Highly dispersed Mo-doped graphite carbon nitride: Potential application as oxidation catalyst with hydrogen peroxide

Article abstract of DOI:10.1039/c8nj00316e

A novel molybdenum-doped graphite carbon nitride (g-C₃N₄) catalyst was successfully prepared by simple calcination and applied to the oxidation of various substrates with hydrogen peroxide or tert-butylhydroperoxide (TBHP) as the oxidant. The morphology, structure, and chemical composition of the catalyst were characterized fully, demonstrating the presence of highly dispersed molybdenum species stabilized by the nitrogen atoms of g-C₃N₄. The Mo(vi) sites stabilized on the triazine ring of g-C₃N₄ interacted with H₂O₂ or TBPH, forming peroxo-Mo(vi) groups, which were active for oxidation. Mo-doped g-C₃N₄ exhibited significantly enhanced activity for the non-selective oxidation of methylene blue (MB) when compared with MoO₃ or pure g-C₃N₄. Furthermore, the catalyst exhibited high efficiency for the selective oxidation of sulfides to sulfoxides, and N-compounds to the corresponding N-oxides, under mild conditions. The catalyst also showed potential application in the epoxidation of olefins. The effects of the homogeneous reaction of leached Mo and the recyclability of the catalyst were also evaluated. The data show that the catalyst exhibits great potential for various industrial applications.

Full text of DOI:10.1039/c8nj00316e

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Gonçalves, R. P.R. Alvim, H. A. Bicalho, A. M. Peres, I. binatti, P. F. R. Batista, L. S. Teixeira, R. R. RESENDE
and E. Lorençon, New J. Chem., 2018, DOI: 10.1039/C8NJ00316E.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Highly dispersed Mo-doped graphite carbon nitride: Potential
application as oxidation catalyst with hydrogen peroxide
Diogo A. F. Gonçalvesa, Raquel P.R. Alvima , Hudson A. Bicalhoa, Anderson M. Peresb, Ildefonso
Binattia, Pablo F. R. Batistaa, Leonel S. Teixeiraa, Rodrigo R. Resendec,d, Eudes Lorençona*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A novel molybdenum-doped graphite carbon nitride (g-C3N4) catalyst was successfully prepared by simple calcination and
applied to the oxidation of various substrates with hydrogen peroxide or tert-butylhydroperoxide (TBHP) as the oxidant.
The morphology, structure, and chemical composition of the catalyst were characterized fully, demonstrating the presence
of highly dispersed molybdenum species stabilized by the nitrogen atoms of g-C3N4. The Mo(VI) sites stabilized on the
triazine ring of g-C3N4 interacted with H2O2 or TBPH, forming peroxo-Mo(VI) groups, which were active for oxidation. Mo-
doped g-C3N4 exhibited significantly enhanced activity for the non-selective oxidation of methylene blue (MB) when
compared with MoO3 or pure g-C3N4. Furthermore, the catalyst exhibited high efficiency for the selective oxidation sulfides
to sulfoxides, and N-compounds to the corresponding N-oxides, under mild conditions. The catalyst also showed potential
application in the epoxidation of olefins. The effects of the homogeneous reaction of leached Mo and the recyclability of
the catalyst were also evaluated. The data show that the catalyst exhibits great potential for various industrial
applications.
