86
4 Application of Biocatalysts for the Production of Methanol …
4.2.3 Production of Methanol Using Methane-Oxidizing
Bacteria
Several studies have revealed various MDH inhibition strategies including, but not
limited to, the addition of ethylenediaminetetraacetic acid (EDTA), cyclopropanol,
or high concentrations of various salts. When MDH is inhibited, methanol accumulation stops within a few hours because of the depletion of the reducing equivalents.
This problem can be resolved by supplying sodium formate as an electron source.
However, the addition of expensive external electron donors such as formate limits
the scalability of the process. Furthermore, the biocatalytic methanol yields have
not yet reached the feasible range [18, 98, 115, 116]. In order to put the methanol
production process into practical use, it will be necessary to optimize several factors
(pH, temperature, cell density, and gas mixture ratio), as well as the electron donation
and MDH inhibition conditions.
4.2.3.1 Bacterial Strains
As shown in Table 4.1, studies have been performed on pure cultures such as Methylosinus trichosporium [7, 8, 10–13, 15, 16, 18, 87, 117–119], Methylosinus sporium
[19, 22], Methylocella tundrae [25], Methylocystis bryophila [24], and strain 14B.
In addition, a consortium of strains (Methylosinus trichosporium OB3b, Methylococcus capsulatus, and Methylosinus sporium) and a combination of Methylocella
silvestris, Methyloferula stellate, and Methylomonas methanica were also tested for
methanol production [23]. All the studied strains except strain 14B are classified as
type II methane-oxidizing bacteria.
However, Type I strains have higher growth rates as shown in Table 4.2, exhibit
higher methane affinity, and require less energy for growth, making them more
promising for methanol production [75]. In addition, Type I methane-oxidizing bacteria are more energy efficient. The RuMP pathway in Type I methane-oxidizing
bacteria requires only one mole of ATP to assimilate three moles of formaldehyde
as shown in Fig. 4.3. Compared to the RuMP pathway, the serine pathway in Type
II methane-oxidizing bacteria requires more energy; three moles of ATP and two
moles of NADH are needed to assimilate two moles of formaldehyde and one mole
of carbon dioxide, as shown in Fig. 4.4 [88].
However, from another perspective, Type II methane-oxidizing bacteria show an
advantage for the production of methanol from methane. Up to 50% of the carbon
dioxide produced by Type II methane-oxidizing bacteria can be incorporated into
cell synthesis, compared to only 15% in Type I methane-oxidizing bacteria. Type II
methane-oxidizing bacteria were found to be suitable when biogas generated from
an anaerobic digester composed of about 40% carbon dioxide was used for methanol
production [75].
4 Application of Biocatalysts for the Production of Methanol …
4.2.3 Production of Methanol Using Methane-Oxidizing
Bacteria
Several studies have revealed various MDH inhibition strategies including, but not
limited to, the addition of ethylenediaminetetraacetic acid (EDTA), cyclopropanol,
or high concentrations of various salts. When MDH is inhibited, methanol accumulation stops within a few hours because of the depletion of the reducing equivalents.
This problem can be resolved by supplying sodium formate as an electron source.
However, the addition of expensive external electron donors such as formate limits
the scalability of the process. Furthermore, the biocatalytic methanol yields have
not yet reached the feasible range [18, 98, 115, 116]. In order to put the methanol
production process into practical use, it will be necessary to optimize several factors
(pH, temperature, cell density, and gas mixture ratio), as well as the electron donation
and MDH inhibition conditions.
4.2.3.1 Bacterial Strains
As shown in Table 4.1, studies have been performed on pure cultures such as Methylosinus trichosporium [7, 8, 10–13, 15, 16, 18, 87, 117–119], Methylosinus sporium
[19, 22], Methylocella tundrae [25], Methylocystis bryophila [24], and strain 14B.
In addition, a consortium of strains (Methylosinus trichosporium OB3b, Methylococcus capsulatus, and Methylosinus sporium) and a combination of Methylocella
silvestris, Methyloferula stellate, and Methylomonas methanica were also tested for
methanol production [23]. All the studied strains except strain 14B are classified as
type II methane-oxidizing bacteria.
However, Type I strains have higher growth rates as shown in Table 4.2, exhibit
higher methane affinity, and require less energy for growth, making them more
promising for methanol production [75]. In addition, Type I methane-oxidizing bacteria are more energy efficient. The RuMP pathway in Type I methane-oxidizing
bacteria requires only one mole of ATP to assimilate three moles of formaldehyde
as shown in Fig. 4.3. Compared to the RuMP pathway, the serine pathway in Type
II methane-oxidizing bacteria requires more energy; three moles of ATP and two
moles of NADH are needed to assimilate two moles of formaldehyde and one mole
of carbon dioxide, as shown in Fig. 4.4 [88].
However, from another perspective, Type II methane-oxidizing bacteria show an
advantage for the production of methanol from methane. Up to 50% of the carbon
dioxide produced by Type II methane-oxidizing bacteria can be incorporated into
cell synthesis, compared to only 15% in Type I methane-oxidizing bacteria. Type II
methane-oxidizing bacteria were found to be suitable when biogas generated from
an anaerobic digester composed of about 40% carbon dioxide was used for methanol
production [75].