SO42 -/ZrO2-titania nanotubes as efficient solid superacid catalysts for selective mononitration of toluene

Article abstract of DOI:10.1016/j.catcom.2011.10.035

SO₄^{2 -}/ZrO₂-titania nanotubes-X (SO ₄^{2 -}/ZrO₂-TNTs-X; X of Ti/Zr) solid superacid catalysts with different atom ratios (Ti/Zr) were successfully prepared. The SO₄^{2 -}/ZrO₂-TNTs-2 (Ti/Zr of 2) have a specific surface area of 394 m²/g and an acid intensity of 1.839 mmol/g. The SO₄^{2 -}/ZrO₂-TNTs-2 show excellent selectivity (p/o of 3.34) and activity (yield of 98%) in the selective mononitration of toluene. Importantly, the SO₄^{2 -}/ZrO₂-TNTs-2 can be efficiently recycled and regenerated by simple sulfuric acid soaking.

Full text of DOI:10.1016/j.catcom.2011.10.035

Catalysis Communications 18 (2012) 47–50
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
SO42−/ZrO2–titania nanotubes as efficient solid superacid catalysts for selective
mononitration of toluene
⁎
Pengfei Chen, Mingxing Du, He Lei, Yan Wang, Guoliang Zhang, Fengbao Zhang, Xiaobin Fan
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
SO42−/ZrO2–titania nanotubes–X (SO42−/ZrO2–TNTs–X; X of Ti/Zr) solid superacid catalysts with different
atom ratios (Ti/Zr) were successfully prepared. The SO42−/ZrO2–TNTs–2 (Ti/Zr of 2) have a specific surface
area of 394 m2/g and an acid intensity of 1.839 mmol/g. The SO42−/ZrO2–TNTs–2 show excellent selectivity
Article history:
Received 13 August 2011
Received in revised form 6 October 2011
Accepted 12 October 2011
Available online 17 November 2011
(p/o of 3.34) and activity (yield of 98%) in the selective mononitration of toluene. Importantly, the SO42−
/
ZrO2–TNTs–2 can be efficiently recycled and regenerated by simple sulfuric acid soaking.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
SO42−/ZrO2–titania nanotubes–X
Superacid
Toluene
Mononitration
1. Introduction
reaction, a white paste was filtered and washed with distilled water
and 0.1 M HCl successively until the filtrate became neutral. The de-
posits were then dried at 343 K and calcined at 573 K to obtain titania
nanotubes (TNTs). The TNTs were then added to a certain amount of
ZrOCl2·8H2O aqueous solution (0.1 M), and the suspension was then
heated at 363 K under intensity magnetic stirring until all the water
was completely evaporated. Then, the dried solid was treated by sul-
furic acid (0.5 M, 50 ml/g) and calcined at 573 K for 3.5 h to obtain
SO42−/ZrO2–TNTs–X.
Selective mononitration of toluene to p-nitrotoluene is one of the
most widely studied organic reactions. To date, industry still largely
relies on the conventional mixed acids method with notorious corro-
sion, over-nitration and low para-selectivity (o/m/p=58/4/38[1]).
Among them, low para-selectivity is the crucial problem. During the
past decades, researchers had introduced various nitrating reagents
and catalysts to improve the para-selectivity [2–5]. However, search-
ing for more efficient protocols and catalysts is still an urgent task.
In this study, we focused on ZrO2 and TiO2, because these two ox-
ides contain both Brønsted and Lewis acid sites and have many other
unique properties [6,7]. Many researchers had devoted to the acidic
properties of their particle forms [8–15]. Whereas titania nanotubes
[16,17] which have larger specific surface area were seldom applied
as solid acid catalysts [18], and no study reaches the catalytic perfor-
mances of the composites (ZrO2 and titania nanotubes) in the selec-
2.2. Mononitration of toluene
To a flask, catalysts (0.2 g), nitric acid (1.068 g, 69 wt%), acetic an-
hydride (1.52 ml) and toluene (10 mmol) were added successively in
an ice bath and kept for 5 min. The mixture was then reacted for a
certain time under required temperatures. Along the whole reaction,
0.95 ml of acetic anhydride was added additionally to remove the
water produced during the reaction. After the reaction, the only by-
product (acetic acid) was separated by distillation, and the product
was analyzed by GC (6890N, Agilent). The catalysts were washed
with CCl4 and then recycled. When the catalysts were recycled for
four to five runs, they were immersed by sulfuric acid (0.5 M,
50 ml/g) again and calcined at 573 K for 3.5 h to regenerate.
tive mononitration of toluene. Here we prepared the new SO42−
ZrO2–titania nanotubes–X (SO42–/ZrO2–TNTs–X;
of Ti/Zr) solid
superacid catalysts and investigated their catalytic performances in
the selective mononitration of toluene.
/
X
2. Experimental
2.1. Preparation of SO42−/ZrO2–TNTs–X (X of Ti/Zr)
2.3. Characterization
In an autoclave, TiO2 powder (anatase, 0.6 g) and NaOH aqueous
solution (10 M, 50 ml) were heated at 393 K for 12 h. After the
The samples were characterized by transmission electron microsco-
py (TEM; Tecnai G2 F20), X-ray diffraction (XRD; Rigaku D/Max–2500
X-ray diffractometer, Cu Kα, λ=0.15418 nm), scanning electron mi-
croscopy (SEM; Hitachi S4800) and corresponding energy dispersive
⁎
Corresponding author. Tel.: +86 22 27408778; fax: +86 22 27408778.
E-mail address: xiaobinfan@tju.edu.cn (X. Fan).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.10.035

