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Pervaporation Separation of WaterEthanol Mixtures Using OrganicInorganic Nanocomposite Membranes Veeresh T. Magalad,†,§ Gavisiddappa S. Gokavi,*,† Mallikarjuna N. Nadagouda,‡ and Tejraj M. Aminabhavi† †
Department of Chemistry, Shivaji University, Kolhapur 416 004, India The United States Environmental Protection Agency, ORD, NRMRL, WSWRD, 26 West MLK Drive Cincinnati, Ohio 45268, United States § Department of Chemistry, Tontadarya College of Engineering, Gadag 582 101, India ‡
ABSTRACT: Nanocomposite membranes (NCMs) of chitosan were prepared by incorporating Preyssler type heteropolyacid, namely, H14[NaP5W30O110], nanoparticles by solution casting and the solvent evaporation method. The nanoparticles as well as the membranes were characterized by a variety of physicochemical techniques. The membranes were employed in pervaporation separation of waterethanol mixtures at varying feedwater compositions and temperatures. The filler nanoparticles (58 nm size), also functioning as cross-linking agents, are responsible for increasing the thermal stability and mechanical strength of the NCMs over that of the nascent chitosan membrane. Furthermore, a dramatic increase of separation factor of 35 991 for NCMs from a base value of 96 for nascent chitosan membrane demonstrates the influence of filler nanoparticles in the matrix due to their favorable interactions with chitosan. Pervaporation performance of the NCMs showed a decrease with increasing feedwater composition and temperature. Diffusion and permeability data of water and ethanol molecules were analyzed by Fick's equation, and temperature dependence was studied through the Arrhenius relationship. Pervaporation data were analyzed on the basis of sorption-diffusion model. The FloryHuggins theory was used to understand sorption phenomenon and to estimate thermodynamic interaction parameters.
’ INTRODUCTION Pervaporation (PV) is a unique membrane-based separation technique used widely for the dehydration of organic solvents in view of its distinct advantages such as ease of operation, ecofriendly nature, energy intensive nature, and its effective separation ability for azeotropic mixtures compared to conventional processes.1 The technique has received a widespread importance in recent years due to the availability of organicinorganic nanocomposite membranes (NCMs), which seemingly offer improved separation performances over those of the conventional type membranes for aqueousorganic mixtures.27 Among the variety of polymers employed in PV dehydration of waterorganic mixtures, chitosan (CS) has been widely used.810 However, the polymer exhibits poor chain packing ability due to its rigid backbone, low interchain cohesion energy, and high free volume. Even though CS was one of the earliest membranes used in ethanol dehydration,11 its low selectivity to water has been widely explored by cross-linking,8 blending, and polyelectrolyte complex linkage formation.9,10 Ongoing efforts26 to develop NCMs by incorporating inorganic nanoparticles (58 nm) into the dense polymeric matrices have demonstrated further improvements in their mechanical strength properties in addition to barrier performances. The incorporation of such nanofillers would increase the diffusion path length to the penetrant molecules facilitating an easy transport of liquids due to the tortuous pathway created around the filler particles.12,13 Even though the NCMs of CS prepared by r 2011 American Chemical Society
incorporating inorganic nanoparticles have been of immense interest in membrane separations,1416 yet to the best of our knowledge, no reports are available on the NCMs of CS prepared by incorporating a Preyssler type heteropolyacid, H14[NaP5W30O110], that is, H14P5 nanoparticles used in the PV dehydration of ethanol. Adding H14P5 nanoparticles into the host CS matrix will improve the low water-selective nature of amorphous CS, due to favorable interactions of filler nanoparticles with the host CS matrix, thus leading to the creation of additional free volume spaces and thereby tortuous pathways in the matrix, facilitating easy molecular diffusion of liquids. The H14P5 nanoparticles have a strong Bronsted acidity function with high hydrolytic and thermal stability, and these will act as efficient “super acids”. They have