Extraction of l-Tryptophan by Hydroxyl-Functionalized Ionic Liquids

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Extraction of L‑Tryptophan by Hydroxyl-Functionalized Ionic Liquids Yunchang Fan,† Xing Dong,† Yun Li,† Yingying Zhong,‡ Juan Miao,*,† Shaofeng Hua,† and Yongchun Sun† †

College of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454003, China Technology Center of Ningbo Entry−Exit Inspection and Quarantine Bureau, Ningbo 315012, China

‡

S Supporting Information *

ABSTRACT: In this work, several hydroxyl-functionalized ionic liquids (ILs) were synthesized and used to extract tryptophan from the aqueous phase. The experimental results indicated that the hydroxyl-functionalized ILs presented better extraction ability than the dialkylimidazolium-based ILs. The extraction efficiency was significantly influenced by the pH of the aqueous phase. Moreover, the cationic form of tryptophan preferred to transfer into the IL phase, thereby suggesting a cation-exchange extraction mechanism. Driving forces of the extraction processes included hydrogen bonding, hydrophobicity, π−π stacking, and the screening effect. Among these factors, hydrogen bonding was the main parameter, as suggested by the results of thermodynamic analysis. After extraction, [C1C6OHim]NTf2 could be reused by using the pH swing effect. These findings demonstrate that [C1C6OHim]NTf2 is a good alternative to conventional solvents for recovering tryptophan from the aqueous phase. ILs with special chemical properties.14 In this context, Li and co-workers designed a new amide-based functionalized IL with NTf2− as the anion; the D value of this functionalized IL for tryptophan is as high as 10.07, much higher than those of NTf2−-based dialkylimidazolium ILs but still similar to those of BF4−-based dialkylimidazolium ILs.14 Although the use of an IL-based aqueous biphasic system (ABS) exhibits better extraction ability than the hydrophobic IL-based LLE, where D values of tryptophan ranging from 0.145 to 124 are obtained,15−19 the extraction of amino acids with hydrophobic ILs is still generally preferred in practice because the IL-based ABS usually involves the use of some inorganic salts, thereby complicating the extraction processes and increasing the operation costs for the wastewater treatment due to the disposal of the additional salts.13 Therefore, the design of hydrophobic functionalized ILs with high extraction ability is of considerable importance. Amino acids contain a carboxyl group that readily forms hydrogen bonds; the hydroxyl group is also capable of forming hydrogen bonds. Thus, it can be predicted that hydroxylfunctionalized ILs will present great extraction ability for amino acids. On the basis of this prediction, a series of hydroxylfunctionalized ILs were synthesized in this work, and the extraction ability of these ILs for tryptophan was examined. The extraction mechanism and reuse of the proposed hydroxylfunctionalized ILs were also studied. For clarity, the chemical structures of the ILs used in this work are shown in Figure 1.

1. INTRODUCTION As their name suggests, ionic liquids (ILs) are totally composed of ions with a melting point close to or below room temperature. They have been regarded as promising alternatives to conventional organic solvents in the chemistry,1,2 chemical engineering,3,4 and materials5,6 fields because of their low vapor pressure and tunable physicochemical properties. Amino acids are biologically important organic compounds and the basic building blocks of proteins. Fermentation7 and protein hydrolysis8 are the common biotechniques used to produce amino acids. A separation procedure is generally required to ensure the purity and quality of amino acids in the biotechnological processes. Liquid−liquid extraction (LLE) is one of the widely used separation methods due to its low cost, ease of operation, high capacity, and good selectivity.9 However, the conventional organic solvents commonly used in the LLE processes are usually volatile and hazardous. Environmental considerations have caused ILs to emerge in the literature as possible more environmentally friendly potential candidates for LLE.10−19 Wang and collegues11 used four 1,3dialkylimidazolium-based ILs with PF6− and BF4− anions to extract amino acids from water. It was found that the partition coefficients were strongly influenced by the pH of the aqueous phase and the ILs with BF4− exhibited better extraction ability than ILs with PF6−. Similar phenomena were also reported by Absalan’s group.12 Owing to the instability of the BF4− anion, which hydrolyzes to release highly toxic HF, Tomé et al. recommended the use of ILs with the bis[(trifluoromethyl)sulfonyl]imide anion (NTf2−) to extract tryptophan.13 However, one of the main disadvantages of using NTf2−-based ILs is their limited extraction ability; their distribution ratios (D) for tryptophan, for example, range from 0.03 to 4.5.13 The design of functionalized ILs may be an ideal method to further improve the extraction efficiency of tryptophan because incorporation of specific groups into the IL cation endows © XXXX American Chemical Society

