Rational Design of Cobalt-Platinum Alloy Decorated Cobalt Nanoparticles for One-Pot Synthesis of Imines from Nitroarenes and Aldehydes

Article abstract of DOI:10.1002/cctc.202001331

Developing high-performance heterogeneous catalysts for one-pot reductive amination reactions is critically important for pharmaceutical and agrochemical synthetic industries. In this work, N-doped carbon nanotubes supported CoPt alloy decorated Co nanoparticles (NPs) are successfully fabricated. As a consequence, the resultant catalyst exhibits desirable activity, selectivity and stability toward the one-pot synthesis of imines. More importantly, the extensive experimental studies and density function theory (DFT) calculations results reveal that the high catalytic activity of the catalyst is mainly due to the co-existence of CoPt alloy NPs and Co NPs with Co?N_x active sites. The high selectivity of imines could be ascribed to the following aspects: (1) competitively preferred adsorption of nitroarenes to avoid side-hydrogenation of aldehydes; (2) weakened adsorption of imines to minimize its over-hydrogenation. This work may provide a promising direction and strategy to design high-performance reductive amination catalysts for industrial applications.

Full text of DOI:10.1002/cctc.202001331

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Rational Design of Cobalt-Platinum Alloy Decorated Cobalt
Nanoparticles for One-Pot Synthesis of Imines from Nitroarenes
and Aldehydes
Authors: Wanbing Gong, Miaomiao Han, Chun Chen, Yue Lin,
Guozhong Wang, Haimin Zhang, and Huijun Zhao
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To be cited as: ChemCatChem 10.1002/cctc.202001331
Link to VoR: https://doi.org/10.1002/cctc.202001331
01/2020

10.1002/cctc.202001331
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Rational Design of Cobalt-Platinum Alloy Decorated Cobalt Nanoparticles for One-Pot
Synthesis of Imines from Nitroarenes and Aldehydes
Wanbing Gong, + Miaomiao Han, + Chun Chen, Yue Lin,* Guozhong Wang, Haimin Zhang*
and Huijun Zhao*
Dr. W. Gong, Dr. M. Han, Dr. C. Chen, Prof. G. Wang, Prof. H. Zhang, Prof. H. Zhao
Key Laboratory of Materials Physics, Centre for Environmental and Energy Nanomaterials,
Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in
Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, 350 Shushanhu
Road, Hefei 230031, P. R. China
E-mail: zhanghm@issp.ac.cn, h.zhao@griffith.edu.au
Dr. Y. Lin
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and
Technology of China, 96 Jinzhai Road, Hefei 230026, P. R. China
E-mail: linyue@ustc.edu.cn
Prof. H. Zhao
Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus,
Queensland 4222, Australia
+ These authors contributed equally to this work.
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Abstract
Developing high-performance heterogeneous catalysts for one-pot reductive amination
reactions is critically important for pharmaceutical and agrochemical synthetic industries. In
this work, N-doped carbon nanotubes supported CoPt alloy decorated Co nanoparticles (NPs)
are successfully fabricated. As a consequence, the resultant catalyst exhibits desirable activity,
selectivity and stability toward the one-pot synthesis of imines. More importantly, the
extensive experimental studies and density function theory (DFT) calculations results reveal
that the high catalytic activity of the catalyst is mainly due to the co-existence of CoPt alloy
NPs and Co NPs with Co-Nx active sites. The high selectivity of imines could be ascribed to
the following aspects: (1) competitively preferred adsorption of nitroarenes to avoid side-
hydrogenation of aldehydes; (2) weakened adsorption of imines to minimize its over-
hydrogenation. This work may provide a promising direction and strategy to design high-
performance reductive amination catalysts for industrial applications.
2
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Introduction
Imines, a class of versatile nitrogen-containing organic intermediate, have received great
attentions in the field of synthesis chemistry for the production of various pharmaceuticals,
agrochemicals, dyes, polymers and resins.[1] Compared with traditional synthesis methods, the
reductive amination process shows some incomparable advantages such as mild reaction
conditions, limited byproducts, low pollution and high atomic efficiency.[1c] Although the
reductive amination with primary amines has been extensively studied,[1d, 2] it would be one of
the most promising strategies for the one-pot tandem amination directly from nitro
compounds, which avoids the separation and purification of the primary amines (Scheme
1).[3] In recent years, some efforts have been devoted to preparing highly efficient
heterogeneous catalysts for the one-pot tandem reductive amination system.[1a] Nevertheless,
the simultaneous realization of high conversion and excellent imine selectivity under mild
conditions remain a huge challenge. Typically, the Pt-based catalysts exhibit the high catalytic
activity, however, suffer from poor selectivity for imines due to the competitive side-
hydrogenation of aldehydes to alcohols and over-hydrogenation of imines into amines.[3b, 4]
Besides, the high cost and scarcity of such Pt metal limit its industrial applications. Although,
the low cost Pt-free catalysts can achieve high imine selectivity, nonetheless, require harsh
reaction conditions (e.g., high temperature, high pressure and longer reaction time).[1a, 1c]
Recently, the development of low cost Pt-based bimetallic composites has demonstrated great
potentials to meet the challenge.[1b, 3b, 5] For instance, Qin’s group demonstrated a new
approach to tailor the Pt-Fe3+-OH sites by depositing Pt on the surface of Fe2O3 nanoparticles
(NPs), which weakens the Pt-H bond to suppress the further hydrogenation of imine to amine
for the reductive coupling of nitrobenzene and furfural.[1b] Zhang et al. recently demonstrated
the use of bimetallic PtCo NPs to facilitate the unparalleled activity and selectivity towards
imines for one-pot cascade reductive amination. The experimental and density function theory
(DFT) results suggested that the competitive molecular adsorption on the catalyst is a crucial
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factor contributing to the high selectivity towards imines.[3b] In addition, the Pt-based
bimetallic alloy catalysts have shown high catalytic performance due to the synergistic effects
between two active sites in other catalytic systems.[6] For example, Sun’s group recently
demonstrated that the ultrafine PtCo NPs and synergistic effects between PtCo and Co greatly
promoted the hydrogen evolution activity in alkaline media.[6d] Yang et al. also reported the
PtCo alloy NPs embedded in the capsule walls were highly active, selective and recyclable
catalysts for the hydrogenation of nitroarenes to anilines.[6e]
Scheme 1. Various synthesis strategies for imines in the previous works and this work.
