Cobalt Nanoparticles Apically Encapsulated by Nitrogen-doped Carbon Nanotubes for Oxidative Dehydrogenation and Transfer Hydrogenation of N-Heterocycles

Article abstract of DOI:10.1002/cctc.201901304

It is important to develop a highly active and stable transition-metal catalyst with dual-functional properties in the reversible transformations between various saturated and unsaturated N-heterocycles. Herein, we prepared the cobalt nanoparticles (Co NPs) apically encapsulated by the N-doped carbon nanotubes catalyst (Co@NCNTs) via a multiple pyrolysis of low-cost dicyandiamide and cobalt (II) acetylacetonate. The catalyst shows excellent activity and recyclability towards the oxidative dehydrogenation (ODH) and the catalytic transfer hydrogenation (CTH) for various N-heterocycles. The structure of outer N-doped carbon nanotubes (NCNTs) can protect Co NPs from aggregation and leaching. Moreover, the encapsulated Co NPs and the NCNTs may generate a synergistic effect. Both of them facilitate the high performance. The poisoning tests with KSCN were to clarify the different active sites for ODH and CTH reactions: the Co NPs could modify the NCNTs through electrons redistribution, therefore the NCNTs could directly activate O₂ in ODH. The encapsulated Co NPs is enhanced by the doped N atoms which is good for the H₂ activation in CTH. What's more, the mechanisms of ODH and CTH reactions were also proposed. This work provides a facile and low-cost method to design catalysts, which are dual-functional, highly active and stable, for industrial applications.

Full text of DOI:10.1002/cctc.201901304

Accepted Article
Title: Cobalt Nanoparticles Apically Encapsulated by Nitrogen-doped
Carbon Nanotubes for Oxidative Dehydrogenation and Transfer
Hydrogenation of N-heterocycles
Authors: Dan Xu, Hong Zhao, Zhengping Dong, and Jiantai Ma
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To be cited as: ChemCatChem 10.1002/cctc.201901304
Link to VoR: http://dx.doi.org/10.1002/cctc.201901304
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ChemCatChem
10.1002/cctc.201901304
Cobalt Nanoparticles Apically Encapsulated by Nitrogen-doped
Carbon Nanotubes for Oxidative Dehydrogenation and Transfer
Hydrogenation of N-heterocycles
Dr. Dan Xu, Hong Zhao, Prof. Zhengping Dong* and Prof. Jiantai Ma*
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering
Laboratory for Chemical Catalysis, Laboratory of Special Function Materials and
Structure Design of the Ministry of Education, Lanzhou University, No.222, South
tianshui road, Lanzhou (PR China)
E-mail: dongzhp@lzu.edu.cn (Prof. Zhengping Dong), majiantai@lzu.edu.cn (Prof.
Jiantai Ma), Tel: 86 931 8912577.
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Abstract
It is important to develop a highly active and stable transition-metal catalyst with
dual-functional properties in the reversible transformations between various saturated
and unsaturated N-heterocycles. Herein, we prepared the cobalt nanoparticles (Co
NPs) apically encapsulated by the N-doped carbon nanotubes catalyst (Co@NCNTs)
via a multiple pyrolysis of low-cost dicyandiamide and cobalt (II) acetylacetonate.
The catalyst shows excellent activity and recyclability towards the oxidative
dehydrogenation (ODH) and the catalytic transfer hydrogenation (CTH) for various
N-heterocycles. The structure of outer N-doped carbon nanotubes (NCNTs) can
protect Co NPs from aggregation and leaching. Moreover, the encapsulated Co NPs
and the NCNTs may generate a synergistic effect. Both of them facilitate the high
performance. The poisoning tests with KSCN were to clarify the different active sites
for ODH and CTH reactions: the Co NPs could modify the NCNTs through electrons
redistribution, therefore the NCNTs could directly activate O
2
in ODH. The
encapsulated Co NPs is enhanced by the doped N atoms which is good for the H
2
activation in CTH. What’s more, the mechanisms of ODH and CTH reactions were
also proposed. This work provides a facile and low-cost method to design catalysts,
which are dual-functional, highly active and stable, for industrial applications.
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10.1002/cctc.201901304
Introduction
The reactions of catalytic oxidative dehydrogenation (ODH) and transfer
hydrogenation (CTH) of N-heterocycles are significant transformations in synthetic
chemistry, given that saturated and unsaturated N-heterocycles are crucial building
blocks in pharmaceutical and chemical fields.[1] In the traditional dehydrogenation
methods to obtain the unsaturated N-heterocycles, excessive strong oxidants (e.g.
TBHP, sulfur, metal oxides) are desired, which leads to the production of toxic
substances in large quantities.[2] Therefore the participation of molecular oxygen in
the ODH reaction is highly necessary. It is an efficient way to lower the
thermodynamic energy barrier with the help of activated oxygen. At the same time,
the water was generated from the process as the only by-product.[3] Due to the high
atomic efficiency, the hydrogenation of unsaturated N-heterocycles has possibility to
produce saturated N-heterocycles derivatives.[4] Traditional hydrogenation routes
generally require a high pressure of hydrogen, probably forming the overreduction
products.[5] So CTH may be a fascinating alternative to solve this problem. Formic
acid (HCOOH) always serves as a hydrogen source for CTH reaction via the
following dehydrogenation reaction: HCOOH→H
2
+CO
.[6] Much work has focused
2
on the homogeneous[7] and noble metals-based heterogeneous catalysts (e.g. Pt,[8]
Pd,[9] Rh,[10] Au, and Ru[12]), however, most of them concentrated on the single
hydrogenation or dehydrogenation reaction. Additionally, the researches of ODH and
CTH on dual-functional catalysts catalyzing are rare. Although these catalysts show
high activity, high price and complicated recovery greatly restrict their development
in large-scale applications. It is imperative to develop a low-cost transition metal
catalyst presenting dual-functional catalytic abilities in realizing the reciprocal
transformations between various saturated and unsaturated N-heterocycles.
