Co-MOF-Derived Hierarchical Mesoporous Yolk-shell-structured Nanoreactor for the Catalytic Reduction of Nitroarenes with Hydrazine Hydrate

Article abstract of DOI:10.1002/cctc.201900714

Porous nanoreactors demonstrate immense potential for applications in heterogeneous catalysis due to their excellent mass-transfer performance and stability. The design of a simple, universal strategy for fabricating nanoreactor catalysts is of significance for organic transformation. In this study, a nanoreactor with a hierarchical mesoporous yolk-shell structure was successfully prepared by the high-temperature carbonization of a ZIF-67@polymer composite. The core of the resultant Co@ZDC@mC material comprised Co NPs anchored in the ZIF-67-derived carbon framework, while the shell comprised resin-polymer-derived mesoporous carbon. The as-obtained Co@ZDC@mC-700 catalyst enriched reactants, efficiently catalyzed the reaction in the core, and permitted the desorption of the product from the nanoreactor. In the catalytic reduction of nitrobenzene with N₂H₄?H₂O, Co@ZDC@mC-700 exhibited superior catalytic efficiency (TOF=1136.3 h^?1). In addition, Co@ZDC@mC-700 exhibited excellent performance for the catalytic reduction of various functionalized nitroarenes, as well as good reusability and recyclability. Hence, a simple, useful approach for fabricating a metal-organic-framework-derived non-noble metal-based yolk-shell nanoreactor for effective catalytic transformation is proposed.

Full text of DOI:10.1002/cctc.201900714

Accepted Article
Title: Co-MOFs-derived hierarchical mesoporous yolk–shell–structured
nanoreactor for the catalytic reduction of nitroarenes with
hydrazine hydrate
Authors: Man Yuan, Hongbo Zhang, Chen Yang, Fanhao Wang, and
Zhengping Dong
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201900714
Link to VoR: http://dx.doi.org/10.1002/cctc.201900714
A Journal of
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ChemCatChem
10.1002/cctc.201900714
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Co-MOFs-derived hierarchical mesoporous yolk–shell–structured
nanoreactor for the catalytic reduction of nitroarenes with hydrazine
hydrate
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Mr. Man Yuan,[a]✝ Dr. Hongbo Zhang,[b]✝ Mr. Chen Yang,[a] Mr. Fanhao Wang[a]
[
a]
and Prof. Dr. Zhengping Dong *
[
a]
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering
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Laboratory for Chemical Catalysis, Laboratory of Special Function Materials and
Structure Design of the Ministry of Education, Lanzhou University, Lanzhou 730000,
PR China
[
b]
Institute of Nanoscience and Nanotechnology, School of Physical Science and
Technology, Lanzhou University, Gansu 730000, China
✝
Mr. Man Yuan and Dr. Hongbo Zhang contributed equally to this work.
E-mail address: dongzhp@lzu.edu.cn (Zhengping Dong), zhb19870831@163.com
(Hongbo Zhang), Tel: 86 931 8912577.
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Abstract
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Porous nanoreactors demonstrate immense potential for applications in
heterogeneous catalysis due to their excellent mass-transfer performance and stability.
The design of a simple, universal strategy for fabricating nanoreactor catalysts is of
significance for organic transformation. In this study, a nanoreactor with a hierarchical
mesoporous yolk–shell structure was successfully prepared by the high-temperature
carbonization of a ZIF-67@polymer composite. The core of the resultant
Co@ZDC@mC material comprised Co NPs anchored in the ZIF-67-derived carbon
framework, while the shell comprised resin-polymer-derived mesoporous carbon. The
as-obtained Co@ZDC@mC-700 catalyst enriched reactants, efficiently catalyzed the
reaction in the core, and permitted the desorption of the product from the nanoreactor.
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In the catalytic reduction of nitrobenzene with N
2
H
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·H O, Co@ZDC@mC-700
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exhibited superior catalytic efficiency (TOF=1136.3 h ). In addition,
Co@ZDC@mC-700 exhibited excellent performance for the catalytic reduction of
various functionalized nitroarenes, as well as good reusability and recyclability.
Hence, a simple, useful approach for fabricating a metal-organic-framework-derived
non-noble metal-based yolk–shell nanoreactor for effective catalytic transformation is
proposed.
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Keywords: Co-MOFs; Yolk-shell structured nanoreactor; Catalytic reduction;
Nitroarenes
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Introduction
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Yolk–shell-structured materials demonstrate immense potential in drug delivery,[1, 2]
energy storage and conversion,[3, 4] and catalysis[5-7] due to their customizable physical
and chemical properties. Compared to solid core–shell-structured materials, yolk–
shell-structured materials with a mesoporous wall provided an additional specific
space between the shell and core, which is beneficial for catalysis.[8-10] In addition, the
permeable shell enables the facile mass transfer of the substrate.[11] Moreover, the
active sites of the catalysts confined in the core can be protected from leaching and
agglomeration, thereby improving the stability of the yolk–shell nanoreactor
catalysts.[12-14] Hence, mesoporous yolk–shell materials that can mimic the structural
ability of living cells to achieve high catalytic efficiency are considered to be ideal
nanoreactors.[15] In recent years, some different synthetic methods have been
investigated for the preparation of yolk–shell nanoreactors, such as soft templating,
Kirkendall-effect-based methods, Ostwald ripening, selective etching, ship-in-bottle
methods, and the galvanic replacement method.[16-23] Notably, these methods afford
yolk–shell nanostructures comprising mononuclear nanocrystal and porous shells.[23]
However, for nanoreactor catalysts, it is preferable to have a large number of active
sites in the core rather than a large single core in the cavity. Besides, if the ultrafine
nanocrystals can move in the cavity, they will inevitably agglomerate into a single
large particle at high temperature during the preparation,[24] and such agglomeration is
negative for heterogeneous catalysts.[25-27] In liquid- and gas-phase catalytic
applications, catalysts with a hierarchical porous structure exhibit advantages,
including larger surface area, and render excellent mass transfer of the reactants and
products between the active sites located in the framework.[28] Hence, it is challenging
and attractive to develop a simple, cost-effective method to prepare hierarchical
mesoporous yolk–shell-structured nanoreactor catalysts.
