Carbon supported Pt colloid as effective catalyst for selective hydrogenation of nitroarenes to arylhydroxylamines

Article abstract of DOI:10.1039/b916686f

The Pt colloid supported on carbon is an active and selective catalyst for the partial hydrogenation of nitroaromatics with electron-withdrawing substituents to the corresponding N-arylhydroxylamine, indicating an additive-free green catalytic approach fo

Full text of DOI:10.1039/b916686f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Carbon supported Pt colloid as effective catalyst for selective
hydrogenation of nitroarenes to arylhydroxylaminesw
Zeming Rong, Wenqiang Du, Yue Wang and Lianhai Lu*
Received (in Cambridge, UK) 13th August 2009, Accepted 7th December 2009
First published as an Advance Article on the web 8th January 2010
DOI: 10.1039/b916686f
The Pt colloid supported on carbon is an active and selective
catalyst for the partial hydrogenation of nitroaromatics
with electron-withdrawing substituents to the corresponding
N-arylhydroxylamine, indicating an additive-free green catalytic
approach for arylhydroxylamine synthesis.
reaction kinetics of 2,4-dinitrotoluene over Pd/C11 and found
that during the reaction the yield of intermediate arylhydroxyl-
amine was about 82%. Oemer et al. studied the hydrogenation
kinetics of 2,6-dinitrotoluene using Pt/C and Pt/Al2O3
as catalysts and got 2-amino-6-nitrotoluene as the main
intermediate.12 Westerterp et al. studied the kinetics of the
catalytic hydrogenation of 2,4-dinitrotoluene over a 5 wt%
Pd/C using methanol as solvent. The reaction was conducted
in a batch slurry reactor under the conditions of 2 MPa and
308 K, and the maximum selectivity of 4-hydroxylamino-2-
nitrotoluene was 79.3%.13 It is obvious that the direct
synthesis of HA through selective catalytic hydrogenation of
NA is a difficult task. Therefore, it is important to have a deep
understanding of the reaction mechanism and to design
an active, selective and additive-free hydrogenation catalyst
system for the clean and efficient synthesis of HA.
N-Arylhydroxylamine (HA) usually exists as an intermediate
during catalytic hydrogenation of various nitroarenes (NA) to
aromatic amines.1 The amount of HA accumulated in the
hydrogenation process changes dramatically according to the
properties of substrates, reaction conditions and different
catalyst systems. Since HA is generally very active and can
be converted to many high-valued fine chemicals,2,3 the direct
synthesis of HA has been an attractive topic to synthetic
chemists for decades.4 However, it is extremely difficult to
get high HA yield by conventional catalytic hydrogenation
techniques.
Pt/C is
a well-known catalyst material and a solid
knowledge has been accumulated on it as a mild selective
hydrogenation catalyst and effective electrocatalyst. It has
been well accepted that the preparation method and the
dispersion of Pt particle on C play very important roles in
its catalytic performance. Pt colloid has recently become
attractive.14–16 The Pt/C prepared from colloid method
showed unique properties and excellent electrocatalytic activity.16
In this communication, we report at the first time a green
and highly efficient catalytic methodology for chemoselective
hydrogenation of various nitroaromatic compounds into the
corresponding HA using Pt colloid on carbon as catalyst. The
catalyst is especially effective for those NA containing
electron-withdrawing substituents. Without adding any
chemical promoters, high HA selectivity (maximum 96.5%)
and almost 100% NA conversion were obtained, indicating a
general catalytic methodology for HA production.
