Highly regioselective dinitration of toluene over reusable zeolite Hβ

Article abstract of DOI:10.1016/j.jcat.2012.10.017

A nitration system comprising nitric acid, propanoic anhydride, and zeolite Hβ has been developed for dinitration of toluene to give 2,4-dinitrotoluene in 98% yield, with a 2,4-:2,6-dinitrotoluene ratio of over 120. This represents the most selective quantitative method for 2,4-dinitration of toluene; the catalyst is reusable, solvent is not needed, and an aqueous work-up is not required.

Full text of DOI:10.1016/j.jcat.2012.10.017

Journal of Catalysis 297 (2013) 244–247
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Highly regioselective dinitration of toluene over reusable zeolite Hb
,1
⇑
⇑
Keith Smith , Mohammad Hayal Alotaibi, Gamal A. El-Hiti
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A nitration system comprising nitric acid, propanoic anhydride, and zeolite Hb has been developed for
dinitration of toluene to give 2,4-dinitrotoluene in 98% yield, with a 2,4-:2,6-dinitrotoluene ratio of over
120. This represents the most selective quantitative method for 2,4-dinitration of toluene; the catalyst is
reusable, solvent is not needed, and an aqueous work-up is not required.
Received 15 August 2012
Revised 11 October 2012
Accepted 13 October 2012
Available online 16 November 2012
Ó 2012 Elsevier Inc. All rights reserved.
Keywords:
Nitration
Acidic zeolite catalysis
Propanoyl nitrate
Mono-substituted benzenes
Zeolite Hb
Regioselectivity
1. Introduction
and fragrances [29,30]. Unfortunately, dinitration of toluene with
mixed acids produces 2,4- and 2,6-dinitrotoluenes in a ratio of only
The synthesis of valuable chemicals frequently involves electro-
philic aromatic substitution reactions. However, the commercial
syntheses of these compounds commonly suffer serious disadvan-
tages, including: a requirement for large quantities of mineral or
Lewis acids as activators, which on work-up may generate large
quantities of corrosive and toxic waste by-products; the use of
stoichiometric quantities of toxic reagents; poor yields; or produc-
tion of mixtures of regioisomers with low selectivity [1–4].
Major efforts are therefore being made to develop clean and
environmentally benign processes, particularly for the regioselec-
tive production of para-substituted products. It is well recognized
that zeolite catalysts can play an important role through their abil-
ities to act as recyclable heterogeneous catalysts that can enhance
product selectivities [5–11]. We have investigated the use of zeo-
lites in electrophilic substitution of aromatic compounds [12–14]
and have shown that they can be used successfully in cleaner
para-regioselective reactions including nitration [15–22], alkyl-
ation [23], acylation [24,25], halogenation [26,27], and methane-
sulfonylation [28] reactions.
4:1. There have been several attempts to improve the process by
use of solid catalysts [31–36]. For example, the 2,4-:2,6-dinitrotol-
uene ratio can be improved to 9:1 (in 85% yield) when nitration is
carried out over claycop, using nitric acid and acetic anhydride
[29,30], but the method requires a large excess of nitric acid and
the use of carbon tetrachloride as a solvent. A better ratio (14:1)
was reported using nitric acid over a zeolite b catalyst [37].
We have previously investigated the dinitration of toluene with
nitric acid and trifluoroacetic anhydride (TFAA) over zeolite Hb,
which gave a 92% yield of 2,4-DNT with a 2,4-:2,6-dinitrotoluene
(2,6-DNT) ratio of 25 [21]. Even greater selectivity (70:1; in 96%
yield) was obtained when the reaction was conducted in two
stages, with trifluoroacetic anhydride added only in the second
stage [21]. However, trifluoroacetic anhydride’s volatility, toxicity,
and cost may render it unattractive for commercial processes. We
have therefore sought to find a more attractive alternative [38].
