Nanoclusters of Cu(ii) supported on nanocrystalline W(vi) oxide: A potential catalyst for single-step conversion of cyclohexane to adipic acid

Article abstract of DOI:10.1039/c5gc00379b

A facile hydrothermal synthesis method is developed to prepare Cu(ii) nanoclusters supported on a nanocrystalline W(vi) oxide catalyst with sizes between 35 and 65 nm. Detailed characterization of the material was carried out by XRD, XPS, SEM, TEM, STEM-mapping, Raman, FTIR, TGA, UV-visible spectroscopy and ICP-AES. The catalyst was highly active for selective oxidation of cyclohexane to adipic acid with H₂O₂. The influence of reaction parameters such as temperature, substrate to oxidant molar ratio, reaction time, etc. was investigated in detail. The reusability of the catalyst was tested by conducting the same experiments with the spent catalyst and it was found that the catalyst did not show any significant activity loss even after 4 reuses. A cyclohexane conversion of 75% with 88% adipic acid selectivity was achieved over this catalyst at 70°C temperature.

Full text of DOI:10.1039/c5gc00379b

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. S. Acharyya, S.
Ghosh and R. Bal, Green Chem., 2015, DOI: 10.1039/C5GC00379B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
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.
www.rsc.org/greenchem

Page 1 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
Nanoclusters of Cu (II) Supported on Nanocrystalline W (VI)
Oxide: A Potential Catalyst for Single-Step Conversion of
Cyclohexane to Adipic Acid
a
a
a,
Shankha S. Acharyya, Shilpi Ghosh, and Rajaram Bal *
a
Catalytic Conversion & Processes Division, CSIRꢀIndian Institute of Petroleum, Dehradun
2
48005, India
Corresponding author. Tel.: +91 135 2525917; Fax: +91 135 2660202
Eꢀmail addresses: raja@iip.res.in

Green Chemistry
Page 2 of 35
View Article Online
DOI: 10.1039/C5GC00379B
Keywords: Cu (II) nanoclusters • nanocrystalline W (VI) oxide • cyclohexane • adipic acid
selective oxidation
•
Abstract: A facile hydrothermal synthesis method is developed to prepare Cu (II)
nanoclusters supported on nanocrystalline W (VI) oxide catalyst with size between 35ꢀ65 nm.
Detailed characterization of the material was carried out by XRD, XPS, SEM, TEM, STEMꢀ
Mapping, Raman, FTIR, TGA, and ICPꢀAES. The catalyst was highly active for selective
2 2
oxidation of cyclohexane to adipic acid with H O . The influence of reaction parameters such
as temperature, substrate to oxidant molar ratio, reaction time, etc. were investigated in detail.
The reusability of the catalyst was tested by conducting same experiments with the spent
catalyst and it was found that the catalyst did not show any significant activity loss even after
4
reuses. A cyclohexane conversion of 75% with 88% adipic acid selectivity was achieved
over this catalyst at 70 ºC temperature.
1. Introduction
1
Adipic acid (AA) is one of the most important commodity chemicals worldwide. With a
projected global market size of more than 6 billion pounds by 2017, AA serve as a versatile
2
building block for an array of processes in the chemical, pharmaceutical and food industries.
1
,3
Primarily, it is used as a precursor for the synthesis of the polyamide Nylonꢀ6,6.
Additionally, AA is widely used for the production of polyester and polyurethane resins, as a
plasticizer in the production of polyvinyl chloride (PVC) and polyvinyl butyral (PVB), and as
an approved additive in cosmetics, gelatins, lubricants, fertilizers, adhesives, insecticides,
1
,3
paper and waxes. In 2013, ~3.5 million metric tons of adipic acid were produced, reflecting
4
a ~5% growth rate per year over the past 5 years. Nearly 95 % of the world wide industrial
4
production of adipic acid employed the nitric acid oxidation method. The first step is air

Page 3 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
oxidation of cyclohexane, where high temperature (125° ꢀ 165°C) and high pressure (8ꢀ15
atm) are employed to produce KA oil/“olone” (i.e., a mixture of cyclohexanone and
4
cyclohexanol) with 4 ꢀ11% conversion and ~85% selectivity. In the second step, nitric acid
is applied as an oxidant: the conversion is ~100%, and the selectivity for adipic acid is 93ꢀ
9
5% with some other shortꢀchain acids are produced as side products. Demerits of the current
industrial process involve low overall product yield, corrosion of reaction vessels by nitric
acid, emission of N O, which is ozoneꢀdepleting greenhouse gas and finally, high energy
consumption. It was estimated that ~0.3 kg of N O gas is formed per kilogram of AA
2
2
produced. After energyꢀconsuming recovery and recycling, the amount of N O gas released
2
to the atmosphere still accounts for ~5 to 8% of annual anthropogenic N O emission
2
4
worldwide. Many efforts have been devoted to developing more efficient and
environmentally friendly processes for industrial production of AA that devoid N
emission.
2
O
The hydrogen peroxideꢀbased catalytic conversion has received much attention; it has the
5
high content of active oxygen species (47 wt%) as it generates water as the sole by product.
Moreover, H
2
O
2
is cheaper and safer to use than the other peroxides and peracids; so many
as
researchers came forward to produce adipic acid by oxidation of cyclohexane, using H
2
O
2
oxidant. The first successful oxidation of cyclohexene to AA was reported in a seminal
contribution by Sato et al., who developed a biphasic system using 30% H O in the presence
2
2
of Na
2
4 3 8 17 3 4
WO as a homogeneous catalyst and [CH (nꢀC H ) N]HSO as a phaseꢀtransfer
6
catalyst. An AA yield of 90% was furnished after 8 h. Freitag et al. then improved this
biphasic system using an Na
2 4
WO catalyst and microwave radiation, reducing the reaction
7
time to 90 min with 68% yield. Bohstro¨m et al. demonstrated the conversion of cyclohexene
8
to AA in aqueous dispersions of mesoporous oxides with builtꢀin catalytic WO sites.
3
Several other tungstateꢀbased catalysts have been reported as well, including oxoꢀtungstenꢀ

