Cooperative Cage Filling Enables Mild-Condition CH4 Storage in Structure II Hydrates

This study proposes sulfur hexafluoride (SF6) as a promising gaseous promoter for methane (CH4) storage in clathrate hydrate. Phase-equilibrium measurements of binary SF6 + CH4 hydrate revealed that increasing the SF6 fraction shifted the equilibrium boundaries toward higher temperatures and lower pressures, indicating enhanced thermodynamic stability. Unlike conventional mixed-gas hydrate systems, the equilibrium curves of SF6 + CH4 hydrates were shifted to lower pressures and higher temperatures than that of pure SF6 hydrate. Powder X-ray diffraction confirmed that all binary hydrates crystallized in structure II, with the fitted lattice parameters showing a slight increasing trend with increasing SF6 content. Raman spectroscopy further supported preferential SF6 occupation of the large cages and CH4 occupation of the small cages, together with an increased relative contribution of CH4 at higher SF6 fractions. Deconvolution of the O–H stretching region showed the increase in strong hydrogen bonding and decrease in weak hydrogen bonding as SF6 composition increased. The binary SF6 + CH4 hydrates exhibited stronger hydrogen bonding than the pure SF6 hydrates. These findings establish the thermodynamic and structural basis for using SF6 as a potential gaseous promoter for hydrate-based CH4 storage.