Cool-flame chemistry of the representative bio-hybrid fuel 1,3-dioxane
Résumé
The low-temperature oxidation of 1,3-dioxane was systematically investigated in two atmospheric pressure jetstirred reactors (JSRs) at temperatures ranging from 450 to 850 K and equivalence ratios ranging from 0.25 to 0.5. A suite of oxidation intermediates, including carbonyl compounds, conjugated olefins, cyclic ethers, and reactive hydroperoxides, were identified and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). The experiments reveal strong low-temperature reactivity and a pronounced negative temperature coefficient (NTC) behavior, that had not previously been reported for 1,3-dioxane to such an extent. A detailed kinetic model was developed to interpret the observed phenomena. Rate constants for hydrogen abstraction reactions by OH radicals, identified as the dominant fuel consumption pathways, are calculated using ab initio methods. The model also incorporates theoretically derived rate constants from the literature for key beta-scission ring-opening reactions and first-stage oxygen addition pathways of three 1,3-dioxanyl radicals. These inclusions improve the model's predictive capability and highlight the complex cool-flame chemistry associated with 1,3-dioxane. Model validation against experimental datasets from this study and literature-including JSR oxidation, flow reactor pyrolysis and oxidation, and ignition delay time (IDT) measurements-demonstrates good agreement. Mechanistic insights reveal that the ether group in the 1,3-dioxane ring facilitates hydrogen abstraction at the ortho positions (e.g., methylene bridge and ortho-CH2-) while suppressing abstraction at meta sites. These structural effects also influence intramolecular hydrogen shifts in ROO and OOQOOH radicals. Additionally, the presence of ring oxygen atoms weakens radical stabilization through inductive effects, promoting ring-opening reactions of R and QOOH species. Collectively, these factors contribute to the observed NTC behavior and the unique cool-flame characteristics of 1,3-dioxane.
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