Facile synthesis of a highly efficient Co/Cu@NC catalyst for base-free oxidation of alcohols to esters

Article abstract of DOI:10.1039/d0nj00172d

The direct oxidation of alcohols to esters is an environmentally benign and cost-effective organic synthetic strategy, but it is still a great challenge to discover an economic, highly active, and long-term stable catalyst for efficient transformation of alcohols to esters under milder conditions. Herein, we developed cobalt and copper nanoparticle -co-decorated nitrogen-doped carbon catalysts (CoCu@NC_n) through two steps of ball milling and calcination. It was found that CoCu@NC_n could catalyze the oxidation of alcohols to esters effectively in the absence of basic additives. The catalytic activity was much higher than those of monometallic Co@NC₂ and Cu@NC₂ samples, and the catalyst can be conveniently recovered and quite steadily reused. Through a series of control experiments and characterizations, it is concluded that the remarkable catalytic performance of CoCu@NC₂ was associated with the synergistic effect between the two metal components, the enhanced basic active sites and the active surface area.

Full text of DOI:10.1039/d0nj00172d

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DOI: 10.1039/D0NJ00172D
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Facile Synthesis of Highly Efficient Co/Cu@NC Catalyst for Base-
free Oxidation of Alcohols to Esters
Received 00th January 20xx,
Accepted 00th January 20xx
Jiusheng Jiang, Xiang Li, Shengyu Du, Langchen Shi, Pingping Jiang, Pingbo Zhang, Yuming Dong,
Yan Leng*
DOI: 10.1039/x0xx00000x
The direct oxidation of alcohols to esters is an environmentally benign and cost-effective organic synthetic strategy, but it
is still a great challenge to explore an economic, highly active, and long-term stable catalyst for efficient transformation of
alcohols to esters under milder conditions. Herein, we developed cobalt and copper nanoparticles-co-decorated nitrogen-
doped carbon catalysts (CoCu@NCn) through two steps of ball milling and calcination. It was found that CoCu@NCn could
catalyze the oxidation of alcohols to esters effectively in the absence of basic additives. The catalytic activity was much
higher than those of monometallic Co@NC2 and Cu@NC2 samples, and the catalyst can be conveniently recovered and
quite steadily reused. Through a series of control experiments and characterizations, it is concluded that the remarkable
catalytic performance of CoCu@NC2 was associated with the synergistic effect between two metal components, the
enhanced basic active sites and active surface area.
favorable catalytic materials.20-23 Because of the nanoscale
features as well as tunable properties such as compositions,
porosity, surface area, and accessible active sites, M@C have
Introduction
Esters are a class of most important compounds in organic
transformations and have been wildly used in the field of fine
chemicals, natural products, pharmaceuticals, and polymers.1-3
Traditionally, esters are prepared by the Fischer esterification
reaction of carboxylic acids or activated acid derivatives with
alcohols, or transesterification reactions.4-8 However, strong
acidic or basic reaction conditions in those processes always
result in large amounts of undesired byproducts and limit the
reaction scope. With the developments in metal-catalyzed
oxidation reactions,9-11 the direct oxidation of alcohols to
esters (also known as oxidative esterification) has been
recognized as an environmentally benign and cost-effective
alternative to traditional ester synthesis.12-16 For this purpose,
various metal catalysts (e.g., Pd, Au, Pt) have been identified,
but most of the known catalysts for such reactions are based
on previous metals and mostly in homogeneous systems.17-19
Moreover, a base additive was required in these cases, which
is economically and environmentally unfavorable. Therefore,
the development of inexpensive, non-noble metal, and easily
recyclable heterogeneous catalysts for the oxidation of
alcohols to esters becomes an attractive subject in sustainable
chemical conversion process.
shown great potential applications in heterogeneous catalysis.
