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

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 mu s. 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 equivalent to 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 beta 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 Delta H-f(298)(CH3COOCH3) = -98.7 +/- 0.2 kcal mol(-1), Delta H-f(298)(CH3CO2) = -45.7 +/- 0.3 kcal mol(-1), and Delta H-f(298)(COOCH3) = -38.3 +/- 0.4 kcal mo1(-1).