Aluminum-rich zeolite beta incorporated sodium alginate mixed matrix membranes for pervaporation dehydration and esterification of ethanol and acetic acid

Journal of Membrane Science 318 (2008) 233–246

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Aluminum-rich zeolite beta incorporated sodium alginate mixed matrix membranes for pervaporation dehydration and esterification of ethanol and acetic acid夽 Susheelkumar G. Adoor, Lata S. Manjeshwar, Santoshkumar D. Bhat, Tejraj M. Aminabhavi ∗ Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India

a r t i c l e

i n f o

Article history: Received 6 October 2007 Received in revised form 16 February 2008 Accepted 21 February 2008 Available online 4 March 2008 Keywords: Pervaporation Sodium alginate Zeolite beta Mixed matrix membranes

a b s t r a c t Mixed matrix membranes of sodium alginate (NaAlg) were prepared by solution casting by incorporating 2.5, 5, 7.5 and 10 wt.% of zeolite beta particles. The membranes thus prepared were crosslinked with glutaraldehyde and tested for the pervaporation (PV) dehydration of ethanol and acetic acid at 30–60 ◦ C. The aluminum-rich zeolite beta, with its hydrophilic nature as well as molecular sieving effect and its favorable interaction with hydrophilic NaAlg, was responsible to enhance the PV dehydration of acetic acid and ethanol in terms of separation factor, flux, pervaporation separation index (PSI) and enrichment factor (ˇ). Thermodynamic model for sorption process was investigated typically for water + ethanol mixtures based on Flory–Huggins theory to explain the PV performance. Based on these results, permeance and driving force mechanisms were also elucidated. Extraction or dissolution of zeolite beta from mixed matrix membranes is confirmed by equilibrium sorption. Additionally, the changes in the dimension of pristine and mixed matrix membranes are compared to provide the influence of swelling on the stabilities of mixed matrix membranes. Arrhenius parameters for the process of permeation were calculated using these data at different temperatures to investigate their effects on the nature of the mixed matrix membrane. The plots of ln Jp vs. 1/T were constructed and found to follow the linear trends in the studied range of 30–60 ◦ C for both the feed mixtures, indicating that flux followed the Arrhenius trend. PV experiments were also carried out for 5 and 10 wt.% incorporated NaAlg mixed matrix membranes at 70 ◦ C to verify the suitability of the membranes at the esterification temperature. PV-aided catalytic esterification of acetic acid with ethanol was studied at 70 ◦ C, which led to a considerable increase in ethyl acetate conversion with a reduction in reaction time as compared to the blank reaction due to continuous removal of water permeating through the barrier membrane. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Of the various types of membrane-based separations, PV has been widely used in chemical and biochemical industries for the selective removal of solvents from the mixed media in view of its low energy consumption, high separation characteristics and lower environmental pollution as compared to other conventional type techniques [1–3]. PV has also been used for organic–organic mixture separations [4,5]. The chemical potential gradient across the membrane acts as a driving force for molecular transport to occur. PV dehydration membranes should have high hydrophilicity, good mechanical strength and thermal stability [6]. In this sense, alginic

夽 This paper is Center of Excellence in Polymer Science communication # 171. ∗ Corresponding author. Present address: Reliance Industries Limited, Navi Mumbai, India. Tel.: +91 836 2215372; fax: +91 836 2771275. E-mail address: [email protected] (T.M. Aminabhavi). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.043

acid, which is a highly hydrophilic polysaccharide polymer that is present in some seaweeds is quite useful to develop membranes. But, its high swelling nature in the presence of hydrophilic solvents is a major concern in its usage as a membrane in PV. The alginate form of alginic acid fulfils both the requirements of hydrophilicity as well as mechanical strength properties in PV dehydration of organic solvents [2]. In the recent past, researchers have successfully used sodium alginate, since it possesses the better mechanical strength as compared to other PV membranes for the dehydration of aqueous–organic mixtures [7–10]. It is a linear chain structure of (1–4)-linked ␤-d-mannuronic acid (M) and ␣-l-guluronic acid (G) residues arranged in a blockwise fashion. These blocks are constructed in three different ways: homopolymeric MM blocks, homopolymeric GG blocks and heteropolymeric sequentially alternating MG blocks [11,12]. The presence of ␣-l-guluronic acid in various ratios and molecular weights alters the physico-chemical properties of the polymer [13]. Sometimes, its performance as a

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membrane exceeds those of poly(viny1 alcohol) (PVA) in a filled matrix form [14], ion-exchange resins [15] and other polysaccharides such as chitosan [16] and natural cellulose [17]. Aluminum-rich zeolite beta has the three-dimensional 12membered ring interconnected channel system (tortuous channel along the c direction and straight channels along a and b directions), like faujasite and EMC2, but its framework is very different from the structures of these zeolites. Aluminum-rich samples can easily be interpreted as mechanical mixtures between zeolite beta with five or six aluminum atoms per unit cell (64 tetrahedra) and an amorphous phase with nearly one aluminum atom per three tetrahedra (Si + Al) [18]. Earlier literature suggests that by increasing the Al content, the stability of beta particles can be increased [19–22]. Zeolite beta is thus having potential industrial applications as a catalyst in fluid catalytic cracking, hydrotreating, dewaxing and alkylation [23–28]. Zeolite beta is also used in hollow fiber membranes for gas separation applications [29,30]. PV esterification coupling has attracted much attention due to its efficiency over the conventional method. Hydrophilic membranes would allow the selective removal of produced water molecules such that successful esterification can be achieved [31]. PV membrane reactors have also been studied for esterification of oleic acid and ethanol [32], propionic acid and propanol [33,34], tartaric acid and ethanol [35], oleic acid and butanol [36] with various acids or lipases as the catalysts. In some cases, membrane itself can be catalytically active [37,38]. Waldburger et al. [39] studied the heterogeneously catalyzed esterification of acetic acid and ethanol, and proposed the cascade arrangements of membrane reactors for a continuous operation. Dehydration of ethanol and acetic acid are the most difficult tasks using the conventional methods. In order to overcome this problem, in this paper, we have reported the dehydration and esterification coupled pervaporation of acetic acid with ethanol using the NaAlg-beta filled mixed matrix membranes. In the earlier literature, many researchers have successfully employed such mixed matrix membranes for aqueous–organic mixture separation [40–43] by the PV method. The present study demonstrates the applicability of zeolite beta filled NaAlg membranes for the effective dehydration of acetic acid and ethanol from their aqueous streams as well as study of esterification of acetic acid with ethanol for the enhanced conversion of ethyl acetate as compared to the conventional esterification. PV performance has been discussed in terms of polymer–zeolite and membrane–solvent interactions as well as thermodynamic interaction and driving force mechanism. 2. Experimental 2.1. Materials Sodium alginate, acetic acid, ethanol, glutaraldehyde (GA), acetone and hydrochloric acid (AR) 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 the Permionics pilot plant (Vadodara, India) by employing the reverse osmosis membrane module. The zeolite beta samples were kindly supplied by Dr. S.B. Halligudi, Scientist, Catalysis Division, National Chemical Laboratory, Pune, India. Sulfonated cation exchange resin (Dowex-50) used in the esterification reaction was kindly provided by Dr. G.S. Gokavi, Shivaji University, Kolhapur, India. 2.2. Membrane fabrication NaAlg (4 g) was dissolved in 80 mL of water under constant stirring. Different amounts of zeolite beta particles were dispersed

