Chemoselective Hydrogenation of Nitroaromatics by Supported Gold

Sep 16, 2009 - Department of Molecular Design and Engineering, and Department of Electrical Engineering and Computer Science, Graduate School of ...
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J. Phys. Chem. C 2009, 113, 17803–17810

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Chemoselective Hydrogenation of Nitroaromatics by Supported Gold Catalysts: Mechanistic Reasons of Size- and Support-Dependent Activity and Selectivity Ken-ichi Shimizu,*,† Yuji Miyamoto,† Tadahiro Kawasaki,‡ Takayoshi Tanji,§ Yutaka Tai,| and Atsushi Satsuma† Department of Molecular Design and Engineering, and Department of Electrical Engineering and Computer Science, Graduate School of Engineering, Nagoya UniVersity, Nagoya, 464-8603, Japan, EcoTopia Science Institute, Nagoya UniVersity, Nagoya, 464-8603, Japan, and Materials Research Institute for Sustainable DeVelopment, Chubu Research Base of National Institute of AdVanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: July 27, 2009

Supported Au nanoparticles (NPs) prepared by colloid deposition method were well characterized, and their catalytic performance was tested for chemoselective reduction of a nitro group of substituted nitroaromatics by H2. Systematic studies on the effects of NPs size and support show small size of Au NPs, and acid-base sites of supports are required for high activity. The Au/Al2O3 catalyst with Au particle size of 2.5 nm selectively hydrogenates a nitro group in the presence of various other reducible functional groups, and it shows higher intrinsic activity than the state-of-the-art catalyst (Au NPs on TiO2). In situ FTIR studies provide a reaction mechanism, which explains fundamental reasons of the observed structure-activity relationship. Cooperation of the acid-base pair site on Al2O3 and the coordinatively unsaturated Au atoms on the Au NPs are responsible for the H2 dissociation to yield a H+/H- pair at the metal/support interface. High chemoselectivity could be attributed to a preferential transfer of the H+/H- pair to the polar bonds in the nitro group as well as a preferential adsorption of nitroaromatics on the catalyst through the nitro group. 1. Introduction Since Haruta discovered a remarkable activity of supported gold nanoparticles (NPs) in CO oxidation, many publications have been devoted to clarify the factors controlling the activity of gold catalysts mostly for oxidation reactions.1-8 Among them, the most relevant appear to be particle size, the nature of the support, and preparation method. For many catalytic systems, including oxidation and hydrogenation reactions, Au NPs on inert oxides (SiO2 and Al2O3) exhibit lower activity for various reactions than those on reducible semiconductor oxides (TiO2 and Fe2O3),1,2b,6 and it is widely believed that this tendency is due to the oxygen vacancies at metal support interface. The low activity of Au NPs on inert oxides should be partly due to difficulty in controlling size and oxidation state of gold using the conventional deposition-precipitation method. For example, when Al2O3-supported gold catalysts are prepared by the deposition-precipitation method,1a small Au NPs (2-3 nm) as major species, larger Au particles (4.3-20 nm),4a,5a and slightly oxidized gold species4a coexist on the support, and such nonuniform structural features can lead to lower catalytic activity. Recently, one of the authors8a-c and other research groups8d,e have reported that the colloid deposition method allows dispersion of metallic gold NPs with a narrow size distribution on various supports, including inert oxides such as SiO2.8 Hence, supported Au catalysts prepared by the colloid deposition method are suitable to investigate the effect of the * Corresponding author. Fax: +81-52-789-3193. E-mail: kshimizu@ apchem.nagoya-u.ac.jp. † Department of Molecular Design and Engineering, Nagoya University. ‡ Department of Electrical Engineering and Computer Science, Nagoya University. § EcoTopia Science Institute, Nagoya University. | AIST.

nature of support materials on the catalytic activity and to discuss whether reducible semiconductor oxides are needed in a Au NPs-catalyzed reaction. Selective hydrogenation of the nitro group in the presence of other reducible functional groups is an important reaction to produce functionalized anilines as industrial intermediates for a variety of specific and fine chemicals.9 The reaction is performed with a stoichiometric amount of reducing agents, producing a large number of byproducts. Catalytic hydrogenation by H2 is more ideal, but conventional platinum-group metal (PGM) catalysts simultaneously hydrogenate both the nitro and the olefinic or carbonyl functions. Recently, new catalytic systems for the selective reduction of nitroaromatics have been reported,2,10 but functional groups tolerated are limited; only a few reports2,10j,k succeeded in the selective hydrogenation of nitrostyrene. Corma et al.2 reported that Au NPs (3.5-4.0 nm) on TiO2 and Fe2O3 catalyzed the selective hydrogenation of nitrostyrene, whereas Au NPs on inert supports (SiO2 and carbon) as a catalyst resulted in the reduction of both the CdC double bond and the nitro functions. The unique behavior of Au/TiO2 was explained by a cooperative effect between gold and TiO2; H2 is dissociated on Au, and nitrostyrene is adsorbed on the boundary between TiO2 and Au through the nitro group. Theoretical and experimental studies established that dissociative chemisorption of H2 does not occur on clean gold extended surfaces but can occur on low coordinated Au atoms at corners or edges of Au NPs.7 Based on these backgrounds, it is of fundamental as well as industrial importance to study the effects of gold particle size and support material on the title reaction. Herein, we report that gold NPs (2.5 nm) on Al2O3, prepared by the colloid deposition method, catalyze the selective hydrogenation of nitro group in the presence of CdC or CdO groups. Effect of gold particle size and support materials on the catalytic

