Diffusion and adsorption in covalent organic frameworks (COFs) probed by nuclear magnetic resonance methods

Molecular diffusion and surface dynamics within two covalent organic frameworks (COFs) have been investigated using nuclear magnetic resonance (NMR) pulsed-field gradient (PFG) and relaxation. The effect of chemical functionalities of the COFs on the effective self-diffusivity of the probe molecules within the pore space and the adsorbate/adsorbent interactions were investigated. In particular, diffusion and interaction of water, methanol, -octane, and 1,3,5-triisopropylbenzene (1,3,5-TIPB) within COF–SIOC and COF-DHTA were assessed. The two types of COFs used in this study possessed a dual pore Kagome structure consisting of larger hexagonal and smaller triangular pores. The PFG NMR results show the presence of two distinct diffusion coefficients for small probe molecules, such as water, methanol, and -octane. This behaviour is attributed to their relatively smaller kinetic diameters, allowing them to access both smaller and larger pores in the COFs. In contrast, the PFG diffusion plot for 1,3,5-TIPB showed a single component linear behaviour, which is attributed to diffusion through the larger hexagonal pores only, as a result of a much larger kinetic diameter of 1,3,5-TIPB compared to the other probe molecules, which prevents access to the smaller triangular pores. The presence of functional groups affects surface interactions between the probe molecules and the surface of the COFs. The NMR / relaxation measurements reveal a higher strength of surface interaction for water molecules in COF-DHTA compared to COF–SIOC, which is attributed to the presence of hydrophilic –OH groups in COF-DHTA. Conversely, a higher strength of surface interactions was achieved for -octane in COF–SIOC, due to the hydrophobic nature of this material. This work reports new insights into transport and dynamics of molecules confined in COFs, which can help design and optimisation of such pore structures in applications such as separation and catalysis.