CoPd Nanoalloys with Metal–Organic Framework as Template for Both N-Doped Carbon and Cobalt Precursor: Efficient and Robust Catalysts for Hydrogenation Reactions

Article abstract of DOI:10.1002/chem.202003640

In this work, a series of metal–organic framework (MOF)-derived CoPd nanoalloys have been prepared. The nanocatalysts exhibited excellent activities in the hydrogenation of nitroarenes and alkenes in green solvent (ethanol/water) under mild conditions (H₂ balloon, room temperature). Using ZIF-67 as template for both carbon matrix and cobalt precursor coating with a mesoporous SiO₂ layer, the catalyst CoPd/NC@SiO₂ was smoothly constructed. Catalytic results revealed a synergistic effect between Co and Pd components in the hydrogenation process due to the enhanced electron density. The mesoporous SiO₂ shell effectively prevented the sintering of hollow carbon and metal NPs at high temperature, furnishing the well-dispersed nanoalloy catalysts and better catalytic performance. Moreover, the catalyst was durable and showed negligible activity decay in recycling and scale-up experiments, providing a mild and highly efficient way to access amines and arenes.

Full text of DOI:10.1002/chem.202003640

Accepted Article
Accepted Article
Title: CoPd Nanoalloys with MOF as Template for Both N-doped
Carbon and Cobalt Precursor: Efficient and Robust Catalysts for
Hydrogenation Reactions
Authors: Jie Zhu, Deng Xu, Lu-jia Ding, and Pengcheng Wang
This manuscript has been accepted after peer review and appears as an
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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
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003640
Link to VoR: https://doi.org/10.1002/chem.202003640
01/2020

10.1002/chem.202003640
Chemistry - A European Journal
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CoPd Nanoalloys with MOF as Template for Both N-doped Carbon
and Cobalt Precursor: Efficient and Robust Catalysts for
Hydrogenation Reactions
Jie Zhu,†[a,b] Deng Xu, †[a] Lu-jia Ding,[a] and Peng-cheng Wang*[a]
[a]
J. Zhu, D. Xu, L.-J. Ding, Prof. P.-C. Wang
School of Chemical Engineering
Nanjing University of Science & Technology
Nanjing, 210094, China
E-mail: alexwpch@njust.edu.cn
J. Zhu
[b]
College of Sciences
Nanjing Agricultural University
Nanjing 210095, China
†
These authors contributed equally
Supporting information for this article is given via a link at the end of the document.
Abstract: In this work, a series of MOF-derived CoPd nanoalloys
have been prepared. The nanocatalysts exhibited excellent activities
in the hydrogenation of nitroarenes and alkenes in green solvent
(ethanol/water) under mild conditions (H2 balloon, room temperature).
Using ZIF-67 as template for both carbon matrix and cobalt precursor
coating with a mesoporous SiO2 layer, the catalyst CoPd/NC@SiO2
were smoothly constructed. Catalytic results revealed the synergistic
effect between Co and Pd components in the hydrogenation process
due to the enhanced electron density. Mesoporous SiO2 shell
effectively prevented the sintering of hollow carbon and metal NPs at
high temperature, furnishing the well-dispersed nanoalloy catalysts
and better catalytic performance. Moreover, the catalyst was durable
and showed negligible activity decay in recycling and scale-up
experiments, providing a mild and highly efficient way to access
amines and arenes.
Traditional synthesis of Pd NPs relies on thermal (pyrolysis or
calcination) or chemical reduction of the respective metal
precursors on heterogeneous supports.[5] Recently, the use of
metal organic frameworks (MOFs) as versatile support materials
has emerged as a rational method to avoid sintering of the active
species and regulate selectivity due to their well-ordered
crystalline structure, high porosity and tunable pore size.[6]
Moreover, MOF is of particular interest when serving as structure
controlling templates[7] to design heterogeneous catalysts via
direct pyrolysis.[8] Zeng and Wang further extended the strategy
independently with MOF as sacrificial precursors.[9] In their
elegant works, mesoporous silica shell covered ZIF-MOF was
used as precursor to prepare supported Pd, Pt or other
nanoparticles, which prevented nanoparticles from sintering at
high temperature and leaching in catalytic process (Scheme 1a).
