Formation and fine-tuning of metal–organic frameworks with carboxylic pincers for the recognition of a C₂H₂ tetramer and highly selective separation of C₂H₂/C₂H₄

Abstract

Highly efficient ethylene (C₂H₄) and acetylene (C₂H₂) separation is a great challenge and an important process in current industries. Herein, we finely tune a new family of 6-c metal–organic frameworks (MOFs) with crab-like carboxylic pincers for the recognition of a C₂H₂ tetramer and afford NTU-72 with high adsorption C₂H₂/C₂H₄ selectivity (56–441, 298 K) as well as unprecedented recovery of both highly pure C₂H₄ (99.95%) and C₂H₂ (99.36%). Furthermore, the effective binding of a C₂H₂ tetramer by NTU-72's carboxylic pincers has been revealed by gas-loaded crystallography and Raman spectral studies. Our work provides a novel approach for the selective binding of a small molecular cluster for designing high-performance MOFs.

---

Introduction

As components of cracked gas, C₂H₄ and C₂H₂ are important raw materials for the synthesis of various organic compounds such as polyethylene, α-ethynyl alcohols, and acrylic acid derivatives.^1–3 The preparation of these value-added products generally requires a high-purity feed gas. In industry, pure C₂H₄ is realized by solvent absorption or selective catalytic hydrogenation from cracked gas, which are energy consuming processes.^4,5 Meanwhile, the recovery of high-purity C₂H₂ is generally ignored in this process.

Adsorptive separation using porous materials is more environmentally friendly and energy efficient, and is regarded as a promising future separation technology.^6–11 As emerging porous materials, metal–organic frameworks (MOFs), derived from the self-assembly of organic ligands and inorganic nodes, demonstrate great opportunities to distinguish these two kinds of gas molecules by dedicated framework regulations, despite them having nearly identical molecular dimensions and physical properties.^12–15 For example, the MOF materials SIFSIX-Cu-i⁷ and NKMOF-1-Ni¹⁶ exhibit minimal trade-off between adsorption capacity and selectivity, while the flexible UTSA-300 (ref. 17) demonstrates gating separation of these mixtures. However, they all produce lower purity C₂H₂ during desorption, even for the sieving materials, as both C₂H₂ and C₂H₄ molecules can interact with strong binding sites, or the more or less flexibility of the sieving frameworks allows a certain amount of co-adsorption.^18,19

MOFs with free carboxylic acids might be potential light hydrocarbon separation materials due to their remarkable characteristics of being a hydrogen bonding acceptor (O–H) and donor (C=O).^20,21 However, these MOFs have never been systematically investigated or intentionally designed for high-performance gas separation,^22–24 due to the significant coordination activity of carboxylic acids in MOF construction. We are interested in finely tuning MOFs to achieve efficient gas separations.^25–28 Herein, evolved from the interpenetrated dia topology,²⁹ a new family of 6-c MOFs (termed NTU-71 to NTU-73) with carboxylic pincers were designed based upon a C_2v symmetric ligand of 3,5-di(1H-imidazol-1-yl)benzoic acid (HL) and the hybrid ions with different radii (SiF₆²⁻ (d_Si–F: 1.690 Å), TiF₆²⁻ (d_Ti–F: 1.886 Å) and ZrF₆²⁻ (d_Zr–F: 1.983 Å)). Among them, the distance of their carboxylic pincers has been finely tailored, and thus, they may approach those found in crab-like systems (Scheme 1). Very interestingly, NTU-72 exhibits a high C₂H₂/C₂H₄ selectivity and great ability for recovery of both highly pure C₂H₄ (99.95%) and C₂H₂ (99.36%). This excellent separation performance is due to the stabilization of a C₂H₂ tetramer by a couple of carboxylic pincers in the optimized pore geometry and size in NTU-72.

Scheme 1 Crab-inspired MOF design incorporating carboxylic pincers for selective recognition of a C₂H₂ cluster.

Experimental section

General procedures of the experiment and simulation are available in the ESI.†

Synthesis of NTU-70 to NTU-73

Synthesis of NTU-71. HL (4 mg, 0.015 mmol), CuSiF₆ (10 mg, 0.04 mmol) and H₂SiF₆ (100 μL) were mixed in DMA(dimethylacetamide)/H₂O/EtOH (1 : 1 : 1, 3 mL) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 95 °C for 48 h. After cooling to room temperature, the resulting blue-green crystals were harvested and washed with fresh DMA three times. Yield is ∼49% (based on the ligand).

