RhNPs/SBA-NH2: A high-performance catalyst for aqueous phase reduction of nitroarenes to aminoarenes at room temperature

Article abstract of DOI:10.1039/c4cy00143e

A RhNPs/SBA-NH₂ catalyst with <3 nm sized rhodium nanoparticles has been synthesized using amine functionalized SBA-15 (SBA-NH₂) as support, rhodium acetyl acetonate as a Rh precursor and sodium borohydride (NaBH₄) as a re

Full text of DOI:10.1039/c4cy00143e

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RhNPs/SBA-NH : a high-performance catalyst for
2
aqueous phase reduction of nitroarenes to
aminoarenes at room temperature
Cite this: DOI: 10.1039/c4cy00143e
Saidulu Ganji, Siva Sankar Enumula, Ravi Kumar Marella, Kamaraju Seetha Rama Rao
and David Raju Burri*
2
A RhNPs/SBA-NH catalyst with <3 nm sized rhodium nanoparticles has been synthesized using amine
Received 5th February 2014,
Accepted 12th March 2014
functionalized SBA-15 (SBA-NH ) as support, rhodium acetyl acetonate as a Rh precursor and sodium
2
borohydride (NaBH
aminoarenes with nearly 100% conversion and selectivity using in situ produced H
hydrate (NH –NH ·H O) at room temperature in water medium and the estimated turnover frequency is
as high as 6117 h .
4
) as a reducing agent. RhNPs/SBA-NH
2
effectively reduced the nitroarenes into
2
from hydrazine
DOI: 10.1039/c4cy00143e
2
2
2
−
1
www.rsc.org/catalysis
1
6
stability. These features provide a consistent and well-
isolated environment for the guest molecules. Due to porous
silica support characteristics, agglomeration of nanoparticles
gets minimized. Furthermore, the intrinsic advantages of
SBA-15 supported nanoparticles catalysts are the ease of cata-
lyst recovery and product separation.
Introduction
The reduction of nitroarenes to aminoarenes is an indus-
trially important reaction because of the wider utility of
aminoarenes in the production of pharmaceuticals, dyes, pig-
1
–4
ments and pesticides.
Particularly, the aminoarenes are
used as intermediates for the synthesis of a variety of biologi-
cally active compounds including chemopreventive drugs,
like Glu-P-1 and Trp-P-2, which can protect against several
Of late, simultaneous production and utilization of hydro-
gen in hydrogenation reactions has recently been emerging
as a prominent area of research in heterogeneous catalysis
5
,6
11a,17
types of cancers. Despite a considerable amount of work
devoted to the selective reduction of nitroarenes over Pd, Pt,
such as transfer hydrogenation,
coupling of hydrogena-
1
8
tion and dehydrogenation reactions,
decomposition of
7
–11
19
Ru, Rh, Au etc.,
some of the drawbacks like usage of envi-
reducing agents into H2 and its simultaneous utilization.
Recently, the generation of H by the decomposition of
hydrazine over Rh nanoparticles at room temperature has
ronmentally harmful organic solvents, long reaction times,
and high temperature and high pressure operations are still
preventing their potential applications.
2
2
0
been described. In general, the decomposition of hydrazine
takes place in two pathways, which are given as follows:
(i) Complete decomposition
Among the principles of Green Chemistry, the use of sol-
vents is a key aspect. However, the use of volatile organic sol-
vents gives rise to toxic and hazardous waste. At this
juncture, the use of water instead of volatile organic solvents
as reaction medium is an important aspect to gain the eco-
H
2
NNH
(ii) Incomplete decomposition
3 H NNH → 4NH + 2N (g)
2
→ N
2
(g) + 2H
2
(g)
(i)
1
2,13
nomical and ecological benefits,
and in addition, the
time taken for heterogeneous type of organic reactions in
water is dramatically shorter compared to the homogeneous
(ii)
2
2
3
2
1
4
type of reactions, which is of great industrial importance.
