Amorphous, Porous Heterogeneous Catalysts and Supports

The catalytic activity of amorphous silica-alumina ([Si—Al]) in reactions via carbonium ions is due to the existence of Bronsted acid sites on their surface. Consequently, amorphous [Si-Al] acid catalysts provide acid sites and transport to the active sites easily. As a result, amorphous [Si-Al] acid catalysts have been widely operated as cracking catalysts. Acid zeolites have been successfully applied as cracking catalysts. However, in some industrial applications of acid catalysts, for example, in the cracking of hydrocarbons of high molecular weight, zeolites are not useful, since 

The method generally used for the synthesis of amorphous [Si-Al] is by the appropriate combination of sodium silicate and sodium aluminate, as explained for the synthesis of aluminosilicate zeolites in Section 3.4.1. But, in this case, the gel is not hydrothermally treated in an autoclave in order to crystallize a zeolite. It is instead thermally dried (see Section 2.7.1) to obtain an amorphous [Si-Al], The produced amorphous material is then exchanged with NH4, using a method similar to that previously explained for zeolites  

To conclude this section, it is necessary to state that besides their application in catalytic cracking, amorphous silica-alumina acid catalysts have been applied in other hydrocarbon transformations, such as isomerization of olefins, paraffins, and alkyl aromatics, the alkylation of aromatics with alcohols and olefins, and in olefin oligomerization [55], 

---