Preparation and Characterization of Polysulfone-graft-4

Mar 17, 2015 - A novel polymer polysulfone (PSF)-graft-4′-aminobenzo-15-crown-5-ether (AB15C5) (PSF-g-AB15C5) for lithium isotope separation was ...
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Preparation and Characterization of Polysulfone-graf t-4′aminobenzo-15-crown-5-ether for Lithium Isotope Separation Feng Yan,†,‡ Hongchang Pei,†,§ Yanchun Pei,†,§ Tuanle Li,†,§ Jianxin Li,*,†,§ Benqiao He,†,§ Yu Cheng,†,§ Zhenyu Cui,†,§ Dongfa Guo,∥ and Jianyong Cui∥ †

State Key Laboratory of Separation Membranes and Membrane Processes, ‡School of Environmental and Chemical Engineering, and School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, People’s Republic of China ∥ Center of Analysis, Beijing Research Institute of Uranium Geology, Beijing 100029, People’s Republic of China §

ABSTRACT: A novel polymer polysulfone (PSF)-graft-4′-aminobenzo-15-crown-5-ether (AB15C5) (PSF-g-AB15C5) for lithium isotope separation was prepared from PSF and AB15C5 as starting materials via nucleophilic substitution reaction. The chemical structure and properties of PSF-g-AB15C5 polymers were characterized by FT-IR, 1H NMR, XPS, and TGA. The polymers obtained were used for lithium isotope separation by solid−liquid extraction. The effects of the immobilization amount of crown ether grafting on PSF, the type of counteranion of lithium salt, and the kind of solvent on the single stage separation factor were explored. Results showed that the single stage separation factor was only 1.002 ± 0.002 for AB15C5 in the traditional liquid−liquid extraction system of H2O−LiCl/CHCl3−AB15C5, whereas the single stage separation factor increased from 1.003 ± 0.001 to 1.015 ± 0.002 with the increase of the immobilization amount of crown ether from 0.23 to 0.79 mmol g−1 on PSF-gAB15C5 polymers in the solid−liquid extraction system of CH3OH−LiCl/PSF-g-AB15C5 polymers. The order of the single stage separation factor obtained in different lithium salts was LiI > LiBr > LiClO4 > LiCl. Further, the maximum value of single stage separation factor was 1.031 ± 0.002 for the extraction system of CH3NO2−LiCl/PSF-g-AB15C5 polymers. Moreover, 6Li and 7Li were enriched in the polymer phase and the solution phase, respectively.

1. INTRODUCTION Natural lithium is composed of two stable isotopes, lithium-6 (6Li, 92.5%) and lithium-7 (7Li, 7.5%).1 Both lithium isotopes play a key role in the nuclear power industry. 6Li is indispensable for nuclear fusion reactions due to the fact that 6 Li generates tritium (T) and helium (He) by neutron bombardment, and 7Li is commonly used as pH controller and coolant of the nuclear fusion reactor.2 Therefore, the development of a green and highly efficient lithium isotope separation method is of great importance.3 The mercury amalgam method is the first industrial-scale application for lithium isotope separation.4,5 However, the huge volumes of toxic mercury used in this technique bring about serious biological and environmental problems. To avoid the use of toxic mercury, many other techniques such as laser,6,7 electromigration,8−10 solvent extraction,11−15 chromatography,16−22 and membrane separation23,24 have been developed. Among them, solvent extraction and chromatography methods using crown ethers or polymerized crown ethers as lithium isotope receptor have been proven to be an alternative method with favorable separation factor.15,25,26 For example, benzo-15-crown-5 ether is widely chosen as an extractant of lithium isotope separation because its cavity size is close to the ionic diameter of Li+.27 Nishizawa et al.28 reported that the single stage separation factors of lithium isotope obtained by using liquid−liquid extraction with benzo-15crown-5 ether in chloroform (CHCl3) as the extractant under the extraction temperature of 25 °C were 1.002 ± 0.002 for LiCl, 1.014 ± 0.002 for LiBr, and 1.026 ± 0.002 for LiI. Although single stage liquid−liquid extraction is a simple process, the factors such as the large volume use of organic © 2015 American Chemical Society

