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The effect of intermediate layer on synthesis and gas permeation properties of NaA zeolite membrane.

Abstract NaA zeolite membrane coating was successfully synthesized on an alumina porous disc by hydrothermal treatment. The effects of synthesis parameters, such as seeding condition (in situ, ex situ), synthesis time, synthesis stages, application of intermediate layer, etc., on membrane characteristics were investigated. Surface seeding accelerates the zeolite crystallization process on the support surface, and also enhances the formation of homogeneous NaA zeolite layer. But the main problem associated with membrane coating synthesis is crack formation. Formation of crack was reduced by applying intermediate layer, between the support surface and seed layer. A thin Boehmite layer was applied to the support surface before applying seed crystals to enhance the adherence between zeolite seed layer and boehmite layer by hydrogen bonding and also to increase the mechanical strength of the membrane layer. The quality of the membrane layer can be improved by employing the multi-stage coating methods. The permeance of [O.sub.2], [N.sub.2] decreased as kinetic diameter of gases increased, which shows the molecular sieving effect of the NaA membrane. The permselectivity of [[O.sub.2]/[N.sub.2]] was 1.9-2.0. This value of permselectivity ratio is higher than Knudsen diffusion ratio 0.94; it was also confirmed the molecular sieving properties of synthesized NaA zeolite membrane.

Keywords NaA zeolite, Membrane coating, Thin film, Gas separation

Introduction

In recent years, attempts to develop zeolite membranes for separation and catalytic applications have been intensified. Considering their molecular sieving properties and uniform pore size, high thermal resistance, and high mechanical strength, zeolite membrane has attracted great interest for application in many important industrial processes. (1), (2)

Among the various types of zeolite species, especially NaA membrane has the potential to sieve out molecules in a continuous process due to its hydrophilicity, making the electrostatic interaction between ionic sites in the water molecule stronger. As a result it has been commercially applied to alcohol dehydration. (3), (4)

For application in separation processes, the membrane must be defect free and continuous. The synthesis of defect-free membrane is still challenging although different methods have been developed to improve the quality of the membrane. It was proposed that the formation of zeolite membrane included three stages: sol-gel adhesion on the substrate to form the gel layer, the nucleation and crystallization and the coating formation. (5)

There are many reports on the improvement of the formation stages of the membranes like microwave heating, (6) addition of intermediate silane layer to increase the adhesion between gel layers and supporting substrate, (7) vacuum seeding, (8) brush seeding, cross-flow filtration seeding, (9) etc.

The in situ hydrothermal synthesis appears to be the best studied method, in which the porous support is immersed into the synthesis solution, and then the membrane is formed by direct crystallization. However, it is difficult to prepare high-quality membranes by this in situ crystallization method directly. (10) Coating the zeolite seed on the support surface before hydrothermal synthesis, which is also called a secondary growth method, is an effective approach to develop a high-quality zeolite membrane. It is well known that the presence of seed on the support surface plays an important role in membrane formation. Synthesis with seeds gives a better control of the membrane formation process by separating the crystal nucleation and growth with a shortened crystallization time. (11) In addition, the secondary growth ensures the formation of the zeolite crystal on the support. The secondary growth method which was proposed by Lovello el al. (12) exhibits many advantages such as better control over membrane microstructure and higher reproducibility. (13)

According to literature, NaA zeolite membrane is a good candidate for the separation of several industrially important gases. (14), (15) The pore size of NaA zeolite is 0.41 nm. Therefore, small molecules, such as [O.sub.2] and [N.sub.2], are expected to be separated by utilizing the molecular sieving or configurational diffusion properties of the NaA zeolite membrane. The molecular kinetic diameters of [O.sub.2] and [N.sub.2] are 0.346 and 0.364 nm, respectively, which are close to the pore size of NaA zeolite. Due to the configurational diffusion, the small difference in molecular size between [O.sub.2] and [N.sub.2] results in a big difference in the rate of diffusion through the NaA zeolite channels; the diffusion of [O.sub.2] is faster than that of [N.sub.2]. (16) Therefore, [O.sub.2] and [N.sub.2] can be separated by a NaA zeolite membrane.

However, very high selectivity is also possible with the mixture of gases when the zeolite pore size is large enough compared to gas molecules.

In this work, zeolite membrane layer was synthesized on alumina substrate by the secondary growth method, The substrate surface was modified in order to facilitate the adsorption of seed crystals. An intermediate boehmite layer was applied between seeds and support substrate of the membrane to increase the mechanical strength and adherence of the seeds to the supports.

