English

• 1. INTRODUCTION

  Metal-organic frameworks (MOFs) are crystalline materials consisting of metal ions or clusters and organic ligands. Owing to their inherent porosity and high specific surface area, they have wide applications in many fields such as adsorption, purification, separation, sensing, catalysis and drug delivery^[1-8]. It is well known that the properties of MOFs are mainly determined by their structures including pore size and chemical environment. In order to achieve satisfactory properties, chemists not only select ligands with different sizes and metal ions with specific symmetry to tailor the pores of MOFs^{[9, 10]}, but also introduce extra functional groups into the pores of MOFs to realize corresponding applications^[11-14]. Up to now, the synthesis of MOFs with suitable pore size and chemical environment for directional application is one of the hot topics in the field of materials and chemistry.

  Using MOFs materials to selectively capture carbon dioxide from exhaust gas of power plants and purify highvalue hydrocarbons such as acetylene has attracted considerable attentions^{[4, 15-18]}. In order to improve the adsorption capacity and selectivity, it is necessary to introduce open metal ions (OMS) and Lewis base sites (LBS) into the pores of MOFs. Recently, we have prepared a porous framework FJI-H14 based on H₂BTTA (H₂BTTA = 2, 5-di(1H-1, 2, 4-triazol-1-yl)terephthalic acid) ligand and Cu(NO₃)₂. It showed high adsorption capacity for carbon dioxide due to the unusual synergistic effect of OMS and LBS^[17], so we intend to assemble H₂BTTA (Fig. 1a) with other metal ions lighter than Cu(Ⅱ) ion to construct potential MOFs for carbon dioxide uptake. Herein, a novel metal-organic framework FJI-H24 has been prepared, which has one-dimensional (1D) quadrilateral channels with high density of open metal sites and free Lewis base sites. The gas adsorption tests demonstrate that FJI-H24 has moderate CO₂ (34.0 cm³·g^–1) and C₂H₂ (53.0 cm³·g^–1) adsorption capacity and high selectivity of CO₂/N₂ (87) and C₂H₂/CH₄ (66) under ambient conditions (298 K, 1 atm).

  2. EXPERIMENTAL

  2.1 Materials and methods

  All chemicals are purchased from commercial sources and used without further purification. Elemental analyses for C, H and N are performed on an elemental Vairo EL analyzer. Thermogravimetric (TG) analysis experiment is carried out on a Netzsch STA 449C instrument from room temperature to 900 ℃ under N₂ atmosphere at a heating rate of 10 ℃·min⁻¹. Fourier transform infrared spectra are recorded in 4000~400 cm^–1 on a Bruker Optics VERTEX 70 FT-IR spectro-photometer with KBr pellets. The powder X-ray diffraction patterns (XRD) are recorded on a RIGAKU-DMAX2500 X-ray diffractometer using CuKα radiation (λ ＝ 0.154184 nm). The gas sorption experiments are performed by using the Accelerated Surface Area and Porosimetry measurement 2020 system (ASAP2020), and all measurements are implemented by using extremely high purity gases N₂, CO₂, C₂H₂ and CH₄ (> 99.999%).

  2.2 Synthesis of FJI-H24

  A mixture of anhydrous CoCl₂ (0.007 g, 0.05 mmol), H₂BTTA (0.030 g, 0.1 mmol), DMF (2.0 mL), and H₂O (0.5 mL) was added into a 20 mL high-pressure autoclave, heated at 110 ℃ for 72 h and cooled down to room temperature. Finally, the pink block crystals of FJI-H24 were obtained with 80% yield. IR (KBr, cm⁻¹): 3437, 3141, 1666, 1577, 1533, 1404, 1361, 1278, 1137, 1055, 985, 813, 651, 595 and 540. Elemental analysis calcd. (%) for FJI-H24 (C₁₂H₁₀CoN₆O₆): C, 36.66; H, 2.56; N, 21.37. Found: C, 36.98; H, 2.29; N, 21.12%.