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of Fe species in the g-C3N4 structure.5 Fe/g-C3N4 has been
shown to exhibit high activity in the Fenton-like reaction and
benzene oxidation, along with improved photodegradation
1. Introduction
In recent years, graphitic carbon nitride (g-C3N4) has emerged
as a promising material for application in photocatalysis,
catalysis, and adsorption owing to its good chemical stability,
low cost, and visible-light response. Moreover, its properties
can be tuned by doping with metal/non-metal elements, in
addition to achieving morphological control, etc. The use of g-
C3N4 as a substrate to support metal species is particularly
interesting for the creation of active catalysts. The
characteristic structure of g-C3N4 comprises a high content of
uniformly distributed N-pyridinic cavities, so-called “nitrogen
pots” with six nitrogen lone-pair electrons, which are ideal
sites for metal coordination/stabilization.1,2 It has been
reported that Fe-doped g-C3N4 can easily be produced by
calcination of Fe+3 salts with melamine or dicyandiamide at
550 °C.3,4 In this approach, the Fe species are stabilized by
chelation with pyridinic nitrogen atoms during monomer
thermo-condensation, which results in homogeneous inclusion
activity.4,6−8 Bimetallic Fe/Co doped g-C3N4 was found to be an
active catalyst for the selective oxidation of cyclohexene using
molecular oxygen as an oxidant.9 López et al.10 synthesized
highly dispersed single Pd atoms supported on g-C3N4, thereby
achieving enhanced activity for the hydrogenation of alkynes
and nitroarenes. Mo(VI)-based materials have also shown
potential as catalysts for a variety of oxidation reactions.11,12
Mo(VI)-polyoxometalates have been applied as active catalysts
in the oxidation of pyridines,11 S-compounds,13,14 and
olefins15-17
with
hydrogen
peroxide. Homogeneous
molybdenum(VI) complexes showed very high efficiency for
epoxidation reactions under mild conditions. Recently,
molybdenum oxide interlaced with N-donor molecules showed
interesting catalytic performance for the oxidation of amines,
alcohols, and sulfides when tert-butylhydroperoxide (TBHP) or
aqueous H2O2 was used as an oxidant. However, the structure,
stability, and catalytic behaviour of Mo species anchored in the
priridinic groups of g-C3N4 have not been studied. Herein, we
report the synthesis of a new Mo-doped g-C3N4 compound
through a simple calcination approach. Characterization of the
prepared material suggests that the Mo species are well
dispersed and stabilized in the g-C3N4 structure. Catalytic tests
indicate that Mo/g-C3N4 exhibits excellent activity and
selectivity for both the oxidation of sulfides and pyridinic
compounds with H2O2 and has potential application in the
epoxidation of olefins with TBHP.
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reaction was stopped at suitable times by adding MnO2 and
the resulting solids were removed from the solution by
centrifugation. A 1 mL aliquot of the MB solution was diluted
to 25 mL with methanol prior to analysis.
2. Experimental
All chemicals were of reagent grade and were used without
further purification. Pure Mo/g-C3N4 was synthesized
according to previous reports.5,18 Briefly, 6.3 g of melamine (50
mmol) and 1.63 g of MoO2(acac)2 (5 mmol) were dissolved in
100 mL of hot distilled water with 5 mL of HCl (37%) under
Recycling tests with Mo/g-C3N4 were carried out under similar
conditions to those described above. At the end of the
reaction, the catalyst was removed from the reaction mixture
by centrifugation and used in the next catalytic run without
purification. The concentration of leached molybdenum was
analyzed by atomic absorption spectrometry.
magnetic stirring for 30 min. The solution was dried at 80
overnight to remove the liquid phase. The resultant solid was
placed in a semi-closed alumina crucible and heated to 550
at a rate of 10
C min-1 and maintained isothermal for 2 h in a
°C
°
C
°
muffle furnace under ambient atmosphere. After cooling to
ambient temperature, the powder obtained was ground in an
agate mortar for further use. Pure g-C3N4 was prepared in a
similar manner to that described above for Mo/g-C3N4, but
without addition of the MoO2(acac)2 precursor.
2.3. Oxidation of sulfur compounds
The selective oxidation of diphenyl sulfide and methyl phenyl
sulfide was carried out in a closed 50 mL flask at room
temperature (25 °C) under magnetic stirring. In the standard
procedure, 25 mg of catalyst was dispersed in 15 mL of
methanol with 1 mmol of substrate. The reaction was initiated
by adding 0.1 mL of H2O2 (10 mol L-1) to the mixture
(H2O2/substrate molar ratio = 1/1). Reaction samples were
taken at specified time intervals and were treated with a few
milligrams of manganese dioxide to decompose H2O2 and with
anhydrous sodium sulfate to remove the residual water,
followed by centrifugation. The products were identified by
GC-MS (Agilent 6890N) and quantified by GC-FID (Agilent
6890) using authentic standards. The recycling tests were
performed under similar conditions to those reported above.
