Isolators Acidic and Basic Catalysts

Catalysts belonging to this group are less common, and their activity for redox reactions is relatively low, at least at low temperatures. The solid oxides of the third period Na20, MgO, AI2O3, Si02, and P2O5 are insulators, and they exemplify the transition from basic through amphoteric to acidic character. The oxides of the elements of other periods behave similarly. 

Since the catalytic properties can not be explained directly by means of electronic properties, it is appropriate to introduce another catalyst concept. In this case, the acid/base concept is suitable. Well-known catalysts with insulator properties are AI2O3, aluminosilicates, Si02/Mg0, silica gels, phosphates such as AIPO4, and special clays activated by chemical treatment. All these catalysts have acid centers on their surface. 

A special class of crystalline aluminosilicates are the highly active and selective zeolites, which are discussed separately in Chapter 7. 

In comparison, the basic catalysts play only a minor role. Well-known acidic/basic catalysts are listed in Table 5-27. 

Mixed oxides such as Al203/Si02, MgO/ Si02, Zr02/Si02, heteropolyacids 

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