Diffusion is an omnipresent phenomenon in nature. In the world of molecules, it describes their irregular thermal motion. The interplay of diffusion and interaction of molecules with pore walls of nanoporous materials constitutes the benefit of using such materials in applications of separation and catalysis. The need for understanding the rate-limiting mechanisms, further optimization and development of new processes makes this topic subject of continued fundamental research [1].

This cumulative dissertation is a compilation of eight peer-reviewed, published scientific papers on the subject of two microscopic techniques of diffusion measurement, namely infrared (IR) microimaging and pulsed field gradient (PFG) NMR. The dissertation contains mainly five chapters. The first chapter introduces diffusion phenomena in general and concisely explains the importance and the current challenges of the investigation of molecular diffusion in nanoporous materials, which are the primary motivations behind the present work. To rise the challenges, it proposes an option of employing IR microimaging in parallel with PFG NMR in the measurement of the molecular diffusion. The second chapter describes the basic principles of the two diffusion measurement techniques and what they are capable of. Chapters 3 and 4 deliver convincing demonstrations of their applicability and potential in diffusion studies. Lastly, Chapter 5 concludes the present work by discussing complementary benefits of the two techniques, along with the novel application of the two-region model for assessing mass transfer in hierarchically porous materials.:Table of Contents CHAPTER 1. Introduction CHAPTER 2. Basics of diffusion measurement techniques 2.1. Introduction to infrared microscopy (IRM) 2.1.1. Working principle 2.1.2. Experimental setup 2.2. Introduction to pulsed field gradient nuclear magnetic resonance (PFG NMR) 2.2.1. Self-diffusion and propagator 2.2.2. Theory of PFG NMR CHAPTER 3. Applicability and potential of IRM • Publication 3.1. Anomaly in the chain length dependence of n-alkane diffusion in ZIF 4 metal-organic frameworks • Publication 3.2. Metal-organic framework Co-MOF-74-based host-guest composites for resistive gas sensing • Publication 3.3. Revealing the transient concentration of CO2 in a mixed-matrix membrane by IR microimaging and molecular modeling • Publication 3.4. IR microimaging of direction-dependent uptake in MFI-type crystals CHAPTER 4. Importance of PFG NMR in diffusion studies • Publication 4.1. NMR diffusometry with guest molecules in nanoporous materials • Publication 4.2. Structural characterisation of hierarchically porous silica monolith by NMR cryo-porometry and -diffusometry CHAPTER 5. Complementary benefits of IR microimaging and PFG NMR • Publication 5.1. Diffusion in nanopores: correlating experimental findings with 'first-principles' predictions • Publication 5.2. Diffusion analysis in pore hierarchies by the two-region model Bibliography Appendix A. Supporting information Appendix B. Author contributions