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

摘要

Biodiesel mechanism development poses interrelated challenges. Kinetic parameters of interest for thenumerous steps with implications for low-temperature biodiesel combustion can be determined largely viacomputational means, but the computational approaches that best balance expense and accuracy in generatingthese parameters have not been systematically determined. A CO2 production pathway recentlyproposed in the literature to occur in the methyl esters commonly used as surrogates for complex biofuelchemistry provides an opportunity to explore both these areas. We quantified this previously-proposedCO2 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 andsuggest 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 quantitativelyand qualitatively well with benchmark G3B3 data, while density functional theory (DFT) approaches generallyfailed to achieve the accuracy or precision desirable in determining reaction barriers. Potential energysurfaces generated via G3MP2B3 calculations were used to explore kinetic parameters for reactions implicatedin early CO2 production and competing pathways for methyl esters; these parameters were comparedto extant mechanistic data.