Kinetics and mechanism of the thermal chlorination of chloroform in the gas phase

Article abstract of DOI:10.1002/1097-4601(2000)32:8<466::AID-KIN3>3.0.CO;2-4

The gas-phase thermal chlorination of CHCl₃ has been studied up to high conversions by photometry and gas chromatography in a conditioned static quartz reaction vessel between 573 and 635 K. The initial pressures of both CHCl₃ and Cl₂ ranged from about 10-100 Torr, and the initial total pressure was varied between about 30-190 Torr. The reaction is rather complex because the produced CCl₄ is not stable. The rate of consumption of Cl₂ therefore increases in the course of time. This acceleration is explained quantitatively in terms of a radical mechanism and its kinetic and thermodynamic parameters. This reaction model is based on a known model for the pyrolysis of CCl₄ to which only one reaction couple involving CHCl₃ has been added. Analyses of the rates of the homogeneous elementary steps show that the primary source of Cl atoms is the second-order dissociation of Cl₂, which is rapidly superseded by a secondary source, the first-order dissociation of the CCl₄ primary product.

Full text of DOI:10.1002/1097-4601(2000)32:8<466::AID-KIN3>3.0.CO;2-4

Kinetics and Mechanism of
the Thermal Chlorination of
Chloroform in the Gas
Phase
G. HUYBRECHTS, Y. HUBIN,* B. VAN MELE
Laboratorium voor Fysische Scheikunde en Polymeren, Vrije Universiteit Brussel, 1050 Brussels, Belgium
Received 9 October 1998; accepted 6 March 2000
ABSTRACT: The gas-phase thermal chlorination of CHCl3 has been studied up to high con-
versions by photometry and gas chromatography in a conditioned static quartz reaction vessel
between 573 and 635 K. The initial pressures of both CHCl3 and Cl2 ranged from about 10–
100 Torr, and the initial total pressure was varied between about 30–190 Torr. The reaction is
rather complex because the produced CCl4 is not stable. The rate of consumption of Cl2 there-
fore increases in the course of time. This acceleration is explained quantitatively in terms of
a radical mechanism and its kinetic and thermodynamic parameters. This reaction model is
based on a known model for the pyrolysis of CCl4 to which only one reaction couple involving
CHCl3 has been added. Analyses of the rates of the homogeneous elementary steps show that
the primary source of Cl atoms is the second-order dissociation of Cl2, which is rapidly su-
perseded by a secondary source, the first-order dissociation of the CCl4 primary product.
᭧ 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 466–472, 2000
INTRODUCTION
CCl4 produced. The reaction model used is an entirely
homogeneous radical mechanism and its known as-
sociated kinetic and thermodynamic parameters. It in-
volves the reaction couple CHCl3 ϩ Cl ϭ CCl3 · ϩ
Despite conditioning ofthe reactors, the gas-phase
chain thermal chlorinations and the chlorine-catalyzed
pyrolyses ofhydrocarbons or partially chlorinated hy-
drocarbons are likely initiated by the heterogeneous
dissociation ofCl 2 on the wall [1,2]. The thermal chlo-
HCl and the thermal reactions ofCCl . These latter
4
have recently been modeled [8] and the values oftheir
parameters are thus checked in this work.
rination ofCHCl in flow systems [3–7] is one ex-
3
ample amongst others.
This work aims to present new experimental results
for this reaction. They have been obtained in a con-
ditioned static system and show a typical increase of
the reaction rate with time that is not reported in pre-
vious works [1,3–7]. This auto-acceleration can be
quantitatively explained by the decomposition ofthe
EXPERIMENT AND MODELING
CHCl3 (Merck) was purified by preparative gas chro-
matography and contained less than 0.005% impurity.
Cl2 (Matheson, ultra-high purity), 99.9% pure, was
used without further purification. Both reagents were
degassed at Ϫ196ЊC on the vacuum line at pressures
5
below 10Ϫ Torr before their introduction into the re-
Correspondence to: G. Huybrechts (ghuybrec@vub.ac.be)
* Present address: Exxon Chemical Polymers International,
Canadastraat 20, B-2070 Zwijndrecht, Belgium
᭧ 2000 John Wiley & Sons, Inc.
action cell. The conventional static system, procedure,
and photometric and gas chromatographic analyses

