Characterization of cyclopentane clathrates with gaseous guests for gas storage and separation

Abstract

In this study, stability conditions and characterization of guest gas (CH4 and N-2)+ CP hydrates were investigated with a primary focus on phase equilibria and microscopic analyses. Four-phase (H-L-W-L-CP-V) equilibria of the CH4 + CP and N-2 + CP hydrates measured at the pressure range of 0-11 MPa demonstrated that CP could significantly reduce hydrate equilibrium pressure at any given temperature and, thus, could be used as a powerful thermodynamic promoter for gas hydrates containing CH4 and N-2. Powder X-ray diffraction revealed that guest gas (CH4 and N-2)+ CP hydrates are indexed using a cubic sII unit cell (space group Fd3m) and that the inclusion of guest gases does not affect the original structure of CP hydrates. It was confirmed from C-13 NMR spectra that CH4 molecules are enclathrated only in the small 512 cages of sII CH4 + CP hydrates and that the cage occupancy of CH4 molecules in the small 512 cages of sII CH4 + CP hydrates is relatively lower than that of sI CH4 hydrate. Time-dependent in situ Raman spectra also indicated the occupation of guest gases in the CP hydrate and an absence of structural transformation caused by the inclusion of guest gases. The findings of this study can broaden our understanding of cage-specific occupation of guest molecules in gas hydrates and, thus, provide important information on the application of CP hydrates to gas storage and separation.