Praseodymium incorporated alpo-5 molecular sieves for aerobic oxidation of ethylbenzene

Article abstract of DOI:10.1166/jnn.2013.7415

PrAlPO-5 with (Al+P)/Pr ratios of 25, 50, 75 and 100 molecular sieves were successfully synthesized by hydrothermal method. These molecular sieves were characterised using XPS, TPD-NH₃, ex-situ pyridine adsorbed IR, TPR, TGA, ²⁷Al and ³¹P MAS-NMR and ESR studies. The incorporation of praseodymium in the framework of AlPO-5 was confirmed by XRD, DRS UV-vis and ²⁷Al and ³¹P MAS-NMR analysis. ESR spectrum showed the presence of adsorbed oxygen. The nature and strength of acid sites were identified by ex-situ pyridine adsorbed IR and TPD-NH₃. The BET surface area was found to be in the range of 238-272 m² g ^-1. The catalytic activity of the molecular sieves was tested for the liquid phase aerobic oxidation of ethylbenzene. Acetophenone was found to be the major product with more than 90% ethylbenzene conversion. ICP-OES analysis revealed the presence of praseodymium intact in the framework of AlPO-5 up to five cycles. Copyright

Full text of DOI:10.1166/jnn.2013.7415

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 2507–2516, 2013
Praseodymium Incorporated AlPO-5 Molecular
Sieves for Aerobic Oxidation of Ethylbenzene
B. Sundaravel, C. M. Babu, B. Palanisamy, M. Palanichamy,
∗
K. Shanthi, and V. Murugesan
Department of Chemistry, Anna University, Chennai 600025, India
PrAlPO-5 with (Al+P)/Pr ratios of 25, 50, 75 and 100 molecular sieves were successfully synthe-
sized by hydrothermal method. These molecular sieves were characterised using XPS, TPD-NH ,
3
2
7
31
ex-situ pyridine adsorbed IR, TPR, TGA, Al and P MAS-NMR and ESR studies. The incorpo-
ration of praseodymium in the framework of AlPO-5 was confirmed by XRD, DRS UV-vis and Al
and P MAS-NMR analysis. ESR spectrum showed the presence of adsorbed oxygen. The nature
27
3
1
and strength of acid sites were identified by ex-situ pyridine adsorbed IR and TPD-NH . The BET
3
2
−1
surface area was found to be in the range of 238–272 m g . The catalytic activity of the molecular
sieves was tested for the liquid phase aerobic oxidation of ethylbenzene. Acetophenone was found
to be the major product with more than 90% ethylbenzene conversion. ICP-OES analysis revealed
the presence of praseodymium intact in the framework of AlPO-5 up to five cycles.
Keywords: Zeotypes, PrAlPO-5, Framework Incorporation, Aerobic Oxidation, Ethylbenzene.
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1
1–13
1. INTRODUCTION
(air) as the oxidant.
Solvent plays an important role in
1
4ꢀ15
the catalytic oxidation reactions.
However, the use of
Zeolites and zeotypes molecular sieves find applications
in the field of catalysis, adsorption, separation and envi-
ronmental pollution control. These crystalline microporous
materials are extensively used as catalysts for oil refining,
petrochemicals, organic synthesis and production of fine
and specialty chemicals. Recently, researchers developed
novel multifunctional metal organic framework (MOF)
materials and they exhibited potential applications in the
field of catalysis, drug delivery, gas storage and bio-
medicine.1 Selective catalytic oxidation of alkyl aromat-
ics is a viable technology to functionalize saturated and
solvent also leads to environmental problems. Hence, it is
a challenge for the researchers to develop a solvent free
catalytic oxidation, and a few systems have been already
16ꢀ17
reported for catalytic oxidation reactions.
Transition
metal oxide based catalysts have been reported for oxi-
dation reaction but sintering of the catalyst and leaching
18ꢀ19
of metal ions are serious drawbacks.
Transition metal
incorporated molecular sieves are also reported for selec-
20–24
tive oxidation.
Such catalysts are more attractive than
–4
bulk metal oxides or supported metal oxides, as the active
sites are isolated and possess uniform activity both in
slurry and vapour phase reactions. Such properties prompt
the researchers to concentrate mainly on framework incor-
porated active sites for oxidation. The aluminophosphate
5
–7
unsaturated hydrocarbons. Organic peroxides, hydrogen
peroxide and molecular oxygen are cost effective oxidants
for the oxidation of alkyl aromatics in the presence of
8
–10
a suitable catalytic system.
