Crystallization Behavior of Zeolite Beta in OSDA-Free, Seed-Assisted

Dec 27, 2010 - In this study, the crystallization process of zeolite beta in the OSDA-free, seed- embedded Na. +-aluminosilicate gel system, which nev...
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J. Phys. Chem. C 2011, 115, 744–750

Crystallization Behavior of Zeolite Beta in OSDA-Free, Seed-Assisted Synthesis Yoshihiro Kamimura,† Shinya Tanahashi,† Keiji Itabashi,† Ayae Sugawara,† Toru Wakihara,‡ Atsushi Shimojima,† and Tatsuya Okubo*,† Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed: October 15, 2010; ReVised Manuscript ReceiVed: NoVember 26, 2010

Recent reports on the organic structure-directing agent (OSDA)-free synthesis of some zeolites with the aid of seed crystals have opened a new way to the robust and environmentally friendly production of industrially valuable zeolites. However, the details on the crystallization behavior as well as the role of the seeds have not been fully clarified yet. In this study, the crystallization process of zeolite beta in the OSDA-free, seedembedded Na+-aluminosilicate gel system, which never yields beta in the absence of the seeds, is investigated in detail. The XRD and TEM studies of the solid aluminosilicate products in the course of the hydrothermal treatment suggest that the crystallization of zeolite beta proceeds on the outer surface of amorphous aluminosilicates. The Raman spectroscopy, solid-state 27Al and 23Na MAS NMR and high-energy XRD analyses of seeded and nonseeded amorphous materials just before crystallization reveal that the beta seeds induce no major changes in their structures, implying that the nucleation of beta does not occur directly from the amorphous phase. The intermediate addition of the seeds after prehydrothermal treatment of a nonseeded gel enhances the crystallization rate and results in the increased number of beta crystals with smaller size. It is elucidated that, during the hydrothermal treatment, the beta seeds embedded in the gel provide crystal growth surface after they are exposed and/or released to the liquid phase by partial dissolution of the amorphous aluminosilicates. These findings provide a promising approach to the designed syntheses of valuable zeolites in the completely OSDA-free system. Introduction Zeolites are crystalline microporous aluminosilicates that have been widely used as catalysts, ion-exchangers, and adsorbents because of their well-defined micropores, large surface areas, high acidities, and high hydrothermal stabilities.1-4 Zeolite beta is one of the most industrially important zeolites because of its three-dimensional, 12-ring channel system (6.6 × 6.7 Å2 along a- and b-axes and 5.6 × 5.6 Å2 along c-axis) with high thermal stability.5-12 Because of its unique pore structure and excellent catalytic properties, zeolite beta has been widely used in the petroleum industry, especially in the alkylation process of benzene for the production of ethylbenzene and cumene.12-14 In the general synthesis of zeolite beta,15 tetraethylammonium (TEA+) cation has been used as organic structure-directing agent (OSDA). Although alternative OSDAs for TEA+ cation have been found,16-18 it has long been recognized that the use of an OSDA is crucial for the formation of zeolite beta framework. From a practical viewpoint, however, the OSDA-free synthesis of zeolite beta is strongly desired because the use of OSDA makes the industrial processes expensive, complex, energy consuming, and environmentally unfriendly. Recently, Xiao and co-workers19,20 reported the first OSDAfree synthesis of zeolite beta, in which calcined TEA+-directed zeolite beta seeds were added to an OSDA-free Na+aluminosilicate gel. Mintova and co-workers21 have extended this seed-assisted method to the synthesis of aluminum-rich * Corresponding author. E-mail: [email protected]. Tel.: +81-3-5841-7348. Fax: +81-3-5800-3806. † The University of Tokyo. ‡ Yokohama National University.

