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poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes
Journal of Membrane Science 287 (2007) 230–236
Chitosan/poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes Ying-Ling Liu ∗ , Chung-Hao Yu, Kueir-Rarn Lee, Juin-Yih Lai R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chungli, Taoyuan 32023, Taiwan Received 28 August 2006; received in revised form 16 October 2006; accepted 21 October 2006 Available online 27 October 2006
Abstract Chitosan/PTFE composite membranes were prepared from casting a ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS)-containing chitosan solution on poly(styrene sulfuric acid) grafted expended poly(tetrafluoroethylene) film surface. The adhesion between the chitosan skin layer and the PTFE substrate was pretty good to warrant the high performance of chitosan/PTFE composite membranes using in pervaporation dehydration processes on isopropanol. The chitosan/PTFE membrane exhibited a permeation flux of 1730 g/m2 h and a separation factor of 775 at 70 ◦ C on pervaporation dehydration of a 70 wt% isopropanol aqueous solution. The membrane also survived after a long-term operation test in 45 days. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(tetrafluoroethylene); Chitosan; Composite membrane; Pervaporation
1. Introduction Pervaporation dehydration process shows great achievements on separation of organic solvent–water azeotropic mixtures, which are hard being separated by conventional distillation processes. In most cases, water-selective membranes are utilized and the dehydration is performed through a solution-diffusion mechanism. Hydrophilic membranes, such as poly(vinyl alcohol) and chitosan based membranes, are good candidates owing to their water-permselectivity and high permeation fluxes [1–14]. On the other hand, uses of hydrophilic membranes in pervaporation dehydration usually suffer with poor stability due to the high swelling ratios of membranes in aqueous solutions. Membrane stabilizations are therefore attractive and necessary in developments of high performance membranes using in pervaporation dehydration processes. Introduction of cross-linked structures to hydrophilic membranes can significantly depress their swelling ratios in aqueous solutions [15–20]. Formation of organic–inorganic hybrids and nanocomposites also shows certain achievements on membrane stabilization [20–23]. Moreover, uses of hydrophilic polymer blends, which are stabilized with hydrogen bonds and ionic
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linkages, are another approach to prepare stable pervaporation dehydration membranes [24–28]. However, reduction of permeation flux is usually accompanied with above-mentioned modified membranes. To overcome this problem, composite membranes prepared from casting hydrophilic polymers on porous substrates are utilized [29–36]. The porous substrate provides mechanical strength and the casting layer provides separation efficiency to the membrane. Compared to conventional signal-layer membranes, the separation layer thickness in composite membrane is greatly reduced so as to significantly decrease its water-permeation resistance and to increase its permeation flux. Polyacrylonitrile [29], cellulose acetate [30], and poly(vinylidene fluoride) [32–34] were reported using as support layers in composite membranes. However, the stability of these organic-supported composite membranes was still not good enough for most organic solvent aqueous solutions. One attempt to overcome this problem is using ceramic support layers [35,36]. Unfortunately, employment of ceramic membranes usually accompanies with poor processing properties and high costs. Polytetrafluoroethylene (PTFE) is attractive in membrane applications basing on its superior chemical resistance, good thermal stability, and high mechanical strength. One major problem of using PTFE membranes for pervaporation dehydration is their poor surface hydrophilicity and adhesive compatibility to other polymers. In 1972, Aptel et al. [37]
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grafted N-vinylpyrrolidone and 4-vinylpyridine onto PTFE membranes through 60 Co irradiation-initiated polymerization. The permselectivity of the surface grafted PTFE membrane was very low. After, Tealdo et al. [38] prepared sulfonatedpolystyrene grafted PTFE membranes for pervaporation dehydration. With increasing the surface hydrophilicity of PTFE membranes, a significant increase of permeation flux was achieved. However, the water permselectivity was still not high enough. Our previous work reported surface-modified PTFE membranes, which showed very high permselectivities and moderate permeation fluxes in pervaporative dehydration on some organic aqueous solutions [39]. Continuing our studies on pervaporation dehydration, this work explores the first attempt on developments of PTFEsupported composite membranes for pervaporation dehydration processes. Surface-modification on porous expanded-PTFE was done through surface-initiated polymerization of styrene sulfuric acid to incorporate hydrophilicity and compatibility to PTFE film surface [40]. Chitosan/PTFE composite membranes were then obtained from casting a chitosan dense layer on modified PTFE film surface. The –SO3 H groups on the poly(styrene sulfuric acid)-modified PTFE film surface could react with the amino groups of chitosan chains, to provide inter-facial linkages between chitosan and PTFE layers [3,22]. The chitosan/PTFE composite membranes, which possess thin chitosan dense layer and stable PTFE supporting layer, showed high performance of pervaporation dehydrations for various aqueous solutions of organic compounds, including hydrocarbon alcohols, tetrafluoropropanol (TFP), and N,N-dimethylformamide (DMF). 2. Experimental 2.1. Materials Isotropic expanded polytetrafluoroethylene (e-PTFE) films with a thickness of 500 m and pore sizes of 20–40 m were received from Yu-Min-Tai Co., Ltd., Taiwan. Sodium 4styrenesulfonate (NaSS) was purchased from Fluka Chemie. Poly(styrene sulfuric acid) grafted PTFE films (PTFE-g-PSSA) were prepared according to the reported method [39,40]. The PSSA grafting amount was 1.50 mg/cm2 . Chitosan with a degree of deacetylation of 85% (Sigma Chemical Co.) and ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS, United Chemical Technologies Inc.) were used without further purification. 2.2. Preparation of chitosan/PTFE composite membranes Chitosan was dissolved in 2 wt% acetic acid aqueous solution. Three chitosan solutions with chitosan concentrations of 1.5, 2.0, and 3.0 wt% were prepared. After stirring at room temperature for 24 h, the solution was filtered, bathed in an ultrasonic bath, and stood for 24 h. GPTMS was then added into chitosan solution with stirring. The GPTMS fractions in GPTMS/(GPTMS + chitosan) were 0–15 wt%. The mixture was cast on PTFE-g-PSSA film surfaces with a casting knife. The casting thickness was 100 m. The cast membrane was
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then heated in an oven at 40 ◦ C for 1 h, 60 ◦ C for 1 h, and 100 ◦ C for another 36 h to result in chitosan/PTFE composite membranes. 2.3. Instrumental analysis Water contact angles were measured with an angle-meter (Automatic Contact Angle Meter, Model CA-VP, Kyowa Interface Science Co. Ltd., Japan) at room temperature. Distilled water (5 L) was dropped on the sample surface at ten different sites. The average of ten measured values for a sample was taken as its water contact angle. Scanning electron micrographs (SEM) were recorded with a Hitachi S-3000N SEM. 2.4. Pervaporation dehydration operation Pervaporation was conducted with a conventional process [41] with the conditions of effective membrane area: 6.7 cm2 , temperatures of feeding solutions: 25–70 ◦ C, and downstream pressure: 667–1067 Pa (5–8 Torr). The compositions of feeding solutions and permeates were determined with a gas chromatography (China Chromatography GC-8700T). The separation factor (αwater/organic compound ) was calculated from the concentration ratio of permeate solution over feeding solution. αwater/organic compound =
(Ywater /Yorganic compound ) , (Xwater /Xoragnic compound )
where Y and X are the concentrations of permeate and in feeding solutions, respectively, and the subscription (water and organic compound) indicates the species. Permeation flux was determined by measuring the weight of permeate liquid through the membrane at given time. Data were obtained from the average of measuring results from four pieces of separate membranes. 3. Results and discussion 3.1. Preparation of chitosan/PTFE composite membranes Since chitosan solutions were not compatible to the hydrophobic and inert pristine PTFE film surface, pristine PTFE films were not suitable using in preparation of chitosan/PTFE composite membranes. Surface modification on PTFE films with grafting poly(styrene sulfuric acid) (PSSA) chains was done to increase its surface hydrophilicity with a water contact angle change from 125◦ to 38◦ [40]. Casting chitosan solution on the modified hydrophilic PTFE film surface became practicable. In addition, the –SO3 H groups of PSSA chains grafted on PTFE film surface might react with the amino groups in chitosan chains, to provide inter-facial linkages between chitosan and PTFE layers [3,22] and consequently increase the adhesion strength between these two layers. On the other hand, GPTMS was added into chitosan solution to in situ cross-linking chitosan and formation of chitosan-silica nanocomposites. In the acidic aqueous solution, GPTMS reacted onto chitosan chains through the acid-catalyzed amino-oxirane addition reaction. Simultaneously the methoxysilane groups hydrolyzed to form silanol groups. The condensation reaction of the silanol group
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Fig. 1. Preparation and structure of chitosan/PTFE composite membranes.
