A highly efficient and green method for catalyzing the Hantzsch reaction under solvent-free conditions using a seashell/Kaolin calcined system

A green method for synthesizing 1,4-dihydropyridines (1a-j) by a one-pot reaction of various aldehydes, ethyl acetoacetate, and ammonium carbonate, catalyzed by a seashell and kaolin powders, mixed and calcined at 500 and 800 °C, under solvent-free conditions, has been developed. The catalyst was characterized by physicochemical techniques, namely, X-ray diffraction (XRD), electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The mixture composed of 20% seashell and 80% Kaolin (Q3-500), calcined at 500 °C, gave the best results (78 to 96% yields). Q3-500 separation from the reacting medium was carried out by filtration and reused several times with a slight decrease in yields. Compared with other conventional methods, the present method is inexpensive and offers advantages, namely: the catalyst is eco-friendly, recyclable, and gives high yields with shorter reaction time. Graphical Abstract: A seashell/Kaolin calcined system (20/80) at 500°C was elaborated to catalyze the Hantzsch reaction under solvent-free conditions. Using Q3-500 six times gave dihydropyridines with excellent yields with a slight loss of activity. The improvement of the catalytic properties of the kaolin surface is due to the increase of cationic content. [Figure not available: see fulltext.]