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para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol
CLAY-03140; No of Pages 11 Applied Clay Science xxx (2014) xxx–xxx
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Research paper
para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol D.P. Suhas a, T.M. Aminabhavi a,b, A.V. Raghu a,⁎ a b
Materials Science Division, Poornaprajna Institute of Scientific Research, Bengaluru 562 110, India Department of Pharmaceutical Engineering and Chemistry, Soniya College of Pharmacy, Dharwad 580 002, India
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 16 August 2014 Accepted 20 August 2014 Available online xxxx Keywords: p-TSA Acid modified clay Pervaporation Isopropanol dehydration Clay composite
a b s t r a c t The para-toluene sulfonic acid (p-TSA) treated clay (5, 10 and 15 wt.% loadings) particles were inserted into sodium alginate (NaAlg) matrix to derive the composite membranes that showed enhanced pervaporation performance for isopropanol separation. The 10 wt.% modified clay-loaded NaAlg membranes showed the highest selectivity of 5781 with a permeance of 3423 GPU for 10 wt.% water containing isopropanol feed mixture at 30 °C, showing a 140% higher selectivity compared to pristine clay-loaded NaAlg composite membrane. This effect is ascribed to an enhanced surface area and hydrophilicity after p-TSA treatment of clay, as verified by BET and TGA studies. The membranes were characterized by a variety of physico-chemical techniques and their pervaporation performance was assessed in terms of selectivity, permeance, with extent of variations in filler loading, temperature and feed water composition. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, pervaporation (PV) technique has gained a much wider acceptance as an energy efficient method for obtaining pure alcohols (Hoof et al., 2004). For separating alcohol–water mixtures, distillation is effective only to a certain extent viz., 80–85 wt.%, after which separation is done by evaporating the entire alcohol (usually any process involving phase change from liquid to gas consumes large quantity of energy). Since the number of evaporating molecules is large, naturally energy consumption is also large. On the other hand, PV selectively removes the minor components from such mixtures and thus limiting the energy burden only to minor components. PV is also an environmentally benign technique, since it does not involve an entertainer like benzene, as applicable in azeotropic distillation (Jonquières et al., 2002; Lipnizki et al., 1999; Shao and Huang, 2007; Widagdo and Seider, 1996). In this research, isopropanol (IPA) dehydration is considered in view of its importance in cosmetics, gums, waxes, cleansing agent in industries and as a blend in gasoline (Papa, 2000). High purity grade IPA is required in all these areas. IPA forms an azeotrope at 12.3 wt.% of water, making it difficult to separate by conventional distillation (Ghazali et al., 1997). Hydrophilic polymers like poly(vinyl alcohol), sodium alginate (NaAlg), chitosan, polyacrylonitrile, and polyimide have been reported for IPA dehydration (Chapman et al., 2008; Suhas et al., 2013, 2014). NaAlg is a naturally occurring anionic bio⁎ Corresponding author at: Centre for Emerging Technologies, Jain Global Campus, Jain University, Kanakapura, Ramanagara 562 112, India. E-mail addresses: [email protected], [email protected] (A.V. Raghu).
compatible material with good film forming ability, moderate mechanical strength and good affinity to water (Bhat and Aminabhavi, 2007a, 2007b). However, nascent NaAlg suffers from uncontrolled swelling, thus exhibiting low PV performance. To circumvent these problems, composite membranes have been prepared by dispersing suitable fillers into NaAlg matrix. Such membranes have emerged to be the best for PV dehydration of aqueous–organic mixtures (Adoor et al., 2013; Chung et al., 2007; Zhao et al., 2013). Typical fillers used in developing composite membranes are nonporous, porous or intercalated materials, whose presence creates torturous pathways in the matrix favoring the selective permeation of water molecules. Examples of the widely used fillers include heteropolyacids (Teli et al., 2007), titanium dioxide (Liu et al., 2011), zeolites (Bhat and Aminabhavi, 2007a, 2007b), mesoporous silica (Patil et al., 2007), and clays (Anilkumar et al., 2008). Among these, montmorillonite clays, owing to their layered structure with a high aspect ratio and hydrophilic surface chemistry, have gained much wider acceptance as fillers in barrier membranes (Avagimova et al., 2013). Clays are the important class of layered silicates consisting of alumina octahedra sandwiched between two sheets of silica tetrahedra interconnected by sharing of O2− atoms at the polyhedral corners and edges (Chen et al., 2008). Earlier studies on sodium montmorillonite clay incorporated PVA and NaAlg membranes have shown improved separation performance (Adoor et al., 2006; Bhat and Aminabhavi, 2006). Acid treatment is the most actively pursued method to improve the surface property of the clay. The technique involves leaching of the clay with suitable acids leading to deagglomeration of clay particles, elimination of mineral impurities as well as causing structural and chemical changes in the clay.
http://dx.doi.org/10.1016/j.clay.2014.08.017 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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The acid treated clay has several attributes like high surface area and large number of hydrophilic sites compared to the nascent clay. It was found that the degree of clay structure decomposition depends on several factors like the type of acid used, concentration of acid, duration of treatment and experimental conditions (Madejova et al., 1998; Panda et al., 2010; Siddiqui, 1968). Earlier (Ramesh et al., 2012), it was shown that p-TSA treated smectic clay enhanced both the surface area and the catalytic activity compared to nascent clay. Moreover, to enhance the efficiency, we have incorporated a microwave irradiation technique, which is environmentally friendly. In this work, organically modified clay particles prepared by treating with p-TSA were incorporated into the NaAlg matrix to derive composite membranes that were tested for PV dehydration of IPA–water mixture. Performance of the modified clay-loaded NaAlg composite membrane was compared with that of pristine clay-loaded NaAlg composite membrane for IPA dehydration. The composite membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetry (TGA) and contact angle measurements. Sorption and diffusion parameters along with their temperature dependencies have been used to estimate the Arrhenius activation parameters for permeation and diffusion processes. These along with selectivity and permeance data have been used to discuss the PV results for IPA dehydration. To the best of our knowledge, this is the first study on the use of p-TSA treated clay loaded NaAlg membranes used in PV dehydration of IPA.
water for 30 min followed by 1 h of sonication. The suspension obtained was slowly added to the NaAlg solution and mixed over a magnetic stirrer for 24 h. The resulting uniform suspension was poured onto a perfectly aligned flat clean glass plate dried at ambient temperature. The dried membranes were peeled off from the glass plate and were further dried in a vacuum desiccator at ambient temperature before use. The membranes formed were cross-linked by immersing in a crosslinking solution bath for 6 h that contained water–acetone (30:70) mixture with 2 mL of GA as a cross-linking agent and 2 mL of conc. HCl as a catalyst. The cross-linked membranes were alternatively rinsed with water and methanol for at least ten times followed by soaking in methanol for 24 h and then preserved in a desiccator before further testing. Using the above procedure, 5, 10 and 15 wt.% of p-TSA treated clayloaded NaAlg composite membranes were prepared and are designated as: PCL-5, PCL-10 and PCL-15, respectively. As a control, the nascent NaAlg (cross-linked, but without adding fillers) and 10 wt.% pristine clay loaded NaAlg (designated as CL-10) membranes were fabricated. 2.4. Membrane characterization
In a typical procedure, 20 g of clay and 200 mL of 2 molar p-TSA were taken in a 500 mL round bottom flask connected to a double walled reflux condenser and digested under a microwave irradiation (Microware Lab Station START-5 Italy, courtesy of Dr. Y.S. Bhat, BIT, Bangalore) for 10 min at 1000 W power and heated to 80 °C. The reaction mixture was centrifuged and the residual solid was washed thoroughly with water until the solution acquired neutral pH. The resulting solid was dried at 120 °C for 2 h in a vacuum oven and ground to fine powder. The increase in surface area as measured by BET isotherm was 113 m2 g−1 as against the original value of 29 m2 g−1 for pristine clay, confirms the modification of clay. Additionally, thermogravimetry showed an increase in water retention capacity of the modified clay, suggesting increased hydrophilicity (see the Thermogravimetry (TGA) section). The pristine clay used in this study was also prepared by the same approach, except that only double distilled water was used instead of p-TSA.
