M. Ribaucour

A rapid compression machine investigation of oxidation and auto-ignition of n-heptane: measurements and modeling

n-Heptane oxidation and auto-ignition in a rapid compression machine is studied in the low and intermediate temperature regimes at high pressures. Experimental ignition delay times and some phenomenological aspects related to knock in engines are presented, providing additional information at lower temperatures on previously published delays from shock tube experiments. The products of oxidation are identified and time profiles are measured during a two-stage ignition process. Eight C7 heterocycles, heptenes, lower 1-alkenes, aldehydes, and carbon monoxide are the main species. Their origin is discussed in relation to the isomerization and decomposition of heptylperoxy radicals. The high selectivity observed in the formation of lower 1-alkenes is explained by the scission of the β CC bond of the β-hydroperoxyheptyl radicals weakened by the presence of oxygen atoms. Numerical simulation of the experiments with Warnatzs comprehensive chemical mechanism gives satisfactory results for cool flame and total ignition delays, but fails to reproduce the detailed chemistry before auto-ignition.

The production of benzene in the low-temperature oxidation of cyclohexane, cyclohexene, and cyclohexa-1,3-diene

Abstract The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa-1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to benzene from cyclohexane. Auto-ignition delay times were measured and concentration-time profiles of the C 6 intermediate products of oxidation were measured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neither a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3-diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The identified C 6 products belong to three families: the bicyclic epoxides and cyclic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes, which are always the major products. The formation of C 6 products from cyclohexane is explained by the classical scheme for low-temperature oxidation, taking into account addition of O 2 to cyclohexyl radicals and the various isomerizations of the resulting peroxy radicals. Most of the C 6 products from cyclohexene are predicted by the same scheme, beginning with the formation of the allylic cyclohexenyl radical. However, addition of HO 2 to the double bond has to be included to predict the formation of 1,2-epoxycyclohexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C 6 oxygenated products are only formed by addition of HO 2 to the double bond. For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO 2 to double bonds, or by addition of O 2 to the initial cyclic radical.

Autoignition Delays of a Series of Linear and Branched Chain Alkanes in the Intermediate Range of Temperature

A set of ignition data of linear and branched chain alkanes (n-butane, n-pentane, neopentane, n-heptane, and isooctane) measured in an original rapid compression machine is provided. It allows a comparison of the ignition conditions of pressure, temperature and equivalence ratio for these hydrocarbons. Detailed mechanisms from different research groups based on a similar generic scheme of hydrocarbon oxidation are tested against the experimental ignition delays. The differences between experimental and modeling results are discussed.

The low-temperature autoignition of alkylaromatics: Experimental study and modeling of the oxidation of n-butylbenzene

The low-temperature oxidation of n -butylbenzene, an intermediate structure between alkanes and short-chain alkylaromatics, was studied between 640 and 840 K by rapid compression and by modeling. Delay times of one- and two-stage autoignitions were measured, and intermediate species after the cool flame were analyzed. First, a detailed mechanism for n -butane was developed with existing material. Then, an n -butylbenzene mechanism was built by taking into account the change of reactivity due to the introduction of the aromatic nucleus. Both mechanisms have been validated by simulations of the delays and the product concentrations. Finally, the n -butylbenzene mechanism was used to analyze the main low-temperature reaction pathways. The comparative calculation of the concentrations of alkyl, alkylperoxy, and hydroperoxyalkyl radicals in the cool flame of n -butane and n -butylbenzene illustrates the effects of the aromatic nucleus on the first steps of oxidation. A study of the competitive channels to the main molecular intermediate species shows that the internal transfer of a benzylic hydrogen to the peroxy sites is a major event in the development of reactions leading to branching and ignition. This can explain a previous observation that alkylaromatics with two oitho -alkyl groups or a long single lateral chain have the possibility of an internal transfer of a benzylic hydrogen and manifest a greater low-temperature reactivity than aromatics that have neither oitho -alkyl groups nor a long lateral chain.

A theoretical study of the kinetics of the benzylperoxy radical isomerization.

The rate constant of the benzylperoxy isomerization reaction has been computed using 54 different levels of theory and has been compared to the experimental value reported at 773 K. The aim of this methodology work is to demonstrate that standard theoretical methods are not adequate to obtain quantitative rate constants for the reaction under study. The use of the elaborated CASPT2 method is essential to estimate a quantitative rate constant. Geometry optimizations and vibrational frequency calculations are performed using three different methods (B3LYP, MPW1K, and MP2) and six different basis sets (6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), and cc-pVDZ). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the 6-31G(d,p) basis set, and with the CASPT2 level of theory with the ANO-L-VDZP basis set. Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. We recommend the use of the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory to compute the temperature dependence of the rate constant of the four-center isomerization of the benzylperoxy radical. It is given by the following relation: k(600-2000 K) (in s (-1)) = (1.29 x 10 (10)) T (0.79) exp[(-133.1 in kJ mol (-1))/ RT]. These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. This computational procedure can be extended to predict rate constants for other similar reactions where no available experimental data exist.

Theoretical study of H-abstraction reactions from CH3Cl and CH3Br molecules by ClO and BrO radicals.

