Phosphoric acid modified montmorillonite clay: A new heterogeneous catalyst for nitration of arenes

Article abstract of DOI:10.1016/j.catcom.2014.08.019

The easily available montmorillonite clay is treated with phosphoric acid and 10 wt.% is found to be the optimum concentration of phosphoric acid that can be adsorbed chemically on the surface of the clay. Acidity of this phosphoric acid treated montmorillonite clay (PAM) is determined by volumetric as well as potentiometric titration and characterized. Catalytic efficacy of PAM in nitration of various aromatic compounds is reported.

Full text of DOI:10.1016/j.catcom.2014.08.019

Catalysis Communications 57 (2014) 124–128
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Catalysis Communications
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Short Communication
Phosphoric acid modified montmorillonite clay: A new heterogeneous
☆
catalyst for nitration of arenes
a,b,
b
b
Saitanya K. Bharadwaj ⁎, Purna K. Boruah , Pradip K. Gogoi
a
Department of Chemistry, Pragjyotish College, Guwahati, 781009 Assam, India
Department of Chemistry, Dibrugarh University, Dibrugarh, 786004 Assam, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2014
Received in revised form 11 August 2014
Accepted 12 August 2014
Available online 20 August 2014
The easily available montmorillonite clay is treated with phosphoric acid and 10 wt.% is found to be the optimum
concentration of phosphoric acid that can be adsorbed chemically on the surface of the clay. Acidity of this
phosphoric acid treated montmorillonite clay (PAM) is determined by volumetric as well as potentiometric
titration and characterized. Catalytic efficacy of PAM in nitration of various aromatic compounds is reported.
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Heterogeneous catalysis
Montmorillonite
Phosphoric acid
Nitration
1
. Introduction
benign catalyst for nitration reactions, solid acids are found to be
more efficient. Several solid acid catalysts such as sulfonated poly-
organosiloxanes [15], acidic resins [16], zeolites [17] and supported
sulfuric acids [18] are used in nitration reactions. Regioselectivity in
nitration reaction is observed while using polyoxometalates (POMs)
Heterogeneous catalysis is receiving significant interest over homo-
geneous counterpart due to the advantages like a) good dispersion of
active sites, b) easier and safer handling, c) easier separation from the
reaction mixture, and d) reusability [1]. Among heterogeneous catalysis,
solid acids, prepared by adsorbing mineral acids onto a solid surface, are
introduced mainly to replace highly corrosive mineral acids in the
reaction medium and used in many important large-scale industrial
processes [2]. In this regard, nitration of organic compounds is one of
the most important industrial reactions and nitro-derivatives are exten-
sively utilized as a chemical feedstock for wide range of useful materials
or heteropolyacids [19–22]. Modification of solid oxides such as TiO
SiO , and MoO with phosphoric acids was reported [23–25] and the
modified MoO in the presence of silica support was applied for toluene
2
,
2
3
3
nitration [25]. Also ‘phosphate impregnated titania or alumina’ catalyst
was prepared and found to be efficient in nitration of various organic
compounds [26–29]. Many acidic zeolites have been used for para-
selective vapor phase nitration of toluene. However, the process could
not be commercialized due to deactivation of catalyst. In order to
achieve efficient and selective process zeolites are modified, for
instance, ZSM-5 was modified with phosphoric acid and found to be
highly selective for vapor phase nitration of toluene [30]. The impact
of modification of ZSM-5 with phosphoric acid was also studied [31].
Although, there was no change in the hydrothermal stability of the
zeolite, variation on the Brønsted acid site was observed which is
responsible for the efficient catalytic activity [31]. Para-selectivity in
nitration of toluene was also achieved by using montmorillonite clay
as catalyst [32]. The clay minerals have been modified with acid as
well as other reagent and applied in various organic transformations
[
3]. However, nitration of organic compounds is still being done by
using fuming nitric and concentrated sulfuric acids in the industry.
Hence, development of an efficient and eco-friendly protocol for
nitration of organic compounds without using sulfuric acid is still
desirable. Very recently, advances in sulfuric acid free toluene nitration
are briefly reviewed [4]. Although a large number of reagents and
catalysts are known [4–14], they suffer from limitations like longer
reaction time, tedious work-up procedure and involvement of huge
expenses for cleaning up. In an effort to develop environmentally
[
33,34]. However, to the best of our knowledge, no report related to
☆
Dedicated to Prof. M. K. Chaudhuri, Vice Chancellor, Tezpur University, India on
occasion of his 67th birthday.