Recently, N-doped carbon supported transition metal (e.g., Fe, Co, Ni) catalysts have
showed great potential for the reductive amination process.[1a, 3a, 7] Liu et al. revealed the Co-
N-C/CNT@AC was highly active for the reductive coupling of structurally diverse
nitroarenes and alcohols to imines and secondary amines under exogenous base- and
solvent-free conditions due to synergistic effect between its nanostructure, Co-N and basic N
species.[7a] Meanwhile, N-doped carbon supported Co NPs were observed to be active for
one-pot reductive amination with nitro compounds by transfer hydrogenation with
8b, 9]
CO/H2O[8] or HCOOH[3a,
as the hydrogen donors. For instance, Zhou et al.
demonstrated N-doped carbon supported Co catalysts were active toward the one-pot
reduction amination process, affording the structurally diverse secondary amines with
excellent amines yields (85~99%).[8a] Based on the above considerations, the N-doped
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carbon supported Pt-transition metal (Pt-M, M = Fe, Co, Ni) alloy catalysts are considered as
highly active and selective catalyst for the one-pot synthesis of imines.
We recently have successfully developed new N-doped carbon nanotubes (N-CNTs)
confined Co NPs derived from ZIF-67, and the as-synthesized catalyst showed excellent
activity and perfect selectivity for a wide range of biomass-derived molecules with diverse
functional groups under very mild conditions.[10] Herein, for the first time, we present ZIF-
67-derived N-CNTs supported CoPt alloy decorated Co NPs, the novel hybrid structure is
highly active and selective for the one-pot tandem reductive amination with nitro compounds
and aldehydes. Further studies reveal that the co-existence of CoPt alloy NPs and Co NPs
with Co-Nx active sites greatly improve the catalytic activity. Additionally, the high
selectivity of imines could be attributed to the preferred adsorption of nitroarenes to avoid
side-hydrogenation of aldehydes and weakened adsorption of imines to minimize its over-
hydrogenation simultaneously.
Results and Discussion
Firstly, the N-CNTs supported Co NPs with ~12 nm (denoted as Co-800) were obtained by
two-step pyrolysis of the ZIF-67 precursors according to our previous study (Figure S1).[10]
Subsequently, the CoPt alloy decorated Co NPs catalyst (denoted as 2% CoPt) was
synthesized by encapsulating Pt ions as the guest metal source. The field-emission scanning
electron microscopy (FESEM) images (Figure 1a) show the size and polyhedral shape of the
2% CoPt were well retained after alloying treatment. The transmission electron microscopy
(TEM) images (Figure 1b) show the sample was observed to be several hundred nanometers
in length with the homogeneous distribution of ~20 nm NPs over the hollow N-CNTs.
Furthermore, the high-angle annular dark-field scanning transmission electron microscopy
(HAADF-STEM) and energy dispersive X-ray (EDX) elemental mappings (Figure 1c) further
demonstrate that Co and Pt were evenly dispersed within the entire architecture. More
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importantly, we found that the Co species has two forms: CoPt alloy and metallic Co NPs.
The aberration-corrected high resolution TEM (HRTEM) image of representative CoPt alloy
NPs in Figure 1c reveals that the NP had a CoPt alloy core and a thin Pt shell (Figures 1d-e).
The lattice distance of 0.216 nm was observed which represents the (111) planes of CoPt
alloy (Figure 1e). In addition, Figures 1f-g show the rest Co NPs with a spacing of 0.204 nm,
consistent with the (111) lattice plane of the metallic Co. Therefore, these results confirm the
co-existence of CoPt alloy NPs and Co NPs in this architecture.
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Figure 1. (a) The SEM images. Inset: high magnification image; (b) TEM images. Inset: high
magnification image; (c) HAADF-STEM image and EDX mappings; (d-e) HRTEM images
of representative CoPt NP in (c); (f-g) HRTEM images of representative Co NP in (c) of 2%
CoPt catalyst.
The X-ray diffraction (XRD) pattern of representative 2% CoPt (Figure 2a) displays
characteristic peaks at 44.2, 51.5 and 75.8o which are assigned to the crystal planes (111),
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(002) and (022) of cubic Co, respectively (JCPDF No. 96-901-0969). Moreover, the two weak
diffraction peaks at 41.7 and 48.5o can be assigned to the crystal planes (111) and (200) of
cubic CoPt, respectively (JCPDF No. 03-065-8970), which identify the formation of CoPt
alloy. In addition, the high resolution Pt 4f spectra (Figure 2b) have two characteristic peaks
at 71.6 and 75.0 eV, which are attributed to the 4f7/2 and 4f5/2 of metallic Pt, respectively.[11]
Notably, the Pt 4f7/2 peak has a positive shift (~ 0.4 eV) compared to that of pure Pt (71.2 eV)
of the 2% Pt-NC sample.[6d] It is known that the binding energy is strongly correlated with the
adsorption/desorption capability of reaction species,[6c] and such blue shift of the binding
energy illustrates a strong electronic effect between Pt and Co, thus improving the catalytic
performance of alloy sample.[6c, 6d] Compared with Co-800 sample, the decrease of metallic
Co peak and the increase of Co-Nx peak (Figure 2c) were mainly due to the hybridization of
Co orbitals by Pt after alloying treatment and thermal activation of some Co sites to Co-Nx
sites.[12] In addition, the peak shift around 796 eV was detected because of the enhancement of
metal-N interaction.[13] The N 1s spectra demonstrate the pyridinic-N (~ 398.7 eV) and
graphitic-N (~ 401.1 eV) are existed in the N-CNTs, and the decrease of pyridinic-N and the
increase of graphitic-N can be observed after the alloying treatment (Figure 2d and Table S1).
