Na4W10O32/ZrO2 nanocomposite prepared via a sol-gel route: A novel, green and recoverable photocatalyst for reductive cleavage of azobenzenes to amines with 2-propanol

Article abstract of DOI:10.1016/j.molcata.2009.11.010

Sodium decatungstate-zirconia (Na₄W₁₀O₃₂/ZrO₂) nanocomposite was prepared through entrapment of Na₄W₁₀O₃₂ into zirconia matrix by a sol-gel route. This new nanocomposite was characterized by means of X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), UV-vis spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and surface area measurement. The nanocomposite was used as a green and recyclable heterogeneous photocatalyst for rapid and efficient reductive cleavage of azobenzenes into their corresponding amines with 2-propanol as a hydrogen donor at room temperature under N₂ atmosphere. The reductive cleavage occurs without hydrogenolysis or hydrogenation of reducible moieties, such as -NO₂, -OH, -CH₃, -OCH₃, -COOH, -COCH₃, halogen and -CN. The photocatalyst has been reused several times, without observable loss of activity and selectivity. According to the azobenzene reduction, the Na₄W₁₀O₃₂/ZrO₂ nanocomposite is more effective as a photocatalyst than pure Na₄W₁₀O₃₂, showing that the nanocomposite approach could be an excellent choice to improve the photoactivity of POMs.

Full text of DOI:10.1016/j.molcata.2009.11.010

Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Na4W10O32/ZrO2 nanocomposite prepared via a sol–gel route: A novel, green and
recoverable photocatalyst for reductive cleavage of azobenzenes to amines with
2-propanol
Saeid Farhadi∗, Shahnaz Sepahvand
Department of Chemistry, Lorestan University, Khoramabad 68135-465, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Sodium decatungstate–zirconia (Na4W10O32/ZrO2) nanocomposite was prepared through entrapment of
Na4W10O32 into zirconia matrix by a sol–gel route. This new nanocomposite was characterized by means
Received 9 August 2009
Received in revised form
12 November 2009
Accepted 15 November 2009
Available online 16 December 2009
of X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), UV–vis spectroscopy, scan-
ning electron microscopy (SEM), transmission electron microscopy (TEM) and surface area measurement.
The nanocomposite was used as a green and recyclable heterogeneous photocatalyst for rapid and effi-
cient reductive cleavage of azobenzenes into their corresponding amines with 2-propanol as a hydrogen
donor at room temperature under N2 atmosphere. The reductive cleavage occurs without hydrogenolysis
or hydrogenation of reducible moieties, such as –NO2, –OH, –CH3, –OCH3, –COOH, –COCH3, halogen and
–CN. The photocatalyst has been reused several times, without observable loss of activity and selectivity.
According to the azobenzene reduction, the Na4W10O32/ZrO2 nanocomposite is more effective as a pho-
tocatalyst than pure Na4W10O32, showing that the nanocomposite approach could be an excellent choice
to improve the photoactivity of POMs.
Keywords:
Photocatalytic reduction
Azobenzenes
Nanocomposite
Sodium decatungstate
Amines
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Utilization of heterogeneous catalysts seems to be one of the
most promising solutions to avoid the above-mentioned problems.
Azobenzene and its derivatives are a important class of organic
materials which are extensively used in textile, printing, leather,
papermaking, agrochemical, drug and food industries [1,2]. Due
to wide applications, substantial quantities of toxic and carcino-
genic azo compounds are dumped into environment as industrial
wastewaters. Thus, it is essential to develop methods for the elimi-
Heterogeneous catalysts offer several advantages over homoge-
neous systems with respect to easy recovery and recycling of
catalysts as well as minimization of undesired toxic wastes. In this
framework, several transition metal-based mesoporous silicates
or aluminophosphate molecular sieves (e.g. PdMCM-41, FeHMA,
NiHMA and NiMCM-41) have been applied as recyclable heteroge-
neous catalysts for the reductive cleavage of azo compounds into
amines by hydrogen donors such as alcohols [8–12]. However, each
of these methods has its own advantages and limitations, so that
introduction of an inexpensive, green and novel method for this
purpose is still in much demand.
Photocatalytic materials play a very important role for selective
organic transformations in an economically and environmentally
friendly way [13–15]. Among them, polyoxometalates (POMs)
which constitute a large variety of oxygen bridged metal clusters
well-known for their rich photocatalytic action [16–18]. Irradia-
tion of the POMs results in the formation of the O → W CT excited
state with considerable oxidizing capacity (2.63 eV versus NHE
[19]), which plays a key role to their photochemical behavior. The
outstanding photocatalytic ability of these non-toxic and green
compounds is attributed to this strong oxidant excited state which
is able to oxidize a great variety of organic compounds and pollu-
tants [20–32]. After oxidizing organic substrate, the photoreduced
POM is usually reoxidized to its original oxidation state without
nation of these compounds for environmental reasons. The N
N
bonds in azo compounds are good electron/hydrogen acceptors
with relatively high reduction potential around 0.76 eV versus
NHE [3], so that their reduction into aromatic amines, which are
less toxic and easily biodegradable, is preferable. Several methods
including electrolytic reduction [4] or chemical reduction by sul-
fides [5], metal iron [6], and Zn/HCO2NH4 [7] as reducing agents
are available for this transformation but most of them are homoge-
neous, non-recoverable and suffer from one or more disadvantages
such as prolonged reaction times, low yields, harsh conditions, use
of harmful organic solvents, tedious work-up procedures, require-
ment of excess of reducing agents which generate equal amounts
of metal waste.
