English

• As the basic raw materials of petrochemical industry, acetylene (C₂H₂) and ethylene (C₂H₄) are usually produced by thermal cracking of hydrocarbons or combustion of natural gas [1]. In the process of producing C₂H₂, carbon dioxide (CO₂) is often contained as an impure substance, and traces of acetylene are often contained in the process of C₂H₄ production [2-4]. Therefore, it is very important to separate C₂H₂/CO₂ and C₂H₂/C₂H₄ binary gas mixtures. At present, distillation technology is used to separate mixed gases in industrial gas purification, but this technology has high energy consumption and high cost, which is not conducive to large-scale use [5]. Physical adsorption and separation based on porous adsorbents is supposed to greatly cut down energy consumption while achieving environmental protection.

  Metal-organic frameworks (MOFs), also denoted as porous coordination polymers (PCPs), have emerged as a class of novel porous material self-assembled with metal ions/clusters and organic ligands [6, 7]. In recent years, due to the distinctive characteristics of compositional diversity, large surface area, adjustable pore structure, and functional pore surface [8-12], MOFs have appeared as a promising class of porous materials in many fields, especially for the storage, separation and purification of small molecular gas [13-18]. According to the difference in gas mixtures (including size, shape and polarity of gas molecules) to design and synthesize MOF adsorbents is the key to achieve efficient gas separation performance [19, 20]. For example, for C₂H₂ and C₂H₄ with different kinetic diameters (C₂H₂: 3.3 Å and C₂H₄: 4.2 Å), the size-based sieving strategy is effective for C₂H₂/C₂H₄ separation [21-24]. However, for CO₂ and C₂H₂ with similar kinetic diameters, it is necessary to introduce other functional sites such as functional ligands, open metal sites (OMS) and Lewis N sites to enhance the interaction between the C₂H₂ and skeleton [25-30]. Therefore, suitable pore size combine with the special binding sites in the frameworks of MOFs are considered to be more useful to enhance the adsorption discrimination for C₂H₂ over other gas.

  Pore space partition (PSP) strategy is an effective method to control the pore size of MOFs. Herein, based on PSP strategy, a series of novel pacs (partition acs) MOFs were successfully synthesized by inserting symmetrical matching ligand into the hexagonal channel of MIL-88-type (acs topology) framework (Scheme 1). By modulating the functional groups of dicarboxylic acid ligands of prototype framework, four isoreticular MOFs namely, [Co₃(OH)L₃(TPA)] (SNNU-278: L = BDC; SNNU-279: L = NH₂BDC; SNNU-280: L = OHBDC; SNNU-281: L = DHBDC) were obtained. The combination of these structural features make these MOF materials not only show the excellent gas uptakes for C₂H₂, but also exhibit remarkable separation performance for C₂H₂/CO₂ and C₂H₂/C₂H₄ binary gas mixtures. The proper pore size coupled with high-density –OH groups make SNNU-281 demonstrates best dynamic breakthrough results for C₂H₂/CO₂ and C₂H₂/C₂H₄ separation.

  Scheme 1

  

  Scheme 1.  Schematic representation of the PSP strategy.

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  All target MOF adsorbents were synthesized by solvothermal reactions with the mixtures of linear dicarboxylate ligands (BDC, NH₂BDC, OHBDC or DHBDC), tripyridine partitioner (TPA) and Co(NO₃)₂·6H₂O ultrasonically dissolved in mixed solvents of DMF, methanol and HBF₄ in a 20 mL glass vial. The mixtures were sealed and heated at 100 ℃ for 48 h. SNNU-278–281 were thus obtained as pink shuttle-shaped, red octahedral, light pink rod-shaped, and brown long shuttle-shaped crystals, respectively.

