Langmuir 2000, 16, 3993-4000
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Key Role of the Interface Gel-Support in the Synthesis of Zeolitic Coatings Guillaume Clet,*,† Joop A. Peters, and Herman van Bekkum Laboratory of Applied Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received October 15, 1999. In Final Form: January 17, 2000 The synthesis of zeolite Y on stainless steel using a seeded synthesis mixture brings new information on the initial steps of the formation of a zeolitic coating. Initially, an amorphous gel layer is deposited on the support. The modifications carried out on the interface constituted by this layer on the support induce direct changes on the coating. The coverage and the final crystal size for instance can be tuned with the seed content. Crystal growth is thought to occur directly from the seeds present on the support. The zeolite layer is stabilized when crystals grow individually and perpendicularly to the support and bind with the surface via a hematite interface. This initial zeolite layer can then be used in a secondary growth process during which intergranular voids can be closed. To induce growth on the support, the seeds must be well dispersed in the synthesis gel. Here seeds are thought to be constituted of a preliminary local ordering of the synthesis gel in which nanoaggregates of gel are formed from the Q0 and Q1 silicate species.
Introduction Supported zeolites have a potential for a wide range of applications1,2 including chemical sensors,3-5 membranes,6-9 and catalytic systems.10-12 Two main approaches are currently used in the preparation of these systems. The first one involves the preadsorption of seeds on the support, followed by secondary growth under hydrothermal conditions in a common synthesis mixture. Deposition of seeds for Y crystallization has once been achieved on alumina by rubbing it with zeolite X powder,13 but usually, supports are contacted with colloidal suspensions of the zeolite. The crystallites are attached to the support with a binder, like boehmite,14,15 by dipping followed by calcination16,17 or by mere drying.18 Alternatively, the support is pretreated with a solution of a polymer to reverse its surface charge and then brought in contact with the colloidal * To whom correspondence should be addressed. † Present address: Laboratoire de Catalyse et Spectrochimie, University of Caen, ISMRa, 6 Bd. du Marechal Juin, 14050 Caen Cedex, France. Fax +33/2 31 45 28 22; E-Mail Guillaume.Clet@ ismra.fr. (1) Bein, T. Chem. Mater. 1996, 8, 1636. (2) Mizukami, F. Stud. Surf. Sci. Catal. 1999, 125, 1. (3) Yan, Y.; Bein, T. J. Phys. Chem. 1992, 96, 9387. (4) Yan, Y.; Bein, T. J. Am. Chem. Soc. 1995, 117, 9990. (5) Alberti, K.; Fetting, F. Sens. Actuators, B 1994, 21, 39. (6) Geus, E. R.; van Bekkum, H.; Bakker, W. J. W.; Moulijn, J. A. Microporous Mater. 1993, 1, 131. (7) Yan, Y.; Davis, M. E.; Gavalas, G. R. Ind. Eng. Chem. Res. 1995, 34, 1652. (8) Nishiyama, N.; Ueyama, K.; Matsukata, M. Microporous Mater. 1996, 7, 299. (9) Coronas, J.; Falconer, J. L.; Noble, R. D. AIChE J. 1997, 43, 1797. (10) Antia, J. E.; Govind, R. Ind. Eng. Chem. Res. 1995, 34, 140. (11) Calis, H. P.; Gerritsen, A. W.; van den Bleek, C. M.; Legein, C. H.; Jansen, J. C.; van Bekkum, H. Can. J. Chem. Eng. 1995, 73, 120. (12) Oudshoorn, O. L.; Janissen, M.; van Kooten, W. E. J.; Jansen, J. C.; van Bekkum, H.; van den Bleek, C. M.; Calis, H. P. A. Chem. Eng. Sci. 1999, 54, 1413. (13) Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Ind. Eng. Chem. Res. 1997, 36, 649. (14) Lovallo, M. C.; Tsapatsis, M. AIChE J. 1996, 42, 3020. (15) Lovallo, M. C.; Tsapatsis, M.; Okubo, T. Chem. Mater. 1996, 8, 1579. (16) Boudreau, L. C.; Tsapatsis, M. Chem. Mater. 1997, 9, 1705. (17) Lai, R.; Gavalas, G. R. Ind. Eng. Chem. Res. 1998, 37, 4275. (18) Gouzinis, A.; Tsapatsis, M. Chem. Mater. 1998, 10, 2497.