∗
Corresponding author. Tel.: +98 6612202782; fax: +98 6616200088.
E-mail address: sfarhad2001@yahoo.com (S. Farhadi).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.010

76
S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
structural change by an electron acceptor such as dioxygen, result-
according to the modified published procedure and characterized
by spectral data as follows [50]. Na4W10O32/ZrO2 photocata-
lyst was also synthesized on the basis of the sol–gel method
reported in literature for preparation of composites such as
Li5PW11TiO40/ZrO2 and K7PW10Ti2O40/ZrO2 with minor modifica-
tion as follows [51]. The residual azobenzene concentration during
irradiation was determined by monitoring the decrease in its
absorption peak (i.e. ꢀmax = 316 nm; ε = 2.35 × 104 dm3 mol−1 cm−1
at pH 2). Samples (ca. 5 mL) for UV–vis analysis were with-
drawn from suspension at given intervals of illumination through
pipette and immediately centrifugated. The clear solution was
analyzed by UV–vis spectroscopy using a UV-2401PC UV–vis
spectrophotometer. GC–MS analysis was carried out on a Shi-
madzu QP 5050 GC-MS instrument. Pure nitrogen (99.99%) and
dioxygen (99.90%) were used for deaeration or oxygenation of
solutions.
ing in closing the photocatalytic cycle.
4−
Although, photoreduced form of POMs such as W10O32
,
4−
6−
PW12O403−, PMo12O403−, SiW12O40
and P2W18O62
are well-
known strong reducing agents but their application has been only
limited to reduce several simple chemical species, i.e. H+ [33], O2
[34] and metal ions [35–39]. Recently, several groups have reported
4−
that photoreduced POMs such as PW12O40 and SiW12O405−, can
efficiently reduced naphthol blue black and acid orange 7 dyes in
the presence of 2-propanol as the electron donor [40–43]. How-
ever, the use of photoreduced POMs as the reducing agents in the
conversion of organic compounds to their corresponding reductive
products has not been extensively explored.
In spite of numerous advantages of POMs, several problems
associated with the use of this type of materials as photocatalysts.
On the one hand, their hydrolytic stability is limited to narrow
3−
specific pH ranges. For example, PW12O40
whereas, W10O32
is stable at pH ca. 1,
4−
4−
is stable up to pH 2.5, and SiW12O40
and
2.1. Preparation of Na4W10O32 [50]
P2W18O626− are stable up to pH ca. 5.5 [43]. On the other hand, spe-
cific surface area of POMs is very low (<10 m2 g−1), leading to very
few active sites on their surfaces. Moreover, it is difficult to sepa-
rate POMs from reaction mixture because of their high solubility in
polar media such as water and acetonitrile, which impedes ready
recovery and reuse of them. Therefore, heterogenization of POMs
for photocatalytic purposes has attracted particular attention, since
the solid supports: (i) make the handling and recycling of the sys-
tem easier and (ii) provide an opportunity to POMs to be dispersed
over a large surface area, which increases their photocatalytic activ-
ity. Various supports like silica, alumina, active carbon, NaY zeolite,
layered double hyroxide, MCM-41 and titania have been used for
supporting POMs.
In continuation of our interest in the use of heterogeneous POMs
as the photocatalyst in organic transformations [46], in this paper
we report on the preparation of sodium decatungstate–zirconia
(Na4W10O32/ZrO2) nanocomposite by the sol–gel process and
its application as an efficient, green and recyclable hetero-
geneous photocatalyst in the presence of 2-propanol as the
electron/hydrogen source for the reductive cleavage of azo aro-
matic compounds into their corresponding amines under mild
conditions (atmospheric pressure, room temperature). As far as
we know, there is no report regarding the photocatalytic appli-
cation of photoreduced POMs, especially as heterogeneous for
the reduction of organic substrates such as azobenzenes by an
alcohol. In contrast to acidic or neutral solid supports, which
interact weakly with POMs, the surface hydroxyl groups of
amphoteric ZrO2 are able to undergo a chemical reaction or
strong interaction with incorporated components. In addition, as
same as TiO2, this oxide is an n-type semiconductor with high
band-gap energy of 5.0 eV [47]. Thus, it is expected that hetero-
genization of photoactive Na4W10O32 into the ZrO2 framework
can improve their photocatalytic activity through the synergistic
effect.
To a boiling solution containing Na2WO4·2H2O (66 g) in distilled
water (400 mL) was added 400 mL of a boiling aqueous 1 M HCl
solution. The resulting solution was allowed to boil for 10 s, divided
into two equal portions in 2-L beakers, and rapidly cooled to 30 ◦C
in dry a dry ice/methanol bath with stirring. Solid NaCl was added
to near saturation, and the mixture was cooled further to 0 ◦C. The
formed precipitate was collected and dried on a fritted funnel (33 g).
