Article pubs.acs.org/crystal
Hollow SAPO-34 Cubes with Hierarchically Organized Internal Structure Jie Gong,† Fei Tong,† Xiaobo Ji,† Changfeng Zeng,‡ Chongqing Wang,† Yinong Lv,§ and Lixiong Zhang*,† †
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China ‡ College of Mechanic and Power Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China § State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China S Supporting Information *
ABSTRACT: We herein report novel hollow SAPO-34 cubes with a hierarchical internal structure. They are prepared from dry gels containing gelatin by a vapor-phase transport method. Microscopic characterization reveals that the cubes are constructed by symmetrically packed four two-top-point truncated octahedrons encapsulated by dense crystalline shells, exhibiting a structure-in-structure formation. The cubes show either +-shaped or the butterfly like X-shaped black spots on their faces under light. Investigation of the formation process of the hollow SAPO-34 cubes demonstrates a surface to core crystal growth route, which has not been found in vapor-phase transport synthesis of zeolites. A possible formation mechanism is proposed.
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INTRODUCTION Zeolitic molecular sieves (ZMSs) are important microporous crystals with uniform and intricate channels, high specific surface area and adsorption capacity, and high thermal and hydrothermal stabilities, which have been widely used in catalysis, adsorption, ion-exchange, separation, etc.1−7 They are usually synthesized by the hydrothermal synthesis method as crystals with well-defined geometry and uniform structure. In last two to three decades, many efforts have been made to synthesize hierarchical ZMSs, which are broadly defined as ZMSs integrating multiple levels of porosity (micro- and mesopores).7−11 These hierarchical ZMSs exhibit great advantages over their microporous counterparts by providing fast mass transport rate and easy accessibility to the active sites.5,8,12−16 One kind of them is ZMSs with hierarchical porous structure, including mesporous zeolite crystals, aggregated zeolitic systems, and supported zeolite composites.5,7,17 Another kind of them is ZMSs with hierarchically organized structure.7,18,19 They are constructed by irregularly assembled20−23 or regularly stacked zeolite nanosheets to ordered multilamellar structure,22,24−26 decussate zeolite slices united to sphere-like particles,27 nanosheets assembled to ballshaped house-of-cards-like assemblies,25,28 thin-zeolite plates with enhanced 90° rotational intergrowth to unusually shaped structures,11,29,30 or nanorods or nanofibers constructed by oriented assembly to a specific structure.31,32 Most of their syntheses use special synthesized organic surfactants. Preparation of this kind of hierarchical ZMSs can provide a new way to synthesize hierarchical porous structured ZMSs as well as more © XXXX American Chemical Society
understanding of zeolite crystallization mechanisms. Thus, more strategies are expected to be developed to synthesize ZMSs with hierarchically organized structure. Herein, we report novel hollow SAPO-34 cubes with unique internal structure prepared from solid precursor gels containing gelatin by a vapor-phase transport method. SAPO-34 is a silicon-substituted aluminophosphate molecular sieve that possesses small pore size and medium acidic strength. It has been commercially used for the conversion of methanol to olefins33 and successfully employed to separate carbon dioxide and hydrogen from different gases.34 Generally, SAPO-34 is synthesized as solid cubic crystals, although hierarchically structured SAPO-34 has been synthesized by using layered kaolin27 as the raw material or polystyrene spheres as the template.35 To the best of our knowledge, no SAPO-34 or other ZMSs with such a hollow and unique internal structure have been reported. The hollow SAPO-34 cubes are constructed by symmetrically arranged four two-top-point truncated octahedrons with layered square-like nanoplates encapsulated by dense crystalline shells. Thus, the SAPO-34 cubes exhibit a hierarchically organized or structure-in-structure arrangment. Use of gelatin in the synthesis is found to be critical to the formation of this unique structure, suggesting the importance of biominerals, application of which in zeolite synthesis is limited. This biomineralization preparation method Received: March 18, 2014 Revised: June 12, 2014
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RESULTS AND DISCUSSION XRD and FT-IR characterizations of the as-synthesized sample (Figure S1, Supporting Information) exhibit characteristic peaks and absorption bands of SAPO-34, indicating the CHA structure.36 The scanning electron microscope image of this sample (Figure 1a) shows a typical cubic shape morphology for
may provide a new route to synthesize ZMSs combining micro-, meso-, and macropores or with unique structure and to explore the synthesis of zeolites and other crystals by biomineralization.
