Tandem dehalogenation-hydrogenation reaction of halogenoarenes as model substrates of endocrine disruptors in water: Rhodium nanoparticles in suspension vs. on silica support

Article abstract of DOI:10.1016/j.apcata.2011.01.004

This paper focuses on the removal of mono- and poly-halogenated arenes, as model substrates of endocrine disrupting compounds from aqueous effluents. The efficiency of catalysts based on metallic nanoparticles in the tandem dehalogenation-hydrogenation reaction as a pre-treatment for the remediation of organic halogenated pollutants was studied. Two catalytic systems - aqueous colloidal Rh⁰ dispersions and SiO₂-supported Rh ⁰ nanoparticles - were easily prepared under mild conditions and were compared in the hydrodehalogenation reaction of the model substrates. Interesting results in terms of catalytic activity were obtained, particularly with Rh⁰@SiO₂ catalyst, which proved to be more stable and efficient under the reaction conditions.

Full text of DOI:10.1016/j.apcata.2011.01.004

Applied Catalysis A: General 394 (2011) 215–219
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Tandem dehalogenation–hydrogenation reaction of halogenoarenes as model
substrates of endocrine disruptors in water: Rhodium nanoparticles in
suspension vs. on silica support
Claudie Huberta,b, Elodie Guyonnet Biléa,b, Audrey Denicourt-Nowickia,b,∗, Alain Roucouxa,b,∗
a
Ecole Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35 708 Rennes Cedex 7, France
Université Européenne de Bretagne, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2010
Received in revised form
This paper focuses on the removal of mono- and poly-halogenated arenes, as model substrates of
endocrine disrupting compounds from aqueous effluents. The efficiency of catalysts based on metallic
nanoparticles in the tandem dehalogenation–hydrogenation reaction as a pre-treatment for the reme-
2
0 December 2010
0
diation of organic halogenated pollutants was studied. Two catalytic systems – aqueous colloidal Rh
Accepted 3 January 2011
Available online 8 January 2011
dispersions and SiO2-supported Rh0 nanoparticles – were easily prepared under mild conditions and
were compared in the hydrodehalogenation reaction of the model substrates. Interesting results in terms
0
of catalytic activity were obtained, particularly with Rh @SiO2 catalyst, which proved to be more stable
Keywords:
Nanoparticles
Hydrogenation
and efficient under the reaction conditions.
©
2011 Elsevier B.V. All rights reserved.
Dehalogenation
Halogenoarenes
Endocrine disrupting compounds
1
. Introduction
of appropriate methods that could be integrated into treatment
facilities to avoid the release of EDCs into natural waters. Among
them, advanced oxidation processes are relevant techniques for
the removal of some EDCs [3,4] but have proved to be difficult in
the case of some refractory compounds. In that context, the pre-
treatment of aqueous effluents to modify the chemical structure
of these recalcitrant chemicals could be a promising alternative.
For that purpose, the reductive hydrodehalogenation reaction [5],
which involves the reductive cleavage of a C–X bond, represents a
viable, low cost and environment friendly method [6]. This reaction
has appeared in the environmental field as a potential technique
to eliminate organic chlorinated compounds, competing with oxi-
dation processes. In the case of halogenoarenes compounds, the
tandem dehalogenation–hydrogenation reaction seems promising
for the detoxification of these hazardous compounds into less
harmful products.
In that context, noble metal nanoparticles are considered as
robust, high surface area heterogeneous catalysts [7,8], which are
particularly pertinent for the hydrogenation of arene derivatives
[9,10] and also efficient in the hydrodehalogenation reactions [11].
A few examples of the tandem dehalogenation–hydrogenation
reactions have already been reported with Rh0 nanoparticles in
aqueous solution [12] or on support [13]. Indeed, the deposit
of metallic nanospecies on inorganic supports has emerged as a
powerful means for the development of new, highly active
and easily recyclable catalysts [14,15]. The colloidal suspensions
Endocrine disrupting compounds (EDCs) from domestic, agri-
cultural or industrial sources have been widely released into the
aquatic environment [1]. These substances could alter endocrine
functions and adversely disrupt growth, development and repro-
duction through interactions with the endocrine system [2,3]. Due
to their persistence in aquatic environment and potential adverse
health effects, the removal and the degradation of EDCs from
aqueous effluents have been an object of public concern. Among the
numerous EDCs, the four major classes that have recently received
scientific and public interest as the most potential disruptors are
polyaromatic compounds, alkylphenols, organic oxygenated com-
pounds such as dioxins, phthalates and pesticides (Fig. 1). The
common feature of these compounds is the presence of one or
more aromatic rings in their structure, which could sometimes be
halogenated.
