Co-N-C catalysts synthesized by pyrolysis of Co-based deep eutectic solvents for aerobic oxidation of alcohols

Article abstract of DOI:10.1039/C8NJ03446J

The selective oxidation of alcohols to the corresponding aldehydes and ketones is of great importance in the academic and industrial fields. A series of excellent nanostructured catalysts comprising cobalt nanoparticles supported on nitrogen-doped carbon (Co-N-C) were thus prepared by pyrolysis of a deep eutectic solvent Co(NO₃)₂·6H₂O/[Bmim]Br supported on commercial carbon. The catalytic activity of the Co-N-C materials was studied in the selective aerobic oxidation of alcohols with molecular oxygen under base-free conditions. The results indicated that the optimized Co-N-C/700 catalyst exhibited excellent catalytic performance in the selective oxidation of both aryl and alkyl alcohols, giving their corresponding aldehydes and ketones in good to excellent yields. Furthermore, the combination of the catalytic results of the control group and the different characterization methods showed that such high catalytic activity is due to the synergistic interaction between the nitrogen-doped carbon support and Co-N species in Co-N-C/700. In addition, the magnetically recoverable Co-N-C catalyst could be easily separated from the reaction system by using an external magnetic field and reused at least five times without an obvious decrease in the catalytic efficiency.

Full text of DOI:10.1039/C8NJ03446J

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Co-N-C Catalysts Synthesized by Pyrolysis of Co-based Deep
Eutectic Solvents for Aerobic Oxidation of Alcohols
Xin Zhaoa, Yan Zhoua, Ai-Ling Jina, Kuan Huangb, Fujian Liuc,*, Duan-Jian Taoa,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The selective oxidation of alcohols to corresponding aldehydes and ketones is of great importance in academic and
industrial fields. A series of excellent nanostructured catalysts comprising cobalt nanoparticles supported on nitrogen-
doped carbon (Co-N-C) were thus prepared by pyrolysis of a deep eutectic solvent Co(NO3)2·6H2O/[Bmim]Br supported on
commercial carbon. The catalytic activity of Co-N-C materials was studied in the selective aerobic oxidation of alcohols
with molecular oxygen under base-free conditions. The results indicated that the optimized Co-N-C/700 catalyst exhibited
excellent catalytic performance in the selective oxidation of both aryl and alkyl alcohols, giving their corresponding
aldehydes and ketones in good to excellent yields. Furthermore, combination of catalytic results of the control group and
the different characterization methods showed that such high catalytic activity is owing to the synergistic interaction
between nitrogen-doped carbon supporter and Co-N species in the Co-N-C/700. In addition, the magnetically recoverable
Co-N-C catalyst could be easily separated from the reaction system by using an external magnetic field and reused at least
five times without obvious decrease in catalytic efficiency.
www.rsc.org/
catalytic performance is still not satisfactory because the
additional base additive was usually required, peroxides were
employed as terminal oxidants, and even, metal nanoparticles
Introduction
The selective oxidation of alcohols to corresponding
aldehydes/ketones is an important reaction in chemical
industry, to build up complex molecules from readily available
substrates.1,2 Recently,
are prone to leaching or/and aggregation, leading to the
deterioration of the catalytic activity in some cases.9-11 Given
the demand for a sustainable and environmentally friendly
reaction process with zero additives and by-products, the
construction of an environmentally benign base-free aerobic
oxidation system is highly desirable. In addition, further
promoting the catalytic activity of the transition-metal
nanoparticle-based catalysts should be the most direct
approach to meet the requirements of green chemistry.12
Therefore, it is essential to develop a novel synthesized
strategy for metal nanoparticles catalysts for efficient
heterogeneous catalysis with high recyclability.
