Kinetic study of the hydrogenolysis of polychlorobenzenes over a Pd/C catalyst in an alkaline aqueous-n-hexane system

Article abstract of DOI:10.1016/j.catcom.2010.02.018

The kinetics of the hydrogenolysis of chlorobenzene, dichlorobenzenes and some trichlorobenzenes over a 10% Pd/C catalyst was studied using a multiphase system. The reactions were carried out in a batch reactor with an aqueous NaOH/n-hexane solution of chloroaromatic compound as the liquid phase. Benzene was the final product of the hydrogenolysis of all the compounds studied. Hydrogenolysis was more effective in the presence of in situ generated hydrogen than gaseous hydrogen. The initial reaction rates and TOFs of dichlorobenzenes and trichlorobenzenes were slightly lower than those of chlorobenzene. The position of the chlorine atoms in trichlorobenzenes affects the kinetics of the removal of the first chlorine from these molecules. The differences in chlorine reactivity were explained by the inductive and steric effects induced by the benzene-Cl bonds.

Full text of DOI:10.1016/j.catcom.2010.02.018

Catalysis Communications 11 (2010) 797–801
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Kinetic study of the hydrogenolysis of polychlorobenzenes over a Pd/C catalyst
in an alkaline aqueous-n-hexane system
b
Iwona Anusiewicz a, Tadeusz Janiak a, , Janina Okal
*
a University of Gdan´sk, Faculty of Chemistry, J. Sobieskiego 18, 80-952 Gdan´sk, Poland
b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of the hydrogenolysis of chlorobenzene, dichlorobenzenes and some trichlorobenzenes over
a 10% Pd/C catalyst was studied using a multiphase system. The reactions were carried out in a batch
reactor with an aqueous NaOH/n-hexane solution of chloroaromatic compound as the liquid phase. Ben-
zene was the final product of the hydrogenolysis of all the compounds studied. Hydrogenolysis was more
effective in the presence of in situ generated hydrogen than gaseous hydrogen. The initial reaction rates
and TOFs of dichlorobenzenes and trichlorobenzenes were slightly lower than those of chlorobenzene.
The position of the chlorine atoms in trichlorobenzenes affects the kinetics of the removal of the first
chlorine from these molecules. The differences in chlorine reactivity were explained by the inductive
and steric effects induced by the benzene-Cl bonds.
Received 11 December 2009
Received in revised form 9 February 2010
Accepted 15 February 2010
Available online 19 February 2010
Keywords:
Hydrodechlorination
Polychlorobenzenes
Pd/C catalyst
Ó 2010 Elsevier B.V. All rights reserved.
Kinetics
1. Introduction
appear to be useful in the treatment of wastes containing chloroa-
romatic constituents. There are reports that the hydrogen chloride
Organochlorine compounds, being persistent pollutants, are of
great environmental concern; therefore, wastes containing them
should be properly treated before final disposal to avoid further
unnecessary contamination of the environment [1,2]. One possible,
promising solution to this problem is to convert aromatic chlorides
to dechlorinated compounds by hydrogenolysis [2] and to use the
product as a fuel. Hydrogenolysis of organohalogen compounds
has been investigated using different reducing reagents, catalysts
and substrates [2,3]. The usual aim of the procedures described
in the literature [2–5] is to convert a particular organochlorine
compound to the desired reaction product. For example, in the
Rosenmund reduction, acid chlorides are converted to aldehydes
without further reduction to alcohols [4]. But in the treatment of
hazardous wastes, it is effectiveness of halogen removal and
simplicity of the reaction system that are in fact required. The hyd-
rodechlorination of chloroaromatic compounds is known to pro-
ceed over various transition metal catalysts [2,4,5]. In particular,
Pd, Rh and Pt supported on carbon, alumina or Raney nickel have
been reported to promote hydrogenolysis of the carbon–chlorine
bond [2,3,6,7]. Palladium on activated carbon appears to be the cat-
alyst of choice [5], and to the best of our knowledge, it is indeed the
most effective in hydrodechlorination. Dechlorination in the pres-
ence of supported Pd has been studied in the gaseous [8–14] and
liquid phases [15–29]. Heterogeneous multiphase liquid systems
produced during the hydrodehalogenation of organochlorine
compounds deactivates the catalyst surface [9,26]. However, in li-
quid multiphase heterogeneous catalytic hydrogenolysis a base is
easily added to restore catalyst activity by reacting with the hydro-
gen chloride [23–26]. Moreover, aqueous and nonpolar phases
facilitate desorption of both hydrogen chloride and dechlorinated
compound from the catalyst surface, each one to a different phase.