zeolite beta (Si/Al ) 3.9) by adding noncalcined, TEA+containing zeolite beta seeds into a Na+-aluminosilicate gel. Most recently, we have studied the effects of various parameters on the OSDA-free, seed-assisted synthesis of zeolite beta.22 Zeolite beta was successfully synthesized with a wide range of chemical compositions of the initial Na+-aluminosilicate gel (SiO2/Al2O3 ) 40-100, Na2O/SiO2 ) 0.24-0.325, and H2O/ SiO2 ) 20-25) by adding calcined beta seeds with the Si/Al ratios in the range of 7.0-12.0. It is noteworthy that beta is crystallized from the OSDA-free gel in which a different type of zeolite, mordenite, crystallizes in the absence of the seeds. Furthermore, the obtained beta can be used as seeds for the consecutive synthesis of zeolite beta, termed Green Beta. Still, the crystallization behavior has not been reported in spite of the remarkable effect of the beta seeds. The seed-assisted synthesis of various zeolites has been widely studied since the early days23-29 to establish efficient mass-production technology.26 In addition to zeolite beta,19-22 the OSDA-free, seed-assisted syntheses of other important zeolites such as ZSM-34,30 nanosized ZSM-5,31 RTH-type zeolite,32 and MTW-type zeolite33 have been reported. However, similar to the case of zeolite beta, the crystallization behavior and the exact role of seeds have not been fully understood yet.29-33 During the hydrothermal treatment, seed crystals are dispersed in the Na+-aluminosilicate gel under strongly basic condition, and such a severe environment hampers the in situ microscopic observations of the seed-assisted crystallization process. Further investigations are therefore required to enhance the applicability of the seed-assisted method to various practical zeolites.

10.1021/jp1098975  2011 American Chemical Society Published on Web 12/27/2010

Crystallization of Zeolite Beta in OSDA-Free, Seed-Assisted Synthesis In the present study, we report, for the first time, the crystallization behavior of zeolite beta in the OSDA-free Na+-aluminosilicate gel system in the presence of seed crystals. Important insights have been obtained by monitoring the structural properties of the solid constituents in the seedembedded system before and during crystallization by using transmission electron microscope (TEM), Raman spectroscopy, solid-state 27Al and 23Na MAS NMR, high-energy X-ray diffraction (HEXRD), along with powder XRD and fieldemission scanning electron microscope (FE-SEM). The effect of the hydrothermal treatment of the aluminosilicate gel prior to the addition of the seeds was also examined. On the basis of the results, the characteristic features of the crystal-growth behavior are reported. Experimental Section Materials. The following raw materials were used as provided: Mizukasil (Grade P707, SiO2 ) 96.92%, Al2O3 ) 0.08%, Mizusawa Industrial Chemicals Ltd.) and Cab-O-Sil (Grade M5, Cabot) as silica sources, sodium aluminate (NaAlO2, Wako) as an aluminum source, and sodium hydroxide solution (NaOH, 50w/v% in water, Wako) as an alkali source. Tetraethylammonium hydroxide (TEAOH, 35 wt % in water, Aldrich) was used as OSDA for the synthesis of beta seeds. OSDA-Free, Seed-Assisted Synthesis of Zeolite Beta. Zeolite beta seeds with Si/Al ) 12.0 and Na/Al ) 0.04 were synthesized by the hydrothermal treatment of an aluminosilicate gel containing TEAOH with the following chemical composition: 1.25Na2O:6.12TEA2O:1Al2O3:35SiO2:490H2O. Detailed procedures are reported in our recent paper.22 OSDA-free synthesis was carried out by the addition of the calcined beta seeds (Si/Al ) 12.0) to an OSDA-free Na+-aluminosilicate gel before the hydrothermal treatment (i.e., initial addition) or after 5 h of hydrothermal treatment (i.e., intermediate addition). The chemical composition of the Na+-aluminosilicate gel was adjusted to13Na2O:1Al2O3:40SiO2:1000H2O, which led to a shorter crystallization time than that tested in the previous paper.22 The procedure for the OSDA-free synthesis by the initial addition of the beta seeds is as follows; sodium aluminate was dissolved in purified water, followed by the addition of NaOH to form a clear solution. Then, Cab-O-Sil and the beta seeds were slowly and simultaneously added to the solution, and the mixture was homogenized by using a mortar and pestle. Here, the quantity of the beta seeds was 10 wt% relative to silica source, and the total weight of the Na+-aluminosilicate gel was adjusted to 18 g. The seed-embedded Na+-aluminosilicate gel was transferred into a 60 mL stainless-steel autoclave and subjected to the hydrothermal treatment at 140 °C for different periods of time under static condition under autogenous pressure. The resulting solid products were filtrated, washed with purified water and dried at 60 °C. The fully crystallized sample obtained after 40 h of hydrothermal treatment is designated as Crystal1. The solid yield (g/g) of the product was defined as the ratio of the weight of dried solid product to the dry weight of SiO2, NaAlO2, and calcined beta seeds in the initial gel. The solid yield of Crystal-1 was ca. 26%. The synthesis of zeolite beta by the intermediate addition of the calcined beta seeds (Si/Al ) 12.0) into a Na+aluminosilicate gel was performed to obtain further information on the crystallization behavior. Initially, the nonseeded Na+-aluminosilicate gel (13Na2O:1Al2O3:40SiO2:1000H2O) was hydrothermally treated at 140 °C for 5 h, followed by rapid quenching of the autoclave. Subsequently, the autoclave was

