Properties of acid and base sites on zirconium oxides having monoclinic and tetragonal structures formed by calcination at low temperature

A relationship between acid-base properties on ZrO2 catalysts prepared by hydrolysis of ZrOCl2 adjusted to various pH values with NH3 solution and their crystalline structures determined by XRD measurements was examined. When the hydrolysis was carried out at pH 7 and pH 3, followed by the calcination at low temperature of 723 K in air, it was confirmed that the obtained ZrO2 had a simple structure of monoclinic and tetragonal, respectively. The acid and base amounts per unit surface area of the ZrO2 were scarcely different in the type of the crystalline structures, indicating that the densities of acid and base sites present on the surfaces were not affected by the crystalline structures. However, the strength of both acid and base sites was found to significantly depend on the crystalline structures. The strong acid and base sites were observed in TPD spectra of NH3 and CO2 to appear on the monoclinic ZrO2 obtained by the calcination at low temperature, while only weak acid and base sites were observed on the tetragonal ZrO2 surface as well as on the monoclinic ZrO2 calcined at high temperatures. The higher specific activity and the lower activation energy for 1-butene isomerization were observed on monoclinic ZrO2 catalysts. It is suggested that the strong acid and base sites are formed on monoclinic ZrO2 with small particle size, and this is due to the structural defect.