Magnetic nanoparticles: Improving chemical stability via silica coatingand organic grafting with silanes for acidic media catalytic reactions

tMagnetic nanoparticles based on Fe3O4(MNP) were coated with a silica (with thin or thick layers) priorto grafting with (3-aminopropyl) triethoxysilane (APTES) and octyltriethoxysilane (OTES) to increasetheir hydrophobicity and their pH stability. The synthetic conditions for the APTES grafting were chosento achieve either mono- or polylayers of chemically attached silane on the surface. The synthesizedFe3O4@SiO2- APTES and Fe3O4@SiO2-OTES core-shell particles were characterized by different methodsincluding XRD, TEM, FTIR-ATR, N2physisorption, ICP AES and XPS. The stability tests showed that coatingwith silica shell and grafting with organosilanes considerably improve the stability of MNP at pH 3; thethicker the silica layer the less iron oxide dissolved. The MNP-based materials showed catalytic activity inthe esterifications of propionic acid (PA) with n-butanol and of oleic acid (OA) with trimethylolpropane.In the PA containing medium the particles partly leach into a liquid medium contrary to the OA containingmedium where they seemed to be resistant to leaching