Preparation of Silicalite Membranes on Stainless Steel Grid Supports

Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain. Silicalite membranes have been prepared by seeded...
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Ind. Eng. Chem. Res. 2005, 44, 7627-7632

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Preparation of Silicalite Membranes on Stainless Steel Grid Supports Fausto Lo´ pez, M. Pilar Bernal, Reyes Mallada, Joaquı´n Coronas, and Jesu ´ s Santamarı´a* Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain

Silicalite membranes have been prepared by seeded liquid-phase hydrothermal synthesis on alternative supports, namely, two types of stainless steel grids with opening sizes ranging from 2 to 5 µm. The influence of the gel composition, the use of surfactants, and the synthesis time have been studied. The silicalite membranes prepared on stainless steel grids were tested in the separation of n-/i-butane mixtures, showing performances which are comparable to those of good quality membranes prepared on conventional supports. Introduction Zeolite films and membranes constitute a new kind of technical material with potential applications in multiple fields.1 In particular, zeolite membranes and films can be efficiently used in a variety of processes such as (i) pervaporation and gas separation processes, where they often compete favorably with traditional separation technologies; (ii) reaction processes, where membrane reactors allow process intensification and frequently lead to a higher level of conversion or selectivity; and (iii) sensors (electrochemical sensors, quartz crystal microbalances, surface acoustic wave sensors, cantilevers, and optic fibers), where zeolite films can provide higher selectivity and/or sensitivity. Self-supporting zeolite membranes larger than a few square millimeters do not exist, and therefore, the zeolite membranes to be used in applications of industrial or commercial interest need to be supported on a suitable material. The choice of support is critical, not only with regard to the characteristics of membrane performance (permeation flux and selectivity) but also with regard to the process configuration that can be used and the final cost of the membrane system. Most of the zeolite membrane systems developed to date at the laboratory scale involve flat disks and tubular supports made of either porous stainless steel or ceramic materials. However, the high price and lack of flexibility of these systems have prompted research on alternative materials that can be used as membrane supports. Thus, among other attempts, it is worth mentioning the use of steel2 and ceramic3 monoliths (which may also be shaped as wheels or rotors4), stainless steel grids,5 wire gauze packings,6 glass fibers,7,8 nonporous ceramic9 and metal10 plates, glass, and steel beads.11 Monoliths are attractive as supports because of the same characteristics that made them good candidates for catalytic applications (flexibility of operation, good tolerance to the presence of dust, low pressure drop, reasonable cost, and relatively easy scale-up12). In * To whom correspondence should be addressed: Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain. E-mail: [email protected]. Phone: +34 976 761153. Fax: +34 976 761879.

addition, zeolite-coated cordierite monoliths can be prepared by liquid-phase hydrothermal synthesis,3,13,14 without recourse to a binder, meaning that a greater load of active material can be achieved. In the case of ZSM-5 films on cordierite monoliths, loadings of up to ∼30 wt % have been obtained in our laboratory,3 while for the case of mordenite, the load of zeolite in some cases exceeded 50 wt %.15 ZSM-5 coatings have also been prepared on mullite honeycomb supports,16 ceramic slabs,9 and foams.17 BEA (β-zeolite) coatings (loadings of up to 9 wt %) have been prepared by dipping monolithic and wire gauze packings in a BEA slurry.6 Zeolite monoliths have found use in rotatory adsorbers for dehumidifiers and desiccant cooling processes4 or VOC treatment systems.18 Alumina-coated, silicon carbide monoliths have also been employed as supports for B-ZSM-5 membranes,19 providing a larger surface area per unit volume than traditional membrane supports. With these membranes, Kalipcilar et al.19 reported n-/i-butane and H2-/i-butane separation selectivities of 35 and 77, respectively.20 A different approach used in our laboratory used laser-perforated 75 µm thick stainless steel sheets as the support for silicalite membranes displaying propane/N2 selectivity.21 This work reports on the preparation of silicalite membranes on stainless steel grids. Earlier investigations on the deposition of zeolite material on stainless steel grids and wire gauze5,6 were not aimed at the preparation of membranes, but to the development of reactors with structured catalytic packing. Thus, ZSM-5 coatings on stainless steel grids were used for partial oxidation of benzene by N2O,5 and the deposition of BEA on wire gauze was carried out to perform acylation of anisole with organic acids. In this work, silicalite membranes have been prepared by seeded liquid-phase hydrothermal synthesis on stainless steel grids and have been tested in the separation of n-/i-butane mixtures. Grids made of stainless steel wires constitute an inexpensive and highly flexible support that seems to be well suited to the industrial development of zeolite membranes. To our knowledge, this is the first investigation in which synthesis of zeolite material on this type of support has produced a selective membrane.

