Photocatalytic reduction of nitro aromatic compounds to amines using a nanosized highly active CdS photocatalyst under sunlight and blue LED irradiation

Article abstract of DOI:10.1515/chempap-2015-0231

A variety of aromatic nitro compounds were chemoselectively reduced to the corresponding anilines using conveniently prepared nanosized CdS as a photocatalyst under the sunlight and blue LED irradiation. The results demonstrated that synthesized CdS nanostructures have the potential to provide a promising visible light driven photocatalyst for chemoselective reduction of nitro aromatics in the presence of nitrile and carbonyl groups to the corresponding amines under both sunlight and blue LED irradiation. Photoreduction of nitro aromatics by the prepared nanosized CdS with high surface area was faster than when using the commercially available CdS under both sunlight and LED irradiation. Nanosized CdS photocatalyst was prepared by a simple method without any capping agent. X-ray diffraction (XRD), energy dispersive spectrometry (EDAX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N₂ absorption-desorption, diffuse reflectance spectroscopy (DRS), and flat band potential methods were employed for the characterization, which revealed that the prepared CdS nanoparticles have a well-resolved cubic structure with the size of around 10-30 nm and a band gap of 2.37 eV.

Full text of DOI:10.1515/chempap-2015-0231

Chemical Papers
DOI: 10.1515/chempap-2015-0231
ORIGINAL PAPER
Photocatalytic reduction of nitro aromatic compounds to amines
using a nanosized highly active CdS photocatalyst under sunlight
and blue LED irradiation
a
a,b
Ali Asghar Safari, Foad Kazemi*
aDepartment of Chemistry, bCenter for Climate and Global Warming (CCGW), Institute for Advanced Studies in Basic
Sciences (IASBS), Gava Zang, Zanjan, 45137-66731, Iran
Received 28 January 2015; Revised 12 August 2015; Accepted 22 August 2015
A variety of aromatic nitro compounds were chemoselectively reduced to the corresponding ani-
lines using conveniently prepared nanosized CdS as a photocatalyst under the sunlight and blue
LED irradiation. The results demonstrated that synthesized CdS nanostructures have the poten-
tial to provide a promising visible light driven photocatalyst for chemoselective reduction of nitro
aromatics in the presence of nitrile and carbonyl groups to the corresponding amines under both sun-
light and blue LED irradiation. Photoreduction of nitro aromatics by the prepared nanosized CdS
with high surface area was faster than when using the commercially available CdS under both sun-
light and LED irradiation. Nanosized CdS photocatalyst was prepared by a simple method without
any capping agent. X-ray diffraction (XRD), energy dispersive spectrometry (EDAX), transmis-
2
sion electron microscopy (TEM), scanning electron microscopy (SEM), N absorption–desorption,
diffuse reflectance spectroscopy (DRS), and flat band potential methods were employed for the
characterization, which revealed that the prepared CdS nanoparticles have a well-resolved cubic
structure with the size of around 10–30 nm and a band gap of 2.37 eV.
ꢀ
c 2015 Institute of Chemistry, Slovak Academy of Sciences
Keywords: photoreduction, nanosized CdS, aromatic nitro compounds, LED, sunlight
Introduction
under visible light irradiation (In et al., 2011; Schultz
Yoon, 2014). In this connection, CdS with a suit-
&
Synthesis of organic compounds via heterogeneous
photocatalysis using semiconductor materials is an un-
conventional technology and an attractive field for
many scientists (Abedi et al., 2013; Herrmann, 2005;
Kakroudi et al., 2014; Ohtani et al., 2003; Wang et
al., 2013). Recently, design and synthesis of new vis-
ible light-driven photocatalysts for organic transfor-
mations have attracted the attention of researchers
because of their uncomplicated application, low costs
and favorable environmental aspects of sunlight and
visible light (Ananthakrishnan & Gazi, 2012; Prier et
al., 2013).
able band gap (2.4 eV) corresponding well with the
spectrum of sunlight has been studied for many pho-
tocatalytic applications such as sensors (Wang et al.,
2003), solar cells (Britt & Ferekides, 1993; Chun et al.,
2011), hydrogen evolution (Bao et al., 2008; Li et al.,
2011), and pollutants decontamination (Veréb et al.,
2012).