Since conventional wastewater treatments are inefficient for the
remediation of EDCs and particularly in the case of high concen-
trations, numerous efforts have been devoted to the development
∗ Corresponding authors at: Ecole Nationale Supérieure de Chimie de Rennes,
CNRS, UMR 6226, Avenue du Général Leclerc, CS 50837, 35 708 Rennes Cedex 7,
France.
E-mail address: alain.roucoux@ensc-rennes.fr (A. Roucoux).
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.01.004

2
16
C. Hubert et al. / Applied Catalysis A: General 394 (2011) 215–219
Polyaromatic compounds
Alkylphenols
5
n(Cl)
(Cl)n
HO
HO
Pesticides
Cl
OH
Polychlorobiphenyle
Benzo[a]pyrene
Nonylphenol
Bisphenol A
Cl
Organic oxygenated compounds
CCl3
DDT
COOR
COOR
Cl
Cl
O
Cl
Cl
N
N
O
Dioxin
Cl
N
H
N
NHEt
Phthalate
Atrazine
Fig. 1. Some endocrine disrupting compounds.
0
of zerovalent rhodium nanoparticles (Rh @HEA16Cl), present-
ing sizes around 3.0 nm, were easily prepared by a bottom-up
approach based on the chemical reduction of an aqueous
solution of RhCl ·3H O in the presence of N,N-dimethyl-N-
previously characterized by transmission electron microscopy
(TEM) [16,17,21], showing that the particles are well-dispersed on
the grid with an average diameter of 2.1 nm.
3
2
cetyl-N-(2-hydroxyethyl)ammonium chloride HEA16Cl salt as
protective agent [16]. The silica-supported rhodium nanoparticles
Rh @SiO ), having sizes around 5.0 nm, were obtained by a simple
impregnation of the inorganic support by the previously described
aqueous colloidal suspensions without any particular treatment
0
2
.2.2. Preparation of Rh @ SiO catalyst by a wet impregnation
2
0
method
0
(
0
2
The Rh @SiO2 catalyst was prepared according to a proce-
dure developed in the laboratory [17,18] and based on the simple
impregnation of the inorganic support by the previously described
[
17,18]. In this paper, we compare the catalytic performances of
0
Rh @HEA16Cl colloidal suspension [16,21]. Silica (18.5 g) was
0
rhodium nanoparticles in aqueous suspension or supported on
silica towards the tandem dehalogenation/hydrogenation reaction
of various halogenoarenes (Cl, Br), which were chosen as model
substrates for endocrine disruptors.
vigorously stirred in 40 mL of deionized water for 2 h. Furthermore,
0
−4
5
0 mL of Rh @HEA16Cl (1.9 × 10 mol of Rh) was added under
vigorous stirring and the system was kept under stirring for 2 h. The
coloration of the aqueous phase turned from black to colourless,
evidencing the adsorption of the rhodium0 nanoparticles on the
silica surface. The catalyst was recovered by filtration, then washed
2
. Experimental
◦
several times with distilled water and dried at 60 C overnight. The
2.1. General
rhodium loading of the materials, which was evaluated by ICP-
AES, was of 0.08 ± 0.01 wt% and near the theoretical value (0.1 wt%).
0
RhCl ·3H O was obtained from Strem Chemicals. All the
Previous TEM analyses [17] have evidenced well-dispersed Rh par-
3
2
organic compounds were purchased from Acros Organics or
Sigma–Aldrich–Fluka and used without further purification. The
silica Si80 was obtained from Merck and presents the following
characteristics: mean diameter: 80 ␮m, pore diameter 5.4 nm,
ticles with an average size of 5 nm, which is limited by the pore
diameter.