Green Chemistry
Page 4 of 35
View Article Online
DOI: 10.1039/C5GC00379B
9
10
11
containing SBAꢀ15, [BMIm]
2
WO
4
supported on silica sulphamic acid,
H
3
PW12
O
40 and
12
13
combinations of Na WO with H
2
4
2
SO
4
2 4
or H WO with acidic resins, thus emphasizing the
remarkable catalytic efficiency of tungstate in this oxidation reaction. Lee et al. explored
titanium frameworkꢀsubstituted aluminophosphate (TAPOꢀ5) as another catalyst for the
1
4
oxidation of cyclohexene to AA. Chavan et al. have used a Co/Mn cluster complex as a
1
5
catalyst to convert cyclohexanone to AA in acetic acid medium with a selectivity of 86%.
Recently, our group reported conversion of cyclohexene to AA with very high yield (~94%)
16
over silver supported tungsten oxide nanorod catalysts using H
2 2
O as oxidant. Although
17
there are several reports on preparation of AA by various processes from various substrates,
Production of AA by direct oxidation of cyclohexane has been rarely reported; moreover,
none of the reported processes have been developed, largely because either high reaction
temperatures have been employed or poor selectivity of AA was observed, despite higher
conversion, or severe leaching of the catalyst was observed. Therefore, there is substantial
interest in the development of more efficient, readily separable, reusable catalysts for the
catalytic oxidation of cyclohexane.
18
Supported Cu (II) species employed catalytically in many important reactions and the
catalytic properties of the active copper phase can be greatly influenced by the nature of the
19
support and the dispersion of the active component. We reported that Cu (II) nanoclusters
supported on nanocrystalline Cr catalyst efficiently converts cyclohexane to
cyclohexanone with high yield at room temperature, but the catalyst failed to show any
2
O
3
20
activity in AA production by direct oxidation of cyclohexane, even at drastic condition. In
the path of our research, we synthesized Cu (II) nanoclusters supported on nanocrystalline W
(VI) oxide with size 25ꢀ65 nm in hydrothermal process, promoted by cationic surfactant

Page 5 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
cetyltrimethylammonium bromide. To our delight, this catalyst accomplished >88%
conversion of cyclohexane with >75% AA selectivity by virtue of Csp3ꢀCsp3 bond rupture.
2
. Experimental
.1 Materials
2
Hydrogen Peroxide (50 wt % in water) was purchased from Merck KGaA, Darmstadt,
Germany. Ammonium metatungstate, hydrate, anhydrous cupric chloride,
cetyltrimethylammonium bromide (CTAB), hydrazine (80% aq. solution), ammonium
hydroxide, cyclohexane (purity > 99.9%), acetonitrile (HPLC grade) were purchased from
SigmaꢀAldrich Co. All the chemicals were used without further purification.
2
.2 Synthesis of the catalyst
Cu (II) nanoclusters supported on nanocrystalline WO
process, taking CuCl and (NH ) H W O • xH O as precursor of Cu and W respectively.
3
was prepared modifying our own
20
2
4 6
2
12 40
2
In a typical procedure 5. 3 g (NH ) H W O • xH O and 0.40 g of CuCl were dissolved in
4
6
2
12 40
2
2
4
16 g deionized water. Then the solution was made basic to a pH range of 8 using NH OH
solution dropwise. After that, a transparent and homogeneous precursor solution formed, 0.8
g CTAB was added into the solution. After the solution has been stirred for half an hour,
aqueous hydrazine solution (0.4 g) was added to it. All the reagents were added maintaining
the molar ratio: Cu: CTAB: H O: N H = 1: 0.75: 300: 1. The resulting precursor solution
2
2
4
was transferred into a Teflonꢀlined autoclave with a stainless steel shell. The autoclave was
sealed and maintained at 180 °C for 24 h. After cooling naturally to room temperature, the
white products were collected after centrifuging, washed sequentially with distilled water and
absolute ethanol to remove ions possibly remnant in the products, and dried in an oven at