HA has been synthesized from NA by chemical reduction
using Zn/NH4Cl(HCOONH4) in aqueous suspension,5
biocatalytic reduction using baker’s yeast or plant cells,6 and
selective hydrogenation using noble metal catalysts with
a large amount of organic additives. Typical additives
are DMSO,7 divalent sulfur compounds,8 organic bases
(monoamine, pyrrolidine, piperidine, pyridine)9 and organic
phosphorus compounds.10 However, the conventional
chemical reduction method has been challenged due to serious
pollution, and the biological reduction has the shortcoming of
low efficiency with long reaction time using a large quantity of
biocatalyst. The selective catalytic hydrogenation reduction
seems to be a better choice for HA preparation, which is
environmentally benign and offers several advantages
over chemical and biochemical reductions. However, the
knowledge on the chemoselective catalytic hydrogenation of
NA to HA has been so far very limited. Only some patents
claimed Pd/C and Pt/C as catalyst with the help of a large
amount of selectivity-improving organic additives. Despite the
serious shortcoming of separation, purification and further
application of HA in the additive-containing system as
reported in patents, the HA yield is still low and far from ideal.
There have been some publications concerning the organic
additive-free hydrogenation of NA. Neri et al. studied the
The catalytic hydrogenation reaction was carried out in a
70 ml stainless-steel autoclave. All products were analyzed by
HPLC and LC-MS. The Pt colloid on carbon catalyst was
prepared according to the literature.16 Briefly, H2PtCl6
was quantitatively dissolved in deionized H2O containing
Tween-20 and Brij-35. Aqueous NaBH4 was added dropwise
to the solution which soon turned into a dark black stable
colloid solution. Under vigorous stirring, active carbon was
added to the colloid solution. The Pt/C catalyst was obtained
by filtration, washing with hot H2O to remove the surfactant
from the carbon surface. The particle size of Pt is about 3 nm.
The surface morphology and structural characteristics of the
Pt/C as well as experimental details are described in the
supporting information (ESI)w and in the literature.16
State Key Laboratory of Fine Chemicals, School of Chemical
Engineering, Dalian University of Technology, Dalian 116012, China.
E-mail: lianhai@dlut.edu.cn; Fax: +86-411-3989 3800;
Tel: +86-411-3989 3836
w Electronic supplementary information (ESI) available: Catalytic
hydrogenation experiment, analytical and spectral characterization
data, preparation and characterization of catalyst. See DOI:
10.1039/b916686f
Table 1 summarizes the results of the catalytic hydrogena-
tion of several substituted NA to the corresponding HA over
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1559–1561 | 1559

View Article Online
Table 1 Chemoselective hydrogenation of different NA to HA over
carbon supported Pt colloid
t/
min
Conv./ Sel./
%
Entry Substrate
Product
%
1a
190 100
170 100
92.3
Scheme 1 Reaction pathway for the hydrogenation of m-DNB.
unchanged. Di- and tri-substituted NA were also investigated.
3,5-Dinitrobenzoic acid (entry 7) gave the highest selectivity of
96.5% at 100% conversion. The hydrogenation of 1-chloro-
2,4-dinitrobenzene (entry 8) resulted in two isomeric HA
products. The para-nitro group in 1-chloro-2,4-dinitrobenzene
is easier to be hydrogenated than the ortho-nitro group due to
a steric effect.
2b
91.9
3b
4b
115 100
65 100
86.3
86.3
We have recently reported the highly selective partial hydro-
genation of m-dinitrobenzene (m-DNB) to m-nitroaniline (c2)
over Ru/C.17 In order to explore the hydrogenation reaction
mechanism for the formation of N-(3-nitrophenyl)hydroxyl-
amine (b), m-DNB was chosen as a model substrate. The
whole reaction pathway is illustrated simply in Scheme 1.