At the end of the reaction, the catalyst was removed from the
reaction mixture by centrifugation and reused without
purification.
2.1. Characterization techniques
X-ray diffraction patterns were obtained with a Rigaku D/MAX
2400 instrument with a Cu source (λ = 1.54 Å, Kα). Fourier
Transform infrared (FTIR) spectra were recorded on a
Shimadzu IRAffinity-1 spectrophotometer. Scanning electron
microscopy images (SEM) and energy-dispersive X-ray
spectroscopy (EDX) elemental mapping analyses were
performed with
a FEI model Quanta microscope at
accelerating voltages of 15–30 kV. High-resolution
transmission electron microscopy (HR-TEM) images were
obtained with a FEI Tecnai G2 microscope at an acceleration
voltage of 100 kV. The Brunauer–Emmett–Teller (BET) specific
surface areas were determined by the multipoint BET method
using Autosorb equipment from Quantachrome Corporation.
CHN elemental analysis was performed on a Vario EL Cube
elemental analyzer. Visible absorption spectra were obtained
with a Shimadzu UV-vis 2101 spectrometer. TGA data were
acquired using a Shimadzu DTG60/60H instrument at a
2.4. Oxidation of nitrogen compounds
Pyridine and quinoline oxidation was performed at room
temperature (25 °C). Typically, 50 mg of catalyst was dispersed
in 2 mL of acetonitrile and 3 mL of water with 1 mmol of
substrate. The reaction was initiated by adding 0.5 mL of H2O2
(10 mol L-1; H2O2/substrate molar ratio = 5/1) to the mixture
under constant magnetic stirring. After the reaction, the
organic products were separated from the aqueous phase by
extraction with dichloromethane and analyzed using GC-FID
and GC-MS.
constant heating rate of 10 °
C min-1 under synthetic air at a
flow rate of 50 mL min-1 at 700 °C. X-ray photoelectron
spectroscopy (XPS) analysis was conducted with an Al-Kα X-ray
source (1486.6 eV) and a hemispherical concentric analyzer
(CLAM2 – VG Microtech). Electrospray ionization mass spectral
(ESI-MS) data were obtained on a LCQ Fleet (Thermo Scientific)
ion trap mass spectrometer.
2.5. Epoxidation
The catalytic epoxidation of cyclooctene and styrene was
carried out in a closed 50 mL flask at 65 °C under magnetic
stirring. The catalyst (100 mg) was dispersed in 5 mL of
acetonitrile with 1 mmol of olefin. The reaction was initiated
by adding 0.2 mL of a 5.0 M solution of TBHP in decane to the
mixture (TBHP/substrate molar ratio = 1/1). After 24 h of
reaction, the catalyst was removed from the suspension by
centrifugation. The products were identified by GC-MS and
quantified by GC-FID using authentic standards.
2.2. Methylene blue oxidation
Tests for the adsorption/oxidation of methylene blue (MB)
were conducted in a reactor at room temperature under
magnetic stirring at 250 rpm. The catalytic activity of the
prepared materials was evaluated by periodically monitoring
the intensity of the absorbance of MB at 664 nm. In a typical
reaction, 100 mg of catalyst was dispersed in 100 mL of a
methylene blue solution (50 mg L-1). The dye oxidation
reaction was initiated by adding suitable amounts of H2O2 (10
M) to the suspension. The catalytic oxidation reaction was
performed without pH adjustment (initial pH of 6.5). The
intermediates formed from MB oxidation were evaluated
3. Results and discussion
using ESI-MS in positive extraction mode. For the analysis, the 3.1. Catalyst characterization
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band was most pronounced in the spectrum of Mo/g-C3N4,
Figure 1a shows the X-ray diffraction (XRD) patterns of g-C3N4 suggesting that the Mo ions may partially inhibit melamine
and Mo/g-C3N4. Two characteristic diffraction peaks were condensation, leading to the formation of g-C3N4 with a low
observed for pure g-C3N4 at 13.1° and 27.4°, indexed to the degree of polymerization and more defect sites, in agreement
(100) and (002) planes of g-C3N4, respectively.19 The (100) peak with the XRD results.
