title: Novel Methodologies based on a Combination of Nuclear Magnetic Resonance Relaxometry and Gas Adsorption for Advanced Characterization of Nanoporous Materials and Powders in the Wet and Dry State
Creators: name: Schlumberger, Carola
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: NMR Relaxometry
subject: Textural Characterization
subject: Gas Adsorption
subject: Nanoporous Materials
subject: Surface Area Assessment

description: Tuning of textural properties and the surface chemistry of porous materials is essential to enhance their selectivity to certain compounds and the resulting process efficiency, e.g. in chromatographic separation processes or catalysis. Hence, the valid assessment of textural properties such as surface area and pore size is crucial for these applications. Within this context, major progress has been made concerning the characterization of porous materials in the gas/dry state by gas physisorption and mercury porosimetry. However, these methods are not sufficient for the characterization of wet porous materials used in liquid phase processes. Considering this is particularly important for functionalized nanoporous materials, where the interaction of surface functional groups with the chosen liquid may affect the effective surface area and pore characteristics. For this, the application of nuclear magnetic resonance (NMR) relaxometry has been considered in the past, but a systematic and rigorous assessment of its validity is missing so far. Within this context, this thesis focuses on the development of novel methodologies by combining NMR relaxometry with gas adsorption for the reliable and advanced characterization of nanoporous materials in the wet and dry state. First, the thesis particularly focuses on NMR relaxometry for surface area assessment. A systematic study is presented, which assesses the applicability of NMR relaxometry for valid surface area assessment, utilizing for the first time true surface area benchmark data based on argon 87 K adsorption of silica and carbon model materials. This is coupled with an advanced method development including the investigation of the choice of probe molecule and the effect of its accessibility to the pore system. This thesis demonstrates that the method can provide valid and fast surface areas for these model materials. Furthermore, it shows the potential of NMR relaxometry for the targeted surface area assessment of defined pore classes and proposes a novel methodology for the determination of pore window sizes. The effect of confinement on the applicability of NMR relaxometry for surface area assessment is systematically investigated, which also indicates its potential for pore size analysis. Related to that, this work shows a first attempt to apply NMR relaxometry for pore size analysis by comparing NMR relaxometry for the first time with benchmark pore size data determined with Ar 87 K adsorption coupled with advanced statistical mechanics approaches. These experimental studies are coupled with the implementation of a molecular simulation framework for complementary simulations of the NMR relaxation times of confined pore fluids. This thesis further investigates the potential of NMR relaxometry to assess wettability effects. NMR relaxation times are affected by an interplay of textural properties and wettability effects. To distinguish between these effects, a novel methodology is suggested to quantify the wetting behavior of fluids on (pore) surfaces and its applicability is demonstrated coupled with water vapor adsorption and intrusion experiments to assess the hydrophilicity/hydrophobicity of functionalized mesoporous silica. In this sense, the combination of NMR relaxometry and gas adsorption builds the basis for a unified approach for the comprehensive characterization of nanoporous materials.