Green Chemistry
Page 6 of 35
View Article Online
DOI: 10.1039/C5GC00379B
100°C for overnight. Finally, the white powders were undergone calcinations under oxygen
atmosphere at 550°C for 6h to get canaryꢀyellow products (catalyst).
2.2 Catalyst characterization techniques
Xꢀray power diffraction (XRD): Powder Xꢀray diffraction patterns were collected on a
Bruker D8 advance Xꢀray diffractometer fitted with a Lynx eye highꢀspeed strip detector and
α
a Cu K radiation sourceusing Cu Ka radiation with a wavelength of 1.5418 Å. Diffraction
°ꢀ
°
patterns in the 2 80 region were recorded at a rate of 0.5 degrees (2θ) per minute. The
resulting XRD profiles were analyzed to identify the crystal phase of the compound using
reference standards. The line width of the most intense XRD peak was taken for estimation of
crystallite size by the Scherrer equation. Scanning electron microscopy (SEM): Scanning
electron microscopy images were taken on a FEI Quanta 200 F, using tungsten filament
6
doped with lanthanum hexaboride (LaB ) as an Xꢀray 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. Samples were
subjected to scanning electron microscope analysis to understand the shape, size, and
morphology properties. Transmission electron microscopy (TEM): The particle size and
distribution of the samples were analyzed by TEM, JEOL JEM 2100 microscope, and
samples were prepared by mounting an ethanolꢀdispersed sample on a lacey carbon Formvar
coated Cu grid. Xꢀray photoelectron spectroscopy (XPS): XꢀRay photoelectron spectra were
recorded on a Thermo Scientific KꢀAlpha XꢀRay photoelectron spectrometer and binding
energies (±0.1 eV) were determined. The resulting spectra were analyzed to identify the
different oxidation states of the copper and chromium ions present in the sample. Prior to the

Page 7 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
analysis, the spectra were calibrated with reference to C1s observed at a binding energy of
84.5 eV. Inductively coupled Atomic Absorption Spectroscopy (ICPꢀAES): Chemical
2
analyses of the metallic constituents were carried out by Inductively Coupled Plasma Atomic
Emission Spectrometer; model: PS 3000 uv, (DRE), Leeman Labs, Inc, (USA).
Thermogravimetric Analyses (TGA) of the uncalcined catalyst were carried out in a Pyris
Diamond, Perkin Elmer Instruments and Technology by SII (Seiko Instruments INC), USA]
°
ꢀ1
instrumentꢀbalance by heating 2.15 mg samples at 5 C min in flowing air atmosphere.
Raman spectra was measured at 298K by using a Laser Raman Spectrometer (JASCO, NRSꢀ
3100) with the 532 nm line from a diodeꢀpumped solidꢀstate laser for excitation. Fourier
transform infraꢀred (FTꢀIR) spectra were recorded on a Thermo Nicolet 8700 (USA)
ꢀ1
instrument with the operating conditions: resolution: 4 cm , scan: 36, operating temperature:
ꢀ1
2
3–25 °C and the frequency range: 4000ꢀ400 cm . Spectra in the lattice vibrations range
were recorded for wafers of sample mixed with KBr.
2
.3 Reaction setup
Liquid phase oxidation reaction was carried out in a two neck round bottom flask, equipped
with refrigerant, containing 0.1 g catalyst, 10 ml solvent and 1 g benzene to which H
50% aq. solution) was added dropwise to prevent immediate H decomposition. The flask
2
O
2
(
2 2
O
was then emerged in a preheated oil bath and vigorously stirred with a magnetic stirrer. The
reaction temperature was ranged between RT and 90 °C. 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 Chromatograph (GC, Agilent 7890) connected with a HP5 capillary
column (30m length, 0.28 mm id, 0.25 µm film thickness) and flame ionisation detector
(FID). Chem Station software was used to collect and analyze the respective GCꢀdata. The
relative error of product determination did not exceed ±5%. The benzene conversion and

Green Chemistry
Page 8 of 35
View Article Online
DOI: 10.1039/C5GC00379B
phenol formation were calculated using a calibration curve (obtained by manual injecting the
authentic standard compounds). An anisole solution with a known amount was used as an
external standard for analysis. 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. For the reusability test, the catalyst was
repeatedly washed with acetonitrile and acetone and dried overnight at 110 °C and used as
such, without regeneration. In order to check the metal leaching the mother liqor was then
analyzed using ICPꢀAES.
2.4 Material Balance
We have performed the Cꢀbalance for the most of the experiments and have also done the
material balance for few experiments. The estimated error in analysis arising due to sampling
and handling losses was + 5%.
3. Results and discussion
3.1 Generation of CuW Nanoparticles
It is believed that, the precursors of copper and tungsten ionizes and counter ions diffuse into
the aqueous solution, maintaining the homogeneity of the medium. When concentration of
2
+
6+
the micelles was less than that of critical micelle concentration (CMC), Cu and W ions
–
22
(
hydrated) are preferentially located at the micellar surface due to presence of Br ions. The
growth of the particles occurs along the side of the particles, which is exposed to water.
When concentration of the surfactant becomes > CMC, the aqueous medium is largely
populated by surfactant micelles. The Cu and Wꢀprecursor ions approach towards the
micellar headgroup region. In this situation, growth of the particles in a particular direction is
prohibited due to the micellar steric effects and the nanoparticles are allowed to grow to form