Although hydrogenolysis of aromatic chlorides with gaseous
hydrogen is far from uncommon, the available literature data on
the kinetics of the process in the liquid multiphase system are rather
sparse; the most comprehensive reports in this respect are those of
Marques et al. [21,22] and Janiak et al. [24–26]. Recently, we inves-
tigated the kinetics of the hydrogenolysis of o-chlorotoluene [25]
and some meta-substituted chlorobenzenes [26]. Generally, we
found that in meta-substituted chlorobenzenes the effectiveness
of chlorine removal does not depend essentially on the type of sub-
stituent [25,26].
In the present work we studied the kinetics of the dechlorina-
tion of mono, di and some trichlorobenzenes in a multiphase sys-
tem in the presence of a 10% Pd/C catalyst. In particular, we
investigated how the number of chlorine atoms in a molecule
and their position in polychlorobenzenes influences the suscepti-
bility of these heteroatoms to dechlorination. There are few data
on the hydrogenolysis of dichlorobenzenes [10,14,26] and trichlo-
robenzenes [28,30,31] in the literature, in contrast to the hydrog-
enolysis of monochlorobenzene [9–11,14,15,17,23,24,28]. The
hydrogenolysis of 1,2,4-trichlorobenzene, as well as 1,2,3,4- and
* Corresponding author. Tel.: +48 58 523 53 21; fax: +48 58 523 53 57.
E-mail address: janiak@chem.univ.gda.pl (T. Janiak).
1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.02.018

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I. Anusiewicz et al. / Catalysis Communications 11 (2010) 797–801
1,2,4,5-tetrachlorobenzene was investigated in a fixed bed circular
reactor over a supported Pd catalyst and hydrogen gas [30]. The
hydrogen transfer hydrogenolysis of 1,2,4-trichlorobenzene was
also studied over a Rh/C catalyst [28]. In the gaseous phase, the
hydrogenolysis of 1,2,4-trichlorobenzene and 1,2,4,5-tetrachloro-
benzene was investigated in the presence of a Ni–Cr catalyst
[31]. Despite these efforts, however, there is some uncertainty sur-
rounding the relation between carbon–chlorine reactivity and the
position of chlorine in polychlorinated benzenes.
We also carried out a number of experiments to improve the
effectiveness of our hydrogenolysis system using chlorobenzene
as a model compound: for example, we wanted to see how the
addition of 2-methoxyethyl ether or 2-methoxyethanol affects
the hydrogenolysis reaction. Generally, these solvents are totally
miscible with water at room temperature, and totally (2-methoxy-
ethyl ether) or partially (2-methoxyethanol) miscible with n-hex-
ane. Both compounds are non-ionic surfactants, strongly
adsorbing on polar-nonpolar interfaces and modifying (lowering)
the corresponding surface tension values. It was found that these
substances may improve the hydrogenolysis of chloroaromatic
compounds [32]. The effectiveness of hydrogenolysis with gaseous
and in situ generated hydrogen was also investigated. This is a no-
vel, innovative contribution in this field.
and analysing the n-hexane layer. In all experiments, on comple-
tion of H2 uptake, the reaction mixture was filtered and separated
into to the aqueous and n-hexane layers. Chloride anions were ana-
lysed in the aqueous layer (by the Volhard method) and the
hydrogenolysis products in the n-hexane layer. The analyses were
performed on a PYE Unicam 104 gas chromatograph equipped with
a flame ionisation detector (FID) and packed columns. Analyses of
the n-hexane layers were preceded by a test to evaluate the accu-
racy and precision of the chromatographic method. In this test,
15 ml 10% aq NaOH, 100 mg 10% Pd/C and 10 ml of an n-hexane
mixture containing known quantities of 1,2,3-trichlorobenzene,
1,2-dichlorobenzene, 1,3-dichlorobenzene, chlorobenzene, ben-
zene and m-xylene (as internal standard) were stirred in the reac-
tor. The n-hexane layer was then analysed by gas chromatography.