Synthesis of NTU-72. HL (4 mg, 0.015 mmol), Cu(BF₄)₂ (80 mg, 0.29 mmol), (NH₄)₂TiF₆(10 mg, 0.05 mmol) and H₂TiF₆ (100 μL) were mixed in DMA/H₂O/EtOH (1 : 1.5 : 1, 3.5 mL) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 95 °C for 48 h. After cooling to room temperature, the resulting blue-green crystals were harvested and washed with fresh DMA three times. Yield is ∼53% (based on the ligand).

Synthesis of NTU-73. HL (4 mg, 0.015 mmol), Cu(BF₄)₂ (80 mg, 0.29 mmol), (NH₄)₂ZrF₆(10 mg, 0.04 mmol) and H₂ZrF₆ (100 μL) were mixed in DMA/H₂O/EtOH (1 : 1 : 1, 3 mL) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 90 °C for 48 h. After cooling to room temperature, the resulting blue-green crystals were harvested and washed with fresh DMA three times. Yield is ∼32% (based on ligand).

Results and discussion

Synthesis and structure characterization

A reaction of copper(II) fluorosilicate with HL afforded polyhedron-shaped crystals (NTU-71). It crystallizes in a tetragonal I4₁22 space group with the formula of [Cu(HL)₂SiF₆]·xguest (Table S1†). Its asymmetric unit includes one ligand, half a Cu²⁺ ion and half a SiF₆²⁻ ion (Fig. S1†). The charge of all the Cu²⁺ ions in NTU-71 is balanced by SiF₆²⁻ ions, and thus, all the HL ligands are neutral. The single Cu node here is coordinated by four imidazole nitrogen atoms from four independent HL and two F atoms from two SiF₆²⁻ anions. Meanwhile, the three general branching points of the ligand take part in the coordination with the Cu nodes by two imidazole N atoms, while the carboxylate sites are free. As a third component of this framework, two F atoms from the SiF₆²⁻ anion join the coordination of the adjacent Cu node, yielding two helical chains in this 3D framework (Fig. S2†). Obviously, two channels with densely incorporated carboxylic groups can be observed along the b- and c-axis (Fig. 1a and S3 and S4†). Like crab pincers, these carboxylate groups are packed in pairs throughout the whole structure, and thus form a defined nanospace of about 4.5 × 5.2 Ų (Fig. 1b and c). In addition, these carboxylic pincers are also available in another semiregular channel with an aperture size of 3.9 × 5.6 Ų (Fig. 1d). To understand this structure better, topology analysis was performed. Cu(HL)₂ is an interpenetrated framework, of which each net can be simplified as a classical dia topology. Interestingly, these interpenetrated dia nets are then bridged by a 2-connected SiF₆²⁻ anion, yielding a new 6-c topology with a point symbol of {4^8.6^7} (Fig. S5†).

Fig. 1 Structure of NTU-71: view of the 1D channel that features carboxylic pincers along the b-axis (a); view of the pincers arranged alternately along the a-axis (b); local view of the nanospace defined by carboxylic pincers (c and d). Color codes: C, grey; N, blue; F, cyan; Si, yellow; O, red. The blue red polyhedron represents an octahedral coordinated Cu atom.

Nanospace tuning

Encouraged by this structure and reticular chemistry,³⁰ the fine pore size engineering of the carboxylic-functionalized MOFs was performed, in which the balancing SiF₆²⁻ ions were replaced by TiF₆²⁻ and ZrF₆²⁻, respectively. With a general formula of [Cu(HL)₂MF₆], the other two iso-reticular MOFs (M = Ti²⁺ and Zr²⁺, termed NTU-72 and NTU-73) were obtained (Fig. S6–S8†). Compared with that of NTU-71 (d_O⋯O: 5.021 Å), the pincer distance increased to 5.583 Å and 5.911 Å in NTU-72 and NTU-73, respectively (Fig. 2). These fine changes may offer a fitted nanospace for carboxylic pincers to recognize C₂H₂ clusters. The phase purity of these MOFs was examined by powder X-ray diffraction (PXRD). The diffraction peaks of the as-synthesized NTU-series are in good agreement with the simulated patterns, as well as the activated phases (Fig. S9–S11†).

