Synthesis of all-silica BEA-type material in basic media

Microporous and Mesoporous Materials 93 (2006) 55–61 www.elsevier.com/locate/micromeso

Synthesis of all-silica BEA-type material in basic media Olivier Larlus, Valentin Valtchev

*

Laboratoire de Mate´riaux a` Porosite´ Controˆle´e, UMR-7016 CNRS, ENSCMu, UHA, 3 rue Alfred Werner, Haute Alscae, 68093 Mulhouse, France Received 16 November 2005; received in revised form 30 January 2006; accepted 1 February 2006 Available online 20 March 2006

Abstract Previous investigations have shown that an all-silica BEA-type (Si-BEA) material is difficult to be nucleated in basic media and that the fluorosilicic precursors formed in near neutral media play an important role in its formation. The present study reports on the synthesis of this microporous material under basic conditions by using TEA2SiF6 as an additional silica source that provides preformed fluorosilicate species. The effect of the amount of TEA2SiF6 on the purity of the final product, crystallization kinetics and morphological features of Si-BEA were systematically studied. The investigation showed that above a certain concentration corresponding to TEA2SiF6/SiO2 = 0.07 pure Si-BEA was obtained and further increase in the concentration improved substantially the crystallization kinetics. The increase in TEA2SiF6 content led also to a gradual increase of the size of the pyramidal (h 0 l) face coupled with the decrease of pinacoidal (0 0 1) one. Thus, the morphology of the zeolite crystals can be varied from well developed truncated bipyramids to plate-like crystals with dominating pinacoidal faces. A consequence of this morphological changes is the number the pore openings of the channel systems running along the a- and c-axes per total external surface. The framework defectness and water content in the synthesized materials were studied by 29Si NMR spectroscopy and TG analysis, respectively. This combined study revealed that materials with a high level of hydrophobicity, due to the specific effect of fluorine anions, have been synthesized under basic conditions.  2006 Elsevier Inc. All rights reserved. Keywords: Zeolite; Synthesis; Si-BEA; Basic media

1. Introduction The unique properties of microporous zeolite-type materials have attracted much attention from both academic and industrial points of view [1–4]. The control of their properties and the development of new applications is closely related with the synthetic methods employed for their production. Hence, the synthesis of zeolitic materials over the past decades was comprehensively studied and new approaches have been developed. Industrially important zeolites, for instance the MFI-, BEA-, FAU-, MOR-types, were amongst the most widely studied and thus new formulations or synthesis conditions were developed. Great potential of BEA-type framework topology in the area of catalysis [5,6], separations and new advanced *

Corresponding author. Tel.: +33 3 89 33 67 08; fax: +33 3 89 33 68 85. E-mail addresses: [email protected], [email protected] (V. Valtchev). 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.02.003

applications as for instance the low-k films [7,8], makes the zeolite particularly interesting from an application point of view. The BEA-type framework has a relatively ˚ 3) and a threelow framework density (15.1 T per 1000 A dimensional large pore system that make the material particularly attractive when relatively bulky molecules are processed [9]. The Si/Al framework ratio can be easily controlled in the range 6.5–100, thus providing a material with a wide variation in the concentration of the active sites [10,11]. The extension of the Si/Al framework composition over 100 is relatively difficult in basic media and usually seeding is required. For instance, van Bekkum and coworkers successfully obtained an aluminum-free Ti-beta form of the zeolite by using seeds and a specific template (di(cyclohexylmethyl)dimethylammonium) [12]. Recently, the synthesis of all-silica BEA-type nanocrystals using the conventional structure-directing agent (tetraethylammonium) was reported [13]. The latter investigation, however, was essentially devoted to the synthesis of thin films and no

