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appears from the limited data presented here that the grown oxides are fairly good substrates for ionic absorption. In the language of hard and soft acids,14AP+ is a very hard acid and prefers hard bases. This explains the preference for A1-OCN (rather than A1-NCO) and A1-NCS (rather than A1-SCN). It does not, however, completely explain the weak presence of A1-SCN and the complete absence of A1-CN. We infer from these data that ionic species can be stably adsorbed to A1203from solution provided they present an external surface which may act as a hard base without modifying the ion. We may use the previous conclusions to address the second question posed. If one is to successfully incorporate transition metal complexes into A1-A1203-Pb diodes from fluid solution, it will be helpful to have one or more ligands which present a hard base “external surface.” That is, one would expect the complex CH3Hg-SCN to be much more stable on, and adsorb better to, aluminum oxide than the complex (CH3)3Si-NCS. As to the stability of metal cyanide complexes, we have gained pertinent, though negative, information. Our results indicate that the cyanide ion is not dissociating from the metal to preferentially bond to the aluminum. This leaves two other possibilities which we are investigating: (a) the CNpreferentially bonds to the “soft” lead top electrode, or (b) the decomposition is due to an oxidation-reduction process. This work has definitely identified the 2145-cm-l band
(observed by IR spectroscopy) of A120,-supported noble metal catalysts exposed to CO, NO, and N2 at elevated temperatures as the CN stretching mode of A1-OCN. We have not observed any sign of the AI-NCO species. There are several possible reasons for the absence of this species in our systems and they have been presented earlier. We are in the process of determining which of the possible explanations is correct.
Acknowledgment. We thank the National Science Foundation for supporting this research in the form of Grant No. DMR-7820251. References and Notes (1) P. K. Hansma and R. V. Coleman, Science, 184, 1369 (1974). (2) M. G. Simonsen, R. V. Coleman, and P. K. Hansma, J . Cbem. Pbys., 81, 3789 (1974). (3) U. Mazur and K. W. Hipps, forthcoming work. (4) M. L. Unland, Science, 179, 567 (1973). (5) M. L. Unland, J . Phys. Cbem., 77, 1952 (1973). (6) F. Solymosl, L. Volgyesl, and J. SBrkBny, J . Cafal., 54, 336 (1978). (7) F. Solymosi and T. BBnsBgl, J . Pbys. Cbem., 83, 552 (1979). (8) R. A. Dalla Betta and M. Shelef, J . Mol. Cafal., 1, 431 (1976). (9) P. K. Hansma, Phys. Rep., 30C, 145 (1977). (10) W. M. Bowser and W. H. Weinberg, Surf. Sci., 84, 377 (1977). (11) Kazuo Nakamoto, “Infrared Spectra of InorganicCompounds”, Wiley, New York, N. Y., 1970, p 87. (12) F. Sobmosl, J. Kiss, and J. SBrkBny, “Proceedlngs, 3rd International Conference on Solids”, R. Dobrozemsky, Ed., Vienna, 1977. (13) E. Maslowsky, Jr., “Vibrational Spectra of Organometallic Compounds”, Wiley, New York, N. Y., 1977, p 93. (14) R. G. Pearson, Ed., “Hard and Soft Acids and Bases”, Druden, Hutchinson and Ross, Stroudsburg, Pa., 1973.
ZSM-5-Type Materials. Factors Affecting Crystal Symmetry E. L. Wu,* S. L. Lawton, D. H. Olson, A. C. Rohrman, Jr., and G. T. Kokotailo Mobil Research and Development Corporation, Paulsboro Laboratory, Paulsboro, New Jersey 08066, and Mobil Research and Development Corporation, Princeton, New Jersey 08540 (Received March 9, 1979) Publication costs assisted by Mobil Research and Development Corporation
Powder X-ray diffraction patterns of as-synthesized ZSM-5-type materials are consistent with idealized orthorhombic symmetry. A change to apparent monoclinic symmetry is observed upon certain treatments, such as calcination and ion exchange. Numerous pieces of evidence, including simulated powder patterns, show that this is a reversible, displacive transformation between the two symmetry forms causing no change in framework topology. Factors affecting the symmetry change are described.
Introduction The crystal structure of ZSM-5 has been rep0rted.l The framework of ZSM-5 contains a novel configuration of linked tetrahedra, consisting of eight five-membered rings. Such units are linked to form chains which in turn interconnect to form a unique three-dimensional framework structure with a dual-interconnecting channel system. The X-ray diffraction patterns of as-synthesized ZSM-5 materials are consistent with the maximum topological symmetry of the described framework having orthorhombic space group Pnma. Upon certain treatments (vide infra), the apparent symmetry changes to monoclinic with small changes in lattice parameters. We report below a detailed study of this symmetry change.
