L'étude théorique de la co-oxydation de deux substances oxydables RH et R'H est effectuée systématiquement à partir d'un schéma cinétique général et en envisageant successivement les différents cas possibles. Dans cette étude, on admet que RH s'oxyde, en l'absence de R'H, selon un mécanisme radicalaire en chaînes longues, alors que R'H peut, en l'absence de RH, s'oxyder selon un mécanisme radicalaire en chaînes longues, courtes ou par stades. A partir de cette étude systématique, on propose des techniques simples qui permettent de comparer des résultats expérimentaux et le mécanisme proposé. Les conclusions déduites de ce dernier sont confrontées aux résultats expérimentaux de la co-oxydation du cumène et-de la décaline. The theoretical study of the cooxidation of two oxidizable compounds, RH and R'H, is systematically carried out by using a general kinetic scheme and considering the different possible cases. In this study, it is assumed that the oxidation of RH in the absence of R'H takes place according ta a long-chain radical mechanism, whereas the oxidation of R'H in the absence of RH takes place according ta a radical mechanism by long chains, short chains or by steps. Based on this systematic study, some simple techniques are put forward for comparing experimental results and the proposed mechanism. The conclusions deduced from the latter are compared with the experimental results from the cooxidation of cumere and decalin.

To determine the relative importance of gas-phase and surface reactions in the oxidative coupling of methane (OCM), experimental investigations were performed. Our experimental results were compared to simulated values derived from a kinetic model taking into account heterogeneous and gas-phase reactions. We propose an original approach derived from Benson's techniques to estimate the kinetic parameters of surface reactions.

The different behaviour of hydrocarbons with respect to autoignition phenomena is, at present, not yet fully explained. We have therefore investigated the oxidation of two alkanes that have different octane numbers: neopentane (85.5) and isopentane (92.3), to obtain a better understanding of their reaction mechanisms, in particular, those reactions that are responsible for the onset of knock in spark ignition engines. The experimental study was performed at 873 K in a jet-stirred flow reaction vessel. The oxidation mechanisms were simplified by using the CHEMKIN programme of simulation of reaction mechanisms. These mechanisms were compared to those accounting for the oxidation of n-pentane, cyclopentane, n-heptane, and isooctane that we have previously studied. This comparison shows that the different behaviour of these hydrocarbons can be explained, at least in part, by the presence, in the reaction medium, of resonance-stabilized radicals.

The thermal reaction of propene was examined around 800 K in the presence of less than 20% oxygen. At initial time, the production of H-2, CH4, C2H4, C2H6, allene, C3H8, 1,3-butadiene, butenes, 3- and 4-methylcyclopentene, a mixture of 1,4- and 1,5-hexadienes, methylcyclopentane (or dimethylcyclobutane), 4-methylpent-1-ene, and hex-1-ene, was observed along with hydrogen peroxide, CO, and small quantities of ethanal and CO2. Oxygen increases the initial production of hydrogen and of most hydrocarbons and, particularly, that of C(6)( )dienes and of cyclenes. However, the production of aliene, methylcyclopentane (or dimethylcyclobutane), and 4-methylpent-1-ene is practically not affected. A kinetic study confirms the mechanism proposed for the thermal reaction of propene. Formation of aliene, thus, involves a four-center-unimolecular dehydrogenation of propene, that of 4-methylpent-1-ene is explained by an ene bimolecular reaction while methylcyclopentane (or dimethylcyclobutane) probably arises from a bimolecular process involving a biradical intermediate. Other products arise from a conventional chain radical mechanism. A kinetic scheme is proposed in which chains are primarily initiated by the bimolecular step:C3H6 + O-2 --> HO2 + C3H5which competes with the second-order initiation of propene pyrolysis. Since aliene production is not affected by oxygen, it is concluded that allyl radicals are not dehydrogenated by oxygen; but they oxidize in a branching step involving allylperoxyl radicals; r. radicals other than methyl, and allyl are dehydrogenated according to the conventional process:r. + O-2 --> unsaturated + HO2.and account for the production of a large excess of C-6 diolefins, methylcyclopentenes, and hydrogen peroxide, when r. stands for C6H11, the allyl adduct. Hydrogen peroxide gives rise to a degenerate branching of chains. Based on the proposed scheme, a modeling of the reaction is shown to account fairly well for the concentration-time profiles. Rate constants of many steps are evaluated and discussed. (C) 1998 John Wiley & Sons, Inc.

