Room temperature activation of oxygen by monodispersed metal nanoparticles: Oxidative dehydrogenative coupling of anilines for azobenzene syntheses

Article abstract of DOI:10.1021/cs300707y

It is highly challenging but desirable to develop efficient catalysts for the activation of oxygen under mild conditions. Here, we report that various monodispersed metal nanoparticles (Ag, Pt, Co, Cu, Ni, Pd, and Au) efficiently activated molecular oxygen under mild conditions, illustrated by the aerobic oxidation of anilines to form either symmetric or asymmetric aromatic azo compounds. This discovery indicates that exploiting the catalytic power of nanoparticles could enable sustainable chemistry suitable for important oxidation reactions.

Full text of DOI:10.1021/cs300707y

Research Article
pubs.acs.org/acscatalysis
Room Temperature Activation of Oxygen by Monodispersed Metal
Nanoparticles: Oxidative Dehydrogenative Coupling of Anilines for
Azobenzene Syntheses
Shuangfei Cai,† Hongpan Rong,† Xiaofei Yu,† Xiangwen Liu,† Dingsheng Wang,† Wei He,*,‡
and Yadong Li*,†
‡
†Department of Chemistry and Tsinghua-Peking Joint Center for Life Science, Tsinghua University, Beijing, 100084, People’s
Republic of China
S
* Supporting Information
ABSTRACT: It is highly challenging but desirable to develop
efficient catalysts for the activation of oxygen under mild
conditions. Here, we report that various monodispersed metal
nanoparticles (Ag, Pt, Co, Cu, Ni, Pd, and Au) efficiently
activated molecular oxygen under mild conditions, illustrated
by the aerobic oxidation of anilines to form either symmetric
or asymmetric aromatic azo compounds. This discovery
indicates that exploiting the catalytic power of nanoparticles
could enable sustainable chemistry suitable for important
oxidation reactions.
KEYWORDS: nanoparticle, oxygen activation, oxidation coupling, azobenzene
facile access to reactant molecules as well as easy separation and
recycling of the catalysts, are emerging as one of the most
promising novel catalyst classes.23 Recently, we have developed
a systemic synthetic technique for monodispersed metal
NPs.24−26 In our effort to probe their advantages in catalysis
based on the above rationale, we discovered that many types of
metal NPs could efficiently activate molecular oxygen in air at
room temperature, leading to the synthesis of azo compounds
via oxidative coupling of anilines.
Aromatic azo compounds have found important applications
in many industry sectors ranging from dyes, pigments, and food
additives to pharmaceuticals.27−31 Reflecting their oxidation
state, azo compounds are most conveniently accessed by
oxidative coupling of anilines. Classic methods entail
stoichiometric reactions that employ 1 or more molar
equivalents of environmentally unfriendly oxidants, such as
Mn, Pb, and Hg salts.32−34 In light of the sustainable chemistry,
catalytic dehydrogenative coupling of anilines using molecular
oxygen as the oxidant not only is attractive but also has met
with some exciting successes.21,35 Notably, unsymmetric
aromatic azo compounds are typically obtained through the
coupling reaction of aryl diazonium salt with electrophilic
aromatics36−38 or the Mills reaction.39,40 There is currently a
lack of a generally applicable catalyst that is capable of oxidative
coupling of aniline under mild conditions. It is in this context
that we wish to report the discovery of such a catalyst: Ag/C
1. INTRODUCTION
Oxidation reactions are of fundamental importance in nature.