a cyclic assembly of five PW6O22 units, each derived from the Keggin's anion [PW12O40]3 by the removal of two sets of three corner-shared WO6 octahedra.17 The bridging and terminal oxygen atoms are available to associate with water molecules to form hydrates that are thought to enhance selectivity to water. For these reasons, we thought of preparing NCMs of CS by incorporating different amounts of H14P5 into the host CS matrix and use these for PV dehydration of ethanol, since ethanol dehydration is an important unit operation. Ethanol is a useful solvent in chemical and Received: February 4, 2011 Revised: May 25, 2011 Published: June 20, 2011 14731
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The Journal of Physical Chemistry C pharmaceutical industries and is also a renewable biofuel derived from fermentation; its separation by the conventional route at its azeotropic composition with water is difficult.11 The present study deals with the development of NCMs of CS prepared by taking different amounts of H14P5 for applications in PV separation of waterethanol mixtures as a function of feedwater composition and temperature. The associated Arrhenius activation parameters for diffusion and permeation have been estimated to understand the sorptiondiffusion anomalies. Furthermore, PV separation results were analyzed on the basis of the principles of a sorptiondiffusion model.18 Surface free energies of the membranes have been calculated from the experimental results of a contact angle. Thermodynamic interaction parameters have been evaluated from the well-known FloryHuggins theory.19,20 These results along with the experimental data have been discussed in terms of the efficiency of the membranes used in ethanol dehydration.
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Figure 1. Particle size analysis of H14P5 nanoparticles.
’ EXPERIMENTAL METHODS Materials. Chitosan (Mw = 200 000 and degree of deacetylation = 7585%) was obtained from SigmaAldrich Chemicals, USA. Orthophosphoric acid, sodium tungstate, potassium chloride, and ethanol were all purchased from S. D. Fine Chemicals, Mumbai, India. All other chemicals were of reagent grade samples used without further purification. Double-distilled water was used throughout the study. Methods. Preparation of the Preyssler Type Heteropolyacid. Orthophosphoric acid (75 cm3, 90%, and 1.2 mol) was added slowly to a solution of 99 g of sodium tungstate (0.3 mol) in 50 cm3 water at 45 °C. The solution was refluxed for 5 h, cooled to ambient temperature (30 °C) and diluted with 15 cm3 of water. To this, potassium chloride (22.5 g, i.e., 0.32 mol) was added and stirred vigorously for 35 min at ambient temperature. The pale green solid precipitate formed was filtered and washed with 0.1 M potassium acetate. The white needle-like crystals of potassium salt of Preyssler anion obtained was recrystallized from hot water. The free acid was prepared by adding a solution of potassium salt of Preyssler anion in 20 cm3 water through a column (50 1 cm) of Dowex-50WX in the form of H+. Evaporation of elute to dryness under vacuum afforded the desired heteropolyacid,21 H14[NaP5W30O110] with a yield of 19%. Particle Size Measurement. The zeta average diameter of H14P5 particles dispersed in paraffin was measured using Zetasizer laser light scattering equipment (model 3000HS, Malvern, Buntsford, U.K.), which showed a 58 nm size (see Figure 1). A JEOL JEM-2100 STEM with a side mounted Gatan digital camera was used for the imaging nanoparticles. The 15 μL of water dispersed solution was placed on a Formvar-carbon coated nickel/copper grid and allowed to air-dry. Images were captured at an accelerating voltage of 200 kV and collected using Gatan software. The sizes of H14P5 particles were found to be 50 nm from the obtained transmission electron microscopy (TEM) image (see Figure 2). Membrane Preparation. Chitosan (3 g) was dissolved in 100 mL distilled water containing 2% acetic acid by stirring for 24 h at ambient temperature. The solution was filtered using a fritted glass disk filter to remove residual particles and kept overnight to release effervescence. The resulting homogeneous solution was spread onto a dust-free perfectly aligned glass plate using a doctor's knife at ambient temperature. After drying
Figure 2. TEM image of H14P5 nanoparticles.
