144
6 Conversion of Methane to Aromatic Hydrocarbons
H
Si
O
O
O
O
O
Al
O
O
+
Si
O
O
O
O
O
Al
O
O
Si
O
O
O
O
O
Al
O
O
Mo
O
O
Species (III)
Si
O
O
O
O
O
Al
O
O
MoOH
Species (I)
O
O
+
-
+ H 2 O
(6.12)
The formation of Species (II) and Species (III) depends on the Si/Al ratio of the
zeolite; that is, they are strongly influenced by the distance between neighboring Al
sites in the zeolite lattice. The relationship between the formation of these molybdenum species and the distance between neighboring Al sites will be discussed in the
following section.
Vahel et al. reported the chemical properties of MoO 3 , including its reversible
reaction with H 2 O to form MoO 2 (OH) 2 [75].
MoO 3 + H 2 O
MoO 2 (OH) 2
(6.13)
In the preparation of Mo/H-ZSM-5, H 2 O is formed during the calcination of the
Mo-modified H-ZSM-5, and MoO 2 (OH) 2 has a vapor pressure of 4.9 Pa at 973 K.
Therefore, MoO 2 (OH) 2 can react with the acidic protons in H
+ -exchanged zeolites
as follows: