Credit: Argonne National Laboratory
Chemists spend a lot of time and energy trying to get the chemical reactions to start or accelerate – but sometimes it can be just as important to stop them before they go too far.
In a recent study by the US Department of Energy's (DOE) National Argonne Laboratory, chemists have identified a way to convert cyclohexane to cyclohexene or cyclohexadiene, important chemicals in a wide range of industrial processes. It is important to note that this process occurs at low temperatures, eliminating the creation of carbon dioxide that would result from an unwanted breakdown of carbon-carbon bonds.
Cyclohexane is an important initial molecule in a wide range of chemical reactions, according to Argonne chemist Stefan Vajda, now at the J. Heyrovský Institute of Physical Chemistry in Prague. However, without a suitable catalyst to initiate the reaction, the conversion of cyclohexane into useful products typically requires high temperatures generated through the expenditure of a large amount of energy, and the process may also suffer from low selectivity.
In the study, Larry Curtiss, a chemist from Vajda and Argonne, and his international team of researchers examined a type of reaction called oxidative dehydrogenation, in which hydrogen molecules are removed from a larger molecule. By cutting a limited number of hydrogen-carbon bonds, the reaction can produce cyclohexene and cyclohexadiene before combustion occurs in carbon dioxide.
The work has improved in earlier studies by the Argonne team on the dehydrogenation of cyclohexane and cyclohexene by introducing two major components: a nanometer-sized cobalt oxide catalyst in an aluminum oxide carrier and a controlled oxygen environment.
The researchers employed X-ray scattering techniques in Argonne's Advanced Photon Source (APS), a DOE Office of Science user facility, to monitor the nature and stability of catalysts during catalytic testing of clusters in real-time. They found that the agglomerates performed the partial dehydrogenation of cyclohexane at temperatures close to 100 degrees Celsius – far below what had previously been observed for this type of reaction, and the agglomerates retained their oxidized nature and stability at reaction temperatures of up to 300Â ° W. .
"The fact that we can make this conversion happen at lower temperatures protects the intermediate dehydrogenation products, cyclohexene and cyclohexadiene from being converted into unwanted products," Vajda said.
Vajda and Curtiss have observed that the highly selective catalyst is long-lasting and is not poisoned or degraded by the reaction. In theoretical and experimental investigations of catalyst size, the researchers found that clusters of size four and twenty-seven atoms were equally efficient in the execution of the reaction. "It seems that since the catalyst is below about one nanometer in size, this composition works well – an important factor for the potential increase of this class of catalysts by more traditional routes of synthesis, though less selective in size." Vajda said.
To better understand the basic mechanisms behind the activity and selectivity of cobalt catalysts, the researchers used the calculations of the density functional theory to model the reaction pathways. "The excellent performance of cobalt clusters can be explained by theoretical calculations that reveal highly active cobalt atoms in the clusters and show that the oxidized nature of the clusters causes the formation of the product's low temperature," Curtiss explained.
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