Synergistic effect between Cu-Cr bimetallic oxides supported on g-C3N4 for the selective oxidation of toluene to benzaldehyde

Article abstract of DOI:10.1039/c9cy00743a

A well-defined catalyst with Cu-Cr bimetallic oxides supported on g-C₃N₄ was prepared via direct heating treatment of metallic precursors incipiently decorated with melamine. The obtained catalysts were used in the selective oxidatio

Full text of DOI:10.1039/c9cy00743a

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Xu, X. wang, Y.
Chen and L. Dai, Catal. Sci. Technol., 2019, DOI: 10.1039/C9CY00743A.
Volume
Number
21 January 2017
Pages 313-534
7
2
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Catalysis
Science &
Technology
rsc.li/catalysis
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
rsc.li/catalysis

Please do not adjust margins
Catalysis Science & Technology
Page 1 of 12
View Article Online
DOI: 10.1039/C9CY00743A
Journal Name
ARTICLE
Synergistic effect between Cu-Cr bimetallic oxides supported on g-
C3N4 for the selective oxidation of toluene to benzaldehyde
Received 00th January 20xx,
Accepted 00th January 20xx
Cai Xu, Xiaozhong Wang*, Yingqi Chen and Liyan Dai*
DOI: 10.1039/x0xx00000x
A well-defined catalyst with Cu-Cr bimetallic oxides supported on g-C3N4 was prepared via direct heating treatment of
metallic precursors incipiently decorated with melamine. The obtained catalysts were used in the selective oxidation of
toluene to benzaldehyde with H2O2 as the oxidant, the prepared bimetallic Cu-Cr catalysts outperformed the monometallic
counterparts, the highest yield of bezaldehyede can reach 42.9% within 5 h under mild reaction conditions. It was found
that some metal oxides were coated by g-C3N4 to form a core/shell-like structure, which hindered the metal leaching. XPS
analysis indicated the presence of partial low valence Cu species and the electronic interactions between the Cu and Cr
species. The bimetallic Cu-Cr catalysts possessed larger specific surface area and pore volume compared to the
monometallic ones. Besides, effects of the reaction parameters on catalytic activity were investigated in detail. The
kinetics study indicated that toluene oxidation was the first order reaction and the apparent activation energy was
calculated to be 24.3 kJ mol−1. The recycle tests verified the good reusability of the prepared catalyst, making it promising
in the future practical catalysis application. Finally, a possible reaction mechanism for the selective oxidation of toluene
www.rsc.org/
over
the
supported
catalyst
was
proposed.
along with some undesirable problems regarding economical
cost and environmental pollution. As an eco-friendly oxidant,
H2O2 is usually used in oxidation reactions since it generates
H2O as the only by-product.1,12,13 In this regard, the production
of benzaldehyde from toluene using H2O2 as the oxidant under
mild reaction conditions is consistent with the development of
green chemistry.
Introduction
The direct oxidation of toluene to oxygenated products
generally involves two competitive reactions of methyl sp3 C
H activation and ring sp2 C H activation.1 Among them, the
former one to produce benzaldehyde is usually desirable, since
it is
a valuable chemical intermediate and has wide
To date, considerable efforts have been devoted in
exploring catalytic systems for the selective oxidation of
toluene, some homogeneous catalysts, such as metal (Fe, Mn,
Cu) complexes with different ligands were extensively
developed.14,15 Recently, Capocasa et al. synthesized an
iminopyridine Fe(II) complex via an in-situ self-assembly
method, the prepared catalyst showed a good conversion of
toluene.16 Detoni et al. modified the Cu(II) complexes by
varying the number of 1,10-phenanthroline ligands, which
allowed to tailor the catalytic activity and selectivity in toluene
oxidation.17 Though good catalytic activity can be achieved
over these homogeneous catalysts, the tedious separation
process, difficulty in ligand synthesis and catalyst recovery still
constitute the major barriers to large-scale practical
applications. Therefore, many researchers have engaged in
developing heterogeneous catalytic systems for this
transformation because of the easily separated characteristic
and excellent recyclability of the catalyst.18,19 The
heterogeneous catalysts used in this transformation are mostly
based on modified porous zeolite materials, such as alkali-
treated HZSM-5,20 MnOx/SBA-15,21 Ti/Si-1,1 Fe2O3/HZSM-5,22
KMn/SBA-15,23 MCM-41.24,25 However, zeolite-based catalysts
have some unfavorable problems including relatively
applications in pharmaceutical, perfumery and agrochemical
industries.2,3 Around the world, benzaldehyde has great
market prospects and the annual demand exceeds 90 000
tons.4 Inserting oxygen in
C
H
bonds of aromatic
hydrocarbons has been considered as a hard task in chemical
synthesis and catalysis Traditionally,
community.5
stoichiometric amount of reagents and additives (such as
sulfuric acid,6 N-hydroxyphthalimide,7 KOAc,8 acetic
anhydride,9 etc.) were added to promote the oxidation
process, which would undoubtedly bring a large amount of
chemical waste. In addition, some strong oxidizing
reagents,10,11 such as KMnO4, peracids and organic peroxides
were widely used to achieve higher conversion, but generally
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 12
ARTICLE
Journal Name
complicated preparation processes and unsatisfactory The g-C3N4 supported Cu-Cr bimetallic oxides catalysts were
View Article Online
DOI: 10.1039/C9CY00743A
selectivity. Moreover, the catalysts obtained from the prepared via an in-situ heating treatment method. Briefly, 1 g
traditional wet impregnation method often suffer from metal of melamine was dispersed in 50 ml of deionized water and
leaching owing to the weak interaction between the metal and then heated to 70 °C under vigorous magnetic stirring until it
zeolite support.26 Other heterogeneous catalysts, such as dissolved completely, 1 mmol of Cu(OAc)2∙H2O and 2 mmol of
metals/metals oxides (Pd,27 Au,28 Au-Pd alloy,29 Co/ZnO,30 Cr(NO3)3∙9H2O were dissolved in 20 ml of deionized water and
MgAlOx,31 MnWO4,2 etc.) were also developed. Recently, Wang subsequently added to the above hot solution dropwise, and
et al. prepared a series of Zr/V-based catalysts for the toluene then stirred for another 30 min. Afterwards, 28 wt.% ammonia
oxidation with H2O2 in the presence of acetic acid, but the aq. solution was added in the mixture until the pH was around
conversion only reached around 20%.32 Sun et al. investigated 10, a blue-gray suspension was then formed. The excess water
the catalytic activity of Ag-Cu-BTC composite and achieved was evaporated at approximately 85 °C under continuous
12.7% conversion, the oxidation reaction was conducted in stirring. Subsequently, the solid sample was collected and
relatively harsh conditions (160 °C, 1.0 MPa O2).33 In general, transferred into a crucible with a cover, and then calcined at
highly selective conversion of toluene to benzaldehyde over 550 °C for 3 h with a ramping rate of 10 °C min−1. Finally, the
heterogeneous catalysts under mild reaction conditions still obtained sample was ground into powder, for convenience,
remains a challenging task, hence it is desirable to develop an the prepared bimetallic catalysts were labelled as CuxCr3-x/CN
efficient catalyst with good stability for this transformation.
