A new advanced method for heterogeneous catalysed dechlorination of polychlorinated biphenyls (PCBs) in hydrocarbon solvent

Article abstract of DOI:10.1016/S0040-4039(02)02390-0

The dechlorination of PCBs with solid hydrazine hydrochloride catalysed by palladium, in an organic solvent, yields biphenyl in short reaction times. The catalyst system can be efficiently reused for several cycles. Ultrasonication of the heterogeneous catalysed reaction increases the dechlorination rate remarkably. The reactivity of the C-Cl bond on the PCB ring are in the order meta>para?ortho.

Full text of DOI:10.1016/S0040-4039(02)02390-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9581–9583
A new advanced method for heterogeneous catalysed
dechlorination of polychlorinated biphenyls (PCBs) in
hydrocarbon solvent
J. Gonzalo Rodr´ıguez* and Antonio Lafuente
Departamento de Qu´ımica Orga´nica, Universidad Auto´noma, Cantoblanco 28049 Madrid, Spain
Received 23 October 2002; accepted 24 October 2002
Abstract—The dechlorination of PCBs with solid hydrazine hydrochloride catalysed by palladium, in an organic solvent, yields
biphenyl in short reaction times. The catalyst system can be efficiently reused for several cycles. Ultrasonication of the
heterogeneous catalysed reaction increases the dechlorination rate remarkably. The reactivity of the CꢀCl bond on the PCB ring
are in the order meta>paraꢀortho. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Polychlorinated biphenyls (PCBs) have been classified
(EPA), as priority pollutants due to their widespread
presence and persistence. The PCBs have been applied
in a wide variety of industrial uses including heat
transfer, flame retardation, and dielectric fluids,1 but
have actually received attention as endocrine disrup-
tors, human carcinogenes,2 and environmental
estrogens.3
Samples of 2,4,5-trichlorobiphenyl (PCB-29) or 2,5-
dichlorobiphenyl (PCB-9) with well-known concentra-
tions in hexane, were prepared for the dechlorination
reaction. Thus, PCB-29 (12.5 mg, 0.05 mmol) or PCB-9
(11.0 mg, 0.05 mmol) were dissolved in hexane (40 g)
and were treated with a mixture of hydrazine hydro-
chloride (moderately toxic reagent7) (250 mg, 3.65
mmol) and sodium carbonate (390 mg, 3.65 mmol) and
palladium on carbon (100 mg, Pd/C 10%). A multifre-
quency generator and ultrasonic reactors have been
designed (with frequencies centred at 250, 475, 650 and
800 kHz) to improve the heterogeneous catalytic
dechlorination.
The most widely accepted method for the destruction of
the PCBs consists of incineration,4 although this is
expensive and may produce undesirable byproducts
such as polychlorinated dibenzofurans and polychlori-
nated dibenzodioxins.4b Catalytic dehydrochlorination
has proved to be an efficient method for the detoxifica-
tion of hazardous chlorinated wastes,5 but the practical
application of catalysts to the dechlorination of organic
halides was always accompanied by the problem of the
deactivation of the catalyst.6
The reaction was run in a bath with stirring at variable
temperature or in a reactor with ultrasonication at
room temperature, finally yielding biphenyl as the sole
aromatic product (HPLC). The dechlorination at 60°C
(external bath) shows the highest rate. The rate of the
reaction increases remarkably at room temperature
with ultrasound radiation at all of the frequencies
experimented, but proceeds to completion for the short-
est times at 487 kHz versus 250 and 650 kHz. The
analysis of all the reaction products was carried out by
HPLC with a UV–vis detector, with an error of <0.1%.
Hexane solutions of the required dechlorinated com-
pounds such as dichlorobiphenyl, monochlorobiphenyl
and biphenyl were used as reference.
We have found a new heterogeneous catalytic method
for the complete dechlorination of the PCBs, in an
organic solvent under mild conditions of temperature
or using an ultrasound source.
Keywords: dechlorination; polychlorinated biphenyl; PCBs; palla-
dium; hydrazine; sonochemistry.
* Corresponding author. E-mail: gonzalo.rodriguez@uam.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02390-0

