Facile fabrication of a Cu@g-C3N4 nanocatalyst and its application for the aerobic oxidations of alkylaromatics and the reduction of 4-nitrophenol

Article abstract of DOI:10.1039/c6ra18288g

In this study, the preparation of a Cu@g-C₃N₄ nanocatalyst with highly active sites was developed in a facile precipitation method by the in situ reduction of Cu²⁺ adsorbed on the surface of layered graphitic carbon nitride (g-C₃N₄). The catalyst was characterized by FT-IR, XRD, SEM, TEM, ICP, XPS and BET, verifying that the Cu nanoparticles with a mean particle size of 35.89 nm were uniformly distributed onto the surface of g-C₃N₄. Alkylaromatics oxidation reactions and 4-nitrophenol reduction reactions were investigated on the Cu@g-C₃N₄ nanocatalyst with superior catalytic activity. The aerobic oxidation of ethylbenzene (EB) to acetophenone (AP) by tert-butyl hydroperoxide (TBHP) was conducted readily, and a conversion and selectivity of 98.8% and 94.0%, respectively, could be reached under optimal conditions. Moreover, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) employing NaBH₄ has a kinetic rate constant of 1.86 × 10^-2 s^-1 and an activity parameter of 1.134 s^-1 mM^-1 in the presence of Cu@g-C₃N₄, which are higher values than previous results described in the literature. Repeated cycles demonstrated the outstanding recyclability and stability of the catalyst for at least four runs without a considerable loss in catalytic activity.

Full text of DOI:10.1039/c6ra18288g

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Facile fabrication of the Cu@g-C3N4 nanocatalyst and its
application for the aerobic oxidations of alkylaromatics and
the reduction of 4-nitrophenol
Shen Huang, Yukai Zhao, Ruiren Tang*
School of Chemistry and Chemical Engineering, Central South University, Hunan, 410083, China
*Corresponding author. Tel.: 86ꢀ731ꢀ88836961 Fax: 86ꢀ731ꢀ88879616
*Eꢀmail: trr@mail.csu.edu.cn
Abstract: In this study, the preparation of Cu@gꢀC3N4 nanocatalyst with
highlyꢀactive sites was developed in a facile precipitation method by in situ reduction
of Cu2+ adsorbed on the surface of layered graphitic carbon nitride (gꢀC3N4). The
catalyst was characterized by FTꢀIR, XRD, SEM, TEM, ICP, XPS and BET, verifying
that Cu nanoparticles with a mean particle size of 35.89 nm were uniformly
distributed on the surface of gꢀC3N4. The reactions of alkylaromatics oxidation and
4ꢀnitrophenol reduction were investigated on the Cu@gꢀC3N4 nanocatalyst with
superior catalytic activity. The aerobic oxidation of ethylbenzene (EB) to
acetophenone (AP) by tertꢀbutyl hydroperoxide (TBHP) was conducted readily, and
the conversion and selectivity could be reached to 98.8 % and 94.0 % under the
optimal condition, respectively. Moreover, the reduction of 4ꢀnitrophenol (4ꢀNP) to
4ꢀaminophenol (4ꢀAP) employing NaBH4 owns a the kinetic rate constant of
1.86×10ꢀ2 sꢀ1 and an activity parameter of 1.134 sꢀ1·mMꢀ1 in the presence of
Cu@gꢀC3N4, which are higher than previous results described in the literatures.

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Repeated cycles demonstrated the outstanding recyclability and stability of the
catalyst for at least four runs without considerable loss of catalytic activity.
Key word: aerobic oxidation, catalytic reduction, gꢀC3N4, nanoparticles.
Introduction
Copper nanoparticles have opened great spirits in miscellaneous technological
applications like optoelectronics, chemical sensing and catalysis owing to the
1
properties of inexpensive, supernormal optical behaviors and easy accessibility.
2
Huang et al. prepared Cu nanoparticle sensors and applied it to explore plasmon
peak sensitivity. Du and coworkers 3 found that copper as a transparent electrocatalyst
expanded application for water oxidation. In catalytic applications, a precise
correlation between catalytic performance and particle size, however, the intractable
problems that nanoparticles may be aggregated or grow during the reactions could
4
lead to the decrease of the catalytic activity and further confine applications.
Therefore, dispersing metal particles on appropriate supporting materials with large
surface areas have been applied to restrain the reunion of particles.
At present, graphitic carbon nitride (gꢀC3N4), an organic polymer semiconductor
consisting mainly of carbon and nitrogen with band gap energy of 2.7 eV, has drawn
extensive attention owing to large surface areas, particular physical features, high
thermal stability, easily recycled, promising electronic and catalytic properties. 5, 6 It is
formed by Nꢀbridged triꢀsꢀtriazine repeating units which are generated by

