Mesoporous Zeolitic Imidazolate Frameworks

Abstract

Open AccessCCS ChemistryCOMMUNICATION5 Sep 2022Mesoporous Zeolitic Imidazolate Frameworks Zhi Xu, Le Li, Xiaoxia Chen, Chenhong Fang and Guyu Xiao Zhi Xu Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Le Li Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Xiaoxia Chen Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 , Chenhong Fang Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 and Guyu Xiao *Corresponding author: E-mail Address: [email protected] Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240 https://doi.org/10.31635/ccschem.021.202101430 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The zeolitic imidazolate frameworks (ZIFs) with three-dimensional periodic micropores encounter the barrier of mass transfer in the diffusion-limited processes. To solve this problem, fabricating the ZIFs combined intrinsic micropores with ordered mesopores (mZIFs) is highly desirable. Herein, mZIFs were synthesized for the first time, achieved by employing a polymer-micelle template strategy. The chemical structure of mZIFs could be readily regulated via the coordination of different transition-metal ions and imidazolate linkers, whereas their mesopore size could be tuned by the length of the hydrophobic block within polymer micelles. mZIFs remarkably accelerated the diffusion-limiting processes such as catalysis because of the hierarchical pore structures, accessible active sites, and high accommodation of the reactants. In addition, their pyrolytic carbons inherit the original pore features; thus, exhibiting excellent electrocatalytic performances for the oxygen reduction reaction. Download figure Download PowerPoint Introduction The zeolitic imidazolate frameworks (ZIFs) are porous crystals constructed from the tetrahedral coordination of transition-metal ions and imidazolate bridges, showing the zeolite-like topological structures, and belong to a subclass of metal–organic frameworks (MOFs).1 ZIFs have attracted ever-increasing interest in recent years because of facile synthesis, high porosity, large specific surface area, tunable properties, and high thermal/chemical stability.2,3 Based on these advantages, ZIFs exhibit promising applications in the fields of molecular sieve and gas separation/storage.4,5 Besides, owing to the natural metal-N4 coordination, ZIFs and their derivates have been extensively developed as superior catalysts.6–12 However, ZIF catalysts are restricted to the micropores, which usually lead to severe flooding due to capillarity action, thereby hindering reactant accessibility toward the active sites.13,14 As a result, ZIFs generally encounter the barrier of mass transfer in the diffusion-limited processes because of the intrinsic microporous structure.15–18 Therefore, much effort has been devoted to constructing the hierarchical pore structures of ZIFs by incorporating the mesopores or macropores, including templating,19–23 etching,24,25 coating,26,27 trapping,28 and competing coordination.16 However, these attempts fail to yield the ordered distribution of the meso/macroporous structures.29 Superior to the random porosity frequently met in the nanomaterials, the ordered meso/macropore structures contribute to the effective utilization of the smooth diffusion pathways, high surface area, and evenly dispersed active sites.30–33 Recently, ordered macroporous ZIFs were designed and synthesized successfully using the polystyrene-sphere monolith template.15,34 Strikingly, the introduction of ordered macropores facilitates the preparation of the hierarchically porous ZIFs, which exhibit much better overall performances in the diffusion-limited catalytic reactions compared with conventional ZIFs.34–39 It was anticipated from these studies that the mesopores were incorporated into the inherent microporous ZIFs to construct the ordered mesoporous ZIFs (mZIFs). It is predicted that these mZIFs would display superior physical and chemical properties for the diffusion-limited processes.40 However, such mZIFs have not been practically achieved to date. Therefore, it is imperative to develop a reliable strategy to prepare mZIFs. Herein, an amphiphilic polymer-micelle template was adopted to fabricate mZIFs. As a demonstration, the preparation process of the ordered mesoporous ZIF-8 (mZIF-8) is shown in Figure 1a. First, the block polymer of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) was assembled into a spherical micelle with a hydrophobic PS-block as core and a hydrophilic PEO-block as corona in a 2-methylimidazole (2-MI) solution. The 2-MI molecules were adsorbed on the PEO coronae of the PS-b-PEO micelles via inter- and