Lamellar porous mo-modified carbon nitride polymers photocatalytic epoxidation of olefins

Article abstract of DOI:10.1016/j.mcat.2021.111441

A lamellar porous Mo-modified carbon nitride polymers (CN-Mo) was successfully prepared by a simple bottom-up method, which involves molecule self-assembly by hydrogen bonding between uracil and melamine. Analysis Results of X-ray diffraction (XRD), scann

Full text of DOI:10.1016/j.mcat.2021.111441

Molecular Catalysis 504 (2021) 111441
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Lamellar porous mo-modified carbon nitride polymers photocatalytic
epoxidation of olefins
,
Qian Gua, Ping Ping Jianga *, Yirui Shenb, Kai Zhanga, Phyu Thin Waia, Agus Haryonoc
a Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, PR
China
b College of Materials and Chemical Engineering, Ningbo University of Technology, Ningbo, 315211, PR China
c Research Center for Chemistry, Indonesian Institute of Science, Serpong, 15314, Indonesia
A R T I C L E I N F O
A B S T R A C T
Keywords:
A lamellar porous Mo-modified carbon nitride polymers (CN-Mo) was successfully prepared by a simple bottom-
up method, which involves molecule self-assembly by hydrogen bonding between uracil and melamine. Analysis
Results of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM)
and X-ray photoelectron spectra (XPS) show that Mo element has been doped in the skeleton of carbon nitride.
The optical characterization results of the as-prepared photocatalysts show that, CN-Mo has a suitable band
structure that can promote the separation of carriers and expands the absorption of light. Compared with CN, CN-
Mo has better photocatalytic activity for epoxidation of olefins. When the mass value (relative to melamine) of
ammonium molybdate tetrahydrate is 2 %, the photocatalytic efficiency is the best for the epoxidation of styrene.
And the conversion rate of styrene reaches 38.4 % and the selectivity of styrene oxide is up to 83.2 %. In addition,
it can maintain the better photocatalytic performance for the epoxidation of styrene after five reaction-
regeneration cycles. This finding provides an effective development direction for photocatalytic olefins epoxi-
dation under mild conditions.
Molybdenum
C3N4
Photocatalytic
Epoxidation
1. Introduction
photocatalytic systems is limited by poor circularity and extra additives,
so more concerns have been paid to heterogeneous photocatalysts.
Chemical reactions driven by solar energy have attracted wide
attention because of the global energy crisis and environmental pollu-
tion [1,2]. So far, the solar energy has been applied in various chemical
systems, such as photocatalytic hydrolysis [3,4], reduction of CO2 [5],
photodegradation of organic pollutants [6], bacterial disinfection [7],
photocatalytic organic synthesis [8,9], photocatalytic hydrogen (oxy-
gen) evolution [10–13]. Among them, Photocatalytic selective oxidation
is an important branch of photocatalytic research, especially photo-
catalytic epoxidation. The obtained epoxides are widely used in organic
synthesis, pharmaceutical industry, materials and life sciences [14,15].
Since Inoue [16] et al. reported photochemical epoxidation of olefins at
first, researchers have developed a series of photocatalytic epoxidation
systems. For example, homogeneous systems: mononuclear chiral
manganese (Mn) with non-heme [17], Mn (IV) nitro complex [18], iron
complex [19] and several poly-pyridinium complex photocatalysts [20],
heterogeneous photocatalysts: TiO2 [21], Cu@TiN [22], g-C3N4 [23],
Pt/ZnO [24]. However, the development of homogeneous
Metal-free semiconductor materials carbon nitride (g-C3N4) polymer
has become excellent photocatalytic material in the field of photo-
catalysis due to its light activity, low lost, chemical stability and non-
toxicity [25–27]. Compared with the traditional photocatalyst TiO2,
g-C3N4 has a weaker oxidation capacity and a higher selectivity due to
its narrow band gap (Eg = 2.7 eV) and low valence band position of 1.4
V [28]. Therefore, the g-C3N4 is one of the excellent photosensitizers for
photocatalytic selective oxidation. However, the development of bulk
g-C3N4 is greatly limited by its low carrier separation rate, fast
electron-hole recombination, low electrical conductivity, poor light
utilization rate, etc. [29]. The optical properties of g-C3N4 can be
effectively improved through its modification (doping [30,31], loading
[32–34], mechanical composite [35], morphology control [6,36,37],
etc.). The research results of Zhengxin Ding et al. [23] confirmed that
doping metals into g-C3N4 is one of the simple and efficient ways to
enhance the optical properties of the semiconductor materials. The
transition metals such as Fe, Co, V, Cu, Mn, Ti and Mo can be used to
* Corresponding author.
