Preparation and catalytic properties of Pd nanoparticles supported on micro-crystal DUT-67 MOFs

Abstract

Zr-based MOFs usually feature exceptionally high thermal and chemical stability, which suggests that composites of noble metals and Zr-based MOFs could have wide industrial applications. In this work, we report the synthesis and characterization of Pd nanoparticles (of three different loadings: 0.3%, 0.5% and 1.0%) supported on micro-crystal DUT-67 MOFs. Via SEM, TEM and XPS characterization methods, it was found that the Pd nanoparticles were well dispersed on the interface of the MOF micro-crystals, with a diameter of 3.5 nm, and both the dangling organic groups and the cavities of the MOFs play important roles. Furthermore, PXRD, IR, TGA and N₂ adsorption measurements confirmed that the composites are very robust. Studies on the catalytic properties indicated that they have good catalytic performance with a conversion of 99% and selectivity of 89% in the Suzuki coupling reaction. By a series of explorations, we found the best catalytic conditions are an ethanol–water mixed solvent as the medium, K₂CO₃ as the base and a temperature of 70 °C. Moreover, good catalytic properties were also shown for the hydrogenation of nitrobenzene, where the optimum temperature was 60 °C, and a conversion of 99% and selectivity of 99% were achieved.

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Introduction

In the last two decades, metal–organic frameworks (MOFs)¹ have attracted enormous attention owing to their regular porous structure and potential applications in adsorption,² separation,³ catalysis,⁴ luminescence,⁵ magnetism⁶ and nonlinear optics.⁷ Arguably most notable in this context is the research on heterocatalysis.⁸ By design or serendipity, many catalytically-active MOFs have been realized for acid catalysis,⁹ base catalysis¹⁰ and asymmetric catalysis.¹¹ The active sites mainly originate from moieties of linking units and unsaturated metal sites. However, owing to the coordination geometry requirements of metal ions and changeable coordination geometry of ligands, these active MOFs usually exhibit huge synthetical challenges, which may hinder the further development of MOFs in industrial applications.

Noble metal nanoparticles (NMNPs) usually show good catalytic performance. When they feature dangling organic groups in the interface, MOF micro-crystals and nano-crystals are good supports and are capable of stabilizing nanosized novel metal clusters.¹² Since the first report of Pd/MOF-5 by Fischer,¹³ efforts made by many chemists have resulted in a series of studies,^12,14 including various MOFs (e.g. MOF-74,¹² ZIF-8,^14g,15 MIL-101,¹⁶ MOF-177 (ref. 17) et al.) and single or binary metal clusters. Within this context, Zr-based MOFs usually exhibit exceptionally high thermal and chemical stability,¹⁸ which promotes their potential use as substrates in industrial applications. Recently, great attention was drawn to NMNP/Zr-MOFs. These studies involved two Zr-based MOFs: UIO-66 and UIO-67. DUT-67 prepared by I. Senkovska et al.¹⁹ is a MOF with a large inner pore of 14.2 Å, which can be regarded as an assembly of Zr₆O₆(OH)₂ building blocks connected by thiophenedicarboxylic acid. However, corresponding studies of NMNP/DUT-67 have not been reported so far.

In this study, we report the synthesis and characterization of a Pd nanoparticle supported DUT-67 catalyst, where Pd nanoparticles with a diameter of 3.5 nm are well dispersed on the interface of the DUT-67 micro-crystals. Catalytic studies showed that they exhibit good performance in the Suzuki coupling reaction and for nitrobenzene hydrogenation.

Experimental details

Structure characterization and property measurements

All reagents and chemicals were purchased from Aladdin and used without further purification. The obtained structure was measured using powder X-ray diffraction (PXRD) patterns in a Panalytical X-Pert pro diffractometer with Cu-Kα radiation. The shapes and morphologies of the catalysts were observed using scanning electron microscopy (SEM, Hitachi S4700) and transmission electron microscopy (TEM, Tecnai G2 F30). A thermal gravimetric analyzer (SDT Q600, TA Instruments Co.) was applied for thermogravimetric analysis (TGA), where samples were heated at a rate of 10 °C min⁻¹ from room temperature to 800 °C in an ambient atmosphere. X-ray photoelectron spectroscopy (ESCALab220i-XL) was carried out using an instrument from VG Scientific with 300 W Al Kα radiation. Fourier transform infrared (FT-IR) spectroscopy was performed using a Nicolet 6700 FT-IR spectrometer (Thermo). The surface area was measured from nitrogen adsorption isotherm experiments with a surface properties analyzer instrument (3Flex, Micromeritics). The resultant products were confirmed using gas chromatography-mass spectrometry (GC/MS, GCT Premier).

