Hydrodechlorination of chlorophenols at low temperature on a novel Pd catalyst

Article abstract of DOI:10.1039/b907846k

Pd/mesoporous silica-carbon nanocomposites with 3.2 nm Pd particles, prepared by a simple wetness impregnation method, have demonstrated high activity at 258 K for the hydrodechlorination of chlorophenols with a high selectivity. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b907846k

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Hydrodechlorination of chlorophenols at low temperature on a novel
Pd catalystw
Zhonghao Jin,a Chao Yu,b Xingyi Wang,*a Ying Wan,b Dao Lia and Guanzhong Lua
Received (in Cambridge, UK) 20th April 2009, Accepted 28th May 2009
First published as an Advance Article on the web 12th June 2009
DOI: 10.1039/b907846k
Pd/mesoporous silica–carbon nanocomposites with 3.2 nm Pd
particles, prepared by a simple wetness impregnation method,
have demonstrated high activity at 258 K for the hydro-
dechlorination of chlorophenols with a high selectivity.
HDC of chlorinated aromatics at low temperature. In this
work, Pd/MSCN was used for the HDC of 4-chlorophenol
(4-CP) and 2,4-dichlorophenol (2,4-DCP), as models of chlori-
nated organic pollutants. The reaction proceeded within the
temperature range of 258–313 K, of which 258 K is lower than
the reaction temperature reported in the literature. Meanwhile,
the activity of Pd/MSCN was evaluated through different
addition modes of triethylamine (Et3N) into the reaction mass
of chlorophenols.
Aryl chlorides are hazardous pollutants that are considered
among the most harmful organic contaminants due to their
acute toxicity and strong bioaccumulation potential,1 and
therefore, the safe disposal of aryl chloride pollutants has
acquired great importance with the ever increasing concern
of environmental protection.2 Among the various available
detoxification techniques, catalytic hydrodechlorination
(HDC) is an interesting one that can be employed to treat
streams containing concentrated or dilute chlorinated organic
pollutants under mild conditions.
The carrier material, MSCN, with a C : SiO2 molar ratio of
3.85 : 1, prepared by the method as reported in ref. 7, was
impregnated in 0.48 M (0.051 gPd mlÀ1) H2PdCl4 aqueous
solution for 24 h at 298 K, and then dried at 353 K for 8 h in a
vacuum oven. The resulting dry solid was reduced under H2
flow (30 ml minÀ1) at 473 K for 3 h in a tubular furnace, and
finally, 5 wt% Pd/MSCN catalyst, as determined by ICP, was
obtained. The same method was employed in preparation of
5 wt% Pd catalyst supported on active carbon (Pd/AC), which
was used as a reference sample to compare with Pd/MSCN.
The Pd/MSCN catalyst was characterized by XRD, XPS, N2
adsorption–desorption isotherms and TEM (ESIw).
Supported Pd catalysts are receiving attention for the treat-
ment of wastewater containing chlorinated organic pollutants,
especially in HDC reactions.3 High catalytic performance of
Pd catalysts in the HDC of chlorinated aromatics has been
observed at ordinary temperature.4 However, the HDC at low
temperature on Pd catalysts has seldom been reported. The
liquid phase catalytic HDC of 2,4-dichlorophenol on Pd/C
and Pd/Al2O3 was investigated at 273 K.5 Limited by the
freezing point, the HDC in aqueous solution below 273 K was
not feasible. In organic solutions, 10 wt% Pd/C showed
almost no activity in the HDC of 4-chlorobiphenyl at 253 K.6
Mesoporous silica–carbon nanocomposites (MSCNs) are
potentially useful materials for creating a high performance
heterogeneous catalyst, because of their unique properties
unachievable in traditional materials, with well-controlled
pore structures, high surface areas, and large and tunable pore
sizes, which facilitate the diffusion of reactant and product
inside the pores.7 Recently, MSCNs have been used as support
materials for metal catalysts in water-mediated coupling
reactions of chlorobenzene, without the presence of any
phase-transfer catalysts.8 The size of the metal particles
supported on MSCNs can be controlled on a B3 nm scale,
and a higher activity of catalysts than that so far reported in
literatures was observed. This may offer an opportunity for
establishing novel Pd catalysts with high performance for the
N2 sorption isotherms of the MSCN exhibit type-IV curves
with a very sharp capillary condensation step at P/P0
0.46–0.80 and an H1-type hysteresis loop featured by large-
pore mesoporous materials with cylindrical channels (Fig. 1).
