Preparation of Zeolite ANA Crystal from Zeolite Y ... - ACS Publications

Apr 13, 2010 - A new method has been explored to synthesize zeolite ANA crystals with regular icositetrahedron in aqueous media via transformation of ...
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J. Phys. Chem. B 2010, 114, 5747–5754

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Preparation of Zeolite ANA Crystal from Zeolite Y by in Situ Solid Phase Iso-Structure Transformation Yi Wang,‡ Xuguang Li,† Zhiyuan Xue,‡ Linsen Dai,‡ Songhai Xie,*,† and Quanzhi Li*,† Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China, and Center of Analysis and Measurement, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: March 26, 2010

A new method has been explored to synthesize zeolite ANA crystals with regular icositetrahedron in aqueous media via transformation of zeolite Y under the conditions of low temperature, short reaction time, and without organic template. The products are perfect, almost 100% crystals. The samples prepared at different crystallization stages are measured by XRD, TEM, and SEM to investigate the transformation mechanism from zeolite Y to zeolite ANA. It has been demonstrated for the first time that the mechanism of forming a zeolite ANA polycrystal with sphere or shell morphologies is the in situ solid phase iso-structure transformation (Is-SPIST) of zeolite Y. The Is-SPIST mechanism is also supported by the results of steam-induced crystallization experiments and other assistant means, including the same Si/Al ratio, the same weight, the same particle size, and the same morphology before and after transformation of zeolite Y to zeolite ANA. It is also observed that a spherical or shell ANA polycrystal is constructed via the reconstruction from its exterior to interior, to form an ANA single crystal with a solid or hollow icositetrahedron. The main driving force of the reconstruction is considered to be the grain boundary energy existing between polycrystalline grains. This process also obeys the mechanism of in situ solid phase reconstruction (Is-SPR). Furthermore, the size and morphology of the zeolite ANA single crystal can be modified by surfactants. Introduction Zeolite-type materials, as a kind of crystalline microporous aluminosilicate, have been used as high-performance materials such as catalysts, ion exchangers, and adsorbents.1 They are usually synthesized under hydrothermal conditions by a solutionmediated process.2-7 Therefore, the nucleation (formation of the first and very small crystalline entities) and crystal growth around these nuclei are the common liquid phase mechanisms proposed for zeolite synthesis.7 Subsequently, the solid phase mechanism and both phases of liquid and solid mechanisms are also proposed one after the other by scientists.8-12 These mechanisms of zeolite formation proposed from different points of view are based on some experimental facts. Recently, many new mechanisms or models of zeolite synthesis have been suggested under given experimental conditions, along with the progress of modern analytical techniques, such as high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering, and atomic force microscopy, etc.13-17 For examples, homogeneous18,19 and heterogeneous nucleation14,20 are proposed in the view of nucleation, while the oriented aggregation mechanism15,21 and epitaxial growth mechanism22 are presented in the view of growth. More recently, a reversed crystal growth process has been observed during the self-construction of the core-shell and hollow zeolite crystals.23,24 Undoubtedly, all these studies are very important for a deep understanding of the formation mechanism of zeolite. However, there are still plentiful works to be done to approach the ultimate goal for * Corresponding authors. E-mail: [email protected]; [email protected]. † Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials. ‡ Center of Analysis and Measurement.

