Regioselective preparation of 4-nitro-o-xylene using nitrogen dioxide/molecular oxygen over zeolite catalysts. remarkable enhancement of para-selectivity

Article abstract of DOI:10.1246/cl.140031

In the presence of molecular oxygen and zeolite H-β with Si/Al ₂ = 500, o-xylene reacted regioselectively with liquid nitrogen dioxide at 35 °C to yield mononitro-o-xylenes as the main product, where the 4-nitro-o-xylene isomer predominated up to 89% and the 4-nitro-/3-nitro-o- xylene isomer ratio improved to 7.8. The process is eco-friendly, less expensive, and the zeolite could be easily regenerated by a simple workup to afford results similar to those obtained with the fresh catalyst.

Full text of DOI:10.1246/cl.140031

Received: January 17, 2014 | Accepted: February 10, 2014 | Web Released: June 5, 2014
CL-140031
Regioselective Preparation of 4-Nitro-o-xylene Using Nitrogen Dioxide/Molecular Oxygen
over Zeolite Catalysts. Remarkable Enhancement of para-Selectivity
Hongtao Liu, Cheng Ji, Xiongzi Dong, Xinhua Peng,* and Chunjie Shi
School of Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. China
(
E-mail: xinhpeng@mail.njust.edu.cn)
In the presence of molecular oxygen and zeolite H-β with Si/
Al2 = 500, o-xylene reacted regioselectively with liquid nitrogen
dioxide at 35 °C to yield mononitro-o-xylenes as the main
product, where the 4-nitro-o-xylene isomer predominated up to
Table 1. Regioselectivity in the preparation of nitro-o-xylene using
nitrogen dioxide and molecular oxygen over various zeolite catalysts
a
CH3
CH3
CH3
CH3
nitration
CH3
+
CH3
89% and the 4-nitro-/3-nitro-o-xylene isomer ratio improved to
.8. The process is eco-friendly, less expensive, and the zeolite
7
NO2
could be easily regenerated by a simple workup to afford results
similar to those obtained with the fresh catalyst.
NO2
1
2
ortho
3
para
d
Nitrating
Isomer proportion /%
c
Entry
Catalystb Yield /%
3:2 ratio
4
-Nitro-o-xylene is an industrially important intermediate for
agent
3
2
the manufacture of pharmaceuticals, agrochemicals, fragrances,
and dyestuffs. Industrial preparation employs nitric and sulfuric
1
2
3
4
5
6
7
8
9
HNO3
HNO3
NO2­O2
NO2
NO2­O2 HZSM-5
NO2­O2 Y-type
®
H SO
®
®
67
81
64
61
63
70
76
75
72
75
77
45
52
66
60
66
67
76
66
73
75
78
55
48
34
40
34
33
24
34
27
25
22
0.8
1.1
1.9
1.5
1.9
2.0
3.1
2.9
2.7
3.0
3.5
1
2
4
acids to produce approximately equal amount of isomer mixtures
involving 3-nitro-o-xylene and 4-nitro-o-xylene. However, this
time-honored reaction suffers from several disadvantages such
as corrosiveness, potential danger of explosion, poor selectivity,
overnitration, oxidative degradation by-products, and waste
NO2­O2 HBEA30
NO2­O2 HBEA60
2
acids. Thus, it is urgent to develop an environmentally friendly,
NO ­O
HBEA130
2
2
commercially viable process for the selective preparation of nitro
compounds.
10
11
NO2­O2 HBEA260
NO2­O2 HBEA500
In recent years, some clean approaches are employed with the
aReactions conditions: o-xylene:acetic anhydride = 0.6:5 (v/v), o-xylene:
nitrogen dioxide = 1:5 ratio, catalyst (0.11 g, 20 wt % based on o-xylene) at
3,4
aim to use nitrating agents such as nitrogen dioxide, dinitrogen
tetraoxide,5 and dinitrogen pentaoxide.6 Various catalysts and
b
room temperature for 12 h under oxygen atmosphere. Zeolites were
c
7
,8
calcined at 550 °C for 2 h in air prior to use. Combined yield of 2 and 3
supports promote the nitrating activity and regioselectivity. The
d
Kyodai nitration9
­11
based on consumed 1 was given. Proportion of products was determined
using lower nitrogen oxides/ozone system
by GC with 4-nitrotoluene as internal standard.
has demonstrated an excellent conversion of various aromatic
compounds into the corresponding nitro derivatives.12 It is noted
that zeolites are very appealing catalysts to make positive
contributions to the desirable selectivity in the nitration processes.