On the other hand, the availability and high price of Pd as a
noble metal also hinder its widespread applications and long-term
use. In this regime, intensive efforts have been made to design
lower-cost alternatives and in particular, alloying an inexpensive
and earth-abundant metal with the precious metal seems
attractive to minimize the usage of noble metal and retain the
activity at the same time.[10] Moreover, the hybrid structure of the
alloyed catalyst provides much promise to possess not only the
properties of individual constituents but also synergistic effect
leading to improved catalytic activity, selectivity and stability.[11]
Piccolo reported that nanolayers obtained by segregation of
palladium at an alloy surface displayed increased catalytic activity
with respect to bulk materials by maintaining hydrogen atoms in
the near-surface region.[12] The challenge may mainly lie in the
complicated fabrication process for alloy and uneven distribution
of each metal constituent. Considering the templating method as
a straightforward and facile way, the utilization of MOF as
precursor provides great opportunities for the controllable
fabrication of functional bimetallic NPs embedded within a support.
Li’s group has performed representative works in this area with
the construction of low-cost bimetallic nanocatalysts,[8h,13] for
Introduction
Hydrogenation process, especially for nitroarenes and alkenes to
access amines and arenes which arguably finds the most
application in pharmaceutical and fine chemical industries, has
always been the center of attention of both academic and
industrial researchers.[1] Considering the atom economy, it is not
surprising that transition metal catalyzed direct hydrogenation
based on gaseous H2 continue to attract interest, since the
pioneering work by von Wilde using Pt/H2 tracing back to 1874.[2]
The vast majority of catalysts were then extended to other
precious metals, such as ruthenium and rhodium with palladium
being the most common one due to its unique activity and
selectivity for the hydrogenation.[3] Heterogeneous catalysis is
thus highlighted to recover and reuse the precious transition
metal; and catalysts based on stabilized nanoparticles (NPs) have
proven to be a powerful tool with the consensus that catalytic
reactivity could be enhanced by increasing the number of
exposed active sites.[4]
1
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example, nanostructured PtNi catalyst on nitrogen-doped carbon
and NiCo/Fe3O4 heteroparticles within MOF-74.
The study was initiated by preparing the MOF derived CoPd
nanoalloys as illustrated in Figure 1. ZIF-67 nanocubes which act
as both Co precursors and morphological carbon matrix were first
prepared by mixing cobalt(II) nitrate hexahydrate and 2-
methylimidazole in deionized water followed by crystallization.
After the impregnation process with palladium solution, Pd2+ was
introduced into ZIF-67 to form Pd2+/ZIF-67 as another component
for the alloy. To increase the surface area and achieve better
distribution of CoPd/NC upon calcination, a thin layer of silica was
then coated on the surface of the Pd2+/ZIF-67 with hydrolysis of
TEOS to afford Pd2+/ZIF-67@SiO2. Finally, the as-synthesized
Pd2+/ZIF-67@SiO2 was treated with high temperature under inert
atmosphere which led to the in-situ formation of alloy
nanoparticles supported on nitrogen-doped porous carbon as
ultimate catalyst denoted as CoPd/NC@SiO2. After the
carbonization process, hollow ZIF-67 containing metal ions and
heteroatoms could be topotactically transformed into heteroatom-
doped porous carbon materials containing homogeneously
embedded metal alloy NPs, which brings out unique chemical and
physical properties that are not attainable from single MOF or Co
precursors.[15]
Scheme 1. (a) previous work on ZIF-MOF derived NPs within SiO2 shell; (b)
previous work on cobalt catalysed Hydrogenation Reactions; (c) this work.
Despite of the efforts made in this area, there is yet much work
to be done. For example, although MOFs have been extensively
used as sacrificial templates to fabricate hollow carbon materials,
the feasibility that MOFs paly roles in delivering both carbon
support and metal constituent for alloy have rarely been
reported.[10a,10b] Besides, when utilization Pd species in
hydrogenation process, elevated hydrogen pressure, high
temperature, safety measures and special reactors are usually
required for hydrogenation process which is difficult and
dangerous to control. It is envisaged that combining Pd with a low-
cost metal constituent may be achieved with the MOF derived
strategy and presenting a highly active and versatile nano-alloy.
Recently, there is a general agreement that cobalt plays an
indispensable role in enhancing hydrogenation activity. Several
N-doped carbon supported cobalt materials prepared by the
carbonization of MOFs or a mixture of cobalt salts and organic
complex has been demonstrated which exhibited good activity in
hydrogenation reactions (Scheme 1b).[10c,14]
Figure 1. Schematic illustration of the process for the synthesis of
CoPd/NC@SiO2.