Page 9 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
spherical nanoparticles with limited size. Basic medium influences the role of CTAB and
23
hydrazine prohibits Ostwald ripening process to a certain extent.
3.2 Catalyst characterization
The Xꢀray diffraction analyses (XRD) of the catalyst was carried out to determine the
crystallite phase of the CuꢀW nanocomposites as shown in Fig. 1. Sharp peaks at 2θ = 23.1º,
3
23.7º, 24.4º, 33.3º and 34.0º, confirm the formation of monoclinic WO (JCPDS No. 43ꢀ1035,
space group: P21/n). In addition, XRD reflections with comparatively low peak strength due
to crystalline CuO (monoclinic) were also noticed at 2θ = 35.57°, 38.75°, 48.7°, 68.2° etc.
[
JCPDS card no. 89ꢀ2530]. Applying DebyeꢀScherrer equation, the average crystallite sizes
of the particles was determined to be 48 nm (based on 2θ = 23.1°) which is actually near the
size of the crystallites as seen under HRTEM imaging. Moreover, other crystalline byꢀ
products or other phases of copper/tungsten oxides were not detectable by XRD, indicating
the high purity of our synthetic procedure.
XPS was utilized to detect the surface composition and the chemical state of the catalyst. The
XPS binding energies (BE) of some characteristic core levels of Cu and W in the CuꢀW
samples have been presented (Fig. 2 & 3 respectively). The Cu 2p XPS results indicate that
the binding energy of the Cu 2p electron in the sample, and the peaks center at approximately
24
9
35.7 eV corresponding to Cu2p3/2. The existence of satellite peaks in the spectra suggests
that copper exists in the samples as Cu (II). The value of the binding energy (~935.7 eV) is
much greater than that of pure CuO (933.8 eV), which indicates that the Cu (II)ꢀspecies are
surrounded by tungsten oxide. The W 4f5/2 and 4f7/2 spectra attributed to the binding energies
37.9 eV and 35.8 eV respectively suggesting that the tungsten in the tungsten oxide sample

Green Chemistry
Page 10 of 35
View Article Online
DOI: 10.1039/C5GC00379B
+6 16, 25
exists as W .
XRD and XPS data support the fact that, the main phase is comprised of
monoclinic W (VI) oxide and the supported metal is of Cu (II) oxide.
To further investigate the surface property and to detect subtle phase information of the
composite, we conducted Raman (Fig. 4) and FTIRꢀspectrum (Fig. 5) analyses of the sample.
ꢀ1
Generally, the 950–1050 cm Raman wavenumbers of the transition metal oxide are assigned
to be the symmetric stretching modes of metal and oxygen bonds [short terminal W=O,
ꢀ1
υ
s
(W= O) terminal bands], and 750–950 cm bands were either the antisymmetric stretching
of W–O–W bonds [υ (W–O–W)] or symmetric stretching of–O–W–O– bonds [υ (–O–W–O–
s
s
26
)
]. The Raman spectrum of the Cu/WO NP catalyst detected vibrational peaks at 806, 718,
3
ꢀ1
ꢀ1
6
86, 326, 274, 134, 76 and 60 cm . The two main intense peaks at 806 and 718 cm , and the
ꢀ1
3
shoulder at 686 cm , are typical Raman peaks of crystalline WO , which correspond to the
26
stretching and bending vibrations of the bridging tungsten and oxygen atoms. They are
assigned to be the W–O stretching (υ), W–O bending (δ) and O–W–O deformation (γ)
ꢀ1
modes, respectively. Two peaks at 326 and 274 cm are assigned to be the bending δ(O–W–
26
ꢀ1
26
O) vibrations. Those below 200 cm modes were attributed to the lattice vibrations.
This assignment is tentative and requires further confirmation by FTIR measurements (Fig.
ꢀ1
+
5
). The peaks of the uncalcined sample at 812, 1062 cm can be assigned to the CꢀN
26d,27
ꢀ1
stretching modes of CTAB molecules.
The peak at 1378 and at 1462 cm are assigned to
+
symmetric mode of vibration of the head groups of the methylene moiety (N ―CH
3
) and
2
ꢀ1
ꢀ1
CH
CH
2
2
scissoring mode respectively. The frequencies above 1600 cm to 3000 cm are due to
symmetric and antisymmetric vibrations, respectively. It is to be noted that, the shift of
vibrations to lower frequency occurred as the alkyl chains experienced a more hydrophobic
environment in CuꢀW blocks upon the surface of which the CTAꢀmoieties were supposed to