Raman spectra show the graphitization degree of N-CNTs is not been much effected by
alloying treatment (Figure S2). The porous texture was characterized by N2 sorption at 77 K
(Figure S3), which reveals its large surface area (150 m2 g−1, Table S1). It would be beneficial
to the exposure of active sites and rapid transportation of relevant species. The inductively
coupled plasma (ICP) analysis data shows that the Pt content was 1.9 wt%, suggesting Pt ions
have been fully converted to alloy NPs. Meanwhile, Co content was 42.2 wt% for 2% CoPt
(Table S1). For comparison, it can be found that the deposited NPs were more and more
clearly observed and the sizes increased with the Pt content from the SEM and TEM images
(Figures S4-S5). Additionally, it can be observed that the diffraction peaks of CoPt alloy were
also more and more obvious with the Pt content (Figure S6), indicating an increase of alloy
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NPs with the Pt content. The control 2% Pt-NC sample was same as those of the original ZIF-
8 (Figure S7), and no diffraction peaks from Pt NPs were observed, which could be related to
the high dispersion and the small particle size (Figure S6).
Figure 2. (a) XRD pattern of 2% CoPt catalyst. (b) Pt 4f spectra of 2% Pt-NC and 2% CoPt
catalysts. (c) Co 2p and (d) N 1s spectra of Co-800 and 2% CoPt catalyst.
The one-pot tandem reductive amination of benzaldehyde and nitrobenzene was used to
evaluate the catalytic performance of the as-synthesized catalysts. First, the parent ZIF-67
NPs were actually inactive at 60 oC and 0.5 MPa H2 within 2 h (entry 1, Table 1). Then, only
a 0.1% benzaldehyde 1 conversion was obtained on Co-440 (entry 2, Table 1), indicating that
Co-Nx-free NPs catalyst was also not active in this reductive animation process. After that, the
Co-800 catalyst gave a 38.4% benzaldehyde 1 conversion with superior selectivity of imine 4
(n-benzylideneaniline, 98.6%) (entry 3, Table 1). Based on the previous study, it can be easily
inferred that the high catalytic activity of Co-800 is due to the presence of Co NPs with Co-Nx
active sites[10] For comparison purpose, the 2% Pt-NC catalyst gave a 50.8% benzaldehyde 1
conversion and 73.3% imine 4 selectivity, respectively (entry 4, Table 1). This result indicates
that Pt can increase reaction efficiency but decrease the selectivity of imine 4. Interestingly,
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the combination of Pt and Co can give a high benzaldehyde 1 conversion and imine 4
selectivity simultaneously. It can be noted that the benzaldehyde 1 conversion drastically
increased to 82.6%, and the imine 4 selectivity was almost unchanged (90.2%), indicating an
enhanced catalytic activity of CoPt alloy (entries 5-6, Table 1). However, by increasing the
loading amount of Pt, the benzaldehyde 1 conversion increased gradually but the imine 4
selectivity decreased (entries 7-8, Table 1). So, the optimal loading amount of Pt was 2 wt%.
For comparison, physically mixing 2% Pt-NC and Co-800 did not result in an enhanced
activity owing to the lack of synergistic effect in the alloy system (entry 9, Table 1). Recently,
Liu’s group found that close proximity between Pt-Co NPs and Co-Nx-Cy sites could promote
synergistic catalysis in a fuel cell.[12] Meanwhile, the Co-Nx sites on the carbon support are
widely identified as the active sites responsible for the catalysis such as hydrogen evolution
reaction,[14] or hydrogenation reactions.[15] Based on above results and discussions, the
excellent catalytic performance of this 2% CoPt catalyst is mainly due to the co-existence of
CoPt alloy NPs and Co NPs with Co-Nx active sites.
Table 1. Catalytic performance of various catalysts for the reductive animation of
benzaldehyde and nitrobenzene.a
Sel. (%)
Conv. 1
(%)
Entry
Catalysts
3
--
0
4
5
1
2
3
4
5
6
7
ZIF-67
Co-440
--
--
--
0.1
98.9 1.1
98.6 1.4
Co-800
38.4
50.8
56.9
82.6
86.9
0
2% Pt-NC
1% CoPt
2% CoPt
4% CoPt
19.2 73.3 7.5
7.2 90.6 2.2
7.7 90.2 2.1
10.1 78.6 11.3
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8
9
8% CoPt
100
12.2 2.5 85.3
11.1 84.2 4.7
2% Pt-NC+Co-800
64.2
a Reaction conditions: Catalyst = 20 mg, Benzaldehyde = 1 mmol, Nitrobenzene = 1 mmol, 2-
propanol = 10 mL, Reaction temperature = 60 oC, Reaction time = 2 h, H2 pressure = 0.5 MPa.
