Enhanced catalytic activity of natural hematite-supported ppm levels of Pd in nitroarenes reduction

Article abstract of DOI:10.1007/s13738-020-01908-z

In this work, Pd NPs supported on amine-modified natural hematite have been prepared and characterized. Using this simple catalyst, nitroaromatic compounds as a major cause of industrial pollution were reduced to corresponding amines with ppm levels of Pd in the presence of designer surfactant TPGS-750-M and NaBH₄ at room temperature in aqueous media. Synergistic effect between hematite and Pd is responsible for the observed enhanced catalytic activity. This catalyst was recycled for at least four times with a small decrease in the activity.

Full text of DOI:10.1007/s13738-020-01908-z

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-01908-z
ORIGINAL PAPER
Enhanced catalytic activity of natural hematite‑supported ppm levels
of Pd in nitroarenes reduction
Mohammad Gholinejad1,2 · Mohammad Shojafar1 · José M. Sansano3
Received: 22 September 2019 / Accepted: 10 March 2020
© Iranian Chemical Society 2020
Abstract
In this work, Pd NPs supported on amine-modifed natural hematite have been prepared and characterized. Using this simple
catalyst, nitroaromatic compounds as a major cause of industrial pollution were reduced to corresponding amines with ppm
levels of Pd in the presence of designer surfactant TPGS-750-M and NaBH4 at room temperature in aqueous media. Syner-
gistic efect between hematite and Pd is responsible for the observed enhanced catalytic activity. This catalyst was recycled
for at least four times with a small decrease in the activity.
Keywords Hematite · Pd · Heterogeneous · Nitroarenes · Synergistic efect
Introduction
manufacturing explosives, pesticides, dyes, pharmaceuticals,
and plastics gives rise to the huge discharge into the envi-
ronment [6, 7]. One of the efcient methods for the removal
of nitro compounds is the reduction of them to the corre-
sponding amines, which are versatile precursors in organic
synthesis as well as in the preparation of various compounds
such as pharmaceuticals and agrochemicals [8–11]. In recent
years, catalytic hydrogenation of nitroaromatic compounds
employing different noble transition metals such as Pd
[12–14] and Au [15–22] was investigated well. Moreover,
cheap alternative metals such as Fe [23–32], Zn [33–38],
and Sn [39, 40] have been widely used for reduction of nitro
group. However, these reactions often require stoichiometric
(or super stoichiometric) amounts of metals and/or harsh
reaction conditions. Recently, a great deal of attention has
been paid to use of bimetallic catalysts in diferent organic
reactions. It is expected that synergistic electronic efect in
bimetallic catalysts enhances their catalytic activity com-
pared to the corresponding monometallic systems [41, 42].
In this way, using ppm levels of precious transition metals
such as Pd in the presence of abundant metals and studying
their synergistic efect in the reduction of nitroarenes are
a key objective of sustainable chemistry [43, 44]. Lately,
synergistic efects of Fe nanoparticles doped with ppm lev-
els of Pd- or Pd/Ni-catalyzed reduction of aromatic nitro
compounds were studied under homogeneous conditions
at room temperature [45–47]. In this work, we endeavored
to carry out further study into the reduction of nitroarenes,
using hematite-supported Pd NPs as green heterogeneous
Iron is one of the naturally occurring abundant elements in
the Earth’s crust which is considered as important energy
sources for biogeochemical and geochemical processes such
as nitrogen fxation, oxygen delivery, and photosynthesis. On
the other hand, sorption of toxic elements such as arsenic,
antimony, and phosphorus on iron (oxy)hydroxides is vital
as it ofers a means of controlling the mobility, toxicity, and
availability of these elements in natural environments [1–4].
Hematite, one of the most abundant natural minerals on
Earth’s surface, is mined as the main ore of iron in the Fe2O3
form [5]. On the other side, the worldwide use of nitroaro-
matic compounds, known as major industrial pollutants, in
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-020-01908-z) contains
supplementary material, which is available to authorized users.
