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Quest for the Ncb-type Metal−Organic Framework Platform: A Bifunctional Ligand Approach Meets Net Topology Needs Di-Ming Chen, Nan-Nan Zhang, Jia-Yue Tian, Chun-Sen Liu,* and Miao Du* Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, People’s Republic of China S Supporting Information *

usually require multistep complex organic synthesis, which is of high cost. Ncb-type MOFs constructed from one simple single linker have not been explored, and it has been reported that it is very difficult for a single kind of ditopic ligand to fulfill the requirement for building the ncb network.13 This might be due to the particular requirement of the coordination model of the ditopic ligand: to achieve both three-coordinating sites (pyridyl carboxylate) and four-coordinating sites (ditopic dicarboxylate). It has been reported that the bifunctional ligand synthetic strategy was a good choice to design and synthesize porous MOFs, in which two kinds of coordinating groups embedded in one single ligand were used to bind with the in situ generated metal clusters.16−21 In this regard, such a type of ligand may exhibit the coordination models of the both functional groups. Bearing this in mind, in this work, we select a simple bifunctional ligand 4-(4-carboxyphenyl)-1,2,4-triazole (HCPT, Figure S1) as the linker for building a highly porous ncb-type MOF. The two distinct coordinative functionalities of the HCPT ligand were chosen for their propensities to bind in a monodentate or bidentate fashion and for their high possibility to form the Co3(OH) cluster. The self-assembly of HCPT with the CoCl2·6H2O under solvothermal conditions afforded a robust and porous MOF {(CoCl4)0.25[Co3(μ3-OH)(CPT)4.5](DMA)5(DEF)-(CH3CN)6}n (1, DMA = N,N-dimethylacetamide, DEF = N,N-diethylformamide), which represents the first case of ncb-type networks constructed from a single kind of ditopic ligand. The network of 1 contains three different types of cages to generate a polyhedron-stacked 3-periodic network with open channels in three orthogonal directions. Noticeably, the activated MOF shows high performance for C2H2 uptake at room temperature, and the selectivity of C2H2/CO2 and C2H2/ CH4 has also been appraised using the ideal adsorbed solution theory (IAST). Polyhedral dark purple crystals of 1 were harvested by reacting the bifunctional ligand HCPT with CoCl2·6H2O in a mixed solvent of DMA, DEF, and CH3CN with the 2fluorobenzoic acid (2-FA) as the additive. It is worth noting that the presence of 2-FA is key to the formation of 1. The Xray single crystal structural determination discloses that compound 1 belongs to the cubic space group I4̅3m. The foundational unit of 1 consists of one Co(II) center (half occupancy), two CPT− ligands (half occupancy), one μ3-OH atom (a sixth occupancy), and half of a CoCl42− anion (1/32; Figure S2). As shown in Figure 1a, all the six-coordinated

ABSTRACT: A custom-designed bifunctional ligand was used to connect an in situ formed Co3(OH) cluster affording a porous metal−organic framework, which represents the first case of ncb-type networks constructed from a single kind of ditopic ligand. Noticeably, the activated MOF shows high volumetric C2H2 uptake and excellent adsorption selectivity for C2H2/CO2 separation at room temperature with a low sorption heat.

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he past two decades have witnessed a burgeoning evolution of a kind of emerging porous material named metal−organic frameworks (MOFs) which facilitates the construction of a large variety of molecular architectures applied in numerous useful domains such as gas storage/ separation, heterogeneous catalysts, fluorescent sensors, and so on.1−6 The inherent structural modularity (such as various metal clusters and organic ligands with different geometries and lengths), high crystallinity, and amenability to fine-tuning of properties place MOFs as an advantageous material over other traditional porous materials such as active carbon and zeolite.7,8 The network topology is a powerful tool to provide the blueprints for rationally designing and even predicting the final structures of MOFs.9 Different MOFs with the same network topology might be obtained from different building blocks, and this further contributes to the modifiable functionalities of this material. Polynuclear clusters are very useful secondary building units (SBUs) for rational construction of MOFs with specified network topologies for their well-defined geometry, which could serve as structurally directing nodes.10 Among different forms of clusters, [M3(O/OH)] (M = Cr, In, Co, Ni, etc.), a well-known type of trinuclear cluster, has been used in the construction of many highly porous crystalline frameworks. For instance, Fèrey et al. reported the famous Cr(III)-MOF MIL101 constructed from the 6-connected Cr3O SBU and BDC ligands; Bu and co-workers have succeeded in the fabrication of a series of pacs-type MOFs by using the 9-connected In3O SBU and various mixed ligand systems.11,12 The ncb-type MOFs, which are usually constructed by a combination of pyridyl carboxylate and ditopic dicarboxylate ligands with the In3O or Ni3O clusters, represent a unique hierarchical channel-cavity biporous system, and its pore size and shape could be simply modulated by choosing linkers with different lengths.13−15 However, such a mixed-ligand system suffers from an uncertain construction possibility induced by the linker lengths and length ratios.14 In addition, the organic ligands with large length © XXXX American Chemical Society

