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

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, beta-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.