Effect of framework rigidity in metal-organic frameworks for adsorptive separation of ethane/ethylene

Ethane-selective adsorbents having high adsorption capacities along with high selectivity toward C2H6 are required to develop a cost-effective adsorption process for the facile separation of ethane (C2H6) from the ethane and ethylene (C2H4) mixture. Understanding the roles of physicochemical properties of the C2H6 -selective adsorbent in terms of its C2H6/C2H4 separation performance is especially essential for the design of novel advanced materials. Herein, we selected the known metal-organic framework structure, namely, the pillared three-dimensional framework DUT-8, which is synthesized by incorporation of 2,6-naphthalenedicarboxylic acid as a linker. We compared physicochemical properties of DUT-8 according to the framework flexibility and the type of metal ion. Even though the framework stability of DUT-8 is very low in the presence of water vapor, we further evaluated the C2H6/C2H4 separation performance of isomorphous DUT-8(M) adsorbents with different metal ions (M = Co, Ni, Cu, and Zn) by means of single-gas isotherm measurements, ideal adsorbed solution theory calculations, grand canonical Monte Carlo simulations, and density functional theory calculations. Separation results showed that preferential uptakes of C2H6 were observed from the C2H6/C2H4 mixture particularly over the rigid framework of DUT-8 (Cu and Ni) through the C-H center dot center dot center dot pi interaction close to the metal node. Thus, high selectivity of C2H6 was achieved over DUT-8(Cu and Ni) along with high uptakes of C2H6 (up to 11.2 mmol/g) at a temperature range of 283-303 K and pressure of 10 bar. The breakthrough separation results on DUT-8 (Cu) showed that the binary mixture of C2H6/C2H4 (1 : 9 v/v) can be effectively separated at pressures of 1 and 5 bar with high productivity of high purity ethylene (20.8 L/kg at 1 bar and 45.0 L/kg at 5 bar). The facile regeneration possibility over DUT-8 is confirmed by the dynamic desorption studies, expecting continuous production of high purity ethylene.