a large number of metal-based
heterogeneous catalysts have been developed to promote the
chemical transformation from benzyl alcohol to benzaldehyde,
since heterogeneous catalysts are preferred industrially for
their low cost and ease of separation and reuse. For example,
Beller et al. reported that Co3O4−N@C was a promising catalyst
in the oxidation of alcohols, in which the cross and self-
esterification of alcohols to esters proceed in good to excellent
yields.3 Li et al. prepared MOF-derived Co catalysts, which
exhibited excellent catalytic performance in selective oxidation
of benzyl alcohol.4 Also, many other supported cobalt-based
catalysts such as N-doped cobalt-graphene composite, Co-N-C
immobilized on CNTs, and Co3O4/AC (activated carbon) were
found to have good catalytic activities.5-8 Nonetheless, the
Ionic liquids (ILs) typically served as excellent solvents and
functional materials have been attracted much more attention
in diverse areas such as organocatalysis, gas separation,
biomass degradation and electrolytes in batteries and fuel
cells.13-17 Deep eutectic solvents (DESs) represent a new class
of ILs analogues have attracted great concern because of their
diverse and tunable compositions, structures, and properties,
as well as various potential applications and can be
acknowledged as promising alternatives to ILs.18 They are
usually obtained by the complexation of a hydrogen bond
receptor (HBR) with a hydrogen bond donor (HBD).19 Notably,
metal-containing DES, is a novel represent with abundant
carbon and nitrogen ligands, and high metal ion contents.
Benefit from the strong hydrogen bonding between the HBR
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and HBD, the metal-containing DES is expected to be good
candidates as the precursor template or sacrificial agent to
design various porous nanostructured metal and nitrogen bi-
modified carbon material (M–N–C).
Table 1. Optimization of reaction conditions for oxidation of benzyl alcohol to
benzaldehyde.
Based on the above considerations and inspired by the
strategy that construction of carbon supported metal
nanocomposites via the pyrolysis of metal-coordinated
copolymers,20-22 thus we designed and prepared several metal-
containing DESs and fabricated corresponding M–N–C
materials, which are expected to endow the following
advantages. First, compared with other traditional metal
solutions, the metals in the DESs are uniformly dispersed and
have strong hydrogen bonding with the ligand, affording stable
microenvironments of encapsulated metals. Second, the rich
nitrogen species could partially dope into the as-prepared M–
N–C materials during pyrolysis, and the evaporation of non-
metallic elements would contribute to a small portion of the
pore structures in such materials. And all these features render
the metal-containing DES derived M–N–C an exciting materials
via a novel pyrolysis method.
Entry
1
Catalyst
Solvent
Toluene
Toluene
Toluene
Toluene
Octane
DMF
Conv. (%)
57
Yield (%)
56
Co-N-C/600
Co-N-C/700
Co-N-C/800
Co-N-C/900
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
2
92
92
3
74
72
4
67
66
5
99
15
6
52
51
7
DMSO
50
50
8
Hexanol
Toluene
Toluene
Toluene
Toluene
38
38
9a
10b
11c
12d
57
57
26
26
56
55
10
7
Reaction conditions: 1a (1.0 mmol), catalyst (10 mol% Co), solvent (5 mL), O2
balloon, 100 °C, 36 h. And the main primary by-product for the selective
oxidation of benzyl alcohol is benzoic acid. a 12 h. b 50 °C. c Catalyst (5 mol% Co). d
Filled with N2 balloon.
Herein, we designed and prepared a series of highly
efficient DES-derived cobalt catalysts (nitrogen-doped carbon
embedded cobalt nanopaticles, Co-N-C). They were prepared
simply by pyrolysis of the precursor commercial carbon
impregnated a DES consisting of 1-butyl-3-methylimidazolium
bromide and cobalt salts. Combining abundant exposed Co-N
active sites, fast diffusion of reactant/product, and improved
structural stability offered by the sacrificed DES retained rich
pores and given the catalyst a relatively high surface area, they
gave rise to largely enhanced catalytic activity as well as high
durability in the selective aerobic oxidative reaction of benzyl
alcohols. Furthermore, the prepared Co-N-C catalytic system
features a broad substrate scope for aromatic and aliphatic
alcohols, giving their corresponding aldehydes in good to
excellent yields. And these catalysts could also be quickly
recovered by magnetic separation and reused without losing
activity. It is believe that this pyrolyzing strategy of the Co-
containing DES can be expanded to construct other catalytic-
active M-N-C catalysts for a wide range of applications.
Inspired by these intriguing results, we further prepared a
series of C-containing, N-containing, and Co-based materials
and employed them as reference catalysts to support the
outstanding catalytic property of the Co-N-C/700 composite in
aerobic oxidation. The results are listed in Table 2. It is
indicated that the oxidation reaction hardly occurred without
any catalysts, while Vulcan XC72R carbon and [Bmim]Br were
inactive for oxidation of benzyl alcohol (Table 2, entries 1-3).
