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Autumn 2015

Getting More and Getting Better

Optimizing yields of sugars and high-value chemicals from plant biomass for biofuels

Kirsten Chojnicki & Ryan Patet

Delivering iron directly to cellulose. (A) Image of standard (EV) and ferritin-enhanced (FerIN(2)) poplar plants shows that their relative growth rates and outward appearances are unchanged. Cross sections of the (B) standard and (C) ferritin-enhanced poplar stems viewed under a stereomicroscope with Prussian blue dye reveal the differences in these plants, as an accumulation of the Prussian blue dye in the ferritin-enhanced poplar stem indicates an accumulation of iron in the stem.

The complex structure of lignin before it is broken down into its component sugar molecules. This complex structure is what makes it difficult for the cellulose enzyme to efficiently produce sugar without first breaking down the lignin.

You do not have to look very far to find people who are making the most out of what they have. Used vegetable oil from restaurants is processed into biodiesel to power vehicles and generators. Old sneakers are ground up and used as a soft alternative to gravel and sand on school playgrounds. Even our garbage is mined for value as microorganisms are used to break down the garbage and produce methane that is then burned in homes as a heat source or in power plants as a replacement for coal to generate electricity. It is no surprise, therefore, that researchers at the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio) would want to get more out of a fuel source such as biomass and get better at using that resource.

Biomass refers to any plant, in part or whole, that can be used as a source of fuel or chemicals. Burning trees for cooking or home heat has been used since the discovery of fire. A more modern example is using corn to produce ethanol fuel to power vehicles. In this process, starch from the corn grains is broken down into simple sugar molecules such as glucose before being biologically converted to a simple alcohol, ethanol, by microorganisms.

Current practices are somewhat inefficient in the total amount of plant carbon that is converted to fuels when compared to the total amount of biomass used as the source, a situation that researchers at C3Bio are working to change. Their approach addresses the issue by investigating how the plant itself is assembled at the molecular level, how to break down the structure using chemicals and heat (getting rid of the microbial middleman), and the subsequent molecular reassembly into fuels and high-value products. In two recent publications, C3Bio scientists discuss their advances in modifying plant DNA and making better use of non-food parts for fuel.

In Biomass and Bioenergy, C3Bio scientists describe how they increased the amount of extracted sugars (from non-food parts) by delivering iron directly to cellulose, the main component of plant cell walls, while the plants grow. Expressing a gene encoding ferritin, a protein that binds iron, Arabidopsis plants enhanced the accumulation of iron within the plant cell walls. Breaking down plants with more iron yields more sugar. When the ferritin-enriched plants were broken down with a dilute acid pre-treatment, they released 18 to 19 percent more glucose and 13 to 14 percent more xylose, another simple sugar molecule, than control plants that didn’t express ferritin. Increasing the amount of iron in plant biomass with the ferritin protein, rather than the current practices of adding iron externally after milling, also may reduce equipment costs, water usage, and other practical limitations of the biomass conversion process. C3Bio scientists are extending this new methodology to other plants, such as poplar trees, which would produce the larger amounts of biomass necessary for operational processes.

Another way in which scientists are attempting to optimize their yield of fuels from plants is by using the non-food portion of the plant. Returning to the example of corn-grain ethanol, the starch-based sugars are found only in the corn kernels leaving a large amount of the corn biomass, such as cobs, stems, and leaves, unused. In addition to cellulose, these woody, or non-food, parts of plants contain lignin (material that provides plant structure), which is difficult to break down into uniform products because of its branched and interconnected structure. The only major use of biomass lignin has been to burn it for process heat, which has comparatively limited value.

In the journal Green Chemistry, C3Bio researchers set out to make better use of this lignin. While sugars may be extracted from plant cellulose using enzymes, biological catalysts called cellulases found in cells that help to break down foods into chemicals, the extraction process is very inefficient. Lignin molecules limit access of the enzymes to the desired sugars. When intact poplar biomass was fed to cellulase enzymes, only 11 percent of the glucose and xylose sugars were collected. To improve that yield, C3Bio researchers used small particles (chemical catalysts) of carbon with zinc and palladium metal clusters to chemically break the lignin down into a liquid phase and a solid phase. When they fed the solid phase to the same cellulase enzyme, 95 percent of the possible sugars were extracted, eight times more than before!

