Selective photocatalytic reductions of nitrobenzene derivatives using PbBiO2X and blue light

Article abstract of DOI:10.1039/c0gc00857e

Blue light irradiation of heterogeneous photocatalysts PbBiO₂X (X = Cl, Br) in the presence of triethanolamine as an electron donor leads to hydrogen evolution, and the selective, clean and complete reduction of nitrobenzene derivatives to their corresponding anilines. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00857e

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PAPER
Selective photocatalytic reductions of nitrobenzene derivatives using
PbBiO X and blue light†
2
a
b
a
b
b
Stefan F u¨ ldner, Patrick Pohla, Hanna Bartling, Stephan Dankesreiter, Roland Stadler,
c
b
a
Michael Gruber, Arno Pfitzner* and Burkhard K o¨ nig*
Received 28th November 2010, Accepted 5th January 2011
DOI: 10.1039/c0gc00857e
Blue light irradiation of heterogeneous photocatalysts PbBiO X (X = Cl, Br) in the presence of
2
triethanolamine as an electron donor leads to hydrogen evolution, and the selective, clean and
complete reduction of nitrobenzene derivatives to their corresponding anilines.
Sunlight is the only sustainable energy resource on earth, and
photovoltaic systems for the conversion of solar energy into
electrical power have evolved into a wide range of applications.
the green light-induced reductions of nitrobenzene derivatives
6
with dye-sensitized TiO .
2
1
Here, we report a method for the selective reduction of
nitroarenes with blue light using PbBiO X particles. These
effective heterogeneous photocatalysts are prepared from sto-
ichiometric amounts of PbO, Pn and PnX (Pn = Sb, Bi, X =
Cl, Br, I; ratio 3 : 1 : 1) in evacuated silica ampoules by annealing
Photocatalysts, converting visible light energy into chemical
energy, are known but they are less well-developed. Recent
examples of homogeneous photocatalysts include the hydro-
2
2
O
3
3
2
3
genation of alkynes, the asymmetric alkylation of aldehydes
4
◦
8
and the dehalogenation of a-alkylated esters. Heterogeneous
at 500–650 C for several days. They show the quantitative
photoreduction of nitrobenzenes to anilines when irradiated
with a blue high powered LED light or sunlight. The observed
photoconversions of organic substrates are very clean, as verified
by gas chromatographic monitoring, and in many cases are
quantitative.
photocatalysts are typically semiconductors based on modified
or unmodified TiO
2
or CdS. Unmodified TiO has been investi-
2
gated for the reduction of nitrobenzenes to their corresponding
5
anilines under UV light irradiation, whereas modified and dye-
sensitized TiO
2
reduces these molecules or oxidizes alcohols with
6
blue or green light. CdS quantum dots have been found to be
suitable photocatalysts for the reduction of azides to anilines,
All PbPnO X photocatalysts absorb in the blue range of the
2
7
visible spectrum (Fig. 1).
but these systems have disadvantages: UV light is only a small
part of the solar spectrum, which diminishes the efficiency of
unmodified TiO
in the visible range require special preparation. Oxide halides,
such as PbPnO X (Pn = Bi, Sb; X = Br, Cl), have been structurally
2
in sunlight. Modified CdS and TiO
2
absorbing
2
characterized and applied to the oxidative photodegradation
8
of organic dyes, e.g. methylene blue and methyl orange. These
materials are potential heterogeneous photocatalysts for organic
chemical synthesis using visible light and may nicely complement
a
Institute of Organic Chemistry, University of Regensburg,
Universit a¨ tsstrasse 31, 93040, Regensburg, Germany.
E-mail: Burkhard.koenig@chemie.uni-regensburg.de; Fax: +49 941
Fig. 1 Absorption spectra of PbPnO X (Pn = Bi, Sb; X = Br, Cl, I).
