Porosity Properties of the Conformers of Sodalite-like Zeolitic

ABSTRACT: The conformational isomers of zeolitic imidazolate frameworks (ZIFs) can have their own unique porosity and structural stability. We report ...
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Porosity Properties of the Conformers of Sodalite-like Zeolitic Imidazolate Frameworks Yejin Choi, Kyungkyou Noh, Jisu Lee, and Jaheon Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08997 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Journal of the American Chemical Society

Porosity Properties of the Conformers of Sodalite-like Zeolitic Imidazolate Frameworks Yejin Choi,†,§ Kyungkyou Noh,‡,§ Jisu Lee,† and Jaheon Kim†,‡,* †Department

of Chemistry, Soongsil University, 369 Sangdo-Ro, Dongjak-Gu, Seoul 06978, Republic of Korea of Information Communication, Materials, and Chemistry Convergence Technology, Soongsil University 369 Sangdo-Ro, Dongjak-Gu, Seoul 06978, Republic of Korea ‡Department

Supporting Information Placeholder ABSTRACT:

The conformational isomers of zeolitic imidazolate frameworks (ZIFs) can have their own unique porosity and structural stability. We report that a new sodalite-like ZIF (termed β-ZIF-65(Zn)) is polymorphous with as the existing ZIF-65(Zn) (Zn(nIm)2, nIm = 2-nitroimidazolate) but has a different linker conformation in the six-membered rings of sodalite cages. This conformational isomerism leads to distinctive permanent porosity for each conformer, which has been verified by gas adsorption measurements. In addition, variabletemperature X-ray diffraction analyses indicate that β-ZIF-65(Zn) is more resistant to displacive phase transitions than ZIF-65(Zn). The activated β-ZIF-65(Zn) conformer adsorbs 2.8 times more benzene than the activated ZIF-65(Zn) at P/P0 = 0.3 and 298 K. This work suggests that other types of ZIF conformers can be discovered.

Conformational isomers can produce different mechanical properties, electrical conductivity, and chemical reactivity.1 Since metal-organic frameworks (MOFs) contains molecular struts, they also show a variety of conformational changes such as gateopening2 and swelling3 triggered by applied heat or pressure.4 Moreover, without involving these stimulus-driven conformational changes, MOFs can exist as supramolecular conformational isomers.5 That is, as-synthesized MOFs can adopt different ligand conformations, leading to conformers. Compared with molecular atropisomers such as BINAP,6 the possible rotational isomers in IRMOFs have usually smaller rotational barriers, which is reflected on the structural disorders in crystal lattice.7 Compared with MOFs having metal clusters as nodes, zeolitic imidazolate frameworks (ZIFs) have much shorter metalmetal distances, and thus are more likely to form conformational isomers due to effective imidazolate-imidazolate interactions.8 Thus far, only ZIFs having compositions of Zn-(mnIm)2 (mnIm = 4-methyl-5-nitroimidazolate)9 and Zn(dcim)2 (dcim = 4,5dichloroimidazolate)10 are known as conformational isomers. It was also predicted that a variety of sod ZIF conformers can be achievable for 4,5-substituted imidazolates whereas 2-substituted imidazolates would not.10 Here, we report two ZIF conformers comprising of Zn ion and a 2-substituted imidazolates, nIm (2nitroimidazolate). These conformers present different crystal structures, permanent porosities, and phase transition behaviors as well, which has not been observed in MOFs and ZIFs thus far. We conducted 16 different solvothermal reactions employing various combinations of three amine solvents, N,N-

dimethylformamide (DMF), N-methylpyrrolidone (NMP), and 1,3-dimethyl-2-imidazolidinone (DMI) (Fig. S1). In more detail, 2-nitroimidazole (0.40 mmol) was reacted with Zn(NO3)2·6H2O (0.10 mmol) in 4.0 mL of a solvent at 120 °C for 48 h; only the solvent condition was varied systematically by combinations of the three amine solvents. Crystalline solids produced from the 16 setups gave similar PXRD patterns as the existing sod ZIF-65(Zn) (Figs. S2,3). However, we found that the intensities of the three peaks at 2θ = 24-27° for the products of the setups 7 and 11 were notably weaker than others. Therefore, we determined their crystal structures using single-crystal XRD (SCXRD) methods including the products from the setups 8 and 16 were also analyzed (Tables S2,3). All the crystals taken from the setups 8, 11 (three crystals), and 16 had the same structure as that of the known ZIF-65(Zn) with the cubic non-centrosymmetric I(-4)3m space group with a = 17.302(2) Å (the setup 16); ZIF-65(Zn) is known as NOF-111 or ZIF-10812. However, the crystal from the setup 7 crystallized in the cubic centrosymmetric Ia(-3) space group with a = 34.503(4) Å (Table S2). As in the measured PXRD patterns, many peaks in the simulated PXRD patterns generated from the crystal structures appeared at almost same diffraction angles due to their structural similarity in crystal lattices (Fig. S7). However, ZIF-65(Zn) gives greater intensities of the three peaks 2θ = 24-27° than the crystal 7 (Figs. S2,3); hereafter, the crystals from the setups 7 and 16 are designated as β-ZIF-65(Zn) (or β-sod) and ZIF-65(Zn) (or α-sod), respectively. Due to the severe disorder of the occluded solvents, their chemical compositions were determined by 1H-NMR analyses of the dissolved crystals in DCl/DMSO (Table S1, Fig. S9). Crystal structures revealed the delicate differences in the orientations of nIm linkers in sodalite-like frameworks. In both sod ZIFs, the four nIm linkers in the four-membered rings (4MRs) of sodalite cages adopt same alternating orientations (Fig. 1). The 4MRs in α-sod overlap by one-unit cell translations. In β-sod, the orientations of the imidazolates in two diagonally-facing 4MRs of a sodalite cage are inverted due to the crystallographic inversion center. More important structural difference is found in the local structures of 6MRs. In α-sod, all eight 6MRs in a sodalite cage have the same C3-6MR structures, where three in-plane and three perpendicularly out-of-plane imidazolates are related by a threefold rotational symmetry (C3). By contrast, among the eight 6MRs in β-sod, only two have the C3-6MR structures and the other six 6MRs have an irregular orientation of nIm linkers (C1-6MR), which is not shown in other ZIFs such as frl ZIF-77,13 rho CoNIm,14 Zn-mnIm,9 or Zn(dcim)210 (Fig. S8). The less

