Novel polyelectrolyte complex membranes containing free sulfate groups with improved pervaporation dehydration of ethanol

Journal of Membrane Science 452 (2014) 73–81

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Novel polyelectrolyte complex membranes containing free sulfate groups with improved pervaporation dehydration of ethanol Xue-San Wang a, Quan-Fu An a,n, Tao Liu a, Qiang Zhao a, Wei-Song Hung b,n, Kueir-Rarn Lee b, Cong-Jie Gao c a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China b R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan c The Development Center of Water Treatment Technology, Hangzhou 310012, China

art ic l e i nf o

a b s t r a c t

Article history: Received 30 August 2013 Received in revised form 10 October 2013 Accepted 13 October 2013 Available online 22 October 2013

Novel polyelectrolyte complexes containing free sulfate (SO3) groups (PECSs) were synthesized, with the sulfation of NH2 groups in the soluble chitosan (CS)/sodium carboxymethyl cellulose (CMC) complexes, and their membranes (PECSMs) were subjected to pervaporation dehydration of ethanol. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the chemical structure and the composition of PECSs. Zeta (ξ) potential and field emission scanning electron microscopy were used to investigate the surface charge density of the PECS particles and the morphology of their membranes. The effects of the chemical composition on the swelling degree, the hydrophilic property, and the pervaporation dehydration performance of PECSMs were determined. It was found that free SO3 groups were successfully incorporated into PECSMs. Both the flux and the separation factor of PECSMs increased with increasing SO3 groups. The permeation flux of PECSMs increased, and their selectivity was almost invariable with increasing temperature. A high separation performance of PECSMs was achieved in the dehydration of 10 wt% water–ethanol mixtures at 70 1C, yielding a flux and a separation factor for the PECSM-20 at 1385 g/m2 h and 1571, respectively. These results indicated that the introduction of free SO3 groups into PECSMs was an effective strategy to improve the pervaporation dehydration performance of PECSMs. & 2013 Elsevier B.V. All rights reserved.

Keywords: Polyelectrolyte complex Free SO3 groups Pervaporation Dehydration

1. Introduction Pervaporation utilized as a membrane separation technology has advantages over the traditional distillation or adsorption process, especially for separating an azeotropic mixture or a mixture of liquids with close boiling points [1]. Pervaporation features energysaving characteristics and high efficiency in separating liquid–liquid molecules, and has been attracting growing interests [2]. There is a wide range of applications for pervaporation, such as the dehydration of organics, the recovery of organics, and the separation of organic mixtures. The dehydration of organics is always viewed as a major part of industrial usage [3]. Polymeric materials are excellent candidates for the dehydration of organics, and their membranes are lower-cost and improved process-ability [4]. A variety of polymeric materials, including poly (vinyl alcohol) [5–7], thin-film composite membranes [8], organic–inorganic hybrids [9–11], and charged

n

Corresponding authors. Tel./fax: þ86 571 87953780. E-mail addresses: [email protected] (Q.-F. An), [email protected] (W.-S. Hung). 0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.10.028

polymers such as chitosan (CS) and other synthetic polyelectrolytes as well as their corresponding complexes [12–14], have been subjected to pervaporation dehydration of alcohols. Polyelectrolyte complexes (PECs) have been considered as a promising candidate for pervaporation dehydration due to their ionic-crosslinking structure and highly hydrophilic property [15]. PEC membranes (PECMs), including multilayered PECMs [16,17], two-ply PECMs [18], and blend PECMs [19,20], were prepared by different methods and used for pervaporation dehydration. Homogenous PECMs were prepared with weak poly-carboxylate (COO
) and positively charged polyelectrolytes in acidic conditions, and dispersed in alkaline solutions [21]. The homogenous PECMs exhibited good performance in the dehydration of aqueous alcohols, which was arisen from the unique water channel structure contributed by the charged PEC particles, based on the positron annihilation lifetime spectroscopy (PALS) analysis [22]. Due to the higher hydrophilicity of sulfonate or sulfate groups compared with carboxylate groups [23], PECMs containing sulfonate groups exhibited a better pervaporation dehydration performance than those containing carboxylate groups [24–26]. These homogenous PECMs composed of complexed sulfate (SO3) groups were also

