N-Doped Sub-3 nm Co Nanoparticles as Highly Efficient and Durable Aerobic Oxidative Coupling Catalysts

Article abstract of DOI:10.1002/asia.201600921

A nano-coating associated with sulfuric acid leaching protocol was developed to prepare N-doped sub-3 nm Co-based nanoparticle catalyst (Co?N/C) using melamine–formaldehyde resin as the N-containing precursor, active carbon as the support, and Co(NO₃)₂ as the Co-containing precursor. By thermal treatment under nitrogen atmosphere at 800 °C and leached with sulfuric acid solution, a stable and highly dispersive Co?N coordination structure was uniformly dispersed on the formed Co?N/C catalyst with a Co loading of 0.47 wt % and Co nanoparticle size of 2.55 nm. The Co?N/C catalyst was characterized with XRD, XPS, Raman, SEM, TEM, ICP, and elemental analysis. The Co?N/C catalyst showed extremely high catalytic efficiency with a TON of 257 for the aerobic oxidative coupling of aldehydes with methanol to directly synthesize methyl esters with molecular oxygen as the final oxidant. The Co?N/C catalyst also showed broad substrate range and stable recyclability. After recycling for 7 times, no obvious deactivation was detected. It was confirmed that the sub-3 nm Co?N coordination structure formed between metallic Co nanoparticles and pyridinic nitrogen doping into graphitic layers functions as the active site to activate molecular oxygen for the β-H elimination from generated hemiacetal intermediates to produce methyl esters. The nano-coating associated with acid leaching protocol provides a novel strategy to prepare highly efficient non-precious metal-based catalysts.

Full text of DOI:10.1002/asia.201600921

CHEMISTRY
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Accepted Article
Title: N-Doped Sub-3 nm Co Nanoparticles as Highly Efficient and Durable Aerobic
Oxidative Coupling Catalysts
Authors: Junxing Han; Feifei Gu; Yuchao Li
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N-Doped Sub-3 nm Co Nanoparticles as Highly Efficient and
Durable Aerobic Oxidative Coupling Catalysts
Junxing Han,*[a] Feifei Gu,[b] and Yuchao Li[a,c]
Abstract: A nano-coating associated with sulfuric acid leaching
protocol was developed to prepare N-doped sub-3 nm Co-based
nanoparticle catalyst (Co-N/C) using melamine-formaldehyde resin
as the N-containing precursor, active carbon as the support, and
Co(NO3)2 as the Co-containing precursor. By thermal treatment
under nitrogen atmosphere at 800℃ and leached with sulfuric acid
solution, a stable and highly dispersive Co-N coordination structure
was uniformly dispersed on the formed Co-N/C catalyst with a Co
loading of 0.47 wt% and Co nanoparticle size of 2.55 nm. The Co-
N/C catalyst was characterized with XRD, XPS, Raman, SEM, TEM ,
Scheme 1. Oxidative coupling of aldehydes with methanol to produce methyl
esters.
ICP, and elemental analysis. The Co-N/C catalyst showed extremely
high catalytic efficiency with a TON of 257 for the aerobic oxidative
coupling of aldehydes with methanol to directly synthesize methyl
esters with molecular oxygen as the final oxidant. The Co-N/C
generates a large amount of toxic pollutants. In recent years,
catalyst also showed broad substrate range and stable recyclability.
aerobic oxidative coupling of aldehydes with methanol using
After recycling for 7 times, no obvious deactivation was detected. It
molecular oxygen as the final oxidant has been actively pursued.
was confirmed that the sub-3 nm Co-N coordination structure formed
Up to now, several kinds of precious metal-based catalysts,
between metallic Co nanoparticles and pyridinic N doping into
such as Pd,[5] Ru,[6] Ir,[7] and Au-based catalysts,[8] have been
graphitic layers functions as the active site to activate molecular
developed and used for the aerobic oxidative coupling reaction
oxygen for the β-H elimination from generated hemiacetal
(Scheme 1). But the high cost and scarcity of precious metals
intermediates to produce methyl esters. The nano-coating
limit their industrial applications.
