Microporous Yolk-Shelled

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Energy, Environmental, and Catalysis Applications

Synthesis of 3D Ordered Macro/Microporous Yolk-Shelled Nanoreactor with Spatially Separated Functionalities for Cascade Reaction Yingchun Guo, Lei Feng, Changcheng Wu, Xiaomei Wang, and Xu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11578 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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ACS Applied Materials & Interfaces

Synthesis of 3D Ordered Macro/Microporous YolkShelled Nanoreactor with Spatially Separated Functionalities for Cascade Reaction Yingchun Guo, Lei Feng, Changcheng Wu, Xiaomei Wang,* and Xu Zhang* National-Local Joint Engineering Laboratory for Energy Conservation in Chemical Process Integration and Resources Utilization, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, China. KEYWORDS: Yolk-shell, Hierarchical MOF skeleton, Multifunctional nanoreactor, Cascade reactions, Bifunctional catalyst

ABSTRACT: Constructing three-dimensional (3D) hierarchical materials with spatial compartmentalization of multiple catalytic functionalities effectively facilitates the chemical processes intensification, especially for bulky-molecule-involved cascade reaction. Herein, a facile and novel core-shell colloidal crystal templating strategy was developed to synthesize highly ordered arrays of integrated yolk-shelled nanoreactor consisting of monolithically interconnected ZIF-8 shell and sulfonated polystyrene yolks decorated with rhodium

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nanoparticles. The obtained nanoreactor achieves efficient catalytic one-pot cascade Knoevenagel condensation-hydrogenation reactions for larger molecules, by taking advantage of the superior mass diffusion properties of hierarchical macro/microporous metalorganic framework (MOF) skeleton, robust monolith nature as well as the spatially separated functionalities. This work offers an important strategy for preparing MOF-based (MOF-based) composites with a hierarchical framework, accelerating various applications of MOF, such as electrochemical applications, photothermal conversion, and heterogeneous catalysis.

INTRODUCTION Nature’s strategy to utilizing multistep enzymatic reactions for the production of complex molecules in biological systems has been an origin of enlightenment for fabricating artificial cooperative catalysts.1,2 To closely mimic various enzymes, nanomaterials can be incorporated with multiple catalytic functionalities (e.g., acidic and basic sites, metal nanoparticles and acidic or basic sites, metal nanoparticles and enzyme) that are spatially compartmentalized to promote cascade

chemical

transformations.3-8

Recently,

metal-organic

framework

(MOF,

all

abbreviations are shown in Table S1) as a kind of crystalline material, with well-ordered tunable microporous structures and exceptional textural properties, has spurred great interest for their promising applications pertaining to gas storage, gas separation, sensing, catalysis, and many more.9,10 Particularly, the synergistic combinations of mixed ligands, mixed metal centres, metal centres and ligands in MOFs lead to a diverse range of MOF-based multifunctional materials with enhanced catalytic performance for cooperative catalysis and cascade catalysis.11,12


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The rational integration of metal nanoparticles (NPs) and MOFs together with the hierarchically porous structures is an effective strategy to achieve the multistep catalytic reactions.13-15 Enhanced catalytic performance is obtained due to synergistic effects and confinement effects. The structure of ZIF-8 is characterized by large pores of 11.6 Å connected through small apertures of 3.4 Å, making them relatively inert for bulky-molecule reactions.16-18 To address this issue, fabricating ordered macro/microporous structure via colloidal crystal templating method is a judicious solution.19-22 The macropores greatly facilitate the transport and diffusion of guest molecules, whereas the micropores allow large pore volumes that renders all active sites readily accessible.23 More interestingly, a novel structure of three-dimensional integrated yolk-shell (3D-IYS) structure is elaborately designed using core-shell colloidal crystal templates (CS-CCTs) strategy.24-26 3D-IYS structure has yolk-shelled 3D interconnected skeletons with open windows between adjacent spherical voids. The yolk-shell structure is capable of integrating various functionalities into a nanoreactor to accomplish cascade reactions with superior catalytic performance, which avoids wasting time, cumbersome isolation and purification steps required, significantly reduce waste generation, and to savings in energy.27,28 From the perspective of green and sustainable chemistry, cascade catalysis is efficient and ecofriendly chemical synthesis methods.29-33 Therefore, the sensible design and preparation of MOFbased 3D-IYS nanoreactor with hierarchical structures and spatial compartmentalization of multiple catalytic functionalities is greatly desirable. In this work, we demonstrate an efficient and versatile strategy to fabricate novel bifunctional integrated yolk-shell nanoreactor composed of sulfonated cross-linked polystyrene (CLPS-SO3H) yolks decorated with rhodium nanoparticles (Rh NPs) and 3D ordered macro/microporous ZIF-8 shell, denoted as IY-SO3H/Rh@S-ZIF-8. This bifunctional integrated yolk-shell was constructed


