MOF-Derived Cu-Nanoparticle Embedded in Porous Carbon for the Efficient Hydrogenation of Nitroaromatic Compounds

Article abstract of DOI:10.1007/s10562-020-03244-6

Abstract: Novel Cu-nanoparticles (NPs) embedded in porous carbon materials (Cu@C-x) were prepared by one-pot pyrolysis of metal–organic frameworks (MOF) HKUST-1 at different temperatures. The obtained material Cu@C-x was used as a cost-effective catalyst for the hydrogenation of nitrobenzene using NaBH₄ as the reducing agent under mild reaction conditions. By considering the catalyst preparation and the catalytic activity, a pyrolysis temperature of 400?°C was finally chosen to synthesize the optimal catalyst. When the aromatic nitro compounds with reducible groups, such as cyano, halogen, and alkyl groups, were tested in this catalytic hydrogenation, an excellent selectivity approaching 100% was achieved. In the recycling experiment, a significant decrease in nitrobenzene conversion was observed in the third cycle, mainly due to the very small amount of catalyst employed in the reaction. Hence, the easily prepared and cost-effective Cu@C-400 catalyst fabricated in this study demonstrates potential for the applications in selective reduction of aromatic nitro compounds. Graphic Abstract: The catalyst Cu@C-400 exhibited 100?% conversion and high selectivity for the hydrogenation of industrially relevant nitroarenes.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03244-6

Catalysis Letters
https://doi.org/10.1007/s10562-020-03244-6
MOF‑Derived Cu‑Nanoparticle Embedded in Porous Carbon
for the Eꢀcient Hydrogenation of Nitroaromatic Compounds
1
1
1
1
1
1
1
Chenxia Qiao · Wenlan Jia · Qiming Zhong · Bingyu Liu · Yifu Zhang · Changgong Meng · Fuping Tian
Received: 14 January 2020 / Accepted: 28 April 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Novel Cu-nanoparticles (NPs) embedded in porous carbon materials (Cu@C-x) were prepared by one-pot pyrolysis of
metal–organic frameworks (MOF) HKUST-1 at diꢀerent temperatures. The obtained material Cu@C-x was used as a cost-
eꢀective catalyst for the hydrogenation of nitrobenzene using NaBH as the reducing agent under mild reaction conditions.
4
By considering the catalyst preparation and the catalytic activity, a pyrolysis temperature of 400 °C was ꢁnally chosen to
synthesize the optimal catalyst. When the aromatic nitro compounds with reducible groups, such as cyano, halogen, and alkyl
groups, were tested in this catalytic hydrogenation, an excellent selectivity approaching 100% was achieved. In the recycling
experiment, a signiꢁcant decrease in nitrobenzene conversion was observed in the third cycle, mainly due to the very small
amount of catalyst employed in the reaction. Hence, the easily prepared and cost-eꢀective Cu@C-400 catalyst fabricated in
this study demonstrates potential for the applications in selective reduction of aromatic nitro compounds.
Graphic Abstract
The catalyst Cu@C-400 exhibited 100 % conversion and high selectivity for the hydrogenation of industrially relevant
nitroarenes.
Keywords Cu@c-400 · Reduction · Aromatic nitro compounds
1
Introduction
Chenxia Qiao and Wenlan Jia have contributed equally to this
paper.
Selective reduction of nitro compounds into amines is
one of the most important chemical reactions in synthetic
organic chemistry [1, 2]. Amines have been widely used
for the production of bulk and ꢁne chemicals, such as dyes,
agrochemicals, pharmaceuticals, pesticides, and polymers
[3–5]. The extensively employed method for the produc-
tion of anilines is the direct reduction of the corresponding
aromatic nitro compounds [6–10]. Generally, the catalytic
reduction of nitro compounds with hydrogen or hydro-
gen donors has been mainly performed over noble metal
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03244-6) contains
supplementary material, which is available to authorized users.
*
Fuping Tian
fptian@dlut.edu.cn
1
State Key Laboratory of Fine Chemicals & Department
of Chemistry, Dalian University of Technology, 2 Linggong
Road, P. O. Box 288, Dalian 116024, China
Vol.:(0
1
123456789)
3

C. Qiao et al.
catalysts (for example, Ru, Pt, Pd, etc.) [11, 12], but the
scarcity of these noble metals and the high price heav-
ily limit their large-scale industrial applications. Moreo-
ver, the presence of other reducible functional groups in
the nitroarenes makes the selective reduction of the nitro
group quite challenging. As an alternative choice, the
heterogeneous catalysts based on the earth-abundant and
cost-eꢀective non-noble metals (for examples, Fe, Co, Ni
and Cu) are attracting more and more attention, but the
leaching and sintering of the non-noble metals during the
reaction readily cause an irreversible deactivation [13].