2
9
434566; Tel: +49 941 9434575
b
Institute of Inorganic Chemistry, University of Regensburg,
Universit a¨ tsstrasse 31, 93040, Germany.
The reduction of nitrobenzene derivatives was chosen to
monitor the photocatalysts’ efficiency, because of the transfer
E-mail: arno.pfitzner@chemie.uni-regensburg.de; Fax: +49 941 943
9
4
983; Tel: +49 941 943 4552
of six equivalents of hydrogen. The selective photoreduction
c
Department of Anaesthesia, University Hospital Regensburg, 93051,
Regensburg, Germany.
of nitrobenzenes has been described using TiO2 and UV
10
6
irradiation or dye-sensitized TiO and green light irradiation.
2
E-mail: michael.gruber@klinik.uni-regensburg.de; Fax: +49 941 944
7
All oxide halides have been investigated for their ability
to photoreduce nitrobenzenes, but the irradiation of samples
801; Tel: +49 941 944 7870
†
1
Electronic supplementary information (ESI) available: See DOI:
0.1039/c0gc00857e
containing PbSbO
2
X (X = Br, Cl, I) as the catalyst did not
6
40 | Green Chem., 2011, 13, 640–643
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Table 1 Photocatalytic reduction of nitrobenzene to aniline via
Photocatalytic measurements with PbO, which has a layered
PbPnO
2
X
2
+
structure similar to the polycationic sheets [PbBiO
2
] in the title
•
compound, did not show any catalytic activity at all, although
the absorption spectra were quite similar. Therefore, activity due
2
+
to Pb can be ruled out.
As the surface is only built up of Pb (in the antimony-
2
+
2
+
3+
Nitrobenzene
conversion (%)
containing materials) or by both Pb and Bi (in the bismuth-
b
c
Entry
PbPnO
2
X
LED wavelength/nm
3+
containing materials), it is likely that Bi is responsible for
the photocatalytic properties in this system. Surprisingly, only
1
2
3
4
5
6
7
PbSbO
PbSbO
PbSbO
2
2
2
Br
Cl
I
Br
Cl
I
440
440
440
440
440
440
530
£4
PbBiO
of nitrobenzene to aniline, while no conversion occurred for
PbBiO I when irradiated under comparable conditions. Fur-
2
X where X = Cl and Br catalyzed the photoreduction
£4
£4
PbBiO
PbBiO
PbBiO
PbBiO
2
>99
>99
£1
2
2
2
2
thermore, the latter materials absorb a significant part of the
visible spectrum (see Fig. 1). The experimentally determined
I
£1
optical band gaps were 2.55 eV (PbBiO
2
Cl), 2.47 eV (PbBiO Br)
2
a
b
c
5
0 mg oxide halide. LED with 3 W electrical power, 80 Lumen. From
11
and 2.39 eV (PbBiO
2
I). It seems that the gaps of all these
the integration of signals in the GC chromatogram.
semiconductors are in the right range to catalyze the observed
reaction. Therefore, changes in the structural nature of the
materials may be decisive; increasing the iodine content in
lead to any conversion (Table 1, entries 1–3). The exchange of
Sb for Bi in the composition of the photocatalyst lead to an
almost quantitative conversion of nitrobenzene to aniline within
PbBiO
directions. Thus, PbBiO
along (001), whereas PbBiO
2
X leads to a change of the preferred crystal growth
Cl and PbBiO Br form layered crystals
I crystallizes as rods with [001] as
3
+
3+
2
2
2
2
0 h of blue light irradiation for X = Cl or Br. In case of X =
the rod axis (see the ESI for pictures of the crystals†). Changes
in the bonding interactions between the halide and metal atoms
indicate this clearly. In the case of rod-like crystals, the surface
of the material is significantly different; the surface changes
from mainly (001) for chloride and bromide to mainly (100)
and (010) for iodide, which may alter the catalytic activity. The
stronger bonds of iodine to Pb or Bi, well known for the binary
metal halides, change the crystal growth and may deactivate the
surface.