To determine the pyrolysis temperature and analyze its
influence on catalyst preparation, TGA-DSC study of Pd2+/ZIF-
67@SiO2 was performed (Figure 2a). A slight weight loss together
with an endothermic peak were witnessed at 330oC, which was
assigned to the decomposition of the ligand for Pd. ZIF-67
precursor was stable up to 540oC where it began to decompose.
To achieve fully carbonization, the pyrolysis temperature was thus
started from 700oC and different calcination temperatures (700,
800 and 900oC) were tried, denoted as C700, C800
(CoPd/NC@SiO2) and C900 respectively.
Inspired by the leading reports, herein, we describe the
successful synthesis of CoPd alloys as a versatile nanocatalysts
for hydrogenation of nitroarenes and alkenes with substantially
enhanced catalytic activity (Scheme 1c). Co-zeolitic imidazole
MOF (ZIF-67) was utilized as Co precursor and acted as sacrificial
template as well after coating with a silica layer. The resulting
bimetallic nanocatalyst supported on N-doped carbon not only is
effectively prevented from sintering upon high temperature
calcinations, but also exhibiting great synergistic effect between
Co and Pd when applied for catalytic hydrogenations under mild
conditions for nitroarenes and alkenes (EtOH/H2O solvent,
hydrogen balloon and room temperature).
X-ray diffraction (XRD) of the prepared samples were
investigated to reveal the phase structure and composition
(Figure 2). Peaks located at 7.2°, 10.3°, 12.7°, 18.0° were
observed in XRD patterns of ZIF-67 (curve a), correspond exactly
to the (011), (002), (112) and (222) plane of its standard
simulation (CCDC-671074).[16] When comparing the results of
ZIF-67 and Pd2+/ZIF-67@SiO2, no obvious difference was
detected except for the broad peak appeared around 20-30°
attributed to the existence of SiO2,[17] which indicated the
successful coating of silica. It also disclosed that the structure of
the ZIF-67 remained intact during the introduction of Pd2+ and
coating process. After calcination, CoPd/NC@SiO2 exhibited
diffraction peak at two theta of 41.1° which was ascribed to (111)
plane of Pd metal.[9b,18] It was suggested that the reduction of Pd2+
to metallic Pd NPs through pyrolysis under inert atmosphere was
feasible. Moreover, peaks at 44.1° and 51.5° appeared, assigning
Results and Discussion
2
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10.1002/chem.202003640
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to the (111) and (200) planes of Co respectively (JCPDS-15-
0806).[19] The slightly shift of the peaks to lower values compared
with face-centered cubic pattern of crystalline Co further
confirmed that the starting Pd2+ and Co2+ have been converted
into CoPd alloys successfully through the carbothermic
reduction.[10c,20] The peaks for Pd and Co were relatively weak and
wide, resulting from the encapsulation of silica and small particle
size of catalysts. When increasing the calcination temperature
from 700 to 900oC (inserted Figure 2b), the signals tended to be
stronger and sharper due to the better crystalline at higher
temperature.
100
0.0
C900
90
80
70
60
50
40
(II)
C800
C700
-0.1
-0.2
-0.3
30 35 40 45 50 55 60
2 Theta (degree)
(III)
(I)
(II)
(I)
Figure 3. SEM images of (a) ZIF-67, (b) Pd2+/ZIF-67@SiO2, (c) CoPd/NC@SiO2,
(d) CoPd/NC.
100 200 300 400 500 600 700
Temperature (oC)
10 20 30 40 50 60 70 80
2 Theta (degree)
300
250
200
150
100
50
CoPd/NC@SiO2
CoPd/NC
The morphology and microstructure of the samples were further
monitored by transmission electron microscopy (TEM) and high-
resolution transmission electron microscopy (HRTEM). As
illustrated in Figure 4(a), Pd2+/ZIF-67@SiO2 was of a core-shell
cubic shape with SiO2 coating of about 33 nm thickness. The
structure maintained for CoPd/NC@SiO2 but a slightly smaller
size was observed, which was attributed to the partial surface
collapse of Pd2+/ZIF-67@SiO2 during the pyrolysis process,
resulting in a shrinked overall structure (Figure 4b). The alloy
nanoparticles of about 10-15 nm were homogeneously dispersed
within the SiO2 surface on nitrogen doped carbon matrix and with
no considerable formation of aggregates. HRTEM images of
CoPd/NC@SiO2 revealed the lattice fringes with an interplanar
distance of 0.225 nm and 0.210 nm which were corresponding to
the (111) plane of Pd and (111) plane of Co respectively,
indicating the existence of Pd and Co after carbothermic reduction
under inert atmosphere. The relative element mapping analysis
was performed and demonstrated the well distribution of Pd, Co,
C, N and Si. It was found that the Pd species spatially distributed
with Co species in the nitrogen doped carbon matrix, which
indicated that with the protection of SiO2, Co and Pd formed
bimetallic alloys on the NC support. The line scanning in Figure
4(e) and (f) also revealed the coexistence of Co and Pd in the
particle, and their superficial ratia was about 3:1. Further
investigation found that changing the thickness of SiO2 shell
would result in different particle sizes. When increasing the
loading of TEOS, the SiO2 shell thickened from 33 nm to 67 nm
with larger nanoparticle obtained from 12 nm to 18 nm owing to
“Ostwald ripening” without additional passivation (Figure S1). The
combined results of SEM and TEM indicated that SiO2 shell
effectively prevent the sintering of hollow carbon and metal NPs
at high temperature, furnishing the well-dispersed nano-alloy
catalyst. As also evidenced clearly by the TEM images in Figure
S2, the alloy catalysts tended to aggregate into larger
nanoparticles with uneven distribution when increasing the
calcination temperature. The C700, C800 and C900 catalyst
exhibited size distributions of 12, 18 and 30 nm respectively.