Several methods were reported in the literature to obtain
eꢂcient and stable non-noble metal-based catalysts. For
example, atomic layer deposition (ALD) was one method
employed by some researchers to stabilize metal NPs on
the support to enhance the stability of the catalyst [14],
and the pyrolysis of MOFs (metal–organic frameworks)
materials was another eꢀective method to prepare carbon-
encapsulated metal/metal oxide NPs catalysts [15–22].
Actually, there were a bunch of examples in the latter case,
and the obtained catalysts were extensively investigated in
the chemoselective hydrogenation of nitroarenes. Li et al.
2 Experimental
2.1 Materials
Analytical grade reagents, such as copper nitrate
(Cu(NO ) ·3H O, > 99.5%), ethanol (> 99.7%), nitroben-
3
2
2
zene (>99.0%), N,N-dimethyllformamide (DMF,>99.5%),
and sodium borohydride (> 98%) were purchased from
Tianjin Damao Chemical Co. Ltd. Azobenzene(>97.0%)
was purchased from Shanghai Macklin Biochemical Co.
Ltd. Azoxybenzene(> 98.0%) was purchased from Shang-
hai Alfa Aesar Co. Ltd. 1-Bromo-4-nitrobenzene(>98.0%)
was purchased from Beijing Innochem Technology Co. Ltd.
1,3,5-Benzenetricarboxylic acid (H BTC, > 98%), other
3
nitroarenes and intermediates (> 98.0%) were purchased
from Aladdin Chemistry Co. Ltd. All chemicals were used
as purchased without further puriꢁcation.
2.2 Synthesis of HKUST‑1
HKUST-1 was synthesized by following the proce-
dure reported in our previous work [24]. Typically,
Cu(NO ) ·3H O (2.00 g, 8.4 mmol) and H BTC (1.00 g,
[
20] reported the preparation of an active Co–N–C mate-
3
2
2
3
rial by the pyrolysis of well-deꢁned Co-MOF (ZIF-67).
The catalyst pyrolyzed at 600 °C was found to be the most
active and selective, achieving 97% selectivity in 3-nitros-
tyrene hydrogenation at full conversion. A γ-Fe O based
4
.8 mmol) were added together in 51 mL of solution consist-
ing of equal volume of DMF, deionized water, and ethanol.
The obtained mixture was ꢁrstly treated by ultrasonication
for 30 min and then transferred into vials. The vials were
sealed and placed in an oven, heated to 85 °C at a ramping
rate of 0.5 °C/min and maintained for 20 h. The product
was separated by ꢁltration, washed with absolute ethanol
for several times.
2
3
catalyst derived from the pyrolysis of MIL-88A MOF was
reported by Jiang [21]. The resultant catalyst exhibited
excellent catalytic activity, chemoselectivity and magnetic
recyclability for the hydrogenation of diverse nitro com-
pounds under mild conditions using hydrazine hydrate as
the reductant. Yang et al. [22] reported Ni NPs encapsu-
lated in graphitic carbon layers, which was derived from
the thermolysis of MOF built up with terephthalic acid
and NiCl ·6H O. The substrates under investigation were
2.3 Preparation of Cu@C‑x
1.0 g HKUST-1 was placed in a quartz boat, which was
heated in a tube furnace in Ar atmosphere at a ꢃow rate of
100 mL/min. The sample was ꢁrstly heated from room tem-
perature to 250 °C at a ramping rate of 10 °C/min, followed
by a further heating to diꢀerent temperatures at a ramping
rate of 2 °C/min, and maintained at the ꢁnal temperature for
2 h. The obtained sample was cooled down in Ar atmosphere
and labeled as Cu@C-x, where x refers to the ꢁnal calcina-
tion temperature.
2
2
successfully hydrogenated with very high conversions and
selectivities under a condition of 140 °C and 5 bar H .
2
Cu nanocatalysts, as one of the most important earth-
abundant and inexpensive metals, have attracted consid-
erable attention because of their potential as alternatives
to the rare and expensive noble-metal catalysts in many
chemical processes [17]. However, the report on the
investigation of Cu nanocatalysts in the hydrogenation
of nitrobenzene and its derivatives was very limited [23].