is attributed to the interlayer stacking structure of the tris- The morphology of Mo/g-C3N4 was investigated by scanning
triazine units and the (002) peak represents the interlayer and transmission electron microscopy (SEM and TEM), as
reflection of the graphitic-like structure, as reported shown in Fig. 2. The SEM image (Fig. 2a) shows that Mo/g-C3N4
elsewhere.20,21 An inversion of the intensity of the (100) and presented the typical sheet-like structure with pores resulting
(002) peaks was clearly observed upon the insertion of from gas bubbles during the pyrolysis of melamine.25,26 The
molybdenum into g-C3N4 and the position of the (100) peak TEM image (Fig. 2b) demonstrates that Mo/g-C3N4 exhibits a
shifted to a lower diffraction angle. Moreover, no peaks layered platelet-like morphology with micrometer size. No
associated with isolated molybdenum phases such as MoO3, evidence of any segregated Mo phases was found in the SEM
molybdenum carbide, or molybdenum nitride, were detected. or TEM images.26
These observations indicate that molybdenum were most
likely present as isolated and chemically coordinated species
by the N-pyridinic in-plane cavities of g-C3N4, thereby
enlarging the (100) intra-planar separation. The changes in
the intensity of the (100) and (002) peaks implies that the
incorporated Mo ions had a considerable influence on the
crystal growth of g-C3N4 during the thermo-condensation of
melamine. The FTIR spectra of g-C3N4 and Mo/g-C3N4 are
shown in Fig. 1b. The materials exhibited similar spectra,
indicating that the basic g-C3N4 network was maintained upon
Mo incorporation.21 The typical breathing mode of the tris-s-
Figure 2. (a) SEM and (b) TEM images of prepared Mo/g-C3N4.
triazine units was indicated by the peak at 805 cm-1 for both
materials. The peaks between 1200 cm−1 and 1650 cm−1 are
Furthermore, Fig. 3 shows the TEM image and elemental
due to the stretching vibration modes of the CN aromatic
mapping data for Mo, C, and N in Mo/g-C3N4. Although no Mo
heterocycles.22 These bands became weaker in the profile of
species were visually detected in the bright-field TEM image
the Mo-doped sample, indicating possible interaction of the
(Fig. 3a), the maps of the three elements showed a similar
Mo ions with the nitrogen groups of the tris-s-triazine units.
distribution and location of the elements, revealing the
Close inspection revealed a new band at 954 cm-1 in the IR
homogeneous distribution of these elements, especially of
spectrum of Mo/g-C3N4, which may be assigned to the oxo-
molybdenum, throughout the g-C3N4 matrix. These
molybdenum group (Mo=O).23,24 The weak band at 2138 cm-1
observations are consistent with the XRD and FTIR results, and
is ascribed to the stretching vibration of defect terminal –C≡N
confirm that highly dispersed Mo-doped g-C3N4 was
bonds.5
successfully synthesized.
Figure 3. (a) TEM image and elemental mapping analysis of (b) molybdenum, (c)
carbon, and (d) nitrogen in Mo/g-C3N4.
Figure 1. (a) XRD patterns and (b) FTIR spectra of g-C3N4 and Mo/g-C3N4.
The broad band around 3180 cm-1 can be assigned to the N‒H
stretching vibration of non-condensed amino groups. This
The elemental analysis and surface area data for pure g-C3N4
and Mo/g-C3N4 are presented in Table 1. The elemental
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composition (CHN) of both samples was close to that reported
in the literature.27 The atomic C/N ratio of about 0.66 (the
theoretical value for g-C3N4 is 0.75) can be attributed to partial
condensation of the amino groups and incomplete
crystallization of the structure, as reported elsewhere.5,9 The
Brunauer-Emmett-Teller (BET) surface area of pure g-C3N4 was
14 m2 g-1, whereas that of Mo-doped g-C3N4 was maintained at
ca. 16 m2 g-1, even with the high amount of molybdenum (18.7
%).
