Thermal oxidative etching method derived graphitic C3N4: Highly efficient metal-free catalyst in the selective epoxidation of styrene

Article abstract of DOI:10.1039/c6ra26137j

A series of g-C₃N₄ nanosheets were synthesized by thermal oxidation with long time heating and etching. With an increase in heating time from 1.0 to 3.0 h, the g-C₃N₄ nanosheet was obtained with a thinner layer

Full text of DOI:10.1039/c6ra26137j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Thermal oxidative etching method derived
graphitic C3N4: highly efficient metal-free catalyst
in the selective epoxidation of styrene†
Cite this: RSC Adv., 2017, 7, 5340
a
a
a
a
b
a
*
*
Jia Ren,‡ Xin Liu,‡ Lu Zhang, Qianqian Liu, Ruihua Gao and Wei-Lin Dai
A series of g-C3N4 nanosheets were synthesized by thermal oxidation with long time heating and etching.
With an increase in heating time from 1.0 to 3.0 h, the g-C3N4 nanosheet was obtained with a thinner
layer thickness, larger BET surface area and higher graphic nitrogen ratio. As
a metal-free
heterogeneous catalyst, the g-C3N4 NS-3 h nanosheets show a superior catalytic performance for the
epoxidation of styrene to styrene oxide than that of the bulk g-C3N4 and other recently reported
metal-free materials. The higher activity of the g-C3N4 NS-3 h synthesized by long time thermal
oxidative etching might be ascribed to the enlarged specific surface, pore volume and higher graphic
nitrogen ratio with the loose and soft laminar morphology. The graphitic nitrogen species play a key
role in the catalytic reaction based on a good linear correlation between the content of these species
and the activity results. The g-C3N4 nanosheets are very stable and can be reused 5 times without
obvious loss of catalytic activity.
Received 1st November 2016
Accepted 13th November 2016
DOI: 10.1039/c6ra26137j
www.rsc.org/advances
species dispersed in this molecular sieve was identied as the
key active center of the catalyst.5 Wang and coworkers found
1. Introduction
that g-C3N4 could promote the catalytic epoxidation of styrene
using molecular oxygen as the primary oxidant aer modica-
tion with Co and Fe.6 In our previous work,7 the catalytic
performance of the Zr–CeO2 is mainly attributed to the oxygen
vacancy and the Ce3+.
Alkene epoxidation reaction is a rst and important step in
industrial processes because the products are versatile inter-
mediates in the synthesis of ne chemicals such as plasticizers,
perfumes, epoxy resin, pharmaceuticals, etc. Traditionally,
epoxidation reaction was carried out by directly using stoi-
chiometric peracids as the oxidants,1 which produced a large
amount of by-products. Even worse, the peracids were expen-
sive, unsafe and hazardous. Due to the increasing attention
paid to the environmentally sustainable growth, much more of
an attempt has been made to develop various new processes for
alkene epoxidation reaction. The current research is being
concentrated on the metal-related catalysts, such as metal-
embedded molecular sieves, heteropolyacid modied
complexes and noble metal supported catalysts which are
usually synthesized though complicated process.2–4 For these
catalytic systems, the metals play an irreplaceable role in the
adsorption and activation of the alkenes and oxidants. Partic-
ularly, titanium substituted silicalite (TS-1) has been demon-
strated to be very effective for styrene epoxidation, and Ti
As sustainable development was hindered by the issue of
metal-based catalysts, such as their scarcity in nature, deactiva-
tion, high cost in disposal of waste catalysts, and secondary
pollution,8 it is potential to develop metal-free catalysts for alkene
epoxidation reactions. Recently, graphene-based non-metal
materials have gained more and more attention in the realm of
catalysis. Garcia and co-workers achieved a breakthrough by
synthesizing nitrogen- and boron-doped graphene, which can
efficiently activate di-oxygen in the epoxidation of benzylic
styrene compounds.9 In addition, Ma and co-workers discovered
that nitrogen-doped graphene can effectively catalyze the C–C
double bond epoxidation reaction, and the nitrogen dopants at
the quaternary site were proven to be the active species.10
However, there are still few studies reported on the application of
non-metal catalysts in the alkene epoxidation reaction. These
interesting results motivate us to pioneer green nitrogen-rich
carbon materials applying in the alkene epoxidation.
aDepartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Material, Fudan University, Shanghai 200433, P. R. China. E-mail:
wldai@fudan.edu.cn; Fax: +86 021 5566 5572; Tel: +86 021 5566 4678
bState Key Laboratory of Molecular Engineering of Polymers, Department of
Macromolecular Science, Fudan University, Shanghai, 200433, P. R. China. E-mail:
ruihuagao@fudan.edu.cn; Tel: +86 021 5566 5626
It is well known that graphitic carbon nitride materials are
a kind of nitrogen rich material which has similar sheet-like
structure with graphene. In addition, polymeric graphitic carbon
nitride materials (g-C3N4) have attracted much attention owing to
their similarity to graphene.11 Meanwhile, the g-C3N4 materials are
especially promising catalysts for introducing N into carbon
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra26137j
‡ These authors contribute equally to the paper.
