A wide variety of carbon materials is known for their applicability as a catalyst or support of it. Especially the activated carbons with their large specific surface can be employed. But also carbon nanotubes and, to some degree, the fullerenes may be used to the same end. Besides, carbon onions and onion-like carbons are promising for catalytic applications, too. They feature a considerable specific surface, bear little structural defects (at suitable preparation), and they are stable over a wide range of temperatures.
The catalytic production of styrene by dehydrogenation of ethylbenzene constitutes an example of carbon onions being applied in the process of industrial relevance (Figure 4.43) . Styrene is a basic chemical that is prepared on a scale of millions of tons. Normally, it is generated by thermal dehydrogenation, which, for being endothermic, requires supply with large amounts of energy. Furthermore, the catalyst (hematite with added potassium) is quickly deactivated, so altogether the process efficiency is limited. Hence, it is worthwhile searching for alternative procedures and catalysts. The oxidative dehydrogenation to styrene, for example, is exothermic and consequently far less demanding with regard to its energy consumption. A series of catalyst materials such as alumina, various phosphates, or metal oxides was found indeed. It turned out, however, that the actual catalytically active substance was a film of carbon that formed on the surface of the respective support. Therefore, it was self-suggesting to directly employ carbon materials themselves.
Still, when using porous carbons, the desorption of newly generated styrene from the catalyst surface was found to be hindered, and so the conversion was limited. The lack of any porosity in carbon onions should clearly be beneficial here, and indeed the conversion and the yields of styrene could be increased when using nano-onions or onion-)ike material. In samples of catalyst that had already been used, the onion structure was found to be partly destroyed. Actually, the catalyst reaches full activity only in this state, which also accounts for the induction period observed. On the onions' surface, presumably carbonyl groups and quinoid structures constituting the real active sites are formed.
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