Synthesis of DUT-67. DUT-67 was synthesized according to the references as follows: ZrCl₄ (167 mg, 0.5 mmol) solid powder was dissolved in a mixture of DMF (6.25 mL) and NMP (6.25 mL) by sonication for 10 min. Subsequently, 2,5-thiophenedicarboxylic acid (H₂TDC) (56 mg, 0.34 mmol) was added to the mixture and sonicated for 5 min. Then, acetic acid (3.5 mL) was added to the solution and the solution was sonicated for a further 10 min. The resulting mixture was transferred to a 25 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 2 days under autogenous pressure. The obtained powder was filtrated and washed several times with DMF and ethanol until the filtrate became colorless. The resultant product was filtered and dried at 80 °C under a vacuum for three hours.

Synthesis of Pd/DUT-67. Firstly, the as-prepared DUT-67 (0.1 g) was dissolved in 8 mL DMF through sonication for 10 min. Meanwhile, different amounts of PdCl₂ powder (1.0 mg/0.01 mmol for 0.3%Pd/DUT-67; 1.7 mg/0.01 mmol for 0.5%Pd/DUT-67; 3.4 mg/0.02 mmol for 1.0%Pd/DUT-67) were dispersed in another 4 mL DMF by stirring in a 25 mL beaker. Then, both solutions were mixed in a flask and stirred for a further 5 h. NaBH₄ (12 mg for 0.3%Pd/DUT-67; 20 mg for 0.5%Pd/DUT-67; 40 mg for 1.0%Pd/DUT-67) was added dropwise to the mixture. Finally, the color of the solution changed from orange to black. The product was collected by filtration and washed with DMF and diethyl ether several times. The resultant products were dried under a vacuum at 80 °C for 3 hours.

Suzuki coupling reaction using Pd/DUT-67 as catalyst. 0.1 mmol iodobenzene, 0.6 mmol R-substituted phenylboronic acid and 1 mmol base were dissolved in a specific solvent (see the following discussion). Subsequently, 20 mg Pd@DUT-67 was added to the mixture. The catalytic reaction was conducted at 30–80 °C for an hour. The final product was analyzed by GC (FuLi-9790II) with a flame-ionization detector using a weak polarity column (AT.SE-54).

Nitrobenzene hydrogenation using Pd/DUT-67 as catalyst. All catalytic reactions were carried out for 3 hours in a steel autoclave with 2 bar H₂ and a temperature range from 30 to 60 °C. A typical experiment consisted of catalyst (20.0 mg), nitrobenzene (0.3 mL) and ethanol (15 mL). The resultant product was identified by GC (FuLi-9790II) with a flame-ionization detector using a weak polarity column (AT.SE-54).

Results and discussion

Structural characterization

The DUT-67 MOF crystallizes in the Fm3̄m space group and exhibits two different inner pores: 14.2 for the cuboctahedral cage and 11.6 Å for the octahedral cage.¹⁹ The whole framework indicates a binodal 8-connected net, which can be regarded as an assembly of Zr₆O₆(OH)₂ units connected by TDC²⁻ ligands.¹⁹ As shown in Fig. 1, the SEM images show the polyhedral micro-crystals of DUT-67, where the crystal planes have two different shapes: regular hexagon and square. In the face, the edge size is about 1 μm. This phenomenon corresponded well to the reported article about DUT-67. Furthermore, the TEM images indicate that the two different polyhedral micro-crystals feature good crystalline phases.

Fig. 1 Representative SEM (a and b) and TEM (c and d) images for DUT-67.