Experimental methods and materials

A porous alumina disc of diameter 22 mm (cut from 47 mm disc) and thickness 3 mm (5 KD, median pore size 1.5 nm) collected from Sterlitech USA was used as substrate for the preparation of zeolite membrane. Before coating, the substrates were cleaned with acetone in a ultrasonic cleaner (Vibracell, USA) for 5 min just to remove dust particles and oily matter. The cleaned substrates were coated with boehmite sol followed by attachment of zeolite seeds. The boehmite layer was applied by dip coating method. Initially the porous alumina substrate was dipped into boehmite sol. Dipping time was varied from 1 to 30 s and repeated 5 times. Then, the substrate was dried at 200[degrees]C for 2 h. A thin layer of boehmite was formed on support surface.

The modified support was coated with NaA zeolite crystals as nucleation seed. To get a uniform membrane layer on the support surface, the nucleation seeds should be very small and uniform in size. The desired nucleation seeds were synthesized by hydrothermal synthesis technique having small particle size and narrow size distribution. (17) The particle size of the seed crystals was 50 nm (average particle size measured from SEM). In order to get a uniform seed layer on the support surface, the seeds should be dispersed homogeneously on the support surface, and the amount of nucleation seeds should not be too large otherwise the membrane layer will be too thick and uneven.

In this nucleation seeds coating process, the support substrate was dipped in a 1-3% NaA zeolite suspension in deionized water 5 times for a duration of 15 s. After the dipping procedure, the seeded supports were dried al 150[degrees]C for 24 h.

The zeolite A membranes were synthesized hydrothermally on porous modified, seeded support discs. The materials used for the synthesis were sodium meta silicate nonahydrate (Qualigens Fine Chemicals, India) aluminum chloride (S. d Fine Chemicals, India), sodium hydroxide (Merck India), and distilled water. Two reactant mixtures were prepared respectively dissolving sodium meta silicate and aluminum chloride in freshly made sodium hydroxide solution. The total amount of sodium hydroxide was distributed between the corresponding silicate and aluminate solutions while the water was divided evenly. After aging each mixture separately for 1 h, the two mixtures were mixed slowly under stirring at room temperature. The resulting mixture was stirred vigorously for 15-30 min to produce a homogeneous gel. The molar composition of the gel used for the synthesis was [Al.sub.2][O.sub.3]:[SiO.sub.2]:[Na.sub.2]O:[H.sub.2]O 1:5:50:1000. The seeded substrate was placed vertically in an autoclave. Crystallization was continued under autogenous pressure in a hot air oven al 65[degrees]C for 2-6 h. Repeated crystallization for second stage and third stage was carried out to improve the quality of the membrane. After synthesis, the zeolite coated membrane was washed thoroughly with deionized water until the pH of the washing liquid became neutral. They were then dried in air at ambient temperature and were stored in a desiccator.

The crystalline structure of the as-synthesized membrane was determined by XRD pattern. XRD was carried out on a Philips 1710 diffractometer using [CuK.sub.[alpha]] radiation ([alpha] = 1.541 [Angstrom]).

The microstructure and morphology of growth layer was examined using scanning electron microscopy (SEM: model Leo, S430i, UK). Gas permeation is also a common method of membrane characterization. Ideal selectivity is defined as the ratio of single-gas permeance, and is often used as an indication of membrane quality. (18) High selectivity is also possible for the gases having smaller size than pore sizes of zeolite membrane. Permeation depends on both adsorption and diffusion, but molecules close to or larger than the zeolite pore size have difficulty entering zeolite channels. Thus for characterization of small channel sized NaA membrane, single gas permeation measurement of small gases like [O.sub.2], [N.sub.2] is suitable. Gas permeation was measured by a specially designed permeation cell developed in our laboratory. The gas permeance of the membranes was measured by soap film flow meter under the feed pressure of 1-4 kg/[cm.sup.2] and at 25[degrees]C temperature. The permselectivity of two gases [G.sub.1]/[G.sub.2] was defined as the permeance ratio of [G.sub.1] and gas [G.sub.2]. The gas permeation measurement of each single gas was repeated until the permeance data for the successive 10 tests were closed. The single gas permeance was the average of 10 successive tests.

