Polymorphism/pseudopolymorphism of metal–organic frameworks composed of zinc(II) and 2-methylimidazole: synthesis, stability, and application in gas storage

Abstract

This paper reports on the synthesis and stability of a polymorphic system of a metal–organic framework (MOF) composed of zinc(II) and 2-methylimidazole, as well as its potential applicability in gas storage. Three polymorphs/pseudopolymorphs, ZIF-8, ZIF-L, and dia(Zn), are discussed in this work. It was found that the synthesis of dia(Zn) with a crystal morphology of hexagonal nanosheets requires a catalyst (NH₄OH, CH₃COOH, or HCOONa), and a synthesis temperature of 60 °C. In contrast, the synthesis of ZIF-8 and ZIF-L can be conducted in the absence of a catalyst and at room temperature. This suggests that the activation energy of dia(Zn) exceeds that of ZIF-8 and ZIF-L. The three crystals were subjected to hydrothermal treatment at 100 °C to evaluate their stability. ZIF-8 presented the highest hydrothermal stability, whereas ZIF-L presented the lowest. Nitrogen physisorption performed at 77 K suggests that the microporosity of ZIF-8 exceeds that of ZIF-L and dia(Zn), which were almost nonporous. Interestingly, CO₂ thermogravimetric analysis revealed that the CO₂ adsorption of ZIF-L and dia(Zn) at 323.15 K is on par with that of ZIF-8, which implies that the flexibility of the ZIF-L and dia(Zn) framework increased considerably with temperature. Our results suggest that nonporous MOFs might be useful for gas adsorption or gas separation at ambient or high temperature.

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1. Introduction

Metal–organic frameworks (MOFs) are crystalline nanoporous materials comprising metal ions coordinated to organic ligands.¹ In the last decade, MOFs have attracted considerable attention from researchers due to their applications in catalysis,^2–4 gas storage^5–7 and separation.^8–10 The pore topology of MOFs, which is determined by their crystal topology, is a dominant factor in the above applications. One effective strategy to engineering the pore topology of MOF materials deals with their polymorphism, which means that the same pair of metal ions and organic ligand(s) can form multiple distinct MOF crystals.^11,12

UHM-8 and UHM-9 are polymorphs formed by copper and the linker 5,5′-(1,4-phenylenedi-2,1-ethinyl)bis(1,3-benzenedicarboxylic acid).¹³ UHM-8 is synthesized using dimethylacetamide as a solvent, which results in a topology with a window for the molecular transport of 11 × 13 Å.¹³ UHM-9 can be obtained from a mixed solvent composed of dimethylformamide, dioxane, and water, the resulting crystal of which possesses a window of 9 × 13 Å.¹³ MIL-101(V), MIL-88B(V), and MIL-47 are polymorphs consisting of vanadium and 1,4-benzenedicarboxylate, corresponding to mtn-e-a net, acs-a net, sra net, respectively.¹⁴ MIL-101(V) and MIL-88B(V) can be converted into MIL-47 through hydrothermal treatment at approximately 200 °C.¹⁴ Thus, MIL-47 is considered the thermodynamic phase among the three polymorphs. DUT-67, DUT-68, and DUT-69 represent another polymorphic system composed of zirconium/hafnium and 2,5-thiophenedicarboxylic acid with acetic acid.¹⁵ DUT-68 and DUT-69 are both synthesized in dimethylformamide but at different ligand/metal ratios, whereas DUT-67 is synthesized in a mixed solvent composed of dimethylformamide and N-methyl-2-pyrrolidone.