E-mail address: ppjiang@jiangnan.edu.cn (P.P. Jiang).
https://doi.org/10.1016/j.mcat.2021.111441
Received 24 September 2020; Received in revised form 16 January 2021; Accepted 25 January 2021
Available online 13 February 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Scheme 1. Synthesis and catalytic process of lamellar porous Mo-modified Carbon Nitride for the epoxidation of styrene.
catalyze epoxidation reactions. Among them, the molybdenum-based
catalyst exhibits better catalytic performance for epoxidation of olefins
[38–41]. Gang Bian et al. [42] demonstrated the Mo loaded the lamellar
graphene oxide/g-C3N4 (Mo-GO/g-C3N4) has photocatalytic activity for
epoxidation of olefins. Additionally, in order to provide more active sites
and a larger specific surface area, researchers often peel the bulk C3N4
into C3N4 nanosheets [43,44]. Nevertheless, this top-down process is
complicated, time-consuming, and costly [45]. In contrast, the
bottom-up approach is easier to operate and the process is simple, but
rarely produces porous and metal-doped C3N4 sheets.
dissolved in 60 mL of deionized water under ultrasound for 0.5 h. 60 mg
of uracil was added into the above homogeneous solution, which was
continuously dispersed by ultrasound until a homogeneous transparent
solution was formed. Next, 4 g of melamine was added and dispersed by
ultrasound over 1 h. Then, the mixture solution was transferred to me-
chanical stirring and stirred vigorously for another 8 h. After that, the
mixture slurry was transferred to 200 ml Teflon reactor with stainless
steel housing and kept the temperature at 170 ◦C for 12 h. The mixture
slurry was separated by high speed centrifugation after cooled to room
temperature, washed 2–4 times by deionized water, and then dried at 60
◦C for 12 h. The dried samples after grinding were put into a tube
furnace and calcined in the atmosphere of nitrogen. The annealing
temperature of the tubular furnace is set at 550 ◦C for 4 h with a heating
rate of 2 ◦C /min. The resulting samples were marked as CN-Mo-X (X is
the mass percentage of ammonium molybdate tetrahydrate.). For com-
parison, pure carbon nitride polymers were prepared in the same
experimental scheme except the addition of ammonium molybdate tet-
rahydrate, and the obtained catalyst was named as CN. The bulk g-C3N4
powder was prepared by thermal treatment of melamine according to
the previous paper [47].
In this work, porous Mo-modified Carbon Nitride nanosheets was
synthesized by a bottom-up strategy, which was applied to catalytic
epoxidation of olefins under simulated solar irradiation. Hydrogen
bonds are formed in the precursors of uracil, ammonium molybdate and
melamine. And then, Hydrogen bonds are released during calcination,
resulting in a high degree of freedom and a porous structure. In addition,
because ammonium molybdate can form hydrogen bonds with both
melamine and uracil in the precursors, Mo element was doped into the
C3N4 skeleton during calcination. Compared with C3N4 nanosheets, Mo-
modified C3N4 show better photocatalytic activity for epoxidation of
olefins. The detailed prepared and photocatalytic process was exhibited
in Scheme 1.
2.3. Photocatalytic test
The photocatalytic reactions were treated in a Quartz single diameter
flask (25 ml). A Xe lamp (300 W) with an AM 1.5 G simulated sunlight
irradiation (85 mW cmꢀ 2) was used as the sunlight source. Typical
conditions for the epoxidation were as follows: olefin (0.1 mmol),
acetonitrile (3 mL), chlorobenzene (0.05 mmol, a GC internal standard),
and photocatalysts (20 mg) were added to the quartz reactor with a
magnetic stirrer. After ultrasonic dispersion for 15 min, 50 wt.% H2O2
(17 mg, served as oxidant) was added into the above mixture and then
stirred continuously under the simulated sunlight irradiation for 6 h at
the room temperature. After the reaction, the photocatalysts was
centrifuged, washed twice by deionized water and acetonitrile respec-
2. Experimental section
2.1. Chemicals
Ammonium molybdate tetrahydrate, melamine, uracil, acetonitrile,
1,4-dioxane, 1,2-dichloroethane and chlorobenzene were derived from
Sinopharm Chemical Reagent Co., Ltd. Cis-Cyclooctene, Cyclohexene,
Cyclopentene, 1-Octene, 1-Pentene, 1-Hexene, 4-methyl-1-cyclohexene,
1-Phenyl-1-cyclohenxene, Styrene were purchased from Sigma-Aldrich.
