Synthesis and characterization of hybrid membranes using chitosan and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane for pervaporation dehydration of isopropanol

Journal of Membrane Science 441 (2013) 83–92

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Synthesis and characterization of hybrid membranes using chitosan and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane for pervaporation dehydration of isopropanol P.S. Rachipudi a, A.A. Kittur b, A.M. Sajjan a, M.Y. Kariduraganavar a,n a b

Department of Chemistry, Karnatak University, Dharwad 580 003, India Department of Chemistry, SDM College of Engineering & Technology, Dharwad 580 002, India

art ic l e i nf o

a b s t r a c t

Article history: Received 5 May 2012 Received in revised form 13 February 2013 Accepted 17 March 2013 Available online 2 April 2013

Chitosan based hybrid membranes were prepared by incorporating 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (ETMS) into chitosan matrix using a sol–gel technique. The resulting membranes were studied using Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). After measuring the swelling data at different mass% of water, membranes were employed for pervaporation separation of water–isopropanol mixtures in a temperature range of 30–50 1C. The experimental results demonstrated that membrane containing 40 mass% of ETMS showed the highest separation selectivity of 17,990 with a flux of 2.92
10−2 kg/m2h at 30 1C for 10 mass% of water. The values of total flux and flux of water are found to be overlapping particularly for hybrid membranes, suggesting that the developed hybrid membranes could be effectively used 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) are two times lower than those of isopropanol permeation (EPIPA), suggesting that the developed membranes have higher separation efficiency for water–isopropanol system. The estimated Ep and ED values were ranged between 21.64 to 33.26, and 22.62 to 33.49 kJ/mol, respectively. The negative heat of sorption (ΔHS) values was observed in all the hybrid membranes, indicating that Langmuir's mode of sorption is predominant. & 2013 Elsevier B.V. All rights reserved.

Keywords: Chitosan 2-(3,4-Epoxycyclohexyl) ethyltrimethoxysilane Pervaporation Selectivity Activation energy

1. Introduction Membrane separation processes have been offered many advantages over the existing separation processes. Pervaporation (PV) is one such type of membrane separation processes with a wide range of uses such as solvent dehydration and separation of aqueous organic mixtures. It has significant advantages particularly in azeotropic systems where traditional distillation is only able to recover pure solvents with the use of entrainers. The removal of entrainers takes additional separation step and tedious as well [1,2]. Therefore, PV is accepted as environment-benign and energy-saving technology, and has become a promising alternative to conventional technologies in separation of liquid mixtures due to its high separation efficiency coupled with energy savings [3]. The dehydration of isopropanol (IPA) has received a great attention from industries due to the escalation cost of isopropanol

n

Corresponding author. Tel.: +91 836 2215286x23; fax: +91 836 2771275. E-mail address: [email protected] (M.Y. Kariduraganavar).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.03.055

and its importance in various industries. It is one of the important solvents used on a large scale in chemical industries as well as in pharmaceutical laboratories. It is generally used as solvent for oils and gums, as a cleaning agent in the semi-conductor and liquid crystal display industries. However, IPA forms an azeotrope at 12.4 mass% of water and hence, the separation of water-IPA mixture by conventional methods such as solvent extraction and rotavapor or by distillation could prove uneconomical [4–6]. To overcome this, polymeric materials are overwhelmingly explored as PV membranes because of easy fabrication, reasonable separation performance and low cost. Based on the solution-diffusion model, a highly selective PV membrane could be developed if membrane possesses high selective sorption and diffusion. As far as dehydration of organic solvent is concerned, the ideal materials are the long chain rigid-network polymers, which have active groups of establishing some interactions with water (ion-dipole or hydrogen bonding) [7]. In the light of this, several hydrophilic polymers such as poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA) and polysaccharides like hydroxyl ethyl cellulose (HEC), sodium alginate and chitosan [8–11], demonstrated strong affinity towards water.

