APTEOS hybrid membranes

Journal of Membrane Science 287 (2007) 237–245

Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes Qiu Gen Zhang a , Qing Lin Liu a,∗ , Zhong Ying Jiang b , Yu Chen a a

Department of Chemical and Biochemical Engineering, Collage of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China b Department of Physics, Nanjing University, Nanjing 210093, China Received 13 May 2006; received in revised form 18 October 2006; accepted 23 October 2006 Available online 28 October 2006

Abstract Novel organic–inorganic hybrid membranes were prepared through sol–gel reaction of poly(vinyl alcohol) (PVA) with ␥-aminopropyltriethoxysilane (APTEOS) for pervaporation (PV) separation of ethanol/water mixtures. The membranes were characterized by FTIR, EDX, WXRD and PALS. The amorphous region of the hybrid membranes increased with increasing APTEOS content, and both the free volume and the hydrophilicity of the hybrid membranes increased when APTEOS content was less than 5 wt%. The swelling degree of the hybrid membranes has been restrained in an aqueous solution owing to the formation of hydrogen and covalent bonds in the membrane matrix. Permeation flux increased remarkably with APTEOS content increasing, and water permselectivity increased at the same time, the trade-off between the permeation flux and water permselectivity of the hybrid membranes was broken. The sorption selectivity increased with increasing temperature, and decreased with increasing water content. In addition, the diffusion selectivity and diffusion coefficient of the permeants through the hybrid membranes were investigated. The hybrid membrane containing 5 wt% APTEOS has highest separation factor of 536.7 at 50 ◦ C and permeation flux of 0.0355 kg m−2 h−1 in PV separation of 5 wt% water in the feed. © 2006 Elsevier B.V. All rights reserved. Keywords: Poly(vinyl alcohol); Hybrid membranes; Pervaporation; ␥-Aminopropyl-triethoxysilane; Composite membranes

1. Introduction Membrane technology is a novel and promising separation method that has been widely used in industrial processes due to its easy operation, effective and high energy savings. PV being one of the membrane processes, which has been most actively researched, has application in separation and purification of liquid mixtures, in particularly for dehydration of organic compounds, separation of azeotropic or close-boiling point mixtures. PVA is one of the most important materials for the dehydration of organic mixtures owing to its good chemical stability, film-forming ability and high hydrophilicity, especially high water permselectivity in PV separation of aqueous ethanol solutions [1–3]. However, the swelling of the PVA membrane in an aqueous solution results in increase in both the solubility and



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diffusivity of the organic component, consequently lowers the water permselectivity, and simultaneously reduces the strength of the PVA membranes [4]. To improve the membrane stability and permeation properties, firstly, the PVA membranes were crosslinked with an organic chemical, for example, aldehyde and organic acid, glutaraldehyde, polyacrylic acid, maleic acid, formaldehyde, fumaric acid [4–13]; or modified by a metal salt agent, such as sulfated zirconia [14] and lithium chloride [15]. Secondly, adding filler such as zeolite to the polymer membranes was studied [16]. Thirdly, blending with other polymers [4,17], and others, such as grafting [2], preparing semi- and interpenetrating polymer were also reported [18,19]. In recent years, some researchers have prepared PVA/tetraethoxysilane (TEOS) organic–inorganic hybrid membranes through sol–gel reaction for PV separation [20–22]. These hybridizations effectively controlled the swelling of PVA-based membranes in aqueous solutions. The hybrid materials have film-forming properties, chemical and physical stabilities, and high permselectivity. But permeation flux and hydrophilicity are usually excessively decreased. However, through chemical crosslinking

