A Systematic Theoretical Kinetics Analysis for the Waddington Mechanism in the Low-Temperature Oxidation of Butene and Butanol Isomers

Characterization of the Autoxidation of Terpenes at Elevated Temperature Using High-Resolution Mass Spectrometry: Formation of Ketohydroperoxides and Highly Oxidized Products from Limonene

Low-temperature experiments on the oxidation of limonene-O₂-N₂ mixtures were conducted in a jet-stirred reactor (JSR) over a range of temperatures (520-800 K) under fuel-lean conditions (equivalence ratio φ = 0.5) with a short residence time (1.5 s) and a pressure of 1 bar. Collected samples of the reaction mixtures were analyzed by (i) online Fourier transform infrared spectroscopy (FTIR) and (ii) Orbitrap Q-Exactive high-resolution mass spectrometry after direct injection or chromatographic separation using reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) and soft ionization (with positive or negative heated electrospray ionization and atmospheric-pressure chemical ionization). H/D exchange using deuterated water (D₂O) and a reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) were performed to probe the presence of OH, OOH, and C═O groups in the oxidized products. A broad range of oxidation products ranging from water to highly oxygenated products containing five and more O atoms were detected (C₇H₁₀O_4,5, C₈H₁₂O_2,4, C₈H₁₄O_2,4, C₉H₁₂O, C₉H₁₄O_1,3-5, C₁₀H₁₂O₂, C₁₀H₁₄O_1-9, C₁₀H₁₆O_2-5, and C₁₀H₁₈O₆). Mass spectrometry analyses were only qualitative, and quantification was performed with FTIR. The results are discussed in terms of reaction routes involving the initial formation of peroxy radicals, H atom transfer, and O₂ addition sequences producing a large set of chemical products, including ketohydroperoxides and more oxygenated products. Carbonyl compounds derived from the Waddington oxidation mechanism on exo- and endo-double bonds (C═C) were observed in addition to their products of further oxidation. Products of the Korcek mechanism (carboxylic acids and carbonyls) were also detected.

Combustion of n-C3-C6 Linear Alcohols: An Experimental and Kinetic Modeling Study. Part I: Reaction Classes, Rate Rules, Model Lumping, and Validation

This work (and the companion paper, Part II) presents new experimental data for the combustion of n-C3-C6 alcohols (n-propanol, n-butanol, n-pentanol, n-hexanol) and a lumped kinetic model to describe their pyrolysis and oxidation. The kinetic subsets for alcohol pyrolysis and oxidation from the CRECK kinetic model have been systematically updated to describe the pyrolysis and high- and low-temperature oxidation of this series of fuels. Using the reaction class approach, the reference kinetic parameters have been determined based on experimental, theoretical, and kinetic modeling studies previously reported in the literature, providing a consistent set of rate rules that allow easy extension and good predictive capability. The modeling approach is based on the assumption of an alkane-like and alcohol-specific moiety for the alcohol fuel molecules. A thorough review and discussion of the information available in the literature supports the selection of the kinetic parameters that are then applied to the n-C3-C6 alcohol series and extended for further proof to describe n-octanol oxidation. Because of space limitations, the large amount of information, and the comprehensive character of this study, the manuscript has been divided into two parts. Part I describes the kinetic model as well as the lumping techniques and provides a synoptic synthesis of its wide range validation made possible also by newly obtained experimental data. These include speciation measurements performed in a jet-stirred reactor (p = 107 kPa, T = 550-1100 K, φ = 0.5, 1.0, 2.0) for n-butanol, n-pentanol, and n-hexanol and ignition delay times of ethanol, n-propanol, n-butanol, n-pentanol/air mixtures measured in a rapid compression machine at φ = 1.0, p = 10 and 30 bar, and T = 704-935 K. These data are presented and discussed in detail in Part II, together with detailed comparisons with model predictions and a deep kinetic discussion. This work provides new experimental targets that are useful for kinetic model development and validation (Part II), as well as an extensively validated kinetic model (Part I), which also contains subsets of other reference components for real fuels, thus allowing the assessment of combustion properties of new sustainable fuels and fuel mixtures.

Pressure-dependent kinetics of peroxy radicals formed in isobutanol combustion

Bio-derived isobutanol has been approved as a gasoline additive in the US, but our understanding of its combustion chemistry still has significant uncertainties. Detailed quantum calculations could improve model accuracy leading to better estimation of isobutanol's combustion properties and its environmental impacts. This work examines 47 molecules and 38 reactions involved in the first oxygen addition to isobutanol's three alkyl radicals located α, β, and γ to the hydroxide. Quantum calculations are mostly done at CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor corrections obtained at B3LYP/6-31G(d). The resulting potential energy surfaces are the most comprehensive isobutanol peroxy networks published to date. Canonical transition state theory and a 1-D microcanonical master equation are used to derive high-pressure-limit and pressure-dependent rate coefficients, respectively. At all conditions studied, the recombination of γ-isobutanol radical with O₂ forms HO₂ + isobutanal. The recombination of β-isobutanol radical with O₂ forms a stabilized hydroperoxy alkyl radical below 400 K, water + an alkoxy radical at higher temperatures, and HO₂ + an alkene above 1200 K. The recombination of β-isobutanol radical with O₂ results in a mixture of products between 700-1100 K, forming acetone + formaldehyde + OH at lower temperatures and forming HO₂ + alkenes at higher temperatures. The barrier heights, high-pressure-limit rates, and pressure-dependent kinetics generally agree with the results from previous quantum chemistry calculations. Six reaction rates in this work deviate by over three orders of magnitude from kinetics in detailed models of isobutanol combustion, suggesting the rates calculated here can help improve modeling of isobutanol combustion and its environmental fate.