Hydrogenation of p-chloronitrobenzene over nanostructured-carbon-supported ruthenium catalysts

Article abstract of DOI:10.1002/cssc.201000335

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been used for the first time to support ruthenium nanoparticles for the hydrogenation of p-chloronitrobenzene (p-CNB) to produce selectively p-chloroaniline. The preparation of well-dispersed ruthenium catalysts from the [Ru ₃(CO)₁₂] precursor required activation of the purified supports by nitric acid oxidation. The supports, purified and functionalized, and the supported catalysts have been characterized by a range of techniques. The catalytic activity of these materials for the hydrogenation of p-CNB at 35bar and 60°C is shown to reach as high as 18mol_p-CNB g _Ru^-1 h^-1, which is one order of magnitude higher than a commercial Ru/Al₂O₃ catalyst. Selectivities between 92 and 94 % are systematically obtained, the major byproduct being aniline. Carbon nanotubes and carbon nanofibers act as supports for ruthenium nanoparticles in the hydrogenation of p-chloronitrobenzene to selectively produce p-chloroaniline. The preparation of well-dispersed ruthenium catalysts from a [Ru₃(CO)₁₂] precursor requires activation of the purified supports by nitric acid oxidation. The catalytic activity is one order of magnitude higher than that of a commercial Ru/Al₂O₃ catalyst.

Full text of DOI:10.1002/cssc.201000335

DOI: 10.1002/cssc.201000335
Hydrogenation of p-Chloronitrobenzene over
Nanostructured-Carbon-Supported Ruthenium Catalysts
Mustapha Oubenali,[a, b] Giuditta Vanucci,[c] Bruno Machado,[d] Mohammed Kacimi,[b]
Mahfoud Ziyad,[b] Joaquim Faria,[d] Anna Raspolli-Galetti,[c] and Philippe Serp*[a]
Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have
been used for the first time to support ruthenium nanoparti-
cles for the hydrogenation of p-chloronitrobenzene (p-CNB) to
produce selectively p-chloroaniline. The preparation of well-dis-
persed ruthenium catalysts from the [Ru3(CO)12] precursor re-
quired activation of the purified supports by nitric acid oxida-
tion. The supports, purified and functionalized, and the sup-
ported catalysts have been characterized by a range of tech-
niques. The catalytic activity of these materials for the hydro-
genation of p-CNB at 35 bar and 608C is shown to reach as
high as 18 molp-CNB gRuꢀ1 hꢀ1, which is one order of magnitude
higher than a commercial Ru/Al2O3 catalyst. Selectivities be-
tween 92 and 94% are systematically obtained, the major by-
product being aniline.
Introduction
The role of catalysis in the development of clean chemical pro-
cesses with limited hazardous/toxic by-products/wastes is now
well established as an essential element of a sustainable
chemistry. Aromatic haloamines are important intermediates
for the synthesis of many fine chemicals, such as dyes, herbi-
cides, pesticides, pharmaceuticals, and cosmetic products. A
current method for producing these haloamines is reduction
of the corresponding nitro-compounds, either with a metal
acid or by selective hydrogenation over metal supported cata-
lysts.[1] Thus, p-chloroaniline (p-CAN),
a high production
volume compound, can be synthesized via Bechamp reduction
(Fe/HCl) of p-chloronitrobenzene (p-CNB) or by selective hydro-
genation using metal supported catalysts.[1,2] Industrial imple-
mentation of the Bechamp reduction is no longer viable due
to the generation of metal oxide residues and acid effluents,
and liquid-phase selective catalytic hydrogenation has
emerged as a cleaner alternative, with little impact on the en-
vironment and higher product yields. However, the economic
viability depends on catalyst selectivity because unwanted
CꢀCl bond hydrogenolysis is difficult to fully circumvent. Thus,
with a variety of catalysts (e.g., Pt, Pd, Ni, Rh) the hydrogena-
tion of halonitroaromatics to the corresponding haloanilines is
always accompanied by some dehalogenation reaction. Reac-
tion selectivity is therefore critical, as p-CNB hydrotreatment
can generate a range of intermediates and by-products,[3] as
shown in Figure 1, which presents the reaction pathways pro-
posed for batch liquid-phase hydrogenation. Platinum and
nickel are the metals most widely used for the hydrogenation
of halonitroaromatics. In this process, the solvent often used is
methanol or ethanol. Metals with favorable selectivity patterns
are ruthenium,[4] rhodium,[5] iridium,[6] palladium,[7] and gold.[8]
Among these catalysts, ruthenium is cheaper than the other
noble metals and more selective for this reaction, but its cata-
lytic activity is generally low. Therefore, to achieve high yields
of haloanilines, many approaches have been developed by
Figure 1. The hydrogenation reaction pathways of p-chloronitrobenzene.
