- Email: [email protected]
Crosslinked organic–inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol–water mixtures with a long-term stability
Journal of Membrane Science 251 (2005) 233–238
Crosslinked organic–inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol–water mixtures with a long-term stability Ying-Ling Liua,b , Yu-Huei Sua,b , Kuir-Rain Leeb,c , Juin-Yih Laia,b,∗ a b
Department of Chemical Engineering, Chung Yuan University, #200, Chung-Pei Road, Chungli, Taoyuan 320, Taiwan R&D Center for Membrane Technology, Chung Yuan University, #200, Chung-Pei Road, Chungli, Taoyuan 320, Taiwan c Department of Chemical Engineering, Nanya Institute of Technology, Chungli, Taoyuan 320, Taiwan Received 30 August 2004 Available online 13 January 2005
Abstract Crosslinked organic–inorganic hybrid chitosan membranes were obtained from blending chitosan and ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS) in acetic acid aqueous solution. The hydrophilicity of the modified membranes was not significantly decreased, so as to result in good permselectivity and high permeation flux in pervaporative dehydration on a 70 wt.% isopropanol–water (IPA–H2 O) solution. A flux of 1730 g/(m2 h) and a separation factor of 694 were found with the chitosan membrane containing 5 wt.% GPTMS. Both of the crosslinked and organic–inorganic hybrid structures contribute to stabilize the membranes to maintain the performance of the membranes in a 140 days long-term operation. © 2004 Elsevier B.V. All rights reserved. Keywords: Chitosan; Pervaporation; Crosslinked membrane; Isopropanol–water separation
1. Introduction Separation of azeotropic mixtures or liquid mixtures containing substances of close boiling points is hard to achieve by conventional distillation process. To overcome this problem, pervaporation separation has showed its attractive performance and has received considerable investigations [1–8]. Membranes for pervaporation separation of aqueous alcohol mixtures, like ethanol–water [7–15] and isopropanol–water (IPA–H2 O) [16–20] solutions, are especially focused on. In the dehydration operation, hydrophilic membranes are good candidates owing to their high water-permselectivity. Chitosan is widely used in membrane applications based on its high hydrophilicity, good film-forming character, and excellent chemical-resistant properties [9–14,16–18]. However, chitosan membranes are highly swollen in water and ∗
Corresponding author. Tel.: +886 3 4535525; fax: +886 3 4356760. E-mail address: [email protected] (J.-Y. Lai).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.003
alcohol solutions, and the swollen membranes usually lose their permselectivity and show poor long-term stability of operation. Great efforts are therefore attempting to improve the stability and mechanical properties of chitosan-based membranes, such as bringing crosslinked structure to membranes [13,16–17,21–23], blending chitosan with other polymers [11–12,19], casting chitosan on another polymer substrate to form composite membrane [17,18], and adding inorganic reinforcements into chitosan membranes [15]. However, the reduction in permeation flux is always accompanied with these modifications. Concentration and purification of isopropanol from its water solution are necessary for many chemical processes, such as acetone production, solvent extraction, and manufacture of hydrogen peroxide. Isopropanol is also used as a cleaning agent in modern semiconductor and electronic industries, where recycling of wasted isopropanol is essential from environmental and economical point of view. On the other hand, the azeotropic concentration of isopropanol–water is
234
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
87.8 wt.%. The relatively low azeotropic concentration indicates that membranes for pervaporative dehydration of isopropanol–water solutions are operated with solutions of relatively high water contents. The stability of membranes against swelling in water becomes critical. In addition, the long-term stability of operation is another key factor for membranes applied to commercial practice. Although there have been a lot of works on chitosan membrane stabilization being published, however, long-term stability of membranes received little discussion. In this work, ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS) was used to crosslink chitosan. Membranes with organic–inorganic hybrid structure were obtained by means of the conventional fabrication process of preparing chitosan membrane. Both crosslinked structure and reinforcement from organic–inorganic hybrid structure contribute to the stabilization of the chitosan-based membranes [23]. The membranes were applied to pervaporative dehydration of a 70 wt.% isopropanol–water solution. Improvement on longterm stability as well as the maintenance of high permselectivity and flux is observed, which suggests their potential in practical applications.
