Cordierite-supported ZSM-5 membrane: Preparation and pervaporation properties in the dehydration of water–alcohol mixture

Cordierite-supported ZSM-5 membrane: Preparation and pervaporation properties in the dehydration of water–alcohol mixture

Separation and Purification Technology 44 (2005) 266–270 Short communication Cordierite-supported ZSM-5 membrane: Preparation and pervaporation prop...

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Separation and Purification Technology 44 (2005) 266–270

Short communication

Cordierite-supported ZSM-5 membrane: Preparation and pervaporation properties in the dehydration of water–alcohol mixture Lizhi Zhou a , Tao Wang a , Quang Trong Nguyen c , Jun Li b , Yingcai Long b , Zhenghua Ping a,∗ a

Department of Macromolecular Science, Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, PR China b Department of Chemistry, Fudan University, Shanghai 200433, PR China c Rouen University-UFR Sciences-UMR 6522, 76821 Mont St. Aignan, France Received 17 August 2004; accepted 24 December 2004

Abstract The ZSM-5 type membrane was prepared on a flat cordierite support by low temperature chemical vapor deposition (LTCVD) of a silica layer, followed by an in situ crystallization in vapor phase. The membrane obtained consisted of loosely-packed zeolite crystals in the pores of the support. The silica–alumina ratio of the membrane zeolite layer determined by EDX was low, indicating a hydrophilic nature of the membrane. The performances of the ZSM-5 membrane in the separation of water–alcohol mixtures by pervaporation were studied and discussed with respect to the membrane structure and the alcohol nature. The membrane showed very high selectivity towards water in the water–ethanol mixture. The transport mechanism of the membrane has also been discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: ZSM-5 membrane; Vapor deposition; Cordierite; Pervaporation; Water–alcohol separation

1. Introduction There has been an increasing interest in zeolite membranes due to their strong potential in the separation of liquid mixture by pervaporation [1–4]. Zeolite membrane can be prepared with different methods: in situ hydrothermal synthesis [5]; chemical vapor phase method [6] and spraying seed coating [7], etc. Whatever the method, an inorganic porous support is required and its nature and structure may affect the quality of a composite zeolite membrane [8,9]. A popular support is that made of sintered Al2 O3 . Nevertheless, this support is expensive and contributes for a large part to the high price of the membranes. It is thus important to study the possibility of obtaining membranes with a lowprice support. Cordierite, a structural ceramic with a general formula of Mg2 Al4 Si5 O18 , exists as an ore in the nature. Considering its abundant resource and its low cost, its easy processing into a support with a regular structure by sinter∗

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

1383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.12.016

ing, this material is a good candidate for the zeolite membrane support. Dehydration of organic solvents is presently the major market for the pervaporation process. Zeolite NaA membrane were reported to be excellent materials for the solvent dehydration by pervaporation [2]. But under slightly acid conditions and under hydrothermal stresses, zeolite A membranes turned out to be unstable due to hydrolysis. There are only a few attempts to develop hydrophilic highly siliceous zeolite membrane of different Si/Al ratios with improved hydrothermal stabilities [10,11]. In this paper, we report the preparation of a hydrophilic ZSM-5 zeolite membrane on a cordierite support by a novel method which consists of low temperature chemical vapor deposition (LTCVD) of a silica layer, and then the MFItype zeolite layer is hydrothermally obtained by in situ crystallization of the silica layer with water vapor. The surface structure and the morphology had been determined by different techniques. The membrane performances in the water–alcohol separation by pervaporation were studied.