THERMAL CHLORINATION OF CHLOROFORM IN THE GAS PHASE
467
were similar to those in [9,10]. Reactions were carried
out in two cylindrical quartz reaction vessels unpacked
(100 ml) and packed (70 ml) with quartz tubing with
surface-to-volume ratios, S/V, of about 1.4 and 10
these compounds do not absorb at this wavelength and
thus do not affect the Cl2 measurement. Due to the
presence ofquartz tubes, the experiments perofrmed
in the packed reactor had to be carried out without
photometric monitoring ofthe Cl 2 pressure. The vari-
ation ofthe total pressure was measured with a Pyrex
Bourdon gauge, capable ofdetecting pressure changes
of0.1 Torr. The gas chromatographic analyses (ka-
tharometer detector) were carried out on a 3-m column
containing 30% by weight ofSilicone SE 30 on 60–
80 mesh Chromosorb PAW. After 14 min, the tem-
perature ofthe column was raised rfom 80 to 140 ЊC
1
cmϪ , respectively. The reactor walls were conditioned
by pyrolyzing pure CHCl3 at high temperature (729
K) until reproducible kinetic results were obtained in
the absence as well as in the presence ofCl 2. The ki-
netics ofthe reaction were of llowed by continuously
monitoring the absorption ofCl 2 at 350 nm in the un-
packed reaction cell. Introduction ofamounts of
CHCl3, CCl4, and HCl into the cell showed that
Figure 1 Pressure ofCl (p(Cl2) in Torr) vs. reaction time (t in s) at 573 K for varying initial
2
pressures ofCl 2.
᭝: p(Cl2)0 ϭ 10.5 and p(CHCl3)0 ϭ 90.4 Torr
᭜: p(Cl2)0 ϭ 19.7 and p(CHCl3)0 ϭ 92.1 Torr
ٗ: p(Cl2)0 ϭ 48.5 and p(CHCl3)0 ϭ 91.7 Torr
ࠗ: p(Cl2)0 ϭ 92.1 and p(CHCl3)0 ϭ 93.0 Torr
Full curves: simulations with the reaction model given in Tables I and II wherein only parameters
involving CHCl3 are slightly adjusted. Dotted curves: simulations with a model wherein some pa-
rameters ofthe thermal reactions ofCCl 4 are slightly adjusted, too (see text)
.

468
HUYBRECHTS ET AL.
Figure 2 Pressure ofCl (p(Cl2) in Torr) vs. reaction time (t in s) at 615 K for varying initial
2
pressures ofCl 2 and CHCl3.
ϩ: p(Cl2)0 ϭ 15.2 and p(CHCl3)o ϭ 14.0 Torr
ࠗ: p(Cl2)0 ϭ 22.4 and p(CHCl3)o ϭ 92.0 Torr
᭝: p(Cl2)0 ϭ 51.2 and p(CHCl3)0 ϭ 96.4 Torr
᭜: p(Cl2)0 ϭ 93.3 and p(CHCl3)0 ϭ 49.2 Torr
ٗ: p(Cl2)0 ϭ 95.7 and p(CHCl3)0 ϭ 95.4 Torr
Full curves: simulations with the reaction model given in Tables I and II wherein only parameters
involving CHCl3 are slightly adjusted. Dotted curves: simulations with a model wherein some pa-
rameters ofthe thermal reactions ofCCl are slightly adjusted, too (see text). Dashed curve: simu-
4
lation with the reaction model given in Tables I and II wherein the Step Ϫ4 has been suppressed.
at 20ЊC/min. H2 was used as a carrier gas with a flow
EXPERIMENTAL RESULTS
of3 L/h. The simulated and optimized Cl -pressure-
2
time curves were obtained using OPTKIN, a user-
friendly PC program for mechanistic modeling of re-
actions by kinetic and thermodynamic parameter
optimization [11]. Applications ofthis program and
examples ofthe modeling strategy are to be found in
[8,10,12,13]. The sensitivity analyses were also per-
formed with OPTKIN.
The thermal chlorination ofCHCl has been studied
3
between 573 and 635 K with initial pressures ranging
from 10.5 to 95.7 Torr for Cl2 and 14.0 to 97.2 Torr
for CHCl3. The total pressure was varied between 29.2
and 191.1 Torr. No inert gases were used. As shown
by the photometric and gas chromatographic analyses,
the reaction stoichiometry is

