Palladium nanoparticles embedded in metal–organic framework derived porous carbon: synthesis and application for efficient Suzuki–Miyaura coupling reactions

Abstract

A nanoporous carbon (NPC) material was prepared by one-step direct carbonization of a metal–organic framework, MOF-5, without additional carbon precursors. Pd nanoparticles were immobilized on the MOF-5-derived NPC by an impregnation method coupled with subsequent reduction with NaBH₄. The prepared catalyst was in-depth characterized by X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, and N₂ adsorption. The catalyst was used to catalyze the Suzuki–Miyaura coupling reactions and exhibited high catalytic efficiency with yields ranging from 90% to 99% under mild conditions. The results demonstrated the great application potential of MOF precursor-based metal nanoparticle composites in catalysis.

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Introduction

The palladium-catalyzed Suzuki–Miyaura cross-coupling reaction plays a key role in the synthesis of pharmaceuticals, herbicides, polymers and so on.^1–3 Palladium organic complexes containing ligands such as phosphine, dibenzylideneacetone, and carbenes have been widely used as homogeneous catalysts for the Suzuki–Miyaura cross-coupling reaction.^4,5 Although homogeneous catalytic systems are known to exhibit high activity, problems associated with the separation and recovery of the expensive catalysts limit their large scale application in industry.^6,7 On the contrary, heterogeneous catalysts can be easily recovered from the reaction system and readily reused. Recently, as a new generation of heterogeneous catalysts, Pd nanoparticles supported on solid supports has gained much attention due to their ease of separation and satisfactory reusability, superior catalytic performance, good stability in comparison to the traditional homogeneous catalysts.⁸ The supports for metal nanoparticles also play an important role to prevent the aggregation of metal nanoparticles, which would result in loss of their catalytic activity. Therefore, suitable supports for Pd nanoparticle catalysts are highly desirable.

Several solid materials, such as active carbon,⁹ zeolites,¹⁰ mesoporous silica¹¹ and polymers,^12,13 have been used as heterogeneous catalyst supports for C–C coupling reactions. However, some of the supported catalysts involve disadvantages such as, uneven distribution of the active sites, tedious preparation of the catalyst, lower catalytic activity and so on. Therefore, the development of highly active and selective heterogeneous catalysts for catalyzing C–C coupling reactions under mild conditions is required for practical use.

Recently, nanoporous carbons (NPCs) materials, have been touted as ideal catalyst support for heterogeneous catalyst because of their high specific surface area and porosity¹⁴ in combination with high chemical (acid and base resistant), thermal (>1000 K) and excellent mechanical stability.^15,16 Highly porous carbons can be prepared via a variety of methods, including direct pyrolysis of organic precursors, activation (physical or chemical) of carbon,¹⁷ hard-template or soft-template route,¹⁸ and nanocast method.¹⁹ However, these methods often encounter one or more shortcomings, such as the obtained carbon materials often contain disordered structures, the templates need to be removed after carbonization, the preparation process is tedious and so on. Therefore, the development of novel synthesis strategy based on facile, versatile, and reproducible method is highly desirable.

As an emerging class of highly porous material, metal–organic frameworks (MOFs) have gained particular attention in recent years mainly due to their ease of preparation, designable framework structures and multifunctional nature.²⁰ MOFs with fascinating diverse structures, topologies, permanent nanoscale porosity, high surface area and uniform structured cavities, have been demonstrated to be ideal templates for fabricating nanoporous carbon. Highly nanoporous carbon can be fabricated by direct carbonization of different MOFs without the need of additional carbon precursors because MOFs was taken as both a sacrificial template and a secondary carbon precursor. Several MOFs, such as UiO-66,²¹ MIL-100,²² and Al-PCP,²³ have recently been demonstrated as promising templates to construct porous carbons. Compared with the conventional nanocasting technique with several steps, the merit of this approach lies in that it is a facile, controllable, and single-step procedure method. NPCs derived from MOFs have found a wide range of applications in different fields, such as water treatment, contamination removal,²⁴ separation, electrode materials,¹⁸ gas storage²⁵ and carriers for drug delivery systems,²⁶ due to their high specific surface area, large pore volume, high thermal stability and excellent electrochemical performance.