* Mohammad Gholinejad
gholinejad@iasbs.ac.ir
1
Department of Chemistry, Institute for Advanced Studies
in Basic Sciences (IASBS), P. O. Box 45195-1159,
Gavazang, Zanjan 45137-66731, Iran
2
Research Center for Basic Sciences & Modern Technologies
(RBST), Institute for Advanced Studies in Basic Sciences
(IASBS), Zanjan 45137-66731, Iran
3
Departamento de Química Orgánica, Instituto de Síntesis
Orgánica, Centro de Innovación en Química Avanzada
(ORFEO-CINQA), Universidad de Alicante, Alicante, Spain
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
catalyst under enhanced catalytic activity and recoverable
conditions.
with centrifugation and it was washed with ethyl acetate
(2×15 mL) and dried in the oven at 80 °C.
Procedure for the preparation of Hematite@NH2/Pd
Experimental
Hematite@NH2 (1 g) was sonicated in water (10 mL) for
15 min. Then, a solution of Pd(OAc)2 (0.0093 mmol, 2 mg)
in 2 mL H2O/THF (1:1) was added and then solution of
NaBH4 (0.08 mmol, 3 mg) in H2O (1 mL) was added slowly
and the mixture was stirred for 24 h at room temperature
under argon atmosphere. The resulting solid was separated
with centrifugation, washed with water (3×10 mL) and eth-
anol (3×10 mL) and dried in an oven at 60 °C. The loading
of Pd on the obtained material was determined by atomic
absorption spectroscopy (AAS) and inductively coupled
plasma mass spectrometry (ICP) to be 0.001 mmol g−1.
Materials and methods
All materials were purchased from Sigma-Aldrich, Acros
and Merck Millipore. Reactions were monitored by gas
chromatography (GC) using Varian CP 3800 and thin-layer
chromatography (TLC) using Merck silica gel 60F254 glass
plate with 0.25 mm thickness. Hematite was purchased from
Daneshmand chemical trading co (Iran). Column chroma-
tography was carried out on silica gel 60 Merck (230–240
mesh) in a 2-cm-diameter column. 1H NMR and 13C NMR
spectra were recorded at 400 MHz and 100 MHz, respec-
tively, on a Bruker Avance HD apparatus in d6-DMSO and
CDCl3. Chemical shifts are given on the δ-scale in ppm,
and residual solvent peaks were used as internal stand-
ards. X-ray difraction (XRD) patterns were recorded using
Philips X’Pert Pro instrument. The TEM and SEM images
were captured with EOL JEM-2010 and JEOL JSM 840,
respectively. FTIR study of samples was performed using
FTIR spectrophotometer (Bruker vector 22 spectrophotom-
eter, Germany) by preparing their KBr pellets from 400 to
4000 cm−1. The weight loss of samples was measured using
thermogravimetry (NETZSCH STA 409) under an O2 fow
rate of 20 mL min−1 with a heating rate of 10 °C min−1
from 30 to 800 °C. Energy-dispersive X-ray analysis (EDX)
results were obtained using Carl Zeiss Sigma instrument.
General procedure for the catalytic reduction of nitro
compounds using Hematite@NH2/Pd as the catalyst
To a 5-mL fask, nitroarene (0.4 mmol), NaBH4 (1.6 mmol,
60.5 mg), Hematite@NH2/Pd (100 mg, 0.01 mol% of Pd,
Table 1 Reduction of 4-nitrotoluene using Hematite@NH2/Pd in dif-
ferent solvents
General procedure for catalytic reduction of nitro
compounds using hematite and ppm levels of Pd
Entry
Solvent
Yield (%)[a]
Nitroarene (0.4 mmol), NaBH4 (1.6 mmol, 60.5 mg), hema-
tite (100 mg), Pd (ppm levels in 1.65 mL) and H2O/THF
(1.5:0.15 mL) were added to a 5-mL fask, and the reaction
was stirred for appropriate reaction time at room tempera-
ture. The progress of the reactions was monitored by GC
analysis. After completion of the reaction, the crude prod-
uct was extracted with ethyl acetate (3×5 mL) and purifed
using column or plate chromatography.