in 20 mL of water separately, sonicated for 1 h, added to the previously prepared NaAlg aqueous solution and the mixture was stirred for about 24 h. The above mixture was poured on a clean glass plate to cast the membranes. The membranes formed were allowed to dry at the ambient temperature. Dried membranes were peeled off from the glass plate and immersed in a crosslinking solution bath containing water and acetone (30:70) mixture with 2.5 mL of GA and 2.5 mL of conc. HCl. After 12–14 h, the crosslinked membranes were removed from the bath, washed repeatedly with deionized water and dried in a hot air oven at 40 ◦ C. The pristine NaAlg membrane was prepared in a similar manner without the addition of filler zeolite particles. Membrane thicknesses as measured by the micrometer screw gauge were around 50 ± 1.0 ␮m. Thus prepared pristine NaAlg, 2.5, 5, 7.5 and 10 wt.% zeolite beta filled NaAlg membranes were designated as NaAlg, NaAlg-1, NaAlg-2, NaAlg-3 and NaAlg-4, respectively. It is important to note that for mixed matrix membranes with the incorporation of more than 10 wt.% of zeolite beta, phase segregation was observed and membranes were brittle which could not be employed for the PV and/or esterification experiments. 2.3. Particle size measurement of zeolite beta Particle size and size distributions of zeolite beta have been measured by 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 cuvette subjected to laser beam radiation. Series of readings were taken and the average distribution histograms were considered to measure the particle size range. 2.4. Scanning electron microscopy (SEM) Cross-sectional SEM micrographs of NaAlg-2 and NaAlg-4 mixed matrix membranes were obtained under similar resolutions (Mag. 10KX) using Leica Stereoscan-440 scanning electron microscope equipped with Phoenix energy dispersive analysis of X-rays (EDAX). Since these films were nonconductive, gold coating (15 nm thickness) was done on samples and measurements were done at Indian Institute of Science, Bangalore, India. 2.5. Universal testing machine (UTM) Tensile strength and % elongation at break of the pristine NaAlg, NaAlg-1, NaAlg-2, NaAlg-3 and NaAlg-4 filled mixed matrix membranes were measured using the universal testing machine (Model H 25 KS, Hounsfield, Surrey, United Kingdom). Test specimens were prepared in the form of dumbbell shapes as per ASTM D-638 standards. Films of gauge length of 50 mm and width of 10 mm were stretched at the crosshead speed of 10 mm/min. 2.6. Fourier transform infrared spectroscopy (FTIR) FTIR spectral measurements were performed using Nicolet, model Impact 410, USA spectrophotometer to confirm the presence of zeolites in NaAlg matrix. Membranes were finely ground with KBr to prepare the pellets under a hydraulic pressure of 400 kg and spectra were scanned between 400 and 4000 cm−1 . 2.7. Pervaporation–esterification coupling experiments Pervaporation–esterification coupling experiments are the same as originally described before for PV separation of aqueous–organic mixtures [44]. The permeation cell was made of stainless steel and the effective membrane area of 28.3 cm2 was

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sticked on a porous stainless steel support plate using 704 silicone rubber. PV experiments were performed by maintaining on one side of the membrane (feed) a mixture of acetic acid–water or ethanol–water, which was fed batchwise at the atmospheric pressure and on the other (permeate or product) side, a reduced pressure of 6.66 Pa through a vacuum pump (Toshniwal, Mumbai, India) was maintained. Capacity of the PV cell was 250 mL, but to start with, 50 g of the feed mixture was added on the upper side of the membrane and stirred at the constant temperature by circulating water through the thermostatic water jacket. Before starting the PV experiment, 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 collected was measured on a Mettler Balance (model B 204-S, Greifensee, Switzerland: accuracy 10−4 g) to determine the flux, J (kg/m2 h) using the weight of liquids permeated, W (kg), effective membrane area, A (m2 ) and measurement time, t (h) as J=

W At

(1)

The analyses of feed and permeate samples were done using a Nucon Gas Chromatograph (GC) (Nucon, model 5765, Mumbai, India) provided with a Thermal Conductivity Detector (TCD) equipped with a DEGS or Tenax packed column of 1/8 in. internal diameter having 2 m length. The oven temperature was maintained at 70 ◦ C (isothermal), while the 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 0.073 MPa. The GC response was calibrated for column and for known compositions of water + organic (acetic acid or ethanol) mixtures. Calibration factors were fed into the GC software to obtain the analysis for unknown samples. Pervaporation separation factor ˛ was calculated using: ˛=

 P 1 − F  A A 1 − PA

(2)

FA

Pervaporation separation index (PSI) and enrichment factor (ˇ) were calculated as PSI = J(˛ − 1) ˇ=

(3)

P Cw

(4)