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information about the factors controlling the formation of the BEA-type material was provided. Thus, the synthesis of very high silica (Si/Al > 100) and all-silica BEA-type materials under the conventional basic conditions for zeolite synthesis is far from being well controlled. The zeolite yielding systems are fairly complex, they contain a large diversity of aluminosilicate species, solvent, structure directing and mineralizing agents. The nucleation and crystal growth depend on the balance between these components and include numerous simultaneous equilibrium and condensation steps [2,14,15]. Amongst these species the role of the mineralizing agent consists in the dissolution and transport of silicate precursors. Consequently the concentration of (alumino-)silicate species in the solution and thus the supersaturation depends on the concentration and nature of the mineralizing agent. The mineralizing agents commonly used in the zeolite syntheses are the fluoride and hydroxide anions. Although, the function of these anions in zeolite formation is similar, substantial differences for the microporous crystalline materials produced in hydroxide and fluoride media have been observed [16–18]. In general, the zeolite crystals resulting from fluoride syntheses are larger in size and contain less framework defects in respect to corresponding products obtained in hydroxide media. Such crystals exhibit higher hydrophobicity and increased diffusion limitations than the smaller particles obtained in basic media. These features of the fluoride syntheses are due to the relatively low supersaturation degree and a consequence of limited nucleation that provides smaller crystals. Besides, the physico-chemical properties, a substantial difference in crystallization time using these mineralizing agents have been observed, that is, the fluoride route requires a much longer crystallization time. Another specificity of fluorine mediated syntheses is the specific interactions between the silica precursors and F anions that make possible the formation of some materials only in such media. For instance, the all silica BEAtype material readily crystallizes when fluoride is used as mineralizing agent [19], whereas its formation in hydroxide media is difficult, as mentioned above. Recently, we have used this specificity of fluorosilicate species to synthesize an all-silica BEA-type material in basic media, which was nucleated under near neutral conditions [20]. In other words, a two-step synthesis comprising formation of the fluorosilicate gels under near neutral conditions, followed by the increase of the pH and synthesis under basic conditions was developed. This approach allowed the synthesis of highly crystalline BEA-type products in basic media and was further used to control the morphology of the zeolite Beta crystals. This investigation revealed the paramount importance of fluorosilicate precursors for the nucleation of the all-silica BEA-type material. The aim of the present investigation was the nucleation and growth of Si-BEA in basic media without using a preliminary step. In order to achieve the goal, a silica precursor comprising fluorosilicic species was employed. Amongst the goals of the investigation were also the

control of the morphology and the study of the physicochemical properties of the Si-BEA material synthesized in basic media. 2. Experimental section 2.1. Synthesis The reactants used in the present investigation were pyrogenic silica (Aerosil 130, Degussa), tetraethylammonium fluoride dihydrate (TEAF Æ 2H2O, Aldrich), tetraethylammonium hydroxide (TEAOH 40% in water, Fluka), potassium hydroxide (KOH 86%, Fluka), ammonium hexafluorosilicate ((NH4)2SiF6, 98%, Aldrich) and distilled water. The TEA2SiF6 solution was prepared by dissolving (NH4)2SiF6 in TEAOH under stirring at room temperature. In order to remove the remaining traces of ammonia, the solution was treated three times at room temperature in a low pressure (0.2 atm) chamber for 5 min. After each treatment the gel was stirred for 5 min. Further, the as-prepared TEA2SiF6, which was employed as an additional silica source, was added to the clear solution prepared by dissolving the pyrogenic silica into a TEAOH or TEAOH/KOH solution. The gel was outgassed for 5 min under vacuum and aged at 25 C for 24 h. The hydrothermal syntheses were performed at 150 C in a 20 mL Teflonlined autoclave. 2.2. Characterization The materials were characterized by powder X-ray diffraction (XRD) with a STOE STADI-P diffractometer in Debye–Scherrer geometry equipped with a linear position-sensitive detector (6 in 2h) and employing Ge monochromated Cu Ka1 radiation. Micrographs of the samples were taken with a Philips XL 30 LaB6 scanning electron microscope (SEM). The thermogravimetrical analysis of the samples was performed with a Setaram TG-ATD LABSYS thermal analyzer at a heating rate of 5 C min 1 in an atmosphere containing 80% N2 and 20% O2. 29Si NMR spectra were collected with a MSL 300 Brucker spectrometer at a resonance frequency of 59.6 MHz and magic angle spinning at 4 KHz. The contact time was 4.4 ls for both the as-synthesized and the calcined samples. The recycle times were 10 and 80 s for the as-synthesized and the calcined samples, respectively. 19F NMR experiments were carried out on a DSX 400 Brucker spectrometer at a resonance frequency of 376.3 MHz. Magic angle spinning was performed at 25 KHz, with 4 ls of contact time and 15 s of recycle time. The elemental analysis of the solids was performed with a X-ray fluorescence spectrometer MagiX (Philips). 3. Results and discussion Different synthesis formulations were used in order to study the effect of basicity, TEA2SiF6 and potassium