1
2
3
4
5
6
7
8
Ethyl acetate
1,4-Dioxane
H2O/THF
PEG200
DMF
THF
76
5
97
21
6
5
71
3
Ethanol
Toluene
9
Acetone
DMSO
Dichloromethane
H2O/Ethanol (10:1)
H2O
67
7
8
65
58
Procedure for the preparation of Hematite@NH2
10
11
12
13
To a 50-mL fask, hematite (2 g) and dry toluene (20 mL)
were added and the mixture was sonicated for 10 min at
room temperature. Then, (3-aminopropyl) triethoxysi-
lane (10 mmol, 2.3 mL) was added slowly under an argon
atmosphere and the resulting mixture was stirred at 110 °C
for 24 h. Afterward, the reaction mixture was cooled down
to room temperature, the obtained solid was separated
Reaction conditions: nitroarene (0.4 mmol), Hematite@NH2/
Pd (100 mg, 0.01 mol% of Pd, 6 ppm in 1.65 mL solvent), solvent
(1.65 mL), NaBH4 (1.6 mmol) at room temperature
aGC yields, octadecane was used as an internal standard
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Journal of the Iranian Chemical Society
Table 2 Reduction of 4-nitrotoluene using Hematite@NH2/Pd in dif-
ferent reducing agents
in the presence of diferent amounts of hematite and ppm
levels of Pd. Results indicated that using 150 mg of hema-
tite in the presence of 2 ppm Pd, 68% 4-aminotoluene was
obtained (Table 1, ESI). By increasing Pd contents to 3 ppm
and using 150 mg of hematite, 81% yield was achieved
(Table 2, ESI). Using 4 ppm of Pd and 100 mg of hematite,
97% yield was also obtained during 1 h (Table 3, ESI). It
should be noted that in the absence of hematite and using
2,3 and 4 ppm of Pd, reaction proceeded very slowly and low
yields were obtained. Furthermore, the reaction gave very
poor yields in the absence of Pd using diferent amounts of
hematite (Table 4, ESI). Therefore, 4 ppm of Pd and 100 mg
of hematite was found as optimum amounts for the reduction
of 4-nitrotoluene. In the next step, we functionalized hema-
tite with an amino group (Hematite@NH2), via reaction of
hematite with (3-aminopropyl) triethoxysilane under refux
toluene (Scheme 1).
Entry
Reductant
Time (h)
Yield (%)a
1
2
3
4
5
6
Ammonium formate
Formic acid
Isopropyl alcohol
Hydrazine
Glycerol
Sodium borohydride
4
4
4
4
4
1
3
6
2
4
5
FTIR spectrum of Hematite@NH2 showed the –CH2
stretching and –CH2 bending vibrations at 2897 cm−1 and
1435–1473 cm−1, respectively. Bending vibrations of N–H
in NH2 group were also observed at 1532 cm−1 (Fig. 1).
FTIR spectrum of the Hematite@NH2/Pd is very similar to
Hematite@NH2, and a low amount of Pd did not afect the
structure [48–50] (Fig. 1).
97
Reaction conditions: nitroarene (0.4 mmol), Hematite@NH2/Pd
(100 mg, 0.01 mol% of Pd, 6 ppm in 1.65 mL solvent), H2O: THF
(10:1, 1.65 mL), reducing agent (1.6 mmol) at room temperature
aGC yields, octadecane was used as an internal standard
6 ppm in 1.65 mL solvent), and 2 wt% TPGS-750-M in
H2O/THF (1.5:0.15 mL) were added and the reaction was
stirred for an appropriate reaction time at room tempera-
ture. The progress of the reactions was monitored by GC or
TLC. After completion of the reaction, the crude product
was extracted by ethyl acetate (3×5 mL) and purifed using
column or plate chromatography.
The X-ray difraction (XRD) spectrum of natural hematite
was in agreement with the standard data given in its JCPDS
card (33-0664) [51, 52] (Fig. 2). However, due to low con-
centration of Pd, related pattern of Pd was not observed in
the XRD of Hematite@NH2/Pd. It should be noted that func-
tionalization of hematite with amine group did not afect the
hematite, and its structure was preserved (Fig. 2).
Thermogravimetric analysis (TGA) of Hematite@NH2
was achieved in the temperature range 20 to 800 °C under
air fow. The TG spectra showed a small weight loss between
20 and 160 °C, which could be related to the desorption of
water and solvents. Furthermore, the samples displayed a
major second weight loss of 5.23% at 300–800 °C, which
was correlated to the presence of organic group of (3-ami-
nopropyl) triethoxysilane with calculated loading of 0.9
mmol g−1 (Fig. 3).
