Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Fast Synthesis of Hybrid Zeolitic Imidazolate Frameworks (HZIFs) with Exceptional Acid−Base Stability from ZIF‑8 Precursors Ying Wang, Fei Wang,* and Jian Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
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S Supporting Information *
ABSTRACT: Facile, fast methods have been developed to synthesize high crystalline hybrid zeolitic imidazolate frameworks (HZIF-1Mo) from ZIF-8 precursors via reflux and solvent-free method. Photocatalysis results in the combination of powder XRD and BET experiments, proving that the obtained product from ZIF-8 was highly crystalline and defect-free. Furthermore, HZIF-1Mo exhibits better strong acid stability than that of ZIF-8 and is among the highest level of MOFs.
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Zn4(mim)6TO4].30 HZIFs integrate compositional and structural features of zeolites and ZIFs by combining TO4 tetrahedra with zinc-imidazolate units, which own both the merits of zeolites and ZIFs, for example, possession of catalytic active TO4 (T = Mo6+ or W6+) sites of zeolites and the high porosity of ZIFs. However, very little research attention has been paid so far to such impressive materials. The main reason may be attributed to its complex reaction system, long reaction time, high reaction pressure, and temperature. For example, HZIF-1Mo was first reported from reaction of zinc acetate (Zn(CH3COO)2·2H2O), 2-methylimidazole (2-mim), and molybdic acid (H 2 MoO 4 ) in N,N-dimethylformamide (DMF) in a 23 mL Teflon-lined airtight reactor at 160 °C. We noticed that the structure similarity between ZIF-824 and HZIF-1 is the presence of truncated octahedral cages of [Zn24(2-mim)36] (SOD cage). Differently, for ZIF-8, such SOD cages link each other by sharing their four-ring and sixring windows to form SOD topology, while the inorganic MoO4 or WO4 units isolate and connecte SOD cages to generate the sdt-type framework of HZIF-1. Inspired by this, we reported here the facile, fast synthesis of HZIF-1Mo from ZIF-8 precursors. Highly crystalline HZIF-1Mo was obtained in pure phase via heating the mixture of ZIF-8 and H2MoO4 in DMF for a short time (50 min). In addition, solvent-free synthesis of HZIF-1Mo from ZIF-8 can also be realized. More importantly, as-synthesized HZIF-1Mo exhibits better strong acid stability than that of ZIF-8 and is among the highest level of MOFs.
INTRODUCTION Porous coordination polymers (PCPs) or metal−organic frameworks (MOFs) are a new class of crystalline porous Scheme 1. Synthesis Routes of HZIF-1Mo from ZIF-8
materials consisting of metal ions or clusters coordinated by organic ligands.1−7 MOFs have received extensive attention not only for their fascinating structure features but also for their potential applications in gas adsorption/separation, catalysis, and so on.8−16 Despite numerous advantages, applications of many MOFs are ultimately limited by their stability under harsh conditions. Hence, design and synthesis of stable MOF architectures have become one of the most active research fields.17−28 Considering the intrinsic stability of MOFs, stable MOFs can be classified into two categories: highvalent metal−carboxylate frameworks and low-valent metal− imidazolate frameworks, represented by MILs (Materials Institute Lavoisier)23 and zeolitic imidazolate frameworks (ZIFs) series, respectively.24−26 ZIFs have gained wide attention due to their high thermal/chemical stability, easy crystallization, and cheap prices of reactants as well as the high surface area and porosity.27,28 Many efforts have been devoted to developing novel approaches (i.e., microwave, mechanochemical, and flow chemistry synthesis for large scale syntheses).29 Recently, we have reported one kind of MOF with zeolitelike topology, hybrid zeolitic imidazolate frameworks (HZIFs, © XXXX American Chemical Society
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EXPERIMENTAL SECTION
Materials and Instrumentation. All reagents were purchased commercially and used without further purification. All powder X-ray diffraction (PXRD) analyses were recorded on a Rigaku Dmax 2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a step size Received: March 8, 2019 Revised: April 29, 2019 Published: May 13, 2019 A
