Phase transition of cubic structure II to tetragonal structure in monofluoromethane + 2-propanol clathrate hydrate

The thermal properties of clathrate hydrates are determined by the combination of water molecules and encapsulated types of guest molecules through the resulting crystal structure. Takaya et al have reported that the 2-propanol + CH₄ hydrate possesses a tetragonal structure II’ (sII′) below 110 K. This finding shows that thermodynamic conditions ( T, P ) could change the crystal structural transition in the same chemical system. This study investigated crystallographic characteristics of the HFC-41 + 2-propanol hydrate and demonstrated that a single-atom fluorination stabilizes the sII’, replaced with methane in small cages. We have employed variable-temperature powder X-ray diffraction (PXRD) measurement, and the results show that the binary HFC-41 + 2-propanol hydrate exhibits canonical cubic sII hydrate with a lattice constant of a=17.2259(5) Å. Cooling down to 93 K, HFC-41 + 2-propanol hydrate transits to a tetragonal structure (sII’) from sII with lattice constants a t =12.304(1) Å and c t =16.925(2) Å. The transition temperature from sII′ to sII is near 153 K, 40 K higher than that of the methane + 2-propanol hydrate. This result demonstrated that a single-atom fluorination stabilizes the sII’, which was replaced with methene in the small cage.