A hydrophobic pervaporation membrane with hierarchical microporosity for high-efficient dehydration of alcohols

Chemical Engineering Science 206 (2019) 489–498

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A hydrophobic pervaporation membrane with hierarchical microporosity for high-efficient dehydration of alcohols Peng Liu 1, Mengmeng Chen 1, Yiqiang Ma, Chuan Hu, Qiugen Zhang ⇑, Aimei Zhu, Qinglin Liu Department of Chemical & Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A synthetic strategy for hydrophobic

polymer pervaporation membrane is presented.
The membrane has the hierarchical structure of micropores-microporesmesopores.
The membrane shown high permeability and selectivity for dehydration of alcohols.

a r t i c l e

i n f o

Article history: Received 12 March 2019 Received in revised form 28 May 2019 Accepted 31 May 2019 Available online 1 June 2019 Keywords: Pervaporation Organic dehydration Polymeric membrane Polymer of intrinsic microporosity

a b s t r a c t Pervaporation is a promising membrane technology for separation of aqueous organic solution particularly azeotrope and close-boiling point compounds. However, most of pervaporation membranes are based on hydrophilic polymers that suffer decline of separation performance due to their swelling in aqueous solution. Here we present a copolymer synthetic strategy for preparation of hydrophobic polymer pervaporation membrane with hierarchical porosity via uniting both rigid and flexible segments. The synthesized membrane has well-defined micropores and high free volume faction of 8.03% with cavity size of 0.476 nm, and demonstrate exceptional performance with high permeability and selectivity for dehydration of alcohols. In the pervaporation of ethanol-water azeotrope, the membrane displays the high permeabtion flux of 74.6 lm kg m2 h1 and water selectivity of 34 (separation factor of 732) at 30 °C. The developed strategy has excellent potential for synthesizing copolymers for high-efficient pervaporation membranes. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Pervaporation is a promising separation technology for liquid mixture particularly azeotrope and close-boiling point compounds. Compared with conventional processes like distillation, the pervaporation dehydration of organics has been proved to be a more favorable industrial process due to several advantages like easy

⇑ Corresponding author. 1

E-mail address: [email protected] (Q. Zhang). These authors contributed equally to this work.

https://doi.org/10.1016/j.ces.2019.05.057 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

operations and effective energy saving (Mahdi et al., 2015; Ong et al., 2016). Generally, hydrophilic polymer membranes are employed in this process due to the preferential adsorption of water molecules. However, they easily swell in aqueous solution, leading to reducing the water selectivity and the mechanical strength (Chapman et al., 2008). To control their swelling, various methods have been developed, such as chemical cross-linking, blending with other polymers and filling with inorganic fillers (Cheng et al., 2017; Deng et al., 2016; Li et al., 2015a,b; Liu and Kentish, 2018). Unfortunately, the ‘‘trade-off” relationship usually exists between the selectivity and permeability, and results in a

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Fig. 1. The structure of PIM-PSF copolymer. (a,b) The chain structure of PIM-1 and PSF segments respectively. (c) A molecular model of PIM-PSF copolymer.

