Preparation and Pervaporation Property of Chitosan Membrane with

Oct 12, 2010 - chitosan membrane for separation of ethanol/water mixtures by pervaporation. The pristine MWNTs were treated by mixed acid and then ...
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Ind. Eng. Chem. Res. 2010, 49, 11667–11675

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Preparation and Pervaporation Property of Chitosan Membrane with Functionalized Multiwalled Carbon Nanotubes Shi Qiu,† Liguang Wu,‡ Guozhong Shi,† Lin Zhang,*,† Huanlin Chen,† and Congjie Gao†,§ Department of Chemical and Biological Engineering, Zhejiang UniVersity, Hangzhou 310027, China, College of EnVironmental Science and Engineering, Zhejiang Gongshang UniVersity, Hangzhou, 310035, China, and National Engineering Research Center for Liquid Separation Membrane, Hangzhou, 310012, China

This study was designed to prepare functionalized multiwalled carbon nanotubes (MWNTs-COOH) incorporated chitosan membrane for separation of ethanol/water mixtures by pervaporation. The pristine MWNTs were treated by mixed acid and then functionalized by diisobutyryl peroxide. The structure and property of the functionalized MWNTs were characterized by Fourier transform infrared and Raman spectroscopies and transmission electron microscopy. A series of functionalized MWNTs incorporated chitosan membranes were prepared by solution blending method. The swelling degree of the resulting membranes in ethanol/water mixtures was 6 times that of the pristine chitosan membrane. The permeation flux of the membranes increased significantly with increasing functionalized MWNTs content in blend membrane matrix in pervaporation. On the basis of the experiments of sorption equilibrium, the solubility and the diffusion coefficient of membranes in water, ethanol, and 90% ethanol/water mixtures were obtained. Compared with the calculated diffusion coefficient (D90), the measured diffusion coefficient (D90T) in 90% ethanol/water mixtures was higher, taking M(2) for an example, D90 was 0.193 × 10-6 m2/s, and D90T was 0.41 × 10-6 m2/s, which indicated the functionalized MWNTs were more prone to increased water permeation when ethanol and water penetrated into the membrane simultaneously. In addition, effects of MWNTs content in the membrane matrix and operating temperature on pervaporation performances were investigated. After introducing functionalized MWNTs, the Arrhenius activation parameters for the total permeation decreased from 28.15 to 12.91 kJ/mol, which indicated that the carbon nanotubes filled membranes were easier to penetrate and exhibited higher flux performance than a pristine membrane. 1. Introduction Carbon nanomaterials such as fullerenes, single-walled and multiwalled carbon nanotubes, carbon nanofibers, carbon nanoparticles, and nanodiamonds are currently attracting a lot of attention due to expected outstanding physical, electrical, and thermal properties.1,2 One promising application of those materials is as reinforcing additives for various matrices. Especially, multiwalled carbon nanotubes (MWNTs) are of particular interest because of their relatively low cost and availability in larger quantities as the result of their more advanced stage in commercial production. Recently, considerable research has been focused on the incorporation of inorganic carbon nanotubes into polymers to prepare membranes.3-14 These so-called mixed matrix membranes combine useful properties of inorganic materials with the desirable mechanical and processing properties of polymers.3,4 In addition, several studies have proved that the incorporation of carbon nanotubes or porous fillers into dense membranes could improve the separation performance of the membrane due to the combined effects of molecular sieving action, selective adsorption, and difference in diffusion rates.15-17 Kim et al.18 mixed carbon nanotubes which were functionalized with long-chain alkyl amines with a polysulfone matrix to form a novel composite membrane. Both permeability and diffusivity of the membranes increased with increasing weight fraction of carbon nanotubes in the membrane. Our previous work19 on the polysulfone * To whom correspondence should be addressed. E-mail: linzhang@ zju.edu.cn. Tel.: +86-571-87952121. † Zhejiang University. ‡ Zhejiang Gongshang University. § National Engineering Research Center for Liquid Separation Membrane.

