Hongyu Wua,
Xukun Qian*ab,
Haiping Zhuc,
Songhua Maa,
Guangshan Zhud and
Yi Long*b
aSchool of Engineering and Design, Lishui University, Lishui 323000, PR China. E-mail: qianxukun@126.com
bSchool of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. E-mail: LongYi@ntu.edu.sg
cSchool of Ecology, Lishui University, Lishui 323000, PR China
dState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
First published on 12th January 2016
In this work, dodecahedral microcrystals of zeolitic imidazolate framework-67 (ZIF-67) with good uniformity and exceptional stability have been synthesized by simply controlling the mole ratio of reactants at room temperature. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) characterization indicates that the as-synthesized ZIF-67 microcrystals are highly uniform and have perfect rhombic dodecahedral morphology with 12 exposed {110} faces. The X-ray diffraction (XRD) and thermogravimetry analysis (TGA) investigations demonstrate that they display high thermal stability up to ∼350 °C in air and ∼425 °C in vacuum, as well as remarkable chemical resistance to boiling organic solvents and room-temperature water. Through a two-step oxidative thermolysis process, these ZIF-67 dodecahedra can be further converted into hollow Co3O4 architectures consisting of nanosized building blocks. Owing to their large specific surface area and unique structure, the hollow Co3O4 catalyst obtained at the calcination temperature of 425 °C exhibits high catalytic activity and stability for gas-phase CO oxidation.
In the past few years, many efforts have been carried out to control the size22 and shape23,24 of MOFs crystals since theses parameters can significantly influence their properties such as gas sorption25 and indeed are critical for biological applications.26 ZIF-8 (Zn(mim)2, mim = 2-methylimidazolate) and ZIF-67 (Co(mim)2) are two most representative members of the large ZIFs family.2 They share quite similar crystal structure with a zeolite sodalite topology, which indicates they have similar crystallization mechanism. Syntheses of ZIF-8 with controlled particle sizes3,9,27 and shapes28,29 have been achieved in aqueous, methanol or dimethylformamide (DMF) solutions at room temperature. The obtained ZIF-8 particles usually are uniform and monodispersed and have large surface area of 800–1671 m2 g−1. Nevertheless, to our knowledge, only a few efforts have been devoted to the development of uniformly well-defined ZIF-67 so far. ZIF-67 has been synthesized under hydro-/solvothermal reaction between a cobalt salt and Hmim in solvents such as DMF2 or water.28,30,31 Nano-sized ZIF-67 crystals (∼228 nm) were synthesized in aqueous solutions at room temperature.30 The formation of ZIF-67 was accelerated and completed within 10 min by adding triethylamine to deprotonate the 2-methylimidazole (Hmim) linker. Unfortunately, these work failed to deliver uniform ZIF-67 crystals under the investigated conditions. To further tune the properties of ZIF-67 for specific applications as well as to make them available as novel building blocks for advanced nanodevices, it is necessary to develop synthetic routes for the production of homogeneous crystals. Furthermore, controlling the shape and size of ZIF-67 particles will facilitate the fundamental understanding of the crystallization mechanism.
Chemical stability of ZIFs against aqueous or organic solutions is an important criterion for practical applications. One of the main advantages of ZIFs materials is their remarkable thermal and chemical stability when compared with other MOFs, a fact that undoubtedly expands their industrial applications in large scale.1 The high chemical stability of ZIF-8 has been primarily attributed to the strong bonding between Zn(II) and mim(I). Another possible reason is that the hydrophobic pore and surface structure repels water molecules, preventing the attack of ZnN4 units and dissolution of the framework.1 In this regard, however, rather limited data have been reported on ZIF-67 in the scientific literatures up to now. This prompted us to make a systematic study on the stability of ZIF-67.
In this work, we report the facile synthesis of uniform and nonaggregated ZIF-67 dodecahedra. The thermal and chemical stability of the as-synthesized ZIF-67 microcrystals are also systematically examined using powder X-ray diffraction (XRD) and thermogravimetric analysis (TGA). By a two-step oxidative thermolysis of ZIF-67 templates, hollow Co3O4 catalysts with high specific surface area can be obtained, and they demonstrate remarkable catalytic activity for low-temperature CO oxidation.
Hollow Co3O4 dodecahedral catalysts consisting of nanoparticles were prepared via a two-step oxidative thermolysis of ZIF-67 templates. First, the purple ZIF-67 powders were heated at a slow ramp of 5 °C min−1 and then calcined in an inert atmosphere of flowing nitrogen for 30 min at 425, 500 and 575 °C, respectively. Subsequently, they were continuously calcined in an oxidative atmosphere of flowing air for another 30 min at the same temperature. Finally, the powders were cooled down naturally and the derived black products are correspondingly referred to as Co3O4-425, Co3O4-500 and Co3O4-575.
The thermal stability of the as-synthesized ZIF-67 microcrystals in vacuum was monitored by in situ Siemens D5005 high-temperature XRD diffractometer from RT to 450 °C. The diffractometer is equipped with monochromatic Cu Kα radiation (λ = 1.540 Å). The heating rate is 10 °C min−1 and the scan speed 5° min−1 in 2 theta.