24-26
In particular, Co NPs-based M@C (Co@C) are found to be
useful and reusable catalysts for the oxidation of alcohols to
esters.27-29 In recent years, several strategies for turning the
catalytic performance have been established. It is reported
that doping carbon with heteroatoms could alter the physical
properties of M@NC (acid-base property, electronic density,
etc) and facilitate strong interaction with metal species, which
might boost their final catalytic activities. For example, Co@NC
catalysts activated by N-doping or S-doping have displayed
improved catalytic activity toward the oxidative esterification
of alcohols.30-33 Very recently, the incorporating deliberately
the second metal into M@C (M1M2@NC) has been shown to
be advantageous for enhanced catalytic performance
particularly in electro-catalysis due to the modified electrical
conductivity and oxidation status of bi-metal elements.34-38
Despite the versatile applications of bimetallic M1M2@NC in
many OER, HER, and ORR processes, surprisingly, their use as
catalysts for oxidative esterification has been seldom reported.
Herein, Co and Cu NPs-co-decorated N-doped carbon
(CoCu@NCn) catalysts were prepared by a facile and effective
mechanical mixing and calcination method using chitosan
(CTS) and melamine as carbon and nitrogen sources,
respectively. The synthesized CoCu@NCn were applied for
direct oxidation of alcohols to esters under atmospheric
conditions without the assistance of any base additives,
presenting good to excellent catalytic activity and selectivity.
Investigations on the correlation between CoCu@NCn
structure and catalytic activity demonstrated that the
Transition metal nanoparticles (NPs), especially Co, Cu, Ni,
Fe NPs, encapsulated in carbon materials (M@C) are becoming
School of Chemical and Material Engineering, Jiangnan University, Lihu Road
1800#, Wuxi 214122, Jiangsu, China. E-mail: yanleng@jiangnan.edu.cn.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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synergistic effect between Co and Cu NPs, together with The oxidative esterification of benzyl alcohol was carried out in
View Article Online
abundant basic sites arising from N-doping may be responsible a 15 mL vial sealed with rubber plug. TyDpOicIa: 1ll0y.,10b3e9n/Dz0yNl Ja0lc0o17h2oDl
for its good catalytic performance.
(0.5 mmol), CH3OH (5 mL), and catalyst CoCu@NCn (40 mg)
were added into the reactor, followed by heating to 60°C with
continuous stirring over oil bath. The vial was degassed and
purged with pure oxygen for 10 min, and a balloon filled with
O2 was inserted with a needle. After the reaction, the solid
catalyst was collected by centrifugation. The liquid was
identified by GC-MS equipped with a FID detector and a
capillary column (SE-54 60 m × 0.32 mm × 0.25 mm). The
recovered catalyst was washed with 0.1 M NaOH and pure
CH3OH, respectively, dried at 60°C, retreated in N2 atmosphere
at 500 °C for 2 h, and then reused in the next run.
Experimental
Materials
Chitosan was purchased from Macklin. Melamine, CoCl2.6H2O,
and CuCl2.2H2O were purchased from Sinopharm Chemical
Reagent Co., Ltd., China. All the chemicals used in the
experiment without further purification.
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Synthesis of CoCu@NCn
In a typical procedure, chitosan (0.5 g), melamine (0-1.0 g),
and metal salts (0.17 g), (the molar ratio of Co/Cu was 1:1)
were added into a 4.5 × 5.5 cm (diameter × height) screw-
capped stainless steel reactor along with six stainless steel ball
Results and discussion
bearings. The reactor was ball milled for 30 min in a high- As a kind of cheap and easily obtained renewable biomass, CTS
speed vibrating ball miller (AC 220V, 50 HZ). The obtained was chosen as a carbon source and melamine as a nitrogen
precursor mixture was carbonized at 800 °C for 2 h with 5 source to ball mill with metal salts (CoCl2 and CuCl2) for 30 min
°C/min heating rate in N2 atmosphere to give the final catalyst to form a mixture containing CTS, melamine, Co2+, and Cu2+.