further performed to form Si–O–Si linkages and to provide inter-chain covalent bonds to result in a cross-linked structure [19]. The GPTMS cross-linked chitosan membranes showed attractive pervaporation dehydration performance and long-term operation stability for 70 wt% isopropanol aqueous solution [20]. Basing on above, chitosan/PTFE composite membranes were prepared from casting chitosan/GPTMS solutions onto PSSA-modified PTFE film surfaces. Preparation and structure of chitosan/PTFE composite membranes are illustrated in Fig. 1. The surface morphologies of chitosan/PTFE composite membrane in various preparation stages are shown in Fig. 2. Pristine e-PTFE film exhibits porous surface. After grafting polymerization some PSSA polymer covering on PTFE film surface was observed. The covering material cannot be washed out with ultrasonic bathing in water and acetone, to demonstrate the formation of chemically covalent linkages between PSSA chains and PTFE surface. Fig. 2c shows the SEM micrograph of chitosan-5/PTFE composite membrane surface, which was made from casting a chitosan solution containing 5 wt% GPTMS (the chitosan solution concentration was 3 wt%) on PTFE-g-PSSA film surface. The PTFE surface was completely covered with chitosan-GPTMS and the resulting surface was very smooth. In a blank test chitosan solution was cast on pristine PTFE surface and a shrunk chitosan layer formed on top of PTFE film. The strong adhesion between chitosan and e-PTFE layers was investigated with a 180◦ peel strength test. A high peel strength of 2.1 N/cm2 was observed for the chitosan/PTFE com-
posite membranes to demonstrate the formation of some strong interactions between the membrane interfaces. Fig. 2d shows the cross-sectional SEM micrograph of the same chitosan/PTFE membrane shown in Fig. 2c. The lower part exhibited the porous structure of e-PTFE film. The chitosan layer was observed at the e-PTFE film top with a layer thickness of about 1.26 m. The chitosan layer thickness play an important role on pervaporation separation performance. An optimum layer thickness is to show the highest permeation flux with maintaining separation ability. In this work the chitosan layer thickness was tailored with casting solution concentrations. Fig. 3 shows the effect of chitosan casting solution concentrations on the pervaporation performance on a 90 wt% isopropanol aqueous solution. The permeation flux decreased and separation ability increased dramatically with increasing the casting solution concentrations, i.e. the chitosan layer thickness in composite membranes. The composite membrane made with 3 wt% chitosan casting solution showed the optimum performance. The GPTMS contents in chitosan casting solution brought different cross-linking densities to chitosan membranes [20]. Fig. 4 shows the relationship between GPTMS contents in chitosan solutions and the pervaporation dehydration performance on a 70 wt% isopropanol aqueous solution. Increases of GPTMS contents depressed chitosan swelling behavior in isopropanol aqueous solution, so as to increase separation ability of membranes. However, the permeation flux also decreased with increasing GPTMS contents. The membrane cast from
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Fig. 2. SEM micrographs on preparation of chitosan/PTFE composite membranes. (a) Pristine e-PTFE top surface, (b) PTFE-g-PSSA top surface, (c) chitosan-5/PTFE composite membrane top surface and (d) chitosan-5/PTFE composite membrane cross-section.
a chitosan solution containing 5 wt% GPTMS exhibited the best pervaporation dehydration performance among the testing formulations. Basing on the above discussion, the effective composite membrane (chitosan-5/PTFE) was made from casting a 3 wt% chitosan solution which contained 5 wt% GPTMS on a PTFE-g-PSSA film. 3.2. Pervaporation dehydration performance of chitosan-5/PTFE membrane on isopropanol aqueous solutions
Fig. 3. The effect of chitosan casting solution concentrations on the pervaporation dehydration performances (90 wt% isopropanol aqueous solution at 25 ◦ C) of chitosan/PTFE composite membranes.
Fig. 4. Effect of GPTMS contents in 3 wt% chitosan casting solution on the pervaporation dehydration performances (70 wt% isopropanol aqueous solution at 70 ◦ C) of chitosan/PTFE composite membranes.
Fig. 5 shows the pervaporation dehydration performance of PTFE-g-PSSA membrane on aqueous isopropanol solutions in various concentrations. The results were similar to that obtained with other membranes, i.e. the permeation fluxes increased and the separation abilities decreased with increasing the water contents in feeding solutions. The membrane showed poor water
Fig. 5. Pervaporation dehydration performances of chitosan-5/PTFE composite membrane on isopropanol aqueous solutions in various concentrations at 70 ◦ C.
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Fig. 6. Effect of feed IPA aqueous solution (70 wt%) temperatures on the pervaporation performance of chitosan-5/PTFE composite membrane.
selectivity for 10 wt% IPA feed solution (separation factor = 1.3), as the water concentration in permeate was about 92 wt%, which was close to the feed solution concentration. Increases of IPA concentrations in feed solutions enhanced the membrane water selectivity. The 30 wt% IPA feed solution exhibited a water concentration in permeate of about 88 wt%. Although this concentration was lower than that observed with 10 wt% IPA solution in feed, however, the separation factor increased from 1.3 to 3.2. For inherent chitosan membranes, feeding solutions of high water contents seriously swelled the membranes, so as to result in a dramatic increase of flux and lose of permselectivity. These membranes were therefore only suitably used in low-water-content solutions. However, this phenomenon was not observed with chitosan-5/PTFE composite membrane, as high permselectivity was observed with the 40 wt% isopropanol aqueous solution. Therefore, chitosan-5/PTFE composite membrane is suitable for pervaporation dehydration of aqueous isopropanol solutions in wide-range concentrations. Since the azeotropic concentration of isopropanol–water is 87.8 wt% and the waste isopropanol solution from semiconductor manufacturer was about 70 wt%, the dehydration performance on 70 wt% isopropanol aqueous solution is especially interested in [20]. The temperature effects on permeation dehydration performance of chitosan-5/PTFE composite membrane on 70 wt% isopropanol aqueous solution are shown in Fig. 6. The permeation fluxes dramatically increased from 580 to 1730 g/m2 h with increasing the operation temperatures from 25 to 70 ◦ C High temperature enhanced chitosan polymer chain motion and increased swelling degree of chitosan layer, both attributing to increases in permeation fluxes. On the other hand, the increases in permeation fluxes are generally effective for both organic compound (IPA) and water, so as to result in a decrease in separation ability. However, decrease in the water concentrations in permeates was almost not observed for chitosan-5/PTFE membrane operated at high temperatures. The activation energy of pervaporation dehydration was obtained from Fig. 7 according to the Arrhenius equation of Ep ln Q = ln A − (1) RT
Fig. 7. Arrhenius plots of pervaporation dehydration of chitosan-5/PTFE composite membrane and chitosan-5 membrane [20] on a 70 wt% aqueous isopropanol solution.