The catalysts Co salts Co(NO3)2 and Co3O4 also induced low
benzaldehyde yields of 33% and 47%, respectively (Table 2,
entries 4,5). Notably, the Co-C/700 and N-C/700 catalysts
displayed very poor catalytic performance under identical
conditions (Table 2, entries 6,7), showing the importance of N-
doping and metallic Co in the composite for the oxidation
reaction. Meanwhile, Co-N/700 had few activity in the
oxidation of benzyl alcohol because of the absence of Vulcan
XC72R carbon as supporter in the pyrolysis process (Table 2,
entry 8). Co-C prepared by impregnation method exhibited
good conversion but poor selectivity as well as Co(NO3)2 did,
whereas Co-C lost the catalytic activity after the reduction with
Results and discussion
Alcohol oxidation reactions were performed under a base- NaBH4 (Table 2, entries 9,10). Furthermore, various Co and N-
free condition using an atmospheric pressure of oxygen as the containing precursors using other cobalt salts and N-doping
terminal oxidant. The experimental results are summarized in sources were studied for the preparation of Co-N-C/700
Table 1. The Co-N-C materials were assessed as catalysts for catalysts, in which moderate to good yields of benzaldehyde
the aerobic oxidation of benzyl alcohol in toluene at 100 °C. It could be achieved (Table 2, entries 11-13). Thus, these control
can be seen that Co-N-C materials displayed various catalytic experiments suggested that the synergic interaction between
activities in the oxidation of benzyl alcohol (Table 1, entries 1- Co species and N-doped carbon played a significant role in
4), in which Co-N-C/700 was the best active catalyst to obtain determining the excellent performance of Co-N-C/700 in the
a
92% conversion of benzyl alcohol and 92% yield of oxidation of benzyl alcohol.
benzaldehyde. After screening the reaction parameters, the
optimized conditions can be set as follow: 10 mol% Co-N-
C/700, toluene as the solvent, O2 as the oxidant, temperature
of 100 °C and reaction time of 36 h (Table 1, entries 5-12).
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the face-centered cubic structure of metallic cobalt (PDF#15-
0806), respectively, indicating the existence of metallic Co.17,
22-24
Table 2. Selective oxidation of benzyl alcohol catalyzed by various catalysts
Entry
1
Catalyst
-
Conv. (%)
<1
Yield (%)
<1
<1
14
The metallic Co might be formed by reducing Co-based
precursors with carbon species at high temperatures.
Moreover, it is seen that with the increase of pyrolyzing
temperature, more intense diffraction peaks of the cobalt
species were detected, implying the presence of the high
proportion of reduced Co at high temperatures.
2
Vulcan XC72R
[Bmim]Br
Co(NO3)2
Co3O4
<1
3
18
4
98
33
5
47
47
6
Co-C/700
N-C/700
<1
<1
<1
9
7
<1
8
9a
Co-N/700
Co-C
9
99
28
10b
11c
12d
13e
14f
Co-C (NaBH4)
Co-N-C/700
Co-N-C/700
Co-N-C/700
Co-N-C/700
<1
<1
89
90
87
86
61
61
73
71
Reaction conditions: 1a (1.0 mmol), catalyst (10 mol% Co), Toluene (5 mL), O2
balloon, 100 °C, 36 h. And the main primary byproduct for the selective oxidation
a
of benzyl alcohol is benzoic acid. Co-C was prepared by an impregnation
method. b Reduction with NaBH4. c Co(OAc)2·4H2O. d CoCl2·6H2O. e [Bpy]Br. f TBAB.
In order to clarify the remarkable catalysis behavior of Co-
N-C/700 in the aerobic oxidation of alcohols, a series of
characterization techniques were carried out to get the
characteristics of the catalysts. As shown in Table 3, the Co
content of the composites was 8−11 wt %, and the nitrogen
content decreased from 4.5% to 1.4% when the pyrolysis
temperature increased from 600 °C to 900 °C. Compared with
other Co-based materials, the Co-N-C/700 had a higher surface
area, higher pore volume, and larger pore size. This shows that
Figure 1. XRD patterns of (a) Co-N-C/600, (b) Co-N-C/700, (c) Co-N-C/800, (d) Co-N-
C/900.
