Oxygen-Rich Carbon Quantum Dots as Catalysts for Selective Oxidation of Amines and Alcohols

Article abstract of DOI:10.1002/cctc.201701148

Metal-free carbocatalysis has been widely utilized for aerobic oxidative reactions. Here, we report that oxygen-rich carbon quantum dots (O-CQDs) demonstrate a catalytic performance superior to graphene oxide, if used as a metal-free nanocatalyst for the direct transformation of amines and alcohols, under mild and solvent-free conditions. O-CQDs show a yield of 75 % for the oxidative coupling of amine to imine (with 5 wt % catalyst loading) and a conversion of 3.8 % for benzyl alcohol (with 2 wt % catalyst loading). The catalytic activities of thermally treated O-CQDs are further improved for benzylamine, for example, indicated by a yield of up to 98 % with 4 wt % catalyst loading. In addition, O-CQDs show a photoenhanced catalytic ability of amine (98 % yield with 5 wt % catalyst loading for 6 h reaction). Characterizations and simulations show that numerous carboxyl oxygen functional groups and unpaired electrons at the edge sites of O-CQDs are likely involved in the aerobic oxidation of amines.

Full text of DOI:10.1002/cctc.201701148

Accepted Article
Title: Oxygen-rich carbon quantum dots as catalysts for selective
oxidation of amines and alcohols
Authors: Jianglin Ye, Kun Ni, Jie Liu, Guanxiong Chen, Mujtaba Ikram,
and Yanwu Zhu
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of the final Version of Record (VoR). This work is currently citable by
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Oxygen-rich Carbon Quantum Dots as Catalysts for Selective
Oxidation of Amines and Alcohols
[
a]
[a]
[a]
[a]
[a]
[a,b]
Jianglin Ye, Kun Ni, Jie liu, Guanxiong Chen, Mujtaba Ikram and Yanwu Zhu*
Abstract: Metal-free carbocatalysis has been widely utilized for
aerobic oxidative reactions. Here we report that oxygen-rich carbon
quantum dots (O-CQDs) demonstrate catalytic performance superior
to graphene oxide, when being used as metal-free nanocatalyst for
the direct transformation of amines and alcohols, under mild and
solvent-free conditions. O-CQDs show a yield of 75% for the
oxidative coupling of amine to imine (with 5 wt.% catalyst loading)
and a conversion of 3.8% for benzyl alcohol (with 2 wt.% catalyst
loading). The catalytic activities of thermally treated O-CQDs are
further improved for benzylamine, e.g., indicated by a yield of up to
(CQDs), including graphene quantum dots (GQDs), have
exhibited unique physical and chemical properties, and found
[8]
applications in catalysis and bio-imaging.
demonstrated that, CQDs decorated with hydrogen sulfate
groups give new aspect in enhanced photo-catalyzed
performance for the ring-opening of epoxides.
photoluminescence (PL) and photo-induced
Previous work
a
[9]
With
electron
transfer/reservoir properties, CQDs have been used as
heterogeneous catalysts for H-bond catalysis, showing excellent
photo-enhanced catalytic abilities with a yield of 89% in the aldol
condensation when 4-cyanobenzaldehyde was used as
98% with 4 wt.% catalyst loading. In addition, O-CQDs show a
[10]
photo-enhanced catalytic ability of amine (98% yield with 5 wt.%
catalyst loading for 6 h reaction). Characterizations and simulations
show that numerous carboxyl oxygen functional groups and
unpaired-electrons at the edge sites of O-CQDs are likely involved in
the aerobic oxidation of amines.
reactant.
Hybrids of CQDs and semiconductors were also
prepared, demonstrating further improved photocatalytic
[11]
activities.
