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

Abstract

Ethane-selective adsorbents having high adsorption capacities along with high selectivity toward C 2 H 6 are required to develop a cost-effective adsorption process for the facile separation of ethane (C 2 H 6 ) from the ethane and ethylene (C 2 H 4 ) mixture. Understanding the roles of physicochemical properties of the C 2 H 6 -selective adsorbent in terms of its C 2 H 6 /C 2 H 4 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 C 2 H 6 /C 2 H 4 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 C 2 H 6 were observed from the C 2 H 6 /C 2 H 4 mixture particularly over the rigid framework of DUT-8 (Cu and Ni) through the C–H‧‧‧π interaction close to the metal node. Thus, high selectivity of C 2 H 6 was achieved over DUT-8(Cu and Ni) along with high uptakes of C 2 H 6 (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 C 2 H 6 /C 2 H 4 (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. • Rigid and flexible DUT-8 MOFs were investigated for separation of ethane/ethylene. • Flexible DUT-8 MOFs with Ni, Zn, and Co exhibited low uptakes for both two gases. • Rigid DUT-8 MOFs with Cu and Ni showed exceptional uptakes particularly for ethane. • Rigid DUT-8 frameworks have good selectivity by CH···π interaction with ethane. • High purity ethane over 99.95% could be efficiently separated over DUT-8(Cu).