Investigation for flavanone synthesis in subcritical water with La-ZrO2, Mg-ZrO2, and Ca-ZrO2 as solid base catalysts

In this study, flavanone was synthesized via a two-step process, Claisen–Schmidt condensation between 2’-hydroxyactophenone and benzaldehyde followed by isomerization, in subcritical water with ZrO₂ based catalysts. Kinetic analysis was conducted to quantitatively show the catalytic activity of different modified ZrO₂. The activity of ZrO₂ was improved upon incorporating with active metal species. Compared with La-ZrO₂ and Mg-ZrO₂, Ca-ZrO₂ showed the highest activity attributed to strong basicity, but substantial formation of side product and loss of product were observed. La-ZrO₂ showed weaker activity than Mg-ZrO₂, though La-ZrO₂ had higher basicity. One explanation was the dissociation of water to provide OH⁻ on the catalyst surface was more challenging since containing significantly more active sites with strong water affinity. Moreover, such active sites contributed less to reaction due to dense water surrounding the catalyst surface. The reaction catalyzed by Mg-ZrO₂ reached flavanone selectivity of 72.2 % in 3 hours at 150 °C.