pergamon S0045-6535(96)00038-0

Chemosphere, Vol. 32, No. 7, pp. 1261-1273, 1996 Copyright © 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0045-6535/96 $15.00+0.00

E X P E R I M E N T A L S T U D Y O N T H E T H E R M A L O X I D A T I O N O F 1 , 3 - H E X A C H L O R O B U T A D I E N E A T 500 - 1100 ° C

C.BAILLET, A. FADLI and J-P SAWERYSYN*

Laboratoire de Cinrtique et Chimie de la Combustion URA-CNRS 876 Universit6 des Sciences et Technologies de Lille

59655, Villeneuve d'Ascq-France

(Received in Germany 9 September 1995; accepted 15 January 1996)

ABSTRACT

Thermal degradation processes of 1,3-hexachlorobutadiene (C4C16) have been studied using

a tubular flow reactor at 1 atm over the temperature range 500-1100°C for residence times of 2 seconds.

Kinetic studies were performed with mixtures of 1000 ppmV of C4C16 in air. Overall Arrhenius parameters

for the destruction of C4CI 6 were determined between 700 and 850°C. About 30 molecular halogenated

products from C1 to C8 formed by pyrolysis or oxidation of 1,3-hexachlorobutadiene over the investigated

temperature range were identified. Concentration profiles of major products ( CO2, CI 2, CO, COCI2, C2CI4,

CCI 4 ) and some minor products (C3CI40, C4C140, C6CI6, C3CI 4 and C3C16) have been measured as a

function of temperature. Phosgene is the major chlorinated intermediate product. Detection of some aromatics

such as hexachlorobenzene and octachlorostyrene show the importance of the molecular growth pathways in

the chemical mechanism. Reaction pathways of main products are proposed and corresponding reaction

enthalpies are estimated. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION

Waste feed to both municipal and hazardous waste incinerators is usually in solid or liquid

form. Initial exposure of this feed to the incinerator environment results in thermal heating on surfaces of this

material, where vaporization or polymer fragmentation occurs along with pyrolysis reactions. Once in the

vapor phase these fragment molecules and radicals undergo reactions with the oxidizing medium (air) before a

uniform combustion environment is achieved. The initial reaction on surfaces may be closely represented by

pyrolysis, and the initial reactions in the vapor, but near the solid or liquid surfaces may be described as

oxidation of the neat material, as opposed to well mixed combustion. It is of value to study reactions

representative of these regimes, in order to obtain knowledge about products that may enter subsequent

1261

1262

incinerator processes. Over the last five years, numerous laboratory studies have been focused on the thermal

degradation processes of chlorinated compounds. Most studies were essentially devoted to chlorinated

hydrocarbons (CHCs) in CI [1-6] and C2 [1,2,7-14] because of their industrial interest and the relative

simplicity of their pyrolysis and oxidation processes. In contrast, very few experimental investigations or

modelling studies were performed on the higher molecular weight CHCs due to the complexity of their

chemical processes and the lack of kinetic and thermodynamic data characterizing, the elementary reactions

involved in their decomposition, pyrolysis and oxidation. Some investigations were reported on the thermal

degradation processes of chlorobenzenes [3,15-17].

The purpose of this work is to contribute to a better understanding of high temperature oxidation

processes of chlorinated species without hydrogen. 1,3-hexachlorobutadiene (C4C16) was selected for several

reasons : (i) its identification as a pyrolysis byproduct of trichloroethene [8,14], tetrachloroethene [9,10] and

1,2-dichlorobenzene [17], (ii) the lack of experimental data related to its thermal degradation processes, (iii)

as an unsaturated chlorinated species, the potential production of chlorinated aromatics and other toxic

components such as phosgene by thermal oxidation or combustion.

Relatively few studies have been reported on the thermal degradation processes of 1,3-

hexachlorobutadiene. The oxidation of this compound has been investigated in liquid phase over the

temperature range 120-210°C for residence times between 1 and 7 hours [18]. Under these experimental

conditions, the main detected products were oxygenated species such as pentachloroacetoacetyl chloride

(C4CI602), dichloromaleic chloride (C4CI402), dichloromalonyl dichloride (C3C1402) and

tetrachlorosuccinyl chloride (C4C1602). Gas phase thermal decomposition data, including temperatures

required for destroying 99.9% and 99.99% of 1,3-hexachlorobutadiene at 2 seconds with an equivalence ratio

of 3 yielded empirical kinetic parameters and conversion data relative to other hazardous organic compounds

[ 19]. An incinerability ranking was developed [20] based on the type of mechanism which generally dominates

compound decomposition. Here, 1,3-hexachlorobutadiene has been ranked in the stability family

corresponding to species with dominant unimolecular mechanism involving bond fission or concerted

molecular elimination [20].

