two separation agents: (a) [N2222][L-Ala], and (b) [N2222][Gly]. Conditions: initial phenol content, 100 g/dm3; temperature, 298.2 K; model oil, toluene + phenol.

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Energy & Fuels

200

Initial phenol content: 3 50 gdm 3 100 gdm 3 200 gdm

150 －3

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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100

50

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

[N2222][L-Ala]/phenol mole ratio

Figure 6. Effect of initial phenol concentration on phenol separation. Conditions: initial phenol content, 100.0 g/dm3; temperature, 298.2 K; model oil, toluene + phenol.

32


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100 80

SE/%

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

ro] ][L-P [N 2222

r] ][Sa [N 2222

] ys] la] ][Gly ][L-L ][L-A [N 2222 [N 2222 [N 2222

TAAAIL type

Figure 7. Effect of TAAA type on phenol separation. Conditions: initial phenol content, 100.0 g/dm3; temperature, 298.2 K; TAAA:phenol mole ratio, 0.60 – 0.61; model oil, toluene + phenol.

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50

Phenol type: Phenol content o-Cresol content m-Cresol content

40 －3

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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30 20 10 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Mole ratio of [N2222][L-Ala]:phenols

Figure 8. Effect of phenol types on phenol separation. Conditions: initial phenol content, 150.0 g/dm3 (50.0 g/dm3 for each phenol); temperature, 298.2 K.

34


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100 80

SE/%

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

10

20

30

40

50

Mass percent of water in ILs/%

Figure 9. Effect of water content in [N2222][L-Ala] on phenol separation. Conditions: initial phenol content, 100.0 g/dm3; temperature, 298.2 K; [N2222][L-Ala]:phenol mole ratio, 0.60; model oil, toluene + phenol.

35


Energy & Fuels

[N2222][L-Ala]

[N2222][Gly]

100 80

SE/%

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 39

60 40 20 0

1

2

3

4

Cycle time

Figure 10. The separation efficiency of phenol as a function of cycle time. Conditions: initial phenol content, 100.0 g/dm3; temperature, 298.2 K; TAAA:phenol mole ratio, 0.60.

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(b)

(a)

3.77

Regenerated

Regenerated

Original

3.42

Original 4

3

2

1

4

0

3

2

1

0

Chemical shift/ppm

Chemical shift/ppm

Figure 11. 1H NMR spectra of original and regenerated (a) [N2222][L-Ala] and (b) [N2222][Gly].

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Energy & Fuels 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 38 of 39

160

240

220

200

180

160

140

Chemical shift/ppm

Figure 12. 13C NMR spectrum of regenerated [N2222][L-Ala].

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Energy & Fuels

441x255mm (72 x 72 DPI)


Tetraethylammonium Amino Acid Ionic Liquids and CO2 for

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