Other Types of Non-homogeneous Catalysts

In addition to the classical heterogeneous catalysts such as zeolites, or metals on supports, there are now a host of other catalysts which are not truly homogeneous and where the catalyst, the reactants and the products are in different phases. Such catalysts can offer the advantages of high activity and selectivity associated with the more usual homogeneous catalysts, as well as 

Active sites in zeolites are located in a regular network of channels of uniform dimensions, whose shapes and sizes are characteristic of the individual zeolite. The diameter of the pore ports are of similar magnitude to the molecular dimensions of smaller organic compounds. Shape selectivity is the first and best known consequence of this. It can be defined as the capacity of zeolites to discriminate reactants, products and intermediates on the basis of their sizes and shapes (see also p 77). 

The absence of shape selectivity in both homogeneous and conventional heterogeneous catalysts leads to very different results in the above reactions. 

The term adsorption is the complex phenomenon of the interaction of an organic or inorganic molecule with a surface. Physisorption and chemisorption are generally distinguished according to the nature of forces involved. 

Physisorption results from the interaction of a sorbent and a sorbate mostly through Van der Waals forces. Generally such an interaction is relatively weak, but there are notable exceptions, e.g., the physisorption of hydrocarbons in zeolites which have heats of adsorption up to several tens kJ/mol. 

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