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Millimeter-scale magnetic spherical metal-organic framework core-shell structured composites for recyclable catalytic applications
Microporous and Mesoporous Materials 300 (2020) 110152
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Millimeter-scale magnetic spherical metal-organic framework core-shell structured composites for recyclable catalytic applications €ser b, *, Chang-jun Liu a, ** Wenlong Xiang a, Sebastian Gebhardt b, Roger Gla a b
Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China Institute of Chemical Technology, Leipzig University, Leipzig, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Spherical activated carbons Metal-organic frameworks Magnetic responsivity Core-shell structures Millimeter-scale composite beads
A facile in situ growth approach for preparing magnetic spherical activated carbon (MSAC)-metal organic framework (MOF) core-shell composite beads with millimeter size is described. This approach overcomes the challenge of processing MOF nanoparticles into shaped bodies with fast magnetic responsivity. The size of the MOF@MSAC beads can be easily tuned by changing the activated carbon core size. Furthermore, the thickness of the MOF shell can be designed by controlling the assembly cycle number and the precursor concentration of forming the MOF. In addition, different types of MOF, including monometallic (e.g., ZIF-67 and HKUST-1) as well as bimetallic ones (e.g., CoZn-ZIF) can be successfully self-assembled to form the shell. As a model catalyst, ZIF-67@MSAC displays satisfying activity in the Knoevenagel condensation reaction, good recyclability and remarkable convenience in separation and recovery. The applicability of the catalyst is contributed to its unique structural features, such as nanoscale active MOF, magnetic responsivity and the dispersing and stabilizing ef fects of the activated carbon support.
1. Introduction Metal-organic frameworks (MOFs), assembled by the coordination of metal ions with organic ligands, are a class of porous crystalline mate rials [1,2]. MOFs have attracted considerable attention over the last decade due to their high porosity and specific surface areas, tunable and uniform pore systems, and easily tunable functionality. MOFs are promising for numerous applications in gas storage [3], separation [4, 5], molecular sensors [6], drug delivery [7], and heterogeneous catalysis [8–10]. MOFs are very attractive for catalytic applications in the liquid phase, since they possess both the catalytic site characteristics of mo lecular organometallic catalysts and recyclability of solid catalysts [9, 11]. For example, several MOFs have been investigated as solid catalysts for efficient catalytic organic transformations [10], such as Knoevenagel condensation [12,13], Aldol condensation [14], Friedel-Crafts reactions [15], Diels-Alder reactions [16] and CO2 cycloaddition [17,18]. How ever, MOFs prepared by conventional methods are typically nano- or microcrystalline powders. A further expense is necessary for the sepa ration and recovery of the MOF catalyst powders from the reaction so lutions. To provide MOF catalysts that can easily be separated from
reaction solutions, the combination of magnetic separation with MOFs is a viable solution [19–22]. Magnetic MOF catalysts may not only possess high catalytic activity, but also show the advantages for recycling in a liquid system [19,23]. The separation and recovery of magnetic MOF catalysts can be easily performed by an external magnetic field, which is energy-saving, rapid and convenient. In particular, magnetic MOFs with core-shell structures have several advantages such as preventing ag gregation of magnetic cores, stabilized magnetic cores and high catalyst stability [24]. Nevertheless, up to date, magnetic MOF materials are mostly applied as powders or small particles with sizes ranging from nanometers to dozens of microns [20–24], which are difficult to handle and not promising for applications [25]. If these reported micrometer-scale magnetic MOF composites would be used for applications in the liquid phase, a significant difficulty in the separation still arises. Increasing efforts have been recently devoted to shape engineering of MOFs. MOFs powders have been shaped into spheres [26,27], fibers [28], thin films [29,30], foams [25], and other forms of structures [31–33]. At present, one commonly used method is the in situ growth of MOFs on chemically stable and cost-efficient substrates, including synthetic polymers [27,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (R. Gl€ aser), [email protected], [email protected] (C.-j. Liu). https://doi.org/10.1016/j.micromeso.2020.110152 Received 24 November 2019; Received in revised form 9 February 2020; Accepted 5 March 2020 Available online 9 March 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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29] and porous supports (e.g., γ-Al2O3 [26], silica foam [34] or zeolites [35]). Porous activated carbons have been recognized as one of the widely used versatile materials for numerous practical applications, such as adsorption, filtration or as a substrate for catalytically active compounds. Surface modified activated carbons can be developed by various methods like acid or plasma treatment [36]. Prior to the growth of MOFs, surface modification of the substrate can be useful in order to facilitate the binding of metal ions to the substrate surface [29,37]. In this work, we report on the preparation of millimeter-scale mag netic spherical activated carbon-MOF core-shell composite beads (denoted as MOF@MSAC). Importantly, the size of the core-shell structured composite beads and the thickness of MOFs shells are widely adjustable. Moreover, this synthesis strategy can be expanded to both monometallic (e.g., ZIF-67 and HKUST-1) and bimetallic (e.g., CoZn-ZIF) MOFs. This synthetic method is expected to be extended for other materials for MOF shaping. In addition, ZIF-67@MSAC composite beads are investigated as model catalysts for Knoevenagel condensation. An excellent catalytic performance with remarkable convenience in separation, recovery and recycling has been confirmed.
methanol rinsing for 5 min. Then the beads were dried in ambient air at 60 � C for 0.5 h. After that, beads were immersed into a mixture of 2methylimidazole (Hmim) and methanol (20 mL, CHmim: CCo(NO3)2 ¼ 4:1) and 0.3 wt% PSS aqueous solution (20 mL) at 60 � C for 1 h. Then beads were separated by a magnet and washed three times with meth anol and dried in ambient air at 60 � C for 0.5 h. The treatment of beads by Co(NO3)2 and Hmim solutions was repeated four times. The resulting products are referred to as ZIF-67@MSAC. As a reference, pure ZIF-67 was prepared by mixing the Co(NO3)2 and Hmim methanol solutions in the absence of activated carbon beads. HKUST-1@MSAC and CoZnZIF@MSAC composite beads were prepared using a similar procedure as that described above. Further details can be found in the Supporting Information. 2.4. Characterization Powder X-ray diffraction (XRD) measurement was conducted on a Rigaku D/max-2500 with Cu Kα radiation (λ ¼ 1.54056 Å) using the finely crushed samples. The data were collected in the 2θ range of 5–70� with a step size of 0.02� and a time per step of 0.6 s. The microscopic features of the samples were characterized by scanning electron mi croscopy (SEM, S-4800) equipped with an energy-dispersive X-ray (EDX) analyzer. Thermogravimetric analysis (TGA) was conducted with a Netzsch STA 449 F3 system at a heating rate of 10 � C min 1 from 35 � C to 800 � C under an air flow of 100 mL min 1. Fourier transform infrared (FTIR) spectra were obtained on a ThermoNicolet Nexus 6700 system. The grinding powder samples were uniformly mixed with KBr powder and then pressed into a disk for FTIR analysis. For each spectrum, 64 scans were collected with a resolution of 4 cm 1 over the range 400–4000 cm 1. N2 sorption isotherms were recorded at liquid nitrogen temperature ( 196 � C) on a Quantachrome Autosorb-1C instrument. Prior to the measurement, the samples were outgassed under vacuum (<10 Pa) at 120 � C for 24 h. The specific surface area was calculated by the multipoint Brunauer-Emmett-Teller (BET) method. The total pore volume was estimated at P/P0 ¼ 0.99. The micropore and mesopore volumes were determined by the t-plot method and Barrett-JoynerHalenda (BJH) method, respectively. The magnetization curve was measured at room temperature on a commercial magnetometer with the superconducting quantum interference device (MPMS SQUID-VSM, Quantum Design) with applied magnetic fields between 1.5 T and 1.5 T. The metal contents of the composites were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700x). The solution 1H and 13C NMR spectra were recorded in CDCl3 on a Bruker 400 MHz spectrometer. HRMS was performed on a SolariX70FT-MS Fourier transform ion cyclotron resonance mass spectrom eter (Bruker Daltonics, Germany) in electrospray ionization (ESI) posi tive ion mode.