THERMAL CHLORINATION OF CHLOROFORM IN THE GAS PHASE
469
CHCl3 ϩ Cl2 !: CCl4 ϩ HCl
(A)
Experimental Cl2 profiles for a series of experi-
ments carried out in the unpacked cell are represented
in Figures 1 (T ϭ 573 K), 2 (T ϭ 615 K), and 3 (T ϭ
635 K). As can be seen from Figures 2 and 3, these
profiles show an S-shape at the higher temperatures.
The first part ofthe S-shape corresponds to an auto-
acceleration. This auto-acceleration seems to be neg-
ligible at the lower temperature (see Fig.1). Notice,
however, that the chlorine consumption rate depends
on the initial chlorine pressure (the initial CHCl3 pres-
sure is almost constant) but that it remains almost con-
stant in the course oftime while the chlorine pressure
decreases. All these findings are too complicated to be
up to conversions close to 100%. No pressure change
is observed (within the detection limits) in agreement
with equation (A). In some experiments, trace amounts
ofC 2Cl6 (less than 0.01%) are formed. No other prod-
ucts are detected.
The kinetics ofthe reaction were studied by follow-
ing the consumption ofCl . The chlorination in the
2
clean-walled reaction vessel is very fast and strongly
heterogeneous. All the results reported hereafter refer
to reproducible slower reactions in totally conditioned
reaction cells.
Figure 3 Pressure ofCl (p(Cl2) in Torr) vs. reaction time (t in s) at 635 K for varying initial
2
pressures ofCl 2 and CHCl3.
᭺: p(Cl2)0 ϭ 20.1 and p(CHCl3)0 ϭ 97.2 Torr
᭝: p(Cl2)0 ϭ 91.0 and p(CHCl3)0 ϭ 91.3 Torr
ٗ: p(Cl2)0 ϭ 94.4 and p(CHCl3)0 ϭ 22.3 Torr
Full curves: simulations with the reaction model given in Tables I and II wherein only parameters
involving CHCl3 are slightly adjusted. Dotted curves: simulations with a model wherein some pa-
rameters ofthe thermal reactions ofCCl 4 are slightly adjusted, too (see text).