DOI: 10.1021/acs.cgd.9b00306 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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of 0.05°. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer with a heating rate of 10 °C/min under an air atmosphere. The gas sorption studies were performed on a Micromeritics ASAP 2020 surface-area and pore-size analyzer. Synthesis and Activation of ZIF-8. Method A. Methanol solution (70 mL) of 2-methylimidazole (2-mim, 3.300 g) was added into methanol solution (70 mL) of Zn(NO3)2·6H2O (1.500 g), and the mixture solution was stirred at ambient temperature. The precipitate was separated by centrifugation, washed by methanol, and dried at 80 °C. The sample was added to ethanol solution at ambient temperature for 3 days for guest exchange. Prior to gas sorption measurements, the dried samples were directly loaded in sample tubes and activated under high vacuum (10−3 Torr) at 80 °C for 12 h. Method B. Zn(NO3)2·6H2O (0.210 g) and 2-methylimidazole (2mim, 0.060 g) were dissolved in N,N-dimethylformamide (DMF,18 mL) in a 20 mL vial. The vial was capped and heated at 140 °C for 24 h and then cooled to room temperature. After removal of mother liquor from the mixture, chloroform (20 mL) was added to the vial. Colorless polyhedral crystals were collected from the upper layer, washed with DMF (10 mL), and dried. The sample was added to ethanol solution at ambient temperature for 3 days for guest exchange. Prior to gas sorption measurements, the dried samples were directly loaded in sample tubes and activated under high vacuum (10−3 Torr) at 80 °C for 12 h. Synthesis and Activation of HZIF-1. Reflux Method. ZIF-8 (0.0900 g) and molybdic acid (H2MoO4, 0.0164 g) were combined, and the solid was heated with N,N-dimethylformamide (DMF, 20 mL) at 160 °C for 50 min. The precipitate was separated by centrifugation, washed by DMF, water, and ethanol, and dried. Then, the samples were sieved by the mesh and added to ethanol solution at ambient temperature for 3 days for guest exchange. Prior to gas sorption measurements, the dried samples were directly loaded in sample tubes and activated under high vacuum (10−3 Torr) at 80 °C for 12 h. Solvent-Free Method. ZIF-8 (0.0090 g) and molybdic acid (H2MoO4, 0.0017 g) were combined, and the solid was grinded at ambient temperature for 20 min; the precipitate was heated at 120 °C for 48 h without solvent and then washed by DMF, water, and ethanol, and dried. Gas Sorption Measurements. Low-temperature gas sorption isotherms were measured on a Micromeritics ASAP 2020 surfacearea and pore-size analyzer up to 1 bar of gas pressure by the static volumetric method after the sample was activated. The gas sorption isotherms for N2 were measured at 77 K with liquid nitrogen.
Figure 1. Powder XRD patterns at different reaction times of the reflux method.
SEM results. As shown in Figure 2, after 50 min, the original cubic ZIF-8 (ca. 100 nm) was transformed into HZIF-1Mo with truncated octahedron morphology. Acid and Base Stability of HZIF-1Mo. As mentioned above, the thermal and chemical (especially acid−base) stabilities of MOFs play an important role in their potential applications including gas separation and catalysis. For example, many catalytic processes are executed at high temperatures and varying pH ranges. Our former results showed that HZIF-1Mo exhibited high thermal stability (up to 550 °C).30 Herein, we also checked the acid−base stability of HZIF-1Mo as well as of ZIF-8 for comparison. The samples of HZIF-1Mo were soaked in aqueous solutions with various pH values for 7 days. PXRD patterns confirm that HZIF-1Mo can maintain its framework in aqueous solutions with pH values of 1−14 after 1 week (Figure 4). For ZIF-8, only the base stability has been reported before.26 By contrast, we performed similar experiments by suspending ZIF-8 samples in aqueous solutions with pH values of 1−7. After 7 days, most samples of ZIF-8 were dissolved in the solution with pH = 2, and the remaining samples sustained their structure (Figure S1). However, all the samples are completely dissolved in the pH = 1 solution at the short time. In addition, the acid−base stabilities of some representative stable MOFs are listed in Table S1 for easy comparison. It