Composite nanocatalysts of amino-organoalkoxysilanes functionalized palygorskite for Knoevenagel condensation reaction

The development of sustainable heterogeneous basic catalysts is of significant importance in organic synthesis. In this work, three different silane coupling agents—Sil-1, (3-aminopropyl)trimethoxysilane; Sil-2, [3-(2-aminoethylamino)propyl]trimethoxysilane; and Sil-3, (3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane)—were grafted onto palygorskite (Pal) via a silanization strategy to prepare three high-performance, cost-effective, and reusable amino-organoalkoxysilane functionalized Pal composite nanocatalysts, namely Pal-Sil-1, Pal-Sil-2, and Pal-Sil-3. Comprehensive characterization (FT-IR, TGA, XRD, XPS, and SEM) confirmed the intact structure of modified catalysts and the successful introduction of amino active sites. Catalytic experiments show that Pal-Sil-3 exhibited the best catalytic performance among the three different nanocatalysts, achieving a conversion exceeding 98.5 % within 5 min for the Knoevenagel condensation of benzaldehyde and malononitrile in ethanol. It exhibited broad substrate adaptability for various substituted aromatic aldehydes and enabled the efficient synthesis of the drug intermediate isomer (Z)-5-benzylidene-1,3-thiazolidine-2,4-dione. Notably, Pal-Sil-3 maintained stable catalytic efficiency (> 97 % conversion) in a continuous flow system and retained high activity after five reuse cycles. This study provides a novel strategy for developing environmentally friendly heterogeneous catalytic systems and green synthesis of α,β-unsaturated molecules.