Progress of crystallization into zeolite beta in the absence of steam

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkov~iand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.

343

Progress of crystallization into zeolite beta in the absence of steam S. Inagaki a, K. Nakatsuyama a, E. Kikuchi a'b and M. Matsukata a'b aDepartment of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555 Japan bAdvanced Research Institute of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555 Japan The role of steam on the crystallization of zeolite beta by the steam-assisted crystallization (SAC) method was studied. It was found that after an aluminosilicate dry gel with SIO2/A1203 molar ratio = 100 was treated with steam in an autoclave at 130~ and autogenous pressure, thermal treatment of a resultant product at 130~ without the addition of water in an autoclave gave zeolite beta with a high crystallinity. Zeolite beta obtained by this method consisted of nano-particles with a diameter of ca. 30 nm. It was presumed that intermediates of zeolite beta was formed during steam treatment, and the dehydration between silanols in the intermediates during the thermal treatment in the absence of steam possibly caused the transformation from ill-ordered structures to zeolite beta crystals with a periodicity in long-range order. 1. INTRODUCTION Zeolite beta possessing a 3-dimensional 12-membered ring micropore system was found to a promising catalyst for the cracking of paraffins [1], isomerization of n-heptane [2] and n-hexane [3], and the disproportionation and transalkylation of toluene and C9 aromatics [4]. We have developed a dry gel conversion (DGC) technique, where a silicate or aluminosilicate hydrogel is dried and the resultant dry gel is converted into microporous crystals in vapor. We have proposed classifying the DGC method as two separate methods [1 ]: (1) the vapor-phase transport (VPT) method in which a dry gel is crystallized in the vapor of steam and volatile structure-directing agents (SDA) like ethylenediamine, and (2) the steam-assisted crystallization (SAC) method in which a dry gel containing a non-volatile SDA like tetraethylammonium (TEA) hydroxide is crystallized in steam. Kim et al. [5] and our previous research [6] confirmed that the VPT method could be applied to synthesis of various zeolites such as ANA, FER, MFI and MOR. The VPT method has also been applied to the synthesis of zeolite membranes supported on porous alumina [7] or self-supported [8]. We recently reported that MCM-22 zeolite obtained by the VPT method showed a higher selectivity to p-xylene in the alkylation of toluene with methanol than that obtained by the hydrothermal synthetic method [9]. Zeolite beta can be obtained from a dry gel containing TEA + cation as an SDA by the SAC method [10]. In comparison with the conventional hydrothermal synthetic method, the SAC method enables us to rapidly produce highly crystalline zeolite beta from dry gels with a wide range of SIO2/A1203 ratio from 7 to infinity [ 11-13]. It was remarkable that the contents of Si and A1 atoms in the solid products were almost the same as those in the dry gels [ 11].

344 We investigated the crystallization mechanism of zeolite beta under the SAC conditions on the basis of the results of XRD, FE-SEM and 29Si MAS NMR measurements [12-13], and concluded that the crystallization of zeolite beta from a dry gel in the SAC method occurs via solid intermediates made of nanoparticles [13]. In this study, we found that well-crystallized zeolite beta with a diameter of ca. 30 nm was obtained by the thermal treatment of solid intermediate in the absence of steam. The influence of steam on the crystallization of zeolite beta will be discussed. 2. E X P E R I M E N T A L

2.1. Synthetic procedure of zeolite beta Zeolite beta was crystallized from a dry gel having a composition of SiO2: AleO3: Na20: TEAOH = 1.0: 0.010: 0.030: 0.37. A parent gel was prepared by mixing colloidal silica (Snowtex-S, SiO2, 30 wt%; NaeO, 0.6 wt%, Nissan Chem.), NaOH (>99.0%, Kanto Chem.), anhydrous A12(SO4)3 (>99%, Kanto Chem.) and TEAOH (20% in water, Aldrich). The mixture was heated to 80~ after stirring at 20~ for 2 h and then dried while stirring. The dry gel powder was obtained by crushing the dried solid in an alumina mortar. A dry gel weighing 1.5 g was placed in a Teflon | cup, and the cup was set in 50 cm 3 of a Teflon| stainless autoclave in which 0.50 cm 3 of distilled water was poured at the bottom as a source of steam, as shown in Fig. la. Crystallization was carried out at 130~ and autogenous pressure for 1-24 h. After the crystallization, a solid product was recovered without filtration and rinsing, and dried in a vacuum desiccator at room temperature. The dried product was then treated at 130~ for 12 h in the autoclave without the addition of water, as shown in Fig. lb. The products obtained after the thermal treatment were also kept in a vacuum desiccator at room temperature.

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- - Water Fig. 1. Setups of autoclaves a) for the steam-assisted crystallization method and b) for the thermal treatment used in this study.

2.2. Characterization The dried and thermally treated products were characterized by using X-ray diffractometer (XRD, Miniflex, Rigaku) at 30 kV and 15 mA. The morphologies of the products were observed using field emission scanning electron microscopy (FE-SEM, S-4500S, Hitachi) at 15 kV of ACC voltage. The contents of water in the as-made products were measured in flowing air with a heating rate of 10 ~ min-lusing thermal gravimetry (TG, DTG-50H, Shimadzu).

