TiO2-SiO2 mixed oxide supported MoO3 catalyst: Physicochemical characterization and activities in nitration of phenol

Article abstract of DOI:10.1016/j.molcata.2010.03.017

12 wt% MoO₃/TiO₂-SiO₂ solid acid catalyst was prepared and calcined at various temperatures. The calcined catalyst was characterized by XRD, FT-IR, BET, SEM, NH₃-TPD and pyridine adsorbed FT-IR methods. The effect of calcination temperature on activity of catalyst was studied by choosing liquid phase nitration of phenol as a model reaction. For the same reaction effect of various solvents, effect of reaction time and reusability of the catalyst was also studied. Catalyst calcined at 500 °C temperature showed highest phenol conversion whereas greater o-nitrophenol selectivity is claimed over catalyst calcined at 700 °C. It was observed that high phenol conversion co-relates with the presence of greater number of strong Bro?nsted acid sites over the catalyst surface whereas the selectivity of o-nitrophenol is related to the pore size of the catalysts. No use of sulfuric acid along with the nitric acid used in its diluted form in the reaction makes the process safe and environmentally friendly.

Full text of DOI:10.1016/j.molcata.2010.03.017

Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
TiO2–SiO2 mixed oxide supported MoO3 catalyst: Physicochemical
characterization and activities in nitration of phenol
S.M. Kemdeoa, V.S. Sapkalb, G.N. Chaudharia,∗
a Department of Chemistry, Shri Shivaji Science College, Amravati, Maharashtra 444601, India
b University Department of Chemical Technology, SGB Amravati University, Amravati, India
a r t i c l e i n f o
a b s t r a c t
Article history:
12 wt% MoO3/TiO2–SiO2 solid acid catalyst was prepared and calcined at various temperatures. The cal-
cined catalyst was characterized by XRD, FT-IR, BET, SEM, NH3-TPD and pyridine adsorbed FT-IR methods.
The effect of calcination temperature on activity of catalyst was studied by choosing liquid phase nitra-
tion of phenol as a model reaction. For the same reaction effect of various solvents, effect of reaction time
and reusability of the catalyst was also studied. Catalyst calcined at 500 ◦C temperature showed highest
phenol conversion whereas greater o-nitrophenol selectivity is claimed over catalyst calcined at 700 ◦C.
It was observed that high phenol conversion co-relates with the presence of greater number of strong
Brönsted acid sites over the catalyst surface whereas the selectivity of o-nitrophenol is related to the
pore size of the catalysts. No use of sulfuric acid along with the nitric acid used in its diluted form in the
reaction makes the process safe and environmentally friendly.
Received 14 January 2010
Received in revised form 11 March 2010
Accepted 12 March 2010
Available online 17 March 2010
Keywords:
MoO3/TiO2–SiO2
TiO2–SiO2
NH3-TPD
Nitration
© 2010 Elsevier B.V. All rights reserved.
o-Nitrophenol
1. Introduction
nitrated in water–ether system at room temperature [12]. More-
over, catalyst suspended in acetonitrile and treated with nitronium
Nitration of aromatic compound is widely used in organic
syntheses and industrial application. Among the various nitro-
aromatic compounds nitrophenols are important intermediate as
they are greatly utilizable in synthesis of drugs, pharmaceuticals,
dyestuff, perfumes, plastics and explosives [1]. Earlier industrial
nitration processes involve the use of nitric acid and sulfuric acid
mixtures (liquid phase) which are responsible for the major diffi-
culties like corrosion of processing equipments and generation of
large amount of hazardous waste [1,2]. Therefore lots of efforts had
been directed to seek environmentally friendly and reusable alter-
natives for such acid combination in recent year. Nitration using
solid acid may be answer to these problems as solid acid effectively
plays the role of sulfuric acid in the reaction, assisting the formation
of nitronium ions. Dedicated to above issue, many researchers tried
nitration of phenol using AcONO2/HNO3 [3], TfONO2 [4], N2O4 [5]
and N-nitropyrazole/BF3 [6]. Recently the nitration of phenol has
been attempted also on zeolites [7,8] modified silica [9], sulfated
zirconia [10], Mo/SiO2, H-Y, H-Beta and other solid acid catalysts
[11].
tetrafluroborate was also come up as excellent formulation in favor
of good o/p nitrophenol ratio [13]. But instability of active phase
and tedious workup procedures with these catalysts demands high
regioselective catalyst operable with simpler workup procedure in
nitration of phenol.
MoO3 supported on TiO2 is reported as excellent catalyst for the
selective oxidation of hydrocarbon [14], selective catalytic reduc-
tion of NOx by ammonia [15], alkene metathesis [16] and sulfided
hydrodesulfurization [17]. MoO3/SiO2 is another candidate suc-
cessfully tested for selective catalytic oxidation of ammonia to N2
and the direct conversion of methane to methanol [18,19]. Their
performance is also well known for transestrification of dimethyl
oxalate with phenol and nitration of benzene [20,21].
The type of support plays an important role on the catalytic
properties and for a given reaction the activity and selectivity of the
catalyst can be improved by the use of an appropriate support [22].
