Silver-based ionic liquid as separation media: Supported liquid membrane for facilitated methyl linolenate transport

Journal of Membrane Science 585 (2019) 218–229

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Silver-based ionic liquid as separation media: Supported liquid membrane for facilitated methyl linolenate transport

T

Xianghong Lu∗, Qianxia Chen, Dehua Zhao, Jiajian Zhu, Jianbing Ji Zhejiang Province Key Laboratory of Biofuel, Biodiesel Laboratory of China Petroleum and Chemical Industry Federation, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, 310014, PR China

ARTICLE INFO

ABSTRACT

Keywords: Supported ionic liquid membrane Methyl linolenate Complexation DFT AgBF4

Methyl linolenate (C18-3) is a type of unsaturated fatty acid methyl ester with high biological activity. Membrane separation of C18-3 from tallow seed oil methyl ester by supported ionic liquids membranes (SILM) containing carrier Ag+ was successfully achieved in this study. The interaction between Ag+ and C18-3 was explored by density functional theory (DFT). High selectivity to C18-3 was mainly due to the π/inverse π complexation interactions between Ag+ and C=C bonds. The effects of ionic liquid structure, carrier Ag+ concentration, adjacent phase flow rates and membrane pore size on permeability and selectivity to C18-3 were examined. The existence of 1-hexene in the stripping phase and carrier Ag+ can facilitate C18-3 transport. Good separation performance was achieved by the 0.45 μm nylon membrane impregnated with ionic liquid [BMIm] BF4 containing AgBF4. C18-3 was enriched from 16.7% in the feed phase to higher than 75% in the product. Moreover, adding a hydrophobic PVDF protective membrane to form a composite membrane was an effective method to increase the SILM stability and membrane lifetime by about twice compared with a single nylon membrane.

1. Introduction Linolenic acid, an omega-3 polyunsaturated fatty acid, is nutritionally important to form cell membranes and biological enzymes. It has functions of lowering the level of blood pressure, maintaining lipoprotein balance and regulating cholesterol metabolism. Unfortunately, humans can't biosynthesize omega-3 fatty acids which must be taken from the outside [1–3]. Linolenic acid (in the form of glyceride) presents in plants such as linseed oil, perilla seed oil, and eucalyptus oil with oleic acid, linoleic acid and palmitic acid together. Due to the similarity of molecular size, polarity and structure, separation of linolenic acid from unsaturated analogues remains challenging. Several methods were proposed to separate polyunsaturated fatty acids (esters), including freezing crystallization [4], molecular distillation [5], urea complexation [6], supercritical fluid extraction [7] and silver ion complexation [8]. Silver ion complexation extraction is a separation method based on the complexation between Ag+ and C=C bonds. Compared with other methods, the silver ion complexation extraction has advantages of high selectivity, mild separation conditions and low reaction energy, which has been widely used to separate unsaturated fatty acids (esters) by

liquid-liquid extraction [9]. Li et al. [10] reported that silver salt/ionic liquid (IL) was an excellent extraction solution to separate and enrich polyunsaturated fatty acid methyl esters. C20-5 (EPA) and C22-6 (DHA) were largely enriched from 18.0% in the original cod liver oil to 74.2% with [HMIm]PF6 containing AgBF4 as the extractant. However, liquid-liquid extraction consumes large quantities of expensive silver salts and needs multi-step operations to fully separate target product. To reduce the consumption of silver salts and simplify manipulations, liquid membrane separation technology is a promising alternative. The liquid membrane separation technology combines two processes of extraction and stripping in a single operation simultaneously, with target product directly obtained from stripping solvent. Besides, small quantities of membrane solvent and carrier could achieve efficient separation. It also has advantages of larger specific surface area for extraction, one-step mass transfer operation, better energy efficiency, lower consumption of organic solvents and much lower environmental impact, which has been suggested for many industrial fields [11]. In general, there are different types of liquid membranes such as supported liquid membranes (SLM), bulk liquid membranes (BLM), and emulsion liquid membranes (ELM) [12]. Emulsion liquid membrane, known as emulsion surfactant liquid membrane, usually has

Corresponding author. E-mail addresses: [email protected] (X. Lu), [email protected] (Q. Chen), [email protected] (D. Zhao), [email protected] (J. Zhu), [email protected] (J. Ji). ∗

https://doi.org/10.1016/j.memsci.2019.05.027 Received 31 January 2019; Received in revised form 10 May 2019; Accepted 10 May 2019 Available online 17 May 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

stability issues. In addition, making and breaking emulsion is a difficult and costly process [13]. As to the BLM, Zhang et al. [14] selected methanol-AgNO3 aqueous solution as BLM to separate methyl linolenate from tallow seed oil methyl ester. Under the optimal conditions, methyl linolenate with purity of 94.1% could be extracted from the tallow seed oil methyl ester, and the yield was approximately two times higher than that of fraction extraction. Baylan et al. [15] used imidazolium-based ionic liquids as a bulk liquid membrane, tributyl phosphate as a carrier in the membrane to remove levulinic acid. However, the extreme thickness of the membrane phase (thickness ≥ 0.1 cm) results in large mass transfer resistance and low mass transfer rate of BLM in the separation process. Therefore, BLM is difficult to upgrade from lab scale to pilot/commercial scale. SLM is a liquid membrane prepared by impregnating porous supports in the membrane solvent with dissolved carriers by capillary forces [16]. Compared with BLM, the thickness of SLM is extremely minimal, therefore, the mass transfer rate could be effectively increased and the consumption of membrane solvents and carriers would be greatly reduced. The main drawbacks of SLM are low stability and short lifetime resulting from leakage or evaporation of membrane phase, which limit the scale up of the SLM technique [17]. An effective approach to enhance the stability of SLM is to appropriately increase the viscosity of membrane phase. Relatively high viscous ILs with negligible vapor pressure is a promising membrane solvent. Furthermore, ILs have excellent solvent properties [18,19]. In previous studies, supported ionic liquid membrane (SILM) was used to separate light organic compounds such as alcohols, organic acids, and aromatic hydrocarbons [17,20,21], mixed gases [22,23] and metal ions [24,25]. However, to our best knowledge, no studies on the separation of unsaturated fatty acid methyl esters (UFAMEs) by SILM were reported. In this work, SILM containing carrier Ag+ was designed for methyl linolenate (C18-3) extraction from tallow seed oil methyl ester mixtures. The effects of different ILs and operating variables (Ag+ concentration, adjacent phase flow rates and pore size of supported membrane) on the separation process were investigated. DFT calculation was conducted to explore the action mechanism between Ag+ and UFAMEs at the molecular level. In addition, a 24 h long-term operation verified the stability of SILM and then an effective procedure was proposed to increase the membrane lifetime.

