Solving the Trade-off Phenomenon Between Permeation Flux and

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Ind. Eng. Chem. Res. 2009, 48, 4002–4013

Organic-Inorganic Hybrid Membranes: Solving the Trade-off Phenomenon Between Permeation Flux and Selectivity in Pervaporation Mahadevappa Y. Kariduraganavar,*,† Jolly G. Varghese,† Santosh K. Choudhari,† and Robert H. Olley‡ Center of Excellence in Polymer Science and Department of Chemistry, Karnatak UniVersity, Dharwad 580 003, India, and Polymer Science Centre, UniVersity of Reading, Whiteknights, Reading RG6 6AF, U.K.

To solve the trade-off phenomenon existing between permeation flux and selectivity of membranes in pervaporation, organic-inorganic hybrid membranes composed of chitosan and TiO2 were prepared using a solution technique. The resulting hybrid membranes were characterized by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Compared to pure chitosan membrane, the hybrid membranes exhibited high thermal stability and low crystallinity. These membranes were tested for their ability to separate water-isopropanol mixtures by pervaporation in the temperature range of 30-50 °C. The experimental results demonstrated that both flux and selectivity increased simultaneously with increasing TiO2 content in the membrane. The permeation flux of pure chitosan membrane increased dramatically from 3.06 to 12.17 × 10-2 kg/(m2 h) when 40 mass % of TiO2 was incorporated, and correspondingly its separation factor increased from 509 to 94 984 at 30 °C for 5 mass % of water in the feed. The total flux and flux of water were found to be almost overlapping particularly for hybrid membranes, suggesting that these membranes could be used effectively to break the azeotropic point of water-isopropanol mixtures. From the temperature dependent diffusion and permeation values, the Arrhenius activation parameters were estimated. The activation energy values obtained for water permeation (Epw) were significantly lower than those of isopropanol permeation (EpIPA), suggesting that the developed membranes have higher separation efficiency for water-isopropanol systems. The negative heat of sorption (∆HS) values were observed in all the membranes, indicating that Langmuir’s mode of sorption is predominant. 1. Introduction Isopropanol (IPA) is an important solvent widely used in electronic and liquid crystal display industries for cleaning and drying operations during the production of semiconductors, flatpanel displays, disks, optoelectronics, and other electronic components.1,2 Concentration and purification of IPA from its water solution are essential for many chemical processes such as acetone production, solvent extraction, and manufacture of hydrogen peroxide.3 Because of escalating virgin-chemical costs, disposal costs, and safety-environmental concerns, the reprocessing of IPA has become attractive, practical, and costeffective for recycling IPA. Obtaining IPA in its ultrapure form is difficult as it forms an azeotrope at 12.2 mass % of water4 and hence, its separation by conventional methods such as solvent extraction and rotavapor or by distillation could prove uneconomical. Under these circumstances, exploration of energysaving and environmentally benign processes such as pervaporation (PV) is thus urgently needed. It is well-known that in PV the phase change takes place from liquid to vapor. The processes involving phase changes are generally energy-intensive, and distillation is a notorious example of them. Use of PV cleverly surmounts the challenge of phase change by two features: (i) PV deals only with the minor components of the liquid mixtures, and (ii) PV employs on the most selective membranes. The first feature effectively * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +91-836-2771275. Tel.: +91-8362215286, x23. † Karnatak University. ‡ University of Reading.

reduces the energy consumption of PV process. Because of the characteristics of PV operation, it is essentially true that only the minor component in the feed consumes the latent heat. The second feature generally makes PV the most efficient liquid separating technology. Therefore, combination of these two features ranks PV as the most cost-effective liquid separation technology.5,6 The application of PV as a means to achieve dehydration of IPA has received widespread attention due to a so-called synergic effect: water is both preferentially dissolved and transported in the hydrophilic membranes because of its much small molecular size. However, the technical feasibility of this process largely depends on the membrane and its properties. In regard to this, several hydrophilic polymers like poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), and polysaccharides such as hydroxyethyl cellulose [HEC], sodium alginate, and chitosan demonstrated strong affinity toward water, and have been widely investigated for PV separation of alcohol/ H2O mixtures.2,7-10 Among these, chitosan [poly-β(1f4)-Dglucosamine], an aminopolysaccharide, has attracted a great interest as a basic membrane material because of its natural occurrence, high abundance, hydrophilicity, chemical resistance, adequate mechanical strength, good membrane forming properties, functional groups that can be easily modifie, and ease of processing.11-13 In addition to these, it possesses a Hansen’s solubility parameter value (43.04 J1/2/cm3/2) which is close to that of water (47.9 J1/2/cm3/2).14,15 However, the swelling of chitosan membrane in an aqueous solution results to an increase of both solubility and diffusivity of alcohols, and consequently lowers the water permselectivity. Considerable efforts have been made in order to improve its performance through blending with other hydrophilic polymers such as sodium alginate,2 HEC,9

