Extra-framework Composition

Partial exchange of Brpnsted acid sites by alkaline, alkaline earth, transition or rare earth cations can modify the acidity of the final zeolite. The local geometry of the Brpnsted acid site is affected by the nature of the cation and this consequently causes a change in the value of the proton affinity of the site [17,18]. 

When zeolites are dealuminated by steam-calcination part of the framework A1 is extracted and generates extra-framework species (EFAL) that can be cationic, anionic or neutral. Some of these EFAL species can act as Lewis acid sites [19] or can influence the Brpnsted acidity, by either neutralizing Brpnsted acid sites by cation exchange, or by increasing the acidity by a polarization effect and/or by withdrawing electron density from lattice oxygens [20-22]. However, under mild steaming the A1 can also become partially, and reversibly, disconnected from the lattice [23]. This opens the way to Lewis acid catalysis by the A1 [24]. 

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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 Unpublished measurements by J.J. Pluth and G.D. Price of a hydrated Na-A (i.e. as-synthesized) showed strict obeyance of Fm3c except for a very weak (111) diffraction, and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 ... 

The framework types discussed in the last section describe only the connectivities of the frameworks. While these characterize the basic framework structure in terms of approximate pore opening, cage arrangement and channel system, and facilitate comparison of related materials, they do not describe real materials. That is, the influence of framework composition, extra-framework cations, organic species, sorbed molecules, or structural defects, is not considered. These aspects are addressed in the following sections. 

The X-ray diffraction patterns of the AlPO and CoAPO samples indicate pure phases, except for some samples of CoAPO —34, which might contain amorphous material. However, in view of the product compositions (Table 1) it must be concluded that many samples are not pure since the (Al+Co)/P ratio is considerably greater than 1. In these cases, part of the aluminium and/or cobalt must be present in extra-framework species. In particular, samples prepared with high cobalt concentrations in the gel and samples of CoAPO -34 exhibit unfavourable element ratios. These samples are also often less homogeneous in that they can contain white particles, whereas the major phase exhibits a blue colour. 

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. 

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