intra-molecular hydrogen bonding.41 Then Zn(NO3)2·6H2O was added, and the zinc ions chelated with the PEO coronae to induce the nucleation and crystal growth of ZIF-8 around the PS core42 to form the [email protected] assemblies. After removing the PS-b-PEO micelle templates with tetrahydrofuran (THF) as a solvent, the mZIF-8 with a rhombic dodecahedron morphology was obtained. To the best of our knowledge, our laboratory is the first to fabricate the ordered mZIFs. Moreover, the chemical structure and mesopore size of mZIFs were readily tuned, suggesting that this strategy could be employed to synthesize various mZIFs conveniently. Figure 1 | Synthesis and structure of mZIF-8. (a) Schematic illustration of mZIF-8 preparation, (b) SEM image, (c) TEM image, (d) model diagrams from three typical perspectives and corresponding SEM/TEM images, (e) XRD pattern, (f) SAXS profile, and (g) N2 adsorption/desorption isotherms and Barrett–Joyner–Halenda (BJH) pore-size distribution. Download figure Download PowerPoint Results and Discussion Following the preparation procedure in Figure 1a, mZIF-8 was fabricated with the PS76-b-PEO114 spherical micelles as the template ( Supporting Information Figure S1). The scanning electron microscopy (SEM) image (Figure 1b) revealed that mZIF-8 displayed a typical rhombic dodecahedron morphology with an average size of 250 nm, according to the Wulff's construction rule, reviewed by Troyano et al.2 Uniform mesopores could be clearly observed on each of the 12 equiv rhombic faces of mZIF-8 (Figure 1b). The pore diameter and the wall thickness were estimated to be 13 and 9 nm, respectively. Images obtained from transmission electron microscopy (TEM) (Figure 1c) suggested that the 13 nm diameter mesopores were evenly distributed inside the mZIF-8 crystal, consistent with the diameter of PS76-b-PEO114 micelles ( Supporting Information Figure S1). To better show the rhombic dodecahedron morphology and mesoporous structure of mZIF-8, three typical models of the mZIF-8 crystal and the corresponding SEM and TEM images from three viewpoints are shown in Figure 1d. Such morphologies indicated that the mesopores were uniformly dispersed in the mZIF-8 crystal. Element mapping images reveal the homogeneous dispersion of the C, N, and Zn elements within mZIF-8 ( Supporting Information Figure S2). The X-ray diffraction (XRD) pattern of mZIF-8 agreed with the simulated profile of conventional ZIF-8 (cZIF-8; Figure 1e), verifying that mZIF-8 possessed the pure-phase crystalline structure of ZIF-8. The small-angle X-ray scattering (SAXS) was also employed to characterize mZIF-8. Its SAXS profile in Figure 1f shows a peak centered at 0.27 nm−1, indicating a mesopore-to-mesopore distance of 23 nm (calculated by the equation: d = 2π/q), matching the SEM results (Figure 1b). These pieces of evidence illustrated that the mesopores within mZIF-8 were ordered periodically.43,44 The particle size of mZIF-8 was small, so there was no apparent highly ordered diffraction peak of the mesopores in its SAXS profile.45,46 As shown in Figure 1g, the N2 adsorption/desorption isotherms of mZIF-8 presented a hysteresis loop, indicating the presence of the mesopores.41mZIF-8 exhibited high specific surface area (1562 m2 g−1) and external surface area (300 m2 g−1). The pore-size distribution curve of mZIF-8 showed a peak centered at 12 nm (inset of Figure 1g), consistent with the TEM results. In contrast to mZIF-8, cZIF-8 only showed microporous features ( Supporting Information Figure S3 and Table S1). The PS-b-PEO content in the reaction mixtures markedly affected the formation of the mesopores within the ZIF-8 crystals. The morphology of the ZIF-8 crystals synthesized with different PS-b-PEO content is displayed in Supporting Information Figure S4. Only conventional ZIF-8 crystals could be prepared in the absence of PS-b-PEO, implying that it is indispensable regarding the formation of the mesopores within ZIF-8 ( Supporting Information Figure S4a). When a small amount (3 mg) of PS-b-PEO was added to the reaction system, the microporous and mesoporous ZIF-8 particles coexisted ( Supporting Information Figure S4b). Interestingly, when a moderate amount (10 mg) of PS-b-PEO was added, uniform mesopores were apparent within each ZIF-8 particle ( Supporting Information Figure S4c). Further, increasing the PS-b-PEO content to 20 mg, mZIF-8 crystals with reduced size were acquired, accompanied by a partially destroyed shape ( Supporting Information Figure S4d), attributable to the fast nucleation and crystal growth of ZIF-8 at high PS-b-PEO concentration.42,47 Benefiting from the controllability of the PS-b-PEO molecular weight, the mesopore