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through CS-CCTs and in-situ nanocasting method. The overall synthesis procedure for the 3D ordered macro/microporous yolk-shelled nanoreactor includes four steps as illustrated in Scheme 1. Firstly, the CLPS-SO3H@SiO2 microspheres were synthesized by coating CLPS-SO3H microspheres with silica and were packed into CS-CCTs via centrifugation. The monolithic CSCCTs were impregnated with ZIF-8 precursor solution and subsequent a double-solvent-induced heterogeneous nucleation approach was performed, resulting in the interstitial voids of the monolithic CS-CCTs filled with ZIF-8. The CH3OH/NH3·H2O solution was used as the solvent. NH3·H2O can stimulate the quick crystallization of the precursor while CH3OH acts a role in effectively stabilizing the precursor and regulating the balance between nucleation and growth of ZIF-8.19 Finally, the SiO2 middle layer in the composite was selectively etched with sodium hydroxide, and Rh nanoparticles were loaded into the CLPS-SO3H yolks via an in situ reduction method. The obtained integrated yolk-shell nanoreactor possesses movable yolks (~230 nm), monolithically interconnected macropores (~50 nm), highly accessible micropore channels (~0.67 nm), high surface area (~435 m2/g), and spatially isolated functionalities. The obtained IY-SO3H/Rh@S-ZIF-8 is utilized as nanoreactors for a prototypical Knoevenagel condensationhydrogenation cascade reaction with high activity and stability. Such excellent performance is greatly attributed to the following characteristics: (i) the interconnected macropores allow an effective mass transfer; (ii) the hierarchically porous structure is enabling abundantly exposed active sites; (iii) the integrated yolk-shell units can not only provide sufficient space for cascade reaction but also facilitate good cycling stability.


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Scheme 1. Synthesis procedure for the IY-SO3H/Rh@S-ZIF-8 EXPERIMENTAL SECTION Chemicals. Tetraethoxysilane (TEOS, 99.5%), sodium hydroxide (NaOH), Styrene (St) and divinylbenzene (DVB, containing 80% divinylbenzene isomers) were received from Tianjin Guangfujingxi Chemical Corporation. St and DVB monomers were purified by distillation under reduced pressure and stored nitrogen atmosphere before their polymerization. Rhodium (III) chloride hydrate, malononitrile (99%), benzaldehyde (99%), 9-anthracenecarboxaldehyde (99%), 1-naphthaldehdye

(99%),

zinc

nitrate

hexahydrate

(Zn(NO3)2·6H2O,

98%)

and

2-

methylimidazole (2-MeIM, 99%) were obtained from Aladdin Industrial Corporation and used without further purifying. All other chemicals were utilized as received. Preparation of CLPS-SO3H@SiO2 CS-CCTs. The monolithic CLPS-SO3H@SiO2 CS-CCTs was fabricated according to the previous report.25 Typically, The CLPS microspheres prepared by emulsion polymerization reacted with H2SO4 at 40 °C for 12 h to gain CLPS-SO3H


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microspheres. The CLPS-SO3H microspheres (0.5 g) were dispersed in TEOS/ ultrapure water/ ethanol (10/10/50 mL) and stirred for 24 h at 25 °C. The powders (CLPS-SO3H@SiO2) were gained by centrifugation, treated thoroughly with ethanol. The sol-gel process was repeated to increase the thickness of the silica shell. The resulting CLPS-SO3H@SiO2 microspheres were dispersed in ethanol (250 mL). Then, TEOS (5 mL) and ammonia solution (15 mL) were added into the mixture and reacted at 25 °C for 12 h. The CLPS-SO3H@SiO2 solution was centrifuged, treated thoroughly with ethanol, and dispersed in ethanol under ultrasonic. The resulting uniform emulsion was centrifuged at 1000 rpm for 12 h. Finally, the obtained precipitations were then dried at room temperature for 5 days to form the monolithic CLPS-SO3H@SiO2 CS-CCTs. Fabrication of IY-SO3H@S-ZIF-8. In a normal procedure, 8.15 g of Zn(NO3)2·6H2O and 6.75 g of 2-MeIM were mixed in 45 mL of methanol, and the mixture was stirred 30 min at 25 °C. Then a piece of monolithic CLPS-SO3H@SiO2 CS-CCTs was impregnated into the above solution for 1 h and further treated with vacuum degassing for 10 min to guarantee all voids inside the opal-structured frame completely filled with precursor solution. The impregnated composite monolith was transferred into a beaker and then dried at 50 oC for 6 h. After that, the obtained monolith was fully immersed with a CH3OH/NH3·H2O (1:1 v/v) mixed solution at room temperature and subsequently treated with vacuum degassing for 3 min to achieve the efficient infiltration of solution into the CS-CCTs. The resulting mixture was allowed to crystallize at room temperature for 24 h. The obtained sample was filtrated and dried in air. In order to fill all interstitial space with ZIF-8, this impregnation process was repeated three times. IY-SO3H@SiO2/S-ZIF-8 composite monolith was obtained. Finally, the as-synthesized IYSO3H@SiO2/S-ZIF-8 composites were completely immersed into NaOH solution (0.5 M) to selectively etch the middle silica layer. Thus, the IY-SO3H@S-ZIF-8 was prepared.


