Wei-Hong Zhang, Ya-Nan Ma, Guo-Tong Du, Ping Wang, and Dong-Xu Xue*. Chem. Sci., 2025, DOI: https://doi.org/10.1039/D5SC06836C

Ethylene is a key feedstock in the chemical industry and must be purified to polymerization grade (>99.9%) from either ethane/ethylene binary or acetylene/ethane/ethylene ternary mixtures. Conventional separation processes such as low-temperature distillation are extremely energy-intensive; adsorptive separation with porous solids offers a far more efficient alternative. Nevertheless, the rational design and synthesis of novel MOFs that combine exceptionally high selectivity with high gravimetric uptake is still strongly desired. Face-transitive-topology MOFs—whose nets contain only one kind of window—show exceptional promise for discriminating among the structurally similar C₂ hydrocarbons. However, existing MOFs still struggle to achieve simultaneous, high-efficiency separation of both binary and ternary C₂ mixtures.

Leveraging reticular chemistry and a mixed-ligand strategy, we report the synthesis of two nia-d-type trinuclear manganese-based MOFs using the same metal source and a tridentate triazine ligand, as well as a hetero-functional linear linker bearing carboxylic acid and tetrazole motifs. One linker is fluorine-free, while the side arm of the carboxylate in the other linker is methods inefficient. nia-d-TZB preferentially adsorbs ethane and can purify ethylene in one step from ethane/ethylene binary mixtures, nia-d-FTZB preferentially adsorbs both acetylene and ethane, achieving one-step polymerization grade ethylene from acetylene/ethane/ethylene ternary mixtures.

Fig. 1 Schematic representation of the two MOFs constructed from a trinuclear manganese cluster and respective mixed linkers associated with corresponding cage sizes.

Two distinct cage motifs are present in both compounds, i.e., a triangular bipyramidal cage (cage-I) and an anti-prismatic cage (cage-II). The dimensions of cage-I are 9.96 × 16.15 Ų in both structures. The presence of fluorine in the linear linker slightly perturbs cage-II, changing its dimensions from 13.17 × 6.30 Ų in nia-d-TZB to 12.62 × 6.23 Ų in nia-d-FTZB . At the same time, the triangular windows, defined by portions of the TPT linker and two linear linkers, serve as the sole entrances/exits for the framework. The aperture size modestly decreases from 5.16 Å in nia-d-TZB to 5.0 Å in nia-d-FTZB . Nevertheless, these window dimensions remain sufficient to permit the passage of C₂ gas molecules.

After sequential solvent exchange with acetonitrile and acetone for three days, the samples were activated under dynamic vacuum at 120 °C. Nitrogen sorption isotherms recorded at 77 K exhibit reversible Type-I profiles, At 1 bar, nia-d-TZB (546 cm³ g^-1) exhibits a higher nitrogen uptake than nia-d-FTZB (503 cm³ g^-1) .Correspondingly, The BET surface areas and total pore volumes are 2,029 m² g^-1 and 0.85 cm³ g^-1 for nia-d-TZB, and 1,987 m² g^-1 and 0.78 cm³ g^-1 for nia-d-FTZB, respectively.

Fig. 2 (a) C₂H₂, C₂H₆, and C₂H₄ sorption isotherms of nia-d-TZB at 298 K; (b) C₂ sorption isotherms of nia-d-FTZB at 298 K; (c) Q_st of C₂H₂, C₂H₆ and C₂H₄ for nia-d-TZB and nia-d-FTZB; (d) IAST selectivity of nia-d-TZB and nia-d-FTZB for C₂H₂/C₂H₄ (50/50, v/v) and C₂H₆/C₂H₄ (50/50, 10/90, v/v) at 298 K and 1 bar.

Single-component C₂-hydrocarbon isotherms reveal that both frameworks consistently take up more C₂H₂ and C₂H₆ than C₂H₄ For nia-d-TZB, the guest-host interaction hierarchy is C₂H₆ > C₂H₂ > C₂H₄, whereas nia-d-FTZB displays the highest uptake for C₂H₂, followed by C₂H₆ and then C₂H₄. Zero-coverage isosteric heats of adsorption (Q_st) mirror these trends. Ideal adsorbed solution theory (IAST) calculations at 298 K and 1 bar give selectivities of 1.11 (nia-d-TZB) and 1.45 (nia-d-FTZB) for an equimolar C₂H₂/C₂H₄ mixture (50/50, v/v). Under identical conditions, the C₂H₆/C₂H₄ selectivity is 1.58 for nia-d-TZB and 1.41 for nia-d-FTZB (50/50 and 10/90, v/v). Dynamic breakthrough and GCMC studies of binary/ternary C₂ feeds confirm the two MOFs’ exceptional one-step ethylene purification and reverse C₂H₆/ C₂H₄ separation.

In summary, the results establish a critical paradigm for the deliberate design of face-transitive MOFs that achieve advanced gas-separation performance, underscoring the potential of crystalline porous materials to contribute to energy savings and emission reductions.

Fig. 3 Experimental breakthrough curves for nia-d-TZB at total gas flow of 1.0 mL min^-1 of (a) C₂H₂/C₂H₄ (50/50, v/v) and C₂H₆/C₂H₄/C₂H₂ (1/1/1, v/v/v), (b) C₂H₆/C₂H₄ (50/50, v/v) and (c) C₂H₆/C₂H₄ (10/90, v/v) at 298 K and 1 bar; Experimental breakthrough curves for nia-d-FTZB at total gas flow of 1.0 mL min^-1 of (d) C₂H₂/C₂H₄ (50/50, v/v), (e) C₂H₆/C₂H₄ (50/50, v/v) and (f) C₂H₆/C₂H₄/C₂H₂ (1/1/1, v/v/v) at 298 K and 1 bar.

First Author: Zhang Weihong, master’s student, Shaanxi Normal University

Correspondence Author: Prof. Xue Dongxu, Shaanxi Normal University

Full Text Link: https://pubs.rsc.org/en/Content/ArticleLanding/2025/SC/D5SC06836C