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ISSN 1998-0124 CN 11-5974/O4 2019, 12(2): 437–440 https://doi.org/10.1007/s12274-018-2235-1 Research Article Facile synthesis of ultrathin metal-organic framework nanosheets for Lewis acid catalysis Xiaofei Zhang 1,2,§ , Lin Chang 2,§ , Zhongjie Yang 2 , Yanan Shi 2 , Chang Long 1,2 , Jianyu Han 2 , Binhao Zhang 2 , Xueying Qiu 2 , Guodong Li 2 ( ), and Zhiyong Tang 1,2 ( ) 1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China 2 CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China § Xiaofei Zhang and Lin Chang contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Received: 12 October 2018 / Revised: 23 October 2018 / Accepted: 27 October 2018 ABSTRACT Ultrathin metal-organic framework (MOF) nanosheets are attracting great interest in catalysis due to their unique and intriguing two-dimensional (2D) features. Although many progresses have been achieved, it is still highly desirable to develop novel strategies for controllable synthesis of the well-defined ultrathin MOF nanosheets. Herein we report a polyvinylpyrrolidone (PVP)-assisted route to synthesize the ultrathin Ni-MOF nanosheets characteristic of 1.5 nm in thickness, in which PVP is reacted with 2-aminoterephthalic acid (H 2 BDC-NH 2 ) via formation of C=N bond, followed by coordination with Ni 2+ ions to form the ultrathin MOF nanosheets. Impressively, when used in the Knoevenagel condensation reactions of propane dinitrile with different aldehydes, ultrathin Ni-MOF nanosheets display the significantly enhanced catalytic activity and good stability in respect with the bulk Ni-MOF, mainly owing to the exposed active sites as well as facile mass transfer and diffusion of substrates and products. KEYWORDS ultrathin nanosheet, metal-organic framework, Lewis acid site, Knoevenagel condensation Metal-organic frameworks (MOFs), which are synthesized by self- assembly of metal ions or clusters with polytopic organic linkers, have emerged as novel porous materials and shown outstanding advantages of periodically distributed metal centers, ordered porosities and tunable functional groups [1–7]. Generally, the metal nodes in MOF structures could be controlled to become coordinatively unsaturated [8, 9], which are used as Lewis acid catalysts for condensation reactions, oxidation reactions, and so on. Furthermore, the high surface area and ordered porosity of MOF benefit the mass transfer of reactants from the exterior to the interior of MOF crystallites as well as the desorption of products from the catalytic sites in liquid phases compared with conventional porous materials like carbons and zeolites. These characteristics make them as good candidates for catalysis. To further improve their catalytic efficiency, an effective route is to synthesize ultrathin MOF nanosheets characteristic of well-defined nanostructure, which would generate more exposed active sites and greatly facilitate mass transfer and diffusion of substrates and products [10–13]. It is known that there are two typical strategies for synthesis of ultrathin nanosheets, i.e. top-down or bottom-up techniques [14, 15]. Until now several types of ultrathin MOF nanosheets have been developed via balling- or sonication-based exfoliation of the bulk materials with layered structure [16–18], or assembly on the flat metal surfaces or grown at the air/liquid or liquid/liquid interfaces [19–21]. Also, surfactant or microemulsion assistant protocols have been applied for synthesis of ultrathin MOF nanosheets with high yield and good dispersibility [22–24]. Although many preparation methods have been reported by far, it is still imperative to develop novel bottom-up strategies for synthesis of ultrathin MOF nanosheets for heterogeneous catalysis. In this work, we report a polyvinylpyrrolidone (PVP)-assisted strategy to fabricate the ultrathin Ni-MOF nanosheets of 1.5 nm in thickness. The PVP molecules firstly bind with 2-aminoterephthalic acid (H2BDC-NH2) ligands via formation of C=N bond, and the subsequent coordination of Ni 2+ ions with carboxylic acid groups in the modified H2BDC-NH2 results in formation of the ultrathin MOF nanosheets under solvothermal conditions (Scheme 1). As comparison, if PVP is mixed with terephthalic acid (H2BDC) that is absent of amine group, only bulk crystal materials are obtained. When used in catalyzing Knoevenagel condensation reactions, ultrathin Ni-BDC MOF nanosheets display the much improved activity and good stability. In brief, synthesis of ultrathin Ni-BDC MOF nanosheets involves reaction of Ni(NO3)2·6H2O, H2BDC-NH2 and PVP in the mixed solution of N,N-dimethylformamide (DMF) and ethanol via one-pot solvothermal method. The morphology and structure of as-prepared Scheme 1 Scheme of synthesis of ultrathin Ni-BDC MOF nanosheets. Address correspondence to Zhiyong Tang, [email protected]; Guodong Li, [email protected]