suspension, dried, and calcined.19-21 After the secondary growth step, the adsorbed seeds are converted into intergrown zeolite layers. For these studies glass plates, silicon wafers or alumina was mainly used, and thin films or membranes could be formed efficiently. However, this preparation mode involves multiple-step syntheses and in some cases the use of a binder that might affect the layer properties. The second approach for the preparation of supported zeolites utilizes the in-situ crystal growth: the support is brought in contact with the synthesis mixture, and synthesis occurs under hydrothermal conditions. By this method various zeolites, mainly ZSM-5, could be supported on a wide range of materials,10,22,23 notably metallic supports.11,12,24,25 This method is easier to operate than the one mentioned previously. However, films of low densities are often obtained due to random nucleation on the support. It has been reported that, with this preparation procedure, the crystallization occurs on the surface by heterogeneous nucleation after deposition of an initial amorphous gel layer. Crystals can then nucleate in this layer and start growing close to the support.26 Nucleation is also possible on the interface of this layer with the synthesis gel.27 Homogeneous nucleation in the bulk of the synthesis gel followed by the deposition of the crystals has also been suggested.8 The distinction between the possible mechanisms is often difficult as the partial dissolution of the support used (alumina or silicon) can mask the influence of the gel-support interface. The use of a stable support would eliminate this problem. We (19) Hedlund, J.; Schoeman, B. J.; Sterte, J. Stud. Surf. Sci. Catal. 1997, 105, 2203. (20) Hedlund, J.; Schoeman, B. J.; Sterte, J. Chem. Commun. 1997, 1193. (21) Valtchev, V.; Hedlund, J.; Schoeman, B. J.; Sterte, J.; Mintova, S. Microporous Mater. 1997, 8, 93. (22) Jansen, J. C.; Kashchiev, D.; Erdem-Senatalar, A. Stud. Surf. Sci. Catal. 1994, 85, 215. (23) Anderson, M. W.; Pachis, K. S.; Shi, J.; Carr, S. W. J. Mater. Chem. 1992, 2, 255. (24) Davis, S. P.; Borgsted, E. V. R.; Suib, S. L. Chem. Mater. 1990, 2, 712. (25) Valtchev, V.; Mintova, S. Zeolites 1995, 15, 171. (26) Yan, Y.; Chaudhuri, S. R.; Sarkar, A. Chem. Mater. 1996, 8, 473. (27) Koegler, J. H.; van Bekkum, H.; Jansen, J. C. Zeolites 1997, 19, 262.
10.1021/la991362r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/21/2000
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Table 1. Seeds Formulations Applied in the Different Experiments
code
weight of seeds addeda (g)
wt % seeds added/total mixture
% Na2O contributed by seeds
W1 W2 W3 W4
1.4 2.8 4.1 5.5
2.8 5.4 7.9 10.3
4.7 8.2 12.8 16.4
a
In 38 g feedstock solution.
showed recently that coating of stainless steel with zeolite Y could be achieved with an alternative method combining the advantages of both fore-mentioned methods. The synthesis was achieved in situ, using a seeded synthesis mixture.28 In this case we also observed the deposition of an amorphous gel layer as had been previously shown in the ZSM-5 system.26,27 Therefore, the interface between the gel and the support will probably determine the features of the final coating. To control more efficiently the coating of solid surfaces with zeolites, the specific role of the gel and its interaction with the support, especially at the interface, should be better understood. Here we report on zeolite Y supported on stainless steel, as an example to clarify the zeolite crystal growth on a support and the formation of a supported layer, notably by evidencing the type of nucleation involved. Experimental Section Plates (1 cm × 1 cm × 51 µm) of nonporous stainless steel AISI316L (Fe ∼66%, Cr ∼17%, Ni ∼12%, Mo ∼2%, C e0.03%) and AISI302 (Fe ∼71%, Cr ∼18%, Ni ∼9%, Mo ∼0%, C e0.15%) were used. Prior to the synthesis, the supports were etched successively in 2.2 M KOH and 37% HCl, then washed with water, and dried (393 K). Some plates were also calcined at 813 or 923 K for 10 h in air. The zeolite coating weight was measured using a metal strip (∼4 cm × 25 cm × 51 µm) for the synthesis. The zeolite Y synthesis mixture was prepared in two steps. A first gel (seeding mixture) was prepared with a molar composition of 10 SiO2-1 Al2O3-10.7 Na2O-180 H2O. It was stirred for 1-48 h at 500 rpm and further aged for 24 h static at room temperature. A second mixture (feedstock gel) was prepared with a composition 10 SiO2-1 Al2O3-5.2 Na2O-180 H2O and stirred for 24 h. Part of the seeding mixture was incorporated into the feedstock gel in amounts accounting for 0-20 wt % of the final Na2O