2
For example, CQDs modified MoS has been
explored in the catalysis of hydrogen evolution reaction (HER)
[12]
under visible light,
ascribed to the high charge transfer efficiency between CQDs
and MoS . Relatively, there are few reports demonstrating
and the improved HER activity has been
2
aerobic oxidation reactions using CQDs. The abundant edge
active sites and rich photochemical properties due to the
quantum size effect in CQDs have prompted us to investigate
the possibility of using the oxygen-functionalized CQDs for direct
transformation of reactant by aerobic oxidation. However, the
typical content of oxygen-containing functional groups in CQDs
Introduction
Catalytic oxidation of chemicals using oxygen molecular as
oxidant for high-quality products remains a significant challenge
[
1]
in fine chemical processes and green chemistry. Compared to
traditional metal-based catalysts, carbon-based materials,
including graphene oxide, carbon nanotubes, amorphous carbon,
mesoporous carbon and graphitic carbon nitride, have been
developed as potential catalysts owing to their relatively low cost
and natural abundance. In many heterogeneous oxidation
reactions, carbon materials serve both as supporting scaffolds
for metal catalysts as well as carbocatalyst in its own right.
Among the potential carbocatalysts, graphene oxide (GO)
produced by exfoliation of graphite oxide has been investigated
for the oxidation of alcohol,
oxygen-containing functional groups and rich defects in the
carbon backbone of GO provide active sites to trap and activate
target molecular species. In addition, the conjugated π electron
system leads to the co-existence of both aromatic and olefin
[13]
cannot reach to that of GO.
Herein we present
a detailed study on the catalytic
performance of an oxygen-rich carbon quantum dots (O-CQDs)
as metal-free nanocatalyst for aerobic oxidation of amines and
alcohols. With oxygen-containing functional groups and possible
curvatures due to presence of pentagons, O-CQDs have
demonstrated excellent catalytic abilities for the selective
oxidation of alcohols to aldehyde; a yield of up to 98% has also
been obtained with a relatively low catalyst loading (4 wt.%) for
the oxidative coupling of amine to imine under mild conditions
[
2]
[3]
[4]
[5]
alkane and amines.
The
(
90 °C and solvent-free). It is suggested that numerous carboxyl
oxygen functional groups coupled with spins of π-electrons from
the atoms located at the edge sites of O-CQDs are most likely
involved in the aerobic oxidation of amines. In addition, photo-
induced electron-accepting in O-CQDs has led to further
improvement in the efficient catalytic oxidation performance of
amines under light illuminations.
[
6]
[
7]
properties in graphene-based catalysts. The high catalyst
loading (typically 20-400 wt.%) for typical oxidative reactions,
however, hinders the further application of GO-based
[3a]
catalysts.
With a typical size of below 10 nm, carbon quantum dots
Results and Discussion
[
a]
b]
Jianglin Ye, Kun Ni, Jie liu, Dr. Guanxiong Chen, Mujtaba Ikram and
Prof. Yanwu Zhu
Key Laboratory of Materials for Energy Conversion, Chinese
Academy of Sciences, & Department of Materials Science and
Engineering, University of Science and Technology of China, Hefei,
Anhui, 230026, P. R. China
O-CQDs were prepared from fullerenes (C60) using
a
modified Hummers’ method and the preparation was described
[14]
in the supporting information and our previous report.
To
ensure the removal of metallic residuals, the as-obtained O-
CQDs suspension was dialyzed for 8 days before the catalysis
E-mail: zhuyanwu@ustc.edu.cn
Prof. Yanwu Zhu
2
-
[
studies. The content of Mn, Fe and SO
4
in the dialyzed
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), University of Science and Technology of China, Hefei,
Anhui, 230026, P. R. China
suspension was all found to be lower than 10 ppm based on an
inductively coupled plasma mass spectrometry (ICP-AES)
analysis. The typical diameter of O-CQDs falls in the range of
Supporting information for this article is given via a link at the end of
the document.
0.6-2.2 nm with an average height of ≈1.2 nm (Figure S1). The
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Raman spectra of O-CQDs clearly show the presence of G band
variations in the O-CQDs loadings and reaction times have
significant impacts on the yield of the imine product (Table 1,
entries 2-5). For the O-CQDs catalyst loading of below 2 wt.%,
the reaction has exhibited a first order rate dependence with
−1
−1
(
(
≈1580 cm ) and D band (≈1360 cm ) of graphitic carbon
Figure S2). The Raman band near 1470 cm may be ascribed
−1
to the Ag(2) vibration mode of pentagon pinch remained in O-
CQDs. A broad PL peak through down-conversion process was
-
1
[17]
respect to amine with a rate constant of 0.029 h (Figure S3).