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7
twoꢀdimensional conjugated planes packed together via van der Waals interactions.
In view of its unique structure and preponderance, gꢀC3N4 has been widely employed
8
9
in various fields, for instance, oxygen reduction reaction,
water splitting,
10
11
SuzukiꢀMiyaura coupling reaction, bioimaging. Up to now, Chen and coworkers
12 empolyed gꢀC3N4 to catalyze oxidation of benzene to phenol. Su 13 reported aerobic
oxidative coupling of amines using gꢀC3N4 as photocatalyst. Nevertheless, the
catalytic property of gꢀC3N4 was limited due to low separation efficiency of
photogenerated electrone hole pairs and wideꢀband gap. 6 So strategies that depositing
nanoparticles onto gꢀC3N4 is regard as a convenient, straightforward, but effective
strategy to enhance the performance of gꢀC3N4.
Inspired by the unique architecture of gꢀC3N4 and the outstanding catalytic
performance of Cu nanoparticles, the gꢀC3N4 as support material judiciously
hybridized with copper nanoparticles to obtain a highly active and green
heterogeneous catalyst. More interestingly, the copper nanoparticles and gꢀC3N4 are
interdependent and mutually reinforcing. On the one hand, metal particles can
promote charge separation at the interface of the metalꢀsemiconductor heterojunction,
14, 15
which in turn lead to the improvement of catalytic performance.
On the other
hand, the gꢀC3N4 as “active support” also can restrict the aggregation of metal
15
particles, thus further enhancing the selectivity and activity.
This enabled us to
explore the catalytic performance of Cu@gꢀC3N4 for alkylaromatics oxidation and
4ꢀnitrophenol reduction. While the products of alkylaromatics oxidation as valuable
intermediates are pervasively applied in diversified areas like pharmaceuticals,

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16ꢀ18
surfactants, agrochemicals and spice industry,
the transformation of
4ꢀaminophenol (4ꢀAP) by the catalytic hydrogenation of 4ꢀnitrophenol (4ꢀNP) is
considered to be the most green, environmental, convenient way for the degradation
of industrial effluents which contain mainly 4ꢀNP without complicated composition.
19
The major challenge for the oxidation of alkylaromatics and the reduction of
4ꢀnitrophenol concerns tedious workup procedures, poor activity, harsh conditions
20, 21
and the overuse of organic solvents in the presence of traditional catalysts.
This
demands the synthesis of novel catalysts to improve reaction properties. In this work,
the nanocatalyst Cu@gꢀC3N4 was fabricated via a straightꢀforward chemical reduction
method with ultraꢀhigh catalytic activity, and also can be reused without great loss of
activity at least four runs.
2 Experimental
2.1 Materials
All chemicals were commercially available and were used without further purification.
Deionized water was used throughout the experiments. Solvents were purified and
dried according to standard methods. ICP were measured with Optima 5300DV, and
the heterogeneous samples were preprocessed by acid digestion with concentrated
nitric acid. FTꢀIR spectra were determined on a Nicolet NEXUS 670 FTꢀIR
spectrophotometer using KBr discs in the 400–4000 cmꢀ1 region under atmospheric
conditions. The products of oxidation reactions were monitored using Techcomp
GCꢀ7890II gas chromatograph with a OVꢀ1701 column (50 m × 0.25 mm × 0.25 ꢁm)
and flame ionization detector (FID). Scanning electron microscopy (SEM) were