associated with acid leaching protocol provides a novel strategy to
Since Jasinski’s pioneering work of using earth-abundant
prepare highly efficient non-precious metal-based catalysts.
transition metal macrocyclic compounds as the electrolyte of
polymer electrolyte membrane cells,[9] M-N/C hybrids (M
represents transition metals, especially iron and cobalt)
prepared by hydrothermal reaction,[10] high-temperature
Introduction
pyrolysis,[11] or interfacial coordination using various
combinations of transition-metal inorganic salts, nitrogen
Oxidation reactions play significant roles in producing highly
valuable fine chemicals.[1] Among them, oxidative coupling of
containing compounds and carbon have attracted much
attention as promising materials to substitute precious metal
catalysts in the electrochemical oxygen reduction and hydrogen
methyl esters in the absence of carboxylic acids or carboxylic
aldehydes with methanol is of high efficiency to directly produce
evolution reactions,[12] the selective oxidation of alcohols to
nitriles,[13] the oxidative dehydrogenation of N-hererocycles,[14]
the epoxidation of alkenes,[15] and the selective hydrogenation of
nitroarenes.[16] Beller and co-workers successfully designed a
series of novel CoOx-N/C and FeOx-N/C catalysts by pyrolyzing
organometallic Co(II) and Fe(II) complexes.[13-16] For the first
time, they realized the aerobic oxidative coupling of benzyl
alcohol with methanol using CoOx-N/C as the catalyst.[17]
However, owing to the large size of CoOx nanoparticles, much
excess methanol, high Co loadings, and long reaction time are
needed to maintain high product yields.
Although the nature of the active sites remains unclear, it
has been verified that the performance of M-N/C catalysts are
closely related to the structure of N-containing ligands, transition
metal precursors, carbon support morphology, and thermal
treatment method.[18] Herein, we describe a N-doped sub-3 nm
Co nanoparticle catalyst with a Co loading of 0.47 wt% and Co
nanoparticle size of 2.55 nm by using a melamine-formaldehyde
resin (MF resin) nano-coating associated with sulfuric acid
acid derivatives.[2] One of the most representative applications is
the oxidative coupling of methylacrolein with methanol to
synthesize methyl methacrylate (MMA).[3] Conventionally,
oxidative coupling of aldehydes with methanol is conducted with
stoichiometric amount of heavy metal salts, such as CrO3,
KHSO5, KMnO4, or V2O5 (Scheme 1).[4] However, the process
[a]
Dr. J.X. Han, Dr. Y.C. Li
Institute of Process Engineering
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
E-mail: junxing_han@zju.edu.cn
F.F. Gu
Beijing City University
Beijing 101399 (P. R. China)
Dr. Y.C. Li
[b]
[c]
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (A) XRD patterns of CoOx-N/C pyrolyzed at different temperatures;
and (B) Raman spectra of CoOx-N/C pyrolyzed at different temperatures.
based materials. Analysis to the element contents demonstrated
that elevating the pyrolyzing temperature resulted in the
increased content of carbon accompanied by the decreased
contents of Co, O, and N (Table S1).
Scheme 2. Schematic diagram for the synthesis of Co-N/C catalyst.
Hence, the pyrolyzing temperature is an important parameter
for the synthesis of N-doped Co-based hybrids. At low
pyrolyzing temperatures, Co-based species could not be
reduced to metallic Co and the degree of graphitization of
carbon materials was at a low level. On the contrary, at high
pyrolyzing temperatures, the leaching of Co and N occurred
accompanied by the aggregation of Co-based nanoparticles.
Therefore, in the further work, the pyrolyzing temperature of
800℃ was used to maintain the reduction of Co-based species
and the doping of N species into graphitic layers.
leaching protocol. The prepared Co-N/C catalyst showed high
activity and stability for the aerobic oxidative coupling of
aldehydes with methanol to directly produce methyl esters. It
was confirmed that the sub-3 nm Co-N coordination structure
formed between metallic Co nanoparticles and pyridinic N
doping into graphitic layers functions as the active site to
activate molecular oxygen for the β-H elimination from
generated hemiacetal intermediates to produce methyl esters.
