Preparation of the CuCr2O4 spinel nanoparticles catalyst for selective oxidation of toluene to benzaldehyde

Article abstract of DOI:10.1039/c3gc42369g

CuCr₂O₄ spinel nano.particles with size between 30 and 60 nm were prepared by a hydrothermal synthesis method in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). It was found that the catalyst is highly active in

Full text of DOI:10.1039/c3gc42369g

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Acharyya, S. Ghosh, R. Tiwari, B. Sarkar, R. K. Singha, T. Sasaki and C. Pendem, Green Chem., 2014, DOI:
0.1039/C3GC42369G.
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Preparation of CuCr O Spinel Nanoparticles Catalyst for Selective
2
4
†
Oxidation of Toluene to Benzaldehyde
a
a
a
a
Shankha Shubhra Acharyya, Shilpi Ghosh, Ritesh Tiwari, Bipul Sarkar, Rajib Kumar
a
a
b
a
Singha , Chandrasekar Pendem, Takehiko Sasaki and Rajaram Bal*
a
Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum, Dehradun
248005, India
b
Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The
University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan
[
*] Mr. Shankha Shubhra Acharyya, Ms. Shilpi Ghosh, Mr. Ritesh Tiwari, Mr. Bipul Sarkar,
Mr. Rajib Kumar Singha, Mr. Chandrashekar Pendem, Prof. Dr. Takehiko Sasaki, Dr. Rajaram
Bal
Corresponding author. Tel.: +91 135 2525797; Fax: +91 135 2660202
Eꢀmail addresses: raja@iip.res.in

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CuCr O spinel nanoparticles with the size between 30ꢀ60 nm was prepared by a hydrothermal
2
4
synthesis method in the presence of surfactant, cetyltrimethylammonium bromide (CTAB). It
was found that the catalyst is highly active for the selective oxidation of toluene with H O at 75°
2
2
C. The catalyst was characterized by XRD, ICPꢀAES, XPS, TPR, BETꢀsurface area, SEM, TEM
and EXAFS. Factors effecting reaction parameters, such as the substrate to oxidant molar ratio,
weight of catalyst, reaction time, etc. were investigated in detail. The investigation revealed that
the size of the catalyst as well as the spinel phase plays a crucial role towards the activity by
favoring the oxidation of toluene. The reusability of the catalyst was examined by conducting
repeat experiments with the same catalyst; and it was observed that the catalyst displayed no
significant changes in its activity even after 5 reuses. The toluene conversion of 57.5 % with
8
4.4% selectivity towards benzaldehyde was observed after 10 hours over CuCr O spinel
2 4
nanoparticles catalyst.
1
. Introduction
Controlled synthesis of inorganic nanoparticles is of both fundamental and technical interest due
to their size and shape dependent physical properties and diverse applications. The simple
preparation method is required for easy manipulation of the nucleation and growth kinetics for
1
,2
tunable shape and size of the nanoparticles. Bimetallic nanoparticles (NP), emerging as a new
class of materials, are of much potential interest compared to the monometallic ones as they
3
,4
generate new properties and capabilities due to a synergy between the two metals. Moreover,
bimetallic NPs usually show compositionꢀdependent surface structure and atomic segregation
behavior, and thus bear attracted attention in multidisciplinary fields. Compared to the

