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Visible-Light-Driven Photocatalytic Suzuki−Miyaura Coupling Reaction on Mott−Schottky-type Pd/SiC Catalyst Zhifeng Jiao,† Zhaoyang Zhai,†,‡ Xiaoning Guo,*,† and Xiang-Yun Guo† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of the Chinese Academy of Sciences, Beijing 100039, China

‡

ABSTRACT: The Mott−Schottky heterojunction in Pd/SiC can continuously transfer photogenerated electrons to Pd nanoparticles and leave holes in SiC under irradiation. The electrons in Pd particles and holes in SiC played different roles in the photocatalytic Suzuki−Miyaura coupling reaction, respectively, for cleaving the C−Br or C−I bond in benzene halides and the C−B bond in phenylboronic acids. Therefore, the intrinsic activity of Pd was dramatically enhanced when employing SiC-supported Pd nanoparticles as the photocatalyst for the coupling reaction. The Mott− Schottky-type Pd/SiC catalyst in the coupling of iodobenezene and phenylboronic acid showed a high turnover frequency of 1053 h−1 and a selectivity of nearly 100% under visible-light irradiation at 30 °C. This provides a green photocatalytic route for synthesizing biaryl compounds and a facile strategy for designing novel photocatalysts for a wide range of organic transformations driven by visible light.

1. INTRODUCTION Suzuki−Miyaura reaction can link an aryl halide and an aryl boronic acid under mild conditions to achieve C−C coupling.1 Because the coupling reaction can be conveniently used to synthesize a wide range of organics, including many natural products and pharmaceuticals, it has been recognized as one of the most important transformations in organic chemistry.2,3 At present, the reactions via Suzuki−Miyaura cross-coupling are realized mainly via homogeneous processes using soluble Pd complexes with various ligands as catalysts.4 The Pd complexes exhibit high catalytic efficiency, but the difficulty in separating the catalysts from the reaction mixtures seriously hinders their application in the chemical industry.4,5 Compared with homogeneous catalysis, heterogeneous processes using supported Pd as solid catalysts are expected to overcome the separation problem,2,3,6 but the overall catalytic activity is still relatively low in most cases. To obtain the desired activity, one has to elevate the temperature of the reaction systems. However, an elevated temperature will speed the aggregation of Pd nanoparticles dispersed on solid supports and thus damage the recyclability of the catalysts.3 By employing a light-harvesting catalyst or photocatalyst, the reaction temperature for Suzuki−Miyaura coupling can be lowered to room temperature.7−12 Wang et al. reported a nanostructured Au−Pd photocatalyst for the coupling reaction and obtained an elevated yield under the irradiation of a 809 nm laser at 1.68 W.7 Zhu and co-workers studied Au−Pd alloy nanoparticles on a ZrO2 support as a catalyst for the crosscoupling reaction under visible light (intensity, ∼0.30 W/ cm2).8,9 In the above Au−Pd catalysts, Au nanoparticles absorb visible light via localized surface plasmon resonance13 and provide energetic electrons to Pd. This group also found that © 2015 American Chemical Society

irradiation with light can significantly enhance the intrinsic catalytic performance of Pd nanoparticles at ambient temperatures for Suzuki−Miyaura coupling.10 Pd nanoparticles can strongly absorb the light through interband electronic transitions, and the excited electrons interact with the reactant molecules on the particles to accelerate these reactions. Li et al. reported a Mott−Schottky-type photocatalyst, C3N4-supported Pd nanoparticles, for Suzuki−Miyaura coupling reactions under UV irradiation.11,12 In the Pd/C3N4 catalyst, Pd nanoparticles obtain photogenerated electrons via the Mott−Schottky heterojunction between Pd and C3N4. Cubic silicon carbide (β-SiC) possesses a suitable band gap (∼2.4 eV) and can absorb visible light.14 Moreover, SiC has excellent chemical stability and thermal conductivity, so it can be employed under many harsh reaction conditions, such as strong acid or alkali. Recently, we found that Pd nanoparticles could continuously obtain photogenerated electrons from β-SiC when they form a Mott−Schottky contact.15 Electron-enriched Pd nanoparticles showed high photocatalytic activity for the hydrogenation of furan derivatives under irradiation with visible light. Herein we report that the Mott−Schottky-type Pd/SiC catalyst exhibits excellent activity for Suzuki−Miyaura coupling under visiblelight irradiation at 30 °C.

