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Unraveling the science behind biomass breakdown

The deconstruction of lignocellulose is one of the most complex processes in bioenergy technologies. Although researchers at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) had already uncovered information about how woody plants and waste biomass can be converted into biofuel more easily, they have now discovered the chemical details behind that process.

A researcher team led by Jeremy Smith, a University of Tennessee (UT)–ORNL Governor’s Chair and the director of the UT–ORNL Center for Molecular Biophysics (CMB), used computer simulations to investigate the chemistry of biomass deconstruction. Smith’s collaborators Charles Cai and Charles Wyman at the University of California Riverside and in the BioEnergy Science Center, a DOE Bioenergy Research Center led by ORNL, previously developed a pretreatment method for breaking down biomass that initiates lignin removal, delignification.

The co-solvent enhanced lignocellulose fractionation pretreatment involves aqueous solutions of tetrahydrofuran (THF), a versatile organic solvent. This co-solvent mixture uniquely interacts with cellulose to enable its breakdown, which is essential for its conversion into ethanol. A better understanding of the process of breaking down cellulose will enable improvements in the current pretreatment method or lead to finding new solvents more efficiently.

Super-computing

In a project through the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) programme, the researchers created models of up to 330 000 atoms and ran simulations on a supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility earlier this year. The results, “Local Phase Separation of Co-solvents Enhances Pretreatment of Biomass for Bioenergy Applications” have been published in Journal of the American Chemical Society.

They found that the THF–water co-solvent phase separates on the faces of the crystalline cellulose fibre. These faces are distinct regions with which certain enzymes or molecules can interact. During the phase separation, THF preferentially binds to the hydrophobic faces of cellulose while water binds to the hydrophilic faces. THF enhances the binding of water molecules to the bonds that link two sugar molecules, which can potentially increase hydrolysis.

An illustration that demonstrates how THF (orange) and water (blue) phase separate on the surface of cellulose (green), thus facilitating its breakdown (image courtesy Barmak Mostofian).

An illustration that demonstrates how THF (orange) and water (blue) phase separate on the surface of cellulose (green), thus facilitating its breakdown (image courtesy Barmak Mostofian).

– We saw this phase separation, and we knew it might mean there was chemistry that was taking place on the surface that we hadn’t observed, that we hadn’t considered before, said Micholas Smith, another CMB postdoctoral researcher.

The team also found that when they broke the cellulose apart, single chains of cellulose became surrounded primarily by water, while THF—because of its molecular structure—remained bound to the hydrophobic surfaces of cellulose.

Identification and deconstruction pathway

With this new higher-resolution understanding of the chemical properties behind lignocellulosic deconstruction, researchers say that it will help them identify new co-solvents in the future.

– This information will help us find the minimum number of things we need to calculate to tell if a solvent is good for lignocellulose. Hopefully, we will eventually be able to write a programme that creates a better screening process for solvents and automatically selects the best ones, Micholas Smith said.

The finding also represents a first step toward determining the full pathway the THF–water co-solvent takes to break down cellulose.

– Now we are limited to looking at the two end states of the cellulose deconstruction process. If we can map out the full pathway, that will be more relevant. In the future, with more computing power, we will be able to simulate the degradation pathway of the lignocellulosic biomass to understand what happens between the two endpoints, said CMB researcher Xiaolin Cheng.

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