Author: Chen, Qinxue
Title: Influence of torsional anharmonicity and addition reactions on hydrogen abstraction reactions of methylbutanoate with hydroperoxyl radical
Advisors: Zhang, Peng (ME)
Degree: M.Sc.
Year: 2015
Subject: Biodiesel fuels -- Research.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xiii, 113 leaves : color illustrations ; 30 cm
Language: English
Abstract: Methyl butanoate (MB) is a representative surrogate fuel for biodiesel and its combustion chemistry has been gaining increasing interest in recent years. This work presents an ab initio chemical kinetics study of hydrogen abstraction reactions of MB with hydroperoxyl radical (HO₂). Important addition reactions followed by isomerization and β-scission reactions were also investigated. The geometry optimization and vibrational frequency calculation of all the molecular structures on the potential energy surface of MB + HO₂ were conducted with Density Functional Theory using B3LYP functional. Higher level stationary point energies were obtained at the level of QCISD(T)/CBS. The H-abstraction on β site was predicted to have the lowest energy barrier. Transition states with hydrogen bonding or not were considered as distinctive and non-swapable by simply internal rotation according to discrepancies of the energy, entropy and 1-D hindered rotor treatment. The transition state theory (TST) was used to calculate the high-pressure limit rate constants over a board range of temperature (500-2000 K). Phenomenological rate coefficients for temperature- and pressure-dependent reactions were calculated by the time-dependent multiple-well master equation. Low-frequency torsional modes of reactants and transition states were replaced by one-dimensional hindered rotors approximation and internal-coordinate multi-structural (MS) approximation, respectively. Comparison of the results show the ratio of calculated pre-exponential factor by using these two approximations can be larger than one order of magnitude. M06-2X functional and the Moller-Plesset (MP2) method employed to attain the uncertainty of frequency calculation on pre-exponential factor, exhibited one order of magnitude smaller value than present theory using B3LYP functional. The predicted rate constants were compared with the available theoretical data, and agreed well with total high-pressure rate constants obtained from the widely used reaction mechanism of MB. All the calculation details were considered carefully and were expected to be useful in understanding the future theoretical study of combustion modeling of methyl butanoate.
Rights: All rights reserved
Access: restricted access

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