However, organic perox-
molecular sieves known as AlPO -n (n refers to a dis-
4
ides are not environmentally benign as they leave large
volume of organic waste. Hydrogen peroxide decomposed
tinct structure type) are built from strict alternation of
3+
AlO and PO tetrahedra. The AlPO contains Al and
4
4
ꢀ
(
above 70 C) rapidly; water formed in the reaction mix-
5+
P in tetrahedral position and the resultant framework
is neutral. By careful incorporation of the metal ion
transition metal ions and lanthanides) it is possible to
ture shall decrease the activity of the catalyst by decreas-
ing the interaction between the substrate and the catalyst
surface. Molecular oxygen (air) is the cheapest and clean
oxidizing agent. Recently, researchers developed a few het-
erogeneous catalytic systems employing molecular oxygen
25
(
develop weak and moderate acid sites in AlPO-5 frame-
work. These active sites favour side chain oxidation rather
26–30
than ring hydroxylation.
Hence, there has been a con-
tinued interest in search of environmentally benign catalyst
for the selective synthesis of aromatic ketone from the
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 4
1533-4880/2013/13/2507/010
doi:10.1166/jnn.2013.7415
2507

Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
Sundaravel et al.
corresponding alkyl aromatics by catalytic oxidation in
which the catalytic system does not depend on the electron
density of the substrate.
2.3. Catalyst Characterization
The X-ray diffraction patterns were recorded on a PAN-
alytical X pert PRO diffractometer equipped with Cu Kꢂ
In this study, we report the oxidation of alkyl aromatics
over praseodymium incorporated AlPO-5 molecular sieves
(
ꢃ = 1ꢄ5418 Å) as the radiation source in the 2ꢅ range
ꢀ
ꢀ
3
+
4+
5–80 with a step size of 0.02 and a count time of 20 s
at each point. The DRS UV-vis absorption spectra were
recorded between 200 and 800 nm on a UV-Vis spectropho-
for the first time. Since Pr and Pr are paramagnetic
in nature they can activate molecular oxygen and oxidize
alkyl aromatics. Further, we also attempted to understand
the effect of change of electron density in the alkyl aro-
matics by introduction of different functional groups in the
para position of ethylbenzene.
tometer (Shimadzu model 2450) using BaSO as the ref-
4
erence. The ATR-IR spectra were recorded on an ATR-IR
spectrometer (Perkin Elmer, Spectrum Two) in the range
−
000–400 cm . The N adsorption–desorption isotherms
2
1
4
were recorded at 77 K using a volumetric adsorption ana-
lyzer (Micromeritics ASAP 2020). Prior to each adsorption
2
. EXPERIMENTAL DETAILS
.1. Preparation of the Catalysts
Aluminium isopropoxide (Merck), ortho-phosphoric acid
ꢀ
measurement, the samples were degassed at 200 C under
−
5
2
vacuum (p < 10 mbar) in the port of adsorption analyzer.
The specific surface area was calculated by Brunauer–
Emmett–Teller (BET) method, and the pore size distribu-
tion was calculated using Barrett–Joyner–Halenda (BJH)
method. Temperature programmed reduction (TPR) exper-
iments were carried out using a Micromeritics chemisorb
(Merck), praseodymium nitrate hexahydrate (Indian rare
earths Ltd.), hydrofluoric acid (Merck) and triethy-
lamine (Merck) were used in the synthesis of PrAlPO-
5
. PrAlPO-5 molecular sieves with different (Al + P)/Pr
2
750 insrument. The calcined sample (10 mg) was heated
ratios were synthesized in fluoride medium using the
−1
ꢀ
in a flow of helium (30 ml min ꢁ at 150 C for 30 min
and then cooled to 40 C. TPR signals were recorded for
every 0.1 s up to 900 C under the gas flow (30 ml min ꢁ
following gel composition: xPr(NO ꢁ · 6H O:1.0Al O :
ꢀ
3
3
2
2
3
3
1
1
.3P O :1.6TEA:1.3HF:425H O. The typical synthesis
ꢀ
−1
2
5
2
procedure is as follows: Aluminium isopropoxide (7.15 g)
was soaked in 50 ml deionised water for 24 h. Then
of 10% H /Ar mixture. The X-ray photoelectron spec-
2
tra (XPS) of the samples were recorded using MS Omi-
ortho-phosphoric acid (D4 .e9 li vg e) ,r et dr i ebt hy y Pl aumb il ni se h i(n2 g. 8T ge )c ha nn do logy to: Purdue University Libraries
cron nanotechnology using monochromator XM-1000 with
praseodymium nitrate hexahydr aI Pt e: (20 1. 39 8. 1g 2) 9 w. 1e 4r e5 .d 1i s0 s 1o l Ov end : Tue, 22 Mar 2016 05:34:19
AlKꢂ as the radiation source operated at 200 W and
Copyright: American Scientific Publishers
in 15 ml water in a polypropylene bottle and stirred for
h. This was labelled as solution A. The aluminium pre-
a hemispherical electron energy analyzer. The morphol-
2
ogy of the materials were examined by scanning electron
microscope (SEM) after gold coating using SEM-JEOL,
JSM-5600 model. Cressington 108 auto sputter coater was
used for coating. The accelerating voltage of the SEM
cursor stirred for 2 h was added to solution A and the
resultant mixture was stirred for 2 h. This was labelled as
solution B. Then hydrofluoric acid (0.98 g) was diluted in
5
ml water. This dilute hydrofluoric acid was added slowly
was 20 keV. TPD-NH desorption study was carried out
3
to solution B and stirred for 2 h. Finally, the gel was trans-
ferred to a teflon lined stainless steel autoclave and heated
at 180 C for 12 h. After quenching the hot autoclave in
ice-cold water, the crystallized product was filtered and
washed several times with water. The crystalline product
obtained was dried in an oven at 120 C and calcined at
using Micromeritics Chemisorb 2750 instrument. In a typ-
ꢀ
ical run, the calcined sample was pretreated at 250 C
ꢀ
ꢀ
for 30 min, cooled down to 100 C and saturated with
ammonia by passing 10% NH –He mixture. Then isother-
3
mal desorption of ammonia from dead volume and phys-
ically adsorbed ammonia was carried out in a flow of
helium for 20 min. Temperature programmed desorption
ꢀ
ꢀ ꢀ −1
5
50 C at a heating rate of 1 C min in air for 6 h. The
same procedure was adopted for the synthesis of AlPO-5
without the precursor for praseodymium.