Thus intrinsic being characteristics of both titania and silica sup-
port can be explored fully by using them in combination. Therefore
the combined TiO2–SiO2 mixed oxide has attracted much attention
recently as a support [23–25].
In this context, we strive to introduce TiO2–SiO2 mixed oxide
supported MoO3 solid acid catalyst for nitration reaction of phenol
in the hope of avoiding the use of large volume of sulfuric acid and
adjusting the isomer proportion of nitrophenol in order to meet the
desired markets. This solid acid is also expected to offer significant
industrial benefits due to the simpler work up procedure, easy sep-
Concentrating upon regioselectivity, La (NO3) as a catalyst in the
presence of hydrochloric acid gives o/p ratio of 2.1 when phenol is
∗
Corresponding author.
E-mail address: nano.d@rediffmail.com (G.N. Chaudhari).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.03.017

S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
71
aration of catalyst, the nature of non-corrosiveness and superior
recycling.
Therefore in the present investigation 1:1 mole ratio TiO2–SiO2
mixed oxide support obtained via co-precipitation method was
impregnated with 12 wt% MoO3 using ammonium heptamolyb-
date and subjected to thermal treatment at various temperatures
in order to understand the dispersion and its subsequent effect on
the activity of catalyst. In this study thermal effects are examined
by different characterization techniques including XRD, FT-IR, SEM,
BET surface area, NH3-TPD and pyridine adsorbed FT-IR and activ-
ities of the catalysts are monitored by choosing nitration of phenol
as a model reaction.
2. Experimental
2.1. Catalyst preparation
1:1 mole ratio of TiO2–SiO2 mixed oxide support was prepared
by co-precipitation method using ammonia as a precipitating agent
[26]. Appropriate amount of cold TiCl4 (Loba chemi, AR grade) was
initially digested in cold conc. HCl and then diluted with doubly
distilled water. To this aqueous solution the required quantity of
Na2SiO3 (Loba chemi, AR grade) dissolved separately in deionized
water was added, excess ammonia solution (25%) was also added
to this mixture solution for better control of pH 8 and heated to
115 ◦C with vigorous stirring. Instantly a white precipitate formed
in the solution is stirred for 6 h additionally and the precipitate
was allowed to stand at room temperature for 24 h to facilitate
the aging. The precipitate thus obtained was filtered off and thor-
oughly washed with deionized water until no chloride ion could be
detected with AgNO3 in the filtrate. The obtained sample was then
oven dried at 120 ◦C for 16 h and finally calcined at 500 ◦C for 6 h in
an open air atmosphere.
Fig. 1. XRD pattern of calcined samples: (ꢀ) lines due to anatase phase; (ꢀ) rutile
phase of TiO2; (᭹) MoO3 and (ꢁ) SiO2.
chromatograms were recorded on Perkin-Elmer Autosystem XL,
with PE-1 column.
Molybdena (12 wt%) was deposited on 1:1 mole ratio of
TiO2–SiO2 mixed oxide support by adopting incipient wetness
method. To impregnate MoO3, calculated amount of ammonium
heptamolybdate was dissolved in doubly distilled water and few
drops of dilute NH4OH were added to make the solution clear and
to keep pH constant (pH 8). Finally, powdered calcined support was
then added to this solution and the excess of water was evaporated
on water bath with continuous stirring. The resultant solid is then
dried at 110 ◦C for 12 h, part of the obtained catalyst powder is again
calcined at 500, 600, and 700 ◦C for 5 h.
2.3. Nitration of phenol
Liquid phase, batch process nitration of phenol was carried out
at atmospheric pressure, using series of MTS catalysts, according to
the procedure described elsewhere [11]. In the particular experi-
ment, 2 g of phenol was taken in a round bottom flask containing
3 ml of solvent to which 1 g of catalyst pre-dried at 110 ◦C and
4 ml of 30% nitric acid were added and the reaction mixture was
stirred at room temperature. After the completion of reaction cata-
lyst was filtered off and products were extracted with diethyl ether
and analyzed offline by gas chromatography and identified by their
GC-retention times.
For the simplicity in discussion, catalyst with 12 wt%MoO3 sup-
ported on TiO2–SiO2 mixed oxide support, calcined at 500, 600,
and 700 ◦C are coded as MTS-5, MTS-6, and MTS-7, respectively
and commonly referred as ‘Series of MTS catalysts’.
