Study in Surface Acidity and Catalytic Behavior of Mesoporous Zirconia-supported Catalysts

Mesoporous zirconia, synthesized from surfactant templating, was employed to disperse alumina and tungstate (AW/m-ZrO2) and then used as a catalyst for the isomerization of n-butane. After calcination at 700 oC, tetragonal phase of zirconia was formed with high surface area. Furthermore, the catalyst AW/m-700 performed exceptionally well, and gave maximum rate and stability at reaction temperature of 250 oC, compared to other tungstated zirconia-based catalyst reported so far. Moderate calcination temperature leads to the presence of more Brönsted acid sites with higher acid strength. In comparison with the normal dense phase zirconia catalyst, AW/m-ZrO2 showed higher activity than the corresponding AW/d-ZrO2. The difference in catalytic activities was examined by various surface characterization techniques, and was attributed to different degree of dispersion of tungstate on the surface of m-ZrO2.

In the second part, we focused on the alumina sulfate supported on mesoporous zirconia (AS/m-ZrO2). Our primary goal was to understand the origin of the catalytic active sites and the roles of these sites in the various stages of reaction. Three kinds of acid sites were determined by pyridine adsorption with XPS (N 1s core-level). The remarkable performance of the Al-promoted catalysts is due to an optimum balance of these acid sites, and the catalytic activity seems to correlate directly with the amount of weak Brönsted acid sites. 27Al MAS-NMR and in-situ DRIFT results elucidate the origin and the role of these acid sites in the various stages of isomerization reaction. EPR experiments explore the existence of O2-; the active species eliminates hydrogen atoms to form n-butene and starts the overall reaction in our experiment.
The promotion effect of Ga on sulfated zirconia catalyst was studied in n-butane isomerization reaction and compared with Al-promoted catalyst. Our NH3-TPD and DRIFT studies reveal that the enhanced acid sites are due to the addition of Al and Ga promoters to mesoporous zirconia. AS/m-ZrO2 promoted by Al seems to bring greater contribution to the acidity of catalyst than that by Ga. Therefore, the improved activity of AS/ZrO2 over S/ZrO2 catalyst is related to the increased acidity created by the Al promoter, while the Ga promotion effect is then through a redox mechanism in the acid catalyzed initiation mechanism.

Finally, highly dispersed biomimetic catalysts tungstated zirconia (WZ) and mesoporous ziconia-supported rhenium oxide (Re/m-ZrO2) were prepared by a simple impregnation method and excellent catalytic performances were obtained in liquid phase bromination of phenol red using hydrogen peroxide and ammonium bromide under neutral condition. It was shown that the activity of phenol red bromination reaction depends directly on the dispersion of WOx or ReOx species over support surfaces.