Co-N-C supported on SiO2: A facile, efficient catalyst for aerobic oxidation of amines to imines

Article abstract of DOI:10.1039/c7ra09516c

We developed a novel, facile preparation method of Co-N-C/SiO₂, which was the pyrolysis of silicone gel containing metal ion and triethanolamine (TEA) prepared by sol-gel process. N₂ adsorption-desorption characterization displayed the sample had high specific surface area and pore volume (220.9 m² g^-1, 0.67 mL g^-1). The active Co appeared to be small particles with size of about 5 nm and was well dispersed on SiO₂. And the highly dispersed cobalt and nitrogen-doped carbon in Co-N-C/SiO₂ served as active phase for the oxidation of amines to imines, thus Co-N-C/SiO₂ could efficiently catalyze the oxidation of amines to imines in solvent-free, air atmospheric conditions, avoiding the use of large excesses of additives, specialized oxidant and solvent.

Full text of DOI:10.1039/c7ra09516c

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Co–N–C supported on SiO : a facile, efficient
2
catalyst for aerobic oxidation of amines to imines†
Cite this: RSC Adv., 2017, 7, 47366
a
a
b
a
a
Chenghui Zhang, Pengshan Zhao, Zongliang Zhang, Jingwei Zhang, Ping Yang,
b
a
a
Peng Gao, Jun Gao
and Di Liu
*
2
We developed a novel, facile preparation method of Co–N–C/SiO , which was the pyrolysis of silicone gel
containing metal ion and triethanolamine (TEA) prepared by sol–gel process. N
2
adsorption–desorption
2
ꢀ1
characterization displayed the sample had high specific surface area and pore volume (220.9 m
g
,
ꢀ1
0
.67 mL g ). The active Co appeared to be small particles with size of about 5 nm and was well
dispersed on SiO . And the highly dispersed cobalt and nitrogen-doped carbon in Co–N–C/SiO served
as active phase for the oxidation of amines to imines, thus Co–N–C/SiO could efficiently catalyze the
Received 28th August 2017
Accepted 3rd October 2017
2
2
2
DOI: 10.1039/c7ra09516c
oxidation of amines to imines in solvent-free, air atmospheric conditions, avoiding the use of large
excesses of additives, specialized oxidant and solvent.
rsc.li/rsc-advances
the preparation of M–N–C materials was mainly by the pyrolysis
of metal–organic framework (MOF) and the complex of metal
Introduction
13
Imines are a class of reactive nitrogen-containing compounds component–nitrogen compounds (phenanthroline, melamine,
14
which are widely utilized in aziridination, reduction, cycliza- etc.) supported on carbon nanotubes, CMK-3, etc. Obviously,
1
tion, condensation and addition reactions. The traditional these methods were not suitable for large-scale preparation due
preparation of imines by the condensation of aldehydes or to the use of expensive and fabricating complex materials.
ketones with primary amines cannot meet the requirements of
Here, we developed a N-doped carbon based metal sup-
2
green chemistry, great efforts have been made to develop more ported by SiO
2
material, which is easily obtained by the pyrol-
3
facile and green synthetic strategies. The oxidation coupling of ysis of silicone gel containing metal ion and triethanolamine
4
amines to imines as a effective method has been developed.
(TEA) prepared by sol–gel process, and the material has low cost
Traditionally, noble–metal complexes (e.g., Ru–, Au–, Pd–) were without the use of expensive materials. In this work, the M–N–
widely used for the oxidation of amines to imines. Although C/SiO
was employed to catalyze aerobic oxidation of amines to
2
5
these complexes exhibited good yields and selectivity, they had imines, and the catalytic performance, applicability and reus-
disadvantages of high cost and poor reusability. In order to ability of the catalyst were investigated in detail.