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Synthesis of IY-SO3H/Rh@S-ZIF-8. Rh NPs were loaded into the obtained IY-SO3H@SZIF-8 via an in-situ reduction method. Typically, the white IY-SO3H@S-ZIF-8 (100 mg), deionized water (20 mL) and fresh RhCl3·3H2O aqueous solution (15.0 mL, 1.0 mg/mL) were mixed, and sequentially fresh trisodium citrate aqueous solution (12 mL, 1.0 mg/mL) were added. The above mixture was kept at 25 °C with stirring for 4 h. Afterward, the NaBH4 aqueous solution (4.0 mL, 0.1 M) was added dropwise, and the mixture was reacted for 5 h. The pink mixture transformed into the black in this process. The resultant mixture was isolated via filtration, treated with ethanol and dried at 60 °C for 12 h. Finally, the rhodium-loaded products IY-SO3H/Rh@S-ZIF-8 was obtained. Synthesis of three-dimensionally ordered microporous ZIF-8 (3DOM-ZIF-8). The linear polystyrene (LPS) microspheres were prepared by emulsion polymerization according to the previous report.34 LPS microspheres were centrifuged and dried to form the monolithic LPSCCTs. The synthesis of 3DOM-ZIF-8 was almost the same as that of IY-SO3H/Rh@S-ZIF-8 described above except with a change of the CCTs. For 3DOM-ZIF-8, the LPS-CCTs were used in place of the CS-CCTs in the process of material preparation. Finally, the LPS-CCTs were removed via N,N-dimethylformamide (DMF). Synthesis of CLPS-SO3H/Rh@ZIF-8 core-shell microspheres. The synthesis of CLPSSO3H/Rh was almost the same as that of IY-SO3H/Rh@S-ZIF-8 described above except with a change of the supporter. For the CLPS-SO3H/Rh, the obtained CLPS-SO3H microspheres were used in place of the IY-SO3H@S-ZIF-8 in the preparation process. The obtained CLPS-SO3H/Rh microspheres (50 mg) were mixed with 8.3 mL methanol solution of 2-methylimidazole (0.69 g) and polyvinylpyrrolidone (PVP-30, 1.0 g). After that, a methanol solution of Zn(NO3)2·6H2O (8.3 mL, 48.2 mg/mL) was introduced dropwise into the above mixture, which was standing for


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12 h after 20 min stirring. Then the resultant composite was separated and treated with methanol. Ultimately, the product was dried at 60 oC overnight. Catalytic activity test. Knoevenagel condensation-reduction cascade reaction was investigated in a 20 mL pressure vessel. Typically, IY-SO3H/ZIF-8@S-ZIF-8 (50 mg), toluene (5 mL), aldehyde (0.3 mmol), and malononitrile (0.4 mmol) were added into the pressure vessel. The reaction mixture was stirred at 30 °C for 2 h under air atmosphere conditions. Subsequently, the vessel was pressurized with hydrogen to 2 MPa and 80 °C for 12 h. Finally, the IYSO3H/ZIF-8@S-ZIF-8 was filtered off, treated with methanol, and then reused in next run. The filtrate was analyzed via GC-MS. For the mixture of CLPS-SO3H/Rh and 3DOM-ZIF-8, CLPSSO3H/Rh, 3DOM-ZIF-8, and CLPS-SO3H/Rh@ZIF-8, the catalytic conditions are the same as above. Characterization. Fourier transform infrared (FT-IR) spectroscopy was performed on a Bruker VECTOR-22 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo Scientific ESCALab250Xi spectrometer with monochromatic Al Kα radiation (1486.6 eV) and spot size 650 µm, and the binding energies were calibrated using the C 1s peak at 284.6 eV. Scanning electron microscopy (SEM) images were obtained on FEI Nova Nano-SEM450 field-emission scanning electron microscope at different accelerating voltages. Transmission electron microscopy (TEM) images were taken on a JEM-2100F electron microscope. The content of Rh in the obtained material was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES). Nitrogen adsorption-desorption isotherms were performed at 77 K on a surface area and