F Cw

where FA is the wt.% of water in feed and PA is the wt.% of water in permeate, P and F are the weight fractions of permeate and feed, respectively. Subscript, w stands for water, J is the water flux and C is the concentration in wt.%. A minimum of three independent measurements of flux and separation factor were taken under similar conditions of temperature and feed compositions to confirm the steady-state pervaporation. Standard errors in computing different mixture compositions were less than 3%, since all the weight measurements were done within ±0.01 mg. Triplicate measurements of flux and separation factor were reproducible within 3% standard errors and average values were considered in all the computations. Esterification reaction accompanied by pervaporation was performed in the same membrane reactor. Sulfonated cation exchange resin (Dowex-50) was used as a catalyst for the reaction. Product samples were drawn periodically and analyzed to follow the reaction course. The conversion of membrane catalytic reaction was determined by ethanol conversion. This type of calculational method was reasonable because acid was always excessive, but alcohol permeation was less. Thus % Conversion (X) =

 W − (W + W )  1 2 W

× 100

(5)

235

where W is the initial weight of alcohol (g), W1 is the weight of excess alcohol in the reaction mixture (g) and W2 is the weight of the permeated alcohol (g). A general kinetic expression for second order reversible reaction was used to find the kinetic parameters of esterification reaction. The reaction rate with respect to acetic acid can be written as k Ca Cb − kf Cc Cw −dCa = f Ke dt

(6)

where Ca is the molar concentration of acetic acid, Cb is the molar concentration of ethanol, Cc is the molar concentration of ethyl acetate and Cw is the molar concentration of water. The equilibrium constant, Ke was calculated from the equilibrium conversion using the equation: Ke =

X2 (1 − X)(2.88 − X)

(7)

Here, X represents the equilibrium conversion in terms of molar concentration. Under initial conditions: Cb0 = 2.88Ca0 and Cc0 = Cw0 = 0 at t = 0 such that by writing all the concentrations in terms of conversion and initial concentration of acetic acid, we get Ca = Ca0 (1 − X);

Cb = Ca0 (2.88 − X);

Cc = Cw = Ca0 X

(8)

Then, transforming the above equation in the rate equation and simplifying, we get f (X) =

−2.0 ln (1 − X) + 2 ln (2.88 − X) = kf Ca0 t (1 − (1/Ke ))

(9)

where X represents the conversion in terms of molar concentration with respect to time, t and kf represents the rate of forward reaction. Thus, by plotting f(X) vs. time and by computing slopes of the lines, we have calculated the term, kf Ca0 . The straight lines obtained demonstrate the applicability of rate law equation for the studied esterification reaction. 2.8. Sorption experiments Sorption experiments were performed gravimetrically [45] typically on NaAlg, NaAlg-2 and NaAlg-4 membranes taken in 10, 15, 20, 25 and 30 wt.% water containing feed mixtures at 30 ◦ C. Initial weight of the circularly cut (diameter = 2.5 cm) NaAlg, NaAlg-2 and NaAlg-4 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 solvent. Test bottles were transferred to a WTB binder incubator (model BD-53, Tuttlingen, Germany) maintained at the desired constant temperature. Dry membranes were equilibrated by soaking in different compositions of the feed mixture in a sealed vessel at 30 ◦ C for 48 h. The sorbed membranes were weighed immediately after careful blotting with the soft tissue papers on a digital single pan microbalance. The % sorption was then calculated as % sorption =

W − W  ∞ 0 W0

× 100

(10)

where W∞ and W0 are, respectively the weights of sorbed and dry membranes. Sorption experiments were also carried out for pristine as well as all mixed matrix NaAlg membranes in water, ethanol, acetic acid, 10 wt.% water + ethanol and 10 wt.% water + acetic acid mixtures following the procedure as discussed above. Sorption selectivity experiments for water + ethanol with different feed compositions were measured for the pristine NaAlg and NaAlg-4 loaded NaAlgmixed matrix membranes by the procedure described elsewhere [46].

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2.9. Thermodynamic model influencing PV performance

the well-known Flory–Huggins equation [47]:

Sorption selectivity values of the pristine NaAlg and NaAlg-4 loaded mixed matrix membranes are explained in terms of the thermodynamic interactions using Flory–Huggins theory [47]. For this, we have typically selected water + ethanol mixture. Aminabhavi and Munk [48] derived the computation model for a threecomponent system using Gibbs free energy of mixing (Gmix ), notice that a similar approach was extended for analyzing the PV data [49]. Following these approaches, thermodynamic equation for sorption selectivity, ˛S can be derived as

iP =



ln ˛S =

ln

i j



  vi vj

− ln



=



Vi −1 Vj

ij (j − i ) − ij (vi − vj ) − P





ln

iP −

j



vj Vi  Vj jP

−



(11)

Here, ϕi and ϕj are volume fractions of water and ethanol, respectively in the swollen polymer membrane, vi and vj are respective volume fractions of water and ethanol in the external liquid phase, Vi and Vj are individual molar volumes of water and ethanol. The volume fraction, P , of the pristine and mixed matrix membranes in the swollen state was calculated using [50]:



P = 1 +

P S

 M    −1 a P Mb

−

S

(12)

where p is the density of the pristine polymer and mixed matrix membranes, s is the density of solvent and 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. Initially, the benzene-filled bottle and empty 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 was calculated as [51] V=

xi Mi + xj Mj

(13)

sm

where xi and xj are the mole fractions of water and ethanol, respectively, Mi and Mj are the corresponding molecular weights, sm is density of the solvent mixture (measured by Anton Paar digital density meter, Model DMA 4500, K.G. Austria). The interaction parameter, ij between water and ethanol was calculated using the equation [52]: ij =

xi ln (xi /vi ) + xj ln (xj /vj xi vj

) + (GE /RT )

(14)

Excess Gibbs free energy, GE , was calculated from the activity coefficients, , of the mixtures as E

