Dynamics of gas hydrates

Clathrate (gas) hydrate is a class of materials consists of small nonpolar molecules (< 0.9 nm) hosted in three-dimensional array of cages formed by water molecules connected together through hydrogen bonds. Methane and carbon dioxide clathrate hydrates have brought a great deal of attention because a large fraction of earth’s natural gas is stored in clathrate hydrates, much of which was found in marine sediments, and methane and carbon dioxide are major greenhouse gases.

Many of the equilibrium properties of clathrate hydrates were quite well understood. However, very little about hydrate kinetics has been known. We study the guest and water molecular kinetics of clathrate hydrates by means of dynamic NMR and quantum mechanical calculation.

Cages and unit cell structures of the three common clathrate hydrates. Numbers on the left sides of the cages indicate the number of cages included in one unit cell, 51264 below the cages, for example, indicates a water cage composed of 12 pentagonal and 4 hexagonal faces, and 46 H2O on the right side of a unit cell, for example, shows that there are 46 water molecules in a unit cell of structure I.

To facilitate NMR study, we have developed method of Temperature-Gradient Assisted Gas-Dissolved Liquid Phase Synthesis of Clathrate Hydrates.

Photo pictures of the D2O/Xe system in a 5mm NMR tube. The metal plate under the NMR tube shows the top of the thermostage and the hole on it is where the NMR tube sitting in. The Teflon O-ring on the NMR tube was used to seal the chamber of the thermostage. (a) shows the gas hydrate after its crystallization on the bottom of the NMR tube for a short while, (b) the growth of the hydrate for 3 days, (c) for 6 day, and (d) for 7 days.

129Xe NMR spectra of the D2O/Xe gas hydrate (right) and the D2O/THF/Xe double gas hydrate (left).

Photo pictures of the D2O/propane system in the NMR tube during the formation of the gas hydrate.

13C NMR spectrum of the D2O/propane gas hydrate at -10°C.

Experimental 2H Quadrupolar Echo spectra of D2O/propane gas hydrate at -60°C,
-10°C and -5°C, respectively, from the bottom to the top.