Page 11 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
26d
be bound. It can be inferred that, the mutual interactions between CTAB and the CuꢀW
ꢀ1
surface have taken place. Some bands were detected at 2800–3020 cm , which can be
attributed to the CTAB surfactant. The FTIR spectrum of CTAB shows two intense bands at
ꢀ1
2
918 and 2846 cm , corresponding to the asymmetric and symmetric stretching vibrations of
ꢀ1
C–CH in the methylene chains. The sharp bands at 1450–1500 cm were specified as the
1
deformation of –CH
2
– and –CH , and the weak band at 3011 cmꢀ as the C–CH
3
asymmetric
26d
stretching and N–CH symmetric stretching vibrations of the solid surfactant. These typical
frequencies were absent when the material was calcined at 500 °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. In the crystalline structure
3 6
of WO , W atoms are located in the centre of WO octahedra with O at the vertices forming
28
WꢀOꢀW connections. For such an arrangement, the IR active bands are fundamental
vibrations of W=O, WꢀO and WꢀOꢀW. These can be stretching (υ) or inꢀplane bending
29
vibrations (δ) and outꢀofꢀplane wagging (γ). The main WO vibrations are found in the
3
ꢀ1
1
700–380 cm IR region. All synthesis temperatures present characteristic peaks at 570, 800,
1
8
95, 964, 1045, 1404 and 1608 cmꢀ associated with γ(WꢀOꢀW), υ(WꢀOꢀW), υ(WꢀOꢀW),
υ(WꢀO, W=O), δ(WꢀOH), υ(OH) and δ(OH) in WꢀOH respectively. The ꢀOH bands are
associated with surface hydroxyl groups and weakly bound solvent, ethanol/water. A stronger
ꢀ1
broad band around 3350 cm should appear from a WꢀOH mode if intercalation of H O had
2
29
ꢀ1
occurred. The band at 517 cm is also marked with asymmetric line broadening indicating
ꢀ1
the presence of a band at 500 cm , which is the characteristic band of Cu–O. The three
ꢀ1
absorption peaks at 601, 508 and 432 cm are assigned to the A
u
u
mode and 2B modes,
ꢀ1
respectively. The peak at 601 cm is likely to be from CuꢀO stretching along the [ꢁ101]
ꢀ1
direction and the peak at 508 cm is likely to arise from CuꢀO stretching along the [101]

Green Chemistry
Page 12 of 35
View Article Online
DOI: 10.1039/C5GC00379B
30
direction. Moreover, it was also confirmed from the FTIR diagram that, there was no
structural deformation in the catalyst even after 4 recycles.
The topology of the catalyst was studied by scanning electron microscopy (SEM, Fig. 6)
which showed a typical sample composed of throughout uniform spherical peaꢀshaped
nanoparticles in the range of 35ꢀ65 nm. SEM images revealed that, all the nanoparticles
organized into spherical assemblies, owing to the structure directing effect of the template.
SEMꢀEDX analysis of the composite revealed that, there appears a distribution of Cu, W and
O only, and no sort of C or Br, which are visualized from the SEMꢀEDX image of the
uncalcined sample (Fig. S1, ESI†). This observation indicated the complete removal of the
structureꢀdirecting template (s). This experimental finding was further supported from TGA
analysis of the unalcined catalyst (Fig. S2, ESI†). It was also speculated from SEM diagram
that, increment in Cu loading spoils the uniformity of the samples and the particle size of the
catalyst also increases (Fig. S3, ESI†).
However, transmission electron microscopy (TEM) image revealed that the sample is
comprised of several spherical peaꢀshaped particles (Fig. 7) with 55 nm in size in average. To
get further information about the catalyst structure, a representative high resolution TEM
(
HRTEM) image taken from a nanoparticle is shown in Fig.7c. The lattice fringes with a dꢀ
spacing of 0.23 nm, corresponding to the spacing of the [111] planes of monoclinic CuO is
conveniently discriminated from that of the [020] planes of monoclinic WO with a dꢀspacing
Typically, the dispersion of Cu, W and O atoms in the catalyst were also
3
1
6,24,31
of 0.30 nm .
analyzed by STEMꢀelemental mapping (Fig. 8). It indicated that each of Cu, W and O species
was homogeneously dispersed.

Page 13 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
The UV–vis absorbance for Cu/WO
3
NP catalyst (Fig. 9) shows an absorption band in the
200–450 nm. No absorption band in the visible range was detected. The chargeꢀtransfer (CT)
2
+
bands give us an idea of the environment in the neighborhood of Cu ions. According to the
literature, the band at 210ꢀ270 nm indicates the O ligandꢀtoꢀmetal chargeꢀtransfer (LMCT)
transition, where the Cu ions occupy isolated sites over the support. The band at 355 nm
indicates the formation of (CuꢀOꢀCu) clusters in a highly dispersed state, which is not
24
detectable by XRD.
The corroboration of all the catalystꢀcharacterization data led to the conclusion that, the main
phase of the catalyst is comprised of monoclinic W (VI) oxide, whereas a small amount of
CuO is dispersed (as active component) on this phase of the catalyst surface.
3.3 Catalytic activity
Table 1 shows the catalytic activity of differnt catalysts in direct AA production from
cyclohexane. Blank reaction was performed (Table 1, entry 1) and we noticed very poor
conversion of cyclohexane with cylohexanone as the only product. This indicated the
necessity of catalyst. Notably, commercial CuO and WO catalyst did not show any activity
3
(Table 1, entry2, 3). Conventional catalyst prepared by impregnation method also showed
negligible activity (Table 1, entry 4). The poor catalytic activity of the impregnated catalyst
may be attributed to their irregular shape and larger particle size, which limits the
accessibility of the catalyst towards the reacting substrates. We introduced our so prepared
Cu/WO nanoparticles catalyst and we noticed 66% yield of AA. Various reaction parameters
3
that regulated the yield of AA were studied (like temperature, time etc.). It was found that

