Catalytic Performance of Solid Base Na-ZrO2 in Subcritical Water

Abstract

In this study, Na-ZrO2 was prepared from two types of ZrO2 as solid base catalyst and the catalytic activity, selectivity, and mechanism of Na-ZrO2 for base catalyzed reactions in subcritical water were elucidated using aldol condensation between acetophenone and benzaldehyde as a model reaction. Based on the different selectivity for Cannizzaro reaction of benzaldehyde, a side reaction accompanied with the aldol condensation, two plausible mechanisms were proposed. One was that the doped Na2O could directly act as a base, deprotonating the acetophenone for catalyzing aldol condensation, and the other was that the doped Na2O could dissociate the water to provide OH− for catalyzing both aldol condensation and Cannizzaro reaction. The dominant mechanism varied depending on the nature of the ZrO2 support. The effect of the amount of subcritical water was also examined, showing that as reducing the loading of water, the yield of byproduct from the Cannizzaro reaction decreased due to the lower amount of dissociated OH−. On the other hand, the presence of water adversely affected the reaction equilibrium, but meanwhile sufficient water in the system also prevented the carbonization of organics and polymerization reaction on the catalyst surface, favoring for a ring balance.

Keywords

• Subcritical water
• Solid base catalyst
• Modified zirconium oxide
• Aldol condensation
• Cannizzaro reaction