2. Experimental section 2.1. Materials The spherical activated carbon (SAC, dsphere ¼ 0.45–0.50 mm) was obtained from Blücher GmbH (Germany). Poly(sodium 4-styrenesulfo nate) (PSS, average Mw is 70,000) and zinc nitrate hexahydrate (99%) were purchased from Alfa Aesar. 3-Aminopropyl-trimethoxysilane (APTES, 98%), ethyl cyanoacetate (99%) and n-dodecane (�99.5%, GC) were purchased from Shanghai Macklin Inc. Ferric acetylacetonate (98%), concentrated nitric acid (65 wt%) and 1, 3, 5-benzenetricarbox ylate (H3BTC, 98%) were obtained from Shanghai Aladdin Biological Technology Co., Ltd. Benzaldehyde (99%), 4-methylbenzaldehyde (99%), 4-bromobenzaldehyde (99%) and 4-nitrobenzaldehyde (99%) were purchased from Adamas Reagent, Ltd. Cobalt(II) nitrate hexahy drate (99%), 2-methylimidazole (Hmim, �99.0%), benzyl alcohol (�99.0%), methanol (99.9%), ethanol (99.8%) and dichloromethane (�99.5%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All materials were used without further purification. 2.2. Preparation of magnetic spherical activated carbon (MSAC) 0.1 g SAC was firstly treated with 2 mL of concentrated nitric acid for 10 h at 60 � C. Afterwards, the beads were washed with deionized water until the wash water was neutral, then dried in ambient air at 110 � C for 2 h. Subsequently, the beads were added to a solution of 1 g Ferric acetylacetonate in 20 mL of benzyl alcohol. The resulting mixture was transferred into a 50 mL polytetrafluoroethylene (PTFE)-lined autoclave and sealed for 48 h at 175 � C under static condition. After cooling, the magnetic spherical activated carbon beads (denoted as MSAC) were collected by decantation and sieving, washed with 20 mL ethanol three times to remove the remaining organics and subsequently dried in ambient air at 110 � C for 2 h. As a reference, Fe3O4 powder was prepared under the same conditions in the absence of activated carbon beads. The MSAC beads were immersed in a mixture of 100 mL of 95 wt% ethanol aqueous solution and 8 g L 1 APTES for 20 h. After decantation, the MSAC beads were rinsed two times with ethanol and dried in ambient air at 110 � C for 2 h. After that, the MSAC beads were immersed into 100 mL of 0.3 wt% PSS aqueous solutions for 1 h at 60 � C. After decantation, the MSAC beads were washed again two times with deionized water and then dried in ambient air at 110 � C for 2 h.
2.5. Catalytic reaction Prior to the reaction, ZIF-67@MSAC beads were pretreated under vacuum (�1 kPa) at 120 � C for 6 h. In a typical reaction, 50 mg of the pretreated ZIF-67@MSAC beads were added into the reaction mixture composed of benzaldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), and n-dodecane (0.88 mmol) as an internal standard in liquid ethanol (5 mL) in a 25 mL round-bottom flask. The resulting mixture was sealed and shaken in a shaker incubator at room temperature for a given time. The reaction progress (conversion based on the amount of benzalde hyde) was followed by withdrawing aliquots (10 μl) from the reaction mixture, diluted in 1 mL of ethanol. Subsequently, the diluted samples were analyzed on a gas chromatography (Agilent 7890B) equipped with a flame ionization detector and a HP-5 column (30 m � 0.25 mm � 0.25 μm). The bead catalysts were easily separated from the reaction mixture by a magnet. To remove the product remaining in the pores of the catalyst, the catalyst was washed with dichloromethane three times. The residual solvents were removed under vacuum (�1 kPa) at 80 � C for 10
2.3. Preparation of ZIF-67@MSAC beads 0.1 g Surface-modified MSAC beads were immersed in Co(NO3)2 methanol solution (20 mL, 0.2 M or 0.5 M) for 1 h at 60 � C, followed by 2
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h. The bead catalysts were then collected for reuse. The reaction solution was dried by removing the volatile components using a rotary evapo rator (RE-52AA, Yarong Co. Ltd., Shanghai, China) under reduced pressure at 50 � C. The solid products were purified by recrystallization from 95 wt% ethanol. NMR (1H and 13C) and high-resolution mass spectrometry (HRMS) were performed to further characterize the ob tained solid products. All the characterization data are listed in Sup porting Information.
styrenesulfonate (PSS). It has been reported that the amino group (-NH2) in silane coupling agents can combine with sulfonic acid group (-SO3H) in PSS through the electrostatic interaction/acid-base reaction, as depicted in Fig. 1B [29]. Confirmation of the surface modification was achieved by FTIR and EDX analysis (Figs. S2 and S3). Metal ions were then anchored on the surface of the MSAC through coordination with the sulfonate groups [29,41]. When the substrate was immersed into a mixed solution of 2-methylimidazole and PSS, the fixed metal ions could effectively coordinate with organic ligands, resulting in in situ formation of MOF nanoparticles. After the in situ growth of ZIF-67, the black MSAC beads changes to the characteristic purple color of ZIF-67 (Fig. 3A and B), directly indi cating the successful synthesis of ZIF-67 on MSAC beads. The charac teristic reflexes of ZIF-67 observed from the XRD pattern of ZIF67@MSAC beads (Fig. 2A, blue line) are in accordance with those re ported in the literature [42]. This clearly confirms the successful growth of ZIF-67 crystals. The formation of ZIF-67 crystals was further confirmed by the FT-IR analysis (Fig. S4). The presence of the charac teristic band at 422 cm 1 assigned to Co–N stretching vibration in the ZIF-67 framework [42,43], clearly proofs the formation of ZIF-67 crys tals on the MSAC. The Fe3O4 and ZIF-67 content in the ZIF-67@MSAC composite beads, calculated based on the ICP tests and weight changes before and after the growth, are 12.8 wt% and 25.1 wt%, respectively. The values are basically in line with the loading values calculated by TGA (Fig. S5). The textural properties of the ZIF-67@MSAC composite beads and their parent materials analyzed by N2 sorption isotherms are presented in Fig. 3C and Table 1. All samples clearly exhibit a combination of types I and IV with a hysteresis loop at p/p0 > 0.6. After depositing magnetic nanoparticles, MSAC has a lower specific surface area of 1120 m2 g 1 than that of initial SAC (~1677 m2 g 1). The Fe3O4 content in MSAC composite beads was found to be 17.0% as determined by ICP. Due to the negligible specific surface area of Fe3O4 (102 m2 g 1, Fig. S6), the specific surface area of SAC in the MSAC composite is estimated to 1349 m2 g 1. The value is significantly lower as compared to that of bare SAC. It indicates that part of magnetic nanoparticles is penetrated into the pores of SAC, which agrees with the EDX mapping results in Fig. 2B. As shown in Fig. 2B, the Fe species are concentrated on the surface of SAC and penetrates into about a 10-μm layer of SAC. In addition, the ZIF67@MSAC composite beads show reduced specific surface area and micropore volume in comparison to MSAC and bare ZIF-67 (Table 1). This can be attributed to the formation of crystals generated at the interface of MOFs and carbon support and even in the support pore channel [35], leading to the partial pore blockage of the carbon support. An advantage, however, is that the part of ZIF-67 nanocrystallites are firmly embedded in the pore of SAC, which is