5340 | RSC Adv., 2017, 7, 5340–5348
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
materials, due to their high N content, high thermal, chemical
stability, low price, and easily tailorable structure.12 The catalytic
activity of g-C3N4 has recently been reported for hydrogen evolu-
tion,12 oxidation of alkanes, including benzene,14 cyclohexane,15
toluene,16 and alcohols,17 and photodegradation of pollutants,18
etc. However, it is worth noting that bulk g-C3N4 shows a low
catalytic activity by far. It is necessary to optimize the structure of g-
C3N4 for getting a better catalytic performance. For example,
exfoliating bulk g-C3N4 into nanosheets have been proved to be
able to enhance the efficiency of hydrogen evolution by one order
of magnitude,13 offer novel activity in photocatalytic disinfection19
and bio-imaging application.20 Currently, the g-C3N4 materials can
also be used as photodegradation catalysts in the eld of photo-
catalysis. However, scarce efforts have been taken to employ g-C3N4
as the metal-free catalysts for the epoxidation of olens.
Herein, the g-C3N4 nanosheets with varying thickness of
layer, surface chemistries and surface areas were obtained by
a simple method, namely, thermal oxidative etching of bulk g-
C3N4 in air with various time, leading to different catalytic
performances. The g-C3N4 was rstly used as a metal-free
catalyst in the epoxidation of styrene with a stable and excel-
lent catalytic performance. The g-C3N4 nanosheets demonstrate
morphology-dependent activity in the epoxidation of styrene,
which is much higher than that of bulk g-C3N4. By detailed
characterizations, the graphitic nitrogen rich species and high
surface areas are responsible for the epoxidation reaction. The
excellent catalytic performances as well as the wide applicability
make g-C3N4 nanosheets a promising catalyst for “green”
oxidation of styrene.
Scheme 1 Schematic diagram of the synthesis process of bulk g-C3N4
and the thermal etching process.
(2q) range of 10–90ꢀ at a scanning speed of 8ꢀ minꢁ1, operated at
40 kV and 40 mA with nickel-ltered Cu Ka radiation (l ¼
0.15406 nm).
The Brunauer–Emmett–Teller (BET) specic surface areas
and pore volume of the catalysts were characterized by nitrogen
adsorption–desorption (Micromeritics Tristar ASAP 3000)
employing BET method. The samples were pre-treated at 250 ꢀC
in vacuum for 3 h before test to desorb moisture adsorbed.
Transmission electron micrographs (TEM) images were ob-
tained on a JOEL JEM 2010 transmission electron microscope.
The samples were supported on carbon-coated copper grids.
Fourier transform infrared spectra (FT-IR) were gained from
Nicolet Avatar-360 FT-IR spectrometer. Scanning electron
micrographs (SEM) were obtained using a PHILIPS XL 30
microscope operating at an accelerating voltage of 20 kV.
Tapping-mode atomic force microscopy (AFM, Nanoscope
Multimode 8) with Si-tip cantilever was used to evaluate the
morphology and thickness of the obtained nanosheets on the
mica substrate.
2. Experimental
2.1 Preparation and activity test
The X-ray photoelectron spectroscopy (XPS) were determined
on a RBD-147 upgraded Perkin-Elmer PHI 5000C ESCA system
equipped with a dual X-ray source and a hemispherical electron
energy analyzer. The Mg Ka (1253.6 eV) anode was operated at
14 kV and 20 mA. The spectra were performed in the constant
pass energy mode with a value of 46.95 eV, and all binding
energies were calibrated referenced to the carbonaceous C1s at
284.6 eV.
Synthesis of the bulk g-C3N4 and g-C3N4 nanosheets. The
bulk C3N4 was prepared according to a procedure reported by
Wang et al.20 In detail, 20 g of dicyandiamide (Sinopharm
Chemical Reagent Co., Ltd. China.) was heated in muffle
ꢁ1
ꢀ
ꢀ
furnace at 550 C for 4 h with a ramping rate of 2.3 C min
.
The resultant yellow agglomerates were collected and grounded
into powder in an agate mortar for further catalytic perfor-
mance measurements and structural characterization.
The g-C3N4 nanosheets with different morphologies and
sizes can be directly synthesized via thermal oxidative etching
method (denoted as g-C3N4 NS-t h, where t represents the
thermal oxidative etching time). Typically, a certain amount of
as-prepared bulk g-C3N4 was plaꢀced in a crucible and heated for
minꢁ1. Aer cooling to room temperature, a light yellow powder
of g-C3N4 NS-2 h was nally obtained. In addition, thermal
oxidative etching time is a key factor in controlling the forma-
tion and mass yield of the obtained g-C3N4 nanosheets
(Scheme 1).
2.3 Activity measurements
The activity measurement for styrene epoxidation was carried
out in a closed 25 mL regular glass reactor with a reux
condenser using styrene as a substrate, 70 wt% aqueous t-butyl
hydroperoxide (TBHP) as the oxidant and acetonitrile as
solvent. In a typical oxidation reaction, 50 mg of catalysts, 10 mL
of acetonitrile and 8.7 mmol of TBHP (70 wt% aqueous) were
introduced into the regular glass reactor at 80 ꢀC with magnetic
stirring. Aer the reaction, the catalyst was separated and the
quantitative analysis of the reaction products was carried out by
gas chromatography (GC), and the determination of different
ꢀ
2 h in a muffle furnace at 500 C with a ramping rate of 5 C
2.2 Catalyst characterization
The samples were characterized by wide-angle XRD patterns on products in the reaction mixture was performed by GC-mass
a Bruker D8 Advance X-ray diffractometer in a scanning angle spectroscopy (GC-MS).