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Meanwhile, due to their simple preparation, good morphology control, uniform
pore size, and good chemical stability, metal organic frameworks (MOFs) are
typically utilized in heterogeneous catalysis. [29-31] Meanwhile, MOF-derived porous
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carbon materials have also attracted great attention.[32-34] For example, Li et al. have
fabricated Co NPs embedded in nitrogen-doped graphite by the pyrolysis of ZIF-67
for the oxidation of alcohols to esters under base-free conditions.[35] Zang’s group has
fabricated apically Co-NP-wrapped nitrogen-doped carbon nanotubes in situ from
single-source Co-MOFs for efficient oxygen reduction.[36] Jiang et al. have reported
ZIF-67-derived Co-CoO@N-doped porous carbon for the efficient tandem
dehydrogenation of ammonia borane and the hydrogenation of nitro compounds.[37]
These catalysts exhibit excellent performance by the in situ encapsulation of metal
NPs in the carbon framework. As a result, porous carbon materials encapsulated with
non-noble metal NPs derived from MOFs as the polycrystalline core inside
nanoreactors for heterogeneous catalysis are considerably attractive. In addition,
inspired by the Stöber-type synthesis of colloidal polymer spheres,[38] it is feasible to
coat the resin polymer on the MOF outer layer to form an MOF@resin polymer,
followed by the calcination of the MOF@resin polymer to prepare a mesoporous
multi-core yolk–shell-structured nanoreactor.
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Meanwhile, aromatic amines are important fine organic intermediate chemicals for
the synthesis of dyes, pharmaceuticals, and agricultural chemicals.[39, 40] With regard
to catalysts used for the catalytic reduction of nitroarenes, heterogeneous catalysts are
widely accepted due to their facile separation and recycling.[41] Recently, several
non-noble metal-supported catalysts have been reported.[42-46] However, few
applications for the use of yolk–shell–structured nanoreactor catalysts in the catalytic
reduction of nitroarenes have been reported. In this study, a mesoporous multi-core
yolk–shell–structured nanoreactor (Co@ZDC@mC) was fabricated in which
ZIF-67-derived porous carbon material confined with Co NPs (Co@ZDC) and
resin-derived mesoporous carbon (mC) comprised the core and shell, respectively.
The gap between Co@ZDC and the mC shell provided a unique chemical
environment for the accumulation of reactants; the polycrystalline core of the Co NPs
grown in situ facilitated the contact between the active site and substrate molecules.
These special structural characteristics enhanced the transfer of mass and energy
between the substrate molecule and active site. Moreover, the external carbon shell
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effectively inhibited the agglomeration and leaching of Co NPs. The as-obtained
Co@ZDC@mC nanoreactor catalyst exhibited superior efficiency in the catalytic
reduction of nitrobenzene with N ·H O. Furthermore, Co@ZDC@mC exhibited
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H
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good reusability and recyclability. Hence, this study should provide a new platform
that contributes to the construction of mesoporous yolk–shell-structured nanoreactors
for efficient, sustainable organic transformation.
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Experimental
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Chemicals
Co(NO
3
)
2
·6H O, cetyltrimethylammonium bromide (CTAB), 2-methylimidazole,
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m-aminophenol, nitrobenzene, and other nitroarenes were purchased from Aladdin
Industrial Corporation. Methanol, ethanol, formaldehyde solution (37%), formic acid
(FA), ammonia solution (25 wt%), and N
2
H
4
·H O (80%) were purchased from
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Sinopharm Chemical Reagent Co., Ltd., China. All reagents were used as received.
Preparation of ZIF-67
ZIF-67 was prepared according to a previously reported method, albeit with some
modification.[47] Typically, Co(NO
)
2
·6H O (4 mmol) and 2-methylimidazole (4
3
2
mmol) were dissolved in methanol (150 mL) to form two transparent solutions,
followed by mixing and stirring for 24 h. The bright purple powder obtained by
centrifugation was washed with water and methanol, followed by drying for 12 h in
vacuum.
Preparation of ZIF-67@polymer
First, CTAB (1 g) was dissolved in 160 mL of a mixed solvent (water/ethanol, v:v,
1:1), followed by the addition of an ammonia solution (2 mL). Second, after stirring
for 0.5 h at room temperature, the treated suspension containing ZIF-67 (1 g) and
water (20 mL) was added to the above mixed solution, followed by stirring for 30 min.
Next, 0.8 g of m-aminophenol was added in the mixture and stirred for 30 min. Then,
after adding a formaldehyde solution (1.1 mL), the mixture was transferred to the
crystallization kettle and maintained at 110 °C for 24 h. The final product was
obtained by filtration after cooling to room temperature, followed by washing with
water and ethanol. A brown solid was obtained by drying at 100 °C for 24 h, referred
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to as the ZIF-67@polymer.
Preparation of Co@ZDC@mC
The as-obtained ZIF-67@polymer was heated to 700 °C at a heating rate of 2.5 °C
min 1 under N
−
and maintained constant for 4 h. The obtained black material was
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denoted as Co@ZDC@mC-700. Co@ZDC@mC-500, Co@ZDC@mC-600,
Co@ZDC@mC-800, and Co@ZDC@mC-900 were prepared by the same method
and subjected to carbonization at 500, 600, 800, and 900 °C, respectively. The
prepared catalysts were stored in air for use.
Catalytic reduction of nitroarenes
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Typically, nitro compounds (1 mmol), Co@ZDC@mC-700 (10 mg), and ethanol (2
mL) were first added to a 10-mL flask. Second, after maintaining the temperature at
80 °C for 5 min, N
2
H
4
·H O (200 μL) was added to start the reaction. Equivalent
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reaction samples were collected with syringes at regular intervals. Third, after the
reaction, Co@ZDC@mC-700 was recovered using an external magnet, washed
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several times with water and ethanol, and dried for subsequent cycles.
