Preparation of novel composite membranes for the pervaporation separation of water–acetic acid mixtures

Journal of Membrane Science 285 (2006) 420–431

Preparation of novel composite membranes for the pervaporation separation of water–acetic acid mixtures Srikant S. Kulkarni, Subhashchandra M. Tambe, Arjumand A. Kittur, Mahadevappa Y. Kariduraganavar ∗ Department of Chemistry and Center of Excellence in Polymer Science, Karnatak University, Dharwad 580 003, India Received 7 July 2006; received in revised form 8 September 2006; accepted 14 September 2006 Available online 16 September 2006

Abstract Novel composite membranes were prepared involving “class I” and “class II” hybrid materials. The membranes obtained were characterized by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Membranes exhibited a remarkable increase in degree of swelling with increasing zeolite loading in “class II” hybrid material. The pervaporation performance of these membranes for the separation of water–acetic acid mixtures was investigated in terms of feed composition and zeolite loading. Both the permeation flux and selectivity increased simultaneously with increasing zeolite content in the membrane matrix. This was explained on the basis of enhancement of hydrophilicity, selective adsorption and the establishment of molecular sieving action. While assessing the membranes’ efficiency, it was noticed that both total flux and flux of water are overlapping each other, signifying that the membranes developed in the present study involving “class I” and “class II” are highly selective towards water. Among the membranes developed, the membrane containing 15 mass% of zeolite exhibited the highest separation selectivity of 2423 with a flux of 8.35 × 10−2 kg/m2 h for 10 mass% of water in the feed at 30 ◦ C. Comparison was also made to justify the efficiency of the developed membranes with respect to efficiency of the membranes reported in the literature. From the temperature dependent diffusion and permeation values, the Arrhenius activation parameters were estimated. The resulting activation energy values obtained for water diffusion being lower than those of acetic acid diffusion values, suggest that the composite membranes exhibit higher separation efficiency. The negative heat of sorption values (Hs ) obtained for all the membranes, suggesting that Langmuir’s type of sorption is predominant in the process. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(vinyl alcohol); NaY zeolite; Composite membranes; Pervaporation; Activation energy

1. Introduction Acetic acid is an important basic chemical in the chemical industry, ranking among the top 20 organic intermediates. The current processes for acetic acid production include the carbonylation of methanol, the liquid-phase oxidation of hydrocarbons and the oxidation of acetaldehyde. One process for reducing the cost of acetic acid is fermentation of biomass, forestry residues, municipal wastes and other byproducts [1]. The concentration of acetic acid obtained in this process is usually lower than 5 wt% [2,3]. Azeotropic distillation and extractive distillation have been developed for acetic acid recovery, but distillation is energy-intensive, because of the small differences in the volatil-

∗

Corresponding author. Tel.: +91 836 2215372; fax: +91 836 2771275. E-mail address: [email protected] (M.Y. Kariduraganavar).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.09.021

ities of water and acetic acid in dilute aqueous solution [4,5]. Exploring energy-saving and environmental-benign processes, such as pervaporation (PV) is thus urgently needed. As a novel and efficient separation technology, PV has been found wide applications in chemical industry. At present, majority of the large-scale PV units are employed for the dehydration of ethanol, isopropanol and other organic solvents [6–8]. Therefore, development of PV processes for acetic acid recovery, which may fully or partly substitute distillation processes, is quite promising. Many reports can be found in seeking robust membranes materials with higher permeability and permselectivity. Despite of concentrated efforts to innovate polymer type and tailor the polymer structure to improve separation properties, current polymeric membrane materials commonly suffer from the inherent drawback of trade-off effect between permeability and selectivity [9–11]. On the other hand, although some inorganic materials have shown rather good separation properties

