Gold-supported magnetically recyclable nanocatalysts: a sustainable solution for the reduction of 4-nitrophenol in water

In this work mesoporous silica-coated manganese(II) ferrite (MnFe2O4) magnetic nanoparticles functionalized with amine and thiol groups were prepared and used as supports for the in situ immobilization of gold nanoparticles (Au NPs). The resulting Au-supported magnetic nanocatalysts, denoted as Mn@SiO2-NH2@Au and Mn@SiO2-SH@Au, were tested in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), at room temperature in water, in the presence of NaBH4 reducing agent. This organic compound is typically used in the production of pesticides and dyes and commonly found in the resulting wastewaters. The chemical, morphological, textural and magnetic properties of the nanosupports and resulting Au-supported nanocatalysts were characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray diffraction, N2 adsorption-desorption isotherms at -196°C and SQUID magnetometry. The influence of the type of organosilane linker between the magnetic nanosupport and the Au NPs on the in situ immobilization of the Au NPs was evaluated: Mn@SiO2-NH2@Au presented lower Au loading than Mn@SiO2-SH@Au, but the anchored Au NPs showed a higher degree of crystallinity. The magnetic Au-supported nanocatalysts led to almost 100% reduction of 4-NP to 4-AP, monitored by UV-vis spectroscopy, with the reaction time depending on the type of nanocatalyst/linker: 12 and 17 min, for the amine- and thiol-based nanocatalysts respectively, what corresponded to pseudo first-order rate constants normalized for Au loading of K = 6117 mmol-1 min-1 and 827 mmol-1 min-1, respectively. Both catalysts could be efficiently recovered by magnetic separation and were highly stable upon reuse in four further cycles, preserving their catalytic performance with negligible Au leaching.