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2. Experimental 2.1. Materials The cordierite support was kindly provided by East China University of Science and Technology. Its average pore size is 2 ␮m and its porosity ca. 40%. It appeared as a white plate of ca. 3 mm thick. Si(OC2 H5 )4 (TEOS) was purchased from Shanghai Baihe Chemical Factory and tetrapropylammonium hydroxide (TPAOH) from Aldrich Chemical Company. 2.2. Synthesis of zeolite membrane The preparation of the ZSM-5 layer on a cordierite support comprises three steps. In the first step, the support, after being dipped in a 40 wt.% aqueous solution of TPAOH, was exposed to the TEOS vapor for 8 h in an autoclave at 180 ◦ C. The hydrolysis of TEOS and the polycondensation of the hydrolysis product by the adsorbed TPAOH solution gave rise to a silica layer on the support surface. In the second step, the substrate was wetted with an aqueous solution (1.5 NaOH:1.0 TPAOH:70 H2 O in weight), and then treated in water vapor at 180 ◦ C for 72 h in an autoclave. After the treatment, the deposited silica layer crystallized and transformed into ZSM-5 type, which was tightly bound to the support. The membrane was next taken out of the autoclave, cooled to room temperature, washed with distilled water, and dried at 120 ◦ C. In the third step (template agent removal), the dried membrane was calcined by heating to 500 ◦ C at a rate of 0.1 ◦ C min−1 in a muffle furnace, followed by a temperature plateau of several hours. After the calcinations, the membrane was cooled to room temperature at a rate of 0.1 ◦ C min−1 . All these processes were carried out under nitrogen atmosphere. 2.3. Structure and morphology determination The membrane surface and section were examined in SEM (HITACHI S-520) to visualize the membrane morphology and the crystals of the zeolite layer. The crystalline structure of the membrane was determined by X-ray diffraction (Rigaku D-MAX/IIA). The Si/Al ratio of the zeolite layer was analyzed by a SEM equipped with an energy-dispersive X-ray analysis device (EDX, Oxford).

Fig. 1. XRD spectrum of cordierite support (a) and zeolite membrane (b).

densed in a trap cooled by liquid nitrogen and was weighed to determine the total flux. The permeate composition was determined by gas chromatography under the following condi◦ tions: Porapak-Q filled column, temperature: 150 C, carrier gas: hydrogen, flow rate: 20 ml min−1 . The permeation flux J (kg m−2 h−1 ) was calculated from the permeate condensed in the cooled trap for a known pervaporation time t: J=

W A×t

where W is the permeate weight and A is the membrane area. The selectivity is in general represented by the parameter α. α=

c /(1 − c ) c0 /(1 − c0 )

where c0 and c are weight fraction of water in the feed and in the permeate, respectively. We prefer to use c as selectivity, because α varies greatly with the feed composition; their value is not meaningful for very selective membranes and for the feed mixtures at high fast-component contents in the feed. For instance, a hydrophilic zeolite membrane shows a sharp decrease in the α value when the water content increases, while the selectivity remains perfect, i.e. the permeate consists of pure water (c = 1). In this case, one cannot say that the membrane selectivity towards water has decreased.

2.4. Pervaporation apparatus and procedure Pevaporation experiments were carried out in a cell made of stainless steel with effective surface area of 1 cm2 . The feed of 100 mL was continuously circulated from the feed tank by a pump. The amount of permeate extracted was so small compared with that of the feed (<0.1 wt.%) that the feed concentration was practically constant during the pervaporation batches. The pervaporation was carried out at 30 ◦ C under a vacuum better than 100 Pa. The permeate vapor was con-

3. Results and dicussion 3.1. Characterization of the cordierite-supported ZSM-5 zeolite membrane by SEM, XRD, and EDX Fig. 1 shows the XRD spectrum of the cordierite support and that of the zeolite layer. None of the eight diffraction peaks of the cordierite XRD spectrum (Fig. 1a) was found in

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Fig. 2. SEM photograph of ZSM-5 membrane surface.

the XRD spectrum of zeolite layer (Fig. 1b), whose most intense peak is located at ca. 2θ = 24◦ . We infer that the cordierite surface was perfectly covered by a ZSM-5 layer. Fig. 2 shows the SEM photograph of ZSM-5 membrane surface. Most of the crystals lie disorderly on the surface. The average size of the crystal is 8–12 ␮m long, 2–4 ␮m width and 1–2 ␮m thick. The SEM photograph of ZSM-5 membrane section shows that the cordierite surface (c) is completely covered by a ZSM-5 crystal layer (Fig. 3), whose thickness is larger than 40 ␮m. The crystal layer is composed of two layers, the top layer consists of pure ZSM-5 crystals (a), and the intermediate one, of ZSM-5 crystals grown in the cordierite pores (b). The latter should be formed by the penetration of TEOS by capillary condensation in the support pores. Although holes of micrometer size are visible between the crystal packs on the surface, they are not through-holes, as we will show in the