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symmetric C1-6MR gives distorted but slightly larger pore openings than the C3-6MR in α-sod having a circular opening of 3.0 Å in diameter.

and β-sod samples activated at 150 °C were 470 and 530 m2/g, respectively (Fig. S13). Analyses on the pore-size distribution showed the presence of the ~0.8 nm pores in α-sod and ~0.7 and ~1.2 nm pores in β-sod (Fig. S12c), suggesting that two activated conformers would have different framework structures and pore environments from each other.

Fig. 1. Comparison of the crystal structures for (a) ZIF-65(Zn) (αsod) and (b) β-ZIF-65(Zn) (β-sod): C3-6MRs and C1-6MRs are orange and cyan hexagons, respectively. Only the 4MRs are shown to highlight their orientation difference between two conformers. In the rightmost models of (a) and (b), fragments of a 4MR and 6MR are shown, where the orange and cyan spheres represent the centers of C6- and C1-6MRs, respectively. In (c), one of the fused 4MR and 6MR fragments in β-sod is displayed with ball-and-stick and space-filling models, respectively. The meaning of the blue and red arrows are explained in the main text.

Fig. 2. N2 adsorption-desorption isotherms measured at 77 K for the activated (a) α- and (b) β-sod ZIFs at the temperatures in parentheses. The PXRD patterns of the activated (c) α- and (d) βsod samples and after immersion in organic solvents.

The nIm linkers adopt a restricted conformation in the C1-6MR of β-sod toward interconversion to C3-6MR (Fig. 1d). When a nIm linker (A) in a 4MR rotates to make its imidazolate ring parallel to the 6MR plane, the adjacent two nIm linkers (B) in the 4MR and one nIm (C) in the 6MR must rotate accordingly in order to avoid unfavorable steric repulsion (the blue arrows). However, at the same time, the rotation of B would make C rotate in the opposite direction (a red arrow). In other words, C is in a locked conformation, and thus the conversion of the C1-6MR to the C3-6MR is not likely to happen easily; moreover, all the three 4MRs in β-sod (green squares in Fig. 2b) should be inverted in a sodalite cage. As mentioned above, this difference in local structures is hard to detect with laboratory PXRD data alone. For example, the product in the setup 11 gave a similar PXRD pattern as that of β-sod, but the selected single crystals turned out to be αsod (Table S3). The porosity of two conformers was analyzed by N2 adsorption measurements (Fig. 2a). The α-sod sample activated at 50 °C under vacuum for 10 h adsorbed a small amount of N2 gas with very long equilibrium time probably due to the unremoved solvent molecules in pores based on 1H-NMR analyses (Fig. S10). The activated sample at 150 °C under vacuum for 10 h gave a hysteretic isotherm similar to that of ZIF-65(Zn) nanocrystals.15 By contrast, the β-sod samples activated respectively at 50 and 150 °C adsorbed similar amounts of N2 gas with small hysteresis. The calculated Brunauer–Emmett–Teller surface areas for the α-

