Effect of Support Material on the Performance of Chromia Dehydrogenation Catalysts

Satu Korhonen
Abstract & Cover

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

Source of Information
FinALD40 exhibition material, http://www.aldcoe.fi/events/finald40.pdf
Helsinki University of Technology, Faculty of Chemistry and Materials Sciences, Department of Biotechnology and Chemical Technology, Industrial Chemistry
(Espoo, Finland)
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