In this work, the main molecular chlorinated products generated by pyrolysis or oxidation of 1,3-

hexachlorobutadiene are identified and monitored as a function of temperature over the range 500 - 1100 °C

for residence times of 2 seconds. Time decays of C4C16 are measured and Arrhenius parameters

characterizing its overall rate constant of consumption are estimated between 700 and 800 ° C Reaction

pathways for main products are also proposed and discussed.

EXPERIMENTAL

High temperature oxidation reactions of 1,3-hexachlorobutadiene were performed at 1 atm total

pressure using a tubular flow reactor. The experimental set-up consisted of three main parts respectively

devoted to :

- the preparation of C4CI6/air mixtures,

- the thermal treatment,

- the analysis of exhaust gas effluents.

Air was obtained from the laboratory network of compressed air. It was purified by passage through

three Balston filters and a molecular sieve trap. Its flow rate was controlled and measured using the sonic

1263

throat method. C4CI 6 liquid at room temperature was directly injected by a syringe pump into a vaporization

chamber similar to a gas chromatograph injector. The chamber was heated at 200 ° C and the cartier gas (air)

flow was controlled to obtain the desired ratios. All experiments described in this work were performed using

a C4CI6/air mixture containing I000 ppmV of C4CI 6. To improve the homogeneity of the gases, the mixture

flaw through a 8 cm i.d x 10 cm length tube, also heated at 200 ° C prior to the thermal treatment. The purity

ofC4CI 6 supplied by Aldrich was > 98 %. The main impurities were C2C14, C2C16, C5CI 6 and an isomer of

1,3- hexachlorobutadiene.

Thermal treatment was carried out using a quartz cylindrical reactor (o.d = 5 cm, length = 15 cm). The

reactor was heated by a temperature controlled electrical furnace. The variation of temperature measured

along the axis of the reactor did not exceed 5 ° C at 1000 ° C.

Chemical and chromatographic methods were used for analyzing the exhaust gas effluents. Organic

chlorinated species were quantified using a gas capillary chromatography on line. The chromatograph was

equipped with a gas sampling valve with a loop of 5 cm3 volume. Gas chromatographic conditions were as

follows : column, HP5 fused silica capillary (50 m x 0.32 mm i.d. x 1.05/am) ; column temperature, 80°C hold

for 4 minutes then ramped to 260°C at 5 ° C/min. ; detector, FID ; detector temperature, 260°C; carrier gas

(He), flow rate, 2.5 cm3/min. Inorganic species such as CO, CO 2 and COCI 2 were also quantified by gas

chromatography with batch samples. The reaction effluent was cooled to room temperature and collected in a

PVC bag. Carbon oxides and phosgene were separately analyzed. Gas samples were withdrawn from the bag

by a gas syringe and injected into specific chromatographs. CO and CO 2 chromatograph was equipped with a

thermal conductivity detector and two concentric columns Porapak Q/Molecular sieve (Alltech).

Chromatographic conditions were : injector and detector temperatures, 150°C ; initial column temperature

40°C, with a ramp of 10°C/min to 90°C ; carrier gas (He), flow 30 cm3/min.

COCI 2 was analyzed using a second chromatograph equipped with a HP5 fused silica capillary column

(50 cm x 0.32 mm i.d. x 1.05/am) and a quadrupole mass spectrometer as a detector, I-IP 5890/5971 GC/MS.

Chromatographic and detection conditions were as follows : injector temperature 200°C ; column

temperature, 40°C hold for 4 minutes then ramped at 5°C/min to 260°C ; carrier gas (He), flow rate 1

cm3/min ; temperature detector, 280°C ; source, electron impact (E.I) at 70eV. The detection limit for COCI 2

determined from standard injection was 20 ppmV.

CI 2 was quantified using a chemical method based on the classical oxidation reaction of I- to 12. The

iodide solution was contained in three impingers in series through which the effluent gas was passed.