2. EXPERIMENTAL METHODS 2.1. Preparation of SiC. The preparation of high surface area SiC can be found elsewhere.16 The preparation process includes the xerogel preparation and subsequent carbothermal Received: December 17, 2014 Revised: January 20, 2015 Published: January 21, 2015 3238

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The Journal of Physical Chemistry C reduction. The xerogel was prepared as follows. First, 6 g of phenolic resin and 0.5 g of nickel nitrate were dissolved in 18 mL of ethanol (AR) and then mixed with 25 mL of tetraethoxysilane (TEOS, AR) with stirring. Second, 0.5 mL of hydrochloric acid was added into the mixture, and the mixture was then stirred for 24 h to enhance the hydrolysis of TEOS. Finally, 5 mL of hexamethylenetetramine (HMTA, 35.8%) aqueous solution was dropped into the above mixture for rapid gelation. The xerogel was obtained by drying the gel at 110 °C for 12 h. The carbothermal reduction was carried out in a horizontal alumina tubular furnace. The xerogel was placed in a small alumina boat and then put into the heating zone of the furnace. The xerogel was heated in Ar flow (40 mL/min) to 1000 °C at a rate of 10 °C/min and then to 1300 °C at a rate of 2 °C/min and maintained at this temperature for 5 h. After the furnace was cooled down to room temperature, the as-prepared product containing SiC was collected. The raw product was heated in air at 700 °C for 3 h to remove the residual carbon and subsequently treated with nitric acid (HNO3) and then hydrofluoric acid (HF) to eliminate the unreacted silica and other impurities. A light-green powder was obtained after washing with distilled water. 2.2. Preparation of 3% Pd/SiC Catalyst. The catalyst was prepared by a one-step impreganation−reduction method. A 2 g portion of β-SiC powder with a specific surface area of ∼50 m2/g was dispersed to 56.4 mL of Pd(NO3)2 aqueous solution (0.01 M). After 30 min of stirring, 20 mL of lysine aqueous solution (0.53M) was added drop-by-drop into the above mixture. After another 30 min of stirring, 10 mL of NaBH4 solution (0.35 M) was added dropwise, and 10 mL of 0.3 M HCl was then added to the above suspension. The mixture was separated and put into a vacuum-drying chamber at 60 °C for 24 h. The microstructures of the catalysts were investigated by high-resolution transmission electron microscopy (HRTEM, JEM-2010). X-ray photoelectron spectroscopy (XPS) was measured on a Kratos XSAM800 spectrometer, using Al Kα (hk = 1486.6 eV) X-ray source as the excitation source. 2.3. Photocatalytic Coupling Reaction. The reactions were conducted in a 50 mL reactor with a quartz window for light transmission; 4 mmol of iodobenzene or one of its derivatives, 8 mmol of phenylboronic acid, 3.91 g of Cs2CO3 (12 mmol), 9 mL of DMF, 3 mL of water, and 10 mg of 3% Pd/SiC catalyst were put into the reactor, and the reactant mixture was protected by an Ar atmosphere. The mixture was irradiated under a 300 W Xe lamp for 80 min, and the light intensity was 0.35 W/cm2. The temperature of the reaction system was precisely controlled at 30 °C by an oil bath. The dependence of the catalytic performance on the light wavelength was investigated by employing various optical filters to allow the transmission of specific-wavelength light while the light intensity was kept the same as with the reaction system without an optical filter. After reaction, 2 mL aliquots were collected, centrifuged, and then filtered through a Millipore filter (pore size 0.22 μm) to remove the catalyst particulates. The filtrates were analyzed by BRUKER SCION SQ 456 GC−MS to measure the concentration changes of reactants and products. The quantitative analysis of specific analytes was detected by SIM mode in GC−MS. Conversions were based on the amount of substituted iodobenzene or its derivatives used. The turnover frequency (TOF) in our case was calculated as the following:

TOF = [amount of iodobenzene or one of its derivatives (mol) × conversion (%) × selectivity (%)]/[mass of Pd/SiC (g) × Pd loading (%) × reaction time (h) /M[Pd] (g/mol)]

3. RESULTS AND DISCUSSIONS 3.1. Catalyst Characterization. Figure 1A,B shows the transmission electron microscopy (TEM) images of 3 wt % Pd/

Figure 1. TEM images (A, B), XRD patterns (C), and UV−vis absorption spectra (D) of the 3 wt % Pd/SiC catalyst; the inset pictures in A and B are the size distribution of Pd nanoparticles and the HRTEM image of Pd/SiC, respectively.

