Zeolites extra framework composition

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

A rough estimation of the concentration of potential active sites is often possible from the composition of the catalyst. Thus the maximum value of the concentration of the protonic sites of zeolites, which are active in most of the acid catalysed reactions, can be estimated from the complete unit cell formula of the zeolite (with distinction between framework and extra-framework A1 species). The actual concentration of these sites is often lower (two times or more). 

Because of the chemical implausibility of some speculations about zeolite A, high-precision X-ray analyses were made of dehydrated crystals whose chemical composition was checked by electron microprobe analysis. Data were collected for the 24 A superstructure. All diffractions are consistent with space group Fm3c except for a few weak ones. Unpublished measurements by J.J. Pluth and G.D. Price of a hydrated Na-A (i.e. as-synthesized) showed strict obeyance of Fm3c except for a very weak (111) diffraction, and the inconsistent diffractions of the dehydrated crystals are attributed tentatively to minor positional disorder of extra-framework cations (1). 

Zeolites are crystalline microporous aluminosilicates possessing an anionic framework the electroneutrality of the crystals requires the presence of cations in extra-framework positions, such as Na+, K+, H+, A Ca++, A Mg++, 1/3 La+++, 1/3 Ce+++, etc. By extension, molecular sieves are defined as microporous crystalline structures with a variable elemental composition leading to frameworks with variable charge as outlined in Table 1. 

The collapse to an amorphous phase is the most pronounced effect accompanying treatments of zeolites at elevated temperatures. In addition, heating results in changes in the framework composition, caused by movement of the tetrahedral aluminum into extra-lattice positions. 

Results from this and other studies [22] have indicated that FCC deactivation by V contaminants can occur by two different mechanisms depending on the way the FCC cracking component (the zeolite) has been stabilized. Residual Na-ions, RE cations, framework Si, A1 composition and extra framework A1 are parameters believed to influence FCC resistance to V-induced deactivation. During thermal treatment (in air) of V contaminated faujasite crystals, the following have been observed ... 

Zeolites can be classified in many ways. Two convenient methods are on the basis of pore size and chemical composition, that is, the Si/Al ratio. The pore diameter is determined by the size of the free apertures in the structure, which is dependent on the number of T atoms (T = Si or Al) that form the aperture. Table 10.1 summarizes some examples of zeolites based on pore size classification. It should be noted that the values typically reported in the literature are determined by crystallographic studies. While these numbers are good guides, it is important to note that the actual pore size depends on many factors, including temperature, firamework composition, and the type of extra-framework cations present in the zeolite. These factors can lead to subtle changes in effective pore sizes and subsequently large changes in material properties (adsorption/reactivity). 

In general, the surface of pure silicate mesostructures is weakly acidic. It is found that the incorporation of metal ions into the framework can introduce acidic and ion-exchange functionality and catalytically active sites. Various metal ions, such as Al +, Ti " ", V +, Ga +, and Fe +, have been incorporated into S BA-15 to enhance its catalytic performance. In contrast to zeolites, which have crystalline structures, the incorporation of metal ions in mesoporous silicates caimot be strictly defined as intra- or extra-framework incorporation since these ions are highly dispersed on the framework. A wide range of compositions with different coordination numbers and chemical environments can contribute to amorphous framework structures. For example, both tetrahedrally and octahedrally coordinated aluminum in S BA-15 are involved in the formation of the amorphous pore walls, and may be defined as intraframework Al. The former may exist inside the pore walls, while the latter may be located on the pore surface. 

H-ZSM-5 zeolites were synthesized, with Fe concentrations ranging from a trace level of 30-2,000 ppm and a Si/Al framework composition from 12.5 to 300. The samples were investigated in dehydrated, dehydroxylated, and steamed forms. Together with the changes in concentration of the protonic as well as Al-Lewis sites, changes in the structme and concentration of extra-framework Fe species sites also occurred. The high activity of the H-zeolite can be ascribed to some extra-framework Fe species rather than to the protonic sites. The differences in catalytic activity of the variously treated zeolites should mostly account for the differences in the structure and concentration of Fe species and not for those of the protonic or Al-Lewis sites (Fig. 26.4). ... 

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