DOI: 10.1002/cctc.201900714
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Full Papers
Co-MOF-Derived Hierarchical Mesoporous Yolk-shellstructured Nanoreactor for the Catalytic Reduction of
Nitroarenes with Hydrazine Hydrate
Man Yuan+,[a] Hongbo Zhang+,[b] Chen Yang,[a] Fanhao Wang,[a] and Zhengping Dong*[a]
Porous nanoreactors demonstrate immense potential for applications in heterogeneous catalysis due to their excellent masstransfer 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 ZIF67-derived carbon framework, while the shell comprised resinpolymer-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.
Introduction
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.
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
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
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,
[a] M. Yuan,+ C. Yang, F. Wang, Prof. Dr. Z. Dong
College of Chemistry and Chemical Engineering
Gansu Provincial Engineering Laboratory for Chemical Catalysis
Laboratory of Special Function Materials and Structure Design of the
Ministry of Education
Lanzhou University
Lanzhou 730000 (P.R. China)
E-mail: dongzhp@lzu.edu.cn
zhb19870831@163.com
[b] Dr. H. Zhang+
Institute of Nanoscience and Nanotechnology
School of Physical Science and Technology
Lanzhou University
Gansu 730000, (P.R. China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201900714
ChemCatChem 2019, 11, 3327 – 3338
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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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 Stöbertype 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 yolkshell-structured nanoreactor.
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 resinderived 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 effectively
inhibited the agglomeration and leaching of Co NPs. The asobtained Co@ZDC@mC nanoreactor catalyst exhibited superior
efficiency in the catalytic reduction of nitrobenzene with
N2H4·H2O. Furthermore, Co@ZDC@mC exhibited 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.
Experimental Section
Chemicals
Co(NO3)2·6H2O, cetyltrimethylammonium bromide (CTAB), 2-methylimidazole, 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 N2H4·H2O (80 %) were purchased from
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(NO3)2·6H2O (4 mmol) and 2methylimidazole (4 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
ChemCatChem 2019, 11, 3327 – 3338
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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 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 N2 and maintained constant for 4 h. The
obtained black material was 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
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, N2H4·H2O (200 μL)
was added to start the reaction. Equivalent reaction samples were
collected with syringes at regular intervals. Third, after the reaction,
Co@ZDC@mC-700 was recovered using an external magnet,
washed several times with water and ethanol, and dried for
subsequent cycles.
Characterization
Fourier transform infrared (FTIR, Nicolet FT-170SX) spectroscopy
was employed for the functional-group identification of the ZIF67@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 N2. 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. Energydispersive 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
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
3328
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Scheme 1. The preparation procedure of Co@ZDC@mC catalyst.
Figure 1. (a) FTIR spectra of ZIF-67@polymer and Co@ZDC@mC-700; (b) TGA curve of the ZIF-67@polymer.
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).
Results and Discussion
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 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 Figure 1a, the bond
changes of the ZIF-67@polymer during high-temperature
pyrolysis and the formation of Co@ZDC@mC-700 were clearly
distinguished. FTIR peaks observed at 1143.8 and 1284.1 cm 1
corresponded to the C N and C O bonds in the shell,
respectively, which were retained after the polymerization of maminophenol. In the core, C=N, which served as the marker for
the imidazole framework, was observed at 1437.2 cm 1. Notably, the C=C bond in the imidazole framework of ZIF-67 in the
core and the aromatic C=C bond in the shell were observed at
1617.5 cm-1. Compared to the FTIR spectra of Co@ZDC@mC700, a majority of these bonds were pyrolyzed at 700 °C, and
only a few key C=C, C=N, and C N bonds were retained. The
difference is that the bond observed at 2850.2 cm 1, correChemCatChem 2019, 11, 3327 – 3338
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sponding to the methyl group in the core, and the bond
observed at 2937.4 cm 1, 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 ZIF67@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 (Figure 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 carbonization temperatures of Co@ZDC@mC
nanoreactor catalysts are investigated from 500 to 900 °C.
Figure 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 (Figure 2a). This
3329
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Figure 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 histogram of the Co NPs in Co@ZDC@mC-700; (h) elemental mapping images of Co@ZDC@mC-700.
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 Figure 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 (Figure 2b). The increase of 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. Figure 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 enlargement in Figure 2d. After carbonization, Co2 + in ZIFChemCatChem 2019, 11, 3327 – 3338
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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 ZIF67@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. Figure S2 shows the TEM images of
Co@ZDC@mC prepared at different carbonization temperatures.
