Zeolitic Tetrazolate-Imidazolate Frameworks with High Chemical

May 10, 2016 - Zeolitic Tetrazolate-Imidazolate Frameworks with High Chemical Stability for Selective Separation of Small Hydrocarbons. Min-Yu Li, Fei...
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Zeolitic Tetrazolate-Imidazolate Frameworks with High Chemical Stability for Selective Separation of Small Hydrocarbons Min-Yu Li, 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 S Supporting Information *

ABSTRACT: Presented here are two zeolitic tetrazolate-imidazolate frameworks (ZTIFs) with SOD and cag topologies (ZTIF6 and -7) directed by link−link interaction. Furthermore, the SOD-type ZTIF-6 shows high chemical stability and permanent porosity, as well as selective separation of C2 hydrocarbons over C1 methane. Zn2(tz)2(2-mbim)2·DMF (ZTIF-7, cag, tz = tetrazole). Similarly, the steric interference between the substitute groups (link−link interaction)25 leads to distinct topologies in the final structures. Furthermore, ZTIF-6 exhibits permanent porosity and selective separation of C2 hydrocarbons over C1 methane. ZTIF-6 and -7 were solvothermally synthesized by mixing Zn(CH3COO)2·2H2O and 2-methylbenzimidazole with 5methyltetrazole (5-Hmtz) or tetrazole (tz) in N,N-dimethylformamide (DMF) and ethanol solvents, respectively (see Supporting Information for details). X-ray single crystal structural determination reveals that ZTIF-6 crystallizes in trigonal R3m, which is similar to the homochiral framework HZIrF-1.29 The big difference between the 5-mtz and 2-mbim ligands reduces the whole symmetry (from cubic to trigonal). There are one crystallographically independent Zn(II) atom, one and a half 5-mtz, and half 2mbim ligands (Figure 1a). Each Zn2+ center has tetrahedral geometry and is coordinated by three N atoms from three 5mtz ligands and one N atom from the 2-mbim ligand. Either 5mtz or 2-mbim ligand is a μ 2 -linker similar to 2methylimidazole in ZIF-8. Each 2-mbim bridges two zinc centers with the Zn···Zn distance of 5.884 Å, and the Zn···Zn distances separated by 5-mtz are from 6.020 to 6.077 Å. The bond angle of Zn−2-mbim−Zn is ca. 140°, and the bond angles of Zn−mtz−Zn are ca. 152° and 153°. A (46·64) SOD cage

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etal−organic frameworks (MOFs) with zeolitic topologies, integrating the high porosity of MOF materials and high thermal and chemical stability of zeolites, have received extensive attention.1−33 A common but most useful strategy to construct zeolitic MOFs is the combination of tetrahedrally coordinated divalent cations (M2+ = Zn2+ or Co2+) and the uninegative azolate ligands, such as imidazolate, triazolate, and tetrazolate derivatives.16−35 Correspondingly, zeolitic imidazolate frameworks (ZIFs), tetrahedral tetrazolate frameworks (TTFs), or zeolitic tetrazolate frameworks (ZTFs)16−25 have been developed. Among them, ZTFs or TTFs26−33 with uncoordinated N-heteroatom sites from the μ2tetrazole ligand show higher CO2 uptake than that of ZIFs, and potential application to carbon dioxide capture and separation. However, such materials display non-zeotype topologies and have less chemical stability than that of ZIFs. So it is still an intriguing challenge to create metal-tetrazole frameworks with exceptional stability and zeolitic topologies. Recently, we have developed one kind of zeolitic tetrazolateimidazolate framework (ZTIF) with uncoordinated N-heteroatom sites and zeolite-type topology. In such a system, imidazoles act as the structure-directing agents (SDAs) and colinkers to construct a zeolite-type host framework, and tetrazoles cooperate on the construction of the framework and also functionalize the pore surface by providing uncoordinated N-heteroatom sites. As a continuous work, we report here another two ZTIFs, namely, Zn2(5-mtz)3(2-mbim)·guest (ZTIF-6, SOD, 5-mtz = 5methyltetrazole, 2-mbim = 2-methylbenzimdazole) and © XXXX American Chemical Society

Received: March 16, 2016 Revised: May 5, 2016

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DOI: 10.1021/acs.cgd.6b00422 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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X-ray single crystal structural determination reveals that ZTIF-7 crystallizes in orthorhombic Pbca. Each zinc center is tetrahedrally coordinated by two tz and two 2-mbim ligands (Zn−N bond length 1.983−2.044 Å) (Figure 2a), which leads

Figure 2. (a) The coordination mode of zinc atoms of ZTIF-7; (b) the simplified (42.68) cage in ZTIF-7; (c) the 3D framework of ZTIF-7 along the b-axis; (d) the cag topology of ZTIF-7.