10.1021/ie048972t CCC: $30.25 © 2005 American Chemical Society Published on Web 02/17/2005

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Experimental Section The stainless steel wire grids used in this work are the so-called Twilled Dutch Weave (TDW), where the wires are woven in such a way that tiny curved triangular openings are left among the wires. These grids are thick, strong, and light, and are used primarily in fluid, vacuum, and pressure filtration of liquids and gases. Two different kind of grids were used, 325 × 2300 mesh (type A grid) and 200 × 1400 mesh (type B grid). Type A grids contain more openings, and their nominal aperture is 2.03 µm; on the other hand, the nominal aperture of type B grids is 5 µm. The grids, 25 mm in diameter, were cleaned using the following procedure. First, they were immersed in a HNO3 solution (1 wt %) for 4 h at 60 °C; then they were cleaned with distilled water and dried at room temperature. Finally, the supports were cleaned in acetone in an ultrasonic bath for 1 h. The clean supports were stored at 100 °C. To seed the grids, a colloidal suspension containing silicalite particles of ca. 100 nm was prepared according to the procedure described by Lovallo et al.22 The concentration of the solution was adjusted to 20 g/L and the pH to 8, using tetrapropylammonium hydroxide (TPAOH). The seeding step was carried out as follows. The face of the grid to be seeded was slowly moved toward the surface of the colloidal suspension until the support contacted the liquid surface. This allowed the face in contact to be wetted by capillarity. The support was then dried at room temperature, and the seedingdrying cycle was repeated four times to ensure a homogeneous result. Finally, the grid support was dried overnight at 80 °C. This seeding procedure concentrates the seeds in one side of the grid, while the other side is kept almost seed free. The gels used for hydrothermal synthesis had the following molar compositions: 4.5:1:1:1000 SiO2:TPABr: KOH:H2O ratio for gel A23 and 0.32:1:165 SiO2:TPAOH: H2O ratio for gel B.24 In both cases, tetraethylortosilicate (TEOS) was used as the silicon source. The hydrothermal synthesis was carried out in Teflon-lined stainless steel autoclaves, where the grid supports, seeded or not, were introduced together with the corresponding gel. The autoclaves were heated inside a forced convection stove to temperatures in the 150-175 °C range, and they were kept at this temperature for 2-15 h at a time. To activate the grid membranes for permeation, the template was removed from the zeolitic micropores by calcination in air at 480 °C for 8 h, using heating and cooling rates of 0.5 °C/min. The experimental setup for single gas permeation measurements and for the separation of n-/i-butane mixtures was described previously.25 Basically, the gridsupported membrane was sealed in a flat membrane module by means of Viton gaskets, and then one side of the membrane was put in contact with a mass-flow controlled stream (100 NmL/min) of an equimolar n-/i-butane mixture, while the other was swept with a He stream (100 mL/min). Results and Discussion Characterization of the Silicalite Grid Membranes. Figure 1 shows the evolution of the weight gain with synthesis time for both of the supports employed in this work and for different synthesis conditions (seeding and gel composition). In all cases, there is a

