Selective nitration of aromatic compounds by solid acid catalysts

Article abstract of DOI:10.1039/a908011b

High activity and para-selectivity in the nitration of aromatic compounds is achieved by a high density of acidic sites and ready formation of the para-isomer in the pores of zeolite beta with low Si/Al ratio as revealed by molecular modeling studies.

Full text of DOI:10.1039/a908011b

Selective nitration of aromatic compounds by solid acid catalysts
B. M. Choudary,* M. Sateesh, M. Lakshmi Kantam, K. Koteswara Rao, K. V. Ram Prasad, K. V. Raghavan
and J. A. R. P. Sarma
Indian Institute of Chemical Technology, Hyderabad-500 007, India. E-mail: Choudary@iict.ap.nic.in
Received (in Cambridge, UK) 5th October 1999, Accepted 22nd November 1999
High activity and para-selectivity in the nitration of aro-
matic compounds is achieved by a high density of acidic sites
and ready formation of the para-isomer in the pores of
zeolite beta with low Si/Al ratio as revealed by molecular
modeling studies.
zeolite beta-acetic anhydride-nitric acid system developed later
by Smith et al.,4b,c afforded good space time yields but required
great care in large scale operations. Barrett and coworkers4d
employed La(III) triflates as recyclable catalysts for aromatic
nitrations but such systems required longer reaction times to
obtain good yields without any explicit selectivity towards o-/
p-isomer.
Nitration of aromatic compounds is a ubiquitous reaction to
realise organic intermediates required in large tonnages for the
fine chemical industry. The conventional nitration process,1
employing a nitrating mixture of nitric and sulfuric acid, for the
last 150 years has remained unchallenged in the commercial
arena owing to uneconomical alternative options. Selective
synthesis of the desired isomer in the nitration of substituted
aromatic compounds is of topical interest to reduce pollution
caused by unwanted isomers and specifically to respond to
market demand. Earlier we reported selective nitrations by
employing a solid acid, Fe3+ montmorillonite in the presence of
acetic anhydride–nitric acid.2 Later, various novel options,
meeting international environmental laws, and also avoiding the
recovery of the spent acid in the conventional mixed acid
process have been explored that include Laszlo’s3 claycop
(copper nitrate impregnated on K10 montmorillonite) acetic
anhydride and recently claycop–nitric acid–acetic anhydride,
Smith’s4a mordenite–benzoyl nitrate, Kwok’s5 ZSM-5-propyl
nitrate and Olah’s6 Nafion-H–n-butyl nitrate. However these
options fall short of expectations to commercial realisation in
view of low space time yields, high dilution, and use of
expensive mixtures in the form of acetic anhydride–nitric acid
or expensive acyl or alkyl nitrate whereas Sato’s7 and Prins’s8
vapour phase nitrations of benzene conducted employing clays
and mordenite zeolite provided low space time yields. The
Herein, we present a unique methodology for the nitration of
aromatic compounds that offers high space time yields, near
zero emission of effluents, especially high p-selectivity for
monosubstituted products, essential for commercial processes,
employing nitric acid at a concentration of 60–90%, with reuse
of the solid acid catalysts being possible by azeotropic removal
of water formed in the reaction and present in nitric acid.†
We executed a two-fold approach employing modified clays
and zeolites to realise a high space time yield as well as high
selectivity of the desired p-isomer as indicated in Table 1. In an
earlier investigation,9 a series of different metal-exchanged and
pillared clay catalysts were prepared10 and employed in the
nitration reactions of aromatic compounds under identical
conditions. Fe3+ montmorillonite catalyst was found to be the
most active owing to its high metal content and highly acidic
sites. This catalyst (entry 1) is far superior in terms of space time
yield, when compared even with gas phase reactions conducted
with various metal-exchanged montmorillonites7 and morde-
nite.8 In the nitration of benzene, mononitrobenzene is obtained
selectively without formation of any di- or poly-nitrobenzenes
and the p-selectivities in the nitration of monosubstituted
benzenes are improved.
Our main emphasis is to achieve higher p-selectivity to meet
market demand. Accordingly, various zeolites which have well
Table 1 Selective nitration of aromatic compounds
Product distributionc
Product
Entry
Arene
Catalyst
HNO3 (%)
yielda (%)
STYb
o-
m-
p
1
2
3
4
5
6
C6H6
C6H6
C6H5Me
C6H5Me
C6H5Me
C6H5Me
Fe3+ mont.
Al3+ mont.d
Mixed Acid
Fe3+ mont.
K10 mont.
Zeolite beta-Ig
5th recycle
Zeolite beta-IIg
ZSM-5
Mordenite
HY
TS-1
Fe3+ mont.
Zeolite beta-I
Zeolite beta-I
Zeolite beta-I
Zeolite beta-I
Zeolite beta-I
90
60
90
90
60
60
60
60
60
60
60
60
90
60
70
70
70
70
88
22.8
0.6
—
26.0
9.1
10.6
10.3
6.8
5.6
6.0
5.4
5.0
26.0
12.0
15.8
19.8
12.8
11.4
—
—
58
53
45
30
33
41
49
56
54
56
36
10
18
28
19
25
—
—
4
3
3
3
3
4
7
6
7
6
—
—
—
—
—
—
—
—
38
44
52
67
64
55
44
38
39
38
64
90
82
72
81
75
48,e 95f
—
89
53
68, 96f
67, 95f
40, 95f
32
35
32
30
77
51, 98h
51, 96h
52
7
8
9
C6H5Me
C6H5Me
C6H5Me
C6H5Me
C6H5Me
C6H5Cl
C6H5Cl
C6H5Bri
C6H5Ii
10
11
12
13
14
15
16
17
C6H5Pri
C6H5OMe
51
49, 96h
a Product yield based on total amount of HNO3 used. b Space time yield: kg product/kg catalyst/h. c By GC. d Results from ref. 7. e Yields based on
benzene. f Yields calculated based on HNO3 actually consumed, i.e. subtracting the amount of nitric acid azeotropically distilled out from the reaction vessel
and collected in the Dean–Stark trap from the total amount of nitric acid used. The nitric acid collected is reused after adjusting to the desired concentration.
g Zeolite beta-I, II+SiO2/Al2O3 molar ratio = 22, 27. h Yields based on aromatics with aromatic compound and HNO3 in 1+2 ratio. i Dichloroethane used
as solvent.
Chem. Commun., 2000, 25–26
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25

