ACIDIC AND BASIC PROPERTIES OF TITANIA-ALUMINA MIXED OXIDES - ACTIVE-SITES FOR PROPAN-2-OL DEHYDRATION

TiO2-Al2O3 mixed oxides have been prepared by cohydrolysis of Ti4+ and Al3+ isopropoxides. Textural and acid-base properties were investigat ed using nitrogen physisorption, adsorption of probe molecules followe d by FTIR spectroscopy, and test-reactions. Catalysts were found to ex hibit high surface areas and pore volumes. Results obtained from adsor bed CO2 probe molecules and COS hydrolysis activity agreed well and sh owed that the hydroxy basicity of mixed oxides was lower than that of the two oxide components. Pyridine and 2,6-dimethyipyridine were used to titrate Lewis and Bronsted acidic sites, respectively. A linear inc rease of the number of strong Lewis acid sites with alumina content wa s observed, whereas the number of Bronsted acid sites reached a maximu m for 30% TiO2-70% Al2O3 compared with that expected from the composit ion 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 si tes into Bronsted sites is proposed when propan-2-ol is chemisorbed on Lewis acid sites.