decrease of the permeation flux or selectivity (Chapman et al., 2008; Park et al., 2017). Although some works have overcome this weakness, the membranes still have low permeability due to inherent packing of flexible chains (Cheng et al., 2017; Khoonsap et al., 2017; Ong et al., 2016). Therefore, it is strongly required for industrial dehydration process to develop a high permeable membrane with good water selectivity and structure stability (Ong et al., 2016; Park et al., 2017). Microporous materials with well-defined size-selective channels and pores are considered as the promising next-generation molecular sieving materials for separation membranes (Gin and Noble, 2011; Koros and Zhang, 2017; Paul, 2012). Among of them, inorganic and organic ordered open frameworks are representative microporous molecular sieving materials with precisely defined pore architecture, such as zeolites and metal-organic frameworks (Li et al., 2017; Richter et al., 2017; Varoon et al., 2011). Due to their tailored pore size, high surface area and chemical functionality, they potentially have excellent intrinsic selectivity and permeability in membrane separation. However, they are usually brittle and difficult to prepare large scale separation membranes (Li et al., 2017). To utilize their high permeability, mixed matrix membranes containing these ordered frameworks are developed by many researchers for pervaporation (Fan et al., 2014; Li et al., 2015a,b; Li et al., 2016; Liu et al., 2011). In contrast, microporous polymers have the great advantage over classical inorganic molecular sieve materials due to solution-processable capacity (Budd et al., 2004; Carta et al., 2013). However, most microporous polymers with high permeability show insufficient selectivity for molecule separation, because they possess ill-defined voids that effectively limits size selectivity (Budd et al., 2004; Carta et al., 2013; Holst et al., 2010; Qiao et al., 2014). Therefore, it remains a great challenge to develop a general, effective approach for preparing polymeric pervaporation membranes with well-defined microporosity that own both the permeability and selectivity (Song et al., 2014). As a representative class of microporous polymers, polymers of intrinsic microporosity (PIMs) have attracted great attention recently in the preparation of high performance membranes for fuel cells, gas separation, nanofiltration, pervaporation (Budd et al., 2004; Carta et al., 2013; McKeown and Budd, 2006).

Currently, the PIMs membranes have been devolved to remove volatile organic compounds (VOCs) from aqueous solutions via pervaporation due to their strong hydrophobicity (Gao et al., 2017; McKeown et al., 2005; Song et al., 2014; Wu et al., 2016). They generally show the high permeability and moderate VOCs selectivity. According to the sorption-diffusion mechanism, the water selectivity of the PIMs membranes should be determined by the diffusion process in the dehydration of alcohols, that is, affected by the membrane’s microstructure. For instance, the sorption selectivity is 0.11 whereas the diffusion selectivity is 390 for the PIM-1 membrane in the pervaporation of 81.5 wt% ethylene glycol aqueous solution (Wu et al., 2015). Therefore, the PIMs membrane with well-defined free volume cavities is expected to provide the high flux as well diffusion selectivity for separation of water/alcohols. In this work, we report a versatile strategy for preparation of polymer pervaporation membrane by introducing flexible segments to tune the microporosity of PIMs. The resultant membrane has hierarchical porosity and shows high-efficient separation performance for pervaporation dehydration of alcohols. As shown in Fig. 1, the PIM-PSF copolymer is comprised of hard PIM-1 and flexible polysulfone (PSF) segments, in which the latter is employed to adjust the microporosity and mechanical property of the PIM-1 membrane. Effect of PIM-1 segment on membrane structure and performance was investigated in detail. 2. Material and methods 2.1. Materials The monomers of PIM-1, TTSBI (5, 50 , 6, 60 -tetrahydroxy-3, 3, 30 , 30 -tetramethyl-1, 10 -spirobisindane) and TFTPN (2, 3, 5, 6-tetra fluoroterephthalonitrile) were purchased from J&K Scientific Ltd. and Sigma-Aldrich, respectively. The former was purified by recrystallization with methanol and chloroform. The monomers of PSF, FPS (bis (4-fluorophenyl) sulfone) and BPA (bisphenol A) were purchased from Tokyo Chemical Industry and Aladdin Ltd., respectively. Besides, boron tribromide, 3, 4-dimethoxyphenol, K2CO3 and solvents (analytical grade) were obtained from Shanghai Chemical Reagent Store (Shanghai, China).