CoCu@NCn. When the chitosan/melamine quality ratio were The obtained precursor mixture was then carbonized at 800°C
adjusted to 0.5/0, 0.5/0.25, 0.5/0.5, 0.5/1.0 during the ball in N2 atmosphere to afford the catalysts CoCu@NC2. During
milling, the obtained samples were denoted as CoCu@NC0, this process, the Co2+ and Cu2+ ions were reduced to Co and Cu
CoCu@NC1, CoCu@NC2, and CoCu@NC3, respectively.
Characterization
NPs while the surrounding organic ligands were polymerized to
NC, resulting in Co and Cu NPs embedded in NC matrix. TG
The nitrogen sorption isotherms and pore size distribution analysis shows that the stability of CoCu@NC2 is high up to
curves were measured at the temperature of liquid nitrogen 300°C in air (Figure S1). The Co and Cu contents in CoCu@NC2
(77 K) using a BELSORP-MINI analyzer. The metal content was are approximately 9.0 wt% and 9.1 wt%, respectively, as
measured using a Jarrell-Ash 1100 ICP-AES spectrometer. X-ray determined by ICP analysis. For the sake of comparison, a
photoelectron spectroscopy (XPS) was conducted on a PHI control catalyst (CoCu@NC0) was prepared without adding
5000 Versa Probe X-ray photoelectron spectrometer equipped melamine, and monometallic Co@NC2 and Cu@NC2 samples
with Al Karadiation (1486.6 eV). XRD patterns were collected were also prepared by carbonization of monometallic
on a Bruker D8 Advance powder diffractometer using a Ni- precursors.
filtered Cu/Kα. TGA was performed using an STA 409
instrument from 50 to 800 °C in air at a heating rate of 10 peaks at 44.1°, 51.2°and 75.8° corresponding to face-centered
°C/min. Raman experiments were performed using cubic Co (PDF # 15-0806) and peaks at 43.3°, 50.5°, and 74.0°
The XRD diffraction pattern of CoCu@NC2 exhibit a group of
a
PerkinElmer-Raman Station 400 F spectrometer equipped with corresponding to face-centered cubic Cu (PDF # 04-0836)
a liquid N2 cooled charge-coupled device detector and a (Figure 1A), confirming the formation of Co and Cu NPs-co-
confocal microscope. A 350 mW near infrared 785 nm laser decorated NC. The N-free sample CoCu@NC0 also reveals the
was used for analysis under ambient conditions. The basicity of formation of cubic Co and Cu. The relative strong peak
samples was determined through temperature-programmed intensity was probably due to larger crystallite size of metal
desorption of CO2-TPD by
a Micromeritics BelCata II NPs in CoCu@NC0 compared with CoCu@NC2. The existence of
equipment. The sample was pre-treated in He flow at 150 ºC NC was revealed by Raman spectrum, which shows obvious D
for 3 h and cooled to room temperature. After being saturated and G bands located at 1318 and 1574 cm-1 (Figure S2). The
with CO2, the sample was purged with He for 1h at room porous structure of CoCu@NC2 was evaluated by N2 sorption
temperature to sweep the physical molecule. Then TPD signals
(B)
(A)
CoCu@NC0
CoCu@NC2
were recorded by performing CO2 desorption from 50 to 900
°C at the rate of 10 ºC/min. The morphology was studied by
field emission scanning electron microscopy (FESEM; Hitachi S-
4800, accelerated voltage: 5 kV). Scanning electron microscopy
(SEM) images were acquired on a SUPERSCAN SSX-550
electron microscope. Transmission electron microscopy (TEM)
Co PDF#15-0806
Cu PDF#85-1326
10
Pore diameter dp (1n0m0 )
SBET= 575.92 m2/g
D = 3.06 nm
Pore volume =0.306 cm3/g
images were recorded with
a JEOL JEM-2100 electron
0.0
0.2
0.4
0.6
0.8
1.0
30
40
60
70
80
2-The5t0a (degree)
Relative pressure (p/p0)
microscope operated at 200 kV. The Co and Cu contents in
CoCu@NC2 were measured using a Jarrell-Ash 1100 ICP-AES
spectrometer.
