Mechanism of PCBs formation from the pyrolysis of chlorobenzenes

Article abstract of DOI:10.1016/S0045-6535(00)00434-3

Polychlorinated biphenyls (PCBs) were formed by pyrolysis of chlorobenzenes in a HCl atmosphere. Varieties of substituted chlorobenzenes were used as model compounds to optimize the condensation reaction. Lower chlorinated benzenes produce more PCBs than that of higher ones. Up to 0.7 μg PCBs/mg 1,2,3-trichlorobenzene was produced through the condensation reaction.

Full text of DOI:10.1016/S0045-6535(00)00434-3

Chemosphere 43 !2001) 783±785
Mechanism of PCBs formation from the pyrolysis of
chlorobenzenes
*
Peng-Yan Liu, Ming-Hui Zheng , Bing Zhang, Xiao-Bai Xu
Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085,
People's Republic of China
Abstract
Polychlorinatedbiphenyls !PCBs) were formedby pyrolysis of chlorobenzenes in a HCl atmosphere. Varieties of
substituted chlorobenzenes were used as model compounds to optimize the condensation reaction. Lower chlorinated
benzenes produce more PCBs than that of higher ones. Up to 0.7 lg PCBs/mg 1,2,3-trichlorobenzene was produced
through the condensation reaction. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Polychlorinatedbiphenyls; Chlorobenzenes; Formation; Condensation; Mechanism
1. Introduction
phase oxychlorination of chlorobenzenes, but few sys-
tematic experiments on formation of PCBs have been
reported.
Formation of polychlorinatedibdenzo-
p-dioxins
!PCDDs) andpolychlorinateddibenzofurans !PCDFs)
in combustion processes of chlorine containing organic
materials has been frequently studied. The most im-
portant precursors for PCDD/Fs are aromatic structures
like chlorobenzenes andchlorophenols !Buser, 1979;
Sommeling et al., 1994; Born et al., 1993). A better un-
derstanding of the mechanism of PCDD/Fs formation
provides a better guidance for devising technical control
measures andimproving the performance of practical
combustion equipment to reduce PCDD/Fs emissions.
However, few studies on the formation of polychlori-
natedbiphenyls !PCBs) from pyrolysis or combustion
processes of chlorine containing organic materials have
been published. Buser, 1979 observed the formation of
small quantities of higher chlorinatedbiphenyls from the
pyrolysis of chlorobenzenes at 620°C, but the mecha-
nism of the formation of PCBs hadnot been investi-
gated. Dorrepaal and Louw !1978) and Sommeling et al.
!1993) provedthat formation of PCBs from the gas-
Bao et al. !1993) reported®ndings of PCBs in the
waste from reaction stills of a chemical factory in China.
The factory produces trichlorobenzenes by pyrolysis of
hexachlorocyclohexane. Since HCl atmosphere exits in
the reaction still, this made us think HCl may play a role
in formation of PCBs. At this study, pyrolysis of chlo-
robenzenes takes place in a HCl atmosphere.
2. Materials and methods
The experiments were carriedout in glass tubes. HCl
was prepared by adding about 5 ll sulfuric acidand15
mg anhydrous cupric chloride in a one-end sealed cap-
illary tubing !1.5 mm i.d.) and then inserted the capillary
tubing into another tube !50 Â 4:2 mm2 i.d.) that con-
tained 4 mg of di€erent kinds of chlorinated benzenes.
The glass tube was sealedunder atmospheric pressure,
andheatedin an oven. When the tube was removed
from the oven, it cooledat room temperature. After
cooling, the tube was crushedandthe reaction products
were extractedwith 25 ml 5% !v/v) toluene in n-hexane
under ultrasonication. The extract was cleaned up on an
* Correspondence author. Fax: +86-10-6292-3563.
E-mail address: zhengmh@mail.rcees.ac.cn !M.-H. Zheng).
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ! 0 0 ) 0 0 4 3 4 - 3

