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

This study answers several pending questions about alumina-catalyzed epoxidation with aqueous 70 wt% H2O2. 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-Al2O3's with distinct textural and acidic properties. A combination of 27Al MAS NMR and TPD-NH3 results of calcined aluminas allowed us to assign the type Ia AlOH sites as the catalytic sites for epoxidation. The type Ib AlOH sites have no function in catalytic epoxidation, because ethyl acetate poisons these sites. The strong acid sites of types IIa, IIb, and III AlOH groups are responsible for the undesired H2O2 decomposition and decreased oxidant selectivity.