Mechanism and Kinetics of Low-Temperature Oxidation of a Biodiesel Surrogate: Methyl Propanoate Radicals with Oxygen Molecule

Abstract

This paper presents a computational study on the low-temperature mechanism and kinetics of the reaction between molecular oxygen and alkyl radicals of methyl propanoate (MP), which plays an important role in low-temperature oxidation and/or autoignition processes of the title fuel. Their multiple reaction pathways either accelerate the oxidation process via chain branching or inhibit it by forming relatively stable products. The potential energy surfaces of the reactions between three primary MP radicals and molecular oxygen, namely, C(•)H2CH2COOCH3 + O2, CH3C(•)HCOOCH3 + O2, and CH3CH2COOC(•)H2 + O2, were constructed using the accurate composite CBS-QB3 method. Thermodynamic properties of all species as well as high-pressure rate constants of all reaction channels were derived with explicit corrections for tunneling and hindered internal rotations. Our calculation results are in good agreement with a limited number of scattered data in the literature. Furthermore, pressure- and temperature-dependent rate constants for all reaction channels on the multiwell-multichannel potential energy surfaces were computed with the quantum Rice-Ramsperger-Kassel (QRRK) and the modified strong collision (MSC) theories. This procedure resulted in a thermodynamically consistent detailed kinetic submechanism for low-temperature oxidation governed by the title process. A simplified mechanism, which consists of important reactions, is also suggested for low-temperature combustion at engine-like conditions.

Theoretical studies on the hydrogen abstraction reactions of methyl esters with HO2radical and the following β-scission reactions

“…The PESs are explored by using the CBS‐QB3 composite method because this method was shown to be able to obtain potential energy barrier and enthalpy of formation for a large set of molecules with a mean absolute error about 1.0 kcal/mol in previous studies and has been widely used to study mechanisms and kinetics in combustion and atmosphere chemistry. Specifically, geometry optimization for all the species along the PES are performed at the B3LYP/CBSB7 level implemented in the CBS‐QB3 method.…”

“…Further, the following β‐scission reactions of the forming methyl ester radicals via abstraction reactions play a critical role in the description of high‐temperature combustion processes because the products from the β‐scission reactions completely determine the combustion characteristics of the fuels . Although a series of theoretical studies were performed for hydrogen abstraction reactions through radical mechanism for typical compounds and thermal decomposition reactions of saturated methyl esters, little attention has been focused on the effect of the C = C double bond on the elementary abstraction reactions of methyl esters. Moreover, the following β‐scission reactions of the forming radicals are also not considered in previous theoretical studies.…”

Influence of the double bond on the hydrogen abstraction reactions of methyl esters with hydrogen radical: an ab initio and chemical kinetic study

“…Under combustion relevant conditions, hydrogen atom abstraction reactions by small radicals (H, OH, O, HO 2 , and CH 3 ) from fuel molecules are always important in the oxidation of fuels. 16,17 Although a series of theoretical and kinetic studies have been performed on the elementary reactions of saturated esters, [18][19][20][21][22][23][24] however, little attention has been given to obtain a better understanding of the effect of the double bond on the elementary reaction kinetics of methyl esters. In particular, the hydrogen abstraction reactions by H radical for methyl esters, which are one of the major fuel consumption pathways during oxidation, are not well understood at combustion relevant conditions.…”

Ab Initio Kinetics of Initial Thermal Pyrolysis of Isopropyl Propionate: A Revisited Study

“…14 The geometry optimization and frequency calculations employ the hybrid density functional theory, namely, the B3LYP/CBSB7 15−17 level of theory. Found to be suitable for investigating the detailed kinetics of the similar reactions of methyl propionate radicals 18 /methyl acetate radicals 19 with an O 2 molecule, the composite method CBS-QB3, 20 based on the B3LYP/CBSB7 geometry, was used to obtain the energies of all species involved. The HIR corrections were thoroughly taken into account in thermodynamic/kinetic calculations, in which the hindrance potentials of the rotation along the C−C and C−O "single" bonds were calculated at the B3LYP/CBSB7 level via relaxed surface scans (cf.…”