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Pervaporation dehydration of aqueous ethanol solution using H-ZSM-5 filled chitosan membranes
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Separation and Purification Technology 58 (2008) 429–436
Pervaporation dehydration of aqueous ethanol solution using H-ZSM-5 filled chitosan membranes Honglei Sun, Lianyu Lu, Xue Chen, Zhongyi Jiang ∗ Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Received 11 May 2007; received in revised form 9 September 2007; accepted 10 September 2007
Abstract H-ZSM-5 filled chitosan (CS) membranes were prepared by incorporating H-ZSM-5 into chitosan for pervaporation dehydration of aqueous ethanol solution. Characterization results revealed that hydroxyl groups on H-ZSM-5 could form hydrogen-bonding interaction with the hydroxyl and amino groups of chitosan, which lowered the crystallinity of membranes and consequently improved the interface morphology. The sorption experiments showed increased degree of swelling (DS) and sorptivity selectivity with increase of H-ZSM-5 content and decrease of Si/Al ratio in H-ZSM-5, respectively. Decreased separation factor of the filled membranes in pervaporation process was mainly attributed to the formation of nanometer-scale voids, and enhancement of hydrogen-bonding interaction by decreasing Si/Al ratio in H-ZSM-5 reduced the amount of nonselective voids accordingly. Compared with chitosan control membrane (permeation flux 54.18 g/m2 h and separation factor 158.02 for 90 wt.% aqueous ethanol solution at 80 ◦ C), the H-ZSM-5(50)-CS-08 membrane (mass ratio of H-ZSM-5(50) to chitosan is 8 wt.%) exhibited the remarkably improved pervaporation performance with permeation flux 230.96 g/m2 h and separation factor 152.82 under the identical experimental condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Chitosan; H-ZSM-5; Filled membrane; Pervaporation; Ethanol dehydration
1. Introduction Recently, continuous alert for the serious shortage of fossil resource as well as the increased concern for negative impact of fossil fuel on the environment, particularly greenhouse gas emissions, has put great pressure on our society to find renewable fuel alternatives [1]. The most common renewable biofuel today is ethanol produced from sugar or grain, which can be blended with petrol or used as neat alcohol in dedicated engines taking advantage of the higher octane number and higher heat of vaporization [1]. In the process of ethanol fermentation production, ethanol dehydration has been proved to be a difficult task due to formation of ethanol/water azeotrope. The prevailing industrial technologies such as azeotropic distillation and extractive distillation suffer from high-energy consumption and need for an auxiliary agent. In comparison, pervaporation (PV), due to its distinct advantages including low-energy cost, environment-friendly and unrestraint of vapor–liquid equilibrium, is considered as a promising alternative [2]. The first
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industry scale pervaporation plant was put into operation in 1988 in France with a daily capacity of 15 m3 refined ethanol, and there have been more than 60 pervaporation units operating around the world so far [3]. Chitosan (CS) is the deacetylated form of chitin, which is the second most abundant biopolymer in nature [4]. Due to the rich hydrophilic groups such as hydroxyl and amino groups on chitosan chains, water can be preferentially adsorbed and diffused in chitosan membrane during the pervaporation process of ethanol dehydration [4]. Chitosan membrane was first used in ethanol dehydration by Masaru et al. [5]. Subsequently, cross-linking [6,7], blending [8] and polyelectrolyte complex linkage [9] strategies were employed to enhance the pervaporation performance and stability of chitosan membranes. In recent years, the organic–inorganic hybrid materials, which combined the superior separation performance of rigid adsorptive inorganic materials and ideal membrane-forming property of organic materials, attracted rapidly increased attention [10–12]. However, nonideal defects (often nonselective voids) usually occurred at the organic–inorganic interface, mainly due to poor interaction between organic–inorganic phases [13]. Consequently, the rational selection of inorganic materials is quite crucial to the development of the organic-inorganic hybrid
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materials with desirable interfacial morphology and separation performance. H-ZSM-5 is a kind of common employed hydrophilic zeolite. The most important acid-type center is the bridged framework formed by hydroxyl groups with Si–OH–Al configuration, which ensures the high acid strength of the H-ZSM-5 [14]. Due to the contribution of framework Al atoms to the hydroxyl groups of the Si–OH–Al configuration, the ratio of Si atoms to Al atoms will significantly influence the hydrophilicity of H-ZSM-5 [15]. In addition, H-ZSM-5 possesses the MFI-type structure with two kinds of channel structures. The straight channels are elliptical with an opening of 0.51 nm × 0.57 nm, and the sinusoidal channels are almost circular with a diameter of 0.54 nm [16]. The calculated molecular diameters of water and ethanol are 0.26 and 0.52 nm, respectively [17], which are appropriate for exploitation of the size-selective effects of H-ZSM-5 to increase selective diffusion of ethanol and water. Finally, it can be naturally expected that hydrogenbonding interaction will be established between H-ZSM-5 and chitosan. In this study, the H-ZSM-5 filled chitosan membranes were prepared, the structural morphology, dynamic mechanical property and thermal stability of the filled membranes were characterized in detail. The effects of H-ZSM-5 content, Si/Al ratio in H-ZSM-5, operation temperature and feed composition on sorption, diffusion and pervaporation performance of the filled membranes were investigated thoroughly. 2. Experimental 2.1. Materials Chitosan (the degree of deacetylation was 90.2%) was purchased from Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan, China). H-ZSM-5 (Si/Al ratio = 25, 38 and 50) was purchased from Tianjin Nankai Catalyst Ltd. (Tianjin, China) and grinded by ball mill for 6 h before using. Ethanol and acetic acid were from Tianjin Kewei Ltd. (Tianjin, China). All the chemicals were of analytical grade and were used without further purification. Double distilled water was used throughout the study. 2.2. Membrane preparation Chitosan was dissolved in 2 wt.% acetic acid solution and stirred at 80 ◦ C for 1 h to obtain 2 wt.% chitosan solution. Then a certain amount of H-ZSM-5 was added into the solution under stirring for 1 h. The suspension mixture was under ultrasonic treatment for 0.5 h [18] and then filtered to remove air bubbles. The treated suspension was cast onto a glass plate with a casting knife. The formed membranes were dried at room temperature for about 48 h. The average thickness of the final membranes was around 25 m. The filled membranes were designated as HZSM-5(X)-CS-Y, where X indicates the Si/Al ratio in H-ZSM-5 and Y indicates the mass ratio of H-ZSM-5 to chitosan varied as 2, 4, 6, 8, and 10 wt.%.