[15]
observed from O-CQDs;
excitation wavelength for the excitation wavelength of lower than
60 nm, then decreases with excitation wavelength for longer
the PL intensity increases with
Higher concentration of catalyst and longer reaction duration
resulted in significantly improved catalysis performance.
Compared to GO and multi-walled carbon nanotubes
(MWCNTs) catalysts, O-CQDs show a much higher yield under
the same conditions, indicated by, e.g., yield of 75% for O-CQDs,
remarkably higher than that of 47% for GO and that of 6% for
MWCNTs for the same catalyst loading of 5 wt.% (Table 1,
entries 5, 7, 8).
3
excitation wavelengths (Figure S2).
a)
Table 1. Oxidation of benzylamine to N-benzylidene benzylamine.
Entry
Catalyst
No catalyst
O-CQDs
O-CQDs
O-CQDs
O-CQDs
GO
Loading (wt.%)
Yield (%)
< 0.5
14
1
-
2
b)
2
3
2
31
c)
4
2
43
5
6
7
8
5
75
2
21
GO
5
47
Figure 1. Recyclability of HO-CQDs in benzylamine oxidation. Reaction
conditions: 90 °C, oxygen condition, 4 wt.% catalyst loading, 12 h.
MWCNTs
O-CQDs (hv)
O-CQDs (hv)
RO-CQDs
5
6
In addition, it has been reported that light-induced electrons
d)
9
2.5
5
54
-
(
e ) can activate molecular oxygen during the synthesis of
[10,18]
d)
organic compounds.
Thus the oxidative reaction was also
10
98
conducted under irradiation of visible light using O-CQDs
catalyst; a notable yield of up to 98% for the oxidation of amine
to imine is obtained with 5 wt.% catalyst loading for 6 h reaction
(Table 1, entry 10), higher than those from 5 wt.% O-CQDs
1
1
2
3
4
5
7
1
1
1
O-CQDs (N
HO-CQDs
HO-CQDs
2
)
5
5
(
Table 1, entry 5) or 5 wt.% GO (Table 1, entry 7) measured
2
54
under the same conditions except without irradiation. No
observation of enhanced catalytic ability in the oxidative coupling
of amines to form imines under visible light irradiation has been
previously reported for heterogeneous aerobic oxidation using
GO-based catalysts. Control experiments, performed in nitrogen
atmosphere or using reduced carbon quantum dots (named RO-
4
98
d)
1
5
HO-CQDs (hv)
2
56
e)
1
1
6
porous-GO
5
98
o
CQDs, obtained by annealing O-CQDs at 800 C for 2 h in Ar)
f)
7
Au/Al
2
O
3
5
92
with much less amount of oxygen functional groups as catalysts
(Figure S4), show dramatic decreases in the yield (Table 1,
entries 11, 12). The results indicate that oxygen is the terminal
oxidant and the major catalytically active sites are lost in RO-
CQDs. To further investigate the contribution of oxygen
functional groups in the catalyst, O-CQDs was gently reduced by
heating at 200 °C, resulting in a sample named as HO-CQDs in
which some oxygen functional groups like C-O are partially
removed. As shown in Table 1, entry 14, HO-CQDs surprisingly
show an improved conversion of 98% from amine to imine, with
a catalyst loading of 4 wt.%. Such a conversion efficiency from
HO-CQDs is higher than that from porous GO catalyst (5 wt.%
a)
Standard reaction conditions: 25 mg catalyst, 90 °C under solvent-free and
oxygen condition for 12 h. Catalyst loading wt.% is the ratio of weight of
catalyst to that of amine.
but for 6 h and 20 h, respectively. Reactions were run under the standard
conditions but for 6 h with visible light irradiation.
b), c)
Reaction was run under the standard conditions
b
c
d)
e), f)
e
f
Refer to ref. 5b and ref.
