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Introduction
What are Zeolites?
Zeolites is a crystalline microporous materials with a tetrahedral framework structure enclosing cavities occupied by cations and water molecules, both of which have enough freedom of movement to permit cation exchange and reversible dehydration.
Zeolites are ideal catalytic materials for this research because they have nanostructure on the same scale as the molecules that react within their pores. Their nanostructure is a function of the conditions under which they are created, and because of the molecular scale of the catalytic surfaces, their macroscopic properties are thus a function of those same conditions.
Zeolites are microporous crystalline oxides with a high surface to volume ratio. Strictly speaking, they are aluminosilicates of form Mx/n[(AlO2-)x(SiO2)y]wH2O.
Structures of Zeolites
Zeolites are complex materials, chemically and structurally comprising the framework aluminosilicates. Structural classifications of zeolites have been proposed by Smith, Fischer and Meier, and Breck. A new system of zeolite classification is not based on morphologies as before, but based on framework topologies instead. The classification consists of seven group; within each group, zeolites have a common subunit of structure which is a specific array of (Al,Si)O4 tetrahedra. In this classification, the Si-Al distribution is neglected.
Lim’s group is especially interested in Zeolite A and Faujasite (Zeolite X and Y) which have a large single crystals available with reasonable size and low Si/Al ratio.
Zeolite A belongs to Group 3. The aluminosilicate framework of zeolite A can be described in terms of two types of polyhedra; one is a simple cubic arrangement of eight tetrahedral, the D4R; the other is the truncated octahedron of 24 tetrahedra or b-cage.
Uses of Zeolites
Normal crystalline zeoslites contain water molecules, which are coordinated to the exchangeable cations. These structures can be dehydrated by heating under vacuum; in these circumstances the cations move position, frequently settling on sites with a much lower coordination number. The dehydrated zeolites are very good drying agents, adsorbing water to get back to the preferred high coordination condition.
Zeolites are used as
(a) Dehydrating agents
(b) Adsorbent
(c) Ion Exchanger
(d) Catalyst (shape selectivity, cracking, isomerization etc)
(e) Separation of gases and liquids
(f) Remover radioactive materials
(g) Forage
(h) Natural air fresheners
(i) Deodorizers
(j) Storage of gas
Research Scheme
