Chemoselective hydrogenation of nitroarenes catalyzed by cellulose-supported Pd NPs

Article abstract of DOI:10.1016/j.catcom.2017.09.024

Cellulose-supported palladium nanoparticles (NPs) were prepared by straightforward deposition of metal NPs on modified cellulose. The catalyst exhibited excellent catalytic activity and selectivity in room-temperature hydrogenation of various nitroarenes to arylamines under atmospheric hydrogen pressure in neat water without any additives. High chemoselectivity was also achieved in the hydrogenation of substituted nitroarenes with multiple reducible groups. The catalyst can be recycled by simple centrifugation and reused for at least 4 times without significant decline of yields.

Full text of DOI:10.1016/j.catcom.2017.09.024

CatalysisCommunications103(2018)47–50
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Chemoselective hydrogenation of nitroarenes catalyzed by cellulose-
supported Pd NPs
Dan-dan Li, Jia-wei Zhang, Chun Cai⁎
Chemical Engineering College, Nanjing University of Science & Technology, 200 Xiaolingwei, Nanjing 210094, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Biomass supported catalyst
Nitroarenes
Hydrogenation
Chemoselectivity
Cellulose-supported palladium nanoparticles (NPs) were prepared by straightforward deposition of metal NPs on
modified cellulose. The catalyst exhibited excellent catalytic activity and selectivity in room-temperature hy-
drogenation of various nitroarenes to arylamines under atmospheric hydrogen pressure in neat water without
any additives. High chemoselectivity was also achieved in the hydrogenation of substituted nitroarenes with
multiple reducible groups. The catalyst can be recycled by simple centrifugation and reused for at least 4 times
without significant decline of yields.
1. Introduction
the application of cellulose in catalytic systems has already been rea-
lized. A cellulose supported Pd complex catalyst catalyzed Tsuji–Trost
Arylamine is one of the most important intermediates in the man-
ufacture of pharmaceutical preparation, dyes, polymers, and biologi-
cally active substances [1–3]. Arylamines can be synthesized by several
methods, such as reduction of aryl azides [4], amination of aryl halides
[5], catalytic reduction of nitroarenes [6] and so on. Among them, the
metal-catalyzed reduction of nitroarenes is one of the most cost-effi-
cient methodologies as well as a fundamental transformation in organic
chemistry [7–9]. Numerous metals have been reported for catalyzing
the reduction of nitroarenes, including noble metals (Pd [10], Rh [11],
Ir [12]) and earth-abundant metals (Fe, Co [8,13], Ni [13–14]). Com-
pared with unrecoverable homogeneous metal catalysts, heterogeneous
catalysts are more favorable in industry for economic and environ-
mental consideration.
Heterogeneous catalysis is at the heart of the modern energy and
chemical industries [15–17]. Heterogeneous catalysts, especially sup-
ported metal catalysts play decisive roles in both industry and aca-
demic. A plenty of materials have been applied as supports of different
catalysts, such as SiO2 [18–20], TiO2 [21], carbon materials [22–23]
and so on. The use of biopolymers as support materials for metal cat-
alysts has been widely developed since the 1990s [24–25]. Cyclodex-
trin, chitosan, cellulose and some other biomass materials have been
used for their biodegradability. Here we focus on the use of cellulose,
for this material is a kind of earth-abundant and environmental-friendly
biopolymer and bears many hydroxyl groups which make it can be
easily modified and stabilize metal NPs effectively. Furthermore, cel-
lulose is insoluble and stable in water and common organic solvents, so
it can be separated by simple centrifugation as catalyst support. Indeed,
reaction was reported in 2001 [26]. Various mono-dispersed metal NPs
immobilized on cellulose fibers via NaBH4-mediated reduction of cel-
lulose fibers immersed in a metal salt was reported by Kunitake and co-
workers [27]. After these reports, the capability of cellulose as catalyst
support attracted much attention from many researchers. Various metal
nanoparticles supported on cellulose have been reported to catalyze
different reactions, for example Pd NPs [28], Au NPs [29] and Cu NPs
[30]. What's more, cellulose supported metal NPs can even combine
with chiral ligands to catalyze synthesis of chiral products [31]. How-
ever, surface of cellulose without any modification is smooth, catalyst
can only reuse for no more than 3 times because of metal leaching and
aggregation. Various surface modifications have been made to cellulose
to improve its ability of stabilizing NPs. Previously, both Vallribera's
group and our group showed that metal nanoparticles could be stabi-
lized by fluorinated compounds and immobilized on fluorous silica gel
by fluorous–fluorous interactions, and the catalyst could also be applied
in a series of coupling reactions [32–33]. Along this line, fluorinated
cellulose can be synthesized to stabilize and immobilize NPs directly.
Recently, great progresses had been made in the hydrogenation of
nitroarenes catalyzed by heterogenous catalysts. Zhang and co-workers
reported a Pd@zeolite catalyst for nitroarene hydrogenation with high
chemoselectivity, arising from sterically controlled adsorption in the
zeolite micro-pores [34]. Nevertheless, high hydrogen pressure is
needed. Indeed, robust reducing reagent, high hydrogen pressure or
high temperature are often required in previous works. The critical
issue in the synthesis of amines is to choose an efficient and environ-
mental-friendly heterogenous catalyst with high activity, selectivity and
⁎
Corresponding author.
E-mail address: c.cai@njust.edu.cn (C. Cai).
http://dx.doi.org/10.1016/j.catcom.2017.09.024
Received 13 July 2017; Received in revised form 8 September 2017; Accepted 26 September 2017
Availableonline28September2017
1566-7367/©2017PublishedbyElsevierB.V.

