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M. Stekrova et al. / Applied Catalysis A: General 470 (2014) 162–176
and silica and alumina and the Dubinin’s equation was used for
calculation of the specific surface area of microporous zeolites.
frequency pulse (0.6 s), and about 1000 scans were accumulated
with a 0.5 s recycle delay. 29Si MAS NMR spectra were recorded with
a /2 excitation pulse of 5.0 s duration and 20 s repetition time,
and 1000 scans were acquired for signal accumulation. Both 27Al
and 29Si NMR spectra were recorded with use of 4 mm rotors and a
spinning rate of 10–15 kHz.
2.2.3. Pyridine adsorption–desorption with FTIR
The acidity of the proton and Fe modified catalyst was mea-
sured by infrared spectroscopy (ATI Mattson FTIR) using pyridine
(≥ 99.5%) as a probe molecule for qualitative and quantitative deter-
mination of both Brønsted and Lewis acid sites. The samples were
pressed into thin pellets (10–25 mg). The pellets were pretreated
at 450 ◦C before the measurement. Pyridine was first adsorbed for
30 min at 100 ◦C and then desorbed by evacuation at different tem-
peratures. Three different temperatures were used for desorption
of pyridine, defined as 250 ◦C–350 ◦C as weak, medium and strong
sites, 350 ◦C–450 ◦C as medium and strong sites as well as pyri-
dine which stays adsorbed after desorption at 450 ◦C as strong sites
[20]. The amount of Brønsted and Lewis acid sites were calculated
from the intensities of the corresponding spectral bands, 1545 cm−1
and 1450 cm−1, respectively, using the molar extinction parame-
ters previously reported by Emeis [21]. The catalysts weights were
taken into account in the calculations.
2.2.7. X-ray diffraction
Powder X-ray diffraction (XRD) of the samples was measured
using Philips XꢀPert Pro MPD using monocrhomated CuK␣ radi-
ation at 40 kV/50 mA. The divergence slit was 0.25◦ with a fixed
20 mm mask. The diffractograms were analyzed by Philips XꢀPert
HighScore MAUD programs.
2.3. Catalytic tests
Liquid phase isomerization of ␣-pinene oxide over the Fe
modified catalysts was carried out in the batch-wise operating
glass reactor. In a typical experiment using toluene as a solvent
(VL = 150 ml) the initial concentration of ␣-pinene oxide and the
catalyst mass were 0.013 mol/l and 75 mg, respectively. The kinetic
experiments were performed at 70 ◦C under the following con-
ditions to avoid external mass transfer limitation: the catalysts
particle size below 90 m and the stirring speed of 390 rpm. The
catalyst was activated in the reactor at 250 ◦C under an inert argon
atmosphere for 30 min before the reaction. The samples were taken
at different time intervals and analyzed by GC using a DB-Petro
column with a capillary column of 100 m x 250 m x 0.50 m nom-
inal (Agilent 128-1056) and with a FID detector. The products were
confirmed by GC-MS.
2.2.4. X-ray absorption spectroscopy measurements
FeK edge (7111 eV) XAS measurements were carried out at
HASYLAB on the beamline X1 and on CLÆSS beamline of ALBA
synchrotron facility. Si (1 1 1) double crystal monochromator was
used for the energy scan along with Rh-coated toroid mirror for
unwanted harmonics elimination. The spectra were recorded in the
transmission mode at ambient temperature. For the measurements
samples were pressed in self supporting pellets and wrapped with
Kapton tape. Spectra were measured simultaneously with the ref-
erence spectrum Fe foil placed between second and third ionisation
chambers, so that the absolute energy calibration is performed. Fe
foil and ␣-Fe2O3 (hematite), which were used as references, were
collected at the same conditions. All spectra were measured two
times to ensure their reproducibility.
Analysis of the EXAFS spectra was performed with the software
VIPER for Windows [22]. In the spectra of the absorption coeffi-
cient ꢀ, a Victorian polynomial was fitted to the pre-edge region
for background subtraction. A smooth atomic background ꢀ0 was
evaluated using a smoothing cubic spline. The Fourier analysis of
k2-weighted experimental function ꢁ = (ꢀ-ꢀ0)/ꢀ0 was performed
with a Kaiser window. The required scattering amplitudes and
phase shifts were calculated by the ab initio FEFF8.10 code [23] for
␣-Fe2O3 (hematite) structure. The fitting was done in the k- and r-
spaces simultaneously. The shell radius r, coordination number N,
Debye–Waller factor ꢂ2 and adjustable “muffin-tin zero” ꢃE were
determined as fitting parameters. The errors of the fitting param-
eters were found by decomposition of the statistical ꢁ2 function
near its minimum, taking into account maximal pair correlations.
In order to evaluate a possibility of catalyst reuse, the spent cat-
alyst was filtered from the reaction mixture after isomerization,
washed by acetone and dried overnight at 100 ◦C and reused in the
reaction. In the case of catalyst regeneration, calcination at 400 ◦
was done prior to reaction.
C
3.1. Catalyst characterization
3.1.1. Morphological studies by scanning electron microscopy
were studied by scanning electron microscopy (Figs. 2–5). Fe mod-
tal morphology were observed due to the loading various amount
lyst were observed (Fig. 5). Fig. 3 compares crystal morphologies
of the fresh Fe-MCM-41-SSIE and the catalyst after isomerization.
distribution (Fig. 3a). On the other hand, the spent catalyst contains
larger particles with various sizes which is caused by agglomera-
tion of particles during the reaction. As it is discussed below, the
regained (Table 11). The catalyst deactivation in the isomerization
reaction of ␣-pinene oxide is attributed to the pore blockage of the
Fe-MCM-41-SSIE by carbon residuals and not due to morphology
changes in MCM-41 or Fe species (Table 2). So there should not be
any changes in the morphology or sintering of Fe species, otherwise
it would not have been possible to regenerate the catalyst. It can be
conclude, that the MCM-41 crystals are kept intact after the isom-
erization reaction of ␣-pinene and only reversible agglomeration
of catalyst particles occurred during the reaction.
2.2.5. XPS analysis
The photoemission spectra were measured using a Perkin-Elmer
PHI 5400 spectrometer with a monochromatized Al K␣ X-ray
source that was operated at 14 kV, 300 W. The analyzer pass energy
was 17.9 eV and the energy step was 0.1 eV. The vacuum chamber
base pressure was 10−9 mbar. The use of charge neutralizer was
necessary and its power was set so that Si 2p peak was at 103.5 eV,
corresponding to SiO2 binding energy. The studied peaks were Fe
2p, Si 2p and Al 2p. Also, a 1400 eV survey spectrum was taken for
each sample. The step length was 0.5 eV. The peaks were calibrated
using binding energy (BE) of SiO2 which is 103.3–103.7 eV.
2.2.6. 29Si MAS NMR and 27Al MAS NMR
NMR spectra were recorded at 9.4 T on a Bruker Avance-400
spectrometer equipped with broad-band double-resonance-MAS
probe. 27Al MAS NMR spectra were acquired with a short /12 radio