English

• Propylene (C₃H₆), another important olefin feedstock in the petrochemical industry besides ethylene, is widely used to produce polypropylene, propylene oxide, acrylonitrile, etc^[1-5]. It is mainly produced through naphtha cracking or hydrocarbon fractionation, as well as propane steam cracking^{[4, 6-9]}. However, C₃H₆ obtained in these processes often contains some impurities such as propyne (C₃H₄) or propane^[10-17]. Polymer-grade purity (> 99.5%) propylene requires an impurity content (C₃H₄, etc.) of less than 1 μL·L^{-1[5, 10, 12, 16, 18-21]}. At present, the main technology to remove trace amounts of C₃H₄ is selective hydrogenation by noble metal catalysts, which has the disadvantages of high catalyst cost, short lifespan, low efficiency, and potential secondary pollution^{[4, 6, 14, 22-23]}. Recently, physical adsorption-based separation technology, due to its environmental friendliness and energy efficiency, has received widespread attention^{[4, 7, 9-12, 14, 16, 19-20, 23]}.

  Metal-organic frameworks (MOFs), which possess tunable structures and properties, have been investigated for the trace C₃H₄ capture^{[1, 4-9, 11, 14, 18-19, 21-23]}. By introducing strong adsorption sites, such as open metal sites (OMSs) (MOF-74^[1], etc.) or anion pillars (SIFSIX-2-Cu-i, etc.)^{[5, 24]}, MOFs can exhibit high adsorption uptake for C₃H₄; however, these MOFs also usually show high adsorption enthalpies. Until now, MOFs reported for C₃H₄/C₃H₆ separation (SIFSIX-2-Cu-i^[24], ZU-62^{[5, 7]}, NKMOF-1-Cu/Ni^[4], Co-gallate^[25], etc.) display the chemical adsorption of C₃H₄. Low adsorption enthalpy is preferred for the easy regeneration of MOFs, but it usually results in low adsorption uptakes for C₃H₄ at ultra-low pressures. It is rather difficult to achieve MOFs with both high C₃H₄ adsorption uptake and moderate adsorption enthalpy. Notably, designing MOFs with the pore size approaching the kinetic diameter of C₃H₄, an alternative way to increase the affinity of the framework toward guest molecules^[26], may be beneficial to balance these two parameters. In this work, it is speculated that if OMSs are occupied by solvent molecules, strong adsorption sites in MOFs will be eliminated and pore sizes will consequently be decreased, which may be favorable to achieve the target MOFs. The acs topological MOFs based upon trinuclear [M₃(μ₃-OH)(COO)₆] (M=Fe, Co, Mn, etc.) clusters and linear dicarboxylic acids are usually decorated by OMSs^[27]. Thus, herein, based upon the trinuclear [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] cluster and bicyclo[2.2.2] octane‑1, 4-dicarboxylic acid (H₂L), a new acs‑MOF, [Fe^ⅢFe_1.9^Ⅱ Mn_0.1^Ⅱ(μ₃-OH)(C₁₀O₄H₁₂)₃·xGuest] (NJTU-Bai88, NJTU-Bai=Nanjing Tech University Bai′s group), has been successfully synthesized under solvothermal conditions. Confirmed by thermogravimetric (TG) curves and elemental analysis, the coordinated N, N-dimethylformamide (DMF) molecules were still retained in activated NJTU‑Bai88, occupying the potential OMSs. Very interestingly, its HK pore size was 0.47 nm, which is very close to the kinetic diameter of the C₃H₄ molecule. NJTU‑Bai88 exhibited high C₃H₄ adsorption uptake at 1 kPa, large C₃H₄/C₃H₆ and C₃H₄/C₃H₈ selectivities, and a moderate C₃H₄ adsorption enthalpy. This may be attributed to the effective synergy of suitable pore size and nonpolar pore surfaces.