D.-d. Li et al.
CatalysisCommunications103(2018)47–50
reusability that can catalyze the reaction under mild conditions.
Herein we report a perfluorobutyl modified cellulose supported Pd
catalyst for hydrogenation of nitroarenes in water at room temperature.
Instead of robust or toxic reducing reagent (NaBH4 or N2H4·H2O),
molecular hydrogen was chosen as hydrogen donor. This protocol al-
lows an efficient transformation of nitroarenes under mild conditions.
2. Experimental section
2.1. Cellulose modification
All chemicals were purchased from commercial sources and used
without further treatment. In a typical procedure, α-cellulose (500 mg)
was dispersed in dry DMF (10 ml). Then the mixture was sonicated for
30 mins to obtain equally distributed cellulose suspension. The sus-
pension was cooled to 0 °C and NaH (900 mg) was added in two por-
tions. After stirring for 15 min, perfluorobutyl iodide (500 mg) was
slowly added and the reaction mixture was warmed to room tempera-
ture. After stirring overnight, the reaction mixture was filtrated. The
solid was washed with ethyl acetate (3 × 15 ml), deionized water
(3 × 15 ml) and ethanol (3 × 15 ml) before dried at 60 °C for 5 h. The
final product was obtained as yellowish solid.
Fig. 1. IR spectrum of cellulose (Cell) and C4F9-Cell.
2.2. Catalyst preparation
Pd catalyst was prepared by chemical reduction with NaBH4 as re-
ductant. 1 mg PdCl2 was dissolved in 0.5 ml water and 1 mg NaCl was
added to accelerate dissolution. To C4F9-Cell (200 mg) in ethanol (6 ml)
in a round-bottom flask (50 ml), the PdCl2 solution was added and the
mixture was stirred at room temperature for 5 h. Then a fresh prepared
ethanol solution of NaBH4 (0.8 mg, in 0.5 ml ethanol) was added into
the reactor quickly under vigorous stirring at room temperature under
argon atmosphere. The color of the mixture would turn to black im-
mediately which indicates that metal salts have been reduced to metal
particles. After stirring for another 5 h, the mixture was filtrated and
the solid was dried in vacuum oven at 25 °C for 12 h and stored in a
desiccator.
Fig. 2. (a) TEM image of Pd/C4F9-Cellulose. (b) Size distribution of Pd NPs of Pd/C4F9-
Cellulose.
2.3. Catalytic reduction of nitroarenes
catalyst is rod shaped with rough surface. The deposition of Pd NPs on
C4F9-Cell is examined by TEM (Fig. 2a). The TEM micrograph indicates
that the prepared Pd NPs are well-proportioned anomalous spherical
with an average diameter of 2–3 nm. Pd NPs on unmodified cellulose
was also characterized by TEM image (Fig. S2), it is obviously that Pd
NPs aggregated on cellulose surface.
Hydrogen was chosen as hydrogen donor for the hydrogenation of
nitroarenes. The hydrogenation reactions were carried out in a Schlenk
tube. Typically, the reactant and catalysts were dispersed into the sol-
vent, then the Schlenk tube was purged with H2 four times to replace
air. Then the mixture was stirred at room temperature for a desired
period. After reaction, the reaction mixture was extracted by ethyl
acetate. The product and unreacted reactant were analyzed by GC–MS.
The catalysts were separated by centrifugation and washed with ethyl
acetate (3 × 15 ml), deionized water (3 × 15 ml) and then used in the
next cycle.
Fig. 3 depicts the XPS spectrum in the Pd 3d region for Pd/C4F9-Cell,
3. Results and discussion
3.1. Characterization of catalyst
Cellulose and C4F9-Cell were characterized by IR spectrum (Fig. 1).
The strong peak around 3300 cm−1 is due to the stretching vibration of
the OeH bonds. It is an obvious decrease in the peak intensity at
3300 cm−1 after cellulose was modified, showed that some of the OeH
bonds in cellulose reacted. And there is also an increase of peak in-
tensity at 1600 cm-1 which may be attributed to the stretching vibra-
tion of the CeF bonds. The peak around 1050 cm−1 due to the
stretching vibration of the C-O-C bonds increased slightly, indicating
perfluorous compounds was attached to cellulose by ether linkage.
Scanning electron microscopy (SEM) image of Pd/C4F9-Cell de-
monstrated the morphology of cellulose after modified (Fig. S1). the
Fig. 3. XPS spectrum of Pd 3d region of Pd/Cell-C4F9.
48