Fourier transform infrared (FTIR) spectroscopy (Bruker Alpha-T spectrometer) was used to analyze the samples after mixing with KBr and pellets were made under applied hydraulic pressure (180 kg/cm2). Scanning was done in the range of 400–4000 cm−1 for 32 times with a resolution of 4 cm−1 and these data were accumulated in a transmission mode. Wide-angle XRD analysis of the membranes was carried out in a powder form using Bruker D-2α Phaser X-ray diffractometer. The Xrays of wavelength 1.54 Å were generated by nickel filtered CuKα radiation source and samples were scanned in the 2θ range of 2–60° at the scanning rate of 2°/min. Micro-structural morphology of the nascent NaAlg as well as composite NaAlg membranes were examined by a field emission scanning electron microscope (FE-SEM) using an FE-SEM ZEISS Ultra-55 instrument (available at CeNSE, Indian Institute of Science, Bangalore). To improve the image quality, a conductive layer of sputtered gold was coated prior to these measurements. Thermal stability of nascent NaAlg and its composite membranes was examined by thermogravimetry (Universal V3.9A TA Instruments, Bangalore, India) over the temperature range of 40–650 °C at the heating rate of 10 °C/min. Analysis was carried out under an inert atmosphere at the nitrogen gas flow rate of 10 mL/min. For each analysis, about 6–9 mg of the sample was taken in aluminum pans. Similarly, TGA analysis of the pristine clay and p-TSA treated clay was performed from 40 to 800 °C in an ambient atmosphere. Water contact angle on the surface of the membrane was measured as per the Sessile drop method using the Data Physics OCA 20 at 25 °C (courtesy of Prof. S.K. Biswas, Indian Institute of Science, Bangalore). Prior to these measurements, all the membranes were vacuum dried and kept in a desiccator. Then, a piece of membrane (1 cm × 7 cm) was adhered to clean and smooth glass slide for contact angle measurements. A 2 μL of deionized water droplet was placed on the sample surface and droplet image was captured by a microscope coupled to a charge coupled device (CCD) camera. All the measurements were done within 10 s to minimize the errors due to evaporation losses. A total of six measurements were taken at different locations for each membrane sample and the average value was taken with N3% standard deviation.
2.3. Membrane fabrication
2.5. Equilibrium swelling
The solution casting method was adopted to prepare composite membranes from a uniform suspension of p-TSA treated clay-loaded NaAlg. In a typical procedure, 4 g of NaAlg was dissolved in 100 mL of water with constant stirring followed by 24 h of degassing. In a separate flask, the required amount of clay was stirred by taking into 20 mL of
Equilibrium swelling experiments were performed gravimetrically at 30 °C for 10 wt.% water containing IPA feed mixture. Initial weights of all the circularly cut membranes were measured after vacuum drying in a hot air oven and the weight of dry membrane was noted as Wd. Dry membranes were placed inside the specially designed air tight test
2. Experimental 2.1. Materials Sodium montmorillonite clay was purchased from Sigma Aldrich, USA, while sodium alginate (NaAlg), isopropanol (IPA), hydrochloric acid (35%) (HCl), p-toluene sulfonic acid (p-TSA) and glutaraldehyde (GA) were all purchased from s.d. fine chemicals, Mumbai, India. Absolute ethanol (EtOH) (99.9%) was purchased from Commercial Alcohols, Brampton, Canada. All other chemicals were of analytical reagent (AR) grade samples, used without further purification. Double distilled water was used throughout the experiments. 2.2. Preparation of p-TSA treated clay
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
bottles containing 30 cm3 of the test liquid at 30 °C. All the membranes were soaked in the test solvent for 48 h and before measuring the swollen weight (Ws), membranes were gently pressed between the filter paper wraps to remove surface-adhered solvent molecules and weighed immediately to minimize the weight loss due to evaporation. Membranes after a thorough drying attained almost same as the initial weight (within a small deviation of ± 3%). Equilibrium swelling (%) was calculated using: Equillibrium swelling ð%Þ ¼
W s −W d 100: Wd
ð1Þ
3
2.6. Pervaporation experiment PV experiments were performed in an indigenously fabricated stainless steel unit described elsewhere (Aminabhavi and Naik, 2002), which consisted of two compartments viz., feed tank and permeate cell as shown in Fig. 1. The membrane with an effective area of 3.84 × 10− 3 m2 was held tight with the help of two O-shaped Teflon rings supported on a porous stainless steel support, which was placed between two cells. The feed tank was a double walled cell with a feed capacity of 500 mL. The desired temperature in the test cell was maintained by circulating water using a thermostatic bath
1 – Thermostac bath 2 – Pump 3 – Srrer 4 – Thermometer 5 – Outer Jacket 6 – Feed Chamber 3
7 – Membrane
4
8 – Porous stainless steel support
5
9 – Permeate Chamber
6
10 – Valve
2
11 – Pressure gauge 7
12 – Glass trap
8
13 – Liquid Nitrogen Dewar 14 – Vacuum pump
1
10
9
11
10
14 12
13
Fig. 1. Schematic diagram of pervaporation system.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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(Grant, UK, model GD-120) at the desired temperature into the outer jacket. Further, the feed tank was provided with a thermometer to monitor the feed temperature and the stirrer was rotated constantly. The feed mixture was maintained at atmospheric pressure, while permeate pressure was (measured by a piezometer) maintained at b5 mbar with the help of a double stage vacuum pump. The feed mixture at different temperatures was first equilibrated for 2 h before collecting the sample, after which a constant flux was produced. Permeate samples were collected at regular intervals of time in two glass traps (fitted in the condenser assembly) immersed in a liquid nitrogen containing Dewar flask. All the sample measurements (viz., weight and concentration) were done at ambient temperature and samples were weighed on a Sartorius BSA224 balance. The compositions of liquid feed mixture were analyzed by measuring the refractive index using a refractometer (Mettler, Toledo) within the accuracy of ±0.0001 units that was calculated from the previously established standard graph of refractive index vs known mixture composition. The refractometer prism was maintained at 25 ± 0.1 °C. Simultaneously, the composition of permeate was determined by gas chromatography (Trace 700, Thermo-Fisher) equipped with a TCD detector and a Parapak-Q packed column. The same disk was re-used for subsequent tests at increasing water composition of the feed mixture and also at increasing temperature. The membranes were found to be stable without showing any signs of degradation during the PV experiments. The feed mixture was maintained at constant composition by compensating for any loss of water. At least three independent readings were taken for each membrane and calculations were done by taking
the average data within ± 3% standard errors. All the membranes showed good performances up to 8 h of continuous PV operation. Following the suggestion by Wijmans (2003), all the PV data are reported in a driving force normalized form viz., permeance (Pi/l) and selectivity (αij), since these calculations are free from the influence of driving force and reveal the intrinsic membrane performance. Thus, molar flux for each component (ji) was calculated as: ji ¼
J i vi mi
ð2Þ
where Ji is the flux (g/m2 h) obtained from the PV data, vi is the molar volume [22.4 L (STP)/mol] of the ith component and mi is the molecular weight. Partial vapor pressure (pfi) was calculated from the van Laar equation as: f
s
pi ¼ xi γ i pi
ð3Þ
where xi is the molar concentration and γi is the activity coefficient of the individual components. The saturated vapor pressure (psi ) of the ith component was calculated using (Gmehling et al., 1977): s
logpi ¼
A−B : T þC
ð4Þ
Here, A, B and C are the Antoine constants taken from the literature (Holmes and Winkle, 1970) and T is the temperature in Kelvin. The
Fig. 2. (a) FTIR spectrum of nascent clay and p-TSA treated clay. (b) FTIR of nascent NaAlg and NaAlg with different loadings of p-TSA treated clay showing an increase in red-shift with an increase in clay loading.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
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molar flux together with the vapor pressure of the ith component of the feed (pfi) and permeate (ppi ) allowed for the calculation of permeance (Pi/l) as: P i Di K i ji : ¼ ¼ l l pif −ppi
ð5Þ
Here, Di, Ki and l refer to the diffusion coefficient, sorption coefficient and membrane thickness, respectively. Similarly, membrane selectivity, which is the ratio of permeabilities or permeances of components, i and j through the membrane, was calculated as: α ij ¼
P i =l : P j =l
ð6Þ
These calculated parameters are used in the discussion of PV results. 3. Results and discussion 3.1. Characterization of modified clay and NaAlg composite membranes 3.1.1. FTIR FTIR spectra of clay, p-TSA treated clay, nascent NaAlg and composite NaAlg membranes are displayed in Fig. 2(a) and (b). FTIR spectra of both clay and p-TSA treated clay shows characteristic absorption peaks at 3450, 1060 and 550 cm− 1 corresponding to \OH, Si\O\Si and Al\O\Al vibrations, respectively (Madejová, 2003), but p-TSA treatment has resulted in some minor changes in FTIR spectra of montmorillonite clay. Absorption peaks at 3400 cm−1 and 3626 cm−1 correspond to the \OH stretching of absorbed water molecules and structural \OH, respectively. The intensity of these peaks has increased after p-TSA treatment, which can be attributed to the removal of some of the structural alumina and its subsequent occupation at clay intercalation with the hydration shell. The presence of an extra peak (in p-TSA treated clay) at 750 cm−1 may be attributed to the influence of p-TSA molecules. Similar results were obtained by Tong et al. (2013). The spectra for nascent NaAlg shows the absorbance between 1620 and 1077 cm− 1 that corresponds to symmetric C_O and asymmetric O\C\O stretching vibrations of the carboxyl group, respectively. The characteristic peak around 3300 cm−1 is attributed to the stretching vibration of the \OH group of NaAlg membranes. After the incorporation of clay into these membranes, the peak was strengthened and broadened, due to high density of \OH groups available on the clay surface. Moreover, FTIR spectra show a systemic decrease in \OH absorption (red-shift) with an increase in clay loading showing the sequence: 3480 cm−1 (nascent NaAlg) N 3440 cm−1 (PCL-5) N 3428 cm−1 (PCL10) N 3410 cm− 1 (PCL-15). In general, red shift refers to the shift in the absorption band to a higher wavelength, which otherwise refers to the decrease in energy required for stretching. This is observed because the \OH group of NaAlg interact with the \OH group of clay with hydrogen bonding, thereby reducing the electron density on the O\H bond. As a consequence, lesser energy is required for stretching. 3.1.2. Wide angle X-ray diffractograms (W-XRDs) The XRD of p-TSA treated clay shows a peak at 2θ of 6.6° and clearly the same peak has disappeared in PCL-5 and PCL-10 (Fig. 3(a)), which reveals the molecular level interaction between clay and NaAlg or possible delamination of montmorillonite clay in the NaAlg matrix. These results are in good agreement with the previous report by Hua et al. (2010). The 6.6° peak appears in PCL-15 showing particle agglomeration. Fig. 3(b) displays the W-XRD pattern of p-TSA treated clay, nascent NaAlg and all the clay-loaded composite membranes. Fig. 3(b) (inset) shows the typical diffraction pattern of p-TSA treated clay with its characteristic crystalline peak observed at 2θ of about 20° that is in good agreement with the earlier report (Ramesh et al., 2012), suggesting
Fig. 3. (a) XRD from 2θ = 2–10° of p-TSA treated clay and its composite membranes. (b) Powdered XRD of nascent NaAlg and MMMs, showing a decrease in intensity with an increase in clay loading (inset: XRD p-TSA treated clay).
