Reactions of methyl radicals with isobutane at temperatures between 800 and 950 Kelvin

Article abstract of DOI:10.1002/kin.1070

The methyl radical induced pyrolysis of isobutane has been studied in the temperature range of 800-950 K and at pressures of 80 and 130 mbar. We used CO₂-laser induced heating by fast vibrational-translational (V-T) energy transfer from SF₆ to the reaction mixture. This is a convenient method to study homogeneous high temperature kinetics since the reactor walls remain cold. The radial temperature distribution in the reactor has been investigated by four different methods: stationary heat balance, optical absorption, pressure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl radicals were produced in the presence of isobutane via the fast thermal decomposition of di-tert-butyl-peroxide and the products were analyzed using gas chromatography and mass spectroscopy. The main products of the overall reaction were propene (C₃H₆) and isobutene (i-C₄H₈), whereas ethene (C₂H₄), propane (C₃H₈), neo-pentane (neo-C₅H₁₂), and isopentane (i-C₅H₁₂) were minor products. The product distribution showed strong temperature dependence. The product yields and their temperature dependence were analyzed by a kinetic model developed for the high temperature oxidation of n-butane and the low temperature oxidation of n-pentane, and isopentane. We included a few missing reactions and slightly adjusted some rate constants to obtain a better agreement between the modeling and the experiments. The extended mechanism consists of 63 species and 171 reactions to be evaluated further in forthcoming experiments.

Full text of DOI:10.1002/kin.1070

Reactions of Methyl
Radicals with Isobutane
at Temperatures Between
800 and 950 Kelvin
1
1
2
2
¨
ELKE GOOS, HORST HIPPLER, KARLHEINZ HOYERMANN, BETTINA JURGES
1Institut fu¨r Physikalische Chemie, Universita¨t Karlsruhe, Fritz-Haber-Weg 4, D-76128 Karlsruhe, Germany
2Institut fu¨r Physikalische Chemie, Universita¨t Go¨ttingen, Tammannstrasse 6, D-37077 Go¨ttingen, Germany
Received 24 May 2001; accepted 5 June 2001
ABSTRACT: The methyl radical induced pyrolysis of isobutane has been studied in the
temperature range of 800–950 K and at pressures of 80 and 130 mbar. We used CO2-laser
induced heating by fast vibrational–translational (V–T) energy transfer from SF6 to the reac-
tion mixture. This is a convenient method to study homogeneous high temperature kinetics
since the reactor walls remain cold. The radial temperature distribution in the reactor has
been investigated by four different methods: stationary heat balance, optical absorption, pres-
sure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl
radicals were produced in the presence of isobutane via the fast thermal decomposition of
di-tert-butyl-peroxide and the products were analyzed using gas chromatography and mass
spectroscopy. The main products of the overall reaction were propene (C3H6) and isobutene
(i -C4H8), whereas ethene (C2H4), propane (C3H8), neo-pentane (neo-C5H12), and isopentane
(i -C5H12) were minor products. The product distribution showed strong temperature depen-
dence. The product yields and their temperature dependence were analyzed by a kinetic model
developed for the high temperature oxidation of n-butane and the low temperature oxidation
of n-pentane, and isopentane. We included a few missing reactions and slightly adjusted some
rate constants to obtain a better agreement between the modeling and the experiments. The
extended mechanism consists of 63 species and 171 reactions to be evaluated further in forth-
C
coming experiments. 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 732–740, 2001
temperatures large alkyl radicals degrade rapidly into
methyl radicals. The fate of the methyl radicals is,
however, imperfectly characterized since the rate co-
efficients for their reactions with many hydrocarbons,
as CnH2n+2 and CnH2n, are not known. The reason is
essentially given by experimental difficulties at tem-
peratures around 1000 K. The general problem of flow
system studies is the heating through the wall, which
results in catalytic wall effects and therefore in compli-
cations of the homogeneous reaction system. Wall free
heating in shock tubes can also not be used because of
INTRODUCTION
The reactions of methyl radicals with alkanes are im-
portant to the combustion of hydrocarbons. At high
Dedicated to the 90th anniversary of Prof. Dr. S. H. Bauer.
