High adsorption and separation performance of CO₂ over N₂ in azo-based (N=N) pillar[6]arene supramolecular organic frameworks*

Azo-based pillar[6]arene supramolecular organic frameworks are reported for CO2 and N2 adsorption and separation by density functional theory and grand canonical Monte–Carlo simulation. Azo-based pillar[6]arene 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 pillar[6]arene_N4 > pillar[6]arene_N2 > pillar[6]arene. Pillar[6]arene_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 pillar[6]arene frameworks are greater than that of N2. The stronger affinity of CO2 with pillar[6]arene_N4 than other structures and the larger isosteric heat differences between CO2 and N2 rendered pillar[6]arene_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 pillar[6]arene for CO2 capture and separation.