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prepared by complexing sulfated sodium carboxymethyl cellulose containing SO3
and COO
groups with polycations in acidic conditions, and then dispersed in alkali solutions. Complexed SO3 groups improved the pervaporation performance of PECMs for the ethanol dehydration [27]. However, homogenous PECMs containing free SO3 groups have not been prepared yet. This is because SO3
units pair with ammonium (NH3 þ ) groups prior to COO
when the co-existence of the dual anionic groups occurs, owing to the stronger binding interaction between SO3
and NH3 þ [28]. Moreover, free SO3 groups exhibit a higher affinity with water than with the ion-pairs between SO3
and NH3 þ [29]. It is readily anticipated that free SO3 groups could enhance the hydrophilicity of the water channels embedded in PECMs leading to the effective improvement of the permeation flux, while maintaining the ionic-crosslinking characteristic of the PECMs, which inhibits excessive swelling and assures high selectivity. Aiming at introducing free SO3 groups into PECs, our approach in this study was through the sulfation of NH2 groups in PECs. CS containing NH2 groups and CMC containing carboxylate (COONa) groups were used as polyelectrolyte substrates, and their soluble CS/CMC complexes were prepared based on the previous “acid protection” strategy. PECs containing free SO3 groups (PECSs) and their homogenous membranes (PECSMs) were prepared by the sulfation of NH2 in the soluble CS/CMC under alkaline conditions. The characterization of the structure and the pervaporation performance of PECSMs were systematically discussed. It was confirmed that free SO3 groups were capable of effectively improving the performance of PECSMs in the dehydration of ethanol and rendering PECSMs to have high pervaporation performance.

2. Experimental 2.1. Materials Chistosan (CS) (Mn¼200,000 g/mol, deacetylation degree¼90%) was purchased from Yuhuan Chemical Company. Sodium carboxymethyl cellulose (CMC), with an intrinsic viscosity of 625.1 mL/g in 0.1 M sodium hydroxide (NaOH) aqueous solutions at 30 1C, was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Trim-ethylamine-sulfur trioxide complex ((CH3CH2)3N  SO3) was purchased from Aldrich and used without further purification. All organic solvents (analytical grade), such as ethanol and acetone, were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and used as received. Hydrochloric acid (HCl), sodium hydroxide (NaOH), and sodium carbonate (Na2CO3) were analytical reagents. Polysulfone ultra-filtration membranes were supplied by the Development Centre of Water Treatment Technology, Hangzhou,

China, which were used as membrane substrates. De-ionized water, with a resistance of 18 MΩ cm, was used in all experiments. 2.2. Preparation of sulfated CS/CMC complexes (PECSs) and their membranes (PECSMs) As shown in Fig. 1, PECSs were prepared by using sulfur trioxide trim-ethylamine complex ((CH3)3N  SO3) to sulfate the NH2 groups of the water-soluble CS/CMC complexes. This sulfation procedure was actual N-sulfated of NH2 groups in CS/CMC complexes according to the method developed by Holme [30]. From Fig. 1, it could be observed that the prepared CS/CMC complexes contained some residual NH2 on CS chains for sulfation reaction. The unionized carboxyl groups (COOH) entitled the CS/CMC complex with the ability to form homogenous solutions under alkali conditions. The water-soluble CS/CMC complexes were prepared based on the previously reported “acid protection” strategy, and dispersed in alkali aqueous solutions [31]. First, 400 mL negatively charged CMC solutions and 400 mL positively charged CS solutions were dissolved in 0.005 M HCl. Then, the molar concentration of both the CS and the CMC monomer unit was kept at 0.01 M. It was speculated that there was 50 mol% of residual NH2 protonated to NH3 þ groups on CS chains and 50 mol% COO
groups protonated to COOH groups on CMC chains. In other words, the charge density of the CMC chains was 0.5, which was equal to that of the CS chains. Subsequently, the CS solutions were dropped into the CMC solutions under vigorous stirring at 600 rpm. Turbidity immediately occurred when the CS solutions were added, and the PECSs were precipitated out when the ionic complexation between CMC and CS was reached. It should be noted that the ionic complexation was reached when approximately 400 mL CS solutions were added into 400 mL CMC solutions. This result was attributed to CMC and CS chains having the same charge density. The obtained CS/CMC complexes were dried at 50 1C for 6 h. Finally, the dried CS/CMC complexes (1.6 g) were dispersed in Na2CO3 aqueous solution (80 mL) due to the existence of unionized carboxyl groups (COOH) in them, and then the sulfated agent ((CH3)3N  SO3) was added. The pH of this aqueous solution was kept at 9 by adjusting the molar ratio of Na2CO3 to (CH3)3N  SO3. After the reaction at 60 1C for 6 h, the product was precipitated in acetone, collected, and washed with acetone three times. The obtained PECSs were dissolved in 80 mL water, and then precipitated in ethanol, collected and washed with 80% (v/v) aqueous ethanol three times to remove the residual salts. The obtained PECSs were dried at 50 1C for 6 h. The compositions of PECSs were tuned by varying the ratio of the (CH3)3N  SO3 monomer to the monomer unit of CS. Three compositions of PECSs were prepared with the ratios of the (CH3)3N  SO3 monomer to the monomer unit of CS in CS/CMC complexes at 1, 2, and 3. The ratios of the Na2CO3 monomer to the monomer unit of CS were 0.5, 1.5,