Characterization
Fourier transform infrared (FTIR, Nicolet FT-170SX) spectroscopy was employed
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for the functional-group identification of the ZIF-67@polymer and
Co@ZDC@mC-700. Thermogravimetric analysis (TGA, TA-Q50) of the
ZIF-67@polymer was carried out from room temperature to 1000 °C under N .
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Transmission electron microscopy (TEM, Tecnai G20 F30) was employed to observe
the morphology of the Co@ZDC@mC catalysts, the size and dispersion of Co NPs in
the core, and the morphology of the porous carbon shell pore. Energy-dispersive
X-ray spectroscopy (EDS) was employed to analyze the elemental composition of
Co@ZDC@mC-700. The chemical and electronic states of the elements in the
prepared catalysts were determined by X-ray photoelectron spectroscopy (XPS,
Perkin-Elmer PHI-5702). In addition, the Raman spectra of the as-obtained catalysts
were recorded on a Jobin Yvon LabRam HR Evolution system. The crystalline phase
structures of the Co@ZDC@mC catalysts and recovered Co@ZDC@mC-700 were
observed by powder X-ray diffraction (XRD) by using a Rigaku D/Max-2400
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diffractometer under Cu-Kα radiation. Brunauer–Emmett–Teller (BET, Micromeritics
ASAP 2010) analysis was employed to record nitrogen adsorption–desorption curves
and pore size distribution of the Co@ZDC@mC-700 catalyst. A vibrating sample
magnetometer (VSM, MicroSense) was utilized to measure the room-temperature
magnetic property of Co@ZDC@mC-700. Inductively coupled plasma optical
emission (ICP-OES, PQ 9000) spectroscopy was employed to calibrate the Co content.
The conversion and selectivity of the reaction were determined by gas
chromatography–mass spectrometry (GC–MS, Agilent 5977E).
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Scheme 1. The preparation procedure of Co@ZDC@mC catalyst.
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Results and discussion
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Fig. 1. (a) FTIR spectra of ZIF-67@polymer and Co@ZDC@mC-700; (b) TGA curve of the
ZIF-67@polymer.
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Scheme 1 shows the preparation of Co@ZDC@mC. Under mild conditions, the
core–shell precursor of the ZIF-67@polymer was formed by the coating of a resin
polymer on the ZIF-67 surface. Then, the ZIF-67@polymer was carbonized at high
temperatures to obtain the desired product Co@ZDC@mC. During the calcination
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process, ZIF-67 formed Co@ZDC core, and the outer layer resin polymer with CTAB
as a pore-forming template formed mC shell.
From the FTIR spectra shown in Fig. 1a, the bond changes of the ZIF-67@polymer
during high-temperature pyrolysis and the formation of Co@ZDC@mC-700 were
−
1
clearly distinguished. FTIR peaks observed at 1143.8 and 1284.1 cm corresponded
to the C–N and C–O bonds in the shell, respectively, which were retained after the
polymerization of m-aminophenol. In the core, C=N, which served as the marker for
−
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the imidazole framework, was observed at 1437.2 cm . Notably, the C=C bond in the
imidazole framework of ZIF-67 in the core and the aromatic C=C bond in the shell
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were observed at 1617.5 cm . Compared to the FTIR spectra of Co@ZDC@mC-700,
a majority of these bonds were pyrolyzed at 700 °C, and only a few key C=C, C=N,
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and C–N bonds were retained. The difference is that the bond observed at 2850.2 cm ,
−
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corresponding to the methyl group in the core, and the bond observed at 2937.4 cm ,
corresponding to the methylene group in the shell, disappeared, indicating that
chemical changes occur during pyrolysis. High-intensity OH and NH stretching
vibrations were observed at 3396.8 cm 1 in the FTIR spectrum of the
ZIF-67@polymer, while the corresponding stretching vibrations in the FTIR spectrum
of Co@ZDC@mC-700 catalyst were weak. Besides, the stretching vibration peak of
the phenolic hydroxyl group at 3637.2 cm 1 completely disappeared after the
high-temperature calcination of the ZIF-67@polymer. FTIR spectra revealed that a
majority of the groups in the core and shell were pyrolyzed at 700 °C. TGA curves
were recorded to analyze the thermal stability and pyrolysis of the ZIF-67@polymer
(Fig. 1b). At a temperature of less than 182.1 °C, the ZIF-67@polymer maintained
good thermal stability, with a marginal weight loss of ~7.3%, possibly related to the
loss of bound water. With the further increase in the carbonization temperature to
507.7 °C, the pyrolysis of the resin polymer shell and ZIF-67 core started, followed
by a weight reduction of 33.5%. At temperatures between 507.7 and 709.3 °C, the
material was further pyrolyzed at a steady rate, leading to a continuous weight loss of
26.4%. The pyrolysis of materials tended to ended, which was characterized by a slow
weight loss in TGA at temperatures greater than 709.3 °C. Hence, different
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carbonization temperatures of Co@ZDC@mC nanoreactor catalysts are investigated
from 500 to 900 °C.
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Fig. 2. TEM images of ZIF-67 (a), ZIF-67@polymer (b), Co@ZDC@mC-700 (c) and (d);
HRTEM images of Co@ZDC@mC-700 for Co NPs (e) and shell (f); (g) particle size
distribution histograms of the Co NPs in Co@ZDC@mC-700; (h) elemental mapping images
of Co@ZDC@mC-700.
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Fig. 2 shows the TEM images of the prepared materials. ZIF-67 was a regular
polyhedron with a particle diameter of ~700 nm and a side length of ~300 nm (Fig.
2a). This observation was consistent with the previously reported regular prismatic
dodecahedron of ZIF-67,[48] indicative of the successful preparation of ZIF-67. And
XRD spectrum of ZIF-67 prepared in this work is shown in Fig. S1. Compared with
ZIF-67, the particle size of the ZIF-67@polymer approximately increased to 900 nm,
and the thickness of the resin polymer shell was ~100 nm (Fig. 2b). The increase of
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particle size indirectly verified the successful preparation of the ZIF-67@polymer.
