J . Phys. Chem. 1992,96, 8206-8209
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(10) Mostafavi. M.; Keghouche, N.; Delcourt, M.-0.; Belloni, J. Chem. Phys. Lett. 1990, 167, 193. (1 1) Hilpert, K.; Gingerich, K. A. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 739. (12) Gingerich. K. A.; Cccke, D. L.; Miller, F. J . Chem. Phys. 1976,64, 4027. (13) Chceacman, M. A,; Eyler, J. R. J . Phys. Chem. 1992, 96, 1082. (14) GantefBr,G.; Gausa, M.; Meiwcs-Brocr, K.-H.; Lutz, H. 0.J . Chem. Soc., Faraday Trans. 1990,86, 2483.
(15) Bauschlicher, C. W.;Langhoff, S. R.;Partridge, H. J . Chem. Phys.
1990, 93, 8133.
(16) In writing the redox equations (electron, e-, on the right side, and using the standard reduction potential without change in sign) we follow the method of: Pourbaix, M. Atlas of Electrochemical Equniibrla; Pergamon Press: Oxford, 1966. (17) Breitenkamp, M.; Henglein, A.; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1976,80, 973. (18) Mulvaney, P.; Henglein, A. J . Phys. Chem. 1990, 94, 4182.
ReaCSpace Imaging of Molecular Sieves Composed of Aluminum Phosphates and Their Metal-Substituted Analogues hatibha
L.Gai-Boyes,
Central Research & Development, E . I. Du Pont de Nemours Experimental Station, Wilmington, Delaware 19880-0356
John Meurig Thomas,* Paul A. Wright, Richard H. Jones, Srinivasan Natarajan, Jiesheng Chen, Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London W l X 4BS
and Ruren Xu Department of Chemistry, Jilin University, Changchun, People's Republic of China (Received: June 23, 1992; In Final Form: August 12, 1992)
High-resolution electron microscopic images of representative title materials, notably JDF-2and CoALPO-5, both of which are extremely beam sensitive, have been recorded and compared with computed images. It is shown that real-space images, coupled with information derived from electron diffraction, distanceleast-squares,and energy minimization procedures, form the basis of a new method for determining the three-dimensional structure of aluminum phosphates (ALPOs) and their metal-substituted analogues (MeALPOs).
Introduction Our understanding of the structures and properties of naturally occurring and synthetic molecular sieves of the aluminosilicate (zeolitic) type has been greatly enlarged by real-space imaging using transmission electron microscopy. From the time when high-resolution elcctron " p y (HREM) was fmt successNly applied to image the structures of zeolite A' and ZSM-5,* considerable progress has been made with this technique. Thus,direct proof for the existence of an infinite family of ordered intergrowths of ZSM-5 and ZSM-11 has becn ~ b t a i n e d ;the ~ structures of several zeolites that had defied solution by X-ray methods (e.g., ZSM-23?y5 Theta- 1: ECR- 1 zeolite beta*) were all established principally through vital clues provided by HREM, and new insights have been gained into the nature of recurrent, as well as nonrecurrent, but nonrandom intergrowths in several distinct families such as zeolites ABC-69,10and ZSM-3/ZSM-20.11-14 If only such HREM studies were applied to the much larger families of ALP&, MeALPOs, and SAPOS (silicon aluminum many of which have not yet had their structures detennined-principally because they do not form crystals suitable for single-crystal X-ray analysissimilar progress could be expected in our appreciation of the properties of this additional family of molecular sieves and catalysts. The application of HREM to these interesting solids has, however, been much hampered by their extraordinary tendency to become amorphous when exposed to electron beams of the intensities required for imaging at the requisite degree of resolution. This kind of experimental difficulty was encountered in the early days of imaging of aluminosilicate zeolites. It was overcome for that class of solid by a combination of approaches: use of higher
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0022-3654/92/2096-8206$03.00/0