membranes filled with functionalizing MWNTs with 5-isocyanatoisophthaloyl chloride, showed that the membrane fouling by protein could be effectively reduced by incorporating MWNTs to the polymer matrix. Peng et al.20 had prepared nanocomposite membranes by incorporating a chitosan-wrapped multiwalled carbon nanotube into poly(vinyl alcohol) (PVA), and the separation factor of the membranes was five times higher compared to pure PVA membrane, while the flux were also remarkably improved. Similarly, Peng et al.21 had prepared another hybrid membrane by incorporating carbon nanotubes which was dispersed by using β-cyclodextrin and had reached the same effects. Among the polymeric materials for a membrane to perform dehydration of organic solvents such as alcohols, chitosan plays an important role for its high hydrophilic, good formation properties, and easily modified functional groups as well as its good mechanical and chemical stability. Feng et al.22,23 revealed that chitosan and cross-linked chitosan membranes were capable of separating methanol/water mixture by pervaporation. Depending on both carbon nanotubes and chitosan sharing some important properties and potential values for dehydration, respectively, the opportunity to combine carbon nanotubes and chitosan appears to be a desirable way to develop nanocomposites with inherent properties of both components for dehydration of ethanol. However, MWNTs generally stay as bundles or an individual tube with some degree of entanglement, so the effective incorporation of carbon nanotubes in mixed matrix membranes is hindered by the agglomeration of carbon nanotubes and poor interfacial compatibility of the carbon nanotubes with the polymer, leading to unselective voids in the membrane.24-26 To enhance the compatibility between the inorganic and

10.1021/ie101223k  2010 American Chemical Society Published on Web 10/12/2010

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Figure 1. Chemical equations for preparing carboxylic MWNTs.

polymeric components, it is necessary to modify MWNTs for providing chemical affinity to polymer matrices without blocking or affecting the pore structure of carbon nanotubes. Until now, there already have been much research focused on MWNTs modification. By a nucleophilic substitution reaction, Gang27 succeeded in covalent functionalization of shortened MWNTs with a natural low molecular weight chitosan. Liu and Chen28 functionalized carbon nanotube through the addition reaction between the initiators of atom-transfer radical polymerization (ATRP) and carbon nanotubes; subsequently the ATRP initiating groups would transfer to the CNT surface with this reaction. In our work, MWNTs were pretreated by mixed acid and then functionalized by diisobutyryl peroxide through radical polymerization to enhance the compatibility between MWNTs and the chitosan matrix. A composite membrane was prepared by incorporating modified MWNTs into the chitosan matrix, and the effect of MWNTs on membrane properties was investigated, from which it was confirmed that functionalized MWNTs are an effective inorganic component for improving the pervaporation properties of membranes for dehydration of ethanol. 2. Experiments 2.1. Materials. The pristine multiwalled carbon nanotubes, of which the main range of diameter is 20-40 nm (inner diameter is about 5 nm) and the purity is 95%, were manufactured by Nanotech Port Co. Ltd., Shenzhen, China. Analytical grade sulfuric acid and nitric acid were purchased from Sanying Chemical Reagent Co., Shanghai, China. Analytical grade ethanol and acetic acid were purchased from Reagent Co., Shanghai, China. Chitosan was obtained from Sigma-Aldrich Chemical Co., St. Louis, MO, USA; double deionized water was used to make the aqueous solutions for membrane preparation and treatment. 2.2. Preparation and Characterization of Functionalized MWNTs. Pristine MWNTs were first treated with a mixed acid solution (H2SO4/HNO3 ) 3/1) at 80 °C for 6 h and then diluted with deionized water. Minor nanotube residue and debris were disposed via reduced pressure distillation, while the bulk MWNTs were rinsed with deionized water repeatedly until there was no more trace of acid. MWNTs were then dried in a vacuum drying oven at 70 °C before further treatment. The oxidated MWNTs were dispersed in DMF and then reacted with diisobutyryl peroxide (synthesized by butanedioic anhydride and oxidol in an ice bath for several hours) in the reflux device at 80 °C for 10 days, then the outcome was washed with abundant DMF and carboxylic MWNTs (MWNTs-COOH) were acquired via reduced pressure distillation and finally dried in the oven. The chemical equations for preparing carboxylic MWNTs have been shown in Figure 1. The morphology of MWNTs and its grafting production were directly observed by transmission electron microscopy (JEM-

