A novel hydrophilic polymer-ceramic composite membrane 1

Available online at www.sciencedirect.com

Journal of Membrane Science 312 (2008) 15–22

A novel hydrophilic polymer-ceramic composite membrane 1 Acrylic acid grafting membrane Xuzhi Cao a , Taozhou Zhang a , Quang Trong Nguyen b , Yuanyuan Zhang a , Zhenghua Ping a,∗ a

Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, 200433 Shanghai, PR China b Laboratoire “Polym` eres, Biopolym`eres, Membranes”, UMR 6522, CNRS, Universit´e de Rouen, 76821 Mont-Saint-Aignan Cedex, France Received 18 July 2007; received in revised form 19 November 2007; accepted 6 December 2007 Available online 15 December 2007

Abstract A novel hydrophilic ceramic/polymeric pervaporation membrane was prepared by free-radical graft polymerization of acrylic acid (AA) onto the ceramic membrane which was pre-coated with a SiO2 active layer by in situ hydrolysis-deposit method. The influence of reaction conditions on the SiO2 layer was studied. The surface property and the morphology of the membrane were analyzed by means of SEM, FT-IR and TGA. Only the AA-chain grafting on the support coated with an active silica layer can lead to high performance membranes for solvent dehydration. The grafted chains imparted a high selectivity to the membrane, but their confinement in rigid ceramic pores led to a singular decrease in the permeation flux when the temperature increases. The membrane performances for the separation of ethanol/water mixtures by pervaporation for different monomer concentrations in the grafting solution were measured. The membrane grafted in 6 wt.% of AA had good separation performances: for the dehydration of 95 wt.% ethanol at 30 ◦ C, the permeation flux was 540 g m−2 h−1 and the water content in permeate was higher than 98.7 wt.%. © 2007 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Composite membranes; In situ hydrolysis-deposit; Poly(acrylic acid); TEOS

1. Introduction Pervaporation has gained increasing attention in many chemical processes as an effective and energy-saving membrane technique for separating azeotropes, close-boiling mixtures, isomers and thermally sensitive compounds [1–3]. Currently, most researches on pervaporation involve the dehydration of alcohols, particularly ethanol [4–8]. For the dehydration purpose, hydrophilic membranes are used. Polymers are the most commonly used membrane materials since they are inexpensive, and can be economically processed into membranes and modules. However, hydrophilic polymeric membranes are highly swollen in aqueous solutions and loose its permselectivity and mechanical stability [9,10]. Thus the zeolite membranes have recently been developed for pervaporation applications due to

∗

Corresponding author. Tel.: +86 21 65642035; fax: +86 21 65640293. E-mail address: [email protected] (Z. Ping).

0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.12.006

its good chemical and thermal stability, and high separation performances [11–14]. Unfortunately; the difficulty in membrane and module preparation, as well as the high fabrication cost limit its industrial applications. A promising approach is to form a covalently bonded polymer layer on a porous ceramic substrate surface. The emergence of polymer/ceramic composite membrane has attracted many attentions for its significant performances in liquid mixture separation by pervaporation. Cohen and co-workers [15,16] grafted poly(vinyl pyrrolidone) and poly(vinyl acetate) onto the microporous alumina or silica supports to obtain membranes with higher performances for the separation of organic mixtures. Zhou et al. [17] prepared a hydrophilic organic–inorganic composite membrane by grafting PAM onto the cordierite membrane, and used it for the dehydration of alcohol and acrylic acid with high selectivity and permeability. In the present work, we developed an in situ hydrolysisdeposit method to prepare an active SiO2 layer on a porous ceramic membrane, then grafted PAA onto the membrane

