Chem. Mater. 2007, 19, 1601-1612
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Pure Silica Large Pore Zeolite ITQ-7: Synthetic Strategies, Structure-Directing Effects, and Control and Nature of Structural Disorder L. A. Villaescusa,† I. Dı´az,‡ P. A. Barrett,§ S. Nair,| J. M. LLoris-Cormano,⊥ R. Martı´nez-Man˜ez,† M. Tsapatsis,X Z. Liu,O O. Terasaki,# and M. A. Camblor*,3 Instituto de Quı´mica Molecular Aplicada, Departamento de Quı´mica, UniVersidad Polite´ cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain, Instituto de Cata´ lisis y Petroleoquı´mica, Consejo Superior de InVestigaciones Cientı´ficas (CSIC), C/Marie Curie 2, Campus Cantoblanco, 28049 Madrid, Spain, Praxair, Incorporated, 175 East Park DriVe, P.O. Box 44, Tonawanda, New York 14150-7891, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe NW, Atlanta, Georgia 30332-0100, AIDICO, Vale` ncia Parc Tecnolo` gic, AVda. Benjamı´n Franklin 17, Paterna, 46980 Valencia, Spain, Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455, Research Center for AdVanced Carbon Materials, National Institute of AdVanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba 305-8565, Japan, Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden, and Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InVestigaciones Cientı´ficas (CSIC), C/Sor Juana Ine´ s de la Cruz, Campus Cantoblanco, 28049 Madrid, Spain ReceiVed September 18, 2006. ReVised Manuscript ReceiVed January 8, 2007
Several organic cations derived from the same rigid bicyclic amine 1,3,3-trimethyl-6-azabicyclo[3.2.1]octane have been used as structure-directing agents for the synthesis of pure silica zeolites in the presence of fluoride. The results are rationalized in terms of the varying size and rigidity of the cations. Both concepts were used in defining a large and rigid spiroammonium derivative which afforded the synthesis of the large pore pure silica zeolite ITQ-7. The existence of structural disorder in ITQ-7, which may be controlled through the water/silica ratio of the synthesis mixture, is confirmed by a number of techniques, but high-resolution electron microscopy results ruled out the existence of stacking faults. Thus, disorder in ITQ-7 does not appear to be due to intergrowths and may correspond to the existence of local defects, probably involving double four ring units, causing lattice strain mainly in the ab plane, according to X-ray diffraction simulations.
Introduction Structure-directing effects during the synthesis of zeolites receive well-deserved attention because of the possibility to synthesize materials with novel structure types amenable to new applications. Thus, efforts to understand why a particular set of synthesis conditions affords the crystallization of a specific zeolite among many other possible structures continue, with the final goal of the a priori design of zeolites for target applications. One of the main structure-directing effects in the synthesis of silica and high silica zeolites is exerted by organic cations that end up occluded within the zeolite void spaces (structure-directing agents, SDA).1-5 Here * Corresponding author. E-mail:
[email protected]. † Universidad Polite ´ cnica de Valencia. ‡ Instituto de Cata ´ lisis y Petroleoquı´mica, Consejo Superior de Investigaciones Cientı´ficas. § Praxair, Incorporated. | Georgia Institute of Technology. ⊥ Vale ` ncia Parc Tecnolo`gic. X University of Minnesota. O National Institute of Advanced Industrial Science and Technology. # Arrhenius Laboratory. 3 Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas.
(1) Gies, H.; Marler, B. Zeolites 1992, 12, 42. (2) Gies, H.; Marler, B.; Werthmann, U. Molecular SieVes; SpringerVerlag: Berlin, 1998; Vol. 1, p 35. (3) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4, 756.
we report studies of the structure-directing ability of a series of organic cations with slightly varying size, shape, and conformational flexibility, all derived from the same rigid amine (1,3,3-trimethyl-6-azabicyclo[3.2.1]octane, ABO). The effect of size and flexibility of the cations and of the degree of concentration of the reaction mixtures is discussed. The studies afforded the synthesis of three silica zeolites with novel topologies (structure codes ITE, STF, ISV) and a number of disordered materials. The rationale for the discovery of the large pore silica zeolite ITQ-7 (ISV) is highlighted. We also discuss structural disorder encountered in ITQ-7 materials and how the degree of disorder may be controlled by varying the water/silica ratio in the synthesis mixture. Experimental Section Synthesis of the Organic Cations. ABO was used as the parent amine, and different alkyl halides were used for its quaternization (Scheme 1, disregarding enantioisomerism, see below). Methylation and ethylation reactions were carried out in refluxing CHCl3 for (4) Lobo, R. F.; Zones, S. I.; Davis, M. E. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 47. (5) Kubota, Y.; Helmkamp, M. M.; Zones, S. I.; Davis, M. E. Microporous Mater. 1996, 6, 213.
