Regioselective nitration of m-xylene catalyzed by zeolite catalyst

Article abstract of DOI:10.1071/CH14551

Nitration with nitric acid and acetic anhydride via acetyl nitrate as nitrating species is efficient with the substrate m-xylene as solvent. Zeolite Hβ with an SiO2/Al2O3 ratio of 500 was found to be the most active of the catalysts tried both in yield and regioselectivity in the nitration of m-xylene. The molecular volume of the reactants was calculated with the Gaussian 09 program at the B3LYP/6-311+G(2d, p) level and compared with the size of the zeolite Hβ channels. A range of other substrates were subjected to the nitrating system under the same conditions as those optimized for m-xylene and excellent selectivity was obtained.

Full text of DOI:10.1071/CH14551

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH14551
Regioselective Nitration of m-Xylene Catalyzed
by Zeolite Catalyst
A B
,
A C
,
Xiongzi Dong
and Xinhua Peng
A
School of Chemical Engineering, Hefei University of Technology, Hefei 230009,
China.
B
School of Chemistry and Chemical Engineering, Hefei Normal University, Hefei
230601, China.
C
Corresponding author. Email: orgpeng@163.com
Nitration with nitric acid and acetic anhydride via acetyl nitrate as nitrating species is efficient with the substrate m-xylene
as solvent. Zeolite Hb with an SiO2/Al2O3 ratio of 500 was found to be the most active of the catalysts tried both in yield
and regioselectivity in the nitration of m-xylene. The molecular volume of the reactants was calculated with the Gaussian
09 program at the B3LYP/6–311þG(2d, p) level and compared with the size of the zeolite Hb channels. A range of other
substrates were subjected to the nitrating system under the same conditions as those optimized for m-xylene and excellent
selectivity was obtained.
Manuscript received: 9 September 2014.
Manuscript accepted: 1 December 2014.
Published online: 3 February 2015.
Introduction
was utilized to determine product isomer compositions. The
temperature program used was as follows: heating from 80 to
1208C at a rate of 108C minꢁ1; hold 1 min, then heating at
208C minꢁ1 to 1708C. Product identities were proved by direct
comparison with authentic samples. All other chemicals were of
analytical grade and were used without any further purification.
Nitro derivatives of m-xylene are important starting materials
for a variety of fine chemical products including pharmaceu-
ticals and agrochemical drugs. Typical nitration requires a
mixture of concentrated nitric acid with sulfuric acid, leading to
excessive acid waste streams, which leads to serious environ-
mental issues. In order to address these problems, many efforts
have been made and some progress has been achieved. Earlier
studies of the nitration of m-xylene catalyzed by boron tri-
fluoride with silver nitrate in acetonitrile solution were reported
by Olah et al.[1] Lanthanide(III) triflates[2] and lanthanide(III)
nosylates[3] were used as new nitration catalysts for the nitration
of m-xylene. Kantam et al.[4] reported the nitration of m-xylene
catalyzed by b-zeolite with nitric acid and acetic anhydride.
Bharadwaj nitrated m-xylene catalyzed by Al(H2PO4)[35] and
acid phosphate-impregnated titania.[6] Recently, Wang and
coworkers[7] reported the regioselective nitration of m-xylene
under phase-transfer catalysis. The results are listed in Table 1
for comparison.
In our previous work,[8–11] a series of aromatic compounds
were nitrated with good regioselectivity catalyzed by zeolites
and hydroxyl-aluminium cross-linked bentonites. To further
improve the conversion of m-xylene and the ratio of 2,4-
dimethylnitrobenzene to 2,6-dimethylnitrobenzene, the nitration
of m-xylene was investigated and excellent conversion and yield
were obtained. The detailed results are reported in this paper.