Fig. 2 Fine distance tuning in NTU-seriesvia the anion planting approach, where d_O⋯O increases at the sub-angstrom level.

Pore evaluation

Permanent porosity of NTU-71 to NTU-73 was further explored by N₂ (77 K) and CO₂ (195 K and 298 K) adsorption measurements. They all display negligible N₂ uptakes, but rapid and revisable type-I CO₂ adsorptions, indicating typical microporous characteristics. Their maximum CO₂ uptakes are 77, 82 and 71 cm³ cm⁻³, respectively (Fig. 3a). Their calculated BET surface areas (based on CO₂, 195 K) reach 242, 315 and 225 m² g⁻¹, respectively, while the analysis of pore size distributions illustrate the systematic change of the nanospace (inset of Fig. 3a).

Fig. 3 Pore evaluation of the NTU-series: CO₂ adsorption isotherms (a), inset is the pore size distribution; C₂H₂ and C₂H₄ isotherms (b); IAST selectivity (c); isosteric heats (d).

Static adsorption selectivity and isosteric heats

Single-component adsorption isotherms of C₂H₂ and C₂H₄ were collected (Fig. 3b and S16–S21†), respectively. At 298 K, they all show relatively higher C₂H₂ uptakes compared with C₂H₄. Interestingly, the C₂H₂ uptakes (10.6 cm³ g⁻¹ and 8.2 cm³ g⁻¹) at 1 kPa are almost 13–14 times higher than that of C₂H₄ (0.8 cm³ g⁻¹ and 0.6 cm³ g⁻¹) in NTU-71 and NTU-73, and this ratio (15.8 cm³ g⁻¹vs. 1.0 cm³ g⁻¹) increases to 16 in NTU-72 (Fig. S22†). These higher uptake ratios reflect preferred affinity of the three frameworks toward C₂H₂ over C₂H₄. The adsorption selectivity of C₂H₂/C₂H₄ (v/v, 1/99) in these three MOFs was predicted by ideal adsorbed solution theory (IAST) (Fig. 3c and S23–S35†).^31,32 The low-pressure C₂H₂ adsorption selectivity of NTU-72 (∼441) is much higher than that of both NTU-71 (∼58) and NTU-73 (∼147), and is also higher than that of ZUL-200 (60),³³ NTU-69 (49),³⁴ SIFSIX-2-Cu-i (45),⁷ and JCM-1 (9).³⁵ In addition, the C₂H₂ adsorption selectivities remain as high as 26, 56 and 38 at 1 bar. Particularly, the selectivity of NTU-72 is only surpassed by the benchmark UTSA-200 (6320),³⁶ ZU-33 (1100),³⁷ and ZUL-series (175 and 114),³³ and is higher than most of the well known MOFs.^7,38

To explore the reason for such high selectivities, isosteric heats of adsorption (Q_st) of C₂H₂ and C₂H₄ were calculated (Fig. 3d and S36–S41†). The Q_st values of C₂H₄ at low pressure (36.8, 36.7 and 35.7 kJ mol⁻¹) are all lower than that of C₂H₂ (42.4, 43.5, and 40.9 kJ mol⁻¹) for the NTU-series. Similar to the higher C₂H₂ uptake, the adsorption enthalpy difference of NTU-72 (6.7 kJ mol⁻¹) is higher than that of NTU-71 (5.6 kJ mol⁻¹) and NTU-73 (5.2 kJ mol⁻¹). This may be due to the fine fitting of the pore geometry and size in NTU-72 to the C₂H₂ molecule, while the relatively larger or smaller nanospace in NTU-71 or NTU-73 provides a small interaction difference.

