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In the present work, the different nanostructures, i.e. carbon black, graphite, nanofilaments, nanotubes, onions, ultra-dispersed diamonds, were tested as catalysts for the oxidative dehydrogenation of ethylbenzene to styrene. The comparative characterizations of carbons before and after catalytic tests with TEM, XPS, Raman- and IR-spectroscopy, TG/DTA, and BET surface area techniques allowed us to develop a structure-activity relationship and to propose a model of the reaction mechanism. The determination of the conditions, under which carbon catalysts develop their activity maximum, was done with Experimental Design. A screening of the experimental parameters was conducted with the theoretically lowest possible number of experiments according to the Box-Behnken Plan and Simplex method. The optimum reaction conditions for all carbons tested lied at the temperature range of 495-515°C. The oxygen content in the feed was found to be an insignificant parameter in the accessible mass flow rates and the chosen temperature region. It was found that sp2-bound carbon is required for the selective styrene formation, since sp3-bound carbon led to the production of benzene instead of styrene. It has been shown that the microstructure of sp2-bound carbon materials is of paramount importance in order to obtain high and stable efficiencies. Carbon nanofilaments have shown the highest styrene yields at the highest ethylbenzene conversions as compared to carbon black and graphite. The comparative study of carbon nanofilaments and nanotubes of different structure has shown that more perfect carbon nanotubes produced by the arc-discharge technique are the most active catalysts in terms of reaction rates. The onion-like carbon was found to be the most efficient catalyst for the oxidative dehydrogenation reaction on a mass-referenced basis. XPS results revealed that the surface of onion-like carbon, being oxygen-free before the reaction, contained surface oxygen groups after the reaction. The experiments with oxygen pretreatment confirmed the creation of functional groups on the onion-like carbon surface at 570°C. Due to the high formation temperature and the XPS binding energy of the oxygenated species, it was proposed that chinoidic carbonyl groups of strongly basic character are generated during the reaction. The reaction model suggested for the oxidative dehydrogenation of ethylbenzene to styrene over sp2 - carbon materials follows a Langmuir-Hinshelwood mechanism, in which both adsorbed ethylbenzene and adsorbed oxygen-species play an important role. According to this model, the reaction might occur via i) ethylbenzene adsorption at the graphite step edges, ii) ethylbenzene reaction with the oxygenated species also located at the graphite step edges leading to the dehydrogenation of ethylbenzene to styrene, iii) the simultaneous transformation of the dehydrogenating oxygen species to hydroxyl groups, which remain at the graphite edges, iv) the styrene desorption from the carbon surface, v) gas-phase oxygen activation on the basal planes of the graphene layers, vi) oxygen diffusion to the prismatic planes with the hydroxyl groups, vii) reformation of the basic, chinoidic oxygen functionalities from the activated oxygen and the hydroxyl groups, iix) water desorption. The catalytic reaction passes these steps cycle by cycle. The establishment of structure-activity relation by the catalytic tests and the characterisation of different carbon nanostructures allowed one to determine carbon nanostructure stable under oxidative reaction conditions. Carbon nanotubes and onions have shown a high and stable efficiency in the ODH reaction. A radius of curvature of the basic structural element of carbon nanotubes and onions and also their high aspect ratio seem to provide a high density of functional surface groups under reaction conditions. The perfectness of these carbon nanostructures provides also enough stability toward oxidation and is essential for gas phase oxygen activation. The simplicity of carbon and its unique property that deactivated surfaces gasify themselves in oxidative dehydrogenation reactions not only renders them well-suited model systems but also allow for realistic expectations for a technical application.