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 5340–5348 | 5341

View Article Online
RSC Advances
Paper
Substrate conversion ¼ moles of substrate converted/moles of
substrate used
Product selectivity ¼ moles of product formed/moles of substrate
converted
The stability was tested as follows: the re-used catalyst was
separated by centrifuge, washed by distilled water and ethanol for
several times, and dried under 80 ꢀC for 12 h for the next run.
3. Results and discussion
3.1 Characterization of the as-prepared samples
The XRD proles of the as-prepared samples before and aer
heat treatment are shown in Fig. 1. All the samples show two
obvious diffraction peaks at 2q ¼ 12.9ꢀ and 27.3ꢀ, respectively.
The strong peak at 27.3ꢀ is ascribed to the (002) plane with d ¼
0.326 nm, which is assigned to a characteristic interlayer
spacing of aromatic systems.11 With increasing the thermal
oxidative etching time, the diffraction intensity of this (002)
peak remarkably decreases and the diffraction peak shis to the
higher angle end, which demonstrates that the increased
heating time during thermal oxidation process may lead to the
exfoliation of bulk g-C3N4 to nanosheets and a denser packing
between the basic sheets.11,21–23 Additionally, Antonietti et al.
reported that undulated single layers in bulk g-C3N4 could be
planarized by further heating, resulting in a denser stacking.21,24
Furthermore, the low-angle reection peaks at 13.1ꢀ, stemmed
from in-planar tris-s-triazine structural packing, are not
changed with different etching time, which indicated that the
planar size of the layers changed little during the thermal
oxidative etching of bulk g-C3N4.13,22
Fig. 2 TEM and SEM images of bulk g-C3N4 (a and c), and g-C3N4 NS-
3 h (b and d).
sheet edges roll up in order to minimize the surface-energy of
the sheet. Simultaneously, very similar trend was observed by
Niu et al.22 To a certain extent, the appearance of g-C3N4
nanosheets in Fig. 2b is quite similar to that of graphene or
graphene oxide. Fig. 2c shows the bulk g-C3N4 display in the
form of irregular blocky agglomerates, and the size distribution
is mainly in several micrometers. While upon thermal treat-
ment at 500 ꢀC for 3 h, the resulting sample g-C3N4 NS-3 h
(Fig. 2d) appeared as loose and so agglomerates consisting of
laminar morphology like petals. Obviously, great numbers of
small gaps between these petals indicate the formation of
mesoporous structure and the exfoliation of bulk g-C3N4, which
is in a good agreement with the XRD and BET results.
To gain insight into the structural features of g-C3N4 nano-
sheets, atomic cross-sectional AFM was conducted. As shown in
Fig. 3, the representative AFM image and thick analyses of the g-
C3N4 NS-3 h show average thickness of ca. 1.301 nm, revealing
the same laminar morphology appeared in the observations
from SEM and TEM. It is well known that the theoretical
The morphology and microstructure of the as-prepared g-
C3N4 NS-3 h and bulk C3N4 were investigated via SEM and TEM.
As shown in Fig. 2b, the as-prepared g-C3N4 nanosheets are
transparent to electron beams owing to their thinner nature
compared with bulk g-C3N4 (Fig. 2a). Furthermore, the basic
Fig. 3 (a) The typical AFM images of the g-C3N4 NS-3 h, (b) height
Fig. 1 XRD patterns of bulk g-C3N4, g-C3N4 NS-1 h, g-C3N4 NS-2 h, analysis of the nanosheets corresponding to the write curves in the
and g-C3N4 NS-3 h.
AFM images.
5342 | RSC Adv., 2017, 7, 5340–5348
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
thickness of monolayer is 0.326 nm.25,26 Besides, Tang et al.
conclude that the thickness analysis of 1-, 2- and 4-layer g-C3N4
nanosheet by AFM revealed an average thickness of ca. 0.4, 0.7
and 1.3 nm, respectively.27 From this point of view, it could be
believed that herein the g-C3N4 NS-3 h fabricated by a simple
thermal oxidative etching process was in form of four C–N
layers.