G = RT (xi ln i + xj ln j )

ln i = Aij

Aji xj Aij xi + Aji xj

2

2

(17)

where ıi is the solubility parameter (J1/2 cm−3/2 ) of ith -component. The solubility parameter, ıp of NaAlg polymer was estimated to be 61.48 J1/2 cm−3/2 from the atomic group contribution method [54]. Solubility parameter values of water (33.02 J1/2 cm−3/2 ) and ethanol (26.5 J1/2 cm−3/2 ) were taken from the literature [55]. These data were fitted into Eq. (11) to compute sorption selectivity, ˛S . Effect of driving force mechanism and permeation due to interactions between feed components are important in PV dehydration [56]. To explain this mechanism in depth, we have selected water + ethanol feed mixture under the operating conditions of 30 ◦ C. Permeation rate (J) can be written according to the theory of Wijmans and Baker [57]: p

Jw = (pfi − pi )

Pi l

p

Jorg = (pfj − pj )

(18)

Pj

(19)

l

Here, the superscripts, f and p refer to feed and permeate, respectively, pi and pj are partial vapor pressures of water and ethanol, respectively, Pi and Pj are membrane permeability coefficients of water and ethanol, respectively, which are the product of diffusion (Dij ) and solubility coefficients (Sij ). Diffusion and solubility coefficients are calculated using the procedure described elsewhere [58], l is the thickness of the membrane and Pi /l and Pj /l are permeances of water and ethanol, respectively. The relationship between partial vapor pressure (pf ), molar concentration of water (xi ) and ethanol (xj ) and activity coefficients of individual components of the feed obtained from Eq. (10) are given as pfi = xi i psi

(20)

pfj = xj j psj

(21) (psi )

(psj )

Saturated vapor pressures of water and ethanol were calculated using the Antonine equation [56] given in the form: A−B T +C

log p0ij =

(22)

where p0ij are vapor pressures of water and ethanol, respectively, A, B, and C are the Antonine constants, T is the temperature in (K). By considering activity coefficient and molar concentrations of individual component, saturated vapor pressure of water and ethanol are written as psi = xi i p0i

(23)

psj

(24)

=

xj j p0j

Compiling Eqs. (12)–(15), we obtain: Jw =

P  l

i

p

(xi i psi − pi )

(25)

 

(15)

For calculating  i and  j , we have used the van Laar equation at 30 ◦ C to compute the activity coefficient,  i of component, i in the mixture as



Vi (ıP − ıi ) RT

Jorg =

Pj l

p

(xj j psj − pj )

(26)

The ratio of membrane permeances is defined as the ideal membrane selectivity, which is given as

 

(16)

The van Laar parameters, Aij for water (1.5054) and Aji for ethanol (0.9994) were taken from the literature [53]. Then, polymer–solvent interaction parameter, iP was calculated using

ˇij =

Pi Pj

(27)

By knowing the molar water concentration in the feed and activity coefficient, the activity of water in water + ethanol feed mixtures can also be calculated.

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Fig. 1. Particle size distribution histogram of zeolite beta particles.

3. Results and discussion 3.1. Particle size of zeolite beta The average size distribution of zeolite beta particles dispersed in water has a diameter of about 0.5 ␮m as shown in Fig. 1. However, the particle size distribution histogram indicates the size distribution in the range of 50–1500 nm, but majority of particles are in the average size range of around 500 nm. 3.2. Membrane morphology Fig. 2(a) and (b) shows the cross-sectional SEM micrographs of NaAlg-2 and NaAlg-4 mixed matrix membranes in which beta particles are distributed throughout the NaAlg matrix. The crosssectional view of the membranes clearly shows a good contact between polymer and the zeolite. A uniform distribution profile of the beta particles with sodium alginate matrix is observed.

Fig. 2. Cross-sectional SEM micrograph of (a) NaAlg-2 and (b) NaAlg-4 mixed matrix membrane.

3.4. FTIR analysis FTIR spectra of the crosslinked NaAlg-2 and NaAlg-4 membranes are shown in Fig. 3. In the case of zeolite beta filled NaAlg membranes, the characteristic broad band appearing at ∼3442 cm−1 corresponds to O–H stretching vibrations of NaAlg backbone. Sharp peaks observed around ∼1628–1634 cm−1 and 1394–1399 cm−1 correspond, respectively to asymmetric and symmetric stretching of carboxyl group of NaAlg. A sharp peak around 1090–1101 cm−1

3.3. Universal testing machine (UTM) The % elongation at break and maximum tensile strengths of pristine and all the mixed matrix membranes are given in Table 1. These data clearly indicate higher mechanical strengths for the mixed matrix membranes as compared to pristine NaAlg membrane.

Table 1 Mechanical strength data of pristine and mixed matrix membranes Membrane

% Elongation at break

Tensile strength (MPa)

Pristine NaAlg NaAlg-1 NaAlg-2 NaAlg-3 NaAlg-4

21.0 18.0 15.0 13.5 12.0

20 22 25 28 30

Fig. 3. FTIR spectra of (a) NaAlg-2 and (b) NaAlg-4 membranes.

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Fig. 5. Bar diagram of membrane sorption vs. % zeolite loading for different media.

Fig. 4. % Sorption curves of pristine NaAlg (), NaAlg-2 (䊉) and NaAlg-4 () mixed matrix membranes for (a) water + ethanol and (b) water + acetic acid feed mixtures at 30 ◦ C.

corresponds to the C–O stretching of NaAlg and also for Si–O bonds in zeolite beta. Peaks appeared between 410 and 1000 cm−1 correspond to Al–O stretching vibrations of the zeolite beta incorporated in the NaAlg matrix. This proves the stronger interaction between the zeolite beta and NaAlg matrix.

the membranes have shown increased values of sorption in water, indicating the hydrophilic–hydrophilic interactions between water and membrane materials. Membranes showed the least sorption in acetic acid. Compared to water + acetic acid feed mixture, higher sorption values were observed for water + ethanol mixture. But, the overall sorption in these mixtures is smaller than pure acetic acid and ethanol components of the feed. These results follow the same trends as those of flux and separation factor values obtained for all the membranes. A dimension factor of the mixed matrix membrane before and after sorption influences its stability. Before sorption the mixed matrix membrane dimension was 28.3 cm2 but after the sorption the membrane dimension increased due to the increased swelling of the mixed matrix membrane. Dimension of the mixed matrix membrane was found to be 30.2 cm2 for water + acetic acid mixture and it is found to be maximum of 33.7 cm2 for water + ethanol mixture after sorption, since the upstream layer of the membrane swells more and downstream layer of the membrane is generally dry. This optimum condition controls the dissolution of zeolite beta from the membrane. Extraction of zeolite beta in acidic media is slightly more than other organic solvent media which in turn affect the PV flux. Therefore, water + acetic acid media gives less flux than water + ethanol media. However, it is interesting to note that since both zeolite and NaAlg are hydrophilic water adsorption–desorption and finally diffusion will be optimum in all the cases. 3.6. Membrane performance