In recent years, mesoporous materials have been widely
Recently, Luo et al. reported that a Rh/PICP nanocatalyst
1
5
used in absorption, separation, catalysis etc. Among them,
there is an increasing interest in SBA-15 type materials as
host materials for the synthesis and spatial dispersion
of metal nanoparticles due to their unique properties like
high surface area, uniform larger pores and high thermal
showed efficient reduction of nitroarenes using hydrazine
2
1
hydrate as a reducing agent at 60 °C. In a recent report,
gold nanoparticles stabilized on nanocrystalline magnesium
oxide actively catalyzed the selective reduction of nitroarenes
with sodium borohydride in aqueous medium at room tem-
2
2
perature. Gao and his co-workers also reported that the
reduced graphene oxide catalyzed the hydrogenation of nitro-
Catalysis Laboratory, Indian Institute of Chemical Technology, Hyderabad 500607,
India. E-mail: david@iict.res.in; Fax: +91 40 27160921; Tel: +91 40 27193232
2
3
20
benzene at room temperature. As reported by Singh et al.,
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hydrazine hydrate is an eco-friendly material, which contains
angles further confirms the existence of smaller metallic
nanoparticles on SBA-15 support as reported in the
literature.
a large amount of H (7.9 wt%) and can be easily decomposed
2
2
4
to H
2
and N
2
at RT with RhNPs. However, to the best of
our knowledge, no catalyst that catalyses the reduction of
nitroarenes to aminoarenes with hydrazine hydrate at room
temperature in aqueous medium with 100% yields at the
Fig. 2 shows the N adsorption–desorption isotherms and
2
pore size distribution curves (inset) of SBA-15, SBA-NH and
2
2
RhNPs/SBA-NH . These isotherms could be classified as type
IV characteristic of mesoporous materials. SBA-15, SBA-NH2
−
1
TOF of >6000 h is reported. Hence, herein, the stabilization
of RhNPs on SBA-NH (RhNPs/SBA-NH ), its characterization
and RhNPs/SBA-NH are uniform in size, which is indicated
2
2
2
and breakthrough catalytic activity including reusability have
been described.
2
by the sharp increase in the amount of N adsorbed. The
pore size distributions of the samples (inset in Fig. 2) were
calculated from the N2 adsorption isotherm following the
Barrett–Joyner–Halenda (BJH) method, revealing the narrow
pore distribution in the range of 5–10 nm.
Results and discussion
The structural aspects of the RhNPs/SBA-NH2 catalyst were
investigated by X-ray diffraction analysis. As manifested in
Fig. 1, three well-resolved reflections of (100), (110) and (200)
could be clearly indexed in the hexagonal space group of
p6mm, exhibiting the well-defined and ordered pore struc-
tures of the SBA-15, SBA-NH and RhNPs/SBA-NH samples.
As shown in Table 1, the gradual decrease in BET surface
area, total pore volume and BJH pore diameter confirms the
functionalization of propylamine and deposition of RhNPs.
No substantial variation is observed in the structural parame-
ters like d spacing, unit cell length and pore wall thickness,
confirming the retention of hexagonally ordered SBA-15
2
2
As inset in Fig. 1, an XRD pattern obtained for a RhNPs/
SBA-NH catalyst in the wide-angle region is displayed, which
shows the characteristic reflection (111) plane of metallic
rhodium according to JCPDS card no. 87-0714. It is important
2 2
structures both in SBA-NH and RhNPs/SBA-NH catalysts.
Transmission electron microscopy (TEM) images were
taken to analyze the distribution of rhodium species and to
2
2
directly observe the morphology of the RhNPs/SBA-NH cata-
to note that the diffraction peak of the RhNPs/SBA-NH
lyst is very broad and weak, suggesting the existence of very
small crystalline RhNps on the surface of SBA-NH . The
absence of large and intense diffraction peaks at higher
2
cata-
lyst. As shown in Fig. 3, RhNPs were highly dispersed in the
mesopore channels of SBA-NH . The size distribution histo-
2
2
grams for the catalyst were obtained by manually measuring
the particles from the TEM images. The RhNPs are
Fig. 2
2
N adsorption–desorption isotherms and pore size distribution
2 2
Fig. 1 XRD patterns of the SBA-15, SBA-NH and RhNPs/SBA-NH catalysts.
curves (inset) of the samples.