Polymorphs comprising zinc(II) and 2-methylimidazole are an emerging family of MOFs for the capture of CO₂ and gas separation. Four polymorphs/pseudopolymorphs from this class of MOF have been identified: ZIF-8, kat(Zn), dia(Zn), and ZIF-L. The crystal topologies are illustrated in Fig. 1. ZIF-8 is the best known of the four polymorphs, possessing a zeolite SOD topology (Fig. 1a) with a window of 3.4 Å for molecular transport.^7,16 ZIF-8 was first synthesized using dimethylformamide as the solvent,¹⁷ and then an aqueous reaction was developed for the synthesis of ZIF-8 crystals^18,19 and membranes.^20–23 Subjecting ZIF-8 to hydrothermal treatment was shown to cause ZIF-8 to decompose²⁴ or partially transform into ZnO.²⁵ A computational study has also been conducted for the diagnosis of potential defect spots within ZIF-8 crystals.²⁶ Two additional polymorphs/pseudopolymorphs of this family, dia(Zn) and ZIF-L, were first synthesized in 2011 (ref. 27) and 2013.²⁸ Dia(Zn) was discovered from a failed attempt to synthesize ZIF-8. It possesses a diamondoid²⁹ crystal topology (Fig. 1b) and is considered nonporous.²⁷ Few studies have been reported on the synthesis of dia(Zn); however, ZIF-L has attracted some attention. It has a layered topology with a cushion-shaped cavity of 9.4 × 7.0 × 5.3 Å (Fig. 1c). ZIF-L possesses a slightly different composition to ZIF-8 and dia(Zn), and thus is technically a pseudopolymorph, instead of polymorph, of ZIF-8 and dia(Zn). The CO₂ adsorption capacity of ZIF-L exceeds that of ZIF-8.²⁸ Pure ZIF-L membranes³⁰ and ZIF-L containing hybrid membranes²³ both outperform pure ZIF-8 membranes with regard to gas separation. The most recently found crystal of these four polymorphs/pseudopolymorphs is kat(Zn) (Fig. 1d), which was discovered during the milling of ZIF-8 in 2015.³¹ It possesses a nonporous crystal topology, and no applications of kat(Zn) have been reported.

Fig. 1 Crystal topologies of (a) ZIF-8, (b) dia(Zn), (c) ZIF-L, and (d) kat(Zn).

This work developed a method of controlling the aqueous synthesis of ZIF-8, ZIF-L and dia(Zn) crystals. This is the first study to develop a synthetic approach to the synthesis of dia(Zn) with a definite crystal morphology of hexagonal nanoflakes. The as-synthesized materials were characterized using a number of solid-state techniques, including scanning electron microscopy (SEM), powder X-ray diffraction (XRD), FT-IR spectroscopy, thermogravimetric analysis (TGA), and nitrogen physisorption. The CO₂ adsorption capacity of the three crystals was investigated using cyclic adsorption/desorption measurements. Hydrothermal treatment was used to evaluate the stability of the three MOF materials, and a reaction coordinate diagram.

2. Experimental

2.1 Chemicals

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, >98%), 2-methylimidazole (C₄H₆N₂, >99%), sodium formate (HCOONa, >99%), and ammonium hydroxide solution (NH₄OH, =30–33%) were purchased from Sigma-Aldrich. Acetic acid (CH₃COOH, >99.7%) was purchased from J. T. Baker. All chemicals were used without further purification. Deionized water used for in the synthesis process was purified using the Thermo Scientific™ Barnstead™ NANOpure® DIamond™ analytical ultrapure water system.

2.2 Synthesis of ZIF-8

ZIF-8 was prepared using the method outlined in previous reports,^23,32 with minor modifications. In a typical synthesis, 2.27 g of 2-methylimidazole were dissolved in 20 mL of deionized water. A separate solution formed by dissolving 0.11 g of zinc nitrate hexahydrate in 20 mL of deionized water was added to the previous mixture and stirred at room temperature for 2 h. The product was collected by repeated centrifugation and washed using fresh deionized water 3 times. The obtained products were then dried in a convection oven at 105 °C for a period of 24 h.

2.3 Synthesis of ZIF-L

ZIF-L was prepared using the method outlined in previous reports,^23,28,33 with minor modifications. Briefly, 0.657 g of 2-methylimidazole was dissolved in 20 mL of deionized water. A separate solution formed by dissolving 0.298 g of zinc nitrate hexahydrate in 20 mL of deionized water was added to the previous mixture and stirred at room temperature for 2 h. The product was collected by repeated centrifugation and washed using fresh deionized water 3 times. The obtained products were then dried in a convection oven at 105 °C for a period of 24 h.