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above the upper-bound trade-off curve, which was constructed on an empirical basis for many gas or liquid pairs using published permeability and selectivity data, their large-scale application is still seriously restricted due to poor processability and high capital cost. Therefore, it can be naturally envisaged that elaboration of hybrid membrane materials by bridging organic and inorganic components will be convenient and efficient approach to cross the trade-off hurdle [12]. Organic–inorganic hybrid materials are promising systems for many applications due to their extraordinary properties arising from the synergism between the properties of these two different building blocks. A general classification to organic–inorganic hybrid materials has been proposed [13–15], distinguishing “class I” materials, in which the inorganic and organic components interact through weak hydrogen bonding, van der Waals contacts, or electrostatic forces, from “class II” materials, in which the inorganic and organic components are linked through strong ionic/covalent bonding. Recently, these membranes have attracted considerable attention as potential “next generation” membrane materials [16–18]. Much research effort is being continued to explore the “class I” organic–inorganic membranes, which are prepared by simply incorporating inorganic particles, such as zeolite [19,20], carbon molecular sieve [21] and silica [17,22,23], into dense polymeric membranes to improve the molecular separation properties. Merkel et al. [17,23] prepared poly(4-methyl-2-pentyne) (PMP) membranes containing nanoscale, nanoporous fumed silica and found that contrary to behavior in traditional filled polymer systems, addition of fumed silica to glassy, amorphous PMP increases penetrant permeability by as much as 240%. They concluded that it was attributed to increased free-volume through disruption of polymer chain packing by inorganic filler. In a previous study [19], we prepared NaY zeolite filled sodium alginate membranes and increased both permeation flux and selectivity simultaneously. This was explained on the basis of enhancement of hydrophlicity, selective adsorption and molecular sieving action including reduction of pore size of the membrane matrix. However, most of the filled polymeric membranes failed to cross the upper-bound trade-off curve mainly due to agglomeration of inorganic particles and formation of nonselective voids, which usually exist at the interface of these two phases, since, the interaction between inorganic particles and polymer is of physical origin [24,25]. In recent years, “class II” materials prepared from tetraethylorthosilicate (TEOS) have also been reported [18,26–28] for PV separation. The silica network formed from self-condensation reaction of the hydrolyzed Si–OH groups. No doubt, these membranes exhibited high selectivity, but failed to give high flux. Similarly, Kusakabe et al. [29] prepared polyurethane (PU) membranes containing tetraethylorthosilicate and applied them to benzene/cyclohexane fractionation. They found that benzene/cyclohexane selectivity in the hybrid membranes was higher than that in the PU counterpart. However, the permeation flux was lower in the hybrid membranes. Liu et al. [30] prepared organic–inorganic hybrid membranes composed of chitosan and ␥-glycidyloxypropyltrimethoxysilane (GPTMS), which exhibited decreased permeability and enhanced selectivity.

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Table 1 Physicochemical properties of hydrophilic NaY zeolite [31,32] Counter ion SiO2 /Al2 O3 Density Pore size Pore volume Topology Nature

Na+ 2.6 1.27 g/ml 0.5–2.0 ␮m 0.47 ml/g FAU Hydrophilic

Based on the solution–diffusion theory and free-volume theory, we believe that as long as we can prepare the organic–inorganic hybrid membranes with larger free-volume and suitable size of the free-volume cavity, the permeability and selectivity will be enhanced simultaneously. However, the successful preparation relies heavily on the appropriate recipe of the organic–inorganic hybrid materials. Keeping these in mind, an effort was made to prepare novel composite membranes involving both “class I” and “class II” hybrid materials so as to enhance both permeation flux and selectivity simultaneously. The composite membrane consists of NaY zeolite and hybrid poly(vinyl alcohol). The resulting membranes were characterized by FTIR, DSC, WAXD and SEM, and were employed for the separation of water–acetic acid 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 ability of the membranes. 2. Experimental 2.1. Materials ¯ w ∼ 125, 000; degree of hydrolysis, Poly(vinyl alcohol) (M 86–89%), acetic acid and hydrochloric acid were purchased from SD Fine Chemicals Ltd., Mumbai, India. Tetraethylorthosilicate (TEOS) was procured from E. Merck (India) Ltd., Mumbai. NaY zeolite was kindly supplied by Indian Petrochemicals Corporation Ltd., Baroda, India. The characteristic properties of NaY zeolite are given in Table 1. All the chemicals were of reagent grade and used without further purification. Double distilled water was used throughout the research work. 2.2. Membrane preparation Poly(vinyl alcohol) (4 g) was dissolved in 100 ml of deareated-distilled water at 60 ◦ C. To the hot solution, 6 g of TEOS and 1 ml of concentrated HCl as an acid catalyst were added for the sol–gel reaction. The reaction mixture was stirred overnight at room temperature. The solution was then filtered using a fritted glass disc filter to remove the undissolved residue particles and the solution was left overnight to release the effervescence. The resulting homogeneous solution was spread 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 peeled-off and was designated as M.

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To prepare the composite membranes, a known amount of NaY zeolite was added into the above sol–gel reaction. The mixed solution was stirred overnight at room temperature, and then it was kept in an ultrasonic bath at a fixed frequency of 38 kHz (Grant XB6, UK) for about 30 min to break the aggregated crystals of zeolite and so as to improve the dispersion of zeolite in the hybrid polymer matrix. The rest of the procedure was followed as mentioned above. The amount of NaY zeolite with respect to PVA was varied from 5, 10 and 15 mass%, and the composite membranes thus obtained were designated as M-1, M-2 and M-3, respectively. 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 ␮m and the average thickness was considered for calculation. The thickness of the membranes was found to be 40 ± 2 ␮m.

2.7. Swelling measurements

2.3. Fourier transform infrared (FTIR) spectroscopy

where Ws and Wd are the masses of the swollen and dry membranes, respectively.