A narrow pore size distribution with a mean value of 6.5 nm
was calculated from the adsorption branch based on the BJH
model. The BET surface area and pore volume of MSCNs
were calculated to be 413 m2 gÀ1 and 0.53 cm3 gÀ1
,
respectively. After supporting Pd, Pd/MSCN also displays
type-IV N2 sorption isotherms with an H1-type hysteresis
loop. The capillary condensation step shifts slightly to low
relative pressure with a wide range of P/P0 0.44–0.75, resulting
a Laboratory for Advanced Materials, Research Institute of Industrial
Catalysis, East China University of Science and Technology,
Shanghai 200237, China. E-mail: wangxy@ecust.edu.cn;
Fax: +86 21-64253372; Tel: +86 21-64253372
b Department of Chemistry, Shanghai Normal University,
Shanghai 200234, China
w Electronic supplementary information (ESI) available: Experimental
section, XRD data, XPS results, HDC of 2,4-DCP over 5 wt% Pd/AC,
and initial HDC rates for 4-CP. See DOI: 10.1039/b907846k
Fig. 1 Nitrogen adsorption–desorption isotherm plots and pore size
distribution curves for MSCN and Pd/MSCN.
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from the reduction of the pore size to 6.2 nm. The BET surface
area and pore volume of Pd/MSCN were calculated to be
345 m2 gÀ1 and 0.46 cm3 gÀ1, respectively. These phenomena
can be attributed to 5 wt% Pd supporting inside the channels
of the MSCNs, which leads to a partial blocking of the pores.
The 2-D hexagonal mesostructures of MSCN and Pd/MSCN
samples were confirmed by XRD (ESIw). For Pd/MSCN, the
face-centered cubic (fcc) Pd lattice is present, as confirmed by
several peaks at 2y = 40.1, 46.5, and 68.01 in wide-angle XRD
patterns (ESIw). It is interesting to find by TEM (Fig. 2(A)) a
narrow Gaussian distribution of Pd particle size of about
3.2 nm, which is consistent with the results either obtained
from H2 titration or estimated through the Scherrer formula.
Compared with the Pd particle size on Pd/AC, as shown in
Fig. 2(C), the dispersion of Pd on Pd/MSCN is much higher.
The possible reason for the high dispersion of small-size metal
particles is the unique hybrid nature of the carrier. In the
carrier, the silica and carbon components uniformly disperse
on the pore wall to construct a continuous framework.7 Pd ions
may be selectively adsorbed on the surface of hydrophilic SiO2,
whose silanol groups can interact with the Pd ions, while the
inert, hydrophobic component carbon possibly plays a role to
separate them. XPS results (ESIw) indicate that the intensity of
Pd for Pd/MSCN is much stronger than that for Pd/AC, which
can be ascribed to higher dispersion of Pd on Pd/MSCN and
more Pd atoms exposed to the surface.
Fig. 3 HDC of 4-CP over 5 wt% Pd/MSCN (Pd : Cl = 1 : 100 mol) at
258 and 278 K under ordinary hydrogen pressure (balloon) using Et3N
(1.0 equiv. vs. the number of chlorine atoms); 4-CP concentration:
5 mmol per 50 ml methanol solution; sp: the addition of 0.25 equiv.
