A novel inverse membrane bioreactor for efficient bioconversion from methane gas to liquid methanol using a microbial gas-phase reaction

<jats:title>Abstract</jats:title><jats:sec><jats:title>Background</jats:title><jats:p>Methane (CH<jats:sub>4</jats:sub>), as one of the major energy sources, easily escapes from the supply chain into the atmosphere, because it exists in a gaseous state under ambient conditions. Compared to carbon dioxide (CO<jats:sub>2</jats:sub>), CH<jats:sub>4</jats:sub>is 25 times more potent at trapping radiation; thus, the emission of CH<jats:sub>4</jats:sub>to the atmosphere causes severe global warming and climate change. To mitigate CH<jats:sub>4</jats:sub>emissions and utilize them effectively, the direct biological conversion of CH<jats:sub>4</jats:sub>into liquid fuels, such as methanol (CH<jats:sub>3</jats:sub>OH), using methanotrophs is a promising strategy. However, supplying biocatalysts in an aqueous medium with CH<jats:sub>4</jats:sub>involves high energy consumption due to vigorous agitation and/or bubbling, which is a serious concern in methanotrophic processes, because the aqueous phase causes a very large barrier to the delivery of slightly soluble gases.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>An inverse membrane bioreactor (IMBR), which combines the advantages of gas-phase bioreactors and membrane bioreactors, was designed and constructed for the bioconversion of CH<jats:sub>4</jats:sub>into CH<jats:sub>3</jats:sub>OH in this study. In contrast to the conventional membrane bioreactor with bacterial cells that are immersed in an aqueous phase, the filtered cells were placed to face a gas phase in the IMBR to supply CH<jats:sub>4</jats:sub>directly from the gas phase to bacterial cells.<jats:italic>Methylococcus capsulatus</jats:italic>(Bath), a representative methanotroph, was used to demonstrate the bioconversion of CH<jats:sub>4</jats:sub>to CH<jats:sub>3</jats:sub>OH in the IMBR. Cyclopropanol was supplied from the aqueous phase as a selective inhibitor of methanol dehydrogenase, preventing further CH<jats:sub>3</jats:sub>OH oxidation. Sodium formate was added as an electron donor to generate NADH, which is necessary for CH<jats:sub>3</jats:sub>OH production. After optimizing the inlet concentration of CH<jats:sub>4</jats:sub>, the mass of cells, the cyclopropanol concentration, and the gas flow rate, continuous CH<jats:sub>3</jats:sub>OH production can be achieved over 72 h with productivity at 0.88 mmol L<jats:sup>−1</jats:sup> h<jats:sup>−1</jats:sup>in the IMBR, achieving a longer operation period and higher productivity than those using other types of membrane bioreactors reported in the literature.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>The IMBR can facilitate the development of gas-to-liquid (GTL) technologies via microbial processes, allowing highly efficient mass transfer of substrates from the gas phase to microbial cells in the gas phase and having the supplement of soluble chemicals convenient.</jats:p></jats:sec>