The change of chemical structure of g-C3N4 with prolong-
ing heat treatment time was further conrmed by the FTIR
spectra shown in Fig. S1.† All samples display similar char-
acteristic IR absorption spectrum. The sharp peak at ca. 810
cmꢁ1 is originated from the characteristic breathing mode of
heptazine units.28 The peaks in the region from ca. 1200 to
1480 cmꢁ1 are assigned to the typical stretching vibrations of
either trigonal C–N(–C)–C or bridging C–N(–H)–C heterocycles
associated with skeletal stretching vibrations of aromatic
rings,29 and these bands become sharper compared with bulk
C3N4, probably due to the more ordered packing of hydrogen-
bond cohered long stands of polymeric melon units survived
aer thermal oxidative etching in the layer of nanosheets.22
The absorption bands at ca. 1633 and 1570 cmꢁ1 is due
primarily to the presence of C]N in the s-triazine. The broad
peaks between 3000 and 3600 cmꢁ1 are originated from the
O–H stretches, suggesting there are adsorbed hydroxyl species
in the surface of the catalysts. The similarity between all the
bands indicates that the heat-treatment of the C3N4 sample
does not lead to signicant changes in the structure of the g-
C3N4 nanosheets. Besides, the most prominent change of the
IR is the band in the ca. 3600–3000 cmꢁ1 region, corre-
sponding to the surface-bonded H2O molecules, which is
much stronger aer the thermal treatment, implying the
enlarger BET surface area.30 The nitrogen adsorption–
desorption isotherms were used to further characterize the
microstructural change of obtained C3N4 with increasing the
thermal etching time. According to IUPAC classication, all
the g-C3N4-t h samples and bulk-C3N4 can be identied as
Type IV adsorption–desorption isotherms with H3 hysteresis
loop in the relative pressure range of 0.6–1.0 (Fig. 4).23,31 The
obtained specic BET surface areas are 22, 52, 151 and 245 m2
gꢁ1, respectively, for bulk-C3N4, g-C3N4-1 h, g-C3N4-2 h, g-
C3N4-3 h, suggesting the increased specic BET surface area
with thermal etching time. The BET surface area of the g-
C3N4-3 h is about 11 times higher than that of bulk g-C3N4.
And the large surface area could provide more reactive sites
and adsorb more reactants. Furthermore, as seen from the
BJH pore-size distribution curve (inset), the distribution of
pores centered between 25 and 50 nm may attribute to the
mesopores originated from the stacking of C3N4 nanosheets,
which also increased with the thermal etching time. These
variation tendencies further conrm that the laminar struc-
ture bulk g-C3N4 can be effectively exfoliated by extending the
thermal etching time. This result supports those from SEM,
TEM and AFM. Most importantly, g-C3N4 material with mes-
opores and large surface area can be obtained by using this
simple etching method even though the nal mass yields
decrease at much longer thermal etching time (as shown in
Table S1†). Therefore, according to the results from SEM,
Fig. 4 The N2 adsorption–desorption isotherm and the correspond-
ing BJH pore-size distribution curve (inset) of bulk g-C3N4, g-C3N4
NS-1 h, g-C3N4 NS-2 h, and g-C3N4 NS-3 h.
TEM, AFM and N2 adsorption, we can conclude that the
increased etching time in our study gives rise to signicant
changes in the textural properties of carbon nitride. The bulk
g-C3N4 in the form of irregular blocky agglomerates was
divided into loose and so agglomerates, consisting of
laminar morphology like petals aer long time heating and
etching process.
To further investigate the compositions and chemical state
of the samples, the XPS measurements were carried out. As
shown in Fig. S2,† only three non-metal elements (C, N, O) are
detected in the XPS spectra. The O1s peak is assigned to the
adsorbed H2O on the sample surface, which appeared at
about 532.5 eV for all samples.29 In order to gain insight into
the chemical bonding between nitrogen and carbon atoms in
the samples, the high resolution C1s and N1s spectra of g-
C3N4 were further deconvoluted into Gaussian–Lorentzian
peaks, respectively. As shown in Fig. S3,† the C1s spectra
shows three peaks centered at ca. 284.6, 287.2 and 287.8 eV.
The peak at 287.2 eV is identied as sp2 hybridized C atoms
bonded to N inside the aromatic structure, while the peak at
287.8 eV is attributed to the sp2 hybridized C atoms bonded to
N inside the aromatic structure.12,31,32 The C1s peak at ca.
284.6 eV is typically originated from surface adventitious
carbon.22
Meanwhile, the high-resolution N1s spectra can also be
deconvoluted into four Gaussian–Lorentzian peaks. The four
peaks centered at about 398.8, 400.3, 400.9 and 404.5 eV are
assigned to N1, N2, N3 and N4, respectively (Fig. 5).33 The
dominate N1 peak is typically assigned to sp2-bonded N(C–N]
C) involved in the triazine rings.34,35 While the N2 is attributed to
the graphite-like N in the form of N–(C)3, and the peak at
400.9 eV (N3) is assigned to bridging N atoms [exocyclic:
NH(C3N3)2, NH2(C3N3)], originating from the defective
condensation of heptazine substructures.36–38 The very weak
peak N4 at 404.1 eV can be indicative of the charging effects or
positive charge localization in the heterocycles.37
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 5340–5348 | 5343

View Article Online
RSC Advances
Paper
Fig. 5 Deconvolution of the XPS N1s peaks for (a) g-C3N4 NS-3 h; (b) g-C3N4 NS-2 h; (c) g-C3N4 NS-1 h; (d) bulk g-C3N4; (e) U-g-C3N4.