ILs with the purity of 99% were purchased from Lanzhou Green Chemistry and Catalysis, LICP, CAS(China). The molecular structures and physical properties of ILs used in this work were listed in Table 2. AgBF4 (≥99%, purity) was obtained from Adamas Reagent Co. Ltd. 1-hexene (≥95%, purity) was obtained from Tokyo Chemistry Industry Co. Ltd. All the materials were used without further purification. 2.2. Methods 2.2.1. Preparation of SILMs All SILMs were prepared at room temperature by impregnating the porous membrane with ILs. Due to the complexation of Ag+ with 1hexene, the addition of 1-hexene as cosolvent could effectively promote the dissolution of AgBF4 in IL. First, a certain amount of AgBF4, 0.6 mL 1-hexene and 0.5 mL ionic liquid were added into a 2 mL centrifuge tube which was shaken for 10 min. Then the mixture was centrifuged at 12000 rpm for 10 min in darkness to ensure the complete stratification of two phases, and the lower liquid was applied to the surface of the support. After the membrane was wetted thoroughly, excess of liquid was removed from the membrane surface using a tissue [26]. To determine the amount of membrane solvent immobilized in the support membrane, all the membranes were weighed before and after impregnation. 2.2.2. Liquid membrane extraction configuration The schematic of experimental setup used in this study was shown in Fig. 1. Membrane contactor consisted of two identical PTFE modules (100 mm × 100 mm × 8 mm), and each module had a cylindrical cavity with a diameter of 47 mm and a height of 2 mm. In the upper PTFE module there were two ports which corresponded with the stripping solvent inlet and outlet. Similarly, in the lower PTFE module there were also two connections which were the feed solution inlet and outlet. The SILM was clamped in the two PTFE modules and sealed by fluoro rubber gaskets hermetically to avoid the leakage of liquid. The stripping solvent and the feed solution were placed in 50 mL sealed centrifugal tubes and delivered by peristaltic pumps (Baoding Longer Precision Pump Co., Ltd.). 2.2.3. Liquid membrane extraction operation The prepared supported ionic liquid membrane was placed into the membrane contactor, and screws were tightened firmly to keep the membrane from moving. FAMEs petroleum ether solution (50 mg/mL) was used as feed solution. 10 wt% 1-hexene petroleum ether solution was used as stripping solvent. Feed solution (20 mL) and stripping solvent (20 mL) were placed in 50 mL centrifugal tubes apart. The stripping solvent and the feed solution were circulated through upper and lower sides of the membrane respectively in a cross flow way. The flow rates were adjusted by peristaltic pumps, and calibrated by measuring the volume of liquid with a graduated cylinder and a stopwatch. Samples (1 mL) were taken from the stripping solvent tube and the feed solution tube respectively per hour, and then analyzed by GC to determine the content of FAMEs. The concentration of FAMEs over time in the stripping solvent and the permeability of FAMEs were used to describe the mass transfer rate of SILM. The separation factor and the purity of C18-3 in the product were used to characterize the separation performance of SILM. The separation factor was calculated by Eq. (1).

2. Materials and methods 2.1. Materials In this paper, the feed phase was fatty acid methyl esters (FAMEs) petroleum ether solution. The stripping phase was 1-hexene petroleum ether solution. Nylon membrane with different pore size of 0.22 μm and 0.45 μm was used as supported membrane. PVDF membrane with pore size of 0.45 μm was used as protected membrane. Nylon membrane and PVDF membrane were purchased from Tianjin Bo Jin Technology Co., Ltd. The FAMEs mixture was prepared with tallow seed oil by transesterification. The composition of FAME mixture was determined by GCFID, as listed in Table 1. Table 1 The composition and content of tallow seed oil methyl ester. FAME

Formula

Content

Methyl-2,4-decandienoate (C10-2) Methyl Palmitate (C16-0) Methyl Stearate (C18-0) Methyl Oleate (C18-1) Methyl Linoleate (C18-2) Methyl Linolenate (C18-3)

CH3CH=CHCH=CH(CH2)4COOCH3

1.4%

CH3(CH2)14COOCH3 CH3(CH2)16COOCH3 CH3(CH2)7CH = CH(CH2)7COOCH3 CH3(CH2)3(CH2CH=CH)2(CH2)7COOCH3 CH3(CH2CH=CH)3(CH2)7COOCH3

15.3% 4.5% 30.2% 31.9% 16.7%

Separation factor Sij =

( i / j ) stripping phase ( i / j ) feed phase

(1)

where Sij stands for the degree of separation between component i and component j; ωi and ωj are the mass fraction of components i and j. The permeability coefficient (P) was calculated by the following method. The C18-3 mass flux J = P (Cf - Cs), where P is the permeability of SILM, Cf and Cs are C18-3 concentrations in the feed phase and the stripping phase respectively. The mass flux was also expressed 219

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Table 2 Molecular structures and physical properties of ILs. Viscosity (cP, 20 °C)

density（g/cm3）

MW

[EMIm]BF4

45

1.21

197.97

[BMIm]BF4

140

1.26

226.02

[HMIm]BF4

266

1.15

254.08

[OMIm]BF4

415

1.09

282.13

[BMIm]PF6

366

1.37

284.18

[BMIm]NTf2

69

1.44

419.36

[BMIm]CF3SO3

93

1.29

288.29

′

Cation

Anion

gas chromatograph (GC) equipped with an auto injector (Model 7683B), a flame ionization detector (FID) and a J&W DB-WAX column (30 m×32 μm×0.25 μm). The temperatures of injector port and FID were held at 280 °C and 300 °C respectively. The oven temperature was initially held constant at 150 °C for 2 min, and increased at 10 °C/min to 200 °C and kept at 200 °C for 4 min, followed by being increased at 40 °C/min to 230 °C, and finally held at 230 °C for 9 min. The injector volume was 1 μL and the split ratio was 15:1 [27]. The absolute correction factor of methyl palmitate, methyl stearate, methyl linoleate and methyl linolenate was regarded as the same

V dC

as J = Ss dts according to the mass balance, where Vs, S and ε are the volume of stripping phase, the membrane area and the membrane porosity respectively. Cf = Cf0 - Cs when the feed phase and the stripping phase have the same volume. Then, ln 1

2Cs C f0

Vs dCs S dt

obtained from the integration ofP (Cf

Cs ) =

termined from the slope of ln 1

over time.

2Cs C f0

=

2 V Pt could be S

s

, and P can be de-

2.2.4. GC analysis The concentration of FAMEs was determined by an Agilent 7890A

Fig. 1. Schematic of membrane module device. 220

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Table 3 Electron configuration of silver ion. UFAME

[C18-2·3Ag]

Ag+ electronic configuration

Functional group

3+

[C18-3·4Ag]4+

Before complexation

After complexation

C=C

[core]5s(0.00)4d(10.00)

C=O C=C

[core]5s(0.00)4d(10.00)

[core]5s(0.11)4d(9.94)5p(0.03) [core]5s(0.09)4d(9.94)5p(0.01) [core]5s(0.07)4d(9.98)5p(0.02) [core]5s(0.09)4d(9.94)5p(0.01) [core]5s(0.08)4d(9.94)5p 0.01) [core]5s(0.09)4d(9.94)5p(0.01) [core]5s(0.07)4d(9.98)5p(0.02)

C=O

interaction between Ag+ and double bonds has already been experimentally investigated (UV–visible spectroscopy and FT-IR analysis) [31], few researchers theoretically studied it at the molecular level. In addition, in our work, both carbonyl group and C=C bonds in the molecule of C18-3 possess large negative electrostatic potentials [27], and thus both structures can interact with Ag+, which was not conducive to improving the selectivity to C18-3. Thus, we investigated the action mechanism by DFT and made a deeper insight into the relationship between Ag+ and UFAMEs. In addition, the performance of carrier Ag+ and the effect of Ag+ concentration on separation of C18-3 were studied.

because of their highly similar molecular structure and physicochemical properties [28,29]. Methyl palmitate with purity of 98% was chosen to obtain the absolute correction factor. Then the concentration of fatty acid methyl ester was calculated using Eq. (2): Ci = f Ai

(2) −3

where f is the absolute correction factor (f = 1.98 × 10 ); Ci stands for the concentration of component i; Ai is the peak area of component i. 2.2.5. X-ray diffraction (XRD) XRD analysis of blank nylon membrane and the support ionic liquid membrane was carried out on an X'Pert Pro Advance diffractometer with Cu-Kα radiation source (λ = 1.5406 Å) at 40 KV and 30 mA. The diffraction patterns were collected in the range of 2θ from 10° to 80° by step of 0.05°.

