High catalytic activity of mesoporous Co-N/C catalysts for aerobic oxidative synthesis of nitriles

Article abstract of DOI:10.1039/c6cy00195e

A high-efficiency and atom-economic synthetic strategy for nitriles by aerobic ammoxidation of alcohols is developed using a novel mesoporous cobalt-coordinated nitrogen-doped carbon catalyst (meso-Co-N/C) fabricated from a cobalt-coordinating polymer, which manifests superior activity towards the target reaction. The catalytic system features a broad substrate scope for various benzylic, allylic as well as heterocyclic alcohols, providing good to excellent yields of the target products with high selectivities, albeit with 0.5 mol% Co catalyst loading. 11,11′-Bis(dipyrido[3,2-a:2′,3′-c]phenazinyl) (bidppz) with extreme thermostability is selected as a robust ligand bridge between cobalt ions, resulting in the homogeneous distribution of active sites at the atomic or subnanoscale level and high catalyst yield. Silica colloid or ordered mesoporous silica SBA-15 is employed to realize the mesoporous structure. The unprecedented performance of the meso-Co-N/C catalyst is attributed to its high Brunauer-Emmett-Teller (BET) surface area (up to 680 m² g^-1) with a well-controlled mesoporous structure and homogeneous distribution of active sites. Kinetic analysis demonstrates that the catalytic oxidation of benzyl alcohol to benzaldehyde is the turnover-limiting step and that the apparent activation energy for benzonitrile synthesis is 61.5 kJ mol^-1 and cationic species are involved in the reaction.

Full text of DOI:10.1039/c6cy00195e

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High Catalytic Activity of Mesoporous Co-N/C Catalysts for
Aerobic Oxidative Synthesis of Nitriles
Sensen Shang,a,b Lianyue Wang,a Wen Dai,a Bo Chen,a,b Ying Lva and Shuang Gao*a
Received 00th January 20xx,
Accepted 00th January 20xx
A high-efficiency and atom-economic synthetic strategy for nitriles by aerobic ammoxidation of alcohols is developed using
a novel mesoporous cobalt-coordinated nitrogen-doped carbon catalyst (meso-Co-N/C) fabricated from an cobalt-
coordinating polymer, which manifests superior activity to the target reaction. The catalytic system features a broad
substrate scope for various benzylic, allylic as well as heterocyclic alcohols, providing good to excellent yields of the target
products with high selectivities, albeit with 0.5 mol % Co catalyst loading. 11,11’-bis(dipyrido[3,2-a:2,3-c]phenazinyl)
(bidppz) of extreme thermostability is selected as a robust ligand bridge between cobalt ions, resulting in the
homogeneous distribution of active sites at atomic or subnanoscale level and high catalyst yield. Silica colloid or ordered
mesoporous silica SBA-15 is employed to realize the mesoporous structure. The unprecedented performance of meso-Co-
N/C catalyst is attributed to their high Brunauer-Emmett-Teller (BET) surface area (up to 680 m2/g) with well-controlled
mesoporous structure and homogeneous distribution of active sites. Kinetic analysis demonstrates that the catalytic
oxidation of benzyl alcohol to benzaldehyde is the turnover-limiting step and that the apparent activation for benzonitrile
synthesis is 61.5 kJ mol-1; cationic species are involved in the reaction.
DOI: 10.1039/x0xx00000x
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been reported.5h-k
Introduction
Recently, carbon materials benefiting from special electronic
property have been widely applied in many catalytic
Nitriles are a very important class of compounds in chemistry
as well as biology, playing a major role in the elaboration and
composition of pharmaceuticals, agrochemicals and fine
chemicals.1 Representative general synthetic strategies for
nitrile synthesis include Sandmeyer’s reaction,2 transition
metal-catalyzed cross-coupling of aryl halides3 or direct C-H
functionalization of arenes4 commonly need highly toxic
cyanides and produce large amounts of inorganic salts as
waste, which limits their application from a green chemistry
perspective. Hence, the development of new environmentally
friendly and atom-economic procedures for nitrile synthesis is
a very important subject in modern organic synthesis. In this
respect, aerobic ammoxidation directly from renewable
feedstock alcohols and aqueous ammonia would be an
alternative approach to generating nitriles which produce
water as the only by-product. Many achievements have been
made in converting alcohols to nitriles under both
homogeneous and heterogeneous systems.5 From an
economic and environmental viewpoint, the careful design of
heterogeneous catalysts instead of homogeneous ones is
prevailing for the ease of their separation and recycling.