The chemical stability of the as-synthesized ZIF-67 was studied by suspending the materials in boiling ethanol, toluene and boiling/RT water, respectively, for up to 7 days. After the treatment, the powders were centrifugated and vacuum-dried at 80 °C overnight before the XRD analysis was performed.
Nitrogen physisorption isotherms were measured on a Micromeritics ASAP 2020 volumetric instrument. Prior to the sorption measurements, ZIF-67 and templated hollow Co3O4 samples were degassed in vacuum at 100 °C for 12 h. Surface areas were estimated by applying the Brunauer–Emmett–Teller (BET) equation. The morphology and microstructure of the samples were characterized by a JEOL 7600F field-emission scanning electron microscopy (FESEM) at an acceleration voltage of 5 kV and a JEOL JEM-2100F high-resolution transmission electron microscopy (HRTEM) at an acceleration voltage of 200 kV.
The CO catalytic oxidation of templated hollow Co3O4 was evaluated in a fixed bed reactor at various temperatures without pre-treatment. 150 mg of the Co3O4 catalyst was sandwiched by two layers of quartz wool, and tested with the reaction gas, 1% CO balanced with air. The flow rate was fixed at 100 ml min−1 (GHSV = 110000 h−1). The effluent gases in the outlet were analyzed online using an Shimadzu-14B gas chromatography equipped with TCD and FID detectors. The CO conversion was calculated from the measured CO concentration by the formula CO conversion = [(COin − COout)/COin] × 100%, where COin and COout are the inlet and outlet CO concentration, respectively.
TGA curve (Fig. 2a) of the as-synthesized material shows only ∼3.1 wt% of guest molecules remained in the crystal pores. It is seen that ZIF-67 show different thermal stability in air and in vacuum. The ZIF-67 microcrystals are thermally stable up to ∼350 °C and undergo fast decomposition at ∼362 °C in air. However, they are thermally stable up to a higher temperature of ∼425 °C in vacuum. The TGA results demonstrate that the as-synthesized ZIF-67 exhibits a higher thermal stability in vacuum than in air, indicating the framework structure is quite sensitive to oxygen at high temperature. Although the thermal stability of ZIF-67 in air is slightly lower than that of ZIF-8 (∼500 °C),12 it still surpasses many MOFs.36 The thermal stability and decomposition in vacuum of ZIF-67 materials were further monitored by in situ high-temperature XRD, as shown in Fig. 2b. Initially, the materials are able to maintain good crystalline up to 350 °C. A decrease in the peak intensity at 400 °C is observed, indicating the crystal decomposition occurred. As the temperature is increased to 425 °C, the diffraction peaks become much weaker. Finally, no diffraction peaks associated with ZIF-67 can be detected at 450 °C, clearly showing the completed decomposition of the material. These results are in accordance with the TGA analysis.
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Fig. 2 (a) TGA curve of the as-synthesized ZIF-67 samples, (b) in situ high-temperature XRD patterns of ZIF-67 microcrystals in vacuum. |
Structural resistance to solvents is critical for the promising application of ZIFs. The chemical stability of dodecahedral ZIF-67 was examined by suspending powder samples in boiling ethanol, toluene and RT/boiling water, conditions that can represent operational parameters of typical chemical processes for industrial applications. The structure integrity of ZIF-67 after chemical treatment was monitored by XRD. As shown in Fig. 3a, there is no significant change in the crystallinity level and peak positions for the tested crystals, nor is structural transformation observed after uninterrupted soaking. This proves that the as-synthesized ZIF-67 materials are stable in boiling ethanol for at least 7 days. In the case of boiling toluene, an obvious decrease in intensity of the (001) reflection (2θ = 7.335°) is observed. However, ZIF-67 still maintains its full crystallinity for 7 days. The highly chemical stability enables ZIF-67 as a potential catalyst or catalyst support for Knoevenagel reaction, where chemical reactions are normally performed in toluene.37 Furthermore, ZIF-67 can also sustain its structure in RT water for 7 days (Fig. S2†). This good resistance to hydrolysis is comparable to that of ZIF-8 and UiO-66,1,38 and better than the well-known HKUST-1 (ref. 39) and MOF-5.40,41 To comprehensively investigate the chemical stability, ZIF-67 was further subjected to boiling water. An extra diffraction peak (2θ = 19.058) appears after soaking for 1 day and becomes stronger after prolonged soaking (Fig. 3c). This suggests that ZIF-67 undergoes a gradual structural transformation and thus exhibits limited structure stability in boiling water. Finally, ZIF-67 crystals are completely transformed to other crystalline materials after 7 days because no peaks associated with them are detected. The wide angle XRD show the final product is composed of a mixture of Co3O4 and Co(OH)2. Combined the FESEM examinations (Fig. 3d) with EDS results (not shown here), it can be concluded that the spherical particle is Co3O4 whereas the plate-like crystal is Co(OH)2. To the best of our knowledge, this is the first systematic study on the chemical stability of dodecahedral ZIF-67 crystals.