1.5
1.0
0.5
0.0
0
4
8
12
16
20
0.0 0.2 0.4 0.6 0.8 1.0
Relative Pressure (P/P0)
Pore Size (nm)
Figure 2. (a) TG(I) and DSC (II) curves of CoPd/NC@SiO2; (b) XRD patterns of
(I) ZIF-67, (II) Pd2+/NC@SiO2 and (III) Pd2+/ZIF-67@SiO2; inserted: XRD
patterns of samples with different calcination temperature; (c) N2-adsorption-
desorption isotherms of CoPd/NC@SiO2 and CoPd/NC; (d) Pore size
distribution of CoPd/NC@SiO2.
The N2 adsorption-desorption isotherms of CoPd/NC@SiO2
and CoPd/NC were shown in Figure 2c, which displayed type IV
curves for both samples. The former one showed a hysteresis
loop at P/P0 = 0.4-0.8, suggesting the presence of mesoporous
structure inside the catalyst. As can be seen from Figure 2(d),
CoPd/NC showed micropores mainly of 1.7 nm, while
CoPd/NC@SiO2 were proved to be mesoporous materials with
pore size of 4.1 nm and few micropores involved. Using the
Brunauer-Emmett-Teller (BET) method, the specific surface area
and total pore volume of the CoPd/NC@SiO2 was calculated to
be 392 m2/g and 0.447 cm3/g. Without the coating of mesoporous
SiO2 shell, the values for CoPd/NC decreased to 259 m2/g and
0.363 cm3/g accordingly. The coating of silica shell led to higer
surface area of the samples, which could furnish better
distribution of CoPd/NC upon calcination. In this way, the
agglomeration of inner metals was prevented and smaller particle
size was obtained.
The samples were further characterized by means of scanning
electron microscopy (SEM) and the images were shown in Figure
3. As can be seen, the as prepared ZIF-67 displayed regular
nanocube structure with diameter of ~200 nm. The cubic structure
was remained almostly after coating of silica shell but the size
increased to about 220 nm, testifying the successful covering of a
thin SiO2 layer. Owning to the protection of SiO2, subsequent
thermal treatment showed no significant influence on the
morphology of CoPd/NC@SiO2, exhibiting similar cubic shape but
with rough facets despite of tiny irregularities. As for comparison,
CoPd/NC without SiO2 showed severe agglomeration and
irregular morphology (Figure 3d).
3
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compensated by the nitrogen doped carbon matrix. Therefore, the
surface electron density of the bimetallic CoPd alloy is enhanced
compared with monometallic Pd.[26] Moreover, the superficial
Co/Pd ratio determined by XPS was about 3:1, similar to the line
scanning result; while the ICP data for the whole CoPd alloy was
8:1. The distinct result could be rationalized that XPS determine
only the superficial elements and may give deviation for the hybrid
catalyst. The ICP result is considered to be more accurate.
1200 1000 800 600 400 200
Binding Energy (eV)
0
300 295 290 285 280 275
390
410
405
400
395
Binding energy (eV)
Binding Energy (eV)
Figure 4. TEM images of (a) Pd2+/ZIF-67@SiO2; (b) and (d) CoPd/NC@SiO2;
HR-TEM images of (c) CoPd/NC@SiO2; (e, f) Line scanning of Co and Pd,
element mapping images of (g) Co, (h) Pd, (i) Co+Pd, (j) C, (k) N and (l) Si for
CoPd/NC@SiO2 catalyst.
