CkEMISTRy Of
MATERIALS
VOLUME 6, NUMBER 12
DECEMBER 1994
0Copyright 1994 by the American Chemical Society
Communications Dealumination of Hexagonal (EMT)/Cubic (FAU)Zeolite Intergrowth Materials: A SEM and HRTEM Study Tetsu Ohsuna,? Osamu Terasaki,*s$ Denjiro Watanabe,t Michael W. Anderson,$ and Stuart W. Carr*J
College of Science and Engineering Iwaki Meisei University, 5-5-1 Iino 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 1 8 0 , U.K. Unilever Research, Port Sunlight Laboratory Quarry Road East, Bebington Merseyside L63 3JW, U.K. 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 ~eolites.l-~ 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. P UMIST.
Unilever Research.
* To whom correspondence should be addressed.
(1) Delprato, E.; Delmotte, L.; Guth, J. L.; Huve, L. Zeolites 1990, 10,546. (2)Anderson, M. W.; Pachis, K. S.; Prebin, F.; Cam, S. W.; Terasaki, 0.; Ohsuna, T.; Alfredsson, V. J . Chem Soc., Chem Commun. 1991, 1660. (3)Terasaki, 0.; Ohsuna, T.; Alfredsson, V.; Bovin, J.-0.; 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.; Schra", S. Zeolites 1991,11, 98. (5)Treacy, M. M. J.; Newsam, J. M.; Deem, M. W. Proc. R . SOC. London A 1991,433,499.
0897-4756/94/2806-2201$04.50/0
The EMT structure can be envisaged as an AT3AB 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 t o 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 treatment^.^ While there are a number of known materials which form FAU/EMT intergrowths (e.g., CSZ-1,6-9 ECR-30,1° ZSM-3," ZSM-2012-16 1 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 {lll}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 t o demonstrate that the intergrowths are spatially (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. US.Patent 4,021,331, 1977. (14)Ernst, S.;Kokotailo, G. T.; Weitkamp, J. Zeolites 1987,7,180. (15)Fiilop, 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.
0 1994 American Chemical Society
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Figure 1. SEM images of FAU (a) before and (c) after dealumination, and EMT (b) before and (d) after dealumination. Table 1 Si/Al ratio sample FAU (calcined) dealuminated FAU EMT (calcined)
from 29Si MAS NMR
unit cell size/A
3.9 9.4 3.6
24.69 24.40 a = b = 17.38
adsorp capacity (NzY(gig) 0.21 0.27
c = 28.34
dealuminated EMT
5.1
a = b = 17.18
0.17
e = 28.10
intergrowth 15-33/18-67 calcined deal. 15-33/18-67
3.1
0.28
5.3
0.28
correlated on a 10-20 nm length ~ c a l e . 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 ze01ite.l~ In particular solid-state NMR, infrared spectroscopy, high-resolution TEM, sorption, and catalytic measurements have been used to study the nature of the dealuminated material^.'^-^^ 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) Seherzer J. Catalytic Materials: Relationships Between Strueture and Reactivity.ACS Symp. Ser. 1984,248, 157. (18) Coma, A. Zeolites: Facts, Figures, Future. Stud. Su$. Sei. Catal. 1989, 49, 49-67 and references therein. (19) Martens, J. A.; Grobet, P. J.; Jacobs, P. A. Preparation of Catalysts V. Stud. Surt Sei. 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 Koishi 1989, 90s
"I".
(23) Chawin, B.; Boulet, M.; Massiani, P.; Fajula, F.; Figuaras, F.; Courieres, T. D. J. Catal. 1990,126,532.
method 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 p r e v i o u ~ l yand ~~~ 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 t o remove the crown ether template (600 "C, flowing air, 16 h) and then exchanged with ammonium ions. The zeolite (6 g) was slumed in ammonium acetate solution (450 cm3, 0.8 M), and to this was added slowly 15.6 cm3 of ammonium hexafluorosilicate solution (0.5 MI. The mixture was stirred a t 75 "C for 3 h. The zeolite was collected and carefully washed with water (3 x 100 cm3). The dealuminated samples were characterized by 29Si and 27Al 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 P A l , zsSiMAS 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? there are distinct changes in the dealuminated material. The surface amorphous material observed in the SEM is apparent as the flu& 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 Athick. 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 technique^.^^-^^ Using a combination of XF'S-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
'(27) Lo& F.;Lunsf&d, J. € J.I. Catel. 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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amorphous amorphous + crystalline crater crystalline
Figure 2. High-resolution TEM images of (a) FAU before dealumination and (h) and (e) after dealumination at different magnifications.Figure 2d is a schematic representation of the image shown in Figure 2c. amorphous layer is a region of high dealumination. craters observed in Figure 2c are schematically represented in Figure 2d. These holes are bounded on all These will be directly confirmed by newly developed EM techniques and on going experiments. The crystalline sides by {ill}, faces. For the dealumination of EMT a similar picture is observed (Figure 3). The HRTEM region shows no indication of the formation of mesopor o w defects or of selective removal of regions of alumiimages of EMT viewed perpendicular to the c axis ([1001) nosilicate framework. The regions in the image which and along the c axis before and after dealumination are shown in Figure 3a-d, respectively. Before dealumiappear to be poorly focused (Figure 2c) are ascribed to 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 surface aluminosilicate framework (Figure 3b). As in the outer amorphous layer (Figure 2c); third, these holes 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-
2204 Chem. Mater., Vol. 6, No. 12, 1994
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