10.1021/cm0622248 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
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Scheme 1. Organocations Used as SDAs in This Work, All Derived from the Central Amine by Quaternization of Its N Atom Using Different Alkyl Halides
Figure 1. Both enantiomers of EABO-5 as found in C14H28BrNO. Two water molecules and two bromide anions are also included. N, blue; O, red; C, gray; H, white; Br, bronze.
several days using K2CO3‚1.5H2O as the base, followed by washing with ethyl acetate and final recrystallization in MeOH/ethyl acetate. The preparation of the spiro-cations required a slightly more involved procedure, which is based on an old method to prepare quaternary ammonium salts for the study of Hoffmann degradation processes.6,7 1,4-Dibromobutane or 1,5-dibromopentane was added to a NaOH solution and allowed to reflux. Then, the parent amine was added dropwise and allowed to react for 3 h. Addition of a solution of concentrated NaOH precipitates the salt, which is recovered by filtration, washed with diethyl ether, and purified by recrystallization from MeOH/ethyl acetate mixtures. Characterization by 1H NMR and chemical analysis is provided as Supporting Information. The nature of the spirocation synthesized with 1,4dibromobutane was further confirmed by the structure solution of the corresponding bromide (see below). Enrichment of the parent amine in one of the enantiomers was carried out by obtaining their solid diastereoisomer salts by using L-tartaric acid in methanol/ethyl acetate mixtures. This procedure was repeated seven times, followed by the measurement of the optical activity in dimethylformamide after each step. The starting racemate was optically inactive, but we observed an increase in activity after each precipitation with L-tartaric acid, confirming that the procedure afforded the enrichment in one enantiomer. However, no plateau was obtained after these cycles, suggesting we have not completely resolved the racemate. After the seventh precipitation the molal optical activity reached a value of +5.7°. Dissolution of the final salt in a basic aqueous solution (pH ∼ 12) and the subsequent extraction in CHCl3 gave the enriched amine, which was quaternized to obtain the spiro derivative in the same way as carried out for the racemic mixture. Structure Determination of EABO-5 (C14H28BrNO). M ) 306.28, monoclinic, space group P21/c, a ) 8.179(2), b ) 14.494(3), c ) 26.543(4), β ) 97.800(13)°, V ) 3117.5(9) Å3, Dcalcd ) 1.621 g cm-3, λ(Mo KR) ) 0.710 69 Å, T ) 293(2) K, µ(Mo KR) ) 3.281 mm-1. Measurements were carried out using a Siemens P4 diffractometer with graphite-monochromated Mo KR radiation on a colorless crystal of dimensions 0.10 × 0.20 × 0.27 mm. A total of 8687 reflections were collected, of which 4077 were independent (Rint ) 0.0641). Lorentz, polarization, and absorption (ψ scan, maximum and minimum transmissions 0.299 and 0.167) corrections were applied. The structure was solved by direct methods (SHELXTL) and refined by full-matrix least-squares (6) Blicke, F. F.; Hotelling, E. B. J. Am. Chem. Soc. 1953, 76, 5099. (7) Jewers, K.; McKenna, J. J. Chem. Soc. 1958, 2209.
