UC Irvine

Clathrate hydrates are a class of compounds in which small guest molecules, such as methane, ethane, tetrahydrofuran, etc. are encapsulated within a hydrogen-bonded host water cages. In the oil industry, vast deposits of methane hydrates in deep ocean and permafrost have received great interest as a potential energy source. However, methane hydrate formation can potentially block the transmission pipelines in deep see drilling which may lead to catastrophic economic losses from the flow assurance failures. Therefore, understanding the fundamental mechanisms of gas hydrates formation, decomposition and inhibition is the key for their safe and economical use as a potential energy source and natural gas storage. The effect of low concentrations of methanol on the clathrate hydrate formation kinetics were studied in this thesis. The catalytic effects from methanol were observed on propane hydrate formation but inhibitory effects were found in fluoromethane hydrate formation. We postulate that the catalytic activity of methanol is dependent on the nature of the guest gas itself. Moreover, one of the fascinating properties of the clathrate hydrate structure is that the water molecules of host lattice completely satisfy the hydrogen-bonding requirement, so the interactions between the guests and host cages are mainly due to van der Waals forces rather than hydrogen bondings. Within the cages, the trapped guest molecules have limited translational mobility but may retain rotational and vibrational freedom. From the standpoint of physical chemistry, the dynamics of the guest and host molecules is an excellent material to study and the motion of the guest molecules within the cages is not well understood. In this thesis, the dynamics of tetrahydrofuran (THF) and cyclopentane (CP) guests in the hydrate cages above 200 K were investigated by magic angle spinning (MAS) solid-state NMR. The barrier to guest motion of CP is much lower than THF indicating the existence of hydrogen bonding interactions between THF guest and cage water molecules above 200 K.