acceleratingvoltages (200,in m e cases lO00, instead of 100 keV) in the electron microscope (so as to minimize the production of inelastic collisions which are principally responsible for the structural degradation of the sample); employing a better quality vacuum in the specimen chamber, so as to reduce greatly the presence of water vapor, a key agent for triggering the loss of crystallinity; and, when possible, dduminating the aluminosilicate, i.e. replacing the AI3+ by Si4+in the framework without loss of structural integrity. (For reasons that are not well understood, A I 4 bonds are more susceptible to rupture under electron irradiation than S i 4 bonds.) The first two options for improving beam stability of ALPOs and MeALPOs have been followed hm; the third is not open to us. Even so, it transpires that ALPOs and MeALPOs are extraordinarily sensitive to electron irradiation, and this is the principal reason why no high-resolution images of these solids have been hitherto reported. Previous studied9 have underlined the magnitude of the experimental difficulties encountered in producing high-resolution, real-space images of ALPOs (contrast the situation for, say, oxide superconductorsm). Experimental Section Using accelerating voltages of 300 keV in a Philip CM30 electron microscope with a super twin (ST)objective lens, we have succeeded in imaging representative ALPOs and MeALPOs in such a manner as to encourage us to believe that, soon, progress in structural elucidation comparable to that achieved for aluminosilicates will also be achievd with ALPOs and related materials. High-precision microanalysis on a nanometer scale was carried out simultaneously using a windowless energy-dispersive X-ray (EDX) detector. The spherical aberration coefficient (C,) 0 1992 American Chemical Society
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Figure 1. (a) The [Ool] projection of the ALPO-5 framework (AI/P disordered, Pblmcc, a = 13.77, c = 8.39 A) showing the main (12-ring) and subsidiary (6- and 4 4 n g ) channels. (b) and (c). Two views of the JDF-2framework (Pbca, a = 10.28, b = 17.06, c = 13.84 A) given as line drawings for simplicity. The [lo01 projection (b), slightly tilted, shows the arrangement of channels. The [OlO]projection (c), which is imaged in this paper, reveals the layered nature of the structure.
of the objective lens pole piece is 1 mm. The resolution of the instrument at the first zero of the contrast transfer function at optimum defocus, Df (where Df = 1.2 X Scherzer defocus (-430 A); Le., 1.2C,1/2I*/*, where I is the electron wavelength), is 1.9 A, and lattice imaging is possible to at least 1.4 A. The limitation on the quality of HREM imaging in these materials is commonly the beam damage caused to the samples. For our experiments the samples were left in the EM column vacuum overnight before examination to eliminate residual gas or water. Suitably reduced electron gun bias setting was used together with smaller condenser apertures (30-50 pm in size), smaller electron beam spot sizes, and defocused illumination in order to reduce the beam dose on the specimens. Crystals were rapidly tilted to symmetrical zone axis orientations with the side entry goniometer. The lattice images were stable for only a few seoonds. Exposure times were typically 1-3 s, using Kodak SO163 electron image film, and a longer film developing procedure was used. This was complemented by printing the lattice images and electron diffraction patterns directly off the TV monitor where possible, using a Sony printer. Attempts were made, in some cases, to subtract the noise (nonperiodic Fourier coefficients) in order to improve the experimental image quality, using a Neotech digitizer on a Macintosh PC, and the images were Fourier transformed (with 256 X 256 arrays) using the Ultimate image processing software. Power spectrum diffraction (which is equivalent to an optical diffractogram) was thus obtained to which suitable masks were applied to filter the noise. The masked diffraction pattern was then inverse Fourier transformed to yield an image with the output on a 35." film. Multislice image simulations were carried cut using 256 X 256 arrays to interpret the HREM images. The standard multislice procedure9was modified to handle a large number of beams in the calculations. For the calculations of the 300-keV images, the electron microscope parameters in its supertwin configuration were used: C, = 1 mm, objective aperture radius R = 0.5 and 0.6 A-', chromatic focus spread of -0.25 mrad. Up to 3000 structure factors were created and lo00 beams were used in the preliminary simulations. Defocus values between the Scherzer defocus and -1100 A were used in steps of --lo0 A, with total crystal
thickness (1) between 20 and 200 A. Images were simulated with and without beam damage. Images were found to be very sensitive to objective lens defocus values, and thickness of the crystals and larger underfocus values between -900 and -1 100 A were necessary to match the images. As pointed out by others? dark patches of intensity appear at the centers of channels of the samples near Scherzer defocus. This is because the contrast transfer function of the objective lens is not optimized for large unit cell structures exhibited by these samples.