(x refers to the amount of metal precursor and x = 1, 1.5, 2
In this work, a well-defined catalyst with Cu-Cr bimetallic mmol). For comparison, pure g-C3N4 was prepared from
oxides supported on g-C3N4 was prepared via a facile in-situ thermal polymerization of melamine at 550 °C for 3 h in a
thermal treatment method. More recently, g-C3N4 has muffle furnace with a ramping rate of 10 °C min−1, g-C3N4
attracted increasing attention in the catalysis community supported monometallic oxide catalysts were prepared via the
(particularly in photocatalysis) due to its excellent visible-light above process using the corresponding metal precursors, and
response (band gap about 2.7ꢀeV), low cost, unique 2D denoted as Cu/CN, Cr/CN, respectively.
structure and high chemical stability.34,35 It is believed that
these advantages of g-C3N4 and its graphene-like π conjugated Characterization
matrix are beneficial to heterogeneous catalysis. The prepared
X-ray diffraction (XRD) patterns were reocrded on
a
catalysts were tested in the selective oxidation of toluene to
benzaldehyde with H2O2 under mild reaction conditions. The
results revealed that the bimetallic catalysts exhibited
improved catalytic activity compared to the monometallic
counterparts, a series of characterizations were used to
investigate the intrinsic correlations between the catalyst
properties and the enhanced catalytic performance.
Moreover, different reaction parameters were optimized, the
kinetics of the oxidation reaction were investigated and the
PANalytical X’Pert PRO diffractometer using Ni-filtered Cu Kα
radiation (λ = 1.5406 Å). The morphology and microstructure
of the catalysts were observed by field-emission scanning
electron microscopy (FESEM, Hitachi SU-70) and transmission
electron microscopy (TEM, FEI Tecnai G2 F20) equipped with
an energy dispersive X-ray spectrometer (EDS, X-MaxN 80T,
Oxford Instruments, UK), and Molybdenum (Mo) grid was used
as the sample support during the TEM characterization. N2
adsorption-desorption measurements were performed at −196
°C with an Autosorb-1-C instrument (Quantachrome Corp.,
U.S.A.). Prior to the measurement, the samples were degassed
under vacuum at 150 °C for 10 h. The surface areas and pore
size distributions were determined by the BET and BJH
method, respectively. X-ray photoelectron spectroscopy (XPS)
apparent activation energy was determined. Finally,
a
plausible mechanism was proposed to better understand the
oxidation of toluene over the supported catalysts.
Experimental
was recorded on
a Thermo Scientific Escalab 250Xi
Materials
spectrometer using monochromatic Al Kα radiation (1486.6
eV), C 1s peak at 284.6 eV was used as a reference to calibrate
the obtained XPS data.
Melamine, cupric acetate monohydrate (Cu(OAc)2∙H2O),
chromium nitrate nonahydrate (Cr(NO3)3∙9H2O), toluene,
benzaldehyde, benzyl alcohol, benzoic acid, (o, m, p)-cresol,
hydrogen peroxide (30 wt.%) and ammonia solution (28 wt.%)
were provided by Sinopharm Chemical Reagent Co., Ltd
(China). Dichloroethane, cyclohexane, tetrahydrofuran, N, N-
dimethylformamide, 1, 4-dioxane and acetonitrile were
purchased from Sinopharm Chemical Reagent Co., Ltd (China).
All the chemicals and reagents in the experiments were of
analytical grade and used as received without further
purification.
Catalytic activity test
The catalytic oxidation of toluene was carried out in a three-
necked flask (25 ml) equipped with a refluxing condenser.
Typically, 2.5 mmol of toluene and 20 mg of catalyst were
added to the solvent (5 ml) under vigorous stirring, the
temperature was raised up to 75 °C and then 10 mmol of
hydrogen peroxide (30 wt.% aq. solution) was added dropwise.
The reaction mixture was refluxed for 5 h and then cooled to
room temperature. At the end of the reaction, the solid
catalyst was collected for the next catalytic use via simple
centrifugation, the filtrate was analyzed by gas
Catalyst preparation
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 12
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
chromatography (GC, Agilent 6820) connected with a 30 m ×
0.32 mm × 0.25 μm HP-5 capillary column and a flame
ionization detector (FID). Conversions and selectivities were
calculated using a calibration curve obtained by external
standard method. The products were identified by comparing
the retention time of the standard samples in GC diagram and
further confirmed by GC-MS (Agilent 6890-5973). In addition,
each experimental result has been repeated for at least three
times to assure its reproducibility.