  The single-crystal structure analysis uncovers that SNNU-278−281 are isostructural frameworks and crystallizes in the hexagonal P6₃/mmc space group. Each Co center is six-coordinated in a octahedral geometry formed by four O atoms from four dicarboxylic acid ligands, one N atom from TPA, and one central μ₃–OH. Three Co atoms form the well-known trigonal prismatic [Co₃(OH)] trimer, which is 9-connected with six dicarboxylic acid ligands and three TPA ligands. The [Co₃(OH)] trimer units are extended to form MIL-88 type structures by six dicarboxylic acid ligands. When the three axial positions occupied by N atoms from TPA ligands, the 1D hexagonal channels along c-axis of MIL-88 were partitioned into numerous cylindrical segments of about 5.8 Å in length (Scheme 1). As shown in Fig. 1, the cylindrical cage (cage A) consist of six [Co₃(OH)] trimer units, six dicarboxylic acid ligands and two TPA ligands. In addition, another kind of trigonal bipyramid-type cage (cage B) consist of five [Co₃(OH)] trimer units, six dicarboxylic acid ligands, and three TPA ligands (Figs. 1d and e). Each cage is adjacent to six another cages by sharing the vertexes and edges. The interlacing of two kinds of cages generate 3D porous framework of SNNU-278−281 (Figs. 1f and g).

  Figure 1

  

  Figure 1.  The crystal structures for SNNU-278−281: (a) [Co₃(OH)(COO)₆] cluster; (b) four linear dicarboxylate ligands; (c) triangular tri(pyridin-4-yl)-amine; (d, e) two types of cages; (f, g) 3D porous architecture viewed along the c-axis and b-axis directions.

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  The phase purity of all samples was determined by the powder X-ray diffraction (PXRD). As shown in Fig. S2 (Supporting information), the PXRD patterns of synthesized SNNU-278−281 MOFs match well with the simulated patterns from single-crystal data, which indicates the high crystallinity and good phase purities of all samples. Moreover, the isostructural architectures of SNNU-278−281 also verified by the PXRD. After soaking in different organic solvents for three days, the PXRD results of SNNU-278−281 compounds still are in good agreement with the as-prepared samples, which demonstrate their good chemical stability (Fig. S3 in Supporting information) Meanwhile, the thermal stability of SNNU-278−281 were measured by thermogravimetric analysis (TGA). As shown in Fig. S4 (Supporting information), the as-prepared and CH₃CN-exchanged samples all can be stable up to at least 300 ℃ under N₂ atmosphere. Compared with the as-prepared samples, the TGA curves of the activated samples have more obvious platform. A continuous weight loss of ~20% from 25 ℃ to ~100 ℃ are observed for activated samples of SNNU-278−281, corresponding to the loss of trapped solvent molecules. Then the MOF skeletons can keep stable until 380, 340, 350 and 300 ℃ for SNNU-278−281, respectively. With the rising of temperature, two continuous weight losses are observed, corresponding to the escape of organic ligands.

  To evaluate the permanent porosities of SNNU-278−281, N₂ adsorption isotherms were measured on their activated samples at 77 K. These as-synthesized materials were activated by immersion in CH₃CN for 4 days and then were heated at 80 ℃ for 10 h under a dynamic vacuum. The PXRD patterns of samples after gas sorption measurements matches well with that of the as-synthesized samples, indicating that the activated samples retained structures stability (Fig. S2). As shown in Fig. 2a, SNNU-278−281 show reversible type-I isotherms of the microporous materials. The maximum N₂ adsorptions for SNNU-278−281 at 77 K are 304, 293, 277 and 254 cm³/g and their pore volumes are 0.46, 0.40, 0.39 and 0.38 cm³/g, respectively. The corresponding Brunauer−Emmett−Teller (BET) surface areas of SNNU-278−281 are estimated to be 830, 750, 730 and 710 m²/g and Langmuir surface areas are estimated to be 1340, 1180, 1150 and 1110 m²/g, respectively. The pore size distributions reveal that sharp peaks centered at 4.25 Å by the Horvath−Kawazoe method.

  Figure 2

  

  Figure 2.  Gas adsorption performance for SNNU-278−281: (a) N₂ sorption isotherms at 77 K; (b) C₂H₂ uptake isotherms at 273 K and 298 K; (c) C₂H₄ uptake isotherms at 273 K and 298 K; (d) CO₂ uptake isotherms at 273 K and 298 K; (e) CO₂, C₂H₂ and C₂H₄ uptake isotherms at 273 K and 298 K for SNNU-278; (f) isosteric heats of gas adsorption (Q_st) for SNNU-278.

