Quantum Chemical Study of Autoignition of Methyl Butanoate

Methyl butanoate is a widely studied surrogate for fatty acid esters used in biodiesel fuel. Here we report a detailed analysis of the thermodynamics and kinetics of the autoignition chemistry of methyl butanoate. We employ composite CBS-QB3 calculations to construct the potential energy profiles of radicals derived from methyl butanoate. We compare our results with recently published G3MP2B3 results for reactions of peroxy (ROO(•)) and hydroperoxy alkyl ((•)QOOH) radicals and comment on differences in barrier heights and reaction enthalpies. Our emphasis, however, is on hydroperoxy alkylperoxy ((•)OOQOOH) radicals that are critical for autoignition of diesel fuel. We examined four classes of reactions: peroxy radical interconversion of (•)OOQOOH ((•)OOQOOH→ HOOQOO(•)), H-migration reactions (from carbon to oxygen), HO2 elimination, and cyclic ether formation with elimination of OH radical. We evaluate the significance of reaction pathways by comparing rate coefficients in the high-pressure limit. Unexpectedly, we find a low activation barriers for 1,8 H-migration of RC(═O)OCH2OO(•). We also find peroxy radical interconversion of (•)OOQOOH radicals from methyl butanoate commonly possess the lowest barriers of any unimolecular reaction of these radicals, despite that they proceed via 8-, 10- and 11-member ring transition states. At temperatures relevant to autoignition, these peroxy radical interconversions are dominant or significant reaction pathways. This means that some (•)OOQOOH radicals that were expected to be produced in negligible yields are, instead, major products in the autoignition of methylbutanoate (MB). These reactions have not previously been considered for MB, and will require revision of models of autoignition of methyl butanoate and other esters.