Enhanced NH₃ uptake and selectivity at low pressure in monolithic MOF-808 metal–organic gels incorporating CuCl₂

Abstract

The capture and separation of trace NH₃ from industrial processes or polluted air remains a significant challenge. Herein, we report a monolithic CuCl₂@G808 metal–organic gel achieving an NH₃ uptake of 2.23 mmol g⁻¹ at 298 K and 0.002 bar: a 79% enhancement compared to pristine G808. The ideal adsorbed solution theory (IAST) selectivity reaches 2.8 × 10³ for NH₃/N₂ and 4.9 × 10⁵ for NH₃/H₂ at 298 K, ranking among the highest reported values. In situ FTIR and XPS analyses reveal that the excellent performance mainly originates from two synergistic mechanisms: (i) coordination between NH₃ and Cu²⁺ sites, and (ii) hydrogen bonding between NH₃ and Cl⁻ sites.

---

Introduction

Ammonia (NH₃) is an important chemical feedstock, mainly produced by the Haber–Bosch process, and widely used in refrigeration, energy, and fertilizer industries.^1,2 At the same time, ammonia is a highly toxic and corrosive gas, mostly derived from the inadvertent or intentional emission, which causes severe damage to human health and the environment even at low concentrations.³ Therefore, efficient capture and separation of trace NH₃ from the NH₃ production process or polluted air is of great significance for improving NH₃ production efficiency and reducing energy consumption, and alleviating impacts on the environment and human health.

Metal–organic frameworks (MOFs) adopt structural designability and diversity, abundant active sites, and high porosity, enabling them prospective candidates for real-world NH₃ capture and separation. Several representative MOFs such as LiCl@MIL-53-(OH)₂-43.4,⁴ LiCl@G66-OH-35.7,⁵ IL@MIL-101(Cr),⁶ Mg₂(dobpdc),⁷ Ni_acryl_TMA,⁸ Cu₂Cl₂BBTA,⁹ MOF-303(Al),¹⁰ MOF-253(Al)-NiCl₂-2,¹¹ MFU-4,¹² Cu(cyhdc),¹³ Co(NA)₂,¹⁴ MFM-300(V^IV),¹⁵ Cu(BDC),¹⁶ UiO-66-Cu^II,¹⁷ DUT-6-(OH)₂,¹⁸ [Mn₂Cl₂BTDD],¹⁹ Fe-soc-MOF,²⁰ and MFM-300(Sc)²¹ have demonstrated outstanding NH₃ capture performance even at low pressure. However, their connatural limitations such as low NH₃ adsorption capacity and separation ability at extremely low pressure, high production cost, limited stability, and powder state problem preclude their potential application for NH₃ efficient capture and separation. Therefore, there is an urgent need to fabricate granular high-performance MOFs adsorbents possessing remarkable NH₃ adsorption and separation ability at ultra-low pressure and facile preparation process.

Metal–organic framework gels (MOGs), as a novel self-shaping material, have come to prominent attention owing to its adjustable aperture from micropore to mesopore/macropore, high adsorptive capacity, and easy large-scale preparation.²² MOGs can be facilely fabricated by regulating reaction conditions such as metal source, reactant concentration, solvent, and temperature.^23,24 Up to now, MOGs have demonstrated excellent capture performance in many fields such as methane storage,^25,26 CO₂ capture and storage,^27,28 volatile organic compounds capture,^29–31 water remediation,^32,33 chemical warfare agents decontamination,^34,35 toxic chemical filtration.^36–38 However, there are few reports on the efficient capture and separation of NH₃ using MOGs, especially under ultra-low pressure.

CuCl₂ possesses an exceptional NH₃ uptake, but its application is greatly limited due to its powder state. Based on the above advantages of MOGs, herein, MOF-808 metal–organic framework gel (labeled as G808) is chosen as the platform due to its excellent stability to NH₃. Thus, CuCl₂@G808 composite is prepared in water by a facile impregnation strategy at 80 °C. CuCl₂@G808 containing 2.55 wt% of CuCl₂ shows excellent low-pressure NH₃ uptake (2.23 mmol g⁻¹) at 298 K and 0.002 bar, which displays enhancement of 79% than that of the pristine G808. Notable, highly selective adsorption of trace NH₃ was also obtained at 298 K. Furthermore, molecular-level insights into the adsorption mechanism were elucidated through combined spectroscopic analyses.