1
1
9. Yield percentage was calculated using H NMR.
We focus on the oxidative coupling of amines to form imines,
which are important electrophilic intermediates in organic
synthesis.
reactions using O-CQDs as catalyst and oxygen as oxidant
under a mild condition (90 °C and solvent-free) resulted in high
yield for N-benzylidene benzylamine. The conversion is lower
than 0.5% in absence of catalyst (Table 1, entry 1). The
[16]
As shown in Table 1, performing oxidation
2 3
loading, 98% yield, 12 h, entry 16) or from Au/Al O (5 wt.%
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ChemCatChem
10.1002/cctc.201701148
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loading, 92% yield, 24 h, entry 17). The recyclability of HO-
CQDs was tested by repeatedly using catalyst recycled by
filtration and rinsing in acetonitrile. As shown in Figure 1, HO-
CQDs show a good recyclability and the decrease in yield with
recycle number is not obvious for 5 cycles. To summarize, the
performance of as-made O-CQDs is superior to that of GO
catalyst; the catalytic activity of HO-CQDs catalyst and
photocatalytic activity of O-CQDs are comparable or even
superior to those of porous GO, doped carbon catalysts or metal
aniline/benzylamine exhibited
unsymmetrical imine. The rate of the reaction can be further
improved under visible light irradiation (Table 2, entry 8).
a
yield of 23% towards the
[5b,16a,19]
catalysts.
Table 2. Oxidative coupling of primary amines to imines catalyzed by O-
a)
CQDs.
Entry
Substrates
Time (h)
Yield (%)
b)
1
10
85
Figure 2. (a) Thermal gravimetry of O-CQDs, GO and HO-CQDs. (b) XPS C
1
s spectra of O-CQDs, GO and HO-CQDs, respectively.
b)
2
10
12
12
20
8
81
91
64
-
To determine why O-CQDs, especially HO-CQDs show
b)
3
catalytic properties superior to GO for the oxidative coupling of
amines to imines, the detailed chemical bonding configuration
was investigated using thermal gravimetry analysis (TGA) and
X-ray photoelectron analysis (XPS). Figure 2a shows TGA
thermograms of O-CQDs, HO-CQDs and GO obtained in
nitrogen atmosphere. About 10% weight loss is found for both
O-CQDs and GO below 100 °C which accounts for the
volatilization of water molecule. While HO-CQDs show a slightly
higher weight loss (14%) below 100 °C, indicating the stronger
hygroscopic nature in HO-CQDs. Comparing with GO, a much
lower weight loss between 100 and 400 °C is shown from O-
CQDs and HO-CQDs, suggesting more pyrolysis of the labile
oxygen-containing groups present in GO. The residual weight of
GO remains 36% of initial mass at 800 °C while that of O-CQDs
has sharply decreased to only 26% under the same conditions.
HO-CQDs show less weight loss than O-CQDs and GO at
800 °C. These results reveal that the concentration of oxygen
functional groups in O-CQDs is more than that in GO and some
unstable oxygen functional groups have been effectively
removed in HO-CQDs. The high-resolution C 1s peaks are
shown in Figure 2b and C/O in different groups are obtained
from the deconvolution (Table 3). The deconvolution of C 1s
peak for all three catalysts shows similar type of oxygen
functional groups. Four main carbon bonding types: C-C (≈284.7
b)
4
b)
5
c)
1: 4
6
2: 96
1
2
d)
1: 23
7
6
2: 77
d)
1: 25
2: 75
8
4 (hv)
6
d)
1: 29
9
2: 71
a)
Standard reaction conditions: 90 °C under solvent-free and oxygen condition.
wt.% loading of O-CQDs. 1 mmol aniline, 1 mmol benzylamine, 10 mg of
O-CQDs. 4 mmol aniline, 1 mmol benzylamine, 20 mg of O-CQDs.
b)
c)
5
d)
The oxidation of different types of amines is explored using
O-CQDs as catalyst. It is proven that benzylamines with the
electron-donating methyl groups are suitable substrates (Table 2,
entries 1, 2). When 4-Chlorobenzylamine was used as reactant
with the electron-withdrawing group, an imine yield of 91% was
obtained (Table 2, entry 3). The catalyst was also applicable for
the oxidation of 2-thiopheneamine containing S as heteroatom
Table 2, entry 4), indicating that the high toleration for electronic
effects of the substituents on the phenyl ring under mild reaction
conditions.
eV), C-O (≈286.9 eV), C=O (≈288.4 eV), and COOH (≈288.9 eV)
can be ascribed.