Fig. 1 (A) XRD patterns of CoCu@NC0 and CoCu@NC2; (B) N2
sorption isotherm curves of CoCu@NC2.
Catalytic tests
2 | J. Name., 2012, 00, 1-3
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3+ Co0(779.2)
Co
Co2+(783.2)
sat.
(A)
(B)
View Article Online
DOI: 10.1039/D0NJ00172D
Co 2p
Cu 2p
Cu 2p3/2(932.6)
5
Cu 2p1/2(952.5)
CoCu@NC2
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Co@NC2
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Binding Energy (eV)
Binding Energy (eV)
(C)
C-C
Co-Nx(398.9)
pyrrolic-N
graphitic-N
(D)
C 1s
N 1s
pyridinic-N
C-N(285.9)
C-O
Co-Cu@NC2
CoCu@NC2
Fig. 2 (A, B, C) TEM images, (D) Diagram of CoCu@NC2 and (E)
EDS maps of C, O, N, Co, and Cu for CoCu@NC2.
Co@NC2
Co@NC2
measurement at 77 K (Figure 1B). CoCu@NC2 exhibited typical
IV isotherm with H3-type hysteresis loops between the
pressure P/P0 =∼0.5-1.0, indicating the presence of mesopores.
The specific surface area was estimated to be 576 m2/g with
an average pore size of 3.0 nm and pore volume of 0.306 m3/g.
The TEM image of CoCu@NC2 exhibits a markedly crumpled
sheets structure with irregular menses and visible pores
(Figure 2A). The HRTEM images demonstrate that metal NPs
have been successfully embedded in NC framework (Figure 2B).
The interplanar spacing were calculated to be 2.05 and 2.11 °A,
corresponding to the (111) and (111) planes of metal Co and
Cu, respectively, which reveals good crystallinity (Figure 2C).
The Co and Cu NPs have been found to be in close contact in
some selected-area, this may facilitate the synergistic effect
for high-efficient catalysis, as has been discussed hereinafter.
The EDS elemental mapping images confirm that C, N, Co, and
Cu were uniformly distributed in CoCu@NC2 (Figure 2E). The
above characterization results verified that the Co and Cu NPs
have been successfully embedded in porous NC, and the
resulting catalyst CoCu@NC2 possesses a reticular structure as
shown in Figure 2D.
To further elucidate the detail chemical composition of
CoCu@NC2 as compared to that of monometallic Co@NC2, XPS
measurements were performed (Figure S3). The Co 2p XPS
spectrum for Co@NC2 can be deconvoluted into four doublets,
where the peaks at 779.2 and 794.5 eV can be assigned to
metal Co0, the peaks at 781.3 and 796.2 eV correspond to Co3+
originating from CoOx or CoCxNy, and the peaks at 783.2 and
798.1 eV are assigned to Co2+ originating from Co–Nx species
(Figure 3A). The formation of Co–Nx can be attributed to the
abundant N-doping. In the Co 2p XPS spectrum of CoCu@NC2,
the peak intensity for Co3+ and Co2+ increased significantly
after the introduction of Cu. In addition, there is a shift in the
peaks of Co3+ and Co2+ towards higher binding energy for
CuCo@NC2 as compared to that for Co@NC2, indicating a
strong electron interaction between Cu and Co species. The Cu
2P spectrum of CuCo@NC2 only show the peaks (932.6 and
952.5 eV) assigned to the Cu metal without observing the
peaks assigned to Cux+ (Figure 3B). This is mainly because of
that the copper was reduced easier during the pyrolysis than
the Co tended to exist in the form of Cu0.39 The N 1s XPS
spectra of Co@NC2 and CuCo@NC2 can be deconvoluted into
graphitic-N (401.3 eV), pyrrolic-N (400.2 eV), M-N (398.9 eV)
295
290
285
280
410
405
400
395
Binding Energy (eV)
Binding Energy (eV)
Figure 3. (A) Co 2p, (B) Cu 2p, (C) N 1s, (D) C 1s XPS spectra of Co@NC2
and CoCu@NC2
and pyridinic-N (398.2 eV) (Figure 3C). The presence of the
peak at 398.9 eV further indicates the generation of Co-Nx. It
has been reported that more likely the metal species next to
pyridinic N are important active sites for many catalytic
processes.40,41 The M-N and pyridinic-N content in CuCo@NC2
is much higher than that of Co@NC2, confirming a high doping
ratio of N, which would help enhance the metal catalytic
activity. The peak situated at 285.9 eV in the C 1s XPS
spectrum further proves the formation of C-N bond (Figure 3D).
