Thermodynamic and Structural Investigation of Xe and Kr Hydrates Containing Tetrahydrofuran for Applications in Gas Capture and Storage

Abstract

Noble gases, such as xenon (Xe) and krypton (Kr), can be captured in hydrate cages, and thus, gas hydrates can be used as capture and storage media for these gases. This study examined the phase equilibria, crystal structure, and guest distributions of Xe and Kr hydrates in the presence of tetrahydrofuran (THF). The three-phase [hydrate (H)-liquid (L)-vapor (V)] equilibria of the Xe + THF + water and Kr + THF + water systems were measured at two different concentrations of THF (1.0 and 5.6 mol %) to examine the thermodynamic promotion effect of THF on Xe and Kr hydrates. The inclusion of THF in the hydrate cages led to significant shifts in the equilibrium curves of the Xe and Kr hydrates to lower pressure and higher temperature regions. The powder X-ray diffraction patterns and the Xe-129 and C-13 nuclear magnetic resonance spectra revealed that THF was enclathrated in the large (5(12)6(4)) cages of structure II hydrates and that the small (5(12)) cages were occupied by Xe (or Kr). In particular, at 1.0 mol % THF, the large 5(12)6(4) cages were partly occupied by Xe (or Kr) as a result of tuning behavior. The experimental results expand the basic understanding of hydrate-based gas capture and storage for Xe and Kr.