analysis on F2. The refinement converged at R1 ) 0.058 [F > 4σ(F)] and wR2 ) 0.195 (all data). Largest peak and hole in the final difference map were +0.73 and -0.43 e Å-3. The bromide salt of EABO-5, C14H26BrN‚H2O, crystallizes as the true racemic single crystalline phase, and both estereoisomers are depicted in Figure 1, where the closure of a five-ring over N can be seen. Further crystallographic information is provided as Supporting Information. Synthesis of Zeolites. The zeolite synthesis followed a welldefined general procedure. Organic cations were used as solutions in their hydroxide form after anion exchange. Tetraethylorthosilicate was hydrolyzed under stirring at room temperature in an aqueous solution of the hydroxide form of the corresponding organic cation. All the ethanol produced plus some water was allowed to evaporate from the gel. The amount of water evaporated was determined by weight, and if necessary, addition of more water allowed the desired water content of the paste to be fixed. Finally, HF (48%) was added while stirring by hand. Preparation of very concentrated gels was accomplished by extensively evaporating water by heating at 80 °C under vacuum after addition of the HF solution. The overall final composition was always 0.5:0.5:1:n ROH/HF/SiO2/H2O, where ROH is the hydroxide of the organic cation R+ and n e 15. The crystallization was carried out in Teflon lined stainless steel autoclaves which were tumbled at 60 rpm while heated. After heating for a selected time the autoclaves were quenched, and the solids were recovered by filtration and exhaustively washed with deionized water. The products were calcined in ambient air at 580 °C for 3 h. Characterization. The recovered solids were identified by powder X-ray diffraction (XRD) recorded in a Philips X’Pert or a Philips PW1829 diffractometer (Cu KR radiation provided by a graphite monochromator). 19F and 29Si magic angle spinning (MAS) NMR spectroscopy of as-made and calcined samples, respectively, was carried out on a Varian VXR 400S WB spectrometer. 29Si MAS NMR spectra of calcined samples were recorded with a spinning rate of 5.5 kHz at a 29Si frequency of 79.459 MHz and a recycle delay of 60 s. The 19F MAS NMR spectra were recorded at a spinning speed of 7.0 kHz, at a frequency of 376.325 MHz with a recycle delay of 5 s. As-made samples were also investigated by 29Si MAS NMR with a π/2 pulse length of 5 µs and a recycle delay of 60 s and by 29Si{1H} cross-polarization (CP) MAS NMR (contact time 1.5 ms) using a MSL-400 Bruker spectrometer at a frequency of 79.491 MHz. References for chemical shifts were trimethylsilane for 29Si and CFCl3 for 19F. High-resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction (SAED) was used to index the crystal faces and study the possible intergrowths or defects. HRTEM images and SAED patterns were taken with a
Pure Silica Large Pore Zeolite ITQ-7
Chem. Mater., Vol. 19, No. 7, 2007 1603
JEOL ARM 1250 microscope operating at 1250kV (Cs ) 1.7 mm). The images were obtained after the samples were crushed, dispersed in acetone, and placed on a holey carbon microgrid. Disorder Simulations. The XRD patterns of ITQ-7 were calculated using the Reflex module of Materials Studio (Accelrys, Inc.). Lattice strain broadening effects (0-2%) were included in the calculations, according to the established theory.8 A pseudoVoigt8,9 profile was adopted. Because these simulations were focused upon explaining the structural disorder, we used only minimal instrumental broadening9 parameters (U ) V ) W ) 0.01) with a Lorentzian-Gaussian mixing parameter η ) 0.5.
Results and Discussion Strategies in the Design of SDA Cations. Although zeolite synthesis is still largely based on trial and error, the empirical knowledge derived over the last few decades, especially from the works of Gies, Davis, Zones, and their respective co-workers, allows synthesis strategies for the discovery of new zeolites to be conceived.1 For this “directed trial and error approach”, the first step is to realize that silica frameworks themselves do not vary much in their relative stabilities10 and that crystallization kinetics is of large importance and depends on a number of factors that are convenient to screen.11 Second, the interaction between the inorganic framework and the organic SDA may help to stabilize a given zeolite.12 And even if the stabilization energy is not large,13 if the SDA is to be occluded in the final material, the size and shape of an SDA may prevent the crystallization of some zeolites while allowing others with larger or more appropriately shaped voids to form. This allows for some rational design of organic cations to be used as SDAs. However, the rigidity of the organic cation needs also to be considered as a very important feature determining its structure-directing ability: a highly flexible cation with multiple possible conformations may more easily adapt to the channels of different zeolites, giving it a low specificity in structure direction and a greater chance of yielding a zeolite that easily nucleates and grows (i.e., the so-called “default zeolites”, such as, for instance, the medium pore ZSM-23 zeolite, which crystallized when linear diquats of varying length were unable to produce an ideal fit inside other frameworks).14 Thus, in the search for new structures the use of rigid SDA cations may represent a meaningful strategy. Also, an SDA with a featureless shape (such as a globular cation that can fit in different void spaces) may be expected to show a lower specificity than a more distinctly shaped cation of similar size. In our work to make new large pore zeolites, we based our strategy on the points above and paid particular attention (8) Young, R. A. The RietVeld Method; IUCr Monographies of Crystallography 5; Oxford University Press: Oxford, 1993. (9) Caglioti, G.; Paoletti, A. B.; Ricci, F. P. Nucl. Instrum. Methods 1958, 3, 223. (10) Piccione, P. M.; Laberty, Ch.; Yang, S.; Camblor, M. A.; Navrotsky, A.; Davis, M. E. J. Phys. Chem. B 2000, 104, 10001. (11) Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials. Zeolites, Clays and Nanostructures; Occelli, M. L., Kessler, H., Eds.; Marcel Dekker, Inc.: New York, 1997; p 1. (12) Burton, A.; Lee, G. S.; Zones, S. I. Microporous Mesoporous Mater. 2006, 90, 129. (13) Piccione, P. M.; Yang, S.; Navrotsky, A.; Davis, M. E. J. Phys. Chem. B 2002, 106, 3629. (14) Moini, A.; Schmitt, K. D.; Valyocsik, E. W.; Polomski, R. F. Zeolites 1994, 14, 504.
