Valorization of Rambutan Seed Waste into Biodiesel via Non-Catalytic Supercritical Ethanol and Ethyl Acetate

Rambutan seed waste from fruit processing remains underutilized, while conventional biodiesel routes face high feedstock costs and food-versus-fuel concerns. This study investigated a novel catalyst-free process for biodiesel production from rambutan seed waste using supercritical ethanol and ethyl acetate as renewable reactants to valorize fruit by-products. Batch reactions on the semi-solid fraction of rambutan seed oil (RSO) were conducted at 15 MPa to evaluate the effects of temperature (275–375 °C), reactant-to-oil molar ratio (20:1–40:1), and reaction time (15–50 min) on fatty acid ethyl ester (FAEE) yield. Under optimal conditions, FAEE yields of 59.92 and 41.92% were obtained using ethanol (350 °C, 40:1, 30 min) and ethyl acetate (350 °C, 30:1, 40 min), respectively. However, severe conditions degraded unsaturated esters, revealing a conversion–stability trade-off. The ethanol system exhibited faster reaction kinetics and lower activation energy than ethyl acetate. Applying the optimized ethanol-based conditions to the liquid fraction of RSO, which contained a lower proportion of saturated fatty acids, resulted in a markedly improved FAEE yield of 94.16%. This study demonstrated a catalyst-free supercritical route for converting rambutan seed waste into biodiesel, advancing waste-to-energy strategies and circular bioeconomy.