2.3. Hydrogenation procedure
2
−1
3
−1
specific area 490 m g , pore volume 0.77 cm g and internal
porosity 58%. Water was distilled twice before use by conven-
tional method. The hydroxyethylammonium chloride salt HEA16Cl,
which was used as a protective agent for the Rh nanoparticles in
aqueous solution, was synthesized as previously described in the
literature and fully characterized [19,20]. All gas chromatography
analyses of catalytic reactions were performed using a Carlo Erba
GC 6000 with a FID detector equipped with a Factor Four column
Catalytic hydrogenations were carried out at room temperature
and at atmospheric hydrogen pressure or under hydrogen pres-
sure according to the catalytic results. The conversions and the
identification of the obtained products were monitored by gas chro-
matography analysis. Turnover frequency (TOF) was determined
for 100% conversion.
0
(
30 m, 0.25 mm i.d.).
2.3.1. Hydrogenation under atmospheric pressure
0
A 25 mL round bottomed flask, charged with Rh @HEA16Cl
2
2
.2. Preparation and characterization of rhodium catalyst
0
(
10 mL) or Rh @SiO (1 g in 10 mL of distilled water) and the sub-
2
strate ([substrate]/[metal] = 100), are connected to a gas burette
and a flask to balance pressure. Then the system is filled with hydro-
gen and the mixture is magnetically stirred.
0
.2.1. Preparation of Rh @HEA16Cl aqueous colloidal suspension
The aqueous colloidal suspension was prepared following the
previously reported method [16,21].
Sodium borohydride (18 mg, 0.48 mmol) was added to an
aqueous solution of ammonium surfactant HEA16Cl (133 mg,
2
.3.2. Hydrogenation under hydrogen pressure
A stainless steel autoclave was charged with the catalyst –
0
.38 mmol in 40 mL H O). Then, this solution was quickly added
2
0
0
Rh @HEA16Cl (10 mL) or Rh @SiO (1 g in 10 mL of distilled water).
under vigorous magnetic stirring to an aqueous solution of the pre-
2
The substrate (100 eq.) was then added and dihydrogen was admit-
ted to the system at constant pressure. Then the reaction was stirred
until completion.
cursor RhCl ·3H O (50 mg, 0.19 mmol in 10 mL H O). The initial red
3
2
2
solution darkened instantaneously. The obtained suspensions are
acidic (pH = 2.4) and are stable for several weeks. They have been

C. Hubert et al. / Applied Catalysis A: General 394 (2011) 215–219
217
Table 1
Hydrogenation of chloroanisoles.
.
0
Rh @HEA16Cla
0
b
P (bar)
Rh @SiO2
Ratioc (%)
t (h)
TOFd (h−1
)
Ratioc (%)
t (h)
TOFd (h−1
)
1
5
53
60
47
40
11
2
34
188
64
64
36
36
5.5
1.5
69
250
1
5
76
68
24
32
7
1.7
54
220
60
62
40
38
5
1
75
375
1
5
48
41
52
59
8
2.4
48
156
65
68
35
32
5.5
1
69
375
a
b
c
−5
−5
−3
Rh (3.8 × 10 mol), HEA16Cl (7.6 × 10 mol), substrate (3.8 × 10 mol), H2O (10 mL), room temperature.
0
−6
−4
Rh @SiO2 (1 g, 0.08 wt%, 7.8 × 10 mol), substrate (9.6 × 10 mol), H2O (10 mL), room temperature.
Determined by gas chromatography.
d
Turnover frequency defined as the number of mole of hydrogen per mole of rhodium per hour for 100% conversion.
3
. Results and discussion
obtained from the tandem dehalogenation–hydrogenation reac-
tions. The cyclohexanone has already been observed in acidic media
via the formation of a hemiacetal during the hydrogenation of
anisole with aqueous colloidal Rh suspensions [23]. In the present
work, the neutralisation of the reaction media, as already described
in the literature [24,25], was not carried out in order not to modify
In the drive towards the remediation of organic chlorinated
0
pollutants from natural environment and industrial sites, the
tandem dehalogenation–hydrogenation reaction seems to be a
promising means to transform endocrine disruptors in less harmful
products. In these preliminary studies, we have chosen halogenated
the ionic force. In contrast, no cyclohexanone was observed in the
0
(
Br, Cl) anisoles as model substrates, which are also considered as
hydrogenation of anisole with the Rh @SiO . In this case, the forma-
2
a family of endocrine disruptors, for this tandem reaction. The use
of metallic nanoparticles as catalysts for this reaction seems to be
relevant, considering their outstanding activity in the hydrogena-
tion of aromatic derivatives.
tion of cyclohexanone during the hydrogenation of chloroanisole
isomers could be explained by the release of HCl in the reaction
media during the dehalogenation, which constitutes the first step
of the tandem dehalogenation–hydrogenation reaction, as shown
in Fig. 2.