They also demonstrated how the lignin portion could be converted to hydrocarbon fuels and high-value chemicals. This new methodology is exciting because it drastically increases the efficiency of cellulose conversion and uses a substantial amount of the previously low-value lignin to make high-value products. This process is the basis for the start-up company, Spero Energy, Inc.

It is clear that C3Bio scientists approach the problem of maximizing the use of biomass resources along multiple, interdisciplinary pathways. Getting more sugars from non-food, or lignocellulosic, plant biomass will advance process efficiencies. Getting better at biomass conversion of lignin will ultimately facilitate more efficient biorefineries. Diversifying biofuel production and reducing fossil fuel consumption will help secure and sustain energy production in the United States.

Acknowledgments

This work was supported as part of the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Parsell T, S Yohe, J Degenstein, T Jarrell, I Klein, E Gencer, B Hewetson, M Hurt, JI Kim, H Choudhari, B Saha, R Meilan, N Mosier, F Ribeiro, WN Delgass, C Chapple, HI Kenttämaa, R Agrawal and MM Abu-Omar. 2015. “A Synergistic Biorefinery Based on Catalytic Conversion of Lignin prior to Cellulose Starting from Lignocellulosic Biomass.” Green Chemistry 17:1492-1499. DOI: 10.1039/c4gc01911c

Wei H, H Yang, PN Ciesielski, BS Donohoe, MC McCann, AS Murphy, WA Peer, S-Y Ding, ME Himmel, and MP Tucke. 2015. “Transgenic Ferritin Overproduction Enhances Thermochemical Pretreatments in Arabidopsis.” Biomass and Bioenergy 72:55-64. DOI: 10.1016/j.biombioe.2014.11.022

About the author(s):

  • Kirsten Chojnicki is a postdoctoral appointee in Geomechanics at Sandia National Laboratories and a member of the Center for Frontiers of Subsurface Energy Security. She is an experimental geophysicist, specializing in the mechanics of geologic fluid flows. She has an M.S. and Ph.D. in geology from Arizona State University and a B.S. in earth and space sciences from the University of Washington. Before joining Sandia, she was a Scripps Postdoctoral Fellow in Geophysics and Planetary Physics at the Scripps Institution of Oceanography at the University of California, San Diego.

  • Ryan Patet is a Ph.D. candidate at the University of Delaware and is a student in the Catalysis Center for Energy Innovation. He is advised by Dion Vlachos and Stavros Caratzoulas. He is using computational tools to fundamentally understand zeolite catalyst effects in reactions used to produce fuels and chemicals from biomass. Ryan holds B.S. degrees in chemical engineering and chemistry from Purdue University.

More Information

Parsell T, S Yohe, J Degenstein, T Jarrell, I Klein, E Gencer, B Hewetson, M Hurt, JI Kim, H Choudhari, B Saha, R Meilan, N Mosier, F Ribeiro, WN Delgass, C Chapple, HI Kenttämaa, R Agrawal and MM Abu-Omar. 2015. “A Synergistic Biorefinery Based on Catalytic Conversion of Lignin prior to Cellulose Starting from Lignocellulosic Biomass.” Green Chemistry 17:1492-1499. DOI: 10.1039/c4gc01911c

Wei H, H Yang, PN Ciesielski, BS Donohoe, MC McCann, AS Murphy, WA Peer, S-Y Ding, ME Himmel, and MP Tucke. 2015. “Transgenic Ferritin Overproduction Enhances Thermochemical Pretreatments in Arabidopsis.” Biomass and Bioenergy 72:55-64. DOI: 10.1016/j.biombioe.2014.11.022

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.