10.1021/jp906044t CCC: $40.75  2009 American Chemical Society Published on Web 09/16/2009

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TABLE 1: List of Catalysts catalysts-xa

supports

DNP/nmb

M/wt %

preparation method

Tcal/°Cc

Au/Al-2.5 Au/Al-6.0 Au/Al-30 Au/Si-1.9 Au/mont-2.0 Au/Mg-3.0 Au/C-2.5 Au/TiWGC-3.6d Pt/Al-1.3

γ-Al2O3 γ-Al2O3 γ-Al2O3 SiO2 montmorillonite MgO carbon TiO2 (anatase) γ-Al2O3

2.3 5.9 2.3 2.3 2.3 2.3 2.3

1 1 1 2.5 1 1 1 3 1

colloid deposition colloid deposition colloid deposition colloid deposition colloid deposition colloid deposition colloid deposition deposition-precipitation impregnation

300 300 1000 300 300 300 300 500

a Average particle size (nm) of metallic Au species estimated from the EXAFS except for Au/Al-6.0 (TEM), Au/Al-30 (XRD), Au/TiWGC-3.6 (from WGC), and Pt/Al-1.3 (CO adsorption). b Mean diameter of gold NPs in gold colloid used for catalyst preparation. c Calcination temperature. d Standard catalyst with Au loading of 3 wt % supplied from WGC.

efficiency as well as kinetic and spectroscopic studies are also presented to show the origin of the high activity and selectivity. To establish a design concept of gold-based selective hydrogenation catalysts, we show detailed mechanistic and structural studies that address the influence of the particle size and support material on the catalytic activity. 2. Experimental Section 2.1. General. The GC (Shimadzu GC-17A) and GC-MS (Shimadzu GC-17A) analyses were carried out with a Rtx-65 or DB-1 capillary column (Shimadzu) using nitrogen as the carrier gas. Commercially available organic and inorganic compounds were used without further purification. 2.2. Catalyst Preparation. The World Gold Council test catalyst, named Au/TiWGC-3.6 (Au ) 3 wt %, average Au particle size ) 3.6 ( 0.28 nm), was purchased from the World Gold Council. γ-Al2O3 with surface area (SBET) of 224 m2 g-1 was prepared by calcination of γ-AlOOH (Catapal B Alumina purchased from Sasol) at 600 °C for 3 h. MgO (JRC-MGO-1, SBET ) 55 m2 g-1) was supplied from the Catalysis Society of Japan. Montmorillonite K-10 clay purchased from Aldrich has an SBET of 220 m2 g-1 and the following chemical composition (average value): SiO2 (73.0%), Al2O3 (14.0%), Fe2O3 (2.7%), CaO (0.2%), MgO (1.1%), Na2O (0.6%), K2O (1.9%). The basal (001) reflection was not observed in the XRD pattern of K-10, which confirms the delamination of the layered structure of the montmorillonite. Carbon (C) purchased from Aldrich (Darco G-60) has an SBET of 580 m2 g-1. To prepare SiO2 aerogel (SBET ) 600 m2 g-1), silica wet-gel was first synthesized by the hydrolysis of silicon tetramethoxide or silicon methoxide oligomer in methanol: 1 mol of silicon methoxide (as monomer) was dissolved in 10 mol of methanol, and 6 mol of water containing 0.1 N NH4OH was added to the solution. A jellylike bulk solid was yielded by gelation in about 30 min. The product was aged for 1 day and then kept in ethanol. The wetgel was placed in an autoclave with pore-filling ethanol solvent, and CO2 was fed into the autoclave. To substitute the solvent to liquid CO2, the pressure of CO2 was then raised and kept for 2 h at above 7 MPa using a high pressure pump, allowing a small amount of CO2 flowing out of the autoclave with the toluene solute. This operation was done three times to complete the substitution. The temperature of the whole system was kept below 24 °C during the procedure. CO2 was then removed at a supercritical condition: the vessel temperature and pressure were above 40 °C and 8 MPa, respectively. Au NPs supported on various supports (Al2O3, montmorillonite, SiO2, MgO, carbon) were prepared by the colloid deposition method, which has been intensively studied by Tai et al.8a-c In a typical preparation of AuNPs, AuCl4- ions were

