One of the biggest challenges biofuel producers face is breaking down energy-containing plant material into simple sugars that can be fermented into fuel. It's particularly difficult to break down the tough cellulose in material like wood chips and switchgrass, which could otherwise produce more energy-efficient ethanol than corn.
A majority of ethanol producers use strong chemicals or heat to dissolve the plant material. But molten salts -- also called ionic liquids -- may provide a better alternative, researchers said at an Australian symposium on ionic liquids. The liquids are made up of highly charged atoms called ions, and the forces exerted by those ions make the liquid an ideal solvent.
"Ionic liquids are the enabling technology to 'crack' biomass efficiently and economically," said Robin Rogers, a chemistry professor at the University of Alabama. "This is really the key to any biomass product."
Scientists have known about the solvent properties of ionic liquids for decades. But only in the past few years have the liquids begun to move from the laboratory to factories and processing plants for use in processes such as textile and paper manufacturing.
Chemical company BASF owns a patent on an ionic liquid-based cellulose processing method developed by Rogers, and is trying to apply that method on a commercial scale. It's already working with AlterVia Fuels, an early stage technology development company, on a smaller-scale version.
Mark Lenhart, COO of AlterVia Fuels said, "By using ionic liquids we have a lower energy input and can use the existing infrastructure [of ethanol refineries], and use fewer raw materials to produce biofuel."
Current chemical solvents generate waste and can take 48 hours to separate the cellulose. Unlike ionic liquids, they cannot be reused, so refineries must continually repurchase them. They also result in ethanol containing water that must be removed, according to Matthias Maase, manager of business development at BASF. The new method works faster, doesn't generate toxic waste and produces a purer ethanol with less water, he said.
"What we have observed about ionic liquids is that they grab the water and release the ethanol, which results in a purer ethanol, an estimated 20 percent gain in purity," Maase said. He estimated that if a biofuel refinery produces 10,000 gallons of ethanol a day they could increase capacity to 12,500 gallons with this new technology.
But not everyone believes that ionic liquids will be the answer to most ethanol producers needs. At the moment, they are more expensive to purchase than the chemicals already used to dissolve cellulose, and they cannot replace other costly chemicals needed in the biofuel refining process.
Although ionic liquids are generally considered less toxic than traditional solvents, some researchers also question their environmental benefits. Imidazolium salts -- the main ingredients in ionic liquids -- do not evaporate, meaning they create no air pollutants, but there is still research being done on their toxicity in aqueous environments, said Ziding Zhang of China Agricultural University.
"In the area of cellulose for biofuel, the ionic liquids used are still within the traditional ionic liquid family, i.e. composed of mostly by imidazolium derivatives that have minor-to-moderate toxicities to the environment and human beings," he said.
Peter Scammells, a chemistry professor at Monash University, has also questioned the green properties of ionic liquids. In a 2005 article, he pointed out that there are still problems with recycling ionic liquids, and maintaining their purity and effectiveness.
Meanwhile, laboratory research on the liquids continues. Scientists at the Joint Bioenergy Institute see ionic liquids as a next step in producing ethanol from wood and grasses -- materials that are much more abundant and cheaper than corn.
"Every process in development goes through a few roadblocks. Corn may not be the best solution for biofuels. I'm very confident that we will be able to produce fuel from biomass with lower carbon emissions associated with production when compared to oil petroleum," Blake Simmons says, a scientist and researcher at Joint Bioenergy Institute.
And the University of Alabama's Rogers believes the technology could be useful in producing other biofuels, such as butanol. "Ionic liquids will allow the cracking of biomass in an economical manner for a number of new businesses. The focus on ethanol is shortsighted," he said.