In continuation of our efforts in the development of a sustainable and active catalyst system,^27–29 in this paper, porous carbon was fabricated by one-step direct carbonization of MOF-5 (Zn₄O(H-BDC)₃, BDC = 1,4-benzenedicarboxylate) without using any additional carbon precursors. The obtained MOF-5-derived NPC exhibited a high specific surface area and large pore volume. Pd nanoparticles were immobilized on MOF-5-derived NPC by an impregnation method coupled with subsequent reduction with NaBH₄. To evaluate the performance the MOF-5-NPC-Pd, Suzuki–Miyaura reaction was selected as the model reaction. The results indicated that the MOF-5-NPC-Pd can catalyzed the Suzuki–Miyaura reaction efficiently under mild conditions.

Experimental

Materials and methods

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), terephthalic acid (H₂BCD), sodium borohydride, potassium carbonate, chloroform, and N-N dimethylformamide (DMF) were all obtained from Chengxin Chemical Reagents Company (Baoding, China). Aryl boronic acids, aryl halides, and palladium chloride (PdCl₂) were purchased from Aladdin Reagent Limited Company and used as received.

X-ray photoelectron spectroscopy (XPS) was performed with a PHI 1600 spectroscope using Mg Kα X-ray source for excitation. Scanning electron microscopy (SEM) studies were conducted on a Hitachi (Model S4800) instrument. The size and morphology of the nanoparticles were observed by transmission electron microscopy (TEM) using a JEOL model JEM-2011 (HR) at 200 kV. The Brunauer–Emmett–Teller (BET) surface areas were determined from the N₂ adsorption at 300 K using V-Sorb 2800P (China). The amount of Pd and Zn was determined by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES) on Thermo Elemental IRIS Intrepid II.

Synthesis of MOF-5-NPC

MOF-5 nanocrystals were prepared according to the reported synthesis protocol.³⁰ Typically, MOF-5 was prepared by mixing ZnNO₃·6H₂O (10.4 g) and H₂BCD (2 g) in DMF (140 mL) until complete dissolution of the solids. Then, the mixture was transferred into a round-bottom flask connected to a condenser and heated at 120 °C for 24 h. After cooling, the supernatant was removed and crystals deposited on the bottom of the flask were collected, washed with DMF, and then immersed in fresh chloroform overnight. The chloroform was changed twice during two days. Finally, crystals were collected and dried in a vacuum at 50 °C for 3 h. For the synthesis of MOF-5-NPC, MOF-5 was directly carbonized at various temperatures (from 700 to 1000 °C) under a flow of nitrogen gas. After the carbonization, the obtained black samples were denoted as MOF-5-NPC-700, MOF-5-NPC-800, MOF-5-NPC-900, and MOF-5-NPC-1000, respectively.

Synthesis of MOF-5-NPC-Pd

Pd nanoparticles were supported on the MOF-5-NPC by an impregnation method. 50 mg of MOF-5-NPC was dispersed in 5 mL (1 mg mL⁻¹) chlorine palladium acid solution. Then the resulting mixture was magnetically stirred in ambient temperature. After 5 hours later, the obtained solution was filtered off and washed three times with distilled water, then dried under vacuum at 60 °C. The resultant was dispersed in 5 mL of water, and 5.5 mg NaBH₄ (0.027 mmol) was added slowly to the resulting mixture. Subsequently, the pH of the mixture was adjusted to 10 with 10% sodium hydroxide solutions and the reaction was stirred for 2 h at 98 °C. Finally, the isolated product was washed with distilled water and dried at 60 °C for 5 hours in vacuum.

To preparation of the Zn-free MOF-5-NPC-900-Pd, 50 mg of MOF-5-NPC-900 powder was dispersed in 20 mL of 10% HCl and standed for 6 h. And then the mixture was filtered and washed with distilled water. Finally, the MOF-5-NPC-900 without Zn or ZnO was obtained by vacuum drying at 60 °C for 2 hours.

The Zn-free MOF-5-NPC-900-Pd was prepared according to the same procedure mentioned above except that 50 mg MOF-5-NPC-900 was replaced by 50 mg Zn-free MOF-5-NPC-900.