size of mZIF-8 could be readily tuned with varying the PS-block length. When the PS58-b-PEO114 with a short hydrophobic PS-block length of 58 units was served as a template, the mesopore size of mZIF-8 decreased to 10 nm (Figure 2a). By contrast, when the PS-block length increased to 97 units, the mesopore size of mZIF-8 increased to 17 nm (Figure 2b). The mesopore size of the above mZIF-8 particles was consistent with the PS-b-PEO micelles ( Supporting Information Figure S5). However, if the PS-block length increased to 252 units, the mesopores were randomly distributed inside the ZIF-8 particles ( Supporting Information Figure S6). This was ascribed to the reason that PS252-b-PEO114 involved lower PEO content than the other three PS-b-PEO, and thus, failed to stabilize enough zinc ions and imidazolate linkers to mediate the crystal growth of ZIF-8 around the oversized PS core.42,48 Figure 2 | Preparation of mZIFs by regulating polymer-micelle templates, metal ions, and linkers. (a and b) TEM images of mZIF-8 templated by (a) PS58-b-PEO114 and (b) PS97-b-PEO114. (c) TEM image and (d) XRD pattern of mZIF-67. (e) TEM image and (f) XRD pattern of mZIF-90. Download figure Download PowerPoint To validate the universality of this strategy, two other mZIFs with different chemical compositions were synthesized by similar procedures, including the ordered mesoporous ZIF-67 and ZIF-90 (mZIF-67 and mZIF-90). mZIF-67 was synthesized by replacing zinc nitrate with cobalt nitrate, according to the preparation procedure of mZIF-8. The TEM image in Figure 2c indicates that the mesopores were evenly dispersed in the crystal particles of mZIF-67. Its element mappings showed that the C, N, and Co elements were uniformly distributed ( Supporting Information Figure S7). The XRD pattern of mZIF-67 coincided with the simulated outcome of ZIF-67 (Figure 2d), implying that mZIF-67 possessed the pure crystal phase of ZIF-67. Apart from the coordination ions, the chemical structure of mZIFs could also be tuned using different imidazolate linkers. For example, mZIF-90 was prepared by replacing 2-MI with imidazole-2-carboxaldehyde as the linker, following the synthesis steps of mZIF-8. The TEM image in Figure 2e indicated that the mesopores were evenly dispersed in the mZIF-90 particles. The crystallographic structure and chemical composition of mZIF-90 were confirmed by the simulated XRD pattern (Figure 2f) and element mappings ( Supporting Information Figure S8), respectively. These results revealed that this strategy could serve as a powerful tool for constructing plentiful mZIFs with various chemical compositions and different mesopore sizes. Since the meso/micropore structures promote the mass transfer and expose numerous active sites, mZIFs could accelerate the diffusion-limited catalytic reactions.49 As a demonstration, the catalytic performance of mZIF-8 was evaluated in a Knoevenagel reaction between malononitrile and benzaldehydes (Figure 3a). For comparison, the catalytic performance of cZIF-8 was also investigated under the same conditions. The reaction of benzaldehyde was completed after 2 h under the catalysis of mZIF-8, whereas it took 5 h for the benzaldehyde conversion reaction to complete under the catalysis of cZIF-8 (Figure 3b). It is well-known that this reaction occurs on the external surface but not the micropore surface of the ZIF-8 particles.50 Moreover, mZIF-8 held a much larger external surface area (300 m2 g−1) than that of cZIF-8 (118 m2 g−1) ( Supporting Information Table S1). Thereby, the former showed much better catalytic activity than the latter. The Knoevenagel reaction between malononitrile and other benzaldehydes catalyzed by mZIF-8 and cZIF-8 was also explored. As shown in Figure 3c, mZIF-8 exhibited much higher catalytic activity than cZIF-8 for all these reactions. The gap of the relative catalytic activity (ratio of reaction conversion) between mZIF-8 and cZIF-8 gradually broadened as the molecular dimension of the benzaldehydes enhanced. These testings demonstrated that the mesopores of mZIF-8 greatly accelerated the catalytic reactions involving bulky-molecule because these mesopores acted as smoother diffusion pathways and possessed more accessible active sites for the reactants compared with the micropores of cZIF-8. Figure 3 | Knoevenagel reaction catalyzed by mZIF-8 or cZIF-8. (a) Reaction scheme. (b) Conversion of benzaldehyde as a function of reaction time. (c) Comparison of the conversion catalyzed by mZIF-8 or cZIF-8 for different benzaldehydes. Download figure Download PowerPoint The pore structures, high specific surface area, and accessible active sites of ZIFs could be inherited to their pyrolytic