English

• 0.   Introduction

  Acetylene (C₂H₂) is a critical precursor for manufacturing essential polymers and industrial chemicals^[1]. In industry, C₂H₂ is produced via methane combustion or hydrocarbon pyrolysis; during this process, carbon dioxide (CO₂) is generated as a major impurity, which significantly affects the purity of the product^[2]. Owing to the similar physical properties (i.e., boiling points, dipole moment, and polarizability), their separation is presently considered as one of the most challenging industrial separations. Although the conventional purification techniques, such as solvent extraction or cryogenic distillation, can achieve the separation, the high energy penalty and the environmental unsustainability of these methods hinder their further development and wide application^[3]. Thus, developing an energy-efficient method for separating the C₂H₂/CO₂ mixture is urgently desirable^[4]. Adsorptive separation by using porous materials (porous carbon and zeolite) has emerged as one of the promising techniques for addressing acetylene purification. However, the almost same molecular geometry structure (linear shape) and the nearly identical molecular dimensions (C₂H₂: 0.332 nm×0.334 nm×0.57 nm; CO₂: 0.318 nm×0.333 nm×0.536 nm) of these two gases bring huge challenges for selective physical adsorption^[5].

  Compared to traditional porous materials, which only rely on physical adsorption mechanisms (e.g., molecular sieve effect), metal-organic frameworks (MOFs) have emerged as an ideal platform for fabricating the new generation adsorption materials owing to their superior structure programmability and pore environment modifiability^[6]. By introducing additional binding sites or functional groups into the inner wall of the MOFs′ channels, it can not only achieve the precise pore size modification of these materials, but also endow these materials with intriguing chemical adsorption capacity based on improved host-guest interactions^[7]. Over the past decade, a large number of porous MOFs with special functional groups have been constructed and exhibit great separation ability of the C₂H₂/CO₂ mixture, relying on discriminatory gate effect, C—H⋯Cl hydrogen bonds, and the electrostatic interaction^[8]. However, in most MOF systems, in an attempt to further enhance the separation performance of MOFs, blindly increasing the density of binding sites or functional groups in the framework always induces a dramatic decrease in the porosity, which finally results in the degradation of MOFs′ performance^[9]. In order to break through the trade-off barrier between the density of functional groups in the framework and the free pore volume, a suitable MOF platform with ultra-high structure tunability is urgently desirable. Since 2013, Feng et al. have depended on the pore space partition (PSP) strategy, encapsulating a series of pore partition agents with C₃ symmetric into the 1D channels of MIL-88 to form the famous pacs-MOFs^[10], which can be an ideal platform for fabricating the MOF-based adsorption materials. Firstly, in this system, the 1D open channel spaces in original MOFs (MIL-88) can be rationally divided into numerous smaller segments to form the confined spaces, which have beneficial effects for enhancing the interactions between the host framework and guest gas molecules^[11]. Secondly, because this type of structure was integrated by the trinuclear metal cluster, linear bidentate carboxyl ligands, and the tridentate N-containing ligands with C₃ symmetric, the pacs-MOFs exhibit some advantages such as the fruitful species of building blocks, the highly structural designability and programmability, which is convenient for subsequent modification of functional groups and active sites^[12]. The last, the synergistic effect between different functional groups or active sites in the regular confined space (cages with trigonal bipyramidal geometry and cylindrical geometry) of pacs-MOFs has significant positive effects for improving the adsorption selectivity of materials^[13]. Up to date, most of the pacs-MOFs were constructed by using organic pyridinyl ligands, which extremely limited the structural diversity of pacs-MOFs. In contrast, utilizing the simple organic molecules (i.e., isonicotinic acid, triazole, and formic acid) to design and construct the novel metal-organic pore-partition ligands not only increases the structural diversity of this type of MOFs, but also improves the modifiability of the framework. However, the relative research remains scarcely reported^[14].

