Beyond Ethanol

Synthetic fuel offers promising alternative

By Davide Castelvecchi
From Science News, June 23rd, 2007; Vol.171 #25 (p. 389)

DMF may not yet be a household name, but thanks to newly improved production processes, this biofuel may some day prove even better than ethanol as a sustainable alternative to gasoline.

Plant-derived biofuels such as ethanol offer renewable-energy alternatives to fossil fuels. Their use could help reduce carbon emissions and greenhouse-gas generation.

Currently, the global leader in biofuel production is Brazil, which focuses on converting energy-rich sugarcane into ethanol. Meanwhile, corn-to-ethanol plants, which are less efficient, are popping up across the midwestern United States.

Even ethanol from sugarcane, however, is far from ideal. A gallon of ethanol yields less energy than a gallon of gasoline, resulting in lower automobile mileage. Moreover, producing ethanol from sugar is slow and inefficient. As in wine and beer making, ethanol plants use yeast fermentation to brew the fuel from sugary liquids. Unfortunately, this process can take several days.

Chemists have long known about a synthetic alternative fuel called 2,5-dimethylfuran, or DMF. Like ethanol, DMF can be derived from sugars, but it has a 40 percent higher energy density, says James Dumesic, a chemical engineer at the University of Wisconsin–Madison. That means cars could get at least as many miles per gallon of DMF as they get from gasoline.

Until now, DMF production has largely been confined to the lab. Typically, acidic catalysts are used to strip oxygen off either of two types of sugar, glucose or fructose, producing an intermediate compound known as 5-hydroxymethylfurfural (HMF). Extracting the HMF has been cumbersome, however. “If you leave it in water, it tends to react with the sugar that’s still there,” Dumesic says.

Now, Dumesic and his colleagues have developed a technology that could extract HMF efficiently enough to make DMF an attractive alternative to ethanol.

The team describes its two-step process in the June 21 Nature. First, the researchers added salt along with the acidic catalyst to a solution of sugars in water. They then exposed the watery mix to a second fluid, a hydrocarbon. The HMF quickly transferred into the hydrocarbon, in which it dissolved more readily than in water. Crucially, the presence of salt sped up the process by making the water more hostile to the HMF.

In the second step, the scientists mixed HMF with hydrogen and a copper-ruthenium-based catalyst. The hydrogen stripped two more oxygen atoms off HMF, producing water and DMF.

It should be relatively easy to apply the new technique on an industrial scale, says Dumesic. Before DMF can be mass-produced, however, further research is needed to explore its toxicity and potential environmental impact, he adds.

The new process is an “interesting piece of technology,” says Lanny Schmidt of the University of Minnesota in Minneapolis. Using catalysts rather than fermentation could dramatically speed and simplify the conversion process, resulting in lower costs, he notes. Conrad Zhang of the Pacific Northwest National Laboratory in Richland, Wash., calls the technique “an important step toward a usable, liquid transportation fuel.”

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