10.1021/ie8016626 CCC: $40.75  2009 American Chemical Society Published on Web 03/23/2009

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 4003 16,17

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PVA, poly(ethylene oxide) (PEO), formation of polyelectrolyte complex,10,19,20 surface modification with maleic anhydride,21 incorporation of selective zeolite into membrane matrix,22 and development of hybrid membranes.3,23,24 Among these, membranes incorporated with selective zeolite (filler) demonstrated an excellent PV performance due to its unique properties. A literature search of similar materials showed that hydrophilic character can be induced on the surface of TiO2 upon irradiation of UV light.25-28 During the process of UV irradiation, the electrons tend to reduce Ti(IV) cations to Ti(III) state, and the holes oxidize the O2- anions. As a result, oxygen atoms are ejected and thereby create oxygen vacancies. Water molecules can then occupy these oxygen vacancies and create absorbed -OH groups, which tend to make the surface hydrophilic.29 Such materials have a unique property of attracting water molecules rather than repelling, which is commonly known as superhydrophilicity.30 Further, it is reported that TiO2 can also be used in harsh conditions because of its high chemical and thermal stability.31 Current investigation focuses on the preparation of TiO2incorporated chitosan hybrid membranes. The content of TiO2 was varied to improve the overall performance of the membranes. The physicochemical changes in the resulting membranes were investigated using FTIR, WAXD, SEM, and TGA, and successfully employed for PV separation of water-isopropanol mixtures. The values of permeation flux, separation selectivity, and diffusion coefficients were evaluated. From the temperature dependence of permeation flux and diffusion coefficients, the Arrhenius activation parameters were estimated. The results were discussed in terms of PV separation efficiency of the membranes. 2. Experimental Section 2.1. Materials. Chitosan (Mw ≈ 200 000; N-deacetylation degree, 75-85%) was obtained from Sigma-Aldrich Chemicals, USA. Titanium dioxide (TiO2), isopropanol, and acetic acid (HAc) were purchased from s. d. Fine Chemicals Ltd., Mumbai, India. All the chemicals were of reagent grade and used without further purification. Double distilled water was used throughout the study. 2.2. Membrane Preparation. Chitosan (3 g) was dissolved in 100 mL of deareated-distilled water containing 2% of acetic acid under constant stirring for about 24 h at 60 °C. The solution was then filtered through a fritted glass disk-filter to remove undissolved residue particles and the filtrate was left overnight such that the effervescence was complete. The resulting clear solution was cast onto a glass plate with the aid of a casting knife in a dust-free atmosphere at room temperature. After being dried for about 48 h, the membrane was subsequently peeledoff and was designated as M-1. To prepare TiO2-incorporated chitosan membrane, a known amount of TiO2 was added into an acidic chitosan solution. The amount of chitosan was kept constant for each membrane. The mixed solution was stirred for about 24 h, followed by sonication for about 30 min at a fixed frequency of 38 kHz (Grant XB6, UK) to break the aggregated crystals of TiO2 and to improve the dispersion of TiO2 in the polymer matrix. It was then filtered and left overnight to obtain a homogeneous solution. The resulting solution was poured onto a glass plate and the membrane was dried as mentioned above. The amount of TiO2 with respect to chitosan was varied as 10, 20, 30, and 40 mass %, and the membranes thus obtained were designated as M-2, M-3, M-4, and M-5, respectively. An attempt was also made to incorporate higher amount of TiO2, but even using 45 mass

Figure 1. Scheme for the preparation of chitosan-based TiO2-incorporated hybrid membranes.

