A study of the low-temperature oxidation of a long chain aldehyde: n-hexanal

Large aldehydes are important species among the oxygenated products formed during the oxidation of fuels (e.g. alkanes or alkenes) and biofuels (e.g. long chain alcohols), but their oxidation chemistry has been scarcely studied up to now. In this work, a study of the oxidation of hexanal has been performed in a jet-stirred reactor over the temperature range 475-1100 K, at a residence time of 2 s, a pressure of 106.7 kPa, an inlet fuel mole fraction of 0.005 and at three equivalence ratios (0.25, 1 and 2). Reaction products were quantified using two analytical methods: gas chromatography and cw-cavity ring-down spectroscopy. In addition to species usually measured during oxidation study (such as CO, CO2, water, H2O2 and C1-C3 olefins, aldehydes and ketones), specific low temperature oxidation species were also observed: C6 species, such as δ- and γ-caprolactones and hexanoic acid, but also C5 species, such as 2-methyl-tetrahydrofuran and pentanal which are typical reaction products detected during the oxidation of n-pentane. A detailed kinetic model has been developed using an automatic generation software. This model was used to highlight the specificities of the oxidation chemistry of long chain aldehydes and to understand the formation routes of the C5 and C6 low-temperature oxidation products. Simulations indicate that the low energy of the aldehyde CH bond has a significant effect on the fuel reactivity and on the distribution of reaction products.