extracted from the water into the toluene phase by excess tetraoctylammonium bromide (TOAB). After the toluene phase was separated, the protecting agent dodecanethiol (DDT, Au/ DDT ) 1:1 mol mol-1) was added to it at 40 °C under vigorous stirring. The obtained solution was then left under stirring for 30 min. A following rapid injection of an aqueous solution of NaBH4 (Wako, 95% purity, Au/NaBH4 ) 1:10 mol mol-1) led to formation of a dark orange-brown solution, indicating the formation of the gold sol.11a Transmission electron microscopy (TEM) analysis showed that the mean particle diameter of Au NPs thus prepared was 2.3 ( 0.41 nm. The support was then added to the colloidal gold solution under stirring and kept in contact until total adsorption (1 wt % of gold on the support) occurred, indicated by decoloration of the solution. The solids were collected by filtration followed by washing the solids with toluene to remove all of the soluble species. The resulting composites were dried at room temperature and calcined at 300 °C for 4 h under air for the combustion of thiols.8a-c The Al2O3supported Au NPs catalysts are designated as Au/Al-x, where x is the mean size of Au NPs (nm) estimated from the characterization results, which will be described in the following section. Irrespective of the support materials, the calcined catalysts thus prepared have similar gold particle size (1.9-3.4 nm) as listed in Table 1. For Al2O3, gold colloid with larger mean diameters (5.9 ( 0.49 nm) prepared through annealing the mixture of as-preapared Au NPs and TOAB at 165 °C11b was also used, and the prepared catalyst was named Au/Al6.0. A sample named Au/Al-30, composed of large gold particles (30 nm), was prepared by calcining the Au/Al-2.5 sample at 1000 °C for 3 h. Pt/Al2O3-1.3 (Pt ) 1 wt %) was prepared by impregnating γ-Al2O3 with an aqueous solution of metal nitrate, followed by drying at 80 °C for 12 h, calcining at 500 °C for 2 h, and reducing at 300 °C for 0.5 h in H2. The average particle size of Pt (1.3 nm) was estimated with the CO uptake of the Pt/Al2O31.3 sample at 25 °C using the pulse-adsorption of CO in a flow of He. 2.3. XAFS. Au L3-edge X-ray absorption fine structure (XAFS) measurements were performed in transmission mode at the BL01B1 in the SPring-8. The storage ring was operated at 8 GeV. A Si(111) single crystal was used to obtain a monochromatic X-ray beam. Samples were sealed in cells made of polyethylene under ambient atmosphere, and XAFS spectra were taken at room temperature. The analysis of the extended X-ray absorption fine structure (EXAFS) was performed using the REX version 2.5 program (RIGAKU). The Fourier transformation of the k3-weighted EXAFS oscillation from k space to r space was performed over the range 30-140 nm-1 to obtain a radial distribution function. The inversely Fourier filtered (in

Chemoselective Hydrogenation of Nitroaromatics a range 0.10-0.35 nm) data were analyzed with a usual curvefitting method in k-space in a range of 39-135 nm-1. For the curve-fitting analysis, the empirical phase shift and amplitude functions for Au-Au and Au-O shells were extracted from the data for Au foil and Au2O3, respectively. 2.4. TEM and STEM. TEM images of the AuNPs and catalysts were recorded using a JEOL 2010 microscope equipped with a LaB6 filament operated at 200 kV. High angle annular dark field scanning TEM (HAADF-STEM) images were recorded using a HD-2300S (Hitachi) microscope operated at 200 kV. 2.5. In Situ FTIR. In situ FTIR spectra were recorded on a JASCO FT/IR-620 equipped with a quartz IR cell connected to a conventional flow reaction system. The sample was pressed into 10 mg of self-supporting wafer and mounted into the quartz IR cell with CaF2 windows. Spectra were measured accumulating 5-20 scans at a resolution of 4 cm-1. A reference spectrum of the catalyst wafer (typically 20 mg) in He taken at measurement temperature was subtracted from each spectrum. Prior to each experiment, the catalyst disk was heated in He flow (100 cm3 min-1) at 300 °C for 0.5 h, followed by cooling to the desired temperature under He flow. For the introduction of nitro compounds to the IR disk, the liquid compound was injected under the He flow preheated at 150 °C, which was fed to the in situ IR cell. Next, the IR disk was purged with He for 600 s. 2.6. Typical Procedures for the Catalytic Test. Catalytic experiments of chemoselective hydrogenation of nitro compounds were carried out in a 30 cm3 autoclave with a glass tube inside equipped with magnetic stirring. In each reaction, 2 mmol substrate and 15 cm3 of tetrahydrofuran as solvent were placed into the autoclave together with typically 0.04-0.2 mol % of catalyst. After being sealed, the reactors were flushed with H2 and then pressurized at 3.0 MPa, and then heated to the required temperature (typically 160 °C). Conversion and yields of products were determined by GC using n-dodecane as an internal standard. The products were identified by gas chromatography/mass spectrometry (GCMS-QP5000, Shimazu) equipped with the same column and in the same conditions as GC and also by comparison with commercially pure products. 3. Results and Discussion 3.1. Characterization of Catalysts. We prepared a series of supported gold catalysts with similar Au particle size using thiol-capped colloidal Au NPs (2.3 ( 0.41 nm) and various supports, γ-Al2O3, SiO2, MgO, montmorillonite clay (mont), and carbon (C). Characterization results of EXAFS at Au L3edge indicate that the metallic Au NPs are the dominant Au species in these catalysts. Figure 1 shows the Fourier transform (FT) of the k3-weighted Au L3-edge EXAFS of gold catalysts and Au foil. A representative result for the curve-fitting analysis for the Au/Al-2.5 sample is shown in Figure 2. A simulated k3-weighted EXAFS oscillation obtained using curve-fitting with parameters of the Au-Au shell extracted from Au foil fitted well with the experimental one. The structural parameters derived from the curve-fitting analysis are listed in Table 2. The EXAFS spectra of the samples consist of a Au-Au contribution except for Au/Al-2.5, Au/mont-2.0, and Au/C-2.5, which have a small contribution of Au-O shell with coordination numbers below 1. This indicates that all supported gold species are metallic states. The Au-Au coordination numbers (8.9-9.9) are lower than that of the bulk Au (12). Mean diameters of gold NPs in these catalysts were estimated from the coordination number using the method by Jentys12 and assuming spherical NPs. Note that Jentys confirmed that the

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Figure 1. Au L3-edge EXAFS Fourier transforms.