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Among them, chitosan [poly-β (1-4)-D-glucosamine] has attracted a great interest as a basic membrane material because of its natural occurrence, high abundance, hydrophilicity, chemical resistance, good membrane forming properties, functional groups that can be easily modified, and ease of processing [12]. However, membranes developed using this type of materials posed a lack of mechanical strength and chemical stability in aqueous solutions mainly due to excessive swelling. This could be overcome by introducing crosslinks in the membrane matrix [13], blending chitosan with other polymers [14,15], casting chitosan on another polymer substrate to form composite membrane [16,17], incorporation of selective zeolite into membrane matrix [18] and the development of organic–inorganic hybrid membranes [19,20]. Among these, organic–inorganic hybrids, especially silica based, are an emerging class of innovative nano-structured materials with tailored properties and unparallel performances suitable for wide range of practical applications [21–23]. This is mainly because; silica-hybrid materials have synergistic effects of stability of inorganic material and film forming ability of a polymer. Therefore, these materials offer consistent and unique opportunities to obtain the specific transport properties by combining organic and inorganic materials, so that membranes with high selectivity and good flux can be achieved [24,25]. Keeping this in mind, we have prepared novel organic–inorganic hybrid membranes via hydrolysis of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane followed by condensation with chitosan using a sol–gel reaction. The 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane is used as a coupling agent and its crosslinking with chitosan chains can effectively reduce the degree of swelling of chitosan and expected to increase the water permselectivity. The physico-chemical properties of the resulting membranes were studied by Fourier transform infrared, wide-angle X-ray diffraction, differential scanning calorimetry and thermogravimetric analysis. The membranes were successfully employed for PV separation of water-isopropanol system at different compositions. The values of permeation flux and separation selectivity were determined. The diffusion coefficient and Arrhenius activation parameters were also estimated. The results were discussed in terms of PV separation efficiency of membranes.

2. Experimental section 2.1. Materials Chitosan (M w  200,000; N-deacetylation degree 75–85%) and 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane (ETMS) were purchased from Sigma-Aldrich Chemicals, USA. Isopropanol and acetic acid were procured 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.

48 h, the membrane was peeled-off and the membrane thus obtained is designated as M-1. To prepare hybrid membrane, a known amount of ETMS and 1 ml conc. HCl were added into a homogenous chitosan solution and stirred for 8 h at room temperature. Before reaction proceeds, added HCl helps to hydrolyze the ETMS. The resulting solution was poured onto a glass plate and the rest of the procedure was followed as similar to M-1. The amount of ETMS with respect to chitosan was varied as 10, 20 and 40 mass%, and the membranes thus obtained were designated as M-2, M-3 and M-4, respectively. We have also prepared membranes with higher mass% of ETMS, but they failed to withstand pervaporation due to their brittle nature and hence, we have restricted only up to 40 mass% of ETMS. The thickness of the membranes was measured at different points using Peacock dial thickness gauge (Model G, Ozaki MFG Co. Ltd., Japan) with an accuracy of 72 μm and it was found to be 4072 μm. 2.3. Membrane characterization 2.3.1. Fourier transform infrared (FTIR) spectroscopy The crosslinking reaction between chitosan and ETMS, was confirmed by FTIR spectrometer (Nicolet, Impact-410, USA). Membrane samples were ground well to make KBr pellets under hydraulic pressure of 400 kg/cm2 and spectra were recorded in the range of 500–4000 cm−1. In each scan, the amount of membrane sample and KBr were kept constant in order to know the changes in the intensities of the characteristic peaks with respect to the amount of ETMS. 2.3.2. Wide-angle X-ray diffraction (WXRD) The crystallinity of pure chitosan and its hybrid membranes was studied at room temperature using a Brucker's D-8 advanced wide-angle X-ray diffractometer. The X-ray source was Ni-filtered CuKα radiation (40 kV, 30 mA). The dried membrane of uniform thickness (40 72 μm) was mounted on a sample holder and the pattern was recorded in the reflection mode at an angle 2θ over a range of 5–451 at a speed of 8 deg/min. 2.3.3. Differential scanning calorimetry (DSC) Thermal properties of chitosan and its hybrid membranes were studied using a differential scanning calorimeter (DSCQ 20, TA Instruments, Waters LLC, New Castle, Delawave, USA). Sample weights ranged from 9–10 mg and heated from ambient to 400 1C at a heating rate of 10 1C/min. 2.3.4. Thermogravimetric analysis Thermal stability of the membranes was investigated using a thermogravimetric analyzer (SDTQ 600, TA Instruments Waters LLC, New Castle, Delawave, USA). The sample weights ranged from 9–10 mg were heated from ambient to 650 1C at a heating rate of 10 1C/min.

