Dynamic Duo: Two catalysts build valuable carbon chains

By combining the power of two well-known reactions, chemists have devised a way to alter the length of carbon chains. The process might someday convert less-valuable carbon chains into a transportation fuel, the researchers say.

As oil supplies shrink, chemical processes that turn coal or biomass such as corn into liquid hydrocarbons will become more important, says chemist Maurice Brookhart of the University of North Carolina at Chapel Hill. Of particular interest are linear alkanes, chains in which single bonds connect carbon atoms and hydrogen atoms fill out the molecules. Diesel engines, for example, run most efficiently on alkanes with 9 to 20 carbons per molecule.

The reaction that converts coal and biomass to alkanes, however, produces carbon chains of many lengths. Included in the mix are alkanes with four to seven carbons, lengths that can’t be used as fuel, says Brookhart.

Brookhart, Alan S. Goldman of Rutgers University in Piscataway, N.J., and their research teams used two catalysts to promote reactions that together reclaimed the short alkanes. The first reaction removes two hydrogen atoms from an alkane, creating a double bond between two of the molecule’s carbons. The second reaction induces two molecules to exchange chain portions from either side of the double-bonded carbons. Then, the first reaction’s catalyst returns the hydrogen atoms, eliminating the double bond.

The reactions convert a starting short alkane into products with two lengths. For example, two 6-carbon-long alkanes—hexanes—would become a 10-carbon alkane—decane—and a 2-carbon alkane—ethane. “Then, you are in great shape: You’ve got the diesel fuel, and you’ve got the ethane,” a gas that can be used as heating fuel, says Goldman. The team describes its work in the April 14 Science.

“It’s a spectacularly clever use of those two reactions,” says John F. Hartwig of Yale University.

But the process is far from ready for industrial applications, Brookhart notes. For example, the number of reactions that each catalyst molecule can perform before becoming unstable must increase from about 1,000 to several million.

The reactions’ selectivity isn’t optimal either, Brookhart says. The catalysts also convert alkanes of desired lengths into other lengths. “What we would really like is, from hexane, to get only ethane and decane,” says Brookhart. However, he notes that with the current procedure, chemists could put the unwanted alkanes “back in the pot” to cycle through the reactions again.

The group is now testing other catalysts for its system.

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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