The influences of reaction temperature, time and H2 pressure on the conversion and
selectivity over the optimum 2% CoPt catalyst were deeply investigated (Figure 3). As shown
in Figure 3a, the benzaldehyde 1 conversion sharply increased from 28.9% at 40 oC to 82.6%
o
o
at 60 C, and then increased to 100% at 80 C. Meanwhile, the imine 4 selectivity decreased
and amine 5 selectivity increased with increasing reaction temperature, and the 90.2%
o
selectivity of imine 4 was obtained at 60 C after 2 h reaction. In addition, the side-
hydrogenation of benzaldehyde into benzyl alcohol was also observed, and some amount of
o
benzyl alcohol (7.7%) was detected at 60 C. From the Figure 3b, the catalytic activity
gradually increased from 27.3% to 100% with reaction time increased from 0.5 h to 3 h at 60
oC and 0.5 MPa H2 pressure. The product selectivity changed very slowly with increasing
reaction time at the fixed temperature. Afterwards, the benzaldehyde 1 conversion was low
(25.5%) with a 99.7% imine 4 selectivity at the low hydrogen pressure of 0.1 MPa. Further
increasing the pressure to 0.5 MPa, a 82.6% conversion with 90.2% imine 4 selectivity was
obtained. When the H2 pressure was increased to 2 MPa, the imine 4 selectivity was sharply
decreased to 8.5% and amine 5 selectivity increased to 75.4%, this may be due to the speed up
of over-hydrogenation of imine 4 to amine 5 at high hydrogen pressure (Figure 3c).
Additionally, the effect of solvent was also studied (Figure S8). The ethanol was best solvent
in terms of reaction conversion. Nevertheless, 2-propanol gave the highest imine 4 selectivity.
The slight difference was likely attributed to the different polarities. Lastly, the recyclability
of the catalyst was tested and the result was shown in Figure 3d. It shows that the catalytic
performance remained almost the same after six cycles, demonstrating a satisfactory stability
of the catalyst. No metal traces in reaction solution were detectable by ICP, indicating the
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leaching of the active site is negligible. The XRD patterns (Figure S9) and TEM images
(Figure S10) of the reused catalyst after six cycles do not show an obvious change compared
with the as-synthesized one, indicating the stability of the catalyst.
Figure 3. The reductive animation of benzaldehyde and nitrobenzene over 2% CoPt catalyst.
o
(a) Reaction time = 2 h, H2 pressure = 0.5 MPa; (b) Reaction temperature = 60 C, H2
pressure = 0.5 MPa; (c) Reaction temperature = 60 oC, Reaction time = 2 h; (d) Reusability of
catalyst: Reaction temperature = 60 oC, Reaction time = 2 h, H2 pressure = 0.5 MPa. All other
reaction conditions: Catalyst = 20 mg, Benzaldehyde = 1 mmol, Nitrobenzene = 1 mmol, 2-
propanol = 10 mL.
We further explored the substrate applicability in the reductive animation of various
aldehydes and nitro compounds to imines. As summarized in Table 2, all reactions were
achieved in excellent conversion (> 95%) and satisfactory selectivity (> 90%). Firstly, the
reductive animation of aromatic benzaldehyde and nitro compounds with diverse substituent
groups were investigated (entries 1−6, Table 2). Thankfully, the activity and selectivity were
o
extremely high at 60 C within the 3 h reaction. Among them, when the substitute was
hydroxyl group on the nitrobenzene, the reaction conversion and corresponding imine
selectivity were 100% and 91.2%, respectively (entry 2, Table 2). For nitroarenes containing
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methyl group, such as 4-nitrotoluene, 3-nitrotoluene and 2-nitrotoluene, all of the substrates
were efficiently converted to the corresponding imines with ideal selectivity (entries 3−5,
Table 2). Due to the steric effect, the benzaldehyde 1 conversion was dependent on the
substituted position on the benzene ring with the order as follows: para > meta > ortho
isomer.[7a] In addition, halogen-substituted nitroarenes such as p-nitrochlorobenzene can be
100% converted to corresponding haloaromatic imines and without any dehelogenation type
byproducts (entry 6, Table 2). Furthermore, the nitro compounds with diverse substituent
groups also successfully underwent the reductive amination with pentose-derived furfural to
produce the corresponding biomass-derived imines (entries 7−12, Table 2). Similarly, the
exceptional conversion efficiencies and selectivities for imines were exhibited under the same
reaction conditions. For instance, the furfural reacted with both electron-donating (e.g., -Me)
and electron-withdrawing groups (e.g., -Cl) substituted nitrobenzene, giving the imines with
high conversion and selectivity (entries 9−12, Table 2). In addition, the benzaldehyde
incorporating electron-donating (-Me, -OMe) such as o-methylbenzaldehyde, p-
methylbenzaldehyde, o-methoxybenzaldehyde and p-methoxybenzaldehyde also gave good
conversions and imines selectivities (entries 13−16, Table 2). Thus, this catalyst is superior
for the reductive animation of various aldehydes and nitro compounds to imines.
Table 2. The reductive animation of various substrates using 2% CoPt catalysta
Substrate
Substrate
Conversion Selectivity
Entry
Product
(%)
100
(%)
1
2
1
2
90.0
100
100
91.2
90.8
3
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4
5
6
7
8
98.3
97.1
100
100
98.5
90.6
90.6
90.1
94.9
90.6
9
100
91.2
10
11
12
96.7
95.9
96.4
91.1
90.8
90.0
13
14
98.9
99.6
92.3
92.5
15
95.3
93.2
16
96.9
93.1
a Reaction conditions: Catalyst = 20 mg, Substrate 1 = 1 mmol, Substrate 2 = 1 mmol, 2-
propanol = 10 mL, Reaction temperature = 60 oC, Reaction time = 3 h, H2 pressure = 0.5 MPa.