P. Liu et al. / Chemical Engineering Science 206 (2019) 489–498

2.2. Synthesis of PIM-PSF copolymer The PSF-OH oligomer was synthesized via the conventional synthetic procedure of polycondensation reaction, as depicted in Fig. 2. Typically, FPS (2.2800 g, 10 mM), BPA (2.2860 g, 9 mM) and K2CO3 (3.7260 g, 30 mM) were added into dimethylacetamide (DMAc) (50 mL) containing toluene (8 mL) as azeotrope with byproduct water contained in a three-neck round bottom flask equipped with mechanical stirrer and water knockout vessel. The resulting mixture was heated to 140 °C and refluxed for 3 h in N2 atmosphere to remove the by-product H2O and toluene. Then the mixture was further heated to 165 °C, and maintained at this temperature for 12 h to produce the PSF oligomer with –F as the terminal groups. Next, 3, 4-dimethoxyphenol (0.4312 g, 2.8 mM) was added and continually reacted at 165 °C for 1 h to transform the terminal groups to –OCH3. The resulting solution was cooled down to room temperature, precipitated in the methanol/water mixture (50:50% v/v, 500 mL). The precipitate was collected and purified through Soxhlet extraction in methanol for 12 h, and then dried in vacuum oven at 80 °C for 12 h. The obtained white powders of PSF-OCH3 were reacted with boron tribromide in chloroform at 0 °C for 6 h to turn –OCH3 to

491

–OH, and then precipitated in methanol (500 mL). The resulting precipitate was purified through Soxhlet extraction in methanol for 12 h, and subsequently dried in the vacuum oven at 80 °C for 12 h. The obtained solids, expected PSF-OH oligomer, were characterized by size exclusion chromatography (SEC) and 1H Nuclear Magnetic Resonance (1H NMR). The average molecular weight (Mw) and molecular dispersity (Mw/Mn) of the PSF-OH, is 5085 Da and 1.51, respectively. The PIM-PSF copolymer was synthesized from PSF-OH oligomer and PIM-1 monomers. Typically, the PSF-OH oligomer (5.085 g,1 mM), TTSBI (3.4000 g, 10 mM), TFTPN (2.2000 g, 11 mM) and K2CO3 (4.554 g, 33 mM) was added into dimethylformanide (70 mL) in a three-neck round bottom flask equipped with a mechanical stirrer and Dean–Stark trap. The resulting mixture was heated to 80 °C and maintained at this temperature for 72 h under N2 atmosphere. After that, the mixture was cooled to the room temperature, and precipitated in the methanol/water mixture (50:50% v/v, 800 mL). The resulting precipitate was purified through Soxhlet extraction in methanol for 24 h, and dried at 80 °C in vacuum oven for 24 h. Finally, the obtained yellowishbrown solids, expected the PIM-PSF copolymer, were characterized by SEC and 1H NMR.

Fig. 2. The synthesis route of PIM-PSF copolymer.

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Ji ¼ C i  JN

2.3. Preparation and characterizations of PIM-PSF membrane The PIM-PSF membrane was prepared by typical casting/ solvent evaporation process. The homogeneous solution of 5 wt% PIM-PSF copolymer was prepared by dissolving the PIM-PSF block polymer in chloroform, and then cast in a Teflon holder. After solvent evaporation at 30 °C in the vacuum oven for 3 days, the formed membrane was peeled off and further dried at 30 °C for 12 h to remove any residual solvent. The morphology and microstructure of the prepared membrane was observed by scanning electron microscope (SEM) (S-4800), Atomic Force Microscopy (AFM) (Nanoscope-MultiMode/Dimension), and Transmission electron microscopy (TEM) (TECNAI F30). X-ray powder diffraction (XRD) was determined on an X’pert PRO for physical structure of the membrane. Positron annihilation spectra (PALS) (ORTEC) was utilized to explore the free volume properties of the membranes. Thermo gravimetric analysis (TGA) was characterized with a TG209F1 at a heating rate of 10 °C min1 in nitrogen atmosphere. Surface hydrophobicity was measured using static water angle measurements (SL200B, Shanghai Solon Tech Inc. Ltd). 2.4. Swelling and pervaporation experiments A dried PIM-PSF membrane was weighed and immersed in the aqueous alcohol solution for 48 h. Then, the swollen membrane was taken out, wiped with tissue paper to remove the residual solution on the membrane surface and quickly weighed. The swelling degree (SD) was calculated by