ꢀ
−1
(
10 C min ꢁ of the sample was recorded in the tem-
ꢀ
perature range 100–600 C. The sample for acidity mea-
surement was prepared in the pellet form by mixing KBr
and the catalyst in 10:1 ratio. The pellet was saturated
with small amount of pyridine followed by degassing at
2
.2. Synthesis of Praseodymium Oxide
ꢀ
Praseodymium nitrate hexahydrate (4.35 g) was dissolved
in 40 ml double distilled water and then 10 ml aque-
ous ammonia solution was added to it slowly with con-
stant stirring. A pale greenish precipitate was formed, and
the stirring was continued for another 2 h. The precipi-
tate was filtered, washed thoroughly with double distilled
water and dried at 120 C. The dried sample was crushed
well and calcined at 550 C for 12 h at a heating rate
of 1 C min .
150 C for 1 h. The FTIR spectra in absorbance mode after
pyridine adsorption were subtracted from that of untreated
catalyst to obtain the peaks due to interaction between
pyridine and acid sites only. ESR spectra were recorded
using Bruker EMX Plus spectrophotometer at room tem-
perature with a microwave power of 3 mW and modulation
frequency of 100 kHz. Chemical analysis of the samples
was performed using ICP-OES Perkin Elmer OPTIMA
7000 DV instrument. Thermogravimetric analysis (TGA)
ꢀ
ꢀ
ꢀ
−1
2
508
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013

Sundaravel et al.
Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
was carried out using Perkin Elmer SII (Diamond series)
model under nitrogen atmosphere in the temperature range
the framework of AlPO-5 without any non-framework
praseodymium species in the materials. Since the intensity
of different reflections of PrAlPO-5 (25 and 50) exhibited
the same order as that of AlPO-5, it is presumed that all
of them possessed the same crystal morphology. Although
the reflections of PrAlPO-5 (75 and 100) exhibited similar
to that of AlPO-5 and PrAlPO-5 (25 and 50), the inten-
sity of reflections changed. For example, the reflection at
ꢀ
ꢀ
−1 27
31
3
0–600 C at a heating rate of 20 C min . Al and
P
MAS-NMR spectra were recorded on a Bruker AV-700
NMR spectrometer.
2
.4. Typical Procedure for Aerobic
Oxidation of Ethylbenzene
2
ꢅ = 18 became intense peak, and based on the increase in
PrAlPO-5 (200 mg) and ethylbenzene (50 mmol) were
taken in a two necked round bottom flask fitted with a
condenser and a magnetic pellet for stirring. To this hetero-
geneous reaction mixture, air was purged through balloon
for 6 h. After the completion of reaction, the reaction mix-
ture was filtered and washed the catalyst with ether for
intensity it could be inferred that praseodymium was bet-
ter located in (210) plane compared to other planes. When
praseodymium content is low as in the case of PrAlPO-
5(75) and PrAlPO-5(100), it became more selective for
specific planes and the intensity of reflections changed
from the parent AlPO-5.
5
to 6 times. The filtrate was poured into ice cold water
and extracted with ether. The ether layer and washings
were combined and dried over anhydrous sodium sulphate.
The ether layer was evaporated to obtain the product. The
products were analyzed by gas chromatograph (Shimadzou
GC-17A model) and also identified by gas chromatograph
coupled with mass spectrometer (Perkin Elmer Clarus 500)
3.2. DRS-UV-Vis Spectroscopy
DRS-UV-Vis spectra of praseodymium oxide, AlPO-5
and praseodymium incorporated PrAlPO-5 are shown in
Figure 2. DRS-UV-Vis spectra were recorded between 200
and 800 nm in order to confirm the frame-work incor-
poration of Pr in AlPO-5. Praseodymium oxide showed
increase in absorbance starting from 600 to 400 nm
and below 400 nm the absorbance decreased. Such an
increase in absorbance at 600 nm was not observed in
all PrAlPO-5 molecular sieves. Hence, free praseodymium
−
1
using helium as the carrier gas at a flow rate of 1 ml min .
