Chemists in the U.S. say they have broken new ground in biofuels, transforming plant sugar into a liquid fuel that packs 40% more energy than ethanol and appears to have fewer of its drawbacks.
Ethanol (ethyl alcohol) is currently the only car fuel made in large quantities from biomass. Biomass sources such as corn, sugarcane and other plants are rich in potential energy, in the form of large chains of carbohydrates. In their plant form, these long sugary molecules comprise six carbon atoms and six oxygen atoms. But car engines like a leaner form of carbohydrate molecule -- one with between five and 15 carbon atoms and with very little oxygen.
Most ethanol facilities harness biology, using enzymes to break down starch and cellulose to glucose, which is then fermented by a common yeast, Saccharomyces cerevisiae, to produce ethanol and carbon dioxide. But the process takes days and the fuel still has relatively high levels of oxygen, which reduces its energy density, makes it evaporate readily and leaves it liable to water contamination by absorbing atmospheric humidity. Distillation is needed to separate the fuel from water, and this process is energy-intensive.
Biofuel engineers at the University of Wisconsin at Madison believe they have found the answer in a process that results in 2,5-demethylfuran, or DMF, which volume for volume delivers 40% more energy than ethanol. In addition, it is not soluble in water and is stable in storage. Under the process, reported on last week in the British journal Nature, enzymes rearrange the plant carbohydrates into a highly oxygenated sugar, fructose. The next step is to turn the fructose into an intermediate chemical, hydroxymethylfurfural, or HMF, by using an acid catalyst and a solvent with a low boiling point. This expels three oxygen atoms. Under the final phase, the HMF is converted to DMF by exposing it to a copper-ruthenium catalyst that kicks out two more oxygen atoms and converts the gas to a liquid at a lower temperature, thus making it easier to use as a general fuel for transport.
More research is needed before the technology can be commercialized, according to lead researcher James Dumesic, a professor of chemical and biological engineering. "There are some challenges that we need to address, but this work shows that we can produce a liquid transportation fuel from biomass that has energy density comparable to petrol," he says.
Copyright Agence France-Presse, 2007