for 48 h, the membrane formed was peeled off. The nascent (unfilled) chitosan membrane was designated as CS. For preparing NCMs, 3 g of CS was dissolved in 100 mL distilled water containing 2% acetic acid by stirring for 24 h at ambient temperature. The solution was filtered using a fritted glass disk filter to remove residual particles and kept overnight to release effervescence; a known amount of H14P5 in finely powdered form was directly added into chitosan solution by keeping the amount of CS the same for each membrane preparation. The mixture was stirred for 24 h and further sonicated in an ultrasonic bath for 30 min to obtain a homogeneous dispersion of H14P5 particles in CS matrix. The mixture was kept overnight to release effervescence, poured onto a clean glass plate, and dried for 48 h at ambient temperature to obtain NCMs of chitosan loaded with H14P5 nanoparticles in 5, 10, and 15 wt % with respect to weight of CS. The NCMs are, respectively designated as: NCM-5, NCM-10, and NCM-15. Membrane thickness was measured by a micrometer screw gauge, and the average thickness of all the membranes was around 40 ( 3 μm. Membrane Characterization. Bruker's D-8 advanced wideangle X-ray diffractometer was used to study solid state morphology of the membranes. X-rays of 1.5406 Å wavelengths were obtained by Cu KR radiation source. Surface morphology of the membranes was assessed using JSM-840A scanning electron microscope (JEOL, Tokyo, Japan) at 10 kV. Since these films were nonconductive, a gold coating of 10 nm thickness was added on all the samples before scanning. Surface imaging of the samples was carried out using a noncontact mode atomic force microscopy (AFM), Ambios XP-1, 14732
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Ambios Technology, Santacruz, CA, USA. Polymer films mounted on mica slides were analyzed at the resonance frequency of 332 kHz. The maximum scan ranges for AFM in x, y, and z axes were 40, 40, and 4 μm, respectively with a resolution of 10 readings was taken to ascertain the accuracy of angle measurement within (3°. Differential scanning calorimetry (DSC) thermograms were obtained using SDT 2960 (TA Instruments, USA). Measurements were performed at the heating rate of 10 °C/min. The sample pan was conditioned in the instrument before performing the actual experiment. Dynamic mechanical testing analysis (DMTA) was done (courtesy of Dr. K.VSN Raju, Indian Institute of Chemical Technology, Hyderabad) using a Rheometric Scientific, USA, DMTA IV instrument operated in a tensile mode at a frequency of 1 Hz and at the heating rate of 6 °C/min. Fractional Free Volume. Positron annihilation lifetime spectroscopy (PALS) was used to estimate the fractional free volume in CS and the NCMs (courtesy of Prof. C Ranganathiah, Department of Physics, Mysore University). Details of the PALS measurements are given elsewhere.22,23 Porosity. Membranes were cut into the desired size, soaked in water for 30 min, and weighed immediately after when the surface-adhered water droplets were carefully blotted with Kim wipe papers. The wet membranes were dried for 2 h at 60 °C and weighed. The volume occupied by water and volume of membrane in the wet state were determined to calculate membrane porosity24 porosity ¼
ðWw Wd Þ FwVT
ð1Þ
where Fw is the density of pure water at 25 °C (g/cm3) and VT is the apparent volume of membrane in the wet state (cm3); Ww and Wd refer to weights of membranes in wet and dry states, respectively. The VT was calculated using ðWw Wd Þ Wd VT ¼ + Fw Fm
ð2Þ
where Fm is the density (g 3 cm3) of the membrane in its dry state. Membrane Swelling and Sorption. Swelling experiments were performed gravimetrically at 30 °C in 10, 20, 30, and 40 wt % water containing ethanol mixtures. Initial masses of the circularly cut (diameter = 3 cm) 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 30 cm3 of the test solvent. Dry membranes were equilibrated by soaking in different compositions of feed mixtures in a sealed vessel at ambient temperature for about 48 h. Swollen membranes were weighed immediately after carefully blotting them with the Kim wipe papers. Sorbed liquids were recovered in a liquid nitrogen trap by desorbing the equilibrated sample in the purge and trap apparatus, then analyzed by gas chromatography (model: Ultima-2100, Netel India Pvt Ltd., Mumbai, India). The % degree of swelling, DS, was calculated as: [(Ws Wd)/Wd] 100, where