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The one-pot reductive animation of benzaldehyde and nitrobenzene is mainly divided into
two steps: (1) the hydrogenation of nitrobenzene to aniline; (2) the condensation of aniline
with benzaldehyde to n-benzylideneaniline.[3b, 8] For the hydrogenation step, the rate constant
k at 60ꢀ°C can be calculated and the results were shown in Figure 4. We can speculate at this
time that the hydrogenation of nitrobenzene to aniline shows pseudo-first-order kinetics at the
condition of low conversion. Furthermore, the rate constant k was 0.024 min-1 for Co-800 and
0.162 min-1 for the 2% CoPt catalyst, which indicates that CoPt alloy greatly accelerates the
hydrogenation reaction.
Figure 4. First order kinetic fit for the hydrogenation of nitrobenzene to aniline: plots of -
ln(1-x) vs. time (min) to calculate rate constant k. Reaction conditions: Catalyst = 20 mg,
o
Nitrobenzene = 1 mmol, 2-propanol = 10 mL, Reaction temperature = 60 C, H2 pressure =
0.5 MPa.
Considering the structure characterizations (Figures 1-2) and experimental results (Table 1),
we used the H2-TPR and H2-TPD experiments to investigate why CoPt alloy decorated Co
NPs are highly active in this reaction. For the unreduced catalysts after the alloy treatment,
H2-TPR patterns (Figure S11) indicate that two peaks at around 250-300 °C and 310-330 °C
are scribed to the reduction of Co3O4 to CoO and CoO to Co°, respectively.[16] Compared to
strong cobalt peaks, the reduction of Pt species is overlapped owing to the low content of Pt.
Impressively, the addition of Pt shifted the peak reduction temperature to lower value,
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indicating that the hydrogen spillover from the CoPt alloy to Co NPs had occurred.[17]
Additionally, the hydrogen adsorption amounts of the catalysts at different temperatures can
be estimated by integration of the peak areas. It demonstrates that the strong interaction
between Pt and Co would significantly improve the increasing of activated hydrogen number,
thus increasing the catalytic activity. The H2-TPD results (Figure 5) show that there are two
H2 desorption peaks in the temperature range of 50–380 °C. Importantly, the amount of H2
desorption in 2% CoPt is much larger than that in Co-800, and the desorption peaks shift to a
lower temperature with the addition of Pt. This suggests that the strong alloy effect between
Pt and Co would improve H2 desorption at lower temperature, thus improving the catalytic
performance.[18] Besides, the DFT calculations found H2 will dissociate to two H* without
energy barrier on these catalysts surface. Furthermore, the calculated adsorption energy of
two H* is -1.165 eV on Co (111) and -0.945 eV on CoPt (111), which indicates that H*
desorbs more easily on CoPt (111) surface (inset of Figure 5). Thus, the less stable adsorbed
H* will be easier to participate in the next hydrogenation of nitrobenzene on CoPt catalyst.[19]
Figure 5. H2-TPD profile of the Co-800 and 2% CoPt catalysts. Inset: adsorption energies of
hydrogen on Co (111) and CoPt (111) surfaces.
Some studies have proved the adsorption behaviors of the reactants and products deeply
affected the selectivity of catalysts.[3b, 20] For instance, Zhang et al. recently showed the
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competitive adsorption of nitroarenes with aldehydes and weakened adsorption of imines
would effectively suppress the side-hydrogenation of aldehydes and over-hydrogenation of
imines, which lead to the significantly improved selectivity to imines by the experimental
results and DFT calculations.[3b] Herein, the ex-situ Fourier Transform Infrared (FT-IR)
spectroscopy was used to study the interaction between various substrates and 2% CoPt
catalyst. As shown in Figure 6, the band located at 1525 cm-1 and 1348 cm-1 could be
attributed to the absorption peaks of nitro group, indicating that the strong interaction between
nitro group and catalyst. Additionally, the weak interactions among the benzaldehyde, imine
and 2% CoPt catalyst were also investigated. Thus, this competitive adsorption of
nitrobenzene over benzaldehyde on CoPt alloy decorated Co NPs surface would reduce
benzaldehyde hydrogenation into benzyl alcohol as side reaction in our catalytic system. DFT
calculations further showed the adsorption energies of nitrobenzene and benzaldehyde on Co
(111) surface are -0.894 eV and -0.368 eV (Figure S12). Meanwhile, the adsorption energies
of nitrobenzene and benzaldehyde on CoPt (111) surface are -0.994 eV and -0.439 eV,
respectively (Figure 7). These results suggest that the induction of Pt to Co catalyst leads to
stronger adsorption of both nitrobenzene and benzaldehyde. More importantly, whether on the
Co (111) surface or CoPt (111) surface, the smaller adsorption energy of nitrobenzene
indicates competitive adsorption of nitrobenzene over benzaldehyde. Finally, the weak
adsorption of imine leads to the decrease of over-hydrogenation of the imine to amine. Based
on this consideration, Zhang et al. have adopted DFT calculations to confirm the bimetallic
PtCo can greatly weaken the adsorption of imine to suppress the over-hydrogenation of
imines, in contrast to the pure Pt or Co.[3b] As a result, the superior imine selectivity of this
catalyst depends on a strong adsorption capability for the nitrobenzene with a weak adsorption
capability for the imine.
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Figure 6. FT-IR spectra of various substrates on the 2% CoPt catalyst.
Figure 7. (a-b) Optimized geometry and (c) adsorption energy for the configuration of
benzaldehyde (C6H5CHO) and nitrobenzene (C6H5NO2) adsorption on CoPt (111) surface.
Blue sphere: Co, grey sphere: Pt, red sphere: O, brown sphere: C, pink sphere: H.