The distributions of the four kinds of nitrogen in the the process of vacuum heat-treatment of carbon nitride.39 As
samples were estimated according to their percent in total XPS well known, reaction path for the formation of graphitic C3N4
peak area. As shown in Table 1, the N1 ratio keeps substan- starting from di-cyanodiamide is a combination of a poly-
tially constant (ca. 76 at%), while the ratios of N2 and N3 addition and poly-condensation scheme, which consist of
change with different thermal oxidative etching time. With the several processes, such as the condensation of precursors
increase of thermal oxidative etching time, the N2 ratio towards melamine, the rearrangements of melamine to form
increases along the decrease of the N3 ratio, which suggests tris-s-triazine, and the condensation of this tris-s-triazine to
that thermal oxidative etching promote the poly-condensation polymers.40 The nal polymeric C3N4 is composed of a series of
and the structural rearrangement of small oligomeric frag- different degrees of polymerization of the polymer, so poly-
ments. These variation trends are similar with those found in condensation and the structural rearrangement of small
5344 | RSC Adv., 2017, 7, 5340–5348
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
Table 1 Distribution of N atoms in the sample
mainly at the cost of benzaldehyde. This result indicates that
the epoxidation process is a thermodynamics controlled
reaction, while the oxidative C]C bond cleavage is a kinetics
controlled reaction. The reusability of a heterogeneous cata-
lyst is a key role for its practical application. For this reason,
the practical reusability of the g-C3N4 nanosheet in the epox-
idation of styrene was tested to evaluate the stability of the
catalyst. The recycling test was performed for continuous ve
reaction cycles under the identical reaction as mentioned
above. The re-used g-C3N4 nanosheet catalyst was separated by
centrifugation from the reaction solution, washed with
distilled water and ethanol several times, and then dried at 353
K for 12 h. It is interesting to nd that the g-C3N4 nanosheet
catalyst also shows desired recyclability. As illustrated in
Fig. S5,† during the six reaction cycles, there is almost no
difference in the conversion and epoxide selectivity of the g-
C3N4 NS-3 h on the epoxidation of styrene. In addition, the
used catalyst kept the same structure and morphology with
fresh g-C3N4 NS-3 h, conrmed by the determination of TEM
and XRD. These results strongly indicate that g-C3N4 nano-
sheets are very stable and active inexpensive catalysts for the
epoxidation reaction, meaning that it can be used for further
industrial applications.
Samples
N states
N1
N2
N3
N4
g-C3N4 NS-3 h B.E/eV
Atom content/%
g-C3N4 NS-2 h B.E/eV
Atom content/%
g-C3N4 NS-1 h B.E/eV
Atom content/%
B.E/eV
Atom content/%
Bulk U-g-C3N4 B.E/eV
Atom content/%
398.4
76.73
398.2
76.66
398.2
76.83
398.3
76.25
398.3
75.27
399.8
11.04
399.6
9.71
399.7
9.18
399.7
8.81
399.5
8.52
400.9
8.97 3.26
400.7
9.75 3.88
400.7
9.99 4.00
400.7
11.03 3.91
403.9
404
404.1
Bulk g-C3N4
404.1
400.6
12.60 3.61
403.9
oligomeric fragments occur in the thermal etching process
where N3 is eliminated.
3.2 Reaction behavior
The reaction behavior of epoxidation of styrene over g-C3N4 NS-
2 h under different reaction conditions was investigated
(Fig. S4†). Fig. S4b† depicts the reaction pathway where the
conversion of (mol%) and selectivity (mol%) of the products are
plotted against reaction time. As expected, styrene conversion
increases steadily in the rst 12 h, and then grows slowly. The
3.3 Catalyst activities of the g-C3N4 nanosheets
selectivity of epoxidation products dropped with the progress of For the rst time, the C3N4 nanosheets were used as metal-free
the reaction, and the selectivity of benzaldehyde changes in the catalyst for the epoxidation of styrene to styrene oxide by using
opposite trend simultaneously. This nding is mainly because TBHP (70 wt% in water) as the oxidant and CH3CN as the
of the further oxidation of primarily formed epoxide into solvent. As shown in Table 2, without any catalyst (Entry 5,
benzaldehyde by TBHP.
Table 2), autooxidation of styrene into styrene oxide and
Fig. S4d† illustrates the effect of reaction temperature on benzaldehyde occurs, whereby the conversion of the substrate
the conversion (mol%) of styrene and the epoxide selectivity is very low (ꢂ5%). This value is much lower than that of bulk
(mol%). As expected, styrene conversion increased from 30% C3N4 with the conversion of 51.6% and the selectivity of 82.1%
to 76% by increasing the reaction temperature from 333 K to (Entry 1, Table 2). It should be noted that the g-C3N4 nanosheet
363 K. Further, selectivity to styrene oxide also increased samples apparently exhibit high styrene oxide yield than the
Table 2 The catalytic performance of samples under different reaction conditiona
Selectivity (%)
Entry
Catal.