2 2 2
Table 1 Textural and structural parameters of the SBA-15, SBA-NH and RhNPs/SBA-NH samples obtained from N sorption data and a low-angle
XRD analysis, respectively
a
2
−1
b
t
−1
c
d
e
0
f
Catalyst
S
BET (m g
)
V
(cc g
)
D (nm)
d
100 (nm)
a
(nm)
t (nm)
SBA-15
SBA-NH
RhNPs/SBA-NH
699
536
451
1.06
0.99
0.84
6.1
5.8
5.8
9.1
9.3
9.8
10.5
11.1
11.2
4.4
5.3
5.4
2
2
a
b
c
d
e
BET surface area. Total pore volume. BJH pore diameter. Periodicity of SBA-15 derived from a low angle XRD. Unit cell length
f
(a
0
= 2d100/√3). Pore wall thickness (t = a − D).
0
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2
Fig. 3 TEM image of a RhNPs/SBA-NH catalyst.
2 2
Fig. 6 FT-IR spectra of the SBA-15, SBA-NH and RhNPs/SBA-NH
samples.
distributed in the range of 1.5–3 nm. A similar kind of high
and uniform dispersion of PdNPs in the pore channels of
2
5
2
SBA-NH was described in one of our recent publications.
Fig. 4 shows the SEM image of RhNPs/SBA-NH , which
indicates that the particles are freely distributed and are
cylindrical in shape. The EDX shows that the amount of Rh
Fig. 6 shows the FT-IR spectra of SBA-15, SBA-NH2 and
RhNPs/SBA-NH , recorded in the region of 4000–400 cm .
2
2
−
1
As shown in Fig. 6, the infrared spectrum of SBA-15
is 2.06 wt% in the RhNPs/SBA-NH sample.
exhibits the typical Si–O lattice vibration in two regions
2
−
1
−1
The XPS spectrum of a RhNPs/SBA-NH
2
sample is shown
between 1450–900 cm and 900–450 cm . Additionally, two
−
1
−1
in Fig. 5, in which the 3d5/2 and 3d3/2 peaks of metallic Rh
appear at binding energies of 307.7 and 312.8 eV, respec-
tively. The XPS results are consistent with the reported litera-
bands appear at 3450 cm and 1635 cm due to the O–H
stretching and H–O–H (physisorbed water) bending vibra-
tions, respectively. Upon amine functionalization, a decrease
2
6
−1
ture for the confirmation of zero valent metallic Rh.
in the intensity of the bands at 3450, 800 and 960 cm is
observed, indicating the occurrence of a reaction between the
isolated silanol groups of SBA-15 surface with the methoxy
groups of aminopropyl trimethoxy silane. The 2950 and
−
1
2
893 cm bands in SBA-NH are assigned to ν of the –CH2
2 CH
−
1
groups, and the bands at 1544 and 1465 cm are attributed
to δNH of the –NH groups.
2
2
7
The catalytic activity of the RhNPs/SBA-NH catalyst in the
2
chemoselective hydrogenation of nitroarenes was carried out
at room temperature with hydrazine hydrate as a hydrogen
source in aqueous medium (Scheme 1).
Fig. 4 SEM image and EDAX of RhNPs/SBA-NH
2
.
Nitrobenzene is used as a model substrate in the chemo-
selective hydrogenation reaction. Different experiments were
conducted using nitrobenzene as a substrate with SBA-15 and
2
SBA-NH and without a catalyst. No substantial amount
of nitrobenzene conversions were obtained with the use of
parent SBA-15 and SBA-NH and without a catalyst (Table 2,
2
entries 1–3), confirming the need for a metal to decompose
the hydrazine hydrate and to perform the hydrogenation of
nitrobenzene as expected.
Fig. 5 X-ray photoelectron spectrum of RhNPs/SBA-NH
2
.
Scheme 1 Chemoselective hydrogenation of nitroarenes to aminoarenes.
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Table 2 Hydrogenation of nitrobenzene with different catalystsa
dispersion of RhNPs occurs in the pore channels rather than
agglomeration due to the confinement effect and electrostatic
interaction of amine functional groups. The RhNPs located
b
S. no
Catalyst
Conv. (%)
Yield (%)
TOF
1
2
3
4
No catalyst
SBA-15
0
0
0
0
0
0
—
—
—
in the mesopore channels of SBA-NH are highly accessible
2
to the reactant molecules while passing through the channels
during the course of the reaction. In other words, the RhNPs
containing mesopore channels are nothing but multi-tubular
reactors during the reaction.
