Zeolites basic units

The zeolites are generally thought of as having three-dimensional rather than sheet or chain structures. The existence of tetrahedral ring linkages that produce chainlike substructures characteristic of the natural zeolitic species found as needles or fibers reinforces the earlier statement fibers can be constructed from any number of basic units that grow in a preferential direction. 

Fig. 5. (a) Site II cation on a six-membered oxygen ring as the basic unit on types A and X zeolites. T denotes Si or Al. (b) Geometry-optimized cluster model to represent the chemistry of Ag/zeolite. 

The zeolite X and Y framework is considered to be topologically built up out of basic units consisting of two sodalite cages that can be connected to other units through double six-rings into 6 directions. In this way, a three dimensional network of T-site connectivities can easily be constructed. Each unit contains 48 T-sites. Most calculations were done on a cluster of 27 (3x3x3) units, i.e., 1296 sites. 

The basic unit of a zeolitic structure is the TO4 tetrahedron, where T is normally a silicon or aluminium atom/ion (or phosphorus in an aluminophosphate). In this section we deal with the aluminosilicate zeolites, which have the general formula M4/ [(A102)J/Si02) ].mH20. The zeolite framework is composed of [(AlO (SiOj) ] and M is a non-framework, exchangeable cation. 

Zeolites occur in many different structures. The basic units are always Si04- and A104-tetrahedra, linked to one another by common oxygen atoms. Fig. 5.1-5 and 5.1-6 show the structures of zeolite A and zeolite X and Y respectively, as examples of zeolite structures. These zeolites are built up of eubo-octahedra ( p-cages ) with Si04- and A104-tetrahedra at their eorners, as shown in Fig. 5.1-4. 

Figure 11.6 Synthesis of a typical zeolites and basic units (bottom) of the zeolites.

Figure 1. Basic units and two tetrahedra model of a zeolite. ...

External templates were used to structure silicates and generate porous materials for the first time in the early 1990s by independent groups in Waseda University in Japan [7] and at Mobil in the United States [8]. These syntheses took place under hydrothermal conditions similar to those for the synthesis of zeolites basic media and in the presence of cationic surfactants such as cetyltrimethylam-monium bromide. After this pioneer work, numerous silicates have been synthesized using a wide variety of surfectants. They have allowed the preparation of many mesoporous silicates such as MCM-41 with a hexagonal pore structure, MCM-48 with a cubic structure, and MCM-50 with a lamellar structure. 

The basic units of the zeolite framework are TO. tetrahedra with silicon atoms at the central... 

Zeolites are prepared by the linking of basic structural units around a template molecule. The structural units are typically based on oxides of silicon and aluminium, and the templates are usually individual small molecules. Under the right conditions, the silicon and aluminium oxide precursors will link up around the template to form a crystalline three-dimensional matrix containing the template molecules. The template... 

Busca, G. (2006) The surface acidity and basicity of solid oxides and zeolites, Chemical Industries (Boca Raton, FL, United States), 108 (Metal Oxides), 247. 

Introduction of zeolites into catalytic cracking improved the quality of the product and the efficiency of the process. It was estimated that this modification in catalyst composition in the United States alone saved over 200 million barrels of crude oil in 1977. The use of bimetallic catalysts in reforming of naphthas, a basic process for the production of high-octane gasoline and petrochemicals, resulted in great improvement in the catalytic performance of the process, and in considerable extension of catalyst life. New catalytic approaches to the development of synthetic fuels are being unveiled. 

Transmission electron microscopy (TEM) can provide detailed stmcture of zeolites. I use the word characterize or characterization for stmctural study on a unit cell scale, such as various kind of stmctural defects and basic stmctural units, and determine or determination for obtaining atomic coordinates within the unit cell for all the atoms of a crystal. A simple text or reviews for stmctural characterization of porous materials can be found in a book or review articles [1-6]. Now, we are in a new era, that is, we can determine new stmctures of micro- and mesoporous materials only by electron microscopy(EM), an area called electron crystallography (EC) [7-11]. 

Introductory Chapter 1 provides a historical overview of molecular sieve materials. Chapter 2 covers the definition of a zeolite and describes their basic and composite building units and how they are linked in zeolite frameworks. It defines pores, channels, cages and cavities and it gives references for finding detailed... 

Materials. A series of zeolites was prepared from NaY by exchanging part of the Na+ ions by NH4+, Ca2+, Ce3+, Mg2+, and La3+. Palladium was exchanged into these zeolites from a Pd(NH3)2Cl2 aqueous basic solution to yield a palladium content of approximately 2 wt % (approximately 2 Pd2+ ions per unit cell). The chemical compositions of the catalysts so obtained are given in Table I. During the exchange by the palladium salt part of the sodium is replaced by ammonium ions. 

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