Catalytic oxidations that use oxygen as green oxidants in
organic synthesis are of the utmost significance.1 To develop
efficient catalysts for activation of oxygen is highly challenging
but desirable.2−4 Metal nanoparticles (NPs) are implicated in
many industrial reactions, predominantly in heterogeneous
catalysis; for instance, oxidation,5−9 olefin polymerization,10
desulfurization,11 ammonia synthesis,12 and so on. For their
ubiquitous role, research on metal nanoparticle catalysis has
been a central focus and has witnessed tremendous
progresses.13−15 On one hand, much effort has been dedicated
to probing the mechanistic profile, partially enabled by
advancements in synthetic methodology16 and characterization
techniques.17 On the other hand, exploiting their catalytic
power in new types of transformations serves as an avenue to
meet the urgent demand of sustainable development.18 One
important front is oxidation reactions utilizing molecular
oxygen as the green oxidant.19 A prominent example is the
Au NPs/metal oxides catalyst reported by Haruta and co-
workers20 for low-temperature CO oxidation. Recently, Corma
and co-workers21 in their seminal contribution described an
inspiring Au NPs/TiO2 catalyst that enabled the first catalytic
synthesis of aromatic azo compounds. In this regard, a generally
applicable, recyclable catalyst that activates oxygen at room
temperature under atmospheric pressure is likely to impact
many important oxidation reactions both in the laboratory and
in industry.
Received: November 1, 2012
Revised: January 6, 2013
Monodispersed metal NPs,22 which combine the advantages
of both homogeneous and heterogeneous catalysts, including
© XXXX American Chemical Society
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(12.4 nm). We demonstrate that it is highly selective, efficient,
and recyclable for the syntheses of both symmetric and
unsymmetrical azobenzenes, featuring air as the sole oxidant
under mild conditions.
2. RESULTS AND DISCUSSION
2.1. Catalyst Preparation. Monodispersed Ag NPs were
obtained by the reduction of AgNO3 in nontoxic octadecyl-
amine (ODA) according to the protocol described in ref 24.
Here, ODA acts as solvent, reducing agent, and surfactant. By
changing the reaction temperature and reaction time, various
Ag NPs with different sizes were synthesized. Ag/C catalysts
were conveniently prepared by an impregnation method.
Other monodispersed NPs were prepared as follows. Pd NPs
were obtained by the reduction of palladium(II) acetyl
acetonate in ODA by a procedure similar to that described
above. Au NPs were prepared by the reduction of
HAuCl4·4H2O in a solution containing oleylamine (OA) and
oleic acid. Pt, Ni, Cu, and Co NPs were obtained by the
reduction of the corresponding acetyl acetonate-based metal
precursors in OA or a solution containing toluene and OA in
the presence of borane-tert-butylamine complex (TBAB).
2.2. Catalyst Characterization. The morphology and size
of the as-prepared NPs were examined by transmission electron
microscopy (TEM). The as-prepared Ag NPs with average sizes
of 7.7, 9.4, 12.4, and 25.3 nm were composed of spherical
particles (Figure 1).
The 12.4 nm Ag NPs can be well supported on carbon by
standard impregnation methods. Figure 2a and 2b show that
the obtained Ag/C catalysts consist of spherical particles of
unchanged average particle size (12.4 nm). The ICP result
shows that the content of Ag in the 12.4 nm Ag/C sample was
3.2 wt %.
The TEM images and the particle size distribution of various
as-prepared Pt, Co, Cu, Ni, Pd, and Au NPs are shown in
Figure 3.
2.3. Catalyst Activity. We detected the formation of
azobenzene when attempting Ag NPs synthesis using aniline as
the growth inhibition agent. We thus quickly identified that
aniline could be converted into azobenzene in moderate yield
in the presence of a catalytic amount of Ag NPs in DMSO
under air. Interestingly, no product was detected in the absence
of Ag NPs. In addition, no product was detected when the
reaction was carried out under a nitrogen atmosphere. This
serendipitous discovery suggested a catalytic oxidative coupling
of aniline, featuring metal NPs as the catalyst and molecular
oxygen as the oxidant. On the basis of this hypothesis, we
screened various metal NPs. To our great delight, mono-
dispersed Ag, Pt, Co, Cu, Ni, Pd, and Au NPs were found to be
active at room temperature under air (Table 1). This general
yet high activity, possibly arising from the metal NPs’ intrinsic
size-/shape-dependent properties, represents the “magic”
power of NPs in chemical reactions.
