An Interpenetrated Pillar-Layered Metal-Organic Framework with

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An Interpenetrated Pillar-Layered Metal-Organic Framework with Novel Clusters: Reversible Structural Transformation and Selective Gate-Opening Adsorption Hong-Yun Yang,†,# Yong-Zhi Li,†,# Chen-Yu Jiang,† Hai-Hua Wang, Lei Hou,*,† Yao-Yu Wang,† and Zhonghua Zhu‡ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China ‡ School of Chemical Engineering, The University of Queensland, Brisbane 4072, Australia S Supporting Information *

ABSTRACT: A new metal−organic framework (MOF), [Co3(tzba)2(bpy)3(F)2]·DMF·C2H5OH·2H2O (1), has been constructed by solvothermal reaction of Co(NO3)2 with 4(1H-Tetrazol-5-yl) benzoic acid (H2tzba) and 4,4′-bipyridine (bpy) mixed ligands. 1 shows an interpenetrated threedimensional (3D) pillar-layered framework with the novel trinuclear [Co3(CN4)2(F2)] clusters. The π···π interactions existing among three bpy give rise to an uncommon tripillar in the framework. The framework reveals a reversible dynamic flexibility related to the solvent molecules. Meanwhile, the adsorption of CO2 exhibits an interesting gate opening−closing phenomenon which features the S-shaped isotherms with dramatic hysteretic desorption. The selective adsorption of CO2 over CH4 as well as the adsorption mechanism for CO2 were also discussed deeply.

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INTRODUCTION The capture and separation of CO2 have been paid extensive attention in many areas because of its wide existence in living and industrial processes, as well as the chief instigator of the greenhouse effect. Many endeavors have been directed toward the synthesis of new materials to capture or exclude CO2 from various gas mixtures, such as flue gas, natural gas, and landfill gas. Compared to the traditional zeolite and organic alkanolamine adsorbents, metal−organic frameworks (MOFs) have emerged as fast-growing crystalline materials.1−4 By the prescient choice of organic and inorganic building blocks, the structure and property of a MOF can be regulated at the molecular level. This endows MOFs with distinctive properties such as designability, tailorability, and high porosity to achieve specific physical characteristics and chemical functionalities.5−10 According to the classification, the flexible and dynamic frameworks are called the third generation of porous coordination polymers11,12 which have received the increasing attention of researchers because they feature reversible structural transformations incurred by guest inclusion and exclusion or the external physical stimuli of temperatures and pressures.13−18 In addition, some reported flexible MOFs also revealed the interesting structural transformations caused by the central metal exchange and guest anion exchange, which is a diffusion-exchange process with the stretch of the skeleton.19−23 In contrast to the rigid framework, the channels or pores in a flexible framework can be closed or opened for only certain guest molecules, generating a selective “gate opening” © XXXX American Chemical Society

phenomenon; for example, the pores open for CO2 but close for other gases.24−26 Therefore, the flexible MOF is more gasselective and possesses great potential in gas mixture separation as well.12,27−30 These advantages, however, the dynamic MOFs triggered by CO2 molecules as well as their application for selective CO 2 capture, were less thoroughly investigated.24−26,31−34 In general, one effective strategy for the formation of flexible MOFs is to build the pillar-layered frameworks employing dicarboxylate and diamine mixed ligands, wherein the layers formed by paddle-wheel M2(CO2)4 clusters and dicarboxylates are pillared by the third dimensional bridging diamine linkers.35−37 This approach commonly gives rise to the different degrees of interpenetrated frameworks by altering the lengths of pillars. The very unique structural feature of pillar-layered frameworks leads to not only the incidental distortion of single subnet but also the movement among the subnets;38−40 as a result, the pore sizes can be tuned for their selective separation application. In this work, we represented an uncommon flexible interpenetrated pillar-layered framework, [Co3(tzba)2(bpy)3(F)2]·DMF·C2H5OH·2H2O (1), assembled from the novel trinuclear [Co3(CN4)2(F2)] clusters with H2tzba (4-(1H-tetrazol-5-yl)benzoic acid) linkers and bpy Received: February 5, 2018 Revised: March 7, 2018

A

DOI: 10.1021/acs.cgd.8b00194 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(4,4′-bipyridine) pillars. Notably, unlike the common pillarlayered framework incorporating dicarboxylate ligands, a tetrazolate-carboxylate ligand was used to replace dicarboxylate ligands. The incorporation of tetrazolate groups leads to different types of pillar-layered frameworks and also forms more stable framework due to relative rigid coordination bond of tetrazolates.41 The CO2 adsorption isotherms of 1 show gate opening−closing phenomenon and hysteretic desorption as well, which allow remarkably selective CO2 adsorption over CH4.