In our sustained attempt, we achieved an efficient and highly
selective preparation of 4-nitro-o-xylene with nitrogen dioxide as
a nitrating agent over catalysts toward industrialization.
The current trends in the preparation of nitro compounds
emphasize minimizing the by-product formation and spent-acid
amount. Moreover, regioselective nitration of the desired
Various zeolites were screened in the nitration of o-xylene
with lower nitrogen dioxide. ZSM-5, Y-type zeolites could
slightly promote regioselectivity toward the 4-nitro isomer in the
4-nitro/3-nitro isomer ratio of 1.9­2.0 (Entries 5 and 6 in
Table 1) compared with those of mineral acids (Entries 1 and 2 in
Table 1). Sengupta and co-workers reported that the HY zeolite
showed para-positional orientation in the nitration of o-xylene
with fuming nitric and polyphosphoric acids. By using nitrogen
dioxide process, we found that zeolite HBEA appear to be
encouraging 4-nitro selectivity (Entries 7­11 in Table 1). Further
studies were extended to the BEA catalysts of various Si/Al2
ratios. It was found that HBEA500 of higher Si/Al2 ratio due to
the dealumination had excellent catalytic para-selectivity up to
the ratios 2.7­3.5 in moderate yields (Entries 7­11 in Table 1).
Moreover, in the hope of improving further reactivity,
the amount of nitrogen dioxide and o-xylene substrate was
discussed. The representative results are summarized in Table 2.
To optimize the reaction temperature for maximum mono-
nitration yield and selectivity, the samples were analyzed at
temperatures ranging from ¹5 to 35 °C (Table 3). At 35 °C, a
higher yield of nitro-o-xylene and a higher selectivity of 4-nitro-
o-xylene were observed. The results showed that the selectivity
for the 4-nitro isomer increased with increasing temperature.
Higher temperatures were preferable because of the faster
1
6
product involving lower nitrogen dioxide of atom economy
was improved by using various zeolite catalysts.1
3,14
Suzuki and
co-workers have reported that ZSM-5 zeolites appear to have
3
good catalytic selectivity toward monosubstituted benzenes,
while Smith et al.5
,15
have found that para-nitro isomer
predominated over the ortho-isomer over β-zeolites with
dinitrogen tetroxide nitration. Generally, o-xylene (1) is com-
mercially nitrated to afford a mixture of 3-nitro-o-xylene (2) and
4
-nitro-o-xylene (3). The traditional process using mixed acids
formed mixtures with the isomer ratio of approximately 0.8 and
.1 (Entries 1 and 2 in Table 1). However, we found that the
1
formation of 3 was highly favored when the substrate was
subjected to the combined action of nitrogen dioxide and
molecular oxygen in the presence of aluminosilicate zeolites,
leading to the substantive reversal of the 3-nitro-/4-nitro-o-
xylene isomer ratios. Some typical results are shown in Table 1.
Chem. Lett. 2014, 43, 817–819 | doi:10.1246/cl.140031
© 2014 The Chemical Society of Japan | 817

Table 2. Effect of nitrogen dioxide/o-xylene molar ratios in the
preparation of nitro-o-xylenea
d
Isomer proportion /%
Entry Nitrating agentb
Yield /%
c
3:2 ratio
3
2
1
2
3
4
5
NO2­O2 (1:2)
NO2­O2 (1:3)
NO2­O2 (1:4)
NO2­O2 (1:5)
NO2­O2 (1:6)
68
76
78
77
63
80
86
88
86
85
20
14
12
14
15
4.0
6.3
7.2
6.4
5.7
a
b
aAll reactions were carried out in acetic anhydride (5.0 mL) using substrate
(5.0 mmol), HBEA500 (0.53 g, 100 wt % based on o-xylene) at room
1
Figure 1. SEM images: (a) fresh HBEA500 and (b) HBEA500 used 4
cycles.
temperature for 12 h under oxygen atmosphere. Zeolite was calcined at
5
50 °C for 2 h in air prior to use. bThe ratio data in parentheses mean a
c
molar amount in nitrogen dioxide to o-xylene. Combined yield of 2 and 3
based on consumed 1 was given. Proportion of products was determined
d
(c)
by GC with 4-nitrotoluene as internal standard.
