Classification of Clathrate Hydrates

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran–CO2 Binary Hydrate

“…The tetragonal hydrate crystal structure obtained here is not included in notations by Jeffrey 34 as well as Dyadin and Belosludov 35 and, based on incorporating H cages as its largest cage type, can be designated as TS-IV according to the systematic scheme of naming clathrate hydrate structures. 17 This thermally induced cubic to tetragonal phase transition is reversible and has been observed earlier for the binary 2propanol + CH 4 hydrate. 36 The low-temperature tetragonal sII′ unit cell shown in Figure 2a is described as 8D′•4H′• 68H 2 O (D′ = 5 12 and H′ = 5 12 6 4 , where the primes indicate geometric distortions compared to the sII cages) while the high-temperature cubic sII unit cell is described as 16D…”

“…The lattice constant of the a -axis in low-temperature sII′ is 1/√2 (= 0.707) times smaller than the sII lattice constant. The tetragonal hydrate crystal structure obtained here is not included in notations by Jeffrey as well as Dyadin and Belosludov and, based on incorporating H cages as its largest cage type, can be designated as TS-IV according to the systematic scheme of naming clathrate hydrate structures . This thermally induced cubic to tetragonal phase transition is reversible and has been observed earlier for the binary 2-propanol + CH 4 hydrate .…”

“…Further understanding of dynamical motion of guest molecules such as the rattling and host–guest hydrogen bonds − is essential for the development of more efficient gas storage technologies using clathrate hydrates. Three different crystal structures of clathrate hydrates are well-known under ambient or moderate pressure conditions depending on type of guest species; cubic structure I (sI) with space group Pm -3 n comprising two small 5 12 (D) cages and six large 5 12 6 2 (T) cages, cubic structure II (sII) with space group Fd -3 m comprising 16 D cages and eight large 5 12 6 4 (H) cages; and hexagonal structure H (sH) with space group P 6/ mmm comprising three D cages, two irregular dodecahedron 4 3 5 6 6 3 cages, and one large icosahedron 5 12 6 8 cage . The simple CO 2 hydrate forms the sI structure: CO 2 occupies 100% of the larger T cages and approximately 70% or less of the smaller D cages at moderate conditions (around 273 K and a few MPa). , In the simple CO 2 hydrate, the CO 2 in the T cage is distributed over a limited angle range around 90° to the short axis of the cage, and the CO 2 in the quasi-spherical D cage is distributed spherically in the center of the small cage. ,, Under low-pressure conditions, the crystal structure and dynamical disorder of CO 2 in its cages remain qualitatively unchanged without any phase transition occurring, down to low temperatures (<10 K) .…”

Microstructural Study on Dissolution of Natural Methane Hydrate by Multicontrast and Multiscale X-ray Computed Tomography

“…These phenomena are important for a fundamental understanding of phase transitions and nonequilibrium phenomena. With regard to natural resources on earth, CH 4 emission from CH 4 hydrates owing to their dissolution is of concern as a contributor to hydrate-related global climate change, along with the CH 4 seepage from the seafloor. − CH 4 hydrates are ice-like inclusion compounds in which CH 4 molecules as guests are encapsulated inside hydrogen-bonded water cages. , A comprehensive understanding of the crystal dissolution through multiscale analysis using experiments and simulations at the molecular , and crystal , levels of CH 4 hydrates , is required.…”

“…1−3 CH 4 hydrates are ice-like inclusion compounds in which CH 4 molecules as guests are encapsulated inside hydrogen-bonded water cages. 4,5 A comprehensive understanding of the crystal dissolution through multiscale analysis using experiments and simulations at the molecular 6,7 and crystal 8,9 levels of CH 4 hydrates 10,11 is required.…”