Table 1. BET surface area and elemental analysis data for
prepared materials
Elemental analysis
SBET Mo*
2 -
Sample
(wt
C
N
H
C/N
m g
1
%)
(wt%) (wt%) (wt%) (atomic)
g-C3N4
Mo/g-
C3N4
14
16
0
35.22 61.29
24.57 43.55
1.30
1.74
0.67
0.66
18.7
*Obtained by TGA analysis
The surface chemical composition of pure g-C3N4 and Mo/g- Figure 4. X-ray photoelectron spectroscopy (XPS) data for g-C3N4 and Mo/g-C3N4:
(a) survey spectrum, (b) N 1s, (c) C 1s, and (d) Mo 3d spectra.
C3N4 was evaluated by X-ray photoelectron spectroscopy (XPS;
see Fig. 4). The full XPS spectra (Fig. 4a) for both samples show
sharp photoelectron peaks at binding energies of 284.5 eV and
3.2. Catalytic activity of Mo/g-C3N4
395.5 eV, attributed to C and N, respectively. The new peak at
Initially, the catalytic activity of the produced Mo/g-C3N4 was
228.6 eV detected in the survey XPS spectrum of Mo/g-C3N4
evaluated in the degradation of methylene blue (MB) with
can be ascribed to Mo. The signal at a binding energy of 528.6
aqueous H2O2 in the absence of light to prevent photochemical
eV can be attributed to O associated with Mo in Mo/g-C3N4 as
reactions (Fig. 5a). Notably, MB degradation reached a stable
this signal is practically negligible in the profile of the non-
state within 30 min when pure g-C3N4 was used as the catalyst
doped sample. Figure 4b shows the high-resolution N 1s
in 400 mM H2O2. However, no appreciable degradation of MB
spectra for the synthesized samples. Three main peaks were
was observed in the dye removal profile (about 14%) for the
observed at 397.6, 399.0, and 401.3 eV for pure g-C3N4. The
signal at 397.6 eV is associated with sp2-bonded N atoms (C–
control experiment with pure g-C3N4 in the absence of
N=C) in the triazine rings.21,28 The peak at 399.0 eV indicates
hydrogen peroxide. The adsorption capacity of g-C3N4 (7 mgMB
g-1) is consistent with previously published results.32,33 On the
in the form of N−(C)3 or H−N−(C)2,29,30 whereas the weaker
other hand, Mo/g-C3N4 exhibited an MB adsorption capacity of
the presence tertiary nitrogen atoms bonded to carbon atoms
27 mgMB g-1 (about 47% MB removal in the absence of H2O2).
This effect can be attributed to an increase in the negative
charge of g-C3N4 upon Mo doping, which favours chemical
adsorption of cationic MB molecules onto the surface of Mo/g-
C3N4. Clearly, the Mo/g-C3N4 catalyst displayed a remarkable
effect on the degradation of MB by H2O2. The addition of 5
mM H2O2 to the dye suspension led to a rapid decrease in the
MB concentration. The dye degradation rate increased with an
increase in the hydrogen peroxide concentration from 5 mM
to 50 mM; the profiles obtained using initial H2O2
concentrations of 100 mM and 400 mM were very similar.