structural integrity after the modification of clay. The NaAlg matrix is partly made up of crystalline regions and partly amorphous regions. The presence of such crystalline region is responsible for the observed peak at 2θ of 15° (Hua et al., 2010), which results from the polar interactions between \OH groups of NaAlg chains. The clay particles act as seeding agent to enhance the crystalline region present in the polymer matrix, by facilitating an ordered arrangement of polymer chains. Thus, increase in clay loading results in enhanced crystallinity of the NaAlg matrix as well as d-spacing between the NaAlg chains gradually decreases from 0.59 nm to 0.56 nm. As shown in the inset of Fig. 3(b), clay particles show an intense crystalline peak at 2θ of 20°. Such peak with less intensity is observed only in clay composites (not in nascent NaAlg), thus it can be attributed to the presence of clay particles. Comparatively, the clay particles show d-spacing of 0.41 nm, while the clay particles in the NaAlg matrix show enhanced d-spacing of 0.45 nm, which is attributed to the favorable interactions between clay and NaAlg matrix. These results suggest the successful incorporation of clay particles into NaAlg matrix and the structure of clay remains intact even after membrane fabrication. 3.1.3. FE-SEM FE-SEM image of nascent NaAlg shown in Fig. 4(a) is smooth, which implies the formation of a uniform dense matrix. However, in the case of
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
Fig. 4. FE-SEM images of (a) nascent NaAlg, (b) PCL-5, (c) PCL-10 and (d) PCL-15 showing an increase in particle agglomeration with loading.
composite membranes as shown in Fig. 4(b–d), the interface defects are not observed, but some rough surfaces are observed with well-wrapped clay particles. Such irregularly raised rough surfaces are attributed to clay-rich regions of the matrix. As shown in Fig. 4(b), the PCL-5 membranes show particle size variation from 400 to 600 nm, but in Fig. 4(c), the PCL-10 shows the size range between 600 and 800 nm, along with a higher number of particle distribution. Fig. 4(d) displays particle agglomeration and the sizes of particles are almost close to 2 μm, which corresponds to PCL-15 membrane. Thus, from the PV data, one can see that by increasing the concentration of clay particles beyond 10 wt.%, particle agglomeration takes place, which might be responsible for a decrease in membrane performance. 3.1.4. Thermogravimetry (TGA) The oxidative thermal degradation profiles of nascent clay and p-TSA treated clay presented in Fig. 5(a) show the weight loss regions between 110 and 150 °C, which can be ascribed to the release of absorbed
water molecules. In this region, p-TSA treated clay shows a higher water loss along with its release at higher temperature, due to enhanced water retention capacity of the composite membranes, an indicator of increased hydrophilicity after the p-TSA treatment. Thermal stability and degradation behavior of the nascent NaAlg and clay-incorporated NaAlg tested in the range of 40–650 °C under nitrogen atmosphere shown in Fig. 5(b) suggests non-oxidative degradation with two major weight loss regions. The first loss occurs around 100 °C due to the release of absorbed water molecules, which progressively increases with increasing loading of clay due to its enhanced water retention capacity. However, a higher weight loss of about 25–30% is observed around 240–400 °C due to thermal decomposition of NaAlg polymer. TGA curves reveal that, with increase in clay loading, both membrane water retention capacity and thermal stability have increased, which can be attributed to favorable interactions between NaAlg and modified clay particles. A large number of clay particles remain un-dissociated even at 600 °C, as a consequence of which all the composite membranes show lesser weight losses in this region.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
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Fig. 7. Equilibrium swelling (%) of membranes at 10 wt.% water–IPA mixture at 30 °C, showing an increase in swelling with an increase in clay loading.
3.1.6. Equilibrium swelling In a PV process, equilibrium swelling studies are helpful to analyze the available free-volume towards solvent diffusion (Aminabhavi et al., 1994). The effect of p-TSA treated clay loading on equilibrium swelling is depicted in Fig. 7. Both NaAlg and clay are hydrophilic in nature (as also supported by contact angle data), so that composite membranes show increased swelling at higher clay loading. Hence, a higher filler loading facilitates larger free-volume towards solvent diffusion. The p-TSA treated clay-loaded membranes show a higher equilibrium swelling compared to the unmodified clay-loaded membranes (due to enhanced surface area and hydrophilicity of the filler), which is further evidenced by an increase in PV selectivity and permeance. However, uncontrolled membrane swelling is checked by glutaraldehyde crosslinking. Fig. 5. TGA curves of (a) clay and p-TSA treated clay, showing an increase in water retention capacity after the p-TSA treatment. (b) Nascent NaAlg, PCL-5, PCL-10 and PCL-15.
3.2. Pervaporation performance
3.1.5. Contact angle studies Water contact angle data offer information on the relative degree of surface hydrophilicity of all the membranes; the smaller the contact angle, the higher the hydrophilicity and vice versa. The results of contact angle studies are shown in Fig. 6. The nascent NaAlg shows a contact angle of 78°, but with clay loading, contact angle continuously decreases to reach a maximum of 37° for PCL-15. Increase in polar groups has resulted in an increase of available polar (hydrophilic) groups, which has contributed towards the formation of more hydrophilic and rougher surfaces (also supported by XRD studies).
3.2.1. Effect of clay concentration PV performance of the nascent NaAlg membrane was enhanced considerably after loading with p-TSA treated clay particles in concentrations of 5, 10 and 15 wt.% (see Fig. 8 and Table 1). The increase in selectivity for modified clay-loaded NaAlg composite membrane is the result of increase in hydrophilicity, which might have increased water sorption capacity of the membranes (sorption selectivity). This observation is also supported by XRD, FTIR, contact angle and equilibrium sorption studies. The favorable physico-chemical interactions between clay
Fig. 6. Change in water contact angle (θ°) of MMMs as a function of wt.% of clay loading, showing an increase in hydrophilicity with increasing clay loading.
Fig. 8. Effect of filler loading on permeance and selectivity for nascent NaAlg and clayloaded NaAlg membranes at 30 °C.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Table 1 Relevant data on membrane performance. Membrane type
Feed temperature (°C)
ES (%)
Jw (GPU)
Je (GPU)
Total permeance (GPU)
αij
Dw × 1012 (m2 s−1)
DIPA × 1014 (m2 s−1)
Nascent NaAlg
30 40 50 30 40 50 30 40 50 30 40 50 30 40 50
9
1730 1951 2169 3012 3311 3588 3422 3858 4100 3832 4098 4369 3244 3553 3972
7.07 11.00 15.00 3.1 5.37 7.67 0.59 4.16 7.85 2.5 5.92 8.85 1.33 4.74 7.87
1737 1962 2184 3015 3316 3595 3423 3862 4108 3835 4104 4378 3245 3557 3979
244 182 147 975 616 497 5781 926 522 1534 692 493 2428 749 504
1.28 1.48 1.67 2.15 2.39 2.62 2.42 2.78 3 2.72 2.95 3.2 2.3 2.57 2.92
6.18 11.0 15.0 2.6 6.5 14.0 0.49 13.0 20.0 2.1 15.0 22.0 1.1 13 23
PCL-5
PCL-10
PCL-15
CL-10
15
22
31
19
Jw = Permeance of water; Je = permeance of alcohol; αij = membrane selectivity to water; Dw = diffusion co-efficient of water; DIPA = diffusion co-efficient of IPA; ES = equilibrium swelling.
particles and NaAlg matrix (see Fig. 9) might create torturous pathways (as per Mc and ve data shown in the Supplementary information), thereby imposing greater barrier resistance to the diffusion of bigger IPA (kinetic diameter = 0.47 nm) molecules than water molecules (kinetic diameter = 0.28 nm). This would also enable easy selective transport of water molecules across the membrane (water diffusion selectivity).
