Preparation and characterization of superparamagnetic nanocomposites of aluminosilicate/silica/magnetite

Three types of SPASMs (i.e., aluminosilicate materials incorporated with superparamagnetic particles) comprised of magnetite, silica and aluminosilicate have been successfully synthesized via three sequential steps: chemical precipitation of nano-sized Fe3O4, coating of SiO2 on Fe3O4 using an acidifying method or sol–gel route, and further surface functionalization to form aluminosilicates adopting sol–gel or aluminum substitution methods. The characteristics of the resulting composites were well identified using scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), superconducting quantum interference device (SQUID), solid nuclear magnetic resonance (solid NMR), nitrogen adsorption and electrophoresis. The IR and XRD spectra can well explain the bonding interaction and crystal structures of various composites respectively. The poor crystallinity of silica in the magnetic carrier can promote the substitution of Al in the silica framework. The SPASM obtained from surface functionalization via the sol–gel route possesses a greater ratio of Al(IV)/Al(VI) confirmed by the solid NMR spectra, larger surface acidity verified by SEM/EDX, and higher sensitivity to the pH value of solution near pHPZC of composites. Furthermore, such superparamagnetic materials can easily overcome the difficulty in the solid–liquid separation process when applied in the heterogeneous system.