Furthermore, no significant decrease in the MB concentration
was observed in the absence of catalyst or with the use of
MoO3 as catalyst obtained by calcination of MoO2(acac)2 at
550 °C in the presence of 400 mM H2O2. The FTIR spectra of
Mo/g-C3N4 before and after H2O2 treatment were used to
identify changes in the chemical composition, possibly related
to the catalytic process (Fig. 5b). All characteristic vibration
modes of g-C3N4, i.e., NH stretch (ca. 3000 cm-1), aromatic CN
stretch (1570−1480 cm-1) and the breathing mode of the
triazine units at 805 cm-1 remained virtually unchanged after
peak at 401.1 eV can be ascribed to –NH2 or NH groups.25,30
Notably, the three corresponding N 1s peaks were shifted to
397.9, 399.2, and 400.6 eV in Mo/g-C3N4, suggesting that Mo
chemically interacts with the N groups via Mo−N bonds. Two
peaks associated with carbon species were observed at 284.6
eV and 288.1 eV in the XPS spectrum of pure g-C3N4 (Fig. 4c);
these peaks can be assigned to the sp2 C=C and N–C=N bonds
in g-C3N4, respectively.3,31 The insertion of Mo ions into g-C3N4
led to a decrease in the binding energy of the N–C=N peak to
287.5 eV. The Mo 3d doublet was strongly broadened due to
electrostatic charging of the Mo/g-C3N4 sample. The spectrum
was deconvoluted into two characteristic doublets related to
the Mo 3d3/2 and Mo 3d5/2 states. The set of peaks at binding
energies of 230.7 and 227.3 eV were assigned to the Mo 3d3/2
and Mo 3d5/2 states of Mo+6, whereas the signals at 229.1 eV
and 226.1 eV were associated with Mo+5. Additionally,
quantitative analysis of the XPS data revealed that the O/Mo
atomic ratio was about 2.3, suggesting that highly dispersed
dioxo-molybdenum(VI) species coordinated to pyridinic
nitrogen were obtained.
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H2O2 treatment. This suggests that the skeletal structure of g- intermediate structures are presented in Figs. S2 and S3.
C3N4 is stable against H2O2. On the other hand, peaks were Initially, the Mo(VI) sites stabilized on the triazine ring of g-
observed at 958, 858, 650, and 572 cm-1 in the FTIR spectrum C3N4 could interact with H2O2 to form peroxo-Mo(VI) groups,
of Mo/g-C3N4 after H2O2 treatment. It has been reported that which are active and responsible for the oxidation (Fig. S4).
catalytically active oxo-peroxo molybdenum(VI) species can be After 10 min of reaction, new peaks were detected at m/z =
produced by treatment of Mo(VI) complexes with TBHP or 318 and 274, suggesting that oxidation of the dye was initiated
hydrogen peroxide. Accordingly, the appearance of the new by transfer of oxygen from the peroxo group to the sulfur
peak at 858 cm-1 can be assigned to O-O stretching vibrations atom, forming a sulfoxide,35 followed by demethylation and
of the peroxo species (Mo-O2), whereas the weak bands nitrogen oxidation. The intermediate could undergo successive
appearing at 650 and 572 cm−1 can be ascribed to asymmetric oxidations given that the molar ratio of H2O2 to MB was above
and symmetric stretching vibrations of the peroxo group of 300. The intermediates with high m/z values of 353 and 363
Mo(O2), respectively. Additionally, the band related to the indicate that non-selective oxidation events could lead to
stretching vibration of the high-valence molybdenum(VI) oxo aromatic ring disruption and even probable mineralization.
(Mo=O) bond, which appeared at 945 cm-1 in the spectrum of
the non-treated sample, shifted to 958 cm−1 owing to changes
in the chemical environmental of the Mo ions upon H2O2
treatment.13 These results suggest that the presence of well-
dispersed Mo sites in the g-C3N4 structure play a fundamental
role in the catalytic activity of the produced material.
Figure 5. (a) Effect of initial H2O2 concentration on MB oxidation by Mo/g-C3N4;
conditions: [MB] = 50 mg L-1, [Cat] = 1 g L-1, initial pH = 6.5, T = 25 °C. (b) FTIR
spectra of pristine and H2O2-treated Mo/g-C3N4.
Figure 6. ESI-MS mass spectra in positive ion mode for the oxidation of MB in
water: unreacted MB and after 10, 30, 60, and 90 min of reaction with H2O2 in
the presence of Mo/g-C3N4. Conditions: [MB] = 50 mg L-1, [Cat] = 1 g L-1, [H2O2]0
50 mM, initial pH = 6.5, T = 25 C.