[11]
Fortunately, transition-metal (e.g. Fe, Co and Ni)-based N-doped carbon materials are
greatly hopeful to replace the Pt-based catalysts in electrochemical reactions,[13]
selective oxidation of alcohols[14] and hydrogenation reactions.[15] The extensive
applications of transition metal-based N-doped carbon materials are attributed to their
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10.1002/cctc.201901304
outstanding abilities to activate O
2
and H
molecules.[16] It is noting that the external
2
carbon shell structure of encapsulated metal NPs could efficiently protect metal NPs
from aggregation, leaching and acid corrasion, thus the catalysts have the high activity
and stability.[17] Although lots of progresses have been made, the affirmation of active
sites for oxidation and hydrogenation reactions is still matter of debate. Bao et al.
have reported the N-doped carbon material and pod-like carbon nanotubes wrapped
Co, Fe NPs catalysts for water electrolysis and oxygen reduction reaction,
respectively. They pointed out that N-doped carbon nanotubes (NCNTs), enhanced by
the electron transfer from Fe NPs, act as active sites to activate O through DFT
2
calculations.[18] Chen’s group designed a Mott-Schottky-type catalyst Co@NC for
aerobic oxidation of alcohols and its activity results from the electron transfer at the
interface of Co NPs and N-doped carbon. This electron redistribution would form rich
oxidative areas around the Co NPs.[19] Beller and co-workers proposed active Co
oxide-N/C catalysts for nitroarene reduction[20] and N-doped graphite carbon enclosed
Co NPs for oxidation of alcohols to esters,[21] while they simply attributed the
high-performance to the synergic interactions between N-doped carbon and
metal/metal oxides. Ding et al. also mentioned that the wrapped Ni NPs tuned the
electronic properties of the inert carbon nitride and enabled the carbon nitride to
adsorb and activate H
for nitro compounds reduction.[22] Even so, all of these studies
2
tend to a speculation that small molecules are isolated by the carbon shell and cannot
get to the surfaces of metal NPs. Another interesting finding has been reported that the
defects and cracks generally generate during the graphitization of amorphous carbon
catalyzed by the metal[23] and the small gas molecules could make use of the van der
Waals gap to penetrate the surface between the graphene carbon shell and metal
NPs.[24] Lately, Li’s group designed a bifunctional Co@N-doped graphene shells. The
Co@N-doped graphene shells has cracks and defects in order to diffuse O
oxidative dehydrogenation and hydrogenation reactions.[25] Corma et al. also prepared
carbon layers wrapped Co NPs catalyst and the defects can help H diffuse easier,
while carbon layers act as an onlooker in the H activation for selective reduction of
2
and H in
2
2
2
nitroarenes.[26] Therefore, fabricating of N-doped carbon materials supported
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10.1002/cctc.201901304
non-noble metal NPs as dual-functional catalysts for ODH and CTH of
N-heterocycles and illuminating their reaction mechanism are meaningful research
work.
In this study, we synthesized NCNTs encapsulated cobalt nanoparticles (Co@NCNTs),
which acted as a low-cost dual-functional catalyst for ODH and CTH reactions. The
catalyst achieves the mutual transformations between saturated and unsaturated
N-heterocycles. The outer NCNTs could protect encapsulated Co NPs from
aggregation and leaching, thus the catalyst shows excellent activity and high stability
towards the ODH and CTH reactions. The formed defects from the doping of N could
facilitate the intercalation of reactive molecules to the surface of Co NPs. Moreover,
the poisoning tests with KSCN were to identify the active sites of the two reactions:
the NCNTs were confirmed to be direct active sites to activate O
enhanced by the electron transfer from Co NPs in ODH reaction; the encapsulated Co
NPs serve as direct active sites to activate H for CTH reaction and the activity could
2
, the activity was
2
be improved by the doped N atoms. On the basis of these results, we also proposed
the possible mechanisms of ODH and CTH reactions.
Results and discussion
Scheme 1 Schematic of the synthesis process of Co@NCNTs-X.
The formation of catalyst undergoes a step-by-step pyrolysis process (Scheme 1).
Under 350 ℃ condition, C
2
H
4
4
N condensated into melem and Co precursor was
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10.1002/cctc.201901304
anchored on the melem skeleton with the formation of Co-O-N through ketoamine
condensation between Co(acac) and melem. With the increasing of temperature to
50 ℃, it would be transformed into Co-g-C
.[28] From Fig. S1, uniform layered
structure of melem skeletons and lots of Co-g-C nanosheets could be observed.
2
6
3
N
4
3
N
4
When the temperature reaches 650 ℃, Co@NCNTs-X was continuously generated.
The process can be illustrated by FT-IR spectrum (Fig. S2). It should be pointed out
that NCNTs were not be generated without the Co with the pure g-C
3
4
N would
absolutely decompose over 650 ℃. The Co NPs are of great importance in the
formation of NCNTs, which takes in the oversaturated carbon into its body and the
carbon subsequently precipitates into multiwalled graphitic shell of nanotubes.[29]
Moreover, the aggregation of Co NPs is well prevented in the formation of
Co@NCNTs-X due to the Co-g-C
3
4
N structure. The Co@NCNTs is thermally stable
at 234 ℃ and pure CNTs starts to decompose at 86 ℃ (Fig. S3) from TGA analysis.