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opened, and the beta seeds (10 wt%) were added to the Na+-aluminosilicate gel, followed by mixing with a spatula. Then, the autoclave was again sealed and hydrothermally treated at 140 °C for different periods of time under static conditions and autogenous pressure. In this system, the fully crystallized product (Crystal-2) was obtained after 30 h of hydrothermal treatment. Similar to Crystal-1, the solid yield of Crystal-2 was ca. 27%. Characterizations. In the present study, zeolite beta seeds were characterized after calcination to remove OSDAs, whereas the products of the OSDA-free synthesis were characterized in the as-synthesized forms. Powder X-ray diffraction (XRD) patterns were collected by using a Mac Science, MO3XHF22 diffractometer with Cu KR radiation (λ ) 0.15406 nm, 40 kV, 30 mA) from 5 to 35° with a scanning step of 0.02° at a scanning speed of 2° per minute. The crystallinity of the obtained beta phase was evaluated by comparing the intensity of the observed diffraction line at 2θ ) 22.18° to that of the fully crystallized beta obtained under identical conditions. The crystal size and morphology of the solid products were observed by a FE-SEM (Hitachi S-4800). Morphological evolution of the aluminosilicate products and crystals of beta were observed by a TEM (JEOL 2000EX) at 200 kV of accelerating voltage. Prior to the TEM observations, all the products were dispersed in ethanol, and a droplet of the suspension was deposited onto carbon-coated Cu grids. Elemental analyses of the products were performed by inductively coupled plasma-atomic emission spectrometer (ICPAES, Varian Liberty Series II). Solid-state 27Al and 23Na MAS NMR spectra were collected with a JEOL CMX-300 spectrometer. 27Al MAS NMR spectra were observed at the resonance frequency of 78.3 MHz, with a pulse width of 1.0 µm and a recycle delay of 5 s. 23Na MAS NMR spectra were observed at the resonance frequency of 52.9 MHz with a pulse width of 1.0 µm and a recycle time of 5 s. The samples were spun at 10 kHz in standard 4 mm zirconia rotors. Prior to the measurements, all zeolite samples were hydrated in a desiccator with saturated ammonium chloride solution for 72 h at room temperature. Here, 1 M aluminum nitrate solution and 1 M sodium chloride solution were used as chemical shift standard references for the 27Al and 23Na MAS NMR, respectively. The ring distribution of the amorphous aluminosilicate were characterized by a laser Raman spectrometer (JASCO Corp., NRS1000) by using argon green laser with the wavelength of 532 nm for excitation. Raman spectra were recorded between 200 cm-1 and 600 cm-1, in which vibration spectra attributed to the ring structure were obtained. The measurements were carried out with the exposure time of 300 or 600 s (twice) for integration. Prior to the measurements, all samples were dried at 60 °C and formed into pellets. Nitrogen adsorption-desorption measurements of fully crystallized beta samples were performed on Quantachrome Autosorb-1 at 77 K. Prior to the measurements, all zeolite products were degassed at 400 °C for 4 h. High-energy XRD (HEXRD) measurements were carried out by using a pressed zeolite disk with 200 mg weight at room temperature on the horizontal two-axis diffractometer, optimized for the structural measurements in glass and liquid, built at the BL04B2 high-energy monochromatic bending magnet beamline of SPring-8 that operates at 8 GeV. A bent Si(220) crystal, mounted on the monochromator stage fixed at a Bragg angle of 3° in the horizontal plane, provided incident photons at 61.63 keV (λ ) 0.2012 Å). Qmax collected in this study was ca. 25 Å-1.34 The obtained data were subjected to well-established analysis procedures, such as absorption, background, and the Compton scattering corrections and then normalized to Faber-

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Kamimura et al.

Figure 1. XRD patterns of (a) beta (polymorph A by simulation),35 (b) calcined beta seeds, and (c) beta (Crystal-1) obtained by the OSDAfree, seed-assisted synthesis.

Ziman total structure factor S(Q). These collected data were used to calculate the total correlation function T(r) by using the following function:

T(r) ) 4πFr +

2 π

Qmax Q[S(Q) - 1] sin(Qr) dQ + 1 ∫Qmin

(1) where F is the atomic number of density. Results and Discussion OSDA-Free, Seed-Assisted Synthesis of Zeolite Beta. By the initial addition of the beta seeds, highly crystallized zeolite beta (Crystal-1) was obtained after 40 h of hydrothermal treatment. The XRD pattern of Crystal-1 (Figure 1c) shows that it is a pure beta phase (cf. Figure 1a for the simulated pattern of beta (polymorph A) as a reference).35 Crystal-1 (Figure 1c) exhibits sharper and better-defined diffraction lines compared with those of the beta seeds (Figure 1b). The Si/Al and Na/Al ratios of Crystal-1 were 5.2 and almost unity, respectively. Figure 2a,b shows the FE-SEM images of the beta seeds and Crystal-1, respectively. In contrast to the irregular, aggregated particle morphology of the seeds, well-defined crystals with truncated octahedral morphology that is analogous to the natural counterpart of zeolite beta, Tschernichite, are observed for Crystal-1.36 The crystal size of Crystal-1 is in the range of 60-300 nm. These characteristics are similar to those for the recently reported OSDA-free beta prepared with different composition of the gel.22 Investigation of Crystallization Process of Zeolite Beta in Seed-Embedded OSDA-Free Gel System. The crystallization processes of the Na+-aluminosilicate gels with and without the beta seeds were monitored by XRD. Figure 3a shows the evolution of XRD patterns of the solid products formed by the hydrothermal treatment of the seeded gel for different periods of time. Before hydrothermal treatment (0 h), a small diffraction line of the beta seeds is observed at 2θ ) 22.18°. The intensity of this diffraction line decreases after 5 h, which is explained by the partial dissolution of the beta seeds embedded in the strongly basic Na+-aluminosilicate gel under hydrothermal condition, although it is difficult to evaluate the amount dissolved. The dissolution rate appears to depend on the Si/Al

Figure 2. FE-SEM images of (a) calcined beta seeds, (b) beta (Crystal1) obtained by initial addition of the seeds, and (c) beta (Crystal-2) obtained by intermediate addition of the seeds.

ratio of the seeds: Mintova and co-workers21 and we22 have recently shown that the addition of the beta seeds with the Si/ Al ratio greater than 26 does not give beta phase, which should be due to the complete dissolution of the seeds. The diffraction lines due to the beta phase show a slight increase after 10 h, and the highest crystallinity is achieved after 40 h. Figure 3b shows the XRD patterns of the solid products formed in the nonseeded gel after different periods of time. In this case, only the amorphous product is observed even after 20 h. As the time is further prolonged, the beta phase never appears, but the diffraction lines attributed to mordenite (MOR), analcime (ANA), and unidentified phases are observed. This is somewhat different from our recent study, in which only mordenite was formed in the nonseeded aluminosilicate gel with a slightly lower

