Surface characterization and catalytic activity of sulfated-hafnia promoted zirconia catalysts for n-butane isomerization

Sulfated-hafnia promoted zirconia containing various compositions of hafnia (1-10 wt.%) were prepared by precipitation method in an attempt to ultimately carry n-butane isomerization reaction. The catalyst samples were calcined under dry air flow at 600 °C. The structural and crystal changes were monitored by FTIR and XRD whereas the textural changes were estimated by low temperature N2 adsorption. FTIR spectroscopy has been used to characterize the hydroxyl groups and to determine the concentration of Brönsted and Lewis acid sites from pyridine adsorption. The catalytic activity, stability and selectivity of the catalyst samples were tested for n-butane isomerization at 250 °C. The results reveal that the existence of a small content of hafnia increases the surface sulfate density, stabilizes the tetragonal phase of zirconia, increases the amount and strength of Brönsted acid sites and enhances the initial catalytic activity of the samples. Increasing hafnia content to 5 and 10 wt.% is accompanied with the formation of hafnia monoclinic phase which causes a drastic decrease in the amount of Brönsted acid sites and the initial activity of the catalyst. The isomerization of n-butane occurred through bimolecular pathway with the formation of iso-butane, propane and pentanes as primary products. In spite of the high activity of the samples, the catalyst deactivates rapidly during the first hour of the reaction due to coke formation, retreating the catalyst at 450 °C in the presence of dry air before the catalytic reaction lead to regeneration of the initial catalytic activity of the catalyst which implies that the coke formation is the main source for catalyst deactivation.