Functionality over Order: Decoupling Chemical and Structural Contributions in CO2/CH4 Separation by Covalent Organic Frameworks

Abstract

Porosity and crystallinity are commonly regarded as key parameters governing gas adsorption and separation in porous materials. However, their relative importance compared to chemical functionality in determining gas selectivity remains insufficiently understood. Here, we investigate the interplay between structural order and chemical functionality in covalent organic frameworks (COFs) for CO2/CH4 separation. A series of four porous frameworks spanning a wide structural landscape, from highly crystalline and porous azine-linked COFs to poorly crystalline and non-crystalline polymeric networks enriched with CO2-philic functional groups, were synthesized. This series enables decoupling the roles of porosity, crystallinity, and chemical functionality for selective CO2 capture. Single-component adsorption isotherms, extended isotherm model-based binary predictions, and dynamic column breakthrough (DCB) experiments reveal that COFs containing polar adsorption sites exhibit significantly enhanced CO2/CH4 selectivity despite possessing lower surface areas and reduced structural order. Molecular dynamics (MD) simulations further identify the specific interaction motifs responsible for preferential CO2 binding within the COFs. These results demonstrate that chemical functionality plays a dominant role over surface area and crystallinity in governing selective CO2 capture and provide key insights for the rational design of next-generation COFs for gas separation.