Crystallization of Zeolite Beta in OSDA-Free, Seed-Assisted Synthesis

Figure 3. Evolution of XRD patterns of the solid aluminosilicate products with increasing time for hydrothermal treatment (a) with and (b) without the beta seeds. The product after 40 h in (a) corresponds to Crystal-1.

Na2O/SiO2 ratio.22 Importantly, the crystallization rates of these phases are much slower than that of beta. Figure 4 shows the TEM images of the solid products formed in the seed-embedded system after hydrothermal treatment for different times. After 5 h (Figure 4a), only the amorphous material with oval-shape morphology is observed. No seed particles with a size of 300-500 nm (Figure 2a) are observed, which is consistent with the XRD result showing the partial dissolution of the seeds (Figure 3a). Most of the residual, small beta seeds are supposed to be embedded in the amorphous material, because no such crystalline particles are found outside of the amorphous material. After 20 h (Figure 4b), small beta crystals are formed on the surface of the amorphous material. As heating time is further prolonged to 30 h, the number of beta crystals increased with a relative decrease in the amount of the amorphous material (Figure 4c). After 40 h, the

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Figure 5. Raman spectra of the (a) seeded amorphous material and (b) nonseeded amorphous material formed after 5 and 20 h of hydrothermal treatment, respectively.

amorphous material is no longer observed, and intergrown beta crystals (60-300 nm) are present (Figure 4d). Structural Characterizations of the Seeded and NonSeeded Amorphous Materials. As confirmed by XRD, the beta seeds in the OSDA-free Na+-aluminosilicate gel system play a crucial role in the crystallization of beta. In this section, Raman spectroscopy, solid-state 27Al and 23Na MAS NMR, and HEXRD are employed to investigate the effect of the seeds on the aluminosilicate structure in the solid products. We focus on the amorphous materials just before crystallization, that is, after 5 and 20 h of heating with and without the beta seeds, respectively. Both samples show no diffraction lines attributed to any crystalline phase (Figure 3a,b). Figure 5 shows the Raman spectra of the amorphous materials obtained with (Figure 5a) and without (Figure 5b) the beta seeds.

Figure 4. TEM images of the solid aluminosilicate products obtained after (a) 5 h, (b) 20 h, (c) 30 h, and (d) 40 h of hydrothermal treatment. The product after 40 h corresponds to Crystal-1.

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Figure 6. Solid-state (a) 27Al and (b) 23Na MAS NMR spectra of the seeded and nonseeded amorphous materials formed after 5 and 20 h of hydrothermal treatment, respectively.