View Article Online
DOI: 10.1039/C9CY00743A
Results and discussion
Catalyst characterization
Fig. 1 shows the XRD patterns of pure g-C3N4 and a series of
metal oxides/g-C3N4 composites. The characteristic diffraction
peak at 2θ = 27.6° is assigned to (0 0 2) facet of g-C3N4, which
is resulted from the interlayer stacking of conjugated aromatic
systems on the g-C3N4 matrix.36 As regards Cu/CN, the
diffraction peaks at 2θ = 35.5°, 38.6° and 48.7° can be assigned
to the diffractions of the (1 1 -1), (1 1 1) and (2 0 -2) planes of
the monoclinic-phase CuO (JCPDS No. 48-1548), respectively.
As for Cr/CN, the several strong diffraction peaks are
consistent well with the hexagonal-phase Cr2O3 (JCPDS No. 38-
1479), and the peaks at 2θ = 24.5°, 33.6°, 36.3° and 54.8° can
be respectively ascribed to the (0 1 2), (1 0 4), (1 1 0) and (1 1 6)
planes of the hexagonal-phase Cr2O3. The diffraction peaks of
the supported bimetallic catalysts clearly reveal the
Fig.1 XRD patterns of the prepared catalysts.
coexistence of CuO and Cr2O3. It can be observed that with the
increasing Cu/Cr molar ratio, the intensity of diffraction peaks
assigned to CuO gradually strengthens and that of Cr2O3
gradually weakens. However, there are no characteristic peaks
of g-C3N4 can be found over the composite catalysts, which
might be attributed to the low content of g-C3N4.37 Hence, XPS
analysis below was used to confirm the presence of g-C3N4
among the composites.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 12
ARTICLE
Journal Name
The morphological structure of the prepared catalysts were presents no uniform and regular morphologies, only a few
View Article Online
investigated by SEM and TEM. Fig. 2a shows the TEM image of Cr2O3 particles show quasi-rectangularDOnaI:n10o.r1o03d9s/.C9TChYe00T7E43MA
Cu/CN, it can be found that CuO mainly exhibits typical square images of CuCr2/CN in Fig. 2c and d reveal that the binary
nanoplates with an average particle size of around 50 nm, and oxides particles are successfully loaded on the surface of g-
Fig. 2 TEM images of (a) Cu/CN and (b) Cr/CN, (c–e) TEM images of CuCr2/CN, (f) HRTEM image and (g) FESEM image of
CuCr2/CN, (h) the dark field TEM image of CuCr2/CN and its corresponding elemental mappings, (i) EDS spectrum of CuCr2/CN.
An inset in panel (c) is the corresponding SAED pattern of CuCr2/CN.
the CuO particles are distributed on the wrinkled and layered C3N4, and some metal oxide particles at the edges of the g-
g-C3N4 nanosheets. The Cr/CN sample in Fig. 2b mostly C3N4 support are also observed. The inset in Fig. 2c shows the
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 12
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
consistent with the XRD results. FESEM image in Fig. 2g gives a
View Article Online
general surface topography of theDOCI:u1C0.r120/3C9N/C9CcYa0t0a7ly4s3tA.
Moreover, the distribution of the elements over CuCr2/CN
were analyzed by EDS, the HAADF-STEM image and the
elemental mappings (C, N, O, Cu, Cr) in Fig. 2h suggest that the
Cu-Cr oxides are anchored on the g-C3N4 support. The EDS
results in Fig. 2i also verify the presence of the five elements of
C, N, O, Cu, Cr and no other impurity elements are introduced
into the catalyst.
Fig. 3 shows the N2 adsorption-desorption isotherms and
pore size distributions of the prepared catalysts. It can be
found that the nitrogen sorption isotherms of all the samples
displayed a typical type IV isotherms with a hysteresis loop,
implying the presence of
a mesoporous structure, the
corresponding BJH pore size distribution curves in Fig. 3b
reveal that the pore diameters of the prepared catalysts
mainly center at around 5–30 nm. The detailed structural
properties of the catalysts are listed in Table 1, it can be seen
that the specific surface areas of Cu/CN and Cr/CN are
respectively 8.2 and 13.4 m2 g–1, and both of which are a little
smaller than that of CuCr2/CN (15.7 m2 g−1). In addition,
compared to that of the monometallic counterparts, the
prepared bimetallic CuCr2/CN catalyst possesses a larger pore
volume (0.120 cm3 g–1). It is reasonable to deduce that an
increase in the specific surface area and pore volume is
favorable for the enhancement of catalytic activity, since
which can offer more active sites and facilitate the mass
transfer during the reaction process.
Fig. 3 (a) N2 adsorption-desorption isotherms and (b) pore size
distributions of the prepared catalysts.
corresponding selected area electron diffraction (SAED)
pattern, indicating the CuCr2/CN catalyst is close to
polycrystalline characteristic. As depicted in Fig. 2e, it can be
found that the metal oxide nanoparticle is enwrapped by g-
C3N4 (the yellow dashed lines represent the identified edges of
the metal oxide), forming a unique core/shell-like structure. It
is believed that such a well-defined structure play a crucial role
in enhancing the stability of the catalyst and hindering the
metal leaching during the reactions.38 Fig. 2f shows the high-
resolution TEM (HRTEM) image of CuCr2/CN, a lattice space of
0.233 nm corresponds to the (111) plane of the monoclinic
phase CuO, and the other lattice space of 0.365 nm is assigned
to the (012) plane of the hexagonal phase Cr2O3, which are
Fig. 4 (a) XPS survey of the bimetallic catalysts, high resolution XPS spectra of (b) C 1s, (c) N 1s and (d) O 1s of CuCr2/CN,
high resolution XPS spectra of (e) Cr 2p and (f) Cu 2p.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 6 of 12
ARTICLE
Journal Name
Table 1 The structural properties of the prepared catalysts
Catalytic activity test
View Article Online
The selective oxidation of toluene to beDnOzaI:ld10e.h10y3d9e/Cw9CitYh00H724O3A2
was selected as the probe reaction to evaluate the activity of
the prepared catalysts. Fig. 5 shows the catalytic performance
of both the supported monometallic and bimetallic oxides
catalysts for this reaction, the former with single Cu or Cr oxide
as the main active component showed relatively poor
conversions of toluene and low selectivities to benzaldehyde.
