1.4 Direct Conversion of Methane for C–O and C–C …
7
been investigated [31–37]. Additionally, some heterogeneous catalysts for the
selective production of methanol from methane, such as FeO 2 /SiO 2 [48], inverse
CeO 2 /Cu 2 O/Cu(111) [49], Ni/CeO 2 (111) [50], and copper-exchanged zeolites
[51], have recently been developed; however, their catalytic performance cannot yet
match that of MMO. The mechanism of the conversion of methane to methanol is
discussed in detail in Chapter 2, and the practical use of MMO as a biocatalyst for
methane production is described in Chapter 4.
The methyl radical (•CH 3 ) is considered a key reaction intermediate in the partial
oxidation of methane. This intermediate has not been observed experimentally under
an oxygen atmosphere, but was detected via electron paramagnetic resonance (EPR)
spectroscopy at 77 K using molybdenum supported on silica as a heterogeneous
catalyst and N 2 O as the oxidant [52]. Furthermore, several theoretical calculations
have suggested the involvement of •CH 3 [31, 33]. Based on these experimental and
theoretical results, reaction (1.4) is believed to proceed via •CH 3 in the presence of
oxygen when catalyzed by heterogeneous or biological catalysts.
Additionally, theoretical investigations have predicted that this reaction could be
catalyzed by homogeneous organometallic catalysts [53, 54]. A theoretical investigation of the oxidation of methane to methanol on organometallic compounds such
as copper and nickel complexes suggested that metal–methoxy species (M–OCH 3 ,
M: metal cations) could act as the key intermediate in reaction (1.4) [19, 54]. The
theoretical reaction of M–OCH 3 with methane via the oxidative addition pathway is
shown in reaction (1.5) below.
L n M
+ CH 4
L n M
+ CH 3 OH
OCH 3
CH 3
(1.5)
In reaction (1.5), L represents a ligand. However, such catalytic reactions have not
been experimentally observed yet.
1.4.1.2 Conversion of Methane to Methanol Derivatives
The second category of C–O bond formation reactions involves the production of
methanol derivatives from methane. For example, the reaction of methane with strong
acids such as H 2 SO 4 has been reported.
CH 4 + 2 H 2 SO 4
CH 3 OSO 3 H + 2 H 2 O + SO 2
(1.6)
In this reaction, the produced methanol derivative, methyl sulfate, must be hydrolyzed
as below to obtain methanol [24].
CH 3 OSO 3 H + H 2 O
CH 3 OH + H 2 SO 4
(1.7)
7
been investigated [31–37]. Additionally, some heterogeneous catalysts for the
selective production of methanol from methane, such as FeO 2 /SiO 2 [48], inverse
CeO 2 /Cu 2 O/Cu(111) [49], Ni/CeO 2 (111) [50], and copper-exchanged zeolites
[51], have recently been developed; however, their catalytic performance cannot yet
match that of MMO. The mechanism of the conversion of methane to methanol is
discussed in detail in Chapter 2, and the practical use of MMO as a biocatalyst for
methane production is described in Chapter 4.
The methyl radical (•CH 3 ) is considered a key reaction intermediate in the partial
oxidation of methane. This intermediate has not been observed experimentally under
an oxygen atmosphere, but was detected via electron paramagnetic resonance (EPR)
spectroscopy at 77 K using molybdenum supported on silica as a heterogeneous
catalyst and N 2 O as the oxidant [52]. Furthermore, several theoretical calculations
have suggested the involvement of •CH 3 [31, 33]. Based on these experimental and
theoretical results, reaction (1.4) is believed to proceed via •CH 3 in the presence of
oxygen when catalyzed by heterogeneous or biological catalysts.
Additionally, theoretical investigations have predicted that this reaction could be
catalyzed by homogeneous organometallic catalysts [53, 54]. A theoretical investigation of the oxidation of methane to methanol on organometallic compounds such
as copper and nickel complexes suggested that metal–methoxy species (M–OCH 3 ,
M: metal cations) could act as the key intermediate in reaction (1.4) [19, 54]. The
theoretical reaction of M–OCH 3 with methane via the oxidative addition pathway is
shown in reaction (1.5) below.
L n M
+ CH 4
L n M
+ CH 3 OH
OCH 3
CH 3
(1.5)
In reaction (1.5), L represents a ligand. However, such catalytic reactions have not
been experimentally observed yet.
1.4.1.2 Conversion of Methane to Methanol Derivatives
The second category of C–O bond formation reactions involves the production of
methanol derivatives from methane. For example, the reaction of methane with strong
acids such as H 2 SO 4 has been reported.
CH 4 + 2 H 2 SO 4
CH 3 OSO 3 H + 2 H 2 O + SO 2
(1.6)
In this reaction, the produced methanol derivative, methyl sulfate, must be hydrolyzed
as below to obtain methanol [24].
CH 3 OSO 3 H + H 2 O
CH 3 OH + H 2 SO 4
(1.7)