Figure 2. Analytical device for concentration in swollen membrane: (1) test tube; (2) valve; (3) swollen membrane.

1230, JEOL). The structure was characterized by FTIR (Nexus670, Nicolet Co.) and Raman spectroscopy (LabRAM HR UV). Dispersibility of carbon nanotubes in water was analyzed by particle size analyzer (Brookhaven Corp.). 2.3. Preparation and Characterization of Membrane. A preweighed chitosan powder was dissolved in 2 wt % acetic aqueous solution for about 24 h at room temperature to produce a casting solution consisting of 2 wt % chitosan. A certain amount of MWNTs-COOH was added into a chitosan solution. The mixed solution was stirred for about 24 h, and then, it was kept in an ultrasonic bath at a fixed frequency for about 30 min to improve the dispersion of MWNTs-COOH in the polymer matrix. The mixtures were then filtered and left overnight to get a homogeneous solution. The resulting solution was poured, cast onto a polyacrylonitrile (PAN) UF membrane in a dustfree atmosphere at 50 °C, dried for about l-2 h below an infrared lamp, and then put in an oven at 50 °C for another 24 h. The amounts of functionalized MWNTs with respect to chitosan were 0, 0.5, 1, 1.5, and 2%, and the membranes thus obtained were designated as M(0), M(0.5), M(1), M(1.5), and M(2), respectively. The resulting membranes were observed by transmission electron microscopy (TEM; JEM-1230, JEOL), X-ray diffraction (XRD; PANalytical, X’Pert PRO), and scanning electron microscopy (SEM; JSM-5610LV). 2.4. Swelling and Sorption Measurements. After being kept in desiccators to desorb any moisture sorbed from the air, the preweighed membranes were immersed in a known composition of ethanol/water mixtures in a closed bottle at room temperature for over 48 h for an equilibrium swelling. The membranes were periodically weighed until the mass had been constant. Then, the membrane sample was taken out from the liquid bath, with the surface solution wiped off carefully with tissue paper, and weighed in a tightly closed bottle. The amount of absorbed liquid in the membranes was expressed as the degree of swelling (DS), which was calculated by the following equation: DS(%) )

Ws - Wd × 100 Wd

(1)

where Ws and Wd are the weights of the swollen and dry membranes, respectively. A parallel membrane sample was immediately placed in an analytical device, shown in Figure 2. The liquid sorbed by the membranes in one test tube were desorbed under vacuum and collected in the other test tube. The collected liquid was then weighed and analyzed for composition by gas chromatography (Agilent Model 1790, Agilent Technology, Shanghai). The individual sorbed amount was calculated from the total sorbed amount and the sorbed composition. Solubility selectivity (Rs) was calculated as follows:

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Rs )

Cwater /CEtOH Xwater /XEtOH

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(2)

where Cwater and CEtOH are the weight fractions of the ethanol and water in the membrane, and XEtOH and Xwater are the weight fractions of ethanol and water in the feed solution, respectively. According to the solution-diffusion model, diffusivity selectivity RD was calculated by RD )

R Rs

(3)

where R is ath emembrane separation selectivity. 2.5. Pervaporation Separation Experiments. Pervaporation separation experiments were carried out by a laboratory-made apparatus, which is shown in Figure 3. Membrane performance in PV experiments was studied by calculating the total flux (J), separation factor (R), and pervaporation separation index (PSI). These were calculated, respectively, using the following equations: W At

(4)

Ywater /YEtOH Xwater /XEtOH

(5)

J) R)

PSI ) JR

Figure 3. Schematic of pervaporation apparatus.