Received: September 29, 2015 Revised: November 24, 2015 Accepted: December 8, 2015

A

DOI: 10.1021/acs.iecr.5b03651 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

variable-wavelength detector (VWD) and an autosampler. An Amethyst C18-H column (4.6 mm × 150 mm, 5 μm, Sepax Technologies Inc., Newark, DE) was used to separate tryptophan. The mobile phase was a mixture of acetonitrile and 0.10% (v/v) phosphoric acid aqueous solution (20:80, v/ v), the flow rate was 0.90 mL min−1, the injection volume was 5.0 μL, and the detection wavelength was set at 270 nm. For the separation of tryptophan, tyrosine, and phenylalanine, a gradient elution mode was adopted for the mobile phase: 5% acetonitrile kept constant from 0 to 3.5 min, then increased to 15% acetonitrile in 0.1 min and kept constant until 10.5 min, and thereafter restored to 5% in 0.1 min, followed by a 3.4 min re-equilibration time. Other chromatographic conditions were the same as mentioned above. 2.3. Measurements of the Nonaromatic Amino Acids. The concentrations of nonaromatic amino acids, including Lhistidine, L-glutamic acid, L-threonine, L-alanine, L-glycine, and L-valine, in the water phase were determined by an ion chromatograph (IC-2001, Tosoh Corp., Tokyo, Japan) equipped with a conductivity detector. Chromatographic conditions were as follows: separation column, SuperICCation/P (4.6 mm × 15.0 cm, Tosoh Corp.); mobile phase, 1.0 × 10−3 mol L−1 H2SO4; flow rate, 0.7 mL min−1; injection volume, 30 μL; detection mode, direct conductivity. 2.4. Extraction Procedure. Extraction of tryptophan was performed at 298 ± 1 K. Typically, 0.20 mL of an IL was mixed with 3.0 mL of tryptophan solution (0.1 g L−1). The resulting mixture was vigorously stirred for 5.0 min, which was the minimum time established by the preliminary experiments to achieve phase equilibrium for the extraction processes. After stirring, the phase separation was achieved by centrifugation. The extraction efficiency (E) was calculated by the following equation:

Figure 1. Chemical structures of the ILs used in this work.

2. EXPERIMENTAL SECTION 2.1. Material. N-Methylimidazole (99%) and N-butylimidazole (99%) were obtained from Alfa. 6-Chloro-1-hexanol (98%), lithium bis[(trifluoromethyl)sulfonyl]imide (LiNTf2; 98%), potassium hexafluorophosphate (KPF6; 99%), L-histidine (99%), L-glutamic acid (99%), L-threonine (99%), L-alanine (99%), L-glycine (99%), L-valine (99%), L-tyrosine (99%), Lphenylalanine (99%), and L-tryptophan (99%) were obtained from Energy Chemical Co. (Shanghai, China). 11-Bromo-1undecanol (≥99%) was purchased from Fluka. Activated carbons (200 mesh), 1-bromoheptane (98%), and 3-chloro-1propanol (98%) were obtained from Aladdin Reagent Co. (Shanghai, China). 2-Chloroethanol (99%) was obtained from Maya Reagent Co. (Jiaxing, China). Acetonitrile (HPLC grade) was purchased from Fisher Scientific. 1-Benzyl-3-methylimidazolium bromide ([Bzmim]Br) was obtained from the Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences (Lanzhou, China). The ILs 1-butyl-3-(6-hydroxyhexyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4C6OHim]NTf2) and 1butyl-3-(11-hydroxyundecyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4C11OHim]NTf2) were synthesized as described in our previous work.20 The synthetic procedures and characterization of 1-butyl-3-(3hydroxypropyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C4C3OHim]NTf2), 1-methyl-3-(3-hydroxypropyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1 C 3O H i m ] N T f 2 ) , 1 -m e t h y l - 3 - h e p t y l i m i d a z o l iu m b is [(trifluoromethyl)sulfonyl]imide ([C1C7im]NTf2), 1-methyl3-(6-hydroxyhexyl)imidazolium chloride ([C1C6OHim]Cl), 1methyl-3-(6-hydroxyhexyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C 1 C 6 OHim]NTf 2 ), 1-methyl-3-(6hydroxyhexyl)imidazolium hexafluorophosphate ([C1 C 6 OHim]PF6), 1-methyl-3-(2-hydroxyethyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C 1C 2OHim]NTf2 ), 1butyl-3-(2-hydroxyethyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C 4 C 2 OHim]NTf 2 ), 1-methyl-3-(11hydroxyundecyl)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C11OHim]NTf2), and 1-methyl-3-benzylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([Bzmim]NTf2) are shown in the Supporting Information. All the other reagents were of analytical grade unless stated otherwise. Ultrapure water (18.2 MΩ·cm) produced by an Aquapro purification system (Aquapro International Co., Ltd., Dover, DE) was used throughout the experiments. 2.2. Measurements of the Tryptophan Concentration. Before and after extraction, the concentration of tryptophan was measured by a high-performance liquid chromatograph (Agilent 1200 model, Santa Clara, CA) equipped with a

