High performance composite membranes with a polycarbophil calcium transition layer for pervaporation dehydration of ethanol

Journal of Membrane Science 429 (2013) 409–417

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High performance composite membranes with a polycarbophil calcium transition layer for pervaporation dehydration of ethanol Cuihong Zhao a, Hong Wu a,b, Xianshi Li a, Fusheng Pan a, Yifan Li a, Jing Zhao a, Zhongyi Jiang a,n, Peng Zhang c, Xingzhong Cao c, Baoyi Wang c a

Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, China c Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b

a r t i c l e i n f o

abstract

Article history: Received 18 September 2012 Received in revised form 24 November 2012 Accepted 24 November 2012 Available online 3 December 2012

A new kind of composite membranes, consisting of a polyacrylonitrile (PAN) support layer, a polycarbophil calcium (PCP) transition layer and a glutaraldehyde cross-linked chitosan (CS) active layer, was prepared by sequential casting and spin coating methods. A dense region was formed between the active layer and the transition layer as confirmed by positron annihilation spectroscopy. No distinct boundary between these two layers was observed by field emission scanning electron microscope and the total thickness of these two layers was around 200 nm. The static water contact angle revealed that the PCP layer promoted interfacial compatibility between the PAN layer and the CS layer. T-peel test verified that the interfacial adhesion strength of the composite membrane was enhanced by the incorporation of PCP transition layer. The composite membranes were utilized for pervaporation dehydration of ethanol. Even when the PCP concentration was only 0.05 wt%, the CS/PCP/ PAN membrane exhibited a high performance with the separation factor of 1279 and the permeation flux of 1390 g/m2h for 90 wt% ethanol aqueous solution at 353 K with a flow rate of 60 L/h and a pressure of 0.1 kPa at the downstream side of the membrane. Furthermore, the composite membrane displayed desirable stability during the long-term continuous operation. & 2012 Elsevier B.V. All rights reserved.

Keywords: Polycarbophil calcium Transition layer Composite membrane Pervaporation Dehydration

1. Introduction The pervaporation (PV) technology has demonstrated distinct advantages in separating thermal-sensitive compounds, azeotropes, close-boiling mixtures, and isomers owing to its mild operation conditions, relative simplicity and low energy cost [1–3]. Among the diverse applications of pervaporation technology, dehydration of organics is most widely developed. It is well known that one of the key targets in pervaporation R&D is to fabricate membranes with high permeability, selectivity, and structural stability [4]. Compared with the homogeneous membrane, the composite membrane which bears a thin, dense active layer and a thick, porous support layer has the greater potential to offer higher separation performance, as well as higher mechanical stability, and thus more competitive for industrial applications [5]. In recent years, tremendous efforts have been dedicated to design and fabricate polymeric composite membranes with novel and

n Correspondence to: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, 92# Weijin Road, District Nankai, Tianjin 300072, China. Tel./fax: þ86 22 23500086. E-mail address: [email protected] (Z. Jiang).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.11.063

controlled structure for efficient pervaporation dehydration from organic–water mixture. Throughout the numerous existing research works, most attention has been paid to constructing the active layer of composite membrane with relatively high water sorption capacity [6,7]. Thus selecting a hydrophilic active layer or modifying an existing membrane to include such feature is often desirable. Several kinds of methods such as cross-linking [8–12], grafting [13], blending [14–16], filling [17,18] and layer-by-layer assembly [6,19–21] are utilized to tune the physicochemical properties and microstructure of the active layer, in particular, to increase the hydrophilicity. To ensure the long-term utilization of the composite membrane, the support layer is often made of relatively hydrophobic materials. However, the more hydrophilic the active layer is, the weaker interfacial compatibility between the active layer and the support layer will become, resulting in greater difficulties and challenges in fabricating composite membrane with ultrathin and robust active layer. Therefore, the appropriate physicochemical property matching of active layer and support layer becomes one of the major concerns in designing and fabricating high performance composite membranes. One feasible strategy to enhance interfacial compatibility is to modify the support layer. Since the support layer is relatively

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hydrophobic, hydrophilic modification such as hydrolysis [14,22,23] or grafting [13,24] is often conducted. However, the majority of modification reactions occurs heterogeneously and may lead to pore shrinkage or collapse of support layer [25]. Another strategy is to introduce a transition layer between active layer and support layer. In our previous study, due to its excellent adhesive capacity, polydopamine (PDA) was introduced into chitosan/polyethersulfone (CS/PES) composite membrane as the transition layer [26]. The thickness of the PDA layer was only 182 nm and the permeation flux of the membrane was as high as 2280 g/m2h. However, the PDA only offered little assistance to enhance the selectivity of the membrane. In another work, carbopol (CP), a mucoadhesive polymer, was chosen as the transition layer for chitosan/polyacrylonitrile (CS/PAN) membrane [27]. The CP layer was relatively thick (775 nm), and the composite membrane displayed the permeation flux of 1247 g/m2h and the separation factor of 256. It can be naturally conjectured that an eligible transition layer should have the following features: (i) the hydrophilicity of the transition layer should lie between that of active layer and that of support layer; (ii) the transition layer should be thin and robust enough; (iii) the transition layer should have moderate interaction with the active layer and the support layer. In the present study, polycarbophil calcium (PCP) is introduced as the transition layer of the CS/PAN composite membrane. As one of the most potent mucoadhesive polymers, PCP is the calcium salt of polyacrylic acid cross-linked with divinyl glycol, and the numerous carboxylic groups on the polymer backbone impart PCP with multifunctionality and rich properties. Besides, PCP as a polyanion could form polyelectrolyte complex with CS polycation. It has been recognized gradually that preparation of polyelectrolyte complex membranes is one of the effective ways to drastically enhance the pervaporation performance [28–30]. However, there have been few reports on the preparation of the polyelectrolyte membranes with PCP as the polyanion and no PCP-containing membranes have been utilized for alcohol dehydration by pervaporation process. In this study, the membrane performance was evaluated by pervaporation separation of ethanol from aqueous solution. The effects of PCP concentration, feed concentration, operation temperature and operation stability on membrane permeability and selectivity were investigated, respectively.

