Catalytic applications of aminopropylated mesoporous silica prepared by a template-free route in flavanones synthesis

Aminopropylated silica containing mesopores was prepared by a simple sol–gel process with tetraethyl orthosilicate (TEOS) and aminopropyltriethoxysilane (APTES) under strong acidic condition. The materials were characterized by N2 sorption, TGA, FTIR, and solid-state NMR measurements. It was proved that the amount of APTES in the initial mixture and the preparation process had a great influence on the textural properties of the hybrid organic–inorganic materials. The samples prepared by TEOS prehydrolysis and an appropriate amount of APTES (equal to or less than 10 mol%) contained mesoporous structures of narrow pore size distribution and possessed a larger surface area and pore volume than that prepared without TEOS prehydrolysis. The materials were used as catalysts in the Claisen–Schmidt condensation between substituted benzaldehydes and substituted 2′-hydroxyacetophenones and the subsequent isomerization of the 2′-hydroxychalcone intermediates in the liquid phase. The results showed that the presence of an appropriate amount of aminopropyl groups and narrow distributed mesopores was important for good performance of the catalysts. Good catalytic activities and very high selectivities for flavanones were obtained in solvent-free reactions. The influence of the substituting groups in the aromatic rings of benzaldehyde and 2′-hydroxyacetophenone was investigated under solvent-free conditions. The influence of electron-donating and electron-withdrawing groups at the para position of benzaldehyde was contrary to the results obtained over other base catalysts. The possible reaction mechanism involving the formation of imine intermediates was suggested for the synthesis of flavanone over aminopropylated silica.