(322d) Printable Bioreactors for Bioconversion of Methane to Value-Added Chemicals

An industrial process for the
selective oxidation of methane under mild conditions would be highly valuable
for controlling emissions to the environment and for utilizing vast new sources
of natural gas. Methanotrophic bacteria utilize methane under nearly ambient
conditions, suggesting a biochemical process for methane conversion could be
more efficient, less capital intensive, and more scalable compared to current
industrial practice. However, the industrial use of bioconversion for
large-scale applications, especially those involving gas-to-liquid reactions,
has been limited by poor mass transfer within stirred-tank reactors. We have embedded
the enzyme particulate methane monooxygenase (pMMO) within a 3D printed
hydrogel to create a mechanically robust and gas permeable material that
overcomes mass transfer limitations to convert methane to methanol. Building up
those findings, we also explored encapsulation of whole cells within our
material to utilize the entire cell proteome to further oxidize methanol into
additional chemicals and expand our production capabilities. 

Methods: Biocatalysts including enzymes and/or dehydrated cells
were immobilized in poly(ethylene glycol) (PEG) hydrogel during crosslinking by
UV-initiated photopolymerization. The utilization of methane by the
encapsulated biocatalysts was measured by an infrared-based methane detector
over time and high pressure liquid chromatography was used to detect production
of lactate and methanol. The stability of the encapsulated biocatalysts was
investigated for at least 72 hours.   

Results and Discussion: We demonstrated
the ability to encapsulate biocatalysts within a gas-permeable hydrogel
material while retaining catalytic activity. Encapsulation within our material
allowed an increased surface area to volume ratio, thereby overcoming mass
transfer limitations and improving methane utilization. We investigated the stability of the encapsulated biocatalysts
over time as well as the effect of loading, hydrogel weight fraction, and surface
area to volume ratio on biocatalyst activity.  We look forward to improving the activity of
the encapsulated biocatalyst, and eventually curing the hydrogel material within
a poly(dimethyl siloxane) scaffold as part of a flow-through reactor, shown in
Figure 1, with which we have demonstrated continuous methane conversion using
encapsulated pMMO.

Figure 1: Schematic of enzyme immobilization within hydrogel
in a structured polymer scaffold to create a gas-permeable biocatalytic
material in a flow-through bioreactor. 

This work was performed under the auspices of the U.S. Department
of Energy by Lawrence Livermore National Laboratory under Contract
DE-AC52-07NA27344.