3.1.1. The action mechanism between Ag+ and UFAMEs NBO analysis could be used to investigate the electron transfer and orbital action in the molecule, which is beneficial to the analysis of the action mechanism between molecules. To explore the interaction site between Ag+ and UFAMEs, Ag+ electronic configurations before and after complexation were obtained by NBO calculation. The calculation was performed using the NBO 3.0 program as implemented in the Gaussian 09 package at 311 + G (2d, p) +LANL2DZ level while silver ions were placed near the carbon-carbon double bonds and carbonyl groups of unsaturated fatty acid methyl ester molecules. Three silver ions coordinated respectively with two C=C bonds and a carbonyl group of C18-2, while four silver ions respectively coordinated with three C=C bonds and a carbonyl group of C18-3. The electron configurations of silver ions were shown in Table 3. Before complexation, the 5s orbit of Ag+ was empty, while 4d orbit was full. In Table 3, regardless of Ag+ interacted with a carbonyl group or C=C bonds, the electron quantity of the 5s and 4d orbitals increased and decreased, respectively, which indicated the UFAMEs provided electrons to form σ coordination bond with the 5s orbital of Ag+, while Ag+ provided 4d electrons to UFAMEs and formed π back bonding coordination. Compared with the complexation between C=C bonds and sliver ions, the number of electrons increased in the 5s and decreased in the 4d was fewer than silver ions coordinated with carbonyl group. It showed that the interaction between Ag+ and C=C bond was stronger than that between Ag+ and carbonyl group, in other words, Ag+ preferentially coordinated with the C=C bonds. Zhu et al. [27] calculated the molecular electrostatic potential of C18-3 molecule at the level of DFT/B3LYP/6-311G (d, p) using the SMD solvent model of Gaussian view. The results showed that the negative electrostatic potential of the carbonyl group was higher than the C=C bond, but the stability of the functional groups was not investigated. Since both the molecular electrostatic potential and stability of functional group affected its donating electrons ability, the stability of Ag+ interacted with these two functional groups was further investigated in this work based on frontier molecular orbital analysis. The molecular orbitals and the energy levels of C18–2 and C18-3 molecules were displayed in Fig. 2 by single-point calculation using the SMD solvent model at the level of B3LYP/6-311 + G (2d, p) +LANL2DZ of Gaussian view. HOMO is the highest occupied molecular orbital with the donating

2.2.6. Computational details The structure optimization of C18–3 and C18-2 was performed at B3LYP/6-31G (d, p) level of theoretical method using SMD model in Gaussian program. The electronic properties of these two compounds, such as HOMO, LUMO and NLUMO, were also recognized by DFT approach. The structures of the two complexes of C18-3·Ag+ and C18-2·Ag+ were optimized at the level of B3LYP/6-311 + G (d, p) +LANL2DZ. The atoms of C, H and O have been described with 6–311 + G (d, p) basis sets, and Ag+ with LANL2DZ basis sets. Vibrational analyses were performed at the same level to confirm the stationary of the optimized geometries. The results showed that the optimized structures corresponded to a minimum of potential energy surface with no imaginary frequency. Natural bond orbital (NBO) analysis was also carried out for the evaluation of the orbital contribution of all possible donor atoms of ligands to Ag+ for C18-3·Ag+ and C18-2·Ag+, and the bonding and anti-bonding charge transfer was clarified quantitatively using second order perturbation energy. 3. Result and discussion 3.1. Interaction between carrier Ag+ and UFAMEs Silver ions (or other transition metal ions such as cuprous) could form reversible π-bond complexation with C=C bonds [30], which could effectively improve the extraction efficiency and selectivity to unsaturated materials [31,32]. Li et al. [10] illustrated the extraction of UFAMEs predominantly resulted from the coordinative interaction between silver ions and UFAMEs, and the distribution ratios of C18–3 and C18-2 reached 42.0 and 18.4, respectively, at AgBF4 concentration of 0.042 mmol/mL. Dou et al. [33] used SILMs with carrier Ag+ to carry out efficient ethylene/ethane separation, and ethylene permeability and ethylene/ethane selectivity were significantly elevated. These works showed that Ag+ is an effective carrier to promote the separation of saturated/unsaturated compounds. However, although the 221

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 2. Frontier orbitals of methyl linoleate and methyl linolenate molecule.

the complexation with Ag+ by C=C bonds in their molecules. The competition mechanism can be explained by Eq. (4). On the feed side, the reaction proceeded in a positive direction due to stronger complexation ability of C18-3. On the stripping side, the high concentration of 1-hexene prompted the reaction to proceed in the opposite direction. The C18-3·Ag+ was decomplexed and the 1-hexene·Ag+ was formed. The formed 1-hexene·Ag+ transported to the feed side to carry C18-3 again. Thus, we chosen 10 wt% 1-hexene petroleum ether as stripping solvent to increase the transport rate of C18-3.

electrons ability, LUMO is the lowest unoccupied molecular orbital with the accept electrons ability, and NLUMO is the next lowest unoccupied molecular orbital with the accept electrons ability just below LUMO. According to the frontier molecular orbital calculation, it could be seen that HOMO and NLUMO charge density almost covered the C=C bond, and the LUMO density was scattered only over C=O bond. Since HOMO had the strongest electron-donating ability, silver ions preferentially formed π orbitals with C=C bonds. Although the LUMO orbit had the strongest electron accepting ability, the inverse π complexation of Ag+ with the carbonyl group was unstable because the HOMO was far apart from the LUMO located at C= O bond. Through further orbital calculations, the energy level of NLUMO orbital was almost the same as that of LUMO (Table 4), particularly for the C18-3 molecule, indicating that NLUMO had the same ability to accept electrons. Furthermore, NLOMO and HOMO charge density both covered the C=C bond, which indicated that Ag+ could form stable complexation and anti-collaboration on the C=C bond. Thus, based on the NBO analysis and frontier molecular orbital analysis, Ag+ preferentially combined with C=C bonds and the interaction force between them was stronger.