2. Experimental 2.1. Materials Chitosan with a degree of deacetylation of 85% (Sigma Chemical Co.) and ␥-(glycidyloxypropyl)trimethoxysilane (United Chemical Technologies Inc.) were used without further purification. Isopropanol aqueous solution used in pervaporation experiments was prepared in the laboratory by mixing isopropanol (Echo Chem. Co., Miaoli, Taiwan) with distilled water. Glutaraldehyde (GA, Fluka, 50 vol.% in water) and sulfuric acid (>95%, Nihon Shiyaku Industries Ltd., Japan) were used as crosslinking agents for chitosan. 2.2. Measurement of contact angle The contact angles of water on the membranes were measured at room temperature using the sessile drop method by means of an angle-meter of Automatic Contact Angle Meter, Model CA-VP (Kyowa Interface Science Co., Japan). Distilled water was dropped onto at least 10 different sites on each sample. An average value was obtained for the measured contact angle. 2.3. Measurement of degree of swelling The membrane was completely dried under reduced pressure at room temperature and then weighed. The dried membrane was immersed in the testing solvent for 24 h. Then the membrane was taken out of the solvent, wiped quickly with filter paper, and weighed. The degree of swelling of the mem-
brane was determined from W s − Wd DS = , Wd where DS is the degree of swelling of the membrane, Ws the weight of swollen membrane and Wd is the dried membrane. 2.4. Preparation of chitosan membranes Chitosan–silica hybrid materials (CSHM) based membranes were obtained from the procedures described below [23]. A 1.5 wt.% chitosan solution was prepared from dissolving chitosan in a 2 wt.% acetic acid aqueous solution. After stirring at room temperature for 24 h, the solution was filtered, bathed in an ultrasonic bath for 20 min, and stood for 24 h. Certain amount of GPTMS was then added into the solution with stirring. The mixture was stirred for 1 h, and then cast on a glass plate with a casting knife. The casting thickness was 1200 m. The cast membrane was then heated in an oven at 40 ◦ C for 1 h, 50 ◦ C 1 h, 60 ◦ C 1 h, and then 80 ◦ C for another 36 h. The membrane was peeled off the glass after immersion in a water bath. The membranes were not treated with NaOH (aq) to increase their hydrophilicity, since they were stable in alcohol due to its crosslinked and organic–inorganic hybrid structures. The membranes obtained are coded as CSHM-X, where X (wt.% based on chitosan) indicates the amount of GPTMS added into the preparation solution. Sulfuric acid crosslinked chitosan membrane (CS–SA) [13,24] and glutaraldehyde crosslinked chitosan membrane (CS–GA) were prepared according to the previous report [25]. 2.5. Pervaporative dehydration operation Pervaporation was conducted with a conventional process [26]. The feed is 70 wt.% of isopropanol (IPA) aqueous solution. The effective membrane area is 7.0 cm2 . The feed temperature studied was in the range of 25–70 ◦ C, and the downstream pressure was kept at 6.67 × 102 –10.67 × 102 Pa (5–8 Torr). The compositions of the feed solutions and permeates were measured by gas chromatography (China Chromatography GC-8700T). The separation factor (αwater–isopropanol ) was calculated from the concentration ratio of the permeate solution over that of the feed solution. Ywater /Yisopropanol , αwater−isopropanol = Xwater /Xisopropanol where Y and X are the concentrations in permeate and in feeding solutions, respectively, and the subscription (water and isopropanol) indicates the species. The 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 separated membranes.
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
2.6. Measurements on the long-term stability of pervaporative dehydration operation The membrane was mounted onto the equipments for pervaporation tests. While the measurement was completed, the tested membranes were removed from the equipments and then immersed in a 70 wt.% IPA–H2 O solution at 25 ◦ C for storage. At desired time the membrane was taken out for repeating pervaporative dehydration measurements. Four pieces of membranes were performed in one test. The test was terminated while one piece of the tested membranes failed, or the water concentration in the permeate liquid lower than 95 wt.%.
3. Results and discussion 3.1. Membrane preparation Crosslinked chitosan membranes for pervaporative dehydration of isopropanol–water mixtures were obtained from casting the chitosan acetic acid aqueous solution contain-
235
ing ␥-(glycidyloxypropyl)trimethoxysilane. GPTMS reacted with the amino group on chitosan chains through its oxirane ring. Meanwhile, hydration of the trimethoxy groups of GPTMS moiety formed silantriol pendent through acidcatalyzed sol reaction. Dehydration reaction among the silantriol groups occurs during the drying process and forms inter-chain linkages between chitosan chains, so as to bring crosslinked structures into the chitosan membranes (Fig. 1). In this process, in situ crosslinking and membrane preparation were performed with the conventional fabrication process, so as to provide a convenient way of preparing cross-linked chitosan membranes [23]. In addition to crosslinked structure, chitosan–silica hybrid structure was also introduced into the membranes obtained. The membranes obtained were transparent and exhibited good mechanical strength for pervaporation tests [23]. 3.2. Pervaporative dehydration of isopropanol–water mixtures The high hydrophilicity of chitosan contributes to its high permselectivity and flux in the pervaporative dehy-
Fig. 1. Chemical structure and reaction model for crosslinking chitosan.
236
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Fig. 2. Plots of permeation flux and separation factor vs. GPTMS contents of CSHM membranes: pervapoartion separation of 70wt.% aqueous isopropanol solution at 70 ◦ C.
dration of alcohol–water solutions. Unfortunately, high hydrophilicity of chitosan also brings high degree of swelling as water content higher than 20%, so as to reduce its stability in alcohol–water mixtures. Considerable efforts, including crosslinking the membranes and formation of hybrid/composite membranes, are done to improve the swelling property of chitosan membranes and enhance their permselectivity and stability. However, significant decrease of permeation flux usually accompanies such modification and crosslinking of chitosan membranes. Fig. 2 showed the pervaporative dehydration performance of the CSHM membranes for separating a 70 wt.% isopropanol–water solution. As seen in the figure the flux decreased as the added amounts of GPTMS increased. The CSHM membranes showed relatively high fluxes while compared with other crosslinked membranes. Flux of 1730 g/(m2 h) and separation factors of 694 were found for CSHM-5. Generally, crosslinking on chitosan membranes decreases their hydrophilicity and chain mobility to lower the permeation flux. The high fluxes of the CSHM membranes might come from their high hydrophilicity, as the contact angles of CSHM membranes were similar to that
Fig. 3. Plots of water contact angle and degree of swelling in 70 wt.% IPA–water solution of CSHM membranes vs. GPTMS contents in CSHM membranes.