Figure 1. TEM images and the particle size distributions of Ag NPs
with average particle diameter d = 7.7 (a, b), 9.4 (c, d), 12.4 (e, f), and
25.3 nm (g, h).
We sought to understand the general activity of the NPs by
probing their catalytic behavior and, in turn, to query their
advantage as the catalyst in azobenzene synthesis. To this end,
we chose Ag NPs (12.4 nm) as a model catalyst because of their
higher activity, convenient syntheses, and reactivity.
The key to the process is the activation of molecular oxygen
in air. We found that Ag NPs is critical to the generation of
superoxide anion radical (Figure 4a, top) using electron spin
resonance (ESR) because no superoxide radical anion was
detected in the absence of Ag NPs (Figure 4a, bottom).
Figure 2. TEM image and the particle size distribution of Ag/C with
average Ag particle diameter d = 12.4 nm.
Further, we identified the presence of aniline radical cation
intermediate (Figure 4b) together with superoxide radical anion
(asterisked, Figure 4b) by in situ ESR. No aniline radical cation
intermediate was observed in the absence of Ag NPs (not
shown). This evidence suggested that Ag NPs catalyzed the
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Table 1. Oxidative Coupling of Aniline with Different Metal
a
NPs
b
entry
catalyst
size/nm
yield/%
1
2
3
4
5
6
7
Ag NPs
Pt NPs
Co NPs
Cu NPs
Ni NPs
Pd NPs
Au NPs
12.4
5.7
58
46
45
28
42
52
38
13.3
23.9
14.4
10.9
10.1
a
Standard reaction conditions: 1a (1 mmol), KOH (1 equiv), metal
b
NPs (1 mol %), DMSO (2 mL), air (1 atm), RT, 24 h. Isolated
yields.
formation of an aniline radical cation intermediate through the
superoxide anion radical.
To further elucidate the NPs role in the reaction, we studied
the size effect of Ag NPs. To this end, Ag NPs with average
sizes ranging from 7.7, 9.4 to 25.3 nm were examined under
standard conditions (Table 2). There is no significant
difference in the activities of the first two catalysts at 1 mol
% loading (entries 1 and 2, Table 2), in accordance with the
result obtained by Ag NPs of 12.4 nm. When the average size of
the Ag NPs increased to 25.3 nm, however, the reaction rate
decreased drastically (entry 3, Table 2). These observations
substantiated that the surface area is critical to the adsorption
and activation of molecular oxygen and the success of the
reaction is largely dependent on the NPs’ ability to activate
oxygen.
To probe the reaction mechanism, several control experi-
ments were carried out. Only a trace amount of product was
obtained when the reaction was performed under N2,
underscoring the participation of dioxygen in the reaction (eq
1). We also established that nitrosobenzene was not a viable
intermediate for this reaction because no crossover product was
observed, as indicated by eq 2. We further discovered that
hydrazine was oxidized rapidly into the azo product (eq 3),
which explains the fact that we did not detect any hydrazine
intermediate during the course of the reaction.
On the basis of the above evidence, we postulated the
reaction mechanism (Scheme 1).21 Oxidation of the nitrogen
atom of aniline 1a by A gave rise to aniline radical cation B.
Subsequent coupling between B and an aniline molecule
probably led to the formation of a three-electron σ bond in
intermediate C, which was the rate-limiting step of the overall
reaction, evident by the accumulation of aniline radical cation B
Figure 3. TEM images and the particle size distributions of 5.7 nm Pt
(a, b), 13.3 nm Co (c, d), 23.9 nm Cu (e, f), 14.4 nm Ni (g, h), 10.9
nm Pd (i, j), and 10.1 nm Au (k, l) NPs.