Figure 1. Weight gain as a function of synthesis time under different synthesis conditions: (9) type B (5 µm) grid, gel B, seeded; (0) type B (5 µm) grid, gel B, not seeded; (O) type A (2 µm) grid, gel B, not seeded; and (2) type B (5 µm) grid, gel A, seeded.

rapid weight gain during the initial 5-10 h of synthesis, followed by a plateau and even a small decrease of weight in some cases. This can be explained on the basis of the growth mechanism of zeolites: initially, the high reactant concentration in the gel gives rise to intense nucleation and crystal growth; eventually, the gel becomes depleted of nutrients, and crystal growth slows and finally stops. With a diluted solution surrounding the supports, dissolution of part of the existing mass of zeolite crystals and zeolite precursors may occur, leading to a decrease in the weight gain. On the other hand, the maximum weight gains observed are higher than those reported by Louis et al.,5 with ZSM-5 deposited on stainless steel grids. In that case, they obtained a coverage of 95 g/m2, after three synthesis steps at 170 °C totaling 40 h. In this work, the coverage on 5 µm supports varies from 50 to 140 g/m2 after synthesis for 10 h at 175 °C. It is interesting to note that gel B, though more diluted than gel A, gives rise to a higher zeolite weight gain thanks to its higher TPA+:SiO2 molar ratio (3.1, vs 0.22 for gel A). Also, type A grids, with a smaller (2 µm) nominal diameter, produce lower weight gains compared to grids with 5 µm openings. This is an expected result, since more porosity per unit volume of grid means more volume to be filled with zeolitic material. Finally, seeding has only a limited influence in the weight gain achieved, as can be seen by comparing the curves corresponding to seeded and unseeded synthesis on 5 µm supports. This is in contrast with the results obtained on smoother ceramic surfaces (where seeding has a clear effect on the rate of crystal growth) and probably reflects the strong influence of the stainless steel surface in the process of heterogeneous nucleation of zeolite crystals. SEM observations were used to assess the morphological differences between the different zeolite films. Figure 2 shows SEM micrographs taken on 5 µm grids after seeded synthesis for 2 and 14 h (only one side was seeded) using gel A. As can be seen from Figure 2a, 2 h is enough to completely cover the grid with silicalite crystals which are very well attached to the wires of the grid. However, despite its well-intergrown appearance, the voids between zeolite crystals are not completely closed, and the 2 h membrane is not selective for gas separation. As the synthesis time increased from 2 to 14 h, the weight gain increased from 25 to ca. 60 g/m2, and the

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Figure 2. SEM micrographs corresponding to membranes prepared on type B (5 µm) grids, in seeded syntheses using gel A, with synthesis times of (a) 2 and (b) 14 h.

thickness of the zeolite layer increased correspondingly. After synthesis for 14 h, the cross section of the film (Figure 2b) evidences a fully grown zeolite layer with a thickness of ∼4 µm. This layer follows well the curvature of the wires, and presents the typical columnar structure of c-oriented silicalite. On the unseeded face (not shown here), some randomly oriented crystals can be observed, but these are isolated and do not form a layer. For comparison, Figure 3 shows the results obtained using the same synthesis conditions on grids with 2 µm openings. These (type A) grids contain more wires per unit area, and these are thinner than for the 5 µm grids (compare Figures 2a and 3a). In this case, after 7.5 h (Figure 3b) the wires also appear to be fully covered with a zeolite film, the amorphous-looking deposits between wires seem to have disappeared, and the wires appear to be thicker than they were after 2 h (Figure 3a). Figure 3c shows the unseeded side of the grid, where the crystals grown are not preferentially oriented, unlike those on the seeded face. The XRD results of Figure 4 confirm these SEM observations. For a membrane prepared using gel A on a 2 µm grid, the XRD intensities corresponding to the seeded side [see the peak at 8° for the (101) plane in Figure 4a] reveal that the crystals grow preferentially along their c-axes, while multiple orientations are present on the unseeded face. Figure 5 shows the evolution of the XRD patterns with synthesis time for a membrane prepared using gel B without a seeding step. After synthesis for 2 h, the intensities corresponding to (200) and (020) planes appear as a single peak at 9.05°, and the reflection corresponding to the (101) plane at 8° (c-orientation) is not discernible. After 5.5 h, a- and b-orientations still predominate over c. It is only after synthesis for 15 h that the c-orientation becomes more important, an expected result given the fact that this is the thermodynamically favored direction for silicalite growth. However, even after 15 h it is not possible to characterize the membrane as a c-oriented film.