similar to the work re ported earlier [44]. In order to investigate the morphology of the magnetic composite beads, SEM studies were performed. ZIF-67@MSAC beads remain an almost spherical shape of active carbon beads with particle sizes be tween 0.4 and 0.5 mm (Fig. 4A and Fig. S7). The size distribution of SAC beads is plotted in Fig. S7. The magnified image of ZIF-67@MSAC sur face revealed the growth of rhombic dodecahedral ZIF-67 crystals (Fig. 4B). From the cross-section images (Fig. 4C–D), a clear interface between the ZIF-67 dense layer and the activated carbon layer was observed, indicating the growth of the ZIF-67 crystals on the outer surface of entire MSAC. The thickness of ZIF-67 layer is approximately 11 μm. The EDX mapping of ZIF-67@MSAC beads further illustrates the growth of the ZIF-67 crystals on the outer surface of MSAC (Fig. 4E). These results show that the finally formed ZIF-67@MSAC composite beads are composed of a MSAC core and a MOF shell, clearly demon strating the formation of a core-shell structure. It should be noted that the surface modification of MSAC is essential for the growth of the MOF shell layer. The images visually indicate the obvious difference of the MOF shell layer on the modified MSAC and unmodified MSAC, as shown in Fig. S8. The SEM images further confirm that only a small amount of
3. Results and discussion 3.1. Characterization of MSAC and ZIF-67@MSAC The preparation procedure of the ZIF-67@MSAC composite beads is shown in Fig. 1A. The SAC (SBET ¼ 1748 m2 g 1, Vtotal ¼ 2.0 cm3 g 1, dparticle ¼ 0.45–0.50 mm, Blücher GmbH) was firstly pretreated with nitric acid to increase surface acidic oxygen-containing groups to facil itate grafting silane coupling agents. Magnetite nanoparticles readily deposit on the SAC support via a facile one-pot solvothermal reaction [38]. The successful deposition of Fe3O4 on SAC is confirmed by the results of magnetic separation test and XRD patterns. The MSAC beads have a fast optical response to the external magnetic field (Fig. S1). The XRD reflexes (2θ) of the MSAC at 18.3� , 30.1� , 35.4� , 37.1� , 43.1� , 53.5� , 57.1� and 62.6� are in good agreement with the standard XRD data of cubic Fe3O4 (JCPDS 19–0629) (Fig. 2A, black line). The average Fe3O4 nanoparticle size of MSAC can be estimated to be 13 nm by using the Scherrer equation from the (311) reflection (Table S1). The value is similar to those reported in the literature [38]. Besides, Fourier trans form infrared (FTIR) spectroscopy analysis provides further evidence of the formation of magnetite nanoparticles (Fig. S2). The broad band at 579 cm 1 corresponds to the vibration of the Fe–O bonds in the crys talline lattice of Fe3O4 [39,40]. In order to further investigate the dis tribution of Fe element in the MSAC, energy-dispersive X-ray (EDX) mapping was conducted. As shown in Fig. 2B, the magnetic nano particles are evenly distributed over the surface of SAC. A small amount of the magnetic nanoparticles penetrates below the surface. Afterwards, the MSAC beads were modified successively with 3-ami nopropyl-trimethoxysilane (APTES) and poly(sodium 4-
Fig. 1. Schematic illustration of (A) synthesis procedure for the ZIF-67@MSAC and (B) the applied force between ZIF-67 and MSAC. 3
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Fig. 2. (A) XRD patterns of MSAC and ZIF-67@MSAC composite beads. (B) SEM images of MSAC and the corresponding EDX mapping images of C and Fe elements (the top: surface morphology; the bottom: cross-section morphology).
3.2. Tuning the thickness and composition of the MOF shell The size of magnetic core-shell structured MOF beads is controllable by using MSAC core with different sizes (Fig. S9). The SAC with diameter ranging 0.6 from 1 mm was used as core to prepare magnetic core-shell structured MOF composite beads (Fig. S9A). The characteristic purple color and magnetic response behavior suggest the successful fabrication of magnetic composite beads (Fig. S9B). The SEM images further provide evidence for the formation of ZIF-67 shell on the surface of MSAC (Figs. S9C–D). These results indicate that it is feasible to tune the size of magnetic composite beads by simply changing the carbon core size. In addition, the MOF shell thickness of ZIF-67@MSAC beads is found to be tunable. As clearly shown in Fig. S10, the thickness of the MOF shell increases with the increasing number of assembly cycles. It suggests that the MOF shell can be tuned by varying the assembly cycle number. It is also found that the thickness of the MOF shell can be tuned by varying the precursor concentration of the MOF to be synthesized. For example, the MOF shell thickness of the ZIF-67@MSAC beads prepared at 0.2 mol L 1 cobalt ion is approximately 1.5 μm after four-time assembly cycles (Fig. S11), whereas the MOF shell thickness of ca. 11 μm is achieved with the same assembly cycle number, as can be seen from SEM results (Fig. S10). These results demonstrate that the MOF shell thickness of the ZIF-67@MSAC beads can be effectively regulated by altering the as sembly cycle number and the precursor concentration. Importantly, the synthetic approach is applicable to prepare shells with other MOF-type materials, such as HKUST-1 or bimetallic CoZn-ZIF [43]. The formation of HKUST-1 and CoZn-ZIF shells is confirmed by XRD patterns (Fig. S12). All composite beads have similar spherical morphologies as the initial SAC beads. The octahedral HKUST-1 and rhombic dodecahedron CoZn-ZIF crystals can be seen on the surface of the composite beads, as revealed by SEM characterization (Fig. S13). All these results revealed that the HKUST-1@MSAC and CoZn-ZIF@MSAC composite beads were successfully prepared. Further, the EDX elemental mapping confirms that the HKUST-1@MSAC and CoZn-ZIF@MSAC composite beads possess the MSAC core-MOF shell structures (Figs. S14 and S15).
Fig. 3. (A–B) Optical micrographs of ZIF-67@MSAC composite beads. The horizontal length of the pink bar is 0.25 mm. (C) N2 sorption isotherms of the SAC, MSAC and ZIF-67@MSAC composites. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Table 1 Textural properties of the composites and their parent materials determined by N2 sorption. Sample
SBET [m2 g 1]
Vtot [cm3 g 1]
Vmicro [cm3 g 1]
Vmeso [cm3 g 1]
mares [wt %]
ZIF-67 SAC MSAC ZIF67@MSAC
1324 1677 1120 976
0.72 1.39 1.22 0.83
0.60 0.54 0.36 0.27
n.d. 0.10 0.08 0.06
36.3 n.d. 21.8 25.8
3.3. Analysis of magnetic properties The magnetic properties of the ZIF-67@MSAC composite beads and synthetic Fe3O4 powder have been investigated using a vibrating sample magnetometer at room temperature (Fig. 5A). The magnetic hysteresis loops show no obvious remanence or coercivity at room temperature. It suggests the superparamagnetic feature, which is essential for the magnetic separation. The saturation magnetization value of pristine Fe3O4 and ZIF-67@MSAC beads are estimated to be 66.6 emu g 1 and 8.2 emu g 1, respectively. According to these saturation magnetization values, Fe3O4 content in the ZIF-67@MSAC composite beads can be
n.d.: not determined. a Residual mass (mres) obtained from TGA at 800 � C.
separate ZIF-67 crystals and crystal islands rather than a continuous MOF shell layer is formed if the MSAC surface was not treated with APTES and PSS (Fig. S8).
