Zeolite K-LTL-loaded sodium alginate mixed matrix membranes for pervaporation dehydration of aqueous–organic mixtures

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Journal of Membrane Science 306 (2007) 173–185

Zeolite K-LTL-loaded sodium alginate mixed matrix membranes for pervaporation dehydration of aqueous–organic mixtures夽 Santoshkumar D. Bhat, Tejraj M. Aminabhavi ∗ Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India Received 30 March 2007; received in revised form 23 August 2007; accepted 26 August 2007 Available online 30 August 2007

Abstract In this work, zeolite K-LTL-loaded sodium alginate (NaAlg)-mixed matrix membranes were prepared by solution casting and cross-linked with glutaraldehyde. The pervaporation (PV) dehydration of isopropanol, 1,4-dioxane and tetrahydrofuran (THF) was tested at 30, 40, 50, 60 and 70 ◦ C as a function of membrane thickness and feed compositions. Activation parameters for permeation were evaluated from the temperature-dependent pervaporation flux data. The results showed a simultaneous enhancement in flux and selectivity at azeotropic compositions of the mixtures due to the addition of K-LTL particles of different silica–alumina ratios in NaAlg matrix. Flux and selectivity values to water were higher for water–1,4dioxane azeotrope than those of water–isopropanol and water–THF mixtures. Pervaporation results were discussed in terms of sorption–diffusion principles. Molecular sieving effect created due to uniform distribution of K-LTL zeolite particles and its hydrophilic nature in addition to its interaction with hydrophilic NaAlg is responsible to appreciably increase the membrane performance than pristine cross-linked NaAlg membrane. Thermodynamic treatment of sorption process was investigated typically for water + 1,4-dioxane mixtures based on Flory–Huggins theory to explain the PV performance. Based on these results, permeance and driving force mechanisms were elucidated. The present membranes could withstand the repetitive cyclic PV runs on the laboratory level module. © 2007 Elsevier B.V. All rights reserved. Keywords: K-LTL zeolite; Sodium alginate; Pervaporation; Aqueous/organic separation; Membrane thickness; Flory–Huggins approach; Driving force; Permeation activation energy

1. Introduction Pervaporation (PV) is one of the widely studied techniques for the separation of aqueous–organic mixtures [1–4]. Natural polymer such as sodium alginate (NaAlg) has been widely explored as a membrane in PV dehydration, even though it has the drawback of yielding low flux and selectivity [5–7]. Crosslinked NaAlg, its blend and composite membranes have also shown good stability towards liquids like tetrahydrofuran (THF), 1,4-dioxane, and isopropanol, but their intrinsic transport properties are inferior to many commercial membranes [8–17]. In PV dehydration of alcohols, membranes with hydrophilic groups are required that tend to absorb water preferentially giving 夽

This article is Center of Excellence in Polymer Science Communication # 197. ∗ Corresponding author. Present address: Reliance Industries Limited, Navi Mumbai, India. E-mail address: [email protected] (T.M. Aminabhavi). 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.08.040

increased flux and selectivity. The introduction of hydrophilic groups would result in plasticization effect due to excessive membrane swelling, which can be minimized by either crosslinking the polymer or by adding the appropriate filler particles to improve membrane performance. In our previous studies [18–22], modification of NaAlg was attempted by loading different inorganic fillers such as zeolites, mesoporous materials, clays and alumino-phosphates, which have shown pronounced increase in membrane performance due to their unique structural characteristics and hydrophilicity, resulting in appreciable separation performance and permeation flux. Zeolite-filled membranes were proved to be better than the pristine (unfilled) membranes due to their uniform molecular size pores that provide improved transport rates due to molecular sieving effects and better chemical stability than the pristine polymers [23–25]. In effort to explore further in this area, we have now chosen the zeolite-type K-LTL, a versatile molecular sieve available from zeolite family. It has a onedimensional pore of about 0.71 nm aperture, leading to cavities

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of 0.48 nm × 1.24 nm × 1.07 nm and a framework of Si/Al ratio, typically about 3 [26]. By virtue of its unique structural and compositional peculiarities, zeolite K-LTL has attracted much greater interest in its synthesis, characterization and utilization as filler in polymeric matrices; hence, study of its sorptive properties and catalytic applications in various hydrocarbon conversion reactions [27]. The size and shape of zeolite crystals are important to display its performance during PV dehydration studies. Since the change in crystal size and morphology could alter the performance in terms of flux and selectivity, the rate of adsorption would influence diffusion through crystallite regions [28]. Various physico-chemical parameters also could influence the crystal size and properties of the zeolites [29]. Even though, the shape selectivity is higher for larger crystals, yet the compromise between selectivity and effectiveness should be considered for optimization of crystal size and shape. A perusal of literature indicates that no results are available on K-LTL zeolite-mixed matrix membranes of NaAlg or any other polymers for PV dehydration of aqueous–organic mixtures. Realizing this, we have decided to investigate the possible improvement on NaAlg membrane performance by adding KLTL zeolite particles in PV dehydration of three industrially important organics mentioned before. Zeolite-type K-LTL, chosen in this study, was synthesized by hydrothermal route and was incorporated into NaAlg matrix to obtain mixed matrix membranes to boost flux and selectivity values simultaneously for water from aqueous mixtures of isopropanol, 1,4-dioxane, or tetrahydrofuran (THF) at the respective azeotropic compositions over and above that of the pristine NaAlg membrane. K-LTL zeolite containing different ratios of silica–alumina when incorporated into NaAlg matrix have shown better membrane performance in terms of flux and selectivity to water over the pristine NaAlg membrane. The experimental results of this study were analyzed by Flory–Huggins theory. The driving force mechanism was used to understand thermodynamic interactions between the polymer membranes and solvent molecules. The PV separation achieved for the chosen mixtures at azeotropic compositions would be otherwise difficult by conventional distillation method and hence, the present study is undertaken to address the separation by PV technique. 2. Experimental 2.1. Materials Sodium alginate (NaAlg), isopropanol, 1,4-dioxane, THF, glutaraldehyde (GA), acetone, hydrochloric acid and sulfuric acid were all purchased from s.d. Fine Chemicals, Mumbai, India. Deionized double distilled water having a conductivity of 20 ␮S/cm was produced in the laboratory itself using Permionics pilot plant (Vadodara, India) by the reverse osmosis membrane module. KOH (85%, Aldrich), silica sol (40% SiO2 , V.P. Chemicals, India) and pseudoboehmite (70% Al2 O3 , Condea, Germany) were the silica, alumina and potassium sources were used to synthesize the K-LTL.

