NMR spectroscopy in adsorption studies of metal-organic frameworks

The present thesis deals with the investigation of liquid-phase and gas-phase adsorption of metal-organic frameworks (MOFs) using NMR spectroscopy. NMR-based methods are developed and then applied to characterize the surface polarity of studied materials and the influence of structural flexibility on the adsorption selectivity. In the first part of the thesis a new methodology for surface polarity screening of MOFs is proposed based on quantitative 1H NMR spectroscopic measurements of liquid-phase adsorption. The influence of the surface polarity on the adsorption process was studied on several materials, e.g. activated carbons, PAF-1, MIL-101(Cr), HKUST-1, and UiO-67-series. The surface polarity was characterized through the difference in the 1,4-dioxane adsorption uptake from two solvents of opposite polarity, namely n-heptane and N,N-dimethylformamide. An NMR-derived surface polarity index was defined where the polarity of the MOF corresponds to its affinity to polar substances. It was demonstrated that the structural modifications of MOF materials, which should affect the polarity of these MOFs, are indeed reflected by the proposed polarity index. The second part of the thesis focuses on gas mixture adsorption experiments. Metal-organic frameworks are promising candidates for selective separation processes such as CO2 from methane. Framework flexibility is observed only for some special MOFs. The main question is: Does framework flexibility influence the adsorption selectivity? High-pressure in situ 13C NMR spectroscopy was used to monitor the adsorption of 13CO2/13CH4 gas mixtures. This method allows to distinguish between the two gases as well as between adsorbed molecules and the interparticle gas phase. Gas mixture adsorption was performed under isothermal conditions. The selectivity factor for CO2 adsorption from CO2/CH4 mixtures was measured as a function of total gas pressure. The flexible material SNU-9 as well as the flexible and the non-flexible forms of DUT-8(Ni) were compared. Maximum selectivity factors for CO2 were observed for the flexible form of DUT-8(Ni) in its open-pore state. In contrast, the rigid form of DUT-8(Ni) and SNU-9 especially in its intermediate state exhibit lower adsorption selectivity factors. This observation indicates significant influence of the framework flexibility on the adsorption selectivity