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Article Cite This: ACS Omega 2017, 2, 7066-7070
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Gold Nanoparticles Supported on Alumina as a Catalyst for Surface Plasmon-Enhanced Selective Reductions of Nitrobenzene Kittichai Chaiseeda,†,‡ Shun Nishimura,† and Kohki Ebitani*,† †
School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan ‡ Natural Products Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand S Supporting Information *
ABSTRACT: Au nanoparticles supported on alumina (Au/Al2O3) with average particle size of 3.9 ± 0.7 nm and surface plasmon band centerned at 516.5 nm were prepared by deposition−precipitation method, and their photocatalytic activities for the reduction of nitrobenzene using either formic acid in acetonitrile (method A) or KOH in 2-propanol (method B) were investigated. Even at room temperature, the Au/Al2O3 was found to be highly active and selective for conversion of nitrobenzene to aniline when used with formic acid in acetonitrile or to azobenzene when performed with KOH in 2-propanol under irradiation with green light-emitting diode (517 nm).
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INTRODUCTION Since Haruta et al. reported their pioneering work on Au catalysts for the oxidation of carbon monoxide in 1987,1 research on Au nanoparticles has grown exponentially. The developments of Au nanoparticles including their preparation, characterization, theoretical interpretation, and utilization have produced numerous breakthrough discoveries that will serve as fundamentals for future innovations.2−14 Au nanoparticles possess surface plasmon resonance (SPR) property in which their electrons oscillate in harmony with the irradiating light. This unique behavior, only found in a few metals such as Au, Ag, Cu, and Pt has been employed for many applications15 including surface-enhanced Raman scattering,16 spectroscopy,17 biosensor,18 and photocatalysis.19,20 The SP band is generally in the visible light region, hence, allowing the utilization of sunlight as a clean and sustainable source of energy. Despite having been exploited for various fields for a long time, SPRmediated catalysis of organic reactions has just been picking up steam in the last few years and a number of researches have sprung up as illustrated in various reviews.21−27 Reduction of aromatic nitro compounds is important because it can provide many products including aromatic amines and azo dyes, which are essential for the pharmaceutical, pigment, and agrochemical industries.28 Industrially, aniline is produced by catalytic hydrogenation of nitrobenzene using Cu, Pd, and Fe as a catalyst or amination of phenol using ammonia in the presence of a silica−alumina catalyst,28 while the production of azo dyes includes diazotization and coupling, condensation of nitro compounds with amines, reduction of nitro compounds, and oxidation of amino compounds.29 When nitrobenzene is reduced, aniline is the final product, while intermediates that © 2017 American Chemical Society
include nitrosobenzene, N-phenylhydroxylamine, azoxybenzene, azobenzene, and hydrazobenzene could also be obtained depending on the reaction conditions. Several methods for reduction of nitroarenes have been reported as reviewed by Kadam and Tilve.30 Some examples of photocatalysts are Au/ ZrO2,31 Au/TiO2,32 Au/CeO2,33 Au−Cu alloy,34 Au/TiO2 photocatalyst with an Ag co-catalyst,35 Pt-deposited aminofunctionalized Ti(IV) metal−organic framework (Pt/Ti-MOFNH2),36 CdS/g-C3N4,37 uniform CdS nanospheres/graphene hybrid nanocomposites,38 Ce2S3,39 CdS nanowires,40 and PbBiO2X (X = Cl, Br).41 The majority of researches of nitroarenes reduction has been using hydrogen gas, borohydride, and ammonium formate as a hydrogen donor. Recently, formic acid (FA, HCO2H) has also been demonstrated to be used as a hydrogen source for the reduction of nitrobenzene.42−44 Formic acid, industrially manufactured by methyl formate hydrolysis and hydrolysis of formamide, is inexpensive, easily handled, and abundantly available with nearly a million tons produced worldwide each year.45 A number of researches have also been focusing on producing FA sustainably from biomass.46−50 These make FA another ideal source of hydrogen. In this study, gold nanoparticles supported on γ-Al2O3 (Au/ Al2O3) were prepared, characterized, and explored for their photocatalytic activity under light-emitting diode (LED) irradiation at room temperature for the reductions of Received: August 24, 2017 Accepted: October 9, 2017 Published: October 23, 2017 7066
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For method A, as shown in Figure 2, using 1 mmol nitrobenzene, 3.5 mmol FA,b and 100 mg Au/Al2O3 at 23 °C,
nitrobenzene using either FA in acetonitrile or KOH in 2propanol.