X-ray photoelectron spectroscopy (XPS) was further carried out
to analyze the composition and chemical state of the catalysts.
Figure 5a showed the wide-scan spectra which indicate the
existence of Pd, Co, N, O, C, and Si elements in CoPd/NC@SiO2
without other impurities. In the N 1s XPS spectra (Figure 5b),
peaks located at 399.0 eV and 401.1 eV were identified as
pyridine type N and graphitic type N respectively,[21] suggesting
the successful doping of N into carbon support with ZIF-67 as a
sacrificial template. The C 1s spectra could be deconvoluted into
two peaks at 286.7 eV and 284.8 eV respectively which was
attributed to the graphitic type C-C bond and C-N bond.[22] The
high-resolution XPS spectrum of the Pd 3d region (Figure 5b)
presented doublet peaks at 335.4 eV and 340.8 eV,
corresponding to the Pd 3d5/2 and Pd 3d3/2 respectively for metallic
state Pd(0) species.[23]
360 355 350 345 340 335 330
Binding Energy (eV)
810
800
790
780
770
Binding Energy (ev)
Figure 5. XPS spectrum of (a) full scan; (b) N1s; (c) C 1s; (d) Pd 3d and (e) Co
2p respectively.
With the synthesized CoPd alloy catalyst in hand, initial
attempts to perform hydrogenation reactions were conducted in
EtOH/H2O system using nitrobenzene as the substrates. To our
delight, over 95% conversion of nitrobenzene toward the desired
aniline was obtained in the presence of only 0.4 mol% catalyst
and a H2 balloon within 120 min at room temperature. The
catalytic activity employing other samples including monometallic
Pd/NC@SiO2, Co/NC@SiO2, CoPd/NC with no SiO2 shell and
commercial Pd/C were further tested and compared as outlined
in Figure 6a. To be noted, CoPd/NC@SiO2 outperformed both
Pd/NC@SiO2 and Co/NC@SiO2, which gave only 65% and trace
conversion after 180 min of reaction respectively. When
compared with the result of commercial Pd/C in this system (14%,
180 min), the advantage of using the nanoalloy was still
highlighted. It was indicated that Pd serve as the active centre and
Co was supposed to accelerate process through the synergistic
effect with Pd. The enhanced electron density of the bimetallic
CoPd alloy (as illustrated above) which would be favorable for H2
adsorption and activation may account for the improved catalytic
performance. In terms of CoPd/NC as catalyst, the conversion of
nitrobenzene decreased to only 25% after 180 min. The effect of
SiO2 shell was thus manifested that prevented the alloy
nanoparticles on carbon support from agglomeration.
In case of Co 2p (Figure 5d), the core level spectra could be
deconvoluted into eight peaks indicating the existence of Co(0),
Co(II), Co(III) and satellite peak. The characteristic peaks at 778.3
eV and 792.6 eV could be attributed to Co(0) 2p3/2 and Co(0)
2p1/2,
[9c,24] which were in accordance with the formation of metallic
Co in CoPd alloy. The peak at 780.9 eV and 782.5 eV can be
assigned to Co(II) and Co(III) peak in Co-N species and Co
oxides.[25] The content of Co(0) vs total Co(II) and Co(III)
estimated by the area ratio of two doublet peaks was
approximately 1:4. XPS for metallic Pd(0) and Co(0), together with
the TEM and line scanning results confirmed the formation of
CoPd alloy. As Pd is generally more electronegative compared
with Co, it would gain electrons from Co, which is supposed to be
4
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without cleavage or hydrogenation of C=O bonds, exhibiting great
selectivity of the system (2i-2l). As for amino group, the
hydrogenation process was retarded significantly and prolonged
reaction time was needed to achieve the process (2m-2o). This
differentiation might be rationalized by relative differences in the
stability and reactivity of intermediates affected by the substitution
groups. Tri-substituted substrates also performed well under the
standard conditions and notably, dinitro compound was evaluated
which turned into the desired 1,3-diamine smoothly in good yield
(2p). For substrate 1q, cascade coupling reaction was occurred
in the system furnishing 2q in 57% yield, probably due to the
activation of ortho- methyl for the leaving of Cl. To understand the
origin of product 2q, 3-chloro-2-methylaniline was tested in the
system, which delivered the coupling product 2q under the Co-Pd
catalysis.