2.2. Membrane preparation 2.4. Swelling measurements Chitosan (2.5 g) was dissolved in 100 ml of deaerated-distilled water containing 2% of acetic acid with a constant stirring for about 24 h at room temperature. The solution was filtered through a fritted glass disc filter to remove the undissolved residue particles. It was then kept in an ultrasonic bath at a fixed frequency of 38 kHz (Grant XB6, UK) for 30 min so as to break the possible aggregated molecules and also to remove the air bubbles. The resulting homogeneous solution was then spread onto a glass plate with the aid of a casting knife in dust-free atmosphere. After being dried at ambient temperature for about

The equilibrium sorption experiments were performed in different compositions of water–isopropanol mixtures using an electronically controlled oven (WTB Binder, Germany). The masses of the dry membranes were first determined and kept for equilibrium by soaking in different compositions of the feed mixtures in sealed vessels at 30 1C for 24 h. The swollen membranes were weighed as quickly as possible after careful blotting on a digital microbalance (Mettler, B204-S, Toledo, Switzerland) within an accuracy of7 0.01 mg. The percent degree of swelling

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Fig. 1. (A). Schematic representation of pervaporation apparatus: (1) pervaporation cell; (2) vacuum control valves; (3) permeate cold trap; (4) moisture cold trap; (5) pressure sensor; (6) vacuum pump. and (B). Schematic diagram of pervaporation cell: (1) water inlet; (2) water outlet; (3) feed inlet; (4) thermometer jacket; (5) stirrer; (6) O-ring; (7) membrane; (8) sintered disk; (9) permeate outlet.

(DS) was calculated as:   W s −W d
100 DS ð%Þ ¼ Wd

ð1Þ

where Ws and Wd are the masses of the swollen and dry membranes, respectively. 2.5. Pervaporation experiments PV experiments were performed using the in-house designed apparatus illustrated in Fig. 1(A and B). 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 apparatus was maintained [1.333224
103 Pa (10 Torr)] using a two-stage vacuum pump (Toshniwal, Chennai, India). The water composition in the feed mixture was varied from 10 to 30 mass%. While maintaining the temperature, the test membrane was allowed to equilibrate for about 2 h in the feed compartment with a known volume of feed mixture before performing the PV experiment. After a steady state was attained, the permeate was collected in a trap immersed in the liquid nitrogen jar on the downstream side at fixed intervals of time. The experiments were carried out at 30, 40 and 50 1C. The flux was calculated by weighing the collected permeate on a digital microbalance. The composition of water and isopropanol in the

permeate was estimated by measuring the refractive index of the permeate within an accuracy of7 0.0001 units using an Abbe's refractometer (Atago-3 T, Japan), and by comparing it with a standard graph that was established previously with the known compositions of water-isopropanol mixtures. All the experiments were performed at least three times, and the results were averaged. The results of permeation for water–isopropanol mixtures during the pervaporation were reproducible within an admissible range. From the PV data, separation performance of the membranes was assessed in terms of total flux (J), separation selectivity (α) and pervaporation separation index (PSI). These were respectively calculated using the following equations: J¼

W At

ð2Þ

α¼

P w =P IPA F w =F IPA

ð3Þ

PSI ¼ Jðα−1Þ

ð4Þ

where W is the mass of permeate (kg); A, the effective membrane area (m2); t, the permeation time (h); Pw and PIPA are the mass percent of water and isopropanol in the permeate, respectively; Fw and FIPA are the respective mass percent of water and isopropanol in the feed.