Raman spectroscopy is commonly used to measure carbon-
based catalysts. D band at 1325 cm-1 and G band at 1590 cm-1
of the Co-N-C composites were detected in the Raman spectra
(Figure 2). It is known that the D band is related to the
disordered structure of the materials, including the defects in
a
relative high temperature would probably cause the
decomposition of the mesoporous structure and leads to the
lower surface area.23 The large surface area and pore size of
the Co-N-C/700 composite was beneficial to the Co metal
dispersions and could promote the mass transfer process of
reactants and thus resulted in a good reactivity, which could
be observed by SEM and XRD results.
graphene lattice and amorphous carbons. The
G band
corresponds to the vibration peak of the ideal sp2 hybridized
carbon atom, showing the integrity of graphene layer. Then
the intensity ratio of D band to G band, termed ID/IG, is used to
estimate the defects of carbon-based nanostructures.25-27 It is
showed that increasing the pyrolyzing temperature, the ID/IG
ratio decreased slightly from 1.36 at 600 °C to 1.09 at 800 °C,
indicating the increased crystallization degree of graphitic
carbon, which was in good agreement with those results
observed in XRD patterns.
Figure 1 shows XRD patterns of the Co-N-C composites. A
broad peak at 25° could be assigned to the (002) interlayer of
graphite-type carbon sheets. For Co-N-C/600, no diffraction
peak of cobalt was found from the XRD patterns, showing that
the cobalt species were amorphous at 600 °C. For other three
Co-N-C composites, the peaks at 44.1°, 51.4°, and 75.9°
indexed to the (111), (200), and (220) of crystalline planes of
Table 3. Properties of Co-N-C catalysts
Sample
SBET (m2 g-1)
Pore volume (cm3
Pore size (Å)
Content (wt %)
g-1)
Ca
Na
Ha
Cob
8.1
Co-N-C/600
Co-N-C/700
Co-N-C/800
Co-N-C/900
96
0.03
0.07
0.03
0.04
2.4
3.3
2.0
1.8
75.6
76.3
84.2
86.3
4.5
3.3
1.5
1.4
0.8
0.9
0.6
0.6
216
165
177
9.6
10.5
11.6
a Measured by elemental analysis. b Measured by ICP-OES.
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comparison, the HRTEM image indicated that Co nanoparticles
in the Co-C/700 material were seriously aggregated with much
bigger clusters (Figure S3). This finding demonstrated that the
N-containing precursor [Bmim]Br plays an important role in
highly dispersing of Co particles on graphitized carbon and
reducing the particle size of Co particles.
Figure 2. Raman spectra of (a) Co-N-C/600, (b) Co-N-C/700, (c) Co-N-C/800, (d) Co-N-
C/900.
Figure 4. TEM images and the size distribution of Co particles for Co-N-C/700.
In order to gain the further insight into the valence states
of Co, N, and O, XP Spectra of N 1s and Co 2p for Co-N-C
catalysts are shown in Figures 5 and 6. As regards the N 1s XP
spectra (Figure 5), three distinct peaks were observed of the
Co-N-C materials with electron binding energies of pyridinic N
(398.9 eV), pyrrolic N (400.4eV), and graphitic N (401.3 eV).29-31
It is found that the percentage of pyridinic N decreased as the
pyrolysis temperature increased, due to the graphitization of
the precursor DES and commercial carbon. The relative
content of pyridinic N in Co-N-C/700 was higher than that in
the other two samples Co–N-C/800 and Co–N-C/900. Some
studies indicated that pyridine-type nitrogen can stabilize the
metal particles and provide the required Lewis basicity to
improve the catalytic activity for the oxidation of hydrocarbons
and alcohols.32-36 Thus, the catalyst Co-N-C/700 showed good
catalytic performance in aerobic oxidation of benzyl alcohol.
Figure 3. SEM images of (a) Co-N-C/600, (b) Co-N-C/700, (c) Co-N-C/800, (d) Co-N-
C/900.