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Table 1. Crystallographic Data and Structure Refinement Summary for Complex 1

EXPERIMENTAL SECTION

Materials and General Methods. All chemicals are commercially available and were used without further purification. An infrared (IR) spectrum was obtained through an EQUINOX-55 FT-IR spectrometer together with a KBr pellet. Elemental analyses for C, H, and N were recorded on a PerkinElmer 2400C Elemental Analyzer. Thermogravimetric analyses (TGA) were carried out in a N2 stream using a Netzsch TG209F3 instrument at a heating rate of 5 °C min−1. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was tested by an Axis Ultra spectrometer. Gas sorption isotherms were measured by an ASAP 2020 M adsorption equipment. Synthesis of [Co3(tzba)2(bpy)3(F)2]·DMF·C2H5OH·2H2O (1). A mixture of Co(NO3)2·6H2O (0.0291 g, 0.1 mmol), H2tzba (0.015 g, 0.08 mmol), bpy (0.0156 g, 0.1 mmol) in DMF (1 mL), ethanol (2 mL), water (4 mL), and two drops of HBF4 was sealed in a 25 mL container. The container was heated at 115 °C for 72 h and then cooled to room temperature at a rate of 10 °C h−1 to get pink prism crystals of 1. Yield: 45.7 mg (75.2%, based on H2tzba). The existence of F atoms in 1 that originate from the hydrolysis of HBF442,43 under solvothermal conditions, which was demonstrated by XPS spectra measurement (Figure S1). Anal. Calcd for C51H49Co3N15F2O8: C, 50.30; H, 3.88; N, 17.39; Found: C, 50.42; H, 4.07; N, 17.29%. IR (KBr, cm−1, Figure S2): 3430(m), 1668(m), 1611(m), 1543(m), 1442(s), 1218(w), 1068(w), 864(w), 811(m), 743(w), 631(w), 494(w). Crystallography. The single-crystal diffraction data were collected on a Bruker SMART APEX II CCD detector at 296(2) K using Mo Kα radiation. The structures were solved using direct methods and refined by the full-matrix least-squares method based on F2 by using SHELXL-97.44 Non-hydrogen atoms were refined with anisotropic displacement parameters. The disordered solvent molecules cannot be identified, and the SQUEEZE routine of PLATON was used in refining.45 Relevant crystallographic data was shown in Table 1. Selected bond lengths and angles were listed in Table S1. The solvent molecules in 1 were determined by elemental analysis results and TGA.

a

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complex

1

chemical formula formula weight T (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) μ (mm−1) reflns collected/unique GOF Rint R1a [I > 2σ] wR2b [I > 2σ]

C46H32Co3F2N14O4 1059.65 296(2) orthorhombic P2221 21.514(4) 11.443(2) 22.579(4) 90 90 90 5558.6(17) 4 1.266 0.942 34927/13467 0.948 0.0554 0.0538 0.1261

R1 = Σ(|Fo|−|Fc|)/Σ|Fo|. bR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.

Figure 1. Trinuclear cluster in 1.

RESULTS AND DISCUSSION Crystal Structure of 1. Complex 1 crystallizes in the orthorhombic P2221 space group, and shows a 3D pillar-layered framework with the trinuclear clusters. As shown in Figures 1 and S3), the asymmetric unit is composed of four independent Co2+ ions, four bpy ligands, two depronated tzba ligands, and two coordinated F− anions. 1 possesses two similar linear trinuclear metal-azolate-halide clusters, [Co3(CN4)2(F2)], formed by the bridging of F− anions and tetrazolate groups of tzba with three Co atoms. The cluster has a twofold symmetry of axis at the central Co atom, in which all Co atoms show the distorted octahedral coordination environments. Notably, the coordination polymers based on the metalazolate-halide trinary building blocks were frequently reported; however, the corresponding clusters-based compounds were rarely recorded.46−51 The cluster in 1 differs from some known trinuclear cluster containing azolates and halide anions, such as