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examined by FEI QUANTA 200 microscope. Transmission Electron Microscope
(TEM) images were recorded with a JEOL JEMꢀ2100F microscope. The ultraviolet
visible spectra (UV–Vis) were conducted by Shimadzu UVꢀ2600. N2 gas adsorption
and desorption analyses were taken using Quantachrome instruments, BoyntonBeach,
o
FL33426 at constant temperature (ꢀ196 C). The power Xꢀray diffraction (XRD)
patterns were conducted using a Rigaku D/Max III VC diffractometer in the range of
2θ = 10–80o. Xꢀray photoelectron spectroscopy (XPS) measurements were
characterized by ThermoFisherꢀVG Scientific ESCALAB 250Xi. 1H NMR
measurement were performed on a Brukerꢀ400 MHz nuclear magnetic resonance
spectrometer with deuterated chloroform as solvent and TMS as internal reference.
2.2 Synthesis of Cu@gꢀC3N4 Catalyst
The support gꢀC3N4 was prepared through the heat polymerization of melamine on the
5, 22
basis of the previously reported methods.
Typically, melamine was put into a
ceramic crucible with a cover and placed into the muffle furnace, heated at a rate of 3
oC/min until the temperature up to 550 oC, and kept at this temperature for 4 h. When
cooling to room temperature, the primrose yellow polymer was milled to obtain
6
gꢀC3N4 powder. The Cu@gꢀC3N4 was synthesized by a facile precipitation method.
As described below: 500 mg of asꢀprepared gꢀC3N4 was dispersed in 100 mL
deionized water to obtain the uniformity of dispersion by ultrasonic method. Then 10
mL CuCl2 (84 mg) aqueous solution was added dropwise under strong stirring. The
mixed solutions was stirred at room temperature for 10 h. 1 M NaOH aqueous
solution was added dropwise until the pH of mixture was approximately equal to 10.

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After an hour, 30 mL NaBH4 (1 M) aqueous solution was added to reduce Cu (Ⅱ) to
Cu (0). The greyishꢀgreen precipitate was filtrated, washed with deionized water
(3×25 mL) and alcohol (3×25 mL), and dried under vacuum at 70 oC for 8 h.
2.3 Typical Procedure for ethylbenzene oxidation
The oxidation of ethylbenzene was chosen as a model reaction. Reactions were
conducted by suspending the Cu@gꢀC3N4 catalyst in a acetonitrile solution (10 mL)
containing ethylbenzene (10 mmol). The catalyst was stirred and swollen. Then
TBHP (a number of 70% aqueous solution) was added dropwise, and the mixture was
stirred at a certain temperature for the required time. After the reaction finished, the
catalyst was filtrated, washed with acetonitrile and 1 M NaOH solution. The saturated
aqueous NaHSO3 solution was added into the filter liquor to remove the extra TBHP.
The filtrate was extracted with ethyl acetate (3×10 mL). The organic phase was
washed with saturated aqueous EDTA solution, saturated NaHCO3 solution, deionised
water and dried over anhydrous Na2SO4.The products in reactions were analyzed by
GC.
2.4 Typical Procedure for the Reduction of 4ꢀnitrophenol
In a typical reaction, 2.8 mL deionized water and 40 ꢁL 4ꢀNP (0.01 M) were mixed to
a colorimetric ware, following 80 ꢁL freshly prepared NaBH4 solution (0.5 M) was
added, resulting in the appearance of a bright yellow solution. After that, 10 ꢁL
Cu@gꢀC3N4 catalyst (5 mg/mL) was added to the above mixed solution, and the color
of solution faded from bright yellow to colorless slowly. The reaction conversion was
monitored by ultraviolet visible spectra (UV–Vis).

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2.5 Catalyst Recycling Test
The recovery of the Cu@gꢀC3N4 catalyst for the oxidation of ethylbenzene (EB) is
represented below. EB (10 mmol), TBHP (50 mmol), acetonitrile (10 mL) and
Cu@gꢀC3N4 (80 mg) were mixed into a 50 mL round bottomed flask. The mixture
o
was stirred at 80 C for 6 h. At the end of the reaction, the catalyst was separated by
filtration and washed with acetonitrile and 1 M NaOH solution, dried under vacuum at
50 oC. The obtained catalyst was used for the next cycles.
3 Results and Discussion
3.1 Characterization of catalyst
3.1.1 Fourier Transform Infrared
The FTꢀIR spectra of gꢀC3N4 and Cu@gꢀC3N4 are shown in Figure 1. It is clear that
the sharp peak at 806 cmꢀ1 comes from outꢀof plane bending vibration of triazine
cycle. The peak at 1645 cmꢀ1 was referred to heterocyclic C=N stretching vibration,
while the peak at 1238, 1317, 1405 and 1543 cmꢀ1 were considered as C–N stretching
of triꢀsꢀtriazine. The broad bands ranging from 2800 to 3400 cmꢀ1 assigning to NꢀH
(uncondensed terminal amino groups) stretching vibration were observed in gꢀC3N4.
23 After Cu was loaded, almost nothing has essentially changed in the FTꢀIR spectrum,
making clear that there was no covalentꢀbond interaction between gꢀC3N4 and Cu
nanoparticles.

