Effect of binary antioxidant heterodimer properties on the oxidation stability enhancement of palm oil biodiesel

Palm Biodiesel is one of the renewable energy sources which offers many advantages over petroleum diesel such as high biodegradability, feedstocks sustainability, cetane number, and lower exhaust emissions. It has also been widely used as substitute for diesel fuel which consumption has reached more than 30% of total oil and gas fuel consumption in Indonesia. However, biodiesel degrades over time due to autoxidation and may adversely affect its desired properties. Addition of synthetic antioxidant, e.g., Tert-Butylhydroquinone (TBHQ) and Butylated Hydroxytoluene (BHT) into biodiesel has been successfully reduced the low oxidation stability problem. Furthermore, binary mixture of these two antioxidants (BHT:TBHQ) has been proven to have synergism with the addition of 500 ppm concentration and ratio of 1:3 due to heterodimer formation, thus yields higher biodiesel oxidation stability than those with individual antioxidant. Though the synergism of these antioxidants has been previously discovered, the cause of this synergism in biodiesel has still yet to be fully understood. Dispersion test with measurement of biodiesel samples upper-to-lower density ratio and UV-Vis Spectrophotometry delta absorbance was carried out for sample which has synergism which is BHT:TBHQ (1:3). It is found that palm biodiesel with 1:3 BHT:TBHQ antioxidant ratio has higher dispersion than those with its individual antioxidant with delta absorbance of 0.025 for 1000 ppm of antioxidant and 0.000 for 2000 ppm of antioxidant, along with density of 1.000577 g/ml for 1000 ppm of antioxidant and 1.000866 g/ml for 2000 ppm of antioxidant, while also possess synergism. This implies that the higher dispersion of the binary antioxidant is significant for the occurrence of the binary antioxidant synergism, and subsequently higher oxidation stability of the biodiesel which might be due to its heterodimer formation.