Refinement, zeolite structure

Other exciting applications involved using parallel tempering in connection with available experimental data. For example, Falcioni and Deem [57] used X-ray data to refine structures of zeolites, and Haliloglu et al. [58] refined NMR structural data for proteins (in particular using residual dipolar coupling constraints). 

According to the chemical analysis and coordination distances, the Rietveld refinement of the crystal structure at room temperature revealed 1.2 Co2+ atoms per unit cell at the Col and Co2 sites, whereas the 1.4 Ag+ cations are spread over the Co3 site, from now on referred to as Ag5 for clarity, and two new sites, Ag2 and Ag3, located near Co2 in the 10-membered ring (Fig. 3). In addition, for this catalyst the presence of Ag° clusters outside the zeolite structure was recognized by the detection of a strong reflection at about 40° 28. In agreement with the lower Ag content, in Ag2.7Co2.8AF the Ag3 site... 

Structure refinement based on kinematical scattering was already applied by the Russian scientist 60 years ago. Weirich et al. (1996) first solved the structure of an unknown TinSe4by HREM combined with crystallographic image processing. Then they used intensities extracted from selected area electron diffraction patterns of a very thin crystal and refined the structure to a precision of 0.02 A for all the atoms. Wagner and Terasaki et al. (1999) determined the 3D structure of a new zeolite from selected area electron diffraction, based on kinematical approach. 

The crystal structure of hydrated NaA a detailed refinement of a pseudosymmet-ric zeolite structure. Z. Kristallogr., 133, 134-149. 

Zeolite structures pose unconventional problems for crystal structure refinement. These problems arise from positional disorder pseudo-symmetry, twinning, high mobility of some atoms, and (sometimes) the inaccessibility of single-crystal data. Methods are discussed for investigating split atoms, Si-Al distribution, pseudo-symmetry, and for dealing with parameter correlation and limited data sets. Some additional techniques which have not been applied to zeolite structures are mentioned. 

X-ray powder diffraction has been the primary tool used in zeolite structure research. With new high-flux sources, the size requirement of useful single-crystals for structure determination studies has decreased significantly. In addition, refinements of atomic coordinates of known structures using Rietveld powder techniques have become common (24). The solution of a dozen or more new zeolite structure types within the last several years has added to our knowledge base for looking at unknowns (for examples, see references 25-31), and has made us better able to characterize catalyst materials and to correlate synthesis, sorptive, catalytic, and process parameters to their structures (32,33). 

Baerlocher, Ch.,"Zeolite Structure Refinements Using Powder Data", Zeolites 1986, 6, 325. 

Even within the unit cell and symmetry constraints of this system, there are two ways to interconnect mazzite and mordenite sheets in three dimensions - one related to the other by a shift of a/2. Differentiation of the two models will be best resolved by full Rietveld refinement of the observed data. This situation of several related structures having the same two dimensional projections but different three dimensional connectivity is common in zeolite structural chemistry ( eg. mazzite - omega several members of the ABC-6 family of structures ). 

The driving force behind the rapid development of powder diffraction methods over the past 10 years is the increasing need for structural characterization of materials that are only available as powders. Examples are zeolite catalysts, magnets, metal hydrides, ceramics, battery and fuel cell electrodes, piezo- and ferroelectrics, and more recently pharmaceuticals and organic and molecular materials as well as biominerals. The emergence of nanoscience as an interdisciplinary research area will further increase the need for powder diffraction, pair-distribution function (PDF) analysis of powder diffraction pattern allows the refinement of structural models regardless of the crystalline quality of the sample and is therefore a very powerful structural characterization tool for nanomaterials and disordered complex materials. 

Catalysts based on molecular sieve zeolites will certainly continue to be of much interest in petroleum refining. Improvements can be expected as time goes on. Radically different zeolite structures offer the most chance of major improvements. Several authors have indicated the likelihood of obtaining radical, carbanion, and other noncarbonium ion reactions over zeolite catalysts. Pursuit of this possibility may prove fruitful. 

Computer modeling techniques are a substantial aid in zeolite structure solutions or refinements, and a means of extracting structural insight from difiraction or other anal ftical experiments. Sorption results, particle shapes in some cases, diffraction or scattering data, as well as optical, NMB and EXAFS spectra can all be simulated based on an atomic structure and, conversely, analytical data of these various types can be used to guide the development or detailing of an appropriate structural model. 

The widespread use of their ion-exchange properties in the water-softening/detergent industry and their strong acid catalysis properties in petroleum-refining have made zeolites the workhorse materials in both applications. While this has attracted many researchers to the zeolite field, it has also had the effect of typecasting these remarkable materials into a limited number of chemical roles. The work reported below is a brief review of our own work here at Du Pont which attempts to dispel this stereotype by demonstrating that there are many remarkable similarities between zeolite structures and those of protein portions of natural... 

Zeolite Structural Chemistry. The elucidation of the intricate details of zeolite structures is intriguing in a scientific sense as well as important in providing an understanding for industrial applications. The basic framework types of zeolites can be grouped into classes by common subunits of their structure. Classifications of this type were proposed by Smith (71) and refined by Meier (59). 

W. M. Meier and D. H. Olson, Atlas of Zeolite Structure Types, Structure Commission of the International Zeolite Association, 1978 It has recently been demonstrated that the use of synchrotron x-ray sources permits single crystal studies to be carried out on much smaller samples. P. Eisenberger, J. B. Newsam, M. E. Leonowicz, and D. E. W. Vaughan, Nature 309 45 (1984) The use of Rietveld refinement techniques allows better. structural information to be obtained from powder diffraction data. W. I. F. David, W. T. A. Harrison, and M. W. Johnson, in High Resolution Powder Diffraction, Materials Science Forum, vol. 9 (C. R. A. 

A number of zeolite structures have been refined to reasonably precise levels based on synchrotron powder X-ray diffiaction data, including those of ZSM-11 [57,58] and silica-ZSM-12 [18] where, in both cases, the previously reported atontic coordinates had been c timized solely by distance least squares. Complete stracture determinations of... 

Considerable progress has been made in recent years in developing the complementary relationship between the NMR and XRD techniques in the investigation of zeolite structures. 2D Si-O-Si connectivity experiments are particularly successful and have been refined to the stage where they may be reliably applied to the study of unknown structures. In the future it should be possible to combine experiments of this type with the 0 DOR/DAS described in the earlier lecture to obtain 0-Si heteronuclear connectivities. 

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