Results and Discussion The two aluminophosphate molecular sieves that we have investigated are CoALPO-5 and a new one, synthesized using methylamine as a template and designated JDF-2 (JDF = Jilin Davy Faraday), the structure of which has recently been determinds2' It is a centrosymmetric variant of ALPO-EN322(possessing the same framework topology). The structure of ALPO-5 (Figure la) is well-known and has been established by singlecrystal X-ray diffract~metry~~ and in the calcined form by neutron powder d i f f r a ~ t i o n . ~We ~ have shown by X-ray absorption spectroscopy and other means that the cobalt in as-prepared C O A L P O -is~ in ~ ~the +2 oxidation state and in tetrahedral coordination, replacing A13+ions uniformly within the framework. Upon calcination, the occluded template is removed with partial loss of cobalt from the framework, although element mapping by EDX reveals that it still remaips uniformly distributed throughout the sample. JDF-2 has a structure (Figure 1b.c) that is quite different from that of most other ALPOs so far reported, not least because of the presence of some 5-coordinated framework A13+ ions. High-resolution experimental images obtained down the [Ool] axis of CoALPO-5, both before and after noise filtering, and down [OlO]of JDF-2 are given in Figures 2 and 3, respectively, along with simulated images and superimposed representations of structural projections down those axes. It is clear that the quality of the images now attainable is already sufficient to reveal both the main (12-ring) channel system of the ALPO-5 molecular sieve and the larger (Le. the 6-ring) of the two subsidiary channel systems. It is also clear that the connectivity of the secondary
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Figure 2. (upper) The [OOl] diffraction pattern and corresponding experimental high-resolution image. (lower) The noise-filtered image, simulated image (inlaid), and structural projection (also inlaid) of calcined CoALPO-5 (Co/(Co + AI) = 0.036). H¶ b % a d - -
,
----
% 1 I
P
Figure 4. (a, top) The [00 1 ] diffraction pattern and corresponding experimental image of MeALPO-36 (Me = Zn, Zn/(Zn + AI) = 0.04)) with, inlaid, images simulated with (above) and without (below)taking
d
Figure 3. The [OIO] diffraction pattern and corresponding experimental high resolution image, simulated image (avowed), and the structure projection of the framework of the as-prepared aluminophosphate JDF-2.
building units that make up the structure of JDF-2 is faithfully reflected in the real-space image (Figure 3)-compare the observed image with that computed by the multislice procedure. This degree of resolution in the real-space images, coupled with the unit cell and symmetry information retrievable from the electron diffraction patterns down a series of zone axes, will undoubtedly enable fundamental contributions to be made to the determination of
electron beam damage into account. These show the arrangement of the 12-ring channels parallel to [loo]. (b, bottom) The [ 1001 diffraction pattern, noise-filtered image, and simulated image of the same sample, showing the arrangement of six-membered rings in projection.
ALPO or MeALPO structures that have hitherto defied solution by any other single (or combination of) means. Once the broad details of the framework structure have been obtained, refinements in the atomic coordinates may be achieved by employing the well-known DLS26(distance-least-squares)procedure. Further refinements and the solution of the crystal structure come from the use of energy minimization computation^.^^*^* Since reliable atomatom potentials are now available for the constituents of ALPOs and most MeALPOs, it is a relatively straightforward matter to arrive at the atomic coordinates.29To test the trustworthiness of the resulting structures, determined by a combination of electron microscopy and computation, the theoretically determined X-ray powder diffraction pattern based on it may be compared with the measured pattern. We have applied such a combination of electron microscopy, electron diffraction, distance-least-squares ( D E ) procedures, and
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Acknowledgment. We gratefully acknowledge support from the SERC (UK) and the Royal Society (UK). Registry No. Co, 7440-48-4; Zn, 7440-66-6; Mg, 7439-95-4.