The reaction between PVA and TEOS, and the incorporation of different amounts of NaY zeolite into hybrid polymer matrix were confirmed by FTIR spectroscopy (Nicolet, Impact410, USA). Membrane samples were ground well to make KBr pellets under a hydraulic pressure of 400 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 characteristic peaks with respect to the amount of NaY zeolite. 2.4. Wide-angle X-ray diffraction (WAXD) The morphology of the TEOS crosslinked PVA membrane and its zeolite incorporated membranes was studied at room temperature using a Brucker’s D-8 advanced wide-angle Xray diffractometer. The X-ray source was nickel-filtered Cu K␣ radiation (40 kV and 30 mA). The dried membranes of uniform thickness (∼40 ␮m) were mounted on a sample holder and the patterns were recorded in the reflection mode at an angle 2θ over a range of 5–45◦ at a constant speed of 8◦ /min. 2.5. Differential scanning calorimetry (DSC) Thermal properties of the membranes were measured using a differential scanning calorimeter (Stanton, Redcroff DSC 1500). The samples weight ranged from 5 to 8 mg and they were heated from ambient temperature to 300 ◦ C at a heating rate of 10 ◦ C/min. The intercept point of the slopes was taken as the glass transition temperature (Tg ). A repeat run, following cooling at 10 ◦ C/min exhibited reproducibility within ±1.5 for Tg values. 2.6. Scanning electron microscopy (SEM) Membranes’ surfaces were observed at 15 kV using a JSM840A scanning electron microscope (JEOL, Tokyo, Japan). All ˚ of sputspecimens were coated with a conductive layer (400 A) tered gold.

The degree of membranes swelling was carried out with different compositions of water–acetic acid mixtures using an electronically controlled oven (WTB Binder, Germany). The masses of the dry membranes were first determined and these were equilibrated by soaking in different compositions of the feed mixture in a sealed vessel at 30 ◦ C 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 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:   Ws − Wd DS (%) = × 100 (1) Wd

2.8. Pervaporation experiments PV experiments were performed using an indigenously designed apparatus reported in our previous articles [26,27]. The effective surface area of the membrane in contact with the feed mixture was 34.23 cm2 and the capacity of the feed compartment is about 250 cm3 . The vacuum in the downstream side of the apparatus was maintained [1.33224 × 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 corresponding temperature before performing the PV experiment with fixed compositions of the feed mixture. After attaining a steady state, the experiments were carried out at 30, 40 and 50 ◦ C and the permeate was collected in a trap immersed in the liquid nitrogen jar on the downstream side at fixed intervals of time. The water composition in the feed mixture was varied from 10 to 50 mass%. The flux was calculated by weighing the permeate on a digital microbalance. The compositions of water and acetic acid were estimated by measuring the refractive index of the permeate within an accuracy of ±0.0001 units using an Abbe’s refractometer (Atago-3T, Japan) and by comparing it with a standard graph that was established with the known compositions of water–acetic acid mixtures. All the experiments were performed at least three times and the results were averaged. The results of permeation for water–acetic acid mixtures during the PV were reproducible within admissible range. From the PV data, separation performance of the membranes was assessed in terms of total flux (J), separation selectivity (αsep ) and pervaporation separation index (PSI). These were calculated respectively using the following equations: J=

W At

αsep =

(2) Pw /PHAc Fw /FHAc

(3)

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Fig. 2. Wide-angle X-ray diffraction patterns of hybrid PVA membranes with and without zeolite: (M) 0 mass%; (M-1) 5 mass%; (M-2) 10 mass%; (M-3) 15 mass%. Fig. 1. FTIR spectra of hybrid PVA membranes with and without zeolite: (M) 0 mass%; (M-1) 5 mass%; (M-2) 10 mass%; (M-3) 15 mass%.

PSI = J(αsep − 1)

(4)

where W is the mass of permeate (kg), A the effective area of the membrane (m2 ), t the permeation time (h), and Pw and PHAc are the mass percent of water and acetic acid in the permeate, respectively. Fw and FHAc are the respective mass percent of water and acetic acid in the feed. 3. Results and discussion 3.1. FTIR studies Fig. 1 illustrates the FTIR spectra of hybrid PVA membrane and its zeolite-incorporated membranes. A characteristic strong and broad band appeared at around 3400 cm−1 and multiple bands appeared between 1000 and 1100 cm−1 in hybrid PVA membrane (M), are respectively assigned to –OH stretching vibrations and Si–O–C bonds formed between PVA and TEOS. All these peaks match with those reported by Kariduraganavar et al. [27] and Robertson and Mauritz [33]. This is clearly evident to the occurrence of reaction between PVA and TEOS. The intensity of –OH band marginally decreased from membrane M-1 to M-3 with increasing zeolite content. On the contrary, the intensity of multiple bands correspond to Si–O–C bonds increased systematically from membrane M-1 to M-3. This is expected as due to increase of Si–O bond character by the incorporation of zeolite and increase of its mass [34]. 3.2. X-ray diffraction studies The WAXD patterns of all the membranes including the pattern of pure NaY zeolite are presented in Fig. 2. All the peaks respectively match with those reported by Kita et al. [35] and