content and stirred for 1 h. The average final gel composition was 10 SiO2-1 Al2O3-5.6 Na2O-180 H2O. Sodium silicate (Aldrich; 27% SiO2, 14% NaOH), sodium aluminate (Riedel de Haen; 54% Al2O3, 41% Na2O), sodium hydroxide, and demineralized water were used as reactants. The synthesis procedure will be coded SmWn, where m is the stirring time of the seeding mixture and n is the relative amount of seeds added. The corresponding weight percentages are reported in Table 1. Alternatively, a usual unseeded synthesis mixture was used with a composition of 10 SiO2-1 Al2O3-4 Na2O-180 H2O using colloidal silica (Ludox HS-40, DuPont). It was stirred for 24 h and further aged for 24 h before synthesis. Synthesis occurred under hydrothermal conditions in a 50 mL autoclave with a Teflon lining heated at 373 K and rotated at 180 rpm. The support was inserted on a Teflon holder in the middle of the autoclave. After typically 7 h synthesis and subsequent cooling, zeolite was always obtained both on the support and as a loose powder in the autoclave. The powder and the support were then washed separately with demi-water and dried at 393 K. Powders were characterized by XRD (Philips PW1840, Cu KR radiation), SEM (Philips XL20 operated at a tension of 15-25 kV), and ICP-AES. Before characterization, supports were sonicated for 20 min (50 W, 50 kHz). Parent metal support and zeolite-covered supports were characterized by XRD (θ-θ dif(28) Clet, G.; Jansen, J. C.; van Bekkum, H. Chem. Mater. 1999, 11, 1696.
fractometer AXS D5005, Cu KR radiation), SEM, and EDS coupled with a SEM (JEOL JSM-6400F; tension used for the electron beam, 12 kV). The quasi-elastic light scattering measurement was carried out on a Coulter N4 apparatus after filtration of the mixture on a 3 µm pore membrane. 29Si NMR measurements were carried out on a Varian Unity Inova 300. A solution of the sodium 3-(trimethylsilyl)propionate in D2O was used as an external standard. Crystal sizes were obtained from the SEM pictures. The theoretical coverage of the supports was calculated assuming a cubic volume for the crystals and a zeolite density of 1.27 g cm-3.
Results and Discussion I. The Gel Side of the Interface. When stopping a synthesis after only 1 h, SEM and EDS evidenced on the support an amorphous layer containing silicon and aluminum in a ratio close to that of the synthesis mixture. Yan et al. have observed the presence of globular species on their glass support when a synthesis was stopped early.26 In our case, however, no indication could be obtained either from SEM or from EDS measurements that individual globules were formed on the stainless steel. Therefore, it may be concluded that deposition of a homogeneous gel on the support took place. After 7 h synthesis, pure zeolite Y was already formed, and zeolite P appeared after 18 h synthesis. The relatively short times indicate the presence of nucleation particles in the synthesis mixture. This will probably also be the case in the gel layer deposited initially on the support. Hence, we first studied the influence of the seeding treatment on the final coating. For this part of the study we made use of uncalcined supports to be able to distinguish between the contributions of the gel and of the support. A. Influence of the Seeding Procedure. Quantitative Influence of the Seed Content. The amount of seeding solution added to the feedstock gel was found to be quantitatively related to the amount of crystals finally attached on surface. With an uncalcined stainless steel, a monolayer was not formed, although zeolite crystals were homogeneously dispersed on both sides of the support. An increase of the amount of the seeding solution added to the feedstock resulted in a coherent increase of the number of attached crystals. The amount of supported crystals obtained under various seeding conditions was calculated from the SEM data. This is shown in Figure 1 for seeding mixtures stirred for 24 h (S24) or 48 h (S48). In both cases the density of the crystals on the surface increased by a factor of about 2 when the amount of seeding solution added was doubled (Table 2). For the same amount of seeding solution added, the number of supported crystals was approximately equal regardless of the stirring time of this solution and the final crystal size. Therefore, the presence of nucleating particles in the synthesis gel is the main factor that directs the formation of supported crystals. The relationship between the amounts of seeds added and the coverage highlights the importance of the gel interface on the support and of its initial composition. In an in-situ synthesis, the