The seeded and nonseeded amorphous materials show similar Raman spectra, where broad asymmetric bands at 300-600 cm-1 correspond to a typical disordered structure composed of predominantly four-membered aluminosilicate rings (4R)37,38 It is thus likely that the addition of beta seeds causes no significant change in the ring structure of the amorphous aluminosilicate. The solid-state 27Al MAS NMR spectra of the seeded and nonseeded amorphous materials are shown in Figure 6a. Both samples exhibit only one signal centered at δ ) 53 ppm that corresponds to the tetrahedrally coordinated aluminum.21,39 In addition, the fully crystallized beta (Crystal-1) also shows only one signal centered at δ ) 53 ppm (data not shown), which is similar to our previous report on the beta obtained at different gel compositions.22 Thus, no signal that corresponds to the octahedrally coordinated aluminum (δ ) 0 ppm) is observed in the as-synthesized aluminosilicate products obtained from various stages of the hydrothermal treatment. Barrer40 claimed that under sufficiently high alkaline conditions in the initial gel, only tetrahedrally coordinated aluminum species comprised the aluminosilicate networks in the amorphous material. Valtchev and co-workers41 have investigated the crystallization stages of zeolite A (LTA), and reported that only tetrahedrally coordinated aluminum was present in the aluminosilicate gel and in the products obtained at various stages in the crystallization of zeolite A. On the other hand, calcined beta seeds show the presence of tetrahedrally and octahedrally coordinated aluminums, like we reported recently.22 The octahedrally coordinated aluminum in the seeds is formed by the dealumination upon the calcination for the removal of TEA+ cations. The 23Na MAS NMR spectra of these samples show only one signal centered at δ ) -15 ppm as depicted in Figure 6b, suggesting that only one environment of Na+ cation exists in the Na+aluminosilicate gel.42-44 According to Valtchev and co-workers,44 23Na chemical shift is sensitive to the structural change in the amorphous aluminosilicate. Although detail of the Na+ environment in the present amorphous aluminosilicates is still under investigation, the results of 27Al and 23Na MAS NMR strongly suggest that there is no critical difference in the structures of the seeded and nonseeded amorphous aluminosilicates. In order to obtain a better understanding of the structure of seeded and nonseeded amorphous aluminosilicates, HEXRD measurements were performed. Figure 7 shows the total correlation functions, T(r), of seeded and nonseeded amorphous materials and fully crystallized beta. From the T(r) curves, it is possible to identify the various distances associated with several

Kamimura et al.

Figure 7. Total correlation functions, T(r), (1.0-6.0 Å), of (a) nonseeded amorphous material, (b) seeded amorphous material, and (c) fully crystallized beta (Crystal-1).

Figure 8. Evolution of XRD patterns of the solid aluminosilicate with increasing time after intermediate addition of the seeds. The product after 30 h corresponds to Crystal-2.

features. The first peak in the T(r) is related to Si-O (ca. 1.61 Å) and Al-O (ca. 1.71 Å) distances, although the Q range obtained here is not sufficient to resolve the two distances. Peaks at 2.6 and 3.1 Å are related to O-O and Si-Si(Al) distances, respectively. No major changes in these three peaks are observed in Figure 7. Peaks at 3.7-3.9 and 4.1-4.2 Å are mainly due to the second nearest neighbor of Si(Al)-O in 4R and rings larger than 4R, respectively.34,45 The T(r)s of seeded and nonseeded amorphous materials (Figure 7a,b) are similar to each other, indicating that the effect of the addition of beta seeds on the medium-range order is limited. It is noteworthy that T(r) curves of seeded and nonseeded amorphous materials have a shoulder peak at 3.8 Å, whereas the peak is less pronounced in the fully crystallized beta (Figure 7c). Therefore, it is suggested that the fraction of 4R in the amorphous aluminosilicate is larger than that in crystalline beta, and the fraction of 4R decreases with increasing the crystallinity. On the basis of the results of Raman spectroscopy, solid-state 27Al and 23Na MAS NMR, and HEXRD, it is concluded that the presence of beta seeds gave no crucial effect on the ring structure of amorphous aluminosilicate before the appearance of Bragg diffraction lines in the XRD. Effect of Intermediate Addition of Zeolite Beta Seeds. Figure 8 shows the evolution of the XRD patterns of the solid products obtained by the intermediate addition of calcined beta seeds to a prehydrothermally treated Na+-aluminosilicate gel. At 0 h (just after the seed addition), a small diffraction line of

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Figure 10. Schematic illustrations of the crystallization processes in the OSDA-free, seed-assisted synthesis of beta (Crystal-1).

Figure 9. Crystallization curves of (a) Crystal-1 and (b) Crystal-2 plotted from the evolution of XRD patterns.