Kariduraganavar et al. [27] for X-type crystals of NaY zeolite and hybrid PVA membrane (M), with respect to the positions and intensities of the observed reflections. After incorporating 5 and 10 mass% of NaY zeolite into hybrid PVA matrix (M-1 and M-2), no characteristic peaks of zeolite were observed in the patterns. This suggests that the hybrid membrane matrix is completely accommodated the zeolite particles without affecting much on its morphology. However, when the zeolite content was increased to 15 mass% (M-3), the pattern exhibited only the characteristic peaks of zeolite, suggesting that hybrid membrane matrix no longer retains its crystallinity. In spite of this, the resulting membranes tend to have a more rigid structure owing to the restriction of polymer chain mobility with increasing zeolite loading. 3.3. Glass transition temperature (Tg ) In an effort to study the effect of zeolite incorporation on the hybrid membrane, the glass transition temperatures were measured for all the membranes and are shown in Fig. 3. The Tg of the hybrid membrane decreased almost linearly as the zeolite content was increased in the membrane. It manifests that the incorporation of zeolite decreases the crystallinity of the hybrid membrane, eventhough the segmental motions of the polymer chains are restricted due to a reduction of free-volume in the membrane matrix with increasing the zeolite content. This is in good agreement with the X-ray patterns. Generally, a decrease in free-volume leads to an increase in selectivity and a decrease in permeation flux. But in the present study, it is demonstrated that both permeation flux and selectivity increased significantly with increasing zeolite content in the membrane matrix (see Fig. 9). These observations suggest that after incorporation of a large amount of zeolite into a membrane matrix, the packing density of the hybrid membrane increased apart from increasing its hydrophilicity, leading to selective

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Fig. 3. Effect of NaY zeolite on the glass transition temperature of hybrid PVA membranes.

adsorption and establishing molecular sieving action. These in turn responsible for the enhancement of both permeation flux and selectivity simultaneously. 3.4. SEM studies Fig. 4 presents SEM photographs of surface views of hybrid membranes and its zeolite incorporated membranes. The micrographs confirm that the distribution of zeolite increased from membrane M-1 to M-3 with increasing zeolite loading. The zeolite was distributed evenly throughout the membrane matrix with no apparent clustering. This ensures that the zeolite incorporated membranes obtained here are free from possible defects. 3.5. Effects of feed composition and NaY zeolite content on membrane swelling It has been realized since the pioneering work of Flory and Rehner reported in the early 1950s [36,37] that membrane swelling in certain liquids depends on the extent of crosslinking, morphology of the membrane and the free-volume available within the membrane matrix, which strongly influence on the sorption mechanism. Therefore, the degree of membrane swelling is an important factor in PV process that controls the transport of permeating molecules under the chemical potential gradient. In order to study the effects of feed composition and zeolite loading on the membrane swelling, the percent degree of swelling of all the membranes was plotted with respect to water composition in the feed at 30 ◦ C as shown in Fig. 5. It is observed that the degree of swelling increased for all the membranes with increasing water composition in the feed. This is because of strong interactions occurring between water molecules and the membrane, since membrane contains –OH groups and Na+ ions. The interaction becomes more predominant particularly at higher composition of water, since water causes a greater degree of swelling than those of organic liquids. When the polymer matrices are filled with NaY zeolite, the degree of swelling increased more than that of the hybrid PVA membrane throughout the range of water composition. This effect becomes

Fig. 4. SEM photographs of hybrid and composite membranes.

more predominent as the zeolite content in the membrane was increased. This may be due to the fact that incorporation of zeolite not only increases the free-volume in the membrane matrix, but also increases the electrostatic force of attraction

Fig. 5. Variation of degree of swelling with different mass% of water in the feed for hybrid and composite membranes.

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Fig. 6. Variation of total flux with different mass% of water in the feed for hybrid and composite membranes.

between water molecules and the membrane due to the presence of Na+ ions in the cages. As a consequence, adsorption of water molecules increases remarkably and this, in turn becomes responsible for an enhanced swelling with increasing zeolite content in the membrane. 3.6. Effects of feed composition and NaY zeolite content on pervaporation properties Fig. 6 demonstrates the effects of feed composition and zeolite 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 almost in accordance with the results observed in swelling study. This is due to increased selective interactions between water molecules and the membrane. However, this tendency becomes more predominant for the zeolite-incorporated membranes (M-1 to M-3). This is mainly attributed to a combined influence of ionic species (Na+ ) present in the zeolite cages and establishing the free-volumes in the membrane matrices by the incorporation of zeolite. The presence of ionic species greatly responsible for the increase of adsorption process through an establishment of electrostatic force of attraction between water molecules and the membrane, whereas creation of pores (freevolume) increases the molecular sieving action, which makes the diffusion process easier. In order to assess the extent of permeation of individual components, we have plotted the total flux, and fluxes of water and acetic acid as a function of zeolite content in the membrane for 10 mass% of water in the feed as shown in Fig. 7. From the plot, it is clearly demonstrated that the total flux and flux of water are almost overlapping each other for all the membranes, and thereby the flux of acetic acid is negligibly small, indicating that the composite membranes developed in the present study involving “class I” and “class II” materials are highly selective towards water with a tremendous improvement in the flux compared to hybrid PVA membrane (class II).