solvent and a low distribution coefficient limit its commercial application. For this reason, a solid−liquid extraction method by column chromatography packed with appropriate resins of polymerized crown ethers as well as the relative multistage system was developed for the practical enrichment of isotopes. For instance, Kim et al.29 reported a separation factor of 1.053 obtained using styrene-divinylbenzene copolymer with monobenzo-15-crown-5 ether upon column chromatography with 0.9 cm (ID) × 25 cm (height) and 5% (v/v) H2O in acetonitrile as an eluent. Furthermore, Kim et al.30 had also investigated the lithium isotope separation using aminobenzo-15-crown-5 ether bonded Merrifield peptide resin. By column chromatography using 1.0 mol L−1 NH4Cl solution as an eluent and LiCl as lithium salt, a single stage separation factor of 1.026 was obtained. What is more, silica beads were also used as substrates for immobilization of crown ethers. Ban et al.31 synthesized a benzo-15-crown-5 resin by condensation polymerization with phenol and formalin in high porous silica beads for lithium isotope separation. The separation factor obtained was 1.0127 at 35 °C. Similarly, Zhou et al.13 prepared benzo-15-crown-5 and imidazolium doped mesoporous silica for separating lithium isotope. The maximum single stage isotope separation factor in the solid−liquid extraction was up to 1.046 ± 0.002. Obviously, the solid−liquid extraction method using the resins bonded benzo-15-crown-5 ether exhibits a good Received: Revised: Accepted: Published: 3473

January 23, 2015 March 17, 2015 March 17, 2015 March 17, 2015 DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

Article

Industrial & Engineering Chemistry Research Scheme 1. Synthesis Route of Chloromethylation PSF Polymer

methanol under stirring and the polymer was precipitated from methanol. The separated precipitate was redissolved in dichloromethane, and then reprecipitated to remove inpurities. CMPSF was obtained after filtering and drying under reduced pressure at 80 °C at least 24 h to reach a constant weight. 2.3. Synthesis of 4′-Aminobenzo-15-crown-5-ether. 4′-Aminobenzo-15-crown-5-ether (AB15C5) was prepared by the Ungaro method36 as shown in Scheme 2. 0.9 g (2.88 mmol)

separation efficiency for lithium isotope. At the same time, the resin bonded crown ether can be reusable. However, the resins usually have a small porous structure with nanoscale leading to a slow diffusion rate and low mass transfer efficiency. This is the bottleneck of resins bonded crown ether for lithium isotope. Consequently, an alternative approach is to prepare a new polymeric material bonded crown ether with a microporous structure so as to improve mass transfer during the solid−liquid extraction. Polysulfone (PSF) as an amorphous thermoplastic polymer with high tensile strength, good chemical stability, and easy ability to form a penetrating pore structure has been widely used as membrane material and supporting membrane material.32 In addition, PSF is easy to chemically modify. Trisca-Rusu et al.33 prepared formyl modified PSF membrane at first, and then immobilized crown ethers onto the PSF membrane by combining the formyl group of PSF membrane with amino crown ether. Finally, a type of membrane obtained could be used for the detection of interest cations from biological fluids (Na+ and K+). Similarly, Cozan et al.34 synthesized a new copolyether sulfone having copper-(II) chelate units as pendant groups by a chemical modification reaction of chloromethylated PSF with sodium salt of copper(II) bis(2,4-dihydroxybenzaldehyde). PSF could be grafted to target groups after being modified through chloromethylation previously. It provided a new method for synthesizing functionalized PSF polymers for lithium isotope separation. The aim of the present study is to prepare a novel PSF-graf t4′-aminobenzo-15-crown-5-ether (PSF-g-AB15C5) for lithium isotope separation. The chemical structure and properties of PSF-g-AB15C5 polymers were characterized by FT-IR, 1H NMR, XPS, and TGA. Meanwhile, PSF-g-AB15C5 polymers obtained were used for lithium isotope separation by solid− liquid extraction. The effects of the immobilization amount of crown ether grafting on PSF, the type of counteranion of lithium salt, and the kind of solvent on the single stage separation factor were explored.

