Nitrogen-coordinated cobalt nanocrystals for oxidative dehydrogenation and hydrogenation of N-heterocycles

Article abstract of DOI:10.1039/c9sc00475k

To endow non-noble metals with the high catalytic activity that is typically exhibited by noble metals is the central yet challenging aim for substituting noble metals. In this regard, by exploiting the coordination effect of nitrogen, we prepared cobalt nanocrystals stabilized by nitrogen-doped graphitized carbon (Co NCs/N-C). The obtained Co NC/N-C catalyst showed extraordinary performances toward both oxidative dehydrogenation of N-heterocycles and its reverse hydrogenation process under extremely mild conditions. A nearly quantitative conversion could be achieved for oxidative dehydrogenation even at room temperature (25 °C), for which the coordination effect of nitrogen is responsible: the interaction of Co-N induces a partial positive charge on the Co surface, thereby promoting the reaction. In contrast, cobalt nanocrystals supported by pristine carbon (Co NCs/C) proved to be inactive for oxidative dehydrogenation, owing to the lack of nitrogen. Moreover, in Co NCs/N-C, the N-doped graphitized carbon formed a protective layer for Co NCs, which preserved the active valence of Co species and prevented the catalyst from leaching. It was found that the catalyst still retained its excellent catalytic activity after five regeneration cycles; in comparison, its cobaltous oxide counterpart (CoO_x/N-C) was barely active. As for the mechanism, electron paramagnetic resonance (EPR) analysis revealed the formation of superoxide anion radicals during the dehydrogenation process. Interestingly, the pressure of feed hydrogen had little effect on the hydrogenation process. Our Co NC/N-C catalyst is capable of activating molecular oxygen and hydrogen as effectively as noble metals; the coordination effect of nitrogen and the protection by the carbon layer in combination confer tremendous potential on the Co NCs/N-C for substituting noble-metal-based catalysts and soluble catalysts for homogeneous reactions.

Full text of DOI:10.1039/c9sc00475k

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Nitrogen-Coordinated Cobalt Nanocrystals for Oxidative
Dehydrogenation and Hydrogenation of N-Heterocycles
Yue Wu,a‡ Zheng Chen,a‡ Weng-Chon Cheong,a Chao Zhang,a Lirong Zheng,b Wensheng Yan,c Rong
Received 00th January 20xx,
Accepted 00th January 20xx
Yu,d Chen Chen,a* Yadong Lia
DOI: 10.1039/x0xx00000x
To endow non-noble metals with the high catalytic activity that is typically featured by noble metals is the central yet
challenging pursuit for substituting noble metals. In this regard, by exploiting the coordination effect of nitrogen, we pre-
pared cobalt nanocrystals stabilized by nitrogen-doped graphitized carbon (Co NCs/N-C). The obtained Co NCs/N-C catalyst
showed extraordinary performances toward both oxidative dehydrogenation of N-heterocycles and its reverse
hydrogenation process under extremely mild conditions. A nearly quantitative conversion could be achieved for oxidative
dehydrogenation even at room temperature (25 oC), for which the coordination effect of nitrogen is responsible: the
interaction of Co-N induces a partial positive charge on the Co surface, thereby promoting the reaction. In contrast, cobalt
nanocrystals supported by pristine carbon (Co NCs/C) proved to be inactive for oxidative dehydrogenation, owing to the lack
of nitrogen. Moreover, in Co NCs/N-C, the N-doped graphitized carbon formed a protective layer for Co NCs, which preserved
the active valence of Co species and prevented the catalyst from leaching. It was found that the catalyst still retained its
excellent catalytic activity after five regeneration cycles; in comparison, cobaltous oxide counterpart (CoOx/N-C) was barely
active. As for the mechanism, electron paramagnetic resonance (EPR) analysis revealed the formation of superoxide radical
anions during the dehydrogenation process. Interestingly, the pressure of feeding hydrogen showed little effect on the
hydrogenation process. Our Co NCs/N-C catalyst is capable of activating molecular oxygen and hydrogen as effectively as
noble metals; the coordination effect of nitrogen and the protection of carbon layer in combination confer on the Co NCs/N-
C tremendous potential for substituting noble-metal-based catalysts and soluble catalysts for homogeneous reactions.