The Co@NCNTs are totally decomposed at 712 ℃, the weight loss of 99.5% results
from the decomposition and volatilization of many unstable components, C and
N-containing groups. The Co@NCNTs is not as stable as the pure CNTs, probably
2
because the doping of Co and N disrupts the regular sp structure of carbon.
Morphologies of the Co@NCNTs-800 were observed by TEM and SEM. It is obvious
that Co NPs are wrapped in the apexes of most bamboo-shaped carbon nanotubes (Fig.
1
a) and the SEM exhibits an identical tubular morphology (Fig. 1b). For
Co@NCNTs-700, the residual nanosheets exists much and few NCNTs are formed,
indicating incomplete graphitization of amorphous carbon under this temperature.
And average size of the Co NPs was obtained to be 9.1 nm in the Co@NCNTs-700
(Fig. S4a-c) via randomly counting 100 Co NPs. Under the 800 ℃ pyrolysis
temperature, amorphous carbon was nearly graphitized into uniform NCNTs. The
diameters of NCNTs are uniform and the average size of Co NPs is 8.0 nm (Fig.
S4d-f). The diameter of NCNTs is increasing and larger average size of Co NPs is
2
6.6 nm when overheated at 900 ℃ (Fig. S4g-i), which reduced the metal utilization.
HRTEM image shows that Co NPs are wrapped in N-doped carbon layer with 2-6 nm
thickness (Fig. 1c). The legible lattice distances of the C (002) facet and cubic Co
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ChemCatChem
10.1002/cctc.201901304
(111) plane are 0.34 nm and 0.20 nm, which is in accordance with XRD results. The
structure of NCNTs encapsulated Co NPs could protect Co NPs from aggregation and
leaching during the process of the reaction and the inner Co NPs could also activate
2
O molecules through the outer N-doped carbon layer. As can be seen from Fig. S5,
the direction of carbon layer with interlayer spacings is not always along the axis of
the NCNTs. This phenomenon reveals that the abundant defects and edges exist in the
NCNTs, which is significant to the improved catalytic property. The TEM element
mappings (Fig. 1d) clearly show that C, N are uniformly distributed in the skeleton of
NCNTs and the Co NPs are encapsulated by the N-doped carbon layer.
Fig. 1. (a) TEM, (b) SEM and (c) HRTEM images of Co@NCNT-800. (d) the EDX mapping
images of orange rectangular area.
The formation of metallic Co and crystalline carbon was observed by the XRD
patterns (Fig. 2a). The typical diffraction peak at 26.2° belongs to the (002) plane of
graphitic carbon. Without graphitic carbon signal of Co@NCNTs-700 indicates that
carbon substrate could not be graphitized under this temperature. With the increasing
of pyrolysis temperature, the degree of graphitization is improved and
Co@NCNTs-800 has the highest graphitization. Besides, the diffraction peaks at
4
4.3°, 51.2° and 75.6° are ascribed to (111), (200) and (220) planes of metallic Co.
The process of amorphous carbon graphitization is also catalyzed by the metallic
Co.[30] The values of D and G bands are 1354 cm and 1589 cm from Raman spectra
-1
-1
(
Fig. 2b), according to the lattice defects and the graphitic phase.[31] The nanotube
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skeleton of Co@NCNTs-800 (I
D
/I =0.93) has the highest degree of regularity. The
G
Co@NCNTs-900 catalyst has a more disordered structure (I
D
/I =1.07) than that of
G
Co@NCNTs-800, the reason is that the removal of unstable atoms forms more defects
under higher temperature. The mixed phases of amorphous carbon, noncrystalline Co
and remanent g-C
3
N
4
lead to low signal to noise ratio of Co@NCNTs-700
(
D G
I /I =1.02). The BET isotherms of Co@NCNTs-700, -800, and -900 belong to a
Type IV category. And the hysteresis loop occurs in the medium and high pressure. It
means that the abundant mesoporous exist in the catalysts (Fig. 2c). This is confirmed
by the relatively intensive pore size distributions in Fig. 2d. From Table S1, we could
2
-1
obtain that the BET surface areas of Co@NCNTs-700, -800, and -900 are 267.9 m g ,
2
-1
2 -1
1
75.2 m g and 174.4 m g . The medium surface area and mesoporous structure
could efficiently weaken the negative effect of the mass transport.
Fig. 2. (a) XRD patterns, (b) Raman spectrum, (c) N adsorption/desorption isotherms and (d)
2
pore size distributions of Co@NCNTs-700, Co@NCNTs-800 and Co@NCNTs-900.
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XPS was performed to determine the composition and bond states of the catalyst, the
C, N, O and Co coexisted in the catalysts (Fig. 3a), which is also confirmed by EDX
spectrum (Fig. S6). The high resolution C1s spectra (Fig. S7) could be divided into
C-C (284.62 eV), C-N (285.45 eV) and C-O (288.21 eV), the C-N bond indicates N
atoms were successful doped in the carbon nanotubes. Fig. 3b exhibits high resolution
Co 2p spectra of Co@NCNTs-700, -800, and -900, the peaks centered at 778.26 eV,
79.41 eV, 780.42 eV and 782.38 eV are ascribed to Co (C-C), Co-C, Co2+ and
0
7
Co-N.[32] The presence of Co-N bond proves the strong interaction between NCNTs
2+
0
and Co. Additionally, the proportion of Co-N and Co reduced while Co increased
with the incremental temperatures (Table S2), indicating that a higher calcination
temperature is helpful to reduce Co species and improve the crystallization of Co NPs.
