Chitosan supported biosynthesized gold nanoparticles as catalyst for oxidation and reduction reactions

Abstract

For many years, gold was considered as an inert catalyst. However, gold nanoparticles (AuNPs) are now attracting significant attention as they show different chemical and physical properties depending on size and shape. AuNPs have been recognised as an active and effective catalyst for organic transformations. Currently, there is a growing need to develop environmentally benign metal nanoparticle synthesis process that does not use any toxic chemicals. In this regard, biological approach for metal nanoparticle synthesis using microorganisms, enzymes, plants or plant extracts have been applied as possible eco-friendly alternatives to chemical and physical methods. In this research, an environmental friendly method for the synthesis and stabilization of AuNPs by the reduction of aqueous AuCl4 - ions using leaf extract of Psidium guajava is reported. The optimized parameters for the formation of AuNPs are 10 mL of 1 mM Au(III) ions, 1 mL of 1% leaf extract at pH 6 and at 30 minutes reaction time. UV-Vis spectroscopic analysis of the reaction mixture confirmed the successful reduction of Au(III) ions to Au(0). Transmission electron microscopic (TEM) analysis revealed a dominant spherical morphology of the AuNPs with average size of 7.6 nm. X-ray diffraction (XRD) analysis of the purified gold nanoparticles showed five Bragg reflection peaks at 2? of 38.42 Å, 44.45 Å, 64.71 Å, 77.61 Å and 81.87 Å corresponding to the (111), (200), (220), (311) and (222) lattice planes respectively that could be indexed to the face centred cubic (fcc) structure of gold. Fourier transform infrared (FTIR) spectroscopic analysis on the purified AuNPs revealed the presence of surface adsorbed biomolecules during the preparation. A chitosan supported AuNPs heterogeneous catalyst has been synthesised and characterized by various physicochemical techniques such as UV-Vis spectroscopy, XRD, TEM analysis, FTIR spectroscopic analysis and X-ray photoelectron spectroscopy (XPS). The catalytic performance of the chitosan supported AuNPs was examined in two types of organic reactions. The catalyst showed excellent catalytic activity in oxidation reaction with complete conversion of benzyl alcohol with 97% selectivity to benzaldehyde at 80?C after 6 hours. The catalyst could be recycled at least four times without significant loss in the conversion. Catalytic activity of the chistosan-GLA/AuNPs was also tested for the reduction of 4-nitrophenol to 4-aminophenol using an excess of Psidium guajava leaf extract as a reducing agent instead of NaBH4 complete reduction of 4- nitrophenol occurred within 15 minutes at room temperature.