(6)

where W is the permeate mass flux (g/(m2 · h)); t is the permeation time (h); A is the membrane area (cm2); Y and X are the mass fractions of the permeation and feed, respectively; and subscript EtOH denotes ethanol. 3. Results and Discussion 3.1. Characterization of Functionalized MWNTs. 3.1.1. FTIR. The FTIR spectrum of pristine and carboxylic MWNTs was shown in Figure 4. The spectrum of pristine MWNTs exhibits weak sp2 C-H and sp3 C-H stretching bands at 2935 and 2860 cm-1, respectively. They are attributed to the defects at the sidewalls and open ends of MWNTs. The defects provide sites for the electrophilic substitution reaction. From the FTIR spectrum of MWNT-COOH, the bands at 1715 and 1581 cm-1 were present which are characteristic peaks of a -CO- group; meanwhile, the band at 3434 cm-1 was the characteristic peak of an -OH group and the band at 1211 cm-1 represented a -CH2- group. All the existing bands indicated the carboxyl group had been grafted on the MWNTs. 3.1.2. Raman. Raman spectroscopy was employed to identify the formation of defects on the MWNTs. In Figure 5, two peaks are clearly visible in the spectra of both samples: (i) disordered carbon band (D-band) at approximately 1353 cm-1 and (ii) graphitized band (G-band) at approximately 1586 cm-1. The D-band typically corresponds to defects in the walls of the MWNTs, whereas the G-band is a characteristic of MWNTs (a form of sp2-bonded crystalline carbon). The D-band and G-band peaks observed in the spectra of both samples suggest the presence of MWNTs with wall defects which were made by the strong acid and ultrasonic treatment. Furthermore, it is calculated that the ratio of D-band to G-band intensity (D/G ratio) for the pristine MWNTs was about 0.952. After functionalization, some sp2-hybridized carbons in MWNTs converted to be sp3 hybridization. Therefore, the D/G ratio of MWNTsCOOH increased to 1.31. This change in D/G ratio indicates the success of bonding carboxyl to MWNTs side walls.

Figure 4. FTIR spectra of pristine MWNTs and carboxylic MWNTs.

Figure 5. Raman spectra of pristine MWNTs and functionalized MWNTs (excitation at λ ) 632.8 nm).

3.1.3. TEM. The morphologies of pristine and carboxylic MWNTs were observed by TEM as shown in Figure 6. The pristine MWNTs are present as bundles or an individual tube with some degree of entanglement in the solvent, and it was difficult to observe a single one without disturbance in the picture. After functionalization, the carboxylic MWNTs were well-dispersed in solvent, and they were much shorter than original ones because of treatment by ultrasound and mixture acid. More importantly, compared to the smooth wall of pristine MWNTs, that of carboxylic MWNT was rough, and there were some grafted points which complemented the results of FTIR

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Figure 6. TEM pictures of pristine MWNTs and MWNTs-COOH dispersed in water: (a) pristine MWNTs; (b) MWNTs-COOH.