3. Results and discussion
2.2. Catalyst characterization
3.1. Surface study of catalysts
XRD analysis was carried out with Phillips Holland, XRD sys-
tem, PW1710 Using Cu-k␣ (1.54056 Å) radiation, the diffractograms
were recorded in 10–60◦ range of 2ꢀ. The XRD phases present in the
samples were identified with the help of JCPDS card files. Fourier
transform infrared spectroscopy (FT-IR) spectra were recorded on
Perkin-Elmer 1720 single beam spectrometer at ambient condi-
tions using KBr disks, with a nominal resolution of 4 cm−1. The
mixed samples were pressed into a 10 mg/cm2 self-supporting
wafers before measurements were conducted at room temperature
in the range of 1500–400 cm−1. Temperature programmed desorp-
tion of ammonia (NH3-TPD) was carried out using Micromeritics
Autochem 2920 instrument. Scanning Electron Micrograms (SEM)
was obtained using Instrument, JEOL JSM-6380. BET surface area
and pore size was estimated by, Quantachrome Autosorb Auto-
mated Gas Sorption System, using N2 as a probe molecule. Gas
3.1.1. XRD measurements
Powder XRD patterns of 12 wt% MoO3/TiO2–SiO2 catalyst cal-
cined at different temperature are shown in Fig. 1. At lower
calcination temperature (500 ◦C) i.e. for MTS-5 catalyst, a broad
peak due to anatase phase of TiO2 (JCPDS File No. 21-1272) and
peak of negligible intensity due to (0 2 1) plane of MoO3 at d = 3.26
(JCPDS File No. 5-508) was observed. This suggests the monolayer
accumulation of molybdena over TiO2–SiO2 mixed oxide support.
The observation also indicates that MoO3 phase is present in highly
dispersed or amorphous state on the surface of support material.
With increase in calcination temperature corresponding increase in
crystallinity of anatase phases is seen and partial transformation of
TiO2 anatase in to TiO2 rutile (JCPDS File No. 21-1276) phases starts,
along with this, small peak attributed to MoO3 (0 2 0) crystalline
phase is observed due to structural rearrangements of the solid cat-

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S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
alyst. Therefore it can be concluded that amorphous TiO2 enhances
the association between MoO3 and SiO2 and improve the dispersion
capacity of MoO3 on the surface of SiO2 whereas crystalline TiO2
may not effectively contribute further to the interaction of MoO3
with SiO2. The above experimental observation also agrees with
the assumptions of monolayer dispersion theory [27], according to
which, amorphously dispersed TiO2 has a much stronger interac-
tion with the SiO2 support and other metal oxides than crystalline
TiO2. At high temperature the interaction of MoO3 with support
weakens which causes three-dimensional oxide agglomerations;
due to which MoO3 exists over the surface of MTS-6 catalyst in its
crystalline form. At calcination temperature of 700 ◦C, XRD lines
due to rutile phase become very strong with no indication of any
anatase phase. Thus, a complete transformation of anatase into
rutile can be noted at this temperature. Moreover very new peak in
diffractogram of MTS-7 corresponds to (1 0 1) plane of SiO2 (JCPDS
File No. 11-695) at 2ꢀ = 21.910◦ provides an additional evidence
for lowered interaction of MoO3 and TiO2 with SiO2. Because of its
closer melting point (MoO3, m.p. = 795 ◦C) to calcination tempera-
ture of catalyst (700 ◦C), MoO3 is expected to be present as a spread
layer over the support species or crystallite exist may be of size less
than 4 nm (beyond the detection capacity of XRD technique) hence,
no any peaks were observed due to MoO3 phase in diffractogram
of MTS-7 catalyst.
Experimental results revealed that calcination temperature is an
important factor in anatase-to-rutile phase transformation. Gen-
erally the anatase-to-rutile phase transformation of impurity free
TiO2 occurs at 400 ◦C (673 K) and above temperatures [28]. There-
fore from the XRD results of 12 wt% MoO3/TiO2–SiO2 catalyst
calcined at different temperatures it can be inferred that, presence
of SiO2 with TiO2 inhibits the anatase-to-rutile phase transforma-
tion and secondly, at higher temperature the lowered interaction of
rutile phase and MoO3 with SiO2 phase separates some SiO2 species
from TiO2–SiO2 matrix.
Fig. 2. FT-IR spectra of MTS series of catalyst samples.
of the structural reordering of the solids. In this regard, lowering of
the intensity of band attributed to Ti–O–Si, upraise of new bands
in the region 800–550 cm−1 and around 474 cm−1 and broaden-
ing of band at 1101 cm−1 can be seen. This happening is correlated
to the formation of Ti–O–Ti and Si–O–Si bonds due to breakage of
Ti–O–Si bonds at high temperature. This experimental observation
is similar to the conclusions made by Gunji et al. [36] in his inves-
tigation regarding the thermal stability of Ti–O–Si structure. The
fall in the intensity of the band at 987 cm−1 with progress in calci-
nation temperature is seen which may be because of the increase
in mutual interaction of molybdena with TiO2 phase. Moreover
the dominant existence of TiO2 rutile phase over the catalyst sur-
face might have masked the band corresponds to molybdena. The
present IR results also show the evidence for phase transformation
of titania anatase into rutile with progress in calcination temper-
ature of catalyst hence, all these observations are inline with the
XRD results.
3.1.2. FT-IR
In order to get detailed information about the surface struc-
ture through the modes of vibration offered by different species
on the surface of catalyst, FT-IR spectroscopic analysis of 12 wt%
MoO3/TiO2–SiO2 catalysts were done and results are displayed in
Fig. 2. Generally the IR band at 1097 and 467 cm−1 corresponds
to asymmetric stretching and bending modes of Si–O–Si linkage,
respectively [29,30]. Whereas anatase and rutile phase exhibit the
bands in 850–650 and 800–600 cm−1 region, respectively [31]. The
Ti–O–Si infrared vibration is generally observed between 910 and
960 cm−1 [32,33] with the exact band position depending on the
chemical composition of the sample as well as calibration and res-
olution of the instrument whereas terminal M O stretch of MoO3
phase can be identified from a band at 1000 cm−1 [31,34,35].
