Nitration of phenol over silica supported H4PW 11VO40 catalyst

Article abstract of DOI:10.1016/j.catcom.2011.07.030

Vanadium incorporated tungstophosphoric acid (TPAV₁) supported on silica was synthesized and characterized by BET-surface area, Fourier transform infrared spectroscopy, X-ray diffraction and Laser Raman techniques. Nitration of phenol was studied at room temperature (25 °C) using HNO ₃ in the presence of 0-20 wt.% TPAV₁/SiO₂ catalysts taking 1, 2-dichloroethane as solvent. The effects of various parameters such as phenol/HNO₃ mole ratio, reaction time, catalyst weight, and stirring speed on the catalyst activity were studied. 8 wt.% TPAV₁/SiO₂ has shown the best activity, regioselectivity and reusability in the nitration of phenol, with a conversion of 92.6% and o-nitrophenol selectivity of 97.9%.

Full text of DOI:10.1016/j.catcom.2011.07.030

Catalysis Communications 18 (2012) 37–40
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Nitration of phenol over silica supported H PW VO catalyst
4
11
40
a
a
a
b
a
a,
A. Sri Hari Kumar , K.T. Venkateswara Rao , K. Upendar , Ch. Sailu , N. Lingaiah , P.S. Sai Prasad ⁎
a
Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500 607, India
University College of Technology, Osmania University, Hyderabad-500 607, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2011
Received in revised form 16 July 2011
Accepted 29 July 2011
Available online 18 August 2011
Vanadium incorporated tungstophosphoric acid (TPAV ) supported on silica was synthesized and
characterized by BET-surface area, Fourier transform infrared spectroscopy, X-ray diffraction and Laser
1
Raman techniques. Nitration of phenol was studied at room temperature (25 °C) using HNO
of 0–20 wt.% TPAV /SiO catalysts taking 1, 2-dichloroethane as solvent. The effects of various parameters
such as phenol/HNO mole ratio, reaction time, catalyst weight, and stirring speed on the catalyst activity
were studied. 8 wt.% TPAV /SiO has shown the best activity, regioselectivity and reusability in the nitration of
phenol, with a conversion of 92.6% and o-nitrophenol selectivity of 97.9%.
© 2011 Elsevier B.V. All rights reserved.
3
in the presence
1
2
3
1
2
Keywords:
Phenol
o-Nitrophenol
Vanadium
Phosphotungstic acid
Silica
1
. Introduction
3
the Keggin-based super acids phosphotungstic acid (H PW12O40; TPA)
is the most preferred one based on its thermal stability [15] and acidity
[19].
Nitration of phenol to ortho nitrophenol (ONP) is an industrially
important process for which a corrosive mixture of nitric acid and
sulfuric acid is used. However, over nitration and oxidation of by-
products result inpoor selectivityand generatinghuge amount of waste.
The ortho/para ratio of products also remains as 1:2 [1,2]. Alternate
In the present communication, we disclose an efficient environ-
mentally benign, reusable TPAV /SiO catalyst for phenol nitration at
1 2
RT. The advantage of the present catalyst is that its preparation is
simple, inexpensive and the catalyst offers high product yield within
reasonable reaction time.
technologies carrying the nitration with N
ammonium nitrate [5], metal nitrates [6], nitronium tetrafluoroborate
7] and zirconyl nitrate [8] were partially successful. For the nitration of
2 4 3
O [3], NaNO [4], ceric
[
phenol solid acid catalysts are found to be good alternatives for sulfuric
acid. The catalysts include Claycop (copper (II) nitrate supported on
K10-Montmorillonite) [9], zeolites-based solid acids [10], ionic liquids
2. Experimental
H
4
PW11VO40 (TPAV
WO ·2H O (Loba Chemie, India). The preparation procedure reported
by Tsigdinos et al. [20] for vanadium incorporated molybdophosphoric
acid was adopted for the synthesis of H PW11VO40. NaVO (7.33 mmol)
dissolved in 15 ml of deionized water at 80 °C was mixed with aqueous
Na HPO (7.3 mmol in 20 ml of water) and cooled to room temperature.
Concentrated H SO (5 ml) was then added to give a red solution.
Aqueous Na WO ·2H O (79.32 mmol dissolved in 50 ml of water) was
then added to the above solution drop-wise with vigorous stirring
followed by slow addition of concentrated H SO . TPAV was obtained
1 3 3 4
) was prepared using NaVO , Na PO , and
[
11], sulfated titania [12] and TBAB as transfer catalyst [13]. Recently, the
application of nano sized tungsten oxide supported on sulfated SnO
14] has also been reported. However there is a necessity for the
Na
2
4
2
2
[
4
3
development of new class of catalysts that could operate at room
temperature (RT).
2
4
Heteropolyacids (HPAs) with Keggin structure have been used as
catalysts for various acid-catalyzed reactions [15,16]. Bulk HPAs have
2
4
2
4
2
2
low thermal stability, low surface area (2 m /g) and solubility in polar
solvents. However, in some supported systems, like phosphotungstic
acid supported on silica, the surface area, thermal stability and water
resistance can be enhanced. Parida et al. [17] and Heravi et al. [18] have
carried out this reaction on silicotungstic acid supported on zirconia and
vanadium incorporated molybdophosphoric acid, respectively. Among
2
4
1
with ether extraction followed by evaporation and recrystallization. The
catalyst mass was kept for further drying in an air oven at 120 °C for 12 h
and finally calcined at 300 °C for 2 h.
For the preparation of supported catalysts, calculated amounts of
1
TPAV (0–20 wt.%) were dissolved in deionized water and added to
appropriate amounts of silica support adopting the wet impregnation
method. The excess water was evaporated on a water bath, the masses
were air dried at 120 °C for 8 h and then calcined at 300 °C for 4 h.
⁎
Corresponding author. Tel.: +91 40 27193163; fax: +91 40 27160921.
E-mail address: saiprasad@iict.res.in (P.S.S. Prasad).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.07.030

