Indian Journal of Chemistry
Vol. 50A, August 2011, pp. 1050-1055
Notes
Surface acidic properties of sulphated
Fe2O3-TiO2
K Joseph Antony Raj, M G Prakash, R Shanmugam,
K R Krishnamurthy & B Viswanathan*
National Centre for Catalysis Research,
Indian Institute of Technology Madras, Chennai 600 036 India
Email: [email protected]
Received 5 May 2011; revised and accepted 11 July 2011
Mixed iron and titanium oxides with varying quantities of
sulphate species have been prepared by sulphation of ilmenite
followed by calcination. A combined approach involving DFT
and DRIFT spectroscopy has been adopted to determine the
appropriate structure for adsorption of pyridine on sulphated
Fe2O3-TiO2 (SFT). The DRIFT spectra obtained for pyridine
adsorbed SFT samples show equal number of Brønsted and Lewis
acid sites. The TG-DTG analysis reveals the greater affinity for
water and hydroxyl groups on SFT. Evaluation of IR frequencies
for various structures based on DFT demonstrates that the
adsorption of water/hydroxyl groups on S and/or Fe leads to the
conversion of Lewis acid sites into Brønsted acid sites.
Investigation of Mulliken charges for Fe, Ti, S and N for the
pyridine adsorbed SFT structures demonstrates the preferable
adsorption of pyridine on sulphur rather than on Fe.
Keywords: Catalysis, Acidity, Density functional calculations,
Sulphated iron oxide, Iron oxide, Titania
Acid catalysis is of fundamental industrial
importance. Amongst the many solid acid catalysts,
the sulphate promoted metal oxides such as ZrO2,
TiO2 and Fe2O3 have gained importance due to their
high catalytic activity, selectivity, thermal stability
and reusability1,2. Although the life of the catalyst is
limited, there is scope for saving of energy as these
sulphated metal oxides can catalyze reactions at low
temperatures.
The esterification of oleic acid with glycerol3,
preparation of dioctyl phthalate4, butyl phenols5,
liquefaction of coal6-8, ring-opening isomerisation of
cyclopropane9,
dehydration
of
2-butanol10,
polymerization of alkyl vinyl ether11, and
isomerisation of n-butane12,13 can be effectively
catalysed by sulphated metal oxides. The doping of
sulphated zirconia with transition elements such as Fe
and Mn can favorably influence the rate of
isomerisation of n-butane14. Tanabe et al.9 showed
that the generation of strong acidity in sulphated
metal oxides was due to the existence of covalent
S=O bonds on the surface of metal oxides. The
pyridine molecule which is employed for identifying
the Brønsted and Lewis acid sites in sulphated metal
oxides may get adsorbed either on the metal or on the
sulphur. Lee et al.15 showed the adsorption of pyridine
on sulphur while Jin et al.16 suggested that pyridine is
coordinated to metal sites. Saur et al.17 reported that
the generation of Brønsted acid sites on sulphated
Fe2O3 was due to the presence of moisture in the
sample. Yamaguchi et al.18 showed the high catalytic
activity of sulphated Fe2O3 to the presence of S(IV) in
SO bonds. Hua et al.19 reported the enhancement in
catalytic activity of sulphated metal oxides when
modified with alumina. Ramadan et al.20 reported the
preparation of sulphated TiO2-ZrO2 and the variation
of the acidity of the system with temperature. Das et al.21
studied the synthesis of sulphated TiO2-ZrO2 and its
catalytic activity for isopropylation of benzene.
York et al.22 examined the synthesis of sulphated
Fe2O3-TiO2 and its activity for the photodegradation
of 4-chlorophenol.
In the present work, the synthesis of sulphated
Fe2O3-TiO2 using ilmenite and sulphuric acid is
reported. The sulphation of ilmenite generates
interesting structural effects in terms of acidity. The
discrimination of Brønsted and Lewis acid sites on the
surface of sulphated Fe2O3-TiO2 adsorbed with
pyridine is studied with DRIFT spectroscopy and
density functional calculations (DFT). The structures of
pyridine adsorbed sulphated Fe2O3-TiO2 is necessary
for an understanding of how these materials would
perform as catalysts. Hence, the various possible
structures of pyridine adsorbed sulphated Fe2O3-TiO2
are studied with DFT to identify the appropriate site of
adsorption of pyridine on Fe/Ti or S.
Experimental
Ilmenite ore (ball milled, 10 g) was homogenously
mixed with conc. H2SO4 (20 g) and aged for 2 h at
30°C. To this mixture, 10 g of water was added with
stirring to initiate the reaction. The reaction mass was
subjected to constant stirring for about one hour.
