Identification of a mechanism of transformation of clathrate hydrate structures I to II or H

Binary mixed-gas hydrates including methane and other guest gases demonstrate a structural transition between the sI and sII phases. Under increasing pressure pure methane hydrate exhibits a phase transition first from sI to sII and then to sH. But the mechanism of the transformation from sI to sII or sH has not yet been identified. Recently, molecular dynamics simulations of methane hydrates suggest there may exist uncommon 15-hedral cages (51263), linking the sI and sII cages. In addition, xenon hydrate involving 15-hedral cages has been synthesized and named an hsI hydrate. Based on the hsI cages, we propose a mechanism for the transition of sI to sII or sH at atomic level resolution. The sI hydrate is first transformed to hsI, and hsI is further transformed to sII. Upon compression, hsI is transformed to sH owing to depletion of atomic layers. The mechanism of transformation speculated here calls for experimental verification.

A mechanism for the transition of sI to sII at the atomic level is proposed. The sI hydrate is first transformed to hsI, and hsI is further transformed to sII. 15-Hedral cages of hsI mediate the transition between sI and sII. We show visually the transition of sI to hsI to the right: In the first step, two waters transferred from one 14-hedron of sI transform the adjoining 14-hedron in sI into one 15-hedron of hsI. In the second step (not shown here), two waters transferred from one 14-hedron of hsI transform the adjoining 15-hedron in hsI into one 16-hedron of sII. The existence of 14-hedron in sI or hsI is essential in the transition.Figure optionsDownload high-quality image (267 K)Download as PowerPoint slideHighlight
► Three-dimensional structures of clathrate hydrates (sI, sII and sH) are constructed.
► These are based on the coordinates obtained by the X-ray crystallographic analysis.
► We analyze the transformations of these structures at the atomic level.
► A mechanism for the transition of sI to sII or sH is elucidated.