For the catalyst calcined at 500 ◦C (MTS-5) a prominent band
appeared at 1101 cm−1 that corresponds to Si–O–Si linear stretch in
SiO2 and another band at 987 cm−1 is related to stretching vibration
mode of M O due to impregnated MoO3 phase. The IR band at
929 cm−1 is assigned to Ti–O–Si species which clearly indicates the
union of TiO2 and SiO2 counterparts of support material and the
3.1.3. BET surface area and pore size
TiO2–SiO2 mixed oxide support having N2 BET surface area of
218 m2/g was impregnated with 12 wt% MoO3 using calculated
amount of ammonium heptamolybdate and the corresponding
results of BET surface area, pore volume and pore size of 12 wt%
MoO3/TiO2–SiO2 catalyst calcined at different temperature are
summarized in Table 1. As soon as the TiO2–SiO2 support is impreg-
nated with MoO3 and subsequently activated at 500 ◦C, the drastic
decrease in surface area for MTS-5 (106 m2/g) sample was noted
due to penetration of the active component into the pores of the
support. Further progress in calcination temperature of catalyst up
TiO2 anatase phase can be detected from the IR band at 782 cm−1
.
Increase in calcination temperature brings out some modifica-
tion in IR spectra of corresponding samples (MTS-6, MTS-7) because
Table 1
Surface area and acidity measurement of the catalysts.
´
˚
Catalyst
Surface area (m2/g)
Average pore diameter (A)
Average pore volume (cm3/g)
Amount of NH3 desorbed (mmol/g)
MTS-5
MTS-6
MTS-7
106
39
15
68.52
26.69
13.03
0.1817
0.0268
0.0051
6.440
0.901
0.074

S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
73
Fig. 4. NH3-TPD profiles for samples: MTS-5, MTS-6 and MTS-7.
for MTS-7. Beside this, the surface of MTS-7 catalyst appears to
be sintered and reduced indicating the effect of higher calcina-
tion temperature (700 ◦C) on morphology of the catalyst. SEM
result provides the back up to the trend observed in surface area
measurements.
3.1.5. Ammonia desorption
MoO3/TiO2–SiO2 catalyst calcined at various temperatures were
subjected to temperature programmed desorption of ammonia
(NH3-TPD) to survey the acid amount and acid strength of cata-
lysts. In NH3-TPD profile, peaks are generally distributed into two
regions, high temperature (HT) region (T > 400 ◦C) and low temper-
ature (LT) region (T < 400 ◦C). Peaks in high temperature (HT) region
are ascribed to desorption of ammonia from strong Brönsted and
Lewis acid sites, while peaks in low temperature region are assigned
to desorption of ammonia from weak acid sites [37,38]. NH3-TPD
profiles for the samples are shown in Fig. 4 and amount of ammonia
desorbed is given in Table 1.
Fig. 3. Scanning Electron Micrographs of MTS series of catalysts.
MTS-5 (6.44 mmol/g) has highest acidity among all MTS series
of catalysts. NH3-TPD profile for sample MTS-5 suggests the avail-
ability of greater number of weak acid sites as maxima of the broad
ammonia desorption peak is centered at temperature 114 ◦C in LT
region whereas another strong peak in HT region at 810 ◦C indi-
cates the presence of strong acid sites as well. So, weak acid sites
contribute mainly in total acid amount of MTS-5 sample. Weak acid
sites are generally associated with terminal hydroxyl (–OH) groups
of support and presence of Si–O–Ti species in TiO2–SiO2 matrix [39].
Rise in calcination temperature cause dehydroxylation of the sur-
face and reduces the relative abundance of Si–O–Ti due to breaking
of Ti–O–Si species in to Ti–O–Ti and Si–O–Si individual structures
which results in to decrease in weak acid sites from the catalyst sur-
face. Beside that, TiO2 rutile crystallites formed (as evident from
XRD and FT-IR analysis) exhibits strong Lewis acidity only [40].
Inline with this justification, sample MTS-6 showed lowered acid-
ity (0.9017 mmol/g) as compared to that of MTS-5. At calcination
temperature 700 ◦C, the above phenomenon becomes more severe
to 700 ◦C indicated the rapid fall in surface area (MTS-7 = 15 m2/g)
as well as the reduction in pore volume and pore diameter. This is
mainly due to phase transformation of the titania anatase into rutile
which is accelerated by calcination temperature. The calcination of
catalyst at high temperature induces the crystalline growth in cat-
alyst that results in to a formation of a denser solid material with
narrow channels. Hence catalyst calcined at progressive tempera-
ture indicated the gradual loss in pore volume and pore size, intern
in surface area.
3.1.4. SEM
Fig. 3 represents the results of scanning electron micrograms
(SEM) recorded in order to demonstrate the morphology and
particle size of the catalysts. As can be seen, with progress of
calcination temperature, agglomerates are formed due to mutual
interaction of small crystallites of the catalyst so the average par-
ticle size of catalyst MTS-5 was shifted from 15–25 to >30 ␮m

74
S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
Scheme 1. Different MoO3 species proposed to exist on TiO2–SiO2 support.
compared to that of Lewis acidity. A similar trend was observed for
MTS-7 catalyst but the intensities of the respective peaks are very
low due to sharp decrease in the population of acid sites at high
calcination temperature.
