Experimental confirmation of the low-temperature oxidation scheme of alkanes

Article abstract of DOI:10.1002/anie.200906850

(Chemical equation presented) Intermediate detected: The design of internal combustion engines relies on a good understanding of the mechanism of the auto-ignition of hydrocarbons. A key assumption of this mechanism, which was commonly accepted but never proven, has now been experimentally demonstrated: the formation of ketohydroperoxides has been observed under conditions close to those actually observed before the auto-ignition.

Full text of DOI:10.1002/anie.200906850

Angewandte
Chemie
DOI: 10.1002/anie.200906850
Hydroperoxides
Experimental Confirmation of the Low-Temperature Oxidation
Scheme of Alkanes**
Frꢀdꢀrique Battin-Leclerc,* Olivier Herbinet, Pierre-Alexandre Glaude, Renꢀ Fournet,
Zhongyue Zhou, Liulin Deng, Huijun Guo, Mingfeng Xie, and Fei Qi*
The control of auto-ignition can be used to increase the
efficiency of internal combustion engines which has clear
potential positive implications for the problem of global
warming.[1] The design of internal combustion engines,[2] as
well as improved safety in oxidation processes,[3] rely on a
good understanding of the kinetic mechanism of the auto-
ignition of organic compounds. Here we experimentally
demonstrate a key assumption of this mechanism that has
governs many features of the combustion in internal combus-
tion engines (e.g. knock and related phenomena, octane and
cetane rating).[2] The potential explosive properties of mix-
tures of organic compounds with oxygen seldom has cata-
strophic consequences, but it very often imposes stringent
constraints on the operating conditions of partial-oxidation
processes.[3]
In addition to their propensity to auto-ignite, mixtures of
organic compounds with oxygen possess the following specific
reactive features. Single or multiple small temperature
pulsations (a few tenths of K) accompanied by weak emission
of blue light due to excited formaldehyde, so-called “cool
flames”,[7] are often observed under conditions preceding
those of auto-ignition (usually from 550 K). Closely linked
with “cool flames”, a zone of temperature (usually around
650 K) where the reactivity decreases with temperature,
commonly called “negative temperature coefficient” (NTC)
zone, is a second well-known characteristic of these sys-
tems.[2,7] These intriguing features have made the gas-phase
low-temperature oxidation of organic compounds a fascinat-
ing field of investigation for kineticists since the end of the
19th century. It was proven early on that the auto-ignition of
hydrocarbons cannot be explained by solely thermal theory,[8]
but is mainly a result of free-radical chain reactions.[9]
Following Semenov,[9] who proposed the concept of degener-
ate branched-chain reactions (production of free radicals
from products formed in a chain reaction) to explain
explosive reactions, much work has been devoted to elucidate
the species responsible for degenerate branched-chain reac-
tions during the oxidation of organic compounds.[7]
been accepted for more than 20 years but never proven.[4–6]
A
detailed speciation of the hydroperoxides responsible for the
gas-phase auto-ignition of organic compounds has been
achieved for the first time, thanks to the development of a
new system coupling a jet-stirred reactor to a molecular-beam
mass spectrometer combined with tunable synchrotron
vacuum ultraviolet (SVUV) photoionization. The formation
of alkylhydroperoxides (ROOH) and carbonyl compounds
having a hydroperoxide function (ketohydroperoxide) has
been observed under conditions close to those actually
observed before the auto-ignition. This result gives the
experimental confirmation of an assumption made in all the
detailed kinetic mechanisms developed to model auto-
ignition phenomena.
A good understanding of the kinetic mechanism govern-
ing the oxidation and the auto-ignition of organic compounds
is important in two significant applications: the design of
combustion engines and the safety of oxidation processes in
the chemical industry (e.g. petrochemistry). Auto-ignition
[*] Dr. F. Battin-Leclerc, Dr. O. Herbinet, Dr. P.-A. Glaude,
Prof. R. Fournet
Scheme 1 presents the current commonly accepted mech-
anism for the oxidation of hydrocarbons at low temperatures
(below 1000 K).[4] After a short initiation period, a hydro-
carbon (RH) mainly reacts with a hydroxyl radical (COH) to
give an alkyl radical (RC) through reaction (1), followed by the
formation of alkylperoxy radicals (ROOC) after a barrierless
reaction with an oxygen molecule (reaction (2)). The exis-
tence of the NTC zone is mainly due to the enhanced
reversibility of this reaction as the temperature increases.