Fig. 4. Arrhenius plot of feed solution temperature on pervaporation dehydration performance of CSHM-5 membrane on 70 wt.% IPA–H2 O solution.
of pristine chitosan membrane (Fig. 3). On the other hand, for all of the membranes excepting CSHM-50, the water concentration in permeate is above 99%. The high water concentration in permeate indicates the high permselectivity of the membranes. While plotting separation factor versus GPTMS contents in membranes (Fig. 2), separation factors between 225 and 694 are observed. However, there is no tendency being observed between separation factor and GPTMS contents. Additionally, as the swelling behavior of the chitosan membranes was significantly exhibited by the modification with GPTMS (Fig. 3), improvements of the stability of the CSHM membranes were reasonably expected. Experimental data of temperature dependence of permeation flux were obtained in the tested temperature range of 25–70 ◦ C for CSHM-5. The permeation flux of 70 wt.% IPA–H2 O solution is plotted against the reciprocal of the operating temperature (K) in Fig. 4. The flux increases with the feed solution temperature increasing. High operating temperature would increase the amounts of water dissolved in the membrane. The plasticizing effect of the dissolved water enhanced the chain mobility of the membrane, so as to increase the permeation fluxes. The activation energy of permeation for CSHM-5 membrane (23.0 kJ/mol) and the preexponential factor A (6.97 × 103 kg/(m2 h)) were obtained from Fig. 4 according to the Arrhenius equation of Ep , ln Q = ln A − RT 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. On the other hand, the permselectivity often decreases with increasing permeation temperatures [22]. The decrease of permselectivity was due to increase of alcohol permeation rate at high temperatures. However, no obvious decrease of the water concentration in permeate was observed with CSHM-5 in the permeation temperatures of 25–70 ◦ C. The maintenance of separation ability could come from the high hydrophilicity of CSHM-5. The
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Fig. 5. Long-term operation results of chitosan-based membranes for pervaporation dehydration on 70 wt.% IPA–H2 O solution at 70 ◦ C. (A) Stability of permeation flux and (B) permeation water concentration.
high hydrophilicity of membrane increases the water flux at high temperatures to offset the increase of isopropanol fluxes. 3.3. Long-term stability of membranes Long-term stability of operation is critical for membranes in commerce. However, few studies discussed the membrane long-term stability. Therefore, long-term stability test on pure chitosan and CSHM membranes containing GPTMS contents of 2.5–10 wt.% was examined in this study (Fig. 5), and the data are summarized in Table 1. Membrane was judged to lose its efficiency while the water concentration in permeate was less than 95 wt.%. Pristine chitosan membrane, Table 1 Long-term stability test on various membranes for pervaporation dehydration of 70 wt.% IPA–H2 O solution at 70 ◦ C Membrane
Thickness (m)
Pure CS CSHM-2.5 CSHM-5 CSHM-7.5 CSHM-10 CS–SA CS–GA
11.8 12.8 12.0 13.8 14.2 12.4 13.6
± ± ± ± ± ± ±
1.4 1.0 1.2 0.5 0.4 1.4 0.4
Initial flux (g/(m2 h))
Membrane life (day)
4470 2000 1730 1560 1240 2370 1430
1 47 141 70 4 2 116
237
showing a very high permeation flux of 4470 g/(m2 h), lost its efficiency only after one day of operation. The membranes crosslinked with GPTMS exhibited reduced initial permeation fluxes. However, the long-term stability of the CSHM membranes was significantly improved. CSHM-5, which had an initial flux of 1730 g/(m2 h), showed the best long-term stability on pervaporative dehydration of 70 wt.% IPA–H2 O solution among the tested membranes. The dehydration performance of CSHM-5 was maintained after 140 days. CSHM-10 failed in 4 days in the test owing to its brittleness. To further examine the performance of CSHM-5 on pervaporative dehydration of 70 wt.% IPA–H2 O solution, chitosan membranes crosslinked with sulfuric acid (CS–SA) and glutaraldehyde (CS–GA), respectively, were prepared for comparison. Both membranes showed good performance on pervaporative dehydration of alcohol aqueous solutions in previous work [24,25]. CS–SA showed an initial permeation flux of 2370 g/(m2 h). This high flux resulted from its loose crosslinked structure originating from sulfuric acid–amine linkages. However, this membrane lost its efficiency of pervaporative dehydration after 2 days. The ionic sulfuric acid–amine linkages were easily broken while contacting with the water in the membrane. Therefore, the crosslinked structure of CS–SA membrane was broken down to lose its performance during the pervaporation operation. On the other hand, CS–GA showed a relatively good stability of long-term operation, as it maintained the dehydration performance after a 116-day operation. One of the drawbacks of CS–GA membrane was its low permeation flux (1430 g/(m2 h)) due to its high crosslinking density. Another disadvantage of CS–GA membrane was to employ the toxic agent of glutaraldehyde. In addition to the crosslinked structure, the organic– inorganic hybrid structure in the CSHM-5 membrane might also contribute to its superior long-term stability over the reported CS–SA and CS–GA membranes. The effect of silica hybrid structure on stabilizing organic membranes was also observed in other works [4,27,28]. From the above discussion it was concluded that CSHM-5 membrane prepared in this study had high permeation flux, good separation performance, and superior long-term operation stability on pervaporative dehydration of 70 wt.% IPA–H2 O solution. 4. Conclusions Chitosan membranes crosslinked with GPTMS showed good performance of pervaporative dehydration of a 70 wt.% IPA–H2 O solution. A chitosan membrane crosslinked with 5 wt.% of GPTMS (CSHM-5) showed a permeation flux of 1730 g/(m2 h) and a separation factor of 694. The separation performance could be maintained in a 140-day longterm operation. The performance of the CSHM-5 membrane was superior over other reported chitosan-based membrane to ensure its potential of commercial application.
