High-Resolution Electron Microscopic Studies of Zeolite ZSM-48

Chapter 39

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High-Resolution Electron Microscopic Studies of Zeolite ZSM-48 Angus I. Kirkland, G. Robert Millward, Stuart W. Carr, Peter P. Edwards, and Jacek Klinowski Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England We have prepared large spherulitic crystals of ZSM-48 from silica-rich gels containing amines and structure-directing agents (templates). Thermally treated (steamed) crystallites have been studied by high-resolution electron microscopy (HREM) and x-ray diffraction (XRD). At an interpretable point resolution of ca. 3 Åin tandem with image simulations carried out according to established procedures, HREM enabled us to establish the imaging conditions under which the two previously proposed topologies can be distinguished. The majority of crystallites of ZSM-48 exhibit extensive (100) twinning.

Since the discovery of ZSM-48 as an impurity phase i n ZSM-39 (1) and its subsequent synthesis i n an impurity-free form (2) the zeolite has been a subject of structural studies by powder x-ray diffraction (3,4). These studies have indicated a structure based on ferrierite sheets linked by oxygen atoms located on mirror planes, and an orthorhombic lattice w i t h pseudo I or pseudo C centering. Schlenker et al. (5) reported that structures i n which the four independent Τ atoms in the ferrierite sheet are i n the U U D D (U = up, D = down) w i t h C m c m symmetry or i n the U D U D w i t h Imma sym metry show the best agreement between calculated Smith plots (6) and the experimental x-ray powder patterns. Schlenker et al. (5) also report that a structure based on random intergrowths of these two basic structures gives rise to even better agreement. W e have investigated the structure of Z S M 48 by high resolution electron microscopy supported by image simulation in order to test these conclusions and identified the optimum conditions for imaging the zeolite. Experimental Zeolite ZSM-48 was synthesised according to ref. 7. A solution prepared by mixing 0.56 g of sodium hydroxide, 0.43 c m 30% aqueous ammonia and 3

0097-6156/89/0398-O574$06.00/0 ο 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Electron Microscopic Studies ofZeolite ZSM-48

3

10 c m of water was added with vigorous stirring to the thick slurry made by mixing 5 g fumed silica (Cab-O-Sil M5), 1.78 g hexamethonium bromide ( H M B r 2 > and 60 c m water. This gave a free-flowing gel of composition 5 (ΗΜΒΓ2) : 5 N a 2 U : 5 ( N H ^ O : 60 S1O2 : 3000 H 0 . The mixture was heated i n a Teflon-lined autoclave at 200°C for 72 hrs. The spherical crystallites (yield 4.35 g) were collected, washed with water and dried at 80°C for 16 hrs. They were larger than reported i n ref. 7 and had a different crystal habit from those studied i n ref. 5. The as-prepared ZSM-48 was calcined i n air at 500°C for 48 hrs to give H,Na-ZSM-48. This was washed with 1.0 M HC1 at 25°C for 3 hrs to give H-ZSM-48. The material was then heated at 450°C for 30 hrs i n a tubular quartz furnace (Hamdan, H . ; Sulikowski, B.; K l i n o w s k i , J. T. Phys. Chem., i n press) with water being injected at a rate of 12 m l per hour into the tube by a peristaltic pump so that the partial pressure of H 2 O above the sample was 1 atmosphere. It was then exchanged with 1 M N H 4 N O 3 and steamed at 650°C for a further 16 hrs. The ammonium exchange was repeated and the zeolite steamed at 650°C for 16 hrs and at 800°C for 8 hrs. X R D diffraction patterns (not shown) d i d not indicate any loss of crystall inity upon thermal treatment. High-resolution electron microscopic studies employed a modified JEOL-JEM200CX (8) operated at 200 k V with objective lens characteristics C = 0.52 m m , C = 1.05 m m leading to a theoretical point resolution as defined by the first zero i n the phase contrast transfer function of 1.95 Â at the optimum or Scherzer (9) defocus position (400 Â underfocus). Samples for electron microscopy were prepared by dispersing the assynthesized zeolite i n acetone and allowing one drop of the resultant suspension to dry onto an E M grid coated with a thin holey amorphous carbon film. Suitable crystallites were oriented along high symmetry zone axes i n the selected area diffraction mode and subsequent high resolution images were recorded at electron optical magnifications of ca. 120,000x after correction of illumination system misalignments and residual object ive lens astigmatism (10). In view of the established beam sensitivity of zeolites all images were recorded under conditions of l o w incident electron flux (11,12). Simulated images were computed on an I B M 3084 mainframe computer using the multislice method (13) for [00l], [OlO] and [lOO] project ions, using programs written i n this laboratory (14), and employing the atomic coordinates of Schlenker et al. (5) for both C m c m and Imma space group symmetries. For all calculations electron optical parameters applic able to our microscope were employed with images being calculated for crystal thicknesses of 20,40,60,80 and 100 Â and for defoci of between -29 and -1029 Â i n increments of 200 Â. N o account was taken of focal spread or beam divergence envelopes. The calculations were performed for an aperture limited resolution of either 3.1 or 3.5 Â, which entailed using structure factors extending into reciprocal space to 0.7525 or 0.875 À and evaluating the Fourier transform i n the phase grating to 1.55 Â or 1.75 À. 3