Moreover the smectic intercalation in clay structure would act as water selective channels. Due to the aforementioned factors, the p-TSA treated clay loaded membranes show a better sorption and diffusion selectivity towards water than IPA. The maximum selectivity of 5781 with a moderate permeance of 3423 GPU at 10 wt.% clay loading, and straight increase
Fig. 9. Interaction model between clay particles and the NaAlg matrix (showing H-bonding linkage between NaAlg and clay).
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
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Fig. 10. Permeance (GPU) and selectivity of PCL-10 and CL-10 (effect of p-TSA treatment on membrane performance).
of permeance with a decrease in selectivity is observed at higher loading (15 wt.%). These observations can be explained by considering the fact that membranes attain the saturation limit near 10 wt.% of clay loading, above which the clay particles start percolating (self-agglomeration), leading to a reduced exposure to NaAlg. As a consequence of this, void size increases, resulting in a decrease of barrier resistance towards IPA molecules. 3.2.2. Effect of p-TSA treatment The PV results of 10 wt.% p-TSA treated clay-loaded NaAlg (PCL-10) and 10 wt.% untreated clay-loaded NaAlg (CL-10) presented in Fig. 10 show enhanced selectivity of 5781 for the former compared to a value of 2428 for the latter (almost 140% increase in selectivity), but with only a slight improvement in the permeance value of 3423 GPU from its original value of 3244 GPU. Such an enhancement in selectivity is the result of increased hydrophilicity and surface area of the clay after modification, resulting in enhanced interfacial compatibility of clay particles with NaAlg, along with an increase in membrane hydrophilicity. The p-TSA treatment etches the structural alumina, which eventually forms an aqueous alumina, thereby replacing Na+ ions from the clay intercalation. This might result in two effects; firstly, it increase the overall surface area from 29 to 113 m2 g−1 (as shown by BET studies) and secondly, it enhances the surface free hydroxyl groups, which along with the replaced aqueous alumina are responsible to increase the surface hydrophilicity (as shown by TGA studies) (Ramesh et al., 2012). The aforementioned changes could be attained without disturbing the original crystal structure of the clay (as shown by XRD studies). Therefore, increase in these factors have a direct impact on the performance of composite membranes in terms of sorption and diffusion selectivity, since modified clay composite membranes show 20% increase in water sorption selectivity along with 5% increase in water diffusion selectivity compared to unmodified clay loaded membranes (see Tables S-1 and S-2 in the Supplementary information). 3.2.3. Effect of feed composition and feed temperature Increase in feed water composition has an adverse effect on the PV performance of the membranes. For instance, increase of feed water composition from 10 to 40 wt.% for PCL-5 membranes (see Fig. 11(a)) resulted in an increase of total permeance from 3015 to 3937 GPU; on the other-hand, selectivity decreased from 975 to 176. Such a decrease in membrane performance is mainly driven by the increase in driving force followed by subsequent increase in membrane swelling. As the feed water composition increases in the feed side, while that on permeate side, it does not vary much, due to reduced pressure (5 mbar) maintained at the permeate side. As a consequence, chemical potential
Fig. 11. (a) Concentration dependence of permeance (GPU) and selectivity for PCL-5 at 30 °C. (b) Temperature dependence of permeance (GPU) and selectivity for PCL-5 for feed mixture containing 10 wt.% of water in IPA.
difference (driving force) increases, leading to enhanced dissolution of water molecules in the membrane matrix, which contributes towards enhanced available free-volume for permeation. Through enhanced free volume, the permeation of water molecules increases along with a parallel increase in IPA permeation, resulting in enhanced permeance at the cost of selectivity. The effect of temperature on PV performance studied at 30, 40 and 50 °C for 10 wt.% water in IPA feed mixture presented in Fig. 11(b) suggests increased permeation rate at the cost of selectivity with an increase in feed temperature. At higher temperature, polymer thermal mobility (segmental motion) increases, leading to enhanced free volume of the composite matrix, resulting in a non-selective permeation of IPA molecules through enlarged void channels. These observations are in accordance with the reported results of Huang and Yeom (1991).
4. Literature comparison Several studies have been reported on the PV dehydration of IPA. Majority of literature data reported in terms of flux and separation factor are converted into permeance in GPU and selectivity as per Eqs. (5) and (6) and these are compared in Table 2. See for instance, a very high permeance of 73,393 GPU and a selectivity of 453 are observed for GOTMS (3-glycidoxypropyltrimethoxysilane) loaded polyimide membranes (Teoh et al., 2008). In the case of 20 wt.% 13X zeolite-loaded blend membranes of sodium carboxymethylcellulose and poly(vinyl alcohol) (Prasad et al., 2012), a selectivity of 5138 and a permeance of 2587 GPU are observed. Compared to some of these reported data, the present membranes display a balanced separation performance in terms of permeance and selectivity.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Table 2 Comparison of permeance and selectivity of present membranes with literature reports. Membranes
Feed water (wt.%)
Temperature (°C)
Water permeance (GPU)
Selectivity
Ref.
PAI—polyamide TFC F2PVGLU6 GOTMS incorporated polyamide membranes Tarlon/P84 blended hollow fiber 6 FDA-ODA-NDA/Ultem hollow fiber PEC+/PEC− layer by layer film NaCMC/PVA and 20 wt.% of 13X zeolite NaAlg and 10 wt.% p-TSA treated clay
15 12.3 15 15 15 10 10 10
50 40 50 60 60 50 35 30
27,206 2122 73,393 4009 10,053 21,502 2587 3423
1019 96 453 314 5781 1043 5138 5781
Zuo et al. (2012) Das et al. (2011) Zuo et al. (2013) Teoh et al. (2008) Widjojo and Chung (2009) Zhao et al. (2010) Prasad et al. (2012) Present work
PAI—polyamide; TFC—thin film composite; F2PVGLU—2 wt.% of glutaraldehyde in polyvinyl alcohol and 6 wt.% of glutaraldehyde; GOTMS—3-glycidyloxypropyltrimethoxy-silane; NaCMC—sodium carboxymethylcellulose.