After leaching CoOx-N/C with sulfuric acid, the Co-N/C
catalyst was obtained. Figure 2A shows the SEM image of Co-
N/C catalyst. The rough surface of active carbon indicates the
formation of Co-N-C hybrid structure (Figure S1). Diffraction
peaks belonging to metallic Co, CoO, and Co3O4 particles could
be detected on the CoOx-N/C hybrid (Figure 1A, 800℃). The
sharp diffraction peaks indicates the high contents of Co-
containing species and large particle size. After sulfuric acid
leaching, only a weak diffraction peak ascribed to metallic Co
was detected at 44.0° on Co-N/C catalyst (Figure 2B, red),
suggesting the low Co content and small particle size. The Co
contents for CoOx-N/C and Co-N/C measured by ICP-AES were
4.9 wt% and 0.47 wt%, respectively (Table S2).
Large CoOx particles were observed on CoOx-N/C hybrid
with some CoOx particles larger than 150 nm (Figure S2). After
sulfuric acid leaching, large CoOx particles disappeared. The
remaining Co-based nanoparticles with a particle size of 2.55
nm uniformly dispersed on the active carbon support (Figure 2C
and inset). In the HR-TEM image, metallic cobalt nanoparticles
with a (200) d-spacing of 0.176 nm could be clearly observed,
which is in line with the XRD measurements. Meanwhile, Co
nanoparticles were coated with graphitic layers derived from
pyrolyzed MF resin, which might be the reason that these highly
dispersed Co nanoparticles could be preserved after sulfuric
acid leaching. The d-spacing of generated graphitic layers is
0.336 nm, corresponding to the (002) basal plane of carbon
materials.
Results and Discussion
As shown in Scheme 2, Co-N/C catalyst was prepared as
follows: (A) adsorbing Co(II) cations onto active carbon; (B)
nano-coating Co(II)/C with MF resin; (C) pyrolyzing under
nitrogen atmosphere to form the primary CoOx-N/C hybrid; and
(D) leaching with sulfuric acid solution to obtain the final Co-N/C
catalyst.
Figure 1A shows XRD patterns of primary CoOx-N/C hybrids
pyrolyzed in the range of 600 to 900℃. It could be seen that
when the pyrolyzing temperature increased to 700℃, diffraction
peaks belonging to Co3O4 and CoO appeared at 31.0, 36.6, 42.4,
and 61.3°, respectively. After treated at 800℃, besides Co3O4
and CoO, new diffraction peaks at 44.0 and 51.4° could be
ascribed to the metallic cobalt. The metallic Co might be
generated by reducing Co-based precursors with carbon-based
species at high temperatures. Further increasing the pyrolyzing
temperature to 900℃, no new diffraction peaks appeared. But
the diffraction peaks became sharp, indicating the aggregation
of Co-based nanoparticles at high temperatures. Meanwhile, a
N-doped graphitic layers derived from MF resin formed and
coated on the surface of CoOx/C.
D band at 1353 cm-1 and G band at 1598 cm-1 of the
pyrolyzed materials could be detected in the Raman spectra
(Figure 1B). With the increment of pyrolyzing temperature, the
ID/IG value declined from 1.27 at 600 ℃ to 1.09 at 900 ℃ ,
indicating the increased degree of graphitization of carbon-
Directly pyrolyzing MF resin under nitrogen atmosphere
generated a N-doped carbon material, denoted as CN. The XRD
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Figure 2. (A) SEM image of Co-N/C; (B) XRD patterns of fresh Co-N/C (red),
Co-N/C further pyrolyzed under nitrogen atmosphere at 800℃ for another 2 h
(blue), and Co-N/C used for 7 times (black); (C) TEM image of Co-N/C with
the inset showing the particle-size distribution histogram; and (D) HR-TEM
image of metallic Co and N-doped graphitic layers on Co-N/C.
pattern of CN showed a diffraction peak at 26.1° (Figure S3).