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to control the swelling degree, the permselectivity of the membrane increased, but the permeation flux depressed remarkably [23]. Thus, how to control the swelling degree of PVA-based membranes, and to increase or retain the permeation flux is our focus on the modification to the PVA membranes. In this study, novel PVA/APTEOS organic–inorganic hybrid membranes were prepared through sol–gel reaction. APTEOS, a widely used inorganic crosslinking agent, crosslinking with PVA chains can effectively reduce the swelling degree of PVA and increase water permselectivity. On the other hand, amido in APTEOS will decrease the hydrophilicity of the hybrid membranes as less as possible. The larger aminopropyl groups in APTEOS that dispersed in the membrane will prevent crystalline region from forming in it and increase the permeation flux. The chemical and physical structures were studied by FTIR and WAXD. PV characteristics of separation for ethanol/water mixture were studied. The effects of APTEOS content, the feed concentration and temperature on the sorption and diffusion were investigated. 2. Experimental 2.1. Materials PVA, polymerization degree of 1750 ± 50 and degree of hydrolysis of 98%, was supplied by Sinophatm Chemical Reagent Co. Ltd. (China). APTEOS, was purchased from Shanghai Yaohua Chemical Plant (China). All other solvents and reagents, of analytical grade, were purchased from Sinophatm Chemical Reagent Co. Ltd. and were used without further purification. 2.2. Membrane preparation PVA of 4 g was dissolved in 100 ml of double-distilled water at 90 ◦ C for 3 h. After the hot solution was filtered, a measured amount of APTEOS was then added at 30 ◦ C, and 1 ml HCl solution (2.0 mol/l) was added 10 min later to the PVA/APTEOS mixture with stirring for 10 h. The resulted homogeneous solution was cast onto a clean glass plate with the aid of a casting knife. The membranes were allowed to dry at room temperature for 36–48 h and were subsequently peeled off, and then allowed the solvent to evaporate completely at 80 ◦ C for another 24 h. The obtained membranes were transparent, thickness of 18 ␮m. The mass ratio of APTEOS to PVA was varied at 0.0, 2.5, 5.0, 7.5 and 10.0, and the resulted homogenous hybrid membranes were designated M-1, M-2, M-3, M-4, and M-5, respectively. 2.3. Characterization of the hybrid membranes The FTIR spectra of the hybrid membranes were measured using a Nicolet spectrometer (FT-IR 740SX, USA) equipped with total reflectance (ATR) accessories. The samples had dried in vacuum at 80 ◦ C for 8 h before experiment; the spectra were recorded in the range of 400–4000 cm−1 . The morphology of the PVA and the hybrid membranes was studied at room temperature using CAD4 - PDP11/44 X-ray diffractometer (Enraf

- Nonious Co. Holland). And the membrane samples were scanned in the reflection mode at an angle 2θ in a range from 5◦ to 35◦ at a speed of 8◦ /min. Element mapping was conducted with a Philips LEO1530 scanning electronic microscope equipped with energy-dispersive X-ray spectroscopy (EDX) of ISIS300 (Oxford). PALS experiments were performed by using an EG&G ORTEC fast-fast coincident positron lifetime spectrometer with a time resolution of about 270 ps at 18 ◦ C in air. The source 22Na with an activity of approximate 5.0 × 105 Bq was deposited between two 6 ␮m thick Ti foils. All spectra were collected with total counts of about 2.5 × 106 . The well-known program POSITRONFIT was used to analyze the spectra into three mean lifetime components, in which no constraint or no source correlation were applied. The variations of the fit χ2 were some what smaller than 1.2, and the shifts of time-zero channel were less than 8 ps for all PALS measurements. 2.4. Sorption measurements The PVA and the hybrid membranes were dried completely at 80 ◦ C for 8 h and weighed. Then these membranes were immersed into an aqueous ethanol solution with various water concentrations in a sealed vessel at a desired temperature for 24–48 h to allow reaching an equilibrium swelling. The swollen membranes were weighed using a digital microbalance as quickly as possible after wiped with filter paper. Each run was performed at least three times, until the weight of the membranes kept constant, and the results were averaged. The degree of swelling (DS) of the membrane was calculated by   Ws − Wd × 100 (1) DS (%) = Wd where Ws and Wd are the mass of the swollen and the dry membranes, respectively. After weighing, the swollen membrane was placed into a dry flask, which was connected to cold traps and a vacuum pump. The sorbed solvents in the swollen membrane were desorbed under reduced pressure and were collected. The concentration of the collected solution was measured by an Abbe’s refractometer (accuracy is ±0.0001 units, WAY-2S, China). And the sorption selectivity in an aqueous ethanol solution αsorp , is expressed by αsor =

SH2 O /SETOH FH2 O /FETOH

(2)

where SH2 O and SETOH are the mass fractions of water and ethanol in the collected solution, respectively; FH2 O and FETOH are the mass fractions of water and ethanol in the feed, respectively. 2.5. Pervaporation separation of ethanol/water mixtures PV experiments were carried out on the PERVAP 2201 (SULZER CHEMTECH, Germany) at 50, 60 and 70 ◦ C, respectively. The effective surface area of the membrane in contact with the feed is 70.88 cm2 . The pressure in the permeate side was maintained 20 mbar using a vacuum pump. The membrane