  1.   Experimental

  1.1   Synthesis of [Fe^ⅢFe^ⅡMn^Ⅱ(μ₃‑OH)(COO)₆] cluster

  A solution of sodium acetate (25.4 g, 0.31 mol) in 40 mL of H₂O was mixed with iron(Ⅲ) nitrate nonahydrate (8.0 g, 0.02 mol) and manganese nitrate tetrahydrate (25.1 g, 0.10 mol) in 30 mL of H₂O. The mixture was stirred at room temperature overnight. The black-brown precipitate obtained was filtered and washed alternately with water and ethanol. Yield: 70 %. The results of ICP showed that the molar ratio of Fe to Mn was 2∶1.

  1.2   Synthesis of NJTU-Bai88

  A solution of [Fe₂Mn(μ₃‑OH)(COO)₆] (8.0 mg, 0.018 mmol) in 0.5 mL of DMF was mixed with H₂L (8.0 mg, 0.04 mmol) in 1.5 mL of DMF. To this was added 0.1 mL of acetic acid and 0.08 mL of trifluoroacetic acid with stirring. The mixture was sealed in a Pyrex tube and heated to 140 ℃ for 48 h. The reddish-brown block crystals obtained were filtered and washed with DMF. Yield: 40 %. ICP analysis of the crystal resulted in a molar ratio of Fe to Mn of 27∶1. According to this molar ratio and the total number of metal ions in the trinuclear [Fe_27xMn_x(μ₃-OH)(COO)₆] cluster as 3, it can be calculated that x=0.107 and 27x=2.89, which gave the estimated molecular formula of the trinuclear cluster as [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆]. Combined with the single-crystal X-ray diffraction data, TG, and charge balancing, the molecular formula of NJTU-Bai88 was determined to be [Fe^ⅢFe_1.9^ⅡMn_0.1^Ⅱ(μ₃-OH)(C₁₀H₁₂O₄)₃·xGuest].

  1.3   Activation of NJTU-Bai88

  The as-synthesized sample of NJTU-Bai88 was activated by heating under vacuum conditions, with gradient heating at 80, 100, 120, and 140 ℃ for 3 h at each temperature to give the activated NJTU‑Bai88, [Fe_2.9Mn_0.1(μ₃‑OH)(C₁₀O₄H₁₂)₃]. Elemental analysis Calcd. for C₃₉H₆₈N₃Fe_2.9Mn_0.1O₂₁ [Fe_2.9Mn_0.1(μ₃‑OH)(C₁₀O₄H₁₂)₃(DMF)₃·5H₂O](%): C 43.28, H 6.33, N 3.88; Found(%): C 43.12, H 5.92, N 3.48. The water in the pores was due to moisture absorbed from the air during experimental operations. After thoroughly removing the coordinated DMF molecules under high temperature and vacuum, powder X‑ray diffraction (PXRD) patterns indicated that the framework had severely collapsed and was almost amorphous. In addition, the CO₂ adsorption capacity at 195 K was very low, indicating that the sample does not have permanent porosity after complete activation.

  1.4   Single-crystal X-ray diffraction analysis

  A single crystal of NJTU-Bai88 with the size of 0.080 mm×0.040 mm×0.030 mm was selected and mounted on a CrysAlisPro 1.171.42.95a (Rigaku OD, 2023). Single-crystal X-ray diffraction data were measured at about 300 K using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) with the absorption correction type of multi-scan. Data reduction was performed with the CrysAlisPro 1.171.42.95a (Rigaku OD, 2023). The structures were solved by direct methods and refined with the full-matrix least-squares technique using the Olex package. Due to the very low content of Mn, inducing its ADP to be N.P.D., the occupancy rate of the metal atom in the structure was refined according to the fact that the sites of the metal atom were all occupied by the Fe atom. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2U_eq of the attached atom. The unit cell includes a large region of disordered solvent molecules, which could not be modelled as discrete atomic sites. We employed PLATON/SQUEEZE to calculate the diffraction contribution of the solvent molecules and, thereby, to produce a set of solvent-free diffraction intensities; structures were then refined again using the data generated by the SHELXTL package. The crystallographic data and structure refinement parameters are listed in Table S1 (Supporting information).