3.5. Sorption Fig. 4 displays typical data of % sorption of pristine NaAlg, NaAlg-2 and NaAlg-4 filled matrix membranes at 30 ◦ C for 10, 15, 20, 25 and 30 wt.% water-containing feed mixtures. Sorption of water + ethanol mixture in NaAlg membranes is higher than that observed for water + acetic acid mixture due to higher-level hydrophilic interactions with the membranes. Sorptions of NaAlg-2 and NaAlg-4 mixed matrix membranes in both the feed mixtures are higher than the pristine NaAlg membrane. At 10 wt.% loading of zeolite beta into the NaAlg matrix increased sorption is observed due to larger number of hydrophilic beta particles present than when compared to 5 wt.% beta loaded NaAlg mixed matrix membrane. The increased sorption is due to the presence of beta zeolite particles that possess three-dimensional channel network with an asymmetrical aperture, which helps to increase water uptake capacity of the membrane compared to the organic components of the feed. This also would result in increased separation factor for filled matrix membranes compared to the pristine NaAlg membrane. Fig. 5 displays the bar diagram of % sorption data of all the membranes in water, acetic acid, ethanol and 10 wt.% water-containing feed mixtures of acetic acid and ethanol. It is observed that all

PV performances of pristine NaAlg, NaAlg-1, NaAlg-2, NaAlg-3 and NaAlg-4 mixed matrix membranes have been studied by calculating flux, separation factor, pervaporation separation index and enrichment factor. However, a detailed study was carried out typically in case of NaAlg-2 and NaAlg-4 mixed matrix membranes. The mixed matrix membranes exhibited better separation characteristics than the pristine membrane. Fig. 6 shows the variation of flux and separation factor as a function of wt.% of zeolite beta particles into NaAlg matrix. Pristine NaAlg membrane exhibited the lowest separation factor of 23 with a flux of 0.039 kg/m2 h at 10 wt.% water-containing feed mixture of ethanol. However, the flux and separation factor values of NaAlg increased after incorporation of beta particles. For NaAlg-1 membrane, the separation factor has increased to 256, while that for NaAlg-2, it was increased further to 540 at 10 wt.% of water in the feed mixture of ethanol; fluxes were 0.052 and 0.095 kg/m2 h, respectively for both the NaAlg mixed matrix membranes. A further enhancement in flux and separation factor values was observed for NaAlg-3 and NaAlg-4 mixed matrix membranes. Separation factor values of 774 and 1598 with fluxes of 0.112 and 0.132 kg/m2 h were observed for both these membranes that contained higher amounts of zeolite beta particles. A similar trend was also observed for

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239

of beta particles and hence, these were not useful in the PV study. Therefore, we restricted our PV experiments with those membranes with the loadings up to 10 wt.% of beta particles into NaAlg matrix. Since both NaAlg polymer and zeolite beta particles are hydrophilic in nature and therefore, the overall membrane performance can be explained on the basis of solution–diffusion theory [62,63] in addition to the adsorption–diffusion–desorption concepts [64]. In the pristine NaAlg membrane, permeating water molecules first get adsorbed inside the microvoids and then diffuse out on the permeate side due to the existence of concentration gradient. In case of filled matrix membranes, the overall separation can be explained by hydrophilic interactions between beta particles and the NaAlg matrix. Beta particles are distributed throughout the NaAlg matrix, thus forming a strong mixed matrix. Adsorption through the zeolites of the matrix under pervaporation conditions, being physical in nature is a non-activated exothermic process that is reversible. Molecules get adsorbed into the zeolite pores because of the intermolecular attractive forces between the adsorbent and the adsorbate [65].

Fig. 6. Water flux and separation factor vs. wt.% of beta in the membrane for 10 wt.% water-containing (a) ethanol and (b) acetic acid feed mixtures at 30 ◦ C. Symbols: () flux and () separation factor.

water + acetic acid feed mixture as shown in Fig. 6, wherein pristine NaAlg membrane exhibited the separation factor of 109 with a lowest flux of 0.030 kg/m2 h at 10 wt.% water, whereas NaAlg-1 and NaAlg-2 membranes showed increased separation factors of 612 and 1355 with subsequent increase in the fluxes, i.e., 0.050 and 0.078 kg/m2 h, respectively for 10 wt.% water-containing feed. NaAlg-3 and NaAlg-4 mixed matrix membranes showed separation factors of 5991 and 12,848 with flux values of 0.091 and 0.109 kg/m2 h, respectively at 10 wt.% of water in the feed, suggesting much better values as compared to all other membranes of this study. Incorporation of zeolitic particles into the continuous polymer phase is shown to be an attractive method of coupling an easy processability of polymers with superior separation properties due to the adsorptive nature of the fillers and hence, zeolite–polymer mixed matrix membranes are quite useful in PV dehydration studies. Expectedly, zeolites, through the molecular sieving effect, would also allow selective sorption and diffusion of liquid molecules as a result of decrease in the mobility of the less permeable component of the feed mixture [59]. Besides the molecular sieving effect, zeolitic hydrophilic/hydrophobic interactions would also contribute to the facile transport. Zeolite hydrophilicity/hydrophobicity ratio is generally controlled by the Si/Al ratio. For instance, hydrophilicity increases as the aluminum content of zeolite framework increases [60,61]. In the present study, Si/Al ratio being very small, i.e., 10, makes the zeolite rich in Al and, hence it is hydrophilic in nature. When the beta particles are added to the NaAlg matrix, the mixed matrix membrane becomes hydrophilic and thus, the organic components of the feed are retained back at the feed side of the membrane. At 10 wt.% incorporation of zeolite beta into NaAlg matrix, membrane sorption capacity has increased. Membranes prepared by adding more than 10 wt.% of beta particles were found to be more brittle than those containing lower amounts

Fig. 7. Water flux and separation factor vs. wt.% of water for (a) pristine NaAlg, (b) NaAlg-2 and (c) NaAlg-4 mixed matrix membranes for water + ethanol feed mixtures at 30 ◦ C. Symbols: (䊉) flux and () separation factor.