www.rsc.org/
metals through interaction with exotic atoms are the two promising
strategies. Starting from the first report of metal phthalocyanine
complexes tested as cathode catalysts in fuel cells to replace
Introduction
With the rapid progress in the field of catalysis, noble metal catalysts
have established dominance in both homogeneous and
heterogeneous catalytic systems.1-3 However, the limited reserves
and high cost of noble metals severely hinder their large-scale
applications and industrialization. To address this issue, extensive
attention has been paid to exploiting non-noble metal catalysts4 as
substitutes, by virtue of their relatively high abundance and low cost.
The key concerns on substituting noble metals with non-noble
metals are to achieve the superior catalytic activity (generally
featured by noble metals) under mild conditions, and in the
meantime to keep the non-noble metals intact during the reaction
process, since non-noble metals are commonly susceptible to
oxidation and leaching, particularly under harsh reaction conditions.
To protect the relatively unstable non-noble metals, structural
protection by inert layers and the charge balance of non-noble
expensive Pt,5 numerous transition metal catalysts have emerged in
attempts to achieve high catalytic activity like noble metal ones.
From the perspective of alternative energy technologies and
hydrogen storage, catalytic oxidative dehydrogenation and
hydrogenation have received significant attention.6 N-heterocycles,
a class of pharmacophores broadly present in numerous FDA-
approved pharmaceuticals and other bioactive compounds,7 are an
important research object in this field. In particular, the catalytic
oxidative dehydrogenation of saturated N-heterocycles provides one
of the most atom-efficient routes8 towards yielding high-value
products such as indoles, quinolones, and other N-heterocycle
aromatic compounds.
To optimize the catalytic system for oxidative dehydrogenation of N-
heterocycles and its reverse process, extensive researches have been
carried out in homogeneous and heterogeneous catalysis. In the
most typical homogeneous catalytic system, Cp*Ir complexes9 gave
almost quantitative yields for both oxidative dehydrogenation and
hydrogenation. Inspired by this work, subsequent reports still
focused on noble metals, such as Ir,6, 10-11 Ru,12-14 Pt,15 Pd,15 Rh.15 In
spite of the development of noble metal catalytic systems, great
efforts are devoted to exploiting efficient catalysts from sustainable
a.
Department of Chemistry, Tsinghua University Beijing 100084 (China). E-mail:
cchen@mail.tsinghua.edu.cn
b. Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese
Academy of Sciences, Beijing 100049 (China)
c. National Synchrotron Radiation Laboratory (NSRL), University of Science and
Technology of China, Hefei, Anhui 230029, China
d. National Center for Electron Microscopy in Beijing, School of Materials Science
and Engineering, Tsinghua University, 100084 Beijing, China.
‡ These authors contributed equally.
metals so as to circumvent the high cost, high toxicity and low
16
abundance of noble metals. Iron-based7,
and cobalt-based17
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
catalysts are two typical cases, performing at relatively high
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temperatures (100 oC or even higher), or under pressurized dehydrogenation of N-heterocycles and its reverseVipewroAcrteicslse.OnTlhinee
condition, or for long reaction time. However, some of them still gave
barely satisfactory yields. Therefore, up to date it seems that non-
DOI: 10.1039/C9SC00475K
catalyst exhibits excellent catalytic activity for both oxidative
dehydrogenation and hydrogenation. Remarkably, even at room
noble metal catalysts typically require relatively harsh reaction
temperature without extra O2 or additional pressure, the Co NCs/N-
conditions to deliver acceptable activities. In addition, homogeneous
C can effectively facilitate the oxidative dehydrogenation with a
catalysis has an innate limitation in recycling.18 Whereas for
nearly complete conversion. The Co NCs/N-C catalyst is readily
heterogeneous systems, there still exist plenty of challenges to
recyclable by applying magnetic field and can almost retain its
excellent catalytic activity after 5 recycling rounds. It is the first
catalytic system for the efficient oxidative dehydrogenation-
hydrogenation reaction that proceeds under ambient conditions,
which is a fundamental improvement for traditional catalytic
methods for organic transformations of N-hetercycles.
develop inexpensive, earth-abundant metal catalysts instead of the
noble metal ones, and meanwhile, to retain superb performances
with high activity (under reaction conditions as mild as possible) and
robust stability for recycling.
