Fe₃O₄@Silica sulfuric acid core–shell composite as a novel nanomagnetic solid acid: synthesis, characterization and application as an efficient and reusable catalyst for one-pot synthesis of 3,4-dihydropyrimidinones/thiones under solvent-free conditions

Abstract

A simple and convenient method was used to prepare Fe₃O₄@Silica sulfuric acid core–shell composite using Fe₃O₄ spheres as the core and silica sulfuric acid nanoparticles as the shell. Magnetite nanoparticles were synthesized by the co-precipitation of FeCl₂ and FeCl₃ in ammonia solution. To improve the chemical stability of magnetite nanoparticles, its surface engineering was successfully performed by the suitable deposition of silica onto nano-particles’ surface by the ammonia-catalyzed hydrolysis of tetraethoxysilane. Next, the SiO₂ spheres served as a support for the immobilization of SO₃H groups by simple mixing of the core–shell composite and chlorosulfonic acid in CH₂Cl₂. The resulting solid acid nanoparticles were characterized by infrared spectroscopy, scanning electron microscope, thermogravimetric analysis, and vibrating sample magnetometer. The catalytic activity of this solid acid nanocomposite was probed through one-pot synthesis of 3,4-dihydropyrimidinones via three-component couplings of aldehydes, β-diketone, and urea or thiourea under solvent-free conditions. In this reaction, Fe₃O₄@Silica sulfuric acid shows a highly catalytic nature, easy to handle procedure, short reaction time, recycle exploitation, and excellent isolated yields. The nanomagnetic catalyst could be readily separated from the solution via application of an external magnet, allowing straightforward recovery and reuse.