to ascertain the structure of carboxylic MWNTs. In other respects, the results of distributions of particle size were in excellent agreement with TEM pictures. From Figure 7a, The apparent particle size of pristine MWNTs in water was about 1700 nm, and it was obviously seen that some MWNTs precipitate in the bottom of water even after ultrasonic treatment. The main apparent diameter of carboxylic MWNTs-COOH shown in Figure 7b reduced to 183 nm, which proved the dispersibility of MWNTs-COOH was remarkably improved. The photographs of pristine MWNTs and MWNTs-COOH dispersed in water for a week were shown in Figure 7c. The pristine MWNTs almost precipitated in the bottom, and MWNTs-COOH could still evenly disperse in water. 3.2. Characterization of MWNT-COOH Incorporated Chitosan Membrane. The compatibility between the polymer and the surface of the inorganic filler is a key issue in determining the final membrane property and performance. To improve the interfacial morphology in hybrid membrane, a transitional phase is expected to be created between the organic and inorganic phases, mitigating or eliminating the nonselective voids. In this study, the interaction between carboxyl group (-COOH) grafted on the side wall of carbon nanotubes and hydroxyl groups (-OH) or amino groups (-NH2) on chitosan chains created the desired transitional phase as illustrated in Figure 8. In the FTIR spectra of chitosan and chitosan-MWNTs (Figure 9), the characteristic band of the glucopyranose rings appeared at approximately 1060 cm-1; moreover, the new band occurring at 1645 cm-1 (amide I) was assigned to amide groups between chitosan and MWNTs-COOH. The 1560 cm-1 bond which was assigned to N-H bending in the amino become weaker, suggesting that some amino groups were converted into amide groups. In addition, the new bands at 1384 cm-1 (C-O) and

Figure 8. Schematic illustration of interfacial interaction existing in MWNTs-COOH incorporating chitosan membranes.

Figure 9. FTIR spectra of (a) chitosan and (b) chitosan-MWNTs.

the very weak band at 1310 cm-1 suggested that the primary hydroxyl groups participated partly in the reaction. Furthermore, the broad O-H and N-H absorption band of the chitosan at approximately 3373 cm-1 shifted to 3328 cm-1 and became sharp, not only supporting the discussions above but also implying that hydrogen bonds have formed between the functionalized surface of the MWNTs and chitosan chains. In the other hand, the crystallinity of chitosan membrane reduced because of such strong interactions between MWNTsCOOH and chitosan molecules, shown in Figure 10. The chitosan membrane was found to be semicrystalline and the typical peaks of the chitosan membrane appeared at 2θ ) 15.5 and 20.4°,; after mixing MWNTs-COOH in membrane, the intensity of the original crystal peaks at 15.5 and 20.4° appeared

Figure 7. Distributions of particle size for carbon nanotubes. The average diameters of (a) pristine MWNTs and (b) MWNTs-COOH are 1700 and 183 nm, respectively. (c) Photographs of pristine MWNTs (left) and MWNTs-COOH (right) dispersed in water for a week.

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Figure 10. XRD patterns of original chitosan membrane and chitosanMWNTs blend membranes (2 wt %). Figure 12. Variation of swelling degree with different ethanol concentrations in the feed for M(2) membrane.

Figure 13. Variation of swelling degree with different contents of MWNTCOOH incorporated chitosan membranes at 90% of ethanol in the feed. Figure 11. TEM micrographs of (a) pristine MWNTs and (b) MWNTsCOOH incorporated chitosan membrane and SEM micrographs of (c) surface and (d) cross-section of MWNTs-COOH incorporated chitosan membrane. The MWNTs loading content in membrane matrix were 0.5%.

to be reduced, and a new peak at 2θ ) 9.6°, which represents the MWNTs, appeared. That is to say the crystallinity of chitosan was decreased, and the mixed MWNTs well-combined with chitosan molecules. As the content of MWNTs increased, the effect of MWNTs, which through strong interactions with chitosan molecules, made chitosan molecules not easy to pack together to form crystals; meanwhile, the MWNTs randomly distributed through the chitosan matrix and, interacting with the chitosan molecules, induced the chitosan molecules to orient one another in a limited area and resulted in the decrease in the crystallinity. Figure 11 showed the micrograph of pristine MWNTs and MWNTs-COOH filled membranes. From parts a and b, the obvious differences could be observed. The pristine MWNTs were long enough and had entanglements in the chitosan matrix; therefore, it was easy to create defects in the membrane surface. On the contrary, a fairly large number of cut MWNTs-COOH were well-dispersed through a chitosan matrix without apparent clustering. From parts c and d, the pictures of both surface and cross-section proved that the formed membrane was homogeneous and defects-free, and further revealed that, by incorporation with the polymeric matrix, the defects and functional groups of MWNTs provided stronger interactions with hydroxyl groups