General procedure for Suzuki–Miyaura reactions

Phenylboronic acid (0.6 mmol), aryl halide (0.5 mmol), base (1.5 mmol), solvents (EtOH/H₂O = 1 : 1, v/v, 4 mL) and a certain amount of the catalyst was placed in a 25 mL round-bottom flask, then the reaction mixture was shaken in an air atmosphere for an appropriate time at room temperature. After completing of the reaction, the reactor was diluted with 10 mL of H₂O and the organic products were extracted with ethyl acetate (3 × 10 mL). Then the combined organic solution dried over anhydrous MgSO₄ and filtered. The pure products were obtained by flash chromatography using petroleum ether:ethyl acetate as the eluent. The preparation process of the catalyst and the Suzuki coupling reaction were illustrated in Scheme 1.

Scheme 1 The preparation process of the catalyst and the Suzuki coupling reaction.

Results and discussion

Characterization of the catalyst

In order to explore the pore structure of MOF-5-NPC samples, the nitrogen adsorption and desorption isotherms were recorded at 77 K. As we can see from Table 1, the sample carbonized at 700 °C and 800 °C, the BET surface area is just 222 and 400 m² g⁻¹ and the total adsorption average pore width is 9.1 nm to 5.47 nm, respectively. These results indicate that the heat treatment at 700 °C and 800 °C is insufficient to carbonize MOF-5 into porous carbon. When the temperature increased to 900 °C, the BET surface area is 2478 m² g⁻¹ and the total adsorption average pore width is 4.8 nm. As displayed in Fig. S1 (see ESI†), the sample MOF-5-NPC-900 showed typical type IV isotherm with sharp uptakes at low relative pressure (P/P⁰ < 0.1) and slight uptakes at high relative pressure (P/P⁰ > 0.9), indicating that they are totally microporous materials with some macropores probably forming between particles. However, when the carbonization temperature was raise to 1000 °C, the BET surface area decreased to 1778 m² g⁻¹ and the total adsorption average pore width rise to 5.47 nm. The reason may be ascribed to that the carbon framework of MOF-5 has already graphitization at high temperature. In addition, it also worth mentioned that the surface area of Zn-free MOF-5-NPC-900 raised to 2700 m² g⁻¹ after the removal of Zn or ZnO impurities through washing with HCl solution. After embedded Pd NPs in the Zn-free MOF-5-NPC-900, the BET surface area of the sample deceased to 2200 m² g⁻¹, the result indicated that Pd NPs were embedded in the Zn-free MOF-5-NPC-900 successfully. The amount of Zn in Zn-free MOF-5-NPC-900 was determined by means of ICP-AES and the amount is <0.001 wt%.

Table 1 The nitrogen adsorption–desorption isotherm and pore size distributions of the MOF-5-NPC

Catalysts	SBET/m^2 g^−1	Pore width^a/nm	V^b/cm^3 g^−1
a Total adsorption average pore width.b Total pore volume.
MOF-5-NPC-700	222	9.10	0.51
MOF-5-NPC-800	400	6.60	0.56
MOF-5-NPC-900	2478	4.8	2.96
MOF-5-NPC-1000	1778	5.47	2.93
Zn-free MOF-5-NPC-900	2700	4.35	2.88
Zn-free MOF-5-NPC-900-Pd	2200	4.83	2.65

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The TEM investigations are carried out to observe the size, and the distribution of Pd nanoparticles embedded in Zn-free MOF-5-NPC-900-Pd. The highly existence of Pd nanoparticles, which embedded in Zn-free MOF-5-NPC-900 was clearly observed in Fig. 1A and B. The density of Pd nanoparticles in Zn-free MOF-5-NPC-900-Pd nanocatalyst is abundant and these nanoparticles did not form clusters. Fig. 1B and C showed the high-resolution TEM (HRTEM) image of the as-prepared catalyst, wherein the lattice spacing is 0.22 nm, which is just the lattice spacing of face centered cubic (fcc) Pd (0.22 nm), further indicating that is Pd nanoparticles. Besides, the mean diameter of the particles was just 2.7 nm and was almost similar to the pore diameter of the nanoporous carbon support. It also can be clearly seen from the SEM image (Fig. 1D) that some of the Pd NPs supported on the surface of the Zn-free MOF-5-NPC-900, and the most of them inserted into the cavities of Zn-free MOF-5-NPC-900. The element mapping images (Fig. S2†) clearly showed the uniform distribution of fine Pd nanoparticles throughout the Zn-free MOF-5-NPC-900-Pd composite.