Correspondence to: Prof. Dr. Horst Hippler; e-mail: horst.-
hippler@chemie.uni-karlsruhe.de.
Contract grant sponsor: Deutsche Forschungsgemeinschaft.
Contract grant number : SFB 551, 357.
Contract grant sponsor: Fonds der Chemischen Industrie.
2001 John Wiley & Sons, Inc.
c

REACTIONS OF METHYL RADICALS WITH ISOBUTANE BETWEEN 800 AND 950 K
733
the limited observation time of about 1 ms compared
to the small reaction rates of the system.
production of olefins [25]. There has been a lasting con-
troversy about the mechanism of self-inhibition. Inhi-
bition could either arise from the addition of hydrogen
atoms to olefins
The best-suited technique for studying the reaction
of methyl radicals with hydrocarbons at temperatures
below 1000 K is the laser-powered homogeneous py-
rolysis, pioneered and further developed by Shaub and
Bauer [1]. We present an application of this method for
the investigation of the methyl radical induced pyroly-
sis of isobutane.
H + i-C4H8 → t-C4H9
(R3)
or from abstraction reactions [26]
The main advantage of laser-powered pyrolysis is
the ability to heat rapidly a sample using IR laser ra-
diation without the complicating effects of hot sur-
faces. By using modulated laser-radiation, time scales
on the order of milliseconds are easily realized, re-
stricting the chemistry primarily to initial reactions and
only a few subsequent reactions. The homogeneous
nature of laser-powered heating minimizes wall reac-
tions and improves reproducibility by eliminating ef-
fects of surface contamination in the pyrolysis reac-
tor. The flow and temperature fields of the sample gas
in laser-powered homogeneous pyrolysis are compu-
tationally analyzed [2,3]. Therefore this technique is
used in the wide field of determination of rate coeffi-
cients [4], for example the methyl-induced pyrolysis of
n-butane [5] or propene [6], the decomposition of 1,3-
butadiene [7] or silane [8] or in the synthesis of soot
containing fullerene [9].
H + i-C4H10 → H2 + t-C4H9
CH3 + i-C4H10 → CH4 + t-C4H9
H + i-C4H8 → H2 + C4H7
(R4)
(R5)
(R6)
(R7)
CH3 + i-C4H8 → CH4 + C4H7
giving progressively more tert-butyl and 2-methylallyl
radicals (C4H7, CH2C(CH3)CH2), which are unreac-
tive and increase the rate of chain termination and, thus,
account for self-inhibition. The relative importance of
the competing addition and abstraction mechanisms is
still subject to debate.
Bradley [18] has studied the pyrolysis of isobutane
in argon in the temperature range from 1200 to 1500 K
at total pressures between 6.5 and 9 bar using a single-
pulse shock tube at reaction times of 1 ms or below. The
major products were methane, ethane, ethene, propene,
and isobutene, with acetylene appearing only at the
highest temperatures. Very small amounts of allene,
propyne, and but-1-ene were detected. The absence of
propane in their products was confirmed both by gas
chromatography and mass spectroscopy.
Isobutane pyrolysis has been extensively studied
around 800 K in conventional static systems [10–16]
and in an unpacked Pyrex reactor at 26–133 mbar [17].
This system has also been investigated in shock tubes
[18,19], very low pressure pyrolysis (VLPP) [20], and
in a “quasi-wall-free” reactor [21,22].
Konar et al. [15] investigated the pyrolysis of pure
isobutane (26–200 mbar) between 770 and 855 K up to
4% decomposition. They estimated rate constants for
theinitiatingreactionsforthemethyl-inducedpyrolysis
of isobutane
At low conversions, pyrolysis of isobutane may be
described by the following main global reactions
i-C4H10 → CH4 + C3H6
i-C4H10 → H2 + i-C4H8
(R1)
(R2)
CH3 + i-C4H10 → CH4 + i-C4H9
CH3 + i-C4H10 → CH4 + t-C4H9
(R8)
(R9)
and
where reaction (R2) should dominate at 763 K [23].