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3. Results and discussion 3.1. Synthesis of hybrid membranes The hydrolysis of ETMS and its possible reaction with chitosan are respectively illustrated in Fig. 2(A and B). From the Fig. 2A it is noticed that trimethoxy groups of ETMS underwent hydrolysis in the presence of acid catalyst, and yielding silanol groups. These hydroxyl groups were condensed with hydroxyl groups of chitosan. Similarly, under the same acidic condition, the epoxy rings of ETMS were opened and the resulting –OH groups subsequently reacted with hydroxyl groups of chitosan to form C–O–C bonds (Fig. 2B). The reaction possibility of –OH groups of silanol or terminated by epoxy groups with –NH2 groups of chitosan is ruled out since all the –NH2 groups were converted into –NH3+ by the addition of acetic acid. During the formation of siloxanes, the co-condensation reaction was occurred between the silanols and/ or hydroxyls by dehydration. The flexible –OH groups arising from ETMS could bridge between the silica particles and chitosan chain, thereby making the hybrid matrix more rigid and compact.

3.2. Membrane characterization 3.2.1. FTIR studies The chemical structure of the chitosan–siloxane hybrid membranes was analyzed by FTIR spectrometer and the spectra thus obtained are presented in Fig. 3. A characteristic strong and broad band appeared at around 3400 cm−1 in all the membranes corresponds to O–H stretching vibrations of the hydroxyl groups. The bands appeared at around 1650 and 1570 cm−1 are respectively assigned to amide I (CQO) and amide II (NH) functional groups of chitosan [26,27]. The multiple bands appeared between 900 and 1200 cm−1 were assigned to C–O stretchings. Upon condensation of silanol or epoxy terminated –OH groups with chitosan matrix, the intensity of C–O stretching was increased and this has become predominant with increasing the amount of ETMS as clearly demonstrated from the spectra of M-2 to M-4. This is

Fig. 3. FTIR spectra of pure chitosan and its crosslinked membranes: (M-1) 0 mass %; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

Fig.4. Wide-angle X-ray diffraction patterns of pure chitosan and its crosslinked membranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

Fig. 2. (A and B). Synthesis of chitosan-based hybrid membrane.

expected due to Si–O stretching of silanol as it overlaps at the same frequency of C–O [28]. In addition to this, the –OH band appeared at around 3400 cm−1 was also correspondingly shifted to lower frequency with increasing the content of ETMS, which supports the enhanced hydrogen bonding occurring between ETMS and the chitosan. All these evidences confirm the incorporation of silanol and its interaction with the chitosan.

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3.2.2. WAXD studies To study the effect of crosslinking on the membrane morphology of chitosan, X-ray diffraction was employed and the patterns thus obtained for the chitosan and its hybrid membranes are presented in Fig. 4. The diffraction pattern of pure chitosan membrane exhibits two sharp peaks at around 2θ¼10 and 151, and a broad peak at around 211. The sharp peaks correspond to semicrystalline part while a broad peak corresponds to amorphous part [29]. Upon increasing the content of ETMS, the intensity of broad peak was decreased gradually owing to the formation of crosslinks in the membrane matrix. This becomes responsible for decreasing the crystallinity. Further, it is observed that a broad peak appeared at 2θ¼211 was shifted towards higher degree with increasing the content of ETMS and this is very well supported by the calculated d-spacing values, which were decreased from 2.14 Å to 1.93 Å. The decreased d-spacing values give a clear evidence of shrinkage in cell size or inter-segmental spacing, which would improve the selective permeation of water through the hybrid membranes. 3.2.3. DSC studies Any change in physical properties in the polymer membranes due to crosslinking can be reflected in glass transition temperature (Tg) and melting temperature (Tm). This has prompted us to measure the DSC of pure chitosan and its hybrid membranes at a heating rate of 10 1C/min in nitrogen atmosphere and the resulting thermograms are illustrated in Fig. 5. The DSC thermogram of virgin chitosan membrane shows a Tg at 70.1 1C. It is evident from the thermograms that Tg values of hybrid membranes were increased with increasing the ETMS content in the membrane. This is due to increased crosslinking density by the incorporation of ETMS content, resulting to decreased free-volume in the membrane matrix. This is clearly reflected in the d-spacing values, which are calculated from the WAXD data. The DSC patterns also reveal about the melting temperature Tm, which were gradually shifted from 205 to 181 1C by the incorporation of ETMS into a chitosan matrix. The shifts in melting temperature are

