The role of reduction in the deactivation of potassium-promoted iron oxide dehydrogenation catalysts.

摘要

One of the largest commercial application for potassium promoted iron oxide catalyst (K-Fe2O3) in petrochemical industry is in the dehydrogenation of ethylbenzene (EB) to styrene (ST). It is generally accepted that the active sites on the K-Fe2O3 catalyst is potassium ferrite (KFeO2), which resides on the surface of a bulk magnetite phase and potassium polyferrite (K2Fe22O 34). This dehydrogenation reaction is typically performed in excess steam and the catalyst is known to experience short-term deactivation when the steam-to-hydrocarbon molar ratio (S/EB) is lowered. While possible causes for the deactivation phenomena are coking or reduction of the reactive site, the relative importance of the two mechanisms is not known.; Understanding of the relative contributions of active site loss by coking or reduction is important for developing catalysts with improved performance at low S/EB operation. Presented were results from decoupling the potential deactivation mechanisms with emphasis on the reduction behavior of the K-Fe 2O3 catalysts. Reducibility of the K-Fe2O 3 catalyst system included presence of the Cr and V promoters typically used in the model dehydrogenation catalyst. The reduction performance towards K-Fe2O3 with or without V/Cr promoters was evaluated in three separate studies. First at low hydrogen partial pressures, followed by mixed steam-hydrogen conditions, and finally using a mixed hydrogen-steam-hydrocarbon condition. Characterization techniques included Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and an isothermal reactor packed with a model dehydrogenation catalyst.; At TGA low hydrogen partial pressures the addition of K to the Fe 2O3 was found to increase the onset temperature for Fe 3O4 formation, and also impacted on the apparent reduction-activation energy. The role of steam in delaying the rate of iron oxide reduction was confirmed using TGA at isothermal steam to hydrogen molar ratio (S/H 2). At S/H2, maghemite (gamma-Fe2O3) was found to be a kinetic stable phase for K-Fe2O3. Addition of Cr/V promoter at reducing conditions confirmed their structural properties typically observed during dehydrogenation reactions.; When compared to the synthetic KFeO2, the synthetic K 2Fe22O34 phase was shown to be less resistant under reducing conditions. The K2Fe22O34 phase was reformed by oxidizing either in air or steam. Overall catalytic properties provided by the K-Fe2O3 with Cr/V promoters were validated using an isothermal reactor that was packed with a model dehydrogenation catalyst.