The N1s spectrum (Fig. 3c) was divided into pyridinic N (398.61 eV) and graphitic N
(400.98 eV). It can be inferred that a higher pyrolysis temperature is helpful to form
the graphitic N from the content of N species (Fig. 3d) and Co@NCNTs-800 has the
highest graphitization, XPS shows the similar trend (Table S3, entries 1-3). O1s
spectrum demonstrates that the O element in the catalyst mainly results from O atoms
doped carbon matrix ((Fig. S8), because the strong peak located in 532.2 eV belongs
to the O species of carbon matrix while the typical peak of Co oxide (529.3 eV) is not
observed. This result matches XRD analysis. Additionally, the oxygen concentration
gradually decreased with the increasing of pyrolysis temperature, which means that a
higher temperature contributes to the reduction of Co precursor and graphitization of
materials (Table S3).
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Fig. 3. (a) XPS wide scan spectra, (b) high-resolution Co2p spectrum, (c) high-resolution N1s
spectrum, (d) the percentages of pyridinic N and graphitic N in Co@NCNTs-700,
Co@NCNTs-800 and Co@NCNTs-900.
Oxidative dehydrogenation reaction
The Co@NCNTs-800 catalyzing ODH reaction, the 1,2,3,4-tetrahydroquinoline as the
substrate, was conducted. Firstly, the hot filtration test was performed to verify the
heterogeneity nature of the Co@NCNTs-800 catalyst (Fig. S9). The conversion is
6
2.78% after reaction for 5 h and the catalyst was removed from the solution. Then
the solution was purged with pure O and proceeded for another 8 h at 80 ℃ (Table S4,
2
entry 10). No extra quinoline was produced and Co element was not detected by the
ICP, which confirms that the reaction is inherently heterogeneous. Different pyrolysis
temperatures were screened (Table S4, entries 1-3), the Co@NCNTs-800 has the
highest conversion (99.90%) compared with Co@NCNTs-700 (73.20%) and
Co@NCNTs-900 (51.57%), which results from the combination of several factors,
such as the states of Co and N, particle size of Co NPs, degree of graphitization and
the surface areas. The Co@NCNTs-700 (7.28%) has the highest N content compared
with Co@NCNTs-800 (2.60%) and Co@NCNTs-900 (2.75%). In spite of the highest
surface area and Co content, it is mainly existed in the form of pyridinic N (Table S3),
which indicates it is the graphitic N not the N content that promotes the ODH reaction.
Accordingly, several groups have proved that graphitic N contributes to activating the
O
2
via theoretical and experimental methods.[33] Additionally, the Co2+ in
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Co@NCNTs-700 is much more than two other catalysts, which also has a negative
effect on the ODH reaction. Given that the particle size of Co NPs in
Co@NCNTs-900 is much larger than that in Co@NCNTs-800, which weakens the
ODH activity of Co@NCNTs-900. The reaction was difficult to proceed without the
K
2
CO
3
or under N
2
/air atmosphere (Table S4, entries 4-6). So the K
2
CO plays an
3
important role in the reaction. It is reported that the base could activate the substrate
molecules and it also facilitates deprotonation of the H-metal intermediates to
regenerate the catalytic active surface of metal nanocatalysts, accelerating the whole
reaction process.[19] Different oxidants were also studied, the conversions are 22.51%
and 89.61% when using the H
selectivity of the latter is quite low (Table S4, entries 7-8). It is widely accepted that
peroxide species ( ) act as O
intermediates to complete the ODH reaction.[34] To
find out how O is involved in the reaction, the butylated hydroxytoluene (BHT) was
2
O
2
and TBHP as the oxidants under N
2
, while the
2
2
added into the reaction system as a radical scavenger. Though extending the reaction
time to 24 h (Table S4, entry 9), the conversion was only up to 20.81%. This result
suggests that the ODH reaction goes through a radical-driven course. Subsequently,
+
the activity of the Co@NCNTs-800-H stays unchanged (Table S4, entry 11),
implying that the content of Co would not directly influence the reaction and the
graphitic N might be the active sites to take part in the ODH reaction instead of Co
NPs. To prove this conjecture, several control experiments were conducted. The
Co(acac)
2
(Ⅱ), dicyandiamide and Co(acac) (Ⅱ) firstly catalyzed the ODH reaction, the
2
conversions were all less than 1% (Table S4, entries 12-13), these results show that
Co species make a small contribution for the reaction. The reaction does not proceed
without the catalyst (Table S4, entry 14). The activities of CNT and NCNTs were then
studied, the conversion of NCNTs was improved after the doping of N atoms (Table
S4, entries 15-16). Furthermore, the Co NPs supported on the NCNTs was also
studied and it reached a conversion of 28.62% (Table S4, entry 17), the activity of
Co/NCNTs is much less than the Co@NCNTs. On the basis of these results, we
concluded that the peculiar structure of NCNTs wrapped Co NPs plays vital roles in
the ODH reaction. The N-doped graphene shell is regarded as active sites for the
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10.1002/cctc.201901304
ODH reaction and the participation of Co cores could enhance the activity.[20, 35]
Furthermore, compared with the former literatures, the ODH reaction of the
Co@NCNTs-800 catalyst possesses a relatively mild reaction conditions with the low
reaction temperature and short reaction time, and an excellent yield (Table S5). The
desired reaction temperature is the result of many factors, such as the formed defects,
the synergistic effect of Co NPs and the high average pore size.
Catalytic transfer hydrogenation reaction
The CTH of quinoline was chosen as a model reaction. The HCOOH is the hydrogen
source in this reaction. As can be seen from Table S6, entries 1-4, a higher reaction
temperature makes contribution to the conversion. Different solvents were also
evaluated (Table S6, entries 5-7). All of them have low conversions and selectivities.