  Inspired by the above mentions, in this work, a novel pacs-MOF (NH₂Me₂)[Fe₃(μ₃-O)(bdc)₃][In(FA)₃Cl₃] (denoted Fe-FAIn-bdc; bdc^2-=terephthalate) was constructed successfully by using [In(FA)₃Cl₃]^3- (FA^-=formate) as pore-partition agents through the solvothermal method. Distinguished from the common planar triangular organic N-containing ligands, the [In(FA)₃Cl₃]^3- exhibits a stereo configuration and two-fold orientational disordered state in the network of Fe-FAIn-bdc. The abundant Cl^- anions in this unit can act as potential binding sites, which could significantly enhance the adsorption capacity for C₂H₂ via host-guest interactions, finally improving the selectivity of Fe-FAIn-bdc for C₂H₂/CO₂. The adsorption data illustrate that the adsorption capacity of Fe-FAIn-bdc for C₂H₂ and CO₂ was about 50.79 cm³·g^-1 and 29.99 cm³·g^-1 at 298 K and 100 kPa with a high C₂H₂/CO₂ selectivity of 3.08 in a volume ratio of 50∶50.

  1.   Experimental

  The reagents and instruments are listed in the supporting information, as well as the activation and gas adsorption tests of the materials.

  Synthesis of Fe-FAIn-bdc. FeCl₂·4H₂O (0.2 mmol, 40 mg), InCl₃ (0.1 mmol, 22 mg), and H₂bdc (0.2 mmol, 33 mg) were dissolved in the mixture of 4.0 mL of N, N-dimethylformamide (DMF) with 0.3 mL 50% HBF₄. The mixture was sealed in a 10 mL glass vial and was heated in an oven at 120 ℃ for 2 d without disturbance. After the reaction system was cooled to room temperature, the orange-red strip-shaped crystals were obtained after filtration, washed with DMF several times, and dried in air. Yield: 23% (based on InCl₃).

  2.   Results and discussion

  The orange-red strip-shaped crystals of Fe-FAIn-bdc, formulated as (NH₂Me₂)₂[Fe₃(μ₃-O)(bdc)₃][In(FA)₃ Cl₃], was synthesized by mixing a solution FeCl₂·4H₂O, InCl₃, H₂bdc and HBF₄ under solvothermal reaction at 120 ℃ for 2 d. Single-crystal X-ray diffraction analysis reveals that Fe-FAIn-bdc crystallizes in the hexagonal P6₃/mmc space group belonging to 6/mmm point group (Table 1). The asymmetric unit contains one Fe³⁺ ion (Fe1), one-third of a bridged μ₃-O^2- ion, one bdc^2- anion, one formate anion, 1/3 of a In³⁺ cation, one Cl^- anion, and 2/3 of a dimethyl ammonium cation with three-fold disorder. To confirm the valence state of Fe elements, the bond valence calculation was carried out, demonstrating that the bond valence sum analysis of Fe1 in Fe-FAIn-bdc was 3.028 7, which was very close to 3, by using r₀=0.176 5 nm for Fe³⁺ ions and 2.785 3 by using r₀=0.173 4 nm for Fe²⁺ ions^[15-16]. This result indicates that the valence state of Fe1 should be trivalent. Each Fe³⁺ ion exhibits a distorted octahedral coordination sphere with four O atoms from four bdc^2- ligands in the equatorial plane, one O atom from the formate anion, and one μ₃-O^2- anion on the axial positions. Three crystallographically equivalent Fe³⁺ ions are interconnected via a μ₃-O^2- group and further coordinate with the carboxyl groups of bdc^2- ligands to form the classical [Fe₃(μ₃-O)(COO)₆]⁺ trinuclear metal cluster (Fig.1a and 1b). The adjacent trinuclear in space are connected with each other by bdc^2- ligands to construct the famous MIL-88-type framework with acs topology, featuring the large 1D hexagonal channels and fruitful open metal sites along the c-axis (Fig.1c and 1e). Then, the open metal sites on the inner wall of the channels are occupied by the metal-organic pore-partition ligands [In(FA)₃Cl₃]^3-. In this unit, the In³⁺ ions (In1) exhibit an octahedral coordination model with three O atoms from three formate anions and three Cl^- anions. Along the c-axis direction, the whole geometric structure of [In(FA)₃Cl₃]^3- units is like an equilateral triangle with C₃-symmetric, which is very similar to the classical pore-partition organic ligands [i.e., tri(pyridin-4-yl)amine (TPA), 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT)]. After that, the 1D open channels of MIL-88 were fragmented into numerous segments to form the typical structure of pacs-MOFs (Fig.1d and 1f).

