Kinetic modeling of methyl pentanoate pyrolysis based on ab initio calculations

Abstract

Recently, methyl pentanoate (MP) was proposed as a viable biodiesel surrogate to petroleum-based fuels. To better understand the pyrolysis chemistry of MP, the unimolecular decomposition kinetics of MP is theoretically investigated on the basis of ab initio calculations; ten primary channels, including four intramolecular H-shifts and six C–C and C–O bond fissions, are identified. The geometries are optimized at the M06-2X/cc-pVTZ level of theory, and accurate barrier heights are determined using the DLPNO-CCSD(T)/CBS(T-Q) method, which shows a good performance against the CCSD(T)/CBS(T-Q) method with an uncertainty of 0.5 kcal mol⁻¹ for small methyl esters. The atomization enthalpy method is adopted to obtain the thermodynamics of involved species. The Rice–Ramsperger–Kassel–Marcus/master equation theory coupled with one-dimensional hindered rotor approximation is employed to calculate the phenomenological rate constants at 500–2000 K and 0.01–100 atm. The branching ratio analysis indicates that two reactions, MP ↔ CH₃OC(O)CH₃ + CH₂CHCH₃ and MP ↔ CH₃OC(O)CH₂ + CH₂CH₂CH₃, are the dominant channels at low and high temperatures, respectively. The model from Diévart et al. [Proc. Combust. Inst., 2013, 34(1), 821–829] is updated with our calculations, and the modified model can yield a better prediction in reproducing the ignition delay times of MP at high temperatures. This work provides a comprehensive investigation of MP unimolecular decomposition, and can serve as a prototype for understanding the pyrolysis of larger alkyl esters.