carbons.23,51mZIF-8 was initially pyrolyzed and transformed into mesoporous nitrogen-doped carbon (mNC). Its SEM image revealed that mNC kept the rhombic dodecahedron morphology decorated with uniform mesopores (Figure 4a). The mesopores with a diameter of 9 nm could be confirmed in the TEM image of mNC (Figure 4b), consistent with the result of nitrogen physisorption measurement in Supporting Information Figure S9. The mesopore size of mNC was less than that of mZIF-8 (∼13 nm) because of pyrolysis-induced shrinkage. The SAXS profile of mNC exhibited a peak centered at 0.34 nm−1, implying that its mesopores were periodic and the mesopore-to-mesopore distance was 18 nm (Figure 4c). For comparison, cZIF-8 was calcined to prepare the conventional nitrogen-doped carbon (cNC), which showed similar chemical compositions to mNC ( Supporting Information Figure S10 and Table S2) but negligible mesopores ( Supporting Information Figure S11). Their electrocatalytic performances for the oxygen reduction reaction (ORR) were evaluated by linear sweep voltammetry (LSV) in O2-saturated 0.1 M KOH using commercial Pt/C as the benchmark (Figure 4d).52mNC, cNC, and Pt/C displayed a half-wave potential (E1/2) of 0.85, 0.73, and 0.84 V, respectively. Meanwhile, they exhibited a limiting current density (JL) of 5.59, 3.34, and 5.42 mA cm−2, respectively. Therefore, mNC exhibited the best ORR catalytic activity among them. Furthermore, their Tafel slopes in Figure 4e revealed that mNC showed a higher kinetic rate for ORR than cNC and Pt/C. Additionally, the rotating ring-disk electrode (RRDE) measurements were further performed to assess their catalytic pathway and selectivity. The electron transfer number (n) of mNC was calculated to be 3.90 (Figure 4f), whereas that of cNC and Pt/C is 3.37 and 3.92, respectively. On the other hand, the H2O2 yield of mNC was close to that of Pt/C and much lower than that of cNC in the range of 0.2–0.8 V. These results indicated that mNC followed a favorable 4e− pathway for ORR.52 After 5000 continuous cycles, the E1/2 decay of mNC was negligible, while that of Pt/C was quite obvious ( Supporting Information Figure S12), illustrating that mNC also featured an excellent long-term catalytic stability. As a result, thanks to the superior meso/micropore structures inherited from mZIF-8, mNC exhibited much better ORR catalytic performances than cNC and most metal-free doped carbons ( Supporting Information Table S3). Figure 4 | Structure characterization and ORR catalytic performances of mNC. (a) SEM image, (b) TEM image, and (c) SAXS profile of mNC. (d) LSV curves, (e) Tafel plots, and (f) electron transfer number (n) and H2O2 yield of mNC and the reference catalysts. Download figure Download PowerPoint Conclusion The ordered mZIFs were synthesized for the first time, achieved by the amphiphilic PS-b-PEO micelle template strategy. The chemical structure of mZIFs was adjusted readily by coordinating different transition-metal ions and imidazolate linkers. Meanwhile, the mesopore size of mZIFs was tunable by changing the length of the PS-block within PS-b-PEO. mZIFs remarkably accelerated the diffusion-limited processes such as catalytic Knoevenagel reaction because of the ordered meso/micropore structures, accessible active sites, and high accommodation of the reactants. Additionally, their pyrolytic carbons inherited the pore-structure features of mZIFs, exhibiting excellent electrocatalytic performances for ORR. This micelle-template strategy paves a facile avenue to fabricate the ordered mZIFs. Moreover, this strategy could be extended to synthesize the ordered mesoporous MOFs (mMOFs). These achievable ordered mZIFs and mMOFs could transfer their pore-structure features to the corresponding pyrolyzed carbons. Further, these ordered mesoporous materials exhibit promising application prospects due to their ability to markedly promote diffusion-limited processes. Supporting Information Supporting Information is available and includes an experimental section, XPS spectra, TEM images, N2 physisorption isotherms, LSV curves, structural parameters, and electrochemical performances. Conflict of Interest There is no conflict of interest to report. Funding Information The authors thank the National Natural Science Foundation of China (nos. 21774073 and 51690151) and the Special Fund for Science and Technology of Guangdong Province (no. 1908 0515 5540 379) for their financial support. Acknowledgments The authors appreciate Dr. Hui Pan at Shanghai Jiao Tong University for the help with SAXS measurements. References 1. Phan A.; Doonan C. J.; Uribe-Romo F. J.; Knobler C. 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