Cellulose can be mighty tricky.
Cellulose is very crystalline,"" said Joe Binder, a doctoral student at UW-Madison working on a new method of breaking down cellulose. The most abundant organic molecule on earth, cellulose has been targeted as a source of energy because of its ubiquitous nature.
Most biofuels today are derived from easily transformable materials. Biodiesel, for instance, has corn kernels, cane sugar, palm oil and soy beans as ingredients. But these sources only make up a small fraction of plant matter.
The United States produced about nine billion gallons of ethanol in 2008, and ethanol producers expect the demand to increase with the passage of the 2007 Energy Independence and Security Act. It calls for an increase of 36 million gallons of biofuels added to gasoline by 2022.
Only 15 million gallons can be provided by traditional materials. The rest will need to come from cellulose.
But the cellulose needs to be broken apart.
""If you think of cellulose, you can think of cotton balls. They don't dissolve in water, they're really tough,"" Binder said.
""On top of that, the cellulose is wrapped in other polymers called hemicelluloses and lignin, which really stick it together and really glue it and make it tough to get at,"" he said.
Plants use cellulose to build their cell walls; it provides structure and support, so it has evolved to resist efforts to break it down. Lignin, in particular, has long vexed researchers, as it essentially creates a wall between the cellulose and any potential solvents.
""We worked out a way to convert the biomass in one step into a chemical precursor that you could use to make fuels and chemicals,"" said Binder, who worked with UW-Madison biochemistry and chemical engineering professor Ronald Raines. The research appeared in a recent issue of the Journal of the American Chemical Society.
""The reason cellulose is so hard to dissolve is that the molecules in the cellulose chain are bound to each other by what are called hydrogen bonds. Those hydrogen bonds are very sticky,"" Binder said.
""Typical solvents can't break the hydrogen bonds. But we used special solvents that can make even stronger hydrogen bonds to the cellulose than the cellulose can make by itself,"" he said.
The chemicals Binder and Raines used are small enough to fit between lignin molecules and cellulose molecules.
""These solvent molecules wiggle in there and break up the hydrogen bonds between the cellulose chains, which allows the cellulose to dissolve,"" Binder said.
The solvent dissolves these cotton balls, 90 percent cellulose, into a clear, slightly viscous liquid. The team of researchers has also tested its method on corn stover, a common source of biomass in Wisconsin, and pine sawdust. Broken down, they resemble molasses.
This method requires no pretreatment of the biomass. The resulting molecule at the end of this first step is abbreviated as HMF.
""It's a molecule you can imagine converting into a lot of different molecules, which could then be used to make plastics, fuels or all these different things that we need for modern-day life,"" Binder said.
""And it could be a replacement for the stuff that we get petroleum from today. So then we would have bio-based plastics and bio-based fuels,"" he said.
Breaking down the cellulose into HMF is the first step. In the second step, HMF is converted to another compound, DMF. This would serve as an alternative to gasoline.
According to Raines, it has the same energy content as gasoline and is already in use as a fuel additive. Binder also noted that with a boiling point higher than ethanol's, DMF won't evaporate at the filling station, nor will it absorb water.
""Our process is so general I think we can make DMF or HMF out of any type of biomass,"" Raines said in a UW-Madison news release.
In a test case with corn stover, 9 percent of the cellulose was converted to DMF.
""That was just to demonstrate that we could do it from corn stover,"" Binder said.
""If we really worked on optimizing all the steps with the way we know how to do it right now, we could certainly get three or four times that yield, easily. And with further process improvements we could certainly boost those yields even further,"" Binder said.
Further process development is definitely still needed. One of the solvents in the cellulose-to-HMF conversion is chromium, a toxic metal.
The team also needs to develop a method for recycling the solvents. Still, they've already come a long way toward creating a viable source of energy.
Continued research, at the Great Lakes Bioenergy Center, may some day lead to Wisconsinites filling up their cars with a fuel developed in their own state.
""It's really cool, all the different options people are coming up with,"" Binder said.
