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Where Are the Active Sites in Zeolites? Origin of Aluminum Zoning in ZSM-5 Nadiya Danilina,† Frank Krumeich,‡ Stefano A. Castelanelli,† and Jeroen A. van Bokhoven*,† Institute for Chemical and Bioengineering and Laboratory of Inorganic Chemistry, ETH Zurich, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland ReceiVed: January 21, 2010; ReVised Manuscript ReceiVed: February 18, 2010
We visualized the aluminum zoning in TPA+-templated crystals of ZSM-5 by EDX mapping of cross sections obtained by milling of the crystal with a focused ion beam. Aluminum was preferentially located in the rim, while silicon and oxygen were well distributed. Because of its limited penetration, EDX of the large uncut crystals failed to detect the enrichment. Desilication and dealumination by base and acid leaching supported the results of FIB/EDX mapping, showing that the crystals have a dissolved interior or etched rims. To understand the origin of the heterogeneous distribution of aluminum within the crystal, the location of aluminum and silicon after the synthesis of crystals of TPA+-templated ZSM-5 was determined by analyzing the crystals at different stages of synthesis. The crystals had different sizes, and all showed aluminum zoning. The catalytically active sites are located almost exclusively at the rim of the zeolite crystals. These results have improved our understanding of the mechanism of aluminum incorporation into the ZSM-5 framework. Our results may contribute to the controlled locating of active sites within zeolite crystals. Introduction The physical properties and the catalytic behavior of crystalline aluminosilicates are closely related to the aluminum content and its distribution in their framework.1 The structure, shape, and size of zeolite crystals as well as their hydrophobicity and acidic properties are all affected. The spatial distribution of aluminum or aluminum zoning should be differentiated from the distribution of aluminum and silicon over the crystallographic T sites within the zeolite structure.2–5 A T site is a crystallographic position, which is occupied by a tetrahedrally coordinated atom, usually silicon and aluminum. Relatively little attention has been paid to aluminum zoning within a zeolite crystal, even though the phenomenon is well-known. Aluminum zoning is important, not only for the physicochemical characterization of the zeolite crystals but also from the point of view of their application. Because the presence and distribution of aluminum in the zeolite framework are related to the number and strength of the catalytically active acid sites, which affect the effective diffusion path of reactants before they reach an active site, its spatial distribution directly affects catalytic performance. Spatial distribution of aluminum also impacts dealumination and desilication, which improves the molecular transport through the zeolite crystal. Having control of aluminum distribution will enable us to tune catalytic performance and mesoporosity. In the past few years, base-assisted desilication has become an important method for the creation of mesoporosity in the zeolite crystals.6,7 Base leaching of zoned crystals can lead to the dissolution of silicon-rich parts of the crystal without creating mesopores in the aluminum-rich parts of the crystal, which contain the catalytically active sites. Thus, aluminum zoning significantly affects the catalytic properties of the zeolites. In situ optical and interference microscopies have already shown that various acid-catalyzed reactions take place preferentially in the rim of large crystals of ZSM-5.8–15 Styrene * To whom correspondence should be addressed. Fax: +41 44 6331361. Tel: +41 44 6325542. E-mail:
[email protected]. † Institute for Chemical and Bioengineering. ‡ Laboratory of Inorganic Chemistry.
