Laser powered homogeneous pyrolysis of ethyne, propyne, and propadiene initiated by methyl radicals: Formation and degradation of hydrocarbons at 800-950 K

Article abstract of DOI:10.1039/b111643f

Applying laser heating by fast vibrational-translational energy transfer in a quasi-wall-free reactor the pyrolysis of ethyne (C₂H₂), propyne (p-C₃H₄), and propadiene (a-C₃H₄) was studied experimentally at 0.13 bar in the medium temperature range of 800-950 K with respect to the degradation and formation of hydrocarbons. The radical/hydrocarbon chemistry was chemically induced via CH₃ radicals produced by the fast thermal dissociation of di-tert-butyl-peroxide DTBP ((tert-C₄H₉O)₂ → 2 CH₃ + 2 CH₃COCH₃). Complete analysis of the product yields was achieved by means of GC-MS with special attention to isomeric product and benzene formation. The product distribution, the temperature dependence and the underlying reaction schemes were analyzed by kinetic models developed for high temperature alkane oxidation/pyrolysis and aromatic formation in premixed ethene and ethyne flames. The primary attack of the unsaturated hydrocarbons by CH₃ radicals in the studied temperature range occurs via the addition to the double/triple bond and via hydrogen atom abstraction, leading to different classes of radicals. For the reaction system C₂H₂ + CH₃ high yields of C₆H₆ with a marked negative temperature dependence were observed. A semi-quantitative description of the C₆H₆ yield was obtained by a reaction sequence of successive addition of C₂H₂ to the radicals C₂H₃ (from C₂H₂ + H) and C₄H₅, being consistent with recent discussed reaction networks. For the reaction systems p-C₃H₄ + CH₃ and a-C₃H₄ + CH₃ only qualitative agreement between measured and modelled product yields was found, pointing to a lack of reliable data of the reactions of p-C₃H₄/a-C₃H₄/C₃H ₃/H. Modified mechanisms are presented for the radical rich reaction systems C₂H₂ + CH₃, p-C₃H₄ + CH₃, and a-C₃H₄ + CH₃ experimentally studied.

Full text of DOI:10.1039/b111643f

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Laser powered homogeneous pyrolysis of ethyne, propyne, and
propadiene initiated by methyl radicals: formation and degradation
of hydrocarbons at 800–950 K
a
Elke Goos, Horst Hippler, Karlheinz Hoyermann and Bettina J u¨ rges
a
b
b
a
Institut f u¨ r Physikalische Chemie, Universit a¨ t Karlsruhe, Fritz-Haber-Weg 4,
D-76128 Karlsruhe, Germany. E-mail: horst.hippler@chemie.uni-karlsruhe.de
Institut f u¨ r Physikalische Chemie, Universit a¨ t G o¨ ttingen, Tammannstr. 6, D-37077 G o¨ ttingen,
Germany. E-mail: jkupfer@gwdg.de
b
Received 2nd January 2002, Accepted 8th March 2002
First published as an Advance Article on the web 1st May 2002
Applying laser heating by fast vibrational–translational energy transfer in a quasi-wall-free reactor the pyrolysis
of ethyne (C H ), propyne (p-C H ), and propadiene (a-C H ) was studied experimentally at 0.13 bar in the
medium temperature range of 800–950 K with respect to the degradation and formation of hydrocarbons. The
2
2
3
4
3
4
radical/hydrocarbon chemistry was chemically induced via CH radicals produced by the fast thermal
3
dissociation of di-tert-butyl-peroxide DTBP ((tert-C
4
H
9
O)
2
! 2 CH
3 3 3
+ 2 CH COCH ). Complete analysis of
the product yields was achieved by means of GC-MS with special attention to isomeric product and benzene
formation. The product distribution, the temperature dependence and the underlying reaction schemes were
analyzed by kinetic models developed for high temperature alkane oxidation/pyrolysis and aromatic formation
in premixed ethene and ethyne flames. The primary attack of the unsaturated hydrocarbons by CH
the studied temperature range occurs via the addition to the double/triple bond and via hydrogen atom
abstraction, leading to different classes of radicals. For the reaction system C + CH high yields of C
with a marked negative temperature dependence were observed. A semi-quantitative description of the C H
3
radicals in
2
H
2
3
6 6
H
6
6
2 2 2 3 2 2
yield was obtained by a reaction sequence of successive addition of C H to the radicals C H (from C H + H)
and C H , being consistent with recent discussed reaction networks. For the reaction systems p-C H + CH
4
4
5
3
3
and a-C
pointing to a lack of reliable data of the reactions of p-C H /a-C H /C H /H. Modified mechanisms are
3 4 3
H + CH only qualitative agreement between measured and modelled product yields was found,
3
4
3
4
3
3
presented for the radical rich reaction systems C
studied.
