Solid solvents, zeolite shape selectivity

Many reactions in the fine-chemical industry are performed with organic liquids or in organic solvents. When the surface of a solid-acid catalyst is hydrophilic, the presence of small amounts of water can completely block the surface of the catalyst. Carefully dried catalysts and reagents must then be employed. It may be noted that zeolites with a high silica-to-alumina ratio are hydrophobic. The (internal) surface of the zeolite is, therefore, not readily poisoned by accumulation of (traces of) water. With zeolites, moreover, use can be made of the uniform size of the pores to perform shape-selective reactions after passivation of the catalytic sites on the external surface of the zeolite crystallites. In the bulk-chemical industry several interesting reactions have been developed with zeolite catalysts [27] (cf. the review of Tanabe and Hdlderich [28]). 

When compared with other solid acids, zeolites (and, more generally, zeotypes) possess certain peculiarities which make them a unique type of material. These arc some of the properties high surface areas ability to control the number and strength of acid sites high adsorption capacity, and important media (solvent) effects. The control of the molecular dimensions of the pores can result in shape selective effects and picactivation of the molecules inside the pores by strong dielectric fields and molecular confinement. 

Obviously, the catalytic future of zeolite-occluded clusters and organometallics is two-fold. They can be used as coordination-type catalysts or act as zeolite-entrapped metal precursors. In both cases, the zeolite will provide its solid solvent property, its bifunctional capability (in a broad sense, either its acidic or macroanionic properties), and its unique shape-selective properties. 

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