SDð%Þ ¼

mw  md  100 md

ð1Þ

where md and mw is the weight (kg) of membrane before and after swelling, respectively. The pervaporation measurements were performed by the method described in our previous work (Wu et al., 2016). The ethanol solution of 4.4–30 wt% and other two kind of alcohol solution of 10 wt% water was used as feed. Both feed composition and temperature were investigated for separating the ethanol aqueous solution. The permeate was collected in a liquid nitrogen cold trap and determined by gas chromatography (GC) with a thermal conductivity detector (TCD) (GC-950, Shanghai Haixin Chromatographic Instruments, China). The thickness-normalized total flux (JN, kg lm m2 h1), partial fluxes (Ji) and separation factor (b) were calculated by

J N ¼ Dm  l=ðA  Dt Þ

a

ð2Þ

b¼

ð3Þ


C w;p = 1  C w;p
C w;f = 1  C w;f

ð4Þ

where Dm and l denote the mass of the permeation (kg) and the membrane thickness (lm), A is the effective area of the membrane (m2), and Dt is the permeation time (h), Ci is the concentrations of component i in the permeate (wt%), Ci,f and Cw,p is the water concentration in the feed and permeate respectively. The membrane permeance (Pi/l, gpu) (1 gpu = 1  106 cm3 (STP) cm2 s1 cmHg) and water selectivity (aij) are calculated by

Pi ji ¼ l P i0  Pil

aij ¼

ð5Þ

Pi Pi =l ¼ P j Pj =l

ð6Þ

where ji is the molar flux of component i (cm3 (STP) cm2 s1), Pi0 and Pil are the partial pressure of component on the feed side and permeate side of the membrane, which were obtained from the ASPEN software 11.1. 3. Results and discussion 3.1. Synthesis and characterization of the PIM-PSF copolymer The synthetic route of the PIM-PSF copolymer is displayed in Fig. 2. Firstly, the PSF oligomer (PSF-OCH3) was produced by condensation between bis (4-fluorophenyl) sulfone (FPS) and bisphenol A (BPA), and terminated by 3, 4-dimethoxyphenol. Then the hydroxyl-terminated PSF oligomer (PSF-OH) was obtained from PSF-OCH3 by reacting with boron tribromide. The 1H NMR spectrum is showed in Fig. 3a. The peaks emerging from 6.92 to 7.86 ppm are in response to the characteristic aromatic protons. The signal at 1.61 ppm is attributed to the methyl group which is associated with monomer BPA. The peaks at 9.01 and 9.29 ppm is associated with the protons of hydroxyl on the benzene rings (ArOH). The results of gel permeation chromatography (GPC) reveal the average molecular weight (Mw) of the PSF-OH is 5.1 kDa, that is, the average degree of polymerization (DP) is about 9. Finally, the PIM-PSF copolymer was synthesized by polymerizing of PSF-OH with PIM-1 monomers based on the synthesis of PIM-1. The yellowish brown powder was obtained after further purification, which is expected PIM-PSF copolymer.

b

2

5

1

7 6

43

DMSO H2O

9

H2O

85 6

1

6 3,4

7

1,2

9

89

CDCl3 5

7

56 7

4 2 1 3

3 4

8

7

6

5

ppm

4

3

2

1

9

8

7

2

6

5

4

3

2

1

ppm

Fig. 3. The 1H NMR spectra of (a) PSF-OH oligomers and (b) PIM-PSF copolymer. DMSO and CDCl3 were used as solvents in this characterization.

