Fatty acid methyl esters (FAME) represent a promising alternative to fossil middle distillates. The availability of predictive tools for the design of cutting-edge combustion technologies is strongly dependent upon a comprehensive understanding of ester oxidation kinetics. However, gas-phase oxidation of FAME at low temperatures (600-900 K) remains poorly understood. Due to the significant impact of chain length and unsaturation of esters on their low-temperature combustion and pollutant formation pathways , it is desirable to consider C5+ unsaturated esters as biodiesel surrogate compounds. Previous studies mainly focused on H-abstraction from the alkyl chain and the subsequent long-debated O2 addition , overlooking the impact of ester function on abstraction kinetics [3,4]. The reaction of esters with HO2 is expected to have a significant role in autoignition under low temperature conditions. This system has been widely studied either in the context of olefin oxidation [5-6] or as a product channel for alkylperoxy decomposition [7-8]. To our knowledge, no exhaustive kinetic modeling study on HO2 interaction with an unsaturated ester has been carried out.
The present study aims at unraveling the reaction mechanism involved in the interaction of a biodiesel surrogate, methyl trans-3-hexenoate, with hydroperoxy radicals using a bottom-up methodology, involving ab initio electronic structure calculations and a theoretical kinetics study. Using post-Hartree-Fock methods coupled with extended basis sets, an accurate potential energy surface was obtained for the major reaction channels identified. The corresponding kinetic rate constants were obtained by solving the master equation using an efficient eigenvalue based method . The addition of HO2 to the carbonyl and oxygen of the ester and the impact of the ester function on the H abstraction kinetics were extensively studied. This study helps resolve the current controversy regarding the effect of stereoisomerism coupled with an ester moiety . The obtained rate constants were finally implemented into several existing kinetic mechanisms, resulting in overall better agreement with experimental concentration profiles [11-12].
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