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Fig. 7. Variation of total flux, and fluxes of water and acetic acid with different mass% of NaY zeolite-incorporated hybrid PVA membranes at 10 mass% of water in the feed.

In PV process, the overall selectivity of a membrane is generally explained on the basis of interaction between membrane and the permeating molecules, molecular size of the permeating species and pore diameter of the membrane. Fig. 8 displays the effects of both water composition and zeolite content on the selectivity. It is observed that the selectivity of all the membranes decreased drastically from 10 to 20 mass% of water in the feed and then, it was decreased gradually with further increasing the water concentration. At higher concentration of water in the feed, the membranes swell greatly due to establishing strong interactions between membrane and the water molecules. This suppresses the interaction within the membrane material (i.e., between NaY zeolite and hybrid PVA) matrix, since the interaction between inorganic particles and the polymer is of physical origin [24,25]. As a result, selectivity decreases drastically at higher concentration of water in the feed, irrespective of the loading of zeolite in the membrane matrix.

Fig. 8. Variation of separation selectivity with different mass% of water in the feed for hybrid and composite membranes.

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Fig. 9. Variation of total flux and separation selectivity with different mass% of NaY zeolite-incorporated hybrid membranes at 10 mass% of water in the feed.

On the contrary, the selectivity increased significantly from membrane M-1 to M-3 upon increasing the zeolite content in the membrane matrix. This is attributed to increased selective interaction between membrane and the water molecules. This can be clearly demonstrated from Fig. 9, in which the flux and selectivity were plotted as a function of zeolite 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 crosslinking density or due to the incorporation of zeolite into the membrane matrix, the permeation flux decreases and selectivity increases [38,39]. However in this study, both permeation flux and selectivity increased simultaneously with increasing zeolite content in the membrane matrix. Although, this is contrary to a trade-off phenomenon existing between flux and selectivity in PV process, a significant enhancement of hydrophilicity, selective adsorption and the establishment of a molecular sieving action in the membrane matrix, overcome the situation.

Fig. 10. Variation of facilitation ratio with different mass% of NaY zeoliteincorporated hybrid membranes at 10 mass% water in the feed.

We have also made an attempt to calculate the facilitation ratio to understand the molecular sieving effect on the permeabilities for zeolite incorporated membranes using the following equation proposed by Jia et al. [40] at 10 mass% water in the feed: Pz+p − Pp Pp

(5)

where Pz+p and Pp are the permeability of zeolite-filled and zeolite-free membranes, respectively. The resulting facilitation ratios are presented in Fig. 10. The facilitation ratio increased almost linearly with increasing zeolite content in the membrane matrix, and this clearly justifies that the molecular sieving effect significantly contributed to the permeability. This is in good agreement with the results observed in Figs. 6 and 7. The results for total flux and selectivity, fluxes of water and acetic acid measured at 30 ◦ C for all the membranes with the

Table 2 Pervaporation flux and separation selectivity data for different membranes measured at 30 ◦ C for different mass% of water in the feed Mass% of water

10 20 30 40 50

αsep

J (×102 kg/m2 h) M

M-1

M-2

M-3

M

M-1

M-2

M-3

2.45 3.85 4.57 5.56 6.74

4.11 5.22 5.99 7.72 9.13

5.51 6.53 7.45 9.33 11.22

8.35 9.45 10.54 12.53 15.49

1102 472 231 106 62

1277 496 243 135 70

1627 567 257 141 82

2423 663 331 165 99

Table 3 Pervaporation fluxes of water and acetic acid for different mass% of water in the feed at 30 ◦ C for different membranes Mass% of water

10 20 30 40 50

Jw (×102 kg/m2 h)

JHAc (×102 kg/m2 h)

M

M-1

M-2

M-3

M

M-1

M-2

M-3

2.43 3.81 4.53 5.48 6.64

4.08 5.18 5.93 7.63 8.99

5.48 6.49 7.38 9.23 11.00

8.32 9.39 10.46 12.42 15.33

0.02 0.04 0.04 0.08 0.10

0.03 0.04 0.06 0.09 0.14

0.03 0.04 0.07 0.10 0.22

0.03 0.06 0.08 0.11 0.16

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investigated feed compositions are presented in Tables 2 and 3, respectively. There is a systematic increase in total flux with respect to both water composition and zeolite loading. Similarly, the selectivity was increased systematically with increasing the amount of zeolite loading throughout the investigated range of water composition, but it was decreased with respect to water composition. With regard to individual fluxes, the flux of water was increased considerably from membrane M to M-3, but the acetic acid flux was constant in some cases or increased marginally for others. The marginal increase of acetic acid flux is unusual. But this cannot be ruled out, when there is a considerable increase of water flux, which normally allows to permeate the associated molecules, and therefore, the marginal increase of flux of acetic acid occurs. On the other hand, both the fluxes of water and acetic acid increased with increasing water concentration in the feed. This was expected because of greater swelling of membranes at higher concentration of water in the feed. Fig. 11. Variation of pervaporation separation index with different mass% of NaY zeolite-incorporated hybrid membranes at 10 mass% of water in the feed.