The complete reaction is generally described by a long
chain radical mechanism. Chains are initiated by the
dissociationofaC Cbondofisobutaneandterminated
by the bimolecular combination of chain carriers.
The nature of termination steps was discussed by
Konar et al. [12,14,15]. At temperatures above 700 K
and initial pressures of isobutane below 133 mbar they
have shown that chains are essentially terminated by
methyl recombination. However at slightly lower tem-
peratures and larger pressures, they found mainly chain
termination by tert-butyl combination. Isobutane py-
rolysis is strongly self-inhibited. This feature was first
reported by Dintzes et al. [24] and was ascribed to the
The major products were methane, propene, isobutene,
and hydrogen. The formation of the minor products
ethene and propane, which arise from the initiation re-
action, has been discussed [13]. Additionally Zhang
et al. [27] obtained the rate constant for abstraction
of hydrogen atoms from isobutane by methyl radicals
evaluating experiments at temperatures above 660 K.
The aim of the present study is to investigate the
methyl-inducedpyrolysisofisobutaneina“quasi-wall-
free” reactor and to analyze the temperature and pres-
sure dependence of the product distribution. These ex-
periments will be used to evaluate the current chemical
mechanism for hydrocarbon pyrolysis and oxidation.

734
GOOS ET AL.
The rationale is as follows:
The CO2-laser (Apollo 560) was operated in its
pulse mode at a repetition rate of 1–100 Hz. The laser
was stabilized at the P20 line (10.59 ␮m) and the beam
can be satisfactorily described by a Gaussian profile.
When a laser pulse of 50 ms duration is applied,
the temperature of the given mixture rises due to vi-
brational to translational energy transfer from vibra-
tionally excited SF6 to the reaction mixture. A station-
ary temperature profile is reached within 1–3 ms with
a maximum temperature along the optical axis of the
cell. The energy absorbed by the gas mixture was var-
ied with the partial pressure of the SF6 and the incident
laser power, which was monitored continuously. For
the 50 ms duration of the CO2 laser pulse the tem-
perature profile remains constant and decays quickly
after the laser pulse within 1–3 ms. Cycles of heating
and “cooling” were obtained by repeated application
of laser pulses, resulting in a complete mixing of the
heated gas near the axis of the cell with the cold gas
close to the wall by convection, which homogenizes
the composition between two succeeding laser pulses.
The time for diffusion from the center to the wall is
in the order of 30 ms, so that radicals produced in the
center are removed mainly by homogeneous reactions.
Convective flow into the nonisothermal region leads to
mixing with the gas in the dead space. No deposit was
found on the reactor’s walls.
(1) The use of the laser powered homogeneous gas
heating via fast vibrational to translational en-
ergy transfer avoids heterogeneous pyrolysis at
the cold walls of the reactor.
(2) The competing routes for CH3 radicals of ab-
stracting tertiary or primary hydrogen atoms
from isobutane can be easily studied experimen-
tally at different temperatures.
(3) Applying GC–MS analysis allows the identifi-
cation and quantification of the products, espe-
cially of the different isomers.
(4) The comprehensive mechanisms for the oxida-
tion and pyrolysis of n-butane, n-pentane, and
isopentane, derived and applied preferentially
for the oxidation, are used to model our ex-
perimental results, thus validating, falsifying, or
identifying missing reactions in the mechanism
reported so far.
EXPERIMENTAL PROCEDURE
The general experimental set-up and the temperature
determination have been described previously [5] and
are therefore only summarized here. A schematic rep-
resentation of the experimental arrangement is given in
Fig. 1.
As described in Ref. [5], the radial temperature pro-
files were deduced by four different methods:
Before a series of experiments reaction mixtures
were prepared and stored in the dark. Their compo-
sition and the experimental conditions are summarized
in Table I. The commercial grade chemicals ((CH3)3
COOC(CH3)3, DTBP: 95%; SF6: 99.9%; c-C3H6:
99.0%; all Merck; CF4: 99.8%; Linde; Ar: 99.9998%,
Messer-Griesheim) were used without further purifica-
tion and liquids were carefully degassed before usage.