Notwithstanding, only few heterogeneous examples have
transformations in modern organic chemistry.6 Furthermore,
the introduction of heteroatoms (B, N, P, S, etc.) and non-
noble transition metals (Fe, Co, etc.) would be crucial for
improving catalytic efficiency of the materials by tuning their
electronic properties and generating active sites.7 Typically,
metal-nitrogen-doped carbon materials (M-N/C) were
prepared from direct pyrolyzation of coordinating compound
precusors containing Fe or Co salts and small molecular
nitrogen-containing ligand adsorbed on carbon black. However,
random location on carbon black and poor thermostability of
these coordinating compounds undoubtedly result in the
agglomeration of active metal species during the subsequent
pyrolyzation, which largely decreases atomic catalytic
efficiency. Moreover, the traditional method for preparing M-
N/C materials frequently fails in controlling specific surface
area and porous structure, leading to inadequate exposure
of the active sites and relatively poor transport properties.
In this contribution, we developed a novel mesoporous
cobalt-nitrogen-doped carbon materials (meso-Co-N/C)
derived from
c]phenazinyl)) coordinating
Co-bidppz (11,11’-bis(dipyrido[3,2-a:2,3-
polymer using template
synthesis. The Co-bidppz polymer consists of alternating chains
of N-containing ligand and cobalt ions. The existence of robust
ligand bridge between cobalt ions efficiently confines
cobalt ions at molecular level, resulting in the homogeneous
distribution of active sites at atomic or subnanoscale level,
responsible for the excellent catalytic performance. Silica
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colloid or ordered mesoporous silica SBA-15 is employed to Note that, the extraordinary high catalyst yield was achieved
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DOI: 10.1039/C6CY00195E
realize the mesoporous structure. It was worth noting that up owing to the extreme thermostability of Co-bidppz polymer, as
to 80 % catalyst yield was achieved resulting from extreme exhibited in thermo-gravimetric analysis (TGA) curve of the Co-
o
thermostability of Co-bidppz polymer. The as-fabricated bidppz/template composites (weight loss only 10% from 25 C
mesoporous catalyst exhibits unprecedented superior activity to 900 oC, Fig. S1†). For comparison, Co-N/C catalyst was
towards the aerobic oxidative synthesis of nitriles. To the best prepared by direct pyrolysis of Co-bidppz polymer without
of our knowledge, this is the first time to apply such a using any template.
Exploratory catalytic experiments with different catalytic
materials were performed for the aerobic oxidative synthesis
of benzonitrile (2a) from benzyl alcohol (1a) and aqueous
ammonia using molecular oxygen. Typically, this model
reaction was performed at 130 °C using simply 5 bar of
dioxygen. Blank runs (without any catalysts) gave essentially
no activity under identical conditions (Table 1, entry 1). The
Co-bidppz polymer exhibited hardly any conversion of benzyl
alcohol, suggesting that cobalt ions coordinated with the
bidppz ligand were inactive for alcohol oxidation under the in-
vestigated conditions (Table 1, entry 2). The Co-N/C (800)
catalyst obtained by direct pyrolysis of Co-bidppz polymer at -
mesoporous functional carbon materials as catalysts for the
target reaction. The proposed catalytic system features a
broad substrate scope for various benzylic, allylic as well as
heterocyclic alcohols, providing good to excellent yields of the
target products with high selectivities. The relationship
between the physicochemical property and catalytic behavior
of the catalyst is discussed. The catalyst is readily separated
and reused five times without significant change in activity,
albeit with 0.5 mol % Co catalyst loading.
Results and discussion
Table 1 Aerobic oxidative synthesis of benzonitrile over different catalystsa
Fig. 1 depicts the chemical structure of bidppz, the synthetic
process of the meso-Co-N/C catalysts (the details of fabrication
process in Supporting Information). The readily rotatable C-C
single bond linking two dppz moieties in the bidppz molecule
(marked in red, Fig. 1a) resulted in numerous configurations of
bidppz and a network structure of Co-bidppz, which can be
utilized as a self-supporting catalyst excluding the requirement
of a carbon support. Moreover, nearly 20 wt% concentration
of stable pyridinic nitrogen of bidppz also allows for a high
degree of Co-N coordination at high pyrolysis temperatures, b-
enefiting the overall catalytic activity. To synthesize the meso-
Co-N/C catalysts, various template materials were first
dispersed in the Co-bidppz polymer under vigorous stirring
before evaporation of DMF. The obtained Co-bidppz/template
composites were then thermally treated under flowing nitrog-
en at 700-900 oC and finally etched in hydrogen fluoride to
remove the templates, resulting in the meso-Co-N/C catalysts.
