Adsorptive separation of C₂H₆/C₂H₄ on metal-organic frameworks (MOFs) with pillared-layer structures

Adsorptive separation is a promising alternative to the energy-intensive cryogenic distillation process for separating the ethane/ethylene (C₂H₆/C₂H₄) mixtures. Herein, two pillared-layer metal-organic frameworks (MOFs), Ni(HBTC)(bipy) and Ni₂(HBTC)₂(bipy)_0.6(dabco)_1.4, are prepared as the C₂H₆-selective adsorbents for C₂H₄ purification from C₂H₆/C₂H₄ mixtures. The effect of the type of pillars on the framework structure, thermal and moisture stability, as well as the C₂H₆ and C₂H₄ adsorption propertiy, of the MOFs was studied. The use of the longer pillars (i.e. bipy versus dabco) to scafolld the Ni(HBTC) improved the pore size (5.5 Å versus 5.3 Å), specific surface area (1474 m²/g versus 1070 m²/g) and moisture stability in the relative humidity range of 0–90%, but compromised the thermal stability (267 °C versus 278 °C). Both MOFs were C₂H₆-selective, which was evidencd by the single component adsorption experiments using C₂H₆ and C₂H₄. The ideal adsorbed solution theory (IAST) selectivity for C₂H₆/C₂H₄ mixtures (1:1 and 1:15, v/v) is in the range of 1.4–1.7 at 25–50 °C and 0–1 bar. The preferential adsorption towards C₂H₆ over C₂H₄ on both MOFs is then explained by the isosteric heat of adsorption. Additionally, Ni(HBTC)(bipy) also shows the best capacity of up to 6.6 mmol/g for C₂H₆ adsorption in comparison with other C₂H₆-selective MOFs at 25 °C and 1 bar. Both MOFs showed the excellent recyclability, with the negligible reduction in the gas uptake observed during four cycles of adsorption/desorption tests. Besides, breakthrough experiments demonstrated that both MOFs can achieve efficient separation of an equimolar C₂H₆/C₂H₄ mixture. The findings suggest that Ni(HBTC)(bipy) and Ni₂(HBTC)₂(bipy)_0.6(dabco)_1.4 can be further considered as C₂H₆-trapping adsorbents for the C₂H₆/C₂H₄ separation applications in practice.