The corresponding lattice spacing is 0.341 nm in accordance
with TEM observations. It should be noted that CN showed a
positive shift in the C (002) peak in comparison with active
carbon (Figure S3), indicating that N-doping promoted the
graphitization degree of carbon materials and decreased the
lattice spacing of the (002) crystal planes.[19] When MF resin was
supported on active carbon and pyrolyzed under nitrogen
atmosphere, the obtained CN@C material showed the same
XRD pattern as the active carbon (Figure S3) and no peak
position shift was observed.
XP spectra of Co 2p and N 1s are shown in Figure 3. For
CoOx-N/C, the Co 2p peaks could be fitted into three peaks
located at 780.6, 782.3, and 783.7 eV (Figure 3B),
corresponding to metallic Co, Co3O4, and CoO, respectively. For
Co-N/C, the peak intensity of Co 2p became weak after sulfuric
acid leaching, suggesting the low Co content. In addition, the
fitted spectrum showed that zero-valenced metallic Co is the
main Co-containing species (Figure 3A), indicating that most
cobalt oxides (Co3O4 and CoO) have been leached in the course
of sulfuric acid treatment. Analysis to element contents showed
that after sulfuric acid leaching, the contents of Co and O
dropped from 1.24 and 7.18 atomic% to 0.34 and 4.85 atomic%,
respectively (Table S3). For CoOx-N/C, the N 1s peaks could be
fitted into pyridinic N (398.9 eV), pyrrolic N (400.2 eV), and
quaternary N (401.3 eV) (Figure 3E). For Co-N/C, the fitted N 1s
spectrum was almost the same as that of CoOx-N/C. No obvious
changes in the peak intensity and the peak numbers were
observed, indicating the doped N into graphitic layers are stable.
For CN material, XP spectrum of N 1s could also be fitted
into pyridinic N (398.1 eV), pyrrolic N (399.5 eV), and quaternary
N (400.7 eV) (Figure 3C), which indicates that the addition of Co
did not affect the doping of N into graphitic layers. However,
compared with CN, the binding energy of pyridinic N of Co-N/C
Figure 3. Fitted XPS: (A) Co 2p on Co-N/C; (B) Co 2p on CoOx-N/C; (C) N 1s
on CN; (D) N 1s on Co-N/C; and (E) N 1s on CoOx-N/C.
shifted to 398.9 eV. The positively shifted binding energy
suggests the presence of a chemical interaction between highly
dispersed metallic Co and pyridinic N,[20] which preserves the
sub-3 nm Co nanoparticles in the course of sulfuric acid leaching.
The catalytic performance of Co-N/C catalyst as well as
other relevant catalysts was evaluated by using the aerobic
oxidative coupling of benzaldehyde with methanol as a model
reaction. The results were summarized in Table 1. The blank
experiment showed that almost no benzaldehyde was converted.
In the absence of catalysts, both the benzaldehyde conversion
and the methyl benzoate yield are negligible, indicating that
catalysts play important roles in activating oxygen molecules. In
the presence of active carbon supports or CN, the conversion of
benzaldehyde was less than 10%, while using CN@C as the
catalyst the substrate conversion dropped to less than 2%,
though it was reported that the presence of quaternary N on CN
facilitates the activation of molecular oxygen.[22] Calcinating
Co(NO3)2 under nitrogen atmosphere at 800 ℃ generated a
mixture of CoO and Co3O4 (Figure S4), denoted as CoOx. No
diffraction peaks belonging to metallic Co could be detected.
The catalytic activity of CoOx was low with a benzaldehyde
conversion of 6%. No improvements were observed using the
combination of CoOx and CN@C as the catalyst. Calcinating
active carbon supported Co(NO3)2 produced a hybrid material
denoted as CoOx@C. Metallic Co, CoO and Co3O4 were
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detected in the XRD pattern with metallic Co as the main crystal
phase (Figure S4), suggesting that active carbon could reduce
the Co precursor at high temperatures. Compared with CoOx-
N/C, the sharper diffraction peaks of metallic Co on CoOx@C
indicate the larger metallic Co particle size on CoOx@C, which
manifests that the coating of MF resin on Co(II)/C could
effectively inhibit the aggregation of Co-based species. The
large Co-based particle size of CoOx@C resulted in the low
catalytic activity with a substrate conversion less than 10%.