Styrene conversion (%)
Styrene oxide
Benzaldehyde
Others
Yield of styrene oxide (%)
1
2
3
4
Bulk-g-C3N4
g-C3N4-NS-1 h
g-C3N4-NS-2 h
g-C3N4-NS-3 h
—
g-C3N4-NS-3 h
g-C3N4-NS-3 h
g-C3N4-NS-3 h
g-C3N4-NS-3 h
N-OLC-3
51.6
56.8
64.0
81.2
5.4
82.1
81.9
82.7
82.2
81.0
—
14.6
12.6
13.9
12.9
19.0
—
88.6
25.9
11.3
75.8
60.0
27.2
10.1
3.3
5.5
3.4
4.9
—
42.4
46.5
52.9
66.7
4.37
—
5b
6c
7d
8e
9f
—
—
13.0
12.1
80.5
92.9
58.0
87.6
36.1
7.8
3.6
13.2
8.0
—
—
—
1.0
7.4
60.9
80.7
24.2
40.0
72.8
85.6
65.0
22.5
23.2
63.8
30.9
10g
11h
12i
13j
N-Graphene
NG-800
U-Bulk-g-C3N4
4.3
a
b
Reaction condition: 4.35 mmol styrene, 50 mg catalyst, 5 mL solvent (CH3CN), 8.7 mmol TBHP (70% aqueous solution), 80 ꢀC, 12 h. Without
Catal. c Without TBHP. H2O2 as oxidant. e 2,6-Di-tert-butyl-4-methylphenol was added as a free-radical quencher. f Tributyl phosphate is added
d
as an polymerization inhibitor. g Ref. 10. h Ref. 36. Ref. 9 (the error range is below ꢃ0.5%). j The bulk g-C3N4 from urea as the catalyst.
i
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 5340–5348 | 5345

View Article Online
RSC Advances
Paper
bulk C3N4. The thermal etching of bulk C3N4 improves the and lower N2 ratio. As shown in Table 2 (Entry 13), the
catalytic performance signicantly. Among all the catalysts, g- conversion of styrene on the g-C3N4 from urea is lower that of g-
C3N4 NS-3 h exhibits an optimal conversion of 81.2% with the C3N4 derived from di-cyanodiamine. This nding implies that
highest yield of 66.7% (Entry 4, Table 2). This result is superior graphitic nitrogen species N2 is critical for the epoxidation
to those recently reported metal-free catalysts (Entry 10–12, activity.
Table 2) under similar conditions.9 In order to exclude the self-
Based on the discussion above and the reported litera-
polymerization of styrene in the strong oxidizing environment, ture,10,43,47 the possible catalytic reaction mechanism of the g-
tributyl phosphate is added to the reaction system employed C3N4 nanosheets were proposed and illustrated in Scheme 2.
as an effective polymerization inhibitor.41 The result (Entry 9, Catalytic reactions traditionally using TBHP have been re-
Table 2) reveals that high conversion for styrene arises from ported to proceed via a free radical mechanism, which was
catalysis rather than polymerization.
indirectly veried in this reaction system by using 2,6-di-tert-
The remarkably improved catalytic activities of g-C3N4 butyl-4-methylphenol as a free-radical quencher (Entry 8,
nanosheets caused by thermal oxidative etching can be mainly Table 2). Due to the formation of a metal-like d band electronic
attributed to the thin lamella conjugate structure with the structure, the nitrogen-neighboring carbon atom in g-C3N4
larger surface area and porous structures, which provide more nanosheets (Cat) may be more able to interact with TBHP to
surface active sites for promoting the adsorption and more form Cat-OH species, which can be further converted to
channels for mass transfer. In addition to the BET area Cat-OOH species under the oxidation conditions. In addition,
affecting the catalytic activity of g-C3N4, the graphitic nitrogen based on the big p system of g-C3N4, styrene with p orbitals
species N2 is one of the main factors. As shown in Fig. 6, the are able to adsorb on g-C3N4 surface via a strong p–p inter-
catalytic performance is in line with the N2 ratio which action, subsequently, on account of the specic properties of
increases with the thermal oxidative etching time. Similarly, free radicals,48 carbon–carbon double bond of styrene is
the linear relationship between the catalytic activity and the attacked by the a-oxygen of the Cat-OOH species, broke up to
concentration of graphitic nitrogen is also reported by Su and carbon–carbon single bond and then formed a transition
coworkers.10
state with a new C–O bond and C6H5ꢁcCH–CH2 inter-
Ma et al. reported that graphitic nitrogen modulated the mediate species. Subsequently, since the ring-closure barrier
electronic structure of the adjacent sp2 carbon atoms, nally is much lower than the C–O bond formation barrier, a ring-
confers the nitrogen-neighboring carbon a metal-like d-band closure process essentially leads to the formation of the
electronic phenomenon.42,43 These unique electronic structure target epoxide product. In the nal step, the surface reactive
of carbon atoms make them more capable of adsorbing the oxygen species can be recovered by the reaction between
peroxide-like species such as –OOH.10,42,44,45 To conrm that the tBuOOH, which completed the reaction cycle. In addition, the
catalytic performance of the g-C3N4 is not only decided by the abundant existence of tBuOH aer reaction by using gas
BET area but also the concentration of graphitic nitrogen, we chromatography-mass spectrometry veried indirectly the
prepared the bulk g-C3N4 using urea as precursor, where the rationality of the proposed mechanism process.