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EXPERIMENTAL SECTION
Preparation. Hollow SAPO-34 cubes were synthesized from solid precursor gels containing gelatin by a vapor-phase transport (VPT) method. First, a synthesis solution with a molar composition of 0.75 SiO2/1 Al2O3/0.94 P2O5/0.59 TEA/37.5 H2O was prepared by mixing 10 g of aluminum isopropoxide (Aladdin), 5.30 g of phosphoric acid (85 wt %, Sinopharm Chemical Reagent Co., Ltd.), 3.67 g of 30% colloidal silica sol (Ludox HS-30, Sigma−Aldrich), and 1.46 g of triethylamine (99 wt %, Sinopharm Chemical Reagent Co., Ltd.) successively in 13.2 g of deionized water at room temperature. After being stirred for 2−3 h, the solution was heated to 60 °C and charged with a 2 wt % gelatin solution. The mass ratio of the gelatin solution to the synthesis solution was 0.5:10. The gelatin solution was prepared by adding 0.2 g of gelatin (Sinopharm Chemical Reagent Co., Ltd.) to 10 mL of deionized water, which was preheated to 60 °C. The mixture was vigorously stirred for 30 min before it was poured into a Petri dish and frozen at 0 °C for 12 h. Afterward a proper amount of 5 wt % glutaraldehyde aqueous solution was poured into the Petri dish to cross-link the gelatin. The resulting suspension was transferred into a centrifuge tube, and the solid precursor gels were obtained by centrifugation at 10000 rpm and drying at 50 °C overnight. Finally, the solid precursor gel was transferred into a Teflon holder that was shelved in an autoclave for VPT synthesis. The autoclave was precharged with about 40 g of a mixed solution of 1 TEA/10 H2O (molar ratio). The VPT synthesis was carried out at 180 °C for 72 h to obtain the final product. Instruments and Characterization. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance powder diffractometer using a Ni-filtered Cu Kα radiation source at 40 kV and 20 mA and a Braun position sensitive detector at a scan rate of 5°/ min and a step size of 0.05°. The Fourier transform infrared spectra (FT-IR) were obtained on a Nexus 870 FT-IR spectrometer. Samples were mixed and ground with KBr (in a mass ratio of 1:10) for FT-IR measurement in the wavenumber range of 4000−400 cm−1. N2 adsorption/desorption measurements were performed at 77 K on a Micromeritics ASAP 2020 instrument. The samples were all outgassed at 200 °C for 3 h before measurement. An optical microscope (Pentax), metallographic microscope (DM 4000M Leica), scanning electron microscope (SEM, Hitachi S-4800, Quanta 200 and FEI Quanta 3D FEG), and transmission electron microscope (TEM, JEL200CX) were used to investigate the particle size and morphology and the microstructure of the samples. To investigate the internal structures of the products by SEM and TEM, we ground the samples with moderate strength by pestle in a mortar for 5 min in ethanol. Additionally we treated some of the ground samples and the products synthesized at different stages of the reaction times in an acetic acid solution at room temperature overnight. About 0.1 g of the solid sample was dispersed in 20 mL of the acid solution. After the treatment, the suspension was filtered, and the obtained solids were washed with deionized water and dried at 50 °C overnight. The C, Al, Si, and P contents on the surface and at the edge of X-shaped solid triangles of the hollow SAPO-34 were analyzed by an energy dispersive X-ray analyzer (EDX, Sigma) attached to the SEM (Quanta 200). Moreover, several hollow SAPO-34 cubes were cut by a dual beam high resolution focused ion beam (Ga FIB) miller. The sample was directly attached onto conductive double side carbon tapes, followed by coating with an ∼40 nm thick layer of gold. The selected particles were sectioned with a fine Ga ion beam in the Dualbeam FIB. The electron-beam-generated secondary electron images were recorded at 52° with the built-in function of tilt correction.
