Tuning the acidic properties of aluminas via sol-gel synthesis: New findings on the active site of alumina-catalyzed epoxidation with hydrogen peroxide

Abstract

This study answers several pending questions about alumina-catalyzed epoxidation with aqueous 70 wt% H 2O 2. To evaluate the effect of the water-to-aluminum tri- sec-butoxide molar ratio, this was systematically changed from 1 to 24. The xerogels were calcined at 450 °C and gave different γ-Al 2O 3's with distinct textural and acidic properties. A combination of 27Al MAS NMR and TPD-NH 3 results of calcined aluminas allowed us to assign the type Ia Al OH sites as the catalytic sites for epoxidation. The type Ib Al OH sites have no function in catalytic epoxidation, because ethyl acetate poisons these sites. The strong acid sites of types IIa, IIb, and III Al OH groups are responsible for the undesired H 2O 2 decomposition and decreased oxidant selectivity.