General Procedure
A typical nitration of m-xylene over zeolite with nitric acid and
acetic anhydride was carried out as follows. Zeolite (0.1 g,
calcinated at 5508C for 2 h before use), m-xylene (5 mL), and
acetic anhydride (0.613 g, 6 mmol) were mixed under stirring at
08C. Then nitric acid (0.199 g, 95 %, 3 mmol) was added drop-
wise. The mixture was stirred for 30 min and then allowed to
warm to room temperature and allowed to stand for 24 h. The
catalyst was removed by filtration and washed with dichloro-
methane (10 mL). The organic layers were washed with water
(10 mL ꢀ 2), NaHCO3 solution (10 %, 10 mL), and water
(10 mL ꢀ 2), and dried with MgSO4. A known amount of
p-nitrotoluene was added as internal standard. Then the solution
was analysed by gas chromatography to get the total yield and
ratio of 2,4-dimethylnitrobenzene to 2,6-dimethylnitrobenzene.
1
The products were characterized by H NMR.: 2,4-dimethyl-
nitrobenzene: dH (600 MHz, CDCl3) 2.39 (s, 3H), 2.58 (s, 3H),
7.12 (d, J 8.0, 2H), 7.91 (d, J 8.2, 1H); 2,6-dimethyl-
nitrobenzene: dH (600 MHz, CDCl3) 2.31 (s, 6H), 7.12 (d, J 7.6,
2H), 7.26 (t, J 7.6, 1H).
Experimental
Materials
Results and Discussion
Solid catalysts were purchased and calcinated at 5508C for 2 h
before use. Gas chromatographic analysis was carried out with a
Shimadzu 2014-C (Wondacap-1; film thickness Df ¼ 1.5 mm ꢀ
0.53 mm inner diameter ꢀ 30 m) and a flame ionization detector
To improve the conversion of m-xylene and the regioselective
formation of 2,4-dimethylnitrobenzene, different conditions
were investigated. The detailed results are listed in Table 2.
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

B
X. Dong and X. Peng
Table 1. Nitration of m-xylene with different catalysts in the literature
CH3
CH3
CH3
CH3
NO2
HNO3
ϩ
ϩ
O2N
CH3
CH3
CH3
CH3
NO2
m-xylene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
3,5-dimethylnitrobenzene
Entry Catalyst
Conversion Yield
Proportion [%]
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
[%]
[%]
2,4-
2,6-
dimethylnitrobenzene
dimethylnitrobenzene
1
2
3
4
5
6
Boron trifluoride[1]
–
.95
.98
69
–
–
87
85
86
86
90
90
13
15
14
14
–
6.69
5.67
6.14
6.14
–
Lanthanide(III) triflates[2]
Lanthanide(III) nosylates[3]
b zeolite[4]
Al(H2PO4)[35]
–
–
99
90
90
Acid phosphate-impregnated
titania[6]
Sodium dodecyl sulfonate[7]
–
–
–
7
97.8
–
90.4
9.2
9.83
Table 2. Nitration of m-xylene under different conditions
Catalyst Conversion Yield Proportion [%]
Entry Solvent
Acetic
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
anhydride
[%]
[%]
2,4-
2,6-
dimethylnitrobenzene dimethylnitrobenzene
1A
2A
3A
4A
5C
6
Dichloromethane None
Dichloromethane None
None
HbB
None
HbB
HbB
HbB
18.5
48.5
98.3
100.0
100.0
–
12.4
24.8
98.1
98.9
83.7
88.4
84.3
89.8
90.2
93.5
16.3
11.6
15.7
10.2
9.8
5.13
7.58
5.36
8.78
9.25
14.4
Dichloromethane 6 mmol
Dichloromethane 6 mmol
Acetic anhydride 5 mL
100
100
m-xylene
6 mmol
6.5
ADichloromethane was used as solvent and m-xylene (0.319 g, 3 mmol) was added.
BRatio of SiO2 to Al2O3 was 500.
CAcetic anhydride was used as solvent and m-xylene (0.319 g, 3 mmol) was added.
Under catalysis with Hb, the conversion, yield, and regio-
selectivity were all improved (Table 2, entry 2) owing to the
participation of Brønsted acid centres and the three-dimensional
networks of the catalyst.
