Understanding Water Adsorption and the Impact on CO₂ Capture in Chemically Stable Covalent Organic Frameworks

In this work, a diverse set of chemically stable covalent organic frameworks (COFs) has been studied to understand their water adsorption behavior and how water adsorption will affect CO₂ capture performance in the COFs using grand canonical Monte Carlo (GCMC) simulations. Our results revealed three different types of water adsorption behavior in the COFs: (1) initial small water cluster formation was followed by early monolayer coverage in COFs with a smooth hydrophilic surface, such as TpPa-1, TpPa-NO₂, and DAAQ-TFP; (2) hydrophobic functional groups presented in the hydrophilic surface of the COFs, such as COF-42, COF-43, and TpBD, could disrupt the water monolayer formation, which would delay the pore filling of water in the COFs; and (3) instantaneous pore filling of water molecules could happen without any nucleation stage for COFs with relatively small pores, such as NPN-COFs. In general, hydrophilic surface and small pore sizes are two good features for COFs to be used in water adsorption applications at low relative humidity (RH). Geometric steric effect may disrupt the pore filling of water in COFs. On the other hand, NPN-COFs provide good CO₂ capture performance and water tolerance to a low RH (p/p₀ = 0.1) due to their small pores with hydrophobic surface. It is also exciting to find out that CO₂ capture in COF-300 can tolerate a high RH of 0.8 due to its superhydrophobic surface and the unique interplanar space for CO₂ uptake.