oligomerization in TPA+-templated aluminum-rich ZSM-5 (Si/Al 17) was followed by in situ optical microspectrometry.10 The color of the edges differed from the color of the main body of the crystal because the reaction products were more concentrated in the crystal rim. This was attributed to diffusion limitation; upon dimerization, the carbocation is trapped in the straight pore near the surface and blocks further access of the monomer to the zigzag pores in the interior of the crystal. An acid-catalyzed reaction of two furfuryl alcohol molecules in ZSM-5 crystals was studied by electron backscattering diffraction and fluorescence microscopy.11,13 A negative gradient of product concentration in the interior of the crystal was observed. All the above-mentioned observations were explained by diffusion limitation These reports show that catalytic reactions in large ZSM-5 crystals occur preferentially at the edges of the crystals and, thus, the importance of determining the properties of an individual zeolite crystal. We determined the spatial distribution of aluminum in crystals of ZSM-5 by analyzing individual crystals of varying size. The distribution of aluminum in the bulk and on the surface of zeolites, such as zeolites A, X, Y, mordenite, and ZSM-5, has been studied by means of numerous, preferentially surfacesensitive, techniques. The effects of the synthesis route,16–19 crystal size,20 Si/Al ratio,21 and counter cations22 on the aluminum distribution on the surface and in the bulk were determined. Homogeneous distribution of aluminum in different zeolite crystals of various sizes was revealed by Auger electron spectroscopy (AES), atomic absorption spectrometry (AAS),23 fast atom bombardment mass spectrometry (FABMS),24 electron microprobe analysis (EPMA), X-ray fluorescence (XRF),18,25 and induced-coupling plasma atomic spectroscopy (ICP).17 On the other hand, it was reported based on the results obtained with X-ray photoelectron spectroscopy (XPS) that polycrystalline ZSM-5 consists of a siliceous surface and an aluminum-rich interior.26 A silicon-rich surface was also observed in large ZSM-5 crystals (12 µm × 10 µm × 10 to 280 µm × 130 µm × 130 µm) by means of XPS. Sputter-depth profiling of about 1 µm showed that the outer crystal crust (about 200 nm) is
10.1021/jp1006044 2010 American Chemical Society Published on Web 03/04/2010
Origin of Aluminum Zoning in ZSM-5 aluminum-poor or aluminum-free.27 Enrichment of aluminum on the rim of the zeolite crystals was observed for zeolite powders and single crystals by means of XPS,22 XRF,18 scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX),28 proton-induced gamma-ray emission (PIGE),16 and EPMA.29 Differences in element zoning occur with synthesis. The aluminum-rich rim is often observed for ZSM-5 crystals synthesized in the presence of TPA+ as the template, whereas 1,6-hexanediol leads to homogeneous elemental distribution within the individual crystals.18 Relatively few studies report the distribution of aluminum throughout the crystal volume and not only the composition of the bulk or the surface.20,30,31 This is due mainly to a lack of appropriate techniques for analyzing the interior of a zeolite crystal. The upper layers of the ZSM-5 crystal were rubbed off and polished with diamond paste, and the surface composition was analyzed by EPMA.20 A drawback of the method is that the resulting surface was rough, and the composition was shown only for selected points of the surface. There are reports of SEM and EDX analysis of desilicated Al-ZSM-5 crystals.30,32 An aluminum-rich external surface of ZSM-5 crystals was observed. A promising technique is focused ion beam (FIB) milling used particularly in the semiconductor and materials science fields for site-specific analysis, deposition, and ablation of materials. A previous application of FIB combined with electron backscattering diffraction on zeolites aimed at unraveling the orientation of the zeolite channel network.33 We studied the spatial distribution of aluminum and silicon in TPA+-templated Al-ZSM-5. The zeolite crystals were cut with FIB, and then, EDX mapping of the smooth cut was performed. Furthermore, we desilicated and dealuminated the crystals by treating them with base and acid to establish textural changes during these processes and compare these changes with the results obtained by FIB. To understand the origin of the heterogeneous distribution of aluminum in the crystal, aluminum and silicon were tracked during the synthesis of crystals of ZSM-5 of different sizes by analyzing the crystals at different stages of the synthesis. Experimental Section Al-ZSM-5 crystals were synthesized from a mixture of sodium hydroxide, sodium aluminate, silica sol, and tetrapropylammonium