Beads Comprising a Hierarchical Porous Core and a Microporous Shell

Mar 7, 2007 - Arian Ghorbanpour , Abhishek Gumidyala , Lars C. Grabow , Steven P. ... Hierarchical Porous Zeolite Composite with a Core−Shell Struct...
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J. Phys. Chem. C 2007, 111, 4535-4542

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Beads Comprising a Hierarchical Porous Core and a Microporous Shell Younes Bouizi,† Gerardo Majano,‡ Svetlana Mintova,‡ and Valentin Valtchev*,† Laboratoire de Mate´ riaux a` Porosite´ Controˆ le´ e, UMR-7016 CNRS, ENSCMu, UniVersite´ de Haute Alsace, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France, and Department of Chemistry and Biochemistry, Ludwig Maximillians UniVersity, Butenandtstr. 11, 81377 Munich, Germany ReceiVed: December 1, 2006; In Final Form: January 31, 2007

Two different types of zeolitic materials were structured in core-shell composites possessing a large core (BEA-type) and a thin shell (MFI-type). The core structure comprised a hierarchical porous organization where the access to the zeolite micropores was enabled by macro- and mesopores. This structure was prepared by macroporous anion exchange resin templating, that is, zeolite β was crystallized within the pores of the resin beads. The organic macrotemplate was then removed by combustion leaving stable 300-500 µm macrospheres of zeolite β. Preliminary seeding of the calcined beads with silicalite-1 nanocrystals induced the formation of a well-intergrown silicalite-1 shell during the hydrothermal treatment. The shell thickness did not exceed 1.0 µm, thus providing a material with a very high core-shell aspect ratio. The integrity of the shell layer was tested by N2 adsorption measurements on materials comprising a calcined core and a noncalcined organic templatescontaining silicalite-1 film, thus providing information about the percentage of composite beads possessing defect-free shells. Complementary techniques, such as X-ray diffraction, thermogravimetry/differential thermal analysis, scanning electron microscopy/transmission electron microscopy, energy-dispersive spectrometry, and X-ray fluorescence analyses were employed in order to fully characterize the composites and their intermediates.

Introduction A large variety of zeolitic materials possessing different framework compositions, pore diameters, and channel types offers properties for various applications. Thus, besides the traditional uses in the area of gas separation, cation elimination, and numerous catalytic transformations, a number of new applications ranging from data storage and optical antennas to medical devices involve zeolites.1 The enlargement of zeolite applications requires new materials with specific properties that the known zeolites cannot face. Hence, the quest for new zeolitic materials is still urgent, although 174 structure types are already known.2 Two main approaches are employed in this quest: (i) the discovery of new framework topologies and (ii) the synthesis of known structure types with compositions beyond the limits imposed by the framework. Substantial progress was achieved in both directions during the past several years. Many new structure types were obtained by using the fluoride route of synthesis, new organic structure-directing agents, and germanium as a costructuring agent forming double-four ring units.3 Probably the most amazing discovery lately was the synthesis of pure silica LTA-type material, achieved by Corma and coworkers.4 Without any doubt, these new materials open new horizons for zeolite science and practice. However, they are still limited to the properties of the particular structure type. New prospects would be envisaged if a single material combines the characteristics of two different zeolite types. The opportunities that would open such materials is exemplified by the MWW structure type, where the access to 12 MBr pores goes by 10 MBr windows.5 Consequently, one could imagine materials * Corresponding author e-mail: [email protected]. † Universite ´ de Haute Alsace. ‡ Ludwig Maximillians University.