5. Conclusions The p-TSA treated clay-loaded NaAlg composite membranes were prepared and characterized to understand the effect of organic acid treatment on clay; such membranes offer improved PV dehydration of IPA from its mixture with water. It was demonstrated that after p-TSA treatment of clay, both its surface area and hydrophilicity have increased (without compromising on its structural integrity), thereby improving the interface compatibility between clay and NaAlg matrix, thus showing enhanced permeance and selectivity values compared to unmodified clay-loaded membrane. Further, PV studies were performed at varying feed temperatures and feed compositions, where the best performance was observed at lower water composition and temperature. Increase of filler loading enhanced the membrane hydrophilicity as per contact angle studies; these effects are also confirmed by FTIR, TGA and equilibrium sorption studies. The modified clay loading up to 10 wt.% was sufficient to obtain optimum values of permeance and selectivity, but any further increase in clay loading resulted in a decline of selectivity due to particle agglomeration (as supported by FE-SEM). The Arrhenius energy barrier values revealed similar trends. In addition, experimentally determined packing density parameter confirms an increase in packing density with an increase in filler loading. The diffusion coefficient values calculated from the Fick's equation also support these observations. Overall, p-TSA treatment is an effective method to improve the membrane performance of clay-loaded NaAlg membranes for dehydrating isopropanol. Acknowledgment Financial assistance from the Department of Atomic Energy (DAE), Board of Research in Nuclear Sciences (BRNS) (Grant No. 2013/34/4/ BRNS); Vision Group of Science and Technology (VGST) (GRD No. 91) and Admar Mutt Education Foundation (AMEF), Bangalore are gratefully acknowledged. DPS is thankful to Manipal University for permitting to publish this research as a part of Ph.D. program. We also thank Shubha and Varadharaju at the Indian Institute of Science, Bangalore for their kind assistance with contact angle and FE-SEM analysis respectively. Appendix A. Supplementary data All the theoretical calculations are presented in the Supplementary information. References Adoor, S.G., Sairam, M., Manjeshwar, L.S., Raju, K.V.S.N., Aminbhavi, T.M., 2006. Sodium montmorillonite clay loaded novel mixed matrix membranes of poly(vinyl alcohol) for pervaporation dehydration of aqueous mixtures of isopropanol and 1,4-dioxane. J. Membr. Sci. 285, 182–195. Adoor, S.G., Rajineekanth, V., Nadagouda, M.N., Chowdoji Rao, K., Dionysiou, D.D., Aminabhavi, T.M., 2013. Exploration of nanocomposite membranes composed of
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Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Research paper
para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol D.P. Suhas a, T.M. Aminabhavi a,b, A.V. Raghu a,⁎ a b
Materials Science Division, Poornaprajna Institute of Scientific Research, Bengaluru 562 110, India Department of Pharmaceutical Engineering and Chemistry, Soniya College of Pharmacy, Dharwad 580 002, India
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 16 August 2014 Accepted 20 August 2014 Available online xxxx Keywords: p-TSA Acid modified clay Pervaporation Isopropanol dehydration Clay composite
a b s t r a c t The para-toluene sulfonic acid (p-TSA) treated clay (5, 10 and 15 wt.% loadings) particles were inserted into sodium alginate (NaAlg) matrix to derive the composite membranes that showed enhanced pervaporation performance for isopropanol separation. The 10 wt.% modified clay-loaded NaAlg membranes showed the highest selectivity of 5781 with a permeance of 3423 GPU for 10 wt.% water containing isopropanol feed mixture at 30 °C, showing a 140% higher selectivity compared to pristine clay-loaded NaAlg composite membrane. This effect is ascribed to an enhanced surface area and hydrophilicity after p-TSA treatment of clay, as verified by BET and TGA studies. The membranes were characterized by a variety of physico-chemical techniques and their pervaporation performance was assessed in terms of selectivity, permeance, with extent of variations in filler loading, temperature and feed water composition. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In recent years, pervaporation (PV) technique has gained a much wider acceptance as an energy efficient method for obtaining pure alcohols (Hoof et al., 2004). For separating alcohol–water mixtures, distillation is effective only to a certain extent viz., 80–85 wt.%, after which separation is done by evaporating the entire alcohol (usually any process involving phase change from liquid to gas consumes large quantity of energy). Since the number of evaporating molecules is large, naturally energy consumption is also large. On the other hand, PV selectively removes the minor components from such mixtures and thus limiting the energy burden only to minor components. PV is also an environmentally benign technique, since it does not involve an entertainer like benzene, as applicable in azeotropic distillation (Jonquières et al., 2002; Lipnizki et al., 1999; Shao and Huang, 2007; Widagdo and Seider, 1996). In this research, isopropanol (IPA) dehydration is considered in view of its importance in cosmetics, gums, waxes, cleansing agent in industries and as a blend in gasoline (Papa, 2000). High purity grade IPA is required in all these areas. IPA forms an azeotrope at 12.3 wt.% of water, making it difficult to separate by conventional distillation (Ghazali et al., 1997). Hydrophilic polymers like poly(vinyl alcohol), sodium alginate (NaAlg), chitosan, polyacrylonitrile, and polyimide have been reported for IPA dehydration (Chapman et al., 2008; Suhas et al., 2013, 2014). NaAlg is a naturally occurring anionic bio⁎ Corresponding author at: Centre for Emerging Technologies, Jain Global Campus, Jain University, Kanakapura, Ramanagara 562 112, India. E-mail addresses: [email protected], [email protected] (A.V. Raghu).
compatible material with good film forming ability, moderate mechanical strength and good affinity to water (Bhat and Aminabhavi, 2007a, 2007b). However, nascent NaAlg suffers from uncontrolled swelling, thus exhibiting low PV performance. To circumvent these problems, composite membranes have been prepared by dispersing suitable fillers into NaAlg matrix. Such membranes have emerged to be the best for PV dehydration of aqueous–organic mixtures (Adoor et al., 2013; Chung et al., 2007; Zhao et al., 2013). Typical fillers used in developing composite membranes are nonporous, porous or intercalated materials, whose presence creates torturous pathways in the matrix favoring the selective permeation of water molecules. Examples of the widely used fillers include heteropolyacids (Teli et al., 2007), titanium dioxide (Liu et al., 2011), zeolites (Bhat and Aminabhavi, 2007a, 2007b), mesoporous silica (Patil et al., 2007), and clays (Anilkumar et al., 2008). Among these, montmorillonite clays, owing to their layered structure with a high aspect ratio and hydrophilic surface chemistry, have gained much wider acceptance as fillers in barrier membranes (Avagimova et al., 2013). Clays are the important class of layered silicates consisting of alumina octahedra sandwiched between two sheets of silica tetrahedra interconnected by sharing of O2− atoms at the polyhedral corners and edges (Chen et al., 2008). Earlier studies on sodium montmorillonite clay incorporated PVA and NaAlg membranes have shown improved separation performance (Adoor et al., 2006; Bhat and Aminabhavi, 2006). Acid treatment is the most actively pursued method to improve the surface property of the clay. The technique involves leaching of the clay with suitable acids leading to deagglomeration of clay particles, elimination of mineral impurities as well as causing structural and chemical changes in the clay.
http://dx.doi.org/10.1016/j.clay.2014.08.017 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
The acid treated clay has several attributes like high surface area and large number of hydrophilic sites compared to the nascent clay. It was found that the degree of clay structure decomposition depends on several factors like the type of acid used, concentration of acid, duration of treatment and experimental conditions (Madejova et al., 1998; Panda et al., 2010; Siddiqui, 1968). Earlier (Ramesh et al., 2012), it was shown that p-TSA treated smectic clay enhanced both the surface area and the catalytic activity compared to nascent clay. Moreover, to enhance the efficiency, we have incorporated a microwave irradiation technique, which is environmentally friendly. In this work, organically modified clay particles prepared by treating with p-TSA were incorporated into the NaAlg matrix to derive composite membranes that were tested for PV dehydration of IPA–water mixture. Performance of the modified clay-loaded NaAlg composite membrane was compared with that of pristine clay-loaded NaAlg composite membrane for IPA dehydration. The composite membranes were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetry (TGA) and contact angle measurements. Sorption and diffusion parameters along with their temperature dependencies have been used to estimate the Arrhenius activation parameters for permeation and diffusion processes. These along with selectivity and permeance data have been used to discuss the PV results for IPA dehydration. To the best of our knowledge, this is the first study on the use of p-TSA treated clay loaded NaAlg membranes used in PV dehydration of IPA.
water for 30 min followed by 1 h of sonication. The suspension obtained was slowly added to the NaAlg solution and mixed over a magnetic stirrer for 24 h. The resulting uniform suspension was poured onto a perfectly aligned flat clean glass plate dried at ambient temperature. The dried membranes were peeled off from the glass plate and were further dried in a vacuum desiccator at ambient temperature before use. The membranes formed were cross-linked by immersing in a crosslinking solution bath for 6 h that contained water–acetone (30:70) mixture with 2 mL of GA as a cross-linking agent and 2 mL of conc. HCl as a catalyst. The cross-linked membranes were alternatively rinsed with water and methanol for at least ten times followed by soaking in methanol for 24 h and then preserved in a desiccator before further testing. Using the above procedure, 5, 10 and 15 wt.% of p-TSA treated clayloaded NaAlg composite membranes were prepared and are designated as: PCL-5, PCL-10 and PCL-15, respectively. As a control, the nascent NaAlg (cross-linked, but without adding fillers) and 10 wt.% pristine clay loaded NaAlg (designated as CL-10) membranes were fabricated. 2.4. Membrane characterization
In a typical procedure, 20 g of clay and 200 mL of 2 molar p-TSA were taken in a 500 mL round bottom flask connected to a double walled reflux condenser and digested under a microwave irradiation (Microware Lab Station START-5 Italy, courtesy of Dr. Y.S. Bhat, BIT, Bangalore) for 10 min at 1000 W power and heated to 80 °C. The reaction mixture was centrifuged and the residual solid was washed thoroughly with water until the solution acquired neutral pH. The resulting solid was dried at 120 °C for 2 h in a vacuum oven and ground to fine powder. The increase in surface area as measured by BET isotherm was 113 m2 g−1 as against the original value of 29 m2 g−1 for pristine clay, confirms the modification of clay. Additionally, thermogravimetry showed an increase in water retention capacity of the modified clay, suggesting increased hydrophilicity (see the Thermogravimetry (TGA) section). The pristine clay used in this study was also prepared by the same approach, except that only double distilled water was used instead of p-TSA.