3.7. Pervaporation separation index (PSI) PSI is the product of permeation and separation factor, which determines the membrane separation ability. This index can be

Table 4 Comparison of PV performance of PVA-based membranes reported in the literature for water–acetic acid mixtures Membrane

Thickness (␮m)

Temperature (◦ C)

Mass% of water in feed

Flux (kg/m2 h)

Separation factor

Reference

PVA/PEI PVA/PAA PVA/PHC PVA/PVP PVA with 100% hydroxyl content PVA with 96% hydroxyl content PVA/MA (Xcr = 0.05) PVA/MA (Xcr = 0.1) PVA/MA PVA/MA PVA/amic acid (12 wt%) PVA crosslinked with p-phenylene diamine PVA crosslinked with m-phenylene diamine PVA crosslinked with GA (5 vol.%) PVA crosslinked with GA (0.1 wt%) PVA crosslinked with formaldehyde PVA/PAA (75/25) NaAlg/PVA NaAlg/PAN/PVA (composite) PVA-g-PAAm (48%) PVA-g-PAAm (93%) PVA-g-AAm (48%) PVA-g-AAm (93%) PVA-g-AN (52%) PVA/TEOS (0.5 wt%) PVA/TEOS (1.0 wt%) PVA/TEOS (1.5 wt%) PVA/TEOS (2.0 wt%) PVA/TEOS (M) PVA/TEOS (M-1) (composite) PVA/TEOS (M-2) (composite) PVA/TEOS (M-3) (composite)

30 30 30 30 30 30 NR NR 50 50 14–18 36–46 39–47 12–14 NR NR NR NR NR NR NR NR NR 15-40 40 40 40 40 40 40 40 40

25 25 25 25 25 25 25 25 25 40 30 30 30 35 30 30 30 40 50 25 25 35 35 30 30 30 30 30 30 30 30 30

10 10 10 10 10 10 10 10 13 20 10 10 10 10 10 10 10 15 10 10 10 20 20 10 10 10 10 10 10 10 10 10

1.40 0.30 0.25 0.80 0.05 0.50 ∼0.40 ∼1.0 0.35 0.05 0.079 0.079 0.012 0.029 ∼0.0001 ∼0.0001 0.0056 0.044 0.094 0.008 0.042 0.086 0.094 0.090 0.1127 0.0700 0.0481 0.0333 0.0245 0.0411 0.0551 0.0835

1.8 6.6 5.0 2.4 7 4.5 ∼7.8 ∼6.8 6.22 670 42 42 176 420 ∼9.0 ∼5.5 795 46 18.0 8.53 5.36 5.38 3.81 14.60 36 441 741 1116 1102 1277 1627 2423

[41] [41] [41] [41] [41] [41] [42] [42] [42] [43] [44] [44] [45] [46] [47] [47] [48] [49] [49] [50] [50] [50] [50] [51] [27] [27] [27] [27] Present work Present work Present work Present work

GA: glutaraldehyde; PEI: polyethyleneimine; PAA: poly(acrylic acid); NaAlg: sodium alginate; PHC: poly(hydroxycarboxylic acid); PVP: poly(N-vinyl-2pyrrolidone); MA: malic acid; PAAm: poly(acryl amide); PAN: poly(acrylonitrile); AN: acrylonitrile; AAm: acryl amide; Xcr : reaction density; NR: not reported in the literature.

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used as a relative guideline index for the design of a new membrane in PV separation processes and also for the selection of a membrane with an optimal combination of flux and selectivity. Fig. 11 illustrates the effect of NaY zeolite content on the pervaporation separation index at 30 ◦ C for 10 mass% of water in the feed. The PSI values increased exponentially with increasing the zeolite content, signifying that the membranes incorporated with a higher amount of zeolite, exhibited an excellent performance while separating the water–acetic acid mixtures. This was attributed to the incorporation of zeolite into membrane matrix that was changed 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; in the second step of diffusion, a unique property of zeolite and its significant role in the membrane matrix enhanced the overall performance of the membrane.

where J is the permeation flux per unit area (kg/m2 s), D the diffusion coefficient (m2 /s), C the concentration of permeant (kg/m3 ), subscript i stands for water or acetic acid, 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 [56]:

