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

Abstract

BackgroundMethane (CH4), 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 (CO2), CH4 is 25 times more potent at trapping radiation; thus, the emission of CH4 to the atmosphere causes severe global warming and climate change. To mitigate CH4 emissions and utilize them effectively, the direct biological conversion of CH4 into liquid fuels, such as methanol (CH3OH), using methanotrophs is a promising strategy. However, supplying biocatalysts in an aqueous medium with CH4 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.ResultsAn inverse membrane bioreactor (IMBR), which combines the advantages of gas-phase bioreactors and membrane bioreactors, was designed and constructed for the bioconversion of CH4 into CH3OH 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 CH4 directly from the gas phase to bacterial cells. Methylococcus capsulatus (Bath), a representative methanotroph, was used to demonstrate the bioconversion of CH4 to CH3OH in the IMBR. Cyclopropanol was supplied from the aqueous phase as a selective inhibitor of methanol dehydrogenase, preventing further CH3OH oxidation. Sodium formate was added as an electron donor to generate NADH, which is necessary for CH3OH production. After optimizing the inlet concentration of CH4, the mass of cells, the cyclopropanol concentration, and the gas flow rate, continuous CH3OH production can be achieved over 72 h with productivity at 0.88 mmol L−1 h−1 in the IMBR, achieving a longer operation period and higher productivity than those using other types of membrane bioreactors reported in the literature.ConclusionsThe 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.