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Cite This: Inorg. Chem. 2017, 56, 14767−14770

Microporous Cobalt(II)−Organic Framework with Open O‑Donor Sites for Effective C2H2 Storage and C2H2/CO2 Separation at Room Temperature Di-Ming Chen, Xiao-Hui Liu, Jia-Yue Tian, Jia-Hui Zhang, Chun-Sen Liu,* and Miao Du* Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, P. R. China

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S Supporting Information *

high C2H2 uptake capacity and excellent C2H2/CO2 separation performance. In this work, we seek to an alternative approach to constructing a porous MOF with high C2H2 uptake capacity, excellent C2H2/CO2 separation performance, and low preparation cost by appealing to the open O-donor-functionalized rodshaped secondary building unit (SBU) and a custom-designed bifunctional organic ligand. It has been well documented that employing rod-shaped SBUs could avoid interpenetration in MOFs, and the triazolcarboxylate ligand 4-(4-carboxyphenyl)1,2,4-triazole (HCPT; Figure S1) is a versatile organic building block for constructing functional MOFs.20−25 Bearing these in mind, we successfully obtained a new microporous cobalt(II)based MOF, [Co2(HCOO)2(CPT)2](NMF)5(H2O)2 (1; NMF = N-methylformamide), with 1D rod-shaped SBUs and the bifunctional organic ligand by a direct solvothermal reaction. Given its high pore volume and polar O-atom-functionalized pores, we study its gas sorption behavior toward C2H2 and CO2 around room temperature, and the results show that this MOF demonstrates a moderately high C2H2 storage capacity (145 cm3/g) and excellent adsorption selectivity for C2H2/CO2 (11) at room temperature. Furthermore, grand canonical Monte Carlo (GCMC) calculation results illuminate that the open O atoms on the formate groups account for the strong binding strength of the C2H2 molecules in the framework. Red polyhedral crystals of 1 were obtained by reaction of the bifunctional ligand HCPT with Co(NO3)2·6H2O in a mixed solvent of NMF/water (H2O) with fluoroboric acid (HBF4) as the additive at 80 °C. The X-ray study demonstrates that complex 1 belongs to the trigonal space group R3c̅ with three CoII ions, three CPT− ligands, and two formate groups in the structural unit (Figure S2). As shown in Figure 1a, all of the CoII (Co1, Co2, and Co3) ions exhibit six-coordinated octahedral coordination geometry, among which the Co2 and Co3 ions are on the 21 axis with half-occupancy. The three CoII ions are alternately bridged by the carboxylic, triazolate, and the formate groups originating from the decomposition of NMF to afford the 1D rod-shaped SBU. A prominent structural feature of 1 is the presence of octahedral cages that are comprised of 12 CoII ions and 12 CPT− ligands with an inner hole of 11 Å (considering the van der Waals radius; Figure 1b). Each octahedral cage has eight trigonal windows formed by three CoII ions and three CPT−

ABSTRACT: The self-assembly of a bifunctional organic ligand with a formate-bridged rod-shaped secondary building unit leads to a new microporous metal−organic framework (MOF). This MOF shows a moderately high C2H2 storage capacity (145 cm3/g) and an excellent adsorption selectivity for C2H2/CO2 (11) at room temperature. Furthermore, its discriminatory sorption behavior toward C2H2 and CO2 was probed by computational analysis in detail.

A

s an emerging type of crystalline adsorbent materials, metal−organic frameworks (MOFs) have undergone a booming development in the last 20 years because of their unique pore structures and precise design of pore functionality.1−3 The highly ordered structural features as well as modifiable inorganic and organic building blocks endow these materials enormous opportunities to be applied in gas storage/separation domains such as C2H2 storage and C2H2/CO2 separation, via adjustment of their pore sizes/shapes, framework topology, and chemical functionality.4,5 Furthermore, their well-characterized structures by means of single-crystal X-ray analysis make the sorption behavior of these materials understandable at the molecular level through computational analysis or an in situ neutron powder diffraction study, which could further guide the synthesis of new MOFs with improved performances.6−9 Being the simplest alkyne, acetylene (C2H2) has been widely used in various industrial applications such as cutting metals and organic synthesis.10 High-purity C2H2 is generally required in those applications; nevertheless, impurities such as carbon dioxide (CO2) always exist in C2H2 production. Their similar geometrical dimensions and boiling points make C2H2/CO2 separation a challenging task.11 To boost the C2H2/CO2 separation performance in MOFs, two strategies have been commonly used: (i) tuning the cross-sectional size of the pore; (ii) introducing functional organic linkers to provide strong binding sites.12−16 However, these strategies usually suffer from some drawbacks such as low C2H2 uptake capacity or the high cost of targeted organic ligands for multistep organic syntheses. According to the literature, ideal porous materials for column breakthrough gas separations are those that can not only take up a large amount of the preferred gas molecule but also display significantly high gas separation selectivity.17−19 In this case, it is still desirable to explore new ways to construct the MOFs with © 2017 American Chemical Society