Identification of organic chlorinated species was achieved by different procedures depending on the

nature and the concentration obtained at the treatment temperature. To make the concentration level of minor

species compatible with the detection limit of the used method, the reaction products were accumulated in a

cold trap at - 100 ° C. After heating the trapped products to room temperature, a fraction of them was injected

into the chromatograph to be separated and identified by mass spectrometry (GC/MS HP 5890/5971). Mass

spectra were obtained at 70 eV by electron impact. Identification was achieved by comparing the experimental

mass spectra to reference mass spectra memorized in a data base (NBS 49 K or Wiley) and to spectra of

purchased reference compounds.

In order to have an overview of most species separated by gas capillary chromatography, Figure 1

combines the most significant parts of five different chromatograms obtained under different experimental

conditions. The interest of this figure is to show the great variety of species (at least 40) produced by the

thermal degradation of 1,3-hexachlorobutadiene. Most reaction products were identified at a high level of

confidence since the comparison with both reference mass spectra and chromatographic retention time of

1264

purchased reference compounds were in very good agreement. The identification of such compounds was

denoted by ***. In the case where experimental and reference mass spectra were characterized by a high level

of coincidence but a reference species was not commercially available to check retention time, the

identification of this species was denoted by **. For other products presenting no acceptable similarities

between experimental and reference mass spectra, detailed interpretation of experimental mass spectra were

performed and-their identification denoted by *. Only a few species were not identified. Unexpectedly some

products in very small concentrations were identified as hydrogenated species whereas the treated chlorinated

species did not include any hydrogen. This observation is likely due to some water traces present in the inlet

air.

AbundRnce

~3 17}

:1 ii It I. i~ il 'J

23

21

20 25

_ _ ~ i 2 6 2 7

TIME (rain) S.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 48.00

Figure 1 : Juxtaposition of 5 different chromatograms illustrating the variety of intermediate products formed by the thermal degradation of 1,3-hexachlorobutadiene. Peaks : 1) phosgene (COCI2)***; 2) dichloroethyne (C2CI2)**; 3) trichloromethane (CHCI3)**; 4) tetrachloromethane (CC14)***; 5) dichloroacetyl chloride (C2HCI30)**; 6) trichloroacetic acid (CCI3CO2H)**; 7) trichloroacetyl chloride (C2CI40)**; 8) tetrachloroethene (C2CI4)***; 9) tetrachlorocyclopropene (C3C14)**; 10) tetrachloroetheneoxide (C2C140)**;11) tetrachloropropadiene (C3CI4)**; 12) isomers of (C2CI40)*; 13) trichloropropenoyl chloride (C3CI40)***; 14) 3,4dichloro 2,5 furanedione (C4CI203)**; 15), perchloro2cyclobutenel-one (C4C140)***; 16) tetrachlorobutenyne (C4C14)**;17) isomer of the species 15 (C4Ci40)**; 18) hexachlorethane (C2CI6)***; 19) C4C1402"; 20) , C3C1602"; 21) isomer of the 1,3-hexachlorobutadiene (C4CI6)*; 22) hexachloropropene (C3CI6)***; 23) 1,3-hexachlorobutadiene (C4CI6)***; 24) CxCIyO z (unidentified); 25) , C4CI60*; 26) isomer of the previous species (C4CI60)*; 27) and 28) , unidentified species; 29) C5CI602"; 30) C5CI602"; 31) hexachlorocyclopentadiene (C5C16)***; 32) unidentified species; 33) pentachlorobenzene (C6HCI5)**; 34) unidentified species ; 35) and 36) (CxCly) (unidentified); 37) octachlorolbutene (C4C18)*; 38) 5,dichloromethylenetetrachlorol,3cyclo pentadiene (C6CI6)**; 39) hexachlorobenzene (C6C16)***; 40) unidentified species (CxCly); 41) octachlorohexatriene (C6CI8)*; 42) octachlorostyrene (C8CI8)** *.

1265

RESULTS AND DISCUSSION

Kinetic study of the overall consumption rate of C4C16

Figure 2 shows time decay plots of C4CI 6 determined over the temperature range 700 - 850 °

C. In a first step, let us assume that the overall destruction rate o f C4CI 6 can be expressed by two terms : a

first term due to the thermal degradation by pure pyrolysis, a second one due to the oxidation processes :

d [C4C161 = k, [C4C16] °~1 + k 2 [C4CI6] °~2 [ 0 2 ] 132

dt

with k I and k 2 the overall rate coefficients governing the processes 1 and 2, cq, ct 2 and 132 denoting the

partial reaction orders with respect to the reactants occuring in these processes. As 0 2 was in large excess

over the initial concentration of C4CI6, the consumption of 0 2 by the processes 2 can be neglected.