50 wt. % H2O2 was derived from Arkema. All chemicals were used
directly without any further purification. Deionized water comes from
the laboratory.
◦
tively, as well as dried at 60 C for 12 h to next use. The stability of
photocatalysts was investigated with the recovered photocatalyst.
Products of epoxidation were analyzed with a gas chromatograph
(Shimadzu GC-8A). The conversion of olefins and products selectivity
were calculated via the internal standard method according to the GC
data, the calculation method was as follows:
2.2. Photocatalyst preparation
Mo modified g-C3N4 was prepared by improved on previous report
[46]. A certain mass percentage (1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, 6 wt.%
relative to melamine) of ammonium molybdate tetrahydrate was
2

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 1. (a) Wide angle XRD patterns of photocatalysts; (b) high-resolution (002) diffraction peak of photocatalysts.
Fig. 2. SEM images ((a)-(c)) and TEM ((d)-(f)) of the photocatalysts; SEM-EDS elemental mapping images of CN-Mo-2 for elements C, Mo and N ((h)-(j)).
[
]
[
]
2.4. Characterizations
A(olefin)
A(olefin)
ꢀ
A(chlorobenzene)
A(chlorobenzene)
t0
tx
[
]
[Conversion (%) ] =
[Selectivity (%) ] =
× 100
(1)
(2)
See the Supporting Information.
3. Results and discussion
3.1. Characterization of photocatalysts
A(olefin)
A(chlorobenzene)
t0
A(product)
× 100
[A(product) + A(by ꢀ products)]
—A: Peak area of detecting object
—t0: Initial time
The phase structure information of the as-prepared photocatalysts
was researched by power XRD. The typical XRD diffraction pattern of g-
C3N4 were observed in the Fig.1 a [48]. Two diffraction peaks
—tx: Reaction time
3

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 3. Survey scan XPS spectra of CN and CN-Mo-2; (b) C 1s, (c) N 1s high-resolution XPS spectra in the CN and CN-Mo-2; (d) high-resolution XPS spectra of Mo 3d
in the CN-Mo-2.
corresponding to diffraction angles of 13.0◦ and 27.2◦ were considered
to the (100) and (002) planes, respectively. In detail, the (100) plane is
associated to the in-plane structural packing motif of the trigonal N
linkage, and (002) plane reflects the interlayer-stacking structure of
aromatic units [49]. Compared with CN, XRD patterns of CN-Mo do not
appear diffraction peak of Mo element, which may be caused by the high
dispersion of Mo and the size is less than 5 nm (5 nm is the detection
limit of XRD [50]). The diffraction peaks of Mo species is not observed in
Fig. 1a, indicating that the doping of Mo element does not form a new
crystalline phase in the g-C3N4. Additionally, it is worth noting that the
(002) diffraction peak of CN-Mo-X is shifted (Fig. 1b), which may be
caused by the addition of Mo. Thus, it can be considered that Mo element
is evenly distributed in the skeleton of g-C3N4.
content percentages of C, Mo and N are listed on the Table S1 (Sup-
porting information). The EDS results are considered to be the stronger
evidence for Mo element doping into g-C3N4. According to the above
analysis, it is assumed that the Mo element may be doped into CN
skeleton.
