Photoinduced Reduction of Nitroarenes Using a Transition-Metal-Loaded Silicon Semiconductor under Visible Light Irradiation

Article abstract of DOI:10.1021/acscatal.6b00886

We investigated transition-metal-loaded silicon nanoparticles for the photocatalytic reduction of nitroarene derivatives in the presence of formic acid under visible light irradiation. Formic acid assumes the role of both a hydrogen source and a sacrificial reagent for the introduction of electrons into the generated holes of semiconductors. As such, in the presence of formic acid, photocatalytic reactions smoothly proceed under mild conditions without gaseous hydrogen. In particular, palladium-loaded silicon (Pd/Si) was the most suitable catalyst for the conversion of nitrobenzene to aniline, compared to Pt/Si, Ru/Si, and Pd/C.

Full text of DOI:10.1021/acscatal.6b00886

Letter
pubs.acs.org/acscatalysis
Photoinduced Reduction of Nitroarenes Using a Transition-Metal-
Loaded Silicon Semiconductor under Visible Light Irradiation
Ken Tsutsumi,*,† Fumito Uchikawa,‡ Kentaro Sakai,§ and Kenji Tabata*,‡
†Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami Osawa,
Hachioji, Tokyo, 192-0364, Japan
‡Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, 1-1, Gakuen-Kibanadai-Nishi, Miyazaki-shi,
889-2155, Japan
§Center for Collaborative Research & Community Cooperation, University of Miyazaki, 1-1, Gakuen Kibanadai-Nishi, Miyazaki,
889-2155, Japan
S
* Supporting Information
ABSTRACT: We investigated transition-metal-loaded silicon nano-
particles for the photocatalytic reduction of nitroarene derivatives in
the presence of formic acid under visible light irradiation. Formic acid
assumes the role of both a hydrogen source and a sacrificial reagent
for the introduction of electrons into the generated holes of
semiconductors. As such, in the presence of formic acid, photo-
catalytic reactions smoothly proceed under mild conditions without
gaseous hydrogen. In particular, palladium-loaded silicon (Pd/Si) was
the most suitable catalyst for the conversion of nitrobenzene to
aniline, compared to Pt/Si, Ru/Si, and Pd/C.
KEYWORDS: transition-metal-loaded semiconductor, silicon, formic acid, reduction, visible-light responsiveness
emiconductor materials have been extensively researched as
photocatalysts for the conversion of light into chemical
S
energy, allowing for economical and environmentally friendly
organic syntheses.1 In particular, visible-light-responsive semi-
conductors have great potential in this regard owing to their
sustainable and clean characteristics as well as their efficient use
of solar energy.2 Silicon is one of the most versatile
semiconductors and displays photocatalytic activity under
visible light irradiation.3 We recently reported photoassisted
hydrogen evolution in an aqueous solution of formic acid with
silicon particles under visible light irradiation, which is
enhanced by the support of noble metals on the silicon
surface.4 The pioneering work on photocatalytic hydrogen
evolution by Yoneyama et al. indicated that formic acid was the
most efficient sacrificial reagent when using platinum-loaded
silicon.5
Figure 1. Mechanistic design of photoinduced reduction of organic
compounds using formic acid as a hydrogen source via a transition-
metal-loaded Si semiconductor.
Based on these studies, photocatalytic reduction of organic
compounds using a transition metal-loaded silicon semi-
conductor is reasonable, as shown in Figure 1. Conventional
metal catalysts, such as palladium on carbon, dissociate
molecular hydrogen to produce atomic hydrogen (hydride)
on the metal surface in the catalytic reduction of organic
compounds using hydrogen gas.6 Metal-loaded silicon can
photochemically produce atomic hydrogen from formic acid for
the reduction of organic molecules in the following manner.
The semiconductor absorbs supra-bandgap photons to generate
photoexcited electron (e−) and hole (h+) pairs in its
conduction band (CB) and valence band (VB), respectively.