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Fig. 4. SEM images of (A–B) surface morphologies and (C–D) cross-section morphology of ZIF-67@MSAC after four times of ZIF-67 growing cycles. (E) The elemental mapping images of ZIF-67@MSAC for C, Fe, and Co.
estimated to be about 12.3 wt%, which is basically consistent with the loading content obtained by TGA and ICP results. Taking advantage of the magnetic properties, the magnetic composite beads are expected to be easily separated from the solution. As can be seen from the inset in Fig. 5A and Video S1, the ZIF-67@MSAC beads are attracted toward the magnet within a few seconds and move flexibly following the magnet. This clearly demonstrates the convenient separation of the ZIF67@MSAC beads using an external magnetic field. In addition, we compared their dispersion state of the ZIF-67@MSAC composite beads and as-synthesized Fe3O4 powder when they were separated from ethanol by a magnet (Fig. 5B). ZIF-67@MSAC composite beads are dispersed individually. On the contrary, Fe3O4 powder looks like the mud and forms a pile of powder after drying naturally. It is important to mention that the obvious mass loss of ZIF-67 was observed in the weighting and recycling process. As shown in Fig. S16, powder ZIF-67 adhered to the weighing paper and glass container walls due to strong electrostatic forces. It can be concluded that millimeter-scale ZIF67@MSAC beads are more convenient in separation and recycling than Fe3O4 and ZIF-67 powder particles. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.micromeso.2020.110152
ZIF-67@MSAC magnetic beads. The reaction was carried out using ethanol as a solvent at room temperature. Fig. 6A shows the catalytic results of the Knoevenagel condensation over various catalysts. Only trace conversion (<5%) of benzaldehyde was observed in the absence of the catalyst. In contrast, ZIF-67 shows a high conversion (75%) after 3 h. It suggests that ZIF-67 can effectively catalyze the Knoevenagel condensation reaction [45]. Nevertheless, it was found that ZIF-67@MSAC achieves a higher catalytic activity than pure ZIF-67 with 86% conversion after 3 h and up to 97% conversion after 6 h. Without anchored ZIF-67 nanoparticles, the activity of MSAC is significantly lower than that of ZIF-67@MSAC and ZIF-67 under the same reaction condition and gave only 16% conversion after 6 h. These results provide strong evidence that ZIF-67 acts as the catalytically active component for this reaction. The higher activity of ZIF-67@MSAC than pristine ZIF-67 is mainly attributed both to a small amount of free amino groups of MSAC and to the advantageous stabilizing effect of the support. The existence of free terminal amino groups was confirmed by FTIR analysis (Fig. S2). The amine functional groups have been reported previously to catalyze the Knoevenagel condensation reaction to different extents [46, 47]. In general, the active sites located at the external surface of ZIF materials are responsible for the catalysis [48]. As expected, smaller size crystals normally exhibit higher catalytic activity because of the increased external surface of smaller crystals [12,49]. Compared to the ZIF-67 nanoparticles anchored in the ZIF-67@MSAC, the freestanding ZIF-67 nanoparticles much more easily form bulkier crystal agglomer ates, resulting in a smaller number of exposed active sites. As a result, ZIF-67 exhibits lower catalytic activity than ZIF-67@MSAC. The similar
3.4. Evaluation of catalytic activity To investigate the applicability of ZIF-67@MSAC beads, the Knoe venagel condensation reaction of benzaldehyde and ethyl cyanoacetate was chosen as a model reaction to evaluate the catalytic activity of the 5
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particles. In contrary, no obvious color can be observed in the decant ing solutions separated from the ZIF-67@MSAC system, indicating the strong adhesion of ZIF-67 to MSAC beads. To further evaluate the capability of ZIF-67@MSAC, the Knoevena gel condensation of several benzaldehyde derivatives having different substituents with ethyl cyanoacetate was carried out employing ZIF67@MSAC beads as catalysts. It was found that all the reactions pro ceed smoothly to give the corresponding products (Table 2). The re actions can proceed well for benzaldehyde derivatives bearing electronwithdrawing substituents such as -Br, and –NO2, with 96% conversion after 5 h for the case 4-bromobenzaldehyde and 99% conversion after 1 h for the case of 4-nitrobenzaldehyde, respectively (Table 2, entries 1–2). As for benzaldehyde derivatives with electron-donating sub stituents such as –CH3, the ZIF-67@MSAC beads can also give the product in 93% conversion, but with a longer reaction time of 6 h (Table 2, entry 3). These results are consistent with the expected higher reactivity of benzaldehyde derivatives containing electron-withdrawing groups than those with electron-donating groups [12,50]. Besides, as Table 2 The Knoevenagel reaction employing various aldehydes.a
Entry
Fig. 5. (A) Magnetization curve of pristine Fe3O4 nanoparticles and ZIF67@MSAC composite beads. The Inset shows the separation of ZIF-67@MSAC under the external magnetic field. (B) The dispersion state of ZIF-67@MSAC composite beads and magnetic powder separated by a magnet from ethanol.
dispersing and stabilizing functions from the support have been recently reported in the literature [49]. In addition, we added ZIF-67 and ZIF-67@MSAC into ethanol and observed the color of decanting ethanol after shaking the mixture for 6 h. As showed in Fig. S17, it can be seen clearly that the solution shows purple due to the dispersion of ZIF-67
Time/h
Conversion/%
Yield/%
1
Product
5
96
95
2
1
99
99
3
6
93
92
4
6
97
96
a Reaction conditions: aldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), ethanol (5 mL), ZIF-67@MSAC (50 mg, 3 mol% Co), room temperature under atmospheric conditions.
Fig. 6. (A) Catalytic performances of various catalysts for the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate. Reaction conditions: benzaldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), ethanol (5 mL), catalysts (50 mg, 3 mol% Co), room temperature under atmospheric conditions. For the MSAC and ZIF-67 cases, 50 mg and 13 mg solid catalysts were used, respectively. (B) Catalyst recycling studies of ZIF-67@MSAC catalyst. (C) XRD patterns and (D) magnetization curves of the fresh and the fourth reused ZIF-67@MSAC. 6
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reported in the literature, steric and size selective effects were observed over the MOF catalysts [51,52]. This substrate selectivity of MOF plays an important role in catalyst design. Wang et al. [53] reported the MOF-catalyzed Knoevenagel condensation, providing evidence for the substrate selectivity. We expect that the substrate selectivity of MOFs remains in the magnetic MOF composite beads.
Appendix A. Supplementary data
3.5. Recycling test
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110152. References
Recyclability is an important and essential feature for solid catalysts. Herein, the ZIF-67@MSAC composite beads were examined for recov erability and reusability in the Knoevenagel condensation of benzalde hyde with ethyl cyanoacetate. After the completion of the reaction, the composite bead catalysts can be easily and conveniently separated from the solution. It was found that the beads can be recovered and reused without a significant degradation in catalytic activity (Fig. 6B). The XRD, SEM and magnetization curves were conducted to investigate the structural stability of the reused composite beads catalysts. As shown in Fig. 6C, the XRD pattern of the fourth reused catalyst confirms that ZIF67@MSAC maintains its initial structure. The saturation magnetization value of reused composite beads is basically consistent with that of the fresh ZIF-67@MSAC (Fig. 6D), suggesting no obvious mass loss of the composites. The cross-section SEM image and optical micrograph of the fourth reused ZIF-67@MSAC further show a good stability of the coreshell structured composites (Fig. S18). 4. Conclusions This study reports a facile and general approach to fabricate sizetunable spherical activated carbon-MOF core-shell structured magnetic composite beads in the millimeter scale. The obtained beads show excellent catalytic performance for the Knoevenagel condensation re action. The size of the MOF@MSAC beads can be easily tuned by changing the size of the activated carbon core. Furthermore, the struc ture, composition, and thickness of the MOF shell are tunable by design. Different types of activated carbon-MOF magnetic composite beads, including HKUST-1@MSAC, ZIF-67@MSAC and CoZn-ZIF@MSAC, were successfully prepared by varying the MOF precursor. Compared to other solid catalysts, the resulting MSAC-MOF core-shell structured magnetic beads combine the superior properties of MOFs with the unique mag netic responsivity. Capitalizing on its unique structural features, such as large specific surface area, nanoscale active phase, magnetic respon sivity and the dispersing and stabilizing effects of carbon support, the ZIF-67@MSAC displays high activity, recyclability, easy separation and remarkable convenience in the separation and recovery processes. Furthermore, the magnetic composite beads are reusable without a significant loss of catalytic activity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Wenlong Xiang: Conceptualization, Methodology, Investigation, Writing - original draft. Sebastian Gebhardt: Conceptualization, €ser: Conceptualization, Super Methodology, Investigation. Roger Gla vision, Resources, Writing - review & editing. Chang-jun Liu: Concep tualization, Supervision, Resources, Writing - review & editing. Acknowledgments This work was supported by the National Natural Science Foundation of China (#21536008 and #21621004). 7
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Millimeter-scale magnetic spherical metal-organic framework core-shell structured composites for recyclable catalytic applications €ser b, *, Chang-jun Liu a, ** Wenlong Xiang a, Sebastian Gebhardt b, Roger Gla a b
Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China Institute of Chemical Technology, Leipzig University, Leipzig, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Spherical activated carbons Metal-organic frameworks Magnetic responsivity Core-shell structures Millimeter-scale composite beads
A facile in situ growth approach for preparing magnetic spherical activated carbon (MSAC)-metal organic framework (MOF) core-shell composite beads with millimeter size is described. This approach overcomes the challenge of processing MOF nanoparticles into shaped bodies with fast magnetic responsivity. The size of the MOF@MSAC beads can be easily tuned by changing the activated carbon core size. Furthermore, the thickness of the MOF shell can be designed by controlling the assembly cycle number and the precursor concentration of forming the MOF. In addition, different types of MOF, including monometallic (e.g., ZIF-67 and HKUST-1) as well as bimetallic ones (e.g., CoZn-ZIF) can be successfully self-assembled to form the shell. As a model catalyst, ZIF-67@MSAC displays satisfying activity in the Knoevenagel condensation reaction, good recyclability and remarkable convenience in separation and recovery. The applicability of the catalyst is contributed to its unique structural features, such as nanoscale active MOF, magnetic responsivity and the dispersing and stabilizing ef fects of the activated carbon support.