Result and discussion

Structure and morphology of CuCl₂@G808-X composites

G808 is firstly synthesized according to our previous work.³⁹ Then, CuCl₂@G808 composite is obtained by a facile impregnation strategy in water solution containing CuCl₂ at 80 °C for 24 h. The content of CuCl₂ loaded on G808 is 2.55% measured by ICP-OES. As observed in Fig. 1a, the PXRD pattern of G808 matches well with the simulated MOF-808 (CCDC: 1002672), as evidenced by the characteristic peaks at 2θ = 4.4° and 8.6°.⁴⁰ Furthermore, the crystallinity of CuCl₂@G808 decreases compared with that of G808, suggesting that the crystal structure of the composite collapses after loading CuCl₂. SEM results also further confirm this conclusion (Fig. S1). In addition, no significant additional peaks are observed in the PXRD of the composite, indicating that CuCl₂ is evenly anchored in the nanopores of G808. Meanwhile, the uniform distribution of Cu and Cl elements in CuCl₂@G808 is verified by energy-dispersive X-ray spectroscopy (Fig. S2). Fig. 1b illustrates the TGA profiles of the samples. The initial mass reduction (303–373 K) is due to residual solvent volatilization, followed by two distinct degradation stages: framework collapse at 473–673 K and organic ligand decomposition at 673–1073 K.⁴⁰ Compared with G808, the thermal stability of CuCl₂@G808 slightly decreases from 743 K to 703 K, indicating that the introduction of CuCl₂ has a certain influence on the thermal stability of the composite. This result is in coincidence with that of XRD. Comparative FT-IR analyses reveal negligible spectral differences between pristine G808 and CuCl₂@G808 (Fig. 1c). Compared with G808, the peak of CuCl₂@G808 stemmed from the carboxylate (–COO⁻) group undergoes some degrees of red shift from 1572 cm⁻¹ to 1562 cm⁻¹, possibly through partial charge transfer from –COO⁻ groups to Cu²⁺.⁴¹ Like G808, the CuCl₂@G808 composite also display characteristic IV isotherms for N₂ adsorption (Fig. 1d), indicating the existence of microporous and mesoporous features. This result is also validated by the pore size distribution (Fig. S3 and S4). Compared to G808, pore volume, surface area, and pore width of CuCl₂@G808 composite dramatically decrease due to the occupation of the pore space by CuCl₂ (Table S1).

Fig. 1 (a) PXRD patterns, (b) TGA curves, (c) FT-IR spectra, and (d) N₂ adsorption–desorption isotherms of CuCl₂@G808.

NH₃ capture and separation

To study capture performance, NH₃ adsorption isotherms (Fig. 2) of G808 and CuCl₂@G808 composite are collected at 298 K and 1 bar. The isotherms show obvious hysteresis loops, suggesting the existing of strong interaction between NH₃ and active sites. Compared with G808 (9.6 mmol g⁻¹), the NH₃ capture capacity of the CuCl₂@G808 (8.1 mmol g⁻¹) slightly decreases due to the decrease of porosity at 298 K and 1 bar. However, the CuCl₂@G808 manifests a significantly enhanced uptake at low pressure. For example, the NH₃ adsorption capacity of CuCl₂@G808 can reach approximately 5.44 mmol g⁻¹ at 0.1 bar, 3.65 mmol g⁻¹ at 0.01 bar, and 2.23 mmol g⁻¹ even at 0.002 bar (Fig. 2a), Compared to pristine G808, these values represent enhancement of 37%, 74%, and 79%, respectively. These results are comparable to the best-behaving values reported recently (Table S2), and indicate that our composite is conducive to the capture of low concentration NH₃. To validate this, N₂ and H₂ isotherms of CuCl₂@G808 are measured at 273 K and 298 K (Fig. 2b, S5 and S6). Notably, the adsorption capacities of N₂ (0.056 mmol g⁻¹) and H₂ (0.016 mmol g⁻¹) on CuCl₂@G808 at 298 K and 1 bar are significantly lower than that of NH₃ (8.1 mmol g⁻¹). This may be attributed to their weak affinity with the composite originated from low Q_st values (Fig. S7 and S8). The IAST-predicted selectivity values of CuCl₂@G808 reach 2.8 × 10³ for NH₃/N₂ and 4.9 × 10⁵ for NH₃/H₂ at 298 K, respectively (Fig. S9 and 10), surpassing most reported MOFs.^11,42 Furthermore, granular CuCl₂@G808 exhibits excellent self-shaping ability, facile synthesis, and low production cost, demonstrating its competitive and practical potential in low concentration NH₃ capture and separation.

