Novel atom-economic nitration of benzene with a nitrogen dioxide-oxygen system using cocatalyst of solid oxides and lanthanide(III) metal triflates

Article abstract of DOI:10.1080/00397910701569502

Benzene is nitrated by a novel atom-economic nitration procedure with a NO2-O2 system in the presence of a mixture of solid oxides and Ln(OTf)3. The only by-product in this novel method is water. The efficiency of NO2 is much high than 50%, the theoretica

Full text of DOI:10.1080/00397910701569502

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Novel Atom‐Economic Nitration of Benzene
with a Nitrogen Dioxide–Oxygen System using
Cocatalyst of Solid Oxides and Lanthanide(III)
Metal Triflates
Xiaohua Du a , Xiaoqing Li a & Zhenyuan Xu a
a State Key Laboratory Breeding Base of Green Chemistry–Synthesis
Technology , Zhejiang University of Technology , Hangzhou, China
Published online: 19 Oct 2007.
To cite this article: Xiaohua Du , Xiaoqing Li & Zhenyuan Xu (2007) Novel Atom‐Economic Nitration of
Benzene with a Nitrogen Dioxide–Oxygen System using Cocatalyst of Solid Oxides and Lanthanide(III) Metal
Triflates, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 37:21, 3741-3749, DOI: 10.1080/00397910701569502
To link to this article: http://dx.doi.org/10.1080/00397910701569502
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Synthetic Communicationsw, 37: 3741–3749, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701569502
Novel Atom-Economic Nitration of Benzene
with a Nitrogen Dioxide–Oxygen System
using Cocatalyst of Solid Oxides and
Lanthanide(III) Metal Triflates
Xiaohua Du, Xiaoqing Li, and Zhenyuan Xu
State Key Laboratory Breeding Base of Green Chemistry–Synthesis
Technology, Zhejiang University of Technology, Hangzhou, China
Abstract: Benzene is nitrated by a novel atom-economic nitration procedure with a
NO2-O2 system in the presence of a mixture of solid oxides and Ln(OTf)3. The only
by-product in this novel method is water. The efficiency of NO2 is much high than
50%, the theoretical efficiency of NO2 of the known methods. Among the solid
oxides and Ln(OTf)3 studied, HZSM-5 and Sm(OTf)3 were the most efficient
catalysts in this process. Therewith, the yield of the benzene nitration reached
72.5%, calculated by NO2.
Keywords: benzene, lanthanide(III) metal triflate, nitration, nitrogen dioxide,
oxygen, solid oxides
INTRODUCTION
Nitration of organic compounds is one of the most important and extensively
studied organic reactions. The nitroaromatic compounds so produced are
themselves widely utilized and act as chemical feedstocks for a great range
of useful materials such as dyes, pharmaceuticals, perfumes, and plastics.[1]
Unfortunately, classical nitration has long been carried out using concentrated
or fuming nitric acid with sulfuric acid on a high-volume scale, which leads to
Received in Japan January 22, 2007
Address correspondence to Xiaohua Du, State Key Laboratory Breeding Base of
Green Chemistry–Synthesis Technology, Zhejiang University of Technology,
Hangzhou 310014, China. E-mail: greensyn@zjut.edu.cn
3741

