Microwave-assisted, Ni-induced Fabrication of Hollow ZIF-8

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Microwave-assisted, Ni-induced Fabrication of Hollow ZIF-8 Nanoframes for Knoevenagel Reaction Pengfang Zhang, Yun Xiao, Hui Sun, Xiaoping Dai, Xin Zhang, Haixia Su, Yuchen Qin, Daowei Gao, Axiang Jin, Hai Wang, Xiubing Wang, and Shi-Gang Sun Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00045 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Crystal Growth & Design

Microwave-assisted, Ni-induced Fabrication of Hollow ZIF-8 Nanoframes for Knoevenagel Reaction Pengfang Zhang,a† Yun Xiao,a† Hui Suna, Xiaoping Dai,a Xin Zhang,*a Haixia Su,a Yuchen Qin,a Daowei Gao,a Axiang Jin,b Hai Wang,b Xiubing Wang,a and Shigang Sunc a

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum Beijing 102249 (China) b

National Institute of Metrology, Beijing 100013, China

c

College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

†

These authors contributed equally to this work and should be considered co-first authors.

ABSTRACT: Morphology-controlled synthesis of metal organic frameworks (MOFs) is of tremendous importance to understand the morphology-structure-property relationships. Although great emphasis has been placed on the controllable synthesis of MOFs, the preparation of hollow and nanoframe-like MOFs, especially using microwave irradiation method, remains a great challenge. In the present work, we report microwave-assisted synthesis of hollow butterfly and ring ZIF-8 with the stimuli of Ni ions. Interestingly, the sphere, butterfly and fiber ZIF-8 catalysts were also fabricated when adjusting the weight ratio of Zn and Ni resources. Specifically, sphere, butterfly, hollow butterfly and ring particles have maintained Zeolitic imidazolate framework - 8 (ZIF-8) structure. Based on the time sequential experiments we inferred that the formation of hollow butterfly and ring particles is driven by the change of surface energy. Knoevenagel reaction was carried out to explore the catalytic activity of these ZIF materials. The ring particles with ZIF-8 structure showed the highest catalytic activity. The open structure, thin walls and the amount of basicity have been extensively discussed to shed light on the superior catalytic performance of ring particles. characterized crystalline architectures, controllable pore size, and suitable chemical functionalities, the MOFs have been widely applied in gas separation,6,7 sensors,8 catalysis,9 drug delivery,10 and energy storage.11 In recent years, the synthesis of nanoframe-like MOFs attracted extensive interest because of unique

INTRODUCTION MOFs, assembled by metal ions and organic ligands through coordination bonds, are a class of highly crystalline hybrid porous materials.1−5 Owing to their superior properties such as exceptionally high specific surface area, well1

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Crystal Growth & Design 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

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microwave irradiation method assisted by two kinds of metal ions to synthesize different morphologies of MOFs materials including hollow and nanoframe-like structures highly efficient for Knoevenagel reaction. In the present work, we report microwave irradiation method to produce a series of well-defined morphologies including sphere, butterfly, hollow butterfly, ring and fiber using 2-methylimidazole (2-MeIM) as organic ligands, zinc and nickel ions as mixed metallic nodes, poly (vinylpyrrolidone) (PVP) as dispersive and protecting reagent and N,Ndimethylformamide (DMF) as solvent. Interestingly, the morphological evolution process can be facilely controlled by just changing weight ratio of Zn and Ni salts. The structure of ZIF-8 could be retained when zinc ions are used as metallic nodes. However, the asprepared fibers exhibits non-MOF structure when nickel ion was used as single metal resource. The Knoevenagel condensation is an excellent tool for the formation of C–C structure unit from an active methylene compound and a carbonyl.41,42 This reaction could be used to the synthesis of some drug intermediates such as anticancer, antimicrobial and antituberculosis agents. The catalyst containing basic group is the most used catalyat for this reaction. To explore the catalytic applications of as-synthesized ZIF-8 nanostructures, Knoevenagel reaction was carried out. The ring particles exhibit the highest catalytic activity among these five samples. The open structure, thin walls and the amount of basicity play crucial effects on the different catalytic activities.