2. Experimental 2.1. Materials Polycarbophil calcium was purchased from Hubei Longtai BioPharmaceutical Co. Ltd. (Hubei, China). Chitosan with 90.2% Ndeacetylation degree (a viscosity-average molecular weight of 450,000) was supplied by Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan, China). PAN ultrafiltration membranes with a molecular weight cut-off of 100,000 were gained from Shanghai MegaVision Membrane Engineering & Technology Co. Ltd. (Shanghai, China). Glutaraldehyde with 50 wt% and hydrochloric acid (HCl) standard solution with 0.10 M were gained from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Acetic acid (glacial) and absolute alcohol were purchased from Tianjin Guangfu Science and Technology Development Co. Ltd. (Tianjin, China). All the reagents were of analytical grade and were not further purified. Deionized water was used through the whole experiments.

2.2. Preparation of the membranes In this study, CS/PCP/PAN composite membranes were fabricated by sequential casting and spin coating methods. PAN ultrafiltration membranes (the area of 9 cm  9 cm) were soaked in deionized

water for 24 h and then fully dried at room temperature to be the support layers of the composite membranes. Six polymer solutions of different PCP concentration (0.05, 0.1, 0.5, 1, 3, 5 wt%) were prepared by dissolving the PCP in deionized water at room temperature. The pH value of each solution was regulated to 3.0 by HCl. After filtration, the solution was cast on the surface of the pretreated PAN support layer and then dried at room temperature for 24 h. The as-prepared membranes were denoted as PCP(X)/PAN, where X meant the mass percentage of PCP in its aqueous solution, ranging from 0.05 wt% to 5 wt%. 2 wt% CS solution was obtained by dissolving CS into 2 wt% acetic acid solution with stirring at 353 K for 1 h. After cooling to room temperature, the given amount of cross-linker GA (2.5 wt%) was added under stirring for 1 h. Upon filtration, the solution was spin-coated onto the prepared PCP(X)/PAN membrane, and the specific operation as follows: firstly, the CS solution was spread on the PAN substrate until the surface was entirely covered, and the substrate was then accelerated to the desired rotation rate: the rotation rate was set at 500 rd/min for 20 s, then increased to 1000 rd/min for 40 s, and finally increased to 2000 rd/min for 20 s. Ultimately, the membranes were dried at room temperature for 24 h. The resultant composite membranes were denoted as CS/PCP(X)/PAN, where X meant the mass percentage of PCP in its aqueous solution, ranging from 0.05 wt% to 5 wt%. For comparison, CS/PAN membrane was also prepared by directly spin coating CS onto the PAN substrate. 2.3. Membrane characterization 2.3.1. Fourier transform infrared (FT-IR) spectroscopic analysis FT-IR spectra for each layer of the composite membrane were acquired by using a Nicolet-560 Fourier transform infrared spectrometer with scan range of 4000–400 cm  1 and resolution of 1.93 cm  1. 2.3.2. Field emission scanning electron microscope (FESEM) PCP(0.05)/PAN and CS/PCP(0.05)/PAN composite membranes were cryogenically fractured in liquid nitrogen and then sputtered with gold. The cross-section morphologies of the membrane samples were examined with a Nanosem 430 field emission scanning electron microscope operated at 10 kV. 2.3.3. Contact angle Contact angle goniometer (JC2000C Contact Angle Meter, Powereach Co., Shanghai, China) was utilized to measure the static contact angles of CS active layer, PCP transition layer and PAN support layer with the probe liquid of water. Water droplets (about 5 mm in diameter) were dropped onto at least 6 different sites of each membrane surface. An average value was obtained for the measured contact angle with an error of 5%. 2.3.4. Positron annihilation spectroscopy (PAS) Currently, positron annihilation spectroscopy technique with variable mono-energy slow positron beam is capable of probing the depth profile of defects in the membrane. The Doppler broadening energy spectra as one of positron annihilation spectrometers can be characterized by the S parameter, which is defined as the ratio of the central part of the annihilation spectrum to the total spectrum [31]. The S parameter is measured as a function of positron implantation energy, which can be converted to the mean implantation depth by using the following empirical equation [32]:   40 1:6 Re ¼ ð1Þ E

r

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

2.3.5. T-peel test The interfacial strengths of CS/PAN and CS/PCP(0.05)/PAN composite membranes were tested by a Micro-Uniaxial Fatigue Testing system (MUT-1020, CMC). The size of sample was 8 mm (width)  30 mm (length). The displacement-dependent load curves were recorded with a loading rate of 0.5 mm/s at room temperature.

1100cm-1

3420cm-1 Transimitance (%)

where Re represents the mean implantation depth (nm), r is the target density (g/cm3) and E is the positron incident energy (keV). The S parameter versus the converted depth reflects the relative value of the free volume depth profile in membrane. In this study, PAS was used to probe the free volume for the composite membrane by Beijing high-purity Ge detector with a 22Na slow positron beamline at the room temperature, and the energy of the positrons could be continuously varied in the range of 0.18–20 keV.