48
P. Chen et al. / Catalysis Communications 18 (2012) 47–50
100
ZrO2-TNTs-2
95
90
85
80
1400
SO42-/ZrO2-TNTs-2
997
1040
1137
1242
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Fig. 1. XRD patterns of the prepared samples with different atom ratios of Ti/Zr.
Fig. 3. FT-IR spectra of ZrO2–TNTs–2 and SO42−/ZrO2–TNTs–2.
spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR; Ni-
colet Nexus FTIR), temperature programmed desorption of NH3 (NH3–
TPD; AutoChem II 2920) and N2 isotherm ad–desorption (Gemini V
rapid specific surface area).
on catalytic activities of all these samples demonstrated that the
SO42−/ZrO2–TNTs–2 showed the best catalytic performance. There-
fore, we focused on the SO42−/ZrO2–TNTs–2 in later studies.
3. Results and discussions
3.2. Morphology analysis
3.1. XRD
Fig. 2a and b shows the transmission electron microscopy (TEM)
images of TNTs and SO42−/ZrO2–TNTs–2. The TNTs are open-ended
with an outer diameter of about 9.3 nm and an inner channel of
about 5 nm (Fig. 2a). The specific surface area of TNTs is about
412 m2/g, similar to previous studies [18]. After loading with ZrO2
and SO42−, the TNTs become amorphous (Fig. 2b). The inner channel
of TNTs should be filled with ZrO2, and the specific surface area of
TNTs decreases along with the increasing of ZrO2 (Table 1). SEM
image (Fig. 2c) and corresponding EDS mapping analysis (Fig. 2d, e
and f) of the SO42−/ZrO2–TNTs–2 show that the elements (Ti, Zr and
Fig. 1 compares the XRD patterns of SO42−/ZrO2–TNTs–X (X of Ti/
Zr) with the X of ∞, 5, 2, 0.5, 0.2 and 0.1. The TNTs (X=∞) show
two distinct peaks at 2θ=25.43 and 48.27, indicating the TNTs are
consisted of pure anatase crystal. With the decrease of X (adding of
ZrO2), a new shoulder at 2θ=30.8 corresponding to the amorphous
ZrTiO4 [19,20] gradually emerges and becomes prominent when the
X is 2. However, when the ratios of Ti/Zr decrease further, tetragonal
phase of ZrO2 becomes dominant. Interestingly, preliminary studies
Fig. 2. TEM images of (a) TNTs and (b) SO42−/ZrO2–TNTs–2; SEM image (c) and corresponding EDS mapping of (d, e, f) SO42−/ZrO2–TNTs–2.