combining zeolites with different catalytic functionality or separation properties. In order to obtain composite materials with interdependent properties, a particular spatial organization is necessary. For materials with layered organization, for instance, the consecutive deposition of zeolite layers of different functionality might be suitable.6 In the case of bulk threedimensional solids, the core-shell organization seems to be the most appropriate since a shell layer with required characteristics can control the access to the core structure. Shell zeolite layers have been grown on different types of core, followed by the elimination of the core and formation of hollow zeolite capsules. For instance, polystyrene beads of various sizes were employed in the preparation of hollow zeolite spheres7a and bodies with a regular system of macrocavities.7b The preparation of these hollow materials was based on electrostatic adsorption of zeolite seeds, followed by secondary growth under hydrothermal conditions and elimination of the sacrificial core. Hollow zeolite microcapsules have also been prepared employing a layer-by-layer approach, that is, consecutive adsorption of layers of zeolite nanocrystals on a sacrificial template.8 Tang and co-workers9 developed an original procedure, where sacrificial templating mesoporous spheres were employed. After seeding, the spheres were subjected to vaporphase transport synthesis. During the growth of a zeolite shell, the mesoporous silica was consumed, thus leaving a hollow zeolite replica of the templating mesoporous material. This approach enables an easy encapsulation of catalytically active metals deposited on the inner part of the shell.10 The same group reported the preparation of zeolite microcapsules by using CaCO3 and Fe3(SO4)2‚2H2O cores that later on were eliminated using wet chemistry methods.11 Hollow microcapsules offer certain advantages in the development of optical, sensing, semiconducting, and low-weight materials.12 Further, such

10.1021/jp068240+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007

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Figure 1. Schematic presentation of envisaged core-shell materials comprising a large core with a hierarchical porous organization and a very tiny shell built up of intergrown zeolite crystals.

microcapsules might be employed for storage and controlled drug release or as catalytic microreactors. The latter applications require shells with controlled porosity; consequently, the preparation of microcapsules built up of microporous7,8 or mesoporous13 materials with well-defined pore systems is highly desired. However, substantial disadvantages of hollow microcapsules are the low mechanical strength and the absence of functionality of the empty core. Core-shell structures where the access to a large core with specific properties is controlled by a tiny shell with different functionalities would increase considerably the application areas of such materials. In addition, core-shell materials are expected to show much higher mechanical stability in respect to hollow microcapsules. Such a core-shell material, possessing a core and a shell of different porosities, was reported by Yu et al.14 A material comprising a zeolite core and a mesoporous shell was further employed in the preparation of hollow mesoporous carbon microcapsules. The formation of carbon microcapsules was due to the fact that the carbon is difficult to infiltrate in zeolite micropores. Consequently, during the dissolution of the inorganic template, only the carbon replicating the mesopore shell remains. For catalytic and separation purposes, a material combining a microporous shell that could control the access to a microporous or mesoporous core would be much more interesting. For instance, zeolite-zeolite microcomposites might be used for the simultaneous separation and storage of small molecules. Separation by the shell structure and further catalytic transformation in the core might also be performed on such microcomposites. Very recently, the preparation of core-shell zeolitezeolite composites possessing a core and shell of different structural types was reported.15 The preliminary adsorption of nanoseeds on the core crystals and their further secondary growth allowing the formation of core-shell materials was exemplified by the preparation of BEA-MFI15a and mordenitesilicalite-115b core-shell structures. These investigations proved the feasibility of the seeding approach in the preparation of core-shell materials without providing, however, the limits of the method. As is known, the zeolites are metastable compounds that may easily be transformed into other porous or nonporous materials under hydrothermal conditions. Factors controlling the formation of core-shell zeolite composites such as framework