D.-d. Li et al.
CatalysisCommunications103(2018)47–50
that only one nitro group was reduced into the respective amine with
little diamino product (Table 2 entry 10). What's more, the 1,3-dini-
trobenzen can be completely reduced to provide the 1,3-diaminobenzen
by prolonging reaction time to 12 h (Table 2 entry 11).
Table 1
Optimization of nitrobenzene hydrogenation conditionsa.
Entry
Cat. (mol%)
Sol.
Time (h)
Yieldb (%)
Chemoselective hydrogenation of nitroarenes with other reducible
groups has been investigated. The catalyst exhibited high chemoselec-
tivity in hydrogenation of 2-acetylnitrobenzene, although the activity is
relatively low. There is no byproduct like 1-(2-aminophenyl) ethanol in
this reaction (Table 2 entry 9). The nitro groups were reduced into
amino groups with good selectivity in the presence of C]C and C^C
(Table 2 entries 12–13), which are considered as the main challenge in
the chemoselective reduction of nitroarenes.
Importantly, halogenated anilines, which are commonly used as
precursors for agrochemicals, were obtained by reduction of haloge-
nated nitroarenes without the formation of dehalogenation products
(Table 2 entries 2–3). As often reported in the literature, in the hy-
drogenation of 4-chloronitrobenzene, Pd nanoparticle-based catalysts
almost lead to considerable amounts of dehalogenation product. Higher
selectivity toward the sole reduction of the nitro group was often
achieved using supported Pt, Ru, or Ni nanocatalysts. Fortunately, little
dehalogenation product was obtained in Pd/Cell-C4F9 catalyzed hy-
drogenation of 4-chloronitrobenzene. To investigate the reason for the
excellent selectivity, higher temperature was applied, and nearly 10%
byproduct aniline was obtained besides of major product 4-chloroani-
line without 4-chloronitrobenzene left.
1
2
3
4
5
6
7
8
Pd0/Cell-C4F9(0.7)
Pd0/Cell-C4F9(0.7)
Pd0/Cell-C4F9(0.5)
Pd0/Cell-C4F9(0.3)
Pd0/Cell-C4F9(0.2)
Pd0/Cell-C4F9(0.07)
–
EtOH
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
5
5
5
5
5
5
5
1
3
4
4
99
99
98
95
80
33
0
20
Pd0/Cell-C4F9(0.3)
Pd0/Cell-C4F9(0.3)
Pd0/Cell-C4F9(0.3)
Pd0/Cell(0.3)
9
10
11
76
99(90c)
96(82d)
a
Reaction conditions: 0.2 mmol of nitroarenes, 2 ml H2O as solvent, The reactions
were carried out at room temperature in a sealed tube under H2 atmosphere with a
balloon filled with H2 for a desired period.
b
Yields were determined by GC.
Catalyst after 5 runs.
Catalyst after 2 runs.
c
d
the intense doublet centering at 333.7 and 338.8 eV belongs to metallic
Pd. And there is no other doublets ascribed to the +2 oxidation state of
Pd, illustrating no metal salts exists on nanoparticle surface.
The amount of the palladium supported on cellulose was 2.433 mg/
g determined by ICP, which was a little lower than the theoretical value
(2.82 mg/g).
4. Reusability study
3.2. Catalytic reduction of nitrobenzene
Pd/Cell-C4F9 could be recovered by centrifugation and the reusa-
bility of the catalyst was tested in the model reaction. The recovered
catalyst was washed with ethyl acetate (3 × 15 ml) and deionized
water (3 × 15 ml). Then the recovered catalyst was used directly in
next run without further drying. After 5 runs, no significant decrease of
yields was observed (Table 1 entry 9c). The stability of the catalyst
could be ascribed to the mild reaction conditions.
The amount of the palladium supported on cellulose after 5 runs
was 1.963 mg/g determined by ICP, indicating metal leaching during
the reaction procedure,which might be the main reason for the decrease
of catalytic activity. The recovered catalyst was examined by TEM
analysis and a slight increase of palladium particle size was observed
(Fig. S3), which was believed to be another reason for the de-activation
of the catalyst. Meanwhile, XPS spectrum of Pd 3d region of Pd/Cell-
C4F9 after 5 runs showed Pd nanoparticles maintained Pd0 state (Fig.