Scheme 2. Synthesis Route of 4′-Aminobenzo-15-crown-5ether

of 4′-nitrobenzo-15- crown-5-ether, 10% Pd/C catalyst (0.36 g), and hydrazine (7.5 mL) were dissolved in 150 mL of ethyl alcohol. The mixture was stirred at 80 °C for 6 h. The mixture then was filtered, and the solvent was removed by vacuum. AB15C5 was obtained as a brown solid in the yield of 70%. 2.4. Synthesis of Polysulfone-graft-4′-aminobenzo15-crown-5-ether Polymers. Polysulfone-graft-4′-aminobenzo-15-crown-5-ether (PSF-g-AB15C5) polymers were prepared through the unimolecular nucleophilic substitution reaction between the chloromethyl groups of CMPSF and the amino groups of AB15C5 as shown in Scheme 3. In a typical reaction, 2.0 g of CMPSF was dissolved in 40 mL of N,Ndimethylformamide (DMF) in a round-bottom flask. 0.6 g of AB15C5 and 0.15 g of anhydrous potassium carbonate (acid acceptor) were added into the CMPSF solution. After being stirred at 60 °C for 9 h, the mixture was precipitated in ethanol. The brown resin CMPSF-g-AB15C5 polymers were obtained after being washed by DMF three times and drying under reduced pressure at 80 °C at least 24 h. 2.5. Characterizations of CMPSF, AB15C5, and PSF-gAB15C5. The 1H NMR spectra of CMPSF and CMPSF-gAB15C5 were recorded on a Bruker DRX-500 NMR instrument (300 MHz). Deuterated chloroform was used as solvent, and tetramethylsilane as internal standard. The chemical composition and structure of PSF, CMPSF, and PSF-g-AB15C5 polymers were further characterized by FT-IR spectrometer (Tensor 37, Bruker) with a resolution of 2 cm−1 and a spectral range of 3500−400 cm−1 and X-ray photoelectron spectroscopy (XPS) (K-alpha, ThermoFisher) employing a monochromated Al Kα X-ray source (hν = 1486.6 eV). The thermal behaviors of PSF, CMPSF, and PSF-g-AB15C5 polymers were analyzed with thermogravimetric analysis (TGA) (STA409 PC, Netzsch) at a heating rate of 10 °C min−1 under nitrogen flow. The immobilization amount (IA) of crown ether was determined by Vario EL/micro cube elemental analyzer. IA is defined as eq 1:

2. EXPERIMENTAL SECTION 2.1. Materials. PSF with a molecular weight cutoff of 81 kDa was purchased from Dalian Polysulfone Plastic Co. Ltd. (Dalian, China). 1,4-Bis(chloromethoxy) butane was purchased from Xi’an Lanjing Biotechnology Co., Ltd. (Xi’an, China) and used as received without further purification. All other reagents were of analytical grade and obtained from Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin, China) and were used as received. 2.2. Synthesis of Chloromethylated Polysulfone. Chloromethylated polysulfone (CMPSF) was prepared following the procedures described by Du et al.35 as illustrated in Scheme 1. PSF (5 g) was dissolved in dichloromethane (90 mL) into a round-bottom flask equipped with a magnetic stirrer and thermometer. The chloromethylation reagent, 1,4-bis(chloromethoxy) butane (21 mL), and the catalyst, SnCl4 (2 mL), were added to the PSF solution to react for 3 h at 25 °C. Once the reaction was completed, the mixture was poured into 3474

DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

Article

Industrial & Engineering Chemistry Research Scheme 3. Synthesis Route of PSF-g-AB15C5 Polymers