345

3. RESULTS AND DISCUSSION

3.1. Crystallization of zeolite beta nano-particles in the absence of steam Fig. 2 shows the XRD patterns of the dry gel and the products crystallized at 130~ for different periods from 1 to 24 h by using the SAC method. While zeolite beta was formed after 4 h of crystallization, crystallization prolonged up to 24 h was difficult to give well-crystallized zeolite beta crystals. As shown in Figs. 3b and c, amorphous products obtained by the steaming treatment for 1 and 3 h were transformed into zeolite beta crystals by the thermal treatment at 130~ for 12 h. Fig. 4b shows that zeolite beta obtained by this method was composed of nano-particles with a diameter of ca. 30 nm, whereas the particles size of the dry gel was ca. 1.0 lam, as shown in Fig. 4a. We have previously reported [4] that the dry gel particles with micron-order sizes were transformed to zeolite beta crystals with a size of several hundred nano-meters via intermediates consisting of nano-particles. We considered that whereas the intermediates consisting of nano-particles were formed in the presence of steam, such intermediates were transformed to zeolite beta crystals without fusion of the nano-particles during the thermal treatment in the absence of steam.

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Fig. 2. XRD patterns for (a) the dry gel and (b-g) the as-made products treated at 130~ in the presence of steam. Treatment periods were (b) 1 h, (c) 3 h, (d) 4 h, (e) 8 h, (f) 16 h and (g) 24 h.

346

Figs. 3d-f indicate that ill-crystalline zeolite beta obtained by the steam treatment at 130~ for 8-24 h was converted to well-crystallized zeolite beta by the thermal treatment in the absence of steam. We presumed that removal of water from an ill-crystalline product having a structure resembling zeolite beta by the thermal treatment helps the transformation of the ill-ordered structure to well-crystallized zeolite beta. Fig. 3a shows that an amorphous phase was obtained by the thermal treatment of the dry gel at 130~ without the addition of water in an autoclave. This result strongly suggested that the crystallization of zeolite beta in the SAC method requires a steaming treatment of a dry gel at 130~ to form the intermediates having ill-ordered structures.

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2 0 (CuKcJ.) / degree Fig. 3. XRD patterns for the products obtained by the treatment at 130~ for various periods in the presence of steam and successive treatment at that temperature for 12 h without the addition of water in an autoclave. The periods of steam treatment were (a) 1 h, (b) 3 h, (c) 4 h, (d) 8 h, and (e) 24 h.

347

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Fig. 4. Typical FE-SEM images for (a) the dry gel and (b) as-made products crystallized at 130~ for 3 h in the presence of steam and successively at that temperature for 12 h without the addition of water in an autoclave.

348 3.2. Influence of steam on the crystallization of zeolite beta

As mentioned above, the amorphous product obtained by the steaming treatment of a dry gel at 130~ was transformed to zeolite beta crystals by the thermal treatment at 130~ in the absence of steam. In this section, we discuss the influence of steam on the crystallization of zeolite beta. Fig. 5 shows the XRD patterns for the products obtained by the steaming treatment at 130~ for 3 h and successive thermal treatment at that temperature for various periods. Fig. 5d shows that zeolite beta was formed after 6 h of the thermal treatment, whereas Figs. ga-c indicate that the products up to 4 h were still amorphous. Crystallization of zeolite beta proceeded from 6 to 12 h of the thermal treatment, as shown in Fig. 5d-g. The XRD pattern shown in Fig. 4g suggests the formation of a highly crystalline zeolite beta after 12 h of the thermal treatment. As show in Fig. 6, the content of water in the aluminosilicate gel rapidly decreased from 19 wt% to 9 wt% after 2 h of the thermal treatment, and was permanent at ca. 9 wt% by further thermal treatments. During the thermal treatment, removal of water from an aluminosilicate solid occurred earlier than the formation of zeolite beta. We supposed that the dehydration condensation of silanols in the aluminosilicate solid causes the transformation from the intermediates to highly crystalline zeolite beta by the thermal treatment. Although zeolite beta formed in the steaming treatment at 130~ its crystallinity was still low after 16 h of crystallization, as shown in Fig. 2g. In the steaming conditions, the content of water in the solid increased from 15 wt%, reached a maximum of ca. 28 wt% after 4 h of crystallization, and leveled off at ca. 22 wt% by further steaming treatment, as shown in Fig. 7. We presumed that the existence of a high content of water in the solid obstruct the dehydration condensation of silanols in the solid, resulting in the retardation of the progress of the crystallization of zeolite beta. IIJJl. lllllllJJJ

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Fig. 5. XRD patterns for the products obtained by the treatment at I30~ for 3 h in the presence of steam and successive treatment at that temperature for various periods without the addition of water in an autoclave. The periods of thermal treatment were (b) 2 h, (c) 4 h, (d) 6 h, (e) 8 h, (f) 10 h and (g) 12 h. (a) No thermal treatment was carried out.

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Crystallization period / h Fig. 7. Relative crystallinity (o) and contents of water (o) of the dry gel and products crystallized at 130~ in the presence of steam.

350 4. C O N C L U S I O N S Well-crystallized zeolite beta with a diameter of ca. 30 nm can be crystallized at 130~ without the addition of water in an autoclave from an aluminosilicate solid treated at 130~ for 3 h under the SAC conditions. We supposed that an aluminosilicate solid having an ill-ordered structure can be transformed into zeolite beta crystals by the dehydration of silanols in the solid during the thermal treatment. This method would reduce the production cost of well-crystallized zeolite beta.

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