Improvement of the oxidation stability of biodiesel using natural antioxidants

Abstract

Efforts to meet the constantly increasing global energy demand without adverse environmental impacts have led to the development of alternative energy sources. Biodiesel, a biomass alternative, has been identified as a source with a potential substitute for fossil fuel-derived diesel for transportation purposes. Oxidation degradation is one of the main drawbacks to large-scale commercialisation of biodiesel. This study investigated the effect of three natural antioxidants, vitamins A, C and E (OA, OC and OE, respectively), on the oxidation stability of biodiesel derived from waste cooking oil (WCO). The antioxidants were first investigated individually and then in pairs. The effect of antioxidant concentration on oxidation stability was studied using the Rancimat method, which measures the biodiesel's induction period. When employed individually, it was shown that all of the antioxidants, with OC being the most effective, enhanced the oxidative stability of WCO biodiesel. At a concentration of 1000 ppm, OC lengthened the Rancimat induction duration of the WCO-biodiesel from 0.79 to 7 hours. Moreover, using OC as an antioxidant resulted in biodiesel meeting the EN14112/IS-15607 standard. This was explained by OC’s low bond dissociation energy (BDE, 318.5 kJ/mol), low molecular weight (Mw, 176.16 g/mol), and acetyl palmitate production, which is known to exhibit potent antioxidant behaviour in oils. Following OC were OA and OE, which improved the oxidation stability of biodiesel. However, biodiesel prepared with added OA or OE did not meet the ASTM 6751 standard of 3 hours. The combined effects of antioxidants in a 1:1 ratio demonstrated a synergistic influence of a mixture of OA and OC, OC and OE, and OA and OE with induction periods of 12, 7, and 0.33 hours, respectively. It can be concluded that combining natural antioxidants may have more excellent beneficial effects on the oxidation stability of biodiesel prepared from waste cooking oil than using them individually. This study has revealed the potential benefits associated with the use of selected natural antioxidants. Moreover, it offers valuable insight into using natural antioxidants in the biodiesel industry faced with the oxidation stability challenge when considering waste cooking oil as feedstock. Further studies should consider investigating the mechanistic phenomena leading to the observed antioxidant activity at the molecular level to understand and improve antioxidant activity.