Pore
SBET
Pore diameter
(nm)
Entry Sample
volume
1
(m2 g− )
(cm3 g− )
1
1
2
3
Cu/CN
Cr/CN
8.2
13.4
CuCr2/CN 15.7
0.056
0.098
0.120
26.5
21.7
17.5
XPS was performed to investigate the surface chemistry and
chemical states of the prepared catalysts. The full range XPS
spectrum in Fig. 4a clearly reveals the existence of the
elements C, N, O, Cu and Cr among the prepared bimetallic
catalysts. It is noteworthy that the measured Cu/Cr molar
ratios via XPS of these bimetallic catalysts are consistent well
with the nominal values (Table S1). C 1s spectrum in Fig. 4b
can be fitted for three peaks at 284.6, 285.7 and 288.3 eV. The
first peak at 284.6 eV can be assigned to the surface
adventitious carbon, the second peak at 285.7 eV can be
ascribed to the sp2 C atoms bonded to the nitrogen atoms
inside the aromatic structure , and the third peak at 288.3 eV
corresponds to the tertiary carbon C (N)3 in the g-C3N4
lattice.39,40 As shown in Fig. 4c, N 1s spectrum can be divided
into three peaks at 398.6, 399.6 and 400.7 eV, which are
attributed to the sp2-hybridized nitrogen atoms in triazine
rings, the tertiary nitrogen N (C)3 and the amino groups (
Fig. 5 Catalytic performance of toluene oxidation over the different
supported mono-/bi- metallic oxides catalysts. Reaction conditions:
acetonitrile (5 ml), toluene (2.5 mmol), H2O2 (10 mmol), catalyst
(20 mg), 75 °C, 5 h.
NH2 or
NH), respectively.38,41 Fig. 4d shows the O 1s
spectrum of CuCr2/CN, one dominant peak at 529.6 eV is
relevant to the lattice oxygen atoms in the metal oxides, the
other peak at 531.4 eV is ascribed to the presence of hydroxyl
groups ( OH) on the catalyst surface.42 In the high resolution
XPS spectra of Cr 2p (Fig. 4e), as for Cr/CN, two obvious
characteristic peaks at 576.6 and 586.0 eV are respectively
attributed to Cr 2p3/2 and Cr 2p1/2 doublets, which verifies the
chemical valence state of Cr element is +3.43 Notably, with the
addition of Cu, the Cr 2p binding energies of the bimetallic Cu-
Cr/CN catalysts both obviously shift to a higher position
compared to that of Cr/CN. Such a change (shift of 0.5–0.7 eV)
reflects the electron loss of Cr and the electronic interaction
between the Cu and Cr species among the composites. As
shown in Fig. 4f, with respect to Cu/CN, two principal peaks at
954.6 and 934.4 eV with several satellite peaks are observed,
which respectively correspond to the Cu 2p1/2 and Cu 2p3/2
doublets and also verify the presence of CuO (Cu2+).44 As
regards the several bimetallic Cu-Cr/CN catalysts, the Cu 2p3/2
spectra can be clearly fitted for two peaks, the peaks at around
934.4 eV are assigned to Cu2+, and the other peaks at around
932.5 eV are attributed to Cu+.45 In addition, two peaks at
568.5 and 569.8 eV in Auger spectra of Cu LMM (Fig. S1) also
reveals the co-existence of Cu2+ and Cu+.46 It can be inferred
that with the addition of element Cr among the bimetallic
catalysts, the Cu2+ species got electrons and was partially
reduced to Cu+ species, such results are in good agreement
with the XPS results of Cr 2p (Fig. 4e), further demonstrating
the electronic interaction between the Cu and Cr species
among the bimetallic catalysts.
However, it was delighted to find that the supported bimetallic
catalysts displayed much better catalytic activity and achieved
greatly increased yields to the target product (benzaldehyde),
directly demonstrating the promotional synergistic effect
between the Cu and Cr species for this oxidation reaction. The
highest benzaldehyde yield (42.9%) could be achieved over the
CuCr2/CN, which was about 8.6 times that of Cu/CN and 5.0
times that of Cr/CN. In addition, Cu-Cr bimetallic oxides
supported on other carriers, such as Al2O3, MgO, SiO2,
activated carbon (AC), were also prepared and used in the
selection oxidation of toluene to benzaldehyde, the results
were given in Table 2. Interestingly, it was found that when g-
C3N4 was chosen as the support, the catalytic activity was
comparable or even superior to that of the other metal/non-
metal supports. It is generally supposed that g-C3N4 possesses
a
graphene-like conjugated structure, its π-character is
beneficial to the adsorption of aromatic reactant molecules
and accelerate the mass transfer during the reactions.47,48
Table 2 also shows a comparison with the previously published
works in the selective oxidation of toluene to benzaldehyde, it
can be found that the present catalytic system displays some
advantages in terms of the yield of the target product and the
mildness of the reaction conditions.