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Figure 4. ESR spectra of (a) superoxide anion (g = 2.0020, aN = 1.45 mT, aH = 1.40 mT, aHα = 1.40 mT, and aHβ = 0.08 mT) obtained from a
mixture of Ag NPs (5 μmol), DMSO (2 mL), and spin trap DMPO (6 μmol) and (b) the mixture of Ag NPs (5 μmol), aniline (1 mmol), KOH (0.5
equiv), DMSO (2 mL), and spin trap NBP (15 mg). The spectra showed the superposed signal of superoxide anion and aniline free radical cation
during the reaction course.
a
a
Table 2. Particle Size Effect on the Catalyst Performance
Table 3. Catalyst Loading and Recycle Experiments
b
b
entry
size/nm
yield/%
entry
catalyst
Ag NPs
temp./°C
yield/%
1
2
3
7.7
9.4
61
59
31
1
2
25
40
60
25
40
60
110
60
60
60
60
51
78
97
55
81
97
95
94
94
94
93
Ag NPs
25.3
3
Ag NPs
a
4
Ag/C
Standard reaction conditions: 1a (1 mmol), KOH (1 equiv), Ag NPs
(1 mol %), DMSO (2 mL), air (1 atm), RT, 24 h. Isolated yields.
b
5
Ag/C
6
Ag/C
7
Ag/C
observed by the ESR experiment. In addition, one-electron
oxidation of intermediate C led to hydrazine intermediate D,
which was rapidly oxidized to the azo product 2aa.
8
reused Ag/C cycle 2
reused Ag/C cycle 3
reused Ag/C cycle 4
reused Ag/C cycle 5
9
10
11
The high activity of Ag NPs in oxygen activation necessitates
very low catalyst loading (Table 3). Thus, 6 mol ‰ Ag NPs
was capable of effecting the reaction at room temperature in 24
h in 51% yield (entry 1, Table 3). At 40 °C (entry 2, Table 3)
and 60 °C (entry 3, Table 3), the reaction proceeded faster to
give 78% and 97% yields, respectively. In fact, this facile
activation of oxygen by the Ag NPs was successfully applied to
the catalytic oxidation of N−H or C−H bond(s) to form C−N,
a
Standard reaction conditions: 1a (1 mmol), KOH (1 equiv), Ag NPs
b
(6 mol ‰), DMSO (2 mL), air (1 atm), 24 h. Isolated yields.
C−O, CC, and CN bonds, which will be reported
elsewhere in detail because of space restraints.
Scheme 1. The Proposed Mechanism for Ag NPs-Catalyzed Oxidative Coupling of Aniline
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Figure 5. XPS spectrum of catalyst: (a) the fresh Ag/C and (b) a catalyst sample taken during the reaction course (contaminated by KOH).
Figure 6. Characterization of catalyst: (a) HRTEM image of a catalyst sample taken during the reaction course and (b) the corresponding SAED
pattern; (c) XRD patterns of the fresh Ag/C (red line) and a catalyst sample taken during the reaction course (black line); (d) UV−vis spectra of the
fresh Ag nanoparticles.
Notably, the activity of our Ag/C catalysts of 12.4 nm
(entries 4−6, Table 3) is very similar to that of unsupported Ag
NPs of 12.4 nm (entries 1−3, Table 3) despite the reaction
temperature. The results showed that the solid support was
unlikely to play a role in the activation of molecular oxygen,
indicating that the NPs, but not the support, was responsible
for oxygen activation. Importantly, the Ag/C catalyst can be
recycled and reused for several cycles by simple filtration with
no loss in catalytic activity (entries 8−11, Table 3). After the
fifth cycle of reaction, the Ag concentration in the filtrate was
found to be 0.001 mg/mL (6 ‰ of the total amount of Ag in
the reaction mixture), which is consistent with the good
recyclability of the Ag/C catalyst. Further reaction carried out
with the filtrate did not result in any detectable amount of
azobenzene. These results suggested that the reaction was
catalyzed by Ag NPs, but not the leached Ag ions.