Calcinating the combination of MF resin and Co(NO3)2 under the
same conditions generated a hybrid material denoted as CoOx-
CN. The diffraction peaks in the XRD pattern of CoOx-CN could
be attributed to metallic Co, CoO, and Co3O4, respectively
(Figure S4). The CoOx-CN catalyst provided a benzaldehyde
conversion of 50% with a product selectivity of 74%, which
suggests that after N doping the catalytic performance of Co-
based catalyst could be greatly improved. Using Co(NO3)2, the
combination of Co(NO3)2 and CN, or the combination of
Co(NO3)2 and CN@C as the catalyst, almost no substrate
conversion occurred.
In a series of CoOx-N/C hybrids treated in the range of 600
to 900℃, the CoOx-N/C pyrolyzed at 800℃ showed the best
performance with a benzaldehyde conversion of 94% and a
methyl benzoate selectivity of 96%. According to the discussion
on XRD and Raman spectra in Figure 1, the optimal pyrolyzing
temperature of 800℃ benefits for the reduction of Co precursor
to metallic Co and the doping of N into graphitic layers.
We then evaluated the performance of Co-N/C catalyst for
the aerobic oxidative coupling reaction. To our surprise, no
obvious decline in catalytic activity was observed, though the Co
loading dropped from 4.9 to 0.47 wt% after sulfuric acid leaching.
The Co-N/C catalyst offered a benzaldehyde conversion of 92%
with a methyl benzoate selectivity of 89%. The calculated TON,
defined as the total number of product moles formed per mole of
Co, of Co-N/C reached up to 257, nearly 10 times higher than
that of CoOx-N/C (Table S2). The much higher TON of Co-N/C
demonstrates that the leached large Co-based particles had
marginal effect on the aerobic oxidative coupling of
benzaldehyde with methanol and the remaining sub-3 nm Co-N
coordination structure functions as the active site for the aerobic
oxidative coupling reaction.
Table 1. Aerobic oxidative coupling of benzaldehyde with methanol to
synthesize methyl benzoate.[a]
O
H
O
O
O2
+ CH3OH
+ H2O
Catalyst
Entry
1[c]
2
Catalyst
None
None
C
T [℃][b]
Conv. [%]
< 2
< 2
7
Selec. [%]
< 2
< 2
46
28
< 2
33
20
51
74
< 2
< 2
< 2
93
74
82
96
90
89
36
81
90
3
4
CN
800
800
800
800
800
800
8
5
CN@C
CoOx
< 2
6
6
7
CoOx+ CN@C
CoOx@C
8
8
8
9
CoOx-CN
50
< 2
< 2
< 2
29
16
67
94
88
92
3
10
11
12
13
14
15
16
17
18
19[c]
20[d]
Co(NO3)2
Co(NO3)2+ CN
Co(NO3)2+ CN@C
Co(NO3)2+ C
CoOx-N/C
800
800
600
700
800
900
800
800
800
CoOx-N/C
CoOx-N/C
CoOx-N/C
Co-N/C
Co-N/C
Co-N/C-FT
Pd2Pb8/alumina
33
94
21[e]
In the absence of K2CO3, the benzaldehyde conversion
declined to 3%, which demonstrates that the alkalinity of K2CO3
also plays an important role in improving the oxidative coupling
reaction rate. Based on the proposed oxidative coupling
mechanism,[2a] this reaction begins with the deprotonation of
methanol. The formed methoxy group nucleophilically attacks
the aldehydic carbon atom to generate the hemiacetal
intermediate. The β-H elimination of hemiacetal by activated
oxygen species yields the methyl ester product. Hence, the Co-
N coordination structure functions as the active site for the
activation of molecular oxygen to generate activated oxygen
species and K2CO3 acts as an alkali to promote the formation of
methoxy groups by deprotonation of methanol. The combination
of Co-N coordination structure of Co-N/C and K2CO3 exhibited
extremely high efficiency for the oxidative coupling of
benzaldehyde with methanol and provided a TON of 257.