The pervaporation experiment was conducted according to our previous work [33]. The core part of the pervaporation equipment was the P-28 membrane module made by CM-Celfa AG Company, Switzerland. During the experiment, the effective membrane area in contact with feed was 2.56  10  3 m2. The feed flow rate was 60 L/h, and the downstream pressure was maintained about 0.1 kPa by a vacuum pump. After the pervaporation experiment achieved the steady state (about 2 h later), the permeate vapor was collected in traps immersed in liquid nitrogen. The concentrations of water and ethanol in collected samples and feed solution were analyzed by HP4890 gas chromatography (GC) [27]. Unless particularly specified, all the pervaporation experiments were performed with 90 wt% ethanol aqueous solution at 353 K. Permeation flux J, separation factor a and pervaporation separation index (PSI) were used to evaluate pervaporation performance of membrane, and were defined by the following equations: J¼

M At

ð2Þ

a¼

P W =PE F W =F E

ð3Þ

PSI ¼ J ða1Þ

ð4Þ

where M is the mass (g) of permeate in operation time t (h), and A is the effective membrane area (m2); P and F represent the weight fractions of water (W) and ethanol (E) in the permeate solution and feed solution, respectively.

3. Results and discussion 3.1. Membrane characterization 3.1.1. FT-IR The FT-IR spectra for PAN, PCP(0.05)/PAN and CS/PCP(0.05)/ PAN membranes are shown in Fig. 1. Comparing the FT-IR spectrum of PCP(0.05)/PAN membrane with that of PAN membrane, the appearance of characteristic peaks of PCP at 3420 cm  1, 1740 cm  1, 1420 cm  1, and 1100 cm  1 which were ascribed to –OH, –CQO, –COO and C–OH stretching vibration respectively verified that PCP had been coated on PAN support layer. Whereas, the characteristic peak of PAN at 2242 cm  1 corresponding to the –CRN stretching vibration still existed. The reason was that some of PCP chains may intrude into the pores of PAN membrane, resulting in the incomplete coverage of

CS/PCP (0.05)/PAN

1560 cm-1

1740cm-1 3420cm-1

PCP (0.05)/PAN

PAN

4000 2.4. Pervaporation experiment

411

3500

3000

1420cm-1

2242cm-1

2500 2000 Wavenumber (cm-1)

1500

1000

Fig. 1. FT-IR spectra of PAN, PCP(0.05)/PAN and CS/PCP(0.05)/PAN membranes.

PCP layer on PAN layer. After coating CS on PCP/PAN membrane, the intensity of the characteristic peaks of PCP at 1740 cm  1 and PAN at 2242 cm  1 displayed an apparent decline. Meanwhile, the characteristic CS peak of N–H in plane deformation at 1560 cm  1 can be observed clearly. These phenomena implied that CS layer covered the majority surface of PCP/PAN membrane. 3.1.2. FESEM FESEM was used to observe the cross-section morphology of composite membrane. Fig. 2(a) and (b) presented that there was no interspace between PCP layer and PAN layer; Fig. 2(c) and (d) showed that there was no apparent boundary between CS layer and PCP layer. These implied the favorable compatibility between the adjacent layers, and the total thickness of CS layer and PCP layer was only about 200 nm. The proposed interfacial interaction and schematic structure for CS/PCP/PAN composite membrane are shown in Fig. 3. As a kind of polyanion, PCP could form ionic bonds with CS (polycation) via the carboxy group (–COOH) of PCP and the amino group (–NH2) of CS [34]. Upon the contact of CS and PCP, the strong interaction between these two polymers could prompt the formation of entangled network, and no apparent boundary between PCP layer and CS layer could be observed. In addition, hydrogen bonds could be formed between the carboxy group (–COOH) of PCP and the cyano group (–CN) of PAN. 3.1.3. Contact angle Contact angle could indicate relative hydrophilicity or hydrophobicity of membrane material. In this study, deionized water was utilized as the probe liquid. The smaller the contact angle was, the more hydrophilic the membrane material was. As shown in Table 1, the contact angle of PCP was between that of PAN and that of CS, verifying that the introduction of PCP contributed to reducing the difference in the hydrophilicity of CS layer and PAN layer, which was helpful to improve the interfacial compatibility of these two layers. 3.1.4. PAS To establish the correlation between membrane structure and separation performance, positron annihilation spectroscopy was utilized in this study. The S parameter was highly sensitive to the presence of free volume cavities in polymeric systems. A decrease in the S parameter indicated the size or concentration of the positron trapping cavities decreased [35], which could reflect the

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Fig. 2. FESEM images of cross-section: (a) and (b) PCP(0.05)/PAN membrane at different scales; (c) and (d) CS/PCP(0.05)/PAN membrane at different scales. CH2OH O

CS

CH2OH

H

O

O

OH

H

H

O

H

CS

OH

H H

+

NH3

n

H

-

COO

+

NH3

COOH

n

H

PCP

-

COOH

COO

PAN n

n

PCP

COOH CN

PAN

H2 C

C H

H

OH

H

OH

n

n COOH

COOH

CN H2 C

C H

CN H2 C

C H

COOH

CN H2 C

C H

CN H2 C

C H

n

Fig. 3. (a) Proposed interfacial interaction and (b) schematic structure for CS/PCP/PAN composite membrane.