bromide at 175 °C. The growth of the crystals was studied by interrupting the synthesis after 5, 8, 14, 18, 22, 38, 48, 69, 84, and 96 h by quenching the autoclaves with cold water. The zeolite was calcined in flowing air at 500 °C for 12 h. Silicalite was synthesized by the same procedure without adding aluminum. (Details can be found in the Supporting Information.) After quenching, some of the synthesis mixtures were reheated to 175 °C and crystallized for 22 h. To desilicate or dealuminate the crystals, they were treated with NaOH or oxalic acid. For the base leaching, about 250 mg of the crystallites was suspended in a mixture of 4 mL of 30% H2O2 solution and 10 mL of 1 M NaOH solution. For the acid treatment, about 250 mg of the sample was reacted in 25 mL of 0.5 M oxalic acid. The mixtures were transferred to Teflon containers, heated to 120 °C, and kept at this temperature for 20-120 min, depending on the size of the crystals. The resulting solid was separated by filtration, washed extensively with deionized water, and dried at room temperature. X-ray powder analysis was conducted with a STOE STADIP2 diffractometer in transmission mode (flat sample holder, Gemonochromated Cu KR1 radiation) equipped with a positionsensitive detector with a resolution of ∼0.01° in 2θ. Nitrogen
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6641 TABLE 1: Characterization of Zeolite Samples after Different Crystallization Times crystallinity, BET Sext Vmicro % Si/Albulka Si/Alfrb (m2/g) (m2/g) (cm3/g) ZSM-5_5h ZSM-5_8h ZSM-5_14h ZSM-5_18h ZSM-5_22h ZSM-5_38h ZSM-5_48h ZSM-5_69h ZSM-5_84h ZSM-5_96h
11 54 87 95 100 100 100 100 100 100
43 42 41 41 44 40 37 35 34 34
c c c
27 28 34 37 30 34 34
88 156 377 371 371 374 365 355 336 325
74 101 122 120 119 111 135 124 107 90
0.01 0.02 0.11 0.11 0.11 0.11 0.10 0.10 0.10 0.10
a Determined by AAS. b Determined from 29Si MAS NMR (S3 in the Supporting Information). c Quantification was not possible.
physisorption isotherms were collected at the temperature of liquid nitrogen with a Tristar 3000 apparatus from Micromeritics. Prior to the measurements, the samples were degassed for at least 2 h at 10 Pa and 250 °C. The specific surface area was determined by means of the BET method and the specific pore volume by the t-plot method. AAS measurements were performed with a Varian SpectrAA 220 FS spectrometer. The samples were dissolved in an HF/HNO3/water matrix overnight. Quantification of the aluminum content was done by standard addition. For silicon, individual calibration curves were determined for the quantification. 27Al MAS NMR experiments were carried out with a Bruker Avance 700 NMR spectrometer using a 2.5 mm double-resonance probe head. The resonance frequency for 27Al was 182.4 MHz, and the pulse length was 6 µs. All the spectra were obtained at a spinning speed of 15 kHz, and scans were added with a recycle delay of 1 s. The 27Al chemical shifts were referenced to (NH4)Al(SO4)2 · 12H2O. 29Si MAS NMR measurements were performed with a Bruker Avance 500 NMR spectrometer using a 4 mm probe head. The spinning frequency was 8 kHz, and the relaxation delay was 10 s. The 29Si chemical shifts were referenced to octakis(trimethylsiloxy)silsesquioxane. To measure SEM and EDX, the sample was deposited onto a conductive carbon foil supported on an aluminum stub. The investigations were performed with an LEO 1530 Gemini (Zeiss, operated at 1 kV) or with a Quanta 200 (FEI, operated at 5 kV) equipped with an EDX spectrometer (EDAX). Prior to milling with a focused ion beam (FIB), the crystals were coated with a layer of gold. The chosen crystals were cut on a Gemini Zeiss NVision 40 FIB workstation. The instrument incorporated a FIB gallium ion column and a scanning electron microscope (SEM) column tilted to each other at an angle of 52°. The FIB column was adjusted to 3 nA at 30 kV with a specified resolution of 3 nm. The instrument was equipped with four 30 mm2 SUTW detectors (SE + SE In-lens, ESB, EDX, and EBSD) for image acquisition. The EDX mappings of the crystal cuts were performed with the Ametec/EDAX Analytical System Genesis Version 5 apparatus. The voltage was 5 kV in high-current mode. The crystal was positioned at 54° to the detector. The spatial resolution of the EDX mapping was in the micrometer range. Results Table 1 gives the characteristics of the zeolite samples obtained by quenching the synthesis mixture after 5, 8, 14, 18, 22, 38, 48, 69, 84, and 96 h. Figure 1 shows their X-ray diffractograms. All reflections corresponded to the MFI zeolite structure. In the diffractograms of ZSM-5_5h, ZSM-5_8h, ZSM-
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Figure 1. X-ray diffractograms of ZSM-5 after 5, 8, 14, 18, 22, 38, 48, 69, and 84 h of crystallization.