3
2
3. RESULTS AND DISCUSSION
3
.1. X-Ray Diffraction Analysis (XRD)
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oxide phase was established to be absent in all the sam-
ples. These samples showed an increase in absorbance
at 350 nm which was assigned to f –f transition. Simi-
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The XRD patterns of calcined AlPO-5, praseodymium
incorporated AlPO-5 molecular sieves and praseodymium
oxide are shown in Figure 1. Pure praseodymium oxide
showed reflections close to 2ꢅ at 28, 33, 47 and 56 which
are in agreement with already reported in the literature.
The XRD patterns of AlPO-5 also matched with the one
Copyright: American Scientific Publishers
larly, the absorbance bands around 210 and 252 nm were
2
−
3+
2−
4+
due to O to Pr and O to Pr charge transfertran-
sitions. This confirmed the framework incorporation of
praseodymium in AlPO-5.
3
2ꢀ33
3
4ꢀ35
reported in the literature.
2
PrAlPO-5 ((Al + P)/Pr =
5, 50, 75 and 100) exhibited reflections characteristics
3.3. Scanning Electron Microscope (SEM) Analysis
of AlPO-5 and not that of praseodymium oxide. This
result established the incorporation of praseodymium in
The SEM image of calcined AlPO-5 is shown in Fig-
ure 3(a). The image exhibited hexagonal rods of smooth
Fig. 1. XRD patterns of (a) Praseodymium oxide, (b) AlPO-5,
Fig. 2. DRS UV-vis spectra of (a) Praseodymium oxide, (b) AlPO-5,
(c) PrAlPO-5(25), (d) PrAlPO-5(50), (e) PrAlPO-5(75) and (f) PrAlPO-
5(100).
(
c) PrAlPO-5(25), (d) PrAlPO-5(50), (e) PrAlPO-5(75) and (f) PrAlPO-
(100).
5
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013
2509

Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
Sundaravel et al.
(
a)
(b)
(
c)
(d)
(
e)
Fig. 4. N2 adsorption–desorption isotherms and pore size distribution
of (a) AlPO-5, (b) PrAlPO-5(100), (c) PrAlPO-5(75), (d) PrAlPO-5(50)
and (e) PrAlPO-5(25).
Table I. Sorption results of AlPO-5 and PrAlPO-5 (25, 50, 75 and 100).
Surface area,
Pore volume,
Vp (cm /g)
Pore diameter,
Fig. 3. SEM images of (a) AlPO-5, (b) PrAlPO-5(100), (c) PrAlPO-
(75), (d) PrAlPO-5(50) and (e) PrAlPO-5(25).
Catalyst
ABET (m
2
/g)
a
3
b
Dp (Å)
c
5
AlPO-5
238
272
265
258
245
0.23
0.28
0.27
0.25
0.24
4.36
5.18
5.03
4.89
4.69
PrAlPO-5(25)
PrAlPO-5(50)
PrAlPO-5(75)
PrAlPO-5(100)
surface, most of the rods were aggregated and grown from
a common base. Some of them were broken along the lon-
gitudinal direction, and such a cleavage could be occurred
during pulverization. The SEM image of PrAlPO-5(100)
shown in Figure 3(b), illustrates similar features as shown
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a
b and c
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:3ad4so:1rp9tion–desorption isotherm;
Determined by
2
BJH method.
Copyright: American Scientific Publishers
in Figure 3(a). There were hexagonal aggregated rods and
most of the rods were cleaved along the longitudinal direc-
tion. The SEM image of PrAlPO-5(75) (Fig. 3(c)) was
nearly the same as that of Figures 3(a) and (b). Both
PrAlPO-5(25) and PrAlPO-5(50) also exhibited similar
morphology (Figs. 3(d) and (e)). Although the XRD pat-
tern of PrAlPO-5 ((Al + P)/Pr = 75 and 100) exhibited
change in the order of reflections from that of parent
AlPO-5, the morphology of the material did not change.
This revealed that the concentration of praseodymium in
the synthesis gel didn’t affect the nucleation process. The
appearance of tiny crystallites on the surface of hexago-
nal rods was due to slow condensation of hydroxides of
praseodymium with phosphates.
angle resulted increase in surface area, pore size and pore
volume. This increase in surface area and pore size also
supported framework incorporation of praseodymium in
AlPO-5.
3.5. Electron Spin Resonance Spectroscopy (ESR)
The ESR spectrum of PrAlPO-5(25) recorded at room
temperature is shown in Figure 5. The ESR signal
3
.4. N Sorption Studies
2
The nitrogen adsorption–desorption isotherms of calcined
AlPO-5 and PrAlPO-5 ((Al+P)/Pr = 25, 50, 75 and 100)
molecular sieves are shown in Figure 4. All of them
exhibited type III adsorption isotherm similar to the one
3
6
reported in the literature. The BET surface area, pore
size and pore volume of the materials are presented in
Table I. PrAlPO-5 showed increase in surface area with
increase in praseodymium content. The larger ionic radii
of Pr(III) and Pr(IV) than that of Al(III) and P(V) resulted
change of O Pr O bond angle. Such increase in bond
Fig. 5. ESR spectrum of PrAlPO-5(25) at room temperature.