Results and discussion

The formation of NaA zeolite membrane on the unseeded alumina support with different synthesis time was investigated and the as-synthesized membranes were characterized by XRD and SEM. Figure 1 shows the XRD pattern of membrane synthesized for 4 h and that of zeolite powders formed in bulk medium for 2, 4 and 6 h. Comparing the diffraction pattern for coated membrane and bulk powders, it is clear that in bulk medium zeolite was formed after 2 h of synthesis. But in case of membrane formation, a few zeolite crystals were formed on the support surface and it was too low to detect by XRD. Since uniform, continuous coating throughout the support was not formed, the diffraction intensity of the NaA zeolite was weak and it was difficult to detect the zeolite phase on the support by XRD. Figures 2a-c show the top surfaces of the membrane formed without seeding the surface. SEM images indicate that NaA zeolite grains were scattered on the substrate surface after 2 h of synthesis and up to 4 h quantity of crystals were increased but not well crystalline. No continuous, dense, interlocked membrane structure was formed up to 6 h. According to the formation mechanism of zeolite membrane on the porous support, the nucleation of zeolite on the support/gel interface and in the bulk synthesis mixture are competitive processes. (19) In order to form a continuous zeolite membrane on the support surface, the nucleation of zeolite in the bulk synthesis mixture must be inhibited, while the nucleation of zeolite on the support surface must be increased. Comparing XRD and SEM, it can be seen that a few crystals are formed on the support surface, whereas at the same time NaA zeolites were formed in the bulk solution. Thus, in case of synthesis of continuous membrane layer, seed crystals must be added before synthesis.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Figure 3 shows the XRD pattern of as-synthesized membrane on the seeded alumina support with different synthesis time. Under the same condition NaA zeolite was not formed on the unseeded (without zeolite seed crystals) support. Thus, seed crystals enhanced the formation of zeolite layer on support surface.

[FIGURE 3 OMITTED]

The formation of zeolite membrane on seeded support under the same above-mentioned condition is depicted in Figs. 4a-d. The SEM images were in accordance with the XRD results. The support surface was covered with NaA zeolite after 2 h of synthesis. When the synthesis time increased to 4 h, the coverage of the NaA zeolite on the support increased. From these figures it is clear that after 4 h of synthesis, the zeolite crystals grew and interlocked to form dense membrane structure. Comparing the formation of NaA zeolite membrane on the seeded support with that of the unseeded support, surface seeding cannot only accelerate the formation of the NaA zeolite on the support, but also inhibit the transformation of the NaA zeolite into other types of zeolites. A continuous NaA zeolite membrane can be formed on the seeded support. The main advantage with seed crystals is that they are easily inter-grown. In seed film method, the final film thickness can be controlled by varying crystallization time and size of the seed crystals. It is also possible to control the preferred orientation of the crystalline materials constituting the films as the orientation was found to be dependent on the amount and size of the seed crystals and the film thickness. (20), (21) The SEM images showed that although a continuous NaA zeolite membrane can be obtained after coating the seeds, a crack of 2-3 [micro]m still existed in the obtained membrane after interlocking of zeolite crystals of synthesis. The films are fragile since zeolite grains were not strongly bonded to the support surface. However, XRD and SEM can only indicate whether a continuous membrane formed on the support, they cannot confirm whether a high-quality zeolite membrane formed. The quality of zeolite membrane can only be evaluated by gas permeation properties of the zeolite-coaled membrane.

[FIGURE 4 OMITTED]

Molecular sieving results when the size of the pores of the membrane material is similar to the size of the gas molecules. NaA zeolite membrane is capable of separating gases by molecular sieving effect. The gas permeation decreases in the order: [H.sub.2] > [O.sub.2] > [CH.sub.4] > [N.sub.2]. (17) Here, we have measured single gas permeation for [N.sub.2] and [O.sub.2] separately. The sizes of the gas molecules are less than the pore size of the NaA zeolite (0.41 nm). The small pore sized zeolite NaA (0.41 nm) could improve separation of small gas molecules such as nitrogen (0.364 nm) and oxygen (0.346 nm) by exploiting the differences in size between them. Xu et al. (5) made use of this fact in order to separate nitrogen and oxygen and achieved a permselectivity of 1.8 for [O.sub.2]/[N.sub.2]. Poly crystalline zeolite membrane contains defects, i.e. inter-crystalline pathway which is also called non-zeolitic pores. The synthesis procedure, type of zeolite and heat treatment condition affect the concentration of defect. The transport mechanism of gas molecules through non-zeolitic pores is difficult and it is difficult to quantify because the pore size of these non-zeolitic pores is not well defined. Usually non-zeolitic pores are larger than zeolitic pores. The presence of non-zeolitic pores reduces the selectivity of the gases.