where Q is the permeation flux, A the pre-exponential factor, Ep the activation energy of permeation, R the gas constant, and T is the operating temperature. A value of 21.0 kJ/mol was found for the activation energy of permeation for chitosan-5/PTFE membrane. This activation energy is close to the value (23.0 kJ/mol) found for chitosan-5 membrane (11.2 m in thickness) [20] in same pervaporation condition. Therefore, the presence of ePTFE substrate layer in chitosan-5/PTFE composite membrane did not bring much barrier in permeation. Moreover, reduction of chitosan separation layer thickness from 11.2 (pure chitosan-5 membrane) to 1.26 m (chitosan-5/PTFE composite membrane) did not significantly contribute to permeation promotion. The operation durability of chitosan-5/PTFE membrane was examined with immersing the membrane in a 70 wt% aqueous isopropanol solution at 25 ◦ C [20]. Pure chitosan lost its pervaporation separation ability in a 2-day test. However, with formation of composite membrane with PTFE, the operation durability was extended to 15 days. In the previous work, a chitosan-5 (chitosan cross-linked 5 wt% GPTMS) membrane in 11.2 m thickness exhibited a long-term stability of 141 days [20]. The chitosan-5/PTFE membrane, which possessed chitosan-5 in 1.26 m thickness, survived after a 45-day test in this work. The results suggest that formation of composite membrane with PTFE film can effectively improve the longTable 1 Pervaporation dehydration performance of chitosan-5/PTFE membrane on various aqueous solutions of organic compounds (90 wt%) at 25 ◦ C Organic solvent
Molar volume of organic solvent (mL/mol)
Permeation flux (g/m2 h)
Separation factor (α)
Methanol Ethanol n-Propanol Isopropanol TFP DMF
40.7 58.6 75.1 76.4 88.9 81.3
557 446 427 409 314 317
14 81 740 1490 –a 8990
a
100 wt% of water in permeate side.
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Table 2 Pervaporation dehydration performance of chitosan-5/PTFE composite membrane and other reported membranes Membrane material
Feeding solution
Operation temperature (◦ C)
Permeation flux (g/m2 h)
Separation factor (α)
Reference
Chitosan-5/PTFE composite PSSA surface-modified PTFE membrane Cross-linked chitosan/alginate composite Chitosan/NaY zeolite composite Chitosan-5/PTFE composite GPTMS cross-linked chitosan (chitosn-5) (Glutaraldehyde + H2 SO4 ) cross-linked chitosan Hexamethylene diisocyanate cross-linked chitosan Chitosan-5/PTFE composite PSSA surface-modified PTFE membrane Chitosan-5/PTFE composite membrane PSSA surface-modified PTFE membrane Poly(vinylalcohol)-g-poly(acrylonitrile
90 wt% IPA(aq) 90 wt% IPA(aq) 90 wt% IPA(aq) 90 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 90 wt% TFP(aq) 90 wt% TFP(aq) 90 wt% DMF(aq) 90 wt% DMF(aq) 90 wt% DMF(aq)
25 65 60 50 70 70 50 30 25 25 25 25 25
409 422 324 546 1730 1730 1000 400 317 319 317 277 9
1490 4490 3205 271 775 694 800 190 Infinite Infinite 8990 Infinite 18
This work [39] [9] [7] This work [20] [16] [15] This work [39] This work [39] [42]
term operation stability of chitosan membranes in pervaporation dehydration process.
demonstrated, as chitosan-5/PTFE membrane exhibited superior performance for all of the tested solutions.
3.3. Pervaporation dehydration performance of chitosan-5/PTFE membrane on various aqueous solutions of organic compounds
4. Conclusions
To probe the application scope of chitosan-5/PTFE membrane, the performance of pervaporation dehydration of chitosan-5/PTFE membrane on aqueous solutions of other organic compounds were examined, and the results are shown in Table 1. This membrane is not effective to small molecular solvents, as very low separation factors of 14 and 81 were found for methanol and ethanol solutions, respectively. The order of separation factors shows same tendency to that of the molar volume of organic solvents. The permeation process in the studied cases might be diffusion-dominant according to the sorption-diffusion model of permeation. The solvents possessing small molar volumes diffused through the membrane much easier, to result in poor separation abilities. On the other hand, the large molecular solvents dissolved in the membrane would inhibit the permeation of water, so as to reduce permeation flux. Therefore, the large molar volume of tetrafluoropropanol and N,N-dimethylformamide bring about much steric hindrance to water permeation, so as to reduce the total flux. Both of the high separation factors for TFP and DMF aqueous solutions are also attributed to the low permeability of TFP and DMF due to their large molar volumes. It is noteworthy that chitosan-5/PTFE membrane can be applied to pervaporation dehydration on TFP and DMF solutions, as chitosan based membranes were not reported to these highly polar solvents before due to their poor stability. Table 2 collects some data for further comparison the pervaporation dehydration performances of chitosan-5/PTFE and some reported membranes. It can be seen that chitosan5/PTFE membrane showed superior or comparable performance to other membranes, especially for the relatively critical solutions like IPA (70 wt%), TFP (90 wt%), and DMF (90 wt%) aqueous solutions. Moreover, the wide applications of chitosan5/PTFE membrane for various organic compound solutions is
This work reported the first attempt on preparation of chitosan/PTFE composite membranes for pervaporation dehydration processes. The membrane was prepared from formation of GPTMS cross-linked chitosan layer on poly(styrene sulfuric acid) modified PTFE film surface. The membranes showed high interlayer peel strength and significant improvements on membrane stability in organic aqueous solutions. The membrane exhibited great performance for applying to pervaporation dehydration on various organic aqueous solutions In conclusion the chitosan/PTFE membrane developed in this work showed superior performances in pervaporation dehydration to other reported membranes. Acknowledgements Financial supports on this work from the Ministry of Economic Affairs, Taiwan (Grant No. 94-EC-17-A-10-S1-0004) and the National Science Council, Taiwan (Grant No. NSC 942216-E-033-002) is highly appreciated. This work is also under support of the Center-of-Excellence Program on Membrane Science and Technologies, the Ministry of Education, Taiwan. References [1] T. Uragami, M. Saito, K. Takigawa, Studies on syntheses and permeabilities of special polymer membranes 69. Comparison of permeation and separation characteristics from aqueous alcoholic solutions by pervaporation and new evapomeation methods through chitosan membranes, Makromol. Chem. Rapid Commun. 9 (1983) 361–368. [2] T. Uragami, T. Matsuda, H. Okuno, T. Miyata, Structure of chemically modified chitosan membranes and their characteristics of permeation and separation of aqueous ethanol solutions, J. Membr. Sci. 88 (1994) 243–251. [3] J. Ge, Y. Cui, Y. Yan, W. Jiang, The effect of structure on pervaporation of chitosan membrane, J. Membr. Sci. 165 (2000) 75–81. [4] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of waterisopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241–249.
