MOF Hollow Nanospheres with Double

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Functional Nanostructured Materials (including low-D carbon)

Fabrication of Magnetic Pd/MOFs Hollow Nanospheres with Double-Shell Structure: Towards High-Efficient and Recyclable Nanocatalysts for Hydrogenation Reaction Yicheng Zhong, Yuelin Mao, Shunli Shi, Mingming Wan, Chong Ma, Shuhua Wang, Chao Chen, Dan Zhao, and Ning Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07864 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Fabrication

of

Magnetic

Pd/MOFs

Hollow

Nanospheres with Double-Shell Structure: Towards High-Efficient and Recyclable Nanocatalysts for Hydrogenation Reaction Yicheng Zhong, Yuelin Mao, Shunli Shi, Mingming Wan, Chong Ma, Shuhua Wang, Chao Chen*, Dan Zhao* and Ning Zhang

Key Laboratory of Jiangxi Province for Environment and Energy, College of Chemistry, Nanchang University,

Nanchang, Jiangxi 330031, P. R. China.

E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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KEYWORDS: magnetic reusable, hollow MNPs@MOFs, double-shell structure, hydrogenation of styrene, heterogeneous nanocatalysis

ABSTRACT: MNPs@MOFs catalysts obtained by encapsulating Metal nanoparticles (NPs) into metal-organic frameworks (MOFs) show fascinating performance in heterogeneous catalysis. The improvement of catalytic activity and reusability of MNPs@MOFs catalysts has been a great challenge for a long time. Herein, we demonstrate a well-designed Pd/MOFs, featured hollow double-shell structure and magnetic property, exhibiting high reusability, efficient catalytic activity and sizeselectivity for hydrogenation reaction. The as-synthesized Pd/MOFs, denoted as Void nFe3O4@Pd/ZIF-8@ZIF-8, possesses diverse functional structural features. The hollow cavity can improve mass transfer; superparamagnetic Fe3O4 NPs embedded in the inner MOF shell can enhance the separation and recyclability; Pd NPs are highly-dispersed in the matrix of the inner MOF shell, and the outer MOF shell acts as a protector to prevent the leaching of Pd NPs and a sieve to achieve size-selectivity. As a proof of concept, Void nFe3O4@Pd/ZIF-8@ZIF-8 catalyst exhibited excellent performance for the hydrogenation


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of styrene at room temperature. The activity only reduced 10% after 20 cycles for the higher conversions (>90%), and the lower conversion only decreased 3.6% (from 32.5% to 28.9% conversion) after twenty consecutive cycles, indicating the good and intrinsic reusability of the catalyst. The proposed structure in this work provides a strategy to effectively improve the reusability of MNPs@MOFs catalysts, which would increase their practical applications.


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1. INTRODUCTION

In recent years, metal–organic frameworks (MOFs), which are constructed from metal ions or metal ion clusters and bridging organic linkers, have been a research hotspot in the field of chemistry and material science owing to their high tunable porosity, large internal surface areas and adjustable chemical properties.1,2 These characteristics endow MOFs with a broad range of applications in different areas such as catalysis, gas storage/separation, drug delivery, sensing and so on.3-8 Notably, the tunable pores of MOFs can not only serve as templates for regulating the size of guest species, but also provide great microenvironments which could improve the selectivity and activity of the encapsulated nanoparticles in catalytic reactions.9-11 These features enable MOFs to become a suitable carrier to load metal nanoparticles (MNPs). The obtained multifunctional composite materials, called MNPs@MOFs, show fascinating properties in heterogeneous catalysis.12,13 At present, various MNPs@MOFs composite materials have been designed and synthesized, mainly through “ship-in-a-bottle” and “bottlearound-a-ship” approaches.14-16 Based on the structural morphology and the location of


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metal nanoparticles, MNPs@MOFs composites could be divided into three main types: “dispersion”, “core-shell” and “hollow” structures (Table S1). Among them, the “hollow” type of MNPs@MOFs composites (including “yolk-shell” and “reverse bumpy ball”) has captured widespread attention because of the various superiorities of hollow nanomaterials such as larger surface areas, lower densities and quicker mass transfer than their solid counterparts.17-19

In our previous work, a new type of hollow double-shell MNPs@MOFs nanosphere had been designed and fabricated, and exhibited efficient catalytic activity and sizeselectivity.20 However, the synthesis process of the inner MOF shell of the hollow MNPs@MOFs is very complicated and time-consuming due to the layer-by-layer synthetic method. Besides, when the catalyst is applied in the catalytic reaction system, the inner fragile HKUST-1 shell causes the instability of catalyst, and it is difficult to recover the catalyst completely during the complex centrifugation process. All these shortcomings also often occur in other MNPs@MOFs catalysts and normally lead to the poor reusability of catalysts.