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013
2
510

Sundaravel et al.
Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
at g = 2ꢄ00284 is attributed to oxygen chemisorbed
(a)
on praseodymium species. Similar observations were
3
7
reported for metal incorporated aluminophosphates. The
broad and unsymmetrical ESR signals suggested that
praseodymium species exist in different environment. It is
difficult to distinguish the distinct signals for oxygen
3
+
4+
adsorbed on Pr and Pr at room temperature. The signal
around g = 3ꢄ42431 is due to paramagnetic praseodymium
species. This spectrum clearly indicated the presence of
adsorbed oxygen, and hence the catalyst can be exploited
for oxidation reaction at ambient temperature.
(
b)
3
.6. X-Ray Photoelectron Spectroscopy (XPS)
The XPS spectrum (Fig. 6) of calcined PrAlPO-5(25)
revealed the oxidation state of praseodymium in AlPO-5.
It showed peaks for Pr 3d5/2 and Pr 3d3/2 around 938 eV
and 959 eV respectively. De-convolution of Pr 3d5/2 peak
yielded two peaks around 933 eV which is assigned to
Fig. 7. 27Al MAS-NMR spectra of (a) AlPO-5 and (b) PrAlPO-5(25).
3+
4+
Pr , and the peak around 935 eV is assigned to Pr
oxidation state. Similar de-convolution of Pr 3d3/2 peak
aluminium in AlPO-5 is due to delocalization of unpaired
electron of praseodyium. The appearance of negative sig-
nals thus established large delocalization of unpaired elec-
resulted two peaks around 953 and 955 eV. This confirmed
3
+
4+
the presence of both Pr and Pr species in the frame-
work of PrAlPO-5(25).
31
tron over the near aluminium sites.
31
The P MAS-NMR of AlPO-5 and PrAlPO-5 ((Al+P)/
Pr = 25) are shown in Figures 8(a) and (b) respectively.
AlPO-5 showed an intense sharp peak at −24.62 ppm.
.7. 27Al and 31P Magic Angle Spinning-NMR
3
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This confirmed only one type of crystallographic phospho-
2
7
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Al MAS-NMR of AlPO-5 and PrAlPO-5 ((Al+P)/Pr =
rous in AlPO-5. This peak is shifted to −27.86 ppm in
Copyright: American Scientific Publishers
2
5) are shown in Figures 7(a) and (b) respectively.
The Al MAS-NMR of AlPO-5 exhibited a broad peak
between 30 and 50 ppm. It showed seven overlapped peaks
PrAlPO-5(25). A shoulder to the main peak observed for
PrAlPO-5(25) suggesting different environment of frame-
work phosphorous. The MAS-NMR studies thus confirmed
the framework incorporation of praseodymium by replac-
ing both Al and P ions in the framework of AlPO-5.
27
2
7
due to quadrupolar interactions. The Al MAS-NMR of
PrAlPO-5(25) showed an intense peak at 37.93 ppm due to
tetrahedral aluminium, and a weak peak at 4.96 ppm due to
aluminium in distorted trigonal bipyramidal co-ordination.
The downfield peak (−14 ppm) may be assigned to alu-
minium with praseodymium nearest neighbors. The high
shielding of aluminium in PrAlPO-5 compared to that of
(a)
(
b)
Fig. 6. XPS spectrum of calcined PrAlPO-5(25).
Fig. 8. 31P MAS-NMR spectra of (a) AlPO-5 and (b) PrAlPO-5(25).
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013
2511

Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
Sundaravel et al.
Fig. 11. Ex-situ pyridine adsorbed IR (a) PrAlPO-5(75), (b) PrAlPO-5
Fig. 9. TPR profile of (a) Praseodymium oxide, (b) PrAlPO-5(25),
c) PrAlPO-5(50) and (d) PrAlPO-5(75).
(50) and (c) PrAlPO-5(25).
(
(
(Al + P)/Pr = 25, 50, 75 and 100) showed desorption of
3
.8. Temperature Programmed Reduction (TPR)
ꢀ
ammonia between 130 and 250 C suggesting the pres-
ence of weak acid sites in these molecular sieves. Des-
orption of ammonia between 310 and 450 C is attributed
to moderately strong acid sites. Praseodymium in +3 oxi-
dation state might replace aluminium + 3, and + 4 oxi-
dation state of praseodymium might replace phosphorous.
