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Framework Determination of a Polytype of Zeolite Beta by Using Electron Crystallography Tetsu Ohsuna,*,† Zheng Liu,‡,§ Osamu Terasaki,‡,| Kenji Hiraga,† and Miguel A. Camblor⊥ Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan, CREST, Japan Science and Technology Corporation, Tohoku UniVersity, Sendai 980-8578, Japan, Department of Physics and CIR, Tohoku UniVersity, Sendai 980-8578, Japan, and Industrias Quimicas del Ebro, Poligono de Malpica, calle D, no 97, 50057 Zaragoza, Spain ReceiVed: August 7, 2001; In Final Form: December 23, 2001
During a trial of synthesizing pure silica zeolite beta, we found recently ITQ-14 was a multiphase crystal and contained small peculiar pillars overgrown on ordinary beta zeolite. We have developed a new method applicable for solving a structure of this kind, i.e., Fourier reconstruction of high resolution transmission electron microscopy (HRTEM) images and enhancement of the atomic positions in blurred potential map obtained by the reconstruction with a support of Patterson map derived from selected area electron diffraction (SAED) patterns, which contain structural information with much higher resolution than that of HRTEM images. The new framework solution obtained by this method turned out to be a structure type C of beta, which was hypothetically proposed by Newsam et al., and we reported it briefly with observation results of surface structure of the framework. In the present paper, basic idea and the determination process are fully discussed.
Introduction Because the framework structure was independently solved by Newsam2 and Higgins3, zeolite beta, the first synthetic high Si/Al ratio large pore zeolite with a three-dimensional(3d-) channel system,4 has attracted much interest and has been demonstrated to be useful in a number of acid-catalyzed reactions. Both Newsam and Higgins proposed the framework structure models based on HREM images, ED patterns, and the comparison of observed powder XRD patterns with the simulated ones. They found that the structure of beta contained planar faults and that the model was described by polytypic series of layers. Two different ordered “end members” of the series, polytype A (with space group symmetries P4122 and P4322) and B (with space group C2/c) were proposed, and a third polytype was also suggested as a hypothetical structure without experimental evidence. Recently, we studied the structure of silica ITQ-14 synthesized by the same way as previously reported5 and found a new phase which is neither polytype A nor B. To solve a structure of the new phase, we should develop a new method which was suitable for the small pillared crystal. Then we have reported the framework structure of the new phase and the surface termination of the framework without details of the structure determination process1. In this report, the process using electron crystallography is described. Framework structure determination or characterization of zeolites is usually carried out by the single crystal X-ray diffraction (XRD) method, if large single crystal can be synthesized. In most cases, however, synthesized crystallites are in the range of microns in size and contain planar defects. They * To whom correspondence should be addressed. E-mail: ohsuna@ imr.tohoku.ac.jp. Phone: +81-22-215-2128. Fax: +81-22-215-2126. † Institute for materials research, Tohoku University. ‡ Japan Science and Technology Corporation, Tohoku University. § Present address: Institute for multidisciplinary research for advanced materials, Tohoku University, Sendai 980-8577, Japan. | Department of Physics and CIR, Tohoku University. ⊥ Industrias Quimicas del Ebro.