The rate constants of the H-abstraction reactions from CH(3)Cl and CH(3)Br molecules by ClO and BrO radicals have been estimated over the temperature range of 300-2500 K using four different levels of theory. Calculations of optimized geometrical parameters and vibrational frequencies are performed using B3LYP and MP2 methods combined with the cc-pVTZ basis set. Single-point energy calculations have been carried out with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (perturbatively) electron excitations CCSD(T) using the cc-pVTZ and cc-pVQZ basis sets. Canonical transition-state theory combined with an Eckart tunneling correction has been used to predict the rate constants as a function of temperature. In order to choose the appropriate levels of theory with chlorine- and bromine-containing species, the reference reaction Cl ((2)P(3/2)) + CH(3)Cl → HCl + CH(2)Cl (R(ref)) was first theoretically studied because its kinetic parameters are well-established from numerous experiments, evaluation data, and theoretical studies. The kinetic parameters of the reaction R(ref) have been determined accurately using the CCSD(T)/cc-pVQZ//MP2/cc-pVTZ level of theory. This level of theory has been used for the rate constant estimation of the reactions ClO + CH(3)Cl (R(1)), ClO + CH(3)Br (R(2)), BrO + CH(3)Cl (R(3)), and BrO + CH(3)Br (R(4)). Six-parameter Arrhenius expressions have been obtained by fitting to the computed rate constants of these four reactions (including cis and trans pathways) over the temperature range of 300-2500 K.

A CASPT2 Theoretical Study of the Kinetics of the 2-, 3-, and 4-Methylbenzylperoxy Radical Isomerization

The rate constants of the 2-, 3-, and 4-methylbenzylperoxy isomerization reactions have been computed using the elaborated CASPT2 method. Geometry optimizations and vibrational frequency calculations are performed with two methods (B3LYP and MPW1K) combined with the cc-pVDZ and 6-31+G(d,p) basis sets, respectively. Single-point energy calculations are performed at the CASPT2/ANO-L-VDZP//B3LYP/cc-pVDZ level of theory as recommended by Canneaux et al. (J. Phys. Chem. A 2008, 112, 6045). Canonical transition-state theory with a simple Wigner tunneling correction is used to predict the high-pressure limit rate constants as a function of temperature. They are given by the following relations for the 2-, 3-, and 4-methylbenzylperoxy (MBP) (1,3s) isomerizations and for the 2-methylbenzylperoxy (1,6p) isomerization, respectively: k(2-MBP(1,3s))(600-2000 K) (in s(-1)) = (3.33 x 10(10))T(0.79) exp((-142.6 in kJ mol(-1))/RT); k(3-MBP(1,3s))(600-2000 K) (in s(-1)) = (0.74 x 10(10))T(0.79) exp((-130.7 in kJ mol(-1))/RT); k(4-MBP(1,3s))(600-2000 K) (in s(-1)) = (1.12 x 10(10))T(0.79) exp((-133.6 in kJ mol(-1))/RT); k(2-MBP(1,6p))(600-2000 K) (in s(-1)) = (5.10 x 10(8))T(0.85) exp((-87.1 in kJ mol(-1))/RT). These parameters can be used in the thermokinetic models involving aromatic compounds at high pressure. In the case of the 2-methylbenzylperoxy radical, the (1,6p) H-atom transfer reaction is consistently the most important channel over the studied temperature range, and the (1,3s) H-atom transfer reaction is not energetically favored.

Thermochemical data and additivity group values for ten species of o-xylene low-temperature oxidation mechanism.

o-Xylene could be a good candidate to represent the family of aromatic hydrocarbons in a surrogate fuel. This study uses computational chemistry to calculate standard enthalpies of formation at 298 K, Δ(f)H°(298 K), standard entropies at 298 K, S°(298 K), and standard heat capacities C(p)°(T) over the temperature range 300 K to 1500 K for ten target species present in the low-temperature oxidation mechanism of o-xylene: o-xylene (1), 2-methylbenzyl radical (2), 2-methylbenzylperoxy radical (3), 2-methylbenzyl hydroperoxide (4), 2-(hydroperoxymethyl)benzyl radical (5), 2-(hydroperoxymethyl)benzaldehyde (6), 1-ethyl-2-methylbenzene (7), 2,3-dimethylphenol (8), 2-hydroxybenzaldehyde (9), and 3-hydroxybenzaldehyde (10). Δ(f)H°(298 K) values are weighted averages across the values calculated using five isodesmic reactions and five composite calculation methods: CBS-QB3, G3B3, G3MP2, G3, and G4. The uncertainty in Δ(f)H°(298 K) is also evaluated. S°(298 K) and C(p)°(T) values are calculated at B3LYP/6-311G(d,p) level of theory from molecular properties and statistical thermodynamics through evaluation of translational, rotational, vibrational, and electronic partition functions. S°(298 K) and C(p)°(300 K) values are evaluated using the rigid-rotor-harmonic-oscillator model. C(p)°(T) values at T ≥ 400 K are calculated by treating separately internal rotation contributions and translational, external rotational, vibrational, and electronic contributions. The thermochemical properties of six target species are used to develop six new additivity groups taking into account the interaction between two substituents in ortho (ORT/CH2OOH/ME, ORT/ET/ME, ORT/CHO/OH, ORT/CHO/CH2OOH) or meta (MET/CHO/OH) positions, and the interaction between three substituents (ME/ME/OH123) located one beside the other (positions numbered 1, 2, 3) for two- or three-substituted benzenic species. Two other additivity groups are also developed using the thermochemical properties of benzenic species taken from the literature: the C/CB/H2/OO and the CB/CO groups. These groups extend the capacities of the group additivity method to deal with substituted benzenic species.