Exploring the oxidative decompositions of methyl esters: Methyl butanoate and methyl pentanoate as model compounds for biodiesel

Biodiesel mechanism development poses interrelated challenges. Kinetic parameters of interest for the numerous steps with implications for low-temperature biodiesel combustion can be determined largely via computational means, but the computational approaches that best balance expense and accuracy in generating these parameters have not been systematically determined. A CO2 production pathway recently proposed in the literature to occur in the methyl esters commonly used as surrogates for complex biofuel chemistry provides an opportunity to explore both these areas. We quantified this previously-proposed CO2 production pathway using composite (G3B3) calculations and identified areas for potential side reactions, in both methyl butanoate and methyl pentanoate. Alternative isomerizations were also examined and suggest that side reactions may play an increasingly larger role in the chemistry of long-chain methyl esters, compared to methyl butanoate. In methodological terms, G3MP2B3 calculations matched quantitatively and qualitatively well with benchmark G3B3 data, while density functional theory (DFT) approaches generally failed to achieve the accuracy or precision desirable in determining reaction barriers. Potential energy surfaces generated via G3MP2B3 calculations were used to explore kinetic parameters for reactions implicated in early CO2 production and competing pathways for methyl esters; these parameters were compared to extant mechanistic data.