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Fig. 1 FTꢀIR spectra of gꢀC3N4 and Cu@gꢀC3N4
3.1.2 Xꢀray diffraction
The Xꢀray diffraction (XRD) patterns of pure gꢀC3N4 and Cu@gꢀC3N4 were
performed in Figure 2. The sharp and strong peak at 27.5o was observed in pure
gꢀC3N4, corresponding to the characteristic (002) interlayer stacking peak of gꢀC3N4.
24
The (100) weak diffraction peak at 13.1o was attributed to the inꢀplane repeated
25
units in the gꢀC3N4. After loading Cu, the intensity peak at 27.5o decreased mildly
6
as a result of dilution effect. Moreover, new typical peaks appeared in the XRD
pattern of Cu@gꢀC3N4. The (111), (200), (220) another diffraction peaks located at
24
43.3o, 50.4o and 74.1o represented the planes of the face centered cubic Cu,
suggesting that asꢀprepared sample takes on a twoꢀphase composition.

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Fig. 2 XRD patterns of pure C3N4 and Cu@gꢀC3N4
3.1.3 Xꢀray photoelectron spectroscopy
The Xꢀray photoelectron spectroscopy (XPS) of Cu@gꢀC3N4 was shown in Figure 3.
The coexistence of elements C, N, O and Cu in the Cu@gꢀC3N4 catalyst were
observed from the survey spectrum. The C 1s signal could be identified with two
main peaks at binding energies of 284.8 and 288.3 eV, which corresponded to sp2
26
carbon (C−C) and sp2ꢀbonded carbon (N−C=N), respectively. The three main N 1s
peaks were exhibited at 398.8, 400.1 and 401.3 eV, which assigned to sp2ꢀhybridized
nitrogen (C=N−C), sp3ꢀhybridized nitrogen tertiary nitrogen (Nꢀ(C)3), and amino
27
functional groups with one hydrogen atom (C−N−H) respectively.
The peaks
located at 935.3 and 955.2 eV attributed to Cu2+ species 2p3/2 and 2p1/2. Meanwhile,
two shakeꢀup lines centered at 944.1 and 963.0 eV are measured, meaning the
28
existence of paramagnetic chemical state of Cu2+. Indeed, the binding energies of
Cu 2p3/2 and Cu 2p1/2 shift to 932.7 and 952.7 eV, respectively, indicating the
6
formation of metal Cu. The Cu(0)/Cu(Ⅱ) ratio was calculated as 1.869, suggesting
that Cu(Ⅱ) can be partially reduce to Cu(0) and Cu(0) takes up a majority.

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Fig. 3 The XPS spectrum of Cu@gꢀC3N4: (a) survey, (b) C 1s, (c) N 1s and (d) Cu 2p
3.1.4 Surface area and porosity measurements
The surface area and porous architecture of the gꢀC3N4 support and the asꢀmade
Cu@gꢀC3N4 catalyst were measured. As you can see in Figure 4, the surface area and
pore size of gꢀC3N4 were calculated to be 10.015 m2/g and 3.816 nm, respectively.
Clearly, the gꢀC3N4 has a type IV adsorption−desorption isotherms, which conforming
29
the representative mesoporous structure.
There was no significant change in the
isotherm profile and the pore diameter for Cu@gꢀC3N4, suggesting that the Cu
nanoparticles did not break the pore distribution, which is in accordance with the
presence of the plane structure after the loading of Cu nanoparticles. In addition, the
content of Cu in the Cu@gꢀC3N4 is determined to be 6.34% by ICPꢀAES.
Fig. 4 N2 adsorption−desorption isotherms and the corresponding poreꢀsize distribution curves of