Furthermore, ifct I and ct 2 are assumed to be equal to 1, the equation of the consumption rate can be simplified

as follows :

d [C4C16] dt - k°bs [C4C16 ]

where kobs = k ! + k 2 [ 0 2 ]~2 is defined as a pseudo-first order rate coefficient. By integrating,we obtain

In [C4CI6] = - kob s t + c st

1000 1

900

8 0 9

7 0 0

6 0 0

500 .

4 0 0 .

3 0 0

2 0 0 .

100.

0

0 0,5 1 1,5 2 2,5 3

RESIDENCE TIME (S)

° 700oc

o 725oC

• 750oc

• 800oC

o 850oc

Figure 2: Time decay plots of 1,3-hexachlorobutadiene at various temperatures.

The plots In [C4C16] versus reaction time correspond to the straight lines the slope of which

gives by least squares linear regression the value of kob s at various temperatures. The plot In kob s vs (l/T)

(Fig. 3) using the least squares treatment lead to the following Arrhenius expression •

1266

k ( 7 0 0 - 8 5 0 ° C ) = 107'8+05 exp [ - (18000 _+1250) / T] s -I

Uncertainties represent only statistical errors (+_~) of the data treatment.

This Arrhenius expression is significantly different from the one previously determined by Dellinger et

al. [19] • k ( s - ' ) = 1012±287 exp [ - (22500 + 6500) / T] which predicts decomposition rates of 1,3

hexachlorobutadiene 150 to 300 faster than the ones measured over the 700-850°C range in this work. We

note that the data of Dellinger et al. were obtained in a reactor with a ratio surface/volume (S/V) equal to 40

relative to 0.8 in this study and with initial C4CI 6 concentration of 10 ppmV. Similar effect of S/V ratio has

been previously observed in the pyrolysis mechanism of trichloroethene by Taylor et al. [14]. For example, at

700 K, 95 % conversion of C2HCI 3 was measured using a 0.1 cm i.d reactor whereas the conversion was <

0.1% with a 1 cm i.d reactor. There is also a likely more important difference : that of fuel equivalence ratio (

~). The study of the University Dayton Group used • =3 while this study uses no supplemental hydrogen

containing fuel (hydrogen = 0). Decomposition rates of 1,3 hexachlorobutadiene determined by Dellinger et

al. are much faster than the ones measured over our temperature range because their system contains

hydrogen in excess. We conclude that the assumption of the primary unimolecular dissociation path in

destruction of 1,3 hexachlorobutadiene is not justified in reference 20.

3-

O

..E

2-

1-

0 0.

-1-

-2 1000IT (K-~)

Figure 3 • Arrhenius plot of the overall reaction C4CI 6 + 0 2 ---> products over the temperature range 700- 850°C.

Temperature dependence of the concentration levels of molecular species

Figure 4 displays the decay of 1,3-hexachlorobutadiene as a function of temperature for

residence times of 2 seconds. At 600 ° C, its conversion rate is only 10 %. The maximum conversion yield is

observed near 700 ° C. At 800 ° C, the conversion yield of 1,3-hexachlorobutadiene is close to 99 %. This

value of temperature is higher than the value T99,2 (750°C) previously determined under different operating

conditions [19]. As expected, increased temperature favors destruction of 1,3-hexachlorobutadiene and the

build up of final products carbon dioxide and molecular chlorine. CI 2 concentration increases with the

1267

temperature and reaches its maximum value (3000 ppmV) at around 800 ° C. The profile of CO 2

concentration as a function of temperature is more complex. It slowly increases till 900°C above which it

increases at a much higher rate.

The intermediate products formed in the thermal oxidation of 1,3 hexachlorobutadiene over the

temperature range 500 - 1100 ° C and for residence times of 2 seconds can be ranked in three groups :

- the major species (maximum concentration > 100 ppmV) : carbon monoxide (CO) , phosgene

(COC12), tetrachloromethane (CC14), tetrachloroethene (C2CI4),

- the minor species (maximum concentration included between 0.3 and 10 ppmV) : hexachloropropene