Received: October 28, 2017 Published: December 7, 2017 14767

DOI: 10.1021/acs.inorgchem.7b02764 Inorg. Chem. 2017, 56, 14767−14770

Communication

Inorganic Chemistry

(Figure S8). The solvent-free 1 (1a) was obtained by solvent exchange with CH2Cl2 and then activated at 70 °C under high vacuum, and its porosity was checked by N2 adsorption at 77 K. 1a shows a stepwise adsorption behavior with a noticeable hysteresis (Figure 2a). The hysteretic adsorption/desorption profiles of 1 resemble the type IV isotherm or type H2 hysteresis based on IUPAC classifications.25 The former could be attributed to the presence of a hierarchical pore structure induced by the crystal defect, and the latter is related to the sequential filling of the pores of different sizes. From the N2 sorption isotherm, the calculated Brunauer−Emmett−Teller surface area and pore volume of 1a were estimated to be 973 m2/ g and 0.463 cm3/g, respectively, which are similar to the values (1021 m2/g and 0.479 cm3/g) derived from the crystal data via the software Poreblazer V3.0.2.26 On the basis of the results above and the crystal data, the possibility for the presence of a hierarchical pore could be ruled out. Instead, a stepwise filling might account for the abnormal sorption behavior in 1a.27 The N2 molecules first diffuse into the cages, resulting in the initial uptake, then they diffuse slowly into the small passages to finish the second uptake. In this case, desorption of N2 in the hindered passages would occur at pressures lower than they were adsorbed, similar to the case of capillary condensations in mesoporous materials, leading to a hysteresis.28−30 The available void space and open O-donor-functionalized pores of 1a promote us to study its C2H2, CO2, and CH4 sorption behavior around room temperature. As shown in Figure 2b, the C2H2 uptake capacity of 1a is 178 cm3/g at 273 K/1 bar and 145 cm3/g at 298 K/1 bar, which is much higher than those of CO2 and CH4 under the same conditions. It should be noted that the C2H2 uptake capacity of 1a at room temperature is comparable to that of UTSA-74a (145 cm3/g) and Zn-MOF-74 (150 cm3/g) with comparable surface areas (830 m2/g for UTSA-74a and 816 m2/g for Zn-MOF-74), but its CO2 sorption capacity (60 cm3/g) is much smaller than theirs (95 cm3/g for UTSA-74a and 146 cm3/g for Zn-MOF-74) under the same conditions.31,32 This also indicates that 1a would have a better performance in the C2H2/CO2 separation than UTSA-74a and Zn-MOF-74, although they have similar surface areas. To validate this

Figure 1. (a) Coordination surroundings for the CoII ions and the 1D rod-shaped SBU of 1. (b) Octahedral cage in 1. (c) View for the cagebased framework of 1. (d) Natural tiling for the network of 1.

ligands, and the aperture of these windows is about 6 Å (Figure S3). Each octahedral cage is connected to six other ones through the small passages in the ab plane, and these cages are further stacked along the c axis via sharing of the trigonal windows to give rise to the 1D nanosized channels (Figures 1c and S4). The linking of the 1D rod-shaped SBUs and CPT− ligands results in the formation of a 3D noninterpenetrating framework with a 48.6% solvent-accessible void (Figure S5). Topologically, the network of 1 could be viewed as a 4,4,4,6,6,6-connected net by considering the CPT− ligand as a 4-connected node and the CoII ion as a 6-connected node (Figures 1d and S6). The phase purity of 1 has been confirmed by power X-ray diffraction (PXRD) measurements of its as-prepared samples and corresponds well with the simulated one from the crystal data (Figure S7). Furthermore, the thermogravimetric analysis (TGA) curve of 1 shows a continuous weight loss of 35.9% from room temperature to 237 °C, corresponding to the loss of five NMF and two H2O molecules (calcd: 36.2%) in the lattice

Figure 2. (a) N2 sorption isotherm at 77 K. (b) C2H2 and CH4 sorption isotherms for 1a. (c) IAST adsorption selectivities for an equimolar C2H2/CO2 gas mixture at 298 K. (d) Comparison of the Qst values of C2H2 and CO2 for 1a. 14768

