Complete and truly catalytic degradation method of PCBs using Pd/C-Et3N system under ambient pressure and temperature

Article abstract of DOI:10.1016/S0040-4039(02)01620-9

Since PCBs are persistent toxic pollutants and do not degrade easily, the development of a safe and perfect methodology for destruction of such remains is of great importance. We have found that the catalyst activity of Pd/C toward the hydrodechlorination of PCBs was outstandingly activated by the addition of triethylamine. PCBs could be thoroughly dechlorinated under ambient temperature and pressure, and no product other than biphenyl has been detected by GC/MS.

Full text of DOI:10.1016/S0040-4039(02)01620-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7251–7254
Complete and truly catalytic degradation method of PCBs using
Pd/C–Et N system under ambient pressure and temperature
3
a,
a
a
b
a,
Hironao Sajiki, * Akira Kume, Kazuyuki Hattori, Hisamistu Nagase and Kosaku Hirota *
a
Laboratory of Medicinal Chemistry, Gifu Pharmaceutical University, 5-6-1 Mitahora-higashi, Gifu 502-8585, Japan
b
Laboratory of Hygienics, Gifu Pharmaceutical University, 5-6-1 Mitahora-higashi, Gifu 502-8585, Japan
Received 13 June 2002; revised 22 July 2002; accepted 26 July 2002
Abstract—Since PCBs are persistent toxic pollutants and do not degrade easily, the development of a safe and perfect
methodology for destruction of such remains is of great importance. We have found that the catalyst activity of Pd/C toward the
hydrodechlorination of PCBs was outstandingly activated by the addition of triethylamine. PCBs could be thoroughly
dechlorinated under ambient temperature and pressure, and no product other than biphenyl has been detected by GC/MS.
©
2002 Elsevier Science Ltd. All rights reserved.
6
7
8
Polychlorinated biphenyls (PCBs) were commercially
produced as complex mixtures beginning in 1929
through the mid 1970s and they are a class of nonpolar
chlorinated hydrocarbons with a biphenyl nucleus on
which one to ten of the hydrogens have been replaced
by chlorine. Commercial PCBs were manufactured and
sold as complex mixtures containing multiple isomers at
different degrees of chlorination. PCBs are widespread,
persistent, bioaccumulate and pose a risk of causing
adverse effects to human health and the global ecosys-
dechlorination using metals, photolysis, g-radiolysis,
9 10 11
oxidation, electrolysis,
supercritical degradation
Some of them are used commercially
1c,2a,12
and so on.
to treat mainly liquid PCBs, and PCB-contaminated
oils (4), however, the low reactivity and/or selectivity of
most reagents is manifested by the low applicability to
dechlorination of especially multichlorinated aromatic
compounds, and most processes require high heat, high
pressure, radiation, stoichiometric reagents, vast
amounts of catalyst and/or strongly-basic conditions
1
13
tem, while some part of the world production (ca. 1.5
and many of them are very frequently incomplete.
million tons) has been destroyed, the major part still
remains in use or awaits destruction, whilst a substan-
tial proportion has been released to the environment.
Because of the high thermodynamic, chemical and bio-
logical stability of PCBs, all degradation methods are
In our preceding paper on chemoselective hydrogena-
tion using the Pd/C–Et N system, we reported that the
3
catalytic activity of Pd/C toward the hydrodechlorina-
tion of only aromatic chlorides was outstandingly and
1
c,d
14
extremely difficult.
Furthermore, as is clear from the
selectively activated by the addition of Et N. To
3
recent report commissioned by the Environment Pro-
gramme of the United Nations, many countries still do
explore the application of our dechlorination method,
the hydrodechlorination of some PCBs which are com-
plex mixtures of multichlorinated biphenyls was investi-
1d
not have suitable PCB disposal or treatment facilities.
15
Therefore, an effective remediation methodology for
PCBs is a critical factor for future protection from
improper disposal or an accidental leak (e.g. fire, earth-
quake, war and so on) of stored PCBs. Currently they
are principally being destroyed by high-temperature
incineration, but public concerns about discharges from
incinerators are strongly forcing a move to chemical
gated. Triethylamine was used in 1.2 equiv. versus the
4c
average chlorination number (ACN) of the PCBs and
10% Pd/C was employed in 10% of the weight of the
substrate in the case of PCBs degradation for safety
reasons, although 3% of the weight of the substrate was
used for the hydrodechlorination of mono-chloronated
13
aromatic substrates. Degradation of a 10% PCBs
1
c,d,2
®
methods.
rently under development include substitution of chlo-
5
Chemical remediation techniques cur-
(
Aroclor 1254, Monsanto Chemical Co., ACN=5.0)
solution in methanol using 10% Pd/C and triethylamine
under ambient temperature and hydrogen pressure is
shown in Fig. 1. As expected, GC peaks of the PCB
3
4
rides,
hydride reduction,
hydrodechlorination,
®
congeners of Aroclor 1254 disappeared completely
*
0
Corresponding authors.
after 15 min, while a small amount of intermediates,
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01620-9