[a] M. Oubenali, Prof. Dr. P. Serp
Laboratoire de Chimie de Coordination UPR CNRS 8241
composante ENSIACET
Universitꢀ de Toulouse UPS-INP-LCC
4 allꢀe Emile Monso B.P. 44362, 31030 Toulouse Cedex 4 (France)
Fax: (+33)05 34 32 35 96
E-mail: philippe.serp@ensiacet.fr
[b] M. Oubenali, Dr. M. Kacimi, Prof. Dr. M. Ziyad
Facultꢀ des Sciences, Dꢀpartement de Chimie
Laboratoire de Physico-chimie des Matꢀriaux et Catalyse
Avenue Ibn Battouta, B.P. 1017, Rabat (Morocco)
[c] G. Vanucci, Dr. A. Raspolli-Galetti
Dipartimento di Chimica e Industriale
Universitꢁ di Pisa
Via Risorgimento 35, 56100 Pisa (Italy)
[d] Dr. B. Machado, Dr. J. Faria
Laboratorio de Catalise e Materiais
Universidade do Porto
Rua Dr. Roberto Frias, 4200-465 Porto (Portugal)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201000335.
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Hydrogenation over Nanostructured-Carbon-Supported Ruthenium
either dedicated preparation of the catalysts (colloids,[9–11] bi-
metallic catalysis,[4,12,13] controlling the metal particle dispersion
and metal–support interaction[14,15]) or the use of specific addi-
tives such as water.[16] One of the most important aspects con-
cerns the catalyst support. Thus, the catalytic performances
can be altered significantly by the use of suitable supports
that control metal particle dispersion and metal–support inter-
actions. Modification of the electronic state and/or morpholo-
gy of ruthenium crystals through metal–support interactions
creates a chance of obtaining a catalyst with unique catalytic
properties. Carbon nanotubes (CNTs) and nanofibers (CNFs)
can show unusual behavior compared to classical supports, es-
pecially for liquid-phase reactions, and some interesting results
have already been obtained.[17–20] According to these reports
the catalytic activity in hydrogenation reactions can be en-
hanced either due to electronic effects or to the absence of
diffusion limitations. Such supports have already been used for
the hydrogenation of chloronitrobenzenes.[21–24] Over PtM/CNT
catalysts both catalytic activity and yield of p-chloroaniline are
improved.[21,22] Ni/CNF catalysts show very good activity and
selectivity for the hydrogenation of chloronitrobenzenes to the
corresponding chloroanilines.[23,24] To the best of our knowl-
edge, no attempt has yet been made to improve the catalytic
activity of supported ruthenium catalysts for the selective hy-
drogenation of p-CNB by using CNTs or CNFs as support.
The aim of the present work is to determine the influence of
the support and ruthenium precursor on the hydrogenation of
p-chloronitrobenzene over Ru/CNT and Ru/CNF catalysts. Pres-
sure and reaction temperature effects in the hydrogenation of
p-CNB over Ru/CNT and Ru/CNF catalyst in the liquid phase are
also studied. The presented results establish a basis for the de-
velopment of a sustainable catalytic route for the production
of haloamines.
carbon nanostructures with oxygen-containing surface
groups.[25,26]
The as-synthesized carbon nanostructures have been puri-
fied by HCl treatment to remove the remaining catalyst. The
purity of the purified supports, CNTP and CNFP is 92% and
84%, respectively. The remaining impurities are iron and nickel
particles. HRTEM images of the purified supports (CNTP and
CNFP) are shown on Figure 2, where the orientation of the gra-
phene layers with respect to the fiber axis can be measured:
~ 08 for CNTP and between 188 and 258 for CNFP.