2.2. Hydrothermal synthesis of zeolite-type K-LTL The synthesis of K-LTL was carried out by following the method similar to the one described before [30]. In the present study, the substrates having oxide molar gel compositions as xK2 O:ySiO2 :Al2 O3 :zH2 O, where 4 ≤ x ≤ 8, 10 ≤ y ≤ 20 and 100 ≤ z ≤ 200 were prepared separately in such a way that the initial gel composition having the molar ratio of SiO2 /Al2 O3 varied individually in the given range by keeping other initial molar ratios constant. The kinetics of crystallization was studied by comparing the extent of crystallization of the gel mixture at different stages in the process at 170 ◦ C under static condition. The final homogeneous reaction mixture was prepared by (i) adding required amounts of KOH and selected alumina source into deionized water under stirring, (ii) optionally heating the reaction mixture until a clear solution of potassium aluminate is formed, (iii) cooling the solution to ambient temperature (30 ◦ C) and compensating water loss due to boiling, (iv) adding the clear potassium aluminate solution to aqueous suspension of the desired silica source under vigorous stirring, and (v) further stirring to form a homogeneous gel mixture. The final homogeneous reaction mixture was equally distributed on weight basis into several 100 mL stainless steel autoclaves, sealed and then placed in an air-heated oven maintained at 170 ± 3 ◦ C. The time when the autoclave attained the temperature of 170 ◦ C (as sensed via thermocouple inserted into thermo-well of one of the autoclaves) was taken as the zeropoint in the crystallization process. The autoclaves were then taken out of the oven one by one at the scheduled time intervals, quenched to the ambient temperature and monitored to know the extent of crystallization as a function of crystallization time. The solid products were separated by filtration, washed with deionized water and dried at 120 ◦ C to obtain the final product. 2.3. Membrane preparation NaAlg (4 g) was dissolved in 80 mL of water under constant stirring. Then, 0.4 g of the synthesized zeolite K-LTL particles of different SiO2 /Al2 O3 ratios (10 and 20) were weighed separately and dispersed in 20 mL of water, sonicated for 30 min, and added to NaAlg solution (already prepared) with further stirring for 24 h. The solution was poured on a glass plate to cast the membranes. Dried membranes were peeled off from the glass plate and immersed in a cross-linking bath containing (30:70) water:acetone mixture along with 2.5 mL of GA and 2.5 mL of conc. HCl. After keeping the membranes in a crosslinking bath for about 12–14 h, they were removed, washed repeatedly with deionized water and dried in an oven at 40 ◦ C. Acetone being a non-solvent prevented the initial dissolution of the membrane and water present in the feed mixture caused membrane to swell thereby facilitating an easy penetration of glutaraldehyde into the membrane matrix to establish an effective cross-linking. Crosslinking reaction took place between the –OH group of NaAlg and the –CHO group of glutaraldehyde due to the formation of ether linkage by eliminating

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water. The pristine cross-linked NaAlg membrane was also prepared in the same manner except that no particles of zeolite K-LTL were added. Membranes with different thicknesses ranging from 30 to 50 within ±2 ␮m were prepared. However, a thickness of 50 ± 2 ␮m was found to be ideal for performing the repetitive PV experiments, which withstood the high-vacuum pressures. 2.4. Particle size measurement of zeolite-type K-LTL Zeta average diameter of K-LTL particles dispersed in water was measured using Zetasizer, Model 3000HS, Malvern, UK. Zeolite particles were dispersed in 10 mL of water. After sonicating for 1 h, the solution was taken in a couvette subjected to laser beam radiation. Series of readings were taken, but average distribution histograms were considered to measure the particle size range. 2.5. Scanning electron microscopic (SEM) studies Cross-sectional SEM micrograph of the K-LTL-mixed matrix membranes of NaAlg were obtained under high resolutions (mag. 10 kV) using Leica Stereoscan-440 scanning electron microscope (SEM) equipped with Phoenix energy dispersive analysis of X-rays (EDAX). Since these films were nonconductive, gold coating (15 nm thickness) was done before subjecting to SEM analysis. 2.6. Mechanical properties The equipment used to perform out the mechanical properties of the membranes was that of universal testing machine (UTM) (Model H25 KS Hounsfield, Surrey, UK). Test specimens were prepared in the form of dumbbell-shaped objects as per ASTM D-638 standards. Films of gauge length of 50 mm and width of 10 mm were stretched at the cross-head speed of 10 mm/min. Cross-sectional area of the sample of known width and thickness was calculated and tensile strength was calculated using the equation: tensile strength (N/mm2 ) =

maximum load cross-sectional area

(1)

2.7. Pervaporation experiments Pervaporation experiments were carried out in a 100 mL batch level instrument with an indigenously constructed manifold operated at a vacuum level as low as 6.67 Pa in the permeate line as described before [31]. The effective membrane area was 20 cm2 and weight of the feed mixture taken in the PV cell was 70 g. Pervaporation experiments were performed at 30, 40, 50, 60 and 70 ◦ C. Temperature of the feed mixture was maintained constant by a thermostatic water jacket. Before starting the PV experiment, the test membrane was equilibrated for about 2–4 h with the feed mixture. After establishment of a steady state, permeate vapors were collected in cold traps immersed in liquid nitrogen up to 4–5 h. Weight of the permeate col-

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lected was measured on a Mettler Balance (model B 204-S, Greifensee, Switzerland: accuracy 10−4 g) to determine the flux, J (kg m−2 h−1 ) using the weight of liquids permeated, W (kg), effective membrane area, A (m2 ) and measurement time, t (h) as J=