% of TiO2 led to embrittlement and lost the membrane property. Hence, the loading of TiO2 was restricted up to 40 mass %. The scheme for the preparation of chitosan-based hybrid membranes is illustrated in Figure 1. The thickness of these membranes was measured at different points using a Peacock dial thickness gauge (model G, Ozaki Mfg. Co. Ltd., Japan) with an accuracy of (2 µm and the average thickness was considered for the calculation. Thickness of these membranes was found to be 40 ( 2 µm. 2.3. Fourier Transform Infrared (FTIR) Spectroscopy. The incorporation of different amounts of TiO2 in chitosan was confirmed by FTIR spectroscopy (Nicolet, Impact-410, USA). Membrane samples were ground well to make KBr pellets under a hydraulic pressure of 600 kg/cm2 and spectra were recorded in the range of 400-4000 cm-1. In each scan, the amount of membrane sample and KBr were kept constant in order to estimate the changes in the intensities of the characteristic peaks with respect to the amount of TiO2 loading. 2.4. Wide-Angle X-ray Diffraction (WAXD). The morphology of the pure chitosan and its TiO2-incorporated hybrid membranes was studied at room temperature using a Bruker’s D-8 advanced wide-angle X-ray diffractometer. The X-ray source was Ni-filtered Cu KR radiation (40 kV, 30 mA). The dried membranes of uniform thickness (40 ( 2 µm) were mounted on a sample holder, and the patterns were recorded in the reflection mode at an angle 2θ over a range of 5-50° and at a speed of 8°/min. 2.5. Scanning Electron Microscopy (SEM). The morphology of the pure chitosan and its TiO2-incorporated hybrid membranes was investigated at 10 kV using a JSM-840A scanning electron microscope (JEOL, Tokyo, Japan). All the specimens were coated with a conductive layer (400 Å) of sputtered gold. 2.6. Thermogravimetric Analysis (TGA). Thermal properties of the membranes were analyzed using a Perkin-Elmer

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TGA/DTA thermogravimetric analyzer at a heating rate of 10 °C/min under nitrogen atmosphere. The weight of the samples taken for each record was about 5-9 mg. 2.7. Swelling Measurements. The degree of membranes swelling was performed with different compositions of waterisopropanol mixtures using an electronically controlled oven (WTB Binder, Jena, Germany). The masses of the dry membranes were first determined and these were equilibrated by immersion in different compositions of the feed mixtures in sealed vessels at 30 °C for 24 h. The swollen membranes were weighed as quickly as possible after careful blotting on a digital microbalance (model B204-S, Mettler-Toledo International, Zurich, Switzerland) with an accuracy of (0.01 mg. All the experiments were performed at least three times and the results were averaged. The percent degree of swelling (DS) was calculated as DS (%) )

(

)

Ws - Wd 100 Wd

(1)

where Ws and Wd are the masses of the swollen and dry membranes, respectively. 2.8. Pervaporation Experiment. PV experiments were carried out using the in-house designed apparatus reported in our previous articles.32,33 The effective surface area of the membrane in contact with the feed mixture was 34.23 cm2 and the capacity of the feed compartment was about 250 cm3. The vacuum in the downstream side of the PV cell was maintained [1.333224 × 103 Pa (10 Torr)] using a two-stage vacuum pump (Toshniwal, Chennai, India). The test membrane was allowed to equilibrate for about 2 h in the feed compartment at the appropriate temperature before performing the experiment. The experiments were carried out at 30, 40, and 50 °C. The water composition in the feed was varied from 5 to 25 mass %. The permeate was collected in a trap immersed in liquid nitrogen jar on the downstream side of the PV cell at fixed time intervals. The permeate thus obtained was weighed on a digital microbalance to determine the flux. The compositions of water and IPA were estimated by measuring the refractive index of the mixture within an accuracy of (0.0001 units using Abbe’s refractometer (Atago-3T, Tokyo, Japan) and by comparing it with a standard graph of refractive index, which was established with the known compositions of water/IPA mixtures. All the experiments were performed at least three times and the results were averaged. The results of permeation for water-IPA mixtures during the pervaporation were reproducible within the admissible range. The efficiency of the membranes in PV experiments was assessed in terms of total flux (J), separation factor (Rsep) and pervaporation separation index (PSI). These were respectively calculated using the following equations: W At Pw /PIPA ) Fw /FIPA

J) Rsep

PSI ) J(Rsep - 1)

(2) (3) (4)

where W represents the mass of permeate (kg); A is the effective membrane area (m2); t is the permeation time (h); P and F are the mass fractions of permeate and feed, respectively; subscripts w and IPA, respectively denote water and isopropanol. 3. Results and Discussion 3.1. Membrane Characterization. 3.1.1. FTIR Studies. The incorporation of TiO2 into chitosan matrix was established

Figure 2. FTIR spectra of pure chitosan and its TiO2-incorporated hybrid membranes: (M-1) 0 mass %; (M-2) 10 mass %; (M-3) 20 mass %; (M-4) 30 mass %; (M-5) 40 mass % of titanium dioxide.

