Acidic and basic properties of titania–alumina mixed oxides; active sites for propan-2-ol dehydration

Abstract

TiO2–Al2O3 mixed oxides have been prepared by cohydrolysis of Ti4+ and Al3+ isopropoxides. Textural and acid–base properties were investigated using nitrogen physisorption, adsorption of probe molecules followed by FTIR spectroscopy, and test-reactions. Catalysts were found to exhibit high surface areas and pore volumes. Results obtained from adsorbed CO2 probe molecules and COS hydrolysis activity agreed well and showed that the hydroxy basicity of mixed oxides was lower than that of the two oxide components. Pyridine and 2,6-dimethylpyridine were used to titrate Lewis and Bronsted acidic sites, respectively. A linear increase of the number of strong Lewis acid sites with alumina content was observed, whereas the number of Bronsted acid sites reached a maximum for 30% TiO2–70% Al2O3 compared with that expected from the composition and the activity of pure oxides. Comparison between probe-molecule adsorption and test reaction, points out that both types of site are involved in propan-2-ol dehydration. A transformation of Lewis acid sites into Bronsted sites is proposed when propan-2-ol is chemisorbed on Lewis acid sites.