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Effect of Support Material on the Performance of Chromia Dehydrogenation Catalysts

Satu Korhonen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Chemistry and Materials Sciences for public examination and debate in Auditorium KE2 (Komppa Auditorium) at Helsinki University of Technology (Espoo, Finland) on the 21st of November, 2008, at 12 noon.

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Dissertation is also available in print (ISBN 978-951-22-9551-7)

Abstract

The effect of support material on the dehydrogenation performance of chromia catalysts was studied with zirconium oxide (zirconia), aluminum oxide (alumina), and zirconia/alumina as the support materials. The dehydrogenation performance of the supports and chromia catalysts was studied by in situ infrared and Raman spectroscopies and activity measurements. The active surface sites of the supports and catalysts were characterized by in situ infrared and Raman spectroscopies using probe molecules, and the surface of zirconia was studied in more depth by modeling with density functional theory (DFT).

The characterization experiments and DFT calculations revealed the amphoteric character of the hydroxyl groups of zirconia and the presence of coordinatively unsaturated (c.u.s.) acid–base pairs. The hydroxyls of alumina exhibited similar basicity to those of zirconia, but their acidity was lower. Lewis acid and Lewis base sites were observed for alumina, but they did not form c.u.s. acid–base pairs as on zirconia. The deposition of zirconia on alumina decreased the Lewis acidity of the c.u.s. sites and the acidity of the hydroxyls, while the total basicity of the material appeared to increase. The addition of chromium also appeared to increase the basicity.

Zirconia was the most active and selective support material in the dehydrogenation of isobutane. The high activity was suggested to originate from the acid–base pairs that were present only on the zirconia surface. The performance of zirconia/alumina resembled that of alumina more than that of zirconia. The benefit of zirconia deposition on alumina was a lower coke deposition rate than on alumina due to the lower Lewis acidity of the zirconia/alumina. However, the cracking activity of alumina was not influenced by zirconia deposition and this was attributed to the presence of similar hydroxyls.

The chromia/zirconia catalyst was the most active dehydrogenation catalyst. The deposition of zirconia on alumina decreased the activity of the chromia catalysts. This was attributed to an incomplete monolayer of zirconia on alumina, which enabled chromium to interact with both zirconia and alumina. In contrast to the supports, the rate of coke deposition was not influenced by the acid–base properties of the catalysts but followed the dehydrogenation activity, and the formation of cracking products was due to thermal cracking. It was concluded that deposition of zirconia on alumina is not beneficial for chromia dehydrogenation catalysts. High surface area zirconias should be investigated instead.

This thesis consists of an overview and of the following 5 publications:

  1. Satu T. Korhonen, Monica Calatayud, and A. Outi I. Krause. 2008. Stability of hydroxylated (1̅11) and (1̅01) surfaces of monoclinic zirconia: A combined study by DFT and infrared spectroscopy. The Journal of Physical Chemistry C, volume 112, number 16, pages 6469-6476. DOI: 10.1021/jp8008546.
  2. Satu T. Korhonen, Monica Calatayud, and A. Outi I. Krause. 2008. Structure and stability of formates and carbonates on monoclinic zirconia: A combined study by density functional theory and infrared spectroscopy. The Journal of Physical Chemistry C, in press. DOI: 10.1021/jp803353v.
  3. Satu T. Korhonen, Miguel A. Bañares, José L. G. Fierro, and A. Outi I. Krause. 2007. Adsorption of methanol as a probe for surface characteristics of zirconia-, alumina-, and zirconia/alumina-supported chromia catalysts. Catalysis Today, volume 126, pages 235-247. DOI: 10.1016/j.cattod.2007.01.008. © 2007 Elsevier Science. By permission.
  4. Satu T. Korhonen, Sanna M. K. Airaksinen, and A. Outi I. Krause. 2006. In situ characterization of carbonaceous deposits formed on chromia/zirconia during isobutane dehydrogenation. Catalysis Today, volume 112, pages 37-39. DOI: 10.1016/j.cattod.2005.11.053. © 2006 Elsevier Science. By permission.
  5. Satu T. Korhonen, Sanna M. K. Airaksinen, Miguel A. Bañares, and A. Outi I. Krause. 2007. Isobutane dehydrogenation on zirconia-, alumina-, and zirconia/alumina-supported chromia catalysts. Applied Catalysis A: General, volume 333, pages 30-41. DOI: 10.1016/j.apcata.2007.08.040. © 2007 Elsevier Science. By permission.

Keywords: dehydrogenation, zirconia, alumina, chromia, infrared, Raman, DFT

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© 2008 Helsinki University of Technology

Last update 2011-05-26