In order to further illustrate the chemical state of Mo in CN, XPS
analyses were conducted. As displayed in Fig. 3a, a weak peak is
observed in CN-Mo-2 but not in CN, suggesting that Mo element is
successfully doped into the g-C3N4 skeleton. The high-resolution spec-
trum of C 1s, N 1s and Mo 3d are shown in the Fig. 3 b–d. For CN, the
signal spectrum of C 1s (Fig. 3b) is formed by the fitting of three peaks
using XPS peak fit software and the binding energies at 284.7, 286.6 and
–
288.2 eV, which is considered to be the C C bond of the amorphous
2
–
–
–
SEM and TEM measurements were carried out to observe the
microscopic morphologies of the photocatalysts. All the corresponding
images are shown in Fig. 2. In the SEM images (Fig. 2 a–c), it can be
found that the g-C3N4 obtained by direct calcination of melamine is bulk
(Fig. 2a). The g-C3N4 sheets can be obtained with the addition of uracil
(Fig. 2 b). Notably, when an ammonium molybdate is introduced, the
lamellar g-C3N4 produce a raised knot. In the TEM images (Fig. 2 d–f),
the CN exhibits a very thin lamellar structure (Fig. 2 e), and the CN-Mo-2
shows a lamellar porous structure with addition of Mo. In addition, Mo
nanoparticles are observed in the Fig. 2f, indicating Mo has been doped
into the skeleton of g-C3N4, which is similar to the results of XRD. The
SEM- EDS images were applied to present the elemental distribution of
the photocatalyst CN-Mo-2 (Fig. 2 h–j). The elemental C, N and Mo are
distributed uniformly in the surface of CN-Mo-2. Moreover, the specific
carbon, C-NH2 bond in the framework of g-C3N4 and N CN by sp
hybridization, respectively [51]. However, in the C 1s high-resolution
spectrum of CN-Mo-2, the binding energies of the C-NH2 peak is going
to move the higher binding energy and the peak area is significantly
increased. This may be caused by the Mo doping into the skeleton of
g-C3N4. Similarly, the spectrum of N 1s (Fig. 3c) can be divided into four
– –
peaks, corresponding to charging effect (404.5 eV), C NH (400.9 eV)
–
–
–
bond, the tertiary nitrogen state N-(C)3 (399.8 eV) and C NC (398.6
eV) formed by sp2 hybridization, respectively [52–54]. Compared to
pure CN, the fitted peaks of N 1s in CN-Mo-2 show a shift at higher
binding energies, resulting from the addition of Mo element. The char-
acteristic peaks of Mo 3d is observed in the Fig. 3d, the binding energies
of two major peaks at 232.1 and 235.2 eV associated with Mo 3d5/2 and
Mo 3d3/2, respectively, revealing that the mainly chemical state of Mo
4

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 4. (a) Thermogravimetric curves for photocatalysts; (b) nitrogen adsorption-desorption isotherms curve of photocatalysts.
Fig. 5. (a) UV–vis diffuse reflectance spectrum of photocatalysts; (b) PL spectra of photocatalysts; (c) transient fluorescence decay of photocatalysts; (d) EPR signal of
photocatalysts in light and darkness.
element in the skeleton of g-C3N4 is Mo (VI). But there is little Mo
element appeared in the chemical state of Mo (IV) (230.0 eV) [55],
which may be caused that Mo (VI) exposed on the surface of g-C3N4 has
been reduced due to the release of a large amount of NH3 during
calcination. To sum up, Mo element is successfully attached to the CN
skeleton.
photocatalysts all experience significant weight loss with the increasing
of temperature, which results from carbonization of C3N4 skeleton. Bulk
g-C3N4 and CN has been completely carbonized at 750 ◦C. Nevertheless,
the tris-structure of CN-Mo-2 is fully carbonized at 735 ◦C, which may be
caused by the presence of Mo element. Additionally, the initial thermal
decomposition temperature of CN-Mo-2 is lower than that of CN and
bulk g-C3N4, which may be due to the porous structures of CN-Mo-2. In
the diagram of N2 isothermal adsorption-desorption curves (Fig. 4b), it
TG analysis of photocatalysts was applied to study the thermal sta-
bility of the photocatalysts in N2 atmosphere. As shown in Fig. 4a,
5

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 6. Photocatalytic performance histogram of different photocatalysts.
can be observed that typical IV curve and a noticeable H3-type hyster-
esis loop at the relative pressure ranging from 0.45 to 1.0, the former
indicating the presence of mesopores and macropores in the materials
[56] and the latter suggesting the formation of slit-like pores. According
to the BET experimental parameters (Table S2, Supporting information)
and the pore size distributions curves (Fig. S1, Supporting information),
compared with CN, the increasing of specific surface area and pore
volume in the CN-Mo-2 indicates that it can provide more active sites.
All photocatalysts have an absorption edge at ~ 460 nm in the
UV–vis absorption spectra (Fig. 5a). Compared with CN, CN-Mo-X ex-
hibits a stronger absorption in ultraviolet wavelengths and a red shift of
the absorption edge, indicating the enlarged band gap. This phenome-
non may be caused by the doping of Mo element. The steady-state
fluorescence photoluminescence (PL) was applied to explore the
charge migration and separation of photocatalysts [57]. In the PL
spectra (Fig. 5b), the peak centre of CN-Mo-2 appears at the wavelength
of 486 nm, while the peak centre of CN appears at the lower wavelength
of 476 nm. It shows that the peak centre of CN-Mo-2 has a red shift,
2.004) is higher than that of CN (g = 2.003), implying the existence of
defects, which may be caused by the doping of Mo element into the
g-C3N4 skeleton.
