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12-Tungstophosphoric acid (PW) immobilized on carbon-coated Fe 3 O 4 nanoparticles (Fe 3 O 4 @C-PW) was prepared through a combination of hydrothermal and chemical co-precipitation. The intermediate carbon layer, which was produced from starch as a green material, protects the magnetic core and also improves the dispersion and catalytic activity of the nanoparticles. Characterization of this catalyst was investigated by high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), vibrating sample magnetometry, and the acidic properties were studied by NH 3 -temperature programmed desorption and potentiometric titration. The HRTEM image showed that the catalyst had a well-defined core-shell structure with an average particle size of 50 nm. The characterization data derived from FT-IR reveal that basic structure and geometry of the Keggin anion are preserved after synthesis of Fe 3 O 4 @C-PW. The as-prepared Fe 3 O 4 @C-PW was used as a nanocatalyst for the synthesis of various bis(indolyl)methanes and b-functionalized indoles in water. The catalyst can be recovered simply using an external magnetic field and reused several times without appreciable loss of its catalytic activity.

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