Figure 3. Aluminum (dashed line) and silicon (solid line) distribution along a horizontal (a) and vertical (b) line in the cut crystal of ZSM5_96h; all y axes represent normalized intensity.
Figure 2. Scanning electron micrograph of the FIB-cut crystal of ZSM5_96h (a) and EDX maps representing aluminum (b), silicon (c), and oxygen (d).
5_14h, and ZSM-5_18h, there was a broad visible background, which decreased in intensity with crystallization time. This is attributed to the amorphous part of the sample. The crystallization was complete after 22 h. Figure S1 (Supporting Information) shows the 27Al MAS NMR spectra of the samples. In line with the XRD patterns, the spectra of samples, which were synthesized more than 14 h, showed only one peak at 57 ppm, corresponding to the tetrahedral aluminum in the framework. This peak was also present in the spectra of ZSM-5_5h and ZSM-5_8h together with a broad shoulder between 0 and 45 ppm, which is attributed to various nonframework aluminum species in the amorphous part of the sample. AAS showed that the Si/Al ratio in the bulk varied from 44 to 34, decreasing with increasing crystallization times. The Si/Al ratio in the framework obtained from 29Si MAS NMR was between 27 and 37, more or less corresponding to the values for the bulk. As crystallization time increased, the Si/Al ratio in the framework corresponded more accurately to the bulk values. For samples that were synthesized for less than 22 h, the Si/Al ratio in the framework could not be determined accurately as the samples were still partially amorphous. Nitrogen physisorption revealed that, after 14 h, the BET surface area, the micropore volume, and the external surface area had reached almost constant values of 375 m2/g, 0.11 cm3/g, and 120 m2/g, respectively. Figure 2a shows an FIB cut of a ZSM-5 crystal synthesized for 96 h. The size of the crystals was typically 45 µm × 10 µm × 10 µm. The surface generated by FIB milling was very smooth. Figure 2b-d shows the aluminum, silicon, and oxygen EDX maps of the crystal cut. The edges of the crystal appeared sharp, which indicates the good resolution of the mapping. The
Figure 4. X-ray diffractograms of ZSM-5_96h and ZSM-5/NaOH and a simulated diffractogram of MFI.
