Cracking reactions using clays

There is some evidence that carbonium ions are also formed on clay surfaces, and the carbonium ion theory has been used to explain cracking reactions. Evans (121) reports that a solution of 1,1-diphenylethylene,... 

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare... 

Clays are the most universal of all the minerals occurring at the surface of the earth and consequently allow both cheap and environmentally friendly organic chemistry [56, 58]. Natural clays were among the earliest solid acid catdysts used to promote cracking and isomerization reactions in the oil industry [59]. 

Until 1960s, proton exchanged or acid treated clays were used as cracking catalysts. They have also proved to be good catalysts for many industrially important reactions like alkylation [1], dimerization and polymerization of unsaturated hydrocarbons [2], Diels Alder condensation [3] etc. Moreover acid treated clays are commonly used commercially for decolourising oils [4] and also in colour formation with leuco dyes in pressure sensitive recording paper [5]. 

It should be noted that, at this point, it is unlikely that pillared clays will replace zeolites for fluidized catalytic cracking, the reason being the hydrothermal instability of the clays at the conditions typically used in a modem riser cracker. Nevertheless, there is ongoing interest in clays and pillared clays as shape-selective catalysts for other, more specific reactions or separations. 

---