Magnetically recoverable biosynthesised gold nanoparticles as catalysts for oxidation of benzyl alcohol and reduction of 4-nitrophenol

In recent years, gold nanoparticles (AuNPs) have received considerable attention owing to their unique properties which are promising in diverse fields and applications such as biomedical science and catalysis. AuNPs has been considered as the catalyst of choice for numerous organic reactions. Vast numbers of chemical, physical and biological strategies have been employed to synthesise AuNPs. Among these approaches, the biological method employing plant extract is gaining attention as it is simple and environmentally friendly. In this research, a green biosynthetic approach for the preparation of AuNPs using aqueous leaf extract of Polygonum minus as reducing and stabilising agent is described. The reduction of Au(III) ions to elemental Au occurred rapidly and it was completed within 20 min at room temperature as monitored by ultraviolet-visible (UV-Vis) spectroscopy. High resolution transmission electron microscopy/energy-dispersive X-ray (HRTEM/ EDX) and X-ray diffraction (XRD) analytical data indicated that the nanoparticles were in fcc crystalline shape, mostly icosahedral and nearly monodispersed with an average size of 23 ± 5.1 nm. Fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV) analyses of the AuNPs and the leaf extract revealed that the oxidised (quinone) form of quercetin and myricetin were presumably the main stabilising agents in the formation of stable nanoparticles. The biosynthesised AuNPs showed good catalytic activity, with a turnover frequency (TOF) of 85.2 h-1 for the oxidation of benzyl alcohol and a normalised rate constant, Knor of 0.06 s-1 mmol-1 in the reduction of 4-nitrophenol. The same bioreduction process was employed in the preparation of AuNPs catalysts supported on highly branched metforminfunctionalised silica-coated magnetite (Fe3O4-SiO2-Met). The structural, surface and magnetic properties of the support material (Fe3O4-SiO2-Met) was investigated by FTIR, XRD, HRTEM/EDX, elemental carbon-hydrogen-nitrogen (CHN), thermogravimetry (TGA) and vibrating sample magnetometry (VSM) analyses. The XRD, HRTEM/EDX, X-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (AAS) analytical data revealed that AuNPs with smaller average sizes (6.1 ± 2.2 nm and 16.2 ± 8.3 nm) were well-dispersed on the Fe3O4- SiO2-Met support. Under optimum benzyl alcohol oxidation reaction conditions, 0.3% Au/Fe3O4-SiO2-Met catalyst displayed an enhanced catalytic performance as compared to the unsupported AuNPs, with a TOF improvement factor of 2.5. Meanwhile, the catalytic performance of the 6% Au/Fe3O4-SiO2-Met catalyst showed enhancement with an increase in the normalised rate constant, Knor value by a factor of 8.8 as compared to the unsupported AuNPs under an optimised 4-nitrophenol reduction reaction conditions. The supported AuNPs catalyst could be easily recovered magnetically and reused for at least four times and three times in the oxidation and reduction reactions, respectively, without significant loss of activity