Table 1 lists the catalytic performance of CoCu@NCn for the
oxidation of benzyl alcohol to ester with 1 bar O2 balloon
under base-free conditions, together with comparisons with
various control catalysts. The reaction could not proceed in the
absence of either catalyst or O2, or with metal-free NC as the
catalyst (entry 1). It indicates that metal active centers are
required to overcome the kinetic barrier to promote the
benzyl alcohol conversion. CoCu@NC2 offered the highest
Table 1 Oxidative esterification of benzyl alcohol over the
synthesized catalysts.
O
O2
C
O
OH
OCH3
+
CH3OH
+
60 o
C
Entry
Catalyst
Time (h)
Con. (%)
Sel. (%)
-
1
2
NC2
12
12
12
12
12
12
12
12
2
-
100
78
50
5
CoCu@NC2
CoCu0.5@NC2
Co@NC2
100
89
3
4
77
5
Cu@NC2
--
6
Co@NC2 + Cu@NC2
CoCu@NC0
Co@NC0
42
53
8
65
7
73
8
12
9
CoCu@NC1
CoCu@NC2
CoCu@NC3
54
67
44
74
10
11
2
83
2
71
Reaction conditions: 0.5 mmol benzyl alcohol, 5 mL CH3OH, 40
mg catalysis, 60 °C, O2 ball. Catalyst mmol/benzyl alcohol mmol =
0.11/0.50. The main byproduct is benzaldehyde. The conversion
and selectivity were determined by GC.
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Table 2 CoCu@NC2-catalyzed oxidative esterificDatOioI:n1o0f.1v0a3r9io/uDs0aNlcJo0h0o17ls2.D
O
Co-Cu@NC0
Co-Cu@NC1
Co-Cu@NC2
60-110 o
C
C
O
OH
2
OR
+
R
OH
R
R
CoCu@NC2
6
O
O
O
7
O
O
O
8
O
Cl
Br
9
3a (60°C, 12h）
1a (60°C, 12h）
2a (60°C, 12h）
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C: 99.0% S: 73.6%
C: 99.5% S: 99.6%
C: 91.4% S: 99.5%
O
F
O
O
O
O
O
O2N
6a (60°C, 12/24h）
C: 65.8%/92.4%
4a (60°C, 12h）
C: 98.5% S: 98.9%
5a (60°C, 12h）
100 200 300 400 500 600 700 800 900
Temperature (oC)
C: 95.8% S: 84.9%
S: 90.4%/98.5%
Figure 4. CO2-TPD profiles of CuCo@NC0, CuCo@NC1, and
O
O
O
O
CuCo@NC2
O
O
O
O
O
catalytic activity in a solid-liquid heterogeneous catalysis
system (entry 2). GC spectra for this reaction performed at
different reaction time indicate a complete transformation of
benzyl alcohol to methyl benzoate with excellent selectivity of
100% (Figure S4). In contrast, the monometallic Co@NC2
exhibited a low conversion of 50% and selectivity of 77% (entry
4). Moreover, only a trace amount benzyl alcohol was oxidized
to benzaldehyde over Cu@NC2 (entry 5). The above results
indicate that the incorporation of Cu into Co@NC2 (nCo:nCu =
1:1) could significantly promote the aerobic oxidative
esterification of benzyl alcohol. Change the Cu content in
CoCu@NC2 resulted in lower conversion and selectivity
(CoCu0.5@NC2, entry 3) probably due to the weakened
synergetic effect between Co and Cu.42,43 Interestingly,
Cu@NC2 as an additive for the reaction with Co@NC2 was less
effective, indicating that the decoration of Co and Cu NPs into
a single NC support was important to enhance the catalytic
activity (entry 6). Similar kind of high catalytic activity of
