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Zeolite Structures: Some Common Misconceptions and Pitfalls
217 11 Drzaj, S. Hocevar and S. Pejovnik (Editors). Zeolites ~) 1985 Elsevir Science Publishers B. V. Arnsterdarn- Printed in Yugoslavia
ZEOLITE STRUCTURES: SOME COMMON MISCONCEPTIONS AND PITFALLS W.M. MEIER Institute of Crystallography, ETH Zurich, Switzerland
The past decade has seen great advances in zeolite structural research. This has been brought about by significant improvements of existing techniques and the advent of new methods including 27A1 and 29Si magic angle NMR and X-ray Rietveld refinement using high-resolution powder diffractometer data. The available structural information on zeolites and zeolite-like materials is very substantial indeed and raises the need for data compilations like those issued by the IZA Structure Commission (refs.l,2,3). The general aims of these compilations are (a) to provide frequently needed structural information on zeolites (which has been checked), and (b) to promote some necessary uniformity in recording such data. Some common misconceptions relating to major features of zeolite structures and frequently encountered pitfalls include the following points: 1) The terms zeolites and molecular sieves are not synonymous. Not all zeolites are molecular sieves,for the channel dimensions of some zeolite structures (like that of natro1ite e.g.) undergo marked and irreversible changes on dehydration. 2) A and faujasite type zeolite molecular sieves happen to be cubic, but contrary to widespread belief cubic symmetry does in no way imply structural rigidity. Structural studies of synthetic zeolite Rho (ref.4) serve as an illustration that the apparent rigidity of a zeolite framework has more to do with with the availability (or non-availability) of cation sites than cubic symmetry. 3) The assignment of the structure type based on similarity of XRD powder patterns is full of pitfalls as has become evident in a number of instances including that of synthetic zeolite E (ref.5). 4) Another common misconception is that R-va1ues are the principal or even the only criteria in assessing the reliability of structural results. It must be noted that there are several different types of R-va1ues (based on structure
218
factors, integrated intensities or profile values etc., either weighted or unweighted). In addition, R-va1ues depend on the type of data used (powder! single crystal), the 28-range, the mode of refinement and other factors. 5) The term lattice denotes the periodicity of a crystal structure and not the structure itself or the alumino-silicate framework. Lattice defects are irregularities with respect to the periodicity and comprise various faults. Stacking faults are conservative and have been known for a long time. Nonconservative faults in zeolites have been described only recently (ref.6). Hypothetical structures are a constant challenge in the field of synthesis. Remarkably enough the number of known network or structure types is quite small compared to the vast number of hypothetical frameworks. The 1atterare at least 4, possibly 5 orders of magnitude greater than the number of observed network types. This raises the question: Just how feasible is a particular hypothetica: framework structure? The following criteria are proposed for assessing the feasibility of hypothetical zeolite structure types: Bond distances and angles must all be within acceptable limits. i) ii) It must be possible to build the framework structure from ~ kind of secondary building unit. iii) In each family of networks those of highest topological symmetry are the most likely to occur. A fair percentage of the hypothetical zeolite frameworks which have been enumerated appear in fact improbable on the basis of these rules.
REFERENCES
2 3 4 5 6
W.M. Meier and D.H. Olson, "Atlas of Zeolite Structure Types", IZA Structure Commission (1978). W.J. Mortier, "Compilation of Extra Framework Sites in Zeolites", IZA Structure Commission (Butterworth, 1982). R. von Ba11moos, "Collection of Simulated XRD Powder Patterns of Zeolites", IZA Structure Commission (Butterworth, 1984). L.B. McCusker, Zeolites, 4 (1984) 51-55; ibid., Proc. 6th Int. Zeolite Conf. (in press). W.M. Meier and M. Groner, J. Solid State Chem., 37 (1981) 204-218. R. Gramlich, W.M. Meier and B.K. Smith, Z. Krista110gr. (1985) in press.
ZEOLITE STRUCTURES: SOME COMMON MISCONCEPTIONS AND PITFALLS W.M. MEIER Institute of Crystallography, ETH Zurich, Switzerland
The past decade has seen great advances in zeolite structural research. This has been brought about by significant improvements of existing techniques and the advent of new methods including 27A1 and 29Si magic angle NMR and X-ray Rietveld refinement using high-resolution powder diffractometer data. The available structural information on zeolites and zeolite-like materials is very substantial indeed and raises the need for data compilations like those issued by the IZA Structure Commission (refs.l,2,3). The general aims of these compilations are (a) to provide frequently needed structural information on zeolites (which has been checked), and (b) to promote some necessary uniformity in recording such data. Some common misconceptions relating to major features of zeolite structures and frequently encountered pitfalls include the following points: 1) The terms zeolites and molecular sieves are not synonymous. Not all zeolites are molecular sieves,for the channel dimensions of some zeolite structures (like that of natro1ite e.g.) undergo marked and irreversible changes on dehydration. 2) A and faujasite type zeolite molecular sieves happen to be cubic, but contrary to widespread belief cubic symmetry does in no way imply structural rigidity. Structural studies of synthetic zeolite Rho (ref.4) serve as an illustration that the apparent rigidity of a zeolite framework has more to do with with the availability (or non-availability) of cation sites than cubic symmetry. 3) The assignment of the structure type based on similarity of XRD powder patterns is full of pitfalls as has become evident in a number of instances including that of synthetic zeolite E (ref.5). 4) Another common misconception is that R-va1ues are the principal or even the only criteria in assessing the reliability of structural results. It must be noted that there are several different types of R-va1ues (based on structure
218
factors, integrated intensities or profile values etc., either weighted or unweighted). In addition, R-va1ues depend on the type of data used (powder! single crystal), the 28-range, the mode of refinement and other factors. 5) The term lattice denotes the periodicity of a crystal structure and not the structure itself or the alumino-silicate framework. Lattice defects are irregularities with respect to the periodicity and comprise various faults. Stacking faults are conservative and have been known for a long time. Nonconservative faults in zeolites have been described only recently (ref.6). Hypothetical structures are a constant challenge in the field of synthesis. Remarkably enough the number of known network or structure types is quite small compared to the vast number of hypothetical frameworks. The 1atterare at least 4, possibly 5 orders of magnitude greater than the number of observed network types. This raises the question: Just how feasible is a particular hypothetica: framework structure? The following criteria are proposed for assessing the feasibility of hypothetical zeolite structure types: Bond distances and angles must all be within acceptable limits. i) ii) It must be possible to build the framework structure from ~ kind of secondary building unit. iii) In each family of networks those of highest topological symmetry are the most likely to occur. A fair percentage of the hypothetical zeolite frameworks which have been enumerated appear in fact improbable on the basis of these rules.
REFERENCES
2 3 4 5 6
W.M. Meier and D.H. Olson, "Atlas of Zeolite Structure Types", IZA Structure Commission (1978). W.J. Mortier, "Compilation of Extra Framework Sites in Zeolites", IZA Structure Commission (Butterworth, 1982). R. von Ba11moos, "Collection of Simulated XRD Powder Patterns of Zeolites", IZA Structure Commission (Butterworth, 1984). L.B. McCusker, Zeolites, 4 (1984) 51-55; ibid., Proc. 6th Int. Zeolite Conf. (in press). W.M. Meier and M. Groner, J. Solid State Chem., 37 (1981) 204-218. R. Gramlich, W.M. Meier and B.K. Smith, Z. Krista110gr. (1985) in press.