Acidic properties of Si- and Al- promoted TiO₂ catalysts: Effect on 2-propanol dehydration activity

The effect of Si and Al promoted in TiO₂ catalysts on both acid-base properties and 2-propanol dehydration was studied. The sol-gel method was used to prepare pure TiO₂ and TiO₂-Al₂O₃ and TiO₂-SiO₂ mixed oxides with 1%, 3% and 10% of Al₂O₃ or SiO₂. The structure and surface characterization (XRD, TG-DTA, N₂ sorption, ²⁷Al and ¹⁹Si MAS NMR) revealed that the promoted cations were dispersed in the TiO₂ lattice. The acid and basic sites density was measured by NH₃-TPD and CO₂-TPD. FTIR-pyridine-TPD revealed predominantly Lewis-type acidity for all these catalysts and a low presence Brønsted-type acidity was detect only for 10% SiO₂ promoted catalyst. The change in the activity of 2-propanol decomposition was useful to detect the effect of type and content of cation in the acid-base properties of promoted catalysts. The acid-base molar ratio of the catalytic surface was increased as a function of content of Al and Si and it was related to the increase in the alcohols decomposition activity. Dehydration activity was more sensitive for the Si-promoted catalysts than the Al-promoted ones, showing an important role of cation coordination in the acidic and catalytic properties of these mixed oxides.