When the amount of HCOOH was gradually decreased, the conversions have the
same trend (Table S6, entries 8-10). The CTH reaction would not be triggered without
the HCOOH (Table S6, entry 11). The metal center sites of the catalyst are beneficial
for activation of H , so several control experiments were conducted to analyze the
2
roles of N and Co during the CTH reaction. Firstly, the reaction would not happen
without the catalyst (Table S6, entry 12) and the result is the same using the unburned
mixture precursor, so the Co in the coordination form is inactive (Table S6, entry 13).
Furthermore, the reactions did not happen when using CNT or NCNTs as the catalysts
(
Table S6, entries 14-15), it is inferred that Co NPs play a role in activating H .
2
Moreover, the catalysts with different Co content were performed to further verify this
result, the conversion significantly increased from 59.31% to 85.59% with the
increasing of Co active sites (Table S6, entries 16-17). After acid treatment to
eliminate the influence of Co NPs on the surface of the catalyst, the CTH reaction still
maintains excellent conversion of 99.87% and selectivity of 99.92% (Table S6, entry
1
8). Combining with the content of Co after the acid treatment (7.56% in Table S2)
and TEM image in Fig. S10, it is concluded that only half of Co NPs in the catalyst
are encapsulated by the NCNTs and they probably work as active sites to participate
in the reaction. Similarly, compared with the previous studies, the Co@NCNTs-800
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10.1002/cctc.201901304
catalyst also presents high activity when using HCOOH as the hydrogen source (Table
S7).
Table 1. The ODH reaction of various saturated N-heterocycles onCo@NCNTs-800
catalyst.a
Entry
1
Substrate
Product
t (h)
13
Conv. (%) b Sel. (%) b
TOF (h-1) c
0.71
>99.9
87.27
>99.9
>99.9
>99.9
>99.9
2
3
24
25
0.34
0.37
4
15
15
>99.9
>99.9
>99.9
>99.9
0.62
5
0.62
a
Reaction conditions: 0.5 mmol 1,2,3,4-tetrahydroquinoline, 0.5 mmol K
2
CO
3
, 20 mg catalyst,
b
5
3
mL CH OH, 13 h, O
2
balloon, 80 ℃. The conversion and selectivity were determined by
mole of converted substrate
c
GC-MS. Turnover frequency (TOF) =
.
mole of Co · t (h)
Table 2. The CTH of various saturated and unsaturated N-heterocycles over
a
Co@NCNTs-800.
or
Entry
1
Substrate
Product
t (h) Conv. (%) b
Sel. (%) b TOF (h-1)c
1/6
>99.9
>99.9
111.39
2
1/6
>99.9
>99.9
111.39
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ChemCatChem
10.1002/cctc.201901304
3
1/6
0.5
0.5
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
111.39
37.13
37.13
4
5
6
7
8
6
6
>99.9
>99.9
99
99.83
81.53
97.44
3.09
2.53
1.50
12
9
30
12
77.80
>99.9
>99.9
>99.9
0.48
1.55
10
1
1
2
3
18
19
18
>99.9
>99.9
15
98.83
>99.9
>99.9
1.02
0.98
0.15
1
1
1
1
4
5
25
17
>99.9
>99.9
>99.9
>99.9
0.74
1.09
aReaction conditions: 0.5 mmol quinoline, 10 mg catalyst, 4 mmol HCOOH, 2 mL Toluene,
1
30 ℃, 6 h. b The conversion and selectivity were determined by GC-MS. c Turnover
mole of converted substrate
frequency (TOF) =
.
mole of Co · t (h)
Kinetic studies of the ODH reaction over Co@NCNTs-800 and Co/NCNTs
catalysts
Temperature generally makes a great difference on the reaction rate. When the
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10.1002/cctc.201901304
temperature rises from 25℃ to 80 ℃, the rate of reaction correspondingly has
increased (Fig. 4a) and the inset also shows the TOF values increase from 0.29 to 0.71
-1
-1
h . The activation energy (Ea) was calculated to be 12.03 kJ mol based on the
Arrhenius plots (Fig. 4b). The conversions versus reaction time over Co@NCNTs-800
and Co/NCNTs were studied, the conversion of the former is always higher than the
latter (Fig. 4c). A pseudo first-order reaction was confirmed from Fig. 4d of
ln[1,2,3,4-tetrahydroquinoline] versus reaction time, from which the rate constants k
1
and k
2
-1
-1
were calculated to be 0.21 h and 0.05 h .
Fig. 4. (a) Plots of time versus conversions of 1,2,3,4-tetrahydroquinoline using the
Co@NCNTs-800 catalyst under different temperatures. (b) The Arrhenius plots for
calculating the activation energy. (c) Time courses of ODH reaction of
1,2,3,4-tetrahydroquinoline over Co@NCNTs-800 and Co/NCNTs catalysts. (d) Kinetic
studies of ODH reaction of 1,2,3,4-tetrahydroquinolinewith a pseudo first-order reaction
model over Co@NCNTs-800 and Co/NCNTs catalysts based on the time course plots.
Reaction conditions: 0.5 mmol 1,2,3,4-tetrahydroquinoline, 0.5 mmol K
mL CH OH, 13 h, O balloon.
2
CO , 20 mg catalyst,
3
5
3
2
Recycling performance of Co@NCNTs-800 in ODH reaction of
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ChemCatChem
10.1002/cctc.201901304
1
,2,3,4-tetrahydroquinoline and CTH reaction of quinoline
The recyclability and stability of the Co@NCNTs-800 were evaluated to confirm that
whether the encapsulation structure that decreases the deactivation of the catalyst in
the ODH and CTH reactions. In the ODH of 1,2,3,4-tetrahydroquinoline, the activity
of Co@NCNTs-800 decreased after six cycles, because each collection of the catalyst
would lead to the mass loss. However, the Co/NCNTs was almost inactive after four
cycles (Fig. 5a). Compared with the fresh Co/NCNTs catalyst from TEM, Co NPs of
recycled catalyst seriously aggregated into larger NPs (Fig. S11) and the content of Co
also decreased from 18.28% to 8.943% by ICP analysis, all of these lead to a
significant decline in activity. Additionally, the recyclability of the Co@NCNTs-800
was also tested in CTH of quinoline. The reaction over Co@NCNTs-800 catalyst kept
high conversion and selectivity even after ten cycles (Fig. 5b). The Co NPs average
size of recycled catalyst is 8.2 nm, which is close to the fresh catalyst of 8.0 nm (Fig.