bimetallic catalysts was also observed in many electrochemical
reactions and organic transformations.44-46
To check the role of N-doping on the catalytic performance
of CoCu@NC2 in the oxidative esterification, the reaction was
carried out over a N-free control sample CoCu@NC0. However,
a relative low conversion of 53% and selectivity of 73% was
obtained (entry 7), which is much lower than that of
CoCu@NC2. This implies that the N-doping also play an
essential role in improving the overall activity of CoCu@NC2.
Notably, another monometallic N-free sample Co@NC0 was far
less active than that of N-containing sample Co@NC2,
presenting only 8% conversion and 12% selectivity (entry 8),
further confirming the catalytically promotional role of N
dopant for base-free oxidative esterification. By tuning the
melamine content during the CoCu@NCn synthesis, the
catalytic activity and selectivity can be adjusted. According to
the initial activity in 2 h (entries 9-11), CoCu@NC1 offered 54%
conversion with 74% selectivity. CoCu@NC2 exhibited the best
performance, giving 67% conversion and 83% selectivity. The
further increase of melamine content for CoCu@NC3 synthesis
resulted in partial transformation of Co and Cu NPs into metal
oxides as suggested by XRD patterns (Figure S5), and thus do
not make a further increase in catalytic activity.
9a (60°C, 24/48h）
C: 65.4%/83.9%
S: 90.2%/95.8%
8a (60°C, 24/48h）
C: 80.4%/96.2%
S: 71.5%/82.7%
7a (60°C, 12/24h）
C:82.8%/93.7%
S:92.6%/96.5%
O
O
O
O
O
O
10b (110°C, 24/48h）
C: 90.7%/95.3%
S: 42.4%/54.5%
12b (70°C, 24/48h）
C: 95.3%/99.4%
S: 72.1%/83.4%
11b (90°C, 24/48h）
C: 94.6%/98.2%
S: 48.7%/55.4%
Conditions: 0.5 mmol substrates (0.1 mmol for 10b-12b), 5 mL CH3OH, 40
mg CoCu@NC2, O2 ball. The main byproducts are the corresponding
aldehyde (1a-9a) and benzaldehyde (10b-12b). The conversion (C) and
selectivity (S) were determined by GC.
As has been reported, the N-doped carbon could act as
built-in and “strengthened” Lewis base to accelerate the
deprotonation process.47-49 Therefore, we used CO2-TPD
analysis to determine the basic properties of CuCo@NCn
samples (Figure 4). The N-free sample CuCo@NC0 has weak
peaks in the CO2-desorption temperature range of 250-600 °C.
Generally, CO2-TPD shows a positive relationship between the
amount of basic sites and the area of peaks. The peak intensity
increased obviously after the N-doping in CuCo@NC1,
revealing that the basic sites were mainly ascribed to the N-
doping. CuCo@NC2 possessed the strongest and maximum
peak area, directly confirms the formation of abundant basic
sites. As a result, CuCo@NC2 not only enable the oxidative
esterification of alcohol to proceed in a base additive-free
reaction system, but also exhibit a high activity and selectivity.