  2.3 Structure determination

  Single-crystal X-ray diffraction data are collected on a Super Nova diffractometer with CuKα radiation (λ ＝ 1.54184 Å) by using the ω-scan technique at 164.0(9) K. Data collection and reduction are measured on CrysAlisPro 171.37.35 software. The structure of FJI-H24 is solved by SHELXT (direct methods) and refined by SHELXL (fullmatrix least-squares techniques) in the Olex-2 package^[19]. The non-hydrogen atoms are refined anisotropically, and the hydrogen atoms of organic linker are refined isotropically as riding atoms. The guest solvents are removed through the SQUEEZE option of PLATON^[20]. FJI-H24 crystallizes in monoclinic system with space group I2/a, a = 11.1096(5), b = 15.5982(7), c = 16.7051(9) Å, β = 108.394(6)°, V = 2746.9(2) ų, Z = 4, D_c = 0.951 g·cm^–3, F(000) = 796 and μ = 5.147 mm^–1. A total of 9151 reflections and 2458 unique ones (R_int = 0.0366) are measured in the range of 3.98≤θ≤67.05º, of which 2128 with I > 2σ(I) are used in the succeeding refinement. The final R = 0.0590, wR = 0.1804 (w = 1/[σ²(F_o²) + (0.1159P)² + 4.9603P], where P = (F_o² + 2F_c²)/3), (Δρ)_max = 0.533, (Δρ)_min = –0.469 e/ų, (Δ/σ)_max = 0.000 and S = 1.060. The final chemical formula of FJI-H24 is deduced by the crystal data after SQUEEZE process. Partial bond lengths and bond angles of the structure are summarized in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(2)	2.101(2)		Co(1)–O(1)	2.117(2)		Co(1)–N(3)b	2.119(3)
  Co(1)–O(2)a	2.101(2)		Co(1)–O(1)a	2.117(2)		Co(1)–N(3)c	2.119(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Co(1)–O(2)a	180.0		O(2)a–Co(1)–O(1)a	90.13(9)		O(2)–Co(1)–O(1)a	89.87(9)
  O(2)a–Co(1)–O(1)	89.87(9)		O(2)–Co(1)–O(1)	90.13(9)		O(2)–Co(1)–N(3)b	88.90(10)
  O(2)–Co(1)–N(3)c	91.10(10)		O(2)a–Co(1)–N(3)b	91.10(10)		O(2)a–Co(1)–N(3)c	88.90(10)
  O(1)–Co(1)–O(1)a	180.0		O(1) –Co(1)–N(3)b	89.95(11)		O(1)–Co(1)–N(3)c	90.05(11)
  O(1)a–Co(1)–N(3)b	90.05(11)		O(1)a–Co(1)–N(3)c	89.95(11)		N(3)b–Co(1)–N(3)c	180.0
  Symmetry transformation: a: 1–x, 1–y, 1–z; b: –1/2+x, 1–y, z; c: 3/2–x, y, 1–z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure description

  The single-crystal X-ray diffraction shows that FJI-H24 crystallizes in the monoclinic space group I2/a. Its asymmetric unit contains one Co(Ⅱ) ion, one H₂BTTA ligand and two water molecules. As shown in Fig. 1b, each Co(Ⅱ) ion adopts an octahedral coordination geometry through coordinating with four O and two N atoms. Four positions of the equatorial plane are occupied by two N atoms from different triazolyl groups and two O atoms from coordinated water, and two axial positions are occupied by two O atoms from different carboxyl groups. Then each Co(Ⅱ) ion coordinates with neighboring BTTA^2– ligands to form a three-dimensional (3D) porous framework with 1D quadrilateral channel along the a axis (Fig. 1c). Interestingly, the pore walls are densely populated with the uncoordinated N atoms from triazolyl groups and Co(Ⅱ) ions with two coordinated water molecules. After activation, such Co(Ⅱ) ions will be transformed into active open Co(Ⅱ) ions, generating a polar environment for gas adsorption. Such potential open Co(Ⅱ) ions and neighboring free N atoms from triazolyl groups may synergistically improve the gas adsorption. Both Co(Ⅱ) ions and BTTA^2– ligands can be considered as 4-connected nodes, so the framework of FJI-H24 can be simplified to a 4-connected lvt network with the point symbol of (4².8⁴) (Fig. 1d). Very recently, a compound with similar coordination environment has been reported, but it displays a typical two- dimensional layered structure rather than a three-dimensional porous structure^[21] mainly due to the different conformation and orientation of ligands in these two compounds. The porosity of FJI-H24 is estimated to be 50% according to PLATON calculation with a probe radius of 1.8 Å. Revealed by Zeo++ software, the limiting pore diameter and the maximum pore diameter are 4.7 and 5.7 Å, respectively^[22].