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As it is well known, magnetic materials can be readily handled and recovered from the heterogeneous reaction mixture.21-23 Until now, there are two types to introduce the magnetism to MNPs@MOFs composite catalysts. In the first type, magnetic Fe3O4 NPs of small particle size disperse inside the matrix of MOFs.24,25 The other type is like the core-shell structure, in which the Fe3O4 nanoparticle with big particle size works as the core and MOFs is the shell meanwhile the metal NPs are encapsulated in MOFs or deposited on the surface of Fe3O4 core.26,27 Although the recyclability of both types of catalysts has been enhanced, the catalytic activity is dramatically reduced because they lose lots of cavities that can improve mass transfer.28 Therefore, the fabrication of magnetic MNPs@MOFs composite catalysts with both excellent reusability and remarkable catalytic activity remains meaningful, but challenging. Inspired by this, herein, we have made great improvements to break this deadlock based on the hollow doubleshell MNPs@MOFs nanosphere in our previous work. As shown in Figure 1, ZIF-8, a famed MOF with nano-porous structure and high stability,29-31 was synthesized and employed as the inner shell by facile self-assembly at the room temperature to replace unstable HKUST-1. Then, Fe3O4 NPs of small particle size are embedded in the inner


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ZIF-8 shell to provide magnetism. The high-dispersedly Pd NPs are confined into the cavities of the inner ZIF-8 shell to avoid the agglomeration; the outer ZIF-8 shell serves as a protective shield and a sieve with size-selectivity for molecules; the whole internal hollow cavity contributes to facilitate the diffusion of chemicals. As expected, the result of olefins hydrogenation demonstrates the superiority of such structure. In particular, it exhibits efficient catalytic activity and excellent reusability for the hydrogenation of styrene. The activity was only reduced 10% after 20 cycles.

2. EXPERIMENTAL SECTION

2.1 Materials and Instruments.

All chemicals were obtained from commercial sources and used as received. Sodium hydroxide (NaOH), magnesium sulfate anhydrous (MgSO4), acrylic acid (AA), ammonium persulphate (APS), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), hydrochloric acid (HCl, 37%), ammonium hydroxide (NH3·H2O), methanol (MeOH), ethanol (EtOH) and N,NDimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Styrene (St) and 2-methylimidazole (2-MeIM) were obtained from Beijing J&K Scientific


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Co., Ltd. Palladium chloride (PdCl2) was purchased from Hans PGM. Ltd. Ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), dodecane, cisstilbene and tetraphenylethylene was obtained from Aladdin Scientific Co., Ltd.

Transmission electron microscopic (TEM) images were recorded by Talos F200x and JEOL, JEM-2100F instruments with an energy dispersive X-ray spectrometry (HRTEMEDX, Oxford INCA). The Scanning electron microscopy (SEM) images were carried out using scan electronic microscopy (ZEISS Sigma 300, and JSM-6701F). The powder Xray diffraction patterns were collected on a Puxi DX-3 diffractometer with Cu Kα radiation (λ = 0.15418 nm), with a scan speed of 1 °/min, a step size of 0.02 ° in 2θ, and a 2θ range of 4°-70°. The thermogravimetric experiments were performed on Q600 SDT from 30 ℃ to 800 °C, with a heating rate of 10 °C /min in nitrogen flow. The Pd content of the samples was measured by inductively coupled plasma-optical emission spectroscopy (Agilent, ICP-OES-5100). N2 adsorption-desorption isotherms were tested by autosorb iQ/AsiQwin analyzer (Quantachrome) at liquid nitrogen temperature (77K). Before the adsorption, the samples were degassed at 150 ℃ under vacuum overnight. The specific surface areas


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were calculated by multi-point BET (Brunauer-Emmett-Teller) method while the pore diameter and total pore volumes were calculated by DFT method. X-ray photoelectron spectra (XPS, Thermo Scientific ESCALAB 250 Xi) using Al Kα radiation (1486.6 eV) operated at 12.5 kV and 12 mA. And the binding energy was calibrated by C 1s peak at 284.8 eV. The conversions of styrene and cis-stilbene were measured using GC (Agilent, GC-7820A) equipped with a flame ionization detector (FID) and a Agilent 19091J-413 capillary column (HP-5, 5% Phenyl Methyl Siloxan, 30 m × 0.32 mm × 0.25 μm). The conversions of tetraphenylethylene were analyzed by NMR spectrometer (Agilent DD2 400-MR, 400MHZ).

2.2 Synthesis of polystyrene-co-acrylic acid (PS-co-AA) nanospheres.

Styrene was purified briefly by washing (five times) with NaOH solution (5 wt %) to remove the inhibitor, followed by washing with pure water. After the pH was close to neutral, the solution was drying with Magnesium sulfate anhydrous (MgSO4). Then, 4.95 mL St, 0.47 mL AA, 0.1g APS, a magnetic stirrer and 100 mL deionized water were added in a 250 mL three-neck round bottom flask. After being stirred for 30min in N2 atmosphere at room


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temperature, the mixture was heated to 70 ℃ and reacted for 24 h. Afterwards, the mixed solution cooled to room temperature. Finally, the obtained product was separated from mother liquor through centrifugation by washing with ethanol, and vacuum drying at 40 ℃.