For the Pd NP-supported DUT-67, we loaded Pd on DUT-67 at the ratios of 0.3%, 0.5% and 1% (see Fig. S1–S3†). The SEM image of the 0.5%Pd@DUT is shown in Fig. S4a.† Fig. 2a and b show the TEM images of the 0.5%Pd@DUT-67, where most of the Pd NPs prefer to disperse on the crystal planes of the DUT-67 than in the porous matrices of the framework. It was observed that some Pd NPs concentrated on the edges of the micro-crystals, suggesting a chemical adsorption interaction between the Pd NPs and the interfaces of the MOFs. From Fig. 2b, we can see the Pd NPs on the interface partially permeated into the cavities of the MOFs. Thus, it was concluded that both the cavities and the interface of the MOFs play important roles for the adsorption of Pd NPs. Moreover, lattice fringes of Pd NPs were observed in Fig. 2b, leading to an interplanar spacing of 2.37 Å. Accounting more than 100 particles in the TEM images revealed that the average diameter of palladium was ca. 3.5 nm (see Fig. 2c). Energy dispersive X-ray spectroscopy (EDS) spectra also confirmed the constituents of Pd and Zr elements on the DUT-67. To further confirm the chemical state of the loaded Pd on the DUT-67, X-ray photoelectron spectroscopy was carried out. Fig. 3a shows the two peaks of Pd (338.4 and 332.9 eV) and Zr (343.9 and 330.3 eV), respectively, which can be assigned to the 3d_3/2 and 3d_5/2 of Pd(0), and the 3p_1/2 and 3p_3/2 of Zr(IV). Thermogravimetric analysis results showed that after loading the Pd NPs the whole framework maintained its original thermal stability (see Fig. 3b). As displayed in Fig. 3c, the FT-IR spectra show that there is no obvious difference between them, which also validates the stability after loading with Pd NPs. Nitrogen adsorption isotherms before and after loading the Pd NPs are shown in Fig. 3d. The specific surface areas, as estimated by Brunauer–Emmett–Teller (BET) methods, were 967 m² g⁻¹ for DUT-67 and 301 m² g⁻¹ for the 0.5%Pd/DUT-67, respectively. Obviously, the surface area decreasing after the Pd NPs loading may be attributed to the fact that the Pd NPs covered the holes or were inset in the cavities. As shown in Fig. 4, the PXRD patterns of the as-synthesized DUT-67 and 0.5%Pd/DUT-67 are in good agreement with the simulated pattern of DUT-67, indicating the whole framework stays stable after the loading of Pd NPs. Meanwhile, no apparent peaks of Pd NPs can be found, which can be attributed to the short-range order of the Pd NPs.

Fig. 2 TEM images (a and b), Pd-NP distribution curve (c) and EDS analysis result (d) for the 0.5%Pd/DUT-67.

Fig. 3 (a) XPS curve of 0.5%Pd/DUT-67; (b) TG curves of DUT-67 and 0.5%Pd/DUT-67; (c) IR curve of DUT-67 and 0.5%Pd/DUT-67 and (d) nitrogen adsorption isotherms of DUT-67 and 0.5%Pd/DUT-67 at 77 K.

Fig. 4 PXRD curves of theoretical DUT-67, experimental DUT-67, and 0.5%Pd/DUT-67 before and after the catalytic reaction.

Catalytic activity for the Suzuki coupling reaction

As the Suzuki coupling reaction is a classical model reaction of Pd catalysis in basic conditions (Scheme 1), we carried out a series of reactions between iodobenzene and phenylboronic acid to identify the catalytic performance of Pd/DUT-67. Firstly, the best reaction conditions, consisting of temperature, solvent and base, were identified. The reaction temperature is an important factor. By changing the temperature from 30 to 80 °C at 10 °C intervals under the medium of a water–ethanol mixed solvent and using the base K₂CO₃, the effects of temperature on the conversion and selectivity were obtained, as shown in Fig. 5. It was found that 70 °C is the optimum temperature. The solvent is also critical for Suzuki coupling reactions. The effect of the reaction solvent often plays an important role. The reaction was carried out with a series of solvents from the solvent mixture of ethanol–water to pure water and DMF under the base K₂CO₃. As can be seen from Table 1, in water, the selectivity (27.33%) was very low in spite of having a high conversion of 99%. For the case of ethanol as solvent, comparing the pure water to the solvent mixture, it was observed that although the conversion remained high, the product yield decreased rapidly. However, both the conversion and selectivity of the catalytic reaction were zero in DMF. Therefore, it was found that the mixed solvent of water–ethanol is the most appropriate solvent. Meanwhile, the base is also a critical factor in Suzuki coupling reactions. In this regard, different kinds of base, consisting of inorganic bases (K₂CO₃, CsF) and organic bases (triethylamine, triethanolamine), were examined. The organic bases exhibited low selectivity regardless of high conversions. Among the two kinds of inorganic base, CsF had a low conversion and low selectivity, while K₂CO₃ showed high conversion and selectivity. Generally, K₂CO₃ is the best base for this catalytic reaction.

Scheme 1 Scheme of the Suzuki coupling reaction.

Fig. 5 The relationship of conversion or selectivity vs. temperature under the medium of a water–ethanol mixed solvent and the base K₂CO₃.

Table 1 Effect of solvent and base on the Suzuki coupling reaction of iodobenzene and phenylboronic acid using 0.5%Pd/DUT-67 as the catalyst at 70 °C

Exp. ID	Solvent	Base	Conversion (%)	Selectivity (%)
1	Water + ethanol	Triethylamine	98	34.2
2	Water + ethanol	Triethanolamine	77.4	21.0
3	Water + ethanol	CsF	37.7	0
4	Water + ethanol	K2CO3	99.0	89.0
5	DMF	K2CO3	0	0
6	Water	K2CO3	99.0	27.0
7	Ethanol	K2CO3	98.0	84.0