2. Experimental
2,4-Dinitrotoluene (2,4-DNT) is a crucial intermediate for the
production of many industrially important chemicals, including
polyurethanes, explosives, dyes, pharmaceuticals, agrochemicals,
2.1. Materials
Commercial Hb zeolite was purchased from Zeolyst Interna-
tional and was freshly calcined at 450 °C for a minimum of 6 h
prior to use. Toluene was purchased from Fisher Scientific, and ni-
tric acid (100%) was purchased from BDH Laboratory Supplies.
Other chemicals were purchased from Aldrich Chemical Company
and used without further purification except for toluene and prop-
anoic anhydride, which were distilled.
⇑
Corresponding authors. Fax: +44 (0)2920870600 (K. Smith).
E-mail addresses: smithk13@cardiff.ac.uk (K. Smith), el-hitiga@cardiff.ac.uk
(G.A. El-Hiti).
1
Address: Chemistry Department, Faculty of Science, Tanta University,
Tanta 31527, Egypt.
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.10.017

K. Smith et al. / Journal of Catalysis 297 (2013) 244–247
245
2.2. Analysis and characterization of the products
3. Results and discussion
All GC analyses were carried out on a PU 4400 Gas Chromato-
graph (Philips) using a capillary ZB Carbowax column (30 m,
0.32 mm ID). The GC conditions used for analysis were as follows:
35 °C for 0.5 min, ramped to 240 °C at 20 °C/min and held for
15 min. The injection temperature was 300 °C and the detection
temperature 300 °C. Hexadecane was added as an internal stan-
dard to allow quantification. All of the expected products from
nitration of toluene were purchased from Aldrich Chemical Com-
pany and used to determine retention times and response factors
relative to hexadecane (average from four injections) for each
product.
Zeolite b shows high para-selectivity in mono-nitration of
toluene with acetyl nitrate [22]. The high para-selectivity has been
attributed to steric hindrance of the surface-bound acetyl nitrate
complex [39,40]. Therefore, we first attempted direct dinitration
of toluene over zeolite Hb (SiO2:Al2O3 = 25) with HNO3 and acetic
anhydride, which generates acetyl nitrate in situ. However,
although we varied the stoichiometry, amount of zeolite, reaction
temperature, and reaction time, the maximum yield of 2,4-DNT
obtained was 89%, though with only trace amounts of 2,6-DNT,
along with 9% of 4-nitrotoluene (4-NT) and ca. 2% of 3-nitrotoluene
(3-NT). Although the reaction was highly selective, we were unable
to cause the reaction to go to completion. It was thought that nitra-
tion of toluene using propanoyl nitrate as nitrating reagent instead
of acetyl nitrate could enhance the production of 2,4-DNT as a re-
sult of both additional steric hindrance of the surface-bound re-
agent, which could lead to even greater selectivity, and its lower
volatility, which might reduce losses of reagent during the reaction
period. Therefore, we decided to try the use of propanoic anhydride
and nitric acid, which should generate the propanoyl nitrate in situ
(Scheme 1). We now report success with this system.
Initially, we attempted a single step double nitration of toluene
(Scheme 2) using toluene (35 mmol), propanoic anhydride
(70 mmol), and nitric acid (100%, 70 mmol) in the presence of zeo-
lite Hb (2.0 g), but even for a reaction carried out at 50 °C for 4 h,
the yield of dinitrotoluenes (DNTs) was only 57%, with a 61:1 ratio
of 2,4-:2,6-isomers. However, when the amounts of nitric acid and
propanoic anhydride were increased to 120 mmol each under iden-
tical conditions, the reaction gave 94% of DNTs, with a 85:1 ratio of
2,4-:2,6-isomers, along with around 5% of 4-NT and 1% of 3-NT.