Thereafter it was treated with 100 g of water to
remove any soluble residues. The solid mass obtained
after several washing with distilled water was dried at
NOTES
100°C for 12 h. The samples were calcined in air at
500°C (SFT-500) and 700 °C (SFT-700) to prepare
Fe2O3-TiO2 containing various levels of sulphate.
The acidity of the samples was measured with
DRIFT spectroscopy using pyridine as the probe
molecule. The DRIFT spectra were also recorded for
the samples without pyridine adsorption using a
Bruker Tensor-27 instrument. TG/DTG analyses were
made using a Perkin-Elmer instrument and the
measurements were run under air with a temperature
ramp of 2 °C/min between 40 and 900 °C using
alumina as the reference. The composition of the
catalysts was analyzed using an XRF spectrometer. A
sulphate content of 5.1 wt % and 0.4 wt % were
obtained for SFT-500 and SFT-700 respectively.
Quantum chemical computations were performed
on the structures of pyridine adsorbed sulphated
Fe2O3-TiO2 in order to obtain their IR frequencies and
thereby to determine the optimum geometry. The
molecular geometries of the model structures were
fully optimized by density functional theory using
Gaussian 03 software23 without any geometrical
restrictions. The effects of electron correlation on the
geometry optimization were taken into account by
using Becke’s three parameter exchange functional
with Lee-Yang-Parr gradient-corrected correlation
functional (B3LYP)24 in conjunction with the Los
Alamos ECP plus DZ basis sets (Lanl2DZ)25. The
vibrational frequencies were calculated for the
optimized structures to ascertain that the structures
correspond to the potential minimum.
Results and discussion
The characteristics of sulphate species formed on the
surface of Fe2O3-TiO2 during calcination were
examined using DRIFT spectroscopy. Figure 1 shows
the DRIFT spectra of the SFT samples obtained in the
range of 600-4000 cm-1. A peak observed at 1160 cm-1
for SFT-500 is generally attributed to asymmetric
stretching characteristic of sulphate vibrations26,27.
The other absorption bands around 840, 940, 1050
and 1235 cm-1 are assigned to S=O symmetric and
S-O asymmetric stretching27. The SFT-700 sample
showed only two broad bands at 840 and 1170 cm-1.
The absence of other peaks is attributed to the
removal of (92 wt %) sulphates from the sample when
the calcination temperature is increased beyond
500 °C. Generally, polynuclear complex sulphates of
S2O72- and/or S3O102- types are characterized by
absorptions between 1400 and 1600 cm-1 (ref. 28).
This region is devoid of any peaks for the SFT
1051
samples indicating the absence of such polynuclear
sulphates. The absorption peaks around 3400 and
1640 cm−1 are due to the stretching and bending
modes of adsorbed water and hydroxyl groups.
The calcined SFT samples were evacuated at
300 °C for 2 h before the adsorption of pyridine. The
adsorption was performed by exposing the samples to
pyridine vapor at 150 °C and 0.01 bar pressure for
30 min. The desorption of pyridine was carried out at
150 °C for 1 h and thereafter the samples were cooled
to room temperature and the DRIFT spectra were
recorded. Figure 2(a) shows the DRIFT spectra for the
pyridine adsorbed on SFT-500 and SFT-700 samples.
The spectrum of SFT-500 shows bands both due to
pyridinium ion (1540 cm-1) and coordinated pyridine
(1485 cm-1) which indicates, respectively, the
presence of Brønsted and Lewis acid sites. In
addition, the intensity of the band at 1540 cm-1 is
moderately greater than the band at 1485 cm-1 which
suggests the presence of Brønsted acid sites in greater
amounts in SFT-500 sample. On the other hand, for
SFT-700, no peaks are observed at 1485 and 1540 cm-1
which reveals the absence of acid sites and sulphates
on the surface of the sample. An increase in calcination
temperature from 500 to 700 °C leads to the removal of
sulphate species and the accompanied loss of acid sites.
The sulphate groups are removed to the tune of about
92 %. Hence, it is apparent from the pyridine
adsorption studies that the presence of sulphate on the
Fig. 1  DRIFT spectra of the sulphated Fe2O3-TiO2 (SFT)
samples calcined at (1) 500 oC (SFT-500) and (2) 700 oC
(SFT-700).
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INDIAN J CHEM SEC A, AUGUST 2011
Fig. 2  (a) DRIFT spectra measured for pyridine adsorbed
sulphated
Fe2O3-TiO2
(SFT-500
and
SFT-700) and
(b) IR frequencies of the pyridine adsorbed sulphated
Fe2O3-TiO2 structures (a) – (f) obtained by DFT studies
[a-f structures as given in Fig. 3].
surface of Fe2O3-TiO2 causes the generation of acid
sites. The loss of acid sites when the SFT sample was
calcined at 700 °C suggests that the catalysts can be
effectively used only at less than 500 °C.