Entry
1
2
3
4
Catalyst
–
Co-bidppz
Conv.b (%)
Yieldb (%)
<1
<1
<1
19
93
95
93
93
26
52
88
70
90
0
10
90
95
89
93
15
52
Co-N/C (800)
Meso-Co-N/C (700)
Meso-Co-N/C (800)
Meso-Co-N/C (900)
Meso-Co-N/C (800, SBA-15)
Meso-Co-N/C (800, H+)
Meso-Co-N/C (800)
Meso-Co-N/C (800)
Meso-Co-N/C (800)
Meso-Co-N/C (800)
5
6
7
8
9c
10d
11e
12f
84
61
84
a
Reaction conditions (unless specified otherwise): 1a (0.5 mmol), catalyst (0.5
mol % Co), 3.7 equiv. aq. NH3 (25-28% NH3 basis), 5 bar O2 , 1 ml t-amyl alcohol, 130
oC, 18 h. Conversions and yields were determined by GC (using biphenyl as
b
standard) and confirmed by GC-MS. In case of lower yields, benzaldehyde was
detected as a minor product. c DMF. d 1,4-dioxane. e THF. f 5 bar air.
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
Running cycle
Fig. 1 Schematic illustration of the synthetic process of Co-N/C, meso-Co-N/C
catalyst: chemical structure of (a) bidppz, (b) the corresponding Co-bidppz
polymer.
Fig.
2 Meso-Co-N/C(800)-catalyzed oxidative synthesis of benzonitrile. Reaction
conditions: 1a (0.5 mmol), catalyst (0.5 mol % Co), 3.7 equiv. aq. NH3 (25-28% NH3
basis), 5 bar O2, 1 ml t-amyl alcohol, 130 oC, 18 h. Yields were determined by GC.
2 | J. Name., 2012, 00, 1-3
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800 °C without using any template afforded only 10 % yield of
2a (Table 1, entry 3). However, to our delight, the prepared
meso-Co-N/C catalysts using silica colloid as hard template,
showed excellent activity for the model reaction, and meso-
Co-N/C (800) exhibited the best catalytic performance to give
2a in 95% yield at 130 °C (Table 1, entries 4-6), such a
substantial performance improvement ensuring that the
presence of porous structures was very essential for the
reaction. Upon employing ordered mesoporous silica SBA-15
instead of silica colloid as hard template, meso-Co-N/C (800,
SBA-15) was also found to be an excellent catalyst producing
2a in 93% yield (Table 1, entry 7).
To unveil whether transition metal acts as the activity site or
it only facilitates the formation of active nitrogen-carbon
functional sites, we examined the catalytic efficiency of meso-
Co-N-C (800) after reducing the Co content under identical
conditions. The as-synthesized meso-Co-N/C (800) was
immersed in aqua regia to reduce the surviving Co species
(from 0.44 wt % to 0.09 wt %) and the resulting material is
denoted as meso-Co-N/C (800, H+). Meso-Co-N/C (800, H+)
showed a sharp performance decline on the conversion of
benzyl alcohol, demonstrating that the Co species may serve
as active sites to catalyze the reaction (Table 1, entry 8).
bidppz polymer consists of alternating chains of ligand and
cobalt, which hinders the agglomeration of cobalt species
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DOI: 10.1039/C6CY00195E
during the subsequent heat treatment. It was reasonable to
speculate that the active cobalt species that survived from acid
leaching may exist at the atomic or subnanoscale level. In
addition, element mapping (Fig. 3c) revealed that C, N, Co and
O species were homogeneously distributed throughout the
whole mesoporous structure, undoubtedly signifying that the
active cobalt species were uniformly distributed in the N-
doped carbon matrix. Fig. 3d deduced well-defined linear array
of mesoporous structure of Meso-Co-N/C (800, SBA-15),
reflecting the successful introduction of SBA-15-type
mesoporosity. For comparision, bulk aggregations were
Besides t-amyl alcohol, a series of solvents including N,N-
dimethylformamide (DMF), 1,4-dioxane and THF, were also
examined for the reaction, but the yield of benzonitrile (2a) all
decreased to some extent, suggesting that t-amyl alcohol was
the best solvent for the target reaction among the investigated
solvents (Table 1, entries 9-11). The conversion under 5 bar air
proceeded in a comparable result to that under 5 bar of pure
oxygen (Table 1, entry 12). Furthermore, the meso-Co-N/C
(800) catalyst could be reused at least five times with a slight
decrease in activity, which was probably due to physical loss
during catalyst recovery and recycling (Fig. 2). The results were
in accordance with the inductively coupled plasma (ICP) mass
analysis experiments, in which only traces of Co (less than 0.4%
of the total cobalt) were detected in the solution collected by
hot filtration after reaction.
The surface morphology of meso-Co-N/C (800) was
determined by SEM (Fig. 3a), which clearly indicated the
introduction of spherical pores into the resulting material.
Representative transmission electron microscopy (TEM) for
meso-Co-N/C (800) was displayed in Fig. 3b. The
interconnected vesicle-like frameworks of the catalyst were
obviously observed, and the pore sizes had a range from
several to dozens of nanometers. However, we hardly
observed any metal-containing nanoparticles by careful
HRTEM inspection because the etching agent (hydrogen
fluoride) dissolves both the inorganic templates and
metal/metal oxide nanoparticles (Fig. S2a†), although there
was 0.43 at% Co species survived from acid leaching, as
indicated by X-ray photoelectron spectroscopy (XPS) analysis.