3.8. Comparison of PV performance of PVA-based membranes

Di =

The flux and separation selectivity for water–acetic acid mixtures measured through PVA-based membranes during PV process are summarized in Table 4. The separation factors of the composite membranes developed in the present study are significantly much higher than those of other PVA-based membranes. Similarly, the permeation flux values are no way imperior to the membranes developed by others. Although, the permeation flux values reported by others are close to the values of our membranes, but they have achieved this by sacrificing the selectivity to a greater extent. This is really a significant achievement made by the composite membranes reported by us through a combination of “class-I” and “class-II” hybrid membrane materials. This has possible just because of the combination of these two materials, which overcomes the problems associated by the individual hybrid membranes during the PV process.

where δ is the membrane thickness. The calculated values of Di at 30 ◦ C are presented in Table 5. Similar to PV study, the diffusion coefficients of water increased significantly from membrane M to M-3 with marginal increase of diffusion coefficients of acetic acid. Although, marginal increase of diffusion coefficients of acetic acid is unusual, but it cannot ruled out since the considerable increase of diffusion coefficients of water allows to diffuse the associated molecules through the membrane matrix. Despite of this, the membranes developed in the present study have remarkable separation ability for the separation of water from the acetic acid. This was attributed to increased hydrophilicity, selective adsorption and the establishment of molecular sieving action by the incorporation of zeolite into “class II” hybrid material. On the other hand, with increasing water concentration in the feed for all the membranes, the diffusion coefficients of water decreased while increasing the diffusion coefficients of acetic acid. This is expected due to decrease in membrane selectivity as described in pervaporation study due to formation of nonselective voids at the interphases, since the interaction between inorganic particles and polymer is of physical origin. However, all the membranes exhibit a considerable increase in diffusion coefficients of both water and acetic acid with increasing water concentration in the feed. This is expected because of a considerable deterioration of membrane selectivity as discussed in PV study for establishing a strong interaction between water molecules and the membrane.

3.9. Diffusion coefficient The transport of binary liquid molecules through a polymer membrane in PV process is generally described by the solution–diffusion mechanism, which occurs in three steps: sorption, diffusion and evaporation [52]. Thus, the permeation rates and selectivity are governed by the solubility and diffusivity of each component of the feed mixture to be separated. In PV process, because of the establishment of fast equilibrium distribution between the bulk feed and the upstream surface of

a membrane [53,54], the diffusion step controls the transport of penetrants. Therefore, this has prompted us to estimate the diffusion coefficient, Di of penetrating molecules to understand the mechanism of molecular transport. From Fick’s law of diffusion, the diffusion flux can be expressed as [55]: Ji = −Di

dCi dx

(6)

Ji δ Ci

(7)

Table 5 Diffusion coefficients of water and acetic acid for different membranes calculated at 30 ◦ C from Eq. (7) for different mass% of water in the feed Mass% of water

10 20 30 40 50

Dw (×108 m2 /s)

DHAc (×109 m2 /s)

M

M-1

M-2

M-3

M

M-1

M-2

M-3

4.89 3.84 3.03 2.75 2.66

8.28 5.22 3.98 3.83 3.61

11.20 6.56 4.96 4.63 4.45

17.10 9.55 7.06 6.24 6.17

0.147 0.269 0.436 0.866 1.440

0.213 0.348 0.542 0.944 1.71

0.225 0.381 0.639 1.090 1.800

0.229 0.473 0.703 1.250 2.070

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429

Table 6 Pervaporation flux and separation selectivity for different membranes at different temperature for 10 mass% of water in the feed Temperature (◦ C)

αsep

J (×102 kg/m2 h)

30 40 50

M

M-1

M-2

M-3

M

M-1

M-2

M-3

2.45 4.52 5.34

4.11 7.13 8.28

5.51 7.68 10.00

8.35 9.68 12.28

1102 554 400

1277 599 465

1627 683 491

2423 741 554

3.10. Effect of temperature on membrane performance The temperature is another important factor affecting the PV process in all the steps: sorption, diffusion and desorption. Generally, the flux increases as the temperature increases and the overall temperature effect on the partial flux at a fixed condition can usually be described with an Arrhenius expression. The separation efficiency of the membrane will thus be dependent on the overall activation energy of each permeant. The effect of operating temperature on the PV performance was studied for water–acetic acid mixtures at 10 mass% of water in the feed and the resulting values are presented in Table 6. The permeation rate was increased from 30 to 50 ◦ C for all the membranes while decreasing the separation factor remarkably. This is because of decreased interaction within the permeants, and permeants and membrane at higher temperature, which predominate the plasticizing effect on the membrane. Therefore, permeation of diffusing molecules and the associated molecules through the membrane becomes easier, resulting in an increase of total permeation rate while suppressing the selectivity. Nevertheless, the transport of water molecules through the membrane is predominant at lower temperature. The temperature dependency of permeation and diffusion rates can be expressed using the Arrhenius type equation [44]:  X = X0 exp

−Ex RT

Fig. 12. Variation of log J with temperature for different mass% of NaY zeoliteincorporated hybrid membranes at 10 mass% of water in the feed.