overcome these drawbacks, some supported noble-metal cata-
6
lysts were employed to achieve this transformation, but harsh
reaction conditions were usually required, and the cost of such Results and discussion
7
noble-metal catalysts was still unacceptable. In addition, Cu-
Initially, the self-oxidative coupling reaction of benzylamine
8
9
10
and Fe -based catalysts, bioinspired catalysis, photocatalysis
was selected as the test reaction to optimize of the reaction
conditions. We rstly investigated four N-doped carbon based
transition metal catalysts, which exhibited from poor to
moderate catalytic properties (Table 1, entries 1–4). Co–N–C
possessed the best catalytic performance among the M–N–C
screened. In order to further improve the activity of Co–N–C,
Co–N–C supported by SiO (8.8 wt% load of Co) was prepared to
2
catalyze the model reaction under the same condition and gave
imine in 92% yield (Table 1, entries 5). Meanwhile, Co–N–C/
11
and mesoporous carbon were developed. These catalytic
systems showed varying degrees of success as well as limitations
like laborious work-up procedures, use of oxidant and/or toxic
organic solvents. Therefore, research of efficient catalyst system
for the oxidation of amines is desirable.
Recently, N-doped carbon base metal-based catalysts (M–N–
C, M ¼ Co, Fe, Ni) were found to be effective for a variety of
12
organic redox transformations. According to the current study,
SiO
gated. Unfortunately, they did not exhibit higher activity (Table
, entries 6–7). The pyrolysis temperature also affected the
catalytic efficiency of Co–N–C/SiO . The activity of Co–N–C/SiO
2
catalysts with 5 wt% and 15 wt% load of Co were investi-
a
College of Chemical and Environmental Engineering, Shandong University of Science
and Technology, Qingdao, 266590, P. R. China. E-mail: ld002037132@163.com
b
1
State Key Laboratory of Bioactive Seaweed Substances, Qingdao Brightmoon Seaweed
Group Co Ltd, Qingdao, 266400, P. R. China
2
2
ꢁ
1
13
pyrolyzed at 500 C signicantly decreased, and the yield of the
†
Electronic supplementary information (ESI) available: IR, H NMR, C NMR
and MS spectral data for compounds. See DOI: 10.1039/c7ra09516c
expected product did not further increased when the pyrolysis
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a
Table 1 Optimization of the reaction conditions
ꢁ
b
Entry
Catalysts
Conditions
Pyrolysis temperature ( C)
Yield [%]
1
2
3
4
5
6
7
8
Cu–N–C
Ni–N–C
Fe–N–C
Co–N–C
Co–N–C/SiO
Co–N–C/SiO
Co–N–C/SiO
Co–N–C/SiO
Co–N–C/SiO
8.8 wt% Cu/6 mol%/solvent-free
8.8 wt% Ni/6 mol%/solvent-free
8.8 wt% Fe/6 mol%/solvent-free
8.8 wt% Co/6 mol%/solvent-free
8.8 wt% Co/6 mol%/solvent-free
5 wt% Co/6 mol%/solvent-free
15 wt% Co/6 mol%/solvent-free
8.8 wt% Co/6 mol%/solvent-free
8.8 wt% Co/6 mol%/solvent-free
Solvent-free
600
600
600
600
600
600
600
500
700
600
600
600
600
600
40
34
35
57
92
81
70
78
92
68
99
27
34
70
2
2
2
2
2
9
c
1
1
1
1
1
0
1
2
3
4
N–C/SiO
2
Co–N–C/SiO
Co–N–C/SiO
Co–N–C/SiO
Co–N–C/SiO
2
2
2
2
8.8 wt% Co/8 mol%/solvent-free
8.8 wt% Co/8 mol%/ethanol
8.8 wt% Co/8 mol%/toluene
8.8 wt% Co/8 mol%/DMF
a
ꢁ
b
Reaction conditions: amine (10 mmol), catalyst (amount of catalyst was calculated based on metal), 110 C, 24 h, 1 atm air. Yield of isolated
c
2
product. Amount of N–C/SiO : 0.424 g.