Fig. 5. Differential scanning calorimetry patterns of pure chitosan and its crosslinked membranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

87

ascribed to reduce the crystallinity of chitosan through crosslinking reaction with ETMS, which breaks the molecular symmetry of chitosan matrix to marginally. Thus, by incorporation of ETMS content into a chitosan matrix, the mechanical property of chitosan was enhanced while suppressing the crystallinity and which are responsible for increasing the overall PV performance. 3.2.4. TGA studies The thermal stability and degradation behavior of chitosan– siloxane hybrid membranes were investigated under nitrogen atmosphere using TGA and the resulting thermograms are displayed in Fig. 6. It is observed that all the membranes underwent decomposition in two stages. The first stage of decomposition occurred between ambient and 200 1C is accounted to around 12 to 16% weight loss, corresponding to dehydration of membranes. The second stage of decomposition which starts from around 200 to 550 1C was attributed to a weight loss due to the decomposition of polymeric network. When we compared the TGA patterns of all the membranes, it is clear that the weight loss of hybrid membranes at the first stage was correspondingly decreased with increasing the content of ETMS. Similarly, the weight loss of hybrid membrane at the second stage was drastically reduced. This suggests that water retention capacity of the hybrid membranes was significantly affected by the crosslinks. This is clearly reflected in both swelling and pervaporation study. This signifies that incorporation of crosslinks was highly influenced on the thermal stability of the chitosan matrix. 3.3. Effects of feed composition and ETMS content on membrane swelling The characteristics of the sorption (swelling) mechanism depend on the structure of the membrane, the affinity of the permeants towards membrane and the mutual interaction between the permeants [30–32]. In PV process, sorption property of a membrane plays a key role in achieving good separation performance, which is generally evaluated by studying the membrane swelling in different feed mixtures. Fig. 7 shows the swelling behavior of all the membranes in different mass% of water–IPA mixtures at 30 1C. It is observed that the degree of swelling was increased remarkably for all the membranes with increasing the mass% of water in the feed. This is because of increased interaction

Fig. 6. Thermogravimetric analysis of pure chitosan and its crosslinked membranes: (M-1) 0 mass%; (M-2) 10 mass%; (M-3) 20 mass%; (M-4) 40 mass%; of 2-(3,4-epoxycyclohexyl) ethyltrimethoxysilane content.

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Fig. 7. Variation of degree of swelling with different mass% of water in the feed for all the membranes.

Fig. 8. Variation of total pervaporation flux with different mass% of water in the feed for all the membranes.

between water molecules and the membrane, owing to the presence of large number of hydrophilic groups (–NH3+ and –OH) in the membrane matrix. This is expected since water being more polar than IPA, which preferentially interacts with membrane resulting to increased degree of swelling. On the contrary, the degree of swelling was decreased with increasing the content of ETMS (M-1 to M-4), and this trend remains same throughout the investigated feed compositions. This is mainly because of increased crosslinks between siloxane and chitosan matrix, which minimize the free-volume in the membrane matrix. Secondly, the establishment of hydrogen bonds also responsible for decreasing the membrane swelling as it was increased owing to the increase of silanol and epoxy terminated –OH groups in the membrane. This was clearly demonstrated in both WAXD and TGA patterns. 3.4. Effects of feed composition and ETMS content on pervaporation Fig. 8 illustrates the effects of feed composition and ETMS content on the total permeation flux for all the membranes at 30 1C. It is observed that total permeation flux was increased for all the membranes with increasing the amount of water in the feed. This is more predominant at higher composition of water. The increased flux with increasing mass% of water in the feed is not merely dependent on the hydrophilic groups that are present in the membrane, but also on the increased chemical potential. This is expected since the composition of water increases the vapor pressure, and hence the driving force due to chemical potential. As a result of both interactive groups and chemical potential, the permeation flux was increased with increasing the water composition in the feed. On the other hand, the permeation flux was decreased as expected from membrane M-1 to M-4. This is due to increased crosslinking and hydrogen bonding as discussed in swelling study. In order to see the extent of permeation of individual components, we have plotted the individual fluxes as a function of mass% of ETMS at 10 mass% of water in the feed as shown in Fig. 9. From the plot, it is clearly noticed that except for pure chitosan membrane (M-1) the values of total flux and water flux are almost overlapping each other while suppressing the flux of IPA, signifying that the crosslinked hybrid membranes developed in the present study are highly water selective. The overall selectivity of a membrane in PV process is generally described on the basis of interaction between membrane and the permeating molecules, molecular size of the permeating species and

