Controllable Adsorption of CO₂ on Smart Adsorbents: An Interplay between Amines and Photoresponsive Molecules

Photoresponsive adsorbents for CO₂ capture attract significant attention, because the variation of adsorption capacity can be tailored conveniently by light irradiation. However, all photoresponsive adsorbents reported until now capture CO₂ via physical interaction (weak sites). Thus, the selectivity of CO₂ over other gases like CH4 is low, and the variation of CO₂ adsorption capacity originates from pure structural change via isomerization of photoresponsive units. Despite great challenges, photoresponsive adsorbents for CO₂ capture based on chemical interaction (strong sites) are extremely desirable. Here, we report for the first time the construction of photoresponsive adsorbents with chemical adsorption sites for CO₂. The control of adsorption capacity is based on the interplay between active amine sites {A, [3-(methylamine)propyl]trimethoxysilane} and photoresponsive azobenzene molecules [P, 4-(3-triethoxysilylpropyl-ureido)azobenzene]. Density functional theory (DFT) calculations reveal that the negative surface potential of amines correlate with the adsorption capacity on CO₂. Upon visible light (450 nm) irradiation, trans-isomers of azobenzene are formed and interact with amines, which leads to decreased surface electrostatic potential of amines, and CO₂ can thus be adsorbed freely on exposed active sites. In contrast, ultraviolet light (365 nm) irradiation results in the isomerization of azobenzene from trans to cis conformation. The surface electrostatic potential of amines increases obviously, and the active sites are thus blocked. A maximum variation amount is obtained on the adsorbent with comparable density of A and P (0.34 group nm^-2), which confirms the interplay between amine sites and photoresponsive molecules. Because amines are specific active sites for CO₂, high selectivity of CO₂ over CH₄ is obtained, and such selectivity is tunable upon irradiation with UV-vis light. We thus demonstrate the successful control of CO₂ captured by chemical sites through the interplay between amines and azobenzene molecules, which is impossible or difficult to realize via conventional structural change caused by isomerization of photoresponsive units.