Partial oxidation of toluene by O
2
over mesoporous Cr–AlPO
Ch. Subrahmanyam
a
, B. Louis
b
, F. Rainone
b
, B. Viswanathan
a,
*
, A. Renken
b
,
T.K. Varadarajan
a
a
Department of Chemistry, Indian Institute of Technology – Madras, Chennai 600 036, India
b
Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
Received 30 July 2001; accepted 14 December 2001
Abstract
The synthesis of mesoporous Cr–AlPO was carried out under hydrothermal conditions. Characterisation of the
catalyst was done with low angle XRD, N
2
adsorption, UV–VISDRS, ESR and thermal analysis. The catalytic activity
was tested for partial oxidation of toluene with molecular oxygen in vapour phase. It was observed that Cr–AlPO
functions both as acid and redox catalyst. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
The incorporation of transition metal ions into
the framework sites of microporous aluminosili-
cates and aluminophosphates has been reported in
the literature and resulting systems are potential
catalysts for various selective redox reactions [1–
3]. Oxidation of alkylbenzenes is one of the inter-
esting reactions because of the importance of the
products for various industrial applications. Both
homogeneous and heterogeneous catalysts have
been used for these reactions [4,5]. But for the
large-scale production of fine chemicals, replace-
ment of conventional homogeneous systems by
heterogeneous catalysts will be advantageous in
the sense of catalyst recovery and reduction of
undesirable side reactions. In liquid phase, these
reactions have been catalysed by variety of heter-
ogeneous microporous catalysts like TS-1, VS-1,
CrS-1 with peroxides as oxidants [1,4,6]. Cr–
AlPO-5 was shown to be an active and recyclable
catalyst for the oxidation of alkylbenzenes with
either TBHP or O
2
[7]. However, the activity of
microporous catalyst is limited due to the pore size
and stability of the active metal ion in the mic-
roenvironment. With the recent discovery of mes-
oporous materials, the activities of chromium
containing MCM-41, MCM-48, HMS have been
tested for the oxidation of alkylbenzenes with
peroxides as oxidants [8–10]. However, the po-
tential of chromium containing porous solid cat-
alysts in liquid phase is limited because of the
leaching of the active metal from framework.
Moreover, it is always desirable to choose molec-
ular oxygen as oxidant due to its availability in the
nature. The potential of chromium containing
Catalysis Communications 3 (2002) 45–50
www.elsevier.com/locate/catcom
*
Corresponding author. Tel.: +91-44-4458250; fax: +91-44-
2350509.
E-mail address: bviswanathan@hotmail.com (B. Viswana-
than).
1566-7367/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 566 - 7 3 6 7 ( 0 1 ) 0 0 0 7 0 - X
catalysts can be increased by using molecular ox-
ygen as oxidant and thereby carrying the reaction
in vapour phase it is possible to control the
leaching of chromium from the framework. In this
communication, the synthesis, characterisation
and catalytic activity of mesoporous Cr–AlPO for
toluene oxidation with molecular oxygen are re-
ported.
2. Experimental
2.1. Synthesis
Mesoporous chromiumaluminophosphates were
prepared using cetyltrimethylammoniumbromide
(CTAB) as surfactant and with the following gel
composition. Al
2
O
3
:xP
2
O
5
: yCr
2
O
3
:zCTAB:TM
AOH:wH
2
O, where x ¼ 2:0–2:5; y ¼ 0:01–0:2, z ¼
0:4–0:5 and w ¼ 300–350. Various sources of alu-
minium have been tried and aluminium hydroxide
has been chosen as the source. Chromium nitrate
was used as transition metal precursor. The pH of
the gel was maintained at 9.5 with tetra methyl
ammonium hydroxide, as the use of other sources
like NaOH and NH
4
OH resulted only in the
amorphous materials. The resulting gel was aged at
room temperature for 3 h, transferred into a stain-
less steel autoclave and hydrothermally treated at
423 K for 24 h. The solid was filtered, washed several
times with deionised water and calcined at 773 K for
6 h to remove the organic template.
2.2. Experimental setup and procedure
The experimental setup consisted of three parts:
the gas supply system, the reactor and the analysis
system. The gasses O
2
(99.995%), and Ar
(99.998%) (Carba-Gas, Lausanne, Switzerland)
were used without further purification. The feed
was regulated through mass flow controllers. Flow
A contained oxygen, argon and toluene vapour.
Flow B was used for the pretreatment of the cat-
alyst with oxygen diluted in argon. Gas mixtures A
and B were mixed at a pressure of 101 kPa. The
loading of the catalyst and the gas flow were
maintained constant throughout the study at 0.2 g
and 1 ml/s. The catalyst was diluted with quartz
powder in a 1:1 ratio. All lines and valves were
heated up to 423 K in order to avoid the con-
densation of the products. The catalyst was pre-
treated in O
2
(40-vol% O
2
, rest Ar) at 573 K before
the reaction. The temperature was decreased and
the flow was switched to the mixture of 2 vol%
toluene plus 40-vol% O
2
in Ar.
