Phase behavior and structural characterization of coexisting pure and mixed clathrate hydrates.

Abstract

Clathrate hydrates are crystalline compounds formed by a physically stable interaction between water and relatively small guest molecules. Under suitable conditions of guest pressure and temperature, water molecules, via hydrogen bonds, form into polyhedral cavities that connect to give space-filling frameworks. Because of partial cage filling, these crystalline compounds are nonstoichiometric. They can be divided into three distinct structural families I (sI), II (sII), and H (sH), which differ in their combination of cavities of different sizes and shapes. Although the above hydrate structures with natural gas components as guest molecules are widely known, new structures continue to be reported, such as a new, highly complex structure, known as structure T (sT) hydrate, that contains two cages of unusual geometry, and a new sII ± SH polytype. As well, a metastable sII hydrate of xenon has been observed: when a sII hydrate of tetrahydrofuran (THF) was placed in contact with hyperpolarized xenon. It is clear that many of the physical attributes of clathrate hydrates remain unknown and need to be identified in more detail. In our previous work, multiphase equilibria were measured to examine the complex phase behavior in the hydrate stability region for aqueous solutions containing carbon dioxide and methane as secondary guests. Although normally a single phase is expected to be stable in such systems, here we report the coexistence of sI and sII phases in a ternary system consisting of methane, THF, and water. This system was chosen especially to examine which structure would appear, depending on the relative concentration of the water-soluble THF molecules. THF by itself forms sII hydrate in a completely miscible aqueous solution, and in this structure, because of their size, THF guests occupy only the large 56 (H) cages. Furthermore, the mixed hydrate formed by THF and methane guests has been identified as sII, and this is independent of the THF to water ratio. However, stable hydrates in THF ±methane systems may have widely varying overall compositions, as methane may well occupy some of the large, as well as small, cages. In order to determine this, we followed two specific THF concentrations, 5.6 and 3.0 mol%, during hydrate formation. As an indirect measure of hydrate formation, the pressure ± temperature (P ± T) trajectory, which represents hydrate nucleation, growth, and dissociation stages, was measured experimentally and the resulting closed loop made by the hydrate formation and dissociation processes is shown in Figure 1a. The mixed CH4 THF hydrate started to form directly after going through the three-phase equilibrium boundary, and rapid pressure reduction occurred immediately