Zeolites structural determination

Zeolite structure determination using electron crystallography... 

A central problem in zeolite structure determinations is characterization of the Al, Si distribution. A central question is that of A1 avoidance (Lowenstein, 1954), the infrequent occurrence of Al-O-Al linkages. This question has been treated using an MO approach by Hass et al. (1981). They obtained an energy difference of about 120 kcal/mol between the following two reactions ... 

Powder X-ray Diffraction and Solid State NMR Techniques for Zeolite Structure Determination... 

Focus Zeolite structure determination from powder diffraction data Applications of the FOCUS method, R.W. Grosse Kunstleve, L.B. McCusker, Ch. Baerlocher, J. Appl. Crystallogr. 1999, 32, 536 542 Chemical Information, Zeolites... 

The advances of our capability in this area were aided by a number of remarkable new developments in analytical instrumentation. These new characterization tools include, among many, NMR, which provides information on the local environment of atoms in the structure (16-17), synchrotron XRD, which provides very high resolution x-ray data from powders (18-19), the application of Rietveld analysis of x-ray or neutron diffraction data to zeolite structure determination (20), and STEM, which provides atomic resolution of the crystal structure and its chemical composition (21), to name just a few. 

Koningsveldvan W, Bennett JM (1999) Zeolite Structure Determination from X-Ray Diffraction. 2 1-29... 

Electron crystallography offers an alternative approach in such cases, and here we describe a complete structure determination of the structure of polymorph B of zeolite beta [3] using this technique. The clear advantage of electron microscopy over X-ray powder diffraction for elucidating zeolite structures when they only occur in small domains is demonstrated. In order to test the limit of the structural complexity that can be addressed by electron crystallography, we decided to re-determine the structure of IM-5 using electron crystallography alone. IM-5 was selected for this purpose, because it has one of the most complex framework structures known. Its crystal structure was solved only recently after nine years of unsuccessful attempts [4],... 

Given the fact that structure determination by electron crystallography is general, success with these two zeolite structures also has implications for other materials, where... 

For normal zeolite beta samples, the domains of polymorphs A and B are only a few nanometers in size and heavily intergrown. This is not large enough for a full structure determination by electron microscopy. However, in a polymorph B enriched zeolite beta sample [2], we could find areas of sufficient size. 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

Using the more advanced quantum chemical computational methods it is now possible to determine the fundamental electronic properties of zeolite structural units. The quantum chemical basis of Loewenstein s "aluminum avoidance" rule is explored, and the topological features of energy expectation value functionals within an abstract "nuclear charge space" model yield quick estimates for energy relations for zeolite structural units. 

Key words microporous materials, zeolites, electron microscopy, structure determination... 

X-Ray powder diffraction is a powerful tool for characterization of zeolites. The basic experiment is relatively easy to perform and can be done in most labs on standard diffractometers and the data obtained is easy to analyze for many applications. Powder diffraction can be used to determine whether a new zeolite has been synthesized, whether a desired zeolite has been made or whether a crystallization process has completed. As noted in Section 4.2, X-ray powder diffraction can be an integral tool in determining the details of the structure of a newly synthesized zeolite. In addition, it is a critical characterization technique that can be routinely used, for example, to identify contaminants present in a synthesis, to determine how much zeolite has been bound into a catalyst or adsorbent pellet, or to ascertain if heat treatment alters the zeolite structure. Of the techniques described in this chapter, powder diffraction is probably the most commonly used. Additional details can be found elsewhere [15-19]. 

The utilily of measuring lattice vibrations for obtaining information about zeolites has been widely demonstrated. Applications include determining the structure of zeobtes by the identification of the structural units present, measuring changes in the framework Si/Al within materials with the same zeolite structure and tracking the formation of zeolite during synthesis. 

Zeolites can be ion-exchanged with cations or impregnated with various metals to modify their performance for use in applications such as separations, adsorption and catalysis. For example, faujasite zeolites exchanged with Na, Li, K, Ca, Rb, Cs, Mg, Sr, Ba, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Pd, Ag, Cd, In, Pt, H, Pb, La, Ce, Nd, Gd, Dy and Yb have been made and studied due to their use in separation and catalysis [135]. The ability to determine the distributions of these cations in the zeolitic structure is one of the key parameters needed in understanding adsorption mechanisms and molecular selectivities. Little has compiled an excellent reference... 

McCusker, LB. (1991) Zeolite crystallography, structure determination in the absence of conventional single-crystal data. Acto Cryst., A47, 297-313. 

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