Plants grown for biofuel have the potential to power our travel industry, but an important fraction of their chemical power has remained stubbornly difficult to recover. New research from the DOE-funded Center for Advanced Biofuel and Bioproduct Innovation on the University of Illinois campus has demonstrated a way to preserve native lignin structure, a key component of plant matter for conversion to other valuable products, putting an improved pathway for biofuel and bioproduct production within reach.
The project, led by postdoctoral research associate Tirath Raj and Integrated Bioprocessing Research Laboratory Executive Director Vijay Singh, demonstrated that a new method of pretreating harvested biofuel crops resulted in higher yields of valuable materials, while requiring much lower energy and chemical inputs. The study, which was supported by the USDA in addition to the DOE, was published in Chemical Engineering Journal Advances . Its central finding is a high-priority step not only for CABBI, but for the other three DOE Bioenergy Research Centers as well.
Biofuels offer the possibility of powering our society on plants that we can grow and harvest year after year. Any plant matter can provide energy when it is burned; to make efficient and versatile biofuel, the different components of a plant's physical structure must first be separated, then fermented or otherwise chemically transformed into familiar forms of fuel like ethanol or biodiesel, or other useful aromatic and aliphatic compounds. This process is called conversion.
Singh, Raj, and their collaborators set out to improve the preprocessing step of conversion. In this step, cellulose and related plant fibers that give stems and branches their strength are separated from lignin, a distinct fiber type that helps fill in a plant's structure and keep water out.
The structure of cellulose, long straight fibers made out of glucose, make it easy to break down and ferment into fatty acids. In contrast, lignin is referred to as "recalcitrant:" its fibers are made of several phenolic structural units (e.g., syringyl, guaiacyl, and p-hydroxyphenyl groups), branched and cross-linked to produce a web of strong, sticky, water-resistant material that is good for making a plant strong but bad for easy conversion. Each of these different chemical groups gives different potentially valuable properties to lignin while also making it difficult to purify. Hydrothermal treatment is typically used for preprocessing, but it has significant drawbacks.
"When you treat with water at a very high temperature and pressure, it starts to break down that recalcitrant structure" of lignin, Singh said. "But what happens is if you use a hydrothermal treatment process, you tend to melt down the amorphous polymeric [multipart] lignin, which is the glue that holds the cellulose, the hemicellulose together. It is like a cement—and the hydrothermal process is able to break that cement down and release those polymeric cellulosic sugars. But the lignin in that process also gets broken down." Singh is also a Founder Professor of Agricultural and Biological Engineering in the College of Agricultural, Consumer & Environmental Sciences and the Grainger College of Engineering.
When lignin is broken down and separated by hydrothermal treatment, it tends to condense into an even more solid and impenetrable mass. This represents two losses of energy in the conversion process, both from generating the high temperature and pressure for the pretreatment and from losing part of the valuable biomass of the plant. By finding a different way to separate out lignin from the other plant components, the research team aimed to avoid these losses.
They focused on a special type of salt solution, called natural deep eutectic solvent, to more gently loosen lignin's hold. Despite their industrial-sounding acronym, NADES are naturally-derived compounds that are relatively safe to handle, active at room temperature, and recyclable. Raj worked with the rest of the research team to demonstrate that the right NADES combinations were able to gently free lignin from the plant structure, keeping its original branches and cross-links intact and preventing it from condensing.
"After processing, we separated the lignin out, we characterized it, and tried to find the overall chemical structure, how it behaves, whether the native structure is preserved or not," Raj said. "We proved that the lignins coming out of this process have preserved their native structure."
The high yields of more accessible lignin and more pure cellulose sugars are exactly the starting point needed to make the rest of the biofuel conversion process successful and profitable. High purity lignin can be much more readily chemically convertible to aromatics and aliphatic compounds; further special strains of yeast can ferment the sugars into not only ethanol, but oil for biodiesel and sustainable aviation fuel production. In addition, the operational costs of NADES pretreatment are lower than hydrothermal treatment, and the solutions can be recovered and reused as many as five times without major loss of effectiveness.
"This effort also feeds into a shared research objective among all four DOE Bioenergy Research Centers related to lignin," Singh said. "Other BRCs are taking this native lignin and catalytically breaking it down and feeding it to other microorganisms to produce other chemical products . . . our NADES pretreatment is able to preserve that lignin, which can then go for these other high value products. This is CABBI's contribution to the shared research objective."
A final strength of the team's innovation is its flexibility. Although it was tested in this study on a bioenergy grass called Miscanthus, the pretreatment process can be applied to a wide variety of biofuel crops, agricultural residues and woods also known as feedstocks, and preserves any oils produced by the wild type or engineered plant that can be recovered for an additional fuel source, as well as other useful substances.
"It's what we call a feedstock agnostic process," Singh said. "The goal is to develop a biorefinery where we can use this feedstock and produce all kinds of products that consumers can use."