structures, intriguing properties that differ from their solid counterparts. Surfactants,12 metal salts,13 lingands,14 solvents15,16 and even the synthesized methods, such as solvothermal method,17−19 microwave irradiation,20 microemulsion21 and electrochemical deposition methods22, have been tried to control the size and morphology of hollow MOF materials. It has been well established that mixed solvents, postsynthesis modification, sacrificing templates are the most used methods to prepare hollow MOF materials.23−26 For example, Wang and coworkers synthesized hollow MOF-5 and MOF-2 by adding two kinds of solvents including N,Ndimethylformamide and ethanol.23,24 Lou et al prepared nickel sulfide nanoframes using structure-induced anisotropic chemical 25 etching/anion exchange methods. Li and coworkers produced hollow ZIF-8 nanospheres using silica as the sacrificing template.26 While the reaction time of these methods was at least six hours or even more. The microwave irradiation method is on behalf of a very energy efficient method of heating with rapid heating rate, shorter reaction time and higher selectivity and yield in contrast with conventional solvothermal methods.27 The synthesis of MOFs assisted by microwave radiation technology has been extensively used and emphasized on four aspects: (I) the acceleration of crystallization, (II) the preparation of nano-ranged particles, (III) the improvement of product purity and (IV) the selective synthesis of polymorphs.28 ZIF is a subclass of MOFs. ZIF8 is one of the most representative ZIF materials due to its robust chemical and thermal stabilities.29,30 Although many efforts have been made on the morphology-controlled synthesis of MOFs31-35, to our knowledge, it is barely reported

EXPERIMENTAL SECTION

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detector (SS420x), ultraviolet spectrophotometer, and C18 chromatographic column (silica, 250 mm X 4.6 mm). The methanol (MeOH) and acetonitrile were chosen as mixed solvents with volume ratio of 6:4. The flow rate of mixed solvent is 1 mL·min−1. The differential refractive index detector was used to detect the different materials. The retention time of malononitrile, benzaldehyde, final product, and methylbenzene are 3.2, 6.3, 8.9, 19.2 min, respectively. The extent of conversion was calculated based on the ratio of benzaldehyde and final product. Characterization. The size and morphology of the particles were analyzed by JEM 2100 transmission electron microscope (TEM) at 200 kV. The samples were conducted by dropping ethanol dispersion onto carbon-coated copper grids with a pipettor, and the solvent was allowed to evaporate for TEM detection. The scanning electron microscope (SEM) images were determined by ULTRA 55 scanning electron microscope at 200 K. The X-ray diffraction (XRD) patterns of samples were recorded on a Bruker D8-advance X-ray diffractometer operated at a voltage of 40 kV and current of 40 mA with CuK radiation (λ = 1.5406 Å). The elemental analysis was done using Rigaku Nanohunter total-reflection X-ray fluorescence spectroscopy (TXRF) (Excitation line: Mo-Ka 50 kV, 0.60 mA; Peak: net; Angle (deg): 0.050; Time (sec): 300; Dead rate (%): 0.2). A certain amount of samples was first dissolved in aqua regia and then was dropwise added on the silica wafer to dry for TXRF detection. Thermogravimetric analyses (TGA) were measured on a Mettler Toledo TGA/SDTA 851 instrument in flowing air (50 mL min−1) with a rate of 10 K min−1. The Fourier-transformed infrared resonance (FTIR) spectra were obtained on a TENSORII