Fig. 1. Schematic diagram for fabricating PECs containing free SO3 groups (PECSs).

X.-S. Wang et al. / Journal of Membrane Science 452 (2014) 73–81

Table 1 Composition of PECSs determined by XPS. Sample

O (at%)

N (at%)

S (at%)

CMC:CS (molar ratio)

DS (%)b

X (%)

PECS-0a PECS-7 PECS-13 PECS-20

38.05 38.13 39.84 40.37

3.55 3.53 2.57 1.98

– 0.46 0.79 0.97

0.96 – – –

0 13 30 49

0 7 13 20

a PECS-0 refers to PEC containing no SO3 groups, and the PECS-0 was made from CMC and CS. b Degree of sulfation.

75

For the swelling measurement, free-standing PECSMs were prepared by casting PECS solutions (2 wt%) onto a Teflon sheet, and were then dried under vacuum at 35 1C for 24 h. The dried PECSMs with a pristine weight (M0) were immersed in a 10 wt% ethanol solution and equilibrated for 12 h at a designed temperature. Finally, the PECSMs were taken out from the solution and weighed (M1) after the superfluous liquid was wiped out. The equilibrium swelling degree (ESD) was calculated according to the equation: ESD ð%Þ ¼ ðM 1
M 0 Þ=M 0 . The ethanol and water in the swollen PECSMs were determined through a sorption experiment, which is described in Section 2.4. 2.4. Pervaporation experiment

and 2.5 to maintain the pH of the reactive solutions at 9. The compositions of the as-prepared PECSs were determined by XPS, and the result is shown in Table 1. The sulfation of CS procedure was conducted with the above-mentioned method, and the ratio of (CH3)3N  SO3 to the monomer unit of CS was 3. The molar percent of SO3 groups (mol%) in PECSs was denoted by X. X in PECSs was denoted as PECS-X, and their corresponding membranes were designated as PECSM-X. CS/CMC complexes containing no SO3 groups and their membranes were designated as PECS-0 and PECSM-0, respectively. PECSs were dispersed in aqueous alkali sodium hydroxide solutions (2 wt%) with pH of 8. PECSMs were prepared by casting these dispersed solutions of PECS on polysulfone ultra-filtration supporting membranes, followed by drying them at 35 1C for 8 h and then at 60 1C for 2 h to remove any residual solvent.

The pervaporation experiment was carried out on the same equipment used in a previous report [21], and a schematic diagram of an apparatus for pervaporation process was given in Fig. 2. The downstream pressure measured by a piezometer was maintained at about 180 Pa, and the feed temperature was with an accuracy of 0.3 1C. The feed composition was maintained stable by circulation. The concentration of the permeate condensed by liquid nitrogen was determined by a GC1690A gas chromatograph (Ke Xiao Chemical Instrument Co., Ltd., Hangzhou, China). The permeation flux (J), separation factor (α), and the pervaporation separation index (PSI) were all used to evaluate the pervaporation performance; each of these was defined by the following equations: J¼

2.3. Characterization

α¼

Fourier transform infrared spectroscopy (FT-IR) for solid PECSs was obtained using a BRUKER VECTOR 22 FT-IR spectrometer (Germany), with the PECSs dispersed in KBr and made into pellets. A 90 plus particle size analyzer was employed to measure the zeta (ζ) potential; the concentration of the PECS aqueous solutions was 0.02 wt%, and its pH was 8. pH values were determined by a digital pH meter (pHS-25). Surface morphologies of the PECS particles were observed with a Hitachi S4800 Field Emission Scanning Electron Microscopy (FESEM, SIRION-100, USA). Samples of PECS particles were prepared by drying the PECS aqueous solutions (0.02 wt%) on a silicon wafer at 30 1C for 24 h, and they were coated with gold before the FESEM examination. The concentrations of the S, O, and N atoms in the PECSs were analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5300 ESCA) with Mg/Al Dual Anode Hel/Hell ultraviolet source (400 W, 15 kV, and 1253.6 eV). X, denoting the molar percent of SO3 groups (mol%) in PECSs, was determined by the following equation: ½O ½CMC ¼7 þ4 ½N ½CS DS ¼

