Immobilization of MetalâOrganic Framework Nanocrystals for

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Immobilization of Metal-Organic Framework Nanocrystals for Advanced Design of Supported Nanocatalysts Ping Li, and Hua Chun Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11775 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Applied Materials & Interfaces

Immobilization of Metal-Organic Framework Nanocrystals for Advanced Design of Supported Nanocatalysts Ping Li and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

KEYWORDS: layered double hydroxides (LDH), metal–organic frameworks (MOFs), integrated nanocomposite, thermal conversion, heterogeneous catalysis

ABSTRACT: In recent years, metal-organic frameworks (MOFs) have been employed as heterogeneous catalysts or precursors for synthesis of catalytic materials. However, conventional MOFs and their derivatives usually exhibit limited mass transfer and modest catalytic activities owing to lengthy diffusion path and less exposed active sites. On the other hand, it has been generally conceived that nanoscale MOFs are beneficial to materials utilization and mass transport, but their instability poses a serious issue on practical application. To tackle above challenges, herein we develop a novel and facile approach to design and synthesis of nanocomposites through in-situ growth and directed immobilization of nanoscale MOFs onto 1

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layered double hydroxides (LDH). The resulting supported nano-MOFs inherit advantages of pristine MOF nanocrystals, and meanwhile gain enhanced stability and workability under reactive environments. A series of uniform nanometer-sized MOFs, including monometallic (ZIF-8, ZIF-67 and Cu-BTC) and bimetallic (CoZn-ZIF), can be readily synthesized onto hierarchically structured flowerlike MgAl-LDH supports with high dispersion and precision. Additionally, the resultant MgAl-LDH/MOFs can serve as a generic platform to prepare integrated nanocatalysts via controlled thermolysis. Knoevenagel condensation and reduction of 4-nitrophenol (4-NP) are used as model reactions for demonstrating the technological merits of these nanocatalysts. Therefore, this work elucidates that the synthetic immobilization of nanoscale MOFs onto conventional catalyst supports is a viable route to develop integrated nanocatalysts with high controllability over structural architecture and chemical composition.

1. Introduction Metal–organic frameworks (MOFs), or porous coordination polymers (PCPs), constructed through formation of coordination bonds between metal ions (or clusters) and organic linkers, emerge as a new class of nanoporous crystalline materials.1,2 In recent years, development of MOFs becomes one of the most attractive research areas, since flexible and versatile choices of metal centers and organic linkers allow the advanced design and synthesis of inorganic-organic materials with novel structure, topology and functionality which are unachievable by conventional materials and methods.3 These features, along with their high porosity and specific surface area, make MOFs have unprecedented opportunities for applications in a multitude of fields including gas adsorption and separation,4 sensing,5 and energy conversion and storage.6-9 2


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In particular, utilization of MOFs and their corresponding derived products in catalysis has attracted continuously increasing research interest over the last two decades.10,11 With wellorganized active functionalities from metallic centers and organic linkers, MOFs themselves are fascinating molecular solids to catalyze a variety of organic reactions.12-14 Furthermore, MOFs are found to be capable of serving as a class of versatile precursors for fabrication of catalytic materials. Under certain conversion conditions (e.g., controlled pyrolysis or reductive calcination), for instance, a broad range of solid catalysts including metals, metal oxides, and doped-carbon can be prepared.15-17 However, at present, the reported MOFs are largely lacking size-control and morphological manipulation in this type of applications.18 The usage of largesized MOFs as catalysts or catalyst precursors bring some disadvantages. On the one hand, their catalytic activity and materials utilization rate are not high owing to less catalytic active sites are exposed in the large particles. On the other hand, the mass transfer is also limited because of lengthy diffusion paths for reaction substrates and product molecules. Therefore, to overcome the above drawbacks, it is proposed that nanoscale MOFs could be a type of promising alternative catalysts and catalyst precursors. Nevertheless, free-standing nanoscale MOFs are unstable, prone to agglomeration and/or deterioration.19 When acting as precursors to undergo high-temperature heat treatment, for example, nano-MOFs tend to suffer from sintering or fusion into a bulk phase.20 Besides, from the perspective of applications, nanoMOFs are not convenient to handle, due to difficulty in recovery and toxicity of nanomaterials. In this regard, fabrication of MOFs-based catalysts with small dimensions for abundant active sites and easy mass transfer, and long durability for low operating cost and environmental impact is highly desirable yet faces many challenges. If an appropriate support material is introduced to act as a directing matrix for in-situ synthesis of size-controlled MOFs and at the same time to 3



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disperse and stabilize forming products, we might be able to enhance the workability of general nano-MOFs while keeping their inherited properties unaltered. Such an immobilization concept inspires us to search for suitable supports for the integrative construction of catalytic MOFsbased nanocomposites in the present work. Concerning potential support materials, layered double hydroxides (LDH) are a class of economical anionic clay compounds, consisting of positively charged brucite-like host layers, and exchangeable anions situated in the interlayer space to balance the positive charges of host layers.21 Because of their unique compositional and structural flexibility (i.e., ion-exchange ability and memory effect etc.), LDH have received lots of attention.22-24 Particularly, LDH and their calcined products (layered double oxides, LDO) are found to act as low-cost, effective carriers for loading catalytic components, such as noble metals and transition metal oxides.25,26 In view of their richness and easiness in chemical modifications and usefulness and importance in heterogeneous catalysis, we chose LDH/LDO as supporting materials for testing the above hypothesis. Herein we develop a facile approach to design a class of integrated MOFs-based nanocomposites via immobilization of nanoscale MOFs onto flowerlike MgAl-LDH micro/nanoparticles with hierarchical structures. With the aid of support, monodisperse MOFs nanocrystals can in-situ nucleate and grow on the surface of LDH nanosheets. This approach is applicable to mount both monometallic (e.g., ZIF-8, ZIF-67 and Cu-BTC) and bimetallic (e.g., CoZn-ZIF) MOFs onto the LDH phase. Importantly, the as-prepared MgAl-LDH/MOFs nanocomposites can serve as generic precursors to form various LDH/LDO-supported ultrafine metal and metal-oxide nanocomposites (e.g., MgAl-LDO/MxOy, MgAl-LDO/M and MgAlLDH/M) with high controllability over structural architecture and chemical composition. The 4


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MgAl-LDH/MOFs as well as their corresponding derivatives promise practical applications in heterogeneous catalysis. To elucidate their workability, in this study, two of such nanocomposites MgAl-LDH/ZIF-8 and MgAl-LDH/Cu are investigated as model catalysts for Knoevenagel condensation and reduction of 4-nitrophenol (4-NP), respectively.