Fig. 1 TEM (A), HRTEM (B and C) and SEM (D) images of Zn-free MOF-5-NPC-900-Pd.

The X-ray diffraction (XRD) pattern in Fig. 2 showed the crystalline structure of the Zn-free MOF-5-NPC-900-Pd samples. The wide diffraction peak at 2θ = 25° can be indexed to porous carbon.¹⁹ Besides, we characterized the XRD of the catalyst, the broadly peak appear at 40°, and the peaks at 47° and 68° were very poor, which suggests the well dispersion of Pd nanoparticles.^31,32 The three peaks appear at 40°, 47° and 68° can be assigned to (111), (200) and (220) crystal planes of Pd⁰, respectively.

Fig. 2 The XRD image of Zn-free MOF-5-NPC-900-Pd.

The XPS spectra (Fig. 3) demonstrated that the Pd species embedded in the sample Zn-free MOF-5-NPC-900 was present in metallic state rather than in oxidation state with the bond energies of 335.4 and 341.0 eV in the Pd 3d_5/2 and 3d_3/2 levels. As displayed in Fig. 3, the Pd 3d binding energy region was deconvoluted into four peaks with binding energies of 335.6, 337.1, 340.9 and 342.0 eV. The two minor peaks in the Pd 3d spectrum centered at 342.0 and 337.1 eV were assigned to Pd²⁺ 3d_3/2 and Pd²⁺ 3d_5/2, and the other two peaks centered at 340.9 and 335.6 eV are very sharp with a huge intensity. These peaks are assigned to Pd⁰ 3d_3/2 and Pd⁰ 3d_5/2, respectively.^33,34

Fig. 3 The XPS image of Zn-free MOF-5-NPC-900-Pd.

Suzuki–Miyaura reactions catalyzed by the prepared catalyst

In order to investigate the catalytic activity of the different catalysts for Suzuki–Miyaura coupling reaction, the coupling of phenyl boronic acid and phenyl bromide was chosen as model reaction. As we can see in Table 2, among all the catalysts tested, Zn-free MOF-5-NPC-900-Pd was found to be the efficient catalyst for it gave the highest yield of product. Besides, as the data in Table 3, comparing with the other reported Pd nanoparticles catalysts in terms of the reaction temperature, the dosage of catalyst and the yield of the product, the catalyst of Zn-free MOF-5-NPC-900-Pd can efficiently catalyze the Suzuki–Miyaura coupling reaction with comparable or higher yield, especially the coupling reaction which catalyzed by the MOF-5-NPC-900-Pd can be performed efficiently at room temperature. Good dispersion of the Pd NPs on the Zn-free MOF-5-NPC-900 is the key factor for the high catalytic activity of the catalyst. This is mainly owing to the high surface area of Zn-free MOF-5-NPC-900, which leads to high dispersion of Pd on the surface of support.

Table 2 Suzuki–Miyaura coupling reactions catalyzed by different catalysts^a

Entry	Catalysts	Yields/%
a n(K2CO3) : n(aryl iodide) : n(phenylboronic acid) = 4 : 1 : 1.3; solvent: 2 mL ethanol + 2 mL H2O; the dosage of catalyst was 0.1 mol%; room temperature.
1	MOF-5-NPC-700-Pd	68
2	MOF-5-NPC-800-Pd	73
3	MOF-5-NPC-900-Pd	92.1
4	MOF-5-NPC-1000-Pd	90.3
5	Zn-free MOF-5-NPC-900-Pd	98.6

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Table 3 Suzuki coupling reactions of aryl halide with phenylboronic acid catalyzed by different catalysts