Aromatic amines are important key intermediates
in the production of dyes, antioxidants, pharmaceu-
tics, and agricultural chemicals which can be obtained
by the reduction of aromatic nitro compounds (Ku-
mar et al., 2001). Applications of heterogeneous pho-
tocatalysis for chemoselective reduction of nitro aro-
matic compounds to the corresponding amines have
also been studied using a variety of semiconductors
Yet, the stringent UV light limitations in semicon-
ductors caused by the wide band gap such as in TiO2
motivate the researchers to find new reaction routes
*Corresponding author, e-mail: kazemi f@iasbs.ac.ir
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and modified semiconductors such as: TiO2 (P-25 and
rutile) (Flores et al., 2007; Imamura et al., 2011; Kaur
&
Pal, 2014), N-doped TiO2 (Wang et al., 2009) or
dye-sensitized TiO2-P25 (F u¨ ldner et al., 2010, 2011),
TiO2 co-catalyst (Hakki et al., 2009), Ru(II) com-
plex immobilized ZnO hybrid in the presence of a
Pt(II) co-catalyst (Bhar & Ananthakrishnan, 2015),
and ZnS nanostructures (Pal & Pal, 2015). Low band
gap, which leads to fast recombination of photogen-
erated electron–hole pairs, is the main limitation of
the commercial CdS particles and limits their applica-
tion in semiconductor photocatalysis (Li et al., 2011;
Sheeney-Haj-Ichia et al., 2005). Employing the nano-
sized CdS is one of the approaches developed in order
to widen the CdS band gap and avoid the fast re-
combination of electron–hole pairs (Eskandari et al.,
Fig. 1. X-ray powder diffraction patterns of CdS-NP.
2
013).
Very recently, chemoselective photocatalytic re-
blue LED (4 × 1 W, λ ≥ 420 nm, intensity: 80 lu-
duction of nitroaromatics by various nanostructured
photocatalysts under visible blue LED irradiation has
been reported (Eskandari et al., 2014; Kakroudi et al.,
men) or sunlight (of daily ambient temperature and
3
sunlight intensity range of 80–100 × 10 lux).
The reaction conversion was monitored by thin
layer chromatography (TLC). After completing the
reaction, the mixture was centrifuged and super-
natant was removed and analyzed on a GC Alient
gas chromatograph (Nonpolar CP-Sil 8 column (30
m × 0.32 mm), Varian Chrompack (Middelburg, The
Netherlands).
2
014; Zand et al., 2014). In continuation of our studies,
considering the great importance of visible light appli-
cations in photocatalytic reactions as a green method,
chemoselective reduction of nitro aromatics to the cor-
responding amines was also investigated using a sim-
ply prepared nanosized CdS (CdS-NP) photocatalyst
in propan-2-ol as an appropriate solvent under the
sunlight and blue LED irradiation. Photocatalytic ac-
tivity of the catalyst was compared with that of com-
mercial Aldrich CdS (CdS-Ald) as a typically available
photocatalyst.
Results and discussion
In continuation of our previous studies on the en-
hancement of the nitroaromatics photocatalytic re-
duction yield using CdS based photocatalysts, highly
photoactive CdS nanoparticles prepared according to
literature (Hopfner et al., 2002).
Experimental
All solvents, sodium sulfide (Na2S), nitro com-
pounds, and cadmium sulfate (CdSO4 · 8/3H2O) were
purchased from Merck (Germany). Cadmium sulfide
Nanosized CdS was synthesized using the very easy
method and was able to effectively reduce high concen-
trations of nitroaromatic compounds (0.01 M) via blue
LED and sunlight irradiation in the absence of any ad-
ditives (ammonim formate, oxalates, . . .) in short reac-
tion times compared with other reported CdS based
photocatalysts at the same reaction conditions (Es-
kandari et al., 2013, 2014).
(CdS-Ald) was supplied by Sigma–Aldrich (UK). All
other chemicals were used as received without further
purification.
CdS nanoparticles were prepared according to lit-
erature (Hopfner et al., 2002) and were characterized
in detail by TEM, porosimetry, powder XRD, dif-
fuse reflectance spectra (DRS), and EDAX elemental
analysis. Band edge positions of both CdSs were es-
timated by measuring the photovoltage as a function
of the suspension pH according to the reports of Roy
et al. (1995) and White and Bard (1985). The sur-
face area and pore volume of the photocatalysts were
determined from the nitrogen adsorption–desorption
isotherm by the BET and BJH methods, respectively.
