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Faujasite - A.K.A. Zeolite Y
Structure and Characterization
Zeolite Y exhibits the FAU (faujasite) structure. It has a 3-dimensional pore structure with pores running perpendicular to each other in the x, y, and z planes similar to LTA, and is made of secondary building units 4, 6, and 6-6. The pore diameter is large at 7.4Å since the aperture is defined by a 12 member oxygen ring, and leads into a larger cavity of diameter 12Å. The cavity is surrounded by ten sodalite cages (truncated octahedra) connected on their hexagonal faces. The unit cell is cubic (a = 24.7Å) with Fd-3m symmetry. Zeolite Y has a void volume fraction of 0.48, with a Si/Al ratio of 2.43. It thermally decomposes at 793ºC. [1,2,3]Synthesis
Zeolite Y, like zeolite A, is synthesized in a gelling process. Sources of alumina (sodium aluminate) and silica (sodium silicate) are mixed in alkaline (NaOH) aqueous solution to give a gel. The gel is then usually heated to 70-300ºC to crystallize the zeolite. The zeolite is present in Na+ form and must be converted to acid form. To prevent disintegration of the structure from acid attack, it is first converted to the NH4+ form before being converted to acidic form. If a hydrogenation metal such as platinum is needed, it is deposited via impregnation or ion exchange. [3]Application
The most important use of zeolite Y is as a cracking catalyst. It is used in acidic form in petroleum refinery catalytic cracking units to increase the yield of gasoline and diesel fuel from crude oil feedstock by cracking heavy paraffins into gasoline grade napthas. Zeolite Y has superseded zeolite X in this use because it is both more active and more stable at high temperatures due to the higher Si/Al ratio. It is also used in the hydrocracking units as a platinum/palladium support to increase aromatic content of reformulated refinery products. [3,4]Zeolite Products
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