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surface to prepare a hydrophilic composite membrane. The membrane performances were studied in the dehydration of ethanol by pervaporation. 2. Experimental 2.1. Materials Acetic acid and potassium persulfate (KPS) were purchased from Shanghai Chemical Reagent Corp. Tetraethyl orthosilicate (TEOS) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. ␥-Methacryloxy propyltrimethoxy silane (KH-570) was purchased from Nanjing-Crompton Shuguang Organosilicon Specialties Co., Ltd. Silica particles, with diameter range from 200 to 300 mesh, was supplied by Qingdao Ocean Chemical plant. Porous ceramic supports with an average pore size 2 ␮m were obtained from Jiangsu Shenglaite ceramic Co., Ltd. 2.2. SiO2 active layer preparation The SiO2 active layer was prepared by the in situ hydrolysisdeposit method. First, a ceramic disc (30*3) was dipped into a dilute solution of HCl (5 wt.%) for a night. The disc was then immersed into 6.3 wt.% TEOS in ethanol and heated for a fixed time (ca. 30 h) at 70 ◦ C. After the reaction, the disc was washed in an ultrasonic bath for 10 min, and then calcined for 3 h at 550 ◦ C to form a SiO2 active deposit on the ceramic membrane. When cooled to room temperature, the disk was ultrasonically washed with distilled water to remove the unbound species. The treated disk was immersed in 1 wt.% solution of HCl for 24 h in order to activate the silanol groups, then rinsed with distilled water till neutral pH and dried in vacuum oven at 80 ◦ C. 2.3. Silylation and graft polymerization The silylation of the silica layer on the porous ceramic support with a silane-borne methacrylate monomer was performed at 110 ◦ C for 5 h in a flask equipped with a reflux condenser and containing 10 wt.% KH-570 in toluene. Free-radical graft polymerization of acrylic acid onto silylated ceramic support was initiated with potassium persulfate dissolved in the aqueous solution of monomer under a nitrogen atmosphere. 2.4. Analysis Because the amount of grafted polymer on the porous ceramic support was very low, it was not possible for us to detect the chemical change on the membrane surface by surface analysis techniques. We had to study the silica particles treated under the same silanization and grafting conditions. We assumed that the reaction extent of the material grafted onto the silica particles is the same as that obtained with the porous ceramic substrate. This assumption may be wrong from the quantitative viewpoint due to the difference in specific surface areas of the two substrates, but should be qualitatively valid due to the similar chemical nature of the substrate surfaces to be grafted.

The chemical composition and the grafting yield of the silica particles after each reaction were studied with an infrared spectrometer (Nicolet Magna 550) and a thermogravimeter (TGA, Pyris 1, PerkinElmer), respectively. The elemental analysis was performed with a VARIO EL III element analyzer (Elementar Co.). Scanning electron micrographs (SEM) of the membranes were taken with a JEOL JSM 25800. TGA analysis was performed from 100 to 700 ◦ C at a heating rate of 25 ◦ C/min in a nitrogen atmosphere, i.e. conditions that ensure the total decomposition of the organic segment grafted on the silica surface. 2.5. Pervaporation The pervaporation experiments were carried out in an equipment described in Ref. [17]. The membrane area was about 1.23 cm2 . The downstream pressure was below 150 Pa, and the fixed operation temperatures were in the range 30–70 ◦ C. The composition analysis was carried out on a FDD-1A gas chromatograph equipped with a column packed with Porapak Q and a thermal conductivity detector. The selectivity is defined as the mass fraction of water in the permeate (c w ) for each mass fraction of water in the feed. The permeation flux (J) is defined as J = W/At, where W, A, and t represent the weight of permeant (g), effective membrane area (m2 ), and operating time (h), respectively. 3. Results and discussion 3.1. In situ hydrolysis-deposition 3.1.1. Principle of in situ hydrolysis-deposition Since the graft polymerization cannot be directly carried out on a raw ceramic porous support, it is necessary to prepare an active intermediate interface which contains free acrylate groups. The interface preparation consisted of two steps: formation of a silica layer by chemical deposit and silanization with a silane compound bearing an easily activated vinyl group (methacryloxy propyltrimethoxy silane, or KH-570) on the membrane surface. The silica layer formation can be performed by chemical vapor deposit (CVD), of TEOS from a vapor phase on the external and inner surface of the porous ceramic support. Compared with the sol–gel process, CVD needs more expensive equipments, although it provides a better control of the silica deposit thickness [18]. The successive pre-treatments of the ceramic substrate with aqueous HCl, then with an ethanolic solution of TEOS makes it possible to in situ hydrolyze TEOS into silica. This in situ hydrolysis-deposit (IHD) differs from the classical CVD in that the IHD reaction occurs in the successive immersions in dilute solutions, thus does not require expensive equipment as in CVD. The main reactions for IHD are as follows: hydrolysis reaction: Si(OR)4 + H2 O → Si(OH)n + nHOR

(1)

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condensation reactions: Si(OH)n + Si(OH)n → (HO)n−1 Si–O–Si(OH)n−1 + H2 O (2) Si(OH)n + Si(OR)n → (HO)n−1 Si–O–Si(OR)n−1 + HOR

