Experimental characterization of n-heptane low-temperature oxidation products including keto-hydroperoxides and highly oxygenated organic molecules (HOMs)

Abstract

The oxidation of n-heptane was performed in a jet-stirred reactor (JSR) at 10 atm, an equivalence ratio of 0.5, and an initial fuel concentration of 1000 ppm at a residence time of 1 s in the temperature range 580–790 K (from cool-flame, negative temperature coefficient NTC, to intermediate temperature oxidation regime), and with 5000 ppm of fuel at 647 K and a residence time of 1.5 s. Low-temperature products formed in JSR were characterized using high-resolution mass spectrometry analyses (HRMS). Atmospheric pressure chemical ionizations (APCI) was used in positive and negative modes for MS analyses. Both flow injection analyses (FIA) or ultra-high-pressure liquid chromatography-OrbitrapⓇ coupling were used to characterize hydroperoxides (C7H16O2), keto-hydroperoxides (C7H14O3), cyclic ethers (C7H14O), carboxylic acids (C2H4O2, C3H6O2, C4H8O2), ketones (C3–5H6–10O), diones (C7H12O2), and highly oxygenated molecules (C7H14O5, C7H14O7, C7H14O9, C7H14O11) resulting from the addition of up to six O2 molecules on fuel radicals. H/D exchange with D2O was used to confirm the presence of –OH or –OOH groups in the products. Several available kinetic reaction mechanisms were tested against the present measurements of keto-hydroperoxides showing significant discrepancies.