BET area (84.5 m2 gꢁ1) is about 4 times of that bulk g-C3N4
derived from di-cyanodiamine. Compared with the sample
using di-cyanodiamine as precursor which goes through a direct
poly-addition process to melamine, the urea precursor
undergoes a series of gas-releasing stages to form melamine,
leading to the formation of a less condensed and porous g-C3N4
with more edge excited amino groups,46 namely higher N3 ratio
Fig. 6 The relationship between the catalytic performance (yield of
styrene oxide) and the concentration of graphic nitrogen (N2) in g- Scheme 2 The possible reaction mechanism for the epoxidation of
C3N4 nanosheet.
styrene on g-C3N4 nanosheet.
5346 | RSC Adv., 2017, 7, 5340–5348
This journal is © The Royal Society of Chemistry 2017

View Article Online
Paper
RSC Advances
14 X. F. Chen, J. S. Zhang, X. Z. Fu, M. Antonietti and
X. C. Wang, J. Am. Chem. Soc., 2009, 131, 11658–11659.
15 Y. Wang, J. S. Zhang, X. C. Wang, M. Antonietti and H. R. Li,
Angew. Chem., Int. Ed., 2010, 122, 3428–3431.
16 X. H. Li, X. C. Wang and M. Antonietti, ACS Catal., 2012, 2,
2082–2086.
17 F. Z. Su, S. C. Mathew, G. Lipner, X. Z. Fu, M. Antonietti,
S. Blechert and X. C. Wang, J. Am. Chem. Soc., 2010, 132,
16299–16301.
18 S. C. Yan, Z. S. Li and Z. G. Zou, Langmuir, 2009, 25, 10397–
10401.
19 H. Zhao, H. Yu, X. Quan, S. Chen, Y. Zhang, H. Zhao and
H. Wang, Appl. Catal., B, 2014, 152–153, 46–50.
20 X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan and Y. Xie, J. Am.
Chem. Soc., 2013, 135, 18–21.
4. Conclusions
In summary, the g-C3N4 nanosheets were prepared by a simple
thermal oxidative etching process. Thermal oxidative etching
time has a signicant effect on the exfoliation process of g-C3N4,
lamellar thickness, morphology evolutions and chemical state
of nitrogen on the g-C3N4. As a metal-free heterogeneous cata-
lyst, the g-C3N4 NS-3 h nanosheet shows more superior catalytic
performance for the epoxidation of styrene to styrene oxide than
that of the bulk g-C3N4 and other recently reported metal-free
materials. The thin lamellar structure with large specic
surface area and pore volume, graphitic nitrogen in g-C3N4
nanosheets contribute to the superior catalytic performance.
The optimal styrene conversion and epoxide selectivity reach
81.2% and 82.2%, respectively. The possible reaction mecha-
nism is proposed on the basis of the unique properties of
graphitic nitrogen and the free radical theory. This nding put
forward a new and eco-friendly methodology for olen epoxi-
dation using the metal-free catalysts.
21 X. Chen, J. Zhang, X. Fu, M. Antonietti and X. C. Wang, J. Am.
Chem. Soc., 2009, 131, 11658–11659.
22 P. Niu, L. L. Zhang, G. Liu and H. M. Cheng, Adv. Funct.
Mater., 2012, 22, 4763–4770.
23 Q. Gu, Z. Gao, H. Zhao, Z. Lou, Y. Liao and C. Xue, RSC Adv.,
2015, 5, 49317–49325.
24 M. Groenewolt and M. Antonietti, Adv. Mater., 2005, 17,
1789–1792.
Acknowledgements
We would like to thank nancial support by the Major State
25 H. Zhao, H. Yu, X. Quan, S. Chen, H. Zhao and H. Wang, RSC
Adv., 2014, 4, 624–628.
Basic
Resource Development
Program
(Grant
No.
2012CB224804), NSFC (Project 21373054, 21303023, 21173052),
the Natural Science Foundation of Shanghai Science and
Technology Committee (08DZ2270500).
26 K. Schwinghammer, B. Tuffy, M. B. Mesch, E. Wirnhier,
C. Martineau, F. Taulelle, W. Schnick, J. Senker and
B. V. Lotsch, Angew. Chem., Int. Ed., 2013, 52, 2435–2439.
27 J. Jiang, L. Yang, L. Zhu, A. Zheng, J. Zou, X. Yi and H. Tang,
Carbon, 2014, 80, 213–221.
Notes and references
1 A. Murphy, G. Dubois and T. D. P. Stack, J. Am. Chem. Soc., 28 Q. Xiang, J. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115,
2003, 125, 5250–5251. 7355–7363.
2 J. M. Thomas and G. Sankar, Acc. Chem. Res., 2001, 34, 571– 29 F. Dong, Z. Zhao, T. Xiong, Z. L. Ni, W. D. Zhang, Y. J. Sun
581.
and W. K. Ho, ACS Appl. Mater. Interfaces, 2013, 5, 11392–
11401.
30 Q. Han, B. Wang, J. Gao, Z. Cheng, Y. Zhao, Z. Zhang and
L. Qu, ACS Nano, 2016, 10, 2745–2751.
31 C. Yuan, X. Zhang, L. Su, B. Gao and L. Shen, J. Mater. Chem.,
2009, 19, 5772–5777.