W At

(2)

The analyses of feed and permeate samples were done using Nucon Gas Chromatograph (model 5765) provided with Thermal Conductivity Detector (TCD) equipped with DEGS or Tenax packed column of 1/8 internal diameter having 2 m length. The oven temperature was maintained at 70 ◦ C (isothermal), while injector and detector temperatures were maintained at 150 ◦ C. The sample injection volume was 1 ␮L. Pure hydrogen was used as a carrier gas at a pressure of 10.67 psi. The GC response was calibrated for the column and for known compositions of water + organic mixtures. Calibration factors were fed into the GC software to obtain the analysis for unknown samples. The selectivity, α was calculated as    Forg PW α= (3) Porg FW Here, PW and Porg are the wt.% of water and organic component, respectively in permeate; FW and Forg are the wt.% of water and organic component, respectively in the feed. Minimum of three independent readings on flux and selectivity were taken under identical conditions of temperature and feed compositions to confirm the steady-state pervaporation. The % error values in computing different mixture compositions were less than 3%, since all the weight measurements were done within ±0.01 mg. Triplicate measurements on three membranes prepared differently gave the flux and selectivity data reproducible within 3% of standard errors. The mean values were considered in all calculations and graphical display. 2.8. Sorption Sorption experiments were performed gravimetrically [32] on all the membranes at the azeotropic compositions of isopropanol, 1,4-dioxane and THF at different temperatures. Sorption was also studied for different feed compositions at the ambient temperature (30 ◦ C). Initial mass of the circularly cut (diameter = 2.5 cm) pristine NaAlg and K-LTL-type zeolite incorporated NaAlg-mixed matrix membranes were placed on a single-pan digital microbalance (model AE 240, Mettler, Switzerland) sensitive to ±0.01 mg. Samples were placed inside the specially designed airtight test bottles containing 20 cm3 of the test solvent. Test bottles were transferred to an oven maintained at constant desired temperature. Dry membranes were equilibrated by soaking in different feed mixtures in a sealed vessel at 30 ◦ C for 48 h. Sorbed membranes were weighed immediately after careful by blotting on a digital microbalance. The % sorption was calculated as   W∞ − Wo %sorption = × 100 (4) Wo

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where W∞ and Wo are the weights of sorbed and dry membranes, respectively. Sorption selectivity experiments for water + 1,4-dioxane with different feed compositions were measured for the pristine NaAlg and K-LTL (SiO2 /Al2 O3 -10)-loaded NaAlgmixed matrix membranes by the procedure described before [17]. 3. Results and discussion 3.1. Particle size analysis The average size distribution of K-LTL-type zeolite particles dispersed in water has a diameter of about 1 ␮m as shown in Fig. 1. However, the particle size distribution histogram indicates the size distribution in the range of 150–2000 nm, but majority of particles are in the average size range of around 1000 nm. 3.2. Membrane morphology Fig. 2 shows the cross-sectional SEM image of 10 wt.% KLTL-type zeolite (SiO2 /Al2 O3 -10)-loaded NaAlg-mixed matrix membrane. Notice that K-LTL zeolite particles are uniformly distributed in the NaAlg matrix. It is important to have uniform dispersion of particles in the NaAlg matrix to exhibit enhanced effects of flux and selectivity during PV dehydration experiments. 3.3. Mechanical properties Tensile strength at break of 10 wt.% K-LTL-loaded NaAlg membrane (SiO2 /Al2 O3 -10) as well as pristine NaAlg membrane in dry state given in Table 1 suggests that mixed

Fig. 2. Cross-sectional scanning electron micrograph of K-LTL (SiO2 /Al2 O3 10)-filled NaAlg-mixed matrix membrane.

matrix membranes exhibit higher tensile strengths than the pristine NaAlg. This enhancement is attributed to the interaction of filler particles with NaAlg matrix. NaAlg chains in the presence of zeolite filler particles will experience a restriction in chain segmental mobility, resulting in an increase of rigidity or tensile strength, thereby reducing elongation at break. Such enhancement will be more pronounced for K-LTL zeolite (SiO2 /Al2 O3 -10, 20)-loaded mixed matrix membranes than the pristine NaAlg membrane. However, the KLTL zeolite (SiO2 /Al2 O3 -10)-loaded mixed matrix membranes exhibited a high-mechanical strength due to higher interaction/compatability with NaAlg matrix. 3.4. Pervaporation performance and sorption Extensive studies have been made to understand the relationship between sorption and diffusion anomalies during PV dehydration using the membranes [33]. PV results have been generally explained in terms of the well-known solution–diffusion model [34], according to which the solubility and diffusion could affect the permeability of liquids through the membrane. These parameters can be controlled by modifying the pristine NaAlg membrane with the addition of hydrophilic K-LTL zeolite filler in different ratios such that in the presence of a hydrophilic NaAlg, the mixed matrix membranes would be more selective to water than organics. These effects have been

Table 1 Tensile strength and % elongation of pristine NaAlg- and K-LTL-filled NaAlgmixed matrix membranes

Fig. 1. Average particle size distribution histogram of K-LTL zeolite particles.

Membrane type

Tensile strength (N/mm2 )

Elongation at break (%)

NaAlg 10 wt.% Zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 10 wt.% Zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg

19.18 55.37

7.3 6.9

71.32

6.1

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investigated with the help of thermodynamic theories. Sorption, i.e., solubility of the membrane is caused by the interaction of the penetrating species with the membrane and is a dynamic process where the solvent molecules are distributed randomly within the solid membrane. This implies that solvent molecules would interact differently, depending upon their dielectric constant or polar nature. In addition, the nature of filler particles, polymer and mixed matrix membranes are equally important to induce improved performance of the membrane. In PV dehydration, the separation can be achieved when the barrier membrane has the ability to transport or discriminate a particular component of the mixture (water) more readily than the other viz., organic component. Sorption tendencies of the pristine NaAlg and K-LTL zeoliteloaded mixed matrix membranes for each of the feed mixture compositions are displayed in Fig. 3 at the ambient temperature.