½S  100% ½N

Δg AΔt P w =P E F w =F E

PSI ¼ J  ðα
1Þ

ð4Þ

where Δg was the weight of the permeate collected in traps cooled by liquid nitrogen during the operation time Δt; A 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 (F) and permeate (P) sides. 2.5. Sorption experiment The sorption experiment was performed as it was in our previous report to further investigate the swelling behavior of PECSMs [27]. Specifically, the PECSMs were weighed after they were dried in a vacuum oven, and were then immersed in a 10 wt% water–ethanol solution at 50 1C for 2 days to achieve sorption equilibrium. Then the solution remaining on the surface of the PECSMs was wiped off with a tissue paper quickly. The absorbed

ð1Þ

4

5

ð2Þ

10

7

3

X¼ ¼

½SO3  ð½SO3 =½CSÞ ¼ ½CMC þ ½CS þ ½SO3  ð½CMC=½CSÞ þ1 þ ð½SO3 =½CSÞ ð½S=½NÞ ð½CMC=½CSÞ þ 1 þ ð½S=½NÞ

8 6

ð3Þ

where [CMC]/[CS] in PECS-0 represented the molar ratio of the monomer unit of CMC to that of CS. [O] and [N] in Eq. (1) were the contents of O and N atoms in the pristine PECS-0. [S] and [N] in Eqs. (2) and (3) were designated as the content of S and N atoms in PECSs. DS was designated as the degree of sulfation.

1

2

9

Fig. 2. Schematic diagram of an apparatus for pervaporation process. 1—Feed pump, 2—Feed solution, 3—Membrane cell, 4—Temperature controller, 5—Stirrer, 6—Cold trap, 7—Valve, 8—Vacuum buffer, 9—Vacuum pump, and 10—Vacuum gauge.

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liquid in PECSMs was condensed in a liquid nitrogen trap by a vacuum pump used in the pervaporation experiment, and the condensate composition was analyzed by gas chromatography. All the experiments were repeated three times, and the results were averaged. The sorption selectivity (αs) was calculated and the diffusion selectivity (αd) was obtained, on the basis of the solution–diffusion principle, by the following equations:

αs ¼

M w =M E F w =F E

αd ¼

α αs

ð5Þ

where α and αs were the separation factor and the sorption selectivity of the PECSMs, respectively. M and F were the weight fractions of water (W) and ethanol (E) in the PECSMs and the feed solution. The contents of water and ethanol in the swollen PECSMs, which were equilibrated in 10 wt% water–ethanol mixtures, were determined by multiplying ESD with the MW and MF, respectively.

3. Results and discussion 3.1. Characterization of PECSs and their corresponding membranes XPS was employed to determine the chemical composition of PECSs, and the results are given in Table 1. The molar ratio of the monomer unit of CMC to that of CS in PECS-0 is approximately 1 according to Eq. (1). The result is ascribed to the same charge density of CMC and CS chains in PECSs, which equals to 0.5. It is speculated that there is 50 mol% residual NH2, and 50 mol% NH3 þ groups in CS is complexed with COO
groups in CMC. The degree of sulfation (DS) in PECSs is less than 50%, as shown in Table 1, and the result is ascribed to the limited NH2 amount in PECS-0. It is seen that the content of SO3 groups (i.e. X) exhibits an increase with increasing degree of sulfation. It can be concluded that PECSs containing SO3 groups are synthesized, and their chemical compositions are tunable. Among the prepared PECSs, PECS-20 had the highest content of SO3 groups. Therefore, it is seen that SO3 groups have been successfully incorporated into PECSs. Fig. 3 shows the FT-IR spectra of the sulfated CS in comparison with the pristine CS and PECSs with different degrees of sulfation. On the basis of Fig. 3(a) (b), the net decrease in the intensity of the peak 1590 cm
1 (NH2) [32], the apperance of a large shoulder at

