Magnetic nanoparticles-supported tungstic acid (MNP-TA): an efficient magnetic recyclable catalyst for the one-pot synthesis of spirooxindoles in water

This paper reports the preparation, characterization and catalytic application of a novel, magnetically separable catalyst consisting of tungstic acid supported on silica coated magnetic nanoparticles. To obtain this new catalyst system, first, (3-chloropropyl) triethoxysilane was reacted with silica coated magnetic nanoparticles to generate a 3-chloropropyl magnetic nanoparticle (3-CPMNP) substrate. Subsequently, the addition of sodium tungstate to 3-CPMNP resulted in the stabilization of tungstic acid species on the surface of the magnetic nanoparticles (MNP-TA). The synthesized catalyst was characterized using some different microscopic and spectroscopic techniques such as X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and FTIR spectroscopy. The catalyst nanoparticles were obtained with near spherical morphology and an average size of similar to 45 nm. The W content of the catalyst was determined by ICP analysis to be 82.5 ppm (82.5 mg L-1), which was equal to 8.25% w/w (0.45 mmol g(-1)). The catalyst was successfully used for the one-pot synthesis of spirooxindoles via the multicomponent reaction of isatins and dicarbonyl compounds in water, which was used as a green solvent. The results revealed that this new catalyst showed high catalytic activity in this protocol and that it can be reused at least 5 times without any change in its catalytic activity.