2. Experimental Section 2.1. Materials and reagents. The following chemicals were used as received without any further purification: Zn(NO3)2·6H2O (99%, Sigma−Aldrich), Co(NO3)2·6H2O (98%, Sigma−Aldrich), Cu(NO3)2·3H2O (99.5%, Merck), MgCl2·6H2O (99+%, Sigma−Aldrich), AlCl3·6H2O (99+%, Sigma−Aldrich), urea (99%, Fluka), 2-methylimidazole (HMIM, 99%, Sigma−Aldrich), trimesic acid (95%, Sigma−Aldrich), triethylamine (TEA, 99+%, Acros Organics), methanol (99.99%, Fisher), ethanol (99.99%, Fisher), methyl cyanoacetate (98%, Sigma−Aldrich), benzaldehyde (99%, Sigma−Aldrich), 4-methylbenzaldehyde (99%, Sigma−Aldrich), 2-hydroxybenzaldehyde (98%, Merck), 4-chlorobenzaldehyde (99%, Sigma−Aldrich), 4-bromobenzaldehyde (99%, Sigma−Aldrich), 4-nitrophenol (4-NP, 99%, Sigma−Aldrich), sodium borohydride (99.99%, Sigma−Aldrich). Deionized water was collected through the Elga Micromeg purified water system. 2.2. Preparation of flowerlike MgAl-LDH support. Flowerlike MgAl-LDH support was synthesized by our previously reported method with some modifications.26,27 In a typical procedure, MgCl2·6H2O, AlCl3·6H2O and urea with a molar ratio of 3: 1: 7 were dissolved in 30 mL of methanol. The above mixed solution were stirred for 2 h and then transferred into a Teflon-lined autoclave, sealed and heated at 150 °C for 10 h. The product was centrifuged, rinsed with deionized water and ethanol for several times and dried in an oven at 70 °C for 12 h. 5



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2.3. Preparation of MgAl-LDH/MOFs nanocomposites. Preparation of MgAl-LDH/ZIF-8: Briefly, MgAl-LDH particles (100 mg) were firstly dispersed in methanol (40 mL). Then a mixed methanolic solution containing HMIM (0.1 mmol) and TEA (0.1 mmol) was added into the above homogeneous suspension under vigorous stirring. After ultrasonic treatment for 0.5 h, zinc nitrate (0.05 mmol) metanolic solution was added. The as-obtained mixture was stirred for another 2 h. Finally, the final product was collected, washed with methanol, and dried overnight under vacuum. The MgAl-LDH/ZIF-8 with different ZIF-8 loadings can be obtained via adjusting the amount of HMIM, TEA and zinc nitrate added during synthesis. Preparation of MgAl-LDH/ZIF-67: MgAl-LDH particles (100 mg) were firstly dispersed in methanol (40 mL). Then a mixed methanolic solution containing HMIM (0.4 mmol) and TEA (0.4 mmol) was added into the above suspension under vigorous stirring. After ultrasonic treatment for 0.5 h, cobalt nitrate (0.067 mmol) methanolic solution was added. The as-obtained mixture was stirred for another 2 h. The final product was collected, rinsed with methanol, and vacuum-dried overnight. Preparation of MgAl-LDH/Cu-BTC: MgAl-LDH support (100 mg) was firstly dispersed in methanol (40 mL). Then trimesic acid (0.05 mmol) methanolic solution was added into the above suspension under vigorous stirring. After ultrasonic treatment for 0.5 h, copper nitrate (0.05 mmol) metanolic solution was added. The as-obtained mixture was stirred for another 2 h. Finally, the product was collected, washed with methanol, and dried overnight under vacuum condition. Preparation of MgAl-LDH/CoZn-ZIF: 100 mg of MgAl-LDH sample were firstly dispersed in methanol (40 mL). Then a mixed methanolic solution containing HMIM (0.1 mmol) and TEA (0.1 mmol) was added into the above homogeneous suspension under vigorous stirring. After 6


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ultrasonic treatment for 0.5 h, the mixed methanolic solution containing cobalt nitrate (0.025 mmol) and zinc nitrate (0.025 mmol) was added. The as-obtained mixture was stirred for another 2 h. The product was then collected, washed with methanol, and dried overnight in vacuum. 2.4. Preparation of MgAl-LDH plates/MOFs nanocomposites. Firstly, the hexagonal MgAlLDH nanoplates were fabricated via the same preparation procedure of flowerlike MgAl-LDH support while using deionized water as solvent. Then, MgAl-LDH plates/MOFs was prepared via the same preparation procedure of the case of MgAl-LDH/MOFs while using hexagonal MgAlLDH nanoplates instead of the flowerlike MgAl-LDH support. 2.5. Preparation of MgAl-LDO/MxOy nanocomposites. Three representative nanocomposites of this type, MgAl-LDO/ZnO, MgAl-LDO/Co3O4 and MgAl-LDO/CuO, were produced via calcining their corresponding MgAl-LDH/MOFs at 400 °C under air atmosphere for 1 h with a heating rate of 1 °C/min. 2.6. Preparation of MgAl-LDH (or LDO)/M nanocomposites. Preparation of MgAl-LDO/Co. The above as-prepared MgAl-LDH/ZIF-67 was heated at 600 °C under H2 atmosphere for 2 h with a heating rate of 2 °C/min, followed by natural cooling to room temperature. Preparation of MgAl-LDH/Cu. The MgAl-LDH/Cu-BTC prepared above was heated at 250 °C under H2 atmosphere for 2 h with a heating rate of 2 °C/min, followed by natural cooling to room temperature. 2.7. Preparation of ZIF-8 microcrystals. In a typical synthesis, zinc nitrate (10 mmol) was dissolved in 40 mL of methanol to form a clear solution. HMIM (40 mmol) was dissolved in another 40 ml of methanol. The above two solutions were then mixed. After thorough mixing, the resulting solution was transferred into an autoclave and heated at 120 °C for 8 h. The white

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powder was collected by centrifugation, washed with methanol for several times, and dried in an oven at 80 °C for 12 h. 2.8. Catalytic activity evaluations. Knoevenagel condensation reactions: MgAl-LDH/ZIF-8 (10 mg, 0.77 mol% Zn) was added into the reaction mixture composed of methanol (3 mL), aldehyde (0.5 mmol), methyl cyanoacetate (1 mmol), n-dodecane (0.5 mmol) in a glass vial. The catalyst concentration was calculated with respect to the Zn/aldehyde molar ratio. The resulting mixture was vigorously stirred at room temperature for a given time. Then the solid catalyst was separated via centrifugation, and the supernatant was analyzed by gas chromatography (GC, Agilent-7890A) equipped with a capillary column (HP-5, 30.0 m × 320 µm × 0.25 µm) and a flame ion detector (FID). The products were further confirmed by gas chromatography-mass spectrometry (GC-MS, Agilent, 7890A-5975C) with a capillary column (DB-5 ms, 30.0 m × 320 µm × 0.25 µm). Catalytic reduction of 4-NP: In a typical catalytic reduction reaction, 1.5 mL of freshly prepared NaBH4 aqueous solution (20 mM) was added into a quartz cell containing 1.5 mL of 4NP aqueous solution (0.1 mM), resulting in immediate color change from light yellow to bright yellow. Then 0.05 mL of the colloidal suspension containing 0.025 mg of MgAl-LDH/Cu (Cu loading: 1.90 wt%) was added to start the reaction. The UV-Vis spectrometry was employed to in-situ monitor the reaction process by measuring the absorbance of solution as a function of reaction time; the wavelength scanning range was set at 250−550 nm. 2.9. Characterization. The microscopic features of the samples were characterized by scanning electron microscopy (SEM, JEOL-6700F) equipped with an energy-dispersive X-ray (EDX) analyzer (Oxford INCA), transmission electron microscopy (TEM, JEOL JEM-2010, 200 kV), and high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, 200 kV). 8


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The elemental mapping was done by energy-dispersive X-ray spectroscopy (EDX, Oxford Instruments, model 7426). The wide-angle X-ray diffraction patterns were taken on Bruker D8 Advance system (Cu Kα radiation). Nitrogen adsorption–desorption isotherms were obtained on Quantachrome NOVA-3000 system at 77 K. The specific surface area of the materials was measured by the Brunauer–Emmet–Teller (BET) method. Inductively coupled plasma (ICP) analysis (Dual-view Optima 5300 DV ICP-OES) was used to measure the elemental compositions of the above studied samples. The catalytic reduction of 4-NP to 4-AP was monitored with an UV−Vis spectrophotometer (UV-2450, Shimadzu).

