Homogenous polyelectrolyte complex membranes incorporated with strong ion-pairs with high pervaporation performance for dehydration of ethanol

Homogenous polyelectrolyte complex membranes incorporated with strong ion-pairs with high pervaporation performance for dehydration of ethanol

Journal of Membrane Science 435 (2013) 71–79 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science journal homepage: www.el...

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Journal of Membrane Science 435 (2013) 71–79

Contents lists available at SciVerse ScienceDirect

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

Homogenous polyelectrolyte complex membranes incorporated with strong ion-pairs with high pervaporation performance for dehydration of ethanol Xue-San Wang a, Quan-Fu An a,n, Qiang Zhao a, Kueir-Rarn Lee b, Jin-Wen Qian a, Cong-Jie Gao c,d a

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan c Department of Chemical Engineering and Bioengineering, Zhejiang University, Hangzhou 310027, China d The Development Center of Water Treatment Technology, Hangzhou 310012, China b

a r t i c l e i n f o

abstract

Article history: Received 20 October 2012 Received in revised form 15 January 2013 Accepted 4 February 2013 Available online 17 February 2013

Solution-processable polyelectrolyte complexes (PECs) incorporated with both sulfate (OSO3) groups and carboxylic acid groups (COOH) were prepared from sulfated sodium carboxymethyl cellulose (SCMC) and poly(diallydimethylammonium chloride) (PDDA). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were utilized to characterize the chemical structures of SCMC and the compositions of SCMC/PDDA PECs. Membranes were made from PECs (SPECMs), and their surface properties were studied by field emission scanning electron microscopy and contact angle characterizations. Moreover, SPECMs were subjected to pervaporation dehydration of aqueous ethanol solution. Effects of SPECMs’ chemical compositions and operation conditions on pervaporation performance were studied and discussed in terms of membrane structures. It was found that OSO3 groups were incorporated into SPECMs in the form of sulfate–amine ion-pairs, and they improved both the selectivity and flux in the dehydration of aqueous ethanol solution. For instance, the flux and water content in permeate for SPECM-0.34 in dehydration of 10 wt% water–ethanol mixture at 70 1C were 1760 g m  2 h  1 and 98.7 wt%, respectively, both of which were much higher than those for pristine PEC membranes with no sulfate groups. Moreover, the high separation performance of SPECMs was stable and both the tensile strength and elongation at break increased with increasing strong ion-pairs content. For instance, the SPECM-0.34 tensile strength and elongation at break were 64.5 MPa and 9.33%. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.

Keywords: Sulfated sodium carboxymethyl cellulose Polyelectrolyte complex Pervaporation membrane Dehydration

1. Introduction Pervaporation is a membrane separation technology that is characteristically advantageous because of its energy-saving characteristic and ability to efficiently separate of azeotropic mixtures or liquid mixtures with close boiling points [1,2]. Major applications of pervaporation include dehydration of organics, recovery of organics, and separation of organic mixtures [3]. Recently, much interest has been focused on renewable biofuels due to the limited fossil fuels resources and the global climate challenge [4]. Bio-fuels consist of biodiesel, bio-alcohol, biogas and some other minor components. Ethanol is one of the major bio-fuels that can substitute for petroleum, which is being usage globally [5]. Consequently, the dehydration of ethanol by pervaporation is attracting increasing interest. Over the past decades, extensive endeavor has been made on exploring novel membrane

n

Corresponding author. Tel./fax: þ 86 571 87953780. E-mail address: [email protected] (Q.-F. An).

materials for the pervaporation dehydration of aqueous ethanol mixtures [6]. Polymeric membranes are widely used today for ethanol dehydration. They include crosslinked PVA [7,8], organic– inorganic hybrids [9–11], thin-film composite membranes [12] and charged polymers such as chitosan, sodium alginate and other synthetic polyelectrolytes [13–16]. Polyelectrolyte complexes (PECs), being a type of multicomponent polymeric material formed by ionic complexation between oppositely charged polyelectrolytes [17], have been proved as versatile membrane materials in pervaporation [18,19], nanofiltration [20], and fuel cell applications [21,22]. In 2008, our group explored a so-called ‘‘acid protection’’ strategy for preparing solution-processable PECs and their homogenous PEC membranes with very high pervaporation performance in organics dehydration [23–26]. One essential prerequisite of this strategy is the use of weak poly-acid as component polyelectrolytes, because the protonation and de-protonation of COOH groups can be easily tuned via pH values [23]. Indeed, it has been revealed that PEC membranes prepared via the ‘‘acid protection’’ method show both high permeability and selectivity in dehydration of aqueous isopropanol.