⎛ C ⎞ E = ⎜1 − wo ⎟ × 100 Cw ⎠ ⎝

(1)

where Cwo and Cw indicate the concentrations of tryptophan in the aqueous phase before and after extraction, respectively. The distribution ratio (D) is defined as D=

C IL Cw

(2)

where CIL and Cw are the concentrations of tryptophan in the IL and water phases, respectively. The concentration of tryptophan in the aqueous phase was analyzed by the aforementioned high-performance liquid chromatography (HPLC) method, and the tryptophan concentration in the IL phase was calculated by mass balance. To accurately determine the D values, the aqueous and IL phases were mutually saturated with each other before extraction to reduce their volume changes. Finally, the relationship of E and D can be described by the equation E=

D D+

Vw VIL

× 100 (3)

where Vw and VIL are the volumes of the aqueous and IL phases, respectively. 2.5. Measurements of the Octanol−Water Partition Coefficients (log Pow) of the ILs. The log Pow value is usually used as a measure to estimate the hydrophobicity of chemicals. B


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2.9. Determination of Fluoride (F−) in Water. The concentration of F− in water was measured by an ion chromatograph (IC-2001, Tosoh Corp., Tokyo, Japan) equipped with a conductivity detector. A TSKgel SuperIC-AZ anion-exchange column (4.6 mm × 15.0 cm, 4 μm, Tosoh Corp.) was used to separate F−. The mobile phase was an aqueous solution containing 6.3 × 10−3 mol L−1 NaHCO3 and 1.7 × 10−3 mol L−1 Na2CO3, and the flow rate was 0.6 mL min−1. The injection volume was 30 μL, and the suppressed conductivity was adopted as the detection mode. All experiments were carried out in triplicate, and the data are presented as average values. Experimental errors were not more than 7.1% and are indicated in the form of error bars in all figures.

The log Pow values of the ILs used in this work were determined as described in our previous work.20 Briefly, the IL solutions (7.1 × 10−4 mol L−1 for each) were prepared with water saturated by octanol; the octanol was also saturated by water before extraction. A 10.0 mL volume of a given IL solution and 10.0 mL of octanol were mixed under stirring for 30 min at 298 ± 1 K. After phase separation by centrifugation, the IL concentrations in both the octanol and water phases were analyzed by HPLC. The chromatographic conditions were as follows: injection volume, 2.0 μL; detection wavelength, 220 nm; flow rate, 1.0 mL min−1; mobile phase, a mixture of acetonitrile and 2.0 × 10−3 mol L−1 aqueous sodium 1heptanesulfonate solution; 50% (v/v) acetonitrile was used for determining [C4C11OHim]NTf2 and [C7mim]NTf2, and 40% acetonitrile was adopted for analyzing the remaining ILs. The Pow value was calculated by the equation C Pow = octanol Cwater

