Cool Flame of Methylcyclohexene Isomers in a Jet-Stirred Reactor: Orbitrap Characterization of Highly Oxidized Products

Terpenes which could be used as biofuels have relatively complex chemical structures and oxidation pathways. Therefore, studying the oxidation of simpler model-fuels, such as substituted cyclohexene isomers, could be useful. In this work, the oxidation of three isomers of methylcyclohexene (MCHX) was performed in a jet-stirred reactor over a temperature range of 500–1150 K under fuel-lean conditions (equivalence ratio φ = 0.5), with a residence time of 2 s and a pressure of 10 bar. The mole fractions of the MCHX isomers were measured using gas chromatography and flame ionization detection. A relatively strong cool flame was observed under the present experimental conditions (60–70% fuel conversion), allowing the specific low-temperature oxidation products to be analyzed qualitatively using Orbitrap Q-Exactive high-resolution mass spectrometry. This was done after direct injection, or chromatographic separation via reversed-phase ultrahigh-performance liquid chromatography (RP-UHPLC), and soft ionization using atmospheric pressure chemical ionization in both positive and negative modes. H/D exchange using deuterated water (D2O) was performed to assess the presence of OH or OOH groups in the products. The reaction of 2,4-dinitrophenylhydrazine (2,4-DNPH) with oxidation samples was used to assess the presence of C═O containing compounds. A large number of oxidation products, including highly oxygenated organic molecules (HOMs) with more than five oxygen atoms, were observed. Unexpectedly, numerous products containing carboxylic acid and carbonyl functional groups, as well as aromatic compounds, were detected. According to this study, the MCHX isomers oxidize in a very similar manner, sharing many common oxidation products and species pertaining to the same chemical classes. Routes for the formation of oxidation products were proposed to rationalize the results.