Table 1 Contact angles of water probe liquid on the PAN, PCP, CS layers. Layer

Contact angle (1)

PAN PCP CS

55.7 42.5 36.9

increase of membrane compactness. In Fig. 4(a), the S–E curve of CS/PCP(0.05)/PAN membrane had the lowest value at the positron incident energy of 1.9 keV, which corresponded to the mean depth of about 167 nm. There was strong interaction between PCP and CS, and the total thickness of the CS layer and PCP layer was around 200 nm, it was thus assumed that the lowest S location occurred at the interfacial region.

According to the mean depth profile and curve characteristics, three major zones for CS/PCP(0.05)/PAN could be proposed—(1) the surface and the bulk of CS active layer (zone I), (2) the dense interfacial region including PCP layer (zone II), (3) the PAN substrate (zone III). Since the PCP(0.05)/PAN membrane contained only two layers, in order to have a direct comparison of the free volume characteristics in the corresponding zones, we intentionally translated the origin of the curve of PCP(0.05)/PAN membrane to the boundary between the zone I and zone II (the original curve of PCP(0.05)/PAN membrane was not shown here). It is worth mentioning that since the thickness of the PCP layer was too thin, PAS measurement could not give the exact location of the origin of the PCP(0.05)/PAN curve. In zone I, with higher positron energy, the S parameter value of CS/PCP(0.05)/PAN membrane firstly increased and then declined, indicating that the compactness of the CS active layer became lower when approaching the interfacial region. In zone II, the S value of CS/

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

Mean depth (nm) 0

181

549

1050 1663 2377 3182 4072 5042

0.480 III 0.475

I

0.470

PCP(0.05)/PAN CS/PCP(0.05)/PAN

0.465 0

2

4

6

8

10

12

14

16

Positron incident energy (keV) Mean depth (nm) 0

181

549

1050 1663 2377 3182 4072 5042

3.1.5. T-peel test To investigate the effect of introducing PCP on interfacial adhesion strength of composite membrane, T-peel test was carried out for CS/PAN, CS/PCP(0.05)/PAN membranes. As shown in Fig. 5, the highest peeling strength for CS/PCP(0.05)/PAN was 1.8 N, which was about 1.5 times larger than that for CS/PAN ( o1.2 N). Moreover, when the peeling strength increased from 0.25 N to 0.8 N, the displacement of CS/PCP(0.05)/PAN membrane was nearly zero, while the displacement of CS/PAN membrane was about 2 mm. Therefore, it can be derived that the interaction between the adjacent layers arising from PCP incorporation could strengthen peeling resistance of the composite membrane and enhance the stability of the composite membrane. 3.2. Pervaporation performance of CS/PCP/PAN composite membranes

0.480 III II

0.475 S parameter

the PCP concentration, the increase rate of S value in PAN layer (zone III) slowed down. The reason was that the PCP might intrude into the pores of PAN layer, interacting with PAN chains and filling some pores therein. Since the viscosity of PCP was low, when the PCP concentration was in the range of 1 to 5 wt%, more PCP intruded into the pores of PAN layer at more intruding depth, causing that the compactness of PAN layer increased more slowly.

I 0.470

CS/PCP(5)/PAN CS/PCP(1)/PAN CS/PCP(0.1)/PAN CS/PCP(0.05)/PAN

0.465

0.460 0

2

4

6

8

10

12

14

16

Positron incident energy (keV) Fig. 4. S parameter as a function of the incident positron energy for (a) CS/ PCP(0.05)/PAN and PCP(0.05)/PAN membranes and (b) CS/PCP(5)/PAN, CS/PCP(1)/ PAN, CS/PCP(0.1)/PAN and CS/PCP(0.05)/PAN membranes.

PCP(0.05)/PAN membrane was much lower than that of the PCP(0.05)/PAN membrane. This could be explained by the strong interaction between CS and PCP, which caused interface densification and consequently compromised the membrane compactness. In zone III, the S value of the CS/PCP(0.05)/PAN and PCP(0.05)/PAN membranes both gradually increased with higher positron energy, indicating that the compactness of PAN layer decreased gradually. In Fig. 4(b), comparing the S–E curves of CS/ PCP/PAN composite membranes, with the increase of PCP concentration, the S value in zone I and zone II decreased, implying that the structure in these two zones became less compact. This result could be ascribed to the more interaction sites and higher interaction strength between CS and PCP with the increase of PCP concentration. Meanwhile, since the thickness of the CS active layer was only 200 nm, its structure was prone to be affected by the adjacent layer(s), especially the PCP layer interacting strongly with CS layer [36]. Thus the CS bulk-like properties may cease to govern the film behavior [37]. With the increase of PCP concentration, the effect of PCP layer became more pronounced, and consequently the compactness of the CS layer increased with the increase of the PCP concentration. In addition, with increasing

3.2.1. Effect of PCP concentration on pervaporation performance To illustrate the effect of PCP concentration on membrane pervaporation performance, CS/PCP/PAN membranes with different PCP concentration ranging from 0 to 5 wt% were fabricated for pervaporation. In Fig. 6, with the increase of the PCP concentration, the permeation flux showed a descending trend, while the separation factor first increased and then decreased. It was confirmed by PAS that the compactness of CS/PCP/PAN composite membrane increased with the increase of PCP concentration, which resulted in the permeation flux continuously decreasing. When the PCP concentration was in the range from 0.1 to 1 wt%, the membrane microstructure may be detrimental to ethanol permeation through the membrane, leading to the higher separation factor. With increasing the PCP concentration, the membrane compactness further increased which may hinder the water permeation to a certain extent. Moreover, when the PCP concentration was in the range of 1 to 5 wt%, the intrusion of PCP into the pores of PAN substrate became more severe, which also

2.0 CS/PCP (0.05)/PAN 1.5 Load (N)

S parameter

II

413

1.0 CS/PAN 0.5

0.0 0

2

4 6 Displacement (mm)

8

10

Fig. 5. T-peel load versus displacement curves for CS/PAN and CS/PCP(0.05)/PAN membranes.