composition, matching of the crystallization fields, structural correspondence, and the rapidity of shell formation were studied in detail employing several zeolite couples.15c All these investigations concern the preparation of a polycrystalline zeolite shell on a single crystal core. The core material, that is, the active part of the composite, was relatively large zeolite crystals (1020 µm). A tiny shell (400-500 nm) was grown on them. However, in order to reach complete coverage of the core, it was necessary to grow two or three consecutive shell layers. Thus, the ratio active part (core)/separating part (shell) decreased dramatically. Larger zeolite crystals could be employed as core materials in order to increase the ratio of the active part, but on the other hand, such crystals would impose considerable diffusion limitations. In addition, the synthesis of single zeolite crystals much larger than 10-20 µm is not a conventional task, and few successful syntheses have been reported.16 Polycrystalline aggregates with regular shape could be employed in order to circumvent these difficulties. Again, a much larger size and the polycrystalline nature might provoke diffusional problems. Therefore, the employment of large polycrystalline structures requires close control of porous organization; namely, a hierarchical porosity comprising macro- and/or mesopores enabling the access to zeolite micropore systems is necessary. The objective of the present investigation was to design microcomposites able to combine a large polycrystalline core structure possessing hierarchical porous organization and a tiny separating shell of different functionality in order to effectively control the access to the core but ensuring rapid diffusion kinetics. Such a microcomposite was realized in the form of a core-shell structure, where the polycrystalline core structure built of zeolite nanocrystals and possessing textural macro- and mesoporosity is used to ensure effective substance diffusion while a thin zeolitic shell is used to provide further control on substance transport. A schematic representation of the envisaged core-shell microcomposites is shown in Figure 1. Materials and Methods Synthesis of Core Structure. Macroporous strongly basic styrene-divinylbenzene anion exchange resin beads (Dowex MSA-1) were used as a shape-directing macrotemplate to produce the zeolite β spheres. The beads were hydrothermally

Beads Comprising a Hierarchical Porous Core treated with a clear solution with the following composition: 0.3 Na2O/9 tetraethylammonium hydroxide (TEAOH)/0.5 Al2O3/ 25 SiO2/300 H2O, where Na2O is a consequence of the use of colloidal silica as a silica source. The syntheses were performed at 100 and 170 °C for durations ranging between 1 and 7 days. The synthesis solution/resin beads ratio was varied between 5 and 50. The reactants used in the synthesis of the zeolite beads were TEA hydroxide (20%, Fluka), colloidal silica (Ludox HS30, Aldrich), aluminum isopropoxide (Aldrich), and distilled water. Silica and alumina solutions were prepared by dissolving respectively a freeze-dried silica sol and the aluminum source in portions of TEAOH. The two solutions were then mixed, stirred for 15 min, and transferred to an autoclave containing the required amount of beads. After the synthesis, the resin-zeolite β composites were separated by filtration from the zeolite crystallized in the bulk, treated with a 0.1 M ammonia solution in an ultrasonic bath for 5 min, rinsed several times in distilled water, decanted, and dried at 90 °C. Finally, the organic macrotemplate was removed by combustion at 600 °C for 8 h, after heating to this temperature with a heating rate of 1 °C min-1. Synthesis of Silicalite-1 Nanocrystals. The reactants used in the synthesis of the zeolite seeds were tetrapropylammonium (TPA) hydroxide (20%, Fluka), tetraethylorthosilicate (TEOS; Aldrich), and distilled water. Silicalite-1 nanocrystals were synthesized from a prehydrolyzed solution with the molar composition 4.5 (TPA)2O/25 SiO2/480 H2O/100 ethanol (EtOH), where the presence of EtOH is a consequence of the use of TEOS as a silica source. The synthesis was performed at 90 °C for 24 h. The zeolite suspensions were purified by four series of high-speed centrifugation (20 000 rpm, 1 h), decanting, and redispersion in distilled water. The resulting suspensions were diluted to 3 wt %, and the pH was adjusted to 9.5 by using 0.1 M NH3. Preparation of the Core-Shell Material. The negative surface charge of the zeolite β beads was reversed using a 0.5 wt % aqueous solution of a polycation agent [poly(diallyldimethylammonium chloride); Aldrich], and then, the negatively charged nanoseeds were adsorbed. Calcination of the resulting pretreated core crystals at 500 °C for 8 h provided a material with seeds firmly fixed to the surface. The secondary growth of the silicalite-1 seeds adsorbed on the zeolite β crystals was performed at 200 °C for different periods of time with a solution having the molar composition 1.5 (TPA)2O/25 SiO2/1500 H2O/ 100 EtOH. After cooling, the composites were treated with a 0.1 M ammonia solution in an ultrasonic bath for 10 min to remove the loosely attached silicalite-1 crystals, rinsed repeatedly with distilled water, and dried at 80 °C. The composites were calcined at 450 °C for 6 h in air. All secondary growth experiments were performed on doubly calcined core materials. A first calcination was used to burn the large amount of anion exchange resin while the second was to provide seeds firmly stuck to the surface of the support. The preparation of composites comprising calcined cores and noncalcined shells was necessary to evaluate the adsorption capacity of the as-synthesized composites and thus the integrity of the shell. Characterization. The obtained zeolite β/silicalite-1 coreshell materials were studied by X-ray diffraction (XRD) using a STOE STADI-P diffractometer in Debye-Scherrer geometry equipped with a linear position-sensitive detector (6° in 2θ) and Ge monochromated Cu KR1 radiation. Electron micrographs were taken on a Philips XL30 FEG scanning electron microscope (SEM) equipped with an EDAX energy dispersive X-ray

