Structure-reactivity trends of C1-C4 alkanoic acid methyl esters

Structure-reactivity trends are investigated by means of high temperature shock tube ignition and quantum chemical calculations for four alkanoic acid methyl esters-methyl formate (MF), methyl acetate (MA), methyl propanoate (MP), and methyl butanoate (MB). Ignition delay times are compared at constant argon/oxygen ratios, equivalence ratios and average pressures. It is observed that MA consistently shows longer ignition delay times than the other three esters, while MF and MB have comparable ignition delay times but different activation energies. MP ignition delay times are also comparable with MB in most cases but are found to be slightly shorter, especially under lean conditions. Simulated ignition delay times using the chemical kinetic model by Westbrook et al. [1] also show that MA has longer ignition delay times than MF, although the agreement between model prediction and experiment for MA is not as good as that for MF at 10. atm. Simulated ignition delay times for MB using two recent MB chemical kinetic models do not predict the same ignition delay times as the small ester mechanism under conditions where MF and MB experimental ignition delay times are found to be comparable. Ab initio quantum mechanical calculations are performed using the composite method CBS QB3, in order to determine bond dissociation energies for the four esters, as well as activation barriers for possible fuel H-abstraction reactions by H atoms. The concerted unimolecular decomposition of the esters to yield methanol and a ketene (or CO in the case of MF) is also studied. The relative reactivity observed in experiments for the four esters can be partially attributed to differences in bond energies and the calculated rates obtained in this study. Further work on possible reaction pathways and subsequent reactions of resulting primary radicals from small methyl esters is motivated by this study.