Solid-state NMR study of adsorbed water molecules in covalent organic framework materials

This study investigates the locations and interactions of adsorbed water molecules within a porous COF material, TpPa-1. The TpPa-1 samples include the as-synthesized powder samples and the samples first subjected to compression at high pressure and then ground into a powder form. The investigation applies a variety of advanced solid-state NMR techniques. The results indicate that the adsorbed water molecules in the as-synthesized and compressed samples can exist in different locations and bonding states. Here, the adsorbed water molecules in the as-synthesized samples are found to be mainly located in the pore spaces and interact only loosely with the chemical groups of the material. In contrast, a proportion of the water molecules in the compressed samples enter into the interstices between the layers and interact with the amino groups and the carbonyl groups of the material. The relocation of the water molecules in the compressed samples provides a plausible explanation for the molecular origin of the water adsorption capability of TpPa-1. In addition, the revealed weak interactions between the water molecules and the chemical groups of TpPa-1 are correlated with the molecular origin of the high water permeance.