References and Notes L. A.; Lodge, E. A.; Thomas, J. M. Nuture 1980, 286, 11. (2) Thomas, J. M.;Millward, G. R.; Bursill, L. A. Philos. Truns. R. Soc. London 1981, A300,43. (3) Thomas, J. M.;Millward, G. R. J. Chem. Soc., Chem. Commun. 1982, 1380. (4) Wright, P. A.; Millward, G. R.; Thomas, J. M.;Barri, S.A. I. J. Chem. Soc., Chem. Commun. 1985, I 1 17. (5) Millward, G. R.; White, D.; Thomas, J. M.;Barri, S.A. I. J. Chem. Soc., Chem. Commun. 1988,434. (6) Barri, S . A. I.; Smith, G. W.; White, D.; Young, D. Nuture 1985,312, 533. (7) Leonowicz, M.E.; Vaughan, D.E. W. Nuture 1987,329, 819. (8) Treacy, M.M. J.; Newsam, J. M.Nature 1989, 332, 249. (9) Terasaki, 0.; Millward, G. R.; Thomas, J. M.Proc. R. Soc. London 1984, A395, 153. (10) Terasaki, 0.;Thomas, J. M.;Watanabe, D.; Millward, G. R. J. Chem. Soc., Chem. Commun. 1984,77. (11) Audier, M.; Thomas, J. M.;Klinowski, J.; Jefferson, D. A.; Bursill, L. A. J. Phys. Chem. 1982,86, 581. (12) Millward, G. R.;Thomas, J. M.;Ramdas, S.;Barlow. M. T. Proceedings of the 6th Internutional Zeolite Conference, Reno, 1983; Olson, D. H., Bisio, A., Eds.; Butterworthx London, 1984; p 793. (13) Treacy, M.M.J.; Newsam, J. M.;Vaughan, D.E. W.; Beyerlein, R. A.; Rice, S.B.; DcGruyter, C. B. In Microstructure und Properties of Cutulysts; Trcacy, M.M.J., Thomas, J. M., White, J. M.,Eds.; Mater. Res. Soc. Symp. Proc. No. 11 1; Materials Research Society: Pittsburgh, 1988; p 177. (14) Anderson, M.W.; Pachis, K. S.; Prebin, F.; Carr, S.W.; Terasaki, 0.; Ohsuna, T.; Alfrcddson, V. J. Chem. Soc., Chem. Commun. 1991,1660. (15) Wilson, S. T.; Lok, B. M.;Flanigen, E. M.US Patent 4,310440,1982. (16) Lok, B. M.; Messina, C. A,; Patton, R. L.;Gajek, R.T.; Cannan, T. R.; Flanigen, E. M.US Patent 4,440,871, 1984. (17) Wilson, S. T.; Flanigen, E. M.US Patent 4,567,029, 1986. (18) Flanigen, E. M.;Lok, B. M.;Patton, R. L.; Wilson, S.T. Pure Appl. Chem. 1986,58, 1351. (1 9) Richardson, Jr.; J. W.; Vogt, E. T. C. Zeolites 1992, 12, 13. (20) Gai-byes, P. L.; Thomas, J. M.Supercond. Reu. 1992, 1, 1. (21) Chectham, A. K.; Chippindale, A. M.; Huo, Q.;Jones, R. H.; Powell, A. V.; Thomas, J. M.;Xu, R. Proceedings of the 9th Internutionul Zeolite Conference (Montreuf);Higgins, J. B., van Ballmoos, R., Treacy, M.M. J., -( 1) Bursill,
a
5
15
25 2 0 (degrees)
35
45
Figure 5. (a) The X-ray diffraction (XRD) pattern of the 'ALPO" equivalent of MeALPO-36, simulated using atomic coordinates derived from an idealized trial structure by energy m i n i i t i o n procedures (see text). (b) The experimental XRD of dehydrated MeALPSO-36 (Me= Mg, Mg/(Mg AI) = 0.04,Si/(Mg A1 + P + Si) = 0.07) collected at room temperature. Peaks below around 13" 28 arc lcss than expected because the size of the X-ray beam (divergence = 1") at these low angle? far exceeds the sample size. The shift of the peaks to lower angles in pattern (a) a r k from an increase in the lattice parameters of the energy minimizedstructurc(u= 1 3 . 4 6 A , b = 2 2 . 1 7 A , ~ = 5 . 2 9 A , a = 9 0 . 2 ~ , 4 = 92.0°, y = 89.9').