Q.G. Zhang et al. / Journal of Membrane Science 287 (2007) 237–245

was swelled in the feed for 2 h before PV test, and the permeate was collected in a liquid nitrogen cold trap. The concentration of the feed and the permeate was measured by the Abbe’s refractometer. The permeation properties of the hybrid membranes characterized by total permeation flux (Jp ), separation factor (αsep ) and PV separation index (PSI), can be calculated by the following equations respectively: Wp At PH2 O /PETOH = FH2 O /FETOH

Jp =

(3)

αsep

(4)

PSI = Jp (αsep − 1)

(5)

where Wp is the mass of the permeate (kg); A the area of the membrane in contact with the feed mixture (m2 ); t the permeation time (h); PH2 O and PETOH are the mass fractions of water and ethanol in the permeate, respectively. 3. Results and discussion 3.1. Formation of the hybrid membranes The chemical structure and reaction routes for the hybrid membranes are displayed in Fig. 1. In preparing the hybrid membranes, APTEOS was hydrolyzed in the presence of an acid catalyst (HCl) leading to the formation of silanol groups. The resulting silanol dispersed in the PVA solution, the hydroxyl groups in one silanol formed siloxane bonds with these in another silanol or with the hydroxyl groups in PVA through a dehydration or a dealcoholysis reaction during the membrane drying. In addition, the hydroxyl groups and amine groups in the silanol can form hydrogen bonds with the dissociative hydroxyl groups in PVA amorphous region, and these hydrogen and silox-

Fig. 1. The chemical structure and the reaction scheme for the hybrid membranes.

239

ane bonds are the crosslink spots in the hybrid membranes. These spots cause the mobility of PVA chain to decrease and the stability of the membrane’s structure and the density of the membrane to increase. Moreover, the amino group of APTEOS in the reaction process will reduce the self-condensation of APTEOS since amino groups are larger groups that have size exclusion effect. 3.2. Structure characteristic of the hybrid membranes The FTIR spectra of the PVA and the hybrid membranes are shown in Fig. 2. A characteristic strong and broad band appeared around 3300 cm−1 in PVA spectra (M-1) corresponding to O–H stretching vibrations of the hydroxyl groups. This broad band gradually shifted to the left, and its intensity increased from M-1 to M-3. This is because the stretching of N–H bonds at 3500–3300 cm−1 occurred with the incorporation of APTEOS. And the dissociative hydroxyl groups (3650–3580 cm−1 ) increase since the amorphous region of PVA increased with increasing APTEOS content. It indicates that the hydrophilicity of M-2 and M-3 increases. However, the change from M-3 to M-5 is contrary, because some of the –OH groups in PVA involved in the condensation reaction with the silanol groups in APTEOS with its further addition, forming covalent crosslinks between polymer segments. Furthermore, the absorption band at 1650 cm−1 increased owing to the distorting vibrations of N–H (1650–1560 cm−1 ) with increasing APTEOS content. And multiple bands appeared in the spectra around 1000–1100 cm−1 . Increase in the intensity of these bands from M-1 to M-5 with increasing APTEOS content, suggests the formation of Si–O–C bonds, since Si–O stretching also appears at the same wavelength of C–O stretching (1020–1095 cm−1 ) [21,24]. And the stretching of C–N at 1250–1020 cm−1 in the amine groups also increases the intensity of these bands. The WAXD measurements were mainly performed for the crystalline diffraction in the hybrid membranes. The WAXD patterns of the PVA and the hybrid membranes are shown in Fig. 3. The typical peak of the PVA and the hybrid membranes appeared at 2θ ≈ 20◦ , and the intensity of the peak decreased continuously form M-1 to M-5. This suggests that the crystalline region in the

Fig. 2. FTIR spectra of the hybrid membranes.