3742
X. Du, X. Li, and Z. Xu
excessive acid waste streams, overnitration, and oxiding by-products. The
obvious disadvantages associated with this procedure prompted research on
alternative methods to classical nitration such as using solid acid catalysts,
strong Lewis acid catalysts, and other nitrating agents.
The nonacid method for aromatic nitration using a NO2/O3 system as
nitrating agent has demonstrated an excellent conversion of a wide variety
of aromatic compounds to the corresponding nitro derivatives under mild con-
ditions (Kyodai nitration).[2] However, because ozone used in the nitration
reaction can only be obtained by discharging oxygen at a high voltage, an
expensive ozone generator and large amounts of electricity are needed for
the mass production of ozone. Further efforts to find an alternative means to
solve the problom has led to the Fe(III)-catalyzed nitration of aromatic
compounds with an NO2-O2 system.[3] Rather unfortunately, a long reaction
time (e.g., 12 to 36 h) is needed even though large amounts of nitrogen
dioxide (31 equivalents) and catalyst are used. In anothor modified means,
zeolite was used as catalyst for this process, and high yields and significant
selectivities were obtained. However, a long reaction time and large
amount of nitrogen dioxide were necessary.[4] Furthermore, according to the
reaction equation [Scheme 1, Eq. (1)], the theoretical efficiency of NO2 is
only 50%. The other 50% of NO2 converts into nitric acid as a waste acid,
which is an disadvantage of this procedure.
On the other hand, it has been reported that lanthanide(III) metal triflate
[Ln(OTf)3][5] and other perfluorinated rare-earth metal salts[6] can be used as
recyclable catalysts for the nitration of simple arenes with stoichiometric
quantities of nitric acid. The only by-product is water, and the catalyst may
be readily recycled and reused repeatedly.
To develop an atom-economic process for aromatic nitration, we applied
the mixture of solid oxides and Ln(OTf)3 as catalyst for NO2-O2 system
nitration. As shown in Scheme 1, our approach to the nonacid process for
Scheme 1. Proposed mechanism for nitration of benzene with NO2-O2 system as
nitrating agent catalyzed by solid oxides and Ln(OTf)3.

Atom-Economic Nitration of Benzene
3743
nitration of benzene consisted of two steps: first, a solid oxide catalyzed the
NO2-O2 system to nitrate benzene and generated a by-product of nitric acid
at the same time. Second, Ln(OTf)3 catalyzed nitric acid to nitrate
benzene in situ. The total reaction is shown in Eq. (3). The theoretical effi-
ciency of NO2 is 100%, and the only by-product is water. To the best of our
knowledge, such cocatalyst is rare in nitration methodology. Therefore, the
obtained facts may open new alternatives for atom-economic aromatic
nitration.
RESULTS AND DISCUSSION
Initially, HZSM-5 and Sm(OTf)3 were investigated in our approach. To
optimize the amount of Sm(OTf)3 for maximum yield, nitration of benzene
was studied with varied Sm(OTf)3 amounts ranging from 0 to 2 mol% and
2.0 g of HZSM-5. The results (Table 1) showed that the yield of nitrobenzene
increases with the amount of Sm(OTf)3 until Sm(OTf)3 loading reaches
1 mol%, and more Sm(OTf)3 was not beneficial to yield.
A similar process was studied with varied HZSM-5 amounts ranging from
0 to 2 g with Sm(OTf)3 loading of 1 mol% to optimize the amount of HZSM-5
for maximum yield. The results (Table 2) showed that the yield of nitroben-
zene increases with HZSM-5 until HZSM-5 loading reaches 1.0 g. More
HZSM-5 has no obvious effects on the yield.
Varied pressure ranging from 0.2 to 0.8 MPa was studied to determine the
most suitable reaction pressure. The data (Table 3) show that the highest yield
was obtained at 0.5 MPa. Pressure higher than 0.5 MPa was not beneficial to
yield.
To optimize the reaction temperature, varied temperature ranging from 85
to 1158C was studied. The results (Table 4) show that at low temperature, the
Table 1. Effects of Sm(OTf)3 amount on the yield of
nitrobenzenea
Entry
Sm(OTf)3 (mol%)
Yieldb (%)
1
2
3
4
5
0
51.4
59.5
65.4
65.8
65.6
0.5
1
1.5
2
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene ¼
9 mL (0.105 mol); HZSM-5 ¼ 2.0 g; liquid nitrogen dioxide ¼
3.2 g (0.07 mol); reaction temperature ¼ 858C; reaction
pressure ¼ 0.8 MPa; reaction time ¼ 7 h.
bIsolated yields based on the amount of NO2.