[Cu3(CN4)4(Cl2)],52 [Cu3(C2N3)4(Cl2)],49,50 [Co3(C2N3)4F2],53 [Cd(C2N3)2(Cl)4],54 [Cd(C2N3)4(Cl)2].47,55 In the cluster of 1, the outer Co atom is coordinated by one chelating carboxylate group and one tetrazolate N atom of two tzba, two pyridine N atoms of two bpy, and one F atom. The central Co atom is ligated by two tetrazolate N atoms of two tzba, two pyridine N atoms of two bpy, and two F atoms. In 1, the cluster and the hung tzba are coplanar; meanwhile one cluster is linked by four tzba to form a square grid layer parallel to the ac plane. Interestingly, one cluster in the layer along the b axis is extended by two sets of three bpy that are located at the two opposite sides of the cluster, giving rise to a 3D pillar-layered framework (Figure 2a). Meanwhile, the same direction of three bpy pillars at one cluster are parallel and form B

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Figure 2. (a) 3D open framework of 1 viewed along the b axis. (b) Twofold interpenetrated framework of 1 viewed along the b axis.

strong π···π interactions with the short centroid-to-centroid distances of 3.680 and 3.721 Å for their pyridyl rings. This tripillar between the clusters in 1 is very favorable for the framework stability, which is unprecedented in complexes and is also very unique compared to the common monopillar.35−37 In addition, the layers in 1 are superposed along the b axis to afford a large grid. As a result, the two identical subnets are interpenetrated each other (Figure 2b), leading to the narrow intersected channels along the b and c axes with the dimensions of ca. 7.3 × 5.0 Å2 and 7.8 × 7.7 Å2, respectively (Figure 3).

removal of all H2O and DMF molecules (calcd 9.0%), followed by an abrupt weight loss due to framework decomposition. 1 was soaked in CH3OH for 72 h and then heated at 150 °C for 4 h under vacuum to remove solvent molecules. The activated 1 showed the disappearance of CO vibration at 1668 cm−1 from DMF existing in 1 (Figure S2), illustrating the removal of nonvolatile DMF molecules. TGA curve of the activated 1 showed the complete removal of all solvent molecules (Figure S5) which was also further confirmed by elemental analysis result (Anal. Calcd for activated 1, C46H32Co3F2N14O4: C, 52.14; H, 3.04; N, 18.50; Found: C, 52.07; H, 3.16; N, 18.39%). It is noted that the activated 1 shows the slight changes of some diffraction peaks in PXRD pattern, but still maintaining narrow and strong peaks, indicating the flexibility of the framework to a certain extent. The activated sample was soaked in original solution (DMF/ ethanol/water = 1:2:4) for 24 h, PXRD proved that the framework reversibly transfers back to the initial structure (Figure S4). This phenomenon reveals remarkable flexibility of 1, which is possibly caused by the movements of interpenetrated subnets accompanying with the release or inclusion of guest molecules.34 Gas Sorption. The gas adsorption isotherms were measured for N2 at 77 K, and CO2 and CH4 at 195 K on the activated 1, respectively (Figure 4). Low uptakes for CH4 and N2 were found, with the corresponding adsorption amounts of

Figure 3. 2D pore system in 1 formed by the intersected channels.

Structural Stability and Flexibility. PXRD demonstrated the purity of experimental samples of 1 (Figure S4). Moreover, after 1 was soaked in water for 24 h, the obtained PXRD is unchanged, suggesting the good stability toward water, which is an important prerequisite for the application of MOFs in practice (Figure S4). This stability should result from the formation of tripillars in the framework as well as the robust coordination bonds between Co atoms and tetrazolate groups. TGA of 1 exhibits a rapid weight loss of 3.7% from 30 to 61 °C (Figure S5), caused by the release of one C2H5OH solvent molecule per formula unit (calcd 3.8%). The continuous slow weight loss of 8.8% from 61 to 285 °C corresponds to the

Figure 4. Sorption isotherms of 1 for CO2 (195 K), CH4 (195 K), and N2 (77 K) (inset: an enlarged adsorption isotherm of CO2 at 195 K). C