synthesizing zeolite by rational design due to the complicacy of zeolite synthesis under hydrothermal conditions.25 The main purpose of this paper is to investigate the isostructure transformation of zeolite. Generally, the transformation of one zeolite into another is often encountered when the synthesis conditions changed. To obtain the pure phase of zeolite, this phenomenon of iso-structure transformation should be avoided. In our lab, it is found that the zeolite ANA crystals with a perfect solid or hollow icositetrahedron can be synthesized by iso-structure transformation of zeolite Y under the conditions of lower temperature (100 °C), shorter reaction time (2-3 days), and without organic templates. These conditions excel the conventional methods (140-200 °C, 12 h∼31 days).26-32 Thus, the route of iso-structure transformation might offer a method for zeolite synthesis under mild conditions; i.e., one zeolite as a precursor can be used to synthesize another zeolite. Then, a more interesting problem is what the mechanism of iso-structure transformation of zeolite is and what the intrinsic driving force of this kind of transformation is. We carefully investigated the process of iso-structure transformation using various modern techniques including the thermodynamic analysis. A new mechanism of in situ solid phase iso-structure transformation (designated as Is-SPIST) is proposed, in which the nucleation like in the liquid phase could be eliminated, and it is also found that the ANA polycrystal transformation into an ANA single crystal from its exterior to interior also obeys the mechanism of in situ solid phase reconstruction (designated as Is-SPR). Subsequently, an approach was designed to modify the size and the morphology of zeolite ANA with surfactants through the Is-SPIST method. ANA crystals with a larger size and the morphology of truncated icositetrahedron are obtained, which have never been observed and reported. The scientific information presented in this study may be helpful to understand

10.1021/jp907706c  2010 American Chemical Society Published on Web 04/13/2010

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deeply the iso-structure transformation phenomenon of zeolite observed in the laboratory and existing in nature. Experimental Section 1. Chemicals. Ultrastable Zeolite Y with a high Si/Al ratio of 6.7 (the Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC) was used as a previous source. CTAB (analytical purity, Mingzhi Chemical Co., Shanghai) and OP-10 (chemical purity, First Chemical Co. Shanghai) were used as additive. Sodium hydroxide (analytical purity) was used as an alkaline source. All the chemicals were used without further purification. Deionized water was used in all experiments. 2. Synthesis. The typical synthetic procedure is as follows: 0.88 g of ultrastable zeolite Y was added into the solution of sodium hydroxide (0.54 mol/L) with or without surfactant additive (CTAB + OP-10 ) 0.17 mol/L) and then stirred for 2 h. The gel was heated under static hydrothermal conditions at 100 °C for 72 h in a stainless steel autoclave. The precipitated product was filtered, washed with deionized water, and dried in air at 80 °C overnight. For the as-synthesized samples with surfactant additive, they were calcined at 550 °C (10 °C/min) in air for 5 h to remove the adsorbed organic compound. To observe the growth of ANA crystals, the synthesis was carried out at 100 °C for different crystallization times (28-288 h). 3. Characterization. The X-ray powder diffraction (XRD) measurements of samples were carried out on a Rigaku D/MAXRB X-ray powder diffractometer with Ni-filtered Cu Ka radiation at 30 kV and 20 mA. Scanning electron micrographs (SEM) were obtained from a Philips XL-30 scanning electron microscope, and the energy dispersive X-ray analyzer (EDX) was also used to detect the chemical compositions of individual particles. The transmission electron microscope (TEM) images and the selected area electron diffraction (SAED) patterns were obtained on JEOL JEM-2010 instruments. When preparing the samples, the products were well dispersed in ethanol ultrasonically. Then the ethanol suspensions of samples were dropped on copper grids coated with amorphous carbon film for TEM and SAED measurements. The Si/Al ratio of samples was measured by 29Si-MAS NMR. The resonance frequency is 59.63 MHz, and the magnetic field is 7.0 T. Results and Discussion Structure and Morphology of Zeolite ANA Polyhedrons Synthesized in Hydrothermal Conditions by Transformation of Zeolite Y. The XRD pattern of the product after crystallization at 100 °C for 72 h is shown in Figure 1. All the diffraction peaks are very strong and can be indexed, which confirms that the product is pure zeolite ANA with high crystallization. The space group of zeolite ANA is Iad by PDF card. The calculated unit cell parameter (a0) by XRD is 1.374 nm, and the Si/Al ratio is 2.2 by 29Si NMR and EDX analyses (see the Supporting Information). Although the Si/Al ratio of the prepared ANA is a little higher than the ideal value (2.0), it is still within the range of the Si/Al ratio of zeolite ANA.33,34 These results illustrate that zeolite Y with a high Si/Al ratio of 6.7 could change into zeolite ANA with low Si/Al ratio in the hydrothermal alkaline solution without an organic template. More interesting is the prepared sample’s morphology characterized by SEM. The zeolite ANA sample is uniform and almost 100% icositetrahedral crystals with particle size about 3 µm in diameter (Figure 4, 72 ha, b). These crystals can be characterized as icositetrahedron {112} with 24 identical (211) faces of a cubic structure, which possesses four 3-fold axes and three 4-fold axes (Figure 2a). It has been reported that the (211)