ꢁ
ꢁ
temperature rose from 600 C to 700 C (Table 1, entries 5, 8–9). pyrolysis for both Co–N–C and Co–N–C/SiO . The size of cobalt
2
ꢁ
Thus, 600 C should be a reasonable choice. The above results particle calculated from the broadening of the Co (111) plane
0
demonstrated the introduction of SiO
catalytic performance of Co–N–C. To clarify the mechanism of the Co–N–C/SiO
Co–N–C/SiO with high activity, the catalysts were characterized corroborated by TEM. Two fringes spacing of 0.205 nm corre-
2
could greatly improve using the Scherrer equation is 3.28 nm, which signied Co in
2
was highly dispersed. This was further
2
0
and analyzed.
sponding to the (111) plane of Co in HRTEM image of Co–N–C/
16
0
Firstly, elemental compositions of Co–N–C and Co–N–C/SiO
2
2
SiO were observed (Fig. 2b, inset), the Co appeared to be ne
were determined, it could be seen from Table 2 that the main particles with size of about 5 nm and was well dispersed on the
components were Co, C, N in the Co–N–C catalyst and Co, C, N, surface of silica (small dark dots in Fig. 2b). And the cobalt
Si, O in the Co–N–C/SiO
detected in small quantities. XRD pattern of the Co–N–C concentrated state (Fig. 2a). Obviously, support-SiO
Fig. 1b) showed ve diffraction peaks. Two weak reections to hinder the Co nanoparticles growth.
2
catalyst. Also hydrogen was also phase in Co–N–C was present in larger, non-uniform and
2
was helpful
(
ꢁ
ꢁ
appeared around 26 and 43 attributed to characteristic of the
15
graphitic-type lattice. The other three diffraction peaks at
ꢁ
ꢁ
ꢁ
around 44.2 , 51.6 , and 76.0 could be assigned to the (111),
(
0
200) and (220) planes of metallic Co (PDF 15-0806), while only
a weak and wide characteristic peak of metallic Co in XRD
ꢁ
pattern of the Co–N–C/SiO
2
could be detected at around 44.2 ,
ꢁ
except for a broad halo peak around 22 ascribed to the (111)
2
reection of the SiO lattice (PDF 76-0931) in Fig. 1a. XRD
0
analysis manifested that the Co phase was produced in the
a
Table 2 Analysis of catalyst composition
Catalysts
H%
C%
N%
O%
Co%
Si%
b
Co–N–C
Co–N–C/SiO
0.45
0.54
54.83
23.06
5.79
4.22
0.56
31.62
36.45
9.79
nd
2
28.57
a
The elements of C, N and H were determined by using CHNS
elemental analyzer and the elements of Co was determined by using Fig. 1 XRD patterns of the as-prepared (a) Co–N–C/SiO
2
, (b) Co–N–
b
X-ray uorescence spectrometer (XRF). nd ¼ not detected.
C.
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2
Fig. 2 TEM images of (a) Co–N–C and (b) Co–N–C/SiO .
Moreover, textural properties of Co–N–C and Co–N–C/SiO2
were analyzed by N adsorption–desorption isotherms (Fig. 3). A
2
type IV adsorption isotherm followed by a well-developed
hysteresis loop was observed for Co–N–C/SiO
mesoporous structure. The specic surface area and pore
volume of Co–N–C/SiO calculated by BET method were 220.9
2
, suggesting the
2
ꢀ1
2
ꢀ1
m g and 0.67 mL g , respectively (Table 3). The pore size
distribution estimated by applying BJH method presented
double pore distribution (3.81 nm, 17.53 nm) (Fig. 3, inset).
Nevertheless, Co–N–C exhibited a type IV-like sorption isotherm
with a poor hysteresis loop. The specic surface area and pore
volume were much smaller than that of Co–N–C/SiO
2
. These
analyses showed that the support SiO facilitated the formation
2
of developed porous organization. Apparently, the developed
porous organization was benecial to the exposure of more
catalytic active sites, and promoted the catalytic activity
accordingly.
Fig. 3
2
N adsorption–desorption isotherms and pore size distribution
of Co–N–C and Co–N–C/SiO
2
.