I, no conversion was observed (Table 1). Previous reports have
discussed mainly electronic reasons for the different catalytic
activities of layered PbPnO X-type materials (Pn = Sb, Bi;
X = Cl, Br, I). However, crystal-chemical arguments should
also be taken into consideration to explain the photocatalytic
properties of the compounds. A possible reason for the different
catalytic activity of the oxides may be derived collectively from
their crystal structures, their optical and their redox properties.
All the solid materials under discussion crystallize in a layered
2
8,11
Moreover, redox quenching by iodine may intercept the
catalysis as well. Therefore, the redox potentials of the oxide
halides were measured, revealing multiple redox processes that
2
•
+
structure. They exhibit covalent metal oxygen layers [PbBiO
2
]
8
separated by halide layers, which are stacked along [001]. One
can assume that the crystal surface consists of metal oxygen
layers, i.e., the metal atoms are expected to form the (001)
surfaces. In case of the bismuth compounds, the metal position
is statistically occupied by lead and bismuth in the ratio 1 : 1,
16
did not allow the derivation of exact values. However, iodide
inactivation has been investigated with the semiconductors
PbBiO
2
Br
n
I
m
2
n m
and PbBiO Cl I with varying ratios of their
halogen content and analyses of their photocatalytic activities
in nitrobenzene reduction. With an increasing amount of
whereas in the synthetic antimony compounds PbSbO
PbSbO I, the antimony atoms are slightly displaced parallel
001] towards the oxygen atoms. By way of contrast, PbSbO Cl
2
Br and
2
iodine in PbBiO
2
Br
n
I
m
2
n m
and PbBiO Cl I , the photocatalytic
[
2
activity decreased dramatically (Fig. 3). Similar effects have
been reported for the photodegradation of Rhodamine B with
has an ordered structure. However, therein, the antimony atoms
are also displaced towards the oxygen atoms. Therefore, the
antimony atoms do not reach the outer crystal surface and thus
are not accessible for catalytic processes (Fig. 2).
12
BiOX (X = Cl, Br and I). These materials are isostructural,
Fig. 2 Crystal structures of PbSbO
Ellipsoids correspond to a probability of 70%. The position of the metal
is occupied statistically by 50% Bi and 50% Pb in the case of PbBiO X.
In the case of PbSbO X, the positions of Pb and Sb can be resolved.
2 2
Br (right) and PbBiO Br (left).
2
Fig. 3 Photoconversion of nitrobenzene after 20 h, depending on the
iodine content, for PbBiO Br (green) and PbBiO Cl (blue).
2
2
n
I
m
2
n m
I
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a
their optical band gaps are in the right region for visible light
photocatalysis, but only BiOBr and BiOCl showed activities
towards the oxidative photodegradation of Rhodamine B.
These semiconductors were investigated in the photoreduc-
tion of nitrobenzene to aniline as well, but only in the case
of BiOBr and BiOCl were full conversions observed under
identical experimental conditions. Irradiation of BiOI did not
induce any photocatalytic activity for nitrobenzene reduction.
Such electronic quenching in semiconductor photocatalysis has
already been reported for the UV photodegradation of acid
2
Table 2 Photoreduction of nitrobenzene derivatives via PbBiO Br
b
c
Entry
1
Starting material
Product (%)
2
3
4
13
14
orange and curcumin with TiO in the presence of iodide.
2
Furthermore, the UV light-induced reduction of nitrobenzene to
aniline has been inhibited by adding small amounts of iodide to
TiO
the investigated PbBiO
redox potentials, band gaps and crystal structures. We therefore
relate the observed dependence of the PbBiO X photoactivity
on the iodine content to electronic quenching and changes in
their surface properties.
