Selected Zeolite Framework Structures

Up to now, 167 zeolite framework types have been identified. Here, only some typical framework types with specific structural features will be described. 

Type material Nax Cl2 AI6Si6024]-S0D, cubic P-43n, a = 8.870 A.1411 Na+ and Cl-ions in the structure both can be exchanged. The composition of synthetic sodalite is Na6 AI6Sif,02/il- 

Type material Na12+(H20)27 g[Al12Sii204g]g-LTA, cubic Fm-3c, a = 24.61 A1521. Its framework density is 12.9T/1000 A3. 

Typically, zeolite LTA is synthesized with a framework Si/Al ratio of 1. By using tetramethylammonium cation (TMA+) as the structure-directing agent, the Si/Al ratio of 

LTA framework could be increased up to about three. By using a supramolecular synthon as organic structure-directing agent, it has been possible to synthesize the Al-free as well as the pure-silica forms of zeolite ITQ-29 with the LTA structure.[58] 

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In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. 

The following sections of this chapter will cover (1) descriptions of selected zeolite framework types, (2) a discussion of some aspects of real zeolite structures, and (3) a summary of the information that can be extracted from a powder diffraction pattern. 

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],... 

Figure 10c. Typical HREM image (with associated selected area diffraction pattern and schematic illustration of framework structure) after dealumination of a faufasitic zeolite by exposure to SiCl4.

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

The isomorphous replacement of aluminum by gallium in the framework structure of zeolites (beta, MFI, offretite, faujasite) offers new opportunities for modified acidity and subsequently modified catalytic activity such as enhanced selectivity toward aromatic hydrocarbons [249,250]. The Ga + ions in zeolites can occupy tetrahedral framework sites (T) and nonframework cationic positions. 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

A notable exception are chemisorbed complexes in zeolites, which have been characterized both structurally and spectroscopically, and for which the interpretation of electronic spectra has met with a considerable success. The reason for the former is the well-defined, although complex, structure of the zeolite framework in which the cations are distributed among a few types of available sites the fortunate circumstance of the latter is that the interaction between the cations, which act as selective chemisorption centers, and the zeolite framework is primarily only electrostatic. The theory that applies for this case is the ligand field theory of the ion-molecule complexes usually placed in trigonal fields of the zeolite cation sites (29). Quantum mechanical exchange interactions with the zeolite framework are justifiably neglected except for very small effects in resonance energy transfer (J30). ... 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

The choice of a theoretical model usually depends on the goals of the study, but sometimes other considerations, such as computer resources available, can also play a role in this selection process. A study of zeolite framework acidity or catalytic activity requires an explicit consideration of the electronic structure of the system, and quantum mechanical (QM) models are best suited for such investigations. Since the high CPU demands greatly limit the size of the systems that can be simulated with quantum mechanical models, the zeolite framework is often represented in these simulations by a cluster that presumably resembles the active site. The cluster approximation has the obvious drawback that influences of the crystal lattice are neglected. 

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