2.3. Membrane characterization FTIR spectra were recorded on a Nicolet-560, 5DX instrument equipped with both horizontal attenuated total reflectance (HATR) accessories. Thirty-two scans were accumulated with a resolution of 4 cm−1 for each spectrum. The microstructures of H-ZSM-5 filled chitosan membranes were examined by scanning electron microscope (SEM) using a Philips XL30ESEM instrument. Crystallinity of the membranes was studied using a Rigaku D/max 2500v/pc X-ray diffractometer in the range of 3–50◦ at the speed of 8◦ /min (Cu KR 40 kV/200 mA). Dynamic mechanical data were obtained with a Perkin-Elmer DMA instrument. All samples were tested at a heating rate of 5 ◦ C/min, and a frequency of 10 Hz was selected for all the experiments. Thermogravimetric analysis (TGA) was conducted with a Perkin-Elmer TG/DTA thermogravimetric analyzer at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. The surface hydrophilicity of membranes was evaluated by the measurement of methylene iodide contact angle using a goniometer (ERMA Contact Angle Meter, Japan). 2.4. Pervaporation experiments The pervaporation experiments were the same as that previously reported [19]. The downstream pressure was about 1 kPa and the feed flow rate was 60 L/h. After obtaining a steady-state (about 1 h after start-up), the permeate was collected in cold trap immersed in the liquid nitrogen. The concentrations of feed and permeate were determined by gas chromatography (Agilent 4890, USA) equipped with a 2.0 m long column packed with 10% SE30 and TCD detector with the column temperature 120 ◦ C. Permeate flux (J) and separation factor (α) were calculated according to the following equations: J=
W At
(1)
α=
PW /PE FW /FE
(2)
where W is the mass of permeate (g); A the effective area of the membrane in contact with the feed (m2 ); t the permeation time (h); PW and PE are the weight fractions of water and ethanol in permeate, respectively; FW and FE are the weight fractions of water and ethanol in feed, respectively. 2.5. Sorption experiments A piece of membrane sample (about 0.3 g) was dipped into 90 wt.% aqueous ethanol solution at 80 ◦ C for 12 h for equilibrium. Then, the membrane sample was taken out and wiped off the surface solution with tissue paper carefully, and weighed as quickly as possible. The adsorbed liquid was collected in a liquid nitrogen trap by desorbing the equilibrated sample in the purgeand-trap apparatus, and analyzed by gas chromatography. The degree of swelling (DS) and the sorptivity selectivity αS were
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calculated by: DS(%) = αS =
Ws − Wd × 100 Wd
MW /ME FW /FE
(3) (4)
where Ws and Wd are the weights of the swollen and dry membranes, respectively, MW and ME are the weight fractions of water and ethanol in the membrane, and FW , FE are the weight fractions of water and ethanol in the solution, respectively. According to the solution–diffusion model, diffusivity selectivity αD was calculated by: αD =
α αS
(5)
Considering the volatile characteristic of ethanol, all the sorption experiments were repeated at least three times, and the experiment errors were all within 5%. 3. Results and discussion 3.1. Membrane characterization 3.1.1. FTIR analysis The FTIR spectra of H-ZSM-5, H-ZSM-5-CS-08 membranes and chitosan control membrane are shown in Fig. 1. The strong peak at 3427 cm−1 in Fig. 1a clearly shows that there are abundant hydroxyl groups on the H-ZSM-5. With decrease of Si/Al ratio in H-ZSM-5, the peak strength of the hydroxyl groups on the H-ZSM-5 increases accordingly, resulting in the higher affinity between H-ZSM-5 and water molecules [20]. In Fig. 1b, the chitosan control membrane shows a broad band at 3100–3500 cm−1 which is attributed to the hydroxyl groups stretching vibrations. Coincidence with the results reported by Kittur et al. [18], this band of H-ZSM-5 filled chitosan membranes obviously become weaker and wider, which indicates the hydrogen-bonding interaction between the hydroxyl groups of H-ZSM-5 and chitosan. Besides, the band strength of methylene bending (1380 cm−1 ) [4] decreases due to the vibration limitation of hydroxyl groups on chitosan chains. The characteristic band at 1550 cm−1 is assigned as amino groups [18], which apparently decreases after the incorporation of H-ZSM5. The peak strengths mentioned above all become smaller with increase of Si/Al ratio in H-ZSM-5, suggesting the increase of hydrogen-bonding interaction. 3.1.2. SEM analysis In order to analyze the dispersed phase morphology of membranes, SEM analysis was introduced and the SEM graphs of H-ZSM-5, chitosan control membrane and H-ZSM-5 filled chitosan membranes are shown in Fig. 2. The average particle size of H-ZSM-5 is around 0.5 m (shown in Fig. 2a) and the homogeneous chitosan control membrane is obtained as shown in Fig. 2b. When H-ZSM-5 content is less than 8 wt.%, the HZSM-5 can uniformly distribute in the chitosan bulk (shown in Fig. 2c), and no obvious defects are found at the interface. But
Fig. 1. FTIR spectra of (a) H-ZSM-5 (b) chitosan control membrane and HZSM-5 filled chitosan membranes.