Based on the above experimental and DFT results, a proposed reaction pathway for the
selective synthesis of imine over CoPt alloy decorated Co NP catalyst is shown in the Scheme
2. Firstly, the nitrobenzene is intensively adsorbed on the surface of alloy decorated Co NPs
from the FT-IR and DFT results (Figures 6-7). Just like any other hydrogenation processes,
the H2 is easily dissociated and diffused in 2-propanol to produce the activated hydrogen
species (H*) on the surface of active sites. Next, the activated hydrogen species attack quickly
the adsorbed nitrobenzene to generate aniline (Figure 4). Afterwards, the generated aniline
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condensates with benzaldehyde to produce the imine (n-benzylideneaniline). Finally, the
desorption of imine can be easily occurred on the catalyst. At the same time, the side-
hydrogenation of benzaldehyde to benzyl alcohol and over-hydrogenation of imine to amine
is not preferred due to the weak interactions, thus increasing the selectivity of imine (Figure
6).
Scheme 2. Proposed reaction pathway over the as-synthesized catalysts.
Conclusions
In summary, we have developed a strategy to synthesize N-doped carbon nanotubes
supported CoPt alloy decorated Co NP catalyst, which was used in the one-pot reductive
amination with nitro compounds to imines under mild conditions. Benefiting from the
synergistic effect between CoPt alloy NPs and Co NPs with Co-Nx moieties, the resultant
catalyst exhibited remarkable reactivity toward the synthesis of imines. The high selectivity of
imines was due to the competitive adsorption of nitrobenzene over benzaldehyde would
reduce side-hydrogenation into benzyl alcohol and weak adsorption of imine leaded to the
decrease of over-hydrogenation of the imine to amine. Furthermore, it provided a general
method to design of bimetallic nanocatalysts for chemical reactions with desired activity and
selectivity.
Experimental Section
Materials
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Methanol, ethanol, 2-propanol and cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were
purchased from Sinopharm Chemical Reagent Co. Ltd. Chloroplatinic acid hexahydrate
(H2PtCl6·6H2O) and 2-methylimidazole were purchased from Aladdin Reagent Company.
Unless otherwise stated, all other solvents and chemicals were of commercially available
analytical grade and used without any further treatment.
Synthesis of ZIF-67
ZIF-67 precursors were synthesized according to previous reports with some
modifications.[21] In a typical synthesis, 2-methylimidazole (1.97 g) was dissolved in a mixed
solution of 20 mL methanol and 20 mL ethanol. Co(NO3)2·6H2O (1.746 g) was dissolved in
another mixed solution of 20 mL methanol and 20 mL ethanol. The above two solutions were
then mixed under continuous stirring for 10 s, and the final solution was kept for 20 h at room
temperature. The purple precipitate was collected by centrifugation, washed in ethanol several
times and dried at 70 °C in air for 12 h.
Synthesis of Co-800
The Co-800 was synthesized according to the literatures with some modifications.[21b, 22]
First, 1.0 g ZIF-67 particles were mounted in a ceramic boat in a tube furnace and heated to
440 °C with the heating ramp of 2 °C min−1 and held for 8 h under continuous flow of 10
vol % H2/Ar. The obtained sample is denoted as Co-440. Afterwards, the temperature in the
furnace was further raised to 800 oC at a heating rate of 2 oC min−1 and maintained for 2 h in a
tube furnace and allowed to cool down naturally. The obtained black powdered sample is
denoted as Co-800.
Synthesis of x% CoPt catalysts
In a typical synthesis, the mixture of Co-800 (100 mg), a certain aqueous solution of
H2PtCl6·6H2O (0.135 M) and H2O (5 ml) were added into the stainless steel autoclave with a
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mechanical stirrer. After vigorous stirring for 6 h at room temperature, the sample was
collected by centrifugation, washed in ethanol several times and dried at 70 °C for 12 h. The
obtained samples are denoted as x% CoPt-u, where x is the mass fraction of Pt. The Co-800-u
catalyst was prepared by the same procedure without Pt sources. Afterwards, the solid was
placed in a ceramic boat and heated to 440 °C with the heating ramp of 2 °C min−1 and held
for 8 h under continuous flow of 10 vol % H2/Ar. The final samples were denoted as x% CoPt.
For comparison, the 2% Pt-NC catalyst was fabricated by the same procedure aforementioned
except ZIF-8 derived N-doped carbon instead of Co-800.
Catalyst characterization
Field emission scanning electron microscopy (FESEM) images of the samples were taken
on a FESEM (SU8020) operated at an accelerating voltage of 10.0 kV. Transmission electron
microscopy (TEM) images of the samples were recorded by a high resolution TEM (Philips
TecnaiG2 F20) operated at an acceleration voltage of 200 kV. The high-angle annular dark-
field scanning transmission electron microscopy (HAADF-STEM) images, energy dispersive
X-ray (EDX) elemental mappings and aberration-corrected high resolution TEM (HRTEM)
were recorded by a JEOL ARM-200F field-emission transmission electron microscope
operating at an accelerating voltage of 200 kV. Powder X-ray diffraction (XRD) patterns were
analyzed on a Philips X-Pert Pro X-ray diffractometer with using the Ni-filtered
monochromatic Cu Kα radiation (λKα1 = 1.5418 Å) at 40 keV and 40 mA. The surface area
and porosity of samples were measured at 77 K using a Surface Area and Porosity Analyzer
(Autosorb iQ Station 2). XPS analysis was performed on an ESCALAB 250 X-ray
photoelectron spectrometer (Thermo, USA) equipped with Al Kα1,2 monochromatized
radiation at 1486.6 eV X-ray source. The Fourier Transform Infrared Spectroscopy (FT-IR)
measurements were carried out on a Nexus spectrometer (Thermo Nicolet Corporation, USA)
in a KBr pellet at room temperature. Specifically, the various substrates and the sample were
o
performed in 2-propanol for 1 h at 60 C. Afterwards, the solids were washed by H2O, and
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then dried. Raman spectra of the samples were recorded on a LabRAM HR800 confocal
microscope Raman system (Horiba Jobin Yvon) using an Ar ion laser operating at 532 nm.