The hydrogenation of m-DNB should be practically more
complicated, nitroso compounds and trace amounts of dimer
may also exist in the reaction system. However, only the
compounds illustrated in Scheme 1 were detected by HPLC
and GC. The material balance was good, indicating that other
impurities were negligible. Effects of different catalyst and
reaction parameters were studied. It was proven that mild
reaction conditions favour the formation of HA (b) and THF
is a suitable solvent with better effect than ethanol and
methanol. Table 2 showed the typical comparison results of
different catalysts and solvents. Under the reaction conditions,
the maximum HA (b) yield over colloid Pt/C is 92.3% at 100%
conversion, while the maximum yield over Ru, Pd and Ni
catalysts were notably lower than Pt. Though we tried to
increase the performance of Ru, Pd and Ni catalysts by
optimizing the reaction parameters, the HA (b) yield could
not be further increased. For all the catalyst systems, different
amount of byproducts m-dihydroxylaminobenzene (c1),
m-nitroaniline (c2), m-hydroxylaminoaniline (d) and complete
hydrogenation product m-diaminobenzene (e) were observed.
Several commercial Pt/C and Pt/Al2O3 catalysts were also
tested and typical results are listed in Table 2. It is obvious
that our colloid Pt/C showed the highest yield of HA.
5c
6c
210 100
120 100
93.4
90.7
7a
120 100
96.5
51.1
38.2
93.7
8a
115 100
9a
135 100
a
Reaction conditions: 283.15 K, 0.1 MPa. 10 mmol substrate,
b
30 ml THF, 50 mg 2 wt% Pt/C. 2 mmol substrate, 10 ml THF,
c
10 mg 2 wt% Pt/C. 10 mmol substrate, 30 ml THF, 100 mg 2 wt%
Pt/C. The conversion and selectivity were determined by HPLC.
carbon supported Pt colloid catalyst. Since HA was generally
unstable and can be easily further hydrogenated to aromatic
amine, mild reaction conditions of 283.15 K and 0.1 MPa were
adopted in this study to retard the formation of amine. The
Pt/C prepared from the colloid route showed excellent activity
and selectivity under mild conditions. As a contrast, several
commercial Pt/C samples prepared by the impregnation
method by different makers showed lower selectivity under
the same reaction conditions. It is reasonable to infer that the
excellent catalytic performance of colloid Pt/C is due to the
function of finely dispersed and unformed Pt nano particle on
carbon. Nine kinds of NA with different substituents including
nitryl, carbonyl, ester and carboxyl were investigated.
As shown in entries 1–3, the complete reduction of m-, o-
and p-dinitrobenzene gave the HA yield of 92.3%, 91.9% and
86.3%, respectively. The selectivity for the partial reduction
of 1-(4-nitrophenyl)ethanone (entry 4) was 86.3% at 100%
conversion and the reducible carbonyl group remains
Table 2 Chemoselective hydrogenation of m-DNB to m-hydroxyl-
aminonitrobenzene over different catalyst and solvent
Weight
(g)
t/
min
Maximum
yield/%
Catalyst
Solvent
3.5 wt% Ru/C
3.5 wt% Pd/C
RANEYs Ni
2 wt% Pt/Al2O3
2 wt% Pt/Cb
2 wt% Pt/C
0.5
0.2
0.5
0.2
THF
THF
THF
THF
THF
THF
Ethanol
Methanol
475
80
66.6
86.7
86.4
90.7
90.9
92.3
78.3
91.0
180
150
240
190
160
73
a
0.2
0.05
0.05
0.05
2 wt% Pt/C
2 wt% Pt/C
Reaction conditions: 283.15 K, 0.1 MPa, 10 mmol substrate, 30 ml
a
solvent. Commercial Pt/Al2O3 from Dalian Tongyong Chemicals,
b
China. Commercial Pt/C from Donggang Chemicals, China.
ꢀc
This journal is The Royal Society of Chemistry 2010
1560 | Chem. Commun., 2010, 46, 1559–1561

View Article Online
We are grateful to the National High Technology Research
and Development Program of China (863 Program, Grant No
2007AA03Z345) and the Program for Changjiang Scholars
and Innovative Research Team in University (IRT0711).
Notes and references
1 (a) A. Corma, P. Concepcion and P. Serna, Angew. Chem., Int. Ed.,
2007, 46, 7266–7269; (b) E. A. Gelder, S. D. Jackson and
C. M. Lok, Chem. Commun., 2005, 522–524.
2 (a) K. Shudo and T. Okamoto, Tetrahedron Lett., 1973, 21,
1839–1842; (b) K. Shudo, T. Ohta and T. Okamoto, J. Am. Chem.
