12
1 Overview of Direct Methane Conversion to Chemicals …
1.6.1 Methyl Radical (•CH 3 ) Formation
The methyl radical (•CH 3 ), which is generated by the hemolytic scission of a C–H
bond in CH 4 , is an important reaction intermediate for the production of methanol
and/or C 2
+ hydrocarbons both in the presence and absence of oxygen, especially
when heterogeneous or biological catalysts are used. For example, over metal oxides
or metal–oxo groups, the formation of •CH 3 often proceeds as shown in reaction
(1.15).
+ CH 4
+ •CH 3
M O
M OH
(1.15)
Using biological catalysts such as methane monooxygenase, •CH 3 may be generated and then participate in methanol formation. This overall reaction is shown
below.
CH 4 + O 2 + 2 H + + 2 e –
CH 3 OH + H 2 O
(1.16)
In this reaction, one oxygen atom of the oxygen molecule is incorporated into the
methanol molecule, while the other forms H 2 O, and the supply of H
+ and e
− is
essential for the production of methanol. The mechanism of biological catalysts
seems to be essentially different from that of heterogeneous catalysts. Chapters 2
and 3 discuss the mechanisms of methanol formation via •CH 3 in biological and
heterogeneous catalysis, respectively, and the different mechanisms of these catalysts
are explained in terms of their properties.
1.6.2 Formation of Carbene-like Species CH x (0 < x < 3)
for Dehydroaromatization
Carbene-like species, CH x (0 < x < 3), are key reaction intermediates in dehydroaromatization reactions involving metal ions, such as Mo and W ions loaded
on or exchanged into various zeolites, as mentioned previously. For example, CH x
species can be generated at the pores and surface of a zeolite via the reaction of
Mo 2 O 3 with CH 4 followed by calcination; these species act as active sites. During
the initial stage of reaction, ethylene is formed; the ethylene is then converted into
aromatic hydrocarbons in a stepwise fashion. The details of this reaction and the
properties of the catalysts are discussed in Chapter 6.
1 Overview of Direct Methane Conversion to Chemicals …
1.6.1 Methyl Radical (•CH 3 ) Formation
The methyl radical (•CH 3 ), which is generated by the hemolytic scission of a C–H
bond in CH 4 , is an important reaction intermediate for the production of methanol
and/or C 2
+ hydrocarbons both in the presence and absence of oxygen, especially
when heterogeneous or biological catalysts are used. For example, over metal oxides
or metal–oxo groups, the formation of •CH 3 often proceeds as shown in reaction
(1.15).
+ CH 4
+ •CH 3
M O
M OH
(1.15)
Using biological catalysts such as methane monooxygenase, •CH 3 may be generated and then participate in methanol formation. This overall reaction is shown
below.
CH 4 + O 2 + 2 H + + 2 e –
CH 3 OH + H 2 O
(1.16)
In this reaction, one oxygen atom of the oxygen molecule is incorporated into the
methanol molecule, while the other forms H 2 O, and the supply of H
+ and e
− is
essential for the production of methanol. The mechanism of biological catalysts
seems to be essentially different from that of heterogeneous catalysts. Chapters 2
and 3 discuss the mechanisms of methanol formation via •CH 3 in biological and
heterogeneous catalysis, respectively, and the different mechanisms of these catalysts
are explained in terms of their properties.
1.6.2 Formation of Carbene-like Species CH x (0 < x < 3)
for Dehydroaromatization
Carbene-like species, CH x (0 < x < 3), are key reaction intermediates in dehydroaromatization reactions involving metal ions, such as Mo and W ions loaded
on or exchanged into various zeolites, as mentioned previously. For example, CH x
species can be generated at the pores and surface of a zeolite via the reaction of
Mo 2 O 3 with CH 4 followed by calcination; these species act as active sites. During
the initial stage of reaction, ethylene is formed; the ethylene is then converted into
aromatic hydrocarbons in a stepwise fashion. The details of this reaction and the
properties of the catalysts are discussed in Chapter 6.