OlEMISTRyof
MATERIALS
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6,
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DECEMBER 1994
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Copyright 1994 by the American Chemical Society
Communications The EMT structure can be envisaged as an ABAB stacking sequence of the faujasite sheets, while the cubic material arises from an ABCABC stacking sequence. In the case of EMT this gives rise to a large straight channel (7.4 A) with side pockets of 9.9 x 7.4 A, while the FAU consists of a three-dimensional channel system with large 12.5 A cages. It is expected that the pure end members and intergrowths should show different properties due to the subtle alteration of the pore and channel network. A number of groups have made initial studies on the functionality of these materials and have begun to look at the properties as a consequence of various treatments.* While there are a number of known materials which
Dealumination of Hexagonal (EMT)/Cubic (FAU) Zeolite Intergrowth Materials: A SEM and HRTEM Study Tetsu Ohsuna,* Osamu Terasaki,*’* Denjiro Watanabe,* Michael W. Anderson,§ and Stuart W. Carr*1 College of Science and Engineering Iwaki Meisei University, 5-5-1 lino Chuoudai Iwaki, Fukushima 970, Japan Department of Physics, Tohoku University Aramaki Aoba, Sendai 980, Japan Department of Chemistry, UMIST, P.O. Box 88 Manchester M60 1QD, U.K. Unilever Research, Port Sunlight Laboratory Quarry Road East, Bebington Merseyside L63 3JW, U.K.
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form FAU/EMT intergrowths (e.g., CSZ-1,6-9 ECR-30,10 ZSM-3,11 ZSM-2012-16 ) using a variety of templates; in all these cases it is not possible to control the proportion of the two polymorphs. It has also been established that the intergrowths are very disordered as the twinning occurs on the four different {111}C faces. This leads to the formation of many kinds of defect sites. However, with mixtures of 18-crown-6 and 15-crown-5 it is possible to prepare well defined and ordered intergrowths relatively free from defects. We have recently reported the characterization of some of these materials and were able to demonstrate that the intergrowths are spatially
Received July 6, 1994 Revised Manuscript Received August 30, 1994 Recently it has been possible to prepare well-defined intergrowths of the hexagonal (EMT) and cubic (FAU) polymorphs of faujasite zeolites.1-4 The two polymorphs arise as a consequence of the different stacking sequence of layers which arise from different connectivities of the secondary building units, namely, the sodalite cages.5 Iwaki Meisei University. Tohoku University. UMIST. 1 Unilever Research. * To whom correspondence should be addressed. (1) Delprato, E.; Delmotte, L.; Guth, J. L.; Huve, L. Zeolites 1990,
(6) Treacy, M. M. J.; Newsam, J. M.; Beyerlein, R. A.; Leonowicz, M. E.; Vaughan, D. E. W. J. Chem. Soc., Chem. Commun. 1986, 1211. (7) Audier, M.; Thomas, J. M. J. Chem. Soc., Chem. Commun. 1981. (8) Martens, J. A.; Jacobs, P. A.; Cartilidge, S. Zeolites 1989, 9, 423. (9) Treacy, M. M. J.; Newsam, J. M.; Vaughan, D. E. W.; Beyerlein, R. A.; Rice, S. B.; deGruyter, C. B. MRS Symp. Proc. 1988, 111, 177. Vaughan, D. E. W. European Patent Appl. 315,461, 1986. (10) Vaughan, D. E. W. European Patent Appl. 315,461, 1988. (11) U.S. Patent 4,415,736. (12) Ciric, J. U.S. Patent 3,972,983, 1976. (13) Ciric, J. U.S. Patent 4,021,331, 1977. (14) Ernst, S.; Kokotailo, G. T.; Weitkamp, J. Zeolites 1987, 7,180. (15) Ftilop, V.; Borbely, G.; Beyer, H. K.; Ernst, S.; Weitkamp, J. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2127. (16) Newsam, J. M.; Treacy, M. M. J.; Vaughan, D. E. W.; Strohmaier, K. G.; Mortier, W. J. J. Chem. Soc., Chem. Commun. 1989, 493.
+
* *
10, 546. (2) Anderson, M. W.; Pachis, K. S.; Prebin, F.; Carr, S. W.; Terasaki, O.; Ohsuna, T.; Alfredsson, V. J. Chem Soc., Chem Commun. 1991, 1660. (3) Terasaki, O.; Ohsuna, T.; Alfredsson, V.; Bovin, J.-O.; Watanabe, D.; Carr, S. W.; Anderson, M. W. Chem. Mater. 1993, 5, 452. (4) Annen, M. J.; Young, D.; Arhancet, J. P.; Davis, M. E.; Schramm, S. Zeolites 1991, 11, 98. (5) Treacy, M. M. J.; Newsam, J. M.; Deem, M. W. Proc. R. Soc. London A 1991, 433, 499.
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as it is mild and does not lead to the formation of extraframework aluminium. The aim of this paper is to describe the effects of mild dealumination of the end members (FAU, EMT) and intergrowths produced using crown ethers on the nature of the resultant zeolite. The end members and intergrowths were prepared as described previously2,3 and dealuminated according to the following procedure. The intergrowth was prepared from a mixture of crown ethers; 66% 18crown-6 and 33% 15-crown-5 to give an ordered intergrowth. The zeolites were first calcined to remove the crown ether template (600 °C, flowing air, 16 h) and then exchanged with ammonium ions. The zeolite (6 g) was slurried in ammonium acetate solution (450 cm3, 0.8 M), and to this was added slowly 15.6 cm3 of ammonium hexafluorosilicate solution (0.5 M). The mixture was stirred at 75 DC for 3 h. The zeolite was collected and carefully washed with water (3 x 100 cm3). The dealuminated samples were characterized by 29Si and 27A1 MAS NMR, X-ray powder diffraction and adsorption measurements. These data indicate that the resultant materials are highly crystalline and showed no signs of structural degradation both in short-range order (27A1,29Si MAS NMR) and long-range order (XRD). Data summarizing this are shown in the Table 1. However, the SEM (Hitachi S-800) and high-resolution TEM (JEM-4000EX with Cs 1.0 mm, 400 kV source) images were particularly informative. The SEM images of the end member FAU and EMT samples before and after dealumination are shown in Figure 1. In all cases the morphology was retained, but there is evidence for amorphous material on the surface which had not been readily removed during the washing process. The corresponding high-resolution TEM images are shown in Figure 2. The images of FAU phase before and after dealumination are shown in Figure 2a,b. While the parent material before dealumination was almost defect free,3 there are distinct changes in the dealuminated material. The surface amorphous material observed in the SEM is apparent as the fluffy material of low density on the outer surface. The edges of the crystallites are bound by a dense noncrystalline layer approximately 20 A thick. Between this and the crystalline phase is a layer of less-dense amorphous material approximately 80 A thick. This amorphous layer retains the surface integrity or the crystals and thus can only be seen by high-resolution TEM not by SEM. At this stage we cannot give a decisive conclusion about the nature of the dense layer on the outer surface. Several groups have studied dealumination using the hexafluorosilicate method via a range of chemical and spectroscopic techniques.24-28 Using a combination of XPS-SIMS, chemical analysis, XRD, and NMR, it was concluded that the resultant zeolites have a silicon-rich surface. These observations were explained by selective surface dealumination and by the deposition of silica onto the surface of the crystallites. We may suggest that our TEM images support these conclusions and the dense outer layer is deposited silica while the less dense inner
method
1. SEM images of FAU (a) before and (c) after dealumination, and EMT (b) before and (d) after dealumination.
Figure
=
Table
sample
FAU (calcined) dealuminated FAU EMT (calcined)
1
Si/Al ratio from 29Si MAS NMR 3.9 9.4 3.6
unit cell size/A
5.1
0.21
a=6
0.27
o c
intergrowth 15—33/18--67 calcined deal. 15-33/18-67
(N2)/(g/g)
24.69 24.40 c
dealuminated EMT
adsorp capacity
= = =
= 17.38 28.34 6= 17.18 28.10
0.17
3.1
0.28
5.3
0.28
correlated on a 10- 20 nm length scale.2,3 The crystals, which are synthesized using a mixture of crown ethers and have regular morphologies, and the intergrowth boundaries are introduced in a controlled fashion. Dealumination of faujasite zeolites has been widely studied by a variety of techniques to ascertain the nature of the framework and nonframework species, and to ascertain the resultant effect on the properties of the zeolite.17 In particular solid-state NMR, infrared spectroscopy, high-resolution TEM, sorption, and catalytic measurements have been used to study the nature of the dealuminated materials.18-23 These various studies have indicated that the method of the dealumination strongly influences the reactivity of the resultant zeolite. While there are several methods of dealumination, we have chosen to use the ammonium hexafluorosilicate (17) Scherzer J. Catalytic Materials: Relationships Between Structure and Reactivity. ACS Symp. Ser. 1984, 248, 157. (18) Corma, A. Zeolites: Facts, Figures, Future. Stud. Surf Sci, Catal. 1989, 49, 49-67 and references therein. (19) Martens, J, A.; Grobet, P. J.; Jacobs, P. A. Preparation of Catalysts V. Stud. Surf. Sci. Catal. 1991, 65, 355. (20) Lynch, J.; Raatz, F.; Dufresne, P. Zeolites 1987, 7, 333. (21) Lutz, W. Cryst. Res. Technol. 1990, 25, 921. (22) Horikoshi, H.; Kasahari, S.; Fukushima, T.; Itabashi, K.; Okada, T.; Terasaki, 0.; Watanabe, D. Nippon Kagabu Kaishi 1989, 398. (23) Chawin, B.; Boulet, M,; Massiani, P,; Fajula, F.; Figuaras, F.; Courieres, T. D. J. Catal. 1990, 126, 532.