are sometimes multiphase (including polytypes). In these cases, the crystal structure is hardly determined by using the ordinary XRD method. Transmission electron microscopy is the only method for solving framework structure in the above cases, because both SAED pattern and HRTEM image can be easily obtained from a single crystalline region. Some framework structures were speculated from the HRTEM image3,4,6,7 without quantitative intensity analysis. Because recent high resolution type electron microscopes have a point resolution of ca. 0.18 nm or higher, determination of framework structures from HRTEM image seems to be easy. However, zeolites are so sensitive under a high density of electron irradiation, which is necessary for taking images at high magnification, that the point resolution of an observed HRTEM image of a zeolite is lower than 0.2 nm. Therefore, the positions of the T atoms could not be derived directly from HRTEM images. In the previous works, T-atom networks were extracted from HRTEM images by trial and error method together with deep insights. Electron crystallography, a method to solve crystal structure by using quantitative analysis of electron diffraction and/or HRTEM image intensity, has been developed for the last 20 years.8-16 Especially, Fourier reconstruction of HRTEM images to retrieve 3d-crystal structure was applied to inorganic crystals, Staurolite,8 and Pd3P11 and also succeeded in solving some unknown structures of mesoporous materials.15,16 According to the previous works, at least five HRTEM images are required to retrieve atom positions of the inorganic crystals by Fourier reconstruction. When it is difficult to take enough of the HRTEM images, quantitative SAED intensity measurements coupled with the direct method is sometimes useful. Recently, Wagner et al. solved the crystal structure of the new zeolite, SSZ-48 (now framework type was coded as SFE by IZA), using the direct method with SAED intensities.14 Furthermore, we have confirmed that it is sometimes possible to find a framework topology of a zeolite, such as FAU, by the same way of Wagner’s work when the specimen thickness is less than ca. 50 nm. It is surprising that such a thick specimen is allowed
10.1021/jp0130463 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002
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Ohsuna et al. The conception of the SOMEM is a peak enhancement in a potential distribution by multiplication with the same distribution shifted by a vector between two atom positions. Figure 1a-c show schematic drawings of the idea for one-dimensional(1d-) case. A potential distribution V(x) has two atom peaks at x ) x1 and x2 with a vector d12 between them (Figure 1a), and V(x - d) has the same profile of the V(x) but shifted by a vector d (Figure 1b). When d is the same as d12, a multiplied distribution of V(x)‚V(x - d) has only one peak at x2, and the peak has sharp profile more than that of V(x). If d is not close to d12, no peaks appear in the multiplied distribution. Figure 1c shows a distribution of V(x)‚{V(x - d12) + V(x + d12)}. One can see in the figure that there are two enhanced peaks at x ) x1 and x2. Namely, when a vector between two atom positions is known, the peak profiles corresponding to the two atoms can be enhanced by this treatment. As far as position and visibility of the peak are concerned, but not absolute value (atomic species), the SOMEM makes big improvement for retrieving the atomic positions. If the V(x) is blurred and the two shifted profiles are overlapped too much, an artificial peak may appear at the center of the two atom positions in the final distribution, and two peaks corresponding to the atom positions might disappear. In such cases, the summation should be replaced with the maximum value of the shifted two, what is max[V(x - d12),V(x + d12)]. The 3d potential V(x) is an inverse Fourier transform of crystal structure factor F(h):
Figure 1. Schematic drawings of (a) V(x), (b) V(x - d12), and (c) V(x)‚{V(x - d12)+V(x + d12)}.
for the electron crystallographic method to retrieve the framework structure of the zeolite. That is to say, although the diffraction intensities include significant dynamical scattering effects, the framework topology can be obtained from the intensity data set (in other words, the intensity data set keeps information about the feature of the framework). Also, the effect of dynamical scattering on the Patterson map was simulated by using the multislice method, and then it was confirmed for the most important part that the peak positions in the map corresponding to nearest neighbor T-T pairs did not move for the thickness range we discussed. We guess that the low density of the zeolite and the characteristic structural feature of the framework topology are the main reasons of the success in finding its framework structure. However, for the present case, the structure of the new phase of zeolite beta could not be solved by either Fourier reconstruction or direct method. Therefore, we took a new approach, which consists of Fourier reconstruction of the HREM image and the Patterson map derived from SAED patterns for retrieving T-atom positions in 3d-framework topology. To obtain a reasonable framework structure, a refinement was carried out for T- and O-atom positions using a similar way as that in the DLS program.17 All of the calculations were carried out by a software package developed by one of the authors (T.O.). New Approach to Enhancement of the Atom Position by Using Patterson Map When source HRTEM images have low resolution and/or only a few HRTEM images are available, a potential density map derived from Fourier reconstruction of the HRTEM images becomes blurred (low resolution), and it is difficult to find atom positions in the potential map directly. To retrieve atom positions from the blurred density map, we introduced a shift-overlap multiply enhancement method (SOMEM).
