Charge compensating cations three-dimensional structures

In some ways zeolites are similar in their properties in that they also provide size and shape selectivity because of the presence of pores within the three-dimensional structure. However in the zeolites these pores are generally much larger so that larger molecules can be incorporated into the structure. Zeolites are crystalline aluminosilicates which occur both in nature and as synthetic structures [204]. The zeolite framework is made up of aluminum and silicon atoms tetrahedrally coordinated by oxygen, giving a framework stoichiometry of MO2. For each aluminum atom within the framework there is a formal charge of —1 which is compensated by counterbalancing cations within pores in the structure. Typically these may be cations such as Na" ", Ca ", NH4, or HsO". ... 

Zeolites are hydrated alcaline or alcaline earth aluminosilicates with the general formula M"+.v/ [(A102).v(Si02)jJ , -, w H20, where x indicates the number of M"+ cations which are necessary to compensate the negative charge of the framework, and w the number of water molecules. From the structural point of view, zeolites exhibit a three dimensional network of corner sharing T04 tetrahedra (T = Al,Si) which delimits interconnected tunnels or cages in which water molecules and M ions are inserted. The elimination of the water molecules keeps intact the framework and the solid becomes porous. 

Much more attention has been given to zeolites. Zeolites are crystalline microporous materials whose structure is based on a three-dimensional tetrahedral network of AlO and SiO (the Al/Si ratio can be varied from 1 to 0). The excess negative charges carried by AIO4 units are compensated by cations (Na, H+) which ensure the high hydrophilicity of the aluminated zeolites. The crystallinity of zeolites ensures also a very precise pore size. Typically, the size of zeolite pores ranges from 3 to 8 A, and the inner diameter of the interior spaces from 5 to 13 A. 

Crystalline aluminosilicate-zeolites consist of frameworks of three-dimensionally connected SiO and A104 tetra ders, which enclose cavities containing exchangeable cations and water molecules. The cations compensate the negative charges of the AlO units the water can be reversibly removed without a change of the framework structure. 

Zeolites [64] are crystalline aluminosilicates with a three dimensional micro-porous framework formed by corner sharing SiO and AIO4. (i.e. TO4) tetra-hedra. A framework with Si02 composition is stoichiometrically neutral. The substitution of Si by Al in such a silicate framework, results in an excess negative charge, which is compensated by cations or protons. Zeolites have unique adsorption and catalytic properties. Their diversity in framework composition and structure type leads to almost unlimited design opportunities. 

---