Figure 2. k3-weighted Au L3-edge EXAFS spectrum of Au/Al-2.5 (solid line) and its best fit derived from curve-fitting analysis (dotted line).

TABLE 2: Au L3-Edge EXAFS Analysis of Various Catalysts catalysts-xa Au/Al-2.5 Au/Al-6.0 Au/Si-1.9 Au/mont-2.0 Au/Mg-3.0 Au/C-2.5

shell

CNb

R/Åc

(σ2)/Å2d

Rf/%e

Au O Au Au Au O Au Au O

9.6 0.3 10.7 8.9 9.5 0.3 9.9 9.6 1.0

2.83 2.06 2.86 2.83 2.87 2.09 2.84 2.87 2.14

0.100 0.050 0.087 0.090 0.072 0.040 0.088 0.082 0.048

1.2 1.6 0.8 0.9 1.3 0.5

a Average particle size (nm) of metallic Au species estimated from the EXAFS. b Coordination numbers. c Bond distance. d Debye-Waller factor. e Residual factor.

shape of the particles has a minor influence on the metal particle size; the differences in the size are within the typical limits of accuracy in the EXAFS coordination numbers ((10%).12 The average particle size x (nm) of metallic Au species is in the catalyst name as Au/Al-x (Table 1). Figure 3A and B shows a HAADF-STEM image of Au/Al2.5 and a TEM image of Au/Al-6.0, respectively. Size distributions of Au particles of Au/Al-2.5 and Au/Al-6.0 are in Figure 3C and D, respectively. Au particles in Au/Al-2.5 mostly ranged from 1.0 to 4.0 nm, and the mean diameter was 2.6 ( 0.73 nm. This value is consistent with the mean diameter estimated from EXAFS analysis (2.5 nm) within the standard deviation of size (0.73 nm) and the experimental error of EXAFS coordination numbers (10%), which confirms that the mean particle size from EXAFS analysis is accurate for the samples of small Au size. Au particles in Au/Al-6.0 ranged from 3.25 to 7.75 nm, and the mean diameter was 6.0 ( 1.1 nm. Note that the mean diameter of Au/Al-6.0 was not estimated from EXAFS because

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Figure 4. Au L3-edge XANES spectra of Au foil (dashed line) and Au catalysts (solid lines).

TABLE 3: Hydrogenation of 4-Nitrostyrene by Various Catalystsa

catalysts-xb

t/h

conv./%

2 (3) sel./%

V/mmol h-1 g-1

Au/Al-2.5 Au/Al-6.0 Au/Al-30c Au/Si-1.9d Au/Mg-3.0c Au/mont-2.0 Au/C-3.4c Au/TiWGC-3.6e Pt/Al-1.3

1 2 3 1 3 1 3 1 1

100 85 58 79 90 73 28 100 100

89 (3.8) 82 (2.5) 6 (2.8) 76 (0) 59 (3.3) 48 (0) 6 (0) 95 (0.5) 0 (100)

153 18 0.5 17 4.2 10 0.2 76 0

a 4-Nitrostyrene (2 mmol), THF (15 mL). Conversion of 1 and selectivities of 2 and 3 were determined by GC. b Mean particle size of Au (nm). Au loading was 1 wt %. c Au ) 0.2 mol %. d Au loading is 2.5 wt %. e Standard catalyst with Au loading of 3 wt % supplied from WGC.

Figure 3. (A) HAADF-STEM image of Au/Al-2.5, (B) TEM image of Au/Al-6.0, and size distributions of Au particles in (C) Au/Al-2.5 and (D) Au/Al-6.0.

the Au-Au coordination number of Au/Al-6.0 (10.7) is too large to estimate the particle size.12 For the Au/Al-30 sample, the average particle size was estimated from the XRD line broadening using the Scherrer equation. Figure 4 shows Au L3-edge X-ray absorption near-edge structures (XANES) spectra, which are known to be sensitive to the 5d electron density of Au species; the larger intensity of the white line feature centered around 11 922 eV corresponds to the lower number of 5d electrons. The XANES spectrum of Au/Al-6.0 is basically identical to that of Au foil, which indicates that Al2O3-supported gold NPs with mean diameter of 6.0 nm

have the electronic structure identical to that of bulk gold. The white line intensity for Au/Al-2.5 is lower than that for Au foil, indicating that Al2O3-supported gold NPs with mean diameter of 2.5 nm have fewer holes in the d band than does Au foil. This result is in good agreement with previous reports by van Bokhoven and Miller.7e They performed a comprehensive XANES study of Au catalysts with different particle sizes and supports and concluded that white line intensity did not depend on support but only depended on Au particles size; 5d electron density increased with decrease in the size when Au particles were smaller than 3 nm. 3.2. Catalytic Properties. It has been already shown by Corma et al.2b,10j that hydrogenation of nitrostyrene by Pt- or Pd-based conventional hydrogenation catalysts results in nonchemoselective reduction to ethylaniline rather than selective reduction to aminostyrene. For example, 5%Pt/C showed 29% and 63.1% selectivities to aminostyrene and ethylaniline, respectively, at 95.6% conversion even under a relatively mild condition (40 °C). To date, a few reports2,10j,k succeeded in the selective hydrogenation of nitrostyrene. Therefore, selective reduction of 4-nitrostyrene to 4-aminostyrene was chosen as the model reaction (Table 3). Pt/Al-1.3 as a conventional hydrogenation catalyst shows 100% selectivity to the undesirable byproduct (4-ethylaniline, 3). In contrast, most of the Au catalysts produced 4-aminostyrene (2) as a main product and much less amount of 3. A typical time-course of the reaction with Au/Al-2.5 is shown in Figure 5. The GC/MS experiment showed that only 3 was a detectable byproduct, but hydroxylamine styrene, azostyrene, and azoxystyrene were not detected.

