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%.