236
Y.-L. Liu et al. / Journal of Membrane Science 287 (2007) 230–236
[5] W. Won, X. Feng, D. Lawless, Pervaporation with chitosan membranes: separation of dimethyl carbonate/methanol/water mixtures, J. Membr. Sci. 209 (2002) 493–508. [6] P. Kanti, K. Srigowri, J. Madhuri, B. Smitha, S. Sridhar, Dehydration of ethanol through blend membranes of chitosan and sodium alginate by pervaporation, Sep. Purif. Technol. 40 (2004) 259–266. [7] 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) 75–86. [8] R.Y.M. Huang, G.Y. Moon, R. Pal, N-Acetylated chitosan membranes for the pervaporation separation of alcohol/toluene mixtures, J. Membr. Sci. 176 (2000) 223–231. [9] 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) 17–27. [10] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of waterisopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241–249. [11] N.D. Hilmioglu, S. Tulbentci, Separation of IPA/water mixtures by pervaporation: sorption and pervaporation results, Vacuum 72 (2004) 35–40. [12] N. Isiklan, O. Sanli, Permeation and separation characteristics of acetic acid/water mixtures through poly(vinyl alcohol-g-itaconic acid) membranes by pervaporation, evapomeation, and temperature-difference Evapomeation, J. Appl. Polym. Sci. 93 (2004) 2322–2333. [13] M. Rafik, A. Mas, M.F. Guimon, C. Guimon, A. Elharfi, F. Schu, Plasmamodified poly(vinyl alcohol) membranes for the dehydration of ethanol, Polym. Int. 52 (2003) 1222–1229. [14] N.D. Hilmioglu, A.E.A.S. Sakaoglu, S. Tulbentci, Acetic acid dehydration by pervaporation, Chem. Eng. Process. 40 (2001) 263–267. [15] M.G.M. Nawawi, R.Y.M. Huang, Pervaporation dehydration of isopropanol with chitosan membranes, J. Membr. Sci. 124 (1997) 53–62. [16] 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) 17–30. [17] J. Yu, C.H. Lee, W.H. Hong, Performances of crosslinked asymmetric poly(vinyl alcohol) membranes for isopropanol dehydration by pervaporation, Chem. Eng. Process. 41 (2002) 693–698. [18] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes, J. Membr. Sci. 262 (2005) 91–99. [19] Y.L. Liu, Y.H. Su, J.Y. Lai, Preparation and characterization of a chitosansilica hybrid membrane, Polymer 45 (2004) 6831–6837. [20] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Lai, Crosslinked organic-inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol-water mixtures with a long-term stability, J. Membr. Sci. 251 (2005) 233–238. [21] E. Okumus, T. Gurkan, L. Yilmaz, Effect of fabrication and process parameters on the morphology and performance of a PAN-based zeolite-filled pervaporation membrane, J. Membr. Sci. 223 (2003) 23–38. [22] Y.L. Liu, C.H. Hsu, Y.H. Su, J.Y. Lai, Chitosan-silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol-water solutions, Biomacromolecules 6 (2005) 368–373. [23] S.D. Hat, T.M. Aminabhavi, Novel sodium alginate-Na+ MMT hybrid composite membranes for pervaporation dehydration of isopropanol, 1,4dioxane and tetrahydrofuran, Sep. Purif. Technol. 51 (2006) 85–94. [24] U.S. Toti, T.M. Aminabhavi, Pervaporation separation of water-isopropyl alcohol mixtures with blend membranes of sodium alginate and poly(acrylamide)-grafted guar gum, J. Appl. Polym. Sci. 85 (2002) 2014–2024.
[25] J.J. Shieh, R.Y.M. Huang, Chitosan/N-methyol nylon 6 blend membranes for the pervaporation separation of ethanol-water mixtures, J. Membr. Sci. 148 (1998) 243–255. [26] A. Chanachai, R. Jiraratananon, D. Uttapap, G.Y. Moon, W.A. Anderson, R.Y.M. Huang, Pervaporation with chitosan/hydroxyethylcellulose (CS/HEC) blended membranes, J. Membr. Sci. 166 (2000) 271– 280. [27] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of dimethylformamide/water mixture through poly(vinyl alcohol)/poly(acrylic acid) blend membranes, Sep. Purif. Technol. 51 (2006) 104–111. [28] A. Svang-Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815– 823. [29] X.P. Wang, Z.Q. Shen, F.Y. Zhang, Y.F. Zhang, A novel composite chitosan membrane for the separation of alcohol-water mixtures, J. Membr. Sci. 119 (1996) 191–198. [30] 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) 143–151. [31] P.D. Chapman, X. Tan, A.G. Livingston, K. Li, T. Oliveira, Dehydration of tetrahydrofuran by pervaporation using a composite membrane, J. Membr. Sci. 268 (2006) 13–19. [32] 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) 275–289. [33] C.L. Chang, M.S. Chang, Preparation of multi-layer silicon/PVDF composite membranes for pervaporation of ethanol aqueous solutions, J. Membr. Sci. 238 (2004) 117–122. [34] C.L. Chang, P.Y. Chang, Performance enhancement of silicon/PVDF composite membranes for pervaporation by reducing cross-linking density of the active silicone layer, Desalination 192 (2006) 241–245. [35] W. Yoshida, Y. Cohen, Removel of methyl tert-butyl ether from water by pervaporation using ceramic-supported polymer membranes, J. Membr. Sci. 229 (2004) 27–32. [36] T.A. Peters, C.H.S. Poeth, N.E. Benes, H.C.W.M. Buijs, F.F. Vercauteren, J.T.F. Keurentjes, Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm, J. Membr. Sci. 276 (2006) 42–50. [37] P. Aptel, J. Cuny, J. Jozefowicz, G. Morel, J. Neel, Liquid transport through membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. I. Some fractionations of liquid mixtures by pervaporation, J. Appl. Polym. Sci. 16 (1972) 1061–1076. [38] G.C. Tealdo, P. Canepa, S. Munari, Water-ethanol permeation through grafted PTFE membranes, J. Membr. Sci. 9 (1981) 191–196. [39] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Hydrophilic surface-grafted poly(tetrafluoroethylene) membranes using in pervaporation dehydration processes, J. Membr. Sci. 274 (2006) 47–55. [40] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Surface grafting polymerization and modification on poly(tetrafluoroethylene) films by means of ozone treatment, Polymer 46 (2005) 3976–3985. [41] Y.C. Wang, S.C. Fan, K.R. Lee, C.L. Li, S.H. Huang, H.A. Tsai, J.Y. Lai, Polyamide/SDS–clay hybrid nanocomposite membrane application to water–ethanol mixture pervaporation separation, J. Membr. Sci. 239 (2004) 219–226. [42] M.D. Kurkuri, T.M. Aminabhavi, Polyacrylonitrile-g-poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures, J. Appl. Polym. Sci. 91 (2004) 4091–4097.