EDX mapping of silicon (Figure 2c) and oxygen (Figure 2d) showed a homogeneous distribution of color, whereas the color revealed by aluminum EDX mapping was heterogeneous (Figure 2b). Aluminum tended to be present on the rim of the crystal in a zone about 3 µm thick. A few bright pixels outside the crystal originate from scatter or noise. Figure 3 shows the silicon and aluminum signals along a horizontal (a) and a vertical (b) line. The outer edges of the crystal contained more aluminum than silicon. There was very little aluminum in the core of the crystal. Figure 4 shows the XRD diffractograms of the parent ZSM5_96 and after base leaching (ZSM-5/NaOH). Both diffractograms show sharp reflections, especially for the parent ZSM5_96h; both corresponded to the MFI structure. Base leaching caused preferential dissolution of silica, shown by the decrease in the bulk Si/Al ratio from 34 to 17, as determined by AAS; after treatment with oxalic acid, the ratio remained constant or increased slightly to 35 (Table 2). The textural properties of all the samples were very similar, with the exception of an increase in the external surface area from 90 to 130 m2/g after acid leaching and doubling of the mesoporous volume for the baseleached sample (Table 2). The isotherm of ZSM-5_96h was
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TABLE 2: Chemical Composition of the Bulk and Results of Nitrogen Sorption Measurements for Parent and Baseand Acid-Treated ZSM-5_96h Vmeso Si/Albulk BET (m2/g) Sext (m2/g) Smicro (m2/g) (cm3/g) ZSM-5_96h ZSM-5/NaOH ZSM-5/ox
34 17 35
325 351 350
90 97 130
225 260 220
0.05 0.18 0.08
typical of microporous materials; ZSM-5/NaOH contained a hysteresis of type IV, which is typical of mesoporous materials (Figure 5).34 Figure 6 shows the 27Al MAS NMR spectra of ZSM-5_96h and ZSM-5/NaOH. Both spectra showed a peak at about 57 ppm, which is attributed to the tetrahedrally coordinated framework aluminum in ZSM-5. There is a small broad peak at 30 ppm in the spectra of ZSM-5/NaOH, which originated from an impurity in the probe and/or rotor. The absolute intensity of the spectra of ZSM-5/NaOH was larger based on sample weight, in agreement with the higher Si/Al ratio. No octahedrally coordinated extraframework species, which would have appeared at around 0 ppm, were detected. Figure 7 shows the SEM of ZSM-5_96h, ZSM-5/NaOH, and ZSM-5/ox. The crystals were typically 45 µm × 10 µm × 10 µm. The crystals of the parent sample (Figure 7a) showed the typical twinned coffin-shaped morphology.35 The crystals of ZSM-5/NaOH (Figure 7b) also had this shape but were hollow. The silicon-rich cores of the crystals were leached completely and only 1-3 µm broad rims of the crystals remained. The acidtreated crystals sometimes showed rough surfaces (Figure 7c). Figure 8 illustrates the Si/Al ratios of the surface of the parent and base-leached crystal, as determined by EDX spot analyses.
Figure 7. SEM images of large crystals of ZSM-5_96h (a) and of the NaOH-treated (b) and acid-leached (c) ZSM-5_96h crystals.
Figure 8. EDX analysis and approximate Si/Al ratios on the surface of the parent and base-treated ZSM-5_96h crystals. Figure 5. Nitrogen sorption isotherms of ZSM-5_96h and ZSM-5/ NaOH.
Figure 6. NaOH.
27
Al MAS NMR spectra of ZSM-5_96h and ZSM-5/
The Si/Al ratio for the parent crystal was always between 21 and 24. Due to the enrichment of the surface with aluminum and the limited probing depth, the values were lower compared with the bulk. Thus, within the limits of accuracy and the depth that was probed, no variation was observed. The NaOH-treated crystal consistently showed higher Si/Al ratios (9-12), indicative of the removal of silicon from the volume, as proven by EDX. Figure 9 shows SEM and Si, O, and Al EDX maps of an FIB cross section of a crystal of ZSM-5_48h, 22 µm × 7 µm × 7 µm in size. The SEM image included two diagonal light lines, indicating the presence of intergrowth structures. The Al EDX map barely revealed a positive gradient of aluminum concentration from the middle to the rim of the cross section. Aluminum enrichment occurred in the crystal rim. The aluminumrich rim was not as sharp as for ZSM-5_96h. This is explained by the spatial resolution of EDX for smaller crystals. The Si and O EDX maps showed a homogeneous element distribution.
