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June 27, 2003 The Structure Solution of SSZ-58: a Novel Two-Dimensional 10-Member-Ring-Pore Zeolite with Previously Unseen Double Five-Member-Ring SubunitsA. Burton, S. Elomari, R.C. Medrud, I.Y. Chan, C.Y. Chen, L.M. Bull, and E.S. Vittoratos Researchers at ChevronTexaco have recently prepared a novel zeolite (designated SSZ-58) with a two-dimensional system of ten-membered ring pores. SSZ-58 is the first zeolite to possess a previously unobserved double five-membered ring unit. The structure of SSZ-58 was determined using the ab initio method FOCUS with powder diffraction data collected at beamline X7A. To date, SSZ-58 is the most complex zeolite (in terms of the number of atoms in the crystallographic asymmetric unit) to be solved from powder diffraction data.
Although structure solutions of zeolites from single crystals have been reported, it is rare that zeolite crystals are large enough for single crystal structure analyses of novel zeolite phases. Investigators must therefore rely on powder diffraction data. Ab initio structure solution from powder data is difficult because of the high degree of peak overlap inherent to most powder patterns.
In the past, there was often a long time between the initial discovery of a zeolite and its structure elucidation. The investigators hoped to construct a model that was consistent with the unit cell parameters, the possible space group symmetries, and other physicochemical (i.e., adsorption, catalytic, and density) data. These structure solutions often depended on simple modifications of previously known zeolite topologies. Model building continues to be an important tool to the zeolite crystallographer, particularly for the investigation of disordered materials. However, if an unsolved structure contains previously unobserved secondary building units, and especially if it possesses a large number of tetrahedral (T) atoms in its asymmetric unit, the success of model building is more dubious. Recently, with the advent of the FOCUS and ZEFSAII algorithms, there have been significant advances in ab initio structure solution of zeolites from powder diffraction data. These methods incorporate the crystal chemical information inherent to most zeotype structures: i.e., a fully connected, tetrahedral arrangement of the dominant x-ray scatterers. In this report we discuss the structure solution of the novel zeolite SSZ-58 by the FOCUS method. SSZ-58 was synthesized using the 1-butyl-1-cyclooctylpyrrolidinium cation as a structure directing agent (SDA). Samples for detailed structural analysis were examined at beamline X7A. The synchrotron powder diffraction pattern (Figure 1) of SSZ-58 could be indexed in an orthorhombic unit cell in space group Pmma: a = 25.112 Å, b = 12.498 Å, c = 12.860 Å. Figure 2 shows the topological structure of SSZ-58 (oxygen atoms have been omitted for clarity) determined from the FOCUS algorithm. This structure has 12 tetahedral (T) atoms and 26 oxygen atoms per asymmetric unit. In terms of the number of unique T atoms, SSZ-58 is the most complex zeolite or zeotype structure solved from powder diffraction data.
SSZ-58 possesses a two-dimensional system of pores that intersect to form large cavities that are bound by two pairs of opposing 10-membered rings (10MRs). The pores along the c-direction are straight channels bound by 10-MRs with dimensions of 5.7 x 5.2 Å assuming an oxygen radius of 1.35 Å. The sinusoidal pores along the a-direction are bound by distorted elliptical 10MRs with dimensions of 4.8 x 5.7 Å. The structure of SSZ-58 is composed of columns that possess double five-member rings (D5MRs). Although the D5MR is a simple building unit, it surprisingly has not been observed in any other zeolite structures. The structure solution of SSZ-58 highlights the advances that have been made in structure elucidation of complex materials from powder data. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Zeolites are crystalline microporous aluminosilicates which find widespread
use as catalysts, adsorbents, and ion-exchangers. New zeolites
continue to emerge at an increasingly rapid pace. The syntheses of
high-silica zeolites are usually performed hydrothermally in the
presence of an organocationic structure directing agent (SDA). The
size and shape of the SDA often correlate well with the dimensions
of the zeolite micropores or cages. An understanding of molecular
recognition effects in zeolite synthesis begins with identification
of the structure or structures promoted by certain SDAs. A detailed
knowledge of zeolite structure is also crucial for understanding
catalytic and adsorptive properties. Once the structure of a zeolite
is determined, applications based upon size or shape-selective
processes may be targeted, or known properties of the material may
be rationalized.
