Experimental Investigation of the Low Temperature Oxidation of the Five Isomers of Hexane

Abstract

The low-temperature oxidation of the five hexane isomers (n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethylbutane, and 2,3-dimethylbutane) was studied in a jet-stirred reactor (JSR) at atmospheric pressure under stoichiometric conditions between 550 and 1000 K. The evolution of reactant and product mole fraction profiles were recorded as a function of the temperature using two analytical methods: gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental data obtained with both methods were in good agreement for the five fuels. These data were used to compare the reactivity and the nature of the reaction products and their distribution. At low temperature (below 800 K), n-hexane was the most reactive isomer. The two methyl-pentane isomers have about the same reactivity, which was lower than that of n-hexane. 2,2-Dimethylbutane was less reactive than the two methyl-pentane isomers, and 2,3-dimethylbutane was the least reactive isomer. These observations are in good agreement with research octane numbers given in the literature. Cyclic ethers with rings including 3, 4, 5, and 6 atoms have been identified and quantified for the five fuels. While the cyclic ether distribution was notably more detailed than in other literature of JSR studies of branched alkane oxidation, some oxiranes were missing among the cyclic ethers expected from methyl-pentanes. Using SVUV-PIMS, the formation of C2-C3 monocarboxylic acids, ketohydroperoxides, and species with two carbonyl groups have also been observed, supporting their possible formation from branched reactants. This is in line with what was previously experimentally demonstrated from linear fuels. Possible structures and ways of decomposition of the most probable ketohydroperoxides were discussed. Above 800 K, all five isomers have about the same reactivity, with a larger formation from branched alkanes of some unsaturated species, such as allene and propyne, which are known to be soot precursors.

Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

“…The classical low-temperature auto-oxidation reaction scheme presented in Scheme 1 has been confirmed via gas-phase measurements of the key intermediates such as hydroperoxyalkyl radicals [30], alkylhydroperoxides [31,32], large alkenes [32][33][34][35], cyclic ethers [31,34,36,37], and keto-hydroperoxides [33,35,[38][39][40][41][42]. Experiments on liquid-phase auto-oxidation by Korcek and coworkers [43][44][45][46] identified the presence of monohydroperoxides, dihydroperoxides, and keto-hydroperoxides, while recent computational studies [47,48] have shown that subsequent decomposition pathways of keto-hydroperoxides to acids are favorable.…”

“…To provide additional support to the proposed species, the adiabatic ionization energies (IEs) were calculated using the CBS-QB3 method [53] implemented in Gaussian 09 [54]. This method has been used to calculate the adiabatic IEs of species containing one to three oxygen atoms in the low-temperature oxidation of n-butane [41], hexane isomers [33], n-heptane [35], and 2,5-dimethylhexane [34]. Recent work by Moshammer et al [42] on the keto-hydroperoxide formed during dimethyl ether oxidation showed that different conformers need to be considered in the theoretical calculation of species' IEs to correctly interpret the PIE curves.…”

An experimental and kinetic modeling study of n -hexane oxidation

“…In comparison, intermediate length alkanes such as the hexanes have been less studied [23][24][25][26][27].…”

“…Previous work has focused mainly on the high-temperature oxidation of hexane, however few studies have been conducted at low-temperature conditions [25,[27][28]. In 1996, Burcat et al measured ignition delay times for n-hexane-oxygen-argon mixtures in a shock tube over the temperature range of 1020-1725 K at pressures of 1-7 atm [23].…”

“…A recent study was reported by Wang et al [27] where the low-temperature oxidation of the five isomers of hexane were investigated in a jet-stirred reactor (JSR) between 550-1000 K at 1 atm.…”