IA =

wN % × 1000 14

(1)

where wN% is the nitrogen content of the graft polymers determined by elemental analyzer, and 14 is the atomic weight of nitrogen. 2.6. Lithium Isotope Separation. To determine the single stage separation factor of lithium isotope separation by PSF-gAB15C5 polymers, a batch method of solid−liquid extraction was employed. PSF-g-AB15C5 polymers were washed with ultrapure water until no lithium ions were detected in the supernatant by ICP-AES (Varian 715) before they were used. A certain amount of the PSF-g-AB15C5 polymers was added into a 30 mL polyethylene vial with a polyethylene screw cap. Next, 1 mol L−1 lithium salt of methanol solution (20 mL) was added for solid−liquid extraction on a reciprocal shaking bath at 100 strokes min−1 for 5 h at room temperature. After complexing with lithium ion, the PSF-g-AB15C5 polymers were taken out and dried under vacuum at room temperature. A certain amount of PSF-g-AB15C5 polymers complexed with lithium ion then was carbonized in a muffle furnace at 380 °C for 4 h. The residue was dissolved in deionized water. The ratio of isotopic 6Li/7Li in the solution could be measured by highresolution inductively coupled plasma-mass spectrometry (HRICP-MS) (ELEMENT) (Finnigan MAT, Germany). The single stage separation factor of lithium isotope, α, is defined by eq 2:37 α=

Figure 1. FT-IR spectra: (a) PSF, (b) CMPSF, and (c) PSF-gAB15C5 (KBr pellet method).

successfully grafted onto the macromolecular chains of PSF polymer. To further confirm the above observations, the chemical structures of CMPSF and PSF-g-AB15C5 were characterized by 1 H NMR as illustrated in Figure 2. As shown in Figure 2a, the

([6 Li]/[7Li])s ([6 Li]/[7Li])o

(2)

where [6Li]/[7Li] represents the isotopic ratio. The subscripts of s and o refer to the stationary phase (polymer) and the initial solution phase, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of PSF-g-AB15C5. As mentioned in section 2.4, the PSF-g-AB15C5 polymers were obtained by the unimolecular nucleophilic substitution reaction of CMPSF and AB15C5. The FT-IR spectra of PSF, CMPSF, and PSF-gAB15C5 are shown in Figure 1. All characteristic absorption bands of PSF were displayed in Figure 1a. The peaks at 1580 and 1478 cm−1 were ascribed to the skeletal vibration of benzene ring. The peaks at 1325 and 1298 cm−1 were identified to be the asymmetric stretching of SO bond. The antisymmetric vibration of C−O−C was at 1239 cm−1. The peaks at 1378 and 966 cm−1 were assigned to the bending vibration of −CH3 groups of the aliphatic chains. In the spectra of CMPSF (Figure 1b), a weak absorption peak appeared at 750 cm−1, which was ascribed to the characteristic peak of −CH2Cl. In addition, in the spectra of PSF-g-AB15C5 as shown in Figure 1c, the characteristic bands of N−H appeared at 3427 cm−1 (stretching vibration) and 1670 cm−1 (in-plane bending vibration). These results indicate that crown ethers have been

Figure 2. 1H NMR spectra: (a) CMPSF in CDCl3 and (b) PSF-gAB15C5 in DMSO. 3475

DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

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Industrial & Engineering Chemistry Research

