Brønsted acid property of alumina-based mixed-oxides-supported tungsten oxide

Abstract

The effect of alumina-based supports (Al2O3, Al2O3–TiO2, Al2O3–ZrO2, SiO2–Al2O3, and SiO2) on the structure and acid properties of supported tungsten oxide catalysts was investigated. Among the tested supported tungsten oxide catalysts, WO3/Al2O3–TiO2 (Al2O3/TiO2 = 9, Al–Ti9) showed the highest activity for reactions catalyzed by Brønsted acid sites and the largest Brønsted acidity. Structural characterization revealed that Brønsted acid sites on WO3/Al–Ti9 were generated at the boundaries between domains of amorphous monolayer WO3 and Al2O3–TiO2 support and WO3/Al2O3 reported previously, and that Al2O3 and Al–Ti9 with a high density of Lewis acid and surface hydroxyl groups resulted in the formation of Brønsted acid sites. Crystalline WO3 species formed mainly on SiO2–Al2O3 and SiO2 with a low density of Lewis acid sites and a high density of surface hydroxyl groups. These results suggested that a high density of Lewis acid sites and surface hydroxyl groups are important factors to form amorphous WO3 monolayer domains and to generate Brønsted acid sites on alumina-based supports.