38
A.S.H. Kumar et al. / Catalysis Communications 18 (2012) 37–40
peak also appeared at 1056.8 cm− (Fig. 1) confirming the vanadium
1
The reactor consisted of a mechanically agitated flat-bottom glass
vessel of 100 ml capacity. The assembly was kept in an isothermal oil
bath at 25 °C. The typical reaction mixture consisted of 10 mmol of
phenol and 10 mmol of nitric acid with 0.05 g of the catalyst, and
incorporation. Pure silica support exhibited three main bands at
−
1
− 1
1100 cm
(broad and very strong), 800 cm
(medium) and
−
1
470 cm
(strong), which partly overlapped with the characteristic
3
0 ml of 1, 2-dichloroethane (DCE) taken as solvent. The reaction was
bands of the Keggin unit [21]. This is an important observation
compared with pure TPA. It should be emphasized that some bands
had overlapped with those of the support. The weak absorption band at
carried out at a stirring speed of 800 rpm. The product mixture was
filtered, an aqueous mixture of ethyl acetate and sodium bicarbonate
was then added to the filtrate, the two immiscible layers separated
and the organic layer concentrated by evaporation. Isolation of the
product was carried out with the reaction mixture obtained after 3 h
of reaction. The products were analyzed in a GC-MS, (Shimadzu,
Model QP 2010 S) using a 5 wt.% SE-30 on Chromosorb WHP capillary
column.
−
1
1640 cm
present case, the strong bands of silica masked the band at 1056.8 cm
in supported TPAV catalysts. However, information can still beobtained
from the less affected regions. The bands placed at 980 and 800 cm
2
(H O bending) indicates the presence of moisture. In the
−
1
1
−
1
can be ascribed to Keggin ion. These small non-overlapped bands also
confirm the presence of the Keggin ion on the support.
BET surface areas of the catalysts were determined on a Micromeritics
The Laser-Raman spectra of TPAV
TPA/SiO exhibited bands around 1009, 992 cm
around 905 cm
showed an extremely strong W_O stretching band with its maximum
1 2
/SiO catalysts are shown in Fig. 2.
−
1
(
Auto Sorb-2910) instrument with nitrogen physisorption at −196 °C.
XRD patterns of the catalysts were obtained with a Rigaku Miniflex
Rigaku Corporation, Japan) using Ni filtered Cu Kα radiation
λ=1.5406 Å). The FT-IR spectra were recorded on a DIGILAB (USA)
2
and a broad peak
[22]. The Raman spectrum (Fig. 2) of TPAV /SiO
−
1
1
2
(
(
−
1
−1
at 1000 cm
loadings, i.e. 12 and 20 wt.%. The peaks in the bands of TPAV
shifted toward lower wave numbers compared to TPA/SiO [22] due to
and other components at 981, 903 cm
at higher
−
1
spectrometer, with a resolution of 1 cm using KBr disk method.
Raman spectra of the samples were collected on a UV–vis Raman