Subsequently, XPS was performed to acquire more details on
Co–N–C/SiO catalyst structure. As shown in Fig. 4a, the peaks
2
Table 3 The textural properties of Co–N–C and Co–N–C/SiO
catalysts
2
0
15
around 793.2 and 778.4 eV were ascribed to Co , while those
2+
around 786.3, 783.3 and 781.5 eV were in accordance with Co
2
+
2
ꢀ1
ꢀ1
phase, which demonstrated Co was not thoroughly reduced in
the pyrolysis. And the N 1s spectrum could be deconvoluted
Sample
S
BET (m g
)
Pore volume (mL g
)
Pore size (nm)
Co–N–C
Co–N–C/SiO
58.1
220.9
0.15
0.67
3.82
3.81/17.53
into four peaks, including graphitic N (400.7 eV), pyrrolic N
11,17,18
2
(
399.6 eV), and pyridinic N (398.8 eV, 398.3 eV) (Fig. 4b).
Fig. 4 XPS spectra of the Co 2p (a), N 1s (b), and C 1s (c).
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a
2
Table 4 Aerobic photocatalytic oxidation of various amines over Co–N–C/SiO
be
cf
d
Entry Substrate 1
Substrate 2
N.A.
Molar ratio
Time (h) Product 3
Conversion (%) Selectivity (%)
1
2
N.A.
24
26
3a 99
100
100
N.A.
N.A.
3b 96
3c 97
3
4
N.A.
N.A.
N.A.
N.A.
28
30
100
100
3e 95
5
N.A.
N.A.
30
3d 93
3h nd
100
6
7
N.A.
N.A.
N.A.
N.A.
24
24
nd
nd
28
3f
32
8
9
N.A.
N.A.
1/1
24
24
24
24
24
3g 13
62
38
69
82
89
3i
3i
3i
3j
69
1
1
1
0
1
2
1/1.5
1/3
76
100
100
1/3
1
3
4
1/3
1/3
24
24
3k 100
93
35
1
3l
100
a
ꢁ
2
Reaction conditions: benzylamine and other aromatic and aliphatic amines (10 mmol), Co–N–C/SiO (8 mol%), air, 110 C. Molar ratio of
e
b
c
d
benzylamine and different amines. Conversion was calculated based on substrate 1. Selectivity was calculated based on products 3. N.A. ¼
f
no available. nd ¼ not detected.
In addition, the C 1s spectrum was tted with four peaks metal-free N–C/SiO
2
(Table 1, entry 10), which manifested
centered at 288.7, 285.8, 285.0, and 284.5 eV (Fig. 4c), corre- N–C possessed catalytic activity. In sum, the highly dispersed
1
9
2
sponding to the –C]O, N–C, C]C, and C graphite. This was cobalt and nitrogen-doped carbon supported by SiO served
a powerful evidence that carbon was successfully doped with as active phase for oxidation of amines to imines, they played
nitrogen. Previous studies have shown that carbon material superimposing effect in the catalytic oxidation system, which
doped with nitrogen was able to activate the oxygen molecules seemed to explain why Co–N–C/SiO possessed high catalytic
2
2
0
and substrates in oxidation reactions, and our test also activity.
conrmed imine was produced with the yield of 68% over
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a
Table 5 Recyclability of the Co–N–C/SiO
2
catalyst
selectivity of unsymmetric imine (38%) was observed at the
molar ratio of hexylamine to benzylamine of 1 : 1 (Table 4, entry
9). Increasing the molar ratio of hexylamine to benzylamine
could remarkably improve the selectivity to the unsymmetric
imine. Good selectivity for quantitative conversion of benzyl-
amine was achieved when the molar ratio of hexylamine to
benzylamine is 3 : 1 (Table 4, entry 11). Then, the oxidation
cross-coupling of structurally different amines were investi-
gated with this ratio. The tested aliphatic amines gave the cor-
responding unsymmetric imines in high selectivity (Table 4,
entries 12, 13), while the very low selectivity (35%) of unsym-
metrical imine for the reaction between benzylamine and
aniline was obtained (Table 4, entry 14). These data of activity
test demonstrated that cross-coupling of amines could produce
the corresponding imines, but an excess of reagent was
required, which meant the condensation of amines was not an
efficient way for the preparation of cross-coupled imines.