The accuracy of these chromatographic assays was 5%. The
precision of the chromatographic assays of the hydrogenolysis
products was better than 8%, and that of the chloride ion was bet-
ter than 0.2%.
3. Results and discussion
The commercial 10% Pd/C catalyst was previously well charac-
terised by different techniques [26]. Prior to characterisation the
catalyst was treated in a stream of hydrogen at 400 °C for 2 h. It
then displayed the following characteristics: particle size distribu-
2. Experimental
tion 90% <60 lm; 10% <5 l
m; BET surface area 880 m2/g cat.; the
average particle size of Pd evaluated by high resolution transmis-
sion electron microscopy (HRTEM) was 5.3 nm, and by H2 chemi-
sorption was 12.1 nm; the hydrogen uptake (H/Pd) was 0.09. The
discrepancy between HRTEM and the adsorption data was attrib-
uted to some blockage of the Pd surface by carbon atoms [26].
Preliminary experiments were carried out to test how treat-
ment of the catalyst prior to the reaction would influence the
process. We compared the kinetics of the hydrogenolysis of
1,2,4-trichlorobenzene in the presence of the Pd/C catalyst pre-
treated by soaking and stirring in n-hexane solution with that in
the presence of the catalyst pre-treated in 10% aq NaOH. These
experiments provided insight into the role of solvent–catalyst
interaction in a multiphase system (results – see Fig. 1). It is evi-
dent that the Pd/C catalyst pre-treated in the n-hexane phase is
much more active than the one pre-treated in the aqueous phase
– the rate of reaction was ca 38.3ꢀ faster. It is very likely that when
2.1. Materials and chemicals
10% Pd/C catalyst, n-hexane (99%, HPLC), 1,2-dichlorobenzene
(99%), 1,3-dichlorobenzene (99%), 1,4-dichlorobenzene (99%),
2-methoxyethanol (99.3%), 2-methoxyethyl ether (99%) and
aluminium granules (99%) were from Aldrich. Benzene (99%) and
m-xylene (99%) were supplied by POCH Gliwice (Poland), while
1,2,3-trichlorobenzene (99%) was from Fluka and 1,2,4-trichloro-
benzene (99%) from BDH (England). All reagents and solvents were
used without further purification.
2.2. Apparatus and procedures
The hydrogenolysis reactions were carried out in a 50 ml
thermostatted three-necked round-bottomed flask at 20 °C. Gas-
eous hydrogen was introduced by gas burette at atmospheric pres-
sure. In a typical hydrogenolysis procedure, 100 mg of the 10% Pd/C
catalyst was first pre-treated by soaking it in 10 ml of an n-hexane
solution of 2.45 mmole of chlorinated compound and 0.25 mmole
m-xylene (internal chromatographic standard), after which 15 ml
10% aq NaOH was added. The reactor was flushed twice with
hydrogen and the mixture magnetically stirred at 1300 rpm with
a 12 ꢀ 5 mm stirring bar. The instant the stirrer was turned on
was taken to be starting point of the reaction. The hydrodechlori-
nation was also conducted with the addition of 2-methoxyethanol
or 2-methoxyethyl ether (diglyme) (0.6 ml), which were added
with the n-hexane.
The hydrogenolysis reaction, with hydrogen generated in situ by
the reaction of Al granules with aq NaOH in the reaction flask, was
performed in the same apparatus. First, the flask containing the re-
agents as above was flushed with nitrogen, then 0.07 g Al granules
was added and the reactants stirred at the same rate as above. The
gas burette served as a buffer gas holder to store the surplus of
hydrogen generated.