Besides, from the SEM image of ZIF-67, ZIF-67 clearly exhibited some changes
during the hydrothermal process, which was intuitively reflected by the slight
deformation and distortion of its morphology. In fact, the deformation of this shape
did not adversely affect the encapsulation of the resin polymer on its surface; however,
it served as a positive response to the encapsulation behavior, which was more
conducive to the uniform shell thickness. Fig. 2c shows the TEM image of the
Co@ZDC@mC-700 nanoreactor catalyst calcined at 700 °C. From the TEM images,
the ZIF-67@polymer shrank inward, and by high-temperature carbonization, it was
converted into approximately spherical yolk–shell Co@ZDC@mC-700 NPs with a
size of 250 nm. The morphology of Co@ZDC@mC was clearly observed after
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enlargement in Fig. 2d. After carbonization, Co in ZIF-67 formed Co NPs, and it
was anchored by porous carbon formed from the imidazole framework, affording a
Co@ZDC core. Numerous Co NPs that were formed in situ from ZIF-67 together
constituted the polycrystalline core, which increased the contact probability between
the substrate molecule and active site after the substrate molecule entered into the
nanoreactor. Furthermore, porous carbon derived from the imidazole framework of
ZIF-67 immobilized Co NPs, making them no erosion and agglomeration. Meanwhile,
the mC shell thickness was ~20 nm; this thickness is ~5 times less than that of the
polymer shell precursor. The change in the shell thickness before and after
carbonization further verified that the ZIF-67@polymer shrinks to the center and is
carbonized at high temperature rather than the destruction and crushing of the
complete structure. Moreover, the mC shell further protected Co from loss during the
reaction, and the gap between the mC shell and core provided a special space to
enrich the substrate molecules. Hence, the Co@ZDC@mC-700 nanoreactor catalyst is
successfully prepared. Fig. S2 shows the TEM images of Co@ZDC@mC prepared at
different
carbonization
temperatures.
Similar
to
Co@ZDC@mC-700,
and
Co@ZDC@mC-500,
Co@ZDC@mC-600,
Co@ZDC@mC-800,
Co@ZDC@mC-900 exhibited approximately spherical yolk–shell structures. At high
carbonization temperatures, the yolk–shell nanoreactor still maintained fine shape.
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The shell was less affected by temperature; the change based on the temperature was
mainly reflected in the core. For a more detailed investigation of Co@ZDC@mC-700,
HRTEM images were recorded (Fig. 2e and f). The Co(111) crystal lattice of metallic
Co and C(002) crystal lattice of the Co@ZDC core were observed,[49] indicating that
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the reduction of Co leads to the formation of Co and the strong pyrolysis of the
organic framework leads to the formation of graphitic C (Fig. 2e). In addition, the
lattices of Co NPs in the core of the Co@ZDC@mC nanoreactor obtained at other
temperatures were analyzed. The (220) and (311) planes of Co
3
4
O in the HRTEM
images of Co@ZDC@mC-500 were clearly observed (Fig. S3a).[49] Notably, the
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Co(111) crystal lattice of metallic Co was hardly observed in the HRTEM image of
2+
Co@ZDC@mC-500, suggestive of the partial reduction of Co , albeit not completely
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reduced to Co at 500 °C. With the increase in the carbonization temperature to
0
600 °C, a part of Co
3
O
4
was further reduced to Co , which was reflected by the
appearance of the Co(111) crystal plane corresponding to metallic Co, while
Co (220) did not disappear (Fig. S3b). Similar to Co@ZDC@mC-700, only
3
O
4
Co(111) was observed in the HRTEM images of Co@ZDC@mC-800 and
Co@ZDC@mC-900 (Fig. S3c and d, respectively). Several microporous channels
were observed in the mC shell, providing qualifications for the entry of the substrate
molecules and the release of the product molecules over the nanoreactor (Fig. 2f).
Similarly, the same phenomena were observed in the HRTEM images for the shell of
Co@ZDC@mC carbonized at other temperatures (Fig. S4). Fig. 2g and Fig. S5 show
the particle size distribution of Co NPs. The average diameters of Co NPs in the core
were 5.2, 6.0, 8.9, 19.4, and 36.3 nm for Co@ZDC@mC-500, Co@ZDC@mC-600,
Co@ZDC@mC-700, Co@ZDC@mC-800, and Co@ZDC@mC-900, respectively.
The size of the Co NPs increased at high carbonization temperature. The
agglomeration of Co NPs in the Co@ZDC core possibly led to decrease in the number
of crystal cores in the nanoreactor. Meanwhile, larger particles were not conducive to
catalytic reactions. Thus, the Co@ZDC@mC nanoreactor catalysts obtained at higher
temperatures (>700°C) might exhibit poor catalytic efficiency. The element mapping
image of Co@ZDC@mC-700 clearly revealed that the C shell exhibits a larger
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coverage area and encapsulates the Co cores, while Co NPs are evenly dispersed in
the core (Fig. 2h). In addition, EDS analysis verified the presence of C, N, O, and Co
in Co@ZDC@mC (Fig. S6).
4
5
Fig. 3. Wide-range XPS spectra of Co@ZDC@mC obtained at different calcination
6
7
temperatures.
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Fig. 3 shows the XPS wide spectra of the Co@ZDC@mC nanoreactor catalysts
prepared at different temperatures. In accordance with the results obtained from EDS,
Co, C, N, and O were observed. The N 1s signal corresponding to the Co@ZDC@mC
catalyst calcined at high temperature was because of the deepening of pyrolysis and
the decrease of the N content, which was also observed from the elements analysis
results (Table S2). Similarly, the O 1s signal also exhibited the same regularity. Owing
to the fact that Co NPs were wrapped in the nanoreactor core, the signal intensity of
Co 2p was extremely weak. Fig. 4a shows the C 1s spectra of different
Co@ZDC@mC catalysts. Signals characteristic of C in all materials were mainly
observed at 284.6 eV, indicating that C in the as-obtained catalysts mainly exists as
C=C. The weak signal intensity corresponded to C=O (286.2 eV) and O-C=O (288.7
eV), suggestive of the pyrolysis of oxygen-containing groups during carbonization.