spectrometer system (Horiba-Jobin Yvon LabRam-HR) equipped with
a confocal microscope, 2400/900 grooves/mm gratings, and a notch
filter. The UV laser excitation at 325 nm was supplied by a Yag
doubled-diode pumped laser (20 mW).
1
/SiO
2
2
−
1
vanadium incorporation. The bands at 1000 and 981 cm
assigned to the terminal W\O (O -terminal oxygen atom) symmetric
and asymmetric stretching modes, respectively [23]. The band at
can be
t
t
−
1
903 cm
is the characteristic band for the asymmetric stretching
\W (O -corner-sharing bridging oxygen
vibration of bridging W\O
b
b
3
. Results and discussions
The FTIR spectra of SiO , TPAV
atom). Thus, the Raman spectra also confirm the incorporation of V in
TPA.
The X-ray diffractograms of the samples are shown in Fig. 3. The
patterns reveal the amorphous nature of the catalysts at lower
loadings due to high dispersion of the acid on silica. At higher loadings
2
1
and TPAV
1
/SiO
2
catalysts are shown
in Fig. 1. The Keggin structure was intact after the incorporation of V in
the tungsten matrix of TPA, asshown in the inset of Fig. 1. However, such
clarity could not be obtained from the spectra of the supported catalysts.
With the substitution of a V atom for W in the primary structure of the
(N10 wt.% TPAV
might be due to the increase in crystallinity of TPAV
TPAV /SiO . With increasing loading of TPAV there may be a chance
for agglomeration of TPAV particles on the surface of the support.
1
) the Keggin peaks were clearly observable. This
1
beyond 10 wt.%
oxoanion (TPAV
1
catalyst), the P\O and W_O bands shifted toward
1
2
1
lower wave numbers due to a reduced structural symmetry [14]. A new
1
The appearance of peaks at 2θ=10.24, 20.50, 23.26 and 25.47°
strongly suggests that the Keggin structure was intact in the catalysts
[24].
HPAs by themselves have very low surface area. When impregnated
2
on silica (490 m /g) they can be distributed on the surface of the
support. The surface area (Table 1) showed a decreasing trend with
1
increase in the amount of TPAV . It may be expected that plugging of
g
f
f
e
d
e
d
c
c
b
a
b
a
2500
2000
1500
1000
500
200
400
600
800
1000
1200
-
1
Raman shift (cm-1)
Wave number (cm )
Fig. 1. FT-IR spectra of SiO
SiO (c) 8 wt.% TPAV /SiO
TPAV /SiO (g) 20 wt.% TPAV
2
, TPAV
(d) 10 wt.% TPAV
/SiO
1
and TPAV
1
/SiO
/SiO
2
catalysts. (a) SiO
2
(b) 5 wt.% TPAV
1
/
Fig. 2. Laser-Raman spectra of TPAV
TPAV /SiO (c) 10 wt.% TPAV /SiO
(f) 20 wt.% TPAV /SiO
1
/SiO
2
catalysts. (a) 5 wt.% TPAV
1
/SiO
2
(b) 8 wt.%
2
1
2
1
2
(e) 12 wt.% TPAV
1
/SiO
2
(f) 15 wt.%
1
2
1
2
(d) 12 wt.% TPAV
1
/SiO (e) 15 wt.% TPAV /SiO
2
1
2
1
2
1
2
.
1
2
.