Herein, we reported a facile one-step pyrolysis method
at modest temperature to prepare Cu NPs embedded in
porous carbon employing Cu (BTC) MOF as a template/
2.4 Characterization
3
2
The X-ray diffraction (XRD) patterns of the prepared
Cu@C-x samples were collected in 2 range of 5–80° using
precursor. The resultant composite catalyst was found to
be highly eꢂcient, chemoselective and recyclable for the
hydrogenation of various nitro compounds to anilines
under mild conditions using sodium borohydride as the
reducing agent.
a X’pert diꢀractometer with Cu K radiation (λ=1.5418 Å),
α
operated at 40 kV and 40 mA. N adsorption–desorption
2
isotherm of Cu@C-400 was measured on a Quantachrome
1
3

MOF‑Derived Cu‑Nanoparticle Embedded in Porous Carbon for the Eꢀcient Hydrogenation of…
apparatus at 77 K. Prior to the measurement, the sample
was dried at 423 K overnight in a vacuum oven. Then
about 100 mg of the sample was degassed at 473 K for 5 h.
The total surface area was determined by BET (Brunau-
ere–Emmette–Teller) method. The micropore volume and
the micropore surface area were obtained by t-plot method.
The scanning electron microscopy (SEM) image of the sam-
ple was performed on a Quanta 450 instrument operated
with an acceleration voltage of 20 kV. Prior to the obser-
vation, the sample was sputter-coated with a gold layer to
increase its conductivity. Transmission electron microscopy
(
TEM) measurements were carried out using a Tecnai G2
Spirit (FEI) microscope operating at 200 kV. The content of
Cu in the catalyst was measured by ICP-AES (inductively
coupled plasma-atomic emission spectrometry) on a Perkin
Elmer Nex ION 300D instrument. The X-ray photoelectron
spectrometer (XPS) measurements were conducted on a
commercial XPS system ESCALAB250 (angle-resolved
XPS using a theta probe) equipped with a hemispherical
electron analyzer and monochromatic Al Kα X-ray exciting
Fig. 1 XRD patterns of Cu@C-x samples
and (220) crystal planes of metallic Cu species (PDF no.
04-0836). It is worth noting that a weak peak at 2θ around
36° was also observed in Cu@C-400, which can be attrib-
uted to the characteristic peaks for Cu O (PDF no. 05-0667).
2
–
8
source (1486.6 eV). The degree of vacuum is 1× 10 Pa.
The intensities of the diꢀraction peaks observed on Cu@C-
2
.5. General procedure for the reduction of nitroarenes.
In a typical run, nitroarene (2 mmol), EtOH (10 ml) and
400 clearly suggest that the Cu species are dominant in the
0
state of Cu with the presence of trace amount of Cu O. For
2
the catalyst Cu@C-x (15 mg) were charged successively
into a glass test tube (20 ml) equipped with magnetic stir-
the catalysts prepared at higher calcination temperatures,
two phenomena were observed: (1) several new diꢀrac-
tion peaks at 2θ of 35°and 38°attributed to the existence of
rer. The mixture was ꢁrstly stirred at 45 °C, and then NaBH
4
(
4 mmol) was added in two times, that is, 2 mmol of NaBH
CuO (PDF no. 48-1548, 45-0937) emerged, indicating that
4
0
was added at the initial time, and the other 2 mmol of NaBH
besides Cu and Cu O, CuO was produced at higher calci-
4
2
was added when the reaction proceeded 15 min. After the
designated reaction time, the solution was analyzed by gas
chromatography (GC, Tianmei 7890F) with a SE-54 column
nation temperatures; (2) The intensity of the peaks became
stronger as the calcination temperature increased, suggesting
0
that the particle size of Cu and its oxides was getting larger.
(
30 m×0.32 mm×0.5 μm) to monitor the concentrations of
The morphology of HKUST-1 and Cu@C-400 was shown
in Fig. 2. It can be seen that the as-prepared HKUST-1 pre-
cursor (Fig. 2a) are typical octahedral shape with a smooth
surface. After thermolysis at 400 °C for 2 h under Ar atmos-
phere, no change in the morphology was detected in Cu@C-
400 (Fig. 2b). This phenomenon was also observed in other
literature [25]. However, the surface of the octahedron
became rough, being covered with lots of highly dispersed
and nano-scaled particles.
reactants and products. Products were identiꢁed by com-
paring the retention times with the authentic chemicals and
further conꢁrmed by GC–MS (HP6890). The conversion
and selectivity were calculated using the area normaliza-
tion method.
