Regioselective nitration of naphthalene over HZSM-5-supported phosphotungstic acid

Article abstract of DOI:10.1007/s11164-014-1549-1

HZSM-5-supported phosphotungstic acid (PW) has been prepared and characterized. The results indicate that 5 wt% PW/HZSM-5 calcined at 773 K has the largest surface area and high dispersion of PW. It was confirmed that Keggin units are present inside the catalyst and that the proton of PW and the silanol of HZSM-5 interact via bridging structure. The catalytic properties of PW/HZSM-5 in the regioselective nitration of naphthalene to dinitronaphthalene were investigated. The best results, i.e. 48.5 % yield of 1,5-dinitronaphthalene, 49.2 % yield of 1,8-dinitronaphthalene, and the highest ratio of 1,5-dinitronaphthalene to 1,8-dinitronaphthalene, were obtained by use of 5 wt% PW/HZSM-5.

Full text of DOI:10.1007/s11164-014-1549-1

Res Chem Intermed
DOI 10.1007/s11164-014-1549-1
Regioselective nitration of naphthalene over HZSM-5-
supported phosphotungstic acid
•
Pingle Liu Wei Xiong Xiaofei Wang Kun Huang
•
•
•
•
Fang Hao Liangjie Wang He’an Luo
•
Received: 3 November 2013 / Accepted: 25 January 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract HZSM-5-supported phosphotungstic acid (PW) has been prepared and
characterized. The results indicate that 5 wt% PW/HZSM-5 calcined at 773 K has
the largest surface area and high dispersion of PW. It was confirmed that Keggin
units are present inside the catalyst and that the proton of PW and the silanol of
HZSM-5 interact via bridging structure. The catalytic properties of PW/HZSM-5 in
the regioselective nitration of naphthalene to dinitronaphthalene were investigated.
The best results, i.e. 48.5 % yield of 1,5-dinitronaphthalene, 49.2 % yield of 1,8-
dinitronaphthalene, and the highest ratio of 1,5-dinitronaphthalene to 1,8-dinitro-
naphthalene, were obtained by use of 5 wt% PW/HZSM-5.
Keywords Regioselective nitration ꢀ Naphthalene ꢀ Dinitronaphthalene ꢀ
HZSM-5-supported phosphotungstic acid catalyst
Introduction
Nitration of aromatic compounds is a very important industrial process for
production of intermediates which are widely used in the manufacture of
pharmaceutical products, dyes, polyurethane pesticides, and perfumes. Naphthalene
nitration can produce 1-nitronaphthalene, 1,5-dinitronaphthalene, and 1,8-dinitro-
naphthalene. The product from reduction of 1,8-dinitronaphthalene is an important
intermediate in the production of dyes, and that from reduction of 1,5-dinitronaph-
thalene can be used to produce 1,5-naphthalene diisocyanate (NDI), an important
raw material for production of advanced polyurethane with perfect performance
P. Liu (&) ꢀ W. Xiong ꢀ X. Wang ꢀ K. Huang ꢀ F. Hao (&) ꢀ L. Wang ꢀ H. Luo
College of Chemical Engineering, Xiangtan University, Xiangtan 411105, China
e-mail: liupingle@xtu.edu.cn
F. Hao
e-mail: haofang.happy@163.com
123

P. Liu et al.
(high hardness, excellent elasticity, heat resistance, good dynamic capability, and
wear resistance) [1]. Polyurethane elastomers based on 1,5-naphthalene diisocya-
nate have attracted much research interest.
Nitration of naphthalene is mainly achieved by classical processes involving
large quantities of nitric acid and sulfuric acid; these have many disadvantages,
including generation of large quantities of corrosive waste, low regioselectivity, and
inefficient atom economy. The yield of the most desired product, 1,5-dinitronaph-
thalene, is low, approximately 25–30 %, whereas yields of 1,8-dinitronaphthalene
and 1-nitronaphthalene are approximately 60 and 10 % [2]. An environmentally
benign and economical method for preparation of dinitronaphthalene and improving
the yield of 1,5-dinitronaphthalene is, therefore, urgently required.