This precipitate was suspended in hot acetonitrile (200 mL) and
filtered. The filtrate was placed in a freezer overnight. Large pale-
lime crystalline rectangular blocks of Na4W10O32 were collected
and dried on a fritted funnel (11.2 g). The absorbance spectrum of
Na4W10O32 in acetonitrile consisted of well-defined maximum at
323 nm assigned to an O → W CT transition. FT-IR (KBr, cm−1): 958
(W = Ot,); 895 and 806 (W–Ob–W) [50].
2.2. Preparation of Na4W10O32/ZrO2 nanocomposite
A solution of zirconium(IV) n-butoxide (100 mmol, 38.32 g; ZrO2
content: 12.32 g) in EtOH (25 mL) was stirred at 70 ◦C, and then
the mixture was slowly cooled to ambient temperature. Afterward,
the acidity of the mixture was adjusted to pH 2 by using HCl. To
the resulting mixture was dropwise added Na4W10O32 (1.68 mmol,
4.20 g) dissolved in the mixed solution of 25 mL EtOH and 7.5 mL of
water, which was maintained under constant stirring for 2 h until
gelling. After gelation, the solid was filtered and dried in air at
100 ◦C for 24 h. The dried gel was calcined in a vacuum at 380 ◦C
for 4 h to fasten the zirconia network and washed with hot water
(90 ◦C) three times to give ca. 16.3 g of Na4W10O32/ZrO2. The load-
ing of Na4W10O32 in the Na4W10O32/ZrO2 nanocomposite was ca.
24.88 wt%, estimated by ICP-AES analysis.
The crystal structure of the obtained nanocomposite was char-
acterized by a Bruker D8 Advance X-ray diffractometer using Cu K␣
radiation (ꢀ = 0.15418 nm). Optical absorption spectrum of the solid
photocatalyst was recorded in the range of 200–800 nm using Shi-
madzu UV-2401PC UV–vis spectrophotometer, fitted with a diffuse
reflectance chamber with inner surface of BaSO4. Infrared spectrum
was recorded on a Shimadzu system FT-IR 8400 spectrophotome-
ter using KBr pellet method. The morphology of nanocomposite
revealed by a scanning electron microscope (SEM, Philips XL-30)
and a transmission electron microscope (TEM, LEO-906E). The
TEM image of product was obtained at the accelerating voltage
of 200 kV. TEM sample was prepared by dropping the ethanol
dispersion on a carbon-coated copper grid. Specific surface area
was calculated by the BET method using N2 adsorption–desorption
experiments carried out at −196 ◦C on a Micromeritics ASAP 2010.
Before each measurement the sample was outgassed at 200 ◦C
for 3 h.
2. Experimental
All azo compounds were either purchased commercially or
synthesized as reported in the literature [48]. Alcohols, solvents
and reagents were purchased from Merck company in the high-
est purities available (≥98%) and used without further purification.
Photocatalytic reactions were performed with an osrum 400 W
high-pressure mercury lamp equipped with a cool water circu-
lating filter to absorb the near IR and a 320 nm cut-off filter
in order to avoid direct photolysis of organic substrates. The
total incident light flux was 6.88 × 10−4 Einestein min−1 as deter-
mined by ferrioxalate actinometry [49]. Na4W10O32 was prepared

S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
77
Table 1
Table 3
Photocatalytic reductive cleavage of azobenzene in various solvents.a
Photocatalytic reductive cleavage of azobenzene in the presence, of various alcohols
as the hydrogen donors.a.
Entry
Alcohol
Time (min)
Yieldb (%)
1
2
3
4
5
6
2-PrOH
EtOH
1-PrOH
1-BuOH
2-BuOH
tert-BuOH
35
35
60
60
60
60
93
90
56
45
55
∼0
.
Entry
Solvent
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
CH3CN
n-Hexane
Toluene
CHCl3
CH2Cl2
CH2ClCH2Cl
THF
35
80
70
90
90
60
60
60
93
67
58
48
52
56
∼0
∼0
a
Reaction conditions: azobenzene (0.5 mol L−1), Na4W10O32/ZrO2 (10 g L−1),
alcohol-acetonitrile (1:1 (v/v); 20 mL), N2 atmosphere, rt.
b
Yields are for isolated pure aniline.
H2O
Table 4
Recyclability of photocatalyst.a.
Reaction conditions: azobenzene (0.5 mol L−1), 2-propanol-solvent (1:1 (v/v);
a
20 mL), Na4W10O32/ZrO2 (10 g L−1), under N2 atmosphere at rt.
Cycle
0
1st
91
2nd
90
3rd
90
4th
88
b
Yields are for isolated pure aniline.