TABLE 1: Pore Characteristics of Beta (Crystal-1) Obtained by Initial Addition of Seeds, Beta (Crystal-2) Obtained by Intermediate Addition of Seeds, and Calcined Beta Seeds (Previously Reported Value)22

sample

BET surface area [m2 g-1]

micropore surface area [m2 g-1]a

micropore volume [cm3 g-1]a

Crystal-1 Crystal-2 calcined beta seedsb

498 459 627

428 393 303

0.23 0.21 0.16

a

Determined by t-plot method. b Previously reported value.22

the beta seeds is clearly observed at 2θ ) 22.18°. Then, after 5 h, a weak increase in the diffraction intensity indicates that the crystal growth has occurred, which is in contrast to the crystallization process of Crystal-1, where the dissolution of the seeds appeared to be predominant during the first 5 h. After heating for 10-25 h, rapid crystallization of the beta phase occurred, and highly crystalline beta without any detectable impurities is obtained after 30 h. Figure 9 shows the crystallization curves of Crystal-1 and Crystal-2 obtained from the XRD results. Notably, when the beta seeds were added after prehydrothermal treatment of the nonseeded Na+-aluminosilicate gel, zeolite beta crystallized faster than when synthesized without pretreatment. Crystal-2 consists of the crystals with a size of 20-50 nm (Figure 2c), which is much smaller than that of Crystal-1 (60-300 nm). When considering that the yields of the crystals are similar in both cases (see Experimental Section), the number of the crystals should be larger for Crystal-2. In the case of the initial addition, a portion of the beta seeds might be completely dissolved during the early stage of the hydrothermal treatment (∼5 h). Even when the beta seeds are added after the prehydrothermal treatment of the gel, partial dissolution and/or disaggregation of the beta seeds should occur during the additional hydrothermal treatment; however, it is plausible that more seed crystals effectively serve as growth center to form beta crystals because the crystal growth starts as soon as the seeds are added to the prehydrothermally treated gel. Thus, an increased number of smaller particles is obtained in the case of Crystal-2. The Brunauer-Emmett-Teller (BET) surface areas, micropore surface areas, and micropore volumes of the fully crystallized samples are listed in Table 1. Crystal-1 and Crystal-2 show pore characteristics similar to those of the previously reported beta prepared with different chemical compositions of the gel22 and show higher micropore surface areas (393-428

m2 g-1) as well as micropore volumes (0.21-0.23 cm3 g-1) than those of calcined beta seeds (303 m2 g-1 and 0.16 cm3 g-1). The intermediate addition of the seeds has no effect on the pore characteristics of the final product. Crystallization Behavior of Zeolite Beta in OSDA-Free, Seed-Assisted Synthesis. The evolution of the OSDA-free, seed-embedded Na+-aluminosilicate gel system during the hydrothermal treatment is summarized here. The initial gel was like a hard paste without fluidity, and after 5 h of heating, the gel partially dissolved to form solid amorphous aluminosilicate and liquid phase. As the heating time was prolonged, crystallization of beta proceeded, and the amount of amorphous aluminosilicate decreased. Finally, fully crystallized beta with a wide size distribution was obtained when the amorphous phase disappeared. As mentioned before, a similar aluminosilicate gel without the beta seeds did not yield zeolite beta. In addition, the beta seeds induced no obvious structural change of the ring structures in the amorphous aluminosilicate. These results confirm that seed growth without nucleation is predominant in this system. TEM observations suggested that the beta was mainly crystallized on the surface of amorphous material. This fact implies that the growth of the beta seeds can hardly occur when they are embedded in the amorphous aluminosilicate but proceeds when they become in contact with the liquid phase containing dissolved aluminosilicate precursors. In the case of initial addition of the seeds, the embedded seeds seem to be exposed and/or released to the liquid phase at different times during the hydrothermal treatment, and eventually, this will result in the crystallization of beta with a wide size distribution. In the case of intermediate addition of the seeds, most of the beta seeds are supposed to be on the surface of amorphous material, and as a result, more seeds can act as liberated nuclei to enhance the crystallization rate of beta. Because all of the seed crystals can start to grow at the same time, a narrower size distribution of the beta crystals compared to the case of initial addition is achieved. The crystallization behavior of beta is illustrated in Figure 10. When the seed-embedded Na+-aluminosilicate gel (Figure 10a) turns into amorphous aluminosilicate and liquid phase at the early stage of the hydrothermal treatment, the beta seeds are partly dissolved and disaggregated into small pieces, and most of them are embedded in the amorphous aluminosilicate (Figure 10b). Upon dissolution of the amorphous aluminosilicate, the embedded beta seeds should be exposed on the surface of the amorphous material and/or released to the liquid phase, and these beta seeds contacting with the liquid phase will provide a surface for crystal growth by consuming aluminosilicate precursors in the liquid phase (Figure 10c). Eventually, all the amorphous aluminosilicate is dissolved, and crystal growth is completed (Figure 10d). A similar crystal-growth behavior has been observed for zeolite A (LTA) from nonseeded Na+-aluminosilicate gel. Valtchev and co-workers46 reported that most of the zeolite nuclei formed in the early stages of amorphous aluminosilicate were released to the liquid phase,

