Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
Photocatalytic Reduction of Nitroaromatic Compounds Using TiO2 Nanoparticle by Solar
Light
Z. Zand, F. Kazemi*, A. Safari
Institute of Advanced Studies in Basic Science (IASBS), Chemistry Departement, Zanjan, 45195-159, Iran
kazemi_f@iasbs.ac.ir
Abstract: Aromatic nitro compounds were chemoselectively reduced to the corresponding anilines by using TiO 2
nanoparticle as photocatalyst under sunlight irradation. This method is highly efficient with excellent yields (>90%)
and wide functional group tolerance such as carbonyl, halogen, amino, cyanide groups. In this study TiO2 nanoparticle
with a bicrystalline (anatase and brookite) framework was synthesized directly under high intensity ultrasound
irradiation without thermal treatment, by the agglomeration of monodispersed TiO 2 sol particles. The resulting material
was characterized by XRD, N2 adsoption-desorption and diffuse reflectance spectroscopy.
Keywords: Photocatalysis, Nitroaromatic compounds, Solar light, Ultrasound
Introduction
Solar energy is an inexhaustible resource. The sun
produces vast amounts of renewable solar energy that can
be collected and converted into heat and electricity [1].
Semiconductor photocatalysis is an efficient method for
the chemical utilization of solar energy. It is based on the
surface trapping of light generated charges which induce
interfacial electron transfer reactions with a great variety
of substrates [2].
Titanium dioxide seems to have few competitors to date
in semiconductor photocatalysis since it combines several
advantages such as low cost, chemical and thermal
stability as well as nontoxicity. Investigations on the
active application of the photocatalysis of titanium
dioxide were begun in 1950s. The beginning of
practically applicable photocatalysis of titanium dioxide
originates with a discovery by Fujishima and Honda in
1972 of photocatalytic splitting of water on titanium
dioxide electrodes [3]. However, as a wide band gap
semiconductor (3.20 eV), allows only absorption of
ultraviolet irradiation, which contains of about 5% of
solar energy [4]. To extend the absorption of titanium
dioxide into the visible range, TiO2 doping with metals
[5], carbon, nitrogen [6] or sulfur and other specious and
also surface modifications with dye are used [7].
The photoreduction of nitroarenes to anilines is an
application of such semiconductor photocatalysis to
organic synthesis. Li first reported a photreduction of
nitro compounds to anilines using TiO2 Degussa P25
under UV irradiation [8].
Recently, examples of the application of TiO2 [9], Ndoped TiO2 [10] or dye-sensitized TiO2 [11] and
PbBiO2X [12] under UV, green and blue light irradiation
for the reduction of nitroaromatic compounds to anilines
have been reported.
Here, we describe photocatalytic reductions of
nitrobenzene derivatives with sunlight using TiO2
nanoparticle. The photo-conversions of organic substrates
996
are very selective and verified by gas chromatography
monitoring.
Experimental
Catalyst preparation:
Following the typical TiO2 synthesis reported by Yu et al
[13], a slightly modified procedure of nano-sized TiO2
particles synthesis was employed: titanium tetra
isopropoxide (TTIP, Aldrich) was used as a titanium
source. TTIP (0.0125 mol) was added dropwise to 100 ml
deionized water mixed solution under vigorous stirring at
room temperature.
Sol samples formed by the hydrolysis process were
treated with ultrasonic irradiation in an ultrasonic
cleaning bath (Bransonic ultrasonic cleaner, model DT
102 H, 35 kHz, 120/450 W, German) for 1.5 h, followed
by ageing in a closed beaker at room temperature for 36 h
in order to further hydrolyze the TTIP and form monodispersed TiO2 particles. After the ageing, these samples
were dried at 100 °C for ca. 8 h in air and then ground to
a fine powder to obtain dried gel samples. The dried gel
samples were calcined at 500 °C in air for 1 h to obtain
TiO2 photocatalysts.
Photocatalytic synthesis of aromatic amines:
In a 5ml shell vial equipped with a magnetic stir bar were
mixed 2 ml of a 0.01 M alcoholic solution of a nitro
compound and 2 ml of a TiO2 suspension (5 g/l) in the
same solvent. The reaction mixture purged with nitrogen
for 15 min. The sample was stirred magnetically during
reaction and illuminated with sunlight. Reaction products
analysed by GC.
Instruments:
The X-ray diffraction (XRD) pattern was recordrd on
Philips Xpert X-ray diffractometer with Cu Kα radiation
(λ=0.15406 nm), employing scanning rate of 1º/min in
the 2θ range from 20º to 60º. UV-vis diffusive reflectance
spectra (DRS) were obtained using varian cary 100
spectrophotometer.
Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
Results and Discussion
Fig. 1 shows X-ray powder diffraction patterns of the
prepared TiO2. From the intensity ratios between the
diffraction appearing at 2=25.5 (anatase 101) and 2=31
(brookite 121) it can be conclude that the TiO2 contains
anatase, as the main phase along with about 10 percent of
brookite phase. The average crystalline size of TiO2 was
calculated using the Scherrer equation. TiO2 prepared had
a particle size of 14.5 nm.