2
H
2
+ CH
3
, p-C
3
H
4
+ CH
3
, and a-C
3
H
4
+ CH
3
experimentally
our topic are listed. Experimental and detailed chemical kinetic
modelling of benzene formation in propane–air and butane–air
1
. Introduction
2
,3
The formation of higher hydrocarbons from smaller ones has
attracted attention both technically and scientifically. Partial
burning under fuel rich conditions, cracking, and reforming
of petroleum are used to convert natural gas (methane),
alkanes, and high molecular weight hydrocarbons to chemicals
on a large industrial scale. Emissions of polycyclic aromatic
hydrocarbons as by-products of the combustion in internal
combustion engines are of vital environmental concern. Scien-
tifically the mechanisms of the formation of the first aromatic
ring (benzene) from aliphatic and unsaturated non-cyclic
hydrocarbon fuels are of central interest, especially in the
description of the steps from fuel to soot. This is summarized
flames point to the main routes via propargyl recombination
(C + C ) and propargyl + allyl combination to
! C
fulvene (C + C + 2 H). Detailed modelling of
3
H
3
3
H
3
6 6
H
3
H
3
3
H
5
! C
6 6
H
aromatic formation in premixed ethyne and ethene–air flames
over a wide range of initial composition revealed the interest-
4
ing result that benzene formation in flames is governed by dif-
ferent reactions in the different flame zones. The maximum
reaction rates at the temperatures are: T ¼ 600 K (n-
C
C
(C
4
H
H
3
+ C
2
H
2
! C
6
H
5
), T ¼ 700
K
(n-C
4
H
5
+ C
2
H
2
!
+ H), T ¼ 1000 K (c-C
H
4
+ H ! C
H ), T ¼ 1600 K
6 5
6
6
6
3
H
3
+ C ! C H ). As the propargyl radical plays a vital
3
H
3
6 6
role in aromatic formation the reactions of possible precursors
like propyne and propadiene are of special interest. The studies
of the pyrolysis of propyne show fast mutual isomerization to
propadiene and high amounts of benzene. Moreover, a pyroly-
sis study of propyne in a flow reactor points to the reactions p-
1
in a most recent review by Richter and Howard. Different
5,6
experimental and theoretical approaches are to be noted. At
high temperatures shock tube and flame studies have been used
to identify important pathways, whereas at low temperatures
isothermal flow reactors and photolysis reactors with their spe-
cific detection systems for labile and stable species were
applied. Flame studies together with quantum chemical calcu-
lations and modelling have the advantage of covering the
whole temperature field from room temperature to 2500 K
but with the inherent coupling to the oxidation processes.
Some flame and pyrolysis investigations with relevance to
7
C
3
H
4
+ H ! a-C
3
H
4
+ H and p-C
The main objective of the present paper is to add new experi-
3
H
4
+ H ! C
2 2 3
H + CH .
mental results on the formation of higher hydrocarbons like
benzene obtained in the limited temperature region of 750–
1000 K (i.e. the low temperature preheat zone of a flame) for
the fuels ethyne, propyne, and propadiene in the presence of
methyl radicals in higher concentrations than in flames. The
DOI: 10.1039/b111643f
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applied method of laser powered homogeneous pyrolysis of a
fuel and a radical precursor for methyl radicals together with
detailed species analysis via GC-MS is a powerful addition
to the experimental methods previously available. The existing
reaction networks optimized for similar reaction systems
found in the literature will be used complementarily to inter-
pret our experimental findings in the lower temperature region,
possibly pointing to necessary refinements of the numerous
reaction mechanisms with several estimated rate coefficients
and reaction routes.