DOI: 10.1021/acs.inorgchem.7b02764 Inorg. Chem. 2017, 56, 14767−14770

Communication

Inorganic Chemistry

rod-shaped SBUs and a custom-designed bifunctional ligand. This MOF not only shows a moderately high C2H2 storage capacity at room temperature but also demonstrates highly selective C2H2/CO2 separation at ambient temperature and pressure. Compared with a traditional pure organic linker approach to construct MOFs with polar atom sites, the employment of functionalized metal SBUs is more operable and powerful to control the framework porosity and reduce the preparation cost of the targeted MOFs. This work also illuminates that the incorporation of uncoordinated open Odonor sites and rod-shaped SBUs into MOFs is unique for the construction of porous MOFs with high C2H2 uptake capacity and C2H2/CO2 separation performance.

speculation, the selectivities for C2H2/CO2 mixtures of 1a with different gas ratios at 298 K were calculated using ideal adsorbed solution theory (IAST) after fitting isotherms to the dual-site Langmuir−Freundlich equation. As shown in Figure 2c, the simulated adsorption selectivity (Sads) for the C2H2/CO2 binary equimolar mixture is 11, which exceeds those observed in other MOFs such as UTSA-74a (9), UTSA-50a (5), and Zn-MOF-74 and ZJU-60a (less than 5).13,31 These results indicate the potential in a practical procedure for C2H2/CO2 separation. Furthermore, the isosteric heats of adsorption (Qst) derived from virial analysis illuminates that C2H2 molecules could form a stronger guest−host interaction with 1a than CO2 (33 vs 27 kJ/ mol). In order to understand the unique sorption performance of 1a toward C2H2 and CO2, we carried out detailed GCMC simulation and density functional theory (DFT) calculation studies. The simulated sorption isotherms of C2H2 and CO2 correspond well with the experimental ones, indicating that the force-field-based simulation can exactly predict the sorption behavior in 1a (Figure S12). For C2H2 adsorption in 1a, the simulated preferred location of adsorbed C2H2 at zero loading is near four HCOO− groups in the small passages connecting the two octahedral cages, at which four C−H···O hydrogen bondings notably occur between the linear C2H2 and four bridged HCOO− groups (H1···O1 2.563 Å, H1···O2 3.374 Å, H2···O3 2.973 Å, and H2···O4 2.728 Å; Figure 3a). It could be noted that

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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.7b02764. Additional structural pictures, PXRD data, TGA curve, and sorption data fittings (PDF) Accession Codes

CCDC 1491598 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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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chun-Sen Liu: 0000-0002-5095-7359 Miao Du: 0000-0002-1029-1820 Figure 3. (a) Simulated favorable sorption site for C2H2 at zero loading and 298 K. (b) Simulated C2H2 density distribution at 298 K and 1 bar.

Notes

the distances of H1···O1 and H2···O4 are near to the sum of the van der Waals radius of the H and O atoms (2.6 Å), indicating that corresponding strong O···H−C hydrogen bonding exists. As for CO2, it is preferentially absorbed in the octahedral cages, with the O atoms forming weak supramolecular interactions with the H atoms on the CPT− ligand, and the large O···H distances indicate a weak interaction between CO2 and the framework (Figure S13). The static C2H2 and CO2 binding energies derived from the DFT calculation are 36 and 23 kJ/mol, which are similar to the values obtained from the sorption isotherms. Such a distinct sorption behavior can be attributed to the enhancement of polarization on the C2H2 molecule by the open O atoms on the HCOO− groups. Our calculations demonstrate that the open O donor is a powerful C2H2 recognition site and MOFs incorporating open O sites are promising materials for C2H2 capture and separation. The simulated distribution of C2H2 at 298 K and 1 bar also demonstrates the high density of C2H2 molecules located near the rodlike SBUs, indicating that the open O-donor groups contribute to the high C2H2 uptake capacity in 1a (Figures 3b and S14). In summary, we have targeted a new porous 3D MOF with a high density of open O-donor sites by using the formate-bridged

ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21471134, 21571158, and 21601160), Plan for Scientific Innovation Talent of Henan Province (154200510011), Program for Science & Technology Innovative Research Team in University of Henan Province (15IRTSTHN-002), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (152101510003), and the doctoral program of Zhengzhou University of Light Industry (2015BSJJ042).

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02764 Inorg. Chem. 2017, 56, 14767−14770