Table 1 shows the gas permeance properties of membrane prepared on seeded layer at room temperature (25[degrees]C). The permeance of [N.sub.2] and [O.sub.2] and permselectivity of [O.sub.2]/[N.sub.2] of NaA zeolite membranes with different synthesis stages was measured. The SEM and XRD results were consistent with the results of gas permeation measurements. The [N.sub.2] and [O.sub.2] permeances of the substrates were 17.82 x [10.sup.-6] and 17.67 x [10.sup.-6] mol * [s.sup.-1] * [m.sup.-2] * [Pa.sup.-1]. As the reaction progressed for first-, second-, and third-stage coating, the permeance of [N.sub.2] changed to 12.46 x [10.sup.-6], 6.41 x [10.sup.-6], and 6.06 x [10.sup.-6] mol [s.sup.-1] * [m.sup.-2] * [Pa.sup.-1]. The [O.sub.2] permeance through the membranes changed to 15.78 x [10.sup.-6], 6.48 x [10.sup.-6], and 6.23 x [10.sup.-6] mol * [s.sup.-1] * [m.sup.-2] * [Pa.sup.-1] with first-, second-, and third-stage coating. The permeance due to molecular sieving may be decreased as the kinetic diameter of the molecules increased. But here no such phenomenon was observed. These results confirm that in non-zeolitic pores mainly cracks were present. In case of single-stage synthesized membrane, the inter-crystalline diffusion through the defects in the structure of zeolite membrane increases the flux but reduces the selectivity. With increasing "stage" of synthesis, the extent of defects was reduced and selectivity was increased. Actually the presence of non-zeolitic pores cannot be removed completely but can be reduced so that selectivity can be increased.
Table 1: Gas permeance of seeded membrane without intermediate layer

Synthesis time      Permeance (mol/[m.sup.2] s Pa)

                     [N.sub.2]            [O.sub.2]

Uncoated        17.82 x [10.sup.-6]  17.67 x [10.sup.-6]

One stage       12.46 x [10.sup.-6]  15.78 x [10.sup.-6]

Two stage        6.41 x [10.sup.-6]   6.48 x [10.sup.-6]

Three stage     6.016 x [10.sup.-6]   6.23 x [10.sup.-6]

Synthesis time  Ratio [O.sub.2]/[N.sub.2]

Uncoated                    0.99

One stage                   1.27

Two stage                   1.01

Three stage                 1.04


The main cause of crack of membrane layer was lack of good adherence between zeolite layer and substrate layer. Another cause of cracking of zeolite layer was mismatch of thermal expansion between zeolite layer and support alumina layer. Considering those facts and to obtain a crack-free membrane layer on the substrate, an intermediate layer (buffer layer) was applied. Addition of a buffer layer can enhance the mechanical strength and integrity of the film. It also reduced the effect of thermal expansion "mismatch" of membrane layer and support layer. The formation temperature of zeolite was low (65[degrees]C), still we consider this factor because NaA zeolite is highly hydrophilic in nature; before gas permeation measurement, membranes were heated strongly to remove the adsorbed water. The layer was made of boehmite sol and seed sol (considering as graded layer). Figure 5 shows the SEM micrograph of seeded support with buffer layer.

[FIGURE 5 OMITTED]

Hydroxyl group of boehmite layer acts as binder between zeolite and support layer. The structural element in boehmite crystals consists of double chain of [AlO.sub.6] octahedra giving double molecules.

[EQUATION]

These chains are parallel, forming layers with OH group outside. These hydroxyl groups will form hydrogen bonds between hydroxyl ion and neighboring group. Therefore, an increase in hydroxyl group density causes a better attachment between the zeolite and the support. By increasing boehmite concentration, i.e. from 1% sol to 3% (by weight) sol, the attachment of the seeds with the substrate increases and as a result ultimate membrane quality was enhanced. Figure 5 depicts the effect of buffer layer concentration on the seeding of the substrate. Figure 5a shows the surface of the coated substrate with 3% boehmite sol and 3% seed solution (by weight) and Fig. 5b shows the same with 3% boehmite sol and 2% seed solution (by weight). Figure 6 shows the membrane layer prepared with 5% boehmite sol and 3% seed solution. The thickness of the NaA zeolite membrane layer was approximately 5 [micro]m (measured from SEM micrograph). Boehmite concentration was changed from 1 to 5% (wt%) and dipping time 1 to 5 times (30 s each). Simultaneously, seed concentration was changed from 1 to 3% (wt%) solution. Table 2 describes the detailed effect of boehmite and seed layer concentration. This table describes that combination of 3% boehmite and 5% seed concentration with 5 times dipping gave the best result. It is well known that alumina can be leached from an alumina substrate at high alkaline pH during the zeolite crystal growth procedure. But the quantity is very small. The leached aluminum has dual roles during zeolite membrane growth: to facilitate the formation of the gel layer on the substrate and to retard zeolitization in this gel layer. (16) From Table 2, it is shown that at above 3% boehmite concentration, i.e. in case of 5% boehmite sol, no crystalline phase appeared on the upper surface of the support. It may be because the quantity of leached alumina was higher, it retarded the zeolization process. Thus at 65[degrees]C and 4 h, no prominent microstructure was formed. Figure 7 confirms the phenomenon.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]
Table 2: Details of application of boehmite layer and seed layer on
membrane substrate