Figure 1. XRD pattern of the product zeolite ANA.

Figure 2. Morphologies corresponding to the monotypes of zeolilte ANA crystals synthesized without surfactants (a) and with surfactant (b).

facets of zeolite ANA have the lowest energy.34,35 Thus, the icositetrahedron consisting of 24 (211) facets is the most possible morphology for zeolite ANA. In addition, it is noted that some zeolite ANA crystals are hollow from the SEM images of the sample (Figure 4, 44 hb, 48 hb). All the results, particularly the shorter crystallization time, the lower crystallization temperature, and the hollow zeolite ANA crystals, suggest that the growth of the zeolite ANA crystal using our method is unusual. Formation Processes of Zeolite ANA Single Crystal from Zeolite Y. The XRD patterns of the samples prepared in different crystallization times are shown in Figure 3. From Figure 3, 32 h, it can be seen that the sample exhibits strong XRD diffraction peaks indexed as zeolite Y (Figure 3, 0 h) and only the intensity of peaks is weaker than that of the zeolite Y precursor after hydrothermal synthesis for 32 h. With the prolongation of the hydrothermal crystallization time, the intensities of the diffraction peaks of the typical zeolite Y (for example: (111), (220), (311)) gradually reduce until they disappear (see Figure 3, 32-38 h), whereas those characteristic diffraction peaks of the typical zeolite ANA (for example: (211), (400), (322)) gradually build up, which shows that zeolite Y gradually transforms into zeolite ANA. No detectable crystalline phase of zeolite Y is observed after hydrothermal synthesis for 42 h, and the product is pure zeolite ANA. SEM images of the samples collected at different growth stages are presented in Figure 4. The zeolite Y as a precursor is irregular polyhedrons with particle size about 0.3-1.5 µm, and their surface is smooth (Figure 4, 0 h). At the early stage

Preparation of Zeolite ANA Crystal from Zeolite Y

Figure 3. Powder XRD patterns of the samples prepared at different hydrothermal synthesis stages without surfactants.