Figure 3. SEM micrographs corresponding to membranes prepared on type A (2 µm) grids, in seeded syntheses using gel A, with synthesis times of (a) 2 h, (b) 7.5 h (seeded side), and (c) 7.5 h (unseeded side).

Figure 4. XRD patterns of the seeded and unseeded sides of a membrane prepared using gel A on a type B (5 µm) grid, with a synthesis time of 14 h.

Figure 6 shows SEM micrographs corresponding to samples prepared using gel B on unseeded type B grids. After 2 h, b-oriented crystals can be observed, in agreement with the discussion of the XRD results presented in Figure 5. After synthesis for 15 h (Figure 6c), the grid is fully covered with crystals in different orientations, although c-oriented crystals are now more frequent. In a separate synthesis experiment, the use of surfactants to promote the growth of the zeolite layer was also investigated. Commonly, seeding is carried out with a colloidal suspension of silicalite seeds whose pH is

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Figure 5. XRD patterns of samples prepared at different synthesis times. Unseeded syntheses on type A grids (2 µm) using gel B.

Figure 7. SEM micrographs of a silicalite membrane prepared using gel A on type A grids (2 µm) that had been pretreated by impregnation with the cationic surfactant, with a synthesis time of 7.5 h: (a) seeded side and (b) unseeded side. Table 1. Nitrogen and Butane Permeances and n-/ i-Butane Separation Factors for Membranes Prepared in Unseeded Synthesis Using Gel B on 5 µm Grids synthesis time (h) 2 7.5 15

N2 permeance (mol m-2 s-1 Pa-1) >10-5 7.8 × 10-7 2.7 × 10-8

butane permeance (mol m-2 s-1 Pa-1)

Rn-/i-butane

3.6 × 10-8 2.2 × 10-9

4.2 24

Table 2. Nitrogen and Butane Permeances and Separation Factors before and after the Second Hydrothermal Treatmenta synthesis N2 permeance n-butane permeance time (h) (mol m-2 s-1 Pa-1) (mol m-2 s-1 Pa-1) Rn-/i-butane 5.5 14 5.5 + 14 14 + 14

2.4 × 10-6 8.3 × 10-6 2.7 × 10-8 1.6 × 10-7

1.6 × 10-6 9.6 × 10-6 2.2 × 10-9 7.9 × 10-9

0.9 0.9 5.7 27

a Membranes prepared in seeded synthesis using gel A on 5 µm grids.

Figure 6. SEM micrographs of silicalite membranes prepared on type B grids (5 µm) without seeding and using gel B, with synthesis times of (a) 2, (b) 7.5, and (c) 15 h.

close to 8, while the isoelectric point of silicalite occurs at pH 7.26 This means that the seeds are negatively charged, and therefore, coating the support with a cationic surfactant, CTABr (cetyltrimethylammonium bromide), should favor the adhesion of the seeds and promote the growth of the zeolite layer. Figure 7a shows a membrane prepared on a type A grid after impregnation of both sides of the support with the cationic surfactant as explained above. The grid was seeded on only one side and then subjected to hydrothermal synthesis at 150 °C. Figure 7 shows that a layer consisting of closely packed crystals is obtained not only on the seeded surface but also at the unseeded side,

