Smart adsorbents for CO₂ capture: Making strong adsorption sites respond to visible light

Due to the good controllability and high energy efficiency in adsorption processes, photoresponsive adsorbents are intriguing for CO₂ capture. Nevertheless, most reported photoresponsive adsorbents are designed based on weak adsorption sites, regulating CO₂ adsorption through structural change or steric hindrance. In addition, ultraviolet (UV) light is commonly involved in the regulation of adsorption capacity. Here we report for the first time the smart adsorbents for CO₂ capture, which makes strong adsorption sites respond to visible (Vis) light. The adsorbents were fabricated by introducing primary amine and Dispersed Red 1 (DR1, a kind of push-pull azobenzene that responds to Vis light rapidly) units to mesoporous silica, which act as strong adsorption sites and triggers, respectively. The primary amine sites make the adsorbents highly selective in the adsorption of CO₂ over CH₄. Without light irradiation, azobenzene is in the form of trans configuration, which leads to decreased electrostatic potential of primary amines and subsequently, exposure of active sites and liberal adsorption of CO₂. Upon Vis-light irradiation, cis isomers are formed, which results in increased electrostatic potential of primary amines and subsequently shelter of active sites. Even on such strong adsorption sites, the alteration of CO₂ adsorption capacity can reach 40% for the adsorbent with and without Vis-light irradiation. Moreover, the trans/cis isomerization of DR1 units can be triggered reversibly by Vis light. The present smart system endows adsorbents with selective adsorption capacity and avoids the employment of UV light, which is unlikely to be achieved by conventional photoresponsive adsorbents.