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The uniformity of the framework is also maintained at the molecular level by the formation of strong Si-O and Al-O bonds. These bonds are formed in two ways: (1) Si-O-Si and Al-O-Si bonds are formed by sharing of two oxygens to create an Si-O or Al-O-Si unit, and (2) Si-O-Al bonds are formed in tetrahedral sites of the zeolite framework by sharing of two oxygens to form an Si-O-Al unit.
Zeolites consist of SiO4 tetrahedra, AlO4(OH) tetrahedra, and other tetrahedral units (fig. 17). The tetrahedron size is the same for all the tetrahedra in a zeolite, and the zeolite framework is therefore uniform. The uniformity of the structure is maintained to the highest crystallographic details by the sharing of oxygen atoms, or polyhedra.
Zeolite-catalyzed reactions are often milder than those involving other catalysts, and zeolites are widely used as catalysts for reactions such as the production of gas, the conversion of waste materials to new products, and the conversion of energy or chemicals in combustion processes.
The chemical component of zeolite, aluminosilicate, is a framework formed by a three-dimensional network of SiO4 tetrahedra and AlO4(OH) tetrahedra linked by the sharing of oxygen atoms. The aluminum is a discrete element because it has a significantly smaller radius than silicon. In the framework, aluminum atoms are distributed among tetrahedral sites (Si-O-Al) and tetrahedral sites (Al-O-Al) (fig. 16). The proportion of aluminum atoms in the tetrahedral sites, and hence the proportion of Si-O-Al bonds in the framework, is referred to as the Si/Al ratio. The Si/Al ratio can be varied from about 2 to over 100 and is often controlled by the addition of alkali or alkaline-earth metal ions to the reaction mixture. Each tetrahedral Al atom has a negative charge, so the Si/Al ratio controls the charge density, ranging from negative, neutral, to positive.
A silica-alumina cracking catalyst of the type developed at the University of Chicago is composed of about 55% of a molecular sieve known as an intermediate pore zeolite (IPZ) and 45% of a pore-size directed crystalline aluminosilicate, known as FAU. The IPZ is prepared from a mixture of Na2O-K2O-SiO2-Al2O3-H2O. The zeolite is prepared by mixing these three sources. The aqueous suspension is dried, heated, and hydrothermally treated at 200-300° C. for two to three weeks. The product is screened to collect crystals of a desired particle size and then crystallized at room temperature using a first alkali metal cation or ammonium cation followed by NH4F. The resulting product, FAU, is filtered and washed. The resulting catalyst is processed and used as described above.
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