Table 1 Relative product yields for the reaction C
position of the reaction mixtures, experimental conditions: ptotal
33 mbar; p(di-tert-butyl peroxide) ¼ 4.5 mbar; p(CF ) ¼ 0.79 mbar;
p(C ) ¼ 1.3 mbar (leading to Tav ¼ 795 K);
) ¼ 40.3 mbar; p(SF
p(SF
2 2 3
H + CH (com-
¼
1
4
2
H
2
6
6
) ¼ 3.9 mbar (leading to Tav ¼ 945 K); p(Ar) ¼ added up to pto-
tal ; number of pulses: 10 000 (Tav ¼ 795 K), 2000 (Tav ¼ 945 K), pulse
duration ¼ 50 ms)
2 2 3
C H + CH
Relative product yield (%)
Products
n-C
T ¼ 795 K
T ¼ 945 K
4
H
10
—
—
2
. Experimental
C
C
5
3
H
H
10
Trace
Trace
Trace
Trace
The experimental approach has been described in a recent
paper in this journal. Therefore only a brief summary is given
illustrating the experimental set up, the evaluation procedure,
and the limitations in the application.
The arrangement is shown schematically in Fig. 1. A cylind-
rical cell (id 2.0 cm, length 7.8 cm, stainless steel) with KCl-
windows under the Brewster angle was connected to a gas
handling system, pressure transducers, pumps, and a port for
a septum for taking samples with a syringe for the GC-MS
8
8
C H
3
29.5/28.2
14.2/15.3
1.3/1.4
3.9/4.2
1.7/2.0
1.6/1.8
6.8/8.0
12.0/14.6
29.0/24.5
28.0/26.0
23.4/24.1
1.5/1.5
5.4/5.6
1.3/1.3
1.0/1.0
8.1/9.4
15.2/17.9
16.1/13.2
6
p-C
a-C
-C
3 4
H
3
H
H
4
1
4
8
trans-2-C
cis-2-C
,3-C
4 8
H
4
H
8
1
4 6
H
C
C
4
6
H
H
4
6
analysis. The CO
2
-laser (CW or pulse mode, pulse duration
ꢀ
0 –1 s) was operated at the P20 line (10.59 mm) with an out-
4
1
put up to 40 W and a Gaussian beam profile of 4 mm (1/e-
value). Samples were withdrawn with a syringe (100 ml) and
analyzed by GC-MS analysis (Finnigan MAT 9611, 5100)
using two different capillary columns (Poraplot Q (Chrom-
pack); combination of FS-OV-1CB, 25 m ꢁ 0.25 mm, id df
in the Tables 1 and 2 are essentially those deduced by the che-
mical thermometer. As the decomposition of the CH radical
precursor DTBP occurs at the highest temperatures caused
by the high activation energy (156.5 kJ mol ), the reactions
under study are switched off automatically in the cold parts.
The uncertainty of the cited average temperatures is estimated
3
ꢀ1
0
.5 mm and FS-SE-54-CB, 25 m ꢁ 0.25 mm, id df 0.25 mm).
Species were identified by the retention time and the mass spec-
trometric fragmentation pattern (ionization energy: 70 eV) and
the concentration was obtained by calibration against authen-
tic samples. The non-reactive gas CF4 was admixed in all
experiments as an internal concentration reference. Commer-
cial grade chemicals were used without further purification,
the liquids were carefully degassed before use, and gaseous
reaction mixtures were stored in the dark. ((CH ) -
as ꢂ40 K. Initiation of the hydrocarbon + CH reactions was
3
performed by heating mixtures of di-tert-butyl peroxide
((CH ) COOC(CH ) ), leading to 2CH COCH + 2CH , and
3
3
3 3
3
3
3
the hydrocarbon under study. The experimental conditions
are listed in the Tables 1 and 2.