XRD, and UV−vis experiments to further characterize the
catalyst. XPS experiments41,42 were carried out with the fresh
Ag/C catalyst and with a Ag/C sample taken during the
reaction course. The results showed that the fresh catalyst
(Figure 5a) was composed of Ag0. Figure 5b suggested that the
catalyst was not oxidized during the reaction course.
High resolution transmission electron microscopy
(HRTEM) was performed on a catalyst sample taken during
the reaction course (Figure 6a). Figure 6b shows the
corresponding selected area electron diffraction (SAED)
pattern. The electron diffraction rings obtained with the same
sample could be indexed to the (111), (200), (220), and (311)
planes of face-centered cubic Ag.43 It also revealed the
polycrystalline structure of the catalyst, which could also be
seen from the HRTEM.
The XRD pattern was obtained with a catalyst sample taken
during the reaction course and compared with that of the fresh
Ag/C (Figure 6c). The XRD patterns of both samples exhibited
To shed light on the active catalytic species as well as the
recyclability of the Ag/C, we carried out XPS, HRTEM, SAED,
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diffraction peaks at 2θ = 37.9, 43.9, 64.3, and 77.3°,
representing (111), (200), (220), and (311) planes of Ag
nanoparticles, respectively.44,45
In accordance with XPS observation, this evidence indicated
that the oxidation state of the Ag/C catalyst did not change
during the reaction course.
UV−vis spectroscopy of the fresh Ag nanoparticles showed
an absorption band at 445.5 nm that could be assigned to Ag
nanoparticles (Figure 6d), close to literature reported values of
characteristic Ag0.45−47 Because of the interference of the active
carbon support, however, we failed to obtain results of any Ag/
C sample.
2.4. Reaction Scope. We reasoned that anilines should
have minimum influence on the reaction outcome because Ag
NPs’ high reactivity in oxygen activation would lead to
sufficiently fast generation of aniline radical cation. We thus
tested various anilines with different substitutions. As shown in
Table 4, they could all be smoothly converted into the
importantly, their unprecedented ability in oxygen activation
might facilitate other important oxidation reactions that have
been awaiting sustainable methods. Indeed, research in our
laboratories has successfully harnessed their power in oxygen
activation for oxidative C−N, C−O, CC, and CN bond
formation reactions utilizing molecular oxygen as the green
oxidant. We envision that intensified investigation would open
a new chapter of research that brings more reactions to the
benefit of metal nanoparticle catalysis.48
4. EXPERIMENTAL SECTION
Preparation of 7.7 nm Ag NPs. In a typical synthesis of
7.7 nm Ag NPs, 0.034 g of AgNO3 was added into 1.5 g of
ODA at 90 °C, then the resulting solution was injected into 6 g
of ODA at 210 °C under vigorous stirring. After stirring for 5
min, the product was precipitated by ethanol and collected by
subsequent centrifugation. The precipitate was then washed by
ethanol several times, and the final product was dispersed in
cyclohexane for further use.
Preparation of 9.4 nm Ag NPs. In a typical synthesis of
9.4 nm Ag NPs, 0.034 g of AgNO3 was added into 1.5 g of
ODA at 90 °C, then the resulting colorless and clear solution
was injected into 6 g of ODA at 210 °C under vigorous stirring.
After stirring for 7 min, the product was precipitated by ethanol
and collected by subsequent centrifugation. The precipitate was
then washed by ethanol several times, and the final product was
dispersed in cyclohexane for further use.
Table 4. Symmetric Aromatic Azo Compounds under
a
Standard Conditions
Preparation of 12.4 nm Ag NPs. In a typical synthesis of
12.4 nm Ag NPs, 0.034 g of AgNO3 was added into 1.5 g of
ODA at 80 °C. The resulting colorless and clear solution was
injected into 6 g of ODA at 200 °C under vigorous stirring.