X-ray diffraction (XRD) pattern also further proved the absence
of discernable metallic Co phase and CoOx species (Fig. S3†).
The selected-area electron diffraction (SAED) image revealed
the multicrystalline structure of the sample (Fig. S2b†).
Whereas, the energy-dispersive X-ray (EDX) spectrum proved
clearly the presence of cobalt (Fig. S2c†). Given that the Co-
Fig.
3 (a) SEM image, (b) TEM image, (c) large-area SEM image along with the
corresponding C-Ka, N-Ka, Co-Ka and O-Ka elemental maps of the meso-Co-N/C (800)
catalyst; d) TEM image of the meso-Co-N/C (800, SBA-15) catalyst; e) SEM image of Co-
N/C (800) catalyst.
a
b
c
1600
1400
1200
1000
800
600
400
200
0
500
450
400
350
300
250
200
150
100
50
780
680
580
480
380
280
180
80
1
2
3
4
5
1
2
3
4
5
0
10
20
30
40
Pore diameter / nm
Pore diameter / nm
Pore diameter / nm
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative Pressure (P/Po)
1 1.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative Pressure (P/Po)
1 1.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative Pressure (P/Po)
1 1.1
Fig. 4 N2 sorption isotherms of (a) meso-Co-N/C (800), (b) meso-Co-N/C (800, SBA-15)
and (c) Co-N/C (800) catalyst. Insets show the pore size distribution from the BJH
method of corresponding samples.
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a
b
presented in Co-N/C (800) prepared without using any
template, and the smooth surface implied their poor porosity
(Fig. 3e). From a significant performance improvement on the
aerobic ammoxidation of alcohols achieved by mesoporous
catalysts, it could be deduced that the microstructure of the
catalysts played pivotal roles in the target reaction. Notably,
the mesoporous catalysts of discrepant morphologies
prepared via different templates under the same pyrolysis
temperature presented parallel activities towards the target
reaction, which clearly demonstrated that the aerobic
ammoxidation of alcohols was not very sensitive to special
pore structure.
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Co-O
DOI: 10.1039/C6CY00195E
Pyridinic N/N-Metal
(398.6 eV)
(779.8 eV)
Co-N
(781.6 eV)
Graphitic N
(401.0 eV)
Spinels
406
404
402
400
398
396
791 789 787 785 783 781 779 777 775 773
Binding Energy (eV)
Binding Energy (eV)
Fig. 5 XPS spectra of (a) N 1s, (b) Co 2p3/2 in meso-Co-N/C (800).
samples were less than 0.6% because of acid leaching.
To undrape the underlying factors affecting the catalytic
activity, the textural properties of all Co-N/C samples were
investigated by N2 adsorption-desorption isotherms at 77 K
(Fig. 4), and the results were summarized in Table 2. The
remarkable hysteresis loop indicated the mesoporous nature
of meso-Co-N/C (800) (Fig. 4a). The mesopore size distribution
was centered at 12.6 nm according to the Barrett-Joyer-
Halenda (BJH) model (inset in Fig. 4a). The BET surface areas of
meso-Co-N/C (700), (800), (900) and (800, SBA-15) were 506
m2/g, 680 m2/g, 641 m2/g and 649 m2/g, respectively (Table 2).
Notably, major surface areas of the templated catalysts
comprised mesopores, the corresponding mesoporous surface
aera 424 m2/g, 523 m2/g, 502 m2/g, 642 m2/g, respectively
(especially the meso-Co-N/C (800, SBA-15), nearly 99% of total
surface area, in spite of ununiform pore size). Whereas, it was
microporosity that dominated in Co-N/C (800) catalyst, of
which the mesoporous surface aera was only 67 m2/g, despite
of the total BET surface area 227 m2/g. Therefore, it could be
speculated that the mesoporous structures exert
overwhelming influence on the excellent performance of the
meso-Co-N/C catalysts, which was attributed to the easy
access to active sites and improved masstransport properties.
In addition, the chemical compositions of all Co-N/C samples
were also characterized by elemental analysis and inductively
coupled plasma atomic emission spectroscopy (ICP-AES). Co, C,
N, and H elements were mainly detected, and their weight
contents were shown in Table 2. The N contents were up to
12.3%, which could assist in a stronger coupling between
between metal species and N-doped carbon, and thus lead to
higher catalytic activity. While, the quantities of Co in the
XPS analysis was performed to analyze the chemical state of
cobalt and nitrogen in the meso-Co-N/C samples (Fig. 5, Fig.