2.3. Characterisation of the catalyst and reaction
products
Various techniques have been used for the
characterisation of the materials synthesised.
The low angle X-ray diffraction pattern of the
sample was recorded on a Siemens D 500 ðh=2hÞ
using monochoromatised Cu-Ka radiation ðk ¼
1:5406
AAÞ with a scan speed of 1°/min over the
range 2 < 2h < 10°. Thermal analyses of the
samples were made with thermal analyser (Perkin–
Elmer model TGA 7) at a heating rate of 20 °C/
min. Diffuse reflectance UV–VIS spectroscopy was
carried out on a Cary 5E UV–VIS–NIR spectro-
photometer. ESR spectra were recorded with
Varian E-112 spectrometer at room temperature.
N
2
adsorption–desorption measurements at 77 K
were made using CE instruments, Sorptomatic
1990. The sample was out gassed at 473 K for 12 h.
X-ray photoelectron spectroscopic measurements
(XPS) for Cr
2p
were performed on a PHI-550
ESCA-System (Perkin–Elmer GmbH). A Blazers
QMG-421 mass-spectrometer and a Perkin–Elmer
Autosystem XL gas chromatograph were used for
the gas phase analysis. Toluene and organic
products were separated in an SPB-5 capillary
column and analysed by a FID. Ar=O
2
, CO, CO
2
and H
2
O were analysed in a Carboxen-1010 cap-
illary column and analysed by a TCD.
3. Results and discussion
3.1. XRD
XRD patterns of as-synthesised and calcined
AlPO, Cr–AlPO are shown in Fig. 1. XRD pat-
terns of AlPO show low angle peaks typical for
hexagonal phase. The as-synthesised Cr–AlPO
material shows a maximum intense peak corre-
46 C. Subrahmanyam et al. / Catalysis Communications 3 (2002) 45–50
sponding to ð100Þ reflection followed by weaker
but clear peaks corresponding to ð110Þ and ð200Þ
reflections that can be indexed on the basis of a
hexagonal lattice. After calcination, broadening of
the maximum intense ð100Þ reflection with a slight
decrease in d spacing value was observed. In the
case of Cr–AlPO, intensity of the XRD peak is less
compared to AlPO material. However, the sample
retained its hexagonal structure after calcination.
An organic base, tetramethylammonium hy-
droxide (TMAOH), was used to maintain the pH
of the gel. However, with NaOH and NH
4
OH
only amorphous materials were formed. The
function of organic ammonium cation from
TMAOH is probably to modify the strength of the
electrostatic interactions between the alumino-
phosphate species and the cationic surfactant mi-
celle assembly to form the S
þ
I
=TMA
þ
ion pair. If
either NaOH or NH
4
OH is used, the smaller ca-
tions Na
þ
,NH
þ
4
compete with the aluminophos-
phate species and thus restrict the interaction with
the positively charged cationic surfactant. As sta-
ted earlier, aluminium hydroxide may form a less
polymerised aluminophosphate with many hy-
droxyl groups and favour the assembly of the
mesostructure compared to other aluminium
sources [11,12].
3.2. Thermal analysis
Thermogram of as-synthesised ALPO shows
mainly two weight loss regions one corresponding
to loss of physi-sorbed water below 373 K. The
second and the main weight loss was observed in
the temperature range 450–550 K corresponding
to loss of organic template. It was observed that
the AlPO is stable up to 1073 K.
3.3. N
2
adsorption
Nitrogen adsorption–desorption isotherms of
the Cr–AlPO are shown in Fig. 2. For all samples,
the isotherms are similar having inflection around
p=p
0
¼ 0: 2–0.3 characteristic of mesoporous ma-
terials. BET surface area and BJH pore size dis-
tribution for various catalysts are given in Table 1.
It was observed that Cr–ALPO exhibit type IV
isotherm with a hysteresis characteristic of meso-
porous material. BET surface areas of the AlPO
and Cr–AlPO are 645 and 500 m
2
=g, respec-
tively. The decrease in the surface area of the Cr–
AlPO compared to AlPO could be due to partial
loss of crystallinity, which is in agreement with the
observation from XRD. In both the cases the pore
size is around 29
AA.
3.4. UV–VIS DRS
UV–VIS spectroscopy is a technique for the
characterisation of transition–metal-incorporated
zeolites [13–15]. Fig. 3 shows UV–VISDRS spectra
of as-synthesised and calcined Cr–AlPO samples.
As-synthesised material shows bands around 610,
440 and 270 nm. These bands are characteristic of
Fig. 2. N
2
adsorption–desorption isotherms of Cr–AlPO.
Fig. 1. XRD pattern of Cr–AlPO: (a) uncalcined, (b) calcined.
