Synthesis of zeolite beta in fluoride media under microwave irradiation

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1 Microporous and Mesoporous Materials 68 (2004) Synthesis of zeolite beta in fluoride media under microwave irradiation Dae Sung Kim a, Jong-San Chang a, *, Jin-Soo Hwang a, Sang-Eon Park b, *, Ji Man Kim a,1 a Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Taejon , South Korea b Department of Chemistry, Inha University, Incheon , South Korea Received 9 June 2003; received in revised form 28 November 2003; accepted 29 November 2003 Abstract Zeolite beta has been successfully prepared at 150 C within 4 h by direct synthesis under microwave irradiation. Addition of seeds into the synthesis solution under microwave irradiation did not affect overall synthesis time of the material significantly, while addition of ammonium fluoride accelerated the crystallization of zeolite beta. In particular, microwave technique combined with fluoride species and seeding led to more rapid synthesis of crystalline zeolite beta. Upon microwave irradiation fluoride species and the microwave-activated water in the synthesis solution might be ascribed to shortening an induction period at the nucleation step, resulting in the rapid synthesis of the material. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Microwave synthesis; Zeolite beta; Fluoride mineralization; Rapid synthesis; Nucleation and crystallization 1. Introduction Microwave techniques have attracted growing attention for the rapid synthesis of nanoporous materials requiring several days to prepare under hydrothermal conditions [1 14]. Potential advantages include a rapid and more simultaneous nucleation and growth, homogeneous heating throughout the reaction vessel, superheating, and suppression of undesired phases compared with conventional hydrothermal techniques. To date, several types of mesoporous materials as well as microporous zeolites such as zeolite A [1], zeolite Y [2], ZSM-5 [1,3], AlPO-5 [4,5], SAPO-5/SAPO-34 [6], MCM-41 [7 10], SBA-15 [11], SBA-16 [12] etc. have been synthesized by microwave irradiation. Microwave techniques in the synthesis of inorganic materials are * Corresponding authors. Tel.: ; fax: (J.-S. Chang); Tel.: ; fax: (S.-E. Park). addresses: jschang@krict.re.kr (J.-S. Chang), separk@inha. ac.kr (S.-E. Park). 1 Permanent address: Department of Molecular Science and Technology, Ajou University, Suwon , South Korea. generally known to be faster and simpler than conventional methods [15]. Energy transfer from microwaves to the materials is believed to occur either through resonance or relaxation, which results in rapid heating. Furthermore, the employment of the microwave technique in the synthesis of nanoporous materials have been shown to provide versatile effects, for instance, short heating times, inductive heating through the conducting properties of the synthesis mixture, specific energy dissipation via microwave energization of the hydroxylated surface or associated water molecules in the boundary layer, and formation of the high potential of the active water molecules [1,8,13,14]. Zeolite beta (BEA) is a wide-pore and high-silica microporous material where the structure consists of an intergrowth of two or more polymorphs comprising a three-dimensional system of 12-membered ring channels [15 17]. It has recently been recognized as an interesting catalyst for chemical synthesis due to its high thermal and chemical stability, strong acid sites, hydrophobicity and large pore size. One of polymorphs in zeolite beta shows chirality, making it an attractive material from the view of its potential for catalytic conversions and separation of chiral components [18,19] /$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi: /j.micromeso

2 78 D.S. Kim et al. / Microporous and Mesoporous Materials 68 (2004) Zeolite beta is usually obtained by hydrothermal reaction in a period of several days using amorphous silica, colloidal silica sol or tetraethylorthosilicate as the silica source and tetraethylammonium (TEA) or diaza-1,4- bicyclo[2,2,2]octane as the structure-directing agent. In spite of numerous studies on the synthesis of zeolite beta, rapid synthesis has not been achieved so far due to long induction period for nucleation compared with other zeolites. Cundy and co-workers demonstrated effective microwave synthesis of microporous zeolites including zeolite beta [20]. However, at best it took 14 h to synthesize the crystalline zeolite beta even using the microwave method. In this work, we focus on the development of an effective way to provide facile and fast synthesis of zeolite beta through microwave irradiation. In the synthesis of zeolites, fluoride ions are generally known to play a mineralizing role, as does OH in alkaline synthesis conditions [21 23]. In particular, zeolite synthesis in fluoride-containing media has been studied in connection with the possibility to give larger zeolite crystals than those without the fluoride species in the synthesis solution and to increase the ph range over which zeolites may be synthesized. Additionally, several groups have reported a catalytic role of fluoride species for silicate hydrolysis and condensation [24]. We present here another beneficial effect of fluoride in the synthesis of zeolite beta through microwave irradiation. In an attempt to achieve fast synthesis of zeolite beta, we find an efficient and fast route to the synthesis of zeolite beta using a fluoride-containing medium with microwave irradiation. 