S12) and the conent of Co slightly decreases from 15.84% to 14.05%. The XRD
pattern of recycled catalyst also remains unchanged (Fig. S13). From the
high-resolution Co2p spectrum of Co@NCNTs-800-reused catalyst in Fig. S14, the
proportions of the four types Co species varied slightly (Table S2, entry 5). The
superparamagnetic property of Co NPs makes the catalyst easily separated from the
reaction system for the next use. And the magnetic saturation value of recycled
-1
catalyst is very close to the fresh catalyst of 9 emug from VSM results (Fig. S15).
These results lead us to conclude that the stable encapsulation structure could protect
Co NPs from aggregation and leaching, thus the catalyst shows excellent stability.
Notably, the Co@NCNTs-800 catalyst was confirmed to be an outstanding
dual-functional catalyst and exhibits a wide applicable scope in the ODH and CTH
reactions. Firstly, the ODH reaction of 1,2,3,4-tetrahydroquinoline and indoline
derivatives were studied (Table 1). The 1,2,3,4-tetrahydroisoquinoline and
1
-methyl-1,2,3,4-tetrahydroisoquinoline were slowly transformed into the
corresponding quinoline products over 24 h (Table 1, entries 2-3). Indoline and
-methylindoline were also converted with excellent conversions and selectivities
2
(Table 1, entries 4-5). Additionally, various saturated and unsaturated N-heterocycles
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ChemCatChem
10.1002/cctc.201901304
were also investigated in the CTH reaction (Table 2). Compared with the unsaturated
N-heterocycles, the saturated indoline and quinoline derivatives were rapidly
converted into the target products with high conversions and selectivities (Table 2,
entries 1-5). In the CTH reaction of unsaturated N-heterocycles, the isoquinoline
showed the conversion of 99.9 % and selectivity of 81.53 %, the low selectivity
results from the formation of 3-methyl-1,2,3,4-tetrahydroisoquinoline (Table 2, entry
7
). When the electron-donating groups (-CH
N-heterocycles, the reaction times were prolonged (Table 2, entries 8-11). The
,3-benzodiazine, with two N atoms, was completely converted into the
corresponding products (Table 2, entry 12). Moreover, the high selectivity of 99.9 %
Table 2, entries 13-14) could be reached even the presence of electron-withdrawing
groups (-Br and -NO ). The 8-quinolineboronic acid also had a high conversion and
selectivity (Table 2, entry 15).
3
and -OCH
3
) exist in the unsaturated
2
(
2
Co@NCNTs-800
Co/NCNTs
Conversion
Selectivity
(a)
(b)
1
00
100
8
6
4
2
0
0
0
0
0
7
5
2
5
0
5
0
1
2
3
4
5
6
1
2
3
4
5
6
7
8
9 10
Recycle times
Recycle times
1
20
00
120
100
(c)
without KSCN
with KSCN
(d)
without KSCN
with KSCN
1
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
1
,2,3,4-tetrahydroquinoline Benzyl alcohol
Quinoline
Nitrobenzene
Fig. 5. Recycling tests of Co@NCNTs-800 in (a) the ODH reaction for
,2,3,4-tetrahydroquinoline, (b) the CTH reaction for quinoline. Poisoning tests of
Co@NCNTs-800 and additional substrates with KSCN in (c) the ODH reaction for
,2,3,4-tetrahydroquinoline and benzyl alcohol, (d) the CTH reaction for quinoline and
nitrobenzene.
1
1
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ChemCatChem
10.1002/cctc.201901304
Discussions of active sites for ODH and CTH reactions
Based on the excellent activity, selectivity and stability, Co@NCNTs-800 was
confirmed to be an outstanding dual-functional catalyst. Efforts are underway to
clarify what directly these reactions matters, namely NCNTs for the ODH reaction
and Co NPs for the CTH reaction.
The original carbon nanotubes with sp hybridized structure are chemically inert.[32, 36]
The electron density of C atoms could be tuned by the adjacent doping of
electronegative N atoms, which improves the activity of pristine CNTs. Although
2
much work so far has verified that NCNTs favor the O activation for catalysis, which
2
type of nitrogen acts as real active sites is under way to examine.[37] In addition, the
participation of transition metal into the NCNTs could further enhance the ODH
activity via a synergetic effect, because the N-neighbored carbon atoms receive the
electrons from the Co NPs and further help the O
kind of electron transfer.[38] The FT-IR spectra (Fig. S16) proves the O
the Co@NCNTs-800 catalyst. Compared with Co@NCNTs-800+N
intensity at 1462 cm-1 and 804 cm-1 of Co@NCNTs-800+O
is higher. These
absorption bands are ascribed to bonded with carbon atoms and is
2
adsorption/dissociation through this
2
activation by
2
, the peaks
2
considered as active oxygen intermediates in ODH reaction,[34] in accordance with the
results obtained from BHT experiment and EPR characterization. On the basis of
these analysis, we considered that Co NPs modified NCNTs serves as direct active
sites to activate O
2
into
intermediates to take part in the ODH reaction.