Zolfigol et al. designed 3-methyl-1-sulfonic acid imidazolium nitrate
([Msim]NO3), a new, organic, acidic ionic liquid and nitrating agent, which
enables highly efficient nitration of a variety of aromatic compounds without the
need for a co-catalyst [3]. Methods for nitration of naphthalene or 1-nitronaphtha-
lene to dinitronaphthalene in nitric acid or in a nitrating mixture of nitric acid and
sulfuric acid over a well-defined catalyst have been patented. The yield of 1,5-
dinitronaphthalene from nitration of 1-nitronaphthalene in nitric acid and nitro-
methane catalyzed by zeolite is 30.6–57.3 %. Recently, Wang et al. [4] reported that
78 % yield of dinitronaphthalene can be achieved by nitration of 1-nitronaphthalene
over HZSM-5 with 95 % fuming nitric acid as nitration reagent. However, the
1-nitronaphthalene must be obtained by nitration of naphthalene. In the nitration of
naphthalene in nitric acid and 1,2-dichloroethane over zeolite catalyst, yields of 1,5-
dinitronaphthalene, 1,8-dinitronaphthalene, and 1-nitronaphthalene are only 37.9,
46.3, and 1.0 %, respectively. The main product in the absence of catalyst is
1-nitronaphthalene (yield 62.2 %) [5].
Researchers have devoted much attention to regioselective nitration of naphtha-
lene, phenol, toluene, and other aromatic compounds over solid acid and ionic liquid
catalysts [5–9]. However, selectivity for 1,5-dinitronaphthalene in the catalytic
nitration of naphthalene is only approximately 30–40 %; more 1,8-dinitronaphtha-
lene is formed [10, 11].
Zeolites are well known for their shape-selective behavior and phosphotungstic
acid is widely used as a catalyst because of its strong acidity, high oxidizing ability,
and thermal stability [12–17]. In this paper, we report the nitration of naphthalene
with nitric acid over a zeolite-supported phosphotungstic acid catalyst without use
of sulfuric acid. This is a mild, environmentally benign, and economical method for
preparation of dinitronaphthalene.
Experimental
Catalyst preparation
NaZSM-5 zeolite was ion-exchanged in 1.0 M NH4Cl aqueous solution at 298 K for
12 h, washed, dried at 293 K for 10 h, and calcined at 723 K for 12 h in air to
obtain HZSM-5 zeolite.
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Regioselective nitration of naphthalene over HZSM-5
HZSM-5-supported PW catalysts were prepared by impregnation. Phosphotung-
stic acid solutions were prepared by dissolving appropriate amounts of the acid in
30 mL of a 1:1 (v/v) mixture of ethanol and water. The HZSM-5 zeolite was then
impregnated in this solution, with stirring, for 4 h. The mixture was then dried at
393 K for 12 h and calcined at different temperatures in the range 623–823 K for
4 h.
Catalyst characterization
Specific surface area, pore volume, and pore size distribution of the samples were
determined by use of a NOVA-2200e automated gas sorption system to study
nitrogen adsorption–desorption. X-ray diffraction (XRD) data in the range
2h = 10–50° were acquired by use of a Rigaku (Japan) D/Max 2550 VB? X-ray
diffractometer; the scanning rate was 1° min-1. Fourier-transform infrared (FT-IR)
spectra in the wavenumber range 400–4,000 cm-1 were recorded on a Nicolet 380
spectrometer. Ultraviolet–visible spectra in the range 100–500 nm were recorded on
a UV-2550 spectrometer.