Yieldb (%)
93
Reaction conditions: azobenzene (0.5 mol L−1), Na4W10O32/ZrO2 (10 g L−1), 2-
propanol-acetonitrile (1:1 (v/v); 20 mL), N2 atmosphere, rt.
a
Table 2
The effect of amount of Na4W10O32/ZrO2 nanocomposite on photocatalytic reduc-
b
Yields are for isolated pure aniline.
tive cleavage of azobenzene.a.
Entry
Catalyst (g L−1
)
Time (min)
Yields (%)b
28
3. Results and discussion
1
2
3
4
5
6
7
8
2.5
5
7.5
10
12.5
15
60
60
60
35
35
35
35
60
54
62
93
93
94
90
5
3.1. Characterization of photocatalyst
The aim of this work was to examine the ability of photore-
4−
duced form of W10O32 polyanion in the catalytic transformation
17.5
0
of azobenzenes to the corresponding amines. For this pur-
pose, Na4W10O32/ZrO2 nanocomposite containing ca. 25 wt% of
Na4W10O32, was prepared through entrapment of Na4W10O32 into
a zirconia matrix via a sol–gel technique involving the hydrolysis
of zirconium (IV) n-butoxide, Zr(n-OBu)4, as the ZrO2 source. Dur-
ing the hydrolysis of Zr(n-OBu)4 to hydrous zirconia gel at pH 2,
the Na4W10O32 molecules are entrapped into the network of zir-
conia. In this process, Na4W10O32/ZrO2 nanocomposite is formed
via electrostatic interaction and hydrogen bonding. In electrostatic
interaction, ≡Zr–OH groups in zirconia are protonated in the acidic
medium to form ≡Zr–OH2+ groups, which should act as the counter
a
Reaction conditions: azobenzene (0.5 mol L−1), 2-propanol-acetonitrile (1:1
(v/v); 20 mL), N2 atmosphere, rt.
b
Yields are for isolated pure aniline.
2.3. General procedure for photocatalytic reductive cleavage of
azobenzenes over Na4W12O32/ZrO2 nanocomposite
In
a
Pyrex flask equipped with
a
magnet bar,
a solu-
tion of the azo compound (0.5 mol L−1
)
was prepared in
a
+
ions for polyanion, and give ≡Zr-OH2 W10O324−. This interaction
2-propanol–acetonitrile mixture (1:1 (v/v); 20 mL). To this solution
was added Na4W10O32/ZrO2 (10 g L−1). The pH of reaction suspen-
sion was adjusted at 2 with HClO4 to keep POM stable. The stirred
suspension was irradiated with a 400 W high-pressure mercury
lamp under nitrogen atmosphere. The temperature of suspension
was maintained at 25 2 ◦C by circulation of water through an
external cooling coil. After the completion of the reaction (moni-
tored by TLC, UV–vis and GC–MS), the photocatalyst was separated
by centrifuging and reaction mixture was washed with solvent.
The combined filtrate and washings were concentrated under vac-
uum. The residue was taken into 15 mL chloroform or ether, washed
twice with 15 mL saturated brine solution and finally washed with
water. The organic layer was dried over anhydrous magnesium sul-
fate and the solvent was removed using a rotary evaporator. For
further purification and separation of products, the residue was
purified either by preparative TLC or by column chromatography.
The results of initial experiments to obtain the reaction conditions,
solvent effect and recyclability of photocatalyst were summarized
in Tables 1–4. The general results were given in Table 5. All products
are commercially available and were identified through compari-
son of their physical and spectral data (m.p., TLC, IR and GC–MS)
with those of authentic samples.
also existed in Na4W10O32/SiO2 and H3PW12O40/SiO2 composites,
of which their preparation has been reported in literature [45].
The hydrogen bonding is formed in the nanocomposite between
the oxygen atoms of the W10O32
the zirconia network, which can be expressed in the forms of
4−
and the ≡Zr–OH groups of
W = Ot. . .HO–Zr and W–Ob. . .HO–Zr≡, where Ot and Ob refer to
the terminal and the bridge oxygen atoms, respectively, in the
4−
W10O32
unit. These two interactions would ensure fixation of
4−
the W10O32
unit into the ZrO2 support firmly, so that leaching
4−
W10O32 in the liquid phase may be avoided.
X-ray diffraction was used to identify the phase structures of
Na4W10O32/ZrO2 nanocomposite (Fig. 1). The XRD pattern of pure
ZrO2 prepared via hydrolysis of Zr(n-OBu)4 after calcining at 380 ◦C
is shown in Fig. 1a. The peaks at scattering angle (2Â) values of
30◦, 34.5◦, 50◦ and 60.1◦ correspond to the (1 1 1), (2 0 0), (2 2 0)
and (3 1 1) crystal planes of ZrO2 with tetragonal structure (JCPDS
file No. 17-0923), respectively. Fig. 1b recorded after calcining
the nanocomposite at 380 ◦C, which shows only the pattern cor-
responding to the tetragonal ZrO2 phase. No indication of any
crystalline phase related to Na4W10O32 appears. This result implies
4−
that the W10O32
units homogeneously disperse into the ZrO2
framework, which will be benefit to enhance the catalytic activ-
ity of nanocomposite. The broad diffraction peaks indicate that the
composite is composed of very small particles. The average size
of Na4W10O32/ZrO2 particles was calculated from XRD line broad-
ening using the Debye–Scherrer equation, D = (0.89ꢀ)/(ˇ1/2 cosÂ),
Irradiations in homogeneous solution were carried out dissolv-
ing Na4W10O32 (2.5 g L−1) in 2-propanol–CH3CN (1:1 (v/v); 20 mL)
and adding azo compound (0.5 mol L−1) under the experimental
conditions described for the heterogeneous systems.

