Catalytic cracking
Catalytic cracking is defined as a process of converting where higher molecular weight and boiling hydrocarbons to lower molecular weight and boiling hydrocarbons in presence of a catalyst
Heavy oil (450-550 0C) gasoline
Catalytic cracking was first proposed by A.G Obald in 1936. The process is carried out in presence of catalysts.
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- Earlier, acid clay was used.
- Nowadays, crystalline alumina silicates (zeolites) having a small amount of Ca, Mg, Na, Fe, cr and rare earth oxides are used as catalysts because of its high activity, good fluidization properties, high gasoline and low coke yields. The binder (silica sol) and filler (kaolin ) provide the physical strength to the catalyst.
Structure of the catalyst.
The zeolite is silica and alumina tetrahedral with each tetrahedron having either aluminium or a silicon atom at the centre and four oxygen atoms at the corners. It is a molecular sieve with a distinctive lattice structure that allows only a certain size range (8to 10nm)of hydrocarbon molecules to enter the lattice.
The catalytic sites in the zeolite are strongly acidic. The aluminium atom at the centre of each alumina tetrahedral is at a +3 oxidation state surrounded by four oxygen atoms at the corners which are shared by the neighbouring tetrahedral. So, the net charge of the alumina tetrahedral is -1 which is balanced by a positive ion like ammonium ion. Ammonium ion vaporizes when the catalyst is dried, forming Lewis and boosted acidic sites. The broasted sites may be replaced by rare earth metals to provide alternative activity and stability.
Reactions taking place
During cracking stable and long intermediate cations are collected on the catalysts active sites. (if these deposits are not removed by burning then the catalytic activity is decreased).
The main reactions are acid catalyzed cracking and hydrogen transfer by carbonium ion mechanism
Lewis acidic zeolite (AB) takes H from hydrocarbon to form carbonium ion and then C-C bond breaks (cracking) to form olefins and shorter carbonium ion. The shorter carbonium ion accepts H to from shooter cracked alkenes.
