Application of Zeolites to Catalysts

Abstract

The catalysis by zeolites has attracted the attention of many researchers in the fields of chemistry of catalysis and industrial chemistry. The active species can be introduced in the zeolite by means of cation exchange. Zeolite has several physical properties, such as molecular sieving effect and hydrophilic or hydrophobic property. When these properties are composed with catalytic activities, the catalysts with high activity and selectivity can be designed.
When proton or polyvalent cation is introduced in zeolite by cation exchange method, the solid acidity appears. The acidic zeolites show very high catalytic activity in the reactions which proceed via carbonium ion intermediates. The transition-metal cations can be supported on the zeolite in relatively stable state. Such transition-metal cations show the high catalytic activity as like as the complex or oxide catalysts. When the noble metal cations supported on zeolite are reduced under the suitable conditions, the very small metal crystals can be obtained in the zeolite cage.
The shape-selective catalysts can be designed by the combination of molecular sieving effect. If most catalytic active sites are inside of zeolite pore, most reactions must be occurred in zeolite pore. Reactant selectivity occurs in the case which only one kind of molecule in the reactant can pass through the pore of zeolite. Product selectivity occurs when only one kind of molecule in the product formed in the pore can go out from the catalyst. In transition state seletivity, certain reactions are prevented because the transition state requires more space than that in the pore of zeolite. For the improvement of the shape-selectivity, (1) the effective pore dimension is more fitted to the molecular dimension of reactant or product, (2) the catalytic activity (ex. solid acidity) are controlled to prevent the undesirable side reactions, and (3) the active sites on the outer surface of zeolite crystal on which the reaction occurs non shape-selectively are poisoned or eliminated.
Zeolite with the low atomic ratio of silicon to aluminum shows the hydrophilic property. The highly silicious zeolites, however, show the hydrophobic property. So, the acidic highly silicious zeolites can be used in the presence of water as solid acid catalyst.