SO3SO-3

SO-3

SO-3

Absorbance

f e d c

1125 cm
1 (OåSåO) [33] and the absence of peak at 800 cm
1 for the C–O–S symmetric vibration confirm a selective sulfation of the NH2 groups not the hydroxy in CS [34]. These results are also observed for PECSs compared with the pristine PECS-0, which further verify that the PECSs were successfully sulfated. Another important feature in Fig. 3 is that the characteristic absorption of SO3 groups appears at 625 cm
1 for sulfated CS while that at 750 cm
1, 700 cm
1 and 625 cm
1 for PECSs. This observation is attributed to the different states of SO3 groups in sulfated CS and PECSs. The absporbtion bands at 750 and 700 cm
1 are attributed to the complexed SO3 groups, and the band at 625 cm
1 is associated with free SO3 groups, as shown in the sulfated CS. This result indicates that the transfer of ion-pairs occurs in PECSs upon the incorporation of SO3 groups, i.e. the polycations from COO
groups are transferred to SO3
groups. This observation is in accordance with the previous literature [35]; it is attributed to the stronger binding of SO3
with NH3 þ than that of COO
groups [36]. The partial transfer of ion-pairs for SO3 groups in PECSs is irrevisible, and this transfer is inhibited by unfavorable thermodynamic factors as a result of the greater diffusion engergy of SO3 groups [37]. By using the absorption intensity at 625 cm
1 (free SO3 groups) and that at 750 cm
1 (complexed SO3 groups), in terms of their peak areas, the content of the different states of SO3 groups in PECSs is calculated. Combined with the XPS result (Table 1), the content of free and complexed SO3 groups is obtained, as shown in Table 2. It is seen that both the free and the complexed SO3 groups exhibit an increase with increasing degree of sulfation. This result also confirms that free SO3 groups are incorporated into PECSMs and their compositions are tunable. To determine the stability of free SO3 groups in PECSs, the content of free SO3 groups in PECS-20 versus time was traced by FTIR, as shown in Fig. S1. It can be observed that the content of free SO3 groups in PECS-20 is maintained at almost the same amount, indicating that the degree of the transfer of ion-pairs is maximum, and PECS-20 preserves its structural stability versus time. The absorption at 1615 cm
1 is due to the CåO stretching of COONa groups [21]. It is because PECS-0 was prepared under acidic conditions and then dissolved in alkali aqueous solutions, as shown in Fig. 1, where COONa groups on CMC chains were partially protonated, leaving a part of the residual COOH groups, and COONa was formed in the alkali solutions [21]. As shown in Table 2, the prepared PECS-0 exhibits highly negative ζ potential, owing to the exsistence of COONa groups, as proved by FTIR. It can also be observed that the ζ potential becomes more negative with the increasing content of SO3 groups in PECSs, as shown in Table 2. This observation is ascribed to the increasing charge density of PECSs with the incorporation of free SO3 groups. Combined with the Table 2, Fig. 3 confirms that PECSMs containing free SO3 groups were successfully prepared. Fig. 4 shows the dynamic water contact angle of PECSMs. It is seen that PECSMs containing free SO3 groups are endowed with lower water contact angle and higher hydrophilicity than the pristine PECSM-0. Moreover, this hydrophilicity of PECSMs is much higher than that of the previously reported PECMs containing complexed SO3 groups [27]. These results arise from the hydrophilic property of free SO3 groups in PECSMs, and free SO3 Table 2 Sates of SO3 groups and ζ potential of PECSs.

b a 2000

1750

1500

1250

1000

750

500

Wavenumber (cm-1) Fig. 3. FT-IR spectra for CS (a), sulfated CS (b), PECS-0 (c), PECS-7 (d), PECS-13 (e), and PECS-20 (f).

Sample

ζ potential (mV)

Free SO3 groups Complexed SO3 (mol%) groups (mol%)