when the H-ZSM-5 content reaches 10 wt.%, part of H-ZSM5 particles will aggregate at the bottom of membrane (shown in Fig. 2d), which could be disadvantageous for pervaporation performance. 3.1.3. XRD analysis Distinct crystalline structure of H-ZSM-5 with Si/Al ratio 50 can be seen from XRD pattern in Fig. 3a. As we all know, chitosan usually exists in two crystal forms: form I has the major crystalline peaks at 11.2◦ and 18.0◦ , while form II has major crystalline peaks at 20.9◦ and 23.8◦ [21]. XRD pattern of the chitosan control membrane and the filled chitosan membranes are shown in Fig. 3b. After incorporation of H-ZSM-5, the membranes show the characteristic peaks of both chitosan and H-ZSM-5. The crystalline peak intensity of two-crystal forms in filled chitosan membrane decreases obviously compared with those in chitosan control membranes. Moreover, the variation tendency becomes more pronounced with the decrease of Si/Al ratio in H-ZSM-5, which is attributed to the hydrogen-bonding interaction between H-ZSM-5 and chitosan. Certainly, lowering the crystallinity in chitosan membrane is advantageous for the
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Fig. 2. SEM graphs of (a) H-ZSM-5(50); (b) chitosan control membrane (cross-section); (c) H-ZSM-5(50)-CS-08 membrane (cross-section); (d) H-ZSM-5(50)-CS-10 membrane (cross-section).
increase of chitosan chain mobility, for improvement of swelling property and enhancement of the permeation flux of membranes. 3.1.4. DMA analysis We used DMA to measure glass–rubber transition temperature (Tg ). In DMA, it is generally accepted that the variation of tanδ (= loss modulus E / storage modulus E ) corresponds to Tg . Fig. 4 shows the tanδ curves of chitosan control membrane and filled chitosan membranes. The chitosan control membrane shows a peak at 208 ◦ C, which may related to the ␣-relaxation of chitosan [22], and the peak below 100 ◦ C is attributed to absorbed moisture [23]. In comparison, the Tg of H-ZSM-5(50)CS-08 membrane shifts to 232 ◦ C due to the rigidification of polymer at the interface [13] which may result in the formation of voids. With increase of Si/Al ratio in H-ZSM-5, the Tg of membrane decreases to 229 ◦ C (H-ZSM-5(38)-CS-08 membrane) and 225 ◦ C (H-ZSM-5(25)-CS-08 membrane), indicating improvement of interface morphology. 3.1.5. TGA studies In order to determine the thermal stability, thermogravimetric analysis results of H-ZSM-5(50) and membranes are obtained as shown in Fig. 5. The TGA curve of H-ZSM-5(50) decreases rapidly when temperature increases from 35 to 160 ◦ C, due to the loss of bound water molecules on the H-ZSM-5. However, when temperature increases from 160 to 700 ◦ C, the curve decreases more slowly with a total weight loss of about 3 wt.%. From Fig. 4b, the TGA curve of the chitosan control membrane shows a three-stage decomposition. The initial 25 wt.% weight loss takes place between 40 and 245 ◦ C. The highest weight loss of
about 35 wt.% is observed in the second stage of decomposition, which starts just above 245 ◦ C and terminates at around 425 ◦ C. A complete weight loss can not be seen even after heating the material up to 700 ◦ C, with the weight loss of about 13 wt.% in the third decomposition stage starting from 425 ◦ C. The variation tendency of TGA curve of H-ZSM-5(50)-CS-08 membrane is almost the same as that of chitosan control membrane, except for higher residual weight than that of chitosan control membrane. It indicates that the thermal stability of the membranes is not substantially improved after incorporating H-ZSM-5. 3.2. Swelling and sorption properties of membranes Fig. 6 shows the effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the DS values of filled chitosan membranes. The five-pointed star presents the value of chitosan control membrane. It can be seen that, the filled chitosan membranes exhibit better swelling property compared with chitosan control membrane. And the DS values increase as H-ZSM-5 content increases, which indicates the ideal adsorptive characteristics of H-ZSM-5 and less ordered crystalline structure of the filled membranes. Besides, DS values of filled chitosan membranes increase with decrease of Si/Al ratio in H-ZSM-5, which are in good agreement with the XRD results. The results of sorptivity and diffusivity selectivity of filled chitosan membranes are shown in Fig. 7. The sorptivity selectivity increases with increase of H-ZSM-5 content, but when H-ZSM-5 content reaches 10 wt.%, the sorptivity selectivity slightly decreases, which may be attributed to the looser packing structure and larger DS value with H-ZSM-5 content increas-
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Fig. 3. XRD patterns of (a) H-ZSM-5(50) and (b) chitosan control membrane and H-ZSM-5 filled chitosan membranes.
Fig. 4. The tanδ curves of chitosan control membrane and H-ZSM-5 filled chitosan membranes.
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Fig. 5. TGA curves of (a) H-ZSM-5(50) and (b) chitosan control membrane and H-ZSM-5(50)-CS-08 membrane.
Fig. 6. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the DS values of H-ZSM-5 filled chitosan membranes.