The “heteroatom doping” method resorts to replacing certain carbon
lattice atoms with heteroatoms.19 Nitrogen doping is the most
common one, and has proved to be a powerful method to modify
carbon materials for various applications.20 Due to the difference in
electronegativity between carbon atoms and nitrogen, the nitrogen
doping process generally can redistribute the charge density and spin
density of carbon atoms in the lattice, thereby effectively regulating
the work function and strengthening the adsorption of reactants at
specific sites.19, 21 In addition, the local geometry in the proximity of
dopant nitrogen atoms would be altered due to the difference in
atomic sizes between C and the dopant N1, as confirmed by density
functional theory (DFT) calculations.22 N-doped carbon
nanomaterials are now one of the most important catalytic materials
for a wide range of chemical and electrochemical catalytic reactions,
such as selective oxidation of aromatic alkanes,23 reduction of
nitrobenzene,24 and hydration of different alkynes,25 oxygen
reduction reaction (ORR),26-27 hydrogen evolution reaction (HER),28-
29 and overall water splitting.30
Results and discussion
Figure 2. TEM image of ZIF-67 (a) and Co NCs/N-C (b); (c) HAADF-
STEM image, corresponding element maps showing the distribution
of C (red), N (yellow) and Co (green); d, e, f) TEM image and its
magnified images of Co NCs/N-C after acid treatment; g) Magnified
HAADF-STEM image of the Co NCs/N-C. h) Atomic resolution STEM
image of the Co NC encapsulated in nitrogen-doped carbon; i)
Magnified HAADF-STEM image of nitrogen-doped graphitized carbon
substrate.
The pyrolysis of metal-organic frameworks (MOFs) at high
temperature is an effective and fashionable method for the synthesis
of metal/nitrogen-doped carbon composites. Uniform Co-based
metal-organic frameworks ZIF-67 nanopolyhedrons can be
synthesized from cobalt(II) nitrate (Co(NO3)2) and 2-methylimidazole
linkers,31 and particles with average size of 200-300 nm could be
obtained based on reported procedures.32 Then, ZIF-67 was
Figure 1. The scheme of Co NCs/N-C obtained from the pyrolysis of
ZIF-67.
carbonized at high temperature above 700oC for
temperature-programmed furnace under flow of argon.
3 h in a
a
Herein, cobalt nanocrystals stabilized by nitrogen-doped
graphitized carbon (Co NCs/N-C) are developed for both oxidative
Subsequent treatment with HCl under ultrasonic selectively removed
the unprotected cobalt particles generated during the pyrolysis,
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affording the resultant catalyst denoted as Co NCs (Cobalt 900oC, respectively), which was indexed to graphitic carbon. This
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DOI: 10.1039/C9SC00475K
nanocrystals)/N-C (Nitrogen-doped carbon). The entire synthetic result is also consistent to the HAADF-STEM image (Figure 2i). It is
strategy of Co NCs/N-C is shown in Figure S1. The transmission therefore reasonable to infer that acid washing removed not only Co
electron microscopy (TEM) images showed that ZIF-67 nanocrystals nanocrystals on the surface but also some amorphous carbon,
were all in uniform rhombododecahedral shape (Figure 2a), and the yielding sharper graphitic peaks in the XRD pattern. N-doped
corresponding pyrolysis product well inherited the overall graphene is of vital importance for the catalytic performance of the
polyhedron-like morphology (Figure 2b), with many Co nanocrystals whole material, because the N-doped graphene has been
formed on the carboneous support. Elemental mapping results demonstrated to produce defects in graphene support, which result
(Figure 2c) indicated that the Co particles were evenly dispersed in the formation of active sites for O2 adsorption.16, 33-35 Meanwhile,
in/on the support. Subsequent HCl treatment rendered the support N-doped graphene also has the ability to reductively adsorb O236 and
highly porous, with the metallic Co nanocrystals distributed within, has been employed in the catalytic reduction of nitroarenes37 and for
as confirmed by the TEM and its magnified images of a single reductive aminations.38
rhombododecahedron (Figure 2d-2f). It is worth mentioning that the
X-ray photoelectron spectroscopy (XPS) further identified the
Co valence state of Co NCs/N-C products carbonized at different
temperatures (Figure S3); survey spectrum revealed the presence of
Co, C, N, and O. The C 1s peak can be deconvolved into two peaks
located at 284.1 eV (for typical C−C bond of sp3 carbon) and 284.9 eV
(with respect to C−N species), and the one centered at 285.5 eV is
the typical graphitic carbon peak. Deconvolution of the N 1s peak
indicates that there exist three kinds of nitrogen including pyridinic
nitrogen (398.8 eV), pyrrolic nitrogen (400.0 eV), and graphitic
nitrogen (401.2 eV) in Co NCs/N-C.39 As for the Co 2p spectrum which
can be fitted into two major peaks (Co2p3/2 and Co2p1/2), the peaks
around 778.3 eV and 793.3 eV are corresponding to Co0, and the two
peaks at 780.6 eV and 803.5 eV can be ascribed to Co3+, whereas the
peaks around 780.1 eV and 796.4 eV are assigned to Co2+. It is the
two oxidized states not shown in XRD that are the manifestation of
the nitrogen-coordination effect. The coordination interaction
between nitrogen atom and Co leads to the electron drifting from Co
toward nitrogen atom, resulting in slightly positive charge on the
metallic surface. Thus, cobalt existing in multiple valences was
revealed in XPS data, but only Co0 was found in XRD.
Co species are encapsulated in the porous carbon framework (Figure
2g), and the interplanar spacing of Co nanocrystals wrapped in
porous carbon is 0.214 nm, corresponding to the Co (111) lattice
fringes. Under Aberration-Corrected HRTEM, the surface of
encapsulated Co NCs could be clearly observed with atomic-level
resolution (Figure 2h). The porous layers having lattice fringes with
interplanar distance of 0.340 nm are identified as graphitic carbon
(Figure 2i).
We used the synchrotron radiation technique to further
investigate the local atomic and electronic structures of Co NCs/N-C.
From the NEXAFS spectra resulting from 2p→3d dipole transitions,
the Co L3 and L2 absorption lines are located around 773 eV and 787
eV, respectively. As shown in Figure 3a, the Co L-edge XANES spectra
of Co NCs/N-C and Co NCs/C exhibit a peak at the L3-edge, which is
characteristic for 3d-electrons, and no well-defined multiple
structures are observed; the spectral profiles bear a greater similarity
to that of Co rather than Co-based oxides,40-41 and the decrease in
intensity after nitrogen hybridization implies the higher Co 3d
electron occupation in Co NCs/B-C, mainly due to the stronger
electronic interactions introduced by nitrogen to the Co NCs.42 The
X-ray absorption near-edge structure (XANES) curves of Co K-edge
(Figure 3b) show that the intensity of the peak A is stronger than that
of Co foil and Co NCs/C, indicating the valence state of Co in Co
NCs/N-C is slightly higher than that of Co NCs/C and Co foil.43 There
is a strong peak located at 2.0 to 2.4 Å, which is related to Co-Co
single scattering shown in extended X-ray absorption fine structure
(EXAFS) spectra (Figure 3c). From the locally magnified graph (inset
in Figure 3c), it was clear that the main peak of Co NCs/N-C is of the
Figure 3. a) NEXAFS spectra of Co NCs/N-C b) XANES spectra (the blue
area highlights the near-edge absorption energy) and c) Fourier
transform (FT) of the ZIF-67, Co NCs/N-C, Catalysts treated in 500 oC
H2, 300 C Air, and Co-BTC; d) The N K-edge and e) C K-edge of Co
NCs/N-C; f) the diagram of Co NCs encapsulated in N-doped carbon.
o
In the X-ray diffraction (XRD) patterns (Figure S2), the diffraction
peaks at 44.2o, 51.5o and 75.8o were respectively ascribed to the
(111), (200) and (220) reflections of face-centered cubic cobalt. It is
noteworthy that after acid treatment the peak at 26.1o became quite
o
o
prominent for all three samples (carbonized at 700 C, 800 C and
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lowest intensity, suggesting that the Co-Co bond shrinks with the conversions (21.4% and 36.1%, respectively) (FigureV4iebw).ArWticlee Othnleinne
interaction with nitrogen.44 In N K-edge spectrum (Figure 3d), the assessed the recycling stability of Co NCs/N-C catalyst. As a magnetic
DOI: 10.1039/C9SC00475K
peak at 396.3 eV and the peak at 398.8 eV mainly originated from catalyst, Co NCs/N-C is of advantage for the ease of separation by
pyridinic π* and graphitic π* transitions, respectively. The peak C at applying magnetic field. The Co NCs/N-C could retain its catalytic
404.6 eV suggested the formation of C-N-C or C-N σ* bond. The C K- performance after five-cycle measurements (corresponding
edge spectrum displayed in Figure 3e shows three peaks: the peak A morphology shown as Figure S4c, and XRD pattern shown as Figure
at 283.9 eV derived from the C-C π* (ring) excitations, the peak B at S5), and no obvious decline in activity was found (Figure 4c). In
287.0 eV assigned to C-O-C or C-N-C and the peak C at 290.7 eV addition, there was no other by-products observed.
originating from the C-C σ* (ring) transitions. The peak A and C
Table 1. N-heterocycle compounds oxidative dehydrogenation with
Co NCs/N-C as catalyst.a
suggested that graphitization occurred during the pyrolysis process,
while peak B indicated that defect sites existed in the carbon
lattice.45 Therefore, the Co NCs/N-C sample obtained after acid
treatment show a higher degree of graphitization and more defects
in N-C substrate. Based on the above characterizations, it can be
inferred that the Co nanocrystals are stabilized by the coordination
with nitrogen and the coating with graphitized carbon, as illustrated
in Figure 3f. To assess the activity of Co NCs/N-C for the heterocycle
oxidative dehydrogenation, we tested the catalyst obtained at 800
oC (denoted as Co NCs/N-C-800 oC) with 1,2,3,4-tetrahydroquinolone
as model substrate. Different solvents were screened, and we found
that the prepared catalyst in general performed better in polar
Substrate
Product
Yield(%)b
99.9
1
2
N
H
N
H
N
N
N
99.3
N
H
o
solvents. When the model reaction was tested at 80 C in several
3
4
100.0
97.8
N
H
N
H
nonpolar solvents (Table S1) such as p-xylene, 1,4-dioxane, etc., the
highest yield obtained was 24.9% after 4 h using p-xylene as solvent.
By contrast, polar solvents (Table S2) performed better than all the
tested nonpolar solvents. With methanol as solvent, the reaction
achieved approximately complete conversion with 100% selectivity
at very low temperature (50 oC) after only 4 h (Figure 4b). Even more
surprisingly, a conversion of 97.2% could be achieved with longer
reaction time (24 h) under room temperature (Table S3).
Br
Br
N
H
N
MeO
MeO
5
6
7
97.5
N
N
H
Cl
Cl
74.3
N
(93.8)c
N
H
F3C
F3C
99.9
N
N
H
8
9
96.8
17.8
N
N
H
N
N
H
O
Figure 4. a) Reaction for oxidative dehydrogenation of 1,2,3,4-
tetrahydroquinoline; b) Comparisons of conversion and selectivity
for different catalysts towards oxidative dehydrogenation; c)
Recycling test of the Co NCs/N-C catalyst; d) The DMPO spin-trapping
ESR spectra for ·O2− in this catalytic system.
O
MeO
10
11
MeO
58.3
65.1
N
N
H
N
NH
Next, we compared catalysts carbonized at different pyrolysis
temperature with optimized reaction conditions. After 4 h, Co
NCs/N-C-700 oC and Co NCs/N-C-900 oC gave significantly lower
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O2N
95.5% after 12 h, indicating that Co NCs/N-C-HCl retaiVnieewdAartlimcleoOsntlianlel
O2N
DOI: 10.1039/C9SC00475K
the original catalytic activity, even with only one sixth of the initial Co
12
28.14
N
N
content. Therefore, it is the Co NCs encapsulated in N-doped carbon
framework that essentially drive the reaction during the oxidative
dehydrogenation of tetrahydroquinolone, because nitrogen-doped
carbon layer prevents the Co nanocrystals from oxidation and
deactivation with nitrogen coordination.
H
aReaction conditions: 0.5 mmol substrate, 50 mg catalyst, 3 mL
MeOH, air, 50 oC, 12 h; bThe yields are determined by GC-MS and GC
analysis; cReaction conditions: 0.5 mmol substrate, 50 mg catalyst, 3
mL MeOH, air, 50 C, 24 h; The molar ratio of Co: substrate in this
work is 7.6%.
o
d
To determine the relationship between the N-coordination
environment and the catalytic activity, we changed the ligand for
MOFs from 2-methylimidazole into 1,3,5-benzenetricarboxylic acid
(BTC) (Figure S4d). As a controlled experiment, we modelled on the
synthesis method of Co NCs/N-C, and the resulting catalyst was
denoted as Co NCs/C-BTC. As confirmed by XPS data (Figure S3), Co
NCs/C-BTC contained C, O, and Co, while N could barely be detected.
As for the activity test, the Co NCs/C-BTC with 60.0% Co content
(measured by ICP-OES) (Table S4) gave a marginal yield of 0.86%.
Therefore, the N-coordination environment not only protected the
Co NCs from oxidation and deactivation, but also induced electrons
to deviate from Co, leading to slightly positive surface charge on the
metallic surface, as confirmed by XPS data. This coordination effect
further offered an excitation mechanism for the catalytic centers to
activate O2, by which the encapsulated Co NCs delivered a
remarkable catalytic efficiency.
Under the optimized conditions, we tested the catalyst for other
substrates. As shown in Table 1, the catalyst was also efficient for
oxidative dehydrogenation of similar substrates under 50 oC in
methanol. As expected, substrates with an electron-donating group
were active, such as 8-methyl-1,2,3,4-tetrahydroquinoline (entry 1).
In addition, 1,2,3,4-tetrahydroquinoxaline (entry 2) and indoline
(entry 3) were also quantitatively dehydrogenated under model
reaction conditions. Moreover, in spite of the passivation effect of
electron-withdrawing group on aromatic rings, substrates with an
electron-withdrawing group at the 6 position were well tolerated. 6-
bromo-1,2,3,4-tetrahydroquinoline (entry 4) and 6-methoxy-1,2,3,4-
tetrahydroquinoline (entry 5) both gave yields above 97%, while 6-
chloro-quinoline (entry 6) was produced with a yield of 93.8% under
conditions of prolonged reaction time. Surprisingly, 6-
trifluoromethyl-quinoline (entry 7), the substrate with a strong
electron-withdrawing group, was obtain in a yield up to 99.9%.
Finally, we test two benzylamines. Remarkably, N-benzylaniline
transformed into N-(phenylmethylene)-benzenamine with 96.8%
yield (entry 8). Even N-benzylbutan-1-amine obtained in a yield
around 20% (entry 9). 1,2,3,4-tetrahydroquinoline substituted with
ester, nitro, and 1,2,3,4-tetrahydroisoquinoline gave moderate
conversions towards corresponding oxidative dehydrogenation
products (entry 10-12).