(C3CI6) , hexachlorobenzene (C6C16), tetrachloropropadiene (C3CI4) , propenoylchloride (C3C140) and

perchloro-2-cyclobutene- 1 -one (C4CI40),

- the ultraminor species (maximum concentration < 1 ppmV). These species have not been quantified

but only identified by GC/MS : dichloroethyne (C2CI2) , tetrachloroethene oxide (C2C140),

tetrachlorocyclopropene (C3CI4), tetrachlorobutenyne (C4CI4), 5.dichloromethylenetetrachloro- l-pentadiene

(C6C16) and octachlorostyrene (C8C18). The last one corresponds to the heaviest product identified in the

effluents. Octachlorostyrene is only observed between 700°C and 850 ° C.

r.j

4000

3500

3000 I

2500 ~-

2000 4-

1500

1 o 0 0

5oo

0

5OO

dkr

600 700 800 900 1000 1100

TEMPF-RATUKE (°C)

• C4CI6

o CO

• C 0 2

o CI2

Figure 4 • Temperature dependence of the concentration of 1,3-hexachlorobutadiene, CO, CO 2 and CI 2 for residence times of 2 seconds.

CO is the major species of all intermediates. Its concentration as a function of temperature for 2 s

passes through a maximum of 3500 ppmV at 850°C (Fig.4) then, it decreases but the level of its concentration

remains significant at 1100 ° C : 400 ppmV.

COCI 2 represents the major intermediate chlorinated product over the temperature range 600-830°C

(Fig.5). Its concentration, very low below 600°C, passes through a maximum of around 400 ppmV by 725°C.

Compared to the levels of chlorinated organic species, the concentrations of CO and COCI 2 remain largely

dominant over the entire temperature range investigated. At 700°C, 8 1 % of the initial atomic carbon is

converted to 4 7 0 of CO, 16% of COCI 2 and 18% CO 2. While phosgene is known to be a toxic and

1268

undesirable product from any combustion environment, we note that combustion of hexachlorobutadiene in a

hydrocarbon fuel co-fired environment would be expected to convert the chlorine to HCI, and therefore

dramatically reduce formation of phosgene. Phosgene is also effectly converted (removed) to HCI + CO 2 by

conventional liquid scrubbing operations.

CCI 4 and C2C14 are the main chlorinated organic products released from the thermal treatment

of C4Cl 6. Their maximum concentration ( ~ 100 ppmV) is observed at 700-750 °C (Fig.5).

All other chlorinated organic species quantified in this study (Fig.6) have a concentration level

lower to 10 ppmV. Among these intermediate species the most abondant ones are C3CI40 , C4CI40 and

C3CI 4. The concentrations ofC3C16 and C6C16 are close to the limit of the detection of our analysis.

o

tac

LLI f.3

t.D

400 l 350 l

El 200 +

150 +

100 +

50 ~

0 4 -

500 600 700 800 900

TEMPERATURE (°C) lo00

C2CI4 [

Figure 5 Temperature dependence of the concentration of phosgene, tetrachloroethene and tetrachloromethane for residence times of 2 seconds.

,6 L.O (.9 Z O o 2

A

500 600 700 800 ! I - - I

9 0 0 1000 I I 0 0

TEMPERATURE (*C)

• C3C140

C4C140

• C6C16

o C3CI4 (1)

o C3C16

Figure 6 " Temperature dependence of the concentration of the minor products formed by the thermal degradation of 1,3-hexachlorobutadiene for residence times of 2 seconds.

Reaction pathways responsible for the formation of the main products

1269

Although our quantitative understanding of the detailed oxidation mechanism of 1,3-

hexachlorobutadiene is limited, particularly with regard to the estimation of kinetic parameters characterizing

the associated reactions, preliminary remarks can be made with regard to the major reaction pathways based

on thermochemical properties. The computer program THERM developed by Ritter and Bozzelli [21] was

used to calculate thermodynamic parameters of radicals and molecules based on the methods of Benson group

additivity [22]. Using the program THERMRXN, which is a part of THERM, thermodynamic properties of

each reaction were estimated over the temperature range 298-1500K.

If we consider the total conversions of 1,3-hexachlorobutadiene to final products via oxidation

or pyrolysis pathways, we have :

C4C16 + 4 02--> 4 CO 2 + 3 CI 2 Ar H° (298K) = - 369.4 kcal/mol.

C4CI 6 --~ 4 C(sol.) + 3 CI 2 Ar H° (298K) = + 6.6 kcal/mol.