The electron then migrates to a metal particle and is received
by a proton to provide atomic hydrogen on the metal.
Photocatalytic hydrogen evolution occurs via the reduction of
protons by electrons and the oxidation of the formate species
by holes. Prior to molecular hydrogen production, it is
Received: March 26, 2016
Revised: June 2, 2016
© XXXX American Chemical Society
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Letter
proposed that the atomic hydrogen generated on the metal
particles may be utilized for reduction of organic compounds.
To test this hypothesis, we focused on the reduction of
aromatic nitro compounds to produce aromatic amines.
Although this reduction is generally carried out using metal
catalysts,6,7 photocatalytic reductions have been investigated for
green chemical processes for a few decades.8 In particular, TiO2
nanoparticles are widely used for the photocatalytic hydro-
genation of nitro aromatic compounds as they are inexpensive,
abundant, nontoxic, corrosion-resistant, and chemically stable.9
In addition to TiO2, graphene−ZnO−Au nanocomposites,10
images showed the presence of metal nanoparticles on Si
(Figure S3−10 (SI)).
Photocatalytic reductions of nitrobenzene (1a) were
conducted using the M/Si catalysts (Si 99.999%, Aldrich
Chemical Co.) in the presence of HCOOH at room
temperature for 3 h (Table 1). The reaction suspension was
Table 1. Reduction of Nitrobenzene (1a) To Produce
a
Aniline (2a) under Visible Light Irradiation
12
Mo−W-based copper oxides,11 and Ag2Mo4O13 have also
been used for the photocatalytic reduction of nitrobenzene.
However, the above-mentioned photocatalysts require ultra-
violet (UV) light irradiation for hydrogenation of nitro
compounds. As such, the development of visible-light-
responsive catalysts would be beneficial as they could utilize
solar energy. Some types of semiconductors, such as CdS,
WO3,13 and Bi2MoO6,14 were applied to the visible light-driven
hydrogenation of nitrobenzene. Modification of the TiO2
surface with 2,3-dihydroxynaphthalene15 and construction of
a Ti (IV) metal−organic framework16 have been investigated
for their visible-light responsiveness. However, these catalysts
have shown low yields and selectivities, limitation of substrates
and low stability, including photocorrosion. In addition, the key
step in these conventional reduction systems is the electron
transfer from semiconductors to organic substrates.
In an attempt to develop a novel, visible light-promoted
photoreduction, we prepared transition-metal-loaded silicon
(Si) catalysts. As Si has a small band gap (∼1.1 eV), it has been
suggested that they would be ideal candidate materials for solar
energy conversion.3 The conduction band is positioned at
negative potential to reduce protons.3b On the other hand, the
valence band is less positive than those of other semiconductors
such as TiO2, CdS, and WO3, so most organic substrates
including nitroarenes and common organic solvents are not
oxidatively decomposed by Si. The combination of Si
semiconductor and transition metals produces a novel
photochemical reduction system for organic compounds
(Figure 1). Photoexcited Si can produce active hydrogen on
metals, while the metal particles play a role in the absorption
and reduction of organic compounds. Photocatalytic reactions
of nitroarenes were conducted under visible light irradiation in
the presence of formic acid as a hydrogen source, and the scope
and limitation are discussed.