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Notice that in general, sorption of all the membranes increased with decreasing amount of filler SiO2 /Al2 O3 ratio from 20 to 10 for all the mixtures. In case of the membrane containing K-LTL zeolite (i.e., SiO2 /Al2 O3 -10), sorption as well as permeation flux values are higher than the pristine NaAlg membrane. For water–THF feed mixtures, the presence of high-polar THF molecules would allow only a small fraction of water molecules to permeate through, thereby offering reduced flux and selectivity to water. On the other hand, the less polar isopropanol (than THF) would allow higher amount of water to preferentially permeate with a slight increase in flux and selectivity data and with the mixture that contained a less polar 1,4-dioxane more of water molecules got transported with a considerable increase in flux and selectivity. When the filler silica–alumina ratio decreased, the equilibrium total sorption also increased along with flux and selectivity. From these observations, one can conclude that membranes of this study have higher affinity to water than organic components. Sorption data of the membranes for the azeotropic compositions of the mixtures at different temperatures are displayed in Fig. 4. Sorption of organic compound is higher at higher temperature due to the thermal motion of the polymeric chains in relation to solvent migration rate; this motion increased the permeation flux, giving a decreased selectivity to water. 3.5. Effect of feed composition

Fig. 3. Membrane sorption vs. wt.% of water in the feed. Symbols: (䊉) Prisitne NaAlg; () 10 wt.% K-LTL (SiO2 /Al2 O3 -20)-filled NaAlg-mixed matrix membranes; () 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg-mixed matrix membranes.

Table 2 illustrates the relationship between membrane performance and water composition in different aqueous–organic mixtures for the pristine NaAlg and K-LTL zeolite-loaded NaAlg membranes of different silica–alumina ratios at ambient temperature. Permeation flux increases with increasing water content of the feed, which is caused due to higher concentration of water molecules within the membrane. At the same time, wt.% of water in permeate line and selectivity values decreased for all the three aqueous–organic mixtures of this study. There is a two-fold increase in the permeation flux for all the mixtures because of the hydrophilic nature of the zeolite used and its interaction with NaAlg matrix. For all the feed mixtures, flux values of K-LTL zeolite-loaded NaAlg membranes are higher than that of the pristine NaAlg membrane. However, thermodynamic interactions in mixed feed media (organic + water binary) between the feed components would be responsible for such observed variations in the flux. This interaction depends upon the type and nature of the organic molecule in addition to the differences in the effective thermodynamic driving force across the membrane. For instance, an increase in permeation flux for water–1,4-dioxane mixture is purely due to an increase in activity (or chemical potential) of water in the feed mixture due to the unusual interactions between water and 1,4-dioxane. See for example, that in case of water + 1,4-dioxane feed mixture, the flux and selectivity values are higher than those observed for water + isopropanol and water + THF feed mixtures. Thus, thermodynamic interactions between organics and water molecules are the important in controlling the transport and hence, are thus responsible for the observed variations in flux and selectivity data.

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S.D. Bhat, T.M. Aminabhavi / Journal of Membrane Science 306 (2007) 173–185 Table 2 Pervaporation dehydration of different aqueous–organic mixtures at different feed compositions for NaAlg- and K-LTL-filled NaAlg-mixed matrix membranes at 30 ◦ C for membranes of 50 ␮m uniform thickness Feed composition (wt.%)

Permeation flux, J (kg/m2 h)

Water + THF Pristine NaAlg 10 20 30 40 50

0.052 0.067 0.077 0.121 0.123

1160 467 260 117 67

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.124 0.145 0.172 0.203 0.204

1906 674 326 139 87

10 20 30 40 50

Water + isopropanol Pristine NaAlg 10 20 30 40 50

0.062 0.075 0.087 0.127 0.133

Effect of varying membrane thickness on flux and selectivity was studied by varying the thickness of the membrane. K-LTL zeolite-loaded mixed matrix membranes in thicknesses of 30, 40 and 50 ␮m (within ±2 ␮m) were prepared and subjected to PV dehydration studies of the mixtures at their respective azeotropic compositions. Data presented in Table 3 show a gradual increase in flux with a decrease in selectivity for decreasing membrane thickness, keeping other operating parameters constant. In pervaporation, diffusion plays an important role which controls the rate determining step, decreased with increasing membrane thickness, thereby causing a subsequent reduction in permeation flux with increasing the membrane selectivity, a trend that is most commonly observed in all PV experiments. In a PV process, the

653 189 85 38 25

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.089 0.115 0.139 0.175 0.181

3324 1139 346 144 79

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.136 0.154 0.184 0.211 0.213

5991 1901 438 193 90

10 20 30 40 50

3.6. Effect of membrane thickness

213 94 54 29 19

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.082 0.105 0.126 0.164 0.166

10 20 30 40 50

Fig. 4. Membrane sorption vs. feed temperature. Symbols: (䊉) Prisitne NaAlg; () 10 wt.% K-LTL (SiO2 /Al2 O3 -20)-filled NaAlg membranes; () 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg-mixed matrix membranes.

Selectivity (α)

10 20 30 40 50

Water + 1,4-dioxane Pristine NaAlg 10 20 30 40 50

0.075 0.091 0.113 0.149 0.153

711 261 141 71 31

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.098 0.107 0.143 0.185 0.188

6914 2349 628 226 96

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.146 0.162 0.196 0.225 0.227

8991 2663 931 356 102

10 20 30 40 50 10 20 30 40 50

S.D. Bhat, T.M. Aminabhavi / Journal of Membrane Science 306 (2007) 173–185 Table 3 Pervaporation dehydration of different aqueous–organic mixtures at their azeotropic compositions for different uniform membrane thickness for 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg-mixed matrix membranes at 30 ◦ C Membrane thickness (␮m)

Permeation flux, J (kg/m2 h)

Selectivity (α)