Chitosan/poly(tetrafluoroethylene) composite membranes using in pervaporation dehydration processes Ying-Ling Liu ∗ , Chung-Hao Yu, Kueir-Rarn Lee, Juin-Yih Lai R&D Center for Membrane Technology and Department of Chemical Engineering, Chung Yuan University, Chungli, Taoyuan 32023, Taiwan Received 28 August 2006; received in revised form 16 October 2006; accepted 21 October 2006 Available online 27 October 2006
Abstract Chitosan/PTFE composite membranes were prepared from casting a ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS)-containing chitosan solution on poly(styrene sulfuric acid) grafted expended poly(tetrafluoroethylene) film surface. The adhesion between the chitosan skin layer and the PTFE substrate was pretty good to warrant the high performance of chitosan/PTFE composite membranes using in pervaporation dehydration processes on isopropanol. The chitosan/PTFE membrane exhibited a permeation flux of 1730 g/m2 h and a separation factor of 775 at 70 ◦ C on pervaporation dehydration of a 70 wt% isopropanol aqueous solution. The membrane also survived after a long-term operation test in 45 days. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(tetrafluoroethylene); Chitosan; Composite membrane; Pervaporation
1. Introduction Pervaporation dehydration process shows great achievements on separation of organic solvent–water azeotropic mixtures, which are hard being separated by conventional distillation processes. In most cases, water-selective membranes are utilized and the dehydration is performed through a solution-diffusion mechanism. Hydrophilic membranes, such as poly(vinyl alcohol) and chitosan based membranes, are good candidates owing to their water-permselectivity and high permeation fluxes [1–14]. On the other hand, uses of hydrophilic membranes in pervaporation dehydration usually suffer with poor stability due to the high swelling ratios of membranes in aqueous solutions. Membrane stabilizations are therefore attractive and necessary in developments of high performance membranes using in pervaporation dehydration processes. Introduction of cross-linked structures to hydrophilic membranes can significantly depress their swelling ratios in aqueous solutions [15–20]. Formation of organic–inorganic hybrids and nanocomposites also shows certain achievements on membrane stabilization [20–23]. Moreover, uses of hydrophilic polymer blends, which are stabilized with hydrogen bonds and ionic
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0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.10.040
linkages, are another approach to prepare stable pervaporation dehydration membranes [24–28]. However, reduction of permeation flux is usually accompanied with above-mentioned modified membranes. To overcome this problem, composite membranes prepared from casting hydrophilic polymers on porous substrates are utilized [29–36]. The porous substrate provides mechanical strength and the casting layer provides separation efficiency to the membrane. Compared to conventional signal-layer membranes, the separation layer thickness in composite membrane is greatly reduced so as to significantly decrease its water-permeation resistance and to increase its permeation flux. Polyacrylonitrile [29], cellulose acetate [30], and poly(vinylidene fluoride) [32–34] were reported using as support layers in composite membranes. However, the stability of these organic-supported composite membranes was still not good enough for most organic solvent aqueous solutions. One attempt to overcome this problem is using ceramic support layers [35,36]. Unfortunately, employment of ceramic membranes usually accompanies with poor processing properties and high costs. Polytetrafluoroethylene (PTFE) is attractive in membrane applications basing on its superior chemical resistance, good thermal stability, and high mechanical strength. One major problem of using PTFE membranes for pervaporation dehydration is their poor surface hydrophilicity and adhesive compatibility to other polymers. In 1972, Aptel et al. [37]
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grafted N-vinylpyrrolidone and 4-vinylpyridine onto PTFE membranes through 60 Co irradiation-initiated polymerization. The permselectivity of the surface grafted PTFE membrane was very low. After, Tealdo et al. [38] prepared sulfonatedpolystyrene grafted PTFE membranes for pervaporation dehydration. With increasing the surface hydrophilicity of PTFE membranes, a significant increase of permeation flux was achieved. However, the water permselectivity was still not high enough. Our previous work reported surface-modified PTFE membranes, which showed very high permselectivities and moderate permeation fluxes in pervaporative dehydration on some organic aqueous solutions [39]. Continuing our studies on pervaporation dehydration, this work explores the first attempt on developments of PTFEsupported composite membranes for pervaporation dehydration processes. Surface-modification on porous expanded-PTFE was done through surface-initiated polymerization of styrene sulfuric acid to incorporate hydrophilicity and compatibility to PTFE film surface [40]. Chitosan/PTFE composite membranes were then obtained from casting a chitosan dense layer on modified PTFE film surface. The –SO3 H groups on the poly(styrene sulfuric acid)-modified PTFE film surface could react with the amino groups of chitosan chains, to provide inter-facial linkages between chitosan and PTFE layers [3,22]. The chitosan/PTFE composite membranes, which possess thin chitosan dense layer and stable PTFE supporting layer, showed high performance of pervaporation dehydrations for various aqueous solutions of organic compounds, including hydrocarbon alcohols, tetrafluoropropanol (TFP), and N,N-dimethylformamide (DMF). 2. Experimental 2.1. Materials Isotropic expanded polytetrafluoroethylene (e-PTFE) films with a thickness of 500 m and pore sizes of 20–40 m were received from Yu-Min-Tai Co., Ltd., Taiwan. Sodium 4styrenesulfonate (NaSS) was purchased from Fluka Chemie. Poly(styrene sulfuric acid) grafted PTFE films (PTFE-g-PSSA) were prepared according to the reported method [39,40]. The PSSA grafting amount was 1.50 mg/cm2 . Chitosan with a degree of deacetylation of 85% (Sigma Chemical Co.) and ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS, United Chemical Technologies Inc.) were used without further purification. 2.2. Preparation of chitosan/PTFE composite membranes Chitosan was dissolved in 2 wt% acetic acid aqueous solution. Three chitosan solutions with chitosan concentrations of 1.5, 2.0, and 3.0 wt% were prepared. After stirring at room temperature for 24 h, the solution was filtered, bathed in an ultrasonic bath, and stood for 24 h. GPTMS was then added into chitosan solution with stirring. The GPTMS fractions in GPTMS/(GPTMS + chitosan) were 0–15 wt%. The mixture was cast on PTFE-g-PSSA film surfaces with a casting knife. The casting thickness was 100 m. The cast membrane was
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then heated in an oven at 40 ◦ C for 1 h, 60 ◦ C for 1 h, and 100 ◦ C for another 36 h to result in chitosan/PTFE composite membranes. 2.3. Instrumental analysis Water contact angles were measured with an angle-meter (Automatic Contact Angle Meter, Model CA-VP, Kyowa Interface Science Co. Ltd., Japan) at room temperature. Distilled water (5 L) was dropped on the sample surface at ten different sites. The average of ten measured values for a sample was taken as its water contact angle. Scanning electron micrographs (SEM) were recorded with a Hitachi S-3000N SEM. 2.4. Pervaporation dehydration operation Pervaporation was conducted with a conventional process [41] with the conditions of effective membrane area: 6.7 cm2 , temperatures of feeding solutions: 25–70 ◦ C, and downstream pressure: 667–1067 Pa (5–8 Torr). The compositions of feeding solutions and permeates were determined with a gas chromatography (China Chromatography GC-8700T). The separation factor (αwater/organic compound ) was calculated from the concentration ratio of permeate solution over feeding solution. αwater/organic compound =
(Ywater /Yorganic compound ) , (Xwater /Xoragnic compound )
where Y and X are the concentrations of permeate and in feeding solutions, respectively, and the subscription (water and organic compound) indicates the species. Permeation flux was determined by measuring the weight of permeate liquid through the membrane at given time. Data were obtained from the average of measuring results from four pieces of separate membranes. 3. Results and discussion 3.1. Preparation of chitosan/PTFE composite membranes Since chitosan solutions were not compatible to the hydrophobic and inert pristine PTFE film surface, pristine PTFE films were not suitable using in preparation of chitosan/PTFE composite membranes. Surface modification on PTFE films with grafting poly(styrene sulfuric acid) (PSSA) chains was done to increase its surface hydrophilicity with a water contact angle change from 125◦ to 38◦ [40]. Casting chitosan solution on the modified hydrophilic PTFE film surface became practicable. In addition, the –SO3 H groups of PSSA chains grafted on PTFE film surface might react with the amino groups in chitosan chains, to provide inter-facial linkages between chitosan and PTFE layers [3,22] and consequently increase the adhesion strength between these two layers. On the other hand, GPTMS was added into chitosan solution to in situ cross-linking chitosan and formation of chitosan-silica nanocomposites. In the acidic aqueous solution, GPTMS reacted onto chitosan chains through the acid-catalyzed amino-oxirane addition reaction. Simultaneously the methoxysilane groups hydrolyzed to form silanol groups. The condensation reaction of the silanol group
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Fig. 1. Preparation and structure of chitosan/PTFE composite membranes.