is obvious that HZIF-1Mo exhibits better stability in strong acid than ZIF-8. Besides this, the acid−base stability of HZIF-1Mo exceeds those of most of the MOFs and is among the highest level of MOFs. Permanent Porosity of Desolvated HZIF-1Mo. The permanent porosity of desolvated HZIF-1Mo and ZIF-8 was confirmed by reversible N2 sorption experiments at 77 K. Both samples exhibited type I adsorption isotherm behavior (Figure 4). The BET surface areas of HZIF-1Mo and ZIF-8 were 420 and 1389 m2/g, respectively. These values are similar to former reported values. Photocatalytic Activities of HZIF-1Mo. To evaluate the crystallinity of obtained HZIF-1Mo, photocatalytic activities of HZIF-1Mo were also investigated by choosing methyl orange as the substrate. According to the former method, the characteristic absorption of methyl orange (MO) at about 465 nm was selected for monitoring the adsorption and photocatalytic degradation process. As shown in Figure S2a, the adsorption of MO was gradually decreased with time increasing from 0 to 120 min. The degradation ration of methyl orange reaches 77.5%, which is similar to our former
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RESULTS AND DISCUSSION Synthesis of HZIF-1Mo via Reflux Method. ZIF-8 was synthesized according to the former reported facile method (method A).31 The obtained crystalline samples of ZIF-8 were washed by N,N-dimethylformamide (DMF) and ethanol and dried for further experiments. For the convenience of observation, we executed the synthesis experiment by heating the reaction mixture of ZIF-8, molybdic acid, and DMF in a flask at 160 °C (noted as the reflux method). PXRD spectra were recorded via random sampling from the reaction solution at intervals of 10 min. The transformation process was monitored by the first characteristic (110) Bragg reflection peak of HZIF-1Mo at 2Θ = 5° (IHZIF‑1Mo) and ZIF-8 at 2Θ = 7.5° (IZIF‑8). As demonstrated by the PXRD patterns (Figure 1), both IZIF‑8 and IHZIF‑1 were observed when the temperature just reached 160 °C (0 min), suggesting the generation of HZIF-1Mo. The intensities of ZIF-8 became weaker and weaker with time. The yield is calculated as ca. 11.99% after 50 min according to parallel experiments. There is no obvious change in yield to prolong the reaction time at this temperature. This change process can also be proven by the B
DOI: 10.1021/acs.cgd.9b00306 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 2. Scanning electron microscopy (SEM) images of ZIF-8 (a) and HZIF-1Mo (b); particle size distributions of ZIF-8 (c) and HZIF-1Mo (d).
result (81.6%, Figure S2b). In comparison, under similar conditions, ZIF-8 without the MoO42− active site in the framework did not show any catalytic activity. Photocatalysis results in combination with powder XRD and BET experiments can support that the product obtained from ZIF-8 was highly crystalline and defect-free. Solvent-Free Synthesis of HZIF-1Mo. In addition, solvent-free synthesis has been paid much attention, because this synthesis approach is the environmental friendly process to produce MOFs and could reduce the cost of production. On the basis of the above results, we further explored the solventfree method to synthesize HZIF-1Mo from ZIF-8. Big crystals of ZIF-8 with a size of ∼0.2 mm were used for easy observation (method B).22 The mixture of ZIF-8 and molybdic acid was put into mortar. After 20 min of grinding by hand, the IZIF‑8 disappeared, meaning ZIF-8 has poor mechanical stability. Then, the mixture was heated at 120 °C for 2 days. The PXRD results (Figure 6) proved that ZIF-8 was completely transformed to HZIF-1Mo. The results showed that solvent-free synthesis of HZIF-1Mo from ZIF-8 can also be realized.
Figure 3. Powder XRD patterns of HZIF-1Mo at different pH values.
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CONCLUSION We reported the facile synthesis of highly crystalline HZIF1Mo from ZIF-8 precursors via a reflux method. Photocatalysis results in combination with powder XRD and BET experiments prove that the obtained product from ZIF-8 was highly crystalline and defect-free. Furthermore, HZIF-1Mo exhibits better strong acid stability than that of ZIF-8. In addition, solvent-free synthesis of HZIF-1Mo from ZIF-8 can also be realized.
