Enhancing C2H2/C2H4 separation by incorporating low-content sodium in covalent organic frameworks

Abstract

The separation of C₂H₂/C₂H₄ mixtures is of industrial importance in the production of high-purity C₂H₄ and C₂H₂. The primary objective of this work is to enhance the selectivity of C₂H₂/C₂H₄ separation by developing a method for appropriate modification of porous adsorbents. Toward this end, directly doping low-content Na⁺ ions into covalent organic frameworks (COFs) via cation exchange is proposed in this work. Relative to the pristine COF, COF-ECUT-1, with C₂H₂ and C₂H₄ adsorption capacities of 55.39 cm³ g⁻¹ and 28.63 cm³ g⁻¹, the Na⁺-anchored phase with just 4.0 wt% sodium content, Na@COF-ECUT-1, enables largely enhanced adsorption ability with the corresponding values of 89.70 cm³ g⁻¹ and 49.26 cm³ g⁻¹, respectively. As a result, the adsorption selectivity of C₂H₂ over that of C₂H₄ is enhanced from 6.33 to 9.41, and the separation potential estimated by the simulated transient breakthrough is largely improved about three-fold. The density functional theory (DFT) calculation reveals that the trap of Na⁺ within the COF channel affords strong affinity towards C₂H₂ or C₂H₄ through Na–π interactions and relatively stronger contact for C₂H₂ than for C₂H₄ is observed. This result is in line with the experimental results of both enhanced C₂H₂ and C₂H₄ adsorption capacity and C₂H₂/C₂H₄ separation performance. This work outlines a general and simple method to enhance the gas separation performance upon COF materials.