32 J. Zhang, M. Zhang, G. Zhang and Z. Wang, ACS Catal., 2012,
2, 940–948.
33 J. Xu, L. Zhang, R. Shi and Y. Zhu, J. Mater. Chem. A, 2013, 1,
14766–14772.
3 G. D. Yadav and A. A. Pujari, Org. Process Res. Dev., 2000, 4,
88–93.
4 X. Deng and C. M. Friend, J. Am. Chem. Soc., 2005, 127,
17178–17179.
5 S. B. Kumar, S. P. Mirajkar, G. C. G. Pais, P. Kumar and
R. Kumar, J. Catal., 1995, 156, 163–169.
6 Z. X. Ding, X. F. Chen, M. Antonietti and X. C. Wang,
ChemSusChem, 2011, 4, 274–281.
7 X. Liu, J. Ding, X. Lin, R. H. Gao, Z. H. Li and W. L. Dai, Appl.
Catal., A, 2015, 503, 117–123.
34 A. Vinu, Adv. Funct. Mater., 2008, 18, 816–827.
8 X. Duan, H. Sun, Y. Wang, J. Kang and S. Wang, ACS Catal., 35 Y. J. Cui, J. S. Zhang, G. G. Zhang, J. H. Huang, P. Liu,
2015, 5, 553–559.
M. Antonietti and X. C. Wang, J. Mater. Chem., 2011, 21,
13032–13039.
36 W. K. Ho, Z. Z. Zhang, W. Lin, S. Huang, X. W. Zhang,
X. X. Wang and Y. Huang, ACS Appl. Mater. Interfaces,
2015, 7, 5497–5505.
9 A. Dhakshinamoorthy, A. Primo, P. Concepcion, M. Alvaro
and H. Garcia, Chem.–Eur. J., 2013, 19, 7547–7554.
10 W. Li, Y. Gao, W. Chen, P. Tang, W. Z. Li, Z. J. Shi, D. S. Su,
J. G. Wang and D. Ma, ACS Catal., 2014, 4, 1261–1266.
11 Y. Wang, X. C. Wang and M. Antonietti, Angew. Chem., Int. 37 A. Thomas, A. Fischer, F. Goettmann, M. Antonietti,
€
€
Ed., 2012, 51, 68–89.
J. O. Muller, R. Schlogl and J. M. Carlsson, J. Mater.
12 A. Thomas, A. Fischer, F. Goettmann, M. Antonietti,
Chem., 2008, 18, 4893–4908.
J. O. Muller, R. Schlogl and J. M. Carlsson, J. Mater. 38 Y. Shiraishi, S. Kanazawa, Y. Kofuji, H. Sakamoto,
Chem., 2008, 40, 4893–4908.
13 S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, R. Vajtai,
X. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452–2456.
S. Ichikawa, S. Tanaka and T. Hirai, Angew. Chem., Int. Ed.,
2014, 53, 13454–13459.
This journal is © The Royal Society of Chemistry 2017
RSC Adv., 2017, 7, 5340–5348 | 5347

View Article Online
RSC Advances
Paper
39 Y. Cao, Z. Zhang, J. Long, J. Liang, H. Lin, H. Lin and 44 Y. Zheng, Y. Jiao, L. Ge, M. Jaroniec and S. Z. Qiao, Angew.
X. Wang, J. Mater. Chem. A, 2014, 2, 17797–17807. Chem., Int. Ed., 2013, 52, 3110–3116.
40 B. Jurgens, E. Irran, J. Senker, P. Kroll, H. Muller and 45 X. Zhong, H. Y. Yu, G. L. Zhuang, Q. Li, X. D. Wang,
¨
¨
W. Schnick, J. Am. Chem. Soc., 2003, 125, 10288–10300.
41 J. Li, G. Zhao, S. Gao, Y. Lv, J. Li and Z. Xi, Org. Process Res.
Dev., 2006, 10, 876–880.
Y. S. Zhu, L. Liu, X. N. Li, M. D. Dong and J. G. Wang, J.
Mater. Chem. A, 2014, 2, 897–901.
46 Q. Su, J. Sun, J. Wang, Z. Yang, W. Cheng and S. Zhang,
Catal. Sci. Technol., 2014, 4, 1556–1562.
42 Y. J. Gao, G. Hu, J. Zhong, Z. J. Shi, Y. S. Zhu, D. S. Su,
J. G. Wang, X. H. Bao and D. Ma, Angew. Chem., Int. Ed., 47 Y. M. Lin, X. L. Pan, W. Qi, B. S. Zhang and D. S. Su, J. Mater.
2013, 52, 2109–2133. Chem. A, 2014, 2, 12475–12483.
43 J. Ding, Q. Q. Liu, Z. Y. Zhang, X. Liu, J. Q. Zhao, S. B. Cheng, 48 A. J. Catino, R. E. Forslund and M. P. Doyle, J. Am. Chem. Soc.,
B. N. Zong and W. L. Dai, Appl. Catal., B, 2015, 165, 511–518. 2004, 126, 13622–13623.
5348 | RSC Adv., 2017, 7, 5340–5348
This journal is © The Royal Society of Chemistry 2017