The transient photocurrent spectrum and electrochemical imped-
ance spectroscopy (EIS) of photocatalysts were obtained using chro-
noamperometry, as shown in Fig S3 (Supporting information). It can be
seen that the photocurrent response density of CN-Mo-2 is higher than
that of CN (Fig S3 a.), indicating that the CN-Mo-2 has higher conduc-
tivity at the same voltage, which proves that the separation and
migration of charge carriers is enchanced. However, in the Nyquist di-
agram (Fig S3 b), the arc radius of CN-Mo-2 is smaller than that of CN,
inferring that CN-Mo-2 has lower resistance in the process of charge
carrier migration. Thus, the results of photocurrent and EIS also has
suggested that CN-Mo-2 can improve the capacity of carrier separation
and photocatalytic activities. Therefore, combined with the above
characterization results, it is inferred that the doping of Mo promote
separation capacity of photoinduced electron-hole pairs, improve car-
rier separation capability and enhance the range of light absorption. At
the same time, the doping of Mo element leads to a porous structure of g-
C3N4, which provides more active sites. Benefiting from these features,
the lamellar porous Mo-modified C3N4 shows better competence for
diverse photocatalytic application.
which is caused by the extended
π conjugated system. Additionally, the
PL intensity of CN-Mo-2 is lower than that of CN, which is beneficial to
restrain electron-hole recombination and promote the mobility of the
charge carriers [58]. The time-resolved fluorescence spectroscopy was
further used to validate the improvement of charge separation efficacy,
as shown in the Fig. 5c and Table S3 (Supporting information).
Compared with CN, CN-Mo-2 has a slower exponential decay with
average fluorescence lifetimes of 13.05 ns, which means the carrier
lifetime is extended. Increasing the carrier migration rate, inhibiting the
recombination of electron holes and prolonging the carrier lifetime are
beneficial for improving the photocatalytic performance.
3.2. Photocatalytic activities
To explore the optimized Mo content of CN-Mo-X, photocatalytic
epoxidation of styrene was carried out as model reaction (Scheme 1).
The composition of products was determined by gas phase spectrometry,
and the percentage of products was calculated by the peak area in the
spectrum. As shown in Fig. 6, CN-Mo-X displays the better photo-
catalytic performance for epoxidation of styrene under simulated sun-
light irradiation at the room temperature for 6 h with 50 wt. % H2O2 as
oxygen source. Detailed data are presented in Table S5 (supporting in-
formation), the lamellar CN is used as photocatalyst, the conversion rate
of styrene reaches 13.2 % and increase of 29.5 % over the bulk g-C3N4.
When Mo element is introduced into lamellar CN, the conversion rate of
styrene is more than 30 % and 3-fold enhancement over the CN. As the
increasing of Mo content, the conversion rate of styrene increased
slowly, but the selectivity of styrene oxide decreased. The main by-
products are benzyl alcohol and benzoic acid (Fig. S5, Supporting
Electron paramagnetic resonance (EPR) signals were used to explore
the charge effect on the photocatalysts, as illustrated in Fig. 5d. CN and
CN-Mo-2 produce an single Lorentz derived line which is derived from
the signals of the lone pair electrons in the heptazine ring carbon atoms
[59]. More electrons are excited when the sunlight irradiates photo-
catalysts, leading to photocatalysts exhibit an intensive EPR signal under
simulated sunlight illumination. Additionally, CN-Mo-2 exhibits an
obviously stronger signal than CN, indicating CN-Mo-2 has stronger
electron delocalization ability and is easier to generate lone pair elec-
tron. The g value was obtained and shown in Fig S2 (Supporting infor-
mation) by the analysis of EPR signals. The g value of CN-Mo-2 (g =
6

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Table 1
Performance Epoxidation of styrene.
Entry
Catalyst
Oxidant
Solvent
Heating
Light
Conversion (%)
Selectivity (%)
1
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
CN-Mo-2
50 %H2O2
70 %H2O2
30 %H2O2
TBHP
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
1,4- dioxane
1,2-dichloroethane
acetonitrile
(-)
(+)
(+)
(+)
(+)
(+)
(-)
38.4
25.8
32.3
37.1
52.1
0.3
83.6
76
2
(-)
3
(-)
74.6
20.2
61.1
68.1
71.2
92.5
70.8
81.3
51
4
(-)
5
O2
(-)
6
50 %H2O2
50 %H2O2
50 %H2O2
50 %H2O2
50 %H2O2
50 %H2O2
50 %H2O2
(30℃)
(50℃)
(80℃)
(-)
7
(-)
0.4
8
(-)
11.9
0.4
9
(-)
10
11
12
(-)
(+)
(+)
(+)
11.8
23.5
3.1
CN-Mo-2
(-)
(-)
(-)
80.2
Reaction conditions: styrene (0.1 mmol) as substrate, catalyst (20 mg), chlorobenzene (0.05 mmol) as GC internal standard, the slurry was continuously magnetic
stirred under the simulated sunlight irradiation for 6 h.