Results and discussion
Support characterization
Two kinds of nanostructured carbons have been used as a sup-
port for ruthenium: multiwalled carbon nanotubes and carbon
nanofibers. The multiwalled nanotubes are composed of gra-
phene layers that lie perpendicular to the fiber axis; and the
‘‘herringbone’’ carbon nanofibers used present a tilted layer ar-
rangement. These herringbone nanofibers are usually inferior
to CNTs in mechanical strength and conductivity, but possess
exposed graphene edge sites that make them attractive for
some applications, such as catalysis.
Figure 2. HRTEM images of a) CNTP, and b) CNFP. White bar: orientation of
the graphene layers. Black bar: tube or fiber axis.
The textural and chemical characterization of purified and
functionalized supports is given in Table 1. The distribution of
external diameters for CNTP is relatively narrow (7–40 nm),
while for CNFP the distribution is wider and bimodal (5–30 nm
and 50–200 nm). CNTP and CNFP present similar specific surface
areas, and the significant contribution of small-diameter CNFP
can explain the higher surface measured for CNFp.
In order to obtain a well-dispersed metallic ruthenium phase
on CNTs and CNFs some prerequisites have to be fulfilled. First
of all, the solvent containing the metal precursor must wet the
support. Second, the support must contain anchoring sites in
order to immobilize the metal precursor and the subsequent
metallic particles. The hydrophobic as-produced carbon nano-
structures do not fulfill these prerequisites due to the lack of
reactive anchoring sites. Surface activation of CNTs and CNFs
by refluxing the support in nitric acid resulted in hydrophilic
The Raman spectra of these materials reflect the different or-
ganization of the graphene layers in CNTP with respect to CNFP,
as illustrated by the ID/IG values that are indicative of the Csp3
/
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951

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Catalyst characterization
Table 1. Textural and chemical characterization of purified and functionalized supports.
Sample
Purity
[%C]
Fext
d002
[nm]
Raman
(ID/IG)
BET
Pore vol.
PZC
TPD [mmolgꢀ1
]
The ruthenium catalysts have
been prepared by wet impreg-
nation from the Ru0 organome-
tallic cluster [Ru3(CO)12]. This
precursor was chosen to avoid
the effect of chlorine, which
could affect the chemisorption
and therefore the catalytic
[nm]
[m2 gꢀ1
]
[cm3 gꢀ1
]
CO
CO2
CNTP
CNFP
CNTF
CNFF
92
84
92
98
18
15
18
23
3.39
3.41
3.39
3.41
1.5
1.2
1.6
1.3
134
161
172
117
0.302
0.411
0.344
0.263
7.6
7.6
3.5
4.6
1121
601
2248
1053
332
158
1365
652
Csp2 ratio. The lower ID/IG ratio measured for CNFP compared to
CNTP can be attributed either to a higher ordering of the gra-
phene layers and/or to a shorter length for CNFF compared to
CNTF. In addition, the presence of disordered carbon on the
surface of CNTF (Figure 2a) cannot be ruled out.
properties. This precursor has already been used for the prepa-
ration of well-dispersed Ru/CNF catalysts, which showed high
activity for arene hydrogenation.[27] The chemical composition,
adsorption properties, and particle size of the catalysts are pre-
sented in Table 2. The ruthenium loading achieved by the im-
Controlling the point of zero charge (PZC) of CNTs or CNFs is
important for depositing finely dispersed metal nanoparticles
from precursors in solution. The values obtained for the puri-
fied supports point towards a neutral surface, and tempera-
ture-programmed desoprtion (TPD) spectra are characteristic
of a poorly functionalized carbon surface. The CO/CO2 ratios
measured for CNTP and CNFP are 3.37 and 3.81, respectively,
confirming that the surface is not acidic.
Table 2. Characterization of the supported Ru/CNTF and Ru/CNFF catalysts.
Sample Ru loading Ru particle BET[a]
Pore vol.[a] TPR[b] TPD[a] [mmolgꢀ1
]
[%]
size [nm] [m2 gꢀ1] [cm3 gꢀ1
]
[8C] CO
CO2
Ru/
CNTF
Ru/
2
4
3
185
(172)
134
0.599
(0.411)
0.302
105 2618
(2248) (1365)
110 1587
799
1.5
596
CNFF
(117)
(0.263)
(1053) (652)
Due to the different reactivity of CNTs and CNFs surface to-
wards oxidation, we decided to use nitric acid solution (69%
HNO3) and low-temperature activation (608C, 2 h) to activate
CNF surfaces, in order to avoid severe burn-off during the pro-
cess. CNTs were boiled in 69% HNO3 for 4 h. This treatment
permits to increase the CNF purity up to 98%. At this stage,
the remaining impurities consist of encapsulated metal parti-
cles that are not expected to interfere with the catalytic
measurements.