470
HUYBRECHTS ET AL.
simply described by a classical empirical rate equa-
tion. The experimental results will therefore be pre-
sented in terms ofCl 2 profiles obtained by a combined
numerical modeling-optimization procedure.
Note finally that a slight heterogeneous effect is
detected by carrying out experiments in the packed
reactor with a 7-fold increase in the surface-to-volume
ratio. As shown by GC-analyses, the formation of
CCl4 proceeds about 40% faster than in the unpacked
reaction cell for a typical experiment (T ϭ 615 K,
p(CHCl3)o ϭ 93.0 Torr, p(Cl2)o ϭ 92.1 Torr).
thermal chlorination ofCHCl to CCl4 under the ex-
3
perimental conditions ofthis work, CCl 4 is not stable.
With Steps 2 and Ϫ2 as the starting point, all addi-
tional reaction steps involved in the model from [8]
are expected to be important. Elementary reactions
that could be eliminated are those involving C2Cl6 (and
its issuing C2Cl5 radical), which is produced only in
trace amounts in the thermal chlorination ofCHCl .
3
All experimental data from Figures 1 to 3 were in-
volved as one set in the reduction procedure.
The homogeneous radical mechanism and its ki-
netic parameters are given in Table I. Sensitivity anal-
yses as a function of reaction time show that all re-
actions play a significant role. Note that the back
reactions Ϫ1, Ϫ2, Ϫ3, and Ϫ5 are only ofimportance
for those experiments where the simulation is carried
out up to high consumption ofCHCl 3 or Cl2. The ther-
modynamic parameters are given in Table II. Almost
all these kinetic and thermodynamic parameters are
from [8] and are optimized parameters that fit the ex-
perimental kinetic results on the thermal reactions of
CCl4 and C2Cl6. Only those for reaction 2 and com-
pound CHCl3 were slightly adjusted by optimization
NUMERICAL MODELING AND
DISCUSSION OF THE RESULTS
All the experimental results can be quantitatively ex-
plained by the reaction model presented in Tables I
and II. It was obtained using a combined simulation-
optimization-reduction procedure from OPTKIN,
starting from a larger model comprising all the known
steps ofthe pyrolyses ofCCl and C2Cl6 [8] together
4
with Steps 2 and Ϫ2. Indeed, for the homogeneous
Table I Kinetic Model for the Thermal Chlorination of CHCl3
Homogeneous Elementary Reaction Steps
Primary:
E
log10A
Rem.
55.85
3.65
12.94
9.73
a,b
c
Initiation: Cl2 ϩ M 9
Propagation: Cl ϩ CHCl3 9
CCl3D ϩ Cl2 9
9
1: 2Cl ϩ M
9
2: HCl ϩ CCl3D
3: CCl4 ϩ Cl
5.10
8.70
d
9
Ϫ2
11.79
15.66
Ϫ1.63
0.00
8.38
a
Transfer: CCl3D ϩ HCl 9
9
: Cl ϩ CHCl3
Ϫ3
10.82
9.11
a
CCl4 ϩ Cl 9
9
: CCl3D ϩ Cl2
Ϫ1
e
Termination: 2Cl ϩ M 9
CCl3D ϩ Cl 9
CCl3D ϩ CCl3D 9
9
: Cl2 ϩ M
4: CCl4
5: C2Cl6
10.86
9.67
d
9
0.00
d
9
Secondary:
Ϫ4
68.04
67.09
16.82
17.40
a
a
Decomposition: CCl4 9
9
: CCl3D ϩ Cl
Ϫ5
C2Cl6 9
9
: CCl3D ϩ CCl3D
The activation energies E are expressed in kcal/mol and the A factors in L, mole, s-units.
a Calculated using the principle ofmicroreversibility in the computer program from the thermodynamic data given in Table II and the rate
constant ofthe corresponding back reaction at the mean temperature ofall experiments.
b Due to the lack of experimental data for the third body efficiencies, it is assumed that the rate constant of the back reaction is the same
for M ϭ CHCl3, HCl, and CCl4 and equal to that for M ϭ Cl2. Simulations using different values for third body collision efficiencies show
little effect.
c The values ofthe Arrhenius parameters are values optimized in this work. The initial values used in the optimization procedure ( E ϭ
3.35 kcal/mol and log10A ϭ 9.84) are from [14,15].
d Optimized value from [8].
e From [16,17], not optimized in [8].

THERMAL CHLORINATION OF CHLOROFORM IN THE GAS PHASE
471
Table II Heats of Formation ⌬HЊf (kcal/mol), heat capacities CpЊ,T ϭ a ϩ bT ϩ cT2, and entropies SЊ (Gibbs/mol)
based on a 1-atm standard state
CЊp,T
Compound
⌬HfЊ,298
S2Њ98
a
102 · b
105 · c
Rem.
Cl2
HCl
CHCl3
CCl4
C2Cl6
Cl
0.0
Ϫ22.1
Ϫ24.7
Ϫ22.2
Ϫ33.2
29.0
53.3
44.7
70.8
73.8
95.2
39.5
71.2
7.07
7.19
7.49
12.8
19.9
4.72
9.87
0.426
Ϫ0.114
3.12
3.00
5.35
Ϫ0.25
0.161
a
b
c
a
a
a
a
Ϫ1.73
Ϫ1.93
Ϫ3.32
Ϫ0.179
Ϫ1.41
0.225
2.24
CCl3D
18.0
a From [13].
b From [18].
c The values ofa⌬nHd fЊ,298 are Sv2Њa9l8ues optimized in this work. The initial values (
Ϫ24.6 and 70.6, respectively) used in the optimization
procedure are from [19].
in this work (see legends ofTables I and II, respec-
tively).
the kinetic model, for example for the experiment of
Figure 2 with p(Cl2)0 ϭ 22.4 Torr. As can be seen in
Figure 4, the secondary source ofCl atoms (Step Ϫ4)
is already as fast as the primary source (Step 1) after
only about 150 s (0.8% reaction). After 3000 s (50%
Cl2 consumption), it is even more than 100 times
faster. The decomposition in Step Ϫ4 ofthe CCl 4 pro-
duced by the chain propagation Step 3 is therefore the
origin ofthe observed auto-acceleration. This is con-
firmed in Figure 2, which shows that the auto-accel-
eration disappears when Step Ϫ4 is not taken into ac-
count. This occurs for all experiments. It should be
pointed out here that the study ofthe kinetics ofthe
The fitting ofthe experimental Cl profiles is rep-
2
resented by the curves in Figures 1–3 and is quite
satisfactory. Note that the simulation of the experi-
mental results without any adjustments ofthe kinetic
and thermodynamic parameters is scarcely poorer (the
profiles are never lower by more than 10%). As can
also be seen in Figures 1–3, the fitting is much better
ifsome parameters ofthe thermal reactions ofCCl
4
are also slightly modified by optimization (corrections
do not exceed Ϯ0.3 kcal/mol for activation energies
and heats offormation, Ϯ0.3 log units for A factors,
and 0.5 Gibbs/mol for entropies). This illustrates the
reliability ofthe parameters optimized in [8] between
651 and 718 K and used in this study at lower tem-
peratures ranging from 573 to 635 K.
gas-phase photochlorination ofCHCl [20] is not af-
3
fected by the decomposition of CCl4 owing to the
lower temperatures used (303–426 K). The simula-
tions also show that the recombination ofthe CCl
3
A series ofsimulations have been performed using
Figure 4 Rates (in mol/L · s) ofinitiation steps vs. reaction
time (t in s) at 615 K. Initial pressures ofCl 2 and CHCl3 of
22.4 and 92.0 Torr, respectively. Simulations with the re-
action model given in Tables I and II.
Figure 5 Rates (in mol/L · s) oftermination steps vs. re-
action time (t in s) at 615 K. Initial pressures ofCl and
2
CHCl3 of22.4 and 92.0 Torr, respectively. Simulations with
the reaction model given in Tables I and II.