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H. Sajiki et al. / Tetrahedron Letters 43 (2002) 7251–7254
2
-chlorobiphenyl, 2,6-dichlorobiphenyl and a trichloro-
devices. In order to investigate this possibility, we chose
PCBs practically used as high-tension capacitor oil for
over a quarter-century for the substrate of the
hydrodechlorination. The PCBs, amber and viscoid oil,
possess 3.3 chlorine per molecule on average (ACN=
3.3). The results from the hydrodechlorination of the
PCB congeners in the presence of triethylamine are
identical to those of Aroclor 1254. After 1 h, biphenyl
is the sole product from the hydrodechlorination of the
PCB congeners and the degradation is completed (Fig.
biphenyl, was also detected. However, the generated
intermediates were entirely dechlorinated within 1 h
suggesting the complete degradation of Aroclor 1254,
®
and accumulation of biphenyl 1 was observed as th®e
sole product. In contrast, the degradation of Aroclor
1
254 without triethylamine was quite slow. Very little
®
dechlorination was observed after 1 h (ACN=4.5), and
®
approximately 60% of the chlorine of Aroclor 1254
still remained even after 24 h hydrodechlorination
2
).
(ACN=2.9) (Fig. 1).
A major reason for the concern about the regulation of
PCBs in the environment is their effects on biological
systems. Co-planar PCB congeners that possess no
chlorine in the 2,2%,6 and 6% (ortho-) positions of a
biphenyl nucleus can assume a planar dioxin-like con-
formation because they lack the steric hindrance of
ortho-chlorines and elicit serious dioxin-like toxicity
and carcinogenecity. For the reason that a certain
amount of co-planar PCB congeners was contained in
almost all PCB products during the production process,
the degradation of commercially available (AccuStan-
dard Inc.) pure co-planar PCB congeners such as
1
c,d
It is well known
that a preponderance of the PCBs
was used as electrical insulating oil in capacitors and
transformers. Our fundamental concept to develop a
safe and reliable degradation method of PCBs should
be applicable to spent PCBs that have been used in
1c
3
,3%,4,4%-tetrachlorobiphenyl 2, 3,3%,4,4%,5-pentachloro-
biphenyl 3 and 3,3%4,4%,5,5%-hexachlorobiphenyl 4 has
been studied in detail. Under our hydrodechlorination
conditions, each co-planar PCB congener was smoothly
converted to biphenyl 1 within 1 h and a mixture
consisting only of 1 (100%) was obtained (Scheme 1).
In summary, these examples illustrate major improve-
ments in reliability, time, simplicity, safety and cost as
compared with previously proposed chemical remedia-
Figure 1. Formation of biphenyl from a commercial mixture
®
of PCB congeners, Aroclor 1254. GC/MS observation of the
hydrodechlorination process in the presence of Et N: (a)
3
®
GC/MS of Aroclor 1254. (b) GC/MS after 1 h hydrodechlo-
rination (balloon) of Aroclor 1254 (100 mg, 0.31 mmol) at
®
Figure 2. GC/MS observation of the hydrodechlorination
process of PCB congeners used as a high-tension capacitor oil
(standardized voltage=6.6 KV) in practice for over a quarter-
century (manufactured by Shizuki Seisakusho Co. in 1965).
(a) GC/MS of the capacitor oil (PCB congeners, ACN=3.3).
(b) After 1 h hydrodechlorination (balloon) of the PCBs oil
(500 mg) at ambient temperature (ca. 20°C) using 10% Pd/C
(50 mg) in methanol (5 ml) in the presence of triethylamine
(726 ml, 7.2 mmol). Degradation is completed (ACN=0.00).
ambient temperature (ca. 20°C) using 10% Pd/C (10 mg) in
methanol (10 ml) in the presence of Et N (37 mg, 0.37 mmol).
3
®
Degradation of Aroclor 1254 is completed (ACN=0.0).
GC/MS observation of the hydrodechlorination process with-
out triethylamine; (c) GC/MS after 1 h at ambient tempera-
®
ture (ACN=4.5). (d) After 24 h. Degradation of Aroclor
1254 is quite slow and a large amount of PCB congeners still
remains even after 24 h (ACN=2.9).

H. Sajiki et al. / Tetrahedron Letters 43 (2002) 7251–7254
7253
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,2,13
tion processes of PCBs.
All reagents and solvents
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ethylamine is regenerated by neutralization using
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5a,b
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2
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2
4
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gas (1 mL/min).
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(
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2
bottom flask or a high-pressure glass cylinder (Hyper
Glass Cylinder, Taiatsu Techno Co.), a vigorously stirred
mixture of the PCBs (50 mgꢀ50 g), 10% Pd/C (10% of
the weight of the PCBs) and triethylamine (1.2 equiv. for
each mol of chlorine in the PCBs) in methanol (up to
2
40% solution of PCBs) was hydrogenated (H balloon for
2
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2
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h. The reaction mixture was filtered using a membrane

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H. Sajiki et al. / Tetrahedron Letters 43 (2002) 7251–7254
®
filter (Advantec Dismic-13HP, 0.45 mm) or Celite cake
of the hydrodechlorination were analyzed by GC/MS
(Hewlett Packard, Model 5972A, measurable limit=
approx. 0.5 ppm) and H NMR (JEOL, EX-400).
Further, the aqueous layer was evaporated to dryness
to afford triethylammonium chloride also quantita-
tively.
and the filtrate was evaporated to dryness. The residue
was partitioned between hexanes and water, and the
organic layer was dried over anhydrous magnesium sul-
fate. After filtration, the organic solvents were evapo-
rated to afford biphenyl quantitatively. The products
1