Materials and Reagents. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) was purchased from Acros. Malononitrile was purchased from aladdin. All other chemicals such as Nickel (II) acetylacetonate (Ni(acac)2) 2MeIM (99%) PVP (M.W. 30000), benzaldehyde toluene and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Solvents and all other chemicals were obtained from commercial sources and used without further purification. Synthesis of different morphologies of particles from ZIF-8 sphere to non-MOF fiber. A precursor solution was prepared by mixing Zn(NO3)2·6H2O (300, 200, 175, 125 or 0 mg), Ni(acac)2 (0, 100, 125, 175 or 300 mg), 2-MeIM (41 mg, 0.50 mmol) and 1092 mg PVP in 8 mL of DMF. The mixed solution was stirred for 20 minutes and then transited to 100 mL Teflonlined autoclave and microwave irradiated (purchased from Michem Beijing Co. Ltd) for 5 min before it was cooled to room temperature naturally. The products were separated via centrifugation at 7000 rpm for 10 minutes and further purified by ethanol three times. The yield of the five products (Zn(NO3)2·6H2O=300, 200, 175, 125 or 0 mg) are 60%, 63.6%, 65.2%,65% and 54% respectively. The test of catalytic activity of Knoevenagel reaction. To a 30 mL glassy bottle was sequentially added benzaldehyde (63 mg, 0.6 mmol), malononitrile (198 mg, 3 mmol), toluene (5 mL) and the as-prepared catalysts (10 mg). The solution was stirred at room temperature (25 o C) for 2 hours. The reaction mixture was injected to high performance liquid chromatography. High performance liquid chromatography is equipped with a differential refractive index 3



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spectrometer. The scope of wavelength is 400−4000 cm−1. X-ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 Versaprobe system using monochromatic Al Ka radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 285.0 eV. RESULTS AND DISCUSSION Fig. 1a shows a representative scanning electron microscopy (SEM) image of as-prepared hollow butterfly particles when the weight ratio of Zn(NO3)2·6H2O and Ni(acac)2 is 175:125. The high-resolution (HR) SEM images clearly

Figure 2. (a) SEM, (b) HRSEM corresponding model (inset), (c) TEM and distribution (inset), (d) HRTEM images of particles, (e) EDS-mapping images of the particles. Scale bars indicate 100 nm.

and size ring ring

demonstrate the hollow structure, consisting of many small sheet-like materials. The products present shape selective above 95% with an average size of 551 nm between two bottom surfaces (Fig. 1c and d). Fig. 1e shows elemental mapping images of hollow butterfly revealing the uniform dispersion of C, N, Zn and Ni in the particles. The weak signals of Ni in the EDSmapping indicate the much lower surface content. By decreasing the weight ratio of Zn(NO3)2·6H2O and Ni(acac)2 to 125:175 while keeping other synthetic conditions unchanged, the morphology changed from hollow butterfly to ring. The representative SEM, HRSEM, TEM and HRTEM images demonstrate the high-yield formation of the ring particles with an average

Figure 1. (a) SEM, (b) HRSEM and corresponding model (inset), (c) TEM and size distribution (inset), (d) HRTEM images of hollow butterfly particles (e) EDS-mapping images of the hollow butterfly particles. Scale bars indicate 200 nm.


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hollow butterfly and ring particles confirming the formation of ZIF-8 structure (Fig. 4a). The characteristic peak appeared at about 7.3° in the spheres, butterfly, hollow butterflies and rings is gradually enhanced, which may be ascribed to the different preferred orientation.4 The fibers show non-MOF structure which is consistent

outside diameter of about 275 nm (Fig. 2a−d). The elemental analysis of ring particles shown in Fig. 2e confirms the uniform distribution of C, N, Zn and Ni. The atomic ratio of Zn and Ni determined by EDS-mapping is 96.0:4.0. Interestingly, when further varying the weight ratio of Zn(NO3)2·6H2O and Ni(acac)2, the catalyst exhibits an significant morphology change as shown in Fig. 3. The butterfly products were observed when when the weight ratio of Zn(NO3)2·6H2O and Ni(acac)2 is 200:100 in a standard procedure (Fig. 3c and d). The average length between two bottom surfaces is about 510 nm. When use one of Zn(NO3)2·6H2O and Ni(acac)2 as single metal resource, other two morphologies were obtained. Rough spherical particles with an average size of 430 nm were obtained when 300 mg Zn(NO3)2·6H2O was used as single metal resource during the reaction (Fig. 3a and b), While well-defined fibers with an aspect ratio of 300 were obtained when 300 mg Ni(acac)2 was used as the only metal resource during the synthesis (Fig. 3e and 3f). On the basis of the above experiments, it is found that the morphologies have significantly changed from sphere to butterfly, hollow butterfly, ring and fiber when decreasing the weight ratio of Zn and Ni salts. The size of these five samples increases at first and then decreases with increasing the amount of Ni(acac)2. It can be indicated that the ratio of Zn to Ni ions play a very important role on the morphological evolution during the reaction. The structures of these different morphologies of particles were determined by power XRD. It has been well documented that the peaks appeared at 7.3o, 10.4o 12.7o, 15.1o and 16.8o are the characteristic peaks of ZIF-8.15,12 These five typical peaks have been found in sphere, butterfly,