Typical reaction procedure
Nitration of naphthalene was performed in a three-necked, round-bottomed flask
with a reflux condenser. Naphthalene (10 mmol) was placed in a flask containing
0.3 g catalyst, and excess nitric acid (65 wt%) was added. The reaction mixture was
stirred at 338 K for 3 h. When reaction was complete, unreacted nitric acid was
recovered by filtration. The remaining solid mixture was dissolved in acetonitrile
and the catalyst was separated by filtration. The reaction products were analyzed by
liquid chromatography with an LC-20AT solvent-delivery unit, 55:35:10 CH3OH–
C2H3N–H2O as mobile phase, and detection at 254 nm (SPD-20A).
The concentration of the specimen and the selectivity for 1,5-dinitronaphthalene
were calculated by use of the formulas (Eqs. 1, 2):
Ws ¼ As ꢁ Wi=Ai;
ð1Þ
where Ws and WI are the concentrations of the specimen and the standard sample,
respectively, and As and AI are the peak areas for the specimen and the standard
sample, respectively, and
the amount ðmolÞ of dinitronaphthalene
Selectivity ð%Þ ¼
ꢁ 100 % ð2Þ
the amount (mol) of reacted naphthalene
Results and discussion
Characterization of the catalysts
The textures of catalysts with different PW loading calcined at 773 K are
summarized in Table 1. The surface area of the HZSM-5 was 352.61 m2 g-1; this
123

P. Liu et al.
increased when the HZSM-5 was used as a support for PW, possibly because
formation of a secondary pore structure in the alumina framework. Surface area was
largest (443.17 m2 g-1) for 5 wt% PW/HZSM-5 and decreased with increased PW
loading, possibly because of coverage of the pore structure by large amounts of PW.
Hence, pore volume also decreases with increased PW loading.
XRD patterns obtained from catalysts with different PW loading are shown in
Fig. 1 (5 wt% PW/HZSM-5 (a), 10 wt% PW/HZSM-5 (b), and 30 wt% PW/HZSM-
5 Ó)). XRD patterns obtained from 5 wt% PW/HZSM-5 calcined at different
temperatures are shown in Fig. 2 (623 K (a), 673 K (b), 723 K Ó), 773 K (d), and
823 K (e)). The characteristic diffraction peaks at 2h = 22°–25° for all the samples
are indicative of the presence of the HZSM-5 structure, i.e. this structure was not
destroyed during catalyst preparation. For the 5 and 10 wt% PW/HZSM-5 samples,
the characteristic peaks of PW crystals are barely visible, indicating that the PW is
highly dispersed throughout the large-surface-area porous framework of HZSM-5.
However, when PW loading reaches 30 wt%, the characteristic diffraction peaks of
solid-state PW appear at 2h = 10.5°, 20°, and 35° [18, 19], indicating that Keggin
units are present inside the pores and on the walls outside the pores and interact with
the framework for 30 wt% loading.
Table 1 Textural properties of catalysts with different PW loading
Sample
Surface area
(m2 g-1
Pore diameter
(nm)
Pore volume
(cm3 g-1
)
)
HZSM-5
352.61
443.17
397.64
394.42
2.11
1.94
1.80
1.70
0.171
0.078
0.066
0.049
5 wt% PW/HZSM-5
10 wt% PW/HZSM-5
30 wt% PW/HZSM-5
Fig. 1 XRD patterns of PW/HZSM-5 with different PW loading: a 5 wt% PW/HZSM-5; b 10 wt% PW/
HZSM-5; c 30 wt% PW/HZSM-5
123

Regioselective nitration of naphthalene over HZSM-5
Fig. 2 XRD patterns of 5 wt% PW/HZSM-5 calcined at different temperatures: a 623 K; b 673 K;
c 723 K; d 773 K; e 823 K
Fig. 3 UV–visible DRS of
HZSM-5 and PW/HZSM-5 with
different PW loading: a HZSM-
5; b 5 wt% PW/HZSM-5;
c 10 wt% PW/HZSM-5;
d 30 wt% PW/HZSM-5
Figure 3 shows UV–visible diffuse reflectance spectra (DRS) of HZSM-5 and of
HZSM-5-supported PW with different loading (5 wt% (a), 10 wt% (b), 30 wt% Ó)).