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porosity analyzer (ASAP 2020M+C). The pore volume and average pore size, and the specific surface area were respectively computed by using the Horvath-Kawazoe method and the Brunauer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA) was investigated by the TGA/DTA system (TA, SDT-Q600) from ambient temperature to about 800 °C in a 100 mL/min airflow and a heating rate of 10 °C/min. X-ray diffraction (XRD) pattern was measured on an X-ray diffractometer (Bruker D8-Davinci). The obtained liquid solution after cascade reaction was analyzed by GC-MS (GC-2010 Plus gas chromatograph) equipped with a 30 m BPX 5 column and flame ionization detector (FID). The injected liquids were heated from 40 to 280 °C at a heating rate of 10 °C/min. RESULT AND DISCUSSION Initially, sulfonated cross-linked polystyrene (CLPS-SO3H) were prepared by sulfonation of dry CLPS microspheres with sulfuric acid (Figure S1). Subsequently, the monodispersed CLPSSO3H microspheres (230 nm in diameter) acted as cores to prepare CLPS-SO3H@SiO2 coreshell composite microspheres. The SEM and TEM images (Figure S2, Figure 1a) demonstrate that the formed CLPS-SO3H@SiO2 core-shell composite microspheres exhibit silica shells about 40 nm. The resultant CLPS-SO3H@SiO2 core-shell microspheres were self-assembled to form the highly ordered opal macrostructure (monolithic CS-CCTs), which acted as a hard template to fabricate a yolk-shelled 3D interconnected network. The SEM images show the gained monolithic CS-CCTs possess highly ordered opal structure with a homogeneous diameter of approximately 310 nm (Figure 1b, Figure S3). The precursor solution containing Zn(NO3)2·6H2O and 2-MeIM was impregnated into the void space of the opal macrostructure.


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Subsequently, an encompassing periodically ordered macroporous structures can be obtained after the crystallization and removal of the templates. With the increase of the injection process, the void in CLPS-SO3H@SiO2 CS-CCTs is gradually filled by ZIF-8 to form IY-SO3H@SiO2/SZIF-8 composites (Figure S4). Notably, the SEM image indicates that the interstitial voids between the CLPS-SO3H@SiO2 microspheres were completely filled ZIF-8 which forms 3D ordered macro/microporous structure throughout the entire CS-CCTs monolith (Figure 1c, Figure S5). Finally, the SiO2 shell in the composite was removed with an appropriate concentration of sodium hydroxide. The SEM and TEM images of IY-SO3H@S-ZIF-8 demonstrate that each spherical void internally encapsulated with a movable core, which was similar to the bee pupa-nested honeycomb-like architecture (Figure 1d~e, Figure S6). Moreover, these integrated yolk-shell structures are interconnected through open windows. Theoretically, each large pore should possess twelve opened windows since the original CLPS-SO3H@SiO2 core-shell microspheres touched with each other so that the precursor solution did not penetrate those sections.35 The highly interconnected spherical cavities can enhance the interconnectivity of the yolk-shell units and conspicuously facilitate the mass diffusion properties within the ordered porous material. Sulfonic acid-functionalized solid supports have been widely used for stabilizing ultrafine Rh NPs and preventing their agglomeration.36,37 Herein, the Rh NPs were loaded on the IY-SO3H@S-ZIF-8 via impregnation reduction strategy. Strikingly, the white dots in SEM image (Figure 1f) and the black dots in TEM images (Figure 1g~h) the clearly reveals that the Rh NPs are almost homogeneously deposited on the CLPS-SO3H yolks. High-resolution TEM image reveals that the regular distances of lattice fringe emerged in the Rh nanoparticle are 0.194 nm and 0.219 nm, ascribing to the (200) and (111) planes of face-centered cubic (fcc) Rh crystal structure (Figure S7). The size of Rh nanoparticles in the IY-SO3H/Rh@S-ZIF-8 was