238
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Acknowledgement Financial support on this work from the Ministry of Economic Affairs, Taiwan, is highly appreciated (Grant no. 92EC-17-A-10-S1-0004).
[15]
[16]
References [1] 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. [2] W.F. Guo, T.S. Chung, T. Matsuura, R. Wang, Y. Liu, Pervaporation study of water and tert-butanol mixtures, J. Appl. Polym. Sci. 91 (2004) 4082–4090. [3] W. Yoshida, Y. Cohen, Removal of methyl tert-butyl ether from water by pervaporation using ceramic-supported polymer membranes, J. Membr. Sci. 229 (2004) 27–32. [4] Q.L. Liu, H. Xiao, Silicalite-filled poly(siloxane imide) membranes for removal of VOCs from water by pervaporation, J. Membr. Sci. 230 (2004) 121–129. [5] S.P. Kusumocahyo, T. Kanamori, K. Sumaru, T. Iwatsubo, T. Shinbo, Pervaporation of xylene isomer mixture through cyclodextrins containing polyacrylic acid membranes, J. Membr. Sci. 231 (2004) 127–132. [6] L. Liang, J.M. Dickson, J.X. Jiang, M.A. Brook, Effect of low flow rate on pervaporation of 1,2-dichloroethane with novel polydimethylsiloxane composite membranes, J. Membr. Sci. 231 (2004) 71–79. [7] M.Y. Hung, S.H. Chen, R.M. Liou, C.S. Hsu, J.Y. Lai, Pervaporation separation of water–ethanol mixture by a sodium sulfonate polysulfone membrane, J. Appl. Polym. Sci. 90 (2003) 3374– 3383. [8] M.Y. Hung, S.H. Chen, R.M. Liou, C.S. Hsu, H.A. Tsai, J.Y. Lai, Pervaporation separation of water–ethanol mixture by TGN–PSF blending membrane, Eur. Polym. J. 39 (2003) 2367–2374. [9] 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. [10] T. Uragami, S. Kato, T. Miyata, Structure of N-alkyl chitosan membranes on water-permselectivity for aqueous ethanol solutions, J. Membr. Sci. 124 (1997) 203–211. [11] 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. [12] 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. [13] J. Ge, Y. Cui, Y. Yan, W. Jiang, The effect of structure on pervaporation of chitosan membrane, J. Membr. Sci. 165 (2000) 75–81. [14] R. Jiraratananon, A. Chanachai, R.Y.M. Huang, Pervaporation dehydration of ethanol–water mixtures with chitosan–hydroxyethyl-
[17]
[18]
[19]
[20]
[21]
[22]
[23] [24]
[25]
[26]
[27]
[28]
cellulose (CS–HEC) composites membranes II. Analysis of mass transport, J. Membr. Sci. 199 (2002) 211–222. 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. M. Ghazali, M. Nawawi, R.Y.M. Huang, Pervaporation dehydration of isopropanol with chitosan membranes, J. Membr. Sci. 124 (1997) 53–62. 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. 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. 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. U.S. Toti, T.M. Aminabhavi, Different viscosity grade sodium alginate and modified sodium alginate membranes in pervaporation separation of water plus acetic acid and water plus isopropanol mixtures, J. Membr. Sci. 228 (2004) 199–208. T. Uragami, M. Takuno, T. Miyata, Evapomeation characteristics of cross-linked quaternized chitosan membranes for the separation of an ethanol–water azeotrope, Macromol. Chem. Phys. 203 (2002) 1162–1170. T. Uragami, S. Yamamoto, T. Miyata, Dehydration from alcohols by polyion complex cross-linked chitosan composite membranes during evapomeation, Biomacromolecules 4 (2003) 137–144. Y.L. Liu, Y.H. Su, J.Y. Lai, Preparation and characterization of a chitosan–silica hybrid membrane, Polymer 45 (2004) 6831–6837. A. Mochizuki, S. Amiya, Y. Sato, H. Ogawara, S. Yamashita, Pervaporation separation of water–ethanol mixtures through polysaccharide membranes. III. The permselectivity of the neutralized chitosan membrane and the relationships between its permselectivity and solid state structure, J. Appl. Polym. Sci. 37 (1989) 3385–3398. 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. 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. T. Uragami, K. Okazaki, H. Matsugi, T. Miyata, Structure and permeation characteristics of an aqueous ethanol solution of organic–inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane, Macromolecules 35 (2002) 9156–9163. Y.L. Liu, C.Y. 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, in press.