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Fig. 8. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the contact angle of H-ZSM-5 filled chitosan membranes.
that the decrease of nanometer-scale voids is aroused from the enhancement of interfacial interaction. 3.3. Pervaporation performance of membranes
Fig. 7. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the sorptivity selectivity and diffusivity selectivity of H-ZSM-5 filled chitosan membranes.
ing. The continuous increase of sorptivity selectivity with decrease of Si/Al ratio in H-ZSM-5 demonstrates that increase of Al content in H-ZSM-5 is advantageous for enhancement of Si–OH–Al framework amount and increase the adsorption capacity of H-ZSM-5 towards water molecules. The variation of static contact angle (as shown in Fig. 8) offers the further proof. On the other hand, the diffusivity selectivity of filled chitosan membranes is smaller than that of chitosan control membrane. Koros and co-workers [24,25] attributed this phenomenon to the formation of nanometer-scale voids which could not be seen from the SEM graph. The similar results can be found from the work of Duval et al. [26]. Besides, the particle size scale of HZSM-5 may be another reason for formation of nanometer-scale voids [27]. However, with increase of H-ZSM-5 content the diffusivity selectivity increase, which shows that the size-selective effects of H-ZSM-5 become more prominent with larger HZSM-5 content. Due to the serious aggregation of H-ZSM-5, the diffusivity selectivity decreases when H-ZSM-5 content reaches 10 wt.%. It can be found that, with increase of Si/Al ratio in HZSM-5, the diffusivity selectivity increases apparently within the whole H-ZSM-5 content. From these results, we can derive
Fig. 9 shows the effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the pervaporation performance of H-ZSM-5 filled chitosan membranes. From the pervaporation results of H-ZSM-5(50)-CS membranes, it can be seen that, separation factor decreases sharply after filling compared with that of chitosan control membrane. When H-ZSM-5 content increases from 2 to 8 wt.%, due to increase of both sorptivity and diffusivity selectivity, separation factor of the filled membranes increases continuously till 152.82 which is comparable with that of chitosan control membrane (158.02). When H-ZSM-5 content reaches 10 wt.%, separation factor decreases attributed to the decrease of diffusivity selectivity. On the other hand, permeation flux increases dramatically when H-ZSM-5 content increases from 0 to 8 wt.%, which attributes to enhancement of swelling property and formation of nanometer-scale voids, but permeation flux decreases due to the increased diffusion resistance when H-ZSM-5 content reaches 10 wt.%. Furthermore, with decrease of Si/Al ratio in H-ZSM-5, the separation factor of membrane with the same H-ZSM-5 content increases accordingly, and the permeation flux decreases, which is the result of interface morphology’s improvement. 3.4. Effects of operation temperature on pervaporation performance of H-ZSM-5 filled chitosan membranes Fig. 10 shows effects of operation temperature on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane. It can be observed that both the permeation flux and separation factor of H-ZSM-5(50)-CS-08 membrane increase with operation temperature increasing, which can also be seen from [28]. With the increase of operation temperature, the vapour pressures of water and ethanol both increase and the transport driving force through the membranes increases accordingly, leading to
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Fig. 11. Effects of water weight fraction in feed on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane (operation temperature: 80 ◦ C).
3.5. Effects of feed composition on pervaporation performance of H-ZSM-5 filled chitosan membranes
Fig. 9. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the pervaporation performance of H-ZSM-5 filled chitosan membranes (operation temperature: 80 ◦ C, water weight fraction in feed: 10 wt.%).
the increase of permeation flux. The separation factor increases with operation temperature because the rate of water permeation increases faster than the ethanol rate as the operation temperature increases.
Fig. 11 shows effects of water weight fraction in feed on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane at 80 ◦ C. The permeation flux of H-ZSM-5(50)-CS-08 membrane increases from 124.90 to 256.61 g/m2 h when water weight fraction in feed increasing from 4 to 15 wt.%. While the separation factor increases from 48.39 to 152.82 when water weight fraction in feed increases from 4 to 10 wt.%, but the separation factor decreases to 83.87 when water weight fraction increases to 15 wt.%. With water weight fraction in feed increasing, more water molecules can adsorb into membrane and diffuse through membrane. At the same time, the swelling of membrane also increases and the voids’ size in membrane enlarges, making water diffuse through membrane easier. With water weight fraction in feed further increasing, the voids’ size in membrane becomes larger and larger, more and more ethanol molecules win the chance to diffuse through these voids, which resulted in the decrease of separation factor. 4. Conclusions
Fig. 10. Effects of operation temperature on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane (water weight fraction in feed: 10 wt.%).
Incorporation of H-ZSM-5 into chitosan membrane could significantly increase the permeation flux, but the separation factor did not increase in most H-ZSM-5 content due to formation of nonselective voids at the interface. By increase of Si/Al ratio in H-ZSM-5, the hydrogen-bonding interaction between organicinorganic phases increased and the interface morphology of the filled membranes improved, leading to the increase of separation factor in whole H-ZSM-5 content. In addition, H-ZSM-5 exhibited desirable size-selective and affinity effects for aqueous ethanol solution. Compared with chitosan control membrane (permeation flux 54.18 g/m2 h and separation factor 158.02 for 90 wt.% aqueous ethanol solution at 80 ◦ C), the H-ZSM-5(50)CS-08 membrane (mass ratio of H-ZSM-5(50) to chitosan is 8 wt.%) exhibited the remarkably improved pervaporation performance with permeation flux 230.96 g/m2 h and separation factor 152.82 under the identical experimental condition.
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Acknowledgements The author gratefully acknowledges the financial support from Tianjin Science & Technology Research Program (06YFGPSH03400), the Programme of Introducing Talents of Discipline to Universities (no: B06006) and Innovative Research Team in University (PCSIRT).