SiC catalyst, which reveal the morphology and particle size of the catalyst. Spherical Pd nanoparticles with a mean diameter of 3.6 nm are dispersed uniformly on the SiC surface. From the high-transmission electron microscopy image (HTEM), the interplanar crystal spacing of Pd nanoparticles is 0.19 nm, corresponding to the Pd (200) crystal faces. Figure 1C shows the XRD pattern of the catalyst. All the strong diffraction peaks are corresponding to β-SiC. The peak at 2θ = 46.7° is indexed to cubic Pd. Figure 1D shows the ultraviolet (UV) and visible absorption spectra of pure SiC and Pd/SiC catalyst. The spectra indicate that β-SiC has a strong UV and visible absorption. The maximal peak appears at about 375 nm, which is similar to the previously report.17 However, the Pd/SiC catalyst displays stronger absorption in the UV and visible range, indicating that the light energy can be better exploited by the catalyst. The visible absorption spectra of Pd/SiC above 550 nm are gradually stronger, resulting from the infrared absorption of SiC and the reflection of sample in the measurement. 3.2. Photocatalytic Activity. For the coupling reaction of iodobenzene and phenylboronic acid (Scheme 1), the reaction did not occur using only pure SiC as catalyst or without any catalyst under irradiation. However, the Pd/SiC catalyst 3239

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of the thermal reaction, we can get the contribution of irradiation. It is 40%, 20%, 16%, 14%, and 10% for 400−450, 450−500, 500−550, 550−600, and 600−800 nm, respectively. These values agree well with the UV−visble absorption spectrum of Pd/SiC catalyst (Figure. 2B). The 400−450 nm light contributes to the reaction most obviously, and this can be attributed to the strong absorption of Pd/SiC of the light below 460 nm. The 600−800 nm light only has a contribution of 10%, indicating that the infrared region has limited influence on the reaction. An experiment under the irradiation of ultraviolet light was also conducted. The reaction conditions were strictly controlled to be the same as those under visible-light irradiation. The reaction can be finished in 45 min, and a high activity with a TOF of 1854 h−1 was achieved. Usually, short-wavelength light has higher energy, which results in the bound energy of Pd going to higher energy levels to facilitate reactions of molecules on the Pd nanoparticles.10 3.3. Photocatalytic Mechanism. Control experiments were performed to reveal the mechanism of the photocatalyzed Suzuki−Miyaura coupling reaction. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO)18 is an electron-trapping agent that can capture electrons from Pd nanoparticles. When 0.3 mL of DMPO was added to the reaction system, the conversion of iodobenzene decreased to 7% (Figure 3A). The conversion is almost the same as that in the dark reaction, suggesting that the light-driven reaction is completely quenched. Meanwhile, triethanolamine (TEA)11 was employed as a scavenger to trap the photogenerated holes on the surface of SiC. The conversion of iodobenzene also declined to 8% when 1 mL of TEA was added to the reaction system (Figure 3A). These phenomena indicate that the coupling reaction cannot proceed without the reduction of electrons or oxidation of holes. The enhanced catalytic activity of Pd/SiC originates from the Mott−Schottky contact (metal−semiconductor heterojunctions) between Pd and SiC. When Pd nanoparticles are dispersed on the surface of SiC, they can form metal− semiconductor heterojunctions.11 As the work function (WF) of Pd and SiC is 5.12 and 4.0 eV,19 a built-in potential of 1.12 eV can form between Pd and SiC. The potential forces electrons to transfer from SiC to Pd nanoparticles and results in electron-rich Pd nanoparticles. This can be revealed by the Xray photoelectron spectroscopy (XPS) (Figure 4A). Generally, the binding energy (BE) values of metallic Pd are in the range 334.7−335.5 eV for Pd 3d5/2 and 340.3−340.8 eV for Pd 3d3/2. Therefore, the Pd particles in the Pd/SiC are metallic.