O. Larlus, V. Valtchev / Microporous and Mesoporous Materials 93 (2006) 55–61 Table 1 Variations in the composition of the initial system 1.0SiO2:xTEAOH: tKOH:yTEA2SiF6:zH2O, synthesis time and materials obtained in the efforts to synthesize all-silica BEA-type zeolite in basic media Sample

Molar composition x

t

y

z

Synthesis time (days)

Structure type

1 2 3

0.30 0.30 0.30

0.00 0.00 0.00

0.05 0.07 0.09

6.50 7.15 7.80

21 13 9

BEA + MFI BEA BEA

4 5 6

0.40 0.40 0.40

0.00 0.00 0.00

0.05 0.07 0.09

6.50 7.15 7.80

23 11 7

BEA + traces of MFI BEA BEA

7 8 9

0.35 0.35 0.35

0.05 0.05 0.05

0.05 0.07 0.09

6.50 7.15 7.80

17 10 7

BEA + traces of MFI BEA BEA

10 11 12

0.30 0.30 0.30

0.10 0.10 0.10

0.05 0.07 0.09

6.50 7.15 7.80

21 21 7

BEA + MFI BEA BEA

content on the formation of Si-BEA and the kinetics of its growth (Table 1). The first parameter studied was the effect of basicity since a destruction of silicon–fluoride bonds could be expected at high pH. Thus, two systems with OH /Si ratios of 0.3 and 0.4 were employed and the content of TEA2SiF6 was systematically varied. After hydrothermal treatment the pH values of the supernatant solution was about 11.5 and 12.5, respectively. No pH measurements were performed prior to the synthesis because of the high density of initial gel. The two series of experiments did not reveal any specific effect of the OH concentration, whereas the TEA2SiF6 content influenced strongly both the material obtained and the growth kinetics. At low TEA2SiF6 content (Table 1, samples 1 and 4) BEA- and MFI-type materials were obtained, however Si-BEA was dominant in both systems. The increase of TEA2SiF6 content provided pure highly crystalline BEA-type material (Fig. 1) and reduced substantially the crystallization time.

Fig. 1. XRD pattern of samples: (a) 1, (b) 2 and (c) 3 (Table 1) synthesized from an initial system with TEA2SiF6/SiO2 ratio of 0.05, 0.07 and 0.09, respectively.

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This result proves the importance of fluorosilicic species in the formation of Si-BEA. It should also be mentioned that the experiment performed without the addition of TEA2SiF6 yielded a product where the MFI-type material was the dominant phase. All these evidences confirm the predominant role of the fluorosilicic species for the formation of Si-BEA. The basicity of the system in these two series of experiments was controlled by the TEAOH content, which modifies the TEA/SiO2 ratio as well. The TEA content in the systems largely exceeds the stoichiometric amount required. Nevertheless, two series of experiments where different amounts of TEA+ substituted by K+, were performed in order to verify whether the organic structure directing agent influenced the synthesis. As can be seen in Table 1 (experiments 7–9 and 10–12) the total concentration of TEA did not influence the type of crystallizing product. Again the formation of a mixture of BEA- and MFI-type materials at low TEA2SiF6 content (TEA2SiF6/ Si = 0.05) and pure BEA-type material at higher TEA2SiF6 concentration was observed. Experiments 10 and 11 showed a substantial increase of the crystallization time (21 days) in respect to the corresponding experiments in the other series. The experiments were repeated to verify this result and again completely crystalline materials was obtained after three weeks hydrothermal treatment. The prolongation of the crystallization time could be attributed to the structure-breaking role of potassium. In a previous study we have found that potassium influence both the crystallization time and morphology of Si-BEA crystals [20]. These effects were attributed rather to the physical changes in the gel provoked by the potassium than to it structure-directing properties, which is in agreement with other previous investigations that did not find a specific structure-directing effect of potassium in the formation of zeolite beta [21,22]. Crystal growth kinetics of BEA-type material in the present study seems to be influenced by both the total amount of potassium and its aspect ratio with TEA2SiF6. Thus, the crystallization time for samples 10 and 11 was much longer in respect to 7 and 8, respectively. On the other hand sample 12 crystallized within 7 days, most probably due the higher TEA2SiF6 content that compensated the negative influence of potassium. Summarizing these data, it may be concluded that the use of preformed fluorosilicic species in the synthesis of all-silica BEA-type has a pronounced structure-directing effect. Thus, highly crystalline Si-BEA was synthesized in basic media for relatively short periods of time. Further, the effect of this synthetic approach on the physico-chemical properties was investigated. The size and morphology of zeolite crystals may influence strongly their performance since these characteristics are related to the accessibility and the transport via the intra-crystalline volume. The morphological features are particularly important for zeolites possessing different channel systems which, depending on the crystal morphology, could be present at different extents at the crystal