Typical procedure for the one‑pot synthesis
of acetaminophen
4-Nitrophenol (0.4 mmol), NaBH4 (1.6 mmol, 60.5 mg),
Hematite@NH2/Pd (100 mg 0.038 mol% of Pd, 25 ppm in
1.65 mL solvent), and 2 wt% TPGS-750-M in H2O/THF
(1.5:0.15 mL) were added to a 5-mL fask and the reac-
tion was stirred at room temperature. After 30 min, acetic
anhydride (0.76 mmol, 0.08 mL) was added and the reaction
was stirred at 120 °C for 1 h. Subsequently, the crude prod-
uct was extracted with ethyl acetate (3×5 mL) and purifed
using column chromatography afording acetaminophen in
78% isolated yield.
Hematite@NH2 was then treated with Pd(OAc)2 and
NaBH4 to aford Hematite@NH2/Pd with 0.001 mmol g−1
Pd loading. It should be noted that due to very low level
of Pd, it was hard to characterize Pd species of hematite-
NH2@Pd. FTIR spectra of Hematite@NH2 and Hematite@
NH2/Pd were also recorded. Comparison of the Hematite@
NH2/Pd and Hematite@NH2 indicated the similar spectra
that confrmed the structure of catalyst (Fig. 2). Scanning
electron microscopy (SEM) images of Hematite@NH2/Pd
showed the presence of uniform hematite sheets (Fig. 4).
Moreover, transmission electron microscopy (TEM) images
of Hematite@NH2/Pd showed clay sheets as well as small
nanoparticles of Pd (Fig. 5).
Results and discussion
In order to prepare hematite-supported Pd, we did diferent
experiments to fnd approximate minimum amount of Pd in
the catalyst. Initially, we studied reduction of 4-nitrotoluene
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Journal of the Iranian Chemical Society
Energy-dispersive X-ray spectroscopy (EDX) spectrum of
the Hematite@NH2/Pd confrmed the presence of diferent
elements such as Fe, Mg, Ni, Si, Cu, N, C, and O as well as
Pd species (Fig. 6).
We also investigated the efect of diferent types of sur-
factants on the reduction of three diferent nitroarenes in
the presence of Hematite@NH2/Pd. Results indicated higher
efciency of TPGS-750-M than span 80, SDS, and CTAB
(Table 3).
After characterization of Hematite@NH2/Pd, we investi-
gated the efect of diferent solvents and reducing agents on
the efciency of Hematite@NH2/Pd (100 mg 0.01 mol% of
Pd, 6 ppm in 1.65 mL solvent) in the reduction of 4-nitro-
toluene as model reaction (Tables 1 and 2). For this purpose,
the model reaction was studied in diferent solvents such as
ethyl acetate, 1,4-dioxane, H2O/THF, PEG200, DMF, THF,
toluene, EtOH, DMSO, CH2Cl2, and H2O at room tempera-
ture. Results indicated that 97% of the desired product was
achieved in H2O/THF while lower yields were obtained
in other solvents. Generally, these results indicate higher
activity of catalyst in polar solvents compared to nonpolar
solvents.
Having optimized conditions in hand, we investigated
the reduction reaction of variety of amines using 100 mg
Hematite@NH2/Pd, NaBH4 (1.6 mmol), 1.5 mL H2O,
0.15 mL THF, and 2 wt% TPGS-750-M. Results of this
study indicated that using Hematite@NH2/Pd as heteroge-
neous catalyst, a variety of nitro compounds were reduced
to corresponding amines giving high-to-excellent yields at
room temperature (Table 4). It is worth mentioning that in
nitroarenes-containing carbonyl group, both carbonyl and
nitro groups were reduced to alcohol and amine, respectively
(Table 4, entries 21–23, 26). Reduction of 2-nitropyridine
as a heterocyclic nitro compound was proceeded well and
aforded corresponding product in 91% yield (Table 4, entry
27). Reduction of higher molecular weight and pharmaceuti-
cally active nitro compounds under optimized reaction con-
ditions was proceeded slowly. Therefore, we prepared a cata-
lyst with higher loading of Pd and used 100 mg (0.038 mol%
We also studied efect of other reducing agents such as
ammonium formate, formic acid, isopropyl alcohol, hydra-
zine, and glycerol on the reduction of 4-nitrotoluene. How-
ever, results showed the formation of desired product in
lower yields than using NaBH4.