2.3 Synthesis of magnetic Fe3O4 nanoparticles.

The Fe3O4 NPs were synthesized through a published work with a minor modification.32 Firstly, 2.0 g FeCl2·4H2O, 4.8 g FeCl3·6H2O and 40 mL deionized water were added into a 100 mL three-neck round bottom flask under vigorous stirring. Secondly, under nitrogen atmosphere, the mixed solution was purged for 30 min and heated to 90 ℃. Then, with 15 mL ammonium hydroxide (28 wt%) rapidly added into, the mixture immediately turned black and the reaction was kept at 90 ℃ for 3 h. Finally, the product was magnetically separated from the solution and vacuum dried at 40 ℃ for 12 h.

2.4 Synthesis of Fe3O4/PS nanospheres.


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Fe3O4/PS nanospheres were fabricated in accordance with the literature with minor modifications.32 Typically, 0.5 g Fe3O4 nanoparticles and 2 g as-prepared PS-co-AA nanospheres were dispersed in 30 mL and 100 mL hydrochloric acid solution (PH = 2.3), respectively. Then, after the PS-co-AA solution added into the Fe3O4 mixture drop by drop, the reaction mixture was kept in a 250 mL three-neck round bottom flask for 6 h with mechanical stirring at room temperature. After the reaction, the mixture was poured into a 250 mL beaker. The product was sucked by an external magnet, and the mother liquor was poured out. Afterwards, 100mL water was added into the beaker with shaking. The product was washed with water like this until the pH is close to neutral. The as-obtained Fe3O4/PS nanospheres were vacuum dried at 40 ℃ for 6 h.

2.5 Synthesis of Fe3O4/PS@ZIF-8 nanospheres.

Typically, 1 g Fe3O4/PS nanospheres were dispersed in 100 mL methanol. Meanwhile, 1.5 g Zn(NO3)2·6H2O dispersed in 25 mL methanol was added into the Fe3O4/PS solution. After slower mechanical stirring for 5 minutes at room temperature, 3 g 2-methylimidazole dispersed in 60 mL methanol was added into the former mixture with further reaction for


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3 h. The obtained product was separated by magnet, washed with methanol, and subsequently dried in a vacuum oven at 40 ℃ overnight.

2.6

Synthesis of Fe3O4/PS@Pd/ZIF-8 nanospheres via an Incipient Wetness

Impregnation Method.

Firstly, 0.5 g activated Fe3O4/PS@ZIF-8 samples were dispersed in 20 mL ethanol. Secondly, 6.7 mg PdCl2 was dissolved into the mixed solution of 50μL hydrochloric acid (37%) and 10 mL ethanol. Then, the PdCl2 solution was quickly added into the previously formed orange mixture. After mechanical stirring for six hours, the impregnated sample was magnetically collected, washed with ethanol and dried at 40℃ for 6 h to obtain the Fe3O4/PS@Pd2+/ZIF-8. Finally, the as-synthesized sample was treated under a flow of 100% H2 at 200℃ for 3 h to yield the Fe3O4/PS@Pd/ZIF-8.

2.7 Synthesis of Fe3O4/PS@Pd/ZIF-8@ZIF-8 nanospheres.

In a typical run of synthesis, 0.5 g Fe3O4/PS@Pd/ZIF-8 was dispersed in 100 mL methanol with the presence of an ultrasonic dispersion. Meanwhile, 2 g 2-methylimidazole


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and 1 g Zn(NO3)2·6H2O were dispersed in 50 mL methanol with stirring at room temperature. After 5 min, the latter mixture was added into the Fe3O4/PS@Pd/ZIF-8 solution with mechanical stirring for 6 h. The product was magnetically separated, washed with methanol, and subsequently treated by vacuum drying at 40 ℃ overnight.

2.8 Synthesis of Void nFe3O4@Pd/ZIF-8@ZIF-8 nanospheres

Fe3O4/PS@Pd/ZIF-8@ZIF-8 nanospheres were dispersed in N,N-dimethylformamide to eliminate the PS-co-AA templates and synthesis the Void nFe3O4@Pd/ZIF-8@ZIF-8 nanospheres. After being washed several times with DMF and methanol, the final products were magnetically collected and vacuum dried at 40 ℃ for overnight.

2.9 Catalytic liquid-phase hydrogenation of olefins.

Hydrogenation of olefins was carried out in batch mode at a concentration of 6.44 mmol of olefin/mg of Pd. In a typical reaction procedure, a certain amount of sample (8.1 mg for Fe3O4/PS@Pd/ZIF-8, 11.4 mg for Fe3O4/PS@Pd/ZIF-8@ZIF-8 and 5.4 mg for Void nFe3O4@Pd/ZIF-8@ZIF-8, respectively) with the same Pd content (0.06368 mg) was