The spectrum of HZSM-5 (Fig. 3a) contains absorption bands near 200 and 250 nm.
The spectra of the HZSM-5-supported PW samples contain a peak at 260 nm,
indicating the presence of undegraded H3PW12O40 species on the HZSM-5 support,
which means the Keggin phase remains intact after calcination of PW-loaded
HZSM-5 at 773 K [20, 21].
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P. Liu et al.
FT-IR spectra of PW/HZSM-5 with different loading are shown in Fig. 4 (5 wt%
(a), 10 wt% (b), 30 wt% Ó)). All the spectra contain bands at 530 and 802 cm-1
.
The absorption at 530 cm-1 is characteristic of vibration of the heteropolyanion
[22] and the band at 802 cm-1 is assigned to W–O–W vibration of edge-shared
octahedra, indicating the presence of Keggin-type phosphotungstic acid [21, 23].
The absorption peaks at 550 and 450 cm-1 are assigned to the framework vibration
of HZSM-5. The absorption bands at 3,677 and 1,629 cm-1, are assigned to –OH
stretching vibration and H–O–H deformation vibration, respectively, of molecular
water [24, 25]. Presumably the proton of PW and the silanol of HZSM-5 interact via
a bridging structure. A model of the possible structure of the PW/HZSM-5 catalyst
is shown in Fig. 5.
Fig. 4 FT-IR spectra of PW/HZSM-5 with different PW loading: a 5 wt% PW/HZSM-5; b 10 wt% PW/
HZSM-5; c 30 wt% PW/HZSM-5
Fig. 5 Possible structure of the PW/HZSM-5 catalyst
123

Regioselective nitration of naphthalene over HZSM-5
Catalytic performance in the nitration of naphthalene
Liquid phase nitration of naphthalene with nitric acid was performed over HZSM-5-
supported PW catalysts. The main products were 1-nitronaphthalene, 1,5-dinitro-
naphthalene, and 1,8-dinitronaphthalene (Scheme 1). A possible reaction mecha-
nism is proposed in Scheme 2. The reaction proceeds because of the acidity of
Keggin-type phosphotungstic acid, which replaces the sulfuric acid used in
traditional mixed-acid nitration of naphthalene. The mechanism for nitration of
naphthalene is electrophilic aromatic nitration. Under these reaction conditions,
HNO3 forms the nitronium ion (NO2?) in the presence of phosphotungstic acid.
Scheme 1 Nitration of naphthalene
Scheme 2 Possible reaction mechanism for nitration of naphthalene
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P. Liu et al.
This ion then interacts with naphthalene to form a p-complex intermediate; this is
formed rapidly, because of its low activation energy. A second intermediate, a r-
complex, is formed by electron transfer from the aromatic compound to the
nitronium ion in the p-complex intermediate. The r-complex can then participate in
further transformations. The first step is proton elimination leading to 1-nitronaph-
thalene; this reacts further with nitronium ion (NO2?) to form dinitronaphthalene by
the same mechanism. The predominant dinitronaphthalene products under these
reaction conditions are 1,5-dinitronaphthalene and 1,8-dinitronaphthalene.
The effect of PW loading of PW/HZSM-5 on total yield and 1,5-dinitronaph-
thalene selectivity is shown in Table 2. Over HZSM-5 the yield of 1-nitronaph-
thalene is 78.5 % and the total yield of dinitronaphthalene is only 16.8 %. The total
yield of dinitronaphthalene increases substantially when the reaction is catalyzed by
HZSM-5-supported PW catalyst, possibly because interaction between the surface
silanol and the anion of PW furnishes more acidic sites for the reaction. However,
higher loading of PW leads to the poor dispersion of PW on HZSM-5 and lower
surface area, pore diameter, and pore volume, which do not favor the reaction.