3
.1. Hydrogenation of chloroanisoles isomers – comparison of
The chloroanisole regioisomers were totally dehalogenated
and hydrogenated at atmospheric pressure with TOFs ranging
0
0
Rh @HEA16Cl and Rh @SiO
2
−
1
from 34 to 75 h , depending on the halogen position and the
nature of the catalyst (suspension vs. support). The formation
of cyclohexanone, ranging from 24 to 52%, is much higher with
2-and 4-chloroanisoles. No real influence of the pressure was
observed concerning the ratio of obtained cyclohexanone. Finally,
First, we studied the hydrogenation of the different regioisomers
of chloroanisole using the suspension and supported nanocatalysts
0
0
(
Rh @HEA16Cl or Rh @SiO ) at room temperature under hydrogen
2
pressure (1 or 5 bar) in water (Table 1). The turnover frequencies
TOFs) were calculated considering an optimized reaction time for
0
(
the Rh @SiO2 catalyst seems to be more active than the aqueous
0
complete conversion of the substrate and based on the introduced
amount of metal and not on the exposed surface metal and thus
may be underestimated. As previously reported by Janiak et al.
Rh colloidal suspension with higher TOFs and a 35–40% ratio of
cyclohexanone for the three isomers. The increase in pressure up
−
1
0
to 5 bar H2 leads to higher TOFs up to 375 h for Rh @SiO2 catalyst
with quite the same ratio of cyclohexanone.
[
22] the catalytic activity results not only from the exposed surface
metal atoms since the surface can restructure, atoms can shift posi-
tionsduringheterogeneous processesand partial aggregationcould
occur during catalysis, modifying the fraction of surface atoms.
These changes in the surface render the determination of the num-
ber of surface atoms difficult.
The hydrogenation of di- or tri-chloroanisoles isomers was also
investigated in water under 5 bar H2 and at room temperature to
study the influence of the polysubstitution on the aromatic rings
towards the tandem dehalogenation–hydrogenation reaction. The
results are gathered in Table 2.
The reaction leads to the formation of two products:
the expected methoxycyclohexane and also significant quanti-
ties (20–60%) of cyclohexanone. The methoxycyclohexane was
The reaction leads to the formation of the same products
than those previously observed with monochlorinated compounds:
the expected methoxycyclohexane and also cyclohexanone. The
Fig. 2. Mechanism of the formation of cyclohexanone during the chloroanisoles hydrogenation.

2
18
C. Hubert et al. / Applied Catalysis A: General 394 (2011) 215–219
Table 2
Hydrogenation of di- or trichloroanisoles.
0
Rh @HEA16Cla
0
b
Rh @SiO2
Ratioc (%)
t (h)
Conv.c (%)
Ratioc (%)
t (h)
Conv.c (%)
7
6
5
5
15
23
4
4
90d
88d
88
86
12
14
1
1
100
100
71
19
4
90d
86
14
1
100
a
b
c
−5
−5
−3
Rh (3.8 × 10 mol), HEA16Cl (7.6 × 10 mol), substrate (3.8 × 10 mol), H2O (10 mL), 5 bar H2, room temperature.
0
−6
−4
Rh @SiO2 (1 g, 0.08 wt%, 7.8 × 10 mol), substrate (7.8 × 10 mol), H2O (10 mL), 5 bar H2, room temperature.
Determined by gas chromatography.