Breakthrough experiments were performed on NTU-72, in which the binary mixtures of C₂H₂/C₂H₄ (v/v, 1/99) were introduced into the system. Pure C₂H₄ passed through the sample bed as the first component (4.2 min g⁻¹), while C₂H₂ is retained in the column (Fig. 4a). The mixtures of C₂H₂ and C₂H₄ were detected at about 71.1 min g⁻¹. This result shows that highly pure C₂H₄ (99.95%) can be directly collected in 66.9 min g⁻¹ at the adsorption stage. According to the sample weight and gas velocity, NTU-72 could produce 331.1 cm³ (STP) per g of pure C₂H₄ from this mixture. To confirm the selective adsorption, the saturated adsorbents were then swept by helium at 298 K until no signal was detected. Subsequently, C₂H₂ was desorbed by rapid heating (up to 323 K), and its purity is approximately 99.36% (Fig. 4b and S42 and S43†). Therefore, as an emerging example, NTU-72 can deliver both highly pure C₂H₄ and C₂H₂ in one adsorption/desorption cycle, although the recovery of poly-grade C₂H₄ has been widely reported (Table S2†). Structural stability was then confirmed by cycling breakthrough experiments, where nearly the same separation curves were displayed (Fig. 4c). Further experiments with varied gas velocity (2 and 10 cm³ min⁻¹) also showed obvious separation, and the C₂H₄ productivity (319.6 and 323.8 cm³ (STP) per g) has hardly suffered any impact (Fig. 4d and S44†).

Fig. 4 Breakthrough curves of NTU-72 for a C₂H₂/C₂H₄ mixture (1/99, v/v) at 298 K, 1 bar, velocity is 5 cm³ min⁻¹ (a); desorption curves (b); cycle breakthrough tests (c); (d) breakthrough curves at different velocities.

To gain better understanding of the separation performance, C₂H₂- and C₂H₄-loaded NTU-72 crystals were investigated at 298 K.³⁹ As expected, both gases interact with the carboxylic sites (Fig. S45 and S46†). For NTU-72⊃C₂H₂, the occupancy of C₂H₂ was refined to be 1, which corresponds to 30.5 cm³ g⁻¹, slightly lower than the amount in the C₂H₂ adsorption experiment (41 cm³ g⁻¹) at 1 bar. This difference may be caused by two reasons: (1) the C₂H₂ molecules at other free space are strongly disordered and hard to be accurately modelled; (2) part of the C₂H₂ escaped during the sealing of the glass tube by using a hot candle. Despite this, the position and geometry of the adsorbed C₂H₂ molecules can be clearly identified in this confined nanospace. The C₂H₂ molecules are confined in the channel through three types of hydrogen bonds in different binding geometries (Fig. 5a and b). Acting as an acceptor and a donor, the O_COOH and OH_COOH moieties connect the terminal hydrogen and carbon atoms of a C₂H₂ molecule. In addition, the OH_COOH moiety also forms a hydrogen bond with another C₂H₂ molecule, resulting in close packing of the two gas molecules with intermolecular hydrogen bonds. Meanwhile, the terminal H_C2H2 forms hydrogen bonds with C_C2H2 from its symmetrically equivalent molecule, so that a C₂H₂ tetramer with C_2h symmetry is formed periodically in NTU-72 (Fig. S47–S49†), which is different from the S₄ symmetric C₂H₂ tetramer in SIFSIX-Cu-i.⁷ Although the C₂H₂ clusters have been investigated,^20,21 a compact C₂H₂ tetramer with C_2h symmetry being stabilized by chelating carboxylic pincers is found for the first time. Sharply in contrast, two types of hydrogen bonds are observed in NTU-72⊃C₂H₄ (Fig. 5c and d and S50–S52†). Particularly, dC_C2H4⋯H–O_COOH (2.705 Å) is much longer than that of dC_C2H2⋯H–O_COOH (1.871 and 2.588 Å), as well as the longer dO_COOH⋯H–O_C2H4, reflecting relative weak carboxylic–C₂H₄ interaction. Furthermore, no intermolecular interaction is found between the two C₂H₄ molecules, as their distance is far. Therefore, the carboxylic sites contribute mainly to the recognition of the C₂H₂ tetramer, and meanwhile, the finely engineered pore geometry and nano-size confined this cluster to be released in the pure form. Further Raman spectra of gas loaded NTU-72 show that the peak at 1950 cm⁻¹ belonging to the tensile vibration of C₂H₂ was found in NTU-72, and in contrast, no helium peak was found (Fig. 5e).^40,41 In addition, the corresponding peak that belongs to C₂H₄ (theoretical position: 1689 cm⁻¹) can't be observed. Therefore, it is rational to conclude that the C₂H₂ clusters assembled in a periodic configuration in NTU-72, agreeing well with the crystallographic studies.