  Table 1

  

  Table 1.  Crystallographic data of Fe-FAIn-bdc

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  Parameter	Fe-FAIn-bdc
  Formula	C33H36O19N3Cl3Fe3In
  Formula weight	1 167.41
  Temperature / K	293(2)
  Crystal system	Hexagonal
  Space group	P63/mmc
  a / nm	1.320 66(12)
  b / nm	1.320 66(12)
  c / nm	1.827 67(17)
  Volume / nm3	2.760 6(6)
  Z	2
  μ / mm-1	1.392
  F(000)	1 170.0
  Reflection collected	4 404
  Independent reflection	959
  Goodness-of-fit on F2	1.708
  Final R indexes [I≥2σ(I)]	R1=0.10 87, wR2=0.322 9
  Final R indexes (all data)	R1=0.143 6, wR2=0.394 4

  Figure 1

  

  Figure 1.  (a) Connection mode diagram of Fe-FAIn-bdc; (b) Structural schematic diagram of Fe₃-clusters; (c) Peripheral framework structure (MIL-88) constructed by Fe₃-clusters and bdc^2- ligands; (d) Crystal structure of Fe-FAIn-bdc along the c-axis; (e) View of the 1D channels of the peripheral framework along a- or b-axis; (f) Small segments of Fe-FAIn-bdc after fragmented the infinite 1D channel in MIL-88 by [In(FA)₃Cl₃]^3- ligands; (g) Trigonal bipyramid cages in Fe-FAIn-bdc which constructed by six bdc^2- ligands and five Fe₃-clusters

  Symmetry codes: ⁱ 1-y, 1-x, 1/2-z; ⁱⁱ 1-y, 1-x, z; ⁱⁱⁱ x, y, 1/2-z; ^iv 1-y, -1+x-y, z; ^v 2+y-x, 1-x, z.

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  Owing to the two-fold disordered state of the formate anions, the window size of Fe-FAIn-bdc is only about 0.472 nm, which is much smaller than that in traditional pacs-MOFs. The whole framework is anionic, and two dimethyl ammonium cations, which exhibit a three-fold disorder state, are located near the Fe₃-clusters to balance the surplus negative charges. In this structure, two types of nanocage structures with different geometric shapes can be found (Fig.1f and 1g). One cage exhibits trigonal bipyramidal geometry with spherical diameters of ca. 1.827 and 1.452 nm, which was constructed by six bdc^2- ligands and five Fe₃-clusters. Compared to traditional TPA-based pacs-MOFs, the [In(FA)₃Cl₃]^3- units significantly obstruct the triangular apertures of the trigonal bipyramidal cages. The other cage exhibits regular cylindrical geometry, which was assembled by six bdc^2- ligands and two [In(FA)₃Cl₃]^3- units. The height and width of this cage are 0.913 and 1.320 nm, respectively, indicating the abundant free space in this confined space. Besides, all the Cl^- anions in [In(FA)₃Cl₃]^3- are toward the center of this cage, which is beneficial for capturing C₂H₂ molecules in this narrow confined space through multiple C—H⋯Cl interactions. According to the calculation results of PLATON software, the effective free volume of this Fe-FAIn-bdc is about 33.9%, upon removal of internal guest molecules, which accounts for the spatial occupancy of the In-based structures^[17-19]. The fruitful internal confined spaces, which are decorated by Cl^- anions, and the narrow pore size along the a- or b-axis endow this MOF with potential application perspectives in the field of gas separations.