[a] The values between parentheses correspond to the functionalized sup-
ports. [b] Temperature of ruthenium reduction.
pregnation method corresponds to the immobilization of 30–
40% of ruthenium (5% w/w); the remaining ruthenium species
being recovered from the filtrate. The higher loading obtained
for CNTs should be correlated to the higher amount of oxygen-
ated surface groups on this support. The TEM images of Ru/
CNTF and Ru/CNFF show that the ruthenium particles are well-
dispersed, with narrow size distributions (Figure 3). The Ru/
CNFF contains particles in the range 1–9 nm and the Ru/CNTF
in the range 1–16 nm. The larger particle size distribution in
the case of the CNT support can be correlated to the higher
metal loading, and/or to the different orientation of the gra-
phene layers. The deposition of well-dispersed ruthenium
nanoparticles on nanocarbon supports results in an increase of
their specific surface area. Such a phenomenon has already
been observed in the case of 1–5% Ru/CNT catalysts.[28]
The nitric acid treatment performed on purified supports
permits to introduce acidic surface groups, as shown by (i) the
PZC values of functionalized supports (CNTF and CNFF); (ii) the
low CO/CO2 ratios calculated from the TPD spectra (1.65 and
2.33 for CNTF and CNFF, respectively); and (iii) the significant
contribution of the peak at 2858C, associated to decomposi-
tion of carboxylic groups, on the TPD profiles (Supporting In-
formation, Figure SI1). Interestingly, we noticed that the nitric
acid oxidation significantly modified the TPD profiles of CNTs.
Indeed, for the CNTP sample the profile is consistent with a
transformation of CO into CO2 at around 8358C. This can be
explained by the presence of traces of iron on CNTs that can
catalyze the Boudouard reaction 2CO$CO2+C.[25] After func-
tionalization this phenomenon disappears, pointing towards
the total removal of accessible iron upon nitric acid treatment.
This phenomenon was not observed in the case of CNFs. The
lower amount of oxygenated groups measured on the CNF
surface is due to the milder conditions used for activation.
However, the use of these mild conditions has not permitted
to completely avoid CNF damage. Indeed, thinner CNFs are de-
stroyed by the treatment, resulting in an increase of the mean
diameter and a decrease of the specific surface area and pore
volume.
The H2 temperature-programmed reduction (TPR) experi-
ments were performed on samples reduced at 3008C and re-
exposed to the air. The TPR profiles shown in Figure 4 indicate
that the small ruthenium nanoparticles were easily re-oxidized.
The ruthenium species can be fully reduced at relatively low
temperatures (<2008C), without significant differences be-
tween the two supports. The higher-temperature peaks may
correspond to the reductions of carbon-related functional
groups or carbon species on nanocarbon surfaces.[29] Another
possibility is the ruthenium-catalyzed hydrogen gasification of
carbon, producing methane, followed by a catalyst deactiva-
tion. The higher amount of hydrogen needed for the Ru/CNTF
catalyst reduction compared to Ru/CNFF is consistent with the
difference in ruthenium loading. The TPD spectra correspond-
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Hydrogenation over Nanostructured-Carbon-Supported Ruthenium
Figure 5. TPD profiles (CO2 evolution) of a) CNTF and Ru/CNTF, and b) CNFF
and Ru/CNFF.
released between 150 and 7508C with a maximum at around
3008C (2908C for CNTF and 3108C for CNFF). For the Ru/CNTF
and Ru/CNFF catalysts the CO2 is released abruptly at 4708C for
CNTF and 4808C CNFF. The amount of CO2 released from the
Ru/CNTF catalysts is higher for Ru/CNFF, with a ratio of 1.33,
which is similar to the ratio of ruthenium loadings on both
supports. We thus propose that the carboxylic groups present
on the functionalized support act as anchoring sites for the
ruthenium, and that the formation of ruthenium carboxylate
bonds significantly stabilizes these groups. The fast decompo-
sition of these groups should be correlated to an autocatalytic
decomposition involving ruthenium. Indeed, a sudden stop of
release of any gas from a complicated surface (system) is
highly unlikely without a catalytic reaction.