can be seen from Figure 4a that PSF polymer displayed a good thermal stability with a degradation temperature at approximately 520 °C under a nitrogen atmosphere. Obviously, the thermal stability of CMPSF and PSF-g-AB15C5 appreciably reduced as a result of the PSF modification. Two thermal degradation stages of CMPSF could be observed in Figure 4b. The first major mass loss at 300−350 °C was attributed to the decomposition of CH2Cl groups. The second mass loss at 456−530 °C was ascribed to the degradation of PSF main chains. Equally, PSF-g-AB15C5 polymers exhibited a three-step degradation process (Figure 4c). The first mass loss near 120 °C was attributed to the evaporation of water in the polymer. The second mass loss in the range of 250−320 °C was due to the decomposition of the crown ether molecule and the unreacted CH2Cl groups. Similarly to PSF, the third mass loss of PSF-g-AB15C5 polymers at 433−500 °C was ascribed to the decomposition of PSF main chains. Although the thermal stability of the PSF-g-AB15C5 polymers slightly decreased after the introduction of the crown ether molecule, it still keeps the good thermal stability. On the basis of the above analyses, PSF-g-AB15C5 polymers have been successfully synthesized. The following work is to investigate the properties of lithium isotope separation. 3.2. PSF-g-AB15C5 Polymers for Lithium Isotope Separation. It is known that a certain amount of crown ethers shows the ability to form stable complexes with alkali metals. The selectivity of crown ethers for metal ions is based upon the number of ether oxygen atoms in the ring. Benzo15crown-5-ether could form a complex with the Li+ ions having a relatively high stability constant.15 Herein, PSF-g-AB15C5 polymers were employed to investigate its properties for lithium isotope separation. As mentioned in section 2.6, a certain amount of PSF-g-AB15C5 polymers was added to 1 mol L−1 lithium salt of methanol solution (20 mL) for solid−liquid extraction. The effects of the immobilization amount of crown ether on PSF, the kind of lithium salt (type of counteranion), and the kind of solvent on single stage separation factor were explored as follows. 3.2.1. Effect of the Immobilization Amount of Crown Ether. Table 1 showed the effect of the immobilization amount

chemical shift at 4.53 and 7.83 ppm could be assigned to the methylene protons of −CH2Cl and the four meta protons on the phenyl ring closest to the sulfonyl group, respectively. The percent of chloromethylation per repeat unit (degree of substitution, DS) was determined from the area of the proton peak at 4.53 ppm in the 1H NMR spectra, relative to that of the reference peak at 7.83 ppm. The result indicated that the DS value of the PSF-CH2Cl sample was approximately 1.0. That is to say, each repeat unit was grafted a −CH2Cl group. Similarly, the 1H NMR spectra of PSF-g-AB15C5 are shown in Figure 2b. The peaks at 2.50, 2.75, and 2.85 ppm (designated as e, f, and g) were attributed to the ring of the crown ether. The results obtained from 1H NMR confirm the observations from FT-IR. In addition, the chemical compositions of the polymers were further analyzed through XPS analysis as illustrated in Figure 3.

Figure 3. XPS survey spectra: (a) PSF, (b) CMPSF, and (c) PSF-gAB15C5.

It can be seen in Figure 3 that three major emission peaks at 168, 286, and 532 eV were presented for S 2p3, C 1s, and O 1s for PSF (Figure 3a), respectively. The emission peak at 200 eV (Figure 3b) was assigned to Cl 2p3 of CMPSF polymer. As shown in Figure 3c, there was a new emission peak at 400 eV in the spectrum of PSF-g-AB15C5 polymers, which was attributed to the C−N group of AB15C5. In sum, XPS analysis further confirms that PSF-g-AB15C5 polymers were obtained. To investigate the thermal stability of the polymers, the thermal behaviors of PSF, CMPSF, and PSF-g-AB15C5 polymers were analyzed with TGA as present in Figure 4. It

Table 1. Effect of the Immobilization Amount of Crown Ether on Lithium Isotope Separationa immobilization amount of crown ether/mmol g−1 0.23 0.35 0.51 0.64 0.73 0.79

single stage separation factor (α) 1.003 1.007 1.008 1.010 1.013 1.015

± ± ± ± ± ±

0.001 0.002 0.002 0.001 0.003 0.002

abundance of Li in polymer phase/%

6

7.663 7.689 7.698 7.711 7.731 7.747

enrichment isotope in polymer phase 6

Li Li 6 Li 6 Li 6 Li 6 Li 6

The lithium salt solution used was 1.0 mol L−1 LiCl of methanol solution. The abundance of 6Li in original lithium salt was 7.64%.