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dispersed into 8 mL solvent solution (ethanol for styrene and cis-stilbene, ethyl acetate for tetraphenylethylene). After 0.22 mmol dodecane as internal standard and 0.41 mmol olefin added into the above solution, the mixture was transferred into a stainless steel autoclave (YZPR-50M, purchased from Yan Zheng experimental instrument Co., Ltd.). Subsequently, via flushing with hydrogen for three times, the internal air in the micro reactor was replaced by H2 and the final pressure of hydrogen was set at desired pressure value (10 atm). The catalytic reaction was carried out at room temperature and lasted for desired time (5 min for styrene, 6 h for cis-stilbene, and 12 h for tetraphenylethylene). After the hydrogenation of styrene or cis-stilbene, the catalyst fines were filtered out. Then, the filtrate was detected by using a gas chromatograph coupled with a HP-5 (0.25 mm×30 m) column and a flame ionization detector. The hydrogenation of tetraphenylethylene was detected by 1H NMR spectrometer. Additionally, the recyclability tests were performed. Typically, a certain amount of Void nFe3O4@Pd/ZIF-8@ZIF-8 (5.0 mg for 92.6% conversion and 1.8mg for 32.5% conversion, respectively) was added into the reaction system and treated with the same reaction conditions as the hydrogenation of styrene. After a 5 min reaction, the catalyst was collected by a magnet, washed with ethanol, dried


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at 40℃ and then reused for the next catalytic test. The filtration test was performed using the same reaction conditions of styrene hydrogenation except that the amount of Void nFe3O4@Pd/ZIF-8@ZIF-8 used less (1.5 mg). The Void nFe3O4@Pd/ZIF-8@ZIF-8 catalyst was removed after 2 min reaction and the mother liquor continued to react for another 3 min.

3. RESULTS AND DISSCUSION

The Void nFe3O4@Pd/ZIF-8@ZIF-8 samples were fabricated by a series of procedures as shown in scheme 1. Firstly, the PS-co-AA nanospheres were used as supporter to deposit the Fe3O4 NPs on the surface. Secondly, ZIF-8 was grown around the surface of Fe3O4/PS nanospheres by the facile self-assembly at the room temperature, to obtain Fe3O4/PS@ZIF-8 nanospheres. Then, via an incipient wetness impregnation method, Pd NPs were encapsulated into the inner ZIF-8 shell, hence the Fe3O4/PS@Pd/ZIF-8 nanospheres were formed. Through the same solution self-assembly method, the outer ZIF-8 shell was coating on the as-prepared samples, attaining the Fe3O4/PS@Pd/ZIF8@ZIF-8 nanospheres. Finally, the PS-co-AA nanospheres as hard templates were


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removed by dissolving, forming the hollow magnetic Void nFe3O4@Pd/ZIF-8@ZIF-8 nanospheres.

The entire formation process of Void nFe3O4@Pd/ZIF-8@ZIF-8 nanospheres were monitored by photograph, TEM and SEM analyses. Different from the smooth surfaces of the PS-co-AA templates (Figure 2a2, a3), a lot of small Fe3O4 nanoparticles were embedded on the surface of PS-co-AA to form Fe3O4/PS nanospheres (Figure 2b2, b3), and the color of the samples changed from white to brown (Figure 2a1, b1). The HR-TEM image of Fe3O4/PS (Figure 3a) exhibited distinct lattice fringes of 0.21nm corresponded to the (400) plane of Fe3O4.33 Then, the inner ZIF-8 shell with a thickness of approximately 20 nm was coated on the surface of Fe3O4/PS (Figure 2c2, c3), accompanied by the visible change in color from Fe3O4/PS to Fe3O4/PS@ZIF-8 (Figure 2c1). The polyhedral morphology of the ZIF-8 shell was as same as the morphology of ZIF-8 layer in previous report.34 The samples turned black from Fe3O4/PS@ZIF-8 to Fe3O4/PS@Pd/ZIF-8 (Figure 2d1), indicating that Pd species had been embedded into the ZIF-8 shell. Both Pd NPs and Fe3O4 NPs can be observed in the partially enlarged TEM image of


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Fe3O4/PS@Pd/ZIF-8 (Figure 3b), and the average size of Pd NPs was about 2.29 nm, which was shown by statistical analysis (Figure 3c). It was clear that the (111) plane of Pd and (220) plane of Fe3O4 simultaneously existed in the HR-TEM image of Fe3O4/PS@Pd/ZIF-8 (Figure 3d).35,36 Besides, it exhibited uniform distribution of Pd NPs with small size and no undesired agglomeration could be observed in the HR-TEM image, indicating that the Pd NPs were well encapsulated (Figure 3e). And some Pd NPs were partially exposed on the outer surface of the ZIF-8 shell (Figure 3f). Those characters all implied that Pd nanoparticles were successfully generated and encapsulated into the ZIF8 matrix. It is noteworthy that the structure of the material still remained intact, evidenced by the TEM and SEM images (Figure 2d2, d3). After the growth of the outer shell, the same morphology crystal shell of ZIF-8 with a thickness of approximately 35 nm can be observed on the surface of nanospheres (Figure 2e2, e3), indicating that the formation of Fe3O4/PS@Pd/ZIF-8@ZIF-8 was achieved. At last, the PS-co-AA nanosphere as a hard template was removed to create a huge cavity, and the whole morphology was still preserved well. Thus, the final product with complete structure-Void nFe3O4@Pd/ZIF8@ZIF-8 was successfully fabricated (Figure 2f1-f3). Furthermore, the ratio optimization


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experiments on Fe3O4-loading and Pd-loading of Void nFe3O4@Pd/ZIF-8@ZIF-8 were performed, as shown in Figure S1 and Table S2. The results showed that 1.179% was the highest-performing Pd-loading value and 9.03% was the highest-performing Fe3O4loading value, respectively.

