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

Abstract

Understanding the adsorption and diffusional dynamics of CO 2 in metal− organic frameworks (MOFs) is essential in the application of these materials to CO 2 capture and separation. We show that the dynamics of adsorbed CO 2 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 2 adsorbed in the pore hinders the rotation of the ligands. The rate of diffusion of adsorbed CO 2 monitored by 13 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 4 also showed slow diffusion in the MOF, suggesting molecular size-selective effect of the mobile steric hindrance on the rate of adsorbate diffusion.

Cooperative CO2 adsorption mechanism in a perfluorinated CeIV-based metal organic framework

“…58 The scarce CP efficiency found in our case should therefore be ascribed to a reduction of the dipolar interactions and/or of the residence time of CO 2 in the adsorption site due to reorientational and translational motions. [59][60][61] Energetics of the cooperative CO 2 adsorption mechanism. The foregoing discussion on the adsorption properties and structural response of F4_MIL-140A(Ce) to adsorption and desorption of H 2 O and CO 2 suggests that a concerted mechanism takes place when CO 2 is involved, due to the ease of rotation of the peruorobenzene rings.…”

Effects of MOF linker rotation and functionalization on methane uptake and diffusion

“…This observation is also consistent with several other studies that suggest the incorporation of framework flexibility in simulations is only necessary for systems where the guest molecule is similar or larger in size compared to the pore size. 24,43,44…”

“…33 The dynamics of linker rotation have been probed experimentally, both in empty MOFs and in the presence of guest molecules. [21][22][23][24][25] Framework motion can influence guest molecule behavior, and the adsorbed guest molecules can also affect framework motion and pore size, 23,26,27 including through 'gateopening' mechanisms where linkers experience steric effects that lead to larger pores and framework changes. MOF linker functionalization can alternately enhance or suppress linker dynamics, depending on the functional group.…”

Cooperative CO2 adsorption mechanism in a perfluorinated CeIV-based metal organic framework

“…58 The scarce CP efficiency found in our case should therefore be ascribed to a reduction of the dipolar interactions and/or of the residence time of CO2 in the adsorption site due to reorientational and translational motions. [59][60][61]…”