To gain more insight into the roles of Co NPs in the ODH and CTH reactions, the
poisoning tests with KSCN were performed. The Co sites would strongly coordinate
with SCN- ions to form the highly stable metal-ligand complexes, eliminating the
effects of Co NPs on the reaction.[39] From the TEM line scanning (Fig. S17) of
Co@NCNTs-800 treated with KSCN, the trends of Co and S are close, implying that
-
SCN could coordinate with Co NPs by inserting the carbon shell. Bao et al. also
clarified a defect-aided intercalation mechanism that small molecules KSCN, O
2
and
H
2
could intercalate the graphene shell through the van der Waals gap and get to the
surface of Co NPs.[40] It should be pointed that the doping of N could make the outer
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ChemCatChem
10.1002/cctc.201901304
graphene shells imperfect by forming the carbon vacancies,[41] these defects help
small molecules intercalate into the surfaces of encapsulated Co NPs, as is shown in
Fig. S18. Furthermore, the catalyst has a relatively high average pore size (Table S1),
so the N-heterocycles could have access to the surface of Co NPs to participate in the
reaction through the porous structure.
The ODH poisoning tests with KSCN are showed in Fig. 5c. The conversions of
1
,2,3,4-tetrahydroquinoline decreased from 99.9% to 46.85% after the addition of
KSCN (16 equiv. to Co). Compared with the Co NPs, the carbon atoms, which is
adjacent to nitrogen atoms, have a higher work function. It would receive electrons
from the Co NPs until the balance of Fermi level. This kind of electron transfer is
known as Mott-Schottky effect, which could arouse the electron redistribution at the
interface of N-doped carbon and metallic Co.[19] Because of the strong coordination
-
between SCN and Co NPs, the ability of Co NPs is greatly weakened in providing
electrons. So the interactions between Co NPs and NCNTs are weakened, leading to
the partial decreased activity of the catalyst in ODH reaction. The esterification of
benzyl alcohol was also performed and the conversion changed from 81.03% to
7
3.06%. These results help us to conclude that the NCNTs act as direct active sites to
activate O , the electron transfer by the Co NPs could further enhance the
2
efficiency.[18] Fig. 5d shows the CTH poisoning tests with KSCN and conversions of
quinoline sharply decreased from 99.9% to 17.36% after the addition of KSCN. The
surfaces of encapsulated Co NPs were nearly blocked by the excessive SCN-,
accounting for the decreased activity. Furthermore, we discussed that the CTH
activity is closely related to the content of Co (Table S6, entries 16-17). Therefore, we
firmly believe that Co NPs play the part of direct active sites and the identical
conclusion was obtained in the CTH of nitrobenzene.
Plausible reaction mechanisms for ODH and CTH reactions
Scheme 2 reveals plausible reaction mechanisms for the ODH reaction of
1
,2,3,4-tetrahydroquinoline and CTH reaction of quinoline. Primarily, we have
studied the ODH of 1,2,3,4-tetrahydroquinoline with the addition of BHT as a radical
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ChemCatChem
10.1002/cctc.201901304
scavenger in Table S4. The BHT was known to restrain the formation of
and a
small amount of quinoline was generated though the reaction was prolonged to 24
h.[33b] Here we propose that NCNTs modified by the encapsulated Co NPs could act as
active carbon sites to reduce the absorbed O
of H during the reaction has been detected via an iodometric experiment and has
been quickly consumed to form the H
O and quinoline.[2b] Moreover, the EPR in Fig.
S19 reveals the has generated in the process of reaction after adding the capture
2
to
. The accompanying generation
2
O
2
2
agent 5, 5-dimethyl-1-pyrroline N-oxide (DMPO), which is similar to the proposed
ODH reaction pathway. Then the explanation of reasonable mechanism for CTH
reaction of quinoline was also presented, which contains a one-pot of hydrogen
evolution-reduction-formylation domino order. In the initial step, the N basic sites
could be a proton scavenger agent to promote the O-H bond splitting of HCOOH,
resulting in the formation of Co-formate complex. The decomposition of transitory
Co-formate complex is testified to be a rate-determining step via the kinetic isotope
effects experiments.[42] With the help of -NH from the reaction of N basic sites and
HCOOH, the Co-formate complex would dehydrogenate to generate molecular H
+
accompanied by CO
2
via a β-elimination route.[43] And the H
2
desorption is
as an in situ
irreversible which indicates that renewable HCOOH could replace H
2
hydrogen source from a security and sustainable prospective. Throughout the process
of HCOOH dehydrogenation, the synergistic effect between the Co NPs and N sites
makes a contribution to activating the important Co-formate complex under mild
conditions. In the second step, the quinoline is reduced to the
1
1
,2,3,4-tetrahydroquinoline by the H
2
. Then in the third step, the resulting
in situ to obtain the
,2,3,4-tetrahydroquinoline is formylated
N-formyltetrahydroquinoline. We also apply the saturated N-heterocycles to the
reaction and obtained the corresponding N-formylation products (Table 2, entries 1-5).
The reaction time has dramatically decreased compared with the unsaturated
N-heterocycles. So we conclude that reduction procedure is the rate-determining step
in the domino order, and then the formylation occurred sequentially.
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ChemCatChem
10.1002/cctc.201901304
Scheme 2 Reaction mechanisms for the oxidative dehydrogenation of
,2,3,4-tetrahydroquinoline (left) and the transfer hydrogenation of quinoline (right)
1
catalyzed by Co@NCNTs-800.