Fig. 2 (a) Adsorption–desorption isotherms of NH₃, (b) adsorption isotherms of CuCl₂@G808 for NH₃, N₂, and H₂, (c) breakthrough curves of CuCl₂@G808 for NH₃/N₂/H₂ mixtures, (d) NH₃ adsorption–desorption isotherms for different cycles of CuCl₂@G808 at 298 K.

Considering the valuable NH₃ industry synthesis, breakthrough experiments are adopted to validate the practical separation capability of CuCl₂@G808 for NH₃ using actual NH₃/N₂/H₂ mixtures (3% NH₃ in 25% N₂ and 72% H₂) (Fig. 2c). It is clearly observed that N₂ and H₂ are immediately escaped because of their low adsorption capacity, followed by NH₃ after 16.8 min g⁻¹ accompanied by uptake of 3.9 mmol g⁻¹. Thus, CuCl₂@G808 can be considered as a superior adsorbent for selective separation of NH₃/N₂/H₂ at low NH₃ concentration. Five recycling experiment of CuCl₂@G808 is also carried out to study its practical performance (Fig. 2d and S11). NH₃ adsorption capacity of CuCl₂@G808 can be only maintained up to 5.4 mmol g⁻¹ (67.1%) after five cycles at 298 K and 1 bar due to its partial structural collapse (Fig. S12 and S13). Meanwhile, about 50.7% of NH₃ is difficult to desorb due to the strong interaction between NH₃ and CuCl₂@G808.

Mechanism of NH₃ adsorption

To elucidate adsorption mechanism between CuCl₂@G808 and NH₃, a combination of spectroscopic techniques-including in situ FTIR and XPS spectra are employed (Fig. S14). After adsorption of NH₃, a new IR peak attributed to N–H deformation vibration emerges at 685 cm⁻¹ (Fig. 3a), suggesting potential coordination between NH₃ and Cu²⁺.⁴³ Besides, the C=O stretching vibration peak shifts from 1566 cm⁻¹ to 1574 cm⁻¹ (Fig. 3a) and the weak phenyl C–H peak at 3088 cm⁻¹ disappear (Fig. 3b) after NH₃ adsorption, possibly due to hydrogen bonding between NH₃ and these groups.

Fig. 3 (a) and (b) In situ FTIR spectra of CuCl₂@G808 during NH₃ uptake.

The XPS survey spectrum of CuCl₂@G808 over the range of 0–1300 eV is presented in Fig. S15. Characteristic peaks corresponding to C 1s, O 1s, Zr 3d, Cu 2p, and Cl 2p are clearly observed, confirming the presence of these elements in the composite. Furthermore, XPS spectrum of CuCl₂@G808 after NH₃ adsorption is also recorded (Fig. 4). Following NH₃ adsorption, the binding energies of Cu 2p_3/2 and Cu 2p_1/2 shift from 932.55 eV and 952.70 eV to 932.23 eV and 952.06 eV, respectively (Fig. 4a and b). This negative shift suggests enhanced electron density around Cu²⁺ due to coordination with the N atom of NH₃, consistent with electron donation from the adsorbate to the metal center.⁴³ Meanwhile, the Cl 2p peaks of CuCl₂@G808 at 199.94 eV and 198.36 eV decrease to 199.33 eV and 197.93 eV, respectively (Fig. 4c and d), attributed to hydrogen bonding between NH₃ and Cl⁻ sites.^4,43 These findings further validate that CuCl₂ incorporation enhances the NH₃ adsorption capacity of the composite at low pressure.

Fig. 4 XPS spectra of Cu 2p (a) and (b) and Cl 2p (c) and (d) for CuCl₂@G808 before and after NH₃ adsorption.

Conclusion

In summary, we develop a granular CuCl₂@G808 metal–organic gel via a simple aqueous-phase impregnation method. It is interesting to observe that CuCl₂@G808 demonstrates an exceptional NH₃ uptake (2.23 mmol g⁻¹) at 298 K and 0.002 bar, which exhibits 79% enhancement over pristine G808 and maintains 67.1% adsorption capacity after five adsorption–desorption cycles. On the other hand, CuCl₂@G808 achieves outstanding IAST selectivity for NH₃/N₂ (2.8 × 10³) and NH₃/H₂ (4.9 × 10⁵). In situ FTIR and XPS spectra reveal that low NH₃ pressure can be mainly explained by NH₃–Cu²⁺ coordination and hydrogen-bond networks. These findings advance the rational design of robust adsorbents for capturing and separating trace NH₃.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information (SI) files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request. Supplementary information is available. See DOI: https://doi.org/10.1039/d5ra07740k.

