6.7 Active Species and Reaction Mechanism …
151
These metal species can be separated into two categories [11]. The first category
comprises Mo, W, Fe, V, Cr, Re, and Mn, which undergo an activation (induction)
period before the production of hydrocarbon products. Thus, during this period, the
metal ion is reduced to its active phase, most likely via carburization. The second
category is made up of Ag, In, and Zn, in which the active site has been reported to
be a cation that acts as a proton abstraction site for CH 4. Details about this type of
catalyst are given in Chap. 5.
In the MDA reaction, aromatic hydrocarbons are not the initial products. Thus, the
first C–C bond formation to produce C2 hydrocarbons (i.e., ethylene and/or ethane)
should be distinguished from subsequent C–C bond formation to produce aromatic
hydrocarbons.
6.7.1 Active Mo Species and C–C Bond Formation to Give
C 2 H 6 and C 2 H 4 During the Initial Stage
Since CO, CO 2 , H 2 , and H 2 O are produced during the induction period when a
Mo/H
+ -exchanged zeolite is brought into contact with methane at approximately
900 K, various molybdenum species can potentially be formed, including the molybdenum carbides MoC x [21, 93, 101], MoC (1-x) [97], Mo 2 C [88, 93, 96, 100, 102,
103], and/or MoC x O y [98], as well as Mo [90]. When Mo, MoO 2 , MoO 3 , Mo 2 C, and
MoC (1–x) were used as catalysts in the conversion of methane at 973 K, only trace
C 2 H 6 was observed after the induction period [90]. Furthermore, these compounds
interacted with CH 4 to produce H 2 over Mo, while H 2 O and CO 2 were formed over
MoO 2 and MoO 3 . When Mo 2 C and MoC (1–x) were used as catalysts at the same
reaction temperature, small amounts of C 2 H 4 and C 2 H 6 were observed together with
CO and hydrogen (H 2 ). These results strongly suggest that carbonized Mo species
such as MoC x take part in the first C–C bond formation to produce C 2 H 6 and C 2 H 4
from CH 4 .
Several reaction mechanisms for the formation of these C 2 hydrocarbons have
been proposed. However, the experimental evidence up to the present has not always
been clear. The reaction mechanisms considered to be most likely are listed below.
(1) Catalysis of the formation of C 2 H 6 by MoC x
According to ref. [6], during the initial stage, MoC x species react with methane
to produce a specific carbonaceous intermediate (Intermediate (I)) on the catalyst
surface, which is possibly converted to C 2 H 6 by the further reaction of methane with
the CH y and/or CH z species as follows:
151
These metal species can be separated into two categories [11]. The first category
comprises Mo, W, Fe, V, Cr, Re, and Mn, which undergo an activation (induction)
period before the production of hydrocarbon products. Thus, during this period, the
metal ion is reduced to its active phase, most likely via carburization. The second
category is made up of Ag, In, and Zn, in which the active site has been reported to
be a cation that acts as a proton abstraction site for CH 4. Details about this type of
catalyst are given in Chap. 5.
In the MDA reaction, aromatic hydrocarbons are not the initial products. Thus, the
first C–C bond formation to produce C2 hydrocarbons (i.e., ethylene and/or ethane)
should be distinguished from subsequent C–C bond formation to produce aromatic
hydrocarbons.
6.7.1 Active Mo Species and C–C Bond Formation to Give
C 2 H 6 and C 2 H 4 During the Initial Stage
Since CO, CO 2 , H 2 , and H 2 O are produced during the induction period when a
Mo/H
+ -exchanged zeolite is brought into contact with methane at approximately
900 K, various molybdenum species can potentially be formed, including the molybdenum carbides MoC x [21, 93, 101], MoC (1-x) [97], Mo 2 C [88, 93, 96, 100, 102,
103], and/or MoC x O y [98], as well as Mo [90]. When Mo, MoO 2 , MoO 3 , Mo 2 C, and
MoC (1–x) were used as catalysts in the conversion of methane at 973 K, only trace
C 2 H 6 was observed after the induction period [90]. Furthermore, these compounds
interacted with CH 4 to produce H 2 over Mo, while H 2 O and CO 2 were formed over
MoO 2 and MoO 3 . When Mo 2 C and MoC (1–x) were used as catalysts at the same
reaction temperature, small amounts of C 2 H 4 and C 2 H 6 were observed together with
CO and hydrogen (H 2 ). These results strongly suggest that carbonized Mo species
such as MoC x take part in the first C–C bond formation to produce C 2 H 6 and C 2 H 4
from CH 4 .
Several reaction mechanisms for the formation of these C 2 hydrocarbons have
been proposed. However, the experimental evidence up to the present has not always
been clear. The reaction mechanisms considered to be most likely are listed below.
(1) Catalysis of the formation of C 2 H 6 by MoC x
According to ref. [6], during the initial stage, MoC x species react with methane
to produce a specific carbonaceous intermediate (Intermediate (I)) on the catalyst
surface, which is possibly converted to C 2 H 6 by the further reaction of methane with
the CH y and/or CH z species as follows: