Catalytically Active Co?Nx Species Stabilized on Nitrogen-doped Porous Carbon for Efficient Hydrogenation and Dehydrogenation of N-heteroarenes

Article abstract of DOI:10.1002/cctc.202000826

The development of bifunctional, highly active and stable non-noble-metal catalysts is important for synthetic chemistry. In this study, a highly dispersed Co catalyst stabilized on the mesoporous N-doped carbon layers was prepared by adsorption and pyrolysis of cobalt complex on dendritic fibrous silica nanospheres (KCC-1@Co?N?C?T). The characterizations of HAADF-STEM, XRD and XPS together with the KSCN poisoning tests determine the absence of Co⁰ or CoO_x nanoparticles and suggest that the Co?N_x species are the active sites. The formation of Co?N_x species results from the properties of N-rich cobalt-phenanthroline complex and dendritic fibrous silica supports, increasing the original spatial distance between Co atoms and thus preventing them from aggregation. The KCC-1@Co?N?C-800 catalyst showed excellent activity and selectivity for the oxidative dehydrogenation (ODH) of saturated N-heterocycles and base-free catalytic transfer hydrogenation (CTH) of unsaturated N-heterocycles.

Full text of DOI:10.1002/cctc.202000826

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Catalytically Active Co-Nx Species Stabilized on Nitrogen-doped
Porous Carbon for Efficient Hydrogenation and Dehydrogenation
of N-heteroarenes
Authors: Jiantai Ma, Dan Xu, Hong Zhao, and Zhengping Dong
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01/2020

10.1002/cctc.202000826
ChemCatChem
Catalytically Active Co-Nx Species Stabilized on Nitrogen-doped Porous Carbon
for Efficient Hydrogenation and Dehydrogenation of N-heteroarenes
Dan Xu, Hong Zhao, Zhengping Dong* and Jiantai Ma*
Laboratory of Special Function Materials and Structure Design of the Ministry of
Education, College of Chemistry and Chemical Engineering, Gansu Provincial
Engineering Laboratory for Chemical Catalysis, Lanzhou University, No.222, South
Tianshui Road, Lanzhou (P.R. China)
E-mail address: dongzhp@lzu.edu.cn (Zhengping Dong), majiantai@lzu.edu.cn
(Jiantai Ma), Tel: 86 931 8912577.
Abstract
The development of bifunctional, highly active and stable non-noble-metal catalysts is
important for synthetic chemistry. In this study, a highly dispersed Co catalyst stabilized
on the mesoporous N-doped carbon layers was prepared by adsorption and pyrolysis of
cobalt complex on dendritic fibrous silica nanospheres (KCC-1@Co-N-C-T). The
characterizations of HAADF-STEM, XRD and XPS together with the KSCN poisoning
tests determine the absence of Co0 or CoOx nanoparticles and suggest that the Co-Nx
species are the active sites. The formation of Co-Nx species results from the properties
of N-rich cobalt-phenanthroline complex and dendritic fibrous silica supports,
increasing the original spatial distance between Co atoms and thus preventing them
from aggregation. The KCC-1@Co-N-C-800 catalyst showed excellent activity and
selectivity for the oxidative dehydrogenation (ODH) of saturated N-heterocycles and
base-free catalytic transfer hydrogenation (CTH) of unsaturated N-heterocycles. The
turnover frequency (TOF) of base-free CTH reaction was 3-25 times higher than the
most of reported transition-metal-based cataysts for the hydrogenation reactions of N-
heterocycles. The high performance of catalyst roots in the well-dispersed Co-Nx
species supported on 3D mesoporous silica nanosphere-based carbon layers, the
dendritic channel structure and mesoporosity of support could ensure the high exposure
of Co-Nx species to the reactants and facilitate the mass transport between reactants and
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10.1002/cctc.202000826
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products.
KEYWORDS
Nitrogen-doped carbon, cobalt-nitrogen sites, bifunctional catalyst, oxidative
dehydrogenation, transfer hydrogenation
Introduction
Nitrogen-containing heterocycles have been highlighted as building blocks in
pharmaceutical synthesis, biological and material chemistry.[1] Oxidative
dehydrogenation (ODH) and catalytic transfer hydrogenation (CTH) reactions are
promising methods to achieve reversible transformations between saturated and
unsaturated N-heterocycles. Among several unsaturated N-heterocycles, quinolines and
indoles are key constituents in pharmaceuticals, which are typically synthesized by the
ODH of corresponding saturated N-heterocycles.[2] As an atom-efficient process, the
ODH of saturated N-heterocycles has been extensively investigated, while an excess
amount of strong oxidants have been used in conventional methods, which generates a
considerable amount of poisonous waste.[3] As a benign, non-toxic oxidant, molecular
oxygen can replace strong and toxic oxidants. Notably, water is genarated as the only
by-product with the release of H2. Hence, the ODH reaction not only avoids the use of
strong oxidants but also provides a promising application for H2 storage.[4] N-formyl-
1,2,3,4-tetrahydroisoquinoline and relevant derivatives are crutial alkaloids and
medical intermediates.[5] The transformation from quinolines to high value-added N-
formylation products is significant, because quinolines are rich in coal and oil shale,[6]
and generated as by-products from the petroleum refining process. Thus far, a
considerable number of studies have reported the N-formylation of several unsaturated
N-heterocycles with H2 and CO2 as the reductant and carbon source, respectively.[7] As
a by-product derived from the hydrolysis of biomass, formic acid (HCOOH) can replace
H2 as an eco-friendly, renewable reductant and serve as a carbon source. Hence, CTH
can be performed without pressure equipment using HCOOH as the hydrogen and
carbon source. Besides, it is challenging to selectively reduce unsaturated N-
heterocycles bearing different functional groups, because unsaturated N-heterocycles
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10.1002/cctc.202000826
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exhibit high resonance and possibly produce poisonous effect on the catalyst.[8]
Compared with the general catalytic hydrogenation, the CTH of unsaturated N-
heterocycles has been less explored.
Currently, several homogeneous catalysts based on metals such as Rh,[9] Ir,[10] Ru,[11]
Pd,[12] Fe,[13] and Co[14] have been studied for the oxidation and hydrogenation reactions
of N-heterocycles. Although these homogeneous catalysts showed good activity and
selectivity, the introduction of air-sensitive or precious ligands as well as difficult
separation issues considerably restrict their large-scale applications. On the other hand,
heterogeneous catalysts can be fascinating alternatives due to their facile recovery and
cost-effectiveness. Several heterogeneous oxidation systems including Pt/Ir NC,[15]
AuNPs/C,[16] Cu(0)/Al2O3,[17] and RhCNT[18] have been employed in ODH. In addition,
hydrogenation or CTH has been investigated using other heterogeneous catalysts based
on Au0.9Pd0.1/CNR,[19] Pd/HS-NH2,[20] Pt NPs,[21] and Ru/HT-C12A7.[22] These noble-
metal-derived catalytic systems exhibit undesirable functional group tolerance.
Although several catalysts have been employed for oxidation or hydrogenation
reactions, extremely few catalysts have been explored to catalyze both dehydrogenation
and hydrogenation of N-heterocycles. Therefore, it is highly desirable to develop an
efficient transition-metal-based heterogeneous catalytic system for both ODH and CTH
reactions.
So far, great efforts have been made to develop transition-metal-based heterogeneous
catalysts.[23] Considering the reported catalysts, N-doped carbon modified transition
metal materials (M- Nx -C, M=Fe, Co, Ni, etc.) can act as the most promising catalysts
due to the high performance and stability. It is proved that the doped N atoms and metal
could work synergistically to enhance the activity.[24] The M-NX-C materials are usually
obtained from the pyrolysis of transition metal ions coordinated with N-rich ligands.[25]
Due to the excellent ability of activating oxygen and hydrogen, the M-NX-C catalysts
have been applied to many oxidation and reduction reactions.[26] It is accepted that high
explosure of active sites is closely related to the catalytic activity. And silica-based
materials generally serve as robust supports to confine the active sites. The skeleton
structure of silica could be maintained under the high-temperature pyrolysis conditions,
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10.1002/cctc.202000826
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thus these robust supports could effectively stabilize the in-situ formed metal atoms and
impede them aggregation into nanoparticles.
In this study, a Co-Nx species finely supported on dendritic fibrous silica nanospheres
(KCC-1@Co-N-C-T) catalyst was prepared by the adsorption and pyrolysis of metal-
ligand complex on the KCC-1 support. During the pyrolysis, the 1,10-phenanthroline
ligand was converted to the N-doped carbon skeleton and Co ions were in situ
coordinated with N atoms to form Co-Nx species. The high surface area and
mesoporous structure of KCC-1 facilitate the high explosure of active sites and mass
transport of substrates, so the Co-Nx species stabilized on mesoporous N-doped carbon
layer show excellent activity, selectivity and stability towards the ODH and CTH
reactions. The catalytic activity of KCC-1@Co-N-C-800 is much higher than those of
reported transition-metal-based catalysts towards the hydrogenation reactions. Our
study provides a significant method for preparing a high-performance and cost-
effective transition-metal system for the practical apllications.
Experimental Section
Materials
1-Hexadecylpyridinium bromide monohydrate (CPB, ≥98%), urea (≥99.0%), tetraethyl
orthosilicate (TEOS, ≥98%), cobaltous acetate tetrahydrate (Co(OAc)2·4H2O, ≥99.5%),
cyclohexane (≥99.7%), 1-pentanol, and 1,10-phenanthroline (1,10-phen, ≥99.0%) were
purchased from Tianjin Guangfu Fine Chemical Research Institute. 1,2,3,4-
Tetrahydroquinoline (≥99%) and quinoline (≥98%) were purchased from Tianjin
Heowns Biochem LLC. All of the reagents were used without modification.
Synthesis of KCC-1
According to the study reported by Polshettiwar et al.,[27] urea (3.6 g, 0.06 mmol) and
CPB (6 g, 0.0156 mmol) were dissolved in water (180 mL), referred to as solution A.
TEOS (16.14 mL, 0.072 mmol) was added into a solution containing cyclohexane (180
mL) and 1-pentanol (9 mL), referred to as solution B. Then, A and B were
homogeneously mixed, stirred at room temperature for 30 min, and transferred into a
Teflon-sealed hydrothermal reactor at 120 ℃ for 5 h. Finally, after centrifugation,
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10.1002/cctc.202000826
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washing, and drying, the products were pyrolyzed at 550 ℃ for 6 h in air, affording
target product KCC-1.
Synthesis of KCC-1@Co-N-C-T
First, Co(OAc)2·4H2O (0.5 mmol) and 1,10-phen (1.0 mmol) were added into 20 mL
anhydrous ethanol, and the solution was stirred for 15 min at ambient temperature and
60 ℃ for 1 h. Second, after the addition of the KCC-1 support (345.6 mg), the solution
was stirred overnight and evaporated under vacuum. Finally, the pink power was
1
pyrolyzed at 600℃-900 ℃ under N2 for 2 h and at a heating rate of 5 ℃ min− . In the
controlled experiments, the KCC-1@Co-N-C-800 catalyst was further treated with 2 M
HCl at 50 ℃ for 5 h and 0.5 M H2SO4 at 25 ℃ for 24 h, respectively.
Catalytic Oxidative Dehydrogenation of 1,2,3,4-tetrahydroquinoline
1,2,3,4-Tetrahydroquinoline (0.5 mmol), K2CO3 (0.5 mmol), and the catalyst (20 mg)
were added in CH3OH (5 mL) in a two-neck flask. Then, the flask was purged with an
O2 balloon and heated to 80 ℃ for certain hours. Gas chromatography-mass
spectrometry (GC-MS, Agilent 5977E) was employed for product analysis.
Catalytic Transfer Hydrogenation of Quinoline
First, quinoline (0.5 mmol), catalyst (10 mg), and a certain equivalent of HCOOH were
added in toluene (2 mL) in a sealed tube. Second, the reaction was heated at 130 ℃ for
6 h, and the products were analyzed by GC-MS. In the KSCN poisoning test, the molar
amount of KSCN was equal to 16 equiv. to the Co amount.
Catalyst Characterization
Morphologies of catalysts were observed by transmission electron microscopy (TEM),
and scanning electron microscopy (SEM, HITACHI S-4800). HAADF-STEM images
were measured by using a Titan Themes Cubed G2 300 transmission electron
microscope with a probe spherical aberrarion corrector. Power X-ray diffraction (XRD,
Rigaku D/max-2400 diffractometer) patterns with Cu-kα radiation were recorded.
Raman spectroscopy (Jobin Yvon LabRam HR Evolution) was employed. X-ray
photoelectron spectroscopy (XPS, PHI-5702) was to confirm the bonding states of
elements. The surface area, average pore size, and pore volume can be estimated by the
Brunauer–Emmett–Teller
(BET,
Micromeritics
ASAP
2010)
analysis.
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Thermogravimetric analysis of the samples was carried out by heating the samples from
20 ℃ to 1000 ℃ under N2 using a TA-Q50 TGA instrument. The N, C, O, and Co
contents were determined by elemental analysis (EA, CHNS) and inductively coupled
plasma-atomic emission spectrometry (ICP-AES, Optima-4300 DV Perkin Elmer). The
conversion and selectivity of products were determined by GC–MS.
Results and Discussion
The KCC-1@Co-N-C-800 catalyst was prepared using Co(OAc)2·4H2O and 1,10-phen
as the metal precursor and N-rich ligand, respectively. Scheme 1 briefly summarizes
the preparation method. First, the CPB served as a template to form a microemulsion.
The TEOS was gradually hydrolyzed into negatively charged silicates under urea. Then,
silicates were clustered into the interspace between the self-assembled template
molecules and the condensation of silicates happened. After performing calcination to
remove the template molecules, the dendritic fibrous silica nanospheres (KCC-1) were
obtained. In addition, the Co(OAc)2·4H2O can firmly coordinate with 1,10-phen to
form a complex, and the complex could be uniformly adsorbed on the KCC-1 support.
By pyrolysis at 800 ℃ under N2 atmosphere, Co-Nx species were formed and highly
dispersed in the mesoporous N-doped carbon layers. It is reported that coordination
structure could significantly influence the properties of central metal.[28] In this work,
the metal-ligand complex structure can protect the Co precursor from aggregation at a
high pyrolysis temperature, thereby affording highly dispersed active sites.
Scheme 1 Schematic of the synthesis process of KCC-1@Co-N-C-800.
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10.1002/cctc.202000826
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TGA curve of KCC-1@Co-N-C was recorded to confirm the thermal stability (Fig. S1).
Aweight loss of 17.7% at 278 ℃ corresponded to the loss of chemically adsorbed water
on the precursor and hydroxyl groups on the support. When the pyrolysis temperature
was increased to 476 ℃, the pyrolysis of the Co-1,10-phen complex led to a weight
loss of 12.8%. A weight loss of 15% observed from 476 ℃ to 979 ℃ was related to the
further pyrolysis, which led to the volatilization of C- and N-containing groups. The
TEM image shown in Fig.1a revealed that supports are monodisperse dendritic fibrous
silica nanospheres with diameters ranging from 200 nm to 300 nm, and the SEM image
in Fig.1b revealed that central radial channels are formed between the dense nanofolds,
and the pore size gradually increases from the inside to the outside of the nanospheres.
Co0 or CoOx nanoparticles are not observed in the KCC-1@Co-N-C-800 catalyst,
suggesting that metal-ligand complex could effectively protect Co species from
aggregation during pyrolysis (Fig.1c) and Co ions are probably coordinated with N
atoms. The crystal planes of Co are not observed in the HRTEM image (Fig. S2),
indicating the absence of crystalline Co phases or clusters in the KCC-1@Co-N-C-800
catalysts. From aberration-corrected HAADF-STEM (Fig. S3), high-density and
uniformly dispersed Co single atoms (bright dots) were observed, indicating the
existence of the Co-Nx species. The EDX mapping images of KCC-1@Co-N-C-800
revealed that the KCC-1 support is successfully formed, and C, N, and Co are
homogeneously distributed on its surface (Fig.1d). When the pyrolysis temperature was
increased to 900 ℃, Co NPs were formed in the N-doped carbon layer (Fig. S4c and
Fig. S5a-b), probably resulting from the decreased coordinated N content (Table S1).
The average size of Co NPs was obtained to be 4.35 nm in the KCC-1@Co-N-C-900
catalyst (Fig. S5d) via randomly counting 100 Co NPs.
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Fig. 1. (a) TEM and (b) SEM images of KCC-1. (c) TEM and (d) EDX mapping images
of KCC-1@Co-N-C-800.
Fig. 2a shows the XRD patterns of KCC-1 and KCC-1@Co-N-C-T at pyrolysis
temperatures of 600 ℃-900 ℃. A wide hump observed at ~23° was characteristic peak
of amorphous silica in the KCC-1 support.[29] At pyrolysis temperatures of 600 ℃-
900 ℃, no peaks of Co0 or CoOx were observed, proving that highly dispersed Co
1
species are existed. The Raman spectrum (Fig. 2b) revealed D (1351 cm− ) and G (1581
1
cm− ) bands, corresponding to structural lattice defects and characteristic sp2-
hybridized carbon, respectively.[30] With the pyrolysis performed from 600 ℃ to 900 ℃,
ID/IG values were 1.03, 1.02, 0.99, and 0.98, indicative of the increased degree of
graphitization with the increase of the pyrolysis temperature. Type IV BET isotherms
were observed for the KCC-1 and KCC-1@Co-N-C-T catalysts (Fig. 2c). The KCC-1
1
support exhibited the highest surface area of 459 m2g− , facilitating the loading of
channels and modification of surfaces by Co, N co-doped carbon layers. The KCC-
1@Co-N-C-T catalysts possess microporous-mesoporous structure (Fig. 2d). The
presence of micropores is also related to the amorphous structure of the Co-1,10-phen
complex-derived skeleton.[31] The values of BET surface areas of different pyrolysis
1
temperatures are listed in the following sequence: KCC-1@Co-N-C-800 (208 m2g− ) >
1
1
KCC-1@Co-N-C-700 (199 m2g− ) > KCC-1@Co-N-C-900 (120 m2g− ) > KCC-
1
1@Co-N-C-600 (58 m2g− ). The KCC-1@Co-N-C-600 catalyst exhibited the lowest
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10.1002/cctc.202000826
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surface area, probably related to the incomplete graphitization of the Co-1,10-phen
complex precursor and the KCC-1 support surface was mainly covered by the complex,
which was in agreement with the Raman spectroscopy results. KCC-1@Co-N-C-900
exhibited a relatively lower surface area, resulting from the collapse of the partial
structure that blocked the corresponding channels; this result is supported by the TEM
image in Fig. S4c. The high surface areas of KCC-1@Co-N-C-700 and KCC-1@Co-
N-C-800 facilitated the exposure of active sites, abundant dendritic channels reduced
the mass transport effect, and reactants easily had access to the active sites, thereby
enhancing the catalytic activity.
KCC-1@Co-N-C-600
KCC-1@Co-N-C-700
KCC-1@Co-N-C-800
KCC-1@Co-N-C-900
(b)
(a)
G
D
ID/IG=0.98
KCC-1@Co-N-C-900
KCC-1@Co-N-C-800
KCC-1@Co-N-C-700
ID/IG=0.99
ID/IG=1.02
KCC-1@Co-N-C-600
KCC-1
ID/IG=1.03
500 1000 1500 2000 2500 3000
Raman shift (cm-1)
15
30
45
60
75
90
2 Theta (degree)
KCC-1
KCC-1@Co-N-C-800
KCC-1@Co-N-C-900
(c)
KCC-1@Co-N-C-600
KCC-1@Co-N-C-700
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 2. (a) XRD patterns, (b) Raman spectrums, (c) N2 adsorption/desorption isotherms
and (d) pore size distributions of KCC-1, KCC-1@Co-N-C-600, KCC-1@Co-N-C-700,
KCC-1@Co-N-C-800 and KCC-1@Co-N-C-900.
XPS spectroscopy can analyze the chemical states of C, N and Co elements. The C1s
spectra of KCC-1@Co-N-C-T was fitted into C-C (284.6 eV), C-N (285.3 eV), C-O
(286.4 eV) and O-C=O (288.5 eV), as shown in Fig. 3a.[32] The N1s spectrum shown
in Fig. 3b was resolved into Pyridinic-N (398.2 eV), Co-Nx (399.6 eV) and Graphitic-
N (401.0 eV).[33] The presence of Graphitic N was attributed to the graphitization of
1,10-phen ligands that were not coordinated with Co ions. The Co2p spectrum revealed
Co2p1/2 (796.4 eV) and Co2p3/2 (779.4 eV) and Co-Nx (781.8 eV).[34] The Co2p1/2 and
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10.1002/cctc.202000826
ChemCatChem
Co2p3/2 were characteristic peaks of Co2+ (Fig.3c). The Co-Nx species could effectively
prevent metal from aggregation and leaching in the acidic reaction system. From ICP
and EA results in Table S1, with the pyrolysis temperature increased from 600 ℃ to
900 ℃, the N content decreased and the Co content increased. Compared with KCC-
1@Co-N-C-600 and KCC-1@Co-N-C-700, the KCC-1@Co-N-C-800 catalyst had the
highest ratio of Co/N, revealing that the density of Co-Nx active sites was largest.
Fig. 3. XPS analysis of (a) high-resolution C1s spectrum, (b) high-resolution N1s
spectrum, (c) high-resolution Co2p spectrum of KCC-1@Co-N-C-600, KCC-1@Co-
N-C-700, KCC-1@Co-N-C-800 and KCC-1@Co-N-C-900 catalysts.
The catalytic performance of KCC-1@Co-N-C was firstly investigated by the ODH of
1,2,3,4-tetrahydroquinoline for saturated N-heteroarenes. Compared with 1,10-phen,
using 2,2-bipyridine (bipy) and 1,4-naphthalenedicarboxylic acid (NDC) as ligands,
low conversions were observed (Table 1, entries 1-3). As seen from XRD pattern of the
catalyst with bipy as ligand (Fig. S6), the three diffraction peaks were observed,
corresponding to the (111), (200), and (220) planes of metallic Co (JCPDS No. 15-806).
The N element was not contained in NDC ligand. All of these results led to the
undesired activities. In addition, different pyrolysis temperatures were screened (Table
1, entries 4-6). Compared with the KCC-1@Co-N-C-800, the KCC-1@Co-N-C-600
and KCC-1@Co-N-C-700 showed the conversions of 35.9% 58.9% due to the lower
specific surface area and ratio of Co/N. The KCC-1@Co-N-C-900 also had a low
conversion of 38.3%, resulting from the formation of Co nanoparticles. In addition, the
catalysts without Co(OAc)2 or a homogeneous Co(OAc)2-1,10-phen complex without
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10.1002/cctc.202000826
ChemCatChem
pyrolysis exhibited low conversions (Table 1, entries 7-8). It suggests that Co-Nx
species trigger the reaction. Without the K2CO3 or air as the oxidant, the conversions
are 31.0% and 43.5% (Table 1, entries 9-10). So the addition of K2CO3 and high purity
of oxygen are important for high conversion. Using Ni(OAc)2 and Fe(OAc)2 as the
metal precursors, the reaction also exhibited low conversions of 0.9% and 26.2% (Table
1, entries 11-12). A hot filtration test was conducted to confirm the heterogeneity
property of KCC-1@Co-N-C-800 (Fig. 4a). A conversion of 84.4% was obtained after
reaction for 6 h. The catalyst was subsequently seperated from the reaction solution by
the 0.22 μm microporous membrane filter and the filtrate was transferred into another
two-neck flask. Then, the reaction system was purged with O2 and heated to 80 ℃ for
another 10 h and 14 h (Table 1, entry 13). The reaction did not proceed. This result
suggested that the reaction is heterogeneous. After confirming the optimized reaction
conditions, the ODH reactions of other saturated N-heterocycles are tested to confirm
the extensive applicability of KCC-1@Co-N-C-800. 1,2,3,4-tetrahydroisoquinoline[35]
and 1-methyl-1,2,3,4-tetrahydroisoquinoline[36] were converted into the corresponding
products with high conversions (Table 2, entries 2-3). Moreover, the indoline[35] and 2-
methylindoline[37] were also dehydrogenated with high conversions (Table 2, entries 4-
5). In the recycling tests, the catalyst maintained excellent activity after six cycles,
suggesting that the catalyst exhibits high chemical stability (Fig. 4b).
Table 1
The optimization of reaction conditions for ODH reaction of 1,2,3,4-
tetrahydroquinoline. a
Pyrolysis T
(℃)
800
800
800
900
700
600
800
-
Conversion
(%) b
95.0
TOF
(h-1) c
Entry
Precursor
Ligand
1
2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
-
1,10-phen
bipy
1.96
4.6
-
3
NDC
1.6
-
4
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
1,10-phen
38.3
0.76
5
58.9
1.24
6
35.9
0.84
7
0.1
-
8
Co(OAc)2
Co(OAc)2
Co(OAc)2
Ni(OAc)2
Fe(OAc)2
Co(OAc)2
4.4
-
9d
10e
11
12
13f
800
800
800
800
800
31.0
0.64
43.5
0.90
0.9
-
26.2
0.54
84.4/83.8/84.4
1.74/1.73/1.74
a Reaction conditions: 0.5 mmol 1,2,3,4-tetrahydroquinoline, 0.5 mmol K2CO3, 20 mg
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10.1002/cctc.202000826
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catalyst, 5 mL CH3OH, 12 h, O2 balloon, 80 ℃. b The conversion was determined by
mole of converted substrate
c
d
e
GC-MS. Turnover frequency (TOF) =
. Without K2CO3.
mole of Co · t (h)
Under air atmosphere. f Hot filtration test.
Table 2
The ODH reaction of various saturated N-heterocycles on KCC-1@Co-N-C-800
catalyst.
Entry
1
Substrate
Product
Conversion (%) b
95.0
TOF (h-1) c
1.96
2
3
99.9
97.2
2.06
2.00
4
5
99.3
98.8
2.05
2.04
a
Reaction conditions: 0.5 mmol substrate, 0.5 mmol K2CO3, 20 mg catalyst, 5 mL
b
c
CH3OH, 12 h, O2 balloon, 80 ℃. The conversion was determined by GC-MS.
mole of converted substrate
Turnover frequency (TOF) =
.
mole of Co · t (h)
In addition, the CTH of quinoline was carried out using HCOOH as the hydrogen source,
demonstrating the bifunctionality of the KCC-1@Co-N-C-800 catalyst. It is noted that
a majority of the CTH reactions require the participation of bases, which affects the
practicality of reactions. And our reaction had a conversion of 99.9% in the absence of
Et3N (Table 3, entry 1). The following reactions were carried out without the addition
of base. First, with the decrease of the reaction temperature, the conversions
dramatically decreased (Table 3, entries 2-3). The reaction exhibited low conversions
and selectivities using H2O, DMF and EtOH as solvents (Table 3, entries 4-6). With the
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10.1002/cctc.202000826
ChemCatChem
decrease in the amount of HCOOH, the quinoline was not well transformed (Table 3,
entries 7-9). The reaction did not proceed in the absence of HCOOH or the catalyst
(Table 3, entries 10-11). A conversion of 9.2% was obtained using the non-pyrolytic
precursor, indicating that the homogeneous Co(OAc)2-1,10-phen complex does not
exhibit catalytic activity (Table 3, entry 12). Moreover, the reaction was hardly
triggered using HCOONH4 and i-PrOH as the hydrogen donors (Table 3, entries 13-14).
The KSCN poisoning tests were performed to further confirm the active sites, because
a highly stable metal-ligand complex can be formed by coordination between SCN− and
Co ions.[38] Firstly, the KCC-1@Co-N-C-800 catalyst was treated with 2 M HCl at 50 ℃
for 5 h and 0.5 M H2SO4 at 25 ℃ for 24 h, respectively. After the acid treatment to
remove the Co nanoparticles in the catalyst (Table 3, entries 15-16), the catalysts
showed high conversions of 96.1% and 92.8%. It suggests that the Co nanoparticles do
not act as active sites to catalyze the reaction. However, only 3.3% of substrate was
converted when KSCN was introduced into the reaction system (Table 3, entry 17). It
indicates that SCN− could coordinate with Co ions from the Co-Nx species, leading to
the decreased activity of the catalyst. Based on these results, we conclude that Co-Nx
species serve as the main active sites in reaction system. The KCC-1@Co-N-C-800
catalyst was also applied to the CTH of other unsaturated N-heterocycles. Isoquinoline[6]
was converted into the corresponding product with a conversion of 71.8%, while the
reaction time was long (Table 4, entry 2). In the presence of electron-donating groups[6,
8b, 39] (such as -CH3, -OCH3, and -OH) in the quinoline derivatives, the substrates were
converted into products with excellent conversions and selectivities (Table 4, entries 3-
6). A conversion of 98.2% for phthalazine[36] was obtained (Table 4, entry 7).
Furthermore, unsaturated N-heterocycles with electron-withdrawing groups[36] (such as
-Br and -NO2) were also applicable for CTH, conversions of 76.8% for 4-
bromoisoquinoline and 97.9% for 2-methyl-6-nitroquinoline were observed (Table 4,
entries 8-9). The KCC-1@Co-N-C-800 catalyst was also reused for six cycles without
any obvious decrease in the activity (Fig. 4b). Compared with the Co content (5.815%)
from the fresh catalyst, the Co content of the recycled catalyst was 5.265%. A slight
decrease in the Co content probably resulted from the acid corrosion of negligible Co
NPs in the presence of HCOOH. The XRD diffraction peak of the recycled catalyst was
the same as that of the fresh catalyst (Fig. S7) and no Co0 nanoparticles were observed
in the recycled catalyst (Fig. S8). The high activity and recyclability of the catalyst
derive from the two reasons: the unique structure of Co(OAc)2-1,10-phen complex
could protect Co precursor from aggregation at a high pyrolysis temperature and thus
form the highly dispersed Co-Nx active sites. Furthermore, the abundant channels, high
surface area and mesoporosity of the KCC-1 favor the high exposure of the Co-Nx
active sites to the reaction system and facilitate the mass transfer.
Table 3
The optimization of reaction conditions for CTH reaction of quinoline with formic acid.
a
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10.1002/cctc.202000826
ChemCatChem
Conversion
(%) b
1a
Selectivity
HCOOH
(eq.)
T
TOF
(h-1) c
Entry
Catalyst
Solvent
(%) b
(℃)
2a
1
2
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
None
10
10
Toluene
Toluene
Toluene
H2O
130
110
90
99.9
17.3
6.7
99.8
>99.9
>99.9
64.3
47.7
45.7
99.8
99.5
94.4
95.3
>99.9
>99.9
23.0
NR
8.24
1.42
0.55
1.91
0.63
0.21
8.03
4.91
3.72
0.03
-
3
10
4
10
130
130
130
130
130
130
130
130
130
130
130
130
130
130
36.1
16.0
5.6
5
10
DMF
6
10
EtOH
7
8
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
97.7
59.9
47.9
0.4
8
6
9
4
10
11
12d
13e
14f
15g
16h
none
10
2.8
KCC-1@Complex
10
9.2
-
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
KCC-1@Co-N-C-800
none
none
10
6.1
0.12
-
NR
96.1
92.8
3.3
>99.9
>99.9
>99.9
7.91
7.64
0.27
10
17i
10
a
b
Reaction conditions: 0.5 mmol quinoline, 10 mg catalyst, 2 mL solvent, 6 h. The
conversion and selectivity were determined by GC-MS. c Turnover frequency (TOF) =
mole of converted substrate. d No pyrolysis. e-f 10 eq. HCOONH4 and 5 mL i-PrOH as the
mole of Co · t (h)
hydrogen donors. g-h Catalysts washed with 2 M HCl at 50 ℃ for 5 h and 0.5 M H2SO4
at 25 ℃ for 24 h. i With KSCN (16 equiv. relative to Co).
