The sugar from corn kernels can be used to make ethanol, an alternative to fossil fuels that can help reduce greenhouse gas emissions. However, since corn kernels are a major source of food for humans and livestock, many efforts are turning to the evaluation of corn stover — the stalk and leaves left over after the sweet ears have been removed — for its viability as a source of ethanol.

Experiments at two U.S. Department of Energy Office of Science x-ray light source user facilities, including the Advanced Photon Source at Argonne National Laboratory, provided some answers when researchers characterized corn stover after it was chemically treated in preparation for ethanol production. This treatment is intended to fragment cellulose fibrils —polysaccharide chains in corn stover cells which bond together — into individual chains from which glucose molecules can be obtained. The efficacy of these treatments determines the amount of glucose available for conversion to ethanol. These new results show that treatments that break fibrils into fragments yield higher amounts of sugar, and are preferable to those that only remove non-sugar molecules from the fibrils' surfaces to make the glucose molecules more accessible.

The sugar in corn stover comes from its cellulose, the main structural component forming plant cell walls. Cellulose is a long, linear chain of glucose molecules. Each polysaccharide chain bonds side-to-side with other chains into stiff, stable columns (fibrils). These connected chains form a crystalline lattice that breaks only when the strong hydrogen bonds — which hold the chains together as a fibril — are broken. Fibrils are further grouped into the columns that make up the plant's cell wall, with the orientation of the fibrils parallel to the column they comprise.

The process of producing ethanol begins by treating plant cell walls to weaken the hydrogen bonds between their polysaccharide chains in order to partially free individual glucose molecules. The treatment options include hot water, dilute sulfuric acid, and iron sulfate, both separately and in different combinations. The second step is hydrolysis: breaking the hydrogen bonds between chains. After pretreatment and hydrolysis, the glucose molecules are fermented to produce ethanol and carbon dioxide, which are distilled to separate out the ethanol.

To determine how the cellulose is affected by the pretreatments, the research team from Northeastern University, Brookhaven National Laboratory, Argonne National Laboratory, and Rensselaer Polytechnic Institute began by milling, drying, and steam-exploding the corn stover. They pretreated the corn stover, then measured the saccharine yield of each pretreated sample.  The dilute acid with iron pretreatment produced the most sugar, the acid-only treatment the second highest amount, and the hot water-only treatment the lowest amount.

Next, the team characterized the pretreated samples to determine the amount of cellulose degradation that resulted from each pretreatment. The figure shows the extent to which two pretreatments permeated the samples; the iron sulfate remains in the plant cell wall while the dilute acid spreads through the entire cell. Ultra-small angle x-ray scattering (USAXS) at beamline X9 of the U.S. Department of Energy’s National Synchrotron Light Source and scanning electron microscope data showed that none of the pretreatments affected the size of the cellulose fibrils but did change their texture and their orientation with respect to that of their wall column. The dilute acid changed the orientations of the fibrils so they no longer ran parallel to the column. Using wide-angle x-ray scattering (WAXS) at the National Institute of General Medical Sciences and National Cancer Institute (GM/CA-XSD) 23-ID-B x-ray beamline at the Advanced Photon Source, the team discovered that the dilute acid pretreatment partially broke down fibrils into groups of cellulose chains. The dilute acid with iron pretreatment degraded the cellulose further: it removed a sufficient amount of non-cellulose molecules to allow the remaining fibrils to coil around themselves, no longer orienting parallel to their column, and broke some fibrils into fragments.

Neither the pre-treatments of dilute acid nor the dilute acid with iron changed the radial extent of the crystalline lattice of cellulose within a fibril. The team concluded that the sugars produced after these pretreatment methods come from fibril fragments rather than the hydrolysis of the crystalline lattice within intact fibrils. They further concluded that the most effective pretreatments would break down the crystalline lattice within the corn stover's cellulose. These pretreatments would do more damage than the dilute acid with iron, ripping the lattice apart — although not exploding it like popcorn — and through that destruction make corn stover a more viable source of ethanol.— Mary Alexandra Agner

See: Hideyo Inouye¹, Yan Zhang¹, Lin Yang², Nagarajan Venugopalan³, Robert F. Fischetti³, S. Charlotte Gleber³, Stefan Vogt³, W. Fowle¹, Bryan Makowski⁴, Melvin Tucker⁵, Peter Ciesielski⁵, Bryon Donohoe⁵, James Matthews⁵, Michael E. Himmel⁵, and Lee Makowski^1,6, “Multiscale deconstruction of molecular architecture in corn stover,” Sci. Rep. 4, 3756 (2014). DOI: 10.1038/srep03756

Author affiliations: ¹Northeastern University, ²Brookhaven National Laboratory, ³Argonne National Laboratory, ⁴Rensselaer Polytechnic Institute, ⁵Renewable Energy Laboratory, ⁶Northeastern University

Correspondence: * [email protected]

This work was supported as part of the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio) led by Purdue University, an Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science, Basic Energy Sciences under Award No. DE-SC0000997. GM/CA-XSD has been funded in whole or in part with Federal funds from the National Cancer Institute (ACB-12002) and the National Institute of General Medical Sciences (AGM-12006). The National Synchrotron Light Source was supported by the U.S. Department of Energy Office of Science, Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Corn stalk photo on the home page by Charles Knowles, https://commons.wikimedia.org/wiki/File:Corn_stalks_at_sunset.jpg

Argonne National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.