0376-7388/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.02.008

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Recently, we observed that the high performance of these PEC membranes might be reduced somehow under harsh operation conditions such as acidic aqueous feed [27]. Hence, our current research focus was moved to introduce strong acid groups such as sulfate (OSO3) groups into these PEC membranes because it has been documented that sulfate groups have higher hydrophilicity and stronger ionic complexation strength with cationic polyelectrolytes [28]. In our recent study, a poly(sodium acrylate-co-sodium styrenesulfonate) (PAANa-co-PSSNa) copolymer that contained both carboxylate (COONa) and sulfonate (SO3) groups was synthesized, and utilized successfully for preparing solution-processable PECs and their membranes [29]. Besides the improved performance stability versus the feed pH, it has also been found that the chemical structure of anionic polyelectrolyte plays a vital role for tailoring performance. In this regard, we consider that the sodium carboxymethyl cellulose (CMC) could be a better candidate as substrate polymers for incorporating SO3 groups. First, CMC has a polysaccharide-type backbone containing hydroxyl groups, and possesses improved membrane formation ability. Moreover, SO3 groups can be easily introduced into CMC monomer units that already contain COONa groups. Hence, both SO3 and COONa groups coexist in a single CMC monomer unit, which essentially has advantage over PAANa-coPSSNa polymer, because the latter has only one type of charged groups (either SO3 or COONa group) in one monomer repeating unit. Aiming at optimizing the ethanol dehydration performance, our approach in this study was to introduce OSO3 groups into CMC chains and prepare its soluble SPECs with poly(diallydimethylammonium chloride) (PDDA). Structure characterizations and pervaporation studies were carried out systematically. Indeed, it was confirmed that the current strategy is capable of improving the performance of the PEC membrane in dehydration of aqueous ethanol, isopropanol, and butanol mixtures. Especially, for the aqueous ethanol feed mixtures which are relatively more difficult to separate, both the flux and the selectivity are improved compared with pristine PEC membranes and other polyelectrolyte materials.

2. Experimental

(PDDA) (Mw ¼10,000–20,000 g mol  1, 20 wt% aqueous solution) was purchased from Aldrich and sulfur trioxide pyridine (SO3/pyridine) complex was obtained from Aladdin. Both of them were used without further purification. All organic solvents (analytical grade) such as ethanol, acetone, N,N-dimethylacetamide (DMA), isopropanol and n-butanol were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, and used as obtained. Hydrochloric acid (HCl), sodium hydroxide (NaOH), and p-toluenesulphonic acid (p-TsOH) were analytical reagents. As-received p-toluenesulphonic acid (p-TsOH) was dried at 120 1C under vacuum for 3 h to obtain anhydrous p-TsOH. Polysulfone ultra-filtration membranes were kindly provided by Development Centre of Water Treatment Technology, Hangzhou, China, which were used as substrate membranes. De-ionized water with a resistance of 18 MO cm was used in all experiments. 2.2. Synthesis of sulfated carboxymethyl cellulose As shown in Fig. 1a, the sulfated carboxymethyl cellulose (SCMC) was synthesized by using SO3/pyridine complex as sulfating agent. Before being sulfated in DMA solvent, CMC was activated with anhydrous p-TsOH in order to obtain homogenous reaction, as described in the literature [30,31]. First, 0.98 g anhydrous p-TsOH was added into a stirred suspension of 1.82 g CMC in 45 mL DMA, and stirring was continued for 30 min at 60 1C. The highly swollen gel-suspension formed was cooled to room temperature for 8 h. Then, the designed amount of SO3/ pyridine complex was added into the activated gel-suspension under stirring at room temperature. After being stirred for 1 h, the product was precipitated in acetone, collected and washed three times with acetone. The obtained SCMC was dissolved in 150 mL water and the pH of this solution was adjusted to 7 with 1.0 mol L  1 aqueous NaOH solution. The aqueous solution formed was precipitated in ethanol, collected and washed three times with 80% (v/v) aqueous ethanol to remove residual salts. Finally, SCMC was dried at 50 1C for 6 h, and their compositions are shown in Table 1. The compositions of SCMC, denoted as SCMC-X, were controlled by tuning the feed monomer ratio of sulfating agent to CMC. X was OSO3 groups per SCMC or the ratio of OSO3Na groups to COONa groups in SCMC.