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  The porous properties and regulated functional sites prompted us to further investigate the gas adsorption and separation of SNNU-278−281 for small gas molecules. As shown in Fig. 2b, the C₂H₂ uptakes for SNNU-278−281 under 1 atm are 149.7/109.9, 139.5/106.5, 133.4/102.9, and 123.7/92.7 cm³/g at 273/298 K, respectively. Even if the C₂H₂ uptakes for SNNU-278−281 are not the best, these values are much higher than many famous MOF adsorbents, such as UPC-110 (131.7 cm³/g) [31], JU-26 (127 cm³/g) [32], UTSA-222a (103.4 cm³/g) [33], JXNU-5 (70.3 cm³/g) [34]. The C₂H₂ uptake capacities at 1 bar follow the order of SNNU-278 > SNNU-279 > SNNU-280 > SNNU-281, which are consistent with the order of pore volumes. However, the C₂H₂ uptakes of SNNU-279−281 are higher than SNNU-278 at lower pressure (< 0.1 bar), which attributed to the strong force between functional groups (–NH₂ and –OH) and C₂H₂ molecules.

  Furthermore, at 273/298 K and 1 atm, the C₂H₄ uptakes for SNNU-278−281 also show the same tendency. As shown in Fig. 2c, the corresponding values are 119.9/90.0 cm³/g, 110.3/86.4 cm³/g, 103.1/79.4 cm³/g and 93.0/73.4 cm³/g, respectively. Notably, the CO₂ uptake capacity of these four MOFs show very slight difference at 273/298 K and 1 atm (Fig. 2d). They can adsorb 125.8/74.1, 128.1/80.2, 126.5/76.2 and 125.0/78.0 cm³/g, respectively. In detail, the uptake capacity of SNNU-278−281 for C₂H₂ are higher than those for C₂H₄ and CO₂ (Fig. 2e and Fig. S5 in Supporting information). Meanwhile, under low pressure (< 0.2 bar), the gas uptakes for these MOFs follow the order of C₂H₂ > C₂H₄ > CO₂, indicating the immense potential of C₂H₂/C₂H₄ and C₂H₂/CO₂.

  The isosteric heat (Q_st) was further calculated to assess the interaction between SNNU-278−281 materials and small gas molecules. As depicted in Fig. 2f, the -Q_st⁰ values of C₂H₂, C₂H₄ and CO₂ at zero-coverage on SNNU-278 are 28.6, 26.9 and 24.3 kJ/mol. The corresponding values are 33.4/35.5/33.1, 27.6/28.8/26.9 and 25.8/33.7/25.7 kJ/mol for SNNU-279−281 (Fig. S6 in Supporting information). Obviously, the -Q_st⁰ values of these MOFs for C₂H₂ are higher than C₂H₄ and CO₂, which further suggest the frameworks have stronger interaction with C₂H₂ molecules. Moreover, it is noted that SNNU-279−281 show higher -Q_st⁰ values over SNNU-278, proving again the function of –NH₂ and –OH groups on small gas molecules.

  To evaluate the separation performance for SNNU-278−281, the ideal adsorbed solution theory (IAST) method was used to predict the adsorption selectivity. According to Langmuir−Freundlich (LF) approach, the experimental gas isotherms are well fitted with the simulation results at 273 K and 298 K (Fig. 3a). As shown in Figs. 3b and d, under 1 atm, the IAST selectivities of SNNU-278−281 for the equimolar C₂H₂/CO₂ are 8.6, 9.1, 14.0, and 11.3 at 273 K and 4.3, 5.9, 10.2 and 9.6 at 298 K, respectively. The corresponding values are 1.6, 2.8, 4.3 and 4.4 at 273 K and 1.4, 2.2, 4.2 and 2.8 at 298 K for the equimolar C₂H₂/C₂H₄ (Figs. 3c and e). In comparison, the functional groups of –NH₂ and –OH make the selectivities of SNNU-279−281 superior to SNNU-278. And the values of SNNU-280 and SNNU-281 are clearly higher than SNNU-279, which is due to the stronger force of –OH for C₂H₂ over –NH₂ (Table 1).