information). When the mass value (relative to melamine) of ammonium
molybdate tetrahydrate is 2 %, the selectivity of styrene oxide reaches
maximum (83.6 %), and the conversion rate reaches 38.4 %. In order to
obtain the target product with higher purity, the selectivity of styrene
oxide is given priority. Thus, CN-Mo-2 is considered to be the optimal
photocatalyst for epoxidation of styrene in CN-Mo-X.
darkness under the room temperature (entry 9). Additionaly, we have
also studied the photocatalytic efficiency of CN-Mo-2 for styrene under
different solvents, as illustrated in Table 1(entries 1, 10 and 11). A
solvent with high polarity exhibits better efficiency for the epoxidation
of styrene, so we selected acetonitrile as solvent. In addition, we also
find that styrene has a very low conversion rate without CN-Mo-2
photocatalyst. Therefore, optimal reaction conditions are followed:
CN-Mo-2 as photocatalyst, acetonitrile as solvent, 50 % H2O2 as oxidant,
simulated sunlight as light source, reaction temperature at room
temperature.
Reaction conditions: Reaction conditions: styrene (0.1 mmol), pho-
tocatalysts (20 mg), 50 wt. % H2O2 (17 mg), chlorobenzene (0.05 mmol,
GC internal standard), the slurry was continuously magnetic stirred
under the simulated sunlight irradiation for 6 h.
To explore the role of oxidant in the photocatalytic epoxidation
system, we chose multiple oxidants as oxidant source, as given in Table 1
(entries1 to 5). When the oxidant is H2O2, the 50 % H2O2 shows a better
photocatalytic efficiency than 30 % and 70 % H2O2 in the photocatalytic
epoxidation of styrene. This may be because higher concentration of
H2O2 is prone to peroxide for styrene, and lower concentration tends to
open ring of styrene oxide. When O2 and TBHP are used as oxidants, the
selectivity of styrene oxide declined dramatically (Table 1 entries 4 and
5). Thus, 50 % H2O2 is a suitable oxidant for photocatalytic epoxidation
of styrene. We also have studied the photocatalytic performance of the
photocatalyst at different temperatures without light source, as shown in
Table 1 (entries 6, 7 and 8). The results also illustrate that the increasing
of system temperature does not promote the epoxidation of styrene
without light radiation. The conversion of styrene is very low in
Reaction conditions: Reaction conditions: styrene (0.1 mmol), CN-
Mo-2 (20 mg), 50 wt. % H2O2 (17 mg), chlorobenzene (0.05 mmol,
GC internal standard), the slurry was continuously magnetic stirred
under the simulated sunlight irradiation for 6 h.
According to reports, the stability of catalysts is an important index
of heterogeneous photocatalysts [60]. Therefore, leaching experiment is
used to determine the stability of photocatalysts and its results are listed
in Table S4 (Supporting information). The photocatalyst CN-Mo-2 was
removed via hot filtration after two hours of reaction, and the filtrate
was analysed by GC. The conversion rate and selectivity reach 16.2 %
and 84.0 %, respectively. Then, the above filtrate was transferred to the
quartz reactor and continued to react for another 6 h under the same
condition. The conversion rate and selectivity are 16.8 % and 83.4 %,
respectively, indicating that Mo element is not present in the mixture
Fig. 7. Recycle tests of optimal photocatalysts via five consecutive runs in the epoxidation reaction of styrene.
7

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 8. (a) TEM image and (b, c, d, e) SEM-EDS images of the catalyst CN-Mo-2 after five reaction-regeneration cycles.
Fig. 9. (a) Wide angle XRD patterns of fresh and spent CN-Mo-2; (b) Thermogravimetric curves for fresh and spent CN-Mo-2.
solution. Meanwhile, the recycle experiments were used to further
explore the stability of photocatalysts. As shown in Fig. 7, CN-Mo-2
exhibits better reusability in the photocatalytic styrene epoxidation.
