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On the inuence of the position of the double bond on
the low-temperature chemistry of hexenes
G. Vanhove, M. Ribaucour, R. Minetti
To cite this version:
G. Vanhove, M. Ribaucour, R. Minetti. On the inuence of the position of the double bond on the
low-temperature chemistry of hexenes. Proceedings of the Combustion Institute, Elsevier, 2005, 30
(1), pp.1065-1072. �10.1016/j.proci.2004.08.042�. �hal-01738420�
1
On the influence of the position of the double bond on the low-temperature
chemistry of hexenes
G. VANHOVE
1
, M. RIBAUCOUR
1
*, R. MINETTI
2
1,2
UMR CNRS 8522 Physico-Chimie des Processus de Combustion et de l'Atmosphère (PC2A),
1
Laboratoire de Cinétique et Chimie de la Combustion (LC3), Université des Sciences et Technologies de
Lille, 59655 Villeneuve d’Ascq Cedex, France
2
Laboratoire de Chimie Physique Appliquée (LCPA), Université d'Artois, Parc Porte Nord,
62700 Bruay la Buissière, France
* Corresponding author: Tel: 33 3 20 43 65 53 – Fax: 33 3 20 43 69 77
E-mail: marc.ribaucour@univ-lille1.fr
Colloquium: Reaction Kinetics
Short title: Low-temperature chemistry of isomeric hexenes
2
Abstract
The chemistry of oxidation and autoignition of 1-, 2-, and 3-hexene has been studied after rapid
compression between 630 and 850 K for stoichiometric mixtures with "air". The phenomenology of
autoignition has been recognized and intermediate products formed before autoignition have been
identified and analyzed. They comprise mainly hexadienes, O-heterocycles and aldehydes. There are
many common products, because some of the intermediate alkenyl or alkenylperoxy radicals are
delocalized. Saturated O-heterocycles are specific products formed by addition of HO
2
to the double
bond. Unsaturated O-heterocycles are products typical of the long alkenyl chain. Saturated and
unsaturated lower aldehydes are the products of OH addition to the double bond of hexenes and
hexadienes. The relative abundance of the intermediates enables a better insight into the competition
between the reactivity of the double bond and the reactivity of the alkenyl chain. According to the position
of the double bond, the behavior of 3-hexene is dominated by the properties of the double bond whereas
the behavior of 1-hexene is dominated by the properties of the alkenyl chain. The reactivity of the alkenyl
chain is related to the type and number of C-H bonds, the ability of stabilized radicals to react, and the
cyclic strain of the transition state of isomerization reactions. Therefore, 1-hexene reacts much more with
the typical features of alkanes like a two-stage ignition with a cool flame and a negative temperature
coefficient. 3-Hexene does not have typical features and 2-hexene has an intermediate behavior.
243 words
Keywords: autoignition, oxidation, hexenes, low-temperature
3
Introduction
The oxidation and autoignition of aliphatic saturated hydrocarbons in air at high pressures and in
the low- and intermediate-temperature range (600-900 K) has been the object of many experimental and
modeling studies [1]. It appears that the kinetics of oxidation in adiabatic conditions presents complex
patterns such as a non-Arrhenius behavior and thermokinetic interactions. Much fewer studies have been
published on the oxidation of long chain alkenes in the same temperature and pressure conditions [2-7],
although alkenes are major intermediate products of alkane oxidation.
In this work, isomers of hexene have been chosen to recognize the influence of the position of the
double bond on the low-temperature chemistry of oxidation under high pressure. Autoignition delays and
intermediate product concentrations were measured using a rapid compression machine.
1. Experimental
Autoignition delay times of stoichiometric mixtures of 1-hexene, trans-2-hexene, and trans-3-
hexene/"air" mixtures have been measured in the rapid compression machine of Lille between 6.8-8.5
bar, and 630-850 K according to a methodology already used for alkanes and aromatics [8,9]. The
reaction volume is a cylinder of 38 cm
3
and the compression ratio is 9.3. The compressed gas
temperature (T
c
) was varied by changing the composition of the inert gas (N
2
, Ar, CO
2
) and calculated
according to an adiabatic core gas model [10]. In this study, two series of experiments were conducted. In
the first series, the pressure and light emission traces were recorded for 11 different compositions of the
inert gas. In the second series, the chemical composition of the reactive mixtures for each of the hexenes
were analyzed qualitatively and quantitatively by gas chromatography and mass spectrometry at a
selected time before autoignition.
2. Phenomenology of autoignition
Figure 1 presents typical experiments of autoignition by compression of the three hexenes and
cyclohexene [11] for sake of comparison. The initial charges were identical and the inert gas had the
same composition leading to the same temperature T
c
(725 K) and pressure (9.4 bar). The pressure and
the light emission traces exhibit a two-stage ignition with an intense cool flame for 1-hexene, a two-stage
4
ignition with a weak cool flame for 2-hexene, and a one-stage ignition with a very faint light effect for
cyclohexene and 3-hexene.
The general characteristics of autoignition were studied by changing T
c
. Figure 2 presents the
evolution of the delay times versus T
c
. The phenomenology of autoignition depends markedly on the
position of the double bond. For 1-hexene the evolution is not very different from that of alkanes studied
in the same conditions [8]. However, the typical features of thermokinetic interactions are not so well-
marked. Autoignition occurs in two stages with a cool flame pre-ignition up to 800 K and a slight negative
temperature coefficient (NTC) is visible between 750 and 830 K. For 2-hexene, the typical features are
still less marked. There is a two-stage ignition with a weak cool flame, which disappeared above 730 K.
No NTC is observed but rather a very slow decrease of the ignition delay time between 720 and 815 K
with a distinct inflexion point around 780 K. For 3-hexene and cyclohexene, the phenomenology of
autoignition is plainer. There is no cool flame, nor NTC but only a faint inflection point near 730 K for 3-
hexene and 740 K for cyclohexene.
Clearly the position of the double bond has an impact on the reaction pathways responsible for
the onset of the cool flame, the autoignition delay time, and the dependence of the delay time on T
c
.
3. Analysis of intermediates
The analyses were performed during the delay at T
c
= 710 K. A rapid adiabatic expansion of the
gas into a collecting vessel was allowed, so that all reactions were quenched. Samples of 1- and 2-
hexenes were taken after the cool flame. The hydrocarbon consumption was about 10 % for 1-hexene
and 15 % for 2-hexene. In the case of 3-hexene, the time of sampling was fixed at 90 % of the delay time,
corresponding to a consumption of only a few percent of the hydrocarbon. Analyses were made for the
three isomers in strictly identical chromatographic conditions. This procedure made the identification of
the intermediates much easier and the intermediates common to two or three isomers were easily
recognizable. The structures of the intermediates were recognized by analysis of their mass spectrum. All
concentrations were obtained using an internal standard and expressed as the number of C atoms per
100 initial C in the fuel [11].
Table 1 gives the selectivities of C
6
intermediates and C
2
-C
5
aldehydes: the selectivity is the ratio
of the quantity of the intermediate to the total quantity of analyzed intermediates and is expressed in