Fourier transform infrared (FTIR) spectroscopy (Bruker Alpha-T spectrometer) was used to analyze the samples after mixing with KBr and pellets were made under applied hydraulic pressure (180 kg/cm2). Scanning was done in the range of 400–4000 cm−1 for 32 times with a resolution of 4 cm−1 and these data were accumulated in a transmission mode. Wide-angle XRD analysis of the membranes was carried out in a powder form using Bruker D-2α Phaser X-ray diffractometer. The Xrays of wavelength 1.54 Å were generated by nickel filtered CuKα radiation source and samples were scanned in the 2θ range of 2–60° at the scanning rate of 2°/min. Micro-structural morphology of the nascent NaAlg as well as composite NaAlg membranes were examined by a field emission scanning electron microscope (FE-SEM) using an FE-SEM ZEISS Ultra-55 instrument (available at CeNSE, Indian Institute of Science, Bangalore). To improve the image quality, a conductive layer of sputtered gold was coated prior to these measurements. Thermal stability of nascent NaAlg and its composite membranes was examined by thermogravimetry (Universal V3.9A TA Instruments, Bangalore, India) over the temperature range of 40–650 °C at the heating rate of 10 °C/min. Analysis was carried out under an inert atmosphere at the nitrogen gas flow rate of 10 mL/min. For each analysis, about 6–9 mg of the sample was taken in aluminum pans. Similarly, TGA analysis of the pristine clay and p-TSA treated clay was performed from 40 to 800 °C in an ambient atmosphere. Water contact angle on the surface of the membrane was measured as per the Sessile drop method using the Data Physics OCA 20 at 25 °C (courtesy of Prof. S.K. Biswas, Indian Institute of Science, Bangalore). Prior to these measurements, all the membranes were vacuum dried and kept in a desiccator. Then, a piece of membrane (1 cm × 7 cm) was adhered to clean and smooth glass slide for contact angle measurements. A 2 μL of deionized water droplet was placed on the sample surface and droplet image was captured by a microscope coupled to a charge coupled device (CCD) camera. All the measurements were done within 10 s to minimize the errors due to evaporation losses. A total of six measurements were taken at different locations for each membrane sample and the average value was taken with N3% standard deviation.
2.3. Membrane fabrication
2.5. Equilibrium swelling
The solution casting method was adopted to prepare composite membranes from a uniform suspension of p-TSA treated clay-loaded NaAlg. In a typical procedure, 4 g of NaAlg was dissolved in 100 mL of water with constant stirring followed by 24 h of degassing. In a separate flask, the required amount of clay was stirred by taking into 20 mL of
Equilibrium swelling experiments were performed gravimetrically at 30 °C for 10 wt.% water containing IPA feed mixture. Initial weights of all the circularly cut membranes were measured after vacuum drying in a hot air oven and the weight of dry membrane was noted as Wd. Dry membranes were placed inside the specially designed air tight test
2. Experimental 2.1. Materials Sodium montmorillonite clay was purchased from Sigma Aldrich, USA, while sodium alginate (NaAlg), isopropanol (IPA), hydrochloric acid (35%) (HCl), p-toluene sulfonic acid (p-TSA) and glutaraldehyde (GA) were all purchased from s.d. fine chemicals, Mumbai, India. Absolute ethanol (EtOH) (99.9%) was purchased from Commercial Alcohols, Brampton, Canada. All other chemicals were of analytical reagent (AR) grade samples, used without further purification. Double distilled water was used throughout the experiments. 2.2. Preparation of p-TSA treated clay
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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bottles containing 30 cm3 of the test liquid at 30 °C. All the membranes were soaked in the test solvent for 48 h and before measuring the swollen weight (Ws), membranes were gently pressed between the filter paper wraps to remove surface-adhered solvent molecules and weighed immediately to minimize the weight loss due to evaporation. Membranes after a thorough drying attained almost same as the initial weight (within a small deviation of ± 3%). Equilibrium swelling (%) was calculated using: Equillibrium swelling ð%Þ ¼
W s −W d 100: Wd
ð1Þ
3
2.6. Pervaporation experiment PV experiments were performed in an indigenously fabricated stainless steel unit described elsewhere (Aminabhavi and Naik, 2002), which consisted of two compartments viz., feed tank and permeate cell as shown in Fig. 1. The membrane with an effective area of 3.84 × 10− 3 m2 was held tight with the help of two O-shaped Teflon rings supported on a porous stainless steel support, which was placed between two cells. The feed tank was a double walled cell with a feed capacity of 500 mL. The desired temperature in the test cell was maintained by circulating water using a thermostatic bath
1 – Thermostac bath 2 – Pump 3 – Srrer 4 – Thermometer 5 – Outer Jacket 6 – Feed Chamber 3
7 – Membrane
4
8 – Porous stainless steel support
5
9 – Permeate Chamber
6
10 – Valve
2
11 – Pressure gauge 7
12 – Glass trap
8
13 – Liquid Nitrogen Dewar 14 – Vacuum pump
1
10
9
11
10
14 12
13
Fig. 1. Schematic diagram of pervaporation system.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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(Grant, UK, model GD-120) at the desired temperature into the outer jacket. Further, the feed tank was provided with a thermometer to monitor the feed temperature and the stirrer was rotated constantly. The feed mixture was maintained at atmospheric pressure, while permeate pressure was (measured by a piezometer) maintained at b5 mbar with the help of a double stage vacuum pump. The feed mixture at different temperatures was first equilibrated for 2 h before collecting the sample, after which a constant flux was produced. Permeate samples were collected at regular intervals of time in two glass traps (fitted in the condenser assembly) immersed in a liquid nitrogen containing Dewar flask. All the sample measurements (viz., weight and concentration) were done at ambient temperature and samples were weighed on a Sartorius BSA224 balance. The compositions of liquid feed mixture were analyzed by measuring the refractive index using a refractometer (Mettler, Toledo) within the accuracy of ±0.0001 units that was calculated from the previously established standard graph of refractive index vs known mixture composition. The refractometer prism was maintained at 25 ± 0.1 °C. Simultaneously, the composition of permeate was determined by gas chromatography (Trace 700, Thermo-Fisher) equipped with a TCD detector and a Parapak-Q packed column. The same disk was re-used for subsequent tests at increasing water composition of the feed mixture and also at increasing temperature. The membranes were found to be stable without showing any signs of degradation during the PV experiments. The feed mixture was maintained at constant composition by compensating for any loss of water. At least three independent readings were taken for each membrane and calculations were done by taking
the average data within ± 3% standard errors. All the membranes showed good performances up to 8 h of continuous PV operation. Following the suggestion by Wijmans (2003), all the PV data are reported in a driving force normalized form viz., permeance (Pi/l) and selectivity (αij), since these calculations are free from the influence of driving force and reveal the intrinsic membrane performance. Thus, molar flux for each component (ji) was calculated as: ji ¼
J i vi mi
ð2Þ
where Ji is the flux (g/m2 h) obtained from the PV data, vi is the molar volume [22.4 L (STP)/mol] of the ith component and mi is the molecular weight. Partial vapor pressure (pfi) was calculated from the van Laar equation as: f
s
pi ¼ xi γ i pi
ð3Þ
where xi is the molar concentration and γi is the activity coefficient of the individual components. The saturated vapor pressure (psi ) of the ith component was calculated using (Gmehling et al., 1977): s
logpi ¼
A−B : T þC
ð4Þ
Here, A, B and C are the Antoine constants taken from the literature (Holmes and Winkle, 1970) and T is the temperature in Kelvin. The
Fig. 2. (a) FTIR spectrum of nascent clay and p-TSA treated clay. (b) FTIR of nascent NaAlg and NaAlg with different loadings of p-TSA treated clay showing an increase in red-shift with an increase in clay loading.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
5
molar flux together with the vapor pressure of the ith component of the feed (pfi) and permeate (ppi ) allowed for the calculation of permeance (Pi/l) as: P i Di K i ji : ¼ ¼ l l pif −ppi
ð5Þ
Here, Di, Ki and l refer to the diffusion coefficient, sorption coefficient and membrane thickness, respectively. Similarly, membrane selectivity, which is the ratio of permeabilities or permeances of components, i and j through the membrane, was calculated as: α ij ¼
P i =l : P j =l
ð6Þ
These calculated parameters are used in the discussion of PV results. 3. Results and discussion 3.1. Characterization of modified clay and NaAlg composite membranes 3.1.1. FTIR FTIR spectra of clay, p-TSA treated clay, nascent NaAlg and composite NaAlg membranes are displayed in Fig. 2(a) and (b). FTIR spectra of both clay and p-TSA treated clay shows characteristic absorption peaks at 3450, 1060 and 550 cm− 1 corresponding to \OH, Si\O\Si and Al\O\Al vibrations, respectively (Madejová, 2003), but p-TSA treatment has resulted in some minor changes in FTIR spectra of montmorillonite clay. Absorption peaks at 3400 cm−1 and 3626 cm−1 correspond to the \OH stretching of absorbed water molecules and structural \OH, respectively. The intensity of these peaks has increased after p-TSA treatment, which can be attributed to the removal of some of the structural alumina and its subsequent occupation at clay intercalation with the hydration shell. The presence of an extra peak (in p-TSA treated clay) at 750 cm−1 may be attributed to the influence of p-TSA molecules. Similar results were obtained by Tong et al. (2013). The spectra for nascent NaAlg shows the absorbance between 1620 and 1077 cm− 1 that corresponds to symmetric C_O and asymmetric O\C\O stretching vibrations of the carboxyl group, respectively. The characteristic peak around 3300 cm−1 is attributed to the stretching vibration of the \OH group of NaAlg membranes. After the incorporation of clay into these membranes, the peak was strengthened and broadened, due to high density of \OH groups available on the clay surface. Moreover, FTIR spectra show a systemic decrease in \OH absorption (red-shift) with an increase in clay loading showing the sequence: 3480 cm−1 (nascent NaAlg) N 3440 cm−1 (PCL-5) N 3428 cm−1 (PCL10) N 3410 cm− 1 (PCL-15). In general, red shift refers to the shift in the absorption band to a higher wavelength, which otherwise refers to the decrease in energy required for stretching. This is observed because the \OH group of NaAlg interact with the \OH group of clay with hydrogen bonding, thereby reducing the electron density on the O\H bond. As a consequence, lesser energy is required for stretching. 3.1.2. Wide angle X-ray diffractograms (W-XRDs) The XRD of p-TSA treated clay shows a peak at 2θ of 6.6° and clearly the same peak has disappeared in PCL-5 and PCL-10 (Fig. 3(a)), which reveals the molecular level interaction between clay and NaAlg or possible delamination of montmorillonite clay in the NaAlg matrix. These results are in good agreement with the previous report by Hua et al. (2010). The 6.6° peak appears in PCL-15 showing particle agglomeration. Fig. 3(b) displays the W-XRD pattern of p-TSA treated clay, nascent NaAlg and all the clay-loaded composite membranes. Fig. 3(b) (inset) shows the typical diffraction pattern of p-TSA treated clay with its characteristic crystalline peak observed at 2θ of about 20° that is in good agreement with the earlier report (Ramesh et al., 2012), suggesting
Fig. 3. (a) XRD from 2θ = 2–10° of p-TSA treated clay and its composite membranes. (b) Powdered XRD of nascent NaAlg and MMMs, showing a decrease in intensity with an increase in clay loading (inset: XRD p-TSA treated clay).