Green Chemistry
Page 14 of 35
View Article Online
DOI: 10.1039/C5GC00379B
temperature has prominent effect on cyclohexane conversion. The catalyst showed negligible
conversion at RT but with increasing temperature, the conversion of cyclohexane increased
rapidly (Fig. 10), possibly because initially produced cyclohexanol, cyclohexanone and
cyclohexane diol were compelled to undergo further reaction to produce AA. 70 °C was
proved to be the optimum reaction temperature. With further increment in temperature
(
90°C), we noticed that, conversion of cyclohexane decreases, probably due to the fact that,
decomposes rapidly in this temperature. When cyclohexane : H molar ratio was low
1: 1), we noticed very poor conversion of cyclohexane. The optimum cyclohexane : H
molar ratio was proved to be 1: 3; increment of H O concentration (Fig. 11) lead to the
H
2
O
2
2 2
O
(
2 2
O
2
2
decrement in AA yield, probably due to the fact that, decomposition of H O produced excess
2
2
water in the reaction mixture and thereby contact of cyclohexane (immiscible in water),
catalyst and H (highly soluble in water) is interrupted. Maintaining all the optimum
2
O
2
reaction conditions, when the reaction was allowed to run for hours (Fig.12), we noticed
increment of AA yield with time. After 12h, we didnot notice any appreciable increment of
AA yield reflecting the fact that the unreacted H O decomposed after 12h.
2
2
It was also noticed that, higher loading of Cu decreased the yield of AA, probably due to the
fact that loading of Ag lead to the formation of Agꢀagglomerates (as confirmed from SEM
diagram, Fig S3, ESI†), that decreased the active metal dispersion and catalystꢀefficacy.
In the light of the above experimental findings, we can draw the hypothesis that, Cu (II) is the
reactive species in the catalyst and the reaction proceeds through freeꢀradical mechanistic
pathway. We believe that, Cu (II) in the catalyst framework interacts with the C–H bond of
cyclohexane and probably, the flexibility property of the Cu (II)ꢀframework renders the

Page 15 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
catalytic system suitable for lowering the activation energy of the oxidation reaction. At
normal condition, H does not interact with cyclohexane. H dissociation is believed to
occur homogeneously over Cu/WO nanoparticles catalyst and follows the mechanism
proposed by Kazarnovsky. Dissociation of H O over the catalyst generates active •OH free
2
O
2
2 2
O
3
33
2
2
34
radicals that are stabilized by the solvent acetonitrile to a great extent. The electronegativity
of C is 2.55 and that of H is 2.2. Relatively, electron density is accumulated near Cꢀatom
rather than H atom. We believe that, the negative charge density generated on Cꢀatom is
localized by Cu (II) species present in the catalyst. The generated •OH radicals behave as
3
5
electrophiles and proceed towards the Cꢀatom bearing partial negative charge.
Consequently, a homolytic cleavage of the C–H bond occurs to form cyclohexyl radicals. The
concomitantly formed cyclohexyl radicals react with excess •OH radicals to produce
cyclohexanol and the latter gets oxidized to form cyclohexanone and cyclohexanediol over
2 2
the catalyst surface. In presence of excess H O , the Csp2ꢀCsp2 bond rupture takes place and
36
AA is produced (Scheme 1).
Commercial CuO or WO and even CuO supported on WO prepared by impregnation
3
3
method (Fig. S4, ESI†) didnot show any appreciable activity in oxidation of cyclohexane to
AA. These experimental findings can be attributed to the large and nonꢀuniform and irregular
shapes of these catalysts, which limit their catalytic activities. Moreover, nanoclusters of
Cu(II) oxide supported on nanoꢀcrystalline WO3 prepared in our method showed
extraordinary performance in formation of AA from cyclohexane, owing to the synergistic
interaction displayed between these two species in the catalyst.
3.4 Reusability test
At the end of the reaction, the catalyst was filtered in the hot condition and was the resulting
filtrate was independently analyzed by inductively coupled plasma (ICP) (and atomic

Green Chemistry
Page 16 of 35
View Article Online
DOI: 10.1039/C5GC00379B
2
+
absorption spectroscopy for free or dissolved metal ions. Only trace amounts of Cu (<3
ppb) were detected; which indicated the fact that, the catalyst is almost devoid of leaching
2
+
properties; this may be attributed by the fact that, Cu remains strongly anchored upon WO
3
support. Recycling and reusability of the catalyst was examined by introducing the used
catalyst subsequently 4 times to carry out the catalytic oxidation. The reusability of the
catalyst was studied without regeneration of the catalyst in the same experimental condition.
Before each recycle, the catalyst was recovered facilely from the reaction mixture (after 6h)
by centrifugation, repeatedly washed with ethanol and acetone and kept for drying at 120 °C
for overnight. The catalyst did not show any significant loss in its activity even after 4
successive runs (entry 6, Table 1 & Fig. 13) and the catalyst was proved to be truly of
heterogeneous nature.
Acknowledgements
SSA and SG thank CSIR and UGC, New Delhi, India, for their respective fellowships. R.B.
thanks CSIR, New Delhi, for financial support in the form of the 12 FYP Project (CSCꢀ 0125,
CSCꢀ 0117). The Director, CSIRꢀIIP is acknowledged for his help and encouragement. The
authors thank ASD Division, IIP for the analytical services. We gratefully acknowledge Ms.
Nishita Lucas, CSIRꢀNCL, Pune (India), for performing Raman spectroscopy analyses.
References
1
. M. T. Musser, Adipic Acid, Ullmann's Encyclopedia of Industrial Chemistry, 2000,
DOI: 10.1002/14356007.a01_269.
2. Adipic Acid: A Global Strategic Business Report, 2012, Global Industry Analysts, Inc.,
USA.
Available
electronically
at
http://www.strategyr.com/adipic_acid_market_report.asp.

