Insights into the inhibitive mechanism of chelators for biodiesel oxidation induced by Cu2+/Fe2+: An experimental and computational study

Biodiesel has attracted widespread attention as a renewable energy source. However, the presence of metal ion contaminants in biodiesel can lead to a decrease in fuel oxidative stability performance and alter fuel physicochemical properties, greatly limiting its large-scale application. This study proposes an effective method to inhibit biodiesel oxidation catalyzed by Cu 2+ /Fe 2+ via complexation of metal ions by chelators. Two typical chelators (1-Hydroxyethylidene-1,1-diphosphonic acid, HEDP; 2-phosphonobutane-1,2,4-tricarboxylic acid, PBTCA) were tested for their capability to improve the oxidation stability of biodiesel. The addition of HEDP was able to counteract the undesirable catalyzed oxidation effects on biodiesel induced by both Cu 2+ and Fe 2+ , while PBTCA was only effective for the case induced by Fe 2+ . With 0.05% HEDP addition, the oxidation induction times of biodiesel samples contaminated by Cu 2+ and Fe 2+ were extended by about 12 and 20 times, respectively. For the case of 0.05% PBTCA addition, the oxidation induction time of Fe 2+ contaminated biodiesel sample was extended by about 20 times, while that of Cu 2+ contaminated biodiesel sample was only extended by about 37%. Fourier transform infrared spectroscopy analysis indicated that either the addition of HEDP or PBTCA has little effect on the chemical structure of biodiesel. Molecular dynamics calculations were then carried out to unravel the inhibitive mechanism of HEDP and PBTCA on biodiesel oxidation. The complexation of Cu 2+ /Fe 2+ by HEDP and PBTCA can block the chain branching pathways, reduce the generation of active radicals and thus inhibit biodiesel oxidation.