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Figure 5. Yields of (O) 4-aminostyrene and (4) 4-ethylaniline, (+) selectivity of 4-aminostyrene, and (b) conversion of 4-nitrostyrene for 4-nitrostyrene hydrogenation with Au/Al-2.5 (0.2 mol %) at 120 °C.

TABLE 4: Hydrogenation of Nitroaromatic Compoundsa

entry

T/°C

R

t/h

conv./%

sel. (%)b

1 2c 3 4 5d 6d

120 120 120 65 160 80

4-vinyl 4-vinyl 3-vinyl 4-COCH3 4-CONH2 4-CO2Me

2 2 2 4 2 10

100 95 100 100 100 100

92 (3.0) 89 (2.9) 99 (1) 99 (0) 99 (0) 78 (0.9)

Figure 6. (O) TOF based on the number of surface Au atom for the selective 4-nitrostyrene hydrogenation, (0) initial rates of OH/D2 isotope exchange, and rate constants for (4) PhNO2 ad consumption (k1) and (2) PhNH2 ad formation (k2) in H2 as a function of average particle size of Au in Au/Al.

a Substrate (2 mmol), THF (15 mL). b Selectivity of byproducts in which R groups are also reduced are shown in parentheses. c Second cycle after entry 1. d Au ) 0.04 mol %.

Table 3 includes the rate of 4-aminostyrene formation (V) measured under the condition where conversion is below 40%. Note that for the catalyst with high activity, the rates were determined in separate catalytic experiments using a small amount of catalyst (0.01 mol % with respect to 4-nitrostyrene). The rate depends on the nature of the support material, and Au/ Al-2.5 showed the highest rate. For Au/Al-2.5, the selectivity of 2 gradually decreased with time (Figure 5). Although Au NPs supported on SiO2, MgO, clay, and carbon showed lower conversion than did Au/Al-2.5, these catalysts showed lower selectivity to 2 than did Au/Al-2.5. These results indicate that Au/Al-2.5 shows highest selectivity to 2. The reaction rate and selectivity of Au/Al-2.5 are close to those of Au/TiWGC-3.6 supplied from the World Gold Council (WGC), indicating that the catalytic properties of the Au/Al-2.5 catalyst are comparable to the state-of-the-art catalyst for this reaction.2 The catalyst can be easily separated from the reaction mixture by centrifugation. The separated catalyst was washed with ethanol (5 mL) and distilled water (5 mL), followed by drying at 100 °C for 30 min and calcining in air at 400 °C for 30 min. The conversion and selectivity of the reused catalyst were only slightly lower than those of the fresh catalyst (Table 4). To investigate the scope of the reaction, a series of nitroaromatic compounds with alkene, carbonyl, amide, and ester groups were tested for hydrogenation with Au/Al-2.5 catalyst (Table 4). Good to excellent selectivities (78 L-99%) at 100% conversions were obtained in hydrogenation of the substrates. The GC/MS experiment showed that byproducts in which the substituent groups are also reduced were detected in small amount, but there were no other GC/MS-detectable byproducts. 3.3. Size- and Support-Dependent Activity of Au NPs. On the basis of the catalytic data in Table 3 and structural data, we show that the intrinsic activity of the gold NPs on Al2O3 depends strongly on the gold particle size. By using the mean diameter of metallic gold and the atomic diameter of gold (0.288 nm) and assuming that the supported gold NPs can be modeled as a fcc crystal lattice, the number of surface gold atoms in each

Figure 7. (O) TOF based on the number of surface Au atom for the selective 4-nitrostyrene hydrogenation and (0) initial rate of OH/D2 isotope exchange as a function of electronegativity of support oxide.

catalyst can be statistically determined.1d For a series of Au/Al catalysts with different gold particle size, the turnover frequency (TOF) per surface Au sites is plotted as a function of the average particle size in Figure 6. Clearly, the gold NPs with smaller particle size give higher intrinsic activity, indicating that the present reaction is a structure-sensitive reaction demanding a coordinatively unsaturated Au site. The intrinsic activity of the gold NPs having similar mean size (1.9 L-3.6 nm) depends strongly on the acid-base characteristics of support material. In Figure 7, TOF values are plotted as a function of the electronegativity of metal oxide, which has been used, to a first approximation, as a parameter of acidity (or electrophilicity) of metal oxides.13 The electronegativity of metal oxide is calculated according to the concept of Sanderson14a on the basis of the electronegativity of the element defined by Pauling.14b The result shows that the support with strong basic character (MgO) and that with acidic character (SiO2 and montmorillonite clay) resulted in low activity. This result suggests that both acidic and basic surface sites are necessary for this reaction. The result that Au NPs (3.4 nm) supported on carbon gives 2 orders of magnitude lower reaction rate (Table 3) also suggests some important role of acidic and basic surface sites of support material. It should be noted that the TOF of Au/Al-2.5 (6410 h-1) is more than 3 times larger than that of Au/TiWGC-3.6 (1440 h-1), indicating the higher catalytic activity of Au/Al-2.5 than the state-of-the-art catalyst.2 3.4. Reaction Mechanism. To establish the reaction pathway, we studied the hydrogenation of nitrobenzene by in situ FTIR spectroscopy measured at each temperature in a flow of He or H2 under normal pressure. When nitrobenzene was