Aryl halide	Catalyst	Reaction conditions: solvent/temp./time/Pd loading	Yield/%	Ref.
Bromobenzene	MOF-5-NPC-900-Pd	EtOH–H2O/25 °C/1 h/0.1 mol%	98.6	This work
Bromobenzene	Pd/MIL-53-NH2	EtOH–H2O/40 °C/0.5 h/0.5 mol%	94	35
Bromobenzene	Pd-MCM-41	EtOH–H2O/80 °C/12 h/0.05 mol%	90	36
Iodobenzene	Pd/G	EtOH–H2O/60 °C/02–24 h/0.3 mol%	74	37
Iodobenzene	Pd/SBA-15	EtOH–H2O/60 °C/2–24 h/0.3 mol%	42	37
4-Bromophenyl acetaldehyde	Pd@p-SiO2	DMF–H2O/200 °C/3 h/0.003 mol%	65	38
Iodobenzene	Pd/CMK-3	DMF–H2O/150 °C/10 min/0.02 mol%	99	39

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To standardize the reaction conditions, a series of reactions were performed using several bases, solvents, and different catalyst dosage to obtain the best possible combination. Initially, the experiment was performed using Suzuki–Miyaura cross-coupling reaction of bromobenzene with phenyl boronic acid. As we can see from Table 2, when the reaction was carried out in MeOH, EtOH, DMF and H₂O under the same reaction conditions, the products were obtained in poor yields of 38–70% (Table 4, entries 1–4). However, when we adopted the organic/aqueous co-solvent, high yields of 78–98% were obtained (Table 4, entries 5–7). The merit of the co-solvent can be attributed to the good solubility of the organic reactants and the inorganic base, which can accelerate the transmetalation and contributed to fast overall coupling.⁴⁰ The reaction conditions were also optimized with respect to the base. The reactions were carried out under similar conditions using different bases such as Na₂CO₃, KOH, NaOH and K₂CO₃ (Table 4, entries 7–12), it clearly indicated that K₂CO₃ was the optimal base for the reaction. No product was obtained without base (Table 4, entry13). We can also found that the amount of the catalyst has a great influence on the transformation (Table 4, entries 14–16). When the dosage of the catalyst was above 0.1 mol%, the yield of the product was nearly quantitative (Table 4, entry 14 and 15). And yield of the product was decreased to 77% (Table 3, entry 16) with 0.05 mol% Pd. No product was obtained in the absence of Pd NPs (Table 4, entry 17).

Table 4 Optimization of the reaction conditions for the Suzuki reaction of bromobenzene with phenylboronic acid^a

Entry	Solvents	Bases	Pd/mol%	Yield/%
a Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol), base (1.5 mmol), solvents (4 mL), room temperature.
Solvents effect
1	MeOH	K2CO3	0.1	53
2	EtOH	K2CO3	0.1	70.2
3	DMF	K2CO3	0.1	68.1
4	H2O	K2CO3	0.1	38
5	EtOH/H2O (1 : 3)	K2CO3	0.1	78.1
6	EtOH/H2O (1 : 1)	K2CO3	0.1	98.6
7	EtOH/H2O (3 : 1)	K2CO3	0.1	82.3
Bases effect
8	EtOH/H2O (1 : 1)	NaOH	0.1	72.1
9	EtOH/H2O (1 : 1)	KOH	0.1	78.4
10	EtOH/H2O (1 : 1)	Et3N	0.1	65
11	EtOH/H2O (1 : 1)	Na2CO3	0.1	89.7
12	EtOH/H2O (1 : 1)	K2CO3	0.1	98.6
13	EtOH/H2O (1 : 1)	No base	0.1	Trace
Pd mol%
14	EtOH/H2O (1 : 1)	K2CO3	0.15	99.5
15	EtOH/H2O (1 : 1)	K2CO3	0.1	98.6
16	EtOH/H2O (1 : 1)	K2CO3	0.05	77
17	EtOH/H2O (1 : 1)	K2CO3	0	Trace