 (8)

where X represents rate of permeation (J) or diffusion (D), X0 is a constant representing pre-exponential factor of J0 or D0 , Ex represents activation energy for permeation or diffusion depending on the transport process under consideration, and RT is the usual energy term. As the feed temperature increases, the vapour pressure in the feed compartment also increases, but the vapour pressure at the permeate side is not affected. All these result in an increase of driving force with increasing temperature. Arrhenius plots of log J versus 1/T and log D versus 1/T are shown in Figs. 12 and 13 for the temperature dependency of permeation flux and diffusion, respectively. In both the cases, linear behaviour was observed, suggesting that both permeability and diffusivity follow an Arrhenius trend. From least squares fits of these linear plots, the activation energies for permeability (Ep ) and diffusivity (ED ) were estimated. Similarly, we have also estimated the activation energies for diffusion of water (EDw ) and acetic acid (EDHAc ), but the plots are not given to avoid the crowdness. The values thus obtained are presented in Table 7. It is noticed that hybrid membrane (M) exhibits higher Ep and ED values compared to those of composite membranes

Fig. 13. Variation of log D with temperature for different mass% of NaY zeoliteincorporated hybrid membranes at 10 mass% of water in the feed. Table 7 Arrhenius activation parameters for permeation and diffusion, and heat of sorption Parameters (kJ/mol)

M

M-1

M-2

M-3

Ep ED EDw EDHAc Hs

31.91 32.00 31.82 69.54 −0.09

28.67 28.82 28.75 72.84 −0.15

24.29 24.52 24.39 72.80 −0.23

15.69 16.24 15.18 75.69 −0.55

430

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(M-1 to M-3). This suggests that both permeation and diffusion processes require more energy for transport of molecules through hybrid membrane (M) because of rigidity in its structure. Obviously, composite membranes take less energy because of establishing a molecular sieving action attributed to the presence of sodalite cages and super cages in the framework of zeolite [19]. As a result, activation energies of both permeation and diffusion decreased markedly from membrane M-1 to M-3 with increasing zeolite content. On the other hand, a large difference was noticed between EDw and EDHAc values. However, the difference is more significant particularly for the membranes having higher loading of zeolite, suggesting that membranes developed with higher loading of zeolite exhibited remarkable separation selectivity towards water. The Ep and ED values ranged between 15.69 and 31.91, and 16.24 and 32.0 kJ/mol, respectively. Using these values, we have further calculated the heat sorption as: Hs = Ep − ED

increasing temperature. This was attributed to decreased interaction between permeants, and permeants and membrane at higher temperature. A significant difference was noticed between EDw and EDHAc values. However, the difference was more significant particularly for the membranes having higher loading of zeolite, suggesting that composite membranes developed with higher loading of zeolite exhibited remarkable separation selectivity towards water. The Ep and ED values ranged between 15.69 and 31.91, and 16.24 and 32.0 kJ/mol, respectively. The composite membranes exhibited significantly lower activation energies compared to that of a hybrid membrane, indicating that the permeants consumed less energy for composite membranes during the process. This is because of establishing a molecular sieving action in the membrane matrix due to the presence of sodalite cages and super cages in the framework of zeolite. For all the membranes, Langmuir’s type of sorption dominates the process, giving an exothermic contribution.

(9)

The resulting Hs values are included in Table 7. The Hs values give the additional information about the transport of molecules through the polymer matrix. It is a composite parameter involving contributions from Henry’s type of sorption and Langmuir’s type of sorption [57]. Henry’s mode 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. In case of Langmuir’s mode, the site already exists in the polymer matrix and consequently, sorption occurs by a hole-filling mechanism, making an exothermic contribution. The Hs values obtained in this study are negative for all the membranes, suggesting that Langmuir’s type of sorption is predominant, as expected giving an exothermic contribution. 4. Conclusions Novel composite membranes were prepared involving both “class I” and “class II” hybrid materials. The performance of membranes was evaluated for the separation of water–acetic acid mixtures. An increase of zeolite in the membrane matrix results to a simultaneous increase of both permeation flux and selectivity. This was attributed to a significant enhancement of hydrophilicity, selective adsorption and establishment of a molecular sieving action. While assessing the membranes’ efficiency, it was clearly noticed that both total flux and flux of water are overlapping each other, signifying that the membranes developed in the present study by the combination of “class I” and “class II” hybrid materials, are highly selective towards water with a tremendous improvement in the flux compared to hybrid PVA membrane (class II). The PV separation index data also indicated that the higher the loading of zeolite, better is the membrane performance. The highest separation selectivity was found to be 2423 with a flux of 8.35 × 10−2 kg/m2 h for the membrane having the highest loading of zeolite at 30 ◦ C for 10 mass% of water in the feed. With regard to temperature effect, the permeation rate was increased while suppressing the selectivity with

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