+
+
atom-atom computations to MeALPO-36 (Me = Mg, Zn), a uniform heterogeneous catalyst'J that is the subject of other studies in this laboratory. Previous ~ ~ r hasbindicated ~ ~ that , ~ ~ the structure contains large pores bounded by 12-membered rings similar to ALPO-5. High-resolution micrographs taken parallel Eds.: Buttenvorths: London. in mess. and perpendicular to the needle axis of crystals of MeALPO-36 (22) Parise, J. B. Zeolites; Drzaj, B., Hocevar, S., Pejovnik, S., Eds.; Elsevier: Amsterdam, 1985; p 271. (Me = Zn) (Figure 4a.b) show the spatial arrangement of these (23) Bennett, J. M.;Cohen, J. P.; Flanigen, E. M.;Pluth, J. J.; Smith, J. unidimensional channels. Combining this information with a unit V. ACS Symp. Ser. 1983, No. 219, 109. cell derived from electron diffraction suggests a trial structure. (24) Richardson Jr.. J. W.: Pluth.. J. J.:. Smith. J. V. Actu Crvstullow. " Subsequent energy minimization produces atomic coordinates, 198'7, C43, 1469. (25) Chen. J.; Sankar, G.; Thomas, J. M.; Xu, R.; Greaves, G. N.; Waller, from which a powder X-ray diffraction pattern can be computed D. Chem. Muter., in press. (Figure5a). This simulated pattern shows a very clase agrement (26) Baerlocher, C.; Hcppc, A.; Meier, A. DLS-76 Munuul; Institute of with the observed experimental one, e.g., at room temperature Crystallography and Petrology, ETH: Zurich, Switzerland, 1977. for dehydrated MgALPSO-36 (Figure 5b) (CT,a = 13.15, b = (27) Thomas, J. M.; Catlow, C. R. A. Prog. Inorg. Chem. 1987,35, 1. Catlow, C. R. A.; Thomas, J. M.Philos. Trow. R. Soc. London, in press. 21.53, c = 5.15 A; a = 89.8,B = 92.3, y = 90.2O). HREM also (28) Jackson, R. A.; Catlow, C. R. A. Mol. Simul. 1)89,1.207. Bell, R. reveals that the ZnALPO-36 sample contains many planar defects G.; Jackson, R.A.; Catlow, C. R. A. J. Chem. Soc., Chem. Commun. 1990, on (OlO), the structural nature of which is under examination. 782. More details of this structure will be published elsewhere. (29) van Beest, B. W. H.; Kramer, G. J.; van Santen, R. A. Phys. Rev. Lett. 1990,61, 1955. Note Added in Proof. We have recently become aware that (30) Wilson, S.T.; Flanigen, E. M. Zeolite Synthesis; Occclli, M. L., the structure of MAPO-36 has been recently solved by Smith et Robson, H. E.,Eds.;ACS Symp. Ser. 1989, No. 398, 329. al. (Smith, J. V.; Pluth, J. J.; Andries, K. J. Zeolites, to be (31) Nakashiro, K.; Ono, Y. J. Chem. Soc., Furuduy Truns. 1991, 87, published). 3309.