2. Experimental Zeolite beta has been synthesized in the presence or the absence of seed solution (SS: 0 5 wt%) and fluoride under microwave irradiation. The representative gel composition was 1SiO 2 :0.04Al 2 O 3 :0.034Na 2 O:0.35TEAOH: 0 0.1NH 4 F:11.6H 2 O. Colloidal silica (40 wt%, Aldrich, Ludox AS-40) and tetraethyl ammonium hydroxide (TEAOH, 35 wt%, Aldrich) were homogeneously mixed into distilled water with stirring. In a separate vessel, sodium aluminate was dissolved in distilled water and added dropwise into this mixture with vigorous stirring. NH 4 F was then added and the mixture was stirred for 1 h at ambient temperature. The resulting mixture was loaded in a microwave oven equipped with a Teflon autoclave and was irradiated to a constant temperature of 150 C for 0 20 h under W of microwave power with the microwave synthesis system (CEM Corp., MARS-5). For comparison, zeolite beta with a gel composition as described above was prepared at 150 C for 60 h under hydrothermal condition without the fluoride species and the seed. The resulting solid product was isolated and was dried in air at 100 C for 10 h. To remove the organic species occluded in the pores of zeolite beta, the as-synthesized samples were calcined at 540 C for 6 h in air. The SS was prepared by reaction of 1SiO 2 : 0.04Al 2 O 3 :0.034Na 2 O:0.35TEAOH:0.1NH 4 F:11.6H 2 O at 140 C for 8 h under microwave irradiation. The solid product obtained was identified as the BEA structure having approximately 70 nm size of which amount was calculated as 5 wt% based on the total amount of SiO 2. The prepared samples were characterized by several instrumental analysis techniques. Powder X-ray powder diffraction (XRD) patterns were obtained on a Rigaku diffractometer using CuKa radiation (k ¼ 0:1547 nm). For the calculation of the crystallinity, the intensity of a (3 0 2) reflection centered at d ¼ 4:02 A was compared with that of the reference sample. In this case, the best sample in a series of runs of the same composition was assumed as a reference sample. Field emission scanning electron microscopy (FE-SEM) was performed with a scanning electron microscope (Philips, model XL30S FEG). BET measurements including surface areas and pore volumes were performed using a Micromeritics porosimeter (model ASAP-2400). The samples were degassed at 300 C for 3 h. Thermal properties were analyzed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (Setaram, TGDT92). For these analyses, the sample was placed in the analysis chamber and heated with a ramp rate of 10 C/min up to 700 C in a nitrogen flow (30 ml/min). 3. Results and discussion Growth curves of zeolite beta crystallization detected by XRD reveal the additive effect between fluoride ions and the seed solution under microwave irradiation as shown in Fig. 1 and Table 1. Crystalline zeolite beta is obtained after 14 h under microwave irradiation (Fig. 1a). Under microwave irradiation, the addition of 5 wt% seed solution provides the higher crystallinity at 14 h, but it does not have an influence in the induction period of zeolite beta (Fig. 1b), which is consistent with other group s report [25]. By contrast, the addition of fluoride ions with the molar ratio of F/Si ¼ 0.1 under microwave irradiation gives a shorter induction period compared to seeding alone (Fig. 1c). Moreover, a complete crystallization of the BEA structure is achieved for only 4 h with the combination of fluoride ions and seeding under microwave irradiation (Fig. 1d). Regarding the effect of fluoride ions and seeding, we found the same trend in both microwave and hydrothermal syntheses of zeolite beta. In the absence of fluoride ions and seeding, crystallization of zeolite beta is generally complete after approximately 60 h under hydrothermal conditions, while hydrothermal crystallization of zeolite beta in the presence of fluoride ions and seed nanocrystals is