1
CQDs. Further analysis shows that 35.8% of the total oxygen is
in the form of C-O and 5.3% in the form of COOH for O-CQDs,
both higher than the case in GO (30.9% of C-O and 2.7% of
COOH). The fraction of COOH is 12.4% in HO-CQDs. For
comparison, GO was also gently reduced by heating at 200 °C
[21]
The C/O atomic ratio obtained from XPS is
(
.2 for O-CQDs, less than 1.7 for GO catalyst or 2.2 for HO-
[5b]
The Hammett plot can be obtained by the
oxidation of benzylamine and its derivatives with para-position
substituent (Figure S5),
[20]
suggesting that the oxidation is
favored by the presence of electron-donating substituents.
However, the oxidation of aniline lacking α-H atoms did not
proceed at all (Table 2, entry 5). O-CQDs can also be used to
synthesize unsymmetrical imines by the oxidative cross
condensation of benzylamines and amines. Reactions between
aniline and benzylamine were performed at different conditions
(
leading to a sample named as H-GO). The C/O atomic ratio
obtained from XPS is 4.1 for H-GO, and the fraction of COOH
increases to 5.9% in H-GO (Table and Figure S6).
3
Interestingly, H-GO has shown an improved yield (80%) for N-
benzylidene benzylamine when being compared to that of GO
(
Table 2, entries 6-9). The reaction with 4/1 molar ratio of
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10.1002/cctc.201701148
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(
47%, seen in Table S1) for the same loading of 5 wt.%.
[22]
According to a previous report,
hydroxyl group has the lower
decomposition temperature (100-150 °C) compared to carboxyl
and ketones; the thermal treatment at 200 °C gives rise to the
quick decomposition of hydroxyl, leading to a relatively improved
percentage of carboxyl.
Entry
O-CQDs
53.8
46.2
35.8
5.1
HO-CQDs
68.7
GO
63.2
36.8
30.9
3.2
H-GO
80.2
19.8
12.4
1.5
total carbon (C)
total oxygen (O)
oxygen in C-O
oxygen in C=O
oxygen in COOH
31.3
15.8
Figure 3. (a) Structure schematic of 1-pyrene carboxylic acid, O-CQDs and
HO-CQD. (b) EPR measurement of O-CQDs, HO-CQDs and GO. (c) A
proposed reaction process over O-CQDs.
3.1
5.3
12.4
2.7
5.9
Table 3. Fraction of elements in various chemical bondings obtained from the
[25]
According to Lerf-Klinowski model,
epoxy and hydroxyl
deconvolution of the C 1s peak.
groups mainly reside on the O-CQDs basal plane, whereas the
periphery of the O-CQDs sheets is mainly decorated by carboxyl,
anhydride groups. In addition, the carboxylic group in porous GO
has been considered as the active group which serves as
hydrogen bonding sites for the oxidation of amine.
carboxyl-rich nature and nanoscale feature of O-CQDs and HO-
CQDs make them to own more catalytically active edge sites
Figure S8 a). Besides, it has been found that the molecular with
both carboxylic acid groups and large π-conjugation, e.g., 1-
pyrene carboxylic acid, can exhibit high reactivity in the oxidative
coupling of benzylamine; the carboxylic acids without large π-
conjugation did not exhibit catalytic activity.
carboxylic acid has been used as a molecular analogue of the
CQDs made in this work (Figure 3a). Compared to GO, the
much smaller size of O-CQDs results in more periphery area
that is surrounded by conjugated carbons; electron spins
originating from the non-bonding π electron states are found at
the active edges (simulation results in Figure S8 b, c).
1
3
Fourier transform infrared spectroscopy (FTIR) and
C
solid-state NMR spectra (Figure S7) were performed in order to
further analyze the functional groups in O-CQDs, GO and HO-
[5b]
The
[5b,22-23]
CQDs. As previously reported,
infrared spectra of all
types of catalysts indicate contributions from various oxygen
functional groups. Vibrational modes are shown for hydroxyls
(
-
1
-1
(
C-OH, ≈3000-3700 cm ), C-H
x
(≈2800-3000 cm ), carboxyls
and/or ketones within overlapped frequency range (C=O, COOH
-
1
2
-1
≈
1700-1900 cm ), sp -hybridized C=C (≈1550-1650 cm ),
-
1
-1
epoxides (C-O-C, 1100-1400 cm and 800-900 cm ), and
ethers (C-O at 800-1200 cm ).