Corresponding membranes

PECS-0 PECS-7 PECS-13 PECS-20


507 1.8
557 2.1
597 1.2
66 7 1.3

0 4 8 14

PECSM-0 PECSM-7 PECSM-13 PECSM-20

0 3 5 6

X.-S. Wang et al. / Journal of Membrane Science 452 (2014) 73–81

3.2. Pervaporation performance of PECSMs Fig. 6 presents the effect of chemical composition on the pervaporation performance of PECSMs in the dehydration of ethanol at 50 1C. An interesting characteristic in Fig. 6 is that both the flux and the separation factor of PECSMs exhibit an increase with increasing content of SO3 groups. For instance, the flux for PECSM-20 reaches up to 880 g/m2 h, which is 2.2 times higher than PECSM-0. Moreover, the separation factor of PECSM-20 increases to 1627. It should be mentioned that the 35 mol% complexed SO3 groups increase the PECM flux to 2.1 times, and its separation factor is 633 in the dehydration of a 10 wt% water– ethanol at 50 1C [27]. This result indicates that free SO3 groups could effectively improve the pervaporation performance of PECSMs. It is because free SO3 groups could render PECSMs to have higher affinity with water [29], and this would dramatically increase the hydrophilicity of water channels embedded in PECSMs [22]. The anti-trade-off between the flux and the separation factor for PECSMs is because free SO3 groups improve both the charge density and the hydrophilic property of PECMs, as well as resist the organics permeating into the membrane matrix. As shown in Fig. 7(a), the sorption selectivity is larger than the

diffusion selectivity, indicating that the sorption of the feed component governs the separation process for PECSMs. The incorporated SO3 groups facilitate this sorption procedure, resulting in the membrane selectivity increase with increasing SO3 groups. The amount of water in the swollen PECSMs is much higher than the ethanol, and the amount of water experiences a rapid increase while the ethanol retains almost invariable (E0.15 wt%) with increasing SO3 groups (Fig. 7(b)). This observation implies that more water molecules preferentially interact with PECSMs and swell in their matrix during the sorption step with increasing SO3 groups. This property of SO3 groups endows PECSMs with simultaneously improved flux and separation factor. The swelling behavior correlates well with the pervaporation performance in Fig. 6. After being immersed in 10 wt% water– ethanol mixtures, the PECSMs are swollen with substantial amount of water (Fig. 7(b)). ATR-FTIR was used to monitor the water molecules in PECSMs. The relative water content is defined as the ratio of the water absorption area of PECSMs to that of PECSM-0 [29]. From Fig. 8(a), the OH stretching vibration bands at ca. 2920 cm
1 (Type I) and ca. 3405 cm
1 (Type II) are ascribed to the water molecules in the first and the second hydration layers in PECSMs [38] (i.e. bound water), respectively. The water molecules in PECSMs are in essential forms of bound water, which is associated with low swelling degree (Fig. 7(b)) [39]. It should be mentioned that those water molecules could be removed from the membranes to the permeate side by pervaporation, which is due to the existence of driving force (chemical potential) between them [39]. The water molecules of Type II in PECSMs occupy a major part and increase more sensitively than the water molecules of Type I with the growth of SO3 groups. Accordingly, it is concluded that free SO3 groups enhance the affinity of PECSMs

1800

1000

80

1500

PECSM-7 60

PECSM-13 PECSM-20

40

Flux (g/m2h)

Water contact angle (°)

PECSM-0 800

1200 900

600

600 300

20

Separation factor

groups exhibit a higher affinity with water than complexed ones. It has been documented that the mole ratio of water to free SO3 groups gives 56 in polyelectrolytes, while that to complexed ones yields less than 10 [29]. From Fig. 5, it can be seen that the PECSM20 surface exhibits no appreciable pore or phase separation, which is indicative of the formation of a defect-free dense membrane. As shown in Fig. 5(b), the thickness of the PECSM-20 active layer ranges from 4 to 5 μm, as determined by FESEM.

77

400 0 C SM 3

0

-1

-2 SM C

PE

PE -7

Fig. 4. Dynamic water contact angle of PECSMs.

120

SM

80

Time (s)

C

40

PE

0

-0 SM

C PE

0

Fig. 6. Effect of the content of SO3 groups on the flux and separation factor (α) of PECSMs in the dehydration of 10 wt% water–ethanol mixtures at 50 1C.

Fig. 5. FESEM morphology of the PECSM-20 surface (a) and cross-section (b).

X.-S. Wang et al. / Journal of Membrane Science 452 (2014) 73–81

8 6

450

4 300 2 150

0

5

10

15

20

0.15

Absorbance

600

0.20

Diffusion selectivity (d)

Sorption selectivity (

s)

10

PECSM-0 PECSM-7 PECSM-13

0.10

PECSM-20

0.05

0.00

0

3750

3500

20

15

15

10

10

5

5

0

0 5

10

15

20

SO3 groups (mol.%) Fig. 7. Effect of the content of SO3 groups on sorption selectivity (αs) and diffusion selectivity (αd) (a), and the equilibrium swelling degree (ESD) (b) of PECSMs in dehydrating 10 wt% water–ethanol mixtures at 50 1C.

