BERKELEY, CA --
The elusive goal of harnessing the vast
potential of one of the earth's most plentiful materials is another step closer
to realization. Using ultrafast spectroscopic techniques that provide
"stop-action" images within a trillionth of a second, scientists at the U.S. Deparment of
Energy's Lawrence Berkeley National Laboratory have obtained the first detailed
picture of an alkane-activation reaction at room temperature.
Alkanes are compounds of carbon and hydrogen atoms held together by
single bonds. The simplest and most abundant is methane, the primary
constituent of natural gas. Chemists have long coveted the use of alkanes as
environmentally benign feedstock for clean-burning fuels and a host of petrochemicals,
including plastics, solvents, synthetic fibers, and pharmaceutical drugs. The
problem has been that the bonds between an alkane's carbon and hydrogen atoms are
strong enough to render alkanes generally unreactive.
Enigmatic alkane-activation reactions that take place over billionths of a second have been recorded, as depicted here on the cover of Science magazine
In the early 1980s, Robert Bergman, a chemist in Berkeley Lab's
Chemical Sciences Division (CSD) and with the University of California at
Berkeley, led the discovery of a group of organometallic complexes --compounds
of metal atoms, such as iridium or rhodium, sandwiched between organic molecules with a
unique property. Upon irradiation with ultraviolet light, these
organometallics were shown to generate a reaction that is able to break the
carbon-hydrogen bonds in alkanes and insert metal atoms into the mix, creating
new, much more reactive carbon-metal-hydrogen compounds.
Since discovering this alkane-activating reaction, Bergman has been
working to better understand it with the ultimate aim of designing a catalytic
process that could be used in commercial operations. An obstacle has been that
the reaction takes place within 230 nanoseconds (billionths of a second). To
slow it down for detailed study, Bergman and Bradley Moore, another chemist who
also holds a joint Berkeley Lab-UC Berkeley appointment, conducted experiments
in liquefied noble gas solvents under extremely low temperatures.
Since these conditions are far removed from those that might be used in
a commercial process, Bergman sought a means of studying the alkane-activating
reaction under more realistic conditions. A new collaboration was established with Berkeley Lab scientists Charles Harris and Heinz Frei.
Harris provided a special time-resolved infra-red flash kinetics
spectrometer that operates on a femtosecond (millionths of a billionth)
time-scale. This enabled the researchers to irradiate alkanes and the
organometallic complexes with ultraviolet light and measure the kinetics at room
temperature with the compounds dissolved in a hydrocarbon solvent. Frei
supplied a Fourier transform infrared spectrometer (FTIR) that allowed the
researchers to monitor the carbon-hydrogen activation reaction in the
nanosecond regime. These powerul tools made it possible for the scientists to
directly establish the time-scale for alkane bond-activation in a
Says Bergman, "We now have a detailed picture of the activation
reaction. Structures of all the intermediates involved have been identified and
assigned, and energy barriers for each reaction step from solvation to
formation of the final alkyl hydride product have been estimated."
Says Harris, an expert in femtosecond studies, "The femtosecond
technique used in this alkane reaction study in combination with step-scan
infra-red spectroscopy should be applicable to many other problems associated
with the reactions of complex molecules."
While the results of this study do not represent a quantum leap toward
the goal of converting alkanes into chemically useful products, a good
"mechanistic understanding" of the alkane activation reaction brings science closer to
Collaborating with Bergman, Harris, and Frei on this project were
Matthew Asplund, Steven Bromberg, Tianquan Lian, Kenneth Kotz, Bruce McNamara,
Haw Yang, Jake Yeston, and M. Wilkens.
The research was reported as the cover story in the October 10, 1997 issue of the journal Science.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.