Low Temperature Oxidation Kinetics of Biodiesel Molecules: Rate Rules for Concerted HO₂ Elimination from Alkyl Ester Peroxy Radicals

In an attempt to construct detailed kinetic mechanisms for biodiesel fuels on the fly, high-pressure rate rules for the concerted HO2 elimination reaction class were derived using a comprehensive training reaction set of more than 60 reactions involving different alkyl methyl/ethyl ester peroxy radicals (RCOOR′)–OO•. Using the composite electronic structure method CBS-QB3 in combination with classical statistical mechanics and the transition state theory (TST) rate model, high-pressure rate constants for the reactions in the training set as well as thermodynamic properties for the species involved were calculated. The corrections from Eckart tunneling and hindered internal rotation (HIR) treatments were also included in the calculations. The results reveal that the ester group (−COO−) selectively promotes the reaction when compared with the traditional hydrocarbon fuels; thus it is recommended that the seven derived rate rules for the title reaction class (including the thermodynamic data of the species involved in the NASA format) should be used for construction of detailed kinetic mechanisms for real biodiesel molecules