with water molecules (Type II) and effectively improve their separation performance. Fig. 9 gives the effect of feed temperature on the pervaporation performance of PECSMs in the dehydration of 10 wt% water– ethanol mixtures. It is seen that all the PECSMs containing free SO3 groups exhibit higher flux and perm-selectivity than PECSM-0 in the feed temperature ranging from 40 to 70 1C. Moreover, the flux of all the PECSMs increases with temperature, which results from the improved driving force and the increasing diffusion rate of feed molecules as the feed temperature increases [10–12]. This observation is very common in pervaporation dehydration. It is interesting that the separation factor of PECSMs is nearly invariable. This observation is attributed to the stability of ionic crosslinking structure in PECSMs even at 70 1C [16,20]. With the stable separation factor versus feed temperature, higher flux can be achieved. For instance, the flux and the separation factor of PECSM-20 at 70 1C reach up to 1385 g/m2 h and 1570, respectively. Moreover, the pervaporation performance of PECSM-20 versus the operation time was measured, and the result is given in Fig. 10. It is seen that both the flux and the separation factor of PECSM-20 maintain their stability within 30 h of continuous test. This result further confirms that free SO3 groups in PECSMs are stable. For estimating the process of PECSMs, PECSM-20 was subjected to the batch dehydration experiments from ethanol aqueous mixtures. Fig. 11 shows batch pervaporation dehydration of ethanol for PECSM-20 with operation time at 70 1C. The pervaporation dehydration of PECSM-20 were conducted in 10 wt% water/ethanol (40 g/360 g) mixtures, the effective membrane area was 18.09 cm2. As shown in Fig. 11, with the increasing operation time, it can be observed that the ethanol content in feed increases, the flux of PECSM-20 decreases while the water content in

Ralative water (Type II)

20

0

3250

3000

2750

2500

Wavenumber (cm-1)

Ethanol in PECSMs (wt. %)

Water in PECSMs (wt. %)

SO3 groups (mol.%) 3.0

3.0

2.5

2.5

2.0

2.0

1.5

1.5

1.0

1.0

0.5

0

5

10

15

20

Relative water (Type I)

78

0.5

SO3 groups (mol.%) Fig. 8. FT-IR spectra of PECSMs after they are equilibrated with 10 wt% water– ethanol mixtures at 50 1C (a), and the effect of the content of SO3 groups on the two types of water in PECSMs (b).

permeate is maintained above 99 wt%. This result further indicated that the structures of PECSMs are stable. After a continuous test of 68 h, the ethanol content in the feed can be increased to 99 wt%. It should be mentioned that PECSM-20 exhibits flux at 412 g/m2 h while the water content in permeate keeps above 99.5 wt% when the water concentration in feed is 5 wt%. This result is also competitive compared with the previous membranes for pervaporation dehydration of ethanol [12,20,40,41], which give flux less than 300 g/m2 h while the water content in permeate is above 99 wt%. 3.3. Comparison of pervaporation performance of chitosan-based composite membranes Chitosan has been readily utilized as a type of membrane material in the pervaporation dehydration of ethanol, by virtue of its superior ability of membrane formation and excellent hydrophilic property, as well as ease of modification [44]. Table 3 lists the pervaporation performance of recently reported chitosan based membranes and PECMs in our previous work. It can be observed that the PEC membranes show good performance for the dehydration of aqueous ethanol by pervaporation. This is attributed to the merit of high hydrophilicity and ionic crosslinking properties of PECMs [15]. It is seen that PECSM-20 shows higher performance compared with the other polyelectrolyte membranes based on chitosan. PECSM-20 also shows a higher PSI, which is commonly viewed as a vital parameter for evaluating separation performance. It should be mentioned that the thickness of GACS/PCP/PAN membrane is 0.2 μm, whereas that of PECSM-20 is ca. 4.5 μm.

1500

1500

1200

1200

Flux (g/m2h)

Flux (g/m2 h)

X.-S. Wang et al. / Journal of Membrane Science 452 (2014) 73–81

900

600

0

40

50

60

900 600 300

PECSM-0 PECSM-7 PECSM-13 PECSM-20

300

79

0

0

10

20

30

40

50

60

70

Operation time (h)

70

T

Ethanol in feed (wt.%)

Separation factor ( )

1800

1500

1200

PECSM-0

900

b

96

96 92

94 92

88

PECSM-7 90

PECSM-13

600

0

PECSM-20 40

50

60

70

Fig. 9. Effect of feed temperature on the flux (a) and the separation factor (b) of PECSMs in the dehydration of 10 wt% water–ethanol mixtures.