by FTIR studies. Figure 2 illustrates the FTIR spectra of pure chitosan and its TiO2-incorporated hybrid membranes. A characteristic strong and broad band exhibited at around 3400 cm-1 in membrane M-1 corresponds to O-H stretching vibrations of the hydroxyl groups. The bands appearing at around 1650 and 1570 cm-1 are respectively assigned to amino-I and amino-II functional groups of chitosan.22,4 Similarly, the multiple bands appearing between 900 and 1200 cm-1 correspond to C-O stretching vibrations. The intensity of these bands decreased from membrane M-2 to M-5 with increasing TiO2 content. A band resonance at low wavenumber (600 cm-1) in the pure chitosan membrane became wider as the content of TiO2 increased from membrane M-2 to M-5. This is expected owing to the formation of Ti-O-Ti stretchings.34 All this evidence ascertains the increase of TiO2 content in the membrane matrix. 3.1.2. WAXD Studies. To study the effect of TiO2 on the morphology of chitosan membrane, X-ray diffraction was employed and the patterns thus obtained are displayed in Figure 3. In the diffraction pattern, pure chitosan membrane exhibited sharp peaks at around 9° and 12°, and a broad peak at around 21°. The sharp peaks correspond to the semicrystalline part, whereas the broad peak corresponds to the amorphous part, and these are respectively related to crystal 1 and crystal 2.35 By the addition of 10 wt % of TiO2, the intensity of these peaks decreased drastically as can be seen from the patterns of M-2. This is because of the decreased intersegmental spacing due to shrinkage in cell size. This is expected as the functional groups (-CH2OH and -OH) of pure chitosan membrane underwent significant change owing

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Figure 3. Wide-angle X-ray diffraction patterns of pure chitosan and its TiO2-incorporated hybrid membranes: plain TiO2; (M-1) 0 mass %; (M-2) 10 mass %; (M-3) 20 mass %; (M-4) 30 mass %; (M-5) 40 mass % of titanium dioxide.

to the formation of cross-links with TiO2 as shown in Figure 1. The pattern of M-2 also exhibited sharp peaks at around 26° associated to TiO2 crystals. With further increasing TiO2, there was little change in the intensity of the broad peak of chitosan as clearly evidenced by the calculated d-spacing values, which range from 4.23 to 3.91 Å, but the intensity of the sharp peaks of the TiO2 crystals increased. This suggests that no further cross-linking occurs. When more than 10 wt % of TiO2 was incorporated into the membrane, the oxide was mainly dispersed in the polymer matrix as an inorganic phase. This is in good agreement with the SEM results, in which one can actually see the TiO2 particles dispersed in the matrix. The decrease of semicrystalline

domains and shrinkage of cells together contributed in making the structure more compact, which is responsible for the selective permeation of penetrants through the membrane.36 3.1.3. SEM Studies. Figure 4 illustrates the SEM photographs of the surface and cross-sectional views of the pure chitosan and its TiO2-incorporated hybrid membranes. The micrograph confirms that the distribution of TiO2 increased from membrane M-2 to M-5 with increasing the content of TiO2 and is uniform throughout the membrane matrix with no apparent clusterings. Further, the photographs explicitly show that the TiO2 crystals implanted in the membrane matrix have no voids around them. This ensures that the TiO2-

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Figure 4. SEM micrographs of pure chitosan and its TiO2-incorporated hybrid membranes (left) surface view and (right) cross-sectional view.

incorporated hybrid membranes obtained here are free from possible defects. 3.1.4. TGA Studies. The thermal stability and degradation behavior of pure chitosan and its TiO2-incorporated hybrid

membranes were investigated by TGA under nitrogen atmosphere, and the patterns thus obtained are presented in Figure 5. It could be seen that TGA curves of both chitosan and its TiO2-incorporated hybrid membranes decreased when the

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Figure 5. Thermogravimetric analysis of pure chitosan and its TiO2-incorporated hybrid membranes.

decomposition temperature varied from 50 to 100 °C, which resulted from the loss of water molecules attached to membranes. The highest weight loss of about 35% was observed in the second stage of decomposition starting from 200-350 °C and terminated at around 400 °C. A complete weight loss did not occur even after heating the membranes up to 900 °C or slightly above. It was observed that at 10% weight loss, the hybrid membranes exhibited better thermal stability than that of pure chitosan membrane. Further, the temperatures of rapid weight loss of hybrid membranes shifted to higher temperature regions, and this was more significant particularly for the membranes having higher loading of TiO2. These results demonstrated that the thermal stability was improved substantially after incorporation of TiO2 into chitosan matrix. Based on the characterization results, the change in structure of pure chitosan membrane by the incorporation of TiO2 is illustrated in Figure 6. It was demonstrated that with up to 10 mass % of TiO2 loading, TiO2 mainly established cross-links between linear polymer chains, which are responsible for reduction of semicrystalline regions in the membrane matrix. However, when the loading was beyond 10 mass %, TiO2 mainly dispersed in the polymer matrix as an inorganic phase without further establishing cross-links with chitosan. Further, it is understood that hybrid membranes prepared under acidcatalyzed conditions exhibit excellent hardness and elastic modulus (0.135 and 4.48 GPa with H2O/Ti ) 4) due to low phase separation and the open structure of titania in the polymer matrix. This was very well addressed by Clement Sanchez et al. in their review.37 However, the membranes prepared under neutral conditions show a chainlike structure of titania with small size. Moreover, the reaction between the reactive groups of titania and the polymer chain is very slow, leading to a decrease in cross-linking and mechanical properties (0.113 and 4.04 GPa). On the contrary, the base-catalyzed condition dramatically increases phase separation yielding the lowest values of elastic modulus and hardness (0.104 and 3.55 GPa). 3.2. Effects of Feed Composition and TiO2 Loading on Membrane Swelling. The way a membrane swells in certain liquids depends on the chemical composition and microstructure of the polymer, and the incorporated moiety, which can strongly influence the sorption mechanism.38 Therefore, the degree of