V(x) )
∫ F(h) exp(-2πi xh) dh ) N
∑
F(hn) exp(-2πixhn) (1)
n)-N
Where x is a position in real space, and h is a reciprocal vector related to reflection index. If a small number of F(h) is taken to build the V(x), V(x) is not well resolved and is blurred. Then it is difficult to find potential peaks corresponding to atom positions in V(x). In practicality, we can obtain the vectors between two atom positions from a Patterson map, which corresponds to a summation of vectors between any two atoms in the crystal. Patterson map P(x) is defined from inverse Fourier transformation of diffracted intensity I(h) by
P(x) )
∫ I(h) exp(-2πixh) dh ) M
∑
I(hm) exp(-2πixhm) (2)
n)-M
I(h) ) |F(h)|2 Therefore, the map is independent of phases of structure factors, F(h). Some peaks are searched in the P(x), and positions of the peaks are assigned to di. Overlapping of V(x) shifted by di is VSO(x)
VSO(x) ) max[V(x - di)] i and, VSOM(x) is produced by multiplying V(x) and VSO(x)
VSOM(x) ) V(x) VSO(x) Calculated results of the enhancement process in an ideal 1d crystal with lattice parameter a (including a Si-O-Si bonding and a cavity, namely, a zeolite) are shown in Figure 2. Figure
Determination of a Polytype of Zeolite Beta
Figure 2. Schematic drawings of (a) V(x), (b) P(x), (c) VSO(x), and (d) VSOM(x) of 1d zeolite. The 1d crystal consists of two Si atoms (coordinate x ) a/10 and -a/10) and an O atom (x ) 0.0). Peak positions in the P(x) used to calculate VSO(x) are indicated by arrows in part b.
2a shows V(x) with N ) 4 in eq 1. Fourier transformation of fewer numbers of structure factors makes a blurred potential distribution. Figure 2b shows a Patterson map with M ) 20 in eq 2. Two peaks in the Patterson map corresponding to Si-Si distance are searched (indicated by arrows), and vectors from the origin to these peaks are used to the shift of V(x), and then VSO(x) is obtained by overlapping (taking maximum value, not summation) of the two shifted V(x) (Figure 2c). Figure 2d shows the multiplied distribution of VSOM(x) which has two enhanced peaks. After this process, the result VSOM(x) has enhanced peaks, but the quantity of the peak potential is modified. Therefore, this method is effective for such a case, where atom coordination and bond distances are known but only the topological information is required, i.e., framework of zeolite. Demonstration of SOMEM, for the Case of the Well-Known Framework Structure of LTL In this section, SOMEM is demonstrated in 3d by using calculated structure factors obtained from atomic positions of a known framework of zeolite L (LTL). The framework structure of LTL has a hexagonal cell with lattice parameters of a ) 1.84 nm and c ) 0.75 nm, and a space group of P6/mmm. The 12, 8, 6, and 4 membered rings (MRs) exist in the framework, and one can see all of them in the projection along the [001] direction (Figure 3a). Two data sets of calculated structure factors with resolution limits of 0.2 and 0.4 nm, which are higher and lower resolution for T-T distances, are obtained from the atom positions of LTL.18 The numbers of structure factors of the data sets are 1152 for 0.2 nm resolution and 128 for 0.4 nm resolution, and all of reflections in the each reflection sphere are taken into account. Figure 3b,c shows sections parallel to the (001) plane at z ) 0.5 of the inverse Fourier transformed
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Figure 3. (a) Schematic drawings of framework of LTL projected along [001] direction. Section parallel to the (001) plane at z ) 0.5 of V(x) with resolution of (b) 0.2 and (c) 0.4 nm. (d) A contour map of (c) selected region indicated by rectangle. (e) VSOM(x) of calculated from the blurred distribution with resolution of 0.4 nm and Patterson map with resolution of 0.2 nm. (f) A contour map of (e) selected region indicated by rectangle.