To try to improve the yield and selectivity even further, we
investigated the effect of an even larger excess of propanoic anhy-
dride. Just increasing the amount of anhydride to 130 mmol caused
the reaction to go to completion, producing dinitrotoluenes in al-
most quantitative yield (98% 2,4-dinitrotoluene and 0.8% 2,6-dini-
trotoluene) with a 2,4-:2,6- ratio of over 120:1. Pure 2,4-DNT was
isolated in 94% yield by simply filtering, washing the zeolite with
solvent, concentrating the mother liquor, and recrystallization
from diethyl ether (for large scale work solvent extraction of the
zeolite would probably be unnecessary, since residual product
could be removed from the zeolite under reduced pressure, but
to ensure that all products were accounted for, in this work, the
zeolite was fully extracted).
1H NMR Spectroscopy was the method of analysis for substrates
other than toluene; the calculated mass balances were consistent
with the total weights of products obtained.
2.3. Typical experimental procedure for the double nitration of toluene
using nitric acid, propanoic anhydride and zeolite Hb catalyst
Propanoic anhydride (16.90 g, 130 mmol) was added to a stirring
mixture of nitric acid (7.65 g, 100%, 120 mmol), and Hb (2.0 g,
SiO2:Al2O3 = 25) at 0 °C and the mixture was stirred for 5 min at
constant temperature. Toluene (3.22 g, 35 mmol) was then added
dropwise, and the mixture was allowed to warm to room tempera-
ture. The flask was equipped with a water condenser fitted with a
calcium chloride guard tube and was stirred at 50 °C for 4 h. The
reaction mixture was cooled to room temperature, analytical grade
acetone (30 mL) was then added, and the mixture was stirred for
5 min. The zeolite was removed by suction filtration and washed
with copious amounts of acetone. The mother liquors were
combined, hexadecane (1.00 g) was added, and the mixture was
subjected to GC analysis.
2.4. Purification of 2,4-dinitrotoluene (2,4-DNT)
The isolation procedure is for a reaction that did not have hexa-
decane added. At the end of the reaction, the mixture was allowed
to cool to room temperature, and the catalyst was removed by fil-
tration and washed with analytical grade acetone (3 Â 10 mL).
Water (30 mL) and DCM (30 mL) were added to the combined fil-
trate and washings, and the layers were separated. The aqueous
layer was extracted with DCM (20 mL), and the organic layers were
combined. The organic extract was washed with aq. saturated
NaHCO3 solution (30 mL) followed by water (30 mL), dried
(MgS O4), and evaporated to constant weight under reduced pres-
sure to give the crude product (6.34 g). The crude product was
recrystallized from analytical grade acetone to give pure 2,4-
dinitrotoluene (5.99 g, 94%; >99% purity by GC).
In order to check whether the period of 4 h used in reactions
thus far was required, various reaction times were attempted
(Table 1). As can be seen from Table 1, the yield of 2,4-dinitrotolu-
ene increased with increasing reaction time. The yield of dinitrotol-
uenes was 82% even after 5 min; at that point, the 2,4-:2,6-DNT
ratio was around 74, and already, there was virtually no residual
2-nitrotoluene, suggesting that it reacted rapidly to give dinitrotol-
uenes. The 4-nitrotoluene evidently reacted more slowly, so that
the yield of DNTs and the selectivity for 2,4-DNT both rose over
time after the first few minutes. It is not clear whether the
apparent small reduction in the amount of 2,6-DNT over
time was real, as a result of its further reaction (perhaps to produce
(EtCO)2O
+
HNO3
EtC(O)ONO2
+
EtCO2H
Scheme 1. Generation of propanoyl nitrate.
Me
Me
Me
NO2
O2N
NO2
β
HNO3, H
+
+
others
(EtCO)2O
NO2
Scheme 2. Nitration of toluene over a HNO3/(EtCO)2O/Hb system.