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Page 1: Facile synthesis of ultrathin metal-organic …...Facile synthesis of ultrathin metal-organic framework nanosheets for Lewis acid catalysis Xiaofei Zhang 1,2,§ , Lin Chang 2,§ ,

ISSN 1998-0124 CN 11-5974/O4

2019, 12(2): 437–440 https://doi.org/10.1007/s12274-018-2235-1

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Facile synthesis of ultrathin metal-organic framework nanosheets for Lewis acid catalysis Xiaofei Zhang1,2,§, Lin Chang2,§, Zhongjie Yang2, Yanan Shi2, Chang Long1,2, Jianyu Han2, Binhao Zhang2, Xueying Qiu2, Guodong Li2 (), and Zhiyong Tang1,2 ()

1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China 2 CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and

Technology, Beijing 100190, China § Xiaofei Zhang and Lin Chang contributed equally to this work. © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018 Received: 12 October 2018 / Revised: 23 October 2018 / Accepted: 27 October 2018

ABSTRACT Ultrathin metal-organic framework (MOF) nanosheets are attracting great interest in catalysis due to their unique and intriguing two-dimensional (2D) features. Although many progresses have been achieved, it is still highly desirable to develop novel strategies for controllable synthesis of the well-defined ultrathin MOF nanosheets. Herein we report a polyvinylpyrrolidone (PVP)-assisted route to synthesize the ultrathin Ni-MOF nanosheets characteristic of 1.5 nm in thickness, in which PVP is reacted with 2-aminoterephthalic acid (H2BDC-NH2) via formation of C=N bond, followed by coordination with Ni2+ ions to form the ultrathin MOF nanosheets. Impressively, when used in the Knoevenagel condensation reactions of propane dinitrile with different aldehydes, ultrathin Ni-MOF nanosheets display the significantly enhanced catalytic activity and good stability in respect with the bulk Ni-MOF, mainly owing to the exposed active sites as well as facile mass transfer and diffusion of substrates and products.

KEYWORDS ultrathin nanosheet, metal-organic framework, Lewis acid site, Knoevenagel condensation

Metal-organic frameworks (MOFs), which are synthesized by self- assembly of metal ions or clusters with polytopic organic linkers, have emerged as novel porous materials and shown outstanding advantages of periodically distributed metal centers, ordered porosities and tunable functional groups [1–7]. Generally, the metal nodes in MOF structures could be controlled to become coordinatively unsaturated [8, 9], which are used as Lewis acid catalysts for condensation reactions, oxidation reactions, and so on. Furthermore, the high surface area and ordered porosity of MOF benefit the mass transfer of reactants from the exterior to the interior of MOF crystallites as well as the desorption of products from the catalytic sites in liquid phases compared with conventional porous materials like carbons and zeolites. These characteristics make them as good candidates for catalysis. To further improve their catalytic efficiency, an effective route is to synthesize ultrathin MOF nanosheets characteristic of well-defined nanostructure, which would generate more exposed active sites and greatly facilitate mass transfer and diffusion of substrates and products [10–13]. It is known that there are two typical strategies for synthesis of ultrathin nanosheets, i.e. top-down or bottom-up techniques [14, 15]. Until now several types of ultrathin MOF nanosheets have been developed via balling- or sonication-based exfoliation of the bulk materials with layered structure [16–18], or assembly on the flat metal surfaces or grown at the air/liquid or liquid/liquid interfaces [19–21]. Also, surfactant or microemulsion assistant protocols have been applied for synthesis of ultrathin MOF nanosheets with high yield and good dispersibility [22–24]. Although many preparation methods have been reported by far, it is still imperative to develop

novel bottom-up strategies for synthesis of ultrathin MOF nanosheets for heterogeneous catalysis.

In this work, we report a polyvinylpyrrolidone (PVP)-assisted strategy to fabricate the ultrathin Ni-MOF nanosheets of 1.5 nm in thickness. The PVP molecules firstly bind with 2-aminoterephthalic acid (H2BDC-NH2) ligands via formation of C=N bond, and the subsequent coordination of Ni2+ ions with carboxylic acid groups in the modified H2BDC-NH2 results in formation of the ultrathin MOF nanosheets under solvothermal conditions (Scheme 1). As comparison, if PVP is mixed with terephthalic acid (H2BDC) that is absent of amine group, only bulk crystal materials are obtained. When used in catalyzing Knoevenagel condensation reactions, ultrathin Ni-BDC MOF nanosheets display the much improved activity and good stability.

