Pd@ZIF-67 Derived Recyclable Pd-Based Catalysts with Hierarchical

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Pd@ZIF-67 derived recyclable Pd-based catalysts with hierarchical pores for high-performance Heck reaction Awu Zhou, Rui-Mei Guo, Jian Zhou, Yibo Dou, Ya Chen, and Jian-Rong Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03525 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Sustainable Chemistry & Engineering

Pd@ZIF-67 derived recyclable Pd-based catalysts with hierarchical pores for high-performance Heck reaction Awu Zhou, Rui-Mei Guo, Jian Zhou, Yibo Dou,* Ya Chen and Jian-Rong Li* Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, No. 100, Pingleyuan, Chaoyang District, Beijing 100124, China *Corresponding authors E-mail: [email protected] (J.-R. Li); [email protected] (Y. D). KEYWORDS. Metal-organic framework (MOF), heterogeneous catalyst, Heck reaction, hierarchical-pore structure, magnetic separation

Abstract: Hierarchical-pore Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT (CNT = carbon

nanotube)

were

fabricated

via

the

one-step

pyrolysis

of

palladium@zeolitic-imidazolate-framework-67 (Pd@ZIF-67), respectively, which employed as heterogeneous catalysts display excellent performance in Heck reaction. These Pd@ZIF-67 derivatives retain the skeleton of precursor and enable abundant exposed Pd active sites, and the pyrolysis treatment creates rich hierarchical pores affording convenient path for reaction substrates accessing the active sites. As a result, they exhibit significantly enhanced yields (> 99%) in Heck reaction of iodobenzene with styrene, compared with that of Pd@ZIF-67 (78%). In addition, these catalysts can be easily recovered by magnetic separation and reused without losing activity. The Pd@Co/CNT was also used in catalyzing other Heck reaction, showing good

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performance. The work thus indicates a feasible method in the construction of hierarchical-pore catalysts from treating metal@MOF composites for various organic reactions. INTRODUCTION The palladium-catalyzed Heck reaction as one of the most important reactions for the carbon-carbon (C-C) bond formation between aryl halides and olefins has been widely applied in the synthesis of chemical products. Traditionally, palladium (Pd) salts or organopalladium complexes with special ligands are used, acting as homogeneous catalysts.1-5 These homogeneous systems often suffer from difficulty in the separation and recycling of expensive Pd based catalysts, which leads to excessive consumption of Pd. Recently, Pd nanoparticles (NPs) based heterogeneous catalysts have attracted intensive attention.6-9 Then, great efforts have been devoted to explore various supports (e.g., metal oxides, silica, carbon, zeolite, and organic polymers) for Pd NPs to enhance the catalytic activity and recyclability. Nonetheless, the catalytic performance is still not satisfactory because the Pd NPs are prone to leaching or/and aggregation, leading to the deterioration of the catalytic activity in some cases.10-13 Therefore, it is essential to develop suitable supports for Pd NPs catalysts for efficient heterogeneous catalysis with high recyclability, which is significant from an economic and ecological perspective. Metal-organic frameworks (MOFs) as a burgeoning class of porous materials have attracted great concern because of their diverse and tunable compositions, structures, and properties, as well as various potential applications.14-17 Notably, the encapsulation of metal (e.g., Pd, Au, Ru, and Pt) NPs within MOFs to form heterogeneous catalysts has intrigued fascinating properties because the pores of MOFs can not only be served as templates for regulating particle size of uniform metal NPs, but also afford good microenvironments that can generate selectivity and activity control on the encapsulated NPs in catalytic reactions.18-21 Various methods have been developed to incorporate metal NPs into MOFs, which are mainly classified

into

“de

novo

assembly”

(termed

“ship-in-a-bottle”)

and

“bottle-around-a-ship” approaches. As for “ship-in-a-bottle” method, the locations of metal NPs within the MOF crystal are often random and unpredictable though