The TPR profile of Pr O and PrAlPO-5 ((Al + P)/Pr =
2
3
ꢀ
2
5, 50 and 75) molecular sieves (Fig. 10) were recorded
ꢀ
between 40 and 900 C. The Pr O showed two reduction
2
3
ꢀ
peaks around 594 and 700 C. Such reduction peaks were
not observed for PrAlPO-5 molecular sieves. This further
confirmed the absence of non framework praseodymium
species.
4
+
Such isomorphic substitution of P by Pr created Bron-
sted acid sites in the catalysts. The acidity created in
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PrAlPO-5 molecular sieves is mainly due to incorporation
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3
8ꢀ39
Copyright: American S co i fe np rt iaf si ce oPd uy bm l ii su hm e ri sn the framework.
3
.9. Characterization of Acid Sites (TPD-NH and
3
The nature of acid sites were identified and confirmed
by probing ex-situ pyridine adsorbed FT-IR. The pyri-
dine adsorbed IR spectra of PrAlPO-5 (25, 50 and 75)
are shown in Figure 12. All the materials showed intense
peaks at 1445, 1490 and 1600 cm corresponding to
Lewis acid sites. Further, the peak at 1540 cm due to
Bronsted acid sites is very weak compared to the peak at
Ex-Situ Pyridine Adsorbed IR)
The acidity in PrAlPO-5 was probed by temperature pro-
grammed desorption of ammonia (TPD-NH ꢁ and ex-situ
pyridine adsorbed IR. The TPD-NH3 results of AlPO-
5
3
−
1
−1
, PrAlPO-5 ((Al + P)/Pr = 25ꢀ50ꢀ75 and 100) and
praseodymium oxide are depicted in Figure 11. PrAlPO-5
−
1 40ꢀ41
1
445 cm .
Fig. 10. TPD-NH3 profile of (a) Praseodymium oxide, (b) PrAlPO-
5(25), (c) PrAlPO-5(50), (d) PrAlPO-5(75), (e) PrAlPO-5(100) and
Fig. 12. FT-IR spectra of calcined (a) AlPO-5, (b) PrAlPO-5(25),
(
f) AlPO-5.
(c) PrAlPO-5(50), (d) PrAlPO-5(75) and (e) PrAlPO-5(100).
2
512
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013

Sundaravel et al.
.10. FT-IR Spectroscopy
Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
3
(A)
The FT-IR spectra (Fig. 12) of calcined PrAlPO-5 ((Al +
P)/Pr = 25ꢀ50ꢀ75 and 100) and AlPO-5 samples were
−1
recorded between 4000 and 400 cm in order to verify
the complete removal of template. The band just below
−
1
3
000 cm , due to C–H stretching vibration was absent
in all the samples indicating the complete loss of tem-
plate during calcination. The band between 1300 and
−
1
8
50 cm assigned to asymmetric stretching vibration of
TO groups (Al–O–P and Al–O–Pr) and the corresponding
4
−
1
symmetric vibration appeared between 800 and 600 cm .
PrAlPO-5 molecular sieves showed a band between 512
and 580 cm due to Pr–O linkage, which was absent in
AlPO-5. The band appeared below 500 cm is due to
vibrations in double ring region.
−1
−
1
(
B)
3
.11. Thermogravimetric Analysis (TGA)
The thermograms of assynthesised AlPO-5 and PrAlPO-5
(
(Al + P)/Pr = 25ꢀ50ꢀ75 and 100) are shown in
Figure 13(a). All the samples showed a weight loss between
7
weight loss below 100 C increased with increase in the
metal content suggesting the hydrophilic nature of the cat-
alysts. The second weight loss between 200 and 560 C
ꢀ
0 and 100 C due to removal of physisorbed water. The
ꢀ
ꢀ
was due to removal and decomposition of the template. The
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removal of template occurred in two stages in the case of
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PrAlPO-5 molecular sieves. First the template adsorbed at
Copyright: American Scientific Publishers
weak acid sites lost below 400 C, and then the decom-
ꢀ
ꢀ
ꢀ
position of template occurred between 400 C and 560 C
due to the presence of strong acid sites. Similar behavior
Fig. 13. (A) TGA results of as-synthesized (a) AlPO-5, (b) PrAlPO-
(100), (c) PrAlPO-5(75), (d) PrAlPO-5(50) and (e) PrAlPO-5(25).
B) TGA results of calcined (a) AlPO-5, (b) PrAlPO-5(100), (c) PrAlPO-
5
(
4
2
was already reported in the literature. The thermograms
of calcined AlPO-5 and PrAlPO-5 molecular sieves are
shown in Figure 13(b). This confirmed not only the com-
plete removal of template but also the hydrophilic nature
of metal incorporated AlPO-5 molecular sieves.
5(75), (d) PrAlPO-5(50) and (e) PrAlPO-5(25).
to form a tertiary radical which eliminates a molecule of
water resulting acetophenone. The reaction was carried out
ꢀ
between 60 and 140 C. There was practically no reaction
3
.12. Catalytic Activity
in this temperature range in the absence of catalyst. Fur-
ther, the reaction did not proceed in nitrogen atmosphere
instead of air atmosphere. This supported the proposed
reaction mechanism.