In brief, synthesis of ultrathin Ni-BDC MOF nanosheets involves reaction of Ni(NO3)2·6H2O, H2BDC-NH2 and PVP in the mixed solution of N,N-dimethylformamide (DMF) and ethanol via one-pot solvothermal method. The morphology and structure of as-prepared

 Scheme 1 Scheme of synthesis of ultrathin Ni-BDC MOF nanosheets.

Address correspondence to Zhiyong Tang, [email protected]; Guodong Li, [email protected]

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ultrathin Ni-BDC MOF nanosheets are investigated by different characterization techniques, and a number of important characteristics are recognized. Firstly, the ultrathin MOF nanosheets with very uniform thickness of 1.5 nm are clearly discerned (Figs. 1(a), 1(b), 1(d) and Fig. S1 in the Electronic Supplementary Material (ESM)). The ultrathin nature is also confirmed by the fact that the obtained nanosheets are prone to wrinkles on the substrates. As comparison, when H2BDC-NH2 is replaced by H2BDC, only bulk Ni-BDC MOF is obtained (Fig. 1(e) and Fig. S2 in the ESM), in which the structural motifs are the 4-fold coordinated dimetal-centered paddle-wheel secondary building units (SBUs), namely Ni2(BDC)2 [25, 26]. Therefore, it is easily deduced that the grafted PVP effectively restricts the overgrowth of MOF crystals in the vertical direction. Secondly, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) indicates that all the elements including Ni, C, O and N are homogeneously dispersed in the ultrathin Ni-BDC MOF nanosheets (Fig. 1(c)). Thirdly, powder X-ray diffraction (XRD) measurement shows only one typical characteristic peak at 2θ = 9.7°, corresponding to the (100) facet of ultrathin Ni-BDC MOF nanosheets (Fig. 1(e)). Fourthly, Fourier transform infrared (FTIR) spectroscopy indicates that the symmetric and asymmetric stretching vibrations of NH2 group at 3,495 and 3,390 cm−1 are observed in the molecule H2BDC-NH2. After binding with PVP, the characteristic peak of C=N stretching vibration peak at 1,612 cm−1 appears due to formation of imine bond, while the characteristic absorption band of NH2 group disappears (Fig. 2(a)). Lastly, N2 adsorption–desorption isotherms reveal that the specific surface area of the ultrathin Ni-BDC MOF nanosheets is 116 m2·g−1; whereas as for bulk Ni-BDC MOF, the specific surface area is merely 29 m2·g−1 (Fig. 2(b) and Fig. S3 in the ESM).

The ultrathin Ni-BDC MOF nanosheets as Lewis acid catalysts have been examined in a series of Knoevenagel condensation reactions

 Figure 1 Characterization of Ni-BDC MOF nanosheets. (a) Scanning electron microscope (SEM) and (b) transmission electron microscopy (TEM) images of ultrathin Ni-BDC MOF nanosheets. (c) Elemental mapping images of ultrathin Ni-BDC MOF nanosheets. (d) Atomic force microscopy (AFM) of ultrathin Ni-BDC MOF nanosheets. (e) Powder XRD patterns of bulk Ni-BDC MOF, Ni-BDC MOF nanosheets and simulated Ni-BDC MOF.

 Figure 2 FTIR spectra and N2 sorption isotherms. (a) FTIR spectra of H2BDC- NH2, PVP and Ni-BDC MOF nanosheets. (b) N2 sorption isotherms of ultrathin Ni-BDC MOF nanosheets and bulk Ni-BDC MOF.

Table 1 Catalytic Knoevenagel condensation reactions of propane dinitrile with different aldehydes by ultrathin Ni-BDC MOF nanosheets and bulk Ni-BDC MOFa

Entry Substrates Catalyst T (oC) t (h) Conv. (%)

1 a. Citronellal No 25 1 26.8

2 a. Citronellal No 25 2 34.5

3 a. Citronellal Bulk Ni-BDC 25 1 55.5

4 a. Citronellal Bulk Ni-BDC 25 2 73.8

5 a. Citronellal Ni-BDC nanosheets 25 1 82.7

6 a. Citronellal Ni-BDC nanosheets 25 2 98.2

7 b. p-Nitrobenzaldehyde Ni-BDC nanosheets 25 1 99.0

8 c. Butaldehyde Ni-BDC nanosheets 25 1 92.5

9 d. Benzaldehyde Ni-BDC nanosheets 25 1 99.0 aReaction condition: 1 mL DMF, 6 mg catalyst, citronellal (40 μL, 0.22 mmol), p-nitrobenzaldehyde (30 mg, 0.20 mmol), butaldehyde (40 μL, 0.44 mmol), benzaldehyde (40 μL, 0.39 mmol). Yield of the products was determined by gas chromatography.