First, a number of transition-metal-loaded Si catalysts (M/Si;
M: Pd, Pt, Ru) were prepared for the reduction of
nitrobenzene. We previously found that a platinum nano-
particle dispersion liquid is the most suitable metal-loading
source for Si for photocatalytic hydrogen evolution when
compared with platinum inorganic salts such as H2PtCl6 and
(NH3)4PtCl2.4 Therefore, metal nanoparticle dispersion liquids
were used for the preparation of metal-loaded Si catalysts via an
impregnation method. The obtained M/Si catalysts (M: Pd, Pt,
Ru) were characterized by N2 adsorption−desorption, X-ray
powder diffraction (XRD), scanning electron microscopy
(SEM), and transmission electron microscopy (TEM)
techniques (see Supporting Information (SI)). The N2
adsorption−desorption isotherms exhibited IUPAC type II
patterns (Figure S1 (SI)). XRD diffraction pattern assigned to
crystalline silicon was obtained in each catalyst (Figure S2
(SI)). Pd and Pt peaks were also observed. SEM and TEM
b
entry
catalyst
aniline yield (%)
c
1
2
3
4
5
Pd/Si
27
12
12
0
d
Pt/Si
e
Ru/Si
f
Pd/C
Si
1.4
a
Reaction conditions: nitrobenzene (6.2 mg), HCOOH (610 mg),
catalyst (25 mg), 2-propanol (4.5 mL), photoirradiation with a xenon
lamp through an UV cutoff filter (375 nm < λ < 800 nm), time = 3 h.
Determined by GC analysis. 4.5 wt %. 1.6 wt %. 0.3 wt %. 5 wt %
b
c
d
e
f
Pd/C from Wako Pure Chemical Industries, Ltd.
irradiated with visible light (375 nm < λ < 800 nm) from a
xenon lamp through an UV cutoff filter. M/Si catalysts
produced aniline from nitrobenzene without gaseous H2
(Table 1, entries 1−3). Commercially available palladium on
activated carbon catalyst (5 wt % Pd/C from Wako Pure
Chemical Industries, Ltd.) provided no aniline under the same
conditions (Table 1, entry 4). Several methods have been
reported for the palladium-catalyzed transfer hydrogenation of
nitroarenes using formic acid.17 In these cases, catalytic
amounts of base promote the hydrogen transfer through
generation of the active formate ion. Thus, Pd/C catalyst was
not effective under the present reaction conditions. These
results suggest that the reduction of nitrobenzene using M/Si
catalysts proceeds through a photochemical process. The yield
of aniline decreased when using no-metal-loaded silicon catalyst
(Table 1, entry 5).
Next, a number of experiments were conducted to optimize
the reaction conditions (Table 2). Addition of H2O
dramatically increased both conversion and yield (Table 2,
entries 1−3), potentially promoting the dissociation of
HCOOH to H+ and HCOO−. When the ratio of H2O/2-
propanol was 1:9, the nitrobenzene starting material was
completely consumed (Table 2, entry 3). Each reaction gave
moderate selectivity, which was nearly identical even in the
mixture of methanol and 2-propanol (Table 2, entry 4). The
formation of N-phenyl formamide was observed as a byproduct
through GC-MS analyses, which may lead to moderate
selectivity as it is produced through the reaction of aniline
with formic acid.18 Therefore, reduction in the amount of
formic acid was examined, revealing that smaller amounts of
formic acid resulted in good selectivity (Table 2, entry 5). In
addition, reducing the volume of solvent was able to increase
selectivity up to 100% (Table 2, entries 6 and 7). In some cases,
GC-MS analysis showed a small amount of remaining
nitrosobenzene intermediate. Reduction of 1a to 2a proceeded
in a stepwise manner via nitrosobenzene and hydroxylamine.19
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Letter
catalyst (crystalline 99.999%, Alfa Aesar) proceeded effectively
to provide aniline almost quantitatively.
Table 2. Reaction Conditions Screened for the Pd/Si-
Catalyzed Reduction of Nitrobenzene (1a) To Produce
Aniline (2a)
a
In the case of the Pd/Si catalyst (crystalline 99.999%, Alfa
Aesar), increasing the volume of solvent did not significantly
decrease both conversion and yield (Table 2, entry 10). Smaller
amount of the catalyst decreased the conversion as well as the
yield (Table 2, entries 11). When using an aprotic solvent,
CH3CN, the reaction took place in good yield (Table 2, entry
12). Notably, aniline was provided effectively even in H2O
(Table 2, entry 13). However, the reaction hardly proceeded in
the absence of formic acid (Table 2, entry 14). When sulfuric
acid was used instead of formic acid, a minimal amount of
aniline was generated (Table 2, entry 15). These results suggest
that formic acid plays the role of both a hydrogen source and a
sacrificial reagent for the introduction of electrons into the
generated holes of semiconductors, as shown in Figure 1.