Et3N vs. the number of chlorine atoms per 60 min.
of Pd/AC is low at low temperature, and the TOF is 1.9 minÀ1
at 278 K and 21.7 minÀ1 at 313 K.
The activity of Pd/MSCN for the HDC of 2,4-DCP is shown
in Fig. 4. Considering the conversion of 2,4-DCP to 2-CP and
4-CP, which, in turn, are converted into phenol, occurring on
Pd/MSCN, the corresponding TOF is related to the total number
of C–Cl bonds transformed and is estimated to be 3.2 minÀ1
(Table 1), higher than the TOF for the conversion of 4-CP to
phenol. This is in contradiction with the fact that the additional
Cl substituent is deactivating, 9 implying that there may be some
factor(s) that will affect the reaction process. The equivalence of
the initial 2,4-DCP consumption in the production of 2-CP
(5.9 mmol gÀ1 minÀ1) and phenol (2.7 mmol gÀ1 minÀ1) to the
HDC rate (8.6 mmol gÀ1 minÀ1) indicates that the reaction
proceeds predominantly in a stepwise fashion, converting first to
2-CP and then to phenol. However, a significant amount of 4-CP
was formed (with 10% selectivity) on Pd/AC, although its TOF
for 2,4-DCP is 1.1 minÀ1 (Table 1).
The evaluation of the activity of Pd/MSCN for the HDC
of 4-CP and 2,4-DCP was carried out under a hydrogen
atmosphere within the temperature range of 258–313 K
(ESIw). The conversion of 4-CP on Pd/MSCN at different
temperatures as a function of time is shown in Fig. 3, and the
turnover frequency (TOF) for the HDC of 4-CP is listed in
Table 1. At 258 K, the TOF for Pd/MSCN is 2.6 minÀ1, clearly
surpassing the TOF for Pd/AC of 0.9 minÀ1, and it takes
600 min for the HDC of 4-CP on Pd/MSCN to complete in the
presence of Et3N (1.0 equiv. vs. the number of chlorine atoms),
which can combine with the HCl formed during the reaction
so as to remove Cl species from Pd active sites. This result
confirms for the first time that nano-sized Pd particles
supported on MSCN are highly active for HDC in the liquid
phase at 258 K. Phenol was the only product, and no
formation of other species was observed. With raising
temperature, the activity of Pd/MSCN for HDC increases
obviously, and the TOF reaches 2.8 minÀ1 at 278 K and
25.0 minÀ1 at 313 K. Under similar conditions, the activity
Taking the solubility of H2, which has no significant change
within the range of 258–313 K, the temperature dependence of
the initial reaction rate can be used to generate apparent activa-
tion energies. The associated Ea value (with 95% confidence
limits; ESIw) of the HDC of 4-CP is 15.6 kJ molÀ1 within this
temperature range on Pd/MSCN, markedly lower than the value
(40.8 kJ molÀ1) obtained on Pd/AC (Table 1), and even lower
than the value (24.8 kJ molÀ1) that has been quoted for the HDC
of 4-CP promoted by Pd on carbon cloth over the temperature
range 303–358 K.10 This comparison indicates that nano-sized
Pd catalyst supported on MSCN is highly active for the HDC of
chlorophenols.
When Et3N is added in batches of 0.25 equiv. vs. the number
of chlorine atoms per 60 min for the HDC of 4-CP, the
conversion of 4-CP on Pd/MSCN is promoted greatly within
the range of the amount of Et3N added equivalent to the
formed HCl during the HDC, and the TOF at 258 K increases
up to 4.9 minÀ1 from 2.6 minÀ1. Meanwhile, the corres-
ponding Ea is 15.2 kJ molÀ1 within the range of 258–313 K
(ESIw), almost as same as the value for the HDC with one
batch addition of Et3N. This phenomenon indicates that Pd
atoms could interact with Et3N so as to be unavailable to
Fig.