After complete mixing the sample was filled into a
cylindrical cell with KCl windows, which were fixed
at the Brewster angle; thus no distortion of the applied
laser pulse was observed.
1. By measuring of the total energy absorbed in the
volume and analyzing the heat balance consider-
ing the absorbed power and heat conduction.
2. By the determination of the radial distribution
of the absorbed energy using the temperature-
dependent optical absorption coefficient of SF6.
3. By measuring the pressure rise in the total vol-
ume of the cell and its dependence on the energy
absorbed.
4. By the study of the conversion of cyclopropane
to propene via the well established temperature-
dependent rate coefficient for the isomerization.
Figure 1 Schematic representation of the experimental arrangement.

REACTIONS OF METHYL RADICALS WITH ISOBUTANE BETWEEN 800 AND 950 K
735
Table I Composition of the Reaction Mixture, Experimental Conditions, and Relative Product Yields for the Reaction
CH3 + i -C4H10
T (K)
ptotal (mbar)
p(SF6) (mbar)
p(CF4) (mbar)
Educts p (mbar)
DTBP
795
80
1.3
945
80
4
133
1.3
0.8
133
4
0.8
0.5
0.5
2.7
4.5
2.7
4.5
i-C4H10
24.5
40.7
24.5
40.7
ME(C3H6)
Relative product yield (%)
i-C4H8
0.09
0.1
0.09
0.1
0.09
0.1
0.09
0.1
56.8
35.5
5.0
54.6
37.9
4.8
54.9
32.9
7.1
53.0
35.3
6.9
50.6
41.7
3.0
48.3
44.2
2.9
47.8
40.6
4.1
45.7
43.2
3.9
C3H6
neo-C5H12
C2H4
0.8
0.8
1.9
1.8
3.0
2.9
5.2
5.0
C3H8
0.7
0.7
1.4
1.3
0.6
0.6
0.7
0.7
i-C5H12
1.2
1.2
1.8
1.7
1.1
1.1
1.6
1.5
Argon added up to ptotal; pulse duration: 50 ms; accumulation of 1000–10000 pulses; length of cell 7.8 cm; the differences in the relative
product yields are because of the uncertainty in the mass spectroscopic sensitivity of propene relative to CF4.
The chemical thermometer, e.g. the isomerization
reaction of cyclopropane (c-C3H6) forming propene
(C3H6) or the thermal decomposition of di-tert-butyl
peroxide (DTBP), measures essentially the maximum
temperature in the cell. As the rate constants show
considerable activation energies, measurable conver-
sion occurs only at the highest temperature at the cell
axis. We checked the maximum temperature of each
experiment via the conversion of cyclopropane. There-
fore each experiment was done twice, without and
with added cyclopropane, respectively. For this pur-
pose, small amounts of argon have been replaced by cy-
clopropane thus keeping identical total pressure. Both
mixtures were “heated” with identical laser power and
the resulting temperature of both experiments was de-
duced from the conversion of cyclopropane to propene.
We found identical relative product yields (without
considering the propene yield) for each pair of exper-
iments. The measured temperature distributions indi-
cate an average temperature of 795 K for a pressure of
1.3 mbar of SF6 and 945 K in the case of 4 mbar of SF6
with an uncertainty of 4%.
The rate of the reaction of methyl radicals with
isobutane can be adjusted by its partial pressure.
The methyl radical precursor molecules (DTBP) re-
siding at the heated cell axis decompose completely
during a laser pulse. The resulting methyl radicals re-
act essentially with isobutane, which is in excess in the
reaction mixture, or produce ethane in a radical–radical
recombination reaction. Because of cooling and con-
vectional flow between the laser pulses, the sample is
totally remixed before the next laser pulse restarts the
reaction. Reactions of methyl radicals with products
from previous laser heating can be neglected, because
the product concentration is far too low. We conclude,
that the observed product distributions can quantita-
tively be analyzed.
