Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2002, 41 (11), 1885 - 1888
Layered carbon nanostructures
as catalysts for the synthesis of styrene
Ever since the discovery of fullerenes, carbon molecules shaped like tiny soccer balls, researchers have been intensively investigating previously unknown forms of carbon. Now a whole kaleidoscope of related materials has been synthesized. The unique chemical and physical properties of these nanotubes, nanospheres, nanobundles and onion-shaped carbon molecules are expected to lead to many new applications.
Now the properties of the onion-shaped carbons have been scrutinized for the first time: In a collaboration between the Fritz-Haber Institute in Berlin and the Boreskov Institute of Catalysis in Novosibirsk, researchers are examining the catalytic potential of this material. What exactly is meant by "carbon onions"? Onion-shaped carbon is made of particles with a diameter of a few millionths of a millimeter. The particles are built up out of individual closed shells of carbon atoms. And just as with a real onion, shells are wrapped around each other. The assembly of the individual shells corresponds to the order adopted by carbon atoms in graphite layers. This nearly perfect graphitic structure of the "onions", which is put under tension because of the curvature of the shells, was irresistible to the chemists. It promised to have interesting catalytic properties.
The international team of researchers working with Robert Schloegl wanted to know if the little onions are of any use as catalysts for processes such as the synthesis of styrene. This procedure is one of the ten most important industrial processes. Styrene is an important raw material for the manufacture of plastics, and is made from ethylbenzene. The process by which two hydrogen atoms are removed from the starting material is called oxidative dehydrogenation. That catalysts made of carbon are quite effective at this was previously known. So why not test the onion-like carbon?
With a styrene yield of 62%, the little onions produced significantly better results than the industrial catalysts currently in use, as well as all other carbon forms examined thus far. "Future practical application is possible," reports Schloegl. During the reaction, the onion-like structure is broken up. The reason for this is the deposition of oxygen on the surface of the material. Says Schloegl, "These oxygen centers seem to be the actual active centers of the catalyst."