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A. Bordoloi, S.B. Halligudi / Applied Catalysis A: General 379 (2010) 141–147
2. Experimental
2.1. Materials and catalyst preparation
Sodium tungstate, cyclohexanone oxime, tetraethyl orthosil-
icate (TEOS), P123 (poly (ethylene glycol)–poly (propylene
glycol)–poly (ethylene glycol) block copolymer, with average
molecular weight of 5800, and solvents were purchased from
Aldrich Chemicals and used as received.
Hexagonally ordered mesoporous WOx/SBA-15 nanocomposite
materials were synthesized by using TEOS as a silica source and
P123 as a structure-directing agent. In a typical synthesis, 4.0 g of
P123 block copolymer was dissolved with stirring in a solution
of 30 g of water and the required amounts (20, 10, 5 and 2.5 mL)
of aqueous sodium tungstate solution (NaWO4·2H2O, 0.5 M) were
simultaneously and quickly added into the mixture under vigorous
stirring. After 1 h, 120 g of HCl (2 M), and 9.1 g of TEOS were added
with stirring at 40 ◦C. After 24 h of constant stirring, the gel compo-
sition was kept at 100 ◦C under static condition for 48 h. After being
cooled to room temperature, the solid product was recovered by fil-
tering, washing, drying and calcining at 550 ◦C. The nanocomposite
samples were denoted as WOx/SBA-15(x), where x is the volume of
0.5 M sodium tungstate solution used [29].
Scheme 1. Schematic representation of Beckmann rearrangement of cyclohex-
anone oxime.
with CaF2 windows. Each WOx/SBA-15 sample was out gassed at
500 ◦C under vacuum for 2 h. The sample was then cooled to room
temperature and one drop of 2.5% of cyclohexanone oxime ethanol
solution was added under He gas flow. The sample was then heated
at 300 ◦C (reaction temperature) over time under He gas flow while
FTIR spectra was recorded.
2.2. Catalyst characterization
Tungsten content of the catalyst was determined by energy
dispersive X-ray analysis (EDAX) using a Microanalysis Phoenix
system. Nitrogen adsorption and desorption isotherms were mea-
sured at −196 ◦C with a Micromeritics ASAP 2020 adsorption
analyzer. The samples were out gassed for 3 h at 250 ◦C under
vacuum prior to adsorption measurements. The specific surface
area was calculated using the BET model. The pore size distribu-
tions were obtained from the adsorption branch of the nitrogen
isotherms by the Barrett–Joyner–Halenda (BJH) method with
Kruk–Jaroniec–Sayari (KJS) correction [30]. The X-ray diffraction
(XRD) patterns of the samples were collected on a Philips X’Pert Pro
3040/60 diffractometer using CuK␣ radiation (ꢀ = 1.5418 Å), iron
as the filter, and X’celerator as the detector. For high temperature
XRD, the data quality of the scan was comparatively good, because
the X’celerator detector uses real time multiple strip technology to
enhance both the resolution and the intensity of the reflection.
The total amount of acid sites present on the catalysts was eval-
uated by temperature-programmed desorption (TPD) of NH3. The
NH3-TPD measurements were performed on an Altamira instru-
ment (AMI-200). A sample weight of ca. 90 mg was loaded in a
U-shaped quartz reactor and pretreated in flowing He (50 mL/min)
at 300 ◦C for 1 h. After cooling the sample to 100 ◦C, we exposed
it to ammonia flowing at 50 mL/min. The material was heated to
a final temperature of 550 ◦C at a ramping rate of 10 ◦C/min. The
ammonia consumption was measured by a thermal conductivity
detector. Ammonia pulse calibration was performed after each TPD
experiment for the quantification of TPD data.
2.3. Beckmann rearrangement of cyclohexanone oxime
The catalytic reactions were carried out in an up flow fixed bed
tubular stainless steel reactor (i.d. = 10 mm and 24 cm length) at
atmospheric pressure using 2 g of catalyst. The catalyst was com-
pacted in a hydraulic press, and the pellets were broken and then
sieved to 16–20 mesh size prior to use. The reactor was placed
inside a temperature controlled vertical furnace. The thermocou-
ple tip was centered at the middle of the catalyst bed. A solution
of cyclohexanone oxime (2.5 wt.%) in methanol was fed using a
high pressure pump (Eldex, US). The weight hourly space velocity
(WHSV) was calculated based on the oxime solution injected. The
catalyst was activated in situ in a flow of N2 (20 mL/min) at 500 ◦C
for 6 h. The reactor outlet was connected to a cooling trap contain-
ing ice; and the collected liquid effluent taken at specified intervals
was analyzed using a Hewlett-Packard gas chromatograph (5880A)
uct identification was achieved by GCMS (Agilent). Regeneration
of the catalyst was done by calcination at 500 ◦C for 8 h under N2
flow. A schematic representation of the Beckmann rearrangement
of cyclohexanone oxime is given in Scheme 1.
3. Results and discussion
The nature of the acid sites (Brönsted and Lewis) of the cat-
alyst samples were characterized by in situ FTIR spectroscopy of
chemisorbed pyridine. Circular self-supported wafers of the cata-
lyst samples were prepared while applying 60,000 N/m2 pressure.
Each sample was subjected to vacuum in a glass IR cell until a pres-
sure of 10−6 mbar was attained, followed by activation at 300 ◦C
then each sample was cooled to 100 ◦C. Pyridine vapor was admit-
ted in doses until the catalyst surface is saturated. Pyridine was
then desorbed until a pressure of 10−6 mbar at a temperature of
200 ◦C to ensure that there was no more physisorbed pyridine on
the wafers. Fourier transform infrared spectra (FTIR) we used were
recorded in a Nicolet-Magna 550TM. To follow the Beckmann rear-
rangement by FTIR, a high temperature DRIFT cell (spectro tech)
3.1. Catalyst characterization
3.1.1. X-ray diffraction
The wide and low angle XRD patterns of WOx/SBA-15 catalysts
with different loadings are shown in Fig. 1(A and B), respectively.
All wide angle diffraction peaks were indexed to monoclinic WO3
as reported on JCPDS Card No. 83-0951 (Fig. 1(A)). Moreover,
low angle XRD measurements showed that WOx/SBA-15 materi-
als exhibited three peaks at 2ꢁ in the range 0.5–5◦; these can be
indexed to (1 0 0), (1 1 0), and (2 0 0) reflections of the hexagonal
p6mm space group. The observation is in good agreement with the
XRD pattern of pure hexagonally ordered SBA-15 material reported