5.8 Non-oxidative Coupling of Methane
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typically used. Unfortunately, these catalysts suffer from the accumulation of coke
on the catalyst surface.
Koerts et al. reported that CH 4 was dissociatively adsorbed on transition metal
catalysts, such as Ru/SiO 2 , resulting in the formation of carbonaceous species (CH x
species) on the catalyst surface and hydrogen [135]. In a discussion of the high
catalytic performance of Co–Pt/Na–Y and Co–Pt/Al 2 O 3 , Guczi et al. also referred
to adsorbed CH x , discussing several forms of carbon species such as CH 2 and CH.
These CH x species weakly bonded to catalyst surfaces participated in C–C bond
formation over the bi-metallic catalysts [136, 137].
5.8.4 High Catalytic Performance of Catalysts with Single
Iron Sites in the Non-OCM Reaction
As discussed above, high reaction temperatures are essential for non-OCM reactions. In 2014, Guo et al. reported that a single iron site embedded in a silica matrix
enabled the non-oxidative conversion of methane exclusively to ethylene and aromatic hydrocarbons such as benzene [138]. The maximum conversion of methane
(48.1%) was achieved at 1363 K using 0.5 wt% Fe/SiO 2 as the catalyst. Only ethylene, benzene, and naphthalene were produced; neither coke nor CO 2 was detected
despite the relatively high reaction temperature. The selectivities varied from 40.9
to 52.1% for ethylene, from 21.0 to 29.1% for benzene, and from 23.6 to 38.2%
for naphthalene in the temperature range 1223–1363 K. In comparison, a blank test
(empty reactor with no catalyst) under the same conditions gave a CH 4 conversion
of 2.5% with 95% of the product being coke; a test using SiO 2 alone as the catalyst
yielded virtually the same result.
5.9 Summary of OCM Features
OCM is one of the processes for the direct conversion of methane into hydrocarbons.
This reaction requires high temperatures (~1000 K) and affords products limited to
C 2 H 6 and C 2 H 4 (C 2 hydrocarbons). The corresponding mechanism is not simple,
as OCM proceeds both heterogeneously and homogeneously. Once the formation of
•CH 3 is initiated by the catalyst, the gas-phase self-coupling of •CH 3 together with
the coupling of •CH 3 with other alkyl radicals proceeds extensively. Furthermore,
the facile oxidation of •CH 3 on the catalyst surface and in the gas phase affords CO 2
and CO. Therefore, the formation of C 2 hydrocarbons is controlled mainly by the
homogeneous process, and the yield of these hydrocarbons does not exceed 30%.
In the non-oxidative coupling of methane (non-OCM), methane is mainly activated on the catalyst surface to produce •CH 3 , and the formation of higher
hydrocarbons such as benzene proceeds homogeneously.
119
typically used. Unfortunately, these catalysts suffer from the accumulation of coke
on the catalyst surface.
Koerts et al. reported that CH 4 was dissociatively adsorbed on transition metal
catalysts, such as Ru/SiO 2 , resulting in the formation of carbonaceous species (CH x
species) on the catalyst surface and hydrogen [135]. In a discussion of the high
catalytic performance of Co–Pt/Na–Y and Co–Pt/Al 2 O 3 , Guczi et al. also referred
to adsorbed CH x , discussing several forms of carbon species such as CH 2 and CH.
These CH x species weakly bonded to catalyst surfaces participated in C–C bond
formation over the bi-metallic catalysts [136, 137].
5.8.4 High Catalytic Performance of Catalysts with Single
Iron Sites in the Non-OCM Reaction
As discussed above, high reaction temperatures are essential for non-OCM reactions. In 2014, Guo et al. reported that a single iron site embedded in a silica matrix
enabled the non-oxidative conversion of methane exclusively to ethylene and aromatic hydrocarbons such as benzene [138]. The maximum conversion of methane
(48.1%) was achieved at 1363 K using 0.5 wt% Fe/SiO 2 as the catalyst. Only ethylene, benzene, and naphthalene were produced; neither coke nor CO 2 was detected
despite the relatively high reaction temperature. The selectivities varied from 40.9
to 52.1% for ethylene, from 21.0 to 29.1% for benzene, and from 23.6 to 38.2%
for naphthalene in the temperature range 1223–1363 K. In comparison, a blank test
(empty reactor with no catalyst) under the same conditions gave a CH 4 conversion
of 2.5% with 95% of the product being coke; a test using SiO 2 alone as the catalyst
yielded virtually the same result.
5.9 Summary of OCM Features
OCM is one of the processes for the direct conversion of methane into hydrocarbons.
This reaction requires high temperatures (~1000 K) and affords products limited to
C 2 H 6 and C 2 H 4 (C 2 hydrocarbons). The corresponding mechanism is not simple,
as OCM proceeds both heterogeneously and homogeneously. Once the formation of
•CH 3 is initiated by the catalyst, the gas-phase self-coupling of •CH 3 together with
the coupling of •CH 3 with other alkyl radicals proceeds extensively. Furthermore,
the facile oxidation of •CH 3 on the catalyst surface and in the gas phase affords CO 2
and CO. Therefore, the formation of C 2 hydrocarbons is controlled mainly by the
homogeneous process, and the yield of these hydrocarbons does not exceed 30%.
In the non-oxidative coupling of methane (non-OCM), methane is mainly activated on the catalyst surface to produce •CH 3 , and the formation of higher
hydrocarbons such as benzene proceeds homogeneously.