3
3
COOC(CH
C H (propyne): 96.0%; a-C H (allene): 97.0%, all Merck;
3
)
3
(DTBP): 95%; SF
6
: 99.9%; c-C
3
H
6
: 99.0%; p-
Table 2 Relative product yields for the reactions p-C
ne) + CH and a-C (propadiene, allene) + CH (Composition of
the reaction mixtures, experimental conditions (reaction p-C (pro-
pyne) + CH
: ptotal ¼ 133 mbar, p(di-tert-butyl peroxide) ¼ 4.5 mbar;
p(CF ) ¼ 40.3 mbar; p(SF ) ¼ 1.3 mbar
) ¼ 0.79 mbar; p(p-C
leading to Tav ¼ 795 K); p(Ar) ¼ added up to ptotal ; number of
pulses: 10 000; pulse duration: 50 ms; reaction a-C (prop-
adiene) + CH : p(a-C ) ¼ 1.3 mbar (leading
) ¼ 40.3 mbar; p(SF
to Tav ¼ 795 K); p(SF ) ¼ 3.9 mbar (leading to Tav ¼ 945 K), number
of pulses: 10 000 (795 K), 2000 (945 K), pulse duration ¼ 50 ms))
3 4
H (propy-
3
4
3
4
3
3
H
4
3
Ar: 99.9998%; C
2
H
2
: 99.6%, Messer-Griesheim; CF
Linde). The laser powered homogeneous heating was achieved
4
: 99.8%,
3 4
H
3
4
3
H
4
6
via fast vibration-to-translation energy transfer from the laser-
absorbing SF to Ar after applying laser pulses of 50–100 ms
(
6
3
H
4
duration where the temperature profile was reached within
1
3
3
H
4
6
–3 ms with the maximum temperature at the optical axis of
6
the cell. The temperature profiles in the reactor were deduced
by four different methods (heat transfer balance of absorbed
laser power/heat conduction; radial temperature distribution
via the temperature-dependent optical absorption coefficient
p-C + CH a-C + CH
3
3
H
4
3
3
H
4
of SF
meter via the measured conversion of c-C
to C (propene), see ref. 8). The average temperatures given
6
; pressure rise in the total volume; chemical thermo-
Relative product yield (%)
3 6
H
(cyclopropane)
Products
T ¼ 795 K
T ¼ 795 K T ¼ 945 K
3
H
6
C
C
2
3
H
H
4
—
15.6/15.2
—
23.0/20.4
7.5/7.0
38.8/40.3
—
13.9/12.1
2.8/2.6
38.6/39.2
—
6
p-C
a-C
-C
3 4
H
3
H
H
4
3.3/3.2
5.6/5.6
10.3/10.1
14.7/14.4
4.7/5.1
8.8/9.2
20.9/20.4
—
1
4
8
/iso-C
4
H
8
8.8/9.1
—
—
13.1/13.3
—
—
trans-2-C
cis-2-C
4 8
H
4
H
8
1
1
1
2
2
2
,3-C
,2-C
4
4
H
H
6
0.4/0.4
4.7/4.9
2.6/2.7
11.7/12.6
—
0.6/0.6
7.7/7.8
4.5/4.6
17.3/18.3
—
6
-C
-Methyl-1-butene
-C
-Methyl-2-butene
4 6
H
4
H
6
5.3/5.6
8.0/8.4
2.5/2.5
0.3/0.3
—
2.5/2.6
—
—
1.5/1.5
—
C
C
6
6
H
H
6
6
Fig. 1 Experimental arrangement.
Phys. Chem. Chem. Phys., 2002, 4, 2011–2018
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Fig. 2 Reaction system C
in Table 1). Comparison with measured values in parenthesis.
2
H
2
+ CH
3
. Calculated temporal evolution of the mole fractions of the main products (T ¼ 800 K, initial conditions as
3
. Results
lowing we will discuss our results with respect to the temporal
evolution of the main products during the 50–100 ms heating
period, to the sensitivity analysis of the reactions for the main
products and for benzene, and to the extraction of the main
reaction routes in order to derive the central part of the
mechanism for our special conditions of the CH3 initiated
C H , p-C H , and a-C H pyrolysis.