In conventional hydrogenation reactions, a large excess of
H2 will be supplied through an external source, whereas
in the present process the required H2 will necessarily be
generated, i.e., the decomposition of hydrazine necessarily
occurred according to eqn (i) before the commencement of
nitroarene hydrogenation. It is important to note that the
SBA-NH
RhNPs/SBA-NH
2
2
100
99
6117
a
Nitrobenzene = 1 mmol, catalyst = 10 mg, hydrazine hydrate =
b
2
mmol, water = 3 ml, temperature = RT, time = 5 min. TOF =
−
1
turnover frequency, moles of product/moles of catalyst (h ).
Notably, nitrobenzene is completely reduced to aniline
with 99% yield (Table 2, entry 4) when RhNPs/SBA-NH was
2
−
1
used as a catalyst with a high turnover frequency, i.e., 6117 h ,
which is 6 times greater than that of the reported TOF of the
best catalyst, i.e., rhodium nanoparticles containing a porous
present RhNPs/SBA-NH
2
catalyst is capable of catalyzing
both the decomposition of hydrazine and hydrogenation of
nitroarenes simultaneously under mild reaction conditions
(1 atm pressure and room temperature) in aqueous medium
with 100% conversion in 5 min.
2
1
ionic copolymer (Rh/PICP) catalyst. Apart from its lower
activity compared to the present RhNPs/SBA-NH catalyst, the
2
Rh/PICP catalyst was used at a higher temperature (60 °C)
instead of RT with organic solvents (ethanol). However, the
study reveals that the supported rhodium nanoparticles as
In order to investigate the scope of the reaction with the
presented catalyst system, various functionalized nitroarenes
21
a catalyst and hydrazine hydrate as a H source are effective
were subjected to hydrogenation with a RhNPs/SBA-NH cata-
2
2
for the chemoselective reduction of nitroarenes to amino-
lyst under similar reaction conditions. The results are sum-
arenes compared to either noble metal (Pt, Pd, Ru etc.) cata-
2
marized in Table 4, which shows that RhNPs/SBA-NH was
2
8
lysts or base metal (Ag, Cu, Au etc.) catalysts. The activities
of the different important catalysts in terms of TOF are
presented in Table 3.
highly active and extremely selective for the hydrogenation of
all substrates without disturbing other functional groups,
indicating a high versatility of the catalyst.
Table 3 demonstrates that the present RhNPs/SBA-NH2
catalyst is far more active in the chemoselective reduction of
nitroarenes to aminoarenes under mild reaction conditions
in green water medium. The cutting-edge feature of this cata-
lyst is its high activity, which arises from its structural and
textural features including isolated distribution of tiny nano-
particles of rhodium in the mesopore channels of SBA-NH2,
which acts as a multi-tubular reactor in a diffusion-free envi-
ronment. As discussed in the preceding sections, similar to
that of SBA-15, hexagonally ordered mesopore channels exist
In order to understand the process of reaction, either
through the heterogeneous or homogeneous type, a hot filtra-
tion test was carried out with little modification. In general,
the hot filtration tests are being conducted at about 50% con-
version levels, but in the present study, the reaction time is
too short to conduct the hot filtration test. In the present
study, after completion of the reaction, the catalyst was sepa-
rated from the reaction mixture by a simple filtration. Nitro-
benzene and hydrazine hydrate were added to the filtrate,
and the reaction was run without adding a catalyst for 3 h
and then the product was analyzed by GC. No significant
increase in conversion was observed, revealing that the reduc-
tion of nitrobenzene proceeds in a heterogeneous manner,
and the filtrate was analyzed by ICP-AES for the estimation of
Rh, but no Rh species was detected in the filtrate, revealing
the absence of Rh leaching or below the detection limit of
the instrument (<7 ppb).
in the SBA-NH
the internal pore channels of the SBA-NH
than its external pore surface due to usage of a hydrophobic
2
support. Most of the RhNPs prefer to occupy
2
support rather
2
9
solvent
during the deposition of RhNPs, and isolated
Table 3 Efficiency of various reported catalysts including the present
RhNPS/SBA-NH catalyst
2
To verify the scale-up study, the reactants and catalyst were
taken 10 times higher and an extremely high turnover frequency
−1
Catalyst
Solvent
H
2
source T/°C TOF/h
Ref.