Run b
1
98
2
98
3
95
4
94
5
95
Yield (%)
a
Reaction conditions: amine (10 mmol), catalyst (8 mol%, amount of
catalyst was calculated based on metal), 110 C, 24 h, air. Yield of
isolated product.
ꢁ
b
Aer obtaining the optimized catalyst, the effect of the
catalyst dosage was investigated. When the catalyst dosage
increased from 6 mol% to 8 mol%, there was an improvement
92% vs. 99%) in the reaction yield (Table 1, entry 5 and 11). And
an 8% mol amount of Co–N–C/SiO was sufficient to promote
the reaction. Also the reaction was studied catalyzed by Co–N–C/
SiO (8 mol%) using different solvents (Table 1, entries 12–14).
The results displayed the best yield was obtained under solvent-
free conditions although the desired products could be formed
in solvents. Therefore, we decided to carry out the subsequent
reactions of the amines in the amount of 8 mol% Co–N–C/SiO
under solvent-free conditions. Apparently, this catalytic system
was very simple because no special atmosphere and additives
were required compared with other metal-catalyzed reactions.
With this optimized procedure in hand, the applicability of
the heterogeneous Co–N–C/SiO catalyst for the oxidation self-
(
2
2
Moreover, the recyclability of Co–N–C/SiO was checked by
2
separating the catalyst from the reaction mixture by simple
2

ltration (Table 5). The Co–N–C/SiO was washed with ethanol
2
ꢁ
by sonication for two times, then reduced by H
2
ow at 400 C
2
1
for 2 h prior to reuse as the catalyst in the next run. The Co–N–C/
SiO catalyst could be used repeatedly 5 cycles for amine
oxidation without any signicant loss of the activity (98–94%),
indicating the excellent stability of the Co–N–C/SiO catalyst.
2
2
2
coupling of amines was studied using various combinations
of substrates (Table 4). Benzylic amines gave the imines as the
main product with high conversion and high selectivity aer
different times, and all ring-substituted benzylamines coupled
at a slower rate than benzyl amine. Functional groups such as
methyl (Table 4, entry 2), halogens (Table 4, entries 3–5) were
shown to be highly applicable to the present reaction condi-
tions. Electron donating (Table 4, entry 2) and electron-
withdrawing substituents (Table 4, entries 3–5) smoothly pro-
ceeded. The reactivity of the regioisomer decreased in the order
of para > meta > ortho isomer, which indicated the presence of
a steric effect. However, no reaction occured for the oxidation of
aniline lacking a hydrogen atom under the same condition
Further, we performed TEM characterization for the spent
catalyst aer 1 cycle to investigate the Co particle size distri-
bution, showing no signicant change was observed compared
to fresh catalyst (Fig. 2b vs. Fig. 5).
Furthermore, a plausible reaction mechanism for the
oxidation coupling of amines to imines is shown in Scheme 1.
Firstly, water was detected in the reaction medium by GC-MS.
And the above studies have found that Co and N–C in the Co–
N–C/SiO2 have catalytic activity for the aerobic oxidation of
amines, respectively. Based on these facts and previous litera-
2a,22
ture reports,
two pathways are proposed in terms of different
active phase, (1) Co as active phase: a dehydration reaction of
the benzylamine occurs in the presence of oxidized Co to form
the complex A. Then, A is further oxidized leading to benzyli-
mine B, meanwhile the cobalt is regenerated in this step; (2)
N–C as active phase: both benzylamine and molecular oxygen
are activated on the defect sites of N–C and subsequently
transform into benzylimine and H O intermediates. H O can
(Table 4, entry 6). In addition, a primary aliphatic amine was
also converted to the corresponding coupled imine, but the
conversion and selectivity were lower than those of benzylic
homologues (Table 4, entries 7–8).