Fig. 1. X-Ray powder diffraction patterns
Fig. 2 shows the pore size distribution curve according to
the corresponding N2 adsorption-desorption isotherms of
the TiO2 sample. This analyse suggested both macro and
mesoporous structure of the TiO2 sample. Porous
structure is prepared under the intensity of 120Wcm–2
ultrasound. The BJH pore size curves obtained from the
desorption branch of the isotherm is shown in the inset of
Fig. 2, which indicates that the TiO2 has pore size
distribution around 6 nm. Also the BET surface area of
the TiO2 sample was determined about 43.5 m2/g.
Bandgap energies of 3 eV and 3.1 eV, respectively, were
obtained for our TiO2 sample and TiO2 Degussa P25 from
the extrapolation of the linear part of the modified
Kubelka–Munk functions [F(R∞)E]1/2 versus energy (E)
plot.
Fig. 3. DRS spectrum for a: TiO2 P25 b: TiO2
The photocatalytic activity of the TiO2 was tested for the
reduction of nitrobenzene derivatives to the
corresponding anilines under the sunlight irradiation.
Aromatic amines, widely used as the important
intermediates in the synthesis of chemicals such as dyes,
antioxidants,
photographic,
pharmaceutical
and
agricultural chemicals, can be obtained by the reduction
of aromatic nitro compounds. The photocatalytic
nitrobenzene reduction proceeds stepwise with
nitrosobenzene
and
phenylhydroxylamine
as
intermediates [14]. Here, we reported a clean
photocatalytic reduction of nitro aromatic compounds to
their aniline derivetives using the TiO2 nanoparticle by
sunlight. Table 1 summarizes the results for standard
reaction conditions of photocatalytic reduction under the
sunlight irradiation. It could be seen that the reduction of
nitroarenes gave corresponding amines in excellent yield
(above 90%). The catalytic system was efficient in the
reduction of aromatic nitro compounds bearing additional
substituents in aromatic ring.
Conclusions
Fig. 2. N2 adsorption-desorptions and BJH of TiO2
The TiO2 nanoparticle selectively photoreduce
nitrobenzenes to anilines under sunlight irradiation. The
photocatalyst is easy to prepare by ultrasound, also shows
that ultrasonic irradiation obviously enhances the
photocatalytic activity of TiO2. This may be ascribed to
the fact that ultrasonic irradiation enhances hydrolysis of
titanium alkoxide and promotes crystallization of TiO2
gel. Also the use of ultrasound irradiation assisted in the
formation of the brookit phase.
This discovery reveals a new class of useful catalytic
processes for photocatalytic reactions under sunlight.
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Proceedings of the 4th International Conference on Nanostructures (ICNS4)
12-14 March, 2012, Kish Island, I.R. Iran
R
NO2
TiO2
Sunlight R
NH2
Table 1: Photoreduction of nitrobenzenes using TiO2 nanoparticle under sunlight irradiation
Entry
Nitro Compound
Product
Time
Yield (%)c
0
2
Nitrobenzene
3-Nitroacetophenone
Aniline
3-Aminoacetophenone
1.15 (h)
45 (min)
011
011
3
4-Nitroacetophenone
4-Aminoacetophenone
1 (h)
011
4
4-Nitrobenzonitrile
4-Aminobenzonitrile
45(min)
011
5
3-Chloronitrobenzene
3-Chloroaminobenzene
1.5(h)
011
6
4-Nitrobenzophenone
4-Aminobenzophenone
45(min)
011
7
1-Nitronaphtalene
1-Aminonaphtalene
2(h)
011
8
1, 4-Dinitrobenzene
4-Nitroaminobenzene
1, 4-Diaminobenzene
2.5(h)
70
25
9
2-Methoxynitrobenzene
2-Methoxyaminobenzene
3.5(h)
011
01
3-Methylnitrobenzene
3-Methylaminobenzene
4(h)
011
00
4-Nitroaniline
1,4-Diaminobenzene
3(h)
75
02
1, 2-Dinitrobenzene
2-Aminonitrobenzene
1, 2-Diaminobenzene
3(h)
60
30
C: GC yield, Reaction condition: Nitroaromatic (2 ml, 0.01 M in EtOH), TiO2 (2 ml, 5g/l in EtOH), daily
sunlight (10 am-4 pm).
[7] T. L. Thomson, J. T. Yates, Chem. Rev, 106 (2006),
4428.
Acknowledgment
[8] F. Mahdavi, T. C. Bruton, Y. Li, J. Org. Chem. 58
The authors gratefully acknowledge the support provided
(1993), 744.
by Professor Yousef Sobouti, founder of IASBS.
[9] K. Imamura, S. Iwasaki, T. Maeda, K. Hashimoto, B.
Ohtani, H. Kominami. Phys. Chem. Chem. Phys. 13
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