After applying a large number of laser pulses, a
small fraction of the mixture was sampled and an-
alyzed. The products were identified by gas chro-
matography (column: Poraplot Q(Chrompack)) via
their different retention times, as well as by mass
spectroscopy using listed fragmentation spectra,
which were taken from the NBS Library Compila-
tion. The concentrations of the products were de-
termined by integrating the peak area of the gas
chromatogram identified by characteristic fragment
masses. Adding CF4, which is not involved in the re-
action, allowed consideration of the different mass-
spectroscopic sensitivities (ME) of the products. As
shown in Table I the ratio of initial concentrations
[DTBP]0/[i-C4H10]. is in the order of about 10%.
The total consumption of i-C4H10 never exceeded
5%.
Withinourtemperaturerangeof800–950Kthether-
mal decomposition of DTBP ((CH3)3COOC(CH3)3),
the source of CH3 radicals, proceeds on a millisecond
time scale and the CH3 radicals disappear predomi-
nantly by recombining to ethane
(CH3)3COOC(CH3)3 → 2(CH3)3CO
(CH3)3CO → CH3 + CH3COCH3 (R11)
CH3COCH3 → CH3 + CH3CO (R12)
(R10)

736
GOOS ET AL.
activation energy of 156 kJ/mol, substantial DTBP de-
composition only arises when the final temperature has
almost been reached. Consequently we assumed in the
model an instantaneous temperature raise however in-
cluded in the mechanism the thermal decomposition of
the radical precursor. We also disregarded the small
pressure increase because of the large dead volume
of the set-up. The initial concentrations of the reac-
tants DTBP and isobutane as well as the actual pres-
sure of argon were used as inputs for the modeling
calculations.
EXPERIMENTAL RESULTS
The methyl-induced pyrolysis of isobutane was stud-
ied at temperatures between 800 and 950 K and at a
pressure of 80 and 133 mbar. The product distribu-
tion is given in Table I together with the experimental
conditions.
Different yields for the products were obtained us-
ing different ME [28] for propene (C3H6). These un-
certainties have no significant influence on the product
distribution.
Adjustment of the rate constants in the model was
guided by a sensitivity analysis and by the reported
estimated uncertainties of the kinetic parameters.
Because of the chosen kinetic conditions only a lim-
ited number of products are formed. Therefore, the
number of elementary reactions to be considered is re-
stricted and only a small number of parameters have to
be adjusted in the simulation. Statistical methods such
as least square fitting could not be used to check the
reliability of the model due to the small numbers of
experiments and large error margins of the data.
The main products are isobutene (i-C4H8) and pro-
pene (C3H6). As minor products we identified iso-
pentane (i-C5H12), neo-pentane (neo-C5H12), ethene
(C2H4), and propane, (C3H8). Pentene (C5H10) has
been found as a trace. Additionally n-butane (n-C4H10)
has been proven to be an impurity of the used isobutane
(i-C4H10). A quantitative determination of methane and
ethane was not possible with the GC column used since
the analysis was aimed at detecting higher hydrocar-
bons up to the aromatics. No pressure dependence was
observed in the small pressure range of 80–133 mbar.
The relative yields of the products have uncertainties
of 15%, which result from uncertainties in the GC-
analysis and the ME. We estimate the uncertainty of
the temperature to be 4%. This figure reflects the
uncertainty of the chemical thermometer and of the
simplified temperature model.
MODELING RESULTS
The central part of this CH3/i-C4H10 mechanism is
presented in Fig. 2, showing dominant reaction paths
for the formation of the main products. This reaction
scheme was obtained from a sensitivity analysis of the
product yields. Propene (C3H6) is the main product, re-
flecting the intermediate isobutyl radical concentration
(i-C4H9), produced from CH3 abstracting a primary
H-atom from isobutane (i-C4H10). Additionally at
higher temperature propene can be formed through
decomposition of isobutane to propyl (C3H7) and
H-atom loss. The second main product isobutene
(i-C4H8) stems from isobutyl (i-C4H9) and tert-butyl
(t-C4H9) radical decompositions. The t-C4H9 radicals
were formed by methyl abstracting a tertiary hydrogen
atom in i-C4H10.