The reduction of automotive emissions and the reformulation of gasoline composition require a better understanding of the combustion mechanism, in particular aiming at the determination of the influential reactions which are responsible for the onset of knock in spark ignition engines. We have studied the oxidation mechanism of two hydrocarbons of different research octane number: n-pentane (62) and cyclopentane (100). This work follows a previous study on the oxidation of n-heptane and isooctane. The experimental study was performed at 873 K in a jet-stirred flow reaction vessel. The reaction mechanisms of n-pentane and cyclopentane were analysed on a purely kinetic basis, derived from our experimental results, and were finally modelled by using a programme of simulation of reaction mechanisms. The different behaviour of these two hydrocarbons with respect to autoignition phenomena is mainly dependent on the relative concentration of resonance-stabilized radicals in the reaction medium. Keywords: oxidation reaction, n-pentane, cyclopentane, modelling, autoignition.

The oxidation mechanisms of two alkanes that have quite different octane numbers: n-heptane (0) and isooctane (100), were investigated to obtain a better understanding of the chemical mechanism of autoignition phenomena and to improve the compatibility of the available fuels with the engines. The experimental study was performed at 923 K in a setup equipped with a jet-stirred reactor. The oxidation mechanisms of n-heptane and isooctane were simplified first by a purely kinetic analysis based on the product formation and then by using a software of simulation of reaction mechanisms. The very different behaviour of these two hydrocarbons was explained by the presence, in the products of isooctane oxidation, of alkenes, which would have an antiknock effect due to the formation of resonance-stabilized radicals.

This work aims at a better understanding, from the reaction mechanism point of view, of the antiknock effect of two unsymmetrical ethers : tert-amyl methyl ether (TAME) and ethyl tert-butyl ether (ETBE).An experimental investigation of the reactions of equimolecular mixtures of these ethers in the presence of oxygen shows that these ethers oxidize less easily than the alkanes of similar structures.Taking into account the distribution of the primary reaction products at ca. 300 degrees C and by using the techniques of Thermochemical Kinetics due to BENSON et al. for the estimation of kinetic parameters, it has been shown that these ethers do not oxidize according to a ''low'' temperature scheme, like the neighbouring alkanes. Their reaction with O-2 seems to have the characteristics of a thermal degradation and not those of a genuine oxidation. In line with the experimental results, it appears that in the co-oxidation of one of the above-mentioned ethers and a hydrocarbon, the effect of the ether is that of a ''negative catalyst'' transforming reactive radicals, chain carriers of the hydrocarbon oxidation, through a rather complex reaction scheme into unreactive ones, unable to propagate the reaction at a sufficient rate to trigger the auto-ignition reactions, responsible for the occurrence of knock phenomena.

An experimental investigation in a conventional static apparatus of the oxidation of equimolecular mixtures 1,4-dioxane-O2 has shown that 1,4-dioxane reacts with oxygen more readily than most hydrocarbons. Cool flames and ignitions were observed above 200-degrees-C in a pressure range up to 300 torr.The products of the slow reaction and cool flame were analyzed by gas chromatography and GC-MS; the slow reaction gives only CO, CO2, H2CO, H2, C2H4, and H2O.A radical chain mechanism is suggested and discussed by using an evaluation of the rate constants of the possible elementary steps by the methods of thermochemical kinetics.