membrane swelling is of course an important factor in the PV process that controls the transport of permeating molecules under the chemical potential gradient. To study the effects of feed composition and TiO2 loading on the membrane swelling, the percent degree of swelling of all the membranes was plotted with respect to different mass % of water in the feed at 30 °C (Figure 7). It is observed that degree of swelling increased almost linearly for all the membranes as the mass % of water increased. This is due to a strong interaction between the water molecules and the membrane owing to the presence of -NH2, -NH3+, -OH, and Ti-OH in the membrane matrix. The interaction becomes more marked at higher concentration of water, since water causes a greater degree of swelling than those of alcohols due to its higher polarity. When the polymer matrices are loaded with TiO2, the degree of swelling increased relative to pure chitosan membrane; this effect increased proportionately with increasing the TiO2 content. This may be because TiO2 induces superhydrophilicity on the membrane matrix by the irradiation of UV-light, irrespective of its crystalline nature.30 The mechanism of the photoinduced hydrophilicities of TiO2 is due to the structural changes. It is assumed that Ti(IV) cations were reduced to Ti(III) states via the photogenerated electrons, and oxygen vacancies were generated through the oxidation of the bridging O2- species to oxygen via the photogenerated holes. Successively, the dissociated water adsorption on the vacancy sites created hydrophilic -OH groups. As a consequence, adsorption of water molecules increased remarkably, and this in turn becomes responsible for the enhanced swelling with increasing the TiO2 content in the membrane. 3.3. Effects of Feed Composition and TiO2 Loading on Pervaporation Properties. Figure 8 demonstrates the effects of feed composition and TiO2 loading on the total permeation flux for all the membranes at 30 °C. The permeation flux increased almost linearly for all the membranes with increasing water composition in the feed, and this is in good agreement with the results observed in the swelling study. This is due to an increased selective interaction between water molecules and the membrane, since chitosan contains active groups which are capable of forming hydrogen bonds with the water molecules. However, this interaction is more prominent for TiO2-

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Figure 7. Variation of degree of swelling with different mass % of water in the feed for pure chitosan and its TiO2-incorporated hybrid membranes.

Figure 6. Structure of pure chitosan and its TiO2-incorporated hybrid membranes.

incorporated hybrid membranes (M-2 to M-5). This is mainly attributed to the establishment of superhydrophilicity in the membrane matrix by the addition of TiO2, which enhances the greater attraction between water molecules and the membrane. To assess the extent of permeation of individual components, we have plotted the total flux, and fluxes of water and IPA as a function of TiO2 content in the membrane for 10 mass % of water in the feed as shown in the Figure 9. From the plot, it is clear that the total flux and flux of water are overlapping particularly for TiO2-incorporated hybrid membranes, and consequently the flux of IPA is negligibly small, indicating that the membranes developed in the present study by the incorporation of TiO2 are highly selective toward water with a tremendous improvement in the flux compared to pure chitosan membrane. Further, it is noticed that the difference between the total flux and flux of water gradually became very small with increasing TiO2 content in the membrane, signifying that the amount of TiO2 incorporated in the membrane predominantly enhanced the membranes’ efficiency. In PV process, the overall selectivity of a membrane is generally explained on the basis of interaction between mem-

Figure 8. Variation of total pervaporation flux with different mass % of water in the feed for pure chitosan and its TiO2-incorporated hybrid membranes.

brane and the permeating molecules, and the molecular size of the permeating species. Figure 10 displays the effects of both water composition and TiO2 content on the selectivity of the membranes. It is observed that selectivity of all the membranes decreased with increasing mass % of water in the feed. At higher concentration of water in the feed, the membranes swell greatly because of establishing strong interaction between membrane and the water molecules. As a result, a decrease of selectivity is obviously expected at higher concentration of water, irrespective of the amount of TiO2 in the membrane matrix. On the contrary, the selectivity increased from membrane M-2 to M-5 upon increasing the TiO2 content in the membrane matrix. This is attributed to increased selective interaction between membrane and the water molecules because of establishment of superhydrophilicity in the membrane matrix and decreased semicrystalline region due to cross-links (Ti-O-C) between active groups of chitosan (-CH2OH and -OH) and TiO2 (Ti-OH). The later explanation is very well explained by Chen,39 and Kramer and Prudhomme.40,41 This is also clearly

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Figure 11. Variation of total flux and selectivity with different mass % of TiO2-incorporated hybrid membranes at 10 mass % of water in the feed.

