Abstract
In this work, the oxidation of ethylcyclohexane was studied in a jet-stirred reactor at 780 Torr, 480–780 K and equivalence ratios of 0.5, 1.0 and 2.0. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the detection of oxidation products, including a series of reactive intermediates such as cycloalkylhydroperoxides, keto-hydroperoxides, alkenal-hydroperoxides and highly oxygenated molecules. Quantum chemistry calculations were performed to obtain ionization energies for the identification of some important intermediates and energy barriers of several important pathways. On the other hand, this work presents the first efforts on developing a low-temperature oxidation model of ethylcyclohexane. The present model can reasonably capture the low-temperature oxidation reactivities and negative temperature coefficient behaviors observed in both present and previous experimental work of ethylcyclohexane oxidation. Modeling analyses were performed to provide insight into the low-temperature oxidation chemistry of ethylcyclohexane. The two-stage O2 addition mechanism is concluded to dominate the chain-branching process in the low-temperature region. The concerted elimination reactions of cycloalkylperoxy radical and “formally direct” chemically activated reactions of cycloalkyl+O2 result in cycloalkenes and HO2 formation at the negative temperature coefficient region and serve as main chain-termination pathways. Compared with smaller cycloalkanes like cyclohexane and methylcyclohexane, the ethyl sidechain structure in ethylcyclohexane reduces the energy barriers of cycloalkylperoxy radical isomerization and facilitates the formation of keto-hydroperoxides which leads to more pronounced low-temperature oxidation reactivity of ethylcyclohexane.