2


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s

c

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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ZEOLITE SYNTHESIS

The choice of these artificial limits was felt to be appropriate since the theo retical point resolution limit of 1.95 À at the optimum defocus is rarely achieved for zeolitic specimens due to their sensitivity to the electron beam. Typical total computation times were 120 s for each set of 25 images.


Results and Discussion A scanning electron micrograph of as-prepared zeolite ZSM-48 (see Figure 1) shows the typical size of the crystals and their habit. Figure 2 gives a representative l o w magnification high resolution micrograph of a small crystallite of ZSM-48 oriented along the [001] direction with a thin edge. From the micrograph and inset electron diffraction pattern it is clearly evident that the crystal contains a number of twins on (100) planes marked w i t h arrows. These twin planes are found i n almost all crystallites but there appears to be no regular periodicity between them. Figure 3 shows an enlargement of approximately the marked area of Figure 2 together with simulated images calculated for a crystal thickness of 100 Â and an objective lens defocus of -429 Â for both the U U D D (Cmcm) and U D U D (Imma) structural models. From a comparison of experimental and calculated images it is evident that simulations based on both struct ures of Schlenker et al. (5) give excellent agreement with those obtained experimentally at this crystal thickness and objective lens defocus. However, both C m c m and Imma models w o u l d be expected to give rise to very similar image contrasts for thin crystals i n view of the similarity of their projections along the [001] axis. Therefore, i n the absence of addition al experimental images projected along alternative zone axes, exact differ entiation between the two structures is not possible at this resolution. Hence, the proposal that the disorder present i n this system may arise from stacking faults involving a translation of c / 2 cannot be confirmed. In parallel with these preliminary experimental imaging studies we have also calculated additional simulated images for [001], [100] and [010] incidence for both structural models at two different aperture limited resolutions i n order to establish the optimal conditions for high resolution imaging. Figures 4-9 show the results of these calculations for [001], [100] and [010] incidence, respectively, for both C m c m and Imma topologies, calculated i n each instance for a resolution of 3.1 Â and objective lens defoci of between -29 and -1029 Â and crystal thicknesses of 20-100 Â. For simulations calculated for the [001] projection both topologies give rise to very similar image contrasts, at all the objective lens defoci and crystal thicknesses considered for a resolution of 3.1 A (see Figures 4 and 5). Furthermore, it is evident that the image contrast is a true reflection of the projected structure (with both the central 10-membered channel and the satellite 5- and 6-membered channels visible) only for objective lens defoci of between ca. -229 and -429 Â, and that this is true for all crystal thicknesses considered. This is not surprising since it has been established (14-16) that for values of objective lens defocus close to the optimum (-440 Â) that high resolution image contrast should be directly interprétable i n terms of the



39. KIRKLANDETAL.

Electron Microscopic Studies of Zeolite ZSM-48

577

Figure 1. Scanning electron micrograph of as-prepared zeolite ZSM-48.