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Fig. 9. PSI vs. wt.% of water in feed mixture of (a) ethanol and (b) acetic acid for pristine NaAlg (䊉), NaAlg-2 () and NaAlg-4 () mixed matrix membranes at 30 ◦ C.

Fig. 8. Water flux and separation factor vs. wt.% of water for (a) pristine NaAlg, (b) NaAlg-2 and (c) NaAlg-4 mixed matrix membranes for water + acetic acid feed mixtures at 30 ◦ C. Symbols: (䊉) flux and () separation factor.

3.7. Effect of feed water composition on PV performance In the present research, it is demonstrated that by incorporating zeolite beta particles, one can improve the solvent stability of NaAlg membrane, thereby improving the membrane performance. As displayed in Figs. 7 and 8, flux and separation factor values of NaAlg mixed matrix membranes are higher than those of pristine NaAlg. For NaAlg mixed matrix membranes, flux values increased with increasing feed water composition from 10 to 30 wt.%. As the water composition of feed mixture increases, the separation factor decreases, but flux increases. As seen from Fig. 4, NaAlg membrane swells to a greater extent at higher feed water composition, which induces the plasticization to membranes. Thus, sorption results are a good indicator of PV performance of the membranes. At 30 wt.% of water in the feed mixture containing ethanol, separation factor for pristine NaAlg membrane is quite small, i.e., 5 with a flux of 0.134 kg/m2 h, whereas NaAlg-2 and NaAlg-4 membranes showed separation factors of 109 and 249, respectively with the flux values of 0.233 and 0.341 kg/m2 h. On the other hand, for water + acetic acid mixture, the observed separation factor for pristine NaAlg was 14 with a flux of 0.084 kg/m2 h at 30 wt.% of water. NaAlg-2 and NaAlg-4 membranes exhibited separation factors of

226 and 494 with fluxes of 0.214 and 0.313 kg/m2 h, respectively (see Figs. 7 and 8). Flux values for both the feed mixtures at lower water composition are somewhat closer due to lesser swelling of the membranes. As the feed water composition increases, a steady difference between the flux values for both the feed mixtures can be observed as per the solution–diffusion mechanism. PV results have been also discussed by calculating pervaporation separation index (PSI) and enrichment factor (ˇ) values at 30 ◦ C; these data are displayed in Figs. 9 and 10. Results at 40, 50 and 60 ◦ C are summarized in Tables 2 and 3. Both PSI and ˇ values decrease systematically with increasing water in the feed. The values of PSI and ˇ increase with an increasing amount of zeolite beta particles in the NaAlg matrix; the pristine NaAlg membrane exhibited lowest PSI and ˇ values. These trends for all the membranes follow similar trends to those of separation factor values as discussed before. The ˇ values of NaAlg2 and NaAlg-4 mixed matrix membranes are somewhat identical because of the small differences in wt.% of water in permeate. 3.8. Thermodynamic interpretation of the membrane performance Experimental and theoretical plots of sorption selectivity for pristine NaAlg and NaAlg-4 zeolite-loaded mixed matrix membranes are given in Fig. 11. One can observe that experimental and theoretical values of sorption selectivity are quite comparable. Sorption selectivity is higher for NaAlg-4 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 zeo-

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241

Table 2 Pervaporation results for 10 wt.% water-containing feed mixtures of water + ethanol at 30, 40, 50 and 60 ◦ C. Temperature (◦ C)

Wt.% of water in permeate

Water flux (kg/m2 h)

Water + ethanol Pristine NaAlg 30 40 50 60

72.30 65.05 52.85 40.33

0.039 0.055 0.089 0.103

23 17 10 6

0.89 0.87 0.81 0.52

7.23 6.51 5.29 4.03

NaAlg-1 30 40 50 60

96.61 93.84 87.13 83.23

0.052 0.080 0.102 0.117

256 137 61 45

13.29 10.89 6.11 5.15

9.66 9.38 8.71 8.32

NaAlg-2 30 40 50 60

98.36 97.30 95.27 93.37

0.095 0.114 0.131 0.153

540 324 181 127

51.18 36.86 23.62 19.27

9.84 9.73 9.53 9.33

NaAlg-3 30 40 50 60

98.85 98.21 96.51 94.28

0.112 0.125 0.141 0.167

773 494 249 148

86.53 61.60 34.95 24.54

9.89 9.82 9.65 9.42

NaAlg-4 30 40 50 60

99.44 99.10 98.10 97.77

0.132 0.148 0.166 0.178

1598 991 465 395

210.82 146.52 76.97 70.13

9.94 9.91 9.81 9.77

lite particles, making it more hydrophilic, 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. 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.

Separation factor (˛)

PSI

ˇ

Permeances of water and ethanol with respect to feed compositions for pristine NaAlg and NaAlg-4 loaded NaAlg membrane are displayed in Fig. 12. Water permeance is higher than ethanol, which is obvious considering the hydrophilic nature of beta particles. Water permeance increased for NaAlg-4 loaded mixed matrix membrane of NaAlg than the pristine NaAlg membrane. Ideal membrane selectivity for water + ethanol mixture also increased for NaAlg-4 loaded NaAlg membrane than the pristine NaAlg due to a

Table 3 Pervaporation results for 10 wt.% water-containing feed mixtures of water + acetic acid at 30, 40, 50 and 60 ◦ C Temperature (◦ C)

Wt.% of water in permeate

Water flux (kg/m2 h)