The solvent effect on the selective oxidation of toluene was
investigated using CuCr2/CN as the representative catalyst, the
results were given in Table 3. It shows that the conversion of
toluene was poor when dichloroethane and cyclohexane were
used as the solvents (entries 1 and 2), which might result from
the immiscibility of the aqueous H2O2 and organic solvents.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 12
Catalysis Science & Technology
Please do not adjust margins
Journal Name
Under the similar reaction conditions, the catalytic activities of
ARTICLE
Additionally, other reaction parameters such as reaction
View Article Online
this reaction in other organic solvents were higher than those temperature, H2O2 dosage, catalyst amoDuOntI: a10n.1d03r9e/aCc9tCioYn00t7i4m3eA
in dichloroethane and cyclohexane (entries 3–6). Particularly, were also investigated. The reaction temperature plays a
the most satisfying result was obtained with acetonitrile as the crucial role in the catalytic oxidation of toluene (Fig. 6a), poor
solvent, it was reported that acetonitrile can inhibit the conversion of toluene and selectivity of benzaldehyde were
formation of other by-products in alkane oxidation and obtained under 55 °C, and an obvious decrease in the
promote the reaction process.49,50 Moreover, the oxidation of generation of cresols via ring sp2
C
H activation can be
toluene was completely impeded in the presence of water due observed with the increasing reaction temperature. The
to the poor miscibility of the aromatic substrate and water highest selectivity to benzaldehyde was achieved at around 75
(entry 7).
°C and further increase in temperature was unfavorable for
the target reaction. Oxidant (H2O2) dosage also has
prominent effect
a
on
Fig. 6 Effects of (a) reaction temperature and (b) H2O2: toluene
molar ratio on toluene oxidation over the CuCr2/CN catalyst.
Reaction conditions: acetonitrile (5 ml), toluene (2.5 mmol),
catalyst (20 mg), 5 h; H2O2 (10 mmol) for (a) and reaction
temperature (75 °C) for (b).
Table 2 Catalytic activity of different catalysts for the selective oxidation of toluene to benzaldehyde and a comparison with other
published works a
Conv.
(%)
Sel.
(%)
Yield
(%)
Entry
Catalyst
Reaction conditions
Solvent
Oxidant
Ref.
1
2
3
4
5
6
CuCr2/CN
75 °C, 5 h
75 °C, 5 h
75 °C, 5 h
75 °C, 5 h
75 °C, 5 h
80 °C, 18 h
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
2,2,2-trifluor-
oethanol
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
46.5
30.6
34.5
48.4
15.9
59.5
92.3 42.9
87.3 26.7
78.9 27.2
86.8 42.0
94.3 15.0
90.0 53.6
Herein
Herein
Herein
Herein
Herein
2
CuCr2/Al2O3
CuCr2/MgO
CuCr2/SiO2
CuCr2/AC
MnWO4 NB
7
Co(OAc)2·4H2O
25 °C, 24 h
O2
7.0
100
7.0
7
25 °C, 8 h, additive
(acetic anhydride)
25 °C, 18 h
8
9
Co/Mn/Br
CH3COOH
O2
38.6
/
67.6 26.1
24.0
9
Mn(III) corrole
CH3CN/(CH2)2Cl2 t-BuOOH
/
10
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 8 of 12
ARTICLE
Journal Name
25 °C, 24 h, additive
((COOH)2)
50 °C, 24 h, Ar
atmosphere
25 °C, 24 h, visible light
100 °C, 5 h
180 °C, 1 h
90 °C, 4 h
200 °C, 8 h
160 °C, 7 h
View Article Online
DOI: 10.1039/C9CY00743A
10
11
Mn(salen)
CH3CN
CH3CN
t-BuOOH
H2O2
6.6
87.0 5.7
14
Fe(MBMPA)Cl3
29.2
36.0 10.5
100 33.0
25.0 8.0
37.6 6.0
51.4 8.9
20.4 2.1
63.4 1.0
15
12
13
14
15
16
17
18
19
20
CdS-S2
Benzotrifluoride O2
H2O
33.0
32.0
15.9
17.3
10.4
1.5
15.5
11.3
12.7
19
20
21
22
27
29
31
32
33
HZSM-5-0.20
MnOx/SBA-15
Fe2O3/HZSM-5
Pd-PMHS/TiO2
Au-Pd/C
I/MgAlO-1073 K
Zr-VO(OAC)2
Ag-Cu-BTC
H2O2
O2 (1.0 MPa)
H2O2
Air
O2
O2 (1.0 MPa)
H2O2
O2 (1.0 MPa)
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
CH3COOH
120 °C, 6 h
90 °C, 4 h
160 °C, 4 h
100
15.5
82.6 9.3
99.0 12.6
Solvent-free
a Reaction conditions: CH3CN (5 ml), toluene (2.5 mmol), H2O2 (10 mmol), catalyst (20 mg).
the catalytic efficiency of this reaction (Fig. 6b), the conversion
increased gradually with the increasing H2O2: toluene molar
ratio, but the excess oxidant would conversely decrease the
selectivity to the target product. That is because benzaldehyde
would be further oxidized to benzoic acid inevitably with the
excessive addition of H2O2. The catalyst amount was also
varied (Fig. S2), it should be mentioned that negligible catalytic
activity was obtained when the reaction was conducted in the
absence of catalyst (a blank experiment), indicating the
important role of catalyst in this oxidation reaction. Upon
further increasing the catalyst amount (> 20 mg) for this model
reaction, it was found that the conversion of toluene had no
significant increase. Fig. S3 shows the effect of reaction time
on catalytic activity, the conversion of toluene reached 46.5%
when the reaction proceeded for 5 h and remained nearly
constant for the next 2 h, but prolonging the reaction time
Fig. 7 Kinetic plot of ln (1/(1-CA)) versus time. Reaction
conditions: acetonitrile (5 ml), toluene (2.5 mmol), H2O2 (10
mmol), catalyst (20 mg).
would slightly decrease the selectivity to the target product. In
general, satisfactory catalytic activity in the selective oxidation
of toluene to benzaldehyde can be obtained over the prepared
catalyst under mild reaction conditions (H2O2 as the oxidant,
and reacted for 5 h at 75 °C in acetonitrile).