Page 17 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
3
4
5
. A. Castellan, J. C. J. Bart and S. Cavallaro, Catal. Today, 1991, 9, 237.
. K. C. Hwang and A. Sagadevan, Science, 2014, 346, 1495.
. K. Kamata, K. Yonehara, Y. Nakagawa, K. Uehara and N. Mizuno, Nat. Chem., 2010, 2,
78.
4
6
7
8
9
. K. Sato, M. Aoki and R. Noyori, Science, 1998, 281, 1646.
. J. Freitag, M. Nüchter and B. Ondruschka, Green Chem., 2003, 5, 291.
. Z. Bohstrȍm, I. RicoꢀLattes and K. Holmberg, Green Chem., 2010, 12, 1861.
. C.Y. Cheng, K.J. Lin, M. R. Prasad, S. J. Fu, S.Y. Chang, S. G. Shyu, H.S. Sheu, C. H.
Chen, C. H. Chuang and M.T. Lin, Catal. Commun., 2007, 8, 1060.
10. M. Vafaeezadeh, M. M. Hashemi and M. ShakourianꢀFard, Catal. Commun., 2012, 26,
5
4.
1
1
1
1. K. Fujitani, T. Mizutani, T. Oida and T. Kawase, J. Oleo Sci., 2009, 58, 37.
2. P. Jin, Z. Zhao, Z. Dai, D. Wei, M. Tang and X. Wang, Catal. Today, 2011, 175, 619.
3. H. Wei, H. Li, Y. Liu, P. Jin, X. Wang and B. Li, ACS Appl. Mater. Interfaces, 2012, 4,
4
106.
14. S. O. Lee, R. Raja, K. D. M. Harris, J. M. Thomas, B. F. G. Johnson and G. Sankar,
Angew. Chem., 2003, 42,1520.
1
5. S. A. Chavan, D. Srinivas and P. Ratnasamy, J. Catal., 2002, 212, 39.
6. S. Ghosh, S. S. Acharyya, S. Adak, L. N. S. Konathala, T. Sasaki, and R. Bal, Green
Chem., 2014, 16, 2826.
1
1
7. S. V. de Vyver and Y. Roma´nꢀLeshkov, Catal. Sci. Technol., 2013, 3, 1465.
8. (a) V. Z. Fridman and A. A. Davydov, J. Catal., 2000, 195, 20; (b) T. W. Kim, M. W.
Song, H. L. Koh and K. L. Kim, Appl. Catal. A, 2002, 210, 35; (c) G. Centi and S.
Perathoner, Appl. Catal. A, 1995, 132, 179. (d) M. Matsuoka, W. Ju, K. Takahashi, H.
Yamashita and M. Anpo, J. Phys. Chem. B, 2000, 104, 4911.
1

Green Chemistry
Page 18 of 35
View Article Online
DOI: 10.1039/C5GC00379B
19. K. I. Choi and M. A. Vannice, J. Catal., 1991, 131, 22.
20. B. Sarkar, P. Prajapati, R. Tiwari, R. Tiwari, S. Ghosh, S. S. Acharyya, C. Pendem, R. K.
Singha, L. N. S. Konathala, J. Kumar, T. Sasaki and R. Bal, Green Chem., 2012, 14,
2
600.
2
1. S. S. Acharyya, S. Ghosh, R. Tiwari. B. Sarkar, R.K. Singha, C. Pendem, T. Sasaki and
R. Bal, Green Chem.,2014, 16, 2500.
22. S. De and S. Mandal, Colloids Surf., A, 2013, 421,72.
23. S. S. Acharyya, S. Ghosh, S. Adak, T. Sasaki and R. Bal, Catal. Sci. Technol.,2014, 4,
4
232.
4. K. V. R. Chary, G. V. Sagar, D. Naresh, K. K. Seela and B. Sridhar J. Phys. Chem. B,
005, 109, 9437.
5. (a) F. Wang, J. Xu, X. Q. Li, J. Gao, L. P. Zhou and R. Ohnishi, Adv. Synth. Catal., 2005,
2
2
2
3
47, 1987; (b) S. Ghosh, S. S. Acharyya, D. Tripathi and R. Bal, J. Mater. Chem. A,
014, 2,15726.
2
2
6. (a) M. F. Daniel, B. Desbat, J. C. Lassegues, B. Gerand, M. Figlarz, J. Solid State Chem.,
987, 67, 235; (b) C. Santato, M. Odziemkowski, M. Ulmann, and J. Augustynski, J. Am.
1
Chem. Soc., 2001, 123, 10639; (c) I. M. Szilágyi, J. Madarász, G. Pokol, P. Király, G.
Tárkányi, S. Saukko, J. Mizsei, A. L. Tóth, A. Szabó, K. VargaꢀJosepovits, Chem.
Mater., 2008, 20, 4116; (c) O. Yayapao, T. Thongtem, A. Phuruangrat, S. Thongtem, J.
Alloys and Compounds, 2011, 509, 2294.
2
7. (a) W. Cheng, S. Dong and E. Wang, Langmuir, 2003, 19, 9434; (b) S. S. Acharyya, S.
Ghosh and R. Bal, ACS Appl. Mater. Interfaces 2014, 6, 14451.
28. M. GimenoꢀFabra, P. Dunne, D. Grant, P. Gooden and E. Lester, Chem. Eng. J., 2013,
2
26, 22.

