94
4 Application of Biocatalysts for the Production of Methanol …
and the reaction temperature of the biocatalysts are reasonable. To develop such
bioprocesses, the productivity would need to be increased more than ten times.
There are three potential strategies to increase the productivity of biocatalytic
processes. The first is to increase the amount of methanol allowed to accumulate.
However, methanol production does not increase further at concentrations above
~10 mM because of the inhibition of MMO by methanol. This limitation can be
overcome by increasing the cell density at the expense of a decrease in productivity.
The second strategy is to decrease the amount of bacterial cells used for methanol
production. The strategy was investigated by Takeguchi et al. [10], who achieved a
maximum productivity of 0.05 g-methanol per g-dry cell per h. Under these conditions, the highest amount of methanol was obtained after 100 h. The final strategy
is to increase the rate of methanol production per bacterial cell. For this strategy,
the bacterial culture conditions must be optimized, along with the use of additives
to enhance the growth rate of the bacterial culture and the MMO reaction. For this
strategy, a combination of MMO system with other electron donation systems such
as photosynthetic system may be useful [122, 123].
In addition to biocatalysts, heterogeneous and homogeneous catalysts have been
developed with the aim of producing methanol in the liquid and gas phase [120, 121].
Among them, many catalysts require stronger oxidants than molecular oxygen such
as N 2 O, O 3 , SO 3 , K 2 S 2 O 8 , and H 2 O 2 or their mixtures. These catalysts can oxidize
methane to methanol via an active oxygen species on a metal site, but are not capable
of molecular oxygen activation.
Figure 4.8 shows the methanol productivity of various (bio)catalysts, revealing
that although few catalysts achieve commercially attractive performance, biocatalysts are relatively promising. Notably, biocatalytic methanol production proceeds at
ambient temperature and pressure in the aqueous phase and using molecular oxygen
as the oxidant. Thus, the use of biocatalysts is a low-energy and sustainable process.
Taking into account both the productivity and the reaction conditions, the production of methanol using methane-oxidizing bacteria is a promising alternative for the
production of methanol from methane.
The use of not only pure methane, but also its mixtures with several gases, such
as hydrogen and carbon dioxide, in biocatalytic methanol production has been investigated [118] in order to use natural gas or biogas as the methane source. Methanol
can be obtained using such processes; although the productivities are not high, they
are expected to improve in the near future.
The use of recombinant bacteria containing MMO systems may also allow high
methanol production from methane to be achieved. However, this strategy is currently
hampered by the inability to express functional MMO in E. coli and other cells that
can be applied in industrial processes.
4 Application of Biocatalysts for the Production of Methanol …
and the reaction temperature of the biocatalysts are reasonable. To develop such
bioprocesses, the productivity would need to be increased more than ten times.
There are three potential strategies to increase the productivity of biocatalytic
processes. The first is to increase the amount of methanol allowed to accumulate.
However, methanol production does not increase further at concentrations above
~10 mM because of the inhibition of MMO by methanol. This limitation can be
overcome by increasing the cell density at the expense of a decrease in productivity.
The second strategy is to decrease the amount of bacterial cells used for methanol
production. The strategy was investigated by Takeguchi et al. [10], who achieved a
maximum productivity of 0.05 g-methanol per g-dry cell per h. Under these conditions, the highest amount of methanol was obtained after 100 h. The final strategy
is to increase the rate of methanol production per bacterial cell. For this strategy,
the bacterial culture conditions must be optimized, along with the use of additives
to enhance the growth rate of the bacterial culture and the MMO reaction. For this
strategy, a combination of MMO system with other electron donation systems such
as photosynthetic system may be useful [122, 123].
In addition to biocatalysts, heterogeneous and homogeneous catalysts have been
developed with the aim of producing methanol in the liquid and gas phase [120, 121].
Among them, many catalysts require stronger oxidants than molecular oxygen such
as N 2 O, O 3 , SO 3 , K 2 S 2 O 8 , and H 2 O 2 or their mixtures. These catalysts can oxidize
methane to methanol via an active oxygen species on a metal site, but are not capable
of molecular oxygen activation.
Figure 4.8 shows the methanol productivity of various (bio)catalysts, revealing
that although few catalysts achieve commercially attractive performance, biocatalysts are relatively promising. Notably, biocatalytic methanol production proceeds at
ambient temperature and pressure in the aqueous phase and using molecular oxygen
as the oxidant. Thus, the use of biocatalysts is a low-energy and sustainable process.
Taking into account both the productivity and the reaction conditions, the production of methanol using methane-oxidizing bacteria is a promising alternative for the
production of methanol from methane.
The use of not only pure methane, but also its mixtures with several gases, such
as hydrogen and carbon dioxide, in biocatalytic methanol production has been investigated [118] in order to use natural gas or biogas as the methane source. Methanol
can be obtained using such processes; although the productivities are not high, they
are expected to improve in the near future.
The use of recombinant bacteria containing MMO systems may also allow high
methanol production from methane to be achieved. However, this strategy is currently
hampered by the inability to express functional MMO in E. coli and other cells that
can be applied in industrial processes.