−
1
of 6116 h with 100% selectivity for aniline was found.
The recyclability of the RhNPs/SBA-NH sample was exam-
RhNPs/SBA
NAP–Mg–Au
Rh/PICP
Water
Water
Ethanol
Ethanol
N
NaBH
2
H
4
·H
4
2
O
RT
RT
60
6117
1.8
990
103
95
Present work
22
21
10
11a
11i
8h
0
2
N
N
2
H
H
4
·H
·H
2
O
O
ined in the chemoselective hydrogenation of nitrobenzene
under similar conditions and displayed in Fig. 7. After the
catalytic reaction, the catalyst was isolated from the liquid
phase by centrifugation, thoroughly washed with methanol,
and then reutilized as a catalyst in subsequent runs under
identical reaction conditions. The results included in Fig. 7
indicate that no efficiency loss was observed in the hydroge-
nation of nitrobenzene for up to six runs. The study reveals
Rh/Fe O
3 4
2
4
2
80
AuNPs magn. Ethanol TMDS
Au/TiO -VS CO/H EtOH/H
Pt nanowires Xylene
RT
RT
80
2
2
O
2
O
99
57
H
2
Fe(OAc)
Pd/CF
2
THF
Ethanol
THF
Toluene PhMe
THF
N
H
TMDS
2
H
2
4
·H
2
O
100
35
60
3.33 19a
307
7e
0.42 19b
0.79 19c
Fe(acac)
ReIO (PPh
NAP–Mg–Pd
2
2
3
)
2
2
SiH 110
RT
0
H
2
33
7f
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Table 4 Selective hydrogenation of various nitroarenes catalysed by
a
2
the RhNPs/SBA-NH catalyst
−
1
S. no
1
Substrate
Product
Conversion (%)
100
TOF (h
6117
)
2
3
4
100
100
100
6117
6117
3059
2
Fig. 7 Recyclability of the RhNPs/SBA-NH catalyst.
5
6
100
100
3059
3059
1
6,31
procedures.
In
a
typical synthesis,
a
solution of
EO PO EO : 2 M HCl : TEOS : H O = 2 : 60 : 4.25 : 15 (mass
2
0
70
20
2
ratio) was prepared, stirred for 24 h at 40 °C and then hydro-
thermally treated at 100 °C under static conditions for 24 h,
subsequently filtered, dried at 100 °C and calcined at 550 °C
for 8 h to yield mesoporous silica SBA-15.
7
8
100
100
3059
3059
2
SBA-NH was synthesized in accordance with our previous
2
5
publication. Aminopropyl triethoxy silane (APTES) (3 mL)
was added dropwise to a suspension of SBA-15 (1 g) in dry
2
toluene (30 mL), under a N atmosphere. The resulting mix-
9
100
100
3059
3059
ture was refluxed for 24 h. After that, the suspension was fil-
tered and washed thoroughly with toluene and ethanol. The
resulting solid was dried under vacuum at room temperature
10
and labeled as SBA-NH . The amount of nitrogen obtained
2
−
1
from CHN elemental analysis is 1.5 mmol g
RhNPs were prepared by using
.
a
hydrophobic
1
1
1
2
100
100
2039
2039
dichloromethane solvent as transport medium to inject a
rhodium precursor into the pores of the modified SBA-15. In
a typical procedure, a requisite amount of rhodium acetyl
acetone was dissolved in a dry dichloromethane solvent; to
2
this, SBA-NH was added and the solution was stirred at
room temperature, and then the solvent was evaporated.
The resulting solid was dried at room temperature for 12 h.