3. Results and Discussion 3.1. Overall strategy for fabricating MgAl-LDH/ MOFs nanocomposites.

Scheme 1. Synthesis procedure for hierarchical-structured MgAl-LDH/MOFs nanocomposites. The overall synthetic procedure to produce MgAl-LDH/MOFs nanocomposites is schematically presented in Scheme 1 and described in the Experimental Section in detail. Firstly, uniform flowerlike hierarchically structured MgAl-LDH particles were prepared by our previously reported one-pot solvothermal method using metal chlorides as a metal source, urea as a base and methanol as a solvent.26,27 Then the organic linker and the metal salt were added 9



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stepwise to MgAl-LDH suspension for the in-situ nucleation and directed growth of the desired MOFs. Owing to the dispersing and directing effects of surface property of LDH, nanoscale MOFs can be grown and immobilized homogeneously on the LDH, resulting in MgAlLDH/MOFs nanocomposites with targeted structural and chemical properties. This technique is reproducible and generic for anchoring two important families of MOFs, including those with imidazolate linkers (e.g., ZIF-8 and ZIF-67) and those with carboxylate-containing ligands (e.g., Cu-BTC). Apart from the conventional monometallic ones, furthermore, bimetallic nanocrystals of MOFs can also be synthesized and deposited on the same LDH support with the current approach. 3.2. Preparation and characterization of flowerlike MgAl-LDH. The SEM and TEM images (Figure S1) show that MgAl-LDH sample from one-pot solvothermal route is hierarchically structured flowerlike particles which are assembled by interconnecting LDH nanosheets (about 10 nm in thickness); the diameter for the particles ranges from 2 to 3 µm. The powder X-ray diffraction (XRD) pattern (Figure S2) reveals the characteristic reflections of a typical hydrotalcite phase with a series of symmetric (00l) peaks; the atomic ratio of Mg/Al in the sample is 2.07, determined from ICP-AES analysis. In addition, from the nitrogen sorption measurement, the MgAl-LDH displays a type IV isotherm with H3 type hysteresis loop (Figure S3), indicating the presence of mesoporous structure, and the BET surface area is determined to be 93.9 m2/g. It can be expected that the resultant MgAl-LDH with such hierarchical architecture, large specific surface area, mesoporous structure and abundant surface groups present great potential to serve as an efficient catalyst support, which not only can control the size and distribution of nanocrystals of MOFs, but also can disperse and stabilize them from aggregation and degradation. 10


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3.3. Preparation and characterization of MgAl-LDH/ZIFs. As a large subfamily of MOFs, zeolitic imidazolate frameworks (ZIFs) have received great research interest in the past decades.28,29 In general, ZIFs comprise of imidazolate linkers and divalent metal ions in a tetrahedral arrangement, whose structures are similar to the conventional aluminosilicate zeolites.30 They simultaneously possess the chemical and structural characteristics of both MOFs and zeolites, and thus intersect their respective advantages, such as large surface area, tunable porosity, multiple-functionalities and good thermal and chemical stabilities.

Figure 1. Characterization of MgAl-LDH/ZIF-8 nanocomposite: (a) SEM image, (b) XRD pattern, (c, d) TEM images and (e) results of EDX elemental mapping. 11



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To better control the nucleation and the growth of ZIFs and exploit their unique physicochemical properties, we have immobilized ZIFs on MgAl-LDH support via simultaneous introduction of the organic linker (HMIM) and base modifier (TEA) into the MgAl-LDH suspension, followed by the addition of metal salts. For instance, LDH-supported ZIF-8 nanocrystals (denoted as MgAl-LDH/ ZIF-8) can be obtained by using zinc nitrate as metal source. The SEM (Figure 1a and S4a) and TEM (Figure 1c) images demonstrate that the original flowerlike morphology of MgAl-DLH is well-maintained after ZIF-8 anchoring. From the highmagnification TEM image (Figure 1d), individual ZIF-8 nanocrystals (particle diameter d = 10−20 nm) are well dispersed on the surface of MgAl-LDH. The XRD pattern (Figure 1b) also confirms the formation of crystalline ZIF-8 phase. Additionally, EDX elemental mapping (Figure 1e) demonstrates that the sample contains zinc, nitrogen, aluminum, magnesium, which agrees well with the expected chemical composition of MgAl-LDH/ZIF-8. From nitrogen adsorption/desorption measurement (Figure S5), the nanocomposite maintains type IV isotherm with H3 type hysteresis loop, indicating good retention of the textural property. Furthermore, owing to the contribution of nanoscale ZIF-8 phase, the MgAl-LDH /ZIF-8 displays a larger BET surface area (169.3 m2/g) compared with its pristine MgAl-LDH support. The content of MOFs phase can be further tailored with this method. It is found that MgAlLDH/ZIF-8 with various ZIF-8 loadings can be readily produced through adjusting the experimental parameters. As shown in Figure S6, ZIF-8 nanocrystals with different distributional densities are well-dispersed on the LDH nanosheets via adjusting the amount of the organic linkers and metal salts added during the synthetic process. In addition, other types of ZIFs with different metal ions, e.g., ZIF-67 nanocrystals (with cobalt ions as metal nodes), also can be immobilized on the MgAl-LDH. Similar to the case of MgAl12


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LDH/ZIF-8, MgAl-LDH/ZIF-67 maintains the original flowerlike morphology, and tiny ZIF-67 particles (d = 20−30 nm) are dispersed evenly on MgAl-LDH support (see Figure 2a, c, d and S4b for SEM and TEM images). The XRD pattern (Figure 2b) and EDX elemental mapping (Figure 2e) further verify the presence of ZIF-67 phase in the composite. Furthermore, our nitrogen adsorption/desorption measurement (Figure S7) demonstrates that the MgAl-LDH/ZIF67 sample also possesses mesoporous structure with a large specific surface area (182.0 m2/g).