REACTION MECHANISM AND MODELING
The temperature dependence of the product distri-
bution for the methyl-induced pyrolysis of isobutane
was analyzed by a kinetic model developed for the
high temperature n-butane and the low temperature n-
pentane and isopentane oxidation [29]. We included
the di-tert-butyl peroxide decomposition reactions, and
we extrapolated the rate constants from low tempera-
ture (near 400 K) to our conditions between 800 and
950 K. Furthermore, we incorporated a few missing
reactions, mostly for tert-butyl (t-C4H9) and isobutyl
radicals (i-C4H9), e.g. the reactions of tert-butyl and
of isobutyl radicals with methyl radicals (see Table II).
The full model consists of 63 species and 171 reac-
tions. The rate coefficients and their pressure depen-
dence of the important reactions are summarized in
Table II together with the modifications guided by our
experiments.
The model calculations were performed with the
CHEMKIN Code [38] assuming isothermal condi-
tions. The heating period was neglected since the reac-
tion simply starts when the methyl radical precursor
(DTBP) significantly decomposes. Because of the
Figure 2 Central part of the reaction mechanism.

REACTIONS OF METHYL RADICALS WITH ISOBUTANE BETWEEN 800 AND 950 K
737
Table II Important Rate Coefficients of the Mechanism Expressed as k = AT b exp( E/RT )
Reaction
A
b
E
Ref.
Methyl radical production
C8H18O2 → 2C4H9O
2.0E + 15
12E + 15
1.1E + 14
3.3E + 14
7.9E + 17
24E + 17
0.0
0.0
0.0
0.0
0.0
0.0
156.5
156.5
61.2
61.2
353.0
353.0
[30]
this work
[31]
this work
[32]
C4H9O → CH3 + CH3COCH3
CH3COCH3 → CH3CO + CH3
this work
Reactions of methyl radicals with isobutane
i-C4H10 + CH3 = CH4 + t-C4H9
0.9
3.6
7.6E + 11
1.5E + 13
1.4
3.46
3.46
0.0
0.0
3.65
3.65
19.2
19.2
48.4
68.2
29.9
29.9
[33]
this work
[27]
[15]
[33]
i-C4H10 + CH3 = CH4 + i-C4H9
5.4
this work
Decomposition of isobutane
i-C4H10 = CH3 + i-C3H7
i-C4H10 = H + t-C4H9
i-C4H10 = H + i-C4H9
Reactions of methyl radicals
1.1E + 26
1.0E + 15
1.0E + 15
2.61
0.0
0.0
378.0
390.7
410.4
[34]
[29]
[29]
CH3 + CH3 ( + M) = C2H6 ( + M) k
3.6E + 13
3.6E + 41
0.0
7.0
0.00
11.6
[35]
[35]
[35]
∞
k0
Fcenter = (1 0.62) exp( T/73) + 0.62 exp( T/1180)
Reactions of tert-butyl radicals
t-C4H9 = C3H6 + CH3
1.0E + 16
3.0E + 16
8.3E + 13
17E + 13
7.2E + 16
5.4E + 12
3.8E + 15
4.9E + 15
2.5E + 15
5.1E 06
0.0
0.0
0.0
0.0
1.73
0.0
1.0
1.0
1.1
5.17
193.0
198.0
159.0
159.6
0.0
0.0
0.0
0.0
0.0
[29]
this work
[33]
this work
[29]
[33]
[33]
[33]
this work
[33]
t-C4H9 = i-C4H8 + H
t-C4H9 + t-C4H9 = i-C4H10 + i-C4H8
t-C4H9 + H = i-C4H8 + H2
t-C4H9 + CH3 → i-C4H8 + CH4
t-C4H9 + CH3 → neo C5H12
t-C4H9 + i-C4H10 → i-C4H10 + i-C4H9
Reactions of isobutyl radicals
i-C4H9 = C3H6 + CH3
37.9
2.0E + 13
1.0E + 14
0.5E + 00
1.9E + 14
6.0E + 12
0.0
0.0
3.46
0.32
0.32
125.3
151.9
24.9
0.0
[33]
[36]
[33]
[33]
[33]
i-C4H9 = i-C4H8 + H
i-C4H9 + i-C4H10 → t-C4H9 + i-C4H10
i-C4H9 + CH3 → i-C5H12
i-C4H9 + CH3 → i-C4H8 + CH4
Other reactions
0.0
i-C3H7 + C2H5 = i-C5H12
1.2E + 14
0.35
0.0
[37]
The unit of A is a combination of cm3, mol, s, and K, depending on the reaction order and temperature coefficient b. E is given in kJ/mol.
