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Jan 12, 2009 - Synopsis. A core−shell molecular sieve composite comprised of Cr3+ substituted AlPO4-5 core and Ti4+ substituted AlPO4-5 shell was ...
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CRYSTAL GROWTH & DESIGN

Core-Shell Composite of Ti-/Cr-AFI Molecular Sieve via Solvothermal Epitaxial Growth Dayong Tian, Wenfu Yan, Zhouxiang Wang, Yanyan Wang, Zhibin Li, Jihong Yu,* and Ruren Xu

2009 VOL. 9, NO. 3 1411–1414

State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed June 27, 2008; ReVised Manuscript ReceiVed October 16, 2008

ABSTRACT: The core-shell molecular sieve composite comprised of Cr3+ substituted AlPO4-5 (Cr-AFI) as core and Ti4+ substituted AlPO4-5 (Ti-AFI) as shell was prepared via solvothermal epitaxial growth. The presynthesized tabletlike Cr-AFI crystals were dispersed in the reaction system of Al(OPri)3-H3PO4-TEA-(C4H9O)4Ti-H2O-pEG (polyethylene glycol) followed by solvothermal treatment at 170 °C. The sheetlike Ti-AFI microcrystals were epitaxially grown on the surface of the presynthesized tabletlike Cr-AFI core crystals. The growth process of the Ti-AFI microcrystals on the surface of core of Cr-AFI crystal was studied in detail by scanning electron microscopy characterization. The results demonstrate that the Ti-AFI microcrystals nucleate preferentially on the (100) basal face of the Cr-AFI core crystals. The N2 adsorption analysis shows that the channel systems of the core and shell have good communication, which may provide a good mass transport pathway for the catalytic reaction. The core-shell composite of Ti-/Cr-AFI makes it possible to combine two catalytic sites in one catalyst, which may benefit some special catalysis reactions. Introduction Zeolites and related microporous materials that have periodic three-dimensional frameworks and well-defined pore structures are important due to their wide applications in catalysis, ionexchange, adsorption, chemical separation, and host/guest chemistry.1-3 Aluminophosphate molecular sieves (AlPO4-n), which were first discovered by Wilson and co-workers in 1982, is one of the important families of microporous materials.4 In contrast to the traditionally anionic aluminosilicate zeolite frameworks, their structures are typically built up from strict alternation of AlO4 and PO4 tetrahedra through corner sharing of an oxygen atom to form a neutral open framework. The lattice Al and/or P atoms can be partially replaced by silicon and/or other elements to form frameworks with Bro¨nsted acid sites as well as catalytically active metal sites.5 Because the isolated active sites are distributed spatially throughout the open framework and are freely accessible to reactants, the substituted aluminophosphates (MAlPOs where M ) Ti, Co, Mn, Cr,...) are well studied as single-site heterogeneous catalysts (SSHC).6 For example, MAlPO-18 can shape-selectively convert methanol to light olefins;7 MnAlPO-5 affords a higher selectivity to the desired epoxide product in the epoxidation of both cyclohexene and 1-methylcyclohexene.6d Recently, the core-shell zeolite composites, in which the access to a large core with specific properties is controlled by a tiny shell which has different structure and functionality, have attracted considerable attention due to their high mechanical strength and bifunctional property.8 Using epitaxial growth, the EMT/FAU core-shell composite was successfully synthesized by Martens and co-workers.9,10 Furthermore, the synthesis of core-shell zeolite composites such as BEA/MFI,11 MOR/MFI,12 and BEA/LTA8 was achieved by a seed adsorption method. This method employed the preadsorption of the shell zeolite nanoseeds on the surface of the core crystals followed by the hydrothermal treatment in the synthetic gel of shell zeolite. AlPO4-5 (AFI) is one of the most famous members in the family of aluminophosphate molecular sieves.4 The application * To whom correspondence should be addressed. Tel.: +86 431-8516-8608; fax: +86 431-8516-8608; e-mail: [email protected].