1

3

C18

(3)

3·Ag +

To further increase the permeation rate of C18-3, 1-hexene was added into the stripping solvent. 1-hexene and C18-3 can compete for Table 4 Energy of selected molecular orbitals of methyl linoleate and methyl linolenate molecule. Orbital

NLUMO LUMO HOMO

Energy Methyl linoleate

Methyl Linolenate

0.0637 eV −0.1929 eV −6.6151 eV

−0.1254 eV −0.1331 eV −6.5689 eV

3

C18

3·Ag+ + 1

hexene

(4)

3.1.3. Ag+ concentration According to the action mechanism between Ag+ and UFAMEs, + preferentially interacted with the C=C bonds of UFAME. Ag However, if Ag+ concentration is too high, excess Ag+ may react with the carbonyl group to reduce the selectivity of C18-3, and the interaction between Ag+ and UFAME would be enhanced excessively, which is not conducive to the decomplexation. Thus, the effect of Ag+ concentration on the C18-3 extraction should be investigated. In this work, membrane solvent with a concentration of 0–0.5 g/mL AgBF4 was selected to coat on the supported nylon membrane. The concentration of C18-3 over time in the stripping solvent was shown in Fig. 3a. The purity of C18-3 in the product and the separation factor of C18-3 to C18-1 (SC18-3/C18-1) and C18-3 to C18-2 (SC18-3/C18-2) within 6 h were shown in Fig. 3b. Table 5 listed the permeability of C18–1, C18–2 and C18-3 through different SILMs. Experimental conditions: the initial concentration of fatty acid methyl esters in feed phase: 50 mg/mL, stripping phase: 10 wt% 1hexene petroleum ether, the pore size of nylon membrane: 0.45 μm, IL: [BMIm]BF4, flow rate:40.1 mL∙min−1. Ag+ could promote the mass transfer rate and increase the selectivity to C18-3. Compared with the absence of AgBF4, the C18-3 permeability increased by 3.1 times (0.2 g/mL AgBF4), 6.6 times (0.4 g/ mL AgBF4) and 10.1 times (0.5 g/mL AgBF4), respectively. Ag+ had less influence on C18–2 and C18-1 due to their less C=C bonds. The permeability of C18-2 increased slightly with the increase in Ag+ concentration, with that of C18-1 slightly decreased. No obvious change in the interaction between Ag+ and carbonyl group was found in the range of 0–0.5 g/mL AgBF4. The difference in the permeability of C18–3, C18–2 and C18-1 resulted in the increase in C18-3 selectivity and purity. SC18-3/C18-1 and SC18-3/C18-2 reached up to 12.53 and 8.73, respectively using 0.5 g/mL AgBF4, with the purity of C18-3 reaching 75%. In our work, a SILM with about 0.1 mL membrane solvent containing 0.05 g AgBF4 was used to extract C18-3 from 50 mg/mL FAMEs

3.1.2. Fundamentals of transport The complexation between Ag+ and C18-3 shown in Eq. (3) is a reversible reaction, and the reaction direction can be altered by C18-3 concentration swing. On the feed side, the higher C18-3 concentration contributed to coordination between C18-3 and Ag+, and thus C183·Ag+ was formed and diffused through the SILM to the stripping side. On the stripping side, the reaction proceeded in the reverse direction as C18-3 concentration fell down sharply, which resulted in the decomplexation of C18-3·Ag+ and regeneration of SILM. Ag + transported to the feed side to coordinate with C18-3 again.

Ag + + C18

hexene·Ag + + C18

222

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 3. Effect of AgBF4 concentration on (a) C18-3 concentration in stripping phase, (b) separation factors and purity of C18-3 in the product.

the increase in the alkyl side chain length of imidazole ring. The maximum C18-3 permeability was obtained when the substituent group was butyl. As to the separation performance, [BMIm]BF4 also showed the highest selectivity to C18-3, with SC18-3/C18-1 and SC18-3/C18-2 reaching up to 12.53 and 8.73, respectively. Moreover, the purity of C18-3 increased to almost 75% [BMIm]BF4 was used, which was increased by about 20% compared with other ILs. Overall, the alkyl side chain of IL cation had a significant impact on the extraction of C18-3, which would affect the mass transfer rate and the separation performance of SILM. Since UFAMEs could form hydrogen bonds with the H atoms on the imidazole ring, UFAMEs could be separated from the saturated ones [27]. The alkyl side chain on the imidazole ring affected the van der Waals force and hydrogen bond interaction between cations and anions, which influenced the fundamental characteristics of IL such as polarity, hydrogen bond acidity and basicity, viscosity, density and melting point. Therefore, the interaction between ILs and UFAMEs varied for different alky side chains. The IL polarity, hydrogen bond acidity and basicity were usually described by the empirical scales of Reichard's dye ET(30) and Kamlet-Taft using solvatochromic chemical probes [35–38]. The Kamlet-Taft method has three parameters: hydrogen-bond acidity (α), hydrogen-bond basicity (β) and dipolarity/polarizabilty (π*). Considering these parameters, the extraction ability of different SILMs for UFAMEs could be further explained. The mass transfer rate of SILM was mainly affected by the polarity, viscosity and steric hindrance of IL. The polarity of ILs followed the order: [OMIm]BF4＜[BMIm]BF4＜[HMIm]BF4≈[EMIm]BF4 (Table 7). C18-3 was a low polarity material, which was easier to be dissolved into [OMIm]BF4 and [BMIm]BF4. Besides, due to higher viscosity, [OMIm] BF4 had huger resistance against mass transfer than other ILs. The steric hindrance also increased with increasing alkyl chain length, which was unbeneficial to the formation of hydrogen bonds between UFAMEs and imidazole rings [27]. Therefore, [OMIm]BF4 and [HMIm]BF4 as membrane solvents had lower permeability than that of [BMIm]BF4 although they possessed higher C18-3 solubility. The selectivity to C18-3 was mainly affected by the hydrogen bond interaction between ILs and UFAMEs. With the increase in alkyl chain length, α/π* decreased and β increased. Because higher hydrogen-bond acidity (α) could provide more protons to form hydrogen bonds with UFAMEs, [BMIm]BF4 had the higher interaction with C18-3 compared with [OMIm]BF4 and [HMIm]BF4. In summary, when the alkyl side chain of imidazole ring was the butyl group, SILM showed the highest mass transfer rate and best separation performance.

Table 5 Effect of AgBF4 concentration on the permeability of C18–1, C18–2 and C18-3. CAgBF4 (g/mL)

0.0 0.1 0.2 0.3 0.4 0.5

Permeability ( × 10−9 m/s) C18-1

C18-2

C18-3

1.78 1.76 1.62 1.53 1.31 1.15

1.72 1.74 1.95 1.90 2.31 2.69

1.56 1.79 4.82 4.60 10.33 15.77

mixture (16.7 wt% C18-3) in 20 mL feed solution, with the molar ratio of silver ions to C=C bonds of 0.16:1. As reported by Li et al. [9], AgBF4 (5.5 mg) dissolved in [BMIm]BF4 (1 mL) as extraction phase was selected to extract C18-3 (2 mg) dissolved in hexane (1 mL), with the molar ratio of silver ions to C=C bonds of 1.37:1. The results showed that the amount of Ag+ in membrane extraction was much lower than that in liquid-liquid extraction, which demonstrated that the amount of Ag+ needed in the SILM extraction was minimal, and Ag+ as a carrier could facilitate transport repeatedly. 3.2. The structure of ILs Imidazolium ionic liquid is a low-melting salt composed of an imidazolium ring and an organic or inorganic anion. The difference in IL molecular structure leads to different physicochemical properties [34], e.g., viscosity, dissolving capacity for AgBF4, interaction with UFAMEs, which would have influence on SILM stability, permeability and selectivity. In our study, imidazolium ionic liquids with different carbon chain lengths and anions were selected as membrane solvents, and the separation performance of Ag+ in different ILs was investigated. 3.2.1. The alkyl side chain of IL cation Four ionic liquids with the same anion of [BF4]- were selected to investigate the effect of the length of the alkyl side chain at position 3 on the imidazole ring on the extraction separation of C18-3. The permeability and separation ability of different SILMs were shown in Fig. 4 and Table 6. Experimental conditions: the initial concentration of fatty acid methyl esters in feed phase: 50 mg/mL, stripping phase: 10 wt% 1hexene petroleum ether, AgBF4 concentration in membrane solvent: 0.5 g/mL, the pore size of nylon membrane: 0.45 μm, flow rate: 40.1 mL∙min−1. It could be seen that both the concentration of C18-3 in the stripping solvent and C18-3 permeability first increased and then decreased with

3.2.2. IL anion Zhu et al. [27] selected eight kinds of ionic liquids with the same 223

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 4. Effect of the alkyl side chain length of ILs on (a) C18-3 concentration in stripping phase, (b) separation factors and purity of C18-3 in the product.

side) and decomplexation (on the stripping side), and the hydrogenbond basicity (β) had different effects on them. When β was low, the electron donating ability of ILs was relatively poor, and the interaction between carrier Ag+ and IL decreased, which was more favorable for the complexation of Ag+ with UFAMEs but not conducive to decomplexation. A suitable β value facilitated the complexation/decomplexation between Ag+ and UFAMEs. The results showed [BMIm]BF4 had the highest mass transfer rate and best separation performance, which indicated that the IL with β value of about 0.39 would be an appropriate membrane solvent.

