90
4 Application of Biocatalysts for the Production of Methanol …
4.2.3.4 pH and Temperature
All reported biocatalytic oxidations of methane to methanol were performed at pH
and temperatures of 6.40–7 and 25–37 °C, respectively. Outside these ranges, notable
decreases in methanol production were observed, except in the case of M. bryophila,
which has the ability to produce methanol at a higher pH of 8.5 [24].
4.2.3.5 Copper and Iron Ions
As mentioned above, copper ions are crucial in determining the expression of pMMO
and sMMO. The addition of copper ions to the reaction mixture is expected to
increase methanol productivity by increasing the enzyme activity of pMMO, which
is known to show higher methane conversion than sMMO. In fact, at copper ion
concentrations between 1 and 5 μM, the amount of methanol produced increases with
the copper concentration. However, when the copper ion concentration exceeds 10–
20 μM, both methanol production and bacterial growth are suppressed. Additionally,
enhanced MMO activity and increased methanol production were observed in a
reaction mixture containing 10 μM of iron ions [10, 11, 19, 21, 22, 24–26]. The
addition of both 10 μM of iron ions and 5 μM of copper ions to the reaction mixture
has been reported to double the methanol production of M. sporium [22].
4.2.3.6 Cell Density
Theoretically, increased cell density should increase the methanol productivity of a
reactor, which is expressed as the amount of methanol (mol or g) per unit volume of
the reactor (L) per unit reaction time (h). However, according to studies so far, the
methanol productivity of a reactor does not increase linearly with the cell density, as
shown in Fig. 4.6a. The productivity increases linearly until the cell density reaches
approximately 1 mg-dry weight cells per liter, and then remains almost constant
at cell densities above 1 mg-dry weight cells per L. As shown in Fig. 4.6b, the
low productivity above this cell density is due to the lower productivity per unit
biocatalyst, which is expressed as the amount of methanol (g) per unit of biocatalyst
(g-dry weight cells) per unit reaction time (h); this value decreases at cell densities
above 1 mg-dry weight cells per liter.
Methanol production from methane in reactors with cell densities above 10 g-dry
weight cells per liter has been reported by only one research group. They recently
developed a high-cell-density reactor by increasing the concentration of the MDH
inhibitor [16]. Furthermore, another study mentioned that at high methane concentrations, methanol production was 3.2 times higher than in cultures with higher
biomass [18]. These results suggest that increasing cell density requires increasing
the concentrations of methane and MDH inhibitors and overcoming mass transfer
limitations. Duan et al. added 5% paraffin oil to the reaction solution to increase
the mass transfer of methane to the reaction solution, and examined the methanol
4 Application of Biocatalysts for the Production of Methanol …
4.2.3.4 pH and Temperature
All reported biocatalytic oxidations of methane to methanol were performed at pH
and temperatures of 6.40–7 and 25–37 °C, respectively. Outside these ranges, notable
decreases in methanol production were observed, except in the case of M. bryophila,
which has the ability to produce methanol at a higher pH of 8.5 [24].
4.2.3.5 Copper and Iron Ions
As mentioned above, copper ions are crucial in determining the expression of pMMO
and sMMO. The addition of copper ions to the reaction mixture is expected to
increase methanol productivity by increasing the enzyme activity of pMMO, which
is known to show higher methane conversion than sMMO. In fact, at copper ion
concentrations between 1 and 5 μM, the amount of methanol produced increases with
the copper concentration. However, when the copper ion concentration exceeds 10–
20 μM, both methanol production and bacterial growth are suppressed. Additionally,
enhanced MMO activity and increased methanol production were observed in a
reaction mixture containing 10 μM of iron ions [10, 11, 19, 21, 22, 24–26]. The
addition of both 10 μM of iron ions and 5 μM of copper ions to the reaction mixture
has been reported to double the methanol production of M. sporium [22].
4.2.3.6 Cell Density
Theoretically, increased cell density should increase the methanol productivity of a
reactor, which is expressed as the amount of methanol (mol or g) per unit volume of
the reactor (L) per unit reaction time (h). However, according to studies so far, the
methanol productivity of a reactor does not increase linearly with the cell density, as
shown in Fig. 4.6a. The productivity increases linearly until the cell density reaches
approximately 1 mg-dry weight cells per liter, and then remains almost constant
at cell densities above 1 mg-dry weight cells per L. As shown in Fig. 4.6b, the
low productivity above this cell density is due to the lower productivity per unit
biocatalyst, which is expressed as the amount of methanol (g) per unit of biocatalyst
(g-dry weight cells) per unit reaction time (h); this value decreases at cell densities
above 1 mg-dry weight cells per liter.
Methanol production from methane in reactors with cell densities above 10 g-dry
weight cells per liter has been reported by only one research group. They recently
developed a high-cell-density reactor by increasing the concentration of the MDH
inhibitor [16]. Furthermore, another study mentioned that at high methane concentrations, methanol production was 3.2 times higher than in cultures with higher
biomass [18]. These results suggest that increasing cell density requires increasing
the concentrations of methane and MDH inhibitors and overcoming mass transfer
limitations. Duan et al. added 5% paraffin oil to the reaction solution to increase
the mass transfer of methane to the reaction solution, and examined the methanol