of transition metal substituted AlPO4-5 crystals as single-site catalyst has been well studied in catalytic oxidation reactions.5 Herein, we present a solvothermal epitaxial growth method for the preparation of a core-shell molecular sieve composite comprised of Cr3+ substituted AlPO4-5 (Cr-AFI) as core and Ti4+ substituted AlPO4-5 (Ti-AFI) as shell. In our previous work, the tabletlike Cr-AFI crystals with a low aspect ratio and the kiwi fruitlike Ti-AFI microstructure composed of ultra thin sheetlike microcrystals were prepared in the presence of poly ethylene glycol.13 In this work, we aimed to introduce two different active sites into one catalyst through the epitaxial growth of the Ti-AFI microcrystals on the surface of the tabletlike Cr-AFI crystals. The growth process of the Ti-AFI microcrystals on the surface of core of Cr-AFI crystal was further studied by SEM characterization. Experimental Procedures The Ti-/Cr-AFI core-shell composite was prepared by a two-step synthesis procedure: the presynthesized Cr-AFI crystals (as-synthesized or calcined) were dispersed in a starting gel for the synthesis of TiAFI crystals followed by hydrothermal treatment at 170 °C. Reagents were used as received without further purification: aluminum triisopropoxide, phosphoric acid (85 wt % in water), triethylamine (TEA), chromium trichloride, tetrabutyl titanate, poly ethylene glycol 400 (pEG, molecular weight: 380-420), and deionized water. Synthesis of Tablet-like Cr-AFI Crystals. The uniform tabletlike Cr-AFI crystals with a diameter of about 20 µm were synthesized in the presence of cosolvent of pEG according to our previous work.13 Typically, 0.5 g of well ground aluminum triisopropoxide was first dispersed into 10 mL of mixed-solvents (pEG, 5 mL; H2O, 5 mL) with stirring, followed by the successive addition of 0.215 mL of phosphoric acid and 0.273 mL of triethylamine. Finally, 0.0391 g of CrCl3 · 6H2O was added to the above gel. The reaction gel with the molar composition of Al(OPri)3/1.3H3PO4/0.8TEA/0.06CrCl3/113.4H2O (the volume ratio of pEG/H2O was 1/1) was further stirred for 12 h at ambient temperature and was then sealed in a Teflon-lined stainless steel autoclave and heated at 180 °C for 12 h under static conditions. The solid product was filtered, washed with deionized water, and dried at 50 °C overnight. Synthesis of Ti-/Cr-AFI Core-Shell Composite. Typically, 0.3 g of well ground aluminum triisopropoxide was first dispersed into 10 mL of mixed-solvents (pEG, 7 mL; H2O, 3 mL) with stirring, followed by the successive addition of 0.129 mL of phosphoric acid and 0.164 mL of triethylamine. Then 0.035 mL of tetrabutyl titanate was added

10.1021/cg800683m CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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Figure 2. (a) Simulated XRD pattern of the AlPO4-5 molecular sieve; (b) XRD pattern of the Ti-/Cr-AFI core-shell composite.

Figure 1. SEM images of the Ti-/Cr-AFI core-shell composite: (a) the top view of the core-shell composite; (b) of the magnification image (a); (c) the side view of the core-shell composite; (d) the magnification image of (c). to the above gel to give a final gel with molar composition of Al(OPri)3/ 1.3H3PO4/0.8TEA/0.07(C4H9O)4Ti/113.4H2O (the volume ratio of pEG/ H2O was 7/3). The reaction gel was further stirred for 12 h at ambient temperature. Finally, the presynthesized 0.04 g of Cr-AFI crystals (assynthesized or calcined) were dispersed into the gel and the mixture was then sealed in a Teflon-lined stainless steel autoclave and heated at 170 °C for 4 days under static conditions. In the above reaction system, the Ti-AFI particles about 2 µm were also formed along with the core-shell composites (see Supporting Information Figure S1). The core-shell composites could be easily separated from the suspension containing the Ti-AFI particles by decanting. The as-prepared products were treated by ultrasound for 30 min, washed with deionized water, and dried at 50 °C overnight. The core-shell composites were calcined at 550 °C in air for 4 h to remove the template molecules. Characterization. The X-ray powder diffraction patterns of the core-shell composites were recorded by Siemens D5005 diffractometer with Cu KR (λ ) 1.5418 Å) radiation. The electron micrographs were taken on a JEOL JSM-6700F field-emission scanning electron microscopy (FE-SEM). Nitrogen adsorption analyses were carried out on Quantacerome AUTOSORB-1C automated chemisorption/physisorption surface area and pore size analyzer at 77 K. The atomic force microscopy (AFM) images were recorded on a Dimension 3100 AFM (Digital Instruments, Santa Barbara, CA) operating in TappingMode.

Results and Discussion Synthesis of the Ti-/Cr-AFI Core-Shell Composite. The core-shell composite was synthesized from the reaction mixture of Al(OPri)3-H3PO4-TEA-(C4H9O)4Ti-H2O-pEG with the addition of the presynthesized tabletlike Cr-AFI crystals. If the CrAFI crystals were not added to the reaction system, the kiwi fruitlike Ti-AFI microstructure composed of the sheetlike TiAFI microcrystals were formed.13 The growth rate of the TiAFI crystals along the [001] direction was significantly inhibited by the synergetic effect of the Ti ions and pEG molecules. With the presence of the Cr-AFI crystals, the sheetlike Ti-AFI microcrystals grow epitaxially on the surface of the Cr-AFI crystals (Figure 1). Figure 1a shows the top view of the asprepared core-shell composite. The dense growth of Ti-AFI crystals on the surface of Cr-AFI crystals is clearly observed. Figure 1b shows a high-magnification SEM image of the (001)