Crosslinked organic–inorganic hybrid chitosan membranes for pervaporation dehydration of isopropanol–water mixtures with a long-term stability Ying-Ling Liua,b , Yu-Huei Sua,b , Kuir-Rain Leeb,c , Juin-Yih Laia,b,∗ a b
Department of Chemical Engineering, Chung Yuan University, #200, Chung-Pei Road, Chungli, Taoyuan 320, Taiwan R&D Center for Membrane Technology, Chung Yuan University, #200, Chung-Pei Road, Chungli, Taoyuan 320, Taiwan c Department of Chemical Engineering, Nanya Institute of Technology, Chungli, Taoyuan 320, Taiwan Received 30 August 2004 Available online 13 January 2005
Abstract Crosslinked organic–inorganic hybrid chitosan membranes were obtained from blending chitosan and ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS) in acetic acid aqueous solution. The hydrophilicity of the modified membranes was not significantly decreased, so as to result in good permselectivity and high permeation flux in pervaporative dehydration on a 70 wt.% isopropanol–water (IPA–H2 O) solution. A flux of 1730 g/(m2 h) and a separation factor of 694 were found with the chitosan membrane containing 5 wt.% GPTMS. Both of the crosslinked and organic–inorganic hybrid structures contribute to stabilize the membranes to maintain the performance of the membranes in a 140 days long-term operation. © 2004 Elsevier B.V. All rights reserved. Keywords: Chitosan; Pervaporation; Crosslinked membrane; Isopropanol–water separation
1. Introduction Separation of azeotropic mixtures or liquid mixtures containing substances of close boiling points is hard to achieve by conventional distillation process. To overcome this problem, pervaporation separation has showed its attractive performance and has received considerable investigations [1–8]. Membranes for pervaporation separation of aqueous alcohol mixtures, like ethanol–water [7–15] and isopropanol–water (IPA–H2 O) [16–20] solutions, are especially focused on. In the dehydration operation, hydrophilic membranes are good candidates owing to their high water-permselectivity. Chitosan is widely used in membrane applications based on its high hydrophilicity, good film-forming character, and excellent chemical-resistant properties [9–14,16–18]. However, chitosan membranes are highly swollen in water and ∗
Corresponding author. Tel.: +886 3 4535525; fax: +886 3 4356760. E-mail address: [email protected] (J.-Y. Lai).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.12.003
alcohol solutions, and the swollen membranes usually lose their permselectivity and show poor long-term stability of operation. Great efforts are therefore attempting to improve the stability and mechanical properties of chitosan-based membranes, such as bringing crosslinked structure to membranes [13,16–17,21–23], blending chitosan with other polymers [11–12,19], casting chitosan on another polymer substrate to form composite membrane [17,18], and adding inorganic reinforcements into chitosan membranes [15]. However, the reduction in permeation flux is always accompanied with these modifications. Concentration and purification of isopropanol from its water solution are necessary for many chemical processes, such as acetone production, solvent extraction, and manufacture of hydrogen peroxide. Isopropanol is also used as a cleaning agent in modern semiconductor and electronic industries, where recycling of wasted isopropanol is essential from environmental and economical point of view. On the other hand, the azeotropic concentration of isopropanol–water is
234
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
87.8 wt.%. The relatively low azeotropic concentration indicates that membranes for pervaporative dehydration of isopropanol–water solutions are operated with solutions of relatively high water contents. The stability of membranes against swelling in water becomes critical. In addition, the long-term stability of operation is another key factor for membranes applied to commercial practice. Although there have been a lot of works on chitosan membrane stabilization being published, however, long-term stability of membranes received little discussion. In this work, ␥-(glycidyloxypropyl)trimethoxysilane (GPTMS) was used to crosslink chitosan. Membranes with organic–inorganic hybrid structure were obtained by means of the conventional fabrication process of preparing chitosan membrane. Both crosslinked structure and reinforcement from organic–inorganic hybrid structure contribute to the stabilization of the chitosan-based membranes [23]. The membranes were applied to pervaporative dehydration of a 70 wt.% isopropanol–water solution. Improvement on longterm stability as well as the maintenance of high permselectivity and flux is observed, which suggests their potential in practical applications.