2.4 Synthesis of dia(Zn)

We prepared dia(Zn) with a well-defined crystal morphology as follows. 0.657 g of 2-methylimidazole was dissolved in 20 mL of deionized water. Either 0.136 g sodium formate, 0.1 mL acetic acid, or 1 mL ammonium hydroxide solution was added to the 2-methylimidazole solution. A separate solution formed by dissolving 0.298 g of zinc nitrate hexahydrate in 20 mL of deionized water was added to the previously prepared solution. The mixture was stirred at 60 °C for 2 h. The product was collected by repeated centrifugation and washed using fresh deionized water 3 times. The obtained products were then dried in a convection oven at 105 °C overnight. The MOF product prepared with sodium formate is referred to as dia(Zn)-HCOONa; the MOF product with acetic acid is referred to as dia(Zn)-CH₃COOH; and the MOF product with ammonium hydroxide solution is referred to as dia(Zn)-NH₄OH. In the synthesis solution used for dia(Zn)-HCOONa, the mixture had a Zn(II) : 2-methylimidazole : HCOONa : H₂O molar ratio of 1 : 8 : 2 : 2228. In the synthesis solution for dia(Zn)-CH₃COOH, the mixture had a Zn(II) : 2-methylimidazole : CH₃COOH : H₂O molar ratio of 1 : 8 : 1.6 : 2228. In the synthesis solution for dia(Zn)-NH₄OH, the mixture had a Zn(II) : 2-methylimidazole : NH₄OH : H₂O molar ratio of 1 : 8 : 21 : 2271.

We used dia(Zn) with a poorly-defined crystal morphology as a control sample. This was prepared using the method outlined in a previous report³⁴ with minor modification. Briefly, 0.328 g of 2-methylimidazole (4 mmol) was dissolved in 3.454 g of ammonium hydroxide solution (64 mmol of NH₃) and 12.6 mL deionized water. 0.594 g of zinc nitrate hexahydrate (2 mmol) was dissolved in 3 mL of deionized water. The 2-methylimidazole solution was then added to the zinc nitrate solution and subjected to mixing for 10 min at room temperature. The mixture had a Zn(II) : 2-methylimidazole : NH₃ : H₂O molar ratio of 1 : 2 : 32 : 500. The product, referred to as dia(Zn)-control, was collected by repeated centrifugation and washes using fresh deionized water 3 times. The obtained products were then dried in a convection oven at 105 °C for a period of 24 h.

2.5 Test of hydrothermal stability

We dispersed 0.075 g MOF sample (ZIF-8, ZIF-L, or dia(Zn)) in 15 mL deionized water. The mixture was heated under reflux at 100 °C for 24 h. The product was collected by repeated centrifugation and washed using fresh deionized water 3 times. The obtained products were then dried in a convection oven at 105 °C overnight.

2.6 Material characterization

Powder X-ray diffraction (XRD) patterns were obtained using a Rigaku diffractometer with Cu Kα radiation. The measurements were performed at 40 kV and 40 mA with a scanning rate of 2° 2θ min⁻¹. The XRD patterns were collected from 2 to 40° 2θ with a step size of 0.02°.

The morphology of all solid-state samples was characterized using a Hitachi S-4800 field emission scanning electron microscope (SEM). The SEM was operated at an acceleration voltage of 15 kV.

FT-IR spectra were acquired using a JASCO FT/IR-6700 spectrometer equipped with a Ge coated KBr beam splitter. Each FT-IR spectrum was obtained from 100 scans at a spectral resolution of 4 cm⁻¹.

Thermogravimetric analysis (TGA) was performed using a PerkinElmer's Pyris 1. Approximately 5 mg of powder sample was heated under air from room temperature to 800 °C at a ramp rate of 10 °C min⁻¹.

Nitrogen physisorption isotherms were obtained using a Micromeritics 3-Flex analyzer at 77 K. Prior to the measurement, approximately 50–90 mg of powder sample was placed in an analysis tube and degassed under 0.01 mmHg at 100 °C overnight.

CO₂ thermogravimetric analysis (CO₂ TGA) was performed using TA Instruments SDT Q600. The samples were outgassed at 100 °C under flow of pure nitrogen at a flow rate of 100 mL min⁻¹. The CO₂ adsorption was conducted at 50 °C under flow of pure CO₂ at a flow rate of 100 mL min⁻¹. For every tested sample, 8 adsorption–desorption cycles were performed.