Figure 3. N2 adsorption-desorption isotherm of the calcined Cr-AFI crystals used as a core in the preparation of the composites.

face of the core-shell composite. The side views of the assynthesized core-shell composite with different magnification are shown in Figure 1, panels c and d, respectively. It appears that the Ti-AFI microcrystals grow parallel to each other on the (100) basal face of the Cr-AFI substrate. On both of the (001) and (100) basal faces, the sheetlike Ti-AFI microcrystals grow epitaxially with the same crystallographic direction as that of the Cr-AFI substrate. Figure 2 shows the X-ray powder diffraction (XRD) pattern of the as-prepared core-shell composite. The position of the diffraction peaks match well with that of the simulated XRD pattern of AFI framework,14 which indicates that no XRD detectable impurity existed in the products. Integrity Analysis of the Shell. The good communication between the channel systems of the core and shell is very important for the mass transportation in the reaction catalyzed by the core-shell composite. To examine the communication of the channels of the core and shell of Ti-/Cr-AFI composite, the nitrogen adsorption analysis was carried out on the calcined Cr-AFI core, the Ti-/Cr-AFI composite with a calcined Cr-AFI core and TEA-containing Ti-AFI shell, and the calcined Ti-/ Cr-AFI composite, respectively. The N2 adsorption isotherm of the Cr-AFI core in Figure 3 is a typical type I adsorption-desorption isotherm, characteristic of microporous materials. The Langmuir surface area and the pore volume are 488 m2 g-1 and 0.22 cm3 g-1, respectively, which indicates that the product is a highly crystalline AFItype crystal. The integrity analysis of the shell was conducted

Core-Shell Composite of Ti-/Cr-AFI Molecular Sieve

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Figure 4. N2 adsorption-desorption isotherms of (a) Ti-/Cr-AFI composites containing a calcined Cr-AFI core and TEA-containing TiAFI shell; and (b) calcined Ti-/Cr-AFI composites.

Figure 6. SEM images of the Ti-AFI crystals growing on the (001) face of the Cr-AFI crystals taken at different growth stages: (a) 0 h; (b) 8 h; (c) 12 h; (d) 24 h; (e) 28 h; (f) 48 h.

Figure 5. SEM images of the Ti-AFI crystals growing on the (100) basal face of the Cr-AFI crystals taken at different growth stages: (a) 0 h; (b) 4 h; (c) 8 h; (d) 48 h.

by the N2-adsorption analysis of the as-synthesized Ti-/Cr-AFI composite with calcined Cr-AFI core and TEA-containing TiAFI shell. Before the analysis, the sample was dried at 200 °C for 10 h to remove the physically adsorbed water without decomposing the TEA molecules in the channels of the shell. The neglectable pore volume and Langmiur surface area of the 10.4 m2 g-1 indicate that the excellent integrity of the shell and that the pine holes observed in the Figure 1d do not run through the shell (Figure 4a). After the calcination at 550 °C for 4 h, the same sample gives a Langmuir surface area of 540 m2 g-1 and pore volume of 0.23 cm3 g-1 (Figure 4b), which indicates a very good communication of the channels of the core and shell. Compared with the Cr-AFI sample, a hysteresis loop was observed, which is most probably due to the capillary condensation caused by the nitrogen molecules filling in the pine holes. The Growth Process of the Core-Shell Composite. To further investigate the formation process of the Ti-/Cr-AFI core-shell composites, the products produced at different growth stages were studied in detail using SEM analysis. Figure 5 shows the evolution process of the Ti-AFI shell crystals growing on the (100) basal faces of the Cr-AFI single crystals.

Figure 5a shows the smooth (100) basal face of the assynthesized Cr-AFI crystal employed as the core material. After 4 h crystallization, the growth of the Ti-AFI microcrystals on the (100) basal face of the Cr-AFI substrate was observed (Figure 5b) and the sheet shape has been developed for some crystals. By prolonging the reaction time to 8 h, the (100) basal faces are fully covered with sheetlike crystals (Figure 5c) with the same stacking fashion observed in the kiwi fruitlike Ti-AFI microsphere (see Supporting Information, Figure S2).13 Near the final stage of the crystallization (2 days), the highly oriented shell was formed (Figure 5d). Figure 6 shows the different stages of the Ti-AFI microcrystal epitaxial growth on the (001) basal face of the Cr-AFI crystals. Figure 6a shows the smooth (001) basal face of the assynthesized Cr-AFI crystal employed as the core material. After 4 h crystallization, the growth of the Ti-AFI microcrystals on the surface of (001) face was not observed (SEM images not shown), whereas the growth of the Ti-AFI microcrystals on the (100) basal face of the Cr-AFI substrate was observed (Figure 5b) and the sheet shape was developed for some crystals. Prolonging the crystallization time to 8 h, small Ti-AFI crystals start appearing at the edge of the (001) face to form a belt surrounding the hexagon (Figure 6b), while the (100) basal faces are fully covered with sheetlike crystals (Figure 5c) with the same stacking fashion observed in the kiwi fruitlike Ti-AFI microsphere. Further prolonging the crystallization time, the belt becomes broader and the isolated growth sites within the belt area are observed as well (Figure 6c). After the crystallization of 1 day, the belt and the isolated growth sites start touching each other as shown in Figure 6d. The isolated growth site is