Notably, in the C 1s spectrum of Co@ZDC@mC-500, the signal corresponding to the
C=N bond in the imidazole framework of ZIF-67 was not observed, indicating that
ZIF-67 in the precursor undergoes chemical changes during pyrolysis at 500 °C where
the imidazole framework is destroyed. With the increase in the carbonization
temperature to 600 °C, the signal corresponding to the C=N bond (285.3 eV) was
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
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3
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9
0
1
2
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9
0
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2
3
4
5
6
observed,[50] and it did not disappear until 900 °C, possibly related to the chemical
change of N and C in the material at higher temperatures, affording a new C=N bond.
Peaks corresponding to pyridinic N and pyrrolic N were observed at 398.1 and 399.5
eV, respectively,[51] in the N 1s spectra of different Co@ZDC@mC samples (Fig. 4b).
Particularly, a high temperature of 900 °C led to the disappearance of pyridine N and
the increase of graphitization degree, leading to the formation of graphitic N (401 eV)
in Co@ZDC@mC-900.[52] Table S1 summarizes the contents of different N species.
Different carbonization temperatures affected the type and content of N in
Co@ZDC@mC. The relative content of different N species varied with the
carbonization temperature. Fig. 4c shows the high-resolution Co 2p spectra of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
different Co@ZDC@mC catalysts. Peaks observed at 777.8 eV, 779.2 eV, and 782.0
0
N
, Co-N, respectively.[53, 54] The
eV corresponded to Co , Co
x y
O , and CoC
x
y
comparison of the Co 2p spectra of the catalysts calcined at different temperatures
0
revealed that Co is not formed at 500 °C. At a temperature greater than 600 °C, the
0
characteristic signal of Co started to appear. In addition, this change was reflected in
the HRTEM images of Co NPs. Meanwhile, the signal intensities for Co
CoC
decreased until 800 °C, indicating that Co2+ in Co@ZDC@mC can be
reduced to Co by Co-bonded C at a high carbonization temperature and consume the
surrounding C. Correspondingly, Co-N increased during the reduction of Co and
CoC at carbonization temperatures, and its signal intensity increased gradually and
x
O and
y
N
x y
0
x
O
y
N
x y
stabilized after reaching 900 °C. Table S2 summarizes the contents of N and Co in
Co@ZDC@mC. As detected by elemental analysis, contents of N in
Co@ZDC@mC-500,
Co@ZDC@mC-600,
Co@ZDC@mC-700,
Co@ZDC@mC-800, and Co@ZDC@mC-900 were 5.53%, 5.43%, 3.51%, 1.82%,
and 1.19%, respectively. As estimated by ICP-OES analysis, the corresponding
contents of Co were 2.14%, 2.94%, 3.11%, 3.22%, and 3.31%.
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Fig. 4. C 1s spectra (a), N 1s spectra (b) and Co 2p spectra (c) of Co@ZDC@mC obtained at
different calcination temperatures.
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 5a shows the XRD spectra of different Co@ZDC@mC catalysts. A wide
diffraction peak corresponding to the C(002) plane was observed at 26° in the XRD
spectra of all samples.[55] Diffraction peaks characteristic of the (111), (200), and (220)
planes of metallic Co were observed at 44.1, 51.4, and 75.8°, respectively,[56] which
were observed for all samples. Moreover, with the gradual increase in the
carbonization temperature to 800 °C, the peak intensity of Co(111) gradually
1
1
1
1
1
1
1
1
1
1
2
2
+
0
increased, indicating that a high content of Co is reduced to Co . Meanwhile, the
peak intensity of Co@ZDC@mC-900 was similar to that of Co@ZDC@mC-800,
which was consistent with the results obtained from the XPS Co 2p spectra. At a
0
temperature greater than 800 °C, Co
O
x y
and CoC
x
N
y
were reduced to Co without the
0
further increase in the Co content. With the change in the carbonization temperature,
−
1
−1
peaks of D at 1350 cm and G at 1587 cm were observed in the Raman spectra of
the catalyst (Fig. 5b).[57] The I
/I value first increased and then decreased with the
carbonization temperature increasing. The I /I value for Co@ZDC@mC-700 was
D
G
D
G
relatively small. This higher degree of graphitization might be more conducive to the
transfer of electron to the active sites in catalysis.
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Fig. 5. (a) XRD spectra of different Co@ZDC@mC samples; (b) Raman spectra of different
Co@ZDC@mC samples.
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
Fig. 6a shows the nitrogen adsorption–desorption isotherm of Co@ZDC@mC to
examine its porous structure. The surface area of Co@ZDC@mC-700 was 304.1 m2
−
1
g . This large surface area was possibly related to the outer surface area, the pores in
the shell the core. The pore size of Co@ZDC@mC-700 was clearly hierarchical (inset
of Fig. 6a). Mesopore is the main type of pores with some macropores in the
nanoreactor. Moreover, microporous channels were also generated after 700 °C,
which were mainly observed in the mC shells analyzed from HRTEM images.
Mesopores with a large distribution might be produced by ZIF-67 after carbonization.
The void between the core and shell was the main contributor for the formation of
macropores. The hierarchical pore provided favorable physical properties for the
enrichment and catalytic conversion of substrate molecules in the
1
1
1
1
1
1
1
1
1
1
2
2
Co@ZDC@mC-700
nanoreactor,
rendering
superior
efficiency
to
Co@ZDC@mC-700 for the catalytic reduction of nitroarenes. The magnetism of the
catalyst serves as an effective route for the catalyst recovery.[58, 59] In addition,
magnetism is one of the important properties of Co-based catalysts. The magnetism of
Co@ZDC@mC-700 was 13.4 emu g 1 (Fig. 6b). The inset of Fig. 6b shows the
magnetic separation of the Co@ZDC@mC-700 catalyst.
−
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Fig. 6. (a) Nitrogen adsorption/desorption isotherm of Co@ZDC@mC-700, inset: pore width
distribution curve of Co@ZDC@mC-700. (b) Magnetization curve of Co@ZDC@mC-700,
inset: the magnetic separation of catalyst.