Table 3 The efect of diferent surfactants on the reduction of nitroarenes under optimized conditions
Surfactant
–
Time
Yield (%)a
Time (h)
Yield (%)a
Time (h)
Yield (%)b
SDS
Span 80
CTAB
TPGS-750-M
TPGS-750-M/THFc
H2O
1 h
1 h
1 h
1 h
50 min
1 h
67
69
65
83
97
58
7
7
7
7
6
7
64
66
57
85
90
53
3
3
3
3
2.5
3
61
63
59
80
91
50
Reaction conditions: Hematite@NH2/Pd (100 mg, 0.01 mol% of Pd, 6 ppm in 1.65 mL solvent), nitroarene (0.4 mmol), 2 wt% surfactant/H2O
(1.5 mL), NaBH4 (1.6 mmol) at room temperature
aGC yields, octadecane was used as an internal standard
bYields are isolated
cTHF (0.15 mL)
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Journal of the Iranian Chemical Society
Table 4 Reduction of structurally diferent nitroarenes using Hematite@NH2/Pd at room temperature
a
a
d
d
d
e
e
e
d
d
e
e
a
a
a
d
d
e
e
d
d
d
e
e
e
b
d
d
d
d
d
e
e
e
e
e
a
a
d
d
d
e
e
d
e
e
d
e
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Journal of the Iranian Chemical Society
Table 4 (continued)
b
b
b
d
e
d
d
d
e
e
e
b
d
e
d
e
d
e
c
c
c
d
d
e
e
d
e
c
d
e
Reaction conditions: Hematite@NH2/Pd (100 mg, 0.01 mol% of Pd, 6 ppm in 1.65 mL solvent), nitroarene (0.4 mmol), 2 wt% TPGS-750-M/
H2O (1.5 mL) and THF (0.15 mL), NaBH4 (1.6 mmol) at room temperature
Yields are isolated
aYields determined by GC, octadecane was used as an internal standard
bNaBH4 (3.2 mmol)
cNitroarene (0.2 mmol), NaBH4 (0.8 mmol), catalyst (100 mg, 0.038 mol% of Pd, 25 ppm in 1.65 mL), 2 wt% TPGS-750-M/H2O (1.2 mL) and
THF (0.5 mL)
d[TON] values [(mol product/mol catalyst)]
e[TOF] values [(mol product/mol catalyst)/time of reaction (h)
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Journal of the Iranian Chemical Society
Scheme 1 Preparation of
Hematite@NH2
Fig. 3 Thermogravimetric analysis (TGA) of Hematite@NH2
Fig. 1 FTIR spectrum of Hematite, Hematite@NH2, fresh Hema-
of Pd, 25 ppm in 1.65 mL) of this catalyst for the reduction.
Under this condition, reactions proceeded well, and corre-
sponding amines were produced in 80–90% isolated yields
(Table 4, entries 28–31).
tite@NH2/Pd, and reused Hematite@NH2/Pd
Figure 1 ESI represents reduction of 5-chloro-2-nitroan-
ilide (orange color) to 4-chloro-o-phenylenediamine (gray
color) under optimized reaction condition.
As the reduction of nitrophenols was performed quan-
titatively, we carried out a one-pot reduction/amida-
tion sequence of 4-nitrophenol with acetic anhydride. As
depicted in Scheme 2, the one-pot conversion of 4-nitro-
phenol to acetaminophen adduct was performed efciently
and aforded product in 78% isolated yield (Scheme 2, Fig. 2
ESI).