3. RESULTS AND DISCUSSION 3.1. Stability of the ILs. As reported in the literature,21 PF6− is unstable when exposed to moisture and can undergo hydrolysis to produce the dangerous and corrosive HF. The stability of the ILs was therefore studied using [C1C6OHim]NTf2 and [C1C6OHim]PF6 as representative compounds. Experiments were carried out by allowing 1.0 g of an IL to come into contact with 10.0 mL of pure water. After 7 days of contact at room temperature, 1.6 × 10−5 mol L−1 F− was detected in the [C1C6OHim]PF6 + water system, but no F− was found in the [C1C6OHim]NTf2 + water system. These results indicate that NTf2−-based ILs are considerably more stable than the PF6−-based ILs and are superior options for extraction. 3.2. Effect of the Chemical Structures of the ILs on the Distribution Ratio. In this work, 11 ILs were investigated for their extraction ability for tryptophan. Considering the fact that the hydrophobicity of ILs is also a parameter affecting their extraction ability,11,13 the hydrophobicity (log Pow) of the 11 ILs was thus determined. The correlation between extraction ability and hydrophobicity of the ILs is shown in Figure 2. In panel A of the figure, the extraction ability of [C1CnOHim]NTf2 increases with increasing number of carbon atoms (n) from 2 to 6 and significantly decreases with further increases in n to 11. In the case of [C4CnOHim]NTf2, the extraction ability increases with increasing n from 2 to 3 and then remarkably decreases as the length of the alkyl chain on the IL cation is increased (Figure 2B). The effect of the side chain length on the extraction ability of the IL cation depends on two aspects: (I) increases in the length of the side chain increase the hydrophobicity of the ILs, thereby improving the hydrophobic interaction between the IL cation and the hydrophobic moiety of tryptophan and promoting extraction efficiency, and (II) tryptophan bears a large indole side chain; thus, IL cations with long side chains can be expected to hinder the interaction between the ILs and tryptophan, causing a screening effect (also known as a steric hindrance effect) and decreasing the extraction ability of the ILs.11,13 Actually, the screening effect of the alkyl chain on the IL cation has previously been suggested by Lu and coworkers.22 Another interesting phenomenon that may be observed from Figure 2A,B is that the extraction ability of [C1CnOHim]NTf2 is considerably higher than that of the corresponding [C4CnOHim]NTf2 (e.g., [C1C6OHim]NTf2 > [C4C6OHim]NTf2), which suggests that N-butylimidazoliumbased ILs exhibit a more noticeable screening effect than the Nmethylimidazolium-based ILs. This finding explains why the

(4)

where Coctanol and Cwater are the concentrations of a given IL in the octanol and water phases, respectively. 2.6. Measurements of the Concentrations of [C1C6OHim]+ in the Water Phase before and after Extraction. The determination of the concentrations of [C1C6OHim]+ before and after extraction was conducted by HPLC. The chromatographic conditions were the same as described in section 2.5 on measurements of log Pow except that the composition of the mobile phase was 30% (v/v) acetonitrile and 70% (v/v) aqueous sodium 1-heptanesulfonate solution (2.0 × 10−3 mol L−1). The pH values of the aqueous solutions were adjusted by HCl for pH 1.0−2.0, and controlled by phosphate buffers (0.1 mol L−1) for pH 2.5−12. A pHS-3B digital pH meter (Shanghai Leici Instrument Factory, Shanghai, China) was used to measure the pH of the water phase. 2.7. Measurements of the Solubilities of Tryptophan in Water and [C1C6OHim]NTf2. The determination of the solubility of tryptophan in water was as follows: An excess amount of tryptophan was added to water. After the mixture was stirred for 1 h, the water phase was filtered by a 0.45 μm filter to remove undissolved tryptophan. After appropriate dilution by water, the concentration of tryptophan in the water phase (solubility) was measured by the above-mentioned HPLC method. To measure the solubility of tryptophan in [C1C6OHim]NTf2, 5.0 mL of [C1C6OHim]NTf2 was mixed with 30.0 mL of a saturated solution of tryptophan. After the mixture was stirred for 1 h, the [C1C6OHim]NTf2 and water phases were separated by a separation funnel. The [C1C6OHim]NTf2 phase was dried and filtered by a 0.45 μm filter to remove undissolved tryptophan. After appropriate dilution by ethanol, the concentration (solubility) of tryptophan in [C1C6OHim]NTf2 was determined by the above-mentioned HPLC method. 2.8. Measurements of the Solubities of the ILs in Water. The solubilities of the ILs in water were measured according to the reported literature.20 Briefly, an excess amount of a specific IL was mixed with water. After the mixture was stirred for 1 h, the IL and water phases were separated by centrifugation. The concentration (solubility) of an IL in water was measured by a spectrophotometer (TU-1810, Purkinje General Instrument Co., Ltd., Beijing, China); the detection wavelength was 220 nm. C