2.1. Materials 2.3. Preparation of SCMC/PDDA SPECs and their SPECMs Sodium carboxymethyl cellulose (CMC) with an intrinsic viscosity of 625.1 mL g  1 in 0.1 M sodium hydroxide (NaOH) aqueous solution at 30 1C was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Poly(diallydimethylammonium chloride)

As shown in Fig. lb, water-soluble SPECs were prepared with acid protection strategy and dispersed in alkali aqueous solution, as reported in our previous work [29]. In detail, negatively

Fig. 1. Schematic of synthesis of SCMC (a), Schematic diagram for fabricating SCMC/PDDA (SPECs) (b).

X.-S. Wang et al. / Journal of Membrane Science 435 (2013) 71–79

Table 1 Composition of SCMC determined by XPS. Sample

Molar ratio (sulfating agent: CMC)

S atom content (%)

O atom content (%)

S:O

X

CMC SCMC-0.12 SCMC-0.41 SCMC-0.58

0 1 2.5 3

— 0.96 2.65 3.92

— 56.14 52.89 58.77

— 0.0171 0.0501 0.0667

— 0.12 0.41 0.58

Table 2 Composition of SPECs determined by XPS. Samplea

b

SPEC-0 SPEC-0.10 SPEC-0.25 SPEC-0.34

S atom content (%)

N atom content (%)

S:N

— 0.68 1.28 1.95

— 2.63 2.09 2.34

— 0.2585 0.6124 0.8333

Y

— 0.10 0.25 0.34

Corresponding SPECM SPECM-0 SPECM-0.10 SPECM-0.25 SPECM-0.34

a SPEC-0.1, SPEC-0.25, SPEC-0.34 were made from SCMC-0.12, SCMC-0.41, SCMC-0.58, respectively. b SPEC-0 refers to PECs containing no sulfate groups, i.e., PECs made from pristine CMC and PDDA.

charged SCMC solutions (2.5 g L  1) and positively charged PDDA solutions (1.6 g L  1) were prepared and their pH were tuned at 2.5. Subsequently, the PDDA solutions were dropped into SCMC solutions under vigorous stirring at 600 rpm. It was observed that SPEC precipitated out when the ionic complexation between SCMC and PDDA was reached. Finally, SPECs precipitates were obtained by removing the upper liquid, thoroughly washed by de-ionized water and dried at 60 1C for 24 h. The obtained SPECs could be dispersed in 0.1 mol L  1 aqueous NaOH due to the existence of unionized carboxyl groups in them. The SPECMs were obtained by casting SPECs solutions (2 wt%) on polysulfone ultra-filtration supporting membranes. The prepared SPECMs were dried by post-treatment at 35 1C for 8 h and then at 60 1C for 2 h to remove any residual solvent. SCMC/PDDA complexes and their homogenous membranes were denoted as SPECs and SPECMs, respectively. SPECs with OSO3 groups’ molar percent Y were denoted as SPEC-Y, and their membranes designated as SPECM-Y. The compositions of SPECs and SPECMs are shown in Table 2. It was documented that the difference in free energy of association of a quaternized amine group with a sulfate groups and a carboxylate group was as large as 15 kJ mol  1 for the case of PECMs [32]. As result of such strong interaction energy, ionic pairs of quaternized amine group with SO3 or SO4 groups were extremely stable aqueous solution. The strong binding between sulfate–amine ion pairs decreased SPECs dissolubility and the homogenous SPECs dispersion solution could not be prepared. Therefore, the homogenous SPECMs with higher Y, namely higher OSO3 group content, could not be prepared. CMC/PDDA complex with no sulfate groups and its membrane were prepared by the same method and designated as SPEC-0 and SPECM-0, respectively. 2.4. Characterization Fourier transform infrared (FT-IR) for SCMC and solid SPECs were obtained using a BRUKER VECTOR 22 FT-IR spectrometer (Germany) by dispersing SCMC and SPECs in KBr, and they were made as pellets. FT-IR in attenuated total reflection (ATR) mode (ATR-FTIR) measurement was performed using a Nicolet FT-IR/ Nexus 470 spectrometer equipped with an ATR accessory (ZnSe crystal, 451). All spectra were taken by 16 scans at a nominal