As optimized reaction conditions, a solution of ni-
troaromatic compounds (0.01 M) in an appropriate
solvent and 20 mg of CdS-NP were sonicated and
slowly purged with N2 for 5 min. Then, the reaction
vessel was sealed up with a rubber stopper and the
mixture was stirred magnetically and irradiated with
XRD pattern of the prepared CdS nanoparticles
is shown in Fig. 1. According to the main diffraction
peaks it can be well matched with a standard cubic
zinc blend structure (JCPDS card no. 80-0019).
SEM and TEM images indicated inhomogeneous
dispersion of the prepared CdS nanoparticles within
the diameter range of 10–30 nm (Fig. 2). According
to the TEM image, CdS-Ald (commercial CdS) is ag-
glomerated to the size of more than 100 nm (Fig. 2).
EDAX measurements of the nanoparticles indi-
cated the presence of both cadmium and sulfur ele-
ments in a roughly 1 : 1 mole ratio. In order to de-
termine the band gap energy of CdS-Ald and the pre-
pared CdS-NP, the modified Kubelka–Munk function
2
[F(R∞)hυ] was plotted vs. the incident photon en-
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iii
Fig. 2. SEM (a) and TEM (b) image of prepared CdS, and TEM image of CdS-Ald (c).
Fig. 3. Diffuse reflectance spectra of CdS-NP (a) and CdS-Ald (b).
ergy considering diffuse reflectance spectroscopic anal-
ysis data (Fig. 3, Table 1). As shown in Table 1, band
gap energy of commercial CdS-Ald is 2.31 eV whereas
that of the prepared CdS-NP was by 0.06 eV higher
and CdS-Ald were determined from the nitrogen
adsorption–desorption isotherm by the BET and BJH
methods, respectively (Table 1).
For the primary reaction test, the photocatalytic
reduction of nitrobenzene to aniline using CdS-NP
was investigated and the optimum reaction condi-
tions were determined. Typically, photoreduction of
nitrobenzene using CdS under blue LED irradiation
was investigated to determine the optimum concentra-
tion of the photocatalyst and a suitable solvent (Ta-
ble 2).
Our present as well as previous studies (Eskandari
et al., 2014) showed that propan-2-ol is the best sol-
vent for photocatalytic reduction of nitrobenzene to
aniline considering ethanol, methanol and propan-2-ol
(Table 2, entries 4, 6, and 7) under the same conditions
and in the presence of CdS as the photocatalyst.
Under optimized reaction conditions, the results
showed that both CdS-NP and CdS-Ald photocata-
lysts converted nitrobenzene to aniline by 91 % and
46 %, respectively (Table 2, entries 4 and 8), and no
product was detected under catalyst free or dark con-
ditions (Table 2, entries 11 and 12).
(
2.37 eV) due to the quantum size effect (Eskandari
et al., 2014).
Band edge positions of both prepared and com-
mercial CdSs were obtained by measuring the photo-
voltage as a function of suspension pH according to
literature (Table 1) (Roy et al., 1995; White & Bard,
1
–
985). The band edge positions were determined as
0.38 and –0.42 for CdS-NP and CdS-Ald, respec-
tively (Table 1). Since the flat band potential could
be measured only in aqueous suspension, it was ap-
proximately estimated that the value in propan-2-ol
(
solvent used in the photoreduction reaction) changes
similarly at both CdS band edge positions (Hopfner
et al., 2002; Roy et al., 1995; White & Bard, 1985).
The estimated valence band of CdS-NP and
CdS-Ald was calculated by subtracting the band gaps
from the band edge values (EFb – Ebg) (Table 1)
(Hopfner et al., 2002).
The surface area and pore volume of CdS-NP
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Table 1. Characterization, band gap energies, and photoelectrochemical data
a,b
bg
a,b,c
Fb
Surface aread
Pore volumee
E
E
vs. NHE
V
Evb = (EFb – Ebg) vs. NHE
Sample
m2
g
−1
cm3
g
−1
eV
V
CdS-NP
CdS-Ald
2.37
2.31
–0.38
–0.42
1.99
1.88
76.1
23.4
0.162
0.049
a) Mean value of three independent measurements; b) reproducibility was better than ± 0.01 eV; c) measured according to Roy et
al. (1995) and calculated for pH 7; d) BET surface area; e) BJH pore volume.
Fig. 4. Photocatalytic reduction of nitroaromatics under LED and sunlight irradiation.