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reported in the literature,23−25 introduction of a hydroxyl group to the IL cation can significantly increase the hydrogen-bonding ability of the ILs. Therefore, stronger hydrogen-bonding interaction between tryptophan and [C1C6OHim]NTf2 may be expected, and the effect of the hydrogen-bonding interaction is stronger than that of hydrophobicity. Panel C in Figure 2 demonstrates that, despite its lower hydrophobicity, the IL [Bzmim]NTf2 exhibits greater extraction ability in comparison with [C1C7im]NTf2. This superior ability can be attributed to the π−π stacking between the IL cation and the aromatic ring of tryptophan.13 However, the extraction ability of [Bzmim]NTf2 is notably lower than that of [C1C6OHim]NTf2, indicating that the effect of π−π stacking is also weaker than the hydrogen-bonding interaction. As [C1C6OHim]NTf2 possesses the highest extraction ability among the ILs investigated, this IL may be regarded as the best option for subsequent studies. 3.3. Thermodynamic Analysis. As discussed above, hydrophobic interaction, hydrogen bonding, and π−π stacking contribute to the extraction of tryptophan from water; of these possibilities, hydrogen bonding appears to be the dominating parameter affecting the extraction ability. Additional experiments were conducted to support this hypothesis. In general, the thermodynamic parameters enthalpy change (ΔH) and entropy change (ΔS) are the primary evidence cited to confirm the driving forces underlying the extraction processes.20,26 The Gibbs free energy change (ΔG) of the extraction process can be calculated by the following equation:20,26 (5)

ΔG = −RT ln D

where R and T are the gas constant and extraction temperature, respectively. Provided that ΔH and ΔS remain constant over the studied temperature range, their values can be obtained by the van’t Hoff equation:20,26

ln D = −

ΔH ΔS + RT R

(6)

The van’t Hoff plot is shown in Figure 3, and the results are shown in Table 1. Negative ΔG values indicate that the extraction of tryptophan is a spontaneous process. Both ΔH and ΔS are negative, suggesting that the hydrogen bonding is the main driving force governing the extraction processes, which also confirms the aforementioned deduction.

Figure 2. Correlation of the extraction ability and hydrophobicity of the ILs (Ctryptophan = 0.1 g L−1, Vw:VIL = 15:1, pH 2.0).

extraction ability of [C4CnOHim]NTf2 is significantly reduced when n is only greater than 3. Figure 2 reveals that the extraction ability of [C1C6OHim]NTf2 is markedly higher than that of [C1C6OHim]PF6 and that the hydrophobicity of NTf2− is greater than that of PF6−. Generally, the extraction ability of a solvent depends on its hydrogen-bonding ability, hydrophobicity, and polarity. The hydrogen-bonding and polarity parameters of NTf2− and PF6− are listed in Table S1 in the Supporting Information. Because NTf2− and PF6− exhibit similar hydrogen-bonding abilities and polarities, it can be deduced that the difference in the extraction abilities of NTf2− and PF6− can be ascribed to their different hydrophobicities. The extraction ability of [C1C6OHim]NTf2 is notably higher than that of [C1C7im]NTf2 (D = 0), and the hydrophobicity of the former is lower than that of the latter. These findings suggest that the incorporation of a hydroxyl group remarkably improves the affinity between an IL and tryptophan. As

Figure 3. Van’t Hoff plot of [C1C6OHim]NTf2. D


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Industrial & Engineering Chemistry Research AmH+ w + ([C1C6OHim]NTf 2)IL

Table 1. Thermodynamic Parameters for the Extraction of Tryptophan (0.1 g L−1) at pH 2.0 and Vw:VIL = 15:1 T (K)

