Anomalous preservation of CH4 hydrate and its dependence on the morphology of hexagonal ice.

Clathrate hydrates, also called gas hydrates, are crystalline ice-like inclusion compounds consisting of water molecules hydro-gen-bonded to form host cages that contain small molecules as guests. Because guest–host interactions play a crucial role, hydrates are thermodynamically stable only when guest molecules occupy the host cages to a certain minimum level. Some gas hydrates can be stored at atmospheric pressure below the melting point of ice (273.2 K) even though this is well outside the zone of thermodynamic stability of the hydrate, [1] an effect that has been termed self-preservation. [2] A mechanism for the dissociation of CH 4 hydrate (structure I with space group Pm 3 n ) at atmospheric pressure by a temperature-ramping procedure was given in terms of the rate of gas diffusion through an ice layer at temperatures below approximately 200 K. [3] Above approximately 240 K, delays of CH 4 hydrate dissociation due to ice transformations have been suggested. [4] However, the existence of unusually enhanced preservation phenomena of CH 4 hydrate as reported by Stern et al. in the temperature region from 240 K to 273.2 K [5] is one remaining puzzle, a phe-nomenon which has been termed anomalous preservation. The dissociation rates vary in a reproducible way with two minima at around 250 and 268 K whereas the dissociation is fast below 240 K and appears to be thermally activated. This anomalous preservation has been observed in this temperature region only for CH 4 hydrate upon dissociation by rapid pressure release from the high pressures at which it is stable, to an ambient pressure of CH 4 gas. On the other hand, we re-cently found that dissociation of gas hydrates by temperature-ramping depends on the interaction strength between guest molecules and H 2 O molecules, as reflected by the dissociation pressures of the various hydrates at 273 K. vessel kept below 263 K. Then, hydrate-forming gas, CH 4 , C 2 H 6 , CH 4 + C 2 H 6 (CH 4 :C 2 H 6 = 42.5:57.5) and C 3 H 8 was supplied slowly from high- pressure cylinders until the pressure inside the vessel was increased up to 5.0 MPa for CH 4 , 1.0 MPa for C 2 H 6 , 2.2 MPa for CH 4 + C 2 H 6 and 0.5 MPa for C 3 H 8 , respectively. The temperatures of the sample were slowly raised up to 274 K to transform all ice into gas hydrate. Then the temperatures of the sample were set to the temperatures for the decomposition run. After confirming that almost all of the ice sample transformed into hydrate by PXRD, dissociation measurements of each gas hydrates by the rapid pressure-re-lease method were performed at a constant temperature with a maximum deviation of 0.5 K and the pressure was left to be ambient under the hydrate-forming gas. In situ PXRD measurements were done in q / q step scan mode using Cu K a radiation ( l = 1.542 (cid:4)) with a step width of 0.04 8 in the 2 q range of 22–29 8 every 2.0 min (40 kV, 40 mA; BRUKER axs model D8 Advance equipped with solid-state detector model LynxEye). Grazing incident XRD measurements were done using the same diffractometer in detector fix mode. The angle of the incident X-ray tube with 0.10 mm di-vergence slit was fixed at 3.0, 5.0 and 9.0 degree to measure diffraction from different depths within a hydrate layer of 1.0 mm in thickness.