Green synthesis of copper nanoparticles using Cocoa pod extract and its catalytic activity in deep oxidation of aromatic hydrocarbons

An effective and green technique was performed for the synthesis of copper nanoparticles (CuNPs) from an organic resource using the Cocoa pod (CCP) extract as a reducing agent. The formation of CuNPs was confirmed by ultraviolet–visible absorption spectroscopy (UV–Vis) at the wavelength range of 500–600 nm. The optimized conditions for the synthesis of CuNPs using CCP extract as a reducing agent were determined by the volume ratio of Cu(NO3)2 solution/CCP extract of 3.5/1.5, stirring rate of 300 rpm, pH solution of 7.5, the temperature of 75 °C and the synthesis duration within 180 min. At these conditions, the X-ray diffraction result revealed a face-centered cubic structure of zero-valent copper with a highly crystalline and an average size of 34.4 nm. Fourier transform infrared spectroscopy result confirmed the presence of flavonoids, polyphenolic, and alkaloids components in CCP extract which can act as the reducing and stabilizing agents for CuNPs formation. At the optimized synthetic conditions, CuNPs loaded on various supports (Al2O3, CeO2, and TiO2) were prepared following the same protocol and then applied for catalytic deep oxidation of aromatic hydrocarbons (AHs). Among them, CeO2 was the best support in AHs deep oxidation, and the sample of 7.5Cu–Ce (7.5 wt% of CuNPs supported on CeO2) was the most efficient. Compared with 5Cu–Ce and 10Cu–Ce, the 7.5Cu–Ce sample exhibited a higher benzene conversion at a low temperature (275–325 °C) and reached the full conversions of benzene, toluene, ethylbenzene, and xylene (BTEX) to carbon dioxide and water vapor below 450 °C. Furthermore, the 7.5Cu–Ce sample showed great stability for such reactions at 300 °C as proven by the unchanged conversions of BTEX during 48 h.