Water (6.7 wt.%) + THF 50 ± 2 40 ± 2 30 ± 2

0.120 0.137 0.149

3081 485 278

Water (12.6 wt.%) + isopropanol 50 ± 2 0.140 40 ± 2 0.153 30 ± 2 0.162

3847 539 323

Water (18.1 wt.%) + 1,4-dioxane 50 ± 2 0.155 40 ± 2 0.178 30 ± 2 0.185

4109 692 426

upstream layer of the membrane is swollen and plasticized due to absorption of the feed liquid mixture, which would allow an unrestricted transport of feed components. In contrast, the downstream layer is virtually dry due to the continuous evacuation on the permeate side due to vacuum and hence, this layer would act as a restrictive barrier allowing only the transport of water, but not the organic components. It is also expected that the thickness of the dry layer would increase due to the addition of K-LTL zeolite particles of different silica–alumina ratios. However, an increase in the overall membrane thickness has resulted in improved membrane selectivity to water. 3.7. Effect of SiO2 /Al2 O3 ratio of K-LTL-type zeolite in PV at azeotropic composition The effect of SiO2 /Al2 O3 ratio of the K-LTL-type zeolite on PV performance was studied. The SiO2 /Al2 O3 ratio of the zeolite was varied from 10 to 20. A dramatic improvement over that of pristine NaAlg membrane performance was observed due to the addition of K-LTL zeolite particles of different silica–alumina ratios into NaAlg matrix. Zeolite-K-LTL being hydrophilic would adsorb large quantity of water in the pores by restricting the transport of organic components. This effect was more pronounced due to the adsorptive nature of the filler zeolite particles. Flux and selectivity results increased with decreasing silica–alumina ratios due to increased hydrophilicity of K-LTL zeolite. A somewhat stronger interaction between hydrophilic K-LTL zeolite particles and water seem to render more of water molecules to readily transport across the membrane compared to organic component. With high silica–alumina ratio, flux and selectivity results are lower due to decreased hydrophilicity of zeolite in the NaAlg matrix. It is observed that for water (18.1 wt.%) + 1,4-dioxane azeotrope, selectivity to water is higher than observed for water (12.6 wt.%) + isopropanol and water (6.7 wt.%) + THF feeds. Thus, increased selectivity at lower silica–alumina ratio of KLTL zeolite is due to increased hydrophilic interaction of the zeolite-loaded with NaAlg matrix. However, compared to pristine NaAlg membrane, K-LTL-mixed matrix membranes have

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shown unique permeation trends due to (i) adsorption and diffusion of liquid molecules through the molecular sieve pores and (ii) transport along the boundaries between molecular sieve crystals and of the NaAlg segments. The former process would play a dominant role, since K-LTL-loaded NaAlg membrane would form a good adhesion, whereas the latter will become dominant when K-LTL–polymer interface is poor [35]. Thus, the unique properties like molecular sieving effect and adsorption preference would take into effect only when K-LTL zeolite particles are compatible with the NaAlg polymer phase. Hence, interfacial properties of the zeolite-filled mixed matrices are critical to their separation performances. In the present study, K-LTL zeolite particles are able to discriminate distinctly the components of the binary aqueous–organic mixtures either by the exclusion of the competing molecules on the basis of molecular size and shape or by adsorption preference. The incorporation of K-LTL zeolite particles into NaAlg matrix is expected to enhance the separation performance of the NaAlg matrix. 3.8. Thermodynamic interactions Sorption selectivity values of the pristine NaAlg and K-LTLloaded mixed matrix membranes are explained in terms of the thermodynamic interactions using Flory–Huggins theory [36]. For this, we have typically selected water + 1,4-dioxane mixture since, it offered higher flux and selectivity values than the other two mixtures during PV dehydration experiments of the pristine and K-LTL-loaded NaAlg mixed matrix membranes. For the computation using Gibbs free energy of mixing (Gmix ), Aminabhavi and Munk [37] for a three-component system was used. Notice that a similar approach was extended for analyzing the PV data [38]. Following these approaches, thermodynamic equation for sorption selectivity, αS can be derived as         vi Vi ϕi ϕj − ln = − 1 ln ln αS = ln ϕj vj Vj vj   Vi −χij (ϕj − ϕi ) − χij (vi − vj ) − ϕP χiP − χjP Vj (5) Here, ϕi and ϕj are volume fractions of water and 1,4-dioxane, respectively in the swollen polymer membrane; vi and vj are respective volume fractions of water and 1,4-dioxane in the external liquid phase; Vi and Vj are individual molar volumes of water and 1,4-dioxane. The volume fraction, φP , of the pristine and mixed matrix membranes in the swollen state was calculated using [39]:    −1  ρP ρ P Ma φP = 1 + − (6) ρS M b ρS where ρP is density of the pristine polymer and mixed matrix membranes; ρS is density of solvent; Mb and Ma are the weights of membranes before and after swelling. Densities of the membranes were measured by benzene displacement method using the specific gravity bottle, since the sorption capacities of the membranes are better in benzene as shown in Fig. 5 for all the membranes. Initially, the benzene-filled bottle and empty

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Fig. 5. Sorption capacity of benzene for pristine NaAlg, 10 wt.% K-LTL (SiO2 /Al2 O3 -20)-filled NaAlg membranes and 10 wt.% K-LTL (SiO2 /Al2 O3 10)-filled NaAlg-mixed matrix membranes.

bottle weights were taken. The weighed quantity of the polymeric membranes was then introduced into the bottle. Excess benzene was wiped out using soft tissue papers. Weights of the bottle along with benzene and membranes were measured. These weights were also used to calculate the density of the membranes. Molar volume of the binary mixtures of water + 1,4dioxane was calculated using [40]: V =

x i Mi + x j M j ρsm

(7)

where xi and xj are the mole fractions of water and 1,4-dioxane, respectively; Mi and Mj are the corresponding molecular weights; ρsm is density of the solvent mixture (measured by Anton Paar digital density meter, Model DMA 4500, K.G. Austria). The interaction parameter, χij between water and 1,4-dioxane was calculated using the equation [41]: χij =

[xi ln(xi /vi ) + xj ln(xj /vj ) + (GE /RT )] xi vj

(8)

Excess Gibbs free energy, GE , was calculated from the activity coefficients, γ, of the mixtures as GE = RT (xi ln γi + xj ln γj )