Figure 9. Variation of total flux and fluxes of water and isopropanol with different mass % of TiO2 at 10 mass % of water in the feed.

Figure 12. Variation of pervaporation separation index with different mass % of TiO2 at 10 mass % of water in the feed.

Figure 10. Variation of log R with different mass % of water in the feed for pure chitosan and its TiO2-incorporated hybrid membranes.

evidenced by the d-spacing values reported in WAXD section. This is further demonstrated from Figure 11, in which the flux and selectivity were plotted as a function of TiO2 content in the membrane at 10 mass % of water in the feed. Generally, as the packing density of the membrane increases either due to increase of cross-linking density or due to the incorporation of fillers into the membrane matrix, the permeation flux decreases and the selectivity increases.42,43 However, in the present study both the permeation flux and selectivity increased simultaneously with increasing TiO2 content in the membrane. Although, this is in contrast to a trade-off phenomenon existing between flux and selectivity in the PV process, a significant enhancement of hydrophilicity (i.e., superhydrophilicity) caused by the reduction of Ti(IV) to Ti(III) states overcomes the situation.32,44 3.4. Effect of TiO2 Loading on PSI. PSI is the product of permeation and separation factor, which characterizes the membrane separation ability. This index can be used as a relative guideline for the design of a new membrane in the PV separation processes and also for the selection of a membrane with an optimal combination of flux and selectivity. Figure 12 shows

the effect of TiO2 loading on the pervaporation separation index at 30 °C for 10 mass % of water in the feed. The PSI values increased almost exponentially with an increase of TiO2 content, signifying that the membranes incorporated with a higher amount of TiO2 exhibited an excellent performance while separating water-IPA mixtures. This is attributed to the incorporation of TiO2 into the membrane matrix, which changes not only the hydrophilicity of the membranes but also their morphology, which have a significant influence on the diffusion process. Sorption is only the first step, but in the second step of diffusion, the properties of TiO2 and its interaction with chitosan matrix enhance the overall performance of the membrane. On the basis of significant performance and stability of the membranes demonstrated in this system, we plan to use these membranes for the separation of lower alcohols such as methanol and ethanol, including other organic solvents like tetrahydrofuran, dioxan, etc. in our future study. 3.5. Diffusion Coefficients. The mass transport of binary liquid mixtures through a polymeric membrane in the PV process is generally described by the solution-diffusion mechanism, which occurs in three steps: sorption, diffusion, and evaporation.45 Thus, the permeation rate and selectivity are governed by the solubility and diffusivity of each component of the feed to be separated. In PV process, because of the

4010 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 1. Diffusion Coefficients of Water and Isopropanol at Different Mass % of Water in the Feed for Different Membranes mass % of water M-1 5 10 15 20 25

10.5 8.35 7.29 6.79 7.17

Dw × 108 (cm2/s)

Table 2. Pervaporation Flux and Separation Selectivity at Different Temperatures for Different Membranes at 10 Mass % of Water in the Feed

DIPA × 109 (cm2/s)

M-2 M-3 M-4 M-5 M-1 M-2 M-3

M-4

M-5

19.70 11.9 9.96 10.5 11.0

0.022 0.26 1.11 1.93 3.24

0.015 0.18 0.71 2.16 3.42

26.1 16.2 13.8 14.2 14.1

33.6 20.3 17.8 16.5 15.9

46.4 28.1 22.9 18.8 17.6

0.68 1.14 1.62 2.20 3.25

0.16 0.29 0.90 1.62 2.46

0.083 0.27 1.04 1.78 2.98

establishment of fast equilibrium distribution between bulk feed and the upstream surface of a membrane, the diffusion step controls the transport of penetrants.42,46 Therefore, it is of course important to estimate the diffusion coefficient of the penetrating molecules to understand the mechanism of molecular transport. From Fick’s law of diffusion, the diffusion flux can be expressed as:47 Ji ) -Di

dCi dx

(5)

where J is the permeation flux per unit area (kg/m s), D is the diffusion coefficient (m2/ s), C is the concentration of permeant (kg/m3), subscript i stands for water or IPA, and x is the diffusion length (m). For simplicity, it is assumed that the concentration profile along the diffusion length is linear. Thus, Di can be calculated with the following equation:48 Jiδ Ci