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the less-developed kiwi fruitlike Ti-AFI crystals (see Supporting Information Figure S3).13 The crystals within the belt further grow toward to the center of the (001) face and partially cover the isolated growth site with the prolonging of the crystallization (Figure 6e). At the final stage of the crystallization, the surface growth of Ti-AFI crystals (001) face is completed (Figure 6f). The less-developed kiwi fruitlike Ti-AFI crystals at the stage of Figure 6d did not further develop to form the kiwi fruitlike Ti-AFI shape, but form a flat compact shell on the surface of the substrate. These results indicate that the epitaxial growth of Ti-AFI crystals conducts preferentially on the (100) basal face instead of on the (001) basal face of the Cr-AFI crystals. The results observed above clearly show that the epitaxial growth rate of the Ti-AFI on the (100) basal faces is faster than that on the (001) face of the Cr-AFI core. One possible reason is that the (100) basal faces may be much more rough than the (001) face, which results in more nucleation sites on the (100) basal faces than on the (001) face. However, the atomic force microscopy (AFM) analysis shows that there is no obvious difference in the roughness for these faces (see Supporting Information Figure S4). This indicates that the roughness may not cause a significant difference on the epitaxial growth rates on both faces. Furthermore, the control experiments have been done, in which the pure AlPO4-5 crystals were used as the core materials. The results show that the Ti-AFI microcrystals almost grow simultaneously on the (001) and (100) basal faces of the AlPO4-5 crystals (not shown). The same results can also be observed in Qiu’s work of epitaxial growth AlPO4-5 molecular sieve film on the large AlPO4-5 crystals.15 In general, the epitaxial orientation and growth rate of the deposit on the substrate depend on the structure of the crystal planes in contact and the nature of the bonding across the epitaxial interface. In the case of the transition metal substituted aluminophosphates, the incorporation of heteroatoms other than A1 and/or P atoms into the framework may lead to the lattice misfit between the deposit and substrate crystals. It is known that the external surface of (001) basal faces of AFI consisted of AI atoms as revealed by Klap and co-workers,16 whereas the (100) basal faces are terminated by alternate A1 and P atoms. In the Cr-AFI framework, Cr3+ ions substitute A1 atoms, while in the Ti-AFI, Ti4+ ions substitute P atoms. The preferential epitaxial growth of the Ti-AFI shell on the (100) basal face instead of on the (001) face of the Cr-AFI core implies that the (001) basal faces of Cr-AFI may have more lattice misfit with Ti-AFI than (100) basal faces do. The influence of the heteroatom on the epitaxial growth will be further investigated. Conclusions In this study, the combination of two single-site heterogeneous catalysts of Cr-AFI and Ti-AFI was achieved for the first time by forming a core-shell composite via solvothermal epitaxial growth. The channels of the core and shell crystals have a good communication, which will benefit the catalysis and adsorption process conducted in the composite. The growth process of the

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shell on the core was studied in detail with SEM analyses. The results demonstrate that the epitaxial growth of Ti-AFI crystals conducts preferentially on the (100) basal face of the Cr-AFI crystals instead of on the (001) basal face. The same structure type of AFI makes it feasible for the epitaxial growth of two transition metal substituted AlPO4-5 with different morphologies. Further work on the application of this Ti-/Cr-AFI core-shell composite in catalysis is underway. Acknowledgment. We acknowledge the special funding support from the National Natural Science Foundation of China and the State Basic Research Project of China (Grants: 2006CB806103 and 2007CB936402). W.Y. thanks the support by the Program for New Century Excellent Talents in University (NCET) and the Outstanding Youth Fund of Jilin Province (20060120). Supporting Information Available: SEM images of Ti-AFI particles formed in the reaction system of the Ti/Cr-AFI composites, the surface of the kiwi fruitlike Ti-AFI crystal, the less-developed TiAFI crystal, and the AFM images of the surface of Cr-AFI crystals. This material is available free of charge via the Internet at http:// pubs.acs.org.

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