* To whom correspondence should be addressed at the Paulsboro Laboratory. 0022-3654/79/2083-2777$01 .OO/O
Variations in lattice parameters and framework symmetry have been observed and documented for many zeolite systems. For example, the lattice parameter in faujasite was found to vary as a function of aluminum content.2 The chabazite framework undergoes distortion due to dehydration3 A recent study of Baerlocher and Meier4 showed that a synthetic tetramethylammonium gismondine with tetragonal crystal symmetry undergoes a continuous structural change to a pseudocubic form upon thermal decomposition of the organic cation. Herein we discuss the nature of lattice parameter changes in ZSM-5 and present evidence that these are the result of a reversible, displacive transformation. Experimental Section ZSM-5 samples examined in the present study have varying Si02/A1203ratios from about 70 to greater than 0 1979 American Chemical Society
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Flgure 1. X-ray diffraction patterns of ZSM-5: (a) as-synthesized form; (b) ammonium-exchanged form (scale factor changes just below 28' 20).
3000. They were sythesized in the presence of tetrapropylammonium b r ~ m i d e .Those ~ with high SiOz/Al2O3 ratios were prepared without any added aluminum oxidea6 The presence of aluminum in the crystallized products is due to aluminum impurities in the reactants used. Various forms of these samples were examined and have the following designations: as-synthesized, ammonium exchanged, and hydrogen form. An ammonium-exchanged form is one prepared by ammonium exchange of an assynthesized material after removal of organic material by calcination at 538 "C. The hydrogen form is obtained by calcination of the ammonium-exchanged form, generally at 538 "C. Ammonia loading of a sample was carried out by exposing the sample to 1 atm of ammonia at room temperature. X-ray measurements were made with Cu K a radiation on a Philips X-ray diffractometer equipped with a scintillation counter, pulse-height analyzer, and strip-chart recorder. A scan speed of 0.5' 28/min was used.
Results and Discussion As-synthesized forms of ZSM-5, with varying Si02/A1203 ratios, have very similar X-ray powder diffraction patterns. A typical pattern (sample I) is given in Figure la. The most intense diffraction line occurs at about 23.2' 28. Due to the presence of organic material in the intracrystalline voids, the intensities of the first two lines at about 7.9 and 8.8' 20 are lowered. Except for such intensity differences, the powder pattern given in Figure l a is that expected based on the atomic coordinates derived from a singlecrystal structural determination of ZSM-5' in the space group Pnma; this crystal was in the as-synthesized form. X-ray diffraction patterns such as the one given in Figure 1A will be designated as an "orthorhombic" pattern in the discussion below. When sample I is transformed to the ammoniumexchanged form, changes can be observed in its X-ray diffraction pattern (given in Figure Ib). Some of the more prominent changes are (1)relative intensity changes (the first two lines increased greatly in intensity whereas the lines at about 11.9 and 12.5O 20 decreased in intensity),
TABLE I : Lattice Parameters of ZSM-5 a, a
b, c,a deg P , deg 7 ,deg ff,
orthorhombica
monoclinicb
20.07 19.92 13.42 90 90 90
20.11-20.17 19.90-19.94 13.40-13.43 90.4-90.6
90 90
As-synthesized, uncalcined single-crystal data.' Computed for 7 samples.
a
(2) shifts in line positions (the doublet centered at about 14.7' 20 merged to form an apparent singlet line, the doublet nature of the line at about 23.2' became more apparent, and that at about 23.9' 20 became less distinct), and (3) the appearance of doublets in place of singlets (at about 24.4, 29.2, and 48.6' 20). Some of the intensity changes can be explained as the expected consequence of a decrease in loading, i.e., removal of extra framework organic and inorganic species incorporated into the structural voids during synthesis. The appearance of doublets, however, cannot be accounted for as either changes in the lattice parameters a, b, and c or a lowering of symmetry within the orthorhombic crystal system. Therefore, monoclinic symmetry was suspected. All angular lattice parameters in the orthorhombic crystal system are 90'. A test for monoclinic symmetry was conducted as follows. Three sets of least-squares lattice parameter refinements were carried out by using the program L C R - Z . ~ The unit cell lengths (a, b, and c ) together with either the CY, p, or y angle were refined. Diffraction data used for refinements included observed angular positions in the range of 13-25' 20. Miller indices (based on the single-crystal study) were assigned to the input lines; various combinations of 313,313,133, and 133 were used for the 24.3-24.5' 28 doublet. Out of the many refinements, the one giving the lowest standard deviations corresponded with a slightly greater than 90'. The lattice parameters for a number of samples exhibiting the "monoclinic" diffraction pattern, similar to that given in Figure lb, were refined. The lattice pa-
ZSM-5-Type Materials
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The Journal of Physical Chemistry, Vol. 83, No. 21, 1979
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li
Figure 2. Comparison of (a)observed and (b) computed diffraction patterns of ZSM-5 exhibiting monoclinic symmetry. Computed pattern is based on the cell parameters a = 20.17 A, b = 19.93 A, c = 13.42 A, and a = 90.64’.