  2.   Results and discussion

  2.1   Crystal structure

  Solvothermal reaction of [Fe₂Mn(μ₃‑OH)(COO)₆] cluster with bicyclo[2.2.2]octane-1, 4-dicarboxylic acid (H₂L) in DMF containing acetic acid and trifluoroacetic acid afforded a high yield of reddish-brown block crystals of NJTU-Bai88. Single-crystal X-ray diffraction reveals that NJTU‑Bai88 crystallizes in hexagonal space group P62c. In the structure of NJTU‑Bai88, each metal ion is coordinated by four O atoms from four L^2- ions, one O atom from μ₃-OH^-, and one O atom from a DMF molecule, respectively (Fig.1a). Three metal ions are bridged by a μ₃‑OH^- and six carboxyl groups from six ligands, forming a triangular-prismatic trinuclear [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] cluster (Fig.1a).

  Figure 1

  

  Figure 1.  (a) [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] cluster; (b) Ligand L^2-; (c) Edge-missing trigonal bipyramid cage with the pore size of 0.54 nm; (d) Six edge-missing trigonal bipyramidal cages at the vertices of two interlaced triangles in different layers, respectively, surrounding a 1D pore channel; (e) 1D pore channel with a pore size of approximately 0.56 nm; (f) Framework and (g) topology of NJTU-Bai88

  The gray, red, aqua, and blue spheres represent C, O, Fe/Mn, and N atoms, respectively.

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  Five trinuclear [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] clusters are connected by six L^2- ions, leading to the formation of an edge-missing triangular bipyramidal cage with a pore size of approximately 0.54 nm (Fig.1a‑1c). Six edge-missing triangular bipyramidal cages, located at the vertices of two interlaced triangles in different layers, respectively, are connected with each other by sharing the edges, forming the 1D pore channel with a pore size of approximately 0.56 nm (Fig.1d, 1e, and S2). Moreover, the edge-missing triangular bipyramidal cages are interconnected with the 1D pore channel (Fig.S2). Then, through the connection of L^2-, different 1D pore channels are stacked with each other in a hexagonal mode in the cross-sectional direction, which results in the generation of the 3D framework of NJTU-Bai88 (Fig.1f).

  To better understand the structure, the trinuclear [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] cluster is simplified as a 6-connected node, and the L^2- is viewed as a 2-connected linker; the framework of NJTU-Bai88 can be simplified to a 6-connected acs topology with the Schläfli symbol of (4⁹·6⁶ (Fig.1g)). The total potential solvent accessible volume in desolvated NJTU-Bai88 is about 49.0% as determined by the PLATON/SOLV program, giving a calculated density of 0.983 g·cm^-3 for the desolvated framework.

  2.2   Permanent porosity and selective adsorption

  The purity of the bulk sample of NJTU-Bai88 and the integrity of the framework of the activated sample were confirmed by PXRD (Fig.S4). The peaks of PXRD patterns of the as-synthesized and activated samples are consistent with those of the simulated PXRD pattern. In addition, the thermal stability and activation of the NJTU-Bai88 sample were investigated through thermogravimetric analysis. No significant weight loss was observed in the range of 110-220 ℃, which indicates the complete removal of solvent molecules packed in the pores of the NJTU-Bai88 sample, with the coordinating DMF molecules remaining (Fig.S5).

  To investigate the permanent porosity of NJTU-Bai88, its CO₂ adsorption isotherm was measured at 195 K and 0-100 kPa. The CO₂ adsorption isotherm exhibited the Ⅰ-type adsorption isotherm without hysteresis on the desorption isotherm (Fig.2), indicating that NJTU-Bai88 is a microporous material. The Langmuir and Brunauer-Emmett-Teller (BET) surface areas were 758.5 and 554 m²·g^-1, respectively. The HK pore size estimated from the CO₂ adsorption isotherm at 195 K was about 0.47 nm, which was close to the pore size determined from the crystal structure (Fig.S1b and 3b).

