Lawrence Livermore National Laboratory scientists have combined biology and 3-D printing to create the first reactor that can continuously produce methanol from methane at room temperature and pressure.
Methane monooxygenases (MMOs), found in methanotrophic bacteria, are selective catalysts for methane activation and conversion to methanol under mild conditions; however, these enzymes are not amenable to standard enzyme immobilization approaches. Using particulate methane monooxygenase (pMMO), the researchers created a biocatalytic polymer material that converts methane to methanol. They embedded the material within a silicone lattice to create mechanically robust, gas-permeable membranes, and the direct printing of micron-scale structures with controlled geometry. The enzymes retain up to 100% activity in the polymer construct.
An open-access paper on the research, which could lead to more efficient conversion of methane to energy production, appears in journal Nature Communications.
Remarkably, the enzymes retain up to 100 percent activity in the polymer. The printed enzyme-embedded polymer is highly flexible for future development and should be useful in a wide range of applications, especially those involving gas-liquid reactions.—Sarah Baker, LLNL chemist and project lead
Advances in oil and gas extraction techniques have made vast new stores of natural gas, composed primarily of methane, available. However, a large volume of methane is leaked, vented or flared during these operations, partly because the gas is difficult to store and transport compared to more-valuable liquid fuels. Methane emissions also contribute about one-third of current net global warming potential, primarily from these and other distributed sources such as agriculture and landfills.
Current industrial technologies such as steam reformation to convert methane to more valuable products operate at high temperature and pressure, require a large number of unit operations and yield a range of products. As a result, current industrial technologies have a low efficiency of methane conversion to final products and can only operate economically at very large scales.
A technology to efficiently convert methane to other hydrocarbons is needed as a profitable way to convert “stranded” sources of methane and natural gas (sources that are small, temporary or not close to a pipeline) to liquids for further processing, the team reported.
The only known catalyst (industrial or biological) to convert methane to methanol under ambient conditions with high efficiency is the enzyme methane monooxygenase (MMO), which converts methane to methanol. The reaction can be carried out by methanotrophs that contain the enzyme, but this approach inevitably requires energy for upkeep and metabolism of the organisms. Instead, the team separated the enzymes from the organism and used the enzymes directly.
The team found that isolated enzymes offer the promise of highly controlled reactions at ambient conditions with higher conversion efficiency and greater flexibility.
Up to now, most industrial bioreactors are stirred tanks, which are inefficient for gas-liquid reactions. The concept of printing enzymes into a robust polymer structure opens the door for new kinds of reactors with much higher throughput and lower energy use.—Joshuah Stolaroff, an environmental scientist on the team
The team found that the 3-D-printed polymer could be reused over many cycles and used in higher concentrations than possible with the conventional approach of the enzyme dispersed in solution.
This work opens a pathway for high-throughput biocatalysis suitable to the large-scale and gas–liquid reactions in the energy sector. However, several important hurdles remain, such as achieving enzyme longevity, enhancing throughput and situating pMMO within a larger reaction chain that supplies the necessary reducing agent. NADH is costly, unstable and not viable for industrial use. A convenient reductant for industrial use would be hydrogen, which can be used to regenerate NADH catalysed by hydrogenase enzymes, and which can be reformed from a portion of the methane fed to the system. Such a reaction might be achieved by co-immobilization of hydrogenase with pMMO, which would have the further advantage that all the reactants are gas phase and all the products are aqueous, simplifying reactant–product separation.
Other options include supplying the electrons electrochemically, or co-immobilizing pMMO with methanol dehydrogenase to yield formaldehyde as the product for collection. With these challenges in mind, the application of biological tools to address the need to convert fugitive and remote methane streams requires that biologists and materials scientists work together to optimize biocatalysts through protein engineering and to develop accompanying bioreactor materials.—Blanchette et al.
Other Livermore team members include: Jennifer Knipe, Craig Blanchette, Joshua DeOtte, James Oakdale, Amitesh Maiti and Jeremy Lenhardt. The LLNL team collaborated with (link is external)Northwestern University (link is external) researchers Sarah Sirajuddin and Professor Amy Rosenzweig.
The Laboratory Directed Research and Development program funded the research.
Craig D. Blanchette, Jennifer M. Knipe, Joshuah K. Stolaroff, Joshua R. DeOtte, James S. Oakdale, Amitesh Maiti, Jeremy M. Lenhardt, Sarah Sirajuddin, Amy C. Rosenzweig & Sarah E. Baker (2016) “Printable enzyme-embedded materials for methane to methanol conversion” Nature Communications 7, Article number: 11900 doi: 10.1038/ncomms11900