ABSTRACT
Si atoms cannot only be substituted by AlIII, but also by 3-or 5-valent atoms, such as GaIII, FeIII, or PV. If substitutions occur, cations are present to compensate for the
framework charge. The materials are acting as solid acids when the compensating cation is a proton. Zeolites have been used for many years in several areas of chemistry because
of their interesting properties: first, they display ion exchange capability; and second, they have good separation property (4,5). The separation of molecules originates from the
ordered structure of the zeolite micropores (Fig. 1) (6). This relates to one of the most striking feature of this class of catalyst, which is shape
selectivity. The zeolitic selectivity is the result of (a) the difference in diffusivities of reactants and products; (b) the difference in adsorption of reactants in zeolitic cavities of different size and shape; and/or (c) transition state selectivity (6-10). Nowadays zeolite catalysts are used in almost all petrochemical process (11,12). They have been first used as catalyst at a large scale in the 1950s for alkane cracking in petroleum industry (13). They favorably replaced previously employed alumina catalysts because of their better thermal and mechanical stability. Their use in organic and bioorganic chemistry has also been explored (14-17). Zeolites may be used in catalysis as an inert support for other catalytically active components (18-23) or they can be used as catalysts as such. We will consider the latter case in this chapter and will limit ourselves to the case of Brønsted and Lewis acidity (24). However, it must be mentioned that zeolites can display various types of catalytic active sites (e.g., Brønsted acid, Lewis basic, or oxidation reaction active sites) (25-28). Moreover, the possibility of anchoring of catalytic active sites in zeolites virtually expands zeolite application to unlimited fields in catalysis (29-31).