where a uniform c-oriented silicalite film can also be observed. This indicates that the cationic surfactant is effective in promoting migration of the seeds and of crystal nuclei formed in solution to both sides of the support, ensuring a good surface coverage. Permeation Measurements. Table 1 shows the results of permeation experiments for membranes prepared without seeding and using synthesis gel B, after 2, 7.5, and 15 h. The permeation results are consistent with the above discussion concerning SEM observations (Figure 6) on the morphology of the membranes. Thus, after synthesis for 2 h, intercrystalline gaps could still be observed by SEM (Figure 6a), in agreement with the N2 permeance quoted in Table 1, which is clearly above that expected for a continuous zeolite membrane. As the synthesis time increased, a continuous layer of wellintergrown zeolite crystals gives rise to a zeolite membrane capable of separating n-/i-butane mixtures with a separation factor of 4.2 after synthesis for 7.5 h. This value increased to 24 after synthesis for 15 h (Figure

Ind. Eng. Chem. Res., Vol. 44, No. 20, 2005 7631 Table 3. Separation Factors for Membranes Prepared after Impregnation with Cationic Surfactants

Figure 8. SEM micrographs corresponding to the cross section of a silicalite membrane prepared by two successive synthesis steps on a type B grid (2 µm): (a) 5.5 h + 14 h and (b) 14 h + 14 h.

Figure 9. Component permeances and n-/i-butane separation factor as a function of temperature. Membrane prepared on a type B grid (5 µm) in a seeded synthesis with gel A using two synthesis cycles of 14 h each.

6c), when the density of membrane defects had been considerably reduced. In Table 2, poor performance can be observed for the membranes prepared by a single seeded synthesis using gel A on type B grids. This is due to the low TPA+:SiO2 ratio of gel A (10 times lower than that of gel B), which slows the rate of crystal growth and hinders the formation of a high-quality film. Therefore, a second synthesis was carried out to decrease the density of defects, following the procedures used in conventional tubular supports with MFI membranes.25 The permeation results in Table 2 clearly show the improvement induced by the second synthesis step, with a reduction in the concentration of defects that leads to lower permeances and considerably higher separation factors. Both the permeance and the separation factor obtained after the two 14 h synthesis steps are comparable to those reported for good-quality MFI membranes on tubular supports. Figure 8 shows SEM images corresponding to the cross section of these membranes, where it is clearly observed that after the second synthesis

grid size (µm)

synthesis time (h)

Rn-/i-butane

2 2 2 5

5 7.5 15 7.5

7.3 9.0 4.2 7.3

there is zeolite material occupying the interstitial voids among the mesh wires. The effect of temperature on the n-/i-butane separation was studied for the best membrane of Table 2 (two 14 h synthesis steps). Figure 9a shows a much higher level of permeation of n-butane throughout the temperature range studied, due to the preferential adsorption of n-butane that hinders the permeation of i-butane. However, the n-/i-butane selectivity (Figure 9b) presents a maximum in the vicinity of 80 °C, with selectivity values above 50. This maximum in selectivity is a consequence of two conflicting processes: increasing the temperature increases the mobility of adsorbed n-butane molecules, and therefore, the n-butane permeation flux initially increases. On the other hand, higher temperatures decrease the rate of n-butane adsorption, a process that enhances i-butane permeation. Eventually, the second effect becomes dominant, and the selectivity decreases. This behavior was previously described for membranes on conventional supports.25,27 Finally, Table 3 shows the n-/i-butane mixture separation results obtained with membranes whose preparation process involved impregnation with the cationic surfactant, on both types of grids. The comparison with the results in Table 2 shows that, after a single synthesis step, all the membranes in Table 3 present a better performance than the membranes prepared using the same gel and synthesis conditions, but without impregnation with the cationic surfactant. This is also in agreement with SEM observations for these membranes (Figure 7), which indicated that the use of a cationic surfactant promotes uniform growth on both sides of the grid, giving rise to good-quality membranes. Conclusions It has been shown that the synthesis procedures used to prepare zeolite membranes on conventional supports such as alumina and stainless steel porous tubes and plates can be successfully adapted to develop zeolite membranes on stainless steel grids. Seeded and unseeded liquid-phase hydrothermal synthesis gave rise to similar results, with the quality and performance of the final membranes depending mainly on the synthesis time, presence of surfactants, and grid aperture. Grid-supported membranes with good permeation properties and n-/i-butane separation factors comparable to those of high-quality membranes on conventional supports can be obtained. The use of grids impregnated with a cationic surfactant seems to promote the adhesion of seeds and crystal nuclei formed in solution to the stainless steel support, resulting in more compact membranes. This aspect is still under investigation in our laboratory. Acknowledgment Financial support from DGA and CICYT, both in Spain, is gratefully acknowledged. Literature Cited (1) Coronas, J.; Santamarı´a, J. State-of-the-art in zeolite membrane reactors. Top. Catal. 2004, 29, 29.