of chitosan, thus resulting in a significant increase of the compatibility between the MWNTs-COOH and chitosan. 3.3. Swelling and Sorption Properties of Carbon Nanotubes Incorporated Chitosan Membrane. The percent degree of swelling was plotted as a function of different concentrations of ethanol in the feed mixture for M(2) membrane at 25 °C as shown in Figure 12. It is observed that the degree of swelling decreased with increasing weight fraction of ethanol in the feed. When the concentration of ethanol was less than 50%, the swollen membrane was many times more weight than the dry membrane. This indicated that chitosan had strong swelling in water compared to ethanol. Figure 13 showed variation of the swelling degree of hybrid membranes with different MWNTs-COOH contents. The filling of MWNTs-COOH could increase the swelling degree of the membrane and also reflected that carbon nanotubes filling significantly increased the adsorption ability of chitosan membrane toward ethanol/water mixtures. This may be due to the interaction between the carboxyl groups grafted on MWNTsCOOH and hydroxyl groups or amino groups on chitosan chains (in Figure 8), and the hydrogen bond interaction among chitosan chains weakened which resulted in the chitosan chains packing more loosely and decreasing the crystallinity of the blend membranes. Because the crystalline regions are inaccessible to the penetrant, the decrease in the crystallinity of the blend membranes by the MWNTs might induce the increase of the swelling degree. In addition, the decrease of the chitosan

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Figure 14. Variation of diffusion with different contents of MWNT-COOH incorporated chitosan membranes in pure water at 25 °C.

Figure 15. Variation of diffusion with different contents of MWNT-COOH incorporated chitosan membranes in ethanol at 25 °C.

membrane crystallinity and pore effects of carbon nanotubes would create higher free volume in the membranes. Obviously, the changed structure and the increase of free volume were both expected for the enhanced swelling degree with an increase of carbon nanotubes content in the membrane. 3.4. Diffusion Selectivity and Solubility Selectivity. For Fickian diffusion, diffusion coefficient D can be calculated from the following standard equation,29,30 Mt )1M∞

∞

∑ (2n +8 1) π

2 2

(

exp

n)0

-D(2n + 1)2π2t t2

)

(7)

where Mt and M∞ are the equilibrium sorption amounts of the solution per unit mass dried membrane at times (s) t and ∞, respectively, and l is the thickness of the dried membrane (m). At short times (Mt/M∞ e 0.4), this equation reduces to Mt 4 ) M∞ √π



Dt l2

(8)

which can be rearranged to the following form: D)

( )

π Mt /M∞ 16 √t/l

2

(9)

Because the plot of Mt/M∞ against √t/l is equal to the initial slope of sorption curve (tan θ); that is, D)

π (tan θ)2 16

(10)

The relationships of the sorption amount with the sorption time of pure water, ethanol, and a 90% ethanol/water mixture in the membranes at 25 °C have been shown in Figure 14-16 (l is constant). According to the slopes of the lines, diffusion coefficients of pure water (Dwater, m2/s), ethanol (DEtOH, m2/s), and 90% ethanol/water mixtures (D90, m2/s) with different functionalized MWNTs loading in membranes were obtained by formula 10. The diffusion coefficient increased with the increasing MWNTs loading content, so the existence of MWNTs in membrane was in favor of both water and ethanol molecular transportation in chitosan membrane. To ascertain the impact of MWNTs on the diffusion coefficient of ethanol/water mixed solution, the comparison was drawn between the measured

Figure 16. Variation of diffusion with different contents of MWNT-COOH incorporated chitosan membranes in 90% ethanol/water mixture at 25 °C.

diffusion coefficient (D90, m2/s) and the calculated diffusion coefficient (D90T, m2/s) with different functionalized MWNTs loading in membranes in 90% ethanol/water mixtures. The calculated diffusion coefficient was obtained from formula 11, which was deduced from formula 9, D90T ) (0.1Mt(water) + 0.9Mt(EtOH))/(0.1M∞(water) + 0.9M∞(EtOH))