In order to confirm the composition of the Void nFe3O4@Pd/ZIF-8@ZIF-8, HAADFSTEM and EDS mapping experiments were performed, as shown in Figure 2a4-f4. The element distribution areas of Fe (Figure 2b4), Pd (Figure 2c4) and Zn/C/N (Figure 2d4/e4/f4) were all similar to a circle. The Fe element was in the innermost part, uniform Pd element was in the middle and Zn/C/N element was on the outermost layer. The size of different elements changed from 410 nm (Fe of Fe3O4) to 450 nm (Pd) and finally become 520 nm (Zn/C/N of ZIF-8). Thus it was estimated that the thickness of inner/outer ZIF-8 shell was about 20 nm/35 nm, concordant with TEM and SEM characterization results reported above. Compared with the micron-size hollow MOFs catalyst in our previous work,20 Void nFe3O4@Pd/ZIF-8@ZIF-8 had a smaller size (approximately 520 nm), which possessed lower density and more cavities to accelerate mass transfer.


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Furthermore, in the central region, the density of all elements is lower, proving the existence of hollow cavity which is analogous to the composites with hollow structure.37,38 Moreover, the samples exhibited good thermal stabilities as shown in the TGA curves (Figure S2) and each sample’s yield of embedding Pd was higher than 90% (Table S2) calculated from the TGA and ICP data, indicating the loss of Pd element was very small.

The crystal structure and porous nature of samples were investigated by PXRD and N2 sorption measurements. As shown in Figure 3g, the characteristic diffraction peaks of Fe3O4 (labled in black diamond) could be observed at 2-theta values of 35.756, 43.392, 56.946, and 62.851, respectively corresponding to the (311), (400), (511), and (440) planes of magnetite (JCPDS No. 19-0629), indicating that Fe3O4 NPs were successfully introduced into samples. The characteristic diffraction peaks of ZIF-8 in good agreement with the theoretical patterns from the single crystal data39 (labeled in red star) were detected in the PXRD pattern of Fe3O4/PS@ZIF-8, indicating that the ZIF-8 crystal shell was coating onto the Fe3O4/PS. This was also evidenced by the TEM (Figure 2c2) and SEM images (Figure 2c3). During the growing process of Pd NPs, the structure of the


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inner ZIF-8 shell was slightly damaged and some pores were blocked.40 It was confirmed by the significant reduction of BET surface areas (Table S3) and PXRD diffraction peaks intensity (Figure. 3g) from Fe3O4/PS@ZIF-8 to Fe3O4/PS@Pd/ZIF-8. Nonetheless, the main crystal structure of ZIF-8 shell was preserved well, and no clear peaks of Pd can be observed, which suggested a low content of Pd element (approximately 0.8% confirmed by ICP analysis, Table S2) and a small size of the Pd NPs (Figure 3c). Then, the PXRD pattern

(Figure

3g)

and

N2

sorption

(Figure

3h)

experimental

results

of

Fe3O4/PS@Pd/ZIF-8@ZIF-8 showed higher diffraction peaks of ZIF-8 and larger BET surface area in comparison to Fe3O4/PS@Pd/ZIF-8, proving the successful coating of the outer ZIF-8 shell. After the removal of PS-co-AA templates, the diffraction peaks of ZIF-8 and the BET surface area of sample all increased. These demonstrated the formation of hollow structure, consistent with the results of TEM and EDS-Mapping images. In addition, the hysteresis loop observed in the N2 sorption isotherm was attributed to mesopores which were mainly formed due to the hollow structure and the packing of ZIF-8 nanocrystal. And there was a significant amount of N2 adsorption within the high relative pressure range of 0.9–1.0, also implying a large external surface area and the presence


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of macropores. Besides, the NLDFT pore size distribution (Figure S3) suggested a central pore size of 1.076 nm and few mesopores meanwhile the BJH pore size distribution (Figure S4) showed the existence of macropores, in agreement with the above analysis.

To investigate the electronic interaction of the materials, the XPS experiments were performed on Fe3O4/PS@ZIF-8, Fe3O4/PS@Pd/ZIF-8 and Void nFe3O4@Pd/ZIF-8@ZIF8. As shown in Figure 4a, the characteristic peaks for Pd, N, C and Zn were clearly detected, indicating that the Pd nanoparticles had been successfully immobilized on the inner ZIF-8 shell. Especially, the high-resolution XPS spectrum for N 1S (Figure 4b) showed that the binding energy of Fe3O4/PS@Pd/ZIF-8 had shifted to a higher level compared with Fe3O4/PS@ZIF-8, which indicated a decrease in the electron density of N and pointed to the fact that there existed a strong interaction between the Pd and N.14 The interaction between the Pd and N could play a key role in the formation of evenly, well-dispersed and leaching resistant Pd NPs within the ZIF-8 shell,41,42 thus making for the better catalytic performance of catalyst. The XPS spectrum of Pd 3d (Figure 4c) showed two pronounced bands at 341.1 and 335.9 eV which could be assigned to 3d3/2