of crystallization for 32 h, the SEM image (Figure 4, 32 h) shows that the polyhedral particles of zeolite Y have no evident change except a little flabby surface. It indicates that the surface structure of zeolite Y first became noncompact and unstable in the hydrothermal alkaline solution. However, no obvious dissolution occurred since the size of the polyhedron did not decrease. After crystallization for 36 h, macropores emerged in the particles of zeolite Y (Figure 4, 36 h), and after hydrothermal synthesis for 38 h, the particles of zeolite Y gradually became zeolite nanosheets. This process is desilication or dissolution by gravimetric analysis, which is opposite to that for faujasite sheet structure units whcih assembled to bulk zeolite Y (see Supporting Information, Figure S-4).38,39 Then, nanosheets began to aggregate into zeolite solid spheres or shells. To reduce the surface tension, the smaller nanosheets can be compacted into solid spheres (Figure 4, 38 ha), while the bigger ones can first form nanobelts by oriented aggregation (Figure 4, 38 hc, and Figure 4, 40 hb) and then are further aggregated to form a shell zeolite because its stereohindrance effect can prevent the dense aggregation. Combining the XRD and the TEM results of crystallization for 38 h (Figure 3 and Figure 5, 38 hb), it can be concluded that a majority of the solid spheres or shells are still zeolite Y. When the hydrothermal crystallization time was prolonged to 40 h, the nanosheets or nanobelts assembled into zeolite solid spheres or shells further aggregated to form highly compact surfaces. Combining the XRD, SEM, and TEM results of crystallization for 40 h, it can be considered that the solid spheres or shells of zeolite Y have been gradually transformed into those of the zeolite ANA polycrystal, only containing a few zeolite Y. After crystallization for 42 h, almost all the zeolite Y transforms into the flocky spheres of zeolite ANA polycrystal, and a part of them are still hollow. When the crystallization time was prolonged to 44 h, 46 h, and 48 h, the flocky sphere or shell surfaces of zeolite ANA polycrystals first began to reconstruct and gradually formed the single-crystal faces (Figure 4, 44 h∼48 h). After that, the reconstruction continued from the exterior to the interior of the spheres or shells (Figure 4, 48 hb), which were also observed previously by Zhou.23 Until 72 h, the solid and hollow perfect icositetrahedrals of zeolite ANA single crystals with a uniform size of 3 µm were obtained (Figure 4, 72 h). After crystallization for 288 h, the single-crystal faces of zeolite ANA faded a little because of the eroding of the alkaline solution. Figure 5 shows the TEM images and corresponding SAED images of the samples prepared in the crystallization for 36 h,

J. Phys. Chem. B, Vol. 114, No. 17, 2010 5749 38 h, 40 h, and 42 h, respectively. Figure 5, 36 ha,b, further proves that many zeolite Y nanosheets with macropores are formed by desilication when crystallization time was 36 h. Furthermore, the insetted SAED image confirms that the zeolite Y nanosheets are single crystalline. The HRTEM image of the lattice fringes shown in Figure 5, 36 hc, also characterizes the zeolite Y, and the grain boundaries or defects are existent as pointed out by the dark area in the consecutive lattice of zeolite Y nanosheets. When crystallization time was 38 h (Figure 5, 38 ha), the nanosheets began to aggregate, and their SAED image still shows a single crystalline diffraction pattern of zeolite Y. As shown in Figure 5, 38 hb, some nanosheets or nanobelts of zeolite Y further aggregated into microspheres with about 3 µm, and the hollow in the sphere formed due to the stereohindrance effect of large nanosheets. From Figure 5, 38 hc (HRTEM), it can be seen that the aggregated nanosheets or nanobelts are still well microporous zeolite Y, and the consistency of the lattice fringes along the [220] direction located in the common boundary of aggregated nanosheets further demonstrates the oriented aggregation of nanosheets. In addition, as shown in Figure 5, 40 ha (crystallization time 40 h), the microspheres made up of the zeolite Y nanosheets become more solid, and some regular edges appear on their surfaces. Combining the XRD and SEM results, it can be said that the regular microspheres in this stage are typical zeolite ANA polycrystals. However, there still exists a few zeolite Y nanosheets rooted in the microspheres of zeolite ANA, which is proven by the SAED pattern insetted in Figure 5, 40 hb (crystallization time was not enough to transform them). The SAED image (Figure 5, 40 has) of the interface area between the ANA microspheres and the untransformed zeolite Y particles (see the arrow) also proves that this area is the intergrowth of zeolite ANA and zeolite Y. The laminated zeolite Y nanosheets can be clearly observed from Figure 5, 40 hb, and the well-oriented overlap of their lattice fringes along the direction of [220] can be seen from Figure 5, 40 hc, as well. When the crystallization time was 42 h, the polyhedrons with regular edges can be observed more clearly from Figure 5, 42 ha,b, and the octagon and hexagon shown in Figure 5, 42 ha, are the projections separately along the 4-fold axes and 3-fold axes of the icositetrahedron of the zeolite ANA single crystal. In this stage, no obvious nanosheets of zeolite Y are observed, which indicates that almost all zeolite Y nanosheets transformed into a zeolite ANA single crystal. This is in agreement with the results of XRD and SEM. Further detection of the edge of regular hexagonal particles by the SAED (inset in Figure 5, 42 hb) illustrates that the regular hexagonal particles are zeolite ANA single crystals. Formation Mechanism of the Zeolite ANA Polyhedron by the Transformation of Zeolite Y. According to the results of XRD, SEM, and TEM, the Is-SPIST mechanism is proposed. It can be depicted as follows, which is also schematically illustrated in Figure 8: (1) The irregular particles of zeolite Y with different sizes desiliconized in hydrothermal alkaline solution to form macroporous zeolite Y nanosheets. The desiliconization makes the framework Si/Al ratio decrease from 6.7 to 2.2 and markedly increases the surface area as well as the surface energy of the particles. (2) To reduce the surface energy, the macroporous nanosheets of zeolite Y began to link by oriented attachment. The smaller nanosheets aggregated into solid zeolite spheres, while the bigger nanosheets or nanobelts were in favor of forming hollow spheres (shells) due to their stereohindrance effect.

