Facilitating guest transport in clathrate hydrates by tuning guest-host interactions

The understanding and eventual control of guest molecule transport in gas hydrates is of central importance for the efficient synthesis and processing of these materials for applications in the storage, separation, and sequestration of gases and natural gas production. Previously, some links have been established between dynamics of the host water molecules and guest-host hydrogen bonding interactions, but direct observation of transport in the form of cage-to-cage guest diffusion is still lacking. Recent calculations have suggested that pairs of different guest molecules in neighboring cages can affect guest-host hydrogen bonding and, therefore, defect injection and water lattice motions. We have chosen two sets of hydrate guest pairs, tetrahydrofuran (THF)-CO₂ and isobutane-CO₂, that are predicted to enhance or to diminish guest–host hydrogen bonding interactions as compared to those in pure CO₂ hydrate and we have studied guest dynamics in each using ¹³C nuclear magnetic resonance (NMR) methods. In addition, we have obtained the crystal structure of the THF-CO₂ sII hydrate using the combined single crystal X-ray diffraction and ¹³C NMR powder pattern data and have performed molecular dynamics-simulation of the CO₂ dynamics. The NMR powder line shape studies confirm the enhanced and delayed dynamics for the THF and isobutane containing hydrates, respectively, as compared to those in the CO₂ hydrate. In addition, from line shape studies and 2D exchange spectroscopy NMR, we observe cage-to-cage exchange of CO₂ molecules in the THF-CO₂ hydrate, but not in the other hydrates studied. We conclude that the relatively rapid intercage guest dynamics are the result of synergistic guest A–host water–guest B interactions, thus allowing tuning of the guest transport properties in the hydrates by choice of the appropriate guest molecules. Our experimental value for inter-cage hopping is slower by a factor of 10⁶ than a published calculated value.