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monometallic NPs, synthesis of highꢀquality bimetallic ones with controllable size, morphology,
composition, and structure is much more complicated; therefore, a simple, reliable preparation is
5
,6
still remains a challenge to be solved. Copper chromium mixed oxides with a spinel structure
are an important class of biꢀ metallic oxides promise great potentials as versatile catalyst and
7
ꢀ9
thus, bears wide commercial applications.
These catalysts can be prepared by a variety of
Among these methods, the solꢀgel process using metal alkoxide shows
promising potential, owing to its high purity, good chemical homogeneity etc. but suffers due to
7
,10,11
synthetic methods.
1
2
its sensitivity towards moisture and heat. So, controlled syntheses of copper chromite at large
scale, with tunable morphology and its fruitful application in catalytic oxidation is highly
appreciated in the field of catalysis.
Herein, we report for the first time the preparation of CuCr O spinel nanoparticles by
2
4
hydrothermal synthesis method. This preparation method is simple, reproducible and produces
high yield (98%) and can be prepared in a large scale (upto 20 g). To the best of our knowledge,
there is no report for the preparation of CuCr O spinel nanoparticles with size between 20ꢀ40
2
4
nm using surfactant promoted hydrothermal synthesis method.
3
Direct functionalization of sp hybridized inactive hydrocarbons by means of catalytic oxidation
of CꢀH bonds to form oxygenated products under mild conditions is a major challenge from
1
3,14
industrial aspects.
Toluene is a typical aromatic hydrocarbon, that possesses three primary Cꢀ
H bonds and can be oxidized to several oxygenates like benzyl alcohol, benzaldehyde, benzoic
1
5
acid, and benzoate, which are all useful chemical intermediates. Among these, benzaldehyde is
1
6
the most desirable product due to its immense importance in our daily life. It is used in the
manufacture of dyes, solvents, perfumes, flavors, plasticizers, dyestuffs, preservatives and flame

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retardants. In the pharmaceutical industries, it is used for the manufacture of intermediates for
chloramphenicol, analgin, ephidrin, ampiphilicin etc. Traditionally, benzaldehyde is produced by
1
7
chlorination of –CH functionality of toluene followed by saponification. These processes
3
suffer severely from much waste causing environmental problems and poor efficiencies.
Moreover, the products are not prescribable for edible and drug grade specifications owing to the
chlorinated contaminants. Although, there are some reports on vapor phase oxidation of toluene
with oxygen, but the conducting reaction conditions are harsh and the selectivity are not
1
8ꢀ20
satisfactory.
Although, in the Rhodia, Dow and SniaꢀViscosa processes liquid phase
oxidation of toluene is carried out over homogeneous metal salt catalysts using oxygen or
peroxides as oxidants in industrial grade, involved indispensable halogen ions and acidic
2
1ꢀ23
24
25
solvents cause corrosion of the facility.
There are also several reports using H O , TBHP
2 2
and molecular O as oxidant for oxidation of toluene to benzaldehyde but most of the cases yield
2
2
1,26ꢀ28
is very less.
benzaldehyde using photocatalytic routes.
toluene oxidation using Bꢀdoped polymeric carbon nitride (graphitic phase) with H O as
Apart from these, there are also few reports on selective oxidation of toluene to
2
9,30
Recently, Antonietti and his group reported
2
2
3
1
32
oxidant and using mesoporous nanoꢀstructured carbon nitride with O as oxidant. Although,
2
they reported a high selectivity towards benzaldehyde (~99%), but the conversion of toluene was
very poor.
Herein we report the unique catalytic activity of our so prepared CuCr O spinel nanoparticles in
2
4
selective oxidation of toluene to benzaldehyde. A toluene conversion of 57.5 % and 84.4%
benzaldehyde selectivity was achieved with H O as the principal oxygen donor at 75°C in liquid
2
2
phase.

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2
. Experimental
2.1 Material
Hydrogen Peroxide (50 wt % in water) was purchased from Merck KGaA, Darmstadt, Germany.
Cu(NO ) ꢁ5H O, Cr(NO ) ꢁ9H O, cetyltrimethylammonium bromide, hydrazine, ammonium
3
2
2
3 3
2
hydroxide, toluene (purity > 99.9%), acetonitrile (HPLC grade) were purchased from Sigmaꢀ
Aldrich Co. All the chemicals were used without further purification.
2.2 Catalyst Preparation
The CuCr O spinel nanoparticles catalyst was prepared hydrothermally using
2
4
33
cetyltrimethylammonium bromide as the surfactant by modifying our own preparation method.
An aqueous solution of 3.8 g Cu (NO ) .3H O was added with vigorous stirring to 12.6 g Cr
3
2
2
(
NO ) .9H O dissolved in 80 g deionized water. The pH of the medium was made 9 by adding
3 3 2
drop wise ammonia solution. An aqueous solution of 4.3 g CTAB, followed by of 0.98 g
hydrazine was added drop wise to the reaction mixture. The reagents were added maintaining the
molar ratio: Cu: Cr: CTAB: H O: hydrazine = 1: 2: 0.75: 300: 1. After stirring, the so obtained
2
homogeneous solution was hydrothermally treated at 180 °C for 24 h in a teflonꢀlined autoclave
vessel under autogeneous pressure. The product was washed with distilled water, acetone and
ethanol and dried at 110 °C, for 10 h, followed by calcination at 650 °C for 6 h in air.
2.3 Catalyst Characterization
Powder Xꢀray diffraction (XRD) Powder Xꢀray diffraction (XRD) patterns were collected on a
Bruker D8 advance Xꢀray diffractometer fitted with a Lynx eye highꢀspeed strip detector and a
Cu K radiation source. Diffraction patterns in the 2ºꢀ80º region were recorded at a rate of 0.5
α
degrees (2q) per minute. Scanning electron microscopy (SEM) images were taken on a FEI

