Experimental and modeling investigation of the low-temperature oxidation of n-heptane

The low-temperature oxidation of n-heptane, one of the reference species for the octane rating of gasoline, was investigated using a jet-stirred reactor and two methods of analysis: gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) with direct sampling through a molecular jet. The second method allowed the identification of products, such as molecules with hydroperoxy functions, that are not stable enough to be detected using gas chromatography. Mole fractions of the reactants and reaction products were measured as a function of temperature (500–1100 K), at a residence time of 2 s, at a pressure of 800 Torr (1.06 bar) and under stoichiometric conditions. The fuel was diluted in an inert gas (fuel inlet mole fraction of 0.005). Attention was paid to the formation of reaction products involved in the low-temperature oxidation of n-heptane, such as olefins, cyclic ethers, aldehydes, ketones, species with two carbonyl groups (diones), and ketohydroperoxides. Diones and ketohydroperoxides are important intermediates in the low-temperature oxidation of n-alkanes, but their formation have rarely been reported. Significant amounts of organic acids (acetic and propanoic acids) were also observed at low temperatures. The comparison of experimental data and profiles computed using an automatically generated detailed kinetic model is satisfactory overall. A route for the formation of acetic and propanoic acids was proposed. Quantum calculations were performed to refine the consumption routes of ketohydroperoxides toward diones.