7.7 Unique Properties of Silver Cations …
191
at 4 ppm and 109 ppm. These signals were assigned to ethane and ethylene (πcomplex with Ag
+ cations), respectively. When the system was further heated to
723 K, a new broad signal at 128 ppm was observed, while a broad peak at 128–
134 ppm and a peak at ~ 20 ppm were evident at 823 K. These two peaks were
attributed to methyl-substituted aromatics such as toluene and xylenes. The results
described above clearly show that methane is converted into aromatic hydrocarbons
via ethane and ethylene.
Gabrienko et al. also examined the reaction of ethane and ethylene over Ag-ZSM5 via
13 C MAS NMR spectroscopy and proposed that ethane was the primary product
formed in methane conversion [74]. Thus, ethylene was formed stepwise from ethane.
Further experiments demonstrated that methane was more effectively incorporated
into aromatic hydrocarbons in the presence of ethylene. They also investigated the
H/D exchange reaction between CD 4 and the acidic O–H groups on H-ZSM-5 and
Ag-ZSM-5 zeolites by measuring in situ
1 H MAS NMR spectra. The H/D exchange
rate over Ag-ZSM-5 was two orders of magnitude faster than that over H-ZSM-5.
This result indicates that the Ag
+ cation facilitated the reversible dissociation of C–H
bond of methane, which provides further evidence that the –OCH 3 species are the
key reaction intermediates of the H/D exchange of CD 4 and –OCH 3 groups.
7.7.5 Reaction of Methane with Ethylene Over
Ag + -Exchanged Zeolites
The –OCH 3 species on zeolites can react with ethylene to produce propylene, and
regenerate the acidic O–H groups, as shown in reaction (7.34). The –OCH 3 groups
generated on Ag-zeolites act as
+ CH 3 donors, allowing the reaction of methane with
ethylene, propylene, and benzene to proceed [37, 75–78]. In the following sections,
the characteristics of Ag-zeolite catalysts relevant to C–C bond formation in the reaction of methane with ethylene are discussed. In particular, the activation mechanism
of CH 4 over Ag-zeolites is emphasized and compared with that of other metal-cationexchanged zeolites (metal-cation zeolites). Additionally, the differences between the
catalytic properties of Ag-zeolites and those of superacid catalysts in the conversion
of methane in the presence of ethylene are focused upon.
7.7.5.1 Reaction of
13 CH 4 with Ethylene Over Ag
+ -Exchanged Zeolites
In order to examine whether CH 4 reacts with C 2 H 4 to produce propylene (C 3 H 6 )
and H 2 (reaction (7.31)), the reaction of
13 C-labeled methane (
13 CH 4 ) with ethylene
was carried out using Ag-zeolites (Ag-Y, Ag-A, and Ag-ZSM-5) in a closed
gas-circulation reactor at 673 K [75]. To minimize side reactions such as the
oligomerization–cracking of ethylene, a large excess of
13 CH 4 relative to ethylene
was used and the reaction time was shortened to 1 min. Propylene was formed over
191
at 4 ppm and 109 ppm. These signals were assigned to ethane and ethylene (πcomplex with Ag
+ cations), respectively. When the system was further heated to
723 K, a new broad signal at 128 ppm was observed, while a broad peak at 128–
134 ppm and a peak at ~ 20 ppm were evident at 823 K. These two peaks were
attributed to methyl-substituted aromatics such as toluene and xylenes. The results
described above clearly show that methane is converted into aromatic hydrocarbons
via ethane and ethylene.
Gabrienko et al. also examined the reaction of ethane and ethylene over Ag-ZSM5 via
13 C MAS NMR spectroscopy and proposed that ethane was the primary product
formed in methane conversion [74]. Thus, ethylene was formed stepwise from ethane.
Further experiments demonstrated that methane was more effectively incorporated
into aromatic hydrocarbons in the presence of ethylene. They also investigated the
H/D exchange reaction between CD 4 and the acidic O–H groups on H-ZSM-5 and
Ag-ZSM-5 zeolites by measuring in situ
1 H MAS NMR spectra. The H/D exchange
rate over Ag-ZSM-5 was two orders of magnitude faster than that over H-ZSM-5.
This result indicates that the Ag
+ cation facilitated the reversible dissociation of C–H
bond of methane, which provides further evidence that the –OCH 3 species are the
key reaction intermediates of the H/D exchange of CD 4 and –OCH 3 groups.
7.7.5 Reaction of Methane with Ethylene Over
Ag + -Exchanged Zeolites
The –OCH 3 species on zeolites can react with ethylene to produce propylene, and
regenerate the acidic O–H groups, as shown in reaction (7.34). The –OCH 3 groups
generated on Ag-zeolites act as
+ CH 3 donors, allowing the reaction of methane with
ethylene, propylene, and benzene to proceed [37, 75–78]. In the following sections,
the characteristics of Ag-zeolite catalysts relevant to C–C bond formation in the reaction of methane with ethylene are discussed. In particular, the activation mechanism
of CH 4 over Ag-zeolites is emphasized and compared with that of other metal-cationexchanged zeolites (metal-cation zeolites). Additionally, the differences between the
catalytic properties of Ag-zeolites and those of superacid catalysts in the conversion
of methane in the presence of ethylene are focused upon.
7.7.5.1 Reaction of
13 CH 4 with Ethylene Over Ag
+ -Exchanged Zeolites
In order to examine whether CH 4 reacts with C 2 H 4 to produce propylene (C 3 H 6 )
and H 2 (reaction (7.31)), the reaction of
13 C-labeled methane (
13 CH 4 ) with ethylene
was carried out using Ag-zeolites (Ag-Y, Ag-A, and Ag-ZSM-5) in a closed
gas-circulation reactor at 673 K [75]. To minimize side reactions such as the
oligomerization–cracking of ethylene, a large excess of
13 CH 4 relative to ethylene
was used and the reaction time was shortened to 1 min. Propylene was formed over