Chem. Mater. 2009, 21, 4339–4346 4339 DOI:10.1021/cm901883e
Crystal Structure of MCM-22 (MWW) and Its Delaminated Zeolite ITQ-2 from High-Resolution Powder X-Ray Diffraction Data: An Analysis Using Rietveld Technique and Atomic Pair Distribution Function Vaishali V. Narkhede and Hermann Gies* Chair of Crystallography, Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, 44780 Bochum, Germany Received June 30, 2009. Revised Manuscript Received August 5, 2009
The three-dimensional structure of crystalline, calcined pure siliceous MCM-22 (which has a chemical formula of Si72O144) and its delaminated derivative, ITQ-2 (which has a chemical formula of Si144O288), also in its calcined form, have been studied using synchrotron radiation-based powder diffraction experiments employing full pattern Rietveld analysis for structure elucidation. Because of the limited structural coherence in ITQ-2, conventional analysis of the X-ray diffraction experiments using Bragg peaks only is not suitable. Instead, we applied the pair distribution function (PDF) technique to study the structure of ITQ-2, in particular, the termination of the layer slab. Despite of the structural complexity of the zeolite materials, excellent confirmation of the crystal structure based on Rietveld analysis (RBragg=12.6%) was obtained with PDF analysis (Rw=18.3%) for the calcined siliceous MCM-22. Based on results of the crystalline compound, we only succeeded to refine the diffraction data set of the delaminated material applying PDF analysis. 1. Introduction Zeolites are microporous materials that are useful in many processes, including heterogeneous catalysis, separation processes, ion exchange, catalysts, and catalyst supports. Sorption and catalytic properties of zeolites are strongly dependent on the state of order, pore volume, and, in particular, of pore size of the framework structure. Therefore, to understand the properties of new zeolitic materials or to explore the properties of existing zeolites, the determination of their exact structure is inevitable. Some zeolitic materials are found to have ordered crystalline structures; others are structurally disordered, the latter of which makes it difficult to apply conventional X-ray diffraction (XRD) experiments for structure elucidation of the materials. The zeolites, originating from a lamellar precursor (e.g., MCM-22 (MWW),1 ferrierite (FER),2 Nu-6(2) (NSI),3 CDS-1 (CDO),4 ERS-12,5 MCM-65,6 RUB-41 *Author to whom correspondence should be addressed. E-mail: hermann.
[email protected].
(1) Leonowicz, M. E.; Lawton, J. A.; Lawton, S. L.; Rubin, M. K. Science 1994, 264, 1910. (2) Schreyeck, L.; Caullet, P. H.; Mougenel, J. C.; Guth, J. L.; Marler, B. Microporous Mater. 1996, 6, 259. (3) Zanardi, S.; Alberti, A.; Cruciani, G.; Corma, A.; Fornes, V.; Brunelli, M. Angew. Chem., Int. Ed. 2004, 43, 4933. (4) Ikeda, T.; Akiyama, Y.; Oumi, Y.; Kawai, A.; Mizukami, F. Angew. Chem., Int. Ed. 2004, 43, 4892. (5) Millini, R.; Carluccio, L. C.; Carati, A.; Bellussi, G.; Perego, C.; Cruciani, G.; Zanardi, S. Microporous Mesoporous Mater. 2004, 74, 59. (6) Dorset, D. L.; Kennedy, G. J. J. Phys.:: Chem. B 2004, 108, 15216. (7) Wang, Y. X.; Gies, H.; Marler, B.; Muller, U. Chem. Mater. 2005, 17, 43. r 2009 American Chemical Society
(RRO),7 RUB-24,8 EU-20,9 SOD,10 and layered aluminofluorophosphate (AFO)11) are already known, and they are possibly subjected to structural modifications during topotactic condensation reactions (RUB-39).12 With the increase in number, these types of zeolites are gradually forming an important group of microporous materials. MCM-22, which is a representative of the MWW-type zeolite that has found important industrial applications in the selective production of bulk petrochemicals (e.g., alkylation reactions), is one of the most interesting zeolite structures synthesized up to now. In its silicate framework, there are two independent pore systems, one of which can be defined as an intersecting sinusoidal channel system extending in two dimensions, whereas the other consists of a large supercage with an inner diameter of ∼7.1 A˚ and height of ∼18.2 A˚. Both pores are accessible through 10-membered ring (10R) openings (see Figure 1a). The framework silicate MCM-22 has been obtained from a layered precursor, MCM-22(P), through topotactic condensation at elevated temperatures (∼450 °C) and also removing the structure directing agent (SDA) by thermal decomposition. In addition, the layered precursor of the MCM-22 (MCM-22(P)) can be intercalated :: (8) Marler, B.; Stroter, N.; Gies, H. Microporous Mesoporous Mater. 2005, 83, 201. (9) Marler, B.; Camblor, M. A.; Gies, H. Microporous Mesoporous Mater. 2006, 90, 87. (10) Moteki, T.; Chaikittisilp, W.; Shimojima, A.; Okubo, T. J. Am. Chem. Soc. 2008, 130, 15780. (11) Wheatley, P. S.; Morris, R. E. J. Mater. Chem. 2006, 16, 1035. (12) Wang, Y. X.; Gies, H.; Lin, J. H. Chem. Mater. 2007, 19, 4181.
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Figure 1. Schematic representation of (a) MWW-type zeolite MCM-22 (b) ITQ-2 without the supercage, and (c) ITQ-2 with the supercage.
or even delaminated, yielding new materials that show advantages in catalytic properties, compared to amorphous oxides as well as microporous materials (zeolites), because the material, on the one hand, retains its shape selectivity, whereas, on the other hand, there is no diffusion or pore-size limitations, with regard to accessing the reactive centers. In particular, the delaminated material retains (i) high internal order and homogeneous distribution of the slit-pores due to the “crystallinity” of the layers, and (ii) its well-defined microporosity. Because of its corrugated surface structure, shape-selective chemical reactions should occur at active centers in the half cage.13 Therefore, the accessibility of the active sites is no longer restricted by the diffusion process through the volume of the crystal and the corresponding pore size limitation, because it is accessible from the outer surface. MCM-36 is a typical example of such a molecular sieve. A material that is similar to MCM-36, but with a completely disordered structure of layers, has been reported by Corma et al.,14 who observed that the zeolitic layers of the MCM-22(P) precursor can be fully separated by ultrasonic treatment. This procedure yields “crystalline” monolayers. The resulting material has been named ITQ-2 (see Figure 1b), which consists of very thin sheets (∼25 A˚ thick), leading to an extremely high external surface area of g700 m2/g. These thin sheets consist of a hexagonal array of “cups” that penetrate into the sheet from both sides. Because of delamination of the MCM22(P) precursor layers, the large cylindrical supercages of MCM-22 were halved and an increasing number of now half-open supercages (“cups”) are present in a hexagonal array on the new sheet surface. These cups would have an aperture of ∼7.0 A˚, formed by a 12-membered ring (12R). The cups are 7.0 A˚ deep and meet at the center of the sheet, forming a double 6-membered ring (6R) window that connects the cups, bottom to bottom, resulting in a smooth 10R pore system around the cups inside the layer. Thus, reaction sites located previously in (13) Corma, A.; Fornes, V.; Rey, F. Adv. Mater. 2002, 14, 71. (14) Corma, A.; Fornes, V.; Guil, J. M.; Pergher, S. B.; Maesen, Th.L. M.; Buglass, J. G. Microporous Mesoporous Mater. 2000, 38, 301.