With the XRD and XPS results, it was determined that Co species
in the catalyst was mostly at zero valence state, while local charge
existed on the interface between Co NCs and N ligand. For another
proof of the catalytic activity of Co0, we thermally treated the catalyst
(previously carbonized at 800oC) in air at 300 oC and in H2 at 500 oC,
respectively; the products are denoted as Co NCs/N-C-300Air (Figure
S4a and S5) and Co NCs/N-C-500H2 (Figure S4b and S5). After
oxidized in air, Co NCs/N-C-300Air only gave a sharply decreased
yield of 9.47% after 4 h (Figure S6). Valence state of catalytic species
could explain this significant deterioration in performance. After
thermal treatment in air, Co NCs were transformed into Co3O4, as
confirmed by XRD (Figure S5) (so it is also denoted as Co3O4/N-C in
Figure 4b). The activity of Co3+ and Co2+ was far below that of Co0. As
for Co NCs/N-C-500H2, a yield of 68.3% was achieved (Figure S6). The
catalysts gave almost identical XRD patterns (Figure S5) before and
after H2 reduction, and no Co3+ peak was found in the Co spectrum.
We inferred that H2 atmosphere at 500oC damaged the coordination
interaction between N and Co, which caused the decrease in activity.
To verify this inference, Co NCs/N-C-400H2 was prepared and then
tested for comparison. Treated at a lower temperature (400 oC), the
catalyst retained most of the catalytic activity with the yield reaching
93.0%, which confirmed the importance of N-coordination for
catalytic process.
In light of the afore-mentioned excellent catalytic performance
for oxidative dehydrogenation of quinolone, it was imperative to
conduct further discussion on the catalytic mechanism. Herein, we
investigate the mechanism from the following perspectives: 1) to
identify the catalytically active species at the nanoscopic level; 2) to
get insights on the chemical environment around the catalytically
active species; 3) to explore their active valence states; and finally, 4)
to infer the reaction pathway for the oxidative dehydrogenation
process.
As shown the TEM images above, there were two kinds of Co
NCs in the catalysts, some partially embedded on the surface and
some encapsulated in the porous N-doped carbon framework. After
treatment with HCl, the Co nanocrystals on surface were removed,
and the wrapped ones were acid-resistant and therefore remained
intact. The resulting catalyst (denoted as Co NCs/N-C-HCl) was with
a
cobalt content about 4.5%, as determined by ICP-OES
measurement (Inductively Coupled Plasma Optical Emission
Spectrometer), in contrast to the untreated catalyst Co NCs/N-C
(with 30.4% cobalt content) (Table S4). The activity test of 50 mg Co
NCs/N-C-HCl in the model reaction gave a reaction yield as high as
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DOI: 10.1039/C9SC00475K
Figure 6. a) Reaction for hydrogenation of quinoline and conversion
vs. temperature b), time c), and pressure d) for hydrogenation;
Figure 5. The mechanism for oxidative dehydrogenation and
hydrogenation on three kinds of Co NCs: Co NCs exposed on the
nitrogen-doped carbon substrate (left), Co NCs encapsulated with
nitrogen-doped carbon (middle), Co NCs encapsulated with carbon
(right).
