R. Mistri et al. / Applied Catalysis A: General 485 (2014) 40–50
41
generally observed in most systems. Therefore, it is of great prac-
tical interest to develop a more efficient, easily separable, reusable
and thermally stable materials. These materials have attracted
much attention in both fundamental and applied research because
of their versatile applications and excellent catalytic properties
[19–26]. Binary and ternary oxides possessing a spinel structure
have attracted much attention due to their remarkable transport,
magnetic and catalytic properties. Mixed metal oxide materials are
also good alternatives to both zeolites, such as TS-1, TS-2, Ti-MCM-
alkylation reactions [27]. The catalytic effectiveness of this system
is due to the ability of the metal ions to migrate between the sublat-
tices without altering the crystal structure [28–30]. This property
tions and a number of industrial processes [31–35]. To mention a
few, styrene oxidation has been carried out over NiFe2O4, ZnFe2O4
and MgFe2O4 [36,37]. CuFe2O4 is one of the most excellent oxides
for simultaneous catalytic removal of NOx and diesel particulates
[38,39].
In the present work, we report the synthesis, characterization
and catalytic activity of copper ion substituted MAl2O4 (M = Mg,
Mn, Fe, Ni and Zn) spinel oxides. Of all the series of formulations
studied here, the Cu0.03Fe0.97Al2O4 has been found to show the
highest selective oxidation activity of cyclohexane to K–A oil than
other copper ion substituted spinel oxides using hydrogen perox-
ide in acetonitrile as the solvent under mild reaction conditions.
To the best of our knowledge, this is the first report on the synthe-
sis of copper ion substituted hercynite showing excellent oxidation
behavior.
as CuMAl3 and CuMAl5, respectively) were prepared by the com-
bustion of stoichiometric amount of the respective metal nitrates
with ODH at ∼350 ◦C in a similar manner. The Ni and Mn-samples
were black and the other two were white in color.
For comparison, we prepared CuFeAl3 (the best formulation)
by the incipient wetness impregnation (IWI) method. For the
preparation of the impregnated catalyst, the support (combustion
synthesized FeAl2O4) was first calcined in air at 400 ◦C for 3 h and
then impregnated with an appropriate volume of the aqueous solu-
tion of copper nitrate, corresponding to the support pore volume.
The sample was then dried overnight at 110 ◦C, crushed and cal-
cined at 400 ◦C for 3 h in air to get the catalyst (CuFeAl3IWI).
2.2. Characterization of materials
The synthesized materials have been characterized by XRD, N2
sorption analysis, HRTEM and XPS. X-ray powder diffraction pat-
terns were collected in a Bruker D8 Advance diffractometer (40 kV,
40 mA) and operated using CuK␣ radiation (−e1ffective wavelength
˚
1.5418 A) with a scanning time of 0.4 s step and a step size of
0.02◦ in the range 10–100◦.
The N2 sorption isotherms were measured at −196 ◦C using
Autosorb iQ-MP (Quantachrome Instruments, USA). Before each
measurement, the samples were degassed at 300 ◦C for about 7 h.
The specific surface areas were calculated using BET equation over
the pressure range of 0.3–0.08 P/P0.
Microstructural characterization by High Resolution Trans-
mission Electron Microscopy (HRTEM) was performed at an
accelerating voltage of 200 kV in a JEOL 2010F instrument equipped
with a field emission source. The point-to-point resolution was
0.19 nm, and the resolution between lines was 0.14 nm. The mag-
nification was calibrated against a Si standard. No induced damage
of the samples was observed under prolonged electron beam expo-
sure. Samples were dispersed in alcohol in an ultrasonic bath, and
a drop of supernatant suspension was poured onto a holey carbon-
coated grid. Images were not filtered or treated by means of digital
processing, and they correspond to raw data.
2. Experimental
2.1. Preparation of spinel based oxides
Synthesis of pure and copper incorporated spinels was car-
ried out employing a single step solution combustion method in
an open muffle furnace kept in a fume hood by the combustion
(C2H6N4O2 (ODH)) as the fuel. Oxalyldihydrazide was prepared
by the dropwise addition of diethyl oxalate (C2H6N4O2, Sisco
Research Laboratories Pvt. Ltd., 99%) to ice-cooled aqueous solu-
tion of hydrazine hydrate (N2H4·2H2O, Qualizens Fine Chemicals,
99%) [40].
Pure hercynite (FeAl2O4 and named as FeAl) and several cop-
per ion substituted hercynites (CuxFe1−xAl2O4 (x = 0.01, 0.03, 0.05
and 0.07) and named as CuFeAln (n = 1, 3, 5 and 7)) were pre-
pared. Specifically, the preparation of Cu0.03Fe0.97Al2O4 involved
combustion of the metal nitrates Al(NO3)3·9H2O, Fe(NO3)3·9H2O,
Cu(NO3)2·3H2O with ODH, taken in a molar ratio 2:0.97:0.03:4.485,
at the temperature of ignition of the redox mixture (∼350 ◦C). In
a typical preparation, 2 g of Al(NO3)3·9H2O (Merck India, 99%),
1.0449 g of Fe(NO3)3·9H2O (Merck India, 98%), 0.195 mL of 10%
Cu(NO3)2·3H2O (Merck India, 99%) solution and 1.4134 g of ODH
were dissolved in ∼30 mL of double distilled water in a borosil-
icate dish and then transferred to the preheated muffle furnace
for combustion. Initially the solution boils with frothing followed
by complete dehydration and then combustion took place which
was of smoldering type and was associated with a few sparks.
The combustion was completed within 2 min. The colour of the
as-synthesized material was brown that became darker with the
increase of copper loading.
Surface characterization was done with X-ray photoelectron
spectroscopy (XPS) on a SPECS system equipped with an Al anode
XR50 source operating at 150 mW and a Phoibos 150 MCD-9 detec-
tor. The pressurein the analysischamberwas always below 10−7 Pa.
The area analyzed was about 2 mm × 2 mm. The pass energy of the
hemispherical analyzer was set at 25 eV and the energy step was
set at 0.1 eV. Charge stabilization was achieved by using a SPECS
Flood Gun FG 15/40. The sample powders were pressed to self-
consistent disks. The following sequence of spectra was recorded:
survey spectrum, C 1s, Fe 2p, Cu 2p, Al 2p, Cu LMM Auger and
C 1s again to check for charge stability as a function of time and
the absence of degradation of the sample during the analyses. Data
processing was performed with the CasaXPS program (Casa Soft-
ware Ltd., UK). The binding energy (BE) values were referred to
the C 1s peak at 284.8 eV. Atomic fractions (%) were calculated
using peak areas normalized on the basis of acquisition parame-
ters after background subtraction, experimental sensitivity factors
and transmission factors provided by the manufacturer.
2.3. Catalytic test
The oxidation of cyclohexane by H2O2 was carried out from RT
to 80 ◦C at atmospheric pressure. In a typical reaction, the cat-
alyst (0.05 g), reactant (8 mmol (0.865 mL) cyclohexane (Merck
India, 99.5%)), 10 mL acetonitrile (Merck India, 99.5%) and oxi-
dant (24 mmol (2.45 mL) 30% H2O2 (Merck India)) were introduced
into a 250 mL two-necked round bottom flask. Uniform mixing
of the contents was ensured by continuous stirring (rpm = 900)
during the course of reaction by a magnetic stirrer. The reaction
Copper ion substituted other spinels of general composition
CuxM1−xAl2O4 (M = Mg, Mn, Ni and Zn; x = 0.03, 0.05 and named