From ambient temperature up to about 550 K, the only
prevalent reaction of alkylperoxy radicals is with CHO2
radicals (reaction (3)) or H abstraction from an organic
molecule to yield alkylhydroperoxides (ROOH). The fragility
Laboratoire Rꢀactions et Gꢀnie des Procꢀdꢀs
Nancy Universitꢀ, CNRS, ENSIC
BP 20451, 1 rue Grandville, 54001 Nancy (France)
E-mail: frederique.battin-leclerc@ensic.inpl-nancy.fr
Dr. Z. Y. Zhou, Dr. L. L. Deng, Dr. H. J. Guo, Dr. M. F. Xie, Prof. F. Qi
National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230029 (China)
E-mail: fqi@ustc.edu.cn
[**] This work was supported by the European Commission through the
“Clean ICE” Advanced Research Grant of the European Research
Council and by a grant from Rꢀgion Lorraine (Soutien Jeune
chercheur, Olivier Herbinet). F.Q. acknowledges funding from the
Chinese Academy of Sciences, Natural Science Foundation of China
(50925623 and 20533040), and the Ministry of Science and
Technology of China (2007CB815204 and 2007DFA61310). We
thank Prof. J. F. Pauwels and D. Leray for their help in designing the
coupling with the quartz reactor and S. Bax, H. Legall, and P. Aury
for technical assistance in France.
À
of the RO OH bond of hydroperoxides (the bond dissocia-
tion energy is around 43 kcalmolÀ1), which can easily break
when the temperature increases, has made them very good
candidates as species responsible for the degenerate
branched-chain reactions explaining auto-ignition and the
occurrence of cool flames.[4,7]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906850.
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studied as it is the smallest alkane with oxidation behavior
close to that of the species present in gasoline and diesel fuel.
Minimizing the potential reactions of hydroperoxides
between their formation and their detection is possible by
creating a molecular beam from a high-pressure reactive zone
toward a detection device maintained under vacuum; this
results in the “freezing” of the chemical composition of the
sampled gas. Since hydroperoxides are expected to be formed
in very low amounts along with a large number of other
oxidation products, a highly sensitive mass spectrometer with
low photon energy to avoid product fragmentation has been
used, that is, a time-of-flight mass spectrometer combined
with tunable SVUV photoionization. This analytical method
has already been used successfully to investigate many
reactive systems[12] such as laminar premixed flames.[13,14]
Since we must accumulate several scans to obtain good
sensitivity, this analysis device must be coupled with a reactive
system working under steady conditions. Our purpose was
then not to really observe unsteady phenomena, such as cool
flames or auto-ignition, but to mimic the chemistry leading to
these phenomena. We used a heated jet-stirred reactor. This
type of reactor coupled with gas chromatography analyses has
already been used many times for studying the low-temper-
ature oxidation of organic compounds.[15,16] In these experi-
ments the presence of a NTC zone was shown and the
formation of the same type of products was proved as those
actually analyzed prior auto-ignition or during cool flames.[17]
Figure 1 presents a simplified scheme of the jet-stirred reactor
associating a molecular-beam sampling system to a reflectron
time-of-flight mass spectrometer combined with SVUV
photoionization.
Scheme 1. Simplified scheme of the primary mechanism of oxidation
of alkanes at low temperature.
Above 550 K (in the zone of cool flames), a second
consumption pathway becomes faster for alkylperoxy radi-
cals: their isomerization (reaction (4)) by internal transfer of
a hydrogen atom to give hydroperoxyalkyl radicals (CQOOH).