Figure 3. SEM and TEM images of (a, b) sphere, (c, d) butterfly, (e, f) fiber constructed from mlM and various weight ratios of Zn(NO3)2·6H2O and Ni(acac)2. with the previous published paper that it is hard to form MOF structure when using Ni2+ as metal nodes and 2-MeIM as single ligands.16 It should be noticed that there is a very sharp peak at 11.1o which should be attributed to the welldefined fiber morphology. 5



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0.03) manifesting the inherent microporosity. The fibers display type IV isotherm, characteristic of mesoporous capillary condensation. The BET (Langmuir) specific surface areas of the sphere, butterfly, hollow butterfly, ring and fiber particles were 375 (492), 387 (502), 398 (523), 426 (563) and 67 (87) m2/g, respectively. The lower specific surface area of fibers indicates the non-MOF structure. The pore distributions of these three

To further gain insight into the actual molar ratio of Zn and Ni of different samples, TXRF and XPS analyses were performed. The bulk molar ratios of Ni and Zn were 1.0: 99.0, 3.7:96.3, and 9.2: 90.8 tested by TXRF on the butterfly, hollow butterfly and ring particles, respectively (Table S1). It can be seen that the ring particles show the largest value of nNi:nZn is 9.2:90.8 among the butterfly, hollow butterfly and ring particles. While the molar ratio of Ni and Zn of ring particles determined by XPS analysis was 4.1:95.9 (Fig. 4b). The surface molar ratios of Zn and Ni of hollow butterfly and ring particles determined by EDS-mapping are 99.5:0.5 and 96.0:4.0, respectively (Fig. 1e and 2e). The surface content of Zn and Ni of ring particles determined by XPS is consistent with that measured by EDS. Based on the above results, we can know that the content of Zn is always much larger than the content of Ni, indicating the preferential coordination interaction of the nitrogen-containing group to Zn2+ over Ni2+. The variation of Ni content may reveal that Ni2+ ions have lower stability than Zn2+ ions when coordinating with 2-MeIM. XPS analysis also reveals the existence of C (282.4 eV), N (398.2 eV) and O (532.7 eV) (Fig. 4b). The valence states of Zn and Ni of the as-prepared ring particles were measured by XPS analysis. The peaks at 1021.4 and 1044.4 eV correspond to the Zn oxidation state and the peaks appeared at 873.9 and 855.9 eV are assigned to the Ni 2p1/2 and Ni 2p3/2 indicating the existence of Ni(II) ions (Fig. 4c and 4d).24,37,38. Fig. 4e shows the N2 adsorption-desorption isotherms of sphere, butterfly, hollow butterfly, ring and fiber particles. The sphere, butterfly, hollow butterfly and ring particles display type I isotherms. There is a sharp increase in the low relative pressure (P/P0 =

a)

b)

c)

d)

e)

f)

Figure 4. (a) XRD patterns of different morphologies of particles. XPS spectra of ring particles: (b) The survey spectrum, (c) XPS of Zn2p spectrum, (d) XPS of Ni 2p spectrum. (e) Nitrogen adsorption isotherm and desorption isotherm of spheres, butterflies, hollow butterflies, rings and fibers. (f) FT–IR spectra of five different particles, ZIF-8 and 2-MeIM. 6