Acknowledgements

The Shiyanjia Lab (https://www.shiyanjia.com/) and Beishide Instrument technology (Beijing) Co., Ltd is gratefully thanked for providing instrumental facilities. C. Z. thanks National Natural Science Foundation of China Program (no. 22075319 and no. 22472200) for funding.

References

1. X. Tian, J. Qiu, Z. Wang, Y. Chen, Z. Li, H. Wang, Y. Zhao and J. Wang, Chem. Commun., 2022, 58, 1151–1154.
2. F. H. Nowrin and M. Malmali, ACS Sustainable Chem. Eng., 2022, 10, 12319–12328.
3. Y. Chen, Y. Wang, C. Yang, S. Wang, J. Yang and J. Li, ACS Sustainable Chem. Eng., 2017, 5, 5082–5089.
4. Y. Shi, Z. Wang, Z. Li, H. Wang, D. Xiong, J. Qiu, X. Tian, G. Feng and J. Wang, Angew. Chem., Int. Ed., 2022, 61, e202212032.
5. C. Zhou, J. Sun, C. Zheng, C.-A. Tao, L. Li, S. Bai, G. Fu, X. Yang, S. Zhang and S. He, Sep. Purif. Technol., 2025, 374, 133720–133726.
6. G. Han, C. Liu, Q. Yang, D. Liu and C. Zhong, Chem. Eng. J., 2020, 401, 126106–126112.
7. D. W. Kim, D. W. Kang, M. Kang, J.-H. Lee, J. H. Choe, Y. S. Chae, D. S. Choi, H. Yun and C. S. Hong, Angew. Chem., Int. Ed., 2020, 59, 22531–22536.
8. D. W. Kim, D. W. Kang, M. Kang, D. S. Choi, H. Yun, S. Y. Kim, S. M. Lee, J.-H. Lee and C. S. Hong, J. Am. Chem. Soc., 2022, 144, 9672–9683.
9. A. J. Rieth and M. Dincă, J. Am. Chem. Soc., 2018, 140, 3461–3466.
10. Z. Wang, Z. Li, X.-G. Zhang, Q. Xia, H. Wang, C. Wang, Y. Wang, H. He, Y. Zhao and J. Wang, ACS Appl. Mater. Interfaces, 2021, 13, 56025–56034.
11. Y. Wang, Y. Shi, D. Xiong, Z. Li, H. Wang, X. Xuan and J. Wang, Chem. Eng. J., 2023, 474, 145307–145316.
12. R. Cao, Z. Chen, Y. Chen, K. B. Idrees, S. L. Hanna, X. Wang, T. A. Goetjen, Q. Sun, T. Islamoglu and O. K. Farha, ACS Appl. Mater. Interfaces, 2020, 12, 47747–47753.
13. B. E. R. Snyder, A. B. Turkiewicz, H. Furukawa, M. V. Paley, E. O. Velasquez, M. N. Dods and J. R. Long, Nature, 2023, 613, 287–291.
14. Y. Chen, B. Shan, C. Yang, J. Yang, J. Li and B. Mu, J. Mater. Chem. A, 2018, 6, 9922–9929.
15. X. Han, W. Lu, Y. Chen, I. da Silva, J. Li, L. Lin, W. Li, A. M. Sheveleva, H. G. W. Godfrey and Z. Lu, et al., J. Am. Chem. Soc., 2021, 143, 3153–3161.
16. Y. Chen, Y. Du, P. Liu, J. Yang, L. Li and J. Li, Environ. Sci. Technol., 2020, 54, 3636–3642.
17. Y. Ma, W. Lu, X. Han, Y. Chen, I. da Silva, D. Lee, A. M. Sheveleva, Z. Wang, J. Li and W. Li, et al., J. Am. Chem. Soc., 2022, 144, 8624–8632.
18. I. Spanopoulos, P. Xydias, C. D. Malliakas and P. N. Trikalitis, Inorg. Chem., 2013, 52, 855–862.
19. A. J. Rieth, Y. Tulchinsky and M. Dinca, J. Am. Chem. Soc., 2016, 138, 9401–9404.
20. Z. Chen, X. Wang, R. Cao, K. B. Idrees, X. Liu, M. C. Wasson and O. K. Farha, ACS Mater. Lett., 2020, 2, 1129–1134.
21. P. Lyu, A. M. Wright, A. Lopez-Olvera, P. G. M. Mileo, J. Antonio Zarate, E. Martinez-Ahumada, V. Martis, D. R. Williams, M. Dinca and I. A. Ibarra, et al., Chem. Mater., 2021, 33, 6186–6192.