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Quanta 200 F, using tungsten filament doped with lanthanumhexaboride (LaB ) as an Xꢀray
6
source, fitted with an ETD detector with high vacuum mode using secondary electrons and an
acceleration tension of 10 or 30 kV. Samples were analyzed by spreading them on a carbon tape.
Energy dispersive Xꢀray spectroscopy (EDX) was used in connection with SEM for the
elemental analysis. The elemental mapping was also collected with the same
spectrophotometer. Transmission electron microscopy (TEM) images were collected using a
JEOL JEM 2100 microscope, and samples were prepared by mounting an ethanolꢀdispersed
sample on a lacey carbon Formvar coated Cu grid. The surface area was measured using Belsorp
equipment (BEL Japan Inc.) by nitrogen adsorption at ꢀ196 ºC. Prior to measurement, the sample
was pretreated at 200 ºC for 3 h under vacuum. XꢀRay photoelectron spectra were recorded on a
Thermo Scientific KꢀAlpha XꢀRay photoelectron spectrometer and binding energies (±0.1 eV)
were determined with respect to the position C 1s peak at 284.8 eV. Extended Xꢀray absorption
fine structure spectroscopy (EXAFS) measurements of CuꢀK edge were carried out at the High
Energy Accelerator Research Organization (KEKꢀIMMSꢀPF), Tsukuba, Japan. The measurement
was made at transition mode and spectra were taken at BLꢀ7C and BLꢀ9C. The electron storage
ring was operated at 2.5 GeV and 450 mA, synchrotron radiation from the storage ring was
monochromatized by a Si (111) channel cut crystal. Ionized chamber, which were used as
detectors for incident Xꢀray (I ) and transmitted Xꢀray (I), were filled with N mixture gas,
o
2
respectively. The angle of the monochromators was calibrated with Cu foil. The EXAFS raw
data were analyzed with UWXAFS analysis package including background subtraction program
3
4ꢀ36
2
o
AUTOBK and curve fitting program FEFFIT.
The amplitude reducing factor, S was fixed
at 1.0. The backscattering amplitude and phase shift were calculated theoretically by FEFF 8.4
3
7
38
code. ATOMS were used to obtain FEFF input code for crystalline materials. Inductively

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coupled Plasma Atomic Emission Spectroscopic (ICPꢀAES) analysis was carried out by
Inductively Coupled Plasma Atomic Emission Spectrometer; model: PS 3000 uv, (DRE),
Leeman Labs, Inc, (USA). Thermogravimetric Analysis (TGA) and Differential Thermal
Analysis (DTA) of the uncalcined catalyst were carried out in a PYRIS DIAMOND, PERKIN
º
ꢀ1
ELMER INSTRUMENTS. 2.15 mg samples was used and a heating rate of 5 C min
was
carried out in flowing air. The FTIR spectra were recorded on a Thermo Nicolet 8700 (USA)
ꢀ1
instrument with the operating conditions: resolution: 4 cm , scan: 36, operating temperature: 23ꢀ
ꢀ1
2
5 °C and the frequency range: 4000ꢀ400cm .
2.4 Oxidation of toluene
Liquid phase oxidation reaction was carried out in a two neck round bottom flask containing 0.1
g catalyst, 15 ml solvent and 1 g toluene to which H O (50% aq. solution) was added dropwise.
2
2
Small aliquots of the sample were withdrawn from the reaction mixture at regular intervals for
analysis using a syringe. At the end of the reaction, the solid particles (catalyst) were separated
by filtration and the products were analysed by Gas Chromatography (GC, Agilent 7890)
connected with a HP5 capillary column (30m length, 0.28 mm id, 0.25 ꢂm film thickness) and
flame ionisation detector (FID). The toluene conversion and benzaldehyde formation were
calculated using a calibration curve (obtained by manual injecting the authentic standard
compounds). The individual yields were calculated and normalized with respect to the GC
response factors. The product identification was carried out by injecting the authentic standard
samples in GC and GCꢀMS. The Cꢀbalance as well as material balance was carried out for most
of the experiments and it was found between 98ꢀ102%. After completion of the reaction, the