After achieving efficient catalytic oxidative dehydrogenation of
N-heterocycles under relatively low temperatures, we became
interested in whether the reverse process, namely, hydrogenation of
quinolone to 1,2,3,4-tetrahydroquinoline, could be accomplished
using the same catalyst. To evaluate the catalytic activity for
hydrogenation, we chose external hydrogen as a hydrogen source,
and separately explored the effects of temperature, time, and
pressure. We first carried out the hydrogenation reaction in
tetrahydrofuran (THF) under 2 MPa pressure for 6 h (Figure 6b), and
found that with the increase of temperature, the conversion was
obviously elevated. Under ambient temperature (25oC), almost no
transformation was detected. However, when the reaction was
There has been speculation that this reaction progresses via a
radical-mediated pathway, which involves the superoxide radical
−
anion (·O2 ). Subsequently generated H2O2 is quickly consumed,
forming quinoline and water.16 Mechanistic studies on the role of
·O2− in oxidative stress processes are particularly difficult owing to its
low steady-state concentration and the lack of highly specific probes
that allow its unequivocal identification and quantification.46 Herein,
−
we detected ·O2 by means of electron paramagnetic resonance
o
carried out under 60 C, the conversion reached 80.0%, and almost
(EPR). Fortunately, after significantly elevating the concentration of
tetrahydroquinoline in the catalytic system, we captured the sextet
quantitative transformation was achieved under 120 oC. Kinetic
measurement showed that the reaction rate reached maximum from
0.5 h to 1 h, and conversion kept constant after 6 h (Figure 6c). In
addition, the catalytic efficiency is barely affected by the pressure of
H2 (Figure 6d), as confirmed by performance test under different
pressures in the range from 1 MPa to 5 MPa. Under 120 oC,
hydrogenation of quinoline could be effectively driven in H2
atmosphere. Based on the above results, it can be concluded that Co
NCs/N-C could serve as a bifunctional catalyst for both the oxidative
dehydrogenation of N-heterocycles and its reverse process.
−
signal (Figure 4d) that was characteristic for ·O2 . As mentioned
above, in N-doped graphene, nitrogen atom doping leads to defects,
resulting in the formation of the adsorption active sites for O2
adsorption and reduction. Therefore, we speculate that the catalytic
active species, namely, Co NCs encapsulated by N-doped carbon,
could adsorb O2 from the surrounding air, and donate electrons to
−
reduce O2 into ·O2 , which drives the oxidative dehydrogenation of
tetrahydroquinoline. Under N-coordination environment, the
electron “gas” hanging over the surface of Co NCs shifted toward
nitrogen atoms, facilitating the electron transfer for ·O2− production.
As a protic solvent, methanol could serve as an outstanding proton
Conclusions
−
−
donor and transform ·O2 into HO2 , the effective oxidizing species.
The stronger the protic solvent, the more significantly the oxidative
dehydrogenation was facilitated, which explains the experimental
facts that methanol performed best compared with aprotic solvents
and other protic solvents tested in this work. Based on the
experimental results and our discussion, a plausible reaction
mechanism (Figure S7) is proposed.
In conclusion, we developed a bifunctional catalyst, namely, Co
NCs/N-C, which is capable of catalyzing the oxidative
dehydrogenation and hydrogenation of N-heterocycles with high
efficiencies. Particularly for the oxidative dehydrogenation, the
catalyst requires only an extremely mild and simple catalytic system,
without extra O2, relatively high temperature or pressure. Prepared
under optimal conditions, the catalyst can promote oxidative
o
dehydrogenation with methanol as solvent at 50 C in exposure to
air, with a nearly complete conversion within 4 h. The activity shows
barely any loss after 5-round magnetic regeneration. Through the Co-
N coordination effect (nitrogen modulating the electronic structure
6 | J. Name., 2012, 00, 1-3
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ARTICLE
15 Deraedt, C.; Ye, R.; Ralston, W. T.; Toste, F. D.; Somorjai, G. A.,
of Co NCs) and the protection effect of carbon layer, we successfully
substituted noble metals with inexpensive and readily recyclable
transition metal, and significantly elevated the catalytic efficiency. As
a result, the oxidative dehydrogenation of N-heterocycles can now
be carried out under extremely mild conditions. Compared with
conventional catalytic systems, our catalyst offers a highly effective
alternative for N-heterocycle transformation, and stands as a new
model for converting homogeneous catalytic systems into
heterogeneous ones.
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DOI: 10.1039/C9SC00475K
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There are no conflicts to declare.
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Acknowledgements
This work was supported by the Beijing Natural Science
Foundation (JQ18007), National Key R&D Program of China
(2016YFA0202801, 2017YFA0700101), National Natural Science
Foundation of China (21872076, 21573119, 21590792,
21890383), and China Postdoctoral Science Foundation
(2018M631444). We thank Institute of High Energy Physics in
Beijing and National Synchrotron Radiation Laboratory in Hefei
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