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Fig. 2. SEM pictures of typical zeolite beta crystals with: (a) a schematic presentation of the channel system drawn as it is oriented within a crystal; and (b) the crystal edges used to evaluate the aspect ratio between the prismatic and pinacoidal faces.

surface. The BEA-type framework topology possesses two type of twelve-membered rings (12 MRs) channels, with ˚ and 5.6 · 5.6 A ˚ openings, running along size 7.3 · 7.1 A the a- and c-axes, respectively [9]. Typically zeolite beta forms bipyramidal crystals with well developed pyramidal faces and a smaller pinacoidal one (Fig. 2). Consequently, the system parallel to the a-axis is more extended on the crystal surface. In the present study the morphological evolution was followed by measuring the basal (a) and terminal (b) edges and the height (h) of the pyramidal face (Fig. 2(b)). Representative values for different crystal dimensions were obtained by SEM measurements on about 30 crystals of each sample. The morphological evolution of the crystals is best presented by the b/a ratio, which is related with the aspect ratio between the pyramidal and pinacoidal faces. Only materials synthesized from systems with similar basicity (OH /SiO2 = 0.4) were considered in this study (Table 2). As can be seen, all syntheses provided relatively large crystals with the basal edge ranging between 9 and 14 lm, which is typical of the fluoride mediated synthesis. This result is somewhat surprising having in mind that the system is alkaline and thus an abundant nucleation and reduction of overall crystal size could be expected. The absence of such an effect is most probably related to the nucleation-promoting function of fluorosilicate species that control the number of viable nuclei in the system. Besides the nucleation, these species seem to control the growth rate of different faces since substantial variations

Table 2 Variation of the different crystal faces, their aspect ratio and growth rate ratio of the pyramidal (Vpyr) and pinacoidal (Vpin) surfaces Sample

a (lm)

b (lm)

h (lm)