The sulphated metal oxides are anticipated to give
IR absorption band at 1379 cm-1 due to the
asymmetric vibration of the S=O bond which is used
as a fingerprint for moisture-free sulphated metal
oxide samples15. However, the DRIFT spectra
obtained for the SFT samples and IR frequencies
obtained by DFT for various structures of sulphated
Fe2O3-TiO2 samples (vide-infra) showed no peaks in
the region of 1300–1380 cm-1 indicating the
non-removal of the adsorbed hydroxyl groups and/or
water molecules in SFT samples consequent to
vigorous calcination at 500 °C.
The various possible structures of pyridine
adsorbed on sulphated Fe2O3-TiO2 are optimized with
DFT and are shown in Fig. 3. The structures ‘a’ and
‘b’ with bridged bidentate sulphated Fe2O3-TiO2 and
iron oxide, respectively, showed no change in the
bond order for S=O on optimization by DFT. In
structure c, pyridine is attached on Fe of the chelating
bidentate sulphated Fe2O3. Structure d is similar to
structure c; with oxygen on Fe is replaced with H2O.
Fig. 3  Proposed structures of pyridine adsorbed sulphated Fe2O3-TiO2.
NOTES
Structure e is similar to structure c, with the S=O
groups replaced with S-OH groups to study the effect
of hydroxyl groups attached on the sulphur.
Structure f is similar to structure e except that
pyridine is attached on sulphur. Structure g is similar
to structure c except that pyridine is attached on
sulphur. Structure h is a bridged bidentate sulphated
Titania complex in which pyridine is adsorbed on
Ti site.
The IR absorptions of the pyridine adsorbed SFT
(sulphated Fe2O3-TiO2) structures, a – h, obtained by
DFT studies and the DRIFT spectra measured for
pyridine adsorbed on sulphated Fe2O3-TiO2 samples
are shown in Fig. 2(a) and (b). Structure a showed an
absorption band around 1480 cm-1 due to the presence
of Lewis acid sites. Structure b exhibited a band at
1489 cm-1 with a shoulder at 1510 cm-1, which may be
due to Lewis and Brønsted acid sites respectively.
Structure c showed bands at 1480 and 1520 cm-1 due
to Lewis and Brønsted acidity, respectively. However,
the intensity of Lewis acid band is greater than that of
the Brønsted acid band. Structure c and d are similar
except that one of the oxygens on Fe in the latter is
replaced with water molecule in order to study the
effect of moisture in the sample. A perusal of IR data
obtained for the structures c and d reveals that the
adsorption of water largely lowered the Lewis acidity
and enhanced Brønsted acidity for structure d.
Interestingly, structures e and f showed peaks at 1490
and 1540 cm-1 which is similar to the pattern obtained
by DRIFT spectra for pyridine adsorbed on SFT-500.
In structure e, pyridine is adsorbed on Fe while in
structure f, it is adsorbed on S and in both structures
the S=O groups are replaced with S-OH groups. An
assessment of the intensities of these two bands
suggests that structure e possesses moderately higher
Lewis acid sites than Brønsted acid sites. The IR
spectrum of structure f seems to have more
resemblance to the DRIFT spectra of SFT-500
sample. The IR absorptions of structure h are similar
to structure a which possesses only Lewis acidity. The
DFT studies on the structures of pyridine adsorbed
sulphated Fe2O3-TiO2 suggest that the sample is
susceptible to adsorption of hydroxyl groups or water
molecules. More than the adsorption of water, it is the
hydroxyl groups attached to the sulphur that generates
Brønsted acidity in SFT-500 sample. The sulphate
groups tend to lose their covalent character due to the
hydroxyl groups on sulphur thereby generating
Brønsted acid sites.
1053
The TG-DTG profile of SFT-500 is shown in Fig. 4.
The thermogram shows a weight loss of 5.9 % up to
220 °C which may be attributed to the removal of
water molecules on the surface of SFT-500. A weight
loss of 1.5 % obtained between 385 and 475 °C may
be attributed to the partial removal of sulphate and
hydroxyl groups. The final weight loss was observed
as two stages at 540 and 590 °C 4.9 % and 2.8 %
weightloss respectively. Although SFT-500 is a
non-porous material, it exhibited four stages of weight
loss which explains the strong adsorption of hydroxyl
and sulphate groups on its surface.
