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 Brønsted acidic sites, respectively. A linear increase of the number of strong Lewis acid sites with alumina content was observed, whereas the number of Brønsted 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 Brønsted sites is proposed when propan-2-ol is chemisorbed on Lewis acid sites.