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.