For recycle experiments, the catalyst was separated from
the reaction solution by extraction using a metal ꢁlter, and
then the catalyst attached to the ꢁlter was washed down by
ethanol into the original glass test tube. Finally, new reac-
tants were charged into the test tube for the next catalytic run
without drying the catalyst.
The TEM image of Cu@C-400 (Fig. 2c) showed that the
Cu and its oxide NPs are highly dispersed in the carbon
matrix. And it is worth noting that the Cu NPs were embed-
ded in the carbon matrix, rather than dispersed on the sur-
face of the carbon material. The average size of Cu and its
oxide NPs was determined to be 27 nm (Fig. 2d). However,
the average particle size of Cu NPs in Cu@C-700 increased
to 77 nm when the pyrolysis temperature of HKUST-1 was
risen to 700 °C (not shown for brevity), which was in good
accordance with the enhanced intensity of the diꢀraction
peaks in XRD results. The Cu content in Cu@C-400 was
determined to be 64.3 wt% by ICP-AES analysis, equivalent
to 7.6 mol % of Cu@C-400.
3
Results and Discussion
3.1 Properties of the Catalysts
The as-prepared composite was well characterized by XRD,
SEM, TEM, XPS and N adsorption. As shown in Fig. 1,
2
XRD pattern of Cu@C-400 showed three diꢀraction peaks
at 2θ=43°, 51°, and 74°, corresponding to the (111), (200),
1
3

C. Qiao et al.
a
b
Cu 2p 3/2
9
31.7(Cu O )
2
932.2
(
Cu
)
933.8
(
CuO
)
c
d
2
0
0
0
926
928
930
932
934
936
938
940
942
Binding Energy(eV)
1
Fig. 3 Cu 2p3/2 XPS spectrum of Cu@C-400
10
20
30
40
50
increased amount of Cu O (compared with the small amount
diameter / nm
2
of Cu O detected by the XRD technique in Fig. 1) may be
2
attributed to the facile oxidation of the surface metallic Cu
Fig. 2 SEM images of HKUST-1 (a) and Cu@C-400 (b); TEM
image of Cu@C-400 (c) and the corresponding size distribution of
Cu NPs (d)
in Cu@C-400 when exposing to air.
3
.2 Optimal Reaction Conditions for Transfer
Hydrogenation of Nitrobenzene
The speciꢁc surface area and the porosity of Cu@C-
4
00 and its precursor HKUST-1 were then evaluated by N
A series of control experiments were ꢁrstly performed to
clearly understand the role of the catalyst Cu@C-x and
the reducing agent. It can be seen that hydrogenation of
nitrobenzene to aniline did not proceed at all in the absence
2
adsorption–desorption isotherms, and their textural prop-
erties were listed in Table 1. The total pore volume and
the BET surface area of Cu@C-400 were determined to
3
2
be 0.09 cm /g and 42 m /g, respectively, much lower than
of catalyst (Entry 1, Table 2) or NaBH (Entry 3, Table 2),
4
3
2
those of HKUST-1 (0.46 cm /g and 1169 m /g). The BET
surface area of Cu@C-400 was much lower than the values
indicating that both catalyst and NaBH are necessary for
4
the reduction of nitrobenzene. After that, HKUST-1 before
and after pyrolysis at diꢀerent temperatures were screened in
the transfer hydrogenation of nitrobenzene. When HKUST-1
was directly employed as the catalyst (Entry 2, Table 2), the
conversion was only 11% with 68% selectivity to aniline.
And it’s interesting that the pyrolysis temperature showed
little eꢀect on the catalytic activity of Cu@C-x catalysts. All
Cu@C-x catalysts exhibited 100% conversion of nitroben-
zene, except for Cu@C-300 showing a lower conversion of
96% under identical conditions (Entries 4–8, Table 2). It is
worth noting that the selectivity to aniline was 100% over
Cu@C-x in all cases. Compared with HKUST-1 precursor,
the pyrolysis products Cu@C-x showed evidently improved
catalytic activity, even though the pyrolysis temperature was
as low as 400 °C. The signiꢁcant diꢀerence in nitrobenzene
2
2
of 177 m /g and 285 m /g for the samples reported elsewhere
[
18, 25], possibly due to the diꢀerent ramping rates, pyroly-
sis temperatures, and pyrolysis atmosphere.