The metal content in the composite structure was determined by the inductively coupled
plasma spectroscopy after microwave digestion of the samples (ICP 6300, Thermo Fisher
Scientific). The H2 temperature-programmed reduction (H2-TPR) was measured by using a
Micromeritics AutoChem II 2920 equipped with a thermal conductivity detector. In a typical
experiment, 30 mg sample was first loaded into a U-shaped quartz sample tube. Prior to the
measurement, the sample was preheated under a He flow (20 mL min−1) at 300 °C for 0.5 h to
remove adsorbed species on the surface of sample. The reactor was then cooled down to
40 °C , and 10 vol% H2/Ar mixture was introduced. The H2-TPR test was conducted under a
H2/Ar flow by increasing the temperature to 550 °C with a heating rate of 10 °C min−1.
Hydrogen temperature-programmed desorption (H2-TPD) experiments were performed in the
same apparatus as the H2-TPR experiments. The sample was swept with He at a flow rate of
30 ml min−1 at 300 °C for 0.5 h to remove physisorbed and/or weakly bound species. H2-TPD
was performed by heating the sample from room temperature to 400 °C at a ramp rate of
10 °C min−1 in Ar, and the spectra were recorded.
Performance evaluation
The catalytic reactions were carried out in a 25 mL stainless steel autoclave with a
mechanical stirrer, a pressure gauge and automatic temperature control apparatus (Anhui
Kemi Machinery Technology Co., Ltd., China). In a typical experiment, the reaction solutions
of benzaldehyde (1 mmol), nitrobenzene (1 mmol), catalyst (20 mg) and 2-propanol (10 ml)
were loaded into the autoclave. The reactor was sealed, purged three times with N2 at 1 MPa,
and then pressurized with H2 to a setting point. The reactor was then heated to the desired
temperature and the stirring speed fixed to 700 rpm which eliminates the diffusion effects.
After reaction, the autoclave was cooled down quickly. The autoclave contents were
transferred to a centrifuge tube, and the catalyst was separated by centrifugation. The liquid
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product was identified by gas chromatography–mass spectrometry (GC-MS, Thermo Fisher
Scientific-TXQ Quntum XLS), and was quantitatively analyzed by GC (Shimadzu, GC-2010
Plus), equipped with FID. During the catalyst stability test, the catalyst was reused without
any further treatments. More specifically, the reaction mixture was filtered to recover the
catalyst, which was washed first with water then ethanol followed by drying under vacuum
oven at 60 oC and employed for stability test.
DFT calculations
All the calculations have been performed within the framework of density function theory
(DFT), using the Vienna Ab initio Simulation Package (VASP) with Perdew–Burke–
Ernzerhof generalized gradient approximation (PBE-GGA) functional. The projector
augmented wave (PAW) method has been used to describe the inert core electrons. We
relaxed the structures until the total energy changes within 1×10-4 eV per atom and the
Hellmann–Feynman force on each atomic site was less than 0.05 eV/Å, with a 400 eV cutoff
energy. The fcc Co was used for the bulk, CoPt alloy was constructed by replacing one Co
atom by one Pt atom. Co (111) and CoPt (111) slab models were constructed with three
atomic layers and a vacuum space larger than 20 Å. The Gamma (Γ) centered 2×2×1
Monkhorst-Pack sampling was used for those surfaces. The perpendicular adsorption
configurations of hydrogen, benzaldehyde (C6H5CHO) and nitrobenzene (C6H5NO2) were
chosen for qualitative analysis. In all the calculations, the top two atomic layers of the surface
were allowed to relax together with the molecule adsorbate, while the bottom one layer was
keep fixed to present the bulk properties. The binding energy Eb is given by the following
equation: Eb = Eslab/ads – Emolecule – Eslab, where Eslab/ads is the total energy of catalyst surface
adsorbed with molecules.
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
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This work was financially supported by the Natural Science Foundation of China (Grant No.
51871209 and 51902311), the Postdoctoral Science Foundation of China (Grant No.
2019M652223) and Youth Innovation Promotion Association CAS (2020458).
Conflicts of interest
There are no conflicts to declare.
Keywords: Heterogeneous catalysis • Adsorption • Alloy • Reductive amination • Imine
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[1] a) T. Schwob, R. Kempe, Angew. Chem. Int. Ed. 2016, 55, 15175-15179; b) B. Zhang, X.-
W. Guo, H. Liang, H. Ge, X. Gu, S. Chen, H. Yang, Y. Qin, ACS Catal. 2016, 6, 6560-
6566; c) Y. Zheng, K. Ma, H. Li, J. Li, J. He, X. Sun, R. Li, J. Ma, Catal. Lett. 2008, 128,
465-474; d) J. Bu, W. Wang, H. Zhang, C. Wu, B. Zhang, Y. Cao, F. Zhou, Q. Zhang, X.
Zhang, Mol. Catal. 2018, 459, 46-54; e) L. Ye, Z. Li, ChemCatChem 2014, 6, 2540-2543;
f) F. Bottaro, R. Madsen, ChemCatChem 2019, 11, 2707-2712.
[2] J. Deng, L.-P. Mo, F.-Y. Zhao, L.-L. Hou, L. Yang, Z.-H. Zhang, Green Chem. 2011, 13,
2576-2584.
[3] a) P. Zhou, Z. Zhang, ChemSusChem 2017, 10, 1892-1897; b) S. Zhang, Z. Xia, Y. Ma, J.