For comparison, the parent phosphotungstic acid, and HY, ZrO2, and SiO2-
supported PW catalysts were also tested; the results are shown in Table 3. The total
yield of dinitronaphthalene is 65.2 % over PW alone, much more than over HZSM-
5 alone, indicating that the acidity of PW is important in the reaction. Moreover, the
best result for total yield of dinitronaphthalenes, and, in particular, the highest yield
of 1,5-dinitronaphthalene and the highest ratio of 1,5-dinitronaphthalene to 1,8-
dinitronaphthalene, approximately 1:1, is obtained when the HZSM-5 is treated with
PW. The surface areas of HY, ZrO2, and SiO2 range from 120 to 170 m2 g-1, which
are much smaller than that of HZSM-5. The better dispersion of PW on HZSM-5
results in better catalytic performance.
Table 4 shows the effect of the amount of nitric acid on naphthalene nitration.
For comparison, nitration of naphthalene was also conducted in the absence of the
catalyst; the result, also shown in Table 4, indicates that a large amount of
1-nitronaphthalene is produced when the amount of nitric acid or catalyst is
insufficient. In general, when naphthalene is nitrated with mixed nitric and sulfuric
acids, sulfuric acid favors formation of NO2?, which promotes nitration. However,
Table 2 Nitration of naphthalene over PW/HZSM-5 with different PW loading
Catalyst
HZSM-5
Conversion
(%)
Selectivity (%)
1,5-Dinitronaphthalene
1,8-Dinitronaphthalene
1-Nitronaphthalene
100
100
6.2
10.6
20.7
78.5
58.2
5 wt% PW/
HZSM-5
16.0
10 wt% PW/
HZSM-5
100
100
13.2
13.0
18.6
15.5
65.2
69.5
30 wt% PW/
HZSM-5
Reaction conditions: naphthalene: 0.01 mol; 65 wt% nitric acid: 0.1 mol; catalyst: 0.3 g; temperature:
338 K; reaction time: 3 h
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Regioselective nitration of naphthalene over HZSM-5
Table 3 Effect of the catalyst support on nitration of naphthalene
Catalyst
Conversion Selectivity (%)
(%)
1,5-Dinitronaphthalene 1,8-Dinitronaphthalene 1-Nitronaphthalene
PW
100
100
100
100
28.5
37.1
39.8
40.4
48.5
36.7
59.5
55.3
53.7
49.2
31.5
–
PW/HY
PW/ZrO2
PW/SiO2
2.4
2.5
–
PW/HZSM-5 100
Reaction conditions: naphthalene: 0.01 mol; 65 wt% nitric acid: 0.3 mol; catalyst: 0.3 g; temperature:
338 K; reaction time: 3 h
Table 4 Effect of the amount of nitric acid on nitration of naphthalene
Amount of
nitric acid (mol) (%)
Conversion Selectivity (%)
1,5-Dinitronaphthalene 1,8-Dinitronaphthalene 1-Nitronaphthalene
0.1
0.2
0.3
0.3a
100
100
100
100
16.0
31.5
48.5
21.2
20.7
35.7
49.2
33.1
58.2
38.6
–
43.3
Reaction conditions: naphthalene: 0.01 mol; 5 wt% PW/HZSM-5 catalyst: 0.3 g; temperature: 338 K;
reaction time: 3 h
a
Without catalyst
use of mixed acids causes recycling problems and serious environment pollution.
Thus it is necessary to appropriately increase the concentration of nitric acid to
ensure the formation of dinitronaphthalene.
