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

摘要

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 deg C and gave different gamma-Al_2O_3's with distinct textural and acidic properties. A combination of ~(27)Al 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.