To know the sources of Lewis and Brönsted acid sites, one
need to consider the way that MoO3 species anchor to TiO2–SiO2
mixed oxide support. The most probable fittings of MoO3 with
support are shown in Scheme 1 and their relation with acidity is
discussed further. At monolayer loading MoO3 preferably reacts
with TiO2 hydroxyls mainly forming monodentate species I [42].
Such monodentate groups can also appears on the top of mul-
tilayers of amorphous Mo oxide forming species II. In literature,
similar monodentate tetrahedral species were also proposed for
MoO3/Al2O3 and WoO3/Al2O3 [43,44]. These monodentate species
carry one proton (hydrogen) in their structures which act as a Brön-
sted acid center. On the other hand according to Zhao et al. [45]
amorphous MoO3 on SiO2 is octahedrally coordinated species III
and act as Lewis acid centers. From this point of view, catalyst cal-
cined at 500 ◦C exhibited both Brönsted and Lewis acidity. Further,
when catalyst is calcined at higher temperature (600 and 700 ◦C),
hydroxyl groups from species I are removed in the form of water
(shown separately in Scheme 1) due to which MoO3 species on TiO2
starts rearranging themselves forming species that corresponds to
Lewis acidity. Moreover as observed by Liu et al. pure TiO2 and SiO2
show Lewis acidity and no acidity, respectively [40]. That is why
the IR spectrum of pyridine adsorbed on MTS-6 and MTS-7 cata-
lyst indicated the adverse effect of high calcination temperature on
Brönsted acid site population over the catalyst surface.
Fig. 5. FT-IR spectra of pyridine adsorbed catalyst samples (B = Brönsted, L = Lewis).
hence very low acidity is recorded for MTS-7 (0.074 mmol/g) cata-
lyst.
With increase in calcination temperature of catalyst from 500 to
700 ◦C, peak maxima of NH3-TPD profile shifts from 810 ◦C for MTS-
5 to 409 ◦C for MTS-7. This clearly indicates the effect of calcination
temperature on strong acidity over the catalyst surface. Strong
acid sites in HT region are attributed to strong Brönsted and Lewis
acid sites appeared due to differently anchored MoO3 species with
support. Hence in order to understand the mechanism of trans-
formation of weak acid sites in to strong acid sites, a survey of acid
strength distribution was carried out and discussed in next section.
3.1.6. Pyridine adsorbed FT-IR
In order to obtain a clear distinction between Lewis and Brön-
sted acid sites FT-IR analysis of pyridine adsorbed on catalyst
surface were carried out and results are displayed in Fig. 5. Pyri-
dine adsorbed on Brönsted acid sites produce characteristic IR band
around 1540 and 1638 cm−1 due vibration modes of adsorbed pyri-
dine and bands at 1450 and 1480 cm−1 are assigned to pyridine
coordinated with Lewis acid sites [41]. From Fig. 5 it can be seen that
for sample MTS-5 bands are appeared at 1455, 1550 and 1639 cm−1
suggesting the presence of Brönsted as well as Lewis acid sites.
But the greater intensity of band at 1455 cm−1 compared to other
confirms the dominant presence of Brönsted acid over the surface
of catalyst. Further for catalyst MTS-6, it was observed that the
intensities of bands associated with Brönsted acidity decreases as
3.2. Activity measurements
3.2.1. Nitration of phenol
The catalytic activities was tested for the nitration of phenol in
the presence of mixed oxide supported MoO3 catalyst using 30%
Table 2
Liquid phase batch process nitration of phenol over MTS series of catalysts.
Catalyst
Conversion of phenol (%)
Selectivity (%)
ONP
Total yield (%)
o/p ratio
PNP
Benzq.
Blank expt.
MTS-5
MTS-6
14
94
74
58
49
51
59
66
51
48
40
33
–
1
1
1
14
93
73
57
0.97
1.07
1.48
2.00
MTS-7
Operating conditions: phenol/HNO3 = 1; catalyst = 1 g; HNO3 = 30%; solvent = CCl4; temperature: RT— reaction time: ∼60 min. ONP = ortho-nitrophenol; PNP = para-
nitrophenol; Benzq. = Benzoquinone. Total yield = ONP + PNP.

S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
75
nitric acid at room temperature and the results are summarized in
Table 2. Among the catalysts studied MTS-5 showed highest phenol
conversion (94%) and 66% selectivity for o-nitrophenol was noted
for MTS-7 catalyst. When the reaction was carried out without cat-
alyst the conversion of phenol was low (14%) with nearly equal
ortho- and para-isomers, suggests the influence of solid acid cata-
lysts on the conversion and selectivity. To demonstrate the effect
of calcination temperature, nitration is carried out with catalyst
calcined at three different temperatures. No significant change in
o-selectivity is observed when nitration was done using MTS-5 (cal-
cined at 500 ◦C). This is due to occurrence of the reaction inside the
larger pores of the catalyst in the same manner as in the bulk phase.