Similar to Co@ZDC@mC-700, Co@ZDC@mC-500, Co@ZDC@mC600, Co@ZDC@mC-800, and Co@ZDC@mC-900 exhibited ap-
3330
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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proximately spherical yolk-shell structures. At high carbonization temperatures, the yolk-shell nanoreactor still maintained
fine shape. 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 (Figure 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 the reduction of
Co2 + leads to the formation of Co0 and the strong pyrolysis of
the organic framework leads to the formation of graphitic C
(Figure 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 Co3O4 in the
HRTEM images of Co@ZDC@mC-500 were clearly observed
(Figure S3a).[49] Notably, the Co(111) crystal lattice of metallic Co
was hardly observed in the HRTEM image of Co@ZDC@mC-500,
suggestive of the partial reduction of Co2 +, albeit not
completely reduced to Co0 at 500 °C. With the increase in the
carbonization temperature to 600 °C, a part of Co3O4 was further
reduced to Co0, which was reflected by the appearance of the
Co(111) crystal plane corresponding to metallic Co, while
Co3O4(220) did not disappear (Figure S3b). Similar to
Co@ZDC@mC-700, only Co(111) was observed in the HRTEM
images of Co@ZDC@mC-800 and Co@ZDC@mC-900 (Figure 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 (Figure 2f). Similarly, the same
phenomena were observed in the HRTEM images for the shell
of Co@ZDC@mC carbonized at other temperatures (Figure S4).
Figure 2g and Figure 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 coverage area and encapsulates the Co cores,
while Co NPs are evenly dispersed in the core (Figure 2h). In
addition, EDS analysis verified the presence of C, N, O, and Co
in Co@ZDC@mC (Figure S6).
Figure 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. Figure 4a shows the C 1s spectra of different
ChemCatChem 2019, 11, 3327 – 3338
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Figure 3. Wide-range XPS spectra of Co@ZDC@mC obtained at different
calcination temperatures.
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@mC500, 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 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 (Figure 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. Figure 4c shows the high-resolution Co 2p
spectra of different Co@ZDC@mC catalysts. Peaks observed at
777.8 eV, 779.2 eV, and 782.0 eV corresponded to Co0, CoxOy,
and CoCxNy, Co N, respectively.[53,54] The comparison of the Co
2p spectra of the catalysts calcined at different temperatures
revealed that Co0 is not formed at 500 °C. At a temperature
greater than 600 °C, the characteristic signal of Co0 started to
appear. In addition, this change was reflected in the HRTEM
images of Co NPs. Meanwhile, the signal intensities for CoxOy
and CoCxNy decreased until 800 °C, indicating that Co2 + in
Co@ZDC@mC can be reduced to Co0 by Co-bonded C at a high
carbonization temperature and consume the surrounding C.
Correspondingly, Co N increased during the reduction of CoxOy
3331
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Figure 4. C 1s spectra (a), N 1s spectra (b) and Co 2p spectra (c) of Co@ZDC@mC obtained at different calcination temperatures.
Figure 5. (a) XRD pattern of different Co@ZDC@mC samples; (b) Raman spectra of different Co@ZDC@mC samples.
and CoCxNy at carbonization temperatures, and its signal
intensity increased gradually and 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 ICPOES analysis, the corresponding contents of Co were 2.14 %,
2.94 %, 3.11 %, 3.22 %, and 3.31 %.
Figure 5a shows the XRD pattern of different Co@ZDC@mC
catalysts. A wide diffraction peak corresponding to the C(002)
plane was observed at 26° in the XRD pattern 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,
ChemCatChem 2019, 11, 3327 – 3338
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with the gradual increase in the carbonization temperature to
800 °C, the peak intensity of Co(111) gradually increased,
indicating that a high content of Co2 + is reduced to Co0.
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 temperature
greater than 800 °C, CoxOy and CoCxNy were reduced to Co0
without the further increase in the Co0 content. With the
change in the carbonization temperature, peaks of D at
1350 cm 1 and G at 1587 cm 1 were observed in the Raman
spectra of the catalyst (Figure 5b).[57] The ID/IG value first
increased and then decreased with the carbonization temperature increasing. The ID/IG value for Co@ZDC@mC-700 was
relatively small. This higher degree of graphitization might be
3332
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 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.
more conducive to the transfer of electron to the active sites in
catalysis.
Figure 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 g-1. This large
surface area was possibly related to the outer surface area, the
pores in the shell and the core. The pore size of Co@ZDC@mC700 was clearly hierarchical (inset of Figure 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 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 (Figure 6b). The inset of
Figure 6b shows the magnetic separation of the Co@ZDC@mC700 catalyst.
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 Co@ZDC@mC-700, the relatively
low ID/IG value, which represents a high degree of graphitization, and the high number of cores provided by small Co NPs
was possibly the reason for its excellent catalytic performance.