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defined pore structures and channels that are derived from the
networking of SiO2 and Al2O3 making them attractive candi-
dates for shape selective catalysis11 were employed12 to achieve
higher shape selectivity in the nitration of substituted aromatics.
Zeolite beta-I13 (entry 6) is found to the best catalyst for the
nitration of aromatic hydrocarbons to nitroaromatics with high
p-selectivity. When compared with the selectivities obtained
using classical mixed acid, zeolite beta-I catalyst displayed a
major shift towards p-selectivity as is evident, for example; for
toluene: from 38 to 67%, for cumene: 58 to 81%, for
chlorobenzene: 64 to 90%, for anisole: 40 to 75% (Table 1). The
production of the p-isomer in higher ratio for toluene and
cumene meets the timely demand of the market and at the same
time avoids the burning of undesired o-isomers occasionally
resorted to by the manufacturers which causes environmental
pollution. Increased formation of the p-isomer in the nitration of
chlorobenzene eases the isomeric separation to a considerable
extent and helps to save energy. Whereas the change from
methyl (toluene) to the larger isopropyl (cumene) substituent
increases the para selectivity, the reverse applies for halogens,
for which the best para selectivity is obtained with the smaller
substituent, namely chlorine owing to electronic factors.
We carried out nitration of toluene with various zeolite
catalysts and zeolite beta of different Si/Al ratios (Table 1).
Zeolite beta proved to be the best catalyst among the zeolites
used in terms of space time yield and p-selectivity. On
decreasing the Si/Al ratio in zeolite beta, an increase in p-
selectivity as well as overall activity is observed (entry 6 and 7).
An increased number of Bronsted acid sites reduces the
available space in the pore and this result substantiates our
inference that the formation of o-isomer, which requires more
space, is unfavourable. These observations are in accord with
the results observed by Bellussi et al. in alkylation reactions.14
It is also found that with an increase of catalyst concentration
the p-selectivity of toluene nitration increased further to 73%.
Zeolite beta catalyst showed consistent activity and selectivity
even after five cycles (entry 6). Careful analysis of the
regenerated catalyst after recycling it five times showed no
change in the Si/Al composition indicating that no deal-
umination of catalyst has taken place.
ion, as well as acting as an instant adsorbent for water formed
during the reaction.
On the other hand ZSM-5 under such conditions affords low
space time yield and selectivity (entry 8) in the nitration of
toluene when compared to the high p-selectivity obtained by
Kwok et al.5 employing acyl nitrate as nitrating agent. This is
attributed to low diffusion of aqueous HNO3 in the hydrophobic
pores of ZSM-5 and the resulting density of acidic sites is too
low to promote the generation of nitronium ions from nitric
acid. Therefore the reaction on ZSM-5 mainly takes place on
the surface of the zeolite.
In conclusion, a simple methodology for nitration employing
solid acids dispensing with the use of acetic anhydride or acyl
nitrate described here leading to high space time yields and high
p-selectivities, appears to be a promising alternative to the
conventional acid mixture of nitric and sulfuric acid. Enhanced
p-isomer formation to meet market demands, dispensing the use
of sulfuric acid, achieving near zero emission of effluents, non-
corrosivity and low water requirement are other attractions.
Notes and references
† General procedure for the nitration of monosubstituted aromatics: A
mixture of toluene (161 ml; 1.5 mol) and zeolite beta-I catalyst (10 g) were
added to in a 1 litre-reactor flask equipped with stirring rod and Dean–Stark
apparatus. 60% nitric acid (120 ml, 1.5 mol) was added in a controlled
manner with an infusion pump over a period of 80 min to the above mixture,
which was preheated to the refluxing temperature. The liberated water
collected into the Dean–Stark apparatus was continuously removed. After
completion of the reaction, the catalyst was filtered off and the reaction
mixture was concentrated to obtain the mixture of nitrotoluenes. Nitration of