3. Results and discussion

3.1 Morphology and crystallinity

The crystal morphology of ZIF-8, ZIF-L, and the four dia(Zn) samples synthesized under various conditions was investigated using scanning electron microscopy (SEM), the results of which are presented in Fig. 2. ZIF-8 possesses (Fig. 2a) a nearly spherical crystal morphology, whereas ZIF-L (Fig. 2b) possesses a leaf-like topology. Both of these findings are in agreement with what has been reported in the literature.^23,28,33,35 Three dia(Zn) samples, dia(Zn)-HCOONa, dia(Zn)-CH₃COOH, and dia(Zn)-NH₄OH, prepared in this work present a crystal morphology of hexagonal nanoflakes (Fig. 2c–e). The crystal morphology of the three dia(Zn) samples (using the proposed method) is far better defined than that of dia(Zn)-control, which was prepared following the procedure that was previously reported³⁴ (Fig. 2f). Despite the fact that dia(Zn) has been discussed in several previous reports,^27,34,36,37 this is report on the synthesis of dia(Zn) with a well-defined crystal morphology. In our attempts at dia(Zn) synthesis, we identified two factors that are critical to obtaining well-defined crystals: the addition of HCOONa, CH₃COOH, or NH₄OH and maintaining a synthesis temperature of 60 °C (ZIF-8, ZIF-L, and dia(Zn)-control were all synthesized at room temperature). We suspect that HCOONa, CH₃COOH, or NH₄OH could serve as a catalyst in the formation of dia(Zn), wherein the high temperature brings the reaction across the energy barrier of crystallization. Additional SEM images of the four dia(Zn) samples can be found in ESI, Fig. S1.† The crystals of dia(Zn)-HCOONa, dia(Zn)-CH₃COOH, and dia(Zn)-NH₄OH are nearly identical with a size of approximately 3 μm. The crystal dia(Zn)-NH₄OH presents a higher aspect ratio than do the other two dia(Zn) crystals. This is likely due to the addition of NH₄OH, resulting in the selective inhibition of crystal growth along particular lattice plane(s). Another interesting observation is that both dia(Zn)-CH₃COOH and dia(Zn)-control presented signs of severe intergrowth among the nanoflakes, which has been observed in other crystal systems.³⁸ The intergrowth of nanoflakes has also been seen in ZIF-L.^39,40

Fig. 2 SEM images of (a) ZIF-8, (b) ZIF-L, (c) dia(Zn)-HCOONa, (d) dia(Zn)-CH₃COOH, (e) dia(Zn)-NH₄OH and (f) dia(Zn)-control.

Powder X-ray diffraction (XRD) was used to investigate the crystallinity of ZIF-8, ZIF-L, and four dia(Zn) samples synthesized under various conditions. Fig. 3 summarizes the powder XRD patterns of these three crystals obtained in experiments and simulations. The experiment-derived XRD patterns of ZIF-8 and ZIF-L are in good agreement with the simulated patterns and those in previous studies.^18,28,35,41 The experiment-derived XRD patterns of the three dia(Zn) samples presented in this work, dia(Zn)-HCOONa, dia(Zn)-CH₃COOH, and dia(Zn)-NH₄OH, are consistent with the simulated XRD patterns of dia(Zn) crystals. No peaks from impurity phases appear in the three XRD patterns of dia(Zn). However, the XRD pattern of the dia(Zn)-control, which was prepared using the previously reported procedure,³⁴ presented a number of peaks unassociated with dia(Zn). The characterization results from SEM and XRD suggest that the synthesis method developed in this work yields dia(Zn) with a precisely defined crystal morphology and high degree of purity. The FT-IR spectra of dia(Zn) are presented in ESI, Fig. S2.† ZIF-8 and dia(Zn) are polymorphisms with a chemical formula of Zn(C₄H₆N₂)₂, whereas ZIF-8 and ZIF-L are an example of pseudopolymorphism (the chemical formula of ZIF-L is Zn(mim)₂·(MeIm)_1/2·(H₂O)_3/2), in which MeIm represents 2-methylimidazole and mim represents deprotonated 2-methylimidazole. Thus, it would be reasonable to expect the FT-IR spectra of these three crystals to be nearly identical. The resulting spectra revealed only a marginal difference in relative peak intensity between 2800–3200 cm⁻¹, which is consistent with the findings in previous studies.^35,42

Fig. 3 Experimental and simulated powder XRD patterns of ZIF-8, ZIF-L, and dia(Zn) prepared under various synthesis conditions. The asterisk denotes an impurity phase presented in the sample of dia(Zn)-control.