  Figure 3

  

  Figure 3.  The selectivities predicted for equimolar binary mixtures by IAST of SNNU-278−281: (a) adsorption isotherms of simulated by IAST and experimental for SNNU-278; the comparison of selectivities of all MOFs for (b) C₂H₂/CO₂ at 273 K, (c) C₂H₂/C₂H₄ at 273 K, (d) C₂H₂/CO₂ at 298 K, and (e) C₂H₂/C₂H₄ at 298 K; (f) C₂H₂/CO₂ and C₂H₂/C₂H₄ at 273 K and 298 K for SNNU-281.

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  Table 1

  

  Table 1.  CO₂, C₂H₂ and C₂H₄ uptake amounts (cm³/g) and IAST selectivity values of equimolar binary C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures for SNNU-278−281.

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  In addition, all MOFs show better separation performance for C₂H₂/CO₂ than C₂H₂/C₂H₄, demonstrating that the frameworks interact with gas molecules in the order of C₂H₂ > C₂H₄ > CO₂, which is consistent with the gas adsorption C₂H₂ results (Fig. 3f and Table 1). Remarkably, at 298 K and 1 atm, the C₂H₂/CO₂ selectivity values for SNNU-280 and SNNU-281 are much higher than some promising MOF adsorbents under the same conditions, such as Cu(BDC-Br)(H₂O)_0.5(DMF)_2.0 (3.9) [35], Cu₂(ade)₂(PA)₂ (4.2) [36], FJU-90a (4.3) [37], ZJNU-93a (4.5) [38], ZJNU-76a (4.7) [39], MUF-17 (6.0) [2].

  In order to further verify the practical separation performance of C₂H₂/CO₂ and C₂H₂/C₂H₄ for these MOFs, the breakthrough curves were tested. For equimolar binary gas mixtures C₂H₂/CO₂ and C₂H₂/C₂H₄, the experimental results show that CO₂ and C₂H₄ first eluted and then C₂H₂ was retained until its saturated uptake (Figs. S7 and S8 in Supporting information). As shown in Fig. 4a, at 298 K and 1 atm, the breakthrough time toward C₂H₂/CO₂ mixtures are 17, 22, 29 and 31 min/g for SNNU-278−281 with a flow rate of 2.0 mL/min, respectively. The corresponding values are 10, 12, 14 and 17 min/g toward C₂H₂/C₂H₄ mixtures under the same conditions (Fig. 4b). By comparison, the breakthrough time for SNNU-280 and SNNU-281 is longer than that of SNNU-278 and SNNU-279, which indicates the introduction of –OH groups is beneficial to separate C₂H₂/CO₂ and C₂H₂/C₂H₄. The breakthrough residue times of SNNU-281 clearly surpass many other MOFs such as ZJU-196a (6.3 min/g) [40], UTSA-300a (12.8 min/g) [41], FJU-90a (22 min/g) [37] and UTSA-74a (27 min/g) (Fig. 4d) [42].

  Figure 4

  

  Figure 4.  (a) The experimental breakthrough curves of SNNU-278–281 for the C₂H₂/CO₂ (50:50) at 298 K and 1 atm. (b) The experimental breakthrough curves of SNNU-278−281 for the C₂H₂/C₂H₄ (50:50) at 298 K and 1 atm. (c) The cycling test of SNNU-281 at 298 K for the equimolar C₂H₂/CO₂. (d) Comparison of the breakthrough residue times for equimolar C₂H₂/CO₂ at 298 K and 1 atm of some best MOF adsorbents.

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  The reproducibility and recyclability of the adsorbent is another important parameter to evaluate its practical adsorption and separation performance. Therefore, the recycle breakthrough experiments of SNNU-281 for C₂H₂/CO₂ were also tested under a flow rate of 4.0 mL/min. As shown in Fig. 4c, SNNU-281 shows good recyclability after three dynamic breakthrough experiments, indicating the structural stability of the adsorbent. These breakthrough results further demonstrate that these PSP-MOFs are promising porous materials for C₂H₂/CO₂ and C₂H₂/C₂H₄ separation.