Corresponding author at: Department of Chemistry, Pragjyotish College, Guwahati,
modification of montmorillonite clay with phosphoric acid and its
application is available. Herein, the modification of montmorillonite
clay with phosphoric acid and its efficacy in catalytic nitration of various
aromatic compounds are reported.
⁎
781009 Assam, India. Tel.: +91 9859264499; fax: +91 361 2544531.
E-mail address: saitanya.iitg@gmail.com (S.K. Bharadwaj).
http://dx.doi.org/10.1016/j.catcom.2014.08.019
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

S.K. Bharadwaj et al. / Catalysis Communications 57 (2014) 124–128
125
Table 1
analyses were done in Perkin Elmer Pyris Diamond TG/DTA instrument.
The BET surface area was determined by using Quantachrome Autosorb
IQ equipment. The melting points of the solid products were deter-
Nitration of various organic compounds in the presence of PAM-10.
Time
min
Yield (%)[b]
[c]
Entry
1
Substrate
[a]
Product
(O/P ratio)
1
13
mined using Buchi B450 melting point apparatus. The H and C NMR
spectra were recorded in CDCl
spectrometer.
3
on a Bruker Avance II 400 NMR
90
2.1. Preparation of catalyst
2
3
4
97
5
00 mg of montmorillonite clay was taken in a round bottomed flask
(0.78)
fitted with a reflux condenser. To this, a known amount (5–100 wt.%,
abbreviated as PAM-5, PAM-10, PAM-100, etc.) of phosphoric acid and
mL of toluene was added. Then the mixture was refluxed for 5 h and
the toluene was distilled off. The “acid treated clay” was then dried in
an oven (110–120 °C). PAM-10 indicates 10 mg of phosphoric acid
was used for 100 mg of montmorillonite clay.
For the determination of chemically adsorbed phosphoric acid onto
the clay, the samples (50 mg) were taken in the water (10 mL) and
vigorously stirred for 10 min at room temperature, filtered and dried
in the oven (110–120 °C).
5
92
(0.92)
42
(0.84)
2
.2. Determination of acidity
5
6
6
8,72[e]
(0.17)
The acidity of the acid treated clay was determined by acid-base
titration as well as thermo gravimetric analysis (TGA). To 50 mg of the
catalyst, 10 mL of standardized NaOH (0.01 M) solution was added
and stirred for 10 min. Then it was filtered and filtrate was titrated
73
against standardized H
.3. Catalytic nitration reaction
In a typical experiment, bromobenzene (1.5 g, 1 mL, 10 mmol)
2 4
SO solution.
(0.11)
2
35
7
8
(0.25)
and PAM-10 (75 mg, 5 wt.% to the bromobenzene) were taken in a
round-bottomed flask. Then nitric acid (70%) (1.35 mL, 15 mmol) was
added and stirred at room temperature for the specified time period
(Table 1). The reaction was monitored by TLC. On completion of reac-
80
tion, the product was extracted with ethyl acetate, washed sequentially
with 5% aqueous solution of sodium bicarbonate (2.5 mL), and water
(
2 4
5 mL), and then dried with anhydrous Na SO . Evaporation of the
solvent, followed by column chromatography of the crude mixture on
silica gel using n-hexane and ethyl acetate (95:5) as eluent, afforded
9
8
5
5
2
-nitro toluene and 4-nitro toluene in the ratio of 44:56 in pure form.
The overall yield was 97%. In cases of solid substrates, acetonitrile
(3–5 mL) was used as the solvent.
10
11
12
7
3. Results and discussion
3.1. Preparation and characterization of catalyst
76
Initially, various amounts of phosphoric acid were refluxed with
5
00 mg of montmorillonite clay in toluene to evaluate the maximum
amount of acid adsorbed on the clay. It was observed that after treating
with 80 wt.% of acid the product became sticky which was washed with
diethylether to obtain the solid product. Toluene was chosen as solvent
because of its higher boiling point than water. Hence, water from
phosphoric acid as well as produced in the reaction will be distilled off
with toluene to give better modified clay with a minimum of water con-
tent. MK10 was also treated with other mineral acids such as sulphuric
and perchloric acids. However, refluxing the MK10 with sulphuric and
perchloric acids in toluene resulted in a black material which indicates
the destruction of layered structure.
