Computational Kinetic Study for the Unimolecular Decomposition Pathways of Cyclohexanone

There has been evidence lately that several endophytic fungi can convert lignocellulosic biomass into ketones among other oxygenated compounds. Such compounds could prove useful as biofuels for internal combustion engines. Therefore, their combustion properties are of high interest. Cyclohexanone was identified as an interesting second-generation biofuel (Boot, M.; et al. Cyclic Oxygenates: A New Class of Second-Generation Biofuels for Diesel Engines? Energy Fuels 2009, 23, 1808−1817; Klein-Douwel, R. J. H.; et al. Soot and Chemiluminescence in Diesel Combustion of Bio-Derived, Oxygenated and Reference Fuels. Proc. Combust. Inst. 2009, 32, 2817–2825). However, until recently (Serinyel, Z.; et al. Kinetics of Oxidation of Cyclohexanone in a Jet- Stirred Reactor: Experimental and Modeling. Proc. Combust. Inst. 2014; DOI: 10.1016/j.proci.2014.06.150), no previous studies on the kinetics of oxidation of that fuel could be found in the literature. In this work, we present the first theoretical kinetic study of the unimolecular decomposition pathways of cyclohexanone, a cyclic ketone that could demonstrate important fuel potential. Using the quantum composite G3B3 method, we identified six different decomposition pathways for cyclohexanone and computed the corresponding rate constants. The rate constants were calculated using the G3B3 method coupled with Rice–Ramsperger–Kassel–Marcus theory in the temperature range of 800–2000 K. Our calculations show that the kinetically more favorable channel for thermal decomposition is pathway 2 that produces 1,3-butadien-2-ol, which in turn can isomerize easily to methyl vinyl ketone through a small barrier. The results presented here can be used in a future kinetic combustion mechanism

Experimental and Modeling Study of the Oxidation of Two Branched Aldehydes in a Jet-Stirred Reactor: 2‑Methylbutanal and 3‑Methylbutanal

Aldehydes, especially formaldehyde and acetaldehyde, are common intermediates in the oxidation of many hydrocarbons and biofuels. Alcohols are known to produce important quantities of aldehydes. This work describes the oxidation of 2-methylbutanal and 3-methylbutanal (isopentanal) at both low and high temperatures. The former aldehyde is a main intermediate in 2-methylbutanol oxidation while the latter in isopentanol oxidation. Experiments were conducted in a jet-stirred reactor (JSR) at a pressure of 10 atm, for equivalence ratios of 0.35, 0.5, 1, 2, and 4 and over the temperature range 500–1200 K. The mean residence time was kept constant (700 ms). Concentration profiles of stable species were measured using gas chromatography and Fourier transform infrared spectroscopy. A detailed chemical kinetic mechanism including oxidation of various hydrocarbon and oxygenated fuels was extended to include the oxidation chemistry of both aldehydes; the resulting mechanism was used to simulate the present experiments. Both aldehydes showed negative temperature coefficient behavior in lean mixtures mainly due to the production of <i>sec</i>- and isobutyl radicals from 2- and 3-methylpentanal oxidation, respectively

Experimental and Detailed Kinetic Modeling Study of Cyclopentanone Oxidation in a Jet-Stirred Reactor at 1 and 10 atm

Cyclopentanone oxidation was studied in a jet-stirred reactor at 1 and 10 atm and over the temperature range of 730–1280 K for fuel-lean (φ = 0.5), stoichiometric, and fuel-rich (φ = 2) mixtures. A total of 16 reaction intermediates and products were identified and quantified using online Fourier transform infrared spectrometry and offline gas chromatography. A kinetic submodel was developed, supported by theoretical calculations for the rate constants of hydrogen abstraction reactions by H atoms and OH and CH₃ radicals at the MP2/aug-cc-pVDZ level of theory. The resulting model consisting of 343 species involved in 2065 reactions was used to simulate the present experiments and showed good agreement with the data. The main oxygenated intermediates are aldehydes, and cyclopentenone was also found to be an important species for cyclopentanone oxidation. The rate of production analyses showed that cyclopentanone is mainly consumed by a sequence of reactions producing CO and the but-1-en-4-yl radical. Unimolecular reactions reported in the literature were found to have a very low contribution to the fuel consumption in our experimental conditions. It was finally highlighted that some of the discrepancies observed between the simulation and experiments arise from the chemistry of cyclopentenone that would need to be more detailed

Experimental and Modeling Study of the Oxidation of 1‑Butene and <i>cis</i>-2-Butene in a Jet-Stirred Reactor and a Combustion Vessel

Significant amounts of unsaturated hydrocarbons, such as butene isomers, are formed as intermediate products during the oxidation of higher hydrocarbons. In this study, new experimental data were obtained for the oxidation of 1-butene and <i>cis</i>-2-butene. The experiments were conducted in a jet-stirred reactor in the temperature range of 900–1440 K, at atmospheric pressure, for different equivalence ratios (0.25 ≤ φ ≤ 2), and in a combustion vessel at <i>p</i> = 1 atm and unburned gas temperatures in the range of 300–450 K. From gas sampled in the jet-stirred reactor, concentration profiles of stable species were measured by gas chromatography and infrared spectrometry. A combustion vessel was used to determine laminar burning velocities of butene–air mixtures at atmospheric pressure and over the equivalence ratio range of 0.8–1.4. Additional data were obtained over a range of pressure (1–5 atm). A detailed chemical kinetic mechanism based on a previously proposed scheme for the oxidation of hydrocarbons was used to reproduce the present experimental data (201 species involved in 1787 reactions). The present mechanism was also tested against literature data: the structure of 1-butene premixed low pressure flat flames and 1-butene/oxygen/argon mixtures ignition delays were simulated, showing satisfactory agreement. Sensitivity analyses and reaction paths analyses were used to rationalize the results. Finally, the oxidations of <i>cis</i>-2-butene and <i>trans</i>-2-butene were compared and discussed

A Shock Tube Study of <i>n</i>- and <i>iso-</i>Propanol Ignition

An understanding of the ignition and oxidation characteristics of propanol, as well as other alcohols, is important toward the development and design of combustion engines that can effectively utilize bioderived and bioblended fuels. Building upon a database for “first-generation” alcohols including methanol and ethanol, the ignition characteristics of the two isomers of propanol (<i>n</i>-propanol and <i>iso</i>-propanol) have been studied in a shock tube. Ignition delay times for propanol/oxygen/argon mixtures have been measured behind reflected shock waves at temperatures ranging from approximately 1350 to 2000 K and a pressure of 1 atm. Equivalence ratios of 0.5, 1.0, and 2.0 have been used. Pressure measurements and CH* emissions were used to determine ignition delay times. The influences of equivalence ratio, temperature, and mixture strength on ignition delay have been characterized and compared to the behavior seen with a newly developed detailed kinetic mechanism. The overall trends are captured fairly well by the mechanism, which include a greater level of reactivity for the <i>n</i>-propanol mixtures relative to <i>iso</i>-propanol at the conditions used in this study