Fig. 4 summarizes the scheme used in the synthesis of the three crystals with metal–organic frameworks of zinc(II) and 2-methylimidazole, as well as the synthesis conditions of ZIF-8, ZIF-L, and dia(Zn), and the SEM and XRD characterization results. Briefly, obtaining pure dia(Zn) crystals requires a catalyst (either HCOONa, CH₃COOH, or NH₄OH) and synthesis temperature of 60 °C. Our attempt to synthesize dia(Zn) using the same catalyst at room temperature resulted in the formation of ZIF-L. The SEM images and XRD patterns of these three products are presented in ESI, Fig. S3.† Our attempt to prepare dia(Zn) at 60 °C without a catalyst resulted in the formation of an unknown crystal, which does not appear to have been mentioned in previous reports. An SEM image and the XRD pattern of this unknown crystal are also presented in ESI, Fig. S3.† To further confirm the critical role of catalyst for the formation of dia(Zn), two additional batches were performed: one with HCl and the other with CH₃COONa as a catalyst. Pure dia(Zn) phase was obtained from these two batches (ESI, Fig. S4†).

Fig. 4 Schematic illustration showing the synthesis of polymorphs composed of zinc(II) and 2-methyimidazole (abbreviated to 2-MeIm). Room temperature is abbreviated to RT.

3.2 Porosity and CO₂ adsorption

Nitrogen physisorption at 77 K was used to analyze of the porosity of the three MOF materials discussed in this work. The resulting physisorption isotherms are summarized in Fig. 5. The nitrogen physisorption isotherm from ZIF-8 appeared as an IUPAC type I isotherm,⁴³ which suggests that this is a typical microporous material. The nitrogen physisorption isotherms of ZIF-L and dia(Zn) at 77 K appear as IUPAC type II isotherms,⁴³ suggesting that these are nonporous materials. The isotherms from the three dia(Zn) samples (dia(Zn)-HCOONa, dia(Zn)-CH₃COOH, and dia(Zn)-NH₄OH) were nearly identical, suggesting the synthesis conditions had no effect on the porosity of these materials. The BET surface area and micropore volume derived from the isotherms are summarized in ESI, Table S1.† ZIF-8 was shown to have a surface area and micropore volume far exceeding those of ZIF-L and dia(Zn). The porosity of these three crystals can be attributed to their structural topology. ZIF-L at 77 K possesses a crystal topology with interlayered hydrogen bonds formed by structural water molecules and no micropores. Dia(Zn) is a polymorph of ZIF-8 in which the zinc(II) and 2-methylimidazole are more densely packed. This explains why the micropore volume of ZIF-8 far exceeds that of dia(Zn).

Fig. 5 Nitrogen physisorption isotherms of ZIF-8 (circle), ZIF-L (diamond), dia(Zn)-HCOONa (down triangle), dia(Zn)-CH₃COOH (square), and dia(Zn)-NH₄OH (up triangle).

The results of the nitrogen physisorption tests were supported by thermogravimetric analysis (TGA), in which samples were heated under air flow (Fig. 6). ZIF-8 presented weight loss at temperatures below 200 °C, probably due to the loss of adsorbed water. ZIF-L presented a sharp weight loss at approximately 230 °C, which could be attributed to the loss of structural water molecules. Dia(Zn) did not undergo weight loss until 400 °C, which suggests that no water or other guest molecules adsorbed within this materials. This also provides additional evidence to support the results of nitrogen physisorption indicating the nonporosity of this material. Most of the weight loss associated with these three crystals occurred at approximately 400 °C, which can be attributed to the combustion of the organic moiety in these MOF materials. The nearly identical combustion temperature can be attributed to the polymorphism/pseudopolymorphism of these three crystals.

Fig. 6 TGA curves of ZIF-8, ZIF-L, and three dia(Zn) synthesized using various catalysts.