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catalyst was recovered from the reaction mixture via centrifugation, washed thoroughly with
acetone and reused for multiple circles.
3
. Results and discussion
3.1 Catalyst Characterization
The powder XRD pattern of the prepared CuꢀCr catalyst (Fig. 1) revealed the exclusive
formation of CuCr O spinel with high phaseꢀcrystallinity (JCPDS Card No: 05ꢀ0657), while no
2
4
XRD peaks were attributed either to Cu or Cr metal, or their oxides. The particle size was
determined from the full width half maxima of the line broadening corresponding to the
diffraction angle of 53.4° by using the Scherrer equation and a mean particle size of ~28 nm was
observed (Table 1). The amount of copper and chromium present in the sample was estimated by
ICPꢀAES and it was found that the molar ratio of Cu to Cr was 0.5, which is the typical
°
composition of CuCr O spinel. The catalyst was stable above 1000 C as confirmed by TG/DTA
2
4
analysis (Fig. S1 & S2, ESI).
SEM image (Fig. 2) showed the formation of almost homogeneously distributed uniform
particles with size 20ꢀ40 nm. Energy dispersive XꢀRay analysis (EDAX) showed the presence of
Cu, Cr, O (Fig. 3) and the elemental mapping demonstrated the homogeneous distribution of Cu
and Cr in the catalyst (Fig. 4).
Transmission Electron Microscopy (TEM) images of the CuCr O spinel catalyst are shown in
2
4
Fig. 5. The particle size distribution histogram is also presented in Fig. 5a (inset). The
distribution histogram shows that 10% particles have the diameter of 20 nm, 38% particles have
the diameter of 30 nm, 26% particles have the diameter of 40 nm, 10% particles have diameters

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of 50 nm and rest of the particles (16%) having diameter more than 50 nm. The lattice fringes
3
9
with a dꢀspacing of 0.30 nm corresponding to [220] plane of CuCr O spinel with diffraction
2
4
angle (2θ) of 29.6 is also presented. The HRTEM of the spent catalyst (after 5 recycle)(Fig. 5d)
revealed that the particle size of the CuCr O spinel was unchanged during the catalysis.
2
4
The Cu2p XPS spectrum of the fresh catalyst is characterized by two spinꢀorbit doublets with
2
+
strong satellite peaks which are of typical Cu (Fig. 6). The low energy component with Cu
2p3/2
2
+
at 933.8 eV is associated to Cu in octahedral sites, whereas the high energy component at 935.2
2
+
eV is associated with Cu in the tetrahedral sites of the CuCr O spinel structure. The Cr
2p 3/2
2
4
core level spectra of the CuCr O fresh catalyst appeared at 576.5 eV showed the presence of
2
4
3
+
Cr ions and the O binding energies appeared at 530.2 eV and 532.2 eV, revealed the presence
1
s
2
ꢀ
40
of O species in CuCr O spinel (Fig. S3 & S4, ESI).
2
4
CuꢀK edge extended Xꢀray absorption fine structure (EXAFS) analysis of the catalyst supports
the formation of CuCr O spinel and the spinel structure remains unchanged during the catalysis
2
4
(Table 2 & Fig. S5, ESI). The entry for CuO is not for the reference compound. The EXAFS
spectrum was analyzed as the superposition of the crystal phase of CuCr O and the residual
2
4
CuO phase. The small coordination number 2.5 for the CuO phase indicates that the CuO phase
correspond to the surface layer of oxidized Cu on CuCr O crystal or the monatomically
2
4
dispersed CuO on the CuCr O crystal phase. From Cu2p XPS spectra, tetrahedrally coordinated
2
4
Cu and the octahedrally coordinated Cu are assigned to the CuCr O phase and the surface CuO
2
4
phase, respectively. The coordination number for CuO was found to be smaller than the value 4ꢀ
in the present study, because it is minor phase as compared to the CuCr O phase.
6
2
4
It is worth mentioning that, the embedment of the CTAB molecules can be analyzed by the
FTIR analysis (Fig. 7). A comparison of the FTIRꢀ spectra of dried uncalcined sample, with that