b/a ratio

Vpyr/Vpin ratio

4 5 6 7 8 9 10 11 12

9.3 11.2 10.8 11.8 10.2 9.5 11.8 14.1 13.1

6.3 7.6 5.6 8.6 3.7 3.4 8.8 8.0 3.7

3.1 4.2 7.1 4.1 8.5 8.1 2.7 7.2 11.4

0.68 0.68 0.52 0.73 0.37 0.36 0.74 0.57 0.28

7.3 6.5 3.7 7.0 2.9 2.8 10.6 4.8 2.8

of the b/a ratio were observed (Table 2). In particular, the increase of the TEA2SiF6 content leads to a significant development of the pyramidal face and respectively reduction of pinacoidal one, thus providing materials with a b/a ratio varying from 0.68 to 0.52. This effect was more pronounced in the potassium-containing system, where the b/a factor changed from 0.73 to 0.36 (Experiments 7–9) and progressed further (from 0.74 to 0.28) with the increase of potassium content in the system (Experiments 10–12). This spectacular increase of the pyramidal faces is obviously due to the joined action of the potassium and the fluorosilicic species provided by the TEA2SiF6 source (Fig. 3). Thus two extreme morphologies, plate-like crystals and bipyramids with pinacoidal faces reduced to minimum, were obtained (Fig. 3(a) and (c)). In the first case the pore ˚ ) dominates the crystal system along the c-axis (5.6 · 5.6 A surface, whereas the second provides crystal surfaces largely dominated by the parallel to a channel system ˚ ). (7.3 · 7.1 A It is worth recalling that the crystal morphology of a particular crystalline material is a function of the growth rate of different crystal faces [23]. Thus, faces with a high growth rate are less present or completely absent in the ultimate crystals morphology, while the ones with slow growth rate are well developed. In the present study the growth rates (in lm/day) of the pyramidal (Vpyr) and pinacoidal (Vpin) faces were deduced by dividing the ultimate face size by the crystallization time. This approach is not very precise because it includes both the nucleation and crystallization time. Nevertheless, it provides reliable information about the differences in the growth rates of different faces that is summarized as Vpyr/Vpin ratio in Table 2. As can be seen, in all considered systems this ratio decreases with the increase of the TEA2SiF6 content. In other words, the increase of the concentration of fluorosilicic species leads to a substantial increase in the growth rate of the pinacoidal face. The growth rate was further accelerated by the presence of potassium in the system, as mentioned above. Recently, we have described the specific function of potassium in the formation of Si-BEA nucleated in near neutral conditions and crystallized in basic media [20]. In this case, potassium provided a second generation of smaller crystals that, as mentioned above, was related with its specific effect

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59

Fig. 3. Morphological changes of all-silica BEA-type materials as a function of the TEA2SiF6 concentration in the initial gel: (a) sample 10 (0.05TEA2SiF6); (b) sample 11 (0.07TEA2SiF6); and (c) sample 12 (0.09TEA2SiF6) and schematic illustration of the development of the pyramidal face with increase of the TEA2SiF6 content in the system (bottom).

on the physical features of initial gel. Under the conditions used in the present study, that is, direct nucleation in basic media with a specific fluorosilicic source, potassium seems to play a different role since a monomodal particle size distribution was obtained. The chemical analysis revealed that potassium content in the crystals is strongly related to the TEA2SiF6 concentration in the initial gel, i.e., the increase of the TEA2SiF6 content led to a decrease in the incorporation of potassium in the crystals. Thus, samples 9 and 12 (Table 1) synthesized with TEA2SiF6/SiO2 ratio of 0.05, 0.07 and 0.09 contained 0.045, 0.034 and 0.027 potassium atoms per unit cell, respectively. The potassium content in the materials synthesized in Experiments 7–9 was even lower, ranging 0.01–0.02 potassium atoms per unit cell. The low potassium content indicates a relatively low level of framework defectness that was confirmed by combined NMR/TG analysis. It is worth mentioning also that the presence of potassium in the initial system did not change the type of crystallizing solids. Again, at low TEA2SiF6/SiO2 ratio (0.05) a mixture of BEA- and MFI-type materials was obtained (Fig. 4(a)). The increase of TEA2SiF6 content (samples 11 and 12) provided pure highly crystalline Si-BEA (Fig. 4(b) and (c)). It is well known that the partial hydrophilicity of all-silica microporous materials is related to the imperfection of the zeolite framework. As already mentioned, the crystals synthesised in fluoride media under near neutral conditions are almost defect-free and thus highly hydrophobic. The materials in the present investigation were synthesized under fairly specific conditions, i.e., basic media and in the presence of fluorine ions, whose combined impact on the hydrophilic/hydrophobic properties is difficult to predict. In addition, K+ which was found to lead to structural defects in the BEA-type material [20], was employed in some syntheses. Therefore, Si-BEA samples provided by different synthesis formulations were subjected to 29Siand 19F-solid state NMR and TG investigations in order

Fig. 4. XRD pattern of samples: (a) 10, (b) 11 and (c) 12 (Table 1) synthesized in the presence of potassium from an initial system with TEA2SiF6/SiO2 ratio of 0.05, 0.07 and 0.09, respectively.