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The chemical structure was determined by the 1H NMR spectrum, in which all of characteristic peaks are appealed, as shown in Fig. 3b. The peak emerging around 6.88–7.86 ppm is in response to the characteristic aromatic protons on the benzene ring in PSF, and that around 6.44–6.83 ppm belongs to PIM. The peak at 2.18–2.34 ppm is associated with the protons of methylene that comes from monomer 5,50 ,6,60 -tetrahydroxy-3,3,30 ,30 -tetrame thyl-1,10 -spirobisindane (TTSBI). The peaks at 1.65 ppm and 1.13-1.45 ppm are attributed to the methyl group associated with PSF and PIM, respectively. Moreover, according to the integral ratio of peaks 7 to 1, the ratio of PSF to PIM-1 segments was estimated to 11:10, which is close to the mole ratio (1:1) of monomers. From these, it is concluded that the expected PIM-PSF copolymer was synthesized successfully. 3.2. Preparation and physicochemical properties of the PIM-PSF membrane As described above, the PIM-PSF membrane was prepared by the conventional casting/solvent evaporation process. As shown in Fig. 4a, the formed PIM-PSF membrane is deep yellow and semi-transparent. Specially, this membrane exhibits excellent flexibility, which is other than the PIM-1 membrane (Budd et al., 2004). The same to other dense polymer membranes, the membrane has a smooth surface from SEM observation (Fig. 4a). AFM also reveals the same result (Fig. 4b). Obviously, the membrane is very smooth with root mean square surface roughness of 2.61 nm, which is slightly higher than 0.78 nm of the PIM-1 membrane (Chen et al., 2018). Meanwhile, the membrane is homogeneous in the internal matrix from cross-sectional observation (Fig. 4c). Obviously, there are some close mesopores inside the membrane because of the fast evaporation rate in the membrane forming process, which are advantageous for diffusion of penetrants.

493

Fig. 4d shows the static water contact angle on the PIM-PSF membrane. The membrane has strong hydrophobicity with a water contact angle of 110°, which is slightly higher than 102° of the PIM1 membrane (Richter et al., 2017). This hydrophobic membrane will not easily swell in aqueous solution, and thus should has excellent mechanical stability during the pervaporation. Furthermore, the membrane exhibits good thermal stability with an initial decomposition temperature of about 450 °C from thermogravimetric analysis (TGA), as shown in Fig. 5a. 3.3. Hierarchical microporosity of the PIM-PSF membrane TEM was employed to observe the microstructure of the PIM-PSF membrane. Before observation, the membrane was dyed using tungstate ions (1 M Na2WO4 aqueous solution), and sliced into 60 nm-thick sample by a microtome for observation after packaged with epoxy resin. From the TEM images, a regular structure of well-defined micropores is appeared in the membrane (Fig. 6 and Fig. S1). This is attributed to the regular stacking of the copolymer chains, and forming the reticulate free volume cavities. To reveal their size precisely, positron annihilation spectra (PALS) was performed. The factional free volume and cavity size is 8.026% and 0.476 nm (radius), respectively. These well-defined micropores are favorable for selective diffusion of water molecules in the PIM-PSF membrane, just like in the zeolite membranes (Fan et al., 2014). Fig. 5b displays the powder X-ray diffraction (XRD) spectrum of PIM-PSF membrane. Obviously, the PIM-PSF membrane has a characteristic peak around 18°, which corresponds to the d-spacing of the PIM-PSF chains. The d-spacing is 4.9 Å that is smaller than 6.6 Å of PIM-1 chains in the membrane. As a result, compared to the pure PIM-1 membrane, the as-formed membrane has higher diffusion selectivity for smaller molecules. That is, the water permselectivity will improve in the dehydration of alcohols. From

Fig. 4. Surface properties and inner structure of PIM-PSF membrane. (a) A SEM image of the membrane surface with a digital photo of the membrane. (b) A 3D AFM image of the membrane surface. (c) A SEM image of the membrane cross-section. (d) A static water contact angle on the membrane surface at 25 °C.

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b

a 100

PIM-PSF PIM-1

4000

Intensity

80

Weight (%)

5000

PIM-PSF PIM-1

60

3000

40

2000

20

1000 10

~6.6 Å

0

100

200

300

400

500

600

700

800

T (°C)

12

14

~4.9 Å

16

18

~3.8 Å

20

22

24

26

28

30

2 ()

Fig. 5. (a) TGA curves and (b) XRD spectra of the PIM-PSF and PIM-1 membranes.