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and 3d5/2 of metallic Pd, respectively.43 The other two satellite peaks at 343.6 and 338.3 eV were attributed to the 3d3/2 and 3d5/2 of Pd oxide, indicating that the surface of Pd NPs were partially oxidized due to exposure to the air. Interestingly, the XPS wide scan spectra of Void nFe3O4@Pd/ZIF-8@ZIF-8 (Figure 4d) showed strong peaks of Zn, C and N species which belonged to the ZIF-8 shell.44 However, the characteristic peaks of Pd and Fe could not be observed in the high-resolution XPS spectra (Figure 4e and 4f). This is owing to the fact that the XPS analysis is only able to detect the amount and kinetic energy of electrons that escape from the surface (0-10 nm thickness) of the tested material, but the thickness of the outer ZIF-8 shell of Void nFe3O4@Pd/ZIF-8@ZIF-8 is approximately 35 nm. The results fully confirmed that the inner material was completely wrapped by the outer ZIF-8 shell for Void nFe3O4@Pd/ZIF-8@ZIF-8,44 corresponding to the analysis above.

After successfully synthesis and confirmation of Void nFe3O4@Pd/ZIF-8@ZIF-8, we envisioned that it would own the following advantages for heterogeneous catalysis. Firstly, the magnetic Fe3O4 NPs embedded in the inner ZIF-8 shell could improve the


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separation and reusability of catalyst. Secondly, the inner ZIF-8 shell would play an important role in high-dispersedly loading Pd NPs without agglomeration, which would create more active sites for the catalyst. Then, the outer ZIF-8 shell, which possesses well-preserved crystallinity and appropriate thickness (approximately 35 nm), could play a protective role including preventing Pd NPs from leaching and protecting the whole structure from collapsing during the catalytic reaction. Meanwhile, the outer ZIF-8 crystal shell owns unique porous structure, so it would have molecular sieving capabilities.45 At last, the big cavities were favorable for accelerating the diffusion rate of different chemicals, and the whole hollow structure acted as an ideal nanoreactor.46,47 To prove the structure superiorities, liquid-phase hydrogenation of olefins with different molecular sizes (Figure S5) including styrene, cis-stilbene, and tetraphenylethylene were chosen as probe reactions on the basis of the pore size distribution. As shown in Figure 5, the conversion of styrene catalyzed by Void nFe3O4@Pd/ZIF-8@ZIF-8 reached 100% in only 5 minutes at room temperature, while the conversion of cis-stilbene reached 83.6% until 6 h. When the reactant came to tetraphenylethylene with larger molecule size, no conversion was observed even prolonging the reaction time to 12 hours. The sharp


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difference of the catalytic activity was mainly caused by the difference in the molecule size of substrates. Since the styrene molecules had a small size (7.1 Å), it allowed them to move freely in the ZIF-8 shells and achieve good conversion. By contrast, the molecular size of cis-stilbene (10.6 Å) was close to the main pore size of ZIF-8 (10.8 Å, Figure S3), thus the cis-stilbene molecules cannot move freely through the ZIF-8 shells unless they diffused through the limited mesoporous voids between ZIF-8 nanocrystals in the shells, resulting in the lower catalytic activity. However, the molecular size of tetraphenylethylene (12.3 Å) was bigger than the pore size of ZIF-8 shell, hence it was difficult to pass through the ZIF-8 shell and showed the negative catalytic activity. These results of liquid-phase hydrogenation proved the molecular sieving capabilities of the outer ZIF-8 shell. Besides, Fe3O4/PS@Pd/ZIF-8 which did not possess the outer ZIF-8 shell was able to catalyze tetraphenylethylene with conversion of 4.5% after 12 h reaction, consistent with Figure 3f that some Pd NPs were partially exposed outside the inner ZIF-8 shell. The similar molecular sieving behavior had been further demonstrated in the diverse olefins hydrogenation over Pd@UiO-66 and Pd@ZIF-8 materials with different MOF pore sizes.48-50


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Then, we chose the styrene as the reactant to further explore the multifunctional feature of the hollow structure with double shells. As shown in the Figure 6, Fe3O4/PS@ZIF-8 exhibited no catalytic activity in the hydrogenation of olefins, while Fe3O4/PS@Pd/ZIF-8 only took 10 minutes to achieve 100% conversion rate of styrene. It indicated that the Pd nanoparticle was the catalytic active center for the hydrogenation reaction. As for Fe3O4/PS@Pd/ZIF-8@ZIF-8, it had lower catalytic activity, indicating that it was a time consuming process for styrene molecules to diffuse across the outer ZIF-8 shell. Compared with the two structures above, however, the Void nFe3O4@Pd/ZIF-8@ZIF-8 that the PS template had been removed possessed the highest activity and took only 5 min to achieve complete conversion. The difference of catalytic activities demonstrated the advantages of hollow structure including higher surface area and lower density compared to the same mass of bulk supported catalysts. By shorten the distance for diffusion, the large hollow interspace of Void nFe3O4@Pd/ZIF-8@ZIF-8 can significantly improve the mass transfer of reactant and product molecules thus provide adequate space for maximizing the active surface sites accessible to styrene molecules. In brief, these advantages could enhance the diffusion rate and offer more interaction chances for