Next, we extended our studies to the scope of photocatalytic
reductions using the Pd/Si catalyst prepared with Si (crystalline
99.999%) purchased from Alfa Aesar (Table 3). Our experi-
ments demonstrated that reactions of 4- and 3-nitroacetophe-
none (1b and c) went to completion to afford the
corresponding aminoacetophenones (2b and 2c, respectively)
in excellent yield and selectivity even at room temperature
(Table 3, entries 1 and 2). However, a low yield was obtained
for 2-nitroacetophenone (1d) (Table 3, entry 3). The reactivity
showed the following trend: p-CH3CO > m-CH3CO > o-
CH3CO. Placement of an electron-withdrawing substituent at
the para or ortho position can decrease the electron density of
the NO2 group, thus activating the nitroarane toward reduction.
c
,
d
c,e
HCOOH
b
conv.
yield
select.
(%)
entry
solvent (ml)
(equiv )
(%)
(%)
f
1
2
2-propanol 4.5
260
38
86
27
64
72
74
f
H2O/2-propanol 4.5
(5/95)
260
f
3
4
5
6
7
H2O/2-propanol 4.5
(1/9)
260
100
67
78
51
57
64
67
26
99
94
75
84
78
76
f
MeOH/2-propanol
4.5 (1/9)
260
g
H2O/2-propanol 4.5
(1/9)
2.6
68
84
g
H2O/2-propanol 2.25
(1/9)
2.6
64
100
100
71
h
H2O/2-propanol 2.25
(1/9)
3.6
67
i
h
8
H2O/2-propanol 2.25
(1/9)
3.6
37
j
h
9
H2O/2-propanol 2.25
(1/9)
3.6
100
100
89
99
j
h
10
H2O/2-propanol
10.13 (1/9)
3.6
94
k
h
11
H2O/2-propanol 2.25
(1/9)
3.6
84
j
h
12
H2O/CH3CN 2.25
(1/9)
3.6
95
88
j
h
13
H2O 2.25
3.6
100
10
83
3
83
30
j
14
H2O/2-propanol 2.25
(1/9)
0
j
l
15
H2O/2-propanol 2.25 H2SO4 3.6
(1/9)
10
5
50
Johnston and Backvall reported that the reduction rate of
̈
a
Reaction conditions: nitrobenzene (6.2 mg), Pd/Si (25 mg, 4.5 wt %,
Si (99.999%) from Aldrich Chemical Co.), photoirradiation with a
xenon lamp through an UV cutoff filter (375 nm < λ < 800 nm), time
= 3 h. Based on nitrobenzene. Determined by GC analysis.
Nitrobenzene conversion. Aniline yield. HCOOH (610 mg).
HCOOH (6.1 mg). HCOOH (8.3 mg). Pd/Si (25 mg, 3.0 wt %,
Si from Kojundo Chemical Laboratory, Ltd.). Pd/Si (25 mg, 3.4 wt %,
Si (crystalline 99.999%) from Alfa Aesar). Pd/Si (15 mg, 3.4 wt %, Si
nitroacetophenones using Pd nanoparticles on aminopropyl-
functionalized siliceous mesocellular foam as a catalyst showed
the following order: para, ortho > meta, based on the electronic
resonance effect.20 However, the higher reactivity of the para
and meta isomers for our photocatalytic reaction system can be
attributed to the steric effect around the NO2 group, as
opposed to the electronic resonance effect. In the case of the
TiO2 photocatalyst, an electron transfer to nitrobenzene occurs,
followed by protonation,21 which may show some steric effect.