Fig. 5 Structure of C₂H₂- (a and b) and C₂H₄- (c and d) loaded NTU-72. Raman spectra of gas-loaded NTU-72 (e).

Stability

The water/air stability of NTU-72 was then evaluated at room temperature. As shown in Fig. S53,†NTU-72 retained a polyhedral shape after air treatment, but some changes occurred in the shape after water treatment. Single crystal X-ray analysis showed that the water treated crystal (NTU-72⊃H₂O, for 72 h) retains the framework of the as-synthesized phase (Table S1†), confirmed further by PXRD analysis. Additionally, adsorption measurements revealed that the CO₂ capacity of all treated samples is consistent with that of fresh samples, except for a slight decrease in the air treated sample for 72 h. These results confirm that NTU-72 has good water and moisture robustness (Fig. 6a and b). Considering the structure, the maintained crystallinity of NTU-72 is due to the relatively strong Cu–N bond, similar to that of NTU-67, having a highly robust framework with a Cu–N coordination sphere also.³⁹ In addition, the crystal structure of NTU-72⊃H₂O shows that a couple of carboxylic pincers bind a tetra-water cluster via short hydrogen bonds (d_O⋯O: 2.586–2.662 Å), which may provide a blocking effect for other water molecules to reach the coordination bonds (Fig. 6c).

Fig. 6 PXRD (a) and CO₂ adsorption isotherms (b) of water and moisture treated NTU-72; local view of the structure of NTU-72⊃H₂O (c), and the unit of O⋯O distance is Å.

Conclusions

In conclusion, as needed for the pure C2 hydrocarbons, we have finely tuned a new family of carboxylic-functionalized MOFs and afforded NTU-72 with high adsorption C₂H₂/C₂H₄ selectivity as well as unprecedented recovery of highly pure C₂H₄ and C₂H₂. The carboxylic pincers contribute to the recognition of a C₂H₂ tetramer, and the fitted pore geometry/size enforces synergistic binding and restricts this cluster to be released in a pure form. In addition, this material is stable in water and moisture environments. The foregoing results not only present an excellent MOF for energy-saving C2 hydrocarbon purification, but also provide a novel approach for the selective binding of gas clusters for designing advanced MOFs.

Data availability

All data can be found in the main text and ESI.†

Author contributions

J. D. conceived the idea of this work. Y. D. and Y. H. carried out the experiments and analyzed the results. J. D. and J. B. wrote the paper. C. W. performed the repeated adsorption experiments. H. W. prepared the ligands. All authors gave valuable discussion and suggestion for the final draft.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

We are thankful for the financial support of the National Natural Science Foundation of China (22171135 and 22271150), the Young and Middle-aged Academic Leader of Jiangsu Provincial Blue Project, the State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201803) and Open Foundation of Hunan Provincial Key Laboratory of Controllable Preparation and Functional Application of Fine Polymers (E22306).