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(a) gꢀC3N4 and (b) Cu@gꢀC3N4.
3.1.5 Scanning Electron Microscope (SEM) and Transmission Electron Microscope
(TEM)
The SEM and TEM images of gꢀC3N4 and Cu@gꢀC3N4 are shown in Figure 5. The
gꢀC3N4 possesses a typical stacked layered structure, resulting in porous morphology.
It is clear that the shape of gꢀC3N4 was not broken after the introduction of Cu
nanoparticles, verifying the Cu nanoparticles was sedimented on the surface of
gꢀC3N4. The TEM image (Figure 5e) of Cu@gꢀC3N4 exhibited that Cu nanoparticles
were dispersed homogeneously on the surface of the gꢀC3N4 with an average particle
size of 35.89 nm, which is in good consistent with the results of XRD and XPS.
Moreover, the morphology of Cu@gꢀC3N4 remained unchanged after recycling,
which reflects that the Cu@gꢀC3N4 designed and prepared in our research was stable
and robust.

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Fig. 5 SEM images of (a) gꢀC3N4, (b) the fresh Cu@gꢀC3N4 and (c) the reused Cu@gꢀC3N4. TEM
images of (d) gꢀC3N4, (e) the fresh Cu@gꢀC3N4 and (f) the reused Cu@gꢀC3N4.
3.2 Effect of different reaction conditions for oxidation of ethylbenzene
3.2.1 Effect of catalyst amount
As shown in Table 1, the conversion of EB oxidation was extremely low in the
absence of catalyst. When the introduction of Cu@gꢀC3N4, and discovered that the
EB conversion and the AP selectivity both improved markedly. It should be noted that
with 80 mg Cu@gꢀC3N4 conversion reached a maximum. The EB conversion
decreases slightly with the further increase of catalyst, and the reason is that the
carbon or coke deposition at excess catalyst concentrations probably impede the

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active sites of the catalysts. And the AP selectivity increased within a narrow range
with increasing catalyst amount. In view of practical application, 80 mg Cu@gꢀC3N4
was chosen as the suitable catalyst amount.
Table 1 Effect of catalyst amount for the oxidation of EB
Con.
(%)
Sel. (%)
Entry
Catalyst
Catalyst amount (mg)
AP
BAL
32.4
14.3
12.8
12.1
11.5
11.3
39.8
1
2
3
4
5
6
7
Cu@gꢀC3N4
Cu@gꢀC3N4
Cu@gꢀC3N4
Cu@gꢀC3N4
Cu@gꢀC3N4
Cu@gꢀC3N4
gꢀC3N4
0
20
40
60
80
100
80
16.7
83.9
84.1
87.7
92.8
90.4
43.8
67.6
85.7
87.2
87.9
88.5
88.7
60.2
Conditions: 10 mmol ethylbenzene (EB), different amount of Cu@gꢀC3N4, 5 equiv of 70 % TBHP
in CH3CN (10 mL) were stirred at 80 oC for 4 h. AP = acetophenone, BAL = benzaldehyde.
3.2.2 Effect of reaction temperature
Further experiments were performed to explore the effect of reaction temperature.
From Table 2, both the conversion of EB and the selectivity of AP went up with
increase of reaction temperature. The maximum conversion (92.8 %) and selectivity
(88.5 %) was observed at 80 oC and then lower slightly at higher temperature. Hence,
80 oC was used as the optimal temperature in the following reactions.
Table 2 Effect of reaction temperature for the oxidation of EB

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Sel. (%)
Entry
Temperature (oC)
Con. (%)
AP
BAL
1
2
3
4
5
50
60
70
80
90
71.9
78.5
88.7
92.8
86.2
84.0
84.6
87.4
88.5
79.5
16.0
15.4
12.6
11.5
20.5
Conditions: 10 mmol ethylbenzene (EB), 80 mg Cu@gꢀC3N4, 5 equiv of 70 % TBHP in CH3CN
(10 mL) were stirred for 4 h within several temperature. AP = acetophenone, BAL =
benzaldehyde.
3.2.3 Effect of Reaction Time
The following researches under the optimizations mentioned above were carried out
to determine the ideal reaction time. As illustrated, the conversion of EB and the
selectivity of AP increased with prolonging time from 2 to 6 h, then the conversion
dropped slightly in the longer reaction time. Therefore, 6 h was chosen to keep better
efficiency for the conversion, selectivity and reaction time.
Table 3 Effect of reaction time for the oxidation of EB
Sel. (%)
Reaction time
Entry
Con. (%)
(h)
2
AP
BAL
14.4
11.5
10.8
1
2
3
91.3
92.8
97.1
85.6
88.5
89.2
4
6