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of pure CTAB was analyzed, which not only confirmed the presence, but also revealed the nature
of interaction of CTAꢀ molecules with the CuꢀCr surface. The peaks of the sample at 809, 1062
ꢀ1
+
41
cm can be assigned to the CꢀN stretching modes of CTAB molecules. The peak at 1378 and
ꢀ1
at 1462 cm were assigned to symmetric mode of vibration of the head groups of the methylene
+
41
ꢀ1
moiety (N ―CH ) and CH scissoring mode respectively. The frequencies above 1600 cm to
3
2
ꢀ1
3
000 cm can be attributed due to CH symmetric antisymmetric vibrations region. It is to be
2
noted that, the shift of vibrations to lower frequency suggested that, alkyl chains experienced a
more hydrophobic environment in CuꢀCr blocks upon the surface of which the CTAB moieties
4
1
were supposed to be bounded. These typical frequencies were absent when the material was
calcined at 650 °C in air (fresh catalyst)in the case of the prepared catalyst, which indicated that,
the embedded CTAB moieties have been completely removed from the catalyst surface during
calcination. Moreover, in the SEMꢀEDAX diagram of the fresh catalyst (Fig. 3) does not show
any peak for C, N or even Br, which further ascertains the removal of the template (CTAB) by
calcination.
Based on these detailed characterization data, we presented a model diagram of CuCr O spinel
2
4
catalyst (Fig. S6, ESI), which actually reflects the threeꢀdimensional array of Cu, Cr and O atoms
in the catalyst.
3.2 Activity of the catalyst
Table 3 shows the activities of the CuCr O spinel nanoparticles catalyst in the direct oxidation
2
4
of toluene in liquid phase by using H O as oxidant. The catalyst shows 57.5% toluene
2
2
conversion with 84.4% selectivity of benzaldehyde. The remaining major product was
benzylalcohol with 11.2% selectivity. So a total C7 (benzaldehyde and benzylalcohol) selectivity
of 95.6% was achieved over this catalyst. Blank experiment was carried out without the catalyst

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and it was found that there was no conversion of toluene. For comparison several different Cu
supported catalysts were prepared and their activity was tested for toluene oxidation.
Commercial CuO, Cu O, Cr O does not show any activity for toluene oxidation (Table 3, entry
2
2
3
1
ꢀ3). Although, commercial CuCr O , Cuꢀsupported on commercial Cr O , prepared
2 4 2 3
impregnation method and coꢀprecipitation method show some selectivity for benzaldehyde with
negligible activity but the catalysts show serious leaching, as confirmed by ICPꢀAES (Table 3,
entry 4ꢀ6). We have also tested the catalytic activity of the Cuꢀnanoclusters supported on
nanocrystalline Cr O and noticed that this catalyst shows 12.5 % conversion and 65.2%
2
3
benzaldehyde selectivity (entry 7). Although this catalyst shows good benzaldehyde selectivity
but this catalyst shows leaching of Cu during reaction. This result clearly indicates that spinel
2
+
phase (CuCr O ) and small Cu site are essential for selective oxidation of toluene to
2
4
benzaldehyde.
3.3 Effect of reaction parameters
It was found that temperature has prominent effect on toluene conversion. The catalyst showed
negligible conversion at RT but with increasing temperature, the conversion of toluene increases
rapidly (Fig. 8). Presence of both benzyl alcohol and benzaldehyde exclusively was noticed in
the reaction mixture; moreover at 75 °C, low concentration of H O and low catalyst weight
2
2
facilitates the formation of benzyl alcohol on the catalyst surface; but with the increase in
temperature, time and H O concentration, conversion of benzyl alcohol to benzaldehyde occurs,
2
2
thereby selectivity of the latter increases significantly. The observed decrease in selectivity was
due to the formation of para- and ortho-isomer of cresols and overꢀoxidized product, benzoic
acid. It was also found that, the catalyst was active only in acetonitrile solvent Increment in H O2
2
molar ratio allows oxidation of the side chain and in the aromatic system, both and consequently