presence of zeolite crystals on support can be tentatively explained by the deposition of crystals formed in the bulk of the gel. When using an alternative unseeded synthesis mixture that can yield zeolite Y in the powder, hardly any crystals were observed on the support.28 This could be due to a lower number of crystals available. However, in a typical synthesis, as well as in a seeded synthesis, the weight of unattached solid finally obtained was approximately 5.5 g. In the least favorable case, assuming a density of 1.27 g cm-3,29 a crystal size of 350 nm, and a cubic volume for the crystals, the number of crystals present in the autoclave would be definitely greater than 1014, which is much larger than
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Figure 1. Quantitative influence of the seeds: (a) S24W2 (tilt 60°), (b) S24W4 (tilt 60°), (c) S48W2, (d) S48W4. Table 2. Zeolite Y Coverage of Stainless Steel (AISI316L) in Various Seeding Procedures stirring time of the seeds/h seeds formulation supported cryst/cm-2 av cryst size/nm % of a monolayera
24
24
48
48
W2 4.9 × 108 320 50
W4 9.7 × 108 250 61
W2 5.8 × 108 190 21
W4 1.08 × 109 125 17
a As calculated assuming a cubic volume for the crystals and a zeolite density of 1.27 g cm-3.
the amount of attached crystals. As the crystal deposition is not limited by the number of crystals available, it should only be driven by the attraction between the support and these crystals and by possible repulsion between the crystals. According to this hypothesis, crystals would not have bound to the surface when a seeded mixture was used, and variations in the seed content would not affect the coverage, provided zeolite Y would be formed in both cases. Besides, the fact that the same amounts of particles with a substantial difference in size and mass were found on the support (compare W2 and W4 in Table 2) makes the hypothesis of crystal deposition unlikely. Therefore, this hypothesis fails to explain the quantitative influence of the amount of seeds added. By contrast heterogeneous nucleation appears as the most likely. When a seeded synthesis mixture was used, the crystals’ features in the powder and on support, and notably their size dependence on the seeding treatment, showed that they were all grown from the seeds. An experiment was done in which no seeds were added to the synthesis mixture; the support only was dipped in a normal seeded synthesis mixture. XRD of the nonsupported powder showed only amorphous phase and some zeolite P to be present. On the support some P was found in addition to a large number of very large zeolite Y crystals
(1.5 µm). Hence, the crystallization on the support really occurred from the gel layer initially deposited. The fact that zeolite P was also observed on the support and that some Y crystals were seen by SEM of the powder probably means that exchange might occur between the supported gel layer and the synthesis mixture or in the other direction when unstable crystals are detached from the support. This would also explain the large size of the supported zeolite Y crystals, as partial dissolution of the gel layer in the synthesis mixture would lower the number of nucleating particles on the support. It would be attractive to tune the amount of supported crystals by simply increasing the amount of seeding solution added to the feedstock gel. However, for a same number of crystals, the coverage is directly related to the crystal size (Table 2). Moreover, in the case of zeolite Y synthesis, the increase in the seed content also contributes to the increase in the Na2O concentration in the synthesis mixture. When more than 20 wt % Na2O was introduced via the seeding solution, zeolite P also appeared on the support. By contrast, in the case of a more stable zeolite, ZSM-5 for instance, it would probably be beneficial to increase the concentration of seeds in the synthesis mixture. Crystal Size. Figure 2 gives the particle size distribution as calculated from the SEM measurements on the supports obtained from mixtures synthesized under various seeding conditions. We did not find any notable differences in size between the supported crystals and the powder in contrast to the observations of Valtchev and Mintova25 using a copper substrate. For a particular stirring time a decrease of the crystal size was measured when the amount of seeding mixture added to the feedstock gel was increased, as one would expect when the number of nucleating particles is
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Figure 2. Crystal size distribution of zeolite Y attached on stainless steel AISI316L.
