Estudo cinético da abstração do hidrogênio do acetato de metila

Abstract

The need to lower the pollutant emissions from fossil fuel burning motivated the search for alternative fuels to solve this problem and lower our dependency on fossil fuels. Biodiesel is seen as an excellent candidate to replace Diesel oil, and since 2008 a biodiesel fraction is added to the petrol Diesel. The tendency is to raise the biodiesel ratio in the Diesel continuously. Due to the advantages of biodiesel and its complexity, theoretical studies about its ignition seem to be relevant. However, a computational study of fatty acid esters ignition is inviable.For this reason, the study of small methyl esters, like methyl ethanoate can help gain insights into biodiesel ignition and improve the known combustion models. The ignition delay affects the combustion quality related to the fuel ignition. This process occurs by hydrogen abstraction from the fuel, followed by reactions involving free radicals. A detailed description of such reactions is important to improve combustion kinetic models. The present study uses DFT methods and Transition State Theory to investigate the hydrogen abstraction reactions on methyl ethanoate by O, OH, HO2, H radicals, and O2 molecule. The chosen functionals were B3LYP-D3 and M06-2X with cc-pVDZ, cc-pVTZ, aug-cc-pVDZ, and aug-cc-pVTZ to conduct a methodologic evaluation. Further, single-point calculations at CCSD(T)/cc-pVTZ method were conducted considering the geometries optimized at M06-2X/aug-cc-pVTZ level and the kinetic parameters calculated at 250-3500 K, assuming the frequencies obtained at the DFT level. Tunneling hindered rotations, and anharmonic corrections were also included in k(T) calculations. The analysis of the results at 298K and the Arrhenius curves shows that the B3LYP-D3 method overestimates the calculated rate coefficients compared to the M06-2X functional. The evaluation of the level of theory indicated the functional choice is more important than the basis set and double-zeta basis sets are insufficient to calculate precise rate coefficients. The inclusion of vibrational anharmonicity seems necessary for k(T) calculation since this effect significantly lowers the calculated coefficients.