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the concentration of the side products increases (Fig. 10). Increment in time and temperature
both indicates the fact that, the species present in the reaction mixture are allowed to react
without excess oxidant; inevitably, concentration of benzoic acid (a stable oxidation product)
owing to autooxidation of benzaldehyde (and benzyl alcohol) occurs. It seems, acetonitrile
ꢀ
42
(
solvent) activates H O by forming a perhydroxyl anion (HOO , nucleophile) which attacks the
2 2
C―N group of the acetonitrile to generate peroxycarboximidic acid intermediate, a good oxygen
4
3
transfer agent, and thereby explains the solvent efficacy in the reaction. Moreover, conducting
the reaction in different solvents did not show any satisfactory yields. It seems that polarity of
the solvent does not play significant effect in this reaction. The conversion of toluene was
n
extremely poor in DMF and octane; although selectivity of benzaldehyde was ~98% in acetic
acid, conversion was very low.
3.5 Reusablity of the catalyst
The catalyst displayed a highꢀleaching resistance capability. Reuse of the recovered catalyst in 5
consecutive runs did not lead to any significant decline in its catalytic activity in terms of
conversion, yield and selectivity. Recycling and reusability of the catalyst was examined by
introducing the used catalyst subsequently 5 times to carry out the catalytic oxidation reaction
unhindering the optimum conditions; After the first reaction cycles (10 h), the solid catalyst was
recovered by a centrifugation and washed with acetone and dried for the next reaction cycle. The
catalyst was effective enough in each cycle (Fig. 12), which demonstrates that, no structural
modification, like sintering could have hardly hampered the spinel phase and no significant loss
in the catalytic activity was observed; hence recovered catalyst could be reused several times,
establishing the recyclability of the catalyst. Moreover, to examine the heterogeneous nature, we

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tested the activity of the filtration. When a typical oxidation of toluene was proceeded for 5h
when 40% toluene conversion was achieved) the reaction was stopped and the filtrate was
(
immediately collected. No further increase in conversion was observed after the filtrate was
stirred at 75° C for another 5h. These results clearly indicate that the conversion was contributed
exclusively by CuCr O spinel NP catalyst and leaching of the catalyst hardly occurred during
2
4
the reaction, which was furthermore ascertained by ICPꢀAES analyses and XRD plot of the
catalyst during the recycling experiments.
4
Conclusions
We have prepared CuCr O spinel nano particles with size between 30ꢀ60 nm by hydrothermal
2
4
synthesis method in presence of surfactant. This catalyst can efficiently catalyze the oxidation of
toluene through intervening the activation of C–H bonds, and can operate under mild conditions
and can generate selectively benzyl alcohol and benzaldehyde, tuning the reaction parameters
inconveniently, currently remain to be the major research effort in catalysis. The catalyst can be
reused several times without any activity loss, confirming the true heterogeneous nature of the
catalyst.
Acknowledgements
The Director, CSIRꢀIIP, is acknowledged for his help and encouragement. S.S.A. thanks CSIR
and S.G., B.S., R.K.S. thank UGC, New Delhi, India for their fellowships. The authors thank
Analytical Science Division, Indian Institute of Petroleum for analytical services. The XAFS
measurements were performed at KEKꢀIMSSꢀPF, Tsukuba, Japan with the approval of the
Photon Factory Advisory Committee (project 2010G109).

