A theoretical investigation on Bell-Evans-Polanyi correlations for hydrogen abstraction reactions of large biodiesel molecules by H and OH radicals

High-accuracy prediction of activation energies and enthalpies of reaction is theoretically important while computationally challenging for accurately determining the kinetic parameters of chemical reactions in biodiesel combustion. In practice, the Bell-Evans-Polanyi (BEP) correlations between the activation energy and the enthalpy of reaction play an important role in fast estimation with acceptable accuracy. In the present study, the BEP correlations for hydrogen abstraction reactions of biodiesel surrogates by H and OH radicals were theoretically investigated by using high-level orbital-based and ONIOM-based methods. Reaction classes for these reactions were defined based on distinctive electron interactions due to the complex characteristics of esters. Linear BEP correlations were established for each reaction class and validated by the high-level calculations, with deviations being less than 0.90 kcal/mol. The rate constants of some representative hydrogen abstraction reactions were calculated by using the BEP correlations and other approximate theories. These rate constants validate the BEP assumption of similar pre-exponential factors for the reactions in the same group, and they are found to agree reasonably well with the available data in literature. Furthermore, improved predictions to experimental data were obtained by using an existing kinetic model of methyl decanoate oxidation updated with the calculated rate constants. The present BEP correlations are believed to provide effective solutions to some kinetic issues of real biodiesel combustion.