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estimated to be approximately 3.8 nm from the TEM image via NanoMeasurer (Figure S7c). Scanning TEM (STEM) and the elemental mapping analysis in Figure 1i clearly reveals the uniform distribution of the elements S and Rh in the yolks, and the elements N and Zn in the 3D interconnected framework. Furthermore, the energy-dispersive X-ray spectrometer (EDX) analysis was gained from IY-SO3H@S-ZIF-8 before and after deposition of Rh NPs (Figure 2). Both IY-SO3H@S-ZIF-8 and IY-SO3H/Rh@S-ZIF-8 composite obviously exhibit that the materials consist of C, N, O, Zn and S, but the EDX spectra of the IY-SO3H/Rh@S-ZIF-8 additionally contains Rh, further indicating that Rh was successfully immobilized on the surface of CLPS-SO3H yolks.


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Figure 1. (a) TEM image of CLPS-SO3H@SiO2 microspheres. (b) SEM image of CLPSSO3H@SiO2 CS-CCTs. (c) SEM image of IY-SO3H@SiO2/S-ZIF-8. (d) SEM image of IYSO3H@S-ZIF-8 and the photograph of bee pupa infilled honeycomb. (e) TEM image of IYSO3H@S-ZIF-8. (f) SEM image of IY-SO3H/Rh@S-ZIF-8. (g) TEM images of IYSO3H/Rh@S-ZIF-8. (h) high magnification TEM image of IY-SO3H/Rh@S-ZIF-8 and the corresponding magnified image of Rh nanoparticles (inset). (i) STEM image and corresponding elemental mappings of overall, Zn, S, Rh, and N of IY-SO3H/Rh@S-ZIF-8.


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Figure 2. the EDX spectra of (a) IY-SO3H@S-ZIF-8 and (b) IY-SO3H/Rh@S-ZIF-8. The chemical identities of the materials were also investigated via FT-IR and XPS. The FT-IR spectra of CLPS, CLPS-SO3H, IY-SO3H@SiO2/S-ZIF-8, and IY-SO3H@S-ZIF-8 are demonstrated in Figure 3a. Compared with the pure CLPS, the additional peaks at 1173, 1222, and 1128 cm-1 are originated from the sulfonic acid groups (-SO3H) and the sulfonyl groups (SO2-), respectively.38 In the spectrum of the CLPS-SO3H@SiO2, the peak at 1100 cm-1 is assigned to the Si-O-Si stretching vibration. The new peak at 1387 cm-1 in the spectrum of the IY-SO3H@SiO2/S-ZIF-8 is derived from the imidazole ring stretching vibration, suggesting the successful infiltration of ZIF-8 into the void of CS-CCTs. After the removal of the SiO2 layer by alkaline solution, the peak at 1100 cm-1 arising from Si-O-Si disappeared completely and the characteristic bands derived sulfuric acid groups are still well retained. Moreover, the peaks at 1305 and 421 cm-1 are attributed to the in-plane bending of the 2-MeIM and the Zn-N stretch mode, respectively.39 These results prove the successful encapsulation of sulfonated polystyrene microspheres within an ordered porous ZIF-8 framework. The XPS wide scan spectra show that IY-SO3H/Rh@S-ZIF-8 contains C, N, S, O, Zn, and Rh (Figure 3b), while Rh does not exist in IY-SO3H@S-ZIF-8 (Figure S8). The N 1s situated at 398.8 eV is assigned to the 2-MeIM linker (Figure 3c). The binding energy (BE) peaks at 1021.7 eV and 1044.7 eV are derived from Zn


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2p3/2 and 2p1/2 orbitals (Figure 3d). These results illustrate that ZIF-8 could retain effectively in the final material. Two peaks at 168.9 and 168.0 eV in the S 2p spectra are corresponding to SO3H and -SO3Na, which suggested that Na+ attaching to the part of the -SO3H after alkaline solution etching (Figure 3e). Figure 3f shows the high-resolution Rh 3d peaks before and after Rh3+ reduction, both the signals can be fitted into two peaks. Peaks at 315.2 eV and 310.4 eV are assigned to Rh3+ (3d3/2) and Rh3+ (3d5/2), respectively. After reduction using NaBH4 as reducing agent, the peaks at 315.2 eV and 310.4 eV disappeared and two peaks at 311.9 eV and 307.1 eV appeared ascribing to Rh0 (3d3/2) and Rh0 (3d5/2), respectively.40 These results indicate that Rh3+ can be completely reduced to Rh nanoparticles. The ICP-AES analysis shows that the weight fraction of Rh in the obtained IY-SO3H/Rh@S-ZIF-8 is 0.53 wt%.