5
6
7
Scheme 2. Schematic illustration of the catalytic reduction of nitrobenzene over
Co@ZDC@mC-700.
8
9
0
1
2
3
4
5
6
7
8
9
To investigate its catalytic performance, the as-prepared Co@ZDC@mC-700
nanoreactor was applied in the catalytic reduction of nitrobenzene (Scheme 2). Table
1 summarizes the optimum process of the catalytic reaction. Under the same reaction
conditions, the catalytic performance of the catalysts obtained at different calcination
temperatures was examined (Table 1, entries 1–5). Co@ZDC@mC-700 exhibited the
best activity for the catalytic reduction of nitrobenzene, which converted 73.91% of
nitrobenzene to aniline in 5 min. By analyzing the characteristics of
1
1
1
1
1
1
1
1
1
1
Co@ZDC@mC-700, the relatively low I
D
/I value, which represents a high degree of
G
graphitization, and the high number of cores provided by small Co NPs was possibly
the reason for its excellent catalytic performance. When the metal component in the
catalyst was changed to zinc, marginal amounts of aniline were produced under the
16
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10.1002/cctc.201900714
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
same reaction conditions (Table 1, entry 6), indicating that Co in the nanoreactor is
the active component of the catalyst, and the outer mC shell does not exhibit a
catalytic effect on the reaction. Although most of nitrobenzene was reduced to aniline
in 10 min over Co@ZDC-700, which was obtained from the carbonization of ZIF-67
−
1
at 700 °C, the TOF value (109.6 h ) was considerably less than that of
−
1
Co@ZDC@mC-700 (1136.3 h ) (Table 1, entry 7). This result might be related to the
severe agglomeration of Co NPs without the shell protection during carbonization and
the lower concentration of substrate molecules at the catalyst active site during the
reaction compared with that of the nanoreactor catalysts. The reduction of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
nitrobenzene with N
Effects of the N
investigated (Table 1, entries 9–12). When other conditions are restricted,
nitrobenzene was completely reduced to aniline in 10 min using 200 μL of N ·H O.
With the decrease in the N ·H O volume to 100 μL, the reaction rate decreased to a
yield of 67.58% in 10 min. In addition, the reaction effect was worse with the use of
only inadequate 50 μL N ·H O. In addition, the catalytic reduction of nitrobenzene
over Co@ZDC@mC-700 did not occur under mild conditions with other hydrogen
sources such as H and FA (Table 1, entries 13–14). High reaction temperature is
2
H
4
·H
2
O did not occur without the catalyst (Table 1, entry 8).
2
H
4
·H
2
O volume in the reaction on the yield of aniline were
2
H
4
2
2
H
4
2
2
H
4
2
2
known to be more conducive to the reaction. With the decrease in the reaction
temperature, the conversion of nitrobenzene in 10 min also decreased to a certain
extent (Table 1, entries 15–18). The catalyst and initial substrate concentrations also
affected the reaction rate of catalysis. The concentration of the active component Co
was greater by using a high-quality catalyst, the reaction rate was more rapid, and it
would change with approximately equal spacing (Table 1, entries 19–21). Notably, the
conversion of nitrobenzene reached 100% in 10 min only with the change in the
substrate concentration (Table 1, entries 22–24). From these control experiments, the
Co@ZDC@mC-700 nanoreactor catalyzed the reduction of nitrobenzene to aniline
with N
2
H
4
·H O under mild conditions, related to its unique physical and chemical
2
properties.
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Table 1. The optimal reaction conditions screening for catalytic reduction of nitrobenzene.
m
catalyst
M
nitrobenze
VN2H4•H O
T
(°C)
80
80
80
80
80
80
80
80
80
80
80
80
80
80
70
60
50
40
80
80
80
80
80
80
t
Yield
TOFa
2
Entry
Catalyst
-
1
(
mg)
(mmol)
(μL)
(min) （%） (h )
1
2
3
4
5
6
7
8
9
Co@ZDC@mC-500
Co@ZDC@mC-600
Co@ZDC@mC-700
Co@ZDC@mC-800
Co@ZDC@mC-900
Zn@ZDC@mC-700
Co@ZDC-700
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
8
1
1
200
200
200
200
200
200
200
200
200
100
50
5
27.41
28.91
905.3
695.0
5
1
5
73.91 1679.7
1
5
25.56
18.37
1.21
561.0
1
5
392.3
1
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
-
1
98.93
109.6
-
1
-
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
1
100
67.58
2.53
0
1136.3
1
0
1
1
767.9
1
1
28.7
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
4
1
0
-
-
-
1
1atm (H2)
0.31
0.20
1
300 (FA)
200
200
200
200
200
200
200
200
200
200
1
96.60 1097.7
1
77.12
40.63
35.42
876.3
461.7
402.5
1
1
1
84.06 1194.0
70.25 1330.5
53.35 1515.6
6
1
4
1
10
10
10
1.4
1.2
0.8
100
100
100
1590.9
1363.6
909.1
M
(mmol)×Yield (%)
nitrobenze
a
3
TOF = m catalyst
× 58.9(mg⁄mmol)
.
(mg)×Co (wt%)×t (h)
4
5
6
7
8
9
0
1
Kinetic experiments were carried out to analyze the reaction dynamics (Fig. 7). Fig.
7a plots the yield of aniline from the catalytic reduction of nitrobenzene in presence of
different amounts of Co@ZDC@mC-700 at 80 °C. The initial stage of the reaction is
linear. After processing the original data, Fig. 7b shows the plots of (C
t
−C ) vs. the
0
reaction time for the catalytic reduction of nitrobenzene over different catalyst
concentrations. As can be observed intuitively, the reaction process was linear, with
reaction rate constants of 0.069, 0.05, 0.035, and 0.016 mol L−1 min−1 at
1
1
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Co@ZDC@mC-700 dosages of 10, 8, 6, and 4 mg, respectively. Linear curves for the
conversion of nitrobenzene with the reaction time at different substrate concentrations
were also observed at the initial stage of the reaction (Fig. 7c). This linear relationship
was more clearly observed after visualization (Fig. 7d). The corresponding
relationship between the nitrobenzene concentrations and reaction rate constants k
−
1
−1
−1
−1
−1
−1
were 0.4 mol L to 0.066 mol L min , 0.5 mol L to 0.069 mol L min , 0.6 mol
−
1
−1
−1
−1
−1
−1
L to 0.081 mol L min and 0.7 mol L to 0.093 mol L min respectively. In
conclusion, the catalytic reduction of nitrobenzene with N
2
H
4
·H O is a
2
pseudo-zero-order reaction.