2


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obtained metal NPs in MOFs is small due to the confinement by the frameworks. By contrast, the “bottle-around-a-ship” method can better control over the spatial distribution of the encapsulated metal NPs. In addition, it will not cause damage to the MOF matrix during the formation of the composite.20,21 Despite of great endeavors, the thermal or/and chemical instability of most MOFs usually leads to poor durability of these composites in catalytic reactions under harsh environments.22,23 In addition, a majority of reported MOFs have micropores smaller than 2 nm, which restrict the accessibility of large substrates from outside into the active sites of their metal NPs included composites.24-27 Recently, MOFs derivatives with incorporated functional species from the oxidation, sulfurization, or carbonization have received great attention, particularly for catalytic applications.28-34 For examples, Chen group reported the catalytic reduction of bromate using ZIF-derived nanoscale cobalt/carbon cages.28 Beller group synthesized MOF-derived cobalt NPs as catalysts for preparing various amines.29 Song group prepared ZIF-67 derived Co-based porous carbon catalysts toward the CO2 methanation at low temperature.30 These composites can keep some properties of the precursor MOFs such as porous structure, large surface areas, and uniformly dispersed functional sites.23,35 However, the derivatives of metal NPs included MOFs have been rarely used as heterogeneous catalysts.36-38 The above considerations thus inspired us to fabricate MOF-derivatives supported Pd NPs catalysts by the incorporation of Pd NPs into MOFs and subsequent derivatization treatment, which is expected to endow following advantages. On the one hand, the derivatization treatment enables plenty of micropores of pristine MOFs to transform into mesopores/macropores across the obtained derivatives. These hierarchical pores could facilitate not only the exposure of supported active Pd sites but also the fast diffusion of reactant/product to enhance overall catalytic efficiency. On the other hand, the derivatives would not only have higher structural stability compared with their MOFs precursors but also retain the initial skeletons of parent MOFs, affording stable microenvironments of encapsulated Pd NPs. This would guarantee the resulting catalysts with high durability while remaining good catalytic efficiency to meet practical requirements.

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Scheme 1 Schematic illustration for the fabrication of Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT with hierarchical pores derived from Pd@ZIF-67. Herein,

we

constructed

Pd@zeolitic-imidazolate-framework-67

a

series

(Pd@ZIF-67)

of

hierarchical-pore

derivatives,

Pd@Co3O4,

Pd@Co3O4/C, and Pd@Co/CNT (CNT = carbon nanotube) and evaluated their catalytic activity in Heck reaction. These composite catalysts were feasibly prepared by the pyrolysis treatment of Pd@ZIF-67 under different temperatures (Scheme 1). Combining abundant exposed Pd active sites, fast diffusion of reactant/product, and improved structural stability offered by their retained skeleton with rich hierarchical pores, they give rise to largely enhanced catalytic activity as well as high durability in the Heck reaction of aryl iodide with styrene, compared with pristine Pd@ZIF-67. Furthermore, the Pd@Co/CNT was also checked for catalyzing other Heck reaction, showing good performance. These catalysts can also be quickly recovered by magnetic separation and reused without losing activity. This fabrication strategy of integrating MOFs derivatives with metal NPs can be expanded to construct other hierarchical-pore catalysts for wide range of applications.

EXPERIMENTAL SECTION Synthesis of Pd NPs. Pd NPs were synthesized according to a reported procedure.39 Typically, a mixture of PVP (polyvinylpyrrolidone, 132 mg, Mw = 30 4