The conversion and yield were all greatly increased (Table 2,
entry 3) when acetic anhydride was added because it can convert
all the nitric acid into acetyl nitrate and all the water into acetic
acid. Higher conversion with acetic anhydride is attributed to the
in situ formation of acetyl nitrate as the active nitrating species
in the presence of nitric acid.[12–16]
As expected, the conversion, yield, and regioselectivity were
all greatly improved with the combination of Hb and acetic
anhydride (Table 2, entry 4). Acetic anhydride and m-xylene as
solvents were also investigated. The best ratio of 2,4- to 2,6-
reached up to 14.4 when m-xylene was used as solvent. There-
fore, the substrate m-xylene used as solvent was selected for
more detailed study. Unreacted substrate can be recovered by
distillation. The isomer 3,5-dimethylnitrobenzene was not
found under the reaction conditions selected.
size, channel structure, acidity, and SiO2/Al2O3 ratio were all
assessed. Representative results are summarized in Table 3.
Zeolites are potentially attractive catalysts in the nitration
reaction because of their acidity and shape selectivity and as
they are crystalline aluminosilicates with uniform pore dimen-
sions. Compared with other solid acids, there are both Lewis and
Brønsted acidic sites throughout the inner and outer surfaces of
zeolites. As shown in Table 3, in the presence of all of the
zeolites, the reactions gave higher yields than in the absence of
any catalyst. The selectivity with zeolite HZSM-5 was as low as
the reaction with no catalyst, for the ZSM-5 structure consists
of a three-dimensional channel system with medium channel
intersections that are not large enough to accommodate the
reacting species and little of the reaction occurred inside the
pores as it was occurring at the external surface.
As can be seen from Table 3, zeolite Hb has outstanding
catalytic capacity in the regioselective nitration of m-xylene.
Zeolite Hb has three-dimensional channels and large pore sizes
that assist shape selectivity in the nitration of m-xylene. Zeolite
Hb has been also used to catalyze the nitration of phenol,[17]
benzonitrile,[13] o-xylene,[4,6,18] halogenobenzenes,[19–22] and
toluene[22,23] with enhanced selectivity.
We then carried out reactions in which various zeolites were
used as catalysts in an attempt to determine which zeolite would
be the most effective for m-xylene nitration. The effects of pore

Regioselective Nitration of m-Xylene
C
Table 3. Nitration of m-xylene over various zeolite catalysts
Yield [%] Proportion [%]
Entry
Catalyst
2,4-dimethylnitrobenzene/
(SiO2/Al2O3)
2,6-dimethylnitrobenzene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
1
2
3
4
5
6
7
8
9
10
None
88.5
90.3
84.9
84.9
85.3
85.5
85.3
92.0
91.8
93.5
93.5
85.3
15.1
15.1
14.7
14.5
14.7
8.0
5.62
5.62
5.80
5.90
5.80
11.5
11.2
14.4
14.4
5.80
HZSM-5 (25)
HZSM-5 (40)
HZSM-5 (200)
HZSM-5 (360)
Hb (25)
97.4
99.1
99.8
101.0
101.2
98.8
Hb (150)
Hb (280)
Hb (500)
HY
8.2
6.5
100.0
101.5
6.5
14.7
Table 4. Comparison of surface areas and pore volumes for HZSM-5
and Hb
Entry
Catalyst
Surface area
(BET) [m2 gꢁ1
Total pore volume
[cm3 gꢁ1
]
]
1
2
HZSM-5
386
528
0.20
0.32
Hb
In order to find out the reason of the shape selectivity
of zeolite Hb, the molecular volume of the reactants was
calculated with the Gaussian 09 program[24] at the B3LYP/
6–311þG(2d, p)[25,26] level, and the results are listed in Fig. 1.
The pore size of zeolite Hb is 0.64–0.76 nm. The distance
between 7H and 9H in m-xylene is 0.493 nm. Considering the
diameter of the H atom, the minimum diameter of m-xylene is
0.599 nm. With the same calculation process, considering
the volume of the H atom and O atom, the minimum diameter
of 2,4-dimethylnitrobenzene, 2,6-dimethylnitrobenzene, and
3,5-dimethylnitrobenzene is 0.653, 0.672, and 0.724 nm
respectively.