C. Subrahmanyam et al. / Catalysis Communications 3 (2002) 45–50 47
trivalent chromium in octahedral co-ordination.
Due to the large difference in LFSE values of
Cr(III) for tetrahedral (66.9 KJ/mol) and octahe-
dral (224.5 KJ/mol) geometries, chromium atoms
in as-synthesised material mainly occupy extra
framework sites [16–18]. Upon calcination, a new
charge transfer band at 370 nm along with a
shoulder at 440 nm was observed. This CT band
could be due to O ð2pÞ!Cr
6þ
ð3d
0
Þ and/or
Cr
5þ
ð3d
1
Þ charge transfer transitions viz., chro-
mate or dichromate like species in tetrahedral en-
vironment [19].
3.5. ESR spectroscopy
ESR spectra of as-synthesised and calcined
Cr–AlPO are shown in Fig. 4. As-synthesised
material shows a broad singlet with a g value of
1.98 indicating Cr
3þ
ions in octahedral co-ordi-
nation [20,21]. ESR spectra of calcined Cr–AlPO
shows g value at 1.95 characteristic of pentava-
lent chromium in tetrahedral or distorted tetra-
hedral co-ordination. ESR signal intensity
decreased with increase in sharpness after calci-
nation.
3.6. Catalytic activity
Table 2 gives typical results for the oxidation
of toluene in the temperature range 523–648 K
over Cr–AlPO catalyst. Benzaldehyde, benzene,
CO
2
and CO are the reaction products. The re-
action sequence is given in Scheme 1. The absence
of coupled products indicates that radical mech-
anism is not taking place, rather surface type of
reaction is possible where toluene adsorbs on the
surface parallel to the lattice. It is clear that on
the surface of the catalyst both oxidation and
dealkylation reactions are taking place simulta-
neously leading to benzaldehyde and benzene.
The formation of benzene, as a result of dealky-
lation reaction predominates on acid ðAl
3þ
Þ sites,
whereas on redox sites ðCr
5þ=6þ
Þ oxidation of
toluene is taking place leading to benzaldehyde.
At lower temperatures, redox properties of Cr–
AlPO influence the reaction leading to the for-
mation of benzaldehyde in excess and as the
temperature is increased the selectivity towards
benzene increases, indicating the dominance of
acid sites. It was observed that with the increase
of the temperature, the conversion of the toluene
and selectivity of benzene increased. The catalyst
was reused after calcination in air for 5 h at 673
K, which showed the same activity for the suc-
cessive runs. Coke formation was not detected to
any significant extent.
Fig. 3. UV–VISDRS of Cr–AlPO: (a) uncalcined, (b) calcined.
Fig. 4. ESR spectra of Cr–AlPO: (a) uncalcined, (b) calcined.
Table 1
Physico-chemical properties of AlPO materials
Catalyst d
100
[uncal]
(
AA)
d
100
[cal] (
AA) a ¼ 2
d
100
=
ffiffiffi
3
p
ð
AAÞ
BET surface
area (m
2
=g)
Pore size (
AA) Pore volume
(cc/g)
AlPO 34.5 33.0 38.10 685 28 0.65
Cr–AlPO 35.0 33.4 38.56 490 29 0.51
48 C. Subrahmanyam et al. / Catalysis Communications 3 (2002) 45–50
4. Conclusions
Cr–AlPO materials have been prepared by hy-
drothermal synthesis and found to have hexagonal
MCM-41 like morphology. These materials have
high surface area 500 m
2
=g and pores are in
mesoporous range 29
AA. UV–VIS and ESR
techniques have confirmed the presence of Cr
5þ=6þ
in the framework. Cr–AlPO catalysts have been
shown to be active for the vapour phase oxidation
of toluene with molecular oxygen. It has been
observed that in Cr–AlPO both acidity (due to
Al
3þ
) and redox properties (due to Cr
5þ=6þ
) are
competing leading to benzene and benzaldehyde,
respectively.
Acknowledgements
The authors are grateful to Dr. Kiwi-Minsker
for fruitful discussions and to Mr. Xanthopoulos
and P. Mockli for kindly providing the XRD data
reported in this paper.
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Table 2
Catalytic activity of Cr–AlPO for the oxidation of toluene with molecular oxygen
Temperature (K) Conversion of
toluene (%)
Product selectivity (%)
Benzaldehyde Benzene ðCO
2
þ COÞ
523 0.75 91.0 2.4 6.6
548 0.90 83.5 2.6 13.9
573 1.40 60.4 3.0 36.6
598 2.23 50.6 6.1 43.3
623 4.85 42.4 8.2 49.4
648 9.19 25.3 12.4 62.3
Scheme 1. Proposed reaction scheme for the toluene oxidation
catalyzed by Cr–AlPO.
C. Subrahmanyam et al. / Catalysis Communications 3 (2002) 45–50 49