  Figure 2

  

  Figure 2.  (a) CO₂ adsorption isotherms of NJTU-Bai88 at 195 K; C₃H₄, C₃H₆ and C₃H₈ adsorption isotherms for NJTU-Bai88 at (b) 273 K and (c) 298 K; (d) C₃H₄, C₃H₆ and C₃H₈ adsorption enthalpies; Comparison of (e) C₃H₄ adsorption uptakes at 1 kPa and 298 K, (f) C₃H₄ adsorption enthalpies, and (g) both C₃H₄ adsorption capacities and enthalpies of NJTU-Bai88 with reported representative MOFs; (h) IAST (ideal adsorbed solution theory) predicted selectivities of NJTU-Bai88 for C₃H₄/C₃H₆ (volume ratio of 1/99) and C₃H₄/C₃H₈ (1/99) mixtures at 298 K

  Panels e-g are drawn according to the data in Table S3.

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  The permanent porosity of NJTU-Bai88 prompted us to investigate its selective C₃H₄/C₃H₆ and C₃H₄/C₃H₈ adsorption performances. Gas adsorption isotherms of C₃H₄, C₃H₆, and C₃H₈ were measured under 0-100 kPa at 273 and 298 K (Fig.2b and 2c). In the low-pressure range, especially at 298 K, the C₃H₄ adsorption of NJTU-Bai88 increased rapidly, and the adsorption uptake reached 27.5 cm³·g^-1 at 1 kPa. As shown in Fig.2e and Table S3, although this was lower than the values of benchmark MOFs, ZNU-2 (87.4 cm³·g^-1)^[7] and UTSA-200 (67.0 cm³·g^-1)^[1], it was close to those of representative MIL-100(Cr) (34.0 cm³·g^-1)^[1] and Cu-BTC (32.9 cm³·g^-1)^[1], and even much higher than the data of well-known ZIF-8 (2.9 cm³·g^-1)^[1]. This indicates that the framework of NJTU-Bai88 exhibits a strong affinity for C₃H₄ gas molecules. Differently, the adsorption of C₃H₆ and C₃H₈ increased slightly more slowly, and the adsorption uptakes for C₃H₆ and C₃H₈ at 100 kPa were 47.9 and 45.3 cm³·g^-1, respectively, significantly lower than that of C₃H₄ (63.4 cm³·g^-1). Thus, NJTU‑Bai88 exhibited highly selective C₃H₄/C₃H₆ and C₃H₄/C₃H₈ adsorptions. Based upon adsorption isotherms at 273 and 298 K, the zero-loading adsorption enthalpies for C₃H₄, C₃H₆, and C₃H₈ of NJTU-Bai88 were calculated, using the virial equation, to be -58.8, -87.0, and -60.5 kJ·mol^-1, respectively (Fig.2d). This demonstrates that the affinity of the framework of NJTU-Bai88 for the C₃H₆ or C₃H₈ molecule is slightly higher than that for the C₃H₄ molecule. Consequently, the selective C₃H₄/C₃H₆ and C₃H₄/C₃H₈ adsorptions of NJTU-Bai88 are primarily attributed to the effective synergy of suitable pore size and nonpolar pore surface. Very interestingly, the C₃H₄ adsorption enthalpy of NJTU‑Bai88 belongs to the physical adsorption range and is even lower than those of MOFs with comparable adsorption uptakes, such as Co-gallate (-82.1 kJ·mol^-1)^[25] and Mg‑gallate (-66.8 kJ·mol^-1)^[25]. NJTU‑Bai88 can achieve a certain balance between high low‑pressure C₃H₄ adsorption uptake and moderate adsorption enthalpy, which is rare in MOFs for trace C₃H₄ capture (Fig.2g and Table S3).

