From Microporous to Mesoporous Molecular Sieve Materials and Their Use in Catalysis

Introduction

It is possible to say that zeolites are the most widely used catalysts in industry. They are crystalline microporous materials which have become extremely successful as catalysts for oil refining, petrochemistry, and organic synthesis in the production of fine and speciality chemicals, particularly when dealing with molecules having kinetic diameters below 10 Å. The reason for their success in catalysis is related to the following specific features of these materials:1 (1) They have very high surface area and adsorption capacity. (2) The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials. (3) Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application. (4) The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 Å), and the strong electric fields2 existing in those micropores together with an electronic confinement of the guest molecules3 are responsible for a preactivation of the reactants. (5) Their intricate channel structure allows the zeolites to present different types of shape selectivity, i.e., product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions. (6) All of these properties of zeolites, which are of paramount importance in catalysis and make them attractive choices for the types of processes listed above, are ultimately dependent on the thermal and hydrothermal stability of these materials. In the case of zeolites, they can be activated to produce very stable materials not just resistant to heat and steam but also to chemical attacks.


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