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Making alternative fuel becomes more efficient with dual-catalyst system: UNC-Rutgers study

April 14, 2006

CHAPEL HILL - As the United States' oil reserves dwindle, some say the nation will have to rely on synthetic petroleum fuel made from its large stores of coal.

A two-step chemical process augments a method of making cleaner-burning alternative fuel from coal and other carbon sources by transforming some of its waste products into diesel fuel, researchers from the University of North Carolina at Chapel Hill and Rutgers, the State University of New Jersey, report.

"Two percent of the United States' energy reserves is in oil, 3 percent is in gas, and 95 percent is in coal," said Dr. Maurice Brookhart, W.R. Kenan Jr. professor of chemistry in UNC's College of Arts and Sciences. "Many people in the energy sector think that when oil starts to run out, coal will be a source of transportation fuel for some time before we perfect the science behind solar and hydrogen-based energy. Producing diesel fuels from coal is especially attractive since diesel engines are more efficient than gasoline engines."

The Fischer-Tropsch method of making synthetic liquid fuels from coal and other carbon sources has been used since the 1920s. Today, Fischer-Tropsch fuels power most large vehicles in South Africa, and American companies have expressed interest in these fuels, which emit fewer particulates and less carbon monoxide than conventional diesel fuels. Such fuels have been termed "green diesel."

The cost of making Fischer-Tropsch fuels has been considered prohibitive. "But right now, with oil this expensive, I think it will soon become a competitive process to make liquid fuels," said Brookhart, an author of the study, which is published Friday (April 14) in the journal Science.

Dr. Alan S. Goldman, professor of chemistry and chemical biology at Rutgers, is the lead author of the study.

The Fischer-Tropsch reaction creates hydrocarbon compounds called alkanes. Methane and ethane are examples of alkanes. Some of the alkanes created by Fischer-Tropsch are desirable for use as fuel, but others have low molecular weights that make them unsuitable.

"The process we have developed allows one to convert more of these Fischer-Tropsch materials to usable diesel fuels," said Brookhart.

"It's accomplished by a dual-catalyst system that allows us to take low molecular weight alkanes with between four and nine carbons in the chain and boost their weights up to a range appropriate for diesel fuel (10 to 19 carbons)," he added.

Brookhart said that in the dual-catalyst system, "one catalyst removes hydrogen, converting the alkane to a new material that contains carbon-carbon double bonds." Those double bonds make the new material more reactive, he added.

Then a second catalyst "scrambles" the carbon bonds, creating compounds with higher molecular weights. The first catalyst then returns the hydrogen atoms to the rearranged compounds, yielding alkanes that are usable as fuel.

Currently, a process termed hydrocracking is used to break down hydrocarbons with molecular weights too high for fuel use into lower molecular weight materials, but the process is not very selective.

"The catalyst system we used can combine very low molecular weight and very high molecular weight alkanes to produce alkanes in the diesel fuel range and, thus, may also prove useful for recovering value from high molecular weight materials," Brookhart said.

The investigations are in the early stages, and Brookhart added that "considerable improvements in the catalyst systems are required before they become practical. We are working hard on that."

The other authors of the study, in addition to Brookhart and Goldman, are Rutgers postdoctoral research associate Dr. Ritu Ahuja; postdoctoral research associate Dr. Amy H. Roy and research assistant Dr. Zheng Huang, both of UNC's department of chemistry; and Dr. William Schinski, a chemist with Chevron Research and Technology Company.

The research was sponsored by a grant from the National Science Foundation Center for the Activation and Transformation of Strong Bonds (CATSB), of which Brookhart and Goldman are members.

University of North Carolina at Chapel Hill