3744
X. Du, X. Li, and Z. Xu
Table 2. Effect of HZSM-5 amount on the yield of nitrobenzenea
Entry
HZSM-5 amount (g)
Yieldb (%)
1
2
3
4
5
0
59.4
63.2
65.8
65.7
65.5
0.5
1.0
1.5
2.0
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene ¼
9 mL (0.11 mol); Sm(OTf)3 ¼ 1 mol%; liquid nitrogen dioxide ¼
3.2 g (0.07 mol); reaction temperature ¼ 858C; reaction
pressure ¼ 0.8 MPa; reaction time ¼ 7 h.
bIsolated yields based on total amount of NO2.
yield increased along with reaction temperature, and the highest yield was
obtained at 1008C. However, yield decreased if reaction temperature
increased further.
To optimize the reaction time, the process was studied at varied reaction
times ranging from 3 to 9 h. The results (Table 5) show that the highest yield
was obtained at 7 h. The yield was not changed when the reaction time was
further prolonged.
We then carried out similar reactions in which various solid oxides or
Ln(OTf)3 were used as catalysts instead of HZSM-5 or Sm(OTf)3 to
determine which solid oxide, if any, would be the most applicable to
benzene nitration. As shown in Tables 6 and 7, the reaction occurred in the
presence of all of the solid oxides and Ln(OTf)3. Among the solid oxides
Table 3. Effects of reaction pressure on the yield of
nitrobenzenea
Reaction
pressure (MPa)
Entry
Yieldb (%)
1
2
3
4
5
0.2
0.4
0.5
0.6
0.8
28.5
63
65.7
65.4
65.5
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene¼
9 mL (0.11 mol); Sm(OTf)3 ¼ 1 mol%; HZSM-5 ¼ 2.0 g; liquid
nitrogen dioxide ¼ 3.2 g (0.07 mol); reaction temperature
¼ 858C; reaction time ¼ 7 h.
bIsolated yields based on total amount of NO2.

Atom-Economic Nitration of Benzene
3745
Table 4. Effects of the reaction temperature on the yield of
nitrobenzenea
Reaction temperature
(8C)
Entry
Yieldb (%)
1
2
3
4
5
85
95
65.7
71.8
72
100
105
115
71.8
68.2
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene¼
9 mL (0.11 mol); Sm(OTf)3 ¼ 1 mol%; HZSM-5 ¼ 2.0 g; liquid
nitrogen dioxide ¼ 3.2 g (0.07 mol); reaction pressure ¼ 0.5 MPa;
reaction time ¼ 7 h.
bIsolated yields based on total amount of NO2.
and Ln(OTf)3 studied, HZSM-5 and Sm(OTf)3 gave the most encouraging
yield of 72.5% (entries 1 and 2).
Control experiments were performed according general procedure. The
absence of eitheir solid oxide (entry 9, Table 6) or Ln(OTf)3 (entry 7,
Table 7) decreased the product yields remarkably. The results clearly
indicated that both solid oxide and Ln(OTf)3 are indispensable for nitration
of benzene in our method. Moreover, all entries using catalyst made up of
solid oxide and Ln(OTf)3 gave yields of more than 50%, which means by
cocatalyzing the solid oxide and Ln(OTf)3, the efficiency of NO2 was
increased more than 50%, the theoretical efficiency of NO2 in the prior
methods.
In summary, we have discovered a novel atom-economic aromatic
nitration process with an NO2-O2 system in the presence of a mixture of
solid oxides and Ln(OTf)3. The only by-product in this process is water,
Table 5. Effects of reaction time on the yield of nitrobenzenea
Entry
Reaction time (h)
Yieldb (%)
1
2
3
4
3
6
7
9
41.4
59.4
72.5
72.3
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene ¼
9 mL (0.105 mol); Sm(OTf)3 ¼ 1 mol%; HZSM-5 ¼ 1.0 g; liquid
nitrogen dioxide ¼ 3.2 g (0.07 mol); reaction temperature ¼ 1008C;
reaction pressure ¼ 0.5 MPa.
bIsolated yields based on total amount of NO2.