Spinel cobalt oxide (Co3O4) has been extensively studied as heterogeneous catalysts,42,43 anode materials in supercapacitors and lithium ion battery,44,45 and gas sensing materials.46 Highly symmetric hollow dodecahedra was obtained by a two-step oxidative thermolysis of dodecahedral ZIF-67 templates. In this transformation process, calcination conditions (e.g., temperature and atmosphere) can significantly affect the phase composition and microstructure of the finally derived products. The XRD results (Fig. S4†) show only Co3O4 exists in the final transformed products. There is no structure change in the temperature range of 425 to 575 °C. Therefore the catalysts are at least thermally stable up to 575 °C in air, which would be sufficient for catalytic conversion of CO. Typical morphologies of the hollow Co3O4 calcined at different temperatures are shown in Fig. S5.† After calcining the dodecahedral ZIF-67 microcrystals at the lower temperature of 425 °C (Fig. 4a and S5a†), the obtained Co3O4-425 products perfectly retain the geometries of the ZIF-67 templates. The highly symmetric dodecahedral structures of Co3O4-425 can be observed distinctly and the hollow interiors are clearly revealed by the sharp contrast between the centers and edges of dodecahedra (Fig. 4a). The interior structure of the hollow Co3O4-425 dodecahedra is further studied by TEM and selected area electron diffraction (SAED). The magnified TEM image reveals that the hollow structure is composed of nanoparticles that are identified as polycrystalline Co3O4 by SAED pattern (Fig. 4b). The HRTEM image (Fig. 4c) taken near the dodecahedra edge displays distinct lattice fringes with d spacings of 0.29 and 0.23 nm, which are in good agreement with the (220) and (311) lattice planes of Co3O4, respectively. The elemental mapping results confirm the co-existence and homogenous distribution of Co and O elements within the dodecahedra (Fig. 4d–f). As the temperature increases to 500 °C, the products can only remain a basic framework of dodecahedra (Fig. S5b†). Ultimately, the framework structure completely collapses at 575 °C (Fig. S5c†). Some Co3O4 grains agglomerate and coarsen with irregular shape.
To explore the porosity of the hollow Co3O4 dodecahedra, N2 adsorption–desorption isotherms were recorded (Fig. S6†). As mentioned above, the as-synthesized ZIF-67 microcrystals show typical microporous structure with a high surface area (Fig. 1d). After oxidative thermolysis of ZIF-67 templates, a significant reduction of surface area from 1587 m2 g−1 down to 100 m2 g−1 is observed in comparison with the original ZIF-67. Nevertheless, hollow Co3O4-425 still exhibits a high surface area of 63.85 m2 g−1. The Co3O4-425 and Co3O4-500 products display a typical type IV isotherm with distinct hysteresis loops, which is typical for cage-type pores and indicative of the presence of mesoporous structures. The adsorbed N2 volumes of Co3O4-425 and Co3O4-500 gradually increase in the range of P/P0 = 0.5–0.9, indicating the formation of pores with different sizes. Co3O4-575 has the smallest surface area of 12.65 m2 g−1, which is due to the collapse of hollow structure and grain coarsening.
To investigate the advantages of the hollow Co3O4 dodecahedra, we herein studied their catalytic oxidation of CO. The catalytic activity for CO oxidation as a function of reaction temperature is shown in Fig. 5. Apparently, the catalytic activity follows the order of Co3O4-425 > Co3O4-500 > Co3O4-575. It is concluded that the catalytic activity of the templated Co3O4 catalysts increases with decreasing calcination temperature. Both Co3O4-425 and Co3O4-500 exhibit significant catalytic activity at RT, while the Co3O4-575 catalyst shows negligible activity. It is noticed that CO conversion over three catalysts is increased with the temperature increasing. The temperatures for 50% CO conversion are approximately 309.8, 329.2 and 342.9 K, respectively, for Co3O4-425, Co3O4-500 and Co3O4-575. 100% CO conversion is achieved at 358 K and 368 K for Co3O4-425 and Co3O4-500 catalysts, respectively. In contrast, full CO conversion was obtained only at 378 K for Co3O4-575. Our results demonstrate that the Co3O4-425 sample is highly reactive towards CO oxidation. The better catalytic properties of Co3O4-425 could be attributed to its hollow structure and larger surface area, which offers the largest contact area with CO. The catalytic performances of the present samples are better than that of Co3O4 nanoparticles,47 porous ZnO/Co3O4,48 and Cu/Cu2O composites.49
As Co3O4 crystals may suffer from deactivation after long time reaction, the catalytic durability of the three catalysts was also evaluated (Fig. 6). It is found that there is no significant loss of CO conversion after 20 h. The catalytic activity nearly remains unchanged. Therefore, the ZIF-67 templated Co3O4 catalysts exhibit good long-term stability. After the catalytic experiment, the recovered catalysts are subject to TGA test (Fig. S7†). It is seen there is no significant weight change during heating, which indicates the recovered catalyst is quite stable.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18557b |
This journal is © The Royal Society of Chemistry 2016 |