to a 3D framework with cag topology (Figure 2c,d). Similar to ZTIF-6, both ligands are μ2-linker. The 2-mbim bridges two zinc centers with the Zn···Zn distance being 5.949−5.998 Å, and the bond angle of Zn−2-mbim−Zn is ca. 142.1−145.5°. Each tz bridges two zinc centers with the Zn···Zn distance being 6.095−6.133 Å, and the bond angle of Zn−tz−Zn is ca. 153.6−156.4°. The longer Zn···Zn distances and bigger bond angles of Zn−tz−Zn in ZTIF-7 than that of ZTIF-6 result in the formation of lower symmetry and different topology. The different cage with (42·68) topology in ZTIF-7 is observed (Figure 2b). Each cage encapsulates two ordered DMF molecules. Among it, the four-membered ring is formed by four tz ligands linking four Zn atoms. The six-ring windows are blocked by the substitutes of 2-mbim, generating a 1D channel with irregular windows shape along the b-axis (Figure 2c). The solvent-accessible volume of ca. 28.5% of the crystal volume was calculated with the PLATON program. The chemical stability was examined by suspending samples of ZTIF-6 and ZTIF-7 in common solvents, such as ethanol, methanol, water, DMF, and CH2Cl2 (Figure S2). The assynthesized samples were immersed in the desired solvent for 1−7 days at ambient temperature and the boiling point of methanol. Powder X-ray diffraction (PXRD) patterns collected for each sample showed that the solid samples of ZTIF-6 and ZTIF-7 maintained their frameworks in water and boiling methanol. In comparison, TTFs only constructed by 5-mtz ligands were transformed to unidentified materials when they were soaked in methanol at room temperature,26 and even the SOD-type TTF-4 which can be stable in methanol also become amorphous white powder in water at room temperature.30 To explain this unusual phenomenon, the structural details of ZTIF-6 and -7 were carefully examined. As mentioned above, in all the tetrazolate-based 4-connected frameworks,26−33 longer Zn−N bond lengths and bigger Zn-tetrazolate-Zn bond angles than that of ZIFs are observed, so we can deduce two important structural features of them. First, the ZnN4 units in these compounds are distorted tetrahedra. It is likely that the polar molecules, such as water and methanol, may be attracted by the

Figure 1. (a) The coordination mode of zinc atoms of ZTIF-6; (b) the SOD cage constructed by Zn-tetrazolate-imdazolate; (c) and (d) the comparison of the simplified etb net and SOD cage (e) the 3D framework of ZTIF-6; (f) the topology of ZTIF-6.

consisting of 24 Zn atoms, 9 2-mbim ligands and 27 5-mtz ligands is observed (Figure 1b,d). The four-membered and sixmembered rings along other directions are blocked by the big substitutes of the 2-methylbenzimidazole, just leaving the two six-membered rings along the c-axis (Figure S1). Then these cages link each other based on the edge-expansion to generate a SOD-type three-dimensional (3D) framework including onedimensional (1D) channel along the c-axis (Figure 1e,f). The presence of the uncoordinated N-heteroatom sites from 5-mtz and alkyl groups of the 2-mbim makes the out surface and inner space both hydrophilic and hydrophobic. The solventaccessible volume of ca. 45.7% of the crystal volume was found by the PLATON program.36 These voids were filled by the structurally disordered DMF molecules. Another interesting structural feature of ZTIF-6 is the presence of a 3D framework with rare 3-connected etb topology derived from the 4-connected SOD topology. Without consideration of the 2-mbim ligands, each Zn center is linked by three 5-mtz ligands to form a cationic 3D framework, [(Zn2(5-mtz)3)+]n (Figure 1c). Correspondingly, each Zn center acts as a 3-connected node. While three 5-mtz ligands are classified into two kinds. One is linking the Zn atoms to form 31 helices with both handnesses. Another one is linking such helices to form 3D framework. In this way, the framework [(Zn2(5-mtz)3)+]n could be topologically represented as a 3-connected etb net. On the basis of this result, the etb net can also be considered as the interrupted SOD net. By employing 2-mbim and tz to assemble with zinc salts, another cag-type ZTIF-7 was obtained under a similar reaction condition. For ZTIF-6 and ZTIF-7, their distinct topologies are obviously directed by the link−link interactions. B