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Figure 3. (a) FT-IR spectra of CLPS, CLPS-SO3H, IY-SO3H@SiO2/S-ZIF-8, and IY-SO3H@SZIF-8. XPS spectra of IY-SO3H/Rh@S-ZIF-8 (b) survey scan, (c) N 1s spectra, (d) Zn 2p spectra, and (e) S 2p spectra. (f) Rh 3d XPS spectra of IY-SO3H/Rh@S-ZIF-8 and IY-SO3-Rh3+@S-ZIF8.


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To further investigate the crystal and hierarchical porous structure of the obtained materials, the powder XRD and nitrogen adsorption-desorption isotherms analysis were investigated, respectively. The XRD patterns of IY-SO3H@SiO2/S-ZIF-8, IY-SO3H@S-ZIF-8, and IYSO3H/Rh@S-ZIF-8 are demonstrated in Figure 4a. The well-defined peaks of ZIF-8, including 011, 002, 112, 022, 013, and 222 are both observed. The diffraction intensity of three kinds of materials did not a significant change, indicating the successful generation of phase-pure ZIF-8 with high crystallinity. The porosity of IY-SO3H/Rh@S-ZIF-8 was manifested by nitrogen sorption-desorption measurement (Figure 4b). The sample exhibited similar type I isotherms with a steep slope at very low relative pressure, suggesting the microporous structure was maintained. The BET surface area, average pore size and pore volume of IY-SO3H/Rh@S-ZIF-8 are 435 m2/g, 0.67 nm, and 0.14 cm3/g, respectively. The regular microporous were originated from the intrinsic structural features of ZIF-8, which can boost the surface area and facilitate the accessibility to reactive sites. The BET surface area and pore volume are smaller than 3DOMZIF-8 (~1491 m2/g), which is due to the introduction of CLPS-SO3H microspheres into 3DOMZIF-8 (Table S2, Figure S9 and 10). The TGA curves of 3DOM-ZIF-8, IY-SO3H/Rh@S-ZIF-8, and CLPS are illustrated in Figure 4c. Obviously, TGA curve of IY-SO3H/Rh@S-ZIF-8 shows two weight loss steps and a weight loss of about 68.7 wt% in the heating range from 380 to 500 oC,

ascribing to the structural degradation and decomposition of organic species including the

ZIF-8 and CLPS-SO3H yolks.


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Figure 4. (a) XRD patterns of IY-SO3H@SiO2/S-ZIF-8, IY-SO3H@S-ZIF-8, and IYSO3H/Rh@S-ZIF-8. (b) N2 adsorption-desorption isotherms of IY-SO3H/Rh@S-ZIF-8 and the corresponding pore-size distributions (inset). (c) The TG analysis of 3DOM-ZIF-8, IYSO3H/Rh@S-ZIF-8, and CLPS. Tandem condensation-hydrogenation reactions are the valuable and effective strategy for the synthesis of benzylmalononitrile and benzylmalononitrile derivatives which can be used in the synthesis of antimalarials, and the reactions have attracted substantial attention.41-43 The obtained IY-SO3H/Rh@S-ZIF-8 with high specific surface area, spatially separated functionalities, and ordered macro/microporous structures provide prominent platforms for the bulky-moleculeinvolved cascade catalytic application. The CLPS-SO3H microsphere is nested in each of the ZIF-8 macropores generated by etching of silica, forming 3D ordered macro/microporous yolkshelled nanoreactor. Compared with the previously reported nanoreactors featuring single yolkshell structure unit, this nanoreactor is advantageous to integrate the multiple yolk-shell nanoreactor units into monolithic porous materials. To demonstrate the advantages of the unique hierarchical macro/microporous structure of IY-SO3H/Rh@S-ZIF-8 for bulky-molecule-involved catalysis, the control experiments with the product of CLPS-SO3H/Rh microspheres, coating ZIF-8 layer on the surface of CLPS-SO3H/Rh microspheres (denoted CLPS-SO3H/Rh@ZIF-8), 3DOM-ZIF-8, and the mixture of CLPS-SO3H/Rh and 3DOM-ZIF-8 were conducted (Figure