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DOI: 10.1039/C3GC42369G
1
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DOI: 10.1039/C3GC42369G
g
f
e
d
c
b
a
2
0
40
60
80
2
Theta/ Degree
imp
Fig. 1 XRD patterns of the a) CuO, b) Cu O, c) CrO , d) Cr O , e) Cu/Cr O
(imp:
2
3
2
3
2
3
impregnation method), f) CuCr O (prepared catalyst) and g) CuCr O (spent catalyst, after 5
2
4
2
4
reuse).
a
b
1
µm
2
00 nm
Fig. 2 SEM image of CuCr O nanoparticles: a) low magnification and b) high magnification
2
4

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Fig. 3 SEMꢀEDAX spectra of CuCr O spinel nanoparticles catalyst.
2
4
a
b
Fig. 4 Elemental Mapping of a) Cu atoms and b) Cr atoms in CuCr O spinel nanoparticles
2
4
catalyst.

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a
c
b
5
0
40
30
20
10
0
2
0
30 40 50 60 70
Size/nm
5
nm
d
10 nm
Fig. 5 TEM images of a) ꢀ c) fresh and d) spent catalyst.

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DOI: 10.1039/C3GC42369G
12500
935.3
1
0000
500
000
7
5
933.8
2500
0
945
940
935
930
Binding Energy/ eV
Fig. 6 Cu 2p 3/2 core level spectra

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DOI: 10.1039/C3GC42369G
c
b
1
462
623
2019
1057
812
1
2
917
3237
1378
3
388
a
700
1400
2100
2800
3500
ꢀ1
cm
Figure 7. FTIR spectra of (a) pure CTAB (b) uncalcined and (c) calcined CuꢀCr sample.

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100
80
60
40
20
0
30
40
50
60
70
80
90
0
Temperature/ C
Fig. 8 Effect of temperature on toluene oxidation.
[
[
■ ] Conversion of toluene; [●] Selectivity to benzaldehyde; [▲] Selectivity to benzyl alcohol;
▼]Selectivity to p-cresol; [♦]Selectivity to oꢀcresol; [◄]Selectivity to benzoic acid.
Reaction Condition: toluene =1g; Catalyst = 0.075g; toluene: H O mole ratio =1:3; time= 10 h.
2
2
100
80
60
40
20
0
0.00
0.05
0.10
0.15
0.20
Weight of catalyst/ g
Fig. 9 Effect of catalyst weight on toluene oxidation.
[
[
■ ] Conversion of toluene; [●] Selectivity to benzaldehyde; [▲] Selectivity to benzyl alcohol;
▼]Selectivity to p-cresol; [♦]Selectivity to oꢀcresol; [◄]Selectivity to benzoic acid.
Reaction Condition: toluene =1g; toluene: H O mole ratio =1:3; temperature = 75°C; time= 10
2
2
h.

Green Chemistry
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100
80
60
40
20
0
0
5
10
15
20
H O : Toluene molar ratio
2
2
Fig. 10 Effect of toluene: H O molar ratio on toluene oxidation.
2
2
[
[
■ ] Conversion of toluene; [●] Selectivity to benzaldehyde; [▲] Selectivity to benzyl alcohol;
▼]Selectivity to p-cresol; [♦]Selectivity to oꢀcresol; [◄]Selectivity to benzoic acid.
Reaction Condition: toluene =1g; catalyst weight = 0.075 g; temperature = 75°C; time= 10 h.
100
80
60
40
20
0
0
10
20
30
40
50
Time/h
Fig. 11 Effect of time on stream on toluene oxidation.
[
[
■ ] Conversion of toluene; [●] Selectivity to benzaldehyde; [▲] Selectivity to benzyl alcohol;
▼]Selectivity to p-cresol; [♦]Selectivity to oꢀcresol; [◄]Selectivity to benzoic acid.
Reaction Condition: toluene =1g; weight of catalyst = 0.075 g; toluene: H O mole ratio =1:3;
2
2
temperature = 75°C; time= 10 h.