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Pervaporation dehydration of aqueous ethanol solution using H-ZSM-5 filled chitosan membranes Honglei Sun, Lianyu Lu, Xue Chen, Zhongyi Jiang ∗ Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Received 11 May 2007; received in revised form 9 September 2007; accepted 10 September 2007
Abstract H-ZSM-5 filled chitosan (CS) membranes were prepared by incorporating H-ZSM-5 into chitosan for pervaporation dehydration of aqueous ethanol solution. Characterization results revealed that hydroxyl groups on H-ZSM-5 could form hydrogen-bonding interaction with the hydroxyl and amino groups of chitosan, which lowered the crystallinity of membranes and consequently improved the interface morphology. The sorption experiments showed increased degree of swelling (DS) and sorptivity selectivity with increase of H-ZSM-5 content and decrease of Si/Al ratio in H-ZSM-5, respectively. Decreased separation factor of the filled membranes in pervaporation process was mainly attributed to the formation of nanometer-scale voids, and enhancement of hydrogen-bonding interaction by decreasing Si/Al ratio in H-ZSM-5 reduced the amount of nonselective voids accordingly. Compared with chitosan control membrane (permeation flux 54.18 g/m2 h and separation factor 158.02 for 90 wt.% aqueous ethanol solution at 80 ◦ C), the H-ZSM-5(50)-CS-08 membrane (mass ratio of H-ZSM-5(50) to chitosan is 8 wt.%) exhibited the remarkably improved pervaporation performance with permeation flux 230.96 g/m2 h and separation factor 152.82 under the identical experimental condition. © 2007 Elsevier B.V. All rights reserved. Keywords: Chitosan; H-ZSM-5; Filled membrane; Pervaporation; Ethanol dehydration
1. Introduction Recently, continuous alert for the serious shortage of fossil resource as well as the increased concern for negative impact of fossil fuel on the environment, particularly greenhouse gas emissions, has put great pressure on our society to find renewable fuel alternatives [1]. The most common renewable biofuel today is ethanol produced from sugar or grain, which can be blended with petrol or used as neat alcohol in dedicated engines taking advantage of the higher octane number and higher heat of vaporization [1]. In the process of ethanol fermentation production, ethanol dehydration has been proved to be a difficult task due to formation of ethanol/water azeotrope. The prevailing industrial technologies such as azeotropic distillation and extractive distillation suffer from high-energy consumption and need for an auxiliary agent. In comparison, pervaporation (PV), due to its distinct advantages including low-energy cost, environment-friendly and unrestraint of vapor–liquid equilibrium, is considered as a promising alternative [2]. The first
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industry scale pervaporation plant was put into operation in 1988 in France with a daily capacity of 15 m3 refined ethanol, and there have been more than 60 pervaporation units operating around the world so far [3]. Chitosan (CS) is the deacetylated form of chitin, which is the second most abundant biopolymer in nature [4]. Due to the rich hydrophilic groups such as hydroxyl and amino groups on chitosan chains, water can be preferentially adsorbed and diffused in chitosan membrane during the pervaporation process of ethanol dehydration [4]. Chitosan membrane was first used in ethanol dehydration by Masaru et al. [5]. Subsequently, cross-linking [6,7], blending [8] and polyelectrolyte complex linkage [9] strategies were employed to enhance the pervaporation performance and stability of chitosan membranes. In recent years, the organic–inorganic hybrid materials, which combined the superior separation performance of rigid adsorptive inorganic materials and ideal membrane-forming property of organic materials, attracted rapidly increased attention [10–12]. However, nonideal defects (often nonselective voids) usually occurred at the organic–inorganic interface, mainly due to poor interaction between organic–inorganic phases [13]. Consequently, the rational selection of inorganic materials is quite crucial to the development of the organic-inorganic hybrid
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materials with desirable interfacial morphology and separation performance. H-ZSM-5 is a kind of common employed hydrophilic zeolite. The most important acid-type center is the bridged framework formed by hydroxyl groups with Si–OH–Al configuration, which ensures the high acid strength of the H-ZSM-5 [14]. Due to the contribution of framework Al atoms to the hydroxyl groups of the Si–OH–Al configuration, the ratio of Si atoms to Al atoms will significantly influence the hydrophilicity of H-ZSM-5 [15]. In addition, H-ZSM-5 possesses the MFI-type structure with two kinds of channel structures. The straight channels are elliptical with an opening of 0.51 nm × 0.57 nm, and the sinusoidal channels are almost circular with a diameter of 0.54 nm [16]. The calculated molecular diameters of water and ethanol are 0.26 and 0.52 nm, respectively [17], which are appropriate for exploitation of the size-selective effects of H-ZSM-5 to increase selective diffusion of ethanol and water. Finally, it can be naturally expected that hydrogenbonding interaction will be established between H-ZSM-5 and chitosan. In this study, the H-ZSM-5 filled chitosan membranes were prepared, the structural morphology, dynamic mechanical property and thermal stability of the filled membranes were characterized in detail. The effects of H-ZSM-5 content, Si/Al ratio in H-ZSM-5, operation temperature and feed composition on sorption, diffusion and pervaporation performance of the filled membranes were investigated thoroughly. 2. Experimental 2.1. Materials Chitosan (the degree of deacetylation was 90.2%) was purchased from Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan, China). H-ZSM-5 (Si/Al ratio = 25, 38 and 50) was purchased from Tianjin Nankai Catalyst Ltd. (Tianjin, China) and grinded by ball mill for 6 h before using. Ethanol and acetic acid were from Tianjin Kewei Ltd. (Tianjin, China). All the chemicals were of analytical grade and were used without further purification. Double distilled water was used throughout the study. 2.2. Membrane preparation Chitosan was dissolved in 2 wt.% acetic acid solution and stirred at 80 ◦ C for 1 h to obtain 2 wt.% chitosan solution. Then a certain amount of H-ZSM-5 was added into the solution under stirring for 1 h. The suspension mixture was under ultrasonic treatment for 0.5 h [18] and then filtered to remove air bubbles. The treated suspension was cast onto a glass plate with a casting knife. The formed membranes were dried at room temperature for about 48 h. The average thickness of the final membranes was around 25 m. The filled membranes were designated as HZSM-5(X)-CS-Y, where X indicates the Si/Al ratio in H-ZSM-5 and Y indicates the mass ratio of H-ZSM-5 to chitosan varied as 2, 4, 6, 8, and 10 wt.%.