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reactants and Pd NPs. The similar phenomenon had occurred in some works about hollow structure.51-53

Recovery and reuse of expensive catalysts are important in heterogeneous catalysis due to the economic and environmental reasons. Currently, the recyclability of most MNPs@MOFs catalysts was not far from desirable. We expected it would greatly improve the reusability of Void nFe3O4@Pd/ZIF-8@ZIF-8 nanocatalyst by introducing the magnetic Fe3O4. To prove the concept, the cycle experiments of Void nFe3O4@Pd/ZIF8@ZIF-8 were performed under the same reaction conditions described in the experimental section. As shown in Figure 7a, for the higher conversion of 92.6% (blue columns), there was no significant decrease in the conversion of styrene even after twelve consecutive recycling runs, and the conversion only reduced 10% after twenty consecutive cycles. At the same time, the lower conversion (red columns) only decreased 3.6% (from 32.5% to 28.9% conversion) after twenty consecutive cycles, indicating the good and intrinsic reusability of the catalyst. Afterwards, SEM and XRD experiments were conducted on the spent catalysts after twenty cycles in contrast with the fresh catalysts.


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The results showed that the morphologies of samples (Figure S6) had no observable changes, and the diffraction peaks (Figure S7) did not change significantly after the cycle experiment, suggesting that the structural integrity of Void nFe3O4@Pd/ZIF-8@ZIF-8 had no significant degradation and still retained good crystallinity. Meanwhile, the loss of Pd element content of the spent catalysts was detected by ICP-OES analysis (Table S4). The results indicated that there was less than 0.01 ppm of Pd in the mother liquor of reaction mixture until after 20 cycles. All these characterizations demonstrated the high cyclic stability of Void nFe3O4@Pd/ZIF-8@ZIF-8, which was dramatically enhanced compared with the previous work.20 Furthermore, the filtration experiment was performed, as shown in Figure 7b. After two minutes of reaction, the Void nFe3O4@Pd/ZIF-8@ZIF-8 catalyst was magnetically separated from the reaction solution, and the mother liquor continued to react for another three minutes. The results showed there was no further conversion of styrene tested by GC, indicating that the catalytic process of styrene hydrogenation was truly heterogeneous.


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To verify the protective effects of the outer ZIF-8 shell on the structure and internal Pd NPs, Void nFe3O4@Pd/ZIF-8 were synthesized from Fe3O4/PS@Pd/ZIF-8 by DMF dissolution. And total six recycle experiments of Void nFe3O4@Pd/ZIF-8 were performed under the same reaction conditions with that of Void nFe3O4@Pd/ZIF-8@ZIF-8. Compared with Void nFe3O4@Pd/ZIF-8@ZIF-8 which possessed the outer ZIF-8 shell, the structure of Void nFe3O4@Pd/ZIF-8 collapsed after six consecutive recycling runs (Figure S8). Besides, the PXRD diffraction peaks of Void nFe3O4@Pd/ZIF-8 decreased after six times reaction (Figure S9), and the catalytic activity dropped dramatically with a large loss of Pd element content (Table S5). The results fully demonstrated the important role of the outer ZIF-8 shell in the structural stability of catalyst and the protection of internal Pd NPs. Finally, the catalytic activity and cycle performance of Void nFe3O4@Pd/ZIF-8@ZIF-8 were compared with other palladium based catalyst systems for the hydrogenation of styrene at low temperature (Table S6). Obviously, the Void nFe3O4@Pd/ZIF-8@ZIF-8 catalyst owned remarkable reusability and efficient catalytic activity. Besides, to further evaluate the catalytic performance of Void nFe3O4@Pd/ZIF8@ZIF-8, we choose the practical Heck cross-coupling reactions, which have advanced


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a broad range of organic reactions and been widely used in the agrochemical, pharmaceutical, and fine chemical industries over the past few decades. As shown in the Table S7, the yield of product reached 63.5 % in only 15 min, and the yield only reduced 1.9 % after ten consecutive cycles. The results also fully demonstrated the excellent catalytic activity and reusability of Void nFe3O4@Pd/ZIF-8@ZIF-8, which was concordant with the results of styrene hydrogenation.

4. CONCLUSIONS

In summary, we have successfully synthesized a hollow magnetic double-shell Pd@MOFs composite catalyst. Among the compositions of Void nFe3O4@Pd/ZIF8@ZIF-8, superparamagnetic Fe3O4 NPs can enhance reusability; Pd NPs highlydispersed in the inner ZIF-8 shell act as catalytic sites; the outer ZIF-8 shell contributes to maintaining the structural stability and has size-selective catalytic properties; the big cavity can effectively accelerate mass transfer. The prepared catalyst (Void nFe3O4@Pd/ZIF-8@ZIF-8) exhibited efficient catalytic activity and excellent recyclability for the hydrogenation of styrene at room temperature. These findings provide a strategy


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that can effectively improve the catalytic activity and reusability of MNPs@MOFs catalysts. Furthermore, the hollow magnetic double-shell MNPs@MOFs composite materials could be potential candidates for the other fields by using different MNPs and other porous materials.