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Figure 9. FTIR spectra of adsorbed species on Au/Al-2.5 at 120 °C as a function of time in a flow of H2. Before the measurement (t ) 0 s), 3-nitrostyrene (1.0 mmol g-1) was introduced to the catalyst disk, followed by purging with He for 600 s.

Figure 8. (A) FTIR spectra of adsorbed species on Au/Al-2.5 at 100 °C as a function of time in a flow of H2. Before the measurement (t ) 0 s), nitrobenzene (1.0 mmol g-1) was introduced to the catalyst disk, followed by purging with He for 600 s. (B) Time course of the area of the bands due to PhNO2 ad (1350 cm-1, open symbols) and PhNH2 ad (1603 cm-1, closed symbols) in a flow of (circles) H2 or D2 (triangles).

adsorbed on the Al2O3 support (result not shown) or on the Au/ Al-2.5 catalyst at 100 °C (Figure 8), IR bands due to the νas(NO2) and νs(NO2) of the adsorbed nitrobenzene were observed at 1525 and 1350 cm-1, respectively. The νas(NO2) band has lower frequency than gas-phase PhNO2 (1552 cm-1) and has a frequency close to the νas(NO2) band of nitrobenzene adspecies (PhNO2 ad) on γ-Al2O3 (1524 cm-1) in the literature,15 while very small shifts for the aromatic ring vibration frequencies (1617, 1602, 1583 cm-1) are observed. These results suggest that nitrobenzene on Au/Al-2.5 interacts not with gold surface but with the Al2O3 surface through the nitro group. Reactivity of PhNO2 ad was studied by time-resolved in situ IR measured at 100 °C. The intensity of the band at 1350 cm-1 due to PhNO2 ad on Au/Al-2.5 did not decrease under a flow of He. When the flowing gas was switched to H2, the band due to PhNO2 ad nearly disappeared at t ) 180 s. Simultaneously, new bands due to adsorbed aniline (1498, 1603 cm-1) appeared, and their intensities increased with time. In contrast, PhNO2 ad on Al2O3 did not react under the same condition (not shown). These results indicate that PhNO2 ad is a reactive intermediate and gold NPs play an important role in a H2 activation step. The IR spectrum of 3-nitrostyrene adsorbed on Au/Al-2.5 at 120 °C (Figure 9) showed the bands characteristic to the nitrostyrene interacting with the oxide surface through the nitro group, as observed for Au/TiO2 catalyst.2 When the flowing gas was switched to H2, the band due to 3-nitrostyrene adspecies nearly disappeared at t ) 60 s, and consequently bands assignable to 3-aminostyrene appeared. Therefore, the mechanism obtained from the reaction of nitrobenzene will be applicable to the reaction of substituted nitrostyrenes. To check whether H2 cleavage is involved in the ratedetermining step, the kinetic isotope effect in the hydrogenation of PhNO2 ad under H2 and D2 at 100 °C over Au/Al-2.5 was studied by time-resolved in situ IR. Kinetic curves for PhNO2 ad consumption and PhNH2 ad formation are shown in Figure 8B. The relative amounts of PhNO2 ad and PhNH2 ad were estimated from the area of the IR bands at 1350 and 1603 cm-1, respectivrely, because these bands had the least overlap with other bands and, therefore, were suitable for the quantitative