The XPS technique was used to clarify the chemical
states of surface Cu species in Cu@C-400 and the result was
displayed in Fig. 3. The Cu 2p spectrum of Cu@C-400
3
/2
showed three major peaks at binding energy (BE) of 931.7,
0
9
32.2, and 933.8 eV, suggesting the existence of Cu O, Cu
2
2
+
and CuO, respectively [26]. Since only Cu species was
identiꢁed for HKUST-1 [27], it can be deduced that reduc-
2
+
0
tion of Cu species to Cu O and Cu took place during the
2
process of HKUST-1 pyrolysis. And it is worth noting that
0
nearly equivalent amount of Cu and Cu O was detected
2
on the surface of Cu@C-400. The presence of evidently
Table 1 The textural properties
SBETa (m /g) Smicrob (m /g) Smeso (m /g) VTotal (cm /g) Vmicrob (cm /g) Vmeso (cm /g)
2
2
2
3
3
3
Sample
of diꢀerent samples
Cu@C-400
HKUST-1
42
1169
8
1092
34
77
0.09
0.46
0
0.41
0.09
0.05
a
BET method
t-plot method
b
1
3

MOF‑Derived Cu‑Nanoparticle Embedded in Porous Carbon for the Eꢀcient Hydrogenation of…
Table 2 Screening the catalytic
Entry
Catalyst
NaBH
Conv. (%)
Sel. (%)
Aniline
4
performance of diꢀerent
catalysts with reduction of
nitrobenzene
(
mmol)
Azo
Azoxy
1
2
3
4
5
6
7
8
–
4
4
0
4
4
4
4
4
0
11
2
0
68
0
0
0
0
0
0
0
0
0
32
0
0
0
0
0
0
HKUST-1
Cu@C-400
Cu@C-300
Cu@C-400
Cu@C-500
Cu@C-600
Cu@C-700
100
100
100
100
100
100
96
100
100
100
100
Reaction conditions: Nitrobenzene (2 mmol), EtOH (10 mL), NaBH (4 mmol, added in two times), Cata-
4
lyst (15 mg), 45 °C, 30 min
conversion between HKUST-1 and Cu@C-400 (11% vs.
completely converted within 5 min, with a 100% selectiv-
ity to aniline. (See Fig. S1 in the Supporting Information).
This phenomenon suggested that the reaction rate was much
0
1
00%) clearly demonstrates that metallic Cu or Cu O are
2
the active sites for the reduction of nitrobenzene.
Since both metallic Cu and Cu O were detected on the
faster than the case that 4 mmol of NaBH was added to
2
4
surface of the catalyst (XPS spectrum in Fig. 3), which state
of Cu species plays dominant catalytic role in the reduction
of nitrobenzene is a question needs to be elucidated. It was
reported that the metallic Cu species was acting as the active
sites, on which nitro group was adsorbed and ꢁrstly reduced
to nitroso group [28]. In order to verify the catalytic role
of metallic Cu, the following experiments were designed,
the reaction system at one time. The reason is that the ꢁrst
addition of 2 mmol NaBH reduced the Cu O on the sur-
4
2
face of the catalyst to metallic Cu, exposing more catalytic
active sites, so that the reduction of nitrobenzene proceeded
much quickly. The above results demonstrated that metal-
lic Cu played dominant catalytic role in the reduction of
nitrobenzene.
that is, 4 mmol of NaBH was charged in two steps. Step
Some literatures concerning the hydrogenation of
nitrobenzene using diꢀerent catalysts and reducing agents
were summarized in Table 3[29–37]. It can be seen that a
4
1
: 15 mg of Cu@C-400 was dispersed into EtOH (10 mL),
and then 2 mmol of NaBH was added. The above mixture
4
−
1
was stirred at 45 °C for 30 min. Step 2: 2 mmol of nitroben-
TOF value of 51.0 h was obtained when using Cu@C-
400 as the catalyst in this work, though this number was
lower than some noble metal-based catalysts, such as Ag/
zene and another 2 mmol of NaBH were charged into the
4
above mixture. It can be clearly observed that the reaction
proceeded quite violently in step 2, a lot of bubbles emerged
rapidly, and the GC analysis showed that nitrobenzene was
−
1
−1
Fe O (247.7 h ) [36]and NAP-Mg-Au(0) (112.1 h )[37],
2
3
Cu@C-400 showed higher or comparable catalytic eꢂciency
Table 3 Comparison with previously reported catalysts of the reduction of nitroarenes
Yielda
TOF (h−1)
b
Refs
Catalyst
Mass of cata- Mole of NB Reaction conditions
lyst (mg)
(mmol)
c
Cu-BTC@Fe O4
Cu(10%)-Meso-PANI
15
50
50
2.5
400
40
20
5
1
1
1
0.2
2
1
1
0.2
1.078
2
NaBH (4 ), 45 °C, 3 h
99
100
85
7.05
8.2
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
3
4
NaBH (5), 40 °C, 3 h
NaBH (3), 50 °C, 15 min
NaBH (10), room temperature, 90 min
NaBH (5.3), 0–5 °C ,10 min
HCOOH(3), 110 °C, 3 h
HCOOH(3.5), 100 °C, 5 h
NaBH (10), 100 °C, 30 min
NaBH (46.4), room temp., 60 min
NaBH (2), 45 °C, 30 min
4
Fe O @SiO /EP.EN.EG@Cu
29.7
47.1
36.9
2.25
3.35
247.7
112.1
51.0
3
4
2
4
Cu/SiO @NiFe O
95
2
2
4
4
Ni-Boride-Silica
Co@CN-600-AT
Co O -NGr/C
100
43.1
96
99
85
4
3
4
Ag/α-Fe O3
2
4
NAP-Mg-Au(0)
Cu@C-400
75
15
[37]
This work
4
100
4
a
Yield of aniline
b
the mass of the converted reactant, g
TOF = [
the mass of Cu,g]×[reaction time, h]
cThe number in the brackets represents the molar ratio of the reducing agent to nitrobenzene
1
3