[a]
Typical reaction conditions: Catalyst (0.2 g), benzaldehyde (9.4 mmol),
methanol (625 mmol), K2CO3 (1.4 mmol), 0.1 MPa of oxygen, 30 mL min-1 of
[b]
[c]
oxygen flow rate, 60℃, 2.0 h. T represents the pyrolyzing temperature.
In the absence of K2CO3.
[d]
The obtained Co-N/C catalyst was further
[e]
calcined at 800℃ under nitrogen atmosphere for another 2.0 h.
The
Pd2Pb8/alumina catalyst was prepared according to the literature method.[21]
If the Co-N/C hybrid was further treated under nitrogen
atmosphere at 800℃ for another 2 h, the obtained material,
denoted as Co-N/C-FT, displayed obvious diffraction peaks
belonging to CoO and Co3O4 in the XRD pattern (Figure 2B,
blue). The Co-N/C-FT catalyst showed inferior activity to Co-N/C
with the benzaldehyde conversion dropped to 33%, which
further suggests that the Co-N coordination structure between
metallic Co and pyridinic N functions as the active site for the
oxidative coupling reaction. To be highlighted, the TON of Co-
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O
O
O2
H
OCH3
+ CH3OH
R
+ H2O
R
Co-N/C
O
O
O
O
O
O
37%, 2 h
O
67%, 6 h
O
96%, 1 h
O
O
O
O
80%, 12 h
O
95%, 2 h
O
97%, 2 h
O
O
O
O
Figure 4. Kinetic data for aerobic oxidative coupling of aldehydes with
methanol: benzaldehyde (black), 2-chlorobenzaldehyde (red), and 2-
methylbenzaldehyde (blue).
Cl
Cl
Cl
96%, 2 h
O
84%, 5 h
49%, 2 h
O
O
on the oxidative coupling reaction. The same phenomenon was
also observed by using 2-chlorobenzaldehyde and 2-methoxy
benzaldehyde as the substrate. Kinetic measurements showed
that the oxidative coupling of benzaldehyde with methanol was a
pseudo first order reaction (Figure 4). With the increase of the
steric hindrance of the ortho-position substituent groups (CH3 >
Cl > H), the reaction rate constant decreased from 3.31 to 0.22
h-1 (Figure 4). For 2-methoxy benzaldehyde, the oxidative
coupling reaction no longer displayed the pseudo first order
behavior (Figure S5). Based on the proposed oxidative coupling
mechanism mentioned above,[2a] the nucleophilically attack of
the methoxy group to the aldehydic carbon atom might act as a
rate determining step for the oxidative coupling reaction.
Because the β-H elimination of formed hemiacetal intermediates
by active oxygen species on the Co-N coordination structure is a
fast reaction step.[2a] When the ortho-position H of benzaldehyde
was substituted by a bulk groups, the steric hindrance effect of
the substituent group inhibited the attack of methoxy group to
the aldehydic carbon atom and resulted in the decrease of the
reaction rate. By elevating reaction temperature and prolonging
reaction time, the substrate conversion and product selectivity
could be improved.
On the other hand, no obvious electronic effect was
observed. Co-N/C catalyst showed high activity for the oxidative
coupling of 4-methyl benzaldehyde, 4-chlorobenzaldehyde, and
3-nitrobenzaldehyde with methanol. Complete conversion was
achieved within 2 hours. Due to the stronger electron-donating
effect of the methoxy group, complete conversion of 4-methoxy
benzaldehyde was achieved after reacting for 6 hours. The co-
existence of steric hindrance effect and electron-donating effect
resulted in the conversion of 2-methoxy benzaldehyde no longer
displayed the pseudo first order behavior.