ΔG (kJ mol−1)

288 298 308 318 328

−9.7 −9.3 −9.1 −8.9 −8.8

ΔH (kJ mol−1)

−16.4

⇄ [C1C6OHim]w + + (AmHNTf 2)IL

ΔS (J mol−1 K−1)

where AmH+ and A− refer to the cationic forms of an amino acid and a specific anion, respectively, and the subscripts w and IL indicate the aqueous and IL phases, respectively. Equation 7 corresponds to an ion pair extraction mechanism. If the ion pair extraction occurs, the D values should increase with increasing concentration of A− (Cl− or NTf2−). However, Figures S2 and S3 (log D versus log [A−], shown in the Supporting Information) demonstrate that the D values remain constant with a change in the A− concentration. This result suggests that ion pair extraction is not the appropriate mechanism. Equation 8 illustrates the cation-exchange mechanism. Given an operative mechanism, increases in the [C1C6OHim]w+ concentration in water (performed by adding [C1C6OHim]Cl into water) will result in decreases in the D values. The results shown in Figure S4 (log D versus log [[C1C6OHim]+], shown in the Supporting Information) confirm this mechanism. In another indication of the cation-exchange mechanism, the amount of tryptophan extracted into [C1C6OHim]NTf2 should be equal to that of the IL cations released into the water phase. Therefore, in this work, the amounts of the IL cation ([C1C6OHim]w+) in the water phase before and after extraction under the experimental conditions of pH 2.0 and Vw:VIL = 15:1 were determined by HPLC; here, 5.0 g L−1 tryptophan was selected as the initial concentration to measure concentration changes in tryptophan accurately. The extraction efficiency of tryptophan was found to be 56.4%, corresponding to 1.4 × 10−2 mol L−1 tryptophan to be extracted into the IL phase. After extraction, the increment of [C1C6OHim]w+ was found to be 1.5 × 10−2 mol L−1. These results strongly indicate that the extraction of tryptophan into [C1C6OHim]NTf2 involves a cation-exchange process. 3.5. Effect of the Tryptophan Concentration on the Extraction Efficiency. To investigate the effect of the tryptophan concentration on the extraction efficiency, additional experiments were performed with tryptophan concentrations ranging from 1.0 × 10−3 to 10 g L−1 at pH 2.0 and Vw:VIL = 15:1. As shown in Figure 5, the extraction efficiency remains constant in the concentration range of 1.0 × 10−3 to 1.0 g L−1 and decreases when the tryptophan concentration is increased. To explain this phenomenon, the solubility of tryptophan in [C1C6OHim]NTf2 was measured, and a value of

−23.4

The plot of ln D versus 103/T (Arrhenius equation) was also regressed, and the results are shown in Figure S1 of the Supporting Information. The Arrhenius equation obtained shows good linearity, and an activation energy of −16.4 kJ mol−1 is calculated. A negative activation energy suggests that the transfer of tryptophan from water to [C1C6OHim]NTf2 is highly favorable.27 3.4. Effect of the pH on the Extraction Efficiency and the Extraction Mechanism. The effect of the aqueous pH on the extraction efficiency is shown in Figure 4. The extraction

Figure 4. Effect of the pH on the extraction efficiency (Ctryptophan = 0.1 g L−1, Vw:VIL = 15:1).

efficiency remains constant in the pH range of 1.0−2.0 and then remarkably decreases with further increases in pH, showing a pH swing effect. The dissociation constants (pKa) of the carboxyl and ammonium groups of tryptophan are 2.38 and 9.39, respectively.11 Therefore, when the pH is below the pKa, tryptophan mainly exists in its cationic form and the undissociated carboxyl group is predominant, resulting in stronger hydrogen bonding between the ILs and tryptophan. The resulting hydrogen bonding is responsible for the higher extraction ability of the ILs in the pH range of 1.0−2.0. With increasing pH, the carboxyl group begins to dissociate, which weakens its hydrogen bonding with ILs and thus reduces the extraction efficiency of tryptophan. To reduce the consumption of HCl, pH 2.0 was regarded as the best choice. Since tryptophan can be effectively extracted into the IL phase in the cationic form, it is easy to understand that the transfer of the cationic form of tryptophan to the IL phase should be accompanied by the counterions to maintain electroneutrality of the IL phase. In this context, cationexchange and ion pair extraction mechanisms were used to explain the extraction processes:10,13 AmH+ w + A− w ⇄ (AmHA)IL

(8)

Figure 5. Effect of the tryptophan concentration on the extraction efficiency.