414

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

1800

1200 2800

2500 1000

1500

2400

900 1000

Total flux (g/m2h)

1500

600 500

800

2000

600

1600

400

1200

200

800

0

400

300 0 0.0 0.1 0.2 0.3 0.4 0.5 2 PCP concentration (wt.%)

290

4

where Ap , J p , Ep , R and T represent the pre-exponential factor, permeation flux, apparent activation energy, gas constant and feed temperature respectively. The ln Jp versus 1000/T curves (in Fig. 8) for water and ethanol were very similar to straight lines, implying that the permeability of water and ethanol followed the Arrhenius law. By the further calculation, with the temperature ranging from 323 K to 353 K, the apparent activation energy of water (43.78 kJ/mol) was larger than that of ethanol (20.95

320

330

340

350

360

Water flux (g/m2h)

1200

20

1000

16

800

12

600 8 400 4

Ethanol flux (g/m2h)

200 0 0 290

300

310

320

330

340

350

360

Operation temperature (K) Fig. 7. Effect of operation temperature on pervaporation performance of CS/ PCP(0.05)/PAN membrane.

water ethanol 6 EP,W = 43.78 kJ/mol ln Jp

3.2.2. Effect of operation temperature on pervaporation performance Operation temperature had great impact on small molecule diffusion in the membrane, which directly influenced permeation performance of small molecules through the membrane. As shown in Fig. 7, with the operation temperature increasing from 293 K to 353 K, the total permeation flux and separation factor both increased (in Fig. 7(a)), and water permeation flux increased significantly, while ethanol permeation flux increased slightly (in Fig. 7(b)). With the increase of temperature, partial pressures and mobility of permeation molecules (water and ethanol) increased, and consequently increased the driving force for diffusion. Meanwhile, the increasing temperature could enhance the thermal mobility of polymer chains, leading to the greater free volume cavities. These factors both promoted diffusion of water and ethanol molecules. The trend of permeation flux versus temperature could be described by Arrhenius equation (Eq. (5)) and apparent activation energy also would be figured out according to the equation:   Ep J p ¼ Ap exp  ð5Þ RT

310

Operation temperature (K)

Fig. 6. Effect of PCP concentration on pervaporation performance of CS/PCP/PAN membranes.

increased the resistance of water permeation, leading to the decrease of the separation factor. Considering that high permeation flux is preferred in most cases, the PCP concentration of 0.05 wt% was chosen, where the separation factor was 1279 and the permeation flux was 1390 g/m2h. Compared with the CS/PAN membrane with the separation factor of 73 and the permeation flux of 1635 g/m2h, the CS/PCP/PAN membrane greatly enhanced the separation factor of CS/PAN membrane, without significant decrease of the permeation flux. The interaction between the polyanion and the polycation was utilized to improve the separation performance of composite membranes. PCP as a polyanion with numerous carboxyl groups provided abundant interacting sites with the CS polycation. And the resulting dense interfacial region with even low PCP concentration could play an important role in manipulating membrane separation performance.

300

Separation factor

1200

Separation factor

Flux (g/m2h)

2000

4

2 EP,E = 20.95 kJ/mol 0 2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

1000/T (K-1) Fig. 8. Arrhenius plots of permeation flux for separating ethanol/water mixture by CS/PCP(0.05)/PAN membrane.

kJ/mol), which indicated that water permeability was more sensitive to temperature than ethanol permeability. Hence, with the increase of temperature, the increasing rate of water flux was greater than that of ethanol flux, which would enhance the separation factor [38,39].

3.2.3. Effect of ethanol concentration in feed on pervaporation performance The effect of ethanol concentration in feed on pervaporation performance is presented in Fig. 9. With the increase of ethanol

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

2200

2000

6000

800

400

Flux (g/m2h)

Flux (g/m2h)

1400

5000

85 90 Ethanol concentration in feed (wt.%)

4000 1200

3000

1200

800

2000

1000

400

1000

800 80

1600

Fig. 9. Effect of ethanol concentration in feed on pervaporation performance of CS/ PCP(0.05)/PAN membrane.

concentration in feed, the permeation flux decreased and separation factor increased. A great deal of hydrophilic groups (such as –OH, –NH2) in CS layer would prefer to absorb water molecule rather than ethanol molecule. Meanwhile, the dense region between CS and PCP was in the form of polyelectrolyte complex, which was always sensitive to water but inert to organic solvent [40]. At high water concentration, the CS structure and the interface complex structure were loosened which reduced the permeation resistance. Therefore, the permeation flux increased with the water concentration increasing. However, the looser the membrane structure was, the lower the membrane selectivity was, and namely, the separation factor decreased with increasing the water concentration. 3.2.4. Operation stability To investigate the stability of CS/PCP(0.05)/PAN membrane, the as-fabricated membrane was continuously operated for over 160 h. As shown in Fig. 10, at the beginning, the permeation flux decreased and the separation factor increased. After 48 h, the variation trend of the flux and separation factor both slowed down. It should be noted during the whole operational stability test, the permeation flux and separation factor did not change significantly. Besides, the detachment of the neighboring layers did not occur. These results verified CS/PCP(0.05)/PAN composite membrane possessed good long-term operation stability. 3.3. Comparison of pervaporation performance of chitosan-based composite membranes In recent decades, CS has become a promising membrane material in the dehydration of ethanol, due to its excellent film forming ability, ease of modification and superior affinity towards water [41]. Table 2 compares the pervaporation performance of chitosan-based polymer composite membranes for the dehydration of ethanol reported in the literatures. It could be seen that the PSI of these composite membranes was mostly lower than 5  105, and the permeation flux and separation factor of majority membranes were not simultaneously high. Recently, the polyelectrolyte complexes have been widely used to fabricate pervaporation membranes for dehydration of ethanol. It has been speculated that polysalts, formed from anionic polyelectrolytes and cationic polyelectrolytes, would be appropriate for acquiring membranes with both high permeability and selectivity [42]. As shown in Table 2, since CS and CMCNa, CS and PCP could form polyelectrolyte pairs, the comprehensive pervaporation performances were superior to other kinds of membranes. In this study, the PSI of CS/PCP/PAN