further performed to form Si–O–Si linkages and to provide inter-chain covalent bonds to result in a cross-linked structure [19]. The GPTMS cross-linked chitosan membranes showed attractive pervaporation dehydration performance and long-term operation stability for 70 wt% isopropanol aqueous solution [20]. Basing on above, chitosan/PTFE composite membranes were prepared from casting chitosan/GPTMS solutions onto PSSA-modified PTFE film surfaces. Preparation and structure of chitosan/PTFE composite membranes are illustrated in Fig. 1. The surface morphologies of chitosan/PTFE composite membrane in various preparation stages are shown in Fig. 2. Pristine e-PTFE film exhibits porous surface. After grafting polymerization some PSSA polymer covering on PTFE film surface was observed. The covering material cannot be washed out with ultrasonic bathing in water and acetone, to demonstrate the formation of chemically covalent linkages between PSSA chains and PTFE surface. Fig. 2c shows the SEM micrograph of chitosan-5/PTFE composite membrane surface, which was made from casting a chitosan solution containing 5 wt% GPTMS (the chitosan solution concentration was 3 wt%) on PTFE-g-PSSA film surface. The PTFE surface was completely covered with chitosan-GPTMS and the resulting surface was very smooth. In a blank test chitosan solution was cast on pristine PTFE surface and a shrunk chitosan layer formed on top of PTFE film. The strong adhesion between chitosan and e-PTFE layers was investigated with a 180◦ peel strength test. A high peel strength of 2.1 N/cm2 was observed for the chitosan/PTFE com-
posite membranes to demonstrate the formation of some strong interactions between the membrane interfaces. Fig. 2d shows the cross-sectional SEM micrograph of the same chitosan/PTFE membrane shown in Fig. 2c. The lower part exhibited the porous structure of e-PTFE film. The chitosan layer was observed at the e-PTFE film top with a layer thickness of about 1.26 m. The chitosan layer thickness play an important role on pervaporation separation performance. An optimum layer thickness is to show the highest permeation flux with maintaining separation ability. In this work the chitosan layer thickness was tailored with casting solution concentrations. Fig. 3 shows the effect of chitosan casting solution concentrations on the pervaporation performance on a 90 wt% isopropanol aqueous solution. The permeation flux decreased and separation ability increased dramatically with increasing the casting solution concentrations, i.e. the chitosan layer thickness in composite membranes. The composite membrane made with 3 wt% chitosan casting solution showed the optimum performance. The GPTMS contents in chitosan casting solution brought different cross-linking densities to chitosan membranes [20]. Fig. 4 shows the relationship between GPTMS contents in chitosan solutions and the pervaporation dehydration performance on a 70 wt% isopropanol aqueous solution. Increases of GPTMS contents depressed chitosan swelling behavior in isopropanol aqueous solution, so as to increase separation ability of membranes. However, the permeation flux also decreased with increasing GPTMS contents. The membrane cast from
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Fig. 2. SEM micrographs on preparation of chitosan/PTFE composite membranes. (a) Pristine e-PTFE top surface, (b) PTFE-g-PSSA top surface, (c) chitosan-5/PTFE composite membrane top surface and (d) chitosan-5/PTFE composite membrane cross-section.
a chitosan solution containing 5 wt% GPTMS exhibited the best pervaporation dehydration performance among the testing formulations. Basing on the above discussion, the effective composite membrane (chitosan-5/PTFE) was made from casting a 3 wt% chitosan solution which contained 5 wt% GPTMS on a PTFE-g-PSSA film. 3.2. Pervaporation dehydration performance of chitosan-5/PTFE membrane on isopropanol aqueous solutions
Fig. 3. The effect of chitosan casting solution concentrations on the pervaporation dehydration performances (90 wt% isopropanol aqueous solution at 25 ◦ C) of chitosan/PTFE composite membranes.
Fig. 4. Effect of GPTMS contents in 3 wt% chitosan casting solution on the pervaporation dehydration performances (70 wt% isopropanol aqueous solution at 70 ◦ C) of chitosan/PTFE composite membranes.
Fig. 5 shows the pervaporation dehydration performance of PTFE-g-PSSA membrane on aqueous isopropanol solutions in various concentrations. The results were similar to that obtained with other membranes, i.e. the permeation fluxes increased and the separation abilities decreased with increasing the water contents in feeding solutions. The membrane showed poor water
Fig. 5. Pervaporation dehydration performances of chitosan-5/PTFE composite membrane on isopropanol aqueous solutions in various concentrations at 70 ◦ C.
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Fig. 6. Effect of feed IPA aqueous solution (70 wt%) temperatures on the pervaporation performance of chitosan-5/PTFE composite membrane.
selectivity for 10 wt% IPA feed solution (separation factor = 1.3), as the water concentration in permeate was about 92 wt%, which was close to the feed solution concentration. Increases of IPA concentrations in feed solutions enhanced the membrane water selectivity. The 30 wt% IPA feed solution exhibited a water concentration in permeate of about 88 wt%. Although this concentration was lower than that observed with 10 wt% IPA solution in feed, however, the separation factor increased from 1.3 to 3.2. For inherent chitosan membranes, feeding solutions of high water contents seriously swelled the membranes, so as to result in a dramatic increase of flux and lose of permselectivity. These membranes were therefore only suitably used in low-water-content solutions. However, this phenomenon was not observed with chitosan-5/PTFE composite membrane, as high permselectivity was observed with the 40 wt% isopropanol aqueous solution. Therefore, chitosan-5/PTFE composite membrane is suitable for pervaporation dehydration of aqueous isopropanol solutions in wide-range concentrations. Since the azeotropic concentration of isopropanol–water is 87.8 wt% and the waste isopropanol solution from semiconductor manufacturer was about 70 wt%, the dehydration performance on 70 wt% isopropanol aqueous solution is especially interested in [20]. The temperature effects on permeation dehydration performance of chitosan-5/PTFE composite membrane on 70 wt% isopropanol aqueous solution are shown in Fig. 6. The permeation fluxes dramatically increased from 580 to 1730 g/m2 h with increasing the operation temperatures from 25 to 70 ◦ C High temperature enhanced chitosan polymer chain motion and increased swelling degree of chitosan layer, both attributing to increases in permeation fluxes. On the other hand, the increases in permeation fluxes are generally effective for both organic compound (IPA) and water, so as to result in a decrease in separation ability. However, decrease in the water concentrations in permeates was almost not observed for chitosan-5/PTFE membrane operated at high temperatures. The activation energy of pervaporation dehydration was obtained from Fig. 7 according to the Arrhenius equation of Ep ln Q = ln A − (1) RT
Fig. 7. Arrhenius plots of pervaporation dehydration of chitosan-5/PTFE composite membrane and chitosan-5 membrane [20] on a 70 wt% aqueous isopropanol solution.
