Synthesis, characterization and heterogeneous catalytic activity of diamine-modified silica-coated magnetite-polyoxometalate nanoparticles as a novel magnetically-recoverable nanocatalyst

•Magnetite-polyoxometalate hybrid nanomaterials were synthesized and characterized.•The average size of nanoparticles was approximately 60 nm.•The recoverable nanocatalyst was applied in aqueous catalysis.•The catalyst was used for the synthesis of benzopyrans and Knoevenagel reaction.•The catalyst could be separated by using an external magnetic field and recycled.

Magnetite-polyoxometalate hybrid nanomaterials, Fe3O4@SiO2@NH-NH2-PW, was prepared by grafting of H3PW12O40 on the diamine-functionalized Fe3O4 magnetite nanoparticles. This new heterogeneous nanocatalyst demonstrated a catalytic performance in the synthesis of tetrahydrobenzo[b]pyrans and Knoevenagel condensation in aqueous media. The nanocatalyst could easily be separated from the reaction solution by using an external magnet and recycled several times. The recovered catalyst was reused for at least seven runs without significant loss in catalytic activity. The resulting nanomaterials were characterized with different physicochemical techniques, such as Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), powder X-ray diffraction (XRD), energy dispersive X-ray analysis (EDAX), thermogravimetric analyze (TGA) and alternating gradient force magnetometer (AGFM). SEM and TEM analyses indicated that nanoparticles had relatively uniform spherical nanoparticles and the average size of Fe3O4@SiO2@NH-NH2-PW was approximately 60 nm.

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