Figure 3. TEM images of a) Ru/CNTF, and b) Ru/CNFF.
Obviously, a significant sintering of these catalysts should
occur above a temperature of 470-4808C.
Hydrogenation of p-chloronitrobenzene
The prepared catalysts were tested for the liquid-phase hydro-
genation of p-CNB to p-CAN, and compared to three commer-
cial catalysts: a 5% Ru/Al2O3 catalyst from Fluka, a 0.5% Ru/
Al2O3 catalyst from Aldrich, and a 5% Ru/C catalyst from Engel-
hard. Lower alcohols are generally used as solvents in the
liquid-phase hydrogenation of p-CNB and an examination of
the studies reported by different groups[9–15] prompted us to
use methanol as a solvent in the present study. The catalytic
hydrogenation of p-CNB follows a multi-step reaction pathway
(Figure 1). The main side reaction on ruthenium catalyst is hy-
drodechlorination, leading to aniline (AN) as the ultimate prod-
uct. This may happen either by hydrodechlorination of p-CNB
Figure 4. TPR profiles of Ru/CNTF and Ru/CNFF.
ing to the reduced Ru/CNTF and Ru/CNFF catalysts (Figure SI1)
exhibit different features. First, the amount of CO-releasing
groups has increased for both samples. This might be due to
ruthenium-catalyzed reduction of the CO2-releasing groups
during catalyst reduction. Second, the CO profiles for the sup-
ports and the catalysts are very similar, suggesting that the CO
groups are not involved in ruthenium anchoring. Finally, the
CO2 profiles are very different for the supports and the cata-
lysts (Figure 5). Thus, for CNTF and CNFF the CO2 is formed and
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P. Serp et al.
to nitrobenzene (NB) followed by subsequent hydrogenation
to AN, or by hydrodechlorination of the target product p-chlor-
oaniline (p-CAN) to AN. The selectivity may depend on the
support used; thus, simultaneous hydrogenation of the nitro
group and hydrogenolysis of chlorine in p-CNB was reported
for the reaction over a Pd/C catalyst, whereas on a Pd/polymer
catalyst hydrogenolysis of chlorine was a subsequent reac-
tion.[30] For ruthenium catalysts, the best selectivities (>99%)
towards p-CAN have been obtained on alumina[4] or SnO2 sup-
ports,[14] the other investigated supports such as silica[16] or
[15]
MgF2 permitting to reach high selectivity (>90%).
First, blank experiments using the CNTF and CNFF supports
were performed to evaluate the catalytic performances of the
unloaded supports; indeed the residual iron or nickel particles
might became accessible after reduction at 3008C and present
some activity. After 2 h of reaction at 608C and under 35 bar of
hydrogen the measured conversion and selectivity were 11%
and 50%, respectively, whatever the support used. Thus, the
contribution of the support can be neglected.
The first set of experiments was performed at a temperature
of 608C and a hydrogen pressure of 35 bar. Under these condi-
tions there were remarkable differences in the performances of
the different catalysts (Figure 6 and Table 3). First, we con-
firmed that the highest selectivity can be obtained on alumina
supports (98–100% selectivity towards p-CAN). The only unde-
sirable product was aniline. It is well-known that hydrodehalo-
genation on ruthenium concerns mainly haloanilines. On
carbon supports, activated carbon, or carbon nanostructures,
slightly lower selectivities were achieved (92–96%, C>CNFF >
CNTF). The most remarkable dif-
Figure 6. Catalytic properties of the various ruthenium catalysts: a) conver-
sion, and b) selectivity.
ference between these different
systems concerns the catalytic
Table 3. Catalytic activity of supported ruthenium catalysts in p-CNB hydrogenation.[a]
activity. Indeed, the use of nano-
structured carbon as a support
permits to increase the catalytic
activity by one order of magni-
tude when compared to the alu-
mina support, by a factor of 2 to
3 compared to activated carbon
if we consider the initial activity,
and by a factor of 4 to 10 if we
consider the activity at 100%
conversion. Regarding the struc-
ture of the nanocarbon sup-
ports, the CNFs permit to reach
higher activity and selectivity
[b]
[c]
Catalyst
Ru loading
[%]
Ru
[mg]
A100%
Ain.