The morphologies and metal dispersions of the Co-N-C
composites were investigated using SEM and HRTEM. As
shown in Figure 3, the typical nanosheet structures were
observed in the Co-N-C materials. For the Co-N-C/700 catalyst,
the scanning electron microscopy image was suitably uniform
in size and homogeneous in morphology. At elevated
pyrolyzing temperature (800 and 900 °C), the size of observed
nanosheets decreased and aggregation gave rise to low BET
surface areas, which was in accordance with the results from
nitrogen adsorption/desorption analysis. In addition, it can be
seen from Figure 4 that the Co particles were well-dispersed
on the N-doped carbon in Co-N-C/700 with a uniform particle
size. The (111) and (200) d-spacing of metallic Co nanoparticles
could be clearly measured at 0.208 nm and 0.177 nm,28 which
is in good agreement with the XRD measurements. For
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Figure 5. N 1s spectra for (a) Co-N-C/600, (b) Co-N-C/700, (c) Co-N-C/800, (d) Co-N-
C/900.
Figure 6. Co 2p spectra for (a) Co-N-C/600, (b) Co-N-C/700, (c) Co-N-C/800, (d) Co-N-
C/900.
Moreover, the O 1s spectrum was deconvoluted into the
peak at 532.1 eV (Figure S4). According to the literature37-40, it
could belong to both adsorbed oxygen and a little amount of
C-O group in the graphited carbon, indicating the absence of
any Co−O species in Co-N-C catalysts. In the cobalt region, the
In order to investigate the applicability of the Co-N-C/700
catalyst, the selective oxidation of various alcohols were
further studied with molecular oxygen under base-free
conditions (Table 4). It can be seen that various benzyl alcohols
bearing electron-donating and electron-withdrawing groups
could approach the corresponding substituted benzaldehydes
in good to excellent yields under identical conditions (Table 4,
entries1-11). Moreover, by prolonging the reaction time to 48
h, cinnamyl alcohol was oxidized to cinnamyl aldehyde with a
yield of 76% (Table 4, entry 12). Also, acetophenone was
obtained in 91% via the oxidation of 1-phenylethanol (Table 4,
entry 13). Four p-substituted phenylethanols further afforded
the corresponding substituted ketones in 70-84% yields (Table
4, entries 14-17). The oxidations of α-vinylbenzyl alcohol and
5-hydroxymethylfurfural to α-vinylbenzyl aldehyde and 2,5-
furandicarbaldehyde were observed in 77% and 68% yields,
respectively (Table 4, entries 18,19). 3-Pyridylmethanol gave a
95% yield of 3-pyridinecarboxaldehyde (Table 4, entry 20).
However, it was indicated that the oxidations of primary
alcohols were hard performed to give the corresponding
aldehydes with frustrated yields, although the catalyst
amounts of Co-N-C/700 increased (Table 4, entries 21-23).
binding energies in 781.1 eV and 796.8 eV were assigned to Co
2p3/2 and Co 2p1/2 of Co−N, respecꢀvely.29,
41, 42
The broad
peaks at 785.5 eV and 803.5 eV can be attributed to the shake-
up satellite peaks of Co−N. However, no Co0 species was
detected by XPS, implying that all the Co0 nanoparticles were
probably shielded by the thick graphitic layers encapsulating
the Co0 particles so that they could not be probed by the
surface-sensitive XPS.43 Then, we conducted XPS measurement
for the Co-N-C/700 catalyst using Ar+ ion-beam etching. The
results showed that the Co 2p3/2 peak of metallic Co was
successfully observed and appeared at 778.9 eV18, 44, 45 (Figure
S5). But the peaks at 782.5 eV and 794.5 eV were attributed to
Co3+ because of the oxidation of metallic Co to Co3O4 during
the etching procedure.
Therefore, the XPS characterization results demonstrated
that compared with Co0 nanoparticles, the Co–N species in Co-
N-C/700 was believed to be the active sites for highly efficient
aerobic oxidation of alcohols. The encapsulation of Co0
nanoparticles in N-doped carbon nanosheets is hard to
participate in the aerobic oxidation reaction. Furthermore, the
highly catalytic activity over Co-N-C/700 was also attributed to
the contribution of its highest BET surface area (Table 3). The
large surface area is favorable for exposure of catalytically
active sites to reactants and thereby leads to excellent
catalytic performance.