Ebitani reported that hydrogenation of nitroarenes proceeds in
a stepwise fashion following coordination of the nitro group on
the palladium surface.22 Considering the steric effect of
nitroaranes 1b−d, reductive hydrogen may transfer to the
nitrobenzenes coordinated to the palladium particles similar to
noble metal-catalyzed reductions.
b
c
d
e
f
g
h
i
j
k
l
(crystalline 99.999%) from Alfa Aesar). H2SO4 (18.7 mg).
Considering the stoichiometry of each of the reduction steps,
three equivalents of formic acid are consumed to convert
nitrobenzene to aniline (Scheme 1). Thus, use of 3.6 equiv of
formic acid gave 2a in 67% yield.
Scheme 1. Stepwise Reduction of Nitrobenzene (1a) to
Aniline (2a) Using Formic Acid
Cyano-functionalized nitroarene (1e) can be easily converted
to aniline 2e in good yield (Table 3, entry 4), with a small
amount of 4-methylaniline observed as a byproduct through
GC-MS analysis. Hydrogenolysis of cyanobenzene over Pd/C
to afford toluene was previously reported;23 therefore, a similar
reaction might occur to yield 4-methylaniline under the present
conditions.
3-Nitrostyrene (1f) reacted nearly quantitatively, with trace
amounts of 3-vinylaniline detected (Table 3, entry 5). The
major products were 3-nitroaniline and 3-ethylaniline, which
suggests that the vinyl group is more reactive than the nitro
group. This selectivity is well-known for the palladium-
catalyzed hydrogenation of nitrostyrenes;24 thus, the Pd/Si
catalyst may possess selectivity similar to that of the reported
palladium catalysts. These results imply that the reaction
proceeded on the palladium surface.
We prepared Pd/Si with other silicon materials, such as Si
from Kojundo Chemical Laboratory, Ltd. and Si (crystalline
99.999%) from Alfa Aesar; their photocatalytic activities were
tested for the reduction of nitrobenzene under the same
conditions (Table 2, entries 7−9). Aniline was finally obtained
in high yield and selectivity using Si (crystalline 99.999%)
purchased from Alfa Aesar as a starting material for the Pd/Si
catalyst (Table 2, entry 9). The Pd/Si catalyst (crystalline
99.999%, Alfa Aesar) reveals higher crystallinity compared with
other catalysts, which was supported by XRD analyses (Figure
S2 (SI)). Hence, the photocatalytic reaction using the Pd/Si
Catalytic degradations of 4-nitrophenol (1g) have been
focused because 1g is the biologically stable pollutants under
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For the reaction of 4-bromonitrobenzene (1h), a low yield
was obtained for amino product 2h (Table 3, entry 7).
Increasing the reaction temperature from room temperature to
80 °C and extending the reaction time from 3 to 24 h can
promote the reduction of less-reactive nitro groups (Table 3,
entries 8−11). Finally, 2h was obtained in 75% yield upon
reaction at 80 °C for 12 h (Table 3, entry 10). Further reaction
time resulted in decreased yield (Table 3, entry 11).
Table 3. Reduction of Nitroarenes (1b−g) To Produce
a
Substituted Anilines (2b−g)
Finally, recyclability of the Pd/Si catalyst was tested for the
photocatalytic reduction of 1a to 2b. After completion, the
reaction mixture was centrifuged to separate the solid from the
supernatant. The supernatant was then removed, and the
remaining solid was washed with deionized water, ethanol,
diethyl ether, hexane, and acetone. The recovered catalyst was
dried at 80 °C and reused for further reactions. Upon recycling,
no significant decrease in conversion, yield, or selectivity was
observed for up to four cycles (Figure 2). However, the time
Figure 2. Recyclability of the Pd/Si catalyst for the reduction of
nitrobenzene (1a). Reaction conditions: Pd/Si (25 mg, 3.4 wt % (first
cycle), Si (crystalline 99.999%) from Alfa Aesar), nitrobenzene (6.2
mg, 0.050 mmol), HCOOH (8.3 mg, 3.6 equiv based on nitro-
benzene), H2O/2-propanol (1:9, 2.25 mL), photoirradiation with a
xenon lamp through an UV cutoff filter (375 nm < λ < 800 nm),
temperature = rt, time = 3 h.