For methyl radical recombination the high and low pressure limit of the rate coefficient and Fcenter are given.
The temporal evolution of the products is depicted
in Fig. 3. Clearly a stationary state has been reached for
C3H8 and neo-C5H12. The slight increase of the mole
fractions of the other products can be neglected. The
reason is simply given by the complete consumption
of the radical precursor in the heated reaction zone.
Further heating does not produce new methyl radicals
and therefore the reaction is not promoted any longer.
Remixing of the sample by “cooling” and convectional
flow resets almost the initial condition. Fresh gas is
transported from the “cold” dead space into the “hot”
reaction zone and products are carried away from the
reaction zone into the dead space. It should be noticed
that the volume of the “cold” dead space exceeds the
volume of the “hot” reaction zone by more than a factor
of 100. Therefore, the concentrations of the resulting
products can be increased by multiple repetition of the
experiment (several laser pulses up to 10,000 with a

738
GOOS ET AL.
Figure 4 Relative product yields as a function of tempera-
ture (open symbols: model calculation; closed symbols with
error bars: experimental data from Table I).
Figure 3 Calculated temporal evolution of the mole
fractions of the products (T = 850 K; p((t-C4H9O)2) =
3.4 mbar; p(i-C4H10) = 30.6 mbar; p(Ar) = 66 mbar).
DISCUSSION
pulse width of 50 ms) without changing the product dis-
tribution. In any case, the reaction cannot be initiated
when the radical precursor has been consumed and
therefore additional laser pulses do not alter the prod-
uct distribution. As a result, we can determine relative
product yields by GC and MS.
Isobutane was selected for the present study, because
H-atom abstraction through methyl radical attack
yields isobutyl and tert-butyl radicals. Similarly, in our
previous investigation of the methyl-induced pyroly-
sis of n-butane [5] n-butyl and sec-butyl radicals are
formed. sec-butyl radicals and allyl radicals are pro-
duced in the methyl-induced pyrolysis of propene [6].
Thus this work completes our investigations on butyl
radical reactions.
To model the trends of the relative yields in our ex-
periments, we altered the rate coefficients of some of
the reactions as listed in Table II as guided by sen-
sitivity analysis. In each case the original value from
the literature (mostly estimated) is also given. Probably
other combinations of rate coefficients could represent
the experimental relative product yields, too. There-
fore we listed the sensitive reaction constants, know-
ing that an exact measurement of a rate constant in the
future, which perhaps will not match our used value,
will influence the other rate constants. But the order of
magnitude of the used rate constants should not change.
The resulting relative product ratios have been com-
pared with the experiment. The quality of the fit was
well within the experimental uncertainty of the analyt-
ical measurements.
In Fig. 4 we present the comparison between the
experimental relative product yields and the modeled
values for the main products propene (C3H6) and
isobutene (i-C4H8), as well as for the minor prod-
ucts ethene (C2H4) and neo-pentane (neo-C5H12). The
other minor products propane (C3H8) and isopentane
(i-C5H12) are for clarity not shown, although a similar
agreement between the trend of the experimental data
and the model calculation has been achieved.
H-atom abstraction by methyl radicals is the major
propagation step in hydrocarbon pyrolysis and is the
main source of methane
CH3
+ i-C4H10 → C4H9 + CH4
(R13)
Because the tertiary C H bond in isobutane is weaker
than the primary C H bond [39], abstraction of the
tertiary H-atom is preferred. Given that our gas chro-
matography column was aimed to monitor higher
hydrocarbons and aromatics, methane could not be
detected.
