Pulling water out of thin air: the remarkable adsorption capacity and water transport of continuous films of covalent organic frameworks

Continuous films of high-quality covalent organic framework (COF) films, synthesized from the 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthaldehyde  (PDA), have extraordinary adsorption capacity, gaining as much as 50% wt in water molecules from ambient air and achieving a comparable uptake of other volatile organic compounds in fully saturated ambient air. The high surface area of these films, coupled with the strong binding sites within the COF pores lead to bulk films that are so hygroscopic that they uptake water while stored in a vacuum. Through atomistic simulation, FTIR, and XRD we demonstrate that the absorbed water molecules are largely confined inside of the 3.5nm-diameter pores of TAPB-PDA COF film, behaving like water adsorbate layers despite making up one third the total weight of the saturated film. Molecular dynamics and density functional theory reveal water diffusion pathways and favored physisorption sites within TAPB-PDA respectively. The adsorbed water dramatically change density and optical properties of the COF films without undergoing chemical reactions, and these changes scale with larger populations of confined water. The combination of strong and moderate bonding sites within the COF structures could be exploited to aid diffusion for water filtration and water capture applications. Moreover, the large volume of confined small molecules has implications for unique property control, as well as potentially designing COF films as reactors for altered reaction kinetics.