[

√t/l

]

2

(11)

where Mt(water) and Mt(EtOH) are the sorption amounts of water and ethanol at t time in Figure 14 and Figure 15. M∞(water) and M∞(EtOH) are the equilibrium sorption amounts of water and ethanol in Figure 14 and Figure 15. From the bar picture in Figure 17, it was indicated that D90 (m2/s) was larger than D90T (m2/s) with the higher MWNTs loading content, and the MWNTs were more prone to increased water permeation when ethanol and water penetrated into the membrane simultaneously, and more MWNTs resulted in more differences between calculated and measured diffusion coefficients. In addition, the measured diffusion coefficient and calculated diffusion coefficient were approximate as for M(0) (because of no influences by MWNTs), so it was demonstrated

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Figure 17. Comparison between calculated and measured diffusion coefficient with different functionalized MWNTs loading in membranes in 90% ethanol/water mixture.

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Figure 19. Arrhenius plot of the effect of temperature on permeate flux. Table 1. Arrhenius Activation Parameters for Permeation (kJ/mol) parameters

M(0)

M(0.5)

M(1)

M(1.5)

M(2)

EJ EJ(water) EJ(EtOH)

28.15 26.75 36.54

18.48 17.33 27.23

16.01 13.41 23.48

13.76 11.28 21.65

12.91 9.89 19.8

Table 2. Effect of Nanotube Content on Pervaporation Separation Property of the Membranes at 90% Ethanol in the Feed

Figure 18. Effect of operating temperature on pervaporation separation property at 90% of ethanol in the feed for M(2) membrane.

that, at the beginning, the calculation was reasonable and coincided with the experimental data. 3.5. Pervaporation Property of Carbon Nanotubes Incorporated Chitosan Membrane. 3.5.1. Effect of Temperature on Pervaporation Properties. The effect of operating temperature on pervaporation properties for ethanol/water mixtures was shown in Figure 18. It can be observed that the total permeation flux increased significantly, and separation factor decreased slightly from 30 to 70 °C. This was because of increasing thermal energy; the free volume in the membrane matrix increased on account of the increased frequency and amplitude of the chitosan chain jumping. As a result, the diffusion of both permeating molecules increased, and this led to higher permeation flux, whereas the selectivity was slightly suppressed. The relationship of the permeate flux and feed temperature was analyzed by the Arrhenius equation as follows:

( )

J ) AJ exp -

EJ RT

(12)

where AJ is a frequency factor and EJ is the permeation activation energy. From the Arrhenius relationship, the PV activation energy can be evaluated. From a least-squares fit in Figure 19, the activation energy for total permeation (EJ) was estimated and the results were presented in Table 1. From Table 1, it is noticed that the pure membrane M(0) exhibited a much higher EJ value compared to carbon nanotubes filled membranes (M(0.5)-M(2)).

nanotubes (%)

J (g/(m2 · h))

Α

RS

RD

PSI (g/(m2 · h))