Ws and Wd are the weights of swollen and dry membranes, respectively. Pervaporation Experiments. PV experiments were performed in an indigenously built apparatus described as before.25 The effective area of the membrane in the PV cell was 26.43 cm2 with a liquid volume capacity of 200 cm3. The apparatus consists of a stainless steel cell in which the feed mixture was maintained at a constant desired temperature controlled thermostatically by a circulating water jacket. A PV cell consists of an efficient threeblade stirrer powered by a DC motor in the feed compartment. The feed mixture was agitated at 200 rpm speed by maintaining a downstream pressure of 5 mbar using a vacuum pump (model ED-21, Hindhivac, Bangalore, India). In all of the PV experiments, even after continuously using the membranes up to 8 h, their integrity was retained, and hence, the same membranes were used effectively in repeated PV cycles. Three independent experiments were conducted, but the average data ( NCM-5 > NCM-10 > NCM-15; yet the performance of NCM-5 is regarded as the best. The diffusion coefficient is important in understanding the transport across the barrier membrane during pervaporation; diffusion coefficients were estimated from Fick's theory using eq 7. The diffusion data plotted in Figure 14 as a function of 14740
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Figure 12. Separation factor, flux, selectivity, and permeance of water as a function of feedwater composition at 30 °C.
Figure 13. Swelling vs feedwater composition at 30 °C.
feedwater composition show decreasing trends for water in case of CS, NCM-5, and NCM-10 up to 30 wt % of water, beyond which these slightly increase with increasing feedwater composition. On the other hand, for ethanol, diffusion coefficients increase continuously with increasing feedwater composition. The diffusion of water is much higher than ethanol by 2 orders of magnitude throughout the entire range of feed composition but increases with increasing feedwater content, thus contributing to increased water flux and permeance. For an ideal permeation where the penetrant molecules do not plasticize the membrane, but independently permeate through it, then the permeation of each liquid component would be independent of feedwater composition, thus leading to a constant flux over the entire feedwater composition. On the other hand, increasing the water sorption capacity of NCMs over that of nascent CS is the
result of increased diffusion path lengths for penetrant molecules as a result of increased free volume or porosity as well amorphous regions of the matrix, as discussed before. Influence of Temperature on Pervaporation Performance and Diffusion. The temperature has a significant effect on membrane performance. The influence of temperature (30 60 °C) on separation factor, selectivity, flux, and permeance is shown in Figure 15 for the 10 wt % feedwater mixture. The increase of temperature increases the chain segmental mobility, thereby flux and permeance will increase. On the other hand, the separation factor and selectivity curves show a decline with an increase of temperature as shown before.16,37 At higher temperatures, the diffusion of water is high, restricting the membrane separation ability. In the case of NCM-5, a drastic reduction in the separation factor of 1087 is observed at 60 °C compared to the highest value of 33 651 at 30 °C. Similar observations are seen with NCM-10 and NCM-15 membranes. Here, the higher molecular mobility and vapor pressure of ethanol at a temperature above its boiling point is to be considered. Further, the change in free volume impacts ethanol diffusivity more markedly than water diffusivity, resulting in lower selectivity at a higher temperature. As per free volume theory, an increase in temperature will increase the thermal mobility of polymer chains generating extra void space with increased sorption and diffusion. The driving force for permeation is the concentration gradient, which results from a difference in partial vapor pressure of permeant molecules between feed and permeate mixtures. As the feed temperature increases, the vapor pressure in the feed compartment also increases, but vapor pressure on the permeate side will not be affected, resulting in an increase of driving force, since the latter is closely related to phase transition during the PV dehydration process but is dependent on temperature. These phenomena are caused by the fact that the flux and separation factor depend on 14741
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Figure 14. Effect of feedwater composition on diffusion coefficients.