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Q.G. Zhang et al. / Journal of Membrane Science 287 (2007) 237–245 Table 1 The results for PALS free volume parameters of PVA and its hybrid membranes

Fig. 3. WAXD patterns of the hybrid membranes.

hybrid membranes decrease with increasing APTEOS content. Since the Si–O–C bonds formed between the linear polyethylene segments and the larger aminopropyl groups of APTEOS that dispersed in the membranes exhibiting a size-exclusive effect, which both play a role in preventing crystalline region from forming in the PVA hybrid membranes. Therefore, the amorphous region increase, which favors the diffusion of penetrants, results in increase in permeation flux. EDX Si-mapping of the surface of the hybrid membranes containing 5 wt% of APTEOS is shown in Fig. 4. In the EDX photograph, the bright spots corresponding to silicon were found homogeneously distributing in the matrix. It indicates that APTEOS were dispersed uniformly in the hybrid membranes. 3.3. Free volume analysis Based on the so-called free volume model, PALS method assumes that positronium (Ps) localizes in the free volume elements of the polymer structure because of its repulsion (exchange interaction) from the surrounding molecules. It has emerged as a unique and potent probe for detecting the free

Fig. 4. EDX Si-mapping image of the hybrid membrane M-3.

Membrane

τ 3 (ns)

˚ R (A)

I3 (%)

fv (%)

M-1 M-2 M-3 M-4 M-5

1.225 1.2747 1.344 1.28 1.277

1.993 2.059 2.149 2.067 2.063

15.48 15.087 14.026 15.405 15.28

5.133 5.516 5.831 5.699 5.620

volume properties of polymers in recent years. Positronium can be formed in two states: parapositronium (p-Ps) is formed by two particles with opposite spin and ortho-positronium (o-Ps) is formed if the spins of the particles are parallel. The longestlived component (τ 3 ) results from the pick-off annihilation of ortho-positronium (o-Ps) in the free volume sites. The o-Ps lifetime (τ 3 ) is the measure of size of free volume site in polymers, and intensities (I3 ) are indicative of the number concentration of free volume sites in polymers [25]. The results of PALS about free volume parameters are displayed in Table 1. It suggests that the radius of the free volume increase from M-1 to M-3, and decrease from M-3 and M-5. On the other hand, the change of free volume fraction in the membranes consists with the change in the radius of the free volume. The increase of the free volume favors the permeation of permeants through the membranes. 3.4. Sorption studies In PV, the driving force for transport is the concentration drop across the membranes. The transport process consists of three consecutive steps: (I) feed components sorption into the membranes at the feed side, (II) permeant diffusion through the membranes, and (III) permeant desorption into a vapor phase at the permeate side [26]. Therefore, the sorption properties of the membranes in the feed play an important role in PV processes, which affect the membranes permselectivity and permeation flux. The effects of water content in the feed and the feed temperature on the swelling behavior of the hybrid membranes in aqueous ethanol solutions are displayed in Figs. 5 and 6.

Fig. 5. Variation of the swelling degree (%) of the hybrid membranes under different water content in the feed at 50 ◦ C.

Q.G. Zhang et al. / Journal of Membrane Science 287 (2007) 237–245

Fig. 6. Effects of temperature on the swelling degree (%) of the hybrid membranes in 15 wt% water in ethanol.

It indicates that the swelling degree of the hybrid membranes decreased gradually and slightly from M-1 to M-5. This is owing to the formation of hydrogen and siloxane bonds in PVA matrix. APTEOS crosslinking with PVA chains lead to make the amorphous region of PVA more compact, which reduce the mobility of PVA chains and result in decrease of swelling. Therefore, the degree of swelling decreases with increasing APTEOS content. However, the hydrophilicity and the free volume increase from M-1 to M-3, which make the change of swelling small, especially for M-3. On the other hand, the degree of swelling increased almost linearly for all the membranes with increasing water content. This is due to its inherent hydrophilicity. Fig. 6 shows the degree of swelling increased quickly with temperature. This is due to increase of the mobility of PVA chain with increasing temperature, which leads to increase in the free volume and results in increase in the sorption. Sorption selectivity presents an indication of the membrane permselectivity and it describes how selective the membrane is toward a particular component [27]. Fig. 7 shows the sorption selectivity of the hybrid membranes in aqueous ethanol solutions under various water contents at 50 ◦ C. It is found that the

Fig. 7. Effects of water content in the feed on sorption selectivity of the hybrid membranes at 50 ◦ C.