1. Introduction Metal-organic frameworks (MOFs), assembled by the coordination of metal ions with organic ligands, are a class of porous crystalline mate rials [1,2]. MOFs have attracted considerable attention over the last decade due to their high porosity and specific surface areas, tunable and uniform pore systems, and easily tunable functionality. MOFs are promising for numerous applications in gas storage [3], separation [4, 5], molecular sensors [6], drug delivery [7], and heterogeneous catalysis [8–10]. MOFs are very attractive for catalytic applications in the liquid phase, since they possess both the catalytic site characteristics of mo lecular organometallic catalysts and recyclability of solid catalysts [9, 11]. For example, several MOFs have been investigated as solid catalysts for efficient catalytic organic transformations [10], such as Knoevenagel condensation [12,13], Aldol condensation [14], Friedel-Crafts reactions [15], Diels-Alder reactions [16] and CO2 cycloaddition [17,18]. How ever, MOFs prepared by conventional methods are typically nano- or microcrystalline powders. A further expense is necessary for the sepa ration and recovery of the MOF catalyst powders from the reaction so lutions. To provide MOF catalysts that can easily be separated from
reaction solutions, the combination of magnetic separation with MOFs is a viable solution [19–22]. Magnetic MOF catalysts may not only possess high catalytic activity, but also show the advantages for recycling in a liquid system [19,23]. The separation and recovery of magnetic MOF catalysts can be easily performed by an external magnetic field, which is energy-saving, rapid and convenient. In particular, magnetic MOFs with core-shell structures have several advantages such as preventing ag gregation of magnetic cores, stabilized magnetic cores and high catalyst stability [24]. Nevertheless, up to date, magnetic MOF materials are mostly applied as powders or small particles with sizes ranging from nanometers to dozens of microns [20–24], which are difficult to handle and not promising for applications [25]. If these reported micrometer-scale magnetic MOF composites would be used for applications in the liquid phase, a significant difficulty in the separation still arises. Increasing efforts have been recently devoted to shape engineering of MOFs. MOFs powders have been shaped into spheres [26,27], fibers [28], thin films [29,30], foams [25], and other forms of structures [31–33]. At present, one commonly used method is the in situ growth of MOFs on chemically stable and cost-efficient substrates, including synthetic polymers [27,
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (R. Gl€ aser), [email protected], [email protected] (C.-j. Liu). https://doi.org/10.1016/j.micromeso.2020.110152 Received 24 November 2019; Received in revised form 9 February 2020; Accepted 5 March 2020 Available online 9 March 2020 1387-1811/© 2020 Elsevier Inc. All rights reserved.
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29] and porous supports (e.g., γ-Al2O3 [26], silica foam [34] or zeolites [35]). Porous activated carbons have been recognized as one of the widely used versatile materials for numerous practical applications, such as adsorption, filtration or as a substrate for catalytically active compounds. Surface modified activated carbons can be developed by various methods like acid or plasma treatment [36]. Prior to the growth of MOFs, surface modification of the substrate can be useful in order to facilitate the binding of metal ions to the substrate surface [29,37]. In this work, we report on the preparation of millimeter-scale mag netic spherical activated carbon-MOF core-shell composite beads (denoted as MOF@MSAC). Importantly, the size of the core-shell structured composite beads and the thickness of MOFs shells are widely adjustable. Moreover, this synthesis strategy can be expanded to both monometallic (e.g., ZIF-67 and HKUST-1) and bimetallic (e.g., CoZn-ZIF) MOFs. This synthetic method is expected to be extended for other materials for MOF shaping. In addition, ZIF-67@MSAC composite beads are investigated as model catalysts for Knoevenagel condensation. An excellent catalytic performance with remarkable convenience in separation, recovery and recycling has been confirmed.
methanol rinsing for 5 min. Then the beads were dried in ambient air at 60 � C for 0.5 h. After that, beads were immersed into a mixture of 2methylimidazole (Hmim) and methanol (20 mL, CHmim: CCo(NO3)2 ¼ 4:1) and 0.3 wt% PSS aqueous solution (20 mL) at 60 � C for 1 h. Then beads were separated by a magnet and washed three times with meth anol and dried in ambient air at 60 � C for 0.5 h. The treatment of beads by Co(NO3)2 and Hmim solutions was repeated four times. The resulting products are referred to as ZIF-67@MSAC. As a reference, pure ZIF-67 was prepared by mixing the Co(NO3)2 and Hmim methanol solutions in the absence of activated carbon beads. HKUST-1@MSAC and CoZnZIF@MSAC composite beads were prepared using a similar procedure as that described above. Further details can be found in the Supporting Information. 2.4. Characterization Powder X-ray diffraction (XRD) measurement was conducted on a Rigaku D/max-2500 with Cu Kα radiation (λ ¼ 1.54056 Å) using the finely crushed samples. The data were collected in the 2θ range of 5–70� with a step size of 0.02� and a time per step of 0.6 s. The microscopic features of the samples were characterized by scanning electron mi croscopy (SEM, S-4800) equipped with an energy-dispersive X-ray (EDX) analyzer. Thermogravimetric analysis (TGA) was conducted with a Netzsch STA 449 F3 system at a heating rate of 10 � C min 1 from 35 � C to 800 � C under an air flow of 100 mL min 1. Fourier transform infrared (FTIR) spectra were obtained on a ThermoNicolet Nexus 6700 system. The grinding powder samples were uniformly mixed with KBr powder and then pressed into a disk for FTIR analysis. For each spectrum, 64 scans were collected with a resolution of 4 cm 1 over the range 400–4000 cm 1. N2 sorption isotherms were recorded at liquid nitrogen temperature ( 196 � C) on a Quantachrome Autosorb-1C instrument. Prior to the measurement, the samples were outgassed under vacuum (<10 Pa) at 120 � C for 24 h. The specific surface area was calculated by the multipoint Brunauer-Emmett-Teller (BET) method. The total pore volume was estimated at P/P0 ¼ 0.99. The micropore and mesopore volumes were determined by the t-plot method and Barrett-JoynerHalenda (BJH) method, respectively. The magnetization curve was measured at room temperature on a commercial magnetometer with the superconducting quantum interference device (MPMS SQUID-VSM, Quantum Design) with applied magnetic fields between 1.5 T and 1.5 T. The metal contents of the composites were analyzed using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700x). The solution 1H and 13C NMR spectra were recorded in CDCl3 on a Bruker 400 MHz spectrometer. HRMS was performed on a SolariX70FT-MS Fourier transform ion cyclotron resonance mass spectrom eter (Bruker Daltonics, Germany) in electrospray ionization (ESI) posi tive ion mode.