Figure 1. (a) SEM, (b) optical microscopic, and (c) TEM images of SAPO-34 prepared from precursor gels added with gelatin. (d) TEM image showing the half part of the cube after the sample was crushed. (e) TEM image of the circled part in panel d at a high magnification. (f) SAED pattern of the part in the circle in panel e.
SAPO-34 with particle sizes ranging from 3 to 5 μm. There are some holes with pore sizes ranging from 10 to 90 nm on their surfaces (Figure S2e, Supporting Information). Observation by the optical microscope (Figure 1b) reveals that there are two different kinds of black spots on the SAPO-34 cubes, one +-shaped and the other butterfly-like X-shaped spot. Further observation by the metallographic microscope (Figure S2c,d, Supporting Information) verifies the finding. The TEM pictures of the cube (Figure 1c,d) only show vaguely the X-shaped structure, suggesting a possible hollow structure inside. The Xshaped part is single crystalline, as indicated from the dot pattern of selected-area electron diffraction (SAED) results (Figure 1f) from the circled region in the corresponding TEM image (Figure 1e). No such black spots were found on the SAPO-34 sample prepared from the gel without gelatin (Figure S2b, Supporting Information). These results suggest that the SAPO-34 crystals prepared from the precursor gel containing gelatin possess the common cubic morphology with complex internal structures. To explore the internal structure of the cubes, we first attempted to see the inside by cutting two cubes vertically from the middle into halves by a focused ion beam (FIB) miller with the cutting directions perpendicular to each other. SEM pictures of the remaining crystal show that the cubes are formed by two isosceles triangle-shaped solid parts with their top points opposite to each other and the top points of the two B
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shaped structure, exhibiting the same structure as that of the FIB-cut crystal (Figure 2a). However, the fine structures can be more clearly seen. The solid triangle-shaped parts are composed of well-organized nanofibers with their directions perpendicular to the bottom line of the triangle (Figure S3c, Supporting Information). The vertex angle of the solid isosceles triangle is a little less than 90° (Figure 2c). The two edges perpendicular to the bottom lines of the two solid triangles (Figure S3d, Supporting Information), which exhibit a regular shape with a smooth surface, can be regarded as a characteristic of crystals.37 Looking through the X-shaped hollow, we can see the existence of a highly loose and porous fibrous structure (Figure S3e, Supporting Information). SEM pictures of another broken cube also show peel-off of the wall of one face of the cube (Figure 2d,e and Figure S3f,g, Supporting Information). We can see that the interior of the SAPO-34 cube is constructed by symmetrical packing of four layered trapezoidal prisms, which forms a +-shaped hollow in the middle of the cube. Further investigation reveals that two trapezoidal prisms form a two-top-point truncated octahedron with two side faces of one trapezoidal prism perpendicular to the base of the prism (Figure 2f). The two-top-point truncated octahedron is constructed by layered square-like nanoplates with an average thickness of 100 nm and side lengths of 1.1−2.4 μm from the top to the middle (Figure 2f and Figure S3h−k, Supporting Information). It is this unique structure of the two-top-point truncated octahedron that forms a triangle-shaped structure when looking from the other side of the cube (arrow in Figure 2g). We further treated these broken samples with a pH 2.5 acetic acid solution and found removal of the internal structure by SEM, leaving a cubic microcontainer (Figure 2h). The side length is about 5 μm, and the wall thickness is about 400−500 nm, close to the observed values in Figure S3a,b. This result