6
8
(30, 70)
[
a] The unit of the given in square bracket is hour, [b] isolated yield, [c] the ratio of isomer
o
calculated from NMR and GCMS analysis, [d] reaction at 50 C, [e] reaction in 10 g scale.
2
. Experimental
Montmorillonite K10 (MK10) was obtained from Sigma-Aldrich.
Reagent grade 85% phosphoric acid [Merck] was used as purchased.
Solvents were distilled before used. Organic substrates for nitration
reaction were used as purchased. IR spectra (4000–250 cm ) were
recorded on a Shimadzu Prestige-21 FTIR spectrophotometer. TG
The acidity of these modified montmorillonite clay (PAM) was
obtained by determining the amount of NaOH consumed with
standardized H SO by using phenolphthalein indicator. The results
2 4
are depicted in Fig. 1. The acid equivalent is found to be approximately
in accordance with the phosphoric acid used in preparation. Further, the
−
1

126
S.K. Bharadwaj et al. / Catalysis Communications 57 (2014) 124–128
Fig. 1. The determination of acid equivalent of the catalyst prepared by using different
amounts of phosphoric acid, (a) green = the amount of acid used in preparation,
(b) blue = acid equivalent, and (c) brown = acid equivalent after water.
Fig. 3. XRD pattern of PAM-10.
acid bound to the clay surface, is comparatively lower than that of free
phosphoric acid. This might be due to electron withdrawing effect of
silicon present in PAM-10. The dissociation of the second proton of
PAM-10 did not lead to sharp peaks as compared to phosphoric acid
(Fig. 2). This could be explained by the fact that the attachment on the
surface may impart some stability to the monoanionic species.
acidity of water-washed PAM is also determined and the values are
shown in Fig. 1. Surprisingly, the acid equivalent obtained after washing
is found to be different from that without washing for all the composi-
tion. However, PAM-10 (10 wt.% phosphoric acid), shows minimum
loss of acid equivalent after washing. Hence it can be concluded that
physically adsorbed phosphoric acids are washed out with water and
FTIR spectrum of PAM-10 shows no significant changes compared to
MK10, however, a shoulder peak is observed around 1050 cm− which
can be assigned for P–O bond (see supporting Information). Also, a shift
1
1
0 wt.% is the maximum amount of phosphoric acid that was chemically
adsorbed on the clay surface. Therefore we selected the PAM-10 catalyst
for further studies. In order to study the effect of solvent in the prepara-
tion of catalyst, another reaction was carried out in ethanol (PAM-10 Et)
instead of toluene and the acidity was found to be lower than that of
PAM-10 (Fig. 1). This might be due to the lower boiling point of ethanol
compared to toluene as well as may be the possibility of formation of
hydrogen bonding with the protic solvent. Further loss of acid strength
was observed while heating the catalyst in furnace at 300 °C for 3 h
−
1
−1
from 1061 cm to 1064 cm for Si–O vibration is observed for PAM-
−
1
10. The peak observed around 780 cm is due to symmetric stretching
vibration of alumina octahedra. Thermogravimetric analysis was done
to examine the stability of the acid modified clay and it is observed
that both MK10 and PAM-10 show loss of surface and interlayer water
below 140 °C (see supporting information). A minor weight loss at
around 570 °C is observed which is due to irreversible dehydroxylation
of silane in case of unmodified clay, whereas the same weight loss is not
observed for PAM-10. Differential thermal analysis shows an endother-
mic peak at around 335 °C due to structural change (see supporting
information). The BET analysis shows significant decrease of surface
(
PAM-30F and PAM-40F, Fig. 1) which might be due to destruction of
layered structure at a higher temperature. The acidity of the PAM-10
is also determined by FTIR and TGA by adsorbing pyridine and the re-
sults are found in accordance with that obtained from volumetric
titrations.