Cyclic tests of CO₂ adsorption/desorption were performed on ZIF-8, ZIF-L, and dia(Zn)-HCOONa using thermogravimetric analysis (TGA) under pressure of one atmosphere. The adsorption of CO₂ was conducted at 323.15 K (50 °C), whereas the desorption was performed at 373.15 K (100 °C). The temperature swing cycle and the cyclic adsorption results are summarized in Fig. 7. We performed 8 cycles of temperature swing on the three tested materials, during which all of the materials appeared stable. Our most important finding was that the three crystals shared similar CO₂ working capacities, despite the fact that the porosity of ZIF-8 appeared to be much higher than that of the other two crystals. More interestingly, the CO₂ working capacity of ZIF-L exceeded even that of ZIF-8, which appears counterintuitive. Nonetheless, this can be rationalized by the structural flexibility of MOF materials. Unlike porous materials comprising metal oxides, the major building units of MOFs are organic moieties, which give them a highly flexible framework. The framework flexibility of MOF materials has attracted considerable attention among researchers.^44–47 This flexibility is probably a key reason for the fact that ZIF-L and dia(Zn) are viewed as “nonporous” materials based on nitrogen physisorption at 77 K, but appear “porous” at 323.15 K. The framework in ZIF-L and dia(Zn) lacks flexibility at 77 K, which prevents the gas diffusion or adsorption by these two MOFs. The enhanced flexibility of these two materials at 323.15 K allows for the gas diffusion or adsorption by ZIF-L and dia(Zn). The framework flexibility of ZIF-L and dia(Zn) has yet to be investigated in details; however, the fact that ZIF-L has higher CO₂ adsorption has been reported previously,^24,48 based on measurements obtained using different methods. This paper is the first to describe the CO₂ adsorption properties of dia(Zn), and we determined that the CO₂ working capacity of dia(Zn) is approximately half of that of ZIF-8. This finding could lead to a new route for the design of MOF materials aimed at gas storage or separation. First, MOFs that are deemed “non-porous” at 77 K could become “porous” when the temperature is elevated. Second, the adsorption of CO₂ in dia(Zn) is associated with a gate-opening effect within its framework. The degree of gate-opening may depend on the gas molecules being transported. For example, materials in which CO₂ has a more pronounced gate-opening effect than N₂ would be ideal for the fabrication of membranes selective for flue gas. It is also worth mentioning that it has been known that MOF nanosheets show high performance in gas separation.⁴⁹ Our CO₂ adsorption results suggest that ZIF-L and dia(Zn) nanosheets could become emerging materials for this application.

Fig. 7 (a) Temperature swing cycle of the CO₂ TGA measurements. (b) Normalized CO₂ adsorption quantity, in mmol CO₂ per gram of adsorbent in the three crystals.

3.3 Hydrothermal stability

As discussed in the preceding section, the synthesis of dia(Zn) requires a catalyst and high temperature, which are not required for the synthesis of ZIF-L or ZIF-8. This suggests that the activation energy of dia(Zn) may be higher than that of the other two crystals. Thus, to formulate a diagram of energy versus reaction coordinate requires information pertaining to activation energy as well as the heat of the reaction. Hydrothermal treatment was applied to these three zinc(II)-2-methylimidazole MOFs: ZIF-8, ZIF-L, and dia(Zn). The dia(Zn) sample used in the hydrothermal treatment was dia(Zn)-HCOONa. The hydrothermal process involved refluxing the three MOF samples in water at 100 °C for 24 hours. The resulting MOF materials were then characterized by SEM and XRD, the results of which are summarized in Fig. 8. Changes were observed in the crystal morphology of all samples subjected to hydrothermal treatment; however, the morphological change in ZIF-8 (Fig. 8a and b) was less pronounced than that in ZIF-L and dia(Zn). As shown in the SEM images of ZIF-L and dia(Zn) (Fig. 8c–f), hydrothermal treatment led to the formation of rod-like crystals. The XRD results in Fig. 8g presented three additional peaks between 30–40° 2θ in ZIF-L and dia(Zn) after hydrothermal treatment. These three additional peaks are in agreement with the characteristic diffraction pattern of ZnO.^25,50,51 This suggests that hydrothermal treatment led to the partial decomposition of crystals in ZIF-L and dia(Zn), which then reformed into a more stable crystal of ZnO. Following hydrothermal treatment, the dia(Zn) sample still presented its characteristic XRD pattern, whereas the characteristic pattern of ZIF-L nearly disappeared. This indicates that the hydrothermal stability of dia(Zn) exceeds that of ZIF-L. The XRD pattern of ZIF-8 remained nearly unchanged after hydrothermal treatment, suggesting that it possesses the highest hydrothermal stability among the three polymorphs/pseudopolymorphs. ZIF-8 and dia(Zn) involve only covalent bonds in the structure, so they are expected to be more stable than ZIF-L, which involves both covalent and hydrogen bonds. Our experimental results did suggest that ZIF-L is the least stable crystal among the three MOFs. The reason that ZIF-8 was found to be slightly more stable than dia(Zn) might be due to that ZIF-8 possesses a zeolite SOD topology – the zeolitic MOFs are known to be more stable than non-zeolitic MOFs.⁵² It is worth noting that the arguments about stability of MOF crystals presented above were based upon the hydrothermal tests on pure-phase crystals.