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Figure 4. SEM images of the samples prepared without surfactants at different hydrothermal synthesis stages.

(3) The aggregated spheres or hollows of zeolite Y nanosheets become zeolite ANA polycrystal spheres or shells through the Is-SPIST process. It was carried out under low temperature and in short reaction time and without an organic template. (4) Then, the exterior surface of the zeolite ANA polycrystal spheres or shells first reconstructed into an icositetrahedral single crystal. With the increase of crystallization time, the reconstruction continued from the exterior to the interior until the zeolite ANA polycrystal spheres or shells fully transformed into a solid or hollow icositetrahedral single crystal. It is also the Is-SPR by the driving force of the grain boundary energy existing between polycrystalline grains. For illustrating the rationality of Is-SPIST and Is-SPR mechanisms, two questions should be answered: (1) Why perfect zeolite ANA crystals can be synthesized from zeolite Y through the Is-SPIST mechanism? (2) Why this transformation can be carried out under low temperature and in short reaction time? The reasons are as follows:

(1) The frameworks of both zeolite Y and zeolite ANA are built by the same secondary building units (SBUs) of 6-ring, 4-ring, and 6-2-ring (Figure 6).37 Then the 4-ring and 6-ring of zeolite Y could be transformed into zeolite ANA through the in situ solid phase rearrangement in the hydrothermal system with appropriate pH (Y ≈ 11; ANA ≈ 12), which can significantly shorten the time-consuming inducing period of nucleation and growth of the ANA crystal like in the case of the liquid phase mechanism. So, zeolite ANA crystals can be easily formed under low temperature and in short reaction time through the Is-SPIST process. (2) High pH is beneficial for growing zeolite with high framework density. Since the framework density of zeolite ANA (18.5 T/1000 Å) is higher than that of zeolite Y (12.7 T/1000 Å), zeolite Y can transform into zeolite ANA at the designed high pH (12.0).36 (3) The distorted 8-ring with irregular channels (0.42 × 0.16 nm) of zeolite ANA is more stable than the 12-ring with regular

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Figure 5. TEM images and corresponding electron diffraction images (insets) of the samples prepared at the crystallization for 36 h, 38 h, 40 h, and 42 h, respectively. Photo 40 ha-s is the electron diffraction image of the interface area between the ANA microsphere and the untransformed zeolite Y particles.

channels (0.74 × 0.74 nm) of zeolite Y, which is also beneficial for transforming metastable zeolite Y into stable zeolite ANA.26 To further approve and clarify the mechanism, some assistant evidence was offered as follows. (1) The evidence of solid phase transformation offered by the steam-induced crystallization (SIC) method: the solid phase iso-structure transformation of zeolite Y to zeolite ANA was verified through some experiments. When the hydrothermal synthesis system was carried out at 100 °C for 36 h, the solid products were separated from the mother liquid by filtration, and the filtrate continued to crystallize under 100 °C for 4 days. However, no solid products could be obtained. In contrast, the recovered solid zeolite Y nanosheets (Figure 4, 36 h) can directly transform into zeolite ANA crystal (see Figure 7a,b) by the SIC method under the same synthesis conditions without any liquid phase. This is considered to be the typical solid phase transformation mechanism.