2.3. Analyses
Fig. 1. Variation with time of the amount of 1,2,4-trichlorobenzene consumed
during hydrogenolysis over 100 mg 10% Pd/C catalyst in the presence of 10% NaOH
and gaseous hydrogen; catalyst in the n-hexane phase (s) and the aqueous phase
(j). r30 – reaction rate after 30 min of the reaction.
The course of all the hydrodechlorination reactions was moni-
tored by measuring hydrogen consumption and also by sampling

I. Anusiewicz et al. / Catalysis Communications 11 (2010) 797–801
799
the catalyst is coated with the organic solvent, diffusion of the
1,2,4-trichlorobenzene is much better than through the aqueous
phase. Thus, all the catalytic experiments were performed with
the catalyst soaked in the organic phase.
In separate experiments the influence of the stirring speed
(from 700 to 1300 rpm) on the overall reaction kinetics was tested.
It was found that the reaction rate increased with stirring speed,
reaching a plateau at 1200–1300 rpm. So in our experiments we
set the stirring speed at 1300 rpm, which we think was sufficient
to minimise the influence of mass transfer on the hydrogenolytic
process [14,25]. We therefore adopted the following kinetic model:
(a) hydrogen is adsorbed non-competitively on the Pd surface, (b)
desorption of benzene and HCl is rapid and does not substantially
affect the kinetics of hydrogenolysis, (c) the reaction between
hydrogen and aromatic chloride is the slowest process in the sys-
tem investigated [25].
During hydrogenolysis in an alkaline aqueous-n-hexane liquid
system with gaseous hydrogen, chlorobenzene produces benzene
directly, but di- and tri-chlorobenzenes undergo sequential dechlo-
rination, yielding compounds containing less chlorine. Neverthe-
less, benzene is always the final product of these reactions.
The results of the dechlorination kinetics obtained in this study
are presented in Figs. 2–4 and in Table 1. The initial kinetic rates
(r0) were obtained from the linear fit to the time-dependence of
the amount of chlorinated compound during the initial period of
the reaction, whereas turnover frequencies (TOFs) were calculated
using the initial rates and hydrogen chemisorption data presented
in a previous study [26].
Fig. 3. Variation with time of the amount of 1,2,3-trichlorobenzene (4, 1,2-
dichlorobenzene (s), chlorobenzene (h) and benzene (j) during the hydrogenol-
ysis of 1,2,3-trichlorobenzene over 100 mg 10% Pd/C catalyst in the presence of 10%
NaOH and gaseous hydrogen.
First, we investigated chlorobenzene dechlorination under var-
ious reaction conditions (Table 1, entries 1–5). Fig. 2 shows the
time profile of changes in the amounts of chlorobenzene and ben-
zene during the hydrodechlorination of chlorobenzene with gas-
eous hydrogen, with and without the addition of 2-
methoxyethanol and 2-methoxyethyl ether (diglyme). Some litera-
ture reports indicate that 2-methoxyethanol added to the reaction
mixture may improve the hydrogenolysis of chloroaromatic com-
pounds [32]. On the other hand, 2-methoxyethyl ether was found
to chelate small cations [33]; we presumed this acted as a kind
of phase transfer catalyst. In particular, an improvement in the
neutralisation of HCl, a reaction by-product deactivating the Pd/C
catalyst [26], was expected. The results presented in Fig. 2 and Ta-
Fig. 4. Variation with time of the amount of 1,2,4-trichlorobenzene (D), 1,2-
dichlorobenzene (s), 1,4-dichlorobenzene (d), 1,3-dichlorobenzene (.), chloro-
benzene (h) and benzene (j) during the hydrogenolysis of 1,2,4-trichlorobenzene.