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resolution of 4 cm  1. The dried SPECMs dried under vacuum at 35 1C for 24 h to constant weight were used for FTIR. SPECMs for tensile testing equilibrated with the tensile testing atmosphere were used for FTIR. Stretching tests of the SPECMs were performed on a universal testing machine (SANS CMT4204, Shenzhen, China) at a stretching rate of 1 mm min  1 as previously reported [11]. Tensile strength of the SPECMs was averaged by testing five pieces of SPECMs (20  10 cm2). Note that the humidity of the atmosphere for stretching testing tests maintained at 30% because the SPECMs were sensitive to humidity due to their hydrophilicity. SPECMs for tensile testing should be equilibrated with the atmosphere for ca. 4 h. The ratios of S atom to O atom of SCMC and its SPEC were analyzed by X-ray photoelectron spectra (XPS, PerkinElmer PHI 5300 ESCA), with Mg/Al Dual Anode Hel/ Hell ultraviolet source (400 W, 15 kV, 1253.6 eV). X, Y were determined by the following equation: ½S X 1 ¼ ¼ ½O 3X þ7 3 þ ð7=XÞ Y¼

½OSO3  ½S ½S=½N ¼ ¼ ½SCMC þ ½PDDA ð½S=XÞ þ ½N ð1=X Þ  ð½S=½NÞ þ1

ð1Þ

ð2Þ

wherein [S] and [O] denoted contents of S and O atoms in SCMC in Eq. (1), [S] and [N] designated contents of S and N in SPEC in Eq. (2). Surface morphologies of the SPECMs were observed with a Hitachi S4800 Field Emission Scanning Electronic Microscopy (FESEM, SIRION-100, USA). Samples of SPECMs with 8 mm  8 mm area were dried under vacuum at 35 1C for 12 h and coated with gold before FESEM examination. Free-standing SPECMs prepared by casting SPEC solutions onto a Teflon sheet were subjected to swelling measurements, which were dried under vacuum at 35 1C for 24 h. Dry SPECMs with a pristine weight (M0) were immersed in aqueous ethanol solution and allowed to equilibrate for 12 h at a designed temperature. Each sample was weighed from time to time until no weight change was observed. Finally, these SPECMs were taken out from the solutions and weighed (recorded as MN) after the superfluous liquid was wiped out with a tissue paper. The value of the equilibrium swelling degree (ESD) was calculated according to the following equation: ESDð%Þ ¼

M 1 M 0 M0

ð3Þ

2.5. Pervaporation experiment Pervaporation was conducted on the same equipment as reported previously [23]. The downstream pressure was measured by a piezometer and maintained at about 180 Pa with an accuracy of 0.3 1C by an electric control thermometer; the feed composition was maintained stable by circulation. The permeate was condensed by liquid nitrogen, and its concentration determined by a GC1690A gas chromatograph (made in Hangzhou Ke Xiao Chemical Instrument Co., Ltd., China). Permeation flux (J) and separation factor (a) (water content in permeate) were calculated according to the following equations:

Dg S  Dt P =P a¼ w E F w =F E



ð4Þ

wherein, Dg was the weight of permeate collected in liquid nitrogen traps during the operation time Dt; S was the membrane area (18.09 cm2); Pw, PE, Fw and FE represented the mass fractions of water (W) and ethanol (E) at the feed and permeate sides.

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2.6. Sorption experiment Homogenous SPECMs were weighed after they were dried in a vacuum oven and immersed in a 10 wt% water–ethanol feed solution at 50 1C for 2 days to achieve sorption equilibrium. Then the membrane surface solution was wiped off with a tissue paper quickly. The absorbed liquid in SPECMs was condensed in a liquid nitrogen trap by a vacuum pump used in pervaporation experiment, which was analyzed by gas chromatography. All the experiments were repeated three times, and the average value was obtained. Sorption selectivity (as) was calculated by Eq. (5):

as ¼

M w =M E F w =F E

ð5Þ

wherein M and F were the weight fractions of water (W) and ethanol (E) in the SPECMs and the feed solution, respectively. According to solution–diffusion theory, diffusion selectivity (ad) was calculated by the following equation:

ad ¼ a=as

ð6Þ

where a and as were the separation factor and sorption selectivity of SPECMs or PECM.

