It is a chemical reaction in which the rate depends on the pore size of the catalyst, on the shape and size of the reactant, and product molecules. A shape
−
-
selective catalyst has a variety of active sites of different shape and sizes. Zeolites acts as a shape
−
-
selective catalyst. In the crystal of a zeolite the pore size usually varies from
260
±
260
to
740
±
740
. If the reactant molecules are too large they cannot fit into pores of zeolite and not reaction takes place. On the other hand, if the reactant molecules are too small, they would just slip through the pores in the catalyst without any interactions.
Z
S
M
−
5
Z
is used as a shape
−
-
selective catalyst to produce gasoline of hidh oxtane number from methanol.
x
C
H
3
O
H
Z
S
M
−
5
−−−−→
C
a
t
a
l
y
s
t
(
C
H
2
)
G
a
s
o
l
i
n
e
+
x
H
2
O
x
where
x
x
varies from 5 to 10.
Z
S
M
−
5
Z
stands for zeolite sieve of molecular porosity 5.