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and hence, the crystal growth of zeolite A on the surface of the liberated nuclei was accelerated. Conclusions The crystallization behavior in the seed-assisted synthesis of zeolite beta in the seed-embedded OSDA-free Na+aluminosilicate gel system was investigated in detail. The evolution of XRD patterns and TEM observations of solid aluminosilicate products indicated that zeolite beta could hardly grow when they were embedded in the amorphous aluminosilicate in the early stage of the hydrothermal treatment, and the crystallization of most of beta proceeded on the surface of the amorphous aluminosilicate that was formed by the dissolution of the initial gel. The Raman spectroscopy, solid-state 27Al and 23Na MAS NMR, and high-energy XRD analyses of the seeded and nonseeded amorphous materials revealed that beta seeds induced no structural change of the amorphous aluminosilicate during the hydrothermal treatment. The intermediate addition of the beta seeds after the prehydrothermal treatment of the gel enhanced crystallization rate and resulted in the increased number of beta with a smaller crystal size. These results suggest that the beta seeds in the OSDA-free Na+-aluminosilicate gel system provide a growth surface for new beta crystals. Moreover, during the hydrothermal treatment, zeolite beta is not crystallized directly from the OSDA-free gel, and beta is mainly crystallized on the surface of the residual beta seeds after seeds are exposed and/or dispersed in the interface of the amorphous aluminosilicate and the liquid phase. Acknowledgment. The authors acknowledge Nippon Chemical Industry Co. Ltd. for chemical analysis and FE-SEM observations. This work was financially supported in part by Nippon Chemical Industry Co. Ltd., a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS), and a Grant-in-Aid for Young Scientists from the JSPS. Y.K. gratefully thanks the JSPS for a postdoctorial fellowship and a Grant-in-Aid for Scientific Research. References and Notes (1) Breck, D. W. Zeolite Molecular SieVes; Wiley: New York, 1974. (2) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756–768. (3) Corma, A. Chem. ReV. 1997, 97, 2373–2420. (4) Cundy, C. S.; Cox, P. A. Chem. ReV. 2003, 103, 663–702. (5) Treacy, M. M. J.; Newsam, J. M. Nature 1988, 332, 249–251. (6) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. Zeolites 1988, 8, 446–452. (7) Corma, A.; Llabre´s i Xamena, F. X.; Prestipino, C.; Renz, M.; Valencia, S. J. Phys. Chem. C 2009, 113, 11306–11315. (8) Ogura, M.; Okubo, T.; Elangovan, S. P. Catal. Lett. 2007, 118, 72–78. (9) Kobler, J.; Abrevaya, H.; Mintova, S.; Bein, T. J. Phys. Chem. C 2008, 112, 14274–14280. (10) Holmberg, B. A.; Hwang, S. J.; Davis, M. E.; Yan, Y. Microporous Mesoporous Mater. 2005, 80, 347–356. (11) Corma, A.; Domine, M. E.; Valencia, S. J. Catal. 2003, 215, 294– 304.

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