| Popis: |
Molecular sieve zeolites are a class of stable mineral and synthetic crystalline inorganic compounds characterized by the presence of an open oxide framework structure. This open structure gives rise to a regular network of uniform pores of molecular dimensions that pervades the crystal. The open framework is generally an aluminosilicate one, in which the aluminum and silicon atoms are tetrahedrally coordinated to oxygen atoms in a continuous array. However, they form subunits of rings and cages that can be linked together in a variety of ways to provide, after suitable activation, channels and cavities of predetermined size regularly disposed throughout the crystal. The charge deficiency resulting from the tetrahedral coordination of the aluminum atoms is balanced by the presence of cations positioned by electrostatic forces at relatively fixed locations in the channels and cavities. These charge-balancing cations, however, are not an integral part of the oxide framework. They are, in fact, generally ion-exchangeable by other cations with varying degrees of ease depending on the nature of the cations involved, the conditions under which the exchange is carried out and the location of the various types of ion exchange sites in the structural network, among other things. |
