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Keywords:

  • acid–base catalyst;
  • bifunctional nanocatalyst;
  • heterogeneous catalysis;
  • mesoporous materials;
  • Michael addition

Abstract

We describe the development and application of efficient bifunctional acid–base mesoporous silica catalysts, denoted hereafter as Ext-SBA-15-NMe2, comprising tertiary amine and silanol (weak acid) groups for the Michael addition reaction. The catalysts were synthesized by grafting tertiary amine containing organosilane into the mesoporous channel pores of SBA-15 mesoporous silica in polar protic solvent (isopropyl alcohol) or nonpolar solvent (toluene). The resulting materials, Ext-SBA-15-NMe2-IPA or Ext-SBA-15-NMe2-Tol, respectively, were used as catalysts for the Michael addition reactions between trans-β-nitrostyrene(s) and active methylene compounds, such as malononitrile, acetylacetone, and diethylmalonate, at different temperatures. A variable-temperature NMR-array technique was used to monitor the reactant conversion and the reaction progress. Among the active methylene compounds tested with trans-β-nitrostyrene, malononitrile gave the highest conversion of 90 % in 0.3 h at 0 °C if catalyzed by the catalyst Ext-SBA-15-NMe2-IPA, the silanol groups of which were not passivated by trimethysilyl ([BOND]SiMe3) groups. Compared with Ext-SBA-15-NMe2-Tol, the Ext-SBA-15-NMe2-IPA material (whose the amine groups were grafted in polar-protic solvents), in particular, demonstrated an effective cooperative bifunctional catalysis because of its optimized proportions of tertiary amine groups and a significant number of surface silanol groups that were judiciously left behind on the material with this optimized synthetic strategy. The catalytic activity of this material was found to be significantly higher than that of the corresponding material that was grafted in toluene and that contained less optimized proportions of these two catalytic groups (i.e., amine and silanol groups). The bifunctional catalysts also showed good recyclability upon washing with acetone, which offered an effective way of regenerating the catalyst free from any undesired product/substrate bound on the surface of the catalysts.