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TGA of the as-synthesized sphere, ring and fiber samples was performed under air flowing (Fig. S2 in supporting information). The ring and sphere products, demonstrates negligible weight loss until 400 oC which can be attributed to the release of physically adsorbed solvent molecules, then, followed by a sharp weight loss in the range of 400-535 oC indicating the oxidation process of metal-organic framework. The weight of these samples exhibit only slight decrease (≤ 0.63% weight loss) after the temperature is above 535 o C. In contrast, the significant weight loss of fiber particles was observed when the temperature is over 280 oC. It is evident that the thermal stability of sphere and ring particles is much higher than that of fibers, which should be ascribed to the higher thermal stability of ZIF-8 structure.29,30 The morphologies of material relied on the nucleation and growth rates according to previous reports.9,12,18 Hence, we conducted a series experiments to explore the influences of microwave irradiation and conventional solvothermal methods on the morphologies, the intrinsic differences between the two methods have been firstly comparatively studied. This work chose relatively special morphology of hollow butterfly, ring and fiber for comparison. Products of the solvothermal method using the same raw materials of hollow butterfly, ring and fiber particles are shown in supporting information (Fig. S3). No products were observed when the reaction time was 10 min. Further increasing the reaction time to 4 hours, micrometer-scale particles including irregular spheres and rods were obtained. Not only the morphologies but also the growth rate shows significant differences using these two kinds of methods. Based on the above results, it can be

samples calculated by SF method reveal the coexistence of micropore and mesopore (Fig. S1 in supporting information). The sphere sample has three distinct pores with average diameters of 1.7, 2.4 and 3.1 nm (Fig. S1a in supporting information). For butterflies, pores with average diameters of 1.1, 1.6 and 2.2 nm are present. For hollow butterflies, pores with average diameters of 1.1, 1.7, 2.3 and 2.8 nm are present. For rings, pores with average diameters of 1.1, 1.7, 2.4 and 3.1 nm are present. For fibers, pores distributed from1-4 nm with low pore volume (Fig. S1b in supporting information). It should be noticed that the butterfly, hollow butterfly and ring samples had another kind pore with diameters of 1.1 nm compared to sphere sample, though they all have the ZIF-8 structure (Fig. 4a). The addition of Ni2+ may influence the framework structure and induced the formation of small micropore with diameters of 1.1 nm. The mesoporosity could arise from the secondary particle piled pores of the small crystals. In addition, FT-IR spectra analyses of the as-obtained five samples are measured to determine the surface functional groups (Fig. 4f). The FT-IR spectra exhibits bands at 2930 and 1582 cm−1 corresponding to the C-H and C=N stretch mode, respectively, demonstrating 2-MeIM is the primary linker in these samples.40,41 The bands at 420 cm−1 and 432 cm−1 should be ascribed to the Zn-N and NiN stretch indicating the formation of coordination polymer.44 The fibers possess two unique characteristic bands at 561 cm−1 and 609 cm−1 compared to other samples, which could be resulted from the acetylacetone.42 It can be revealed that the fibers are constructed from Ni ions and two organic ligands, 2-MeIM and acetylacetone. 7



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information). According to the above results, it is demonstrated that the use of particular negative ions such as nitrate and acetylacetonate is essential for the synthesis of ring particles. According to the hard-soft-acid-base theory, the positive and negative ions, owing to their different strengths in electronegativity, have shown various acidity and alkalinity, which further influenced the nucleation and growth process.44 Secondly, the amount of 2-MeIM was changed from 27 and 82 mg to investigate the effect of ligand on the morphology based on the synthesis of ring particles (Fig. S7 in supporting information). With 27 mg 2-MeIM, the irregular particles were obtained (Fig. S7a in supporting information). When the amount of 2-MeIM was 41 mg, well-defined ring particles were produced (Fig. 2). Further increasing the amount to 82 mg, hexagons were observed in TEM images (Fig. S7b in supporting information). It has been reported that it is easier to prepare ZIF-8 particles with higher crystalline when maintaining a larger molar ratio of 2-MeIM and Zn salts, which means more ligands, more stable structure.25-27 However, the morphology often exhibited rhombic dodecahedra which was hard to change because of the stable structure. In this work, with Niinduced synthesis, it is easier to change the morphology with decreasing amount of 2-MeIM, which also demonstrated the morphology could change with various amount of ligands in the present of Ni species. Finally, The addition of surfactant was explored to investigate the effect on the size and morphology of the as-synthesized ring samples. In the standard protocol preparing the ring sample while in absence of PVP (Fig. S8a in supporting information), micrometer-scale particles with ill-designed morphology formed. When the amount of PVP was 500 mg, the