3. Results and discussion 3.1. Characterization of SCMC, SPECs and SPECMs XPS was used to determine the composition of SCMC and its SPECs, and the results are given in Tables 1 and 2. In Table 1, it is seen that the OSO3 per SCMC increases with increasing ratio of sulfating agent to CMC in feed. X values are 0.12, 0.41, 0.58, and SCMC are denoted as SCMC-0.12, SCMC-0.41, SCMC-0.58. Hence, it can be concluded that SCMC with tunable chemical compositions was synthesized. SCMC-0.58’s OSO3 content is the highest among the prepared SCMC. In Table 2, it is seen that the OSO3 molar percent in SPECs increases with increasing content of OSO3 groups in SCMC. The SPEC-0.34 and its corresponding SPECM-0.34 prepared with SCMC0.58 exhibit the most OSO3 groups. Hence, it is seen that sulfate groups have been successfully incorporated into SCMC, SPECs, and SPECMs. Fig. 2 shows the FT-IR spectra of SCMC with different sulfation degree. Absporbtion bands at 815 cm  1 and 1240 cm  1 are attributed to stretching vibration of S–O and the symmetric vibration of S¼O in OSO3 groups, respectively [30]. The peak at 1126 cm  1 is also ascribed to asymnetric vibration of OSO3 groups

a

-

-

-

OSO3

COO

OSO3

in SCMC [33,34]. The absorption at 1615 cm  1 is due to the C¼O stretching of carboxylate groups (COONa) [35]. Hence, Fig. 2 confirms that OSO3 groups have been successfully introduced into SCMC chains. As synthesized SCMC was further utilized to prepare its SPEC and SPECMs, whose FT-IR spectra are shown in Fig. 3. It is seen that characteristic absoprtion peaks for OSO3 (815 cm  1) and COONa (1600 cm  1) groups are observed in both SPEC-0.34 and SPECM-0.34. Combining the XPS results in Table 2, this confirms the coexistence of COONa and OSO3 groups in SPECs and their membranes. On the other hand, there were no OSO3 groups in SPEC-0 or SPECM-0. Moreover, absoprtion peaks at 1745 cm  1 are observed for both SPEC-0 and SPEC-0.34, which are ascribed to carboxylic acid groups (COOH) [21]. This is because the PECs were prepared under an acidic condition, in which COONa groups on SCMC were just partially protonated, leaving a part of residual COOH groups in SPECs. Note that these COOH groups in PECs were protonated again when they were dissolved in alkali aqueous solution prior to the membrane preparation. As a proof of this, the absorption peak at 1745 cm  1 (COOH) is not seen in both SPECM-0 and SPECM-0.34, while the absorption intensity at 1600 cm  1 (COONa) is enhanced considerably. Fig. 4 shows surface and cross-sectional morphologies of SPECM-0.34. As shown in Fig. 4(a), it is seen that SPECM-0.34 exhibits no appreciable pore or phase separation, indicating that a defect-free dense membrane was formed. From the crosssectional view (Fig. 4b), it can be observed also that the thickness of SPECMs ranges from 4 mm to 5 mm. 3.2. Pervaporation and mechanical performance of SPECMs 3.2.1. Effect of chemical composition of SPECMs on their pervaporation performance Fig. 5 shows the effect of chemical compositions of SPECMs on their pervaporation performance in the dehydration of 10 wt% water–ethanol at 50 1C. A unique feature shown in Fig. 5 is that both the flux and the water in permeate of SPECMs increase with increasing OSO3 group content in SPECMs. In detail, the flux and water in permeate for SPECM-0.34 are 1200 g m  2 h  1 and 98.6 wt%, respectively, both of which are higher than those for the pristine SPECM-0. Normally, trade-off relationships between flux and selectivity are frequently observed when pervaporation membranes are modified to optimize their performance [9–11,36,37]. That is, an increase in flux always sacrifices selectivity, or vice versa. Hence, the result in Fig. 5 is unique because both the flux and the selectivity are improved simultaneously.