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Table 4. (Contd.)
Entry
17b
Substrate
Product
Conv. (%)
70
Yield (%)
70
Table 4. Selective oxidation of various alcohols catalyzed by Co-N-C/700
18
77
77
Entry
1
Substrate
Product
Conv. (%)
92
Yield (%)
92
19
20
68
95
20
20
21
68
95
20
20
21
2
3
91
83
91
83
21c
22c
23c
4
5
6
7
8
70
>99
>99
98
70
>99
>99
98
Reaction conditions: 1 (1.0 mmol), catalyst (10 mol% Co), Toluene (5 mL), O2
balloon, 100 °C, 36 h. Conversion and yield were determined by GC-MS. b 48 h.
a
c catalyst (20 mol% Co).
81
81
9
72
88
80
76
91
72
88
80
76
91
10
11
12b
13b
14b
15b
16b
83
84
74
83
84
74
Figure 7. Recovery and reuse of the Co-N-C/700 catalyst.
Finally, the reusability of the Co-N-C/700 catalyst in the
aerobic oxidation of benzyl alcohol was explored. The catalyst
could be separated from the reaction mixture using an
external magnet after reaction. Recycling experiment result is
shown in Figure 7. It is indicated that no obvious decreased in
both the conversion of benzyl alcohol and selectivity of
benzaldehyde was observed during five successive recycles. No
significant change in the morphology and structure of Co-
based species was observed in the SEM image and XRD pattern
of the recycled Co-N-C/700 catalyst (Figures S6,S7). Compared
with the fresh catalyst, the Raman spectra characteristic bands
also appeared in the reused Co-N-C/700 (Figure S8). This
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further demonstrated that Co-N-C/700 could be stable enough flow. The resulting solid was cooled to room temperature to
in the aerobic oxidation for several runs.
give Co-N-C/700.
The procedure for preparing Co-C/700 and Co-N/700 were
same as that of Co-N-C/700, but no [Bmim]Br and Vulcan
XC72R carbon were impregnated in the mixture, respectively.
The prepared material Co-N-C/700 was immersed in aqua
regia to remove Co and thus obtain N-C/700.
Conclusions
In summary, we have reported the employment of Co-N-C
composites as an efficient heterogeneous catalyst for direct
and selective oxidation of alcohols into the corresponding
aldehydes and ketones, with molecular oxygen as a clean
oxidant under base-free conditions. The results of catalytic
studies of the control group and several characterization
investigations demonstrated that such high catalytic efficiency
is resulted from the synergistic interaction between nitrogen-
doped carbon supporter and Co-N species in the Co-N-C/700.
We believe that the utilization of deep eutectic solvent as
precursor provides a novel strategy to prepare highly efficient
non-precious metal-based catalysts and also shows a great
potential for the synthesis of carbonyl compounds from
selective alcohol oxidation.
Catalytic characterization
The cobalt contents in the samples were measured by
Inductively Coupled Plasma Optical Emission Spectrometry
(ICP-OES) using an Agilent 720 instrument after the sample
was completely dissolved in the mixture of HNO3/HCl (1/3
ratio). The CHN elemental contents of catalysts was measured
on an Elementar Vario El III instrument.
The BET surface area, pore volume, pore size distribution,
and average pore diameter of catalysts were characterized by
nitrogen adsorption/desorption analysis at 77
K on a
Quantachrome Quadrasorb SI Instrument. Prior to the
measurements, the samples were degassed at 150 °C for 4 h.
Raman spectra were obtained in
a LabRAM HR
Experimental
spectrometer with an excitation wavelength at 632 nm with 3
cm−1 spectral resolution. Powder X-ray diffraction (XRD)
Materials
studies were conducted on
a Rigaku RINT-2200 X-ray
VULCAN XC72R with Code XVC72R and CAS No. 1333-86-4 was
obtained from Cabot Corporation Prod. 1-Butyl-3-
methylimidazolium bromide ([Bmim]Br, 99%), butylpyridinium
bromide ([Bpy]Br, 99%), and tetrabutylammonium bromide
(TBAB, 99%) were purchased from the Centre of Green
Chemistry and Catalysis, LICP, CAS. Co(NO3)·6H2O (99%),
diffractometer with a Cu Kα source at 40 kV and 20 mA.