The effect of calcination temperature is shown in Table 5. Although the total
yield of dinitronaphthalene is barely affected, selectivity for 1,5-dinitronaphthalene
first increases and then decreases with increasing calcination temperature. The
Keggin structure of PW is destroyed at 801 K, and formation of mixed oxides starts
at approximately 845 K. The decomposition temperature is also reduced somewhat
when PW is loaded on the support. The results from FT-IR spectroscopy show the
Keggin structure is present in 5 wt% PW/HZSM-5 calcined at 773 K. However, the
PW starts to decompose when the calcination temperature is increased to 823 K.
This may be the reason for the decrease in selectivity for 1,5-dinitronaphthalene
when the calcination temperature is close to the decomposition temperature.
The effect of solvent on naphthalene nitration is shown in Table 6. When acetic
acid is used as solvent, a small amount of 1-nitronaphthalene produced, the total
yield of dinitronaphthalene is approximately 75.1 %, and the yield of 1,5-
dinitronaphthalene is only 32.6 %. When nitromethane and 1,2-dichloroethane are
used as solvents, the yield of dinitronaphthalene is little different. Nitration
efficiency and selectivity for 1,5-dinitronaphthalene are highest when no solvent is
used.
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P. Liu et al.
Table 5 Effect of calcination temperature on nitration of naphthalene
Calcination
temperature (K)
Conversion
(%)
Selectivity (%)
1,5-Dinitronaphthalene
1,8-Dinitronaphthalene
623
673
723
773
823
100
100
100
100
100
41.9
43.2
46.0
48.5
43.7
56.5
54.6
50.7
49.2
53.8
Reaction conditions: naphthalene: 0.01 mol; 65 wt% nitric acid: 0.3 mol; 5 wt% PW/HZSM-5 catalyst:
0.3 g; temperature: 338 K; reaction time: 3 h
Table 6 Effect of solvent on nitration of naphthalene
Solvent
Conversion Selectivity (%)
(%)
1,5-
1,8-
Dinitronaphthalene
1-
Nitronaphthalene
Dinitronaphthalene
Acetic acid
100
100
32.6
40.4
42.5
56.8
22.5
–
1,2-
Dichloroethane
Nitromethane
–
100
100
42.8
48.5
54.1
49.2
–
–
Reaction conditions: naphthalene: 0.01 mol; 65 wt% nitric acid: 0.3 mol; 5 wt% PW/HZSM-5 catalyst:
0.3 g; temperature: 338 K; reaction time: 3 h
Finally, 5 wt% PW/HZSM-5 catalyst was used for recycling experiments. After
reaction, the catalyst was separated and washed with acetonitrile, dried at 393 K for
2 h, and used for nitration of naphthalene. It was found the recycled catalyst
retained its original activity and selectivity throughout three cycles.
Conclusions
HZSM-5-supported PW catalysts were prepared and characterized by XRD, BET,
and FT-IR and UV–visible spectroscopy. The results indicate that 5 wt% PW/
HZSM-5 calcined at 773 K has the largest surface area and high dispersion of PW.
Keggin units are present inside the PW/HZSM-5 catalysts and the proton of PW and
the silanol of HZSM-5 interact via a bridging structure. The catalytic properties of
the material in regioselective nitration of naphthalene were investigated. The best
results, i.e. 48.5 % yield of 1,5-dinitronaphthalene and 49.2 % yield of 1,8-
dinitronaphthalene, were obtained by use of 5 wt% PW/HZSM-5, for which the
yield of 1,5-dinitronaphthalene and the ratio of 1,5-dinitronaphthalene to 1,8-
dinitronaphthalene were much higher. It was also found that the catalyst retains its
original activity and selectivity when recycled three times.
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Acknowledgments This work was supported by the Program for New Century Excellent Talents in
University (NCET-10-0168), the National Natural Science Foundation of China (21276218), the
Specialized Research Fund for the Doctoral Program of Higher Education (20124301110007), the
Scientific Research Fund of Hunan provincial Education Department (13K043), and the Project of Hunan
provincial Science and Technology Department (2012FJ1001).
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