A ‘novel’ chemistry to make fuel from sugar

Nano-sized metal particles help convert plant sugar into conventional fuels, such as gasoline

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It’s not alchemy, but it might sound like it: a new way to transform sugars from plants into gasoline, diesel or even jet fuel by passing the sugars over exotic materials.

This chemical trick uses nano-sized particles to produce plant-based gasoline that can be used in existing vehicles in place of petroleum-based fuels. But because they would be made from corn, switchgrass or other plants — which absorb carbon dioxide as they grow — the fuels would emit less net carbon dioxide than normal gasoline.

“You have a conventional fuel that happens to be made from sustainable sources,” says James Dumesic, a chemical engineer at the University of Wisconsin–Madison who led the research, which appears online September 18 in Science.

Ethanol, the most widely used biofuel today, is harder to store and transport than gasoline. Pure ethanol is highly corrosive to rubber tubing and many metals, so the compound must be moved in stainless steel tanks instead of existing pipelines. And engines must be adapted to run on pure ethanol. “Those issues go away when you actually make gasoline or diesel,” Dumesic says.

Also, ethanol is made by fermenting plant sugars in large, microbe-filled vats for hours or days, much like brewing beer. The new process could be simpler because it does not require keeping microbes alive, and it can convert the sugar into fuel in a matter of minutes, the team reports.

Another method for making gasoline from plant sugar exists, but it requires very high temperatures and other energy-consuming steps, making the process inefficient. The new technique requires little energy input and can convert most of the energy in the sugar into useable forms.

While the process is not yet ready for large-scale production, Dumesic’s team was able to convert about 65 percent of the energy in the sugar into gasoline using their laboratory-scale process. Most of the lost energy ends up in gases such as ethane and propane, which if captured could serve as a replacement for natural gas.

An alloy of the precious metals platinum and rhenium triggers the first step of the conversion. Dumesic and his colleagues deposited 2-nanometer-wide specks of this alloy onto surfaces made of pure carbon. When a liquid mixture of water and plant sugar flows over the platinum-rhenium particles at the right temperature and pressure, the metal atoms act as catalysts to cleave chemical bonds in the sugar, releasing oxygen and leaving behind a mixture of molecules containing carbon and hydrogen — the principal elements in gasoline and diesel.

“It’s completely novel chemistry,” comments Manos Mavrikakis, an expert in theoretical catalysis at the University of Wisconsin–Madison who did not collaborate with Dumesic on the new conversion process.

The molecules produced by Dumesic’s catalytic reactions can be used directly to replace petroleum feedstocks that the chemical industry uses to make plastics and other materials. Or, the molecules can pass through another step of previously known catalytic reactions to produce the final fuel.

Cost of the metal catalyst could be an issue, Mavrikakis notes. “The question is how much platinum and rhenium will we need to produce the fuel we need?” he says. “These are among the most expensive metals.”

Studying how the metals trigger the needed chemical reactions could enable scientists to replace the platinum and rhenium with less expensive materials, Mavrikakis suggests.

The other issue is where to obtain the sugar, which for these experiments was the compound sorbitol.

“Using the name ‘biomass’ for simple compounds like … sorbitol is misleading and may raise false hopes,” comments Stefan Czernik, senior scientist at the National Bioenergy Center of the National Renewable Energy Laboratory in Golden, Colo. “They can be obtained from biomass but this is by no means simple and inexpensive.”

Deriving sugar from corn or other food crops ties the food commodity market to the energy market, which has contributed to the global rise in food costs. Using the fibrous stalks and leaves of plants instead could at least partially separate the two markets, and the whole biofuels industry is moving toward using these “cellulosic” plant sources instead of corn. But breaking cellulose down into simple sugars is proving not to be a simple matter.

“We would just intercept the sugar and go to gasoline,” Dumesic says, “but there’s still a lot of work to do on how to go from cellulose to sugar.”

UWM holds a patent on the new technique, Dumesic says.

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