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DOI: 10.1039/C3GC42369G
1
1
20
00
H O CuCr O NP
2
2/
2
4
CHO
CH3
0
MeCN/70 C/10h
80
60
40
20
0
1
2
3
4
5
6
Cycle times
Fig. 12 Recyclability tests of CuCr O spinel nanoparticles catalyst for the oxidation reaction of
2
4
toluene to benzaldehyde.
] Conversion of toluene; [ ] Selectivity to benzaldehyde; [ ] Yield of benzaldehyde.
[

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Table 1. Physicochemical properties of the CuCr O catalyst
2
4
[a]
Entry Catalyst
Cu/Cr
mole
ratio)
BET
Particle size (nm)
[
b]
(
Surface Area
XRD
TEM
2
(m /g)
1
2
CuCr
2
O
4
0.5
92
93
28
40
(fresh)
CuCr
O
4
0.5
28.2
40
2
(spent, after
5
recycle)
[
a] Estimated by ICPꢀAES, [b] measured using Scherrer equation
Table 2. Summary of the EXAFS fitting results for Cu catalysts
Catalyst
Total path
ꢃk
ꢃR
ꢃE0
[eV]
1.0 ꢀ3.6 12.2±1
Rf
ꢀ1
ꢀ1
[
10nm ] [10nm ]
[%]
ꢀ1
Path
R(10 nm)
CN
DW
ꢀ5
2
[
10 nm ]
CuCr O4 CuꢀO 1.938±0.078
2.5±0.4 3.6±0.9
3ꢀ14
3ꢀ14
2.71
2
[
Fresh
CuꢀO 1.886±0.023
4
4
4
13.0
17.0
7.3
catalyst] CuꢀO 3.140
CuꢀCr 3.245
CuCr O4 CuꢀO 1.939±0.061
2.5±0.4 3.6±0.9
1.0 ꢀ3.6 12.2±0.9 2.03
2
[
Spent
CuꢀO 1.885±0.023
4
4
4
14.7
19.1
8.2
catalyst] CuꢀO 3.139
CuꢀCr 3.243

Page 27 of 27
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DOI: 10.1039/C3GC42369G
a
Table 3 Activities of the different CuꢀCr
b
c
d
Entry
Catalyst
Solvent
C
T
(
S
P
(%)
YB
%)
ΦCHO
ΦCH2OH
Others
(%)
[
Φ = Benzene Ring]
COM
1
2
3
4
5
6
7
8
9
1
1
1
1
1
.
.
.
.
.
.
.
.
.
CuO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
COM
Cu O
ꢀ
ꢀ
ꢀ
2
COM
Cr O
ꢀ
ꢀ
ꢀ
2
3
COM
4
CuCr O
6.6
0.8
55.5
53.5
65.2
50.2
ꢀ
11.5
7.8
20.6
14.3
ꢀ
33.0
38.7
14.2
35.5
ꢀ
3.6
0.4
2
IMP
CuOꢀCr O
2
3
e
CuꢀCr O
22.5
1.2
14.7
0.6
2
3
CPM
CuOꢀCr O
2
3
No Catalyst
ꢀ
ꢀ
CuCr O NP MeCN
57.5
55.5
15.5
1.8
84.4
84.5
97.7
55.5
72.3
26.5
11.2
13.4
1.0
10.5
10.8
14.2
4.4
2.1
<1
48.5
46.8
15.1
0.9
2
4
f
0. CuCr O NP
MeCN
AcOH
DMF
2
4
1. CuCr O NP
2
4
2. CuCr O NP
34.0
16.9
59.3
2
4
3. CuCr O NP
DMSO
16.8
16.1
12.1
4.2
2
4
4. CuCr O NP
1,4ꢀ
2
4
Dioxane
n
1
5. CuCr O NP
Octane
2.2
76.5
12.5
11.0
1.6
2
4
a
Reaction conditions: solvent (acetonitrile) = 10ml, substrate (toluene) = 1g, CuCr O
2
4
nanoparticles catalyst = 0.075g, reaction temperature = 75 °C; time = 10 h; toluene: H O mole
2
2
b
ratio = 1: 2.5; C : conversion of toluene based upon the FIDꢀGC results= moles of toluene
reacted/initial moles of toluene used] x 100; S : selectivity of product calculated by total moles
of product formed/total moles of toluene converted; Y = conversion × selectivity/100; Cuꢀ
nanoclusters supported on nanocrystalline Cr O ; catalyst after 5 reuse; COM= commercial;
T
c
P
d
e
B
f
2
3
IMP = impregnation method; CPM= coꢀprecipitation method; NP = nanoparticles