(9)

For calculating γ i and γ j , we have used the van Laar equation at 30 ◦ C to compute the activity coefficient, γ i of component, i in the mixture as  2 Aji xj (10) ln γi = Aij Aij xi + Aji xj The van Laar parameters, Aij for water (2.065) and Aji for 1,4dioxane (1.6999) were taken from the literature [42]. Then, polymer–solvent interaction parameter, χiP was calculated using the well-known Flory–Huggins equation [36]: χiP =

Vi (δP − δi )2 RT

(11)

Fig. 6. Theoretical and experimental plots of sorption selectivity vs. wt.% of water in the feed. Symbols: (black box) theoretical; (plain box) experimental.

where δi is the solubility parameter (J1/2 cm−3/2 ) of ithcomponent. The solubility parameter, δP of NaAlg polymer was estimated to be 61.48 J1/2 cm−3/2 from the atomic group contribution method [43]. Solubility parameter values of water (33.02 J1/2 cm−3/2 ) and 1,4-dioxane (97.82 J1/2 cm−3/2 ) were taken from the literature [44]. These data were fitted into Eq. (5) to compute sorption selectivity, αS . Experimental and theoretical plots of sorption selectivity for pristine NaAlg and K-LTL (SiO2 /Al2 O3 -10) zeolite-loaded mixed matrix membranes are given in Fig. 6. One can observe that experimental and theoretical values of sorption selectivity are quite comparable. Sorption selectivity is higher for K-LTL-loaded mixed matrix membrane than for the pristine NaAlg. This is due to the water selective nature of the mixed matrix membranes and also the molecular sieving effect offered by zeolite particles. Majority of water gets adsorbed by hydrophilic micropores of the zeolite particles, making it more hydrophilic, particularly when silica–alumina ratio was small, thereby extracting the more amount of water on the permeate side, thereby enhancing the permeation flux and selectivity to water. We found that the thermodynamic treatment based on Flory–Huggins theory can be successfully used in explaining PV results. With lower silica–alumina ratios (i.e., SiO2 /Al2 O3 -10) of K-LTL-loaded mixed matrix membranes, due to higher amount of water molecules in permeate, selectivity is also higher. Thus, it is obvious that there is a noticeable difference in sorption and pervaporation selectivity data. However, the trends observed are similar to sorption results explained before.

S.D. Bhat, T.M. Aminabhavi / Journal of Membrane Science 306 (2007) 173–185

181

3.9. Permeation and driving force mechanism Effect of driving force mechanism and permeation due to interactions between feed components are important in PV dehydration [45–47]. To explain this mechanism in depth, we have selected water + 1,4-dioxane feed mixture under the operating conditions of 30 ◦ C. Permeation rate (J) can be written according to the theory of Wijmans and Baker [48]: p

Jw = (pfi − pi )

Pi l

p

Jorg = (pfj − pj )

Pj l

(12) (13)

Here, the superscripts, f and p refer to feed and permeate, respectively; pi and pj are partial vapor pressures of water and 1,4-dioxane, respectively; Pi and Pj are membrane permeability coefficients of water and 1,4-dioxane, respectively, which are the product of diffusion (Dij ) and solubility coefficients (Sij ). Diffusion and solubility coefficients are calculated using the procedure described elsewhere [49]; l is thickness of the membrane and Pi /l and Pj /l are permeances of water and 1,4dioxane, respectively. The relationship between partial vapor pressure (pf ), molar concentration of water (xi ) and 1,4-dioxane (xj ) and activity coefficients of individual components of the feed obtained from Eq. (10) are given as pfi = xi γi psi

(14)

pfj = xj γj psj

(15)

Saturated vapor pressures of water (psi ) and 1,4-dioxane (psj ) were calculated using the Antonine equation [47] given in the form: log p0ij =

A−B T +C

(16)

where p0ij are vapor pressures of water and 1,4-dioxane, respectively; A–C the Antonine constants; T is temperature in Kelvin. By considering activity coefficient and molar concentrations of individual component, saturated vapor pressure of water and 1,4-dioxane can be written as psi = xi γi p0i

(17)

psj = xj γj p0j

(18)

Compiling Eqs. (12)–(15), we obtain:   Pi p (xi γi psi − pi ) Jw = l   Pj p Jorg = (xj γj psj − pj ) l

Fig. 7. Water and 1,4-dioxane permeance vs. wt.% of water in the feed for pristine NaAlg and 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg-mixed matrix membranes.

By knowing the molar water concentration in the feed and activity coefficient, the activity of water in water + 1,4-dioxane feed mixtures can be calculated. Permeances of water and 1,4-dioxane with respect to feed compositions for pristine NaAlg and K-LTL (SiO2 /Al2 O3 10)-loaded NaAlg membrane are displayed in Fig. 7. Water permeance is higher than 1,4-dioxane, which is obvious considering the hydrophilic nature of K-LTL. Water permeance increased for K-LTL (SiO2 /Al2 O3 -10)-loaded mixed matrix membrane of NaAlg than the pristine NaAlg membrane. Ideal membrane selectivity for water + 1,4-dioxane mixture also increased for K-LTL (SiO2 /Al2 O3 -10)-loaded NaAlg membrane than the pristine NaAlg due to a good compatibility between zeolite and NaAlg matrix as can be seen in Fig. 8. Water activity in water + 1,4-dioxane feed mixtures also increased with increasing feed concentration for pristine as well as K-LTL-loaded NaAlg membranes following the

(19) (20)

The ratio of membrane permeances (βij ) is defined as the ideal membrane selectivity, which is given as   Pi (21) βij = Pj

Fig. 8. Ideal membrane selectivity for pristine NaAlg and 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg-mixed matrix membranes.