Solvent free liquid phase aerobic oxidation of ethylben-
zene was carried over PrAlPO-5 molecular sieves in the
temperature range 60–140 C. Acetophenone was found
ꢀ
to be the major product (> 90%) and 1-phenylethanol,
2-phenylethanol, phenyl acetaldehyde and phenyl acetic
acid as minor products. The influence of reaction param-
eters such as temperature, reaction time, substituents and
Pr content was also studied. The plausible mechanism
for the reaction is depicted in Scheme 1. In this mecha-
nism, Pr3 is oxidized to Pr by the chemisorbed oxy-
gen. This then abstracts a hydrogen from the methylene
group of ethylbenzene, thus forming metal hydroperoxide
and phenyl ethyl radical. The formation of 1-phenylethanol
is attributed to the reaction between metal hydroperox-
ide and phenyl ethyl radical. Further, the oxygen radical
+
4+
4
+
present in Pr abstracts a hydrogen from 1-phenylethanol
Scheme 1. Plausible mechanism for the oxidation of ethylbenzene.
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013
2513

Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
Sundaravel et al.
Table II. Effect of temperature on ethylbenzene conversion and selectivity to acetophenone over PrAlPO-5 catalysts.
Selectivity (%)
1-Phenylethanol
ꢀ
a
Catalyst
Temperature ( C)
Conversion (%)
Acetophenone
Others
PrAlPO-5(100)
60
8
17
25
29
31
14
28
39
46
50
22
45
73
86
88
28
49
82
95
95
88
89
88
88
83
89
89
88
89
82
93
91
94
92
85
92
94
93
95
90
10
9
9
10
8
7
8
8
8
8
4
5
3
4
3
5
3
4
2
2
2
2
3
2
9
4
3
4
3
10
3
4
3
4
12
3
3
3
3
80
100
120
140
60
PrAlPO-5(75)
PrAlPO-5(50)
PrAlPO-5(25)
80
100
120
140
60
80
100
120
140
60
80
1
1
1
00
20
40
8
a
Notes: Reaction time: 6 h, Catalyst: 0.2 g, Determined by GC-MS.
3
.12.1. Effect of Temperature
1-phenylethanol selectivity also supported this view. As
-phenylethanol is a neutral compound, and there is also
an appreciable steric hindrance for adsorption on Pr sites
1
Liquid phase oxidation of ethylbenzene in the presence
of air was carried over PrAlPO-5 (25, 50, 75 and 100)
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through its –OH group, it can rapidly diffuse out. Hence,
ꢀ
catalysts in the temperature ran IgP e: 62 01 –9 1. 41 02 9 .C1 .4 T5 h. 1e 0 e1 x pO e nr -: Tue, 22 Mar 2016 05:34:19
increase of framework Pr content in AlPO-5 could rapidly
imental results are presented in Tabl eC oI Ip. y Tr i hg eh t e: t Ah yml eb r ei cn a- n Scientific Publishers
converted 1-phenylethanol to acetophenone. This study
revealed the dependence of acetophenone selectivity on the
framework Pr content in AlPO-5.
zene conversion and acetophenone selectivity were found
to be maximum at 120 C. When the reaction tempera-
ture was increased above 120 C, there was no significant
ꢀ
ꢀ
change in selectivity of 1-phenylethanol whereas selec-
tivity to others increased appreciably. The acetophenone
selectivity also decreased considerably above this reac-
tion temperature. 2-Phenylethanol, phenyl acetaldehyde
3
.12.3. Effect of Reaction Time
The influence of reaction time on conversion and selec-
tivity was studied between 1 and 10 h over PrAlPO-5(25)
at 120 C and the results are depicted in Figure 14. The
ꢀ
and phenyl acetic acid were formed at 140 C due to
activation of methyl group. Since the formation of ace-
tophenone from 1-phenylethanol is a rapid reaction, the
formation of 1-phenyl ethane-1,2-diol is ruled out. The
competition of additional reaction forming phenyl acetic
acid suppressed the 1-phenylethanol formation and this
decreased the selectivity to acetophenone. Hence, ethyl-
benzene conversion remained the same but the selectiv-
ity to acetophenone decreased over PrAlPO-5 catalysts
ꢀ
percentage conversion of ethyl benzene increased from 1
ꢀ
above 120 C.
3
.12.2. Effect of (Al+P)/Pr Ratios
PrAlPO-5 with different (Al+P)/Pr ratios viz. 25, 50, 75
and 100 were used for the aerobic oxidation of ethylben-
zene and the results are presented in Table II. PrAlPO-5
with (Al + P)/Pr ratio 25 showed slightly higher selec-
ꢀ
tivity to acetophenone than others at 120 C. Since Pr
content in this catalyst was high, it could rapidly con-
verted 1-phenylethanol to acetophenone. The decrease of
Fig. 14. Effect of reaction time on ethylbenzene oxidation.