indicates that the above-mentioned internal structure that is constructed by symmetrical packing of four octahedrons is encapsulated by a cubic microbox, thus forming the hollow SAPO-34 cube with hierarchical internal structure. The acid treatment results in separation of the trapezoidal prisms and the microbox (Figure S3l, Supporting Information), which is possibly caused by different growth rates between the prisms and the microbox, as will be discussed later. Based on the above observations, we can draw pictures to schematically illustrate the detailed structure of two two-top-point truncated octahedrons (Figure 2i) and the hollow SAPO-34 cube with hierarchical internal structure (Figure 2j). It is clear that there are four faces of the cube showing an X-shaped structure with the remaining two faces showing a +-shaped structure, similar to the observation by the optical microscope in Figure 1b. A video taken under the optical microscope by soaking the cubes in ethanol shows alternatively the two structures of the faces of a cube during the rolling of the cube driven by evaporation of ethanol (Supporting Information, video 1). In addition, a video of the model shows six faces under 360° rotations (Supporting Information, video 2). The N2 adsorption−desorption isotherm of the calcined hollow SAPO-34 cubes (Figure S4a, Supporting Information) exhibits a type I isotherm, suggesting micropores. The BET surface area is 418 m2 g−1, higher than those of the SAPO-34 synthesized without gelatin (303 m2 g−1) (Figure S4b, Supporting Information) and by VPT method reported in literature.35
bottom lines connected by two walls, leaving an X-shaped hollow in the middle when observed from one side (Figure 2a)
Figure 2. (a, b) SEM images of the hollow SAPO-34 cubes cut by a FIB miller from different directions. (c) The SEM image of a broken cube showing peel-off of the wall of one face of the cube, exposing the inner X-shaped structure. (d) The SEM image of a broken cube showing peel-off of the wall of one face of the cube, exposing the inner +-shaped hollow and four octahedrons. (e−g) SEM images of a broken cube showing a trapezoidal prism, a two-top-point truncated octahedron assembled by two trapezoidal prisms, and half of a cube constructed by two-top-point truncated octahedrons. (h) The SEM image of a SAPO-34 cube after treatment with HAc. (i, j) Schematic illustrations of half of a cube without the shells constructed by twotop-point truncated octahedrons and the hollow cube with open of the shells.
and by four smaller cubes packed together, leaving a +-shaped hollow in the middle (Figure 2b). The observed X-shaped and +-shaped hollows correspond well with the dark spots of the cubes when observed by the metallographic microscope (Figure S2c,d, Supporting Information). Further cut of the remaining crystal into halves from the same cutting direction can help us to observe clearly the shell that encapsulates the two isosceles triangle-shaped parts (Figure S3a, Supporting Information) and the four small cubes (Figure S3b, Supporting Information). The shell thickness is about 1/8 of the side length of the SAPO34 cube. Unfortunately, we could not exploit the fine internal structures of the cut crystal because of technical limitations. We later attempted to break the cubic crystals by grinding. The SEM picture of a broken cube (Figure 2c) clearly shows peel-off of the wall of one face of the cube showing the XC