241

Fig. 8. Effects of temperature on sorption selectivity of the hybrid membranes in 15 wt% water in ethanol.

sorption selectivity increased form M-1 to M-3, and decreased with further APTEOS addition to the hybrid membranes. This is because the hydrophilicity of the hybrid membranes increased and the noncrystalline region of the hybrid membranes became denser with increasing APTEOS content leading to increase in preferential sorption for water. However, the hydrophobicity of the hybrid membranes increased gradually when APTEOS content in excess of 5 wt%. This is confirmed by the FTIR spectra in the previous section. On the other hand, the sorption selectivity decreased gradually with increasing water content. This is owing to that the swelling degree increased linearly with increasing water content in the mixture, and the sorption of ethanol in the membranes increased faster than water. The effects of temperature on the sorption selectivity of the hybrid membranes in an aqueous ethanol solution are displayed in Fig. 8. The sorption selectivity for water increased with increasing temperature. This reason may be that increase in the free volume of polymer leads to increase in the sorption for water. The sorption of water increased faster than ethanol because of both strong affinities of water for membrane and smaller water molecule. 3.5. Pervaporation performances Fig. 9 shows the effect of APTEOS content on the permeation flux and separation factor in PV of 85 wt% ethanol solution through the hybrid membranes. The separation factor increased sharply for the hybrid membranes containing up to 5 wt% APTEOS, but decreased at higher APTEOS contents. The permeation flux increased linearly with increasing APTEOS content in the hybrid membranes. Increase in the separation factor may be because the noncrystalline region in the hybrid membranes became denser and the interaction between water molecules and the hybrid membranes increased from M-1 to M-3. Moreover, decrease in the separation factor from M-3 to M-5 may be that the hydrophobicity of the hybrid membranes increased with an excess of APTEOS content. This is proved by the FTIR spectra of the hybrid membranes and their sorption behavior. The rapid increase of the permeation flux is attributed to the fact that the free volume and the noncrystalline region

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Fig. 9. Effects of APTEOS content in the hybrid membranes on permeation properties in pervaporation of 15 wt% water in ethanol at 50 ◦ C.

of the hybrid membranes increase and the density of the hybrid membranes decrease; this consists with the PALS and WAXD measurements. Membrane performance in PV separation is influenced not only by the membrane properties and characteristic of permeant, but also by the process operating parameters such as feed concentration and temperature. The effect of the feed concentration on the permeation flux and the separation factor is shown in Figs. 10 and 11, respectively. It is found that the permeation flux increased and the separation factor decreased with increasing water content in the feed. The reason is that the swelling degree, the mobility of PVA chains and the free volume of the membranes increased with increasing water content leading the diffusion of both water and ethanol to increase. On the other hand, with increasing water content the sorption selectivity decreased and the diffusion of ethanol increased resulting in decrease in water permselectivity. The effects of the feed temperature on PV performance were investigated by separation of 85 wt% ethanol solution at 50, 60 and 70 ◦ C, as shown in Figs. 12 and 13. The permeation flux increased with increasing temperature due to the free volume increase in the membrane matrix. But it increased remarkably

Fig. 10. Effects of water content in the feed on permeation flux through the hybrid membranes at 50 ◦ C.

Fig. 11. Effects of water content in the feed on separation factor through the hybrid membranes at 50 ◦ C.

Fig. 12. Effects of temperature on permeation flux through the hybrid membranes in pervaporation of 15 wt% water in ethanol.

form 60 to 70 ◦ C because of the fast increase in the swelling degree from 60 to 70 ◦ C resulting in the sharp increase in the diffusion of water and ethanol. On the contrary, the change of separation factor is complicated with the feed temperature.

Fig. 13. Effects of temperature on water permselectivity through the hybrid membranes in pervaporation of 15 wt% water in ethanol.

Q.G. Zhang et al. / Journal of Membrane Science 287 (2007) 237–245

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the permeation process can be separated into three consecutive steps: sorption, diffusion and desorption [28]. The third step has a slight effect on mass transfer owing to a very low concentration of the penetrants in the permeate. Thus, the diffusion of the penetrants through the membranes plays an important role in the permselectivity and the permeation flux. The separation factor can be divided into the sorption selectivity and the diffusion selectivity αdif , and it can be calculated by the following equation: αdif =

Fig. 14. Effects of temperature on pervaporation separation index of the hybrid membrane in pervaporation of the 15 wt% water in ethanol.

