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

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 cm(3) g(-1) and 28.63 cm(3) 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 cm(3) g(-1) and 49.26 cm(3) 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-pi 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.