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Figure 2. X-ray diffraction patterns of an anion exchange resin/zeolite β composite (a), all zeolite β beads obtained after calcination (b), and zeolite β/silicalite-1 composite (c).

spectrometer (EDS; Oxford ISIS-Energy). The EDS analyses were performed on polished samples using standards. Transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) was used to study zeolite crystals building core and shell structures. High-resolution TEM images and SAED patterns were taken in a FEI-Philips CM200 microscope in low-dose mode operating at a 200 kV accelerating voltage, equipped with a LaB6 electron gun. Dynamic light scattering (DLS; Malvern HPPS-ET) was employed for the analysis of the zeolite suspensions used for seeding. Nitrogen adsorption measurements were carried out with a Micromeritics ASAP 2010 surface area analyzer. The as-synthesized and calcined samples were analyzed after outgassing at 130 and 350 °C, respectively. The elemental analyses of the core crystals were performed on an X-ray fluorescence spectrometer MagiX (Philips). Prior to the analysis, the powdery sample was melted with Li2B4O7 at 1300 °C. The resulting glass bead was analyzed under a vacuum with a rhodium anticathode (2.4 kW). Results Core Structure. The zeolite crystallization within macroporous resins is a complex process depending on the synthesis solution employed, the temperature and the duration of the hydrothermal treatment, and the synthesis solution-to-resin weight ratio used. In the present study, we did not change the molar composition of the synthesis mixture. The variation of the duration and crystallization temperature showed that highly crystalline zeolite β beads can be obtained at 170 °C for 24 h or 100 °C for 7 days. The variation of synthesis solution/resin beads ratio showed that high-quality beads can be prepared when this ratio is higher than 10. Brittle and deformed beads were obtained after the calcinations of a sample prepared with a synthesis solution/resin beads ratio of 5. No crystalline material was formed in the bulk solution in this case. Obviously, the available precursor species were not sufficient to pack the entire volume of the resin beads. The XRD patterns of a resin-zeolite β composite and of the resulting all-zeolite β beads obtained after calcinations are shown in Figure 2. The disappearance of the hump in the 15-30° 2θ range after calcination (Figure 2b) of the material clearly shows that the former originates from styrene-divinylbenzene resin. According to the XRD study, the calcined sample contains only pure highly crystalline BEA-type zeolite without traces of amorphous or other crystalline material. The presence of MFI-type material in the core-shell composites was not detected by XRD (Figure 2c). The latter is not surprising, keeping in mind the very high core-shell mass ratio. As can be seen, the XRD patterns of calcined zeolite β beads

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Figure 3. TEM micrograph of the shell layer synthesized at 200 °C for 45 min. Inset: SAED pattern obtained from the shell layer.