2.3. Membrane characterization FTIR spectra were recorded on a Nicolet-560, 5DX instrument equipped with both horizontal attenuated total reflectance (HATR) accessories. Thirty-two scans were accumulated with a resolution of 4 cm−1 for each spectrum. The microstructures of H-ZSM-5 filled chitosan membranes were examined by scanning electron microscope (SEM) using a Philips XL30ESEM instrument. Crystallinity of the membranes was studied using a Rigaku D/max 2500v/pc X-ray diffractometer in the range of 3–50◦ at the speed of 8◦ /min (Cu KR 40 kV/200 mA). Dynamic mechanical data were obtained with a Perkin-Elmer DMA instrument. All samples were tested at a heating rate of 5 ◦ C/min, and a frequency of 10 Hz was selected for all the experiments. Thermogravimetric analysis (TGA) was conducted with a Perkin-Elmer TG/DTA thermogravimetric analyzer at a heating rate of 10 ◦ C/min under a nitrogen atmosphere. The surface hydrophilicity of membranes was evaluated by the measurement of methylene iodide contact angle using a goniometer (ERMA Contact Angle Meter, Japan). 2.4. Pervaporation experiments The pervaporation experiments were the same as that previously reported [19]. The downstream pressure was about 1 kPa and the feed flow rate was 60 L/h. After obtaining a steady-state (about 1 h after start-up), the permeate was collected in cold trap immersed in the liquid nitrogen. The concentrations of feed and permeate were determined by gas chromatography (Agilent 4890, USA) equipped with a 2.0 m long column packed with 10% SE30 and TCD detector with the column temperature 120 ◦ C. Permeate flux (J) and separation factor (α) were calculated according to the following equations: J=
W At
(1)
α=
PW /PE FW /FE
(2)
where W is the mass of permeate (g); A the effective area of the membrane in contact with the feed (m2 ); t the permeation time (h); PW and PE are the weight fractions of water and ethanol in permeate, respectively; FW and FE are the weight fractions of water and ethanol in feed, respectively. 2.5. Sorption experiments A piece of membrane sample (about 0.3 g) was dipped into 90 wt.% aqueous ethanol solution at 80 ◦ C for 12 h for equilibrium. Then, the membrane sample was taken out and wiped off the surface solution with tissue paper carefully, and weighed as quickly as possible. The adsorbed liquid was collected in a liquid nitrogen trap by desorbing the equilibrated sample in the purgeand-trap apparatus, and analyzed by gas chromatography. The degree of swelling (DS) and the sorptivity selectivity αS were
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calculated by: DS(%) = αS =
Ws − Wd × 100 Wd
MW /ME FW /FE
(3) (4)
where Ws and Wd are the weights of the swollen and dry membranes, respectively, MW and ME are the weight fractions of water and ethanol in the membrane, and FW , FE are the weight fractions of water and ethanol in the solution, respectively. According to the solution–diffusion model, diffusivity selectivity αD was calculated by: αD =
α αS
(5)
Considering the volatile characteristic of ethanol, all the sorption experiments were repeated at least three times, and the experiment errors were all within 5%. 3. Results and discussion 3.1. Membrane characterization 3.1.1. FTIR analysis The FTIR spectra of H-ZSM-5, H-ZSM-5-CS-08 membranes and chitosan control membrane are shown in Fig. 1. The strong peak at 3427 cm−1 in Fig. 1a clearly shows that there are abundant hydroxyl groups on the H-ZSM-5. With decrease of Si/Al ratio in H-ZSM-5, the peak strength of the hydroxyl groups on the H-ZSM-5 increases accordingly, resulting in the higher affinity between H-ZSM-5 and water molecules [20]. In Fig. 1b, the chitosan control membrane shows a broad band at 3100–3500 cm−1 which is attributed to the hydroxyl groups stretching vibrations. Coincidence with the results reported by Kittur et al. [18], this band of H-ZSM-5 filled chitosan membranes obviously become weaker and wider, which indicates the hydrogen-bonding interaction between the hydroxyl groups of H-ZSM-5 and chitosan. Besides, the band strength of methylene bending (1380 cm−1 ) [4] decreases due to the vibration limitation of hydroxyl groups on chitosan chains. The characteristic band at 1550 cm−1 is assigned as amino groups [18], which apparently decreases after the incorporation of H-ZSM5. The peak strengths mentioned above all become smaller with increase of Si/Al ratio in H-ZSM-5, suggesting the increase of hydrogen-bonding interaction. 3.1.2. SEM analysis In order to analyze the dispersed phase morphology of membranes, SEM analysis was introduced and the SEM graphs of H-ZSM-5, chitosan control membrane and H-ZSM-5 filled chitosan membranes are shown in Fig. 2. The average particle size of H-ZSM-5 is around 0.5 m (shown in Fig. 2a) and the homogeneous chitosan control membrane is obtained as shown in Fig. 2b. When H-ZSM-5 content is less than 8 wt.%, the HZSM-5 can uniformly distribute in the chitosan bulk (shown in Fig. 2c), and no obvious defects are found at the interface. But
Fig. 1. FTIR spectra of (a) H-ZSM-5 (b) chitosan control membrane and HZSM-5 filled chitosan membranes.