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4
5
8
91.6
89.2
90.5
89.4
9.5
10.6
10
Conditions: 10 mmol ethylbenzene (EB), 80 mg Cu@gꢀC3N4, 5 equiv of 70 % TBHP in CH3CN
(10 mL) were stirred at 80 oC within several time. AP = acetophenone, BAL = benzaldehyde.
3.2.4 Effect of TBHP amount
Subsequently, tests were conducted using different molar equivalents of TBHP in
order to get an optimized value. As expected, the EB conversion improved remarkably
from 57.3 to 97.1 % with increasing TBHP amount from 10 to 50 mmol, while it
changed very slightly in a higher concentration (60 mmol) of TBHP. It is a reason that
at lower TBHP concentration, insufficient TBHP could be activated by catalyst, the
oxidation of EB is difficult. Excessive oxidizing agent has no significant effect to
aerobic oxidation when TBHP reaches saturation. The change rule of the AP
selectivity in accordance with the EB conversion. In view of practical application, 50
mmol TBHP was selected as the best amount for the reactions involved in.
Table 4 Effect of TBHP amount for the oxidation of EB
Sel. (%)
Entry
TBHP amount (mmol)
Con. (%)
AP
BAL
28.2
24.4
16.6
15.1
10.8
1
2
3
4
5
10
20
30
40
50
57.3
70.3
70.8
77.9
97.1
71.8
75.6
83.4
84.9
89.2

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6
60
97.3
89.9
11.1
Conditions: 10 mmol ethylbenzene (EB), 80 mg Cu@gꢀC3N4, different equiv of 70 % TBHP in
CH3CN (10 mL) were stirred at 80 oC for 6 h. AP = acetophenone, BAL = benzaldehyde
.
3.2.5 Effect of different coꢀcatalysts
Some coꢀcatalysts were added to reaction mixture in order to further promote the
catalytic activity. As the Table 5 listed, Nꢀhydroxyphthalimide (NHPI) could improve
markedly the EB conversion and the AP selectivity compared with Cu@gꢀC3N4 as the
sole catalyst. This fact can be explained for the following reasons: the oxidation of EB
is likely to follow the free radical chain mechanism, and NHPI as electron transfer
initiates the radical propagation of autoxidation via one electron transfer and enhances
the catalytic activity. Conversely, the conversion and selectivity actually lowered in
presence of other coꢀcatalysts. The different activation mechanisms of the coꢀcatalysts
may be the reason. Accordingly, NHPI should be the suitable coꢀcatalyst.
Table 5 Effect of coꢀcatalysts for the oxidation of EB
Sel. (%)
Entry
Coꢀcatalysts
Con. (%)
AP
BAL
6.0
1
2
3
4
NHPI
AIBN
CTAB
TBAB
98.8
87.3
73.0
65.1
94.0
77.1
68.3
72.8
22.9
31.7
27.2
Conditions: 10 mmol ethylbenzene (EB), 80 mg Cu@gꢀC3N4, 5 equiv of 70 % TBHP, 1 mmol
different coꢀcatalyst in CH3CN (10 mL) were stirred at 80 oC for 6 h. AP = acetophenone, BAL =

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benzaldehyde, AIBN = α,α'ꢀazobisisobutyronitrile, CTAB = hexadecyl trimethyl ammonium
bromide, TBAB = tetrabutyl ammonium bromide.
3.2.6 Reused of the Cu@gꢀC3N4 catalyst
The stability and reuse of catalyst play a critical role in heterogeneous catalytic
reactions. Further studies were proceeded to examine the recyclability of Cu@gꢀC3N4.
It was clearly seen that, the EB conversion and the AP selectivity decreased
marginally in the second time and changed very slightly after that. The catalytic
activity of Cu@gꢀC3N4 was well retained after 4 runs (the conversion of EB > 93 %
and the selectivity of AP > 84 %). Moreover, the color, FTꢀIR spectra, SEM, TEM,
morphology have no obvious change, indicating that the catalyst Cu@gꢀC3N4 was
stable and robust.
Table 6 The reusability of the Cu@gꢀC3N4 catalyst for the oxidation of EB
Sel. (%)
Entry
Cycle No.
Con. (%)
AP
BAL
10.8
14.5
14.8
15.2
1
2
3
4
Fresh
97.1
94.4
93.7
93.1
89.2
85.5
85.2
84.8
1
2
3
Conditions: 10 mmol ethylbenzene (EB), 80 mg Cu@gꢀC3N4 of different cycles times, 5 equiv of 70 % TBHP in
CH3CN (10 mL) were stirred at 80 oC for 6 h. AP = acetophenone, BAL = benzaldehyde.
3.2.7 Oxidation of different substrates catalyzed by Cu@gꢀC3N4
A series of substrates were used to investigate the generality of catalytic performance