Table 6 Effect of the structure of ILs on the permeability of C18-3. ILs

P × 10−9 (m/s)

[EMIm]BF4 [BMIm]BF4 [HMIm]BF4 [OMIm]BF4 [BMIm]NTf2 [BMIm]PF6 [BMIm]CF3SO3

7.26 15.77 6.92 6.28 5.22 0.65 5.77

3.3. Adjacent phase flow rates

Table 7 Reichard's dye and Kamlet-Taft parameters of imidazolium ionic liquids. Ionic liquid

[EMIm]BF4 [BMIm]BF4 [HMIm]BF4 [OMIm]BF4 [BMIm]NTf2 [BMIm]CF3SO3 [BMIm]PF6

Reichard's dye

Kamlet-Taft

ET(30)

π*

α

β

53.7 52.5 53.7 51.8 51.5 52.3 52.3

– 1.05 0.96 0.93 0.96 1.01 1.03

– 0.627 0.44 0.45 0.63 0.62 0.65

– 0.376 0.6 0.63 0.24 0.47 0.22

The transport process of the FAMEs from the feed phase to the stripping phase involves five consecutives steps [19]: (1) forced convection from the bulk of the feed solution to the feed–membrane interface, (2) partition of the FAMEs between the feed phase and SILM, (3) facilitated transport and molecular diffusion though the SILM to the membrane-stripping interface, (4) partition of the FAMEs between the SILM and the stripping phase, (5) forced convection from the membrane-stripping interface to the bulk of the stripping solvent. The steps of (2), (3) and (4) were affected by the concentration of silver ions. An appropriate Ag+ concentration was beneficial to the increase in partition coefficient of step (2) and (4), as well as the increase in the rate of UFAMEs passing through the pores of SILM via facilitated transport in step (3). The step (4) was also affected by the properties of stripping solution. Besides, increasing forced convection could enhance the mass transfer rate of step (1) and (5). The forced convection could be intensified with the increasing flow rates of the feed solution and stripping solvent. Thus, to investigate the effect of the adjacent phase flow rates on the membrane extraction, the flow rates of the feed phase and stripping phase across the membrane were set to 20 rpm (16.5 mL∙min−1), 40 rpm (26.6 mL∙min−1)，60 rpm (40.1 mL∙min−1), 70 rpm (49.8 mL∙min−1) and 90 rpm (59.3 mL∙min−1).To eliminate the transmembrane pressure and prevent the membrane deformation, the flow rates of adjacent phases were set to the same value. The permeability and separation ability under the different adjacent phase flow rates were shown in Fig. 6 and Table 8. Experimental conditions: the initial concentration of fatty acid methyl esters in feed phase: 50 mg/mL, stripping phase: 10 wt% 1hexene petroleum ether, AgBF4 concentration in membrane solvent: 0.5 g/mL, the pore size of nylon membrane: 0.45 μm, IL: [BMIm]BF4. It could be observed that the concentration of C18-3 in the stripping phase and C18-3 permeability first increased and then decreased with the increasing flow rates, and C18-3 permeability achieved a maximum value of 15.77 × 10−9 m/s at 40.1 mL/min. The change trend of C18-2 permeability was similar to that of C18-3, while the C18-1 permeability

Reference

[37] [37] [37] [37] [37,38] [37,38] [37,38]

cation [EMIm]+ to investigate the effect of anions on the liquid-liquid extraction of C18-3. The results indicated that anions played important roles in extraction of C18-3 in terms of the distribution coefficient and selectivity. In this work, to investigate the effect of anions on the membrane extraction, [BMIm]NTf2, [BMIm]CF3SO3, [BMIm]BF4 and [BMIm]PF6 were chosen as membrane solvents to separate C18-3 from FAMEs. According to Fig. 5, [BMIm]PF6 was not suitable for the separation of C18-3. [BMIm]BF4 as membrane solvent had the highest mass transfer rate, and C18-3 concentration reached 1.24 mg/mL at 6 h which was only 0.50 mg/mL for [BMIm]NTf2 and [BMIm]CF3SO3. Meanwhile, the C18-3 permeability of [BMIm]BF4 membrane was significantly larger than other IL membranes, as shown in Table 6. The selectivity of ILs followed the order: [BMIm]NTf2＜[BMIm]CF3SO3 ＜ [BMIm]BF4. When the anion was BF4−, the purity of C18-3 in the product was increased by 10.66% and 29.48% compared with [BMIm]CF3SO3 and [BMIm]NTf2. The difference in viscosity and HB acidity(α) for [BMIm]NTf2, [BMIm]CF3SO3 and [BMIm]BF4 was slight. However, the difference in hydrogen-bond basicity(β) resulted in the different extraction ability of the three ILs by affecting the interactions between Ag+ and UFAMEs. The liquid membrane extraction contained complexation (on the feed 224

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 5. Effect of IL's anion on (a) C18-3 concentration in stripping phase, (b) separation factors and purity of C18-3 in stripping phase. Experimental conditions: the initial concentration of fatty acid methyl esters in feed phase: 50 mg/mL, stripping phase: 10 wt% 1-hexene petroleum ether, AgBF4 concentration in membrane solvent: 0.5 g/mL, the pore size of nylon membrane: 0.45 μm, flow rate:40.1 mL∙min−1.

Fig. 6. Effect of flow rate on (a) C18-3 concentration in stripping phase, (b) separation factors and purity of C18-3 in the product.

increased with the increase in flow rates. The separation factor and purity of C18-3 in the product also first increased and then decreased, as shown in Fig. 6. The maximum selectivity of C18-3 reached at 40.1 mL/min. The purity of C18-3 in the product at various flow rates was about 71.0% (16.5 mL/min), 75.0% (40.1 mL/min) and 64.8% (59.3 mL/min). The effect of adjacent phase flow rates on the selectivity was mainly due to the effect of flows rates on the mass transfer rate of C18–1, C18–2 and C18-3. As could be seen in Table 8, the flow rates had different effects on their mass transfer ability, and C18-3 permeability was more sensitive to flow rates than C18–1 and C18-2. Increasing adjacent phase flow rates significantly reduced external diffusion resistance. As the flow rates increased to a certain value, the external diffusion resistance was negligible, and the mass transfer resistance in the membrane was dominant. Thus, the permeability of C183 should be stable at relatively high flow rates because the resistance in the membrane was not affected by the external diffusion resistance. However, the C18-3 permeability decreased after the flow rates exceeded 40.1 mL/min, which was due to the change in the SILM at high flow rates. XRD is a powerful tool for detecting the change of membrane solvents’ dosage, which was used to determine the crystalline nature of the blank nylon membrane and the membrane coated with [BMIm]BF4 and Ag+. As shown in Fig. 7a, the diffraction peaks for blank nylon