After calcination, the condensed silicic acid forms on the external and inner surfaces of the porous support a SiO2 deposit which is bound at some spots to the porous support via the activeOH groups present on is surface, (Eq. (2)). 3.1.2. FT-IR characteristics In order to characterize the product obtained by in situ hydrolysis deposition on the external and inner surfaces of the porous support, the raw ceramic support was first ground into powder, then treated under the same deposit conditions as those for the ceramic membrane. The FT-IR spectrum of the raw ceramic powder showed two peaks at 1634 and 1086 cm−1 corresponding to the bending and asymmetric stretching vibration of Si–O–Si, respectively (curve a in Fig. 1). After TEOS treatment, it exhibited a new peak at 2978 cm−1 , which can be assigned to the stretching vibrations of CH2 in the organo-silicon compound (curve b in Fig. 1). After calcination, the peak at 965 cm−1 assigned to the stretching vibration of the Si–OH and the peak at 800 cm−1 assigned to Si–O–Si significantly increased (curve c in Fig. 1). These results provide the evidence of an active SiO2 layer formation and the introduction of reactive Si–OH onto the ceramic support surface during the IHD process. 3.1.3. Effect of reaction time Fig. 2a and b show the SEM photographs of the top surface of the ceramic support obtained at different hydrolysis-deposit times and thoroughly washed in an ultrasonic bath to remove non-tightly bound matters. It shows that the sintered ceramic grains of the disc surface were covered with a thin deposit,

Fig. 1. FT-IR spectra of ceramic particles: (a) before silica deposit; (b) after silica deposit with IHD process; (c) after silica deposit and calcination.

which appears to be thicker for a longer deposit time. In spite of the presence of a truly deposited layer, no integral dense layer was observed on the ceramic disc after the deposit. The fall in the flux of water through the support from 357 kg m−2 h−1 to 23 kg m−2 h−1 , after 44 h of hydrolysis-deposit, indicates a significant reduction in the pore size of the ceramic disc (Table 1), as the inter-grain gaps were far from being closed in the deposition. It can be thus assumed that the deposit is not limited to the outermost grains but occurs equally inside the underneath support pores. 3.1.4. Effect of TEOS content in solution The water permeation flux of the IHD-modified ceramic support for different TEOS concentrations and deposit times are listed in Table 1. Assuming that the ceramic support pores are not plugged but simply coated by a layer that reduced its size, and consequently the support water flux, Table 1 data indicate that the support pore size decreased with increasing TEOS concentration/deposit time. With TEOS/ethanol ratios 1/30 and 1/10, the water permeate flux was 207 kg m−2 h−1 and 42 kg m−2 h−1 ,

Fig. 2. SEM images of the surface of the ceramic support at different times of reaction with TEOS in solution (a) 0, (b) 44 h. TEOS/ethanol = 1:15, T = 70 ◦ C.

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Fig. 3. SEM photographs of the membrane surface with different TEOS/ethanol ratio (a) 1/30, (b) 1/10. t = 34 h, T = 70 ◦ C. Table 1 Effect of reaction conditions on the permeate flux of the ceramic support (70 ◦ C) Deposit time (h)

Water permeation flux J (kg m−2 h−1 )

0 24 34 44

357 252 98 23

TEOS/Ethanol (vol/vol)

Water permeation flux J (kg m−2 h−1 )

0 1/30 1/15 1/10

357 207 98 42

respectively. One can notice, that whatever the IHD conditions, the support grains were surface-coated without being soldered together into a homogenous dense layer (Fig. 3) which can be selective in a transport by a sorption–diffusion mechanism, i.e. there is no chance to obtain a selective pervaporation membrane by the IHD process alone. A high initial TEOS/ethanol ratio which severely alters the support hydraulic permeability would not be good for the membrane performances. The severe alteration of the support permeability would be caused by the high deposit rate so that more pores of the support are covered by a thicker silica layer. Nevertheless, a compromise between a correct support permeability and the reduction to a size small

enough to make possible the formation of a continuous dense layer on the support surface is required for a good membrane. 3.2. Characteristics of graft polymerization 3.2.1. Principle of graft polymerization The ceramic support pretreated with TEOS and KH-570 contains many activable vinyl (methacrylate) groups. The graft polymerization is initiated by potassium persulfate initiator at the vinyl grafts on the silica deposit, where poly(acrylic acid) (PAA) chain grow towards the core of the AA-filled pores. The chemical reactions are illustrated in Fig. 4. 3.2.2. TGA We controlled the grafting ratio by fixing the AA concentration in the solution. The degree of grafting was defined as the weight of polymer attached to the surface per unit area (mg m−2 ) in a similar way as in Cohen et al.’s works [16,19]. It was determined by measuring the dry weight difference calculated from the percentage of weight loss when the polymer in the grafted samples is completely pyrolyzed in the TGA instrument. The typical TGA thermogram is shown in Fig. 5 and the degree of grafting of PAA grafted from the solutions of different AA concentrations is listed in Table 2. The results show that the degree of grafting of PAA on the ceramic support increased with the AA concentration in the

