196
7 C–C Bond Formation via Carbocations in the Methane …
mixtures of methane and benzene, superacids do not catalyze the reaction of methane
with benzene to produce toluene or other aromatic hydrocarbons. This indicates that
superacid catalysis is essentially different from Ag-zeolite catalysis.
7.7.5.4 Mechanism of C–C Bond Formation in the Conversion
of Methane Over Ag
+ -Exchanged Zeolites
As discussed in Sect. 7.7.5, the formation of propylene and toluene by the reaction
of methane with ethylene and benzene, respectively, over Ag-zeolites proceeds via
+ CH 3 carbenium ions. Based on reactions (7.47) and (7.48), the reaction mechanism
for the methane conversion is shown in Scheme 7.4. Ag
+
n cationic clusters activate
methane to generate Ag n –H. The Ag
+
n cationic clusters are then regenerated by the
reaction of Ag n –H with the acidic protons, which is the reverse reaction shown in
reaction (7.47).
Scheme 7.4 Mechanism of
methane conversion over
Ag + -exchanged zeolites
13
CH 4
C 2 H 4
13
CC 2 H 6
H 2
Ag n
+
O
O
O
Si
O
Al
O
O
O
-
Ag
+
-exchanged zeolites
Ag n -H +
O
O
O
Si
O
Al
O
O
O
13 CH 3
Ag n -H +
O
O
O
Si
O
Al
O
O
O
H