Jul
17
Visualizing the Cellulose Change to Sugar
July 17, 2009 | 5 Comments
Researchers at the Joint BioEnergy Institute (JBEI), a US Department of Energy Bioenergy Research Center led by the Lawrence Berkeley National Laboratory have developed a visualization technique, based on the natural auto-fluorescence of plant cell walls. The technique enables researchers for the first time to visualize dynamically the solubilization (dissolving) during an ionic liquid pretreatment of a biomass sample. The study used pristine switchgrass, Panicum virgatum.
The use of ionic liquids – salts that are liquids rather than crystals at room temperature – to dissolve lignocellulose and later help hydrolyze the resulting liquid into sugars, shows promise as a way of pre-treating biomass for a more efficient conversion into fuels. But, the best ionic liquids in terms of effectiveness are also prohibitively expensive for use at commercial scale. The Lawrence Berkley Story author even makes clear, “scientists know little beyond the fact that ionic liquids do work.” So the new technology of seeing the breakdown of the cellulose has real meaning because understanding how ionic liquids are able to dissolve lignocellulosic biomass should pave the way for finding new and better varieties for use in biofuels.
Auto Fluorescence View of EmimAc Solubilization. Click image for more info.
The new technique is based on the natural auto-fluorescence of plant cell walls, so enabling researchers for the first time to dynamically track the solubilization during an ionic liquid pretreatment of a biomass sample, and to accurately and quickly assess the liquid’s performance without the need of labor-intensive and time-consuming chemical and immunological labeling. Imagine the time and personnel that would be involved to get a clear idea of what’s going on.
The technique is successful with the switchgrass using the ionic liquid known as EmimAc (1-n-ethyl-3-methylimidazolium acetate), which is currently the most effective solvent known to date in terms of pre-treating biomass. The researchers observed a rapid swelling of the secondary plant cell walls within ten minutes of exposure at the relatively mild temperature of 120º Celsius, not so much over the boiling point of water. Blake Simmons, Vice President of JBEI’s Deconstruction Division and the principal investigator for this research says, “We attributed the swelling to disruption of inter- and intra-molecular hydrogen bonding between cellulose fibrils and lignin. The swelling was followed by complete dissolution of biomass over three hours. This is the first study to show the process by which biomass solubilization occurs in an ionic liquid pre-treatment using these techniques.” It’s visualized now, which should enlighten the human minds looking for better solutions.
Once the EmimAc had dissolved the switchgrass into its three components – cellulose and hemicellulose sugars, plus the lignin, (woody fiber that gives strength and structure to plant cell walls) the step of adding of an anti-solvent, such as water, resulted in the sugars being precipitated out while most of the lignin remained in solution, a requirement for recovering the sugars separately. This confirmed that the ionic liquid pre-treatment effectively disrupted the resistance to dissolving of the switchgrass and helped liberate the fermentable sugars.
Simmons says, “In comparison to untreated biomass, ionic liquid pretreated biomass produces cellulose that is efficiently hydrolyzed with commercial cellulase cocktail and provides sugar yields over a relatively short time interval. We are now in the process of evaluating other ionic liquids to discover the optimal combination of cost and performance.”
The study results were reported in the journal Biotechnology and Bioengeering in a paper entitled: “Visualization of Biomass Solubilization and Cellulose Regeneration During Ionic Liquid Pretreatment of Switchgrass” Co-authoring the paper with Simmons were his JBEI colleague Seema Singh, and Kenneth Vogel, of the U.S. Department of Agriculture’s Agricultural Research Service, located in Lincoln, NE. Simmons and Singh also hold appointments with Sandia National Laboratories.
The “feature story” at the Lawrence Berkley National Laboratory gets into the details of the technique with a couple of paragraphs. But what matters here is as Simmons says, “The ultimate goal is to find an ionic liquid that can efficiently pre-treat biomass, then scale its use up into a cost-effective process for biorefineries.” Ideally, he and his colleagues would like to identify a single versatile ionic liquid that is capable of producing enriched polysaccharide and lignin output streams irrespective of feedstock and fuel types. That goal points to much more basic research ahead.
Meanwhile Simmons says, “Right now ionic liquids are a bench-top technique, and there are research and engineering obstacles that must be solved before this technology is ready for prime time. But the drivers are clear, and ionic liquids offer processing advantages that no other current commercial pre-treatment technology can provide.”
Exactly, which is just why the biological path to biomass to fuel is so tedious. Cellulose and lignins are billion-year-old evolved designs that are meant to keep plants together and do it well. The notion they are going to be easy to dissolve or deconstruct in a nice, quick and low effort process is a idea worth pursuing, but its going to take a lot of innovation, ingenuity and persistence to get there.
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