analysis. From the slope of the kinetic curves, first-order rate constants for the consumption of PhNO2 ad (k1H or k1D) and for the formation of PhNH2 ad (k2H or k2D) under H2 (k1H or k2H) and D2 (k1D or k2D) were estimated, and the kinetic isotope effects (KIE) at 100 °C for PhNO2 ad consumption (k1H/k1D) and PhNH2 ad formation (k2H/k2D) were determined to be 1.4 and 1.5, respectively. These values are in the range of secondary KIE (1 < kH/kD < 2), which suggests that several kinetically relevant steps, including the H-H bond cleavage step, have similar order of reaction rates. The reaction mechanism of gold NPs-catalyzed hydrogenation of nitrobenzenes has been studied by Corma et al.2 On the basis of kinetic, in situ FTIR, and quantum chemical studies, they concluded that aromatic nitro compounds are hydrogenated on Au/TiO2 dominantly through the direct root, that is, PhNO2 f PhNHOH (phenylhydroxylamie) f PhNH2.2c Assuming that this consecutive reaction mechanism via the hydroxylamine intermediate is also applicable in our system, we propose the mechanism for hydrogenation of nitrobenzenes with Au/Al-2.5 in Scheme 1. Previously, Claus et al.16 studied in situ IR experiments on H2 cleavage over Ag/SiO2 catalyst and reported that the isotopic exchange of OH groups of oxide supports to OD groups in D2 begins with the cleavage of D2. We adopted this reaction as a model reaction to estimate the relative rate of the H2 cleavage step over various gold catalysts. Figure 10A shows the IR spectrum obtained after flowing D2 for 600 s to Au/Al-2.5 at 150 °C. Loss and gain of IR band intensities in the AlO-H (3100-3800 cm-1) and in the AlO-D (1900-2800 cm-1) stretching regions were observed. Kinetic curves for the OD formation are shown in Figure 10B. The OD formation rate for Au/Al-2.5 was 3 orders of magnitude higher than that for γ-Al2O3 (result not shown). This gives direct evidence of gold NPs-catalyzed cleavage of a H-H bond. 3.5. Origin of the Size- and Support-Dependent Gold Catalysis. To obtain mechanistic reasons for the size- and support-dependent activity of gold catalyst, kinetic curves for the OH/D2 exchange reaction at 150 °C monitored by in situ IR are compared in Figure 10B. The initial rates of OD formation (∆AOD/∆t) are estimated from the slope of the curve. For Au/Al catalysts with the same Au loading (1 wt %) but with different mean Au particle sizes, the relative rate of OD formation (∆AOD/∆t) decreased when Au particle size increased from 2.5 to 30 nm (Figure 6). This trend is consistent with the change in TOF for the selective 4-nitrostyrene hydrogenation (Figure 6). Taking into account the fact that no dissociative chemisorption occurs on the extended surface of polycrystalline or single crystal gold surface at low temperature, coordinatively

Chemoselective Hydrogenation of Nitroaromatics

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SCHEME 1: Proposed Mechanism for the Al2O3-Supported Gold Cluster-Catalyzed Hydrogenation of Nitroaromatic Compounds

unsaturated sites on gold NPs are responsible for the H2 dissociation as an important step for the title reaction. This conclusion is consistent with the recent theoretical7a and experimental7c works on H2 adsorption on Au NPs. Corma et al. have shown theoretically7a that low coordinated gold atoms are necessary for exothermic dissociative adsorption of H2 on gold surface. Mliller and van Bokhoven have demonstrated experimentally7c that the average number of dissociatively adsorbed hydrogen atoms per surface gold atom in Au/Al2O3 increased with decreasing particle size and concluded that the dissociation and adsorption of hydrogen was limited to the gold atoms on corner and edge positions. Recently, they performed systematic XANES and theoretical studies on gold catalysts with different particle size and showed that decreasing particle size was associated with an increase in the d-electron density of the Au atoms and the onset of reactivity to oxygen.7d This conclusion agrees with our observation for Au NPs-catalyzed hydrogenation. Au NPs with very small size (2.5 nm) are more effective catalysts than the larger NPs (6.0 nm) because the decrease in the particle size results in an increase in the d-electron density of the Au atoms, which leads to an increased reactivity to H2.

Figure 10. HD exchange of surface OH groups of the catalysts at 150 °C in a flow of D2. (A) Difference IR spectra that were obtained by subtraction of the spectra before the D2 admission to the sample under He flow (100 cm3 min-1) from those recorded under D2 flow (20 cm3 min-1, t ) 600 s). (B) Area of OD band versus reaction time.

TABLE 5: Initial Rates for Hydrogenation of Nitrobenzene (VN) or Styrene (VS)a catalyst

T/°C

VN/mmol g-1 h-1

VS/mmol g-1 h-1

VN/VS

Au/Al-2.5 Au/Mg-3.0 Pt/Al-1.3

120 160 40

271 27 27

7.1 1.8 77

38 15 0.35

a

Substrate (2 mmol), THF (15 mL), catalyst (0.1 mol %).

The effect of support oxide on the rate of OH/D2 exchange reaction was also examined. For the catalysts with similar mean Au particle sizes, the OH/D2 exchange reaction rate for acid-base bifunctional support (Al2O3) was higher than those for basic (MgO) and acidic (SiO2) supports (Figures 7 and 10B). This indicates that the surface acid-base pair sites on the support are also required for the H2 dissociation step. 3.6. Origin of the Chemoselectivity. Finally, the origin of the chemoselectivity is investigated. The IR spectrum of 3-nitrostyrene adsorbed on Au/Al-2.5 (Figure 9) showed the bands characteristic to the nitrostyrene interacting with the oxide surface through the nitro group, as observed for Au/TiO2 catalyst.2 A competitive adsorption of 1:1 mixture of nitrobenzene and styrene on Au/Al-2.5 at 100 °C was performed using the same in situ IR method (result not shown). The bands due to adsorbed nitrobenzene were exclusively observed, indicating a preferential adsorption of the nitro group from nitrobenzene versus the CdC bond in styrene. From these results, one of the reasons is the chemoselective hydrogenation of the nitro group in nitrostyrene is the preferential adsorption of nitrostyrene through the nitro group on Au/Al-2.5. A more important origin of the selectivity is the higher intrinsic rate for the reduction of the nitro group than that of the olefinic group. Table 5 compares the initial rates for hydrogenations of nitrobenzene and styrene. For Au/Al-2.5, the rate for hydrogenation of nitrobenzene (VN) is 38 times higher than that for styrene (VS). In contrast, Pt/Al1.3 exhibits higher activity for styrene. The ratio VN/VS of Au/ Al-2.5 was larger than that of Au/Mg-3.0, suggesting an important role of both acid and base sites in the chemoselective reduction of nitro groups. For the chemoselective hydrogenation of polar bonds (CdO or CdN) with homogeneous metal-ligand bifunctional catalysts, it is widely accepted that the reaction begins with heterolytic cleavage of H2 to yield H+ in a OH or NH ligand and H- in metal hydrides, and the H+/H- pair preferentially transfers to the polar bonds.17 On the other hand, it is established that H2 addition to metal particles on solid acids, such as Pt/SO42--ZrO2, Pt/Al2O3, and Rh/Al2O3, results in the formation of acidic OH groups on the support via hydrogen spillocver.18 Adopting these models, a cooperative mechanism catalyzed by low coordinated Au atoms and acid-base pair site of Al2O3 for the hydrogenation of a nitro group can be proposed as follows. The dissociation of H2 at the gold-support interface yields a Hδ- atom on the low coordinated Au atom and a H atom, which releases an electron to the Lewis acid site of the