Table 2. Photoreduction of nitrobenzene to aniline in the pres-
troaromatics is almost similar in the presence of both
photocatalysts. On the other hand, the valence band
potential position (Evb) of CdS-NP is by 0.11 eV
more positive than CdS-Alds. This observation sug-
gests that the valence band of CdS-NP has stronger
holes for oxidation of the sacrificial solvent compared
with CdS-Ald when using the proposed mechanism
ence of CdS under LED irradiation
(Eskandari et al., 2014). On the other hand, surface
b
area and pore volumes of CdS-NP are by approxi-
mately 3.3 fold higher than those of CdS-Ald (Ta-
ble 1). High surface area of CdS-NP compared with
that of CdS-Ald allows high adsorption of reactants
and enhances the speed of the photocatalytic reaction.
All results of the photocatalytic reactions (Fig. 4) un-
der sunlight and LED irradiation are shown in Table 3.
As shown in Table 3 a variety of aromatic ni-
tro compounds including electron-rich and electron-
deficient substrates proceeded readily under blue LED
and sunlight irradiation and simple experimental con-
ditions to afford good to excellent yields of the corre-
sponding amines.
Entry
1
CdS-NP/mg
8
Solvent
Yield /%
Propan-2-ol
56
2
3
4
5
6
7
12
16
20
24
20
20
20
20
20
0
Propan-2-ol
Propan-2-ol
Propan-2-ol
Propan-2-ol
Methanol
74
87
91
86
48
45
46
89
58
0
Ethanol
c
8
Propan-2-ol
Propan-2-ol
Propan-2-ol
Propan-2-ol
Propan-2-ol
d
9
c,d
e
10
11
12
f
20
0
Under blue LED irradiation, the yield depends
strongly on the electron withdrawing or electron do-
nating nature of the functional groups in the aromatic
rings. The reactions generally take a long time (5–24
h) to complete whereas the same reaction under solar
irradiation decrease to 45–120 min (Table 3).
a) Blue LED (4 × 1 W), 80 lumen; b) GC yield; c) CdS-Ald
as photocatalyst; d) sunlight irradiation (in the range of 80–
3
1
00 × 10 lux) in 60 min; e) without catalyst; f ) in dark.
Reactions under sunlight irradiation were also car-
Nitro compounds with electron withdrawing
groups (CN, NO2, COR) provided high yields in
short reaction times compared with those containing
electron-rich substituents (Me, NH2) (Table 3). Dur-
ing the reactions under both LED and sunlight irradia-
tion, the CN, COR, and CH2OH functionalities in the
aromatic ring remained unaffected (Table 3, entries 2,
3, 4, and 11). As shown in Table 3, sunlight affords
the photoreduction of nitro compounds in high yields
in short times in comparison with the photoreduction
under LED irradiation. Photocatalytic reduction of
1,4-dinitrobenzene under LED irradiation showed of
ried out at the same optimized condition as those un-
der LED irradiation. The reaction under sunlight ir-
radiation was found to be faster than under the LED
irradiation (Table 2, entries 9 and 10).
The reaction of nitroarobenzene using the
CdS-NP photocatalyst under sunlight irradiation was
also found to be faster than when CdS-Ald was used
(
Table 2, entries 9 and 10).
As shown in Table 1, conduction band positions of
two photocatalysts are approximately close together.
It seems that the thermodynamic reduction of ni-
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Table 3. Photocatalytic reduction of nitroaromatics to corresponding amines using CdS-NP photocatalyst under blue LED and
sunlight irradiation in propan-2-ol
Entry
1
Starting material
Product
Time/h (Yield/%)a,c
6 (91)
Time/min (Yield/%)b,c
60 (89)
2
3
5 (98)
8 (87)
45 (99)
60 (92)
4
5
6 (93)
8 (78)
60 (95)
60 (91)
6
7
10 (80)
14 (77)
70 (93)
120 (85)
2
4 (72)
4 (11)
120 (4)
8
9
2
120 (84)
8 (85)
60 (90)
1
0
1
24 (21)
10 (63)
120 (70)
70 (71)
1
3
a) Blue LED (4 × 1 W, 80 lumen); b) sunlight irradiation (in the range of 80–100 × 10 lux); c) GC yield.
1
,4-diaminobenzene and 4-nitroaniline yields of 11 %
Table 4. Reusability of CdS-NP in the reduction of nitroben-
zene to aniline under sunlight and blue LEDs irradi-
ation in propan-2-ol
and 72 %, respectively, in 24 h. The same reaction
under sunlight irradiation yielded 84 % and 4 % of
1
,4-diaminobenzene and 4-nitroaniline, respectively
Run
1
2
3
4
(Table 3, entry 8). The data clearly show that the re-
duction of 1,4-dinitrobenzene to 4-nitroaniline under
LED irradiation can be done chemoselectively.