(Figure 2b) and the core-shell composite (Figure 2c) are fairly similar. This is an indirect proof that during shell formation the core structure stays intact, that is, if there is some dissolution of the core, it is very limited. This result confirmed our previous investigation, where the overlapping of crystallization fields of core and shell materials was found to be very important.15c In other words, MFI- and BEA-type materials crystallize at a similar range of Si/Al ratios and under crystallization conditions that allow overgrowth between them without visible dissolution of the core material. The TEM showed that the shell is built of material with a crystalline appearance; that is, particles with well-developed crystal faces can be seen on the top of the layer (Figure 3). Although only diffusive rings due to the intergrowth of zeolite crystals were observed in the SAED diffraction pattern (Figure 3, inset), the reflections consistent with the MFI-type structure were easily distinguished. Initial anion exchange resin beads and zeolite β/resin composite beads are shown in Figure 4a and b, respectively. The material obtained after the combustion of the resin templates consisted of white spheres with a size similar to that of the starting resin beads used (Figure 4c). Cross-section views of the spheres revealed that the crystallized zeolite is homogeneously distributed in the interior of the beads (Figure 4d). The zeolite β beads represent a negative replica of the polymer network building the resin beads. Therefore, after the combustion of the organic part of the composite, a large (meso-/macro-) pore network enabling the access to the zeolite β crystals was formed. The TEM investigation revealed that the interior of the beads was built up by intergrown 10-40 nm zeolite β nanocrystals (Figure 4c, inset). Chemical analysis of zeolite β synthesized in the bulk and, on the other hand, by using resin templates showed similar results; that is, the Si/Al ratio for both was 22. The fact that a hard tool and substantial pressure were needed in order to break the calcined beads demonstrated the impressive mechanical properties of the obtained material. Core-Shell Composites. Initially, the shell synthesis was performed at 200 °C for 6 h. Under such conditions, zeolite β

Bouizi et al. beads and silicalite-1 crystals formed an intergrown body which was difficult to disintegrate and separate into single core-shell beads. Obviously, sedimented crystals formed in the bulk provoked the intergrowth. Therefore, a compromise between the shell growth and the formation of zeolite crystals in the bulk would have to be found. Generally, secondary growth of zeolite seeds precedes the spontaneous nucleation and growth of zeolite crystals in the bulk.17 This particularity of seeded zeolite synthesis was then used to grow the shell layer and avoid abundant growth in the bulk that leads to the aggregation of zeolite beads. The investigation on the kinetics of shell growth revealed that at 200 °C the crystallization process continues up to 1 h of hydrothermal treatment. During this period, abundant sedimentation of silicalite-1 crystals was not observed. The only extra-shell silicalite-1 was observed in the area where the spheres touch each other. In these areas, a relief structure with a circular form can be seen (Figure 5a). Top-view images showed that after a 30 min hydrothermal treatment the beads’ surface is completely covered by a thick layer of 80-100 nm zeolite crystallites. Such a layer is not typical for electrostatically adsorbed seeds, where the support is usually visible below the monolayer of nanocrystals. Hence, the observed crystals were most likely yielded in the bulk. The DLS analysis of the mother liquor confirmed that after a 30 min hydrothermal treatment 80-100 nm particles exist in the solution. The individual crystallites deposited on the zeolite β beads were loosely attached to each other and did not form an intergrown shell (Figure 5b). However, the growth of a seeded film precedes the growth in the bulk, and thus the formation of an intergrown layer below the observed nanocrystals could be expected. After a 45 min synthesis, the zeolite crystals were embedded in an intergrown film, and individual zeolite crystallites were difficult to observe (Figure 5c). The difference between shells grown for 45 and 60 min was the size of the crystals. Thus, larger zeolite crystals with well-developed crystal faces can be seen on the shell surface after 60 min of crystallization (Figure 5d). The development of larger crystals is usually observed during the final stage of zeolite film formation, when, due to steric hindrances, only a few crystals continue their growth.18 Usually, this part of the layer is not well-intergrown, and gaps arising between crystals can be seen.15a Therefore, further prolongation of the synthesis is not expected to improve the separation properties of the film. Type I adsorption/desorption isotherms typical of microporous materials, where a steep uptake at low relative pressures is followed by nearly horizontal adsorption and desorption branches, were recorded for calcined zeolite β beads (Figure 6a). The specific surface area (SBET) of the zeolite β beads core determined by nitrogen adsorption measurement was 741 m2 g-1. For a BEA-type framework, this value indicates a highly crystalline product. The integrity of the silicalite-1 shell, after 30, 45, and 60 min of crystallization, was evaluated by N2 adsorption measurements (Figure 6b-d). As is known, the tetrapropylammonium-containing MFI-type material does not adsorb the N2 molecule, and thus the obtained value corresponds to the external surface area of the material. The SBET of zeolite β beads was used as a reference and compared to the values measured for different composites containing a TPA-silicalite-1 shell and a calcined β core. The obtained data showed that, after 30 min of hydrothermal treatment, about 78% of the zeolite β beads’ area was covered with a defect-free TPA-silicalite-1 layer. This result confirmed that below the loosely attached nanoparticles (after 30 min of crystallization) shown in Figure 5b there was an intergrown zeolite film. The percentage of