C. Qiao et al.
than some non-noble metal-based catalysts [29–35]. Espe-
cially, when considering the merits of mild reaction condi-
selectivity can be achieved, suggesting that the sterically
hindered chloro- group had no eꢀect on the outstanding
activity and selectivity. It is well known that the most-
challenging substrates are those that bear other easily
reducible groups, and it is satisfying that the catalyst
Cu@C-400 reduced nitro groups in the presence of CN
group to its corresponding aniline product with excellent
conversion and selectivity (Entry 9, Table 4). But for alde-
hyde substituted nitrobenzene, only a conversion of 65%
was achieved, and the aldehyde group was also reduced
at the same time (Entry 10, Table 4). The above results
demonstrated that Cu@C-400 displayed outstanding cata-
lytic activity and chemoselectivity for hydrogenation of
tions, short reaction time, and the low molar ratio of NaBH
4
to nitrobenzene, Cu@C-400 was a potential candidate for
replacing noble metal-based catalysts in the hydrogenation
of nitrobenzene.
3
.3 Catalytic Performance of Cu@C‑400
in the Hydrogenation of Various Nitroarenes
A variety of substituted nitroarenes containing electron-
donating (such as methyl, ethyl) and electron-withdrawing
groups (such as ꢃuoro-, chloro-) at diꢀerent positions were
investigated under the standard test condition to explore
the general applicability of Cu@C-400. As shown in
Table 4, the catalyst Cu@C-400 exhibited high activities
many nitroarenes using NaBH as reducing agent except
4
for aldehyde substituted nitrobenzene.
(
100% conversion) for the hydrogenation of industrially
relevant nitroarenes, such as ꢃuoro-, chloro-, and iodo-
nitrobenzenes, with 100% selectivity to their correspond-
ing anilines without the formation of any dehalogenation
products (Entries 1, 2, 6, Table 4). But for bromo-sub-
stituted nitrobenzene, dehalogenation took place, show-
ing a selectivity of 20% to aniline (Entry 5, Table 4). For
the reduction of para-, ortho-, and meta-chloro-nitroben-
zene (Entries 2–4, Table 4), 100% conversion and 100%
3.4 Recycling Test
The stability and reusability of Cu@C-400 catalyst was
investigated in terms of the reduction of nitrobenzene. As
can be seen in Fig. 4, Cu@C-400 displayed only slight
drop in nitrobenzene conversion in the second cycle, but a
dramatic decline to a conversion of 71 in the third cycle.
Considering the fact that only 15 mg of Cu@C-400 was
employed in the experiment, the poor reusability was ten-
tatively ascribed to the mass loss of the catalyst. To verify
it, the recycle experiments were repeated under the same
conditions except that 50 mg of Cu@C-400 was used. The
results displayed in Fig. S2 in the Supporting Information
clearly showed that Cu@C-400 catalyst was successfully
reused for seven cycles without any drop in nitrobenzene
conversion. Therefore, the decline in catalytic activity with
Table 4 The catalytic performance of Cu@C-400 for various sub-
strates
NO2
NH2
Cu@C-400, NaBH4
1
5 mg of Cu@C-400 mainly arose from the mass loss in
EtOH
the recovery of the catalyst.