To investigate the general applicability of CoCu@NC2 for
oxidative esterification of alcohols to esters, the substrate
scope was surveyed (Table 2). Both electron-donating (Br, Cl,
NO2) and -with drawing (OCH3 and CH3) group substituted
benzyl alcohols were smoothly converted into the
corresponding methyl esters in high conversions (> 90%) and
selectivity at 60°C under the base-free conditions (1a−5a of
table 2). The ortho-substituted benzyl alcohol and
benzenedimethanol were relative tolerated, but the
conversions and selectivity can be optimized by longer
reaction time to 24 h (6a−7a of table 2). Even heterocyclic
alcohols (cinnamyl alcohol and furfuryl alcohol) could be
oxidized over CoCu@NC2 to their corresponding esters with
good conversion and high selectivity with a longer reaction
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time of 48 h (8a−9a of table 2). Finally, CoCu@NC2 was applied
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Notes and references
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DOI: 10.1039/D0NJ00172D
to catalyze the aerobic esterification of benzyl alcohol with
other aliphatic alcohols, including ethanol, propanol and
butanol, the benzyl alcohol showed good to high conversions
with considerable selectivity under the base-free condition
(10a−12a of table 2). However, owing to the steric effect of
fatty carbon chain, the long-chain alcohols have poor
conversions and selectivity relative to methyl alcohol. These
examples demonstrate that CoCu@NC2 is a promising catalyst
for oxidative esterification of a variety alcohols to esters.
Obviously, the catalytic activity and selectivity of CoCu@NC2
are superior to those obtained in base-containing systems
(Table S1, entries 1-5). It is also higher or comparable to those
of previously reported heterogeneous catalysts for oxidative
esterification in base-free systems (Table S1, entries 6-10).
For the durability test, the catalyst CoCu@NC2 was
centrifuged, washed, and thermal treated at 500 °C, then
reused in the next run. The catalyst showed a stable
recyclability, the conversion was still up to 87% with selectivity
maintained at 98% after five runs (Figure S6). The XRD pattern
for the recovered catalyst was similar to that of the fresh one
(Figure S7), and the ICP-AES analysis of the recycled catalyst
showed negligible leaching of either Co or Cu, indicating a
heterogeneous nature of CoCu@NC2.
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Conclusions
In conclusion, we have developed Co and Cu co-embedded N-
doped carbon catalysts (CoCu@NCn) based on a facile one-step
carbonization strategy. With both Co and Cu nanoparticles
embedded in a single NC support, synergetic active sites
produced in the catalyst CoCu@NC2, and therefore caused a
significant increase in catalytic activity and selectivity for
aerobic oxidative esterification of alcohols to esters. N-doping
was also demonstrated to be essential to form highly active
catalysts with abundant basic sites that endow the oxidative
esterification proceeded under base-free conditions. The
CoCu@NC2 exhibited remarkable catalytic performance under
mild conditions for a wide variety of substrates and can be
reusable for 5 times. The present strategy may be suitable for
the exploration of more metal-based catalysts with enhanced
activity and durability that can be applied in various catalytic
processes.
Conflicts of interest
There are no conflicts to declare
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The authors thank the National Natural Science Foundation of
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DOI: 10.1039/D0NJ00172D
Facile Synthesis of Highly Efficient Co/Cu@NC Catalyst for Base-free Oxidation
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of Alcohols to Esters
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Jiusheng Jiang, Xiang Li, Shengyu Du, Langchen Shi, Pingping Jiang, Pingbo Zhang, Yuming
Dong, Yan Leng*
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School of Chemical and Material Engineering, Jiangnan University, Lihu Road 1800#, Wuxi
214122, Jiangsu, China.
E-mail: yanleng@jiangnan.edu.cn.
Co/Cu nanoparticles co-decorated N-doped carbon exhibits excellent activity and stability
for base-free oxidation of alcohols to esters.