0

0

95

Separation factor

1600

1200

2000 Separation factor

1800

7000

2400

2000 1600

415

0

24

48

72 96 120 Operation time (h)

144

168

Fig. 10. Operation stability on pervaporation performance of CS/PCP(0.05)/PAN membrane.

membrane was higher than that of CS–CMCNa/PSf membrane, which may result from the difference in membrane fabrication method.

4. Conclusion In this study, CS/PCP/PAN composite membranes with threelayered structure were prepared by sequential casting and spin coating methods. The hydrophilicity of the PCP transition layer was between that of CS active layer and that of PAN support layer, which was helpful to improve the interfacial compatibility. The total thickness of CS active layer and PCP transition layer was as thin as 200 nm, which added little mass transport resistance. Due to the formation of ionic bond and hydrogen bond, the interfacial interaction between the neighboring layers enhanced, and the dense interfacial region between the transition layer and the active layer was propitious to elevate the separation performance of composite membrane. Compared with the CS/PAN membrane with the separation factor of 73 and the permeation flux of 1635 g/m2h, the CS/PCP(0.05)/PAN membrane exhibited enhanced performance with the separation factor of 1279 and the permeation flux of 1390 g/m2h. During the long-term operation, the composite membrane showed high and stable pervaporation performance and no peeling phenomenon between the neighboring layers occurred. The interaction strength between the active layer and transition layer, the correlation between the interfacial interaction and the membrane separation performance will be explored by molecular simulation and experimental investigations in our future work.

Nomenclature Symbols Re

r E J, Jp A M t

a F, P PSI

Mean implantation depth (nm) Target density (g/cm3) Positron incident energy (keV) Permeation flux (g/m2h) Effective membrane area (m2) The mass of permeate (g) Time (h) Separation factor Weight fractions in the feed and the permeate solutions (wt%) Pervaporation separation index

416

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

Table 2 Comparison of pervaporation performance of chitosan-based composite membranes for dehydration of ethanol. Mass ratio (EtOH/H2O)

Composite membrane

Thickness of Temperature active layer (mm) (K)

Flux (g/m2h)

Separation factor

PSI (  105)

Ref.

90:10 95:5 90:10 90:10 90:10 90:10 95:5 90:10 90:10 90:10 95:5

GACS/PDA/PES CS–PVA/PAN GACS/CP/PAN GACS/PAN CS–HEC/CA GACS/mPAN CS–PAA/PSf GACS/PCP/PAN CS–CMCNa/PSf CS–MWNTs/PAN Ionically surface crosslinked CS/PES

10 15 2 – 4 – 3 0.2 5 3.3 10

2280 1450 1247 456 424 330 132 1390 1140 400 350

56 11 256 54 5469 1410 1008 1279 1062 500 500

1.277 0.145 3.18 0.242 23.18 4.65 1.329 17.76 12.11 2 1.746

[26] [43] [27] [44] [45,46] [44] [47] This work [48] [49] [10]

353 323 353 333 333 343 303 353 343 343 353

GACS: glutaraldehyde crosslinked chitosan; MA: maleic anhydride; PAN: polyacrylonitrile; mPAN: hydrolyzed polyacrylonitrile; HEC: hydroxyethylcellulose; CA: cellulose acetate; PSf: polysulfone; PES: polyethersulfone; PAA: polyacrylic acid; PVA: poly (vinyl alcohol); CMCNa: sodium carboxymethyl cellulose; MWNTs: multiwalled carbon nanotubes; PDA: polydopamine; CP: carbopol.

Ap Ep T R

Pre-exponential factor Apparent activation energy (kJ/mol) Feed temperature (K) Gas constant