structural integrity after the modification of clay. The NaAlg matrix is partly made up of crystalline regions and partly amorphous regions. The presence of such crystalline region is responsible for the observed peak at 2θ of 15° (Hua et al., 2010), which results from the polar interactions between \OH groups of NaAlg chains. The clay particles act as seeding agent to enhance the crystalline region present in the polymer matrix, by facilitating an ordered arrangement of polymer chains. Thus, increase in clay loading results in enhanced crystallinity of the NaAlg matrix as well as d-spacing between the NaAlg chains gradually decreases from 0.59 nm to 0.56 nm. As shown in the inset of Fig. 3(b), clay particles show an intense crystalline peak at 2θ of 20°. Such peak with less intensity is observed only in clay composites (not in nascent NaAlg), thus it can be attributed to the presence of clay particles. Comparatively, the clay particles show d-spacing of 0.41 nm, while the clay particles in the NaAlg matrix show enhanced d-spacing of 0.45 nm, which is attributed to the favorable interactions between clay and NaAlg matrix. These results suggest the successful incorporation of clay particles into NaAlg matrix and the structure of clay remains intact even after membrane fabrication. 3.1.3. FE-SEM FE-SEM image of nascent NaAlg shown in Fig. 4(a) is smooth, which implies the formation of a uniform dense matrix. However, in the case of
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Fig. 4. FE-SEM images of (a) nascent NaAlg, (b) PCL-5, (c) PCL-10 and (d) PCL-15 showing an increase in particle agglomeration with loading.
composite membranes as shown in Fig. 4(b–d), the interface defects are not observed, but some rough surfaces are observed with well-wrapped clay particles. Such irregularly raised rough surfaces are attributed to clay-rich regions of the matrix. As shown in Fig. 4(b), the PCL-5 membranes show particle size variation from 400 to 600 nm, but in Fig. 4(c), the PCL-10 shows the size range between 600 and 800 nm, along with a higher number of particle distribution. Fig. 4(d) displays particle agglomeration and the sizes of particles are almost close to 2 μm, which corresponds to PCL-15 membrane. Thus, from the PV data, one can see that by increasing the concentration of clay particles beyond 10 wt.%, particle agglomeration takes place, which might be responsible for a decrease in membrane performance. 3.1.4. Thermogravimetry (TGA) The oxidative thermal degradation profiles of nascent clay and p-TSA treated clay presented in Fig. 5(a) show the weight loss regions between 110 and 150 °C, which can be ascribed to the release of absorbed
water molecules. In this region, p-TSA treated clay shows a higher water loss along with its release at higher temperature, due to enhanced water retention capacity of the composite membranes, an indicator of increased hydrophilicity after the p-TSA treatment. Thermal stability and degradation behavior of the nascent NaAlg and clay-incorporated NaAlg tested in the range of 40–650 °C under nitrogen atmosphere shown in Fig. 5(b) suggests non-oxidative degradation with two major weight loss regions. The first loss occurs around 100 °C due to the release of absorbed water molecules, which progressively increases with increasing loading of clay due to its enhanced water retention capacity. However, a higher weight loss of about 25–30% is observed around 240–400 °C due to thermal decomposition of NaAlg polymer. TGA curves reveal that, with increase in clay loading, both membrane water retention capacity and thermal stability have increased, which can be attributed to favorable interactions between NaAlg and modified clay particles. A large number of clay particles remain un-dissociated even at 600 °C, as a consequence of which all the composite membranes show lesser weight losses in this region.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Fig. 7. Equilibrium swelling (%) of membranes at 10 wt.% water–IPA mixture at 30 °C, showing an increase in swelling with an increase in clay loading.
3.1.6. Equilibrium swelling In a PV process, equilibrium swelling studies are helpful to analyze the available free-volume towards solvent diffusion (Aminabhavi et al., 1994). The effect of p-TSA treated clay loading on equilibrium swelling is depicted in Fig. 7. Both NaAlg and clay are hydrophilic in nature (as also supported by contact angle data), so that composite membranes show increased swelling at higher clay loading. Hence, a higher filler loading facilitates larger free-volume towards solvent diffusion. The p-TSA treated clay-loaded membranes show a higher equilibrium swelling compared to the unmodified clay-loaded membranes (due to enhanced surface area and hydrophilicity of the filler), which is further evidenced by an increase in PV selectivity and permeance. However, uncontrolled membrane swelling is checked by glutaraldehyde crosslinking. Fig. 5. TGA curves of (a) clay and p-TSA treated clay, showing an increase in water retention capacity after the p-TSA treatment. (b) Nascent NaAlg, PCL-5, PCL-10 and PCL-15.
3.2. Pervaporation performance
3.1.5. Contact angle studies Water contact angle data offer information on the relative degree of surface hydrophilicity of all the membranes; the smaller the contact angle, the higher the hydrophilicity and vice versa. The results of contact angle studies are shown in Fig. 6. The nascent NaAlg shows a contact angle of 78°, but with clay loading, contact angle continuously decreases to reach a maximum of 37° for PCL-15. Increase in polar groups has resulted in an increase of available polar (hydrophilic) groups, which has contributed towards the formation of more hydrophilic and rougher surfaces (also supported by XRD studies).
3.2.1. Effect of clay concentration PV performance of the nascent NaAlg membrane was enhanced considerably after loading with p-TSA treated clay particles in concentrations of 5, 10 and 15 wt.% (see Fig. 8 and Table 1). The increase in selectivity for modified clay-loaded NaAlg composite membrane is the result of increase in hydrophilicity, which might have increased water sorption capacity of the membranes (sorption selectivity). This observation is also supported by XRD, FTIR, contact angle and equilibrium sorption studies. The favorable physico-chemical interactions between clay
Fig. 6. Change in water contact angle (θ°) of MMMs as a function of wt.% of clay loading, showing an increase in hydrophilicity with increasing clay loading.
Fig. 8. Effect of filler loading on permeance and selectivity for nascent NaAlg and clayloaded NaAlg membranes at 30 °C.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Table 1 Relevant data on membrane performance. Membrane type
Feed temperature (°C)
ES (%)
Jw (GPU)
Je (GPU)
Total permeance (GPU)
αij
Dw × 1012 (m2 s−1)
DIPA × 1014 (m2 s−1)
Nascent NaAlg
30 40 50 30 40 50 30 40 50 30 40 50 30 40 50
9
1730 1951 2169 3012 3311 3588 3422 3858 4100 3832 4098 4369 3244 3553 3972
7.07 11.00 15.00 3.1 5.37 7.67 0.59 4.16 7.85 2.5 5.92 8.85 1.33 4.74 7.87
1737 1962 2184 3015 3316 3595 3423 3862 4108 3835 4104 4378 3245 3557 3979
244 182 147 975 616 497 5781 926 522 1534 692 493 2428 749 504
1.28 1.48 1.67 2.15 2.39 2.62 2.42 2.78 3 2.72 2.95 3.2 2.3 2.57 2.92
6.18 11.0 15.0 2.6 6.5 14.0 0.49 13.0 20.0 2.1 15.0 22.0 1.1 13 23
PCL-5
PCL-10
PCL-15
CL-10
15
22
31
19
Jw = Permeance of water; Je = permeance of alcohol; αij = membrane selectivity to water; Dw = diffusion co-efficient of water; DIPA = diffusion co-efficient of IPA; ES = equilibrium swelling.
particles and NaAlg matrix (see Fig. 9) might create torturous pathways (as per Mc and ve data shown in the Supplementary information), thereby imposing greater barrier resistance to the diffusion of bigger IPA (kinetic diameter = 0.47 nm) molecules than water molecules (kinetic diameter = 0.28 nm). This would also enable easy selective transport of water molecules across the membrane (water diffusion selectivity).