2. Experimental 2.1. Materials Chitosan with a degree of deacetylation of 85% (Sigma Chemical Co.) and ␥-(glycidyloxypropyl)trimethoxysilane (United Chemical Technologies Inc.) were used without further purification. Isopropanol aqueous solution used in pervaporation experiments was prepared in the laboratory by mixing isopropanol (Echo Chem. Co., Miaoli, Taiwan) with distilled water. Glutaraldehyde (GA, Fluka, 50 vol.% in water) and sulfuric acid (>95%, Nihon Shiyaku Industries Ltd., Japan) were used as crosslinking agents for chitosan. 2.2. Measurement of contact angle The contact angles of water on the membranes were measured at room temperature using the sessile drop method by means of an angle-meter of Automatic Contact Angle Meter, Model CA-VP (Kyowa Interface Science Co., Japan). Distilled water was dropped onto at least 10 different sites on each sample. An average value was obtained for the measured contact angle. 2.3. Measurement of degree of swelling The membrane was completely dried under reduced pressure at room temperature and then weighed. The dried membrane was immersed in the testing solvent for 24 h. Then the membrane was taken out of the solvent, wiped quickly with filter paper, and weighed. The degree of swelling of the mem-
brane was determined from W s − Wd DS = , Wd where DS is the degree of swelling of the membrane, Ws the weight of swollen membrane and Wd is the dried membrane. 2.4. Preparation of chitosan membranes Chitosan–silica hybrid materials (CSHM) based membranes were obtained from the procedures described below [23]. A 1.5 wt.% chitosan solution was prepared from dissolving chitosan in a 2 wt.% acetic acid aqueous solution. After stirring at room temperature for 24 h, the solution was filtered, bathed in an ultrasonic bath for 20 min, and stood for 24 h. Certain amount of GPTMS was then added into the solution with stirring. The mixture was stirred for 1 h, and then cast on a glass plate with a casting knife. The casting thickness was 1200 m. The cast membrane was then heated in an oven at 40 ◦ C for 1 h, 50 ◦ C 1 h, 60 ◦ C 1 h, and then 80 ◦ C for another 36 h. The membrane was peeled off the glass after immersion in a water bath. The membranes were not treated with NaOH (aq) to increase their hydrophilicity, since they were stable in alcohol due to its crosslinked and organic–inorganic hybrid structures. The membranes obtained are coded as CSHM-X, where X (wt.% based on chitosan) indicates the amount of GPTMS added into the preparation solution. Sulfuric acid crosslinked chitosan membrane (CS–SA) [13,24] and glutaraldehyde crosslinked chitosan membrane (CS–GA) were prepared according to the previous report [25]. 2.5. Pervaporative dehydration operation Pervaporation was conducted with a conventional process [26]. The feed is 70 wt.% of isopropanol (IPA) aqueous solution. The effective membrane area is 7.0 cm2 . The feed temperature studied was in the range of 25–70 ◦ C, and the downstream pressure was kept at 6.67 × 102 –10.67 × 102 Pa (5–8 Torr). The compositions of the feed solutions and permeates were measured by gas chromatography (China Chromatography GC-8700T). The separation factor (αwater–isopropanol ) was calculated from the concentration ratio of the permeate solution over that of the feed solution. Ywater /Yisopropanol , αwater−isopropanol = Xwater /Xisopropanol where Y and X are the concentrations in permeate and in feeding solutions, respectively, and the subscription (water and isopropanol) indicates the species. The 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 separated membranes.
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
2.6. Measurements on the long-term stability of pervaporative dehydration operation The membrane was mounted onto the equipments for pervaporation tests. While the measurement was completed, the tested membranes were removed from the equipments and then immersed in a 70 wt.% IPA–H2 O solution at 25 ◦ C for storage. At desired time the membrane was taken out for repeating pervaporative dehydration measurements. Four pieces of membranes were performed in one test. The test was terminated while one piece of the tested membranes failed, or the water concentration in the permeate liquid lower than 95 wt.%.
3. Results and discussion 3.1. Membrane preparation Crosslinked chitosan membranes for pervaporative dehydration of isopropanol–water mixtures were obtained from casting the chitosan acetic acid aqueous solution contain-
235
ing ␥-(glycidyloxypropyl)trimethoxysilane. GPTMS reacted with the amino group on chitosan chains through its oxirane ring. Meanwhile, hydration of the trimethoxy groups of GPTMS moiety formed silantriol pendent through acidcatalyzed sol reaction. Dehydration reaction among the silantriol groups occurs during the drying process and forms inter-chain linkages between chitosan chains, so as to bring crosslinked structures into the chitosan membranes (Fig. 1). In this process, in situ crosslinking and membrane preparation were performed with the conventional fabrication process, so as to provide a convenient way of preparing cross-linked chitosan membranes [23]. In addition to crosslinked structure, chitosan–silica hybrid structure was also introduced into the membranes obtained. The membranes obtained were transparent and exhibited good mechanical strength for pervaporation tests [23]. 3.2. Pervaporative dehydration of isopropanol–water mixtures The high hydrophilicity of chitosan contributes to its high permselectivity and flux in the pervaporative dehy-
Fig. 1. Chemical structure and reaction model for crosslinking chitosan.
236
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Fig. 2. Plots of permeation flux and separation factor vs. GPTMS contents of CSHM membranes: pervapoartion separation of 70wt.% aqueous isopropanol solution at 70 ◦ C.
dration of alcohol–water solutions. Unfortunately, high hydrophilicity of chitosan also brings high degree of swelling as water content higher than 20%, so as to reduce its stability in alcohol–water mixtures. Considerable efforts, including crosslinking the membranes and formation of hybrid/composite membranes, are done to improve the swelling property of chitosan membranes and enhance their permselectivity and stability. However, significant decrease of permeation flux usually accompanies such modification and crosslinking of chitosan membranes. Fig. 2 showed the pervaporative dehydration performance of the CSHM membranes for separating a 70 wt.% isopropanol–water solution. As seen in the figure the flux decreased as the added amounts of GPTMS increased. The CSHM membranes showed relatively high fluxes while compared with other crosslinked membranes. Flux of 1730 g/(m2 h) and separation factors of 694 were found for CSHM-5. Generally, crosslinking on chitosan membranes decreases their hydrophilicity and chain mobility to lower the permeation flux. The high fluxes of the CSHM membranes might come from their high hydrophilicity, as the contact angles of CSHM membranes were similar to that
Fig. 3. Plots of water contact angle and degree of swelling in 70 wt.% IPA–water solution of CSHM membranes vs. GPTMS contents in CSHM membranes.