Received: April 22, 2017

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DOI: 10.1021/acs.inorgchem.7b01020 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

performed, which exhibits the type-I character with a saturated uptake of 510 cm3/g. Derived from the N2 adsorption data, 1a exhibits a pore volume of 0.795 cm3/g and a BET surface area of 1927 m2/g (Figure 2a). For comparison, we simulated the

Figure 1. (a) The 9-connected Co3(OH) cluster of 1. (b) View of the three different types of cages in 1. (c) View of free volume of 1. (d) Natural tiling for the ncb network of 1. Figure 2. (a) N2 sorption isotherm at 77 K (inset: the pore size distribution analysis based on the Horvath−Kawazoe (HK) method). (b) Gas sorption isotherms around room temperature for 1a. (c) Comparison of Qst of C2H2, CO2 and CH4 for 1a. (d) IAST adsorption selectivities for an equimolar C2H2/CO2 and C2H2/CH4 gas mixture at 298 K.

Co(II) atoms are connected with four O atoms and two N atoms from five different CPT− ligands and one OH group, resulting in an octahedral geometry. Three Co(II) centers are linked together by the OH atom to afford the trinuclear Co3(OH) building units, and the valence of the OH atom has been confirmed by the BVS program.17 The CPT ligands in the network of 1 exhibit two different types of connecting mode (named μ4-CPT and μ3-CPT here): μ4-CPT uses its two N atoms and two O atoms to bind with the adjacent Co3 clusters and the μ3-CPT joins with the neighboring Co3 clusters employing its two N atoms and one O atom. There are three types of cages with different sizes and shapes in the network of 1: a tetrahedral cage with an internal free diameter of 8.1 Å was formed by four Co3 SBUs and six μ4-CPT units (Figure S3); four Co3 SBUs, three μ4-CPT’s, and three μ3-CPT units comprise a trigonal-pyramidal cage with an inner hole of 7.6 Å (Figure S4); and four Co3 SBUs, two μ4-CPT units, and 12 μ3CPT units comprise a distorted cube-like cage (Figure S5). Each tetrahedral cage is capped by four trigonal-pyramidal cages to generate a triakis tetrahedron (Figure 1b), which is further connected with the distorted cube-like cages though small passages (ca. 3.8 Å). The distorted cube-like cage is interconnected to form an NbO-type three-dimensional channel system with a pore diameter of 5.73 Å (Figure 1c). The framework topology of 1 could be viewed as a 9-connected ncb net with a point symbol of (312)(412)(512) by considering the Co3(OH) cluster as a 9-connected node (Figure 1d). The phase purity of the resultant as-synthesized 1 was confirmed by a comparison of the PXRD patterns of the asprepared samples with the simulated one from the crystal data (Figure S7). Thermogravimetry and powder X-ray diffraction showed that 1 can be readily activated without losing its crystalline state by exchanging the guest with CH2Cl2 and then heating at 80 °C for 12 h (Figure S8). The guest-free 1 (1a) is stable up to 300 °C, which was verified by the PXRD patterns at high temperatures (Figure S9). Furthermore, the water stability of 1 has been checked by soaking the as-synthesized crystals in water for 1 day, and the obtained PXRD pattern showed no obvious change compared with the simulated one, revealing its good water stability. To examine the permanent porosity of 1a, the N2 sorption isotherm at 77 K was