ChemCatChem 2019, 11, 3327 – 3338
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Scheme 2. Schematic illustration of the catalytic reduction of nitrobenzene
over Co@ZDC@mC-700.
When the metal component in the catalyst was changed to
zinc, marginal amounts of aniline were produced under the
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 at 700 °C, the TOF value (109.6 h-1) was
considerably less than that of Co@ZDC@mC-700 (1136.3 h-1)
(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 nitrobenzene with N2H4·H2O did not occur without the catalyst
(Table 1, entry 8). Effects of the N2H4·H2O volume in the reaction
on the yield of aniline were investigated (Table 1, entries 9–12).
When other conditions are restricted, nitrobenzene was completely reduced to aniline in 10 min using 200 μL of N2H4·H2O.
With the decrease in the N2H4·H2O volume to 100 μL, the
3333
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Table 1. The optimal reaction conditions screening for catalytic reduction of nitrobenzene.
Entry
Catalyst
mcatalyst
[mg]
Mnitrobenze
[mmol]
VN2 H4 �H2 O
[μL]
T
[°C]
T
[min]
Yield
[%]
TOF[a]
[h 1]
1
2
3
4
5
6
7
8
9
10
11
12
13
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
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
Co@ZDC@mC-700
10
10
10
10
10
10
10
10
10
10
10
10
10
1
1
1
1
1
1
1
1
1
1
1
1
1
200
200
200
200
200
200
200
200
200
100
50
0
80
80
80
80
80
80
80
80
80
80
80
80
80
5
5
5
5
5
10
10
10
10
10
10
10
10
27.41
28.91
73.91
25.56
18.37
1.21
98.93
100
67.58
2.53
0
0.31
905.3
695.0
1679.7
561.0
392.3
–
109.6
–
1136.3
767.9
28.7
–
–
14
15
16
17
18
19
20
21
22
23
24
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
10
10
10
10
10
8
6
4
10
10
10
1
1
1
1
1
1
1
1
1.4
1.2
0.8
80
70
60
50
40
80
80
80
80
80
80
10
10
10
10
10
10
10
10
10
10
10
0.20
96.60
77.12
40.63
35.42
84.06
70.25
53.35
100
100
100
–
1097.7
876.3
461.7
402.5
1194.0
1330.5
1515.6
1590.9
1363.6
909.1
1 atm ðH2 Þ
300 (FA)
200
200
200
200
200
200
200
200
200
200
nitrobenze ðmmolÞ�Yield ð%Þ
[a] T� F ¼ mMcatalyst
ðmgÞ�Co ðwt%Þ�t ðhÞ � 58:9ðmg=mmolÞ.
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 N2H4·H2O. In addition, the catalytic reduction
of nitrobenzene over Co@ZDC@mC-700 did not occur under
mild conditions with other hydrogen sources such as H2 and FA
(Table 1, entries 13–14). High reaction temperature is 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 N2H4·H2O under mild conditions, related
to its unique physical and chemical properties.
Kinetic experiments were carried out to analyze the reaction
dynamics (Figure 7). Figure 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, Figure 7b
shows the plots of (Ct < C!C0) vs. the reaction time for the
catalytic reduction of nitrobenzene over different catalyst
concentrations. As can be observed intuitively, the reaction
ChemCatChem 2019, 11, 3327 – 3338
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process was linear, with reaction rate constants of 0.069, 0.05,
0.035, and 0.016 mol L 1 min 1 at 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 (Figure 7c). This linear relationship was more clearly
observed after visualization (Figure 7d). The corresponding
relationship between the nitrobenzene concentrations and
reaction rate constants k were 0.4 mol L 1 to 0.066 mol L 1 min 1,
0.5 mol L 1
to
0.069 mol L 1 min-1,
0.6 mol L 1
to
1
-1
1
0.081 mol L min and 0.7 mol L to 0.093 mol L 1 min 1 respectively. In conclusion, the catalytic reduction of nitrobenzene
with N2H4·H2O is a pseudo-zero-order reaction.
Besides TOF, activation energy Ea is also an important
parameter for evaluating catalysts. To obtain the Ea for the
catalytic reduction of nitrobenzene over Co@ZDC@mC-700, the
reaction was carried out at different reaction temperatures
(Figure 8). Figure 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 (Figure 8b). This low value of
Ea verified the superior catalytic performance of the
Co@ZDC@mC-700 nanoreactor.
Typically, there are two methods for the hydrogenation of
nitrobenzene (Figure S7): (a) Direct conversion via hydroxyl-
3334
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Figure 7. (a) Curves of aniline yields vs. reaction time with different additions of Co@ZDC@mC-700; (b) plots of (Ct C0) 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 (Ct C0) vs. reaction time for the reduction of nitrobenzene in presence of different initial concentration of nitrobenzene.