246
K. Smith et al. / Journal of Catalysis 297 (2013) 244–247
R
R
R
R
R
R
NO2 O2N
+
NO2
NO2
β
H , HNO3
+
+
NO2
+
(EtCO)2O
NO2
NO2
2.4-DN
2,6-DN
2-N
3-N
4-N
Scheme 3. Nitration of mono-substituted benzenes with a HNO3/(EtCO)2O/Hb system.
Table 1
Effect of reaction time in dinitration of toluene with HNO3/(EtCO)2O/Hb system according to Scheme 2.a
Time (min)
Yields of productsb
3-NT
2,4-:2,6- ratioc
Mass balance (%)d
4-NT
2,4-DNT
2,6-DNT
5
15
30
60
120
240
1.1
1.0
1.1
0.9
0.8
0.4
15
13
11
8.0
4.8
0.8
81
82
84
91
93
98
1.1
1.0
1.1
0.7
0.9
0.8
74
82
76
130
103
123
98.2
97.0
97.2
100.6
99.5
100.0
a
Propanoic anhydride (16.90 g, 130 mmol) was added to a stirring mixture of nitric acid (7.65 g, 100%, 120 mmol) and Hb (2.00 g) at 0 °C, and the mixture was stirred for
5 min. Toluene (3.22 g, 35 mmol) was added dropwise, and the mixture was allowed to warm to room temperature. The mixture was then stirred at 50 °C under a calcium
chloride guard tube for the time indicated.
b
By quantitative GC. Recorded to the nearest 1% (above 10%) or 0.1% (below 10%).
Ratio calculated directly from GC data and not from the rounded figures in the table.
Sum total of the calculated yields of the identified compounds.
c
d
2,4,6-trinitrotoluene, which would not have been detected by the
GC system used) or whether the results reflect experimental error
in the measurement, with all yields of 2,6-DNT being essentially
the same at 0.9 0.2%. Similarly, it is not clear whether the small
reduction in quantity of 3-nitrotoluene over time is real, caused
by its conversion into one or more trace dinitrotoluenes, or reflects
experimental variation in measurement. However, it is clear that
for the reaction to be complete, a 4 h reaction period was
necessary.
chlorobenzene, bromobenzene), were subjected to the nitrating
system under the same conditions as those optimized for toluene.
Reactions were worked up, and product compositions determined
by proton nuclear magnetic resonance (1H NMR) spectroscopy
(since all expected products were not to hand for use as GC
Table 2
Nitration of mono-substituted benzenes according to Scheme 3.a
In order to check on the possibility of reuse of the zeolite, it was
recovered following extraction of the products and was regener-
ated by heating overnight in air at 450 °C. Nitration reactions were
then conducted under identical conditions using the recovered
zeolite. The reaction was repeated seven times using the same
batch of catalyst, but later reactions were scaled down somewhat
due to small losses of zeolite during recovery. The results showed
that under the standard conditions, there was only a slight de-
crease (to 94%) in the yield of 2,4-dinitrotoluene even after using
the same zeolite seven times under identical conditions, and the
selectivity was virtually the same.
Entry
R
Yield (%)b
2-N 3-N
OMe Trace Trace Trace
Mass balance (%)c
4-N
2,4-DN
2,6-DN
1
2
3
4
5
6
7
8
97
96
98
73
1
1
98d
97d
100e
97
OEt
Me
Et
Pr
F
Trace Trace Trace
–
7
0.4
0.8
0.8
Trace
Trace 17
5 (3)f Trace 31 (7)f 61 (89)f Trace
97 (99)f
97
7
2
2
Trace 85
5
2
2
–
2
2
Cl
Br
–
–
92
92
98
98
a
Propanoic anhydride (16.90 g, 130 mmol) was added to a stirring mixture of
Reactions were also conducted in various solvents (cyclohex-
ane, chloroform, 1,2-dichloroethane, propanoic acid or excess
propanoic anhydride). However, use of solvent invariably brought
about a decrease in yield of DNTs, presumably as a result of the
lower concentrations of reacting species, and in all cases except
for propanoic anhydride, the 2,4-:2,6-DNT ratio was also
somewhat lower, consistent with the reduced extent of reaction
of 4-nitrotoluene. Therefore, the addition of solvent to the nitration
system has no advantages on either selectivity or yield and is not
necessary.