Table 3. Effect of temperature and amount of HBEA500 in the
preparation of nitro-o-xylenea
(b)
c
Amount of
HBEA500/g
Isomer proportion /%
b
Temp./°C
Yield /%
3:2 ratio
3
2
(
a)
RT
RT
RT
RT
RT
0.21
0.32
0.42
0.53
0.64
0.53
0.53
0.53
0.53
75
77
79
79
78
81
76
72
64
83
86
87
88
88
89
78
67
58
17
14
13
12
12
11
22
33
42
5.0
6.1
6.9
7.2
7.5
7.8
3.5
2.0
1.4
3
1
5
5
5
5
4
000
3000
Wavenumbers/cm-1
2000
1000
0
¹
Figure 2. FT-IR spectra of (a) fresh, (b) used, and (c) regenerated
aReaction conditions: o-xylene:acetic anhydride = 0.6:5 (v/v), o-xylene:
HBEA500.
nitrogen dioxide = 1:4 ratio, reaction time: 12 h, under oxygen atmosphere.
b
c
Combined yield of 2 and 3 based on consumed 1 was given. Proportion of
decreases in the ring dimensions can effectively hinder or block
the movement of particular molecular species through the zeolite
structure. The effective pore dimensions that control the access
to the interior of the zeolites are determined not only by the
geometric dimensions of the tetrahedra forming the pore
opening, but also by the presence or absence of ions in or near
the pore. The approximate molecular dimensions of the reactant
products was determined by GC with 4-nitrotoluene as internal standard.
diffusion of 4-nitro-o-xylene, and simply raising the temperature
to 35 °C in the sealed system could dramatically reduce the
required reaction time. However, if the temperature was too low,
then a decrease in the mononitration yield was observed. The
lower yield can be explained by formation of a by-product,
which was detected by GC in the reaction at ¹5 °C. This could
be ascribed to the low reaction rate. Hence, the optimal
temperature was 35 °C. The effect of the HBEA500 catalyst
quantity on the selectivity was also investigated (Figure 1a and
Table 3). Increasing the amount of the catalyst improved the
regioselectivity of 4-nitro-o-xylene and the yield of isomer
products. These could be due to the three-dimensional inter-
connecting channel system, in which the accessible amount of
the pores of the catalyst increased.
Nitration reaction is accompanied with the release of water
that will adversely affect the activity of the zeolite. Therefore, the
hydrophobic and hydrophilic character of zeolites must be taken
into account. This is controlled by the Si/Al2 ratio. Zeolites
containing large amounts of alumina show high hydrophilicity,
whereas zeolites containing large amounts of silica tend to be
more hydrophobic. Because of the need to remove water from
the catalyst in this system, hydrophobic zeolites are preferred.
3
o-xylene, 3.6 © 7.1 © 8.1 ¡ , result in very slow diffusion
into the medium-pore-type zeolites such as HZSM-5 (5.6 ©
2
16
5.4 ¡ ); both the selectivity and yield were unsatisfactory
(Entry 5, Table 1). This may be attributed to the medium-
pore-sized structure, which would place more restriction on the
transport of a substrate through the pores. The 4-nitro-o-xylene
to 3-nitro-o-xylene ratio was marginally higher (7.8) over
HBEA500 zeolite as compared to the other zeolites, probably
2
owing to the large pores (pore size: 7.6 © 6.4 ¡ ) of HBEA500
and its open framework structure leading to the faster diffusion
of 4-nitro-o-xylene than 3-nitro-o-xylene because of the differ-
ence in the kinetic diameter. Zeolites possess both the Lewis
1
7
and Brønsted acid sites, which are catalytically active sites.
¹
1
The IR spectrum at 3610 and 3740 cm are assigned to the
acidic bridged OH of Si(OH)Al and isolated silanol group
1
8,19
(Figure 2a).
Highly conspicuous decreases in the number of
acid sites have been observed with the increase in the Si/Al2
ratios (Table 4). Catalytic results showed that zeolites with high
Hydrophobic zeolites generally have greater Si/Al ratios, and
Si/Al ratios improved the yields of isomer products. Zeolites
2
2
hydrophobicity generally increases with increasing Si/Al2
ratios. Hence, HBEA500 zeolite catalyst with a high Si/Al2
ratio showed better activity with the release of water in the
reaction. In practice, it has been observed that very small
with lower Si/Al2 ratios, which have stronger acid strength, are
sufficiently strong to catalyze the substrate to produce by-side as
well, restricting the increase in the yield. Therefore, it could be
expected that HBEA500 with Si/Al2 = 500 is a milder catalyst,
818 | Chem. Lett. 2014, 43, 817–819 | doi:10.1246/cl.140031
© 2014 The Chemical Society of Japan

Table 4. Characteristics of zeolites with different Si/Al2 molar ratios
Table 6. aNitration of various substrates with nitrogen dioxide and
HBEA500
Acid amount
BET surface Pore volume
Catalyst
Si/Al2
Ho
¹1 a
2
¹1
3
¹1
/mmol g
area/m g
/cm g
b
c
d
Entry Substrate Conversion /% Yield /%
Distribution /%
HZSM-5
Y-type
HBEA30
HBEA60
HBEA130 127
HBEA260 256
HBEA500 503
30
4
31
58
0.58
0.37
0.66
0.35
0.15
0.09
0.05
Ho < ¹8.2
¹0.2 < Ho < +0.8
¹4.4 < Ho < ¹0.2
¹0.2 < Ho < +0.8
+3.3 < Ho < +4.8
+3.3 < Ho < +4.8
+3.3 < Ho < +4.8
399
782
615
599
594
587
583
0.20
0.23
0.27
0.27
0.26
0.28
0.29
CH3
CH3
CH3
CH3
NO2
1e
NO2
99.7
100
100
99.8
99.9
87.2
3.4
NO2
7
0.6
26.0
CH3
CH3
CH3
CH3
NO2
2
3
4
5
98.5
1
1.0
89.0
CH3
CH3
NO2
CH3
NO2
CH3
CH3
86.0
aAcid amount and acid strength (Ho) were determined by Hammett
indicator method.