R
R
Entry
R
Conv. (%)
Select. (%)
1
2
3
4
5
6
7
8
9
p-F
100
100
100
100
100
100
100
100
>99
65
100
100
100
100
100
1
00
97
p-Cl
o-Cl
m-Cl
p-Br
p-I
–CH3
–CH CH
71
a
80
5
0
100
100
100
100
2
3
–CN
–CHO
b
1
0
0
0
1
2
3
Reaction condition: substrate (2 mmol), Solvent (10 mL), NaBH
Recycling number
4
(
4 mmol, added in two times), Cu@C-400 (15 mg), 45 °C, 30 min
The selectivity to aniline was 20%
a
Fig. 4 Recycling test for the hydrogenation of nitrobenzene on
b
–
CHO group was reduced simultaneously
Cu@C-400
1
3

MOF‑Derived Cu‑Nanoparticle Embedded in Porous Carbon for the Eꢀcient Hydrogenation of…
Table 5 The catalytic performance of Cu@C-400 using diꢀerent
average size of ca. 27 nm. This non-noble metal Cu based
intermediates as the reactants
catalyst Cu@C-400 was demonstrated to be an eꢂcient and
stable catalyst for the reduction of a variety of substituted
aromatic nitro compounds into their corresponding amines.
This study provides a facile method to prepare catalyst with
highly dispersed active metal species inside the porous car-
bon matrix to improve the catalytic performance toward
diverse catalytic reactions.
Entry
Substrate
Conversion (%)
Selectivity (%)
Aniline
1
2
3
4
Nitrobenzene
100
100
100
100
100
100
100
100
Nitrosobenzene
Hydroxylamine
Nitrosobenzene
+
Hydroxylamine
Acknowledgements We are grateful to the ꢁnancial supports from
Natural Science Foundation of China -Liaoning United Funds (Grant
No. U1508205), Fundamental Research Funds for the Central Univer-
sities (Grant No. DUT15ZD113), and the Key Laboratory of Applied
Surface and Colloid Chemistry (Shanxi Normal University).
5
6
Azoxybenzene
Azobenzene
100
100
100
100
Reaction condition: substrate (1 mmol), Solvent (5 mL), NaBH
4
(
2 mmol, added in two times), Cu@C-400 (7.5 mg), 45 °C, 30 min
3
.5 Study of the Reaction Pathway for Transfer
References
Hydrogenation of Nitrobenzene
1
2
.
.
Tafesh AM, Weiguny J (1996) Chem Rev 96:2035
Adams JP, Paterson JR (2000) J Chem Soc Perkin Trans
So far, the commonly accepted mechanism for the reduction
of aromatic nitro compounds was proposed by Haber [38].
Generally, two diꢀerent reaction pathways, i.e., the direct
and indirect ones, are proposed. In the direct pathway, the
nitro group is reduced to the nitroso group, which is then
transformed into the corresponding hydroxylamine. Subse-
quently, the aniline product is formed by the reduction of
hydroxylamine. While in the indirect pathway, a condensa-
tion takes place between ꢁrstly produced nitrosoarene and
hydroxylamine, forming azoxy compound. And then, the
azoxy group is reduced to primary amine via the azo and
hydrazo interdediates. To elucidate the pathway involved,
several commonly obtained intermediates, such as nitroso-
benzene, hydroxylamine, azoxybenzene and azobenzene,
were used as the individual or mixed reactants under the
identical reaction condition to gain some insight into the
reaction pathways. The results were listed in Table 5. It can
be seen that no matter which intermediate substance was
used as the reactant, the Cu@C-400 catalyst showed an
excellent catalytic performance with 100% reactant conver-
sion and 100% selectivity to aniline, indicating that nitroben-
zene can be converted to aniline through both direct and
indirect pathways.
1
(22):3695
3
.
Figueras F, Coq B (2001) J Mol Catal A 173:223
4. Downing RS, Kunkeler PJ, van Bekkum H (1997) Catal Today
3
7:121
5
6
.
.
Hashmi AS, Hutchings GJ (2006) Angew Chem Int Ed 45:7896
Wienhofer G, Sorribes I, Boddien A, Westerhaus F, Junge K,
Junge H, Llusar R, Beller M (2011) J Am Chem Soc 133:12875
7. Jagadeesh RV, Wienhofer G, Westerhaus FA, Surkus AE,
Pohl MM, Junge H, Junge K, Beller M (2011) Chem Commun
4
7:10972
8
.
