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Making Diesel from CO2 and Sunlight

A new program will develop novel approaches to renewable fuels.
December 10, 2009

A new “electrofuels” program announced this week by the Advanced Research Projects Agency-Energy (Arpa-e) will fund research into novel ways to make renewable fuels. The approaches could prove more efficient than using photosynthetic organisms–such as plants and algae–to make biofuels. And instead of making ethanol, they will make fuels such as diesel, which could be easily distributed and sold with existing infrastructure.

Metal eater: Metallosphaera sedula can draw energy from a copper-iron sulfide called chalcopyrite, the black substance shown here. As it feeds, it produces copper ions (green), iron oxide (orange), and sulfur (yellow). The organism uses the energy from the sulfides to produce acetyl-CoA, a fundamental building block in cells. Researchers have been able to engineer organisms to convert acetyl-CoA into butanol and other liquid fuels.

The idea is to make liquid fuels using organisms that can convert carbon dioxide into organic molecules without using photosynthesis, says Arun Majumdar, the director of Arpa-e. The mechanisms these organisms use could be more efficient than photosynthesis, he says.

Photosynthetic approaches fall into two categories. The most common is to grow plants and then use yeasts or other microorganisms to ferment sugars from the plants to produce fuels–as in converting corn into ethanol. The other approach is to use photosynthetic organisms to make fuels more directly. This might involve using algae that make oils that can then be processed into biodiesel, or even experimenting with microorganisms that make gasoline or other hydrocarbons directly.

One nonphotosynthetic approach Arpa-e may fund is to pair organisms with solar cells. Recent research has suggested that some microorganisms can use electricity to form organic molecules, such as methane, says Bruce Logan, a professor of environmental engineering at the Pennsylvania State University. These microorganisms could be further engineered to make liquid fuels.

A second nonphotosynthetic approach involves microorganisms such as extremophiles. These microorganisms live in extreme environments, such as hot and acidic water. Unlike plants and algae, these organisms don’t use light as an energy source, since they live in dark environments. They also don’t draw energy from organic molecules, such as sugars, because those aren’t available to them. Instead they draw energy from other sources, such as metal sulfides. They also don’t use organic matter for making proteins and lipids, instead using inorganic molecules, such as carbon dioxide.

These organisms have long been studied for the enzymes they produce, since these enzymes can survive at high temperatures, and thus could be useful for industrial processes. But in recent years, researchers’ access to the genomes of organisms has allowed them to identify entire metabolic pathways–series of reactions undertaken by organisms–for converting carbon dioxide into various organic molecules. It may be possible to modify these pathways so that these organisms produce fuels.

One such organism is called Metallosphaera sedula, says Robert Kelly, director of the North Carolina State biotechnology program. It’s often found in mines. “It oxidizes the metal sulfide, and that’s how it gets energy and electrons for cellular processes,” he says. These organisms produce organic molecules that have been used to make butanol and other liquid fuels.

Numerous challenges exist to making a feasible process of the biochemistry. But Eric Toone, the project director in charge of the electrofuels program, thinks it’s worth trying, because photosynthesis is a very inefficient way to make fuels. “Is this going to be more efficient than photosynthesis? I have no idea,” he says. “But the only way we find out is to try. There’s a lot of ‘white space’ here. This is not an area that has been investigated as a possible route toward biofuels. And this is what Arpa-e is supposed to do–try new things.”

Arpa-e’s electrofuels program will not be limited to approaches that use organisms, Majumdar says. The key is to find alternatives to photosynthesis. Another approach is to use solar energy and inorganic catalysts to make fuel from water and carbon dioxide, an approach called artificial photosynthesis.

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