Fig. 4. Arrhenius plot of feed solution temperature on pervaporation dehydration performance of CSHM-5 membrane on 70 wt.% IPA–H2 O solution.
of pristine chitosan membrane (Fig. 3). On the other hand, for all of the membranes excepting CSHM-50, the water concentration in permeate is above 99%. The high water concentration in permeate indicates the high permselectivity of the membranes. While plotting separation factor versus GPTMS contents in membranes (Fig. 2), separation factors between 225 and 694 are observed. However, there is no tendency being observed between separation factor and GPTMS contents. Additionally, as the swelling behavior of the chitosan membranes was significantly exhibited by the modification with GPTMS (Fig. 3), improvements of the stability of the CSHM membranes were reasonably expected. Experimental data of temperature dependence of permeation flux were obtained in the tested temperature range of 25–70 ◦ C for CSHM-5. The permeation flux of 70 wt.% IPA–H2 O solution is plotted against the reciprocal of the operating temperature (K) in Fig. 4. The flux increases with the feed solution temperature increasing. High operating temperature would increase the amounts of water dissolved in the membrane. The plasticizing effect of the dissolved water enhanced the chain mobility of the membrane, so as to increase the permeation fluxes. The activation energy of permeation for CSHM-5 membrane (23.0 kJ/mol) and the preexponential factor A (6.97 × 103 kg/(m2 h)) were obtained from Fig. 4 according to the Arrhenius equation of Ep , ln Q = ln A − RT 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. On the other hand, the permselectivity often decreases with increasing permeation temperatures [22]. The decrease of permselectivity was due to increase of alcohol permeation rate at high temperatures. However, no obvious decrease of the water concentration in permeate was observed with CSHM-5 in the permeation temperatures of 25–70 ◦ C. The maintenance of separation ability could come from the high hydrophilicity of CSHM-5. The
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Fig. 5. Long-term operation results of chitosan-based membranes for pervaporation dehydration on 70 wt.% IPA–H2 O solution at 70 ◦ C. (A) Stability of permeation flux and (B) permeation water concentration.
high hydrophilicity of membrane increases the water flux at high temperatures to offset the increase of isopropanol fluxes. 3.3. Long-term stability of membranes Long-term stability of operation is critical for membranes in commerce. However, few studies discussed the membrane long-term stability. Therefore, long-term stability test on pure chitosan and CSHM membranes containing GPTMS contents of 2.5–10 wt.% was examined in this study (Fig. 5), and the data are summarized in Table 1. Membrane was judged to lose its efficiency while the water concentration in permeate was less than 95 wt.%. Pristine chitosan membrane, Table 1 Long-term stability test on various membranes for pervaporation dehydration of 70 wt.% IPA–H2 O solution at 70 ◦ C Membrane
Thickness (m)
Pure CS CSHM-2.5 CSHM-5 CSHM-7.5 CSHM-10 CS–SA CS–GA
11.8 12.8 12.0 13.8 14.2 12.4 13.6
± ± ± ± ± ± ±
1.4 1.0 1.2 0.5 0.4 1.4 0.4
Initial flux (g/(m2 h))
Membrane life (day)
4470 2000 1730 1560 1240 2370 1430
1 47 141 70 4 2 116
237
showing a very high permeation flux of 4470 g/(m2 h), lost its efficiency only after one day of operation. The membranes crosslinked with GPTMS exhibited reduced initial permeation fluxes. However, the long-term stability of the CSHM membranes was significantly improved. CSHM-5, which had an initial flux of 1730 g/(m2 h), showed the best long-term stability on pervaporative dehydration of 70 wt.% IPA–H2 O solution among the tested membranes. The dehydration performance of CSHM-5 was maintained after 140 days. CSHM-10 failed in 4 days in the test owing to its brittleness. To further examine the performance of CSHM-5 on pervaporative dehydration of 70 wt.% IPA–H2 O solution, chitosan membranes crosslinked with sulfuric acid (CS–SA) and glutaraldehyde (CS–GA), respectively, were prepared for comparison. Both membranes showed good performance on pervaporative dehydration of alcohol aqueous solutions in previous work [24,25]. CS–SA showed an initial permeation flux of 2370 g/(m2 h). This high flux resulted from its loose crosslinked structure originating from sulfuric acid–amine linkages. However, this membrane lost its efficiency of pervaporative dehydration after 2 days. The ionic sulfuric acid–amine linkages were easily broken while contacting with the water in the membrane. Therefore, the crosslinked structure of CS–SA membrane was broken down to lose its performance during the pervaporation operation. On the other hand, CS–GA showed a relatively good stability of long-term operation, as it maintained the dehydration performance after a 116-day operation. One of the drawbacks of CS–GA membrane was its low permeation flux (1430 g/(m2 h)) due to its high crosslinking density. Another disadvantage of CS–GA membrane was to employ the toxic agent of glutaraldehyde. In addition to the crosslinked structure, the organic– inorganic hybrid structure in the CSHM-5 membrane might also contribute to its superior long-term stability over the reported CS–SA and CS–GA membranes. The effect of silica hybrid structure on stabilizing organic membranes was also observed in other works [4,27,28]. From the above discussion it was concluded that CSHM-5 membrane prepared in this study had high permeation flux, good separation performance, and superior long-term operation stability on pervaporative dehydration of 70 wt.% IPA–H2 O solution. 4. Conclusions Chitosan membranes crosslinked with GPTMS showed good performance of pervaporative dehydration of a 70 wt.% IPA–H2 O solution. A chitosan membrane crosslinked with 5 wt.% of GPTMS (CSHM-5) showed a permeation flux of 1730 g/(m2 h) and a separation factor of 694. The separation performance could be maintained in a 140-day longterm operation. The performance of the CSHM-5 membrane was superior over other reported chitosan-based membrane to ensure its potential of commercial application.