course experiments indicated that the reaction rate increased at
second cycle and decreased at fourth cycle (Figure S12−14
(SI)). The palladium content in the recovered catalysts
decreased during the course of the reaction and/or workup
(first: 3.4 wt %, second: 2.0 wt %, third: 0.2 wt %, fourth: 0.2 wt
%). Thus, 2.0 wt % of Pd-content is suitable in the present
photoreaction system.
In conclusion, we have found that transition-metal (Pd, Pt,
Ru)-loaded Si catalysts are effective for the photoinduced
reduction of nitrobenzene to produce aniline. In particular, Pd/
Si (Si: crystalline 99.999% from Alfa Aesar) is able to provide
aniline almost quantitatively (99%), even at room temperature
under visible light irradiation. As formic acid is used as a
hydrogen source, this reaction is safe, allows for easy handling,
and requires no high-pressure equipment. The Pd/Si catalyst is
effective for the conversion of a variety of nitroarenes, showing
selectivity similar to that of conventional palladium catalysts.
The conversion of light to chemical energy using visible-light-
active semiconductors such as silicon can reduce the
consumption of fossil fuels as well as promote various catalytic
reactions. As such, the combination of metal catalysts and
semiconductors will contribute to the development of eco-
friendly organic transformations.
a
Reaction conditions: nitroarene (0.050 mmol), HCOOH (8.3 mg,
3.6 equiv based on nitroarane), Pd/Si (25 mg, 3.4 wt %, Si (crystalline
99.999%) from Alfa Aesar), H2O/2-propanol (1:9, 2.25 mL),
photoirradiation with a xenon lamp through an UV cutoff filter (375
nm < λ < 800 nm), temperature = rt, time = 3 h. Determined by GC
analysis. Temperature = 80 °C, time = 3 h. Temperature = 80 °C,
time = 6 h. Temperature = 80 °C, time = 12 h. Temperature = 80
°C, time = 24 h.
b
c
d
e
f
natural degradation processes.25a The reduced product, 4-
aminophenol (2g), is one of the potent building block for
medicines, agrochemicals, and functional materials.25a,26 CdS
and modified CdS catalysts have been used on the visible light
photocatalytic reduction of 1g.13c,25 The Pd/Si catalyst can
convert 1g to 2g almost quantitatively under visible-light
irradiation at room temperature (Table 3, entry 6).
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ACS Catalysis
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00886.
■
S
(13) (a) Maldotti, A.; Andreotti, L.; Molinari, A.; Tollari, S.; Penoni,
A.; Cenini, S. J. Photochem. Photobiol., A 2000, 133, 129−133.
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Experimental procedures, X-ray diffraction patterns, N2
adsorption−desorption isotherms, SEM and TEM
images, and experimental time courses for the reduction
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: tsutsumi@tmu.ac.jp.
*E-mail: kenjt@cc.miyazaki-u.ac.jp.
Notes
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Mater. Interfaces 2012, 4, 6635−6639.
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ACKNOWLEDGMENTS
■
This work was supported by a Grant-in Aid for Scientific
Research on Priority Areas (C: no. 23550126) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT) of the Japanese Government, and the Iketani
Science and Technology Foundation. We also thank Mr.
Masahiro Fujiwara (Nara Institute of Science and Technology)
for assistance in obtaining ICP/MS. K.T. expresses his heartfelt
thanks to Prof. K. Kakiuchi (Nara Institute of Science and
Technology) and Prof. K. Nomura (Tokyo Metropolitan
University) for fruitful support and discussion.
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ACS Catal. 2016, 6, 4394−4398