The separation factor at 60 ◦ C was higher than others when APTEOS content in the hybrid membrane was less than 4.5 wt%; but it decreased with increasing temperature when APTEOS content was more than 4.5 wt%. This reason may be that the mobility of PVA chains increase with increasing the feed temperature, which favors ethanol molecule diffusion and consequently water permselectivity decrease. However, the sorption selectivity increased sharply for M-1and M-2 with increasing the feed temperature leading the separation factor to increase. On the other hand, with increasing the feed temperature PV separation index increased and it increased with APTEOS content up to 5 wt%, as indicated in Fig. 14. 3.6. Diffusion selectivity and coefficients Mass transfer in PV through a non-porous polymer membrane is generally described by the solution–diffusion model,

αsep αsor

(6)

The effects of the feed concentration and temperature on the diffusion selectivity for water through the hybrid membranes were calculated, as listed in Tables 2 and 3. It is shown that the variation in the diffusion selectivity is contrary to that of the radius of the free volume with increasing APTEOS content. That is, decrease in the diffusion selectivity corresponding to increase in the radius of the free volume from M-1 to M-3; and increase in the former corresponding to decrease in the latter from M-3 to M-5. This is owing to that increasing the radius of the free volume favors the diffusion of ethanol molecules resulting in decrease in the diffusion selectivity for water. On the other hand, the diffusion selectivity of the hybrid membranes decreased gradually with increasing water content and the temperature. This is because the free volume increases with increasing the swelling degree of the hybrid membranes, and favors the ethanol molecules to diffuse through them. Diffusion coefficient is an important factor to estimate the diffusion of penetrants through the membranes and permeation flux. Based on the Fick’s law, the permeation flux of component i can be expressed as Ji = Di

dCi dl

(7)

Table 2 Effects of water content on separation factor, sorption selectivity and diffusion selectivity of the hybrid membranes at 50 ◦ C Membrane

M-1 M-2 M-3 M-4 M-5

αsep

αsor

αdif

15 wt%

30 wt%

45 wt%

15 wt%

30 wt%

45 wt%

15 wt%

30 wt%

40 wt%

50.84 58.03 88.14 68.07 67.41

25.90 29.94 38.31 27.76 21.97

11.18 11.19 11.68 11.46 11.21

8.39 11.07 25.16 11.55 9.79

7.745 9.794 23.29 10.58 8.270

5.31 7.00 18.26 9.80 6.41

6.056 5.242 3.503 5.893 6.885

3.343 3.056 1.645 2.624 2.659

2.104 1.597 0.640 1.170 1.750

Table 3 Effects of temperature on separation factor, sorption selectivity, and diffusion selectivity of the hybrid membranes in pervaporation of 15 wt% H2 O in ethanol Membrane

M-1 M-2 M-3 M-4 M-5

αsep

αsor

αdif

50 ◦ C

60 ◦ C

70 ◦ C

50 ◦ C

60 ◦ C

70 ◦ C

50 ◦ C

60 ◦ C

70 ◦ C

50.84 58.03 88.14 68.07 67.41

56.65 73.97 84.73 66.23 58.52

54.32 59.44 70.50 57.46 46.88

8.39 11.07 25.16 11.55 9.79

22.12 29.36 56.62 19.32 13.84

25.95 41.97 62.28 22.40 14.64

6.056 5.242 3.503 5.893 6.885

2.561 2.519 1.496 3.428 4.229

2.093 1.416 1.312 2.565 3.202

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Table 4 Diffusion coefficients of water and ethanol through the hybrid membranes under different water content at 50 ◦ C Water content (wt%) 15 30 45

DH2 O (×108 m2 /s)

DETOH (×108 m2 /s)

M-1

M-2

M-3

M-4

M-5

M-1

M-2

M-3

M-4

M-5

0.04815 0.33678 1.267155

0.05256 0.309285 1.40706

0.054405 0.3465 1.46088

0.090405 0.404955 1.509795

0.09738 0.701055 1.643355

0.007965 0.10062 0.601335

0.01026 0.113085 0.879165

0.01872 0.18783 1.35342

0.01539 0.179145 1.2906

0.01404 0.15426 0.938835

Table 5 Diffusion coefficients of water and ethanol through the hybrid membranes for 15 wt% water in ethanol at different temperature Temperature (◦ C)

