Crystal- and Pore-Size Dependent Iodine Adsorption Properties in MOFs

Metal-organic frameworks (MOFs) with unique pore architectures and electron-rich sites exhibit an unprecedented iodine capture property. In this contribution, the crystal-size and pore-size-dependent iodine adsorption behavior in MOFs ranging in crystal size from nanometers to micrometers with mesoporous and microporous structures is reported. Nano-sized MOFs have much higher iodine adsorption capacities and rates than Micro-sized MOFs. The pore diffusion for iodine adsorption in MOFs is the rate-controlling step and small-sized MOFs can provide larger external surface areas for iodine transport into adsorbents. Raman spectra suggest the diffusion of iodine molecules is more restricted than iodide species in micropores, and the iodine concentration gradient is more significant in microporous MOF-74 than in mesoporous PCN-222. Molecular simulation indicates iodine transports from mesopores to micropores, and aggregates in micropore cages and framework walls. The electron-rich O/N sites in MOFs attract iodine strongly and restrict iodine diffusion, while the weak interaction between iodine molecules becomes prominent at a high iodine loading amount and thus increases the iodine diffusivity in MOFs. The above result reveals a strong size dependence in iodine adsorption behaviors sensitive to MOFs' intrinsic crystal size, porosity, and guest-host interactions, and MOFs with smaller crystal sizes and mesoporous construction have better iodine capture performance.