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Figure 4. SEM micrographs of (a) initial resin beads, (b) anion exchange beads/zeolite β composite, (c) all zeolite β beads obtained after resin combustion, and (d) a cross-section view of calcined zeolite β beads embedded in an epoxy resin. Inset (c): TEM image of zeolite β crystals building the beads.

Figure 5. SEM micrographs of core-shell material obtained after 60 min hydrothermal treatment of preseeded zeolite β beads in a silicalite-1yielding solution (a). Close SEM views of silicalite-1 shells synthesized at 200 °C for 30 min (b), 45 min (c), and 60 min (d).

covered beads after 45 and 60 min of hydrothermal treatment was 92 and 97%, respectively. However, further prolongation

of the synthesis duration is not likely to reach 100% completed shells since the zeolite film reached a later stage of development

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Figure 6. Adsorption-desorption isotherms of the zeolite β reference sample (a) and zeolite β-silicalite-1 composites comprising a calcined core and a noncalcined shell obtained after shell growth for 30 min (b), 45 min (c), and 60 min (d).

when a few crystals continued growing and thus cannot close the existing pinholes.15a A second intergrown layer will have to be deposited in order to reach 100% beads with accomplished shells. The possibility that the adsorption properties of core-shell composites could be influenced by the adsorption of TPA at the zeolite β pore openings was studied by an experiment where the reference zeolite β sample was hydrothermally treated with a 0.2 M tetrapropylammonium hydroxide solution at 200 °C for 30 h. After rinsing and drying under the conditions used for the composites, the materials were subjected to thermogravimetric (TG) and N2 adsorption measurements. The weight loss in the temperature range of 200-380 °C, attributed to the thermal degradation of the TPA cation, was found to be 4.3%. Indeed, a slight decrease of micropore volume was observed (0.25 cm3 g-1) in respect to the reference sample (0.28 cm3 g-1). However, this value was not significant enough to conclude that the effect was due to pore blocking. If the TPA cations were indeed blocking the pore mouths of zeolite β, a much accentuated decrease would be expected, that is, to a value similar to the core-shell composites (ca. 0.06 cm3 g-1). Thus, the observed decrease in the pore volume is most probably related to a partial dissolution of zeolite β in the highly basic TPAOH solution. Thermal analyses of core-shell composites comprising calcined cores and noncalcined shells showed similar results, for example, a total weight loss of about 12 wt % divided in two temperature ranges. The first one (ca. 3 wt %) was observed in the temperature range 20-200 °C, whereas the second (ca. 9 wt %) was between 230 and 620 °C (Figure 7). The first weight loss coupled to an endothermic effect was attributed to the water desorption of the composite. The series of exothermic peaks corresponding to the second weight loss clearly showed that it is related with the combustion of organic matter. The latter includes the template embedded in the silicalite-1 shell as well as TPA and the charge reversing polymer adsorbed on the surface of zeolite β crystals. Having in mind the coreshell aspect ratio, the contribution of TPA imprisoned in the silicalite-1 shell was considered to be negligible. The major part of the higher-temperature weight loss is most probably due to the species adsorbed on the surface of the nanosized zeolite β