-

COOH

OSO3

-

-

COO OSO3-

OSO3

-

OSO3

a

b b

T

T

c d

2000

c d

1600

1200

800

400

Wavenumber (cm-1) Fig. 2. FT-IR spectra for CMC (a), SCMC-0.12 (b), SCMC-0.41(c), SCMC-0.58 (d).

2000

1600

1200

800

400

Wavenumber (cm-1) Fig. 3. FT-IR spectra for SPEC-0 (a), SPEC-0.34 (b), SPECM-0(c), SPECM-0.34 (d).

X.-S. Wang et al. / Journal of Membrane Science 435 (2013) 71–79

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Fig. 4. FESEM morphology of SPECM-0.34 surface (a), cross-section (b).

1400

1000

97

800

96

600

95 a

b

c

d

Fig. 5. Effect of chemical composition of SPECMs on flux and water in permeate in dehydrating 10 wt% water–ethanol at 50 1C. SPECM-0 (a), SPECM-0.10 (b), SPECM0.25 (c), SPECM-0.34 (d).

It is expected that this is because of the incorporation of OSO3 groups, which improves both the hydrophilicity and the charge density of SPECMs, and also resists the swelling of organics into membrane matrix. Indeed, the dynamic water contact angle (Fig. 6) shows that the hydrophilicity of SPECMs is improved with increasing sulfate groups’ content in them. Fig. 7 (a) shows that the equilibrium swelling degree (ESD) and sorption selectivity of various SPECMs in 10 wt% water–ethanol at 50 1C. Very interestingly, it is seen that both ESD and sorption selectivity increase with increasing sulfate groups content. In other words, this indicates that more water molecules preferentially swell in SPECM matrix during the sorption step with increasing sulfate groups content. Moreover, an increase in sorption selectivity is observed with increasing incorporated OSO3 groups content, whereas the diffusion selectivity is slightly fluctuated as shown in Fig. 7 (b). The much higher sorption selectivity than diffusion selectivity indicates that SPECMs separation performance is mainly governed by the preferential sorption of feed component through membranes, wherein the incorporated OSO3 groups enhance sorption process. This swelling behavior correlates well with the simultaneous increase in flux and selectivity in Fig. 5.

3.2.2. Effect of operation conditions on pervaporation dehydration performance Fig. 8 shows the effect of water content in the feed on pervaporation dehydration performance of SPECM-0.34 and SPECM-0 at 50 1C. It is seen that SPECM-0.34 shows higher flux and selectivity than the pristine SPECM-0 at the same water concentration in feed. Moreover, the flux increases and the water content in permeate drops slightly with increasing water content

SPECM-0

Water contact angle (°)

98

water in permeate (wt %)

1200

Flux (g/m2 h)

80

99

SPECM-0.10 SPECM-0.25

60

SPECM-0.34

40

20 0

40

80

120

Time (s) Fig. 6. Dynamic water contact angle of SPECMs.

in the feed, which is commonly due to the increasing swelling degree [36]. Fig. 9 indeed verifies that the ESD of both membranes increases firmly with increasing water content in the feed. It should be noted that SPECM-0.34 exhibits flux at 2.9 kg m  2 h  1 while the water content in permeate keeps at 98 wt% when the water content in the feed is 20 wt%, which is still competitive compared with the state of art level under similar operation conditions [38]. Fig. 10 shows the effect of feed temperature on pervaporation dehydration performances of SPECMs in the dehydration of a 10 wt% water–ethanol mixture. It is seen that both the flux and the selectivity of sulfate-group-incorporated SPECMs are higher than those of the pristine SPECM-0 in the whole feed temperature range. Moreover, the flux of all membranes increases with increasing feed temperature, which is ascribed to the increasing diffusion rate of feed molecules, as well as the increasing driving force with increasing feed temperature [15,18]. This phenomenon is common and is widely observed in pervaporation dehydration. Interestingly, the water content in permeate of SPECMs is almost invariable with increasing feed temperature. This is because structures of SPECM are ionically crosslinked, and they are stable at even the high feed temperature of 70 1C. With this stable selectivity versus feed temperature, higher pervaporation performance can be achieved at higher temperature. In detail, flux and water in permeate are 1760 g m  2 h  1 and 98.7 wt%, respectively, when the feed temperature is 70 1C. As shown in Fig. 11, this high performance at 70 1C is also stable versus the operation time, since both the flux and the selectivity of SPECM-0.34 are maintained stable after 24 h of continuous pervaporation test.

