Temperature-Dependent Catalytic Performance and Selectivity in the Phase-Transfer Epoxidation of Grapeseed and Sunflower Oils

Recently, there has been an increased interest in more eco-friendly epoxidation using vegetable oils to produce environmentally friendly intermediates. The aim of this study was to determine the optimum epoxidation temperature for grapeseed oil and sunflower oil using phase transfer catalysis (PTC). The reaction was carried out in a system: a quaternary phosphonium salt coupled to a heteropolyacid via a 30% hydrogen peroxide solution under solvent-free conditions. Fourier transform infrared (FTIR) spectra of epoxidized grapeseed and sunflower oils showed oxirane peaks at 823 cm−1. 1H NMR confirmed the presence of an epoxide group at 2.00–3.15 ppm, while epoxide 13C NMR signals were present at 52–60 ppm. The elemental contents, i.e., hydrogen, carbon and oxygen, of the crude grapeseed and sunflower oils and the products after epoxidation clearly showed that oxygen was incorporated. The results demonstrated that the optimum temperature depended on both the vegetable oil and the process criterion considered. For grapeseed oil, the highest epoxide number (0.305 mol/100 g) was obtained at 50 °C, whereas the highest apparent conversion of iodine-reactive unsaturation (88.0%) was observed at 40 °C. For sunflower oil, the highest epoxide number (0.271 mol/100 g) and apparent conversion of iodine-reactive unsaturation (92.6%) were obtained at 55 °C. These results demonstrate that temperature optimization in phase-transfer epoxidation should consider not only the conversion of iodine-reactive unsaturation but also the accumulation of oxirane functionality.