Antioxidative capacity evaluation of imine compounds as metal ions chelators and free radical scavengers in biodiesel

Abstract

Factors affecting the oxidation stability of commercially available biodiesel were primarily investigated using the acid value (AV), peroxide value (PV), 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay, and thermogravimetric (TG) analysis in the presence of imine antioxidants. In this study, the imine compounds N,N′-bis-(4-octadecanate)-salicyl ethylenediamine (stearic dhben) and N,N′-bis-(4-hexadecanate)-salicyl ethylenediamine (palmitic dhben) were synthesized to evaluate their abilities as metal chelators and oxygen scavengers. The AV analysis showed a slight difference between the samples with and without copper. The PV tests demonstrated that both imines were efficient free radical scavengers and copper ion chelators at both room temperature and 50 °C, outperforming the commercial antioxidants butylhydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ). The Derivative Thermogravimetry (DTG) curves indicated that both imines were more effective compared to commercial antioxidants. In the DPPH assay, it was observed that the palmitic dhben imine exhibited the best performance, with an half maximal inhibitory concentration (IC50) of 12.4·10−5 mol L−1. Therefore, both stearic and palmitic dhben imines act as efficient biodiesel antioxidants at room temperature and 50 °C, functioning as excellent metal chelators and free radical scavengers. However, stearic dhben demonstrated better performance as a metal chelator, whereas palmitic dhben was more effective as a free radical scavenger.