Fig. 8 SEM images of ZIF-8 (a) before and (b) after hydrothermal treatment. SEM images of ZIF-L (c) before and (d) after hydrothermal treatment. SEM images of dia(Zn) (e) before and (f) after hydrothermal treatment. (g) XRD patterns of the three crystals before and after hydrothermal treatment.

We assumed that for the MOF system presented in this study, hydrothermal stability is positively correlated to the enthalpy of the MOF crystals. Specifically, crystals with higher hydrothermal stability were assumed to possess a lower enthalpy. According to the results of the hydrothermal tests, ZIF-8 has the lowest enthalpy among the three tested samples, and ZIF-L has the highest. The situation involving enthalpy and the activation energy of crystal formation (discussed in the preceding section) is summarized in Fig. 9 as a diagram of energy depending on the reaction coordinates. The proposed scheme suggests that the energy barrier to the formation of dia(Zn) exceeds that of ZIF-8 or ZIF-L, and that ZIF-8 is the most stable crystal among the three MOFs. This observation is in agreement with previous findings, which suggest that ZIF-L can be transformed into ZIF-8 under specific solvothermal conditions.³⁵ However, in another study, the crystal stability of ZIF-8 and dia(Zn) was investigated by inducing the crystal transformation of ZIF-8 using a milling operation as well as a computational study using density functional theory (DFT).³¹ The milling operation revealed that ZIF-8 is less stable than dia(Zn), which is inconsistent with the finding in this work. This disagreement may be due to differences in the means by which crystal stability was evaluated. Nonetheless, their DFT simulations using general gradient approximation (GGA) did suggest that ZIF-8 is more stable than dia(Zn), which agrees with our findings.

Fig. 9 Reaction coordinate diagram of the polymorph system composed of zinc(II) and 2-methylimidazole.

4. Conclusion

This paper discusses the synthesis, hydrothermal stability, and CO₂ adsorption properties of MOF polymorphs/pseudopolymorphs, ZIF-8, ZIF-L, and dia(Zn), which are composed of zinc(II) and 2-methylimidazole. We identified the criteria for the synthesis of dia(Zn) with definitive crystal morphology (hexagonal nanosheets); i.e., an appropriate catalyst and temperature of 60 °C. The catalyst for synthesizing dia(Zn) could be NH₄OH, CH₃COOH, or HCOONa. Based on the fact that ZIF-8 and ZIF-L can be synthesized in the absence of a catalyst at room temperature, we can deduce that the activation energies involved in the formation of ZIF-8 and ZIF-L are probably lower than that of dia(Zn). We also conducted hydrothermal treatment (at 100 °C) to evaluate the hydrothermal stability of these three crystals. ZIF-8 was identified as the most stable phase, and ZIF-L as the least. In nitrogen physisorption tests, ZIF-L and dia(Zn) both appeared to be nonporous; however, the CO₂ adsorption quantities of these two materials are of the same order of magnitude with that of ZIF-8, when measured using CO₂ TGA. The fact that nitrogen physisorption was performed at 77 K and CO₂ adsorption was performed at 323.15 K suggests that the frameworks of ZIF-L and dia(Zn) lack mobility at 77 K but might become flexible at 323.15 K. This observation suggests that even MOFs that are deemed non-porous in nitrogen physisorption testing at 77 K may in fact be useful for gas adsorption and separation at ambient temperatures. In addition, as MOF nanosheets have demonstrated their high performance in gas separation, dia(Zn) nanoflakes could become an emerging material for this application.

Acknowledgements

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan (MOST 104-2628-E-002-009-MY3 and MOST 105-2221-E-002-056-MY2) and National Taiwan University (NTU-CDP-105R7814). We would like to thank Chin-Yan Lin and Ya-Yun Yang in the Electron Microscope Unit of Instrument Center at National Taiwan University for assistance in obtaining SEM images. We thank Chin-Jung Lin at National Ilan University for conducting CO₂-TGA measurements. Valuable assistance from Cedric Po-Wen Chung and Hao-Ju Chou in the measurement of nitrogen physisorption is acknowledged. We would also like to thank Professor Huanting Wang at Monash University for generously providing the CIF file of ZIF-L crystal.

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Footnote

† Electronic supplementary information (ESI) available: Additional SEM images, FT-IR spectra, and pore volume and surface areas of materials derived from nitrogen physisorption isotherms. See DOI: 10.1039/c6ra19437k

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