Reaction conditions as in Fig. 3. The inset is an enlarged part of the main diagram.
ble 1 (compare entries 1 and 2) show that the addition of 2-
methoxyethanol did not change the kinetics of chlorobenzene
hydrogenolysis, and no enhancement of the reaction rate was ob-
served. In both cases (with and without the additive), the amount
of chlorobenzene decreased linearly with time, suggesting an
apparent zero-order kinetic process; after about 60 min most of
the chlorobenzene had been dechlorinated. Also, the initial kinetic
rates were similar – 3.6 and 3.9 mmole gꢁ1 minꢁ1 – with and with-
Pd
out 2-methoxyethanol respectively. In contrast, the addition of 2-
methoxyethyl ether to the reaction mixture did affect the nature
of the overall hydrogenolytic process. The changes in the amount
of chlorobenzene seem to be exponential, a behaviour that implies
apparent first-order kinetics. After 60 min of reaction, only about
50% of the chlorobenzene was dechlorinated (Fig. 2). Presumably,
the addition of this compound inhibited to some extent the
adsorption of chlorobenzene on the catalyst surface and, contrary
to expectations, the dechlorination rate was lower at
1.8 mmole gꢁ1 minꢁ1 (compare entries 1 and 3). In order to explain
Pd
the different influence of 2-methoxyethanol and 2-methoxyethyl
ether on dechlorination, we performed some additional experi-
ments to test the partition of these compounds between the two
phases used. It appears that the former was present almost 100%
in the aqueous phase, whereas 70% of the latter was in the aqueous
Fig. 2. Variation with time of the amount of chlorobenzene and benzene during
hydrogenolysis over 100 mg 10% Pd/C catalyst in the presence of 10% NaOH and
gaseous hydrogen; without additives (D,N), with added 2-methoxyethanol (s,d)
and 2-methoxyethyl ether (h,j). The open symbols represent chlorobenzene, the
solid symbols stand for benzene.

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I. Anusiewicz et al. / Catalysis Communications 11 (2010) 797–801
Table 1
Kinetics characteristics of hydrogenolysis of chlorobenzene and some polychlorobenzenes in10% NaOH aq/n-hexane in the presence of 100 mg 10% Pd/C catalyst. Hydrogen
pressure 1 atm, magnetic stirrer 12 ꢀ 5 mm, stirring rate 1300 rpm.
Entry
Compound
Temperature
(°C)
Additive
Initial kinetic rate r0
Initial TOF0
(minꢁ1
)
ꢁ1
ꢁ1
(mmole g min
Pd
)
1
2
3
4
5
6
7
8
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
20
20
20
20a
40
20
20
20
20
20
20
–
3.9
3.6
1.8
7.6a
5.4
2.7
1.8
3.2
2.5
2.6
0.8
2.3b
46.7
43.1
21.5
89.4a
63.7
32.3
21.5
38.3
30.1
30.3
9.4
2-Methoxyethanol
Diglime
–
–
Chlorobenzene
1,2-Dichlorobenzene
1,2-Dichlorobenzene
1,3-Dichlorobenzene
1,4-Dichlorobenzene
1,2,3-Trichlorobenzene
1,2,4 Trichlorobenzene
–
Diglime
–
–
–
–
9
10
11
28.0b
a
Reaction performed in the presence of in situ generated hydrogen.
Reaction rate r30 and TOF30 after 30 min run of reaction.
b
phase and the other 30% in the n-hexane phase. Thus, the differ-
ence between the effects of the two glycoethers could be due to
their different repartition between the two phases used in our cat-
alytic experiments.
Another interesting observation is the large increase in the reac-
tion rate as the hydrogen was switched from gaseous phase to in
situ generated. This hydrogen was generated in the reaction mix-
ture by the following reaction:
hydrogenolysis (Table 1, entry 11, and Fig. 4). As shown on the in-
set in Fig. 4, larger amounts of 1,2-dichlorobenzene and 1,3 dichlo-
robenzene than of 1,4-dichlorobenzene were detected during the
hydrogenolysis of this compound. Again, its clear that the reactiv-
ity is governed by the position of chlorine in the benzene ring.