to study the characteristic features of BEA-type materials synthesized under these specific conditions. The 29Si NMR spectra of the as-synthesized forms of samples 6, 9 and 12 are shown in Fig. 5(a). In addition to the well established peaks between 100 and 116 ppm assigned to 4Q sites, a weak shoulder between 95 and 100 ppm characteristic of 3Q sites, is visible. The area of this shoulder increases with the potassium concentration in the system (sample 12, Table 1) and reveals the presence of a few structural defects. The increase of potassium content was confirmed by the chemical analysis, but as discussed above its amount was below 0.05 K per unit cell. This material, which is potentially the most hydrophilic, was subjected to TG analysis to evaluate the water content. The weight loss in the temperature range 25–200 C was below 1.0 wt.% for the as-synthesized material (Fig. 6), whereas the calcined and re-hydrated material showed even a lower (0.4 wt.%) weight loss. These data clearly show that

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-80

(a)

-90

-100 29

-110

-120

-130

Si /ppm from TMS

-100

-105

-110 29

(b)

-115

-120

-125

Si /ppm from TMS

Fig. 5. 29Si NMR spectra of: (a) as-synthesized and (b) calcined materials obtained from the systems with TEA2SiF6/SiO2 = 0.09. From bottom to top: sample 6 (0.4TEAOH); sample 9 (0.35TEAOH + 0.05KOH); and sample 12 (0.30TEAOH + 0.10KOH).

* *

Fig. 6. TG/DTG analysis of an as-synthesized potassium-containing SiBEA material (sample 12, Table 1).

40

20

0

* *

-20 19F

relatively highly hydrophobic BEA-type materials were obtained in basic media. The 29Si spectra of calcined samples were in agreement with this conclusion, they exhibit peaks between 109 and 117 ppm that can only be assigned to 4Q sites (Fig. 5(b)). Gel preparation includes mixing of TEA2SiF6 with a highly alkaline solution that might easily hydrolyse the hexafluorosilicate precursor. Hence, it is interesting to study the behaviour of the fluorine under such conditions. In particular, whether the fluorine is incorporated in the zeolite structure as can been observed in the synthesis performed under near neutral conditions [24,25]. The same series of as-synthesized samples was studied by 19F NMR and the corresponding spectra are shown in Fig. 7. All three spectra showed distinctive peaks evidencing high fluorine incorporation in the BEA-type material. The peaks at 58, 65, and 70 ppm were attributed to fluorine into [4354] cages [24,25]. In addition, the material synthesized in

-40

-60

-80

-100 -120 -140

/ppm from CFCl3

Fig. 7. 19F NMR spectra of as-synthesized materials obtained from the system with the higher TEA2SiF6 concentration. From bottom to top: sample 6 (0.4TEAOH); sample 12 (0.30TEAOH + 0.1KOH) and sample 9 (0.35TEAOH + 0.05KOH) (the spinning bands are marked by asterisk).

potassium-free system possesses a peak ( 38 ppm) that might be attributed to F in a D4R cage. It should be mentioned that such a peak was observed in Si-BEA materials nucleated under near neutral and then synthesized in basic media [20]. Such a framework unit is typical of polymorph C (BEC-type), whose presence in the material could not be completely excluded [26]. However, neither in the previous study nor in the present investigations this suggestion was supported by XRD evidence. Obviously the phase leading to this peak was present in an amount below the detection limit of the XRD technique. On the other hand, the results unambiguously show the incorporation of the fluoride anions in the zeolite structure which explains the low

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hydrophilicity of the synthesized materials. In other words, the incorporated F anions counterbalance the TEA+ cations and thus limit the interaction of the structure-directing agent with the framework. The most significant result is that F incorporation was achieved under basic conditions and thus the range of fluorine-based synthesis substantially expanded. 4. Conclusions All-silica BEA-type materials were synthesized in basic media by the addition of a co-silica source (TEA2SiF6) to the initial gel. The role of this silica source was to provide preformed fluorosilicate species that over a given concentration induced the formation of a pure BEA-type material. Thus, for TEA2SiF6/SiO2 = 0.05, a mixture of BEA- and MFI-type materials was obtained, when this ratio was increased to 0.07 pure Si-BEA was obtained. A further increase of TEA2SiF6 concentration (TEA2SiF6/SiO2 = 0.09) led to faster formation of Si-BEA. These data show that TEA2SiF6 prepared by mixing stoichiometric amounts of ammonium hexafluorosilicate and tetraethylammonium hydroxide provides fluorosilic species that are sufficiently stable in basic media and can induce the Si-BEA nucleation. These species play also a certain role in the crystal growth as revealed by the morphological changes observed for the synthesized materials, i.e., the increase of the fluoride content in the gel leads to a preferential growth of the pinacoidal face and as a consequence to a largely developed pyramidal one. Thus, a series of morphological types starting from plate-like crystals with well developed pinacoidal faces, via those comprising equally developed pyramidal and pinacoidal faces, to bipyramidal crystals where the pinacoidal face was almost absent, were synthesized. The consequence of these morphological varieties is the number of pore openings per unit external surface of parallel to aand c-axes channel systems. The framework imperfection of Si-BEA synthesized in basic media was studied by 29Si NMR combined with TG analysis of the water content of the materials. The obtained data revealed the high degree of hydrophobicity of the synthesized materials, which was attributed to the incorporation of fluorine in the zeolite structure, the latter being proven by the 19F NMR analysis.

