7.7 Unique Properties of Silver Cations …
201
was 41% (97−56 = 41%). CH 3 –
13 CC 5 H 5 was produced by the isomerization of the
13 CH 3 -C 6 H 5 initially produced by the reaction of
13 CH 4 with benzene.
The reaction of CH 4 with benzene over In-ZSM-5 was carried out at 623 K in a
flow reactor [90]. The conversions of CH 4 (33.8 kPa) and benzene (33.8 kPa) were
2.0% and 1.8%, respectively, at 1 h time on stream and a W /F of 8.2 g h mol
−1 . The
main product was toluene (87% selectivity), while xylenes (13% selectivity) were
also produced. Xylenes were presumably formed by the methylation of toluene with
CH 4 , even though the disproportionation of toluene proceeded to produce xylenes
and benzene.
7.7.6.4 Catalytic Performance of Metal-Cation-Exchanged ZSM-5
Zeolites for the Reaction of Methane with Ethylene
M-cation zeolites with metals other than Ag also showed catalytic activity in the
reaction of methane with ethylene [75]. Their catalytic activities at 1 h time on stream
are summarized in Table 7.7. The reactions were carried out at 673 K using various
M-cation ZSM-5 zeolites with metal-to-Al molar ratios of 0.17. The conversion of
CH 4 strongly depended on the metal cations, showing that metal cations, as well as
cationic silver clusters such as Ag
+
n , can activate CH 4 . However, H-ZSM-5 cannot
activate CH 4 , as discussed above.
The conversions of CH 4 over almost all of the M-cation ZSM-5 zeolites decreased
with time on stream. However, In-ZSM-5 showed much more stable catalytic activity,
with the conversion actually increasing from 8.1% at 1 h to 8.6% after 5 h, due to the
fact that In cations are more resistant to reduction than Ag
+ cations. Furthermore, the
catalytic activity of In-ZSM-5 strongly depended on its preparation conditions, i.e.,
the temperature used for calcination before hydrogen treatment and the pretreatment
of In cations with hydrogen [90]. The highest CH 4 conversion was 11.8% for InZSM-5 pretreated with hydrogen at 723 K after calcination at 903 K. The CH 4
conversion remained constant, as shown in Fig. 7.7 [90].
7.7.7 Differences Among the Catalytic Properties of Ag + -,
Zn 2+ -, and H + -Exchanged Zeolites in the Activation
of Lower Alkanes Including CH 4
As discussed in Sect. 7.7.6, M-cation zeolites with metals other than Ag can activate
methane, as shown in reactions (7.51) and (7.52). The generation of [M–CH 3 ]
(n−1)+
and [M–H]
(n−1)+ leads to the formation of CH 4 and H 2 , respectively, as reported
in references 93 and 94. However Ag-zeolites selectively produce silver hydride
species, such as Ag n –H, as well as [M–H]
(n−1)+ , which are converted to H 2 .
The generation of [M–H]
(n−1)+ and [M–CH 3 ]
(n−1)+ can be applied to activate
higher alkanes than CH 4 , such as butenes, to produce many kinds of carbenium ions.
201
was 41% (97−56 = 41%). CH 3 –
13 CC 5 H 5 was produced by the isomerization of the
13 CH 3 -C 6 H 5 initially produced by the reaction of
13 CH 4 with benzene.
The reaction of CH 4 with benzene over In-ZSM-5 was carried out at 623 K in a
flow reactor [90]. The conversions of CH 4 (33.8 kPa) and benzene (33.8 kPa) were
2.0% and 1.8%, respectively, at 1 h time on stream and a W /F of 8.2 g h mol
−1 . The
main product was toluene (87% selectivity), while xylenes (13% selectivity) were
also produced. Xylenes were presumably formed by the methylation of toluene with
CH 4 , even though the disproportionation of toluene proceeded to produce xylenes
and benzene.
7.7.6.4 Catalytic Performance of Metal-Cation-Exchanged ZSM-5
Zeolites for the Reaction of Methane with Ethylene
M-cation zeolites with metals other than Ag also showed catalytic activity in the
reaction of methane with ethylene [75]. Their catalytic activities at 1 h time on stream
are summarized in Table 7.7. The reactions were carried out at 673 K using various
M-cation ZSM-5 zeolites with metal-to-Al molar ratios of 0.17. The conversion of
CH 4 strongly depended on the metal cations, showing that metal cations, as well as
cationic silver clusters such as Ag
+
n , can activate CH 4 . However, H-ZSM-5 cannot
activate CH 4 , as discussed above.
The conversions of CH 4 over almost all of the M-cation ZSM-5 zeolites decreased
with time on stream. However, In-ZSM-5 showed much more stable catalytic activity,
with the conversion actually increasing from 8.1% at 1 h to 8.6% after 5 h, due to the
fact that In cations are more resistant to reduction than Ag
+ cations. Furthermore, the
catalytic activity of In-ZSM-5 strongly depended on its preparation conditions, i.e.,
the temperature used for calcination before hydrogen treatment and the pretreatment
of In cations with hydrogen [90]. The highest CH 4 conversion was 11.8% for InZSM-5 pretreated with hydrogen at 723 K after calcination at 903 K. The CH 4
conversion remained constant, as shown in Fig. 7.7 [90].
7.7.7 Differences Among the Catalytic Properties of Ag + -,
Zn 2+ -, and H + -Exchanged Zeolites in the Activation
of Lower Alkanes Including CH 4
As discussed in Sect. 7.7.6, M-cation zeolites with metals other than Ag can activate
methane, as shown in reactions (7.51) and (7.52). The generation of [M–CH 3 ]
(n−1)+
and [M–H]
(n−1)+ leads to the formation of CH 4 and H 2 , respectively, as reported
in references 93 and 94. However Ag-zeolites selectively produce silver hydride
species, such as Ag n –H, as well as [M–H]
(n−1)+ , which are converted to H 2 .
The generation of [M–H]
(n−1)+ and [M–CH 3 ]
(n−1)+ can be applied to activate
higher alkanes than CH 4 , such as butenes, to produce many kinds of carbenium ions.