S4† to S6†). The high-resolution N 1s spectrum of the meso-
Co-N/C (800) catalyst was fitted with only two different signals
having binding energies of 398.6 and 401.0 eV (Fig. 5a). The
peak at a binding energy of 398.6 eV could be attributed to
pyridinic N, which should also include a contribution from
nitrogen bound to the metal (Me-N), due to the small
difference between binding energies of N-Me and pyridinic N.8
The peak at 401.0 eV might be assigned to nitrogen in a
9
graphitic structure.8a,
Moreover, the ratio of graphitic
nitrogen to pyridinic nitrogen in the meso-Co-N/C catalysts in-
creased when the pyrolysis temperature was enhanced (e.g.,
o
o
o
1.0 for 700 C, 1.3 for 800 C, and 1.7 for 900 C), implying a
higher graphitization degree at higher temperatures (Fig. S5†).
The high-resolution Co 2p3/2 spectrum of meso-Co-N/C (800)
can be deconvoluted into two peaks with binding energies of
781.6 and 779.8 eV (Fig. 5b), which correspond to nitrogen and
oxygen-coordinated metals, respectively,10 while the weak
shakeup satellite at around 786.0 eV from the main spin-orbit
component was attributed to the surface hydroxyl species (i.e.
Co-OH) (Fig. 5b).10 Previous studies have revealed that the
cobalt species stabilized by nitrogen were of vital importance
in determining the activity of catalysts in oxidation reactions.7b,
11 Therefore, the surviving cobalt species may be stabilized and
actived by nitrogen-coordination which were uniformly
distributed in the N-doped carbon matrix at the atomic or
subnanoscale level, combined with easily accessible
Table 2 Surface Areas, Micropore Surface Areas, Mesopore Surface Areas and Chemical Compositions of all Co-N/C Samples.
sample
SBET
(m2/g)
227
SMicro
(m2/g)
160
SMeso
(m2/g)
67
contents (wt %)
Na
Ca
63.2
Ha
Cob
5.70
Co-N/C (800)
meso-Co-N/C (700)
meso-Co-N/C (800)
meso-Co-N/C (900)
meso-Co-N/C (800, SBA-15)
9.1
12.1
12.3
9.8
1.2
1.3
1.2
1.1
1.0
506
680
641
649
82
157
139
7
424
523
502
642
62.3
69.8
74.7
65.9
0.27
0.44
0.59
0.28
10.4
a Measured by elemental analysis. b Measured by ICP.
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a
b
Table 3 Catalytic aerobic oxidative synthesis of structurally diverse nitriles over meso-
benzonitrile
benzonitrile
100
100
90
80
70
60
50
40
30
20
10
0
Co-N/C(800) catalysta
benzaldehyde
benzyl alcohol
benzaldehyde
90
80
70
60
50
40
30
20
10
0
6
5
4
3
2
1
0
0.6
0.4
0.2
0
slope = 0.29
slope = 2.89
0
1
t (h)
2
0
1
2
Entry
Alcohol
Product
Yieldb
(%)
t (h)
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
Time (h)
0
0.5
1
1.5
2
Time (h)
1
2
95
Fig. 6 Kinetic study for the catalytic aerobic oxidative synthesis of benzonitrile over
meso-Co-N/C (800) employing (a) benzyl alcohol and (b) benzaldehyde as substrate.
Reaction conditions: substrate (0.5 mmol), 35 mg meso-Co-N/C (800) (0.5 mol % Co),
3.7 equiv. aq. NH3 (25–28 % NH3 basis), 5 bar O2, 1 ml t-amyl alcohol, 130 oC. Yields
were determined by GC using biphenyl as an internal standard.
1a
2a
96
2b
1b
3
4
>99
>99
1c
2c
mesoporous structure, were highly beneficial for the catalytic
performance.
After having demonstrated the excellent activity of the
meso-Co-N/C catalysts in the model reaction, it was then
extended to the transformation of a series of structurally
diverse benzylic alcohols under similar reaction conditions. As
can be seen from Table 3, various benzylic alcohols were
selectively converted into the corresponding benzonitriles in
good to excellent yields, albeit with 0.5 mol % Co catalyst
loading. The desired nitriles were obtained in >96% yields
when benzylic alcohols substituted with strongly electron-
donating groups such as p-OMe were employed (Table 3,
entries 2-5). The different substituted positions on the phenyl
ring had a marked influence on the reactivity of the reaction.
As an example, the methyl group at ortho position showed a
little lower activity to the desired nitrile in comparison with
that in the para and meta positions (Table 3, entries 6-8).