78
S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
Table 5
Results of photocatalytic reductive cleavage of azo compounds with 2-propanol catalyzed by Na4W10O32/ZrO2.
Entry
1
Azo compound
Producta
Time (min)b
35
Yieldc (%)
93
2
3
45
42
92
90
4
48
88
5
6
45
50
90
85
7
8
55
40
78
85
9
35
44
86
84
10
11
56
79
12
13
40
35
84
83
14
40
78
15
16
46
40
76
82
17
18
19
45
40
50
86
90
86
84
90
20
48
88

S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
79
Table 5 (Continued )
Entry
Azo compound
Producta
Time (min)b
Yieldc (%)
21
22
23
24
25
26
50
40
46
30
30
34
92
94
76
84
92
88
90
86
86
92
90
92
84
27
30
88
92
28
40
a
All products are commercially available and were characterized through comparison of their physical and spectral data (m.p., TLC, FT-IR and GC–MS) with those of
authentic samples.
b
Irradiation time.
c
Isolated yields for symmetric azo compounds and GC–MS yields for unsymmetric azo compounds.
where ꢀ is the wavelength for Cu K␣ radiation, ˇ1/2 is the full width
at half maximum (FWHM) and  is the Bragg angle [53]. The aver-
age particle size calculated is about 5.3 nm which is in accordance
with the SEM and TEM observations (vide infra).
However, these bands have some shifts compared with those of
W10O32 [50], confirming that a strong chemical interaction, not
simple physical absorption, exists between this polyanion and zir-
conia network.
4−
4−
In order to confirm the connection of W10O32 cluster to the
The optical properties of the nanocomposite were character-
ized by UV–vis spectroscopy (Fig. 3). The electronic spectrum of
nanocomposite shows a broad and strong absorptive band in the
range from 200 to 500 nm with an absorption maximum at around
335 nm, which shows a red shift compared with pure Na4W10O32
(ꢀmax = 323 nm). It is clear that the nanocomposite can absorb the
light up to visible region. The spectrum allows us to estimate the
optical band gap (Eg) which to be 2.48 eV corresponding to absorp-
tion edge close to 500 nm. This indicates that the nanocomposite
prepared by this method could be a kind of photocatalytic material
with high activity.
zirconia matrix, the nanocomposite was analyzed by FT–IR spec-
troscopy (Fig. 2). In the FT–IR spectrum of the ZrO2 (Fig. 2a), there
are two strong absorptive bands at about 1625 and 1385 cm−1
which correspond to bending vibration of H–O–H and O. . .H–O
bonds. The broad and strong band at 600 cm−1 is attributed to
Zr–O stretching vibration of tetragonal ZrO2. In addition to these
bands, the FT-IR spectrum of Na4W10O32/ZrO2 nanocomposite
(Fig. 2b) shows bands at about 950 and 890, 800 cm−1 which can
be assigned to the stretching vibrations of W = Ot and W–Ob–W,
4−
respectively [50]. The IR characteristic bands of W10O32
in the
Na4W10O32/ZrO2 nanocomposite are similar with the data reported
for pure Na4W10O32 [50]. This indicates that the primary struc-
ture of POM is retained after immobilizing into the zirconia matrix.
The catalytic activity of composites is strongly dependent on the
shape, size and size distribution of the particles. Therefore, it is of
paramount importance to characterize the microstructure of the

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S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
Fig. 1. XRD patterns of (a) ZrO2 and (b) Na4W10O32/ZrO2 nanocomposite.
Fig. 4. SEM and TEM images of Na4W10O32/ZrO2 nanocomposite.
obtained composite. Fig. 4 shows the SEM and TEM images of the
Na4W10O32/ZrO2 powder. SEM image reveals that powder is com-
posed of aggregated extremely fine particles. From this image, it is
evident that particles have a narrow size distribution and homoge-
neous shape. TEM confirms that Na4W10O32/ZrO2 particles possess
semi-spherical morphology and a narrow distribution of sizes in the
range of 4–6 nm. This is consistent with the average size obtained
from the peak broadening in X-ray diffraction studied.
Fig. 2. FT-IR spectra of (a) ZrO2 and (b) Na4W10O32/ZrO2 nanocomposite.
The specific surface area of the Na4W10O32/ZrO2 nanocomposite
measured by the BET method to be 346 m2 g−1. The high spe-
cific surface area of Na4W10O32/ZrO2 nanocomposite is benefit for
improving its photocatalytic activity.