This assumption is supported by the observation of closely simi-
lar distribution of products (o/p ratio) in the absence of catalyst.
Nitration when performed using MTS-7, increase in o-nitrophenol
selectivity was noted which may be because of preferred orienta-
tion of phenol inside the narrow pores increasing the accessibility
of ortho-position to nitronium ion leading to selective formation
of o-nitrophenol. Conversion (94%) of phenol in case of MTS-5 is
attributed to the presence of more Brönsted acid sites over its sur-
face. Such Brönsted acid sites are responsible for the generation
of nitronium ion in the reaction [46]. When the catalyst is cal-
cined at higher temperatures (MTS-6 and MTS-7) the number of
Lewis acid sites increases which eventually decreases the Brön-
sted acid sites. Because of this reason the lowered conversion of
phenol was observed when the nitration was performed with cat-
alyst MTS-6 and MTS-7. 30% nitric acid limits the formation of
di-nitrophenols and MTS series of catalysts are associated with very
little or negligible formation of side product, generally which is
benzoquinone.
In general, the change in conversion and selectivity of respective
nitro-derivatives of these aromatic substrates can be well corre-
lated to the change in the acidity and pore size distribution rather
than to the change in the surface area of the catalysts.
3.2.3. Mechanism for nitration of phenol
Generally nitration requires aggressive mixture of acids (nitric
and sulfuric acid) to generate NO2+, which is true nitrating inter-
mediate. Brei et al. also explained that strong Brönsted acid sites
+
are necessary for an effective formation of intermediate NO2 ions
from nitric acid [47]. Nitronium ion can be generated by the interac-
tion of nitric acid with the Brönsted acid sites of solid acid catalyst.
HNO3 molecules when adsorbed on active Brönsted acid sites of
catalyst, dissociates in to water, nitrate and nitronium ion. Because
of electronegative hydroxyl group of phenol, the electron cloud of
aromatic ring is pulled towards this group that makes ortho and
+
para sites more favorable for NO2 attack. During the propagation
of such reaction steps, one H+ ion is released out from aromatic sub-
strate which recombines with the nitrate ions to form HNO3 again.
In this way reaction continues to form the nitrated aromatic com-
pounds and generate water as a by-product. The plausible reaction
mechanism for nitration of phenol over MTS series of catalyst is
shown in Scheme 2.
3.2.4. Effect of Solvent
In order to search the best entariner, nitration of phenol over
MTS-7 catalyst was performed using different solvents and their
effect on phenol conversion and o-nitrophenol selectivity was stud-
ied. Fig. 6 clearly indicates that other solvents suffered with lower
yields and o/p ratio where as chlorinated solvents showed the satis-
factory results. Among them CCl4 was found the best in all respects.
3.2.2. Nitration of different aromatic substrates
3.2.5. Effect of reaction time
Successful regioselective nitration of phenol has promoted us
to extend the nitration studies to some other aromatic substrates.
Hence, substrates like benzene, toluene and o-xylene were sub-
jected to the nitration reaction and results are presented in Table 3.
Exclusive formation of nitrobenzene, faster reaction rates and fairly
good conversion of benzene are some of the advantages of carrying
out the nitration of benzene with MTS series of catalyst. Though, the
selectivity is not as much significant as in case of phenol nitration,
the present method of nitration of toluene has advantage of sup-
pression of polynitrated products. The lower selectivity in toluene
nitration may be attributed to the weakly polar nature of the alkyl
group which destabilizes any interaction with the catalyst leading
to continue the reaction as like in the bulk phase. For MTS cat-
alyzed o-xylene nitration conversions of o-xylene are also found
low whereas selectivity did not alter considerably. Lowered con-
versions are mainly due to the formation of side products in the
initial stage that deactivates the catalyst for further reaction.
Fig. 7 shows the influence of time on phenol nitration using
MTS-7 catalyst. As seen in Fig., the conversion of phenol took place
at a faster rate up to 40 min of reaction time and the saturation
stage was reached in next 10 min. The faster reaction rate in the
initial stage can be justified by the formation of more number of
nitronium ions progressively in the reaction. o-Nitrophenol was the
major product whereas para-nitrophenol form in fewer amounts.
The selectivity towards o-nitrophenol does not affect significantly
with increase in reaction time. Importantly, the increase in reaction
time did not show the formation of any di-substituted nitrophenols.
3.2.6. Regeneration and reusability of the catalyst
After completion of the nitration of phenol over MTS-7 catalyst,
it was filtered off and partly given wash with acetone and water
and some part of spent catalyst was heated in an oven at 250 ◦C for
1 h. Thermal analysis of spent and regenerated catalyst was done
Table 3
Nitration of different aromatic substrates.