Figure 15. Separation factor, flux, selectivity, and permeance of water as a function of temperature.
both intrinsic properties of the membrane as well as the experimental conditions, while permeance and selectivity exclude these effects.38,39 In the present work, one can observe a similar changing trend for both permeances and fluxes with increasing temperature. It is reasonable because temperature affects the intrinsic properties of the membranes. As per the sorptiondiffusion model,19 permeance and selectivity reflect the true membrane performance, while selectivity is the selectivity of the membrane and separation factor is the separation factor of PV process.38 Thus, using permeance and selectivity instead of flux and separation factor can significantly decouple the effect of operating conditions on the performance evaluation. As a result, normalizing the flux with respect to driving force would clarify and quantify the contribution by the nature of the membrane to its separation performance. Figure 16 displays increasing diffusion coefficients of water as well as ethanol with increasing temperature from 30 to 60 °C.
Notice smaller values of diffusion coefficients for ethanol than water. The Arrhenius equation was used to fit diffusivity and permeation flux data to calculate the activation energies. From the estimated results presented in Table 3, we find positive values for activation energy, which suggests the increase of permeation flux and diffusion coefficient with increasing temperature. Activation energies of permeation and diffusion for water (Epw) are much lower than those of ethanol (Epe), suggesting an easy permeation of water molecules through the membranes as well as the water selective nature of NCMs compared to CS. In the case of NCMs, the activation energy values are higher than the unfilled CS, but for NCM-5, activation energies of flux and diffusion are lower than those of NCM-10 and NCM-15, suggesting an efficient PV separation by NCM-5 membrane. Activation energies increase with increasing wt % loading of the nanofiller, which reveals that the rate at which water molecules transport through the NCMs increases much faster than ethanol and that water is separated selectively. 14742
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Figure 16. Diffusion coefficient vs temperature.
Table 3. Arrhenius Activation Parameters activation energies in kJ/mol
CS
NCM-5
NCM-10
NCM-15
Ep
21.01
40.24
46.09
47.69
Epw Epe
19.41 38.19
40.08 133.6
45.82 111.6
47.49 109.8
Ed
19.72
40.11
45.87
47.53
’ CONCLUSIONS NCMs of chitosan loaded with heteropolyacid exhibit improved performance compared to unfilled chitosan membrane. The results are supported by increased surface free energy, free volume spaces, and membrane hydrophilicity as well as favorable interactions between nanoparticles and chitosan. Results of sorption and diffusion selectivity as well as diffusion coefficients of water and ethanol are affected by feedwater composition and temperature. At higher feedwater compositions and temperatures, the membrane performance declined. The sorption selectivity was dominant over that of diffusion, suggesting the waterselective nature of the membranes; these results are in accordance with the sorptiondiffusion model. The FloryHuggins theory enabled an accurate assessment of the binary interaction parameters used in understanding the PV process. Feedwater composition plays a significant role in permeation flux, but with little effect on ethanol. The NCMs exhibit significantly lower Arrhenius activation energies for water than ethanol, suggesting their higher separation abilities and water-selective nature. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: 91-231-2609167. Fax: 91-231-2692333 (G.S.G). Email:
[email protected] (T.M.A).
’ ACKNOWLEDGMENT Authors (G.S.G. and V.T.M.) thank the Department of Science and Technology, New Delhi, India (No. SR/S1/PC31/2006) for financial support of this study. ’ REFERENCES (1) Shao, P.; Huang, R. Y. M. J. Membr. Sci. 2007, 287 (2), 162–179. (2) Sairam, M.; Patil, M. B.; Veerapur, R. S.; Patil, S. A.; Aminabhavi, T. M. J. Membr. Sci. 2006, 281 (1), 95–102.
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