30 40 50

4.87 6.67 9.04 11.17 15.24 241 1315 1906 2491 4728 6.59 8.05 10.27 12.34 16.37 207 717 882 948 1756 8.78 10.18 12.35 14.38 18.33 129 424 455 505 980

out since the experiments were performed well below the glass transition temperature of chitosan. Therefore, the viscosity of permeating molecules played a vital role in controlling the transport of selective permeants through the membrane. This in turn results in an increase of total permeation flux, while suppressing the selectivity. Thus, the temperature dependence of permeation and diffusion has prompted us to estimate the activation energies for permeation and diffusion using the Arrhenius type equation:45

( )

X ) Xo exp 2

Di )

J × 102 (kg/(m2 h)) Rsep temp °C M-1 M-2 M-3 M-4 M-5 M-1 M-2 M-3 M-4 M-5

(6)

where δ is the membrane thickness. The calculated values of Di at 30 °C are presented in Table 1. Similar to PV study, the diffusion coefficients of water increased significantly from membrane M-1 to M-5, while the diffusion coefficients of IPA were suppressed. This further confirms that the membranes developed in the present study have remarkable separation ability with respect to water-IPA mixtures. As discussed above, this was attributed to increased selective adsorption due to the establishment of superhydrophilicity and decreased semicrystalline region due to cross-links by the incorporation of TiO2 in the membrane matrix. However, it is found that there was a considerable decrease in diffusion coefficients for all the membranes when the amount of water increased in the feed. This is expected, because of the observed deterioration of membrane’s selectivity as discussed in the PV study. Nevertheless, the magnitude of the diffusion coefficients of water was quite high in comparison with that of IPA, suggesting that the membranes developed in this study are still selective toward water molecules even at higher concentrations of water in the feed. 3.6. Effect of Temperature on Membrane Performance. To study the effect of operating temperature on the performance of the membranes, pervaporation experiments were carried out at different temperatures for water-IPA mixtures at 10 mass % of water in the feed and resulting values are presented in Table 2. It is observed that the permeation rate was increased from 30 to 50 °C for all the membranes, while the separation selectivity decreased remarkably. Generally, this happens because of two major reasons. First, as the temperature increases the viscosity of the permeating molecules decreases due to decrease of cohesive forces between the permeants. Second, an increase of thermal energy intensifies the motions of polymer chain segments, creating more free-volume in the polymer matrix. However in the present study, the latter is ruled

-Ex RT

(7)

where X represents permeation (J) or diffusion (D), Xo is a constant representing pre-exponential factor of Jo or Do, Ex represents the activation energy for permeation or diffusion depending upon the transport process under consideration, and RT is the usual energy term. As the feed temperature increases, the vapor pressure in the feed compartment also increases, but the vapor pressure at the permeate side is not affected. This results in an increase of driving force with increasing the temperature. Arrhenius plots of log J and log D versus temperature are shown in Figures 13 and 14, respectively. In both the cases, linear behavior was observed, suggesting that permeability and diffusivity follow an Arrhenius trend. From the least-squares fits of these linear plots, the activation energies for total permeation (Ep) and total diffusion (ED) were estimated. Similarly, we have also estimated the activation energies for permeation of water (Epw) and isopropanol (EpIPA), and diffusion of water (EDw), but the plots are not given to avoid the overcrowding. The values thus obtained are presented in Table 3. From Table 3, it can be seen that pure chitosan membrane (M-1) exhibits higher Ep and ED values compared to those of

Figure 13. Variation of log J with temperature for pure chitosan and its TiO2-incorporated hybrid membranes at 10 mass % of water in the feed.

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that Langmuir’s sorption is predominant, giving an exothermic contribution. 4. Conclusions

Figure 14. Variation of log D with temperature for pure chitosan and its TiO2-incorporated hybrid membranes at 10 mass % of water in the feed. Table 3. Arrhenius Activation Parameters for Permeation and Diffusion, and Heat of Sorption parameters (kJ/mol)