a

of crown ether on PSF on lithium isotope separation. It can be seen from Table 1 that the single stage separation factor increased from 1.003 ± 0.001 to 1.015 ± 0.002 with an increase of the immobilization amount of crown ether from 0.23 to 0.79 mmol g−1. At the same time, the polymers showed the selective enrichment of 6Li. The reason is that the lithium ions adsorbed in the resins had lower hydratability than those in the solution

Figure 4. TGA results of PSF (a), CMPSF (b), and PSF-g-AB15C5 polymers (c). 3476

DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

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Industrial & Engineering Chemistry Research

coordinating complex. Thus, Li+−crown ether complex is more easily formed if the counteranion is softer. 3.2.3. Effect of the Kind of Solvent. During the liquid−solid extraction of PSF-g-AB15C5 polymers in various organic solvents, the performance of lithium isotope separation is listed in Table 3 (LiCl as lithium salt). The order of the single

phase.20 Meanwhile, the abundance of 6Li in polymer phase increased from 7.663% to 7.731% with an increase of the immobilization amount of crown ether on PSF. It is relative to the stereochemical structure of crown ether on the macromolecular chains. In PSF-g-AB15C5 polymers, crown ether molecules were grafted on the PSF backbone dispersedly, which would increase the chance for each crown ether to complex with lithium ions. For comparison, the lithium isotope separation performance of free crown ether was conducted by a typical liquid−liquid extraction method as reported in the literature.34 The single stage separation factor α obtained was 1.002 ± 0.002 for AB15C5 in the liquid−liquid extraction system of H2O−LiCl/ CHCl3−crown ether, whereas the α increased from 1.003 ± 0.001 to 1.015 ± 0.002 with the increase of the immobilization amount of crown ether (Table 1) in the solid−liquid extraction system. The results indicated that the separation performance of AB15C5 has been improved after the crown ethers were bonded to the macromolecular chains of PSF polymer. 3.2.2. Effect of the Kind of Lithium Salt. PSF-g-AB15C5 polymers with the immobilization amount of crown ether of 0.79 mmol g−1 were added to 1 mol L−1 lithium salt of methanol solution for liquid−solid extraction. The lithium salts used included LiCl, LiClO4, LiBr, and LiI. The effect of the kinds of lithium salts on lithium isotope separation was illustrated in Table 2. It can be found from Table 2 that the

Table 3. Effect of the Kinds of Solvents on Lithium Isotope Separationa

kinds of solvents methanol ethanol propylene carbonate acetonitrile nitromethane

LiCl LiClO4 LiBr LiI

single separation factor (α)

abundance of Li in original lithium salt/%

abundance of 6 Li in polymer phase/%

± ± ± ±

7.64 7.64 7.62 7.72

7.747 7.762 7.726 7.770

1.015 1.017 1.021 1.025

0.002 0.002 0.001 0.002

6

abundance of 6 Li in polymer phase/%

single separation factor

19.0 19.0 15.1

7.711 7.774 7.791

1.010 ± 0.002 1.019 ± 0.002 1.021 ± 0.002

6

14.1 2.7

7.829 7.860

1.027 ± 0.003 1.031 ± 0.002

6

enrichment isotope in polymer phase Li Li 6 Li 6

6

Li Li

The concentration of LiCl was 1.0 mol L−1. The immobilization amount of crown ether was 0.51 mmol g−1. The abundance of 6Li in original lithium salt was 7.64%. a

stage separation factor was nitromethane (CH3NO2) > acetonitrile (CH3CN) > propylene carbonate (CH3C2H3O2CO) > ethanol (CH3CH2OH) > methanol (CH3OH). In other words, the single stage separation factor of the polymer increased from 1.010 ± 0.02 to 1.031 ± 0.02 with the decrease of the solvent donicity from 19.00 for methanol to 2.70 for nitromethane. It also can be seen from Table 3 that the polymer showed the selective enrichment of 6 Li in different kinds of solvents. The abundance of 6Li in polymer phase reached 7.860% when nitromethane was used as a solvent. The result indicated that the lithium isotope separation performance of PSF-g-AB15C5 polymers was related to the donicity of the solvent. It can be explained by entropy and enthalpy changes during the liquid−solid extraction. Shamispur et al.41 investigated the complexation of several macrocyclic compounds with Cs+ in several solvents with low to medium donicity. They found that there existed systematic trade-offs between entropy change and enthalpy change during the liquid−solid extraction in the case of different solvents. The entropy change favored the complexation in the solvents with high solvating power, whereas the entropy change had no effect on the complexation in the case of the solvents with low solvating power. The enthalpy stabilization is explained by the expenditure of less energy in the cation desolvation step in the solvent of lower donicity. As a result, the single stage separation factor of PSF-g-AB15C5 polymers increased with the decrease of donicity of the solvent. The results were in good agreement with the previous work of Kim.42