50 60 70

DH2 O (×108 m2 /s)

DETOH (×108 m2 /s)

M-1

M-2

M-3

M-4

M-5

M-1

M-2

M-3

M-4

M-5

0.04815 0.06381 0.14337

0.05256 0.06849 0.14508

0.054405 0.09306 0.154305

0.090405 0.1134 0.209925

0.09738 0.14067 0.255915

0.007965 0.024525 0.0684

0.01026 0.02709 0.101385

0.01872 0.062505 0.136035

0.01539 0.03321 0.081945

0.01404 0.033165 0.079875

where Ji , Di and Ci are the permeation flux (kg/m2 s), the diffusion coefficient (m2 /s) and the concentration (kg/m3 ) of component i in the membranes which can be obtained form sorption experiments [29]. l is the diffusion length (m). For simplicity, the diffusion coefficient can be calculated by the equation Di =

Ji δ Ci

(8)

where δ is the membrane thickness. The diffusion coefficients of water and ethanol at various concentrations through the hybrid membranes at different temperature are displayed in Tables 4 and 5. It is noted that the diffusion coefficients of water increased with increase in APTEOS content, and that of ethanol increased from M-1 to M-3 and reduced from M-3 to M-5. In general, the diffusion coefficient consists with the radius of the free volume. The larger the radius of the free volume is, and the larger the diffusion coefficient. This is an explanation for the change of the diffusion coefficient of ethanol, but for water is different. This reason may be that increase in the radius of the free volume favors the diffusion of water, but increase in the hydrophilicity of the membrane leads to increase of sorption of water. These result in diminishment of the channels and consequently result in slight increase in the diffusion coefficient of water from M-1 to M-3. Whereas, increase in the diffusion coefficient of water from M-3 to M-5 is owing to increasing in the hydrophobicity of the hybrid membranes instead of the variation of the radius of the free volume. On the other hand, the change of the diffusion coefficient of water and ethanol is similar to the permeation flux. With increasing the feed temperature and water content, the diffusion coefficients of water and ethanol increased quickly owing to the membrane swelling. 4. Conclusion Novel PVA-APTEOS organic–inorganic hybrid membranes were prepared by sol–gel reaction. The chemical and physical structures of the hybrid membranes were studied. With increas-

ing APTEOS content, the amorphous region of PVA and the free volume of the membrane increased owing to the incorporation of aminopropyl groups in APTEOS. The free volume and the hydrophilicity of the hybrid membranes increased from M-1 to M-3. The effects of the feed concentration and temperature on the sorption and PV characteristics of these hybrid membranes were investigated. The anti-trade-off phenomenon in PV of ethanol/water mixture by the hybrid membranes consists with the PALS measurement. With increasing the feed temperature and water content, the degree of swelling, the permeation flux, the diffusion selectivity and the diffusion coefficients of water and ethanol increased. But the addition of excess of APTEOS will reduce the hydrophilicity of the hybrid membranes, and self-condensation reaction among APTEOS will increase. The PVA/APTEOS organic–inorganic hybrid membranes have promising application in PV of aqueous solution of higher ethanol content. Acknowledgements The support of National Nature Science Foundation of China Grant no. 50573063 and the research fund for the Doctoral Program of Higher Education (no. 2005038401) in preparation of this article is gratefully acknowledged. References [1] B. Will, R.N. Lichtenthaler, Comparison of the separation of mixtures by vapor permeation and pervaporation using PVA composite membranes. Part I. Binary alcohol–water systems, J. Membr. Sci. 68 (1992) 119–125. [2] N. Alghezawi, O. Sanlı, L. Aras, et al., Separation of acetic acid–water mixtures through acrylonitrile grafted poly(vinyl alcohol) membranes by pervaporation, Chem. Eng. Process. 44 (2005) 51–58. [3] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation: an analytical review, Desalination 110 (1997) 251–286. [4] P.S. Rao, B. Smitha, S. Sridhar, et al., Preparation and performance of poly(vinyl alcohol)/polyethyleneimine blend membranes for the dehydration of 1,4-dioxane by pervaporation: comparison with glutaraldehyde cross-linked membranes, Sep. Purif. Technol. 44 (2005) 244–254.

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