  The phase purity and the chemical/thermal stability of Fe-FAIn-bdc were confirmed by powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA). As shown in Fig.2a, the experimental PXRD pattern could match well with the simulation result, demonstrating the high purity of the crystal powder sample of Fe-FAIn-bdc. Besides, after being immersed in some common organic solvents (i.e., CH₂Cl₂, MeOH, and acetone) for 24 h, the sample of Fe-FAIn-bdc could still maintain the framework intact. On the contrary, most of the diffraction peaks in the PXRD pattern of the sample immersed in water for 24 h disappeared, suggesting its poor stability in water. According to the TGA curves of Fe-FAIn-bdc before and after solvent exchange by acetone, the framework could remain stable until about 200 ℃. The obvious weight loss before 200 ℃ can be attributed to the removal of the solvent molecules, which are encapsulated in the network of Fe-FAIn-bdc (Fig.2b)

  Figure 2

  

  Figure 2.  (a) PXRD patterns of the crystal powder sample of Fe-FAIn-bdc after being immersed in different common solvents; (b) TGA curves of Fe-FAIn-bdc and the sample exchanged by acetone

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  To investigate the permanent porosity of Fe-FAIn-bdc, the N₂ adsorption-desorption experiment of Fe-FAIn-bdc at 77 K was measured. However, owing to the narrow pore size of Fe-FAIn-bdc and the dramatic thermal vibration of formate anions near the window of the internal space, Fe-FAIn-bdc didn′t exhibit obvious N₂ adsorption behavior at 77 K (Fig.S1). Miraculously, Fe-FAIn-bdc exhibited remarkable CO₂ adsorption performance at 298 K and 100 kPa. The CO₂ adsorption uptake of Fe-FAIn-bdc reached 29.99 cm³·g^-1 (Fig.3a), which can be attributed to the smaller kinetic diameter of CO₂ (0.33 nm) compared to N₂ (0.364 nm) and the larger polarizability and stronger quadrupole moment of CO₂ molecules. These results inspired us to investigate the adsorption capacity of Fe-FAIn-bdc to C₂H₂, which has a similar kinetic diameter to CO₂ (0.33 nm). As shown in Fig.3a, under same condition (298 K and 100 kPa) the adsorption uptake of C₂H₂ on Fe-FAIn-bdc reached up to 50.79 cm³·g^-1, which was much higher than that of CO₂ with a large uptake ratio of 169%, surpassing many benchmark MOFs such as TIFSIX-2-Cu-i (95%)^[20] and SIFSIX-3-Ni (120%)^[20], rivaling UTSA-74-Zn (150%)^[21]. Besides, the adsorption isotherms curve of Fe-FAIn-bdc for C₂H₄ has also been obtained and the adsorption uptake was only about 30.94 cm³·g^-1 at 298 K and 100 kPa (Fig.3b). In addition, compared with the slowly increasing trend of CO₂ or C₂H₄ uptake over the whole pressure region, the C₂H₂ adsorption uptake of Fe-FAIn-bdc increased dramatically in a p/p₀ range of 0-0.3, which indicates the stronger interaction between host framework and C₂H₂ molecules.

  Figure 3

  

  Figure 3.  (a) Adsorption isotherms of Fe-FAIn-bdc for CO₂ and C₂H₂ at 298 K; (b) Adsorption isotherms of Fe-FAIn-bdc for C₂H₂ and C₂H₄ at 298 K

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  To evaluate the separation ability of Fe-FAIn-bdc for the mixture of C₂H₂/CO₂ and C₂H₂/C₂H₄, the ideal adsorbed solution theory (IAST) was employed to estimate the adsorption selectivity for C₂H₂/CO₂ and C₂H₂/C₂H₄ in volume ratio of 50∶50. By fitting the isotherms according to the dual-site Langmuir-Freundlich equation, the selectivity of Fe-FAIn-bdc for C₂H₂/CO₂ (50∶50) was calculated to be 3.08 (Fig.S2) at 298 K and 100 kPa. This value was much higher than some reported benchmark MOFs such as Co-MOF-1 (2.1), SNNU-18 (2.8), and NUM-11 (3) but still lower than some famous MOFs such as UTSA-220 (4.4) and MUF-17 (6.01) (Table S1 and Fig.4a). Compared with the high selectivity of Fe-FAIn-bdc for C₂H₂ /CO₂ (50:50), the selectivity for C₂H₂/C₂H₄ (50∶50) has also been calculated. The value (3.65) (Fig.S3) was higher than some reported MOF materials (i.e., SNNU-95 and MOF-74-Fe), but still couldn′t match well with most top-performing C₂H₂/C₂H₄ separation materials (SIFSIX-2-Cu) (Table S2 and Fig.4b)^[21]. The relatively high selectivity of Fe-FAIn-bdc for C₂H₂/CO₂ (50∶50) and C₂H₂/C₂H₄ (50∶50) makes this MOF a good candidate for application in the field of purification of acetylene in industry.