  Figure 1

  

  Figure 1.  (a) Structure of ligand H₂BTTA. (b) Coordination environment of Co(Ⅱ) ion in FJI-H24. Symmetry codes: A: 1 – x, 1 – y, 1 – z; B: –1/2 + x, 1 – y, z; C: 3/2 – x, y, 1 – z; (c) Framework of FJI-H24. (d) Topology of FJI-H24

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  3.2 PXRD, TGA and FT-IR analyses

  As shown in Fig. S1a, the Powder X-ray diffraction (PXRD) patterns of the as-synthesized sample match well with the simulated one, indicating that the bulk as-synthesized FJI-H24 sample is pure. The framework of FJI-H24 remains stable after soaking in acetone, which reveals that acetone can be used to exchange free solvents inside the pores of FJI-H24. Thermogravimetric analyses (TGA) demonstrate that both free water and N, N-dimethylformamide molecules are comprised in FJI-H24 and its framework can be thermally stable up to about 250 ℃ (Fig. S1b). Fourier transform infrared spectra (FT-IR) analyses further prove the existence of free water and N, N-dimethylformamide molecules, in which the broad peak of 3437 cm⁻¹ belongs to water molecule and the strong peak of 1666 cm⁻¹ attributes to the N, N-dimethylformamide (Fig. S1c).

  3.3 Gas storage and purification

  As mentioned above, the FJI-H24 not only has typical micro-porous structure, but also has high density of metal ions and uncoordinated N atoms; both of them are beneficial to carbon dioxide adsorption. Before adsorption test, the fresh sample FJI-H24 should be soaked in acetone for 3 days to remove less volatile solvents, and activated under dynamic vacuum at 60 ℃ for 12 h. PXRD data indicate that the framework of desolvated FJI-H24 (after activation) is retained. The permanent porosity of FJI-H24 is firstly tested by N₂ adsorption measurement at 77 K; however, no obvious N₂ adsorption at 77 K has been observed. Such almost zero N₂ adsorption at 77 K may result from the relatively narrow pores in FJI-H24 and their polar environment; both narrow pores and polar environment will hinder the diffusion of N₂ (the kinetic diameter of N₂ molecule is 3.6 Å) into pores of FJI-H24 at 77 K. Thus, the permanent porosity of FJI-H24 is tested by CO₂ adsorption at 195 K. As shown in Fig. 2a, the CO₂ sorption isotherm at 195 K exhibits a type-I microporous isotherm with a saturated adsorption of about 84 cm³·g^–1. The Brunauer-Emmett-Teller (BET) and Langmuir surface areas are 259 and 294 m²·g^–1, respectively. To check the CO₂ capture capacity of FJI-H24 at ambient conditions, CO₂ sorption isotherms are further tested at 273 and 298 K, respectively. The CO₂ uptake at 273 K is about 42.7 cm³·g^–1, and its value will reduce to 34.0 cm³·g^–1 when the temperature rises to 298 K (Fig. 2b). Compared with FJI-H14 (the CO₂ capture capacity of FJI-H14 is about 171 cm³·g^–1 at 298 K and 1 atm), the CO₂ adsorption capacity of FJI-H24 is significantly reduced, which may attribute to its more narrow pores and lower BET surface areas than that of FJI-H14. For comparison, N₂ sorption isotherms at different temperature are also measured; corresponding N₂ uptake is only 3.9 and 2.5 cm³·g^–1, separately. According to the ideal absorbed solution theory (IAST), the CO₂/N₂ selectivity for the CO₂/N₂ (15/85) mixture at 1 atm is about 87 at 298 K (Fig. 3a), exceeding that of FJI-H14 (the CO₂/N₂ selectivity of FJI-H14 is 51)^[17] and many recently reported MOF, such as HBU-18 (the CO₂/N₂ selectivity of HBU-18 is 41.79)^[23] and Cu-BTC/GO10 (the CO₂/N₂ selectivity of Cu-BTC/GO10 is 28.1)^[24]. The relatively high CO₂/N₂ selectivity of FJI-H24 may result from its polar environment which can interact with polar CO₂ molecule strongly. To evaluate the interactions between the framework and CO₂ molecules, the isosteric heat of adsorption (Qst) of FJI-H24 has been calculated from their adsorption isotherms at 273 and 298 K based on the Clausius-Clapeyron equation. As shown in Fig. 3b, the Qst value at the onset of adsorption is 24.46 kJ·mol^–1. Such relatively high CO₂/N₂ selectivity demonstrates that FJI-H24 is a potential adsorbent for the removal of carbon dioxide from exhaust gas of power plants.

