Scientists pioneers a new low-temperature chemical conversion process


Scientists pioneers a new low-temperature chemical conversion process

Argonne chemists have identified a way to convert cyclohexane to cyclohexene or cyclohexadiene, two important chemicals in a wide range of industrial processes. The new process takes place at low temperatures, eliminating the creation of unwanted by-products. Credit: National Laboratory of Argonne

Chemists spend a lot of time and energy attempting to trigger or accelerate chemical reactions – but it can sometimes be just as important to stop them before going too far.

In a recent study by the Argonne National Laboratory of the US Department of Energy (DOE), chemists have identified a way to convert cyclohexane to cyclohexene or cyclohexadiene, important chemicals in many industrial processes. It is important to note that this process takes place at low temperatures, thus eliminating the creation of carbon dioxide that would have resulted from an undesired break in carbon-carbon bonds.

Cyclohexane is an important starting molecule in a wide range of chemical reactions, according to the Argonne chemist Stefan Vajda, currently at the J. Heyrovský Institute of Physical Chemistry in Prague. However, without the appropriate catalyst to initiate the reaction, the conversion of cyclohexane to useful products generally requires high temperatures generated by high energy expense, and the process may also suffer from poor selectivity.

In the study, chemist Vajda and Argonne, Larry Curtiss, and his international team of collaborators examined a type of reaction called oxidative dehydrogenation, in which hydrogen molecules are extracted from a molecule more big. By cutting off a limited number of hydrogen-carbon bonds, the reaction can produce cyclohexene and cyclohexadiene before burning to carbon dioxide.

The work has been improved over previous Argonne team studies on the dehydrogenation of cyclohexane and cyclohexene by introducing two key components: a cobalt oxide catalyst of less than a nanometer size on an oxide support. aluminum and an environment containing controlled oxygen.

The researchers used Advanced Photon Source (APS) X-ray scattering techniques from Argonne, a DOE Office of Science user facility, to monitor the nature and stability of catalysts in catalytic clusters testing. in real time. They found that the clusters partially dehydrogenated cyclohexane at temperatures around 100 degrees Celsius, much less than previously observed for this type of reaction. .

"The fact that we can achieve this conversion at lower temperatures prevents dehydrogenation intermediates such as cyclohexene and cyclohexadiene from being further converted into undesirable products," Vajda said.

Vajda and Curtiss noted that the highly selective catalyst has a long life and that it is neither poisoned nor degraded by the reaction. In theoretical and experimental research on the size of the catalyst, the researchers found that clusters of size four and twenty-seven atoms were about as effective in carrying out the reaction. "It seems that as long as the catalyst is less than one nanometer in size, this composition works well – an important factor for the potential scale-up of this class of catalysts by more traditional synthesis routes, although less selective in size. " Said Vajda.

To better understand the basic mechanisms of activity and selectivity of cobalt catalysts, researchers used functional density theory calculations to model the pathways of reaction. "The excellent performance of cobalt clusters can be explained by theoretical calculations, which reveal highly active cobalt atoms in the clusters and show that the oxidized nature of the clusters results in the formation of the product at low temperatures," Curtiss explained.

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More information:
Sungsik Lee et al, Cobalt Oxide Subnanometer Clusters as Selective Oxidative Dehydrogenation Catalysts at Low Temperature, Nature Communications (2019). DOI: 10.1038 / s41467-019-08819-5

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National Laboratory of Argonne

Scientists innovate in the process of chemical conversion at low temperatures (April 4, 2019)
recovered on April 4, 2019

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