After the success in producing a high yield of 2,4-dinitrotoluene
selectively, attention was turned to nitration of various other
mono-substituted benzenes using the same system (Scheme 3).
Several mono-substituted benzenes, including ones that are
activated (anisole, phenetole), moderately activated (ethylben-
zene, propylbenzene), and moderately deactivated (fluorobenzene,
nitric acid (7.65 g, 100%, 120 mmol) and zeolite Hb (2.00 g) at 0 °C, and the mixture
was stirred for 5 min. Substituted benzene (35 mmol) was added dropwise, and the
mixture was allowed to warm to room temperature then stirred at 50 °C for 4 h
under a calcium chloride guard tube. (The order of addition was found not to be
important for halobenzenes, but it was important for the more reactive substrates.)
The mixture was allowed to cool to room temperature, and the catalyst was
removed by filtration and washed with acetone (3 Â 10 mL). Water (30 mL) and
DCM (30 mL) were added to the combined filtrate and washings, and the layers
were separated. The aqueous layer was extracted with DCM (20 mL), and the
organic layers were combined. The combined organic extract was washed with aq.
saturated NaHCO3 solution (30 mL) and water (30 mL), dried (MgSO4) and evapo-
rated under reduced pressure to give the crude product.
b
By weight of products and their 1H NMR spectra.
c
The sum of the calculated percentage yields of nitro products based on weight
of products and their 1H NMR spectra.
d
Traces of other dinitro compounds, possibly arising from 3-nitro derivative
were also seen.
e
Yields of isomers calculated by use of quantitative GC.
When 4.00 g of Hb was used.
f

K. Smith et al. / Journal of Catalysis 297 (2013) 244–247
247
standards). The calculated mass balances were consistent with the
Appendix A. Supplementary material
total weights of products obtained (Table 2).
In no case other than toluene was there evidence of a significant
quantity of 3-nitro compounds, presumably because they were
converted into trace quantities of dinitro derivatives. All reactions
appeared to be extremely selective for formation of 2,4-dinitro iso-
mer or 4-mono-nitro derivative. Highly active anisole and phene-
tole produced excellent yields of the corresponding 2,4-dinitro
derivatives (97% and 96%, respectively). 2,6-Dinitroanisole and
2,6-dinitrophenetole were produced in very low yields (1%) along
with small quantities of other nitrated products.
With ethylbenzene and propylbenzene, significant amounts
(17–31%) of 4-nitro-1-alkylbenzenes remained after 4 h, although
relatively high yields of 2,4-dinitro-1-alkylbenzenes (61–73%) were
obtained. The yield of 2,4-dinitrophenylpropane increased to 89%
under similar reaction conditions when the amount of zeolite used
was doubled to 4 g. However, toluene gave a very high yield of 2,4-
DNT even with 2 g of zeolite, so it seems that longer-chain alkylben-
zenes react more slowly, presumably because of more difficult
diffusion through the pores and/or more restricted transition states
within the pores of the zeolite. Halobenzenes produced very low
yields of dinitro compounds (2–5%). Instead, nitration of fluoroben-
zene, chlorobenzene, and bromobenzene gave 1-fluoro-4-nitroben-
zene (85%), 1-chloro-4-nitrobenzene (92%), and 1-bromo-4-nitro
benzene (92%), respectively, as the main products. We have not
optimized conditions for each substrate but longer reaction times,
additional nitrating reagent, more catalyst and/or higher tempera-
ture might lead to high yields of 2,4-dinitro derivatives for many
substrates. We continue to investigate such reactions.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.10.017.
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We thank the Saudi Government and Cardiff University for
financial support.