R.Y. Pawar et al. / Applied Catalysis A: General 478 (2014) 129–137
131
2.2. Characterization of catalysts
f
+
+
* - Fe-O
$
*
$ - Sr-O
# - Ca-O
The thermal decomposition of the precursor studied by thermo
gravimetric analysis (TG) is performed on a Shimadzu TG/DTA-
60H system with ramp rate of 10 ◦C per minute in flowing air.
The stretching vibration frequencies of the precursor and the cata-
lyst are studied by FTIR (Fourier transform infrared) spectroscopy
on a Shimadzu FTIR-8400 spectrophotometer using the KBr pellet
method. The phases of the synthesized catalysts were characterized
by X-ray diffraction (XRD) using a D-8 Advance Bruker Axs, X-ray
powder diffractometer equipped with a position sensitive detector,
e
d
c
+ - CO3--
#
$
*
+
$
+
#
#
*
b
a
+
$
*
scanning angle ranging from 10◦ to 900◦ at a scan rate 1◦ min−1
.
The mean crystallite size was estimated by high intensity broaden-
ing technique using the Scherrer equation [27]. The morphology
of the particles was examined by scanning electron microscopy
(SEM) using a JEOL-2011 microscope. Chemical analysis of stron-
tium ferrite was carried out by X-ray fluorescence method. Surface
area of the material was determined by using Brunauer, Emmett
and Teller (BET) method using Thermo Fisher surface area analyzer
Model—SURFER.
#
$
*
*
+
#
+
2400 2200 2000 1800 1600 1400 1200 1000 800
600
400
-1
wavenumber (cm )
Fig. 2. Infrared spectra of CaFe2O4 (a), Sr0.2Ca0.8Fe2O4 (b), Sr0.4Ca0.6Fe2O4 (c),
Sr0.6Ca0.4Fe2O4 (d), Sr0.8Ca0.2Fe2O4 (e) and SrFe2O4 (f) calcined at 700 ◦C prepared
by citate gel method.
2.3. Catalytic activities of strontium substituted calcium ferrites
2.3.1. Selective epoxidation of styrene
peaks are found in the range of 418–611 cm−1 in all spectra. The
band appearing around 600–605 cm−1 is attributed to stretching
vibration of tetrahedral groups Ca2+ O2−, while the band appearing
around 552–557 cm−1 (ꢀ1), is attributed to stretching vibration
of tetrahedral groups Sr2+ O2−, and that around 440–457 cm−1
The selective epoxidation of styrene was carried out in a three
neck round bottom flask (100 mL) equipped with X-crossed Teflon
coated magnetic stirrer and a reflux condenser, and two drop-
ping funnel. In typical batch experiment, 5.2 g (50 mmol) of styrene
[99+%, Aldrich], 6.105 g (150 mmol) acetonitrile and 100 mg of
SrxCa1 − xFe2O4 (0.0 ≤ x ≤ 1.0) catalysts were charged to the reac-
tor. The mixture was heated to 343 K; while stirring, a solution
of 2.5 mL (25 mmol) of hydrogen peroxide (30% aq. Merck) was
added followed by the addition of 6.41 g (200 mmol) methanol
dropwise to the above mixture over a period of 45 min. Aqueous
1 N sodium hydroxide was also added simultaneously to main-
tain the pH between 7.5 and 8.0. After 6 h of reaction, the liquid
product was cooled down to room temperature and the catalyst
was separated by filtration. The product was diluted with 25 mL of
water and extracted with three 20 mL portions of dichloromethane
after which the extraction liquid was dried by adding anhydrous
sodium sulphate to it. The extracted solution was concentrated
and analyzed by QP 5050 Shimadzu gas chromatography and mass
spectroscopy (GCMS) equipped with a XE-60 capillary column
(30m × 0.25 × 0.3 m) and a flame ionization detector. The injec-
tor and column temperature were 280 and 140 ◦C, respectively.
(ꢀ2), is attributed to the octahedral group complex Fe3+ O2−
.
The two weak absorption bands found in the tetrahedral region
(552–605 cm−1) indicate the presence of divalent metal ions on
the CaFe2O4 by strontium ion takes place at tetrahedral site only,
which is confirmed by shifting the Sr2+ O stretching band towards
the higher wave number from 446 cm−1, 550 cm−1, 552 cm−1
,
565 cm−1 and 567 cm−1 for sample b–f, respectively, as shown in
Fig. 2 while the Ca2+ O stretching band shifted towards the lower
wave number. The spectrum also reveals that the carboxylates of
the precursor transform into metal carbonate with the character-
.
2.4.1.3. X-ray diffraction analysis of strontium substituted calcium
ferrites. The X-ray diffraction studies of the samples treated at
700 ◦C temperature for 2 h have been carried out using Cu K␣
radiation. Fig. 3 shows the XRD patterns of SrxCa1 − xFe2O4, where
(0.0 ≤ x ≤ 1.0). A pattern (a) is the spinel type CaFe2O4 oxide where
all the d spacing values are in good agreement with the standard
data (JCPDS card no. 46-0100), while a pattern (f) is the XRD pattern
of spinel type SrFe2O4 oxide where all the d spacing values are in
good agreement with the standard data [28]. Patterns (b)–(e) are
the XRD patterns of SrxCa1 − xFe2O4 (0.0 ≤ x ≤ 1.0)
2.4. Results and discussion
2.4.1. Characterization of strontium substituted calcium ferrites
2.4.1.1. Thermo gravimetric analysis. Fig. 1(a–f) shows that the
thermal analysis of the as prepared samples was done to know
the possible changes occurring when they were subjected to heat
treatment and to determine the exact calcination temperature of
ferrites. From TG studies it is seen that the data for the ferrites
synthesized by citrate gel combustion method are featureless in
the temperature range of from 30 to 350 ◦C, except for the water
loss between 100 ◦C and 110 ◦C. Near 350 ◦C, a sudden weight loss
indicates the decarboxylation of the complexes. Above 600 ◦C no
distinct weight loss was observed indicating the crystallization of
ferrite.
As the concentration of strontium ion in SrxCa1 − xFe2O4
increases from (0.0 ≤ x ≤ 1.0) the highest intensity peak is shifted
towards its left from 2ꢁ angle 33.5, 33.1, 33.0, 32.9, 32.8 and 32.7,
respectively (Fig. 3) indicating a higher interplanar distance and
consequently, an increase of the cell volume. The crystallite size
of all the samples (a–f) was calculated using Scherer’s law from
the XRD peak at 33.5–32.7, respectively which is in the range of
49–85 nm. The X-ray pattern of the substituted spinel type ferrites
are monophasic and show the same pattern as that of the calcium
ferrite, although a high intensity line shifts to lower 2ꢁ value when
the strontium doping increases are observed.
2.4.1.2. FTIR study of strontium substituted calcium ferrites. Fig. 2a
and f shows FTIR spectrum of CaFe2O4 and SrFe2O4 powder while
Fig. 2b–e shows the FTIR spectrum of SrxCa1 − xFe2O4 oxide (where
x = 0.2, 0.4, 0.6 and 0.8) calcined at 700 ◦C temperature. Significant
2.4.1.4. Electron dispersive spectra and morphology of strontium sub-
stituted calcium ferrites. The electron dispersive X-ray spectra of