Scheme 1. Photocatalytic Suzuki−Miyaura Coupling of Iodobenzene and Phenylboronic Acid

exhibited excellent photocatalytic activity under the given conditions. The conversion of iodobenzene was 99% and the TOF was 1053 h−1. Without irradiation (dark reaction), the conversion of iodobenzene was only 7% (TOF, 74 h−1), indicating the significant contribution of irradiation to the catalytic activity. Figure 2A shows the influence of irradiation intensity (measured at the quartz window in the reactor) on the iodobenezene conversion. We calculated the contributions of the irradiation to the conversion efficiency by subtracting the conversion of the reaction in the dark from the overall conversion observed when the system was irradiated, with both reactions occurring at identical reaction temperature. Here the conversion of the reaction in the dark is regarded as the contribution of the thermal effect. The increase of irradiation intensity resulted in an almost linear growth of the iodobenezene conversion. The greater contribution of irradiation is achieved at higher light intensity. The contribution from irradiation was 61% at a light intensity of 0.20 W/cm2 and it increased to 93% at a light intensity of 0.35 W/cm2. A stronger light intensity will induce a larger population of photogenerated electrons with high energy to activate reaction. Moreover, high light intensity can create a strong electromagnetic field around the Pd nanoparticles (field enhancement effect).9 The field enhancement effect also contributes to a strong interaction between the Pd nanoparticles and reactant molecules and thus enhanced catalytic activity of the coupling reaction. To investigate the wavelength dependence of the catalytic activity, a series of optical pass filters were employed to allow the transmission of light with a specific range of wavelength. For example, the 450 nm optical filter blocks the wavelength below 450 nm and over 800 nm; in other words, the light irradiating the reactor has a wavelength range from 450 to 800 nm. All the light intensities were strictly the same (0.35 W/ cm2) in every wavelength region. Without any filter, the iodobenzene conversion can reach to 99%. Employing a 450 nm filter, the conversion decreased to 62%. Similarly, the light with a wavelength range from 600 to 800 nm gave an iodobenzene conversion of 16%. By deducting the contribution

Figure 2. Dependences of catalytic activity of 3 wt % Pd/SiC for the coupling reaction on the intensity (A) and wavelength (B) of the irradiation. The pink line in B is the absorption spectrum of the catalyst in the wavelength range of 400−800 nm. 3240

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Figure 3. Dependence of iodobenzene conversion on the volume of DMPO and TEA (A) and the proposed mechanism for the Suzuki−Miyaura coupling reaction on Mott−Schottky-type Pd/SiC catalyst (B).

Figure 4. X-ray photoelectron spectroscopy results of Pd/SiC (A) and photoluminescence spectra of pure SiC and Pd/SiC (excitation wavelength, 320 nm) (B).

However, the lower BE (334.6 and 340.0 eV) of Pd 3d in the Pd/SiC suggests an electron enrichment on the Pd particles. Figure 4B shows the photoluminescence (PL) spectra of pure SiC and Pd/SiC under 320 nm excitation wavelength at room temperature. The PL intensity of Pd/SiC has an obvious decrement compared with that of pure SiC, indicating that the recombination of photogenerated electrons and holes has been effectively suppressed.15,20 The above results suggest that the photogenerated electrons in SiC can continuously move to Pd nanoparticles through the Mott−Schottky contact when the Pd/SiC catalyst is irradiated by light. The electron-rich Pd nanoparticles can activate iodobenzene and produce Pdadsorbed aryl. Meanwhile, the photogenerated holes on the SiC surface can assist in cleaving the carbon−boron bond in phenylboronic acid molecules.9 Then the redox-activated species couple to form the final products. SiC plays an important role in our case; that is, it acts not only as a support in response to visible light but also as active phase to directly participate in the reaction. A schematic mechanism on the photocatalytic coupling reaction is illustrated in Figure 3B. 3.4. General Applicability. The Pd/SiC catalyst can efficiently exploit natural sunlight. Figure 5 shows the evolution of sunlight intensity and ground temperature in 6 h of 1 day. The intensity of sunlight changes from 49 mW/cm2 at 10:00 to the maximum (67 mW/cm2) at 13:00 and then to 28 mW/cm2 at 16:00. The temperature variation ranges from 30 to 35 °C. Under this condition, the coupling reaction of iodobenzene with phenylboronic acid can proceed completely; that is, the