1
0
1
1
1
2
Fig. 7. (a) Curves of aniline yields vs. reaction time with different additions of
Co@ZDC@mC-700; (b) plots of (C −C ) vs. reaction time for the reduction of nitrobenzene
over different additions of Co@ZDC@mC-700; (c) curves of aniline yields vs. reaction time
with different initial concentration of nitrobenzene; (d) plots of (C −C ) vs. reaction time for
the reduction of nitrobenzene in presence of different initial concentration of nitrobenzene.
1
1
1
1
1
3
4
5
6
7
t
0
t
0
1
1
2
8
9
0
Besides TOF, activation energy E
catalysts. To obtain the E for the catalytic reduction of nitrobenzene over
Co@ZDC@mC-700, the reaction was carried out at different reaction temperatures
a
is also an important parameter for evaluating
a
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ChemCatChem
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(Fig. 8). Fig. 8a shows the relationship between the yield of aniline and reaction time
at different reaction temperatures. In accordance with the kinetic conclusion of the
reaction, the plots of different temperatures showed a straight line at the initial stage.
Meanwhile, the reaction rate was rapid at high reaction temperature. The Arrhenius
plot revealed an apparent activation energy of 33.36 kJ mol−1 (Fig. 8b). This low
value of E
a
verified the superior catalytic performance of the Co@ZDC@mC-700
nanoreactor.
8
9
0
1
Fig. 8. (a) Yield versus reaction time graph for the Co@ZDC@mC-700 catalytic reduction of
nitrobenzene at different temperatures; (b) the Arrhenius plot (lnk vs. the reciprocal absolute
1
1
-1
temperature, 1/T (K )).
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
4
Typically, there are two methods for the hydrogenation of nitrobenzene (Fig. S7): (a)
Direct conversion via hydroxylamine intermediates (direct pathway) and (b) indirect
conversion via azobenzene intermediates (indirect pathway).[60] No intermediate was
observed by the GC–MS spectra of all samples. Thus, azobenzene (a stable
intermediate in the indirect pathway) is not produced in the reaction. Furthermore, azo
compound by-products produced from the catalytic reduction of 2,4-dinitroaniline
were not further converted to 2,4-diaminoaniline using excess reductants. Hence, the
hydrogenation of nitrobenzene with N
catalysts follows the mechanism of the “direct pathway” rather than the “condensation
way.” Scheme 3 shows the speculative specific reaction process.[61-63] First, N
·H O
2
H
4
·H
2
O over Co@ZDC@mC nanoreactor
2
H
4
2
entered the nanoreactor and diffused to the Co NP surface, followed by the bond
dissociation to form nitrogen and surface-bound hydrogen as metal hydrides. Next,
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nitrobenzene, which diffused to the Co NP surface, was reduced to nitrosobenzene by
the surface H. Then, highly active nitrosobenzene reacted further with H to form
relatively stable hydroxylamine. Finally, after hydroxylamine was further attacked by
H on Co NPs, aniline was generated and desorbed out of the Co@ZDC@mC
nanoreactor.
6
7
Scheme 3. Possible reaction mechanism for catalytic reduction of nitrobenzene with
8
N H
2 4
·H O over Co@ZDC@mC-700.
2
9
1
1
1
1
1
1
1
1
1
1
2
2
0
1
2
3
4
5
6
7
8
9
0
1
The performance of the as-prepared Co@ZDC@mC-700 catalyst was compared
with that of the reported catalysts (Table S3). Compared with other Co-based catalysts
using N ·H as the hydrogen source, the catalytic efficiency of
2
H
4
2
O
Co@ZDC@mC-700 was considerably higher (Table S3, entries 1–3); in addition, it is
considerably greater than those of the reported Co-based catalysts with other
hydrogen sources (Table S3, entries 4–6). Moreover, Co@ZDC@mC-700 also
exhibited higher catalytic activity than other transition-metal catalysts (Table S3,
entries 7–10). Notably, Co@ZDC@mC-700 exhibited better catalytic performance
than noble-metal catalysts (Table S3, entries 11–13). This high TOF value and better
catalytic performance further suggested that the Co@ZDC@mC-700 nanoreactor,
which can enrich substrate molecules surrounding the active site, can considerably
enhance the catalytic efficiency.
2
2
2
3
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ChemCatChem
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Table 2. Catalytic reaction of nitroarenes over Co@NC-700 catalyst.a
-1
Entry
1
Substrate
Product
t(min)
7
Conv. (%)
100
Sel. (%)
100
TOF (h )
1623.3
2
7
100
100
1623.3
3
4
5
7
7
7
100
100
100
100
100
100
1623.3
1623.3
1623.3
6
7
8
7
7
7
100
100
100
100
100
98.8
1623.3
1623.3
1603.9
9
7
100
96.0
73.4
99.8
100
100
1620.1
839.1
417.0
1
0
1
13
20
1
1
1
1
2
3
4
9
9
99.6
100
100
100
100
1257.5
1262.6
1122.7
10
98.8
1
1
5
6
11
12
100
100
100
100
1033.0
946.9
17
18
19
11
11
29
99.2
97.3
100
100
100
100
1024.8
1005.1
391.8
2
0
1
10
30
23
25
100
92.4
100
100
100
100
1136.3
351.1
361.7
149.5
2
2
2b
3b
73.2
32.9
2
a
3
Reaction condition: 1 mmol of substrate, 10 mg of Co@ZDC@mC-700, 2 mL of EtOH, 200
22
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ChemCatChem
10.1002/cctc.201900714
b
1
2
μL of N H
2 4
·H
2
O, 80 °C; 400 μL of N
H
2 4
·H O.