The acid strength of PAM-10 was determined potentiometrically
and compared with free phosphoric acid. The pH of the PAM-10 and
phosphoric acid was recorded by adding 0.2 mL of standardized NaOH
2
area in PAM-10 (69.67 m /g) as compared to that of montmorillonite
2
(210 m /g) thereby indicating acid adsorption.
The XRD pattern of PAM-10 with the hkl values of highly intense
peaks is shown in Fig. 3. Although there was no significant difference
between the XRD pattern of PAM-10 with that of MK10, the decrease
in intensity for basal (001) peaks indicates the alteration of layered
structure upon acid treatment. The retention of other clay related
peaks with minimum shift of 2θ values observed at 19.84(003),
(
0.01 N) solution. The derivatives of the pH value against the volume
of NaOH added are plotted and shown in Fig. 2. It is observed that the
dissociation constant of the first proton of PAM-10, i.e. phosphoric
2
0.90(020), 26.68(003) and other peaks indicated conservation of
two-dimensional lattice.
Hence the above experiments for characterization of the phosphoric
acid modified montmorillonite clay corroborate chemical adsorption of
acid on the surface. Our next aim was to study the efficiency of this
Table 2
Control experiments.
Entry
Catalyst (5 wt.%)
Conversion (%)
1
2
3
4
5
–
Trace
10
28
35
68
Montmorillonite
Phosphoric acid
Montmorillonite + phosphoric acid
Catalyst (PAM-10)
Fig. 2. Derivative plot of potentiometric titration indicating the first dissociation point.

S.K. Bharadwaj et al. / Catalysis Communications 57 (2014) 124–128
127
newly prepared modified clay in acid-demanding nitration reaction
instead of sulfuric acid.
Table 4
Comparison of catalytic efficiency of PAM-10 with other catalysts in terms of regioselec-
tivity of nitration of toluene.
3
.2. Nitration of different aromatic compounds
Catalyst
Temp. (°C) Yield (%) O/P ratio Ref.
Phosphate impregnated titania RT
87
90
72
97
0.92
1.22
0.78
0.78
[26]
[27]
[32]
This work
In order to examine the efficacy of the catalyst for nitration of aromat-
Al(H
2
PO
4
)
3
RT
Montmorillonite
Present catalyst (PAM-10)
200
RT
ic compounds several reactions were carried out with bromobenzene.
Although the reaction of bromobenzene took longer time, it has been
considered as model substrate for optimization of the reaction conditions
assuming that once this can be nitrated in the presence of the catalyst,
other substrate with electron-donating substituent might undergo nitra-
tion easily. An amount of 5 wt.% of catalyst (with respect to substrate)
and 1.2 equivalent of HNO are found to be the optimum experimental
3
conditions for nitration reaction. Thereafter several aromatic substrates
were nitrated and the results are summarized in Table 1.
d) 5 wt.% montmorillonite and phosphoric acid (entry 4, Table 2),
separately. The conversion in each case was comparatively far less
which demonstrates that neither of them is effective to forward the
desired reaction.
As expected, aromatic compounds with electron donating substitu-
ent gave corresponding nitro compounds with excellent yields in a
very short time (entries 2 and 3, Table 1). In case of aniline, some
oxidized polymeric byproducts were obtained, thereby decreasing the
yield of desired nitro-aniline (entry 4, Table 1). The para selectivity in
the present case is found to be higher than general nitration with
3.2.1. Reusability of the catalyst
The reusability of the catalyst was studied for nitration of
bromobenzene and the results are incorporated in Table 3. After com-
pletion of reaction, the products were extracted with ethyl acetate
(×3) and the remaining aqueous suspension of catalyst was filtered,
washed with water, and finally heated in oven at 100–120 °C. This
dried used-catalyst was reused for nitration reaction with fresh
bromobenzene and nitric acid. The efficiency of the catalyst was found
to decrease significantly and gave 20 and 18% yield in 2nd and 3rd run
respectively. However, when the aqueous suspension was directly
evaporated by heating at 100–120 °C and reused, we obtained 55 and
44% of conversion in 2nd and 3rd run respectively. Thereafter, the
efficiency of the catalyst was lost and very less conversion was
observed. This is due to leaching of phosphate group from the surface
of clay which was confirmed by gravimetric determination. However,
the efficiency can be easily regenerated by refluxing the used catalyst
with 10 wt.% phosphoric acid.