DOI: 10.1021/acs.cgd.8b00194 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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12.7 and 9.4 cm3 (STP) g−1 at 1 atm. In contrast, 1 displays a type-I sorption isotherm for CO2 at 195 K, with the loading of 133.9 cm3 (STP) g−1 at 1 atm, and from which a BET surface area of 321.1 m2 g−1 is obtained. Notably, a non-obvious but intrinsic gate-opening phenomenon existed in the adsorption curve of CO2 in 1 at 195 K. The inflection pressure is about 0.25 kPa; above this point the adsorption amount is rapidly increased from 10 to 65 cm3 (STP) g−1 at 0.4 kPa, while below this point a small quantity of CO2 is loaded. Strikingly, the gate-opening phenomenon is more evident at both 273 and 283 K, which exhibits dramatic steps and Sshaped isotherms with serious hysteretic desorption (Figure 5).

Figure 6. CO2/CH4 selectivity of 1 at 100 kPa with different concentrations of CO2 for CO2−CH4 mixtures.

selectivity for the mixture with a 50% CO2 component is 9.7 at 100 kPa. Although this selectivity value is inferior to the values in [Zn2 (TRZ) 2(NDC)] (24) 65 and Cu-TDPDA (13.8),66 it is very competitive compared to the values reported in most MOFs (Table S2), such as ZIF-97 (9.14),67 NJU-Bai33 (8.9),68 (Cu4I4)[Cu2-PDC2(H2O)2]2 (8),69 UiO-66 (7),70 ZIF100 (5.9),71 JLU-Liu38 (5.6),72 and NOTT-101 (4.5).73 Furthermore, for the mixtures with the concentrations of CO2 increased from 1% to 99%, the CO2/CH4 selectivity shows the overall high values in the range of 9.5−10.8 at 100 kPa. The significant CO2/CH4 selectivity of 1 is mainly attributed to the larger quadrupole moment of CO2 than CH4 (CO2, 1.43 × 10−39 C m2; CH4, 0), which leads to stronger affinity between the framework and CO2 than CH4 molecules. Meanwhile, the smaller kinetic diameter of CO2 (3.3 Å) relative to CH4 (3.8 Å) leads to the easy diffusion of CO2 into the pore. This is very useful for the application in the purification process of natural gas (5% CO2 and 95% CH4), landfill gas (50% CO2 and 50% CH4), as well as various components of CO2−CH4 mixtures. GCMC Simulations. The adsorption sites of CO2 molecules in the framework were deeply studied by GCMC simulation performed at 273 K and 100 kPa (Supporting Information). Two main locations of CO2 molecules, CO2−I and CO2−II, were found existing in the pores of 1. CO2−I is close to the trinuclear cluster, with a vertical molecular axis to the plane of cluster (Figure 7a). Interestingly, due to the planar geometry of the cluster, three electronegative atoms, including one uncoordinated tetrazolate N atom, one F atom, and one carboxylate O atom, exactly chelate with one electropositive C atom of CO2−I, which is very similar to the situation where one metal ion is chelated by three pyridyl rings in metal-terpyridine complexes.74 Consequently, CO2−I is fixed firmly by the cluster through strong C···O, C···F, and C···N interactions, wherein the C···O (3.302 Å), C···F (2.902 Å), and C···N (3.205 Å) distances are comparable with the sum of van der Waals radii of the corresponding two atoms (C 1.70 Å; N 1.55 Å; O 1.52 Å; F 1.47 Å). Notably, the molecular axis of CO2−I is parallel to bpy ligands running along the b axis, as a result two O atoms of CO2−I are further anchored by C−H···O (H···O = 2.543− 3.095 Å) hydrogen bonds with the pyridyl −CH edge groups in two subnets. In contrast to CO2−I, CO2−II is observed in the vicinity of organic fragments of ligands in two subnets. For CO2−II, the C atom situates at the top of one pyridyl ring of bpy and forms C···π interactions with the C···πcentroid distance

Figure 5. Sorption isotherms of 1 for CO2 (273, 283, and 298 K) and CH4 (298 K).