Table 3 Effect of the different solvents on the selective oxidation of
toluene to benzaldehyde a
b
Conversion Selectivity Yield
Entry Solvent
(%)
(%)
(%)
1
2
3
Dichloroethane
Cyclohexane
Tetrahydrofuran
N, N-
dimethylformamide
1, 4-dioxane
Acetonitrile
Water
trace
3.2
14.5
trace
82.0
77.1
trace
2.6
11.2
4
5.8
67.5
3.9
5
6
7
12.7
46.5
trace
98.5
92.3
trace
12.5
42.9
trace
Fig. 8 Arrhenius plot of ln k versus 1000/T for the toluene
oxidation over the CuCr2/CN catalyst.
a
Reaction conditions: solvent (5 ml), toluene (2.5 mmol), H2O2 (10
b
mmol), CuCr2/CN catalyst (20 mg), 75 °C, 5 h. The yields were
determined by GC analysis with external standard method.
Firstly, the apparent reaction order was assumed to be one on
the basis of the references.51,52 Then, we plotted ln (1/(1-CA))
versus time (CA represents the conversion of toluene), it was
found that straight lines can be fitted at different
temperatures ranged from 333.15 to 363.15 K (Fig. 7), which
verified the hypothesis of the first order kinetics, the rate
constant (k) at an indicated temperature could be obtained
Kinetic study on toluene oxidation
In order to understand the reaction pathway of toluene
oxidation more deeply, the kinetics were studied and the
apparent activation energy of this reaction was determined.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 12
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
from the respective fitted lines. Finally, the apparent activation catalytic activity over the bimetallic catalysts (in which both
View Article Online
DOI: 10.1039/C9CY00743A
+
energy can be calculated via the following Arrhenius equation: Cu2+ and Cu exist). Besides, the well-defined nanostructured
ln k = ln A – Ea/RT (A: pre-exponential factor, Ea: apparent catalyst with Cu-Cr oxides distributed on the g-C3N4 support
activation energy). As shown in Fig. 8, the plot of ln k against leads to an increase of the exposed reactive sites, the two
1000/T could be fitted linearly, the corresponding slope was metals (Cu and Cr) also perform synergistically via electronic
−2.92206, from which the apparent reaction activation energy interaction to achieve the enhanced catalytic activity.
(Ea) was calculated to be 24.3 kJ mol−1.
Catalyst reusability
Scheme 1 Possible reaction mechanism for the selective oxidation of
toluene to benzaldehyde with H2O2 over the g-C3N4 supported Cu-Cr
bimetallic oxides catalyst.
Fig. 9 The recycle tests of the CuCr2/CN catalyst in the selective
oxidation of toluene to benzaldehyde. Reaction conditions:
acetonitrile (5 ml), toluene (2.5 mmol), H2O2 (10 mmol), catalyst
(20 mg), 75 °C, 5 h.
Possible reaction mechanism
The recycling stability of the prepared catalyst was studied and
the results were presented in Fig. 9. It was noteworthy that
the spent CuCr2/CN catalyst was regenerated via simple
centrifugation, then washed with deionized water and ethanol
thoroughly, followed by drying at 60 °C overnight for the next
catalytic use under the same reaction conditions. The recycle
tests suggested that there were no significant decrease in
catalytic activity, the conversion of toluene still maintained at
around 45% and the selectivity to benzaldehyde maintained at
around 90% in the consecutive six cycles, the results directly
demonstrated the excellent stability of the prepared catalyst.
Moreover, XRD, SEM and TEM were used to investigate the
structure and morphology of the reused catalyst (Fig. S4), it
can be seen that the intensity of diffraction peaks remained
nearly unchanged and the morphological structure was very
similar to that of the fresh catalyst. The filtrate of the reaction
mixture was also analyzed by ICP-OES (Varian 730-ES) and the
result excluded the leaching of metals. The above results
definitely confirmed the good stability and the remarkable
leaching resistance capability of the catalyst, making it a
promising candidate for the practical catalysis applications.