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600

35

25 300 20 150 15 0

10 5

M-0

M-1

M-2

M-3

Flux (g/m2 h)

450

Sorption selectivity

Swelling degree (wt%)

3200 30

SPECM-0.34

2400

SPECM-0 1600

800

-150 0

5

10

15

20

Water content in feed (wt%) 750

3 300 2

150

M-0

0 M-1

M-2

M-3

Water in permeate (wt%)

450

1

100

600

4

Separation factor

Diffusion selectivity

5

Fig. 7. Equilibrium swelling degree (ESD) and sorption selectivity (as) of SPECMs (a) and diffusion selectivity (ad) and separation factor (a) of SPECMs (b) for 10 wt% water–ethanol at 50 1C. SPECM-0 (M-0), SPECM-0.10 (M-1), SPECM-0.25 (M-2), SPECM-0.34 (M-3).

3.2.3. Mechanical performance of SPECMs Fig. 12 shows the stress–strain curves of SPECMs and the effect of chemical composition of SPECMs on the tensile strength and elongation at break. It is interesting that both the tensile strength and elongation at break increase with the increasing strong ionpairs content. For instance, the SPECM-0.34 tensile strength and elongation at break are 64.5 MPa and 9.33%, both of which are much higher than those for pristine PEC membranes with no sulfate groups (33.5 MPa, 1.95%). It is indicated that the SPECMs flexibility are endowed with better mechanical properties and flexibility compared with the pristine PECMs. The higher tensile

SPECM-0.34 SPECM-0

90 5

10

15

20

Water content in feed (wt%) Fig. 8. Effect of water content in feed ethanol on flux (a) and water content in permeate (b) of SPECM-0.34 and SPECM-0 at 50 1C.

30

ESD (%)

This indicates that the structure of SPECMs are stable as a result of both the ionic crosslinking structures and the incorporation of sulfate groups. Table 3 shows the comparison of SPECMs with other membranes reported recently in the ethanol dehydration by pervaporation. It can be seen that SPECM-0.34 shows higher performance compared with polyvinyl alcohol membranes and other polyelectrolyte membranes such as chitosan and polyethylenimine. Moreover, SPECM-0.34 also shows higher flux compared with another two PEC membranes [24,41] we prepared previously via the same ‘‘acid protection’’ method but with no sulfate groups. We consider that the superior separation performance of SPECM-0.34 is probably because it combines both the ultra-permeable channel structures that are characteristically associated with SPECMs prepared via ‘‘acid protection’’ method [27] and the high hydrophilicity contributed by sulfate groups.

95

20

10 SPECM-0.34 SPECM-0 0

5

10

15

20

Water content in feed (wt%) Fig. 9. Effect of water content in feed ethanol on equilibrium swelling degree (ESD) of SPECM-0.34 and SPECM-0 at 50 1C.

strength for SPECMs is attributed to the stronger interaction of cations and sulfate groups [32]. As a supporting proof of this explanation, attenuated total reflection FT-IR (ATR-FTIR) spectra (Fig. S1) shows that the bound water [42,43] in SPECM-0.34 under the mechanical testing condition (humidity: 30%) is increased

X.-S. Wang et al. / Journal of Membrane Science 435 (2013) 71–79

Table 3 Comparison of pervaporation performance for ethanol dehydration in this study and literature. Membranes

Feed ethanol (wt%)

Feed temperature (1C)

Water in permeate (wt%)

Flux (kg m  2 h  1)

References

PVAa HA/CS/ PANb CMC/P4VPc CSd PEI/PANe CMC/CSf SPECM0.34

90 90

60 80

92.74 98.73

E0.12 1.44

[7] [18]

90 90 95 90 90

70 60 75 70 70

98.86 98.60 97.12 99.1 98.71

1.32 0.30 0.512 1.14 1.76

[24] [39] [40] [41] This study

1500

1000 SPECM-0 SPECM-0.10 SPECM-0.25 SPECM-0.34 500

a

40

50

60

Dimehylolurea crosslinked. The active layer constructed by sequential spin-coating and self-assembly of CS and hyaluronic acid (HA), respectively, on polyacrylonitrile (PAN) support layer. c Poly(N-ethyl-4-vinylpyridinium bromide). d Glutaraldehyde crosslinked, maleic anhydride modification. e Polyethyleneimine (PEI) assembled onto hydrolyzed polyacrylonitrile (PAN). f CMC/CS PEC membranes prepared with ‘‘acid protection’’ method.