ASSOCIATED CONTENT

Supporting Information

The TOF value curve, TGA analyses, NLDFT and BJH pore size distribution curves, molecular size, SEM images, Powder XRD patterns, the tables of major types of MNPs@MOFs, the Yield of each reaction step, summary of surface area, pore volume, ICP data, comparison of different palladium based catalyst systems and Heck reaction of iodobenzene and styrene. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Authors


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* E-mail for C. C.: [email protected] * E-mail for D. Z.: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was financed by the National Natural Science Foundations of China (No. 21561020, 21661020 and 21663016).

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Figure Captions. Scheme 1. Schematic illustration for the fabrication of Void nFe3O4@Pd/ZIF-8@ZIF-8. Figure 1. The structural compositions of Void nFe3O4@Pd/ZIF-8@ZIF-8. Figure 2. The images of photograph (a1-f1, top row), TEM (a2-f2, second row), and SEM (a3-f3, third row) for (a) PS; (b) Fe3O4/PS (c) Fe3O4/PS@ZIF-8; (d) Fe3O4/PS@Pd/ZIF-8; (e) Fe3O4/PS@Pd/ZIF-8@ZIF-8; (f) Void nFe3O4@Pd/ZIF-8@ZIF-8 respectively and the EDS elemental mapping data of Void nFe3O4@Pd/ZIF-8@ZIF-8 nanosphere (a4-f4, bottom row). The scale bars indicate 100 nm for TEM and 200 nm for SEM, respectively. Figure 3. (a). The HRTEM image of Fe3O4/PS. (b). Partial enlarged TEM image of Fe3O4/PS@Pd/ZIF-8. (c) Size distribution of Pd NPs in Fe3O4/PS@Pd/ZIF-8. (d), (e) and (f) The HRTEM image of Fe3O4/PS@Pd/ZIF-8. (g). Powder XRD patterns of PS; Fe3O4; Fe3O4/PS; Fe3O4/PS@ZIF-8; Fe3O4/PS@Pd/ZIF-8; Fe3O4/PS@Pd/ZIF-8@ZIF-8 and Void nFe3O4@Pd/ZIF8@ZIF-8. (h). The nitrogen sorption isotherms of Fe3O4/PS@ZIF-8, Fe3O4/PS@Pd/ZIF-8, Fe3O4/PS@Pd/ZIF-8@ZIF-8, and Void nFe3O4@Pd/ZIF-8@ZIF-8 at 77K. Figure 4. (a) The XPS survey spectra of Fe3O4/PS@Pd/ZIF-8. (b) The high-resolution XPS spectrum for N 1S of Fe3O4/PS@Pd/ZIF-8 and Fe3O4/PS@ZIF-8. (c) The high-resolution XPS spectrum for Pd 3d of Fe3O4/PS@Pd/ZIF-8. (d) The XPS wide scan spectra of Void n Fe3O4@Pd/ZIF-8@ZIF-8. (e) The high-resolution XPS spectrum for Pd 3d of Void n Fe3O4@Pd/ZIF-8@ZIF-8. (f) The high-resolution XPS spectrum for Fe 2P of Void n Fe3O4@Pd/ZIF-8@ZIF-8.


37


Figure 5. Catalytic performance of Fe3O4/PS@ZIF-8

Page 38 of 47

， Fe3O4/PS@Pd/ZIF-8

，

Fe3O4/PS@Pd/ZIF-8@ZIF-8, and Void nFe3O4@Pd/ZIF-8@ZIF-8 for the liquid-phase hydrogenation of styrene, cis-stilbene, and tetraphenylethylene. The catalytic reaction time is 5 min for styrene, 6 h for cis-stilbene and 12 h for tetraphenylethylene, respectively. Figure 6. Conversion versus reaction time curve for styrene hydrogenation catalyzed by Fe3O4/PS@ZIF-8,

Fe3O4/PS@Pd/ZIF-8,

Fe3O4/PS@Pd/ZIF-8@ZIF-8,

and

Void

nFe3O4@Pd/ZIF-8@ZIF-8, respectively. Figure 7. (a) Recyclability of Void nFe3O4@Pd/ZIF-8@ZIF-8 catalyst respectively measured at the conversion of 92.6% (blue columns) and 32.5% (red columns) for the liquid-phase hydrogenation of styrene. (b) Filtration experiment of Void nFe3O4@Pd/ZIF-8@ZIF-8 in the reaction of liquid-phase hydrogenation of styrene.


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Table of Contents Graphic 484x412mm (72 x 72 DPI)



Scheme 1 692x438mm (72 x 72 DPI)


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Figure 1 485x402mm (72 x 72 DPI)





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MOF Hollow Nanospheres with Double

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