Soc., 1981, 103, 645–653; (c) H. C. Ming and L. T. Chu, New J.
Chem., 2000, 24, 859; (d) T. Hiroshi, T. J. Ichi, H. Suguru,
T. Keisuke and O. Yoshinori, Eur. J. Org. Chem., 2003, 3920;
(e) P. M. Vyas, S. Roychowdhury, P. M. Woster and
C. K. Svensson, Biochem. Pharmacol., 2005, 70, 275–286;
(f) R. J. Knox, F. Friedlos, T. Marchbank and J. Roberts,
Biochem. Pharmacol., 1991, 42, 1691; (g) G. M. Anlezark,
R. G. Melton, R. F. Sherwood, W. R. Wilson, W. A. Denny,
B. D. Palmer, R. J. Knox, F. Friedlos and A. Williams, Biochem.
Pharmacol., 1995, 50, 609; (h) E. Johansson, G. N. Parkinson,
W. A. Denny and S. Neidle, J. Med. Chem., 2003, 46, 4009.
3 (a) K. J. Ratnam, R. S. Reddy, N. S. Sekhar, M. LakshmiKantam,
A. Deshpande and F. Figueras, Appl. Catal., A, 2008, 348(1),
26–29; (b) E. Bamberger, Ber. Dtsch. Chem. Ges., 1894, 27,
1347–1350.
4 (a) F. Figueras and B. Coq, J. Mol. Catal. A: Chem., 2000, 173,
223–230; (b) M. Bend, K. Hartmann and K. Reinhard, U.S. Patent
6054592, 2000; (c) T. Hiroshi, T. J. Ichi, H. Suguru, T. Keisuke and
O. Yoshinori, Eur. J. Org. Chem., 2003, 3920; (d) P. M. Vyas,
S. Roychowdhury, P. M. Woster and C. K. Svensson, Biochem.
Pharmacol., 2005, 70, 275–286.
5 (a) S. Ung, A. Falguiers, A. Guy and C. Ferroud, Tetrahedron
Lett., 2005, 46, 5913; (b) O. Kamm and C. S. Marvel, Org. Synth.,
1941, I, 445; (c) P. G. Gassman and J. E. Grandrud, J. Am. Chem.
Soc., 1984, 106, 1498; (d) F. G. Bordwell and W. Liu, J. Am. Chem.
Soc., 1996, 118, 8777; (e) T. Beissel, R. E. Powers, T. N. Parac and
K. N. Raymond, J. Am. Chem. Soc., 1999, 121, 4200; (f) Q. X. Shi,
R. W. Lu, K. Jin, Z. X. Zhang and D. F. Zhao, Chem. Lett., 2006,
35, 226.
Fig. 1 The concentration–time plots for the selective hydrogenation
of m-DNB to HA over Pt/C. Reaction conditions: 283.15 K, 0.1 MPa,
10 mmol m-dinitrobenzene, 30 ml THF, 50 mg 2wt% Pt/C. The
conversion and selectivity were determined by HPLC.
The relationships of component concentration versus
reaction time are shown in Fig. 1. The concentration of
m-DNB decreased almost linearly with reaction time, suggesting
that the reaction has dynamically pseudo-zero order
dependence with m-DNB. At the early stage of the reaction,
the selectivity to N-(3-nitrophenyl)hydroxylamine (b) was very
high only with a trace amount of byproducts, c2. As
the reaction time increase, c1 gradually appeared and
accumulated, then d and e were formed in sequence. During
the whole hydrogenation course, c2 was the main byproduct
and the total amount of c1, d and e was much lower than c2.
As the concentration of m-DNB decrease, the amount of b
increases almost linearly and its yield reaches the highest value
of 92.3% at 100% m-DNB conversion.