Figure 3. Influence of the seeding conditions on the crystal size: (a) effect of the weight added; (b) effect of the stirring time.
increased.30 As can be seen in Figure 3a, the effect on the differences in size is more pronounced for a seeding mixture that has been stirred for a longer time. Another remarkable effect is the crystal size decrease observed when the seeding mixture was stirred for a longer time before adding it to the feedstock gel. From 1 to 48 h the average crystal size decreased nearly by a factor of 2 (Figure 3b). To verify that this effect can be ascribed to the stirring time of the seeds rather than to the longer total aging time, we prepared another seeding mixture by stirring it for 24 h followed by further aging without stirring for 48 h (unlike the other gels). The supported crystals then obtained had exactly the same features as (29) Szostak, R. Handbook of Molecular Sieves; Van Nostrand Reinhold: New York, 1992; p 183. (30) Kacirek, H.; Lechert, H. J. Phys. Chem. 1975, 79, 1589.
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crystals obtained with a normal S24 seeding solution. Upon aging for a longer time, lower amounts of attached crystals and a lower crystallinity of the powder were observed, reflecting a decrease in reactivity of the nucleating particles. When a longer stirring time was applied to the feedstock gel only, no differences could be observed on the crystals. Therefore, crystals derive directly from the seeds. For a short stirring time of the seeds (1 h), a broad crystal size distribution was found, which was undoubtedly due to the lower homogeneity of the solution. When the seeding solution was stirred for a longer time, the particle size distribution was narrow. This means that crystals all grew simultaneously and for the same period and that secondary nucleation occurring in the gel during later stages of the synthesis can be ruled out. This is also consistent with the short synthesis time (7 h) used which makes secondary nucleations unlikely. As a simple stirring applied to the seeding mixture is responsible for the changes in the crystal features, it might contribute to the reduction in size of the seeds. It should also be noted that when the aluminum content of the seeding mixture was lowered, the stirring time had no longer an effect on the final crystal size. Such a seeding mixture was clear after aging, by contrast to the usual seeding mixtures, which were cloudy. On the other hand, when using the low aluminum content, the casually occurring impurities were totally eliminated. Apparently aluminum plays a role in the building of the seeds. Stability. Crystals formed with the present method were found to be firmly bound on the support as they withstood an ultrasonic treatment or a calcination at high temperature. The octahedral crystals were incomplete in shape in the direction of the surface.28 These crystals cannot be formed in the bulk of the synthesis mixture, as nothing in the gel will hamper the crystal growth specifically in one direction. Therefore, the crystal growth of the seeds occurred directly on the support. As on this uncalcined support, the coverage does not reach a complete layer; it can be clearly seen that crystals grow independently from each other. Growth occurs thus “vertically” from the seed to the support and proceeds to the end in the opposite direction, in the gel. Even for larger coverages (Figure 1b), individual crystals are still observed, showing the absence of “horizontal” lateral intergrowth. When no seeds were added to the feedstock gel, zeolite P was the only species obtained, both in the unsupported powder and on the support. With this type of synthesis mixture, and silica source, the presence of nucleating particles is then necessary not only to obtain a coverage on the support but also to get the desired zeolite. This reveals further data on the role of the gel composition at the interface. In the absence of seeds in the synthesis gel, and consequently in the deposited gel layer, the P crystals synthesized were large, 1.5-2 µm in diameter. Unlike what had been seen with zeolite Y, intergrowth of the crystals was observed. The stability was also much lower as the coating remained only partly upon ultrasonic treatment. Furthermore, cracks appeared on the groups of crystals. This was even more obvious when a calcined support was used. A complete and uniform coverage of the support was then achieved. Upon ultrasound treatment cracks appeared in many places on the surface (Figure 4a). Examination of the cross section of the material revealed that the crystals had grown “horizontally”, forming an intergrown layer (Figure 4b). Compared to an uncalcined support, the layer on the calcined support was more stable, probably due to a larger number of crystals bound to the support. However, in any case, at several points and notably on the cracks, the whole layer
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Figure 5. 29Si NMR of the synthesis mixtures: (a) sodium silicate; (b) seeding mixture, (c) feedstock gel.