Apart from electron-donating groups, benzylic alcohols
substituted with various electron-withdrawing groups, such as
−F, −Cl, −Br, −NO2 and –CF3, were selectively transformed
(Table 3, entries 9-14). Notably, even more sensitive allylic
alcohols such as cinnamic alcohol could be converted into the
corresponding nitrile in 90% yield (Table 3, entry 15). It is
worth noting that heterocycles such as pyridine-2-methanol
and thiophene-2-methanol could also be used as substrates in
the title reaction and afforded the heterocyclic nitriles in up to
88% yield (Table 3, entries 16-17). Attempts were made to
oxidize the challenging aliphatic alcohols. Due to the inactive
aliphatic alcohols, a lower yield of the product was observed
(Table 3, entry 18).
1d
2d
5
6
7
>99
>99
>99
1e
2e
1f
2f
1g
2g
8
9
97
94
95
1h
2h
2i
2j
1i
1j
1k
1l
10
11
12
13
>99
97
2k
2l
>99
1m
2m
14
82c
1n
1o
2n
2o
2p
2q
15
16
90
85
1p
1q
17
18
88
After reaction, a trace amount of benzaldehyde was
detected by GC-MS, which clearly exhibited that benzaldehyde
was most likely to be the reaction intermediate. To gain more
insights into the reaction pathway, a kinetic study of the
transformation of benzyl alcohol was carried out, and it was
1r
12d
2r
a Reaction conditions (unless specified otherwise): 1 (0.5 mmol), 35 mg meso-
Co-N/C (800) catalyst (0.5 mol % Co), 3.7 equiv. aq. NH3 (25-28 % NH3 basis), 5
bar O2, 1 ml t-amyl alcohol, 130 oC, 18 h. Determined by GC and confirmed
b
by GC-MS. c 24h. d 70mg catalyst, 1 ml 2:1 water-n-heptane, 140 oC, 24 h
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intermediate; 3) aerobic oxidative dehydrogenation of the
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a
nitrile. Notably, thisDOuI:n1s0t.a10b3l9e/C6imCYi0n0e195iEs
aldimine to
immediately oxidized to give nitrile as the final product. In this
reaction sequence, water is produced as the only by-product.
In a final set of experiments, we examined the synthetic
utility of our catalyst system under scale-up conditions. We
performed gram scale reactions for benzyl alcohol as shown in
Scheme 2, we obtained the corresponding products
benzonitrile in yield of 92%, albeit employing very low amount
of catalyst (0.3 mol % Co).
Scheme 1 Possible reaction pathway for the meso-Co-N/C-catalyzed oxidative synthsis
of benzylic nitriles.
Scheme 2 Gram-Scale Reactions: 10 mmol benzyl alcohol, 2.8 ml aq. NH3 (25-28 % NH3
basis), 430 mg catalyst (0.3 mol % Co), 5 bar O2, 9 ml t-amyl alcohol, 130 oC, 36 h. Yields
were determined by GC.
Conclusions
In summary, the meso-Co-N/C catalyst derived from Co-
bidppzpolymer demonstrates superior catalytic activity in the
observed that benzaldehyde was detected along with the aerobic oxidative synthesis of nitriles from alcohols applying
consumption of the starting benzyl alcohol (Fig. 6a). The plot
for the product can be well fitted with pseudo first-order
reaction kinetics in the range of 0 to 2h, and the corresponding
rate constant was 0.29 h-1. Notably, when benzaldehyde was
subjected as the substrate under the standard condition as
shown in Fig. 6b, the corresponding rate constant was 2.9 h-1,
10-fold bigger than that for the whole reaction. These results
clearly indicated that benzaldehyde is the reaction
intermediate and the rate of the whole reaction depends on
the catalytic oxidation of benzyl alcohol to benzaldehyde. The
apparent activation energy, Ea, is determined from the
Arrhenius plot in the temperature range of 110-140 oC and the
value was 61.5 kJ mol-1 (Fig. 7a-b). Finally, the relative rates for
the aerobic ammoxidation of para-substituted benzyl alcohols
(CF3, Cl, H, Me, OMe) were investigated (Fig. 7c). A reasonable
linear relationship between log(kX/kH) and Brown-Okamoto
constant σ+ was established, and the resulting reaction
constant ρ was -1.2, indicating the intermediary of cationic
species were involved in the reaction.
molecular oxygen and aq. ammonia. A series of benzylic
alcohols, allylic alcohols and also heterocycle alcohols could be
converted to their corresponding nitriles in good to excellent
yields. The outstanding catalytic performance of the prepared
catalysts is strongly associated with their well-defined
mesoporous structures, high mesoporous surface area, and
homogeneous distribution of numerous active cobalt species.
Moreover, the as-prepared catalyst is easily recycled and can
be reused at least five times without a significant loss of
catalytic activity. This work would provide an alternative, high-
efficiency and environmentally benign methodology for the
synthesis of nitriles.
