Thermodynamic Analysis of Distinct Guest Occupation Behavior in Clathrate Hydrates and Its Applications in Sustainable Energy and Environmental Materials

Abstract

A clathrate hydrate is an inclusion compound in which a lattice-like water structure, stabilized by intermolecular hydrogen bonds, enclathrates guest molecules, typically small gases or low molecular weight organic molecules. In this thesis, I examined the thermodynamics and kinetics of various binary clathrate hydrates employing spectroscopic analysis techniques. Notably, I examined the inherent cage occupation behaviors of guest molecules, assessing their potential in specific applications. This thesis is mainly divided into two parts, each consisting of its own chapter. Part 1 and 2 focus on the cage occupation behavior of guest molecules in clathrate hydrates focusing on gas separation and storage applications, respectively. Chapter 1 provides the fundamental knowledge on gas hydrates, such as their physical and structural properties. Chapter 2 and 3 focus on the application of gas hydrate for separating CH4 from gas mixtures containing CH4. Chapter 2 discusses the separation of CH4 from a typical natural gas mixture into solid gas hydrates, utilizing two synthetic methods that exploit differences in cage occupation among guests. Chapter 3 details a hydrate-based multi-stage biogas upgrading where lattice sizes are tailored based on the cage occupancy of guest molecules. From these findings, the thesis suggests suitable strategies for various hydrate-based CH₄ separation applications. Chapter 4 and 5 explore gas hydrate-based natural gas storage, with a focus on enhancing CH₄ uptake with the stabilization of the thermodynamic formation conditions. In Chapter 4, a significant tuning effect is demonstrated using trimethylene oxide (TMO) in binary CH₄ hydrates. Chapter 5 extends this tuning concept to practical natural gas storage applications, identifying key factors for inducing the tuning effect. Based on these findings, a strategy for designing an effective hydrate-based natural gas storage system is suggested. In conclusion, this thesis presents strategies centered on the unique cage occupation of guest molecules, combined with the thermodynamic and kinetic properties of binary clathrate hydrates for gas separation and storage applications. The results suggest the significance of utilizing inherent properties in clathrate hydrates, providing insights for future advancements in clathrate hydrate research.