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S.S. Acharyya et al. / Catalysis Communications 59 (2015) 145–150
preparation and characterization of recyclable CuCr2O4 spinel nanoparti-
cles and its catalytic application on the oxidation of hydrocarbons espe-
cially the C5–C8 cycloalkanes and cycloalkanes containing benzylic C–H
bonds following an environmentally benign oxidation protocol using
H2O2 as oxidant. A cyclohexane conversion of 70% with 85% cyclohexa-
none selectivity was achieved over this catalyst at 50 °C temperature.
The catalyst was proved to be highly efficient for the oxidation of other
cycloalkanes as well.
Instruments INC), USA] instrument-balance by heating 2.15 mg samples
at 5 °C min−1 in flowing air atmosphere. Fourier transform infra-red
(FTIR) spectra were recorded on a Thermo Nicolet 8700 (USA) instru-
ment with the operating conditions: resolution: 4 cm−1, scan: 36, oper-
ating temperature: 23–25 °C and the frequency range: 4000–400 cm−1
.
Spectra in the lattice vibrations range were recorded for wafers of
sample mixed with KBr.
2.3. Liquid phase hydroxylation
2. Experimental
Liquid phase oxidation reaction was carried out in a two neck round
bottom flask, equipped with refrigerant, containing 0.05 g catalyst,
10 ml solvent and 1 g cyclohexane to which H2O2 (50% aq. solution)
was added dropwise to prevent immediate H2O2 decomposition. The
flask was then emerged in a preheated oil bath and vigorously stirred
with a magnetic stirrer. The reaction temperature was ranged between
RT and 100 °C. Small aliquots of the sample were withdrawn from the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 analzed by Gas Chromatograph (GC,
Agilent 7890) connected with a HP5 capillary column (30 m length,
0.28 mm id, 0.25 μm film thickness) and flame ionization detector
(FID). Chem Station software was used to collect and analyze the
respective GC-data. The relative error of product determination did
not exceed 5%. The cyclohexane conversion and cyclohexanone for-
mation 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 GC–MS. The C-
balance as well as material balance was carried out for most of the
experiments. 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 liquor was then analyzed using ICP-AES.
2.1. Preparation of the catalyst
The CuCr2O4 spinel nanoparticles were prepared hydrothermally by
modifying our own preparation method taking nitrate precursors of
copper and chromium [13]. All chemicals were used without further
purification. All solvents used were of reagent grade. All syntheses
were carried out under ambient conditions. In a typical synthesis proce-
dure, an aqueous solution of 2.3 g Cu(NO3)2·3H2O (from Sigma Aldrich)
was added with vigorous stirring to 7.5 g Cr(NO3)3·9H2O (from Sigma
Aldrich) dissolved in 40 g deionized water to give a clear dark blue
homogeneous solution. By gradual addition of a few drops of ammonia
to the solution, the pH of the solution was made 8. An ethanolic solution
(10%) of 2.6 g CTAB (from Sigma Aldrich) was added dropwise followed
by the addition of 0.6 g hydrazine (from Sigma Aldrich) to the reaction
mixture. The reagents were added maintaining the molar ratio of Cu:Cr:
CTAB:H2O:hydrazine = 1:2:0.75:250:1. After stirring, the so obtained
homogeneous solution was hydrothermally treated at 180 °C for 24 h
in a Teflon-lined autoclave vessel under autogeneous pressure. The
solid product was collected by means of centrifugation at 18,000 rpm
and dried at 120 °C, for 10 h, followed by calcination at 750 °C for 6 h
in air (ramped at 1 °C/min) to get CuCr2O4 spinel nanoparticles.
2.2. Catalyst characterization techniques
Powder X-ray diffraction patterns were collected on a Bruker D8
advance X-ray diffractometer fitted with a Lynx eye high-speed strip
detector and a Cu Kα radiation source using 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° (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. Scan-
ning electron microscopy images were taken on a FEI Quanta 200 F,
using tungsten filament doped with lanthanum hexaboride (LaB6) as
an X-ray 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 dis-
persive X-ray 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 elec-
tron microscope analysis to understand the shape, size, and morphology
properties. The particle size and distribution of the samples were ana-
lyzed by TEM, JEOL JEM 2100 microscope, and samples were prepared
by mounting an ethanol-dispersed sample on a lacey carbon Formvar
coated Cu grid. X-ray photoelectron spectra were recorded on a Thermo
Scientific K-alpha X-ray photoelectron spectrometer and binding ener-
gies ( 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 analysis, the spectra were
calibrated with reference to C1s observed at a binding energy of
284.5 eV. Chemical 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 Technolohy by SII [(Seiko
3. Material balance
We have performed the C-balance 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%.
4. Results and discussion
4.1. Catalyst characterization
The crystalline phase, degree of crystallinity and phase purity were
determined by X-ray diffraction (XRD). The X-ray diffraction patterns
of the Cu–Cr catalyst (presented in Fig. 1a) showed the typical diffrac-
tion lines of the bulk, single phased CuCr2O4 spinel (JCPDS. 05-0657).
No impurity phase such as CuCrO2 and not even cubic or monoclinic
CuCr2O4 was found. The particle size was determined from the full
width half maxima of the line broadening corresponding to the diffrac-
tion angle of 35.16° by using Scherrer equation and a mean particle size
of 38 nm was observed. XRD diffractogram (Fig. 1b) also predicts that,
the catalyst retains its spinel phase even after 8 recycles. The CuCr2O4
spinel nanoparticle catalyst, prepared hydrothermally in the presence
of cetyltrimethylammonium bromide (CTAB) surfactant showed a
single-phase morphology reflecting an assembly effect of the surfactant
as imaged by SEM (Fig. 2a) and it showed the formation of almost
homogeneously distributed uniform particles with a size ~35 nm.
From SEM-EDAX image, it can be seen that the sample contains only
Cu, Cr and O (Fig. 2b). The embedment of the surfactant molecules
(CTAB) on the on the uncalcined sample and the generation of stable