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"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. |
