Luc-Sy Tran

Ozone-initiated oxidation of cyclohexene bridges the gap between classical alkene ozonolysis and low-temperature combustion chemistry. In this study, it was investigated in jet-stirred reactors using complementary gas-chromatographic measurements over 350–900 K, temperature-dependent synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS) over 370–800 K for only ozone-containing mixtures, and isomer-resolved synchrotron vacuum-ultraviolet photoelectron-photoion coincidence (SVUV-PEPICO) spectroscopy at 400 K under only ozone-containing conditions. Ozone induces substantial low-temperature oxidation, including about 1,000 ppm 1,6-hexanedial, while synchrotron measurements identify cyclic ethers and ketohydroperoxides (KHPs). The temperature-dependent detection of cyclohex-2-en-1-one provides direct evidence for competition between unimolecular peroxy-radical isomerization and bimolecular radical-recombination pathways: the oxyl-radical channel accounts for 72.3% of the modeled branching at 370 K, whereas hydroperoxide formation and isomerization account for 51.5% and 29.2%, respectively, near 575 K. A kinetic model reproduces cyclohexene consumption and the principal product trends, showing that prompt fragmentation dominates the initial ozonolysis step and that subsequent cyclic-peroxy chemistry is strongly temperature dependent.

L'épuisement des réserves pétrolières et l'augmentation de la concentration du gaz à effet de serre CO2 sont les deux principaux problèmes connus liés à l'utilisation des carburants fossiles. Les biocarburants apparaissent comme un des moyens permettant à la fois une diminution de la dépendance au pétrole et une réduction de l'impact néfaste des moteurs automobiles sur l'environnement. Les biocarburants sont en effet considérés comme une source d'énergie renouvelable. L'objectif de cette thèse était de développer et valider les modèles cinétiques de combustion des composés oxygénés de biocarburants : l'éthanol, les biocarburants de deuxième-génération des familles du furane (furane, 2-méthylfurane, 2,5-diméthylfurane), du tétrahydrofurane (tétrahydrofurane, 2-méthyltétrahydrofurane) et le tétrahydropyrane, en utilisant les nouvelles données obtenues en flamme laminaire pré-mélangée à basse pression. De 20 à 60 produits ont été quantifiés par chromatographie en phase gazeuse et identifiés par couplage avec la spectrométrie de masse. Les résultats obtenus ont ensuite été utilisés pour analyser les voies de consommation des réactifs et de formation des produits, surtout pour les polluants, dans le but de mieux comprendre la chimie de la combustion de ces biocarburants. Ce rapport comprend 5 chapitres et une conclusion. Le premier chapitre présente une revue bibliographique des travaux antérieurs sur l'oxydation de l'éthanol et des éthers cycliques. Dans le second chapitre, le dispositif expérimental est décrit, en détaillant en particulier les nouveaux développements. Enfin les chapitres 3, 4, 5 présentent les résultats de l'étude de la combustion des composés étudiés

In contrast to the extensive research on polycyclic aromatic hydrocarbons (PAHs), the reaction chemistry of oxygenated PAHs (OPAHs) during combustion has received little attention, despite their highly detrimental effects. Among OPAHs, anthraquinone, a tricyclic ring structure with two ketone groups, was selected for focus in this study due to its confirmed production in various combustion processes, and no study on its formation chemistry is available in the literature. To elucidate its reaction chemistry, a multifaceted investigation (experimental, theoretical, and modeling) was employed. Fuel-rich oxidation of toluene and the pyrolysis of anisole and 4-methylanisole using a flow reactor were performed to quantify anthraquinone experimentally. Theoretical calculations were conducted for the rate parameters of molecular growth reactions from p-benzoquinone/p-naphthoquinone via Hydrogen-Abstraction-Carbon-Addition (HACA), and a combination of small oxygenated aromatics—benzaldehyde and its radicals. The sub-mechanism involving anthraquinone formation and consumption was developed and integrated with the existing model. The developed model could satisfactorily replicate both the measured data in a flow reactor and the literature data in a flame. To inspect anthraquinone reaction chemistry, kinetic analysis was conducted, demonstrating that three pathways were important for anthraquinone formation: (1) combination of small oxygenated aromatics; (2) HACA mechanism from small quinones, and (3) oxidation of PAHs. However, the predominant formation pathways varied with conditions and fuel. In a flow reactor, combination and oxidation pathways were the primary contributors to anthraquinone formation during pyrolysis and oxidation. Under flame conditions, the relative importance of these three pathways varied dynamically with the height-above-burner.

Understanding the molecular mechanisms governing soot inception is critical for developing accurate models of particulate formation in combustion systems. Polycyclic aromatic hydrocarbons (PAHs) are well established as key precursors/intermediates that participate to soot formation, but the growth reactions towards the particulate phase are still the subject of active debate. Among the proposed chemical mechanisms, aromatic resonance-stabilized radicals (RSRs) offer a promising alternative pathway, but their in situ detection in flames has proven challenging, limiting further mechanistic insight. Here, we report the direct observation and identification of several polycyclic aromatic RSRs, using a combination of synchrotron-based mass-selected vacuum ultraviolet photoelectron spectroscopy and quantum chemical calculations. The existence of both delocalized and localized π-radicals and their specific implication on the different stages of soot formation, via radical-driven PAH reaction mechanisms, is discussed in the context of combustion chemistry.

Clusters bound by weak, non-covalent forces, such as van der Waals interactions and hydrogen bonds, are ubiquitous in dilute media ranging from aerosols to molecular fluids and biological structures, their interest being not only fundamental as in astrochemistry but also more applied as in organic electronics. Neutral clusters of up to six 1-hexene molecules produced by supersonic expansion of a gas mixture were ionized, mass selected, and spectroscopically characterized using synchrotron-based VUV photoelectron photoion coincidence technique. Ionization energies inferred from these measurements show decreasing trends as the cluster size increases, by about 0.5 eV over the range of 1–6 molecules. Dedicated theoretical DFT-based calculations were performed to unravel the possible structures of these clusters and determine their vertical and adiabatic ionization energies. Our computational search for stable structures considered the possible chirality effects associated with most conformers of the monomer having enantiomers, in an approach with a broad structural sampling employing classical force fields followed by systematic re-optimization using an efficient quantum chemical method. Vertical and adiabatic ionization energies obtained using wavefunction-based methods exhibit significant dispersion due to conformational flexibility already in the monomer, but these effects are magnified in clusters due to their fluxionality at the experimental temperature of about 130 K. Overall, the trends obtained for the calculated vertical ionization energies agree well with the measured data and suggest that possible chiral recognition effects that could stabilize specific structures are likely to be hampered under the present experimental conditions.