100
CH3
O2N
93.2
NO2 64.6
NO2
35.4
O2N
Br
Br
91.9 Br
Br 91.0 Br
Br 9.0
aReaction conditions: substrate:nitrogen dioxide = 1:4 (mole ratio), reaction
time: 12 h, reaction temperature: 35 °C, acetic anhydride as the solvent,
b
b
under oxygen atmosphere. Amount of HBEA500 was 0.53 g. Determined
by GC. cCombined yield of products based on consumed substrate. 4-
d
Nitrotoluene was used as an internal standard. Proportion of products was
e
determined by GC with 4-nitrotoluene an internal standard. Nitrobenzene
was used as an internal standard.
a
biphenyl (Entry 4, Table 6) and 4,4¤-dibromobiphenyl (Entry 5,
Table 6) underwent regioselective nitration under the same
reaction conditions. In the nitration of biphenyl (Entry 4,
Table 6) and 4,4¤-dibromobiphenyl (Entry 5, Table 6), an
unprecedented conversion was shown by this nitration system.
In conclusion, 4-nitro-o-xylene was highly selectively
prepared in moderate yield with a nitrogen dioxide/molecular
oxygen system in the presence of zeolites. Furthermore, the
reaction is environmentally friendly because of atom economy
and easy workup of HBEA500. Moreover, the present process
could be promising for clean and highly selective preparation of
0
5
10 15 20 25 30 35 40 45 50 55 60
θ /°
2
Figure 3. XRD patterns of (a) fresh and (b) 4 cycles HBEA500.
Table 5. Efficiency of recycled HBEA500 in the preparation of nitro-o-
a
xylene
c
Recycle
number
Isomer proportion /%
b
Yield /%
3:2 ratio
3
2
Fresh
81
79
77
76
89
88
88
87
11
12
12
13
7.8
7.3
7.1
6.8
1
2
3
2
0
nitro aromatic compounds.
References and Notes
aReaction conditions: o-xylene:acetic anhydride = 0.6:5 (v/v), o-xylene:
nitrogen dioxide = 1:4 ratio, HBEA500 0.53 g (100 wt % based on o-
xylene), reaction temperature: 35 °C, reaction time: 12 h, under oxygen
1
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2
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2001, 31, 1123.
b
atmosphere. Combined yield of 2 and 3 based on consumed 1 was given.
c
Proportion of products was determined by GC with 4-nitrotoluene as
3
4
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6
7
8
9
1
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reaction and produced less polynitration and oxidation products.
HBEA500 was easily recovered from the reaction mixture by
simple filtration for reuse. The XRD patterns of fresh catalysts
and catalysts used for four cycles displayed two typical peaks
located at 2ª = 22.5 and 7.4, respectively (Figure 3), exhibiting
that the structure of HBEA500 is similar to that of the BEA
zeolite. SEM and FT-IR showed the same results (Figure 1b,
Figure 2b, and Figure 2c). Even after using HBEA500 four
times, only a slight change was observed in the selectivity and
yield with a minor loss of the original activity (Table 5).
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under similar conditions was also described. The results are
summarized in Table 6. The catalyst was very effective toward
activities aromatic compounds (Entries 1­3, Table 6) and the
corresponding nitro products were readily obtained in good to
excellent yields. It is notable that polyaromatic hydrocarbon
0
1
1
1
1
2
3
1
4
5
2
19, 231.
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2
0
Chem. Lett. 2014, 43, 817–819 | doi:10.1246/cl.140031
© 2014 The Chemical Society of Japan | 819