High adsorption and separation performance ofCO₂ over N₂ in azo-based (N=N) pillar6arene supramolecular organic frameworks

Azo-based pillar6arene supramolecular organic frameworks are reported for CO₂ and N₂ adsorption and separation by density functional theory and grand canonical Monte-Carlo simulation. Azo-based pillar6arene provides suitable environment for CO₂ adsorption and selectivity. The adsorption and selectivity results show that introducing azo groups can effectively improve CO₂ adsorption and selectivity over N₂, and both CO₂ adsorption and CO₂ selectivity over N₂ follow the sequence pillar6arene_N4 > pillar6arene_N2 > pillar6arene. Pillar6arene_N4 exhibits CO₂ adsorption capacity of ～ 1.36 mmol/g, and superior selectivity of CO₂ over N₂ of ～ 116.75 with equal molar fraction at 1 bar (1 bar=10⁵ Pa) and 298 K. Interaction analysis confirms that both the Coulomb and van der Waals interactions between CO₂ with pillar6arene frameworks are greater than that of N₂. The stronger affinity of CO₂ with pillar6arene_N4 than other structures and the larger isosteric heat differences between CO₂ and N₂ rendered pillar6arene_N4 to present the high CO₂ adsorption capacity and high CO₂ selectivity over N₂. Our results highlight the potential of azo-functionalization as an excellent means to improve pillar6arene for CO₂ capture and separation.