Fig. 4. Schematic representation of the graft-polymerization of AA on silica membrane. (A) Surface silylation, (B) graft polymerization and (C) resultant covalently bonded PAA chains.

X. Cao et al. / Journal of Membrane Science 312 (2008) 15–22

Fig. 5. TGA thermograms of silica particles obtained in the polymer grafting from solutions of AA of different contents: (a) particles after silylation with KH570; (b–e) after grafting with AA solutions of different contents: (b) AA:4 wt.%; (c) AA:6 wt.%; (d) AA:8 wt.%; (e) AA:10 wt.%.

grafting solution. According to the mechanism of free-radical polymerization, the molecular weight of polymer formed in fixed initiation conditions is proportional to the monomer concentration. Thus the graft length, and consequently the grafting ratios, increases with the monomer concentration in the polymerization medium. One can expect that at high AA concentrations, the homopolymer chains inside the pores coil together with the PAA grafts and make them difficult to be removed from the grafted layer. We noted that at high AA concentration (beyond 10%) the surface of the grafted membranes became gelatinous and the solution much more viscous, indicating the formation of long chain homopolymers both on the surface and in solution. The highest polymer grafting degree obtained in the present work was 1.58 mg m−2 which is smaller than that usually obtained by Cohen et al. for PVAc grafting which was 2.66 mg m−2 and for PVP grafting which was 1.97 mg m−2 . 3.2.3. FT-IR study Fig. 6 shows the FT-IR transmittance spectrum of PAA grafted onto silica particles. After silanization, the appearance of a transmittance peak at 1727 cm−1 (characteristics of C O stretching vibration) and the disappearance of Si–OH stretching peak at 968 cm−1 qualitatively demonstrates the introduction of KH-570. Moreover, after being treated with NaOH, the silica particles grafted with PAA exhibit two strong transmittance peaks at 1409 and 1574 cm−1 (assigned to the symmetric and asymmetric stretching vibrations of –COO− anion, respectively) which clearly confirm the presence of PAA on the ceramic surface. Table 2 Effect of AA monomer concentration on the PAA degree of grafting on the silylated ceramic membrane AA concentration

4%

6%

8%

10%

Percentage of PAA weight loss Polymer degree of grafting (mg m−2 )

7.37 0.41

14.62 0.81

16.68 0.93

28.45 1.58

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Fig. 6. Transmission FT-IR spectra of silica particles at different steps of the PAA grafting process: (a) initial particles; (b) particles after silylation; (c) particles after PAA grafting on silylated particles; (d) PAA-grafted particles after treatment with NaOH.

3.2.4. Configuration of the PAA-grafts on the ceramic membrane In spite of the absence of a continuous dense layer on the ceramic support surface after PAA grafting, the surface of the wet support became shining and slippery. After PAA grafting, the top surface SEM image (Fig. 7a) shows a layer of grafted materials on the sintered particles but not in-between, and the cross-section SEM image (Fig. 7b) shows the presence of PAA grafts deep inside the membrane sections. With the progressive filling of the pores deep inside the membrane section, the pore filling with the PAA grafts, became better close to the surface, to the point that there is an apparently dense layer of about 5–10 ␮m thick in the top section of the ceramic membrane. Pervaporation experiments were needed to see if the apparently dense layer can act as an active separation layer (Fig. 7b). 3.3. Membrane separation capacity 3.3.1. Effect of monomer concentration on membrane performances The aim of this part is to determine the adequate AA monomer concentration which influenced the polymer degree of grafting (Table 2), under our grafting conditions, to obtain membranes of good performances for the dehydration by pervaporation. It should be noted that the ceramic membrane filled with PAA without the pre-activation with KH-570 did not show any separation capacity in the dehydration by pervaporation. In other words, the polymer may be well blocked inside the pores, but there are voids large enough inside the polymer phase, or more likely between the polymer phase and the pore wall, so that the solvent transport occurs in the bulk form, without selectivity. The pervaporation performances of PAA grafted-ceramic membranes have been studied in the dehydration of 95 wt.% ethanol at 30 ◦ C. The results show that the membrane performances depend largely on the AA concentration in the solution used for the grafting, which determines at first glance the over-