Unimolecular decomposition of methyl radicals is
unimportant, however, recombination cannot be ne-
glected and the formation of nearly all products is sen-
sitive to this reaction
CH3 + CH3 + M → C2H6 + M
(R14)
Since the rate constant is well known, we adopted the
recommendation from Baulch et al. [35]. The pres-
sure dependence has been included using Troe’s falloff-
The mechanism and the kinetic modeling explain
the formation of the main products. Within the uncer-
tainties of the experiment reasonable agreement was
found with the model.
formula [40] with the given parameters for k0, k , and
∞
Fcenter
.
In principle, the methyl radicals can also undergo
other radical-radical combination reactions. However,

REACTIONS OF METHYL RADICALS WITH ISOBUTANE BETWEEN 800 AND 950 K
739
the concentration of isobutane is so large compared to
CONCLUSION
the concentration of alkyl radicals (except methyl) that
H-atom abstraction and methyl recombination are the
dominating methyl reactions. The reaction of methyl
and tert-butyl radicals leads to the minor combina-
tion product neo-pentane and by H-atom abstraction
to isobutene and methane, respectively. It is to notice,
that the rate coefficients for these two competing re-
actions from Tsang [33] are cited wrong in the NIST
Chemical Kinetics Database [41].
Although hydrogen atoms are not produced in the
initial step, they are formed in the subsequent radical
decomposition reactions. For isobutyl radicals C H
and C C-bond fission competes
We studied the reaction of methyl radicals with isobu-
tane in the medium temperature range of 800–950 K
by the application of laser induced heat transfer by
fast vibrational to translational energy transfer of SF6
to the reaction mixture. A well characterized individ-
ually checked temperature profile is established. The
reaction products were analyzed using gas chromatog-
raphy and mass spectroscopy. The validated reaction
scheme from oxidation studies of n-butane, n-pentane,
and isopentane has been slightly extended to model the
chemistry in the methyl/isobutane system.
NOMENCLATURE
i-C4H9 → H + C4H8
i-C4H9 → CH3 + C3H6
(R15)
(R16)
CH3
methyl radical
methane
while tert-butyl radicals can only undergo C H-bond
CH4
fission
CF4
C2H4
tetrafluoromethane
ethene
C2H6
C3H6
ethane
propene
t-C4H9 → H + C4H8
(R17)
c-C3H6
C3H7
C3H8
cyclopropane
propyl
propane
As a result, large quantities of isobutene and propene
are formed.
Isobutyl radicals can be transformed into tert-butyl
radicals by an H-atom transfer reaction with isobutane
and vice versa. For the corresponding rate constant
given in the compilation by Tsang [33] we noticed a
discrepancy between the summary table (page 10) and
the recommended value (page 66). We used the expres-
sion from page 66.
C4H7, CH2C(CH3)CH2
i-C4H8, (CH3)2CCH2
i- C4H9, (CH3)2CH CH2 isobutyl radical
t- C4H9, (CH3)2 CCH3 tert-butyl radical
i-C4H10, (CH3)2CHCH3 isobutane
neo-C5H12
i-C5H12
CH3COCH3
SF6
2-methyl-allyl
isobutene
neo-pentane
isopentane
The major consumption of hydrogen atoms will pro-
ceed via H-atom abstraction from the hydrocarbons
propanone, acetone
sulfur hexafluoride
DTBP, di-tert-butyl
peroxide
potassium chloride
mass-spectroscopic
sensitivity
milliseconds
mass spectroscopy
gas chromatography
(CH3)3COOC(CH3)3,
(t-C4H9O)2
KCl
H + i-C4H10 → C4H9 + H2
(R18)
ME
and at low temperatures will generate almost exclu-
sively tert-butyl radicals while at higher temperature
also isobutyl radicals are formed [18].
Only at the highest temperatures thermal decompo-
sition of isobutane has to be included.
ms
MS
GC
i-C4H10 → CH3 + C3H7
(R19)
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