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The developed synthesis route might be useful in the synthesis of other types of highly hydrophobic zeolitic materials. References [1] D.W. Breck, Zeolite Molecular Sieves, John Wiley and Sons, New York, 1974. [2] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. [3] R. Szostak, Handbook of Molecular Sieves, Van Nostrand Reinhold, New York, 1984. [4] H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Studies in Surface Science and Catalysis, vol. 137, Elsevier, Amsterdam, 2001. [5] A. Corma, M.E. Domine, S. Valencia, J. Catal. 215 (2003) 294. [6] J.F. How, Phys. Chem. Chem. Phys. 4 (2002) 5431. [7] Z. Wang, H. Wang, A. Mitra, L. Huang, Y. Yan, Adv. Mater. 13 (2001) 746. [8] S. Mintova, T. Bein, Adv. Mater. 13 (2001) 1880. [9] Ch. Baerlocher, W.M. Meier, D.H. Olson, Atlas of Zeolite Framework Types, fifth revised edition, Elsevier, Amsterdam, 2001. [10] R.L. Wadlinger, G.T. Kerr, E.J. Rossinski, US Patent 3308069, 1967. [11] R.B. Borade, A. Clearfield, Micropor. Mater. 2 (1994) 167. [12] J.C. van der Waal, P.J. Kooyman, J.C. Jansen, H. van Bekkum, Micropor. Mesopor. Mater. 25 (1998) 43. [13] S. Mintova, M. Reinelt, T.H. Metzger, J. Senker, T. Bein, Chem. Commun. (2003) 326. [14] D.P. Serano, P. van Grieken, J. Mater. Chem. 11 (2001) 2391. [15] C.S. Cundy, P.A. Cox, Micropor. Mesopor. Mater. 82 (2005) 1. [16] J.L. Guth, in: R. Aiello (Ed.), Proc. Third Convegno Nazionale di Scienza e Technology delle Zeoliti, Cetrano, Italy, September 28–29, 1995, p. 13. [17] J.L. Guth, H. Kessler, in: J. Weitkamp, L. Puppe (Eds.), Catalysis and Zeolites, Springer, Berlin, 1999, p. 1. [18] M.A. Camblor, A. Corma, S. Valencia, J. Mater. Chem. 21 (1998) 2137. [19] M. Camblor, A. Corma, S. Valencia, Chem. Commun. (1996) 2365. [20] O. Larlus, V.P. Valtchev, Chem. Mater. 17 (2005) 881. [21] R. Mostowicz, F. Testa, F. Crea, R. Aiello, A. Fonseca, J.B. Nagy, Zeolites 18 (1997) 308. [22] M.A. Camblor, J. Perez-Parienter, Zeolites 11 (1991) 202. [23] B. Mutafchiev, The Atomic Nature of Crystal Growth, Springer, Berlin, 2001. [24] M.A. Camblor, P.A. Barrett, M.-J. Fiaz-Cabanas, L.A. Villaescusa, M. Puche, T. Boix, E. Perez, H. Koller, Micropor. Mesopor. Mater. 48 (2001) 11. [25] L.A. Villaescusa, P.A. Barrett, M.A. Camblor, Chem. Mater. 10 (1998) 3966. [26] A. Corma, M.T. Navarro, F. Rey, J. Rius, S. Valencia, Angew. Chem. Int. Ed. 40 (2001) 2277.