A.S.H. Kumar et al. / Catalysis Communications 18 (2012) 37–40
39
*
*
*
*
*
f
8
6
4
2
0
0
0
0
0
e
d
c
b
a
20
40
60
80
0.5
1.0
1.5
2.0
2.5
3.0
2
Theta (degree)
Time (h)
Fig. 3. XRD patterns of silica supported TPAV
TPAV /SiO (c) 10 wt.% TPAV /SiO
f) 20 wt.% TPAV /SiO ; TPAV ).
1
catalysts. (a) 5 wt.% TPAV
1
/SiO
2
(b) 8 wt.%
/SiO
2
Fig. 4. Effect of reaction time on conversion of phenol.
1
2
1
2
(d) 12 wt.% TPAV
1
/SiO
2
(e) 15 wt.% TPAV
1
(
1
2
1 *
(
terms of environmentally benign nature of the catalyst; Jiang et al. [4] in
terms of usage of less number of reactants and solvents; Dagade et al.
[26] in terms of usage of lower catalyst concentration and Esakkidurai et
al. [10] in terms of obtaining higher yield of ONP.
active component in the pores of the support decreased the surface area,
at least in the case of highly loaded catalysts. The significant decrease in
surface area beyond 15 wt.% TPAV
1
can be considered as due to bulk
Fig. 4 shows the effect of reaction time on phenol conversion at
25 °C taking DCE as solvent and 8 wt.% TPAV /SiO . Increase in
TPAV formation on the support surface.
1
1
2
The effect of the speed of agitation on activity was studied from
00 to 1200 rpm. It was observed that the conversion of phenol and
the selectivity to ONP were practically the same in all the cases
above 800 rpm. Hence, all further reactions were carried out at
reaction time increased the conversion giving 90.6% yield of ONP
between 2.5 h and 3 h and the conversion remaining constant
thereafter.
The effect of nitric acid/substrate molar ratio on conversion is
shown in Table 2. With increase in molar ratio the conversion
increased reaching 100% at a ratio of 1.25, with over-nitration beyond
a ratio of 1.25.
6
8
00 rpm.
Nitration of phenol carried out on TPAV
showed excellent catalytic behavior in DCE solvent (Table 1). 8 wt.%
TPAV /SiO catalyzed theformation of ONP efficiently giving a total yield
of 90.6% in DCE. Higher activity as well as selectivity of the catalyst
may be due to high surface area and the well dispersed nature of TPAV
1 2
/SiO catalysts at 25 °C
1
2
The stability of 8 wt.% TPAV /SiO2 was studied to establish its
1
reuse. The catalyst collected by filtration after each use was washed
1
with distilled water and dried at 120 °C. Only a marginal decrease of
3% in the activity was found after four reuses (Fig. 5), which might be
due to the loss of catalyst during handling.
on silica. The activity test was also carried out on unsupported TPAV1
and TPA catalysts. The conversion (30–40%) and ONP selectivity
(
55–65%) were less than that of the supported system. The values
obtained on TPA/SiO2, not containing vanadium, were insignificant. The
main advantage with 8 wt.% TPAV /SiO is the higher ONP/PNP ratio
4
. Conclusions
1
2
(
46.6) obtained at RT. Parida et al. [15] have reported good yield (≈86%)
Regioselective nitration of phenol can be productively carried out
at room temperature but the ONP/PNP ratio was low. Heravi et
al. [18] have carried out phenol nitration on unsupported vanadium
using TPAV
nitric acid in DCE solvent. 8 wt.% TPAV
1
/SiO
2
. Phenol can be selectively nitrated to ONP with
/SiO is found to be the
1
2
(
V)-substituted polyoxomolybdates, H3+xPMo12− x
x
V O
40 (x=1–3), at
was a maximum of
2.1% at 60 °C. Besides there was over nitration to di and trinitrophenol
optimum loading on silica. The reaction can be effectively carried out
at 25 °C with high yields. The catalyst is reusable with consistent
activity.
7
9
3 2
8 °C. The yield on WO supported on sulfated SnO
[25]. Our catalyst compares well with that of Sungajadevi et al. [12] in
Acknowledgments
Table 1
Surface area values, conversion and selectivity data on (0–20 wt.%) TPAV
catalysts during nitration of phenol.
1
/SiO
2
The authors thank the DST, New Delhi, for the financial support.
ASHK and KTVR thank CSIR for the award of Senior Research
Fellowship.
Sample
BET
Phenol
conversion (%)
Selectivity (%)
ONP/
PNP
2
(
m /g)
ONP
PNP
Table 2
Without catalyst
–
9.5
49.3
87.5
97.9
93.8
81.9
59.7
53.7
51.7
12.5
2.1
0.9
7.0
46.6
15.1
4.5
1 2
Effect of nitric acid/phenol mole ratio on the activity of 8 wt.% TPAV /SiO at 25 °C.
5
8
1
1
1
2
TPAV
TPAV
0TPAV
2TPAV
5TPAV
0TPAV
1
/SiO
/SiO
2
410
380
340
318
278
190
60.4
92.6
80.3
64.0
55.0
43.4
1
2
Nitric acid/phenol (mole ratio)
Phenol conversion (%)
1
1
1
1
/SiO
2
2
2
2
6.2
/SiO
/SiO
/SiO
18.1
40.3
46.3
0.50
0.75
1.00
47.5
70.0
92.0
100
1.5
1.1
1.25
(
ONP — Ortho nitrophenol, PNP — Para nitrophenol).

40
A.S.H. Kumar et al. / Catalysis Communications 18 (2012) 37–40
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