The photocatalytic performance of CN-Mo-2 remains stable after recy-
cling 5 times, and the conversion of styrene over 38 % and selectivity of
styrene oxides still over 86 %. A leaching experiment was carried out
after recycling 5 times (Table S4, Supporting information). The spent
photocatalyst CN-Mo-2 was removed by hot filtration after five
reaction-regeneration cycles (named as spent CN-Mo-2), and the filtrate
continued to react for 6 h. The conversion rate and selectivity are 38.2 %
and 82.6 %, respectively, which indicating that there is no Mo element
present in the filtrate. However, in the sixth and seventh cycle experi-
ments, the photocatalytic activity of CN-Mo-2 is significantly reduced
(Fig. S6, Supporting information). Thus, photocatalytic performance of
photocatalyst CN-Mo-2 can maintain a stable in the five cycles of
experiments.
element can also be observed. Phase information of spent photocatalyst
CN-Mo-2 was studied by power XRD, as shown in Fig. 9a. Obviously,
fresh and spent CN-Mo-2 all show the typical diffraction pattern of g-
C3N4, but spent CN-Mo-2 shows a typical diffraction peak of Mo at ~40◦.
This may be caused by the migration of Mo element from the g-C3N4
skeleton after 5 reaction-regeneration cycles. Additionally, the total
carbonization temperature of spent CN-Mo-2 is lower than that of the
fresh CNMo-2 (Fig. 9b), indicating that its thermal stability is reduced.
Optical properties of CN-Mo-2 after five reaction-regeneration cycles
were studied, as shown in Fig. 10. In the UV–vis diffuse reflectance
spectrum (Fig. 10a), spent CN-Mo-2 exhibits a blue shift compared to
fresh CN-Mo-2. In the PL spectra (Fig. 10b), the peak centre of spent CN-
Mo-2 shows a blue shift (474 nm), but the PL intensity is lower than that
of fresh CN-Mo-2 (486 nm). This indicates that the charge separation
efficiency of CN-Mo-2 is reduced after five reaction-regeneration times.
However, compared with fresh CN-Mo-2, the fitted fluorescence decay
To further study the stability of CN-Mo-2, the spent photocatalyst
after cycling 5 times were characterized. In images of TEM and SEM-EDS
(Fig. 8), the spent photocatalyst CN-Mo-2 still has a lamellar porous
structure (Fig. 8a). Based on the SEM-EDS analysis (Fig. 8b-e), it can be
found that molybdenum, carbon and nitrogen are homogeneous, and Mo
lifetimes (τ) of spent CN-Mo-2 has no obvious change (Fig. 10 c), sug-
gesting that the charge carrier separation capabilities of spent CN-Mo-2
has not changed. In addition, the transient photocurrent and electro-
chemical impedance spectroscopy (EIS) of spent CN-Mo-2 were obtained
(Fig S4, Supporting information). After five reaction-regeneration times,
8

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Fig. 10. (a) UV–vis diffuse reflectance spectrum of fresh and spent CN-Mo-2; (b) PL spectra of fresh and spent CN-Mo-2; (c) transient fluorescence decay curve of
fresh and spent CN-Mo-2.
the photocurrent density of spent CN-Mo-2 has greatly been reduced
Table 2
(Fig S4a), indicating that the spent CN-Mo-2 accelerates the electron-
Scope of the CN-Mo-2 photocatalyzed epoxidation.
hole recombination. Moreover, in the Nyquist diagram (Fig S4b), the
Entry
Substrate
Product
Conversion
(%)
Selectivity
(%)
arc radius of spent CN-Mo-2 is larger than that of fresh CN-Mo-2,
inferring that spent CN-Mo-2 has higher charge transfer resistance. To
sum up, After five reaction- regeneration times, the charge separation
ability of photocatalyst CN-Mo-2 does not change significantly, but the
charge separation efficiency and the ability of inhibiting electron-hole
recombination are reduced. Therefore, although spent CN-Mo-2 still
maintains the lamellar porous structure, its thermal stability, optical
performance and electrochemical performance are reduced, which will
eventually affect its photocatalytic activity. If spent CN-Mo-2 continues
to be used, its photocatalytic activity will be sharply reduced.
1
2
3
4
31.2
29.6
21.8
38.4
84.1
77.6
78.8
83.2
5
34.4
45.2
The optimized photocatalyst CN-Mo-2 has been successfully applied
to some other epoxidation reaction of alkenes. Results of photocatalytic
activity are shown in Table 2. The photocatalyst CN-Mo-2 exhibits a
wide selection of substrate scope, being active on cyclic, linear, terminal,
electron-deficient, electron-rich and conjugated alkenes. The terminal
alkenes usually exhibit poor activity than cyclic alkenes, which may be
caused by the low electron density of the terminal alkenes due to a
negative effect on the electrophilic oxygen transfer in the epoxidation
reaction [61]. In this study, higher selectivity of terminal alkenes is
observed (Table 2, entries 1, 2, 3 and 4). Further, the conversion rate of
olefins decreases gradually with the increase of carbon chain, indicating
that the length of carbon chain is one of influencing factors in the
epoxidation reaction. The long carbon chain increases the steric resis-
6
7
42.53
45.8
62.1
70.1
8
9
38.8
13.2
26.1
30.8
Reaction conditions: substrate (0.1 mmol, catalyst (20 mg), acetonitrile (3 ml) as
solvent, chlorobenzene (0.05 mmol) as GC internal standard and 50 wt.% H2O2.