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Figure 9. Scanning electron micrograph of the FIB-cut crystal of ZSM5_48h (a) and EDX maps representing aluminum (b), silicon (c), and oxygen (d).
Figure 10. SEM images of base-leached crystals of ZSM-5 obtained after 22 (a), 48 (b), and 84 (c) h of synthesis and of NaOH-treated silicalite crystals (d).
Figure 10 shows SEM micrographs of base-leached crystals obtained after different crystallization times. The crystals always had the typical coffin shape, and their average size varied with duration of synthesis. After 22 h, the crystals measured about 10 µm × 3.5 µm × 4 µm, after 48 h, they had grown to 22 µm × 7 µm × 7 µm, and after 84 h, to 30 µm × 9 µm × 9 µm. The leaching of each of these crystals with NaOH resulted in characteristic hollow coffin-shaped crystals, in which the siliconrich core was leached and the aluminum-rich rim preserved. Thus, irrespective of the crystal size, most of the aluminum is present in a narrow strip at the edge of the crystal rim. For comparison, base-treated crystals of silicalite are shown in Figure 10d. In contrast to the aluminum-containing samples, the rim of the crystals was leached to form mesopores, and part of the crystal dissolved or broke off. Discussion EDX mapping of the FIB cross section of the large TPA+templated Al-ZSM-5 crystals identified the spatial element distribution and showed that aluminum is present in a narrow rim of about 3 µm. Aluminum enrichment on the surface of large TPA+-templated ZSM-5 crystals has been already observed by means of PIGE,16 XRF,18 XPS,22 EDX,28 and EPMA.29 However, these techniques probed a depth less than 1 µm, giving information about the surface of the crystals only. Several groups have done the elemental analysis of the zeolite sections after
Danilina et al. rubbing off the upper layers of the crystal19 or depth-sputtering.27 EPMA analysis of a hand-polished middle section of a large ZSM-5 crystal showed aluminum zoning. By means of XPS26 and sputtering-depth XPS profiling,27 an about 200 nm siliceous crust was detected. EDX mapping and EPMA point analysis of a middle cross section of a FIB-cut crystal showed results complementary to those of the analysis of the upper layers with XPS. These techniques characterize a zeolite crystal on a different length scale. There are several reports, where homogeneous element distribution in large ZSM-5 crystals was detected by means of EPMA,17 ICP,17 AES,23 FABMS,24 and XRF.16,24 It was claimed that the Si/Al ratio is constant along the surface of a zeolite crystal, which we also observed. EDX analysis of the surface of the parent ZSM-5 crystal (Si/Al ) 21-24) did not show systematic variation in the Si/ Al ratio and gave lower values than those determined by AAS (34) and 29Si MAS NMR (33). The depth of penetration of the electron beam during the EDX measurement is a few micrometers; the signal was thus dominated by the aluminum-rich rim. The bulk Si/Al ratio and the spatial distribution of aluminum in crystals of this size cannot be directly determined by EDX mapping. EDX mapping of an uncut crystal cannot reveal zoning. More accurate results for the elemental distribution throughout the crystal volume are obtained after cutting the crystal with FIB. FIB can lead to amorphization and contamination of the material. However, very careful polishing of the cross section with a beam of low current diminished the amorphization. Contamination would occur only due to incorporation of gallium ions into the zeolite material, which we did not observe. Base leaching removed the silicon-rich core28 and left the aluminum-rich rim, forming a hollow coffin with edges a few micrometers thick. Similar behavior has been already observed for other ZSM-5 crystals with different morphologies, sizes, and Si/Al ratios.30,31 The preferential leaching of the core of a zeolite crystal is in good agreement with the results of FIB/EDX mapping. Aluminum zoning in individual zeolite crystals plays an important role in the generation of mesopores by desilication or dealumination. It has been shown that the desilication of zoned zeolite crystals results in the dissolution of silicon-rich parts, whereas aluminum-rich parts are less affected.28,29,36 In zeolites, aluminum atoms are generally