Thermal Decomposition of Potential Ester Biofuels. Part I: Methyl Acetate and Methyl Butanoate

Abstract

Two methyl esters were examined as models for the pyrolysis of biofuels. Dilute samples (0.06-0.13%) of methyl acetate (CH₃COOCH₃) and methyl butanoate (CH₃CH₂CH₂COOCH₃) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed microreactor were about 20 Torr and residence times through the reactors were roughly 25-150 μs. Reactor temperatures of 300-1600 K were explored. Decomposition of CH₃COOCH₃ commences at 1000 K, and the initial products are (CH₂═C═O and CH₃OH). As the microreactor is heated to 1300 K, a mixture of CH₂═C═O and CH₃OH, CH₃, CH₂═O, H, CO, and CO₂ appears. The thermal cracking of CH₃CH₂CH₂COOCH₃ begins at 800 K with the formation of CH₃CH₂CH═C═O and CH₃OH. By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of CH₃CH₂CH═C═O, CH₃OH, CH₃, CH₂═O, CO, CO₂, CH₃CH═CH₂, CH₂CHCH₂, CH₂═C═CH₂, HCCCH₂, CH₂═C═C═O, CH₂═CH₂, HC≡CH, and CH₂═C═O. On the basis of the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH₂-COOCH₃. The lowest-energy fragmentation will be a 4-center elimination of methanol to form the ketene RCH═C═O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH₂ + CO₂ + CH₃) and (RCH₂ + CO + CH₂═O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH₂═C═O + CH₂═O. The thermochemistry of methyl acetate and its fragmentation products were obtained via the Active Thermochemical Tables (ATcT) approach, resulting in Δ_fH₂₉₈(CH₃COOCH₃) = -98.7 ± 0.2 kcal mol^-1, Δ_fH₂₉₈(CH₃CO₂) = -45.7 ± 0.3 kcal mol^-1, and Δ_fH₂₉₈(COOCH₃) = -38.3 ± 0.4 kcal mol^-1.