References

1. D. S. Sholl and R. P. Lively, Nature, 2016, 532, 435–437.
2. R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota, R. V. Belosludov, T. C. Kobayashi, H. Sakamoto, T. Chiba, M. Takata, Y. Kawazoe and Y. Mita, Nature, 2005, 436, 238–241.
3. Y. Chai, X. Han, W. Li, S. Liu, S. Yao, C. Wang, W. Shi, I. da-Silva, P. Manuel, Y. Cheng, L. D. Daemen, A. J. Ramirez-Cuesta, C. C. Tang, L. Jiang, S. Yang, N. Guan and L. Li, Science, 2020, 368, 1002–1006.
4. K. J. Chen, D. G. Madden, S. Mukherjee, T. Pham, K. A. Forrest, A. Kumar, B. Space, J. Kong, Q. Y. Zhang and M. J. Zaworotko, Science, 2019, 366, 241–246.
5. F. Studt, F. Abild-Pedersen, T. Bligaard, R. Z. Sorensen, C. H. Christensen and J. K. Norskov, Science, 2008, 320, 1320–1322.
6. E. D. Bloch, W. L. Queen, R. Krishna, J. M. Zadrozny, C. M. Brown and J. R. Long, Science, 2012, 335, 1606–1610.
7. X. L. Cui, K. J. Chen, H. B. Xing, Q. W. Yang, R. Krishna, Z. B. Bao, H. Wu, W. Zhou, X. L. Dong, Y. Han, B. Li, Q. L. Ren, M. J. Zaworotko and B. L. Chen, Science, 2016, 353, 141–144.
8. B. Li, X. Cui, D. O'Nolan, H. M. Wen, M. Jiang, R. Krishna, H. Wu, R. B. Lin, Y. S. Chen, D. Yuan, H. Xing, W. Zhou, Q. Ren, G. Qian, M. J. Zaworotko and B. Chen, Adv. Mater., 2017, 29, 1704210–1704217.
9. H. Zeng, M. Xie, T. Wang, R. J. Wei, X. J. Xie, Y. F. Zhao, W. G. Lu and D. Li, Nature, 2021, 595, 542–548.
10. A. Cadiau, K. Adil, P. M. Bhatt, Y. Belmabkhout and M. Eddaoudi, Science, 2016, 353, 137–140.
11. Q. Dong, Y. Huang, J. Wan, Z. Lu, Z. Wang, C. Gu, J. Duan and J. Bai, J. Am. Chem. Soc., 2023 DOI:10.1021/jacs.3c00515.
12. H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Nature, 1999, 402, 276–279.
13. S. Krause, V. Bon, I. Senkovska, U. Stoeck, D. Wallacher, D. M. Tobbens, S. Zander, R. S. Pillai, G. Maurin, F. X. Coudert and S. Kaskel, Nature, 2016, 532, 348–352.
14. S. H. Lo, L. Feng, K. Tan, Z. H. Huang, S. Yuan, K. Y. Wang, B. H. Li, W. L. Liu, G. S. Day, S. S. Tao, C. C. Yang, T. T. Luo, C. H. Lin, S. L. Wang, S. J. L. Billinge, K. L. Lu, Y. J. Chabal, X. D. Zou and H. C. Zhou, Nat. Chem., 2020, 12, 90–97.
15. H. Wang, X. L. Dong, V. Colombo, Q. N. Wang, Y. Y. Liu, W. Liu, X. L. Wang, X. Y. Huang, D. M. Proserpio, A. Sironi, Y. Han and J. Li, Adv. Mater., 2018, 30, 1805088–1805096.
16. Y. L. Peng, T. Pham, P. Li, T. Wang, Y. Chen, K. J. Chen, K. A. Forrest, B. Space, P. Cheng, M. J. Zaworotko and Z. Zhang, Angew. Chem., Int. Ed., 2018, 57, 10971–10975.
17. R. B. Lin, L. B. Li, H. Wu, H. Arman, B. Li, R. G. Lin, W. Zhou and B. L. Chen, J. Am. Chem. Soc., 2017, 139, 8022–8028.
18. J. P. Zhang, P. Q. Liao, H. L. Zhou, R. B. Lin and X. M. Chen, Chem. Soc. Rev., 2014, 43, 5789–5814.
19. N. Behera, J. Duan, W. Jin and S. Kitagawa, EnergyChem, 2021, 3, 100067–100124.
20. K. Shuler and C. E. Dykstra, J. Phys. Chem. A, 2000, 104, 11522–11530.
21. J. G. Yu, S. J. Su and J. E. Bloor, J. Phys. Chem., 1990, 94, 5589–5592.
22. R. Custelcean and M. G. Gorbunova, J. Am. Chem. Soc., 2005, 127, 16362–16363.
