H3PW12O40 supported on silica-encapsulated γ-Fe2O3 nanoparticles: a novel magnetically-recoverable catalyst for three-component Mannich-type reactions in water

A new type of magnetically-recoverable catalyst was synthesized by the immobilization of H3PW12O40 on the surface of silica-encapsulated gamma-Fe2O3 nanoparticles. This catalyst was characterized by transmission electron microscopy (TEM), a laser particle size analyzer, infrared spectroscopy (IR) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results show that the particles are mostly spherical in shape and have an average size of approximately 94 nm. The characterization data derived from IR spectroscopy reveal that H3PW12O40 on the support exists in the Keggin structure. The acidity of the catalyst was measured by a potentiometric titration with n-butylamine. To our surprise, this very strong solid acid catalyst showed an excellent distribution of acid sites, suggesting that the catalyst possesses a higher number of surface active sites compared to its homogeneous analogues. The activity of the catalyst was probed through one-pot three-component Mannich-type reactions of aldehydes, amines and ketones in water at room temperature. The excellent conversions show that the catalyst has strong and sufficient acidic sites, which are responsible for its catalytic performance. After the reaction, the catalyst/product separation could be easily achieved with an external magnetic field, and more than 95% of the catalyst could usually be recovered. The catalyst was reused at least five times without any loss of its high catalytic activity.