Separation of ethane/ethylene gas mixture by ethane-selective CAU-3-NDCA adsorbent

Abstract

Ethane-selective adsorbents for separating ethane (C2H6)/ethylene (C2H4) gas mixtures have attracted considerable interest as promising alternatives to conventional cryogenic distillation because of the possibility of directly producing high-purity C2H4 (>99.95%). Herein, CAU-3-NDCA built by the coordination bond between [Al12(OCH3)24]12+ and 2,6-naphthalenedicarboxylic acid (NDCA) was evaluated for use as a C2H6-selective adsorbent for separating a C2H6/C2H4 gas mixture. The separation performance of the adsorbents was investigated using single-component gas isotherms, ideal adsorbed solution theory calculations, dynamic breakthrough experiments for gas mixture, and molecular simulations, such as Monte Carlo simulation and density functional theory calculation. CAU-3-NDCA displayed exceptional adsorption capacity (9.2–10.2 mmol g−1 at 15 bar) and good C2H6/C2H4 selectivity (1.44–1.56) in the temperature range 293–313 K. In the breakthrough experiments, the C2H6/C2H4 separation performance of CAU-3-NDCA exhibited high-purity C2H4 productivity of 6.6 and 25.7 LSTP kg−1 under a 50/50 (v/v) and 10/90 (v/v) C2H6/C2H4 gas mixture, respectively, at 298 K and 7 bar. Recyclability was evaluated by a multi-cyclic breakthrough test for five cycles with a 10/90 (v/v) C2H6/C2H4 gas mixture. Molecular simulations indicated that the preferential adsorption of C2H6 originates from the higher affinity strength of multiple C–H···π interactions of C2H6 with NDCA linkers, in addition to the stronger methyl group–gas interaction of the methanolate group than those of C2H4. Moreover, it is characterised by thermal and moisture stability. Therefore, CAU-3-NDCA is a promising candidate for pressure swing adsorption operation as a C2H6-selective adsorbent.