9582
J. G. Rodr´ıguez, A. Lafuente / Tetrahedron Letters 43 (2002) 9581–9583
The dechlorination of PCB-29 by ultrasonication at 487
kHz, at room temperature, in general, shows a remark-
ably high efficiency, which is improved in the second
and third catalytic cycles (Table 2).
2.1. Dechlorination of 2,4,5-trichlorobiphenyl (PCB-29)
The dechlorination of PCBs was analysed taking 2,4,5-
trichlorobiphenyl (PCB-29) as a reactive reference
model, which shows three different CꢀCl positions on
the biphenyl ring: (a) ortho, meta and para biphenyl
positions of the chlorine atoms; (b) 1,2-, 1,3- and
1,4-dichloro relative positions, Scheme 1.
2.2. Dechlorination of 2,5-dichlorobiphenyl (PCB-9)
In order to observe the reactivity of the dichlorinated
derivatives, we carried out the dehydrochlorination of
PCB-9. The results of the dechlorination under the
above conditions are summarised in Table 2.
The dechlorination of PCB-29 was carried out with the
heterogeneous hydrazine hydrochloride/palladium (HZ/
Pd) catalyst system above referred at different tempera-
tures (rt, 40 or 60°C), at 60°C it reaches the optimal
kinetic results, which are summarised in Table 1 and
Scheme 1.
After the first catalytic cycle, the palladium catalyst was
recovered by filtration on sodium carbonate and com-
pleted with the necessary hydrazine hydrochloride and
was then reused in the dechlorination of a new sample.
Analysis of Table 2 shows that the dechlorination
happened in shorter times than for PCB-29 and small
differences were observed: (a) At 60°C, the more inac-
tive chlorine atom was also that on the ortho position,
and the PCB-1 was observed; the ortho dechlorinated
PCB-2 was detected in low concentration; (b) the
highest rate for the CꢀCl bond dechlorination was for
that in the meta position; (c) although the recovered
catalyst system exhibits a low efficiency at 60°C, the
kinetic of dechlorination shows the same activity order
observed in the first catalytic cycle.
From Table 1, some aspects of the reaction must be
considered: (a) At 60°C, from the dechlorinated com-
pounds, the more inactive CꢀCl bond was that on the
ortho position (PCB-1, PCB-7 and PCB-9); (b) PCB-29
was transformed into PCB-9 and PCB-7, both having
an ortho chlorine atom. Moreover, the ortho elimina-
tion product PCB-12 was not detected; (c) the 1,3-
dichlorinated product PCB-9 (or PCB-7) shows the
highest rate for the para (or meta) versus ortho CꢀCl
bond dechlorinations, PCB-1 was detected while PCB-2
was never detected. Hence, the meta-rule fails for PCB-
29,8 meaning that the CꢀCl bonds with more neigh-
bouring chlorine atoms in the meta position are
removed preferentially.
The dechlorination of PCB-9 in the second and third
catalytic cycles shows a remarkably high efficiency by
ultrasonication of the reaction at 487 kHz, at room
temperature.
Scheme 1.
Table 1. Dechlorination of the PCB-29
t (min)a
PCB-29
PCB-9
PCB-7
PCB-3
PCB-1
Biphenyl
15
30
25
2
0
20
8
0
7
0
0
0
5
0
0
0
13
6
0
30
84
100
100
45
15b
0
0
0
a At 60°C.
b Ultrasonic radiation (487 kHz).

J. G. Rodr´ıguez, A. Lafuente / Tetrahedron Letters 43 (2002) 9581–9583
Table 2. Dechlorination of the PCB-9
9583
Catalytic cycle
t (min)a
PCB-9
PCB-2
PCB-1
Biphenyl
1
2
15
5b
15
0
0
64
0
38
0
85
52
63
0
0
0
2
0
0
0
2
0
3
0
0
0
3
0
8
0
4
10
6
0
100
100
31
100
54
100
9
38
90
5b
15b
15
3
180
5b
28
100
45b
a At 60°C.
b Ultrasonic radiation (487 kHz).
Dechlorination of an industrial PCBs sample (2768
ppm) in a mineral dielectric oil (100 g) was satisfact-
orily carried out with the HZ/Pd system. By GC analy-
sis, after 15 min, the mineral oil shows <25 ppm of
PCBs. The same experiments, but with sonication,
increases the reaction rate and leads to the practically
complete destruction of the PCBs (after 5 min <4 ppm,
by GC).
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Acknowledgements
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