After stirring for 10 min, the product was precipitated by
ethanol and collected by subsequent centrifugation. The
precipitate was washed by ethanol several times, and the final
product was dispersed in cyclohexane for further use.
a
Standard reaction conditions: 1 (1 mmol), KOH (1 equiv), Ag/C (6
Preparation of 25.3 nm Ag NPs. In a typical synthesis of
25.3 nm Ag NPs, 0.0085 g of AgNO3 was added into 1.5 g of
ODA at 80 °C. The resulting colorless and clear solution was
injected into 5.5 g of ODA at 200 °C under vigorous stirring.
After stirring for 10 min, the product was precipitated by
ethanol and collected by subsequent centrifugation. The
precipitate was washed by ethanol several times, and the final
product was dispersed in cyclohexane for further use.
Preparation of 12.4 nm Ag/C catalysts. In a typical
synthesis of 12.4 nm Ag/C catalyst, 200 mg of charcoal was
dispersed in cyclohexane and stirred with ultrasound for 1 h.
After cooling to room temperature, the solution of the as-
prepared 12.4 nm Ag NPs (10 mg) dispersed in cyclohexane
was added and mixed, then the slurry was stirred for 24 h and
cyclohexane was removed under reduced pressure.
Preparation of Au NPs. In a typical synthesis of Au NPs,
0.02 g of HAuCl4·4H2O was added into 5 mL of OA and 2 mL
of oleic acid, then the solution was stirred for 20 min at 140 °C
and then allowed to cool to room temperature. After washing
with ethanol several times, the final product was dispersed in
cyclohexane.
mol‰), DMSO (2 mL), air (1 atm), 60 °C, 24 h. Isolated yields.
corresponding symmetric azobenzenes. Although the literature
indicates that the electron density of the aniline was essential to
the fate of the reaction,21,35 the Ag NPs catalyst clearly
overcame such an influence.
We envisioned that cross-coupling of anilines would become
practical because the Ag NPs catalyst discriminated anilines to
very little extent. Oxidative cross-coupling between anilines was
problematic because the homocoupling of the more reactive
aniline (electron-rich) usually prevailed. A large excess of the
less reactive (electron-poor) anilines was used to achieve cross-
coupling at the cost of atomic economy and purification. With
Ag NPs catalyst, we needed to use only 1-fold excess of the less
reactive anilines to give very good yields of the desired
asymmetric azobenzenes (Table 5). Clearly, the metal NPs’
power in oxygen activation has significantly improved the
reaction selectivity and atomic economy.
3. CONCLUSIONS
In summary, we discovered that monodispersed metal NPs can
activate O2 in air at room temperature and under atmospheric
pressure, exemplified in the sustainable syntheses of both
symmetric and asymmetric azobenzenes under mild conditions.
By virtue of high reactivity originating from their micro-
structures, we demonstrated their generality in metal sources,
high efficiency, recyclability, and atomic economy. More
Preparation of Pt or Ni NPs. In a typical synthesis of Pt or
Ni NPs, 0.01 g of platinum(II) acetyl acetonate or nickel(II)
acetyl acetonate was added into 5 mL of OA. Under stirring,
0.03 g of TBAB was put into the above system, and then the
solution was transferred into a Teflon cup in a stainless steel-
lined autoclave. The autoclave was maintained at 150 °C for 2 h
and then allowed to cool to room temperature. After washing
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a
Table 5. Asymmetric Aromatic Azo Compounds under Standard Conditions
a
Standard reaction conditions: R1−NH2 (0.5 mmol), KOH (2 equiv), R2−NH2 (0.9 mmol), catalyst loading (6 mol ‰), DMSO (2 mL), air (1
b
c
atm), 60 °C, 30 h. Isolated yields. R1−NH2 (0.5 mmol), R2−NH2 (0.5 mmol). R1−NH2 (0.9 mmol), R2−NH2 (0.5 mmol).