472
HUYBRECHTS ET AL.
Y. Hubin wishes to thank IWONL for a grant.
radicals (Step 5) is the fastest chain termination (see
Fig. 5) and that this step has produced about 0.005%
C2Cl6 at the end ofthe experiment. The calculated ini-
tial chain length (rate ofStep 3/rate ofStep 5) is as
high as 105. The simulations show finally that a surface
controlled removal ofCl atoms would play a role if
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its efficiency ␧(Cl)* is not lower than about 3 ϫ 10Ϫ .
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likely that the experiments were carried out in a well-
conditioned reaction vessel.
Taylor and Hanson [3] and Aver’yanov et al. [5]
suggested that the heterogeneous wall-initiated disso-
ciation ofCl may occur much faster than the homo-
2
geneous initiation step 1 (see Table I). Nevertheless,
Taylor and Hanson [3] observed an increase ofonly
10% of the overall chlorination rate for a seven-fold
increase ofthe surafce-to-volume ratio ofthe reactor
(similar to our observations). If, however, the chain
termination should occur also entirely heterogene-
ously on the wall ofthe reaction cell, a heterogeneous
increase ofthe rate constant ofthe reaction 1 in the
packed cell might be obscured by a greater chain ter-
mination efficiency. The overall reaction rates re-
ported in [3] and [5] are, however, at least 10 times
higher than observed in this work. This may suggest
a difference in the conditioning of the reaction cell or
an effect of impurities. Indeed, we observed that when
a reaction cell has been exposed to O2, the next runs
proceeded much faster. The conditioning of the reac-
tion cell then had to be renewed. Note that simulations
show that the reaction rates observed in [1,3–7] re-
quire a chain initiation step ofthe type Cl : 2Cl,
2
which would be at least 104 times faster than the ho-
mogeneous step 1 and are therefore most probably cat-
alyzed by the wall.
In conclusion, the thermal chlorination ofCHCl
3
studied in this work can be simulated with a kinetic
model that is entirely homogeneous. This model is an
additional check ofthe validity ofa good model ofr
the pyrolyses ofCCl [8] and C2Cl6 [8,10] between
4
651 and 718 K. Indeed, almost all ofits reaction steps
and parameters are the same and where used at lower
temperatures ranging from 573 to 635 K.
* ␧(Cl) ϭ rate ofremoval ofCl atoms per unit surface/rate ofcol-
lision ofCl atoms per unit surface given by kinetic theory ofgases.