P. Chen et al. / Catalysis Communications 18 (2012) 47–50
49
Table 1
deficient metal centers on the metal cations that act as strong Lewis
acid sites. This structure is believed to be a driving force in the generation
of many surface's acidic sites on sulfated metal oxides.
Nitration of toluene over various catalysts.
Catalyst composition
(Ti/Zr)
Time
[min]
Temperature
[K]
Yield
(%)
p/o
Surface area
[m2 g−1
]
A1Control
A2Control
BControl
C1Control
C2Control
C3Control
DControl
TiO2 powder
TNTs
10/1
5/1
3/1
2/1
1/1
1/5
1/7
1/10
2/1
2/1
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
5
343
343
293
343
343
343
343
343
343
343
343
343
343
343
343
343
343
343
298
363
343
1.3
–
0.44
–
–
3.4. NH3–TPD spectrum of SO42−/ZrO2–TNTs–2
–
42
87
81
69
11
88
91
98
97
99
98
95
98
96
89
77
99
99
99
0.69
0.55
0.58
0.51
2.69
1.41
2.80
2.77
2.93
2.89
3.34
3.27
3.01
2.85
2.65
2.47
2.01
3.05
3.25
–
Three types' acid sites of the SO42−/ZrO2–TNTs–2 are observed in
Fig. 4. The peak in the low-temperature range (centered in 500 K) is
believed to be caused by weak adsorption. The second peak centered
at 680 K can be attributed to the strong acid sites. The last peak cen-
tered in 910 K should be the super strong acid sites which are much
stronger than those of SO42−/ZrO2 nanoparticles (815 K) [23]. The
ratio of three acidic centers is about 0.49/1.88/1, and the total amount
of acid sites is about 1.839 mmol/g. This value is much larger than
those of ZrO2 powder (0.138 mmol/g), TiO2 powder (0.173 mmol/g)
and TiO2–ZrO2 powder (1.456 mmol/g) [24]. According to previous
studies [22], the sulfate species modify the electronic environment
around the Ti4+ or Zr4+ by anchoring SO42− in either bridging or che-
lating bidentate complexes. In addition, one bridging complexes can
create three acid sites at most, and one chelating complexes can cre-
ate no more than two acid sites [2]. For the SO42−/ZrO2–TNTs–2, the
theoretical ratio of acid sites to sulfate species should be in the
range of 1–3. Therefore, the calculated ratio of acid sites to sulfate
species here (1.46) is reasonable.
–
–
–
394
156
412
409
432
401
394
367
301
325
249
394
394
394
322
25
45
25
2/1
E2/1
A1HNO3/toluene=2; A2HNO3/toluene=1.
BHNO3/H2SO4/toluene=1/5/1.
C1HNO3/Ac2O/toluene=1/2/1; C2HNO3/Ac2O/toluene=1/1/1;
C3HNO3/Ac2O/toluene=2/1/1.
DHNO3/toluene=1; solid acids (SO42−/ZrO2–TNTs–2, 0.1 g).
EWith fourteen times reused catalyst.
3.5. Selective mononitration of toluene
The catalytic performances of the SO42−/ZrO2–TNTs solid acid cat-
alysts were evaluated by the selective mononitration of toluene, and a
series of control experiments were also carried out (Table 1). Low
mononitration yield (1.1%) and poor p/o selectivity (0.44) were ob-
served when only nitric acid was used. Furthermore, under the
same conditions, the mononitration yield of conventional mixed
acids method is only 42%, and the p/o selectivity is just 0.69, which
is close to the value reported in previous studies [25]. The addition
of acetic anhydride can significantly improve the mononitration
yield of the mixed acids method (increase to 81%), but it contributes
S) are homogeneously distributed, and the atom ratio (Ti/Zr/S) is
about 1.93/1/0.61. This value is close to the stoichiometric dosage
(2/1/0.5), and the mass fraction of S can be calculated to be
4.02 wt% (1.256 mmol/g).
3.3. FT-IR spectra of ZrO2–TNTs–2 and SO42−/ZrO2–TNTs–2
Fig. 3 shows the FT-IR absorption spectra of ZrO2–TNTs–2 before and
after sulfated. Comparing with the pristine ZrO2–TNTs–2, the SO42−/ZrO2–
TNTs–2 show a new band at 1400 cm−1, corresponding to the stretching
frequency of the free sulfate groups [2]. Four new bands at 1242, 1137,
1040, 997 cm−1 are also observed, and they are the characteristic fre-
quencies of a bidentate SO42− coordinated to metals such as Ti4+ or Zr4
+ [21]. According to previous studies [22], the sulfate species modify the
electronic environment around Ti4+ or Zr4+ by anchoring SO42− in either
bridging or chelating bidentate complexes. Such a bridge bidentate or
chelating bidentate structure could strongly withdraw electrons from
the neighboring metal cations, resulting in a number of electron-
slightly to the p/o selectivity (0.6) [26]. As comparison, the SO42−
/
ZrO2–TNTs show impressive p/o selectivity (2.69). Their perfor-
mances can be further improved by adding acetic anhydride, and
the best catalytic performance (yield of 98% and p/o of 3.34) can be
observed in SO42−/ZrO2–TNTs–2 (Ti/Zr of 2). Notably, the SO42−
/
ZrO2–TNTs–2 can be readily regenerated by sulfuric acid and show
stable performance after fourteen runs (yield of 99% and p/o of
3.25). Further systematic studies on the catalytic performance of the
SO42−/ZrO2–TNTs–2 also demonstrate that the optimized temperature
is about 343 K, and the suitable reaction time is about 25 min
(Table 1). Note that the p/o selectivity of conventional mixed acids
method is only 0.69; the new catalysts (SO42−/ZrO2–TNTs–2) here
show impressive para-selectivity. Considering p-nitrotoluene is com-
mercially more desirable than the o-isomer [27], the SO42−/ZrO2–
TNTs solid acid catalysts should be a promising and profitable cata-
lysts for the selective mononitration of toluene.
2.0
1.0
1.5
0.8
0.6
1.0
4. Conclusions
0.4
The SO42−/ZrO2–titania nanotubes–X solid superacid catalysts
with high specific surface area and acid intensity were successfully
prepared. Catalytic investigation demonstrated that excellent selec-
tivity (p/o of 3.34) and activity (yield of 98%) could be achieved in
the selective mononitration of toluene by using the SO42−/ZrO2–
TNTs–2. Importantly, the SO42−/ZrO2–TNTs–2 could be efficiently
recycled and regenerated by simple sulfuric acid soaking. This new
catalyst may also find potential applications in its acid catalytic
reactions.
0.5
0.2
0.0
0.0
900 1000 1100
400
500
600
700
800
Temperature / K
Fig. 4. NH3–TPD spectrum of SO42−/ZrO2–TNTs–2.

50
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