No increase in the conversion is observed after 5 h.
d
conversions of the Rh@HEA16Cl colloidal suspension in the hydro-
genation of di-or tri-chlorinated anisoles are lower than those
observed with monochlorinated compounds, with a 90% con-
version after 4 h. Moreover, the conversion did not increase
significantly with longer times (after 5 h). This could be explained
by a chlorine poisoning effect of the particle surface, leading to
a deactivation of the catalyst, as already observed in the litera-
demonstrated by the addition of 960 ␮l of HCl 1 N (correspond-
ing to one equivalent of formed HCl during the dehalogenation of
dichlorinated compounds) in the hydrogenation of o-chloroanisole
with the Rh@SiO2 system. In this case, the reaction leads to
the formation of methoxycyclohexane (82%) and cyclohexanone
(18%), thus modifying the selectivity observed in Table 1 (36%
cyclohexanone).
ture [26,27]. The Rh@SiO catalyst showed similar kinetic activities
2
with polychlorinated substrates than with monochlorinated ones,
−1
with TOFs ranging around 300 h . This result is quite different
to the literature as usually the increase in chlorine substituents
leads to a significant decrease of the catalytic activity [28]. More-
over, smaller quantities of cyclohexanone, around 10–15% were
formed, compared to those obtained with monochlorinated sub-
strates (30–40%). The higher chlorine content in the reaction
media has an influence on the selectivity of the reaction, decreas-
ing the cyclohexanone ratio. The effect of the chlorines was
3
.2. Hydrogenation of bromoanisoles at 1 bar H – comparison of
2
0
0
Rh @HEA16Cl and Rh @SiO
2
The dehalogenation/hydrogenation reaction was also investi-
gated with bromoanisole regioisomers at atmospheric pressure
and room temperature with the aqueous Rh colloidal suspension
or the SiO -supported catalyst. The results are summarized in
Table 3.
0
2
Table 3
Hydrogenation of bromoanisoles.
.
0
Rh @HEA16Cla
0
b
Rh @SiO2
Ratioc (%)
t (h)
Conv (%)
Ratioc (%)
t (h)
Conv (%)
5
5
5
9
29
11
9
7
24
24
45
41
5
45
29
28
47
22
11
24
24
95
87
17
13
13
58
20
14
8
24
42
10
34
37
19
24
90
a
b
c
−5
−5
−3
Rh (3.8 × 10 mol), HEA16Cl (7.6 × 10 mol), substrate (3.8 × 10 mol), H2O (10 mL), 1 bar H2, room temperature.
0
−6
−4
Rh @SiO2 (1 g, 0.08 wt%, 7.8 × 10 mol), substrate (9.6 × 10 mol), H2O (10 mL), 1 bar H2, room temperature.
Determined by gas chromatography.

C. Hubert et al. / Applied Catalysis A: General 394 (2011) 215–219
219
First, contrary to the results obtained with chloroanisoles, con-
versions were not complete after 24 h whatever the bromide
position. As already described in the literature [28], the lower reac-
tivity of bromoaromatics can be attributed to the lower electron
affinity of Br (3.364 eV) compared to Cl (3.615 eV) that translates
into a less effective activation of the bromo-reactant through sur-
TiO2 anatase [30], which is known for its higher photocatalytic
activity than rutile [31]. Further developments are ongoing con-
cerning the remediation of EDCs from aqueous effluents, combining
tandem dehalogenation–hydrogenation reaction and photocat-
alytic activity using metallic nanoparticles doped-TiO materials.
2
0
face ␴-complex formation. Indeed, in the presence of Rh @HEA16Cl
Acknowledgements
catalyst, only 45% conversion was achieved with the formation of
three products: anisole, methoxycyclohexane and cyclohexanone.
Moreover, the aqueous suspension was slightly destabilized after
This work was financially supported by the Région Bretagne
(PhD fellowship for C. Hubert) and the Université Européenne de
Bretagne.
−
catalysis due to Br formation. To check the detrimental effect
−
0
of the Br ions on the stability and the activity of Rh @HEA16Cl,
the hydrogenation of 2-chloroanisole was carried out in the pres-
ence of HBr and showed the particle aggregation and a decrease
in conversion rate. The supported catalyst seems to be more
active with 87–95% conversion in 24 h with the formation of the
three products. Moreover, the deposit of metallic nanoparticles on
silica increases the stability of the catalytic system. This difference
could be explained by the nature of the halogen. The release of HBr
significantly increases the ionic strength of the media and could
modify the charged double layer organization around the particles
and thus destabilize the aqueous suspensions. As for, in the case
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