2
and TEOA. Hence, the same argument should be valid for
2
X semiconductors, as they show similar
5
6
7
2
A kinetic analysis of the photoreduction of nitrobenzene to
aniline with PbBiO
rate of nitrobenzene to aniline conversion after approximately
h of irradiation with blue light (Fig. 4). This induction period
2
Br and PbBiO
2
Cl showed an increase in the
6
indicates catalyst activation or gas evolution, as reported in the
8
9
photoreductions of nitrobenzenes to anilines via dye-sensitized
6
TiO
2
and metal salts. Head space gas chromatographic analysis
revealed the presence of dihydrogen gas after 14 h of irradiation,
which explains the selectivity for the exclusive reduction of
nitrobenzenes to their corresponding anilines (Table 2).
1
0
1
1
1
2
a
Fig. 4 Kinetics of the reduction of nitrobenzene to aniline via PbBiO
2
X
50 mg and irradiation with a 440 nm LED with 3 W electrical power,
b -4 c
8
0 lumen. 2 ¥ 10 mmol. From the integration of signals in the
(
X = Br, Cl).
GC-/GC-MS chromatogram.
The stability of PbBiO Br has been investigated by XRD
2
analysis of the semiconductor before and after the photore-
action. After irradiation for 20 h, the semiconductor became
grey, but only in the presence of TEOA. The grey color slightly
disappeared when the sample was treated in an ultrasonic bath.
X-Ray powder diffraction patterns clearly show no changes in
the crystal structure under the experimental conditions (Fig. 5).
Similar observations have been described in the photocatalytic
The maximum number of catalyst reuses was determined;
after every catalytic cycle, the grey catalyst was filtered, sonicated
in an ultrasonic bath and used again for the photoreduction
of nitrobenzene to aniline. After five catalytic cycles, the
photocatalytic activity had not decreased; after eight reuses,
the conversion under identical experimental conditions had
decreased to give 50% conversion. Treatment of the catalyst
in an ultrasonic bath is essential to retain its photocatalytic ac-
tivity, which otherwise decreases dramatically within five cycles
(Fig. 6).
12
oxidation of isopropyl alcohol to CO
2
with BiOCl. Therefore,
Br during
we suggest surface passivation processes on PbBiO
2
15
the photocatalysis.
6
42 | Green Chem., 2011, 13, 640–643
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the exclusive selectivity for nitrobenzene reductions. Recycling
experiments revealed a high stability of the photocatalyst over
several reuses, but sonication to remove passivations is essential.
The investigations provide a more detailed mechanistic de-
scription of this class of heterogeneous semiconductor pho-
tocatalysts, which will facilitate the design and prediction of
properties of the next generation of photocatalysts of this type.
Acknowledgements
S. F. is grateful for a scholarship from the Bayerische
Elitef o¨ rderung. This work was supported by the Graduate
Research Training Group 1626 – Chemical Photocatalysis of
the Deutsche Forschungsgemeinschaft.
2
Fig. 5 X-Ray powder diffraction patterns of PbBiO Br before (upper)
and after catalysis (lower).
Notes and references
1
S. Westenhoff, I. A. Howard, J. M. Hodgkiss, K. R. Kirov, H. A.
Bronstein, C. K. Williams, N. C. Greenhorn and R. H. Friend, J.
Am. Chem. Soc., 2008, 130, 13653–13658.
2
S. Rau, B. Sch a¨ fer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich,
H. G o¨ rls, W. Henry and J. G. Vos, Angew. Chem., 2006, 118, 6361–
6
364.
3
4
D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 322, 77–80.
J. M. R. Narayanam, J. W. Tucker and C. R. J. Stephenson, J. Am.
Chem. Soc., 2009, 131, 8756–8757.
5
6
S. O. Flores, O. Rios-Bernji, M. A. Valenzuela, I. Crdova, R. Gmez
and R. Gutirrez, Top. Catal., 2007, 44, 507–511.
(a) S. F u¨ ldner, R. Mild, H. I. Siegmund, A. J. Schroeder, M. Gruber
and B. K o¨ nig, Green Chem., 2010, 12, 400–406; (b) M. Zhang, C.