Li, Y. Qu, J. Catal. 2019, 374, 72-81.
[4] L. Cisneros, P. Serna, A. Corma, Angew. Chem. Int. Ed. 2014, 53, 9306-9310.
[5] A. S. Touchy, S. M. A. Hakim Siddiki, K. Kon, K.-i. Shimizu, ACS Catal. 2014, 4, 3045-
3050.
[6] a) B. Wu, H. Huang, J. Yang, N. Zheng, G. Fu, Angew. Chem. Int. Ed. 2012, 51, 3440-
3443; b) J. Luo, J. D. Lee, H. Yun, C. Wang, M. Monai, C. B. Murray, P. Fornasiero, R. J.
Gorte, Appl. Catal., B. 2016, 199, 439-446; c) B. Y. Xia, H. B. Wu, N. Li, Y. Yan, X. W.
Lou, X. Wang, Angew. Chem. Int. Ed. 2015, 54, 3797-3801; d) Z. Wang, X. Ren, Y. Luo,
L. Wang, G. Cui, F. Xie, H. Wang, Y. Xie, X. Sun, Nanoscale 2018, 10, 12302-12307; e)
24
This article is protected by copyright. All rights reserved.

10.1002/cctc.202001331
ChemCatChem
H. Yang, S. J. Bradley, A. Chan, G. I. Waterhouse, T. Nann, P. E. Kruger, S. G. Telfer, J.
Am. Chem. Soc. 2016, 138, 11872-11881.
[7] a) D. Liu, P. Yang, H. Zhang, M. Liu, W. Zhang, D. Xu, J. Gao, Green Chem. 2019, 21,
2129-2137; b) H. Yang, Y. Zhang, Q. Chi, Z. Zhang, ChemSusChem 2019, 12, 1246-1255.
[8] a) P. Zhou, C. Yu, L. Jiang, K. Lv, Z. Zhang, J. Catal. 2017, 352, 264-273; b) P. Zhou, L.
Jiang, F. Wang, K. Deng, K. Lv, Z. Zhang, Sci. Adv. 2017, 3, 1601945.
[9] L. Jiang, P. Zhou, Z. Zhang, S. Jin, Q. Chi, Ind. Eng. Chem. Res. 2017, 56, 12556-12565.
[10] W. Gong, Y. Lin, C. Chen, M. Al-Mamun, H. S. Lu, G. Wang, H. Zhang, H. Zhao, Adv.
Mater. 2019, 31, 1808341.
[11] Y. Gu, Y. Zhao, P. Wu, B. Yang, N. Yang, Y. Zhu, Nanoscale 2016, 8, 10896-10901.
[12] L. Chong, J. Wen, J. Kubal, F. Sen, J. Zou, J. Greeley, M. Chan, H. Barkholtz, W. Ding,
D. Liu, Science 2018, 362, 1276-1281.
[13] C. Tang, A-E. Surkus, F. Chen, M-M. Pohl, G. Agostini, M. Schneider, H. Junge, M.
Beller, Angew. Chem. Int. Ed. 2017, 56, 16616-16620.
[14] H. W. Liang, S. Bruller, R. Dong, J. Zhang, X. Feng, K. Mullen, Nat. Commun. 2015, 6,
7992.
[15] T. Cheng, H. Yu, F. Peng, H. Wang, B. Zhang, D. Su, Catal. Sci. Technol. 2016, 6,1007–
1015.
[16] a) Z. He, N. Li, K. Wang, W. Wang, Z. Liu, Mol. Catal. 2019, 470, 120–126; b) P. Yang,
Q. Xia, X. Liu, Y. Wang, Fuel 2017, 187, 159-166.
[17] X. Wang, Y. Liu, X. Liang, Green Chem. 2018, 20, 2894-2902.
[18] Y. Li, P.-F. Zhu, R.-X. Zhou, Appl. Surf. Sci. 2008, 254, 2609-2614.
[19] Y. Zhang, Y. Gao, S. Yao, S. Li, H. Asakura, K. Teramura, H. Wang, D. Ma, ACS Catal.
2019, 9, 7967-7975.
25
This article is protected by copyright. All rights reserved.

10.1002/cctc.202001331
ChemCatChem
[20] a) C. Xie, J. Song, H. Wu, Y. Hu, H. Liu, Z. Zhang, P. Zhang, B. Chen, B. Han, J. Am.
Chem. Soc. 2019, 141, 4002-4009; b) G. Liang, A. Wang, L. Li, G. Xu, N. Yan, T.
Zhang, Angew. Chem. Int. Ed. 2017, 56, 3050-3054.
[21] a) P. Xiong, X. Zhao, Y. Xu, ChemSusChem 2018, 11, 202-208; b) B. Y. Xia, Y. Yan, N.
Li, H. B. Wu, X. W. Lou, X. Wang, Nat. Energy 2016, 1, 15006.
[22] J. Meng, C. Niu, L. Xu, J. Li, X. Liu, X. Wang, Y. Wu, X. Xu, W. Chen, Q. Li, Z. Zhu,
D. Zhao, L. Mai, J. Am. Chem. Soc. 2017, 139, 8212-8221.
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A simple and highly efficient CoPt alloy decorated Co nanoparticles catalyst was synthesized
and showed excellent catalytic performance for the one-pot reductive amination with nitro
compounds and aldehydes under mild reaction conditions.
Wanbing Gong,+ Miaomiao Han,+ Chun Chen, Yue Lin,* Guozhong Wang, Haimin Zhang*
and Huijun Zhao*
Rational Design of Cobalt-Platinum Alloy Decorated Cobalt Nanoparticles for One-Pot
Synthesis of Imines from Nitroarenes and Aldehydes
TOC figure
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