density maps obtained from the data sets of 0.2 and 0.4 nm resolution, respectively. A T-atom position in the 8 MR of the framework shown by arrows in Figure 3a is resolved in the density map of 0.2 nm resolution(Figure 3b) but not in that of the 0.4 nm resolution (Figure 3c,d). Using the square of moduli, i.e., intensity, of the data set of 0.2 nm resolution to make the Patterson map, SOMEM was applied to retrieve the missing atom position in the density map of 0.4 nm resolution. After 25 peaks in a distance range of 0.2-0.4 nm from the origin of the Patterson map were chosen, a shifted and overlapped density map VSO(x) was calculated by using the 25 vectors, and then VSOM(x) was obtained. One can see that the atom position is resolved in the VSOM(x) as indicated by an arrow in Figure 3e,f. The choice of the peaks in the Patterson map are essential for the SOMEM. Experimental Section Synthesis of ITQ-14 in pure SiO2 form was the same as in refs 1 and 5. For TEM observation, crystallites of ITQ-14 were crushed and dispersed in ethanol by ultrasonic method. The suspension was dropped onto a holey carbon film. HRTEM observation was performed with a 400 kV electron microscope (JEM-4000EX). HRTEM images and SAED patterns were recorded with a slow scan CCD camera (Gatan 694) and some of the images were taken with a high sensitive film (MEM: Mitsubishi Electron Microscope film). The images taken on the films were digitized as 256 Gy scale data using a film scanner (AGFA SelectScanPlus).
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Figure 4. Low magnification TEM image of ITQ-14 showing the overgrowth of a new phase, overgrown crystals indicated by arrows, on the basal crystallite which is a mixture of polytypes A and B of BEA.
Figure 5. (a) HRTEM image of the overgrown crystal taken with the incident beam parallel to [001] direction and (b) its corresponding ED pattern.
A software package for all of the quantitative calculations in the present works was written in C++ for MacOS by TO. Functions included in the package are acquisition of integral intensity and phase of diffraction beam, Fourier transformation of HRTEM image, 2d symmetric averaging and CTF correction for HRTEM image, unit cell determination from a series of SAED patterns with different incident directions, merging intensities of 2d-data sets, inverse 3d-Fourier transformation, SOMEM process, retrieving frameworks from 3d-peak search result, HRTEM image simulation with weak phase object approximation(WPOA), and refinement of atom positions in a framework. Calculations of each function take from one second to a few minutes by PowerPC-G3 400 MHz processor. Results and Discussions Figure 4 shows a low magnification TEM image of a crystallite of the synthesized ITQ-14. Many small pillars, which grow from basal crystals as indicated by arrows, can be seen in the image. The basal region corresponds to a mixture of polytypes A and B of zeolite beta. The small pillars are the new crystal with new framework structure solved in the present work. 1. Unit Cell and Space Group Determination. Figure 5a shows an HREM image taken with the incident beam parallel to the long direction of the pillars. The image shows planegroup symmetry p4mm. A corresponding SAED pattern is shown in Figure 5b. Both the image and the ED pattern show 4-fold symmetry, and therefore, the crystal is tetragonal or cubic. The tetragonal cell was confirmed by SAED patterns taken from
Figure 6. HRTEM image of the overgrown crystal taken with the incident beam parallel to (a) [100] and (c) [110] directions, and corresponding ED patterns (b) and (d), respectively.
the same area with the incident beam parallel to several other directions, and the unit cell parameters were determined to be a ) 1.31 nm and c ) 1.38 nm. Figure 6a,b shows HRTEM images taken with the incident beam parallel to the [100] and [110] directions, respectively, and the corresponding diffraction patterns are shown in Figure 6c,d. One can see that both images have a plane-group symmetry, p2mm. Reflection conditions of the crystal observed from SAED patterns were (i) hkl, no conditions, (ii) hhl, l ) even, and (iii) 0k0, no conditions. Some reflections of 00l (l ) 2n + 1) indicated by arrows in Figure 6b have strong intensities. However, by comparison with Figure 6d, we can confirm that the reflections are produced by the dynamical scattering effect; thus, the reflections can be regarded as extinctions. From the reflection conditions and the planegroup symmetries of the HRTEM images, the space group of the crystal structure could be determined uniquely to be P42/mmc. 2. Intensities and Phases Acquisition from SAED Patterns and HRTEM Images. Seven SAED patterns were taken with the incident beam parallel to the [100], [210], [310], [320], [001], [014], and [013] directions from the regions thinner than 30 nm in thickness. Integral intensities of diffraction spots, I(h), were obtained with subtracting the planer background, which was derived for each spot using a nonliner least-squares calculation. The seven 2d-data sets of intensities are merged
Determination of a Polytype of Zeolite Beta
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Figure 7. Inverse 2d-FFT image derived from the structure factor data set of (a) {F100(h)} and (b) {F001(h)}, respectively.