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000), methanol (180 mL), and PdCl2 aqueous solution (66 mM, 20 mL) were first refluxed for 3 hours. And the solvent was removed by rotary evaporation. Then, the PVP-stabilized Pd NPs were precipitated by acetone, washed with CHCl3 and hexane to remove excess free PVP, and finally re-dispersed in 200 mL of anhydrous methanol for following application. Synthesis of Pd@ZIF-67. Pd@ZIF-67 was prepared according to the previous reported method with modification.22,40,41 Typically, Co(NO3)2·6H2O (291 mg), PVP (200 mg) and above PVP modified Pd NPs solution (10, 20, or 50 mL) were added into methanol (25 mL). And, 2-methylimidazole (328 mg) was added into methanol (25 mL). Then both of them were mixed and aged at RT for 24 h. After that, purple powders were obtained by centrifugation with methanol to remove un-reacted chemicals, and dried under vacuum at 60 °C overnight. The resulting Pd@ZIF-67 with different mass fractions of Pd (~0.17, ~0.34, and ~0.43 wt %) were collected, which were denoted as Pd@ZIF-67-x (x = 10, 20, and 50), where x indicates the volume (mL) of Pd NPs suspension added in the precursor solution, respectively. The mass fractions of Pd NPs were determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian 710-OES). Synthesis of Pd@ZIF-67 derivatives. Pd@Co3O4-x were synthesized through the calcination of Pd@ZIF-67-x at 300 °C for 3 h. Pd@Co3O4/C-x and Pd@Co/CNT-x were obtained following a similar procedure at different pyrolysis temperature of 600 °C and 800 °C under N2 atmosphere, respectively. Heck reaction. Typically, aryl iodide (1 mmol), styrene (1.5 mmol), NEt3 (triethylamine, 1.5 mmol), nitrobenzene (0.49 mmol, as an internal standard for gas chromatography (GC) analysis) and the catalyst (0.09 mol% of Pd) were added into N,N-dimethylformamide (DMF, 3 mL).12 The reaction mixture was mixed by stirring at 120 °C for different times under N2 atmosphere. After the completion of reactions, the catalyst was filtered from the reaction mixture for the recycling use. The crude products were quantified by GC-MS analysis (Bruker Scion TQ GC-MS/MS equipped with HP-5MS capillary column). For the recyclability tests, the reaction was performed under the same reaction conditions as described above, except using the recovered catalyst. Each time, the catalyst was collected from the reaction mixture by magnetism separation after catalytic reaction, thoroughly washed with methanol for 5



three times and dried at RT. Characterization. Powder X-ray diffraction (PXRD) patterns were recorded on a XRD-6000 (Rigaku Co.) diffractometer with Cu Kα (λ = 1.5418 Å) radiation (40 kV, 30 mA), with a scan step of 0.02°. X-ray photoelectron spectroscopy (XPS) was obtained on an ESCA LAB250 XPS spectrometer (Thermo Electron) using Al Kα radiation. The surface area and pore distribution were measured on a Micrometitics surface area analyzer (ASAP 2020). The morphology of the samples was investigated using a scanning electron microscope (SEM Zeiss SUPRA 55) with an accelerating voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (EDX) for the elemental analysis. Transmission electron microscopic and high resolution transmission electron microscopic (TEM and HRTEM) images were recorded on JEOL JEM-2100 instrument at an accelerating voltage of 200 kV. The contact angle was measured using a Dataphysics-TP50 contact angle equipment.

Figure 1 (A) PXRD patterns of the Pd@ZIF-67-x (x = 10, 20, and 50), (B) SEM image, (C) and (D) TEM images (inset: an HRTEM image), and (E) EDX mapping images of Pd@ZIF-67-50. RESULTS AND DISCUSSION

