Theoretical study on low-temperature oxidation kinetics of methyl pentanoate

Abstract

In order to develop reliable combustion models of biodiesel, the low-temperature oxidation kinetics of methyl pentanoate (MP), the smallest methyl ester with the negative temperature coefficient (NTC), is theoretically investigated. We use the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ and CASPT2/CBS(T-Q)//CASSCF(7e,5o)/cc-pVTZ methods to explore the potential energy surfaces (PESs) of the decomposition of different MP peroxy radicals, including dissociation, isomerization, and concerted HO2 elimination. Comparisons among the PESs of MP and other methyl esters reveal that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. Based on the determined PESs, the pressure- and temperature-dependent rate constants for the low-temperature oxidation of MP are computed at 298−1200 K and 0.01−100 atm. The kinetic calculations suggest that the dissociation rate constants of different methyl ester peroxy radicals are similar at the same active C site, and the channels that lead to tri-molecular products are significant during the low-temperature oxidation of large methyl esters. The literature combustion model of MP is revised with our calculations. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.Novelty and Significance statementMethyl pentanoate (MP) is the smallest methyl ester with the negative temperature coefficient (NTC), but the low-temperature oxidation kinetics in the available combustion model of MP is still insufficient to interpret its NTC behavior. To develop a reliable combustion model for MP, the potential energy surfaces (PESs) of the decompositions of different MP peroxy radicals are investigated. Results show that the ester functional group inherently affects the low-temperature oxidation kinetics of methyl esters, and the energies of the species along the PESs are closely related to the active C site and the alkyl chain length. With our calculated rate constants, the literature combustion model is revised. For the first time, the NTC behavior of MP is theoretically predicted by the revised model, and an excellent agreement between the predictions and the measurements reflects the reliability of the present low-temperature oxidation subset of MP.