M-1

M-2

M-3

M-4

M-5

Ep ED Epw EpIPA EDw ∆Hs

23.99 23.78 22.74 48.17 23.24 -0.50

17.58 17.48 16.99 63.22 17.49 -0.49

12.66 12.83 12.05 70.50 12.50 -0.44

10.26 10.24 9.69 74.85 10.12 -0.42

7.51 7.72 7.21 71.44 7.65 -0.43

TiO2-incorporated hybrid membranes (M-2 to M-5). This suggests that both permeation and diffusion processes require more energy for transport of molecules through the pure chitosan membrane because of its dense nature. Obviously, TiO2incorporated membranes consumed less energy because of decreased intersegmental chains and superhydrophilicity. This has resulted markedly from membrane M-2 to M-5 with increasing the content of TiO2. Although, Ep values are slightly lower than those of ED values in all the membranes, the difference is insignificant, suggesting that both sorption and diffusion contribute equally to the PV process. The same trend is also observed for Epw and EDw values. However, a significant difference was noticed between the apparent activation energy values of water (Epw) and isopropanol (EpIPA), and the difference increased correspondingly with increasing the TiO2 content. This further suggests that membranes incorporated with higher amount of TiO2 demonstrated greater separation efficiency toward water. The Epw and EDw values ranged between 7.51 and 23.99, and 7.72 and 23.78 kJ/mol, respectively. Using these values, we have further calculated the heat of sorption as ∆Hs ) Ep - ED

(8)

The resulting ∆Hs values are included in Table 3. The ∆Hs values give additional information about the transport of molecules through the polymer matrix. It is a composite parameter involving contributions of both Henry’s and Langmuir’s types of sorption.49 The Henry’s type of sorption requires both the formation of a site and the dissolution of chemical species into that site. The formation of a site involves an endothermic contribution to the sorption process. However, Langmuir’s type of sorption requires the pre-existence of a site in which sorption occurs by a hole filling mechanism, giving an exothermic contribution. The ∆Hs values obtained in the present study are negative for all the membranes, suggesting

In this study, chitosan based TiO2-incorporated hybrid membranes were prepared using a solution technique. These membranes were employed for the separation of water-IPA mixtures at 30, 40, and 50 °C. An increase of TiO2 content in the membrane matrix results to a simultaneous increase of both permeation flux and selectivity. This was explained on the basis of superhydrophilicity, and decreased semicrystalline region due to establishment of cross-links by the incorporation of TiO2 in the membrane matrix. While assessing the membranes’ efficiency, it is found that both total flux and flux of water are indistinguishable particularly for TiO2-incorporated hybrid membranes, suggesting that the membranes developed in the present study are highly selective toward water. The PV separation index data also support that the membrane with higher amount of TiO2 demonstrated an excellent PV performance. The membrane containing 40 mass % of TiO2 exhibited the highest separation selectivity of 94 984 with a flux of 12.17 × 10-2 kg/(m2 h) at 30 °C for 5 mass % of water in the feed. With regard to temperature, the permeation rate was increased while suppressing the selectivity when temperature was increased. This was attributed to decreased interaction between permeants, permeant and membrane at higher temperature. The Ep and ED values ranged between 7.51 and 23.99, and 7.72 and 23.78 kJ/mol, respectively. The TiO2-incorporated hybrid membranes exhibited significantly lower activation energy values compared to that of a pure chitosan membrane, indicating that the permeants consumed less energy during the process as a consequence of superhydrophilicity and decreased semicrystalline part, attributed to the presence of TiO2. The membranes showed extremely lower activation energy values for water permeation (Epw) than that of IPA permeation (EpIPA), suggesting that membranes developed here have higher separation ability toward water. All the membranes exhibited negative ∆Hs values, indicating that sorption is mainly dominated by the Langmuir’s mode of sorption, giving an exothermic contribution. Acknowledgment J.G.V. wishes to acknowledge the UGC, New Delhi for awarding the Research Fellowship under meritorious category. The authors sincerely thank the Department of Physics, Indian Institute of Science, Bangalore for extending wide-angle X-ray diffraction facility. Nomenclature Mw ) molecular weight A ) effective membrane area (m2) DS ) degree of swelling (%) Do ) pre-exponential factor for diffusion ED ) activation energy for diffusion (kJ/mol) EDw ) activation energy for diffusion of water (kJ/mol) Ep ) activation energy for permeation (kJ/mol) Epw ) activation energy for permeation of water (kJ/mol) EpIPA ) activation energy for permeation of IPA (kJ/mol) Ex ) activation energy for permeation or diffusion (kJ/mol) ∆Hs ) heat of sorption (kJ/mol) IPA ) isopropanol J ) total flux (kg/(m2 h))

4012 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Jo ) pre-exponential factor for permeation PSI ) pervaporation separation index P and F ) mass percent of permeate and feed R ) gas constant t ) permeation time (h) T ) temperature (K) W ) mass of permeate (kg) Ws and Wd ) mass of the swollen and dry membranes Greek letter δ ) membrane thickness (40 µm) Rsep ) separation factor

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ReceiVed for reView November 1, 2008 ReVised manuscript receiVed January 12, 2009 Accepted March 2, 2009 IE8016626