Biosynthesis of copper (II) oxide nanoparticles using murraya koenigii leaf extract and their catalytic activity in the reduction of nitroaromatic compounds

Abstract

The applications of copper-based nanoparticles have generated a great deal of interest in the field of catalysis. The natural abundance of copper and its relatively low cost make copper-based nanoparticles a viable alternative to catalysts made from expensive precious metals, such as platinum and palladium. In this study, a rapid, simple and green method was developed for the synthesis of copper oxide nanoparticles (CuO NPs) using Murraya koenigii leaves aqueous extract as reducing and stabilising agent. The antioxidant activity of the Murraya koenigii leaves extract was determined using ferric reducing antioxidant potential (FRAP) assay. During the bioreduction, several parameters were optimized, namely the volume of leaf extract, pH, reaction temperature and reaction time. The optimum conditions obtained for the biosynthesis were 3 mL of leaf extract, pH 11, room temperature and reaction time of 50 minutes. The biosynthesised CuO NPs were characterized using ultraviolet-visible (UV-Vis) spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The UV-Vis absorption spectrum confirmed the formation of CuO NPs with the appearance of a surface plasmon resonance band at ?max 634-670 nm. Meanwhile, the FTIR spectroscopic analysis of CuO NPs showed the presence of surface adsorbed biomolecules from the leaf extract that acted as the reducing and stabilizing agents. XRD analysis displayed a series of diffraction peaks that are consistent with monoclinic structure of copper(II) oxide, CuO (JCPDS No. 45-0937). HRTEM images revealed that the CuO NPs were mostly spherical with a mean diameter of 8.4 nm. In the presence of sodium borohydride, NaBH4, the biosynthesized CuO NPs demonstrated good catalytic activity in the reduction of nitroaromatic compounds. The reduction of 4-nitrophenol to 4-aminophenol showed 98% conversion and was maintained for three cycles, indicating no significant loss of catalytic activity. The reaction followed a pseudo-first-order kinetics with a rate constant value of 4.3 × 10-3 s-1. The product of the catalytic reaction was isolated and purified. The purified product was characterized using nuclear magnetic resonance (NMR) and FTIR spectroscopic techniques. The catalytic reduction of other nitroaromatic compounds with various substituents, namely, nitrobenzene, 4-nitrotoluene, 4-nitrobenzaldehyde, 4- nitroaniline, 2-methyl-4-nitrophenol, 2-nitrotoluene and 3-nitrotoluene was also investigated. Nitroaromatic compounds with electron donating group exhibited higher reaction rates compared to nitroaromatic compounds with electron withdrawing group.