The minimum diameter of 2,6-dimethylnitrobenzene at
0.672 nm is longer than 2,4-dimethylnitrobenzene at 0.653 nm.
This means 2,6-dimethylnitrobenzene is more restricted than
2,4-dimethylnitrobenzene in zeolite Hb, and hence a higher
ratio of 2,4-/2,6- was reached. The largest molecule, 3,5-
dimethylnitrobenzene, cannot be formed in the channel owing
to the dimensions of the channels of zeolite Hb.
The SiO2/Al2O3 ratio of the zeolite used influenced the
selectivity of the reactions. The zeolite with the highest SiO2/
Al2O3 ratio gave a higher 2,4-selectivity than that with a lower
ratio. This is in accord with the results observed by Smith et al. in
the nitration of halogenobenzenes.[20] This may be due to dealu-
mination, which would modify the distribution of the size and
shape of the pores in the zeolite.
Zeolite Y, which has the largest cavities, allowed a rapid
reaction, with limited improvement in 2,4-selectivity.
The structure affects the Brunauer-Emmett-Teller (BET)
surface area and pore volume of the zeolite, as indicated in
Table 4. Table 4 shows that Hb had a higher surface area and
total pore volume than HZSM-5, which favoured the nitration
process.
None of the zeolites tried gave better regioselectivity than the
one originally studied and further studies therefore concentrated
on the use of zeolite Hb (SiO2/Al2O3 ¼ 500).
Fig. 1. The reactant molecule entering a zeolite Hb channel with an
opening of 0.64–0.76 nm. The reactant molecules are m-xylene, 2,4-
dimethylnitrobenzene, 2,6-dimethylnitrobenzene and 3,5-dimethylnitro-
benzene, respectively.

D
X. Dong and X. Peng
Table 5. Nitration of m-xylene over various quantities of zeolite Hb
Yield [%] Proportion [%]
Entry
Quantities of
catalyst [g]
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
1
None
0.01
0.03
0.05
0.07
0.10
0.12
0.15
0.20
0.30
0.40
0.50
1.00
1.50
93.8
98.6
99.0
97.0
90.9
100.0
99.3
100.1
97.2
98.4
95.9
97.0
95.9
74.5
85.6
89.0
91.2
91.9
92.7
93.5
93.7
93.9
94.1
94.1
94.3
94.2
94.5
95.0
14.4
11.0
8.8
8.1
7.3
6.5
6.3
6.1
5.9
5.9
5.7
5.8
5.5
5.0
5.94
8.09
10.4
11.3
12.7
14.4
14.9
15.4
15.9
15.9
16.5
16.2
17.2
19.0
2
3
4
5
6
7
8
9
10
11
12
13
14
Table 6. Nitration of m-xylene over various quantities of acetic anhydride
Entry
Quantity of acetic anhydride [mL]
Yield [%]
Proportion [%]
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
1
None
24.3
22.3
41.5
61.3
95.7
100.0
99.9
95.3
92.8
93.1
92.3
88.7
90.1
91.5
91.8
91.5
93.4
93.5
93.4
93.3
92.3
92.2
91.7
92.1
9.9
8.5
8.2
8.5
6.6
6.5
6.6
6.7
7.7
7.8
8.3
7.9
9.11
10.7
11.2
10.8
14.2
14.4
14.2
13.9
12.0
11.8
11.0
11.7
2
0.09 (1 mmol)
0.19 (2 mmol)
0.28 (3 mmol)
0.43 (4.5 mmol)
0.57 (6 mmol)
0.85 (9 mmol)
1.13 (12 mmol)
1.42 (15 mmol)
1.70 (18 mmol)
1.99 (21 mmol)
2.27 (24 mmol)
3
4
5
6
7
8
9
10
11
12
Table 7. Nitration of m-xylene for various reaction times
Entry
Reaction time [h]
Yield [%]
Proportion [%]
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
1
1
2
71.1
84.8
92.5
93.1
92.6
93.2
93.5
93.4
93.5
93.4
93.5
93.5
93.5
7.5
6.9
7.4
6.8
6.5
6.6
6.5
6.6
6.5
6.5
6.5
12.3
13.6
12.5
13.7
14.4
14.2
14.4
14.2
14.4
14.4
14.4
2
3
3
87.0
4
4
83.9
5
6
99.3
6
8
99.7
7
10
12
16
20
24
100.9
99.6
8
9
100.9
100.2
100.0
10
11
Table 8. Recycling of zeolite Hb catalyst in nitration of m-xylene
Cycle
Yield [%]
Proportion [%]
2,4-dimethylnitrobenzene/
2,6-dimethylnitrobenzene
2,4-dimethylnitrobenzene
2,6-dimethylnitrobenzene
Fresh
First
99.3
96.9
94.4
93.5
93.2
93.1
6.5
6.8
6.9
14.4
13.7
13.5
Second

Regioselective Nitration of m-Xylene
E
In order to test the effect of the amount of zeolite Hb,
reactions were carried out with several different quantities of
zeolite Hb. The results are recorded in Table 5.