Figure 2. Characterization of MgAl-LDH/ZIF-67 nanocomposite: (a) SEM image, (b) XRD pattern, (c, d) TEM images and (e) results of EDX elemental mapping. 13



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3.4. Preparation and characterization of MgAl-LDH/Cu-BTC. Apart from the abovementioned supported ZIFs with imidazolate linkers, our strategy is also applicable to the immobilization of other types of MOFs, for instance, metal–carboxylate coordination polymers, another large subfamily of MOFs which are based on carboxylate-containing ligands.31,32 Here we take Cu-BTC as an illustrative example. Using copper nitrate as a metal source and trimesic acid as an organic linker, Cu-BTC nanocrystals can also be integrated onto the above investigated MgAl-LDH support.

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Figure 3. Characterization of MgAl-LDH/Cu-BTC nanocomposite: (a) SEM image, (b) XRD pattern, (c, d) TEM images and (e) results of EDX elemental mapping. As presented in the SEM (Figure 3a and S4c) and TEM (Figure 3c, d) images, the product MgAl-LDH/Cu-BTC inherits the original flowerlike morphology, and the Cu-BTC nanocrystals (d = 10−20 nm) are anchored on the surface of MgAl-LDH with high dispersion. The XRD pattern (Figure 3b) further confirms the existence of Cu-BTC phase. Furthermore, EDX elemental mapping in Figure 3e reveals the presence of copper, aluminum and magnesium, conforming to the chemical composition of MgAl-LDH/Cu-BTC. 3.5. Preparation and characterization of MgAl-LDH/CoZn-ZIF.

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Figure 4. Characterization of MgAl-LDH/CoZn-ZIF nanocomposite: (a) SEM image, (b) XRD pattern, (c, d) TEM images and (e) results of EDX elemental mapping. In addition to the conventional monometallic MOFs, bimetallic ones also can be introduced onto the MgAl-LDH support via our synthetic approach. As a proof-of-concept experiment, the fabrication of MgAl-LDH/CoZn-ZIF is achieved by using cobalt nitrate and zinc nitrate as mixed metal precursors. The SEM (Figure 4a and S4d) and TEM images (Figure 4c, d) demonstrate that MgAl-LDH/CoZn-ZIF nanocomposite preserves the original flowerlike morphology; uniform CoZn-ZIF nanocrystals (d = 10−20 nm) are homogeneously dispersed on the support. The XRD pattern (Figure 4b) and the EDX elemental mapping (Figure 4e) further affirm the presence of bimetallic MOFs (CoZn-ZIF nanocrystals) on the product. 3.6. Preparation and characterization of MgAl-LDH plate/MOFs nanocomposites. Our synthetic method is quite flexible. As for the support materials, in addition to the above MgAlLDH with flowerlike architecture, MgAl-LDH with other morphologies can also be decorated with MOFs NPs. As an illustration, MgAl-LDH with well-known hexagonal platelet morphology is employed to support MOFs. The hexagonal MgAl-LDH plates were prepared by a hydrothermal route. As reported in Figures S8 and S9, the as-obtained hexagonal nanoplates are relatively uniform with a lateral size in the range of 2−3 µm and a thickness of 50−100 nm. The XRD investigation (Figure S10) confirms the hydrotalcite phase with a rhombohedral symmetry (R-3m) (JCPDS card no. 541030). Through the similar method that adopted for flowerlike MgAl-LDH/MOFs nanocomposites (see the Experimental Section for detail), three types of nanoscale MOFs, namely ZIF-8, ZIF-67 and Cu-BTC, have been successfully deposited onto the hexagonal plates

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of LDH. As demonstrated in SEM and TEM images (Figure 5), uniform MOFs nanocrystals are dispersed on the surface of this two-dimensional MgAl-LDH support.

Figure 5. Characterization of MgAl-LDH plate/MOFs nanocomposites: SEM and TEM images of (a, b) MgAl-LDH plate/ZIF-8, (c, d) MgAl-LDH plate/ZIF-67 and (e, f) MgAl-LDH plate/CuBTC.

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3.7. Thermal conversion of MgAl-LDH/MOFs nanocomposites. Owing to the controllable structure and abundant choices of metal centers and organic species in the scaffolds, MOFs is a family of outstanding precursor candidates to synthesize tailorable nanostructured functional materials including porous carbon and related metals/metal oxides under certain controlled conversion conditions.33-35 Nevertheless, as stated in the Introduction, the conversion process, especially concerning heat treatment at high-temperatures, always accompanied by serious aggregation and sintering of primary particles, leading to deteriorated performance of their calcined products. Considering the dispersing and stabilizing effects of the LDH support to the anchored nano-MOFs in our MgAl-LDH/MOFs, it is anticipated that the sintering of particles can be minimized during thermolysis of these precursors, thereby producing nanosized catalytic particles immobilized on the support. Herein, we propose a new approach to use MgAl-LDH/ MOFs nanocomposites as precursors to synthesize a wide range of ultrafine metals and metal oxides supported by LDH/LDO as carriers. For instance, by calcining MgAl-LDH/MOFs in air at moderate temperature, a series of MgAl-LDO/MxOy (where M = Zn, Co and Cu) can be generated. During calcination, MgAl-LDH is converted into MgAl-LDO. Simultaneously, the metal ions in the MOFs NPs are transformed into metal oxides NPs, while carbon and other nonmetal elements (such as N and H) are oxidized. Firstly, TGA characterization was performed. Based on the TGA results (Figure S11), the calcination temperature for MgAl-LDH/ZIF-8, MgAl-LDH/ZIF-67 and MgAl-LDH/Cu-BTC are determined at 400 °C. For all the calcined products, as revealed in Figures 6 and S12, their original hierarchical flowerlike morphology remains essentially, while the nanosheets become very porous. And uniform tiny MxOy NPs (M = Zn, Co and Cu) derived from nanoscale MOFs are highly dispersed on these nanosheets. The XRD patterns of the three calcined products 18


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(Figure S13) only exhibit the peaks indexed to cubic phase of MgO (JCPDS card no. 45-0946), while no peaks assignable to the MxOy phases could be observed owing to their extremely small particle size and low surface contents on the support.

Figure 6. Characterization of MgAl-LDO/MxOy nanocomposites: TEM images of (a, b) MgAlLDO/ZnO, (c, d) MgAl-LDO/Co3O4 and (e, f) MgAl-LDO/CuO.

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Interestingly, supported ultrafine transition metal NPs can also be produced from the MgAlLDH/MOFs by thermolysis under a reductive atmosphere. For instance, MgAl-LDO/Co NPs and MgAl-LDH/Cu NPs can be obtained by thermal treatment of their respective solid precursor MgAl-LDH/MOFs in a hydrogen flow at 600 and 250 °C, respectively. The SEM and TEM images (Figure S14 and Figure 7) exhibit that uniform small-sized Co NPs and Cu NPs are immobilized on the nanosheets of the support. Again, due to the extremely small crystallite size and low surface contents of these metal NPs, no peaks belonging to Co or Cu NPs are observable in their respective XRD patterns (Figure S15 for MgAl-LDO/Co NPs and Figure S16 for MgAlLDH/Cu NPs).

Figure 7. TEM images of (a, b) MgAl-LDO/Co NPs and (c, d) MgAl-LDH/Cu NPs.