Table 8 Effect of flow rate on the permeability of C18–1, C18–2 and C18-3. Flow rate (mL/min)

16.5 26.6 40.1 49.8 59.3

Permeability ( × 10−9 m/s) C18-1

C18-2

C18-3

0.46 0.49 0.79 0.84 1.24

0.83 1.31 2.67 2.28 1.80

4.01 6.78 15.77 9.41 5.06

membrane (A membrane) existed at 2θ = 17.8, 20.4, 23.0, 24.0 and 26.1°. By comparison, the diffraction peaks of the nylon membrane coated with [BMIm]BF4 and Ag+ (B membrane) shifted to lower wavenumbers, and the intensity of the peaks decreased significantly, which indicated that incorporation of [BMIm]BF4 and Ag+ could change crystallinity degree of the membrane and the intensity of the diffraction peaks. This was consistent with the results reported by Mahdavi et al. [39]. In addition, the SILMs after 6 h at 60 rpm (C membrane) and 90 rpm (D membrane) operation were analyzed by XRD. The diffraction peaks of C membrane were similar with B membrane. As to the D membrane, the intensity increased and the position 225

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 7. (a) XRD patterns of (A) the blank nylon membrane, (B) initial SILM, (C) SILM after 60rpm operation and (D) SILM after 90 rpm operation. (b) XRD patterns of the SILM after (C) 60 rpm operation, (D) SILM after 90 rpm operation and PDF card (No.04–0783).

of diffraction peaks moved toward to the higher wavenumbers, which indicated that the D membrane solvent was obviously lost. Moreover, the XRD patterns of the C membrane and D membrane in the 2θ range from 30° to 80° were magnified and presented in Fig. 7b. A diffraction peak at 2θ = 38° was observed on the XRD patterns of both membranes. Based on the PDF card (No.04–0783), the diffraction peak corresponds to metallic silver. Similar result was reported by Fallanza et al. who proposed a partial reduction of the silver ions to metallic silver under the light condition [23]. The dosage of metallic silver on the membrane represented the residual amount of carrier Ag+, which could be judged by the intensity of diffraction peak at 2θ = 38°. The peak intensity for C membrane was larger than D membrane, indicating that the solvent loss of C membrane was much lower than D membrane. Thus, based on the XRD analysis, it can be concluded that the membrane solvent and carrier Ag+ were easy to fall off when the flow rates were higher than 59 mL/min, which would result in the reduction of permeability and selectivity of SILM.

As shown in Fig. 8, the mass transfer rate and separation performance of C18-3 for 0.45 μm nylon membrane were better than 0.22 μm nylon membrane. Due to the larger pore area, 0.45 μm membrane could impregnate more membrane solvent and load more carrier silver ions, resulting in higher permeability and selectivity. The permeability of C18-3 for the 0.45 μm membrane was 1.72 times that of 0.22 μm membrane. C18-3 of higher purity in the product was also obtained differed by about 7% under the different pore size membranes. 3.5. Stability of SILM The stability of membrane is an important factor in practical application. However, a major drawback for SILMs is the lack of stability over time owing to the gradual solubilization of the membrane solvent (carrier and organic solvent) in the adjacent phases [20]. In this work, to study the long-term stability of the 0.45 μm nylon membrane coated with [BMIm]BF4 and Ag+, taking 6 h as a cycle, and after the one cycle, the stripping solution and feed solution were replaced with fresh ones. Data were obtained from a 24 h continuous operation, with the results illustrated in Fig. 9 and Table 9. It could be observed that the permeability ability and selectivity of the SILM declined gradually, and C18-3 basically unable to penetrate the membrane pores after three cycles. As could be seen in Fig. 9b and c, the SILM could maintain relatively high separation factors in the first two

3.4. Pore size of supported membrane The composition and structure of supported membrane have important influence on the separation performance and stability. In this work, the effect of pore size of supported nylon membrane with 0.22 μm and 0.45 μm was studied.

Fig. 8. Effect of membrane pore size on (a) C18-3 concentration in stripping phase, (b) separation factors and purity of C18-3 in the product. Experimental conditions: the initial concentration of fatty acid methyl esters in feed phase: 50 mg/mL, stripping phase: 10 wt% 1-hexene petroleum ether, AgBF4 concentration in membrane solvent: 0.5 g/mL, flow rate: 40.1 mL∙min−1, IL: [BMIm]BF4. 226

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Fig. 9. Comparison membrane stability of single and hydrophilic/hydrophobic composite supported membranes (a) concentration of C18-3 in stripping solvent, (b) the separation factor of C18-3 to C18-1, (c) the separation factor of C18-3 to C18-2. Experimental conditions: feed concentration: 50 mg/mL, stripping phase: 10 wt% 1-hexene petroleum ether, IL: [BMIm]BF4, AgBF4 concentration:0.5 g/mL, flow rate: 40.1 mL∙min−1.

cycles, and then the selectivity declined apparently. The SILM completely lost the selectivity after 24 h operation. Compared with the aqueous solution of liquid supported membrane [40], the stability of SILM was greatly improved due to the relatively high viscosity of ILs, but the membrane lifetime was still short. Further increasing the viscosity of the membrane solvent would be detrimental to the permeability of C18-3. An alternative method is reducing the shear force and solubility between SILM and flowing adjacent phases, which could reduce the loss rate of membrane solvent and carrier Ag+. A 0.45 μm hydrophobic PVDF membrane was selected as a protected membrane to form hydrophilic/hydrophobic composite membrane. According to Fig. 9a and Table 9, C18-3 concentration in the stripping phase and the permeability decreased after one cycle, which indicated that the permeation flux of both the single membrane and the

composite membrane was reduced after one cycle. But the concentration of C18-3 obtained in the stripping phase was greatly increased by using the composite membrane. The permeability of single membrane deceased by 68.57% after first cycle, while the composite membrane only decreased by 35.80%. The permeability of the composite membrane at the fourth cycle was comparable to that of the single membrane at the second cycle. The results indicated that the PVDF membrane can effectively protect the SILM by reducing the shear force and solubility between SILM and flowing adjacent phases. Moreover, the selectivity of the composite membrane was significantly higher than that of the single membrane, as shown in Fig. 9b and Fig. 9c. SC18-3/C18-1 and SC18-3/C18-2 for the composite membrane after fourth cycles were 6.88 and 2.38 times that of the single membrane, respectively. It was mainly due to the reduced loss of membrane 227

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al.

Table 9 The permeability of C18-3 through the single membrane and composite membrane. Permeability × 10−9 (m/s)

First cycle (0∼6 h)

Second cycle (6–12 h)

Third cycle (12–18 h)

Forth cycle (18–24 h)

Single membrane Composite membrane

13.46 23.66

4.23 15.19

1.35 7.50

0.77 3.46

solvent and carrier Ag+. As mentioned in 3.1.3, the permeability of C18-3 increased sharply with the increase in Ag+ concentration in SILM, but Ag+ concentration had less effect on the permeability of C18–1 and C18-2. PVDF membrane effectively reduced the loss of the membrane solvent and kept the permeability of C18-3 at a higher value, while the permeability of C18–1 and C18-2 changed slightly, which resulted in the significant improvement of SC18-3/C18-1 and SC18-3/C18-2. Furthermore, the average SC18-3/C18-1 and SC18-3/C18-2 of the single membrane were 6.90 and 6.88 in the second cycle, while those of the composite membrane were 11.75 and 6.12 in the fourth cycle, respectively. It illustrated the lifetime of the composite membrane was doubled compared with the single membrane. Based on the change in permeability and selectivity of the single and composite membrane within 24 h, the composite membrane could effectively increase the service life and stability.