(7) E


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Industrial & Engineering Chemistry Research 68.8 g L−1 was obtained (the solubility of tryptophan in water is listed in Table S2 in the Supporting Information). Therefore, the observed influence of the tryptophan concentration on the extraction efficiency may be interpreted from two perspectives: (I) under the conditions of Vw:VIL = 15:1 and high tryptophan concentration (5.0 or 10 g L−1), the amount of tryptophan exceeds its solubility in [C1C6OHim]NTf2; (II) with increasing tryptophan concentration, the ionic strength in the aqueous phase increases, which makes the electrostatic interaction between tryptophan cation and chloride anion (existing as a counterion for the tryptophan cation in the water phase) increase, thereby decreasing the electrostatic interaction between tryptophan and [C1C6OHim]NTf2 and thus reducing the movement of tryptophan to the IL phase.28−30 3.6. Effect of the Phase Volume Ratio on the Extraction Efficiency. The phase volume ratio (Vw:VIL) is an important parameter affecting the extraction efficiency. As shown in Figure 6, the extraction efficiency decreases with

those of the dialkylimidazolium-based ILs and [EimCH2CONHBu]NTf2 and approximates that of the [C1C4im]PF6 + dicyclohexano-18-crown-6 system (where dicyclohexano-18crown-6 is used as the chelating agent). These results indicate that [C1C6OHim]NTf2 is a good alternative for recovering tryptophan from water. 3.8. Selective Extraction of Tryptophan. As reported by Wang and co-workers,11 aromatic amino acids, such as phenylalanine and tyrosine, interfere with tryptophan extraction. Therefore, in this work, the possibility of selective tryptophan extraction by [C1C6OHim]NTf2 from a mixture of tryptophan, phenylalanine, tyrosine, histidine, glutamic acid, threonine, alanine, glycine, and valine was investigated. As shown in Figure 7, the extraction efficiencies of glutamic acid,

Figure 7. Extraction efficiencies of [C1C6OHim]NTf2 for amino acids (pH 2.0, 0.1 g L−1 for each amino acid).

threonine, alanine, glycine, and valine are all equal to zero at Vw:VIL = 5:1 and 15:1. The ratio of the extraction efficiencies of tryptophan to tyrosine is 2.2, that of tryptophan to phenylalanine is 1.5, and that of tryptophan to histidine is 7.1 at Vw:VIL = 5:1. When the phase volume ratio is increased to 15:1, the corresponding ratios of extraction efficiency are 5.8, 3.3, and 15.8, respectively. These results suggest that tryptophan can be separated from a mixture of amino acids and that the use of high phase volume ratios, such as 15:1, is a good approach to improving the extraction selectivity. 3.9. Reuse of [C1C6OHim]NTf2. The reuse of [C1C6OHim]NTf2 can offer additional cost and green chemistry benefits. As described in the pH profile, the reuse of [C1C6OHim]NTf2 can be easily achieved by using the pH swing effect. In a typical procedure, 3.0 mL of [C1C6OHim]NTf2 was mixed with 4.0 mL of NaOH aqueous solution (pH 12.0) after extraction. After the mixture was stirred for 5.0 min, the IL phase was washed twice, each time with 2.0 mL of water. After being dried at 70 °C for 3 h, the IL phase can be reused for the next extraction recycle. Figure 8 indicates that [C1C6OHim]NTf2 does not lose its extraction ability after 10 extraction cycles. In addition, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectra were used to investigate the structures of the reused [C1C6OHim]NTf2. The results are shown in Figures S5−S7 in the Supporting Information. As can be seen in the figures, the NMR and FT-IR spectra of the reused [C1C6OHim]NTf2 are identical to those of the fresh IL, thus suggesting that [C1C6OHim]NTf2 retains its original chemical structures after reuse. 3.10. Recovery of [C1C6OHim]NTf2 from Water. As discussed above, [C1C6OHim]NTf2 exhibited the best perform-

