Upcycling Waste to Wealth: CuO-SiO₂/reduced graphene nanocomposite from pomegranate peels for one-pot low-temperature conversion of waste oils into valuable fatty acid monomers

Abstract

The utilisation of heterogeneous catalysts in producing fatty acid monomers can minimise the separation cost and hence reduce the price of the fatty acid monomers. This study reports for the first time a novel, environmentally benign, highly active copper oxide-silica oxide/reduced graphene oxide (CuO-SiO₂/RGO), heterogeneous nano-catalyst derived from waste pomegranate peels, for the one-pot, low-temperature synthesis of fatty acid monomers from high-acid-value waste vegetable oil (WVO). The synthesised nano-catalyst was extensively characterised using XRD, FT-IR, TEM, SEM, EDX and TGA-DTA. Further, it was utilised to synthesise fatty acid-rich oleic phenoxypropyl acrylate (OPA) monomer from high acid value WVO via a single-step reaction. The process parameters for the synthesis of OPA monomer using CuO-SiO₂/RGO catalyst have been optimised using response surface methodology (RSM) and found to be 8.5:1 reactant molar ratio, 3.5 % (w/w) catalyst loading, 54 °C temperature, and 9.5 h reaction time, where the highest OPA monomer yield was 95.73 % under optimum conditions. The CuO-SiO₂/RGO exhibited stable catalytic performance after regeneration with an OPA yield of 93.1 ± 0.37 % after five consecutive runs. The plausible reaction mechanism unveiled that the direct synthesis of OPA monomer from high acid value WVO occurred through both transesterification and esterification reactions simultaneously on the surface of CuO and SiO2 catalyst supported on RGO sheets. The adaptation of waste pomegranate peels into a high-value CuO-SiO₂/RGO nano-catalyst offers a new direction for clean, one-pot and low-temperature production of sustainable fatty acid monomers from high-acid-value WVO.