Unveiling CO2 Dynamics in Perfluorinated Cerium-Based Metal-Organic Frameworks with UiO-66 and MIL-140 Topologies by Solid State NMR

The dynamics of CO2 was investigated in two ultramicroporous perfluorinated metal-organic frameworks (MOFs), F4_MIL-140A(Ce) and F4_UiO-66(Ce), that share the same linker (tetrafluoroterephthalate) and metal (CeIV) but have different topologies. F4_MIL-140A(Ce) displays an S-shaped CO2 adsorption isotherm associated with an adsorption mechanism including CO2 interaction with the CeIV open metal site and a structural rearrangement of the linkers. F4_UiO-66(Ce), belonging to one of the most investigated MOF families in the literature, shows a Langmuir-like CO2 adsorption isotherm hinting at no specific strong interactions with the framework. The structural factors influencing CO2 adsorption properties in these systems were found to affect CO2 dynamics, as revealed by line shape analysis of 13C static solid state NMR spectra of 13C isotopically enriched CO2 (13CO2) adsorbed in the MOFs (1 atm) as well as by the analysis of 13C longitudinal relaxation times (T1), at different temperatures. The evolution of the spectral line shapes of 13CO2 in F4_MIL-140A(Ce) clearly indicated anisotropic dynamics of the gas molecules in the framework, which can be described as a localized wobbling motion on the adsorption site combined with translational hopping from one site to another in the MOF channels. This scenario was corroborated by 13C T1 analysis in terms of a relaxation mechanism governed by CO2 reorientations that modulate chemical shift anisotropy. Rates and activation energies were determined for the two motions. On the other hand, spectral line shape and 13C T1 analyses of 13CO2 in F4_UiO-66(Ce) indicated a fast isotropic reorientational motion, reflecting weak gas/framework interactions and high symmetry of the MOF cavities, and allowed the activation energy for the motion to be determined.