Water + acetic acid Pristine NaAlg 30 40 50 60

92.35 88.10 80.51 76.23

0.030 0.045 0.071 0.083

109 67 37 29

3.23 2.95 2.57 2.32

9.24 8.81 8.05 7.82

NaAlg-1 30 40 50 60

98.15 96.50 93.12 91.17

0.050 0.071 0.085 0.097

612 248 122 93

30.53 17.55 10.27 8.92

9.82 9.65 9.31 9.11

NaAlg-2 30 40 50 60

99.34 98.91 97.70 96.66

0.078 0.092 0.103 0.117

1355 817 382 260

104.22 75.04 39.27 30.31

9.93 9.89 9.77 9.66

NaAlg-3 30 40 50 60

99.85 99.65 99.26 98.93

0.091 0.110 0.128 0.139

5991 2562 1207 832

545.09 281.76 154.40 115.51

9.99 9.97 9.93 9.89

NaAlg-4 30 40 50 60

99.93 99.87 99.60 99.27

0.109 0.129 0.146 0.155

12848 6914 2241 1224

1405.73 891.79 230.72 189.56

9.99 9.99 9.96 9.92

Separation factor (˛)

ˇ

PSI

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Fig. 11. Theoretical and experimental plots of sorption selectivity vs. wt.% of water in the feed for (a) pristine NaAlg and (b) NaAlg-4 mixed matrix membrane. Symbols: (black box), theoretical; (plain box), experimental. Fig. 10. ˇ vs. wt.% of water in feed mixture of (a) ethanol and (b) acetic acid for pristine NaAlg (), NaAlg-2 () and NaAlg-4 () mixed matrix membranes at 30 ◦ C.

good compatibility between zeolite and NaAlg matrix as can be seen in Fig. 13. Water activity in water + ethanol feed mixtures increased with increasing feed concentration for pristine as well as NaAlg4 membranes as shown in Fig. 14 following the transport trends as displayed for water permeance in Fig. 12. The NaAlg-4 loaded mixed matrix may be particularly well suited for the separation of ethanol and water mixtures as supported by experimental flux and selectivity results. 3.9. Effect of temperature on PV performance The membrane performance was studied at higher temperatures, i.e., 40, 50 and 60 ◦ C for 10 wt.% water-containing feed mixtures of ethanol as well as acetic acid to confirm their stability at higher temperatures so as to use the mixed matrix membranes to study the esterification reaction. The present membranes were found to be quite stable in the studied range of temperatures under PV conditions. Results of flux and separation factors at all temperatures are given in Tables 2 and 3. Water flux increased, while the separation factor decreased for pristine as well as mixed matrix membranes at higher temperatures due to excessive swelling of the membranes. The temperature dependency of flux was analyzed using the Arrhenius relationship: Jp = Jp0 exp

 −E  p

RT

(28)

where Jp is the permeation flux of water, Jp0 is the permeation rate constant, Ep is the activation energy for permeation, R is the molar gas constant and T is the temperature in (K). If the activation energy

Fig. 12. Water and ethanol permeance vs. wt.% of water in the feed for (a) pristine NaAlg and (b) NaAlg-4 mixed matrix membrane.

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Fig. 13. Ideal membrane selectivity for pristine NaAlg and NaAlg-4 mixed matrix membrane.

for permeation is positive, then flux increases linearly with temperature [6] as observed in the present study. As the feed temperature increases, vapor pressure in the feed compartment also increases, but the vapor pressure at permeate side is not affected. This would result in increased driving force at higher temperatures. Arrhenius plots of ln Jp vs. 1/T are shown in Fig. 15(a) and (b) for pristine as well as mixed matrix membranes for 10 wt.% of water in the feed mixtures of ethanol and acetic acid. The Ep values for water + ethanol mixture with pristine NaAlg, NaAlg-1, NaAlg2, NaAlg-3 and NaAlg-4 membranes are, respectively 34.03, 27.16, 15.83, 13.26 and 10.29 kJ/mol. The Ep values for water + acetic acid mixture are, respectively, 35.54, 22, 14.41, 13.42 and 11.96 kJ/mol. This indicates a gradual decrease in the Ep values with increased loading of zeolite beta particles into NaAlg matrix.

Fig. 15. Arrhenius plots of ln Jp vs. 1/T for 10 wt.% water in feed mixture of (a) water + ethanol and (b) water + acetic acid for pristine NaAlg (䊉), NaAlg-1 (), NaAlg2 (), NaAlg-3 () and NaAlg-4 () mixed matrix membranes.

3.10. PV performance at the esterification temperature To test the reliability of the present mixed matrix membranes for esterification reaction at 70 ◦ C, PV experiments were carried out for pristine NaAlg, NaAlg-2 and NaAlg-4 membranes. These results are presented in Fig. 16. As expected, flux increased, but separation factor decreased for all the three membranes. The NaAlg-4 membrane showed better results when compared to other two membranes. Flux and separation factor values obtained were sufficient to perform the esterification reaction at this temperature for the conversion of ethyl acetate with a continuous water removal from the permeate side. Esterification results are in accordance with the PV results and NaAlg-4 exhibited the highest conversion

Fig. 14. Water activity for pristine NaAlg and NaAlg-4 mixed matrix membrane.

of ethyl acetate compared to NaAlg-2 and pristine NaAlg membranes. The Ep values for NaAlg, NaAlg-2 and NaAlg-4 membranes at 70 ◦ C are calculated similarly by using the Eq. (28) and is found to be 37.8, 33.9 and 29.0 kJ/mol for water + ethanol mixture, similarly for water + acetic acid mixture it is found to be 40.5, 38.2 and 33.7 kJ/mol, respectively (Arrhenius plots of ln Jp vs. 1/T at 70 ◦ C are not presented here in order to minimize the number of figures). 3.11. Pervaporation-aided esterification As discussed before, water flux has increased with an increase in feed temperature, but separation factor decreased for all the membranes. Pristine NaAlg as well as NaAlg-2 and NaAlg-4 mixed matrix membranes were tested for the PV-aided esterification of ethanol and acetic acid. Such esterification reactions are important in chemical industries [66]. The equilibrium-limited esterification reactions should have different reaction conditions to achieve the enhanced yields. Thus, in the presence of a large excess of one of the reactants, viz., alcohol, the reaction yield of the other component can be enhanced. The esterification of acetic acid (1 M) and ethanol (2.88 M) was attempted at 70 ◦ C in the presence of 6 g of cation exchange resin Dowex-50. The conversion of ethyl acetate with a blank reaction and with PV using different membranes at different time intervals is displayed in Fig. 17. Reactions were allowed to continue until complete conversion of ethyl acetate was achieved. After 18 h, the blank reaction gave 81% conversion of reactants into ethyl acetate. The PV-aided esterification with pristine NaAlg produced a better conversion of 85.7% after 13 h, whereas NaAlg-2 and NaAlg-4 mixed matrix membranes produced 87.2% and 89.5% of ethyl acetate after 10 and 8 h, respectively.