Inspection of the kinetic hydrogenolysis curves in Fig. 4 shows that
the reaction is slow up to 30 min (r0 = 0.8 mmole gꢁ1 minꢁ1), after
Pd
which the reaction rate increases to r30 = 2.3 mmole gꢁ1 minꢁ1, a
Pd
value characteristic of other di and trichlorobenzenes (see Table
1). Additionally, there was hardly any formation of 1,4-dichloro-
benzene up to 30 min (see inset in Fig. 4). We may note in passing
that the molecular bonds between the carbon and chlorine atoms
in polychlorobenzenes are nearly identical. Serguchev and Beloko-
pytov [31] calculated electron densities at the carbon atoms in var-
ious chlorobenzenes and detected only small differences in
electron densities at the carbons bound to chlorine in dichloro-
benzenes, 1,2,3-trichlorobenzene and 1,2,4-trichlorobenzene. In
conclusion, since liquid-phase hydrodechlorination is a stepwise
process for the removal of chlorine species, the lower rates of chlo-
rine elimination from position 2, in both 1,2,3-trichlorobenzene
and 1,2,4-trichlorobenzene, may be related to the inductive and
steric effects induced by the benzene–Cl bonds. Similar conclu-
sions can be drawn from the recent study by Meshesha et al.
[35], who studied the gaseous-phase hydrodechlorination of
1,2,4-trichlorobenzene over Pd/Mg(Al)O catalysts.
2Al þ 2NaOH þ 6H2O ! 2Na½AlðOHÞ4ꢂðaqÞ þ 3H2
ð1Þ
As shown in Table 1, entry 4, the initial rate of reaction
(7.6 mmole gꢁ1 minꢁ1) and initial TOF0 (89.4 minꢁ1) are about twice
Pd
the values in the presence of gaseous H2 (entry 1). The reaction rate
is also higher than that obtained with gaseous hydrogen at a higher
reaction temperature (40 °C, entry 5). In the presence of in situ gen-
erated hydrogen, the concentration of surface or dissolved hydro-
gen atoms in the Pd phase is probably greater than that obtained
with gaseous hydrogen. The hydrogen atoms dissolved in the Pd
crystal are stronger hydrogenating agents than those produced by
the dissociation of the gaseous phase molecules at the metal surface
[34].
The initial reaction rates for the hydrodechlorination of ortho-,
meta- and para-dichlorobenzenes in the presence of gaseous H2
are presented in Table 1, entries 6–9. The reaction rates and TOFs
are slightly lower for these isomers than for chlorobenzene (entry
1). Also, the addition of diglyme decreases the reaction rate (com-
pare entries 6 and 7). The important conclusion is that the position
of chlorine in dichlorobenzenes has only a minor influence on the
removal of the first chlorine (entries 6, 8, and 9) because the initial
kinetic rates are similar, i.e. within experimental error.
Interesting results were obtained for the hydrodechlorination of
1,2,3-trichlorobenzene (Table 1, entry 10, and Fig. 3). This com-
pound undergoes sequential dechlorination with the sorption/
desorption of reagents from/to the liquid phase. The initial rate
of the elimination of chlorine species is very similar to that ob-
tained for dichlorobenzenes, but the ease of chlorine displacement
depends on its position in the molecule. Fig. 3 shows that no mea-
surable quantities of 1,3-dichlorobenzene were detected; the steric
effect is therefore clearly in evidence. The 2-chlorine, located be-
tween the 1- and the 3-chlorine, either does not undergo hydrog-
enolysis or else its initial reaction rate is very low. Our results
are in full agreement with those reported by Marques et al. [22],
who also found a low rate of hydrogenolysis for 2-chloro-1,3-
dimethylbenzene.
4. Conclusions
Multiphase hydrogenolysis of polychlorinated benzenes over a
Pd/C catalyst and gaseous hydrogen is an effective process yielding
benzene as the final product. It was found that the effectiveness of
the Pd/C catalyst improved in the presence of in situ generated
hydrogen. The addition of 2-methoxyethanol or 2-methoxyethyl
ether to the reaction mixture does not accelerate the dechlorina-
tion process.
Acknowledgments
The financing of this work from state Funds for Scientific Re-
search (Grant No. DS/8000-4-0026-7) is gratefully acknowledged.
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