(2) The same Si/Al ratio before and after Is-SPIST: the aggregation of the zeolite Y nanosheets with Si/Al ) 2.3 can transform into zeolite ANA with Si/Al ) 2.2. That is to say, on the eve of iso-structure transformation at crystallization time for 32 h, 36 h, and 38 h, the Si/Al ratio of the samples measured with 29Si-MAS NMR can reach 2.5, 2.3, and 2.2, respectively, by desilication under basic hydrothermal conditions (Table S-1 in Supporting Information). Therefore, the composition (Si/Al) of the products before and after the iso-structure transformation is almost the same. This is also one piece of evidence supporting the solid phase transformation mechanism. (3) The same weight before and after Is-SPIST: it is attested that the weight of the formed zeolite ANA is equal to that of the recovered solid phase zeolite Y nanosheets separated from the mother liquid, and it is also found that the corresponding weights of the recovered solid products are 41.7%, 41.6%, 39.3%, 39.8%, and 39.6%, respectively, at the different crystal-

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Figure 6. Maps of the framework of zeolite Y (a) and ANA (b) and the channels of zeolite Y (c) and ANA (d).

Figure 7. XRD pattern (a) and SEM image (b) of the product synthesized by SIC of the recovered solid zeolite Y nanosheets.

lization stages (36 h, 38 h, 40 h, 42 h, and 44 h). After forming the zeolite Y nanosheets (36 h), the weights of the solid phase are nearly unchanged with the prolongation of crystallization time and the formation of zeolite ANA, which is also equal to the yield of 40% of zeolite ANA product (72 h). The equality of both weights before and after transformation further supports the Is-SPIST. (4) The same particle size and morphlogy before and after Is-SPIST: comparing the SEM images in Figure 4, 38 ha,b,c, with those in Figure 4, 44 ha,b, and comparing the TEM images in Figure 5, 38 ha,b, with those in Figure 5, 42 ha,b, it clearly demonstrates that the zeolite ANA polycrystal and single crystal have the same particle size and morphologies of spheres and shells as the zeolite Y polycrystal. (5) The more interesting event is that the “scars” formed due to the aggregation of the zeolite Y nanobelt (Figure 4, 38 hc, and Figure 4, 40 hb) still exist on the surfaces of the zeolite