6


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The

Pd@ZIF-67

was

synthesized

by

the

previously

reported

“bottle-around-a-ship” method, in the view of this method providing good control in the catalysis because of the efficient control of shape and composition of the embedded metal NPs. 42,43 The structural and morphology of Pd@ZIF-67 synthesized by the solvent thermal method were firstly characterized. As shown in Figure 1A, the PXRD patterns of Pd@ZIF-67-x samples are identical to that of the parent ZIF-67, indicating good crystallization of ZIF-67 in Pd@ZIF-67 composites. Owning to the low loading of the Pd NPs, no obvious diffraction peaks for Pd NPs are detected in those of Pd@ZIF-67-x. The SEM image (Figure 1B) demonstrates well-dispersed Pd@ZIF-67-50 with rhombic dodecahedron, being similar with the pristine ZIF-67 crystals (Figure S1). These similar crystal shapes suggest that the encapsulation of Pd NPs did not break the structure of ZIF-67, being agreement with PXRD results. Moreover, compared with TEM image of ZIF-67 crystals (Figure S2), it is obvious the Pd NPs with a size of ~6 nm are encapsulated by the ZIF-67 (Figure 1C), in which the Pd NPs are homogeneously dispersed without significant aggregation (Figure 1D). In addition, the interplanar distance of 0.23 nm in the HRTEM image (Figure 1D, inset) can be ascribed to the (111) plane of zero-valent Pd.44 Furthermore, the EDX mapping analysis of Pd@ZIF-67-50 illustrates that the Co, Pd and O elements in Pd@ZIF-67-50 were homogeneously distributed on the whole dodecahedron (Figure 1E). All these results account for the successful encapsulation of Pd NPs into the ZIF-67.

Figure 2 PXRD patterns of the Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT.

7



Figure 3 (A) SEM and (B) TEM images of Pd@Co3O4, (C) SEM and (D) TEM images of Pd@Co3O4/C, (E) SEM and (F) TEM images of Pd@Co/CNT, and (G) and (H) HRTEM images of Pd@Co/CNT. Then, the derivatives Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT were obtained by the pyrolysis of Pd@ZIF-67 under various treatment conditions. The corresponding PXRD patterns of them are shown in Figure 2. As for Pd@Co3O4, the diffraction peaks at 2θ of 19.3, 31.6, 37.1, 45.2, 59.7, and 65.5° can be ascribed to the (111), (220), (311), (400), (511), and (440) reflections of cubic Co3O4 spinel phase (JCPDS no. 43-1003). Owning to the Pd NPs were initially modified by PVP, which contains oxygen element, the resulting derivative, even if the pyrolysis of Pd@ZIF-67 under N2 atmosphere, still contains the oxygen element. As a result, similar diffraction peaks are observed for Pd@Co3O4/C, but the intensity of them is relative weak, which is probably because the Co3O4 is covered by derived C. In the case of Pd@Co/CNT, the main diffraction peaks at 44.4 and 51.8° were ascribed to the Co 8


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NPs (JCPDS no. 15-0806) derived from the carbon thermal reduction of Co3O4 under high temperature.36 The Co NPs indeed catalyze the transformation of carbon to CNT,35 which will be confirmed by the following morphology characterization.

The morphologies of three derivatives were investigated by virtue of SEM and TEM. As for Pd@Co3O4 (Figure 3A), though the dodecahedral structure becomes depressed and wrinkled after annealing treatment of the Pd@ZIF-67, the whole morphology skeleton still remains. TEM reveals that the Pd@Co3O4 has porous structure (Figure 3B). The lattice fringe with an interspacing of 0.47 nm can be ascribed to the (111) lattice face of cubic Co3O4 (Figure S3). Similarly, the morphology of Pd@ZIF-67 precursor is also retained for the derived Pd@Co3O4/C (Figure 3C). However, the Pd@Co3O4/C shrinks significantly with distorted and bumpy surface structure. The TEM image shows that the Pd@Co3O4/C has a few cracks on its porous structure (Figure 3D). The lattice fringe of the (111) plane (with an interspacing of 0.47 nm) of cubic Co3O4 was also observed (Figure S4). It should be noted that Pd NPs was hardly observed from the TEM image of Pd@Co3O4/C because of the existence of Co3O4 phase with large size. However, the distribution and morphology of Pd NPs is very important to the subsequent catalytic application. To investigate the changes of the distribution and morphology for Pd NPs before and after the heat treatments, the Co3O4 phase was removed from derivative sample through etching by HCl aqueous solution. As shown in Figure S5, we can find that the size of Pd NPs indeed increased to certain degree because the heat treatment induced the unavoidable integration. However, the resultant Pd NPs were still well-distributed in the hierarchical pores. Similarly, the

Pd@Co/CNT also kept the dodecahedral morphology with porous structures (Figure 3E and 3F). And, the existence of Pd, Co and C elements on the Pd@Co/CNT was also confirmed by the EDX spectrum analysis (Figure S6).