Conclusion
We successfully prepared Co NPs apically wrapped the bamboo-shaped NCNTs
catalyst via a multiple pyrolysis method. In the ODH and CTH reactions of various
N-heterocycles, the Co@NCNTs-800 dual-functional catalyst shows attractive
activity and stability. The structure of NCNTs could efficiently protect wrapped Co
NPs from aggregation and leaching in the process of reaction. Both control
experiments and KSCN poisoning tests demonstrated the different active sites in the
ODH and CTH reactions. The NCNTs work as direct active sites to activate O and
2
the electron transfer, from the encapsulated Co NPs, further promotes the ODH
reaction. At the same time, Co NPs play a key role in the CTH reaction and the
doping of N atoms enhances the activity synergistically. Herein, the doping of N
atoms can redistribute the electrons between the adjacent C atoms and Co NPs. In
addition, the doping of N atoms also makes the small molecules get to the surfaces of
the encapsulated Co NPs easily through the defects of outer NCNTs. This work
provides more hopeful prospects in design of non-precious metal encapsulated
N-doped carbon materials with desired and improved characters for various catalytic
applications.
Experimental section
Chemicals
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ChemCatChem
10.1002/cctc.201901304
Dicyanodiamide (C
2
H
4
N
4
, ≥98.5%), Cobalt(II) acetylacetonate (Co(acac) , ≥97%),
2
benzene (≥99%), formic acid (HCOOH,≥88.0%), 1,2,3,4-tetrahydroquinoline (≥99%)
and quinoline (≥98%) were obtained from Tianjin Heowns Biochem LLC.
Multi-walled carbon nanotubes (MWNTs, >98 wt%) were supplied by Shenzhen
Nanotech Port Ltd. The entire chemicals in the reaction were analytical grade without
further modification.
Characterization
Transmission electron microscope (TEM), high-resolution electron microscope
(HRTEM, Tecnai G2 F30) and scanning electron microscope (SEM, HITACHI
S-4800) were conducted to observe morphologies of the prepared samples. Raman
spectra (Jobin Yvon Lab Ram HR evolution) was performed with a Raman
spectrometer. Powder X-ray diffractometer (XRD) was recorded on a Rigaku
D/max-2400 diffractometer with Cu-kα radiation. The bonding states of elements
were determined by a PHI-5702 X-ray photoelectron spectroscopy (XPS). A Bruker
A300-9.5/12 electron paramagnetic resonance (EPR) was conducted, parameter
setting: center field (3362.000 G), microwave frequency (9.450 G), power (24.689
mW). A Guantum Design vibrating sample magnetometer (VSM) was performed to
determine the magnetic properties. The Brunanuer-Emmett-Teller (BET) method was
used to obtain the specific surface areas and the pore size distributions via the
nitrogen adsorption/desorption measurement (Micromeritics ASAP 2010). A TA-Q50
thermogravimetric analysis (TGA) was performed under N atmosphere and a heating
2
-1
rate of 10 ℃ min . A VERTEX 70 Bruker spectrometer provided the Fourier
transform infrared spectroscopy (FT-IR). Elemental analysis (EA, CHNS) was
conducted to determine the content of C, N, H, O. An inductive coupled plasma
atomic emission spectrometer (ICP-AES) determined the content of Co equipped with
the Optima-4300DV Perkin Elmer. The conversions and selectivities of products were
determined by gas chromography-mass spectrometry (GC-MS, Agilent 5977E).
Catalyst preparation
For the preparation of Co@NCNTs, 109 mg Co(acac) was dissolved in 10 mL
2
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ChemCatChem
10.1002/cctc.201901304
benzene and 5 g C
2
H
4
4
N was dispersed in 10 mL benzene, followed by mixing and
stirring for 12 h. Then, the mixture was filtrated, dried in vacuum and grinded in pink
powder. Finally, the powder was heated to 350 ℃ for 3 h, 650 ℃ for 3 h and a desired
temperature of 700-900 ℃ for 2 h. The pyrolysis was under highly purified N
2
-1
atmosphere and at a heating rate of 5 ℃ min . Accordingly, the obtained material was
donated as Co@NCNTs-X (X representing the calcination temperature). The black
powder was treated with 0.5 M H
2
SO
4
for 24 h at ambient temperature
+
(
Co@NCNTs-H ). For comparison, NCNTs supported Co NPs (Co/NCNT) was also
prepared.[27] 100 mg carbon nanotube was dispersed into 24 mL H
ethanol, followed by adding of 0.6 mL 0.6 M Co(OAc) aqueous solution and 0.5 mL
NH OH. The solution was transferred into a 60 mL autoclave at 150 ℃ for 3 h after
heating and stirring at 80 ℃ for 20 h. The product was collected by filtration, washed
with H O and dried.
Catalytic oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline
.5 mmol 1,2,3,4-tetrahydroquinoline, 69 mg K CO , 5 mL CH OH, 20 mg catalyst
were added into a 10 mL two-neck flask. Then the flask was purged with pure O for
O and 0.5 mL
2
2
4
2
0
2
3
3
2
three times to remove air and heated to 80 ℃ for a certain time. The products were
analyzed via GC-MS.
Catalytic transfer hydrogenation of quinoline
Typically, 10 mg catalyst was dispersed in 1.5 mL toluene and then 0.5 mmol
quinoline, certain equivalent of HCOOH were added into the above solution. The
reaction occurred at 130 ℃ for 6 h and the mixture was analyzed by GC-MS. After
each reaction, the catalyst was collected by magnetic separation, washed and dried for
next ODH or CTH reaction.
Acknowledgements
This work was supported by the Natural Science Foundation of Gansu (No.
1
8JR3RA274).
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ChemCatChem
10.1002/cctc.201901304
Keywords:
Co@nitrogen-doped carbon nanotubes; Dual-functional catalyst; Catalytic transfer
hydrogenation; N-heterocycles; Oxidation dehydrogenation
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