Subscripts E W

Ethanol Water

Acknowledgments The authors gratefully acknowledge the financial support from the National Science Fund for Distinguished Young Scholars (No. 21125627), the National basic research program of China (2009CB623404), Tianjin Natural Science Foundation (No. 10JCZDJC22600), Program for New Century Excellent Talents in University (NCET-10-0623 ) and the State Key Laboratory for Modification of Chemical Fibers and Polymer Materials Dong Hua University, Programme of Introducing Talents of Discipline to Universities (No. B06006). We gratefully thank Professor Xu Chen for his help in the T-peel test. References [1] M. Ulbricht, Advanced functional polymer membranes, Polymer 47 (2006) 2217. [2] B. Smitha, D. Suhanya, S. Sridhar, M. Ramakrishna, Separation of organic– organic mixtures by pervaporation—a review, J. Membr. Sci. 241 (2004) 1. [3] H.L. Castricum, G.G. Paradis, M.C.M. Hazeleger, R. Kreiter, J.F. Vente, J.E. ten Elshof, Tailoring the separation behavior of hybrid organosilica membranes by adjusting the structure of the organic bridging group, Adv. Funct. Mater. 21 (2011) 2319. [4] J. Zhao, J. Ma, J. Chen, F.S. Pan, Z.Y. Jiang, Experimental and molecular simulation investigations on interfacial characteristics of gelatin/polyacrylonitrile composite pervaporation membrane, Chem. Eng. J. 178 (2011) 1. [5] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162. [6] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5. [7] S.G. Adoor, L.S. Manjeshwar, S.D. Bhat, T.M. Aminabhavi, Aluminum-rich zeolite beta incorporated sodium alginate mixed matrix membranes for pervaporation dehydration and esterification of ethanol and acetic acid, J. Membr. Sci. 318 (2008) 233. [8] R.Y.M. Huang, R. Pal, G.Y. Moon, Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes, J. Membr. Sci. 160 (1999) 17.

[9] M. Ghazali, M. Nawawi, R.Y.M. Huang, Pervaporation dehydration of isopropanol with chitosan membranes, J. Membr. Sci. 124 (1997) 53. [10] Y.M. Lee, S.Y. Nam, D.J. Woo, Pervaporation of ionically surface crosslinked chitosan composite membranes for water–alcohol mixtures, J. Membr. Sci. 133 (1997) 103. [11] S.Y. Nam, Y.M. Lee, Pervaporation of ethylene glycol–water mixtures: I. Pervaporation performance of surface crosslinked chitosan membranes, J. Membr. Sci. 153 (1999) 155. [12] G.Y. Moon, R. Pal, R.Y.M. Huang, Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol, J. Membr. Sci. 156 (1999) 17. [13] Y.L. Liu, C.H. Yu, K.R. Lee, J.Y. Lai, Chitosan/poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes, J. Membr. Sci. 287 (2007) 230. [14] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate/chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 167 (2000) 275. [15] Q. Zhao, J.W. Qian, Q.F. An, Q. Yang, P. Zhang, A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration, J. Membr. Sci. 320 (2008) 8. [16] T.R. Zhu, Y.B. Luo, Y.W. Lin, Q. Li, P. Yu, M. Zeng, Study of pervaporation for dehydration of caprolactam through blend NaAlg-poly(vinyl pyrrolidone) membranes on PAN supports, Sep. Purif. Technol. 74 (2010) 242. [17] A.A. Kittur, S.S. Kulkarni, M.I. Aralaguppi, M.Y. Kariduraganavar, Preparation and characterization of novel pervaporation membranes for the separation of water–isopropanol mixtures using chitosan and NaY zeolite, J. Membr. Sci. 247 (2005) 75. [18] S.S. Kulkarni, S.M. Tambe, A.A. Kittur, M.Y. Kariduraganavar, Preparation of novel composite membranes for the pervaporation separation of water– acetic acid mixtures, J. Membr. Sci. 285 (2006) 420. [19] Q. Zhao, J.W. Qian, Q.F. An, Z.W. Sun, Layer-by-layer self-assembly of polyelectrolyte complexes and their multilayer films for pervaporation dehydration of isopropanol, J. Membr. Sci. 346 (2010) 335. [20] Q. Zhao, J.W. Qian, M.H. Zhu, Q.F. An, Facile fabrication of polyelectrolyte complex/carbon nanotube nanocomposites with improved mechanical properties and ultra-high separation performance, J. Mater. Chem. 19 (2009) 8732. [21] J.H. Chen, Q.L. Liu, Y. Xiong, Q.G. Zhang, A.M. Zhu, Composite membranes prepared from glutaraldehyde cross-linked sulfonated cardo polyetherketone and its blends for the dehydration of acetic acid by pervaporation, J. Membr. Sci. 325 (2008) 184. [22] S. Belfer, R. Fainchtain, Y. Purimson, O. Kedem, Surface characterization by FTIR-ATR spectroscopy of polyethersulfone membranes-unmodified, modified and protein fouled, J. Membr. Sci. 172 (2000) 113. [23] W.C. Chao, S.H. Huang, Q.F. An, D.J. Liaw, Y.C. Huang, K.R. Lee, J.Y. Lai, Novel interfacially-polymerized polyamide thin-film composite membranes: studies on characterization, pervaporation, and positron annihilation spectroscopy, Polymer 52 (2011) 2414. [24] Y.L. Liu, C.H. Yu, L.C. Ma, G.C. Lin, H.A. Tsai, J.Y. Lai, The effects of surface modifications on preparation and pervaporation dehydration performance of chitosan/polysulfone composite hollow-fiber membranes, J. Membr. Sci. 311 (2008) 243. [25] Q. Shi, Y.L. Su, S.P. Zhu, C. Li, Y.Y. Zhao, Z.Y. Jiang, A facile method for synthesis of pegylated polyethersulfone and its application in fabrication of antifouling ultrafiltration membrane, J. Membr. Sci. 303 (2007) 204. [26] J. Chen, X. Chen, X. Yin, J. Ma, Z.Y. Jiang, Bioinspired fabrication of composite pervaporation membranes with high permeation flux and structural stability, J. Membr. Sci. 344 (2009) 136.