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constructed by ordered and partly crystallized thin layers with amorphous aggregates stuffed inside. They are still in the form of vesicular textured rectangular boxes but with increasing heights to 2 μm after synthesis for 12 h (Figure 3c and Figure S5b, Supporting Information). Furthermore, some parts of the side rectangular faces become obviously denser than the vesicular textures in other parts, forming an X-shaped structure similar to the above-mentioned one (Figure S7e, Supporting Information). However, the vertex angle of the dense X-shape is nearly 30° (Figure S7f, Supporting Information). The squareshaped face still keeps vesicular textures but becomes much denser (Figure S7g, Supporting Information). The XRD pattern (Figure S6a, Supporting Information) and FT-IR spectrum (Figure S6b, Supporting Information) of this sample show a small peak at 26° and obvious absorption bands at 480, 530, 565, 635, and 1105 cm−1, respectively. These results indicate the formation of CHA crystals at 12 h and fast growth of (211) face at the early stage. After 16 h, the rectangular box continues to grow along the height, leading to an increase in the height to 3 μm and no obvious growth in the width. The solid apex-opposite triangle part is well-crystallized. The area of the triangle part enlarges correspondingly and the angle of the apex on the X-shaped face increases to 70° (Figure 3d; Figures S5c and S9a,b, Supporting Information). The remaining part of this face changes from a vesicular texture to a porous structure filled with fibers. On the square-shaped face, the previous vesicular texture becomes dense (Figure S9b, Supporting Information). The corresponding XRD pattern shows obvious diffraction peaks at 9.3°, 15.9°, and 20.5° (Figure S8a, Supporting Information), in which the peak at 9.3° shows the strongest intensity, suggesting preferential growth along the [001] direction. This is consistent with the increase in the thickness of the rectangular box. These results indicate that both the growth and the crystallization significantly occur at this stage. SEM pictures of this sample treated with HAc indicate formation of four layered structured blocks with a width of 2.0 μm at each corner of the rectangular box (Figure S9c−e) and a +-shaped cavity in the middle of a face (Figure S9f, Supporting Information). The above results suggest that the crystallization starts from the small-side faces of the rectangular box and then extends from the surface inward to the amorphous aggregates stuffed inside. After 18 h, the rectangular box grows to a cube, accompanied by increases in the area of the X-shaped face and the angle of the apex of the triangle to 90° (Figures 3e; Figures S5d and S9g,h, Supporting Information). After 24 h, all the characteristic diffraction peaks of SAPO-34 appear in the XRD pattern of the product (Figure S8a, Supporting Information). SEM pictures show that the size of the cube does not change anymore (Figures 3f and Figure S5e, Supporting Information). The edges of the cube perpendicular to the bottom lines of the X-shaped solid triangles start to crystallize (Figure S9i, Supporting Information), and the surfaces without the X-shaped structure still show a porous structure (Figure S9j, Supporting Information). The optical microscope picture of this sample shows no shadow on the cubes (Figure S9k, Supporting Information), suggesting a solid internal structure. After 36 h, crystallization of the edges perpendicular to the bottom lines of the solid triangles extends to the center of the surface, as observed from the SEM picture of the resultant sample (Figure 3g and Figure S5f, Supporting Information). The surface without the X-shaped structure seems completely crystallized, leaving pores on the surface (Figure 3h). Optical
To investigate the formation process of the hollow SAPO-34 cubes, we examined the products prepared at different times by SEM, XRD, and FT-IR. SEM images reveal that the product obtained after synthesis for 6 h is in the form of irregularly shaped particles with a broad particle size distribution ranging from 2 to 15 μm (Figure 3a). The XRD pattern (Figure S6a,
Figure 3. SEM images of hollow SAPO-34 cubes after crystallization for (a) 6, (b) 10, (c) 12, (d) 16, (e) 18, (f) 24, and (g, h) 36 h. Panels g and h show two different faces of a cube.