On the basis of the above experimental results and the
previous studies, a plausible reaction mechanism for the
toluene oxidation with H2O2 catalyzed by g-C3N4 supported Cu-
Cr oxides was proposed (Scheme 1). The present
heterogeneous catalytic system can be ascribed to a typical
liquid-solid phase reaction catalyzed by the supported metal
catalysts, and the proposed possible mechanism generally
follows the reaction paths of heterogeneous catalysis. On the
one hand, the catalyst support (g-C3N4) possesses an
analogous structure with graphene (a matrix with π electron
conjugated system), which helps the toluene molecules can
easily gain access to the catalyst surface due to the effect of π-
π stacking interaction.53 On the other hand, the distributed
metal oxides (M) interact with H2O2 molecules to generate the
M-peroxo complex (M = Cr, Cu), which is generally considered
as an active oxygen species in the oxidation reaction of organic
hydrocarbons.2,54 Then, the N-neighboring carbon on g-C3N4
skeleton can host the active oxygen species because of its
metal-like d-band electronic structure.55 Afterwards, the
formed active M-peroxo species will attack the methyl C
H
bond of toluene to produce peroxo metallocycle
a
intermediate, which then is broken with the production of
benzaldehyde and simultaneous regeneration of the metal
catalyst. In addition, it is reasonable to believe that the Cu+
species among the bimetallic catalysts, plays an important role
in the improvement of the catalytic activity. It was reported
that the low-valance Cu+ species can provide more adsorption
sites for the reactive intermediate during the oxidation
process,56 giving an possible explanation of the enhanced
Conclusions
In summary, we have developed a heterogeneous process for
the production of benzaldehyde from toluene catalyzed by g-
C3N4 supported Cu-Cr oxides. The prepared bimetallic catalysts
displayed markedly improved catalytic activity compared to
the monometallic counterparts, the highest yield of
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 10 of 12
ARTICLE
Journal Name
16 G. Capocasa, G. Olivo, A. Barbieri, O. Lanzalunga and S. D.
benzaldehyde reached 42.9% within 5 h over CuCr2/CN under
mild reaction conditions. Based on the characterizations, it was
found that the formation of a core/shell-like structure with
metals wrapped by g-C3N4, the existence of electronic
interaction between the two metals, the enlarged specific
surface area and pore volume over the bimetallic catalysts,
such advantages contributed to the good stability and the
enhanced catalytic activity of the prepared catalyst. In
addition, the catalyst can be easily recovered with
centrifugation and reused for six times without obvious loss in
the catalytic activity. Further studies on the reaction
mechanism over the g-C3N4 supported catalysts at molecular
level are currently underway in our laboratory. Overall, the
present work might provide a feasible way for the selective
oxidation of toluene to benzaldehyde over the supported
bimetallic catalysts.
View Article Online
Stefano, Catal. Sci. Technol., 2017, 7, 5D6O77I:–150.6108369./C9CY00743A
17 C. Detoni, N. M. F. Carvalho, D. A. G. Aranda, B. Louis and O.
A. C. Antunes, Appl. Catal. A Gen., 2009, 365, 281–286.
18 B. J. Gallon, R. W. Kojima, R. B. Kaner and P. L. Diaconescu,
Angew. Chem. Int. Ed., 2007, 46, 7251–7254.
19 Y. H. Zhang, N. Zhang, Z. R. Tang and Y. J. Xu, Chem. Sci.,
2012, 3, 2812–2822.
20 B. Du, S. Kim, L. L. Lou, A. Z. Jia, G. X. Liu, B. Qi and S. X. Liu,
Appl. Catal. A Gen., 2012, 425–426, 191–198.
21 W. Z. Zhong, S. R. Kirk, D. L. Yin, Y. Q. Li, R. Zou, L. Q. Mao
and G. Q. Zou, Chem. Eng. J., 2015, 280, 737–747.
22 X. Li, B. Lu, J. Sun, X. Wang, J. X. Zhao and Q. H. Cai, Catal.
Commun., 2013, 39, 115–118.
23 Z. P. Qu, Y. B. Bu, Y. Qin, Y. Wang and Q. Fu, Chem. Eng. J.,
2012, 209, 163–169.
24 X. R. Fu, Y. Liu, W. Y. Yao and Z. B. Wu, Catal. Commun.,
2016, 83, 22–26.
25 M. Popova, Á. Szegedi, Z. Cherkezova-Zheleva, I. Mitov, N.
Kostova and T. Tsoncheva, J. Hazard. Mater., 2009, 168
226–232.
,
26 Y. Qin, Z. P. Qu, C. Dong and N. Huang, Chin. J. Catal., 2017,
38, 1603–1612.
Conflicts of interest
27 B. S. Fu, X. W. Zhu and G. M. Xiao, Appl. Catal. A Gen., 2012,
415–416, 47–52.
There are no conflicts of interest to declare.
28 M. Ousmane, L. F. Liotta, G. D. Carlo, G. Pantaleo, A. M.
Venezia, G. Deganello, L. Retailleau, A. Boreave and A. Giroir-
Fendler, Appl. Catal. B Environ., 2011, 101, 629–637.
29 L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. Saiman, D. I.
Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H.
Taylor, D. W. Knight, C. J. Kiely and G. J. Hutchings, Science,
2011, 331, 195–199.
Acknowledgements
This work was supported financially by the Zhejiang Provincial
Key Laboratory of Advanced Chemical Engineering
Manufacture Technology (Grant No. 2017E10001), Zhejiang
Province (China).
30 G. Huang, F. Xiang, T. M. Li, Y. X. Jiang and Y. A. Guo, Catal.
Commun., 2011, 12, 886–889.
31 X. L. Wang, G. D. Wu, F. Wang, H. Liu and T. F. Jin, Catal.
Commun., 2017, 98, 107–111.
32 X. Q. Wang, X. L. Cao, X. K. Hu, G. Y. Li, L. F. Zhu and C. W. Hu,
J. Mol. Catal. A Chem., 2012, 357, 1–10.
References
1
2
3
4
5
6
7
8
9
C. L. Pang, J. H. Xiong, G. Y. Li and C. W. Hu, J. Catal., 2018,
266, 37–49.
D. D. Mal, S. Khilari and D. Pradhan, Green Chem., 2018, 20
2279–2289.
Z. G. Hu, Y. F. Zhao, J. D. Liu, J. T. Wang, B. Zhang and X.
Xiang, J. Colloid Interface Sci., 2016, 483, 26–33.
S. Pugh, R. McKenna, I. Halloum and D. R. Nielsen, Metab.
Eng. Commun., 2015, 2, 39–45.
S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma,
ACS Sustainable Chem. Eng., 2016, , 2333–2336.
B. B. Sarma, I. Efremenko and R. Neumann, J. Am. Chem.
Soc., 2015, 137, 5916–5922.
E. Gaster, S. Kozuch and D. Pappo, Angew. Chem. Int. Ed.,
2017, 56, 5912–5915.
H. R. Yuan, X. Fang, Q. Y. Ma, J. Y. Mao, K. X. Chen, Z. R. Chen
and H. R. Li, J. Catal., 2016, 339, 284–291.
X. K. Wu, Z. L. Deng, J. J. Yan, F. Zhang and Z. B. Zhang, Ind.
Eng. Chem. Res., 2014, 53, 14601−14606.
33 Z. G. Sun, G. Li, Y. Zhang, H. O. Liu and X. H. Gao, Catal.
Commun., 2015, 59, 92–96.