2000

1200 1200 800 800

400

0

5

10

15

20

25

30

Separation factor ( )

1600

20

30

40

50

60

70

Fig. 11. Batch pervaporation dehydration of ethanol for PECSM-20 with operation time at 70 1C. Flux (a), and ethanol content in feed and water content in permeate (b).

Table 3 Comparison of pervaporation performance of chitosan-based composite membranes.

2000

1600

400

10

Operation time (h)

T

Flux (g/m2 h)

98

Water in permeate (wt.%)

100

100

0

Operation time (h) Fig. 10. Pervaporation performance stability of PECSM-20 in the dehydration of 10 wt% water–ethanol mixtures at 70 1C.

Therefore, PECSM-20 exhibits an obvious advantage over the other membranes. Moreover, PECSM-20 also shows higher separation performance compared with our previously reported PECMs containing no SO3 groups [21] and complexed SO3 groups [27] with the same “acid protection” method. It is proposed that the good separation performance of PECSMs presumably results from the water channel structures that are correlated with the acid protection strategy [17] and the highly hydrophilic property of free SO3 groups.

Membrane

α Feed Feed J ethanol temperature (g/m2 h) (wt%) (1C)

CS/PAN CS-PVA/PAN CS-MWNTs/PAN GACS/PCP/PAN CS-PAA/PSf SCMC-PDDA/PSf CS-CMC/PSf PECSM-20/PSf

90 95 90 90 95 90 90 90

70 60 70 80 30 70 70 70

1247 320 400 1390 1008 1350 1140 1385

PSI References (  105)

256 3.19 93.7 0.30 500 2 1279 17.76 132 1.329 612 8.26 991 11.29 1570 21.67

[42] [12] [43] [16] [19] [27] [21] This study

Note: SCMC-PDDA PECMs contain 25 mol% complexed SO3 groups. PAN: polyacrylonitrile; PSf: polysulfone; GACS: glutaraldehyde crosslinked chitosan; CP: carbopol; PAA: poly (acrylic acid); PVA: poly (vinyl alcohol); SCMC: sulfated carboxymethyl cellulose; PDDA: poly (diallydimethylammonium chloride); and MWNTs: multi-walled carbon nanotubes.

4. Conclusion Polyelectrolyte complexes containing free SO3 groups (PECSs) were successfully prepared by the sulfation procedure. The compositions of PECSs and their corresponding PECSMs were tuned by varying the degree of sulfation. Fourier transform infrared spectroscopy and ζ potential revealed that free SO3 groups were successfully introduced into PECSMs. The pervaporation results showed that free SO3 groups effectively improved the separation performance of

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PECSMs in the dehydration of ethanol. For instance, the flux for PECSM-20 was up to 2.2 times higher than the pristine PECSM-0 in dehydrating 10 wt% water–ethanol mixtures at 50 1C, and the separation factor of PECSM-20 was maintained at a high level. The permeation flux of PECSMs increased along with their separation factor was almost invariable with increasing temperature. PECSM-20 showed a very high pervaporation performance in the dehydration of 10 wt% water–ethanol mixtures at 70 1C, with the flux and the separation factor at 1385 g/m2 h and 1571, respectively. Moreover, both the flux and the selectivity of PECSMs were stable versus the operation time. The improved pervaporation performance of PECSMs was attributed to the incorporation of free SO3 groups, which highly enhanced the hydrophilicity and ESD, as well as the sorption selectivity of PECSMs.

Acknowledgments This research was financially supported by the NNSFC (51173160, 21376206), the Fundamental Research Funds for the Central Universities (No. 2013QNA4049), and the National Basic Research Program of China (No. 2009CB623402).

Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013.10.028.

Nomenclature

α αs αd

separation factor sorption selectivity diffusion selectivity CMC sodium carboxymethyl cellulose SCMC sulfated sodium carboxymethyl cellulose SO3 sulfate ESD equilibrium swelling degree CS chitosan J total flux (g/m2 h) PECs polyelectrolyte complexes PECSs PECs containing SO3 groups PECS-X PECSs with X molar percent SO3 groups PECS-0 PECS containing no SO3 groups PECMs polyelectrolyte complexes membranes PECSMs PECMs containing SO3 groups PECSM-Y PECSM with Y molar percent SO3 groups PECSM-0 PECSM containing no SO3 groups

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