Biosynthesis of Energy-Dense Fuel Alternative Opens Doors toward Fossil Fuel Independence

The Science  

The global economy depends heavily on fossil fuels, a reality that clashes with goals to reduce pollution. In particular, industries such as long-haul cargo transport, some aviation sectors, and rocketry require high-energy fuels that emit large amounts of CO₂ and other pollutants when used. Synthetic fuels with enough energy to replace them are generally costly and produce toxic waste. However, in this study, a multi-institutional team of scientists successfully identified a group of viable, bacterially-derived alternatives that are both energy-rich and can be sustainably produced. 

The Impact 

Freeing key industries like overseas shipping, aviation, and rocketry from fossil fuels has been especially difficult because the energy-rich fuels they require are hard to replicate without petroleum. Using bacteria, researchers produced a special type of hydrocarbon that has more energy than currently used rocket and aviation fuels. These hydrocarbons, which include a fatty acid and an ester, are one of a small fleet of molecules that contain enough cyclopropane rings to meet such specialized energy needs. This new biosynthetic process may be scaled up to commercial levels, opening doors for fossil fuel independence and helping the nation’s economy toward a more sustainable future. 

Summary 

Hydrocarbons are the building blocks of fossil fuels such as crude oil, natural gas, and coal. Cyclopropane-functionalized hydrocarbons offer an exceptionally high energy density, making them excellent fuels for energy-intensive applications such as long-haul shipping, aviation, and rocketry. However, synthesizing these molecules from fossil hydrocarbon feedstocks is complicated, costly, and produces a range of toxic wastes. To overcome this limitation, researchers from Lawrence Berkeley National Laboratory and the Joint BioEnergy Institute produced polycyclopropanated fatty acids (POP-FAs)—molecules made up of multiple cyclopropane rings—using bacteria. The team combed through thousands of bacterial genomes to harness those with naturally occurring cyclopropanated secondary metabolites. From these genomes, they identified genes coding for enzymes that install an energy-dense cyclopropane ring on a fatty acid chain. The researchers hypothesized that by expressing the right genes in the bacterium Streptomyces coelicolor, energy-dense fatty acids with multiple cyclopropane rings, or POP-FAs, could be produced. Then, scientists from the Environmental Molecular Sciences Laboratory, a Department of Energy Office of Science User facility, and Pacific Northwest National Laboratory used advanced nuclear magnetic resonance and mass spectrometry instrumentation to prove that POP-FAs were indeed made by the engineered bacteria. Finally, the team produced POP-FA methyl esters (POP-FAMEs), which can be used directly as fuel. Both the POP-FAs and POP-FAMEs demonstrated net heating values above 50 megajoules per liter (MJ/L), which far exceeds the values of existing fuels (gasoline ~32 MJ/L; and typical jet and rocket fuels ~35 MJ/L). This study shows that these POPs are strong candidates for sustainable and energy-rich fuel alternatives. 

Contacts 

Robert Young,  Environmental Molecular Sciences Laboratory, Robert.Young@pnnl.gov 

Jennifer Kyle, Pacific Northwest National Laboratory, Jennifer.Kyle@pnnl.gov 

John Cort, Pacific Northwest National Laboratory, John.Cort@pnnl.gov 

Funding 

This study was funded by the Department of Energy (DOE) Joint BioEnergy Institute through the DOE Office of Science, Biological and Environmental Research program. Additional support was provided by the Co-Optimization of Fuels & Engines (Co-Optima) project through the DOE Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies and Vehicle Technologies Offices.  

Publication

P. Cruz-Morales, et al., "Biosynthesis of polycyclopropanated high energy biofuels." Joule 6, 1590–1605 (2022). [DOI: 10.1016/j.joule.2022.05.011]