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013
2
514

Sundaravel et al.
Praseodymium Incorporated AlPO-5 Molecular Sieves for Aerobic Oxidation of Ethylbenzene
Table III. Effect of substituent on benzylic oxidation.
was carried out using ICP-OES. The catalyst recovered
from reaction mixture after 2 h of reaction time was
washed with 5% dilute nitric acid, dried and dissolved
completely in aqua regia. The elemental analysis of this
solution was also performed using ICP-OES. The results
showed almost the same praseodymium content in the cat-
alyst before and after the reaction. Hence it is concluded
that praseodymium was intact in AlPO-5 and well incor-
porated in the framework. The recovered catalyst after the
O
PrAlPO-5(25)
R2
R2
air, 120 ºC, 6 h
R1
R1
Conversion Selectivity
a
S. no.
R1
R2
(%)
(%)
Major product
Acetophenone
1
2
3
4
5
6
7
8
9
CH3
CH3
CH3
H
Cl
Br
F
95
94
94
93
91
90
91
94
94
94
96
95
94
95
95
93
94
93
93
93
94
95
4-chloro acetophenone
4-bromo acetophenone
4-fluoro acetophenone
4-iodo acetophenone
4-nitro acetophenone
4-nitro benzaldehyde
4-methoxy acetophenone
Benzaldehyde
ꢀ
reaction was washed well with ether and dried at 200 C.
CH3
The recycled catalyst was used in the reaction for five
times under similar conditions, and it was found that the
catalyst showed similar activity up to five cycles.
CH3
I
CH3 NO2
NO2
CH3 OCH3
H
H
H
H
OCH3
H
4. CONCLUSION
1
1
0
1
4-methoxy benzaldehyde
Benzophenone
C H5
6
The hydrothermal synthesis of PrAlPO-5 with different
(Al + P)/Pr ratios was successfully accomplished in the
fluoride medium. ESR study confirmed the presence of
chemisorbed oxygen in the catalyst which concluded that
this catalyst is found suitable for oxidation reactions. The
TPR study revealed the absence of free Pr O and PrO
which concluded that the oxidation reaction proceeded via
chemisorbed oxygen in the aerobic oxidation of ethyl-
benzene. The decrease of (Al + P)/Pr ratio increased the
conversion and selectivity. This correlated the dependence
a
Determined by GC.
to 6 h reaction time. Though the ethylbenzene conver-
sion remained the same upto 10 h, the selectivity to ace-
tophenone decreased. The concentration of acetophenone
is found to be maximum after 6 h reaction time. It is
2
3
2
presumed that a small amount of acetophenone adsorbed
on the acid sites and further oxidized to benzoic acid.43
Since the selectivity to acetophenone was found to be max-
imum at 6 h reaction time, this was chosen as the optimum
condition.
Delivered by Publishing Technolog yo ft o P: rP uc ro dn ut ee n tU ni niv eA rl sP i Ot y- 5L i ba rn ad ri er es activity. It is also con-
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Copyright: American Scientific Publishers
ated by the framework incorporation of praseodymium
in AlPO-5 favored the side chain oxidation rather than
3
.12.4. Effect of Various Substituents
ring hydroxylation. The electron density in the aromatic
ring did not influence the selectivity to acetophenone.
This study concluded that ethylbenzene and different sub-
stituted ethylbenzenes can be effectively oxidized using
Aerobic oxidation of ethylbenzene over PrAlPO-5(25) at
1
ꢀ
20 C yielded acetophenone as the major product along
with 1-phenylethanol as minor product. The various sub-
stituents in the aromatic ring changed the electron density
around the benzylic hydrogen atom. As stated already the
reaction proceeded via hydrogen abstraction mechanism.
The free radical formed in the proposed mechanism could
be stabilized either by electron releasing or electron with-
drawing substituent. Hence, the electron density around the
benzylic hydrogen may not alter the selectivity to form the
respective carbonyl compound. The benzylic oxidation of
various substituted ethylbenzenes was attempted and the
results are presented in Table III. All the substituted com-
pounds exhibited almost similar selectivity (>90 %) under
the given reaction conditions. Since the free radical formed
at benzylic carbon is resonance stabilized by both electron
releasing and electron withdrawing substituents in the para
position, this catalyst is found to be suitable for the oxi-
dation of different substituted ethylbenzene compounds.
ꢀ
molecular oxygen as oxidant over PrAlPO-5 at 120 C.
Acknowledgment: The authors gratefully acknowledge
the financial support from the Department of Science
and Technology (DST) (Sanction No. SR/S1/PC-10/2009,
Dated 28.10.2009). One of the authors, B. Sundaravel is
thankful to DST for the award of JRF position in this
project.
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IP: 219.129.145.101 On: Tue, 22 Mar 2016 05:34:19
Copyright: American Scientific Publishers
Received: 30 October 2012. Accepted: 9 December 2012.
2
516
J. Nanosci. Nanotechnol. 13, 2507–2516, 2013