The first steps of the thermal degradation of 1,3-hexachlorobutadiene correspond to the

breaking of C-CI bonds much weaker than the C-C bond to generate primary and secondary

hexachlorobutadienyl radicals ( nC4CI 5 and iC4CI 5 respectively) via two unimolecular reactions :

C4C16 --~ nC4CI 5 + CI Ar H° (298K) = + 85.3 kcal/mol.

C4CI 6 ---} iC4CI 5 + CI Ar H° (298K) = + 75.3 kcal/mol.

where the formation of iC4CI 5 radicals is energetically favored.

The dissociation of the C-C bond generate two trichlorovinyl radicals :

C4CI 6 ~ 2 C2CI 3 Ar H° (298K) = + 112.6 kcal/mol.

but this reaction is less important than the two previous ones.

Reactions of hexachlorobutadienyl and trichlorovinyl radicals with 0 2 produce oxygenated

molecular products such as trichloropropenoyl chloride (C3C140) and phosgene (COC12) via a mechanism

including the formation of the corresponding peroxy radicals, their cyclization and fragmentation as previously

reported for the oxidation of vinyl radical [23] :

nC4CI 5 + 0 2 ---} C3C140 + COCI Ar H° (298K) = - 105.5 kcal/mol.

iC4CI 5 + 02 --~ C3CI30 + COCI 2 Ar H° (298K) = - 97.6 kcal/mol.

C2C13 + 02 --~ COCI + COCI 2 Ar H° (298K) = - 1110 kcal/mol.

1270

the major intermediate product i.e carbon monoxide :

COCI ~ CO + CI

C3C130 --+ CO + C2C13

Radical attacks by CI atoms also

hexachlorobutadiene either via abstraction reactions as follows :

C4C16 + CI ~ nC4CI 5 + CI 2

C4CI 6 + CI ~ iC4Cl 5 + CI 2

or via addition on a double bond :

C4C16 + CI --~ C4CI 7

Further thermal decomposition of COCI and C3CI30 radicals contribute to the formation of

Ar H° (298K) = + 6.5 kcal/mol.

Ar H° (298K) = + 31.7 kcal/mol.

contribute to the thermal degradation of 1,3-

Ar H° (298K) = + 27.5 kcaVmol.

Ar H° (298K) = + 17.5 kcal/mol.

Ar H° (298K) = - 16.9 kcal/mol.

Tetrachloroethene (C2C14) and tetrachloromethane (CC14) are the main intermediate organic

species produced by the thermal degradation processes of 1,3 hexachlorobutadiene. C2C14 is generated via

different reaction pathways using C2CI 3 as a precursor species :

C2CI 3 + CI 2 -~ C2CI 4 + Ct

C2C13 + CI ~ C2C[ 4

Ar H° (298K) = - 27.5 kcal/mol.

Ar H° (298K) = - 85.3 kcal/mol.

Addition reaction of CI on C2C14 leads to the formation of C2C15 radicals which reacting with CI

atoms give CCI 3 radicals :

C2CI 4 + CI ~ C2C15

C2C15 + CI --~ CCI 3 +CCI 3

Ar H° (298K) = - 18.0 kcal/mol.

Ar H° (298K) = - 70.2 kcal/mol.

CCI 4 is only formed by the reactions involving CCI 3 radical as a precursor species :

CCI 3 + CI --~ CCI 4

CCI 3 + CI 2 --~ CCI 4 + CI

Ar H° (298K) = - 70.8 kcal/mol.

Ar H° (298K) = - 12.7 kcallmol.

The detection of unsaturated chlorinated products of type CxCly (x = 3 to 8) at very low

concentrations show that radical addition reactions play a significant role in the formation of chlorinated

intermediate compounds :

CCI 3 + C2CI 2 --~ C3C15

C3C15 + CI 2 --~ C3CI 6 + CI

C2C13 + C2C12 --~ C4CI 4 + CI

nC4CI 5 + C2C12 ~ C6C17 ( 1 )

iC4CI 5 + C2C12 ~ C6C17 ( I )

C6C17 ( 1 )---~ C6CI 6 ( I )+ C1

C2CI 3 + C4CI 6 ~ C6C18 ( 1 ) + CI

nC4CI 5 + C4C14 --~ C8CI 8 ( cy ) + CI

iC4CI 5 + C4C14 --~ C8C18 ( cy ) + CI

Ar H° (298K) = - 36.2 kcal/mol.

Ar H° (298K) = - 24.5 kcal/mol.

Ar H° (298K) = - 27.0 kcal/mol.