N2 sorption of 1 via the GCMC simulation, which gave a saturated N2 uptake of 532 cm3/g, confirming its full activation and framework integrity. Meanwhile, the calculated pore-size distribution reveals two types of micropore of 1a are 0.54 and 0.61 nm, respectively, which matches well the results calculated from single crystal data. The surface area of 1a is much higher than the reported ncb-type MOFs with similar ligand length such as Ni3(OH)(Ina)3(BDC)1.5 (1255 m2/g), [Co3(OH)(BDC)1.5(4-Ptz)3](H2NMe2) (1659 m2/g), and [Ni3(OH)(BDC)1.5(4-Ptz)3] (1569 m2/g), highlighting the promise of the bifunctional ligand in constructing porous MOFs.22,23 Furthermore, 1a could also adsorb a large amount of hydrogen at cryogenic temperature, which shows rapid kinetics and good reversibility without any hysteresis (Figure S10). The hydrogen uptake of 1a is 217 cm3/g (1.86 wt %) at 77 K and 1 bar, which exceeds that of MCF-9 (175 cm3/g) with a higher surface area, and is the highest among all the ncb-type MOFs reported. The H2 adsorption enthalpy at zero coverage is calculated to be 8.8 kJ/mol by the virial method (Figure S11), which is comparable with many famous porous materials with a high density of OMSs such as Zn-MOF-74 (8.3 kJ/mol) and UMCM-150 (7.3 kJ/mol).24 To evaluate the gas uptake performances of 1a, the lowpressure adsorption isotherms for C2H2, CO2, N2, and CH4 were collected around ambient temperature. As displayed in Figure 2b, 1a could adsorb 220 and 146 cm3/g of C2H2 at 273 and 298 K, respectively, which is much higher than for CO2 (95 and 47 cm3/g) and CH4 (22 and 13 cm3/g) under the same conditions. In addition, the acetylene storage of 1a is even comparable to some MOFs with a high density of OMSs such as UTSA-34b (121 cm3/g), UMCM-150 (129 cm3/g), and PCP-33 (122 cm3/g).25−27 In addition, the coverage-dependent adsorption enthalpies (Qst) of 1a for C2H2, CO2, and CH4 were also calculated on the basis of the virial method, which is a reliable and widely used methodology for calculating the B

DOI: 10.1021/acs.inorgchem.7b01020 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry *E-mail: [email protected].

sorption heat at zero coverage (Figure 2c). The calculated Qst of adsorption at zero loading for C2H2, CO2, and CH4 is 26.3, 24.2, and 16.8 kJ/mol, respectively. These observed discrepancies among C2H2, CO2, and CH4 absorption properties suggest that 1a might be a promising candidate for C2H2/CO2 and C2H2/CH4 separation. To probe the feasibility of separation mentioned above, the well-known Ideal Adsorbed Solution Theory (IAST) was used to calculate the adsorption selectivity by fitting isotherms of the experimental data for relevant gas mixtures using the single-site Langmuir (SSL) model. The adsorption selectivities of the C2H2/CO2 (50:50) and C2H2/CH4 (50:50) mixture for 1a at 298 K were shown in Figure 2d. It can be seen that the C2H2/CO2 and C2H2/CH4 selectivity of 1a lies in the range of 5−4.8 and 20.3−19 during the entire pressure range. The C2H2/CO2 adsorption selectivity of 1a is comparable to that of some famous MOFs such as UTSA-50a (5.0) and HKUST-1 (5.8), indicating its potential use for C2H2/CO2 separation.28 Moreover, when appraising the performance of a MOF-based adsorbent for practical use, one important factor that should be taken into account is the regeneration energy cost for a fixed bed.29 As mentioned above, the Qst for C2H2 is 26.3 kJ/mol, which is lower than that of many MOFs with a similar C2H2 uptake capacity or C2H2/CO2. The lower Qst value for 1a might be attributed to the lack of strong binding sites (such as open metal sites and free polar groups) in the network.27 The above outcome implies that the energy consumed for the regeneration of 1a should be much lower than those MOFs, leading to a significant energy savings. In conclusion, a new metal−organic framework with a channel-cavity porous network has been successfully prepared by linking the in situ generated trinuclear cluster through the bifunctional linear organic ligand HCPT, which represents the first case of ncb networks constructed from a single kind of ditopic ligand. This MOF shows high H2 uptake capacity and strong H2 binding ability. Moreover, the high volumetric C2H2 uptake, excellent adsorption selectivity for C2H2/CO2 and C2H2/CH4 mixtures, and low C2H2 sorption heat render this MOF a promising material for C2H2 separation with low regeneration cost. This work not only highlights the promise of the bifunctional ligand approach to target porous MOFs but also demonstrates the power of topology-guided synthetic strategies for building functional MOFs for selective gas sorption.

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ORCID

Chun-Sen Liu: 0000-0002-5095-7359 Miao Du: 0000-0002-1029-1820 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471134, 21571158, and 21601160), Plan for Scientific Innovation Talent of Henan Province (154200510011), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (152101510003), Program for Science & Technology Innovative Research Team in University of Henan Province (15IRTSTHN-002), and the Startup Fund for Ph.D.’s of Natural Scientific Research of Zhengzhou University of Light Industry (2015BSJJ042).

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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.inorgchem.7b01020. Additional structural pictures, PXRD data, TGA curve, and sorption data fittings (PDF) Accession Codes

CCDC 1545621 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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*E-mail: [email protected]. C

DOI: 10.1021/acs.inorgchem.7b01020 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b01020 Inorg. Chem. XXXX, XXX, XXX−XXX