  To evaluate the selective C₃H₄/C₃H₆ and C₃H₄/C₃H₈ adsorptions of NJTU-Bai88, the selectivities for C₃H₄/C₃H₆ (molar ratio of 1/99) and C₃H₄/C₃H₈ (1/99) gas mixtures were calculated using IAST (ideal adsorbed solution theory), based upon the single-component gas adsorption isotherms of C₃H₄, C₃H₆, and C₃H₈ at 298 K (Fig.2h). At 100 kPa, the IAST selectivities of C₃H₄/C₃H₆ (1/99) and C₃H₄/C₃H₈ (1/99) reached 3.1 and 5.5, respectively. Although the selectivity for C₃H₄/C₃H₆ was lower than the values of many high-performance MOFs, it is close to those of some representative MOFs, such as MIL-100(Cr) (4.5)^[1] and Cu-BTC (3.2)^[1], and even higher than the data for the well-known ZIF-8 (1.9)^[1] (Table S3). Very interestingly, NJTU‑Bai88 exhibits high C₃H₄ adsorption uptake, selectivity, and moderate adsorption enthalpy, which implies its potential prospect for the trace C₃H₄ capture.

  3.   Conclusions

  In summary, occupying OMSs with solvent molecules in MOFs was proposed to appropriately decrease pore sizes and eliminate strong adsorption sites. A new acs-MOF based upon [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] clusters and bicyclo[2.2.2]octane-1, 4-dicarboxylic acid was successfully synthesized as NJTU-Bai88, in which the potential OMSs are occupied by coordinated DMF molecules after activation. Very interestingly, the HK pore size was 0.47 nm, being close to the kinetic diameter of the C₃H₄ molecule. NJTU-Bai88 exhibited a high C₃H₄ adsorption uptake of 27.5 cm³·g^-1 at 298 K and 1 kPa and a moderate zero‑loading adsorption enthalpy of -58.8 kJ·mol^-1. Moreover, its IAST selectivities for C₃H₄/C₃H₆ (1/99) and C₃H₄/C₃H₈ (1/99) reached 3.1 and 5.5, respectively. This work not only provides a potential new material for the removal of C₃H₄ from C₃H₆, but also offers important insights into the design and synthesis of high-performance MOFs for the trace C₃H₄ capture.

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

  
  
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  Figure 1  (a) [Fe_2.9Mn_0.1(μ₃-OH)(COO)₆] cluster; (b) Ligand L^2-; (c) Edge-missing trigonal bipyramid cage with the pore size of 0.54 nm; (d) Six edge-missing trigonal bipyramidal cages at the vertices of two interlaced triangles in different layers, respectively, surrounding a 1D pore channel; (e) 1D pore channel with a pore size of approximately 0.56 nm; (f) Framework and (g) topology of NJTU-Bai88

  The gray, red, aqua, and blue spheres represent C, O, Fe/Mn, and N atoms, respectively.

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  Figure 2  (a) CO₂ adsorption isotherms of NJTU-Bai88 at 195 K; C₃H₄, C₃H₆ and C₃H₈ adsorption isotherms for NJTU-Bai88 at (b) 273 K and (c) 298 K; (d) C₃H₄, C₃H₆ and C₃H₈ adsorption enthalpies; Comparison of (e) C₃H₄ adsorption uptakes at 1 kPa and 298 K, (f) C₃H₄ adsorption enthalpies, and (g) both C₃H₄ adsorption capacities and enthalpies of NJTU-Bai88 with reported representative MOFs; (h) IAST (ideal adsorbed solution theory) predicted selectivities of NJTU-Bai88 for C₃H₄/C₃H₆ (volume ratio of 1/99) and C₃H₄/C₃H₈ (1/99) mixtures at 298 K

  Panels e-g are drawn according to the data in Table S3.

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