with ethanol several times, the final product was dispersed in
cyclohexane.
temperature, the mixture was purified by flash chromatography
or preparative chromatography on a silica gel. The yields of azo
compounds were calculated with reference to products
Preparation of Cu or Co NPs. In a typical synthesis of Cu
or Co NPs, 0.01 g of copper(II) acetyl acetonate or cobalt(II)
acetyl acetonate was added into a solution containing 10 mL of
toluene and 0.5 mL of OA. Under stirring, 0.03 g of TBAB was
put into the above system, and then the solution was
transferred into a Teflon cup in a stainless steel-lined autoclave.
The autoclave was maintained at 150 °C for 10 h and then
allowed to cool to room temperature. After being washed with
ethanol several times, the final product was dispersed in
cyclohexane.
Preparation of Pd NPs. In a typical synthesis of Pd NPs,
palladium(II) acetyl acetonate (0.043 g) was added into 1.5 g of
ODA at 80 °C. The resulting light yellow and clear solution was
injected into 6 g of ODA at 180 °C under vigorous stirring.
After stirring for 1.5 min, the product was precipitated by
ethanol and collected by subsequent centrifugation, then the
precipitate was washed by ethanol several times, and the final
product was dispersed in cyclohexane.
1
obtained. Product identification was carried out using H and
13C NMR, MS or HRMS, FT-IR, and UV−vis spectra.
Analysis. 1H and 13C NMR spectra were recorded in CDCl3
on a Bruker Avance 400 MHz spectrometer with a 5 mm
PABBO BB-1H/D Z-GRD Z108618/0394 multinuclear probe
at 298 K. ESI spectra were recorded with an Esquire-LC ion-
trap mass spectrometer equipped with an ESI source (Bruker
Daltronik, Bremen, Germany). Samples were injected with a
syringe pump (Cole-Parmer Instrument Company). High
resolution mass spectra (HRMS) of compounds were recorded
with a Fourier transform ion cyclotron resonance mass
spectrometer (Bruker Apex IV). IR spectra were measured on
KBr pellets with a Perkin-Elmer Spectrum One FT-IR
spectrometer in the range 4000−500 cm−1. UV−vis spectra
were recorded in the wavelength range 200−600 nm using a
Shimadzu UV-2100S recording spectrometer. GC chromatog-
raphy was performed on SP-6890 with a FID detector equipped
with a capillary column (SE-54, 30 m × 0.50 μm × 0.53 mm).
Parameters were as follows: initial oven temperature, 60 °C, 3
min; ramp, 15 °C/min; final temperature, 260 °C; final time, 10
min; injector temperature, 260 °C; detector temperature, 250
°C; injection volume, 1 μL. ESR spectra were recorded at room
temperature on a JEOL JES-FA200 ESR spectrometer
Catalytic Experiment. Oxidation reaction of aniline was
carried out in a glass reactor with a water-cooled reflux head
under air (1 atm). For each reaction, a mixture of reactants,
catalyst, base, and solvent was placed into the reactor. The
reaction mixture was vigorously stirred at the designed
temperature for the required time. After cooling to room
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operating at 9.44 GHz. Typical spectrometer parameters were
scan range, 100 G; field set, 3373.91 G; time constant, 30 ms;
scan time, 4 min; modulation amplitude, 1 G; modulation
frequency, 100 kHz; receiver gain, 1.00 × 103; microwave
power, 4 mW. DMPO (5,5,-dimethyl-1-pyrroline N-oxide) and
NBP (N-tert-butyl-α-phenylnitrone) were employed as the
radical traps. TEM observation was performed on a Hitachi
model H-800 transmission electron microscope (JEM 2010F,
JEOL) operated at 80 kV. TEM specimens were prepared via
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21131004), and China Postdoctoral Science Foundation
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