238
Y.-L. Liu et al. / Journal of Membrane Science 251 (2005) 233–238
Acknowledgement Financial support on this work from the Ministry of Economic Affairs, Taiwan, is highly appreciated (Grant no. 92EC-17-A-10-S1-0004).
[15]
[16]
References [1] 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. [2] W.F. Guo, T.S. Chung, T. Matsuura, R. Wang, Y. Liu, Pervaporation study of water and tert-butanol mixtures, J. Appl. Polym. Sci. 91 (2004) 4082–4090. [3] W. Yoshida, Y. Cohen, Removal of methyl tert-butyl ether from water by pervaporation using ceramic-supported polymer membranes, J. Membr. Sci. 229 (2004) 27–32. [4] Q.L. Liu, H. Xiao, Silicalite-filled poly(siloxane imide) membranes for removal of VOCs from water by pervaporation, J. Membr. Sci. 230 (2004) 121–129. [5] S.P. Kusumocahyo, T. Kanamori, K. Sumaru, T. Iwatsubo, T. Shinbo, Pervaporation of xylene isomer mixture through cyclodextrins containing polyacrylic acid membranes, J. Membr. Sci. 231 (2004) 127–132. [6] L. Liang, J.M. Dickson, J.X. Jiang, M.A. Brook, Effect of low flow rate on pervaporation of 1,2-dichloroethane with novel polydimethylsiloxane composite membranes, J. Membr. Sci. 231 (2004) 71–79. [7] M.Y. Hung, S.H. Chen, R.M. Liou, C.S. Hsu, J.Y. Lai, Pervaporation separation of water–ethanol mixture by a sodium sulfonate polysulfone membrane, J. Appl. Polym. Sci. 90 (2003) 3374– 3383. [8] M.Y. Hung, S.H. Chen, R.M. Liou, C.S. Hsu, H.A. Tsai, J.Y. Lai, Pervaporation separation of water–ethanol mixture by TGN–PSF blending membrane, Eur. Polym. J. 39 (2003) 2367–2374. [9] 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. [10] T. Uragami, S. Kato, T. Miyata, Structure of N-alkyl chitosan membranes on water-permselectivity for aqueous ethanol solutions, J. Membr. Sci. 124 (1997) 203–211. [11] 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. [12] 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. [13] J. Ge, Y. Cui, Y. Yan, W. Jiang, The effect of structure on pervaporation of chitosan membrane, J. Membr. Sci. 165 (2000) 75–81. [14] R. Jiraratananon, A. Chanachai, R.Y.M. Huang, Pervaporation dehydration of ethanol–water mixtures with chitosan–hydroxyethyl-
[17]
[18]
[19]
[20]
[21]
[22]
[23] [24]
[25]
[26]
[27]
[28]
cellulose (CS–HEC) composites membranes II. Analysis of mass transport, J. Membr. Sci. 199 (2002) 211–222. 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. M. Ghazali, M. Nawawi, R.Y.M. Huang, Pervaporation dehydration of isopropanol with chitosan membranes, J. Membr. Sci. 124 (1997) 53–62. 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. 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. 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. U.S. Toti, T.M. Aminabhavi, Different viscosity grade sodium alginate and modified sodium alginate membranes in pervaporation separation of water plus acetic acid and water plus isopropanol mixtures, J. Membr. Sci. 228 (2004) 199–208. T. Uragami, M. Takuno, T. Miyata, Evapomeation characteristics of cross-linked quaternized chitosan membranes for the separation of an ethanol–water azeotrope, Macromol. Chem. Phys. 203 (2002) 1162–1170. T. Uragami, S. Yamamoto, T. Miyata, Dehydration from alcohols by polyion complex cross-linked chitosan composite membranes during evapomeation, Biomacromolecules 4 (2003) 137–144. Y.L. Liu, Y.H. Su, J.Y. Lai, Preparation and characterization of a chitosan–silica hybrid membrane, Polymer 45 (2004) 6831–6837. A. Mochizuki, S. Amiya, Y. Sato, H. Ogawara, S. Yamashita, Pervaporation separation of water–ethanol mixtures through polysaccharide membranes. III. The permselectivity of the neutralized chitosan membrane and the relationships between its permselectivity and solid state structure, J. Appl. Polym. Sci. 37 (1989) 3385–3398. 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. 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. T. Uragami, K. Okazaki, H. Matsugi, T. Miyata, Structure and permeation characteristics of an aqueous ethanol solution of organic–inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane, Macromolecules 35 (2002) 9156–9163. Y.L. Liu, C.Y. 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, in press.