784
P.-Y. Liu et al. / Chemosphere 43 12001) 783±785
alumina column !250 Â 10 mm2 i.d., with basic alumina,
which is activatedat temperature of 660 °C for 6 h). We
®rst elute with 50 ml n-hexane, andthen 80 ml 2% !v/v)
methylene chloride in n-hexane. The last elute was
evaporatedto 0.5 ml for GC/ECD or GC/MS analysis of
PCBs.
If HCl is present in a combustion system, various
equilibrium reactions may occur to provide chlorination
agents. These reactions can be catalyzedby CuCl 2. Free-
radical chlorine can be produced as following equations:
2HCl ‡ 1=2 O2 ! H2O ‡ Cl2
Cl2ꢀ‡M† ! 2Cl: ‡ M
Three equipments were usedin this study. Hitachi
663-50 GC equippedwith ECD andDB-5 capillary
column !30 m  0.25 mm i.d., J&W). VG-Trio 2000 GC/
MS equippedwith an EI source andHT-8 capillary
column !30 m  0.25 mm i.d.,SGE). Hewlett±Packard
6890 GC equippedwith 5973 MSD andHP-5 capillary
column !30 m  0.25 mm i.d.).
Identi®cation of the compounds was based on the
selected-ion monitoring !SIM) mode of the GC/MS us-
ing two most abundant ions of PCBs molecular ion
clusters. Congener was identi®ed by comparison to
publishedArcolor mixtures chromatograms !Capel et al.,
1985).
The quanti®cation of PCBs was performedby com-
parison of the response of all peaks in the sample with
those in Aroclor mixtures.
3. Results and discussion
Dorrepaal andLouw, 1978 andSommeling et al.
!1993) reportedthe thermally initiatedfree-radical chain
chlorination of a variety of substitutedbenzenes in the
gas phase. They suggesteda pathway of formation of
PCBs from chlorobenzenes as shown in Scheme 1.
Fig. 2. Mass fragmentogram of PCBs from the condensation
reaction of 1,2,3-trichlorobenzene at 280°C !VG GC/MS); peak
assignment: 1, 2 ˆ penta-, 3 ˆ 2; 20; 3; 30; 4; 40-hexa-, 4 ˆ hexa-
5 ˆ 2; 20; 3; 30; 4; 40,5-hepta-, 6 ˆ hepta-, 7 ˆ 2; 20; 3; 304; 40; 5; 50-
octachlorobiphenyl.
Scheme 1. Formation of PCBs from the pyrolysis of chloro-
benzenes.
Fig. 3. TIC chromatogram of PCBs from the condensation
reaction of 1,2,3-trichlorobenzene at 450°C !HP GC/MS); peak
assignment: 1 ˆ hexachlorobenzene, 2,3 ˆ penta-, 4,5,6 ˆ hexa-,
7,8,11 ˆ hepta-, 9,10,12,13 ˆ octa-, 14 ˆ nona-, 15 ˆ decachlo-
robiphenyl.
Fig. 1. Mass fragmentogram of 2; 20; 3; 30; 4; 40; 5; 50-octachlo-
robiphenyl from the condensation reaction of 1,2,3,4-tetra-
chlorobenzene !HP GC/MS).

P.-Y. Liu et al. / Chemosphere 43 12001) 783±785
785
Factors e€ecting the PCBs formation such as
CuCl2, temperature and reaction time were studied.
NaCl was usedinsteadof CuCl
as chlorine source. It
2
was observedthat less PCBs were proudcedin the
presence of NaCl than that of CuCl2. This indicates
that CuCl2 plays the catalyzing role in converting HCl
into Cl2. It was foundthat the total yieldof PCBs
increasedwith increasing temperature !Fig. 4). As can
be seen from Fig. 5, no signi®cant variances in amount
of PCBs were observedwhen reaction time was longer
than 2 h.
Fig. 4. E€ect of temperature on the yieldof PCBs.
Acknowledgements
This study was supported by National Natural Sci-
ence Foundation of China !Grant no. 29707003) and
Chinese Academy of Sciences !Grant No. KY95T-03-05
andKZ951-B1-209-03).
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