Fig. 9. Variation of total flux, and fluxes of water and IPA with 10 mass% of water in the feed for all the membranes.

Fig. 10. Variation of selectivity with different mass% of water in the feed for all the membranes.

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pore diameter of the membrane. Fig. 10 displays the effects of water composition and ETMS content on the selectivity of all the membranes. It is observed that the selectivity was decreased almost exponentially for all the membranes with increasing the water concentration in the feed. This is because, at higher concentration of water in the feed a small amount of water dissolves in the membrane matrix, which in turn acts as plasticizer for the membrane leading to a greater flexibility of polymeric chains in the matrix. As a consequence of this, the swollen and plasticized upstream surface of a membrane allows some IPA molecules along with the selective permeants, leading to decreased selectivity. However, the selectivity was dramatically increased with increasing the content of ETMS as can be seen from membrane M-1 to M-4. The decrease in free-volume due to an increase of crosslinking and hydrogen bonding increases the rigidity and compactness of the membrane, which becomes responsible for the increase of membrane's selectivity as the content of ETMS increases. This is further demonstrated from the Fig. 11, in which flux and selectivity were plotted as a function of ETMS content in the membranes at 10 mass% of water in the feed. Generally, as the packing density of the membranes increases either due to increase of crosslinks or due to the incorporation of fillers into the membrane matrix, the permeation flux decreases and selectivity increases [33,34]. Obviously, the same trend was observed due to the existence of a trade-off phenomenon in PV process.

Fig. 12. Variation of pervaporation separation index with 10 mass% of water in the feed for all the membranes.

Table 1 Diffusion coefficients of water and IPA at different mass% of water in the feed for different membranes. Mass % of water

3.5. Pervaporation separation index The pervaporation separation index is the product of total permeation flux and separation factor, which determines the membrane separation ability. To implement the PV process, this index is being used as a relative guideline for choosing the suitable membranes based on the optimal combination of flux and selectivity. To demonstrate this, the resulting PSI values were plotted as a function of mass% of ETMS at 30 1C for 10 mass% of water in the feed (Fig. 12). It shows that all the hybrid membranes have shown higher PSI values than that of a pure chitosan membrane M-1. This is because of remarkable selectivity demonstrated by the hybrid membranes. Among the hybrid membranes, membrane containing 40 mass% of ETMS exhibited an excellent overall PV performance.

89

10 15 20 25 30

Dw
108 (cm2/s)

DIPA
1010 (cm2/s)

M-1

M-2

M-3

M-4

M-1

M-2

M-3

M-4

11.4 11.3 11.2 10.7 10.3

8.40 9.16 9.83 9.04 8.31

7.20 7.93 8.33 7.84 7.92

5.90 6.33 6.06 6.17 5.73

0.79 1.10 1.49 2.11 4.36

0.30 0.53 0.96 1.49 2.59

0.21 0.41 0.54 1.03 1.57

0.10 0.22 0.40 0.68 0.65

3.6. Diffusion coefficient The mass transport of binary liquid molecules through a polymer matrix is generally explained by a solution-diffusion mechanism, which occurs in three steps: sorption, diffusion and evaporation [35]. 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 establishment of fast equilibrium distribution between bulk feed and the upstream surface of a membrane, the diffusion step controls the transport of permeants [33,36]. To understand the mechanism of molecular transport, it is therefore important to estimate the diffusion coefficient, Di of permeating molecules. From Fick's law of diffusion, the diffusion flux can be expressed as [37]: J i ¼ −Di