Recycling ability of the Hematite@NH2/Pd was studied
in the reduction of 4-nitrotoluene. Results indicated that the
catalyst was successfully recovered and reused for four con-
secutive runs with a small decrease in the activity. Using
ICP analysis leaching of Pd after the fourth run was studied
indicating 11% leaching of Pd species. However, in the ffth
run, the obtained yield was decreased to 71% (Fig. 7). TEM
Fig. 2 Compared XRD pattern of natural Hematite (black), Hema-
tite@NH2 (red), and Hematite@NH2/Pd (blue)
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Journal of the Iranian Chemical Society
Fig. 4 SEM images of Hematite@NH2/Pd in diferent magnifcations
Fig. 5 TEM images of Hema-
tite@NH2/Pd
images of the reused catalyst after fve runs showed preser-
vation of the structure and very similar pattern to the fresh
catalyst (Fig. 8).
Also, the structure of reused catalyst after four runs was
studied by FTIR and EDX analyses (Figs. 1 and 9). Results
showed that catalyst structure was preserved, and very simi-
lar patterns to fresh catalyst were obtained.
According the common analogies, the proposed mech-
anism for the reduction of nitroarenes is the same as the
known mechanism for the Pd-catalyzed nitroarenes reduc-
tion. Briefy, produced H2 from the reaction of NaBH4 and
H2O was absorbed on the catalyst; then, physically absorbed
Fig. 6 EDX spectra of Hematite@NH2/Pd
Scheme 2 One-pot preparation
of acetaminophen
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Journal of the Iranian Chemical Society
Fig. 9 EDX spectra of Hematite@NH2/Pd
Fig. 7 Recycling of the catalyst (Hematite@NH2/Pd) for the reduc-
tion of 4-nitrotoluene
nitro group on the catalyst was reduced to corresponding
amine (Scheme 3) [53–55].
Comparison activity of Hematite@NH2/Pd with some
palladium catalysts in the reduction of nitrobenzene con-
frmed higher TOF of the presented catalyst (Table 5).
Conclusion
In conclusion, we used natural hematite for stabilization
of ppm Pd NPs for the frst time. Using this clay compos-
ite as a heterogeneous and recyclable catalyst, variety of
nitroarenes were reduced to corresponding amines at room
temperature. Synergetic efect between hematite and ppm
Scheme 3 Mechanism of the catalytic reduction of nitroarene with
Hematite@NH2/Pd
Fig. 8 TEM images of reused
catalyst after fve runs
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Journal of the Iranian Chemical Society
Table 5 Comparison of the catalytic activity of Hematite@NH
2/Pd NPs with that of other Pd catalysts in the nitro reduction reaction
Catalyst
T (°C), t (h)
Solvent
Yield (%)
TOF (h−1
)
Pd cNPs/C@Fe3O4 [56]
Pd@CQD@Fe3O4 [57]
APSNP [58]
PdNP@PPh2-PEGPIILP [59]
NHC-Pd@GO [60]
Pd/CNS [61]
PdNPs/4MePy [62]
Fe3O4@sepiolite-Pd2+ [63]
Pd(II)Pc [64]
Fe2O3@HAP-Pd [65]
Pd-pol [66]
70, 1
r.t, 2
r.t, 2
25, 1.5
r.t, 0.16
160, 0.16
180, 1
70, 0.83
100,12
60, 3
EtOH
H2O:EtOH
EtOH
H2O
MeOH:H2O (1:1)
H2O
H2O:EtOH (1:30)
EtOH:H2O (3:1)
EtOH
EtOH
H2O
99>
96
100
100
95
100
80
82
270
<600
50
<142
1604
38
571
1600
8
98
100
96
6
1000
4000
r.t, 6
r.t, 1
Hematite@NH2/Pd
H2O:THF (10:1)
100
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Acknowledgements The authors are grateful to Institute for Advanced
Studies in Basic Sciences (IASBS) Research Council and Iran National
Science Foundation (INSF-Grant number of 97021804) for support of
this work. We also gratefully acknowledge fnancial support from the
Spanish Ministerio de Economía y Competitividad (MINECO) (pro-
jects CTQ2013-43446-P and CTQ2014-51912-REDC), the Spanish
Ministerio de Economía, Industria y Competitividad, Agencia Estatal
de Investigación (AEI) and Fondo Europeo de Desarrollo Regional
(FEDER, EU) (projects CTQ2016-76782-P and CTQ2016-81797-
REDC), the Generalitat Valenciana (PROMETEOII/2014/017) and
the University of Alicante.
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