It is clear from the results in Table 5 that 2,4-selectivity
increased with the quantity of catalyst. This is consistent with
the toluene nitration reaction.[14] With the amount of catalyst
increasing, the reactant molecules have more chance to diffuse
into the pores of zeolite Hb, on account of the higher interior
surface area and also the more accessible number of Brønsted
acid sites of the catalyst. Meanwhile, a greater proportion of the
acetyl nitrate can be located inside the pores. When the amount
of catalyst increased from 0.10 to 1.50 g, 2,4-selectivity
increased only from 93.5 to 95.0 %, and the total yield of
dimethylnitrobenzene decreased from 100.0 to 74.5 %. From
the point of view of optimal reaction conditions, 0.10 g zeolite
Hb was used in subsequent experiments.
Acetic anhydride reacts with HNO3 to form the acetyl nitrate
(AcONO2) intermediate as nitrating species as follows:
ðCH3COÞ2O þ HNO3 ! CH3COONO2 þ CH3COOH
(b)
(a)
The water released in the nitration reaction will affect the
acidity of the zeolite. It is necessary to scavenge the water
formed during the reaction out of the reaction zone to facilitate
regeneration of active acid sites on the catalyst.[22] With acetic
anhydride present, water molecules released during the reaction
react with acetic anhydride to give acetic acid and thereby the
acidity of the zeolite is retained. The reaction is shown as
follows:
ðCH3COÞ2O þ H2O ! 2CH3COOH
0
5
10 15 20 25 30 35 40 45 50 55 60
2θ [Њ]
To convert all the nitric acid into acetyl nitrate and all
the water into acetic acid, 6 mmol acetic anhydride was used.
To test whether this is the optimum quantity of acetic anhydride,
Fig. 2. Powder X-ray diffraction (XRD) patterns of Hb: (a) fresh; (b) after
three uses.
Table 9. Nitration of various substrates under the optimized conditions
Entry
Substrate
Yield [%]
Proportion of products [%]
NO2
1
87.7
100
CH3
CH3
CH3
CH3
NO2
72.6
2
3
4
90.7
97.3
86.2
2.6
24.8
NO2
NO2
CH3
CH3
73.4
CH3
CH3
CH3
CH3
26.6
NO2
NO2
CH3
CH3
NO2
100
CH3
CH3

F
X. Dong and X. Peng
Table 10. NBO charge of all C atoms in benzene ring
The used zeolite Hb was easily recovered by simple filtration
and regenerated by calcination with little loss of activity
(Table 8).
X-ray diffraction (XRD) patterns (Fig. 2) show that there is
no obvious difference between the fresh Hb and that used for
the third time.
Substrate
Net charge of every carbon atom in benzene ring
C(1) C(2) C(3) C(4) C(5) C(6)
1
6
5
2
3
A range of other substrates were subjected to the nitrating
system under the same conditions as those optimized for
m-xylene. The results are summarized in Table 9.
Using the Gaussian 09 program, the natural bond orbital
(NBO) net charge of each C atom on the benzene ring of the
selected substrates wascalculatedatthe B3LYP/6–311þG(2d, p)
level and the values are listed in Table 10.
