The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Chromium Oxide Catalysts in the Dehydrogenation of Alkanes

Sanna Airaksinen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Chemical Technology for public examination and debate in Auditorium KE2 (Komppa Auditorium) at Helsinki University of Technology (Espoo, Finland) on the 21st of October, 2005, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-7796-4)   [693 KB]
Errata (in PDF format)
Dissertation is also available in print (ISBN 951-22-7795-6)

Abstract

Light alkenes, such as propene and butenes, are important intermediates in the manufacture of fuel components and chemicals. The direct catalytic dehydrogenation of the corresponding alkanes is a selective way to produce these alkenes and is frequently carried out using chromia/alumina catalysts. The aim of this work was to obtain structure–activity information, which could be utilised in the optimisation of this catalytic system. The properties of chromia/alumina catalysts were investigated by advanced in situ and ex situ spectroscopic methods, and the activities were measured in the dehydrogenation of isobutane.

The dehydrogenation activity of chromia/alumina was attributed to coordinatively unsaturated redox and non-redox Cr3+ ions at all chromium loadings. In addition, the oxygen ions in the catalyst appeared to participate in the reaction. The reduction of chromia/alumina resulted in formation of adsorbed surface species: hydroxyl groups bonded to chromia and alumina were formed in reduction by hydrogen and alkanes, and carbon-containing species in reduction by carbon monoxide and alkanes. Prereduction with hydrogen or carbon monoxide decreased the dehydrogenation activity. The effect by hydrogen was suggested to be related to the amount of OH/H species on the reduced surface affecting the number of coordinatively unsaturated chromium sites, and the effect by carbon monoxide to the formation of unselective chromium sites and carbon-containing species.

The chromia/alumina catalysts were deactivated with time on stream and in cycles of (pre)reduction–dehydrogenation–regeneration. The deactivation with time on stream was caused mainly by coke formation. The nature of the coke species changed during dehydrogenation. Carboxylates and aliphatic hydrocarbon species formed at the beginning of the reaction and unsaturated/aromatic hydrocarbons and graphite-like species with increasing time on stream. The deactivation in several dehydrogenation–regeneration cycles was attributed to a decrease in the number of actives sites, which was possibly caused by clustering of the active phase into more three-dimensional structures.

Acidic hydroxyl species of exposed alumina support may have contributed to the side reactions observed during dehydrogenation. Chromium catalysts prepared on unmodified alumina and on alumina modified with basic aluminium nitride-type species were compared in an attempt to increase the activity and selectivity in dehydrogenation. However, the presence of nitrogen in the catalyst was not beneficial for the dehydrogenation activity.

A kinetic model was derived for the rate of dehydrogenation of isobutane on chromia/alumina. The dehydrogenation results were best described by a model with isobutane adsorption, possibly on a pair of chromium and oxygen ions, as the rate-determining step. Satisfactory description of the reaction rate depended upon inclusion of the isobutene and hydrogen adsorption parameters in the mathematical model. The activation energy of the rate-determining step was estimated to be 137±5 kJ/mol.

This thesis consists of an overview and of the following 6 publications:

  1. Puurunen, R. L., Airaksinen, S. M. K., Krause, A. O. I., Chromium(III) Supported on Aluminum-Nitride-Surfaced Alumina: Characteristics and Dehydrogenation Activity, Journal of Catalysis 213 (2003) 281-290.
  2. Airaksinen, S. M. K., Krause, A. O. I., Sainio, J., Lahtinen, J., Chao, K.-j., Guerrero-Pérez, M. O., Bañares, M. A., Reduction of Chromia/Alumina Catalyst Monitored by DRIFTS-Mass Spectrometry and TPR-Raman Spectroscopy, Physical Chemistry Chemical Physics 5 (2003) 4371-4377.
  3. Airaksinen, S. M. K., Bañares, M. A., Krause, A. O. I., In Situ Characterisation of Carbon-Containing Species Formed on Chromia/Alumina during Propane Dehydrogenation, Journal of Catalysis 230 (2005) 507-513.
  4. Airaksinen, S. M. K., Krause, A. O. I., Effect of Catalyst Prereduction on the Dehydrogenation of Isobutane over Chromia/Alumina, Industrial & Engineering Chemistry Research 44 (2005) 3862-3868.
  5. Airaksinen, S. M. K., Kanervo, J. M., Krause, A. O. I., Deactivation of CrOx/Al2O3 Catalysts in the Dehydrogenation of i-Butane, Studies in Surface Science and Catalysis 136 (2001) 153-158.
  6. Airaksinen, S. M. K., Harlin, M. E., Krause, A. O. I., Kinetic Modeling of Dehydrogenation of Isobutane on Chromia/Alumina Catalyst, Industrial & Engineering Chemistry Research 41 (2002) 5619-5626.

Keywords: alkane dehydrogenation, chromia/alumina catalyst

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© 2005 Helsinki University of Technology

Last update 2011-05-26