concluded that it is necessary to use microwave irradiation to prepare the hollow butterfly, ring and fiber samples. Crystals often nucleate near the walls or even on the dusts when using conventional solvothermal method, which may reduce the amount of crystal seeds further lead to the slower growth rate.43 In contrast with solvothermal method, microwave irradiation possesses unique properties: such as rapid heating, faster kinetics and producing more seeds thus resulting faster growth rate and higher yields.12 Once the seeds start to grow, available reactants are quickly depleted, therefore it is much easier to produce nanoscale MOFs with high crystallization in much shorter time.28 To further investigate the influencing factors for various morphology of Ni-induced ZIF-8. The impact of Zn and Ni resource, the amount of 2MeIM and surfactant-PVP were investigated respectively. Firstly, various Zn and Ni resources were explored for the preparation of ring crystals (Fig. S5 and S6 in supporting information). The total molar content of metal salts were always retained identical in this set of experiments. When Zn3(PO4)2 was used as Zn resources, the micrometer-scale products with schistose morphology were produced (Fig. S5a in supporting information). Sea urchins were obtained when replacing Zn(NO3)2·6H2O to ZnCl2 (Fig. S5b in supporting information). With Zn(CH3COO)2 as the Zn resources, the particles with an average size of 127 nm have exhibited well-defined polyhedron morphology as shown insupporting information (Fig. S5c). Furthermore, the Ni salts were tested likewise. The products with morphologies of porous sphere, nanosheet and ill-defined materials were prepared when using Ni(NO3)2, NiCl2 and Ni(CH3COO)2 as Ni resources, respectively (Fig. S6 in supporting 8


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collapsed process of unstable hollow spheres. It has been well documented that microwave radiation could provide high energy to promote the crystal nucleation and growth process.12,44 The seeds were produced rapidly because of the abundant metal ions and organic linkers thus leading to the rapid fusion and growth on the basis of coordination chemistry. The supersaturation of metal ions will result in the formation of particles with high surface-energy faces at the initial stage.24,45When the raw materials were almost used up, the structure was

hollow spherical products with a significant reduced size of 510 nm were observed (Fig. S8b in supporting information). Typical ring particles were obtained when the amount of PVP was 1092 mg (Fig. S8c in supporting information). Further increasing the amount of PVP to 2000 mg, sphere together with nanosheet attached products were obtained (Fig. S8d in supporting information). The morphological evolutional process indicates the PVP plays an important role in morphologycontrolled synthesis of ZIF-8 particles. Also, the tremendous size reduction suggests that PVP acts as stable and dispersive agents in the synthesized process. To investigate the growth mechanism of ring crystals, a series of time-dependent experiments were carried out and analyzed by SEM and TEM characterizations. Initially, when the reaction temperature reached 130 oC at the microwave heating rate of ca. 50 oC/min (the temperature versus time as shown in supporting information (Fig. S9), the maximum temperature could be reached 170 oC for a short while), the reactor was put into cold water and the reaction time was defined as 0 min. Hollow spheres with an average size of 439 nm were obtained as shown in Fig. 5. As the reaction proceeded, both sides of these hollow sphere particles collapsed which was accompanied by curvature variation from spherical surfaces to columnar surfaces. Meanwhile, a size reduction of the external diameter from 439 nm to 352 nm is observed. When further prolonging the reaction time to 5 min, the collapsed part dropped out from its surface and typical ring particles with a high selectivity of 70% formed and the mean size was 275 nm. The time sequential evolution experiments clearly demonstrate that the growth process of the rings could be described as a

b)

Figure 5. (a) TEM (scale bar 500 nm), (b) SEM (scale bar 500 nm) and (c) HRSEM (scale bar 100 nm) images of ring particles synthesized with different crystallization duration time of 0 min, 2 min and 5 min, scale bar (a,b) 500 nm, (c) 100 nm (d) Illustration of the growth process of ring particles.