into one 3d-data set of {I(h)} by excluding saturated spots and those weaker than 3σ and by averaging with Laue group symmetry (4/mmm). Forbidden reflections generated from dynamical scattering were omitted based on the space group P42/mmc. Other intensity corrections, i.e., for the dynamical scattering effect or for Ewald sphere curvature, were not taken into account. The number of reflections in the data set including symmetrically equivalent ones was 1618, the minimum spacing of the reflections was 0.115 nm (corresponding to the 3‚11‚1 reflection), and the density of reflections in the reflection sphere was 26%. Two Fourier diffractograms of the images were obtained from two HRTEM images taken with the incident beam parallel to the [100] and [001] direction of thin regions by two-dimensional fast Fourier transformation (2d-FFT). Intensities and phases of reflection spots in the Fourier diffractograms were measured. The intensities and phases are applied with plane group (p2mm or p4mm) symmetric averaging for the [100] or [001] direction and contrast transfer function (CTF) correction. Optical parameters for calculating CTF were Cs ) 1.0 mm, Cc ) 1.6 mm, semi-angle of beam divergence R ) 0.5 mrad, energy spread of the incident beam dE ) 1.5 eV, and defocuses of both two images were close to Scherzer defocus. Assuming the WPOA, a value of the intensity with phase, which was obtained from the Fourier diffractograms of HRTEM image, can be regarded to be proportional to the crystal structure factor F(h) multiplied by CTF
Iimage(h) ) FT{IHRTEM(x)}(h) ∝ CTF‚F(h)
h*0
where FT means Fourier transformation, IHRTEM(x) is the intensity at x in the HRTEM image, and Iimage(h) is a complex value (with module and phase). In the present work, the complex value of Iimage(h) divided by CTF was regarded to be the structure factor F(h), because the absolute value of the crystal potential was not discussed. Figure 7a,b shows density maps calculated by inverse 2d-FFT of structure factors obtained from HRTEM image of [100] and [001] incidence, respectively. It is clear that artificial contrast, which is dark contrast at the center of channels in the original HRTEM image, was removed by the CTF correction. According to the space group symmetry, the data set of {F100(h)} obtained from the [100] HRTEM image was duplicated as a data set for [010] incidence. Here, {FuVw(h)} means a 2d-data set of structure factors with index h normal to the uVw zone axis. Three data sets, {F100(h)}, {F010(h)}, and {F001(h)} were merged into a 3d-data set of {F(h)} with regard to a consistence of phase matching. Especially, the origin of the data set {F010(h)} duplicated from {F100(h)} had to be moved. The number of structure factors was 220, the minimum spacing of the reflections was 0.254 nm (025 reflection), and the density of reflections in the reflection sphere was 40%. 3. Procedure for Finding Framework Structure. A potential density distribution V(x) was calculated by inverse 3d-FFT of {F(h)}, and seven peaks on asymmetric position were sought
Figure 8. Peak search results of (a) V(x) and (b) VSOM(x), respectively. Circles show peak positions in the both density distributions. Simulated HREM images with the incident direction of (c) [100] and (d) [001] are calculated by using a framework structure made from the peak positions indicated by filled circle in (a). Parts e and f are simulated HREM images of [100] and (d) [001], respectively, which correspond to b.