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RESULTS AND DISCUSSION Au/Al2O3 was prepared by deposition−precipitation method.51 The prepared gold nanoparticles, slightly purple in color, had an average particle size, measured by transmission electron microscopy, of 3.9 ± 0.7 nm (Figure S1, Supporting Information (SI)). The actual gold content, determined by inductively coupled plasma analysis, was 1.85% w/w. The supported Au nanoparticles showed a broad surface plasmon band centered at 516.5 nm typical for Au nanoparticles (Figure 1).3 a X-ray diffraction spectra of both the supporting materials
Figure 2. Time courses of the reduction of nitrobenzene using Au/ Al2O3 and formic acid in acetonitrile under LED irradiation and dark condition. Reaction conditions: 1 mmol nitrobenzene, 3.5 mmol FA, 100 mg Au/Al2O3, 40 mL acetonitrile, 23 °C. The amount of each compound was determined by gas chromatography (GC) (see SI for an example of the chromatogram, Figure S5) using naphthalene as an internal standard.
the reaction in dark condition was much slower and only about 60% of aniline was produced after 56 h while under LED irradiation 96% of aniline was obtained at the same time. Nitrosobenzene as an intermediate gradually increased in both condition and began to decrease after about 36 h under LED irradiation. The reaction without the Au/Al2O3 gave no product at all. Using only Al2O3 also gave no product even under the irradiation. The reaction in acetic acid or propionic acid also gave no product. Corma et al. reported two-step reaction to produce azobenzene from nitrobenzene using Au/TiO2.56 The second step was oxidation of aniline. However, the present Au/ Al2O3 was inactive for oxidation of aniline under oxygen atmosphere. Next, the Au/Al2O3 was later used for reduction of nitrobenzene using KOH in 2-propanol as in method B. Time courses also revealed a significant enhancement by LED irradiation throughout the reaction (Figure 3). The reaction of nitrobenzene into azobenzene under LED irradiation completed in about 11 h while only 10% of azobenzene and 41% of azoxybenzene was produced under the dark condition. The amount of azoxybenzene intermediate rose much faster than that of azobenzene for both conditions and began to decrease after 5 h under LED irradiation. In both methods, the Au/Al2O3 can selectively reduce nitrobenzene to the target products, which are significantly enhanced by the irradiation from green LED. In addition, using only one catalyst, two reactions were successfully and selectively performed to obtain the desired products by changing hydrogen source and reaction conditions. After excitation by light, Au nanoparticles can cause cascading changes in nearby particles through four possible pathways including plasmonic heating, hole transfer, electron transfer, and antenna effects.25,57,58 Several factors could affect the activation efficiency of Au nanoparticles including but not limited to particle size, particle shape, dispersion of particles, amount of Au deposited on the supporting materials, and their
Figure 1. Diffuse reflectance UV−vis spectra of supported gold nanoparticles and the intensity spectrum of green LED used for photocatalysis.
and the prepared Au nanoparticles were measured (Figure S2, SI), but no obvious peaks of Au were found. The X-ray photoelectron spectrum of Au/Al2O3 showed binding energy of Au 4f7/2 of Au(0) at 83.8 eV and of Au 4f5/2 at 87.5 eV (Figure S3, SI). These doublet Au 4f peaks are similar to those of gold foil31,52 indicating that Au(III) used to prepare the gold nanoparticles was reduced to Au(0).53−55 The Au/Al2O3 was used for the reduction of nitrobenzene in two methods, using FA in acetonitrile (method A) or KOH in 2-propanol (method B) as shown in Scheme 1. Each of this method was also performed both under green LED (the spectrum of the green LED emitter is shown in Figure 1 by green line and the LED reactor is shown in Figure S4 in SI) and dark condition to ascertain the enhancement of the reaction by SPR. Scheme 1. Two Methods of Nitrobenzene Reduction