=
°
In order to obtain further information about the reactive
oxygen species related to MB oxidation by Mo/g-C3N4, control
experiments were performed by using different radical
quenchers (Fig. S1). Methanol and dimethyl sulfoxide (DMSO)
were used as the scavengers to quench OH• radicals in the
bulk solution. Iodide ion was used to scavenge hydroxyl
radicals produced at the surface of the catalyst. Benzoquinone
was employed as a scavenger of superoxide radicals, and
2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) was used
to scavenge any radical species. The results showed that when
radical scavengers were introduced into the degradation
systems, even at high concentrations, there were no significant
effects on the dye degradation rate, revealing that MB
oxidation follows a non-radical pathway. The intermediates
formed during the oxidation of MB with Mo/g-C3N4 were
monitored by ESI-MS in positive ion mode (Fig. 6). The
spectrum of the MB solution showed only one signal at m/z =
284, attributed to the cationic MB structure.34 The
characteristic signal of MB gradually disappeared owing to
oxidation, in good agreement with the catalytic behaviour
previously discussed in relation to Fig. 5a. According to the
above results, a scheme for the oxidation of MB and the
It is known that molybdenum(VI)-based catalysts display good
activity and selectivity for epoxidation and oxidation of sulfur
and nitrogen compounds using hydrogen peroxide or TBHP as
the oxidant. Selective oxidation of sulfides to sulfoxides or
sulfones, as well as oxidation of pyridine compounds to N-
oxides, are essential reactions due to their utility for the
production of a wide range of fine chemicals, biological
molecules, agrochemicals, etc.11,36 Additionally, catalytic
epoxidation of olefins is of great importance in the chemical
industry as epoxides are versatile intermediates in organic
synthesis.16,37 In order to evaluate the applicability of Mo-
g/C3N4, the catalytic oxidation of a series of organic substrates
was tested under selected conditions (Table 2). Notably, the
Mo/g-C3N4 catalyst showed high activity and selectivity for
conversion of sulfides to sulfoxides in methanol under mild
conditions, i.e., room temperature with 1 equivalent of H2O2.
As observed in Table 2, in the presence of Mo/g-C3N4, 98%
conversion of methyl phenyl sulfide (entry 1) was achieved in 1
h of reaction, and 98% of diphenyl sulfide (entry 2) in 2 h; in
both cases, there was high selectively to the corresponding
sulfoxide (above 99%). Moreover, oxidation of the sulfur
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compounds could be directed to the formation of sulfones in cyclooctene and styrene over vanadium exchanged faujasite
acetonitrile at 60 °C by using a H2O2/substrate ratio of 4. This zeolite catalysts with 44% conversion and 94% selectivity to
result (entry 3) demonstrates that diphenyl sulfide could be cyclooctene epoxidation and 98% conversion with 53%
completely transformed into diphenyl sulfoxide by the selectivity for styrene. Fernandes et al.43 studied olefin
Mo/catalyst under the given conditions. Comparatively, epoxidation using MoO3 nanoparticles as the catalyst, reaching
oxidation of methyl phenyl sulfide (MPS) in the absence of the 100% conversion and selectivity for cis-cyclooctene
catalyst occurred with poor conversion (18%) and 83% epoxidation, whereas styrene conversion reached 83% with
selectivity. Karimi et al.38 studied the selective oxidation of 23% epoxide selectivity under similar conditions.
sulfides, catalyzed by silica-based ammonium tungstate, These results prove that the synthesized Mo/g-C3N4 is a
demonstrating complete conversion of MPS to sulfoxide with versatile catalyst for both selective and non-selective
82% selectivity after 1.5 h using 3 equivalents of H2O2. oxidation under mild conditions, which can be primarily
Furthermore, the catalyst showed low activity for MFS attributed to the presence of highly dispersed Mo sites
oxidation. The studied catalyst also showed good activity in the stabilized by nitrogen groups on the surface of g-C3N4
.
selective oxidation of N-compounds with 5 equivalents of
H2O2. Notably, 87% of pyridine (entry 4) and 81% of quinoline
(entry 5) were successfully oxidized to N-oxides with high
selectivity after 12 h. These results, comparable to those
obtained with homogeneous polyoxometalate11,39,40 are
attributed to high dispersion of the active catalytic Mo sites on
the surface of g-C3N4.