An experimental study of the homogeneous gas phase oxidation of propane at 350 °C and subatmospheric pressure has been performed in order to identify and to measure the major primary products of the reaction. The experimental results have been interpreted by a chain radical mechanism, deduced from these results and from estimates of the rate constants for the elementary steps obtained by the methods of Thermochemical Kinetics. The proposed elementary steps are discussed and compared with the experimental observations. The results that we have obtained and their interpretation are compared with a similar detailed investigation performed on the oxidation of isobutane. As in the case of isobutane, two parallel reaction pathways appear, a dominant one leading to the conjugated alkene (propylene) and another one leading to the epoxide of this olefin (here propylene oxide). The oxidation of isobutane and that of propane appear to be quite similar, which corroborates the results that we have obtained. Key words: oxidation, kinetics, reaction mechanism, propane, thermochemical kinetics.

An experimental investigation in a conventional static apparatus of gas phase reactions between 1,4 dioxane and chlorine has permitted to observe auto-ignitions above 100°C at subatmospheric pressure and to map out the limits of reaction zones in a pressure-temperature diagram. To achieve a better understanding of the reaction mechanism leading to these auto-ignitions, we have performed an experimental investigation of the slow reaction in a flow system; GC-MS analyses were used to identify the reaction products. Our results show that at high conversion, 1,4-dioxene is the dominant reaction product; we also obtain smaller amounts of chlorinated by-products of 1,4-dioxane. The results of this investigation are used to build a tentative reaction scheme accounting for the experimental features of the reaction.

A mechanistic model of the thermal decomposition reactions of 3-methylpentane at low conversion ( < 5%) using as a basis a complex free-radical chain mechanism has been adjusted and compared to the observed product distribution. A reasonably good agreement between the computed curves and the experimental results obtained from an earlier work has been obtained.

The gas-phase pyrolysis of 1,4-dioxane has been studied in a static reactor at 510 and 550°C below 25 mbar, at reaction times ranging from 1 to 10 minutes, corresponding to conversion rates between 1 and 25%. The main reaction products are CO, H2, C2H4, HCHO, the minor products include C2H6, CH2=CH-CHO, CH3-CHO, CH4 and CH3-CH=CH2. This reaction is markedly inhibited by the addition of toluene, which observation permits us to propose a free radical chain mechanism to account for the experimental results.

L’étude de la pyrolyse du méthyl tert-butyl éther (méthyl-2, méthoxy-2 propane) vers 350-400 °C présente de substantielles difficultés, irreproductibilité en particulier, attribuées à d’importants effets de parois. Ceux-ci ont été mis en évidence par l’utilisation de plusieurs types de réacteurs (vides et garnis, traités et non traités). Pour pouvoir accéder à la part homogène de la réaction, il a été nécessaire de relever les températures d’expérimentation au-dessus de 425 °C et jusqu’à 490 °C et de former un dépôt sur les parois par pyrolyse de bromure d’allyle selon la procédure déjà proposée par Daly et Wentrup.

On the basis of previously reported, as well as new, experimental results, a review is presented of the kinetic and chemical influences of alkenes on the pyrolysis of alkanes. The results confirm that the addition of free H• atoms to the double bonds of alkenes is not sufficient to explain their inhibiting influence on the pyrolysis of alkanes. On the contrary, our results show that the processes of addition can account for the chemical effects of added alkenes on the pyrolysis of alkanes, namely, the modifications brought to the nature and the ratio of the products formed during the reaction. The results presented here confirm that the addition of alkyl free radicals to the double bond of alkenes does not lead to important chemical effects. The previously reported experimental results (pyrolysis of ethane in the presence of ethylene or propene, the pyrolysis of 2,2-dimethylbutane in the presence of propene, isobutene, or two isopentenes), completed by our new results (pyrolysis of ethane in the presence of isobutene, pyrolysis of isobutane in the presence of ethylene, of propene, of trans-2-butene, or of 2-methyl-2-butene), can lead to very simple general rules, on the basis of which it is possible to explain the results and to predict the modifications (relating to the nature of the products and to their ratios) that will result from the addition of an alkene during the pyrolysis of an alkane at a temperature of 500 °C (773 K). Aside from their fundamental aspects, these observations and their mechanistic consequences could be useful for the preparation of models for these thermal reactions, especially for the simplification of complex models. These observations also explain certain facts observed in industry and can help in solving some of the problems associated with the thermal cracking of heavy oils.