Slow CO2 Diffusion Governed by Steric Hindrance of Rotatory Ligands in Small Pores of a Metal-Organic Framework

Understanding the adsorption and diffusional dynamics of CO2 in metal- organic frameworks (MOFs) is essential in the application of these materials to CO2 capture and separation. We show that the dynamics of adsorbed CO2 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 CO2 adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO2 monitored by C-13 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 CH4 also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.

Automated summary

In this study, the dynamics of CO2 adsorption and diffusion in metal-organic frameworks (MOFs) were investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy. The rotational motion of ligands in the narrow pore windows of MOFs was found to affect the dynamics of adsorbed CO2. The diffusion rate of adsorbed CO2 was significantly slower in the MOFs with smaller pores, and it was influenced by the steric hindrance of the rotating ligands. Additionally, the diffusion of adsorbed CH4 was also affected by the steric hindrance, showing a size-selective effect.