The morphology and sizes of the samples were studied by
high-resolution transmission electron microscopy (HRTEM,
JEM-2100 from JEOL). Scanning electron microscope (SEM)
images were carried out on a HITACHI SU8020 cold field-
emission instrument. X-ray photoelectron spectroscopy (XPS)
benzyl alcohol (99%), 1-phenylethanol (98%),
3-
measurements were performed on
a Thermo Scientific
pyridinemethanol (98%), 5-hydroxymethylfurfural (97%),
ethanol (99%), and methylbenzene (99.5%) were purchased
from Adamas Reagent (Shanghai). Other derivatives of
benzene alcohol, phenylethanol, and primary alcohol were
purchased from Energy Chemical (Shanghai). O2 with a purity
of 99.99% was commercially available. All starting materials
and solvents were obtained in the highest purity grade
possible and used as received.
ESCALAB 250Xi system equipped with a hemispherical analyzer
and using monochromatic Al Kα radiation source.
Catalytic reaction
The aerobic oxidation of alcohol was carried out in a 10 mL
round-bottomed flask equipped with magnetic stirrer.
Typically, alcohol (1 mmol), toluene (5 mL), and Co-N-C/700
catalyst (10 mol %) were added into the reactor. The reaction
mixture was stirred at 100 °C under oxygen atmosphere for 36
h. After the reaction, the products were separated from the
catalyst by an external magnet, and then diluted with ethanol
and identified by gas chromatography-mass spectrometry (GC-
MS) (Thermo Trace 1300 GC-ISQ) and quantified by a GC-FID
(Agilent 7890B) equipped with a capillary column HP-5 (methyl
polysiloxane, 30 m × 0.32 mm × 1 μm). Trimethylbenzene was
used as internal standard to quantify the products. The
detailed analysis conditions were described as follows: The
temperatures of injector and detector were 280 and 250 °C,
respectively. The column temperature was increased from 60
to 280 °C at 10 °C·min−1, holding at 280 °C for 2 min.
Catalyst preparation
All of the Co-N-C catalysts were prepared through in situ
pyrolysis methods. The catalysts were labeled as Co-N-C/T
(where T is the pyrolysis temperature, which can be 600, 700,
800, or 900 °C). For example, the typical preparation
procedure of Co-N-C/700 is described as follows:
Co(NO3)·6H2O (1.83 g) and [Bmim]Br (4.38 g) were added to a
20 mL vial. A DES Co(NO3)2·6H2O/[Bmim]Br (Figure S1) was
thus obtained after stirring the mixture for 10 min at room
temperature. Then 2.0 g Vulcan XC72R carbon and 40 mL
ethanol were added to the DES and the whole reaction
mixture was stirred at 25 °C for 12 h. Subsequently, the solvent
ethanol was removed using rotary evaporation and dried at 70
°C for 12 h. After drying, the powdered sample was placed into
a quartz boat and was pyrolyzed at 700 °C for 2 h in a tubular
furnace with a heating rate of 10 °C·min-1 under an argon gas
Recycling of catalyst
After reaction, the catalyst was separated from the reaction
mixture with an external magnet. (Figure S2) Then it was
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ARTICLE
Journal Name
22. A. Zhou, R.-M. Guo, J. Zhou, Y. Dou, Y. Chen, and J.-R. Li,
ACS Sustain. Chem. Eng., 2018, , 2103–2111.
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washed thoroughly with ethanol and dried at 70 °C for 12 h.
After that, the catalyst was used for the next run directly.
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24. X. Yang, S. Wu, J. Hu, X. Fu, L. Peng, Q. Kan, Q. Huo and J.
Guan, Catal. Comm., 2016, 87, 90-93.
25. R. Nie, J. Shi, W. Du, W. Ning, Z. Hou and F.-S. Xiao, J.
Acknowledgements
We thank the National Natural Science Foundations of China
(Nos. 21566011, 31570560), the Jiangxi Province Sponsored
Mater. Chem. A, 2013, 1, 9037.
Programs
20162BCB23026), the Natural Science Foundations of Jiangxi
Province (No. 20151BAB213016), and the Science
for Distinguished
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Technology Programs of Jiangxi Province Department of
Education (No. GJJ160272) for financial support.
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