The formation of the hydrocarbons from ethyne, propyne, and
propadiene initiated by the reactions of methyl radicals was
studied in the temperature range of around 800 K to 960 K
at a pressure of 133 mbar. Here the thermal decomposition
of di-tert-butyl peroxide is complete and the main products
are explained by the reaction scheme
2
2
3
4
3
4
4
2 2 3
.2. The reaction system C H + CH
ðtert-C4H9OÞ ! 2 tert-C4H9O
tert-C4H9O ! CH3COCH3 þ CH3
CH3 þ hydrocarbons ! products
2
The calculated temporal evolution of the mole fractions of the
main products at T ¼ 800 K and the experimental results of
Table 1 are shown in Fig. 2. The experimentally found main
products (C H , p-C H , C H , C H ) are predicted by the
4
4
3
4
6
6
3
6
The experimental conditions and the product yields are listed
in Tables 1 and 2. Methane concentrations were not deter-
mined owing to the use of the special capillary columns
selected for the monitoring of saturated and unsaturated
hydrocarbons of higher molecular weight. The different values
of a product yield in the tables were derived from the determi-
nation of the different mass spectrometric sensitivities and the
integration of the gas chromatographic peaks thereby reflect-
ing the reproducibility and uncertainty of the reported product
yield. Besides the main products only traces of other com-
pounds with a total mole-fraction of less than 1% were
mechanism, their yields have reached a stationary state
after a few ms being constant over a longer time period, and
their appearance is well separated from other products with
respect to concentration. But it is to be noted that C H
4
4
(
overpredicted whereas C H (1 : 0.57) and C H (1 : 0.43)
measured : predicted ¼ 1 : 2.8) and p-C
3 4
H (1 : 1.5) are
6
6
3
6
are underpredicted. The calculated temperature dependence
of the relative product yield is displayed in Fig. 3; the decrease
6 6 3 6 4 4
in the C H and C H yield and the constant C H yield is
reproduced well by the mechanism whereas p-C H and a-
3
4
C
3
H
4
show marked deviations. When reducing the rate coeffi-
2 2 3 3 8
observed. These are for the system C H + CH : C H ,
cient of the reaction C H + C H ! C H by a factor of 10,
3
3
3
3
6
6
C H , C H , C H , for the system p-C H + CH : C H ,
3
5
6
5
8
5
10
4
3
3
8
no changes in the yields of the products, even of C
observed (see Fig. 3). This is taken as supporting evidence that
6 6
H is
C
C H , C H (besides the impurities C H , C H Cl). In the
5
H10 (several isomers), and for the system a-C
3
H
4
+ CH
3
:
3
8
5
8
3
6
3
5
study of the system p-C
C H were detected with slightly different retention times but a
3 4 3
H + CH two isomers of the formula
6
6
fragment ion pattern (at 70 eV ionization energy) of benzene.
4
. Discussion
4
.1. General remarks
The reaction sequences for the formation of the higher hydro-
carbons and their yields were analyzed along existing reaction
networks. The rate coefficients for the decomposition of di-
tert-butyl peroxide and of the tert-C H O radicals are taken
4
9
9
–11
from the literature. The relevant reactions of saturated
alkanes (C –C ), and of the alkyl radicals (C –C ) as well as
1
5
1
5
the chemistry of the unsaturated hydrocarbons, and especially
that of the aromatics were adopted from the detailed work
1
4,15
of.
The kinetic modelling was performed using the CHEM-
1
6
2 2 3
Fig. 3 Reaction system C H + CH . Calculated relative product
KIN programs, assuming isothermal and nearly constant
pressure conditions. The overall procedure has been described
in more detail in our previous papers of the methyl initiated
yields as function of temperature (full symbols: model calculation with
reaction scheme, open symbols: model calculation with reaction
14
1
4
scheme, rate coefficient k(C
3
H
3
+ C
3
H
3
! C
6 6
H ) reduced by factor
8
,12,13
pyrolysis of butane, propene and isobutane.
In the fol-
10). Initial conditions as of Table 1.
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2 2 3 2 2 6 6
Fig. 4 Reaction system C H + CH . Sensitivity analysis of the consumption of C H and formation of C H at 1000 K.
C
6
H
6
formation is dominated by another channel. A sensitivity
analysis for the disappearance of the reactant C H and the
istry has to be revised slightly, including the interplay of p-
C H and a-C H . Therefore, the p-C H and a-C H chemis-
2
2
3
4
3
4
3
4
3
4
formation of C
6
H
6
is given in Fig. 4, indicating a main route
of C H destruction by H atoms via C H and formation of
try was investigated in two separate experimental and compu-
tational studies.
2
2
2
3
C
6
H
6
2 3
via C H . Based on the sensitivity analysis for a large
number of species a reaction network for the main products
was derived (see Fig. 5). In the temperature range of 800–
4
3 4 3
.3. The reaction system p-C H + CH
1
of H and CH
000 K the C H disappearance is governed by the competition
2
In Fig. 6 the predicted mole fraction of the main products are
compared with the measured ones. Only the 1- and 2-butynes
are reproduced well whereas a-C H and C H are drastically
2
3 2 2
addition to C H . Benzene is formed via H
addition to C H followed by successive C H addition and
2
2
2
2
3
4
6
6
partial back-formation of H, making it part of a catalytic
cycle. At higher temperature the redissociation of the C H
radical is more pronounced than that of the C H radical
3 5
overestimated and the 1,2(1,3)-butadienes and propene are
underestimated. The sensitivity analysis shows that according
to the assumed mechanism the disappearance of p-C H is
2
3
3
4
due to the lower number of the degrees of freedom necessary
for stabilization. This competition explains the change in the
C H and p-C H yields with temperature. In order to fit the
governed by the reactions with H and CH , the formation of
3
a-C H by H + p-C H ! H + a-C H and via C H and the
3
4
3
4
3
4
4
6
6
6
3
4
formation of C H by the C H radical combination
3
6
6
3
experimental to the predicted product yields, the p-C
H
3 4
chem-
(see Fig. 7). A modified reaction scheme is presented in Fig. 8,
Fig. 5 Reaction system C
2 2 3
H + CH . Central part of the reaction mechanism (experimental product yields at 795 K and 945 K (italic)).
2
014 Phys. Chem. Chem. Phys., 2002, 4, 2011–2018

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Fig. 6 Reaction system p-C
3
H
4
+ CH
as in Table 2). Comparison with measured values in parenthesis.
3
. Calculated temporal evolution of the mole fractions of the main products (T ¼ 800 K, initial conditions
illustrating the main routes, where the major intermediates are
traced back by the measured stable products as listed in Table 2
and annotated in the figure. At the medium temperature of 800
K both mechanisms of H and CH addition to p-C H and H
3 4 3
4.4. The reaction system a-C H + CH
The temporal evolution of the main products has been derived
from the assumed mechanism for the experimental conditions
in Table 2 (see Fig. 9). Comparing predicted and measured
concentrations a qualitative agreement for p-C H , being the
3
3
4
atom abstraction from the CH
Addition of H atoms and CH radicals to p-C H followed
3
3 4
group of the p-C H occurs.
3
3
4
3
4
by CH
over 50% of the products (alkenes).
The formation of C via H atom abstraction is identified
by the appearance of 1-butyne and the formation of C is
assigned to the reactions of the C radical. This qualitative
scheme has to be substantiated in analyzing the reactions of the
+CH reactions, which have been studied experimen-
tally, too, by the same experimental arrangement and procedure
to be published soon. Moreover, the isomerization of p-C
by H atoms has to be studied experimentally in more detail.
3
combination with the formed radicals accounts for
main product, and for the minor components butadiene, pro-
pene, and butene is observed. But here, as in the p-
C H + CH system, benzene formation is not reproduced ade-
3 3
H
3
4
3
H
6 6
quately (by a factor of 10 too high). Ethene, a major product
of the experiments, is not represented either. In Fig. 10 the
temperature dependence of the product yield is shown. Here
as in the experiment no strong dependence on temperature is
found. The sensitivity analysis for the elucidation of the main
pathways for the formation of p-C H and C H are displayed
3
H
3
C H
4 8
3
3
H
4
3
4
6
6
in Fig. 11, leading to the formation of p-C
3
H
4
via H + a-
Fig. 7 Reaction system p-C
3
H
4
+ CH
3
. Sensitivity analysis of the consumption of p-C
3 4 6 6
H and the formation of C H at 800 K.
