Molecular Origins of CO2 Capture Behavior in Amine-Appended Nanoporous Frameworks

Amine-appended porous frameworks are leading materials for direct air capture of CO 2 . Yet the molecular origins governing CO 2 uptake within their dynamic porous environments remain poorly understood, limiting the design of next-generation frameworks. We selected covalent organic framework-999 (COF-999) as exemplar for investigating CO 2 uptake behavior using large-scale molecular dynamics simulations enabled by machine-learned interatomic potentials and enhanced sampling methods. Mapping the complex free-energy surfaces reveals pocket-like amine constructs which foster CO 2 chemisorption because hydrogen bonding within such pockets lowers activation barriers. We found that the population of active amines doubles under humid conditions because water molecules expand the amine pockets and in turn increase the number of hydrogen-bonding sites. This explains the puzzling experimental observation of doubling uptake in the presence of water. Our findings allow us to propose design principles for next-generation carbon capture materials.