Interestingly, the TEM image of a single Pd@Co/CNT dodecahedron reveals that the Co NPs are homogenously embedded in the CNT. The HRTEM confirms that the derived CNTs have lengths of 100~200 nm and diameters of ~10 nm (Figure 3G). Furthermore, the HRTEM of the derived Co NPs shows an interspacing of 0.20 nm, which can be ascribed to the (111) lattice fringe of Co (Figure 3H), being consistent with the PXRD result (Figure 2).

9



Figure 4 High-magnification XPS spectra in the Pd 3d region of Pd@Co3O4, Pd@Co3O4/C, Pd@Co/CNT, and Pd NPs, respectively. Furthermore, XPS measurements were performed to better understand the heterostructured feature of Pd@ZIF-67 derivatives. As shown in Figure 4, two peaks corresponding to Pd 3d5/2 and Pd 3d3/2 for Pd@Co3O4, Pd@Co3O4/C, Pd@Co/CNT, and bare Pd NPs are observed, which can be ascribed to the Pd and PdOy (0 < y < 2) species, respectively.45 In comparison with that of bare Pd NPs (335.69 eV), a negatively shift of the Pd 3d5/2 peak for the Pd@Co3O4 (335.48 eV), Pd@Co3O4/C (335.43 eV), and Pd@Co/CNT (335.35 eV) was observed. Concomitantly, the binding energy of Co 2p3/2 for Pd@Co3O4 (779.99 eV) exhibits a positive shift compared with that of pristine Co3O4 (779.52 eV, as shown in Figure S7). The increased binding energy of Co2p3/2 indicates an obvious electron transfer from Co3O4 to Pd NPs. Similar behavior was observed for the Pd@Co3O4/C (Figure S8) and Pd@Co/CNT (Figure S9) as well, in which the Co 2p3/2 binding energies shift to the higher values, compared with those of Co 2p3/2 in Co3O4/C and Co/CNT, respectively. These binding energy shifts indicate the existence of electronic coupling and strong interaction between Pd NPs and Co3O4, Pd NPs and Co3O4/C, as well as Pd NPs and Co/CNT, respectively, and further confirm that the derivatives possess a heterostructure feature. Above results thus demonstrate the electron donating effect 10


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from Co based derivatives contribute to the increase of the electron cloud density of Pd, which would impart a more electron richened chemical state of the Pd with improved catalytic activity and stability. Considering the pores of catalysts display a critical role for the accessibility of reactants to contact with catalytic active sites, N2 adsorption-desorption isotherms of the Pd@ZIF-67 precursor and its different derivatives were recorded. The corresponding specific surface areas and total pore volumes are summarized in Table S1. As shown in Figure 5A, a type I isotherm curve was observed for ZIF-67 precursor and Pd@ZIF-67-x, confirming a typical microporous structure for these materials. Compared with that of ZIF-67, the appreciable decrease of surface areas for Pd@ZIF-67-x indicates that the pores of the host ZIF-67 framework are occupied by the encapsulated Pd NPs. After the pyrolysis treatment, the BET surface areas of the derived Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT dramatically decreased (Figure 5B–D). Simultaneously, obvious hysteresis loops in N2 adsorption-desorption isotherms with a type IV behavior were observed in all three cases, which indicates the mesopores and/or even macropores across the derivatives. These are consistent with the TEM observations mentioned above. The pore size distribution curves (inset in Figure 4) further verify the generation of hierarchical pores. In addition, similar transform behavior of porous structures from pristine ZIF-67 to its derivatives, Co3O4, Co3O4/C, and Co/CNT was also observed from their N2 adsorption-desorption isotherm curves (Figure S10). These results illustrate that the generation of hierarchical porosity is attributed to the pyrolysis treatment of the MOF. These hierarchical pores of the resultant derivatives would be favorable for the efficient contact between the reactants and the Pd catalytic centers, and also propitious to the rapid diffusion of both reactants and products, which will be discussed below. The Heck reaction between aryl iodide and styrene was used to evaluate the catalytic activities of the derivatives. Firstly, iodobenzene as the substrate was selected to screen the reaction parameters with various catalysts. The reaction yields and corresponding turnover frequency (TOF) values are summarized in Table 1, from which we can find that all Pd@Co3O4-x, Pd@Co3O4/C-x, and Pd@Co/CNT-x exhibit high catalytic efficiency. The yields of the Heck reaction are improved from 90 to > 99%, 92 to > 99%, and 94 to > 99% with increasing fraction of Pd, respectively on

