A stable metal–organic framework with suitable pore sizes and rich uncoordinated nitrogen atoms on the internal surface of micropores for highly efficient CO₂ capture

Abstract

An air-stable tetrazolate-containing framework, [Zn₂L₂]•2DMF (NENU-520, H₂L = 4-(1H-tetrazole-5-yl)biphenyl-4-carboxylic acid), with uncoordinated N atoms on its internal surface was solvothermally synthesized and structurally characterized. This Metal–organic Framework (MOF) exhibited high CO₂ uptake of 79.9 cm³ cm⁻³ at 298 K and 100 kPa, as well as excellent adsorption selectivity for CO₂ over CH₄ and N₂. Particularly, its exceptionally high selectivity of CO₂ over N₂ at 298 K has ranked NENU-520 among the highest MOFs for selective CO₂ separation. Furthermore, the potential application of NENU-520 for the fixed bed Pressure Swing Adsorption (PSA) separation of CO₂ from CH₄ and N₂ has been validated via simulated breakthrough experiments. The small channel with the size of 3.6 Å, combined with CO₂-accessible free nitrogen atoms directed toward the inner surface, is believed to contribute to its high CO₂ uptake capacity and selectivity. Thus, this work represents a unique way to target MOF materials for highly selective CO₂ separation by incorporating CO₂-philic functional sites on pore surfaces, and at the same time optimizing pore sizes.