Oxidative Dehydrogenation of Ethylbenzene to Styrene Over Graphite Nanofibers
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Oxidative Dehydrogenation of Ethylbenzene to Styrene Over Graphite Nanofibers Xuejun Xu and R. Terry K. Baker Catalytic Materials LLC, 1750 Washington Street Holliston, MA 01746, USA ABSTRACT Tubular and platelet graphite nanofibers were used as catalysts for the oxidative dehydrogenation of ethylbenzene to produce styrene at temperatures between 350 and 577 oC in packed bed tubular quartz flow reactors. The performances of GNF catalysts were evaluated in terms of conversion, selectivity and styrene yield. Tubular GNF showed higher activity than platelet GNF, but the latter showed higher selectivity than the former material. Both types of GNF exhibited significantly better performance than a commercial catalyst when operated under the same conditions. INTRODUCTION The production of styrene from the dehydrogenation of ethylbenzene is one of most important industrial processes. The industrial process is currently carried out at high temperatures (above 600 oC) using promoted iron oxide catalysts in the presence of steam. Their highly endothermic character (∆H = 124.9 kJ/mole) imposes a thermodynamic limitation to the conversion of ethylbenzene that can be attained (about 50%), and requires a great deal of energy consumption. Therefore, alternative approaches would be highly desirable from the economical point of view. The oxidative dehydrogenation of ethylbenzene in the presence of oxygen is one of most promising processes, since the reaction is highly exothermic (∆H = -124.1 kJ/mole). Such a reaction can be carried out at lower temperatures and proceeds to completion. It was proposed that a coke layer generated on various solid inorganic catalysts during the reaction enhanced the activity of the system for the oxidative dehydrogenation of ethylbenzene [1-4]. Recent studies have demonstrated that certain carbon materials exhibit significant activity for the reaction [514]. When dealing with the active carbons, their range of porosity appears to be responsible for obstructing the desorption of styrene, which blocks surface sites and eventually poisons the catalyst [15]. A further shortcoming of active carbons is their propensity to undergo gasification at about 550 oC, a temperature close to that where the oxidative dehydrogenation is conducted. On the other hand, graphitic materials are more resistant to attack by oxygen and furthermore, can be considered nonporous materials. As a consequence, such carbons are stable at 550oC and would not be susceptible to poisoning by adsorption of styrene molecules. Unfortunately, in its conventional form of flat sheets, graphite has an extremely low surface area (about 0.5 m2/g) and this aspect has tended to limit its usefulness as a practical catalyst material. We have attempted to overcome the shortcomings of active carbons and graphite by using graphite nanofibers (GNF). These are new types of fibrous carbon materials that have been developed in our laboratory from the decomposition of certain hydrocarbons and/or CO over selected metal catalysts at temperatures rangin
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