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Obviously, our proposed strategy for the exploration of MgAl-LDH/MOFs nanocomposites as precursors to supported ultrafine metals/metal oxides opens a tremendous opportunity and represents an initial step toward the fabrication of a wide variety of highly tunable functional nanomaterials. Compared to the conventional impregnation approach for catalyst preparation, for instance, our route through synthetic immobilization of MOFs nanocrystals provide a greater flexibility in size control and compositional control for the catalytically active phases. 3.8. Catalytic performance of MgAl-LDH/ZIF-8 nanocomposite. In view of their tailorable compositions and unique structural features, such as hierarchical architecture, high specific surface area and evenly dispersed nanoscale particles on the LDH support, the above MgAlLDH/MOFs and their related derivatives present great potential for applications in a wide range of technological fields, especially in heterogeneous catalysis. In this work, MgAl-LDH/ZIF-8 (Zn loading: 2.5 wt%) was evaluated as a heterogeneous catalyst for Knoevenagel condensation reactions. Knoevenagel condensation, involving the condensation between aldehydes/ketones and compounds containing activated methylene groups, is a useful and widely employed organic reaction for carbon-carbon bond formation.36,37 The corresponding condensation product, α, βunsaturated carbonyl compound, is one of important building blocks for the synthesis of various fine chemicals, drugs as well as functional polymers. Conventionally, Knoevenagel condensation reactions proceed with the assistance of homogeneous catalysts such as organic amines, Lewis acids, ionic liquids and organometallic compounds. From the standpoint of green chemistry, nevertheless, this homogeneous route suffers from many disadvantages, including nonrecoverable catalysts and serious corrosion to the equipment. Thus, design and development of

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environmentally benign, highly active and recyclable heterogeneous catalysts for Knoevenagel condensation is of great significance today.38-41 According to the previous investigation on ZIF-8 as catalyst for trans-esterification reaction, it has been found that the catalytic active sites (e.g., the acid and base sites) are located at the external surface or the defects of ZIF-8, while not in the micropores of ZIF-8.42 In the case of MgAl-LDH/ZIF-8, as we elucidated, ZIF-8 particles are nanometer-sized and uniformly dispersed on the MgAl-LDH support which has a hierarchical architecture and a mesoporous structure. Therefore, we envision that MgAl-LDH/ZIF-8 nanocomposite must be capable of serving as a promising catalyst for organic transformations.

Figure 8. Yield-reaction time curves of the Knoevenagel condensation between benzaldehyde and methyl cyanoacetate using different catalysts: blank, pristine MgAl-LDH, microsized ZIF-8 and MgAl-LDH/ZIF-8. Reaction conditions: benzaldehyde (0.5 mmol), methyl cyanoacetate (1 mmol), methanol (3 mL), solid catalyst (10 mg, 0.77 mol% Zn), room temperature under atmospheric conditions. For MgAl-LDH case, 10 mg of MgAl-LDH was used. 22


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To test our hypothesis, we studied the catalytic behavior of MgAl-LDH/ZIF-8 in Knoevenagel condensation. In view of the economy and green chemistry, in this study, catalytic reactions were conducted using methanol as a solvent at room temperature. The catalytic properties of MgAlLDH/ZIF-8 and a series of control samples were firstly investigated in the reaction between benzaldehyde and methyl cyanoacetate. The corresponding yield-reaction time curves are displayed in Figure 8. The blank experiment shows that indeed the reaction cannot take place without catalysts. In contrast, the reaction can proceed smoothly in the presence of MgAlLDH/ZIF-8, with 76.0% of yield after 1 h, and up to 92.5% of yield after 3 h. Without anchored ZIF-8 NPs, the pristine MgAl-LDH can only exhibit insignificant activity under the same reaction conditions, with 14.3% of conversion after 4 h, confirming that ZIF-8 is the catalytically active component for this reaction. It is noteworthy that the catalytic activity of MgAl-LDH/ZIF8 is comparable or even superior to those of the previously reported MOFs-based catalysts for Knoevenagel condensation reactions (see Table S1 for details). To give a more thorough comparative study, another control sample, conventional crystalline particles of ZIF-8 (ca. 4−8 µm in size) were also synthesized (see the corresponding SEM image, EDX spectrum and XRD pattern in Figure S17) and utilized to catalyze this reaction. As revealed in Figure 8, a much lower activity (53.0% of yield after 4 h) was displayed when using these freestanding ZIF-8 microcrystals. The vastly different results can be mainly attributed to the different particle sizes of ZIF-8 in the two samples and the advantageous dispersing effect of the support. For a particle, the external surface area increases with decreasing particle size (i.e., increase in surface-to-bulk ratio). Compared to the ZIF-8 nanocrystals in the MgAl-LDH/ZIF-8, the ZIF-8 microcrystals own a smaller external surface which leads to less catalytic active sites exposed to reaction species, resulting in a lower activity for the reaction. Furthermore, without 23



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the dispersing and stabilizing functions from the LDH support, the freestanding ZIF-8 microcrystals are much more easily to form bulkier crystal aggregates than the immobilized ones, further decreasing the exposure of the external catalytic active sites, and thereby reducing the catalytic activity. Then more Knoevenagel condensation reactions with a variety of aldehydes were tested to evaluate the generality of MgAl-LDH/ZIF-8. The results are summarized in Table 1. As demonstrated, aromatic aldehydes with electron-withdrawing substituents such as –Cl, –Br and – NO2 can be converted into target products in a high yield within 1 h (entries 1−3, Table 1). As for aromatic aldehydes bearing electron-donating substituents such as –CH3 and –OH, the catalytic composite can also give reasonably good yields (95.2–100%) within 2−4 h (entries 4 and 5, Table 1). Additionally, apart from aromatic aldehydes, the use of this composite catalyst is also feasible for aliphatic aldehydes. For instance, ocatanal can react smoothly with methyl cyanoacetate and give an 88.7% of yield (reaction time = 6 h) in the presence of this nanocatalyst (entry 6, Table 1). All these results reveal that the MgAl-LDH/ZIF-8 is a new type of heterogeneous catalyst with outstanding activity, selectivity and good generality for Knoevenagel condensation reactions. Table 1. Knoevenagel condensation catalyzed by MgAl-LDH/ZIF-8 nanocomposite.[a]

Carbonyl

time

Entry

Yield Product

compound

(h)

(%)

24


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1

1

99.3

2

1

95.0

3

1

≈ 100

4

4

95.2

5

2

≈ 100

6

6

88.7

[a] Reaction conditions: aldehyde (0.5 mmol), methyl cyanoacetate (1 mmol), methanol (3 mL), MgAl-LDH/ZIF-8 (10 mg, 0.77 mol% Zn), room temperature under atmospheric conditions. For practical application, in addition to activity and selectivity, the stability of catalyst is another key consideration for the construction of high-performance catalysts. Here the reusability of MgAl-LDH/ZIF-8 nanocatalyst was examined in the Knoevenagel condensation between benzaldehyde and methyl cyanoacetate. As shown in Figure 9a, the MgAl-LDH/ZIF-8 can be recycled up to 4 times without significant loss of catalytic activity or selectivity. Meanwhile, SEM, TEM images and XRD pattern of the spent catalyst sample reveal that the overall hierarchically structured flowerlike morphology and the structure is still preserved and nanoscale ZIF-8 particles are still dispersed well on the support (Figures 9b, c and S18), further confirming the reasonably good stability and workability of this nanocomposite.