,
34 F. He, G. Chen, Y. G. Yu, S. Hao, Y. S. Zhou and Y. Zheng, ACS
Appl. Mater. Interfaces, 2014, 6, 7171–7179.
35 B. Chai, C. Liu, J. T. Yan, Z. D. Ren and Z. J. Wang, Appl. Surf.
Sci., 2018, 448, 1–8.
36 X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009,
76–80.
8,
4
37 J. Y. Zhang, Y. H. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang and J.
G. Yu, ACS Appl. Mater. Interfaces, 2013, , 10317–10324.
5
38 Y. Z. Hong, C. S. Li, B. X. Yin, D. Li, Z. Y. Zhang, B. D. Mao, W.
Q. Fan, W. Gu and W. D. Shi, Chem. Eng. J., 2018, 338, 137–
146.
39 S. N. Talapaneni, S. Anandan, G. P. Mane, C. Anand, D. S.
Dhawale, S. Varghese, A. Mano, T. Mori and A. Vinu, J.
Mater. Chem., 2012, 22, 9831–9840.
10 S. Bose, A. Pariyar, A. N. Biswas, P. Das and P.
Bandyopadhyay, J. Mol. Catal. A: Chem., 2010, 332, 1–6.
11 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058.
12 C. Xu, L. L. Zhang, Y. An, X. Z. Wang, G. Xu, Y. Q. Chen and L.
Y. Dai, Appl. Catal. A Gen., 2018, 558, 26–33.
13 S. Peta, T. Zhang, V. Dubovoy, K. Koh, M. C. Hu, X. Q. Wang
and T. Asefa, Mol. Catal., 2018, 444, 34–41.
14 T. C. O. M. Leod, M. V. Kirillova, A. J. L. Pombeiro, M. A.
40 S. N. Li, G. H. Dong, R. Hailili, L. P. Yang, Y. X. Li, F. Wang, Y. B.
Zeng and C. Y. Wang, Appl. Catal. B Environ., 2016, 190, 26–
35.
41 Y. Z. Hong, C. S. Li, Z. Y. Fang, B. F. Luo and W. D. Shi, Carbon,
2017, 121, 463–471.
42 A. Arango-Díaz, E. Moretti, A. Talon, L. Storaro, M. Lenarda,
P. Núñez, J. Marrero-Jerez, J. Jiménez-Jiménez, A. Jiménez-
López and E. Rodríguez-Castellón, Appl. Catal. A Gen., 2014,
477, 54–63.
43 J. W. Shi, C. Cheng, Y. C. Hu, M. C. Liu and L. J. Guo, Int. J.
Hydrogen Energy, 2017, 42, 4651–4659.
Schiavon and M. D. Assis, Appl. Catal. A Gen., 2010, 372
191–198.
,
15 G. C. Silva, N. M. F. Carvalho, A. H. Jr., E. R. Lachter and O. A.
C. Antunes, J. Mol. Catal. A Chem., 2017, 426, 564–571.
10 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 12
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
44 A. Y. Kim, M. K. Kim, K. Cho, J. Y. Woo, Y. Lee, S. H. Han, D.
View Article Online
DOI: 10.1039/C9CY00743A
Byun, W. Choi and J. K. Lee, ACS Appl. Mater. Interfaces,
2016, 8, 19514−19523.
45 J. B. Zhao, X. Shu, Y. Wang, C. P. Yu, J. F. Zhang, J. W. Cui, Y.
Q. Qin, H. M. Zheng, J. Q. Liu, Y. Zhang and Y. C. Wu, Surf.
Coat. Tech., 2016, 299, 15−21.
46 P. Liu and E. J. M. Hensen, J. Am. Chem. Soc., 2013, 135
14032–14035.
,
47 J. W. Fu, J. G. Yu, C. J. Jiang and B. Cheng, Adv. Energy
Mater., 2018, , 1701503.
8
48 L. Qin, D. L. Huang, P. Xu, G. M. Zeng, C. Lai, Y. K. Fu, H. Yi, B.
S. Li, C. Zhang, M. Cheng, C. Y. Zhou and X. F. Wen, J. Colloid
Interface Sci., 2019, 534, 357–369.
49 D. X. Yang, T. B. Wu, C. J. Chen, W. W. Guo, H. Z. Liu and B. X.
Han, Green Chem., 2017, 19, 311–318.
50 J. T. Carneiro, C. C. Yang, J. A. Moulijn and G. Mul, J. Catal.,
2011, 277, 129–133.
51 C. Liang, X. F. Li, D. F. Su, Q. Y. Ma, J. Y. Mao, Z. R. Chen, Y.
Wang, J. Yao and H. R. Li, Mol. Catal., 2018, 453, 121–131.
52 Y. Chen, S. S. Jie, C. Q. Yang and Z. G. Liu, Appl. Surf. Sci.,
2017, 419, 98–106.
53 J. W. Sun, Y. S. Fu, G. Y. He, X. Q. Sun and X. Wang, Appl.
Catal. B Environ., 2015, 165, 661–667.
54 J. Y. Liu, T. T. Chen, P. M. Jian, L. X. Wang and X. D. Yan, J.
Colloid Interface Sci., 2018, 526, 295–301.
55 W. Li, Y. Gao, W. Chen, P. Tang, W. Li, Z. Shi, D. Su, J. Wang
and D. Ma, ACS Catal., 2014, 4, 1261–1266.
56 Y. Xie, Y. L. Yin, S. H. Zeng, M. Y. Gao and H. Q. Su, Catal.
Commun., 2017, 99, 110–114.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
Please do not adjust margins

Catalysis Science & Technology
Page 12 of 12
View Article Online
DOI: 10.1039/C9CY00743A
Table of Contents Entry
The supported Cu-Cr/g-C3N4 catalysts were prepared via an in-situ heating treatment
method for the selective oxidation of toluene to benzaldehyde.