6 (a) F. Li, J. N. Cui, X. H. Qian and R. Zhang, Chem. Commun.,
2004, 2338–2339; (b) F. Li, J. N. Cui, X. H. Qian, R. Zhang and
Y. Xiao, Chem. Commun., 2005, 1901–1903.
7 (a) P. N. Rylander, I. M. Karpenko and G. R. Pond, US Patent
3694509, 1972; (b) G. C. Davis, US Patent 4723030, 1988.
8 D. C. Caskey and D. W. Chapman, E.P. Patent 0086363, 1983.
9 J. L. Ludec, U.S. Patent 3927101, 1975.
10 A. H. Sharma, P. Hope and U. S. Patent, 5166435, 1992.
11 (a) G. Neri, M. G. Musolino, C. Milone, A. M. Visco and A. Di
Mario, J. Mol. Catal. A: Chem., 1995, 95, 235–241; (b) G. Neri,
M. G. Musolino, E. Rotondo and S. Galvagno, J. Mol. Catal. A:
Chem., 1996, 111, 257–260; (c) G. Neri, M. G. Musolino,
L. Bonaccorsi, A. Donato, L. Mercadante and S. Galvagno, Ind.
Eng. Chem. Res., 1997, 36, 3619–3624; (d) G. Neri,
M. G. Musolino, C. Milone, D. Pietropaolo and S. Galvagno,
Appl. Catal., A, 2001, 208, 307–316.
The hydrogenation of NA is a typical consecutive reaction
with HA as the first intermediate and amine as the final
product. Pt colloid on carbon is very active and can catalyze
the hydrogenation under very mild conditions, which con-
sequently stop the consecutive hydrogenation reaction in the
middle and accumulates the intermediate HA to be the main
product. In addition, when NA has electron-withdrawing
substituent in the phenyl ring, the N–O bond in the hydroxyl-
amino group was strengthened by the electron-withdrawing
groups through the p-conjugation of the phenyl ring,18 which
effectively stabilize the HA and retard its further converting.
As a matter of fact, the formation rate of HA in the reaction
system is much higher than its disappearing. The reaction
termination for HA production can be easily observed from
the H2 up-taking rate. When NA was completely converted,
the H2 up-taking rate changed to be obviously slow.
12 O. M. Kut, F. Yucelen and G. Gut, J. Chem. Technol. Biotechnol.,
¨
1987, 39(2), 107–114.
13 H. J. Janssen, A. J. Kruithof, G. J. Steghuis and K. R. Westerterp,
Ind. Eng. Chem. Res., 1990, 29(5), 754–766; H. J. Janssen,
A. J. Kruithof, G. J. Steghuis and K. R. Westerterp, Ind. Eng.
Chem. Res., 1990, 29(9), 1822–1829.
14 T. S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein and M. A.
Ei-Sayed, Science, 1996, 272, 1924–1925.
15 R. Narayanan and M. A. Ei-sayed, Nano Lett., 2004, 4, 1343–1348.
16 D. H. Lim, L. H. Lu, D. B. Kim, D. H. Choi, D. R. Park and
H. I. Lee, J. Nanopart. Res., 2008, 10, 1215–1220.
17 J. Hou, Y. H. Ma, Y. H. Li, F. Guo and L. H. Lu, Chem. Lett.,
2008, 37, 974–975.
18 R. K. Rains, E. A. Lambers, R. A. Genetti, F. A. LP and
O. Akron, Catalysis of Organic Reactions, CRC Press, 1996,
pp. 43–52.
In summary, this communication provides an effective
method to obtain HA by the partial hydrogenation of NA
over carbon supported Pt colloid catalyst without using any
chemical promoters. Very good results were obtained with NA
bearing electron-withdrawing substituents. Since NA can be
further converted to highly valuable compounds through
several reactions like Bamberger rearrangement, this result
will generally contribute to the synthetic methodology of NA
derivatives.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 1559–1561 | 1561