2. Experimental section 2.1. Materials The spherical activated carbon (SAC, dsphere ¼ 0.45–0.50 mm) was obtained from Blücher GmbH (Germany). Poly(sodium 4-styrenesulfo nate) (PSS, average Mw is 70,000) and zinc nitrate hexahydrate (99%) were purchased from Alfa Aesar. 3-Aminopropyl-trimethoxysilane (APTES, 98%), ethyl cyanoacetate (99%) and n-dodecane (�99.5%, GC) were purchased from Shanghai Macklin Inc. Ferric acetylacetonate (98%), concentrated nitric acid (65 wt%) and 1, 3, 5-benzenetricarbox ylate (H3BTC, 98%) were obtained from Shanghai Aladdin Biological Technology Co., Ltd. Benzaldehyde (99%), 4-methylbenzaldehyde (99%), 4-bromobenzaldehyde (99%) and 4-nitrobenzaldehyde (99%) were purchased from Adamas Reagent, Ltd. Cobalt(II) nitrate hexahy drate (99%), 2-methylimidazole (Hmim, �99.0%), benzyl alcohol (�99.0%), methanol (99.9%), ethanol (99.8%) and dichloromethane (�99.5%) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. All materials were used without further purification. 2.2. Preparation of magnetic spherical activated carbon (MSAC) 0.1 g SAC was firstly treated with 2 mL of concentrated nitric acid for 10 h at 60 � C. Afterwards, the beads were washed with deionized water until the wash water was neutral, then dried in ambient air at 110 � C for 2 h. Subsequently, the beads were added to a solution of 1 g Ferric acetylacetonate in 20 mL of benzyl alcohol. The resulting mixture was transferred into a 50 mL polytetrafluoroethylene (PTFE)-lined autoclave and sealed for 48 h at 175 � C under static condition. After cooling, the magnetic spherical activated carbon beads (denoted as MSAC) were collected by decantation and sieving, washed with 20 mL ethanol three times to remove the remaining organics and subsequently dried in ambient air at 110 � C for 2 h. As a reference, Fe3O4 powder was prepared under the same conditions in the absence of activated carbon beads. The MSAC beads were immersed in a mixture of 100 mL of 95 wt% ethanol aqueous solution and 8 g L 1 APTES for 20 h. After decantation, the MSAC beads were rinsed two times with ethanol and dried in ambient air at 110 � C for 2 h. After that, the MSAC beads were immersed into 100 mL of 0.3 wt% PSS aqueous solutions for 1 h at 60 � C. After decantation, the MSAC beads were washed again two times with deionized water and then dried in ambient air at 110 � C for 2 h.
2.5. Catalytic reaction Prior to the reaction, ZIF-67@MSAC beads were pretreated under vacuum (�1 kPa) at 120 � C for 6 h. In a typical reaction, 50 mg of the pretreated ZIF-67@MSAC beads were added into the reaction mixture composed of benzaldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), and n-dodecane (0.88 mmol) as an internal standard in liquid ethanol (5 mL) in a 25 mL round-bottom flask. The resulting mixture was sealed and shaken in a shaker incubator at room temperature for a given time. The reaction progress (conversion based on the amount of benzalde hyde) was followed by withdrawing aliquots (10 μl) from the reaction mixture, diluted in 1 mL of ethanol. Subsequently, the diluted samples were analyzed on a gas chromatography (Agilent 7890B) equipped with a flame ionization detector and a HP-5 column (30 m � 0.25 mm � 0.25 μm). The bead catalysts were easily separated from the reaction mixture by a magnet. To remove the product remaining in the pores of the catalyst, the catalyst was washed with dichloromethane three times. The residual solvents were removed under vacuum (�1 kPa) at 80 � C for 10
2.3. Preparation of ZIF-67@MSAC beads 0.1 g Surface-modified MSAC beads were immersed in Co(NO3)2 methanol solution (20 mL, 0.2 M or 0.5 M) for 1 h at 60 � C, followed by 2
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h. The bead catalysts were then collected for reuse. The reaction solution was dried by removing the volatile components using a rotary evapo rator (RE-52AA, Yarong Co. Ltd., Shanghai, China) under reduced pressure at 50 � C. The solid products were purified by recrystallization from 95 wt% ethanol. NMR (1H and 13C) and high-resolution mass spectrometry (HRMS) were performed to further characterize the ob tained solid products. All the characterization data are listed in Sup porting Information.
styrenesulfonate (PSS). It has been reported that the amino group (-NH2) in silane coupling agents can combine with sulfonic acid group (-SO3H) in PSS through the electrostatic interaction/acid-base reaction, as depicted in Fig. 1B [29]. Confirmation of the surface modification was achieved by FTIR and EDX analysis (Figs. S2 and S3). Metal ions were then anchored on the surface of the MSAC through coordination with the sulfonate groups [29,41]. When the substrate was immersed into a mixed solution of 2-methylimidazole and PSS, the fixed metal ions could effectively coordinate with organic ligands, resulting in in situ formation of MOF nanoparticles. After the in situ growth of ZIF-67, the black MSAC beads changes to the characteristic purple color of ZIF-67 (Fig. 3A and B), directly indi cating the successful synthesis of ZIF-67 on MSAC beads. The charac teristic reflexes of ZIF-67 observed from the XRD pattern of ZIF67@MSAC beads (Fig. 2A, blue line) are in accordance with those re ported in the literature [42]. This clearly confirms the successful growth of ZIF-67 crystals. The formation of ZIF-67 crystals was further confirmed by the FT-IR analysis (Fig. S4). The presence of the charac teristic band at 422 cm 1 assigned to Co–N stretching vibration in the ZIF-67 framework [42,43], clearly proofs the formation of ZIF-67 crys tals on the MSAC. The Fe3O4 and ZIF-67 content in the ZIF-67@MSAC composite beads, calculated based on the ICP tests and weight changes before and after the growth, are 12.8 wt% and 25.1 wt%, respectively. The values are basically in line with the loading values calculated by TGA (Fig. S5). The textural properties of the ZIF-67@MSAC composite beads and their parent materials analyzed by N2 sorption isotherms are presented in Fig. 3C and Table 1. All samples clearly exhibit a combination of types I and IV with a hysteresis loop at p/p0 > 0.6. After depositing magnetic nanoparticles, MSAC has a lower specific surface area of 1120 m2 g 1 than that of initial SAC (~1677 m2 g 1). The Fe3O4 content in MSAC composite beads was found to be 17.0% as determined by ICP. Due to the negligible specific surface area of Fe3O4 (102 m2 g 1, Fig. S6), the specific surface area of SAC in the MSAC composite is estimated to 1349 m2 g 1. The value is significantly lower as compared to that of bare SAC. It indicates that part of magnetic nanoparticles is penetrated into the pores of SAC, which agrees with the EDX mapping results in Fig. 2B. As shown in Fig. 2B, the Fe species are concentrated on the surface of SAC and penetrates into about a 10-μm layer of SAC. In addition, the ZIF67@MSAC composite beads show reduced specific surface area and micropore volume in comparison to MSAC and bare ZIF-67 (Table 1). This can be attributed to the formation of crystals generated at the interface of MOFs and carbon support and even in the support pore channel [35], leading to the partial pore blockage of the carbon support. An advantage, however, is that the part of ZIF-67 nanocrystallites are firmly embedded in the pore of SAC, which is