Supporting Information) and FT-IR spectrum (Figure S6b, Supporting Information) of this product indicate that they are amorphous. Prolongation of the synthesis time to 10 h leads to formation of vesicular textured rectangular boxes with the sizes of 5 × 5 × 1.5 μm3, which are constructed by about five layers of plates with thicknesses of 150 nm supported by many pillars with heights of 120 nm (Figure 3b; Figures S5a and S7a, Supporting Information). The XRD pattern (Figure S6a, Supporting Information) shows no obvious diffraction peaks, and the FT-IR spectrum (Figure S6b, Supporting Information) shows almost invisible absorption bands, indicating that the product is still amorphous. We treated this sample with an HAc solution and found from the SEM pictures that the remaining rectangular box is hollow inside (Figure S7b−d, Supporting Information). Furthermore, this treated sample exhibits a small diffraction peak at 26° (corresponding to the (211) face) and weak absorption bands at 480, 530, 635, and 1105 cm−1, respectively, in its XRD pattern (Figure S6a, Supporting Information) and FT-IR spectrum (Figure S6b, Supporting Information). These results imply that the rectangular box is D
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shaped crystal at the early stages of the zeolite synthesis, as revealed by optical and fluorescence microscopy.43,44 Afterward, the big rectangular box further grows, forming a nice cube. The X-shaped parts continue to crystallize, which triggers the crystallization of the other two edges at this face. Thus, the four small rectangular blocks transform to four twotop-point truncated octahedrons with layered square-like nanoplates by consuming the aggregates, leaving a hollow inside (step 4, as shown in Figure 3f,g). This transformation quite possibly resulted from the second crystal growth of the amorphous aggregates stuffed inside, as in the case of the nucleation of sodalite that takes place in the amorphous cores of NaA cubic particles synthesized in the presence of chitosan with increasing synthesis time.40 Furthermore, the interaction between gelatin and faces of the crystallized X-shaped structure may suppress the growth of this structure to form a regular shape, as that in the formation of rhombohedral-shaped calcite and hierarchical ZnO in the presence of chitosan and gelatin, respectively.37,38 To find the evidence of this interaction, we treated gelatin in the vapor of TEA and water at 180 °C for 3 d, following the same procedure for the VPT synthesis of the hollow SAPO-34, and compared the FT-IR spectra of gelatin and treated gelatin (Figure S1c, Supporting Information). We found that the amino band at 1545 cm−1 disappears and the carbonyl band at 1645 cm−1 slightly shifts to 1648 cm−1 after the treatment, suggesting formation of the hydrogen bond between TEA and gelatin molecules.45,46 In addition, we measured the element C contents on the surface and at the edge of X-shaped solid triangles by EDX (Figure S12 and Table S1 in the Supporting Information). Apparently, the C content at the edge is 10% higher than that on the surface. Furthermore, the SEAD result (Figure S2g, Supporting Information) indicates that the circled part at the edge of X-shaped solid triangles shown in Figure 1d is amorphous, possibly due to the existence of gelatin. These results indicate accumulation of gelatin at the edge, which may result from the interaction between TEA and gelatin molecules. Consequently, the gap between the X-shaped structure and shell is not filled, resulting in the formation of a hollow. This process makes the linkage weak between the cubic microbox and the two-top-point truncated octahedrons because of their different growth rates. Thus, they tend to separate from each other after acid treatment (Figure 2h). Finally, crystallization goes on mainly at the faces of the cube from the edge to the center, thus forming the shell of the cube (step 5, as shown in Figure 1a). In the preparation of the hollow SAPO-34 cubes, addition of glutardehyde is quite important because it can cross-link gelatin to form a three-dimensional networked hydrogel with higher strength and stability at high temperatures.47 To verify its importance, we conducted the syntheses by adding gelatin without glutaraldehyde and adding glutaraldehyde without gelatin in the reaction system. The obtained samples are also SAPO-34, as verified by XRD characterization (Figure S13, Supporting Information). Both of them show typical cubic morphology on SEM pictures and do not exhibit any black spots when observed by optical microscopy (Figure S14, Supporting Information). Therefore, glutardehyde plays a role in cross-linking gelatin. The cross-linked gelatin can thus undergo VPT treatment at 180 °C and takes an important role in the formation of the hollow SAPO-34 cubes as a biomineral.
