Biofuel repertoire expanded
New route turns crop sugars into a fuel that beats ethanol.
A biofuel that outperforms ethanol could be easily made from fructose and, in future, glucose derived from the woody parts of plants, researchers claim.
Ethanol has been the biofuel of choice for many years and can be made from corn and other crops. Enzymes are used to break down the plant material to sugar, which is fermented to a boozy fuel. But ethanol has a number of problems, not least its low energy density, its volatility and its water-absorbing nature.
A potential alternative is 2,5-dimethylfuran (DMF) with a 40% higher energy density, a boiling point 20 ºC higher than ethanol and a dislike for water. But DMF has proven hard to make economically from crops and their sugars.
Until now that is. James Dumesic of the University of Wisconsin-Madison and his colleagues have cracked the key step of turning a sugar into the intermediate compound 5-hydroxymethylfurfural (HMF), and then reacting this further to make DMF.
The HMF-creating step involves dehydrating the sugar, and is catalysed by acid. The HMF is turned into DMF thanks to a ruthenium and copper catalyst system that drives the reaction by breaking a carbon-oxygen bond and adding hydrogen.
The process strips the sugar of its oxygen, which decreases the molecule's water solubility and increases its energy density, explains Dumesic. A fructose molecule has six oxygen atoms, HMF has three oxygens, and DMF has just one.
At the moment, Dumesic's process uses fructose as a feedstock, which comes most easily from starchy biomass. But he hopes to tweak the system to work on other sugars directly. "In the long term it would be better to use glucose," he says, which comes from lignocellulosic material — the fibrous parts of plants that normally get discarded.
Fast and simple
Dumesic's system is superior to both the energy-intensive thermochemical routes currently used for making synthetic diesel, and beats the time-consuming enzymatic ethanol-making process, says Paul Dauenhauer, a chemical engineer from the University of Minnesota, Minneapolis. Dumesic's catalytic system is fast, simple and elegant he says: "This new approach is exciting, because it expands the number of processing options and routes to biofuels."
Dumesic says the HMF-producing step has been the problem — and that a number of groups have been working on it for years. Now that step has been cracked, HMF looks like it is not only a precursor to a useful biofuel, DMF, but will also be a useful chemical intermediate across the chemicals industry, says Dumesic, particularly for making polymers including plastics and oils.
It will be a while before we can fill up our cars with DMF at our local petrol station, however. "Naturally, considerable fuel testing must be carried out before DMF could be used commercially," says Dauenhauer.
- Román-Leshkov Y., et al. Nature, 447. 982 - 986 (2007).