Reusability of the CdS-NP photocatalyst was also
investigated by the photocatalytic reduction of ni-
trobenzene as a test model. After each run, the photo-
catalyst was isolated by centrifugation, washed thor-
oughly with propan-2-ol (2 × 2 mL) and used in a new
run.
Conditions
Yield/%
LED
Sunlight
91
90
88
63
75
30
51
13
radiation, respectively. As shown in Table 4 after four
runs, the yields decreased to 51 % and 13 % under
LED and sunlight irradiation, respectively. Results of
Reusability of CdS-NP was also investigated for a
6
0 min and 6 h reaction under sunlight and LED ir-
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atomic absorption spectroscopy showed the presence
Convenient preparation of CdS nanostructures as a highly ef-
ficient photocatalyst under blue LED and solar light irradia-
tion. Separation and Purification Technology, 120, 180–185.
DOI: 10.1016/j.seppur.2013.09.039.
Eskandari, P., Kazemi, F., & Zand, Z. (2014). Photocatalytic
reduction of aromatic nitro compounds using CdS nanos-
tructure under blue LED irradiation. Journal of Photo-
chemistry and Photobiology A: Chemistry, 274, 7–12. DOI:
10.1016/j.jphotochem.2013.09.011.
Flores, S. O., Rios-Bernij, O., Valenzuela, M. A., Córdova, I.,
Gómez, R., & Gutiérrez, R. (2007). Photocatalytic reduction
of nitrobenzene over titanium dioxide: by-product identifica-
tion and possible pathways. Topics in Catalysis, 44, 507–511.
DOI: 10.1007/s11244-006-0098-2.
2+
of low amounts of Cd in the centrifuged solution af-
ter each reaction run under sunlight irradiation that
2+
can be attributed to the release of Cd via photo-
+
2+
corossion reaction (CdS + h → Cd + S) causing
a decrease of the reaction yield. On the other hand,
diffuse reflectance spectroscopy results with CdS-NP
revealed that the photocatalyst band gap energies re-
mained unaffected after four runs under sunlight and
LED irradiation.
Conclusions
F u¨ ldner, S., Mild, R., Siegmund, H. I., Schroeder, J. A., Gruber,
M., & K ¨o nig, B. (2010). Green-light photocatalytic reduction
using dye-sensitized TiO2 and transition metal nanoparticles.
Green Chemistry, 12, 400–406. DOI: 10.1039/b918140g.
F u¨ ldner, S., Mitkina, T., Trottmann, T., Frimberger, A., Gru-
ber, M., & K ¨o nig, B. (2011). Urea derivatives enhance
the photocatalytic activity of dye-modified titanium diox-
ide. Photochemical & Photobiological Sciences, 10, 623–625.
DOI: 10.1039/c0pp00374c.
In conclusion, we have demonstrated that simply
prepared nanosized CdS is an efficient heterogeneous
photocatalyst for the photocatalytic reduction of a va-
riety of nitro aromatics containing electron withdraw-
ing and donating groups to the corresponding amines
under sunlight and blue LEDs irradiation. Photocat-
alytic reduction of amines in sunlight was faster than
that under LEDs irradiation, but the reusability of the
catalyst under LED irradiation was found to be better
than sunlight.
Hakki, A., Dillert, R., & Bahnemann, D. (2009). Photo-
catalytic conversion of nitroaromatic compounds in the
presence of TiO2. Catalysis Today, 144, 154–159. DOI:
10.1016/j.cattod.2009.01.029.
Photocatalytic reaction under sunlight and LED
irradiation showed that CdS-NP is a highly efficient
catalyst for the reduction of nitrobenzenes to the cor-
responding anilines compared with CdS-Ald. High sur-
face area, wide band gap and positive values of the
CdS-NP valence band potential enhanced the photo-
catalytic ability of CdS-NP vs. CdS-Ald.
Herrmann, J. M. (2005). Heterogeneous photocatalysis: state
of the art and present applications. Topics in Catalysis, 34,
49–65. DOI: 10.1007/s11244-005-3788-2.
Hopfner, M., Weiß, H., Meissner, D., Heinemann, F. W., &
Kisch, H. (2002). Semiconductor photocatalysis type B: syn-
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