Figure 4. Zeolite P on steel (AISI302) calcined at 923 K: (a) top view; (b) cross section.
was detached from the support and was only stabilized as crystals were totally intergrown. The specific effect of the support calcination will be discussed later in this paper. Vertical growth thus appears to be a key factor for the stability. By contrast, the horizontal crystal intergrowth, which was observed in the zeolite P system and contributes to a homogeneous layer, also induces new tension-torsion forces which tend to separate it from the support. Seeding with Solid Particles. To determine the nature of the seeds, a seeding was performed with crystallized zeolite Y instead of the usual seeding mixture. Some NaY powder made in a previous synthesis (0.4 µm in size) was added to the feedstock gel in amounts corresponding to the dry weight of the seeding solution. This approach appeared to be unsuccessful as the coverage obtained was very low with particles with a broad range of sizes, and XRD of the powder showed a low crystallinity. On support these crystals were not only unable to act as nucleation sites, but the presynthesized crystals were not attached to the surface either. B. Seed Characterization. The phenomena observed with the crystals used as seeds (see above) and the possibility to tune the final crystal size by the stirring time of the seeding solution raise questions regarding the nature of the seeds and particularly their structure. By quasi-elastic light scattering, no solid particles could be detected in the seeding mixture, which leads to the conclusion that either the particles are below the detection limit (10-3 wt %) or the seeding is just the result of a local ordering in the solution, maybe in the shape of gel aggregates. Liquid-state 29Si NMR spectra of the seeding and feedstock mixtures are shown in Figure 5. Compared to the sodium silicate solution used as a silicon source, the
spectrum of the seeding solution evidenced low intensities of the peaks attributed31 to Q2 (-89.5 ppm), Q3 (-97.8 ppm), and Q4 (-108.3 ppm) and higher relative intensities of the Q0 (-72.4 ppm), Q1 (-80.7 ppm), and Q2∆ (-82.8 ppm). A depolymerization of the silicate species is thus achieved during the seeding process. Unlike what had been observed in solid-state NMR,32 the peaks characteristic of Q4(nAl) were not clearly observed. According to Engelhardt et al.,31 the small peak at -76.3 ppm can be assigned to Q1(1Al). The peak at -82 and the cluster around -90 ppm are probably partly due to Q4(4Al) and Q4(3Al). However, peaks at chemical shifts lower than -95 ppm for Q4(2Al), Q4(1Al), and Q4(0Al) could not be observed. In the seeding mixture aluminate species probably react with the Q0 and Q1 silicon species leading to a specific ordered state. For the feedstock gel, beside the peaks also observed for the seeding mixture, clusters of peaks were observed between -98 and -88 ppm, ascribed to Q3 31 and cyclic Q3∆, 33 respectively, but Q4(2Al) and Q4(1Al) might also contribute to these clusters. In this mixture the number of species is, therefore, much larger than in the seeding mixture. Depolymerization did not occur to such a large extent, probably partly because of the absence of aging time but also due to the lower alkalinity of this mixture.32 In the absence of seeds, this type of species will induce the formation of zeolite P. II. The Support Side of the Interface. The modifications to the gel were shown to have an important influence on the coating features. The final coating is also governed by the spreading of the gel on the support and its interactions with it. Here we deal with the role of the support in the final coating. A. Etching the Metal Surface. After etching successively in aqueous KOH and HCl, the grain boundaries were observed on stainless steel AISI316L. The steel AISI302 was more affected: physical defects (terraces or holes) were observed. However, the Y crystals did not show a specific preferential binding either on the grain boundaries or on other defects. On steel 302 the coverage was slightly better than on 316L, but it never reached a monolayer. As the physical defects did not seem to have much effect on the coating, the differences in composition between the two steels might provoke the difference on (31) Engelhardt, G.; Hoebbel, D.; Tarmak, M.; Samoson, A.; Lippmaa, E. Z. Anorg. Allg. Chem. 1982, 484, 22. (32) Ginter, D. M.; Bell, A. T.; Radke, C. J. In Molecular Sieves; Occelli, M. L., Robson, H. E., Eds.; van Nostrand Reinhold: New York, 1992; p 6. (33) Bell, A. T. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symp. Ser. 398; American Chemical Society: Washington, DC, 1989; p 66.
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Table 3. Influence of the Calcination on the Coverage (Synthesis S24W3 on Steel AISI302) T calcination/K
coverage
density/ g m-3
% of a monolayer a
393 813 923
isolated crystals