Kayo et al.10 showed an increase in intensity of
band in the region of 1340 – 1380 cm-1 and a decrease
in intensity of band in the region of 1240 – 1270 cm-1
when the sample evacuation temperature was
increased from 100 to 500 °C. However, in the
present study no band was observed in the region of
1340 – 1380 cm-1 for SFT-500 and SFT-700 (Fig. 1)
and for the SFT samples adsorbed with pyridine
(Fig. 2). However, the SFT samples adsorbed with
pyridine showed a broad band at 1230 cm-1. This
observation is essentially due to the adsorbed
hydroxyl groups or water molecules on the SFT
samples which could not be completely removed on
calcination.
As demonstrated by the earlier reports10,15,18,
structures a, b and c (Fig. 3) with no water molecules
adsorbed on them were anticipated to show a band in
the region of 1300–1380 cm-1 characteristic of
evacuated sulphated metal oxides. Nevertheless, these
Fig. 4  TG-DTG profile of sulphated Fe2O3-TiO2 calcined
at 500 oC.
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INDIAN J CHEM SEC A, AUGUST 2011
structures displayed a less intense band at 1390 and a
more intense band in the region of 1230 – 1260 cm-1.
Structure d with water molecules adsorbed on Fe as
expected, showed a band at 1250 cm-1. In addition,
structure d with adsorbed water molecule on iron and
structures e and f with hydroxyl groups adsorbed on
sulphur showed band in the region of
1410–1430 cm-1. The band at this region is not
observed for structures a, b and c with no hydroxyl or
water molecules adsorbed on them. However, the
DRIFT spectrum of SFT-500 adsorbed with pyridine
showed a low intense band at 1420 cm-1 (Fig. 2(a)),
which is characteristic of hydroxyl or water molecules
present on the structure.
The adsorption of pyridine on either sulphur or Fe
has been reported15,16. The IR frequencies obtained for
various structures revealed the possibility of
adsorption of pyridine on sulphur of the sulphated
Fe2O3. In addition, the Mulliken charges obtained for
Fe (0.33), Ti (1.15), S (1.23), and N (-0.3) of the
pyridine adsorbed sulphated Fe2O3 suggests a stronger
interaction between S and pyridine than between Fe
and pyridine. The charge on S and Ti is almost the
same as that in sulphated titania and both types of
interactions may be anticipated. The reactivity of the
sulphate group should be taken in to consideration
when pyridine is employed as a probe molecule for
the determination of surface acidity. This also
suggests that interactions between sulphate group and
any other reactants are possible when the SFT sample
is used as an acid catalyst.
The results of the present study show that complete
removal of all the adsorbed hydroxyl groups and/or
water molecules from the sample does not occur
subsequent to the calcination at 500 oC for 2 h. The
calcination at 700°C removes the moisture along with
sulphates present in the sample. In general, the
catalytic reactions are enhanced by the presence of
small quantity of moisture in the sulphated metal
oxide system and water is not known to affect the
stability of sulphated metal oxides. However, the
adsorption of basic molecules such as ammonia or
pyridine may destabilize the sulphated metal oxides as
it enhances the removal of sulphates at lower
temperature.
In the present study, the sulphated Fe2O3-TiO2
obtained by sulphation of ilmenite shows lowering of
covalent character due to adsorption of hydroxyl
groups and pyridine on sulphur. The tendency of
sulphate groups on SFT to adsorb basic pyridine
molecule
demonstrates
the
strong
acidic
characteristics of SFT. The acidic characteristics of
SFT can be enhanced by the charge and co-ordination
number of the metal in addition to the inductive effect
of S=O. The DRIFT spectra obtained for pyridine
adsorbed SFT samples indicate the presence of
Brønsted and Lewis acid sites in equivalent amounts.
The TG-DTG profiles highlight the greater affinity of
SFT for water and hydroxyl groups, while the DRIFT
spectra and DFT studies indicate that calcination of
the samples does not remove the adsorbed
water/hydroxyl groups completely. The evaluation of
IR frequencies for various structures based on DFT
demonstrate that the adsorption of water/hydroxyl
groups on Fe and/or sulphur sites is the origin for the
conversion of Lewis acid sites into Brønsted acid
sites. Investigations of Mulliken charges for Fe, Ti, S
and N for the pyridine adsorbed SFT structures, show
preferred adsorption of pyridine on sulphur rather
than on Fe. It is apparent from the above studies that
adsorption of pyridine or any other reactants takes
place on sulphate groups of SFT rather than on
Fe or Ti.
Acknowledgement
The authors acknowledge the Department of
Science and Technology, Government of India for
funding the National Centre for Catalysis Research
(NCCR) at IIT Madras, Chennai.
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