Experimental
Materials
11,11’-bis(dipyrido[3,2-a:2’,3’-c]phenazinyl
(bidppz)
was
All the above-mentioned experimental evidences indicate
that the present meso-Co-N/C-catalyzed transformation
possibly proceeds through the following sequence of reactions
(Scheme 1): 1) aerobic oxidative dehydrogenation of an
alcohol to an aldehyde, which had been proved to be the
turnover- limiting step; 2) dehydrative condensation of the
aldehyde and ammonia to an aldimine via a hemiaminal
purchased from Jinan Henghua Sci. & Tec. Co., Ltd. All starting
materials and solvents were obtained from commercial
suppliers and used without further purification.
Catalyst preparation
Synthesis of SBA-15.12 A solution of P123 : 2 M HCl : TEOS :
b
c
a
-1
-1.2
-1.4
-1.6
-1.8
-2
0.4
413K
403K
393K
383K
0.6
p-OMe
y = 0.3132x - 0.005
R² = 0.9969
0.2
0.5
0.4
0.3
0.2
0.1
0
p-Me
p-H
0
-0.2
-0.4
-0.6
-0.8
-1
y = -7.404x + 16.754
R² = 0.9952
y = 0.2009x + 0.0019
R² = 0.9953
p-Cl
y = 0.1169x - 0.0053
R² = 0.9921
-2.2
-2.4
-2.6
-2.8
p-CF3
y = 0.0787x + 0.0004
R² = 0.9985
2.4
2.45
2.5
2.55
2.6
2.65
-0.5
-0.25
0
δ+
0.25
0.5
0.75
0
0.5
1
t (h)
1.5
2
103/T
Fig. 7 (a) Time-on-stream course of conversion under different temperatures. Reaction conditions: benzyl alcohol (2 mmol), 35 mg meso-Co-N/C(800), 3.7 equiv. aq. NH3 (25-28%
NH3 basis), 5 bar O2, 1 ml t-amyl alcohol. (b) Arrhenius plot for the catalytic aerobic oxidative synthesis of benzonitrile from benzyl alcohol. Yields were determined by GC using
biphenyl as an internal standard. (c) Hammett plot for the aerobic ammoxidation of the para-substituted benzyl alcohol, the lg(kX/kH) was obtained from the ratio of conversion
with reaction time of 1.5 h.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6CY00195E
Journal Name
ARTICLE
degassed at 300 oC for 6 h before the measurements. The pore
size distribution (PSD) plot was recorded from the adsorption
H2O= 2 : 60 : 4.25 : 15(mass ratio) was stirred for 12 h at 40 oC,
and then hydrothermally treated at 110 oC for 24 h. The
obtained solid was thoroughly washed with water, dried in branch of the isotherm based on the Barrett-Joyner-Halenda
o
o
(BJH) model. Scanning electron microscope (SEM) images were
conducted on a JSM-7800F microscope operating at an
acceleration voltage of 20 kV. Transmission electron
microscope (TEM) images were acquired with JEM-2100
microscope. Surface compositions were determined by X-ray
photoelectron spectroscopy (XPS) using Thermo Scientific
ESCALAB 250Xi instrument with Al Kα radiation anode (hv
=1486.6 eV), and the C 1s line (284.8 eV) was used as the
reference to correct the binding energies (BE).
vacuum at 100 C overnight, and calcined at 550 C for 8 h to
remove the template. (P123: Ttriblock copolymer
EO20PO70EO20, Mw = 5800; TEOS: tetraethyl orthosilicate).
Synthesis of meso-Co-N/C catalysts. The typical meso-Co-N/C
catalysts were prepared as follows: To synthesize Co-bidppz
polymer, 270 mg (0.49 mmol) bidppz and 123.2 mg (0.49 mmol)
Co(OAc)2▪4H2O were added into 40 mL DMF under vigorous
o
stirring. The mixture was refluxed at 160 C for 1h. Then 400
mg template material was added into the mixture under
vigorous stirring. After evaporation of DMF at 180 oC, the
obtained Co-bidppz/template composites were then pyrolyzed
under flowing nitrogen with a heating ramp rate of 5 C/min to
desired temperatures (700-900 C) and the temperature was
kept for 2 h. Finally, 220mg meso-Co-N/C(x) catalysts, where x
indicated the pyrolysis temperature, were obtained after
removal of templates by HF (10 wt.%) etching for 24 h at room
temperature.