Chen, W. Ma and J. Zhao, Angew. Chem., 2008, 120, 9876–9879.
C. Radhakrisham, M. K. F. Lo, M. V. Warrier, M. A. Garcia-Garibay
and H. G. Monbouquette, Langmuir, 2006, 22, 5018–5024.
(a) A. Pfitzner and P. Pohla, Z. Anorg. Allg. Chem., 2009, 635, 1157–
7
8
1
159; (b) Z. Shan, W. Wang, X. Lin, H. Ding and F. Huang, J. Solid
State Chem., 2008, 181, 1361–1366; (c) L. G. Sill e´ n and L. Melander,
Z. Krist., 1941, 40, 537; (d) G. Giuseppetti and C. Tadini, Period.
Mineral., 1973, 42, 335; (e) M. Gilberg, Ark. Mineral. Geol., 1960, 2,
2
Fig. 6 Photocatalyst recycling experiments of PbBiO Br (a) with
sonication of the catalyst before reuse (blue) and (b) reuse without
sonication (red).
5
65; (f) Z. Shan, X. Lin, M. Liu, H. Ding and F. Huang, Solid State
Sci., 2009, 11, 1163.
R. Brosius, D. Gammon, F. Van Laar, E. Van Steen, B. Sels and P.
Jacobs, J. Catal., 2006, 239, 362–368.
9
Conclusions
1
0 (a) H. Tada, A. Takao, T. Akita and K. Tanaka, ChemPhysChem,
006, 7, 1687–1691; (b) Y. Matsushita, S. Kumada, K. Wakabayashi,
2
Semiconductors of composition PbBiO X (X = Cl, Br) selec-
2
K. Sakeda and T. Ichimura, Chem. Lett., 2006, 35, 410–411; (c) H.
Tada, T. Ishida, A. Takao, S. Ito, S. Mukhopadhay, T. Akita, K.
Tanaka and H. Kobayashi, ChemPhysChem, 2005, 6, 1537–1543.
1 P. Pohla, Dissertation, Universit a¨ t Regensburg, 2010.
tively photoreduce nitrobenzenes to anilines in the presence of
triethanolamine if irradiated with blue light of low intensity.
The photocatalysts are easy to prepare by established solid
state chemistry techniques, and allow the complete and clean
conversion of substrates. Structural studies have explained
1
1
2 H. An, Y. Du, T. Wang, C. Wang, W. Hao and J. Zhang, Rare Metals,
2
008, 27, 243–250.
13 Y. Chen, S. Yang, K. Wang and L. Lou, J. Photochem. Photobiol., A,
2005, 172, 47–54.
3
+
3+
the differences in catalytic activity for related Bi and Sb
1
4 U. Singh, S. Verma, H. N. Ghosh, M. C. Rath, K. I. Priyadarsini and
A. Sharma, J. Mol. Catal. A: Chem., 2010, 318, 106–111.
compounds. The missing photocatalytic activity of the iodide-
containing materials may be due to redox quenching and surface
effects, as indicated by the decrease of photocatalytic activity in
15 Activation of band gap decreasing has been excluded by UV-vis
spectra, which is contrary to the reported photocatalysis with BiOCl.
6 (a) A. M. Roy, G. C. De, N. Sasmal and S. S. Bhattacharyya, Int. J.
Hydrogen Energy, 1995, 20, 627; (b) O. L. Stroyuk, O. Y. Rayevska,
A. V. Kozytskiy and S. Y. Kuchmiy, J. Photochem. Photobiol., A,
2010, 210, 209.
1
PbBiO
2
Br
n
I
m
and PbBiO
2
Cl
n
I
m
with increasing iodide content,
in
and an inhibited nitrobenzene photoreduction with TiO
2
presence of iodide. The detection of dihydrogen finally explains
This journal is © The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 640–643 | 643