when the H-ZSM-5 content reaches 10 wt.%, part of H-ZSM5 particles will aggregate at the bottom of membrane (shown in Fig. 2d), which could be disadvantageous for pervaporation performance. 3.1.3. XRD analysis Distinct crystalline structure of H-ZSM-5 with Si/Al ratio 50 can be seen from XRD pattern in Fig. 3a. As we all know, chitosan usually exists in two crystal forms: form I has the major crystalline peaks at 11.2◦ and 18.0◦ , while form II has major crystalline peaks at 20.9◦ and 23.8◦ [21]. XRD pattern of the chitosan control membrane and the filled chitosan membranes are shown in Fig. 3b. After incorporation of H-ZSM-5, the membranes show the characteristic peaks of both chitosan and H-ZSM-5. The crystalline peak intensity of two-crystal forms in filled chitosan membrane decreases obviously compared with those in chitosan control membranes. Moreover, the variation tendency becomes more pronounced with the decrease of Si/Al ratio in H-ZSM-5, which is attributed to the hydrogen-bonding interaction between H-ZSM-5 and chitosan. Certainly, lowering the crystallinity in chitosan membrane is advantageous for the
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Fig. 2. SEM graphs of (a) H-ZSM-5(50); (b) chitosan control membrane (cross-section); (c) H-ZSM-5(50)-CS-08 membrane (cross-section); (d) H-ZSM-5(50)-CS-10 membrane (cross-section).
increase of chitosan chain mobility, for improvement of swelling property and enhancement of the permeation flux of membranes. 3.1.4. DMA analysis We used DMA to measure glass–rubber transition temperature (Tg ). In DMA, it is generally accepted that the variation of tanδ (= loss modulus E / storage modulus E ) corresponds to Tg . Fig. 4 shows the tanδ curves of chitosan control membrane and filled chitosan membranes. The chitosan control membrane shows a peak at 208 ◦ C, which may related to the ␣-relaxation of chitosan [22], and the peak below 100 ◦ C is attributed to absorbed moisture [23]. In comparison, the Tg of H-ZSM-5(50)CS-08 membrane shifts to 232 ◦ C due to the rigidification of polymer at the interface [13] which may result in the formation of voids. With increase of Si/Al ratio in H-ZSM-5, the Tg of membrane decreases to 229 ◦ C (H-ZSM-5(38)-CS-08 membrane) and 225 ◦ C (H-ZSM-5(25)-CS-08 membrane), indicating improvement of interface morphology. 3.1.5. TGA studies In order to determine the thermal stability, thermogravimetric analysis results of H-ZSM-5(50) and membranes are obtained as shown in Fig. 5. The TGA curve of H-ZSM-5(50) decreases rapidly when temperature increases from 35 to 160 ◦ C, due to the loss of bound water molecules on the H-ZSM-5. However, when temperature increases from 160 to 700 ◦ C, the curve decreases more slowly with a total weight loss of about 3 wt.%. From Fig. 4b, the TGA curve of the chitosan control membrane shows a three-stage decomposition. The initial 25 wt.% weight loss takes place between 40 and 245 ◦ C. The highest weight loss of
about 35 wt.% is observed in the second stage of decomposition, which starts just above 245 ◦ C and terminates at around 425 ◦ C. A complete weight loss can not be seen even after heating the material up to 700 ◦ C, with the weight loss of about 13 wt.% in the third decomposition stage starting from 425 ◦ C. The variation tendency of TGA curve of H-ZSM-5(50)-CS-08 membrane is almost the same as that of chitosan control membrane, except for higher residual weight than that of chitosan control membrane. It indicates that the thermal stability of the membranes is not substantially improved after incorporating H-ZSM-5. 3.2. Swelling and sorption properties of membranes Fig. 6 shows the effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the DS values of filled chitosan membranes. The five-pointed star presents the value of chitosan control membrane. It can be seen that, the filled chitosan membranes exhibit better swelling property compared with chitosan control membrane. And the DS values increase as H-ZSM-5 content increases, which indicates the ideal adsorptive characteristics of H-ZSM-5 and less ordered crystalline structure of the filled membranes. Besides, DS values of filled chitosan membranes increase with decrease of Si/Al ratio in H-ZSM-5, which are in good agreement with the XRD results. The results of sorptivity and diffusivity selectivity of filled chitosan membranes are shown in Fig. 7. The sorptivity selectivity increases with increase of H-ZSM-5 content, but when H-ZSM-5 content reaches 10 wt.%, the sorptivity selectivity slightly decreases, which may be attributed to the looser packing structure and larger DS value with H-ZSM-5 content increas-
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Fig. 3. XRD patterns of (a) H-ZSM-5(50) and (b) chitosan control membrane and H-ZSM-5 filled chitosan membranes.
Fig. 4. The tanδ curves of chitosan control membrane and H-ZSM-5 filled chitosan membranes.
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Fig. 5. TGA curves of (a) H-ZSM-5(50) and (b) chitosan control membrane and H-ZSM-5(50)-CS-08 membrane.
Fig. 6. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the DS values of H-ZSM-5 filled chitosan membranes.