H
2
SO
montmorillonite [32], phosphate impregnated titania [26], aluminium
tris(dihydrogen phosphate) [27], and modified MoO [25]. Benzene
4
. Similar observation was observed in the case of unmodified
3
also yielded nitrobenzene efficiently (entry 1, Table 1). Moderately
activated aromatic compound such as acetanilide afforded ortho-
and para-nitro acetanilide in good yields (entry 6, Table 1). The para-
nitroacetanilide was observed as major product due to the bulkiness
of the acetanilide group. Most interestingly, chlorobenzene underwent
nitration in the presence of PAM-10 and gave 35% of corresponding
nitrochlorobenzene.
Notably, several previously reported catalysts failed to nitrate
chlorobenzene [12,26,27]. Due to higher electronegativity of chlorine
than that of bromine, the aromatic ring is more deactivated towards
electrophilic substitution reaction, thereby providing low or almost no
yield. Several polyaromatic hydrocarbons, viz, naphthalene (entry 8,
Table 1) and anthracene (entry 9, Table 1), underwent regioselective
nitration under the same reaction condition. In addition, the presence
of a hydroxyl group, e.g. in 1-naphthol (entry 10, Table 1) further
facilitates the nitration reaction. With the present reaction condition,
we observed some undesired product in the nitration reaction of 1-
naphthol, hence the yield of the reaction is found to be slightly lower
than that of naphthalene (entry 8, Table 1). Nitration reaction with
pyridine yielded pyridine-n-oxide instead of nitro-pyridine (entry 11,
Table 1). Due to the presence of non-delocalized lone pair of electron
on the nitrogen, pyridine prefers oxidation to give pyridine-n-oxide
rather than 3-nitro pyridine with the present reaction condition. A
preparative scale reaction performed with 10 g of bromobenzene
worked well and gave isolated yield of 72% (entry 5, Table 1).
In order to evaluate the efficiency of the present catalyst, we have
compared the results of the nitration of toluene with those of other
catalysts and shown in the Table 4. It is observed that the yields are
2 4 3
almost similar in case of ‘phosphate impregnated titania’, Al(H PO )
and PAM-10, whereas montmorillonite gives comparatively low yield
at high temperature. Montmorillonite and the PAM-10 show similar
selectivity, which is higher than the other two catalysts, indicating
clearly that the present catalyst gives higher yield and better selectivity
at room temperature.
4. Conclusions
A heterogeneous solid acid catalyst is developed by adsorbing
phosphoric acid on montmorillonite clay. 10 wt.% of phosphoric acid
with respect to the weight of clay is found to adsorb chemically with
minimum loss upon water treatment. The phosphoric acid modified
clay is found to be an efficient catalyst for nitration of organic
compounds with nitric acid and a wide range of such compounds
have been nitrated by avoiding the use of sulfuric acid and this is one
of the most significant advantages of the present process. Although
the reusability of the catalyst is poor, it is compensated by efficacy,
ease of handling and scalability which render this protocol very
attractive and useful. Further we hope that gas phase nitration will
provide regio-selective nitration due to layered structure of the clay
which will be carried out in due course. This PAM-10 has been found
to efficiently catalyze several organic transformations, which will be
communicated elsewhere.
The usefulness of the present catalyst is demonstrated by conducting
different control experiments and the results are depicted in Table 2.
Reactions were conducted with bromobenzene and nitric acid in the
presence of a) no catalyst (entry 1, Table 2) b) 5 wt.% montmorillonite
(
entry 2, Table 2), c) 5 wt.% phosphoric acid (entry 3, Table 2) and
Table 3
Reusability of catalyst for nitration of bromobenzene.
Reaction run
Yield (%)
Catalyst is
reused after
filtration
Catalyst is reused
after evaporating
the aq. layer
Catalyst is regenerated
by refluxing with
phosphoric acid
Acknowledgments
1
2
3
4
st
20
18
15
15
55
44
40
38
65
62
63
62
S.K.B. wish to thank Department of Science and Technology (DST/
SERB), New Delhi, India for the Project CS-75-2013. Authors are grateful
to HOD, Department of Chemistry, Dibrugarh University. They are also
thankful to Dr. Papori Goswami (NITS-Mirza, Assam), Dr. Manash R
nd
rd
th

128
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