TiO2-coated magnetite nanoparticle-supported sulfonic acid as a new, efficient, magnetically separable and reusable heterogeneous solid acid catalyst for multicomponent reactions

Abstract

TiO₂-coated magnetite nanoparticle-supported sulfonic acid (nano-Fe₃O₄–TiO₂–SO₃H (n-FTSA)) is synthesized by immobilizing –SO₃H groups on the surface of nano-Fe₃O₄–TiO₂. This catalyst can be isolated readily after completion of the reaction by an external magnetite field. The obtained results demonstrate the use of coated magnetite nanoparticles as an excellent new support for the facile recovery of sulfonic acid catalysts. The newly synthesized heterogeneous solid acid is characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FE-SEM), a vibrating sample magnetometer (VSM), FT-IR spectroscopy, thermal gravimetric analysis (TGA), Hammett acidity function and pH analysis. Significantly, the as-prepared n-FTSA exhibits a high catalytic activity for the synthesis of heterocyclic compounds such as 1,8-dioxo-decahydroacridine, 1,8-dioxo-octahydroxantene, hexahydroquinoline and polyhydroquinoline derivatives in multicomponent reactions. The magnetically separated n-FTSA can be magnetically separated and reused for several times without significant loss of activity. This confirms that the sulfonic acid groups have high stability. TiO₂-coated magnetite nanoparticle-supported sulfonic acid has advantages such as low cost and toxicity, ease of preparation, magnetic separation, high stability, reusability and operational simplicity.