R
H 3 C
H 2 C
R
Pt
H 3 C
R
CH 3
H 3 C
R
CH 3
Pt
Acid
Acid-catalyzed hydrocracking reactions convert paraffins to mostly C 3 and C 4
products. Longer chain paraffins can also produce C 5 and C 6 cracked products. All
of these paraffin reactions are thought to proceed through olefin intermediates as
demonstrated above (bifunctional mechanism). Additional paraffin cracking
(hydrogenolysis or metal-catalyzed cracking) reactions occur on the metal sites
resulting in predominantly C 1 and C 2 products.
H 3 C
CH 3
H 3 C
CH 3
+
CH 3
H 3 C
H 2
+
Acid
The most desired reaction for paraffins is to cyclize to form cyclohexanes and
cyclopentanes as shown below (dehydrocyclization). These reactions are also
believed to proceed through olefin intermediates. The cyclization of the olefin
may be catalyzed by the acidic alumina support. As already shown, after cyclization, cyclohexane rings undergo rapid dehydrogenation to aromatics.
R
H 3 C
H 3 C
H 3 C
R
H 2
+
R
R
H 2
+
Rates and Equilibrium
The dehydrogenation of naphthenes and paraffins is rapid, and equilibrium concentrations are established in the initial portions of a catalyst bed. Isomerization
Catalytic Reforming in Petroleum Processing
245