Microporous Solids as Acid Catalysts

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in 

Analogues of aluminosilicate zeolites, in which the place of aluminium is taken by other Bivalent cations such as B, Ga or Fe, are readily prepared. These give solids with closely similar structure but with weaker acid sites that are also more susceptible to removal of the heteroatom from the framework, for example in the presence of water or steam. Such weakly acidic solids can be selective catalysts for molecular rearrangements that give unwanted by-products in the presence of stronger add catalysts. The Beckmann rearrangement is such a reaction (see Section 8.6.1). 

In selecting a solid acid to catalyse any given reaction, zeolites are not the only candidates. Many solids fulfil the general requirement for solid acidity and some possess favourable attributes of enhanced acid strength and resistance to deactivation. Some of the more important varieties are discussed below. 

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