Effects of structural defects and acid-basic properties on the activity and selectivity of isopropanol decomposition on nanocrystallite sol-gel alumina catalyst

The surface acid-basic properties of sol-gel alumina catalysts were studied by Fourier transform infrared (FTIR) spectroscopy of pyridine adsorption and temperature-programmed desorption of CO₂ and NH₃. The number of acid and basic sites on the samples varied with the calcination temperatures of the samples. The populations of the three different aluminum ions-tetrahedral, pentacoordinated and octahedral, which were identified by the ²⁷Al MAS NMR, were strongly affected by the sample calcination temperature and the crystalline composition. In the reaction temperature range between 100 and 250°C, isopropanol decomposition on sol-gel alumina catalysts was carried out. It was found that isopropanol decomposition on alumina catalyst was a structural-defect sensitive reaction. The dehydrogenation selectivity to acetone depended on the surface basic sites and the concentration of aluminum vacancies in the crystalline structure of γ-Al₂O₃. Bimolecular reaction to isopropylether was largely governed by the pentacoordinated aluminum ions which were related to the coordinately unsaturated aluminum ions. A mechanism for the formation of isopropylether was proposed: oxygen vacancies were suggested to involve the adsorption step of isopropanol, an intermediate species, (CH₃)₂HC⁺, reacted with (CH₃)₂HCO^- yielding isopropylether molecule.