Fig. 6. TEM images of PIM-PSF membrane with (a) low and (b) high magnification.

these, it can be concluded that the PIM-PSF membrane has the hierarchical structure of micropores-micropores-mesopores, as shown in Fig. S2. Firstly, the mesopores formed in the membrane preparation, are dispersed in the membrane matrix (Fig. 4c). Second, the well-defined micropores resulted from the regular stacking of the copolymer chains with the size of about 0.95 nm are also dispersed uniformly in the matrix (Fig. 6a). Further smaller micropores (0.49 nm) of chains packing are existed in the membrane matrix (Fig. 5b). This hierarchical pore structure is favorable for fast diffusion of water molecules through the membrane (Smuleac et al., 2010). 3.4. Dehydration of alcohols using the PIM-PSF membrane The synthesized PIM-PSF membrane was used to separate the water-alcohol mixtures via pervaporation. As shown in Fig. 7a, the membrane shows excellent separating efficiency in the pervaporation of ethanol solution containing 4.4–30.0 wt% water. The water content is more than 97.0% in the permeated stream, which is above the 56.0–64.5 wt% by using the pure PIM-1 membrane, and far higher than that in the gas phase during distillation. For instance, the permeation flux of 74.6 kg lm m2 h1 and water selectivity of 34 was presented in the pervaporation of the

water-ethanol azeotrope. Correspondingly, the separation factor is 732 calculated via the Eq. (4). To further reveal dehydration performance of the membrane, three kinds of alcohols of 10 wt% water were separated, namely methanol, ethanol and isopropanol. The results are showed in Fig. 7b. The membrane shows ultrahigh permeation flux of up to 103.4 kg lm m2 h1. Depending on molecule weight and size of permeants, the permeation flux is highest for separating the methanol solution and lowest for isopropanol solution. Conversely, the membrane has lowest selectivity of 4 for separating the methanol solution while highest of 148 for the isopropanol solution. Specially, the water content is 97.60 and 99.33 wt% in the permeated stream for separating ethanol and isopropanol solutions. Therefore, it can be concluded that the PIM-PSF membrane was successfully used to remove water from alcohols fast and high-selectively via pervaporation. The hierarchical porous structure is an important characteristic of the PIM-PSF membrane, providing molecular-sieving transport channels to the water molecules, as shown in Fig. 8. In the pervaporation of water-ethanol mixture, ethanol molecules preferentially adsorb on the hydrophobic membrane surface due to the strong interaction with the membrane and further diffuse through the membrane matrix. In this process, alcohol molecules will absorb on the surface of well-defined micropores (about 0.95 nm

P. Liu et al. / Chemical Engineering Science 206 (2019) 489–498

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Fig. 7. Separation performance of the PIM-PSF membrane. (a) A phase diagram in the pervaporation of ethanol aqueous solution at 30 °C compared to that of the PIM-1 membrane and the vapour-liquid equilibrium at 1 atm. (b) Separation performance in the pervaporation of alcohols with 10 wt% water at 30 °C.

Fig. 8. Transport mechanism of permeants in the PIM-PSF membrane for separating water-ethanol mixture.

diameter), resulting in the formation of water transport channels at the center of the micropores. As a result, permeant molecules fast diffuse in the membrane matrix, and then are size-sieved by well-defined micropores and chains packing micropores (0.476 nm), leading to ultrafast and highly-selective permeation of water molecules. Furthermore, water molecules will gather in the mesopores during their diffusion through the membrane. Finally, the water molecules are removed fast and highselectively to produce the highly purified ethanol. 3.5. Effects of feed composition and temperature on the separation performance Effect of feed composition on the pervaporation performance of the membrane was investigated by using four proportions of the water content in the ethanol solution, namely 4.4%, 10%, 20%, and 30 wt%, as shown in Fig. 9a. Differently from the great increase in both total and partial fluxes of hydrophilic membranes (Liu and Kentish, 2018), the total flux almost keeps a superior level of 80 kg lm m2 h1 with increasing water content, and the partial flux of water increases slightly while that of ethanol decreases. This is because the swelling degree of the membrane has almost no changed with increasing water content due to its hydrophobicity. Since the water content increases in the feed, the permeance of water increases linearly while that of ethanol almost no changes, as shown in Fig. 9b. As a result, the water selectivity enhances from