Procedure for the synthesis of nitriles
The magnetic stirring bar and corresponding alcohol were
transferred to the glass vial and then solvent was added. The
catalyst was added followed by the addition of aq. NH3. Then
the vial was fitted with septum, cap and needle. The reaction
vials were placed into an autoclave and the autoclave was
pressurized to 5 bar molecular oxygen. The autoclave was
placed into the oil bath to get the required reaction
temperature. The reaction was stirred for required time and
o
o
Synthesis of Co-N/C (800) by direct pyrolysis of Co-dibppz
polymer. In a typical synthesis, 270 mg (0.49 mmol) bidppz and 123 after the completion of the reaction, the autoclave was cooled
to room temperature. The remaining oxygen was discharged
and the samples were removed from the autoclave. To the
individual vials, biphenyl as standard was added and the
reaction product was diluted with t-amyl alcohol followed by
filtration and then analysed by GC and GC mass spectrometry
(GC-MS). Qualitative and quantitative analysis of all products
were made using GC and GC-MS analysis.
mg (0.49 mmol) Co(OAc)2▪4H2O were added into 40 mL DMF under
o
vigorous stirring. The mixture was refluxed at 160 C for 1h. After
evaporation of DMF at 180 oC, the obtained Co-bidppz polymers
were then directly pyrolyzed under flowing nitrogen with a heating
ramp rate of 5 oC/min to 800 oC and the temperature was kept for 2
h.
Synthesis of meso-Co-N/C (800, H+). The perpared meso-Co-
N/C (800) catalyst was dispersed in aqua regia for 24 h. The
solid was washed several times with water and collected by
Recovery and reuse of meso-Co-N/C
o
The recycling of catalyst experiments was carried out using
benzyl alcohol, as model substrate applying standard
procedure under following reaction conditions: 0.5 mmol
benzyl alcohol, 35 mg meso-Co-N/C (800), 140 µl aq. NH3, 130
oC, 18 h, 5 bar O2, 1 ml t-amyl alcohol. After completion of the
reaction, in each run the catalyst was separated by
centrifugation, washed with methanol and calcined at 400 oC
under N2 for 2h. Then the catalyst was used for the next run.
Alternatively, after completion of the reaction, the reaction
solution was carefully decanted and the fresh solvent,
substrate and ammonia were added and the reaction was
performed. Conversion and yield were determined by GC
analysis using biphenyl as standard.
centrifugation, finally dried at 80 C overnight in an oven. The
obtained material was denoted as meso-Co-N/C (800, H+).
Catalyst characterization
Thermogravimetric analysis (TGA) of the samples were
conducted by using a Perkin-Elmer TGA-2 thermo gravimetric
analyzer in N2 from room temperature to 900 oC at a rate of 10
oC/min. The Co loadings of the catalysts were measured by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) using a Perkin-Elmer OPTIMA 3300DV. The detection limit
was 0.10 ppm. The X-ray powder diffraction (XRD) patterns of
the samples were collected on
a Rigaku/Max-3A X-ray
diffractometer with Cu Kα radiation (λ = 1.54178 Å), the
operation voltage and current was maintained at 40 kV and
200 mA, respectively. N2 sorption isotherms were measured at
77K using a QuadraSorb SI4 Station, and the samples were
Procedure for gram scale reaction
To a Teflon-fitted 300-ml autoclave, the magnetic stirring bar and
corresponding alcohol were transferred and then solvent was
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7
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2013, 6, 283 -2855 (b) R. . agadeesh, . unge, .- .
Pohl, . Radnik, A. Br ckner and . BeDlleOrI,: 1J0..A10m39./CCh6eCmY0.0S1o9c5.E,
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Junge, M.-M. Pohl, J. Radnik, J. Rabeah, H. Huan, V.
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Wienhöfer, M.-M. Pohl, J. Radnik, A.-E. Surkus, J. Rabeah, K.
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added. The meso-Co-N/C (800) catalyst was added followed by the
addition of aq. NH3. The autoclave was pressurized to 5 bar
molecular oxygen. The autoclave was placed into a oil bath to get
the required reaction temperature. The reactions were stirred for
required time and after the completion of the reaction, the
autoclave was cooled to room temperature. biphenyl as standard
was added and the reaction product was diluted with t-amyl alcohol
followed by filtration and then analysed by GC.
Acknowledgements
We gratefully acknowledge financial support from the National
8
9
Natural Science Foundation of China (21403219), (21273225).
Y. Nabae, S. Moriya, K. Matsubayashi, S. M. Lyth, M. Malon, L.
Wu, N. M. Islam, Y. Koshigoe, S. Kuroki and M. Kakimoto,
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Catalysis Science & Technology
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DOI: 10.1039/C6CY00195E
Graphic abstract
High Catalytic Activity of Mesoporous Co-N/C Catalysts for Aerobic Oxidative
Synthesis of Nitriles
Sensen Shang,a,b Lianyue Wang,a Wen Dai,a Bo Chen,a,b Ying Lva and Shuang Gao*a
aDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean
Energy, Dalian 116023, China. E-mail: sgao@dicp.ac.cn; Fax: +86-0411-84379728; Tel: +86-0411-84379728
bUniversity of the Chinese Academy of Sciences, Beijing 100049, China.
we report the synthesis and assessment of novel mesoporous cobalt-coordinated nitrogen-doped
carbon catalysts (meso-Co-N/C) for highly effcient synthesis of nitriles.
1