22. B. M. Connolly, D. G. Madden, A. E. H. Wheatley and D. Fairen-Jimenez, J. Am. Chem. Soc., 2020, 142, 8541–8549.
23. J. Hou, A. F. Sapnik and T. D. Bennett, Chem. Sci., 2020, 11, 310–323.
24. B. Bueken, N. Van Velthoven, T. Willhammar, T. Stassin, I. Stassen, D. A. Keen, G. V. Baron, J. F. M. Denayer, R. Ameloot and S. Bals, et al., Chem. Sci., 2017, 8, 3939–3948.
25. T. Tian, Z. Zeng, D. Vulpe, M. E. Casco, G. Divitini, P. A. Midgley, J. Silvestre-Albero, J.-C. Tan, P. Z. Moghadam and D. Fairen-Jimenez, Nat. Mater., 2018, 17, 174–180.
26. B. M. Connolly, M. Aragones-Anglada, J. Gandara-Loe, N. A. Danaf, D. C. Lamb, J. P. Mehta, D. Vulpe, S. Wuttke, J. Silvestre-Albero and P. Z. Moghadam, et al., Nat. Commun., 2019, 10, 2345–2355.
27. C. Zhou, H. Li, H. Qin, B. Yuan, M. Zhang, L. Wang, B. Yang, C.-A. Tao and S. Zhang, Chem. Eng. J., 2023, 467, 143394–143402.
28. L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen and C.-Y. Su, Nat. Commun., 2013, 4, 1774–1782.
29. H. Qin, J. Sun, X. Yang, H. Li, X. Li, R. Wang, S. He and C. Zhou, J. Colloid Interface Sci., 2024, 655, 23–31.
30. X. Zheng, H. Zhang, S. Rehman and P. Zhang, J. Hazard. Mater., 2021, 411, 125057–125067.
31. Z. Xianming, Z. Wu, J. Yang, S. Rehman and R. Cao, ACS Appl. Mater. Interfaces, 2021, 13, 17543–17553.
32. W. Cui, X. Kang, X. Zhang and X. Cui, J. Phys. Chem. Solids, 2019, 134, 165–175.
33. L. Luo, H. Huang, Y. Heng, R. Shi, W. Wang, B. Yang and C. Zhong, J. Colloid Interface Sci., 2022, 628, 705–716.
34. C. Zhou, B. Yuan, S. Zhang, G. Yang, L. Lu, H. Li and C.-A. Tao, ACS Appl. Mater. Interfaces, 2022, 14, 23383–23391.
35. C. Zhou, L. Li, H. Qin, Q. Wu, L. Wang, C. Lin, B. Yang, C.-A. Tao and S. Zhang, ACS Appl. Mater. Interfaces, 2023, 15, 54582–54589.
36. X. Wang, R. Su, Y. Zhao, W. Guo, S. Gao, K. Li, G. Liang, Z. Luan, L. Li and H. Xi, et al., ACS Appl. Mater. Interfaces, 2021, 13, 58848–58861.
37. X. Wang, L. Li, K. Li, R. Su, Y. Zhao, S. Gao, W. Guo, Z. Luan, G. Liang and H. Xi, et al., J. Colloid Interface Sci., 2022, 606, 272–285.
38. X. Wang, Z. Liu, G. Li, G. Jiang, Y. Zhao, L. Li, K. Li, G. Liang, S. Gao and H. Xi, et al., Chem. Eng. J., 2022, 440, 135764–135776.
39. C. Zhou, C. Zheng, F. Liu, B. Yang, S. Zhang, H. Chen, S. Bai, C. Lin, C.-A. Tao and P. Ye, Sep. Purif. Technol., 2025, 378, 134516–134521.
40. H. Furukawa, F. Gándara, Y.-B. Zhang, J. Jiang, W. L. Queen, M. R. Hudson and O. M. Yaghi, J. Am. Chem. Soc., 2014, 136, 4369–4381.
41. I. del Castillo-Velilla, A. Sousaraei, I. Romero-Muñiz, C. S. J. Castillo-Blas, A. Méndez, F. E. Oropeza, V. A. de la Peña O'Shea, J. Cabanillas-González, A. Mavrandonakis and A. E. Platero-Prats, Nat. Commun., 2023, 14, 2506–2515.
42. Z. Wang, Z. Li, H. Wang, Y. Zhao, Q. Xia, J. Qiu and J. Wang, ACS Sustainable Chem. Eng., 2022, 10, 10945–10954.
43. C. Zhou, F. Liu, Q. Wu, H. Chen, L. Li, J. Kang, B. Yang and P. Ye, Chem. Eng. J., 2025, 171484.

---

Footnote

† These authors contributed equally to this work.

---