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3 4 3
Fig. 8 Reaction system p-C H + CH . Central part of the reaction mechanism (experimental product yields at 795 K).
Fig. 9 Reaction system a-C
as in Table 2). Comparison with measured values in parenthesis.
3
H
4
+ CH
3
. Calculated temporal evolution of the mole fractions of the main products (T ¼ 800 K, initial conditions
C
3
H
4
! H + p-C
3
H
4
and benzene via the sequence p-
5. Conclusion
C H + CH ! C H + CH , C H + C H ! C H + H. But
3
4
3
3
3
4
3
3
3
3
6
5
it is to be noted that C
low amounts in the systems p-C H + CH and a-
H
6 6
is experimentally found in equal
The experimental method of laser powered homogeneous pyr-
olysis of the unsaturated hydrocarbons ethyne, propyne, and
propadiene offers the advantage of a wall-free reaction center
at 800–1000 K with high methyl radical concentration and
detailed chemical analysis thus allowing the testing of complex
reaction networks optimized for complex reactive systems like
premixed or diffusion flames.
3
4
3
3 4 3 3 4
C H + CH , possibly via p-C H in both systems. A proposed
modified mechanism based on the experimental findings is
expressed in a short form in Fig. 12.
For the reaction system C
agreement between measured and predicted formation of
higher hydrocarbons is observed. The formation of C in
2 2 3
H + CH semi-quantitative
6 6
H
the temperature range of 800–1000 K is well described by suc-
cessive addition of C H to the radicals C H + H ! C H and
2
2
2
2
2
3
C
4
H
5
making the reactions of the
C
H
3 3
radical
(
C H + C H ! C H , C H + H) less important. This obser-
3
3
3
3
6
6
6
5
vation supports the prediction of ref. 4, but fine tuning of some
rate coefficients is still necessary.
3 4 3 3 4 3
For the reaction systems p-C H + CH and a-C H + CH
only qualitative agreement in the observed and predicted pro-
duct yields is to be stated. A revision in product routes and rate
coefficients seems to be appropriate for the reactions of p-
C
3
H
4
/a-C
3 4 3 3
H /C H . This is illustrated for an important reac-
tion under our conditions, the reaction C H + H ! p-C H /
3
3
3
4
a-C
3 4
H . The reported rate coefficients (high pressure) vary
1
4
3
ꢀ1 ꢀ1 17
13
3
ꢀ1
from 1.4 ꢁ 10 cm mol
s
,
A theoretical study supports the higher value and
to 2 ꢁ 10 cm mol
3 4 3
Fig. 10 Reaction system a-C H + CH . Calculated relative product
yields as function of temperature. Initial conditions as in Table 2.
ꢀ
1 4,18,19
.
s
2
016
Phys. Chem. Chem. Phys., 2002, 4, 2011–2018

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3 4 3 3 4 6 6
Fig. 11 Reaction system a-C H + CH . Sensitivity analysis of the formation of p-C H and C H at 800 K.
3 4 3
Fig. 12 Reaction system a-C H + CH . Central part of the reaction mechanism. (Experimental product yields at 795 K).
2
0
predicts a product branching for p-C
The problems of adequate modelling of the dissociation pro-
cesses of p-C and a-C both on the experimental and
the theoretical sides are discussed in ref. 6. The development
of a quantitative model for the system under study (p-C
a-C H + CH ) is out of the scope of the present paper. But
3
H
4
/a-C
3
H
4
of 1.55.
Prozesse’’). Both groups also acknowledge the financial sup-
port given by the Fonds der Chemischen Industrie.
3
H
4
3 4
H
H
3 4
/
References
3
4
3
1
2
3
H. Richter and J. B. Howard, Prog. Energy Combust. Sci., 2000,
6, 565.
