Letter pubs.acs.org/acscatalysis
Photoinduced Reduction of Nitroarenes Using a Transition-MetalLoaded 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 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. KEYWORDS: transition-metal-loaded semiconductor, silicon, formic acid, reduction, visible-light responsiveness
S
emiconductor materials have been extensively researched as photocatalysts for the conversion of light into chemical energy, allowing for economical and environmentally friendly organic syntheses.1 In particular, visible-light-responsive semiconductors 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 Based on these studies, photocatalytic reduction of organic compounds using a transition metal-loaded silicon semiconductor 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 © XXXX American Chemical Society
Figure 1. Mechanistic design of photoinduced reduction of organic compounds using formic acid as a hydrogen source via a transitionmetal-loaded Si semiconductor.
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
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DOI: 10.1021/acscatal.6b00886 ACS Catal. 2016, 6, 4394−4398
Letter
ACS Catalysis
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
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 hydrogenation 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 Mo−W-based copper oxides,11 and Ag2Mo4O1312 have also been used for the photocatalytic reduction of nitrobenzene. However, the above-mentioned photocatalysts require ultraviolet (UV) light irradiation for hydrogenation of nitro compounds. As such, the development of visible-lightresponsive 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 nanoparticle 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
Table 1. Reduction of Nitrobenzene (1a) To Produce Aniline (2a) under Visible Light Irradiationa
entry 1 2 3 4 5
catalyst c
Pd/Si Pt/Sid Ru/Sie Pd/Cf Si
aniline yieldb (%) 27 12 12 0 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. b Determined by GC analysis. c4.5 wt %. d1.6 wt %. e0.3 wt %. f5 wt % 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 H 2 O 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/2propanol 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 4395
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catalyst (crystalline 99.999%, Alfa Aesar) proceeded effectively to provide aniline almost quantitatively. 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 experiments demonstrated that reactions of 4- and 3-nitroacetophenone (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 > oCH3CO. 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. Johnston and Bäckvall reported that the reduction rate of nitroacetophenones using Pd nanoparticles on aminopropylfunctionalized 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. 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 palladiumcatalyzed 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. Catalytic degradations of 4-nitrophenol (1g) have been focused because 1g is the biologically stable pollutants under
Table 2. Reaction Conditions Screened for the Pd/SiCatalyzed Reduction of Nitrobenzene (1a) To Produce Aniline (2a)a entry
solvent (ml)
1 2
2-propanol 4.5 H2O/2-propanol 4.5 (5/95) H2O/2-propanol 4.5 (1/9) MeOH/2-propanol 4.5 (1/9) H2O/2-propanol 4.5 (1/9) H2O/2-propanol 2.25 (1/9) H2O/2-propanol 2.25 (1/9) H2O/2-propanol 2.25 (1/9) H2O/2-propanol 2.25 (1/9) H2O/2-propanol 10.13 (1/9) H2O/2-propanol 2.25 (1/9) H2O/CH3CN 2.25 (1/9) H2O 2.25 H2O/2-propanol 2.25 (1/9) H2O/2-propanol 2.25 (1/9)
3 4 5 6 7 8i 9j 10j 11k j
12
13j 14j 15j
HCOOH (equivb)
conv.c,d (%)
yieldc,e (%)
select. (%)
260f 260f
38 86
27 64
72 74
260f
100
78
78
260f
67
51
76
2.6g
68
57
84
2.6g
64
64
100
3.6h
67
67
100
3.6h
37
26
71
3.6h
100
99
99
3.6h
100
94
94
3.6h
89
75
84
h
95
84
88
3.6h 0
100 10
83 3
83 30
H2SO4 3.6l
10
5
50
3.6
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. bBased on nitrobenzene. cDetermined by GC analysis. d Nitrobenzene conversion. eAniline yield. fHCOOH (610 mg). g HCOOH (6.1 mg). hHCOOH (8.3 mg). iPd/Si (25 mg, 3.0 wt %, Si from Kojundo Chemical Laboratory, Ltd.). jPd/Si (25 mg, 3.4 wt %, Si (crystalline 99.999%) from Alfa Aesar). kPd/Si (15 mg, 3.4 wt %, Si (crystalline 99.999%) from Alfa Aesar). lH2SO4 (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
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 4396
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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). 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
Table 3. Reduction of Nitroarenes (1b−g) To Produce Substituted Anilines (2b−g)a
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 nitrobenzene), 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-lightactive 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 ecofriendly 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. bDetermined by GC analysis. cTemperature = 80 °C, time = 3 h. dTemperature = 80 °C, time = 6 h. eTemperature = 80 °C, time = 12 h. fTemperature = 80 °C, time = 24 h.
natural degradation processes.25a The reduced product, 4aminophenol (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). 4397
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00886. Experimental procedures, X-ray diffraction patterns, N2 adsorption−desorption isotherms, SEM and TEM images, and experimental time courses for the reduction of 1a (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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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 Technology (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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DOI: 10.1021/acscatal.6b00886 ACS Catal. 2016, 6, 4394−4398
