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.
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 room-temperature solution.
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 that goal.
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 a recent 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.