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Secondly, the effect of the loading value of Pd on the catalytic performance was studied (Table 2). As the loading value of Pd was increased to 1%, the conversion remained unchanged, while the selectivity decreased. This may be due to the fact that the Pd NPs have integrated (see Fig. S1†). As the loading value of Pd decreased to 0.3%, both the conversion and selectivity were reduced to 93.2% and 14.3%, respectively, owing to the fact that the amount of catalyst is insufficient for the whole reaction. Therefore, 0.5%Pd is the optimal loading value. For the case of 0.5%Pd/DUT-67, the influence of the substituent group in phenylboronic acid was further examined. When phenylboronic acid was replaced with 4-chlorophenylboronic acid, the conversion increased to 98% and the selectivity reduced to 76%. When p-tolylboronic acid was substituted for phenylboronic acid, the conversion was unchanged and the selectivity reduced a little to 81.8%. It was found that Pd/DUT-67 prefers phenylboronic acid with electron donating groups. Moreover, a series of blank experiments were also compared. The PXRD curve from after the catalytic reaction indicated that the whole framework remained stable, as shown in Fig. 4. The SEM image (see Fig. S4b†) also confirmed this point. Therefore, it is shown that 0.5%Pd/DUT-67 is a good catalyst for the Suzuki coupling reaction. Recycling experiments showed that at the 4^th cycle the conversion still remains at ca. 80%, as shown in Fig. S5 and S6.†

Table 2 Effect of catalyst on the Suzuki coupling reaction. Conditions: 70 °C, water–ethanol mixed solvent, K₂CO₃

ID	R	Catalyst	Conversion (%)	Selectivity (%)
1	H	1.0%Pd@DUT-67	99.0	78.9
2	H	0.3%Pd@DUT-67	93.2	14.3
3	H	0.5%Pd@DUT-67	99.0	89.0
4	CH3	0.5%Pd@DUT-67	98.0	81.8
5	Cl	0.5%Pd@DUT-67	98.0	76.0
6	H	DUT	48.8	0
7	H	No K2CO3	30.0	42.0
8	H	No catalyst	64.7	0

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Owing to the huge applications of aniline in the areas of colourants, resins and medicine, nitrobenzene hydrogenation is an important catalytic model reaction. In this study, the hydrogenation of nitrobenzene under relatively mild conditions was used to evaluate the catalytic performance of Pd/DUT-67. Firstly, we conducted a blank experiment. When the reaction was carried out without a catalyst, there was no aniline in the product. Then, we investigated the effect of reaction temperature on the catalytic performance. Table 3 illustrates the conversion and selectivity as a function of temperature. At 30 °C, the conversion was only 42%, while the selectivity reached 99%. As the temperature was increased, the conversion gradually enhanced and reached a maximum of 99% at 60 °C. When 1.0%Pd/DUT-67 was used instead of 0.5%Pd/DUT-67, at 60 °C the same catalytic performance was observed, as shown in Table 3. The PXRD pattern measured after the catalytic reaction showed that the whole framework stayed stable, as shown in Fig. 4. However, when the loading value of Pd was reduced to 0.3%, the conversion was very low. Thus, 0.5%Pd is the most suitable loading value.

Table 3 Effect of temperature on nitrobenzene hydrogenation

ID	T/°C	Catalyst	Conversion (%)	Selectivity (%)
1	60	DUT	0	0
2	60	No catalysis	0	0
3	30	0.5%Pd/DUT-67	42.0	99.0
4	40	0.5%Pd/DUT-67	85.1	99.0
5	50	0.5%Pd/DUT-67	92.4	99.0
6	60	0.5%Pd/DUT-67	99.0	99.0
7	60	1.0%Pd/DUT-67	99.0	99.0
8	60	0.3%Pd/DUT-67	10.8	99.0

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Conclusions

In summary, we prepared three Pd NPs supported micro-crystal DUT-67 MOFs with different loadings of Pd. Via various characterization studies, it was observed that the Pd NPs were well distributed in the interface of the DUT-67 and had a diameter of 3.5 nm. Catalytic measurements indicated that they have good catalytic performance in the Suzuki coupling reaction, where the best catalytic conditions were an ethanol–water mixed solvent as the medium, K₂CO₃ as the base and a temperature of 70 °C. Furthermore, the nitrobenzene hydrogenation model reaction was also examined. It was found that when the temperature was 60 °C, both the conversion and selectivity reached 99%. Therefore, it was observed that Pd NPs/DUT-67 shows a high catalytic performance.

Acknowledgements

This work was supported by the 973 project (2013CB733501) and the National Natural Science Foundation of China (21101137, 21136001, 21176221, 21306169 and 91334013).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: TEM image and EDS for 1%Pd/DUT-67 and 0.3% Pd/DUT-67; SEM image of 0.5% Pd/DUT before and after catalytic reaction; reusability results of 5% Pd/DUT-67. See DOI: 10.1039/c5ra03286e

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