[5b]
1-pyrene
-
1 [24]
In contrast to GO, O-CQDs
possess stronger epoxides (O-C-O), C-O and C-OH. However,
the intensity of the carboxyl peak in O-CQDs is lower than that in
HO-CQDs. In addition, C-H
annealing, indicating the further exposure of carbon edges.
NMR spectra of O-CQDs and HO-CQDs reveal broad
x
stretching almost eliminated by the
1
3
C
a
[26]
EPR
2
resonance peak at 138.0 ppm, which suggests the sp
conjugation. The oxygen signals at 73.4 ppm (C-OH) are largely
deteriorated in HO-CQDs, which is consistent with XPS results.
In observation of the fact that HO-CQDs and H-GO
demonstrated the higher conversion efficiency in the oxidation of
amine comparing to O-CQDs and GO, respectively, the
spectroscopic results provide strong evidence that the carboxylic
acid group plays an important role in the aerobic oxidation of
amines.
investigations (Figure 3b) have shown that the concentration of
18
-1
19
-1
spins is about 1.9×10 spins g for O-CQDs, 2.4×10 spins g
for HO-CQDs, both more than 100 times higher than that of GO
16
-1
(
8.2×10 spins g ). In addition, unpaired electrons may be
[27]
highly reactive towards the coupling of diazonium.
We
functionalized O-CQDs with phenyl groups using diazonium
coupling reactions, leading to a sample named as O-CQDs-Ph
(
Figure S9). The catalytic activity of O-CQDs-Ph has been
reduced to 21% from 32% for as-prepared O-CQDs in the
oxidative coupling of benzylamine, suggesting that the high
catalytic ability in O-CQDs and HO-CQDs are related to the
unpaired π-electron.
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a)
Table 4. Oxidative coupling of amine catalyzed by O-CQDs in the presence of
Table 5. Catalytic activity of O-CQDs in the oxidation on different alcohols.
radical quencher and deionized water, respectively.
Conversion
Entry
Substrate
Product
(
%)
Entry
Substrate
Yield (%)
b)
a)
1
3.8
6.7
8.4
ND
1
Benzylamine (with BHT)
32
b)
2
Benzylamine (with H
2
O)
58
c)
2
a)
Reaction conditions: 2 wt.% of O-CQDs catalyst, 90 °C, 12 h. 200 wt.% of
b)
BHT was added. 2 mmol H
2
O was added.
d)
3
The unpaired electrons of CQDs may have activated
oxygen molecules during the oxidation reactions.
[
5b,28]
e)
4
According
to the literatures on metal-free carbocatalysis using oxygen
[5a,29]
molecular as the oxidant,
it is believed that an oxidative
f)
dehydrogenation involving the formation of RCH=NH
intermediate could be the key for achieving high efficient amine
5
3.5
5.6
[
19a,30]
oxidation.
could not quench the oxidation of benzylamine (Table 4, entry 1),
revealing that O is activated directly on the surface of the
catalyst for the oxidation reactions.
conversion efficiency can be enhanced when some amount of
O was introduced at the beginning of the reaction (Table 4,
entry 2). TGA result also shows the stronger hygroscopic nature
in HO-CQDs, indicating that the amount of H O plays an
It was found that a free-radical scavenger (BHT)
g)
6
2
a)
Standard reaction conditions: 25 mg catalyst, different weight of different
[16a]
Interestingly, the
b)
alcohols to make different wt.% of catalyst, 1 atm O
2
.
Reactions were
Reactions were
c), d)
performed at 100 °C for 48 h using 2 wt.% O-CQDs.