Table 4
The CTH reaction of various unsaturated N-heterocycles over KCC-1@Co-N-C-800. a
Entry
Substrate
Product
t (h)
Conv. (%) b Sel. (%) b TOF (h-1) c
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10.1002/cctc.202000826
ChemCatChem
1
6
94.4
>99.9
8.24
2
3
12
7
71.8
98.2
>99.9
97.7
2.96
6.78
4
5
7
7
90.9
99.0
99.0
6.36
6.98
>99.9
6
7
8
10
6
93.9
98.2
76.8
>99.9
>99.9
>99.9
4.63
8.08
3.16
12
9
6
97.9
>99.9
8.06
a
Reaction conditions: 0.5 mmol quinoline, 10 mg catalyst, 5 mmol HCOOH, 2 mL
b
c
Toluene, 130 ℃. The conversion and selectivity were determined by GC-MS.
mole of converted substrate
Turnover frequency (TOF) =
.
mole of Co · t (h)
The best KCC-1@Co-N-C-800 catalyst has a high TOF value of 8.24 h-1, which is 3-
24 times higher than the values reported in the transition-metal-based catalysts towards
the base/base-free catalytic transfer hydrogenation or hydrogenation for quinoline
(Table S2), such as homogeneous Co(BF4)2·6H2O,[40] Mn(CO)5Br,[41] and
[42]
[CoII(L1)(CH3CN)](BF4)2
catalysts and heterogeneous Co@NCNTs-800,[43]
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10.1002/cctc.202000826
ChemCatChem
Co/Melamine-2@C-700,[8b] ISAS-Co/OPNC,[44] Co@OMNC-700,[45] CoOx@CN,[46]
Co@NGS-800[38a] and Co0/ZnCl2 catalysts.[6] A scale-up experiment was performed to
ascertain the practicality of the KCC-1@Co-N-C-800 catalyst. The amounts of
substrate and HCOOH were increased to ten times to be 5 mmol and 50 mmol,
respectively. Then 2 mL of toluene and 10 mg of KCC-1@Co-N-C-800 catalyst were
added into the sealed tube. The reaction system was completed at 130 ℃ for 7 h and
afforded a 98% yield (Fig. S9a). Besides, a 20% NaOH aqueous solution and EtOH
were added into the reaction system, and the reaction was performed at 100 ℃ for 2 h.
A 93.2% of N-formyl-1,2,3,4-tetrahydroquinoline was hydrolyzed into the 1,2,3,4-
tetrahydroquinoline (Fig. S9b).
ODH
CTH
(a)
(b)
100
80
60
40
20
0
100
80
60
40
20
0
Dehygrogenation
Hot Filtration
1
2
3
4
5
6
0
4
12
20
React8ion Time 1(6h)
Cycles
Fig. 4. (a) Hot filtration test of KCC-1@Co-N-C-800. (b) Recycling tests in the ODH
reaction of 1,2,3,4-tetrahydroquinoline and the CTH reaction of quinoline.
Conclusion
In conclusion, a Co-Nx species catalyst without metallic aggregation was prepared via
the cobalt-phenanthroline complex adsorbing on the KCC-1 support followed by the
pyrolysis. The large amount of N atoms in the phenanthroline with strong coordination
ability towards Co2+ and the porosity property of KCC-1 ensured synergistically the
favorable formation and high stability of Co-Nx species, which effectively isolated the
Co atoms and avoided aggregating into Co nanoparticles. Furthermore, benefitting from
the mesoporous structure and high surface area of the 3D silica nanosphere-based
carbon layers, the mass transfer is preferable and the Co-Nx species active sites are
highly exposed to the reaction system. So the Co-Nx species catalyst showed excellent
catalytic activities for both ODH and CTH reactions. The turnover frequency (TOF) of
KCC-1@Co-N-C-800 could reach 3-24 times higher than other reported transition-
metal-based catalysts for the hydrogenation reactions. Most metal center controlled by
organic ligands are applied to the homogeneous catalysis, while less are reported in the
heterogeneous catalysis. So this work provided new opportunities to design
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10.1002/cctc.202000826
ChemCatChem
heterogeneous and high-performance M-N-C catalysts using N-rich precursors.
Supporting Information
Supporting Information is available from the publication website.
Conflicts of interest
The authors declare no conflicts of interest
Acknowledgement
This work was supported by the National Natural Science Foundation of China (China.
21872066)
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Table of Contents Entry
A highly dispersed Co catalyst stabilized on the mesoporous N-doped carbon layers
was prepared by adsorption and pyrolysis of cobalt complex on dendritic fibrous silica
nanospheres. The Co-Nx species catalyst showed excellent catalytic activities for both
oxidative dehydrogenation and catalytic transfer hydrogenation reactions. The turnover
frequency (TOF) of KCC-1@Co-N-C-800 could reach 3-24 times higher than other
reported transition-metal-based catalysts for the hydrogenation reactions.
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