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Fig. 8. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the contact angle of H-ZSM-5 filled chitosan membranes.
that the decrease of nanometer-scale voids is aroused from the enhancement of interfacial interaction. 3.3. Pervaporation performance of membranes
Fig. 7. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the sorptivity selectivity and diffusivity selectivity of H-ZSM-5 filled chitosan membranes.
ing. The continuous increase of sorptivity selectivity with decrease of Si/Al ratio in H-ZSM-5 demonstrates that increase of Al content in H-ZSM-5 is advantageous for enhancement of Si–OH–Al framework amount and increase the adsorption capacity of H-ZSM-5 towards water molecules. The variation of static contact angle (as shown in Fig. 8) offers the further proof. On the other hand, the diffusivity selectivity of filled chitosan membranes is smaller than that of chitosan control membrane. Koros and co-workers [24,25] attributed this phenomenon to the formation of nanometer-scale voids which could not be seen from the SEM graph. The similar results can be found from the work of Duval et al. [26]. Besides, the particle size scale of HZSM-5 may be another reason for formation of nanometer-scale voids [27]. However, with increase of H-ZSM-5 content the diffusivity selectivity increase, which shows that the size-selective effects of H-ZSM-5 become more prominent with larger HZSM-5 content. Due to the serious aggregation of H-ZSM-5, the diffusivity selectivity decreases when H-ZSM-5 content reaches 10 wt.%. It can be found that, with increase of Si/Al ratio in HZSM-5, the diffusivity selectivity increases apparently within the whole H-ZSM-5 content. From these results, we can derive
Fig. 9 shows the effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the pervaporation performance of H-ZSM-5 filled chitosan membranes. From the pervaporation results of H-ZSM-5(50)-CS membranes, it can be seen that, separation factor decreases sharply after filling compared with that of chitosan control membrane. When H-ZSM-5 content increases from 2 to 8 wt.%, due to increase of both sorptivity and diffusivity selectivity, separation factor of the filled membranes increases continuously till 152.82 which is comparable with that of chitosan control membrane (158.02). When H-ZSM-5 content reaches 10 wt.%, separation factor decreases attributed to the decrease of diffusivity selectivity. On the other hand, permeation flux increases dramatically when H-ZSM-5 content increases from 0 to 8 wt.%, which attributes to enhancement of swelling property and formation of nanometer-scale voids, but permeation flux decreases due to the increased diffusion resistance when H-ZSM-5 content reaches 10 wt.%. Furthermore, with decrease of Si/Al ratio in H-ZSM-5, the separation factor of membrane with the same H-ZSM-5 content increases accordingly, and the permeation flux decreases, which is the result of interface morphology’s improvement. 3.4. Effects of operation temperature on pervaporation performance of H-ZSM-5 filled chitosan membranes Fig. 10 shows effects of operation temperature on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane. It can be observed that both the permeation flux and separation factor of H-ZSM-5(50)-CS-08 membrane increase with operation temperature increasing, which can also be seen from [28]. With the increase of operation temperature, the vapour pressures of water and ethanol both increase and the transport driving force through the membranes increases accordingly, leading to
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Fig. 11. Effects of water weight fraction in feed on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane (operation temperature: 80 ◦ C).
3.5. Effects of feed composition on pervaporation performance of H-ZSM-5 filled chitosan membranes
Fig. 9. Effects of H-ZSM-5 content and Si/Al ratio in H-ZSM-5 on the pervaporation performance of H-ZSM-5 filled chitosan membranes (operation temperature: 80 ◦ C, water weight fraction in feed: 10 wt.%).
the increase of permeation flux. The separation factor increases with operation temperature because the rate of water permeation increases faster than the ethanol rate as the operation temperature increases.
Fig. 11 shows effects of water weight fraction in feed on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane at 80 ◦ C. The permeation flux of H-ZSM-5(50)-CS-08 membrane increases from 124.90 to 256.61 g/m2 h when water weight fraction in feed increasing from 4 to 15 wt.%. While the separation factor increases from 48.39 to 152.82 when water weight fraction in feed increases from 4 to 10 wt.%, but the separation factor decreases to 83.87 when water weight fraction increases to 15 wt.%. With water weight fraction in feed increasing, more water molecules can adsorb into membrane and diffuse through membrane. At the same time, the swelling of membrane also increases and the voids’ size in membrane enlarges, making water diffuse through membrane easier. With water weight fraction in feed further increasing, the voids’ size in membrane becomes larger and larger, more and more ethanol molecules win the chance to diffuse through these voids, which resulted in the decrease of separation factor. 4. Conclusions
Fig. 10. Effects of operation temperature on the pervaporation performance of H-ZSM-5(50)-CS-08 membrane (water weight fraction in feed: 10 wt.%).
Incorporation of H-ZSM-5 into chitosan membrane could significantly increase the permeation flux, but the separation factor did not increase in most H-ZSM-5 content due to formation of nonselective voids at the interface. By increase of Si/Al ratio in H-ZSM-5, the hydrogen-bonding interaction between organicinorganic phases increased and the interface morphology of the filled membranes improved, leading to the increase of separation factor in whole H-ZSM-5 content. In addition, H-ZSM-5 exhibited desirable size-selective and affinity effects for aqueous ethanol solution. Compared with chitosan control membrane (permeation flux 54.18 g/m2 h and separation factor 158.02 for 90 wt.% aqueous ethanol solution at 80 ◦ C), the H-ZSM-5(50)CS-08 membrane (mass ratio of H-ZSM-5(50) to chitosan is 8 wt.%) exhibited the remarkably improved pervaporation performance with permeation flux 230.96 g/m2 h and separation factor 152.82 under the identical experimental condition.
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Acknowledgements The author gratefully acknowledges the financial support from Tianjin Science & Technology Research Program (06YFGPSH03400), the Programme of Introducing Talents of Discipline to Universities (no: B06006) and Innovative Research Team in University (PCSIRT).
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