Wang et al. ANA polycrystal and single crystal (Figure 4, 72 h) after transformation, which also confirms the Is-SPIST. Thus, the Is-SPIST mechanism is convincingly confirmed by the analysis of the intrinsic properties of both zeolite Y and ANA, the results of XRD, SEM, and TEM, and some supporting experiments described above. The zeolite ANA can be obtained via directly recomposing the SBUs of zeolite Y such as 6-rings and 4-rings, in which the nucleation like that of the liquid phase was unnecessary. Therefore, the perfect zeolite ANA crystals can be synthesized from zeolite Y through the Is-SPIST mechanism, and this transformation can be carried out under low temperature and in short reaction time. Not only the experimental results but also the thermodynamic discussions illustrate the rationality of the Is-SPIST mechanism. In fact, the Is-SPIST can be divided into three separated states: (1) solid zeolite Y, (2) polycrystal of zeolite ANA, (3) single crystal of zeolite ANA. When the state is changed, the change of Gibbs free energy can be defined as ∆Gn ) ∆GS-S + ∆GS-L + ∆GL-L, where ∆GS-S, ∆GS-L, and ∆GL-L are the solid-solid, solid-liquid, and liquid-liquid interaction energy, respectively, where n represents the reaction stage. For example, ∆G1 represents the change of Gibbs free energy when the state changes from (1) to (2). For the solid phase mechanism, ∆Gtotal ) ∆G1 + ∆G2. In the first stage, ∆G1 is contributed by the ∆G1S-L, ∆G1S-S-A, and ∆G1S-S, and ∆G1S-L is contributed mainly by the desilication and dissolution. The solution heat ∆H and the entropic change of the system T∆S make the ∆G1S-L < 0. However, ∆G1S-L gradually increases with the increase of dissolving of zeolite Y until ∆G1S-L ) 0. In this case, the dissolving of zeolite Y progressed to form the nanosheets. Then, ∆G1S-S-A should also be considered, which is the change of surface energy produced by the aggregation of zeolite Y nanosheets to spherical polycrystal to decrease surface energy (∆G1S-S-A < 0). ∆G1S-S results in solid phase transformation and crystallization. It is equal to the change of lattice energy (∆UA-Y) after the zeolite Y transformed into zeolite ANA. It is known that the crystal cell of zeolite Y is bigger than that of zeolite ANA, and the framework density of Y is smaller than that of ANA. Therefore, the lattice energy value of zeolite Y is less negative than that of zeolite ANA, and ∆G1S-S (∆UA-Y) < 0. As a result, ∆G1 ()∆G1S-L + ∆G1S-S-A + ∆G1S-S) is negative. In the second stage, when zeolite ANA polycrystal transformed into a single crystal, there were no solid-liquid and liquid-liquid interactions. Thus ∆G2 is equal to ∆G2S-S which is the change of grain boundary energy. The spherical polycrystal of zeolite ANA formed by the Is-SPIST process will have many grain boundaries between polycrystalline grains (see Figure 8). To decrease grain boundary energy, the spherical

Figure 8. Scheme of in situ solid phase iso-structure transformation of zeolite Y and the crystal growth process of the solid and hollow ANA polyhedrons.

Preparation of Zeolite ANA Crystal from Zeolite Y

Figure 9. SEM images of sample synthesized in the solution with higher H2O/Y () 79.5 g/g) ratio and surfactants/Y () 0.013 mol/g) ratio, magnified (a) × 13 000 and (b) × 10 000.

polycrystalline aggregates of zeolite ANA would transform into a single crystal with regular icositetrahedron morphology through the migration of atoms on grain boundaries. This process was excited by the heat energy offered by the hydrothermal conditions, and then the grain boundaries were eliminated. Furthermore, the spherical polycrystalline aggregates of zeolite ANA transform into a icositetrahedral single crystal from spherical exterior to interior. The reason is that the grain boundary angles in the exterior of the spherical polycrystal are higher than that in the interior. As known, the higher the grain boundary angle is, the larger the grain boundary energy is. Thus, the atoms located at the high angle grain boundary first migrated and reconstructed the exterior of the zeolite ANA polycrystal to form the shell of the icositetrahedral single crystal. At the same time, this would also form new and larger grain boundary angles between the single crystal shell and the inner polycrystal, which would increase the grain boundary energy. According to the principle of the lowest energy, these atoms located on the new grain boundary also reconstructed to a single crystal to decrease the grain boundary energy. This process would continue until the whole spherical polycrystal was transformed to a single crystal. It is an energy favoring process, i.e., ∆G2 < 0, and is coincident with the experimental results. It is to say that if the synthesis of zeolite obeys the Is-SPIST mechanism, then the evolution from polycrystal to single crystal would also obey the Is-SPR mechanism, and its direction is from exterior to interior, which is different from the classical crystal nucleation and growth theory in solution.23 Therefore, the thermodynamic results (∆G1 + ∆G2 ) Gtotal < 0) also support the Is-SPIST and Is-SPR mechanisms. Effect of Surfactants on the Is-SPIST of Zeolite Y into Zeolite ANA. The iso-structure transformation with surfactants in hydrothermal solution is almost the same as that without surfactants. The XRD and SEM results are shown in the Supporting Information (Figure S-7 and Figure S-8), from which the Is-SPIST process can be visualized clearly. There are three main differences compared with the results without surfactants: First, the transformation speed with surfactants is slightly quicker than that without surfactants, which can be seen from the Supporting Information of XRD and SEM results. Second, the