next sections, i.e. the spaces between the crystal packs must be closed by the other packs beneath. This indicates that the crystallization of zeolite proceeds from the already formed crystals on the support, and tends to close the gap between them. Such a self-closing ability of the zeolite in the process of crystallization makes possible a molecular separation by permeation through a polycrystalline film. The EDX analysis gave the values of Si/Al ratios on the surface of zeolite membranes, which were about 15. The low Si/Al values do not reflect the use of pure TEOS as the silica source, i.e. in the absence of any Al source. It can be speculated that a part of the alumina from the support (as aluminate ions) dissolves in the synthesis solution and is incorporated into the framework during the growth of the zeolite layer [12]. This explains why the Si/Al ratio decreases with the membrane thickness, and the Si/Al ratio is lower while the zeolite layer is closer to the cordierite support (Table 1). As it is well known that the hydrophilicity of a zeolite membrane increases when the Si/Al ratio decreases [13], thus, the membrane prepared with cordierite support by LTCVD could be used for water removal from liquid mixture by pervaporation. 3.2. Pervaporation properties of the cordierite-supported ZSM-5 zeolite membrane It is well known that the pervaporation performances of a dense polymer membrane depend on the ability of the solvent species to dissolve in the membrane at its interfaces, and to their diffusion in the membrane [14]. When a zeolite membrane is used as separation barrier, the solvent species cannot be dissolved in the membrane phase but adsorbed on zeolite sites of the inorganic materials. Its adsorbed capacity depends on the affinity of membrane toward the solvent to be removed. 3.2.1. Effect of the feed ethanol concentration on total flux and selectivity Fig. 4 shows the relationships between the membrane performances and the feed concentration. The membrane exhibited a high selectivity towards water in the water–ethanol mixture. The permeate water content reaches a value as high as 99 wt.% for the feed water content in the range of 1–6 wt.%, i.e. the most interesting composition range for anhydrous ethanol production by pervaporation. The fact that the membrane has a high selectivity to water clearly indicates that the zeolite layer does not have any through-holes (as the SEM picture might suggest), and the transport is diffusive but not Table 1 Si/Al ratio in different depth of the surface of zeolite membrane measured by EDX

Fig. 3. SEM photographs of a cross-section of the ZSM-5 membrane. Pointed zones: (a) active dense layer; (b) zeolite crystals embedded in the cordierite porous matrix; (c) cordierite porous support; 0–3: EDX measuring point.

Measure point (see Fig. 3)

Si (at.%)

Al (at.%)

Si/Al

0 1 2 3

16.97 10.72 8.73 11.25

1.15 1.61 2.13 5.8

14.76 6.65 4.09 1.94

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Fig. 4. Flux and water content in the permeate vs. feed water content in the pervaporation of the water–ethanol mixture at 30 ◦ C.