11



each catalyst. All of the Pd@ZIF-67 derivatives display high conversion efficiency (> 90%), illustrating their hierarchical pores play a very important role. However, Co/CNT, Co3O4/C, and Co3O4 examined under the same conditions promote only tiny conversion in this Heck reaction, confirming that the catalytic sites mainly came from the encapsulated Pd NPs in these derivatives. It should be pointed out that the yields of this reaction on Pd@Co/CNT-x are higher than those on Pd@Co3O4/C-x and Pd@Co3O4-x with the same fraction of the loaded Pd NPs. The reason is probably that the existence of high conductive CNT facilitates the increase of electron density of Pd NPs in Pd@Co/CNT and subsequently the improvement of their catalytic activity.46-48

Figure 5 N2 adsorption/desorption isotherms (insert: pore size distribution) of ZIF-67 and Pd@ZIF-67-x (A), Pd@Co3O4-x (B), Pd@Co3O4/C-x (C), and Pd@Co/CNT-x (D). Most importantly, Pd@Co/CNT-50, Pd@Co3O4/C-50, and Pd@Co3O4-50 lead to higher yields (> 99%) and TOF (185 h−1), compared to Pd@ZIF-67-50 with a relative low yield of 78% and low TOF of 144 h−1 (entry 3, 6, 9, and 10 in Table 1). These results illustrate that these Pd@ZIF-67 derivatives enable uniform dispersed Pd NPs, thus endowing rich active sites in the catalysis. Moreover, the hierarchical pores provide free space for reactants passing through and contacting the active sites, thus significantly reduce the diffusing path of reactants and finally improve the catalytic 12


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conversion efficiency. As a result, the synergistic effect between Co based derivatives and Pd NPs is clearly favorable for the improvement of the reaction rate and TOF value, accounting for their superior activity than Pd@ZIF-67. In addition, the measured contact angles of DMF on the surface of Pd@ZIF-67 derivatives were ~ 0° (Figure S11). This good surface wettability is also favorable for the fast diffusion of reactants and products in the DMF solvent, thus contributing to the improvement of the reaction rate. Table 1 Heck reaction of iodobenzene and styrene with various catalysts[a].

Entry

Catalysts

Yield[b] (%)

TOF (h−1)

1[c]

Pd@Co/CNT-10

94

174

[c]

Pd@Co/CNT-20

98

181

2

3

Pd@Co/CNT-50

>99

185

4[c]

Pd@Co3O4/C-10

92

170

[c]

Pd@Co3O4/C-20

97

179

5

6

Pd@Co3O4/C-50

>99

185

7[c]

Pd@Co3O4-10

90

167

[c]

Pd@Co3O4-20

94

174

9

Pd@Co3O4-50

>99

185

10

Pd@ZIF-67-50

78

144

[d]

Co/CNT

trace

--

[d]

