Chapter 2
Selective Production of Methanol
from Methane and Molecular Oxygen
at Atmospheric Temperature
and Pressure Using Methane
Monooxygenases
2.1 Introduction
Methane monooxygenase (MMO; EC 1.14.13.25) is an enzyme that is biosynthesized
by methane-oxidizing bacteria (methane-utilizing bacteria), which are microorganisms that utilize methane as a carbon and energy source [1, 2]. In bacterial cells,
MMO oxidizes methane to methanol using molecular oxygen and electron donors at
atmospheric temperature and pressure (20–40 °C, 1 bar) as follows [1, 2]:
CH 4 + O 2 + e
−
+ H
+
→ CH 3 OH + H 2 O
(2.1)
Methane conversion via MMO is attractive as a methane-utilization technique.
Firstly, MMO converts methane into methanol at atmospheric temperature and pressure. In principle, the cleavage of a C–H bond in methane requires a large amount of
energy, i.e., high-temperature conditions, because of the high energy of homolytic
C–H bond dissociation (435 kJ mol
−1 ). For example, the production of synthesis
gas (carbon monoxide and hydrogen) from methane requires temperatures of 850 °C
or higher in the presence of a nickel catalyst [2, 3]. The ability of MMO to cleave
the C–H bond of methane under mild conditions is believed to originate from the
generation of extremely reactive oxygen species at the catalytic site of MMO. Therefore, the structure and properties of the catalytic site in MMO and the active oxygen
species involved in the catalysis have been investigated.
The other attractive feature of methane conversion via MMO is its 100% selectivity
toward methanol; the produced methanol is not further oxidized by the reactive
oxygen species. As mentioned in the introduction, methanol is more reactive than
methane itself. The C–H bond energy of methanol is 393–402 kJ mol
−1 [4]; as
a result, the C–H bond of methanol is typically cleaved under methane activation
conditions. That is, in circumstances in which methanol can be produced via the
oxidation of methane, further oxidation of the produced methanol would also be
expected to occur. Thus, the selective production of methanol by MMO indicates
that MMO can control the oxidation reaction in some way. Generally, the protein
© Springer Nature Singapore Pte Ltd. 2020
T. Baba and A. Miyaji, Catalysis and the Mechanism of Methane Conversion to Chemicals,
https://doi.org/10.1007/978-981-15-4132-2_2
23
Selective Production of Methanol
from Methane and Molecular Oxygen
at Atmospheric Temperature
and Pressure Using Methane
Monooxygenases
2.1 Introduction
Methane monooxygenase (MMO; EC 1.14.13.25) is an enzyme that is biosynthesized
by methane-oxidizing bacteria (methane-utilizing bacteria), which are microorganisms that utilize methane as a carbon and energy source [1, 2]. In bacterial cells,
MMO oxidizes methane to methanol using molecular oxygen and electron donors at
atmospheric temperature and pressure (20–40 °C, 1 bar) as follows [1, 2]:
CH 4 + O 2 + e
−
+ H
+
→ CH 3 OH + H 2 O
(2.1)
Methane conversion via MMO is attractive as a methane-utilization technique.
Firstly, MMO converts methane into methanol at atmospheric temperature and pressure. In principle, the cleavage of a C–H bond in methane requires a large amount of
energy, i.e., high-temperature conditions, because of the high energy of homolytic
C–H bond dissociation (435 kJ mol
−1 ). For example, the production of synthesis
gas (carbon monoxide and hydrogen) from methane requires temperatures of 850 °C
or higher in the presence of a nickel catalyst [2, 3]. The ability of MMO to cleave
the C–H bond of methane under mild conditions is believed to originate from the
generation of extremely reactive oxygen species at the catalytic site of MMO. Therefore, the structure and properties of the catalytic site in MMO and the active oxygen
species involved in the catalysis have been investigated.
The other attractive feature of methane conversion via MMO is its 100% selectivity
toward methanol; the produced methanol is not further oxidized by the reactive
oxygen species. As mentioned in the introduction, methanol is more reactive than
methane itself. The C–H bond energy of methanol is 393–402 kJ mol
−1 [4]; as
a result, the C–H bond of methanol is typically cleaved under methane activation
conditions. That is, in circumstances in which methanol can be produced via the
oxidation of methane, further oxidation of the produced methanol would also be
expected to occur. Thus, the selective production of methanol by MMO indicates
that MMO can control the oxidation reaction in some way. Generally, the protein
© Springer Nature Singapore Pte Ltd. 2020
T. Baba and A. Miyaji, Catalysis and the Mechanism of Methane Conversion to Chemicals,
https://doi.org/10.1007/978-981-15-4132-2_2
23