where Q is the permeation flux, A the pre-exponential factor, Ep the activation energy of permeation, R the gas constant, and T is the operating temperature. A value of 21.0 kJ/mol was found for the activation energy of permeation for chitosan-5/PTFE membrane. This activation energy is close to the value (23.0 kJ/mol) found for chitosan-5 membrane (11.2 m in thickness) [20] in same pervaporation condition. Therefore, the presence of ePTFE substrate layer in chitosan-5/PTFE composite membrane did not bring much barrier in permeation. Moreover, reduction of chitosan separation layer thickness from 11.2 (pure chitosan-5 membrane) to 1.26 m (chitosan-5/PTFE composite membrane) did not significantly contribute to permeation promotion. The operation durability of chitosan-5/PTFE membrane was examined with immersing the membrane in a 70 wt% aqueous isopropanol solution at 25 ◦ C [20]. Pure chitosan lost its pervaporation separation ability in a 2-day test. However, with formation of composite membrane with PTFE, the operation durability was extended to 15 days. In the previous work, a chitosan-5 (chitosan cross-linked 5 wt% GPTMS) membrane in 11.2 m thickness exhibited a long-term stability of 141 days [20]. The chitosan-5/PTFE membrane, which possessed chitosan-5 in 1.26 m thickness, survived after a 45-day test in this work. The results suggest that formation of composite membrane with PTFE film can effectively improve the longTable 1 Pervaporation dehydration performance of chitosan-5/PTFE membrane on various aqueous solutions of organic compounds (90 wt%) at 25 ◦ C Organic solvent
Molar volume of organic solvent (mL/mol)
Permeation flux (g/m2 h)
Separation factor (α)
Methanol Ethanol n-Propanol Isopropanol TFP DMF
40.7 58.6 75.1 76.4 88.9 81.3
557 446 427 409 314 317
14 81 740 1490 –a 8990
a
100 wt% of water in permeate side.
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Table 2 Pervaporation dehydration performance of chitosan-5/PTFE composite membrane and other reported membranes Membrane material
Feeding solution
Operation temperature (◦ C)
Permeation flux (g/m2 h)
Separation factor (α)
Reference
Chitosan-5/PTFE composite PSSA surface-modified PTFE membrane Cross-linked chitosan/alginate composite Chitosan/NaY zeolite composite Chitosan-5/PTFE composite GPTMS cross-linked chitosan (chitosn-5) (Glutaraldehyde + H2 SO4 ) cross-linked chitosan Hexamethylene diisocyanate cross-linked chitosan Chitosan-5/PTFE composite PSSA surface-modified PTFE membrane Chitosan-5/PTFE composite membrane PSSA surface-modified PTFE membrane Poly(vinylalcohol)-g-poly(acrylonitrile
90 wt% IPA(aq) 90 wt% IPA(aq) 90 wt% IPA(aq) 90 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 70 wt% IPA(aq) 90 wt% TFP(aq) 90 wt% TFP(aq) 90 wt% DMF(aq) 90 wt% DMF(aq) 90 wt% DMF(aq)
25 65 60 50 70 70 50 30 25 25 25 25 25
409 422 324 546 1730 1730 1000 400 317 319 317 277 9
1490 4490 3205 271 775 694 800 190 Infinite Infinite 8990 Infinite 18
This work [39] [9] [7] This work [20] [16] [15] This work [39] This work [39] [42]
term operation stability of chitosan membranes in pervaporation dehydration process.
demonstrated, as chitosan-5/PTFE membrane exhibited superior performance for all of the tested solutions.
3.3. Pervaporation dehydration performance of chitosan-5/PTFE membrane on various aqueous solutions of organic compounds
4. Conclusions
To probe the application scope of chitosan-5/PTFE membrane, the performance of pervaporation dehydration of chitosan-5/PTFE membrane on aqueous solutions of other organic compounds were examined, and the results are shown in Table 1. This membrane is not effective to small molecular solvents, as very low separation factors of 14 and 81 were found for methanol and ethanol solutions, respectively. The order of separation factors shows same tendency to that of the molar volume of organic solvents. The permeation process in the studied cases might be diffusion-dominant according to the sorption-diffusion model of permeation. The solvents possessing small molar volumes diffused through the membrane much easier, to result in poor separation abilities. On the other hand, the large molecular solvents dissolved in the membrane would inhibit the permeation of water, so as to reduce permeation flux. Therefore, the large molar volume of tetrafluoropropanol and N,N-dimethylformamide bring about much steric hindrance to water permeation, so as to reduce the total flux. Both of the high separation factors for TFP and DMF aqueous solutions are also attributed to the low permeability of TFP and DMF due to their large molar volumes. It is noteworthy that chitosan-5/PTFE membrane can be applied to pervaporation dehydration on TFP and DMF solutions, as chitosan based membranes were not reported to these highly polar solvents before due to their poor stability. Table 2 collects some data for further comparison the pervaporation dehydration performances of chitosan-5/PTFE and some reported membranes. It can be seen that chitosan5/PTFE membrane showed superior or comparable performance to other membranes, especially for the relatively critical solutions like IPA (70 wt%), TFP (90 wt%), and DMF (90 wt%) aqueous solutions. Moreover, the wide applications of chitosan5/PTFE membrane for various organic compound solutions is
This work reported the first attempt on preparation of chitosan/PTFE composite membranes for pervaporation dehydration processes. The membrane was prepared from formation of GPTMS cross-linked chitosan layer on poly(styrene sulfuric acid) modified PTFE film surface. The membranes showed high interlayer peel strength and significant improvements on membrane stability in organic aqueous solutions. The membrane exhibited great performance for applying to pervaporation dehydration on various organic aqueous solutions In conclusion the chitosan/PTFE membrane developed in this work showed superior performances in pervaporation dehydration to other reported membranes. Acknowledgements Financial supports on this work from the Ministry of Economic Affairs, Taiwan (Grant No. 94-EC-17-A-10-S1-0004) and the National Science Council, Taiwan (Grant No. NSC 942216-E-033-002) is highly appreciated. This work is also under support of the Center-of-Excellence Program on Membrane Science and Technologies, the Ministry of Education, Taiwan. References [1] T. Uragami, M. Saito, K. Takigawa, Studies on syntheses and permeabilities of special polymer membranes 69. Comparison of permeation and separation characteristics from aqueous alcoholic solutions by pervaporation and new evapomeation methods through chitosan membranes, Makromol. Chem. Rapid Commun. 9 (1983) 361–368. [2] T. Uragami, T. Matsuda, H. Okuno, T. Miyata, Structure of chemically modified chitosan membranes and their characteristics of permeation and separation of aqueous ethanol solutions, J. Membr. Sci. 88 (1994) 243–251. [3] J. Ge, Y. Cui, Y. Yan, W. Jiang, The effect of structure on pervaporation of chitosan membrane, J. Membr. Sci. 165 (2000) 75–81. [4] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of waterisopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241–249.
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[5] W. Won, X. Feng, D. Lawless, Pervaporation with chitosan membranes: separation of dimethyl carbonate/methanol/water mixtures, J. Membr. Sci. 209 (2002) 493–508. [6] P. Kanti, K. Srigowri, J. Madhuri, B. Smitha, S. Sridhar, Dehydration of ethanol through blend membranes of chitosan and sodium alginate by pervaporation, Sep. Purif. Technol. 40 (2004) 259–266. [7] 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) 75–86. [8] R.Y.M. Huang, G.Y. Moon, R. Pal, N-Acetylated chitosan membranes for the pervaporation separation of alcohol/toluene mixtures, J. Membr. Sci. 176 (2000) 223–231. [9] 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) 17–27. [10] S.Y. Nam, H.J. Chun, Y.M. Lee, Pervaporation separation of waterisopropanol mixture using carboxymethylated poly(vinyl alcohol) composite membranes, J. Appl. Polym. Sci. 72 (1999) 241–249. [11] N.D. Hilmioglu, S. Tulbentci, Separation of IPA/water mixtures by pervaporation: sorption and pervaporation results, Vacuum 72 (2004) 35–40. [12] N. Isiklan, O. Sanli, Permeation and separation characteristics of acetic acid/water mixtures through poly(vinyl alcohol-g-itaconic acid) membranes by pervaporation, evapomeation, and temperature-difference Evapomeation, J. Appl. Polym. Sci. 93 (2004) 2322–2333. [13] M. Rafik, A. Mas, M.F. Guimon, C. Guimon, A. Elharfi, F. Schu, Plasmamodified poly(vinyl alcohol) membranes for the dehydration of ethanol, Polym. Int. 52 (2003) 1222–1229. [14] N.D. Hilmioglu, A.E.A.S. Sakaoglu, S. Tulbentci, Acetic acid dehydration by pervaporation, Chem. Eng. Process. 40 (2001) 263–267. [15] M.G.M. Nawawi, R.Y.M. Huang, Pervaporation dehydration of isopropanol with chitosan membranes, J. Membr. Sci. 124 (1997) 53–62. [16] 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) 17–30. [17] J. Yu, C.H. Lee, W.H. Hong, Performances of crosslinked asymmetric poly(vinyl alcohol) membranes for isopropanol dehydration by pervaporation, Chem. Eng. Process. 41 (2002) 693–698. [18] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes, J. Membr. Sci. 262 (2005) 91–99. [19] Y.L. Liu, Y.H. Su, J.Y. Lai, Preparation and characterization of a chitosansilica hybrid membrane, Polymer 45 (2004) 6831–6837. [20] Y.L. Liu, Y.H. Su, K.R. Lee, J.Y. Lai, Crosslinked organic-inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol-water mixtures with a long-term stability, J. Membr. Sci. 251 (2005) 233–238. [21] E. Okumus, T. Gurkan, L. Yilmaz, Effect of fabrication and process parameters on the morphology and performance of a PAN-based zeolite-filled pervaporation membrane, J. Membr. Sci. 223 (2003) 23–38. [22] Y.L. Liu, C.H. Hsu, Y.H. Su, J.Y. Lai, Chitosan-silica complex membranes from sulfonic acid functionalized silica nanoparticles for pervaporation dehydration of ethanol-water solutions, Biomacromolecules 6 (2005) 368–373. [23] S.D. Hat, T.M. Aminabhavi, Novel sodium alginate-Na+ MMT hybrid composite membranes for pervaporation dehydration of isopropanol, 1,4dioxane and tetrahydrofuran, Sep. Purif. Technol. 51 (2006) 85–94. [24] U.S. Toti, T.M. Aminabhavi, Pervaporation separation of water-isopropyl alcohol mixtures with blend membranes of sodium alginate and poly(acrylamide)-grafted guar gum, J. Appl. Polym. Sci. 85 (2002) 2014–2024.
[25] J.J. Shieh, R.Y.M. Huang, Chitosan/N-methyol nylon 6 blend membranes for the pervaporation separation of ethanol-water mixtures, J. Membr. Sci. 148 (1998) 243–255. [26] A. Chanachai, R. Jiraratananon, D. Uttapap, G.Y. Moon, W.A. Anderson, R.Y.M. Huang, Pervaporation with chitosan/hydroxyethylcellulose (CS/HEC) blended membranes, J. Membr. Sci. 166 (2000) 271– 280. [27] D.A. Devi, B. Smitha, S. Sridhar, T.M. Aminabhavi, Pervaporation separation of dimethylformamide/water mixture through poly(vinyl alcohol)/poly(acrylic acid) blend membranes, Sep. Purif. Technol. 51 (2006) 104–111. [28] A. Svang-Ariyaskul, R.Y.M. Huang, P.L. Douglas, R. Pal, X. Feng, P. Chen, L. Liu, Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol, J. Membr. Sci. 280 (2006) 815– 823. [29] X.P. Wang, Z.Q. Shen, F.Y. Zhang, Y.F. Zhang, A novel composite chitosan membrane for the separation of alcohol-water mixtures, J. Membr. Sci. 119 (1996) 191–198. [30] 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) 143–151. [31] P.D. Chapman, X. Tan, A.G. Livingston, K. Li, T. Oliveira, Dehydration of tetrahydrofuran by pervaporation using a composite membrane, J. Membr. Sci. 268 (2006) 13–19. [32] 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) 275–289. [33] C.L. Chang, M.S. Chang, Preparation of multi-layer silicon/PVDF composite membranes for pervaporation of ethanol aqueous solutions, J. Membr. Sci. 238 (2004) 117–122. [34] C.L. Chang, P.Y. Chang, Performance enhancement of silicon/PVDF composite membranes for pervaporation by reducing cross-linking density of the active silicone layer, Desalination 192 (2006) 241–245. [35] W. Yoshida, Y. Cohen, Removel of methyl tert-butyl ether from water by pervaporation using ceramic-supported polymer membranes, J. Membr. Sci. 229 (2004) 27–32. [36] T.A. Peters, C.H.S. Poeth, N.E. Benes, H.C.W.M. Buijs, F.F. Vercauteren, J.T.F. Keurentjes, Ceramic-supported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm, J. Membr. Sci. 276 (2006) 42–50. [37] P. Aptel, J. Cuny, J. Jozefowicz, G. Morel, J. Neel, Liquid transport through membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. I. Some fractionations of liquid mixtures by pervaporation, J. Appl. Polym. Sci. 16 (1972) 1061–1076. [38] G.C. Tealdo, P. Canepa, S. Munari, Water-ethanol permeation through grafted PTFE membranes, J. Membr. Sci. 9 (1981) 191–196. [39] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Hydrophilic surface-grafted poly(tetrafluoroethylene) membranes using in pervaporation dehydration processes, J. Membr. Sci. 274 (2006) 47–55. [40] C.Y. Tu, Y.L. Liu, K.R. Lee, J.Y. Lai, Surface grafting polymerization and modification on poly(tetrafluoroethylene) films by means of ozone treatment, Polymer 46 (2005) 3976–3985. [41] Y.C. Wang, S.C. Fan, K.R. Lee, C.L. Li, S.H. Huang, H.A. Tsai, J.Y. Lai, Polyamide/SDS–clay hybrid nanocomposite membrane application to water–ethanol mixture pervaporation separation, J. Membr. Sci. 239 (2004) 219–226. [42] M.D. Kurkuri, T.M. Aminabhavi, Polyacrylonitrile-g-poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures, J. Appl. Polym. Sci. 91 (2004) 4091–4097.