rameters for these samples (given in Table I) are strikingly similar to those of an as-synthesized, uncalcined crystal of ZSM-5 from the single-crystal s t ~ d y . The ~ lattice parameter along the a crystallographic axis increases in the monoclinic form but only by 0.5%. The b and c parameters remain essentially unchanged. The a angle increases at most to 90.6’ in the samples examined. With this small change in unit cell shape, symmetry elements like the mirror planes parallel to (010) bisecting the sinusoidal channels are removed. The probable monoclinic space group for treated ZSM-5 samples (like that in Figure lb) exhibiting monoclinic characteristics is P2,/n: (“Monoclinic characteristics” refers to the observation of at least the doublets at about 24.4, 29.2, and 48.6’ 20 mentioned above.) The position of all the lines observed in the “monoclinic” patterns can be accounted for by the introduction of monoclinic lattice parameters. However, it is the diffraction intensity which holds the clue to the atomic arrangements which make up the framework structure. Using the program POWD2,’ we have simulated the “monoclinic” X-ray diffraction pattern of a hydrogen form of ZSM-5 in space group m l / n , using atomic coordinates derived from the single-crystal study for ZSM-5. The computed diffraction pattern is compared to that observed in Figure 2. The match between the computed and observed patterns is extremely good. Many other doublets predicted in the computed pattern, other than those three mentioned above, can clearly be seen in that observed. The accurate prediction of the X-ray diffraction pattern for the “monoclinic” hydrogen ZSM-5 strongly suggests that the materials displaying the monoclinic pattern have the same topological framework structure as the as-synthesized material exhibiting the orthorhombic pattern. This transformation from orthorhombic to monoclinic sym-
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metry involves only very minor displacements of atomic positions. The splitting of peaks and all changes in intensity are accounted for by the lower symmetry and associated lattice parameters. Any variation in the ZSM-5 topology, or tetrahedral linkages, would likely introduce intensity changes which are not observed. As crystallized, the intracrystalline voids are loaded with organic matter and, in many cases, with excess alkali cation species. For example, sample I contains 1.6 wt % of Na in addition to organic species. Since it has a Si02/A1203 ratio of about 1600, the cationic content greatly exceeds that needed to balance the negative charge of aluminum in the framework. Even after calcination to remove the organic matter, the sample still exhibited essentially an orthorhombic pattern; lines at the three diffraction positions where doublets are to be expected if the symmetry was lowered to monoclinic showed only broadening. When the sodium content was lowered, however, to less than 0.02 wt % by contact with NH4C1 solution, the monoclinic pattern appeared as shown in Figure lb. In other preparations where the sodium content is not as high as in sample I, only removal of organic matter is necessary to cause these materials to display the monoclinic pattern. However, sample history is an important factor. Such is shown in Figure 3. Shortly after an air calcination at 600 ‘C for 1h, the X-ray diffraction pattern displayed by a sample with a sodium content of 0.8 wt % appears to be essentially orthorhombic. But, when exposed to the ambient atmosphere for 3 days, its powder pattern (Figure 3b) is definitely that of monoclinic symmetry. In this case, water vapor had presumably caused relocation of the residual sodium species which enabled the framework structure to undergo a symmetry change. These and other similar data showed that the extra framework content and their location are among the critical variables which control the symmetry change of ZSM-5 from apparent orthorhombic to monoclinic. Samples with SiOz/A1203ratios ranging from about 70 to greater than 3000 have been examined. Generally speaking, the higher the SiO2/Al2O3ratio, the easier it is for the ZSM-5 framework to change from apparent orthorhombic to monoclinic symmetry upon treatment. Therefore, aluminum content may be another factor which influences the symmetry change. As is well-known, aluminum content in the framework governs surface polarity of the zeolite, which in turn affects extra framework content such as cations and adsorbed water. In addition, lattice parameters in many zeolites are known to vary with aluminum content which can also affect the resolution of overlapping diffraction lines. Particle size is another factor which affects the resolution of diffraction patterns. Although, in itself, particle size has no relationship to the framework symmetry change, the observation of distinct monoclinic characteristics may be limited in samples where small crystal size contributes to line broadening. The crystallographic computation and powder pattern simulation presented above led to the conclusion that ZSM-5 samples showing the monoclinic powder pattern have the same framework topology as those showing the orthorhombic pattern. The symmetry change appears t o be a consequence of a displacive transformation resulting from changes of the chemical content in the structural voids. The possibility of a reconstructive transformation occurring at the high temperature, resulting in a monoclinic structure, appeared unlikely. Further experiments, conducted at or below 110 O C , gave further support to our conclusion that this is a displacive transformation and showed that it is indeed reversible.