Aromatic substrate
Benzene
Reaction conditions
a
Catalyst
Conversion (%)
Major products (selectivity, %)
MTS-5
MTS-6
MTS-7
100
91
67
NB (100)
NB (100)
NB (99)
Toluene
b
c
MTS-5
MTS-6
MTS-7
82
69
35
ONT (48), PNT (46)
ONT (42), PNT (57)
ONT (40), PNT (59)
O-xylene
MTS-5
MTS-6
MTS-7
23
14
8
3-NOX (39), 4-NOX (44)
3-NOX (33), 4-NOX (43)
3-NOX (33), 4-NOX (42)
a: Catalyst: 0.1 g; benzene: 0.20 moles; nitric acid: 1 equivalent; temperature: room temperature; time: ∼15 min. b: Catalyst: 1 g; toluene/HNO3: 1.0; HNO3: 69%; solvent: 1,
2-dichloroethane; temperature: reflux; time: ∼60 min. c: Catalyst: 0.212 g (20% of o-xylene weight); o-xylene/HNO3 mole ratio:1.0; HNO3: 69%; solvent: carbon tetrachloride;
temperature: 75 ◦C; time: ∼90 min. NB: nitrobenzene; DNB: di-nitrobenzene. ONT: ortho-nitrotoluene; PNT: para-nitrotoluene; 3-NOX: 3-nitro-o-xylene; 4-NOX: 4-nitro-
o-xylene.

76
S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
Table 4
Comparison with various other solid acids under similar reaction conditions.
Catalyst
Conversion of phenol (%)
Selectivity (%)
ONP
o/p ratio
Ref.
PNP
Benzq.
Fe/Mo/SiO2
H-ZSM-5
H-Y
MTS-5
MTS-6
60
60
60
94
74
58
48
55
60
51
59
66
42
37
30
48
40
33
10
8
10
1
1
1
1.14
1.48
2.00
1.07
1.48
2.00
[11]
[11]
[11]
This work
This work
This work
MTS-7
Operating conditions: phenol/HNO3 = 1; catalyst = 1 g; HNO3 = 30%; solvent = CCl4; temperature: RT—reaction time: ∼60 min. ONP = ortho-nitrophenol; PNP = para-
nitrophenol; Benzq. = Benzoquinone.
Scheme 2. The plausible reaction mechanism of nitration of phenol over MTS series of catalysts.
to ensure the amount of organic deposits over the catalyst surface.
Study suggested acetone wash as the favorable method for regen-
eration of the catalyst. Catalyst regenerated by acetone wash when
repeatedly used in the nitration of phenol, retained its activity for
three cycles without affecting the o-nitrophenol selectivity. The 8%
decrease in the yield (ONP + PNP) was observed, when catalyst used
further in reaction.
3.2.7. Comparison
Under similar reaction conditions, the performance of MTS
series of catalysts and other solid acids is compared for the nitra-
tion of phenol and the results are highlighted in Table 4. As could
be noted, MTS-5 shows the best phenol conversion value among
all solid acids and highest o/p nitrophenol ratio was observed for
MTS-7 catalyst which is comparable to that of H-Y type of zeolites.
Fig. 7. Effect of reaction time on the catalytic activity.
But as H-Y zeolite suffers with large amount of side product (bezo-
qinone) formation, MTS-7 can be considered as the best agent in
the respective process.
4. Conclusion
Following conclusions can be drawn from the results of nitration
of phenol over MTS series of catalysts by using 30% nitric acid.
(1) MTS-7 catalyst is found suitable for nitration of phenol to
achieve high ortho-nitrophenol yield at room temperature
whereas MTS-5 gives very high phenol conversion.
(2) Progress in calcination temperature adversely affects the
strength and acid amount of catalyst but change occur in poros-
ity is suitable for ortho-nitrophenol yield.
Fig. 6. Nitration of phenol over MTS-7 using different solvents.

S.M. Kemdeo et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 70–77
77
(3) Activities of these catalysts are more related to nature of
acid sites, acid amount and porosity of catalyst than surface
area.
(4) In the process, no formation by-product and high o/p selectivity
of catalysts at room temperature are some of the advantages.
(5) Avoid of sulfuric acid along with nitric acid used in diluted form
makes the process safe and environmentally friendly.
[19] K. Aoki, M. Ohmae, T. Nanba, K. Takeishi, N. Azuma, A. Ueno, H. Ohfume, H.
Hayashi, Y. Udagawa, Catal. Today 45 (1998) 29–33.
[20] Y. Liu, X. Ma, S. Wang, J. Gong, Appl. Catal. B 77 (2007) 125–134.
[21] S.B. Umbarkar, A.V. Biradar, S.M. Mathew, S.B. Shelke, K.M. Malshe, P.T. Patil,
S.P. Dagde, S.P. Niphadkar, M.K. Dongre, et al., Green Chem. 8 (2006) 488–493.
[22] H. Hu, I.E. Waches, J. Phys. Chem. 99 (1995) 10911–10922, and references
therein.
[23] M.S. Rana, S.K. Maity, J. Ancheyta, G. Murli Dhar, T.S.R. Prasada Rao, Appl. Catal.
A 253 (2003) 165–176.
[24] S.K. Samantaray, K.M. Parida, Appl. Catal. A 211 (2001) 175–187.
[25] M.K. Kobayashi, R. Kuma, S. Masaki, N. Sugishima, Appl. Catal. B 60 (2005)
173–179.
References
[26] D. Mao, G. Lu, Q. Chen, Appl. Catal. A 263 (2004) 83–89.