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detected by temperature controller, is about 50 C/min in the initial stage (Fig. S9 in supporting information). Thus, the heating rate of 5 oC/min was also studied (Fig. S11 in supporting information). There was no products observed when changing the heating rate during the synthesis of ring particles. The butterfly and hollow butterfly particles could be still produced even at different heating rate (Fig. S11a and b in supporting information). And, the fibers changed to micrometer-ranged rods and cubes (Fig. S11c in supporting information). Based on the above results, it can be concluded that the heating rate, to a certain extent, influences the morphology and size. To investigate the catalytic activity of the asprepared five samples, the Knoevenagel reaction was performed. Previous works reported that ZIF-8 possesses a higher activity compared to some Lewis acid catalysts and a lower activity than some base catalysts such as aminefunctionalized superparamagnetic 47 nanoparticles. The tentative mechanism of the ZIF-8 (scheme 1) could be the same as aminegrafted MOFs,48 nitrogen-enriched triazine-based microporous polymeric,49 and nitrogen50 containing ionic liquid. Thus, the nitrogencontaining basic sites of ZIF-8 abstract the acidic proton from the active methylene group of malononitrile to form carbanion. The oxyanion would be obtained when this carbanion intermediate attack the carbonyl-carbon atom of the benzaldehyde. Then, the oxyanion reacts with hydrogen ion from protonated basic sites of ZIF8, as well the ZIF-8 is regenerated. Finally, the products formed by the elimination water molecule. In detail, sphere, butterfly, hollow butterfly, ring and fiber particles exhibit the benzaldehyde conversions of 38.5%, 47.2%, o

Scheme 1. Tentative mechanism for Knoevenagel reaction. gradually stabilized. The dissolution and curvature variation processes tightly followed by the formation of the hollow spheres, which may be attributed to the variation of surface energy. Therefore, the growth mechanism of ring crystal could be ascribed to the surface-energy-driven process. It should be pointed out that the initial morphology is hollow sphere. The microwave heating is able to heat target materials without heating the surrounding medium like air and oil.27 This heating way will lead to high local temperature which may induce the formation of hollow structure. The growth process of hollow butterfly crystals was also investigated by TEM and SEM characterizations (Fig. S10 in supporting information). It can be observed that the morphology evolution of the sample derived from solid butterflies to hollow butterflies. Previous works also have demonstrated that this inside-out formation process is favorable to decrease the high surface energy.23,24,46 Therefore, the synthesis process of hollow butterflies could be also attributed to surface-energy-driven process. As mentioned above, the heating rate, 10


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Scheme 2. Illustration of the Knoevenagel reaction when using sphere and ring samples as catalyst. MeOH with a high conversion of 80%,17 however, the solvent method takes longer reaction time of 24 h, and the product yield was low (ca.43%) compared with our catalyst (ca.65%). It should be noted that quinoline, methyl imidazole, pyridines, and indole were previously used as homogeneous base catalysts for the Knoevenagel reaction.37 Nevertheless, this kind of catalyst could not be recycled and reused, thus they are practically undesirable. MOF materials such as ZIF-8 could be used as heterogeneous catalysts, which are easily separated. Cycling tests were also carried out to study the catalytic stability of ring particles after separated by centrifugation and activation. From the data shown in Fig. 6b, it can be seen that the conversion of benzaldehyde of 55% can be maintained after the 4 recycles, indicating a high stability. The degradation in activity could be ascribed to the mass loss of catalyst and the partially deactivation of active sites. The differences in catalytic activity of these five samples could be attributed to two factors: diffusion rate and the amount of active sites.