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Figure 9. (a) Recycling test of MgAl-LDH/ZIF-8 catalyst in the reaction between benzaldehyde and methyl cyanoacetate. Reaction conditions: benzaldehyde (0.5 mmol), methyl cyanoacetate (1 mmol), methanol (3 mL), MgAl-LDH/ZIF-8 (10 mg, 0.77 mol% Zn), reaction time: 3 h, room temperature under atmospheric conditions. (b) SEM and (c) TEM images of MgAl-LDH/ZIF-8 after being used repetitively for 4 times. The excellent catalytic performance of MgAl-LDH/ZIF-8 can be ascribed to the unique properties of MgAl-LDH and its supported ZIF-8. On the one hand, MgAl-LDH carrier is not only beneficial for good stabilization and dispersion of ZIF-8, but also favorable for the mass diffusion of reacting species during catalysis owing to its large surface area, mesoporous structure and hierarchical architecture. On the other hand, the ZIF-8 particles are finely stabilized in the nanoscale, facilitating the full exposure of catalytic active sites. Although our present 26


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results represent a significant advancement in applying MOFs as novel catalysts, it should be mentioned that the pristine chemical stability of MOFs in this type of heterogeneous catalysis is going to remain as a long-term challenging topic for future investigation. 3.9. Catalytic performance of MgAl-LDH/Cu nanocomposite. 4-aminophenol (4-AP) is an important industrial intermediate for manufacturing various fine chemicals, such as analgesic drugs, corrosion inhibitors and anticorrosion lubricants.43 Thus it is highly desirable to develop efficient low cost catalysts for this reductive reaction. In this work, the nanostructured copper derived from MgAl-LDH/Cu-BTC, e.g., MgAl-LDH/Cu (Cu loading: 1.90 wt%), was investigated as a model catalyst to evaluate its actual catalytic activity in the chemical transformation of 4-nitrophenol (4-NP) to 4-AP. As presented in Figure 10a, aqueous solution of 4-NP shows a strong UV-vis absorbance band at 317 nm under neutral condition. Upon the addition of NaBH4, the absorption peak red-shifts to 400 nm owing to the formation of 4-nitrophenolate ions under alkaline condition, which is accompanied with a color change from light yellow to bright yellow. The absorption peak appearing at around 300 nm is assigned to the formation of product 4-AP (which is colorless). The blank experiment confirms that the reduction reaction cannot take place without adding any catalysts (Figure 10b). Besides, it is found that in the presence of pristine MgAl-LDH alone, 4nitrophenolate ions cannot be reduced either, even with the strong reducing agent NaBH4 (Figure 10c). By contrast, when a small amount of MgAl-LDH/Cu nanocomposite is added into the reaction mixture, as displayed in Figure 10d, the intensity of the absorption peak at 400 nm gradually decreases with increasing reaction time, while the absorbance intensity of the peak at 300 nm increases. The reaction over the MgAl-LDH/Cu can be completed within just 4.5 min. The total reduction of 4-NP to 4-AP also can be affirmed through the color change of the 27



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reaction solution, which changes from originally bright yellow to colorless (inset of Figure 10d). Moreover, catalytic activities of the air-calcined products, MgAl-LDO/ZnO, MgAl-LDO/Co3O4 and MgAl-LDO/CuO nanocomposites were also evaluated in the reduction of 4-NP. As indicated in Figure S19, the MgAl-LDO/ZnO and MgAl-LDO/Co3O4 nanocomposites show negligible catalytic activity. For MgAl-LDO/CuO, it displays moderate catalytic activity, and the reaction can be completed in 15 min, which is still much inferior than MgAl-LDH/Cu.

Figure 10. (a) UV-Vis absorption spectra of 4-NP, 4-nitrophenolate and 4-AP. UV−Vis absorption spectra of the reduction of 4-NP by NaBH4 in the presence of different catalysts: (b) blank, (c) pristine MgAl-LDH support alone and (d) MgAl-LDH/Cu nanocomposite (photo inset: the color of solution before and after reaction). Plots of (e) Ct/C0 and (f) ln(Ct/C0) versus reaction time (t) for the reduction of 4-NP over MgAl-LDH/Cu nanocomposite.

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As the initial concentration of the NaBH4 is much higher than that of 4-NP ([NaBH4]/[4-NP] = 200) in the reaction mixture, it can be considered constant throughout the whole reduction process. Therefore, the pseudo-first-order rate kinetic with respect to the concentration of 4-NP could be used to evaluate the catalytic reaction rate. The reaction kinetics can be described as −ln(Ct/C0) = kt, where k is the reaction rate constant, t is the reaction time, Ct and C0 are the concentrations of 4-NP at reaction time t and at t = 0, respectively. As expected, a good linear correlation of ln(Ct/C0) versus t is obtained (Figure 10f), the corresponding reaction rate constant k is calculated to be 1.0 min. For a better comparison with other previously reported catalysts, turnover frequency (TOF, which is defined as moles of the reactant 4-NP converted by per mole of active metal in catalyst per hour) is introduced. The TOF value is calculated to be 267.6 h−1 for MgAl-LDH/Cu nanocomposite. Such performance is comparable and even better than those of the noble metal-based catalysts, including Au, Pd, and Ag-based ones (see Table S2 for a detailed comparison).44-50 All of these results demonstrate that our MgAl-LDH/Cu nanocomposite with fine Cu NPs highly dispersed on the surface of LDH nanosheets is an effective catalyst for the reduction of 4-NP. Considering its low-cost and high-performance, the developed MgAl-LDH/Cu may serve as a promising candidate to replace the costly noble metalbased catalysts.

4. Conclusions In summary, a novel, facile and general route has been developed to deposit diverse nanoscale MOFs, including monometallic (e.g., ZIF-8, ZIF-67 and Cu-BTC) as well as bimetallic ones (e.g., CoZn-ZIF), on either complex MgAl-LDH nanoflowers or simpler MgAl-LDH nanoplates. The resultant nanocomposites can be used directly as catalysts or as catalyst precursors for


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further chemical modifications. Owing to the dispersing and directing effects of LDH support, the nano-MOFs can be in-situ generated, and simultaneously, size-controlled with high dispersion on the surface of LDH. Furthermore, the resulting MgAl-LDH/MOFs nanocomposites can serve as a generic platform for construction of a variety of integrated functional nanocomposites (supported ultrafine metal and metal oxide nanoparticles; e.g., MgAlLDO/MxOy, MgAl-LDH/M and MgAl-LDO/M) via thermal transformation. With the tunable product morphology and selectable materials combination, these nanocomposites possess great potential for the applications in heterogeneous catalysis. In our proof-of-concept studies, the MgAl-LDH/ZIF-8 sample has been explored as a model nanocatalyst to perform Knoevenagel condensation reactions. Benefitting from its unique structural features, such as the hierarchical morphology, large surface area and nanosized active phase, the MgAl-LDH/ZIF-8 displays satisfying activity, selectivity and recyclability. Moreover, MgAl-LDH/Cu nanocomposite, in which low-cost Cu NPs were derived from Cu-BTC and supported by LDH, exhibits superior catalytic performance for the reduction reaction of 4-nitrophenol (4-NP) to 4-aminophenol (4AP); its catalytic activity is even unmatched by many costly noble metal-based catalysts. Since LDH/LDO have been widely used as carrier materials in heterogeneous catalysis, this approach provides a new synthetic alternative to fabricate LDH/ LDO-supported nanocatalysts with sophisticated compositional and structural designs through synthetic immobilization of MOFs nanocrystals.