3.2. Photocatalytic activity
The photocatalytic activity of the Na4W10O32/ZrO2 nanocom-
posite was first tested via reductive cleavage of azobenzene (1) to
aniline (2) as a model. In order to select the best solvent, we exam-
ined a variety of solvents including acetonitrile, n-hexane, toluene,
chloroform, dichloromethane, 1,2-dichloroethane, THF and water
were investigated in the photocatalytic reduction of azobenzene
(0.5 mol L−1) into aniline using 2-propanol (10 mL) in the presence
of a catalytic amount of Na4W10O32/ZrO2 (10 g L−1) under N2 atmo-
sphere and at room temperature. Acetonitrile provided excellent
yields and proved to be the solvent of choice, whereas toluene
Fig. 3. UV–vis spectrum of Na4W10O32/ZrO2 nanocomposite.

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81
and n-hexane afforded lower yields. The reaction in chlorinated
solvents afforded the product in moderate yield whilst the use of
THF and water did not yield any product. After screening different
solvents, we observed that the best results in terms of yield and
time have been achieved in acetonitrile. This may be attributed
to photochemically inertness of acetonitrile and high solubility of
azobenzene in this solvent as compared to other solvents.
The effect of photocatalyst concentration was also investigated
on the model reaction. As shown in Table 2, the yield of aniline
was increased when the amount of photocatalyst was increased
from 2.5 to 10 g L−1 but the yield remained almost same with fur-
ther increase of catalyst concentration up to 17.5 g L−1 (Table 2;
entries 1–7). It is concluded that the concentration of the pho-
tocatalyst has a significant influence on the efficiency. A higher
efficiency of photocatalytic reduction of azobenzene was obtained
when the concentration of Na4W10O32/ZrO2 was increased from
2.5 to 10 g L−1. More photocatalysts generate more reactive sites
for reaction. However, the efficiency was not enhanced when the
Fig. 5. Variation of UV–vis spectrum of a solution of azobenzene with illumina-
tion time. (azobenzene 0.5 mol L−1, Na4W10O32/ZrO2 10 g L−1, irradiation time is
indicated on spectra).
concentration of photocatalyst was increased from 10 to 17.5 g L−1
.
The saturating photoactivity with increasing Na4W10O32/ZrO2 con-
centration could be understood as the competition between the
surface area and the light scattering loss. While Na4W10O32/ZrO2
with higher concentration provides more active catalytic sites, the
light penetration depth into the suspension decreases due to the
increased light scattering. This reduces the efficiency of the photo-
catalytic reaction. An optimum of 10 g L−1 catalyst in the reaction
mixture is ideal for achieving the best yield. The essential role
played by the photocatalyst is evident from the extremely low
GC-yield (5%) of aniline when omitted from the reaction mixture
(Table 2; entry 8).
In the catalytic reduction of azo compounds the choice of hydro-
gen donor is crucial. In addition to ethanol, the 1:1 (v/v) mixture
of several alcohols with acetonitrile was also investigated as the
hydrogen donors (Table 3). As can be seen in Table 3, only ethanol
was effective and other alcohols required a longer reaction time and
gave a decreased yield. With tertiary butyl alcohol, no observable
reduction occurred and the starting azo compound was recovered
unchanged. This finding confirms that the type of alcohol as the
hydrogen source plays an important role in the rate of reduced
POM production and hence in the reaction rate and yield. The 1:1
ethanol–acetonitrile mixture was chosen as the reducing system in
which higher aniline yield was observed.
than the pure Na4W10O32. (2) The addition of Na4W10O32 nanopar-
ticles to ZrO2 can extend the photoresponse of ZrO2 semiconductor
toward the visible region, and thus increase the efficiency of utiliz-
ing solar energy. (3) In the composite, a coupled effect can exist
between the Na4W10O32 and ZrO2 semiconductor energy bands
due to some differences in their band-gap positions. This possible
coupling effect can play a role in promoting the charge separation of
the generated carriers and interfacial charge transfer, and therefore
improve the photoactivity of Na4W10O32 and ZrO2 semiconductor
Fig. 5 shows the UV–vis absorption spectrum changes of azoben-
zene as a function of irradiation time over Na4W10O32/ZrO2
nanocomposite under the optimized conditions. The azobenzene
has a major absorption peak at about 316 nm, which is attributed
to the conjugated structure (the N N bond). The absorption value
at 316 nm was decreased with the photocatalytic process and the
absorption peak was not obvious after 35 min reaction. It con-
firms that almost complete reduction of azobenzene is achieved
after 35 min light irradiation. The lowering of the absorption peaks
at 316 nm for azobenzene, characteristic of the N N bond, is
attributed to the break of the azo bond. On the other hand, this
result was further confirmed by GC–MS measurement, that is, the
intensity of the chromatography signal of azobenzene disappeared
after 35 min of irradiation.
Recovery of the Na4W10O32/ZrO2 is easy; that is, this photo-
catalyst is separated by centrifuging after the reaction. At the same
time, the concentration of W in the filtrate was founded less than 1%
by ICP-AES. Indeed, the recovered photocatalyst was used for recy-
cling and the deactivation of Na4W10O32/ZrO2 was hardly observed
in the reduction of 1 even after four catalytic cycles (Table 4).