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hydrophilic pore and surface structure and coordinated to the metal centers. The tetrahedral center then transforms into an octahedral one, resulting in the dissolution of the framework. And second, bonding between tetrazolate and metal center is not as stable as that of the ZIFs, which also means the frameworks easily encounter the attack of the polar molecules. In ZTIF-6, similar structural features are observed; differently, each Zn center is linked by three 5-mtz ligands to form a 3D framework, which is further stabilized by 2-mbim ligands. In addition, the small window size and both hydrophilic and hydrophobic surface and inner space are also in favor of the whole stability. In combination, these two features of ZTIF-6 make it more stable than others. As for the ZTIF-7, it may also be attributed to the irregular channel and low pore volume besides the above aspects. Gas-adsorption measurements of ZTIF-6 and ZTIF-7 were performed on a Micromeritics ASAP 2020 surface-area and pore-size analyzer. The samples were activated by solvent exchange with dry ethanol followed by evacuation at 80 °C for 6 h, respectively. The permanent porosity of ZTIF-6 was confirmed by the reversible N2 sorption measurements at 77 K, which showed type I adsorption isotherm behavior (Figure 3).

Figure 4. Gas sorption isotherms of ZTIF-6 at 273 K (a) and 300 K (b). Solid represents adsorption and open represents desorption.

tetrazole ligands can increase the uptake and the enthalpy of CO2. Interestingly, ZTIF-6 exhibits selective separation of the hydrocarbons under environment conditions. At 1 atm, the maximum uptake of C2H6, C2H4, and CH4 are 69.1 cm3/g (3.08 mmol/g), 69.4 cm3/g (3.10 mmol/g), and 13.9 cm3/g (0.62 mmol/g) at 273 K, and 50.3 cm3/g (2.25 mmol/g), 46.5 cm3/g (2.07 mmol/g), and 9.1 cm3/g (0.41 mmol/g) at 300 K, respectively. Obviously, ZTIF-6 exhibits adsorption capacity in the trend with C2 > C1. Correspondingly, the adsorption enthalpies of C2H6 and C2H4 (21.84 and 18.5 kJ/mol) are also higher than that of CH4 (10.26 kJ/mol). These results indicate that ZTIF-6 is a potential candidate for the separation of CH4 from C2 hydrocarbon mixtures. Furthermore, the adsorption selectivities of different hydrocarbons with respect to CH4 were calculated by the ideal adsorbed solution theory (IAST).37 In a range of pressures up to 1 atm at 300 K, the selectivities of C2H6 and C2H4 components with respect to CH4 are 6.7 and 5.4, respectively, which suggests that ZTIF-6 may be a potential candidate material for the separation of C2H4 over CH4. In summary, we report here two new ZTIFs materials with SOD and cag topologies, which are directed by link−link interactions. Both of them exhibit high chemical stability. Furthermore, ZTIF-6 shows selective separation of C2 hydrocarbons over C1 methane. We expect that these findings will lead to many new applications in the energy and environment fields.

Figure 3. N2 sorption isotherms of ZTIF-6 (a) and ZTIF-7 (b) at 77 K. Solid represents adsorption and open represents desorption.

The Langmuir and BET surface areas were 1141 m2/g and 790 m2/g for ZTIF-6, respectively. A single data point at relative pressure at 0.99 gives a pore volume of 0.42 for ZTIF-6 by the Horvath−Kawazoe equation. N2 isotherms for ZTIF-7 were measured to 1 atm at 77 K. It shows that ZTIF-7 almost has no adsorption of N2. The main reason may be that the guest molecules are hard to be exchanged from its narrow channel. The permanent porosity as well as its remarkable chemical stability of ZTIF-6 encouraged us to examine its potential application in gas separation. The pure component sorption isotherms for CO2 and various hydrocarbons at 1 atm under ambient conditions were measured. As shown in Figure 4, the CO2 uptake at 1 atm was 53 cm3/g (2.37 mmol/g) at 273 K and 24 cm3/g (1.07 mmol/g) at 300 K. The adsorption enthalpy (Qst) for CO2 calculated by Clausius−Clapeyron equation is from 18.6 to 19.56 kJ/mol (Figure S7). Although the uptake and enthalpy for CO2 are modest in the tetrazolebased zeolitic MOFs,26−35 it outperforms most of the ZIFs, including the well-known ZIF-8 (also named MAF-4, 29.3 cm3/ g at 273k) with same SOD topology (Table S1).20 These results indicate that the uncoordinated N-heteroatom sites of C

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00422. Experimental details, TGA diagram, powder X-ray diffraction (PDF) Accession Codes

CCDC 1437958−1437959 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support of (2012CB821705), NSFC 21521061), and Chunmiao Chinese Academy of Sciences

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this work by 973 Program (21425102, 21573236 and Project of Haixi Institute of (CMZX-2015-001).

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DOI: 10.1021/acs.cgd.6b00422 Cryst. Growth Des. XXXX, XXX, XXX−XXX