3 D.S. Kim et al. / Microporous and Mesoporous Materials 68 (2004) Crystallinity/ % (d) (c) Time/ h Fig. 1. The combined effect of fluoride and seeding under microwave irradiation on growth curves of zeolite beta crystals: (a) only microwave irradiation, (b) the addition of 5 wt% seed solution, (c) the addition of fluoride species (molar ratio of F/Si ¼ 0.1), and (d) the addition of fluoride species (molar ratio of F/Si ¼ 0.1) and 5 wt% seed solution. See text for detailed preparation conditions. accomplished after 20 h (not shown). To our knowledge, there has been no report on the effect of fluoride ions to reduce the synthesis time in synthesis of zeolite beta (b) (a) except this work. A summary of physicochemical properties of zeolite beta, obtained with microwave and hydrothermal heating, is listed in Table 1. The surface areas and pore volumes of the resulting materials obtained from microwave irradiation are not so different from those of hydrothermal method. However, the faster synthesis of zeolite beta is accomplished through the combination of fluoride mineralization, seeding, and microwave irradiation. It is noted that this method proposed here provides the fastest synthesis time among the synthesis methods of zeolite beta reported so far. Fig. 2 displays FE-SEM micrographs of zeolite beta crystals obtained with and without fluoride ions under microwave irradiation. These pictures reveal that the microwave method provides the smaller zeolite beta crystals (<0.1 lm) in the presence of fluoride ions than those in the absence of fluoride ions. This effect is opposite to a general observation that fluoride-containing medium in the hydrothermal synthesis of zeolites gives larger zeolite crystals than without the fluoride species [26]. This result suggests that the addition of fluoride leads to the smaller particles of zeolite beta due to an increase of the nucleation rate followed by the shortening of the crystallization time. Fig. 3 shows XRD patterns of as-synthesized zeolite beta obtained by varying the molar ratio of fluoride to silicon in the solution at 150 C for 4 h. XRD results clearly reveal that the fluoride species are necessary for the efficient synthesis of zeolite beta under microwave Table 1 Physicochemical properties of zeolite beta prepared by microwave and hydrothermal methods Sample Method a Crystallization conditions Crystallinity (%) S BET b (m 2 /g) V P b (ml/g) I MW with NH 4 F and seed 150 C/4 h II MW with NH 4 F 150 C/8 h III MW with seed 150 C/14 h IV MW 150 C/14 h V HT 150 C/60 h Crystal size c (lm) a Notation: MW, microwave irradiation; HT, hydrothermal heating; seed, 5 wt% zeolite seed solution based on the total amount of SiO 2 ;NH 4 Fas fluoride species with F/Si ¼ 0.1 (molar ratio). b S BET (BET surface area) and V P (pore volume) data were interpreted by the BJH method. c Average diameter of zeolite beta crystals collected by SEM analysis. Fig. 2. SEM images of the as-synthesized zeolite beta prepared by (a) hydrothermal condition at 150 C for 60 h, (b) only microwave irradiation at 150 C for 14 h, and (c) the addition of fluoride species (molar ratio of F/Si ¼ 0.1) under microwave irradiation at 150 C for 9 h. White scale bar is 0.5 lm.

4 80 D.S. Kim et al. / Microporous and Mesoporous Materials 68 (2004) ,000 0 (d) -10 (b) Intensity/ cps 5,000 0 (c) (b) (a) theta/ degree Fig. 3. XRD patterns of the as-synthesized zeolite beta prepared by microwave method at 150 C for 4 h with the addition of 5 wt% seed solution according to the molar ratio of F/Si: (a) F/Si ¼ 0, (b) F/Si ¼ 0.05, (c) F/Si ¼ 0.1, and (d) F/Si ¼ See text for detailed preparation conditions. Weight loss/ % (b') (a') I II (a) III (b") (a") IV Exo Endo DTA/ a.u DTG/ a.u. irradiation. Faster crystallization of zeolite beta is attained at F/Si ¼ (molar ratio), indicating that it is the optimum composition for the microwave synthesis (Fig. 3c). Thermal analysis curves of zeolite beta obtained with and without fluoride ions under microwave irradiation are presented in Fig. 4. The TGA and its first derivative (DTG) curves of the sample in the absence of fluoride ions exhibit three distinct peaks. These peaks could be generally correlated with three different stages of weight loss in the literature [23,27]: , , and C (Fig. 4a 0 ). The first one is due to the desorption of the occluded water (sequence I). The second one can be addressed to the removal of triethylamine and ethylene formed by degradation of TEA cations according to a Hoffmann elimination reaction (sequence II). The peak of the last zone (sequence III) corresponds to the pyrolysis of TEA cations interacting strongly with the framework containing Al species (sequence IV). The peak intensity in sequence I decreases distinctly in the presence of fluoride ions, indicating the hydrophobic nature of zeolite beta obtained in the fluoride-assisted microwave synthesis. It is noted that the DTG curve (Fig. 4b 0 ) of the sample in the presence of fluoride ions presents additional peaks in the range C (sequence III). These peaks are considered to be correlated with the interaction for charge compensation between TEA þ and F ions in the pore structure [23,28]. According to the literature [28], the presence of these peaks indicates the charge balance effect mediated between TEA cations and fluoride ions, and incorporation of fluoride ions located in small cages within the zeolite framework. In addition, the DTA curve (b 00 ) of zeolite beta, obtained with fluoride ions under microwave Temperature/ o C Fig. 4. TGA, DTG and DTA curves of the as-synthesized zeolite beta obtained from only microwave irradiation at 150 C for 14 h (a, a 0, and a 00 ) and the addition of fluoride species (molar ratio of F/Si ¼ 0.1) and 5 wt% seed