Slow CO Diffusion Governed by Steric Hindrance of Rotatory Ligands in Small Pores of a Metal-Organic Framework.

Understanding the adsorption and diffusional dynamics of CO in metal-organic frameworks (MOFs) is essential in the application of these materials to CO capture and separation. We show that the dynamics of adsorbed CO is related to the rotational motion of ligands located in the narrow pore windows of a MOF using solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR analyses of local dynamics reveal that CO adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO monitored by C NMR is much less than that in the larger pores of MOFs and decreases cooperatively with ligand mobility, which indicates that the rate of diffusion is influenced by the steric hindrance of the rotatory ligands. Adsorbed CH also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.