(366c) Combining Novel Methodologies Based on NMR Relaxometry and Gas Adsorption for Reliable Surface Assessment of Nanoporous Materials

In the past, major progress has been made concerning the textural characterization of porous materials in the gas/dry state [1, 2]. The main drawback of these methods stems from their inability to test wet materials, thus providing no direct information concerning the state of the nanoporous materials immersed in a liquid phase. This is particularly important for functionalised materials, where the interaction of surface functional groups with the chosen liquid may affect the effective surface area and pore characteristics for separation processes. Hence, for the optimization of processes in the liquid phase (e.g., liquid chromatography) characterization only based on gas adsorption and liquid intrusion techniques is not sufficient. However, far fewer advances have been made in assessing textural properties in the liquid phase.

One of the important challenges is a reliable and fast assessment of surface area. For this, the application of NMR relaxation measurements has recently received a lot of attention, e.g. [2]. This method is based on the different relaxation behaviour of molecules in contact with a surface compared to those in the bulk phase (or “free” liquid). A material with a high surface area in suspension has a shorter relaxation time, as more fluid is bound to the surface. Despite the fact that the application of NMR relaxometry for textural characterization of various material types (e.g., silicas, carbons and even metal-organic framework materials) has been reported in the literature, a rigorous and systematic validation of the method utilizing materials with true surface area benchmark data has not yet been reported. One problem is that many of the materials used in the reported studies contain a significant amount of microporosity and here the Brunauer-Emmett-Teller (BET) method cannot be applied in a straightforward way. Additional uncertainties result from the choice of nitrogen as the probe molecule, leading to errors in assessing surfaces up to 20-25 %. In contrast to nitrogen, the cross-sectional area of argon does not change depending on the surface chemistry. Recent work in which small angle x-ray scattering (SAXS) and physical adsorption was applied for the surface area determination of silica model materials revealed very good agreement between surface areas obtained from SAXS and argon adsorption, confirming that argon 87 K adsorption allows one to obtain reliable specific surface area information which can then be utilized as benchmark data.

Within this context we report here first results on assessing the surface area of non- and mesoporous silica model materials immersed in a liquid phase by methodologies based on NMR relaxometry in comparison with benchmark surface areas obtained from argon 87 K adsorption. We further investigate the effect of confinement and pore network characteristics on NMR relaxation and consequently on a reliable assessment of surface area, but also explore the potential of NMR relaxation for pore size analysis.

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