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Fig. 17. % Conversion of ethyl acetate vs. time (h) for blank reaction (), pristine NaAlg (䊉), NaAlg-2 () and NaAlg-4 () mixed matrix membranes.

water molecules has increased, which has further enhanced the ethyl acetate conversion at lesser time. Hence, the mixed matrix catalytic membrane containing 10 wt.% of zeolite beta in NaAlg is more favorable for the forward reaction to occur. Equilibrium constant for the forward reaction is higher for PV-aided esterification reaction than the blank reaction, since the reaction rate increases with an increase in the conversion. The values of equilibrium constants, Ke calculated from Eq. (7) are 1.71 for the blank reaction, 2.53 for the pristine NaAlg membrane, 2.99 for NaAlg-2 and 3.88 for NaAlg-4 mixed matrix membranes, respectively. The variation in f(x) with time is displayed in Fig. 18, which shows increased forward reaction for the PV-aided esterification reactions than for blank reaction. Fig. 16. Flux and separation factor at 70 ◦ C (esterification temperature) for 10 wt.% of water in feed. Symbols: (black box) water + ethanol and (plain box) water + acetic acid.

Notice that there is an increase in the conversion of ethyl acetate at lesser time than the blank reaction due to the continuous removal of water from the reactor. The increase in conversion of ethyl acetate from the pristine NaAlg to NaAlg-4 mixed matrix membranes increased continuously due to increasing amount of zeolite beta particles in the matrix, which has acted as a catalyst in addition to the presence of cation exchange resin Dowex-50. In the presence of zeolite beta particles in NaAlg-4, the permeation of

3.12. Comparison with literature data A comparison of the present PV data with those of the published results is made in Table 4. There is a good improvement in the selectivity values of water for both the mixtures for NaAlg-4 membranes compared to similar data published in the literature [9,10,67–71]. In general, one could observe an improvement in both flux and selectivity values. However, there is a decrease in flux for water + acetic acid mixture compared to other results because of the increase in the separation factor. For high separation factor, flux is usually sacrificed as observed in most of the published reports. At

Table 4 Comparison of PV performance of present membranes with literature for ethanol and acetic acid dehydration at ambient temperatures ˛

Membrane type

Flux J (kg/m2 h)

Water + ethanol NaAlg-4a Two-ply composite NaAlg/CSb Cellulose/NaAlg (ca2+ crosslinked) blendc NaAlg/CS blend AlPO4 -5 (20 wt.%)-filled NaAlgd

0.132 0.070 0.068 0.220 0.104

1,598 1,110 1,175 436 980

Present work [67] [9] [10] [68]

Water + acetic acid NaAlg-4 NaAlg composite (crosslinked with HDM)e NaAlg + 5 wt.% PVA + 10 % wt.% PEG Cobolt (III) (3-acetylpyridine-o-aminobenzoyl hydrazine)-filled NaAlg

0.109 0.262 0.071 0.123

12,848 161 40 174

Present work [69] [70] [71]

Reference

CS-chitosan, PHEMA-poly(hydroxyethylmethacrylate); HDM-1,6-hexanediamine; pAAm-poly(acrylamide); GG-Guar gum; PEG-poly(ethylene glycol). a 10 wt.% water in the feed. b 5 wt.% of water in the feed. c At temperature of 60 ◦ C. d 4 wt.% of water in the feed. e 15 wt.% of water in the feed at 70 ◦ C.

S.G. Adoor et al. / Journal of Membrane Science 318 (2008) 233–246

Fig. 18. f(x) vs. time (min) for blank reaction (䊉), pristine NaAlg (), NaAlg-2 () and NaAlg-4 (×) mixed matrix membranes.

any rate, to achieve a proper balance between flux and selectivity in PV dehydration is quite a difficult task. 4. Conclusions Mixed matrix membranes exhibited higher or improved performances than the pristine NaAlg membrane in the PV dehydration of ethanol and acetic acid. Zeolite beta forms the hybrid mixed matrix with NaAlg, which exhibited an improved PV performance. Separation factor and flux values of 10 wt.% beta incorporated NaAlg mixed matrix membrane were superior as compared to lower amount of beta incorporated NaAlg membranes. This could be due to increased hydrophilicity as a result of more number of aluminumrich zeolite beta particles in NaAlg-4; however, molecular sieving effect, selective sorption/adsorption by the filler particles as well as the selective diffusion increased the mobility of the preferentially permeating water molecules in the mixed matrix membranes. Thermodynamic model was also derived to describe the PV performance of water + ethanol mixture. The present experimental data suggest that zeolite beta filled NaAlg mixed matrix membranes can be employed successfully in PV-aided esterification of ethanol and acetic acid to form ethyl acetate. With increasing amount of beta particles in NaAlg, the conversion of reactants into ethyl acetate also increases. However, the use of mixed matrix membranes could yield the higher conversion of ethanol into ester in a short time as compared to blank reaction, which is due to the catalytic effect of beta particles present in NaAlg matrix. Therefore, the rate of forward reaction is favored for NaAlg-4 membrane as compared to the blank reaction, pristine NaAlg and NaAlg-2 membranes employed in esterification reactions. Acknowledgement 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 (CEPS). References [1] 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. [2] Y. Shi, X. Wang, G. Chen, G. Golemme, S. Zhang, E. Drioli, Preparation and characterization of high-performance dehydrating pervaporation alginate membranes, J. Appl. Polym. Sci. 68 (1998) 959.

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