Both the activated α- and β-sod ZIFs at 150 °C produced similar PXRD patterns again (Fig. 2b). Notably, the major peaks at 2θ = 7.2°, corresponding to the reflections (110) for α-sod and (220) for β-sod of the as-synthesized crystals, disappeared and new peaks at 2θ = 6.7° appeared. In contrast, both peaks were observed in the PXRD patterns of the activated samples at 50 °C. In addition to this change, the shift and broadening of the higherangle peaks were indicative of phase transitions to unknown disordered structures. The crystallinity of the conformers could be recovered when the activated samples were immersed in their reaction solvents, that is, mixtures of DMF/DMI/NMP and DMF/DMI respectively for α-sod and β-sod (Fig. 2b). Similar recovery happened in other amine solvents (DMF/DMI for α-sod and DMF/DMI/NMP for β-sod), and benzene, but not in cyclohexane and xylene (Fig. S4). The recovery of the original crystallinity strongly supports that two conformers show displacive phase transitions as observed in amorphized or glassy ZIFs.16 Unfortunately, the activated crystals became opaque and were fragmented (Fig S12d), preventing the collection of SCXRD data. Variable-temperature PXRD (VT-PXRD) data for the samples stored in MeOH indicated that β-sod has a more rigid framework or is more resistant to phase transitions than α-sod (Fig. 3). It should be noted that the patterns are not exactly matched to those obtained with activated samples. The α-sod sample showed a structural change even at 50 °C, which was accompanied with a new peak at 2θ = 6.7° and also peak broadening at higher diffraction angles. This change may be attributed to the transformation to a low-symmetry structure involving structural disorder, which is observed in pressurized ZIFs.17 In the case of the β-sod sample, the PXRD pattern changed abruptly at 125 °C and as temperature increased, became similar to that of the

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Journal of the American Chemical Society activated sample at 150 °C. At the transition temperature at around 125 °C, the C1-6MR might convert to the C3-6MR as in Fig. 2d by overcoming a rotational energy barrier. However, since the ordered structure was recovered just in solvents at room temperature, the C1-6MR conformation was presumed to be maintained during the phase transition (Figs. 2c,d); therefore, the ZIF conformers are similar to resemble atropisomers than common rotational isomers.

Fig. 4. Benzene and cyclohexane adsorption isotherms of (a) αand (b) β-sod measured at 298 K. The PXRD patterns of the samples used for the measurements: (c) α- and (d) β-sod.

Fig. 3. VT-PXRD patterns of (a) α- and (b) β-sod. Before measurements at each temperature, the samples were heated at a rate of 2 °C/min under N2 flow (70 cc/min). While there were no particular differences in the CO2 and CH4 adsorption behaviors (Fig. S14), two conformers showed significantly benzene adsorption behaviors using the activated samples at 150 °C under vacuum for 10 h (Fig. 4). Similar to the N2 adsorption, the activate β-sod showed smaller hysteresis than α-sod. At a relative pressure of P/P0 = 0.3, β-sod adsorbed 2.8 times more benzene than α-sod. Both ZIFs adsorbed only small amounts of cyclohexane as expected from the PXRD patterns for the samples soaked in liquid cyclohexane (Figs. 2c,d). Both ZIF-7 and ZIF-8 coated on silica-coated quartz crystal microbalance (QCM) substrates had a greater benzene/cyclohexane selectivity than ZIF-65(Zn) (α-sod), and can be used for the separation application.18 Since β-sod stores more benzene (4.11 g/g or 3.7 mmol/g at P/P0 = 0.3) than ZIF-7 (1.8 mmol/g) and ZIF-8 (3.2 mmol/g), it may also be utilized for separation of benzene and cyclohexane, which is an important process in petrochemical industry. 19

Unexpectedly, the adsorbed benzene molecules, respectively 79 (α-sod) and 85% (β-sod) relative to the amounts at P/P0 = 0.9, were not able to be desorbed. (Figs. 4a,b). The restored PXRD patterns of the ZIF samples collected after the adsorption measurements indicated the tight encapsulation of benzene in the ZIFs (Figs. 5c,d). The samples heated at 50 °C in an oven for 3 h produced similar PXRD patterns. Interestingly, β-sod released benzene more slowly than α-sod based on 1H-NMR analyses; αand β-sod released 61 and 45% of benzene, respectively (Fig. S15). This phenomenon is similar with commensurate adsorption where tight guest adsorption induces the structural reorganization of flexible frameworks.20 However, the slow release of adsorbed benzene molecules at elevated temperature is rarely observed in MOFs and ZIFs.21 In summary, we presented that two conformational isomers of sodalite-like ZIFs exhibited significantly different permanent porosities and phase transition behaviors. One of the conformers, β-ZIF-65(Zn) stored more benzene than the other conformer, and released more slowly benzene molecules at elevated temperature. Other types of ZIF conformers are anticipated to be discovered if proper reaction conditions are found.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Methods (PDF) Crystal data for α-sod at 100 K (CIF) Crystal data for β-sod at 100 K (CIF) Crystal data for α-sod at RT (CIF) Crystal data for β-sod at RT (CIF)

AUTHOR INFORMATION Corresponding Author [email protected] ORCID Jaheon Kim: 0000-0001-6430-8790

Author Contributions

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§These

authors contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Research Foundation of Korea (NRF2014R1A2A1A11054190, NRF-2016R1A5A1009405). We acknowledge Pohang Accelerator Laboratory (PAL) for the X-ray data collection at the 2D-SMC beamline, and are grateful to Prof. J. H. Kwak for use of the VT-PXRD facility at the UNIST. We also thank Dr. N. Ko and Prof. E. Lee at the POSTECH for the collection of the single-crystal X-ray diffraction data at room temperature.

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