Selectivity at 100%
conversion [%]
[molgRuꢀ1 hꢀ1
]
[molgRuꢀ1 hꢀ1
]
p-CAN
NB
AN
Ru/Al2O3 Fluka
Ru/Al2O3 Aldrich
Ru/C Engelhard
Ru/CNTF
Ru/CNFF
Ru/CNTF
5
0.5
5
2
1.5
2
0.48
0.48
0.48
0.72
0.54
0.72
0.54
0.59
1.3
1.1
0.4
5.8
12.3
18.2
14.1
10.4
100
98
96
92
94
94
93
0
0
0
2
1.7
7.1
18.8
7.6
4.7
0.5
1
0.5
0.5
1
3.5
7
5.5
5.5
7
[d]
[d]
Ru/CNFF
1.5
[a] P=35 bar; T=608C; p-CNB: 2.54 mmol. [b] Activity measured at 100% conversion; [c] Activity measured
after 15 min of reaction. [d] P=10 bar; T=408C; p-CNB: 2.54 mmol.
than the CNTs. In order to increase the selectivity over the Ru/
CNTF and Ru/CNFF catalysts, we performed a second set of ex-
periments at a temperature of 408C and a pressure of 10 bar
(Table 3). No significant improvement of the catalytic perform-
ances was noticed under these milder conditions. The decrease
of the activity in the case of the Ru/CNFF catalyst can be corre-
lated to the different adsorption coefficient of p-chloronitro-
benzene on the small ruthenium particles supported on CNF
surface. It is worth mentioning that the high activity measured
for the Ru/CNFF catalysts, which correspond to a TOF of
1900 hꢀ1, places this systems well above the ruthenium cata-
lysts reported in the literature for the same reaction performed
under similar conditions. Indeed, values of TOF ranging be-
tween 30 and 400 hꢀ1 have been reported for ruthenium-con-
taining catalysts operating between 60-1008C and PH2 of 10 to
40 bars.[12,14,16]
The increased activity obtained over nanostructured-carbon-
supported ruthenium catalyst may have several explanations.
First, the use of CNF or CNT supports, which present a very
open structure permit to avoid diffusion limitations and main-
tain a high hydrogen flux at the catalyst surface and an easy
access to the reaction sites.[19,22] Second, these supports are
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Hydrogenation over Nanostructured-Carbon-Supported Ruthenium
nium was calculated to reach 5 wt%. The impregnated sample was
filtered and washed thoroughly with ethanol, and dried at 1208C
overnight. The catalysts were subsequently reduced in flowing H2/
Ar mixture (80:20 in volume) at 3008C for 2 h.
electron conductors, and charge transfer that modifies the
electronic density at ruthenium centers could occur. A first-
principles study of ruthenium clusters adsorbed on carbon
nanotubes has shown that a charge transfer occurs from metal
to carbon[31] the transition metal atoms adsorbed on carbon
nanotubes donating electrons to the nanotube. It has also
been proposed that the generation of a positive charge on the
ruthenium centers should increase the catalytic activity for p-
CNB hydrogenation.[13] We should also consider the possible in-
fluence of the polarity of the support surface. Indeed, it has
been evidenced that in the case of cinnamaldehyde hydroge-
nation over catalysts supported on CNTs[32] or CNFs,[33] the con-
centration of oxygenated surface species on the nanostruc-
tured supports strongly influences the catalytic activity of
these systems: the lower the concentration of oxygenated sur-
face groups, the higher the activity. Finally, we cannot rule out
an influence of the degree of metal dispersion, even though it
has been demonstrated both for ruthenium and platinum cata-
lysts that the catalytic activity for p-CNB hydrogenation de-
creases when increasing the metal dispersion.[4]
Catalyst characterization
TEM images of the samples were taken on a JEOL 1011 transmis-
sion electron microscope. High-resolution images were obtained
on a JEOL JEM 2100F transmission electron microscope with a field
emission gun (TEM-FEG). Average ruthenium nanoparticle diame-
ters were calculated from statistical distributions of measurements
made on 200 particles per sample and the CNT and CNF diameter
distribution was measured by counting about 200 CNTs or CNFs
on the TEM images. The X-ray diffraction (XRD) patterns were ob-
tained using a modern multipurpose theta/theta powder X-ray dif-
fraction system, equipped with a fast linear detector. Micro Raman
spectra were taken on powder samples on a Perkin–Elmer 400F
Raman spectrometer with 785 nm red laser irradiation. Thermogra-
vimetric analysis (TGA) of the composite materials was carried out
with a SETSYS Evolution (SETARAM Instrumentation) at a heating
rate of 208Cminꢀ1 under air flow.