Figure 2. Summary of synthesis results for DMABO, DEABO, and EABO-5.
to the rigidity and molecular shape aspects that directed our choice of SDA cations. To this end, we synthesized five different cations derived from the rigid polycyclic ABO (Scheme 1). The parent amine, which is no longer commercially available, shows a large conformational rigidity and a relatively big size, with a nonglobular and a nonlinear shape. It also contains two asymmetric centers, and the commercial product did not show any optical activity, actually being a racemic mixture of two (R,S) and (S,R) enantiomers (the methylene bridge preventing the existence of the (R,R) and (S,S) enantiomers). A chiral, enantiomerically pure SDA could eventually be used to synthesize a chiral, enantiomerically pure zeolite, if a good geometric fit between the cation and the zeolite void space was achieved. In the following, the SDA derived from the ABO amine will correspond to a racemic mixture, unless otherwise noted. The SDAs we derived from ABO (Scheme 1) are the dimethyl cation (DMABO), the diethyl cation (DEABO), the spirocation formed by quaternization of the N atom through a chain of four methylene groups to yield a new fivemembered ring cycle (EABO-5), the same cation enriched in the dextro form ((+)EABO-5) by successive recrystallizations of the L-tartrate salt, and the spirocation with a new six-membered cycle around N (EABO-6). The conformational freedom of the SDAs increases in the order DMABO < EABO-5 < EABO-6 < DEABO, and their size increases in the order DMABO < EABO-5 < DEABO < EABO-6. As we have shown previously for other SDA cations,15 the water to silica ratio significantly determines the phase selectivity of the crystallization of silica phases in aqueous fluoride media, and it is, thus, one of the important parameters to be systematically tested. Typically, a large dilution favors the formation of dense phases (tridymite-like solids), while decreasing the water/silica ratio is frequently needed to afford materials with a lower framework density. This is an empirical trend observed by us in the synthesis of silica zeolites using quaternary ammonium cations in fluoride media and15,16 later corroborated by Zones and co-workers.17 (15) Camblor, M. A.; Villaescusa, L. A.; Dı´az-Caban˜as, M. J. Top. Catal. 1999, 9, 59. (16) Villaescusa, L. A. Ph.D. Thesis, Universitat Polite`cnica de Valencia, 1999.
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Table 1. Summary of the Experiments Carried Out in the Synthesis of Zeolites Using EABO-5 as the SDA experiment
a
H2O/Si
T (°C)
t (days)
pH
R (%)
product
17 27 38 19 35 61 21 42 56 16 16 16 16 13 21 4 16 10 10 20 14 7 5 6 12 7 15 25 7 15 30 25 34 53 55 15 15
7.5 7.6 8.0 8.2 8.3 8.7 7.8 7.8 8.2 7.8 7.6 7.0 6.5 8.2 7.9 7.6 7.0 7.0 7.9 7.6 8.8 8.2 7.2 6.9 6.9 7.5 7.0 7.1 7.1 6.3 5.9 7.4 7.6 6.4 6.3 8.2 7.7
12.9 14.2 15.8 14.5 13.8 14.4 14.9 16.3 14.5 18.4 18.6 14.5 15.4 20.5 21.6 19.0 17.5 21.4 24.4 23.8 27.8 27.8 24.9 25.7 26.9 28.7 25.4 26.9 31.1 30.3 30.2 25.4 26.6 27.1 29.5 34.8 34.5
amorphous amorphous MI-3 amorphous MI-2 + ITQ-7 (