A solid catalyst was reduced with NaBH in isopropanol,
4
a
Substrate = 1 mmol, catalyst = 10 mg, hydrazine hydrate = 2 mmol,
water = 3 ml, temperature = RT.
filtered and washed thoroughly with water and methanol,
which was denoted as RhNPs/SBA-NH2.
that the Rh nanoparticles are well stabilized and are uni-
formly distributed on amine functionalized SBA-15 due to the
Characterization of catalysts
SBA-NH
2
pore confinement effect and NH
2
stabilizing nature,
The X-ray diffraction (XRD) patterns were recorded at room
temperature using an X-ray diffractometer (Multiflex, M/s.
Rigaku, Japan) with nickel filtered CuKα radiation. N2
adsorption–desorption isotherms were recorded on a N2
adsorption unit at −196 °C (Quadrusorb-SI V 5.06, M/s.
Quantachrome Instruments Corporation, USA). The samples
were out-gassed at 200 °C for 4 h before the measurement.
Transmission electron microscopy (TEM) analysis was made
using an FEI Technai G2 S-Twin Serial Number D 2083 at an
accelerating voltage of 200 kV. The XPS analysis was made on
which can explain the interaction of the holes formed in the
d-band of a metal with the lone pair electrons of an amine
functional group.
3
0
Experimental
Preparation of the catalyst
Siliceous SBA-15 was synthesized by using P123 as a template
and TEOS as a silica source in accordance with the literature
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a photoelectron spectrometer (KRATOS Axis 165, Shimadzu,
Japan) with Mg Kα radiation (1253.6 eV). Rh contents of the
sample were analysed with a simultaneous ICP-AES allied
analytical system (Perkin Elmer 3100XL). The FT-IR spectra
of the materials were recorded on a Perkin-Elmer FT-IR spec-
trometer under ambient conditions using KBr discs with a
6 T. Sugimura, K. Wakabayashi, H. Nakagama and M. Nagao,
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−
1
nominal resolution of 4 cm and averaging 5 spectra.
Catalytic activity
The catalytic activity of RhNPs/SBA-NH2 in chemoselective
hydrogenation of nitroarenes has been carried out in the
liquid phase at room temperature. In a typical procedure,
8 (a) K. A. Andrianov, V. I. Sidorov and M. I. Filimonova,
Zh. Obshch. Khim., 1977, 47, 485; (b) X. Lin, M. Wu, D. Wu,
S. Kuga, T. Endoe and Y. Huang, Green Chem., 2011, 13, 283;
1
1
mmol of the reactant, 2 mmol of hydrazine hydrate and
0 mg of the catalyst in 3 ml water were added to a 10 ml RB
(
c) V. Pandarus, R. Ciriminna, F. Beland and M. Pagliaro,
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flask and stirred at room temperature. After completion, the
products were extracted with ethyl acetate and the catalyst
was removed by centrifugation. The product analysis was
made by GC-MS (QP-5050 model, M/s. Shimadzu Instru-
ments, Japan) equipped with an equity-1 capillary column
(0.32 mm diameter and 25 m long, supplied by M/s. J & W
Scientific, USA).
3
49, 1955; (h) L. Hu, X. Cao, L. Chen, J. Zheng, J. Lu, X. Sun
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Conclusion
1
305; (b) K. V. R. Chary and C. S. Srikanth, Catal. Lett., 2009,
28, 164.
2
In summary, an efficient and green RhNPs/SBA-NH solid
1
catalyst has been developed with a particle size of 1–3 nm for
aqueous medium chemoselective hydrogenation of nitro-
arenes to the corresponding aminoarenes at room tempera-
ture with short reaction times in excellent yields. Apart from
its high activity and chemoselectivity, ease of separation,
reusability, and facile preparation are its meritorious features.
1
0 Y. Jang, S. Kim, S. W. Jun, B. H. Kim, S. Hwang, I. K. Song,
B. M. Kim and T. Hyeon, Chem. Commun., 2011, 47, 3601.
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1
1
1, 395; (b) Y. Choi, H. S. Bae, E. Seo, S. Jang, K. H. Park and
B.-S. Kim, J. Mater. Chem., 2011, 21, 15431; (c) Y. Zhang,
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Acknowledgements
Saidulu Ganji and Ravi Kumar Marella acknowledge CSIR,
and Siva Sankar Enumula acknowledges UGC, New Delhi, for
a research fellowship.
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