Sample  Boehmite  Dipping   Seed   Dipping   Remark
ID      conc.     time (30  conc.  time (30
        (%)       s each)   (%)    s each)

B01       -                   1      5       Cracks

B12       1         5         2      5       Cracks

B31       3         5         1      5       Small cracks on the
                                             surface

B32       3         5         2      5       No crack on the surface

B33       3         5         3      5       No crack on the surface,
                                             uniform crystal size,
                                             good interlocking of the
                                             crystals.

B53       5         5         3      5       No interlocked
                                             crystalline structure.
                                             Surface cracked severely


In case of boehmite containing membrane layer, the gas permeances of [O.sub.2] and [N.sub.2] decreased as the molecular kinetic diameter increased, which indicates that permeance was controlled by molecular sieving effect. Table 3 shows the permeation results of [N.sub.2] and [O.sub.2] for boehmite-coated NaA zeolite membranes.
Table 3: Gas permeance results of as-synthesized membrane with boehmite
layer

Synthesis time      Permeance (mol/[m.sup.2] s Pa)

                     [N.sub.2]            [O.sub.2]

Uncoated        18.32 x [10.sup.-6]  20.67 x [10.sup.-6]

One stage        5.14 x [10.sup.-7]   6.21 x [10.sup.-7]

Two stage        3.67 x [10.sup.-7]   7.12 x [10.sup.-7]

Three stage      2.32 x [10.sup.-7]   4.70 x [10.sup.-7]

Synthesis time  Ratio [O.sub.2]/[N.sub.2]

Uncoated                    -

One stage                 1.20

Two stage                 1.94

Three stage               2.02


For NaA zeolite (applying intermediate layer) membrane with a single-stage synthesis, perm selectivity was increased from 0.991 to 1.20 compared to that of only seeded membrane (Table 1). It indicates the partial presence of the molecular sieving effect in the as-synthesized zeolite membrane after one stage of synthesis. The permselectivity of [O.sub.2]/[N.sub.2] was increased up to 2.02 for third-stage synthesis, which is much higher than the Knudsen diffusion ratio, which indicates that the quality of the membrane was improved by applying the third-stage coating.

Conclusion

A high-quality NaA zeolite membrane was successfully synthesized on a porous alumina substrate. Application of boehmite layer on support surface improves the quality of the membrane. When support was unseeded (support surface without seed layer), the formation of membrane layer was inhomogeneous. To get a homogeneous, continuous zeolite membrane layer, zeolite seed crystals were coated on the support surface before membrane layer synthesis. Cracking of the zeolite layer was the main problem for the synthesis of membrane layer. To reduce the cracking, boehmite layer was applied on the support layer before zeolite layer synthesis. Gas permeation results indicated that non-zeolitic pores were present after single-stage synthesis. After third-stage synthesis, the quality of the membrane was improved. The permeances of [N.sub.2] and [O.sub.2] increased as the molecular kinetic diameter of the gases increased, which showed the molecular sieving effect of the NaA zeolite membrane. The perm-selectivity of [O.sub.2]/[N.sub.2] was 2.02. The quality of the as-synthesized membrane with three stage synthesis gave the best results.

Acknowledgments The authors thank CSIR, India, for financial support through the project no. SIP 0023 and also thank Dr. H. S. Maiti, Director, CGCRI, for his kind permission to publish the research work.

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N. Das (*), D. Kundu, M. Chatterjee

Sol-Gel Division, Central Glass & Ceramic Research Institute, CSIR, Jadavpur, Kolkata 700 032, India e-mail: dasnandini@cgcri.res.in

J. Coat. Technol. Res., 7 (3) 383-390, 2010

DOI 10.1007/s11998-009-9195-z
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Author:Das, Nandini; Kundu, Debtosh; Chatterjee, Minati
Publication:JCT Research
Date:May 1, 2010
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