At 273 K, 1 shows the low CO2 uptake of 16 cm3 (STP) g−1 at 47 kPa, however, which is increased sharply to 60 cm3 (STP) g−1 at 71 kPa, and then gradually reaches 85 cm3 (STP) g−1 at 1 atm. The adsorbed gas slowly released in the range of 1.0−0.27 atm and is suddenly desorbed at the lower pressures. At 283 K, the inflection points are gradually enhanced for both adsorption and desorption curves, with the gate pressures of 80 and 40 kPa, respectively. Furthermore, the inflection point and desorption hysteresis at 298 K have disappeared, and the uptake of CO2 is 19.4 cm3 (STP) g−1 at 100 kPa (Figure 5). The existing S-shaped isotherms in 1 controlled by CO2 gateopening pressure was not usually observed in MOFs.24−26,56−58 This phenomenon may be attributed to the framework flexibility.38−40,59 On one hand, the pillar-layered structure enables the single subnet in 1 distorted moderately; on the other hand, the movement between two subnets alters the pore sizes in a degree. Thereby it can be speculated that the pores in 1 are closed or opened at a moderate temperature and pressure because of the shrinkage-swelling of the framework accompanying with the release and inclusion of adsorbents.34,60,61 Notably, the inflection pressures are increased from 0.25 kPa at 195 K to 47 kPa at 273 K, and to 71 kPa at 283 K, this fact is ascribed to that the interactions of CO2 molecules with the framework become weak at a relatively high temperature;34,62,63 therefore, more CO2 molecules and higher gate-opening pressure are required to open the pores with the increased temperatures. Notably, 1 shows a trace of CH4 uptake, and the adsorption amount of 2.6 (STP) cm3 g−1 at 298 K and 1 atm is significantly lower than that for CO2 (19.4 cm3 (STP) g−1) at the same condition. The selectivities of 1 for CO2 over CH4 in binary CO2−CH4 mixtures were evaluated by the ideal adsorbed solution theory.64 As shown in Figure 6, the CO2/CH4 D

DOI: 10.1021/acs.cgd.8b00194 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 7. Two different locations of CO2 molecules in 1 with the multiple adsorption sites.

of 3.450 Å (Figure 7b). Two O atoms in CO2−II have the similar environments; both of them are attracted by two electropositive C atoms of phenyl and pyridyl groups from tzba and bpy, respectively, wherein the C···O separations vary from 3.037 to 3.286 Å. In addition, one O atom in CO2−II also contacts with one carboxylate C atom (C···O = 3.204 Å). The simulation clearly reveals that the more accessible porous surface due to the framework interpenetration is very important for CO2 capture, which provides the multiple adsorption sites for CO2 molecules, such as the typical phenyl and pyridyl rings as well as −CH groups in 1. Meanwhile, the unique planar structure of the cluster and the existence of accessible uncoordinated tetrazolate N atoms and F atoms as the direct binding sites play a crucial role in capturing CO2 molecules as well.

ORCID

Lei Hou: 0000-0002-2874-9326 Yao-Yu Wang: 0000-0002-0800-7093 Zhonghua Zhu: 0000-0003-2144-8093 Author Contributions #

Hong-Yun Yang and Yong-Zhi Li contribute equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21471124 and 21531007), NSF of Shannxi province (15JS113), and the Australian Research Council Future Fellowship FT12010072.

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CONCLUSION In summary, a new pillar-layered MOF has been constructed through the combination of H2tzba ligand and bpy mixed ligands, which contains the novel [Co3(CN4)2(F2)] clusters as well as a rare tripillar between the clusters. These specific building blocks contribute to the good chemical stability of the MOF toward water. Strikingly, the MOF reveals reversible structural transfer due to interpenetrated structures, which leads to not only a rare S-shaped isotherm for CO2 adsorption with an interesting gate-opening phenomenon and hysteretic desorption but also good adsorption selectivity for CO2 over CH4. In addition, the molecule simulation evidenced multiple CO2 binding sites in the framework. This contribution presents an alternative and less-adopted approach to enrich flexible pillar-layered frameworks.

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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.8b00194. IR, PXRD, TGA, additional molecule figures, adsorption simulation calculation, and bond lengths/angles (PDF) Accession Codes

CCDC 1818429 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 data_ [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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Corresponding Author

*E-mail: [email protected]. E

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DOI: 10.1021/acs.cgd.8b00194 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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DOI: 10.1021/acs.cgd.8b00194 Cryst. Growth Des. XXXX, XXX, XXX−XXX