,
2001, 11(1), 38–39
Hydrodechlorination of polychlorinated benzenes in the presence of a bimetallic
catalyst in combination with a phase-transfer catalyst
Valentina I. Simagina* and Irina V. Stoyanova
G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.
1
0.1070/MC2001v011n01ABEH001302
Bimetallic supported catalysts (Pd–Ni/C and Ni–Cu/C) in combination with a phase-transfer catalyst were found efficient and
selective in the liquid-phase hydrodechlorination of polychlorinated benzenes under mild conditions.
Polychlorinated aromatic compounds are carcinogenic and muta-
Table 2 Degree of dechlorination (X) of hexachlorobenzene in the presence
genic chemicals, which are environmentally stable and can
of alkylammonium salts; catalyst, Pd Ni98/C; S(C–Cl):Cat ratio, 10:1;
2
1
undergo bioaccumulation in fatty tissues. Presently, catalytic
T = 50 °C; 50% KOH; PH2 = 1 atm; solvent, isopropanol–toluene, 4:7
(15 ml).
hydrodechlorination is a viable alternative for halogenated waste
handling because hazardous materials are transformed into re-
cyclable products in a closed system with no toxic emissions.2
Phase-transfer catalyst
Time/h
X (%)
+
–
–
Hydrodechlorination is known to be promoted by Group VIII
noble metals. Polychlorinated aromatic compounds can be suc-
cessfully dechlorinatied using nickel catalysts under severe reac-
tion conditions (high temperature and high hydrogen pressure).
Palladium and ruthenium catalysts make it possible to perform
the dechlorination of polychlorinated aromatic compounds under
mild conditions; however, they are expensive and hence cannot
Me4N Cl
1.5
1.5
4.0
1.5
98
85
94
90
+
Et N Cl
4
Et N OH
–
+
4
+
–
(
C H ) MeN Cl
8 17 3
of the reaction, samples of the reaction mixture were analysed
by GLC on an LKhM-80 chromatograph using a 2 m×3 mm
stainless-steel column packed with 5% SE 30 on Chromaton
N AW-DMCS, a flame-ionisation detector, and argon as a car-
rier gas at a flow rate of 60 cm min . Undecane was used as
an internal standard.
3
be used on industrial scale.
It was found4 that the complete dechlorination of 1,2,4,5-
tetrachlorobenzene in the presence of a Pd/C catalyst and
Aliquat 336 (tricaprylylmethylammonium chloride) as a phase-
transfer agent with hydrogen can be performed at atmospheric
pressure. However, only isomeric o-, m- and p-dichloroben-
zenes (and dibromobenzenes) can be rapidly reduced with
hydrogen under mild conditions using Raney nickel.
,5
3
–1
The degree of dechlorination (X) was calculated as follows:
6
X = (1 – iC /6C0)×100%
Σ
i
i = 0
In this work, we found that efficient and inexpensive cata-
lytic systems for the hydrodechlorination of hexachlorobenzene
under mild conditions can be designed using a bimetallic
where C is the molar concentration of a substance containing i
i
0
chlorine atoms in the molecule and C is the initial concen-
tration of hexachlorobenzene.
(
Ni–Pd/C or Cu–Ni/C) catalyst in combination with a phase-
Table 1 indicates that the reaction rate was very high in the
presence of a phase-transfer agent. The Pd/C catalyst is most
active among the studied catalysts. The Pd Ni /C bimetallic
catalyst exhibits a higher activity in the hydrodechlorination
than the nickel catalyst and a mixture of the monometallic
catalysts with the same transition metal content as in the bi-
metallic catalyst. It is well known7 that the activity of bimetallic
catalysts is a nonadditive function of the composition. Thus, the
transfer agent.
Mono- and bimetallic catalysts were prepared by the im-
pregnation of a support with aqueous solutions of transition
2
98
metal chlorides followed by reduction with NaBH . The cata-
4
lysts of choice are transition metals (Pd, Ni and bimetallic
,8
Pd–Ni and Ni–Cu catalysts) supported on a Sibunit carbon (C)
6
support. Catalysts with the total metal (Ni, Pd, Ni + Pd or
–
4
Ni + Cu) content 1.7×10 mol per gram of the catalyst were
activity of a Pd–Ni/SiO catalyst for butadiene hydrogenation
2
used. The catalysts were designated as Pd Ni /C and Ni Cu /C,
increases with the content; however, this increase is not pro-
portional to the concentration because palladium demonstrates
a strong tendency to migrate to the surface.
2
98
92
8
where the subscripts indicate the mole ratios between the cor-
responding metals in the catalysts.
The hydrodechlorination reactions of chlorobenzene and hexa-
chlorobenzene were studied in a two-phase system (an aqueous
Table 2 shows that the hydrodechlorination of hexachloro-
benzene with Pd Ni supported on Sibunit is effective in the
2
98
5
0% KOH solution and an organic isopropanol–toluene phase)
at 50 °C. The hydrodechlorination was performed in a thermo-
statically controlled glass reactor with a magnetic stirrer (700 rpm)
under a constant (atmospheric) pressure of hydrogen. Hydrogen
7
100
3
–1
was supplied at a flow rate of about 4 cm min . In the course
Table 1 Degree of dechlorination (X) of hexachlorobenzene in the presence
1
+
–
of mono- and bimetallic catalysts; Me N Cl (phase-transfer catalyst),
4
T = 50 °C, 50% KOH, PH2 = 1 atm, isopropanol–toluene (4:7) solvent
50
3
(15 ml).
Catalyst (Cat)
S(C–Cl):Cat ratioa
Time/h
X (%)
2
b
5
Pd/C
50:1
50:1
10:1
10:1
10:1
10:1
0.5
0.5
1.5
1.5
5.5
6
98
18
98
54
50
43
4
c
6
Pd/C
Pd N98/C
2
Pd/C + Ni/C
0.5
1.0
1.5
Ni Cu /C
9
2
8
Time/h
Ni/C
Figure 1 Hydrodechlorination of hexachlorobenzene with Ni Pd /C and
9
8
2
a
The substrate (S):metal (Cat) ratio was determined as the substrate amount
Me N+Cl– as a phase-transfer catalyst: (1) hexachlorobenzene; (2) penta-
4
b
on a basis of the C–Cl unit. The ratio Pd (Cat):phase-transfer catalyst is
chlorobenzene; (3) tetrachlorobenzene isomers; (4) trichlorobenzene iso-
mers; (5) dichlorobenzene isomers; (6) chlorobenzene; and (7) benzene.
c
1
:200. The reaction was carried out without phase-transfer agents.
–
38 –
Simagina