other aromatic hydrocarbons was similarly carried out on a 76 mmol
scale.
1 G. A. Olah, R. Malhotra and S. C. Narang, Nitration Methods and
Mechanism, VCH, New York, 1989, pp. 5–15; K. Schofield, Aromatic
Nitration, Cambridge University Press, Cambridge, 1980.
2 B. M. Choudary, M. R. Sarma and K. V. Kumar, J. Mol. Catal., 1994,
87, 33.
3 L. Delaude, P. Laszlo and K. Smith, Acc. Chem. Res., 1993, 26, 607; B.
Gigante, A. O. Prazeres, M. J. Marcelo-Curto, A. Cornelis and P.
Laszlo, J. Org. Chem., 1995, 60, 3445; A. Cornelis, L. Delaude, A.
Gerstmans and P. Laszlo, Tetrahedron Lett., 1988, 29, 5657.
4 (a) K. Smith, K. Fry, M. Butters and B. Nay, Tetrahedron Lett., 1989,
30, 5333; (b) K. Smith, A. Musson and G. A. DeBoos, Chem. Commun.,
1996, 469; (c) K. Smith, A. Musson and G. A. DeBoos, J. Org. Chem.,
1998, 63, 8448; (d) F. J. Waller, A. G. M. Barrett, D. C. Braddock and
D. Ramprasad, Chem. Commun., 1997, 613.
5 T. J. Kwok, K. Jayasuriya, D. Reddy and B. W. Brodman, J. Org.
Chem., 1994, 59, 4939.
6 G. A. Olah, R. Malhotra and S. C. Narang, J. Org. Chem., 1978, 43,
4628.
7 H. Sato, K. Hirose, K. Nagai, H. Yoshioka and Y. Nagaoka, Appl.
Catal. A, 1998, 175, 201.
8 L. Bertea, H. W. Kouwenhoven and R. Prins, Appl. Catal. A , 1995, 129,
229.
9 B. M. Choudary, M. L. Kantam, M. Sateesh, K. K. Rao and K. V.
Raghavan, EP 0949240 A, 1999.
10 P. Laszlo and A. Mathy, Helv. Chim. Acta., 1987, 70, 577; E. G. Rightor,
M. Tzou and T. J. Pinnavaia, J. Catal., 1991, 130, 29.
11 S. M. Cseri, Zeolites, 1984, 4, 202; W. Holderich and H. van Bekkum,
Stud. Surf. Sci. Catal., 1991, 58, 631.
12 B. M. Choudary, M. Sateesh, M. L. Kantam, K. K. Rao, K. V. R. Prasad
and K. V. Raghavan, US Pat., Appl. No. 09/188589, 1998.
13 J. Perez-Pariente, J. A. Martens and P. A. Jacobs, Zeolites, 1991, 8,
46.
14 G. Bellussi, G. Pazzuconi, C. Perego, G. Girotti and G. Terzoni,
J. Catal., 1995, 157, 227.
15 Sorption module in Cerius2 implements a rapid Monte Carlo statistical
mechanics method: N. Metropolis, A. W. Rosenbluth, M. N. Rosen-
bluth, A. H. Teller and E. Teller, J. Chem. Phys., 1953, 21, 1087; M. P.
Allen and D. J. Tildesley, Computer Simulation of Liquids, Clarendon
Press, Oxford, 1987.
Sorption studies of the reactant and isomers of the product
and their intermediates (transition states) undertaken using the
sorption module of Cerius2 which implements rapid Monte
Carlo statistical mechanical calculations15 clearly predicted
high p-selectivity possible with ease of formation of the
intermediate of the p-isomer in the pores. The result of shape
selectivity observed employing zeolite beta derives further
support from the results obtained with metal exchanged clays,
wherein mesopores of montmorillonite formed due to acid
treatment also induced some shift in p-selectivity (entries 4 and
5). The higher shape selectivity for the zeolite is ascribed to the
induction of larger steric effects by the 3D structure of zeolite
beta in comparison to the 2D structure of clay catalysts.
Nitration involves electrophilic attack on the aromatic ring by
+
the nitronium ion, NO2 . Bronsted acidic sites are responsible
+
for the generation of NO2 ion from nitric acid. Reaction
conducted with fuming nitric acid without azeotropic removal
of water is totally inhibited after some time owing to a poisoning
effect of water formed in the reaction. It is necessary to
scavenge the water out of the reaction zone formed during the
reaction to facilitate regeneration of active acid sites on the
catalyst. The rate of addition of nitric acid to the reactor
containing powder catalyst and substrate is matched with the
rate of removal of water present in nitric acid and the reaction
zone through extractive distillation with a Dean–Stark appara-
tus using the substrate as the solvent or chlorohydrocarbon
solvent for high boiling aromatic compounds to afford optimum
selectivity, activity and protection of the catalyst from degrada-
tion. This protocol envisages the instant reaction of added nitric
acid and establishes that the solid acid catalysts used here act as
bifunctional catalysts generating the electrophile, nitronium
Communication a908011b
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Chem. Commun., 2000, 25–26