8

11

12

Co3O4/C

trace

--

13[d]

Co3O4

trace

--

14

Pd NPs

18

33

15

Pd@α-Al2O3

62

115

16

Pd@CoAl-LDH

65

120

[a] Reaction conditions: iodobenzene (1 mmol), styrene (1.5mmol), NEt3 (1.5 mmol), DMF (3 mL), catalysts (0.09 mol% of Pd), 6 h, 120 °C, under N2. [b] The yields were determined by GC-MS analysis. [c] 20 mg of catalysts. [d] catalysts ( without Pd)

Furthermore, the same reaction catalyzed by control catalysts Pd NPs, Pd/α-Al2O3, and Pd/CoAl-LDH (LDH = layered double hydroxides) were also conducted (Figure S12 and S13, entry 14, 15, and 16 in Table 1). Compared with those of Pd@ZIF-67 derivatives, the three catalysts all show poor activity with yield of only 18%, 62%, and 65%, respectively. The comparison also demonstrates that the hierarchical pore not only affords highly dispersed Pd NPs but also facilitates the 13



accessibility and adsorption of substrates to active sites and the diffusion of products, which is quite favorable for achieving excellent catalytic activity toward the Heck reaction.

Figure 6 (A) Photograph of magnetic separation and redispersion of Pd@Co/CNT-50, (B) Cycling use of Pd@Co/CNT-50 and Pd@ZIF-67-50 in the Heck reaction of iodobenzene and styrene (reaction condition: Iodobenzene, 1 mmol; styrene, 1.5 mmol; NEt3, 1.5 mmol; DMF, 3 mL; catalysts, 0.09 mol% of Pd; 6 h; 120 °C; under N2).

Moreover, the recyclability of Pd@Co/CNT-50 and its precursor Pd@ZIF-67-50 in catalyzing the Heck reaction was investigated. As shown in Figure 6A, with an external magnet, the Pd@Co/CNT-50 can be quickly recovered from the solution in only ~15 s. After removal of the magnet, it was well re-dispersed in the solution. The result illustrates that the Pd@Co/CNT-50 possesses excellent magnetic responsivity and redispersibility due to the good magnetic property of the Co-based derivatives. The phenomenon that easy magnetic separation indicates that the recyclable catalyst would be sustainable and economical in practical application. Most importantly, the magnetic recovered catalysts, after washed with methanol represent no obvious decay of the activity for the Heck reaction in at least five cycles (Figure 6B). In the contrast, the activity of Pd@ZIF-67-50 decreases dramatically during recycling experiments, with an only ~20% yield of the product after the fifth use. These results illustrate that these derivatives have higher structural stability compared with that of the MOF precursor and the strong interaction with Pd particles, which affords a stable microenvironment for the confinement of Pd NPs, thus accounting for the sustained and stable yields. This illustration can be well supported by the PXRD and SEM characterization results. No apparent difference of the PXRD patterns for the Pd@Co/CNT-50 was observed between the fresh one (Figure 2) and the reused one