associated with catalytically active sites, and their location within a crystal affects the catalytic performance.37 Our TPA+-templated crystals of ZSM-5 have catalytically active sites in the rim of the crystals, which yields shorter diffusion paths from the outer surface to the catalytically active site than is suggested by the crystal size. When the position of aluminum in the crystal is controlled, the catalytically active sites can also be located and controlled. The origin of the heterogeneous aluminum distribution throughout the zeolite crystal is still unclear. Until now, it was presumed that mixing of silica and alumina solutions gives a hydrous aluminosilicate sol, which precipitates to a gel by condensation-polymerization.24 Crystal growth begins initially from the silicate species in the solution. As the latter are used up, continuous dissolution of the silica-alumina gel occurs. Therefore, the concentration of soluble aluminate species in the solution and the aluminum content of the outer layers increase. It is assumed that this mechanism will result in a crystal, in which the aluminum content increases steadily from the core to the rim. However, the sharp change in the aluminum concentration of the ZSM-5 crystals cannot be explained by this mechanism.18,28 Furthermore, the fact that the aluminum profile does not depend on the origin of the aluminum source led to the conclusion that specific interactions of the template
Origin of Aluminum Zoning in ZSM-5 with the silicate and the aluminate species bring about aluminum zoning.18 It is interesting that crystals with different sizes, obtained after different crystallization times, always had the aluminum-rich rim. This is a direct indication that the origin of the zoning is more complex than incorporation of aluminum at the later stages of crystal growth due to the aluminum enrichment in the synthesis gel. This is also in agreement with the aluminum-rich rims in crystals that have a Si/Al ratio above 200.28 The presence of an aluminum-rich rim in the crystals with different sizes is due either to the continuous migration of aluminum to the surface of the growing crystal or to aluminum deposition on the surface of the crystals during cooling of the synthesis mixture. A deposition mechanism would explain the sharp gradient of the aluminum concentration in the crystals. Determining the origin of aluminum zoning enables its control and may lead to the design of zeolite crystals with multiple aluminum-rich and aluminum-poor or even aluminum-free zones. Conclusions We proved in a direct way the presence of aluminum zoning in TPA+-templated ZSM-5 crystals. EDX mapping of FIB-cut cross sections and the desilication of the crystals revealed the preferential presence of aluminum in a 2-3 µm thick rim. EDX analysis of uncut crystals failed to determine aluminum zoning. A sharp gradient of the aluminum concentration from the core to the shell of the crystals and aluminum zoning in crystals of various sizes obtained after different crystallization times suggest that aluminum is deposited during cooling of the synthesis mixture. Locating aluminum within an individual zeolite crystal and understanding the origin of the zoning are crucial to controlling dealumination and desilication processes and to locating the catalytically active zones and sites. Acknowledgment. The work was supported by the Swiss National Science Foundation (SNF). We thank Prof. Detlef Gu¨nther and Prof. Bradley Chmelka for helpful discussions. The authors acknowledge the Electron Microscopy Centre of the Swiss Institute of Technology (EMEZ), especially Philippe Gasser and Dr. Karsten Kunze for assistance with FIB and EDX mapping. We thank Dr. Jeff Miller for providing large crystals of ZSM-5. Supporting Information Available: Additional information about characterization of the material and figures related to the reported findings. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Barthomeuf, D. Stud. Surf. Sci. Catal. 1980, 5, 55. (2) van Bokhoven, J. A.; Koningsberger, D. C.; Kunkeler, P.; van Bekkum, H.; Kentgens, A. P. M. J. Am. Chem. Soc. 2000, 122, 12842. (3) Dedecek, J.; Kaucky, D.; Wichterlova, B.; Gonsiorova, O. Phys. Chem. Chem. Phys. 2002, 4, 5406.
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