  Figure 2

  

  Figure 2.  (a) CO₂ adsorption isotherms of FJI-H24 at 195 K. (b) CO₂ and N₂ adsorption isotherms of FJI-H24at 273 and 298 K. (c) C₂H₂ and CH₄ adsorption isotherms of FJI-H24 at 273 and 298 K

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  Figure 3

  

  Figure 3.  (a) IAST adsorption selectivity of FJI-H24 for different gas mixtures at 1 atm and 298 K.(b) Isosteric heat of adsorption of CO₂ and C₂H₂ for FJI-H24

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  Acetylene is a very significant chemical material in industry, which can be used for manufacturing synthetic rubber, synthetic fiber, etc. However, the safe storage of acetylene still remains a big challenge due to its explosiveness^[4]. Considering that the kinetic diameter of C₂H₂ is the same as CO₂ (3.3 Å), FJI-H24 may be a potential adsorbent for C₂H₂, so C₂H₂ sorption measurements for FJI-H24 are also carried out. As shown in Fig. 2c, the C₂H₂ uptake at 273 K is about 63.2 cm³·g^–1, and its value at 298 K is about 53.0 cm³·g^–1. Acetylene is usually prepared by partial oxidation of methane in industry, so removing unreacted methane from acetylene is very important. A practical C₂H₂ adsorbent usually requires excellent C₂H₂/CH₄ selectivity. For comparison, CH₄ sorption isotherms are measured at 298 and 273cm³·g^–1, separately. Excitedly, the C₂H₂/CH₄ selectivity at ambient conditions (298 K, 1 atm) is up to 66 (Fig. 3a). To the best of our knowledge, the selectivity of C₂H₂/CH₄ exceeds many recently reported MOFs under the same condition, such as ZJNU-100 (22.2)^[25] and Sc-ABTC (14.7)^[26]. The isosteric heat of adsorption (Q_st) also has been calculated based on their adsorption isotherms at 273 and 298 K. At near zero loading, the Q_st of FJI-H24 for C₂H₂ is about 22.30 kJ·mol^–1 (Fig. 3b). Such moderate C₂H₂ adsorption performance and high C₂H₂/CH₄ selectivity may also result from the relatively narrow pores and high density of free N atoms and open metal ions comprised of FJI-H24, which indicates that FJI-H24 is a potential material for purification of raw acetylene from oxidation of methane.

  4. CONCLUSION

  In conclusion, a novel metal-organic framework FJI-H24 with narrow pores and high density of open metal ions and free Lewis base sites has been prepared from H₂BTTA ligand and CoCl₂. FJI-H24 has moderate CO₂ (34.0 cm³·g^–1) and C₂H₂ (53.0 cm³·g^–1) adsorption capacity, but displays relatively high CO₂/N₂ (87) and C₂H₂/CH₄ (66) selectivity under ambient conditions (298 K, 1 atm). The relationship between adsorption and structure has been discussed in detail, which will provide a potential strategy for preparing practical porous metal-organic frameworks for gas adsorption and purification.

  
  
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• 

  Figure 1  (a) Structure of ligand H₂BTTA. (b) Coordination environment of Co(Ⅱ) ion in FJI-H24. Symmetry codes: A: 1 – x, 1 – y, 1 – z; B: –1/2 + x, 1 – y, z; C: 3/2 – x, y, 1 – z; (c) Framework of FJI-H24. (d) Topology of FJI-H24

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  Figure 2  (a) CO₂ adsorption isotherms of FJI-H24 at 195 K. (b) CO₂ and N₂ adsorption isotherms of FJI-H24at 273 and 298 K. (c) C₂H₂ and CH₄ adsorption isotherms of FJI-H24 at 273 and 298 K

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  Figure 3  (a) IAST adsorption selectivity of FJI-H24 for different gas mixtures at 1 atm and 298 K.(b) Isosteric heat of adsorption of CO₂ and C₂H₂ for FJI-H24

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(2)	2.101(2)		Co(1)–O(1)	2.117(2)		Co(1)–N(3)b	2.119(3)
  Co(1)–O(2)a	2.101(2)		Co(1)–O(1)a	2.117(2)		Co(1)–N(3)c	2.119(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Co(1)–O(2)a	180.0		O(2)a–Co(1)–O(1)a	90.13(9)		O(2)–Co(1)–O(1)a	89.87(9)
  O(2)a–Co(1)–O(1)	89.87(9)		O(2)–Co(1)–O(1)	90.13(9)		O(2)–Co(1)–N(3)b	88.90(10)
  O(2)–Co(1)–N(3)c	91.10(10)		O(2)a–Co(1)–N(3)b	91.10(10)		O(2)a–Co(1)–N(3)c	88.90(10)
  O(1)–Co(1)–O(1)a	180.0		O(1) –Co(1)–N(3)b	89.95(11)		O(1)–Co(1)–N(3)c	90.05(11)
  O(1)a–Co(1)–N(3)b	90.05(11)		O(1)a–Co(1)–N(3)c	89.95(11)		N(3)b–Co(1)–N(3)c	180.0
  Symmetry transformation: a: 1–x, 1–y, 1–z; b: –1/2+x, 1–y, z; c: 3/2–x, y, 1–z

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•