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the supercages are more easily accessible, even for larger molecules. Not surprising, ITQ-2 is more active and more selective than a MWW-type framework zeolite, as shown by Corma et al.15 Thus, ITQ-2 should find many applications in petrochemical industries. However, because of the inherent structural complexity of the thin ITQ-2 sheets lacking long-range periodic order, conventional analysis of XRD data provides almost no structural information on ITQ-2. The aforementioned proposed structure of delaminated zeolite (ITQ-2) was deduced solely on the basis of high-resolution electron microscopy images of selected objects,13,15 N2 and Ar adsorption isotherms, and infrared spectroscopy. Still, there is a challenge to determine the exact geometry of the crystal structure of ITQ-2. It is clear from the aforementioned discussion that ITQ-2 possesses short-range order, in comparison to MWW-type zeolite MCM-22. Therefore, it is believed that traditional XRD analysis is not a suitable technique to characterize thoroughly the structure of ITQ-2. Recently, the pair distribution function (PDF) technique has emerged as a powerful and unique tool for the characterization and structure refinement of (non)crystalline materials with intrinsic disorder. The strength of the technique stems from the fact that it takes into account the total scattering information of the powder diffraction experiment (that is, Bragg scattering as well as diffuse scattering). Therefore, it is applicable to both crystalline and amorphous materials. Very recently, it has been shown that the three-dimensional (3D) structure of materials with reduced structural coherence, including nanocrystals, can be determined using the so-called atomic PDF technique.16-20 In this contribution, we present results from the traditional Rietveld and nontraditional PDF techniques to determine the 3D structure of both calcined pure siliceous MCM-22 and its delaminated form, ITQ-2. 2. Experimental Section 2.1. Synthesis. The synthesis of pure siliceous MCM-22 and its delaminated form ITQ-2 zeolite was performed according to the method described in the literature.14,21 In a typical synthesis, the siliceous layered precursor MCM-22(P) has been prepared as follows: 0.635 g of hexamethyleneimine (HMI) and 0.195 g of NaCl were dissolved and stirred for 15 min, and this mixture was added under stirring to a solution made with 0.88 g of a 0.625 M solution of trimethyladamantamonium hydroxide (TMAdaOH) and 13.2 g of deionized water. One gram of silica (Aerosil 200, Degussa) was added to the aforementioned solution, and the final gel was obtained with the following composition: 1 SiO2:0.25 TMAdaOH:0.31 HMI:0.2 NaCl:44 H2O. The resultant (15) Corma, A.; Fornes, V.; Pergher, S. B.; Maesen, Th. L. M.; Buglass, J. G. Nature 1998, 396, 353. (16) Petkov, V.; Bozin, E.; Billinge, S. J. L.; Vogt, T.; Trikalitis, P.; Kanatzidis, M. J. Am. Chem. Soc. 2002, 124, 10157. (17) Petkov, V.; Billinge, S. J. L.; Larson, P.; Mahanti, S. D.; Vogt, T.; Rangan, K. K.; Kanatzidis, M. Phys. Rev. B 2002, 65, 092105. (18) Petkov, V.; Zavalij, P.; Lutta, S.; Whittingham, M. S.; Parvanov, V.; Shastri, S. Phys. Rev. B 2004, 69, 085410. (19) Gateshki, M.; Hwang, S.-J.; Park, D. H.; Ren, Y.; Petkov, V. J. Phys. Chem. B 2004, 108, 14956. (20) Billinge, S. J. L. J. Solid State Chem. 2008, 181, 1695. (21) Camblor, M. A.; Corma, A.; Dı´ az-Cabanas, M. J.; Baerlocher, Ch. J. Phys. Chem. B 1998, 102, 44.
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gel was mechanically stirred for 2 h at room temperature and was then transferred to Teflon-lined autoclaves and heated for 15 days at 423 K while being rotated at 30 rpm. After quenching the autoclave in cold water, the solid was filtered, thoroughly washed with deionized water until pH ca. 9, and dried overnight at 353 K. The MCM-22(P) powder showed an XRD pattern characteristic of the layered precursor of the MWW framework type. A portion of layered precursor MCM-22 (P) was heated to 853 K with a heating rate of 1 K/min and the final temperature was held for 3 h, giving pure siliceous MCM-22; whereas another portion was intercalated, delaminated, and calcined to obtain ITQ-2. The purely siliceous ITQ-2 was prepared starting from the layered precursor MCM-22(P) as follows: 0.5 g of layered precursor MCM-22(P) was dispersed in 2 g of deionized water (slurry pH ,9) and mixed with 9.72 g of an aqueous solution of 29 wt % hexadecyltrimethylammonium bromide and 3 g of an aqueous solution of 40 wt % tetrapropylammonium hydroxide. The resulting slurry was refluxed for 16 h at 353 K. The layers were forced apart by placing the slurry in an ultrasound bath (50 W, 40 kHz) for 1 h. Finally, a few drops of concentrated HCl were added until the pH was
