Spectroscopic Observation of Na Cations Entrapped in Small Cages of sII Propane Hydrate

Abstract

Although numerous studies have been conducted on various ionic clathrate hydrates, in spite of its potential importance the inclusion of metal cations in continuous water-host frameworks has not yet been clearly; identified by direct spectroscopic evidence. Here, a key question arises as to whether the small alkali metals such as Na+ can be entrapped because they are considered to be too small to be stabilized in the hydrate cages. In this study, we first suggested spectroscopic evidence for the enclathration of Na+ in a small cage of sII propane hydrate. First, we checked the overall structure of sII propane hydrate incorporated with NaSO3CH3 and NaSO3NH2 with powder XRD and C-13 NMR. Next, we revealed the difference of chemical shift of Na-23 between Ih and sll hydrate phases with solid-state Na-23 MAS NMR as direct evidence of entrapped Na+ in 5(12) cages. In addition, we also checked C-13 MAS NMR of the CH3SO3- anion and found that Na+ cations in a small cage could be stabilized with an enclathrated CH3SO3- anion in a large cage. To the best of our knowledge, it is the first discovery of small alkali cations stabilized in a continuous hydrate phase. Finally, we would like to emphasize that clathrate hydrate including small alkali metals can be designed and synthesized for its potential applications to various types of energy devices using its ionic mobility through the hydrate channel.