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Figure 4. N2 isotherms of ZIF-8 (a) and HZIF-1Mo (b) by reflux method. Solid shapes represent adsorption, and open shapes represent desorption.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.9b00306. C
DOI: 10.1021/acs.cgd.9b00306 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 5. Optical photographs (a) and plot of degradation of the methyl orange (b). Square represents ZIF-8; triangle represents HZIF-1Mo by reflux method; circle represents HZIF-1 Mo in original method (data from our former work30). (4) Hu, H. C.; Zhao, B. Metal-Organic Frameworks Based on Multicenter-Bonded [M(I)]8 (M = Mn, Zn) Clusters with Cubic Aromaticity. Chem. - Eur. J. 2018, 24, 16702−16707. (5) Wu, X.-Q; Liu, Y.; Feng, P.-Q; Wei, X.-H.; Yang, G.-M.; Qiu, X.H.; Ma, J.-G. Design of a Zn-MOF biosensor via a ligand “lock” for the recognition and distinction of S-containing amino acids. Chem. Commun. 2019, 55, 4059−4062. (6) Wu, Z. L.; Wang, C. H.; Zhao, B.; Dong, J.; Lu, F.; Wang, W. H.; Wang, W. C.; Wu, G. J.; Cui, J. Z.; Cheng, P. A Semi-Conductive Copper-Organic Framework with Two Types of Photocatalytic Activity. Angew. Chem., Int. Ed. 2016, 55, 4938−4942. (7) Huang, W. H.; Ren, J.; Yang, Y. H.; Li, X. M.; Wang, Q.; Jiang, N.; Yu, J. Q.; Wang, F.; Zhang, J. Water-Stable Metal-Organic Frameworks with Selective Sensing on Fe3+ and Nitroaromatic Explosives, and Stimuli-Responsive Luminescence on Lanthanide Encapsulation. Inorg. Chem. 2019, 58, 1481−1491. (8) Wang, H.; Dong, X.; Velasco, E.; Olson, D. H.; Han, Y.; Li, J. One-of-A-Kind: A Microporous Metal-Organic Framework Capable of Adsorptive Separation of Linear, Mono- and Di-branched Alkane Isomers via Temperature- and Adsorbate-Dependent Molecular Sieving. Energy Environ. Sci. 2018, 11, 1226−1231. (9) Qin, J.-S.; Du, D.-Y.; Guan, W.; Bo, X.-J.; Li, Y.-F.; Guo, L.-P.; Su, Z.-M.; Wang, Y.-Y.; Lan, Y.-Q.; Zhou, H.-C. Ultrastablepolymolybdate-based metal-organic frameworks as highly active electrocatalysts for hydrogen generation from water. J. Am. Chem. Soc. 2015, 137, 7169−7177. (10) Hosono, N.; Terashima, A.; Kusaka, S.; Matsuda, R.; Kitagawa, S. Highly responsive nature of porous coordination polymer surfaces imaged by in situ atomic force microscopy. Nat. Chem. 2019, 11, 109−116. (11) Li, P.; Cheng, F. F.; Xiong, W. W.; Zhang, Q. New synthetic strategies to prepare metal-organic frameworks. Inorg. Chem. Front. 2018, 5, 2693−2708. (12) Zhao, M.; Ou, S.; Wu, C. D. Improvement of the CO 2 Capture Capability of a Metal-Organic Framework by Encapsulating Dye Molecules inside the Mesopore Space. Cryst. Growth Des. 2017, 17 (5), 2688−2693. (13) Ugale, B.; Dhankhar, S. S.; Nagaraja, C. M. Exceptionally stable and 20-connected lanthanide metal-organic frameworks (MOFs) for selective CO2 capture and conversion to cyclic carbonates at atmospheric pressure. Cryst. Growth Des. 2018, 18 (4), 2432−2440. (14) Yang, H. Y.; Li, Y. Z.; Jiang, C. Y.; Wang, H. H.; Hou, L.; Wang, Y. Y.; Zhu, Z. An Interpenetrated Pillar-Layered Metal-Organic Framework with Novel Clusters: Reversible Structural Transformation and Selective Gate-Opening Adsorption. Cryst. Growth Des. 2018, 18 (5), 3044−3050. (15) Li, W.; Wang, K.; Qi, X. J.; Jin, Y.; Zhang, Q. Construction of a Thermally Stable and Highly Energetic Metal-Organic Framework as
Figure 6. PXRD pattern under different conditions for solvent-free method.
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Powder XRD results, SEM, the table of representative stable MOFs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Fei Wang: 0000-0001-8432-0009 Jian Zhang: 0000-0003-3373-9621 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the support of this work by NSFC (21573236). REFERENCES
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DOI: 10.1021/acs.cgd.9b00306 Cryst. Growth Des. XXXX, XXX, XXX−XXX
