The effect of solvents on the heterogeneous synthesis of flavanone over MgO

The effect of several solvents on the heterogeneous synthesis of flavanone from benzaldehyde and 2-hydroxyacetophenone over a solid MgO catalyst was examined experimentally through kinetic and FTIR spectroscopic studies. High-boiling-point solvents considered were dimethyl sulfoxide, tetralin, mesitylene, benzonitrile, and nitrobenzene. Kinetic results indicate that the presence of different solvents in this reaction system affects the rate, kinetic dependencies, and selectivities toward flavanone and 2′-hydroxychalcone. Dimethyl sulfoxide (DMSO) in particular, significantly promotes the rates of both steps used in this synthesis (i.e., the Claisen–Schmidt condensation reaction of benzaldehyde with 2-hydroxyacetophenone and the subsequent isomerization of the 2′-hydroxychalcone intermediate to flavanone). The effect is more pronounced for the second reaction. Even the presence of small amounts of DMSO in other solvents, such as benzonitrile and nitrobenzene, results in strong promotion of the flavanone synthesis scheme. Results of FTIR spectroscopic studies indicate that the interaction of MgO with DMSO results in the formation of stable surface sulfate species. The presence of these species on the catalyst surface affects the adsorption behavior of benzaldehyde and 2-hydroxyacetophenone.