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To generalize the application of the Zn-free MOF-5-NPC-900-Pd catalyst, the reaction with a diverse range of arylboronic acids and aryl halides were carried out using 0.1 mol% Zn-free MOF-5-NPC-900-Pd as catalyst at room temperature (Table 5). It was seen that the catalytic system was applicable to various aryl bromides and tolerant to a broad range of functional groups and for most of the substrates, the reaction could be completed in 0.75–1.5 h with high yields, with the substrates having either electron-withdrawing groups (–OH, –COCH₃, –CHO) or electron-donating groups (–OCH₃, –H, –CH₃). It was also found that the yield of reaction para- or meta-substituted aryl bromide is higher than those of ortho-position aryl bromides. To test the feasibility of the aforementioned protocol for challenging substrates, several aryl chlorides with phenylboronic acid were employed and the reaction time was extended to 5 h. And the desired products were obtained in moderate yields (Table 5, entries 13, 14, 15), this just owning to the strength of the C–Cl bond, whose bond dissociation energy was 96 kcal mol⁻¹.⁴¹

Table 5 Suzuki–Miyaura coupling of arylboronic acids and aryl halides catalyzed by Zn-free MOF-5-NPC-900-Pd^a

Entry	X	Y	Z	t/min	Yield^c/%
a Reaction conditions: bromobenzene (0.5 mmol), phenyl boronic acid (0.6 mmol), K2CO3 (1.5 mmol), solvent: 2 mL ethanol + 2 mL H2O, catalyst: Zn-free MOF-5-NPC-900-Pd (0.1 mol%), room temperature.b Aryl chlorides (0.5 mmol), phenyl boronic acid (0.6 mmol), K2CO3 (1.5 mmol), solvent: 2 mL ethanol + 2 mL H2O, catalyst: Zn-free MOF-5-NPC-900-Pd (0.5 mol%), 78 °C.c Yield based on column chromatography.
1	Br	H	H	60	98.6
2	Br	4-OCH3	H	45	93.1
3	Br	4-OH	H	45	95.4
4	Br	4-OH	2-CH3	60	94.6
5	Br	2-CH3	H	90	91.3
6	Br	4-CH3	4-NO2	90	93.8
7	Br	4-OCH3	4-NO2	50	91
8	Br	4-COCH3	H	45	96.3
9	Br	4-OH	3-CH3	60	96.2
10	Br	4-CHO	H	45	96.1
11	Br	4-CHO	4-NO2	50	93.8
12	Br	4-COCH3	4-NO2	50	95.3
13^b	Cl	H	H	300	74.2
14^b	Cl	4-OCH3	H	300	30.7
15^b	Cl	2-CH3	H	330	25.2

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The recovery and reusability of the catalyst were performed, using the reaction of bromobenzene with benzeneboronic acid as the representative reactants, which catalyzed by 0.1 mol% of Zn-free MOF-5-NPC-900-Pd. The results demonstrated that Zn-free MOF-5-NPC-900-Pd can be reused 5 times without significant loss of catalytic activity (Fig. 4). The catalyst after 5 cycles was characterized by TEM (Fig. S3†) and XPS (Fig. S4†), which was similar with that of the fresh prepared catalyst. The amount of Pd in the catalyst after five cycles decreased from 4.85% to 4.4%, which might be the reason for the disability of the catalyst.

Fig. 4 Recyclability of the Zn-free MOF-5-NPC-900-Pd for the Suzuki–Miyaura reaction.

Conclusions

In summary, we have demonstrated that the fabrication of nanoporous carbon derived from a low-cost, facile and readily reproducible MOF and served as the carrier material for palladium nanoparticles. The resultant Zn-free MOF-5-NPC-900-Pd possessing large pore volumes and high surface area, the catalyst is highly efficient in the Suzuki–Miyaura coupling reaction between aryl boronic acid and aryl halide even the reaction was carried out at ambient temperature. Besides, good yield was obtained after the catalyst reused five cycles. The present research might highlight the development of high catalytic activity heterogeneous catalysts by using MOF-derived porous carbon as hosts for ultrafine metal nanoparticles.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), Natural Science Foundation of Hebei Province (B2015204003) and the Natural Science Foundation of Agricultural University of Hebei (LG201404, ZD201506).

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Footnotes

† Electronic supplementary information (ESI) available: The nitrogen adsorption–desorption isotherms and element mappings of the catalyst, TEM and XPS images of the catalyst after 5 cycles, ¹H NMR data of the products. See DOI: 10.1039/c6ra00378h

‡ The authors contribute equally to this work.

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