Shalom M, Molinari V, Esposito D, Clavel G, Ressnig D,
Giordano C, Antonietti M (2014) Adv Mater 26:1272
9. Liu R, Mahurin SM, Li C, Unocic RR, Idrobo JC, Gao H, Pen-
nycook SJ, Dai S (2011) Angew Chem Int Ed 50:6799
1
0. Zhao Z, Yang H, Li Y, Guo X (2014) Green Chem 16:1274
1. Lara P, Philippot K (2014) Catal Sci Technol 4:2445
2. Zhang S, Chang C-R, Huang Z-Q, Li J, Wu Z, Ma Y, Zhang Z,
Wang Y, Qu Y (2016) J Am Chem Soc 138:2629
1
1
1
1
3. Lee J, Jackson DK, Li T, Winans RE, Dumesic JA, Kuech TF,
Huber GW (2014) Energy Environ Sci 7:1657
4. O’Neill BJ, Jackson DK, Crisci AJ, Farberow CA, Shi F, Alba-
Rubio AC, Lu J, Dietrich PJ, Gu X, Marshall CL, Stair PC, Elam
JW, Miller JT, Ribeiro FH, Voyles PM, Greeley J, Mavrikakis M,
Scott SL, Kuech TF, Dumesic JA (2013) Angew Chem Int Ed
5
2:13808
1
1
5. Long J, Zhou Y, Li Y (2015) Chem Commun 51:2331
6. Huang G, Yang L, Ma X, Jiang J, Yu S-H, Jiang H-L (2016) Chem
Eur J 22:3470
1
1
1
7. Formenti D, Ferretti F, Scharnagl FK, Beller M (2019) Chem Rev
1
19:2611
8. Karahan Ö, Biçer E, Taşdemir A, Yürüm A, Gürsel SA (2018) Eur
J Inorg Chem 9:1073
4
Conclusion
9. Sun X, Olivos-Suarez AI, Osadchii D, Romero MJ, Kapteijn F,
Gascon J (2018) J Catal 357:20
A facile and straight-forward strategy was developed to
prepare Cu nanoparticles embedded into carbon mate-
rial (Cu@C-400) by calcining HKUST-1 at 400 °C in Ar
atmosphere. The XRD pattern of Cu@C-400 illustrated that
metallic Cu was in the dominant amount and existed in small
particle size. The catalyst Cu@C-400 showed similarity to
HKUST-1 in the morphology, the TEM image of Cu@C-400
revealed the existence of large amount of Cu NPs with an
2
2
2
0. Wang X, Li Y (2016) J Mol Catal A 420:56
1. Li Y, Zhou Y-X, Ma X, Jiang H-L (2016) Chem Commun 52:4199
2. Tang B, Song W-C, Yang E-C, Zhao X-J (2017) RSC Adv 7:1531
23. Zeynizadeh B, Zabihzadeh M, Shokri Z (2016) J Iran Chem Soc
1
3:1487
2
2
4. Tian F, Fu Z, Zhang H, Zhang J, Chen Y, Jia C (2015) Fuel
1
58:200
5. Tan H, Ma C, Gao L, Li Q, Song Y, Xu F, Wang T, Wang L (2014)
Chem Eur J 20:16377
1
3

C. Qiao et al.
2
2
6. Sasmal AK, Dutta S, Pal T (2016) Dalton Trans 45:3139
7. Niu H, Liu S, Cai Y, Wu F, Zhao X (2016) Microporous
Mesoporous Mater 219:48
35. Jagadeesh RV, Banerjee D, Arockiam PB, Junge H, Junge K, Pohl
M-M, Radnik J, Brückner A, Beller M (2015) Green Chem 17:898
36. Patra AK, Dukjoon Kim NV (2017) Appl Catal A 538:148
37. Layek K, Kantam ML, Shirai M, Nishio-Hamane D, Sasaki T,
Maheswaran H (2012) Green Chem 14:3164–3174
2
2
8. Kadam HK, Tilve SG (2012) RSC Adv 2:6057
9. Yang S, Zhang Z-H, Chen Q, He M-Y, Wang L (2017) Appl Orga-
nometal Chem 32:e4132
38. Haber F, Elektrochem Z (1898) Angew Phys Chem 22:506
3
3
0. Tumma M, Srivastava R (2013) Catal Commun 37:64
1. Rajabzadeh M, Eshghi H, Khalifeh R, Bakavoli M (2016) RSC
Adv 6:19331
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢂliations.
3
3
3
2. Parmekar MV, Salker AV (2016) RSC Adv 6:108458
3. Rahman A, Jonnalagadda SB (2008) Catal Lett 123:264
4. Jiang L, Zhou P, Zhang Z, Jin S, Chi Q (2017) Ind Eng Chem Res
5
6:12556
1
3