similar to the work re ported earlier [44]. In order to investigate the morphology of the magnetic composite beads, SEM studies were performed. ZIF-67@MSAC beads remain an almost spherical shape of active carbon beads with particle sizes be tween 0.4 and 0.5 mm (Fig. 4A and Fig. S7). The size distribution of SAC beads is plotted in Fig. S7. The magnified image of ZIF-67@MSAC sur face revealed the growth of rhombic dodecahedral ZIF-67 crystals (Fig. 4B). From the cross-section images (Fig. 4C–D), a clear interface between the ZIF-67 dense layer and the activated carbon layer was observed, indicating the growth of the ZIF-67 crystals on the outer surface of entire MSAC. The thickness of ZIF-67 layer is approximately 11 μm. The EDX mapping of ZIF-67@MSAC beads further illustrates the growth of the ZIF-67 crystals on the outer surface of MSAC (Fig. 4E). These results show that the finally formed ZIF-67@MSAC composite beads are composed of a MSAC core and a MOF shell, clearly demon strating the formation of a core-shell structure. It should be noted that the surface modification of MSAC is essential for the growth of the MOF shell layer. The images visually indicate the obvious difference of the MOF shell layer on the modified MSAC and unmodified MSAC, as shown in Fig. S8. The SEM images further confirm that only a small amount of
3. Results and discussion 3.1. Characterization of MSAC and ZIF-67@MSAC The preparation procedure of the ZIF-67@MSAC composite beads is shown in Fig. 1A. The SAC (SBET ¼ 1748 m2 g 1, Vtotal ¼ 2.0 cm3 g 1, dparticle ¼ 0.45–0.50 mm, Blücher GmbH) was firstly pretreated with nitric acid to increase surface acidic oxygen-containing groups to facil itate grafting silane coupling agents. Magnetite nanoparticles readily deposit on the SAC support via a facile one-pot solvothermal reaction [38]. The successful deposition of Fe3O4 on SAC is confirmed by the results of magnetic separation test and XRD patterns. The MSAC beads have a fast optical response to the external magnetic field (Fig. S1). The XRD reflexes (2θ) of the MSAC at 18.3� , 30.1� , 35.4� , 37.1� , 43.1� , 53.5� , 57.1� and 62.6� are in good agreement with the standard XRD data of cubic Fe3O4 (JCPDS 19–0629) (Fig. 2A, black line). The average Fe3O4 nanoparticle size of MSAC can be estimated to be 13 nm by using the Scherrer equation from the (311) reflection (Table S1). The value is similar to those reported in the literature [38]. Besides, Fourier trans form infrared (FTIR) spectroscopy analysis provides further evidence of the formation of magnetite nanoparticles (Fig. S2). The broad band at 579 cm 1 corresponds to the vibration of the Fe–O bonds in the crys talline lattice of Fe3O4 [39,40]. In order to further investigate the dis tribution of Fe element in the MSAC, energy-dispersive X-ray (EDX) mapping was conducted. As shown in Fig. 2B, the magnetic nano particles are evenly distributed over the surface of SAC. A small amount of the magnetic nanoparticles penetrates below the surface. Afterwards, the MSAC beads were modified successively with 3-ami nopropyl-trimethoxysilane (APTES) and poly(sodium 4-
Fig. 1. Schematic illustration of (A) synthesis procedure for the ZIF-67@MSAC and (B) the applied force between ZIF-67 and MSAC. 3
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Fig. 2. (A) XRD patterns of MSAC and ZIF-67@MSAC composite beads. (B) SEM images of MSAC and the corresponding EDX mapping images of C and Fe elements (the top: surface morphology; the bottom: cross-section morphology).
3.2. Tuning the thickness and composition of the MOF shell The size of magnetic core-shell structured MOF beads is controllable by using MSAC core with different sizes (Fig. S9). The SAC with diameter ranging 0.6 from 1 mm was used as core to prepare magnetic core-shell structured MOF composite beads (Fig. S9A). The characteristic purple color and magnetic response behavior suggest the successful fabrication of magnetic composite beads (Fig. S9B). The SEM images further provide evidence for the formation of ZIF-67 shell on the surface of MSAC (Figs. S9C–D). These results indicate that it is feasible to tune the size of magnetic composite beads by simply changing the carbon core size. In addition, the MOF shell thickness of ZIF-67@MSAC beads is found to be tunable. As clearly shown in Fig. S10, the thickness of the MOF shell increases with the increasing number of assembly cycles. It suggests that the MOF shell can be tuned by varying the assembly cycle number. It is also found that the thickness of the MOF shell can be tuned by varying the precursor concentration of the MOF to be synthesized. For example, the MOF shell thickness of the ZIF-67@MSAC beads prepared at 0.2 mol L 1 cobalt ion is approximately 1.5 μm after four-time assembly cycles (Fig. S11), whereas the MOF shell thickness of ca. 11 μm is achieved with the same assembly cycle number, as can be seen from SEM results (Fig. S10). These results demonstrate that the MOF shell thickness of the ZIF-67@MSAC beads can be effectively regulated by altering the as sembly cycle number and the precursor concentration. Importantly, the synthetic approach is applicable to prepare shells with other MOF-type materials, such as HKUST-1 or bimetallic CoZn-ZIF [43]. The formation of HKUST-1 and CoZn-ZIF shells is confirmed by XRD patterns (Fig. S12). All composite beads have similar spherical morphologies as the initial SAC beads. The octahedral HKUST-1 and rhombic dodecahedron CoZn-ZIF crystals can be seen on the surface of the composite beads, as revealed by SEM characterization (Fig. S13). All these results revealed that the HKUST-1@MSAC and CoZn-ZIF@MSAC composite beads were successfully prepared. Further, the EDX elemental mapping confirms that the HKUST-1@MSAC and CoZn-ZIF@MSAC composite beads possess the MSAC core-MOF shell structures (Figs. S14 and S15).
Fig. 3. (A–B) Optical micrographs of ZIF-67@MSAC composite beads. The horizontal length of the pink bar is 0.25 mm. (C) N2 sorption isotherms of the SAC, MSAC and ZIF-67@MSAC composites. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) Table 1 Textural properties of the composites and their parent materials determined by N2 sorption. Sample
SBET [m2 g 1]
Vtot [cm3 g 1]
Vmicro [cm3 g 1]
Vmeso [cm3 g 1]
mares [wt %]
ZIF-67 SAC MSAC ZIF67@MSAC
1324 1677 1120 976
0.72 1.39 1.22 0.83
0.60 0.54 0.36 0.27
n.d. 0.10 0.08 0.06
36.3 n.d. 21.8 25.8
3.3. Analysis of magnetic properties The magnetic properties of the ZIF-67@MSAC composite beads and synthetic Fe3O4 powder have been investigated using a vibrating sample magnetometer at room temperature (Fig. 5A). The magnetic hysteresis loops show no obvious remanence or coercivity at room temperature. It suggests the superparamagnetic feature, which is essential for the magnetic separation. The saturation magnetization value of pristine Fe3O4 and ZIF-67@MSAC beads are estimated to be 66.6 emu g 1 and 8.2 emu g 1, respectively. According to these saturation magnetization values, Fe3O4 content in the ZIF-67@MSAC composite beads can be
n.d.: not determined. a Residual mass (mres) obtained from TGA at 800 � C.
separate ZIF-67 crystals and crystal islands rather than a continuous MOF shell layer is formed if the MSAC surface was not treated with APTES and PSS (Fig. S8).