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(16) Katsuki, H.; Furuta, S.; Komarneni, S. Formation of novel ZSM-5/porous mullite composite from sintered kaolin honeycomb by hydrothermal reaction. J. Am. Ceram. Soc. 2000, 83, 1093. (17) Seijger, G. B. F.; Oudshoorn, O. L.; van Kooten, W. E. J.; Jansen, J. C.; van Bekkum, H.; van den Bleek, C. M.; Calis, H. P. A. In situ synthesis of binderless ZSM-5 zeolitic coatings on ceramic foam supports. Microporous Mesoporous Mater. 2000, 39, 195. (18) Shiraishi, F.; Yamaguchi, S.; Ohbuchi, Y. A rapid treatment of formaldehyde in a highly tight room using a photocatalytic reactor combined with a continuous adsorption and desorption apparatus. Chem. Eng. Sci. 2003, 58, 929. (19) Kalipcilar, H.; Falconer, J. L.; Noble, R. D. Preparation of B-ZSM-5 membranes on a monolith support. J. Membr. Sci. 2001, 194, 141. (20) Kalipcilar, H.; Gade, S. K.; Falconer, J. L.; Noble, R. D. Synthesis and separation properties of B-ZSM-5 zeolite membranes on monolith supports. J. Membr. Sci. 2002, 210, 113. (21) Mateo, E.; Lahoz, R.; de la Fuente, G. F.; Paniagua, A.; Coronas, J.; Santamarı´a, J. Preparation of silicalite micromembranes on laser-perforated stainless steel sheets. Chem. Mater. 2004, 16, 4847. (22) Lovallo, M.; Tsapatsis, M. Preferentially oriented submicron silicalite membranes. AIChE J. 1996, 42, 3020. (23) Xomeritakis, G.; Tsapatsis, M. Permeation of aromatic isomer vapors through oriented MFI-type membranes made by secondary growth. Chem. Mater. 1999, 11, 875. (24) Wang, Z.; Yan, Y. Controlling crystal orientation in zeolite mfi thin films by direct in situ crystallization. Chem. Mater. 2001, 13, 1101. (25) Bernal, M. P.; Coronas, J.; Mene´ndez, M.; Santamarı´a, J. On the effect of morphological features on the properties of MFI zeolite membranes. Microporous Mesoporous Mater. 2003, 60, 99. (26) Lai, R.; Gavalas, R. Surface seeding in ZSM-5 membrane preparation. Ind. Eng. Chem. Res. 1998, 37, 4275. (27) Xomeritakis, G.; Nair, S.; Tsapatsis, M. Transport properties of alumina-supported MFI membranes made by secondary (seeded) growth. Microporous Mesoporous Mater. 2003, 38, 61.

Received for review October 21, 2004 Revised manuscript received January 3, 2005 Accepted January 4, 2005 IE048972T