Fluoride-incorporated ionic clathrate hydrates

Ionic clathrate hydrates are promising materials for hydrate-based gas storage and separation processes. Here, we demonstrated that the hydroxide ion in the cubic structure-II (CS-II) and hexagonal structure-III (HS-III) ionic clathrate hydrates can be replaced by fluoride. Me₄N⁺ and Et₂Me₂N⁺ cations were selected as guest species for the CS-II and HS-III hydrates, respectively. The crystal structure of each hydrate was identified through Rietveld analysis of the PXRD pattern. The Fd3 ¯ m structure (CS-II) of Me₄NF+N₂ or O₂ hydrates and the P6/mmm structure (HS-III) of Et₂Me₂NF+CH₄ hydrate were confirmed. We also investigated the phase equilibria of hydroxide or fluoride-incorporated CS-II and HS-III hydrate systems, and found that incorporating fluoride destabilizes the hydrate lattice to a greater extent than hydroxide. The present findings will provide better understanding of the guest-host interactions in ionic clathrate hydrates, and suggest their potential for practical applications in gas storage and separation technologies.