  To clearly know the interaction between gas molecules and MOFs, Grand Canonical Monte Carlo (GCMC) was used to simulate gas molecules distribution and favorable sorption site within the MOF skeleton at 298 K. As depicted in Figs. S9a-f (Supporting information), these results show that the distribution density of C₂H₂ is much higher than that of CO₂ for different frameworks, which is consistent with result of adsorption capacity. For C₂H₂ adsorption of the SNNU-281, main adsorption sites are located at the pore space, indicating that TPA ligand provide affinity site to C₂H₂. In addition, -OH group also provide stronger interactions of OH⋅⋅⋅CH≡CH (Fig. S9g in Supporting information). It is noted that several kinds of hydrogen bonds between C₂H₂ and the MOF skeleton in SNNU-281 are observed. The bond lengths are about 3.99 Å, 3.29 Å, 3.26 Å and 2.57 Å, and bond angles range from 110° to 136°, respectively. But the CO₂ adsorption sites for SNNU-281 are less in the pore compared to C₂H₂ (Fig. S9h in Supporting information). Therefore, it can be concluded that suitable pore space and functional groups play important role to improve separation property of C₂H₂/CO_2.

  In summary, we successfully designed four novel isoreticular MOFs with targeted topological nets by utilizing the pore space partition (PSP) strategy. All MOFs show excellent gas adsorption and separation performance, and the C₂H₂ uptake amount of SNNU-278 is superior to most famous MOFs. The gas adsorption and GCMC simulations all indicate that introduction of Lewis basic groups are clearly beneficial for the C₂H₂/CO₂ and C₂H₂/C₂H₄ separation performance. Specially, the optimized SNNU-281 with high-density –OH groups demonstrates the longest dynamic breakthrough residue time among four pore-space-partitioned metal-organic framework adsorbents.

  Declaration of competing interest

  We declared that we have no conflicts of interest to this work.

  Acknowledgments

  This work is financially supported by the National Natural Science Foundation of China (No. 22071140), the Natural Science Foundation of Shaanxi Province (No. 2021JLM-20), the Fundamental Research Funds for the Central Universities (No. GK202101002).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.019.

  
  
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• 

  Scheme 1  Schematic representation of the PSP strategy.

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  Figure 1  The crystal structures for SNNU-278−281: (a) [Co₃(OH)(COO)₆] cluster; (b) four linear dicarboxylate ligands; (c) triangular tri(pyridin-4-yl)-amine; (d, e) two types of cages; (f, g) 3D porous architecture viewed along the c-axis and b-axis directions.

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  Figure 2  Gas adsorption performance for SNNU-278−281: (a) N₂ sorption isotherms at 77 K; (b) C₂H₂ uptake isotherms at 273 K and 298 K; (c) C₂H₄ uptake isotherms at 273 K and 298 K; (d) CO₂ uptake isotherms at 273 K and 298 K; (e) CO₂, C₂H₂ and C₂H₄ uptake isotherms at 273 K and 298 K for SNNU-278; (f) isosteric heats of gas adsorption (Q_st) for SNNU-278.

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  Figure 3  The selectivities predicted for equimolar binary mixtures by IAST of SNNU-278−281: (a) adsorption isotherms of simulated by IAST and experimental for SNNU-278; the comparison of selectivities of all MOFs for (b) C₂H₂/CO₂ at 273 K, (c) C₂H₂/C₂H₄ at 273 K, (d) C₂H₂/CO₂ at 298 K, and (e) C₂H₂/C₂H₄ at 298 K; (f) C₂H₂/CO₂ and C₂H₂/C₂H₄ at 273 K and 298 K for SNNU-281.

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  Figure 4  (a) The experimental breakthrough curves of SNNU-278–281 for the C₂H₂/CO₂ (50:50) at 298 K and 1 atm. (b) The experimental breakthrough curves of SNNU-278−281 for the C₂H₂/C₂H₄ (50:50) at 298 K and 1 atm. (c) The cycling test of SNNU-281 at 298 K for the equimolar C₂H₂/CO₂. (d) Comparison of the breakthrough residue times for equimolar C₂H₂/CO₂ at 298 K and 1 atm of some best MOF adsorbents.

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  Table 1.  CO₂, C₂H₂ and C₂H₄ uptake amounts (cm³/g) and IAST selectivity values of equimolar binary C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures for SNNU-278−281.

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