34 to 60 with the water content from 4.4 to 30.0 wt%. Moreover, the water content is more than 97% in the permeate stream (Fig. 9a). Besides, the membrane has long-time stability in the pervaporation process. As shown in Fig. S3, the permeaion flux and separation factor almost is no change in the 36 h operation. Therefore, it can be seen that the membrane shows excellent structurestability and high-efficient dehydration performance for aqueous solutions. Furthermore, effect of feed temperature was investigated for separation of water-ethanol azeotrope. As shown in Fig. 9c, the total flux almost keeps consistent as the feed temperature increases from 30 to 60 °C, in which the partial flux of ethanol continually increases while that of water diseases slightly. The increment of the ethanol flux is principally derived from three reasons: the increment of the driving force on the feed side, the accelerated mobility of polymer chains and the free volume, and the improvement of the desorption of the ethanol molecules. The accelerated mobility of the chains and free volume provide an incremental pathway to the ethanol molecules. The increment of the driving force on the feed side and the improvement of the diffusion of the adsorbed molecules have a synergistic effect of the passing for the ethanol molecules. Due to the intrinsic hydrophobicity of the membrane, the swelling degree keep in an inferior level of about 11% and the difference could be negligible with increasing feed temperature. Therefore, the effect of the accelerated mobility of polymer chains and the free volume to the

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Fig. 9. Effects of feed composition and temperature on the pervaporation dehydration of ethanol. (a,b) The pervaporation of ethanol aqueous solution at 30 °C. (c,d) The pervaporation of ethanol-water azeotrope.

increment of the ethanol flux is negligible to the other two reasons. Meanwhile, the increment of ethanol flux leads to a decrease of available diffusion channels for water molecules. This is reason that the water permeance remarkably decreases with increasing feed temperature (Fig. 9d). As a result, the water selectivity

decreases from 34 to 12. However, the water concentration still is more than 92 wt% in the permeated stream (Fig. 9c). Table 1 list separation performance of the PIM-PSF membrane and other membranes reported recently in the pervaporation of water-ethanol mixtures. Obviously, the newly developed PIM-PSF

Table 1 Pervaporation dehydration of ethanol solution using PIM-PSF membranes and other membranes reported recently. Membranes

Ethanol in feed (wt%)

Temperature (°C)

JN (kg lm m2 h1)

b

Reference

T-PESU H-PESU GO filled PVA Matrimid/hPIM-1 Torlon/hPIM-1 Plasma-modified PVA PVA Sericin/PVA blend Crosslinked PVA Crosslinked PAAHCl/PVA Crosslinked PAAHCl/PVA Polybenzoxazole Sulfonated PSF 6FDA-NDA/DABA polyimide Crosslinked CS/PVP blend PIM-PSF PIM-PSF PIM-PSF

85.00 85.00 90.00 85.00 85.00 95.60 93.75 90.00 90.00 95.00 88.40 85.00 90.00 85.00 85.00 90.00 95.60 95.60

60 60 40 60 60 60 60 60 40 70 70 22 25 25 50 30 30 60

0.82 4.35 5.48 6.01 1.38 1.40 2.52 2.93 12.52 2.58 60.60 2.46 210.00 27.45 18.00 78.15 74.60 78.59

936 483 263 10 81 110 430 125 69 1880 1410 85 98 33 170 373 732 263

Xu et al. (2018) Xu et al. (2018) Castro-Muñoz et al. (2019) Yong et al. (2017) Yong et al. (2017) Rafik et al. (2003) Kanse and Dawande (2017) Gimenes et al. (2007) Praptowidodo (2005) Namboodiri and Vane (2007) Namboodiri and Vane (2007) Ong et al. (2012) Chen et al. (2009) Zhang et al. (2013) Le et al. (2012) This study This study This study

T-PESU, polytrimethylphenylethersulfone; H-PESU, hydrophilic polyethersulfone-co-Pluronic; GO, graphene oxide; PVA, poly(vinyl alcohol); hPIM-1, hydrolyzed PIM-1; PAA, poly(allylamine hydrochloride); CS, Chitosan; PVP, polyvinylpyrrolidone.

