Plant cell walls resist chemical or biological degradation, making the breakdown of lignocellulosic biomass into renewable chemical precursors for conversion into chemicals and transportation fuels challenging and costly. As a result, economically viable methods of transforming biomass into biofuels have yet to be realized.

In a step forward in understanding how plant biomass can be more efficiently broken down, a research team led by the University of California, Riverside, has created a chemical roadmap to breach these defenses. A paper on their work is published in the Journal of the American Chemical Society.

In order to access the energy-rich sugars found in the plant cell walls, researchers have renewed focus on solvating lignin, a complex polymer also found in plant cell walls that acts as a natural shield, blocking both chemical and biological attack. Lignin is particularly effective in preventing commercial enzymes from digesting cellulose, which makes up the bulk of sugars found in biomass.

In the past, different specialized chemicals and pretreatment methods have been used to improve enzyme access to cellulose but were ineffective at removing lignin. The use of strong acids, ionic liquids, ammonia, and sulfite treatments have somewhat improved the digestibility of cellulose, but these methods also leave lignin behind, making cellulose expensive to recover. Other methods have applied co-solvents such as ethanol and acetone solvate to remove lignin, but they require very high reaction temperatures that also cause the remaining sugars to degrade.

Charles Cai, an assistant research engineer at the Center for Environmental Research and Technology in the Marlan and Rosemary Bourns College of Engineering at UC Riverside, and Abhishek S. Patri, a doctoral student in chemical and environmental engineering, led a team of researchers taking a new direction to focus on identifying highly specialized co-solvents, substances added to a primary solvent to make it more effective, that can facilitate milder temperature solvation and release of lignin from the plant cell walls. This is known as a “lignin-first” approach to breaking down biomass.

The team constructed a 1.5-million-atom molecular simulation to reveal how the co-solvent pair consisting of tetrahydrofuran (THF), and water are particularly effective at altering the interactions between lignin and cellulose, helping to drive multiple key mechanisms responsible for breaking down biomass.

Co-solvents THF and water cause lignin to dissociate from itself and cellulose, expanding into a random coil. (Charles Cai/UCR)

They discovered that pretreating plant biomass with THF-water caused lignin globules on the cellulose surface to expand and break away from one another and away from the cellulose fibers. The expanded lignin was also more exposed to catalytic fragmentation by dilute acid. As a result, lignin could be more efficiently depolymerized, solubilized, and transported out of the cell wall at milder treatment conditions.

The nearly complete removal of lignin also allowed the remaining cellulose fibers to be more susceptible to enzyme attack. After mild THF co-solvent treatment, the enzymes added to the remaining cellulose-rich solids achieved complete hydrolysis to glucose sugars.

At bulk scale, dilute acid pretreatment of biomass in a co-solvent mixture liberates hemicellulose and depolymerized lignin from cellulose, allowing unfettered access of cellulolytic enzymes that sustain high rates of cellulose hydrolysis to glucose without enzyme deactivation.

Through this multi-scale analysis, synergistic mechanisms can be identified for other multifunctional co-solvents portending a paradigm shift towards first principles design of future biomass deconstruction methods in order to realize lower cost fuels.

—Patri et al.

Resources

• Abhishek S. Patri, Barmak Mostofian, Yunqiao Pu, Nicholas Ciaffone, Mikhael Soliman, Micholas Dean Smith, Rajeev Kumar, Xiaolin Cheng, Charles E. Wyman, Laurene Tetard, Arthur J. Ragauskas, Jeremy C. Smith, Loukas Petridis, Charles M. Cai (2019) “A Multifunctional Cosolvent Pair Reveals Molecular Principles of Biomass Deconstruction” J. Am. Chem. Soc. doi: 10.1021/jacs.8b10242