C. Zhao et al. / Journal of Membrane Science 429 (2013) 409–417

[27] J. Ma, M.H. Zhang, H. Wu, X. Yin, J. Chen, Z.Y. Jiang, Mussel-inspired fabrication of structurally stable chitosan/polyacrylonitrile composite membrane for pervaporation dehydration, J. Membr. Sci. 348 (2010) 150. [28] C.L. Hu, B. Li, R.L. Guo, H. Wu, Z.Y. Jiang, Pervaporation performance of chitosan-poly(acrylic acid) polyelectrolyte complex membranes for dehydration of ethylene glycol aqueous solution, Sep. Purif. Technol. 55 (2007) 327. [29] Z.Q. Zhu, X.S. Feng, A. Penlidis, Layer-by-layer self-assembled polyelectrolyte membranes for solvent dehydration by pervaporation, Mater. Sci. Eng. C 27 (2007) 612. [30] G.J. Zhang, W.L. Gu, S.L. Ji, Z.Z. Liu, Y.L. Peng, Z. Wang, Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures, J. Membr. Sci. 280 (2006) 727. [31] C.H. Lo, J.K. Huang, W.S. Hung, S.H. Huang, M.D. Guzman, V. Rouessac, C.L. Li, C.C. Hu, K.R. Lee, J.Y. Lai, Investigation on the variation in the fine structure of plasma-polymerized composite membrane by positron annihilation spectroscopy, J. Membr. Sci. 337 (2009) 297. [32] P.J. Schultz, K.G. Lynn, Interaction of positron beams with surfaces, thin films, and interfaces, Rev. Mod. Phys. 60 (1988) 701. [33] F.B. Peng, L.Y. Lu, C.L. Hu, H. Wu, Z.Y. Jiang, Significant increase of permeation flux and selectivity of poly(vinyl alcohol) membranes by incorporation of crystalline flake graphite, J. Membr. Sci. 259 (2005) 65. [34] Z.L. Lu, W.Y. Chen, J.H. Hamman, Chitosan-polycarbophil interpolyelectrolyte complex as a matrix former for controlled release of poorly water-soluble drugs I: in vitro evaluation, Drug Dev. Ind. Pharm. 36 (2010) 539. [35] H.M. Chen, W.S. Hung, C.H. Lo, S.H. Huang, M.L. Cheng, G. Liu, K.R. Lee, J.Y. Lai, Y.M. Sun, C.C. Hu, R. Suzuki, T. Ohdaira, N. Oshima, Y.C. Jean, Free-volume depth profile of polymeric membranes studied by positron annihilation spectroscopy: layer structure from interfacial polymerization, Macromolecules 40 (2007) 7542. [36] I. Siretanu, J.P. Chapel, C. Drummond, Substrate remote control of polymer film surface mobility, Macromolecules 45 (2012) 1001. [37] J.N. D’Amour, U. Okoroanyanwu, C.W. Frank, Influence of substrate chemistry on the properties of ultrathin polymer films, Microelectron. Eng. 73–74 (2004) 209.

417

[38] A. Raisi, A. Aroujalian, T. Kaghazchi, A predictive mass transfer model for aroma compounds recovery by pervaporation, J. Food Eng. 95 (2009) 305. [39] P. Wu, R.W. Field, R. England, B.J. Brisdon, A fundamental study of organofunctionalized PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams, J. Membr. Sci. 190 (2001) 147. [40] Y. Kurokawa, N. Shirakawa, M. Terada, N. Yui, Formation of polyelectrolyte complex and its adsorption properties, J. Appl. Polym. Sci. 25 (1980) 1645. [41] D. Xu, S. Hein, K. Wang, Chitosan membrane in separation applications, Mater. Sci. Technol. 24 (2008) 1076. [42] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation: an analytical review, Desalination 110 (1997) 251. [43] B.B. Li, Z.L. Xu, F.A. Qusay, R. Li, Chitosan-poly (vinyl alcohol)/poly (acrylonitrile) (CS–PVA/PAN) composite pervaporation membranes for the separation of ethanol–water solutions, Desalination 193 (2006) 171. [44] X.P. Wang, Y.F. Feng, Z.Q. Shen, Pervaporation properties of a three-layer structure composite membrane, J. Appl. Polym. Sci. 75 (2000) 740. [45] R. Jiraratananon, A. Chanachai, R.Y.M. Huang, D. Uttapap, Pervaporation dehydration of ethanol-water mixtures with chitosan/hydroxyethylcellulose (CS/HEC) composite membranes. I. Effect of operating conditions, J. Membr. Sci. 195 (2002) 143. [46] R. Jiraratananon, A. Chanachai, R.Y.M. Huang, Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethylcellulose (CS/HEC) composite membranes. II. Analysis of mass transport, J. Membr. Sci. 199 (2002) 211. [47] J.J. Shieh, R.Y.M. Huang, Pervaporation with chitosan membranes II. Blend membranes of chitosan and polyacrylic acid and comparison of homogeneous and composite membrane based on polyelectrolyte complexes of chitosan and polyacrylic acid for the separation of ethanol–water mixtures, J. Membr. Sci. 127 (1997) 185. [48] Q. Zhao, J.W. Qian, Q.F. An, C.J. Gao, Z.L. Gui, H.T. Jin, Synthesis and characterization of soluble chitosan/sodium carboxymethyl cellulose polyelectrolyte complexes and the pervaporation dehydration of their homogeneous membranes, J. Membr. Sci. 333 (2009) 68. [49] S. Qiu, L.G. Wu, G.Z. Shi, L. Zhang, H.L. Chen, C.J. Gao, Preparation and pervaporation property of chitosan membrane with functionalized multiwalled carbon nanotubes, Ind. Eng. Chem. Res. 49 (2010) 11667.