  Figure 4

  

  Figure 4.  (a) Comparison about C₂H₂/CO₂ selectivity and C₂H₂ capacity of representative MOFs at 298 K and 100 kPa; (b) Comparison about C₂H₂/C₂H₄ selectivity and C₂H₂ capacity of representative MOFs at 298 K and 100 kPa

  The references are listed in the Supporting information.

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  3.   Conclusions

  In summary, based on the reticular chemistry and the pore space partition (PSP) strategy, a novel pacs-MOFs (Fe-FAIn-bdc) was constructed successfully by solvothermal method. Distinguished from the traditional organic pyridyl pore-partition ligands with C₃ symmetry, in this complex, the metal-formate complex [In(FA)₃Cl₃]^3- unit acts as the pore-partition agent to participate in the self-assembly process of Fe-FAIn-bdc. On account of the small pore size and large pore volume of Fe-FAIn-bdc and the fruitful confined space which was decorated by Cl^- anions, this MOF presented a high C₂H₂ adsorption capacity of 50.79 cm³·g^-1 at 298 K and high IAST selectivity of 3.08 for C₂H₂/CO₂ and 3.65 for C₂H₂/C₂H₄. This work not only provide a new example of C₂H₂ purification adsorbents, but also sheds new light on the design of novel pacs-MOFs with metal-organic pore-partition agents.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  (a) Connection mode diagram of Fe-FAIn-bdc; (b) Structural schematic diagram of Fe₃-clusters; (c) Peripheral framework structure (MIL-88) constructed by Fe₃-clusters and bdc^2- ligands; (d) Crystal structure of Fe-FAIn-bdc along the c-axis; (e) View of the 1D channels of the peripheral framework along a- or b-axis; (f) Small segments of Fe-FAIn-bdc after fragmented the infinite 1D channel in MIL-88 by [In(FA)₃Cl₃]^3- ligands; (g) Trigonal bipyramid cages in Fe-FAIn-bdc which constructed by six bdc^2- ligands and five Fe₃-clusters

  Symmetry codes: ⁱ 1-y, 1-x, 1/2-z; ⁱⁱ 1-y, 1-x, z; ⁱⁱⁱ x, y, 1/2-z; ^iv 1-y, -1+x-y, z; ^v 2+y-x, 1-x, z.

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  Figure 2  (a) PXRD patterns of the crystal powder sample of Fe-FAIn-bdc after being immersed in different common solvents; (b) TGA curves of Fe-FAIn-bdc and the sample exchanged by acetone

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  Figure 3  (a) Adsorption isotherms of Fe-FAIn-bdc for CO₂ and C₂H₂ at 298 K; (b) Adsorption isotherms of Fe-FAIn-bdc for C₂H₂ and C₂H₄ at 298 K

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  Figure 4  (a) Comparison about C₂H₂/CO₂ selectivity and C₂H₂ capacity of representative MOFs at 298 K and 100 kPa; (b) Comparison about C₂H₂/C₂H₄ selectivity and C₂H₂ capacity of representative MOFs at 298 K and 100 kPa

  The references are listed in the Supporting information.

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  Table 1.  Crystallographic data of Fe-FAIn-bdc

  Parameter	Fe-FAIn-bdc
  Formula	C33H36O19N3Cl3Fe3In
  Formula weight	1 167.41
  Temperature / K	293(2)
  Crystal system	Hexagonal
  Space group	P63/mmc
  a / nm	1.320 66(12)
  b / nm	1.320 66(12)
  c / nm	1.827 67(17)
  Volume / nm3	2.760 6(6)
  Z	2
  μ / mm-1	1.392
  F(000)	1 170.0
  Reflection collected	4 404
  Independent reflection	959
  Goodness-of-fit on F2	1.708
  Final R indexes [I≥2σ(I)]	R1=0.10 87, wR2=0.322 9
  Final R indexes (all data)	R1=0.143 6, wR2=0.394 4

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