ASSOCIATED CONTENT


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Supporting Information. Supporting Information. Additional SEM images, TEM images, EDX elemental mappings, XRD patterns, N2 adsorption/desorption isotherms of the samples. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, NUS, and GSK Singapore. This project is also funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

REFERENCES (1) Rowsell, J. L. C.; Yaghi, O. M. Metal–Organic Frameworks: a New Class of Porous Materials. Microporous Mesoporous Mater. 2004, 73, 3-14. (2) Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Introduction to Metal–Organic Frameworks. Chem. Rev. 2012, 112, 673-674. (3) McGuire, C. V.; Forgan, R. S. The Surface Chemistry of Metal-Organic Frameworks. Chem. Commun. 2015, 51, 5199-5217.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

(4) Herm, Z. R.; Bloch, E. D.; Long, J. R. Hydrocarbon Separations in Metal–Organic Frameworks. Chem. Mater. 2014, 26, 323-338. (5) He, L.; Liu, Y.; Liu, J.; Xiong, Y.; Zheng, J.; Liu, Y.; Tang, Z. Core–Shell NobleMetal@Metal-Organic-Framework Nanoparticles with Highly Selective Sensing Property. Angew. Chem. Int. Ed. 2013, 52, 3741-3745. (6) Morozan, A.; Jaouen, F. Metal Organic Frameworks for Electrochemical Applications. Energy Environ. Sci. 2012, 5, 9269-9290. (7) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal–Organic Framework. ACS Nano 2015, 9, 6288-6296. (8) Salunkhe, R. R.; Young, C.; Tang, J.; Takei, T.; Ide, Y.; Kobayashi, N.; Yamauchi, Y. A High-Performance Supercapacitor Cell Based on ZIF-8-Derived Nanoporous Carbon Using an Organic Electrolyte. Chem. Commun. 2016, 52, 4764-4767. (9) Tang, J.; Yamauchi, Y. Carbon Materials: MOF Morphologies in Control. Nat Chem 2016, 8, 638-639. (10) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Metal-Organic Framework Materials as Catalysts. Chem. Soc. Rev. 2009, 38, 1450-1459. (11) Dhakshinamoorthy, A.; Opanasenko, M.; Cejka, J.; Garcia, H. Metal Organic Frameworks as Heterogeneous Catalysts for the Production of Fine Chemicals. Catal. Sci. Technol. 2013, 3, 2509-2540. (12) Gascon, J.; Aktay, U.; Hernandez-Alonso, M. D.; van Klink, G. P. M.; Kapteijn, F. AminoBased Metal-Organic Frameworks as Stable, Highly Active Basic Catalysts. J. Catal. 2009, 261, 75-87.


32

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Opanasenko, M.; Dhakshinamoorthy, A.; Hwang, Y. K.; Chang, J.-S.; Garcia, H.; Čejka, J. Superior Performance of Metal–Organic Frameworks over Zeolites as Solid Acid Catalysts in the Prins Reaction: Green Synthesis of Nopol. ChemSusChem 2013, 6, 865-871. (14) Chughtai, A. H.; Ahmad, N.; Younus, H. A.; Laypkov, A.; Verpoort, F. Metal-Organic Frameworks: Versatile Heterogeneous Catalysts for Efficient Catalytic Organic Transformations. Chem. Soc. Rev. 2015, 44, 6804-6849. (15) Sun, J.-K.; Xu, Q. Functional Materials Derived from Open Framework Templates/Precursors: Synthesis and Applications. Energy Environ. Sci. 2014, 7, 2071-2100. (16) Santos, V. P.; Wezendonk, T. A.; Jaén, J. J. D.; Dugulan, A. I.; Nasalevich, M. A.; Islam, H.-U.; Chojecki, A.; Sartipi, S.; Sun, X.; Hakeem, A. A.; Koeken, A. C. J.; Ruitenbeek, M.; Davidian, T.; Meima, G. R.; Sankar, G.; Kapteijn, F.; Makkee, M.; Gascon, J. Metal Organic Framework-Mediated Synthesis of Highly Active and Stable Fischer-Tropsch Catalysts. Nat Commun 2015, 6, 1-8. (17) Zhan, G.; Zeng, H. C. Synthesis and Functionalization of Oriented Metal–OrganicFramework Nanosheets: Toward a Series of 2D Catalysts. Adv. Funct. Mater. 2016, 26, 32683281. (18) Valvekens, P.; Vandichel, M.; Waroquier, M.; Van Speybroeck, V.; De Vos, D. MetalDioxidoterephthalate MOFs of the MOF-74 Type: Microporous Basic Catalysts with WellDefined Active Sites. J. Catal. 2014, 317, 1-10. (19) Wu, C.-M.; Rathi, M.; Ahrenkiel, S. P.; Koodali, R. T.; Wang, Z. Facile Synthesis of MOF5 Confined in SBA-15 Hybrid Material with Enhanced Hydrostability. Chem. Commun. 2013, 49, 1223-1225.


33


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Page 34 of 38

(20) Shang, L.; Yu, H.; Huang, X.; Bian, T.; Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Well-Dispersed ZIF-Derived Co,N-Co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electrocatalysts. Adv. Mater. 2016, 28, 1668-1674. (21) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124-4155. (22) Song, Q.; Liu, W.; Bohn, C. D.; Harper, R. N.; Sivaniah, E.; Scott, S. A.; Dennis, J. S. A High Performance Oxygen Storage Material for Chemical Looping Processes with CO2 Capture. Energy Environ. Sci. 2013, 6, 288-298. (23) Gu, Z.; Atherton, J. J.; Xu, Z. P. Hierarchical Layered Double Hydroxide Nanocomposites: Structure, Synthesis and Applications. Chem. Commun. 2015, 51, 3024-3036. (24) Li, P.; Yu, Y.; Huang, P.-P.; Liu, H.; Cao, C.-Y.; Song, W.-G. Core-Shell Structured MgAlLDO@Al-MS Hexagonal Nanocomposite: an All Inorganic Acid-Base Bifunctional Nanoreactor for One-Pot Cascade Reactions. J. Mater. Chem. A 2014, 2, 339-344. (25) Fan, G.; Li, F.; Evans, D. G.; Duan, X. Catalytic Applications of Layered Double Hydroxides: Recent Advances and Perspectives. Chem. Soc. Rev. 2014, 43, 7040-7066. (26) Li, P.; Huang, P.-P.; Wei, F.-F.; Sun, Y.-B.; Cao, C.-Y.; Song, W.-G. Monodispersed Pd Clusters Generated in situ by Their Own Reductive Support for High Activity and Stability in Cross-Coupling Reactions. J. Mater. Chem. A 2014, 2, 12739-12745. (27) Li, P.; Liu, W.; Dennis, J. S.; Zeng, H. C. Ultrafine Alloy Nanoparticles Converted from 2D Intercalated Coordination Polymers for Catalytic Application. Adv. Funct. Mater. 2016, 56585668.