O
O
O
O
O
O
15%, 2 h
34%, 12 h
73%, 15 h, 80℃
O
O
O
O
O
O
O
O
NO2
82%, 2 h
66%, 2 h
97%, 6 h
Scheme 3. Catalytic performance of Co-N/C for the aerobic oxidative coupling
of various aldehydes with methanol. Reaction conditions: Co-N/C Catalyst (0.2
g), aldehyde (4.7 mmol), methanol (625 mmol), K2CO3 (1.4 mmol), 0.1 MPa of
oxygen, 30 mL min-1 of oxygen flow rate, 60℃.
N/C was nearly two times higher than the traditional Pd-Pb
bimetallic catalyst. The latter offered a TON of 126 (Table S2).
Besides benzaldehyde, Co-N/C catalyst also showed high
activity for the aerobic oxidative coupling of substituted
benzaldehyde with methanol. The results were listed in Scheme
3. Complete conversion was achieved using 4.7 mmol of
benzaldehyde as the substrate in one hour with a methyl
benzoate yield of 96%. It was found that methyl group
substituted benzaldehyde with the substituent group located at
2-, 3-, and 4-position, respectively, showed different activity.
Complete conversion was achieved in 2 hours for the 3- and 4-
position methyl substituted benzaldehyde, while the yield of 2-
methyl benzaldehyde was only 37%. After prolonging reaction
time to 12 h, the yield of 2-methyl benzaldehyde reached 80%.
The low reactivity of 2-methyl benzaldehyde indicates that the
ortho-substituent groups showed strong steric hindrance effect
Kinetic data manifest that on Co-N/C catalyst the steric
hindrance effect of ortho-substituent groups plays a more
important role than the electronic effect in determining the
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oxidative coupling of benzaldehyde with methanol. The nano-
coating associated with acid leaching protocol provides a novel
strategy to prepare highly efficient non-precious metal-based
catalysts.
Experimental Section
Catalyst preparation
Melamine-formaldehyde resin (MF resin) was synthesized according to
the reported method.[22] Typically, formaldehyde solution (37 wt%, 8.9
mL), deionized water (40 mL) and NaOH aqueous solution (1.0 M, 0.2
mL) were mixed together in a 100 mL round bottom flask and stirred for
10 min at room temperature. Then, melamine (5.0 g) was added to the
solution. The flask was transferred into an oil bath and maintained at
90 ℃for 1 h. The obtained clear MF resin solution was promptly cooled
down to room temperature.
Figure 5. Recycling tests of Co-N/C for the aerobic oxidative coupling of
benzaldehyde with methanol.
Active carbon powder (Vulcan XC72R, 2.0 g), Co(NO3)2•6H2O (1.46
g, 5 mmol) and a HCl aqueous solution (0.5M, 50 mL) were mixed in a
250 mL round bottom flask and stirred for 30 min at room temperature.
Then, the obtained MF resin solution was added into the mixture and
stirred for another 5 h at room temperature. Water was removed slowly
under vacuum. The residual solid sample was dried at 80 ℃for 12 h. The
raw sample was grinded to a fine powder. The obtained fine powder was
pyrolyzed in N2 atmosphere at 800 ℃ for 2 h and cooled down to room
temperature. The obtained sample was denoted as CoOx-N/C. The heat-
treated sample CoOx-N/C (1 g) was leached in a H2SO4 aqueous solution
(0.5M, 75 mL) at 80 ℃for 12 h and thoroughly washed with deionized
water. After dried under vacuum at 80 ℃for 12 h, the Co-N/C catalyst
was obtained.
reaction rates. This phenomenon clearly demonstrates that the
nucleophilic attack of methoxy group to the aldehydic carbon
atoms to form the hemiacetal intermediates acts as the rate-
determining step for the oxidative coupling of aldehydes with
methanol.