Page 19 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
2
9. (a) I. Hargitta, M. Hargitta, V. Spiridon, E. V. Erokhin, J. Mol. Struct., 1971, 8, 31; (b) J.
Pfeifer, G. F. Cao, P. Tekulabuxbaum, B. A. Kiss, M. Farkasjahnke, K. Vadasdi, J. Solid
State Chem., 1995, 119, 90.
30. K. Kliche and Z. V. Popovic, Phys. Rev. B: Condens. Matter, 1990, 42, 10060.
31. S. S. Acharyya, S. Ghosh and R. Bal, R. Chem. Commun. 2014, 50, 13311.
32. E. Roduner, W. Kaim, B. Sarkar, V.B. Urlacher, J. Pleiss, R. Gläser, W.D. Einicke, G.A.
Sprenger, U. Beifuß, E. Klemm, C. Liebner, H. Hieronymus, S.F. Hsu, B. Plietker, S.
Laschat, ChemCatChem, 2013, 5, 82.
33. I. A. Kazarnovsky, Dokl. AN SSSR, vol. 221, 1975. S353 (in Russian).
34. S.S. Acharyya, S. Ghosh, R. Bal, Chem. Commun, 2014, 50, 13311.
35. S. S. Acharyya, S. Ghosh, S. Adak, D. Tripathi, and R. Bal Catal. Commun., 2015, 59,
1
45.
36. A. Corma and H. Garci´a, Chem. Rev., 2002, 102, 3837.

Green Chemistry
Page 20 of 35
View Article Online
DOI: 10.1039/C5GC00379B
(
a)
b)
(
(c)
(d)
(e)
(f)
20
40
60
80
0
2θ/
Fig. 1 XRD diffractogram of (a) fresh and (b) spent (after 4 recycles) Cu/WO nanoparticles
3
catalyst and that of (c) commercial Cu (I) oxide, (d) commercial Cu (II) oxide, (e)
commercial W (IV) oxide, and (f) commercial W (VI) oxide.

Page 21 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
2000
1500
1000
935.7
9
44.4
500
0
9
50
945
940
935
930
Binding Energy/ eV
3
Fig. 2 Cu 2p3/2 coreꢀlevel spectra of fresh Cu/WO nanoparticles catalyst.

Green Chemistry
Page 22 of 35
View Article Online
DOI: 10.1039/C5GC00379B
50000
40000
30000
20000
10000
0
36.2
3
8.3
40
38
Binding energy/eV
36
34
3
Fig. 3 W4f7/2 and W4f5/2 coreꢀlevel spectra of fresh Cu/WO nanoparticles catalyst.

Page 23 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
807
200
1
34
8
8
272
718
326
(a)
(b)
250
500
750
1000
1250
-1
Raman shift/cm
3
Fig. 4 Raman diagram of (a) fresh and (b) spent Cu/WO nanoparticles catalyst.

Green Chemistry
Page 24 of 35
View Article Online
DOI: 10.1039/C5GC00379B
517
432
6
01
(a)
9
64
1404
5
08
1608
1045
5
70
3350
800
895
(b)
3020
1062
2846
(c)
1600
1
378
2800
1500
1
462
812
3000
2918
(d)
3500 3000 2500 2000 1500 1000
500
-1
Frequency/cm
3
Fig. 5 FTIR diagram of (a) fresh and (b) spent (c) uncalcined Cu/WO nanoparticles catalyst
and (d) that of CTAB surfactant.

Page 25 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
a
b
3
Fig. 6 (a) SEM image and (b) SEMꢀEDAX image of Cu/WO nanoparticles catalyst.

Green Chemistry
Page 26 of 35
View Article Online
DOI: 10.1039/C5GC00379B
a
c
b
0
.23 nm
CuO
d
[
111]
0
.38 nm
WO3
[020]
2
nm
3
Fig. 7 TEM image of (aꢀc) fresh and that of (d) spent Cu/WO nanoparticles catalyst.

Page 27 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
a
c
b
d
Fig. 8 (a) STEM image and STEMꢀelemental mapping of (b) Cu, (c) W and (d) O atoms in
the fresh Cu/WO nanoparticles catalyst.
3

Green Chemistry
Page 28 of 35
View Article Online
DOI: 10.1039/C5GC00379B
(
a)
b)
(
200
300
400
500
600
Wavelength/nm
Fig. 9 UVꢀvisible diagram of (a) Cu/WO nanoparticles catalyst and (b) commercial WO .
3
3

Page 29 of 35
Green Chemistry
View Article Online
DOI: 10.1039/C5GC00379B
1
00
80
60
40
20
0
30
45
60
75
90
0
Temperature/ C
Fig. 10 Effect of temperature on cyclohexane oxidation.
■ ] Conversion of toluene; [●] Selectivity to AA; [▲] Yield of AA;
Reaction Condition: cyclohexane =1g; catalyst = 0.15g; cyclohexane: H O molar ratio =1:3;
[
2
2
time= 12 h.