Degree Type

Dissertation

Date of Award

2004

Degree Name

Doctor of Philosophy

Department

Chemical and Biological Engineering

First Advisor

Brent H. Shanks

Abstract

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.

DOI

https://doi.org/10.31274/rtd-180813-8793

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu

Copyright Owner

Sipho C. Ndlela

Language

en

Proquest ID

AAI3158360

File Format

application/pdf

File Size

209 pages

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