Heterogeneous synthesis of flavanone over modified magnesium oxide catalysts

Flavanone is an important member of the flavanoid family, compounds that have been receiving increasing attention due to the medicinal properties that many of them have exhibited. The synthesis of flavanone has been traditionally conducted through homogeneous catalytic routes. Recently, investigations have demonstrated the feasibility of conducting this synthesis under heterogeneous catalytic routes. This thesis focuses on the heterogeneous synthesis of flavanone over modified magnesia catalysts, as an alternative to the homogeneously catalyzed process. More specifically, this work focuses on the nature of the active sites of modified MgO catalysts involved in the synthesis of flavanone. The adopted research strategy includes the controlled modification of a conventional MgO catalyst by both decreasing and increasing its basicity. To decrease the basicity of MgO, various anions (i.e., PO43-, SO 42-, F--, and Cl--) were used. An increase in the basicity of magnesia was achieved by lithium doping. Catalysts were characterized by measurements of the specific surface area, powder X-ray diffraction, HRTEM, CO2-TPD and in situ infrared spectroscopy (FTIR) of adsorbed CO2, benzaldehyde and 2'-hydroxyacetophenone. The results indicate that medium strength basic sites on the magnesia are the most active centers for the reaction. Furthermore, a strong increase of the initial reaction rates was observed at low lithium loadings, correlated to the increase of basicity as indicated by the results of TPD and FTIR studies. However, high lithium loadings cause a dramatic decrease of the specific surface area of the catalysts and consequently lower initial reaction rates per catalyst weight.;ATR-FTIR studies were conducted to study the adsorption of liquid phase benzaldehyde and 2'-hydroxyacetophenone of MgO. Similarities and differences to our findings and assignments from previous gas-phase studies were presented. Multivariate analysis was utilized to further support our assignments. No surface vibrations were identified in the spectra when using DMSO as solvent, confirming a strong interaction between DMSO and the MgO surface, which inhibits the adsorption of 2'-hydroxyacetophenone and benzaldehyde. A promoting effect on the Claisen-Schmidt condensation reaction was observed on activity when using DMSO, which is attributed to its high polarity.