New process helps overcome obstacles to produce renewable fuels and chemicals

The second-most abundant organic polymer in nature may soon be useful for the production of biofuels and renewable chemicals, thanks to a project involving CEE researchers that demonstrated a new method for processing it.

Lignin is an energy-dense polymer present in plants. It is the second most abundant biopolymer on Earth, after cellulose, making it a potentially valuable resource in the manufacture of biofuels. Until recently, though, the biofuels industry had a saying: “You can make anything from lignin except money.” Now new research by CEE researchers and the Department of Energy’s National Renewable Energy Laboratory (NREL) has challenged that adage. The team – which included CEE Ph.D. student Derek Vardon, who has been conducting research at NREL for the past year, and his adviser CEE Professor Timothy Strathmann – has demonstrated a concept that provides opportunities for the successful conversion of lignin into a variety of renewable fuels, chemicals and materials.

The process for converting glucose from biomass into fuels such as ethanol has been well established. However, plants also contain a significant amount of lignin – up to 30 percent of their cell walls. Lignin is a heterogeneous aromatic polymer that plants use to strengthen cell walls, but it is typically considered a hindrance to cost-effectively obtaining carbohydrates, and residual lignin is often burned for process heat because it is difficult to depolymerize and upgrade into useful fuels or chemicals.

“Biorefineries that convert cellulosic biomass into liquid transportation fuels typically generate more lignin than necessary to power the operation,” NREL Senior Engineer and project leader Gregg Beckham said.  “Strategies that incorporate new approaches to transform the leftover lignin to more diverse and valuable products are desperately needed.”

Although lignin depolymerization has been studied for nearly a century, the development of cost-effective upgrading processes for lignin valorization has been limited.

“With regards to lignin ‘valorization,’ we are trying to develop technologies that add value to biomass by converting it to renewable fuels and chemicals in an environmentally sustainable and economically viable fashion,” Vardon said.

In nature, some microorganisms have figured out how to overcome the heterogeneity of lignin. “Rot” fungi and some bacteria are able to secrete powerful enzymes or chemical oxidants to break down lignin in plant cell walls, which produces a heterogeneous mixture of aromatic molecules. Given this large pool of aromatics present in nature, some bacteria have developed “funneling” pathways to uptake the resulting aromatic molecules and use them as a carbon and energy source.

This new study shows that developing biological conversion processes for one such lignin-utilizing organism may enable new routes to overcome the heterogeneity of lignin. That may enable a broader slate of molecules derived from lignocellulosic biomass.

“The conceptual approach we demonstrate can be applied to many different types of biomass feedstocks and combined with many different strategies for breaking down lignin, engineering the biological pathways to produce different intermediates, and catalytically upgrading the biologically-derived product to develop a larger range of valuable molecules derived from lignin,” Beckham said. “It holds promise for a wide variety of industrial applications. While this is very exciting, certainly there remains a significant amount of technology development to make this process economically viable.”

A patent application has been filed on this research and NREL’s Technology Transfer Office will be working with researchers to identify potential licensees of the technology.

The paper describing this work, “Lignin Valorization through Integrated Biological Funneling and Chemical Catalysis,” recently published in the Proceedings of the National Academy of Sciences, is available online. The work initially was financed by the National Advanced Biofuels Consortium via funds from the American Recovery and Reinvestment Act; continuing core project support was provided by the Energy Department’s Office of Energy Efficiency and Renewable Energy.

“By combining novel biological ‘funneling’ pathways with aqueous catalytic upgrading processes, this work points the way to strategies for exploiting the largely overlooked lignin fraction of biomass,” Strathmann said. “The work of our team demonstrates that the combination of biological and catalytic processing can produce valuable materials, such as bioplastics, and fuel-range hydrocarbons.”