H
2
e)
performed at 100 °C for 24 h using 2 wt.% O-CQDs. Reaction was performed
f)
at 90 °C for 48 h using 2 wt.% O-CQDs. Reaction was performed at 75 °C for
2
g)
1
2 h using 2.5 wt.% O-CQDs. Reaction was performed at 100 °C for 24 h
important role in the aerobic oxidation. In addition, ammonia was
also detected in the oxidation reaction. Based on the
observations, a reaction path is proposed and shown in Figure
using 4 wt.% O-CQDs. ND = not determined.
alcohol was used as reactant, the condensation was likely
facilitated by the interaction of the alcohol with the polar groups
3c. The formation of aminal intermediate reacts with H
2
O and
loses NH to produce aldehyde RCH=O; the aldehyde
3
[32]
(
such as C-OH, O-C-O) on O-CQDs by hydrogen bonding.
subsequently reacts with a second amine molecule to give the
final imine coupling product. In addition, when the reactions
were conducted under light irradiation, the conversion efficiency
was further improved, indicating that the extra electrons
generated in carbon dots further activate oxygen molecules for
Our results indicate the sensitivity on the type of surface
functional groups in CQDs for the selective oxidation reaction of
alcohol and amine. Furthermore, the reactions of α-
phenylethanol and cyclohexanol show the similar conversion
efficiency under same conditions, which is higher than that of
[31]
the reaction (Figure S10).
The heterogeneous oxidation
benzyl alcohol (Table 5, entries 2, 3). However, the -NO
2
caused by the edges of defects and photo-induced electrons
work synergistically to trap amine molecules and activate
molecular oxygen, thus improving conversion efficiency of O-
CQDs.
substituted functional groups attached to the aromatic ring of
benzylic alcohols results in a negligible conversion (Table 5,
entry 4), and a modest conversion of 3-thiophenemethanol into
the respective aldehyde without sulfur oxidation is obtained
Meanwhile, the catalytic efficiency of O-CQDs for solvent
free aerobic selective oxidation of alcohols with different catalyst
loadings was investigated (Table 5). For the reactions, neat
alcohols were heated up to given temperature in the presence of
various wt.% O-CQDs, under 1 atm of oxygen. For 2 wt.% O-
CQDs loading, the conversion is about 3.8% with a turnover
(
Table 5, entry 5). Moreover, a treatment of O-CQDs with alkyne
afforded as its respective ketone product, which furthermore
indicates the ability of oxidizing unsaturated hydrocarbons by O-
CQDs catalyst (Table 5, entry 6). The results indicate that O-
CQDs may exhibit broad chemoselectivity.
-
2
number (TON) of 1.9×10 to benzaldehyde (Table 5, entry 1). In
addition, the conversion percentage for O-CQDs catalyst was
found to increase from 1.7% to 6.2% with reaction time for 1
wt.% catalyst loading (Table S2). It is worth noting that the
catalytic efficiency of O-CQDs is much higher than that of GO
Conclusions
In conclusion, we demonstrated that O-CQDs can be used
as selective metal-free catalysts for aerobic oxidation of amines
and alcohols without using any solvents. Based on the studies
on the reaction process of the oxidative coupling of amines to
form imines, it is proposed that the carboxylic groups at the
edges of carbon dots, along with the unpaired electrons, work
synergistically to trap and activate molecular oxygen and the
amine molecules, thus facilitating the conversion of amine. Heat
(
Figure S11), under the same reaction conditions. No benzoic
acid was found in these reactions. On the other hand, reduced
O-CQDs (RO-CQDs) performed unreactive (Table S2, entries 8,
9), and HO-CQDs resulted in negligible conversion (Table S2,
entry 10). From XPS and NMR spectra (Figure S7), the loss of
oxygenated groups such as C-OH may cause the decrease in
the active sites in HO-CQDs. Therefore when primary benzyl
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ChemCatChem
10.1002/cctc.201701148
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treatment has further improved the catalytic activities of O-CQDs
for oxidative coupling of amine to imine because of the increase
in both active sites and unpaired electrons. The mild synthetic
procedure and unprecedentedly low catalyst loading described
herein is expected to find utilization in metal-free catalysis for a
wide range of solvent-free oxidations.
Characterizations and simulation.