[27] Y.C. Xie, Y.Q. Tang, Adv. Catal. 37 (1990) 1–43.
[1] G.A. Olah, R. Malhotra, S.C. Narang, Nitration: Method and Mechanisms, VCH
Publisher Inc., New York, 1989, pp. 201–204.
[2] A. Vogel, A Text Book of Practical Organic Chemistry, fourth ed., Longman,
London, 1978, pp. 745–746.
[3] J.A.R. Rodrigues, A.P. de Oliveira Filho, P.J.S. Moran, R. Custodio, Tetrahedron 55
(1999) 6733–6738.
[4] S.M. Nagy, K.A. Yarovoy, M.M. Shakirov, V.G. Shubin, L.A. Vostrikova, K.G. Ione,
J. Mol. Catal. 64 (1991) L 31–L 34.
[28] S.R. Yoganarasimhan, C.N.R. Rao, Trans. Faraday Soc. 58 (1962) 1579–1589.
[29] M.M. Mohamed, T.M. Salama, T. Yamaguchi, Colloid Surf. A 207 (2002) 25–32.
[30] G.M.S. El Shafei, M.M. Mokhtar, Colloid Surf. A 94 (1995) 267–277.
[31] B.M. Reddy, B. Choddhury, J. Catal. 179 (1998) 413–419.
[32] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233–254.
[33] H. Kochkar, F. Figueras, J. Catal. 171 (1997) 420–430.
[34] R.A. Nyquist, C.L. Putzig, M.A. Leugers, Handbook of Infrared and Raman spectra
of Inorganic Compounds and Organic Salts, Academic Press, New York, 1997,
pp. 295–350.
[5] A. Cornelis, P.P. Laszlo, Synlett (1994) 155–161.
[6] M.J. Thompson, P.J. Zeegers, Tetrahedron 47 (1991) 8787–8790.
[7] T. Esakkidurai, K. Pitchumani, J. Mol. Catal. 185 (2002) 305–309.
[8] L.V. Malysheva, E.A. Paukshits, K.G. Ione, Catal. Rev. Sci. Eng. 37 (1995) 179–226.
[9] E. Suzuki, K. Tohmori, Y. Ono, Chem. Lett. 16 (1987) 2273–2274.
[10] K.M. Parida, P.K. Pattnaik, Catal. Lett. 47 (1997) 255–260.
[11] S.P. Dagade, V.S. Kadam, M.K. Dongare, Catal. Commun. 3 (2002) 67–70.
[12] M. Ouertani, P. Giraro, H.B. Kagan, Tetrahedron Lett. 23 (42) (1982) 4315–4318.
[13] H. Pervez, S.O. Onyisiuka, J.R. Rooney, C.J. Sukling, Tetrahedron 44 (1988)
4555–4565.
[14] D. Vanhove, S.R. Op, A. Fernandez, M. Blanchard, J. Catal. 57 (1979) 253–263.
[15] S. Okazaki, M. Kumasaka, J. Yoshida, K. Kosaka, K. Tanabe, Ind. Eng. Chem. Prod.
Res. Div. 20 (1981) 301–305.
[16] K. Tanaka, K.I. Tanaka, J. Chem. Soc. Chem. Commun. (1984) 748–749.
[17] K. Segawa, T. Soeya, D.S. Kim, Res. Chem. Intermediat. 15 (1991) 129–151.
[18] L. Libetti, G. Ramis, G. Busca, F. Bregani, P. Forzatti, Catal. Today 61 (2000)
187–195.
[35] T. Fransen, P.C. Van Berge, P. Mars, in: B. Delmon, P.A. Jacobs, G. Poncelet (Eds.),
Preparation of Catalysts, Elsevier, Amstardam, 1976, pp. 405–420.
[36] T. Gunji, Y. Nagao, T. Misono, Y. Abe, J. Non-Cryst. Solids 107 (1989) 149–154.
[37] F. Lonyl, J. Valyon, Micropor. Mesopor. Mater. 47 (2001) 293–301.
[38] M. Sawa, M. Niwa, Y. Murakami, Zeolites 10 (1990) 532–538.
[39] J. Ren, Z. Li, S. Liu, Y. Xing, K. Xie, Catal. Lett. 124 (2008) 185–194.
[40] Z. Liu, J. Tabora, R.J. Davis, J. Catal. 149 (1994) 117–126.
[41] A. Rahman, G. Lemay, A. Adnot, S. Kaliaguine, J. Catal. 112 (1988) 453–463.
[42] B.M. Reddy, et al., Appl. Catal. A 211 (2001) 19–30.
[43] C.A. Emis, J. Catal. 141 (1993) 347–354.
[44] J. Bernhole, H.A. Horsley, L.L. Murrell, J. Phys. Chem. 91 (1987) 1526–1530.
[45] B.Y. Zhao, Q. Xu, L.L. Gui, Y.Q. Tang, Acta Chim. Silica 48 (1990) 227–231.
[46] T. Mishra, Catal. Commun. 9 (2008) 21–26.
[47] V.V. Brei, S.V. Prudius, O.V. Melezhyk, Appl. Catal. A 239 (2003) 11–16.