On the other hand, when the photocatalyst was separated from
the reaction mixture shortly (15 min) after the beginning of irradi-
ation and the reaction filtrate was further irradiated under N2, no
extra formation of 2 was observed via GC–MS analysis even after
1 h. All these findings confirm that the present photoreaction cat-
alyzed by Na4W10O32/ZrO2 is heterogeneous in nature. Therefore,
although during this study we observed that the pure Na4W10O32
in homogeneous system also catalyzes the reduction of azo com-
pounds, our results focused on reduction these substrates in the
presence of Na4W10O32/ZrO2 system that permits recycling and
No reduction products were observed when blank experi-
ments were carried out in the absence of Na4W10O32/ZrO2. On
the other hand, when irradiation of azobenzene (0.5 mol L−1
)
in 2-propanol–acetonitrile mixture (1:1 (v/v); 20 mL) over
Na4W10O32/ZrO2 (10 g L−1) was carried out under O2 atmosphere
instead of N2, aniline was formed but in reduced yield (1 h, 48%).
Also, using pure Na4W10O32 (2.5 g L−1) instead of Na4W10O32/ZrO2
(10 g L−1), we found that 86% of 2 were obtained within 1.25 h
under the same conditions. On the other hand, when the pure
ZrO2 (7.55 g L−1) support was used as a photocatalyst for reductive
cleavage of 1, less than 8% of 2 were formed after 1 h irradia-
tion and about 88% of the starting azobenzene was recovered. It
is evident that the pure ZrO2 has little photocatalytic activity for
the azobenzene reduction, whereas pure Na4W10O32 nanoparti-
cles, as well as Na4W10O32/ZrO2 nanocomposite, are quite efficient
photocatalysts. From these findings, it is concluded that the pho-
tocatalytic activity of the Na4W10O32/ZrO2 system is mainly due
to Na4W10O32. Moreover, the Na4W10O32/ZrO2 nanocomposite is
more efficient than the pure Na4W10O32 for the azobenzene reduc-
tion. This is mainly due to higher specific surface area of the
Na4W10O32/ZrO2 nanocomposite than the starting Na4W10O32. The
enhanced photoactivity of the Na4W10O32/ZrO2 composite is not
well understood. We think that this might be related to the follow-
ing factors: (1) The nanocomposite has higher specific surface area
reuse of Na4W10O32
.
Under optimized reaction conditions, the photocatalytic reduc-
tive cleavage of azobenzenes in the presence of Na4W10O32/ZrO2
was investigated. As indicated in Table 5, this new photocat-
alytic system (Na4W10O32/ZrO2/2-PrOH/hv) cleaved with ease a
wide variety of symmetrical and unsymmetrical azobenzenes.
These compounds were selectively converted to the corresponding

82
S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
amines in high to excellent yields (76–94%) within short reac-
tion times (30–55 min). In all cases, azo aromatic compounds
with substituents carrying either electron-donating or electron-
withdrawing groups reacted successfully and gave the products
in high yields. Although, the presence of electron-donating or
electron-withdrawing groups on the aromatic rings did not have
an appreciable effect on the reaction times and yields, however, the
activity was slightly influenced by the nature/position of the sub-
stituents on the aromatic ring. It was shown that the azobenzenes
with electron-withdrawing groups reacted faster than the azoben-
zenes with electron-donating groups as would be expected. In some
cases, the rate enhancement due to electronic effects was substan-
tial. For example, the reduction of 4,4ꢀ-dinitroazobenzene (Table 5;
entry 13) was completed within shorter reaction time in compari-
son with azobenzene with electron-withdrawing substituents, e.g.
4,4ꢀ-dimethoxyazobenzene (Table 5, entry 6) required longer reac-
tion times and increasing the irradiation time did not improve the
yields. The electronic effect observed for symmetric azobenzenes
also seems to be prevalent in the case of unsymmetric ones. While
4-iodo and 4-methoxyazobenzenes (Table 5; entries 20 and 21)
required longer reaction times, 4-cyano and 4-nirtoazobenzenes
(Table 5; entries 26 and 27) were reduced into the corresponding
amines in excellent yields within shorter reaction times.
Scheme 1. The proposed photocatalytic cycle for reductive cleavage of azobenzenes
with 2-propanol over Na4W10O32/ZrO2 nanocomposite.
at room temperature in a short time in the presence of a cat-
alytic amount of Na4W10O32/ZrO2 without affecting of reducible or
hydrogenolyzable substituents such as –NO2, –OH, –CH3, –OCH3,
–COOH, –COCH3, halogen and –CN.
The experiments, usually performed on
a 10 mmol scale,
can be scaled up to 100 mmol without difficulties under same
reaction conditions. For example, a 50 mmol reaction of 4,4ꢀ-
dimethoxyazobenzene provided the corresponding amine in 95%
isolated yield and a 100 mmol reaction of 4,4ꢀ-dichloroazobenzene
gave 4-chloroaniline in 92% isolated yield under the present
method.