Moreover the smectic intercalation in clay structure would act as water selective channels. Due to the aforementioned factors, the p-TSA treated clay loaded membranes show a better sorption and diffusion selectivity towards water than IPA. The maximum selectivity of 5781 with a moderate permeance of 3423 GPU at 10 wt.% clay loading, and straight increase
Fig. 9. Interaction model between clay particles and the NaAlg matrix (showing H-bonding linkage between NaAlg and clay).
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
D.P. Suhas et al. / Applied Clay Science xxx (2014) xxx–xxx
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Fig. 10. Permeance (GPU) and selectivity of PCL-10 and CL-10 (effect of p-TSA treatment on membrane performance).
of permeance with a decrease in selectivity is observed at higher loading (15 wt.%). These observations can be explained by considering the fact that membranes attain the saturation limit near 10 wt.% of clay loading, above which the clay particles start percolating (self-agglomeration), leading to a reduced exposure to NaAlg. As a consequence of this, void size increases, resulting in a decrease of barrier resistance towards IPA molecules. 3.2.2. Effect of p-TSA treatment The PV results of 10 wt.% p-TSA treated clay-loaded NaAlg (PCL-10) and 10 wt.% untreated clay-loaded NaAlg (CL-10) presented in Fig. 10 show enhanced selectivity of 5781 for the former compared to a value of 2428 for the latter (almost 140% increase in selectivity), but with only a slight improvement in the permeance value of 3423 GPU from its original value of 3244 GPU. Such an enhancement in selectivity is the result of increased hydrophilicity and surface area of the clay after modification, resulting in enhanced interfacial compatibility of clay particles with NaAlg, along with an increase in membrane hydrophilicity. The p-TSA treatment etches the structural alumina, which eventually forms an aqueous alumina, thereby replacing Na+ ions from the clay intercalation. This might result in two effects; firstly, it increase the overall surface area from 29 to 113 m2 g−1 (as shown by BET studies) and secondly, it enhances the surface free hydroxyl groups, which along with the replaced aqueous alumina are responsible to increase the surface hydrophilicity (as shown by TGA studies) (Ramesh et al., 2012). The aforementioned changes could be attained without disturbing the original crystal structure of the clay (as shown by XRD studies). Therefore, increase in these factors have a direct impact on the performance of composite membranes in terms of sorption and diffusion selectivity, since modified clay composite membranes show 20% increase in water sorption selectivity along with 5% increase in water diffusion selectivity compared to unmodified clay loaded membranes (see Tables S-1 and S-2 in the Supplementary information). 3.2.3. Effect of feed composition and feed temperature Increase in feed water composition has an adverse effect on the PV performance of the membranes. For instance, increase of feed water composition from 10 to 40 wt.% for PCL-5 membranes (see Fig. 11(a)) resulted in an increase of total permeance from 3015 to 3937 GPU; on the other-hand, selectivity decreased from 975 to 176. Such a decrease in membrane performance is mainly driven by the increase in driving force followed by subsequent increase in membrane swelling. As the feed water composition increases in the feed side, while that on permeate side, it does not vary much, due to reduced pressure (5 mbar) maintained at the permeate side. As a consequence, chemical potential
Fig. 11. (a) Concentration dependence of permeance (GPU) and selectivity for PCL-5 at 30 °C. (b) Temperature dependence of permeance (GPU) and selectivity for PCL-5 for feed mixture containing 10 wt.% of water in IPA.
difference (driving force) increases, leading to enhanced dissolution of water molecules in the membrane matrix, which contributes towards enhanced available free-volume for permeation. Through enhanced free volume, the permeation of water molecules increases along with a parallel increase in IPA permeation, resulting in enhanced permeance at the cost of selectivity. The effect of temperature on PV performance studied at 30, 40 and 50 °C for 10 wt.% water in IPA feed mixture presented in Fig. 11(b) suggests increased permeation rate at the cost of selectivity with an increase in feed temperature. At higher temperature, polymer thermal mobility (segmental motion) increases, leading to enhanced free volume of the composite matrix, resulting in a non-selective permeation of IPA molecules through enlarged void channels. These observations are in accordance with the reported results of Huang and Yeom (1991).
4. Literature comparison Several studies have been reported on the PV dehydration of IPA. Majority of literature data reported in terms of flux and separation factor are converted into permeance in GPU and selectivity as per Eqs. (5) and (6) and these are compared in Table 2. See for instance, a very high permeance of 73,393 GPU and a selectivity of 453 are observed for GOTMS (3-glycidoxypropyltrimethoxysilane) loaded polyimide membranes (Teoh et al., 2008). In the case of 20 wt.% 13X zeolite-loaded blend membranes of sodium carboxymethylcellulose and poly(vinyl alcohol) (Prasad et al., 2012), a selectivity of 5138 and a permeance of 2587 GPU are observed. Compared to some of these reported data, the present membranes display a balanced separation performance in terms of permeance and selectivity.
Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017
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Table 2 Comparison of permeance and selectivity of present membranes with literature reports. Membranes
Feed water (wt.%)
Temperature (°C)
Water permeance (GPU)
Selectivity
Ref.
PAI—polyamide TFC F2PVGLU6 GOTMS incorporated polyamide membranes Tarlon/P84 blended hollow fiber 6 FDA-ODA-NDA/Ultem hollow fiber PEC+/PEC− layer by layer film NaCMC/PVA and 20 wt.% of 13X zeolite NaAlg and 10 wt.% p-TSA treated clay
15 12.3 15 15 15 10 10 10
50 40 50 60 60 50 35 30
27,206 2122 73,393 4009 10,053 21,502 2587 3423
1019 96 453 314 5781 1043 5138 5781
Zuo et al. (2012) Das et al. (2011) Zuo et al. (2013) Teoh et al. (2008) Widjojo and Chung (2009) Zhao et al. (2010) Prasad et al. (2012) Present work
PAI—polyamide; TFC—thin film composite; F2PVGLU—2 wt.% of glutaraldehyde in polyvinyl alcohol and 6 wt.% of glutaraldehyde; GOTMS—3-glycidyloxypropyltrimethoxy-silane; NaCMC—sodium carboxymethylcellulose.
5. Conclusions The p-TSA treated clay-loaded NaAlg composite membranes were prepared and characterized to understand the effect of organic acid treatment on clay; such membranes offer improved PV dehydration of IPA from its mixture with water. It was demonstrated that after p-TSA treatment of clay, both its surface area and hydrophilicity have increased (without compromising on its structural integrity), thereby improving the interface compatibility between clay and NaAlg matrix, thus showing enhanced permeance and selectivity values compared to unmodified clay-loaded membrane. Further, PV studies were performed at varying feed temperatures and feed compositions, where the best performance was observed at lower water composition and temperature. Increase of filler loading enhanced the membrane hydrophilicity as per contact angle studies; these effects are also confirmed by FTIR, TGA and equilibrium sorption studies. The modified clay loading up to 10 wt.% was sufficient to obtain optimum values of permeance and selectivity, but any further increase in clay loading resulted in a decline of selectivity due to particle agglomeration (as supported by FE-SEM). The Arrhenius energy barrier values revealed similar trends. In addition, experimentally determined packing density parameter confirms an increase in packing density with an increase in filler loading. The diffusion coefficient values calculated from the Fick's equation also support these observations. Overall, p-TSA treatment is an effective method to improve the membrane performance of clay-loaded NaAlg membranes for dehydrating isopropanol. Acknowledgment Financial assistance from the Department of Atomic Energy (DAE), Board of Research in Nuclear Sciences (BRNS) (Grant No. 2013/34/4/ BRNS); Vision Group of Science and Technology (VGST) (GRD No. 91) and Admar Mutt Education Foundation (AMEF), Bangalore are gratefully acknowledged. DPS is thankful to Manipal University for permitting to publish this research as a part of Ph.D. program. We also thank Shubha and Varadharaju at the Indian Institute of Science, Bangalore for their kind assistance with contact angle and FE-SEM analysis respectively. Appendix A. Supplementary data All the theoretical calculations are presented in the Supplementary information. References Adoor, S.G., Sairam, M., Manjeshwar, L.S., Raju, K.V.S.N., Aminbhavi, T.M., 2006. Sodium montmorillonite clay loaded novel mixed matrix membranes of poly(vinyl alcohol) for pervaporation dehydration of aqueous mixtures of isopropanol and 1,4-dioxane. J. Membr. Sci. 285, 182–195. Adoor, S.G., Rajineekanth, V., Nadagouda, M.N., Chowdoji Rao, K., Dionysiou, D.D., Aminabhavi, T.M., 2013. Exploration of nanocomposite membranes composed of
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Please cite this article as: Suhas, D.P., et al., para-Toluene sulfonic acid treated clay loaded sodium alginate membranes for enhanced pervaporative dehydration of isopropanol, Appl. Clay Sci. (2014), http://dx.doi.org/10.1016/j.clay.2014.08.017