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support. The latter species becomes a proton stabilized at the oxygen atom (Lewis base) nearby the Lewis acid site. The electrophilic Hδ+ species on the support and nucleophilic Hδspecies on Au atom transfer to the polar nitro group to yield a hydroxylamine intermediate,2 which then undergoes H+/Htransfer to yield the final product (Scheme 1). Conclusion We demonstrated that gold nanoparticles on γ-Al2O3 catalyze highly chemoselective reduction of a nitro group for the reduction of substituted nitroaromatics. The activity depends strongly on the size and support oxides, and small Au NPs (2.5 nm) on the acid-base bifunctional support (Al2O3) give the highest intrinsic activity. Cooperation of the acid-base pair site on Al2O3 and the coordinatively unsaturated Au atoms on the Au NPs are responsible for the H2 dissociation to yield a H+/ H- pair at metal/support interface. High chemoselectivity can be attributed to a preferential transfer of H+/H- to the polar bonds in the nitro group. Fundamental information in this study provides a synthetic strategy of selective hydrogenation catalysis of gold catalysts. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research B (20360361) from the Japan Society for the Promotion Science. References and Notes (1) (a) Haruta, M. Catal. Today 1997, 36, 153. (b) Corma, A.; Garcia, H. Chem. Soc. ReV. 2008, 37, 2096. (c) Hutchings, G. J. Chem. Commun. 2008, 1148. (d) Abad, A.; Corma, A.; Garcia, H. Chem.-Eur. J. 2008, 14, 212. (2) (a) Corma, A.; Serna, P. Science 2006, 313, 332. (b) Corma, A.; Serna, P.; Garcia, H. J. Am. Chem. Soc. 2007, 129, 6358. (c) Corma, A.; Conception, P.; Serna, P. Angew. Chem., Int. Ed. 2007, 46, 7266. (d) Serna, P.; Conception, P.; Corma, A. J. Catal. 2009, 265, 19. (3) (a) Claus, P.; Bruckner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem. Soc. 2000, 122, 11430. (b) Jia, J.; Haraki, K.; Kondo, J. N.; Domen, K.; Tamaru, K. J. Phys. Chem. B 2000, 104, 11153. (c) Okumura, M.; Akita, T.; Haruta, M. Catal. Today 2002, 74, 265. (d) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc. 2003, 125, 1905. (e) Mohr, C.; Hofmeister, H.; Claus, P. J. Catal. 2003, 213, 86. (f) Claus, P. Appl. Catal., A 2005, 291, 222. (g) Panigrahi, S.; Basu, S.; Praharaj, S.; Pande, S.; Jana, S.; Pal, A.; Ghosh, S. K.; Pal, T. J. Phys. Chem. C 2007, 111, 4596. (gg) Bus, E.; Prins, R.; van Bokhoven, J. A. Catal. Commun. 2007, 8, 1397. (h) Zhang, X.; Shi, H.; Xu, B. Q. Angew. Chem., Int. Ed. 2005, 44, 7132. (hh) He, D. P.; Shi, H.; Wu, Y.; Xu, B. Q. Green Chem. 2007, 9, 849. (i) Liu, L.; Qiao, B.; Ma, Y.; Zhang, J.; Deng, Y. Dalton Trans. 2008, 2542. (j) Chen, Y.; Qiu, J.; Wang, X.; Xiu, J. J. Catal. 2006, 242, 227. (k) CrdenasLizana, F.; Gmez-Quero, S.; Keane, M. A. ChemSusChem 2008, 1, 215. (4) (a) Costello, C. K.; Guzman, J.; Yang, J. H.; Wang, Y. M.; Kung, M. C.; Gates, B. C.; Kung, H. H. J. Phys. Chem. B 2004, 108, 12529. (b) Miller, J. T.; Kropf, A. J.; Zha, Y.; Regalbuto, J. R.; Delannoy, L.; Louis, C.; Bus, E.; van Bokhoven, J. A. J. Catal. 2006, 240, 222. (c) van Bokhoven, J. A.; Miller, J. T. J. Phys. Chem. C 2007, 111, 9245. (d) Yang, J. H.; Henao, J. D.; Costello, C.; Kung, M. C.; Kung, H. H.; Miller, J. T.; Kropf,

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