solution under microwave irradiation at 150 C for 4 h (b, b 0, and b 00 ). irradiation, has two exothermic peaks at and C, whereas only one exothermic peak in the absence of fluoride (a 00 ) was observed at C. These results indicate that the peak at C might be due to the decomposition of TEA þ interacted with F anions [28]. Previously, Davis and co-workers have demonstrated that the control of the hydrophobic character of the organic structure-directing agents is necessary for the synthesis of zeolites [29]. During the induction period of zeolite beta, it may require a higher energy to replace water molecules by aluminosilicate oligomers for zeolite beta than for ZSM-5 since the hydrophilic interaction between TEA cations and water molecules in zeolite beta is stronger than the interaction between tetrapropyl ammonium (TPA) cations and water molecules in the synthesis of ZSM-5. Therefore, much longer synthesis time for zeolite beta than other high-silica zeolites is required due to this property. In microwave synthesis of high-silica zeolites, it has been reported on strong synergy between microwave energy and the use of seed nanocrystals [3] so that nanometer-sized seed crystals are generally used to reduce the synthesis time. For example, Cundy and co-workers reported that the microwave crystallization of Na-ZSM-5 zeolite with a TPA template at 175 C in the presence of 70 nm seed

5 D.S. Kim et al. / Microporous and Mesoporous Materials 68 (2004) crystals was almost complete within 3 min [3]. However, there was no seeding effect on reduction of the overall synthesis time in the microwave synthesis of zeolite beta, indicating that the presence of seed crystals in the synthesis mixture do not have influence in nucleation significantly. This effect is probably ascribed to unusually long induction period in zeolite beta compared with other zeolites. Instead, the addition of fluoride ions led to suppression of an induction period and made crystallization process faster, leading to the shortening of the overall synthesis time. Moreover, the faster synthesis of zeolite beta is accomplished through the combination of fluoride mineralization and seeding. It can be concluded that fluoride ions play a crucial role in shortening an induction period at the nucleation step as well as the crystallization step in the synthesis of zeolite beta. It is noted that seed nanocrystals can play a role of synergistic action only in the presence of fluoride ions for the faster synthesis of zeolite beta although the reason to exhibit this behavior is not clear yet. It is assumed that the effect of fluoride mineralization in the synthesis of zeolite beta is due to weakening of hydrogen bonding for hydrated aluminosilicate precursors. Thus, taking into account the relative strength of hydrogen bonding for F H versus O H at induction period, it is expected that F H interaction of the hydrated aluminosilicate oligomers is much weaker than the O H interaction in the absence of fluoride ions, [SiOH OH 2 ] [30]. Hence, fluoride ions in the synthesis of zeolite beta appear to give the effective nuclei formation through its hydrophobic hydration. Koller et al. [22a] has reported that F anions occluded into highsilica zeolite framework accelerate the formation of Si O Si bond as the weakness of the extensive formation on the strong [Si O ] [HO Si] defect. Water molecules strongly absorb microwave energy due to their high dielectric constant, yielding so-called active water with high mobility [2]. In other words, the microwave irradiation destroys the hydrogen bridges of the water molecules by ion oscillation and water dipole rotation, and produce the active water [31]. In microwave synthesis of zeolites, the reduction in the crystallization period was generally considered to result from the high potential of the active water to dissolve the gel constituents under microwave irradiation [2]. The lone pair of a hydroxyl group of the active water molecules was reported to have higher potential to dissolve the gel constituents than normal water [32]. Considering the role of fluoride and active water, the nucleation step of the fluoride-modified zeolite beta appears to be facilitated by rapid replacement of water molecules with TEA cations due to promoted hydrophobic hydration [29] under microwave irradiation, resulting in the suppressed induction period. This step is believed to be greatly accelerated with the superheating effect of microwaves under fluoride mineralization and seeding. 4. Conclusions This work shows that zeolite beta has been rapidly synthesized by microwave method with the help of fluoride species. Upon microwave irradiation, the shortened induction period at the nucleation step with the addition of fluoride species is found to be crucial for rapid synthesis of this material. Fluoride mineralization under microwave and seeding played a role to reduce the particle size due to higher nucleation, allowing the rapid synthesis of the material under microwave energy. 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