Enhancing Ethane/Ethylene Separation Performance in Two Dynamic MOFs by Regulating Temperature-Controlled Structural Interpenetration

Abstract

The separation of ethane from ethylene is an important but challenging process in the chemical industry because of their similar physicochemical properties. Generally, the adsorbents for C 2 H 6 /C 2 H 4 separation require an appropriate and relatively small aperture. Herein, we report two dynamic pillarlayered metal−organic frameworks (MOFs) BUT-111 and BUT-112 with isomorphic frameworks but different degrees of interpenetration for efficient C 2 H 4 purification. The dynamic behavior makes both the activated MOFs exhibit ultramicropores and reversed order adsorption behavior for C 2 H 6 and C 2 H 4 , which could obtain highly purified C 2 H 4 in one step from the C 2 H 6 /C 2 H 4 mixture. BUT-111 and BUT-112 could work in a wide temperature range, and with the decrease in temperature, the C 2 H 6 /C 2 H 4 selectivity would increase. Moreover, the degree of interpenetration could be well controlled by the synthetic temperature, and the increase in the interpenetration degree of BUT-112 enhanced the C 2 H 4 purification effectively.

Ultramicroporous Metal–Organic Framework with Inert Pore Surfaces for Inversed Separation of Ethylene from C2 Hydrocarbons Mixtures

“…The appearance of physical adsorbents is hopeful of greatly reducing the energy consumption of the purification process, which has the potential to replace traditional cryogenic distillation. − Metal–organic frameworks (MOFs) as novel porous materials can be precisely regulated and designed to obtain desired pore sizes and environments at the molecular level through crystal engineering approaches. − In general, most MOF materials exhibit the outstanding adsorption of unsaturated C 2 hydrocarbons with the relationship of adsorption capacities and affinity order of C 2 H 2 > C 2 H 4 > C 2 H 6 because introducing open metal sites/clusters or polar groups on the pore surfaces of MOFs can give priority to binding molecules with high dipole and/or quadrupole moments (quadrupole moments of 7.2 × 10 –26 esu cm 2 for C 2 H 2 , 1.5 × 10 –26 esu cm 2 for C 2 H 4 , and 0.65 × 10 –26 esu cm 2 for C 2 H 6 ). − In addition, the kinetic diameter of C 2 H 4 (4.163 Å) is between C 2 H 6 (4.443 Å) and C 2 H 2 (3.3 Å), so molecular sieving by adjusting the pore aperture cannot be implemented . Therefore, the achievement of direct C 2 H 4 separation in one step is a more challenging and desirable target − because high-purity C 2 H 4 can be directly produced without an extra desorption step, thus avoiding a great deal of energy consumption. Compared with C 2 H 4 and C 2 H 2 , C 2 H 6 has the largest polarizability (39.3 × 10 –25 cm 3 for C 2 H 2 ; 42.52 × 10 –25 cm 3 for C 2 H 4 ; 44.7 × 10 –25 cm 3 for C 2 H 6 ), and adsorbents for C 2 H 6 -selectivity need to contain rich nonpolar/inert surfaces, such as aromatic moieties, which facilitates the capture of gas molecules with high polarizability. − …”