CoPd/NC@SiO
2
100
100
80
60
40
20
0
Pd/NC@SiO
2
CoPd/NC
80
Co/NC@SiO
2
Pd/C
60
0.49% Pd
0.83% Pd
1.37% Pd
1.75% Pd
40
20
0
0
20 40 60 80 100 120 140 160
Reaction Time (min)
0
30
60
90 120 150 180
Reaction Time (min)
Figure 6. (a) Hydrogenation performance of different catalysts for nitrobenzene;
the influence of Pd contents (b), SiO2 thickness (c) and calcination temperature
(d) on hydrogenation of nitrobenzene. Reaction conditions: nitroarene (1 mmol),
CoPd/NC@SiO2 (40 mg, 0.4 mmol% Pd), solvent 10 mL, EtOH:H2O=2:1), H2
balloon, room temperature; conversions were determined by GC-MS.
Table 1. Substrate scope of nitroarenes [a]
To gain some more insights into the catalytic process,
CoPd/NC@SiO2 of different calcination temperature, SiO2
thickness and Pd contents were tested. For comparison, catalyst
loading for all the materials were adjusted to the same Pd
concentration. As shown in Figure 6c and 6d, among the prepared
catalysts, CoPd/NC@SiO2 with a moderate SiO2 thickness and
Pd contents calcinated at 800oC was shown to provide the best
performance. Considering the TEM results, it was suggested that
appropriate pyrosis temperature was necessary to carbonize ZIF-
67 precursor and lead to the alloy with Pd constituent, but higher
temperature may result in inhomogeneous growing of the alloy as
well as larger particle size. The effect of NC@SiO2 support to
stabilize and disperse the alloy particles was weakened at 900oC
and thus decreased catalytic performance was observed. As for
SiO2 thickness, the influence mainly lies in two aspect. Firstly,
different SiO2 thickness means different surface area upon
calcination. With thicker SiO2 shell and larger surface area,
CoPd/NC would disperse better, leading to smaller particle size of
CoPd NPs. Secondly, mass transfer effect would be influenced by
different SiO2 thickness. Excessive SiO2 shell may hinder the
diffusion of substrates during the catalytic process and prevent
the coordination of the substrates on particle surface, thus
resulting in low efficiency. Thus, it was supposed that an
appropriate amount of SiO2 was necessary and catalyst of 67 nm
SiO2 shell gave the best catalytic performance with the smallest
particle size (Figure 6c and Figure S1). Thicker samples were
prepared but afforded decreased catalytic performance, which
further confirmed the conjecture. On the other hand,
CoPd/NC@SiO2 with different Pd contents showed similar
catalytic activities for this transformation, indicating their excellent
efficiency and applicability due to the alloying of Co.
[a] Reaction conditions: nitroarene (1 mmol), CoPd/NC@SiO2 (40
mg, 0.4 mmol% Pd), solvent 10 mL, EtOH:H2O=2:1), H2 balloon,
room temperature; conversions were determined by GC-MS. [b]
Aldimine
condensation
occurred
immediately
after
hydrogenation and conversion is calculated according to the
unreacted substrate isolated by flash chromatography.
Based on the preliminary results, subsequent efforts were
devoted to examine the applicability of this attractive protocol
using a wide range of nitroarenes containing diverse substituent
groups, as listed in Table 1. Generally, nitrobenzenes bearing
either electron-donating (-alkyl, -alkoxyl, hydroxyl) or electron-
withdrawing (-fluoro, -chloro, -bromo, -acyl) substituents
proceeded quite well to afford corresponding anilines with
quantitative yields without observation of obvious electronic effect.
For chlorinated substrates, the influence of the position seems
inconspicuous and ortho-, meta-, and para-substituted
nitrobenzenes were all well tolerated (2b-2d). Moreover, -COOH,
-CHO, -COOEt and -SCH3 remained intact after the reaction
Encouraged by the promising results, the substrate scope was
further extended to evaluate he hydrogenation of various alkenes
using 2 mol% CoPd/NC@SiO2 under a H2 balloon at room
temperature. As the alkenes are difficult to dissolve in H2O/EtOH
solvent, EtOH was used instead to perform the reactions and the
results were summarized in Table 2. It could be observed that the
nano-alloy catalyst still exhibited outstanding catalytic
performance and styrenes containing alkyl, alkoxy, or chloro
substitutions afforded the corresponding hydrogenated products
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with good to excellent conversions (4a-4f).α-methyl styrene,
however, impaired the reaction and afforded cumene in poor yield
(4g). The system shows compatibility for cinnamaldehyde and its
derivatives with great selectivity giving conversions ranging from
91% to 98% (4h-4m). To be noted, 3-(furan-2-yl)acrylaldehyde,
though considered difficult to activate, proceeded smoothly in this
regime with extended reaction time (4n), which further extended
the substrate scope greatly. However, aliphatic substrates were
proved to be inefficient in the system for both long-chain and
cyclic ones with no prodect detected even at refluxing
temperature (4o-4q).
stabilized by the carbon support and SiO2 shell. As for Co,
considering that it was located in both carbon matrix and
nanoalloy, and Co itself could not achieve the catalytic
transformation, the partial loss of Co showed less impact on the
catalyic performance.