S4). This is might because the reaction proceeded under H2 atmosphere.
Nitrobenzene was chosen as the model substrate for the optimiza-
tion of the reaction conditions. The effects of reaction solvents, catalyst
amount and reaction time on the catalytic activity were investigated
(Table 1). First EtOH was used as solvent and resulted in good yield
(Table 1 entry 1). Later when H2O was used as solvent, it also proved to
be good solvent for the hydrogenation (Table 1 entry 2). For economic
and environmental reasons, water was chosen as green solvent. The
amount of catalyst has a huge impact on the hydrogenation of ni-
trobenzene, no reaction took place without catalyst (Table 1 entry 7).
And the yield did not decrease obviously until the amount of catalyst
was decrease to 0.2 mol%, which means only 0.3 mol% catalyst can
catalyze the reaction (Table 1 entries 3–6). Then, we evaluated the
effects of reaction time (Table 1 entries 8–10). It turned out that ni-
trobenzene can be hydrogenated completely in 4 h. To further show the
excellent catalytic activity of Pd0/Cell-C4F9, turn over frequency (TOF)
was calculated on the basis of the conversion at a reaction time of
10 min (TOF = 0.2 s−1).
5. Conclusions
We also examined the support effect by immobilizing palladium
nanoparticles onto unmodified cellulose. An 95% yield of product was
obtained in the first run, while a remarkable loss of activity was ob-
served in the third run (Table 1, entry 11), indicating that interaction of
fluorinated compounds and Pd NPs was crucial for catalyst activity after
recovered.
Cellulose, an earth-abundant and biodegradable material, was
found to be a good support for Pd NPs. The procedure used for the
preparation of Pd/C4F9-Cell is straightforward. Pd NPs can be effi-
ciently stabilized by fluorinated cellulose. The Pd/C4F9-Cell catalyst
showed great performance for chemoselective hydrogenation of ni-
troarenes to arylamines. This catalyst can hydrogenate nitroarenes
without affecting other reducible groups such as carbonyl group and
other unsaturated bonds. And the reaction can occur at room tem-
perature with water as green solvent under atmospheric hydrogen
pressure.
3.3. Hydrogenation of nitroarenes
The functional group tolerance of the hydrogenation process cata-
lyzed by C4F9-Cellulose supported Pd NPs had been demonstrated ap-
plying structurally diverse functionalized nitroarenes under optimized
conditions (Table 2). Most substituted nitrobenzene with electron-do-
nating or withdrawing groups were furnished with good yields. There
are still some substituted nitrobenzenes didn't perform well in the hy-
drogenation reaction, such as 4-chloronitrobenzene, 3-hydro-
xynitrobenzene and 4-aminonitrobenzene with most reactant unreacted
in 5 h. However, good yields can be achieved when reaction time was
prolonged to 12 h. With 1,3-dinitro-benzen as substrate, it was found
Acknowledgments
We gratefully acknowledge the Nature Science Foundation of
Jiangsu Province (BK 20140776) for financial support. This work was
also supported by the National Natural Science Foundation of China
(21402093 and 21476116) and the Chinese Postdoctoral Science
Foundation (2015 M571761 and 2016 T90465) for financial support.
We also gratefully acknowledge the Priority Academic Program
49

D.-d. Li et al.
CatalysisCommunications103(2018)47–50
Table 2
a
Chemoselective Reduction of Nitroarenes Catalyzed by Pd/Cell-C4F9
.
Entry
1
Substrate
Product
Yield (%)b
95
Entry
8
Substrate
Product
Yield (%)b
99
2
3
4
96
9d
10
11
88
84
89
40(99c,90e)
99
5
98
12d
72(95c)
40(96c)
6
7
57(97c)
54(95c)
13
a
Reaction conditions: nitroarenes (0.2 mmol), Pd/Cell-C4F9 (0.3 mol%), room temperature, 1 ml water as solvent, 5 h, H2 (1 atm).
GC yields.
12 h.
Pd/Cell-C4F9(0.5 mol%).
50 °C.
b
c
d
e
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