Figure 2. A typical low magnification HREM image of ZSM-48 project ed along [001] with the corresponding electron diffraction pattern (inset). Several twins on (100) planes are indicated.



578

ZEOLITE SYNTHESIS

Figure 3. A n enlargement from approximately the marked area of Figure 2 together with calculated images (Af = -429 Â, thickness 80 Â) based on (a) the U U D D and (b) U D U D models.



39. KIRKLAND ET AL.

Figure 4.


Calculated images for the C m c m topology of zeolite ZSM-48

for [001] incidence (Af =-29 to -1029 À; thickness 20,40, 60, 80 and 100 Â; resolution 3.1 Â).


579


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ZEOLITE SYNTHESIS

Figure 5. A s for Figure 4, but calculated for the Imma topology.

Figure 6. A s for Figure 4, but calculated for [010] incidence.



39.

KIRKLANDETAL.



Figure 8.

Calculated images for the C m c m topology of zeolite ZSM-48

for [100] incidence (Af =-29 to -1029 Â; thickness 20,40, 60, 80 and 100 Â; resolution 3.1 À). In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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projected charge density of the structure for relatively weakly scattering specimens. In an analogous fashion images projected along [010] of both structures are virtually indistinguishable at 3.1 Â resolution for all crystal thicknesses and objective lens defoci considered (Figures 6 and 7). Again, this is expected given the close similarity of the two projected charge densities and the above criterion. However, images calculated for [100] incidence for the two structures show notable differences i n contrast at all crystal thicknesses and objective lens defoci, with the differences being most marked at defoci close to the optimum (Figures 8 and 9). Again, this difference may be rationalised i n terms of the projected charge density of the structures which i n this projection differ due to the arrangement of the four independent Τ atoms. For the case of the Imma topology all the 4membered rings have the same size projected along the a axis but i n the case of the C m c m structure the 4-membered rings are alternately large or small. Simulations calculated at 3.1 Â resolution show that the [100] projection is the most suitable for differentiation of the two proposed topologies. We have also calculated images for [001] incidence at 3.5 Â resolut ion i n order to ascertain whether differences i n the two structures may be identified at lower resolution as has previously been found for imaging silver clusters i n zeolite Y (17), wherein differences invisible at high resolution became apparent when the resolution was decreased. Figures 10 and 11 show calculations performed for identical conditions to those of Figures 3 and 4 but with an aperture limited resolution of 3.5 Â as opposed to 3.1 Â. From these calculations it is apparent that at this resolution there are notable differences between images calculated for the two structures, w i t h the satellite channels being clearly visible for the C m c m topology but very much weaker for the Imma model. A n examination of the diffracted beam amplitudes for the (001) (1 = 2, 4 or 6) beams reveals the reason for this difference. Table I shows the calculated diffracted beam amplitudes for the (000), (002), (004) and (006) beams for both topologies, for a crystal 60 À thick. The principal difference between the intensities for the two structures lies i n the intensity of the (004) beam relative to (006). [Note that i n both cases the intensity of the central (000) beam is approximately the same and hence the intensities are approximately normalised w i t h respect to each other thus allowing direct comparison]. In the case of the Imma structure the (004) beam is weak w i t h respect to the (006) and hence the (006) must be included to resolve the satellite channels clearly. A t a limited resolution of 3.5 Â the (006) beam lies outside the objective aperture and hence the satellite channels are only very faint i n the image. However, for the C m c m model the relative intensities are reversed w i t h the (004) beam stronger than the (006) and hence even if the (006) beam is excluded from the aperture the satellite channels w i l l be clearly resolved. In conclusion, we have shown that the [100] projection is the most suitable for distinguishing the two structure types at 3.1 Â resolution or



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ZEOLITE SYNTHESIS

Figure 10. Calculated images for the C m c m topology of zeolite ZSM-48 for [001] incidence (Af =-29 to -1029 Â; thickness 20,40, 60, 80 and 100 Â; resolution 3.5 Â).