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up to five runs (Figure S14). And, the SEM image of the recovered Pd@Co/CNT-50 shows a similar morphology to that of the fresh one (Figure S15). However, the structural integrity of Pd@ZIF-67-50 is almost unmaintained after recycle tests (Figure S16). The SEM image also suggests that Pd@ZIF-67-50 suffered serious morphology damage, which renders the Pd leaching and leads to the poor recycling performance consequently (Figure S17). The above recycling tests further illustrate that the hierarchically porous structure of Pd@Co/CNT-50 is capable of reducing the structure strain and buffering the large volume expansion during the diffusion process of the reactants, thus guarantees stable structure and high durability.49 In addition, the reaction after the hot filtration stopped immediately (Figure S18), illustrating no Pd leaching from the composite catalyst into reaction system. The unnecessary Pd leaching illustrates that the Pd@ZIF-67 derivatives as heterogeneous catalysts could reduce the additional purification and avoid the potential heavy-metal pollution to the environment. The Pd@Co/CNT-50 was also applied in catalyzing the Heck reaction on different reactant substrates. As shown in Table 2, various aryl iodides were selected to couple with styrene under optimized reaction conditions with the Pd@Co/CNT-50 as catalyst. The results show the Heck reaction of four different aryl iodides with styrene can proceed smoothly and give corresponding products with high yields of >95% (entry 1-4 in Table 2). As expected, the electron-withdrawing groups (-NO2 and -COCH3) in the para position relative to iodine is favorable for weakening the C–I bond,6,12 thus these substituted iodobenzene compounds react more rapidly with the styrene (with the TOF of 222 h–1) than the iodobenzene (entry 1 and 2 in Table 2). But, the styrene reacts with two electron-rich 4-methy- and 4-ethyl-substituted iodobenzenes gave a decreased yield of 97 and 95%, respectively (entry 3 and 4 in Table 2). It should be noted that 4-iodobiphenyl coupling with styrene gave the yield of only 78%, probably resulting from the sterically hindered effect (entry 5 in Table 2) 6,12

. In addition, the Heck reaction of styrene with bromobenzene also gave a good

yield of 88% by using Pd@Co/CNT-50 as catalyst, though the high bond energy of the C–Br bond renders the bromobenzene difficult to be activated (entry 6 in Table 2). The above results thus demonstrate that these hierarchical-pore Pd@MOF derivatives have excellent catalytic performances, not only in activity but also in stability for the Heck reactions, with great potential for practical application. 15



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Table 2 Heck reactions of selected aryl iodides with styrene over Pd@Co/CNT-50[a].

Yield[c] (%)

TOF (h-1)

1[b]

99

222

2[b]

99

222

3

97

179

4

95

176

5

78

144

6

88

163

Entry

Aryl iodide

Product

[a] Reaction conditions: aryl iodide (1 mmol), styrene (1.5mmol), NEt3 (1.5 mmol), DMF (3 mL), catalysts (0.09 mol% of Pd), 6 h, 120 °C, under N2. [b] 5 h. [c] Yields were determined by GC-MS analysis.

CONCLUSIONS In summary, we have demonstrated a facile and feasible approach to synthesize hierarchical-pore catalysts, Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT by the one-step pyrolysis of Pd@ZIF-67 for Heck reaction. All resulting derivatives retain the skeleton of the MOF precursor, which enables abundant exposed Pd active sites. Most importantly, the derived hierarchical pores can facilitate fast diffusion of substrates, imparting these composites with significantly enhanced catalytic efficiency, compared with that of the Pd@ZIF-67 precursor. In addition, the strong interaction between Pd NPs and the stable structure of the MOF derivatives afford a stable microenvironment for immobilized Pd active sites, accounting for the good reusability. Furthermore, these Pd@Co/CNT can also be widely applied for Heck reactions with different substrates. This feasible strategy for the fabrication of metal@MOF

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derivatives may pave a way to design and construct diverse recyclable catalysts with hierarchical pores and highly dispersed active sites for other reactions. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional figures for PXRD patterns, SEM images, EDX spectrum, TEM images, XPS spectra, N2 adsorption/desorption isotherms, contact angle (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (21576006 and 21606006), the National Natural Science Fund for Innovative Research Groups (51621003), the Beijing Natural Science Foundation (2174064), and the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20150309).

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Table of Contents

A series of recyclable Pd@Co3O4, Pd@Co3O4/C, and Pd@Co/CNT (CNT = carbon nanotube) composites with hierarchical pores were fabricated via the pyrolysis of palladium@zeolitic-imidazolate-framework-67 (Pd@ZIF-67), respectively, which used as heterogeneous catalysts display excellent performance in Heck reaction.

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Pd@ZIF-67 Derived Recyclable Pd-Based Catalysts with Hierarchical

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