in the potential shown in Figure 8a. To find the correct framework, the observed HREM images were compared with simulated ones calculated from several frameworks, which consist of some Si atoms on the positions picked from the seven asymmetric peaks and compensated O atoms. The compensation of O atoms was performed as a procedure; after O atoms were put temporally at the centers of two neighboring Si atoms, all atom positions were refined by using a simple molecular mechanics calculation (a similar way as in the DLS program), which is a least-squares minimization of Si-O bond length and O-O distance in each SiO4 tetrahedron for the given mean bond length, 0.16 and 0.26 nm, respectively. In the present case, no framework structure could be regarded to the correct framework. Figure 8c,d shows simulated images calculated by using the most suitable one of the framework (filled circles in Figure 8a show the selected peak positions to produce the framework), and one can see significant difference between the simulated images and the observed ones. Therefore, SOMEM was applied. A Patterson map P(x) was calculated by inverse 3d-FFT of {I(h)}, and six peak positions di were searched in a distance range of 0.2-0.4 nm from the origin in the P(x). Especially, the origin was added into the set of peak positions di, and it was necessary for the present case. We guess the reason is that the SOMEM effect is too strong for the case. A new distribution VSO(x) was obtained by taking the maximum value of V(x - di):
VSO(x) ) max[V(x - di)] i
i ) 1∼7
VSOM(x) was calculated by multiplying V(x) and VSO(x). A total of six asymmetric peaks were sought in the VSOM(x) shown in Figure 8b, and then several frameworks were made by the same way as described above. By comparing the observed HRTEM images with simulated ones of the frameworks, only one framework had good agreement (Figure 8e,f). The filled circles in Figure 8b showing peak positions in the VSOM(x) made the framework. T-atoms in a 4MR indicated by arrows in Figure 8b were retrieved by the SOMEM.
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Figure 9. Schematic drawing of retrieved structure projected along [100] direction. 12 MR is surrounded by 6, 5, and 4 MR.
TABLE 1: Structural Data of the New Phase of Zeolite Beta atom
Wyckoff notation
Si1 16r Si2 8p Si3 8n O4 16r O5 16r O6 8q O7 8p O8 8o O9 4k O10 4h lattice parameters mean bond length mean bond angle
x
y
z
0.120 1/2 0.312 0.401 0.204 0.141 1/2 0 1/2 1/2
0.377 0.116 0.378 0.115 x 1/4 0.328 0.170 0.325 0.190 0.354 0 0.132 0.629 0.351 0.139 0.352 0 1/2 0.129 a ) b ) 1.31 nm, c)1.38 nm ) 0.164 nm ) 109.4°, ) 157.0°
Figure 9 shows a schematic drawing of the resultant framework projected along the [100] direction. One can see 4, 5, 6, and 12MR corresponding to the bright contrast dots with different sizes in the HREM image shown in Figure 6a. Atom coordinates, lattice parameters, mean bond lengths, and mean bond angles are listed in Table 1. It turns out that the structure solution is the same as that of the hypothetical polytype C proposed by Newsam et al.3 and that the atom coordinates are almost the same as those proposed by them by shifting the origin to the same position. Conclusions Although the C-type framework structure had been hypothetically proposed by Newsam et al., we derived the new structure without using the knowledge. That is, the method can be regarded as an ab initio treatment. It is easier to compare observed HREM images and simulated ones based on the C-type model, but that is only confirmation, and the uniqueness of the solution, which is important, will not be revealed. We believe that an advantage of our approach lies in this point. When several HRTEM images are taken with high resolution and CTF correction is available, a Fourier reconstruction method for making a 3d-potential map from the HRTEM images might be a more robust way to retrieve the crystal structure. However, for zeolites, it is very difficult to take several HRTEM images with higher resolution than the Si-O distance. An advantage of the SOMEM is that the probability of success for retrieving
