Michael Reactions Catalyzed by Basic Alkylamines and Dialkylaminopyridine Immobilized on Acidic Silica–Alumina Surfaces

Acid–base bifunctional catalysts were prepared by immobilization of basic amines on acidic silica–alumina (SA) surfaces. Silane-coupling reagents with various amino-functional groups, such as primary, secondary, and tertiary alkylamines, alkyldiamine, and dialkylaminopyridine, were examined as anchoring reagents for the amines in the preparation of catalysts. The obtained immobilized catalysts (SA–NR′R′′) were characterized by solid-state ¹³C and ²⁹Si MAS NMR and elemental analysis. The catalytic activity of tertiary alkylamines for Michael reactions increased dramatically by the immobilization on silica–alumina, whereas a homogeneous tertiary amine scarcely promoted the reaction. Regarding the kind of amines, the dialkylaminopyridine immobilized silica–alumina with low Al content (SA_L) showed the highest catalytic performance among the amine immobilized catalysts. The solid-state ¹³C NMR analysis revealed the interaction between the nitrogen atom on pyridine ring and a surface strong acid site of the silica–alumina support.