Figure 6. Effect of the phase volume ratio on the extraction efficiency (Ctryptophan = 0.1 g L−1, pH 2.0).

increasing phase volume ratio, in agreement with eq 3 (E = [D/ (D + Vw/VIL)] × 100). In addition, the solubility loss of [C1C6OHim]NTf2 increases with increasing phase volume ratio, leaving a less bulky IL phase that is also responsible for the decrease in the extraction efficiency. Since a phase volume ratio of 5:1 can provide excellent extraction efficiency (89.0%), it is selected as the optimal ratio. 3.7. Comparison with Reported Methods. At present, hydrophobic ILs, such as [C1C6im]BF4, [C1C2im]NTf2, and 1ethyl-3-(N-butylacetamido)imidazolium bis[(trifluoromethyl)sulfonyl]imide ([EimCH2CONHBu]NTf2) are commonly used to extract tryptophan. A comparison of the D values of the aforementioned hydrophobic ILs is provided in Table 2. The D value of [C1C6OHim]NTf2 is markedly higher than Table 2. D Values of Tryptophan in Different Hydrophobic IL/Water Systems D

IL [C1C2im][NTf2] [C1C6im][BF4] [C1C6im][BF4] [EimCH2CONHBu]NTf2 [C1C4im]PF6 + dicyclohexano-18-crown-6 system [C1C6OHim]NTf2

ref

4.5 7.8 8.64 10.02 20.7−75.9

13 13 11 14 10

43.4

this work F



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +863913987821. Fax: +863913987815. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Grants 21307028 and 21401045), Young Backbone Teachers in Colleges and Universities of Henan Province (Grant 2013GGJS-053), Projects of Henan Province (Grants 142102210049, 14B150026, and 12A150010), and Science Projects of Ningbo City (Grants 2012A610149 and 2007C10035).

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Figure 8. Effect of the reuse time on the extraction efficiency (Vw:VIL = 5:1, Ctryptophan = 0.1 g L−1, pH 2.0).

ance among the ILs studied. However, this IL presents certain solubility in water (Table S2 in the Supporting Information) and, therefore, must be retrieved from water. For this purpose, 0.5 g of activated carbons was used to treat 20 mL of the saturated solution of [C1C6OHim]NTf2. After the solution was stirred for 10 min, the removal efficiency of [C1C6OHim]NTf2 from water was 95.6%. To recover [C1C6OHim]NTf2 from activated carbons, 9.0 mL of anhydrous ethanol was used to wash 0.5 g of activated carbons three times (3.0 mL per time), resulting in 101.3% recovery. Lately, [C1C6OHim]NTf2 has been recovered easily from ethanol solution by atmospheric distillation.

4. CONCLUSIONS On the basis of this research, the following conclusions can be obtained: (I) The introduction of a hydroxyl group to the IL cation can remarkably increase the extraction efficiency for tryptophan in comparison with that of the dialkylimidazoliumbased ILs. (II) The extraction efficiency is affected by the aqueous pH, the tryptophan concentration, and the phase volume ratio. (III) Thermodynamic analysis suggests that hydrogen bonding is the main force driving the extraction processes. The hydrophobicity of ILs, π−π stacking between the ILs and tryptophan, and the screening effect also affect the extraction efficiency. (IV) Tryptophan can be effectively extracted from a mixture of nine amino acids. (V) The reuse of [C1C6OHim]NTf2 can easily be achieved by using the pH swing effect.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03651. 1 H and 13C NMR data of synthesized ILs, hydrogenbonding and polarity parameters of NTf2− and PF6− (Table S1), Arrhenius plot of tryptophan (Figure S1), effect of the Cl− (Figure S2), NTf2− (Figure S3), and [C1C6OHim]+ (Figure S4) concentrations on the D values, FT-IR spectra of [C1C6OHim]NTf2 (Figure S5), 1 H NMR (Figure S6) and 13C NMR (Figure S7) spectra of [C 1 C 6 OHim]NTf 2 before and after use, and solubilities of ILs and L-tryptophan in water (Table S2) (PDF) G


Article

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Extraction of l-Tryptophan by Hydroxyl-Functionalized Ionic Liquids

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