Thermochemistry of biodiesel oxidation reactions: A DFT study

Density functional theory (DFT) quantum chemical calculations have been used to evaluate the gas phase electronic and thermochemical properties of fatty acid esters. The calculated low relative energies of the corresponding radicals can explain the large variety of oxidation products observed in experiments. The first oxidation reaction step was determined to be non-spontaneous for all fatty acid esters studied. Ethyl and methyl esters showed similar susceptibilities to oxidation in the gas phase. All subsequent reaction steps leading to the secondary products of oxidation were observed to be spontaneous. The thermochemical stability order of one double bond oxidation in the gas phase was determined to be linoleate < γ-linolenate < α-linolenate < oleate < ricinoleate. Therefore, biodiesel produced from castor oil could be used as an additive to biodiesel produced from other vegetable oil sources in order to improve the oxidation stability properties of the final fuel blend.