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of asꢀmade Cu@gꢀC3N4 under the optimal conditions in Table 7. The reactions
processed smoothly in acetonitrile as the solvent and using TBHP as the oxidizing
agent afforded the desired oxidative products in outstanding conversion (90ꢀ100%)
but αꢀisophorone (the conversion was 55.8 %), with selectivities ranging from 80 to
98 % , confirming a very good catalytic activity. The groups containing either an
electronꢀattracting or an electronꢀdonating group in the aromatic ring did not seem to
have distinct influence on the reaction results for aromatic substrate. Moreover,
cyclohexene was oxidized with 2ꢀcyclohexenꢀ1ꢀol as main product, giving completely
conversion and the selectivity of 83.4 %. In sharp contrast, low αꢀisophorone
conversion (55.8 %) was achieved with moderate selectivity for product
ketoisophorone (80.1 %).
Table 7 Oxidation of different substrates catalyzed by Cu@gꢀC3N4 catalyst
Entry
1
Substrate
Main product
Con. (%)
90.1
Sel. (%)
95.8
2
3
4
94.4
95.6
95.1
97.2
97.7
93.6

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5
6
93.9
100
98.5
83.4
80.1
7
55.8
Conditions: 10 mmol substrate, 80 mg Cu@gꢀC3N4, 5 equiv of 70 % TBHP in CH3CN (10 mL) were stirred at
80 oC for 6 h. The characterization of the products by 1H NMR and the spectra were shown in supporting
information.
3.2.8 Possible reaction mechanism for EB oxidation
30ꢀ32
Based on the experimental results and related published literatrues,
the proposed
reaction mechanism for EB oxidation is thought to proceed via a radicalꢀchain
sequence mechanism and shown in Scheme 1. It can be surmised that firstly TBHP
was adsorbed onto the surface of the Cu@gꢀC3N4 because of its large surface area,
sequentially OꢀO bond in TBHP broken to form tertꢀbutyl oxygen and hydroxyl
radicals due to Cu2+ could activate the lone pair of electrons of the distant oxygen in
TBHP. The above radicals can capture the αꢀH of EB to generate αꢀethylbenzene
radical. Synchronously, the αꢀethylbenzene radical is captured by TBHP producing
tertꢀbutylperoxy ethers. At last, AP as main product and BAL as byꢀproduct can be
formed by two ways.

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Scheme 1 Possible reaction mechanism
3.3 Cu@gꢀC3N4 for Reduction of 4ꢀnitrophenol
The catalytic performance of Cu@gꢀC3N4 was assessed using the reduction of
4ꢀnitrophenol (4ꢀNP) into 4ꢀaminophenol (4ꢀAP) in the presence of excess NaBH4.
The process of this reaction was monitored by measuring the UVꢀVis absorption
intensity. Figure 6 shows the characteristic peak of 4ꢀnitrophenol shifts from 317 to
400 nm after the addition of NaBH4 solution, accompanying with the color changes
from colorless to bright yellow due to the forming of 4ꢀnitrophenolate. 33 The spectral
34
profile at 400 nm remained constant even after 10 h in the lack of a catalyst,
because the reaction is kinetically restricted but thermodynamically feasible. Whereas,
when the introduction of Cu@gꢀC3N4, the intensity of the peak at 400 nm decreased
progressively to be disappeared in 360 seconds with the bleaching of the bright
yellow color of the reaction solution. Simultaneously, a new peak assigning to the
typical absorption of 4ꢀAP increased gradually.