Thermogravimetric analysis (TGA) was carried out using a Q50000
IR (TA, US) in nitrogen for the temperature rising from room temperature
to 850 °C with a step of 10 °C/min. Transmission electron microscopy
(
TEM) images and HRTEM images were taken with a JEM-ARM200F
JEOL, Japan). Atomic force microscope (AFM) images were obtained
(
from an AFM (Nanoscope Icon, Veeco) at the room temperature. Raman
spectra were collected using an inVia Raman microscope (Renishaw,
Experimental Section
532 nm laser, UK). Inductively coupled plasma-atomic emission
spectrometry (ICP-AES) was performed using Optima 7300 DV (Perkin-
Elmer, US). X-ray photoemission spectroscopy (XPS) was carried out on
a Thermo ESCALAB 250 (Thermo Scientific, US) with Al Kα radiation
Preparation of O-CQDs.
The oxygen-rich carbon quantum dots (O-CQDs) powder was first
(hv= 1486.6 eV). Fourier transform infrared spectroscopy (FTIR) were
synthesized by a modified Hummers’ processing of C60 as described in
performed using Nicolet 8700 (Thermo Scientific, US). Electron
a
[
14]
previous report. Briefly, fullerene (500 mg, 99.9%, XF NANO, Inc.) was
mixed with 25 ml concentrated H2SO4 and cooled in an ice water bath.
Under vigorous stirring, KMnO4 (1.75 g, 99.6%) was added gradually,
and the temperature of the mixture was kept below 5 °C. The mixture
was stirred at 25 °C in a water bath for another 24 h after removing the
ice bath. 150 mL H2O was slowly added to the pasty mixture to get a
deep yellow solution. The mixture solution was then purified by dialysis
for 8 days. Finally, the resulting solution was dried by freeze-drying to get
O-CQDs powder. RO-CQDs, HO-CQDs and H-GO were prepared by a
simply heat treatment at 800 °C or 200 °C for 2 h in argon, respectively.
The GO powder was prepared from natural graphite powder using a
paramagnetic resonance (EPR) spectra were obtained using a JES-FA
1
200 (JEOL, Japan). H NMR was performed on an AVANCE AVIII 400
MHz spectrometer (Bruker, Switzerland). The spin calculation was
-6
perform by using Dmol3 software package. the SCF tolerance was 10
hartree for energy with the smearing parameter of 0.005 hartree. The
DNP basis set was used to perform all electron calculation and the spin
was set as unrestricted.
Acknowledgements
[
33]
Hummers’ method.
Multiwalled carbon nanotubes (MWCNTs) with
This work was supported by the China Government 1000 Plan
Talent Program, China MOE NCET Program, Natural Science
Foundation of China (51322204), the Fundamental Research
Funds for the Central Universities (WK2060140014 and
WK2060140017), and funding from Hefei National Synchrotron
Radiation Lab.
diameter of 30-50 nm were obtained from Chengdu Organic Chemicals
Co., Ltd. and used without further treatment.
Solvent-free synthesis of imines.
In a typical reaction, after mixing benzylamine with a specified
amount of catalysts together (as mentioned in Tables 1, 2, catalyst
loading is the ratio of weight of catalyst to that of amine) followed by
ultrasonication for 10 min, the reactor was sealed in desired atmosphere
and heated in oil bath at predetermined temperature for certain reaction
durations. After the reaction, the mixture was further mixed with CDCl3
Keywords: carbon quantum dots • oxygen-rich • oxygen
functional groups • unpaired electrons • aerobic oxidation
(
~0.7 mL), filtered via 20 nm PTFE filter membrane to remove catalyst for
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ChemCatChem
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FULL PAPER
[
a]
[a]
[a]
Oxygen-rich carbon quantum dots
Jianglin Ye, Kun Ni, Jie liu,
[a]
[a]
(
O-CQDs) demonstrate superior
Guanxiong Chen, Mujtaba Ikram
and Yanwu Zhu*
[a,b]
homogeneous catalytic and photo-
enhanced catalytic performance,
when being used as metal-free
Page No. – Page No.
nanocatalyst
transformation
for
of
the
amines
direct
and
and
Oxygen-rich Carbon Quantum Dots
as Catalysts for Selective Oxidation
of Amines and Alcohols
alcohols.
Characterizations
simulations show that numerous
carboxyl oxygen functional groups
and unpaired-electrons at the edge
sites of O-CQDs are likely involved in
the aerobic oxidation of amines.
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