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Figure 3. Effect of molsture on X-ray diffraction pattern of calclned ZSM-5. Exposure to ambient conditions: (a) 1 h; (b) 3 days.
Figure 4. X-ray diffraction patterns of ZSM-5: (a) as-synthesized;
An experiment illustrating this reversibility has been performed on the sample whose powder pattern is shown in Figure 2a. In the as-synthesized form, this sample exhibits sharp singlets at 24.38,29.27, and 48.65' 26 (Figure 4a). Upon transformation of this material into its hydrogen form, doublets are clearly seen a t 24.25-24.50, 29.12-29.35, and 48.42-48.86' 28 (Figure 4b). After this sample was treated overnight with flowing ammonia, at 760 mmHg, and at room temperature, the doublets merged into singlet lines again at 24.37,29.22, and 48.60' 20 (Figure 4c). The positions of the singlet lines in the ammoniatreated sample nearly coincide with those in the as-synthesized form. The ammonia-treated sample in Figure 4c contained excess ammonia. Three experiments were carried out to remove the ammonia: (1)heating under partial vacuum (635 mm Hg) at 28 'C for 4 days, (2) heating at 110 'C overnight, and (3) contacting with 1N NaN03solution at 25 'C. X-ray diffraction patterns of the ammonia-loaded sample and each of the treated samples are given in Figure 5.
Figure 5a is the diffraction pattern of the sample loaded with ammonia showing sharp singlet lines in these three regions of interest. After the mild treatment (l),with
(b) hydrogen form; (c) after subsequent NH3 loading.
removal of only a small amount of ammonia, the 48.6' doublet could be seen in Figure 5b, but the treatment only broadened the singlets at 24.37 and 29.22' 26. With heating at slightly higher temperatures, as in treatment (2), more ammonia was removed. Doublets appeared at all three regions as shown in Figure 5c. The splittings between the lines in each of the doublets are not as great as in the original hydrogen form (Figure 4b). When all the ammonia was removed by treatment (3), the sample exhibited doublets (in Figure 5d) nearly identical with those of the hydrogen form before ammonia treatment. All these treatments were carried out at low temperatures (no highet than 110 'C) to change the hydrogen form of the sample showing doublets to singlets after ammonia loading (in Figure 4) and intermediate stages of return to doublets (in Figure 5). Therefore, in view of the examples of the intermediates, it is highly improbable that the framework undergoes a reconstructive transformation at these low temperatures, Le., breaking and relinking of Si(A1)-0 bonds to interconvert between two distinct framework topologies. The refined lattice parameters for these samples are given in Table I1 and show a continuous change in the cy angle depending on the extent of loading by ammonia. Data in this table reinforce the premise that
The Journal of Pbysical Cbemistty, Vol. 83, No. 21, 1979 2781
ZSM-5-Type Materials
TABLE 11: Variation in Lattice Parameters of ZSM-5 with NH, Loading NH, loading intermed
full 28 “ C
C
23
24
19
30 (291
47
d
48
48
23
24
110°C
none 1N NaNO,
continuously from 90 to about 90.6’, with lowering of loading by ammonia in the structural voids. At low loading, the monoclinic symmetry appears to be favored.
References and Notes (1) G. T. Kokotailo. S.L. Lawton. D. H. Olson. and W. M. Meier. Nature (London), 272,’ 437 (1978). (2) E. Dempsey, G. H. Kuehl, and D. H. Olson, J. Pbys. Cbern., 73, 387 ( 1969). (3) J. V. Smith, Acta Crysta//ogr.,15, 835 (1962). (4) Ch. Baerlocher and W. M. Meier, &/v. Cbirn. Acta, 53, 1285 (1970). (5) R. J. Argauer and G. R. Landolt, U S . Patent 3 702 886. (6) F. G. Dwyer and E. E. Jenkins, US. Patent 3941 871. (7) D. H. Olson et al., unpublished data. \
/a
d9 Lo1 2 3 4’ 7 / -
Flgure 5. X-ray diffraction patterns of ZSM-5 with various NH, loading: (a) full; (b) and (c) intermediate; (d) free of NH,.
when the ZSM-5 material is rendered free of occluded matter, only a change in symmetry, but not in framework topology, occurs. Conclusions The crystallographic computations as well as experi-
I
(8) D. E. Williams, “LCR-2, A Fortran Lattice Constant Refinement Program”, IS-1052, Ames Laboratory, Iowa State Unlverslty, Ames, Iowa, 1964. (9) D. K. Smith, “POWD2, A Revised Program for Calculating X-ray Powder Diffractlon Patterns”, UCRL-50264, 1967; also D. K. Smith, Norelco Rep., 15, 57 (1968).