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Electron Microscopic Studies ofZeolite ZSM-48

T A B L E I. C a l c u l a t e d diffracted b e a m intensities of the (001) (1=2,4,6) b e a m s for the t w o topologies of ZSM-48


Diffracted beam

(006) (004) (002) (000)

Intensity (arbitrary units) Cmcm

Imma

0.000% 0.00144 0.00668 0.50

0.0077 0.000774 0.00816 0.52

d(A)

3.36 5.03 10.07

greater but that at the l o w e r resolution of 3.5 Â differences are also a p p a r e n t for i m a g e s recorded for the [001] projection. Therefore the p r o p o s a l o f Schlenker et al. o f a d i s o r d e r e d structure i n v o l v i n g stacking faults of a translation o f c/2 c o u l d be c o n f i r m e d directly at h i g h resolution for [100] incidence o r at l o w e r resolution for b o t h [100] a n d [001] incidence. F u r t h e r experimental studies are c o n t i n u i n g to v e r i f y these proposals.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Schlenker, J.L.; Dwyer, F.G.; Jenkins, E.E.; Rohrbaugh, W.J., Kokotailo, G.T. Nature (London), 1981, 294, 340. C h u , P. US Patent 4,397,827, 1981. Rollman,C.D.;Valyocsik, E.W. U S Patent 4,423,021, 1983. C h u , P. US Patent No. 4,448,675 (1984). Schlenker, J.L.; Rohrbaugh, W.J.; C h u . , P.; Valyocsik, E.W.; Kokotailo, G.T. Zeolites, 1985, 5, 355. Smith, D.K. A Revised Program for Calculating X-ray Powder Diffraction Patterns, UCRL-502.64, Lawrence Radiation Laboratories, 1967. Dodwell, G.W.; Denkewicz, R.T.; Sand, L.B. Zeolites, 1985, 5, 153. Jefferson, D.A.; Millward, G.R.; Thomas, J.M.; Brydson, R.; Harriman, Α.; Tsuno, K . Nature (London), 1986, 323, 428. Scherzer, C J. A p p l . Phys., 1949, 20, 20. Smith, D.J.; Bursill, L.A.; W o o d , G.J. Ultramicroscopy, 1985, 16, 19. Csencsits, R.; Gronsky, R. Ultramicroscopy, 1987, 23, 421. Treacey, M.M.J.; Newsam, J.M. Ultramicroscopy, 1987, 23, 411. Cowley, J.M.; Moodie, A.F. Acta Cryst., 1957, 10, 609; Goodman, P.; Moodie, A.F. Acta Cryst., 1976, A32, 823. Jefferson, D.A.; Millward, G.R.; Thomas, J.M. Acta Cryst., 1976, A32, 823.


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ZEOLITE SYNTHESIS

Hirsch, P.B.; Howie, Α.; Nicholson, R.B.; Pashley, D.W.; Whelan, M.J. Electron Microscopy of Thin Crystals, Butterworths, London, 1965, and references therein. Cowley, J.M. Diffraction Physics, North Holland, 1975, and references therein. Terasaki, O; Uppal, M.K.; Millward, G.R.; Thomas, J.M.; Watanabe, D. Proceedings of the 11th International Congress on Electron Microscopy, Kyoto, 1986, p. 1777. 1988


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High-Resolution Electron Microscopic Studies of Zeolite ZSM-48

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