Due to the coating of N-doped graphitic layers, Co-N/C
catalyst showed excellent stability and could be recycled for 7
times without deactivation for the oxidative coupling of
benzaldehyde with methanol (Figure 5). No aggregation of Co-
based species was observed in the TEM image of the recycled
Co-N/C catalyst and the Co nanoparticles still uniformly
dispersed on the support (Figure S6). The XRD pattern of the
recycled Co-N/C resembled that of the fresh one. Only a weak
diffraction peak attributed to metallic Co was detected at 44.0°
for the recycled Co-N/C catalyst (Figure 2B, black), further
suggesting the stability of the Co-N/C catalyst.
Characterization
Raman measurements were performed on a LabRAM HR800 equipped
with an excitation wavelength of 514 nm and CCD detector. X-ray
diffraction (XRD) patterns were recorded on
a Rigaku SmartLab
diffractometer using Cu Kα radiation at 45 kV and 200 mA. XPS spectra
were collected on an ESCALAB 250Xi electron spectrometer using 300
W Al Kα X-ray source and the binding energies were referenced with C
1s at 284.6 eV. Field-emission SEM and HR-SEM images were observed
on a JSM 6700F microscope. TEM and HR-TEM images were obtained
on a JEM-2100F microscope operated at 200 kV. The cobalt content was
measured by ICP-AES with an ICP E-9000 spectrometer. The Co loading
in the Co-N/C hybrid is 0.47 wt%. Elemental analysis was conducted on
a Varian EL cube equipment. The contents of C, H, and N in the Co-N/C
hybrid are 88.65, 0.17, and 3.35 wt%, respectively.
Conclusions
In conclusion, a nano-coating associated with acid leaching
protocol was developed to prepare N-doped sub-3 nm Co-based
nanoparticle catalyst using MF resin as the N-containing
precursor, Co(NO3)2 as the Co-containing precursor, and
activated carbon as the support. After pyrolysis under nitrogen
atmosphere at 800℃, a primary CoOx-N/C hybrid material was
formed. Further treating the CoOx-N/C hybrid with sulfuric acid
solution leached unstable Co-based species and obtained the
final Co-N/C catalyst with highly dispersed Co nanoparticles.
After sulfuric acid leaching, the Co loadings dropped from 4.9
wt% to 0.47 wt%. The reserved metallic Co nanoparticles with a
particle size of 2.55 nm chemically interacted with pyridinic N
derived from pyrolysis of MF resin to form a stable Co-N
coordination structure, which functions as the active site for the
activation of molecular oxygen and provides high activity, broad
substrate range as well as stable recyclability for the aerobic
Aerobic oxidative coupling reaction
Co-N/C catalyst (0.2 g), benzaldehyde (1.0 g, 9.4 mmol), methanol (20.0
g, 625 mmol) and K2CO3 (0.2 g, 1.4 mmol) were added to a 100 mL
autoclave sequentially. After filling the autoclave with 0.1 MPa of oxygen,
the reaction was stirred at 60 ℃ for 2.0 h with the oxygen flow rate of 30
mL min-1. After cooling to room temperature, 1 mL reaction mixture and
0.2 mL ethanol (the internal standard) were mixed together. The obtained
sample was directly subjected to GC analysis. Conversion, selectivity
and yields were measured by GC-FID (Agilent 6890 equipped with DB-
624 capillary column). Qualitative analysis of esters and byproducts was
made by GC-MS and identified by comparison with authentic samples.
Catalytic oxidative coupling of other aldehydes with methanol was carried
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Chemistry - An Asian Journal
10.1002/asia.201600921
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Chemistry - An Asian Journal
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FULL PAPER
Heterogeneous Catalysis
Hybrid catalyst: Highly dispersed
N-doped Co nanoparticles with a Co
loading of 0.47 wt% and Co
Junxing Han,* Feifei Gu, and Yuchao
Li
Page No. – Page No.
nanoparticle size of 2.55 nm
function as highly efficient active
sites for the aerobic oxidative
coupling of aldehydes with methanol
and provide an extremely high TON
of 257 as well as stable recyclability.
N-Doped Sub-3 nm Co
Nanoparticles as Highly Efficient
and Durable Aerobic Oxidative
Coupling Catalysts
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