Refining the chain-branching process in the low-temperature oxidation of 1-hexene with synchrotron-based PEPICO spectroscopy

Abstract

Understanding the chemical reactions that occur during the low-temperature oxidation of alkenes is crucial for developing advanced energy conversion devices, as alkenes are significant components of realistic fuels. Despite extensive experimental and theoretical studies, the oxidation chemistry of alkenes remains less understood compared to that of alkanes. The present study investigates the low-temperature oxidation of 1-hexene in a jet-stirred reactor, using synchrotron-based PEPICO spectroscopy at a fixed temperature (T = 625 K) and an equivalence ratio (ϕ=0.5). Previous studies in the literature have suggested that the discrepancy between experiments and predictions from kinetic models might be due to the kinetics considered for the chain branching process. The sensitivity of the PEPICO technique to molecular structure allowed for a comprehensive examination of the conformational landscape, facilitating the differentiation of isomers among the main intermediates: hydroperoxides, cyclic ethers, and ketohydroperoxides. Other intermediates and primary products were also quantified and compared to the predictions of a kinetic model from the literature. The results confirm that kinetic models overpredict the formation of hydroxyl ketohydroperoxides and that alkenyl hydroperoxides are the primary chain-branching agents during 1-hexene oxidation. This work underscores the need for a reassessment of the branching ratio between O2-addition and the isomerization of hydroxy alkyl radicals.