The slurry was continuously magnetic stirred under simulated sunlight irradi-
ation for 6 h at the room temperature.
–
tance of substrate to the catalyst, and even encapsulate C C so that it
–
9

Q. Gu et al.
Molecular Catalysis 504 (2021) 111441
Scheme 2. Overall reaction mechanism of styrene epoxidation over CN-Mo.
cannot be exposed to the photocatalytic system. In addition, aromatic
terminal olefins (Table 2, entry 4) show better conversion and selectivity
50 % H2O2 show the excellent selectivity in the photocatalytic epoxi-
dation system. Sunlight is the driving force for the epoxidation reaction
of styrene. Thus, the solvent, light intensity and oxidant are important
parameters affecting the epoxidation of olefins. The photocatalyst CN-
Mo exhibits better reusability. In a word, our results indicate the Mo
modified g-C3N4 photocatalyst can effectively improve the photo-
catalytic activity of g-C3N4 for olefins epoxidation under mild reaction
conditions.
than aliphatic terminal olefins, resulting from the formation of π-con-
jugated system in the aromatic terminal olefins. The photocatalyst
shows activity for epoxidation of cyclic alkenes, but the selectivity is
poor (Table 2, entries 5, 6 and 7.). As the number of carbon atoms in the
cyclic olefins increases, the conversion rate of cyclic olefins is improved,
which may be caused by a more electron donating (CH2)n cyclic bridge
connects to the double bond. Furthermore, the selectivity is lower for
cyclohexene derivative (Table 2, entries 7 and 8), while the reaction of
1-hexene exhibits better selectivity and conversion (Table 2 entry 2).
The above results indicate that terminal alkenes are more likely to be
highly selective in the photocatalytic systems.
CRediT authorship contribution statement
Qian Gu: Conceptualization, Data curation, Investigation, Method-
ology, Writing - original draft, Writing - review & editing. Ping Ping
Jiang: Project administration, Funding acquisition, Resources, Super-
vision. Yirui Shen: Validation, Visualization, Formal analysis. Kai
Zhang: Validation, Formal analysis. Phyu Thin Wai: Software, Formal
analysis. Agus Haryono: Visualization.
To further analysis the photocatalytic performance of photocatalyst
CN-Mo, based on the results of previous researches and this work, a
conceivable photocatalytic process is proposed [21,22,62,63].
Compared on CN, Mo-modified carbon nitride has higher specific sur-
face area due to its porous structure (Fig. 4b, Table S2). The addition of
molybdenum facilitates the delocalization of photoexcited electrons,
reduces the charge impedance of photocatalysts, ensures effective
migration of photogenerated electrons from valence bland to conduction
bland (Fig. 5). Additionally, the light absorption of g-C3N4 after modi-
fication of molybdenum produces a red shift phenomenon, which leads
to absorption of more visible light. As displayed in Scheme 2, the elec-
trons and holes are generated when the sunlight hits the surface of
CN-Mo-2, the former combines with the oxygen in H2O2 and the latter
oxidizes the carbon atoms in the double bond of styrene, and then form
to styrene oxide. In the process, Mo element in the CN-Mo-2 skeleton
inhibits their rapid recombination and improves the efficiency of charge
separation. Thus, the addition of Mo element improves the optical
properties of g-C3N4.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the financial support from the funda-
mental Research Funds for the Central Universities (JUSRP51507), MOE
& SAFEA, 111 Project (B 13025), and the International Joint Research
Laboratory for Biomass Conversion Technology at Jiangnan University.
International exchange and cooperation projects (BX 2019018), and
Postgraduate Research & Practice Innovation Program of Jiangsu
Province (1042050205205978).
4. Conclusions
In summary, we have developed a lamellar porous CN-Mo photo-
catalyst for olefins epoxidation. Results of photocatalyst characteriza-
tion indicate that the addition of molybdenum changes the physical and
optical properties of g-C3N4. In detail, the lamellar porous structure of
CN-Mo produces more active sites, and the addition of Mo increases the
charge separation efficiency and inhibits electron-hole recombination.
The photocatalyst CN-Mo-2 exhibits photocatalytic activity on both
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111441.
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