3746
X. Du, X. Li, and Z. Xu
Table 6. Nitration of benzene catalyzed by Sm(OTf)3 and var-
ious solid oxidesa
Entry
Solid oxide
Yieldb (%)
1
2
3
4
5
6
7
8
9c
HZSM-5
V2O5
Al2O3
SiO2
72.5
72.2
60.4
65.6
65.4
60.2
64.3
59.4
32.1
Cr2O3
ZrO2
CeO2
WO3
—
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene¼
9 mL (0.105 mol); Sm(OTf)3 ¼ 1 mol%; HZSM-5 ¼ 1.0 g; liquid
nitrogen dioxide ¼ 3.2 g (0.07 mol); reaction temperature ¼ 858C;
reaction pressure ¼ 0.5 MPa.
bIsolated yields based on total amount of NO2.
cNo solid oxide was used.
and the efficiency of NO2 is in excess of 50%. Efforts are under way to
elucidate more mechanistic details of the nitration reaction catalyzed by this
novel mixture catalyst and to identify systems enabling the nitration of
other aromatic compounds and the subsequent transformations thereof.
Table 7. Nitration of benzene catalyzed by HZSM-5 and various
Ln(OTf)a3
Entry
Ln(OTf)3
Yieldb (%)
1
2
3
4
5
6
7c
Sm(OTf)3
Yb(OTf)3
Nd(OTf)3
Gd(OTf)3
Er(OTf)3
Dy(OTf)3
—
72.5
68.4
61.7
60.3
64.5
62.5
27.1
aReaction condition: 1,2-dicholoethane ¼ 50 mL; benzene ¼
9 mL (0.105 mol); Sm(OTf)3 ¼ 1 mol%; HZSM-5 ¼ 2.0 g; liquid
nitrogen dioxide ¼ 3.2 g (0.07 mol); reaction temperature ¼ 858C;
reaction pressure ¼ 0.5 MPa.
bIsolated yields based on total amount of NO2.
cNo Ln(OTf)3 was used.

Atom-Economic Nitration of Benzene
EXPERIMENTAL
General
3747
Sm2O3, Yb2O3, Nd2O3, Gd2O3, Er2O3, Dy2O3, V2O5, Al2O3, Al2O3, Cr2O3,
ZrO2, CeO2, WO3 were procured from Sinopharm Chemical Reagent Co.,
Ltd., China. Silica gel was obtained from Qingdao Haiyang Chemical Co.,
Ltd., China. NaZSM-5 was purchased from Wenzhou Huahua Group
Co., China. Benzene was procured from Quzhou Juhua Chemical Reagent
Co., Ltd., China. 1,2-Dichloroethane was obtained from Shanghai Lingfeng
Chemical Reagent Co., Ltd., China. The purity of the product was analyzed
by GC-MS (Agilent 6890 N/5973) and/or GC (GC-1102, China).
Catalyst Preparation
Ln(OTf)3 (Ln ¼ Sm, Yb, Nd, Gd, Er, Dy)
The triflate salts were prepared following a reported procedure.[7] Typically,
an excess of lanthanide(III) oxide (99.9% purity) was added to an aqueous
solution of trifluoromethanesulfonic acid (50% v/v) and heated at boiling
for 30 min to 1 h. The mixture was filtered to remove the unreacted oxide.
The filtrate was concentrated, and the resulting hydrate was dried by
heating at 1208C for 48 h.
HZSM-5
NaZSM-5 (50 g) was treated with 500 mL of 1 M aqueous ammonium
chloride at 908C for 2 h. The procedure was repeated five times, followed
by filtration of the sample and washing with distilled water until the filtrate
showed the absence of the chloride ions. The obtained sample was oven
dried at 1208C for 10 h followed by calcination at 5508C for 4 h in an air
atmosphere.
General Procedure for Nitration of Benzene
1,2-Dicholoethane (50 mL), benzene (9 mL, 0.11 mol), solid oxides,
Ln(OTf)3, and liquid nitrogen dioxide (2.2 mL, 0.07 mol) were poured into
an autoclave. Next, oxygen was injected at a pressure of 0.2 to 0.8 MPa,
and then the mixtures were reacted at 85 to 1158C for 3 to 9 h while the
same level of pressure was maintained. After completion of the reaction,
the reaction mixture was filtrated to remove the solid oxides, washed with
water to recycled Ln(OTf)3, and dried with MgSO4. The solvent was
removed under reduced pressure to afford nitrobenzene. The purity of the

3748
X. Du, X. Li, and Z. Xu
product was analyzed by GC-MS (Agilent 6890 N 5973) and/or GC (GC-
1102, China).
ACKNOWLEDGMENT
We thank the Zhejiang Natural Science Foundation, China (Y404033), for
financial support.
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