Next, we focused on examining the oxidation cross-coupling
of amines over the same catalytic system. Reactions between
benzylamine and n-hexylamine were rstly performed, a low
2
2
2 2
react immediately with another molecular 1a to obtain
Scheme 1 Proposed mechanism for the aerobic oxidation of amines
Fig. 5 TEM image of spent Co–N–C/SiO
2
catalyst.
2
to imines over Co–N–C/SiO .
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ꢁ
benzylimine B. These two pathways proceed via oxidative mixture was heated to 110 C under vigorous stirring for specic
dehydrogenation of the amine to benzylimine B. As aldehyde in time under an air atmosphere. Aer reaction, the mixture was
ꢁ
reaction mixture was observed, we believe the intermediate B cooled to 80 C, 20 mL of ethanol was added to the mixture
reacts with H O generated in the initial step to give the aldehyde which was vigorously stirred to completely dissolution of
2
C with liberation of ammonia, subsequently a dehydration– product, and the catalyst was removed by ltration. The ltrate
condensation reaction of the aldehyde with reactant amine was determined by Shimadzu GC-2000 Plus and Agilent GC-MS
occurs to obtain the nal product 3a.
7890 to obtain characteristic and yield of product. The solvent
evaporated under reduced pressure to gain crude product which
was puried by silica gel ash chromatography using petroleum
ether and ethyl acetate as eluent to afford the product.
Conclusions
In this paper, a novel, simple preparation strategy of Co–N–C/
SiO
2
catalyst was developed. The obtained catalyst benets from
Conflicts of interest
the use of safe and inexpensive starting materials, and exhibits
high catalytic activity for oxidation coupling of amines to imines There are no conicts to declare.
under mild reaction conditions. More work is under way to
further explore its potential application in the eld of oxidative
esterication of alcohols and dehydrogenative coupling of
Acknowledgements
alcohols with amines.
This work was supported by grants from Open Foundation of
the State Key Laboratory of Bioactive Seaweed Substances,
Qingdao Brightmoon Seaweed Group Co Ltd (No. SKL-
BASS1723).
Experimental
Preparation
Preparation step of the catalyst as follows:
Notes and references
(1) Preparation of silicone gel including Co and triethanol-
amine: to a water solution containing Co(NO ) $6H O (1.79 g,
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3
2
2
6.15 mmol) and nitric acid (2 mL) were added with vigorous
stirring. Then, triethanolamine (TEA, 2.80 g, 18.77 mmol) and
tetraethyl orthosilicate (TMOS, 8 mL) were added dropwise to
the solution. Aer the solution became homogeneous, addition
of a spot of NH
a uniform and stable gel.
2) Pyrolysis of silicone gel: before pyrolysis, a precursor was
4
F caused gelation at room temperature, giving
(
ꢁ
obtained by drying the silicone gel at 80 C for 12 h. Then the
precursor was heated to 600 C at a rate of 3 C min in highly
pure N and held at the nal temperature for 2 h in tube
ꢁ
ꢁ
ꢀ1
2
furnace, followed by naturally cooling to room temperature to
obtain the catalyst.
As a comparison, metal-free N–C/SiO was also prepared by
2
the same method. Unsupported M–N–C catalysts were obtained
by pyrolysis of metal–TEA complex.
Characterization
Powder X-ray diffraction patterns of the catalysts were recorded
on a Rigaku diffractometer (Utima IV, 3 kW) by Cu Ka radiation
5557; (d) A. Maggi and R. Madsen, Organometallics, 2012,
31, 451–455; (e) G. Zhang and S. K. Hanson, Org. Lett.,
2013, 15, 650–653.
(40 kV, 30 mA, 0.1543 nm). The size and morphology of the
catalysts samples was determined by using a Transmission
Electron Microscope (TEM, JEOL, JEM-1400Plus). The determi-
nation of elements of the catalysts samples was determined by
using CHNS elemental analyzer (VARIO EL III) X-ray uores-
cence spectrometer (XRF, AXIOS-Petro, 4 kW) and X-ray photo-
electron spectroscopy (XPS, ESCALAB 250Xi, 150 W).
4
5
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In a typical amine oxidation reaction, a mixture of benzylamine
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