2 TEM images of the mesoporous supported palladium
catalysts: fresh Pd/MSCN (A); used Pd/MSCN (B); Pd/AC (C).
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Table 1 Metallic size and the catalytic performance of hydrodechlorination of chlorophenols
TOF/minÀ1c
4-CP
2,4-DCP
258 K
Sample
Dp/nm
SBET/m2 gÀ1
Vt/m3 gÀ1
dPd/nma
Ea/kJ molÀ1b
258 K
278 K
313 K
MSCN
Pd/MSCN
Pd/AC
6.5
6.2
3.4
413
345
727
0.53
0.46
0.21
3.2
5.1
15.6 (15.1)
40.8
2.6 (4.9)
0.9 (2.2)
2.8 (6.7)
1.9
25.0 (26.5)
21.7
3.2 (3.3)
1.1 (2.7)
a
b
c
Particle size, calculated from the H2 chemisorption. Activation energy for the hydrodechlorination of 4-CP. The turnover frequency, the
number of molecules transformed per surface metal atom and per minute, and the values of the total number of C–Cl bonds transformed in the
case of 2,4-DCP; the values within parenthesis obtained by the addition of 0.25 equiv. Et3N vs. the number of chlorine atoms per 60 min for 4-CP,
and 0.1 equiv. Et3N, per 30 min for 2,4-DCP.
were tested. The TOF for used Pd/MSCN was 2.4 minÀ1
,
almost the same as that of the fresh one. The Pd content of the
used catalyst, after run 5, was 4.9 wt%. A low Pd leaching
amount (less than 0.3% of Pd content of catalyst) was detected
by ICP analysis of the catalyst-free liquor. The mesostructure
stayed unchanged, and the dispersion of Pd particles was still
high (Fig. 2(B)).
In conclusion, a new and stable Pd/MSCN of 3.2 nm Pd
particles has been prepared successfully using a simple
procedure. For the HDC of chlorophenols at 258 K, this
novel Pd catalyst presents higher activity and selectivity than
other Pd catalysts so far reported in the literature.
The authors gratefully acknowledge financial support from
the Commission of Science and Technology of Shanghai
Municipality (No. 0852nm00900nm) and the National Basic
Research Program of China (No. 2006AA06Z379 and No.
2004CB719500).
Fig. 4 The HDC of 2,4-DCP over 5 wt% Pd/MSCN (Pd : Cl =
1 : 100 mol) at 258 K under ordinary hydrogen pressure (balloon)
using Et3N (1.0 equiv. vs. the number of chlorine atoms); 2,4-DCP
concentration: 5 mmol per 50 ml methanol solution; sp: the addition of
0.1 equiv. Et3N vs. the number of chlorine atoms per 30 min.
Notes and references
reactant molecules. For 2,4-DCP, however, the batch addition
of Et3N affects the HDC to a small extent, which implies that
2,4-DCP molecules adsorb more competitively on the active
sites than Et3N does. The different effect of Et3N on the HDC
of 4-CP and 2,4-DCP stems from the fact that Pd atoms favor
reacting with the electron-deficient benzene ring of 2,4-DCP.
By raising the temperature to 278 K, this inhibition effect of
Et3N on the HDC of 2,4-DCP disappears. In the case of the
Pd/AC catalyst, the HDC of both 4-CP and 2,4-DCP is
inhibited obviously by Et3N, even at 278 K. Therefore, the
Pd/MSCN catalyst, with smaller size Pd particles, interacts
more strongly with chlorophenols than Pd/AC, and presents a
high activity for the HDC of chlorophenols. Without Et3N, Pd
catalysts will deactivate and the reaction rate lowers quickly,
because of the strong adsorption of the formed chlorine
species on the active sites. For the recycling study, the HDC
reaction of 4-CP was performed maintaining the same reaction
conditions as described above, and up to five successive runs
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4440 | Chem. Commun., 2009, 4438–4440