Figure 7. TG/DTA analysis of core-shell composite comprising a calcined core and a noncalcined shell (200 °C, 60 min).

crystals building the core. Presumably, the relatively large volume of the core and slow diffusion via the shell shifted the thermal degradation of the organic structure-directing agent and cationic polymer to temperatures higher than those at which they decompose usually. The chemical composition of a given zeolite is an important characteristic which defines its properties. Thus, the preservation of the basic characteristics of two parts, the core and the shell, is of paramount importance for the preparation of desired new materials. During the crystallization process of the zeolite β-silicalite-1 composite, the highly basic silicalite-1 precursor mixture may react with the zeolite β core, and the extraction of different elements may occur. Thus, a partial dissolution and transport of components from the core to the growing shell may be expected. We have performed EDS analysis on the cross section of a composite embedded in an epoxy resin (Figure 8a). SEM images of the analyzed part of the composite with a profile analysis and the concentration profiles of Si and Al across the composite are shown in Figure 8b and c, respectively. The sodium distribution was not studied, because it cannot be determined precisely by the EDS technique due to electron beam induced cation diffusion. The profile analysis (top to bottom) starts from the resin, through the shell, and down to the composite core. The distribution of Si and Al in the core material

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J. Phys. Chem. C, Vol. 111, No. 12, 2007 4541 In other words, if there is some dissolution and transport of aluminum from the core toward the shell, it is very limited and lies below the detection limit. Conclusions The present study demonstrated the possibility to build zeolite-zeolite core-shell materials possessing a large core part (up to 500 µm) and a tiny shell (ca. 1 µm). Thus, a material with a very high aspect ratio between the active part (core) and separating part (shell) was prepared. The preparation of such core-shell composites was exemplified by the synthesis of a zeolite β-core/silicalite-1-shell material. Core zeolite β beads were prepared by the synthesis of zeolite β in the pores of anion exchange resin beads and elimination of the templating resin by combustion. Thus, an all-zeolite β bulk material possessing meso- and macropores that enable fast transport was obtained. The attempts to synthesize the core-shell zeolite composites by direct in situ growth of silicalite-1 on the latter were not successful. Contrarily, a complete overgrowth of the β beads’ crystals by silicalite-1 was achieved through preliminary seeding of the core material followed by secondary growth of the nanoseeds in a silicalite-1 precursor mixture. Nitrogen adsorption experiments performed on calcined-core/noncalcined-shell composites revealed that, after a single crystallization step, about 97% of the beads have accomplished shells. The current approach presents a new alternative for making possible the construction of other core-shell zeolite composites. Zeolite-zeolite core-shell composites are envisioned/expected to find applications as microreactors combining the core and shell of different activities and selectivities, as materials able to separate and store compounds, and also as biocompatible vehicles for the controlled release of drugs. Acknowledgment. The authors acknowledge the financial support of the French-German bilateral program (Procope) and thank Dr. Henri Kessler (LMPC - Mulhouse) for helpful discussions. References and Notes

Figure 8. SEM micrograph of a zeolite β-silicalite-1 core-shell bead embedded in epoxy resin (a), EDS analysis of the silicon and aluminum content in the composite: SEM image of the analyzed area with the profile analysis (white line, b) and graphical presentation of the silicon and aluminum content (c).

is fairly uniform, as can be seen in Figure 8c. Substantial fluctuation of their concentration can be observed only at the interface with the silicalite-1 layer. As can be seen, the sharp uptake of the Si profile precedes the one emanating from Al in the material. Thus, Si and Al concentrations were recorded along the profile line at distances of 8-9 µm and 10-10.5 µm from the beginning of the analysis, respectively. The data clearly showed that the shell consists essentially of silica (silicalite-1).

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