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the reduction of nitrobenzene to N-phenylhydroxylamine and the route skipping nitrosobenzene is a faster step since they could not detect nitrosobenzene during the reaction.64 In our case, however, nitrosobenzene was detected in significant amount during the reduction using the Au/Al2O3 and FA and, therefore, the second step into N-phenylhydroxylamine is the rate determining step (Scheme S2, SI). Based on this information, the overall mechanism for the reduction of nitrobenzene using Au/Al2O3 and FA has been proposed (Scheme S3, SI). In general, hydrides from FA molecules are abstracted by Au nanoparticles forming Au−H species and releasing CO2 in steps 1, 3, and 5.65−67 In step 2, one of the oxygens from nitrobenzene then binds to these hydrides and water and nitrosobenzene are produced. This nitrosobenzene is then hydrogenated to become N-phenylhydroxylamine in step 4. In the last step, one hydride binds to nitrogen while another hydride binds to oxygen of N-phenylhydroxylamine producing aniline and water. Overall, three molecules of FA are required which is consistent with the results (Table S1, SI). The mechanism of the reaction using KOH and 2-propanol is also proposed (Schemes S4−6, SI). With large accumulation of azoxybenzene, the last step of the reaction should be the rate determining step (Scheme S4, SI). KOH is believed to help in deprotonation of 2-propanol to form the corresponding alkoxide binds to the Au nanoparticles. Then β-hydride elimination produces acetone and Au−H (Scheme S5, SI, and steps 1, 3, and 5 in Scheme S6). Once Au−H is formed the remaining steps are similar to those of FA as follows. Oxygen from nitrobenzene receives H from Au nanoparticles and nitrosobenzene and water are formed in step 2. Hydrogenation of nitrosobenzene produces N-phenylhydroxylamine (step 4) which condenses with another molecule of nitrosobenzene producing azoxybenzene (step 6).68 Finally, in step 7, oxygen of azoxybenzene receives hydrogens, water, and azobenzene is produced.
Figure 3. Time courses of the reduction of nitrobenzene using Au/ Al2O3 and KOH in 2-propanol. Reaction conditions: 1 mmol nitrobenzene, 3 mL 0.1 M KOH in 2-propanol, 50 mg Au/Al2O3, 37 mL 2-propanol, 23 °C. The amount of each compound was determined by GC using naphthalene as an internal standard. Note that small amounts of nitrosobenzene intermediate (up to 3%) and aniline byproduct (up to 2%) are not shown.
supporting materials.59,60 The reported band gap of γ-Al2O3 is 8.7 eV.61 However, after Au nanoparticles were deposited on its surface, its band gap was reduced to 2.4 eV,62 making its band gap in the semiconductor range and, therefore, their electrons can be activated from their valence band to conduction band by light with matched wavelength. The excited electrons, also referred to as hot electrons,34 and holes can initiate the organic transformation, in which reduction occurs at the surface of Au nanoparticles while oxidation occurs at the supporting materials. The gap between the calculated lowest unoccupied molecular orbital (LUMO) of nitrobenzene (−2.91 eV) and the Fermi energy level of Au (−5.1 eV) is about 2.2 eV (Figure 4).34 The energy of maximum absorption of the prepared Au/
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CONCLUSIONS In summary, we found that Au nanoparticles supported on alumina act as highly efficient and selective photocatalyst for the reductions of nitrobenzene to aniline using widely available formic acid and to azobenzene using KOH and 2-propanol, respectively. The Au/Al2O3 under the LED irradiation is capable of the reactions at ambient temperatures which avoids generation of byproducts or decompositions of the products or reagents at high temperature.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01248. Chemicals; preparation of supported gold nanoparticles; characterization; fabrication of LED reactor; influence of the amount of FA; GC chromatogram; reaction pathway proposed by Haber; our proposed reaction pathways (PDF)
Figure 4. Hot electron transfer from Au nanoparticles to LUMO of nitrobenzene (modified from Xiao et al. 34).
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Al2O3 is about 2.4 eV. Therefore, the energy absorbed by the Au/Al2O3 would be sufficient to generate hot electrons that will subsequently transfer to LUMO of nitrobenzene adsorbed on the Au surface, breaking the N−O bond and starting the reaction. The mechanism for the electrochemical reduction of nitrobenzene was proposed in 1898 (Scheme S1, SI).63 Corma et al. also reported that two routes are possible for
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kittichai Chaiseeda: 0000-0002-3405-5718 Shun Nishimura: 0000-0003-3084-1444 7068
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Kohki Ebitani: 0000-0002-0262-6029 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.C. gratefully acknowledges support from the grant from the dual Ph.D. program between JAIST and Chulalongkorn University. We also thank Munenori Uno, Machine Shop in Center for Nano Materials and Technology, JAIST, for the fabrication of the LED reactor. This study is supported (in part) by JSPS-KAKENHI (Nos. 17H03455 and 17H04966).
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ADDITIONAL NOTES
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a
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