As the nitration reaction is an electrophilic substitution
process, the reagent preferentially attacks the low-charge-
density position. In addition, steric effects can also affect the
reaction selectivity.
ꢁ0.204 ꢁ0.204 ꢁ0.204 ꢁ0.204 ꢁ0.204 ꢁ0.204
4
CH3
1
6
5
2
3
ꢁ0.035 ꢁ0.206 ꢁ0.196 ꢁ0.214 ꢁ0.196 ꢁ0.205
4
CH3
1
As can be seen from Tables 9 and 10, this method offers
high yield and outstanding regioselectivities for the selected
substrates owing to the combined effects of steric effect, charge
distribution, and shape selectivity of zeolite Hb.
For the nitration of toluene, m-xylene, and o-xylene, the
effects of the three factors are consistent, which leads to
excellent selectivity in the formation of 4-nitrotoluene
(72.6 %), 2,4-dimethylnitrobenzene (93.5 %), and 3,4-dimethyl-
nitrobenzene (73.4 %) respectively.
CH3
6
5
2
3
ꢁ0.033 ꢁ0.033 ꢁ0.199 ꢁ0.205 ꢁ0.205 ꢁ0.199
ꢁ0.044 ꢁ0.198 ꢁ0.198 ꢁ0.044 ꢁ0.195 ꢁ0.195
ꢁ0.027 ꢁ0.207 ꢁ0.027 ꢁ0.216 ꢁ0.187 ꢁ0.213
4
CH3
1
6
5
2
3
4
Conclusion
Zeolite Hb is an effective catalyst for the nitration of m-xylene
in a nitric acid/acid anhydride system. The catalyzed reaction
produces only 2,4-dimethylnitrobenzene and 2,6-dimethyl-
nitrobenzene in quantitative yield, with no 3,5-dimethyl-
nitrobenzene produced under the conditions tested. Zeolite Hb
with an SiO2/Al2O3 ratio of 500 is the most selective of the
catalysts tried and gives a high proportion of 2,4-dimethyl-
nitrobenzene. Acetic anhydride can convert all the nitric acid
into acetyl nitrate and all the water into acetic acid.
CH3
CH3
1
6
5
2
3
CH3
4
A range of other substrates were subjected to the nitrating
system under the same conditions as those optimized for
m-xylene and excellent selectivity was obtained.
This method has several practical advantages that should
make it highly attractive for commercial application. The excess
substrates can be recovered by distillation and the catalyst can be
easily recovered and regenerated.
a series of reactions were conducted in which the ratio of the
anhydride to nitric acid was varied. The results are listed in
Table 6.
As the results in Table 6 show, with 6 mmol acetic anhydride,
the yield and 2,4-selectivity were excellent. Below that amount,
the mechanism presumably switches to that of a nitric acid
nitration, consequently leading to a slower reaction and a lower
degree of 2,4-selectivity.[14] With a greater increase in the
amount of acetic anhydride, the yield and 2,4-selectivity were
both decreased.
References
[1] G. A. Olah, A. P. Fung, S. C. Narang, J. A. Olah, J. Org. Chem. 1981,
46, 3533. doi:10.1021/JO00330A032
[2] F. J. Waller, A. G. M. Barrett, D. C. Braddock, D. Ramprasad, Chem.
Commun. 1997, 613. doi:10.1039/A700546F
Thus far, all experiments were allowed to react for 24 h. It
was of interest to know if such an amount of time was actually
necessary. To optimize the reaction time for maximum yield and
2,4-selectivity, we further investigated the effect of reaction
time in the presence of 0.10 g catalyst. The results are showed in
Table 7.
The results (Table 7) show that the yield and 2,4-selectivity
increased with time. The best yield, near 100 %, and the ratio of
2,4-/2,6- of 14.4 were obtained at 6 h. With time increasing, the
yield and regioselectivity did not change, which suggested that
no reaction occurred after 6 h. Therefore, the total time for the
nitration reaction was optimized to 6 h.
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