framework structure becomes higher, even if only less intensity data observed from HRTEM images and SAED patterns are available. In compensation for the advantage, it is necessary for finding a good condition of the SOMEM process. Therefore, to reduce the time somehow for the trial and error process, we developed the software package. After our previous report, we have noticed that framework topology of (Me3N)6[Ge32O64](H2O)4.5 reported by Conradsson et al.19 is the same although the space group symmetry is different. Also a short time after our publication Corma et al. reported synthesis of the new Si1-xGexO2 partially Ge substituted phase crystal in a single phase and determination of the structure by powder X-ray diffraction.20 The framework topology of our result and of those refs 19 and 20 are exactly the same. The space group of ref 20 is the same as ours, and of course, the values of atom positions by them are more refined than the present result. We can say that our result and existence of the new phase are confirmed by the X-ray experiment by them. Recently the framework-type for the crystals has been coded as BEC by the Structure Commission of International Zeolite Association. Acknowledgment. This work was supported by CREST, Japan Science and Technology Cooperation. References and Notes (1) Liu, Z.; Ohsuna, T.; Terasaki, O.; Camblor, M. A.; Cabanas, M.-J. D.; Hiraga, K. J. Am. Chem. Soc. 2001, 123, 22, 5370. (2) Newsam, J. M.; Treacy, M. M. J.; Koestsier, W. T.; de Gruyter, C. B. Proc. R. Soc. London 1988, A420, 375. (3) Higgins, J. B.; LaPierre, R. B.; Schlender, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T. Zeolites 1988, 8, 446. (4) Wadlinger, R. L.; Kerr, G. T.; Rosinski, E. J. U.S. Patent 3,308,069, 1967. (5) Camblor, M. A.; Barrett, P. A.; Diaz-Cabanas, M. J.; Villaescusa, L. A.; Puche, M.; Coix, T.; Perez, E.; Koller, H. Microporous Mesoporous Mater. 2001, 48, 11. (6) Anderson, M. W.; Terasaki, O.; Ohsuna, T.; Philippou, A.; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (7) Chan, I. Y.; Labun, P. A.; Pan, M.; Zones, S. I. Microporous Mater. 1995, 3, 409. (8) Downing, K. H.; Meisheng, H.; Wenk, H.-R.; O’Keefe, M. A. Nature 1990, 348, 525 (9) Lu, B.; Li, F. H.; Wan, Z. H.; Fan, H. F.; Mao, Z. Q. Ultramicroscopy 1997, 70, 13. (10) Gjonnes, H.; Hansen, V.; Berg, B. S.; Runde, P.; Cheng, Y. F.; Gjonnes, K.; Dorset, D. L.; Gilmore, C. J. Acta Crystallogr. 1998. A54, 306. (11) Carlsson, A.; Oku, T.; Bovin, J.-O.; Wallenberg, L. R.; Malm, J.O.; Schmid, G.; Kubicki, T. Angew. Chem., Int. Ed. 1998, 37, 1217. (12) Sinkler, W.; Marks, L. D. Ultramicroscopy 1999, 75, 251. (13) Weirich, T. E.; Zou, X.; Ramlau, R.; Simon, A.; Cascarano, G. L.; Giacovazzo, C.; Hovmoller, S. Acta Crystallogr. 2000, A56, 29. (14) Wagner, P.; Terasaki, O.; Ritsch, A.;. Zones, S. I.; Davis, M. E.; Hiraga, K. J. Phys. Chem. B, 1999, 103, 8245. (15) Carlsson, A.; Kaneda, M.; Sakamoto, Y.; Terasaki, O.; Ryoo, R.; Joo, S. H. J. Electron Microscopy 1999, 48, 795 (16) Sakamoto, Y.; Kaneda, M.; Terasaki, O.; Zhao, D. Y.; Kim, J. M.; Stucky, G.; Shin, H. J.; Ryoo, R. Nature 2000, 408, 449. (17) Baerlocher, Ch.; Hepp, A.; Meier, W. M. ETH Zurich Report, 1977. (18) Barrer, R. M.; Villiger, H. Z. Kristallogr. 1969, 128, 352. (19) Conradsson, T.; Dadachov, M. S.; Zou, X. D. Microporous Mesoporous Mater. 2000, 41, 183. (20) Corma, A.; Navarro, M. T.; Rey, F.; Rius, J.; Valencia, S. Angew. Chem., Int. Ed. 2001, 40, 2277.