dC i dx

ð5Þ

where J is the permeation flux per unit area, D is the diffusion coefficient, C is the concentration of permeant, subscript i stands for water or isopropanol, and x is the diffusion length. For simplicity, it is assumed that the concentration profile along the diffusion length is linear. Thus, diffusion coefficient, Di can be calculated with the following modified equation [38] Di ¼

Fig. 11. Variation of flux and selectivity with 10 mass% of water in the feed for all the membranes.

Jiδ Ci

ð6Þ

where δ is the membrane thickness. The calculated values of Di at 30 1C are presented in Table 1. Similar to PV study, the diffusion coefficients of both water and IPA molecules were decreased from membrane M-1 to M-4 at all feed compositions. This is because of

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Table 2 Pervaporation flux and separation selectivity at different temperatures for different membranes at 10 mass% of water in the feed. Temp. 1C

30 40 50

J
102 (kg/m2h)

αsep

M-1

M-2

M-3

M-4

M-1

M-2

M-3

M-4

5.60 6.20 7.80

4.14 5.03 7.39

3.55 4.04 6.88

2.92 3.33 6.11

4727 4491 4081

8991 8172 6419

11,241 10,201 8991

17,990 12,847 9991

Fig. 14. Variation of log D with temperature for all the membranes at 10 mass% of water in the feed.

Table 3 Arrhenius activation parameters for permeation and diffusion, and heat of sorption.

Fig. 13. Variation of log J with temperature for all the membranes at 10 mass% of water in the feed.

the establishment of crosslinking and hydrogen bonding in the membrane matrix, which results to decreased free-volume and increased both rigidity and compactness. On the other hand, as we increased the water concentration in the feed for all the membranes, the diffusion coefficients of IPA were increased, but no systematic trend was observed for diffusion of water. Probably, this might be due to hindrance caused by the IPA molecules during the diffusion. In spite of this, the magnitude of diffusion coefficients for water is quite high in comparison with IPA, suggesting that the hybrid membranes developed in the present study have higher separation selectivity towards water even at higher concentration of water in the feed. 3.7. Effect of temperature The effect of operating temperature on the pervaporation performance for water–isopropanol mixtures was studied for all the membranes at 10 mass% of water in the feed and the values thus obtained are presented in Table 2. As we increase the temperature from 30 to 50 1C, the permeation flux was increased while decreasing the separation factor. This was explained on the basis of increased driving force and decreased viscosity of permeating molecules. As the temperature increases the vapor pressure at the feed side increases, but the pressure at the permeate side is not affected. This led to increase the driving force for the mass transport across the membrane. In addition, viscosity of the feed solution decreases with increasing the temperature. All these led to easier transportation of molecules and thereby increasing the permeation flux. Secondly, an increase of temperature decreases the interaction between membrane and the permeants, and thereby membrane allows IPA molecules along with selective water molecules, resulting to a decreased selectivity. This effect prompted us to estimate the activation energies for

Parameters (kJ/mol)