microscopic observation shows that some samples exhibit a +-shaped shadow (Figure S11a, Supporting Information). Further prolongation of the synthesis times leads to crystallization on both the X-shaped (Figure S11b,e,h, Supporting Information) and non-X-shaped (Figure S11c,f,h, Supporting Information) faces from the edge to the center simultaneously, leaving fewer and fewer pores on the faces, as shown in the SEM pictures. More and more samples show a +-shaped shadow in their optical microscope pictures (Figure S11a,d,g,i), which suggests more and more consumption of internal stuffed aggregates. Finally, hollow cubes with a hierarchical internal structure as shown in Figure 1a are formed. Based on the above formation process, we propose the mechanism of formation for the hollow SAPO-34 cubes with a hierarchical structure depicted in Scheme 1. At the beginning, Scheme 1. Schematic Drawing of a Proposed Crystal Growth Route of Hollow SAPO-34 Cubes with Hierarchical Internal Structure
the aluminophosphate gels and gelatin transform to amorphous irregularly shaped aggregates (step 1, as shown in Figure 3a), which later organize to well-shaped rectangular boxes with two square faces and four rectangular faces around the surface of aggregates, thus confining these aggregates inside (step 2, as shown in Figure 3b). They are constructed by layered nanosized squares with pillars between. The layered structure is believed to be formed by connection of gelatin as mineral bridges. That is, the strong adsorption of gelatin on certain surfaces of active aluminophosphate precursor species results in the fomation of thin squares because of oriented attachment of the nonpolar surface on these aluminophosphate precursor species, as in the preparation of hexagonal twin plate shaped ZnO constructed by stacking of ZnO nanoplates in the presence of gelatin.38 The formation of rectangular boxes with the aggregates stuffed inside results from facilitation of zeolite crystallization on the surface of these aggregates, which hinders the crystallization of the aggregates stuffed inside, just like the formation of the typical NaA cubic morphology associated with a very thin crystalline cubic shell and an amorphous core synthesized in the presence of chitosan.39 This formation process should follow the reversed surface-to-core growth mechanism found in the hydrothermal synthesis of zeolites and some other crystals.37−42 The rectangular boxes further grow up mainly at the rectangular faces and obvious crystallization starts also from these faces, thus forming X-shaped crystallinelike dense parts on these faces (Figure 3c,d). At the same time, four small rectangular blocks constructed by layered squares are formed at four corners of the big rectangular box (step 3, as shown in Figure 3d). The formation of X-shaped crystalline-like dense parts can be explained by the formation of dumbbellE
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CONCLUSION We have demonstrated novel hollow SAPO-34 cubes that are constructed by symmetrically arranged four two-top-point truncated octahedrons with layered square-like nanoplates encapsulated by dense crystalline shells. The cubes show either +-shaped or butterfly-like X-shaped black spots on their faces when observed with the optical microscope. For the first time, a complex and unusual structure is observed in zeolites, which has been produced by a simple vapor-phase transport method from dry gels containing a small amount of gelatin. The use of gelatin in the synthesis is thought to be responsible for the formation of the complex hierarchically organized internal structure. And the formation process follows the reversed crystal growth route, which has not been found in the preparation of zeolites by a vapor-phase transport method. Because the hierarchically organized SAPO-34 cubes show a hollow structure, less than 500 nm shell thickness and big holes with pore sizes in the range of 10 to 90 nm on the shells, they may provide better accessibility to the internal area of the crystal and exhibit much higher mass transfer rates than the conventional cubic-shaped SAPO-34 crystals. This work not only provides a new route to synthesize zeolites with hierarchically organized structure but also helps us to understand better the synthesis mechanism of zeolites and other crystals by biomineralization.
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ASSOCIATED CONTENT
S Supporting Information *
XRD patterns and FT-IR spectra of SAPO-34, SEM, optical microscopic, and metallographic microscope images of SAPO34, nitrogen adsorption/desorption isotherms of SAPO-34, EDX analysis results of the as-synthesized samples crystallized for 24 h, video of SAPO-34 cubes showing alternatively the two structures of the facesand video of the model showing six faces under 360° rotations. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Dr. Lixiong Zhang. Mailing address: No 5 Xin Mofan Rd., Nanjing 210009, P. R. China. Tel: +86-25-8317-2265. Fax: +86-25-8317-2263. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Grant 21076107), Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Research and Innovation Program for College Postgraduates of Jiangsu Province (Grant CXLX13_428). We thank the staff at the Monash Center for Electron Microscopy for their technical assistance.
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