0 0.5 1 1.5 2

112 233 293 340 337

580 576 574 573 570

110.3 95.3 85.1 77.6 70.5

5.3 6 6.7 7.4 8.1

64960 134208 168182 194820 192090

It suggested that the permeating molecules require more energy to transport through the pure membrane due to its crystalline nature, whereas, with carbon nanotubes filled membranes, molecules obviously took less energy. This was because of the molecular sieving action, which was attributed to the presence of channels in the framework of the carbon nanotubes.31 Therefore, EJ decreased systematically from M(0) to M(2) with increasing carbon nanotubes content. In addition, the apparent activation energy values of water are significantly lower than those of ethanol, suggesting that membranes have significantly higher separation efficiency. The activation energy values for total permeation and water permeation were found to be almost the same for all membranes, signifying that couple-transport is minimal due to a higher selective nature of membranes.32 3.5.2. Effect of Carbon Nanotubes Content in the Membrane on Pervaporation Property. Table 2 shows the effect of carbon nanotube content on the pervaporation separation property of membranes. From the table, it can be seen that the total permeation flux depended on the composition of the blend membranes and greatly increased with the content of MWNTs increasing, but without a decrease of the separation factor. This is consistent with the swelling experiment results. It is possible to suggest that both the water and ethanol may transport through the inner spaces of MWNTs without serious resistance, helping them to penetrate the blend membranes easily. As mentioned in the Experiments, the inner diameter of the MWNTs is about 5 nm, which is larger than the kinetic diameter of a water molecule (0.30 nm) or an ethanol molecule (0.430 nm), so the water and ethanol molecules can pass through the inner spaces of the MWNTs, improving the flux of the blend membranes.33 Also, another factor that can explain the flux increase is the decrease of the crystallinity. Because the crystalline regions are

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homogeneous membrane and show better pervaporation separation property. Acknowledgment This project is sponsored by the National Basic Research Program of China (Grant 2009CB623402), National Hi-Tech Research and Development Program of China (Grant 2009AA02Z208), and the Fundamental Research Funds for the Central Universities. We are grateful to Prof. J. Y. Lai and Prof. K. R. Lee of the R&D Center for Membrane Technology, Chung Yuan University, Taiwan, for supporting the device of determining the concentration in a swollen membrane.

Figure 20. Relationship between diffusivity selectivity and ratio of activation energy in different MWNTs content membranes.

inaccessible to the penetrant, the decrease in the crystallinity of the blend membranes by the MWNTs might induce the increase in the flux. In Figure 20, the ratio of activation energy (EJ(water)/EJ(EtOH)) increased with more content of MWNTs in the membrane, and it meant that EJ(water) decreased more quickly than ethanol’s (EJ(EtOH)) due to the MWNTs, so the diffusivity selectivity increased for the sake of less activation energy. In addition, it is found that (Table 2) the solubility selectivity decreased accompanied with the diffusivity selectivity increasing. On the basis of the sorption-diffusion mechanism, the decrease of solubility selectivity is not favorable for priority adsorption of water, but the increase of diffusivity selectivity is favorable for priority diffusion of water across the membrane, so the combined effect caused membrane selectivity only decreased slightly. Meanwhile, it is also observed that the PSI value of the chitosan membrane was improved after filling the nanotubes. In a word, the membranes filled with the functionalized carbon nanotubes showed better pervaporation performance for the separation of ethanol/water mixtures compared with that of the chitosan membrane. 4. Conclusions Multiwalled carbon nanotubes were modified by mixed acid and diisobutyryl peroxide and then incorporated into chitosan matrix to fabricate a blended membrane. Several analyses including TEM, Raman, and FTIR demonstrated the active functional groups existed on the wall of carbon nanotubes after modification. Also, the carboxylic MWNTs exhibited well dispersibility in aqueous solution and polymer matrix. Furthermore, the sorption, swelling, and pervaporation properties of the resulting membrane were investigated to show that the membranes incorporated with functionalized carbon nanotubes gained improvement in performance. An increase of carbon nanotubes content in the membrane resulted in the increase of total permeation flux with little decrease of the separation factor. The comparison between the measured diffusion coefficient and the calculated diffusion coefficient demonstrated that functionalized MWNTs in a membrane were more prone to increased water permeation when ethanol and water penetrated into the membrane simultaneously; especially for M(2), D90T (0.41 × 10-6 m2/s) was nearly two times that of D90 (0.193 × 10-6 m2/ s). In addition, after introducing functionalized MWNTs, the Arrhenius activation parameters for total permeation decreased from 28.15 to 12.91 kJ/mol. In summary, the carbon nanotubes filled chitosan membranes have bigger PSI values than chitosan

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ReceiVed for reView June 4, 2010 ReVised manuscript receiVed September 6, 2010 Accepted September 21, 2010 IE101223K