23. X. Duan, R. J. Song, J. C. Yu, H. L. Wang, Y. J. Cui, Y. Yang, B. L. Chen and G. D. Qian, RSC Adv., 2014, 4, 36419–36424.
24. F. Ragon, B. Campo, Q. Yang, C. Martineau, A. D. Wiersum, A. Lago, V. Guillerm, C. Hemsley, J. F. Eubank, M. Vishnuvarthan, F. Taulelle, P. Horcajada, A. Vimont, P. L. Llewellyn, M. Daturi, S. Devautour-Vinot, G. Maurin, C. Serre, T. Devic and G. Clet, J. Mater. Chem. A, 2015, 3, 3294–3309.
25. J. Duan, M. Higuchi, J. Zheng, S. I. Noro, I. Y. Chang, K. Hyeon-Deuk, S. Mathew, S. Kusaka, E. Sivaniah, R. Matsuda, S. Sakaki and S. Kitagawa, J. Am. Chem. Soc., 2017, 139, 11576–11583.
26. Z. Y. Lu, L. T. Du, R. Y. Guo, G. B. Zhang, J. G. Duan, J. F. Zhang, L. Han, J. F. Bai and J. T. Hupp, J. Am. Chem. Soc., 2021, 143, 17942–17946.
27. M. X. Zhang, W. Zhou, T. Pham, K. A. Forrest, W. L. Liu, Y. B. He, H. Wu, T. Yildirim, B. L. Chen, B. Space, Y. Pan, M. J. Zaworotko and J. F. Bai, Angew. Chem., Int. Ed., 2017, 56, 11426–11430.
28. L. Du, Z. Lu, K. Zheng, J. Wang, X. Zheng, Y. Pan, X. You and J. Bai, J. Am. Chem. Soc., 2013, 135, 562–565.
29. M. Li, D. Li, M. O'Keeffe and O. M. Yaghi, Chem. Rev., 2014, 114, 1343–1370.
30. N. W. Ockwig, O. Delgado-Friedrichs, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2005, 38, 176–182.
31. J. G. Duan, M. Higuchi, S. Horike, M. L. Foo, K. P. Rao, Y. Inubushi, T. Fukushima and S. Kitagawa, Adv. Funct. Mater., 2013, 23, 3525–3530.
32. R. Krishna, RSC Adv., 2015, 5, 52269–52295.
33. J. Shen, X. He, T. Ke, R. Krishna, J. M. van Baten, R. Chen, Z. Bao, H. Xing, M. Dinca, Z. Zhang, Q. Yang and Q. Ren, Nat. Commun., 2020, 11, 6259.
34. Y. Huang, Y. Xu, B. Zheng, Z. Wang, Q. Dong and J. Duan, Energy Fuels, 2020, 34, 11315–11321.
35. J. Lee, C. Y. Chuah, J. Kim, Y. Kim, N. Ko, Y. Seo, K. Kim, T. H. Bae and E. Lee, Angew. Chem., Int. Ed., 2018, 57, 7869–7873.
36. H. Li, L. Li, R. B. Lin, G. Ramirez, W. Zhou, R. Krishna, Z. Zhang, S. Xiang and B. Chen, ACS Sustainable Chem. Eng., 2019, 7, 4897–4902.
37. Z. Zhang, X. Cui, L. Yang, J. Cui, Z. Bao, Q. Yang and H. Xing, Ind. Eng. Chem. Res., 2018, 57, 7266–7274.
38. J. Wang, L. Li, L. Guo, Y. Zhao, D. Xie, Z. Zhang, Q. Yang, Y. Yang, Z. Bao and Q. Ren, Chem.–Eur. J., 2019, 25, 15516–15524.
39. Q. B. Dong, Y. H. Huang, K. Hyeon-Deuk, I. Y. Chang, J. M. Wan, C. L. Chen, J. G. Duan, W. N. Jin and S. Kitagawa, Adv. Funct. Mater., 2022, 32, 2203745–2203755.
40. H. Wang, Y. Duan, Y. Wang, Y. Huang, K. Ge, S. Wang, B. Zheng, Z. Wang, J. Bai and J. Duan, ACS Appl. Mater. Interfaces, 2022, 14, 13550–13559.
41. G. C. Guo, S. H. Lin, F. K. Zheng, Z. C. Dong and J. S. Huang, Spectrosc. Spectral Anal., 2000, 20, 830–832.

---

Footnotes

† Electronic supplementary information (ESI) available: Synthesis and characterization of these three crystals, PXRD, TGA, IR, sorption isotherms, IAST, breakthrough experiments and fitting parameters. CCDC 2211444–2211448, 2251139. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00877k

‡ These authors contributed equally to this work.

---