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S11~13). The catalytic performance of IY-SO3H/Rh@S-ZIF-8, the mixture of CLPS-SO3H/Rh and 3DOM-ZIF-8, CLPS-SO3H/Rh, 3DOM-ZIF-8, and CLPS-SO3H/Rh@ZIF-8 were tested in the Knoevenagel condensation-hydrogenation cascade reactions with a variety of substituted benzaldehyde (Table 1 and Table S3). The Knoevenagel condensation reaction is firstly catalyzed by ZIF-8, followed by the hydrogenation reaction catalyzed by Rh NPs. As illustrated in Table 1, the IY-SO3H/Rh@S-ZIF-8 was capable of converting A to the desired product C with 85.3% to nearly 100.0% conversion and 82.7% to nearly 100.0% yield, indicating the good catalytic activity and selectivity. In sharp contrast, CLPS-SO3H/Rh@ZIF-8 led to no yield of the desired product C, although it was able to catalyze the first step reaction to a small extent, which is attributed to the Rh NPs are loaded in the inner layer of microporous ZIF-8. The molecular width of reactant contains benzene ring exceeds the pore size of ZIF-8. Thus, they have the inability to transmit through the ZIF-8 shell and touch the catalytically active Rh nanoparticles, which leads to no conversion. The similar results were also obtained in previous reports.17,18 These results explicitly indicate that the unique macroporous structure plays a critical role in bulky-molecule diffusing to enhance the activity of cascade reaction. Similarly, although 3DOMZIF-8 has 3D ordered macro/microporous ZIF-8 shell, only product B is generated due to the lack of Rh NPs (Table S3). CLPS-SO3H/Rh completely lost its catalytic cascade activity due to its inability to catalyze the first reaction (Table S3). Obviously, the above results highlight the importance of the spatially separated bifunctionalities (ZIF-8 and Rh NPs) on the nanoreactor for cascade reaction. Besides, the physical mixture of 3DOM-ZIF-8 and CLPS-SO3H/Rh with identical active sites concentrations showed much lower catalytic activity, further indicating the advantage of the macro/microporous yolk-shelled structure (Table S3). The reaction path to explain the high performance of IY-SO3H/Rh@S-ZIF-8 is illustrated in Figure 5a. The reagents


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(benzaldehyde and malononitrile) undergo the interconnected macropores shell and are catalytically converted to the intermediate species (2-benzylidenemalononitrile) by ZIF-8. Next, these intermediate species pass through the spherical cavities, which provides sufficient space for the reaction. Finally, they get to the nested yolks and are converted into the final products (2benzylmalononitrile) catalyzed by the Rh NPs on the CLPS-SO3H yolks. Specifically, multiple reactions were performed in this nanoreactor because multiple yolk-shell type reactor units were integrated/assembled. The integration of both the Rh NPs and macro/microporous ZIF-8 framework in one yolk-shelled nanoreactor could shorten the molecules diffusion length and facilitate the availability of the active sites. Based on the preceding results, the tentative cascade reaction mechanism over IY-SO3H/Rh@S-ZIF-8 has been proposed (Figure S14). Table 1. Knoevenagel condensation-hydrogenation cascade reactions catalyzed by different nanoreactors.a

Entry

Substrates A

Products C

Reaction time (h)

IY-SO3H/Rh@S-ZIF-8 a Conv. of Yield of Yield of Ab (%) Bb (%) Cb (%)

CLPS-SO3H/Rh@ZIF-8c Conv. of Ab (%)

Yield of Bb (%)

Yield of Cb (%)

1

2+12

99.9

0

99.9

48.5

48.5

0

2

2+12

97.5

5.0

92.5

43.2

43.2

0

3

2+12

85.3

2.6

82.7

35.8

35.8

0

aReaction

conditions: first step, IY-SO3H/ZIF-8@S-ZIF-8 (50 mg), aldehyde (0.3 mmol), malononitrile (0.4

mmol), toluene (5 mL), 30 °C, 2 h; second step, 2 MPa H2, 80 °C, 12 h. bThe reaction yield was determined by GC-MS. cCLPS-SO3H/Rh@ZIF-8 containing the same amount of Rh NPs as IY-SO3H/Rh@S-ZIF-8.