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Figure 6. (a) UV–Vis spectra of 4ꢀNP before and after addition of NaBH4 solution. (b) The
reduction process of 4ꢀNP to 4ꢀAP in the presence of Cu@gꢀC3N4 catalyst.
The concentration of NaBH4 was very high in comparison to that of 4ꢀNP and thus the
35
reaction process followed pseudoꢀfirstꢀorder reaction kinetics.
Therefore,
pseudoꢀfirstꢀorder kinetics was used to measure the kinetic rate constant (kapp) and the
activity parameter (k) of Cu@gꢀC3N4. We calculated the reaction conversion from
Ct/C0 (Figure 7), which was determined using the relative intensity of the UVꢀVis
absorbance (At/A0) at 400 nm. Ct and C0 are the concentrations of the 4ꢀNP at reaction
time t and the initial stage, respectively.
ln(Ct/C0) = ln(At/A0) = ꢀkapp
t
(1)

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k = kapp / c(Cu)
(2)
The kinetic rate constant (kapp) was calculated to be 1.86×10ꢀ2 sꢀ1 in reduction of 4ꢀNP
using Cu@gꢀC3N4 as catalyst. The activity parameter (k) was designated by
normalizing to the concentration of Cu and the result of k is 1.134 sꢀ1·mMꢀ1,
suggesting that Cu@gꢀC3N4 was excellent catalytic activity for 4ꢀNP reduction.
Figure 7. Plots of Ct/C0 and ln(Ct/C0) versus reaction time for the reduction of 4ꢀNP in the
presence of Cu@gꢀC3N4 catalyst.
In addition, we further compared the catalytic activity of the Cu@gꢀC3N4 with other
catalysts that reported in the reduction of 4ꢀNP. As shown in Table 8, the Cu@gꢀC3N4
catalyst performed the best with the active parameter k as 1.134 sꢀ1·mMꢀ1.
Table 8 Comparison of the apparent kinetic rate constant (kapp) and activity parameter
(k) of different catalysts for the reduction of 4ꢀNP
activation
a
c(4ꢀNP)
kc
Number
of cycle
Samples
CuFe2O4
c(M)a (mM) kappb (sꢀ1)
energy
(kJ·molꢀ1)
ꢀ
Ref.
(mM)
(sꢀ1·mMꢀ1)
34
0.17
1.56×10ꢀ1
1.2×10ꢀ1
0.769
2

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nanoparticles
Pd
36
0.2
2.3
1.6
7.89×10ꢀ4
20×10ꢀ3
0.0005
ꢀ
ꢀ
nanoclusters
Pd/PiHP
37
33
9×10ꢀ2
0.222
0.498
6
5
ꢀ
ꢀ
Pd/FG
5.8×10ꢀ2 4.72×10ꢀ3 2.35×10ꢀ3
p(SPM)ꢀCu
composite
p(AAGA)ꢀAg
p(MAc)ꢀCu
composite
p(TA)ꢀCo
composite ILs
p(AMPS)ꢀCu
38
10
10
10
2.8
4.24×10ꢀ3
1.52×10ꢀ3
1.5×10ꢀ3
7.1×10ꢀ3
0.3556
7
5
4
33.86
33.78
31.33
39
40
2.148
6.77×10ꢀ2 2.41×10ꢀ2
1.475×10ꢀ2 7.167×10ꢀ3
41
10
0.4859
0.005
5
26.19
42
15
3.437×10ꢀ1 1.72×10ꢀ3
1.64×10ꢀ2 1.86×10ꢀ2
5
4
28.16
ꢀ
Cu@gꢀC3N4
0.13
1.134
This work
a
b
c
c: concentration. kapp: pseudoꢀfirstꢀorder rate constant. k: catalytic parameter. Data were
given or calculated in the respective papers.
4 Conclusion
In this study, we described the preparation and characterization of Cu@gꢀC3N4
nanocatalyst, which enabled us to explore its applications in the oxidation of
alkylaromatics and the reduction of 4ꢀnitrophenol. The Cu@gꢀC3N4 exhibited
excellent catalytic performance for the oxidation of ethylbenzene with 98.8 %
conversion and 94.0 % selectivity, and the active parameter k as 1.134 sꢀ1·mMꢀ1 for

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the reduction of 4ꢀnitrophenol. Furthermore, the asꢀprepared catalyst could be
recovered and reused by filtration without considerable loss of catalytic activity,
indicating that Cu@gꢀC3N4 as an efficient, economic and environmental heterogenous
catalyst could provide new insights for green chemistry and industrial applications.
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Graphical Abstract