J. Phys. Chem. B, Vol. 114, No. 17, 2010 5753 surfactants actuate to form the truncated icositetrahedron of the zeolite ANA single crystal with uniform larger size (7 µm). Third, it can stabilize the morphology (see Supporting Information of SEM). In this process, the most special point is the morphology of the truncated icositetrahedron of the zeolite ANA single crystal. As known, the crystal series of the cubic structure contains seven monotypes, and the icositetrahedron of zeolite ANA synthesized is one of them. The truncated icositetrahedron morphology of zeolite ANA studied here is the complex of the icositetrahedron {112} and cube {100} monotype of the cubic crystal series (Figure 2b), which has never been reported, but the (100) facets of truncated icositetrahedron are not very clear. The synthesis conditions were further changed, namely, the higher surfactants/Y (0.013 mol/g) and H2O/Y (79.5 g/g) ratio. Then, the truncated icositetrahedron with more distinct (100) facets is obtained (Figure 9) and the size of (100) facets reaches 1.8 µm × 1.8 µm. The possible reason for the formation of the truncated icositetrahedron morphology is that the surfactants can easily be adsorbed on the (100) facets of zeolite ANA to stabilize them. The amount of surfactants adsorbed on every (100) facet increased with the increase of the surfactants/Y and H2O/Y ratio, which makes the disappearance speed of (100) facets become much slower. Thus, the (100) facets of truncated icositetrahedron are much bigger and more distinct. Conclusions (1) Zeolite ANA with adjustable sizes (3 and 7 µm) and morphology (spherical polycrystal, solid or hollow icositetrahedron, or truncated icositetrahedron single crystal) can be synthesized by Is-SPIST of zeolite Y at 100 °C for 72 h. The intrinsic driving factors, including the common SBUs, the comparability of composition of zeolite Y and zeolite ANA, the low framework density of zeolite Y (12.7 T/1000 Å), and also its metastable crystal lattice would make zeolite Y transform into zeolite ANA with high stability and high framework density. (2) This transformation obeys the Is-SPIST mechanism. Besides the results of XRD, SEM, and TEM, lots of experimental results such as direct synthesis zeolite ANA by the SIC method, the observed sameness of particle size, Si/Al ratio, and the weight between solid phase precursor of zeolite Y and the product of zeolite ANA crystal all convincingly support the IsSPIST mechanism. The nucleation like that of the liquid phase is unnecessary. (3) The grain boundary energy existing between polycrystalline grains is the driving force to reconstruct the spherical polycrystal of zeolite ANA into the regular icositetrahedron or truncated icositetrahedron single crystal ANA. The higher grain boundary energy in the exterior of the spherical or shell zeolite ANA than that in the interior makes the reconstruction start from exterior, then to interior. It also obeys the Is-SPR mechanism. (4) The size and shape of the zeolite ANA crystal can be adjusted by adding surfactants, which can also accelerate the speed of iso-structure transformation of zeolite Y. Acknowledgment. Financial support from National Science Foundation of China (projects 20303005 and 20703011) and Science & Technology Commission of Shanghai Municipality (08DZ2270500 and 09ZR1402300) are greatly acknowledged. Note Added after ASAP Publication. This paper was published ASAP on April 13, 2010. The Acknowledgment was updated. The revised paper was reposted on April 29, 2010.

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