Channel-wall functionalization in covalent organic frameworks for the enhancement of CO2 uptake and CO2/N2 selectivity

Abstract

A series of tailored covalent organic frameworks (COFs), i.e. [NN]_X%–TAPH-COFs and [CC]_X%–TAPH-COFs, were synthesized by post-fabrication of [HO]_X%–TAPH-COFs with 4-phenylazobenzoyl chloride (PhAzo) and 4-stilbenecarbonyl chloride (PhSti), respectively. Powder X-ray diffraction (PXRD), FT-IR, and solution-state ¹H NMR of the digested COFs were applied to clarify the functional groups integrated in the pore channels. Gas sorption isotherms confirmed that the [NN]_X%–TAPH-COFs and [CC]_X%–TAPH-COFs had moderate surface areas, narrow pore sizes, and good physicochemical stability. Compared with [CC]_X%–TAPH-COFs, the [NN]_X%–TAPH-COFs exhibited higher CO₂ uptake capacities of up to 207 mg g⁻¹ (273 K and 1 bar), isosteric heats of adsorption for CO₂ (30.7–43.4 kJ mol⁻¹), and CO₂/N₂ selectivities of up to 78 (273 K) because of the dipole interactions between the azo group and CO₂ as well as the N₂-phobic behavior of the azo group. Furthermore, although the decreased pore size was advantageous for increasing CO₂ adsorption, the decreased surface area of the COFs would undoubtedly decrease CO₂ adsorption if too many functional groups were introduced.