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Fig. 7. SEM photographs of the PAA-grafted ceramic membrane (a) top surface and (b) cross-section. The IHD-modified support was treated for 34 h; the membrane was grafted in AA solution of 6 wt.% at 70 ◦ C.

all grafting ratio of PAA on the membrane (Fig. 8). The higher the AA monomer concentration, the higher the polymer degree of grafting, so the lower the permeability, and the higher the selectivity, which approaches the highest possible value (corresponding to quasi-pure water in the permeate). At a low monomer concentration (4 wt.% or less), the degree of polymer grafting was less than 0.41 mg m−2 , the PAA grafts on the membrane should be short and sparse under this grafting conditions, thus little inclined to form an effective and selective barrier for solvent mixture separation. The grafting ratios increased with the increase of the monomer concentration, and the composite membrane showed gradually separation capability. The polymer degree of grafting reaches 0.81 mg m−2 when the AA concentration in the grafting medium is 6 wt.%. The permeation flux for such a grafted membrane was ca. 540 g m−2 h−1 , and the water content in the permeate exceeded 98.7 wt.% (Fig. 8). A further increase in monomer concentration resulted in a further grafting, but it improved only marginally the membrane selectivity (already very high) while depressed significantly the permeation

flux (down to 250 g m−2 h−1 at 8 wt.% of AA), probably because of an increase in the grafted layer thickness.

Fig. 8. Pervaporation performances (total flux and water content in the permeate) of PAA-grafted membranes prepared with solutions of different AA contents. Pervaporation of 95:5 water/ethanol mixtures at 30 ◦ C.

Fig. 9. Effect of feed water content on the permeate flux and selectivity for PAAgrafted ceramic-supported membranes at 30 ◦ C. Ceramic membrane grafted with a 6 wt.% of AA solution.

3.3.2. Effect of feed water content on pervaporation performances Fig. 9 shows the effect of water content in the water–ethanol mixture on the membrane performances in the dehydration of ethanol by pervaporation. The PAA-grafted ceramic membrane was very selective at all feed compositions: The water content in the permeate was higher than 98.7 wt.% whatever the water content in the feed was. With a water content in feed increasing from 5 to 100 wt.%, the permeation flux increased from 540 to 730 g m−2 h−1 for the membrane grafted with a 6 wt.% of AA solution. 3.3.3. Effect of operation temperature on membrane performances The effect of the pervaporation temperature on the pervaporation performance of the PAA-ceramic membrane measured

X. Cao et al. / Journal of Membrane Science 312 (2008) 15–22

Fig. 10. Effect of the pervaporation temperature on the total flux and selectivity for the PAA-grafted ceramic membrane in the pervaporation of a 95:5 wt.% ethanol–water mixture. Ceramic membrane grafted with a 6 wt.% of AA solution.

with a 95:5 wt.% ethanol/water mixture appears to be singular (Fig. 10). Over the 30–70 ◦ C temperature range, the membrane selectivity, which was high (higher than 98.7 wt.%), did not change very much. The permeation flux exhibited a singular behavior: it decreased significantly with the pervaporation temperature (from 540 g m−2 h−1 at 30 ◦ C to 330 g m−2 h−1 at 70 ◦ C). Yamaguchi et al. [20] studied the swelling behavior of the pore-filling-type membrane, and came to the conclusion that the substrate matrix suppressed the swelling of the filling polymer by a pore confinement effect. The singular behavior of our PAAceramic membrane with the increasing operating temperature can also be explained in a similar way. When the temperature increases, the thermal agitation of polymer segments increase; thus, the polymer grafts tend to expand, but because of their confinement in the rigid pores of the substrate they press more and more each other in the limited space, making the confined polymer grafts more and more compact. As a consequence, the solvent sorption and diffusion in this compacted polymer phase become more and more difficult, leading to a permeation flux decrease. Such a polymer chain compression should occur even when the confined polymer is not covalently grafted onto the pore surface. However, it is very difficult to study the direct effect of chain compaction. In order to indirectly prove the compaction effect of the polymer grafts inside rigid pores due to the temperature increase (i.e. the increase in thermal agitation), a pervaporation experiment was carried out with a PAA gel-filled membrane. The latter was obtained by crosslinking PAA in a free-radical polymerization of a solution of acrylic acid and N,Nmethylenebis-acrylamide inside the support pores. In a similar way as the PAA-grafted membrane, the pore-filled PAA membrane exhibited a steady decrease in the permeation flux with increasing temperature (Fig. 11), while it showed no separation effect towards the 95:5 ethanol water mixtures. The absence of selectivity in the case of the composite membrane in which the inorganic support is in situ filled with a gel of PAA, while the same support but with grafted PAA is highly selective suggests that the covalent binding of PAA to the pore walls is required