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Co@ZDC@mC-700 was used to catalyze the reduction of various nitroarenes to
further confirm its general applicability. For various halogenated nitrobenzenes,
Co@ZDC@mC-700 exhibited excellent conversion and selectivity under the
optimum reaction conditions (Table 2, entries 1–9). Almost all halogenated
nitrobenzenes did not undergo dehalogenation during catalytic reduction. Moreover,
100% selectivity was maintained for polyhalogen-substituted nitrobenzene (Table 2,
entries 10–11). Slight dehalogenation was observed in the catalytic reduction of
m-nitroiodobenzene and p-nitroiodobenzene, but it still maintained extremely high
selectivity (98.8% and 99.8%, respectively). Besides, the increase in the number of
halogenated functional groups led to the decrease of the reaction rate.
Methyl-substituted nitrobenzene perfectly conformed to the catalytic reduction
conditions, which was converted into corresponding amines (Table 2, entries 12–13).
m-Aminonitrobenzene, p-nitrobenzaldehyde and p-nitroacetophenone were also
effectively reduced over Co@ZDC@mC-700 to obtain corresponding products in a
relatively short time (Table 2, entries 14–16). With the change in the substituted
functional groups to hydroxyl, the conversion slightly decreased, albeit with 100%
selectivity in 11 min (Table 2, entries 17–18). Hence, for nitroaromatics with only one
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substituent, it can be well adapted to catalytic reduction with N
2
H
4
·H O over
2
Co@ZDC@mC-700. With the change in the number and type of substituents on
nitrobenzene, the corresponding products were obtained in high yield only by
prolonging the reaction time. 2-Methyl-3-nitroaniline was completely reduced to
2-methyl-3-aminoaniline in 29 min (Table 2, entry 19). 3-Nitro-4-chlorotoluene was
converted to 3-amino-4-chlorotoluene in 10 min (Table 2, entry 20).
2-Amino-5-nitroiodobenzene
(92.4%)
was
converted
to
2-amino-5-amino-iodobenzene in 30 min (Table 2, entry 21). The catalytic reduction
of nitroarenes with a large number of nitro groups not only took a longer time but also
exhibited lower selectivity (Table 2, entries 22–23). The main reason for the decrease
in the selectivity of m-dinitrobenzene was the formation of azo compounds during
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10.1002/cctc.201900714
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complete conversion. In addition, the azo compounds were the main by-products in
the catalytic reduction of 2,4-dinitroaniline. In summary, most nitroarenes exhibit
high conversion and excellent selectivity over Co@ZDC@mC-700 with N
2
H
4
·H O
2
under mild conditions.
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Fig. 9. (a) Reuse test of Co@ZDC@mC-700. Reaction conditions: nitrobenzene (1 mmol),
·H O (200 μL), Co@ZDC@mC-700 (10 mg), EtOH (2 mL), 80 °C, 10 min.
N H
2 4
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To investigate the reusability and recyclability of the Co@ZDC@mC-700
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nanoreactor catalyst, recycling tests were carried out (Fig. 9). After the reaction, the
catalyst was simply recovered using an external magnet. Reused Co@ZDC@mC-700
still maintained its original magnetism (Fig. S8). After six cycles, the catalytic activity
was only slightly decreased. Some characterization for reused Co@ZDC@mC-700
has been measured to verify its stability and durability. Notably, the Co content was
3.03% in Co@ZDC@mC-700 after six cycles, which was similar to the original.
From the TEM image, reused Co@ZDC@mC-700 still maintained the same
multi-core yolk–shell structure as the fresh one (Fig. S9a). Meanwhile, the crystal
lattice of the Co(111) plane corresponding to metallic Co was clearly observed in the
HRTEM image of reused Co@ZDC@mC-700 (Fig. S9b). A large number of
unblocked microporous channels were still observed in the shell (Fig. S9c). Besides,
the average particle size of Co NPs in reused Co@ZDC@mC-700 was 9.1 nm, which
was almost unchanged compared to the original (Fig. S9d). The XRD spectrum of
reused Co@ZDC@mC-700 was the same as that of fresh Co@ZDC@mC-700, in
which Co(111), Co(200), and Co(220) crystal planes were observed (Fig. S10). Hence,
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This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201900714
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the as-prepared Co@ZDC@mC-700 nanoreactor catalyst exhibits excellent
reusability and stability.
3
Summary
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In summary, by adjusting the catalyst structure to promote the contact between the
substrate molecule and active site of the catalyst, the Co@ZDC@mC nanoreactor
catalyst with an enhanced catalytic efficiency was successfully designed and prepared
by the coating of a resin polymer on the ZIF-67 surface and high-temperature
carbonization. The Co@ZDC@mC-700 catalyst was applied to the catalytic reduction
of nitroarenes with N
2
H
4
·H O, which exhibited outstanding catalytic activity and
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superior catalytic efficiency. In addition, the catalysts have higher TOF values and
better catalytic efficiency than other reported catalysts and even noble-metal based
catalysts. The Co@ZDC@mC-700 catalyst was easily recovered and was stable.
Hence, this study provides a new approach for the preparation of a non-noble
metal-based nanoreactor with extremely high catalytic efficiency for organic
transformation.
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Conflicts of interest
There are no conflicts to declare.
Supporting Information
Supporting Information is available from the publication website.
Acknowledgements
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This work was supported by the Natural Science Foundation of Gansu (No.
18JR3RA274).
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Table of Contents Entry
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Co@ZDC@mC nanoreactor with a hierarchical mesoporous yolk–shell structure was
successfully prepared by the high-temperature carbonization of a ZIF-67@polymer
composite. The Co@ZDC@mC-700 exhibited superior catalytic efficiency in the
catalytic reduction of nitrobenzene with N
and recyclability.
2
H
4
·H O, and also showed good reusability
2
1
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This article is protected by copyright. All rights reserved.