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Fig. 4. SEM images of (A–B) surface morphologies and (C–D) cross-section morphology of ZIF-67@MSAC after four times of ZIF-67 growing cycles. (E) The elemental mapping images of ZIF-67@MSAC for C, Fe, and Co.
estimated to be about 12.3 wt%, which is basically consistent with the loading content obtained by TGA and ICP results. Taking advantage of the magnetic properties, the magnetic composite beads are expected to be easily separated from the solution. As can be seen from the inset in Fig. 5A and Video S1, the ZIF-67@MSAC beads are attracted toward the magnet within a few seconds and move flexibly following the magnet. This clearly demonstrates the convenient separation of the ZIF67@MSAC beads using an external magnetic field. In addition, we compared their dispersion state of the ZIF-67@MSAC composite beads and as-synthesized Fe3O4 powder when they were separated from ethanol by a magnet (Fig. 5B). ZIF-67@MSAC composite beads are dispersed individually. On the contrary, Fe3O4 powder looks like the mud and forms a pile of powder after drying naturally. It is important to mention that the obvious mass loss of ZIF-67 was observed in the weighting and recycling process. As shown in Fig. S16, powder ZIF-67 adhered to the weighing paper and glass container walls due to strong electrostatic forces. It can be concluded that millimeter-scale ZIF67@MSAC beads are more convenient in separation and recycling than Fe3O4 and ZIF-67 powder particles. Supplementary video related to this article can be found at https:// doi.org/10.1016/j.micromeso.2020.110152
ZIF-67@MSAC magnetic beads. The reaction was carried out using ethanol as a solvent at room temperature. Fig. 6A shows the catalytic results of the Knoevenagel condensation over various catalysts. Only trace conversion (<5%) of benzaldehyde was observed in the absence of the catalyst. In contrast, ZIF-67 shows a high conversion (75%) after 3 h. It suggests that ZIF-67 can effectively catalyze the Knoevenagel condensation reaction [45]. Nevertheless, it was found that ZIF-67@MSAC achieves a higher catalytic activity than pure ZIF-67 with 86% conversion after 3 h and up to 97% conversion after 6 h. Without anchored ZIF-67 nanoparticles, the activity of MSAC is significantly lower than that of ZIF-67@MSAC and ZIF-67 under the same reaction condition and gave only 16% conversion after 6 h. These results provide strong evidence that ZIF-67 acts as the catalytically active component for this reaction. The higher activity of ZIF-67@MSAC than pristine ZIF-67 is mainly attributed both to a small amount of free amino groups of MSAC and to the advantageous stabilizing effect of the support. The existence of free terminal amino groups was confirmed by FTIR analysis (Fig. S2). The amine functional groups have been reported previously to catalyze the Knoevenagel condensation reaction to different extents [46, 47]. In general, the active sites located at the external surface of ZIF materials are responsible for the catalysis [48]. As expected, smaller size crystals normally exhibit higher catalytic activity because of the increased external surface of smaller crystals [12,49]. Compared to the ZIF-67 nanoparticles anchored in the ZIF-67@MSAC, the freestanding ZIF-67 nanoparticles much more easily form bulkier crystal agglomer ates, resulting in a smaller number of exposed active sites. As a result, ZIF-67 exhibits lower catalytic activity than ZIF-67@MSAC. The similar
3.4. Evaluation of catalytic activity To investigate the applicability of ZIF-67@MSAC beads, the Knoe venagel condensation reaction of benzaldehyde and ethyl cyanoacetate was chosen as a model reaction to evaluate the catalytic activity of the 5
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particles. In contrary, no obvious color can be observed in the decant ing solutions separated from the ZIF-67@MSAC system, indicating the strong adhesion of ZIF-67 to MSAC beads. To further evaluate the capability of ZIF-67@MSAC, the Knoevena gel condensation of several benzaldehyde derivatives having different substituents with ethyl cyanoacetate was carried out employing ZIF67@MSAC beads as catalysts. It was found that all the reactions pro ceed smoothly to give the corresponding products (Table 2). The re actions can proceed well for benzaldehyde derivatives bearing electronwithdrawing substituents such as -Br, and –NO2, with 96% conversion after 5 h for the case 4-bromobenzaldehyde and 99% conversion after 1 h for the case of 4-nitrobenzaldehyde, respectively (Table 2, entries 1–2). As for benzaldehyde derivatives with electron-donating sub stituents such as –CH3, the ZIF-67@MSAC beads can also give the product in 93% conversion, but with a longer reaction time of 6 h (Table 2, entry 3). These results are consistent with the expected higher reactivity of benzaldehyde derivatives containing electron-withdrawing groups than those with electron-donating groups [12,50]. Besides, as Table 2 The Knoevenagel reaction employing various aldehydes.a
Entry
Fig. 5. (A) Magnetization curve of pristine Fe3O4 nanoparticles and ZIF67@MSAC composite beads. The Inset shows the separation of ZIF-67@MSAC under the external magnetic field. (B) The dispersion state of ZIF-67@MSAC composite beads and magnetic powder separated by a magnet from ethanol.
dispersing and stabilizing functions from the support have been recently reported in the literature [49]. In addition, we added ZIF-67 and ZIF-67@MSAC into ethanol and observed the color of decanting ethanol after shaking the mixture for 6 h. As showed in Fig. S17, it can be seen clearly that the solution shows purple due to the dispersion of ZIF-67
Time/h
Conversion/%
Yield/%
1
Product
5
96
95
2
1
99
99
3
6
93
92
4
6
97
96
a Reaction conditions: aldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), ethanol (5 mL), ZIF-67@MSAC (50 mg, 3 mol% Co), room temperature under atmospheric conditions.
Fig. 6. (A) Catalytic performances of various catalysts for the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate. Reaction conditions: benzaldehyde (1.9 mmol), ethyl cyanoacetate (3.8 mmol), ethanol (5 mL), catalysts (50 mg, 3 mol% Co), room temperature under atmospheric conditions. For the MSAC and ZIF-67 cases, 50 mg and 13 mg solid catalysts were used, respectively. (B) Catalyst recycling studies of ZIF-67@MSAC catalyst. (C) XRD patterns and (D) magnetization curves of the fresh and the fourth reused ZIF-67@MSAC. 6
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reported in the literature, steric and size selective effects were observed over the MOF catalysts [51,52]. This substrate selectivity of MOF plays an important role in catalyst design. Wang et al. [53] reported the MOF-catalyzed Knoevenagel condensation, providing evidence for the substrate selectivity. We expect that the substrate selectivity of MOFs remains in the magnetic MOF composite beads.
Appendix A. Supplementary data
3.5. Recycling test
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2020.110152. References
Recyclability is an important and essential feature for solid catalysts. Herein, the ZIF-67@MSAC composite beads were examined for recov erability and reusability in the Knoevenagel condensation of benzalde hyde with ethyl cyanoacetate. After the completion of the reaction, the composite bead catalysts can be easily and conveniently separated from the solution. It was found that the beads can be recovered and reused without a significant degradation in catalytic activity (Fig. 6B). The XRD, SEM and magnetization curves were conducted to investigate the structural stability of the reused composite beads catalysts. As shown in Fig. 6C, the XRD pattern of the fourth reused catalyst confirms that ZIF67@MSAC maintains its initial structure. The saturation magnetization value of reused composite beads is basically consistent with that of the fresh ZIF-67@MSAC (Fig. 6D), suggesting no obvious mass loss of the composites. The cross-section SEM image and optical micrograph of the fourth reused ZIF-67@MSAC further show a good stability of the coreshell structured composites (Fig. S18). 4. Conclusions This study reports a facile and general approach to fabricate sizetunable spherical activated carbon-MOF core-shell structured magnetic composite beads in the millimeter scale. The obtained beads show excellent catalytic performance for the Knoevenagel condensation re action. The size of the MOF@MSAC beads can be easily tuned by changing the size of the activated carbon core. Furthermore, the struc ture, composition, and thickness of the MOF shell are tunable by design. Different types of activated carbon-MOF magnetic composite beads, including HKUST-1@MSAC, ZIF-67@MSAC and CoZn-ZIF@MSAC, were successfully prepared by varying the MOF precursor. Compared to other solid catalysts, the resulting MSAC-MOF core-shell structured magnetic beads combine the superior properties of MOFs with the unique mag netic responsivity. Capitalizing on its unique structural features, such as large specific surface area, nanoscale active phase, magnetic respon sivity and the dispersing and stabilizing effects of carbon support, the ZIF-67@MSAC displays high activity, recyclability, easy separation and remarkable convenience in the separation and recovery processes. Furthermore, the magnetic composite beads are reusable without a significant loss of catalytic activity. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Wenlong Xiang: Conceptualization, Methodology, Investigation, Writing - original draft. Sebastian Gebhardt: Conceptualization, €ser: Conceptualization, Super Methodology, Investigation. Roger Gla vision, Resources, Writing - review & editing. Chang-jun Liu: Concep tualization, Supervision, Resources, Writing - review & editing. Acknowledgments This work was supported by the National Natural Science Foundation of China (#21536008 and #21621004). 7
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