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(28) Chen, B.; Yang, Z.; Zhu, Y.; Xia, Y. Zeolitic Imidazolate Framework Materials: Recent Progress in Synthesis and Applications. J. Mater. Chem. A 2014, 2, 16811-16831. (29) Pimentel, B. R.; Parulkar, A.; Zhou, E.-k.; Brunelli, N. A.; Lively, R. P. Zeolitic Imidazolate Frameworks: Next-Generation Materials for Energy-Efficient Gas Separations. ChemSusChem 2014, 7, 3202-3240. (30) Zhang, J.-P.; Zhang, Y.-B.; Lin, J.-B.; Chen, X.-M. Metal Azolate Frameworks: From Crystal Engineering to Functional Materials. Chem. Rev. 2012, 112, 1001-1033. (31) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. A Chemically Functionalizable Nanoporous Material [Cu3(TMA)2(H2O)3]n. Science 1999, 283, 1148-1150. (32) Jin, L.-N.; Liu, Q.; Sun, W.-Y. An Introduction to Synthesis and Application of Nanoscale Metal-Carboxylate Coordination Polymers. CrystEngComm 2014, 16, 3816-3828. (33) Masoomi, M. Y.; Morsali, A. Applications of Metal–Organic Coordination Polymers as Precursors for Preparation of Nano-materials. Coord. Chem. Rev. 2012, 256, 2921-2943. (34) Hu, L.; Chen, Q. Hollow/Porous Nanostructures Derived from Nanoscale Metal-Organic Frameworks towards High Performance Anodes for Lithium-Ion Batteries. Nanoscale 2014, 6, 1236-1257. (35) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal–Organic Framework Derived Hybrid Co3O4-Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931. (36) Freeman, F. Properties and Reactions of Ylidenemalononitriles. Chem. Rev. 1980, 80, 329350.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

(37) Li, P.; Liu, H.; Yu, Y.; Cao, C.-Y.; Song, W.-G. One-Pot Multistep Cascade Reactions over Multifunctional Nanocomposites with Pd Nanoparticles Supported on Amine-Modified Mesoporous Silica. Chemistry – An Asian Journal 2013, 8, 2459-2465. (38) Kubota, Y.; Nishizaki, Y.; Ikeya, H.; Saeki, M.; Hida, T.; Kawazu, S.; Yoshida, M.; Fujii, H.; Sugi, Y. Organic–Silicate Hybrid Catalysts Based on Various Defined Structures for Knoevenagel Condensation. Microporous Mesoporous Mater. 2004, 70, 135-149. (39) Li, G.; Xiao, J.; Zhang, W. Efficient and Reusable Amine-Functionalized Polyacrylonitrile Fiber Catalysts for Knoevenagel Condensation In Water. Green Chem. 2012, 14, 2234-2242. (40) Li, P.; Cao, C.-Y.; Liu, H.; Yu, Y.; Song, W.-G. Synthesis of a Core-Shell-Shell Structured Acid-Base Bifunctional Mesoporous Silica Nanoreactor (MS-SO3H@MS@MS-NH2) and Its Application in Tandem Catalysis. J. Mater. Chem. A 2013, 1, 12804-12810. (41) Pang, F.; He, M.; Ge, J. Controlled Synthesis of Fe3O4/ZIF-8 Nanoparticles for Magnetically Separable Nanocatalysts. Chemistry – A European Journal 2015, 21, 6879-6887. (42) Chizallet, C.; Lazare, S.; Bazer-Bachi, D.; Bonnier, F.; Lecocq, V.; Soyer, E.; Quoineaud, A.-A.; Bats, N. Catalysis of Transesterification by a Nonfunctionalized Metal−Organic Framework: Acido-Basicity at the External Surface of ZIF-8 Probed by FTIR and ab Initio Calculations. J. Am. Chem. Soc. 2010, 132, 12365-12377. (43) Chang, G.; Luo, Y.; Lu, W.; Qin, X.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. Ag Nanoparticles Decorated Polyaniline Nanofibers: Synthesis, Characterization, and Applications toward Catalytic Reduction of 4-nitrophenol and Electrochemical Detection of H2O2 and Glucose. Catal. Sci. Technol. 2012, 2, 800-806.


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Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Zhang, P.; Shao, C.; Zhang, Z.; Zhang, M.; Mu, J.; Guo, Z.; Liu, Y. In Situ Assembly of Well-Dispersed Ag Nanoparticles (AgNPs) on Electrospun Carbon Nanofibers (CNFs) for Catalytic Reduction of 4-nitrophenol. Nanoscale 2011, 3, 3357-3363. (45) Li, H.; Han, L.; Cooper-White, J.; Kim, I. Palladium Nanoparticles Decorated Carbon Nanotubes: Facile Synthesis and Their Applications as Highly Efficient Catalysts for the Reduction of 4-nitrophenol. Green Chem. 2012, 14, 586-591. (46) Zhou, Y.; Zeng, H. C. Simultaneous Synthesis and Assembly of Noble Metal Nanoclusters with Variable Micellar Templates. J. Am. Chem. Soc. 2014, 136, 13805-13817. (47) Chen, Z.; Cui, Z.-M.; Cao, C.-Y.; He, W.-D.; Jiang, L.; Song, W.-G. TemperatureResponsive Smart Nanoreactors: Poly(N-isopropylacrylamide)-Coated Au@Mesoporous-SiO2 Hollow Nanospheres. Langmuir 2012, 28, 13452-13458. (48) Saha, S.; Pal, A.; Kundu, S.; Basu, S.; Pal, T. Photochemical Green Synthesis of CalciumAlginate-Stabilized Ag and Au Nanoparticles and Their Catalytic Application to 4-Nitrophenol Reduction. Langmuir 2010, 26, 2885-2893. (49) Deng, Y.; Cai, Y.; Sun, Z.; Liu, J.; Liu, C.; Wei, J.; Li, W.; Liu, C.; Wang, Y.; Zhao, D. Multifunctional Mesoporous Composite Microspheres with Well-Designed Nanostructure: A Highly Integrated Catalyst System. J. Am. Chem. Soc. 2010, 132, 8466-8473. (50) Zhang, Z.; Xiao, F.; Xi, J.; Sun, T.; Xiao, S.; Wang, H.; Wang, S.; Liu, Y. Encapsulating Pd Nanoparticles in Double-Shelled Graphene@Carbon Hollow Spheres for Excellent Chemical Catalytic Property. Scientific Reports 2014, 4, 1-5.


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