Thermodynamic analysis and microscopic identification of host-guest interactions occurring in clathrate hydrates

Abstract

Clathrate hydrates, commonly called gas hydrates, are non-stoichiometric crystalline inclusion compounds that are formed by physical interaction between host-water molecules and relatively small gaseous/liquid guest molecules. When water molecules are in a relatively high pressure and low temperature with guest molecules, the water molecules are linked through hydrogen bonding and they form a water framework, namely, “cage” that can stably accommodate the guest molecules. Clathrate hydrates have been applied to many industrial fields of energy and environments, such as energy storage, selective separation of targeted component, carbon dioxide sequestration into deep ocean floor and exploitation of naturally occurring gas hydrates. In this study, thermodynamic and spectroscopic analysis of host-guest interactions on clathrate hydrates were intensively investigated, especially focused on application to recovery and storage systems of gaseous energy such as methane and hydrogen. The scope of this study can be divided into three fields. The first one is the recovery of methane gas from natural gas hydrates by swapping phenomena, the second one is structural and hemodynamic identification of sII double hydrate and its tuning behavior, and the last one is the effect of host-guest interaction on thermal expansivity for $H_2$ double hydrates. For thermodynamic analysis, hydrate phase behaviors of single- or multi-guest systems were carefully measured. For spectroscopic analysis, the structure, distribution of guest molecules and cage occupancies of mixed hydrate were determine through X-ray diffractometer, high-resolution neutron powder diffractometer, Raman spectroscopy and solid-state nuclear magnetic resonance. First, we focused on the direct sequestration of gas mixtures containing $CO_2$ and $N_2$ into the sI $CH_4$ hydrate deposits. A spectroscopic analysis reveals that the external $N_2$ molecules specifically attack $CH_4$ molecules already entrapped in sI-S ...