Various magnetite heterogeneous catalysts for the organic reaction are increasingly recognized while their stability still is a critical challenge. In this regard, SiO2, ionic liquid (IL), and polyvinyl pyrrolidone (PVP) were applied as a three-layer stabilization system for modifying magnetic Fe3O4 to produce Fe3O4@SiO2@IL&#x2010, PVP as a novel nanocatalyst. The Fe3O4@SiO2@IL&#x2010, PVP as stabilized heterogeneous catalysts were utilized in a robust, and environmentally friendly strategy for the preparation of the spirooxindole derivatives by the one-pot condensation of isatin, malononitrile, and 1, 3-dicarbonyl in a water solvent. The proposed method revealed a high yield spirooxindole derivatives preparation (99%) at short reaction times (1. 0 min), low catalyst mass (0. 002 g), and satisfactory temperature (30 °, C) which confirm the lack of any tedious challenge and promising applicability and reusability. This work introduces a rational core-shell nanosystem design with a facile and novel construction strategy to arrive at nonexquisite metal-based composite catalysts with superior catalytic proficiency and prominent long-term stability.

Fe3O4 nanoparticles (NPs) with a continuous and mesoporous silica (m-SiO2) shell were synthesized using a one-step method, sourcing silica from rice husk ash (RHA). The rice husk was thermally treated to obtain ash, from which silica was extracted as sodium silicate and precipitated by pH reduction. This silica powder, combined with iron chloride salts, facilitated the synthesis of the core-shell NPs. Mint extract acted as a capping agent to prevent agglomeration, and CTAB (cetyltrimethylammonium bromide) was used to create the porous SiO2 shell. X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) characterization investigated the structure, size, and shell formation. Coating integrity and suspension stability were assessed through Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS). DLS analysis showed a relatively narrow particle size distribution with an average hydrodynamic size of 72.6 nm. Small-angle X-ray scattering (SAXS) provided insights into the meso- and nanoscale structure, while BET and nitrogen adsorption-desorption isotherms confirmed the mesoporous nature of the silica shell. Magnetization measurements showed superparamagnetic behavior, with specific magnetization values of 57.9 emu/g for Fe3O4 and 27.5 emu/g for Fe3O4@m-SiO2. These results confirm the successful synthesis of superparamagnetic magnetite NPs with a mesoporous silica coating from RHA.

In the present study, Fe3O4@SiO2 core–shell microspheres were prepared via two steps. First, Fe3O4 nanoparticles were synthesized by co-precipitation of Fe+3 and Fe+2 as reaction substrates and NaOH as precipitant. Second, the surface of Fe3O4 was coated with silica by hydrolysis of tetraethylorthosilicate as the silica source. Subsequently, in order to reduce the amount of interaction and the agglomeration of Fe3O4@SiO2 microspheres, the silica shell of these particles was modified by vinyltriethoxysilane as the silane coupling agent. The structural properties of the samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy analyses. The results indicated that the average sizes of Fe3O4 and Fe3O4@SiO2 particles were about 50 and 500 nm, respectively. Also, the surface characterization of Fe3O4@SiO2 microspheres showed that the silane coupling agent was covalently coupled with the silica surface.