N. M. Marinov, J. Castaldi, C. F. Melius and W. Tsang, Combust.
Sci. Technol., 1997, 128, 295.
N. M. Marinov, W. J. Pitz, C. K. Westbrook, A. M. Vincitore, M.
J. Castaldi, S. M. Senkan and C. F. Melius, Combust. Flame,
preliminary simulations with higher rate coefficients for
H + C H and lower for C H + C H ! C H than in ref. 4
2
3
3
3
3
3
3
6
6
6 6
lead to a better description of the observed C H formation.
1
998, 114, 192.
Acknowledgement
4
5
6
H. Wang and M. Frenklach, Combust. Flame, 1997, 110, 173.
C. H. Wu and R. D. Kern, J. Phys. Chem., 1987, 91, 6291.
J. H. Kiefer, P. S. Mudipalli, S. S. Sidhu, R. D. Kern, B. S. Jursic,
K. Xie and H. J. Chen, J. Phys. Chem. A, 1997, 101, 4057.
S. G. Davis, C. K. Law and H. Wang, Combust. Flame, 1999, 119,
EG and HH gratefully acknowledge financial support by the
Deutsche Forschungsgemeinschaft (SFB 551 ‘‘Kohlenstoff
aus der Gasphase: Elementarreaktionen, Strukturen,
Werkstoffe’’). KH and BJ thank the Deutsche Forschungsge-
meinschaft for financial support within the Sonderforschungs-
bereich 357 (‘‘Molekulare Mechanismen unimolekularer
7
8
9
3
75.
E. Goos, H. Hippler, K. Hoyermann and B. J u¨ rges, Phys. Chem.
Chem. Phys., 2000, 2, 5127.
M. J. Perona and D. M. Golden, Int. J. Chem. Kinet., 1973, 5, 55.
Phys. Chem. Chem. Phys., 2002, 4, 2011–2018
2017

View Article Online
1
1
0
1
M. I. Sway, J. Chem. Soc., Faraday Trans., 1991, 87, 2157.
C. Fittschen, H. Hippler and B. Viskolcz, Phys. Chem. Chem.
Phys., 2000, 2, 1677.
E. Goos, H. Hippler, K. Hoyermann and B. J u¨ rges, Faraday Dis-
cuss., 2001, 119, 243.
E. Goos, H. Hippler, K. Hoyermann and B. J u¨ rges, Int. J. Chem.
Kinet., 2001, 33, 732.
Z. Qin, V. V. Lissianski, H. Yang, W. C. Gardiner, S. G. Davis
and H. Wang, Proc. Combust. Inst., 2000, 28, 1663 ; http://
ignis.me.udel.edu/propane/download.html.
16 R. J. Kee, F. M. Rupley, J. A. Miller, M. E. Coltrin, J. F. Grcar,
E. Meeks, H. K. Moffat, A. E. Lutz, G. Dixon-Lewis, M. D.
Smooke, J. Warnatz, G. H. Evans, R. S. Larson, R. E. Mitchell,
L. R. Petzold, W. C. Reynolds, M. Caracotsios, W. E. Stewart
and P. Glarborg, Chemkin Collection III, Reaction Design, Inc.,
San Diego, CA, 1999.
17 D. B. Atkinson and J. W. Hudgens, J. Phys. Chem. A, 1999, 103,
4242.
18 K. H. Homann and Ch. Wellmann, Ber. Bunsen-Ges. Phys.
Chem., 1983, 87, 609.
12
13
14
15
G. P. Smith, D. M. Golden, M. Frenklach, N. W. Moriarty, B.
Eiteneer, M. Goldenberg, C. T. Bowman, R. K. Hanson, S. Song,
W. C. Gardiner, Jr., V. V. Lissianski and Z. Qin, GRI-MECH 3.0;
http://www.me.berkeley.edu/gri_mech/.
19 M. Braun-Unkhoff, P. Frank and Th. Just, Proc. Combust. Inst.,
1988, 21, 1053.
20 L. B. Harding and S. J. Klippenstein, Proc. Combust. Inst., 2000,
28, 1503.
2
018
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