CoPd/NC@SiO
CoPd/NC
Pd/C
2
after recycling for 8 times
after recycling for 4 times
as prepared
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
0
30 60 90 120150180
Reaction Time (min)
Table 2. Substrate scope of alkenes [a]
1
2
3
4
5
6
7
8
0
30
60
90 120 150 180
Run Numbers
Reaction Time (min)
50
40
30
20
10
0
3
6
9
12 15 18 21 24 27 30
Diameter (nm)
Figure 7. The recycling study of catalyst CoPd/NC@SiO2; (a) recycling
performance in hydrogenation of nitroarenes; (b) comparison of kinetic curves
before and after recycling with hot filtration tests inserted; (c) SEM images of
recovered CoPd/NC@SiO2; (d) TEM images of broken part of recovered
CoPd/NC@SiO2 with relative size distribution inserted.
[a] Reaction conditions: alkene (0.5 mmol), CoPd/NC@SiO2 (150
mg, 2 mmol% Pd), EtOH 5 mL, H2 balloon, room temperature;
conversions were determined by GC-MS; n.d.= not detected. [b]
refluxing instead of room temperature.
In terms of a heterogeneous catalyst, the stability and
recyclability always cause concern and thus the recycling test of
the CoPd/NC@SiO2 was performed and compared with CoPd/NC
and Pd/C. As shown in Figure 7, the recovered CoPd/NC@SiO2
catalyst could be reused for at least 8 times in hydrogenation of
nitrobenzene without remarkable loss of activity (96% and 92%
conversion in the eighth run respectively). As for CoPd/NC,
without the protection of SiO2, relatively poor reusability was
shown and the activity dropped obviously. The kinetic curves in
Figure 7(b) further confirmed that the catalyst was reusable but
with lower initial reaction rate, which was due to the gradual
collapse and aggregation of catalyst when increasing the
recycling times. As disclosed by SEM imaged in Figure 7(c),
increased proportion of the broken catalyst was observed after
8th recycling, but CoPd/NC still adheres strongly to the SiO2 wall
which to some certain prevented serious loss or aggregation of
Figure 8. The scale-up experiments.
To demonstrate the practical applications of this attractive
protocol for hydrogenation, scale-up experiments were performed
with nitrobenzene and styrene as substrates. As shown in Figure
8, only slight decline of conversion was observed with 10 mmol
substrate under the standard conditions for both of the
transformations. When the processes further amplified to 100
mmol, high conversions (87% and 82% respectively) were still
maintained for nitrobenzene and styrene with extended reaction
time of 4 h.
the NPs. Hot filtration was further performed as shown in Figure Conclusion
7(b) (inserted). The TEM image of the recovered CoPd/NC@SiO2
after eighth cycle (~15 nm) as well as the hot filtration result and
ICP data of Pd (0.83%) and Co (6.60%) before the reaction and
after the eighth cycle (0.79%, 5.41%) collectively suggested that
leaching of Pd was not obvious during the hydrogenation process,
further displaying the outstanding stability of the nanoalloy
In summary, a CoPd nanoalloy catalyst supported on N-doped
carbon within mesoporous silica shell was successfully developed
with ZIF-67 served as both template for carbon and cobalt
precursor. When applying the catalyst in hydrogenation of
nitroarenes and alkenes, a clean, mild and efficient system was
6
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10.1002/chem.202003640
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centrifugation and washed with ethanol for three times before drying under
vacuum overnight.
disclosed using a hydrogen balloon at room temperature in
ethanol/water solvent. The reaction tolerated various functional
groups for both kinds of substrates, delivering the target products
in good to excellent yields. Notably, due to the synergistic effect
between Co and Pd, the alloy outperformed monometallic Pd and
Co nano-catalyst. The mesoporous SiO2 shell has been proved to
play a key role in preventing the sintering of hollow carbon and
metal NPs at high temperature, furnishing the well-dispersed
nanoalloy catalyst. The present work might open a new avenue to
design MOFs-templated catalyst alloying noble and earth
abundant metals, exhibiting enhanced activity as well as reduced
cost compared with monometallic noble catalyst.