(24) Garralon, A.; Fornes, V.; Corma, A. ZEOLITES 1988, 8, 268. (25) Cruz, J. M.; Corma, A.; Forn4s, V. Appl. Catal, 1989, 50, 287. (26) Wang, Q. L.; Torrealba, M.; Giannetto, G.; Guisnet, M.; Perot, G.; Cahoreau, M.; Caisso, J. ZEOLITES 1990, 10, 703. (27) Lonyi, F.; Lunsford, J. H. J. Catal. 1992, 136, 566. (28) Chawin, B,; Boulet, M.; Massiani, P.; Fajula, F.; Figuaras, F.; Courieres,T. D. J. Catal. 1990, 126, 532.
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2. High-resolution TEM images of (a) FAU before dealumination and (b) and (c) after dealumination at different magnifications. Figure 2d is a schematic representation of the image shown in Figure 2c. craters observed in Figure 2c are schematically repreamorphous layer is a region of high dealumination. These will be directly confirmed by newly developed EM sented in Figure 2d. These holes are bounded on all on and The sides by (111}C faces. For the dealumination of EMT a techniques going experiments. crystalline similar picture is observed (Figure 3). The HRTEM region shows no indication of the formation of mesoporous defects or of selective removal of regions of alumiimages of EMT viewed perpendicular to the c axis ([100]) nosilicate framework. The regions in the image which and along the c axis before and after dealumination are to be focused are ascribed to shown in Figure 3a~d, respectively. Before dealumi2c) appear poorly {Figure the presence of craters on the surface of the individual nation the material consists of a regular defect-free EMT structure, while after dealumination surface craters are crystals. First, the outer amorphous layer is created; second, craters are formed and holes are made below apparent, indicating selective removal of regions of the the outer amorphous layer (Figure 2c); third, these holes surface aluminosilicate framework (Figure 3b). As in the case of FAU the whole surface is covered by a lessexpand below the amorphous outer layer until a subsurface region is completely removed (Figure 2b). The dense amorphous layer with a more dense layer local-
Figure
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f
10011
I 1
amorphous layer (high density) thickness 5 lQnm "
amorphous thickness
]
+
t
crystalline (low density) *~15nm
crystalline
3. High-resolution TEM images of EMT viewed parallel to the c axis (a) before and (b) after dealumination, viewed perpendicular to the c axis (d) before and (e) after dealumination. Schematic representations for (b) and (e) are shown in (c) and (f), respectively.
Figure
on both dealuminated end members. In the case of the intergrown samples the parent nondealuminated samples are defect free and show a regular spatially correlated FAU/EMT intergrowth as described previously.3 Dealumination produces a material which has been modified substantially. In addition to the formation of surface amorphous material after dealumination (observed by SEM but not shown) as is observed for the end-members there are now mesoporous crevices which penetrate deep into the crystals (Figure 4). There is no evidence that a dense amorphous outer crust is formed. As dealumination with hexafluorosilicate is believed to be diffusion controlled25’26 the formation of the mesopores may facilitate the transport of the reagent into the crystallites and thus reduce the deposition of silica onto the outer surface. The amorphous regions occur not just at the surface of crystals but penetrate deep into the crystals and have the appearance of eroded gullies. As it is not observed with the end members, this suggests that dealumination occurs preferentially in regions with a higher level of stacking disorder. Several intergrowths have been examined and similar behavior has been observed. Furthermore the shape of the mesopores suggests that they begin at the surface of crystals and grow into the crystals as dealumination proceeds. These results have implications for the use of intergrown materials where dealumination is required before use. The preparation of controlled amounts of intergrowth may also be used as a method to introduce defects into
ized
Figure 4. High-resolution TEM image of a FAU/EMT intergrowth after dealumination.
the structures. For applications where some mesoporosity is desirable this is potentially a method of producing a controlled amount of larger pores.
Acknowledgment. M.W.A. and O.T. thank the British Council for funding a joint research programme. Support from Grant-in-Aid for New Program from the Ministry of Education, Science and Culture, Japan, Sumitomo Chemical Co. and Iketani Science and Technology Foundation (O.T.), Unilever Research (S.W.C.), and EPSRC (M.W.A.) are gratefully acknowledged.