BET: The textural characterization of the materials was based on
the nitrogen adsorption–desorption isotherms, determined at 77 K
with a Quantachrome NOVA 4200e multi-station apparatus. The
specific surface area (SBET) was calculated by multipoint BET analy-
sis of the isotherm in the relative pressure range from 0.05 to 0.3.
TPR/TPD: Spectra were obtained with a fully automated AMI-200
Catalyst Characterization Instrument (Altamira Instruments),
equipped with a thermal conductivity detector (TCD) and a quad-
rupole mass spectrometer (Dymaxion 200 amu, Ametek). For TPR
experiments, the sample was placed in a U-shaped quartz tube lo-
cated inside an electrical furnace and heated at 5 Kminꢀ1 to 973 K
under a 5% (v/v) H2 flow diluted with He (total flow rate of
30 cm3 minꢀ1, STP); for TPD, the sample was heated to 1373 K
using a constant flow rate of helium (25 cm3 minꢀ1, STP). The H2
consumption was followed by both TCD and mass spectrometry.
The amounts of CO and CO2 released during the thermal analysis
were calibrated at the end of each analysis.
Conclusion
In summary, to the best of our knowledge, this is the first
time that nanostructured carbons (CNFs and CNTs) have been
used as supports for ruthenium nanoparticles for the hydroge-
nation of p-chloronitrobenzene. The preparation of well-dis-
persed Ru/CNTF and Ru/CNFF catalysts by wet impregnation
from the Ru0 organometallic cluster [Ru3(CO)12] required an ac-
tivation of the purified supports with HNO3. These catalysts ex-
hibit excellent performances in the liquid-phase hydrogenation
of p-CNB and produced p-CAN with 94% selectivity and very
high activity for a ruthenium catalyst. The results show that
the as-synthesized Ru/CNFF and Ru/CNTF catalysts have great
potential as cheap and active catalysts for the production of
chloroanilines. Studies to elucidate the details of the catalytic
mechanism of the Ru/CNFs and Ru/CNTF systems and on the
improvement of their catalytic properties are in progress.
pHPZC: The determination of the point of zero charge (pHPZC) of the
sample was carried out according to a procedure described else-
where.[34] Briefly, 50 mL of a NaCl solution (0.01m) was placed in a
closed Erlenmeyer flask; the pH was adjusted to a value between 2
and 12 using HCl (0.1m) or NaOH (0.1m) following which 0.05 g of
each sample was added. The final pH was measured after 72 h
continuous stirring at room temperature.
Experimental Section
Carbon nanostructures synthesis
The carbon nanotubes and carbon nanofibers were produced by
chemical vapor deposition from ethylene in the presence of hydro-
gen on iron and nickel monometallic catalysts, respectively, sup-
ported on hydroxyapatite. The as-produced samples were purified
by HCl washing during 12 h at room temperature, then filtered,
washed with deionized water repeatedly until the pH of the filtrate
was around 7, and dried for 3 days in an oven at 1108C to produce
CNTP and CNFP. The complete characterization of these materials is
given in the Results and Discussion section.
Catalytic activity measurements
The hydrogenation reaction was conducted between 313 and
333 K under a hydrogen pressure of 10 to 35 bar in an autoclave
reactor under stirring (750 rpm). Typically, 0.4 g (2.54 mmol) of p-
chloronitrobenzene and the prereduced catalyst (0.036 g) in 50 mL
methanol were added to the autoclave. The reaction lasted for 2 h.
Samples were extracted every 15 min and the products were ana-
Catalyst preparation
The CNTP and CNFP samples were oxidized in acid HNO3 at 413 K
for 4 h (CNTF) and 333 K for 2 h (CNFF), then washed with deion-
ized water repeatedly until the pH of the filtrate was around 6, and
then dried at 1208C for 12 h. The ruthenium catalysts were pre-
pared by a conventional impregnation procedure from trirutheni-
um dodecacarbonyl, 99% in hexane at 293 K. The loading of ruthe-
lyzed using
a gas chromatograph (Perkin–Elmer Clarus 500,
equipped with
detector).
a
stabilawx DA capillary column and a FID
ChemSusChem 2011, 4, 950 – 956
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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