along with the bulk Ni-BDC MOF [27–31]. Table 1 summarizes the results for catalytic Knoevenagel condensation of propane dinitrile with different aldehydes. It is noted that the condensation reaction of propane dinitrile and citronellal happens without the catalysts; however, the conversion ratio of citronellal is very low to be 34.5% at room temperature (25 °C) for 2 h (Table 1, entries 1 and 2). Bulk Ni-BDC MOF exhibits the improved catalytic performance and corresponding conversion ratio increases to 73.8% (Table 1, entries 3 and 4). As contrast, the ultrathin Ni-BDC MOF nanosheets display the remarkable activity with nearly full conversion of citronellal

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(98.2%) under the same reaction condition (Table 1, entries 5 and 6, Fig. S4 in the ESM). This comparison highlights the unique advantages of ultrathin Ni-BDC MOF nanosheets as the catalysts, for instance, the highly exposure of active Ni sites on the surface and the facile mass transfer and diffusion of substrates and products. In addition, the stability is an important index to evaluate the catalyst’s performance. It can be seen that after five successive catalytic cycles, the conversion rate is kept at 97.7% (Fig. 3(a)), indicating that the catalytic activity of ultrathin Ni-BDC MOF nanosheets remains almost unchanged. To further evidence its high activity and good stability, the amount of ultrathin Ni-BDC MOF nanosheets is reduced from 6 to 3 mg. When used as catalyst, the conversion rate of 71.1% is obtained at same reaction condition, and moreover, the catalytic activity is also well kept during the successive tests, and corresponding fifth conversion rate is 70.4% (Fig. S5 in the ESM). The above stability tests display the high catalytic efficiency of ultrathin Ni-BDC MOF nanosheets. Furthermore, there are no evident structural and morphological destruction of the used catalysts (Figs. 3(b)–3(d)), which confirms the stability of prepared ultrathin MOF nanosheets. To illustrate the high activity of Ni sites in the ultrathin nanosheets, p-nitrobenzaldehyde, butaldehyde and benzaldehyde are also used as the substrates for the Knoevenagel condensation reactions with propane dinitrile (Table 1, entries 7–9, Figs. S6–S8 in the ESM). The conversion ratio is 99.0% for p-nitrobenzaldehyde, 99.0% for benzaldehyde and 92.5% for butaldehyde at room temperature for 1 h. Evidently, the ultrathin Ni-BDC MOF nanosheets exhibit the distinct advantages in catalytic Knoevenagel condensation reactions with various aldehydes.

In summary, we have developed a convenient strategy to synthesize ultrathin Ni-BDC MOF nanosheets. The PVP polymer is bound with the organic linker H2BDC-NH2, which greatly restricts the overgrowth of MOF crystals in the vertical direction. Notably, the ultrathin Ni-BDC MOF nanosheets as Lewis acid catalysts exhibit the high activity and good stability for Knoevenagel condensation reactions of propane dinitrile with different aldehydes under the mild conditions. This work will open up a new avenue towards design and construction of ultrathin MOF nanosheets as high-performance heterogeneous catalysts.

 Figure 3 Stability test of Ni-BDC MOF nanosheets. (a) Catalytic Knoevenagel condensation reaction of propane dinitrile with citronellal by ultrathin Ni-BDC MOF nanosheets for five successive runs. (b) Powder XRD patterns of fresh and used Ni-BDC MOF nanosheet. ((c) and (d)) SEM and TEM images of ultrathin Ni-BDC MOF nanosheets after catalytic reactions.

Acknowledgements This work was supported financially by the National Key Basic Research Program of China (Nos. 2014CB931801 and 2016YFA0200700, Z. Y. T.), National Natural Science Foundation of China (Nos. 21890381, 21721002 and 21475029, Z. Y. T.; 21722102, 51672053 and 21303029, G. D. L.), Beijing Natural Science Foundation (No. 2182087, G. D. L.), Frontier Science Key Project of Chinese Academy of Sciences (No. QYZDJ-SSW-SLH038, Z. Y. T.), K. C. Wong Education Foundation (Z. Y. T.), and Youth Innovation Promotion Association CAS (No. 2016036, G. D. L.).

Electronic Supplementary Material: Supplementary material (experimental details and supplementary figures) is available in the online version of this article at https://doi.org/10.1007/ s12274-018-2235-1.

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