Typical procedure for synthesis of CoPd/NC@SiO2: The prepared
Pd2+/ZIF-67@SiO2 above was calcinated at 800oC with a heating rate of
2oC/min under nitrogen atmosphere and was cooled down to room
temperature naturally, furnishing the final alloy catalyst denoted as
CoPd/NC@SiO2. For comparison, Pd/NC@SiO2 without Co,[9b]
Co/NC@SiO2 without Pd and CoPd/NC without silica shell were prepared
as well using a method similar to the above one. The Pd/NC@SiO2 was
prepared according to a reported literature.
Typical procedure for the hydrogenation of nitroarenes: To a 25 mL
vial equipped with hydrogen balloon was added nitroarenes (1 mmol),
catalyst (0.4 mol % Pd) and mixed solvent (10 mL, EtOH: H2O = 2:1)
respectively. The reaction was then allowed to react at room temperature
for certain time until the complete consuming of starting materials
monitored by TLC. When completed, the catalyst was recovered from the
system through centrifugation; the samples were diluted to 15 mL with
water, extracted with Et2O for three times, and dried over anhydrous
Na2SO4 before being analyzed by GC-MS. Substrate could be recharged
into the aqueous phase to achieve recycling. To achieve recycling, the
recovered catalyst washed with ethanol for several times with ethanol,
dried under vacuum and reused as the catalyst in the next run.
Experimental Section
General information: Solvents and reagents of reagent grade were used
without purification unless otherwise noted. Thermogravimetric analysis
was performed on TGA/SDTA851e under N2 atmosphere from 50 oC to
800 oC, with a heating rate of 10 oC/min and N2 flowing rate of 30 mL/min.
Inductively coupled plasma-mass spectra (ICP-MS) were taken from
Optima 7300 DV (PerkinElmer). XRD data were collected with CuKa
radiation on Bruker C8 ADVANCE. The Brunauer−Emmett−Teller (BET)
specific surface area (SSA) of the samples were determined by collecting
N2 gas adsorption/desorption isotherms on a Micromeritics ASAP 2020
Instrument. All samples were degassed at 200 oC for 18 h before the SSA
determination. Transmission electron microscopy (TEM) pictures were
taken on a FEI-Tecnai G2 F30 at an accelerating voltage of 75 kV with the
chloroform dispersion of catalyst was drop-cast onto a 300 mesh carbon
coated copper grid. The elemental mappings were performed on a
scanning transmission electron microscope (STEM) unit with a high-angle
annular-dark-field (HAADF) detector (HITACHI S-5500) operating at 200
kV.
Typical procedure for the hydrogenation of alkenes: To a 25 mL vial
equipped with hydrogen balloon was added alkenes (0.5 mmol), catalyst
(2 mol % Pd) and 5 mL EtOH respectively. The reaction was then allowed
to react at room temperature for certain time until the complete consuming
of starting materials monitored by TLC. After completion of the reaction,
the catalyst was removed from the solution through centrifugation, and the
liquid phase was subsequently analyzed by GC-MS.
Acknowledgements
Typical procedure for the synthesis of ZIF-67 nanocubes[16a]: In a
typical preparation, 20 mg of hexadecyl trimethyl ammonium bromide
(CTAB) and 580 mg of cobalt(II) nitrate hexahydrate were dissolved into
20 mL of deionized water to form a clear solution; and another solution
was obtained by dissolving 9.08 g 2-methylimidazole in 140 mL of
deionized water. Under vigorous stirring, the cobalt containing solution
was poured into the ligand containing solution and kept stirring at room
temperature for 20 min. Finally, purple precipitates were produced,
centrifuged and washed with ethanol for three times before and drying
under vacuum at room temperature for 12 h.
We thank the National Natural Science Foundation of China (No.
11972195 and 11702141) for support of this work.
Keywords: CoPd nanoalloy • MOF derived • Hydrogenation •
Nitroarenes • Alkenes
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Entry for the Table of Contents
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CoPd nanoalloys were prepared with MOF as Template for Both N-doped Carbon and Cobalt Precursor. The catalyst encapsulated in
silica shell afforded a clean, mild and efficient system for hydrogenation of nitroarenes and alkenes. Improved activity was witnessed
due to the synergistic effect between Co and Pd components as well as improved stability due to the protection of silica shell.
9
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