However, it seems that the efficiency of this photocatalytic
process was significantly influenced by steric factor rather than
electronic factor, so that the number and position of the sub-
Although the details of the mechanism is unknown and inter-
mediates were not observed directly, the our observations in this
study together with literature data [16–45], support which the
events following light absorption by W10O324− in Na4W10O32/ZrO2
nanocomposite can be explained via the photocatalytic cycle in
Scheme 1. According with this cycle, illumination of W10O324− gen-
erates a ligand-to-metal charge transfer excited state, W10O324−*,
which acts as a strong oxidant. In the presence of 2-propanol as
a readily available hydrogen donor, W10O324−* is reduced by a
hydrogen atom transfer to give the one-electron-reduced form
stituents on the aromatic rings play
a crucial role on the
reaction times and yields. The following observations confirm
this statement: (i) among various azobenzenes, unsubstituted
or mono-substituted azobenzenes groups reacted faster than
other ones, (ii) an increase of the reaction rate was observed
with azobenzenes bearing substituent groups in their para-
position whereas the presence of a substituent group, ortho
or meta to the N N functional group, decreased the reaction
rate and yield. (iii) Indeed, 2,2ꢀ,4,4ꢀ-tetramethoxy- and 2,2ꢀ,4,4ꢀ-
tetrachloroazobenzenes afforded lower yields when compared
with most of other azobenzenes (Table 5; entries 7 and 11). This
clearly confirms the crucial role played by the steric factors.
Also, the above findings confirm that the adsorption of azo
compound on the catalyst surface and the interaction of their
(H+ + W10O32 or HW10O324−) [52]. Finally, this reduced species
5−
acts as a strong reducing agent, resulted in the reductive cleavage
of the N N bond to –NH2.
It seems most likely that the primary reaction between
W10O324−* and 2-propanol involves hydrogen-atom abstraction
prior to electron transfer according to the following reactions.
N
N bonds with Na4W10O32/ZrO2 photocatalyst is an important
step of this photocatalytic reaction. Thus, the N N bond in parent
azobenzene, para-substituted and mono-substituted azobenzenes
can easily be adsorbed on the catalyst surface and provided bet-
ter yields of the product when compared to other azobenzenes.
Moreover, high efficiency of Na4W10O32/ZrO2 photocatalyst than
the pure Na4W10O32 can be explained based on these observations.
The former has a very high specific surface area which provides
more contact area for the photocatalytic reaction.
(CH3)2CHOH+W10O43-2∗ → (CH3)2C–OH+W10O35−2 + H+
˙
4-∗
(CH3) C–OH+W
O
→ (CH3)2C = O+W10O532− + H+
˙
10
2
32
High efficiency of 2-propanol in comparison with 1-propanol
is also consistent with a hydrogen-atom abstraction mechanism
(Table 2, entries 1 and 3). Further evidence supporting the above
mechanism was drawn from the fact that when tertiary butyl alco-
hol which possess no ␣-H atom, was used instead of 2-propanol,
no reduction occurred and the starting azo compound was recov-
ered unchanged (Table 2, entry 6). Indeed, the reported deuterium
isotope effects in the photocatalytic oxidation of alcohols by
decatungstate provide strong evidence for a stepwise mechanism,
with a hydrogen-atom abstraction in the rate-determining step
[52].
On the basis of the above observations and the characteristics
of Na4W10O32/ZrO2 nanocomposite, we infer that the following
factors could contribute to the increased photocatalytic efficiency
of this nanocomposite compared with Na4W10O32 alone. On the
one hand, relatively narrow band gap of the nanocomposite and
increase the amount of photochemistry at longer wavelengths play
an important role to this enhanced photocatalytic activity (see
Fig. 3). Therefore, the O → W CT excitation becomes easier, resulting
in more excited states to precipitate in the photocatalytic reaction.
On the other hand, ZrO2 support is a semiconductor and used as
In all cases, the isolated products were characterized by com-
parison of their melting points, TLC, GC–MS and FT-IR spectra
with authentic samples. The disappearance of a strong absorption
band between 1630 and 1575 cm−1 due to the N N stretching and
the appearance of a strong absorption band between 3500 and
3300 cm−1 due to the –NH2 group clearly showed that the azo
compounds had been cleaved into their constituent amines. Fur-
thermore, there was no absorption between 2290 and 2440 cm−1
,
which clearly indicated the absence of the hydrazo (–NH–NH–)
group. The appearance of one spot in the TLC, in the case of symmet-
rical azo compounds, and two spots, in the case of unsymmetrical
azo compounds, clearly confirmed that no hydrazo compounds
were formed during the reductive cleavage of the azo compounds.
Furthermore, Na4W10O32/ZrO2/2-propanol/hv system is more
effective than other reported methods. In contrast with most
of the previously methods [7–12] which require longer reaction
times and high temperatures, the present method accomplishes

S. Farhadi, S. Sepahvand / Journal of Molecular Catalysis A: Chemical 318 (2010) 75–84
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Scheme 2. The synergistic effect between ZrO2 semiconductor and photoactivated
Na4W10O32 in nanocomposite.
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between ZrO2 semiconductor and photoactivated POM as shown
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