51.6%, 68.8% and 11.4%, respectively (Fig. 6a). The catalytic activity of ring particles is about 1.8 times than sphere particles and 6.0 times than the fiber particles. The catalytic performance of our Ni-induced ZIF-8 ring sample synthesized by microwave method for Knoevenagel reaction is compared with other ZIF-8 particles by the common method as summarized in supporting information (Table S2). The performance of our catalyst is comparable to some reported work under the same reaction conditions under room temperature and at the reaction time of 2 h: Simonise et al. synthesized ZIF-8 by solvent method in methanol (MeOH) which exhibit conversion of 63.5%;51 Miao and coworkers synthesized a catalyst encapsulated by ZIF-8 (Fe3O4@P4VP@ZIF-8) exhibit conversion of 66%.52 Meanwhile, our work has outperformed some previous reported catalysts: ZIF-8 prepared by solvent method in PhCH3 exhibit a low conversion of 27% reported by Miao et al.;52 ZIF8 synthesized by solvent method in DMF exhibit conversion of 51% and 54% reported by Uyen

P. N. Tran et al.53 and Lee et al.17 respectively; Lee also synthesized ZIF-8 by solvent method in 11



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Firstly, the ring particles have an open structure and the average thickness of walls is about 40 nm, a)

b)

c)

d)

amount of basicity of sphere, butterfly, hollow butterfly, ring and fiber particles is 3.1, 3.8, 4.6, 6.7, and 1.0 (unit basicity)/g_cat, respectively, by assuming that the amount of basicity of fiber particles is 1.0 (unit baisity)/g_cat. It is evident that the ring particles possess the highest mass concentration of basicity. The rings with the highest concentration of basic sites should be an important factor in the higher catalytic activity. The diffusion rate and the amount of basicity together influence the catalytic activity is illustrated in scheme 2. CONCLUSION

Figure 6. (a) the conversion of benzaldehyde. (b) The recyclability test. (c) CO2-TPD profiles and (d) the calculated basicity content of spheres, hollow butterflies, rings and fibers.

In summary, we have successfully synthesized two kinds of hollow structures, hollow butterfly and ring particles of ZIF-8 materials, assisted by Ni ion stimulus using microwave irradiation method. Sphere, butterfly and fiber samples were also obtained when just changing the weight ratio of Zn(NO3)2·6H2O and Ni(acac)2. Sphere, butterfly, hollow butterfly and ring particles showed ZIF-8 structure while the fibers had nonMOF structure. According to the time sequential experiments, it is concluded that the change of surface energy is an important factor to drive the formation of hollow butterfly and ring particles. The Knoevenagel reaction was investigated to research the catalytic activities of the as-prepared five samples. The ring particles showed the highest catalytic activity which should be attributed to the following three aspects: (1) the highest BET specific surface area, (2) the largest amount of basic sites (3) the rapidest diffusion rate. The synthetic strategy using microwave

which is much smaller than the size of solid spheres with an average size of 430 nm. The thin walls could provide shorter diffusion distance and show higher diffusion rate for the reactants.55,56 Thus, the morphological evolutions of these five catalysts could play a crucial role in catalytic activity. Secondly, fibers have the lower BET specific surface area compared with ring particles, which may suggest that most of active sites have been encapsulated inside and thus decreasing the catalytic activity. Thereby, to further understand the intrinsic catalytic mechanism, it is necessary to calculate the amount of basicity using the CO2TPD analysis. Fig. 6c shows the CO2-TPD profiles of spheres, butterflies, hollow butterflies, rings and fibers and the amount of basicity were calculated and summarized in Fig. 6d. The 12


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For Table of Contents Use Only Microwave-assisted, Ni-induced Fabrication of Hollow ZIF-8 Nanoframes for Knoevenagel Reaction Pengfang Zhang,a† Yun Xiao,a† Hui Sun,a Xiaoping Dai,a Xin Zhang,*a Haixia Su,a Yuchen Qin,a Daowei Gao,a Axiang Jin,b Hai Wang,b Xiubing Wang,a and Shigang Sunc

By varying the weight ratio of Zn and Ni salts, the morphology of ZIF-8 particles could be tuned from sphere to butterfly, hollow butterfly, ring, and fiber. The formation of hollow butterfly and ring particles is driven by the change of surface energy. The ring with short channels and higher amount of basic sites shows the highest catalytic activity. 17 ACS Paragon Plus Environment

Microwave-assisted, Ni-induced Fabrication of Hollow ZIF-8

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