M-1

M-2

M-3

M-4

EP ED EPw EPIPA EDw ΔHs

21.64 22.62 21.01 43.16 22.62 −1.02

23.07 23.79 23.43 51.43 23.89 −0.72

28.77 29.13 29.70 58.63 30.24 −0.36

33.26 33.49 33.61 66.18 34.88 −0.23

permeation and diffusion using the Arrhenius type equation [35]:   −Ex X ¼ X o exp ð7Þ RT where X represents permeation (J), or diffusion (D). Xo is a constant representing pre-exponential factor of Jo or Do. Ex represents activation energy for permeation or diffusion depending upon the transport process under consideration, and RT is the usual energy term. Arrhenius plots of log J and log D versus 1/T are shown in Figs. 13 and 14, for the temperature dependence of permeation flux and diffusion, respectively. In both the cases, a 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 crowdness. The values thus obtained are presented in Table 3. From Table 3, it can be seen that the apparent activation energy values of water (EPw) are almost two times lower than those of isopropanol (EPIPA), suggesting that membranes have higher separation efficiency towards water. The activation energy values of water (EPw) and total permeation (EP) are almost close to each other, signifying that coupled–transport of both (water and isopropanol molecule) is minimal as due to higher selective nature of membranes. The activation energy values were increased from membrane M-1 to M-4, and this is expected due to increased rigidity and compactness in the membrane owing to increased crosslinking density and hydrogen bonding. The estimated Ep and ED values ranged between 21.64 and 33.26, and 22.62 and 33.49 kJ/mol,

P.S. Rachipudi et al. / Journal of Membrane Science 441 (2013) 83–92

respectively. Using these values, we have further estimated the heat of sorption as: ΔH s ¼ Ep −ED

ð8Þ

The resulting ΔHs values are included in Table 3. The ΔHs values give the additional information about the transport of molecules across the polymer membrane. It is a composite parameter involving the contributions of both Henry's and Langmuir's types of sorption [39]. 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 only by a hole filling mechanism, giving an exothermic contribution. The ΔHs values obtained in the present study are negative for all the membranes, suggesting that Langmuir's sorption is predominant, giving an exothermic contribution. 4. Conclusions Using a sol–gel technique, chitosan based hybrid membranes were developed by varying a silica precursor such as 2-(3,4epoxycyclohexyl) ethyltrimethoxysilane. The membranes were successfully employed to separate water–isopropanol mixtures at 30, 40 and 50 1C. The amount of ETMS added in the membrane improved the separation ability of the membranes. The membrane containing 40 mass% of ETMS showed the highest separation selectivity of 17,990 with a flux of 2.92
10−2 kg/m2h at 30 1C for 10 mass% of water in the feed. The PV separation index data also support that membrane with higher amount of ETMS demonstrated an excellent PV performance. Experimental data also reveal that the total flux and flux of water are overlapping each other particularly for the silica precursor incorporated membranes, suggesting that hybrid membranes developed here are highly selective towards water and this is, in accordance with the diffusion data as well. Temperature effect study shows that the permeation flux was increased while decreasing the selectivity when the temperature was increased. This is because of increased driving force and decreased selective interaction between permeants and the membrane. The membranes exhibited almost two times lower activation energy values for water permeation (EPw) than that of isopropanol permeation (EPIPA), again suggesting that membranes developed here have higher separation ability towards water. The estimated EP and ED values were ranged between 21.64 and 33.26, and 22.62 and 33.49 kJ/mol, respectively. All the membranes exhibited negative ΔHs values, indicating that sorption is mainly dominated by the Langmuir's mode of sorption, giving an exothermic contribution. Acknowledgments Authors wish to acknowledge the UGC, New Delhi, Major Research Project (F. no. 37-245/2009, SR) for providing the financial support. One of the authors (Padmeswary S. Rachipudi) wishes to acknowledge the UGC, New Delhi, for awarding the Research Fellowship under meritorious category. Authors sincerely thank the Department of Physics, Indian Institute of Science, Bangalore, for extending wide-angle X-ray diffraction facility.

Nomenclature Mw A DS

molecular weight effective membrane area (m2) degree of swelling (%)

91

Do ED EDw Ep Epw EDIPA Ex

pre-exponential factor for diffusion activation energy for diffusion (kJ/mol) activation energy for diffusion of water (kJ/mol) activation energy for permeation (kJ/mol) activation energy for permeation of water (kJ/mol) activation energy for diffusion of IPA (kJ/mol) activation energy for permeation or diffusion (kJ/ mol) ΔHs heat of sorption (kJ/mol) IPA isopropanol J total flux (kg/m2h) 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 letters δ αsep

membrane thickness (40 μm) separation factor

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