Figure 5. Evolution of sunlight intensity and ground temperature during the sunlight experiment. Reaction conditions: 0.11 mL of iodobenzene (1 mmol), 0.24 g of benzeneboronic acid (2 mmol), 0.98 g of Cs2CO3 (3 mmol), 9 mL of DMF, 3 mL of water, and 35 mg of 3% Pd/SiC catalyst.

yield of biary can exceed more than 99%. This result suggests that the Pd/SiC catalyst has very good sunlight applicability. To investigate the generality of the Pd/SiC photocatalyst in Suzuki−Miyaura cross-coupling, a series of substituted iodobenzenes and phenylboronic acids were tested (Table 1). Iodobenzenes with electron-withdrawing substitutions (entries 1−3) show higher reactivity than those with electron-donating substitutions (entries 4−6). The coupling reactions of 3241

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Table 1. Photocatalytic Performances of Pd/SiC for Suzuki−Miyaura Coupling Reaction of Aryl Halides with Phenylboronic Acidsa

a

Reaction conditions: 4 mmol of iodobenzene or one of its derivatives, 8 mmol of phenylboronic acid, 3.91 g of Cs2CO3 (12 mmol), 9 mL of DMF, 3 mL of water, and 10 mg of 3% Pd/SiC catalyst. Reaction time is 80 min, temperature is 30 °C, and the irradiation intensity is 0.35 W/cm2. bThe reaction time is prolonged to 5 h and the other conditions are the same as in footnote a. cThe other products are the debromination products of brombenzene derivatives.

Figure 6. Recyclability of 3% Pd/SiC catalyst in the Suzuki−Miyaura coupling reaction (A), TEM image of the catalyst after five rounds of recycling (B), and XPS results of the used catalyst (C).

90%, but almost half of them become dehalogenation products rather than coupling with phenylboronic acid. 3.5. Stability. The superiority of heterogeneous catalysts lies in their good stability and recyclability. To investigate the recyclability of Pd/SiC catalyst, the used catalyst was recycled by filtration and drying. Under the same reaction condition, the catalyst was tested for five cycles. Compared with the first cycle, the catalyst showed almost no loss in its catalytic activity in the following four cycles (Figure 6A), suggesting the excellent stability of this catalyst. TEM images of the catalyst after recycling five times also indicate that the catalyst did not show

iodobenzene with substituted phenylboronic acids were also investigated, and nearly quantitative conversion and selectivity were obtained (entries 7−9). Encouraged by these promising results, we also investigated the performances of Pd/SiC for some bromobenzene derivatives. The results suggest that the catalyst also has general applicability to bromobenzene derivatives (entries 10 and 11). However, the catalyst exhibits lower activity and selectivity for the coupling of bromobenzene derivatives with phenylboronic acid. After 5 h of reaction, bromobenzenes show an apparent conversion of more than 3242

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an obvious change in morphology or aggregation of Pd nanoparticles (Figure 6B). The XPS results of the used catalyst also showed that the Pd nanoparticles still existed on the SiC surface as a metallic phase, suggesting that SiC as a support can effectively stabilize Pd nanoparticles (Figure 6C).

4. CONCLUSION In summary, the present work shows that Mott−Schottky-type Pd/SiC catalyst can harvest visible light and catalyze the Suzuki−Miyaura cross-coupling reaction. The catalytic activity of Pd nanoparticles under irradiation can be evidently increased by using the photoactive semiconductor SiC as the support. This can be attributed to the formation of Mott−Schottky heterojunctions between Pd nanoparticles and SiC promoting the transfer of photogenerated electrons in SiC to Pd nanoparticles. By employing this novel heterogeneous photocatalyst, Suzuki−Miyaura coupling reaction can proceed at 30 °C under the irradiation of visible light, even natural sunlight. The work will inspire the further exploitation of semiconductor-supported metal nanoparticles as photocatalysts for a wide range of organic transformations driven by light.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-351-4040468. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by NSFC projects (NO. 21403270) and SKLCC (2013BWZ006 and 2014BWZ006). REFERENCES

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