Selective adsorption of ethane over ethylene on M(bdc)(ted)_0.5 (M = Co, Cu, Ni, Zn) metal-organic frameworks (MOFs)

The application of three-dimensional (3D) pillared-layer metal-organic frameworks (MOFs) M(bdc)(ted)_0.5 (M = Co, Cu, Ni, Zn) in the adsorptive separation of ethane/ethylene (C₂H₆/C₂H₄) mixtures was studied. The effect of the type of metals in M(bdc)(ted)_0.5 on the framework structure regarding the moisture stability and adsorption performance was investigated. M(bdc)(ted)_0.5 MOFs show excellent porous structures with high surface areas of up to 1905 m²/g (M = Ni), as well as being stable under humid conditions (assessed by the dynamic water vapour adsorption tests with maximum 20% relative humidity at 25 °C). M(bdc)(ted)_0.5 exhibits the preferential adsorption of C₂H₆ over C₂H₄ under all conditions (i.e. 25–50 °C and 0–10 bar), although a reversal in adsorption uptake (i.e. C₂H₄ adsorption uptake exceeds the amount adsorbed of C₂H₆) occurs at pressures of >6 bar. The adsorption capacities of C₂H₆ and C₂H₄ on M(bdc)(ted)_0.5 adsorbents are up to 8.63 and 8.79 mmol/g at 25 °C and 10 bar, respectively. The calculated C₂H₆/C₂H₄ selectivity for an equimolar mixture is insensitive to the metal ions in M(bdc)(ted)_0.5, which varies between 1.4 and 1.9. Dynamic breakthrough experiments show that C₂H₆/C₂H₄ mixture can be separated in a fixed bed packed with Ni(bdc)(ted)_0.5. The good adsorption capacity and selectivity, as well as the low isosteric heat of adsorption, make M(bdc)(ted)_0.5 MOFs, especially Ni(bdc)(ted)_0.5, good candidates to be further developed for the effective separation of C₂H₆/C₂H₄ mixtures in the presence of water vapour with a relative humidity lower than 30%.