Page 156 - Fluid Catalytic Cracking Handbook

Chemistry of FCC Reactions 131

exchanged. Ion exchanging sodium with cations, such as hydrogen or
rare earth ions, enhances acidity and stability. The most widely used
3+
3
rare earth compounds are lanthanum (La *) and cerium (Ce ).
The catalyst acid sites are both Bronsted and Lewis type. The
catalyst can have either strong or weak Bronsted sites; or, strong or
weak Lewis sites. A Bronsted-type acid is a substance capable of
donating a proton. Hydrochloric and sulfuric acids are typical Bronsted
acids. A Lewis-type acid is a substance that accepts a pair of electrons.
Lewis acids may not have hydrogen in them but they are still acids.
Aluminum chloride is the classic example of a Lewis acid. Dissolved
in water, it will react with hydroxyl, causing a drop in solution pH.
Catalyst acid properties depend on several parameters, including
method of preparation, dehydration temperature, silica-to-alumina ratio,
and the ratio of Bronsted to Lewis acid sites,

Mechanism of Catalytic Cracking Reactions

When feed contacts the regenerated catalyst, the feed vaporizes.
Then positive-charged atoms called carbocations are formed. Carbo-
cation is a generic term for a positive-charged carbon ion. Carbocations
can be either carbonium or carbenium ions.
+ +
A carbonium ion, CH 5 , is formed by adding a hydrogen ion (H )
to a paraffin molecule (Equation 4-6), This is accomplished via direct
attack of a proton from the catalyst Bronsted site. The resulting
molecule will have a positive charge with 5 bonds to it.

+
R — CH 2 — CH 2 — CH 2 — CH 3 + H (proton attack)
+
-» R — C H — CH 2 — CH 2 — CH 3 + H 2 (4-6)
The carbonium ion's charge is not stable and the acid sites on the
catalyst are not strong enough to form many carbonium ions. Nearly
all the cat cracking chemistry is carbenium ion chemistry.
+
A carbenium ion, R-CH 2 , comes either from adding a positive
charge to an olefin or from removing a hydrogen and two electrons
from a paraffin (Equations 4-7 and 4-8).

R — CH. = CH — CH 2 — CH 2 — CH 3 + H* (a proton @ Bronsted site)

—>_^ jp^ ~™_™. ^^ |"j « v-'Jcin ——• v^JrJ'-j""" v^in.'-) —"""— v^-ii-t \£|.— / j