These radicals can easily decompose to yield oxygenated
products, amongst which cyclic ethers are the most abundant
species (reaction (5)). As cyclic ethers are stable compounds
easily analyzed by gas chromatography, the first steps of the
oxidation of organic compounds through reactions (1)–(4)
(in Scheme 1) are relatively well known.[7]
A second important fate of CQOOH radicals is the second
addition to oxygen (reaction (6)). The formation and the
reactions of the resulting COOQOOH radicals have not yet
been directly experimentally investigated. However, follow-
ing Fish,[10] all the detailed kinetic mechanisms developed
nowadays to model the low-temperature ignition of organic
compounds consider decompositions of COOQOOH radicals
yielding three radicals, amongst them two COH radicals. Cox
and Cole[5] have proposed this global step to occur through
the formation of alkylhydroperoxides including a carbonyl
group (reaction (8)), mainly ketohydroperoxides, which in
turn can easily decompose to give two radicals (reaction (9))
and lead to another possible chain-branching reaction. Many
modeling studies have suggested that the direct formation of
hydroperoxides by reactions of peroxy radicals was not fast
enough and that the inclusion of a chain-branching step
deriving from the second addition to oxygen was necessary to
satisfactorily simulate the experimental observations.
While several detailed models can now reproduce most
features of the oxidation and auto-ignition of alkanes
representative of those present in automotive fuels,[6] exper-
imental evidence supporting this commonly used reaction
scheme is extremely scarce.[11]
The purpose of this study is to give experimental evidence
for the formation of hydroperoxides under conditions similar
to those observed prior to the auto-ignition of alkanes and
using a method that makes it possible to distinguish between
alkylhydroperoxides and ketohydroperoxides. n-Butane was
Figure 1. Simplified illustration of the experimental device.
The study of the oxidation of n-butane was performed at
temperatures between 560 and 720 K, with a mean residence
time of 6 s and for an n-butane/oxygen/argon mixture
composition of 4:26:70 (in mol%), which corresponds to a
stoichiometric mixture for the complete combustion reaction.
Figure 2a presents the evolution of the mole fraction of
n-butane with temperature. The mole fraction of n-butane
(m/z 58) was derived directly from the normalized ion signal
at m/z 58 (obtained with a photon energy of 11.0 eV (1 eV=
96.5 kJmolÀ1)) assuming that no reaction occurred below
590 K. Raw signals of every compound were normalized by
the ion signal obtained for argon (m/z 40) with a photon
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In a system containing only carbon, hydrogen, and oxygen
atoms, m/z 48 can only be related to methylhydroperoxide
(CH3OOH). For m/z 62, 90, and 104, several possibilities
exist. However the mass of ethylhydroperoxide (C2H5OOH)
is 62, that of butylhydroperoxides (C4H9OOH) is 90, and that
of C4 ketohydroperoxides (C4H8O3) is 104. Figure 3 shows a
mass spectrum with signals at m/z 48, 62, 90, and 104. A
substantiation of this identification is given because these four
compounds are the only ones having the same particular
evolution with temperature, with a sharp peak, characteristic
of compounds rapidly decomposing when temperature
increases.
Figure 3. Typical mass spectrum obtained for the oxidation of
n-butane. The temperature in the reactor was 590 K and the photon
energy was 10.0 eV.
Figure 2. Plots of the experimental and simulated mole fractions or
signals of the oxidation of n-butane, the main products and hydro-
peroxides with temperature: a) the experimental (black circles) and
simulated (line) mole fractions of n-butane; b) the experimental ion
signals (in arbitrary units) at m/z 48 (green circles), 56 (white squares,
the signal is divided by 7.5), 62 (purple circles), 72 (black squares), 90
(blue circles, the signal is divided by 4) and 104 (red circles); and
c) the computed mole fractions (in ppm) of CH3OOH (green line,
mole fraction divided by 5), C2H5OOH (purple line), C4H9OOH (blue
line), the C4 ketohydroperoxides (red line), butenes (dash line, mole
fraction divided by 5), and C4H8O products (black line).
