A GLIMPSE ON THE MULTIFACETED WORLD OF POROUS MATERIALS BY HIGH- AND LOW-RESOLUTION SOLID STATE NMR TECHNIQUES

Porous materials have attracted considerable scientific and technological interest due to their critical applications in many fields, such as membrane-based gas separation, building materials, adsorption and storage, catalysis, ion exchange, nanotechnology, etc. From a chemical and structural point of view, the definition of "porous materials" encompasses a wide variety of systems, including inorganic, hybrid organic-inorganic, and polymeric materials. Solid state NMR spectroscopy (ssNMR) has been showing its tremendous potential to clarify the often-intricate behavior of this class of materials at a molecular and nanometric level. Indeed, it provides a wide variety of tools, relying on the observation of different nuclei and the measurement of different spectral and relaxation properties, which can reveal information on structural and dynamic features on wide spatial and time scales, respectively [1,2]. This lecture will cover a selection of ssNMR studies aimed at unravelling these features, and especially the dynamic aspects, on different classes of porous materials. In particular, the case studies presented will concern microporous polymers for solid-state gas separation and ion-exchange membranes, 1D coordination polymers devised to sequester volatile organic compounds, Ce-based metal organic frameworks with potential applications in the field of CO2 capture, and Mg- and Ca-based cement pastes. The main focus will be put on the detailed description of motional processes of polymeric chains and organic ligands and on the interactions and dynamic behavior of adsorbed water and other guest molecules. It will be shown how, depending on the type of material, on the available nuclei, on the desired detail of the information, and on the time scale of the motion, different experimental approaches can be used, combined with different analyses of the nuclear parameters measured in terms of suitable theoretical models. The experiments employed include static and Magic Angle Spinning as well as high- and low-field techniques, and in particular they rely on: 1H and 19F on-resonance FID analysis; 1H, 19F and 13C spin-lattice relaxation times in the laboratory frame and of 1H spin-lattice relaxation times in the rotating frame; 13C chemical shift anisotropy; 2H quadrupolar interaction. References [1] K. Mu?ller, and M. Geppi Solid State NMR: Principles, Methods, and Applications, Wiley ed. (2021). [2] S. Li et al. Adv. Mater. 32, 2002879 (2020).