[2] [3]

[4] [5]

[6]

4. Conclusion

[7]

Facilitated transport supported ionic liquid membrane containing carrier Ag+ was used to separate C18-3 from tallow seed oil methyl ester. The mass transfer rate and separation performance were related to IL structure, Ag+ concentration, adjacent phase flow rates and the pore size of supported membrane. [BMIm]BF4 showed the best permeability and selectivity due to the most appropriate viscosity, polarity and hydrogen bond acidity/basicity. Ag+ preferentially coordinated with C=C bond rather than carbonyl group because of the more stable interaction with the C=C bond. The amount of Ag+ and IL in liquid membrane extraction was much lower than traditional liquid-liquid extraction to achieve satisfactory permeability and selectivity of C18-3. The convection mass transfer between the flow phases and the membrane surface could be intensified by higher flow rates, but part of the membrane solvent and carrier Ag+ would fall off. The highest permeability of 15.77 × 10−9 m/s, SC18-3/C18-1 of 12.53 and SC18-3/C18-2 of 8.73 were obtained at 40.1 mL/min. Larger pores not only reduced the mass transfer resistance but also accommodated more IL and carrier Ag+, which was beneficial to increasing the permeability and selectivity. The proper pore size was 0.45μm for the extraction of C18-3. The stability of membrane is an important factor in practical application. The selectivity of SILM was lost after 24 h operation. Hydrophobic PVDF membrane could be used as protected membrane to form hydrophilic/hydrophobic composite membrane to effectively increase the service life and stability. The lifetime of the composite membrane was doubled compared with the single membrane. For future study, it is crucial to find more effective procedures to further avoid the change of membrane and increase the membrane lifetime. The membrane separation method proposed in this study could be applied to other fatty acid methyl esters.

[8]

[9]

[10] [11]

[12] [13] [14] [15] [16]

[17]

[18]

Acknowledgement

[19]

This research is supported by The National Natural Science Foundation of China (863 Program 2014AA022103).

[20]

References [21]

[1] S. Hernando, C. Requejo, E. Herran, J.A. Ruiz-Ortega, T. Morera-Herreras, J.V. Lafuente, E. Gainza, J.L. Pedraz, M. Igartua, R.M. Hernandez, Beneficial effects

228

of n-3 polyunsaturated fatty acids administration in a partial lesion model of Parkinson's disease: the role of glia and NRf2 regulation, Neurobiol. Dis. 121 (2019) 252–262, https://doi.org/10.1016/j.nbd.2018.10.001. C.E. Roynette, P.C. Calder, Y.M. Dupertuis, C. Pichard, N-3 polyunsaturated fatty acids and colon cancer prevention, Clin. Nutr. 23 (2004) 139–151, https://doi.org/ 10.1016/j.clnu.2003.07.005. V. Ciappolino, G. Delvecchio, C. Agostoni, A. Mazzocchi, A.C. Altamura, P. Brambilla, The role of n-3 polyunsaturated fatty acids (n-3PUFAs) in affective disorders, J. Affect. Disord. 224 (2017) 32–47, https://doi.org/10.1016/j.jad.2016. 12.034. M. Abe, H. Komatsu, K. Yamagiwa, H. Tajima, Effect of nonionic surfactants on the low temperature winterization separation of fatty acid methyl ester mixtures, Fuel 190 (2017) 351–358, https://doi.org/10.1016/j.fuel.2016.10.124. Á.G. Solaesa, M.T. Sanz, M. Falkeborg, S. Beltrán, Z. Guo, Production and concentration of monoacylglycerols rich in omega-3 polyunsaturated fatty acids by enzymatic glycerolysis and molecular distillation, Food Chem. 190 (2016) 960–967, https://doi.org/10.1016/j.foodchem.2015.06.061. D.S. No, T.T. Zhao, Y. Kim, M.R. Yoon, J.S. Lee, I.H. Kim, Preparation of highly purified pinolenic acid from pine nut oil using a combination of enzymatic esterification and urea complexation, Food Chem. 170 (2015) 386–393, https://doi.org/ 10.1016/j.foodchem.2014.08.074. Y. Yang, H. Yan, B. Su, H. Xing, Z. Bao, Z. Zhang, X. Dong, Q. Ren, Diffusion coefficients of C18 unsaturated fatty acid methyl esters in supercritical carbon dioxide containing 10% mole fraction ethanol as modifier, J. Supercrit. Fluids 83 (2013) 146–152, https://doi.org/10.1016/j.supflu.2013.08.009. B. Damyanova, S. Momtchilova, S. Bakalova, J. Kaneti, H. Zuilhof, W.W. Christie, Computational probes into the conceptual basis of silver ion chromatography: I. Silver(I) ion complexes of unsaturated fatty acids and esters, J. Mol. Struct. THEOCHEM. 589–590 (2002) 239–249, https://doi.org/10.1016/S0166-1280(02) 00281-6. M. Li, C.U. Pittman, T. Li, Extraction of polyunsaturated fatty acid methyl esters by imidazolium-based ionic liquids containing silver tetrafluoroborate-Extraction equilibrium studies, Talanta 78 (2009) 1364–1370, https://doi.org/10.1016/j. talanta.2009.02.011. M. Li, T. Li, Enrichment of omega-3 polyunsaturated fatty acid methyl esters by ionic liquids containing silver salts, Separ. Sci. Technol. 43 (2008) 2072–2089, https://doi.org/10.1080/01496390802064174. N.E. Belkhouche, M.A. Didi, R. Romero, J.Å. Jönsson, D. Villemin, Study of new organophosphorus derivates carriers on the selective recovery of M (II) and M (III) metals, using supported liquid membrane extraction, J. Membr. Sci. 284 (2006) 398–405, https://doi.org/10.1016/j.memsci.2006.08.011. M. Mohammadi, M. Asadollahzadeh, S. Shirazian, Molecular-level understanding of supported ionic liquid membranes for gas separation, J. Mol. Liq. 262 (2018) 230–236, https://doi.org/10.1016/j.molliq.2018.04.080. M.A. Malik, M.A. Hashim, F. Nabi, Ionic liquids in supported liquid membrane technology, Chem. Eng. J. 171 (2011) 242–254, https://doi.org/10.1016/j.cej. 2011.03.041. Q. Zhang, X. Lu, J. Zhu, J. Ji, Liquid membrane extraction to separate methyl linolenate from tallow catalpa oil, J. Chinese Cereal. Oils. 32 (2017) 95–99, https:// doi.org/10.13427/j.cnki.njyi.2012.09.027. N. Baylan, S. Çehreli, Ionic liquids as bulk liquid membranes on levulinic acid removal: a design study, J. Mol. Liq. 266 (2018) 299–308, https://doi.org/10.1016/j. molliq.2018.06.075. X. Ren, Y. Jia, X. Lu, T. Shi, S. Ma, Preparation and characterization of PDMSD2EHPA extraction gel membrane for metal ions extraction and stability enhancement, J. Membr. Sci. 559 (2018) 159–169, https://doi.org/10.1016/j. memsci.2018.04.033. I. Cichowska-Kopczyńska, M. Joskowska, B. Debski, R. Aranowski, J. Hupka, Separation of toluene from gas phase using supported imidazolium ionic liquid membrane, J. Membr. Sci. 566 (2018) 367–373, https://doi.org/10.1016/j.memsci. 2018.08.058. B.L. Gadilohar, G.S. Shankarling, Choline based ionic liquids and their applications in organic transformation, J. Mol. Liq. 227 (2017) 234–261, https://doi.org/10. 1016/j.molliq.2016.11.136. L.J. Lozano, C. Godínez, A.P. de los Ríos, F.J. Hernández-Fernández, S. SánchezSegado, F.J. Alguacil, Recent advances in supported ionic liquid membrane technology, J. Membr. Sci. 376 (2011) 1–14, https://doi.org/10.1016/j.memsci.2011. 03.036. Y. Dong, H. Guo, Z. Su, W. Wei, X. Wu, Pervaporation separation of benzene/cyclohexane through AAOM-ionic liquids/polyurethane membranes, Chem. Eng. Process. Process Intensif. 89 (2015) 62–69, https://doi.org/10.1016/j.cep.2015.01. 006. M. Matsumoto, Y. Inomoto, K. Kondo, Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids, J. Membr. Sci.