182

S.D. Bhat, T.M. Aminabhavi / Journal of Membrane Science 306 (2007) 173–185 Table 4 Pervaporation dehydration of different aqueous–organic mixtures at different temperatures for NaAlg- and K-LTL-filled NaAlg-mixed matrix for membranes of 50 ␮m uniform thickness Temperature

Permeation flux, J (kg/m2 h)

Water (6.7 wt.%) + THF Pristine NaAlg 30 0.048 40 0.053 50 0.061 60 0.069 70 0.074 Fig. 9. Water activity for pristine NaAlg and 10 wt.% K-LTL (SiO2 /Al2 O3 -10)filled NaAlg-mixed matrix membranes.

transport trends as displayed for water permeance in Fig. 9. The K-LTL-loaded NaAlg matrix may be particularly well suited for the separation of 1,4-dioxane and water mixtures as supported by experimental flux and selectivity results.

332 227 166 130 101

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.077 0.093 0.111 0.120 0.123

1894 372 241 163 126

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.120 0.143 0.154 0.161 0.165

3081 458 274 184 147

30 40 50 60 70 30 40 50 60 70

Water (12.6 wt.%) + isopropanol Pristine NaAlg 30 0.068 40 0.072 50 0.080 60 0.084 70 0.087

340 243 174 140 109

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.099 0.110 0.118 0.126 0.128

2305 399 255 172 137

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.140 0.151 0.157 0.164 0.166

3847 507 301 196 157

30 40 50 60 70 30 40 50 60 70

Water (18.1 wt.%) + 1,4-dioxane Pristine NaAlg 30 0.082 40 0.088 50 0.094 60 0.097 70 0.100

349 251 214 165 131

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -20) + NaAlg 0.104 0.114 0.123 0.134 0.137

3012 482 310 206 145

10 wt.% zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg 0.155 0.169 0.181 0.189 0.191

4109 714 386 251 186

30 40 50 60 70

Fig. 10. Plots of ln J vs. 1000/T. Symbols: (䊉) Prisitne NaAlg; () 10 wt.% K-LTL (SiO2 /Al2 O3 -20)-filled NaAlg membranes; () 10 wt.% K-LTL (SiO2 /Al2 O3 -10)-filled NaAlg membranes.

Selectivity (α)

30 40 50 60 70

S.D. Bhat, T.M. Aminabhavi / Journal of Membrane Science 306 (2007) 173–185

183

Table 5 Activation energy, EJ for different azeotropic mixtures Azeotropic mixture

Water + THF Water + isopropanol Water + 1,4-dioxane

EJ (kJ/mol) Pristine NaAlg

Zeolite–K-LTL (SiO2 /Al2 O3 -20) + NaAlg

Zeolite–K-LTL (SiO2 /Al2 O3 -10) + NaAlg

79.46 67.45 44.30

73.29 55.66 25.03

67.98 51.66 17.36

3.10. Temperature effect Temperature is an important variable affecting the membrane performance in terms of flux and selectivity as seen from the results given in Table 4. Published reports [22] have shown that variation of flux, J, with temperature can be expressed by the Arrhenius relationship:   EJ J = J0 exp − (22) RT where J0 , EJ , R and T are the pre-exponential factor, apparent activation energy (kJ/mol) for permeation flux, molar gas constant and feed temperature (T) in Kelvin, respectively. From Eq. (22), permeation activation energy was evaluated. Using the respective permeation flux values obtained at 30, 40, 50, 60 and 70 ◦ C, the EJ values for water were computed by the least squares

method from the slopes of ln J vs. 1000/T plots (see Fig. 10). Table 5 presents the comparison of activation energies estimated for K-LTL zeolite-loaded mixed matrix NaAlg membranes with that of pristine NaAlg membrane. It is observed that activation energies for pristine NaAlg membrane are higher than those of the mixed matrix NaAlg membranes for all the feed mixtures at their azeotropic compositions. Therefore, more energy is required for molecules to transport across the membrane under similar conditions. Lower activation energies observed for mixed matrix membranes are desirable and would represent the typical intrinsic properties of such hydrophilic membranes developed in this study, since our objective was to dehydrate organics. Experimental temperature has a significantly stronger effect on water + THF mixture permeation than the other feed mixtures, since EJ is higher for this mixture than observed for the other

Table 6 Comparison of PV performance of the present NaAlg-mixed matrix membranes with other membranes for aqueous–organic mixtures Membrane type Water + THF Zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg NaAlg-HEC-10 (GA + UFS cross-linked) Y-type zeolite with Al2 O3 support Mordenite with Al2 O3 support ZSM-5 zeolite with Al2 O3 support A-type zeolite with Al2 O3 support NaA zeolite with ceramic support Water + isopropanol Zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg Two ply composite membranes of NaAlg and chitosan PVA PVA + KA PVA + NaA PVA + CaA PVA + NaX NaA zeolite with zirconia support ZSM-5 zeolite with ␣-Al2 O3 support Mordenite with ␣-Al2 O3 support Water + 1,4-dioxane Zeolite-K-LTL (SiO2 /Al2 O3 -10) + NaAlg NaAlg (GA + UFS cross-linked NaA zeolite with Al2 O3 support

Thickness (␮m)

Temperature (◦ C)

50

30

50

30

Permeation flux, J (kg/m2 h)

α

Reference

0.120

3081

Present work

10

0.183

1516

[17]

45

10 6.7 6.7 6.7 6.1

0.150 0.031 0.010 0.029 0.023

230 11 3.4 49 2000

50

30

12.6

0.140

3847

Present work

40–50

60

10

0.554

2010

[8]

NA

50

0.146 0.179 0.183 0.190 0.216 0.044 0.180 0.014

233 410 328 233 233 4000 690 330

NA NA NA NA 5–10

60

Wt.% of water in feed 6.7

20

60

[50]

[51]

[52]

NA 1.5 3

75

10 10 10

[53]

50

30

18.1

0.155

4109

Present work

50 30

30 60

10 10

0.115 0.100

268 9000

[17] [55]

[54]

NaAlg, sodium alginate; HEC, hydroxy ethyl cellulose; PVA, poly(vinyl alcohol), GA, glutaraldehyde; UFS, urea–formaldehyde–sulfuric acid; KA, NaA, CaA and NaX, zeolites; NA, not available.