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The stability and reusability of IY-SO3H/Rh@S-ZIF-8 were also investigated. The IYSO3H/Rh@S-ZIF-8 was recovered via filtration and reutilized for the next cycle of cascade reactions. The results confirm that the IY-SO3H/Rh@S-ZIF-8 can be readily reutilized at least five cycles without distinct activity reduction (Figure 5b). The XRD pattern of the recovered nanoreactor in Figure S15 reveals that the crystallinity of the sample was well retained. From the SEM as well as TEM images and EDX elemental mapping in Figure S16, the morphology and chemical compositions of IY-SO3H/Rh@S-ZIF-8 were well retained after being reused for 5 times, indicating the good recyclability of the nanoreactor. The ICP-AES analysis demonstrates that the Rh content in recovered nanoreactor is approximately 0.49 wt%, which is close to 0.53 wt% of the original sample IY-SO3H/Rh@S-ZIF-8. Furthermore, we also investigated the filtration test to verify the nature of heterogeneous catalysis. For Knoevenagel condensation reaction, the solid catalyst IY-SO3H/Rh@S-ZIF-8 was filtered after 0.4 h was carried out, and then the mother liquor was kept on stirring under identical reaction condition. For hydrogenation reaction, the solid catalyst IY-SO3H/Rh@S-ZIF-8 was filtered after 2 h was carried out, and then the mother liquor was kept on stirring under identical reaction condition. Results show that both the conversion of benzaldehyde and the yield of 2-benzylmalononitrile almost remained constantly for the rest of the time, which reflects that the absence of leaching (Figure 5c~d). The above results obviously illustrate the desirable recyclability and stability of the IY-SO3H/Rh@SZIF-8. This can be ascribed to the monolithically interconnected ZIF-8 shell and the -SO3H groups stabilized Rh NPs.


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Figure 5. (a) Illustration of the cascade catalysis process in the IY-SO3H/Rh@S-ZIF-8. (b) Catalytic recyclability of IY-SO3H/Rh@S-ZIF-8 for the one-pot cascade Knoevenagel condensation-hydrogenation

reactions.

(c)

Time-conversion

plots

of

Knoevenagel

condensation reaction for the first run of IY-SO3H/Rh@S-ZIF-8 and CLPS-SO3H/Rh@ZIF-8, and hot filtration experiments of IY-SO3H/Rh@S-ZIF-8. (d) Time-yield plots of hydrogenation reaction for the first run of IY-SO3H/Rh@S-ZIF-8 and CLPS-SO3H/Rh@ZIF-8, and hot filtration experiments of IY-SO3H/Rh@S-ZIF-8. CONCLUSION In summary, 3D ordered macro/microporous yolk-shelled nanoreactor with spatially separated functionalities was synthesized by core-shell colloidal crystal templating strategy and double-


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solvent-induced heterogeneous nucleation strategy. Macropores could tremendously facilitate the diffusion rate of bulky molecules and accelerate the accessibility to the active sites, whereas micropores within macroporous skeleton introduce a high surface area. Particularly, the obtained IY-SO3H/Rh@S-ZIF-8 is capable of catalyzing the cascade reaction of Knoevenagel condensation-hydrogenation. By virtue of the versatility of MOF materials and functional polymers, this strategy is extensively extendable to fabricate various ordered hierarchical macro/micropores materials with programmed positioning of appropriate functionalities for wide promising applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XPS spectra of CLPS-SO3H, IY-SO3H@S-ZIF-8, 3DOM-ZIF-8, and CLPS-SO3H/Rh@ZIF-8; SEM images of CLPS-SO3H, CLPS-SO3H@SiO2, CLPS-SO3H@SiO2 CS-CCTs, monolithic CLPS-SO3H@SiO2 core-shell template, IY-SO3H@SiO2/S-ZIF-8 composites constructed with different impregnation times, IY-SO3H@SiO2/S-ZIF-8 composites, IY-SO3H@S-ZIF-8, LPSCCTs, LPS-CCTs/3DOM-ZIF-8, 3DOM-ZIF-8, CLPS-SO3H/Rh microspheres, and IYSO3H/Rh@S-ZIF-8 after 5 successive cycles; XRD pattern of 3DOM-ZIF-8, CLPSSO3H/Rh@ZIF-8, and IY-SO3H/Rh@S-ZIF-8 after 5 successive cycles; Porosity properties of


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3DOM-ZIF-8 and CLPS-SO3H/Rh@ZIF-8; FT-IR spectra of LPS, LPS-CCTs/3DOM-ZIF-8, 3DOM-ZIF-8, CLPS, CLPS-SO3H, and CLPS-SO3H/Rh@ZIF-8; TG analysis of CLPSSO3H/Rh@ZIF-8; Tentative Mechanism for the Knoevenagel condensation-hydrogenation cascade reaction; EDX elemental mapping and TEM image of IY-SO3H/Rh@S-ZIF-8 after using for 5 times; The size distribution of Rh nanoparticles on the IY-SO3H/Rh@S-ZIF-8 surface; HRTEM images of IY-SO3H/Rh@S-ZIF-8.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] and [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 51573038, 51403049, 50903027) and the Natural Science Foundation of Hebei Province (No. E2016202261 and E2017202036).


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Table of Contents (TOC)


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Microporous Yolk-Shelled

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