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Fig. 11. Effect of the pervaporation temperature on the total flux for the PAAgel filled ceramic membrane in the pervaporation of a 95:5 wt.% ethanol–water mixture.

for a selective permeation. The PAA gel insides the pores acted as obstacles for the permeation; thus, the higher the volume and the density of the gel confined in the rigid pores, the lower the permeation flux due to the pore blocking effect, as indicated by Fig. 11. It should be noted that, for a given feed liquid, the permeation flux in all membrane techniques always increases with increasing temperatures, either due to an increase in the molecular diffusion coefficients or a decrease in the fluid viscosity. Therefore, a decrease in permeation flux with increasing temperatures must stem from a change in the material structure. No other change was seen in the structure of PAA when the temperature increases from 30 to 70 ◦ C. The high permeability of the PAA-grafted ceramic membrane at low temperatures, compared with polymer membranes like poly(vinyl alcohol) [9] makes it attractive in the dehydration at low temperatures of liquids containing thermolabile molecules. As already stated by Cohen et al. [15,16], the stability of the silica-based matrix in organic solvents as well as the covalent fixation of grafts on to the inorganic surface make the membrane suitable for the dehydration of liquid media in which polymer membranes are not stable. The main difference between their membranes and ours lies in the structure of the inorganic support, which was rather common ceramic, with much larger pores (average pore size 2 ␮m for our support). 4. Conclusions A novel process to fabricate poly(acrylic acid)-grafted ceramic membranes for the dehydration of liquids at low temperature by pervaporation was developed. It consists of a graft polymerization of acrylic acid onto a ceramic microporous membrane, which was pretreated with an in situ hydrolysis-deposit process of TEOS, followed by silanization with a methacryliccontaining reactant (KH-570). The obtained membranes showed high performances for the dehydration of organic solvents by pervaporation. When the membrane grafted in AA solution of 6 wt.% is used for the dehydration of 95 wt.% ethanol at 30 ◦ C, a permeation flux higher than 500 g m−2 h−1 , and a permeate containing more than 98.7 wt.% water are obtained.

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The data obtained with different techniques indicated a polymer grafting on the treated ceramic grains on the surface and inside the pores to form a selective barrier whose permeation flux decreases and the water selectivity increases with the monomer content in the grafting medium. The singular decrease in the permeation flux of the membrane, when the pervaporation temperature increases, suggests a compaction of chains at high temperature due to the polymer confinement in the rigid pores. The advantage of the reported process is that it is adapted to an easily controlled production on a large scale for industrial applications. The inorganic support could be a cheap commercial ceramic support, and the equipments required for the membrane fabrication are common. Acknowledgements The financial support was provided by National Basic Research of China (No. 2003CB615702), by National Nature Science Foundation of China (Project No. 20374014) and by Senior Visiting Scholar Foundation of Key Laboratory in Fudan University. References [1] N. Wynn, Pervaporation comes of age, Chem. Eng. Progress 97 (10) (2001) 66. [2] P. Aptel, N. Challard, J. Cuny, J. Neel, Application of the pervaporation process to separate azeotropic mixtures, J. Membr. Sci. 1 (1976) 271. [3] J. Sheng, Separation of dichloroethane–trichloroethylene mixtures by means of a membrane pervaporation process, Desalination 80 (1991) 85. [4] T. Kashiwagi, K. Okabe, K. Okita, Separation of ethanol from ethanol/water mixtures by plasma-polymerized membranes from silicone compounds, J. Membr. Sci. 36 (1988) 353. [5] K.M. Song, W.H. Hong, Dehydration of ethanol and isopropanol using tubular type cellulose acetate membrane with ceramic support in pervaporation process, J. Membr. Sci. 123 (1997) 27.

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