184

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two mixtures. The smaller activation energy values observed for the mixed matrix NaAlg membranes than the pristine NaAlg membrane suggests that the energy required to cross the barrier decreased, thereby allowing more of water molecules to readily permeate across over the Eyring’s energy barrier through the membrane. Due to the addition of K-LTL particles, higher amount of water molecules could permeate compared to organic components across the membrane per unit time, resulting in increasing amount of water in permeate side, giving enhanced selectivity to water. At any rate, flux and wt.% of water in permeate side are strongly dependent upon feed temperature as shown in Table 4. 3.11. Comparison of present membranes with literature data In the literature, many different types of membranes were used to study the PV dehydration of isopropanol, 1,4-dioxane and THF from aqueous mixtures due to their widespread applications in chemical and pharmaceutical disciplines. The present study mainly focuses on PV dehydration of three organics, viz. isopropanol, 1,4-dioxane and THF from the aqueous media. Hence, comparisons are made in Table 6 with other important literature data on blend/composite membranes as well as zeolitemembranes with different supports [50–55]. Flux and selectivity values of the present membranes are comparable with those of other important membranes used in the literature. In some cases, the present membranes gave better performances than those published in the literature. However, small differences in membrane performances are attributed to different experimental conditions used by different authors. At any rate, to achieve a proper balance between flux and selectivity in PV dehydration is a difficult task. 4. Conclusions The present study is a part of our continuing effort on the development of mixed matrix membranes of sodium alginate polymer used in organic dehydration studies. In this study, we have chosen three industrially important liquids such as isopropanol, 1,4-dioxane, and THF for their effectiveness in separating through mixed matrix membranes compared to pristine NaAlg membranes. Hydrophilic–hydrophilic interactions between NaAlg polymer segments and zeolite particles seem to be responsible for increased selectivity to water. Permeation flux and selectivity data were improved due to the addition of K-LTL zeolite particles over and above that of pristine NaAlg membrane due to absorptive nature of the mixed matrix membranes. Efficiency of the mixed matrix membranes in removing water from the mixed aqueous media was quite considerable for isopropanol and 1,4-dioxane than THF. To study this effect in depth, we have used Flory–Huggins thermodynamic theory of polymer solutions as well as the driving force mechanism to narrate the influence of thermodynamic interaction parameters between components of the mixed media and membranes. From the thermodynamic treatment, we conclude that the nature of organic liquid of the mixture

has an important role to play in enhancing the membrane performance. Thermodynamic theory of Flory–Huggins was also used to determine sorption selectivity for pristine and mixed matrix membranes. Arrhenius equation fitted the experimental temperature-dependent data of flux quite well. Estimated Arrhenius parameters varied, depending upon the thermodynamic nature of the mixed feed media and their interactions with the pristine NaAlg as well as the K-LTL-loaded NaAlg mixed matrix membranes. Acknowledgements The authors thank the University Grants Commission (UGC), New Delhi, India for a major funding (F1-41/2001/CPP-II) to establish Center of Excellence in Polymer Science at Karnatak University, Dharwad. Authors wish to thank Dr. S.B. Halligudi and Dr. P.N. Joshi (Scientists at National Chemical Laboratory, Pune, India) for their support in the synthesis of zeolite K-LTL. References [1] T.M. Aminabhavi, R.S. Khinnavar, S.B. Harogoppad, U.S. Aithal, Q.T. Nguyen, K.C. Hansen, Pervaporation separation of organic–aqueous and organic–organic binary mixtures, J. Macromol. Sci. Rev. Macromol. Chem. Phy. C 34 (1994) 139. [2] X. Feng, R.Y.M. Huang, Liquid separation by membrane pervaporation—a review, Ind. Eng. Chem. Res. 36 (1997) 1048. [3] P. Shao, R.Y.M. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162. [4] S.D. Bhat, T.M. Aminabhavi, Pervaporation separation using sodium alginate and its modified membranes—a review, Sep. Purif. Rev. 36 (2007) 203. [5] T.M. Aminabhavi, B.V.K. Naidu, S. Sridhar, R. Rangarajan, Pervaporation separation of water–isopropanol mixtures using polymeric membranes: modeling and simulation aspects, J. Appl. Polym. Sci. 95 (2005) 1143. [6] U.S. Toti, T.M. Aminabhavi, Different viscosity grade sodium alginate and modified sodium alginate membranes in pervaporation separation of water + acetic acid and water + isopropanol mixtures, J. Membr. Sci. 228 (2004) 199. [7] R.Y.M. Huang, R. Pal, G.Y. Moon, Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate/chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane, J. Membr. Sci. 166 (2000) 275. [8] G.Y. Moon, R. Pal, R.Y.M. Huang, Novel two-ply composite membranes of chitosan and sodium alginate for the pervaporation dehydration of isopropanol and ethanol, J. Membr. Sci. 156 (1999) 17. [9] X.P. Wang, Modified alginate composite membranes for the dehydration of acetic acid, J. Membr. Sci. 170 (2000) 71. [10] G. Yang, L. Zhang, T. Peng, W. Zhong, Effects of Ca2+ bridge cross-linking on structure and pervaporation of cellulose/alginate blend membranes, J. Membr. Sci. 175 (2000) 53. [11] P. Kanti, K. Srigowri, J. Madhuri, B. Smitha, S. Sridhar, Dehydration of ethanol through blend membranes of chitosan and sodium alginate by pervaporation, Sep. Purif. Technol. 40 (2004) 259. [12] C.K. Yeom, K.H. Lee, Characterization of sodium alginate and poly (vinyl alcohol) blend membranes in pervaporation separation, J. Appl. Polym. Sci. 67 (1998) 949. [13] C.K. Yeom, K.H. Lee, Characterization of permeation behaviors of ethanol–water mixtures through sodium alginate membrane with crosslinking gradient during pervaporation separation, J. Appl. Polym. Sci. 69 (1998) 1607. [14] Y. Shi, X. Wang, G. Chen, Pervaporation characteristics and solution–diffusion behaviors through sodium alginate dense membrane, J. Appl. Polym. Sci. 61 (1996) 1387.

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