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membranes have the promising separation performance, not only exhibit comparable separation factor but also higher normalized flux than most other membranes, and possibly have great potential for industrialization. 4. Conclusions In summary, the hydrophobic pervaporation membrane of PIM-PSF copolymer was synthesized to high-efficiently dehydration of alcohols. The membrane has the hierarchical micropore structure, allowing fast and selective transport of water molecules. In the pervaporation, the membrane shows ultrahigh water flux of up to 72.45 and 64.49 kg lm m2h1 with the water selectivity of 34 and 148 for separating the water–ethanol azeotrope and isopropanol solution of 10 wt% water, respectively. Compared with hydrophilic polymer membrane, the PIM-PSF membrane has excellent stability of structure and performance in the aqueous solution. The water content is more than 97% and 92% in the permeated stream when separating ethanol solution of above 4.4 wt% water at 30 °C and the water-ethanol azeotrope at less than 60 °C, respectively. The synthesized PIM-PSF membrane has a great potential application in the pervaporation to remove water from organic solutions. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 21676220, 21736009 and 21878253) and the Fundamental Research Funds for the Central Universities (No. 20720170032). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2019.05.057. References Budd, P.M., Elabas, E.S., Ghanem, B.S., Makhseed, S., McKeown, N.B., Msayib, K.J., Tattershall, C.E., Wang, D., 2004. Solution-processed, organophilic membrane derived from a polymer of intrinsic microporosity. Adv. Mater. 16, 456–459. Carta, M., Malpass-Evans, R., Croad, M., Rogan, Y., Jansen, J.C., Bernardo, P., Bazzarelli, F., Mekeown, N.B., 2013. An efficient polymer molecular sieve for membrane gas separations. Science 339, 303–307. Castro-Muñoz, R., Buera-González, J., De La Iglesia, Ó., Galiano, F., Fíla, V., Malankowska, M., Rubioa, C., Figolib, A., Télleza, C., 2019. Towards the dehydration of ethanol using pervaporation cross-linked poly(vinyl alcohol)/graphene oxide membranes. J. Membr. Sci. 582, 423–434. Chen, S.H., Liou, R.M., Lin, Y.Y., Lai, C.L., Lai, J.Y., 2009. Preparation and characterizations of asymmetric sulfonated polysulfone membranes by wet phase inversion method. Eur. Polym. J. 45, 1293–1301. Cheng, X., Pan, F., Wang, M., Li, W., Song, Y., Liu, G., Yang, H., Gao, B., Wu, H., Jiang, Z., 2017. Hybrid membranes for pervaporation separations. J. Membr. Sci. 541, 329–346. Chapman, P.D., Oliveira, T., Livingston, A.G., Li, K., 2008. Membranes for the dehydration of solvents by pervaporation. J. Membr. Sci. 318, 5–37. Chen, M.M., Soyekwo, F., Zhang, Q.G., Hu, C., Zhu, A.M., Liu, Q.L., 2018. Graphene oxide nanosheets to improve permeability and selectivity of PIM-1 membrane for carbon dioxide separation. J. Ind. Eng. Chem. 63, 296–302. Deng, Y.H., Chen, J.T., Chang, C.H., Liao, K.S., Tung, K.L., Price, W.E., Yamauchi, Y., Wu, K.C., 2016. A drying-free, water-based process for fabricating mixed-matrix membranes with outstanding pervaporation performance. Angew. Chem. Int. Edit. 55, 12793–12796. Fan, H., Shi, Q., Yan, H., Ji, S., Dong, J., Zhang, G., 2014. Simultaneous spray selfassembly of highly loaded ZIF-8-PDMS nanohybrid membranes exhibiting exceptionally high biobutanol-permselective pervaporation. Angew. Chem. Int. Edit. 53, 5578–5582.

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