3.3. Recyclability of catalyst
Additional experiments were conducted to examine the
recyclability of the Mo/g-C3N4 catalyst in MB oxidation. The
experiments were repeated for five cycles of 90 min. After the
each reaction cycle was completed, the catalyst was recovered
by centrifugation and reused without any treatment. The
amount of soluble Mo in each cycle was determined by atomic
absorption spectrometry. Figure 7a shows that MB
degradation decreased gradually over the five cycles. The
decrease in the activity was attributed to two factors: (1) loss
of the catalyst during the centrifugation process and (2)
leaching of the active Mo centres from the g-C3N4 matrix
during oxidation. Indeed, molybdenum-supported catalysts
have reportedly been transformed into soluble peroxo species
during reaction with aqueous hydrogen peroxide.11,37,44,45 The
result of Mo analysis for the reacted filtrate shows that about
2.7 wt.% Mo was leached into the solution on the first reuse
and 1.3, 0.80, 0.50, and 0.38 wt.% was leached out of g-C3N4 in
the consecutive reuse cycles.
The potential application of Mo/g-C3N4 was further evaluated
in olefin epoxidation with TBHP (to prevent epoxide
hydrolysis). The material led to 42% conversion of cis-
cyclooctene within 24 h and good selectivity (91%) toward the
epoxide (entry 6).
Table 2. Catalytic activity of Mo/g-C3N4 in selective oxidation of
organic compounds
Mass
(mg)
Temp
(°C)
Time
(h)
Conv.
(%)
Entry
Substrate
Product (s)
Sel.
(%)
O
S
S
1
25
25
25
1
98
89
>99
>99
O
S
S
2
25
60
2
1
O
O
S
S
3a
4
25
50
>99 >99
25 12
25 12
87
81
>99
>99
N
O
N
5
50
N
N
O
O
6
7
100 65 24
100 65 24
42
47
91
62
O
Figure 7. (a) Recycling studies for MB degradation over Mo/g-C3N4. (b)
Contribution of homogeneous reaction. Conditions: [MB] = 50 mg L-1, [Cat] = 1 g
L-1, [H2O2]0 = 50 mM, initial pH = 6.5, T = 25 °C, 90 min.
Reaction conditions: a 15 mL of acetonitrile, H2O2/substrate = 4.
FTIR analysis of the catalyst revealed an increase in the
intensity of the N-H stretching bands after reuse, whereas the
band associated with the aromatic CN stretching and breathing
modes of the triazine units were slightly affected (Fig. S4). This
may be indicative that the Mo species that were more weakly
anchored to the terminal or defect amino groups of g-C3N4
could be leached more easily during the first reuse cycles. The
influence of a possible concurrent homogeneous reaction of
the leached Mo during MB degradation was also evaluated, as
Styrene conversion increased slightly to 46% with 62%
selectivity for the epoxide under the same conditions (entry 7).
Similar results were obtained by Valente et al.41 using a
molybdenum oxide-bipyridine hybrid compound in cis-
cyclooctene epoxidation, where 90–95% conversion was
achieved after 24 h with 100% selectivity towards cyclooctene
epoxide. Yang et al.42 reported the epoxidation of cis-
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shown in Fig. 8b. When Mo/g-C3N4 was removed from the successively declined during recycling in the non-selective
reaction mixture at 10 min, the reaction (catalysis) practically oxidation of MB, no appreciable loss of activity was observed
stopped. Additionally, no appreciable dye reaction was for the selective oxidation of MPS after a five-cycle test,
observed when the leached solution obtained after isolation of demonstrating the great potential of the developed catalyst
the regenerated catalyst was used for MB degradation, for selective oxidation.
indicating that soluble Mo species were not active for dye
oxidation.
Conflicts of interest
On the other hand, the Mo/g-C3N4 catalyst maintained its high
activity and selectivity for oxidation of methyl phenyl sulfide to
sulfoxide (Fig. 8). During the five-run recycling test, the
conversion remained between 90−94%, with a selecꢀvity
exceeding 99% for all tests.
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge financial support from the
CNPq, FAPEMIG (Universal APQ-02616-15), CEFET-MG,
Microscopy centre of UFMG,…..
Notes and references
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Novel Mo-doped g-C3N4 catalyst exhibit high efficiency for selective oxidation of
organic substrates with H2O2 or TBHP.