convective. The result also confirms that the ZSM-5 membrane used behaves as a hydrophilic membrane, probably due to the presence of polar Al atoms in the zeolite crystal structure. At 30 ◦ C, the permeation flux varies from 1 to 0.2 kg m−2 h−1 as the feed ethanol content increases from 0 to 100 wt.%. However, the variation of the flux with the feed composition is quite different from that observed with the NaA zeolite membrane [15] or other zeolite membranes for the same mixture. When the feed water content increases in the narrow high-water content range (>95 wt.% water), the flux increases very sharply, while it remains quasiconstant at ca. 0.32 kg m−2 h−1 in a very large water content range (5–95 wt.%). The high pure water flux value at 1.0 kg m−2 h−1 , suggests that the support layer were not flux limiting for the transport in a very large range of water content. The flux decreases slightly in the low water content range (<5 wt.%). However, its value is still higher than that of the well-known polyvinyl alcohol (PVA) membrane in the water content range of industrial interest. The difference between the flux patterns of this membrane and that of the other membranes is worth a discussion from the point of view of molecular transport mechanism. Although there is also a sharp increase in flux when the feed water content increases in the case of the PVA membrane, the phenomena involved in the two cases are very different. At high water contents, the PVA membrane experiences a strong swelling [16], thus has a high flux due to the large amount of absorbed solvent, while zeolite membranes do not swell. At very low water contents, the PVA membrane is not swollen and remains a glassy material, in which the water diffusivity is very low; and the flux is smaller. On the contrary, the zeolite structure remains unchanged in the liquid media, and the permeation flux depends only on the sorbed–solvent amounts. As the dense layer in composite polymer membranes is generally about 1 ␮m, the fact that the thick ZSM-5 layer (40.2 ␮m) has higher flux than the much thinner dense polymer PVA layer indicates that the resistance of the dense layer to solvent diffusion is much lower for ZSM-5 than for PVA. This

Fig. 5. Flux (a) and water content in the permeate (b) vs. feed water content in the pervaporation of different water–alcohol mixtures at 30 ◦ C.

is a surprising result, considering that the random orientation of ZSM-5 crystals in the layer and the long tortuous path through the ZSM-5 membrane for solvent molecules. The flux increases first with the water activity (or water vapor pressure) in the external phase as a consequence of an increase in the amount of adsorbed water in the zeolite cavities. However, once the saturation level of the zeolite cavities is reached, no further change in the adsorbed amount with the water activity is possible, and the flux remains constant (5–95 wt.% water content range). The sharp increase in the flux at very high water content (more than 95 wt.% water content) can be explained by an additional adsorption, or a capillary condensation, in the intercrystalline phase. 3.2.2. Pervaporation characteristic of different alcohol/water systems Fig. 5 indicates the PV transport characteristics for the mixtures of water with different alcohols. Their molecule size order is methanol < ethanol < isopropanol (IPA), while their polarity order is methanol > ethanol > isopropanol. The

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results indicate that the smaller the molecule size and the higher the molecule polarity, the larger the permeation flux, and the worse the selectivity. This behavior is similar to that of hydrophilic polymer membranes. It can be explained by a stronger sorption of the smaller and more polar organic component, leading to a lower selectivity and a higher total flux. It should be noted that the zeolite pores whose size is ca. 0.55 nm show much lower selectivity to water in methanol than to water in ethanol (and of course, IPA). If we admit that the calculated values of the molecular diameter of the alcohol molecules are correct, i.e. 0.41, 0.52, 0.58 nm for methanol, ethanol, and IPA, respectively [17], then the sharp increase in the selectivity would occur when IPA replaces ethanol or methanol in the water–alcohol mixture, due to the size exclusion of IPA from the zeolite cages. Fig. 5 shows, in fact, a sharp change in selectivity between methanol and ethanol. It seems that the pore sieving action of zeolite pores on linear alcohols in the permeation through the zeolite layer does not correspond exactly to the calculated molecular size. The minimum in the methanol selectivity for the intermediate methanol contents can be explained by a flux coupling. The separation properties of our membrane are determined mainly by the adsorption properties of these permeating molecules in the membrane pores. We speculate that the presence of Al species in the zeolite membrane causes a mass transport in the less-shape selective intercrystalline voids.

4. Conclusions A MFI (ZSM-5 type) zeolite membrane was prepared on a flat cordierite support by a low temperature chemical vapor deposition (LTCVD) of a silica layer, followed by an in situ crystallization in water vapor. The obtained membrane consisted of about 40 ␮m thick zeolite layer with low silica–alumina ratio. Due to this, the membrane exhibits high pervaporation selectivity to water in methanol, ethanol, and isopropanol, and the selectivity increases with the alcohol carbon number. Over a wide feed concentration range, a change

in the feed concentration does not affect the solvent flux through the membrane. This behavior can be explained in the frame of an adsorption–diffusion mechanism in the ZSM-5 membrane.

Acknowledgement Supported by National Basic Research Program of China, No. 2003 CB615702.

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