70

b

Feed temperature (°C)

98

75 60

97 SPECM-0

Stress (MPa)

water in permeate (wt %)

99

SPECM-0.10 96

SPECM-0.25 SPECM-0.34

45 30 SPECM-0 SPECM-0.10 SPECM-0.25 SPECM-0.34

15

95 40

50

60

70

Feed temperature (°C)

0

Fig. 10. Effect of feed temperature on flux (a) and water content in permeate (b) of SPECMs in dehydrating 10 wt% water–ethanol mixture.

3000

0

2000 94 1500 92 1000

90 0

3

6

9

12

15

18

21

8

10

12

24

Operation time (h) Fig. 11. Pervaporation performance stability of SPECM-0.34 in dehydration of 10 wt% water–ethanol mixture at 70 1C.

compared to its dry state, while the bound water in SPECM-0 is not much changed under the two conditions. A direct comparison of SPECM-0.34 and SPECM-0 shown in Fig. S1 (c) also confirm that the former contains more bound water. It is expected that the bound water acts as a plasticizer that enhances both the free volume and lubricity effects among the SPEC aggregates [44] and the elongation at break for SPECMs is improved.

10

Tensile strength (MPa)

96

6

90

Water in permeate (wt%)

98

4

Strain (%)

100

2500

2

75

8

6

60

4 45 2

Elongation at break (%)

Flux (g/m2 h)

2000

Flux (g/m2 h)

77

30 M-0

M-1

M-2

M-3

0

Fig. 12. Stress–strain curves of SPECMs (a) and the effect of chemical composition of SPECMs on the tensile strength and elongation at break (b). SPECM-0 (M-0), SPECM-0.10 (M-1), SPECM-0.25 (M-2), SPECM-0.34 (M-3).

4. Conclusion Sulfated sodium carboxymethyl cellulose (SCMC) with tunable sulfation degree was successfully synthesized. OSO3 groups were incorporated into SCMC/PDDA PECs and their corresponding SPECMs in the form of strong ion-pairs, and their content in

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SPECMs was tunable by controlling the composition of SCMC. Pervaporation performances of SPECMs showed that the incorporation of OSO3 groups into SPECMs improved both the selectivity and flux in the dehydration of aqueous ethanol solution. Moreover, the flux of SPECMs increased with increasing feed temperature while maintaining the water content at a high level above 98.5 wt%. For instance, the flux and the water in permeate of SPECM-0.34 in dehydrating 10 wt% water–ethanol at 70 1C were 1760 g m  2 h  1 and 98.7 wt%, respectively, both of which were much higher than those of the pristine SPECM-0 (1160 g m  2 h  1, 97.8 wt%). Both the flux and selectivity of SPECM-0.34 were stable versus the operation time. Moreover, both the tensile strength and elongation at break increased with the increasing strong ion-pairs content, giving maximum values of 64.5 MPa and 9.33%, respectively, for SPECM-0.34. The improved performance of SPECMs was attributed to the incorporation of sulfate groups and their ionic complexation structures with PDDA, which entitled SPECMs with higher hydrophilicity, and improved ESD as well as sorption selectivity.

Acknowledgements This research was financially supported by the NNSFC (51173160, 21106126), the National Basic Research Program of China (2009CB623402) and the National High Technology Research and Development Program of China (2012AA03A602).

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.02.008.

Nomenclature

a as

separation factor sorption selectivity CMC sodium carboxymethyl cellulose SCMC sulfated sodium carboxymethyl cellulose OSO3 sulfate ESD equilibrium swelling degree PDDA poly(diallyldimethylammonium chloride) J total flux(g m  2 h) SCMC-X SCMC with OSO3 groups per repeat unit X PECs polyelectrolyte complexes SPECs PECs containing OSO3 groups SPEC-Y SPECs with OSO3 groups’ molar percent Y SPEC-0 SPEC containing no OSO3 groups PECMs polyelectrolyte complexes membranes SPECMs PECMs containing OSO3 groups SPECM-Y SPECM with OSO3 groups’ molar percent Y SPECM-0 SPECM containing no OSO3 groups

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