A renewable route to green gasoline
Because they are more attractive to the petroleum industry, "next-generation" biofuels produced from renewable feedstocks may have a brighter future than ethanol.
You can't get it at the pump yet, but a start-up company called Virent Energy Systems, Inc., has found a way to produce gasoline from renewable biomass that "effectively has the same composition [as] standard, unleaded gasoline" made from petroleum, says Mary Blanchard, the company's marketing director. The company expects to commercialize the new "green gasoline," which Blanchard says will cost less than ethanol, within 5 years. It is also working on "green" versions of diesel and jet fuel.
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The promise of Virent's biofuels is one reason why John Regalbuto, director of the U.S. National Science Foundation's (NSF's) Catalysis and Biocatalysis Program, predicts that "next-generation hydrocarbon biofuels" will soon be major competitors to ethanol. Such biofuels can be produced from many of the same renewable materials as ethanol, and they appeal to the petroleum industry because they are compatible with existing infrastructure, according to a technology road map being developed by NSF and the U.S. Department of Energy.
These new biofuels are so similar to conventional petroleum fuel in part because they are being produced via catalytic processes similar to ones used in the petroleum industry, Regalbuto says. However, there are some key differences, notes Brent Shanks, a professor of chemical and biological engineering at Iowa State University who began his career in the oil industry.
"In the petroleum industry, you're basically taking simple molecules [like ethylene] and making them more complicated. With biomass, you start with complicated molecules [such as glucose] and make them simpler," Shanks explains. To Shanks and his peers, trying to find a way to produce the molecules used in conventional fuels from renewable materials such as switchgrass, wood chips, and the leaves and stalks of corn plants, called stover, constitutes a "really interesting problem."
The three main avenues being investigated for creating fuels from biomass via catalysis are liquid-phase processing, pyrolysis, and gasification. The advantage of the latter two routes is that they have the potential to break down the lignin found in the biomass feedstock. Lignin is "especially resistant to chemical or biological breakdown at low temperatures," and this currently hinders the production of lignocellulosic ethanol, as well as green gasoline, Regalbuto says.
For example, George Huber, an assistant professor of chemical engineering at the University of Massachusetts, has demonstrated what he calls a "one-step process to turn cellulose into the aromatic fraction of gasoline" via catalytic pyrolysis. The process operates in equipment similar to the fluidized catalytic crackers used in the petroleum industry, Regalbuto points out. The cellulose gets blasted into sugarlike fragments that can readily react in the zeolite-based catalyst to produce aromatics currently found in gasoline, in addition to CO2 and water, explain Huber and Regalbuto. Huber says that he hopes eventually to use lignocellulose from renewable materials such as waste wood products in the reaction.
Despite their reliance on not-yet-perfected methods for breaking down the lignin in biomass, the liquid-phase approaches for producing biofuels catalytically, including the ones being developed by Virent, are farthest along at this point. Last summer, James Dumesic and colleagues at the University of Wisconsin's chemical and biological engineering department published a letter in Nature (2007, 47, 982–985) describing how fructose derived from biomass could be used to catalytically produce 2,5-dimethylfuran, which can be used as a liquid transportation fuel. In the letter, the researchers point out that 2,5-dimethylfuran has a higher energy density than ethanol (i.e., vehicles will get higher mileage per gallon). They also stress that it is not soluble in water; some environmental scientists believe ethanol's solubility could allow it to pollute groundwater.
In the same month, Conrad Zhang and colleagues at Pacific Northwest National Laboratory's Institute for Interfacial Catalysis published an article in Science (2007, 316, 1587–1600) on a way to produce another biofuel for transportation, hydroxymethylfurfural, from glucose and fructose derived from biomass using an ionic liquid. Zhang says that he and his colleagues are currently investigating how to use what he calls "clean cellulose", which can be obtained from cotton or algae that contain no lignin, in the reaction.
Oil companies are interested in the new biofuels because the end product would work well with all of their existing infrastructure, including refineries, pipelines, delivery systems, and vehicles, Shanks says. The oils produced by these methods self-separate and the processes use less water, giving them an advantage, Regalbuto points out.
