Researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL), the Massachusetts Institute of Technology (MIT), and Washington State University have developed a a process by which lignin can be selectively converted to jet-range aromatics at unprecedented carbon yields, providing a path toward a 100% drop-in sustainable aviation fuel (SAF). A paper on their work is published in Joule.

Stone et al.

Although electrification has shown promise toward reducing the carbon footprint of passenger vehicles, aviation remains dependent on hydrocarbon fuels due to their high energy density relative to even the most advanced battery technologies available today. As such, multiple processes for the generation of sustainable aviation fuels (SAFs) have recently been developed. However, due to their lack of aromatic molecules, they require blending with conventional fuels to achieve the necessary characteristics for their safe use. By using lignin, an abundant, renewable biopolymer that has traditionally been underutilized due to its chemical recalcitrance, an aromatic SAF blendstock could be generated.

—Stone et al.

Jet fuel is a blended mixture of different hydrocarbon molecules, including aromatics and cycloalkanes. Current commercialized technologies do not produce those components to qualify for a 100% SAF. Instead, SAF blendstocks are combined with conventional hydrocarbon fuels. As the largest source of renewable aromatics in nature, lignin could hold the answer to achieving a complete bio-based jet fuel.

This newly published work illustrates the ability of a lignin pathway to complement existing and other developing pathways. Specifically, the lignin pathway described in this new work allows the SAF to have fuel system compatibility at higher blend ratios.

Lignin makes up the rigid parts of the cell walls of plants. Other parts of plants are used for biofuels, but lignin has been largely overlooked because of the difficulties in breaking it down chemically and converting it into useful products.

Because of its recalcitrance, lignin is typically burned for heat and power or used only in low-value applications. Previous research has yielded lignin oils with high oxygen contents ranging from 27% to 34%, but to be used as a jet fuel that amount must be reduced to less than a half-percent.

Other processes have been tried to reduce the oxygen content, but the catalysts involved require expensive noble metals and proved to be low yielding. Researchers at the trio of institutions demonstrated an efficient method that used earth-abundant molybdenum carbide as the catalyst in a continuous process, achieving an oxygen content of about 1%.

The US Department of Energy’s Bioenergy Technologies Office and Center for Bioenergy Innovation funded the research.

Resources

• Michael L. Stone, Matthew S. Webber, William P. Mounfield, David C. Bell, Earl Christensen, Ana R.C. Morais, Yanding Li, Eric M. Anderson, Joshua S. Heyne, Gregg T. Beckham, Yuriy Román-Leshkov (2022) “Continuous hydrodeoxygenation of lignin to jet-range aromatic hydrocarbons,” Joule doi: 10.1016/j.joule.2022.08.005