Table 2. Effect of the Kinds of Lithium Salts on Lithium Isotope Separationa kinds of lithium salts

donicity of the solvent

enrichment isotope in polymer phase 6

Li Li 6 Li 6 Li 6

a

The organic solvent was methanol, the concentration of lithium salt was 1.0 mol L−1, and the immobilization amount of crown ether was 0.79 mmol g−1.

order of the single stage separation factor was LiI > LiBr > LiClO4 > LiCl. For example, the single stage separation factor of the polymer obtained in the solution of LiI was up to 1.025 ± 0.002. Similarly, the polymers showed the selective enrichment of 6Li under the condition of different lithium salt solutions (Table 2). The abundance of 6Li in polymer phase reached 7.770% in the LiI solution. These results obtained are in agreement with the previous liquid−liquid extraction work of Nishizawa.38 This could be explained from the electrostatic interaction of counteranion and from the point of view of the hard and soft acid and base.39 On one hand, the electrostatic interaction of counteranion affected the stability of Li+−crown ether complex. The greater was the charge density, the stronger was the gravitational attraction between Li+ and counteranion, which reduced the coordination bond of the Li+−crown ether complex. On the other hand, Br− and I− are soft bases. The order of softness character is I− > Br− > ClO4− > Cl−.40 That is to say, the soft ion has a greater tendency to stay away from the solvent. Crown ether must exclude the solvent (methanol) action to make an inner sphere

4. CONCLUSIONS A novel polymer PSF-g-AB15C5 was successfully prepared by nucleophilic substitution reaction. The PSF-g-AB15C5 polymers exhibited good performance for lithium isotope separation. The single stage separation factor increased with an increase in the immobilization amount of crown ether on the polymer. Further, PSF-g-AB15C5 polymers performed a high single stage separation factor (1.010 ± 0.002) when the solid− liquid extraction was carried out with CH3OH−Li+ system with a soft counteranion. The order of the single stage separation factor for different lithium salts was LiI > LiBr > LiClO4 > LiCl. 3477

DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

Article

Industrial & Engineering Chemistry Research

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Furthermore, the single stage separation factor of PSF-gAB15C5 polymers increased with the decrease of the solvent donicity during the solid−liquid extraction. The maximum value of single stage separation factor was 1.031 ± 0.002 for the extraction system of CH3NO2−LiCl/PSF-g-AB15C5 polymers. It was also found that the lighter isotope, 6Li, was enriched in the polymer phase, whereas the heavy one, 7Li, was concentrated in the solution phase.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 22 83955798. Fax: +86 22 83955055. E-mail: jxli@ tjpu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the National Natural Science Foundation of China (Grant nos. 51303130, 21376176, and 21174104) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (Grant no. IRT13084).

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NOMENCLATURE codes = full name or meaning PSF = polysulfone AB15C5 = 4′-aminobenzo-15-crown-5-ether PSF-g-AB15C5 = polysulfone-graf t-4′-aminobenzo-15crown-5-ether CMPSF = chloromethylated polysulfone IA = immobilization amount of crown ether α = single stage separation factor of lithium isotope δ = chemical shift in nuclear magnetic resonance spectroscopy

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REFERENCES

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DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479

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DOI: 10.1021/acs.iecr.5b00314 Ind. Eng. Chem. Res. 2015, 54, 3473−3479