Ar H° (298K) = - 61.8 kcal/mol.

Ar H° (298K) = - 51.8 kcal/mol.

Ar H° (298K) = + 38.8 kcal/mol.

Ar H° (298K) = - 28.3 kcal/mol.

Ar H° (298K) = - 78.5 kcal/mol.

Ar H° (298K) = - 68.9 kcal/mol.

1271

Phosgene is the main intermediate chlorinated species produced by the oxidation processes of 1,3-

hexachlorobutadiene. As reported above, its formation is essentially due to the reactions of 0 2 with the

primary radicals iC4CI 5 and C2C13 :

iC4CI 5 + 0 2 --~ C3CI30 + COCI 2

C2C13 + 0 2 ~ COCI + COCI 2

Ar H° (298K) = - 97.6 kcal/mol.

Ar H° (298K) = - 111.0 kcal/mol.

Reaction ofCCl 3 radicals with 0 2 can also contribute to the production of phosgene :

CCI 3 + 0 2 ~ COC12+ CIO Ar H° (298K) = - 47. I kcal/mol

When the temperature is increased, COCI 2 is thermally decomposed as follows :

COCI 2 --~ CO + CI 2 Ar H° (298K) = + 26. I kcal/mol

The destruction ofCOCI 2 can also occur via radical attacks by CI or O atoms :

COCI 2 + CI --~ COCI + CI 2

COCI 2 + O --~ COCI + CIO

Ar H° (298K) = + 19.7 kcal/mol

Ar H° (298K) = + 13.2 kcal/mol

Carbon monoxide represents the main intermediate product formed by the oxidation processes of 1,3-

hexachlorobutadiene. Its production is mainly ensured by the thermal decomposition of phosgene and COCI

1272

radical. In absence of source of hydrogen atoms in the medium, the oxidation of CO to CO 2 is essentially due

to the reactions with 0 2 and O atoms :

CO + 0.5 0 2 ~ CO 2

CO + 0 ---+ CO 2

Ar H° (298K) = - 67.6 kcal/mol.

Ar H° (298K) = - 127.1 kcal/mol.

These oxidation processes are quite slow in comparison with the oxidation of CO by OH radical as

usually observed in the oxidation processes of hydrocarbon species. As assumed in the oxidation of C2HCI 3

[7], CIO radical can also contribute to the oxidation of CO to CO 2 :

CO + CIO --~ CO 2 + CI Ar H° (298K) = - 62.9 kcal/mol.

Taking account into the high concentration of CI atoms released in the thermal degradation of 1,3-

hexachlorobutadiene, molecular chlorine as a final product is generated via recombination of CI atoms :

CI + CI +M --~ CI 2 + M Ar H° (298K) = - 57.8 kcal/mol.

and via abstraction reactions of Cl atoms as listed earlier.

The kinetic parameters for most of these reactions are not experimentally determined. Estimations

based on thermochemical properties and the use of the transition state theory are necessary for modelling the

distribution of the products as a function of temperature and residence time.

C O N C L U S I O N

Experimental flow reactor studies of the thermal degradation processes of 1,3-hexachlorobutadiene in

air for residence times of 2 s over the temperature range of 500-1100°C have been reported. Overall kinetic

parameters governing the destruction ofC4Cl 6 have been determined with mixtures containing 1000 ppmV of

C4C16 in air between the range 700-850°C. Intermediates (stable) and final products generated by pyrolysis

and/or oxidation processes of 1,3-hexachlorobutadiene have been identified by GC/MS and monitored as a

function of temperature. Chromatographic analysis of products has highlighted a great variety of chlorinated

intermediate species from C 1 to C8. Phosgene is the major chlorinated intermediate product passing through a

concentration maximum of around 400 ppmV by 725°C. Some aromatics such as hexachlorobenzene and

octachlorostyrene show the importance of the molecular growth pathways in the chemical mechanism.

Reaction pathways leading to the formation of the main products have been proposed and corresponding

reaction enthalpies estimated. Most kinetic parameters characterizing the behavior of reactions as a function of

temperature are unknown and consequently, must be estimated for modelling the product distribution

experimentally observed. Development of such a model is under way.

1273

ACKNOWLEDGMENTS

This work was financially supported by the Association R.E.C.O.R.D (contract n ° 90.203). We thank

Prof. J.W. Bozzelli for providing a copy of the computer program THERM and for his fruitful help for the

determination of thermodynamic properties of species of interest and useful comments.

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