Journal of Membrane Science 585 (2019) 218–229

X. Lu, et al. 246 (2005) 77–81, https://doi.org/10.1016/j.memsci.2004.08.013. [22] M.Y. Abdelrahim, C.F. Martins, L.A. Neves, C. Capasso, C.T. Supuran, I.M. Coelhoso, J.G. Crespo, M. Barboiu, Supported ionic liquid membranes immobilized with carbonic anhydrases for CO2transport at high temperatures, J. Membr. Sci. 528 (2017) 225–230, https://doi.org/10.1016/j.memsci.2017.01.033. [23] M. Fallanza, A. Ortiz, D. Gorri, I. Ortiz, Experimental study of the separation of propane/propylene mixtures by supported ionic liquid membranes containing Ag +-RTILs as carrier, Separ. Purif. Technol. 97 (2012) 83–89, https://doi.org/10. 1016/j.seppur.2012.01.044. [24] E. Jean, D. Villemin, M. Hlaibi, L. Lebrun, Heavy metal ions extraction using new supported liquid membranes containing ionic liquid as carrier, Separ. Purif. Technol. 201 (2018) 1–9, https://doi.org/10.1016/j.seppur.2018.02.033. [25] S. Agarwal, M.T.A. Reis, M.R.C. Ismael, M.J.N. Correia, J.M.R. Carvalho, Application of pseudo-emulsion based hollow fibre strip dispersion (PEHFSD) for the recovery of copper from sulphate solutions, Separ. Purif. Technol. 102 (2013) 103–110, https://doi.org/10.1016/j.seppur.2012.09.026. [26] A. Khakpay, P. Scovazzo, Reverse-selective behavior of room temperature ionic liquid based membranes for natural gas processing, J. Membr. Sci. 545 (2018) 204–212, https://doi.org/10.1016/j.memsci.2017.09.068. [27] J. Zhu, X. Lu, D. Zhao, Z. Dong, J. Ji, Role of cosolvents in enhancing the performance of ILs for extraction of linolenic acid from tallow seed oil, J. Mol. Liq. 242 (2017) 308–313, https://doi.org/10.1016/j.molliq.2017.07.013. [28] X. Lu, J. Zhu, X. Qian, J. Ji, X. Qian, Separation of methyl linolenate and its analogues by functional mixture of imidazolium based ionic liquid-organic solventcuprous salt, Chin. J. Chem. Eng. 3 (2018) 58–81, https://doi.org/10.1016/j.cjche. 2018.05.009. [29] E.U. EU, 14103 Fat and Oil Derivatives – Fatty Acid Methyl Esters (FAME) – Determination of Esters and Methyl Linoleate, (2003). [30] B.J.G. Traynham, F. Sehnert, Ring Size and Reactivity of Cyclic Olefins : Complexation with Aqueous Silver Ion vol. 78, (1953), pp. 4024–4027, https://doi. org/10.1021/ja01597a042. [31] H. Xiao, Z. Yao, Q. Peng, F. Ni, Y. Sun, C.X. Zhang, Z.X. Zhong, Extraction of squalene from camellia oil by silver ion complexation, Separ. Purif. Technol. 169

(2016) 196–201, https://doi.org/10.1016/j.seppur.2016.05.041. [32] B. Jiang, H. Dou, B. Wang, Y. Sun, Z. Huang, H. Bi, L. Zhang, H. Yang, Silver-Based Deep Eutectic Solvents as Separation Media: Supported Liquid Membranes for Facilitated Olefin Transport, (2017), https://doi.org/10.1021/acssuschemeng. 7b01092. [33] H. Dou, B. Jiang, M. Xu, J. Zhou, Y. Sun, L. Zhang, Supported Ionic Liquid Membranes with High Carrier Efficiency via Strong Hydrogen-Bond Basicity for the Sustainable and Effective Olefin/paraffin Separation vol. 193, (2019), pp. 27–37, https://doi.org/10.1016/j.ces.2018.08.060. [34] L.Z. Cheong, Z. Guo, Z. Yang, S.C. Chua, X. Xu, Extraction and enrichment of n-3 polyunsaturated fatty acids and ethyl esters through reversible π-π Complexation with aromatic rings containing ionic liquids, J. Agric. Food Chem. 59 (2011) 8961–8967, https://doi.org/10.1021/jf202043w. [35] S. Spange, R. Lungwitz, A. Schade, Correlation of molecular structure and polarity of ionic liquids, J. Mol. Liq. 192 (2014) 137–143, https://doi.org/10.1016/j.molliq. 2013.06.016. [36] M.A. Ab Rani, A. Brant, L. Crowhurst, A. Dolan, M. Lui, N.H. Hassan, J.P. Hallett, P.A. Hunt, H. Niedermeyer, J.M. Perez-Arlandis, M. Schrems, T. Welton, R. Wilding, Understanding the polarity of ionic liquids, Phys. Chem. Chem. Phys. 13 (2011) 16831–16840, https://doi.org/10.1039/c1cp21262a. [37] C.F. Poole, N. Lenca, Green sample-preparation methods using room-temperature ionic liquids for the chromatographic analysis of organic compounds, TrAC Trends Anal. Chem. (Reference Ed.) 71 (2015) 144–156, https://doi.org/10.1016/j.trac. 2014.08.018. [38] M.J. Muldoon, C.M. Gordon, I.R. Dunkin, Investigations of solvent–solute interactions in room temperature ionic liquids using solvatochromic dyes, J. Chem. Soc. Perkin Trans. 2 (2001) 433–435, https://doi.org/10.1039/b101449h. [39] H.R. Mahdavi, N. Azizi, M. Arzani, T. Mohammadi, Improved CO2/CH4 separation using a nanocomposite ionic liquid gel membrane, J. Nat. Gas Sci. Eng. 46 (2017) 275–288, https://doi.org/10.1016/j.jngse.2017.07.024. [40] O. Bakhtiari, S. Hashemi Safaee, Industrial grade 1-butene/isobutane separation using supported liquid membranes, Chem. Eng. Res. Des. 123 (2017) 180–186, https://doi.org/10.1016/j.cherd.2017.05.012.

229