Additional supporting evidence for this identification
relies on IEs from measurements of photoionization effi-
ciency spectra. When such values were not available in the
literature,[18] zero-point energy corrected adiabatic IEs have
been calculated from the CBS-QB3 method[19] using Gaus-
sian03.[20] An experimental sweeping of photon energies from
8.5 to 11.5 eV has been made. The obtained experimental IEs
were 9.91, 9.69, 9.37, and 9.29 Æ 0.05 eV for m/z 48, 62, 90, and
104, respectively (see the Supporting Information). The
calculated values for methylhydroperoxide, ethylhydroper-
oxide, butylhydroperoxides, and C4 ketohydroperoxides were
9.83, 9.61, 9.33–9.36, and 9.34–9.39 eV, respectively. For the
last two compounds, a range of energy is given because the
calculations were made for all the expected isomers (see
Table 1). Note that 1,2-dihydroxyethane, another possibility
for m/z 62, has an ionization energy of 10.16 eV.[18]
Finally, even further substantiation of the proposed
assignments is given by simulations performed with a detailed
kinetic mechanism of the oxidation of n-butane previously
proposed[21] (see the Supporting Information). Figure 2a and
c show that, while small differences are encountered for the
start of the reactivity and the extent of the NTC zone, the
model reproduces well the variations of the mole fraction of
n-butane with temperature, as well as the “plateau” obtained
below 680 K for the major products, such as butenes and
C4H8O compounds. As shown in Figure 2c, simulations
reproduce well the particular mole fraction profiles (with a
energy of 16.2 eV which acted as an internal standard. Many
products were obtained; the major C4 products were butenes
at m/z 56 and C4H8O products at m/z 72. Figure 2b also
displays the normalized ion signal at m/z 56 and 72 obtained
with a photon energy of 10.0 eV to avoid ion fragmentation.
The ionization energy (IE) of butenes is between 9.11 and
9.55 eV.[18] The signal at m/z 72 corresponds to the C4H8O
oxygenated products which have an IE below 10 eV, for
example, butanal, butanone, and all cyclic ethers (except
2-ethyloxirane which has an IE greater than 10 eV).
Note the NTC zone obtained above 630 K (see Fig-
ure 2a), which makes the evolution with temperature of the
normalized signal of all the products have the same shape as
those at m/z 72 with a kind of “plateau” between 620 and
680 K. This plateau is not observed in the case of species
corresponding to four specific masses: 48, 62, 90, and 104
(Figure 2b). For these species, a sharp peak is obtained
around 590 K meaning that they are very reactive molecules
that decompose very quickly when the temperature increases.
Angew. Chem. Int. Ed. 2010, 49, 3169 –3172
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Table 1: Ionization energies of most expected isomers of C4 hydro-
peroxides deriving from the oxidation of n-butane.
Keywords: hydroperoxides · mass spectrometry ·
molecular beams · oxidation · photoionization
.
Mass
90
Formula
Structure
IE [eV][a]
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C4H9OOH
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C3H7COOOH
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sharp peak) obtained for the four hydroperoxides detected in
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Experimental Section
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The utilized spherical quartz jet-stirred reactor (diameter around
5 cm), in which a gas mixture is continuously flowing, was operated at
a constant temperature and pressure.[16,24] This type of reactor, which
can be heated up to 1200 K, is well suited for kinetic studies: the gas
phase inside the reactor is well stirred, meaning the concentrations
and temperature are homogenous.[25] Quartz wall effects can usually
be considered as negligible. This study has been performed under
quasi-atmospheric pressure (1.05 atm). As the type of diluent gas has
negligible chemical significance, argon was used instead of nitrogen,
for the ease of the mass spectrometric analysis. The coupling between
the quartz reactor and the mass spectrometer was made through a
quartz conelike nozzle with a 75 mm orifice (the tip of which has been
inserted inside the reactor) generating a molecular beam, which was
skimmed before the entrance in the spectrometer chamber. In situ
detection of intermediates was made possible by combining tunable
VUV photoionization and the molecular-beam-sampling mass spec-
trometer. The produced ions were detected by a reflectron time-of-
flight mass spectrometer which can distinguish between different
masses.[26]
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[26] Y. Y. Li, L. D. Zhang, Z. Y. Tian, T. Yuan, B. Yang, J. Z. Yang, F.
Qi, Energy Fuels 2009, 23, 1473 – 1485.
Received: December 4, 2009
Published online: April 1, 2010
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