Figure 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 temperature, 1/T (K 1)).
amine 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
ChemCatChem 2019, 11, 3327 – 3338
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of 2,4-dinitroaniline were not further converted to 2,4-diaminoaniline using excess reductants. Hence, the hydrogenation of
nitrobenzene with N2H4·H2O over Co@ZDC@mC nanoreactor
catalysts follows the mechanism of the “direct pathway” rather
than the “condensation way.” Scheme 3 shows the speculative
specific reaction process.[61–63] First, N2H4·H2O entered the nano-
3335
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Table 2. Catalytic reaction of nitroarenes over Co@NC-700 catalyst.[a]
Entry
Scheme 3. Possible reaction mechanism for catalytic reduction of nitrobenzene with N2H4·H2O over Co@ZDC@mC-700.
reactor and diffused to the Co NP surface, followed by the bond
dissociation to form nitrogen and surface-bound hydrogen as
metal hydrides. Next, 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.
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
N2H4·H2O as the hydrogen source, the catalytic efficiency of
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@mC700 nanoreactor, which can enrich substrate molecules surrounding the active site, can considerably enhance the catalytic
efficiency.
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 halogenChemCatChem 2019, 11, 3327 – 3338
www.chemcatchem.org
Substrate
Product
t
[min]
Conv.
[%]
Sel.
[%]
TOF
[h 1]
1
7
100
100
1623.3
2
7
100
100
1623.3
3
7
100
100
1623.3
4
7
100
100
1623.3
5
7
100
100
1623.3
6
7
100
100
1623.3
7
7
100
100
1623.3
8
7
100
98.8
1603.9
9
7
100
99.8
1620.1
10
13
96.0
100
839.1
11
20
73.4
100
417.0
12
9
99.6
100
1257.5
13
9
100
100
1262.6
14
10
98.8
100
1122.7
15
11
100
100
1033.0
16
12
100
100
946.9
17
11
99.2
100
1024.8
18
11
97.3
100
1005.1
19
29
100
100
391.8
20
10
100
100
1136.3
21
30
92.4
100
351.1
22[b]
23
100
73.2
361.7
23[b]
25
100
32.9
149.5
[a] Reaction conditions: 1 mmol of substrate, 10 mg of Co@ZDC@mC-700,
2 mL of EtOH, 200 μL of N2H4·H2O, 80 °C; [b] 400 μL of N2H4·H2O.
ated 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
3336
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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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 substituent, it can be well
adapted to catalytic reduction with N2H4·H2O over
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. 2Methyl-3-nitroaniline was completely reduced to 2-methyl-3aminoaniline 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 complete
conversion. In addition, the azo compounds were the main byproducts in the catalytic reduction of 2,4-dinitroaniline. In
summary, most nitroarenes exhibit high conversion and
excellent selectivity over Co@ZDC@mC-700 with N2H4·H2O
under mild conditions.
To investigate the reusability and recyclability of the
Co@ZDC@mC-700 nanoreactor catalyst, recycling tests were
carried out (Figure 9). After the reaction, the catalyst was simply
crystal lattice of the Co(111) plane corresponding to metallic Co
was clearly observed in the HRTEM image of reused
Co@ZDC@mC-700 (Figure S9b). A large number of unblocked
microporous channels were still observed in the shell (Figure 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 (Figure 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 (Figure S10). Hence, the asprepared Co@ZDC@mC-700 nanoreactor catalyst exhibits excellent reusability and stability.
Summary
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@mC700 catalyst was applied to the catalytic reduction of nitroarenes with N2H4·H2O, which exhibited outstanding catalytic
activity and 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.
Acknowledgements
This work was supported by the Natural Science Foundation of
Gansu (No. 18JR3RA274).
Conflict of Interest
The authors declare no conflict of interest.
Figure 9. (a) Reuse test of Co@ZDC@mC-700. Reaction conditions: nitrobenzene (1 mmol), N2H4·H2O (200 μL), Co@ZDC@mC-700 (10 mg), EtOH
(2 mL), 80 °C, 10 min.
recovered using an external magnet. Reused Co@ZDC@mC-700
still maintained its original magnetism (Figure 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 yolkshell structure as the fresh one (Figure S9a). Meanwhile, the
ChemCatChem 2019, 11, 3327 – 3338
www.chemcatchem.org
Keywords: Co–MOFs · Yolk-shell structured nanoreactor ·
Catalytic reduction · Nitroarenes
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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Manuscript received: April 17, 2019
Revised manuscript received: May 25, 2019
Accepted manuscript online: May 27, 2019
Version of record online: June 21, 2019
3338
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
18673899, 2019, 14, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cctc.201900714 by Orta Dogu Teknik Universitesi, Wiley Online Library on [18/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
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